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New SHRIMP U–Pb zircon ages from
the Lachlan Orogen and the New
England Orogen, New South Wales
Mineral Systems Projects, July 2015–June 2016
K. Waltenberg, P. L. Blevin, K. F. Bull, D.E. Cronin and S. E. Armistead
Record 2016/28 | eCat 101681
Geological Survey of New South Wales Report GS2016/0810
APPLYING GEOSCIENCE TO AUSTRALIA’S MOST IMPORTANT CHALLENGES www.ga.gov.au
New SHRIMP U–Pb zircon ages from the
Lachlan Orogen and the New England
Orogen, New South Wales
Mineral Systems Projects, July 2015–June 2016
GEOSCIENCE AUSTRALIA
RECORD 2016/28
GEOLOGICAL SURVEY OF NEW SOUTH WALES
REPORT GS2016/0810
K. Waltenberg1, P. L. Blevin2, K. F. Bull2, D.E. Cronin2 and S. E. Armistead1
1. Resources Division, Geoscience Australia, GPO Box 378, Canberra, ACT, 2601
2. Geological Survey of New South Wales, Division of Resources and Energy, NSW Department of Industry, 516 High Street,
Maitland NSW, 2310
Department of Industry, Innovation and Science
Minister for Resources and Northern Australia: Senator the Hon Matthew Canavan
Assistant Minister for Industry, Innovation and Science: The Hon Craig Laundy MP
Secretary: Ms Glenys Beauchamp PSM
Geoscience Australia
Chief Executive Officer: Dr Chris Pigram
NSW Department of Industry
Minister for Industry, Resources and Energy: The Hon Anthony Roberts MP
Executive Director, Geological Survey of New South Wales: Dr Chris Yeats
This paper is published with the permission of the CEO, Geoscience Australia, and the Executive
Director, Geological Survey of New South Wales, NSW Department of Industry
© Commonwealth of Australia (Geoscience Australia) and State of New South Wales (NSW
Department of Industry) 2016
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ISSN 2201-702X (PDF)
ISBN 978-1-925297-29-4 (PDF)
eCat 101681
Bibliographic reference: Waltenberg, K., Blevin, P. L., Bull, K.F., Cronin, D.E. & Armistead, S.E.
2016. New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, New
South Wales: Mineral Systems Projects, July 2015–June 2016. Record 2016/28. Geoscience
Australia, Canberra; Report GS2016/0810, Geological Survey of New South Wales, Maitland.
http://dx.doi.org/10.11636/Record.2016.028
Contents
Executive Summary .................................................................................................................................. 1
Lachlan Orogen ..................................................................................................................................... 1
New England Orogen ............................................................................................................................. 2
1 Introduction ............................................................................................................................................ 3
2 Lachlan Orogen ..................................................................................................................................... 9
2.1 Babinda Volcanics ........................................................................................................................... 9
2.2 Shuttleton Rhyolite Member...........................................................................................................16
2.3 Unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah .................................................................23
2.4 Unnamed rhyolite, ‘younger volcanics’ at Yerranderie ..................................................................30
3 New England Orogen ..........................................................................................................................36
3.1 Bruxner Monzogranite ....................................................................................................................36
3.2 Jenny Lind Granite .........................................................................................................................42
3.3 Dumbudgery Creek Granodiorite ...................................................................................................47
3.4 Towgon Grange Tonalite ...............................................................................................................53
3.5 Newton Boyd Granodiorite .............................................................................................................58
3.6 Drake Volcanics at Red Rock ........................................................................................................64
3.7 Drake Volcanics at White Rock .....................................................................................................70
4 Discussion ...........................................................................................................................................76
4.1 Lachlan Orogen .............................................................................................................................76
4.2 New England Orogen .....................................................................................................................77
Acknowledgements ................................................................................................................................83
References .............................................................................................................................................84
Appendix A Analytical Procedures .........................................................................................................89
A.1 Summary .......................................................................................................................................89
A.2 Sample Preparation .......................................................................................................................89
A.3 Zircon Data Acquisition and Processing ........................................................................................90
A.4 Session-specific calibrations and data processing ........................................................................94
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 iii
iv New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Executive Summary
This record presents new zircon U–Pb geochronological data, obtained via Sensitive High Resolution
Ion Microprobe (SHRIMP) for eleven samples of plutonic and volcanic rocks from the Lachlan Orogen,
and the New England Orogen. The work is part of an ongoing Geochronology Project (Metals in
Time), conducted by the Geological Survey of New South Wales (GSNSW) and Geoscience Australia
(GA) under a National Collaboration Framework (NCF) agreement, to better understand the geological
and metallogenic evolution of New South Wales. The results herein (summarised in Table 1.1 and
Table 1.2) correspond to zircon U–Pb SHRIMP analyses undertaken on GSNSW mineral systems
projects for the reporting period July 2015–June 2016.
Lachlan Orogen
In the northern part of the central Lachlan Orogen, we have obtained two new ages for volcanic units
within the Cobar Supergroup: 418.9 ± 2.5 Ma for the I-type Babinda Volcanics of the shallow marine
Kopyje Group, and 421.9 ± 2.7 Ma for the S-type Shuttleton Rhyolite Member within the deep water
Amphitheatre Group. The new age for the Babinda Volcanics is indistinguishable from that of the
interfingered and underlying 419.3 ± 2.8 Ma Baledmund Formation (Bodorkos et al., 2015). This age is
also indistinguishable from at least five previously dated I-type volcanic and plutonic units within the
nearby Canbelego-Mineral Hill Volcanic Belt (422.2 ± 3.7 Ma–417.6 ± 3.2 Ma; Black, 2005, Black,
2007, Spandler, 1998, Downes et al., 2016). This may indicate eruption and emplacement of this belt
in a single event.
The new age for the 421.9 ± 2.7 Ma S-type Shuttleton Rhyolite Member within the deep water,
turbiditic Amphitheatre Group in the southern Cobar Basin is indistinguishable from the 422.8 ± 2.6 Ma
S-type Mount Halfway Volcanics (Chisholm et al., 2014a), the 422.5 ± 3.6 Ma S-type Gilgunnia Granite
(Downes et al., 2016), and a 422.8 ± 4.9 Ma inferred equivalent of the Gilgunnia Granite, suggesting
that volcanic activity in the region was accompanied by local coeval plutonism.
The new results in conjunction with previous dating and studies (summarised in Downes et al., 2016)
establish that significant igneous activity occurred between c. 423 and c. 418 Ma within the Cobar
region and comprised two compositionally distinct (I-type vs S-type) but broadly contemporaneous
belts of volcanics and comagmatic granite intrusions.
In the Temora region of the central Lachlan Orogen, the new age of 414.7 ± 2.6 Ma for the ‘Hobbs
Pipe’ quartz monzonite at the Mt Adrah gold prospect establishes an Early Devonian age for the
hosted gold mineralisation. This age is significantly younger than U–Pb SHRIMP zircon ages obtained
from the andesite hosting the Temora (Gidginbung) gold deposit (435.0 ± 5.0 Ma, Perkins et al., 1990;
436.4 ± 3.1 Ma, Lawrie et al., 2007), indicating the variable age of rocks hosting gold mineralisation
along the Gilmore Fault Zone.
In the eastern Lachlan Orogen near Yerranderie, a new age of 413.5 ± 2.3 Ma for an unnamed rhyolite
within the ‘younger volcanics’ is consistent with the existing age of 414.4 ± 2.9 Ma for the underlying
Barrallier Ignimbrite (Black, 2006), and indicates that most of the volcanic units of the Bindook Group
were erupted over a relatively short period. The new age indicates that the epithermal mineralisation
at Yerranderie (muscovite 40Ar/39Ar age = 372.1 ± 1.9 Ma; Downes, 2007) is not genetically related to
the host volcanic succession, but is instead related to a younger event in the eastern Lachlan Orogen,
such as magmatic activity within the Late Devonian Eden-Comerong-Yalwal Rift Zone.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 1
New England Orogen
Four units were dated from the Clarence River Supersuite in the New England Orogen. The
indistinguishable ages of the 256.0 ± 1.4 Ma Bruxner Monzogranite, the 255.3 ± 1.2 Ma Jenny Lind
Granite, the 255.0 ± 1.0 Ma Dumbudgery Creek Granodiorite, and the 255.4 ± 1.2 Ma Towgon Grange
Tonalite demonstrate that these granites are spatially and temporally related. While these four ages
are indistinguishable, the current age span for the Clarence River Supersuite, which currently includes
the 291.2 ± 2.0 Ma Kaloe Tonalite (Cawood et al., 2011), as well as pre-270 Ma phases of the
Barrington Tops Granodiorite (Waltenberg et al., 2015), is more than 40 million years. This wide age
range indicates that classification of granites into the Clarence River Supersuite requires refinement.
The Newton Boyd Granodiorite, dated to test whether it can be classified into the Herries Supersuite,
yielded a magmatic crystallisation age of 252.8 ± 1.0 Ma, similar to some previously dated units within
the Herries Supersuite. However, both the Herries Supersuite and Stanthorpe Supersuite (into which
the Herries Supersuite was reclassified by Donchak, 2013) incorporate units with a broad range of
ages—the age distribution for the Stanthorpe Supersuite currently spans 50 million years. This again
indicates that classification of granites in the New England Orogen in New South Wales needs
revisiting.
Two mineralised intrusions were dated from the Drake Volcanics, which remains nominally in the
Wandsworth Volcanic Group despite the 10 Myr gap to overlying units (Cross and Blevin, 2010). The
new magmatic crystallisation ages of 265.3 ± 1.4 Ma from the Red Rock locality and 265.3 ± 1.5 Ma
for the White Rock locality are the same age as a 264.4 ± 2.5 Ma sill in the middle section of the Drake
Volcanics (Cross and Blevin, 2010). This indicates that the middle to upper section of the Drake
Volcanics, including the mineralising intrusions, were emplaced within the space of 1–2 million years.
These results support a genetic and temporal link between the Au–Ag epithermal mineralisation at
White Rock and Red Rock and their host Drake Volcanic packages rather than to younger regional
plutonism (i.e., Stanthorpe Supersuite) or volcanism (i.e., Wandsworth Volcanics). The almost 10 Myr
gap between the Drake Volcanics and the oldest recognised units of the rest of the overlying
Wandsworth Volcanic Group supports recognition of the Drake Volcanics as a distinct unit.
2 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
1 Introduction
This record presents new zircon U–Pb geochronological data, obtained using a Sensitive High
Resolution Ion Micro-Probe (SHRIMP) for samples of three volcanic and one granitic rock from the
Lachlan Orogen, and two volcanic and five granitic rocks from the New England Orogen, New South
Wales. The work was carried out under the auspices of the National Collaboration Framework (NCF),
as part of the collaborative Geochronology Project between the Geological Survey of New South
Wales (GSNSW) and Geoscience Australia (GA) during the reporting period July 2015 to June 2016.
Table 1.1 Summary of sample identifiers, locations, stratigraphic units and results (magmatic crystallisation age
for all samples and associated 95% confidence limits) for the GSNSW-GA Geochronology Project 2015-2016
(Lachlan Orogen).
GSNSW SiteID
GA
SampleNo
GDA94
Latitude °S
GDA94
Longitude °E
Stratigraphic Name /
‘Informal Identifier’
206Pb/238U Age
(Ma)
NSWSJAF0102
2309502
32.00683
146.45979
Babinda Volcanics
418.9 ± 2.5 Ma
NSWSJAF0103
2309503
32.12456
146.09088
Shuttleton Rhyolite Member
421.9 ± 2.7 Ma
ERIVDEC12.01A 2309504 35.19821 147.91721 Unnamed quartz monzonite,
‘Hobbs Pipe’ at Mt Adrah
414.7 ± 2.6 Ma
PB-15-CHRON-01 2309505 34.12032 150.20607 Unnamed rhyolite, ‘younger
volcanics’ at Yerranderie
413.5 ± 2.3 Ma
Table 1.2 Summary of sample identifiers, locations, stratigraphic units and results (magmatic crystallisation age
for all samples and associated 95% confidence limits) for the GSNSW-GA Geochronology Project 2015-2016
(New England Orogen).
GSNSW SiteID
GA
SampleNo
GDA94
Latitude °S
GDA94
Longitude °E
Stratigraphic Name / ‘Informal
Identifier’
206Pb/238U Age
(Ma)
PB-15-NEO-1
2306495
28.87103
152.49406
Bruxner Monzogranite
256.0 ± 1.4 Ma
PB-15-NEO-4
2306500
28.80391
152.53398
Jenny Lind Granite
255.3 ± 1.2 Ma
PB-15-NEO-5 2306501 29.19510 152.52402 Dumbudgery Creek
Granodiorite
255.0 ± 1.0 Ma
PB-15-NEO-6
2306502
29.43769
152.61710
Towgon Grange Tonalite
255.4 ± 1.2 Ma
PB-15-NEO-7
2306503
29.83772
152.28973
Newton Boyd Granodiorite
252.8 ± 1.0 Ma
PB-15-NEO-9
2550240
28.83892
152.32211
Drake Volcanics at Red Rock
265.3 ± 1.4 Ma
PB-15-NEO-10
2550241
28.93478
152.33869
Drake Volcanics at White Rock
265.3 ± 1.5 Ma
This Record documents detailed results for each sample, encompassing sample location, geological
context, zircon descriptions, an evaluation of the relevant analytical data, and a brief geochronological
interpretation. The data and interpretations are also available via GA’s Geochron Delivery System
(http://www.ga.gov.au/geochron-sapub-web/).
A comprehensive description of sample acquisition and processing procedures, preparation and
analysis of SHRIMP mounts, and data reduction and presentation methods are included in the
Appendix, along with analytical session-specific details of the calibration data collected on the
reference 206Pb/238U and 207Pb/206 Pb zircons. Session specific calibrations are summarised in
Appendix Table A 2.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 3
Figure 1.1 Index map of New South Wales, derived from the Eastern Lachlan Orogen Geoscience Database (Glen
et al., 2006), showing the locations and extents (in pink) of the geological maps comprising Figure 1.2 to Figure 1.5.
Black text and grey lines denote the names and boundaries of 1:250 000 map sheet areas.
4 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 1.2 Geological map of the Cobar–Nymagee area, Lachlan Orogen, showing the locations of two samples
analysed and documented in this Record (yellow squares, labelled with GSNSW SiteIDs). Map data: 1:1 000 000
Surface Geology of Australia (Raymond et al., 2012). Extent of this map is shown in Figure 1.1. Towns are marked
as blue pentagons, localities as blue circles. Pink line = road, black lines = faults, blue pentagons = towns, blue
circles = localities.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 5
Figure 1.3 Geological map of the Cootamundra–Goulburn–Wagga Wagga–Canberra area, Lachlan Orogen,
showing the location of one sample analysed and documented in this Record (yellow square, labelled with GSNSW
SiteID). Map data: 1:1 000 000 Surface Geology of Australia (Raymond et al., 2012). Extent of this map is shown in
Figure 1.1. Pink line = road, black lines = faults, blue pentagons = towns, blue circles = localities.
6 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 1.4 Geological map of the Bathurst–Sydney–Goulburn–Wollongong area, Lachlan Orogen, showing the
location of one sample analysed and documented in this Record (yellow square, labelled with GSNSW SiteID). Map
data: 1:1 000 000 Surface Geology of Australia (Raymond et al., 2012). Extent of this map is shown in Figure 1.1.
Pink line = road, black lines = faults, blue pentagons = towns, grey pentagon = ghost town.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 7
Figure 1.5 Geological map of the Warwick–Grafton area, New England Orogen, showing the locations of seven
samples analysed and documented in this Record (yellow squares, labelled with GSNSW SiteIDs). Map data:
1:1 000 000 Surface Geology of Australia (Raymond et al., 2012). Extent of this map is shown in Figure 1.1. Pink line
= road, black lines = faults, blue pentagons = towns.
8 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
2 Lachlan Orogen
2.1 Babinda Volcanics
Table 2.1 Summary of results: Babinda Volcanics.
GA SampleNo
2309502
GSNSW SiteID
NSWSJAF0102
Parent Unit
Cobar Supergroup, Kopyje Group
Stratigraphic Unit
Babinda Volcanics
Informal Identifier
—
Lithology
Rhyolite
Province
Lachlan Orogen
1:250 000 Sheet
Nymagee (SI/55-02)
1:100 000 Sheet
Nymagee (8133)
Location (GDA94)
32.00683°S, 146.45979°E
Location (MGA94)
Zone 55; 448978 mE, 6458679 mN
Analytical Session No
160009 (see Appendix section A.4 for session details)
Interpreted Age
418.9 ± 2.5 Ma (n = 20)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
2.1.1 Sampling Details and Geological Relationships
The sample of Babinda Volcanics was taken from a large slab disturbed by road works, approximately
10 m from the northwestern side of Nymagee-Hermidale Road, 1.5 km northeast of its intersection
with Peisley Road (Figure 1.2). The sampling location is about 15 km northeast of the township of
Nymagee. The sample is a dark grey, moderately crystal-rich and locally vesicular porphyritic rhyolite
with prominent sub- to euhedral feldspar phenocrysts up to 5 mm in the siliceous matrix (Figure 2.1).
This unit is referred to as unit ‘Dkae’ of the Babinda Volcanics on the Nymagee 1:100 000 map sheet,
where it is described as a “rhyolite vesicular crystal tuff”.
The Babinda Volcanics are part of the Kopyje Group, which represents shallow marine sedimentation and
volcanism on the Kopyje Shelf in the Early Devonian (Gilligan and Byrnes, 1995). A SHRIMP U–Pb
magmatic crystallisation age will refine the understanding of relationships between the Babinda Volcanics
and other units within the Kopyje Group. The Baledmund Formation underlies and interfingers the Babinda
Volcanics (MacRae, 1987), and a texturally immature volcaniclastic sandstone within the Baledmund
Formation yielded a maximum age of deposition of 419.3 ± 2.8 Ma (Bodorkos et al., 2015).
The new age will also clarify the interpreted relationship of the Babinda Volcanics with nearby volcanic
centres and intrusive bodies. The Babinda Volcanics is one of four felsic, I-type, northwest-trending
volcanic centres within the Kopyje Group (the others being the Florida Volcanics to the north, and the
Majuba Volcanics and Mineral Hill Volcanics to the southeast) that are interpreted to be fissure-type
extrusives, formed along a linear fault system known as the Canbelego–Mineral Hill Volcanic Belt
(Felton, 1978; McQueen, 2008).
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 9
Figure 2.1 Hand specimen of the Babinda Volcanics (GSNSW NSWSJAF0102, GA 2309502).
2.1.2 Petrography
The sample of Babinda Volcanics is a monomictic breccia with amygdaloidal, perlitic, plagioclase-
porphyritic clasts. Clasts are <1 to 5 mm, angular to somewhat fluidal, and jigsaw-fit to somewhat
rotated (Figure 2.2). Very small (~0.1 mm long), aligned feldspars make-up the groundmass of the
porphyritic clasts. The breccia matrix comprises altered recrystallised glass.
The clasts, amygdales and matrix within the breccia has undergone extreme alteration to clay
minerals. Alteration to carbonate minerals is minor but consistent throughout the thin-section. One 3
mm long, rounded clast of fine-grained phaneritic granitic rock is included in the thin-section. The
origin of the clast is unclear—whether a cognate or accidental lithic clast.
This rock is interpreted to be an altered autobreccia on or near the margin of an amygdaloidal
intermediate coherent lava or shallow sill.
10 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.2 Thin section photomicrographs of the Babinda Volcanics (GSNSW NSWSJAF0102, GA 2309502). Top
left: Plane-polarised light image showing angular and commonly jigsaw-fit, amygdaloidal and plagioclase-porphyritic
clasts altered to clay minerals (brown) make-up this monomictic breccia interpreted to be an autobreccia. Scale bar
is 1 mm. Top right: Plane-polarised light image showing fluidal-shaped, vesicles filled with sericite and clay minerals
(amygdales) compacted and aligned in a porphyritic clast oriented top-right to bottom-left across the image. Scale
bar is 0.5 mm. Bottom left: Cross-polarised light image of top right image, showing finely felted (aligned) feldspars
within the amygdaloidal and porphyritic coherent clast.
2.1.3 Zircon Description
Mounted zircons (Figure 2.3) are transparent, colourless euhedral to subhedral grains with long axes
of 80–200 µm and aspect ratios of 1–3. Grains are prismatic with pyramidal terminations, though many
are fragmented. Inclusions are round to elongate blebs up to 50 µm in diameter.
Cathodoluminescence (CL) images (Figure 2.3) show that the grains are moderately but variably
luminescent, with diffuse oscillatory or sector zoning. Many grains feature rounded or truncated central
regions, with disconformable overgrowths.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 11
Figure 2.3 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from the
Babinda Volcanics (GSNSW NSWSJAF0102, GA 2309502). SHRIMP analysis sites are indicated and labelled with
grain and spot number. Scale bar is 100 µm.
2.1.4 U–Pb Isotopic Results
Thirty-one SHRIMP U–Pb isotopic analyses were carried out on 31 zircons (Figure 2.4, Table 2.2).
One analysis is characterised by relatively high common 206Pb (>2%): it is interpreted as unreliable,
and is not considered further.
The remaining 30 analyses are characterised by low to moderate U content (106–507 ppm,
median 228 ppm) and overall moderate Th/U ratios (0.15–2.42, median 0.70). Common 206Pb
proportions are predominantly low (maximum 1.24%, median 0.45%). They can be divided into four
groups (Table 2.2) based on textural, chemical and isotopic criteria:
Group 1 comprises 20 analyses derived from domains of moderate to high CL emission,
predominantly on overgrowths or from crystals with no visible core region, with low to moderate U
content (106–399 ppm, median 201 ppm) and variable Th/U (0.31–1.30, median 0.76). Their individual
206Pb/238U dates range between c. 427 Ma and c. 410 Ma, and form a statistically coherent weighted
mean date of 418.9 ± 2.5 Ma (95% confidence, MSWD = 1.05, P = 0.40).
Group 2 comprises five analyses with similar U content (208–422 ppm, median 293 ppm) and Th/U
(0.37–2.42, median 0.63) to Group 1. Two of these analyses are from dull CL emission overgrowths,
two are from zircon fragments with very broad, diffuse zoning and dull CL emission, and one is from
an oscillatory zoned core region with moderate CL emission. Their individual 206Pb/238U dates range
12 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
between c. 516 Ma and c. 503 Ma, and form a statistically coherent weighted mean date of 512 ± 7 Ma
(95% confidence, MSWD = 0.79, P = 0.53).
Group 3 comprises three analyses with similar U contents (209–507 ppm) and Th/U (0.15–1.01) to
Groups 1 and 2. Two analyses are located on broad, diffuse oscillatory overgrowths with dull CL
emission. The third analysis (28.1) is located on a core with bland and dull CL emission. Their
individual 206Pb/238U dates range between c. 584 Ma and c. 564 Ma. They give a combined weighted
mean date of 568 ± 18 Ma (95% confidence, MSWD = 1.05, P = 0.35).
Group 4 comprises two analyses with contrasting textures but similar U contents (133–246 ppm) and
Th/U (0.38–0.60), one from a core and one from a rim of two different disconformably overgrown
zircon grains. The analysis of the core (30.1) is concordant (0% discordant), located on a region of
very diffuse oscillatory zoning and yields a 207Pb/206Pb date of c. 1177 Ma. The analysis (5.1) located
on an oscillatory overgrowth is very discordant (+65%) and has a 207Pb/206Pb date of c. 1795 Ma.
2.1.5 Geochronological Interpretation
The weighted mean 206Pb/238U date of 418.9 ± 2.5 Ma obtained from the 20 analyses in Group 1 is
interpreted as the magmatic crystallisation age of the Babinda Volcanics. The 206Pb/238U dates
between c. 516 Ma and c. 503 Ma obtained from the five analyses in Group 2 are interpreted as
inherited cores and grains of Cambrian age. The 206Pb/238U dates between c. 584 Ma and c. 564 Ma
obtained from the three analyses in Group 3 are interpreted as Neoproterozoic inheritance. The
207Pb/206Pb dates of c. 1795 Ma and c 1177 Ma obtained from the two analyses in Group 4 are
interpreted as pre-Neoproterozoic inheritance.
The new magmatic crystallisation age of 418.9 ± 2.5 Ma for the Babinda Volcanics is indistinguishable
from the 419.3 ± 2.8 Ma maximum deposition age determined on the interfingered and underlying
Baledmund Formation (Bodorkos et al., 2015), indicating that these two units are closely related. The
significance of this new age is discussed in section 4.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 13
Figure 2.4 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Babinda
Volcanics (GSNSW NSWSJAF0102, GA 2309502). Horizontal black line is the mean 206Pb/238U date of Group 1,
grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, red fill: Cambrian
inheritance, orange fill: Neoproterozoic inheritance, yellow fill: pre-Neoproterozoic inheritance. No fill: unreliable
analyses. One discordant analysis from Group 4 is not shown.
14 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 2.2 SHRIMP U–Pb zircon data from the Babinda Volcanics (GSNSW NSWSJAF0102, GA 2309502).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Group 4: Pre-Neoproterozoic inheritance (n = 2; 207Pb/206Pb dates tabulated)
502.5.1
0.21
246
90
0.38
8.872
2.1
0.11133
1.6
1794.9
30.4
502.30.1
0.20
133
78
0.60
4.990
2.0
0.08087
1.4
1176.8
30.8
Group 3: Neoproterozoic inheritance (n = 3)
502.11.1
0.25
507
75
0.15
10.511
2.5
0.06148
0.7
584.4
13.8
502.26.1
0.53
209
204
1.01
10.737
1.4
0.06303
1.0
571.1
7.7
502.28.1
0.28
400
169
0.44
10.898
0.9
0.06147
0.8
564.4
5.1
Group 2: Cambrian inheritance (n = 5)
502.8.1
0.22
317
113
0.37
11.969
1.2
0.06123
0.9
516.2
6.1
502.12.1
0.16
422
210
0.51
11.996
1.1
0.06018
0.8
515.3
5.3
502.15.1
0.10
293
274
0.97
12.062
1.1
0.06005
0.9
513.0
5.5
502.25.1
0.21
271
166
0.63
12.059
1.3
0.06126
1.0
512.5
6.3
502.31.1
0.15
208
488
2.42
12.308
1.3
0.06065
1.1
502.8
6.1
Group 1: Magmatic crystallisation (n = 20)
502.18.1
0.91
336
118
0.36
14.480
0.9
0.06370
0.9
426.7
3.9
502.9.1
0.77
149
81
0.56
14.548
1.3
0.05958
1.4
425.3
5.4
502.22.1
0.99
175
174
1.03
14.566
1.3
0.05920
1.4
423.9
5.5
502.24.1
1.02
106
104
1.02
14.599
1.1
0.06231
1.7
422.9
4.7
502.10.1
0.51
203
73
0.37
14.685
1.0
0.06143
1.3
422.6
4.2
502.13.1
0.21
399
189
0.49
14.744
1.1
0.05693
0.9
422.2
4.7
502.21.1
0.41
145
105
0.74
14.721
1.0
0.05914
1.5
422.0
4.3
502.17.1
0.69
232
160
0.71
14.720
1.3
0.05973
1.2
420.9
5.2
502.2.1
1.01
198
131
0.69
14.675
1.0
0.06074
1.3
420.8
4.2
502.23.1
0.45
187
194
1.07
14.788
1.3
0.05987
1.3
420.0
5.4
502.19.1
0.35
239
119
0.51
14.820
1.2
0.05926
1.2
419.5
5.0
502.1.1
0.20
268
80
0.31
14.845
1.0
0.05897
1.1
419.4
4.0
502.16.1
0.44
309
106
0.35
14.866
1.2
0.05876
1.1
417.9
4.9
502.4.1
1.24
126
95
0.77
14.767
1.1
0.06377
1.6
417.3
4.5
502.7.1
0.56
116
99
0.89
14.902
1.1
0.06469
1.6
416.4
4.5
502.27.1
1.16
162
181
1.15
14.824
1.0
0.06405
1.4
416.1
4.3
502.14.1
0.76
260
283
1.12
14.942
1.0
0.05908
1.2
414.5
4.0
502.29.1
0.42
178
136
0.79
15.061
1.0
0.06085
1.4
412.7
4.1
502.6.1
0.73
253
278
1.13
15.043
1.2
0.05796
1.2
411.9
5.0
502.20.1
0.52
224
282
1.30
15.135
1.0
0.05878
1.2
410.4
4.0
Not considered: Common 206Pb >2% (n = 1)
502.3.1
14.33
326
163
0.52
13.517
1.5
0.17696
0.6
396.1
6.0
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 15
2.2 Shuttleton Rhyolite Member
Table 2.3 Summary of results: Shuttleton Rhyolite Member.
GA SampleNo
2309503
GSNSW SiteID
NSWSJAF0103
Parent Unit
Cobar Supergroup, Amphitheatre Group
Stratigraphic Unit
Shuttleton Rhyolite Member
Informal Identifier
—
Lithology
Rhyolite
Province
Lachlan Orogen
1:250 000 Sheet
Nymagee (SI/55-02)
1:100 000 Sheet
Nymagee (8133)
Location (GDA94)
32.12456°S, 146.09088°E
Location (MGA94)
Zone 55; 414243 mE, 6445395 mN
Analytical Session No
160009 (see Appendix section A.4 for session details)
Interpreted Age
421.9 ± 2.7 Ma (n = 22)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
2.2.1 Sampling Details and Geological Relationships
The sample of Shuttleton Rhyolite Member was taken from low lying outcrop directly adjacent to a
southeast trending dirt track on the northern side of a disused airfield about 1.5 km from the locality of
Shuttleton and about 20 km WSW of the township of Nymagee (Figure 1.2). The sample is a grey-
green, moderately crystal rich, porphyritic, coherent rhyolite (Figure 2.5). It comprises prominent
feldspar and quartz phenocrysts in a fine-grained siliceous matrix, and has a distinctive white
weathering rind. The same outcrop is described in MacRae (1987) as “a large outcrop of massive
rhyodacitic volcanic”, and its stratigraphic thickness is estimated at 180 m at this locality.
The Shuttleton Rhyolite Member is one of the few felsic volcanic units in the deep water turbiditic
Amphitheatre Group. MacRae (1987) included the Shuttleton Rhyolite Member in the Shume
Formation, which separates the upper and lower Amphitheatre Group, and this sample of the
Shuttleton Rhyolite Member is from near the base of the Shume Formation (MacRae, 1987).
A SHRIMP U–Pb magmatic crystallisation age on the Shuttleton Rhyolite Member will test the
relationships between the Amphitheatre Group and related units. The Amphitheatre Group
nonconformably overlies the Thule Granite (Suppel and Gilligan, 1993), which Chisholm et al. (2014a)
dated at 425.7 ± 2.4 Ma. Suppel and Gilligan (1993) suggested that the Amphitheatre Group
sediments were derived from the Erimeran Granite (427.1 ± 2.4 Ma, Black, 2007; 424.5 ± 2.6 Ma,
Downes et al., 2016) to the southeast, based on paleocurrent data, and the quartz-rich nature of the
sediments.
16 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.5 Hand specimen of the Shuttleton Rhyolite Member (GSNSW NSWSJAF0103, GA 2309503).
The new age will also test the relationship between the S-type Shuttleton Rhyolite Member and other
S-type volcanic and intrusive units in the region such as the 422.8 ± 2.6 Ma Mount Halfway Volcanics
(Chisholm et al., 2014a) and the 422.5 ± 3.6 Ma Gilgunnia Granite (Downes et al., 2016).
2.2.2 Petrography
This sample of the Shuttleton Rhyolite Member is a highly altered, felsic porphyritic coherent lava or
high-level sill. Phenocrysts are 1–5 mm, and include embayed quartz (3–5%), K-feldspar altered to
clay minerals (~3%), plagioclase altered to sericite and carbonate minerals (~2%), and biotite altered
to chlorite ± titanite (~3%) (Figure 2.6).
Common throughout the sample are phenocrysts, crystals or clasts altered to pinite and chlorite (~3%)
(Figure 2.6). At least one pinite-rich crystal or clast is hexagonal, suggesting it was cordierite, but the
majority of the objects are wispy and elongate in shape. Another more vaguely hexagonal example
has inclusions of fine-grained, intergrown feldspar within it, suggesting perhaps the altered shapes are
clasts of altered aluminum-rich lithic fragments. The wispy shapes define a flow-foliation in the
coherent rock.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 17
Figure 2.6 Thin section photomicrograph of the Shuttleton Rhyolite Member (GSNSW NSWSJAF0103, GA 2309503)
under plane-polarised light (left) and cross-polarised light (right). Top left, right: Plane- and cross-polarised images,
respectively, showing K-feldspar altered to clay minerals (upper left), plagioclase altered to sericite and carbonate
minerals (bottom and centre), quartz (centre-right), and wispy crystals or clasts altered to pinite and chlorite aligned
with the flow-foliation (top-left to bottom-right) across image. Scale bar is 1 mm. Bottom left, right: Plane- and cross-
polarised images, respectively, showing larger, prismatic to vaguely hexagonal shaped, crystal or clast altered to
pinite and chlorite with fine-grained feldspar inclusions. Scale bar is 1 mm.
2.2.3 Zircon Description
Mounted zircons (Figure 2.7) are transparent, colourless euhedral to subhedral grains, with long axes
of 50–220 µm and aspect ratios of 2–6. Unbroken grains tend to be slightly rounded, with pseudo-
pyramidal terminations. Round to elongate blebs up to 5 µm in diameter are moderately common.
Internal fractures are common, often present on the boundaries between central regions and
overgrowths.
Cathodoluminescence (CL) images (Figure 2.7) show that the grains are moderately luminescent and
have diffuse oscillatory zoning. Many grains feature rounded or truncated central regions, with
disconformable overgrowths.
18 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.7 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from the
Shuttleton Rhyolite Member (GSNSW NSWSJAF0103, GA 2309503). SHRIMP analysis sites are indicated and
labelled with grain and spot number. Scale bar is 100 µm.
2.2.4 U–Pb Isotopic Results
Thirty-three SHRIMP U–Pb isotopic analyses were carried out on 33 zircons (Figure 2.8, Table 2.4).
Three analyses are characterised by relatively high common 206Pb (>2%): they are interpreted as
unreliable, and are not considered further.
The remaining 30 analyses are characterised by moderate U content (193–686 ppm, median
448 ppm) and low Th/U ratios (0.06–0.84, median 0.19). Common 206Pb proportions are predominantly
low (maximum 1.84%, median 0.32%). They can be divided into four groups (Table 2.4) based on
textural, chemical and isotopic criteria:
Group 1 comprises 22 analyses derived from domains of low to moderate CL emission, predominantly on
overgrowths, with moderate U content (326–686 ppm, median 451 ppm) and low Th/U (0.07–0.22, median
0.19). Their individual 206Pb/238U dates range between c. 429 Ma and c. 411 Ma, and form a statistically
coherent weighted mean date of 421.9 ± 2.7 Ma (95% confidence, MSWD = 1.51, P = 0.06).
Group 2 comprises six analyses with similar U content (193–562 ppm, median 415 ppm) and Th/U
(0.06–0.84, median 0.27) to Group 1. These analyses are from core regions or broad, dull CL
emission overgrowths. Their individual 206Pb/238 U dates range between c. 578 Ma and c. 441 Ma.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 19
Group 3 is a single analysis with lower U content (216 ppm) and higher Th/U (0.70) than the bulk of
Group 1. It is located on a euhedral crystal with moderate CL emission and either a very thin or no
overgrowth. This analysis yields a 207Pb/206Pb date of c. 1098 Ma (1% discordance).
Group 4 is a single analysis with similar U content (403 ppm) and higher Th/U (0.48) to Group 1. This
analysis has a common 206Pb content of 1.84%, significantly higher than those in Group 1 (maximum
1.06%, median 0.36%), and is located on a domain that appears cracked under transmitted light. This
analysis yields a 206Pb/238U date of c. 392 Ma.
2.2.5 Geochronological Interpretation
The weighted mean 206Pb/238U date of 421.9 ± 2.7 Ma obtained from the 22 analyses in Group 1 is
interpreted as the magmatic crystallisation age of the Shuttleton Rhyolite Member. The 206Pb/238U
dates between c. 578 Ma and c. 441 Ma obtained from the six analyses in Group 2, and the
207Pb/206Pb date of c. 1098 Ma for the analysis in Group 3 are interpreted as inheritance. The
206Pb/238U date of c. 392 Ma (Group 4) is interpreted as having been affected by post-crystallisation
loss of radiogenic Pb.
The new magmatic crystallisation age of 421.9 ± 2.7 Ma for the Shuttleton Rhyolite Member is consistent
with the nonconformable deposition of the Amphitheatre Group onto the underlying 425.7 ± 2.4 Ma Thule
Granite (Chisholm et al., 2014a). The significance of this new age is discussed in section 4.
20 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.8 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the
Shuttleton Rhyolite Member (GSNSW NSWSJAF0103, GA 2309503). Horizontal black line is the mean 206Pb/238U
date of Group 1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, red
fill: 440-580 Ma inheritance, orange fill: pre-Neoproterozoic inheritance, yellow fill: Pb loss. No fill: unreliable
analyses.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 21
Table 2.4 SHRIMP U–Pb zircon data from the Shuttleton Rhyolite Member (GSNSW NSWSJAF0103, GA 2309503).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Group 3: Pre-Neoproterozoic inheritance (n = 1; 207Pb/206Pb date tabulated)
503.13.1
0.24
216
146
0.70
5.442
1.0
0.07816
0.7
1098.0
17.7
Group 2: 580–440 Ma inheritance (n = 6)
503.28.1
0.16
543
52
0.10
10.635
0.9
0.06445
2.5
578.5
5.1
503.30.1
0.00
193
12
0.06
11.304
2.0
0.06480
1.1
546.5
10.6
503.26.1
0.32
562
457
0.84
12.163
1.0
0.05889
1.1
507.8
4.8
503.16.1
0.26
451
122
0.28
12.460
1.2
0.05931
0.8
496.4
5.6
503.31.1
0.10
364
101
0.29
12.664
0.9
0.05950
0.9
489.5
4.4
503.21.1
0.37
378
96
0.26
14.082
1.4
0.05817
0.9
440.6
5.8
Group 1: Magmatic crystallisation (n = 22)
503.9.1
0.77
588
113
0.20
14.409
1.0
0.06191
0.8
429.3
4.3
503.20.1
0.68
686
46
0.07
14.474
1.2
0.06232
1.2
427.9
4.8
503.29.1
0.21
454
96
0.22
14.542
1.4
0.05705
0.9
427.8
5.9
503.17.1
0.45
393
76
0.20
14.535
1.0
0.05836
0.9
427.1
3.9
503.18.1
0.16
489
86
0.18
14.585
0.9
0.05671
0.8
426.8
3.8
503.24.1
0.10
499
90
0.19
14.631
1.0
0.05713
0.8
425.8
4.3
503.32.1
0.40
468
78
0.17
14.610
1.2
0.05767
0.8
425.1
4.8
503.15.1
0.45
354
66
0.19
14.638
1.2
0.05963
0.9
424.1
4.9
503.33.1
0.43
485
89
0.19
14.655
1.1
0.05821
1.5
423.8
4.4
503.25.1
0.29
542
113
0.22
14.694
0.9
0.05827
0.8
423.2
3.8
503.3.1
0.17
378
75
0.21
14.725
0.9
0.05690
1.0
422.8
3.9
503.19.1
0.41
334
49
0.15
14.693
1.1
0.05884
1.0
422.8
4.7
503.12.1
0.10
440
71
0.17
14.746
1.3
0.05803
0.9
422.6
5.3
503.11.1
0.33
447
79
0.18
14.722
1.4
0.05867
0.9
422.3
5.6
503.27.1
0.20
449
78
0.18
14.745
1.3
0.05833
0.9
422.2
5.2
503.6.1
1.06
376
68
0.19
14.623
1.1
0.06595
1.5
422.0
4.7
503.7.1
0.38
543
82
0.16
14.814
1.0
0.05755
0.8
419.5
4.1
503.8.1
0.33
535
102
0.20
14.824
0.9
0.05727
0.8
419.5
3.7
503.2.1
0.38
326
55
0.17
14.820
1.2
0.05813
1.0
419.4
4.7
503.10.1
0.24
403
81
0.21
14.986
1.1
0.05921
0.9
415.4
4.6
503.23.1
0.21
625
72
0.12
15.163
0.9
0.05767
0.7
410.9
3.7
503.4.1
0.63
418
78
0.19
15.104
0.9
0.06132
0.9
410.7
3.8
Group 4: Affected by loss of radiogenic Pb (n = 1)
503.14.1
1.84
403
187
0.48
15.674
1.4
0.07345
0.9
391.6
5.5
Not considered: Common 206Pb >2% (n = 3)
503.1.1
2.91
303
134
0.46
12.961
1.0
0.07703
1.6
465.7
4.5
503.22.1
11.06
1020
616
0.62
13.754
1.9
0.14687
10.3
403.9
10.1
503.5.1
4.44
342
274
0.83
15.846
0.9
0.09019
4.6
377.5
4.0
22 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
2.3 Unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah
Table 2.5 Summary of results: unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah.
GA SampleNo
2309504
GSNSW SiteID
ERIVDEC12.01A
Parent Unit
Stratigraphic Unit
Informal Identifier
‘Hobbs Pipe’
Lithology
Quartz monzonite
Province
Lachlan Orogen
1:250 000 Sheet
Wagga Wagga (SI/55-15)
1:100 000 Sheet
Tarcutta (8427)
Location (GDA94)
35.19821°S, 147.91721°E
Location (MGA94)
Zone 55; 583496 mE, 6104591 mN
Analytical Session No
160009 (see Appendix section A.4 for session details)
Interpreted Age
414.7 ± 2.6 Ma (n = 28)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
2.3.1 Sampling Details and Geological Relationships
The sample of quartz monzonite was taken from drillhole GHD001 (HQ half core, interval 899.42–
900.35 m) on tenement EL6372 currently held by Mount Adrah Gold Limited, about 17 km northwest
of the township of Adelong (Figure 1.3). The tenement hosts the Hobbs Pipe deposit, and drillhole
GHD001 displays over 1000 m of gold mineralisation. This sample was collected from the base of the
mineralised interval, and is described as a quartz monzonite with trace sulphide minerals (Sovereign
Gold Company Limited, 2016).
The Hobbs Pipe deposit sits on the Gilmore Fault Zone, a well-defined north–northwest structure that
hosts other notable gold deposits, including the Temora Gold Mine at Gidginbung, some 100 km to the
north. Lawrie et al. (2007) interpreted 436.4 ± 3.1 Ma zircons from the Temora mine as representing
the age of mineralisation, while others interpret these zircons to represent the age of host rock
intrusion (Fu et al., 2009).
The Hobbs Pipe deposit has been described as a quartz monzonite intrusion and the host unit a
pyrite-sericite altered felsic biotite quartz monzodiorite (King, 2014). The mineralisation at Hobbs Pipe
is interpreted to be an intrusion-related gold system, so a SHRIMP U–Pb magmatic crystallisation age
on the quartz monzonite host will help to constrain the timing of mineralisation in the area.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 23
Figure 2.9 Hand specimen of the unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah (GSNSW ERIVDEC12.01A,
GA 2309504).
2.3.2 Petrography
The unnamed quartz monzonite from Hobbs Pipe at Mt Adrah is a seriate, sericite and chlorite altered
quartz plagioclase monzonite (Figure 2.9, Figure 2.10). Extensively sericitised plagioclase is dominant
at approximately 50 modal% of original rock volume. Subhedral quartz of 2 mm to less than 1 mm
comprises 30 modal% of the rock. Pyrite (5%) is present interstitially and as phenocrysts and appears
to be associated with micas. Feldspars, occasionally perthitic or myrmekitic in texture, appear to be
compositionally zoned. Extensive sericitisation of cores is suggestive of a more calcic composition to
the cores, while extinction angles on rims suggest a more sodic composition. The texture of the
plagioclase with poikilitic quartz indicates a likely albitisation event. Alteration to sericite, chlorite and
minor to trace carbonate minerals affects approximately 30% of the sample. This particular sample is a
monzogranite or granodiorite based on modal abundances of minerals as per the IUGS QAP system.
24 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.10 Thin section photomicrograph of the unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah (GSNSW
ERIVDEC12.01A, GA 2309504) under cross-polarised light (left) and plane-polarised light (right), showing
mineralisation representative of typical ore zone material at Hobbs Pipe deposit. Note seriate texture, opaque pyrite,
a lack of zoning in feldspars and moderately intense sericitisation and albite alteration. Scale bar is 0.5 mm.
2.3.3 Zircon Description
Mounted zircons (Figure 2.11) are transparent, colourless, stubby, subhedral grains, with long axes of
50–150 µm and aspect ratios of 1–2. Unbroken grains are rare and rounded, with equal abundances
of both pyramidal and bladed terminations. Randomly aligned acicular inclusions are common. Round
to elongate blebs and mineral inclusions up to 10 µm in diameter are also common.
Cathodoluminescence (CL) images (Figure 2.11) show that the grains are moderately luminescent
with diffuse sector zoning and oscillatory zoning. Few grains feature rounded or truncated central
regions, with disconformable overgrowths.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 25
Figure 2.11 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah (GSNSW ERIVDEC12.01A, GA 2309504). SHRIMP
analysis sites are indicated and labelled with grain and spot number. Scale bar is 100 µm.
2.3.4 U–Pb Isotopic Results
Thirty-three SHRIMP U–Pb isotopic analyses were carried out on 33 zircons (Figure 2.12, Table 2.6).
One analysis was characterised by relatively high common 206Pb (>2%): it is interpreted as unreliable,
and is not considered further.
The remaining 32 analyses are characterised by low to moderate U content (93–593 ppm, median
212 ppm) and moderate Th/U ratios (0.27–0.70, median 0.38). Common 206Pb proportions are
predominantly low (maximum 1.65%, median 0.58%). They can be divided into four groups (Table 2.6)
based on textural, chemical and isotopic criteria:
Group 1 comprises 28 analyses derived from domains on centres and edges of crystals with moderate
CL emission and low-contrast oscillatory ± sector zoning, with moderate U content (93–498 ppm,
median 199 ppm) and moderate Th/U (0.27–0.51, median 0.37). Their individual 206Pb/238U dates
range between c. 424 Ma and c. 404 Ma, and display discernible excess scatter, defining a weighted
mean date of 414.7 ± 2.6 Ma (95% confidence, MSWD = 1.71, P = 0.01).
Group 2 is a single analysis derived from a core region with dark CL emission with similar U contents
(519 ppm) and Th/U (0.57) to Group 1. This analysis yields a 206Pb/238 U date of c. 515 Ma.
26 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Group 3 is a single analysis derived from a rounded core disconformably overgrown by broad
oscillatory-zoned zircon. This domain has similar U contents (202 ppm) and Th/U (0.70) to Group 1.
This analysis yields a 207Pb/206Pb date of c. 1621 Ma (0% discordance).
Group 4 comprises two analyses from zircons with domains chemically and texturally similar to Group
1, but with marginally younger dates (c. 394 Ma and c. 380 Ma).
2.3.5 Geochronological Interpretation
The weighted mean 206Pb/238U date of 414.7 ± 2.6 Ma obtained from the 28 analyses in Group 1 is
interpreted as the magmatic crystallisation age of the ‘Hobbs Pipe’ quartz monzonite at Mt Adrah.
Although the analyses display scatter beyond that expected from their analytical uncertainties
(MSW D = 1.71, P = 0.01), the median 206Pb/238U date for the same 28 analyses is identical, and its
95% confidence interval does not display significant asymmetry (415.3 +3.3/-2.9 Ma; 95% confidence),
which supports the interpretation of the 28 analyses in Group 1 as reflecting a single geological event.
The 206Pb/238U date of c. 515 Ma obtained from the analysis in Group 2, and the 207Pb/206Pb date of c.
1620 Ma for the analysis in Group 3 are interpreted as inheritance. The 206Pb/238U dates of c. 394 Ma
and c. 380 Ma (Group 4) are interpreted as having been affected by post-crystallisation loss of
radiogenic Pb.
The new magmatic crystallisation age constrains the maximum age of the hosted gold mineralisation
at Mt Adrah to 414.7 ± 2.6 Ma. The significance of this new age is discussed in section 4.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 27
Figure 2.12 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the
unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah (GSNSW ERIVDEC12.01A, GA 2309504). Horizontal black
line is the mean 206Pb/238U date of Group 1, grey shading is the corresponding 95% confidence interval. Purple fill:
magmatic crystallisation, red fill: Cambrian inheritance, yellow fill: Pb loss, no fill: unreliable analyses. Proterozoic
inheritance (one analysis) not shown on this plot.
28 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 2.6 SHRIMP U–Pb zircon data from the unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah (GSNSW
ERIVDEC12.01A, GA 2309504).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Group 3: Proterozoic inheritance (n = 1; 207Pb/206Pb date tabulated)
504.31.1
0.16
202
136
0.70
3.499
2.6
0.10116
0.9
1620.5
18.1
Group 2: Cambrian inheritance (n = 1)
504.27.1
0.15
519
285
0.57
12.007
1.3
0.05918
1.2
515.0
6.4
Group 1: Magmatic crystallisation (n = 28)
504.20.1
0.60
182
90
0.51
14.604
1.0
0.06064
2.1
424.5
4.2
504.32.1
0.51
230
108
0.49
14.658
1.0
0.05876
1.2
423.3
4.1
504.15.1
1.65
93
37
0.41
14.584
1.1
0.06460
1.8
420.7
4.8
504.28.1
0.29
213
62
0.30
14.791
1.0
0.06011
1.1
420.5
4.0
504.14.1
0.85
104
35
0.35
14.732
1.1
0.06328
2.8
419.9
4.6
504.30.1
0.72
134
43
0.33
14.764
1.5
0.05995
1.5
419.5
6.0
504.6.1
0.28
269
113
0.43
14.847
1.3
0.05959
1.1
419.0
5.4
504.25.1
0.67
183
56
0.31
14.806
1.0
0.06115
1.3
418.6
4.2
504.12.1
0.32
253
103
0.42
14.865
1.0
0.06081
1.2
418.4
4.0
504.10.1
0.63
210
55
0.27
14.823
1.0
0.06010
1.3
418.3
4.1
504.3.1
0.41
133
44
0.34
14.924
1.1
0.06310
1.6
416.4
4.4
504.22.1
0.65
188
67
0.37
14.907
1.0
0.06028
1.3
415.9
4.1
504.17.1
0.81
115
47
0.42
14.886
1.5
0.06376
1.7
415.9
5.9
504.26.1
0.34
310
119
0.40
14.971
1.1
0.05910
1.0
415.5
4.4
504.24.1
0.50
149
62
0.43
14.959
1.4
0.06129
1.4
415.2
5.7
504.21.1
0.53
214
59
0.28
15.000
1.2
0.05960
1.3
413.9
4.8
504.8.1
0.64
171
56
0.34
15.000
1.1
0.06112
1.4
413.5
4.3
504.29.1
0.12
108
43
0.41
15.095
1.6
0.06318
1.7
413.0
6.3
504.7.1
0.57
247
89
0.37
15.032
1.1
0.05975
1.2
412.9
4.6
504.33.1
1.00
217
81
0.38
14.975
1.0
0.06031
1.2
412.7
4.0
504.2.1
0.54
228
71
0.32
15.047
1.3
0.05935
1.3
412.6
5.2
504.23.1
1.37
111
35
0.32
14.932
1.4
0.06280
3.0
412.3
5.8
504.19.1
0.94
165
45
0.28
15.003
1.3
0.06102
1.4
412.2
5.3
504.18.1
0.35
252
97
0.40
15.115
1.0
0.05952
1.1
411.6
4.0
504.11.1
1.52
124
44
0.36
15.023
1.6
0.06374
1.6
409.3
6.5
504.4.1
0.71
248
69
0.29
15.346
1.0
0.05914
1.2
404.1
3.9
504.9.1
0.59
310
142
0.47
15.379
1.0
0.06154
1.0
403.8
3.8
504.5.1
0.37
498
192
0.40
15.416
0.9
0.05727
0.9
403.7
3.7
Group 4: Affected by loss of radiogenic Pb (n = 2)
504.16.1
0.41
593
284
0.49
15.812
0.9
0.05701
0.7
393.8
3.5
504.1.1
0.97
254
86
0.35
16.289
1.0
0.06033
1.3
380.5
3.7
Not considered: Common 206Pb >2% (n = 1)
504.13.1
4.54
636
236
0.38
16.337
0.9
0.09402
3.3
366.1
3.6
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 29
2.4 Unnamed rhyolite, ‘younger volcanics’ at Yerranderie
Table 2.7 Summary of results: unnamed rhyolite, ‘younger volcanics’ at Yerranderie.
GA SampleNo
2309505
GSNSW SiteID
PB-15-CHRON-01
Parent Unit
Bindook Group
Stratigraphic Unit
Informal Identifier
‘younger volcanics’
Lithology
Rhyolite
Province
Lachlan Orogen
1:250 000 Sheet
Wollongong (SI/56-09)
1:100 000 Sheet
Burragorang (8929)
Location (GDA94)
34.12032°S, 150.20607°E
Location (MGA94)
Zone 56; 242312 mE, 6220977 mN
Analytical Session No
160009 (see Appendix section A.4 for session details)
Interpreted Age
413.5 ± 2.3 Ma (n = 28)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
2.4.1 Sampling Details and Geological Relationships
This rhyolite sample was collected from a section of diamond drillhole Y-6 (depth 293–294 m) stored
at the Londonderry Core Library and originally drilled by Catawba International in 1969, about 1 km
west of the ghost town of Yerranderie, in the Yerranderie silver-gold-lead district (Figure 1.4). The
sample is a coherent pale mottled pink and pale cream-green crowded phenocrystic rhyolite
comprising phenocrysts of quartz, pink K-feldspar and greenish plagioclase up to 3 mm in a pale pink-
grey fine-grained groundmass (Figure 2.13).
The volcanic units at Yerranderie are interpreted to represent the final phase of volcanism within the
Bindook Group (Fergusson, 1980; Simpson et al., 1997; Downes, 2007), and overlie the 414.4 ± 2.9
Ma Barrallier Ignimbrite (Black, 2006).
Silver-gold-lead mineralisation at Yerranderie is interpreted to be part of an intermediate sulfidation
epithermal system. Downes (2007) dated the alteration at Yerranderie by 40Ar/39Ar dating of muscovite
grains and reported that the alteration associated with quartz-sulfide vein formation formed at 372.1 ±
1.9 Ma (2σ; MSWD = 1.05).
A new SHRIMP U–Pb magmatic crystallisation age will constrain the age of the final stages of volcanic
activity within the Bindook Group. The age of the host rhyolite will also constrain the maximum age of
mineralisation of the Yerranderie epithermal system.
30 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.13 Hand specimen of the unnamed rhyolite, ‘younger volcanics’ at Yerranderie (GSNSW PB-15-CHRON-
01, GA 2309505).
2.4.2 Petrography
This sample of rhyolite from Yerranderie is an extremely altered, crystal-, pumice- and shard-bearing
volcaniclastic rock (Figure 2.14). Crystals total about 20% of the rock, range from 0.1–1 mm, are
subangular to angular and commonly fragmental, unevenly distributed, and include quartz, feldspar
(altered to clay minerals ± sericite), muscovite (presumably after biotite) and zircon (trace–1%).
Pumice clasts are wispy, quartz-feldspar porphyritic, and completely altered to sericite and clay
minerals ± carbonate minerals. Small, cuspate shapes within the matrix, reminiscent of fragmented
pumice (i.e. altered bubble-wall shards) have been altered to sericite and clay minerals.
Fragmented crystals, pumice, and shards are interpreted to be pyroclasts, suggesting this rock
represents a syn-eruptive deposit. Welding textures are not evident in this sample, although the
shards present suggest a non-welded subaerial pyroclastic flow or fall deposit, or a subaqueous
equivalent.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 31
Figure 2.14 Thin section photomicrograph of the unnamed rhyolite, ‘younger volcanics’ at Yerranderie (GSNSW PB-
15-CHRON-01, GA 2309505). Left: Cross-polarised image showing wispy and porphyritic pumice clasts (left, right,
top), angular and fragmented crystals, and cuspate bubble-wall shards, all altered to sericite and clay minerals.
Scale bar is 1 mm. Right: Closer view in cross-polarised light showing cuspate shards altered to sericite and clay
minerals (left and centre) and poorly sorted and broken quartz, feldspar and muscovite crystals. Matrix consists of
glass altered to clay minerals. Scale bar is 0.5 mm.
2.4.3 Zircon Description
Mounted zircons (Figure 2.15) are transparent, colourless to pale pink, subhedral grains, with long
axes of 35–250 µm and aspect ratios of 1–3. Unbroken grains are slightly rounded with pyramidal
terminations. Inclusions are round to elongate blebs up to 10 µm in diameter, and acicular inclusions
up to 20 µm, a large proportion of which are axially aligned.
Cathodoluminescence (CL) images (Figure 2.15) show that the grains are moderately luminescent
and have fine-scale oscillatory zoning. Many grains feature rounded or truncated central regions,
sometimes with convolute or chaotically disrupted CL zoning with disconformable overgrowths.
2.4.4 U–Pb Isotopic Results
Twenty-eight SHRIMP U–Pb isotopic analyses were carried out on 28 zircons (Figure 2.16, Table 2.8).
These analyses are characterised by low–moderate U content (101–648 ppm, median 178 ppm) and
moderate Th/U ratios (0.55–1.37, median 0.61). Common 206Pb proportions are predominantly low
(maximum 1.7%, median 0.4%).
Group 1 comprises all 28 analyses derived from centres and edges of crystals with moderate CL
emission and low-contrast oscillatory zoning. Their individual 206Pb/238U dates range between c.
427 Ma and c. 404 Ma (Figure 2.16), and form a statistically coherent weighted mean date of
413.5 ± 2.3 Ma (95% confidence, MSWD = 1.26, P = 0.17).
32 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 2.15 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the unnamed rhyolite, ‘younger volcanics’ at Yerranderie (GSNSW PB-15-CHRON-01, GA 2309505). SHRIMP
analysis sites are indicated and labelled with grain and spot number. Scale bar is 100 µm.
2.4.5 Geochronological Interpretation
The weighted mean 206Pb/238U date of 413.5 ± 2.3 Ma obtained from the 28 analyses in Group 1 is
interpreted as the magmatic crystallisation age of the unnamed rhyolite, ‘younger volcanics’ at
Yerranderie. The new SHRIMP U–Pb age is consistent with the existing age of 414.4 ± 2.9 Ma for the
underlying Barrallier Ignimbrite (Black, 2006), and constrains the duration of the bulk of the Bindook
Group to this timespan. The significance of this new age is discussed in section 4.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 33
Figure 2.16 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the
unnamed rhyolite, ‘younger volcanics’ at Yerranderie (GSNSW PB-15-CHRON-01, GA 2309505). Horizontal black
line is the mean 206Pb/238U date of Group 1, grey shading is the corresponding 95% confidence interval. Purple fill:
magmatic crystallisation.
34 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 2.8 SHRIMP U–Pb zircon data from the unnamed rhyolite, ‘younger volcanics’ at Yerranderie (GSNSW PB-
15-CHRON-01, GA 2309505).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Group 1: Magmatic crystallisation (n = 28)
505.19.1
0.56
169
127
0.78
14.537
1.6
0.06191
1.4
427.1
7.6
505.10.1
0.39
155
86
0.57
14.742
1.0
0.05960
1.6
421.8
4.8
505.8.1
0.22
194
111
0.59
14.795
1.0
0.05947
1.3
420.0
4.6
505.24.1
0.40
227
133
0.61
14.879
1.0
0.05999
1.1
418.0
4.4
505.5.1
0.32
123
67
0.56
14.899
1.3
0.06121
1.6
414.6
6.1
505.28.1
0.56
228
157
0.71
14.867
1.5
0.05875
1.2
419.1
7.2
505.6.1
0.22
139
74
0.55
14.923
1.5
0.06200
1.5
412.5
6.9
505.21.1
0.44
175
92
0.55
14.918
1.0
0.06105
1.4
415.9
4.6
505.20.1
0.40
322
288
0.92
14.932
1.0
0.05806
1.0
416.5
4.6
505.11.1
0.34
141
80
0.58
14.944
1.7
0.06266
1.5
414.6
7.8
505.27.1
0.64
233
141
0.63
14.914
1.0
0.05859
1.2
417.4
4.4
505.12.1
0.31
201
118
0.61
14.983
1.0
0.06122
1.3
413.9
4.5
505.23.1
0.08
480
599
1.29
15.034
1.3
0.05795
0.9
413.3
6.4
505.25.1
1.07
145
117
0.84
14.890
1.1
0.06197
1.5
415.4
5.2
505.26.1
0.14
648
858
1.37
15.033
0.9
0.05602
0.7
413.2
4.7
505.9.1
0.50
101
56
0.57
14.990
1.1
0.06414
1.7
411.4
5.1
505.3.1
0.42
248
180
0.75
15.027
1.3
0.05977
1.2
410.6
6.1
505.17.1
0.32
380
220
0.60
15.079
0.9
0.05864
0.9
412.4
4.2
505.1.1
0.33
194
112
0.60
15.096
1.5
0.06249
1.3
411.3
6.7
505.16.1
0.65
230
125
0.56
15.083
1.4
0.05898
1.2
412.1
6.1
505.2.1
0.73
132
77
0.60
15.133
1.8
0.06286
1.6
410.0
8.1
505.18.1
0.85
140
79
0.58
15.116
1.3
0.06477
1.5
409.8
5.9
505.13.1
1.01
129
105
0.84
15.101
1.1
0.06274
1.6
407.2
5.2
505.15.1
0.81
176
100
0.59
15.160
1.0
0.06135
1.3
408.4
4.6
505.22.1
0.63
179
114
0.66
15.235
1.3
0.06148
1.3
406.1
5.8
505.4.1
0.95
159
94
0.61
15.272
1.0
0.06308
1.4
406.9
4.6
505.7.1
0.22
195
106
0.56
15.421
1.0
0.05914
1.3
403.4
4.5
505.14.1
1.73
163
126
0.80
15.194
1.0
0.06504
1.4
403.8
4.9
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 35
3 New England Orogen
3.1 Bruxner Monzogranite
Table 3.1 Summary of results: Bruxner Monzogranite.
GA SampleNo
2306495
GSNSW SiteID
PB-15-NEO-1
Parent Unit
Clarence River Supersuite, Bruxner Suite
Stratigraphic Unit
Bruxner Monzogranite
Informal Identifier
—
Lithology
Granite
Province
New England Orogen
1:250 000 Sheet
Warwick (SH/56-2)
1:100 000 Sheet
Drake (9340)
Location (GDA94)
28.87103°S, 152.49406°E
Location (MGA94)
Zone 56; 450660 mE, 6806197 mN
Analytical Session No
150080 (see Appendix section A.4 for session details)
Interpreted Age
256.0 ± 1.4 Ma (n = 26)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.1.1 Sampling Details and Geological Relationships
The sample was collected along Bruxner Road off the Bruxner Highway, near a Franciscan hermitage
(Figure 1.5). The outcrop comprises about 100 m of bulldozed, disturbed and in-situ granite on the elevated
(south) side of the roadside. The sample is a medium-grained, coherent, inequigranular orange-pink rock
(Figure 3.1). Plagioclase, alkali feldspar and subordinate quartz comprise the bulk of the rock.
Pseudomorphic iron-oxide mineral boxworks suggest the majority of sulphide minerals have been altered,
but minor fresh sulphide minerals remain. The rock is pegmatitic in places, with coarse-grained veins and
zones and minor hematised miarolitic cavities. The rock has experienced low-grade alteration, consistent
with extensive subsolidus low temperature alteration, widespread throughout the granites in the region.
Minor chloritisation (after ferromagnesian minerals and/or plagioclase) and kaolinitisation (after plagioclase)
has occurred in some areas. The weathering rind is only millimetres thick.
The Bruxner Monzogranite is currently classified into the Clarence River Supersuite (Bryant et al.,
1997a). A SHRIMP U–Pb magmatic crystallisation age will allow comparison with previously dated
granites of the Clarence River Supersuite, and refine the regional chronology of magmatism.
36 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.1 Representative photographs of the Bruxner Monzogranite (GSNSW PB-15-NEO-1, GA 2306495) in
outcrop (left) and in hand specimen (right).
3.1.2 Petrography
The sample of Bruxner Monzogranite is medium-grained with early crystallising subhedral tabular
plagioclase with later crystallising interstitial alkali feldspar and anhedral to late quartz (Figure 3.2).
The latter is often present as small blocky quartz crystals along boundaries between the plagioclase
and in an interlocking to occasionally graphic relationship with the similarly late-crystallising alkali
feldspar. Late biotite (2–5 modal %) is almost entirely replaced by chlorite, titanium-oxide minerals and
sericite. Both feldspars are heavily clouded: plagioclase is altered to carbonate minerals, sericite and
clay minerals, and alkali feldspar is very turbid due to very fine-grained pink material. Late interstially
crystallising zones of quartz are somewhat graphic, with intersticies infilled with carbonate minerals.
Iron-oxide stained spots are present after sulphide minerals (also observed in hand specimen). The
texture of the sample is consistent with high level emplacement and crystallisation with late stage
volatile exsolution during crystallisation leading to graphic textures and miarolitic cavities. The rock
has a strong lower temperature overprint of chlorite, carbonate minerals, sericite and hematite, all of
which comprise space infill and also replace magmatic mineral phases.
3.1.3 Zircon Description
Mounted zircons (Figure 3.3) are transparent, colourless or faintly yellow subhedral grains that have
long axes of 80–400 µm and aspect ratios of 1.5–4. Grains are prismatic with pyramidal terminations.
Inclusions are round to elongate blebs up to 30 µm in diameter, and randomly oriented acicular
inclusions.
Cathodoluminescence (CL) (Figure 3.3) images show that the grains are moderately luminescent and
have moderate to diffuse oscillatory zoning, or occasionally sector zoning. Many grains feature central
regions with convolute or chaotically disrupted CL zoning.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 37
Figure 3.2 Representative plane-polarised light photomicrograph of the medium-grained inequigranular to porphyritic
Bruxner Monzogranite (GSNSW PB-15-NEO-1, GA 2306495). The field of view shows cloudy and flecked subhedral
plagioclase (lower central), and areas of later crystallising quartz (white) and alkali feldspar (strongly turbid).
Ferromagnesian minerals are completely replaced by chlorite and/or sericite. Scale is 1 mm.
3.1.4 U–Pb Isotopic Results and Interpretation
Thirty-seven SHRIMP U–Pb isotopic analyses were carried out on 37 zircons (Figure 3.4, Table 3.2).
These analyses are characterised by low to moderate U content (87–609 ppm, median 205 ppm) and
moderate Th/U ratios (0.37–2.62, median 0.56). Common 206Pb proportions are predominantly low
(maximum 1.7%, median 0.2%).
Three spots (24.1, 27.1, 23.1) targeted core regions and yield much older ages (c. 332 Ma, c. 324 Ma,
c. 313 Ma respectively) than the bulk population. These analyses are interpreted to represent
inheritance, and are excluded from the calculation of the magmatic crystallisation age.
When grouped, the remaining 34 analyses are characterised by relatively high MSWD (2.97) and low
P (0.00), which implies the presence of excess scatter in the data, beyond that attributable to the
analytical uncertainties. Pervasive alteration throughout the granite, as indicated by petrographic
evidence (sections 3.1.1, 3.1.2), may have resulted in radiogenic Pb loss from a subset of the zircon
population. Using this justification, data from the younger side of the population was progressively
removed until P >0.05. Eight data points were removed in this manner.
The remainder of the analyses (26 spots from 26 grains) span the range c. 262–250 Ma and form a
statistically coherent population with a weighted mean date of 256.0 ± 1.4 Ma (95% confidence;
MSWD = 1.49, P = 0.06). This is interpreted as the magmatic crystallisation age for the Bruxner
Monzogranite. The significance of this new age is discussed in section 4.2.1.
38 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.3 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from the
Bruxner Monzogranite (GSNSW PB-15-NEO-1, GA 2306495). SHRIMP analysis sites are indicated and labelled with
grain and spot number. Scale bar is 100 µm.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 39
Figure 3.4 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Bruxner
Monzogranite (GSNSW PB-15-NEO-1, GA 2306495). Horizontal black line is the mean 206Pb/238U date of Group 1,
grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, red fill: inheritance,
yellow fill: Pb loss.
40 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 3.2 SHRIMP U–Pb zircon data from the Bruxner Monzogranite (GSNSW PB-15-NEO-1, GA 2306495).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Inheritance (n = 3)
495.24.1
0.04
510
460
0.93
18.949
0.9
0.05300
1.1
331.5
2.8
495.27.1
0.00
609
507
0.86
19.375
0.8
0.05369
1.6
324.4
2.6
495.23.1
0.13
152
386
2.62
20.106
1.6
0.05254
2.4
312.9
5.0
Magmatic crystallisation (n = 26)
495.8.1
0.53
207
102
0.51
24.150
0.9
0.04833
3.5
261.6
2.4
495.5.1
0.83
198
86
0.45
24.165
1.0
0.04657
4.3
261.4
2.6
495.32.1
0.16
222
125
0.58
24.260
1.0
0.05165
2.4
260.4
2.5
495.18.1
-0.20
353
226
0.66
24.273
1.3
0.05300
1.9
260.3
3.3
495.7.1
-0.09
239
129
0.56
24.392
0.9
0.05313
1.9
259.0
2.3
495.2.1
-0.10
116
45
0.40
24.440
1.4
0.05469
2.7
258.5
3.6
495.31.1
0.21
181
142
0.81
24.529
1.0
0.04858
3.0
257.6
2.6
495.37.1
-0.32
219
166
0.78
24.550
0.9
0.05461
2.5
257.4
2.4
495.1.1
-0.12
178
78
0.45
24.581
1.0
0.05186
2.3
257.1
2.4
495.15.1
0.59
260
147
0.58
24.589
1.2
0.04874
3.3
257.0
3.1
495.11.1
-0.44
205
121
0.61
24.593
1.0
0.05540
2.8
256.9
2.4
495.21.1
0.56
216
109
0.52
24.611
1.0
0.04904
3.5
256.7
2.4
495.9.1
0.30
224
147
0.68
24.620
1.1
0.04986
2.7
256.7
2.9
495.13.1
0.04
258
175
0.70
24.723
0.9
0.05076
1.8
255.6
2.3
495.10.1
0.34
170
88
0.54
24.725
1.8
0.04800
3.4
255.6
4.5
495.14.1
-0.71
116
63
0.56
24.764
1.1
0.05865
4.2
255.2
2.8
495.16.1
0.28
216
101
0.48
24.799
1.5
0.05121
2.8
254.8
3.8
495.36.1
0.09
135
74
0.57
24.854
1.0
0.05009
2.8
254.3
2.6
495.4.1
0.28
157
81
0.54
24.892
1.5
0.05087
3.1
253.9
3.8
495.6.1
0.52
104
42
0.42
24.911
1.9
0.04728
6.2
253.7
4.6
495.3.1
-0.05
201
100
0.52
25.020
0.9
0.05270
2.0
252.6
2.3
495.26.1
0.57
218
117
0.55
25.044
1.0
0.04748
3.7
252.4
2.4
495.20.1
0.75
159
80
0.52
25.116
1.3
0.04512
5.0
251.7
3.2
495.28.1
-0.08
431
310
0.74
25.120
0.9
0.05318
1.5
251.7
2.1
495.29.1
0.26
274
144
0.54
25.180
1.5
0.05123
2.4
251.1
3.8
495.30.1
0.28
217
102
0.49
25.252
0.9
0.04841
2.9
250.4
2.3
Pb loss (n = 8)
495.25.1
0.13
294
161
0.57
25.325
0.9
0.05061
2.0
249.6
2.2
495.22.1
1.71
106
48
0.47
25.349
1.8
0.04090
9.6
249.4
4.3
495.33.1
0.11
110
49
0.46
25.353
1.1
0.05268
3.2
249.4
2.7
495.35.1
0.37
332
321
1.00
25.380
0.9
0.04773
2.6
249.1
2.2
495.17.1
0.64
111
63
0.59
25.418
1.9
0.04791
5.3
248.8
4.7
495.34.1
1.15
87
42
0.50
25.428
1.7
0.04560
8.7
248.7
4.2
495.19.1
0.06
202
73
0.37
25.429
1.0
0.05478
2.2
248.7
2.3
495.12.1
0.99
203
132
0.67
26.115
1.0
0.04664
5.9
242.2
2.3
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 41
3.2 Jenny Lind Granite
Table 3.3 Summary of results: Jenny Lind Granite.
GA SampleNo
2306500
GSNSW SiteID
PB-15-NEO-4
Parent Unit
Clarence River Supersuite, Jenny Lind Suite
Stratigraphic Unit
Jenny Lind Granite
Informal Identifier
—
Lithology
Tonalite
Province
New England Orogen
1:250 000 Sheet
Warwick (SH/56-2)
1:100 000 Sheet
Bonalbo (9440)
Location (GDA94)
28.80391°S, 152.53398°E
Location (MGA94)
Zone 56; 454524 mE, 6813650 mN
Analytical Session No
150080 (see Appendix section A.4 for session details)
Interpreted Age
255.3 ± 1.2 Ma (n = 30)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.2.1 Sampling Details and Geological Relationships
The sample of Jenny Lind Granite was collected from boulders 10 metres from the western side of
Hootons Road near a cattle grid and homestead, amongst an area of low outcropping granite
whalebacks (Figure 1.5). The sample is a medium-grained, equigranular, plagioclase-dominated
tonalitic rock (Figure 3.5). The rock contains abundant amphibole to 4 mm, and chloritised biotite.
Although the unit is named granite, this sample is tonalitic in character.
The Jenny Lind Granite is currently classified into the Clarence River Supersuite (Bryant et al., 1997a).
A SHRIMP U–Pb magmatic crystallisation age will allow comparison with previously dated granites of
the Clarence River Supersuite, and refine the regional chronology of magmatism.
Figure 3.5 Representative photographs of the Jenny Lind Granite (GSNSW PB-15-NEO-4, GA 2306500) in outcrop
(left), and hand specimen (right).
42 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
3.2.2 Petrography
The sample of Jenny Lind Granite contains well zoned tabular plagioclase (25–30%) with flecked
cores (Figure 3.6). Amphibole (light brown to dark green pleochroism) is euhedral to subhedral in form
when forming isolated discrete crystals, but also exists as large glomerophyric aggregates, often
felted, which are recrystallised after pyroxene. Biotite is characteristically altered to chlorite. Quartz (10
modal %) is texturally late and blocky to anhedral, and alkali feldspar (<10 modal %) is late and
relatively unclouded. Perthitic exsolutions in the alkali feldspar are not prominent.
Figure 3.6 Representative cross-polarised light photomicrograph of the medium-grained Jenny Lind Granite
(GSNSW PB-15-NEO-4, GA 2306500) that consists of early formed zoned plagioclase crystals and glomerophyric
aggregates of amphibole and biotite (centre right and centre left). Quartz and alkali feldspar is relatively late and
anhedral consistent with the relatively low SiO2 and K2O whole rock composition. Scale is 1 mm.
3.2.3 Zircon Description
Mounted zircons (Figure 3.7) are transparent, colourless or faintly yellow subhedral grains that have
long axes of 75–350 µm and aspect ratios of 1.5–4. Grains are prismatic with pyramidal terminations.
Inclusions are round to elongate blebs up to 50 µm in diameter, and randomly oriented acicular
inclusions.
Cathodoluminescence (CL) images (Figure 3.7) show that the grains are moderately luminescent and
have moderate to diffuse oscillatory zoning, or occasionally sector zoning. Many grains feature central
regions with convolute or chaotically disrupted CL zoning. Some grains have central regions which
appear to be partially resorbed.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 43
Figure 3.7 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from the
Jenny Lind Granite (GSNSW PB-15-NEO-4, GA 2306500). SHRIMP analysis sites are indicated and labelled with
grain and spot number. Scale bar is 100 µm.
3.2.4 U–Pb Isotopic Results and Interpretation
Thirty-one SHRIMP U–Pb isotopic analyses were carried out on 31 zircons (Figure 3.8, Table 3.4).
These analyses are characterised by low to moderate U content (64–479 ppm, median 134 ppm) and
moderate to high Th/U ratios (0.49–1.16, median 0.80). Common 206Pb proportions are predominantly
low (maximum 1.7%, median 0.2%).
A single analysis yields a date (30.1, c. 218 Ma) significantly younger than the bulk population. This
analysis is interpreted to be affected by loss of radiogenic Pb and is not considered for the purposes of
magmatic crystallisation age determination.
The remainder of the analyses (30 spots from 30 grains) span the range 260–248 Ma and form a
statistically coherent population with a weighted mean date of 255.3 ± 1.2 Ma (95% confidence;
MSWD = 0.92, P = 0.58). This is interpreted as the magmatic crystallisation age for the Jenny Lind
Granite. The significance of this new age is discussed in section 4.2.1.
44 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.8 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Jenny
Lind Granite (GSNSW PB-15-NEO-4, GA 2306500). Horizontal black line is the mean 206Pb/238U date of Group 1,
grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, yellow fill: radiogenic
Pb loss.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 45
Table 3.4: SHRIMP U–Pb zircon data from the Jenny Lind Granite (GSNSW PB-15-NEO-4, GA 2306500).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Magmatic crystallisation (n = 30)
500.26.1
-0.93
109
85
0.80
24.340
2.1
0.06222
4.7
259.6
5.4
500.13.1
-0.07
155
139
0.92
24.386
1.5
0.05337
2.4
259.1
3.7
500.21.1
0.21
296
309
1.08
24.395
0.9
0.04933
2.6
259.0
2.3
500.28.1
-0.27
134
108
0.83
24.410
1.0
0.05387
4.4
258.8
2.7
500.2.1
0.67
124
96
0.80
24.444
1.5
0.05022
4.6
258.5
3.9
500.9.1
0.79
133
103
0.80
24.447
1.0
0.04442
5.3
258.4
2.7
500.17.1
-0.69
124
73
0.60
24.461
1.1
0.05947
4.9
258.3
2.7
500.20.1
-0.35
108
56
0.53
24.530
1.1
0.05271
4.1
257.6
2.8
500.18.1
0.00
134
88
0.68
24.562
1.0
0.05498
2.4
257.2
2.6
500.10.1
0.00
213
183
0.89
24.617
0.9
0.05215
2.8
256.7
2.3
500.12.1
0.00
138
75
0.56
24.617
1.0
0.05291
2.2
256.7
2.5
500.11.1
0.24
193
179
0.96
24.668
1.0
0.04929
2.8
256.2
2.4
500.31.1
0.67
110
87
0.82
24.726
1.1
0.04939
5.3
255.6
2.8
500.24.1
0.24
479
540
1.16
24.762
0.9
0.04970
1.8
255.2
2.1
500.3.1
1.43
69
39
0.59
24.767
1.3
0.04454
9.6
255.2
3.3
500.25.1
0.59
189
182
1.00
24.771
1.0
0.04647
4.1
255.1
2.5
500.29.1
0.60
138
113
0.84
24.783
1.4
0.04715
4.7
255.0
3.6
500.7.1
0.51
220
104
0.49
24.803
0.9
0.04786
3.4
254.8
2.3
500.5.1
-0.08
145
87
0.62
24.804
1.0
0.05399
2.5
254.8
2.5
500.6.1
1.30
94
50
0.55
24.894
1.8
0.04201
8.4
253.9
4.4
500.15.1
-0.18
124
76
0.63
24.918
2.5
0.05469
4.1
253.6
6.2
500.16.1
0.26
177
120
0.70
24.926
1.0
0.04998
3.0
253.6
2.4
500.27.1
0.10
131
108
0.85
24.933
2.3
0.05088
3.0
253.5
5.6
500.4.1
1.30
92
54
0.61
24.953
1.2
0.04194
8.4
253.3
2.9
500.14.1
0.67
159
95
0.62
24.956
1.0
0.04794
4.5
253.3
2.5
500.22.1
0.13
192
162
0.87
25.131
1.0
0.05307
2.5
251.5
2.4
500.8.1
0.15
244
253
1.07
25.163
0.9
0.05180
3.0
251.2
2.2
500.19.1
0.21
118
96
0.84
25.172
1.7
0.05347
3.4
251.1
4.3
500.1.1
1.69
64
36
0.59
25.475
1.4
0.04289
11.7
248.2
3.5
500.23.1
1.68
104
71
0.70
25.517
1.8
0.04005
10.0
247.8
4.4
Pb loss (n = 1)
500.30.1
0.08
169
137
0.84
29.067
1.0
0.05253
2.6
218.0
2.1
46 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
3.3 Dumbudgery Creek Granodiorite
Table 3.5 Summary of results: Dumbudgery Creek Granodiorite.
GA SampleNo
2306501
GSNSW SiteID
PB-15-NEO-5
Parent Unit
Clarence River Supersuite, Dumbudgery Creek Suite
Stratigraphic Unit
Dumbudgery Creek Granodiorite
Informal Identifier
—
Lithology
Granite
Province
New England Orogen
1:250 000 Sheet
Grafton (SI/56-6)
1:100 000 Sheet
Coaldale (9439)
Location (GDA94)
29.19510°S, 152.52402°E
Location (MGA94)
Zone 56; 453726 mE, 6770304 mN
Analytical Session No
150080 (see Appendix section A.4 for session details)
Interpreted Age
255.0 ± 1.0 Ma (n = 30)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.3.1 Sampling Details and Geological Relationships
The sample was collected from a roadside outcrop at the base of a 5 m-high cutting on Lionsville
Road, to the west of Baryulgil (Figure 1.5). The material that remains in-situ is deeply weathered,
though scattered boulders of variably textured granite remain. This medium-grained, equigranular rock
contains plagioclase, with alkali feldspar more abundant than quartz. Ferromagnesian minerals
(biotite, amphibole) comprise 5–10 modal % of the rock. Sparse mafic enclaves comprise less than
1% of the outcrop. Zones of pegmatitic rock have prominent acicular amphibole and associated
azurite, malachite and sulphide spotting along thin veins, all of which appears to intrude the main
phase (Figure 3.9).
The Dumbudgery Creek Granodiorite is currently classified into the Clarence River Supersuite (Bryant
et al., 1997a; Henley et al., 2001). A SHRIMP U–Pb magmatic crystallisation age will allow
comparison with previously dated granites of the Clarence River Supersuite, and refine the regional
chronology of magmatism.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 47
Figure 3.9 Hand sample photographs of the Dumbudgery Creek Granodiorite (GSNSW PB-15-NEO-5,
GA 2306501). Top left: outcrop photo showing mafic enclave. Top right: outcrop photo showing zones of pegmatitic
rock with prominent acicular amphibole and associated azurite, malachite and sulphide spotting along thin veins.
Bottom left: Hand sample photo of sampled rock.
3.3.2 Petrography
The sample of Dumbudgery Creek Granodiorite contains tabular and sporadically flecked plagioclase
which is typically zoned out to more sodic compositions around rims (Figure 3.10). Alkali feldspar and
anhedral to blocky quartz is texturally late (alkali feldspar > quartz), and is modally more prominent than
other Clarence River Supersuite plutons. Subequal amounts of biotite (well formed, light brown to chocolate
brown pleochroism) and amphibole (light green-brown to green pleochroism) constitute 10% of the rock.
Amphiboles usually show well developed cleavage traces, while biotite is usually unaltered but occasionally
completely chloritised. Opaque minerals can be prominent associated with clusters of biotite.
3.3.3 Zircon Description
Mounted zircons (Figure 3.11) are transparent-to-cloudy, colourless euhedral to subhedral grains that
have long axes of 50–300 µm and aspect ratios of 1–4. Grains are prismatic with pyramidal
terminations. Inclusions are round to elongate blebs up to 30 µm in diameter, and randomly oriented
acicular inclusions up to 100 µm in length. Occasional zircons have oscillatory zonation patterns
visible in the transmitted light images.
Cathodoluminescence (CL) images (Figure 3.11) show that the grains are strongly variably
luminescent and have high-contrast oscillatory zoning, or occasionally sector zoning. Many grains
feature central regions with convolute or chaotically disrupted CL zoning.
48 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.10 Representative cross-polarised light photomicrograph of the medium-grained Dumbudgery Creek
Granodiorite (GSNSW PB-15-NEO-5, GA 2306501) that consists of early formed zoned plagioclase crystals, biotite
and amphibole. Note in thin section the relative abundance of late crystallising alkali feldspar and quartz relative to
typical more tonalitic Clarence River Supersuite plutons. Scale is 1 mm.
3.3.4 U–Pb Isotopic Results and Interpretation
Thirty-three SHRIMP U–Pb isotopic analyses were carried out on 33 zircons (Figure 3.12, Table 3.6).
These analyses are characterised by low to moderate U content (95–1412 ppm, median 666 ppm) and
moderate Th/U ratios (0.39–0.96, median 0.54). Common 206Pb proportions are predominantly low
(maximum 0.7%, median 0.1%).
A single analysis yields a date (c. 303 Ma) significantly older than the bulk population. This analysis is
interpreted to have come from an inherited core and is not considered for the purposes of magmatic
age determination.
When grouped, the remaining 32 analyses are characterised by relatively high MSWD (2.08) and low
P (0.00), which implies the presence of excess scatter in the data, beyond that attributable to the
analytical uncertainties. Removal of the two youngest (8.1, c. 245 Ma; 19.1, c. 242 Ma) removes the
excess scatter from the population. These analyses are interpreted to be affected by loss of radiogenic
Pb and are not considered for the purposes of magmatic crystallisation age determination.
The remainder of the analyses (30 spots from 30 grains) span the range 262–249 Ma and form a
statistically coherent population with a weighted mean date of 255.0 ± 1.0 Ma (95% confidence;
MSWD = 0.75, P = 0.82). This is interpreted as the magmatic crystallisation age for the Dumbudgery
Creek Granodiorite. The significance of this new age is discussed in section 4.2.1.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 49
Figure 3.11 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the Dumbudgery Creek Granodiorite (GSNSW PB-15-NEO-5, GA 2306501). SHRIMP analysis sites are indicated
and labelled with grain and spot number. Scale bar is 100 µm.
50 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.12 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the
Dumbudgery Creek Granodiorite (GSNSW PB-15-NEO-5, GA 2306501). Horizontal black line is the mean 206Pb/238U
date of Group 1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, red
fill: inheritance, yellow fill: Pb loss.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 51
Table 3.6 SHRIMP U–Pb zircon data from the Dumbudgery Creek Granodiorite (GSNSW PB-15-NEO-5, GA 2306501).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Inheritance (n = 1)
501.26.1
0.06
164
96
0.60
20.802
1.3
0.05332
2.1
302.7
3.8
Magmatic crystallisation (n = 30)
501.16.1
-0.50
95
49
0.53
24.126
1.2
0.05734
4.3
261.8
3.0
501.20.1
0.00
946
632
0.69
24.545
1.0
0.05224
0.9
257.4
2.4
501.12.1
-0.05
899
674
0.78
24.545
0.8
0.05221
1.4
257.4
2.0
501.2.1
0.10
707
432
0.63
24.558
0.9
0.04996
1.2
257.3
2.3
501.10.1
0.10
457
186
0.42
24.600
0.8
0.05016
1.5
256.9
2.1
501.9.1
0.07
1412
1010
0.74
24.642
1.0
0.05200
0.8
256.4
2.4
501.17.1
0.72
164
121
0.76
24.650
1.0
0.04772
4.5
256.4
2.6
501.30.1
0.12
514
219
0.44
24.663
1.0
0.05067
1.5
256.2
2.4
501.28.1
0.14
1150
1003
0.90
24.699
0.8
0.05187
1.0
255.9
2.0
501.3.1
0.07
479
200
0.43
24.713
0.8
0.05137
1.4
255.7
2.1
501.5.1
0.18
849
787
0.96
24.724
0.8
0.05045
1.1
255.6
2.0
501.33.1
0.11
916
519
0.59
24.732
0.8
0.05108
1.1
255.5
2.0
501.6.1
0.27
509
257
0.52
24.752
0.8
0.05281
1.7
255.3
2.1
501.32.1
0.07
484
320
0.68
24.753
0.8
0.05197
1.4
255.3
2.1
501.14.1
0.41
196
86
0.45
24.755
1.0
0.04913
3.3
255.3
2.4
501.21.1
0.08
798
351
0.45
24.774
1.0
0.05141
1.1
255.1
2.5
501.4.1
0.48
301
151
0.52
24.780
1.1
0.04618
3.5
255.0
2.8
501.22.1
0.56
288
118
0.42
24.804
1.3
0.04677
3.2
254.8
3.2
501.11.1
0.10
746
368
0.51
24.825
0.8
0.05070
1.2
254.6
2.0
501.1.1
0.17
652
355
0.56
24.845
0.9
0.05051
1.3
254.4
2.3
501.18.1
0.13
806
396
0.51
24.860
0.8
0.05051
1.2
254.2
2.0
501.7.1
0.01
776
337
0.45
24.900
0.9
0.05205
0.9
253.8
2.3
501.27.1
0.50
729
397
0.56
24.928
1.0
0.05025
2.6
253.6
2.4
501.25.1
0.09
383
202
0.54
24.935
1.3
0.05025
2.9
253.5
3.2
501.15.1
-0.05
270
101
0.39
24.938
1.2
0.05490
1.8
253.4
2.9
501.29.1
0.11
881
462
0.54
24.939
0.9
0.05073
1.1
253.4
2.3
501.23.1
0.08
594
264
0.46
24.963
0.8
0.05142
1.3
253.2
2.1
501.24.1
0.02
666
490
0.76
25.056
0.8
0.05153
1.1
252.3
2.0
501.13.1
0.04
522
330
0.65
25.095
0.8
0.05228
1.2
251.9
2.0
501.31.1
-0.13
676
343
0.52
25.380
1.2
0.05297
1.3
249.1
2.8
Pb loss (n = 2)
501.8.1
0.30
781
374
0.49
25.836
0.8
0.05064
1.4
244.8
2.0
501.19.1
0.25
694
368
0.55
26.162
1.3
0.05189
1.5
241.8
3.1
52 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
3.4 Towgon Grange Tonalite
Table 3.7 Summary of results: Towgon Grange Tonalite.
GA SampleNo
2306502
GSNSW SiteID
PB-15-NEO-6
Parent Unit
Clarence River Supersuite
Stratigraphic Unit
Towgon Grange Tonalite
Informal Identifier
—
Lithology
Granite
Province
New England Orogen
1:250 000 Sheet
Grafton (SI/56-6)
1:100 000 Sheet
Coaldale (9439)
Location (GDA94)
29.43769°S, 152.61710°E
Location (MGA94)
Zone 56; 462863 mE, 6743459 mN
Analytical Session No
150080 (see Appendix section A.4 for session details)
Interpreted Age
255.4 ± 1.2 Ma (n = 31)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.4.1 Sampling Details and Geological Relationships
The sample of Towgon Grange Tonalite was collected from large, slabby in-situ outcrops of granite in
Towgon Creek, from the north side of a creek crossing under a prominent creek bank cutting on Heifer
Station Road (Figure 1.5). The rock is very coherent, tough, fresh, grey and medium-grained (Figure
3.13). This tonalitic rock contains predominantly unclouded plagioclase with biotite, amphibole and
little alkali feldspar. The rock is undeformed, with a very thin (mm-scale) weathering rind.
The Towgon Grange Tonalite is currently classified into the Clarence River Supersuite (Bryant et al.,
1997a; Henley et al., 2001). A SHRIMP U–Pb magmatic crystallisation age will allow comparison with
previously dated granites of the Clarence River Supersuite, and refine the regional chronology of
magmatism.
3.4.2 Petrography
The sample of Towgon Grange Tonalite is a very fresh medium-grained tonalitic rock with well-formed
lathy/tabular well-zoned plagioclase (40 modal %), with later less well-formed quartz and minor alkali
feldspar (Figure 3.14). Amphibole and biotite comprise around 5% of the rock and tend to be evenly
distributed as discrete crystals rather than aggregates suggestive of replacement of earlier pyroxene.
Accessory phases such as zircon and apatite are relatively uncommon.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 53
Figure 3.13 Representative photographs of the Towgon Grange Tonalite (GSNSW PB-15-NEO-6, GA 2306502) in
outcrop (left) and in hand specimen (right).
Figure 3.14 Representative cross-polarised light photomicrograph of the medium-grained Towgon Grange Tonalite
(GSNSW PB-15-NEO-6, GA 2306502) that consists of early-formed, well-zoned plagioclase crystals with biotite and
amphibole. The relative abundance of quartz relative to alkali feldspar is consistent with the unit being
mineralogically tonalitic. Scale is 1 mm.
3.4.3 Zircon Description
Mounted zircons (Figure 3.15) are transparent, colourless euhedral to subhedral grains that have long
axes of 50–350 µm and aspect ratios of 1–3.5. Grains are prismatic with pyramidal terminations.
Inclusions are round to elongate blebs to 50 µm in diameter, and randomly oriented acicular inclusions
up to 50 µm in length.
Cathodoluminescence (CL) images (Figure 3.15) show that the grains are moderately luminescent
and zoning varies from fine-scale to broad-scale oscillatory zoning, with occasional sector zoning.
Some grains feature central regions with convolute or chaotically disrupted CL zoning.
54 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.15 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the Towgon Grange Tonalite (GSNSW PB-15-NEO-6, GA 2306502). SHRIMP analysis sites are indicated and
labelled with grain and spot number. Scale bar is 100 µm.
3.4.4 U–Pb Isotopic Results and Interpretation
Thirty-one SHRIMP U–Pb isotopic analyses were carried out on 31 zircons (Figure 3.16, Table 3.8).
These analyses are characterised by low to moderate U content (92–453 ppm, median 167 ppm) and
moderate Th/U ratios (0.42–0.93, median 0.58). Common 206Pb proportions are predominantly low
(maximum 1.2%, median 0.1%).
All 31 analyses span the range 263–248 Ma and form a statistically coherent population with a
weighted mean date of 255.4 ± 1.2 Ma (95% confidence; MSWD = 1.15, P = 0.26). This is interpreted
to represent the magmatic crystallisation age of the Towgon Grange Tonalite. The significance of this
new age is discussed in section 4.2.1.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 55
Figure 3.16 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Towgon
Grange Tonalite (GSNSW PB-15-NEO-6, GA 2306502). Horizontal black line is the mean 206Pb/238U date of Group
1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation.
56 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 3.8 SHRIMP U–Pb zircon data from the Towgon Grange Tonalite (GSNSW PB-15-NEO-6, GA 2306502).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Magmatic crystallisation (n = 31)
502.7.1
-0.27
239
97
0.42
24.067
0.9
0.05324
2.3
262.4
2.4
502.6.1
0.11
92
51
0.57
24.289
1.1
0.04973
3.3
260.1
2.8
502.12.1
0.21
108
74
0.71
24.443
1.1
0.05014
3.6
258.5
2.7
502.30.1
0.34
183
79
0.44
24.455
1.0
0.04764
4.8
258.4
2.5
502.11.1
0.15
156
77
0.51
24.455
1.4
0.04961
2.8
258.4
3.5
502.22.1
-0.34
166
81
0.51
24.514
1.0
0.05683
3.2
257.7
2.6
502.21.1
-0.13
384
269
0.72
24.582
1.4
0.05331
1.7
257.0
3.5
502.1.1
-0.22
144
81
0.58
24.587
1.0
0.05431
2.8
257.0
2.5
502.16.1
-0.06
199
136
0.71
24.628
1.0
0.05328
2.1
256.6
2.4
502.17.1
0.26
424
310
0.76
24.643
0.9
0.05080
2.0
256.4
2.2
502.23.1
0.05
258
137
0.55
24.654
0.9
0.04990
1.9
256.3
2.3
502.28.1
0.00
157
114
0.75
24.655
1.0
0.05226
2.2
256.3
2.5
502.15.1
0.47
208
169
0.84
24.686
1.0
0.04765
3.5
256.0
2.5
502.31.1
0.39
121
78
0.66
24.688
1.5
0.04991
4.1
256.0
3.9
502.14.1
0.07
167
74
0.46
24.702
1.0
0.05208
2.4
255.8
2.5
502.20.1
0.05
453
348
0.79
24.707
1.0
0.04979
1.4
255.8
2.5
502.2.1
0.00
111
54
0.50
24.727
1.5
0.05191
2.4
255.6
3.6
502.3.1
0.14
237
138
0.60
24.777
0.9
0.05076
2.1
255.1
2.3
502.19.1
0.00
151
65
0.45
24.814
1.0
0.05391
2.2
254.7
2.5
502.4.1
-0.16
132
60
0.47
24.861
1.0
0.05416
2.7
254.2
2.5
502.8.1
0.46
169
137
0.84
24.869
1.0
0.04921
3.6
254.1
2.4
502.26.1
0.39
198
146
0.77
24.872
1.0
0.05245
3.2
254.1
2.4
502.18.1
0.03
450
405
0.93
24.904
1.1
0.05143
1.4
253.8
2.8
502.13.1
0.65
196
89
0.47
24.976
1.4
0.04646
3.8
253.1
3.5
502.10.1
0.06
201
173
0.89
24.982
0.9
0.05161
3.6
253.0
2.3
502.25.1
0.48
160
76
0.49
25.003
1.0
0.04871
4.0
252.8
2.5
502.24.1
0.39
161
80
0.51
25.006
1.7
0.05064
3.6
252.8
4.1
502.29.1
0.48
241
208
0.89
25.090
1.2
0.04963
3.3
251.9
3.0
502.9.1
0.22
156
78
0.52
25.186
1.0
0.05111
3.0
251.0
2.4
502.27.1
0.10
122
85
0.72
25.348
2.0
0.05432
2.9
249.4
4.9
502.5.1
1.15
103
52
0.53
25.501
1.1
0.04617
7.2
248.0
2.8
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 57
3.5 Newton Boyd Granodiorite
Table 3.9 Summary of results: Newton Boyd Granodiorite.
GA SampleNo
2306503
GSNSW SiteID
PB-15-NEO-7
Parent Unit
Stanthorpe Supersuite, Herries Suite
Stratigraphic Unit
Newton Boyd Granodiorite
Informal Identifier
—
Lithology
Granite
Province
New England Orogen
1:250 000 Sheet
Grafton (SI/56-6)
1:100 000 Sheet
Newton Boyd (9338)
Location (GDA94)
29.83772°S, 152.28973°E
Location (MGA94)
Zone 56; 431384 mE, 6698984 mN
Analytical Session No
150080 (see Appendix section A.4 for session details)
Interpreted Age
252.8 ± 1.0 Ma (n = 31)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.5.1 Sampling Details and Geological Relationships
The Newton Boyd Granodiorite is a recessive unit, present as regional lows in topography. Outcrops
are scarce, except in the river bed. The sample was collected from in-situ slabs and outcrops in the
main bed of the Boyd River, behind a public reserve to the south of Old Glen Innes Road (Figure 1.5,
Figure 3.17). The rock is undeformed, felsic, medium- to coarse-grained and equigranular. Quartz and
pink alkali feldspar dominate, with subordinate plagioclase. Biotite comprises 5–8% of the rock. Mafic
microgranular enclaves to 20–30 cm are scarce.
The Newton Boyd Granodiorite was previously classified into the Moonbi Supersuite (Henley et al.,
2001), but was subsequently reclassified into the Herries Supersuite (Donchak et al., 2007), which
was then adjusted to the Herries Suite and grouped into the Stanthorpe Supersuite (Donchak et al.,
2013). A SHRIMP U–Pb magmatic crystallisation age will clarify the age relationship between this unit
and other units in the Herries Suite/Supersuite and Stanthorpe Supersuite.
A new age will also allow comparison with previously dated granites in the region, in particular with
other felsic granites associated with the Demon Fault. The 253.7 ± 1.5 Ma Mount Mitchell
Monzogranite (Chisholm et al., 2014c) abuts the western side of the Newton Boyd Granodiorite,
separated by the Demon Fault. This new age will clarify the relationship between the two plutons.
58 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.17 Representative photographs of the Newton Boyd Granodiorite (GSNSW PB-15-NEO-7, GA 2306503)
showing sampling area (top left), sampled rock (top right), zone of mafic microgranular enclave (bottom left) and
hand specimen (bottom right).
3.5.2 Petrography
The sample of Newton Boyd Granodiorite is very fresh with minor alteration and flecking in the
modestly zoned plagioclases and minor turbidisation in parts of alkali feldspars (Figure 3.18). Quartz
and alkali feldspar are present in subequal proportions in greater general abundance than plagioclase
with biotite generally 5–8% of the rock. Biotite (light straw brown to dark chocolate brown pleochroism)
is mostly fresh or altered in part to apple-green chlorite. Alkali feldspar is moderately perthitic and can
be to a centimetre in length with monzonite texture type inclusions of tabular biotite and plagioclase.
Quartz is anhedral and blocky.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 59
Figure 3.18 Representative cross-polarised light photomicrograph of the medium- to coarse-grained Newton Boyd
Granodiorite (GSNSW PB-15-NEO-7, GA 2306503) showing a larger alkali feldspar studded with smaller, early
crystallised plagioclase and biotite. Included plagioclase has strong sodic rims against the alkali feldspar. Quartz is in
the upper right of the image. Scale is 1 mm.
3.5.3 Zircon Description
Mounted zircons (Figure 3.19) are transparent, colourless euhedral to subhedral grains that are largely
elongate, with long axes of 35–350 µm and aspect ratios of 1–7. Grains are prismatic with pyramidal
terminations. Inclusions are round to elongate blebs up to 10 µm in diameter, and acicular inclusions
up to 50 µm, a large proportion of which are axially aligned.
Cathodoluminescence (CL) images (Figure 3.19) show that the grains are moderately luminescent
and have fine-scale oscillatory zoning. Many grains feature central regions with convolute or
chaotically disrupted CL zoning.
60 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.19 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the Newton Boyd Granodiorite (GSNSW PB-15-NEO-7, GA 2306503). SHRIMP analysis sites are indicated and
labelled with grain and spot number. Scale bar is 100 µm.
3.5.4 U–Pb Isotopic Results and Interpretation
Thirty-four SHRIMP U–Pb isotopic analyses were carried out on 34 zircons (Figure 3.20, Table 3.10).
These analyses are characterised by moderate U content (161–1091 ppm, median 517 ppm) and
moderate Th/U ratios (0.09–1.44, median 0.58). Common 206Pb proportions are predominantly low
(maximum 0.6%, median 0.1%).
Three spots (24.1, 23.1, 26.1) targeted core regions and yield much older dates (c. 642 Ma, c. 317
Ma, and c. 304 Ma, respectively) than the bulk population. These analyses are interpreted to represent
inheritance, and are excluded from the calculation of the magmatic crystallisation age.
The remainder of the analyses (31 spots on 31 grains) span the range 259–248 Ma and form a
statistically coherent population with a weighted mean date of 252.8 ± 1.0 Ma (95% confidence;
MSWD = 1.01, P = 0.44). This is interpreted to represent the magmatic crystallisation age for the
Newton Boyd Granodiorite. The significance of this new age is discussed in section 4.2.2.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 61
Figure 3.20 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Newton
Boyd Granodiorite (GSNSW PB-15-NEO-7, GA 2306503). Horizontal black line is the mean 206Pb/238U date of Group
1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation, red fill:
inheritance.
62 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 3.10 SHRIMP U–Pb zircon data from the Newton Boyd Granodiorite (GSNSW PB-15-NEO-7, GA 2306503).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Inheritance (n = 3)
503.24.1
0.04
750
67
0.09
9.557
2.4
0.06781
2.4
641.5
14.9
503.23.1
0.17
161
91
0.59
19.839
1.0
0.05339
2.4
317.0
3.1
503.26.1
0.28
288
268
0.96
20.733
0.9
0.05259
2.1
303.7
2.7
Magmatic crystallisation (n = 31)
503.19.1
0.00
326
263
0.83
24.439
1.3
0.05062
1.5
258.5
3.4
503.9.1
0.27
603
239
0.41
24.464
1.0
0.04916
1.6
258.3
2.5
503.2.1
0.04
540
271
0.52
24.642
1.0
0.05169
1.2
256.4
2.5
503.1.1
-0.21
239
133
0.58
24.675
1.1
0.05325
2.1
256.1
2.8
503.34.1
0.19
483
350
0.75
24.715
0.8
0.04919
2.6
255.7
2.1
503.4.1
0.28
437
265
0.63
24.731
1.0
0.04914
1.8
255.5
2.4
503.32.1
0.11
689
526
0.79
24.762
1.1
0.05118
1.2
255.2
2.8
503.20.1
0.03
1091
1524
1.44
24.806
0.8
0.05256
0.9
254.8
2.0
503.16.1
0.20
376
212
0.58
24.896
1.4
0.05002
2.0
253.9
3.4
503.30.1
0.31
520
189
0.38
24.902
1.2
0.04973
1.8
253.8
2.9
503.11.1
0.04
516
165
0.33
24.907
0.8
0.05177
1.3
253.8
2.1
503.21.1
0.16
582
381
0.68
24.929
0.8
0.05039
1.5
253.5
2.1
503.33.1
0.20
551
235
0.44
24.937
1.1
0.05025
1.5
253.5
2.7
503.13.1
0.18
618
410
0.69
24.981
1.0
0.05140
1.4
253.0
2.4
503.5.1
0.05
429
128
0.31
25.004
0.8
0.05196
1.4
252.8
2.1
503.17.1
0.30
495
232
0.48
25.007
0.8
0.05307
2.4
252.8
2.1
503.31.1
0.09
530
314
0.61
25.022
1.1
0.05106
1.4
252.6
2.7
503.3.1
0.18
499
395
0.82
25.027
0.8
0.05097
1.6
252.6
2.1
503.10.1
0.08
731
266
0.38
25.057
0.9
0.05125
1.1
252.3
2.2
503.15.1
0.00
519
200
0.40
25.068
1.1
0.05190
1.2
252.2
2.6
503.7.1
0.15
599
353
0.61
25.080
1.0
0.05100
1.4
252.0
2.4
503.18.1
0.10
533
403
0.78
25.083
1.0
0.05076
1.5
252.0
2.4
503.27.1
0.65
293
219
0.77
25.097
0.9
0.05006
4.0
251.9
2.3
503.6.1
-0.02
516
191
0.38
25.135
0.8
0.05149
1.3
251.5
2.1
503.12.1
0.09
589
237
0.42
25.153
0.8
0.05074
1.3
251.3
2.0
503.22.1
0.15
427
232
0.56
25.179
0.9
0.05241
2.5
251.1
2.1
503.14.1
0.30
279
106
0.39
25.308
1.1
0.05026
2.5
249.8
2.8
503.25.1
0.08
317
176
0.57
25.315
1.2
0.04981
1.9
249.7
3.0
503.8.1
0.10
635
495
0.80
25.365
1.0
0.05188
1.2
249.3
2.4
503.29.1
0.48
572
357
0.65
25.425
1.0
0.04948
2.2
248.7
2.5
503.28.1
0.54
491
271
0.57
25.455
0.9
0.05038
2.3
248.4
2.1
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 63
3.6 Drake Volcanics at Red Rock
Table 3.11 Summary of results: Drake Volcanics at Red Rock.
GA SampleNo
2550240
GSNSW SiteID
PB-15-NEO-9
Parent Unit
Wandsworth Volcanic Group
Stratigraphic Unit
Drake Volcanics
Informal Identifier
Red Rock
Lithology
Rhyolite
Province
New England Orogen
1:250 000 Sheet
Warwick (SH/56-2)
1:100 000 Sheet
Drake (9340)
Location (GDA94)
28.83892°S, 152.32211°E
Location (MGA94)
Zone 56; 433871 mE, 6809671 mN
Analytical Session No
160021 (see Appendix section A.4 for session details)
Interpreted Age
265.3 ± 1.4 Ma (n = 28)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.6.1 Sampling Details and Geological Relationships
The sample is from drill hole RRDD003 (191.57 m), from the Red Rock Mining Lease owned by White
Rock Minerals Ltd (Figure 1.5). The sample comprises laminated monomictic rhyolite with 25–30%
quartz and feldspar phenocrysts (1–2 mm) in a palest pink aphanitic groundmass. Zones of
brecciation and alteration are present through the sample. The rhyolite probably has low level phyllic
background alteration with a little pyrite present. This sample comprises the intrusion responsible for
the mineralisation at the Red Rock Mine. The rhyolite dome has intruded the DV4 package (Cumming
et al., 2013) at the top of Drake volcanic pile.
Dating this sample will date the mineralisation and put an age constraint on the top of the volcanic pile
at Drake. The mineralised domes are regarded as being synchronous with the host volcanic
packages. Attempts at 40Ar/39Ar dating at Drake have been relatively unsuccessful due to the very fine
nature of the alteration micas. A coherent sill member of the DV2 package (in the midsection of the
Drake Volcanics) has been previously dated at 264.4 ± 2.5 Ma by SHRIMP (Cross and Blevin, 2010).
64 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.21 Hand specimen of the Drake Volcanics at Red Rock (GSNSW PB-15-NEO-9, GA 2550240).
Figure 3.22 Photomicrographs of the Drake Volcanics at Red Rock (GSNSW PB-15-NEO-9, GA 2550240), intensely
altered porphyritic felsic coherent rock. Top left: Embayed and fractured quartz phenocrysts and intensely sericite-
altered groundmass. Linear alignment of sericite in the groundmass may be after primary flow bands. The small
sericitised prismatic mineral above and right of the upper quartz phenocryst may have been a small ferromagnesian
mineral. The opaque minerals are pyrite. Scale bar is 1 mm; cross-polarised light. Top right: Embayed and fractured
quartz phenocrysts in plane-polarised light. The groundmass is altered to clay minerals and sericite, and contains
several percent disseminated pyrite (opaque minerals). Scale bar is 1 mm. Bottom left: Under cross-polarised light
the high birefringence of carbonate minerals and fine-grained white mica (sericite) delineate the outline of these
former feldspars and/or ferromagnesian minerals. Mosaic or polygonal secondary quartz is also replacing the
minerals. Opaque minerals are pyrite, and the linear feature in the lower left is a quartz veinlet. The groundmass is
completely replaced by sericite and clay minerals. Scale bar is 0.5 mm. Bottom right: Carbonate minerals, sericite
and quartz have replaced this prismatic mineral, likely a former ferromagnesian mineral. Scale bar is 0.5 mm.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 65
3.6.2 Petrography
This sample is an extremely altered felsic porphyritic coherent rock, most likely a rhyolite or rhyodacite
(Figure 3.21, Figure 3.22). The alteration assemblage includes sericite, carbonate minerals, pyrite and
clay minerals, and secondary quartz. The secondary quartz is fine-grained mosaic (polygonal) and is
present inconsistently in altered feldspars or ferromagnesian minerals, and in quartz veinlets.
Secondary quartz is also present in rounded shapes which may have been amygdales or merely small
rounded quartz phenocrysts.
Quartz phenocrysts total approximately 5 modal %, and are 0.5–3 mm. Most are fractured, many are
embayed. Feldspars and ferromagnesian minerals are completely altered to the above mineral
assemblage; only pseudomorphs remain.
The groundmass is completely replaced by sericite and clay minerals. The sericite is aligned within the
groundmass locally, possibly delineating primary flow-banding.
3.6.3 Zircon Description
Mounted zircons (Figure 3.23) are transparent, colourless subhedral grains that have long axes of 70–
200 µm and aspect ratios of 1–3. Unbroken grains are prismatic with pyramidal terminations. A few
grains have oscillatory zoning that is visible through transmitted light. Round to elongate blebs to 20
µm in diameter, and randomly oriented acicular inclusions to 100 µm are common throughout.
Cathodoluminescence (CL) (Figure 3.23) images show that the grains are moderately luminescent
and have moderate oscillatory zoning and occasionally sector zoning. Most grains exhibit a difference
in CL emission between the central and outer grain regions, but there is little evidence of
disconformable overgrowth on most grains. Some grains feature central regions with convolute or
chaotically disrupted CL zoning.
3.6.4 U–Pb Isotopic Results and Interpretation
Thirty SHRIMP U–Pb isotopic analyses were carried out on 30 zircons (Figure 3.24, Table 3.12). One
analysis was characterised by relatively high common 206Pb (>2%): it is interpreted as unreliable, and
is not considered further.
The remainder of the analyses are characterised by low to moderate U content (82–614 ppm, median
150 ppm) and moderate Th/U ratios (0.35–0.92, median 0.48). Common 206Pb proportions are
predominantly low (maximum 1.0%, median 0.2%).
One spot (29.1) yielded a slightly younger date (c. 248 Ma) than the bulk population. This analysis was
affected by poor tuning of the secondary beam as reflected by the poor visual focus of the spot on the
mount surface, and is not considered further.
The remainder of the analyses (28 spots from 28 grains) span the range 270–259 Ma and form a
statistically coherent population with a weighted mean date of 265.3 ± 1.4 Ma (95% confidence;
MSWD = 0.87, P = 0.66). This is interpreted as the magmatic crystallisation age for the Drake
Volcanics at Red Rock. The significance of this new age is discussed in section 4.2.3.
66 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.23 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the Drake Volcanics at Red Rock (GSNSW PB-15-NEO-9, GA 2550240). SHRIMP analysis sites are indicated and
labelled with grain and spot number. Scale bar is 100 µm.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 67
Figure 3.24 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Drake
Volcanics at Red Rock (GSNSW PB-15-NEO-9, GA 2550240). Horizontal black line is the mean 206Pb/238U date of
Group 1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation. No fill:
unreliable analyses. One analysis with common lead of 32% is not shown.
68 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 3.12 SHRIMP U–Pb zircon data from the Drake Volcanics at Red Rock (GSNSW PB-15-NEO-9, GA 2550240).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Magmatic crystallisation (n = 28)
240.26.1
0.38
199
82
0.43
23.287
1.1
0.05135
1.6
270.0
3.0
240.23.1
0.62
155
75
0.50
23.331
0.9
0.05276
1.8
268.9
2.4
240.21.1
0.24
157
140
0.92
23.420
1.2
0.05320
1.8
268.9
3.1
240.19.1
0.23
212
122
0.60
23.434
1.3
0.05764
1.4
268.8
3.4
240.28.1
-0.16
116
53
0.48
23.560
1.3
0.05294
3.6
268.4
3.4
240.18.1
0.14
181
75
0.43
23.544
1.2
0.05126
1.7
267.8
3.1
240.25.1
-0.11
123
47
0.40
23.626
1.2
0.05428
2.0
267.5
3.1
240.6.1
0.40
223
131
0.61
23.539
1.1
0.05331
1.4
267.1
3.0
240.1.1
0.14
614
401
0.68
23.641
0.8
0.05201
0.9
266.7
2.1
240.16.1
0.36
164
68
0.43
23.591
1.2
0.05386
1.6
266.7
3.1
240.27.1
0.11
157
77
0.51
23.653
0.9
0.05314
2.9
266.7
2.4
240.3.1
0.38
133
81
0.63
23.610
0.9
0.05279
1.8
266.4
2.4
240.24.1
0.77
134
50
0.38
23.525
1.2
0.05487
1.9
266.3
3.1
240.13.1
0.39
100
34
0.35
23.643
1.4
0.05897
3.4
266.0
3.7
240.12.1
0.16
241
163
0.70
23.739
0.9
0.05320
1.4
265.6
2.2
240.30.1
0.95
82
31
0.39
23.585
1.0
0.05323
2.7
265.2
2.7
240.10.1
0.31
94
37
0.41
23.756
1.0
0.05963
2.1
265.0
2.6
240.11.1
-0.31
173
101
0.60
23.959
1.3
0.05521
1.7
264.4
3.4
240.20.1
0.20
101
51
0.51
23.850
1.9
0.05273
2.2
264.3
4.9
240.22.1
0.10
246
133
0.56
23.885
1.1
0.05164
1.4
264.1
2.8
240.2.1
-0.12
193
86
0.46
23.990
1.0
0.05087
1.6
263.6
2.7
240.14.1
0.24
98
38
0.40
23.906
1.0
0.05598
3.9
263.6
2.5
240.4.1
0.20
150
81
0.56
24.015
0.9
0.05227
1.9
262.5
2.3
240.5.1
0.34
139
72
0.54
24.007
0.9
0.05418
1.9
262.2
2.4
240.7.1
0.18
120
42
0.36
24.068
1.7
0.05566
3.3
262.0
4.4
240.17.1
0.66
142
57
0.41
23.983
0.9
0.05241
3.2
261.6
2.4
240.9.1
0.61
98
46
0.49
24.009
1.3
0.05501
2.2
261.5
3.3
240.8.1
1.00
156
68
0.45
24.123
1.1
0.05238
1.7
259.3
3.0
Not considered: Poor instrument tuning (n = 1)
240.29.1
0.77
112
40
0.37
25.285
1.0
0.05329
2.3
248.1
2.5
Not considered: Common 206Pb >2% (n = 1)
240.15.1
31.77
277
241
0.90
17.209
2.8
0.31644
4.4
250.7
11.7
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 69
3.7 Drake Volcanics at White Rock
Table 3.13 Summary of results: Drake Volcanics at White Rock.
GA SampleNo
2550241
GSNSW SiteID
PB-15-NEO-10
Parent Unit
Wandsworth Volcanic Group
Stratigraphic Unit
Drake Volcanics
Informal Identifier
White Rock
Lithology
Dacite
Province
New England Orogen
1:250 000 Sheet
Warwick (SH/56-2)
1:100 000 Sheet
Drake (9340)
Location (GDA94)
28.93478°S, 152.33869°E
Location (MGA94)
Zone 56; 435547 mE, 6799060 mN
Analytical Session No
160021 (see Appendix section A.4 for session details)
Interpreted Age
265.3 ± 1.5 Ma (n = 29)
Geological Attribution
Magmatic crystallisation
Isotopic Ratio Used
206Pb/238U (204Pb-corrected)
3.7.1 Sampling Details and Geological Relationships
The sample is from drill core WRDD018 (106.15–106.70 m) within the White Rock Mining Lease
(Figure 1.5). The sample comprises a crowded porphyritic rhyolitic dacite with ~30% phenocrysts
composed of feldspar (often altered to carbonate or clay minerals), lesser quartz and ferromagnesian
minerals (~2%, biotite?) in a grey groundmass (Figure 3.25). Variable background alteration is present
with pyrite.
This intrusive dacite is regarded as the source of mineralisation into the White Rock deposit. It
intrudes the DV2 package (Cumming et al., 2013) within the Drake Volcanics, with which it is
considered to be synchronous. A coherent sill member of the DV2 package has been previously dated
at 264.4 ± 2.5 Ma by SHRIMP (Cross and Blevin, 2010). This sample will date mineralisation at White
Rock and test assumptions regarding the synchronous nature of mineralised domes being emplaced
with successive eruptive volcanic packages.
70 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.25 Hand specimen of the Drake Volcanics at White Rock (GSNSW PB-15-NEO-10, GA 2550241).
3.7.2 Petrography
Less altered than the Drake Volcanics at Red Rock (GA 2550240, this Record), feldspars are still at
least partially present in this porphyritic felsic coherent rock, likely a rhyolite or rhyodacite
(Figure 3.26). Total phenocryst content is approximately 15 modal %. Quartz is 2–3 modal %, 1–5
mm, commonly embayed and fractured. Feldspar phenocrysts are limited to K-feldspar, approximately
7–10 modal %, and are 1–4 mm. Feldspar exhibits altered perthite and simple twinning.
Ferromagnesian minerals are completely altered, and total ~3 modal %.
Similar to GA 2550240 (this Record), the alteration assemblage is sericite-carbonate minerals-pyrite-
quartz. Sphene (titanite) is also present in small, cloudy masses associated with intensely altered and
completely replaced ferromagnesian minerals.
The groundmass in this sample is altered to sericite and clay minerals, but the ‘dotted’ texture may be
reminiscent of granoblastic recrystallisation texture, suggesting it was contact metamorphosed. In
such conditions overgrowths would be expected on the quartz and K-feldspar phenocrysts, but due to
the intense alteration the overgrowths may be difficult to identify.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 71
Figure 3.26 Photomicrographs of the Drake Volcanics at White Rock (GSNSW PB-15-NEO-10, GA 2550241),
altered porphyritic felsic coherent rock. Top left: Perthitic K-feldspar phenocrysts altered to sericite and carbonate
minerals, and embayed and fractured quartz. Secondary polygonal quartz veinlets and pods are also visible in the
groundmass. Cross-polarised light, scale bar is 1 mm. Top right: Phenocrystic ferromagnesian mineral replaced by
sericite, sphene (titanite), and carbonate minerals. Sphene is the dark brown to black cloudy mass in the upper end
of the phenocryst. Also note the acicular apatite inclusion in the centre of the phenocryst. Secondary quartz is visible
at the bottom of the image. Bottom left: The groundmass of this felsic coherent rock has a distinctive ‘dotted’ texture,
commonly associated with granoblastic texture indicative of contact metamorphism. Alteration of the rim surrounding
the feldspar phenocryst to sericite and carbonate minerals may be replacement of the overgrowths commonly
associated with granoblastic texture in contact metamorphosed rocks. Cross-polarised light; scale bar is 0.5 mm.
3.7.3 Zircon Description
Mounted zircons (Figure 3.27) are transparent, colourless subhedral grains that have long axes of 70–
300 µm and aspect ratios of 1–3. Unbroken grains are rounded with pyramidal terminations. Round to
elongate blebs up to 50 µm in diameter, and randomly oriented acicular inclusions up to 100 µm are
occasionally present.
Cathodoluminescence (CL) (Figure 3.27) images show that the grains are moderately luminescent
and have strong oscillatory zoning and occasionally sector zoning. There is little evidence of
disconformable overgrowth on most grains, although some grains feature central regions with
convolute or chaotically disrupted CL zoning.
72 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 3.27 Representative transmitted light (top) and cathodoluminescence (bottom) images of zircon grains from
the Drake Volcanics at White Rock (GSNSW PB-15-NEO-10, GA 2550241). SHRIMP analysis sites are indicated
and labelled with grain and spot number. Scale bar is 100 µm.
3.7.4 U–Pb Isotopic Results and Interpretation
Thirty SHRIMP U–Pb isotopic analyses were carried out on 30 zircons (Figure 3.28, Table 3.14). The
analyses are characterised by low U content (52–234 ppm, median 126 ppm) and moderate Th/U
ratios (0.42–1.01; median 0.62). Common 206Pb proportions are predominantly low (maximum 1.03%,
median 0.15%).
One spot (241.29.1) yielded a slightly younger date (c. 256 Ma) than the bulk population. This analysis
was immediately preceded by a spot that was affected by poor secondary tuning (240.29.1 from
sample GA 2550240), and this analysis is suspected to have been similarly affected. For this reason,
this analysis is excluded from the calculation of the magmatic crystallisation age.
The remainder of the analyses (29 spots from 29 grains) span the range 270–260 Ma and form a
statistically coherent population with a weighted mean date of 265.3 ± 1.5 Ma (95% confidence;
MSWD = 1.04, P = 0.41). This is interpreted as the magmatic crystallisation age for the Drake
Volcanics at White Rock. The significance of this new age is discussed in section 4.2.3.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 73
Figure 3.28 (a) Tera-Wasserburg concordia diagram and (b) 206Pb/238U dates in order of acquisition from the Drake
Volcanics at White Rock (GSNSW PB-15-NEO-10, GA 2550241). Horizontal black line is the mean 206Pb/238U date
of Group 1, grey shading is the corresponding 95% confidence interval. Purple fill: magmatic crystallisation. No fill:
unreliable analysis.
74 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 3.14 SHRIMP U–Pb zircon data from the Drake Volcanics at White Rock (GSNSW PB-15-NEO-10, GA 2550241).
Sample.Grain.Spot
206Pb
c
(%)
U
(ppm)
Th
(ppm)
232Th/
238U
238U/
206Pb
± 1σ
(%)
207Pb/
206Pb
± 1σ
(%)
206Pb/238U
date (Ma)
± 1σ
(Ma)
Magmatic crystallisation (n = 29)
241.24.1
-0.16
150
97
0.67
23.406
1.3
0.05234
1.8
270.7
3.7
241.14.1
0.23
140
85
0.63
23.378
0.9
0.05449
1.8
269.5
2.8
241.27.1
0.37
81
38
0.48
23.357
1.4
0.05347
4.8
269.4
4.2
241.16.1
-0.40
112
76
0.70
23.590
1.0
0.05425
1.9
268.8
2.9
241.28.1
0.14
172
119
0.71
23.483
1.3
0.05159
1.7
267.9
3.9
241.11.1
0.17
140
57
0.42
23.479
0.9
0.05369
1.8
268.3
2.6
241.22.1
0.00
141
84
0.61
23.541
0.9
0.05227
2.9
267.3
2.8
241.30.1
-0.18
157
108
0.71
23.672
1.1
0.05245
1.8
266.6
3.2
241.7.1
0.59
135
90
0.69
23.495
0.9
0.05438
1.9
269.1
3.0
241.19.1
0.02
234
228
1.00
23.697
1.1
0.05274
1.4
266.2
3.4
241.20.1
-0.19
126
57
0.47
23.753
1.2
0.05326
2.0
264.5
3.5
241.6.1
0.36
114
53
0.48
23.634
1.8
0.05579
2.0
266.5
5.2
241.5.1
-0.15
119
79
0.69
23.797
1.5
0.05476
3.8
266.5
4.4
241.25.1
0.79
90
57
0.65
23.585
1.8
0.05565
2.3
265.6
5.4
241.21.1
0.69
113
52
0.47
23.610
1.3
0.05275
2.1
267.3
3.9
241.8.1
-0.20
142
139
1.01
23.838
0.9
0.05402
1.8
265.1
3.4
241.10.1
0.09
149
138
0.95
23.781
1.3
0.05304
1.7
265.8
4.1
241.17.1
0.16
140
112
0.82
23.883
0.9
0.05363
1.8
264.7
3.5
241.2.1
0.19
117
53
0.47
23.883
1.8
0.05261
2.0
264.0
5.1
241.9.1
0.03
147
90
0.63
23.993
0.9
0.05645
1.7
261.5
2.7
241.4.1
0.55
52
29
0.58
23.874
1.8
0.05708
3.0
264.4
5.1
241.23.1
0.95
110
55
0.52
23.825
1.6
0.05392
2.1
265.7
4.6
241.18.1
0.63
75
45
0.62
23.906
1.1
0.05476
5.0
264.1
3.6
241.15.1
-0.18
131
72
0.56
24.124
1.2
0.05457
1.9
260.4
3.7
241.1.1
0.13
89
48
0.56
24.156
1.0
0.05393
2.4
260.2
2.9
241.3.1
0.11
102
70
0.71
24.162
1.6
0.05665
2.1
261.7
4.6
241.13.1
0.61
151
89
0.61
24.071
1.2
0.05320
1.9
263.1
3.4
241.26.1
1.03
75
45
0.61
24.058
1.0
0.05146
2.7
260.4
3.0
241.12.1
0.00
90
48
0.54
24.337
1.5
0.05498
2.4
257.5
4.2
Not considered: poor instrumental tuning (n=1)
241.29.1
0.66
79
44
0.57
24.561
1.0
0.06324
3.7
253.0
2.9
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 75
4 Discussion
4.1 Lachlan Orogen
We have obtained two new ages for volcanic units within the Cobar Supergroup in the northern part of
the central Lachlan Orogen. The first is from the 418.9 ± 2.5 Ma Babinda Volcanics of the shallow
marine Kopyje Group; this age is indistinguishable from the 419.3 ± 2.8 Ma maximum deposition age
determined on the interfingered and underlying Baledmund Formation (Bodorkos et al., 2015),
indicating that these two units are closely related. The new age is also indistinguishable from the
421.7 ± 2.3 Ma Florida Volcanics (Black, 2005), and two of three ages (422.2 ± 3.7 Ma,
428.2 ± 3.9 Ma: Spandler, 1998; 417.6 ± 3.2 Ma, Downes et al., 2016) determined for the Mineral Hill
Volcanics, elsewhere in the Kopyje Group. Blevin and Jones (2004) recognised a link between the
volcanic centres of the Kopyje Group and several felsic I-type intrusive bodies that lie along the same
trend, including the 420.6 ± 2.8 Ma Yellow Mountain Granite (Black, 2007), the undated Mount Walton
Porphyry, the 421.1 ± 3.4 Ma Wilmatha Granite (Spandler, 1998), and unnamed porphyry dykes in the
Mineral Hill region, one of which has been dated at 421 ± 3 Ma (Spandler, 1998). The coeval nature of
the dated I-type volcanic and plutonic rocks within the Canbelego-Mineral Hill Volcanic Belt may
indicate eruption and emplacement during a single event.
The second age is from the 421.9 ± 2.7 Ma S-type Shuttleton Rhyolite Member within the deep water,
turbiditic Amphitheatre Group farther to the west. The Shuttleton Rhyolite Member is interpreted to
nonconformably overlie the Thule Granite (Suppel and Gilligan, 1993), and the new age does not
preclude deposition of the Amphitheatre Group on a pre-exposed, eroded surface of the Thule Granite
(425.7 ± 2.4 Ma, 427.2 ± 3.1 Ma, Chisholm et al.; 2014a; 424.1 ± 2.9 Ma, Downes et al., 2016). This
age also does not preclude the Erimeran Granite (427.1 ± 2.4 Ma, Black, 2007; 424.5 ± 2.6 Ma,
Downes et al., 2016) being the sedimentary source for the Amphitheatre Group as suggested by
Suppel and Gilligan (1993), although this would require the granite to be emplaced, unroofed, eroded
and deposited over an extensive area to a thickness of 2–10 km in a few million years. The Shuttleton
Rhyolite Member is indistinguishable in age with recent U–Pb dating of the 422.8 ± 2.6 Ma S-type
Mount Halfway Volcanics (Chisholm et al., 2014a), which is another volcanic unit which interfingers the
Amphitheatre Group (MacRae, 1987). Further, the Mount Halfway Volcanics is coeval with the
422.5 ± 3.6 Ma S-type Gilgunnia Granite (Downes et al., 2016) as first recognised by the mutual cross-
cutting relationships between the two (MacRae, 1987). By extension, a 422.8 ± 4.9 Ma inferred
equivalent of the Gilgunnia Granite at the R7 prospect to its north (Chisholm et al., 2014a) is also
coeval. This collection of ages suggests that although eruptive volcanic activity is limited (Shuttleton
Rhyolite Member is the only volcanic unit recognised in the Amphitheatre Group), it was accompanied
by local coeval plutonism.
The results for the Babinda Volcanics and Shuttleton Rhyolite Member, in conjunction with previous
dating and studies (summarised in Downes et al., 2016) establish that significant igneous activity
occurred between c. 423 and c. 418 Ma within the Cobar region but comprised two compositionally
distinct (I-type vs S-type) but broadly contemporaneous belts of volcanics and comagmatic granite
intrusions.
In the Temora region of the central Lachlan Orogen, the new age of 414.7 ± 2.6 Ma for the ‘Hobbs
Pipe’ quartz monzonite at the Mt Adrah gold prospect establishes an Early Devonian age for the
hosted gold mineralisation. This age is significantly younger than U–Pb SHRIMP zircon ages obtained
from the andesite hosting the Temora (Gidginbung) gold deposit (435.0 ± 5.0 Ma, Perkins et al., 1990;
436.4 ± 3.1 Ma, Lawrie et al., 2007), indicating the variable age of rocks hosting gold mineralisation
76 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
along the Gilmore Fault Zone. Perkins et al. (1990) analysed a suite of hydrothermal alunites by the
40Ar/39Ar total fusion method, and interpret a 417.3 ± 1.3 Ma minimum alteration age for the
mineralisation at Gidginbung. It is possible that the mineralisation at Gidginbung is related to
emplacement of some of these younger plutons in the Temora region. Other intermediate plutonism of
Early Devonian age in the region includes the Middledale Gabbroic Diorite (416.8 ± 1.3 Ma, Black et
al., 2004; 416.1 ± 1.6 Ma, 414.8 ± 1.8 Ma, 414.8 ± 1.7 Ma, 414.1 ± 1.6 Ma, Iles, 2012; Iles et al.,
2015), the Windover Quartz Monzodiorite (412.9 ± 2.0 Ma, Iles, 2012, Geoscience Australia, 2011:
SampleNo 2129628), and the Woodlands Granite (413.9 ± 2.0 Ma, Iles 2012, Geoscience Australia,
2011: SampleNo 2129626), 20 km east of Temora.
In the eastern Lachlan Orogen near Yerranderie, a new age of 413.5 ± 2.3 Ma for an unnamed rhyolite
within the ‘younger volcanics’ is consistent with the existing age of 414.4 ± 2.9 Ma for the underlying
Barrallier Ignimbrite (Black, 2006). This indicates that most of the volcanic units of the Bindook Group
were erupted over a relatively short period in the latest Silurian to earliest Devonian, rather than the
‘younger volcanics’ at Yerranderie representing a significantly younger event. The new age indicates
that the epithermal mineralisation at Yerranderie (muscovite 40Ar/39Ar age = 372.1 ± 1.9 Ma; Downes,
2007) is not genetically related to the host volcanic succession, but is instead to a younger event in
the eastern Lachlan Orogen, such as magmatic activity within the Late Devonian Eden-Comerong-
Yalwal Rift Zone.
4.2 New England Orogen
4.2.1 Clarence River Supersuite
The Clarence River Supersuite is composed of I-type, isotopically primitive intrusions ranging in
composition from gabbro to monzogranite, and located in either the southern or northeastern areas of
the New England Orogen in New South Wales (Bryant et al., 1997a; Bryant et al., 1997b). Most of the
previous isotopic dates from the Clarence River Supersuite lie between c. 257.5 Ma and c. 246 Ma,
with the exception of the Barrington Tops Granodiorite (c. 281 Ma to c. 258 Ma), and the Greymare
Granodiorite and the Kaloe Tonalite, both early Permian (Table 4.1).
This Record provides SHRIMP U–Pb zircon ages for four members of the Clarence River Supersuite,
all in the northeastern region of the New England Orogen in New South Wales (Figure 4.1). The
reported ages cluster very closely, all between 256 and 255 Ma, demonstrating that these granites are
spatially and temporally related. The closeness of these ages reconciles much of the disparity
between previous ages for these granites which were collected using a range of isotopic systems,
suggesting that the previous ages did not reflect the age of magmatic crystallisation (Table 4.1).
These ages demonstrate that many of the granites of the Clarence River Supersuite have an age
similar to (though slightly older than) most of the dated granite plutons in the southern New England
Orogen. The older ages of the Kaloe Tonalite (shown in Figure 4.1), the Greymare Granodiorite (to the
north in Queensland) and Barrington Tops Granodiorite (to the south) appear to be exceptional, rather
than dominant ages in this supersuite.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 77
Table 4.1 Previous and new isotopic dates determined on members of the Clarence River Supersuite and
potential correlatives discussed herein.
Unit
Age (Ma)
Method
Reference
Duncans Creek Trondhjemite
c. 248–246
249.4 ± 2.9
252.1 ± 3.2
Rb-Sr biotite
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
Shaw and Flood (1993)
Cawood et al. (2011)
Cawood et al. (2011)
Dumbudgery Creek Granodiorite
c. 250–249
253.6 ± 1.0
256.9 ± 1.8
255.0 ± 1.0
Rb–Sr biotite
40Ar/39Ar hornblende (plateau)
40Ar/39Ar hornblende (plateau)
SHRIMP U–Pb zircon
Shaw and Flood (1993)
Bryant et al. (1997a)
Bryant et al. (1997a)
This study
Gogs Top Trondhjemite
c. 255
Rb–Sr whole rock
Hensel et al. (1985)
Jenny Lind Granite c. 249.7
255.3 ± 1.2
40
Ar/
39
Ar hornblende (total fusion)
SHRIMP U–Pb zircon
Bryant et al. (1997a)
This study
Mount Ephraim Granodiorite c. 265
c. 273
255.3 ± 1.3
Sm–Nd whole rock
ID-TIMS Pb–Pb zircon
SHRIMP U–Pb zircon
Hensel et al. (1985)
Kimbrough et al (1993)
Waltenberg et al. (2015)
Towgon Grange Tonalite
c. 257.5
255.4 ± 1.2
40Ar/39Ar hornblende (total fusion)
SHRIMP U–Pb zircon
Bryant et al. (1997a)
This study
Bruxner Monzogranite
256.0 ± 1.4
SHRIMP U–Pb zircon
This study
Barrington Tops Granodiorite
c. 258
c. 265
265 ± 2
269 ± 2
265 ± 8
c. 281
c. 269
267.2 ± 1.4
267.8 ± 1.4
272.5 ± 1.6
277.1 ± 1.9
K–Ar biotite
Sm–Nd whole rock
K–Ar biotite
K–Ar
U–Pb
ID-TIMS Pb–Pb zircon
LA-ICP-MS U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
Cooper et al. (1963)
Hensel et al. (1985)
Roberts et al. (1991)
Roberts et al. (1991)
Collins et al. (1993)
Kimbrough et al. (1993)
Phillips et al. (2011)
Cawood et al. (2011)
Waltenberg et al. (2015)
Waltenberg et al. (2015)
Waltenberg et al. (2015)
Greymare Granodiorite
279.6 ± 2.6
SHRIMP U–Pb zircon
Donchak et al. (2013)
Kaloe Tonalite
293.1 ± 1.8
291.9 ± 2.0
40Ar/39Ar hornblende (plateau)
SHRIMP U–Pb zircon
Bryant et al. (1997a)
Cawood et al. (2011)
Cullens Creek Granite
—
—
No published age data
Linden Hill Monzogranite
—
—
No published age data
Koreelan Creek Granodiorite
246.3 ± 1.4
SHRIMP U–Pb zircon
Chisholm et al. (2014c)
Boxwell Granodiorite
256.6 ± 1.7
SHRIMP U–Pb zircon
Cross and Blevin (2010)
Rocky Creek Granodiorite
257.3 ± 1.5
SHRIMP U–Pb zircon
Waltenberg et al. (2015)
78 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Figure 4.1 Map showing the extent of the Clarence River Supersuite in the northeastern New England Orogen in
New South Wales. Outcropping extents of Clarence River Supersuite members are shaded in blue, and the older
Kaloe Tonalite (also a member of the Clarence River Supersuite) is shown in purple. Other granite plutons are
marked in orange. Sample locations from this Record are shown as circles: yellow for Clarence River Supersuite,
blue for Herries Supersuite, and red for Wandsworth Volcanic Group. The New South Wales – Queensland border is
the grey-black line, pink lines indicate roads.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 79
The Koreelan Creek Granodiorite was formerly classified within the Bruxner Suite in the Clarence
River Supersuite. Chisholm et al. (2014c) determined a distinct Early Triassic age (246.3 ± 1.4 Ma),
which implies that it does not belong in the Clarence River Supersuite. The new age of 256.0 ± 1.4 Ma
for the Bruxner Monzogranite confirms that these two granites are different, and that removal of the
Koreelan Creek Granodiorite from the Bruxner Suite is justified.
Stroud (1992) outlined the similarities between the Boxwell Suite and the Clarence River Supersuite,
and Donchak et al. (2013) classified the Boxwell Suite into the Clarence River Supersuite on
geochemical grounds. The age of the Boxwell Granodiorite (256.6 ± 1.7 Ma; Cross and Blevin, 2010),
and the Rocky Creek Granodiorite (257.3 ± 1.5 Ma; Waltenberg et al., 2015) are not inconsistent with
the 256–255 Ma ages obtained in this Record for members of the Clarence River Supersuite.
Granites of the Clarence River Supersuite currently span more than 40 million years. Shaw and Flood
(1981) intended their classification to distinguish “groups of plutons with distinct mineralogical,
geochemical, isotopic and age characteristics”. The diversity of ages indicates that classification of
granites into the Clarence River Supersuite requires refinement.
4.2.2 Herries Supersuite and Stanthorpe Supersuite
The Newton Boyd Granodiorite was dated to test whether it can be classified into the Herries
Supersuite. The Herries Supersuite as defined by Donchak et al. (2007) comprised 11 moderately to
strongly evolved I-type granites. Nine of these have since been dated, as shown in Table 4.2. These
plutons are characterised by their lack of mineralisation (relative to the Stanthorpe Granite) as well as
a range of geochemical and mineralogical indicators, including low Rb, abundant plagioclase,
prominent hornblende, common mafic enclaves and no miarolitic cavities (Donchak et al., 2007).
Donchak et al. (2013) incorporated the Herries Supersuite (along with the Bullaganang, Ballandean
and Mount You You Supersuites) into the Stanthorpe Supersuite, despite the range in ages of the
various Supersuites, from the Stanthorpe Supersuite at c. 248–245 Ma, to the Mount You You
Supersuite at c. 298 Ma (Donchak et al., 2013). An existing U–Pb SHRIMP zircon age showed that the
Palgrave Granite in the Herries Supersuite (Donachak et al., 2007) is Late Permian. Three other
plutons in the Herries Supersuite have Triassic Rb-Sr ages (Shaw and Flood, 1993), similar to the
known age range of the Stanthorpe Supersuite at the time.
In a subsequent geochronology campaign by Chisholm et al. (2014c), a total of 13 U–Pb SHRIMP
zircon dates from Stanthorpe Supersuite plutons revealed that three fifths are Early Triassic in age. In
contrast, Li et al. (2012), Chisholm et al. (2014c), and this study demonstrate that the Herries
Supersuite plutons are (with the exception of the Boonoo Granite) late Permian (Table 4.2). Seven are
late Permian: five of the granites are in the age range of 253.3–252.0 Ma and two are older, at 258–
255 Ma. The Boonoo Granite is Early Triassic and the Botumburra Range Monzogranite has a Rb–Sr
age of 228–227 Ma.
As for the Stanthorpe Supersuite, the fact that the U–Pb SHRIMP zircon ages of the constituent
plutons span more than 50 million years suggests that the defining criteria for this supersuite need
revisiting.
80 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Table 4.2 Previous and new isotopic dates determined on members of the Herries Supersuite.
Unit
Age (Ma)
Method
Reference
Boonoo Granite
245.6 ± 2.5
SHRIMP U–Pb zircon
Li et al. (2012)
Billys Creek Tonalite
—
—
No published age data
Botumburra Range Monzogranite
c. 228–227
Rb–Sr biotite
Shaw and Flood (1993)
Clare Hills Granite
252.0 ± 2.5
SHRIMP U–Pb zircon
Li et al. (2012)
Dundurrabin Granodiorite
—
—
No published age data
Fairleigh Granite
253.3 ± 2.5
SHRIMP U–Pb zircon
Li et al. (2012)
Four Bull Granodiorite
258.4 ± 1.3
SHRIMP U–Pb zircon
Chisholm et al. (2014b)
Herries Granite
c. 253–248
252.1 ± 2.5
Rb–Sr biotite
SHRIMP U–Pb zircon
Shaw and Flood (1993)
Li et al. (2012)
Palgrave Granite
256.0 ± 1.8
SHRIMP U–Pb zircon
Donchak et al. (2007)
Maryland Granite
c. 242
252.4 ± 1.5
Rb–Sr biotite
SHRIMP U–Pb zircon
Shaw and Flood (1993)
Chisholm et al. (2014b)
Newton Boyd Granodiorite
252.8 ± 1.0
SHRIMP U–Pb zircon
This study
4.2.3 Drake Volcanics and Wandsworth Volcanic Group
The Wandsworth Volcanic Group as defined by Barnes et al. (1991) represents extensive terrestrial
and shallow marine volcanism in the mid–late Permian. The Wandsworth Volcanic Group has been
variably dated to the Permian or Triassic by different investigators (Table 4.3), but modern U–Pb
techniques have established that the Wandsworth Volcanic Group sits firmly in the late Permian.
The Drake Volcanics, defined by Barnes et al. (1991) as the base of the Wandsworth Volcanic Group,
are the exception to the late Permian age for the Wandsworth Volcanic Group. Cross and Blevin
(2010) obtained a U–Pb SHRIMP zircon age of 264.4 ± 2.5 Ma from the middle of the Drake
Volcanics, and the two new ages reported here (265.3 ± 1.4 Ma and 265.3 ± 1.5 Ma) demonstrate that
the Drake Volcanics predate the bulk of the Wandsworth Volcanic Group by approximately ten million
years. Chisholm et al. (2014a) investigated the next-lowest units in the Wandsworth Volcanic Group
and did not date anything older than 256.3 ± 1.5 Ma. On the basis of the age difference, the Drake
Volcanics should be considered a distinct unit from the Wandsworth Volcanic Group (see also Cross
and Blevin, 2010).
The two Drake Volcanics ages reported here are derived from high-level intrusions in the Drake
Volcanics and these intrusions are considered the source of mineralisation for their respective
deposits. The new and existing SHRIMP ages from the dated units in the Drake Volcanics are
indistinguishable and show that the middle to upper section of the Drake Volcanics, including the
mineralising intrusions, were emplaced within the space of 1–2 million years. 40 Ar/39Ar dating of
alteration micas at Drake was unsatisfactory as an age could not be resolved, but we suggest that the
White Rock and Red Rock intrusion ages are a proxy for the mineralisation ages for the epithermal
systems at Drake.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 81
Table 4.3 Previous and new isotopic dates determined on the Drake Volcanics and Wandsworth Volcanic Group
(in stratigraphic order, youngest to oldest).
Unit
Age (Ma)
Method
Reference
Dundee Rhyodacite
c. 242
c. 249–246
257.6 ± 2.5
250.9 ± 1.9
254.1 ± 2.2
254.34 ± 0.34
K-Ar
Rb-Sr biotite
LA-ICP-MS U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
TIMS U–Pb zircon
Evernden and Richards (1962)
Shaw and Flood (1993)
Belousova et al. (2006)
Black (2007)
Brownlow et al. (2010)
Brownlow et al. (2010)
Emmaville Volcanics
- undifferentiated
- Arranmor Ignimbrite Member
- Dellwood Ignimbrite Member
- Magistrate Volcanic Member
252.3 ± 2.0
256.4 ± 1.6
254.8 ± 1.5
252.5 ± 1.5
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
Cross and Blevin (2013)
Black (2006)
Black (2006)
Black (2006)
Wallangarra Volcanics
- undifferentiated
253 ± 2.4
SHRIMP U–Pb zircon
Cross et al. (2009)
Undifferentiated Wandsworth
Volcanic Group
- ‘Uralla volcanics’
- ‘Kurrajong Park volcanics’
- ‘Attunga Creek volcanics’
256.0 ± 1.5
256.3 ± 1.5
255.8 ± 1.5
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
Chisholm et al. (2014a)
Chisholm et al. (2014a)
Chisholm et al. (2014b)
Drake Volcanics
- Intruding DV4 package
- Intruding DV2 package
- DV2 package sill
265.3 ± 1.4
265.3 ± 1.5
264.4 ± 2.5
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
SHRIMP U–Pb zircon
This study
This study
Cross and Blevin (2010)
82 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Acknowledgements
This analytical program was conducted using high-quality zircon separates, mounts, and images
skilfully prepared by David DiBugnara, Joanne Tubby, and Benjamin Linehan (Mineral Separation
Laboratory, GA). Patrick Burke and Chuck Magee (SHRIMP Laboratory, GA) provided valuable
technical support and assistance in optimising analytical conditions during data acquisition. Simon
Bodorkos provided invaluable scientific and technical advice. We thank Natalie Kositcin and Chris
Lewis (GA) for reviews which greatly improved the manuscript.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 83
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88 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Appendix A Analytical Procedures
A.1 Summary
All isotopic analyses reported in this record were undertaken using the SHRIMP IIe at Geoscience
Australia (GA) in Canberra. A summary of key parameters from individual analytical sessions is shown
in Appendix Table A 2. The analytical procedures adopted (outlined in sections A.3–A.4) for zircon
follow those published by Compston et al. (1984), Claoué-Long et al. (1995), Nelson (1997), Williams
and Claesson (1987), and Williams (1998). Parameters specific to this Record are updated below.
A.2 Sample Preparation
Sample preparation methods (sampling, crushing and zircon separation) followed those documented
by Chisholm et al. (2014d). Additional details specific to the reported analyses are outlined below.
A.2.1 Sample Acquisition
The locations of field sites refer to the Geocentric Datum of Australia 1994 (GDA94). Coordinates are
reported as decimal latitude and longitude, and as Map Grid of Australia eastings and northings
(MGA94). Street names were verified using the SIX Maps online service (https://maps.six.nsw.gov.au/)
provided by the Land and Property Information division of the NSW Department of Finance and
Services, New South Wales Government. Site locations are labelled using relevant identifiers in
corporate databases: SITES SiteID for GSNSW, FIELDSITES SampleNo for GA. Samples are located
using GPS (accurate to 10 m) unless otherwise specified.
A.2.2 Mount Preparation
Mount preparation follows the methods documented by DiBugnara (2016). Key details specific to the
reported analyses are outlined here.
Where available, up to 200 handpicked grains for each sample were mounted in a 25 mm epoxy-resin
disc together with calibration and reference zircons (Appendix Table A 1). After cooling, the mount
was placed in an oven at 60°C for 24 hours to assist epoxy curing. Following curing, the mount was
polished on a Struers polisher to expose an equatorial cross-section through the mounted zircon
grains, photographed in reflected and transmitted light using a Leica DM6000 microscope with a
mounted Leica DFC310 FX 1.3 Mp camera, cleaned with ethanol and Milli-Q water and coated with a
2 nm coat of gold prior to cathodoluminescence imaging in a JEOL JSM-6490LV Scanning Electron
Microscope housed at GA. The thin gold coat was then removed using 99.95% ethanol and the mount
cleaned ultrasonically using first RB32 detergent, then petroleum spirit, followed by 99.95% ethanol
and finally triple rinsed in Milli-Q water. The mount was dried in a 30°C oven prior to being coated with
15 nm of 99.999% gold.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 89
Appendix Table A 1 Calibration and reference zircons mounted on epoxy resin discs, together with unknowns.
Zircon
Number
of grains
Purpose
TEMORA2
~100
206Pb/238U reference; 416.8 ± 0.3 Ma, (Black et al., 2004)
OG1
~85
207Pb/206Pb reference; 3465.4 ± 0.6 Ma; (Stern et al., 2009)
QGNG
~10
Lu–Hf reference; alternative 206Pb/238U and 207Pb/206Pb reference
FC1
~10
O-isotope reference
M127
1
Uranium concentration reference; 923 ppm U (Nasdala et al., 2008; 2016)
91500
1
Alternate uranium concentration reference; 81 ppm U (Wiedenbeck et al., 1995)
Metamict
1
Zircon high in non-radiogenic lead, used to calibrate 204Pb peak offset from 196Zr2O
The mount was loaded into the high-vacuum SHRIMP sample lock chamber 48 hours prior to analysis
to enable complete out-gassing of the newly-cured epoxy and lowered into the SHRIMP source
chamber immediately prior to analysis.
A.3 Zircon Data Acquisition and Processing
A.3.1 Data Acquisition
Sensitive High Resolution Ion MicroProbe (SHRIMP) U–Pb zircon analyses were obtained on the
SHRIMP-IIe instrument housed at GA.
A 100 µm Kohler aperture was used to achieve a 20 µm diameter primary beam of O2
ˉ ions at 10 keV
(selected by means of a Wein filter) with total ion count at the mount surface between 2.0 and 5.0 nA
to cause atoms on the crystal surface to sputter and ionise. Approximately 3 ng of sample is sputtered,
forming a flat-bottomed pit 0.5–2 µm deep. Sputtered secondary ions are extracted from the sample
surface and are focussed and steered through an electrostatic analyser and magnet into a collector
through a series of electrostatic lenses and steering plates.
Prior to each analysis the surface of the analysis site was cleaned by rastering the beam for 3 minutes
to reduce the amount of surface common lead. Data acquisition involved 6 magnet cycles through a
ten mass-station run-table with the following counting times in seconds (s): 196Zr2O (2 s), 204Pb (20 s),
background 204.1 (20 s), 206Pb (15 s), 207Pb (40 s), 208Pb (5 s), 238U (5 s), 248ThO (2 s), 254UO (2 s) and
270UO2 (2 s). A full analysis (six scans of the run-table) was set for 25 minutes with these conditions.
Once the instrument was tuned the analytical session commenced with an analysis of M127 zircon for
calibration of the uranium concentration measurement (923 ppm; Nasdala et al., 2008) and data
program inputs for SHRIMP software. This was followed by reference zircon analyses for an early
indication of machine performance: 206Pb/238U was calibrated using the TEMORA2 reference zircon
(206Pb/238U = 416.8 Ma, Black et al., 2004), and 207Pb/206Pb monitored using the OG1 reference zircon
(207Pb/206Pb = 3465.4 Ma, Stern et al., 2009).
Standard procedures in the GA Geochronology Laboratory for analysis site labels are used herein and
take the form: SampleNumber.GrainNumber.SpotNumber; where SampleNumber is the final three
digits of the GA SampleNo, grains are numbered in sequence of analysis, and spots are numbered in
sequence of analysis within a grain (e.g. 495.10.1).
90 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Analyses were collected in a ‘round-robin’ sequence of one measurement from each igneous sample
on the mount in turn with one measurement of TEMORA2 reference zircon made after every third
sample analysis, and a minimum of one OG1 reference zircon measured after every second
TEMORA2 measurement. Over the course of the session periodic data reduction was undertaken to
monitor machine performance and assess the 206 Pb/238U calibration and 207Pb/206Pb ratios.
Appendix Table A 2 Summary of session-specific metadata.
Session 150080
Session 160009
Session 160021
Mount ID
GA6308
GA6317
GA6323
Analyst
Kathryn Waltenberg
Kathryn Waltenberg
Kathryn Waltenberg
Session dates
16/09/2015 to 21/09/2015
01/02/2016 to 06/02/2016
07/03/2016 to 09/03/2016
206Pb/238U reference zircon
TEMORA2: 416.8 Ma
TEMORA2: 416.8 Ma
TEMORA2: 416.8 Ma
Analyses used
66 of 66
64 of 64
31 of 32
Slope of robust regression
(95% confidence): spot age
(Ma) / session duration
(hours)
-0.002 +0.025/-0.024
(no drift correction
applied)
+0.02 +0.03/-
0.03
(no drift
correction
applied)
+0.09 +0.12/-
0.11
(no drift
correction
applied)
206Pb/238U session-to-
session error (2σ)
0.21%
0.28%
0.34%
206Pb/238U spot-to-spot error
(1σ) used [calculated]
0.75% [from 0.55%]
0.85%
0.76%
Mean 207Pb/206Pb age (Ma,
95% confidence)
404 ± 17 Ma
421 ± 12 Ma
425 ± 17 Ma
Mean 204Pb
overcounts/second from
207
Pb (95% confidence)
+0.008 ± 0.014
(no correction)
-
0.006 ± 0.015
(no correction)
-
0.01 ± 0.03
(no correction)
207Pb/206Pb reference zircon
OG1: 3465.4 Ma
OG1: 3465.4 Ma
OG1: 3465.4 Ma
Analyses used
34 of 34
28 of 28
17 of 17
Mean 207Pb/206Pb age (Ma,
95% confidence)
3466.2 ± 1.4 Ma
(no IMF correction)
3465.6 ± 1.5 Ma (no IMF
correction
3466.8 ±
2.1 Ma
(no IMF correction)
Number of samples
analysed
5
4
2
GSNSW SiteID
(GA SampleNo)
PB-15-NEO-1 (2306495)
NSWSJAF0102
(2309502)
PB-15-NEO-9 (2550240)
GSNSW SiteID
(GA SampleNo)
PB-15-NEO-4 (2306500)
NSWSJAF0103
(2309503)
PB-15-NEO-10 (2550241)
GSNSW SiteID
(GA SampleNo)
PB-15-NEO-5 (2306501)
ERIVDEC12.01A
(2309504)
GSNSW SiteID
(GA SampleNo)
PB-15-NEO-6 (2306502)
PB-15-CHRON-01
(2309505)
GSNSW SiteID
(GA SampleNo)
PB-15-NEO-7 (2306503)
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 91
A.3.2 Data Processing
Data from the SHRIMP was processed, calculated and portrayed using Microsoft Excel® 2003, and
the add-ins SQUID 2.50.11.02.03 (Ludwig, 2009) and Isoplot v3.76.12.02.24 (February 2012 revision
of Ludwig, 2003) downloaded from Sourceforge (http://sourceforge.net/). SQUID processes standard
and spot data for evaluation, calculates element concentrations and isotopic ratios required for age
calculation, and propagates the uncertainties associated with the 206 Pb/238U ages. The decay
constants used are those of Jaffey et al. (1971), together with present-day 238U/235U = 137.88,
following the recommendations of Steiger and Jäger (1977).
Common Pb corrections for unknowns were based on measured 204Pb, and a Pb isotopic composition
calculated using the single-stage Pb isotopic evolution model of Stacey and Kramers (1975) at an age
corresponding to the individually estimated age of each unknown analysis. The result of this
calculation is expressed for each analysis in each of the analytical data-tables, in terms of common
206Pb as a percentage of total measured 206Pb. All isotopic ratios and ages cited in this Record are
corrected for common 206Pb.
Ages derived from the pooling of multiple individual analyses are error-weighted means, and their
uncertainties are quoted at the 95% confidence level. Each error-weighted mean has an associated
Mean Square of Weighted Deviates (MSWD) value, which is a measure of the degree of scatter of the
constituent analyses relative to the assigned uncertainties (Ludwig, 2003), and a probability of
equivalence value (P), which is the probability that all of the constituent analyses are equivalent within
their uncertainties. By convention, scatter beyond the assigned uncertainties is assumed to be present
when the probability of equivalence is less than 0.05.
In cases where P is equal to or greater than 0.05, but the MSWD value exceeds 1, the implied
dispersion of the data-points beyond their analytical uncertainties is acknowledged by expanding the
95% confidence interval of the mean, via multiplication of its 1σ uncertainty firstly by Student’s t for n-1
degrees of freedom (where n is the number of constituent analyses), and secondly by the square root
of the MSWD value (Ludwig, 2003). This resulting value, incorporating the effect of the MSWD, is the
95% confidence interval for each new date reported in this Record.
A.3.3 Calibration Procedures
Elemental U concentrations in the unknown zircons were calibrated using the M127 reference zircon
(923 ppm U; Nasdala et al., 2008, 2016), and the power-law relationship of Claoué-Long et al. (1995):
Zr2
196 O+/ U+
238 = A × UO+
254 / U+
238 0.66
(1)
where A is a session-dependent constant determined from measurements on M127. All U
concentration data tabulated for unknowns have uncertainties of the order of 15–20%, based on the
extent of known variations in U abundance in M127.
The values of 232Th/238U in the unknown zircons were calculated using the relationship proposed by
Williams et al. (1996):
Th
232 U
238
=ThO+
248 /UO+
254 ×0.03446× UO+
254 / U+
238 +0.868 (2)
92 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
The values of 206Pb/238U in the unknowns were calibrated using the TEMORA2 reference zircon
(206Pb/238U = 0.0668, corresponding to an age of 416.8 Ma; Black et al., 2004), and a power-law
relationship (Claoué-Long et al., 1995) of the form:
Pb+
206 / U+
238 = B × UO+
254 / U+
238 2
(3)
where B is a session-dependent constant determined from measurements on TEMORA2.
The values of 207Pb/206Pb in the unknowns were monitored using the OG1 reference zircon
(207Pb/206Pb = 0.29907 ± 0.00011, corresponding to an age of 3465.4 ± 0.6 Ma; Stern et al., 2009). For
each session, the error-weighted mean 207Pb/206Pb date for OG1 was calculated (Appendix Table A 2).
Uncertainties on individual 207Pb/206Pb analyses of Phanerozoic zircons are dominated by (relatively)
poor counting statistics, and are almost always large enough that both the instrumental mass
fractionation correction and its associated uncertainty are rendered insignificant. Consequently, the
tabulated values of 207 Pb/206Pb for each sample have not been corrected for instrumental mass
fractionation.
A.3.4 Propagation of Uncertainties
In each session, a ‘calibration constant’ value is determined for each individual analysis of the
206Pb/238U reference zircon TEMORA2 (i.e. bi = [206 Pb+/238U+]i/([254UO+/238U+]i
2). Uncertainties
associated with each of these individual ‘calibration constants’ (i.e. ± bi) are governed primarily by the
counting statistics associated with the constituent isotopic ratio(s). The value of the session ‘calibration
constant’ (B; see equation (3)) is calculated as the error-weighted mean of the session-specific
population of individual calibration constants. However, these populations commonly display
significant excess scatter, manifested as an MSWD value for B that exceeds unity, despite the fact
that most reference zircons are (by definition) characterised by 206Pb/238U homogeneity at a range of
scales. This indicates that the values of ± bi are usually underestimated in going from analysis to
analysis. Consequently, for each session, SQUID calculates the constant additional uncertainty per
spot (expressed as a percentage) that needs to be added in quadrature to each ± bi value, in order to
produce MSWD ~1 for the population of bi values used to calculate B (Ludwig, 2009). This constant
additional uncertainty is termed the external ‘spot-to-spot error’ (or ‘reproducibility’), and its session-
specific 1σ values are presented in Appendix Table A 2. The 1σ spot-to-spot error is added in
quadrature to the other sources of error (principally related to counting statistics and the common Pb
correction) for each value of 206Pb/238U in the unknowns, and thus is incorporated in the uncertainties
for all individual 206Pb/238U values presented in the analytical data-tables.
For each session, SQUID also calculates an uncertainty for the session-specific calibration constant
(i.e. ± B). This uncertainty is termed the ‘session-to-session error’ (or ‘calibration uncertainty’), and its
session-specific 2σ values are presented in Appendix Table A 2. The session-to-session error is not
included in the uncertainties for individual 206 Pb/238U values presented in the analytical data-tables,
and should be neglected when comparing error-weighted mean 206Pb/238 U ages for unknowns co-
analysed in a single analytical session. However, it must be accounted for when seeking to compare
206Pb/238U datasets more widely (e.g. between different analytical sessions), especially when
calculating error-weighted mean 206Pb/238U ages for unknowns, because the session-to-session error
can be of comparable magnitude to the 95% confidence interval arising from population statistics. See
this same section in Waltenberg et al. (2015) for a worked example.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 93
A.4 Session-specific calibrations and data processing
The data reported herein was obtained in three analytical sessions. Session-specific calibrations and
data processing are summarised in Appendix Table A 2 and detailed session-specific information is
below.
A.4.1 Session 150080: Mount GA6308, 16–21 September 2015
This session comprised analysis of five samples:
• PB-15-NEO-1 (2306495) Bruxner Monzogranite
• PB-15-NEO-4 (2306500) Jenny Lind Granite
• PB-15-NEO-5 (2306501) Dumbudgery Creek Granodiorite
• PB-15-NEO-6 (2306502) Towgon Grange Tonalite
• PB-15-NEO-7 (2306503) Newton Boyd Granodiorite
A total of 66 analyses of the TEMORA2 zircon standard were obtained during SHRIMP session
150080. Instrument operating conditions remained stable throughout the session, with the TEMORA2
dataset illustrating no trend as the session progressed. The slope of the robust regression of individual
TEMORA2 206Pb/238U dates (Ma) against the cumulative duration of the session at the time of
acquisition (measured in hours since commencement) was indistinguishable from zero at the 95%
confidence level (-0.002 +0.025/-0.024), and no secular drift correction was applied to the 206Pb/238U
calibration.
All 66 analyses were used to define the calibration (Appendix Table A 2), with a 2σ session-to-session
error of 0.21% and a 1σ spot-to-spot error of 0.55%. The session-to-session error is included, in
quadrature, in the uncertainty of each weighted mean 206Pb/238U date calculated for this session. The
spot-to-spot error, however, is lower than that typically determined for TEMORA2 on the GA SHRIMP
IIe (e.g., Appendix A.3.5 of Bodorkos et al., 2013), which raises the possibility that the value
determined for session 150080 is underestimated. Consequently, the spot-to-spot error for session
150080 was increased to a nominal value of 0.75% (1σ) prior to its incorporation in quadrature into the
uncertainties of individual 206Pb/238U analyses in the unknowns.
Overcounts on mass 204Pb were monitored through reference to the robust mean of the 207Pb/206Pb
ages determined for TEMORA2 (404 ± 17 Ma). This result is marginally younger than the reference
value (416.8 ± 0.38 Ma; Black et al., 2004) at the 95% confidence level; however estimated 204Pb
overcounts based on measured 207Pb are indistinguishable from zero (+0.008 ± 0.014). This suggests
that 204Pb was counted correctly, so no correction was applied.
A total of 34 analyses of the 207Pb/206Pb reference zircon OG1 were obtained over the SHRIMP
session with a weighted mean of 3466.2 ± 1.4 Ma (MSWD = 1.03; n = 34 of 34). As this result is
indistinguishable from the OG1 reference value of 3465.4 ± 0.6 Ma (Stern et al., 2009), no correction
for instrumental fractionation of 207Pb/206Pb was applied to the session.
94 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
Appendix Figure A 1 SHRIMP 206Pb/238U dates on TEMORA2 zircon during session 150080. Blue horizontal line is
the reference age of 416.8 Ma.
A.4.2 Session 160009: Mount GA6317, 1–6 February 2016
This session comprised analysis of four samples:
• NSWSJAF0102 (2309502) Babinda Volcanics
• NSWSJAF0103 (2309503) Shuttleton Rhyolite Member
• ERIVDEC12.01A (2309504) Unnamed quartz monzonite, ‘Hobbs Pipe’ at Mt Adrah
• PB-15-CHRON-01 (2309505) Unnamed rhyolite, ‘younger volcanics’ at Yerranderie
A total of 64 analyses of the TEMORA2 zircon standard were obtained during SHRIMP session
160009. Instrument operating conditions remained stable throughout the session, with the TEMORA2
dataset illustrating no trend to higher or lower values as the session progressed. The slope of the
robust regression of individual TEMORA2 206Pb/238U dates (Ma) against the cumulative duration of the
session at the time of acquisition (measured in hours since commencement) was indistinguishable
from zero at the 95% confidence level (+0.02 +0.03/-0.03), and no secular drift correction was applied
to the 206Pb/238U calibration.
All 64 analyses were used to define the calibration (Appendix Table A 2), with a 2σ session-to-session
error of 0.28% and a 1σ spot-to-spot error of 0.85%. The session-to-session error is included, in
quadrature, in the uncertainty of each weighted mean 206Pb/238U date calculated for this session.
Overcounts on mass 204Pb were monitored through reference to the robust mean of the 207Pb/206Pb
ages determined for TEMORA2 (421 ± 17 Ma). This result is indistinguishable from the reference
value (416.8 ± 0.38 Ma; Black et al., 2004) at the 95% confidence level; and estimated 204Pb
overcounts based on measured 207Pb are indistinguishable from zero (+0.006 ± 0.015). This suggests
that 204Pb was counted correctly, so no correction was applied.
A total of 28 analyses of the 207Pb/206Pb reference zircon OG1 were obtained over the SHRIMP
session with a weighted mean of 3465.6 ± 1.4 Ma (MSWD = 1.10; n = 28 of 28). As this result is
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 95
indistinguishable from the OG1 reference value of 3465.4 ± 0.6 Ma (Stern et al., 2009), no correction
for instrumental fractionation of 207Pb/206Pb was applied to the session.
Appendix Figure A 2 SHRIMP 206Pb/238U dates on TEMORA2 zircon during session 160009. Blue horizontal line is
the reference age of 416.8 Ma.
A.4.3 Session 160021: Mount GA6323, 7–9 March 2016
This session comprised analysis of two samples:
• PB-15-NEO-9 (2550240) Drake Volcanics at Red Rock
• PB-15-NEO-10 (2550241) Drake Volcanics at White Rock
A total of 32 analyses of the TEMORA2 zircon standard were obtained during SHRIMP session
160021. Instrument operating conditions remained stable throughout the session, with the TEMORA2
dataset illustrating no trend to higher or lower values as the session progressed. The slope of the
robust regression of individual TEMORA2 206Pb/238U dates (Ma) against the cumulative duration of the
session at the time of acquisition (measured in hours since commencement) was indistinguishable
from zero at the 95% confidence level (+0.09 +0.12/-0.11), and no secular drift correction was applied
to the 206Pb/238U calibration.
Thirty-one of 32 analyses were used to define the calibration (Appendix Table A 2), with a 2σ session-
to-session error of 0.34% and a 1σ spot-to-spot error of 0.76%. One analysis was excluded as it was
affected by a brief period of instrumental instability. The session-to-session error is included, in
quadrature, in the uncertainty of each weighted mean 206Pb/238U date calculated for this session.
Overcounts on mass 204Pb were monitored through reference to the robust mean of the 207Pb/206Pb
ages determined for TEMORA2 (425 ± 17 Ma). This result is marginally older than the reference value
(416.8 ± 0.38 Ma; Black et al., 2004) at the 95% confidence level; however estimated 204Pb overcounts
based on measured 207Pb are indistinguishable from zero (+0.01 ± 0.03). This suggests that 204Pb was
counted correctly, so no correction was applied.
96 New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016
A total of 17 analyses of the 207Pb/206Pb reference zircon OG1 were obtained over the SHRIMP
session with a weighted mean of 3466.8 ± 2.1 Ma (MSWD = 1.6; n = 17 of 17). As this result is
indistinguishable from the OG1 reference value of 3465.4 ± 0.6 Ma (Stern et al., 2009), no correction
for instrumental fractionation of 207Pb/206Pb was applied to the session.
Appendix Figure A 3 SHRIMP 206Pb/238U ages on TEMORA2 zircon during session 160021. Red analysis was
discarded due to instrumental instability. Blue horizontal line is the reference age of 416.8 Ma.
New SHRIMP U–Pb zircon ages from the Lachlan Orogen and the New England Orogen, July 2015–June 2016 97
