Evaluating the role of deep granitic rocks in generating anomalous temperatures in SW-Queensland

Abstract

Across central Australia and south-west Queensland, a large (~800,000km2) subsurface temperature anomaly occurs (Figure 1). Temperatures are interpreted to be greater than 235°C at 5km depth, ca. 85°C higher than the average geothermal gradient for the upper continental crust (Chopra & Holgate, 2005; Holgate & Gerner, 2011). This anomaly has driven the development of Engineered Geothermal Systems (EGS) at Innamincka, where high temperatures have been related to the radiogenic heat production of High Heat Producing Granites (HHPG) at depth, below thermally insulative sedimentary cover (Chopra & Holgate, 2005; Draper & D’Arcy, 2006; Meixner & Holgate, 2009). To evaluate the role of granitic rocks at depth in generating the broader temperature anomaly in SW-Queensland, we sampled 25 granitic rocks from basement intervals of petroleum drill cores below thermal insulative cover along two transects (WNW–ESE and NNE–SSW — Figure 1) and performed a multidisciplinary study involving petrography, whole-rock chemistry, zircon dating and thermal conductivity measurements.

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Queensland Geological Record 2012/14 1
Evaluating thE rolE of dEEp granitic rocks in
gEnErating anomalous tEmpEraturEs
in south-wEst QuEEnsland
C. Siégel1, 4, S.E. Bryan1, 4, D.J. Purdy2, C.M. Allen3, C.E. Schrank1, 6, T. Uysal4, D.A.
Gust1 and G.R. Beardsmore5
1Earth, Environmental and Biological Sciences, Earth Systems, Queensland University of Technology
2Geological Survey of Queensland
3Research School of Earth Sciences, Australian National University
4Queensland Geothermal Energy Centre of Excellence, The University of Queensland
5Hot Dry Rocks Pty Ltd
6School of Earth and Environment, The University of Western Australia
INTRODUCTION
Across central Australia and south-west Queensland, a large (~800,000km2)
subsurface temperature anomaly occurs (Figure 1). Temperatures are interpreted to
be greater than 235°C at 5km depth, ca. 85°C higher than the average geothermal
gradient for the upper continental crust (Chopra & Holgate, 2005; Holgate &
Gerner, 2011). This anomaly has driven the development of Engineered Geothermal
Systems (EGS) at Innamincka, where high temperatures have been related to the
radiogenic heat production of High Heat Producing Granites (HHPG) at depth,
below thermally insulative sedimentary cover (Chopra & Holgate, 2005; Draper &
D’Arcy, 2006; Meixner & Holgate, 2009). To evaluate the role of granitic rocks at
depth in generating the broader temperature anomaly in SW-Queensland, we sampled
25 granitic rocks from basement intervals of petroleum drill cores below thermal
insulative cover along two transects (WNW–ESE and NNE–SSW — Figure 1) and
performed a multidisciplinary study involving petrography, whole-rock chemistry,
zircon dating and thermal conductivity measurements.
RESULTS
The petrography, composition and degree of alteration vary widely for the sampled
granitic rocks. They range from ne-grained to porphyritic, extensively altered to
fresh, and tonalite to syenogranite in composition (Figure 2). Additionally, S-type
granites (indicated by abundant primary muscovite and a large degree of zircon
inheritance), and I-type biotite granites (indicated by accessory titanite) are both
present. In many instances, factors such as the Aluminium Saturation Index (Chappell
& White, 2001) are not applicable due to the alteration-induced peraluminous nature
of the granites. Ti temperatures calculated for concordant zircons (calculated with
a TiO2 activity of 1; Watson & others, 2006) are generally low and with minimal
variation (from 670 to 720°C). None of the samples consistently plot in the A-type or
within-plate elds of discrimination diagrams based on whole-rock chemistry (Pearce
& others, 1984; Whalen & others, 1987; Bonin, 2007), and A-type mineralogies are
not observed. A-type granites, therefore, are absent from the sample suite.

2 Geological Survey of Queensland
Figure 1. Map indicating the location of granitoid samples selected for this study, as well as the two
transects (WNW–ESW and SSW–NNE). The red contour represents the boundary of the subsurface
temperature anomaly from Chopra & Holgate (2005).
Figure 2. Photographs of granitic rocks from this study, organised by age grouping. A) Proterozoic. B)
Age is unknown, but Rb/Sr geochronology (whole-rock and feldspar concentrate) of a closed granitic
rock (PPC Etonvale 1)(Lewis & Kyranis, 1962) suggests a Late Silurian age. C) Permo-Carboniferous.
D) Mid-Ordovician. E) Late Silurian. F) Mid-Devonian. G) Permo-Triassic. Scale bar is 2cm.

Queensland Geological Record 2012/14 3
New U-Pb (zircon) LA-ICP-MS geochronology indicates a regional trend of
increasing emplacement age from E to W from Permo-Triassic (TEP Jandowae
West 1), Permo-Carboniferous (Roma-Shelf), mid-Devonian (AOP Balfour 1),
mid-Ordovician (AMX Toobrac 1, LOL Stormhill 1 and AOD Budgerygar 1), to
Proterozoic ages (PGA Todd 1). Late Silurian granitic rocks (TEA Roseneath 1 and
DIO Wolgolla 1) occur in southern areas relatively close to the Permo-Carboniferous
granites of the Big Lake Suite (~100km) (Gatehouse & others, 1995). Abundant
zircon inheritance has been detected in AMX Toobrac 1, which is recorded by
xenocrystic cores. Inherited core populations in this sample are generally Proterozoic
with distinct populations at 894±32Ma, 1158±19Ma and 1544±34Ma. Granites
from the Roma Shelf with an interpreted emplacement age of 340Ma exhibit more
subtle inheritance with population ages of ~360 and ~380Ma and minor Proterozoic
inheritance. In contrast, zircon populations from some samples (e.g., TEP Jandowae
West 1 and AOP Balfour 1) are exclusively magmatic, with no inheritance.
The majority of the intrusives are silicic with silica contents ranging from 74 to
78wt%. Calculated heat production values are generally low and range from 0.8 to
5.1µW/m3, with a general enrichment of the Heat Producing Elements with increasing
silica content (from 0.8µW/m3 at 58wt% SiO2 to 4µW/m3 at 76wt% SiO2). However,
granitic rocks with the highest heat production (~ 5µW/m3; >35ppm Th) do not
have the greatest silica content (~73wt% SiO2). These correspond to the Proterozoic
intrusions (PGA Bradley 1 and PGA Todd 1) located at the border between
Queensland and Northern Territory. Interestingly, the Proterozoic to Permo-Triassic
age range and generally low heat production values of our sample suite, both within
and outside the temperature anomaly, contrast strongly to the HHPG and the mainly
Permo-Carboniferous Big Lake Suite which exhibit much higher heat production
values (7 to 9.7µW/m3) (Middleton, 1979; Gatehouse & others, 1995).
Thermal conductivities determined for a suite of 8 samples range from 2.5 to
3.7W/mK and are within the range of published values for similar lithologies (Zoth
& Haenel, 1988). Granitic rocks generally exhibit low porosities (< 5%); therefore,
the variation of thermal conductivity mainly depends on mineralogy. For instance, the
low bulk thermal conductivity (2.5W/mK) of the more mac intrusion (TEP Jandowae
West 1) is explained by the large abundance (~70vol%) of low thermally conductive
plagioclase minerals (~2.1W/mK (Clauser & Huenges, 1995)).
DISCUSSION
To investigate the contribution of granitic rocks to the high crustal temperatures
identied by OzTemp in Queensland (Chopra & Holgate, 2005), we rstly rened
the distribution of anomalous temperatures by restricting data to deep temperature
measurements (i.e. >1000m). This removes climatic and shallow-aquifer advection
effects and reveals several areas of anomalously high temperatures (Figure 3):
• A distinctive NE- trend of high temperatures is apparent in SW Queensland and
correlates with a series of granitic rocks at 1000 to 2500m depth; extrapolation of
this trend along strike to the north-east suggests it may correlate to the Stanage

4 Geological Survey of Queensland
Fault Zone recognised along the central Queensland Coast (e.g., Holcombe &
others, 1997) (Figure 3).
• High temperatures are also identied in the NE part of the subsurface temperature
anomaly, where basement granites are relatively shallow (<1500 m)
• and in the northern part and to the west of the Roma Shelf
• high temperature is also recognised for TEP Jandowae West 1. At this point,
the extrapolated temperature at 5km depth is 248°C, ca. 100°C higher than
the average geothermal gradient of continental crust. The low heat production
(0.8µW/m3) of the intersected granite and the relatively shallow temperature
measurement (64°C at 470m) suggest an advective contribution to the
anomalously high temperature. However, thermal modelling remains to be
undertaken to understand the contribution of the low thermal-conductivity
granitoid to this high temperature.
The origin of these anomalously high temperature areas and the relative roles of
granitic heat production and insulative cover are unclear. Some key points are:
• Areas of anomalously high temperature do not correlate with particular cover
basins.
• Some high temperature areas (e.g., just west of the Roma Shelf) do not correlate
with granite intersections raising the possibility that HHPG’s occur at greater
depth. As observed in Figure 4, most drill cores in SW Queensland have not
drilled deeper than 3km depth, and at Innamincka, HHPG occur between 3 and
5km depth. It is therefore possible that HHPG occur at greater depth but have not
been intersected.
• The broadly linear arrangement of some high temperature zones (i.e. along the
NE-SW trending extension of the Stanage Fault Zone) may suggest a contribution
of high mantle heat ow (as suggested by Italiano & others, 2012) along major
crustal lineaments.
• Some areas (e.g. towards the SW of the Stanage Fault Zone trend) coincide
with abundant granite intersections (Figure 3). However, these granites are not
considered HHPGs and have low to moderate heat production values (2.6, 2.7,
3.2µW/m3). In contrast, some other areas of abundant granite with comparable or
higher heat production values (e.g. southern part of the Roma Shelf area and the
Queensland/Northern Territory border area, respectively) do not correlate with
high temperature zones.
Since there is no compelling correlation between occurrences of granites with
high radiogenic heat production and positive temperature anomalies, an alternative
explanation is required. We suggest that layers of insulating sedimentary cover rocks
combined with the presence of moderately heat producing granites at depth explain
the observed high subsurface temperatures. This hypothesis is tested with multi-layer,
one-dimensional steady-state thermal modelling, which is currently in progress.
Preliminary results from the most well constrained area (far SW Queensland), using
the measured heat production and thermal conductivity values in these granites and
overlying sedimentary cover, yield a relatively high modelled surface heat ow of ca.
85mW/m2. This value is 14mW/m2 higher than the average continental surface heat

Queensland Geological Record 2012/14 5
Figure 3. a) Map indicating high temperature anomaly areas. Data points are extrapolated temperatures
at 5km depth and originate from a selection (temperature agged and >1000m) of the Oztemp database
(Holgate & Gerner, 2011). Major crustal lineaments are also indicated by a dashed line for the
interpreted Stanage Fault Zone and Darling River lineament (Katz, 1976) and a bold grey line for the
Tasman line. Other features as in Figure 1. b) Map indicating the lithology of basement intersected.
Note the WSW–ENE trend of granitic rocks in the SW-corner of Queensland.

6 Geological Survey of Queensland
ow (Davies & Davies, 2010) but still lower than modelled surface heat ow values
at Innamincka (90–110mW/m2 — Middleton, 1979; Beardsmore, 2004). Using the
measured heat production value of the granites and assuming it is constant with depth,
ca. 6.5km of granite thickness is required to explain the higher surface heat ow at
this location. This thickness is plausible and much lower than that predicted by gravity
modelling at Innamincka, where the HHPG plutons have been estimated to be up to
12km thickness (Meixner & Holgate, 2009).
Figure 4. Depth proles of intersected basement rocks along two transects. The green lines join the total
depth of all drill cores. The inset maps indicate the drill cores taken into account in the depth prole. a)
WNW–ESE transect. b) SSW–ENE transect.

Queensland Geological Record 2012/14 7
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