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Structural setting of gold mineralization at Stawell, Victoria, Australia
... The simplest case of paleo-stress determination is where the pronounced structural grain of folds and accompanying cleavages can be related to the regional shortening. In the Stawell Zone there is a pronounced NW-trending structural grain that coincides with the D 1 -D 4a events (Watchorn and Wilson, 1989); that originated in the SE-directed compression during the Delamerian Orogeny. This early compression is likely to have controlled the gross geometries of many subsequent structures and ore zones like the Central Lode system (Miller and Wilson, 2006). ...
... The western Stawell Zone records seven discrete deformation and alteration events ( Watchorn and Wilson, 1989). In the Magdala Mine there are four distinct styles of gold mineralization: (1) Early sediment hosted VMS base metal-gold e.g. the Eastern ore zone (Fig. 17A, B) and 'sulfide-rich Basalt Contact lodes', with an inferred ca. ...
... It has been metamorphosed to middle greenschist facies (Wilson et al., 1992) and comprises flow horizons with pillow layers, minor interflow sediments and flow top breccias (Fig. 3C, D). The unit comprises several smaller basalt sheets or noses (Fig. 18A) that have fold axes parallel to the NNWstructural trend in the Stawell Zone and are separated by synformal-like depressions or reentrant structures known as 'Waterloo' structures (Watchorn and Wilson, 1989), that are infilled with chlorite-altered metasediments ('Stawell Facies', Miller and Wilson, 2002;Robinson et al., 2006). Basalt lobes (e.g. ...
... All ore shoots acted as major fluid conduits during mineralization ; thus, recognition of the controls on their genesis is vital for exploration geologists targeting ore deposits. At the Stawell gold mine in southeastern Australia (Fig. 1), northwest-plunging ore shoots are associated with structures that developed around several >5-m-thick reentrant basalt lobes on the flanks of the Magdala basalt (Watchorn and Wilson, 1989; Wilson et al., 1992; Miller and Wilson, 2002 ). Numerical models have highlighted the importance of the basalt-lobe geometries in controlling the genesis of the ore shoots (Rawling et al., 2006; Robinson et al., 2006b; Schaubs et al., 2006), yet their positions relative to the thick mafic units vary considerably along the >100 km strike length of exposures of the Magdala basalt (Rawling et al., 2006; Squire et al., 2006c). ...
... Despite recognition of the intimate association between the ore shoots and basalt lobes at Stawell, there is uncertainty about how the reentrant features formed, what controlled their geometry and distribution, and why ore shoots are associated with some basalt lobes but not others. The basalt lobes were initially interpreted as fold mullions, formed during intense early (D 2 ) ductile deformation on the limbs of a regionally extensive isoclinal fold (Watchorn and Wilson, 1989; Wilson et al., 1992; Miller and Wilson, 2002; Miller et al., 2005; Miller et al., 2006 ). However, recent reviews of the stratigraphic architecture of the basalt and sedimentary rocks on the southwestern flank of the Magdala basalt at Stawell (Squire and Wilson, 2005; Robinson et al., 2006a) suggest that the lobes are not fold mullions but weakly deformed basalt flows. ...
... The quartz-rich sandstone and mudstone units of the St. Arnaud Group east of the Coongee fault were deposited after the Leviathan formation from about 500 to 490 Ma (Squire and Wilson, 2005), and all the successions were intruded by feldspar ± quartz-phyric felsic intrusions at ca. 413 Ma and the Stawell Granite at ca. 400 Ma (VandenBerg et al., 2000). West of the Coongee fault, the Cambrian successions of the Stawell zone have undergone at least three ductile and two brittle deformation events related to east-northeast–westsouthwest shortening (Watchorn and Wilson, 1989; Wilson et al., 1992). Rare bedding-parallel fabrics represent the earliest FIG. 2. Three-dimensional perspective views showing the geometry and contrasting locations of ore shoots relative to the present-day surface morphology of the Magdala basalt at Stawell and Wildwood. ...
The ability to effectively target ore shoots requires a clear understanding of the numerous parameters that control their formation. However, identification of the nature and timing of these parameters is difficult, particularly in polydeformed deposits with complex stratigraphic and hydrothermal histories. For example, ore shoots at Stawell gold mines Magdala gold deposits in southeastern Australia are hosted by rocks that experienced at least three ductile and two brittle deformation events in the 70 m.y. prior to the principal mineralization event, and have a close spatial relationship to strongly chlorite +/- stilpnomelane-altered sedimentary units near the apex of numerous >5-m-thick reentrant basalt lobes. Whereas the ore shoots at Stawell are located only oil the southwestern flank of the 1-km-thick Magdala basalt, about 1.5 kill along strike at the Wildwood prospect the ore shoots are predominantly restricted to the northeastern flank of the basalt. Here, the important issue of controls oil ore shoot formation and distribution at Stawell is addressed by presenting the results of a multidisciplinary review of the characteristics and architecture of the volcanic and sedimentary facies, the spatial distribution 4 alteration assemblages, and the variations in intensity and vergence relationships of the cleavages associated with ductile deformation. This investigation, undertaken at both a regional and shoot scale, shows that the structures hosting the ore shoots at Magdala formed during preferential and localized reactivation of preexisting faults following rotation of the stress field. Preexisting structures, such as Central Lode fault, host many of the ore shoots and were initially generated during east-northeast-west-southwest compression at ca. 510 to 488 Ma. The geometries of these faults were also influenced by the irregular paleotopography of the basalt lobes, which represent flows of pillow and massive basalt separated by interflow sedimentary units. Coincident with this deformation, locally intense chlorite +/- stilpnomelane alteration indurated the mud-rich units between the basalt flow-lobes and the Central Lode-type structures. Those thick, mud-rich sedimentary units are present oil the southwestern flank of the basalt at Stawell, brit predominantly near the top and on the northeastern flank at Wildwood. The switch to east-west compression at ca. 440 Ma caused localized failure and focusing of mineralized fluids in the indurated sedimentary rocks near the apex of the northwest-plunging portions of the basalt lobes. A second, Subordinate mineralizing event at ca. 425 to 420 Ma utilized the same architecture under a different stress regime. During both mineralizing events, the high-grade ore shoots formed in zones of high fluid flow defined by the complex preexisting architecture.
... The Hangingwall Reefs at Stawell, western Victoria (Figure 1), are high-grade auriferous quartz veins located within and adjacent to the Stawell Fault, about 100 – 300 m above the southwestern shoulder of the gently northwest-plunging Magdala Basalt dome. Early gold production (1856 – 1880) from these quartz – gold reefs totalled about 30 t (1 million ounces) of gold at grades of about 30 g/t (Watchorn & Wilson 1989). Miller and Wilson (2002a) suggested that the structural relationships of the quartz veins, faults and shears hosting the reefs indicated that the mineralisation occurred during east – west compression at ca 440 Ma (i.e. ...
... Despite the similarity in age of mineralisation for the Hangingwall Reefs and nearby Magdala deposit, the two deposits display several strikingly different structural, stratigraphic and hydrothermal-alteration characteristics . For example, the Central Lode and Basalt Contact ore at Magdala occur only in strongly ironaltered (chlorite + stilpnomelane) sediments near the contact with the Magdala Basalt (Watchorn & Wilson 1989; Dugdale et al. 2006), whereas the Hangingwall Reefs occur in muscovite-altered sediments up to 300 m from the basalt contact (Watchorn & Wilson 1989; Miller & Wilson 2002a). We present here the results of over 5 km of detailed lithological and structural drillcore logging (Figures 2, 3) as well as underground mapping of the Hangingwall Reefs in the 212RL drive, together with the results of 3D numerical modeling, in an attempt to determine the controls on the distribution of this important deposit type. ...
... Despite the similarity in age of mineralisation for the Hangingwall Reefs and nearby Magdala deposit, the two deposits display several strikingly different structural, stratigraphic and hydrothermal-alteration characteristics . For example, the Central Lode and Basalt Contact ore at Magdala occur only in strongly ironaltered (chlorite + stilpnomelane) sediments near the contact with the Magdala Basalt (Watchorn & Wilson 1989; Dugdale et al. 2006), whereas the Hangingwall Reefs occur in muscovite-altered sediments up to 300 m from the basalt contact (Watchorn & Wilson 1989; Miller & Wilson 2002a). We present here the results of over 5 km of detailed lithological and structural drillcore logging (Figures 2, 3) as well as underground mapping of the Hangingwall Reefs in the 212RL drive, together with the results of 3D numerical modeling, in an attempt to determine the controls on the distribution of this important deposit type. ...
The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.
... The absence of microfossils in the Cambrian volcanic and sedimentary succession west of the Avoca Fault in western Victoria (see graptolite line in Figure 1) is commonly attributed to their low preservation-potential in generally highly deformed (Watchorn & Wilson 1989), metamorphosed (Kemp & Gray 1999) and hydrothermally altered ) host-rocks. As a result, correlations between Cambrian terranes in southeastern Australia have relied heavily on similarities in magmatic affinities of boninitic and/or tholeiitic lavas and intrusions (Crawford et al. 1984;Crawford & Keays 1987;Kemp 2003;Squire et al. 2006), mica cooling ages (Turner et al. 1993(Turner et al. , 1996Miller et al. 2005), detrital-zircon provenance of the quartz-rich sediments (Ireland et al. 1998; in press), detrital-mica age data (Foster et al. 1998) We present here a diverse range of acritarchs and minor other fossil fragments from siliceous mudstone (chert) in the deformed and strongly altered Albion Formation at Stawell and the less deformed and relatively weakly altered Glenronald Shale Member of the Stavely Volcanics (Buckland 1986;Crawford et al. 1996) (Figures 1, 2). ...
... The Cambrian successions west of the Coongee Fault in the Stawell Zone have undergone at least three ductile and one brittle deformation events related to northeast -southwest shortening (Wilson et al. 1992). Minor cleavage and quartz veins were generated subparallel to bedding by early thrusting (D 1 ) at ca 510 -504 Ma, whereas the more abundant cleavage and quartz veins, also subparallel to bedding, were generated during the main ductile deformation event (D 2 ) at ca 504 -500 Ma (Watchorn & Wilson 1989;Miller et al. 2005). An asymmetric differentiated crenulation cleavage (S 3 ) overprints these earlier ductile fabrics and was developed during southwest-over-northeast shearing and folding at about 500 -492 Ma ( The Cambrian rocks west of the Coongee Fault in the Stawell Zone (Figure 1) also record higher metamorphic grades than the simply deformed, sub-greenschist facies turbidites to the east (including the Heathcote and Mt Wellington Greenstone Belts). ...
... Chert appears to be the best host for preserving microfossils in Cambrian metasedimentary rocks from western Victoria. At Stawell, where the Cambrian rocks were variably modified by hydrothermal fluids and at least three ductile deformation events (Watchorn & Wilson 1989;Dugdale et al. 2006), the chert is variably recrystallised and partially altered to carbonate + chlorite + stilpnomelane. The microfossils may be locally disrupted by quartz crystals (up to 75 mm across) in the recrystallised chert ( Figure 6a) and rare carbonate -quartz veins. ...
The paucity of microfossils in the Cambrian volcanic and sedimentary succession west of the Avoca Fault in western Victoria is generally attributed to their low preservation-potential in the polydeformed and highly altered early Palaeozoic host rocks. The resulting lack of biostratigraphic age control has made temporal correlations between Cambrian successions difficult, leading to a relatively poor understanding of the Cambrian palaeogeographic and palaeotectonic evolution of the region. Here, we present evidence of a diverse range of acritarchs and a rare possible conodont from siliceous mudstone (chert) of the middle Botoman (ca 515 Ma) Albion Formation at Stawell and the middle Middle Cambrian (ca 505 Ma) Glenronald Shale Member near Mt Stavely. Acritarchs are polyphyletic organisms that include the vegetative and resting cysts of unicellular protists, mainly phytoplankton, which underwent rapid diversification during the Early Cambrian. The morphologically heterogeneous populations of Early Cambrian acritarchs enable them to be used as biostratigraphic indicators. Therefore, the discovery of several populations of acritarchs in successions that have relatively good age control represents an important advance, because these fossils may be used to assist with temporal correlations of successions in southeastern Australia. Moreover, the well-preserved nature of these microfossils means that a variety of biota may be preserved in chert in highly deformed and altered Cambrian successions elsewhere in East Gondwana that currently have poorly constrained ages.
... Rocks exposed in the Stawell Zone show a different structural trend and differ in the timing of deformation to similar lithologies in the east (Gray et al., 1988; Gray and Willman, 1991). In other respects, however, there are many features that are comparable to areas in the LFB, namely: (1) there is widespread greenschist facies metamorphism of low pressure/high temperature type, probably synchronous with the earliest recognisable deformations D, and D, (Gray et al., 1988: Cox et al., 1991); (2) granite emplacement postdates the peak of the regional metamorphism and is associated with localised contact metamorphism and the development of schistose rocks (Gibson, 1988); and (3) gold mineralisation may be contemporaneous with granite emplacement but certainly post-dates regional peak-metamorphic conditions (Watchorn and Wilson, 1989; Will, 1990; Quick, 1990). Various tectonic subdi~sions of western Victoria have been proposed by different authors, Fig. 2. Lithological and structural sketch map of the Stawell Zone showing location of main pre-Cretaceous outcrops. ...
... The metavolcanic units occur as five isolated linear fault bounded belts that separate the metasedimenta~ sequence. The metavolcanic sequence includes: (1) the relatively undeformed Mt Stavely Volcanics (Buckland, 1986); (2) isolated volcanic rocks with a strong W-dipping foliation in the Black Ranges (Spencer-Jones, 1965); (3) a narrow volcanic sequence at Moyston transected by faults and joints; (4) the Jallukar Volcanics (Buckland, 1985); and (5) an approximately S-km-wide, highly deformed volcanic sequence (Roder, 1977) at Mt Ararat that is probably equivalent to the metavolcanic sequence at StawelI (Watchorn and Wilson, 1989). The Mt Stavely Volcanic Complex is dominated by andesitic lavas and breccias with minor intrusives ranging in composition from silicic to basaltic (Buckland, 1986). ...
... The volcanic succession at Stawell is internally disrupted by N-to NE-dipping thrusts. At Stawell, slivers of fault-bounded, generally NE-dipping, volcaniclastic and turbiditic material within the Footwall volcanics are referred to as " Waterloo structures " (Watchorn and Wilson, 1989). West of the Footwall volcanics is a mixed unit of cherts and volcanic sediments (volcaniclastics), termed the Magdala volcanogenic unit (Watchorn and Wilson, 1989) that is structurally overlain on the west by a strongly crenulated sequence of shales and sandstones referred to as Mine Schists. ...
The Stawell Zone is interpreted to be part of the eastern extension of the Adelaide Fold Belt into Victoria. The Cambro-Ordovician turbidite sequence of the Stawell and Glenelg zones rests on a pile of Late Proterozoic metavolcanic rocks, rather than Cambrian metavolcanic units that are characteristic of the Lachlan Fold Belt. Early deformation events, D1–D3, in the Stawell Zone pre-date granite emplacement and were synchronous with regional fold-forming events that accompany thrust movements along discrete detachment surfaces. The thrust system follows a NE-SE-trending strike, with an E-directed translation of the tectonic units. The steep (≈ 60°) predominantly W-dipping thrusts represent high-strain zones localised in relatively weak Cambro-Ordovician quartz-rich turbidites, that are sandwiched along the boundaries of the Late Proterozoic metavolcanics. Overprinting the early thrust system are D4–D6 deformation events that include reverse, strike-slip and normal faults.
... The best exposure of the contact aureole is along the northern contact, where the sediments are dominated by quartz-rich sandstone. In this area, the Wonga gold deposit is located within the contact aureole (Fig. l ) , and the Magdala gold deposit is located about 2 km north of the granite, on the western limb of the 'Magdala Anticline' (Watchorn & Wilson, 1989). The local subdivision of the stratigraphic sequence around the Wonga and the Magdala deposits is as follows: the Footwall Volcanics, Magdala Volcanogenic unit, Mine Schist, Wonga Schist and the Eastern Schist (Watchorn & Wilson, 1989). ...
... In this area, the Wonga gold deposit is located within the contact aureole (Fig. l ) , and the Magdala gold deposit is located about 2 km north of the granite, on the western limb of the 'Magdala Anticline' (Watchorn & Wilson, 1989). The local subdivision of the stratigraphic sequence around the Wonga and the Magdala deposits is as follows: the Footwall Volcanics, Magdala Volcanogenic unit, Mine Schist, Wonga Schist and the Eastern Schist (Watchorn & Wilson, 1989). The Wonga Schist is the principal unit in which the contact aureole has been developed. ...
... The Wonga Schist is the principal unit in which the contact aureole has been developed. Two sets of felsic dykes and a small number of basic dykes cross the contact aureole and the granite (Watchorn & Wilson, 1989). ...
The contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low‐grade zone, the medium‐grade zone and the high‐grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low‐ and medium‐ to high‐grade zones are presented in compatibility diagrams projected from biotite, quartz and H 2 O, and biotite, K‐feldspar and H 2 O, respectively. These compatibility diagrams have the advantage of showing both quartz‐bearing and quartz‐absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks:
chl + mu + q = bi + cd + V,
chl + q + cd = g + V,
mu + bi + q = ksp + cd + V,
mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V),
mu + cd = ksp + and + bi + V,
mu + bi + and = ksp + sp + V,
and + bi = ksp + sp + cd + V,
mu + bi = ksp + cor + sp + V,
mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V),
bi + cd + q = g + ksp + V.
The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and pressure of metamorphism.
... The variable distribution of elevated D 4c-5 gold grades in the Magdala lode system is also controlled by preferential and localized reactivation of the pre-existing faults in a progressively rotating stress field (Robinson et al. 2006b). Where mineralization occurs, it is an overprint on a major preexisting structure, with fracture-hosted sulfides (Fig. 3b) and gold overprinting the quartz veins (Watchorn and Wilson 1989;Miller and Wilson 2002). Populations of slickenline lineations on fault surfaces combined with extensional vein arrays associated with the faults (Miller and Wilson 2004b) indicate that the large laminated quartz veins of the Central lode formed initially during D 4a southwest-northeast compression, but were subsequently reactivated during D 5 east-west compression on lower angle fault segments to produce a series of ore-shoots within the overall lode system. ...
... In this paper we will describe analysis of the quartz fabrics using a Fabric Analyzer (Fig. 4) and methods described by Wilson et al. (2003Wilson et al. ( , 2007 where an individual c-axis orientation of selected areas within grains can be obtained down to a scale of approximately 5 µm. A total digital AVA color map is prepared from a thin section (Fig. 3d) by stitching individual rectangular tiles (640×480 pixels), Table 1 Summary of the major deformation and veining events recorded in the Magdala mine (after Watchorn and Wilson 1989;Miller and Wilson 2002) D 1 Bedding parallel foliation and foliation parallel quartz veins (V 1 ). D 2 Upright to slightly inclined, close folds with prominent axial-planar cleavage and thin parallel quartz veins (V 2 ). ...
... The quartz in the laminated vein (vein A) is dominated by blocky grains between thin layers (<1 mm) of wall-rock. c-d Lower hemisphere equal area stereonet plots showing the distributions of caxes and number (N) of data points analyzed in veins A and B. The corresponding histogram shows frequency of adjacent quartz grains whose c-axes occur at angle to the c-axis of adjacent grain, the curve represents the theoretical random distribution of c-axes known to carry significant amounts of gold (Watchorn and Wilson 1989;Miller and Wilson 2002). Laminated textures in these veins are very common, especially proximal to the bounding faults where the vein is thick. ...
Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia,
contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault
rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both
vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant
deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under
conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique
is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic
preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between
the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the
quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution
(pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where
grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution)
developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded
that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid
pressures in a region undergoing deformation at near lithostatic pressures.
... Gold mineralization in the Magdala deposit at Stawell is hosted by a series of complex, generally west-dipping shear zones that developed late in the deformation history of the area on the western flank of a regional anticlinorium during D 4 deformation (Watchorn & Wilson 1989). Continued gold mineralization and/or remobilization associated with post-peak metamorphic sericite-carbonate alteration has been generally attributed to oblique strike-slip faulting during D 5 (Watchorn & Wilson 1989;Mapani & Wilson 1994;Miller & Wilson 2002). ...
... Gold mineralization in the Magdala deposit at Stawell is hosted by a series of complex, generally west-dipping shear zones that developed late in the deformation history of the area on the western flank of a regional anticlinorium during D 4 deformation (Watchorn & Wilson 1989). Continued gold mineralization and/or remobilization associated with post-peak metamorphic sericite-carbonate alteration has been generally attributed to oblique strike-slip faulting during D 5 (Watchorn & Wilson 1989;Mapani & Wilson 1994;Miller & Wilson 2002). D 4 structures are truncated by quartz-feldspar porphyry dykes that were intruded at 413 3 Ma (Arne et al. 1998a), prior to the emplacement of the Stawell granite at c. 400 Ma, thereby placing an upper limit on the timing of gold mineralization. ...
This study investigates whether hydrothermal alteration of metasedimentary rocks associated with orogenic gold mineralization in the Victorian gold province produces systematic changes in oxygen, carbon and sulphur isotope compositions that might provide explorationists with vectors towards ore. The presence of systematic isotopic trends across wallrock alteration haloes in gold deposits in the Stawell, Percydale, Ballarat, Bendigo and Fosterville goldfields clearly supports the interpreted hydrothermal origin of these haloes. General increases in carbonate delta(13)C towards the ore zones in all deposits record the influx of hydrothermal CO2. Whole-rock silicate and quartz delta(18)O, carbonate delta(18)O, and sulphide delta(34)S shifts equally mirror petrological and geochemical changes that resulted from prolonged and extensive interaction between externally derived hydrothermal fluids and the surrounding siliciclastic wallrocks. With the possible exception of sulphur, however, the isotopic enrichment/depletion trends are subtle and also vary significantly from deposit to deposit. Results of this study and comparisons with published data suggest that the oxygen and carbon isotopic ratios of silicates and carbonates in whole-rock samples within the alteration haloes are buffered by the wallrocks. Hydrothermal sulphides in Victorian orogenic gold deposits are characterized by delta(34)S values generally in the range -5 to +5parts per thousand. However, there is relatively little influx of hydrothermal sulphur beyond c. 5 to 10 m from the vein margins and disseminated pyrite well removed from the lode systems is characterized by either greatly negative or positive delta(34)S values typically greater than +10parts per thousand.
... Since the shear Ž . zones in questionWatchorn and Wilson, 1989are major fluid conduits, there is need to identify the sources and composition of fluids that have passed through these structures. The problem of identifying sources of fluids in shear zones is amenable to oxygen isotope studies because isotopic exchange between rocks and an aqueous fluid will reset the initial d 18 O of the rock. ...
... However, at Magdala the volcanogenic metasedimentary rocks do have a wide Ž . range of compositionsWatchorn and Wilson, 1989. The X of stilpnomelane of 0.87 to 0 ...
The mid-greenschist facies turbiditic and subaqueous volcaniclastic metasediments at Stawell, are overprinted by two shear zone systems that host gold mineralization in the Magdala mine. The younger Scotchmans fault zone overprints the older Central lode system and both systems have a reverse sense of movement. There is a systematic reorientation of the Central lode system structures and foliation into parallelism with structures of the Scotchmans fault zone. Microfabrics in the laminated quartz veins indicate vein accretion from either wall or from the center and quartz fibers attesting to vein growth occurring as a series of incremental events rather than being a single event. The gold-bearing fluids overprint earlier laminations but are coeval with later vein accretion associated with sulfides and ankerite. -from Authors
... The volcanic basement of the western subprovince consists of tholeiitic basalts in the western Stawell Zone, including amphibolites of the Moornambool Metamorphic Complex metamorphosed at pressures up to 800 MPa (depth of ~24 km: Watchorn & Wilson 1989; Phillips et al. 2002). Mafic volcanics have also been found along the Avoca Fault from the subsurface (drillcore and from mine dumps) near Pitfield (Morand et al. 1995). ...
... By the Late Cambrian to earliest Ordovician, the thick quartz turbidite succession had prograded across the oceanic basement of the Stawell Zone. The lack of bedded chert between the volcanics and overlying turbidites in the western Stawell Zone implies that backarc spreading only briefly pre-dated turbidite deposition (Watchorn & Wilson 1989). ...
The Lachlan Fold Belt of southeastern Australia contains a remnant ocean basin with Cambrian igneous basement of intra-arc and backarc boninitic to tholeiitic volcanics and island-arc calc-alkaline volcanics overlain by ?Upper Cambrian to dominantly Ordovician siliciclastic turbidite successions derived from Gondwana and deposited in a huge submarine turbidite fan(s). Much of the remnant ocean basin is preserved in accretionary subduction complexes that are characterised by abundant coherent successions with relatively sparse mélanges. The lack of chaotic rocks reflects the accretion of thick (2–5 km) siliciclastic turbidite successions and low rates of underthrusting. Three previously recognised accretionary subduction complexes are located in western Victoria (Stawell and Bendigo Zones), eastern Victoria (Tabberabbera Zone) and along the eastern coastline of southeast Australia (Narooma subduction complex). In addition, a short-lived west-dipping subduction zone is proposed to account for the areally restricted Howqua River Zone along the eastern margin of the Melbourne Zone in eastern Victoria. The Howqua River Zone contains gently dipping mélanges and subduction-related blueschist fragments. The subduction complex in the Tabberabbera Zone is considered to extend throughout the remnant ocean basin succession of the Wagga–Omeo Zone of the central Lachlan Fold Belt. Apart from the Howqua River Zone, these subduction complexes are an end-member in the spectrum of accretionary complexes that contrast with more chaotic assemblages as preserved in southwest Japan.
... The phenocrysts of plagioclase are either locally albitised or replaced by an aggregate of microcrystalline epidote, calcite and chlorite, whereas the pyroxene is typically replaced by ragged actinolitealbite-epidote-chlorite-calcite intergrowths. These alteration assemblages match well with metamorphism of rocks to lower greenschist facies (e.g., Squire et al., 2006;Watchorn and Wilson, 1989). ...
The Weibao copper–lead–zinc deposit, located in the eastern part of the Qimantagh area, East Kunlun Orogenic Belt (EKOB), consists of three skarn ore blocks known as Weixi, Main and Weidong from west to east. The mineralization within the Weibao Cu–Pb–Zn deposit is hosted by the Mesoproterozoic Langyashan Formation. In this study, we describe for the first time basaltic lavas that intruded into this host sequence and chronological, isotopic, major and trace element data of these volcanic rocks are presented here to constrain their eruption age as well as the tectonic setting. Two basaltic lava samples yield sensitive, high-resolution ion-microprobe (SHRIMP) U–Pb zircon ages of 393.0±5.5 Ma–392.0±5.0 Ma, indicating that volcanic rocks in the Weibao deposit erupted in Middle Devonian. The majority of the volcanic rocks have compositions characterized by high potassium, light rare earth element (LREE)-enriched patterns in chondrite-normalized rare earth elements (REE) diagrams, and evident enrichment of Rb, Ba and K and depletion of Th, U, Nb and Ta contents in primitive mantle-normalized patterns, although the degrees of enrichment and depletion are variable. These characteristics of major and trace element data highlight a hornblende-dominated fractionation during ascent of magmas. The εHf(T) values of zircons are relatively scattered and slightly enriched, ranging from -2.6 to +7.5. Modelling the features of the major, trace and isotopic element data indicates a hybrid origin involving combined depleted mantle (and hence asthenospheric mantle) and melts and/or fluids inherited from an early subduction event. Besides, these geochronological and geochemical data, together with previously published data in the EKOB, suggest that the Weibao basaltic lavas formed in a post-collisional setting, and that the Qimantagh area underwent strong interactions between mantle and crust in Early Paleozoic–Middle Devonian.
... The gold present also has a much finer grainsize than in most other deposits in the Stawell Zone (Bierlein & McKnight, 2005). The host schists have been overprinted by contact metamorphic biotite, andalusite and cordierite, which have in turn been overprinted by biotite-quartz-arsenopyrite and sericite-carbonate-arsenopyrite alteration (Watchorn & Wilson, 1988). ...
Gold Undercover Assessment of undiscovered mesozonal orogenic gold endowment under cover in the northern part of the Stawell Zone (Victoria)
... Gold is associated with bismuth, aurostibnite, molybdenite and scheelite (Wilson et al., 1999). In contrast, the Magdala deposit lies outside the contact aureole and is associated with a complex series of generally westdipping thrust faults on the western limb of a regional anticlinorium (Watchorn and Wilson, 1989). The vast majority of production (90,200 kg of gold to 1998; Ramsay et al., 1998) has come from the orogenic gold-style Magdala deposit. ...
It is probable that the most fruitful exploration strategy for discovery of new orogenic gold deposits in Victoria, SE Australia, will be to focus on areas buried by post-mineralization cover rocks. There are several possible approaches in the absence of robust predictive models and detailed subsurface data with which to apply them. This paper focusses on the detection of primary hydrothermal alteration envelopes and presents new data on the mineralogical and bulk-chemical expression of alteration in weathered outcrop around the deposits of Bendigo, Fosterville and Stawell.
... Gold also occurs as a metal of prime economic interest in Au-base metal deposits, such as at Braidwood (•30 t Au), Donkey Hill, and Candoin Creek. Sources of the Au mineralization at Stawell (historical and present production, 57 t; Watchorn and Wilson, 1989) have been related to local derivation from footwall volcanics (Gulson et al., 1988) via metamorphic fluids from a lower crnstal source(Sandiford and Keays, 1986) or from the Stawell granite itself(Watchorn and Wilson, 1989). These latter authors based their conclusions on the presence of hydrofracturing and brecciation and strnctural-metamorphic timing relationships within the Au lodes indicating that mineralization postdated the peak of regional metamorphism and was synchronous with the later stages of contact metamorphism. ...
Granites crop out over 20% of the total exposed area of the Lachlan fold belt and comprise subequal proportions of I and S types. Major Sn mineralization is related to S- and I-type granites that are both reduced and have undergone extended feldspar-dominated crystal fractionation. Mo showings are common but, as for Cu, large deposits are absent. Scheelite skarn mineralization is represented by the major King Island deposits; however, granite related W mineralization of a similar scale is not found elsewhere within the belt. The ore element ratios (Sn/W/Cu/Mo, etc.) of granite-related mineralization in the Lachlan fold belt is a straightforward function of the relative oxidation state and degree of fractionation within the associated granite suites. The progression from Cu-Au, and W to Mo mineralization related to progressively more fractionated, oxidized I-type magmas can be traced within single supersuites. Such a systematic relationship between magma composition, redox state, and ore element ratios is good evidence for the magmatic source of ore elements in granite-related mineral deposits and for the production of the observed ore element ratios dominantly through magmatic processes. -from Authors
... 3). Also, the shear zone-hosted, quartz-sulfide-Au vein network of the Magdala mine, Victoria, Australia (Watchorn & Wilson, 1989, their Figures 13 and 14) is bordered by asymmetrical patterns of sericite-carbonate alteration patterns, with a strong control from the topology of the different vein sets hosted by a sinistral shear zone. Hence, the local topology of veins within mineralized networks may be an important factor controlling the development of hydrothermal alteration haloes, and geochemical models of fluid-rock interaction in such networks should account for the mechanisms governing growth and propagation of veins at a small scale. ...
The Sigma deposit is a large network of Au-mineralized veins that crosscut a sequence of andesitic volcanics and volcanoclastics intruded by porphyritic diorite and feldspar porphyry dikes of the Archean Abitibi greenstone belt of Canada. The network is mainly made of shear zone-hosted, steeply dipping veins and subhorizontal extension veins filled for the most part by quartz and tourmaline. Hydrothermal alteration around these veins is made of coeval albite-rich and tourmaline-rich visible haloes, and formed during multiple stages of shearing of the host rocks, opening, growth, and interconnection of vein segments, and during gold precipitation within veins and wall rocks. Appropriate scale mapping of vein outcrops located within a1 .3 km-deep section of thedeposit is used here to gather quantitative geometrical data on the vein network and the alteration haloes developed at the vein walls. These data show that the albite- and tourmaline-rich haloes have no preferential distribution within the deposit, form patterns that are asymmetrical along and across vein strike, and are mainly controlled by the local three-dimensional arrangement of the veins. Vein gold grades do not correlate with metric properties of veins like thickness and depth. Results suggest that the hydrothermal alteration at Sigma developed mainly from diffusive transport of mass through the wall rocks. They also suggest that the control of local topology of vein arrays on alteration patterns, underestimated by previous studies of Sigma, may be important in other structurally complex vein network deposits. Scale mapping of both mineralized veins and hydrothermal alteration envelopes has proved useful in constraining the structural relations between vein growth and fluid-rock interaction and in determining locations of most intense wall-rock metasomatism, which provides an objective guide to the choice of samples for geochemical studies.
... The domes are made up of massive to pillowed tholeiitic basalt (Squire & Wilson 2005) and are overlain by the Stawell Facies (Dugdale et al. 2006). The latter comprises calcareous and quartzose sandstones , chert and mudstone, some of which is sulfidic and is interpreted to have been altered prior to mineralisation (Watchorn & Wilson 1989). East of the Coongee Fault, the St. Arnaud beds (Figure 2b) are made up of less deformed and less metamorphosed sandstone and shale. ...
A. (2008) 'Predictive targeting in Australian orogenic-gold systems at the deposit to district scale using numerical modelling', This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
... The phenocrysts and microphenocrysts of former plagioclase are either totally albitised or replaced by an aggregate of microcrystalline clinozoisite -epidote and chlorite, whereas the augite is typically replaced by ragged actinolite -albite -chlorite intergrowths. These alteration assemblages are consistent with metamorphism of the basalts to lower greenschist facies (Watchorn & Wilson 1989). ...
Lower Cambrian basalts in the Stawell Zone (e.g. Magdala Basalt) are intercalated with a thick package of continentally derived quartz-rich turbidites, whereas similar basalts in the Heathcote and Mt Wellington Greenstone Belts are underlain by broadly coeval boninites and overlain by Upper Cambrian cherts. These marked differences in the magmatic – stratigraphic associations of the Early Cambrian basalts in Victoria have led to highly varied interpretations of their tectonic setting, which include subduction zones that are east-dipping, west-dipping and even a combination of east- and west-dipping systems. Here, we present new geochemical data for Cambrian basalts from the Stawell Zone and reassess the palaeogeographic and palaeotectonic setting of the region. Our geochemical data show that basalts in the Stawell Zone may be broadly divided into two units: basalts that are LREE-depleted relative to N-MORB and have low (≤3.0 ppm) Nb contents; and basalts that are slightly LREE-enriched relative to N-MORB and have relatively high (>3.0 ppm) Nb contents. The LREE-depleted Nb-poor basalts are the most abundant type in the Stawell Zone and generally occur at lower stratigraphic levels than the Nb-rich basalts. While the slightly LREE-enriched basalts are most similar to the backarc-basin basalts in the Heathcote and Mt Wellington Greenstone Belts, all basalts from the Stawell Zone display characteristics similar to modern backarc-basin basalts from the West Pacific. We propose that basaltic magmatism in the Stawell Zone was generated during a widespread backarc extension event that affected much of western Victoria. However, important stratigraphic – magmatic associations in the Stawell Zone, including the absence of forearc-generated boninites and the presence of a thick (>300 m) package of quartz-rich sandstone and mudstone underlying the Early Cambrian basalts, indicate that the Stawell Zone was relatively close to (or possibly at the outer margins of) the East Gondwanan continental margin and thus always distal to the forearc (cf. Heathcote and Mt Wellington Greenstone Belts). These characteristics indicate that basaltic magmatism in the Stawell Zone occurred in the backarc of a west-dipping subduction zone that initiated (i.e. generated) boninites farther east and southeast before undergoing rapid east-directed rollback.
In this chapter we examine a broad category of hydrothermal mineral deposits whose principal characteristic is their strong structural control, and whose genesis is attributed to the action of deep-circulating crustal fluids. The mineralisation is hosted in a variety of lithologies metamorphosed to lower-upper greenschist facies, and less commonly to lower amphibolite facies. In all cases, however, the mineralising events post-date peak metamorphism. The deposits are found within high strain zones in brittle (lower greenschist facies) to brittle-ductile structures (mid-upper greenschist facies). Nesbitt (1988) and Nesbitt and Muehlenbachs (1989) refer to these deposits, in which Au is the chief economic metal, as “mesothermal lode gold”. In the literature these are also referred to as metamorphogenic or metamorphic vein deposits (Groves and Phillips 1987; Groves et al. 1987; Nesbitt and Muehlenbachs 1989), or solution-remobilisation ores (Boyle 1979, Guilbert and Park 1986). Here we adopt Nesbitt and Muehlenbachs’ terminology and use for all of these deposits the general term mesothermal ore deposits. Within this broad category are included the mineralised quartz vein systems in Archean granite-greenstone terranes (Archean lode Au deposits), the quartz vein lode Ag deposits of Idaho (USA), the lode Au deposits of Pilgrim’s Rest in South Africa and turbidite-hosted Au deposits. The latter are generally of Phanerozoic age, and examples are found in the Canadian Cordillera, Nova Scotia and Alaska in North America, Victoria in Australia, and Otago-Marlborough in New Zealand.
The unpredictability of the location, grade and width of narrow-vein deposits presents difficulties for mine geologists and engineers attempting to estimate, schedule, forecast and mine their deposit profitably. This unpredictability stems not only from the grade distribution within a vein, but also the irregularity of vein geometry in three dimensional space. Traditional deterministic methods of modelling vein geometry do not account for the inherent variability of the veins, resulting in significant errors when using limited drill hole information. This paper presents a probabilistic method of modelling narrow-vein deposits using Sequential Gaussian Simulation, which can quantify this variability and improve production forecasts. Vein geometry, thickness and grade are simulated to provide a number of different equi-probable realisations that each honour the drill hole and face sample input statistics. An example is presented from the Augusta narrow-vein Au-Sb mine in central Victoria, where the simulation results are validated against the input data and survey pickups of the veins along ore drives.
The Wonga gold lodes (∼300,000 oz Au) in the Victorian gold fields of eastern Australia formed at a depth of less than 7 km within the seismogenic zone. The Wonga gold deposit is hosted by mudstone-dominant lithofacies that define the core of the regional synform. These are underlain by sandstone-dominant lithofacies that are interlayered with mafic schist. The lodes occur within the contact aureole of a late-stage granitic body that intrudes an earlier phase of the fractionated pluton. In contrast to the nearby 440 Ma Magdala orogenic gold lodes (∼5 Moz Au), this smaller deposit formed at ca. 400 Ma and has been classified as intrusion related. The orogenic and intrusion-related gold deposits formed at different stages in the structural and magmatic evolution of the fold belt, and at different crustal levels. Compared to the Magdala lodes, the Wonga lodes have a close temporal and spatial relationship with the intrusive rocks, and a distinct structural style and mineralogy, including acicular arsenopyrite, stibnite, gold telluride alloys, bismuth, silver, and late-stage molybdenite. The Wonga lodes formed between two distinct phases of regional faulting, and the kinematics of the mineralized structures cannot be correlated with any preserved, far-field regional paleostresses. Instead, they are inferred to reflect the local state of stress at the point that magmatic-related fluid overpressure occurred. Pervasive fluid overpressure drove deformation across a broad zone, producing massive, mineralized quartz-vein structures that included angular clasts of wall rock. A discrete overpressure event is suggested, rather than repeated crack-seal behavior. The lodes represent an early stage in the development of a fault zone characterized by a series of discontinuous, discrete shear failures. The geometry of the lodes is strongly dependent on structural position and rheology. Northeast-dipping, preexisting weaknesses, such as dike margins and variably developed crenulation cleavages, produced localized shear failure defined by dextral-reverse-slip structures (the hanging-wall structures); 30° to 60° SE-dipping extension veins and extensional shear veins (the link structures) diverge from these, creating en echelon lode geometries. There is a sudden change over the top of a regional plunge reversal defined by the dominant fold axes from shear failure (the hanging-wall structures) to extensional shear and then extensional failure (the links). This occurs along strike in the same structural corridor and within the same stratigraphic unit. The explicit finite difference-code (FLAC) was used to model the local state of stress at the point where an influx of magmatic fluids raised the pore fluid pressure. The model predicts that a zone of slightly lower differential stress was associated with the plunge reversal and this variation in differential stress is interpreted to have caused the drastic change in lode style. The modeling highlights that only subtle differences in the elastic moduli of the rocks are required to produce the observed stress geometry.
The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites. Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation. Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites. Mo is associated with similar granites that are more fractionated and oxidised. W is associated with a variety of granite types and shows little dependence on inferred magma redox state. The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe-Ti phases (e.g. pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2-dependent. Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes. The halogen content is also important. S-type granites are poorer in Cl than I-types. Cl decreases and F increases in both S- and I-type granites with fractional crystallisation. Low Cl contents combined with low magma fO2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites. Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation.
The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near-lithostatic fluid pressure regime. The geometries, distribution and internal structures of veins in the Wattle Gully Fault zone are related to variations in shear stress, fluid pressure, and near-field principal stress orientations during faulting. -from Author
The western margin of the Lachlan Fold Belt contains early ductile and brittle structures that formed during northeast‐southwest and east‐west compression, followed by reactivation related to sinistral wrenching. At Stawell all of these structural features (and the associated gold lodes) are dismembered by a complex array of later northwest‐, north‐ and northeast‐dipping faults. Detailed underground structural analysis has identified northwest‐trending mid‐Devonian thrusts (Tabberabberan) that post‐date Early Devonian plutonism and have a top‐to‐the‐southwest transport. Deformation associated with the initial stages of dismemberment occurred along an earlier array of faults that trend southwest‐northeast (or east‐west) and dip to the northwest (or north). The initial transport of the units in the hangingwall of these fault structures was top‐to‐the‐southeast. ‘Missing' gold lodes were discovered beneath the Magdala orebody by reconstructing a displacement history that involved a combination of transport vectors (top‐to‐the‐southeast and top‐to‐the‐southwest). Fold interference structures in the adjacent Silurian Grampians Group provide further evidence for at least two almost orthogonal shortening regimes, post the mid‐Silurian. Overprinting relationships, and correlation with synchronous sedimentation in the Melbourne Trough, indicates that the early fault structures are mid‐ to late‐Silurian in age (Ludlow: ca 420–414 Ma). These atypical southeast‐vergent structures have regional extent and separate significant northeast‐southwest shortening that occurred in the mid‐Devonian (‘Tabberabberan orogeny') and Late Ordovician (‘Benambran orogeny').
In the internationally significant Victorian goldfields a complex system of faults dismembers the 5 million ounce Magdala gold deposit. These faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio (φ). The fault geometry is strongly controlled by the pre-existing rheology. Faults have propagated around the flanks of an antiformal basalt dome, along earlier ductile cleavages and the margins of porphyry dykes. Many of the faults do not have Andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions. Much of the faulting is associated with the emplacement of porphyry dykes, additional gold mineralisation related to plutonism and late-stage deformation post-dating the intrusion of the Stawell pluton. Systematic mapping of extension veins associated with faults, striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation. This successfully predicted the offset direction of the currently mined Magdala ore body beneath the studied system of faults. The use of extension veins was a critical aspect of the analysis. If striations on the fault surfaces had solely been used, the offset direction of the new Golden Gift orebody would not have been correctly ascertained. The palaeostress history was delineated via use of compression and tension dihedra, stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped. The calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing σ1 and σ3.
Difficulties in correlating the Cambrian magmatic, depositional, structural and metamorphic events along the proto-Pacific margin of East Gondwana have led to subdivision of the region into the Delamerian, Lachlan and Tyennan Orogens in Australia, the Ross Orogen in Antarctica and the Takaka Terrane in New Zealand. As a result, the Cambrian tectonic evolution of the region is poorly understood. We present here a revised lithostratigraphic section from the late Early to Mid-Cambrian rocks exposed near Stawell in western Victoria, which is used as the basis for correlating geological events in East Gondwana. These data show that the Cambrian tectonic evolution of East Gondwana's > 4000 km long proto-Pacific margin involved predominantly compressional orogenesis separated by major short-lived extensional events at c. 516-514 and 504-500 Ma. The most significant extensional event, at c. 516-514 Ma, involved extensive slab rollback along the proto-Pacific margin in response to major changes in global plate motions and plate-boundary stresses following the termination of East-West Gondwana collision. Partial subduction of a ribbon of buoyant continental crust led to localized subduction-zone failure and obduction of the young hot forearc lithosphere in Tasmania at c. 5 10 Ma. Collision of the continental ribbon also significantly modified the architecture of the proto-Pacific margin and ultimately controlled the extent of the second major extensional event associated with slab rollback at c. 504-500 Ma. Tectonic evolution of the proto-Pacific margin of East Gondwana thus involved the complex interaction between convergent-margin processes and collisional orogenesis.
3D models and computer-based numerical simulations have been used in the exploration industry for some time to visualise the geometry and mechanisms resulting in the formation of orebodies. However, due in part to computational limitations, few numerical simulations have been run on complex (real) geometries in order to predict the location of new ore systems. Presented here are the results of an exploration program developed by the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) and MPI (now Leviathan Resources) in the orogenic-gold system of western Victoria that utilised D modelling and numerical finite-element simulations to successfully target several new orebodies and predict their geometries and extent. Existing drillcore databases were utilised to constrain the geometries of known deposits and associated mafic domes, the effects of known post-mineralisation faulting was systematically removed and syndeformation fluid flow was then modelled within the system. The results of these simulations were compared with the known geometry of the mineralised systems about these deposits in order to test the simulation parameters and accuracy. 3D models were also developed of poorly constrained target domes in regions with no outcrop utilising potential-field datasets and limited drilling data. Simulations were then run on these model geometries using the tested parameters in order to predict the likelihood of mineralisation in these systems, its geometry and (most importantly) its location. These targets were then drilled resulting in the discovery of previously unknown gold deposits associated with the Kewell Dome northwest of Stawell.
Variable distribution of elevated gold grades in the Magdala Central Lode system is controlled by preferential and localised reactivation of pre-existing faults in a progressively rotating stress field. Populations of slickenline lineations on fault surfaces in combination with extension vein arrays associated with the faults indicate Central Lode formed initially during southwest – northeast compression, but was subsequently locally reactivated during east – west compression on lower angle fault segments to produce a series of ore-shoots within the overall lode system. A series of 2D cross-section numerical models have been used to examine how deformation and fluid flow are portioned at different stages of the lode evolution. Models have been constructed in the plane estimated to contain maximum and minimum principal stresses during the events leading up to, and during, mineralisation as indicated by the lineation and extension vein data. Results of the models show that during southwest – northeast compression, a major shear zone forms along the boundary between the Stawell Facies and the overlying Albion Formation consistent with the extent and location of the Central Lode Shear Zone. In a second series of models it is shown that during east – west compression, shear failure is considerably more localised to regions overlying lower angle lithological contacts. This localisation of shear failure in these east – west compression models shows strong correlation with the distribution of gold in grade shell models, confirming the structural and lithological factors identified as controlling the mineralisation.
The localised distribution of gold in a series of ore shoots adjacent to the western flank of the Cambrian Magdala Basalt is controlled by formation and partial reactivation of faults and shear zones that closely follow the geometry of the basalt. The recognition of the controls on the geometry of the basalt and overlying metasedimentary units is therefore vital for exploration in the region. Detailed structural mapping around one of the steep, west-dipping basalt sheets that comprise the Magdala Basalt, as well as the overlying metasedimentary units, has been used to compare the recorded structural history in the basalt relative to that in the metasediments to determine how strain has partitioned during the evolution of the deposit, and if the geometry is a product of primary volcanic processes or early ductile deformation. The results of this work show that much of the deformation history at the Magdala mine is recorded within the metasediments, while the basalt itself appears only affected by the late brittle deformation. The lack of evidence for early ductile deformation in the basalt and the partitioning of strain into the Albion Formation early in the deformation process suggests that the variable geometry of the basalt is a product of primary volcanic processes, rather than ductile deformation. The irregular geometry of basalt flows along the southwestern flank of the Magdala Basalt may be related to the dominance of distal volcanic facies, primarily pillow basalt, the variability of which influences localised deformation and gold mineralisation during an episode of low displacement east – west compression.
The initial stage of continental extension between Australia and Antarctica was associated with lateral changes in extension direction along the margin that reflects the three-dimensional nature of strain during continental rifting. In the Cretaceous Otway Basin, this change in extension direction was related to substantial rheological differences in the lithosphere across the boundary between two Paleozoic fold belts, the Lachlan and Delamerian, with the net extension direction at a high angle to this boundary. The initial Early Cretaceous rifting preserved within the onshore Otway Basin has two main structural subdomains in the eastern and western Otway Basins distinguished by different structural trends of Early Cretaceous normal faulting. This is not controlled by a variation in preexisting structural weaknesses within the underlying Paleozoic basement because the same geometry of extensional structures also occurs within the basement to the north irrespective of the preexisting structural grain. The eastern Otway Basin is dominated by NE-striking NW-dipping normal faults. In the western Otway Basin, the faults define arrays of predominantly NE-dipping or SW-dipping faults separated by wide accommodation zones defined by folding and variably striking and dipping faults. The partitioning of strain along the boundary between the eastern and western Otway Basins is accommodated by a progressive change in strike of faults and not via a transfer fault. Younger rifting in the Late Cretaceous had a similar extension direction in the western Otway basin, but had a dominant seaward dip, extension appears to have been hindered in the eastern Otway basin by a Proterozoic/Paleozoic basement feature. These factors produced a region of diverging extension along the lithospheric boundary between the Delamerian and Lachlan Fold Belts that lead to failure along this boundary and the formation of a localized sinistral trans-tensional graben, the Shipwreck trough, in the early Late Cretaceous. As a result, the younger rifting stepped south of the eastern Otway Basin leaving Bass Strait (the strait between the Australian mainland and Tasmania) a failed rift. The formation of oceanic crust in the Mid to Late Eocene followed the boundary of Late Cretaceous rifting, which led to the formation of the Tasman Fracture Zone.
Exploration for orogenic Stawell-type deposits in the northern extension of the Stawell zone under the Tertiary Murray Basin sediments, in southeastern Australia, has produced the discovery of at least two new deposits at Wildwood and Kewell. Previous research on the Stawell gold deposit has highlighted the importance of litho-facies, structural evolution and hydrothermal alteration in the localization of fluid flow at the time of mineralization. Application of geophysical tools including magnetic and gravity surveys at both regional and more detailed scales delineated a number of significant coincident magnetic and gravity anomalies, which upon further investigation through aircore drilling proved to be basalt. Subsequent drilling on the margins of the Wildwood and Kewell basaltic domes delineated areas of prospective litho-facies, hydrothermal alteration, quartz veins and mineralization akin to that found in the Stawell deposit. Analysis of multi-element geochemistry revealed significant gains in Fe in the transition from unaltered to altered unmineralized (1 ppm Au) altered host rock. Magnetic remanence properties of the various lithologies within the Stawell Zone showed that high fluid flow can result in the precipitation of sulfides with fixed stable remanent magnetization the orientation of which may not be parallel to the present day magnetic field, whereas zones of low fluid flow have soft magnetization that can be easily modified. The implication of variation in remanence orientation for exploration is that regional aeromagnetic anomalies, which have opposite polarities, may coincide with zones of high fluid flow and are potential target areas for mineralization.
– data constrain the history of metamorphism, deformation and mineralization in the western subprovince of the Lachlan Fold Belt. Rocks in the Mount Stavely Volcanic Complex and the Bushy Creek Pluton give – ages of about 500 Ma for hornblende and biotite. The dates from the Mount Stavely Volcanic Complex limit the age of mineralization in the metavolcanic rocks of the Stavely Belt. Whereas, the date for the Bushy Creek pluton extends the area of Cambrian–Ordovician plutons, of the Delamerian Orogeny, to east of the Grampians Mountains and helps define the surface expression of the western border of the Paleozoic Lachlan Fold Belt. – dating of metamorphic mica growth (phengite) in cleaved slates and phyllites within the Landsborough Fault, Avoca Fault zone, Heathcote Fault zone and Mount Wellington Fault zone gives ages of 453±2, 440±2, 426±4 and 410–390 Ma, respectively. These data indicate a progression of deformation from west to east starting in Late Ordovician–Early Silurian time in the Stawell and Bendigo-Ballarat structural zones. The major phase of deformation in the western subprovince of the Lachlan Fold Belt is therefore Silurian and not Devonian. Sericites from major gold deposits in the Stawell zone and Bendigo-Ballarat zone give – dates of about 440 Ma, that are coincident with folding and thrusting. A second episode of mineralization and fault reactivation at about 420 to 410 Ma is found in some deposits in these zones, based on data from a previous study. In the Melbourne zone (including the Woods Point Dike Swarm) and locally in the Bendigo-Ballarat zone, some gold deposits give sericite dates of about 380–360 Ma. These ages are consistent with the close association of many Melbourne zone gold deposits with Devonian dikes and plutons.
The Palaeozoic succession of Victoria represents a major world gold province with a total production of 2500 t of gold (i.e. 78 million oz). On a global scale, central Victoria represents one of the most gold mineralized areas outside the Witwatersrand of South Africa, and remains the prime example of a ‘slate belt’ gold province (also known as ‘turbidite-hosted’, or ‘shale-greywacke’ gold province). Production came from quartz veins (1000 t), modern placers (1200 t) and palaeoplacers (300 t). Up to 7000 mines produced gold from quartz veins; however, the 168 mines of this type which produced over 1 t Au contributed 68% of the primary gold production. Twelve goldfields have each produced 30 t Au (approximately one million ounces) from all sources, with Bendigo (697 t), Ballarat (408 t) and Castlemaine (173 t) being the largest: these twelve goldfields have contributed 71% of the combined primary and secondary production. Gold mining commenced in 1851, declined dramatically around 1914, and has not increased significantly since. The Palaeozoic succession of Victoria is part of an extensive fold belt along much of eastern Australia (i.e. the Tasman Orogenic Belt). In Victoria, the succession is dominated by Cambrian to Early Devonian clastic metasedimentary rocks that have undergone major upright deformation and low-grade metamorphism, culminating in the Middle Devonian (Tabberabberan) Orogeny. North-south inliers of Cambrian metavolcanic- and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones. In central Victoria, there are particularly well-developed acid volcanic complexes of Late Devonian age, and synchronous peraluminous granites which represent high-temperature crustal melting. Tertiary to Quaternary basalts have covered some of the Palaeozoic gold deposits, and have also covered several rich gold placer deposits of Cainozoic age. Larger goldfields, including Bendigo and Ballarat, occur in Ordovician slate and greywacke sequences, but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields. Most, if not all, primary deposits demonstrate structural control, although the controlling structural features differ between deposits. Wallrock alteration is strongly influenced by host rock: in metasedimentary rocks, it is limited and subtle, but it is more pervasive in mafic and felsic igneous rocks. Carbonates, muscovite and pyrite are the most widespread alteration minerals in these rocks, representing CO2, K and S addition. Enrichment of As within gold deposits is common, with Sb important in some small deposits, whereas base metals are only abundant locally, and Bi, W, Mo and Te show strong spatial association with granites and are rarely associated with economic gold deposits. Granites are an important component of the Palaeozoic succession in Victoria. No large deposits are within granites, but a few goldfields, including Maldon (65 t Au), are within contact aureoles of either S or I type granites of differing degrees of fractionation and silica content. A single-stage, but protracted and possibly diachronous, period of gold introduction near the time of the Middle Devonian (Tabberabberan) deformation can explain geological relationships at many gold deposits. The possibility that this event was slightly older (i.e. Silurian or Early Devonian) in the west of the state cannot be precluded from the available evidence. The mineralizing event appears linked to the same thermal event as diorite (‘lamprophyre’) dykes and graite intrusions, acidic and basaltic volcanism, deformation and regional metamorphism. The limited amount of fluid chemistry data from Victorian gold deposits suggests low-salinity fluids which contained CO2, and variable methane, with homogenization temperatures from 150 to 300°C. The sulphur isotopic compositions of pyrite are mostly +1 to +5‰, with some values slightly above + 10‰ Mineralizing temperatures are inferred to be near 300°C, and pressure around 1–2 kbar. Sulphide-rich gold deposits in the east of Victoria, many of which contain significant base metals, contrast with the rest of the province (which contains mostly gold deposits with lower sulphide content), and this eastern area should possibly be viewed as separate from the Victorian gold province, per se. The benefits to Victorian gold mining coming from the 1980's global gold boom have been insignificant compared to the historical importance of the Victorian gold province. There seems little justification for assuming that all the large deposits have been discovered, and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types. A major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival.
Age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central Victoria, Australia occurred over a prolonged period of time ranging from prior to the Late Silurian to the Late Devonian. An early phase of gold mineralisation associated with regional deformation pre-dated the emplacement of Late Silurian felsic porphyry dykes at 413±3 Ma in the central lode system of the Magdala deposit, Stawell. Gold mineralisation associated with the final stages of the main regional deformation also pre-dated the intrusion of Early (ca. 400 Ma) to Late Devonian (ca. 370 Ma) plutons at Stawell (Wonga deposit), Mt. Bute, Tarnagulla, Tallangalook, Maldon and Heathcote in both the Stawell, Pyrenees and Ballarat metallogenic provinces, consistent with preliminary dating studies of hydrothermal micas from major gold deposits.
Major 440 Ma orogenic-gold deposits in the western Victorian goldfields formed during east – west shortening but have markedly different structural complexity. These deposits occur in: (i) a Cambrian Delamerian basement block that was substantially reworked and reactivated during the Lachlan Orogeny (Stawell); and (ii) Ordovician turbidites deformed solely by Lachlan-aged deformation (Bendigo, Ballarat, Castlemaine). This produced different structural histories prior to mineralisation, although gold deposits have been localised at the top of regional domal culminations. At Stawell, the 440 Ma gold event reactivated a strike-change along a pre-existing Cambrian fault system above a major lithospheric boundary. In the Bendigo Zone, 440 Ma orogenic-gold deposits have a trend oblique to the dominant structural grain and parallel to the western edge of an inferred crystalline basement block (the Selwyn Block). This gold trend is parallel to metamorphic field gradients and pluton age boundaries, which suggests an underlying basement control on the localisation of these orogenic-gold deposits, even though the exact basement architecture is still unresolved. Major variations in the regional stress fields occurred between 425 and 370 Ma, with large gold deposits [>62 t (2 million ounces) endowments] forming at ca 380 – 370 Ma. These events are not deposit-scale structural anomalies as they also regionally affect overlying cover sequences (e.g. the Grampians Group). Gold deposits that formed in the 425 – 400 Ma period have small endowments, but introduce a marked amount of structural and mineralogical complexity to the gold province. The 425 – 400 Ma period preserved at Stawell records sinistral wrenching associated with gold mineralisation, southeast-directed faulting, intrusion-related gold mineralisation and extensive high-level Early Devonian plutonism.
The Cowarra gold deposit is an epigenetic, structurally controlled, gold‐bearing sulfide vein system in a regional shear zone developed within isoclinally folded Ordovician turbidites. Gold occurs mainly in sulfide‐rich veins parallel to the axial cleavage or shears on fold limbs. There are seven sets of quartz veins, but they contain only minor gold. The mineralised shears lie within a zone of strong fold‐plunge variation, in the chlorite zone of a regional metamorphic complex, and to the west of the Bega Batholith. However, gold is not related to metamorphic outflushing, but more likely to sulfide‐rich fluids derived from depth during intrusion of I‐type granites of the Michelago Igneous Complex. The local control for sulfide veining was decompressional dilation of axial planar structures shortly after folding and shearing; this is unlike the structural control described for most deposits from this and similar terrains. Subsequent normal, strike‐slip and reverse fault movements in the shear zone develop separate quartz‐vein arrays, but these are mostly barren of gold; there are no saddle reefs or fault‐controlled gold veins as are common in other shear‐zone deposits in Australia and Canada.
We present coupled 3D deformation – fluid-flow models which place constraints on the importance of basalt dome shape and interpreted synmineralising shortening direction in localising gold mineralisation around basalt domes in the Stawell corridor, western Victoria. Gold mineralisation in the Magdala orebody at the Stawell mine occurs predominantly within a thin metasomatised unit named the Stawell Facies which blankets the basalt domes and also occurs close to parasitic fold-like basalt lobes on the basalt domes. In dome-scale models that do not contain basalt lobes, areas with the maximum fluid-flow rates occur on the tops of the flanks of the domes where there is a dramatic change in dip of the basalt, and a change from contraction to dilation which creates a significant pore-pressure gradient. In models that contain basalt lobes, the location of high fluid-flow rates is strongly controlled by the presence of these lobes. High fluid-pressure gradients are created between the contracting Stawell Facies in the area between the lobe and the main domes and those areas dilating above. Areas of significant dilation occur on the shallow-dipping portion at the top of the dome and cause fluid to flow towards them. Areas that have significant dilation are also areas of tensile failure in some cases and are coincident with areas of known quartz vein-associated mineralisation. In the Magdala Dome models, only the east-northeast – west-southwest- and east – west-shortened models record high fluid-flow rates in areas of known mineralisation, which is consistent with the interpreted synmineralisation-shortening directions. Therefore in this situation, fluid-flow rates during east-northeast – west-southwest- and east – west-shortening can be used to indicate the potential location of gold mineralisation. In numerical models of the Kewell Dome (a prospect to the north), the position of areas of high fluid-flow rate when shortened in the east-northeast – west-southwest and east – west direction, combined with information from limited drilling, indicated the potential for gold mineralisation at the southwest end of the dome. Diamond drillholes in this area yielded significant gold values.
Central Victoria, in S.E. Australia, is regarded as the classical area for gold mineralization associated with saddle reefs, and with reverse faults, within a turbidite terrane. However, there is also an important contribution by steep, brittle shear zones to the mineralization and two such areas, Fosterville and Heathcote are the subject of this article. The shear zones hosting mineralization are sinistral transpressional structures that, while accommodating regional shortening, do it by both crustal thickening and lateral expulsion. The mineralization occurs at dilational jogs or bends within the zones. The shearing and mineralization postdate the main phase of folding and thrusting but predate widespread post-tectonic granitoid intrusion. It is postulated that the shears are a part of the initial phases of orogenic collapse which culminated in the emplacement of the granites, as well as sedimentation and acid to bimodal vulcanicity. Furthermore, it is argued that mineralization within the saddle reefs and reverse faults that characterize Central Victoria are also related to the initial phase of orogenic collapse. A final postulate is that orogenic collapse provides a common tectonic thread to some turbidite hosted, epithermal and Archean lode types of gold mineralization.
The Magdala deposit in the Stawell goldfield in western Victoria was formed during the 440 Ma gold event of the Lachlan Orogeny and is hosted by Cambrian quartz-rich turbiditic sedimentary rocks (Albion Formation) that onlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). Detailed petrographic and geochemical analyses suggest that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation that lasted at least 10 million years culminated in the formation of the Magdala mineralised system and produced a complex pattern of hydrothermal alteration. Evolution of this alteration occurred over six stages: Stage 1, Fe-enrichment of sedimentary rock adjacent to the basalt pre-D1; Stage 2, chlorite (metamorphism), orbicular carbonate and pyrite, syn-D2; Stage 3, muscovite, siderite, ankerite and pyrrhotite, syn-D3 – D4a – b; Stage 4, stilpnomelane, siderite, pyrrhotite, arsenopyrite and pyrite, syn-D4c; Stage 5, silica, minnesotaite and magnetite, post-D4c – pre-D5; and Stage 6, Fe-rich chlorite, muscovite, calcite, arsenopyrite and pyrite, syn-D5. Comparisons with other turbidite-hosted gold deposits in Victoria (e.g. Bendigo and Ballarat) highlight four major differences: (i) presence of a tholeiitic basaltic pile; (ii) ductile deformation (D1 – 4) over at least 60 million years prior to gold mineralisation; (iii) highly evolved hydrothermal alteration; and (iv) source of sulfur. Of these differences the key element is the basaltic pile and its associated heat, which may have promoted the growth of micro-organisms in, and alteration of, the onlapping sedimentary rocks, thereby creating a basis from which an unusual turbidite-hosted orogenic-gold deposit was formed.
Mineralogical and microfabric studies of the Magdala gold deposit together with isotopic and fluid inclusion studies have shown that the mineralising fluid is externally derived. The host rocks to the quartz veins do not possess any significant amounts of gold, whereas the quartz veins can contain >7 g/t gold. The walls to the quartz veins possess alteration haloes in the form of stilpnomelane porphyroblasts, attesting to the fact that the fluid in the veins was out of equilibrium with the host rocks. The distribution of alteration haloes that occur parallel to the quartz vein margins necessitates that a subvertical fluid gradient, be more or less constant during the deposition process in order to produce the observed architecture of the alteration haloes. Quartz lodes possess δ values that range from 15.8 to 17.3 per mil while associated host rocks range between 9.7 to 12 per mil in the metabasalts and 10.2 to 14.1 per mil in the volcanogenic and metasedimentary rocks. The dissimilarity of δ values between host rocks and quartz veins suggests that there has been a high fluid influx in the veins, implying external buffering. Four types of fluid inclusions were distinguished on the basis of spatial arrangement and composition using the Laser Raman Spectroscopy method. The salinity of the fluid inclusions range from 0.7 to 8.4 wt% NaCl equivalent, suggesting that gold was deposited due to the influx of different external fluids. The association of pyrite–arsenopyrite–pyrrhotite with gold and with PGEs suggests that these fluids have a lower crustal origin and may have originated from the upper mantle and mixed with fluids derived from deep seated metasediments.
Fault zones within turbidite-dominated orogenic systems, typified by the Lachlan Orogen of eastern Australia, are characterised by higher than average strain and intense mica fabrics, transposition foliation and isoclinal folds, poly-deformation with overprinting crenulation cleavages, and steeply to moderately plunging meso- and micro-folds. They have a different character compared to the brittle–ductile fault zones of classic foreland fold-and-thrust belts such as the Appalachians and the Canadian Rocky Mountains. Multiple cleavages and transposition layering record a progressive shear-related deformation history. An intense mica fabric evolves initially during shortening of the overlying sedimentary wedge, but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults. The deformed wedge outside the fault zones generally undergoes one phase of deformation, shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright, subhorizontally plunging chevron-folds. Other faults within the turbidites of the Lachlan Orogen include the steep zones of ‘ductile’ strike-slip deformation that bound a centrally located, high T/low P metamorphic complex. Characterised by S–C mylonites, these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge, with emplacement to higher structural levels along a leading-edge, ductile thrust-fault. Ar–Ar dating constrains the timing of regional deformation to be mostly Late Ordovician through Silurian across the Lachlan Orogen. Faults in the low grade turbidite sequences record the kinematic evolution of the developing Lachlan Orogen and indicate progressive deformation associated with simultaneous, eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt. These deformation fronts are related to ‘accretionary style’ deformation at the leading edges of overriding plates, in an inferred southwest Pacific-type subduction setting from the Late Ordovician to the mid-Devonian, along the former Gondwana margin. The fault zones effectively accommodate and preserve movements within the structurally thickening, migrating and prograding accretionary wedge.
Victoria is a gold province of international stature with primary gold mineralisation richly developed in the pre-Carboniferous rocks of the Lachlan Fold Belt. Total recorded production is in the order of 2,460,000 kg gold of which some 60% was sourced from extensive alluvial deposits, both of deep and shallow lead origin. Eleven mining centres each produced over 30 tonnes of gold with the major producer being Bendigo (684,300 kg).The Palaeozoic sequence in central Victoria is characterised by Cambrian metabasites or greenstones with associated volcanogenic and pelagic sequences followed by a Cambro–Ordovician to Early Devonian sequence of silici-clastic sediments. This succession is characterised by north–south trending open to tight folds and west-dipping thrust faults. Extensive S- and I-type granites, were emplaced from Late Silurian to Late Devonian, with the latter group, in part, associated with acid volcanic complexes.Mineralising styles include mesothermal deposits, either turbidite-hosted or dyke-affiliated, and these account for the bulk of primary gold mineralisation in the State. Less significant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro–Devonian age. Other styles of minor importance include Cambrian volcanogenic exhalative Cu–Zn mineralisation with gold as a minor credit (0.15–0.6 g/t Au), granite-hosted gold, and a potentially important group comprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy with Nova Scotian disseminated intrametallic deposits, may have implications for a hitherto unrecognised style of gold mineralisation in Victoria.A gradation in lode-Au mineralising styles occurs across central Victoria. To the west in the Stawell metallogenic province, mineralisation was emplaced structurally and stratigraphically lowest in the crust. Mesothermal mineralisation is shear-hosted in Cambrian meta-sediments and greenstones exhibiting repeated deformation, multiple foliations, and extensive shearing. Host sediments were dominated by greenschist grade regional metamorphic conditions.Within the Ballarat metallogenic province, mesothermal mineralisation is hosted by dilational-site's style, in Ordovician sediments which were subjected to upper prehnite–pumpellyite facies conditions to the west and zeolite facies conditions to the east adjacent to the Melbourne metallogenic province. Ore fluids were of the low salinity C–O–H type with 1–10 wt% NaCl equivalent. CO2 and CH4 contents of fluid inclusions tend to be low with variable CO2/CH4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that ore fluids were derived either from an evolved igneous or from a devolatilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro–Devonian Melbourne metallogenic province, epithermal style Au mineralisation is commonly emplaced in structurally and stratigraphically higher rocks. Trapping temperatures are lower than in deposits to the west and ore fluids are of low salinity C–O–H–N type with N2>15 vol%. Gangue pyrite has elevated Sb contents and ore fluids were dominated by evolved meteoric water. δ values from vein carbonates suggest their derivation from oxidised organic carbon from within the Siluro–Devonian marine sediments.Alteration styles in gold deposits across central Victoria include sericitisation, carbonatisation, sulphidation, chloritisation, and (de-) silicification. Owing to the primary mineral assemblages and the physico-chemical characteristics of the ore-bearing fluids, visible evidence for wall rock alteration is commonly limited to 5–10 m wide zones of discolouration containing disseminated arsenopyrite and pyrite porphyroblasts and carbonate spotting. However, current petrographical and geochemical studies have demonstrated that the effects of pervasive wall rock alteration are more pronounced and extensive than previously recognised.Structural deformation within the Stawell and Ballarat metallogenic provinces, both from field evidence and / dating of cleavage phyllosilicates, indicates an Early Silurian event which migrated progressively eastwards with time. This Silurian event probably dates accretion of the proto Lachlan Fold Belt with the Kanmantoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests episodic mineralising events 20–70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressure regional metamorphism and/or granite plutonism; the latter being concentrated in two main pulses (400 Ma and 365 Ma) in central Victoria.The source of gold is equivocal with both igneous and devolatilised metamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORB and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical data indicate mean values of 65 ppb Au in these sulphidic interflow sediments which could have reacted with CO2-rich fluids developed during Siluro–Devonian regional metamorphism and magmatism. Subsequent deposition of gold from such fluids could have occurred in structurally higher level dilational traps.
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