Metamorphic Evolution of the Yilgarn Craton

Yilgarn Craton: Latest findings 2015. [1]

The East Yilgarn Craton is a major gold mineralization province, it is also a complex mosaic of different tectono-metamorphic domains and terranes. The thermal and barometric evolution, timing of different metamorphic parageneses and connection between thermal evolution and Au-mineralization has not previously been adequately addressed.

A regional-scale metamorphic project to investigate the above issues, was set up by Bruce Groenewald and Goscombe in 2003, with support of the GSWA. The East Yilgarn Craton Metamorphic Project (EYCMP) became fully funded in 2006 with a 2 years contract for Goscombe to run the metamorphic component within the larger Y4 program. The Y4 program involved [2] some 30 researchers Australia-wide and is funded and supported by pmd*CRC, Geoscience Australia, CSIRO and GSWA. The EYCMP component has been strongly supported by the GSWA, GA and pmd*CRC and is funded until August-2008. Collaborators in the EYCMP are Richard Blewett [3] (Team Leader) and Karol Czarnota at GA, Roland Maas [4] at Melbourne University, Bruce Groenewald at GSWA, Angus Netting [5] and Dr Ben Wade at Adelaide Microscopy and Mike Verrall and Greg Hitchen [6] at CSIRO-AARC Perth.

Ongoing research on the thermo-barometric evolution of the Western Yilgarn Craton is funded by a large research grant from the GSWA [7] (Grant# 20520), from August-2008 to June-2012. The West Yilgarn Craton Metamorphic Project (WYCMP) will involve collaboration with: Richard Blewett [8] (Geoscience Australia), Prof. David Foster [9] (University of Florida) for titanite and zircon geochronology, Dr Ben Wade [10] (Adelaide Microscopy) for monazite geochronology and the GSWA geologists working on the Yilgarn Craton; Steve Wyche, Tim Ivanic and Sandra Romano. Ray Addenbrooke, Daniel Greene and Debbie Caple from the GSWA are acknowledged for there support of the field sampling program.

Below is an extended abstract of a talk presented at Kalgoorlie07 in September 2007. This is work in progress on the Yilgarn at mid-2007. The ideas expressed have evolved substantially since then. Outcomes of the East Yilgarn research were published as GA Record 2009/23 [11], with full datasets and maps. The latest outcomes and models will be published in international journals and conferences in 2010 on release from stake-holders, see abstracts and extended abstracts reproduced below.

• Ma arc related metamorphism [12]

• M1 moderate-P metamorphism [13]

• M2 regional metamorphism [14]

• M3a rift related metamorphism [15]

• M3b late-stage alteration events [16]

(1) BROAD THERMO-BAROMETRIC EVOLUTION OF THE EASTERN GOLDFIELDS SUPERTERRANE.

Ben Goscombe¹, Richard Blewett², Karol Czarnota², Roland Maas³, Bruce Groenewald⁴
¹The University of Adelaide, SA, 5005
²Geoscience Australia, GPO Box 378 Canberra ACT, 2601
³University of Melbourne, VIC, 3010
⁴Geological Survey of Western Australia, 100 Plain St, East Perth, WA, 6004

Introduction:

Geological relationships, relative garnet Lu-Hf chronology, absolute PT calculations and spatial database of metamorphic parameters across all domains, constrain the metamorphic evolution of the Eastern Goldfields Superterrane (EGST). The post-2720 Ma thermo-barometric evolution of the EGST is divided into 5 broad periods of contrasting metamorphic style, conditions and possible thermo-mechanical/tectonic setting. Unlike stress fields, thermal event switching involves slow rates, conductive delays and a history less punctuated than the structural evolution; necessitating broad thermal events and continuums between them, particularly from M2 to M3a and M3b (see below).

Ma = Rare, high-T, high-G arc related metamorphism (2720-2685 Ma):

The period of late crust formation and volcanic stratigraphy involved both multiple events and a spatially complex mosaic of subduction, magmatic arcs, back-arc and plume related environments and processes. Widespread near surface metamorphic parageneses that would have formed at this time are sea floor alteration (e.g. Phillips. 1986). These early alteration assemblages would have been largely obliterated during subsequent events, except possibly in the lowest grade parts of the EGST. Nevertheless, the resulting altered bulk compositions are preserved and these have strongly influenced the subsequent developed metamorphic parageneses in the EYC (e.g. Purvis, 1984).

Granulite metamorphism is recognised in only two localities: a cluster in the northern part of the Southern Duketon Domain and one location in the northern part of Gindalbie Domain. These granulites are statically metamorphosed gabbros, preserving ghosted ophitic textures, recrystallised to granoblastic two pyroxene assemblages. Peak metamorphic conditions of ~730 ºC at low pressures between 2.5-5.0 kbar, indicate very high average thermal gradients (G – simply the ratio T/depth assuming a density of 2.8 gm/cm3) between 45-80 ºC/km. These high-T, high-G conditions in the upper-crust are most possibly formed in a high heat flow magmatic arc environment (Ma). Such a scenario would result in small high-grade domains, widely scattered and of different ages, reflecting multiple magmatic arcs. In contrast, formation in either an extensional or plume setting would result in widespread distribution.

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M1 = Discrete, medium-P, low-G metamorphism (2720-2685 Ma):

Rare, but widely distributed medium-P (>6 kbar), high-T (>600 ºC) and uniquely low average thermal gradient (G ≤20 ºC/km) metamorphic parageneses are found almost exclusively associated with high-strain shear zones (e.g. Ida, Ockerburry, Celia and Hootanui Fault systems). A single Lu-Hf garnet age from one of these high-grade parageneses is 15 Ma older than Lu-Hf ages for garnets from regional metamorphic parageneses and 30-35 Ma older than Lu-Hf ages for metamorphic garnet in a post-volcanic late basin (see below). Consequently, these medium-P high-T parageneses are early formed, prior to pervasive regional metamorphic events. This is substantiated by overprinting of medium-P rocks within shear zones by lower grade shear assemblages and the formation of these rocks in thermal regimes entirely distinct from all younger events (i.e. within late basins) and over-printing (i.e. regional metamorphism and low grade shear events), all of which are characterized by high average thermal gradients (G ≥40 ºC/km). Peak-T was attained at peak-P and was followed by near isothermal decompression, either in the same metamorphic cycle or during a later exhumation event.

The tectonic setting for early, medium-P parageneses is unknown, but can be explained by two competing end-member processes. (1) Either these medium-P, low-G conditions were widespread regional metamorphism and the current restricted distributions are due to late-stage exhumation bringing only a portion into the shallow crustal (i.e. by M3a extensional telescoping). This scenario is supported by exposures within shear zones but nevertheless would predict more widespread relict medium-P parageneses, such as in garnet cores, boudins and xenoliths. (2) Or alternatively, medium-P, low-G conditions were spatially restricted at the time of formation. In which case the current distribution is significant and the tectonic setting would be responsible for discrete, thin, arcuate zones of deep burial followed by rapid exhumation. Possible tectonic settings that match the metamorphic conditions and the apparent restricted spatial distribution (i.e. alternative 2) are shallow and/or slow subduction (typical subduction has G <15ºC/km) or downward advection of greenstone margins by diapiric overturn (e.g. Collins & Van Kranendonk, 1999).

Currently, both the tectonic setting and relative timing of M1 and Ma metamorphic parageneses are unknown. Though both metamorphic events have entirely distinct thermal regimes, it is possible that both formed at similar times in different parts of a tectonic system. For example, both thermal regimes are developed in subduction / magmatic arc pairs; Ma metamorphic conditions at shallow-crustal levels in magmatic arcs and M1 metamorphic conditions at deeper-crustal levels in an associated subduction zone setting.

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M2 = Granite pervaded, moderate-G, regional metamorphism (2685-2665 Ma):

Lu-Hf garnet ages are consistent with stratigraphy; and suggest that regional metamorphic, main phase matrix parageneses in greenstones grew 17-23 Ma before assemblages within turbiditic post-volcanic late basins (PVLB). The regional M2 thermal anomaly possibly continued over a protracted period and into continuum with the thermal event associated with M3a metamorphism of the PVLB’s. Outside of the PVLB’s, it is locally difficult to distinguish the M3a thermal over-print from regional M2 metamorphic parageneses. M2 parageneses range in temperatures from sub-greenschist to mid-amphibolite facies, 300-550 ºC. The pressure range is limited from 3.5 kbar at low grades to 4.0 kbar at highest grades, with elevated average thermal gradients of 30-40 ºC/km. Metamorphic temperatures increase toward granite/gneiss domes (Mikucki & Roberts, 2003) and the moderately elevated thermal gradients are consistent with a “regional-contact” type of metamorphism dominated by conduction and heat supplied by large volumes of (65%) High-Ca granitoid. M2 metamorphic mineral growth accompanied deformation conditions ranging widely from essentially no strain to syn-kinematic well-developed fabrics. M2 metamorphism broadly overlaps in age with the heterogeneously distributed D2 compressional deformation. Ridley (1993) recognised that the universally low pressure and elevated average thermal gradient, mitigate against widespread crustal over-thickening, such as in a collisional orogen as has been previously suggested for the Yilgarn Craton (Groves & Phillips, 1987). The spatial association of M2 field gradients and distribution of High-Ca granites of broadly similar age, infer that metamorphism was in a thermally inversely stratified crust with elevated thermal gradients in the upper crust due to neutral buoyancy pooling of large volumes of granite.

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M3a = Extension related, high-G metamorphism (2665-2650 Ma):

Metamorphism of turbiditic post-volcanic late basin (PVLB), such as Mt Belches, is distinctive, well constrained and characteristic of extensional settings. Peak metamorphism was at 500-580 ºC and 4±1 kbar, indicating high average thermal gradients (G) between 40 and 50 ºC/km. Peak metamorphism accompanied a static stress regime, subsequent to formation of the early burial, bedding-parallel mica foliation. P-T paths are tight, heating followed by cooling paths. Most have up-P prograde-burial trajectories, from early cordierite-andalusite, to peak garnet and post-peak staurolite growth, indicating anticlockwise P-T loops. Recognition of a post-2665 Ma high-heat flow regime accompanying turbiditic basinal infill, has implications for driving basinal brine circulation, and release of large volumes of hydrous fluid by dewatering and dehydration reactions.

The PVLB’s are a crucial time marker that “samples” the PT conditions of the thermal regime in operation between 2665-2650 Ma. Metamorphic parageneses in all older stratigraphy (volcanics) could potentially be a combination of both M2 and M3a metamorphic mineral growth – depending on the extent of M3a thermal effects. There are two possibilities for the extent of the M3a thermal overprint. (1) Either a widespread regional thermal overprint, in which case the EGST metamorphic pattern is a combination of both M2-M3a effects. (2) Or alternatively, M3a thermal effects are restricted to spaced elongate domains of high heat flow associated with maximum rifting, coinciding with exposed and eroded PVLB’s (Fig. 1). Outlining the extent of M3a rifting and thermal overprint is hampered by there being only remnants of PVLB’s and no other widespread stratigraphic marker between M2 and M3a parageneses. If exhumation of medium-P M1 parageneses and old (>2735 Ma) stratigraphic units occurred as a result of M3a extensional telescoping, these rock units will indicate the lower-plate margins to zones of extreme M3a extension. M3a lithospheric extension would be locally controlled by dip-slip reactivation of growth faults that control development of PVLB depositional troughs, some of which may have had earlier histories controlling syn-volcanic late basins (SVLB). Metamorphic patterns are consistent with extensional telescoping of the crust during M3a, with transport away from lower-plate domal cores, stepping outwards to successively younger extensional structures. These geometries result in exhumation of panels of deeper crustal levels and juxtaposition against shallow crustal level rocks, as movements on different extensional structures migrate across this margin. The presence of old (>2735 Ma) stratigraphic units and medium-P M1 parageneses indicate the extended margins of lower-plate domain and PVLB’s occupy upper-plate settings.

Crucially, metamorphism of the PVLB’s by high average thermal gradients (40-50 ºC/km) at 10-12 km depth is diagnostic of lithospheric extensional settings. Other extensional basins such as the Kanmantoo trough and Irindina Province and thermal modelling show that basin fill, burial and metamorphism can all occur in short, 2-10 Ma, time frames. Consequently, M3a metamorphism is interpreted to have essentially accompanied ongoing PVLB fill, with metamorphism peaking somewhere between 2665 and 2650 Ma. The thermal peak recorded in the PVLB may have been more widespread but obscured by the pre-existing M2 parageneses that formed at very similar conditions. If PVLB’s occupied elongate, growth-fault controlled settings coincident with maximum extension, the zones of highest M3a high heat will also be spaced elongate domains mirroring the distribution of exposed and eroded PVLB’s. The ultimate driver for increased M3a lithospheric extension is necessarily speculative, but does post-date volcanism and may be related to cessation of down-going subduction and consequent gravity driven slab roll back.

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M3b = Fault kinematics, high-G alteration and Au-mineralization (2650-2620 Ma):

The M3b period, subsequent to metamorphism of the PVLB’s, is characterised by a switch from dip-slip extensional kinematics to multiple strike-slip kinematic episodes (Blewett & Czarnota, 2005) through a protracted period of high but waning thermal gradients and multiple Au-mineralization events. Metamorphic mineral growth was within hydrothermal alteration assemblages that over-print regional metamorphic parageneses (M2-M3a) and was strongly partitioned into restricted domains and is associated with veining. Hydrothermal alteration associated with main phase Au-mineralization (~2650-2640 Ma) occurred across a wide range of temperatures 250-500 ºC but similar crustal depths of 3.0-3.5 kbar, across most parts of the EGST, indicating moderately elevated thermal gradients of 30-50 ºC/km. This represents crustal levels only 3 km shallower than peak metamorphism during M2 and M3a. Multiple fluid generations and fluid types, ranging from H2O-rich to CO2±CH4-rich are evident in different alteration systems. Alteration assemblages vary on scales too small to have experienced significant differences in T and P and are primarily controlled by pH and redox (e.g. Mikucki, 1997; John Walsh pers. comm.). Later episodes of alteration and Au-mineralization (~2630-2620 Ma) occurred at pressures as low as 1 kbar, while still at relatively elevated alteration temperatures (250-350 ºC) indicating very high average thermal gradients (G >70ºC/km). This event coincides with a secondary peak of shallow crustal, Low-Ca granites, which possibly supplied the heat necessary to drive fluid circulation and gold remobilisation.

Multiple mineral growth events throughout M3b, define a broad continuum of elevated average thermal gradient that persisted from M3a. This long-lived high thermal budget is due to a combination of factors, such as conductive thermal lag from M3a, continuing lithospheric extension or transtension and emplacement of significant (35%) volumes of Low-Ca granites. Low-Ca crustal melts were initiated during M3a lithospheric extension, followed by pooling, emplacement and crystallization in the upper crust ~10 Ma later, representing a causal link between M3a and M3b thermal periods. Similarly, M3a fluid production by dewatering of PVLB brines and dehydration of hydrous minerals in both PVLB sediments and low-grade regional (M2) metamorphic domains sourced the large volumes of hydrous fluid responsible for much of M3b alteration. This causal link is supported by stable isotope and fluid inclusion salinity data indicating meteroric fluid that has undergone exchange in post-volcanic sedimentary basins. (e.g. Mikucki, 1997). The large ambient fluid volumes produced were possibly important in setting up permeability pathways and alteration systems where mixing with secondary, metal-rich, fluids could occur. A third causal link between M3a and M3b is the formation of (1) mantle-derived magmas (Mafic granites and Syenites) and (2) pathways for dry mantle fluids in response to M3a lithospheric extension; both being possible sources for bringing Au into the upper crust.

The strong causal relationships between M3a and M3b intimately links Au-mineralization with the thermal evolution of the crust. This period follows on from a significant switch in lithospheric response to far field tectonic processes: from crustal growth (subduction-arc accretion?) to lithospheric extension. Lithospheric extension was possibly first initiated at ~2685 Ma with the formation of growth fault controlled syn-volcanic late basins (SVLB), steep increase in the volume of High-Ca granitoids and regionally extensive elevated thermal gradient (M2). Coincident with the termination of volcanism, accelerated lithospheric extension from ~2665 Ma generated the M3a thermal perturbation and PVLB rifts. Lithospheric extension may have generated mantle magmas that both transported Au and possibly set up conduits for dry metal-rich fluids to bring Au into the upper crust. M3a rifting generated both the turbiditic PVLB’s that brought large hydrous fluid reservoirs down into the upper crust and resulted in the high thermal gradients that permitted both dehydration and dewatering of juvenile and pre-existing low-grade rocks, and the thermal energy to drive fluid circulation. Modelling indicates that lithospheric extension is conducive to downward fluid flow trajectories, further facilitating movement of basinal brine fluids down into the upper-crust (Sheldon et al., This Volume). The thermally weakened and thinned crust, with a structural architecture established by M2 and M3a growth faults, was conducive to strike-slip and thrust reactivation in appropriate stress fields (D4 to D6), generating dynamic fluid pumping, permeability pathways and deposition sites for Au-mineralization. By comparison to Phanerozoic earth history, a plausible first-order cause for the switch from continental growth to lithospheric thinning is sudden plate reconfiguration caused by the final assembly of a super-continent (e.g. Squire & Miller, 2003). Termination of super-continent assembly may have resulted in slab roll back and lithospheric thinning at ~2665 Ma with similar lagging causal effects resulting in Au-mineralization events from ~2650-2620 Ma in other Neoarchaean terranes worldwide.

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Acknowledgements:

Thanks to the sponsors for their ongoing commitment to the pmd*CRC Y4 project. Special thanks to GSWA for access to legacy thin sections, databases, vehicle for the fieldwork and ongoing support for this project. John Walsh, Martin Van Kranendonk, John Miller, Heather Sheldon, Ned Stolz, Kevin Cassidy and Joe Ogierman are thanked for the very helpful discussions. Angus Netting, Ian Pontifex, Liz Webber, Greg Hitchen and Mike Verrall are thanked for help with lapidary, XRD and mineral analyses. Published with permission of the CEO’s of the pmd*CRC, GSWA and GA.

References:

Blewett R.S. Czarnota K. 2005. Tectonostratigraphic architecture and uplift history of the eastern Yilgarn Craton: Terrane structure. pmd*CRC Y1-P763 Module 3 Final Report.
Collins W.J. & Van Kranendonk M.J. 1999. Model for the development of kyanite during partial convective overturn of Archean granite-greenstone terranes: the Pilbara Craton, Australia. Journal of Metamorphic Geology 17, 145-156.
Purvis A.C. 1984. Metamorphosed altered komatiites at Mount Martin, Western Australia – Archaean weathering products metamorphosed at the aluminosilicate triple point. Australian Journal of Earth Science 31, 91-106.
Phillips G.N. 1986. Geology and alteration in the Golden Mile, Kalgoorlie. Economic Geology 81, 779-808.
Ridley J.R. 1993. Implications of metamorphic patterns to tectonic models of the Eastern Goldfields. Australian Geological Survey Organisation, Record 54, 95-100.
Groves D.I. & Phillips G.N. 1987. The genesis and tectonic controls on Archaean gold deposits of the Western Australian Shield: a metamorphic replacement model. Ore Geology Reviews 2, 287-322.
Mikucki J.A. 1997. Contrasting fluid sources and mineralization styles in the Great Eastern lode-gold deposit, Lawlers, Western Australia. PhD thesis, University of Western Australia.
Mikucki E.J. & Roberts F.I. 2003. Metamorphic petrography of the Kalgoorlie region eastern goldfields granite-greenstone terrane METPET database. GSWA Record 2003/12.
Sheldon H.A., Barnicoat A.C., Zhang Y., Ord A. 2007. Metamorphism in the Eastern Yilgarn Craton: implications for fluid flow and mineralisation. This Volume.

Squire R.J. & Miller J., Mc.L. 2003. Synchronous compression and extension in East Gondwana: Tectonic controls on world-class gold deposits at 440 Ma. Geology 31, 1073-1076.

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(2) Thermobarometric evolution of East Yilgarn crust: constraints on Neoarchaean tectonics and gold mineralization.

B. Goscombe¹, R. Blewett², K. Czarnota², D. Foster³, B. Wade⁴

1. Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd. Aldgate SA 5154.

2. Geoscience Australia, GPO Box 378, Canberra ACT 2601.

3. Dept. of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA.

4. Adelaide Microscopy, University of Adelaide, Adelaide SA 5005.

Submitted to 5^th International Archaean Conference, Perth, September 2010.

The East Yilgarn crust experienced at least five distinct thermal regimes at different times in its evolution (Goscombe et al., 2007, 2009). M1 is recorded by moderate-P (7.5 kb), low-T/depth ratio (20 ºC/km) metamorphism at approximately 2710-2695 Ma accompanying formation of the volcanic sequences and before the high-Ca granite bloom. M1 parageneses are spatially restricted to crustal shear systems and are interpreted to have formed during partial downward advection of magmatic arc margins during docking events, followed by rapid isothermal decompression. Significantly these parageneses predate the granite bloom and doming events and formed in a horizontal tectonics setting (i.e. plate tectonics) and not vertical tectonics settings such as keels adjacent to diapiric domes. Rare low-P (2.5-5.0 kb) granulite parageneses (Ma) are interpreted to have formed in high heat flow settings within the magmatic arcs.

A regional M2 thermal anomaly produced low-P (3.5-5.0 kb), high-T/depth ratio (30-40 ºC/km) metamorphism (M2) associated with emplacement of large volumes of high-Ca granite at 2685-2655 Ma. M2 is associated with termination of volcanism and through-going subduction, with compression at 2670 Ma resulting in minor crustal thickening and tight clockwise P-T paths. The regional M2 thermal anomaly softened the crust, priming it for a marked response from a stress switch to extension at 2665-2645 Ma (M3a). M2 crustal thickening was insufficient to induce gravitationally driven extension, consequently M3a lithospheric extension is interpreted as far field imposed stress such as slab roll-back. Similar horizontal to vertical s1 stress switches are a common feature of Phanerozoic accretionary orogens due to changes in outboard subduction zone dynamics. M3a parageneses indicate a low-P (4.0 kb), very high-T/depth ratio (40-50 º/km) thermal anomaly superimposed on regional M2. The M3a thermal anomaly is spatially restricted to arcuate zones associated with post-volcanic siliciclastic rift basins in the upper-plate of crustal scale extensional shear zones. The extensional upper-plate setting is confirmed by M3a parageneses tracking anti-clockwise P-T paths.

It could be argued that the East Yilgarn Craton would not be as endowed in world class gold mineralization without the late orogenic switch from compression during M2 to lithospheric extension during M3a. All of the processes, architectures and juxtapositions required for world class gold deposits, formed during the M3a period and are a natural response to this significant stress switch. M3a lithospheric extension thinned the crustal lithosphere resulting in a higher flux of energy from the mantle lithosphere that then drove many of the key processes leading to mineralization. Lithospheric extension was heterogeneous, resulting in basinal rifts and partitioned zones of high heat flux resulting in steep thermal gradients that drove fluid flow. The extensional stress field also modified fluid flow trajectories, giving both horizontal and downward paths in part (Sheldon et al., 2007), allowing the mixing of multiple fluid sources in the upper crust. Extensional rifting dropped basinal fluids into the upper crust significantly increasing the ambient fluid volume and introducing a new fluid type into the crust. Lithospheric extension and associated deep penetrating structures facilitated both the generation of mafic granitoid, syenite, lamprophyre and carbonatite melts and dry strongly reduced fluids in the mantle (Walshe et al., 2008a) and transport of these up into the upper crust. These mantle derived melts and fluids are possibly a major primary source of the gold and mixing of this distinct fluid type with ambient hydrous fluids in the upper crust ultimately resulted in precipitation and mineralization. Lithospheric extension both created and reactivated older crustal shear zones, generating suitable structural architectures such as extensional growth faults and footwall domes. These architectures allowed the focusing of fluids from different sources facilitating mixing and ultimately mineralization, both during M3a lithospheric extension and during subsequent reactivation of these structures in D4 and D5 with remobilization associated with low-grade hydrothermal alteration events (M3b) to 2620 Ma. M3a lithospheric extension and consequent high heat flow and decompressing lower-plate domes, facilitated crustal melting and emplacement of low-Ca granites in the upper-crust, which extended the M3a thermal anomaly and perturbed isotherms, further influencing fluid flow and mineralization.

Goscombe, B., Blewett, R.S, Czarnota K., Maas, R. and Groenewald, B.A., 2007. Proceedings of Geoconferences (WA) Inc. Kalgoorlie '07 Conference. Geoscience Australia Record 2007/14, 33-38.
Goscombe, B.D., Blewett, R.S., Czarnota, K., Groenewald, B.A. and Maas, R., 2009. Geoscience Australia Record 2009/23.
Sheldon, H.A., Barnicoat, A.C., Zhang, Y. and Ord, A., 2007. Kalgoorlie'07 Conference, Geoscience Australia Record 2007/14, 138-142.
Walshe, J.L., Neumayr, P., Cleverley, J., Petersen, K., Andrew, A., Whitford, D., Carr, G.R., Kendrick, M., Young, C. and Halley, S., 2008a. pmd*CRC Y4 Project Final Report.

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(3) Intra-orogenic extension in the Archaean Eastern Yilgarn Craton.

R.S. Blewett¹, K. Czarnota¹, B. Goscombe² and P.A. Henson¹

1. Geoscience Australia, GPO Box 378, Canberra City, ACT, 2601, Australia

2. Integrated Terrane Analysis Research, 18 Cambridge Rd, Aldgate, SA, 5154

Submitted to Geological Society of Australia Tectonics Group Conference, 2010.

An orogenic cycle typically follows a sequence of stages, viz basin formation and magmatism during extension, inversion and crustal thickening during contraction, and finally extensional collapse of the orogen. The Archaean granite-greenstone terranes of the Eastern Yilgarn Craton (EYC) record a major deviation in this sequence of events between 2665 Ma and 2655 Ma. Within the overall contractional stage, the EYC underwent a lithospheric-scale extensional event, resulting in major changes to the entire orogenic system. This intra-orogenic extensional event synchronously effected the crustal architecture and structure; greenstone stratigraphy; granite magmatism, and; thermo-barometry of the system. Also synchronous with these system changes was the deposition of the first significant gold, and it is likely that the intra-orogenic extensional event and its resultant impacts was a critical factor in the EYC's world-class gold endowment.

The characteristic map pattern of the EYC comprises elongate domal granite batholiths and intervening synclinal greenstone belts. Many granite-greenstone contacts on the dome flanks are outward dipping shear zones with extensional kinematics (granite up, greenstone down) and lineations partly influenced by the dome geometry, particularly close to dome noses. The long axes of the domes are flanked by relatively linear faults, major gravity gradients, and late basins in the hangingwalls. Seismic reflection images show that these faults and shear zones merge at depth and define an extensional fault controlled architecture transecting the full ~40 km crustal thickness. Meso- to macroscale structural analysis records extension and greenstone basin formation, followed by a series of contractional events of varying intensity and style. Within this contractional stage are major extensional shear zones, horizontal cleavages and folds.

The greenstone stratigraphy for much of the history was dominated by ultramafic to mafic and later felsic volcanism. The youngest preserved greenstones sequences are the so-called Late Basins. These are siliciclastic sediments with the first record of granite detritus being deposited unconformably on pre-folded older (volcanic) stratigraphy. These sequences fine upwards and occur in the hangingwall of extensional shear zones, some of which also transect earlier regional contractional folds. Their sedimentology is inconsistent with a foreland basin setting.

Granite, which comprises 65% of the mapped geology, was dominated by two main types. The early TTG magmas peaked just prior to the first contraction and they make up the bulk of domes in the mid-crustal lower plate. The late granites are crustal melts of the earlier magmas. Temporally between these two types are the Mafic (sanukitoids) and Syenite types, which were uniquely sourced from a metasomatised mantle, and their emplacement controlled by extensional faults that also controlled the Late Basins.

Lithospheric extension also superimposed an additional thermal anomaly after regional metamorphism. This late thermal anomaly occurred in arcuate domains centred on the upper-plate Late Basins, resulting in high average thermal gradients and anticlockwise P-T paths.

The first major gold deposits developed near the largest domes in the oldest parts of the stratigraphy (e.g., Leonora, Lancefield). The locus of extension was greatest at these sites, enhancing dynamic permeability facilitated by high geothermal gradients which allowed for the interaction of strongly contrasting fluids from the neo-formed basins and mantle magmas. This intra-orogenic extension was possibly related to lower crustal delamination and/or subduction roll back around 2665-2655 Ma.

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(4) Geodynamics of the Eastern Goldfields Superterrane.

K. Czarnota, D.C. Champion, B. Goscombe, R.S. Blewett, P.A. Henson, P.B., K.F. Cassidy, and B. Groenewald

Submitted to Economic Geology, 2010

Over the last decade there have been significant advances in our understanding of the stratigraphy, magmatism, deformation, metamorphism and timing of mineralisation, in the Eastern Yilgarn Craton (EYC) of Western Australia. The integration of these disciplines has enabled a holistic review of the tectonic history of the EYC, thereby providing a para-autochthonous geodynamic context for its mineralisation.

A significant advance has been the recognition of a ~2810 Ma rifting event off the eastern margin of the Youanmi Terrane which set up the north-northwest trending architecture of the EYC.

Rifting was followed by the establishment of a convergent margin characterised by a west dipping subduction zone to the east of the EYC. Subduction was associated with the deposition of the 2715-2670 Ma volcanic stratigraphy and the emplacement of voluminous tonalite-trondhjemite-granodiorite magmatism, which resulted in magmatic thickening of the crust. Volcanism was terminated by a ~5 Myr pulse of east-northeast contraction which is inferred to have triggered subsequent mid-orogenic extension possibly linked to lithospheric and lower crustal delamination. The lack of ultra-high pressure metamorphism and the presence of high geothermal gradients preclude this contraction event from recording a continent-continent collision.

Mid-orogenic extension at 2665 Ma resulted in the introduction of metasomatised mantle melts (Mafic-granites and Syenites), deposition of late-stage siliciclastic basins (which record anticlockwise PTt paths) and the start of significant economic gold mineralisation in the EYC. The delamination associated with this event resulted in significant heat input into the base of the crust, which eventually led to the emplacement of Low-Ca (crustal melt) granites and cratonisation of the EYC.

Major gold mineralisation postdates mid-orogenic extension, and was associated with renewed compression-transpression (~2650 Ma) and the development of steep sinistral and later dextral (syn Low-Ca granites) strike-slip fault.

(5) Predictive Mineral Discovery in the eastern Yilgarn Craton, Western Australia: an example of district-scale targeting of an orogenic gold mineral system

K. Czarnota, R.S. Blewett and B. Goscombe

Submitted to Economic Geology, June 2010.

Predictive mineral discovery is concerned with the application of a whole of system process understanding to mineral exploration as opposed to an empirical deposit type approach. A mineral system process understanding can be derived from a consideration of five key questions, namely what is/are the: 1) geodynamic setting; 2) architecture; 3) sources and reservoirs; 4) drivers and pathways, and; 5) depositional mechanisms. The answers to these questions result in the identification of critical processes necessary for the function of a mineral system within a particular terrane, and permit the development of a targeting model. In this contribution we identify district scale critical orogenic gold mineral system processes for the late Archaean eastern Yilgarn Craton of Western Australia. During the geodynamic history of a terrane the critical processes which result in mineralisation change with time resulting in variations in mineralisation style. Proxies for critical processes have been identified and mapped in an integrated GIS and are termed mappable mineral system process proxies (or MMSPP). In recognition of the temporal variation of mineralization processes during the geodynamic history of a terrane MMSPP were identified for three separate geodynamic events and a geochemical theme. Each MMSPP was assigned a weighting factor (WF) which reflects the spatial accuracy/coverage of themdata and process criticality. For each event/theme a separate prospectivity map was generated by summing the WF of overlayed related MMSPP. A final target or prospectivity map was generated by summing the four event/theme prospectivity maps. This final map was tested against the known distribution of deposits resulting in the ‘discovery' of the main gold camps, accounting for over 75% of the known gold in 5% of the area. This result supports the developed mineral system process-based understanding and the applied targeting approach. A further outcome is the identification of a number of new target areas not known for significant gold mineralisation in what otherwise is thought to represent a mature terrane for gold exploration.

(6) Eastern Goldfields structural evolution

Richard Blewett, Karol Czarnota, Paul Henson and Ben Goscombe

Geoscience Australia, GPO Box 378, Canberra, ACT, 2601

Accepted for presentation at the 5th International Archaean Convention, Perth, September 2010.

Introduction

Most mineral deposits of the Eastern Goldfields Superterrane (EGST) are structurally controlled, so knowledge of their structure and tectonics is critical to understand the region's endowment and to predict new resources. A decade ago, at the 4^th International Archaean Symposium (IAS) meeting, the structural terminology for the Goldfields largely followed that of Swager (1997). This terminology consisted of a succession of contractional deformational events labelled 'D1' to ‘D4'. Some authors proposed extension throughout parts of this compressive history although these were not enumerated (other than De or DE), or not particularly emphasised.

Building on the state of knowledge from a decade ago, we present a new structural framework for this meeting, the 5^th International Archaean Symposium. Since the last meeting there has been a vast improvement in geochronology, geochemistry, isotopes, stratigraphy, deep seismic profiles, 3D models, together with new structural mapping from various GA, GSWA, AMIRA and pmd*CRC projects. This new framework integrates the greenstone stratigraphy, granite evolution, metamorphism, structure, tectonic mode, and mineralisation into a coherent structural evolution in time and 3D space.

A New Integrated Tectonic Framework

D1: long-lived extension and granite-greenstone formation

The D1 event was extensional with a dominantly ENE-directed polarity and was likely the result of ENE directed roll-back of a subduction zone(s). Evidence of D1 extension is preserved in: the broadly NNE trending distribution of the greenstone stratigraphy (Swager, 1997); the NNW trends in the granites eNd model age map (Cassidy and Champion, 2004); the subduction signature of the High-Ca granites (Champion and Sheraton, 1997); metamorphic patterns (Goscombe et al., 2009); the presence of unconformities and the excision of stratigraphy in the greenstone sequence (Swager, 1997; Krapěz et al., 2000); and, mesoscale structures in gneisses and older greenstone fragments (Blewett et al., 2004a; Blewett and Czarnota, 2007).

D1 extension in the EGST was active from the earliest greenstone rock record (~2720 Ma) through to the onset of the first significant contraction at around 2665 Ma. Relicts of the older basement (maybe Youanmi Terrane) are preserved as the small slivers of >2750 Ma greenstones at Leonora, Duketon, Dingo Range, and Laverton. These may represent the rifted remnants of the older Youanmi Terrane. The voluminous High-Ca plutonism that occurred during this period likely initiated early elongate domes - ‘sowing the seeds' of the domal architecture seen today.

D2: termination of an arc and ENE-WSW contraction

The first significant contraction (D2) occurred around 2670-2665 Ma, terminating volcanism in the greenstones. During this time interval disparate associations (in chemistry and age distribution) were juxtaposed at a time when the late arcs shut off in the Kalgoorlie Terrane.

In general D2 developed without significant regional foliation development. Although in areas away from late basins and D3 extension, structures here correlated with D4 may well be associated with D2 (as D2 and D4 are co-planar and may lack overprinting relationships). D2 macroscopic structures indicate that shortening was oriented ENE-WSW, perpendicular to the grain of the D1 extensional orogen. Accretion of an external body (an oceanic plateau) into the receding subduction zone is interpreted to have terminated volcanism and sent a wave of D2 contraction across the orogen.

Blewett et al. (2004b) described two examples of regional macroscale F2 folds. In the Kalgoorlie Terrane, a regional S-plunging anticline-syncline pair is overlain by the Kurrawang basin at Ora Banda. In the western Kurnalpi Terrane, the S-plunging upright Corkscrew Anticline is overlain by the Pig Well basin at Welcome Well. These examples show late basins unconformable on pre-folded greenstone sequences, providing an age constraint of >2660 Ma for the development of these folds (Blewett et al., 2004b).

To the east in the Kurnalpi Terrane, the map patterns of the Mt Margaret Anticline around Laverton also show that old greenstone sequences are folded more tightly than the upper surface of the domal batholith and the base of the folded Wallaby late basin (2665 ±5 Ma). This relationship suggests that ENE-oriented shortening had at least commenced before the late basins were initiated. In the southern EGST, the ENE-directed Foster Thrust at Kambalda is interpreted as a D2 structure.

D3: extensional granite doming, Mafic granites and late basin formation

The D2 contraction was followed by a dramatic change in tectonic mode, as well as greenstone and granite type. The D3 extensional event was associated with significant granite doming, a peak in High-Ca granite emplacement, late basin formation, and deformation. The event is characterised by extensional high-strain shear zones which wrap around major granite dome margins.

Late basins (Krapěz et al., 2000) display two geometrical forms, arcuate and elongate. The arcuate late basins are located in the hangingwall of extensional shear zones along the SSE margins of some major granite domes. The elongate late basins strike NNW, are rift-like, and are interpreted to have developed as a result of D3 unidirectional ENE-down, asymmetric extension (inversion of D2 thrusts?).

The D3 event was associated with the introduction of Mafic-type granite magmatism across the EGST. These magmas, with sanukitoid affinity, were derived from a metasomatised mantle source (a good source for gold and sulphur). Syenite magmatism (mantle sourced) commenced in the Kurnalpi and Burtville Terranes at this time. This dramatic change in felsic magmatism suggests that a fundamental geodynamic adjustment occurred, rather than the system returning to the previous D1 extensional setting. Beakhouse (2007) attributed the change from slab melting (TTG) to metasomatised mantle melting in the Superior Province to be a function of slab detachment following collision.

Major granite domes controlled the locus of this D3 extension, with a strong meso- and macro-scale record of extension at Lawlers, Leonora, and Mt Margaret. The accumulation/preservation boundaries of the main greenstone belts also record greenstone down extension along the Ida Fault to the west and the Pinjin shear zone to the east (Swager, 1997). At Leonora on the eastern margin of the large Raeside Batholith, extensional S-C-C' shear zone fabrics are well developed at the meso-scale, and at the macro-scale in seismic reflection images (Czarnota and Blewett, 2007). Furthermore large metamorphic grade jumps consistent with excision of stratigraphy have also been documented across extensional shear zones at Leonora (Williams and Currie, 1993). All scales infer granite-up and greenstone-down sense of movement (down to east), with elongate late basins (Pig Well: <2665 Ma) developed further east in the hangingwall to extensional shear zones.

Czarnota et al. (2010) proposed a model where the D3 extension and its associated rock record in the EGST, were the result of detachment (or delamination) of the D1 slab following D2 collision. This detachment provided drivers, pathways and access to fertile sources for subsequent heat, gold-bearing fluids and magmas. The gross architecture of the EGST developed during the D1 and D3 phases of extension, not during contraction (Drummond et al., 2000).

D4: Sinistral transpression

D4 was a progressive sinistral transpressional event recorded across the terranes of the EGST, both within the granites and the greenstones. It has been subdivided into two distinct stages.

a. The first stage (D4a) involved horizontal compression with σ₁ just north of E-W and a vertical σ₃ (co-planar to the D2 stress field). D4a is characterised by pure shear, basin inversion, NNW-striking upright folding and associated cleavage formation, reverse faulting, and tightening of earlier domes and D2-D3 folds. The geometrical result was the rotation and steepening of stratigraphy (including late basins) along the margins of E-facing granite domes.

b. The second stage (D4b) involved the development of NNW-striking, steeply-dipping ductile sinistral shear zones, associated with a slight clockwise rotation of σ₁ to ESE-WNW and a horizontal σ₃. Sinistral strike-slip shear zones best developed in regions with steep-dipping stratigraphy (where thrusting and flattening ceased to be effective in dissipating the stress) and within the internal granites. This D4b event equates to the ‘D3' deformation of Swager (1997).

Low strain structures associated with the D4b event resolve a locally highly variable stress field with σ₁ ranging from ESE-WNW to N-S in orientation (e.g., main gold at Wallaby, Miller, 2006). This large variation in the local stress field is inferred to be a direct consequence of the development of sinistral strike-slip shear zones on a pre-existing highly anisotropic architecture, primarily composed of doubly plunging granite domes overlain by folded greenstones.

In a significant change from the Swager (1997) framework, the S-over-N thrusts from the Kambalda and Kanowna areas (viz, Foster, Tramways Republican, and Fitzroy), described as ‘D1' (Swager, 1997), are re-assigned to D4b. This is based on regional map pattern superposition relationships where the ‘D1' thrusts overprint NNW-trending upright ‘F2' folds (see Henson et al., 2004). An analogy of how N-directed thrusts developed at a high angle to NNW-trending D4b sinistral strike-slip faults exists in the eastern Gobi-Alty region of Mongolia (Bayasgalan et al., 1999). Thrusts in this example develop at the terminations of major strike-slip faults, acting as accommodation structures to rotational strain, and at restraining step-overs, accommodating displacement between parallel strands of a strikes-slip fault system. Locally σ₁ was oriented NW- to N-striking across these transfer structures and restraining bends despite the regional stress field being oriented ESE-WSW.

D5: dextral transtension and crustal melting

The D5 event was developed in an overall dextral transtensional tectonic mode accompanying the emplacement of Low-Ca granites and characterised by brittle/ductile N- to NNE-striking dextral strike/oblique-slip faults. This D5 event equates to the ‘D4' event of Swager (1997). Many past workers have suggested that it was a progressive event from earlier ‘D2' (e.g., Weinberg et al., 2003 and references therein). However, this study has shown that a significant rotation of the palaeostress field (~60^o) occurred between the D4b sinistral (σ₁ESE-WNW) and the D5 dextral (σ₁NE-SW) events, so the transition was not progressive but probably marked a major plate reconfiguration.

This event is remarkably consistent across the EGST and thereby forms a good marker for structural correlation across the region. D5 ductile high strain and locally transpressional shear zones occur along the most significant terrane boundaries such as Ida-Waroonga, Ockerburry and Hootanui Fault Systems. Distant from these terrane boundaries, the D5 event is expressed as brittle faults with very well-developed quartz-carbonate slicken lines. The development of local transpressional/transtensional structures is controlled by pre-existing fault strike and the geometry of adjacent granite batholiths within a system where σ1 was inclined towards the SW.

D6: low-strain systemic collapse

The last event inferred to be part of the EGST tectonic cycle (cratonisation of the Yilgarn) is systemic collapse. This event is characterised by mostly low strain crenulations, with sub-horizontal axial planes at a range of amplitudes from millimetres to metres. The fold hinges plunge variably. The structural style is brittle to locally brittle-ductile normal faulting. No specific vector of extension has been defined; and the driver for this extension may have been a readjustment of localised topographic highs from earlier events rather than a regional or far-field control. Structures ascribed to this event have been noted previously by Swager (1997), Davis and Maidens (2003) and Weinberg et al. (2003).

D7: Proterozoic contractional events

The D7 event occurred across the EGST and was associated with minor ENE-oriented contraction and the emplacement of dolerite dyke swarms and minor E-W sinistral strike-slip faults. Numerous small displacement faults occur in the granite pavements of the external granites (Blewett et al., 2004a). Swager (2007) also described similar structures. These are likely Proterozoic in age and may reflect events at the craton margin.

Implications for predictive gold discovery

Gold is associated with all of the events throughout the geodynamic history of the EGST, however significant gold mineralisation did not occur until the D3 extension event. The genetic link between D3 extension and late basin formation provides insight into the empirical observation that large gold deposits occur in proximity to late basins (Hall, 2007). This is because late basin distribution is associated with crustal penetrating shear zones developed during D3 extension. These shear zones are necessary to tap deep fluids and metals (from the mantle). The emplacement of mantle-derived Mafic and Syenitic granites into the upper crust during D3 extension reflects this deep connection. Furthermore extension is an efficient way to draw fluids down shear zones to facilitate fluid mixing (Sheldon et al., 2008). Significant gold mineralisation is hosted in high-strain extensional ductile shear zones at Gwalia, Lancefield, and the Lawlers camp. Extensional shear zones occur in other areas of the Yilgarn, so there is significant potential for finding Sons of Gwalia-like ore deposits. The D3 extension is also responsible for setting up the domal architecture of the EGST which is critical for fluid focusing during subsequent events.

The D4 sinistral transpression event was imposed on the highly anisotropic architecture developed largely during D3. This resulted in the creation of numerous depositional sites with significant structural complexity and the development of locally variable and complex stress fields as the anisotropy in the orogen was being ‘ironed out'. Gold is associated with brittle-ductile sinistral strike-slip shear zones at deposits such as Wallaby and Sunrise Dam, St Ives camp, Kalgoorlie, Kanowna Belle, Lawlers and Wiluna (Swager, 1997; Weinberg et al., 2003; Miller, 2006 and references therein).

The final gold event was associated with D5 dextral shearing (brittle transtension), with deposits including Sunrise Dam and Wallaby, Transvaal, Wiluna camp, New Holland, Golden Mile, St Ives camp, and Kundana, being examples. In contrast to the earlier gold-dominated events (D3-4), the mineralogy associated with D5 included base metals and tellurides and may reflect the influence of basinal fluids (Goscombe et al., 2009).

Conclusions

This new tectonic framework will aid researchers to make informed regional correlations, to place local studies in a robust context, to provide input parameters to numerical modelling, and to make predictions for new and different mineral plays (e.g., D3 extensional gold).

Acknowledgements

We thank the pmd*CRC sponsors for their ongoing commitment to the Y4 project and the sponsors of the Y1-P763 project from which this work is based on. Thanks to GSWA for access to their vehicle for the fieldwork. Published with permission of the CEO of Geoscience Australia.

References

Bayasgalan, A., Jackson, J., Ritz, J.F., and Carretier, S., 1999, Field examples of strike-slip fault terminations in Mongolia and their tectonic significance, Tectonics, 18, 394-411.

Barley, M.E., Brown, S.J.A., Krapež, B., and Cas, R.A.F., 2002. Tectonostratigraphic Analysis of the Eastern Yilgarn Craton: an improved geological framework for exploration in Archaean Terranes. AMIRA Project P437A, Final Report.

Beakhouse, G.P., 2007, Structurally Controlled, Magmatic Hydrothermal Model for Archaean Lode Gold Deposits: A Working Hypothesis. Ontario Geological Survey Open File Report 6193, 133 p.

Blewett, R.S., Cassidy, K.F., Champion, D.C., and Whitaker, A.J., 2004a, The characterisation of deformation events in time across the Eastern Goldfields Province, Western Australia. Geoscience Australia Record, 2004/10. https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=47616 [18]

Blewett, R.S., Cassidy, K.F., Champion, D.C., Henson, P.A., Goleby, B.R., Jones, L., and Groenewald, P.B., 2004b, The Wangkathaa Orogeny: an example of episodic regional ‘D2' in the late Archaean Eastern Goldfields Province, Western Australia. Precambrian Research, 130, 139-159.

Blewett, R.S., and Czarnota, K., 2007, The Y1-P763 project final report November 2005. Module 3 - Terrane Structure: Tectonostratigraphic architecture and uplift history of the Eastern Yilgarn Craton, Geoscience Australia Record 2007/15, 113 p. http://www.ga.gov.au/image_cache/GA10678.pdf [19]

Blewett, R.S., Czarnota K., Henson, P.A., 2010. Structural-event framework for the eastern Yilgarn Craton, Western Australia, and its implications for orogenic gold. Precambrian Research, in press.

Cassidy K.F., and Champion D.C., 2004. Crustal evolution of the Yilgarn Craton from Nd isotopes and granite geochronology: implications for metallogeny. In Muhling, J., et al., (Eds), SEG 2004, Predictive Mineral Discovery Under Cover. The University of Western Australia, Publication 33, 317-320.

Champion, D.C., and Sheraton, J.W., 1997. Geochemistry and Nd isotope systematics of Archaean granites of the Eastern Goldfields, Yilgarn Craton, Australia; implications for crustal growth processes. Precambrian Research, 83, 109-132.

Czarnota, K., and Blewett, R.S., 2007, Don't hang it on a foliation to unravel a structural event sequence: an example from the Eastern Goldfields Superterrane, Specialist Group: Tectonics and Structural Geology, Geological Society of Australia Abstracts, Deformation I the Desert, Alice Springs, 9-13 July 2007, 75.

Czarnota, K., Champion, .D.C., Cassidy, K.F., Goscombe, B., Blewett, R.S., Henson, P.A., Groenewald, P.B., 2010. The geodynamics of the Eastern Goldfields Superterrane. Precambrian Research, in press.

Davis, B.K., and Maidens, E., 2003. Archaean orogen-parallel extension; evidence from the northern Eastern Goldfields Province, Yilgarn Craton, Precambrian Research, 127, 229-248.

Drummond, B.J., Goleby, B. R., and Swager, C.P., 2000, Crustal signature of Late Archaean tectonic episodes in the Yilgarn craton, Western Australia: evidence from deep seismic sounding. Tectonophysics, 329, 193-221.

Goscombe, B., Blewett, R.S., Czarnota, K., Groenewald, B., Maas, R., 2009. Metamorphic evolution and integrated terrane analysis of the Eastern Yilgarn Craton: Rationale, methods, outcomes and interpretation. Geoscience Australia Record 2009/23, 270p. http://www.ga.gov.au/image_cache/GA15820.pdf [20]

Hall, G., 2007, Exploration success in the Yilgarn Craton insights from the Placer Dome experience the need for integrated research, this volume.

Henson, P.A., Blewett, R.S., Champion, D.C., Goleby, B.R. and Cassidy, K.F., 2004. Using 3D ‘map patterns' to elucidate the tectonic history of the Eastern Yilgarn. In Barnicoat, A.C., and Korsch, R.J., (Eds), Predictive Mineral Discovery Cooperative Research Centre: Extended Abstracts from the June 2004 Conference. Geoscience Australia, Record 2004/9, 87-90.

Henson, P.A., Blewett, R.S., Champion, D.C., Goleby, B.R., and Czarnota, K., 2007, How does the 3D architecture of the Yilgarn control hydrothermal fluid focussing.

Krapež, B., Brown, S.J.A., Hand, J., Barley, M.E., and Cas, R.A.F., 2000, Age constraints on recycled crustal and supracrustal sources of Archaean metasedimentary sequences, Eastern Goldfields Province, Western Australia, Evidence from SHRIMP zircon dating. Tectonophysics, 322, 89-133.

Miller, J.M., 2006, Linking structure and mineralisation in Laverton, with specific reference to Sunrise Dam and Wallaby. In: A.C. Barnicoat, and R.J. Korsch (eds). Predictive Mineral Discovery CRC- Extended Abstracts for the April 2006 Conference. Geoscience Australia Record 2006/7, 62-67.

Sheldon, H.A., Zhang, Y., Ord, A. 2008. Gold mineralisation in the Eastern Yilgarn Craton: Insights from computer simulations. In: Blewett R.S., (Ed.), Concepts to Targets: a scale-integrated mineral systems study of the Eastern Yilgarn Craton, pmd*CRC Y4 project Final Report, Part III, 177-190. http://www.pmdcrc.com.au/final_reports_projectY4.html [21]

Swager C.P., 1997, Tectono-stratigraphy of late Archaean greenstone terranes in the southern Eastern Goldfields, Western Australia, Precambrian Research, 83, 11-42.

Weinberg, R.F., Moresi, L., and van der Borgh, P., 2003, Timing of deformation in the Norseman-Wiluna Belt, Yilgarn Craton, Western Australia. Precambrian Research, 120, 219-239.

Williams, P.R. and Currie, K.L., 1993, Character and regional implications of the sheared Archaean granite-greenstone contact near Leonora, Western Australia. Precambrian Research, 62, 343-365.

Deformation in quotes are from other workers. The structural nomenclature presented here is not placed in quotes.

(7) Archaean gold mineral systems in the Eastern Yilgarn Craton: New research Contributions from the pmd*CRC

R.S. Blewett and the pmd*CRC team

Geoscience Australia, GPO Box 378, Canberra, ACT, 2601, Australia

Accepted for presentation at the 5th International Archaean Convention, Perth, September 2010.

Introduction

The Eastern Goldfields Superterrane (EGST) in the eastern Yilgarn Craton of Western Australia is Australia's premier gold and nickel province, and has been the focus of geological investigations for over a century. The Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) was a collaborative government-industry initiative (2001-2008) that was designed to answer key questions, test established paradigms, and to advance the geological understanding of this metal-endowed Archaean region. This paper presents a summary of some of the highlights and new findings from this research, many of which challenge established paradigms (Y4 Project team 2008, and references therein). Although a Yilgarn-based study, there are general implications for understanding the tectonics and gold mineral systems of other Archaean terranes.

Geodynamic setting and thermo-baric evolution

In order to better constrain the competing hypotheses of the geodynamics of the Yilgarn Craton, and Archaean tectonics in general, the pmd*CRC synthesised the main elements of the orogenic system into a new integrated framework in time and space. Elements considered included the greenstone stratigraphy, magmatic history, metamorphism, mineralisation, and structural geology, together with extensive geological and geophysical maps. The synthesis highlighted the strong interdependence where a change in one element saw all elements change. The constraints from this integrated 4D framework suggest an overall extensional margin dominated the geodynamic evolution of the eastern Yilgarn Craton.

Fundamental map patterns were revealed with improvement to the Sm-Nd database by the pmd*CRC. A new crustal age map shows the EGST consists of NNW-striking elongate Sm-Nd patterns, where are interpreted to mark variable degrees of crustal contamination from an extended underlying Younami Terrane basement. These fundamental map patterns challenge the exotic strike-slip terrane and allocthonthous obduction settings, and support simpler rifting/back-arc extensional settings.

One of the most significant advances from the pmd*CRC relates to the metamorphic evolution of the EGST. Previously, the EGST was thought to record significant crustal overthickening during collision, developing a single prograde cycle with a post-kinematic peak that was overprinted by retrogression and alteration. In contrast, the pmd*CRC showed that five discrete thermo-baric events occurred in time and space, with large variations in peak metamorphic crustal depths (12 to 31 km). The metamorphic evolution can viewed with stages of crustal growth (Ma and M1), thermal priming of the crust (M2), lithospheric extension (M3b), and finally inversion and reactivation (M3b).

Structural evolution

Taking advantage of the improved geochronological framework, a revised structural/deformational history was developed by the pmd*CRC. The new history also better integrates the stratigraphic evolution and the 3D architecture. The new revised history highlights seven key deformation events (D1 to D7), many of which were of extensional mode. From this new understanding a series of observations and interpretations are made.

D1 extension was dominantly E- to ENE-directed, and likely reflected the shape of the eastern continental margin. Extension dominated the period 2720-2670 Ma, and was responsible for establishing the fundamental NNW-trending architecture of the EGST. The D1 extensional event also influenced all subsequent deformational events, as well as the early Ni and later Au mineralisation.
Short periods of convergence late in the history (<2665 Ma) inverted this system. Most of the convergence direction was ENE perpendicular to the margin (D2, D4a), with a short period (2650 Ma) when there was a far-field stress switch to an ESE orientation (D4b). This stress switch was also responsible for north-directed thrusts (previously called D1), which developed along dome hinges as accommodation of regional sinistral strike-slip faulting (D4b) within mostly inter-dome high-strain shear zones.
Contractional deformation was diachronous with events around 5 My younger in the SW compared to the NE, which was interpreted as oblique far-field convergence zone to the eastern margin of the system.
A major lithospheric extensional event (D3) occurred between two periods of coaxial contraction (D2, D4a). The extension was possibly driven by the delamination of a lower crustal eclogitic restite from the earlier voluminous tonalite-trondjhemite-granodiorite magmatism. The main locus extension was the Ida and Ockerburry Fault Systems (bounding structures of the Kalgoorlie Terrane), with late basins developed in the hanging wall, and intrusion of magmas from a metasomatised mantle source. These late basins are a feature of many granite-greenstone belts (e.g. in Superior, Slave, Pilbara, Barberton, and West African cratons), and they are commonly syn-gold.
The D1/D3 extension ruptured the crust, and developed a deep-penetrating fault system that facilitated access to metasomatised mantle melts (as seen in deep seismic profiles and magnetotellurics). Numerical modelling showed that these extensional events were able to draw fluids downwards, facilitating mixing with chemically contrasting fluids from depth.
The faster the rate and the greater volume of new crust formation and its transition to cratonisation the more favourable the terrane/province (fast/voluminous and slow/less voluminous: cf. EGST and Younami; Yilgarn/Superior and Pilbara/Barberton).
Structurally, texturally and mineralogically these systems record a number of gold mineralisation events and yet commonly only the youngest dated ages are quoted in the literature (e.g. at St Ives, Agnew and Laverton). With a better understanding of the structural and mineralisation paragenesis, a renewed effort should be made to date the various paragenetic stages within the deposits. New techniques, such as 3D scanning of veins, show that some mineralised veins are multiphase, with reactivation throughout the mineralisation history.

Lithospheric architecture

The last decade has seen a dramatic increase in the availability of high quality geophysics, especially seismic reflection and passive seismic data. Software and hardware advances now permit realistic 3D inversions of the potential field data, and the results visualised in 3D on a standard desktop computer. By integrating these geophysical datasets with the geological mapping of the Geological Survey of Western Australia and Geoscience Australia, a series of new 3D geological maps were constructed by the pmd*CRC. These maps delineate the granite-cored domes which dominate the regional architecture. Granite domes also occur beneath the deepest greenstone basins, are no more than 7 km from the surface. Many of these domes nucleated about D1 growth faults that controlled the greenstone stratigraphy. Such faults, when inverted, became the location of major gold deposits, such as Kalgoorlie, Wallaby and Kanowna Belle.

Major crustal breaks are associated with all the main gold districts, with the golden corridor from Kambalda to Wiluna linked to a regional anticlinorium whose limbs are defined by outward dipping shears that connect to deep faults. Convex domes are nested within this regional structure, providing a favourable focussing architecture for deep fluids. This architecture is not the ‘Y-front' shape popularised in the earlier literature. The seismic character of the much of the crust is dominated by extensional features - such as core complexes. This reinterpretation contrasts with earlier seismic interpretations of contraction and thrust duplexes above a contractional detachment.

The architecture of upper mantle was delineated by the pmd*CRC, with tools such as broad-band seismology and magnetotellurics. These datasets show anomalous structure in the upper mantle beneath the main gold camps. The upper mantle fast shear-wave velocity body is interpreted as a delaminated eclogitic lower crust. The delamination was interpreted to have started in the east (2665 Ma), with younging to the west (2655 Ma).

Relationship of gold mineralisation to the structural history

Gold deposits are traditionally described from contractional settings, and late in the tectonic history. However, the pmd*CRC mapped gold in all events up to D5, with the highest grade and tonnage occurring from D3 times onwards. This is the time the metasomatised mantle was accessed and late basins developed. Orogenic gold occurs in extensional shear zones. The pmd*CRC mapped these zones at Leonora, Lancefield, Leinster and at Kunanalling; all developed by extensional exhumation of large granite domes. The deposits are restricted to the shear planes (C and C') of the extensional foliations and have very deep extents down the stretching direction. These are a new gold play and have been under-explored. All of the major deposits/camps have D1 growth faults within them (e.g. Golden Mile, Agnew-Lawlers, Sunrise Dam and St Ives), and these structures were subsequently inverted and mineralised.

Relationship of gold mineralisation to metamorphism

Gold mineralisation and late-stage metamorphism of the greenstone sequences have been traditionally linked in the orogenic gold model. The pmd*CRC has made a significant advance in the thermo-baric evolution of the EGST, with the spatial and temporal definition of five discrete events. The highest pressures attain 8.7 kb, in rocks with low geothermal gradients. These M1 assemblages occur in the oldest greenstone sequences adjacent to granite domes, and in the footwall of extensional detachments. High-temperature (Ma) granulites are likely to have developed beneath an arc, in a region of high geothermal gradients. The main regional moderate pressure and temperature M2 metamorphic event probably accounts for much of the available metamorphic fluid, and was generated before the main gold events (suggesting that this was not a major fluid source). The D3 extensional event was associated with tight anticlockwise M3a PTt paths in the upper plate that exhumed older higher pressure (with clockwise path) assemblages in the footwall. Regional exhumation during D4 to D5 times is recorded by widespread low-pressure M3b assemblages (~1 kb) and was associated with regional retrogression and alteration. Maps of redox of the alteration mineralogy illustrate the regional scale of these hydrothermal systems.

Low-Ca potassic granites are crustal melts that were emplaced at high levels across the entire craton and mark decompression and uplift of the exposed crust to high crustal levels (<1 kb), commencing 10-15 My after the inferred delamination of the eclogitic restite imaged in the tomography, and resulting in the final cratonisation of the craton. This time delay is consistent with the thermal diffusivity through the known crustal thickness of the Yilgarn.

Relationship of gold mineralisation to magmatism

The timing of gold, the number of discrete gold events, and the role of the evolving magmatic system has long been a source of controversy. The first significant gold event was synchronous with the major D3 extensional event (e.g. Lancefield, Kalgoorlie, Sunrise Dam, Leonora and St Ives). This event introduced small volume melts, of syenite and Mafic-type granite (sanukitoids), interpreted to have been sourced from a metasomatised mantle wedge. Many deposits have mineralisation ages younger (10+ My) than the porphyries that host the deposit, implying no direct temporal connection with these intrusions.

3D mapping by the pmd*CRC show that multi-phase granite-cored domes lie at varying depths beneath all the giant gold deposits. Most of the granites (and especially the late-stage Low-Ca granites) do not intrude into the greenstones; rather they crystallise at the lower basalt or komatiite levels in the stratigraphy. These vertically zoned systems may have provided fluids from depth in the cores of the domes through the same pathways that earlier, small-magma volumes (e.g. deposit-scale porphyries) had passed. The early architecture was critical in facilitating early magma emplacement which in turn set up local sites of anisotropy which localised strain during the multiple phases of reactivation.

Gold and fluid source(s)

The suggested gold sources include metamorphic, magmatic and mantle reservoirs. Lamprophyres and metasomatised mantle melts (sanukitoid porphyries and syenites) are temporally associated with major gold mineralisation in other Archaean terranes (e.g. Abitibi, Pilbara). The first significant gold in the EGST was deposited synchronous with the emplacement of metasomatised mantle magmas (2665-2655 Ma). A possible gold source was the metasomatised mantle, and the magmas reflect a melt fraction and common pathway with the gold from this source. Once gold is deposited in the system it can be later remobilised. For example, Proterozoic reworking on the northern margin of the Yilgarn Craton redeposited gold at Plutonic. It is not clear if the remobilisation of D3 gold in the EGST occurred within a few million years (i.e. D4 and D5 events) within the same deposit, or whether multiple gold sources for each event occurred. The addition of base metals together with gold suggests basinal input (late basin inversion) and tellurium suggests magmatic input (Low-Ca granites and High-Ca crustal melts post 2655 Ma) for associated metals. These deposits are the same ones that host classical orogenic gold and yet could be classified as anomalous-metal association and intrusion-related deposits respectively. The coincidence in space (separated in time) suggests that classifying these deposits into different types is not helpful in understanding the gold mineral system.

The fluid associated with orogenic gold has traditionally been considered metamorphic in origin. The pmd*CRC attempted to define the fluid source(s), challenged the traditional view, and showed that at least three fluids were present. Modelling revealed that the devolatilisation of greenstones releases only short-duration low-volume fluids. The resultant rock mass is left dry and unable to subsequently contribute fluid to any later event. The multiple gold events observed mean that metamorphic fluids may have been the source for one of the gold events, but not all. Furthermore, metamorphic studies show that the regional M2 metamorphism of the greenstones occurred prior to the first significant gold event. Considering the mineralogy, simple geochemical modelling required at least two fluids-a mafic fluid and a granitic fluid. Furthermore, the presence of multiple gold events and their differences in PT conditions, redox, mineralogy and metal associations, together with wall rock alteration all indicate an evolving system of fluid sources. This metamorphic-only source contrasts with voluminous magmatic and mantle fluid sources. The pmd*CRC defined three end-member fluids to account for the extreme range of O, S, C stable isotopes, the range of redox inferred from these, and from the chemistry of the alteration mineralogy. Within vein systems, fluid dominates over wall rock so that the chemistry reflects different sources of fluids and not necessarily the influence of reactions with local wall rocks. An emerging picture of the role of possible mantle fluid reservoirs is provided by studies of the noble gases, especially those in D4b gold-bearing veins at St Ives and Sunrise Dam. This is a significant finding. How indicative this is to other deposits remains to be determined.

Depositional mechanisms

There are four main ways to deposit gold (phase separation, fluid-rock reaction, vapour condensation and mixing across chemical gradients). All four processes have been documented in the EGST. The question remains what makes the giant deposits and the high grades. Research from the pmd*CRC suggests that fluid mixing across chemical gradients was the most efficient method, and the range of data showing gradients in operation with multiple fluid sources are now very compelling.

Predictive mineral discovery

The pmd*CRC developed a process-based understanding of the gold mineral system, and translated this understanding into mappable proxies of the process. The maps were then integrated in a GIS, resulting in the generation of a new target map of gold without the input of any gold layer. The map ‘discovered' 75% of the known gold in <5% of the area, a verification of the process understanding. Importantly, the map identified all the major gold camps. This result revealed the critical geological elements in terms of process that are needed to form a giant gold deposit. The necessary datasets for identifying these processes are also defined. The map also revealed a number of areas that were not known for hosting large deposits, but had all the favourable ingredients - these represent new opportunities.

Cooperative not competitive research: a better working model

The pmd*CRC was successful in bringing together government and university researchers with industry. Despite the obvious cultural differences, the working model was very much based on cooperation and collaboration. The results described above stand for themselves. The enduring legacy is also with the participants, the pmd*CRC changed, for the better, the way we think and operate.

Acknowledgements

I wish to acknowledge the team members of the pmd*CRC, the sponsors of the centre, and the centre management. Reviews by David Champion and Dean Hoatson improved the paper. Published with permission of the CEO Geoscience Australia. Geocat number 70128.

References

Y4 Project team, 2008, Concepts to targets: a scale-integrated mineral systems study of the Eastern Yilgarn Craton. pmd*CRC Y4 project Final Report, Parts I-II 162 pp Parts IV-V, 391 pp

(available at http://www.pmdcrc.com.au/final_reports_projectY4.html [21])

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(8) Thermobarometric evolution of sub-domains in the West Yilgarn Craton

B. Goscombe¹, R. Blewett², D. Foster³, B. Wade⁴

1. Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd. Aldgate SA 5154.

2. Geoscience Australia, GPO Box 378, Canberra ACT 2601.

3. Dept. of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA.

4. Adelaide Microscopy, University of Adelaide, Adelaide SA 5005.

The West Yilgarn Craton preserves a long and complex metamorphic history with in the order of >12 distinct periods of metamorphic parageneses growth from the Palaeoarchaean to the Neoarchaean. The dominant regional Neoarchaean events are shared in common with the East Yilgarn Craton (Goscombe et al., 2007, 2009), though in different tectonic settings and resulting in different metamorphic conditions. In addition the north, west and south margins of the craton have been variably reworked (regionally pervasive) and reactivated (shear zones) from the Palaeoproterozoic to the Cambrian in >10 distinct orogenic events producing a wide range of metamorphic parageneses. Peak metamorphic conditions attained in a select number of key domains from the West Yilgarn Craton are summarized below. The peak conditions presented are based on pooling of PT calculations with self-similar results from small geographical sub-domains. Pooled errors are ignored for clarity and are typically ±10-30 ºC and ±0.5-1.0 kb. The metamorphic constraints presented are preliminary and at this stage in the research program, not discussed within a chronologic, structural and tectonic context.

North Southern Cross Terrane

Edale Shearzone in the north Southern Cross Terrane experienced a wide range of low-P peak conditions of 524-607 ºC, 2.2-4.5 kb and temperature over depth ratios of 39-72 ºC/km (n=14) with clockwise P-T paths. Along strike the Illaara Belt experienced peak conditions of 596 ºC, 5.1 kb and temperature over depth ratio of 35 ºC/km (n=2). Further along strike the Mount Ida Belt experienced moderate-P peak conditions of 583-635 ºC, 4.2-6.2 kb and temperature over depth ratios of 29-43 ºC/km (n=3) with clockwise P-T paths.

Marymia Inlier experienced low-P metamorphism in the east and higher-P in the west. Peak conditions in the east are approximately 561 ºC, 3.0 kb and temperature over depth ratio of 53 ºC/km (n=1) and 600 ºC, 8.0 kb and 21 ºC/km in the west (Gazley et al., 2011).

A single sample from the Atley Belt experienced peak conditions of 450 ºC, 4.2 kb and temperature over depth ratio of 31 ºC/km. Gum Creek Belt experienced a moderate range of pressures. Peak conditions in lower-grade rocks are 550 ºC, 4.0 kb and temperature over depth ratio of 39 ºC/km (n=1). Higher-grade samples experienced higher-P peak conditions of 621 ºC, 6.7 kb and temperature over depth ratio of 27 ºC/km (n=2) with anti-clockwise P-T paths. Granitoids crystallized at very low pressures of 1.2 kb. Joyners Find, Red Hooded Bore, Youanmi and Poison Hills Belts experienced similar peak conditions of 588-640 ºC, 4.0-4.6 kb and temperature over depth ratios of 38-44 ºC/km (n=6). Poison Hills Belt has mineral parageneses indicating two metamorphic events. Lower-grade conditions in the Wiluna Belt are 517 ºC, 3.8 kb and 39 ºC/km.

South Southern Cross Terrane

Forrestania and Ironcap Belts preserve a moderate range of pressures. Peak conditions in the lower-P rocks are 503-661 ºC, 2.8-5.2 kb and temperature over depth ratios of 28-57 ºC/km (n=18). A number of samples experienced higher-P peak conditions of 579-676 ºC, 6.3-7.0 kb and temperature over depth ratios of 24-31 ºC/km (n=2) with clockwise and isothermal decompression P-T paths.

Ravensthorpe Belt

Carlingup Terrane within the east Ravensthorpe Belt experienced peak conditions of 566-580 ºC, 4.5-5.0 kb and temperature over depth ratios of 32-38 ºC/km (n=10). Mineral parageneses indicate early anti-clockwise P-T paths followed by probable late clockwise P-T paths and secondary chloritoid growth, which may be due to loading of the Yilgarn margin during the Proterozoic Albany Fraser events. A smaller number of samples experienced low-P peak conditions of 525-548 ºC, 2.2-4.0 kb and temperature over depth ratios of 38-71 ºC/km (n=5) and clockwise P-T paths with isothermal-decompression. Cocanarup Terrane on the west margin of the Ravensthorpe Belt experienced clockwise P-T paths with isothermal decompression and peak conditions of 547-584 ºC, 3.2 kb and temperature over depth ratios of 49-52 ºC/km (n=4). A few samples experienced higher-P peak conditions of 558-620 ºC, 5.9-6.0 kb and temperature over depth ratios of 27-30 ºC/km (n=2). Ravensthorpe Terrane in the central region experienced peak conditions of 559-562 ºC, 3.7-4.1 kb and temperature over depth ratios of 39-43 ºC/km (n=8). A few samples experienced higher-P peak conditions of 572-590 ºC, 5.9 kb and temperature over depth ratios of 28-29 ºC/km (n=3). Both groupings experienced predominantly clockwise P-T paths with isothermal-decompression and a few samples have parageneses indicative of anti-clockwise paths. Manyutup tonolites in the central Ravensthorpe Terrane crystallized at low pressures of 3.0-5.1 kb (n=6) and were subsequently buried and metamorphosed at 614-639 ºC, 6.6-7.0 kb and temperature over depth ratios of 26-27 ºC/km (n=10).

Murchison Domain

Cue and Mount Magnet Belts in the Murchison Domain experienced at least two metamorphic events. Metamorphism associated with Big Bell mineralization experienced a wide range in peak conditions of 543-648 ºC, 4.6-4.9 kb and temperature over depth ratios of 35-38 ºC/km (n=9). Regional metamorphism elsewhere in Cue Belts have typical peak condition of 504-528 ºC, 3.0-4.9 kb and 30-45 ºC/km (n=7), with isobaric cooling P-T paths. A single sample has higher-P peak conditions of 575 ºC, 6.4 kb and 26 ºC/km. Granitoids crystallized at low-pressure of 1.9 kb (n=5).

Dalgaranga Belts experienced a wide range in peak temperature conditions, all at low pressures. Lowest-grade conditions are 494 ºC, 2.0 kb and temperature over depth ratio of 76 ºC/km (n=6) with ambiguous clockwise or anti-clockwise P-T paths. Regional metamorphic peak conditions are 535-566 ºC, 3.0-3.3 kb and 51-53 ºC/km (n=6), with clockwise and isothermal decompression P-T paths. Early gneissic metamorphism on the margins of domes experienced peak conditions of 624 ºC, 4.0 kb and 45 ºC/km (n=4).

Murchison Belts experienced a wide range in peak temperature conditions, all at low pressures. Lowest-grade conditions are 499 ºC, 1.7 kb and temperature over depth ratio of 86 ºC/km (n=4) with isothermal loading and anti-clockwise P-T paths. Regional metamorphic peak conditions are 590-593 ºC, 3.0-3.1 kb and 55-56 ºC/km (n=3). Highest-grade peak conditions are 679 ºC, 4.0 kb and 51 ºC/km (n=3).

Yalgoo Belt experienced a wide range in peak temperature conditions, all at low pressures. Lowest-grade conditions are 469 ºC, 2.9 kb and temperature over depth ratio of 51 ºC/km (n=4) with isobaric heating and isobaric cooling paths. Peak conditions are 552 ºC, 3.3 kb and 50 ºC/km (n=4), with decompressive cooling P-T paths. Early gneissic metamorphism on the margins of domes experienced peak conditions of 624 ºC, 4.0 kb and 45 ºC/km (n=4).

Koonmarra Belt experienced peak conditions are 600 ºC, 3.1 kb and 55 ºC/km (n=2), with decompressive cooling P-T paths. Murgoo, Tardie and Tieraco Belts have similar peak conditions ranging 560-584 ºC, 4.6-5.1 kb and 33-35 ºC/kb. The Perenjori and Bencubbin Belts experienced higher grades at similar pressures, with peak conditions of 631-632 ºC, 4.1-4.7 kb and 38-59 ºC/kb.

Contact aureoles on the margins of mafic/ultramafic intrusive complexes, such as the Windimurra Complex, preserve the earliest and lowest-P metamorphic parageneses in the Youanmi Terrane. Windimurra contact aureoles preserve very low-P conditions ranging 515 ºC, 1.0 kb and 147 ºC/kb to low-P granulite of 709 ºC, 0.2 kb and 844 ºC/km within a raft on the upper surface. Gabanintha contact aureole and elsewhere in the Murchison Domain experienced peak temperatures estimated in literature at 720-860ºC at unknown pressures (n=13).

Narryer Terrane

Narryer Granulites preserve three distinct peak metamorphic conditions. Most PT calculations and sub-domains have peak conditions in the range 665-681 ºC, 4.1-4.5 kb and temperature over depth ratios of 43-48 ºC/km (n=10) with clockwise P-T paths. Higher-T peak conditions in some sub-domains are 759-770 ºC, 5.0-5.2 kb and 42-43 ºC/km (n=6), with clockwise P-T paths. An amphibolite facies sub-domain has peak conditions of 610 ºC, 4.5 kb and 39 ºC/km (n=2) and a clockwise P-T path. Granitoids crystallized at 5.3 kb.

Retrograde shear zones are interpreted to have Proterozoic mineral parageneses. Retrograde shear zones in the granulite terrane have metamorphic conditions of 621 ºC, 5.7 kb and 31 ºC/km (n=3) and 500 ºC, 3.6 kb and 40 ºC/km (n=1). The Jack Hills Belt is interpreted to have Proterozoic mineral parageneses. Peak metamorphic conditions in different parts of the Jack Hills Belt range 498-549 ºC, 2.5-4.3 kb and temperature over depth ratios of 36-61 ºC/km (n=6), with clockwise P-T paths.

Transitional Margins of the Southwest Terrane

Amphibolite gneisses within the east transitional margin of the Southwest Terrane have peak conditions ranging 610-675 ºC, 3.5-5.5 kb and temperature over depth ratios of 34-50 ºC/km (n=4). The north transitional margin experienced both amphibolite and granulite facies metamorphism. Granulite sub-domains including Westonia Belt, experienced peak conditions ranging 760-800 ºC, 2.2 and 4.7 kb and temperature over depth ratios of 103 and 46-49 ºC/km (n=3). Amphibolite gneisses also have a large range in pressures, with peak conditions ranging 536-615 and 630 ºC, 3.6-4.4 and 7.4 kb and temperature over depth ratios of 34-43 and 24 ºC/km (n=26).

Southwest Granulite Terranes

Granulites in the eastern parts of the Southwest Terrane experienced peak conditions ranging 712-784 ºC, 4.2-5.3 kb and temperature over depth ratios of 44-52 ºC/km (n=16) with isobaric cooling P-T paths. Granulites and granitoids in the western parts of the Southwest Terrane crystallized at conditions ranging 700-730 ºC, 4.2 kb and temperature over depth ratios of 48 ºC/km (n=7). Mineral parageneses are ambiguous and may indicate anti-clockwise P-T paths. Boddington Belt experienced peak conditions between 500-600 ºC, 2.0-3.0 kb and 57-71 ºC/km (n=2).

Granulites from the Lake Grace Terrane preserve at least four distinct clusters of peak metamorphic conditions. The highest-P sub-domain experienced peak conditions of 798 ºC, 8.5 kb and 27 ºC/km (n=1) with clockwise and isothermal decompression P-T path. Highest-T sub-domains experienced peak conditions of 754-852 ºC, 5.4-6.9 kb and 31-44 ºC/km (n=8) with clockwise and isothermal decompression P-T paths. Lowest-P sub-domains experienced peak conditions of 771-831 ºC, 3.1-4.8 kb and 50-71ºC/km (n=2) with ambiguous cooling paths. Lowest grade amphibolite facies sub-domains experienced peak conditions of 645-707 ºC, 4.1-6.7 kb and 28-49 ºC/km (n=9) with isobaric cooling P-T path.

Granulites from the Jimperding Belt preserve at least four distinct clusters of peak metamorphic conditions. The highest-grade sub-domains experienced peak conditions of 798-883 ºC, 7.1-8.1 kb and 30-35 ºC/km (n=4) with clockwise P-T paths. Other sub-domains with clockwise P-T paths experienced peak conditions of 810-826 ºC, 5.8 kb and 40-42 ºC/km (n=6). Lowest-grade sub-domains experienced peak conditions of 625-684 ºC, 4.3-6.4 kb and 31-42 ºC/km (n=2) with isobaric cooling paths. Sub-domains with anti-clockwise P-T paths experienced peak conditions of 713-807 ºC, 3.6-5.0 kb and 45-59 ºC/km (n=8).

Chittering Belt

Barrovian metapelite schists within the Chittering Belt are interpreted to have Proterozoic or Cambrian metamorphic parageneses. There is currently no empirical evidence for earlier Archaean parageneses or protoliths for these rocks. Nevertheless, the Barrovian series main foliation envelops large early garnet porphyroclasts, which are tentatively interpreted to be relict Archaean parageneses. Compositional isopleths from these garnet cores indicate low-P conditions of 549-560 ºC, 4.8-4.9 kb and 33 ºC/km (n=7). Main foliation parageneses experienced high-P peak conditions of 565-572 ºC, 8.0-10.6 kb and 16-20 ºC/km (n=9). P-T paths show isothermal loading and very tight anti-clockwise turn around followed by isothermal decompression.

Balingup Terrane

Some rocks in the Balingup Terrane are strongly sheared and almost totally reworked to Proterozoic metamorphic parageneses. Relict primary minerals grains within the mylonites and parageneses in the un-sheared rocks are interpreted to be of Archaean age. Archaean assemblages preserve two peak metamorphic groupings. These are 610-635 ºC, 3.0-3.9 kb and 45-60 ºC/km (n=5) and higher-P conditions of 592, 683 and 718 ºC, 6.1-6.8 kb and 28-31 ºC/kb (n=8). Proterozoic mylonite assemblages preserve peak metamorphic conditions ranging 597-670 ºC, 5.7-6.9 and 23-31 ºC/km (n=38). Proterozoic P-T paths typically involved near isothermal loading and tight clockwise turn around followed by isothermal decompression.

Acknowledgements

Sponsors of the pmd*CRC Y4 project, the Geological Survey of Western Australia, Geoscience Australia and Adelaide Microscopy are gratefully acknowledged for support and contributions. Steve Wyche is thanked for facilitating ongoing support of this long-term research program by the Geological Survey of Western Australia and for samples contributed by staff at the survey.

References

Goscombe, B., Blewett, R.S, Czarnota K., Maas, R. and Groenewald, B.A., 2007. Broad thermobarometric evolution of the Eastern Goldfields Superterrane. Proceedings of Geoconferences (WA) Inc. Kalgoorlie '07 Conference. Geoscience Australia Record 2007/14, 33-38.

Goscombe, B.D., Blewett, R.S., Czarnota, K., Groenewald, B.A. and Maas, R., 2009. Metamorphic evolution and integrated terrane analysis of the Eastern Yilgarn Craton: Rationale, Methods, Outcomes and Interpretation. Geoscience Australia Record 2009/23.

Gazley, M.F., Vry, J.K. and Boorman, J.C., 2011. P-T evolution in greenstone-belt mafic amphibolites: an example from Plutonic gold mine, Marymia Inlier, Western Australia. Journal of Metamorphic Geology 29, 685-697.

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