School of Earth Sciences

The Yilgarn craton, lies in the southern part of Western Australia, became cratonized at around 2600 Ma. Its post-cratonisation history is somewhat fragmentary due to the paucity or absence of a stratigraphic record. However, the exposed Archean crystalline rocks can provide important constraints on the ‘missing’ thermotectonic history if appropriate thermochronological methods are used. Previously reported Rb-Sr biotite cooling ages from the southwestern Yilgarn craton suggest that it was subjected to late ‘Pan-African’ tectonism (~400-600 Ma) and E-W compression, resulting in tectonic loading (thrusting) of sediments onto basement rocks along its western margin. However, this proposed tectonic model is based largely on bulk Rb-Sr biotite analyses with minimal petrological and geochemical control. In order to provide new insights into the thermotectonic evolution of the southwestern Australian crystalline terranes, including the southwestern Yilgarn craton, the Albany-Fraser Orogen and the Leeuwin Complex, this study applied multiple thermochronometers that are sensitive to a broad temperature range (~500-40 ºС). 40Ar/39Ar results of muscovite, biotite and hornblende grains were obtained from well-documented sample sites broadly comparable to those sampled previously for Rb-Sr biotite analysis in the Yilgarn craton and surrounding terranes (e.g. Albany-Fraser Orogen and Leeuwin Complex). Along a north traverse (Perth traverse) extending from W to E across ~80 km in the Yilgarn craton, muscovites record consistent to slightly decreasing ages of ~2450-2220 Ma. However considerably younger muscovite ages of ~600-610 Ma were obtained from a southern traverse (Harvey traverse) across the craton extending from W to E for a distance of ~150 km. Coexisting biotite results from the Perth and Harvey traverses reveal significant age variations, with ages decreasing systematically from east to west. Based on 40Ar/39Ar biotite ages and their chemical composition, three age domains are identified: an easterly biotite domain in the craton interior with ages of ~2500 Ma; a transitional domain with average ages of ~1000-1100 Ma; and a western biotite domain with ages of ~530-860 Ma. It is noted that relatively consistent biotite ages of ~600-630 Ma occur only along the Darling Fault. The transitional zone identified in the Perth Traverse is not revealed along the Harvey traverse, probably due to less comprehensive sampling coverage. Petrographic and chemical studies indicate that the biotite from each domain are distinctly different in composition and origin, i.e. magmatic to the east versus hydrothermal to the west. The more scattered biotite ages in the transitional and western zones are therefore unlikely to represent cooling ages, but rather indicate probable fluid-induced partial or complete biotite recrystallization at ~600-630 Ma. In the adjacent Albany-Fraser Orogen, 40Ar/39Ar ages of ~1100 Ma in the east of orogen decrease to ~650 Ma towards the west. In the Leeuwin Complex however, both biotite and hornblende yield similar 40Ar/39Ar ages of ~500 Ma and these are only marginally younger than coexisting zircon U/Pb ages. Complementary zircon and apatite (U-Th)/He data (ZHe and AHe respectively) were obtained from similar areas in the Yilgarn craton where 40Ar/39Ar dating had been carried out. Zircons yield a wide range of He ages (~400-30 Ma), and only grains with low [eU] (effective uranium contents; [eU] = [U] + 0.235 × [Th]; a proxy for radiation damage) yield relatively similar ages of ~280-350 Ma. For grains with [eU] values of 900-2000 ppm, ZHe ages are negatively correlated with age and range from ~30-200 Ma due to the effect of radiation damage. This age dispersion is not observed in the Albany-Fraser Orogen and the Leeuwin Complex, where ZHe ages cluster around a narrower age range of ~280-380 Ma. AHe ages (~250-330 Ma) yield a broadly similar age range to the ZHe results. 40Ar/39Ar results from the cratonic interior suggest that most of the craton experienced slow cooling soon after initial cratonisation at ~2600 Ma. However, the western margin of the craton seems to have been affected by later tectonic events resulting in young 40Ar/39Ar ages (<1300 Ma). 40Ar/39Ar results from the transitional domain in the Yilgarn craton could be interpreted as partially reset ages due to hydrothermal alteration related to the Pan-African tectonism, as recorded in the Leeuwin Complex. Alternatively these ages may relate to the Pinjarra and/or Albany-Fraser Orogenic events. However, given the E-W strike the Albany-Fraser Orogen is unlikely to have caused thermal/hydrothermal effects along the western margin of the craton in a N-S direction. However, the timing of Pinjarra Orogen that lies to the west of the Yilgarn craton is temporally coincident with 40Ar/39Ar ages from the transitional zone. Therefore, the Mesoproterozoic Pinjarra Orogeny may have affected the western margin of the craton and reset the biotite 40Ar/39Ar ages in the transitional zone. The westernmost biotite recrystallisation ages of ~600–630 Ma support palaeomagnetic indications of oblique collision between Greater India and the Australian continent during Gondwana amalgamation in Late Neoproterozoic time. In view of the aforementioned 40Ar/39Ar data in the western margin of both Yilgarn craton and the Albany-Fraser Orogen, young biotite Rb/Sr ages are interpreted to result mainly from later hydrothermal fluid alteration instead of thermal diffusion process as previously suggested. During Late Palaeozoic, the Yilgarn craton experienced an episode of accelerated cooling (>4 ºС/Myr) indicated by thermal modelling results of (U-Th)/He data and previously unpublished AFT data. This cooling possibly resulted from the removal of several kilometres of sedimentary cover on the craton. Evidence for the sedimentary cover is also inferred from the Collie and Perth basins. The former is a fault-bounded Phanerozoic basin enclosed in the Yilgarn craton, and is assumed to represent an outlier of sediments that once extended over the craton. The Perth Basin, located along the western margin of the Yilgarn craton accumulated a thick sedimentary pile (up to 15 km) during the initial rifting of Gondwana in the Early Permian. However, U/Pb detrital zircons ages in both Collie and Perth basins show few Archaean ages, indicating that the Yilgarn craton was not a major source area despite its close proximity. Therefore, previous and current thermochronological results suggested that the Yilgarn craton may have been covered by early-mid Palaeozoic sedimentary rocks. The inferred sedimentation is also indicated by dynamic topography history, which showed that the craton underwent a history of protracted subsidence since the mid-Palaeozoic, thus providing accommodation space for the accumulation of a sedimentary succession over the craton. These sediments were removed later and caused accelerated cooling during the Late Paleozoic as revealed by thermochronology data in the Late Paleozoic. This accelerated cooling/denudation event may relate to one or all of possibilities listed: 1) mantle flow and the resultant dynamic topography; 2) Permo-Carboniferous glaciation and the isostatic effects of deglaciation; 3) a far-field response to continental collision between Gondwana and Laurussia, followed by intra-Gondwana rifting along the western margin of Western Australia. Therefore, the Late Palaeozoic accelerated cooling in the craton has experienced a more dynamic history than previously envisaged.

The Ballarat East gold deposit (408t) is the second largest orogenic gold deposit in the Western Lachlan Orogen, southeast Australia. The western Lachlan Orogen is characterised by a thick package of Ordovician turbiditic sedimentary rocks overlying Cambrian oceanic volcanic sequences. The region was variably affected by multiple major deformation/metamorphism and magmatism events during the Cambrian to Devonian. The Ballarat East gold deposit is located in the Bendigo structural zone of the Western Lachlan Orogen and is hosted in Ordovician sediments of the Castlemaine Supergroup. Gold mineralisation in the Ballarat East deposit is sited in quartz and quartz-carbonate veins within goldfield-scale, west-dipping reverse faults. Two major lode types are present: 1) lode type ‘1’ is characterised by arsenopyrite-dominated quartz veins associated with early movement on reverse faults, whereas 2) lode type ‘2’ is related to structurally later, shallow east-dipping, pyrite-sphalerite-galena-white-mica dominated veins, emanating from reverse faults. Previous studies have suggested that gold mineralisation in the Western Lachlan Orogen occurred at ~440Ma, as a result of metamorphic devolatilisation reactions in the lower crust. However the age of mineralisation at the Ballarat East deposit is only broadly constrained to a period between 460 and 370 Ma, and the source of the gold-bearing fluids could include metamorphosed volcanic rocks, sedimentary rocks and/or granites. In order to provide a more robust chronological framework for gold mineralisation at the Ballarat East deposit, several samples of detrital and hydrothermal potassium-rich minerals were collected and analysed by 40Ar/39Ar dating. In addition, fluid inclusions in portions of quartz and quartz-carbonate veins were characterised by micro-thermometry and halogen/noble gas isotopic tracer methods to further constrain the source(s) of the gold mineralising fluids. The 40Ar/39Ar data obtained from detrital muscovite grains yield ages between 530 – 460 Ma and are concordant with previously published detrital ages. The vein muscovite/sericite ages fall into three age groupings as follows: 445 – 435 Ma (lode type ‘1’), 420 – 415 Ma (lode type ‘2a’) and 380 – 370 Ma (lode type ‘2b’). The gold-bearing quartz veins (from both lode types) contain low salinity (average 4 wt.% NaCl eq.) aqueous H2O inclusions and mixed H2O-CO2 fluid inclusions. Fluid inclusion 40Ar/36Ar values range from 322 (close to Air Saturated Water; ~296) up to a maximum of 4503, and 40Ar/36Ar is strongly correlated with Cl/36Ar. Fluid inclusions have variable Br/Cl values between 1.66 10-3 and 2.91 × 10-3 and I/Cl values between 153 × 10-6 and 501 × 10-6, with a strong correlation between Br/Cl and I/Cl. The fluid inclusion 84Kr/36Ar and 129Xe/36Ar values are variable but show a systematic enrichment in the heavier noble gases. The 40Ar/39Ar ages suggest gold mineralisation at the Ballarat East deposit occurred in three main episodes at ca. 445 Ma, ca. 420 Ma and ca. 380 – 370 Ma. All episodes of mineralisation are associated with fluid inclusions of similar composition. This fluid is suggested to reflect a deeply sourced fluid, possibly originating by devolatilisation of altered volcanic rocks (e.g. basalts). In this scenario, the fluid would have acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are compatible with genetic models for orogenic Au in which gold mineralisation was initiated by metamorphic devolatilisation in the lower crust, linked to Lachlan Orogenesis at ca. 440 Ma.

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