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    The timing and origin of orogenic gold mineralisation in the western Lachlan Orogen, southeast Australia: constraints from 40Ar/39Ar dating and halogen and noble gas geochemistry
    Fairmaid, Alison Maree ( 2012)
    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.