ALBERT

Description: Gold mineralization at the Tropicana mine occurs within the Plumridge terrane along the eastern margin of the Archean Yilgarn craton in the Albany-Fraser orogen, Western Australia. Mineralization is hosted in a favorable syenitic lithofacies of the Tropicana Gneiss with a minimum igneous age of 2638 ± 4 Ma (2 ) and which was metamorphosed to mid-amphibolite to lower granulite facies in the period ca. 2638 to 2520 Ma. The Tropicana Gneiss was exhumed to crustal levels equivalent to greenschist-facies conditions by the time of economic gold mineralization. The major gold-bearing pyrite-biotite-sericite mineralization formed in association with shear zones during northeast-southwest compression (D 3 ) that postdated W- to NW-verging thrusting (D 2 ). The late fluid-induced event (Tropicana event; Doyle et al., 2013 ; Blenkinsop and Doyle, 2014 ) produced a mineral assemblage indicative of greenschist-facies conditions. The paucity of water in granulite-facies gneisses under retrograde conditions suggests that fluids were introduced from an external source for both mineralization and the younger metamorphic event. This occurred at ca. 2520 Ma as determined from biotite (ca. 2515 Ma, 40 Ar/ 39 Ar age), pyrite (ca. 2505 Ma, Re-Os, Pb/Pb ages), and tungsten-rich rutile (2521 ± 5 Ma, U-Pb age): the latter is considered to provide the best direct measurement age of gold mineralization. The Tropicana Gneiss was derived from the upthrusted easternmost margin of the underlying Yilgan craton, or represents a relatively small (160 km long x 50 km wide) remnant crustal block accreted to the Yilgarn craton sometime between ca. 2.6 and 2.5 Ga. The timing of the Tropicana mineralization is distinctly younger than greenstone-associated gold deposits elsewhere in the Yilgarn craton. As in the Dharwar craton, India, and the Limpopo belt, Zimbabwe, economic gold mineralization at Tropicana postdates the major peak of Late Archean world-class gold mineralization (ca. 2.65 Ga) by more than 100 m.y. The Tropicana Gneiss was relatively unaffected during the younger Albany-Fraser orogeny, with only minor remobilization of gold. Deformation may have been localized at the margins of the domain, between constituent ridged structural blocks, and/or within discrete high-strain shears.

Description: The hydrothermal fluids responsible for gold mineralization at the Ballarat East gold deposit (the second largest orogenic gold deposit in the western Lachlan orogen) are thought to have links to a variety of processes, including metamorphism, sedimentation, and/or magmatism. In the current study, noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold-related fluids at the Ballarat East deposit. Gold-bearing quartz and carbonate veins from the Ballarat East contain low salinity (~4 wt % NaCl equiv) aqueous (H2O) and mixed H2O-CO2 fluid inclusions. These fluid inclusions have variable molar Br/Cl values of between 1.2 x 10-3 and 2.9 x 10-3 and I/Cl values between 150 x 10-6 and 500 x 10-6, and Br is strongly correlated with I, defining a mixing line with a Br/I ratio of 5.6. The fluid inclusions have 40Ar/36Ar ratios ranging from 322 (close to the atmospheric 40Ar/36Ar ratio of ~296) up to a maximum of 4503. 40Ar is strongly correlated with Cl and defines a mixing line with a 40ArE/Cl ratio of 4.6 x 10-4 (40ArE denotes excess 40Ar). The fluid inclusions contain 5.1 to 32 ppm 40ArE (by mass) and exhibit minimum 36Ar concentrations ranging from 3.1 to 11 ppb, which exceed air-saturated water (ASW) levels by several parts per billion (ASW = 1.3-2.7 ppb). Fluid inclusion 84Kr/36Ar and 130Xe/36Ar values are uniformly enriched in Kr and Xe relative to air, but exhibit limited variation. These data provide strong evidence for the involvement of two noble gas and halogen reservoirs. This data is compatible with a deeply sourced fluid, possibly originating by devolatilization of altered volcanic rocks (e.g., basalts) that 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 also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters; however, both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid-rock interaction as controls on fluid compositions in Victorian deposits. The data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust, and was linked to Lachlan orogenesis at ca. 440 Ma.