Alteration patterns linked to high-grade gold mineralization at the Wattle Dam deposit, Western Australia

Abstract

Sharp physio-chemical gradients occur in Archean gold systems, yet they are often poorly defined. These physio-chemical gradients can provide windows into fluids that deposit high-grade gold mineralization. Recognizing different alteration styles and fluid-types in mineral systems helps define processes that led to the deposition of high-grade gold. Here we report on the architecture and chemical gradients across the komatiite-hosted Wattle Dam gold deposit in the eastern Yilgarn Craton, Western Australia, which is one of Australia’s highest grade-gold deposits. The deposit notably contains millimeter- to centimeter-thick gold-only veins with an absence of quartz. Whole rock geochemistry shows that high-grade gold veins are hosted in komatiite flow-margins facies with olivine orthocumulate komatiites in the footwall, interbedded graphitic shale close to the lode and komatiite basalt in the hangingwall. The komatiite flow-margin facies that hosts high-grade gold veins have been pervasively altered by stage 1 biotite-actinolite. Stage 2 gold veins cross-cut the biotite-actinolite altered komatiite and these veins contain thin (<100 µm) selvages of sodicgedrite, riebeckite, aegirine and ullmannite ± sulfates (barite and anhydrite) ± magnetite. Millimeter-thick gold-only veins are associated with the dissolution of zircon and magnetite, and precipitation of catapleiite in veins. These textures are consistent with the contention that fluids mobilized Zr and Fe and were highly reactive. Arsenic- and Sb-bearing alteration in the form of cobaltite, arsenopyrite, nickeline, gersdorffite, breithauptite and ullmannite occur as wide halos (up to 300 m) around the main lode. These As and Sb bearing-phases form in both the hangingwall komatiite basalt unit and footwall olivine orthocumulate komatiites. Arsenides and antinomides contain inclusions of Au and can be directly linked to mineralization. The diverse array of mineral assemblages associated with high-grade and low-grade gold mineralization indicates sharp chemical gradients at the time of gold mineralization. The mobility of Zr and Fe at the time of mineralization in the core of the deposit points towards the presence of alkaline fluids, whereas peripheral zones are associated with minerals associated with the near-neutral H2O-CO2 fluids, commonly linked with Archean gold systems. The generation of carbonate-undersaturated alkaline fluids in the high-grade core of the Wattle Dam deposit, may be explained by phase-separation or a CO2-poor alkaline fluid source. Either way these alkaline fluids played a critical role in mobilizing and concentrating very high amounts of gold into veins.