Granite-pegmatite-related gold skarns and associated Li-Cs-Ta pegmatites in the Archean Yilgarn Craton, Western Australia

Abstract

In greenstones of the east-central Yilgarn Craton, Western Australia, Li-Cs-Ta (LCT) pegmatites of the albite-spodumene type (resources > 300 Mt at 1.4% Li₂O) are spatially associated with high-T (650–450 °C) granite-pegmatite-related skarn and skarn-quartz vein Au-Ag deposits (production 580 t Au). U-Pb and Rb-Sr ages (2.65–2.62 Ga) link both the LCT pegmatites and gold deposits to a suite of peraluminous I-type biotite granites, magnetite-series in least-fractionated K-feldspar megacrystic members, and magnetite- or ilmenite-series in pegmatitic garnet-muscovite intrusions contaminated by melt interaction with continental crust. The granite-pegmatite complexes and the gold skarns formed at 11–15 km crustal depth (300–400 MPa) in a post-orogenic batholith environment at the margin of the 2.7 Ga Eastern Goldfields orogen. Gold occurs in both endo- and exoskarn, and in granite and pegmatite dikes. Calcite carbon isotopes, and initial ⁸⁷Sr/⁸⁶Sr ratios in scheelite relate the skarns to post-orogenic intrusions. Inner calcite, pyroxene, and outer biotite, cordierite, amphibole, and olivine impart a CO₂-Ca-Mg-K metasomatic signature to the hydrothermal system (e.g., Nevoria, Yilgarn Star, Copperhead). An exception is the Ghost Crab deposit, where anthophyllite, cummingtonite, and albite are major constituents in sodic gold ore replacing paragneiss in a zone of magnesian skarn. This deposit is proximal to the albite-spodumene pegmatites of the Mt Marion mine. The skarns in the Yilgarn Craton share the granite-pegmatite association, the gangue mineralogy, the Au-Bi-W geochemical signature, and low- to moderate salinity H₂O-CO₂ fluid inclusions with reduced tungsten skarns, and with world-class gold deposits in other Archean cratons (Kaapvaal, Slave, Superior, Dharwar).