Recognizing hydrothermal alteration through a granulite facies metamorphic overprint at the challenger Au deposit, South Australia

The Challenger Gold Mine, South Australia, is hosted in granulite-facies migmatitic gneisses. Lack of hydrous alteration minerals, evidence for remobilization of Au-rich sulfide melts, and intense stretching of the ore package parallel to the regional gneissic fabric all point to a pre-metamorphic mineralization event. A combination of textural, mineralogical, and lithogeochemical criteria have been used to recognize the effects of partial melting during high-grade metamorphism and mass changes during pre-metamorphic hydrothermal alteration. Accounting for these superimposed processes allows us to document the style of pre-metamorphic hydrothermal alteration that affected the deposit. Alteration in a ∼ 50 m halo surrounding Au-mineralization is characterised by losses of Si, Na, Ca, and Sr and an increase in Al, K, Fe and Mg relative to the mean composition of unaltered distal gneisses. These geochemical fluxes significantly reduced the melting potential of these domains such that the altered rocks were transformed during metamorphism into mottled, granoblastic aluminous gneisses. In contrast, Au-rich zones host abundant relict quartz veins that underwent variable assimilation in voluminous K-feldspar-rich granitic melts generated through dehydration melting of quartz-and mica-rich layers. Geochemical trends attributable to this period of anatexis are distinct from those arising from pre-metamorphic hydrothermal alteration. These physical and geochemical observations are consistent with feldspar-destructive alteration and chloritization of proximal host rocks accompanied by silicification and sericitization (quartz veins + micas) alteration in the Au-rich horizons prior to prograde metamorphism. U-Pb geochronology of pristine magmatic zircon preserved in granoblastic proximal gneisses demonstrate that hydrothermal alteration occurred after deposition of the host volcaniclastic sequence (∼ 2520 Ma) and before the onset of regional metamorphism (∼ 2470 Ma). The evidence gathered here points to an epithermal-style Au deposit that was reworked during collisional orogenesis. The short time interval between Au-deposition and subsequent basin inversion prevented erosive forces from erasing the deposit from the rock record prior to metamorphism.