Biotite and apatite as tools for tracking pathways of oxidized fluids in the Archean East Repulse gold deposit, Australia

Fluid pathways in hydrothermal ore deposits can potentially be traced through alteration zoning patterns and the chemistry of mineral phases (i.e., major, trace elements, and isotopic composition), which provide insight into the physico-chemical gradients within and adjacent to pathways. World-class Neoarchean gold deposits of the eastern Yilgarn can be zoned with respect to alteration assemblages, but in many cases fluid pathways are poorly understood, which makes it difficult to identify the nature and source of fluids. This study conducted detailed core logging, SEM mineral mapping of alteration assemblages, and microprobe analysis of biotite and apatite in order to identify chemical gradients across the East Repulse gold deposit of the St. Ives gold camp, eastern Yilgarn, Western Australia. Results show that the East Repulse deposit is vertically zoned with relatively oxidized sulfate-rich alteration in the footwall of the deposit (anhydrite, celestine and/or barite), sulfide-rich alteration in the core of the deposit (pyrite ± magnetite with minor barite and celestine), and relatively reduced sulfide-only assemblages (pyrite ± pyrrhotite with minor pentlandite, millerite, and cobaltite) in the hanging wall of the deposit. Gold was identified as inclusions in pyrite and associated biotite implying a link between biotite-pyrite alteration and gold mineralization. The abundance of F in biotite and apatite progressively decreases from the footwall of the deposit into the mineralized core. This trend of decreasing F in biotite and apatite with decreasing depth is consistent with an hypothesis that oxidized F- and K-bearing fluids were focused along subvertical pathways beneath the deposit, within or subjacent to granitoid dikes. The biotite-apatite geothermometer shows that these fluids were relatively hot (∼480° ± 60°C) at the time of alteration. These relatively high temperature, oxidized, F- and K-bearing fluids may have been derived from a proximal magmatic source.