Wallrock alteration associated with gold mineralisation in Cohen?s Reef, Walhalla, Victoria

Abstract

Cohen?s Reef in Walhalla is a shear zone-associated, laminated-massive auriferous quartz vein system, and is associated both spatially and temporally with the Woods Point Dyke Swarm. Geochemical analyses of the auriferous Cohen?s Reef and host rocks, have led to a greater understanding of the mechanisms and extent of the alteration and mineralisation events. Geochemical traverses proximal and distal to Cohen?s Reef were based on samples taken from recent exploration drilling. Current research is concentrated on selected gold deposits surrounding Walhalla in response to the latest GeoScience Victoria mapping initiative and current exploration by Goldstar Resources NL. This preliminary geochemical analysis builds on handsample and thinsection studies, and whole rock analyses of the altered and unaltered dyke, and host sediments from drilhole traverses proximal and distal to Cohen?s Reef. Loss on Ignition (LOI) values are a favourable guide to the extent of hydrothermal alteration in dyke and host sediments. Based on the selected samples, broad hydrothermal alteration zones (~5% wt) at least 15 metres from auriferous dyke margins and quartz veins, are characterised by arsenopyrite and pyrite visible in thinsection. More importantly, LOI peaks greater than 12% wt seem to be indicative of greater Au content with narrow alteration zones of less than five metres, and characterised by visible sulphides.