Geochemical exploration around the Harmony gold deposit, Peak Hill, Western Australia

Abstract

Primary Au mineralization at Harmony is associated with sulphide-poor quartz veins in mafic and ultramafic rocks. Gold and W are the most significant indicators of mineralization in fresh rock. Elements associated with the phyllic alteration halo are K, Rb and Ba, and Eu, Lu and Yb; these increase in abundance and range near mineralization. Although As and Sb are elevated, their abundances are far lower than in sulphide-rich mineralization styles. The Harmony gold deposit was completely covered by a blanket of soil and colluvium. Drilling beneath this revealed a complex, partly eroded residual regolith, clay-rich valley fill sediments and colluvium. Sampling and analysis of the top of the basement (ferruginous saprolite, lateritic residuum and mottles washed from the mottled zone) on a 250 m sample spacing showed significant Au and W anomalies in the vicinity of the deposit. Gold dispersion in the ferruginous saprolite is restricted and requires close-spaced sampling (50 m). The unconformity between the stripped basement and colluvium was sampled at a 150 m spacing to capitalize on any mechanical or hydromorphic dispersion along the contact. Dispersion of Au, W, Ta and Nb along this interface, indicating mechanical down-slope migration, produced better and more consistent anomalies than the basement sampling in this stripped environment. There are some very weak Au anomalies in the fine soil fraction (<75 μm), where the colluvial cover over the Harmony deposit was extremely thin (<2 m). However, this cannot be relied upon to locate mineralization.