Regolith mineralogy and geochemical dispersion at the Northparkes Cu–Au deposits, New South Wales, Australia

Abstract

The Cu–Au mineralization at Northparkes, western NSW, Australia, occurs as disseminated or veined sulphides in K-rich trachyandesites intruded by quartz monzonite porphyries. Detailed mapping of the open pits revealed a complex regolith consisting of in situ weathered mineralized trachyandesite and weathered alluvial–colluvial material in a palaeovalley. The kaolinite–smectite-rich sediments have been locally derived, and are marked by development of mottles and ferruginous nodules indicating intense post-depositional weathering. Primary mineralization is expressed in the regolith by anomalous Au, Cu and As contents. Lead and Zn show variable dispersion patterns in the transported regolith of the E22 and E27 deposits. This is probably due to differences in their chemical mobility, provenance and sedimentary environment. Although it occurs at depth (30–50 m), the widespread occurrence of saprock makes it a very good sampling medium in this region.