Contrasting dispersion patterns for gold in stream sediments at Timbarra, NSW, Australia

Abstract

Regional geochemical exploration for granite-hosted Au mineralisation on the Timbarra Plateau has identified poor correlation between bulk cyanide leach (BCL) Au contents in the wet-screened <2000-μm fraction and aqua regia (AR) extractable Au in the wet-screened <180-μm fraction of stream sediments across most parts of the plateau. At James East, there is a weak response to mineralisation in both the BCL and <180-μm fraction Au contents of stream sediments and poor correlation between these fractions. The total Au content of the sediments is primarily contained within the organic-rich <63-μm fraction. There is little visible Au in panned concentrates from stream sediments or adjacent alluvium. Dispersion of Au appears to be predominantly hydromorphic with minor mechanical contributions from a more diffuse palaeo-colluvial source. At the nearby Surface Hill, there is a strong response to mineralisation in stream sediments with contributions to total Au from a wide range of size fractions. The BCL and <180-μm fraction Au contents are closely correlated. Visible Au is present close to bedrock sources in eluvial and alluvial accumulations. Particle size and Au distribution in the stream sediments indicates dispersion of Au to be dominantly by mechanical processes from the proximal part of a colluvial fan on the margin of the plateau. Variations in the concentration, form and distribution of Au in stream sediments at the two sites can be attributed to variations in surficial conditions and the effects of varying amounts of organic material; however, a complimentary factor is the potential loss of colloidal or organic-adsorbed Au through decanting of sieve water during wet-screening of sediments in the field.