Supergene gold transformation: biogenic secondary and nano-particulate gold from arid Australia

Abstract

Biofilms on Au grains from sub-tropical and tropical sites promote the dispersion of gold (Au) by continuously recycling coarse Au and releasing nano-particulate Au. In this study we investigated the influences of (bio)geochemical processes on the transformation of Au grains in arid environments. At these sites water and nutrient availability are limited and episodic, hence abiogenic evaporative mechanisms were thought to control the formation of secondary and especially nano-particulate Au. Gold grains were collected from eight arid sites in three Australian Au provinces, i.e., Lawlers (Western Australia), Tanami (Northern Territory), and Flinders Ranges (South Australia). Sites were chosen based on contrasting deposit styles, i.e., primary underground and epithermal deposits as well as secondary eluvial-, colluvial- and alluvial placers at increasing distances from primary mineralization. Gold grains were studied using optical microscopy, FEG-SEM, FIB-SEM-EDXA, EPMA and PIXE. Gold grains from all surface environments displayed supergene transformation features, i.e., morphotypes indicative of Au and Ag dissolution, Au aggregation and Au neoformation. The latter included spheroidal and bacteriomorphic Au, which increased in abundance with distance to source. Viable biofilms containing abundant Au nano-particles and spheroidal Au μ-crystals were detected on all grains from the Flinders Ranges. Gold grains from the Lawlers and the Tanami provinces are covered by polymorphic layers containing abundant nano-particulate, spheroidal and bacteriomorphic Au. The polymorphic layers consist of vermiform clays and organic matter, suggestive of remnant biofilms. These results demonstrate that biofilms capable of transforming Au grains develop episodically on Au grains in arid environments. In conclusion, this study shows that microbial processes play a critical role for the transformation of Au grains and contribute to the dispersion of Au and the formation of geochemical anomalies in arid environments. © 2012.