Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Dr. Gordon Southam

Abstract

The mobility of gold at near-surface environmental conditions, e.g., supergene weathering environments, lateritic weathering systems, saline to hypersaline systems and placer gold environments, takes place as oxidised, soluble gold complexes and as reduced elemental gold. The transformation between aqueous and solid states of gold is attributed to the varying geochemical conditions that occur in dynamic environments that are catalysed in part by the biosphere. The primary focus of this research is the investigation of biogeochemical processes that contribute to the cycling of gold using laboratory models to represent various natural systems including chemolithotrophic bacteria, e.g., iron-oxidising bacteria, and heterotrophic sulphate-reducing bacteria and nitrifying bacteria. Results from these studies demonstrate that bacteria initiate the gold cycle by liberating gold through the chemical weathering of gold-bearing minerals. Through oxidative-complexation, soluble gold complexes, e.g., gold (I) thiosulphate and gold (III) chloride, could be produced; however, destabilisation of these gold complexes coupled with bioprecipitation and biomineralisation can immobilise gold thereby completing the cycle. Since the biosphere has an influence on the geochemical conditions of natural environments, the duration of the mobility of gold as soluble complexes is finite and represents a brief “snap shot” of gold’s occurrence. Therefore under surface and near-surface environmental conditions gold will predominantly occur as secondary gold. Secondary gold occurs as nanometre-size to micrometre-size colloids, octahedral platelets, euhedral crystals and bacteriomorphic structures. Furthermore, when bacteria develop as a structurally cohesive biofilms, reduction and enrichment of gold can occur and produce macroscopic gold structures including foils, grains and nuggets. Therefore, bacteria can have a profound effect on the occurrence of gold in natural environments as long as nutrients necessary for microbial metabolism are sustained and gold is in the system. The direct and indirect biogenic effects on gold biogeochemistry will persist over geological time forming observed anomalous gold concentrations such as nugget formations and supergene gold enrichment. Characterising the morphology of gold grains and nuggets in association with understanding how biogeochemical conditions contribute to gold immobilisation is important for gold exploration as it has practical application in mineral vectoring.


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