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Abstract
Detrital gold in Late Pleistocene-Holocene placers has been chemically mobilised and redeposited at the micron scale by biologically-mediated reactions in groundwater. These processes have been occurring in a tectonically active semiarid rain shadow zone of southern New Zealand and are probably typical for this type of environment elsewhere in the world. The chemical system is dominated by sulfur, which has been derived from basement pyrite and marine aerosols in rain. Detrital and authigenic pyrite is common below the water table, and evaporative sulfate minerals are common above the fluctuating water table. Pyrite oxidation was common but any acid generated was neutralised on the large scale (tens of metres) by calcite, and pH remained circumneutral except on the small scale (centimetres) around pyritic material. Metastable thiosulfate ions were a temporary product of pyrite oxidation, enhanced by bacterial mediation, and similar bacterial mediation enhanced sulfate reduction to form authigenic pyrite below the water table. Deposition of mobilised gold resulted from localised variations in redox and/or pH, and this formed overgrowths on detrital gold of microparticulate and nanoparticulate gold that is locally crystalline. The redeposited gold is an incidental byproduct of the bacterially-enhanced sulfur reactions that have occurred near to the fluctuating sulfide-sulfate redox boundary.
Highlights
Low-temperature mobility and redeposition of gold are well-established phenomena in the near-surface geological environment, and this gold mobility is commonly mediated by geomicrobiological processes [1,2,3,4,5,6,7,8]
Rise of higher mountains to the west has caused a semiarid rain shadow zone to develop over the area
Late Pleistocene to Holocene alluvial fans (Figure 10a) in which biologically-mediated gold mobility has occurred in groundwater
Summary
Low-temperature mobility and redeposition of gold are well-established phenomena in the near-surface geological environment, and this gold mobility is commonly mediated by geomicrobiological processes [1,2,3,4,5,6,7,8]. Similar processes have been inferred for nanoparticulate and microparticulate crystalline and amorphous gold deposits in the natural environment in a range of geological settings [4,5,6,12,13]. Despite the well-established experimental and observational basis for geomicrobiological mediation of gold mobility, the details of the geochemical environments in which these processes occur are less well understood. This gap in knowledge arises because geomicrobiological agents and processes are ephemeral, and subject to overprinting by later events during the geological evolution of a host environment. We provide some geochemical and mineralogical constraints on geomicrobiological processes of gold mobility in a semiarid environment, where strong evaporative processes have been common
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