Mineralogy and Geochemistry of Biologically-Mediated Gold

minerals Article Mineralogy and Geochemistry of Biologically- Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand Gemma Kerr and Dave Craw * Geology Department, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand; [email protected] * Correspondence: [email protected]; Tel.: +64-3-479-7529 Received: 21 July 2017; Accepted: 13 August 2017; Published: 16 August 2017 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. Keywords: gold; pyrite; sulfate; groundwater; geomicrobiology; authigenic; clay 1. Introduction 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 [1–8]. In particular, bacteria have been shown experimentally to facilitate gold redeposition from encapsulating sulfide minerals, yielding crystalline and amorphous gold deposits at the nanometre scale [4–6,9–11]. Similar processes have been inferred for nanoparticulate and microparticulate crystalline and amorphous gold deposits in the natural environment in a range of geological settings [4–6,12,13]. 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. Recent identification of the products and time scales of bacterially-mediated gold mobilisation in an active river [13] have helped to fill this knowledge gap for moist subtropical environments. In this study, we provide some geochemical and mineralogical constraints on geomicrobiological processes of gold mobility in a semiarid environment, where strong evaporative processes have been common. Minerals 2017, 7, 147; doi:10.3390/min7080147 www.mdpi.com/journal/minerals Minerals 2017, 7, 147 2 of 18 MineralsThis study 2017, 7, provides147 observations on geochemistry and mineralogy of groundwater processes2 of 18 within Pleistocene fluvial sediments and immediately underlying basement rocks, and relates these observationsThis tostudy associated provides gold observations mobility andon geochemi redeposition.stry and These mineralogy processes of havegroundwater occurred, processes and are still within Pleistocene fluvial sediments and immediately underlying basement rocks, and relates these occurring, in an active tectonic environment, where uplift and erosion have caused physical recycling observations to associated gold mobility and redeposition. These processes have occurred, and are of placer gold to younger sediments, followed by renewed gold mobility involving both inorganic and still occurring, in an active tectonic environment, where uplift and erosion have caused physical biologically-mediatedrecycling of placer gold chemical to younger processes. sediments, Evaporation followed has by locallyrenewed enhanced gold mobility solution involving concentrations both so thatinorganic secondary and biologically-mediated minerals have formed chemical withsome processes. of the Evaporation new gold, has providing locally enhanced some useful solution insights intoconcentrations the water geochemistry so that secondary that accompanied minerals have gold formed mobility. with some of the new gold, providing some useful insights into the water geochemistry that accompanied gold mobility. 2. General Setting 2.A General well-defined Setting rain shadow zone has developed to the east of the mountains that form the backboneA ofwell-defined the South Island rain shadow of New zone Zealand has (Figuredeveloped1a). to This the rain east shadow of the mountains results in semiaridthat form climatic the conditionsbackbone with of precipitationthe South Island as lowof New as 300 Zealand mm/year (Figure in inland1a). This areas, rain andshadow the rainresults shadow in semiarid effect has beenclimatic present conditions since at least with the precipitation Pliocene [ 14as, 15low]. Partas 300 of mm/year the rain shadowin inland zone areas, includes and the anrain area shadow of Otago Schisteffect basement has been that present hosts since numerous at least orogenic the Pliocene gold [14,15]. deposits Part that of the formed rain inshadow the Cretaceous, zone includes including an the world-classarea of Otago Macraes Schist depositbasement that that is anhosts active numero mineus on orogenic a regional gold structure, deposits the that Hyde-Macraes formed in the Shear Cretaceous, including the world-class Macraes deposit that is an active mine on a regional structure, Zone (Figure1a,b) [ 16]. This study focuses on placer gold deposits within and near the 500 mm/year the Hyde-Macraes Shear Zone (Figure 1a,b) [16]. This study focuses on placer gold deposits within precipitation contour (Figure1a). The region has potential evaporation greater than precipitation, and near the 500 mm/year precipitation contour (Figure 1a). The region has potential evaporation resultinggreater in than widespread precipitation, evaporative resulting mineral in widespre formationad evaporative at and near mineral the landsurface formation at (Figure and near1a) [the17– 19]. Somelandsurface evaporative (Figure salt deposits 1a) [17–19]. have Some formed evaporative in this region salt deposits from surface have evaporationformed in this of marineregion from aerosols thatsurface were deposited evaporation on of impermeable marine aerosols substrates that were [15 deposited,18,20]. on impermeable substrates [15,18,20]. FigureFigure 1. Location 1. Location images images for sitesfor sites described described in this in studythis study and theirand their regional regional mineralogical mineralogical and climaticand climatic contexts. (a) Digital elevation model map of southern New Zealand, showing part of the contexts. (a) Digital elevation model map of southern New Zealand, showing part of the mountain mountain chain to the west that causes a rain shadow, with the approximate boundary of the chain to the west that causes a rain shadow, with the approximate boundary of the semiarid portion semiarid portion (black dashed contour). Localities referred to in the Table 1 and some other (black dashed contour). Localities referred to in the Table1 and some other associated are indicated. associated are indicated. Inset shows the extent of the Otago Schist belt, which hosts orogenic gold Insetdeposits shows thethat extent form the of thesource Otago for Schistplacers belt, described which herein; hosts orogenic(b) Oblique gold digital deposits elevation that form model the (2 source × for placersvertical describedexaggeration) herein; of the (b eastern) Oblique part digital of the elevationsemiarid rain model shadow (2 × vertical(as indicated exaggeration) with dashed of the easternquadrilateral part of the in semiarida), showing rain tectonic shadow topography (as indicated and withbasement dashed gold quadrilateral deposits. in a), showing tectonic topography and basement gold deposits. Minerals 2017, 7, 147 3 of 18 CoupledMinerals 2017, with7, 147 the rise of the main South Island mountain backbone, parts of3 of the 18 rain shadow area have been progressively uplifted since the Pliocene, but at relatively slow rates (<0.1–1 mm/year)Coupled with [15, 21the]. rise This of uplift the main has South resulted Island in formationmountain backbone, of a set of parts north of to the northeast rain shadow trending antiformalarea have mountain been progressively ranges, and theseuplifted are since separated the Pliocene, by synformal but at basinsrelatively in whichslow rates little (<0.1–1 or no uplift mm/year) [15,21]. This uplift has resulted in formation of a set of north to northeast trending has occurred (Figure1a,b). The antiformal ranges have been rising for at least the past million years, antiformal mountain ranges, and these are separated by synformal basins in which little or no uplift and dominate the topography of the rain shadow area (Figure1a,b) [ 15,21]. The ranges generally

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