Pleistocene paleodrainage and placer gold redistribution, western Southland, New Zealand

Craw, Dave; Kerr, Gemma; Reith, Frank; Falconer, Donna

doi:10.1080/00288306.2015.1007462

Cited by 21 publications

(3 citation statements)

References 19 publications

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“…The overgrowths and the underlying exterior gold flake surfaces have no detectable Ag in EDS spot analyses. Ag-poor overgrowths like those observed on this Waikaia mine gold are found on almost all gold from the Otago Schist goldfield, and have been formed by authigenic Au dissolution and redeposition in situ from groundwater geochemical processes [16,[26][27][28][29]. Similar gold overgrowths are common elsewhere and are thought to be of bacteriogenic origin [30][31][32].…”

Section: External Gold Particle Texturesmentioning

confidence: 83%

Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

et al. 2017

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This study documents the bulk Au fineness (Au parts per thousand) of the bullion from a placer gold mine in southern New Zealand. The compositions of doré bars produced approximately every 10 days over nearly three years is compared to the range of compositions of gold particles which have been extracted. Silver is the principal impurity in the gold, and the doré bars contained 2-3 wt % Ag over the period examined. At the scale of a typical individual 0.5 mm gold particle, there are three different types of gold: an Ag-bearing core (2-9 wt % Ag), a 10-50 µm wide Ag-poor rim (typically <1 wt % Ag), and micron scale overgrowth gold (0% Ag). The overgrowths are volumetrically negligible, and the average Ag content of a gold particle is controlled principally by the proportions of core and rim gold. The rims have been formed by recrystallisation of deformed core gold, with associated leaching of the Ag from the recrystallised gold. The volumetric proportion of cores has decreased with increasing flattening of gold particles, and highly flattened and folded flakes have little or no remnant cores. The bulk Au fineness of doré bars from the mine has decreased from 980 to~970 as the mine progressed upstream in a Pleistocene paleochannel because the upstream gold has been less flattened than the downstream gold.

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Section: External Gold Particle Texturesmentioning

confidence: 83%

Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

et al. 2017

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“…The gold overgrowth textures observed within the rain shadow region are similar at numerous sites in southern New Zealand (Figure 1a) [4,6,12,19,32,39]. The textures are dominated by irregular micron-scale vermiform gold shapes that have precipitated on the exterior of detrital gold particles.…”

Section: Biogenic Gold Texturesmentioning

confidence: 81%

“…Typical basin groundwaters have low dissolved sulfate concentrations (<10 mg/L; Figure 4). This dissolved sulfate is derived from a combination of incursion of marine aerosols in rain and interaction with pyrite-bearing rocks [17,[37][38][39]. Locally, extensive interaction between oxidising groundwater and the widespread pyrite causes elevated dissolved sulfate concentrations in the groundwater, with concentrations reaching ~100 mg/L where groundwaters percolate slowly through relatively impermeable lithic debris such as poorly sorted alluvial fan sediments and landslides (Figures 3c and 4) [19,34].…”

Section: Groundwater Compositionsmentioning

confidence: 99%

Mineralogy and Geochemistry of Biologically-Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand

Kerr

Craw

2017

Minerals

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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.

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Morphology and structural evolution of fine beach gold in comparison to detrital platinum, southern New Zealand

Palmer

Craw

2023

Miner Deposita

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Beach placer gold has been mined around the world historically, but extraction of fine (~ 100 µm) gold particles is notoriously difficult. This study illustrates morphological and mineralogical changes that transform fine gold during aeolian processes on windy beaches and contribute to mine concentration inefficiencies. Sandblasting on exposed beaches in southern New Zealand has caused extreme attenuation of edges of gold flakes that were previously transported in rivers for > 200 km. Flakes have been transformed into complex but compact toroids and spheroids with thin (~ 20 µm) internal and external strands of attenuated gold. Most of the gold within the attenuated strands has recrystallised to fine (micron-scale) undeformed grains with little or no Ag (< 1 wt%). Some coarse (> 40 µm) gold grains remain from the precursor fluvial particles, and these retain original Ag contents (1–10 wt%). These coarse grains show substantial internal crystallographic deformation and sub-grain formation, although some of these strain effects may have been inherited from fluvial transport. Co-existing detrital platinum minerals are much less malleable than gold during sandblasting and have only minor (10-µm scale) toroidal deformation on edges of fluvial flakes. The complex frameworks of the fine toroidal and spheroidal gold particles can include air, water, and clay, which lowers their average density and so they commonly float on water and are readily entrained with other heavy minerals. The fine particle size, compact shapes, and clay coatings also resist mercury amalgamation.

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Pleistocene paleodrainage and placer gold redistribution, western Southland, New Zealand

Cited by 21 publications

References 19 publications

Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

Quantification and Prediction of Bulk Gold Fineness at Placer Gold Mines: A New Zealand Example

Mineralogy and Geochemistry of Biologically-Mediated Gold Mobilisation and Redeposition in a Semiarid Climate, Southern New Zealand

Morphology and structural evolution of fine beach gold in comparison to detrital platinum, southern New Zealand

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