A case study of the postdepositional alteration of the Witwatersrand Basal Reef gold placer

Frimmel, Hartwig E.; Roex, Anton P. le; Knight, J.; Minter, W. E. L.

doi:10.2113/gsecongeo.88.2.249

Cited by 81 publications

(19 citation statements)

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“…Formation of secondary octahedral gold crystals from gold(III)-chloride solution was also promoted by a cyanobacterium (P. boryanum) via an amorphous gold(I)-sulfide intermediate [26,29]. Secondary, bacteriomorphic gold is common in quartz pebble conglomerates deposits (QPC), such as Witwatersrand QPC, which is the world's largest gold deposit [30,31]. Here gold is commonly associated with bituminous organic matter of putative microbial origin.…”

Section: Figurementioning

confidence: 99%

Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions

et al. 2013

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Microbial communities mediating gold cycling occur on gold grains from (sub)-tropical, (semi)-arid, temperate and subarctic environments. The majority of identified species comprising these biofilms are β-Proteobacteria. Some bacteria, e.g., Cupriavidus metallidurans, Delftia acidovorans and Salmonella typhimurium, have developed biochemical responses to deal with highly toxic gold complexes. These include gold specific sensing and efflux, co-utilization of resistance mechanisms for other metals, and excretion of gold-complex-reducing siderophores that ultimately catalyze the biomineralization of nano-particulate, spheroidal and/or bacteriomorphic gold. In turn, the toxicity of gold complexes fosters the development of specialized biofilms on gold grains, and hence the cycling of gold in surface environments. This was not reported on isoferroplatinum grains under most near-surface environments, due to the lower toxicity of mobile platinum complexes. The discovery of gold-specific microbial responses can now drive the development of geobiological exploration tools, e.g., gold bioindicators and biosensors. Bioindicators employ genetic markers from soils and groundwaters to provide information about gold mineralization processes, while biosensors will allow in-field analyses of gold concentrations in complex sampling media.

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Section: Figurementioning

confidence: 99%

Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions

et al. 2013

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“…Compositions of gold particles can be used to elucidate the origin of gold-bearing deposits, by looking at variations in grain compositions on the sample scale (Frimmel et al 1993;Frimmel & Garz 1997) or the regional scale (Knight et al 1999). Most orogenic gold deposits have gold alloy with high fineness (>90% Au), with the remainder of the alloy consisting predominantly of silver and mercury (Morrison et al 1991;Knight et al 1999).…”

Section: Introductionmentioning

confidence: 99%

The mercury and silver contents of gold in quartz vein deposits, Otago Schist, New Zealand

MacKenzie

Craw²

2005

New Zealand Journal of Geology and Geophysics

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A regional study of gold from Otago Schist vein deposits from both Caples and Torlesse Terranes has revealed the presence of both Au-Ag and Au-Ag-Hg alloys in both terranes. Almost all Hg-bearing gold occurs in east Otago vein systems, and Hg-free gold occurs in central and northwest Otago vein systems, irrespective of host terrane. One Central Otago mineralised zone has up to 1.7 wt% Hg in the gold. Au-Ag alloy (3-8 wt% Ag) is typical for gold found in most Torlesse-hosted vein deposits, and Au-Ag-Hg alloys with 1-4 wt% Hg are found in vein material from the Torlesse-hosted Hyde-Macraes Shear Zone. Au-Ag-Hg alloy (3-8 wt% Ag, 2-8 wt% Hg) is found in many Caples-hosted vein deposits. There is no relationship between depth of vein emplacement and Hg content of gold, as both high and low Hg gold are found in shallow-formed (<2 km) and deep-formed (>6 km) deposits. There is no spatial relationship between Hg-bearing gold and cinnabar-bearing veins that occur in Caples Terrane rocks on the southern edge of the schist belt. Mercury-bearing placer gold in eastern Southland may have been derived from erosion of east Otago veins. The source of Hg-bearing placer gold in northern Southland is unknown as yet.

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“…They are also common in metamorphosed Archean quartz pebble conglomerates of the Witwatersrand basin, where the origin of the gold is controversial (e.g., von Gehlen 1983;Oberthür & Saager 1986;Frimmel et al 1993). With the exception of a recent discovery of Au-Ag-Hg alloy in the Golden Cross fossil hot spring deposit in the Coromandel District (J. Mauk pers.…”

Section: Introductionmentioning

confidence: 99%

Au‐Ag‐Hg and Au‐Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer‐source relationships

Youngson

Wopereis²,

Kerr

et al. 2002

New Zealand Journal of Geology and Geophysics

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Gold in Quaternary placers in the Nevis and Nokomai valleys is dominantly a-phase Au-Ag-Hg alloy (c.<10 wt% each of Ag and Hg) with subordinate Au-Ag alloy. The α-Au-Ag-Hg alloy is typically coarse grained (up to 2 cm), angular, and rarely flattened or folded. Crystalline texture, quartz intergrowths, and psuedo-hexagonal crystal pluck cavities are common. Fluvial transport distance estimates based on maximum flatness index of Au-Ag-Hg alloy particles are typically <10-20 km. Coarse (up to 2 cm) crystalline cinnabar is commonly associated with the AuAg-Hg alloy, and both were probably derived from hydrothermal sources in western tributaries of the upper Nevis River and eastern tributaries of the upper Nokomai River. These sources are possibly related to strands of the Nevis-Cardrona Fault System. Secondary Au-Ag-Hg alloy with up to 38 wt% Hg occurs locally in the lower Nokomai alluvial plain, where it coats or cements detrital α-phase AuAg-Hg and Au-Ag alloy particles. The secondary Au-AgHg alloy has formed by diffusion between detrital gold particles and liquid Hg that is either natural or derived from local breakdown of cinnabar.Au-Ag alloy dominates over Au-Ag-Hg alloy in the lower half of the lower Nokomai alluvial plain and in the gravel of a Pleistocene perched channel incised into pumpellyiteactinolite facies semischist basement adjacent to the plain. The Au-Ag alloy is rounded and commonly flattened and folded. Crystalline texture, quartz intergrowths, and pluck cavities are rare in the Au-Ag alloy, and a fluvial transport distance of 25-40 km is estimated from maximum flatness index. Au-Ag alloy, three types of garnet, magnetite, clinozoisite, and well-foliated schist boulders in the abandoned channel gravel are allochthonous. They were derived from greenschist facies schist and sedimentary sources many tens of kilometres north of the Nokomai catchment, and transported either in Wakatipu Glacier till or fluvioglacial outwash gravel that entered the Nokomai valley via Nokomai Saddle, the confluence with the Mataura River, or both. Mercury minerals in hydrothermal deposits within the Otago Schist appear to be restricted to the Caples Terrane. A gold-mercury association is proposed for the Caples Terrane.

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A case study of the postdepositional alteration of the Witwatersrand Basal Reef gold placer

Cited by 81 publications

References 0 publications

Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions

Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions

The mercury and silver contents of gold in quartz vein deposits, Otago Schist, New Zealand

Au‐Ag‐Hg and Au‐Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer‐source relationships

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