Theories on the transport and deposition of gold and PGM minerals in offshore placers near Goodnews Bay, Alaska

Mardock, C.L.; Barker, James C.

doi:10.1016/0169-1368(91)90023-z

Cited by 13 publications

(6 citation statements)

References 8 publications

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“…Similar features were also observed in and on PGM grains of the Bushveld/Great Dyke placer assemblage [27][28][29][30]. Indeed, the above observations [99] pertain to many occurrences of placer PGM. Surface corrosion features are obvious in many localities, and a certain degree of PGE mobility has been documented by newly-formed nm-to µm-sized crystalline PGM [27][28][29][30][100][101][102] or PGE-biofilms [103] on and in cavities of pre-existing residual PGM grains.…”

Section: Pgm In Placerssupporting

confidence: 75%

“…An examination of gold and PGM (Pt-Fe and Os-Ir alloys) in offshore placers near Goodnews Bay, Alaska [99], revealed textures related to both derivation of PGM grains from mechanically weathered primary ore (i.e., typical assemblages of inclusions of PGE-arsenides, -sulfides and -tellurides; exsolution phenomena), and subsequent accretion (i.e., micro-crystalline assemblages of PGM in grain-rim cavities, suggesting leaching and crystallization).…”

Section: Pgm In Placersmentioning

confidence: 99%

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The Fate of Platinum-Group Minerals in the Exogenic Environment—From Sulfide Ores via Oxidized Ores into Placers: Case Studies Bushveld Complex, South Africa, and Great Dyke, Zimbabwe

Oberthür

2018

Minerals

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Diverse studies were performed in order to investigate the behavior of the platinum-group minerals (PGM) in the weathering cycle in the Bushveld Complex of South Africa and the Great Dyke of Zimbabwe. Samples were obtained underground, from core, in surface outcrops, and from alluvial sediments in rivers draining the intrusions. The investigations applied conventional mineralogical methods (reflected light microscopy) complemented by modern techniques (scanning electron microscopy (SEM), mineral liberation analysis (MLA), electron-probe microanalysis (EPMA), and LA-ICPMS analysis). This review aims at combining the findings to a coherent model also with respect to the debate regarding allogenic versus authigenic origin of placer PGM. In the pristine sulfide ores, the PGE are present as discrete PGM, dominantly PGE-bismuthotellurides, -sulfides, -arsenides, -sulfarsenides, and -alloys, and substantial though variable proportions of Pd and Rh are hosted in pentlandite. Pt–Fe alloys, sperrylite, and most PGE-sulfides survive the weathering of the ores, whereas the base metal sulfides and the (Pt,Pd)-bismuthotellurides are destroyed, and ill-defined (Pt,Pd)-oxides or -hydroxides develop. In addition, elevated contents of Pt and Pd are located in Fe/Mn/Co-oxides/hydroxides and smectites. In the placers, the PGE-sulfides experience further modification, whereas sperrylite largely remains a stable phase, and grains of Pt–Fe alloys and native Pt increase in relative proportion. In the Bushveld/Great Dyke case, the main impact of weathering on the PGM assemblages is destruction of the unstable PGM and PGE-carriers of the pristine ores and of the intermediate products of the oxidized ores. Dissolution and redistribution of PGE is taking place, however, the newly-formed products are thin films, nano-sized particles, small crystallites, or rarely µm-sized grains primarily on substrates of precursor detrital/allogenic PGM grains, and they are of subordinate significance. In the Bushveld/Great Dyke scenario, and in all probability universally, authigenic growth and formation of discrete, larger PGM crystals or nuggets in the supergene environment plays no substantial role, and any proof of PGM “neoformation” in a grand style is missing. The final PGM suite which survived the weathering process en route from sulfide ores via oxidized ores into placers results from the continuous elimination of unstable PGM and the dispersion of soluble PGE. Therefore, the alluvial PGM assemblage represents a PGM rest spectrum of residual, detrital grains.

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Section: Pgm In Placerssupporting

confidence: 75%

Section: Pgm In Placersmentioning

confidence: 99%

The Fate of Platinum-Group Minerals in the Exogenic Environment—From Sulfide Ores via Oxidized Ores into Placers: Case Studies Bushveld Complex, South Africa, and Great Dyke, Zimbabwe

Oberthür

2018

Minerals

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“…The Brazilian example has been critically evaluated by Fleet et al (2002), who concluded that the alluvial PGM most likely originated from detrital grains that crystallized from lowtemperature hydrothermal fluids. In their study of gold and PGM (isoferroplatinum and Os-Ir alloy) in offshore placers near Goodnews Bay, Alaska, Mardock and Barker (1991) found textures related to both derivation of PGM grains from mechanically weathered primary ore (i.e., typical assemblages of inclusions and exsolution phenomena) and subsequent accretion (i.e., microcrystalline assemblages of PGM in grain-rim cavities, suggesting leaching and crystallization). This important observation pertains to many occurrences of placer PGM; therefore, a combination of primary and secondary processes has to be considered to explain textural and chemical attributes of PGM nuggets.…”

Section: Discussionmentioning

confidence: 98%

Alluvial and Eluvial Platinum-Group Minerals From the Bushveld Complex, South Africa

Oberthür

Weiser

Melcher

2014

South African Journal of Geology

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The present work provides an initial description of detrital platinum-group minerals (PGM) collected from alluvial sediments of rivers draining the Bushveld Complex, and from eluvial concentrations at the Onverwacht platiniferous pipe. During a field campaign, sediments were sampled at nine localities around the Bushveld Complex, and heavy mineral assemblages were investigated using optical and scanning electron microscopy (SEM) as well as microprobe analysis (EPMA) of PGM. All concentrates from the alluvial samples contain discrete PGM grains with grain sizes in the range from ~50 to 150 µm (maximum 600 µm). The overall PGM proportions are: native Pt, Pt-Pd and Pt-Fe alloys (together 54%), sperrylite (33%), cooperite/braggite (11%), and stibiopalladinite (2%). This PGM assemblage distinctly contrasts to the suite of PGM in the pristine, sulfide-bearing mineralization in the Merensky, UG-2 and Platreef, the assumed sources of the detrital PGM. Specifically, PGE-bismuthotellurides and -sulfarsenides, common in the primary ores, are missing in the assemblage of detrital PGM in the fluvial environment. Nearly all detrital PGM (98%) are Pt minerals, corroborating earlier findings that Pd-dominated PGM are unstable and are dissolved in the supergene environment, and that PGE-bismuthotellurides and -sulfarsenides, common in the PGM assemblages of the pristine ores are unstable during weathering and mechanical transport.The eluvial material collected at Onverwacht contained ca. 150 PGM grains with sizes mainly in the range 100 to 300 µm range (maximum 1.87 mm). The PGM assemblage comprises grains of Pt-Fe alloys (66%), sperrylite (14%), and many rarer PGM including stibiopalladinite, hollingworthite, laurite, PGE arsenides and PGE sulfides. The suite of eluvial PGM observed is similar to the PGM assemblage described previously from the Onverwacht pipe proper, including the type locality minerals genkinite, irarsite and platarsite, as well as some additional and possibly new PGM. Most of the relatively rare PGM detected in the suite of eluvial grains from Onverwacht were also reported in the detrital PGM assemblage from the Moopetsi river, farm Maandagshoek (Oberthür et al., 2004), indicating that many of the latter grains originate from platiniferous pipes and not from the Merensky or UG-2 reefs.Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, this work also highlights the fact that the nowadays somewhat neglected field methods and basic techniques have their merits and value in mineral exploration, especially if they are combined with modern micro-analytical methods. The systematic recovery of PGM from stream sediments, soils and till should regain wider application in mineral exploration as these tools can provide useful indicators of platinum mineralization.

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“…The Brazilian example has been critically evaluated by Fleet et al (2002), who concluded that the alluvial PGM most likely originated from detrital grains that crystallized from low-temperature hydrothermal fluids. In their study of gold and PGM (isoferroplatinum and Os-Ir alloy) in offshore placers near Goodnews Bay, Alaska, Mardock & Barker (1991) found textures related to both derivation of PGM grains from mechanically weathered primary ore (i.e., typical assemblages of inclusions and exsolution phenomena) and subsequent accretion (i.e., microcrystalline assemblages of PGM in grain-rim cavities, suggesting leaching and crystallization). This important observation pertains to many occurrences of placer PGM; therefore, a combination of primary and secondary processes has to be considered to explain textural and chemical attributes of PGM nuggets.…”

Section: Discussionmentioning

confidence: 99%

Detrital Platinum-Group Minerals in Rivers Draining the Great Dyke, Zimbabwe

Oberthür¹,

Weiser²,

Melcher³

et al. 2013

The Canadian Mineralogist

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The present work focuses on the description of the assemblage of detrital platinum-group minerals (PGM) found in rivers draining the Great Dyke. This PGM assemblage distinctly contrasts with the suite of PGM in the pristine, sulfide-bearing Main Sulfide Zone (MSZ) of the Great Dyke, the assumed source of the detrital PGM. Specifically, PGE-bismuthotellurides and-sulfarsenides, common in the MSZ ores, and PGE-oxides or-hydroxides present in the oxidized MSZ, are missing in the assemblage of detrital PGM in the fluvial environment. Instead, conspicuously high proportions of grains of Pt-Fe alloy are common in the sediments, followed by sperrylite, cooperite, braggite, laurite, rare Pd-Sb-As compounds, and Os-Ir-Ru alloys. Possibly, some of the Pt-Fe alloy grains originate from the MSZ, but the majority appear to represent true neo-formations that formed in the course of weathering of the MSZ ores and concomitant supergene redistribution of the ore elements. Sperrylite, cooperite/braggite, and laurite appear to be direct descendants from the primary MSZ. Rare Pd-dominated minerals (Pd-Hg ± As and Pd-Sb ± As) are considered to be neo-formations that probably formed from dispersed elements during supergene processes. Os-Ir-Ru alloy grains, present in the samples from the Umtebekwe River in the Shurugwi area, possibly originate from the Archean chromitite deposits of the Shurugwi greenstone belt and not from the Great Dyke. Geochemically, the Pt/Pd proportions increase from pristine via oxidized MSZ ores to the fluviatile environment, corroborating earlier findings that Pd is more mobile than Pt and is dispersed in the supergene environment. Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, the work also highlights the fact that simple field methods have their value in mineral exploration, especially if they are combined with modern micro-analytical methods. Furthermore, it is established that the PGE-bismu thotel lurides, PGE-sulfarsenides, and PGE-oxides, dominating the PGM assemblages in the pristine and oxidized MSZ, are components that are unstable during weathering and mechanical transport. Including the genetically somewhat disputed Pt-Fe alloys, the order of decreasing stability in the supergene environment is as follows: (1) Pt-Fe and Os-Ir-Ru alloys (very stable) → (2) sperrylite (stable) → (3) cooperite/braggite (variably stable/"metastable") → (4) PGE-bismuthotellurides, PGE-sulfarsenides, and PGE-oxides (unstable).

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Theories on the transport and deposition of gold and PGM minerals in offshore placers near Goodnews Bay, Alaska

Cited by 13 publications

References 8 publications

The Fate of Platinum-Group Minerals in the Exogenic Environment—From Sulfide Ores via Oxidized Ores into Placers: Case Studies Bushveld Complex, South Africa, and Great Dyke, Zimbabwe

The Fate of Platinum-Group Minerals in the Exogenic Environment—From Sulfide Ores via Oxidized Ores into Placers: Case Studies Bushveld Complex, South Africa, and Great Dyke, Zimbabwe

Alluvial and Eluvial Platinum-Group Minerals From the Bushveld Complex, South Africa

Detrital Platinum-Group Minerals in Rivers Draining the Great Dyke, Zimbabwe

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