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

Abstract

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.

Platinum-Group Minerals of Pt-Placer Deposits Associated with the Svetloborsky Ural-Alaskan Type Massif, Middle Urals, Russia

“…The predominance of Pt-Fe alloys with inclusions of Os-Ir-Ru composition, as well as PGE sulfides (laurite-erlichmanite and kashinite-bowieite) in the studied objects is typical for the PGM assemblages from the placers and bedrocks of Ural-Alaskan type massifs in different regions of the world [18][19][20][21][22][23][24][28][29][30][31][32][33][34][35]. The presence of Pt-Ir-Rh thiospinels among the inclusions in Pt-Fe minerals from the placer assemblages is considered as a typomorphic feature of platinum placer objects associated with the Ural-Alaskan type massifs [36].…”

“…4, 5, 8-10) and Table 5 for kashinite (No. 24,25). Isf-isoferroplatinum, Erl-erlichmanite, Os-osmium, CuIrst-cuproiridsite, FeRhst-ferrorhodsite, and CuRhst-cuprorhodsite.…”

“…Furthermore, the changes in the platinum-group minerals occurring during their transport from a lode source to a placer were not studied in detail. Despite the significant amount of work devoted to the Koryak-Kamchatka region, as well as the Bushveld complex and the Great Dyke (South Africa), where the formation of the platinum placers [18][19][20][21][22][23][24] was investigated in detail, similar studies of the Ural Platinum Belt placers have not been conducted yet.…”

“…Isf-isoferroplatinum, Os-osmium, and Chr-chromite.Osmium composition varies considerably (Table 3). Like most Os-Ir-Ru inclusions in Pt-Fe alloy grains from placers associated with the Ural-Alaskan type massifs and their lode sources[6][7][8][9][10][11][12][13][14][18][19][20][21][22][23][24][26][27][28][29][30][31][32][33], Ru is low in the Os-Ir-Ru solid solutions(Figure 9), and its concentration does not exceed 15 at. %.…”

“…The composition of laurite-erlichmanite(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) and kashinite-bowieite(20)(21)(22)(23)(24)(25).PGM from the chromite-platinum ore zones of the Svetloborsky massif (No. 1-4; 20-23), the Vershinniy eluvial placer (No.…”