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

Thomas Oberthür

doi:10.3390/min8120581

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.

Highlights

platinum-group elements (PGE) is taking place, the newly-formed products are thin films, nano-sized particles, smallThe world’s prime sources of platinum-group (PGE)are layered intrusions of Proterozoic crystallites, or rarely μm-sized grains primarily onelements substrates of precursor detrital/allogenicplatinum-group minerals (PGM) and Archean age (e.g., Bushveld Complex, SouthAfrica; the Great Dyke, Zimbabwe; the Stillwater grains, and they are ofthe subordinate significance.In the Bushveld/GreatDyke scenario, and in all Complex, USA).Placer deposits in Russia and in Colombia were the world’s only providers probability universally, authigenic growth and formation of discrete, larger PGM crystals or nuggets of platinum-group up plays to early in the 20throle, century, when the depositsinof in the supergene elements environment no substantial and any proof of richPGMprimary “neoformation” a the BushveldComplex in Africa, of the Sudbury district in Canada, and later
The PGE are bimodally distributed; whereas Pt is dominantly present in the form of discrete PGM like (Pt,Pd)-bismuthotellurides, PGE-sulfides, sperrylite, and Pt–Fe alloys, large though variable proportions of the Pd and Rh are hosted in pentlandite
The behavior of the platinum-group minerals (PGM) in the weathering cycle was studied in the Bushveld Complex, South Africa (Merensky Reef, UG-2 chromitite, Platreef) and the Great Dyke, Zimbabwe (Main Sulfide Zone)

Summary

Introduction

PGE is taking place, the newly-formed products are thin films, nano-sized particles, smallThe world’s prime sources of platinum-group (PGE)are layered intrusions of Proterozoic crystallites, or rarely μm-sized grains primarily onelements substrates of precursor detrital/allogenicPGM and Archean age (e.g., Bushveld Complex, SouthAfrica; the Great Dyke, Zimbabwe; the Stillwater grains, and they are ofthe subordinate significance.In the Bushveld/GreatDyke scenario, and in all Complex, USA).Placer deposits in Russia and in Colombia were the world’s only providers probability universally, authigenic growth and formation of discrete, larger PGM crystals or nuggets of platinum-group up plays to early in the 20throle, century, when the depositsinof in the supergene elements environment no substantial and any proof of richPGMprimary “neoformation” a the BushveldComplex in Africa, of the Sudbury district in Canada, and later. The world’s prime sources of platinum-group (PGE). Placer deposits in Russia and in Colombia were the world’s only providers probability universally, authigenic growth and formation of discrete, larger PGM crystals or nuggets of platinum-group up plays to early in the 20throle, century, when the depositsinof in the supergene elements environment no substantial and any proof of rich. In alluvial gravels on the farm Maandagshoek, The world’s prime sources of platinum-group elements (PGE) are layered intrusions of Proterozoic eastern. Placer deposits in Russia in is Colombia werecountry the world’s onlyand providers of to the world, entirely from the Complex, followed by Russia’s.

Objectives

Methods

Results

Discussion

Conclusion