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

Abstract

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).

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

“…Large idiomorphic to hypidiomorphic grains are common and often show no or only minor physical attrition (Figures 3A; B; 4E; F), indicating short transport distances and/or chemical inertness in the fluvial environment. Cooperite/braggite [(Pt,Pd)S] grains generally display signs of surface dissolution ( Figure 3E) indicating that they are chemically unstable in the fluvial environment, corroborating the statements of Melcher et al (2005) and Oberthür et al (2004Oberthür et al ( , 2013a.…”

“…Sperrylite grains are relict minerals with multiple possible origins (platiniferous pipes, Merensky and UG-2 reefs), thus indicating that sperrylite is generally stable in the supergene environment. Many of the cooperite/braggite grains show distinct features of external corrosion and partial internal leaching along irregular corrosion channels (Oberthür et al, 2004;2013a;Melcher et al, 2005), indicating that cooperite/braggite grains are progressively disintegrated in the course of prolonged oxidation (i.e. they are "metastable" in the placers).…”

“…Their precursor phases are either preexisting, unstable PGM (note Pt coatings and partial replacements of cooperite/braggite and also of sperrylite), or they may even have formed via a solution stage under low-temperature conditions. The assemblage of detrital PGM in alluvial sediments of rivers draining the Bushveld Complex largely mimics that of the Great Dyke in Zimbabwe (Oberthür et al, 2013a), where Pt and Pt-Fe alloys (42% by number of grains) are most common, followed by sperrylite (46%), cooperite/braggite (7%), and stibiopalladinite (1%). Apparently, the detrital PGM assemblages in rivers draining PGE-bearing layered intrusions (Bushveld, Great Dyke) display far-reaching similarities pointing to analogous starting materials (primary sulfidebearing ores), processes of weathering, transport and deposition.…”

“…Accordingly, nearly all detrital PGM (98%) are Pt minerals, which underlines the fact that Pd-minerals are unstable and are dissolved in the weathering environment (e.g. Oberthür et al, 2004Oberthür et al, , 2013aMelcher et al, 2005). Pt-Fe alloy grains are usually unaltered.…”

Surface transformations of platinum grains from Fifield, New South Wales, Australia

“…Schneiderhöhn and Moritz (1939) described native Pt that was entirely porous from the oxidized zone of the Merensky Reef, Bushveld Complex, Republic of South Africa, that they interpreted as a dissolution feature of a pre-existing mineral, such as sperrylite or cooperite. Native Pt as rims was subsequently observed on both sperrylite (Oberthür et al 2003(Oberthür et al , 2013Melcher et al 2005) and on cooperite (Oberthür 2002;Oberthür et al 2004). Platinum mobility can occur in eluvial, alluvial, and lateritic environments in tropical and possibly also semi-arid to arctic climates (Bowles 1986;Gornostayev et al 1999;Barkov et al 2005;Freyssinet et al 2005).…”