Origin and depositional history of platinum-group minerals in placers – A critical review of facts and fiction

Abstract

The present work reviews assemblages of detrital platinum-group minerals (PGM) in placers worldwide with the aim of identifying common and distinctive features related to their depositional history from primary host rocks via weathering into the placers. Special attention is given to the endogenic destruction of primary PGM and the possible formation of PGM from solution under ambient conditions.The dominant PGMs of economic interest occurring in eluvial and alluvial deposits are Pt-Fe alloys and Os-Ir-Ru alloys. Major current and past producing areas are in Russia (Urals, Koryak-Kamchatka, and Aldan Shield areas), Colombia (Chocó area), Canada (Tulameen area), and the USA (Goodnews Bay). The PGE alloys in all these areas have been sourced from zoned mafic/ultramafic Alaskan-Uralian-type intrusions whereby erosion and weathering of enormous volumes of rock resulted in the concentration of the sparsely disseminated alloys. Subeconomic placers in ophiolite terranes such as in Burma (Chindwin area), Papua New Guinea (Aikora river), and the Philippines (Samar Island) are dominated by Os-Ir-Ru alloys.Minor deposits from the Yubdo area (Ethiopia) and the Freetown Layered Complex (Sierra Leone) have been the subject of many studies to prove or disprove authigenic growth of Pt-Fe alloys and other minerals. To scientifically evaluate the genesis of these minerals, we discuss their mineralogy, silicate and sulfide inclusions, isotopic data, and grain sizes, together with the role of weathering, climate, aqueous geochemistry, and biological activity in their formation. Other deposits and relevant platinum-group element (PGE) mineralization are also examined, including the Bushveld deposits (South Africa), Great Dyke (Zimbabwe), Córrego Bom Sucesso (Brazil), Dominican Republic, Rhine River (Germany), and the Witwatersrand paleoplacers (South Africa). No evidence was found for the growth of distinct PGM larger than about 1–5 µm in the surficial environment.The reactivity of the different PGM varies in relation to their mineral chemistry and depends on local environmental conditions (Eh, pH, temperature, solutes) and is possibly aided by bacterial activity. The order of decreasing stability in the supergene environment is: (1) very stable: Pt–Fe alloys, Os-Ir alloys and Os-rich laurite/erlichmanite → (2) stable: sperrylite → (3) variably stable/“meta-stable”: cooperite/braggite → (4) unstable: PGE-(bismutho-) tellurides and PGE-sulfarsenides.Nano- to micrometer-sized grains or small crystallites of authigenic PGM are different to the more common and significant resistate PGM and consist of native platinum and Pt-Pd alloys with ≈< 5 at. % Fe, or Pt-Fe-Ni-rich alloys, predominantly sited on precursor PGM or locked in small veins or in chromite. Under some conditions, microorganisms may play a role in the destruction of primary PGM but have not been proven to form authigenic PGM.It is concluded that grains or nuggets of Pt-Fe alloys in placers are always allogenic constituents and originate from primary magmatic mineralization, as is also the case for Ru-Os-Ir alloys, laurite-erlichmanite and sperrylite grains. Growth of larger millimeter-size crystals or nuggets has never been observed under ambient conditions and proof of PGM “neoformation” to any significant degree is lacking. The missing link, a look into the cradle of PGM creation in the supergene environment, remains outstanding.