Platinum genesis; new mineralogical evidence

Abstract

The two distinct types of platinum deposits in the Bushveld Complex (reefs and pipes) differ in relative age, geological position, and mineralogy. Reefs are characterized by the association of braggite, cooperite, laurite, and sperrylite with platinum-iron alloy and minor Pt-Pd bismuthotellurides and pipes by platinum-iron alloy, sperrylite, geversite, and Rh-Ir sulfarsenides. There are distinct regional variations in platinum mineralogy within the Merensky Reef; these are, however, largely confined to the relative proportions of the constituents. The pipes differ in the actual minerals present.The Sudbury and Stillwater deposits show a prevailingly higher content in compounds of Pd, Pt, Bi, Te, and S. This can be explained by the general tendency of Pd to associate preferentially with sulfides in the earlier stages and to form independent platinum-group minerals in the later stages of the magmatic cycle. Serpentinization plays an active role not only in the conversion of silicate nickel into sulfide nickel, but also in the extraction of platinum-group elements from silicates, oxides, and sulfides, and in their localization as independent and economically significant platinum-group minerals. Hydrothermal copper-platinum ores not spatially related to basic igneous rocks emerge as a separate genetic type.Investigation of placer deposits from worldwide localities shows that (1) placers linked to alpine-type ultrabasic massifs carry significant amounts of Pt as well as iridosmine and osmiridium; (2) inclusions in native alloys and sperrylite may provide information on the mineralogy of primary deposits--michenerite and hollingworthite in Urals placers are a case in point; and (3) alteration of platinum-group element alloys in placers is a widespread process which takes place at surface temperatures. Placers derived from Neogene ultrabasics in Japan carry textural types similar to those found in Precambrian Witwatersrand platinum-group mineral concentrates.The formation of platinum deposits is dominated by polyphase processes which also include transport of platinum-group elements in aqueous solutions.