ORE MINERALOGY OF PGM PLACERS IN SIBERIA AND THE RUSSIAN FAR EAST

Abstract

Platinum-group minerals (PGM) placers of Siberia and the Russian Far East are subdivided into three associations on the basis of mineralogical and geochemical features. The first is a ruthenium–iridium–osmium ophiolitic association from Zolotaya, Suyenga, and Balyksa river placers (A-type), restricted to ultramafic rocks of the ophiolitic belts. Os–Ir–Ru alloys are dominant over Pt–Fe nuggets. Two magmatic parageneses of source rocks are typical in A-type association: early osmium–iridium, and isoferroplatinum–ruthenium, which forms after the Os–Ir–Ru system is saturated with Ru. In early paragenesis, the distribution coefficient of Ru between osmium and iridium (KDRuhex/cube) is 1.35–1.97, with the composition plots of Os–Ir–Ru alloys on an Os–Ru–Ir(+Pt) diagram forming a ruthenium trend The second association is ruthenium–iridium–osmium (B-type), the source of which has not been located, although B-type materials are found along the Ko, Tayenza rivers. In this assemblage, Os–Ir–Ru alloys are dominant over other platinum-group element (PGE) minerals and their compositions vary from Os to Ir on the Os–Ru–Ir(+Pt) diagram with no ruthenium trend. Os–Ir–Ru alloys are characterized by gradual and contrast zoning in composition from osmium in the centre to ruthenium or iridium in the rim. The KDRuhex/cube is less than 1 with an average 0.4. Isoferroplatinum occurs as discrete grains and inclusions in these Os–Ir–Ru alloys. A third association is that of isoferroplatinum and is restricted to the Alaskan-type Inagli, Gal'moenan, Philippa massifs and occurs in the Kuznetsk Alatau along the Kaura, Kaurchak rivers placers. Pt–Fe solid solution, mostly Pt3Fe, is dominant in these placers. Two parageneses are typical in source rocks: an early isoferroplatinum–osmium parageneses and later isoferroplatinum–iridium parageneses. The second one is the result of exsolution structure development from primary Pt–Ir–Ru–Os solid solutions, which formed as the ore-forming system saturated with Ir. Plots of osmium composition form an osmium trend on the Os–Ru–Ir(+Pt) diagram. In the all associations, products of alteration and replacement of primary magmatic minerals caused by late-magmatic S- and As-enriched residual fluids, or by late serpentinization, are observed. The main secondary minerals comprise PGE-sulphoarsenides and laurite in the Os–It–Ru alloys, and cooperite and sperrylite in the Pt–Fe alloys.