Physicochemical conditions of platinoid concentration in stratified ultrabasic-basic massifs

Deposits of low-sulfide ores in stratified ultrabasic-basic intrusions are the main sources of platinum metals: They contain more than 90% of all plat inoid resources. Platinum-metal minerals (PMMs) are concentrated in thin la yers (reefs), which are divided, by the distribution character of platinum group elements (PGE) and sulfur (sulfides), into combined (concordant) and noncombined (discordant) ones. This paper is the first to consider their na ture. The authors carry out a thermodynamic analysis of the temperature and P-O2 effects not only on chromite concentration and sulfidization and ferritizat ion of silicate melts but also on the mechanism of mobilization and concent ration of PGE, leading to formation of platiniferous reefs, and of gold and silver. The physicochemical conditions of PGE concentration are illustrate d by constitution diagrams. At the low values of P-O2, Which are responsible for the stability of a nat ive-metal (lead, copper, iron, nickel, etc.) phase along with early cumulus phases of rock minerals and ores, concentration of noble metals follows ph ysicochemical regularities, which are typical, in particular, of pyrometall urgic processes. These regularities are responsible for the formation of pl atinum-metal reefs in zones of sulfide and chromite metallization. The increase in the redox potential of a system during the formation of str atified ultrabasic-basic massifs is caused by depolymerization processes in silicate melts, which lead to an increase in the activity of bridge, nonbr idge, and free oxygen ions. This activates isolation of a ferrite phase con sisting of magnetite and titanomagnetite in iron-bearing silicate melt syst ems. Since sulfide and native-metal phases are missing in these ferrite acc umulations, no concentration of noble metals takes place in these zones.