Although small quantities of PGMs occur in placer deposits, particularly in the Ural Mountains in Russia, the major commercially mined deposits have nickel-copper sulphides associated with igneous mafic and ultramafic rocks of volcanic origin which are rich in magnesium and iron. The primary silicate mineralogy is feldspathic pyroxenite containing olivine, amphibole, and norite/anorthosite. Base metal mineralogy is dominated by pentlandite (Ni, Fe) S, chalcopyrite (CuFeS2), and pyrrhotite (FeS). The main deposits are in South Africa (Merensky Reef, UG2, and Platreef), Zimbabwe (Great Dyke), North America (Stillwater, Montana, USA and Sudbury, Ontario, Canada) and Russia (Norilsk-Talnakh complex). Around 60% of world primary PGMs production, including 75% of global primary platinum output, comes from South Africa; Russia produces nearly 25% of new PGMs, with palladium predominant.
The Southern African deposits generally contain a higher proportion of platinum than palladium; typically, between 1.4:1 and 2.2:1.
PGMs are the primary economic drivers, with relatively small co-production of copper and nickel. Conversely, the Canadian and Russian deposits are major sources of nickel and copper with comparatively small quantities of PGMs and reversed platinum to palladium ratios typically 1:3 or 1:4. The Stillwater ores have similar PGM ratios to the other northern hemisphere deposits, but far lower base metal content.
These variances in metal ratios can make substantial differences to refining process strategies and economics. Locally Zimbabwe’s Great Dyke has long been recognized as a significant source of primary PGMs. The great Dyke is an igneous intrusion which extends about 550km and a maximum width of just 11km through central Zimbabwe. PGMs occur in a layer known as the Main Sulfide Zone, which is typically about 5mm thick. Platinum-group mineralization occurs within a 4m vertical interval within the stratiform deposit.
The sulphide ore is oxidized at the outcrop but remains un-weathered at depths of 20m or more. The width of the ore zone can be as little as 1m, depending upon the geology, grade, metal prices., and the chosen mining method (HATTON*, n.d.)