Platinum-Group Element Abundances in Pyrite from the Main Sulfide Zone, Great Dyke, Zimbabwe

Abstract

The Main Sulfide Zone (MSZ) of the Great Dyke of Zimbabwe hosts the world’s second largest reserve of platinum-group elements (PGE). It comprises a sulfide assemblage made up of pyrrhotite, pentlandite, chalcopyrite and minor pyrite. Several studies have highlighted that pyrite can be an important carrier of PGE and therefore, we have measured PGE and other trace element abundances in pyrite of the MSZ from the Hartley, Ngezi, Unki and Mimosa mines by LA-ICP-MS. Pyrite occurs as individual euhedral or subhedral grains or clusters of crystals mostly within chalcopyrite and pentlandite (occasionally in form of symplectitic intergrowths) and is generally absent within pyrrhotite. At Hartley and Ngezi, pyrite contains higher Os, Ir, Ru, Rh and Pt contents than co-existing pyrrhotite, pentlandite and chalcopyrite from the same sulfide aggregate. In contrast, at Mimosa and Unki, PGE values in pyrite are low and similar to coexisting sulfides. Pentlandite is always the main sulfide carrying Pd. Although the origin of pyrite in this type of mineralization is commonly attributed to the activity of hydrothermal fluids, we suggest that pyrite may have formed by late, low temperature (< 300ºC) decomposition of residual Ni-rich mss.