Alteration and metasomatism of the UG2 melanorite and its stratiform pegmatoids, Bushveld Complex, South Africa -characteristics, timing and origins.

Abstract

The UG2 chromitite at Two Rivers Platinum Mine (eastern limb, Bushveld Complex) is fully enclosed within a UG2 melanorite that is here divided into five sub-units, all of which crystallised from the same parental magma. From top to bottom, these sub-units are the upper distal, upper stratiform pegmatoid, upper split, lower split and lower stratiform pegmatoid. The upper distal and lower split sub-units exhibit the most primary characteristics whereas hydrothermal fluids and/or hydrous melts infiltrated the remaining three sub-units, as well as the UG2 chromitite. Within the stratiform pegmatoid sub-units, network veins of amoeboidal troctolite, high abundances of secondary silicates, enrichment in incompatible elements and the coarsening of grain sizes indicate this infiltration of fluids and/or melts. The upper split sub-unit is chromite-rich and locally altered to amphibolite. This fluid- and melt-induced alteration/metasomatism occurred at high temperatures and is pervasive along at least 7 km of strike length, suggesting magmatic origins. These magmatic fluids are probably related to the compaction of underlying cumulates or the intrusion of chromite crystal slurries.