PETROGRAPHIC DESCRIPTIONS OF SAMPLES FROM THE MAIN SULFIDE ZONE, DARWENDALE SUBCHAMBER OF THE GREAT DYKE, ZIMBABWE

The mineralogy and textures of samples from Drill Hole UMH 274 that intersected the Main Sulfide Zone (MSZ) in the Ngezi area of the Darwendale Subchamber of the Great Dyke are here described and discussed. Cumulus ortho- and clinopyroxene are the major phases, with plagioclase and sulfides being the minor and postcumulus common minerals. Clinopyroxene and sulfides commonly occur as intercumulus minerals, and less commonly are cumulus minerals. In addition, biotite also occurs as a minor phase in some thin sections. Chlorite, however, occurs in trace concentrations in a few thin sections. The hydrous phases of biotite, chlorite and amphibole are encountered in samples that have been hydrothermally altered. Sulfides typically occur in association with intercumulus phases such as clinopyroxene, plagioclase and quartz. However, in hydrothermally altered samples, sulfides are commonly associated with biotite, chlorite and amphibole. In a few samples with veinlets cutting across the minerals, biotite, chlorite and amphibole are also concentrated along these veinlets. Some irregular globules (discrete sulfide mineral aggregates in magmatic sulfide ores) which were observed and possess partially facetted margins may represent entrainment of previously segregated and sulfides that are partially solidified. Equilibrium dihedral angles of samples under study were >60°, with the walls of the pore being concave into the channel which correspond to pyroxene adcumulates in pyroxenites under study. The occurrence of sulfides as both cumulus and intercumulus phases suggests that sulfides formed during the entire crystallization history of the Great Dyke, indicating a magmatic origin for the sulfide mineralization. Further, the occurrence of some sulfides in association with hydrous phases also suggests that at least some of the sulfide mineralization in the Ngezi Platinum Mines area of the Darwendale Subchamber occurred during a later hydrothermal alteration event likely concurrent with biotite, amphibole, and chlorite alteration. Mineral composition work underway will hopefully help constrain the nature and temperature of this alteration.