HYDROTHERMAL ALTERATION IN THE SHURUGWI SUBCHAMBER OF THE GREAT DYKE, ZIMBABWE: EVIDENCE FROM THE MAIN SULFIDE ZONE AT UNKI MINE

The world famous Great Dyke of Zimbabwe, of Neoarchean age (2.58 Ga), was mostly intruded into Archean granites and greenstones of the Zimbabwean craton. The Great Dyke hosts the second largest reserves of platinum-group element (PGE) deposits in the world. These PGEs are mostly hosted in the Main Sulfide Zone (MSZ), which is a tabular stratabound layer hosted in pyroxenites. A petrographic and silicate composition study across the MSZ at Unki Mine in the Shurugwi Subchamber was conducted to help place some constrains on the origin of the mineralization. The PGE-enriched zone investigated at Unki Mine is a ~10m thick package of rocks which range from gabbronorites, a chromitite stringer, plagioclase websterite, plagioclase pyroxenite (pegmatitic in one narrow zone), a base metal sulfide zone and it is largely located below the contact of the Mafic and Ultramafic Sequences. Pyroxenes have been partially hydrothermally altered to amphibole and chlorite in most lithologies. In addition, sulfides tend to occur as cumulus phases or as inclusions in all the silicate phases encountered. Two generations of sulfide mineralization likely occurred at Unki Mine with primary sulfides occurring in association with cumulus phases, and the relatively finer-grained, often lath-like, sulfides that occur in association with alteration phases of chlorite and amphibole that were likely formed later during hydrothermal alteration. Chlorite thermometry yields temperatures ranging from 241 - 390^oC, and from 491 - 640^oC, and they are interpreted to be temperatures that record the hydrothermal event(s) of magmatic origin which affected the mineralization at Unki Mine. Two-pyroxene thermometry yields temperatures that range from 850-981^oC, and these temperatures are interpreted to indicate a hydrothermal imprint on the minerals that constitute the MSZ at Unki Mine.