Late-magmatic calcite-pyrrhotite-chalcopyrite mineralisation associated with magmatically-evolved diorite, Marble Hall, South Africa

Abstract

Calcite-pyrrhotite-chalcopyrite veins are pervasively developed in a diorite intrusion (~2.06 Ga), associated calc-silicate hybrid rock, and carbonate country rock in the direct vicinity of Marble Hall, Mpumalanga Province, South Africa. The diorite intrusion is probably sill-like in shape and related to a series of sills that formed a ~500m thick package within the impure siliceous carbonate rocks of the Malmani Subgroup, Transvaal Supergroup. The intrusion comprises a hybrid rock at the base, followed upward by cumulate-layered diorite and a capping of diorite pegmatite. Fluid build-up due to crystal differentiation and fractionation of mainly amphibole and plagioclase gave rise to the development of H2S-bearing hydrous fluids that permeated and reacted with the dolomitic country rocks, thereby releasing CO2 into the fluids. Scavenging of aluminium, titanium, iron, calcium, sodium, potassium and zirconium (rare) from the early-formed minerals took place as the fluids channelled into contraction cracks and fissures. Early-formed magmatic and metamorphic (in diorite-related hybrid rocks) amphibole, andesine, magnetite and ilmenite were transformed into secondary assemblages containing actinolite-ferroactinolite, albite, chlorite, calcite, apatite, sphene, zircon (rare), pyrrhotite, chalcopyrite and pyrite. With increasing degree of alteration, magmatic-textured mesocratic diorite went to bleached diorite, allotriomorphic-textured diorite and finally to granoblastic albite-rich diorite. Final crystallisation of the fluids gave rise to aggregates of calcite, pyrrhotite, rare chalcopyrite and sphene. The existence at Marble Hall of coincident gravity and magnetic anomalies, nickeliferous ultrabasic rocks, the presence of sulphur (manifested by the calcite-pyrrhotite veins), and the apparent overprint of a high temperature metamorphic-metasomatic event , collectively enhance the area’s prospectivity for base metal deposits.