The geology of the Shamrocke Mine, Rhodesia; a stratiform copper deposit

Abstract

The Shamrocke mine is a stratiform copper-iron sulfide deposit in a complexly folded and metamorphosed sedimentary sequence of late Precambrian age. It is the northernmost of several producing copper mines that occur in the Deweras and Lomagundi Groups. The mine is located above the contact between orthogneisses of the Rhodesian Craton and paragneisses of the Zambezi mobile belt.The orebody is confined to a fine-grained calcareous biotite-oligoclase rock that forms discontinuous lenses within a unit of graphitic schist. Sedimentation of the host rock took place in a brackish water, nearshore environment as indicated by the high carbon content, the presence of associated coarse sediments, the local development of carbonate structures, and the rapid variations of sediment type. The sulfides are an early constituent of the sediments as they have been dislocated by transverse faults, deformed by longitudinal shear, affected by thermal metamorphism and by an early period of dynamothermal metamorphism. Silicate mineral assemblages of the country rocks indicate metamorphism at pressures in excess of 5 kilobars and temperatures from 525 degrees to 690 degrees C. The temperature estimates for sulfide mineral formation lie between 150 degrees and 550 degrees C.Chemical analyses of the host rock and graphitic schists indicate that the sulfides represent an addition to the sediments and are not the result of replacement. The Co, Ni, Se, and V contents of pyrrhotite indicate that the deposit is sedimentary-volcanogenic in origin and has undergone thermal metamorphism.The copper mineralization displays a close association with specific sedimentary lithologies, mafic intrusives, and a N-S-trending early Proterozoic lineament and a NE-SW-trending Archean lineament. A hydrothermal-sedimentary model is proposed in which Cu, S, and CO 2 were carried by hydrothermal solutions ascending through the unconsolidated sedimentary sequence and were admixed with meteoric water containing sodium as a brine. It is likely that Ag, Au, Fe, Mo, Ni, and Zn were leached from the unconsolidated sediments by the brine and were concentrated contemporaneously with the copper.