The Tsumeb ore body, Namibia, and related dolostone-hosted base metal ore deposits of Central Africa

Abstract

Abstract provided by author:

Geological, geochemical and isotopic data indicate that the "pseudo-aplite" is a sedimentary rock, and that the Tsumeb pipe and similar structures in central Africa formed during subaerial karsting on a regional scale. These structures were mineralized at approximately 600 m. y., prior to deformation of the enclosing rocks or during the early stages of such deformation. The Tsumeb ore body formed from an ore fluid with a large Cu:Fe ratio, an intermediate oxygen fugacity and a large sulphur fugacity. Vertical zoning of rock alteration in the Tsumeb pipe was caused by the mixing of fluids of different chemical composition, and vertical zoning of minerals of the Cu-Pe-As-S system was controlled mostly by variations in the sulphur fugacity of the ore fluid. The Tsumeb ore body and other ore bodies of central Africa contain sulphur of crustal, probably evaporitic, origin. An evaporite horizon in the upper part of the Tsumeb ore body influenced the sulphur concentration (and sulphur fugacity) of the ore fluid. This controlled the above zoning and was the dominant cause of ore deposition. The unique mineralogical and chemical composition of ore of Tsumeb type resulted from a high concentration of both copper and unusual trace elements in the ore fluids

Lead isotope data indicate a genetic relationship between Tsumeb-type mineralization, Berg Aukas-type mineralization (karst-filling lead-zinc mineralization) and Tschudi-type mineralization (stratiform copper mineralization in sandstone). Thedata are most consistent with a model in which the ore fluids which formed each of the above members of the Owambo basin-type ore association were evolved connate brines, which were expelled from evaporite deposits in a graben to the south of the present Otavi Mountain Land. The ore fluids which formed each type of mineralization migrated through separate aquifers at approximately the same time. The fluids were derived from a large area of the graben and migrated more than 60 km up-dip, penetrating sedimentary sequences more than 1000 m thick, to form mineral districts at least5000 sq. km in area. Metals, including lead (as indicated by the lead isotope data), were added to the fluids en route. The time dependence of mineral districts which contain the Owambo basin-type ore association resulted from time-dependent rifting, continental break-up, and the formation of salt basins (which supplied the ore-forming brines)

The ore bodies were investigated using petrographic, mineralogical and geochemical methods. Some fluid inclusion data were available from previous studies, so stable isotope and radiogenic isotope techniques were selected to provide further information about ore forming processes. A reaction line was constructed for the preparation of sulphur isotope samples, and existing facilities were used for lead, carbon and oxygen isotope studies