Aspects of the geology of the mountain ore body, Rosh Pinah Mine, Namibia

Abstract

Abstract by author:

The ore horizon pinches and swells to form discrete ore bodies regarded as primary features reflecting palaeobasin control. Extensive structural control is also apparent. D1 produced layer-parallel foliation coupled with extensive stratigraphic duplication a result of south easterly directed thrusting. D2 produced tight, west-vergent folds; a result of backfolding associated with continued compression from the north west

The ore horizon consists of black, banded, sulphidic cherts overlain by carbonaceous dolomites containing sphalerite, pyrite and galena. Areas are sporadically overlain by massive barite, sulphide or argillite. The underlying breccias consist of angular, heterolithic quartzite blocks hosted in a barium carbonate-rich matrix

Deposition of sulphide and sulphate material occurred as part of an active shallow water hydrothermal system. Mineralization is restricted to synsedimentary hydrothermal events such as silicification, brecciation and sulphidization. Initial mineralization consisted of extensive silicification and Cu introduction. Silica precipitation resulted in the formation of an impermeable self-sealed caprock trapping subsequent hydrothermal solutions. Pressure and temperature build-up resulted in a higher temperature system undergoing hydraulic fracture and brecciating the footwall. Stockwork filling precipitated consists of Mn-rich dolomite with sulphides. A later stage consisted of Ba-Cu-Ag enriched fluids producing rare crosscutting "flow-banded" and Ba-rich veins. Syngenetic Ba-rich ore was deposted over the dolomite ore at this stage. Economic sulphide deposition terminates with the resumption of clastic deposition

Post-lithification reconstitution of ore zone sulphides began during D1. Thrusts developed in the footwall produced extensive structurally-controlled zones of brittle fracture. These zones are concentrated in areas of greatest silicification. Reprecipitation of sulphides, barytes and dolomite into the breccia matrix resulted in coarse veins of chalcopyrite and barite. D2 folding and metamorphism associated with CO2-rich fluids produced pronounced open-system alteration with much of the barite being altered to barium-carbonates. The upper parts of some ore bodies were later affected by supergene alteration