Abstract

Matchless is one of several metamorphosed massive copper-zinc sulfide deposits associated with the Matchless amphibolite belt in the upper Proterozoic Damara orogen (SWA/Namibia). In the footwall of the ore shoots are pyrite-bearing quartz-muscovite-(chlorite) schists with interlayered bands of massive pyrite-quartz rock and a distinctive biotite-rich marker schist. In the hanging wall of the ore shoots the wall rock consists of chloritic schists containing variable proportions of quartz, ankerite, and calcic amphibole, bands of magnetite quartzite, calc-silicate rock, and amphibolite. This rock association is regarded as representing a premetamorphic stratigraphic sequence.The initial deposit was formed in a subsiding trough in which active turbidite sedimentation formed the precursor rocks of the Kuiseb schists. Sedimentation was interrupted by submarine volcanic activity which produced the basaltic protolith of the Matchless amphibolite. Metal sulfides were precipitated from convecting hydrothermal fluids generated by the reaction of seawater with subsea-floor basalts and clastic sediments. This process was accompanied by an intense hydrothermal alteration of the basaltic wall rock, leading to silicification, chloritization, sulfidization, and in part also, to carbonatization. Geochemical changes in the basalts involved depletion in Na, Ca, and Sr and enrichment of Rb, Ba, Fe, S, and the base metals Cu, Zn, and Pb. Among the immobile elements, Y, and to a lesser extent, Ti, seems to be depleted. Compared to the unaffected turbiditic sediments, the altered sedimentary wall rocks of the deposit are depleted in Na, Ca, Sr, Rb, Ni, and the immobile elements Ce, Zr, Ti, V, and Y and enriched in Fe, S, Ba, Cu, and locally, Zn and Pb. These changes can be explained by wall rock-fluid interaction and/or by synsedimentary chemical precipitation of metal sulfides on the sea floor.

First Page Preview

First page PDF preview
You do not currently have access to this article.