The copper-bearing breccia pipes of the Messina district South Africa

Abstract

The Messina Copper Mines comprise a group of breccia pipes, disseminated replacement and fissure deposits within an area 20 km long by 1 km wide, emplaced within high-grade metamorphic rocks of the Limpopo mobile belt. Breccia pipes as well as associated hydrothermal copper deposits are aligned along a northeast trend which forms a southwestern projection of the Nuanetsi Igneous Province. The "L" and West lode breccia columns are known over a vertical extent of 1 250 metres, and are not exposed on surface; they are circular to polygonal in plan outline, as a result of joint controls. Peripheral microbreccia grades downwards and inwards to macrobreccia. Downward displacement of country rock marker zones indicate a maximum 1 – 3 per cent volume increase, while observed interfragmental fill constitutes 15 – 25 per cent. Hydrothermal alteration of wallrock gneiss mantles the breccia columns and does not extend to surface; it is comprised of distinct zones, the outermost characterized by sericitized gneiss, grading to an albitite, and finally to zoisite-quartz rock around the immediate brecciated contacts, and within the pipes. The interfragmental fill consists largely of quartz and sulphides, and grades downwards to albite + sulphides in the West Lode pipe. Sulphide mineral distribution is zoned, pyrite dominating in the apex of the pipes, grading downwards to chalcopyrite-bornite and then bornite-chalcocite at deeper levels. The nature of the fragmentation precludes normal stoping collapse mechanisms for brecciation and suggests a single implosive event. Chemical reaction induced shrinkage of fragments probably created the bulk of the interfragmental volume.