Geology and paragenesis of the Boseto copper deposits, Kalahari Copperbelt, northwest Botswana

Abstract

Detailed lithostratigraphic, structural, and petrographic studies coupled with fluid inclusion and stable isotopic analyses and geochronological studies indicate that the Boseto copper deposits formed initially during diagenesis as metalliferous brines ascended along basin faults and moved along a stratigraphic redox boundary between continental red beds and an overlying reduced marine siliciclastic sequence. The hanging wall rocks to copper-silver ore zones comprise comprises a series of at least three stacked coarsening upwards cycles deposited in a deltaic depositional setting. Early copper mineralization may have been accompanied by regionally extensive albitization. Later multiple pulses of faulting and hydrothermal fluid flow associated with a southeast-vergent folding event in the Ghanzi-Chobe belt resulted in extensive networks of bedding-parallel and discordant quartz-carbonate-(Cu-Fe-sulfide) veins. This contractional deformation-related vein and shear system was responsible for significant remobilization of pre-existing vertically and laterally zoned copper sulfide minerals into high-grade zones by hot (250-300 degrees C), syn-orogenic, metamorphic-derived hydrothermal fluids. Orientation analysis indicates that the mineralized veins probably formed in association with a flexural slip folding processes. Mineralized vein systems display intense carbonate-chlorite-Cu-Fe-sulfide replacement of wall rock slivers within veins and clasts within shear zones, potassic alteration of the surrounding wall rock, and significant remobilization of early diagenetic disseminated copper sulfide minerals. Sulfur isotopic analyses indicate copper sulfides were probably both mechanically and chemically remobilized.