THE GEOLOGICAL SETTING AND STYLE OF COPPER MINERALIZATION AT THE BUSHMAN GROUP OF DEPOSITS, NORTHEASTERN BOTSWANA

The Bushman group of Cu-Pb-Zn deposits occur within the north-northeasterly trending Bushman shear zone in northeastern Botswana. The deposits are hosted by a sliver of sheared and altered, carbonate rocks and chloritic and graphitic schists that resemble and are presumed correlative with metasedimentary rocks of the Matsitama schist belt to the east. This sliver is surrounded by sheared and altered granitic gneisses. Within the sliver, the mineralized zones are a chalcopyrite and pyrite bearing quartz-calcitetalkali feldspar vein breccias near the sheared contact between the carbonate rocks and the chloritic and graphitic schists. Elsewhere within the granitic gneisses, those zones are marked by barren quartz breccias surrounded by a zone dominated by red, turbid orthoclase in turn surrounded by a zone dominated by green epidote. Initially hydrothermal alteration propylitized the host rocks and then porphyroblasts of orthoclase, albite and quartz grew with the introduction of calcite. Away from the mineralized zones, four generations of veins occur, the first two directly traceable to Cu-Pb-Zn mineralization and the third carrying galena and sphalerite. Isotopic data suggest that the protolith of the granitic gneisses was emplaced sometime between about 2.57 Ga and 2.3 Ga ago. After this protolith was deformed and metamorphosed, Cu-Pb-Zn mineralization occurred about 1926 Ma ago from fluids at a temperature of about 360 degrees C. The Pb isotopic compositions of the sulfide minerals indicate that the fluids giving rise to the Cu-Fb-Zn mineralization were possibly the same as those giving rise to Pb and Zn mineralization. In both instances, however, the Pb came from several sources. More recently shear zones parallel to the Bushman shear zone may have controlled in part the emplacement of kimberlite pipes.