[SRD35-02] Multi-stage mineralization of the sediment-hosted stratabound Zone 5 copper-silver deposit, Kalahari Copperbelt, NW Botswana
The Zone 5 copper-silver deposit is located within a series of linked and inverted Meso-Neoproterozoic volcano-sedimentary basins of the Kalahari Copper belt (KCB) of NW Botswana. Zone 5 deposit contains a resource of 91 Mt at 2.13% Cu and 21.9 g/t Ag and it was discovered in 2013 from a soil anomaly 35 km SE of the former Ghanzi Cu mine within the Khoemacau project of Cupric Canyon. At Khoemacau, Zn and Pb form halos around high copper grades and have been identified as fundamental exploration tools. The economic Cu-Ag mineralization in the Zone 5 deposit concentrated at a stratigraphic contact between oxidized, redbed sandstone and overlying reduced shale, siltstone and sandstone, which acted as a chemical trap for copper-bearing fluids that propagated along the stratigraphy and structures such faults, shear zones, and the fold axial planar cleavage. The Zone 5 ore body is characterized by a vertical sulfide zoned sequence consisting, from bottom to top, of chalcocite, bornite, chalcopyrite, pyrite, sphalerite, and galena. The majority of the ore was introduced synchronously with quartz-calcite veining and hydrothermal alteration. Texturally, the sulfide minerals display replacement, intergrowth (co-existence) as well as inclusion-host relationships. Two main hypogene Cu-Ag mineralization episodes occurred at Zone 5. The earlier episode is characterized by fine disseminations of bedding-parallel, syn-sedimentary cubic and framboidal pyrite that occurs interstitially and occasionally overgrows the lithological layering. Fine-grained cubic pyrite is occasionally replaced by bornite, chalcocite, and chalcopyrite, whereas it also exists as inclusions in copper sulfides. The later (main) episode is syn-orogenic, stratabound mineralization represented by coarse-grained sulfides hosted prevalently in fabric sub-parallel quartz-calcite veins, shear fabrics, and axial planar foliation/cleavage lenticles. The bedding-parallel sulfide-bearing veins may be overprinted by crosscutting veins, indicating multiple veining episodes. Crosscutting sulfide-bearing veins often impregnate ore along bedding and foliation planes, suggesting an association between vein-hosted and disseminated ore parallel to fabric. Disseminated and vein-hosted ore is often stretched, recrystallized and transposed into cleavage lenticles suggesting that the mineralization was contemporaneous with deformation. The earlier-stage fine-grained cubic pyrite (n=1) sample has δ34S value of -25.6‰, whereas the late-stage hypogene sulfides (n=14) from quartz-calcite veins display a large variation of generally depleted δ34S values from -28.1 to +2.9‰, which partly overlap with the δ34S signature displayed by the earlier-stage pyrite. Taking into account the sedimentary environment and lack of significant evaporitic sources, these preliminary δ34S values are consistent with a reservoir of bacterially reduced sulfur. The variability from sample to sample may imply that syn-sedimentary sulfur as reflected by a wide range of very light δ34S values was bacterially reduced and δ34S values closer to 0 ‰ resemble sulfur introduced during the hydrothermal event.