The sediment-hosted Ngwako Pan copper deposit, Botswana

Abstract

The strat-bound Ngwako Pan copper deposit is located in the northeastern part of the 250-km-long Neoproterozoic Ghanzi Ridge in northwestern Botswana. The sulfide mineralization occurs in the lowermost 25 m of the Middle Ghanzi Formation, which is composed of reduced facies (gray and greenish-gray) siliciclastic and subordinate carbonate metasediments. The 1- to 3.5-km-thick red-bed metasediments of the Lower Ghanzi Formation underlie the mineralized horizon. The distribution of copper ore in the Middle Ghanzi Formation shows a close spatial correlation with the occurrence of basaltic rocks in the Kgwebe Formation underlying the Lower Ghanzi Formation. The mineralized horizon is hosted by lacustrine or lagoonal sediments that were deposited on a paleohigh in a rift-related basin. The sediments were metamorphosed at lower greenschist facies conditions and tightly folded. The homogenization temperatures of fluid inclusions in secondary quartz are in the range of 57 degrees to 254 degrees C, the median being 179 degrees C. The major sulfide minerals, in order of decreasing abundance, are chalcocite- djurleite, bornite, chalcopyrite, pyrite, sphalerite, and galena. They occur as disseminations related to the intergranular porosity and permeability of the sediment and those related to fracture or shear porosity and permeability. Various load features indicate that at least part of the disseminated mineralization or its precursor sulfide minerals was deposited before major compaction was completed. Sulfurization of the sediment and possibly introduction of copper occurred early in the diagenetic history of the sediment. The mineralizing fluids were probably derived from oxidized ground water passing through red-bed footwall sediments. These fluids were enriched in copper where the aquifer was copper rich, i.e., where high Cu basaltic rocks constitute part of the source region of the siliciclastic sediments.