Mineralizing fluids of the supergene-enriched Mashitu South Cu-Co deposit, Katanga Copperbelt, DRC

Abstract

The sediment-hosted Mashitu South (Sud) Cu-Co deposit situated in the Katangan Copperbelt (DR Congo) hosts hypogene and supergene mineralization in Neoproterozoic age rocks of the Mines and Red R.A.T. Subgroups of the Neoproterozoic Katangan Supergroup. Detailed and systematic petrographic, fluid inclusion and stable isotope studies of minerals from various generations of stratabound metalliferous veins, crackle breccia, and alteration zones placed significant constraints on the intricate fluid dynamics, compositions as well as origins of ore-forming and ore-upgrading fluid systems at the Mashitu South deposit. Our data document large-scale migration of metalliferous (Cu, ±Co, Zn) high-salinity brines, sulfate-rich fluids and meteoric fluids, which can be regionally followed across the Katangan Copperbelt through deposits tens of kilometers apart from each other.The stratabound, hypogene, hydrothermal Cu-Co ore veins precipitated at high temperatures (∼247–316 °C) from mixed high-salinity Mg-rich/Na-K-Ca-Mg-Cl brines and Ba-K-Na-Ca ± Cl-SO42− fluids and locally operating thermochemical sulfate reduction-TSR as indicated by negative δ13CV-PDB ratios (down to −7.4‰). A number of these veins were subjected to dynamic recrystallization, under the influence of low salinity/meteoric fluids, during active deformation at the onset of the Lufilian Orogeny. Isotopic compositions of dolomites from veins containing spectacularly large carrollite, chalcopyrite, bornite and chalcocite crystals from the Kamoya South II deposit overlap with isotope data from hydrothermal veins from the Mashitu South deposit suggesting a similar nature of the parent fluids. Metalliferous (Cu, Zn) Na-K-Cl brines, which provided metals for giant crystals of carrollite in post-tectonic ore veins from the Kamoya South II deposit originated from seawater evaporation. Besides fluctuations of the water table the brecciation, alteration and dissolution/re-precipitation processes in the Red R.A.T. and the R.S.C. units were essential for deposition of supergene ores at Mashitu South. The quartz-specular hematite-dolomite veins sealing the crackle breccia, which resulted from hydraulic fracturing, precipitated from Na-Ca-Cl-SO42− fluids gaining salinity (<12.7 mass% NaCl equiv.) and compositions through pervasive interaction of low salinity meteoric fluids with formation waters stored in hematitic siliciclastic rocks of the Red R.A.T. Subgroup. Primary mono-phase liquid aqueous inclusions and C-O isotopic signatures of free-growing supergene minerals record T < 50 °C and precipitation from predominantly low salinity freshwaters, with local pulses of saline fluids during the formation of decoupled supergene Cu and Co mineralization.