Metal sources and mineralizing fluids characteristics and evolution of the Khoemacau sediment-hosted Cu-Ag deposits, in the Ghanzi-Chobe Belt portion of the Kalahari Copper Belt

Abstract

Khoemacau Mining (KCM) Cu-Ag deposits in the Ghanzi-Chobe Belt (GBC) portion of the Kalahari Cu Belt are hosted in the Neoproterozoic sedimentary succession of the Ghanzi Group. These deposits are characterized by two styles of mineralization including disseminated and structurally-controlled mineralization. A detailed study that combined whole rocks and sulfides Pb isotope characteristics, carbonate rare earth element (REE) analysis, and fluid inclusion examination was carried out in order to constrain the source(s) of Cu amd Ag, characterize the mineralizing fluids and then unravel the processes that converged and led to the genesis of the Khoemacau sediment-hosted Cu-Ag deposits. The Pb isotope compositions of both the sulfides and whole rocks overlap and display a heterogeneous character. Whereas, sulfides yielded (206Pb/204Pb)t of 17.204–67.717, (207Pb/204Pb)t ranging from 15.576 to 19.025 and (208Pb/204Pb)t between 37.026 and 45.911, whole rocks are characterized by (206Pb/204Pb)0 = 16.284–20.897, (207Pb/204Pb)0 = 15.273–15.869 and (208Pb/204Pb)0 = 36.128–42.537. Previous studies regarded the underlying basalts of the Kgwebe Formation as the main metal source; however, our results rule out the possibility of the rhyolites of the Kgwebe Formation as the source of the metals. On the other hand, clastic rocks of the Ngwako Pan Formation (red beds) that underlie the mineralized unit and their precursor felsic basement rocks are likely the main Cu-Ag source rocks for the KCM deposits, with a possible minor contribution from the dolerite of the Kgwebe Formation. Furthermore, the carbonates from the KCM Cu-Ag deposits are characterized by significant variation in their ƩREE contents (26.4–328.5 ppm) and extreme diversity in REE patterns. These results indicate that both the metals of interest (Cu and Ag) and fluids were sourced from multiple sources, and multi-stage hydrothermal activities were involved in the genesis of the KCM Cu-Ag deposits. The negative Eu anomalies (0.28–0.88) of hydrothermal calcite associated with Cu-Ag mineralization are also consistent with the involvement of reduced hot (at temperatures above 200 °C) hydrothermal mineralizing fluids. These fluids are characterized by a wide range of salinities (4.0–23.5 wt% NaCl + CaCl2 equiv.) and homogenization temperatures (Th) (ca. 94–396 °C), thus suggesting that the fluids involved in the mineralization processes of the KCM Cu-Ag deposits are both basinal brines, possibly mixed with seawater and meteoric water, as well as metamorphic fluids. The bimodal distribution (vapor-rich and liquid-rich) of fluid inclusions at room temperature is attributed to the boiling of fluids during entrapment. Meanwhile, the occurrence of both high temperature-high salinity and low temperature-moderate salinity fluids, as well as fluid inclusions characterized by a wide range of salinities but similar Th reflects fluid mixing. A decrease of Th in the fluid inclusions but with very similar salinities also indicates a simple cooling process during the evolution of the ore-forming fluids.