Genesis of sediment-hosted stratiform Cu-Co deposits of the Zambia Copperbelt: Geochemistry and S-Pb isotopes constraints from the Chambishi deposit

In this study, we present new field geology, whole-rock geochemical and S-Pb isotope data for the Chambishi deposit in the Zambia Copperbelt (ZCB), in order to constrain the mineralization process and ore sulphur and metal source. The Lower Roan Subgroup clastic rocks are compositionally similar to the Post-Archean Australian Shale and have low Chemical Index of Alteration, indicating a felsic and slightly weathered source, which we proposed to be the felsic Lufubu Metamorphic Complex (LMC). The Th, Ti, and Zr elemental features suggest that the Lower Roan Subgroup was deposited in a rift basin of passive margin. The S-34 values of early-stage diagenetic Cu-Co sulphide (-6.2 to +6.6 parts per thousand, Vienna Canyon Diablo Troilite [V-CDT]) from the nodular and layered ores are lower than those of the late-stage vein-type hydrothermal Cu-Co sulphides and disseminated molybdenites (3.9-11.9 parts per thousand, V-CDT), indicating different sulphur precipitation mechanism in the two mineralization stages. Integrated with previous fluid inclusion studies, we suggest that the sulphur for the diagenetic sulphides was from bacterial reduction of sedimentary anhydrite and seawater sulphates, whereas that for the orogenic-stage sulphides was from both diagenetic sulphide remobilization and from thermochemical sulphate reduction. The mean U-238/Pb-204 ratios of the diagenetic- and orogenic-stage sulphides are 10.21 and 10.44, respectively, overlapping with those of the average Lufubu schist (10.38) and the Mufulira granodiorite (10.21), and suggest the LMC as the main ore metal source.