Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district, South Africa

Abstract

Stratiform and stratabound barite ± magnetite beds are intimately associated with the polymetallic Broken Hill-type (BHT) massive sulfide deposits of the Aggeneys-Gamsberg Pb–Zn–Cu ± Ag–Ba district in the Northern Cape Province, South Africa. Barite samples were collected and studied from four localities in the district. Although metamorphic water–rock interaction processes have partially altered the chemical and to a lesser degree the isotopic composition of barite, samples identified as being the least altered display distinctly different isotopic compositions that are thought to reflect different modes of origin. All barite samples are marked by low concentrations of SrO (0.5 ± 0.2 wt%), highly radiogenic 87Sr/86Sr ratios, elevated δ 34S and δ 18O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic 87Sr/86Sr signatures (0.7164 ± 0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ 34S values (27.3 ± 4.9‰) indicate that contemporaneous seawater sulfate, modified by bacterial sulfate reduction, was the single most important sulfur reservoir for barite deposition. Most importantly, δ 18O values suggest a lower temperature of formation for the Gamsberg deposit compared with the occurrences in the Aggeneys area, i.e. Swartberg-Tank Hill and Big Syncline. The obvious differences in temperature of formation are in good agreement with the Cu-rich, Ba-poor nature of the sulfide mineralization of the Aggeneys deposits vs the Cu-poor, Ba-rich character of the Gamsberg deposit. In conjunction with this, isotopic and petrographic arguments favor a sub-seafloor replacement model for the stratabound barite occurrences of the Aggeneys deposits, while at Gamsberg, deposition at the sediment–water interface as a true sedimentary exhalite appears more likely.