ARTICLE Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district, South Africa Craig R. McClung & Jens Gutzmer & Nicolas J. Beukes & Klaus Mezger & Harald Strauss & Ellen Gertloff Received: 11 September 2006 / Accepted: 21 December 2006 / Published online: 16 February 2007 # Springer-Verlag 2007 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 partial- ly 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 87 Sr/ 86 Sr ratios, elevated δ 34 S and δ 18 O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic 87 Sr/ 86 Sr signatures (0.7164±0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ 34 S 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 18 O 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. Keywords Aggeneys . Gamsberg . Barite . South Africa . Broken Hill-type Introduction Sediment-hosted, stratabound, polymetallic massive sulfide deposits of the Aggeneys-Gamsberg district of the Northern Cape Province, South Africa (Fig. 1) are spatially associ- ated with and laterally equivalent to stratiform and strata- bound beds of barite ± magnetite hosted by multiply deformed and metamorphosed, amphibolite-facies meta- sedimentary rocks of the Mesoproterozoic Bushmanland Group (Fig. 2). The district consists of five major sulfide deposits (Swartberg, Broken Hill, Broken Hill Deeps, Big Syncline, and Gamsberg) that are regarded as typical examples of Broken Hill-type (BHT) deposits (Parr and Plimer 1993; Walters 1996). Combined, the Aggeneys- Gamsberg district had an initial estimated combined grade Miner Deposita (2007) 42:537–549 DOI 10.1007/s00126-007-0128-4 Editorial handling: B. Lehmann C. R. McClung (*) : J. Gutzmer : N. J. Beukes : E. Gertloff Paleoproterozoic Mineralization Research Group, Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, Johannesburg, South Africa e-mail: crmcclung@juno.com K. Mezger Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany H. Strauss : E. Gertloff Geologisch-Paläontologisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany