Controls on the distribution of the deleterious elements bismuth, cobalt and cadmium in the base metal sulphide deposits of the Mesoproterozoic Aggeneys-Gamsberg Ore District, South Africa

The presence of deleterious elements in sulphide ore mineral assemblages has dire economic and environmental implications. Specifically, we study the distribution of bismuth (Bi), cadmium (Cd) and cobalt (Co) in the Upper- (UOB) and Lower Ore Bodies (LOB) of the four sedimentary exhalative base metal sulphide deposits in the Aggenys-Gamsberg Ore District (South Africa). Whole rock geochemical data from all four deposits, using elemental ratios, reveal broad trends in the distribution of these deleterious elements, with the highest concentrations observed in the Swartberg ore deposit. Further investigation using scanning electron microscopy (SEM) focusses on the distribution and partitioning of these elements within individual co-genetic sulphide mineral parageneses identified in the Swartberg ore deposit. In this ore body, cobalt partitions preferentially into pyrite (average concentration: similar to 4000 ppm), Bi is concentrated predominantly in galena (concentrations can exceed 2 wt. %), and Cd is hosted predominantly in sphalerite (up to 5324 ppm). The distribution of deleterious elements between the different ore bodies (i.e., LOB versus UOB) and across different ore deposits is discussed in terms of hydrothermal ore forming processes and distance from the proposed exhalative source. Furthermore, the implications for this distribution will be discussed in a context of minerals exploration and geo-metallurgy.