The geochemistry of black shales from the Chuniespoort Group, Transvaal Sequence, eastern Transvaal, South Africa

Abstract

Geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the Chuniespoort Group of the Late Archean-Early Proterozoic Transvaal Sequence. The black shales are fine grained and generally argillaceous and contain on average 1.2 percent total C organic , 0.7 percent S, 7.5 ppb Au, 60 ppm Cu, 330 ppm Cr, and 125 ppm Ni. These metal concentrations are similar to, or slightly higher than, the values given for the U.S. Geological Survey black shale standard SDO-1. All other heavy and transition metals, however, are depleted in the Chuniespoort shales relative to SDO-1. It is well known that high concentrations of Cr and Ni occur in Archcan shales. The enhanced Cr and Ni values found in the Chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and/or diagenesis. The Chuniespoort shales are characterized by excess Fe with respect to that required to form pyrite and have extremely variable C/S ratios as well. This, together with the wide-ranging S isotope ratios, is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source. Whole-rock S isotopes further indicate that S could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids. Interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals. Rather, element associations indicate that V and Cr are preferentially contained in illite-hydromuscovite and that Ni, Co, Zn, Cu, Fe, Pb, and Mo probably reside in pyrite and/or chalcopyrite. Gold is only weakly correlated with other elements, which may indicate that its distribution is not solely determined by the presence of sulfide minerals. The fact that Au, Cu, and S/C ratios show an increase in samples with delta 34 S values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin. The limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin. Other factors, such as the low metal potential of the provenance, the rate of sedimentation, and the degree of oxygenation of the bottom waters of the basin, may have been additional factors limiting the concentration in, and extraction of, metals from seawater. The fact that the Chuniespoort shales host significant epigenetic gold occurrences, particularly in the Sabie-Pilgrim9s Rest gold field, is largely related to their chemical characteristics, which promoted permeability and high absorption capacities for metals in hydrothermal fluids. Because of their low metal concentrations elsewhere in the basin, the shales cannot, however, be considered the source rocks for gold mineralization.