Highly siderophile elements in Archaean and Palaeoproterozoic marine shales of the Kaapvaal Craton, South Africa

Abstract

Determination of highly siderophile element (HSE; Au, Pt, Pd, Ir, Os, Rh and Ru) concentrations in relatively unweathered and unaltered marine shales from the Barberton, Witwatersrand and Transvaal supergroups in the Kaapvaal Craton revealed systematic differences, interpreted to reflect secular changes in the HSE content of Mesoarchaean to Palaeoproterozoic seawater. Most of the studied marine shales have HSE concentrations in the range given for average Archaean crustal rocks (0.5–5 ppb), with the exception of the shales in the Witwatersrand Supergroup. These shales contain up to three times more HSE, independent of source rock lithology in the granitoid-greenstone-dominated hinterland. Although sedimentary pyrite incorporated gold from synsedimentary to early diagenetic waters, its modal proportion (<3 vol.%) is too small to account for the total amount of Au and PGE in the studied marine shales. Instead, our results suggest that in addition to contributions from pyrite, some colloidal gold was attached to clay-sized sediments during source area weathering. Probably, colloidal gold and some of the platinum group elements were mechanically aggregated during sediment suspension and deposited synchronously with the host marine sediments.