Lead isotopes and trace elements from sulfides of Archean greenstone belts in South Africa; a contribution to the knowledge of the oldest known mineralizations

Abstract

Lead isotopic investigations of sulfides from gold-sulfide deposits in the Swaziland Sequence of the Barberton Mountain Land indicate that the primary lead of the sulfides is a two-stage lead forming a secondary isochron. From the slope of the secondary isochron a maximum age of 3,800 m.y. and a minimum age of 3,450 m.y. are indicated for the Swaziland Sequence rocks. The two-stage leads show that differentiation of the uranium-lead system had already occurred within this time span of the formation of the early crust. New least radiogenic ore lead was found in the vein-type deposit of the Old Star Gold Mine in the Murchison Range. The variable isotopic ratios of different galena grains also indicate a multistage history for this least radiogenic lead. The trace element patterns observed in pyrites from gold-sulfide deposits in the Barberton Mountain Land support the metallogenetic concepts of their having been formed by lateral secretion of elements from the ultramafic to felsic units of the Onverwacht Group of the Swaziland Sequence during regional metamorphism at the time of the emplacement of the surrounding granites.