Gold distribution in supracrustal rocks from Archean greenstone belts of southern Africa and from Paleozoic ultramafic complexes of the European Alps; metallogenic and geochemical implications

Abstract

Many Archean gold veins are thought to have formed as a result of metamorphic secretion processes, with the source of gold being gold-enriched supracrustal rocks. However, in many gold provinces no elevated gold concentrations can be detected in the proposed source beds.In this study, 98 samples of mafic and ultramafic rocks, and 32 samples of ferruginous chemical sediments from South African greenstone belts and from the Belingwe greenstone belt in Zimbabwe were analyzed for their Mg, Al, Cr, Fe, Co, Ni, and Au contents. The analyses were carried out by means of XRF, AAS, and neutron activation analysis, and the data evaluated by various statistical methods. The gold values of the volcanics range from 0.1 to 372 ppb, with the mean content at 10.8 ppb, and the gold values of the sediments vary from 0.5 to 667 ppb, with the mean content being 129.9 ppb. As a comparative population, 56 samples from Paleozoic European ultramafic complexes were investigated for the same suite of elements. The gold values range from 0.1 to 25 ppb and possess a mean content of 2.0 ppb.For the samples from southern Africa, statistical analyses indicate a complete lack of relationship between gold and rock-forming minerals. Significant differences in gold abundance were observed in samples originating from geologically and geochemically similar greenstone terranes. These differences may be a result of heterogeneous gold abundance in the upper mantle. Differentiation trends of gold in the ultramafic to mafic range were not detected in this study. Ferruginous chemical sediments of the Algoma type were found to contain more gold than younger ones of the Superior type.For volcanic rocks, the analytical data suggest that the principal gold carriers are accessory sulfides and, to a lesser extent, intergranular gold in particulate form. Rock-forming minerals appear to contribute less than 0.5 ppb to the gold in these rocks. Most supracrustal rocks contain sulfides and, thus, contain gold readily accessible to leaching metamorphic hydrothermal solutions which may deposit gold in nearby dilatant zones. It is proposed that in greenstone terranes metamorphic overprint constitutes the key factor determining the formation of gold veins. The conspicuous abundance of such gold mineralizations in Archean greenstone terranes is considered to be a result of the unique evolution of these terranes.