Archaean lode gold deposits in Africa: Crustal setting, metallogenesis and cratonization

Abstract

Archaean rocks in Africa are preserved in seven important tectonic and physiographic terranes: the Zimbabwe craton; the Kaapvaal craton of South Africa; the Tanzanian Shield; the North Zaire Massif which extends westwards into the Central African Republic; the Kasai-Angola Massif of southern Zaire; the Atlantic Rise of Cameroun, Gabon, and Congo extending southwards to the Malanje “Strip” of northern Angola; the Guinea Rise of Guinea, Sierra Leone, and Liberia; and the Reguibat Rise of Mauritania, Morocco, and northwest Algeria. Dilational vein and brittle-to-ductile shear zone mineralization is common in the Zimbabwe craton, distribution of the gold being controlled by the permeability characteristics of the hydrothermal conduits and by the competence and chemistry of the wallrocks. Within the Kaapvaal craton, the Barberton greenstone belt has the highest productivity with a marked concentration of lode-type mineralization at or near the tectonized contact between the basalt-dominated Onverwacht Group and the metasediments of the Fig Tree Group. Smaller lode gold deposits are also present in the greenstone belts of the central and northern Kaapvaal craton, the more important being the stibnite-gold mineralization that accompanied late brittle deformation of carbonatized high-Mg basalts in the Murchison greenstone belt. The Archaean basement and supracrustal sequences of the Guinea Rise, the Reguibat Rise, the Atlantic Rise, and the Kasai-Angola Massif are dominated by medium- and high-grade metamorphic rocks, and, with the exception of small greenschist-facies sequences in central Sierra Leone (the Guinea Rise), are mostly devoid of gold mineralization. However, vein-type gold lodes are more numerous in the low-metamorphic grade greenstone belts of the North Zaire Massif and the Tanzanian Shield. The important gold-producing terranes are characterized by a complex tectonic history of strike-slip and reverse dip-slip shearing, the productive lodes commonly being hosted by second- or higher-order shear zones. These shear zones demonstrate an important component of late reverse, dip-slip activation or reactivation which provided optimum conditions for crustal de-watering during prograde metamorphism and emplacement of tonalitic magmas. The stratigraphic sequences and tectonic evolution are generally compatible with, but do not unequivocally demonstrate, Cordilleran-type accretionary regimes. A link between gold metallogeny and the tectonothermal events that preceded and ultimately culminated in cratonization is commonly evident. However, cratonization in Africa, in a broad sense, was diachronous, ranging from ca. 3.0 Ga in the Barberton greenstone belt, through ca. 2.6-2.5 Ga in the Zimbabwe and Tanzanian cratons, to the ca. 2.0 Ga Eburnean event of West Africa, the latter contributing to the important gold mineralization of southwest Ghana. This multiplicity of tectonothermal events and the evidence for higher and more persistent crustal heat fluxes in the early Precambrian terranes of West and Equatorial Africa have important ramifications for crustal evolution, metallogeny, and gold exploration.