Geology and metallogeny of the Barberton greenstone belt: a survey

Abstract

The principal episodes of the geotectonic history of the Barberton greenstone belt span some 400 Ma from the initial submarine eruption of the Onverwacht laves (at about 3500 Ma) to the compression and inversion of the depository as a fold-and-thrust belt between 3230 and 3215 Ma, followed by late-tectonic hydrothermal Au mineralization at about 3100 Ma. The crust beneath the greenstone belt was fortified by sialic underplating due to episodic felsic plutonism at about 3445 Ma, 3227 Ma and 3105 Ma. The main stratigraphic units of the Barberton Supergroup comprise the Onverwacht Group, a thick assortment of ultramafic and mafic volcaulics including a number of sill-like layered ultramafic complexes, which is overlain by the Fig Tree Group of turbiditic greywacke sandstones and associated mudstones and banded ferruginous shales. These strata are, in turn, paraconformably overlain by continentally-derived shallow-water arenites of the Moodies Group. The broader aspects of the deformation of the Barberton depository can be considered in terms of one of two models: either the structures represent a single compressional episode of upright folding accompanied by high-angle thrusting between 3230 and 3215 Ma, or they resulted from an early episode of tectonic stacking, followed by a later event involving the rotation of early thrust slices into upright folds and accompanied by high-angle thrusting. Substantial deposits of chrysotile asbestos, magnesite and talc were developed in the ultramafic host rocks of the Onverwacht Group and important mesothermal deposits of Au were formed in the reactive host rocks belonging to all three Groups in splays adjacent to regionally-developed shear zones. There is still uncertainty as to the primary metallogenic setting of the belt during Onverwacht times and what part accretionary or collage tectonics may or may not have played in the evolution of the Barberton greenstone belt.