Stress analysis, post-orogenic extension and 3.01 Ga gold mineralisation in the Barberton Greenstone Belt, South Africa

Abstract

Gold deposits in the Barberton Greenstone Belt (BGB) have formed along a common network of brittle–ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at ∼3015Ma. At a deposit scale, mineralised shears occur in en-echelon arrays, distributed as Riedel, P- and anti-Riedel shears, with envelopes that connect groups of mine workings along NNW, ENE, NE and ESE trends. The dip of the mineralised shear envelopes is generally moderate to steep (50–80°). The existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with Fry analysis.Stress inversion for mineralised shear zones from workings near 28 mines using a Bingham axial distribution and an optimized right dihedron approach provide near identical results with σ1 vertical and σ3 horizontal NW. The stress regime is generally radial extensional, or pure extensional. At the scale of the greenstone belt, the mineralised shear envelopes comprise two sets of near-orthogonal trends, positioned symmetrically with respect the measured far field stress: 1. A NW and NE trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike-slip; and 2. A NNW and ESE trending pair accommodating dextral transtensional and sinistral transtensional movements respectively.The fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage. 207Pb/206Pb ages of 3017.1±17.8Ma and 3008.8±16Ma from a syn-mineralisation, porphyry dyke near Golden Quarry indicates gold deposition at ∼3015Ma; i.e. ∼85Ma after the craton had stabilised. Older porphyry dykes intruded near Sheba and Consort mines have been dated at ∼3100Ma and ∼3260Ma respectively, and provide a minimum and maximum age constraint on accretionary deformation in the area.We propose that the main phase of gold enrichment in the BGB occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane. Mineralised brittle–ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension. Mine camps display a cluster spacing of ∼12km, which suggests a regular distribution of controlling shears, possibly as a function of the elastic rigidity of the lithosphere. Rather than linking mineralisation to orogenic processes during greenstone belt accretion, we link gold mineralisation to the opening of fault-bounded intracratonic basins including the Pongola and Witwatersrand Basins, and concomitant igneous activity.