CRATONIC EXTENSION AND ARCHAEAN GOLD MINERALISATION IN THE SHEBA-FAIRVIEW MINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

摘要

Gold mineralisation in the world-class Sheba-Fairview mining district in the Barberton greenstone belt, South Africa, occurs along a fracture network that is arranged in overlapping Riedel, P-shear and anti-Riedel arrays on 10m and 100m scales. The mineralised fracture arrays overprint and fold the Sheba shear zone interpreted by previous workers as a major, strike-parallel thrust thought to have controlled mineralisation in the area. Our results indicate that mineralised structures did not result from movement on the Sheba shear zone, which is not a major thrust. Instead the Sheba shear zone is locally overprinted by the mineralised structures and reactivated as a sinistral normal fault. Mineralisation was accompanied by silicification and the emplacement of porphyries, and occurred after the ductile geometry of the greenstone belt was fully established, i.e. after the emplacement of late-tectonic, 3105 Ma potassic granites and associated doming and constrictional folding. Kinematic analysis shows that the network of mineralising fractures formed in an extensional environment with sigma 1 vertical and sigma 3 orientated in a horizontal northwest to southeast direction. The stress regime was predominantly radial extensional, with sigma 3 orientated at right angles to the long axes of the greenstone belt. Greenstone hosted gold deposits such as the Sheba-Fairview deposits have been classified as orogenic gold deposits invoking a genetic link between mineralisation and compressional or transpressional, accretionary thrusts in a late tectonic environment likened to modern volcanic arcs. Our results suggest that mineralisation in the Sheba area formed in an extensional environment, related to structures that developed after cratonisation of the Kaapvaal Craton had occurred; i.e. the Archaean lode gold deposits at Sheba-Fairview mine are probably not related to orogenic processes linked to formation of the greenstone belt.