Stratigraphic and structural controls on gold mineralization in the Pilgrim's Rest goldfield, eastern Transvaal, South Africa

Abstract

Since 1872, approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the Pilgrim's Rest goldfield. The epigenetic stratiform ore bodies are found primarily in the Malmani Dolomite Subgroup, a laterally extensive Lower Proterozoic carbonate platform succession. A strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation. Analysis of the host sediments indicates deposition in a retrogradational, followed by an aggradational, sequence that comprises supratidal (sabkha) siliciclastic sediments to midshelf carbonates. The retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles. Mineralization is, with few exceptions, restricted to shallow-water siliciclastic sediments; deeper water carbonates are barren. More importantly, siliciclastic mudstones and sandstones have acted as zones of detachment for thin-skinned thrust-fault deformation, resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes. Evidence of shallow hinterland-dipping duplex, antiformal stack, and imbricate thrust systems abounds in several underground and field exposures. The gold-silver emplacement model for the Pilgrim's Rest goldfield infers fluids derived from a deep-seated magmatic source. Oxygen-isotopic compositions of the mineralizing fluids, recalculated from δ 18O quartz (+ 12.1‰ to + 19.5‰), point to a magmatic source and subsequent mixing with saline, evolved formation water. A magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions (100°C/km). The Bushveld Igneous Complex (Rustenburg Layered Suite) to the west of Pilgrim's Rest is thought to be the source of the auriferous hydrothermal solutions, and its emplacement was the driving mechanism of thrust-fault deformation in the Pilgrim's Rest goldfield.