STRATIGRAPHIC AND STRUCTURAL CONTROLS ON GOLD MINERALIZATION IN THE PILGRIMS-REST GOLDFIELD, EASTERN TRANSVAAL, SOUTH-AFRICA

Since 1872, approximately 185 metric tons of gold and a significant am ount of silver have been won from numerous stratiform ore bodies withi n the Pilgrim's Rest goldfield. The epigenetic stratiform ore bodies a re found primarily in the Malmani Dolomite Subgroup, a laterally exten sive Lower Proterozoic carbonate platform succession. A strong relatio nship exists between the presence of stratiform ore bodies and environ ment of deposition and structural deformation. Analysis of the host se diments indicates deposition in a retrogradational, followed by an agg radational, sequence that comprises supratidal (sabkha) siliciclastic sediments to midshelf carbonates. The retrogradational and aggradation al sequences are characterized by numerous higher frequency transgress ive and regressive cycles. Mineralization is, with few exceptions, res tricted to shallow-water siliciclastic sediments; deeper water carbona tes are barren. More importantly, siliciclastic mudstones and sandston es have acted as zones of detachment for thin-skinned thrust-fault def ormation, resulting in the development of conformable passageways in w hich fluids concentrated their auriferous lodes. Evidence of shallow h interland-dipping duplex, antiformal stack, and imbricate thrust syste ms abounds in several underground and field exposures. The gold-silver emplacement model for the Pilgrim's Rest goldfield infers fluids deri ved from a deep-seated magmatic source. Oxygen-isotopic compositions o f the mineralizing fluids, recalculated from delta(18)O quartz (+12.1 parts per thousand to +19.5 parts per thousand), point to a magmatic s ource and subsequent mixing with saline, evolved formation water. A ma gmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions (100 degrees C/km). The Bu shveld Igneous Complex (Rustenburg Layered Suite) to the west of Pilgr im's Rest is thought to be the source of the auriferous hydrothermal s olutions, and its emplacement was the driving mechanism of thrust-faul t deformation in the Pilgrim's Rest goldfield.