A geochemical study of the Theta Reef of the Frankfort Mining Complex in the Sabie-Pilgrim's Rest Goldfield South Africa.

Abstract

For over a century the Sabie-Pilgrim’s Rest Goldfield has been one of the most important gold producers in South Africa. The epigenetic gold deposits are situated at the eastern escarpment of the Eastern Transvaal Drakensberg, approximately 60 km away from the eastern rim of the Bushveld Igneous Complex. The Theta and Bevets Reef of the Frankfort Mining Complex occur within the Neoarchean dolomite of the Malmani Subgroup and the Paleoproterozoic Pretoria Group, respectively, of the Transvaal Supergroup. While the Theta Reef is situated in the dolomites of the Eccles Formation of the Malmani Subgroup, the Bevets Reef is situated at the contact between a conglomerate, also called the Bevets Conglomerate and the shales of the Rooihoogte Formation of the Pretoria Group, therefore providing two different environments for the ore mineralisation. The ore-bearing reefs are represented by quartz-carbonate veins. The ore minerals are pyrite, arsenopyrite, chalcopyrite and minor amounts of minerals of the tetrahedrite-tennantite series. The reefs were emplaced along thrust faults developed parallel to bedding which dips at 4-7° west towards the Bushveld Complex. The thrusts are attributed to forces related to the emplacement of the intrusion. Stable isotope geochemistry revealed that isotopes within a single mineral phase and between two phases are not in isotopic equilibrium. This suggests an episodic mineralisation of the ore-bearing quartz vein. Oxygen and carbon isotopic compositions of 10.9 – 13.8 (fluid) and -4.1 - -2.8 ‰ (calcite), respectively, indicate that the ore forming fluids are most likely of igneous origin and interacted with the rocks of the Transvaal Supergroup. Sulphur isotopic compositions of -1 – 2.3 ‰ (sulphides) show that fluids and metals seem to have originated from the Bushveld Complex. Trace element analysis of ore samples from the most northern section of the Theta Reef shows that the gold content within pyrite is generally low. The common trace elements are Cu, As, Ag Sb, Au, Pb and Bi, occurring as minerals or mineral inclusions of the tetrahedrite-tennantite series. Gold occurs as invisible solid solution and/or as Au-As and/or Au-Sb compound in arsenic rich sulphide minerals and occasionally in association with silver as electrum. The research shows that the intrusion of the Bushveld Complex played a major role in the formation of the gold deposits in the Sabie-Pilgrim’s Rest area being responsible for the mineralizing hydrothermal fluids, the ore metals, the heat budget for the hydrothermal cell and the forces that created the thrust faults that acted as pathways for the circulation of the mineralizing fluids. The circa 2.055Ga age of the Bushveld Complex also provides a maximum age for the Au-mineralization.