ALBITITE FROM THE WEST DRIEFONTEIN MINE, SOUTH-AFRICA

Andesitic magma with alkalic affinity intruded the Upper Jeppestown Su bgroup of the Witwatersrand Supergroup in the West Driefontein Mine as a network of dykes. In the southwestern part of the mine a composite dyke, belonging to the network and consisting of amphibole stilpnomela ne albitite, is exposed in the 23-43 CL Cross Cut, 60 m below the Carb on Leader Reef. The dyke comprises three separate intrusions that are probably related through fractional crystallization at deeper levels. Albitization of feldspar and uralitization of magmatic mafic minerals, as well as the neoformation of actinolitic amphibole and stilpnomelan e transformed the original dioritic rock into amphibole stilpnomelane albitite. Trace-element discrimination diagrams show similarities to G oedgenoegd and Rietgat Formations of the Platberg Group, Ventersdorp S upergroup. Rb-Sr data indicate a date of 1933 +/- 54 Ma, which is rega rded as the age of hydrothermal alteration during a prolonged metamorp hic event related to the intrusion of the Bushveld Complex.