Albitite from the West Driefontein Mine, South Africa

Andesitic magma with alkalic affinity intruded the Upper Jeppestown Subgroup 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 stilpnomelane albitite, is exposed in the 23-43 CL Cross Cut, 60 m below the Carbon 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 stilpnomelane transformed the original dioritic rock into amphibole stilpnomelane albitite. Trace-element discrimination diagrams show similarities to Goedgenoegd and Rietgat Formations of the Platberg Group, Ventersdorp Supergroup. Rb-Sr data indicate a date of 1933 +/- 54 Ma, which is regarded as the age of hydrothermal alteration during a prolonged metamorphic event related to the intrusion of the Bushveld Complex.