Abstract:
Zwartkloof is situated in a structurally complex area where structural control of hydrothermal fluorite mineralisation plays a dominant role in the character, and therefore economic viability, of the deposits in Rooiberg Felsite. A detailed fracture analysis of the mineralised area provides an important initial input into future geological data analysis and indicates a preference for a statistical approach as a guide to reserve calculations. The felsites of the area represent a succession of rocks from the intrusive contact of the Bushveld Granite to the over-lying sediments of the Waterberg Group and have locally been divided into a lower pseudospherulitic and microgranophyric felsite, a marker horizon of pyroclastic sediments and an upper porphyritic red and glassy felsite. Two contemporary, but post-mineralisation faults traverse the farm to the south of the main quarries, one of which has thrown the basal rocks of the Transvaal Supergroup from the floor of the Bushveld Igneous Complex into juxtaposition with the roof-rocks of the latter. A unique occurrence of hydrothermal fayalite (Fa97 ), with ·perfect cleavage and fine-grained magnetite along the cleavage planes, has been found in the Rooiberg Felsite. The fayalite occurs together with fluorite and siderite in the mineralised fractures. Identification of the fayalite was verified by microprobe analysis. An occurrence of altered basic dyke material from old tin workings in Bushveld Granite also contains an altered iron-rich olivine.
