An occurrence of bavenite in the Cape granite suite, southwestern Cape Province, South Africa, and its implication on the formation of the host pegmatite

Bavenite, (Ca (sub 4) [(Al,Be) (sub 4) (Si (sub 9) (O,OH) (sub 26-n) )](OH) (sub 2+n) ), is present in a pegmatite of the Paarl Pluton, a metaluminous I-type granite of Late Precambrian age. We are not aware of any other previous description of a beryllium mineral occurrence in the Cape Granite Suite. The pegmatite consists essentially of quartz and microcline microperthite together with albite, calcite and fluorite. A hydrothermal alteration assemblage of epidote, chlorite and bavenite occurs in vugs and veins within the pegmatite. Stilbite, which is stable below 170 degrees C, is also present, but not texturally related to the alteration assemblage. Microthermometric analyses and mineral chemistry of associated minerals elucidate the conditions of formation for the bavenite. According to primary fluid inclusions in the cores of euhedral quartz, the minimum temperature of crystallization of the pegmatite is 450 degrees C. Homogenization temperatures of later fluids indicate a minimum temperature of 210 degrees C for the main hydrothermal event. Chlorite geothermometry yields crystallization temperatures around 320 degrees C. The bavenite formed between 210 degrees C and 320 degrees C, at a pressure of less than 2 kbar.