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OLLILA, JUSSI TUOMAS, 1984: The crystallization of a tin-bearing granitoid suite: The Bushveld granites in the Zaaiplaata are, South Africa. Bull. Geol. Soc. Finland 56, Part 1—2, 75—88. Alkali feldspars from the tin-bearing Bobbejaankop and Lease granites, and from the surrounding barren Main granite in the Zaaiplaats tin-field were studied by X-ray diffraction method. The temperatures and oxygen fugacities of the lowermost Main granite were calculated using quantitative microprobe analyses from the coexisting iron-titanium minerals. The alkali feldspars in the Bobbejaankop granite and in the upper layers of the Main granite are invariably high ordered maximum microclines whereas those in the coarse-grained bottom layers of the Main granite have a high thermal state, i.e. they are orthoclases. The alkali feldspars in the Lease granite are intermediate microclines. The subsolidus reequilibration of iron-titanium oxides ceased at a very low temperature (382.8°C) in the lower Main granite which crystallized under variable oxygen fugacity in the temperature range 500° to 650°C, and at least partially, under reducing conditions. Oxidizing conditions prevailed at temperatures higher than 650°C. The alkali feldspars in the lower Main granite are obviously original magmatic phases and the absence of a volatile phase at the time of initial crystal growth may have prevented the later development of triclinic alkali feldspars. The feldspars in the upper layers of the Main granite have apparently undergone thorough subsolidus exsolution equilibrium recrystallization due to the progressively more hydrous interstitial melt that contributed towards the high degree of ordering. Maximum microclines in the Bobbejaankop granite are due to the presence of aqueous, high-saline fluid phase during its crystallization. The occurrence of intermediate microclines in the Lease granite indicates quenching and a faster cooling rate than that of the surrounding rocks.

The crystallization of a tin-bearing granitoid suite: The Bushveld granites in the Zaaiplaats area, South Africa