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OLLILA, JUSSI TUOMAS, 1986: Origin of colour zoning in cassiterites from tin deposits within the Bushveld Complex, South Africa. Bull. Geol. Soc. Finland 58, Part 2, 3—11. Cassiterites with striking colour zones from the syngenetic tin deposits at Zaaiplaats and from the epigenetic tin deposits at Rooiberg, Leeuwpoort and Union Mines, in the Central Transvaal, have been analysed by microprobe. The average niobium content (0.13 per cent Nb2Os) is higher, whereas the tantalum (0.05 per cent Ta205) and iron (0.62 per cent FeO) contents are lower in the orthomagmatic cassiterites from the disseminated Bobbejaankop granite than in cassiterites from the greisenized Lease granite (Nb2Os 0.03 per cent, Ta 20 5 0.11 per cent, and FeO 0.79 per cent). The tungsten contents are similar, i.e. 0.49 to 0.47 in dark colour zones and 0.10 to 0.02 per cent W0 3 in light colour zones in all Zaaiplaats cassiterites. The average niobium contents in Rooiberg, Leeuwpoort and Union cassiterites are 0.03, 0.03 and 0.02 per cent Nb2Os and average tantalum contents 0.10, 0.08 and 0.12 per cent Ta2Os, respectively. Generally, the substitution of Sn02 by minor amounts of tantalum, niobium, tungsten and iron oxides is higher in the dark zones than in the light zones of the analysed crystals, which can presumably be attributed to the presence of the colour zoning itself. The very low level of trace and minor element contents is partly due to the great purity of Bushveld cassiterites from mineral inclusions in any scale.