Origin of colour zoning in cassiterites from tin deposits within the Bushveld Complex, South Africa

• Main Author: J.T. Ollila
• Format: Article
• Language: English
• Published: Geological Society of Finland 1986-12-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: cassiterite; zoning; composition; greisen; Bushveld Complex; South Africa
• Online Access: http://www.geologinenseura.fi/bulletin/Volume58/sgs_bt_058_2_pages_003_011.pdf

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 Nb2O5) is higher, whereas the tantalum (0.05 per cent Ta2O5) 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 (Nb2O5 0.03 per cent, Ta2O5 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 WO3 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 Nb2O5 and average tantalum contents 0.10, 0.08 and 0.12 per cent Ta2O5, respectively. Generally, the substitution of SnO2 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.