The Dusi (Garba Tula) sapphire deposit, central Kenya - a unique Pan-African corundum-bearing monzonite

The Dusi sapphire deposit, located in central Kenya, is a unique igneous gem-corundum deposit. Blue and yellow sapphires are recovered from a N-S vertical monzonite dyke, 4-5 in thick, extending over a few kilometres. The rock consists primarily of plagioclase (An(3-9)), orthoclase (Or(82-96)), corundum, zircon and Fe-mica. The colour of corundum is attributed to Fe3+, Fe2+/Fe3+, and Fe2+/Ti4+ charge transfers. Zircons are mostly of the S-11 type of Pupin [Contribut. Mineral. Petrol. 73 (1980) 207], and contain significant traces of Hf (4000-10,000 ppm). The mineralogical and chemical characteristics of this rock are unique. U-Pb geochronology on zircon crystals allows to date the crystallisation of the dyke at 579 +/- 6 Ma, that is during the Barsaloian episode [Precambrian Res. 44 (1989) 197] of the Mozambique Belt in Kenya. Sr-Nd isotopic data indicate that the rock is of mantle origin, with little or no participation of a crustal component. In many respects, the Dusi corundum crystals are comparable to corundum xenocrysts originating from alkali basalt fields, possibly due to a similar crystallisation environment. (C) 2004 Elsevier Ltd. All rights reserved.