THE ARCHEAN VOLCANIC FACIES IN THE MIGORI SEGMENT, NYANZA GREENSTONE-BELT, KENYA - STRATIGRAPHY, GEOCHEMISTRY AND MINERALIZATION

The Migori segment is an 80 by 20 km portion of the Nyanza greenstone belt which forms the northern part of the Archean Tanzanian Craton in western Kenya, northern Tanzania and southeastern Uganda. It consists of two volcanic centres, each with central, proximal and distal volcanic facies, comprising the Migori Group, the Macalder and Lolgorien Subgroups, and eleven volcano-sedimentary formations. The centres are separated by a basin of tuffs and greywacke turbidites. The volcanics are bimodal mafic tholeiitic basalt and dolerite (Zr/Y = 3.8 - 6.5, La(N)/Yb(N) = 1.0 - 2.4), and felsic calc-alkaline dacite-rhyolite (Zr/Y = 10 - 21, La(N)/Yb(N) = 19 - 42) and high-K dacite (Zr/Y = 9 - 16, La(N)/Yb(N) = 21 - 22). Felsic units form approximately three-fourths of the volcanic stratigraphy. Basalts, calc-alkaline dacites and rhyolites were deposited in a submarine environment, but the voluminous high-K dacites were erupted subaerially. The turbidites contain units of iron-formations. Granitic intrusions are chemically continuous with the high-K dacites. The felsic volcanics are anologous to those found at modern volcanic arc subduction settings involving continental crust. The Macalder Zn-Cu-Au-Ag volcanogenic massive sulphide deposit is in central facies basalts-greywacke-rhyolite. Gold mineralisation occurs in proximal facies tuffs and iron formation, and in oblique and semi-conformable quartz veins. Greenstones in the Nyanza belt are dominated by calc-alkaline felsic volcanics in contrast to the komatiite-tholeiitic basalt volcanism in the Kaapvaal Craton of South Africa, and a mixture of the two types in the Zimbabwe Craton.