Exotic origin of the ruby deposits of the Mangari area in SE Kenya

Abstract

The Proterozoic Mozambique Belt contains numerous primary deposits of gem-corundum (i.e., ruby and sapphire). The ruby deposits in the Mangari area of SE Kenya, are among the most productive and best known. Enclosed within a metasedimentary sequence dominated by sillimanite-graphitic gneisses, the Mangari deposits are associated with rootless ultrabasic bodies. Ruby occurs (1) in lenses on the inner side of the contacts between the ultrabasites and the surrounding metasediments and (2) in veins within the ultrabasic bodies or forming their margins. Field relationships reveal that the corundum-bearing rocks are genetically related to the ultrabasic bodies but not to the surrounding gneisses. Laboratory studies and petrographic comparisons indicate that the ruby-bearing rocks crystallized under granulite facies conditions of 700–750°C and 8–10.5 kbar. By contrast, the surrounding gneisses show only amphibolite facies metamorphism with maximum temperatures around 650°C and pressures which did not exceed 7 kbar. At Mangari, the ruby-bearing rocks, as their associated ultrabasites, appear to be exotic with respect to the surrounding metasedimentary rocks. We interpret them as fragments of a deeper crust brought up to their present-day exposure level by the ultrabasic bodies during their emplacement as thrust sheets. Similar reasoning may account for the presence of charnockitic gneisses nearby. By contrast, granulite facies metamorphism characterizes all rock types genetically relatable to the Tanzanian ruby deposits at Longido and Lossogonoi, both also situated in the Mozambique Belt. This suggests that, if associated with ultrabasites, primary deposits of rubies (and of subgem red corundum) formed only under granulite facies conditions in the Mozambique Belt. Some of these deposits were subsequently tectonically inserted into country rocks of lower metamorphic grade. This apparently occurred without the total fracturing of all crystalline gems.