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

The Proterozoic Mozambique Belt contains numerous primary deposits of gem-c orundum (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 M angari deposits are associated with rootless ultrabasic bodies, Ruby occurs (1) in lenses on the inner side of the contacts between the ultrabasites a nd the surrounding metasediments and (2) in veins within the ultrabasic bod ies or forming their margins. Field relationships reveal that the corundum- bearing rocks are genetically related to the ultrabasic bodies but not to t he surrounding gneisses. Laboratory studies and petrographic comparisons in dicate that the ruby-bearing rocks crystallized under granulite facies cond itions of 700-750 degrees C and 8-10.5 kbar. By contrast, the surrounding g neisses show only amphibolite facies metamorphism with maximum temperatures around 650 degrees C and pressures which did not exceed 7 kbar. At Mangari , the ruby-bearing rocks, as their associated ultrabasites, appear to be ex otic with respect to the surrounding metasedimentary rocks. We interpret th em as fragments of a deeper crust brought up to their present-day exposure level by the ultrabasic bodies during their emplacement as thrust sheets. S imilar reasoning may account for the presence of charnockitic gneisses near by. By contrast, granulite facies metamorphism characterizes all rock types genetically relatable to the Tanzanian ruby deposits at Longido and Lossog onoi, both also situated in the Mozambique Belt. This suggests that, if ass ociated with ultrabasites, primary deposits of rubies (and of subgem red co rundum) formed only under granulite facies conditions in the Mozambique Bel t. Some of these deposits were subsequently tectonically inserted into coun try rocks of lower metamorphic grade. This apparently occurred without the total fracturing of all crystalline gems. (C) 1999 Elsevier Science B.V. Al l rights reserved.