Ruby deposits.
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.
Refs.:
Mercier A., Debat P. & Saul J.M. (1999): Exotic origin of the ruby deposits of the Mangari area in SE Kenya. Ore Geology Reviews, 14, 83-104.
