Geology of Matombo ruby prospect, Mororgoro region, Eastern Tanzania

An occurrence of primary ruby mineralization at Matombo, Morogoro region, eastern Tanzania, is described. The Matornbo Ruby Prospect lies within the Mozambique orogenic belt extending along the eastern coast of Africa. Geological field studies followed by petro graphical, whale-rock geochemical and mineral chemical studies in the laboratory were carried out in order to establish the environment of mineralization of gem corundum and its metamorphic history. The major rock types in the Matombo Ruby Prospect are: (i) Quartz-feldspar gneiss; (ii) Biotite-gneiss; (iii) Amphibolite; (iv) Marble and; (v) Calc -silicate rock. The Matonrbo rocks show medium-grade (amphibolite facies) mineral assemblages. Primary ruby mineralization is found in the calc-silicate rock which occurs as lenticular boudins formed at the border of gneiss and marble which is found to be of intense shearing. Geochemical studies have established type SiO2 content be relatively lower in the Calc-silicate booth compared to that in the gneiss. The Cr values are at high in the Calc-silicate boudins varying between 200 and 800 ppm. Mineral chemical composition of ruby from Matombo together with other corundum specimens from other localities within the Tanzanian Mozambique belt has been investigated. The ruby from Matombo Prospect which is pinkish in colour has been found to have Cr2O3 come of between 0.32 and 0.36 %. Inclusions found in Matombo ruby are Fe-oxide (Haematite). The primary ruby mineralization in Matombo is interpreted as being due to desilication caused by the metarnorphism of impure marble.