Geochemical trends within Madagascar chromite ores

The most important chromitite deposits of Africa are mainly located in South Africa, in the well known Bushveld complex, nevertheless Madagascar hosts some important chromitite deposits making Madagascar the world's 10th largest chromite producer. Kraomita Malagasy, the state-owned mining company, produces a chromite concentrate grading between 48 and 50 wt% Cr2O3 with 0.002 to 0.003 wt% P2O5 and lumpy chrome ore grading from 42 to 44 wt% Cr2O3. The main chromitite deposits of Madagascar are located in Andriamena, Befandriana and Tamatave regions and are related to archean basic-ultrabasic intrusions set in the central zone of the island. The mineralized regions present different geochemical and textural features that have influenced the explotaition and the general developing of chromite mining industry of Madagascar. Since strong quality parameters are required for the commercialization of both concentrate and lumpy products, a detailed chemical and textural analysis of such mineralization has been focused on understanding quality profile of these ores in ordrer to maximize future exploitation that strongly depends on chrmical and textural features of the ore itself. The Andriamena complex, located at the southern end of the Tsaratanana Mineral Field, has been the leading chrome producing region of Madagascar for the last decades. In Andriamena complex most of the chromite production comes from three mines: Ankazotaolana mine is characterized by a series of ultramafic lenses hosted by a gneissic rock of archean age. Chromitite shows values of C2O3 wt% between 35 and 40 with a Cr/Fe ratio up to 2.7; Bemanevika mine, characterized by a series of parallel lenses at different depth, has an average C2O3 content of 40 wt% and a Cr/Fe ratio of 2.3. Recent estimates predict 2.6 Mt of reserves for this area; the small deposit of Telomita has been recently reopened to increase the production. Telomita is charachterized by a C2O3 content similar to that of Ankazotaolana and a Cr/Fe ratio of 2.2. Befandriana district represents another strategic zone where chromitite lenses, ranging from few meters up to 10 meters in thickness, are highly massive, with C2O3 close to 53 wt% and Cr/Fe ratio close to 2.6. Tamatave district was the first to be exploited in the ’60 of last century and is characterized by highly deformed and metamorphosed chromitite lenses located in two main sites: Ambodiriana and Ranomena. Generally Tamatave chromitites show a low C2O3 content, close to 30 wt% and a Cr/Fe ratio lower than 1.5. Some important differences among studied ores have been highlightet by textural and chemical analyses. Andriamena chromitites show an average good quality, but with differences between mines. Befandriana ores show the best quality due to their massive texture together with high Cr2O3 content of chromite. Chromitite from Tamatave results to be absolutely unsuitable due to the low C2O3 content of chromite. On the other hand a surplus value of Tamatave chromitite is the presence of Platinum Group Minerals (PGM) mainly of laurite or complex solutions observed both included in chromite grains as well in the matrix. In general main chromite ores of Madagascar show a trend of decreasing Cr2O3 content from north to south, related to differences in ore genesis, that strongly affects ore quality for exploitation.