SURFACE SOURCE OF COROMANDEL DIAMONDS (MINAS GERAIS STATE, BRAZIL) AND THEIR POSSIBLE ORIGIN FROM THE SERRA NEGRA/SALITRE SUPERVOLCANO

The origin of alluvial diamonds in the Coromandel area has been an enigma for many years, in spite of high investment in conventional and high tech prospecting methods by major mining companies for over half a century. The authors review the diamond mining history in this region, and then discuss the two principal hypotheses to explain the source of these alluvial diamonds. After mapping the headwater region of one of the richest alluvial diamond rivers, the Santo Antônio do Bonito River, they reject both principal hypotheses and conclude that the surficial source can be only the Upper Cretaceous Capacete Formation, composed of pyroclastics and epiclastics. Based on geophysical data from the literature, combined with field observations the authors suggest, that the largest alkaline complex, situated within the diamond producing area, the Serra Negra/Salitre Complex has been the primary source for those pyroclastics of the Capacete Formation and the diamonds. The plugs of this complex are 15-30 times deeper than average kimberlites and other alkaline complexes in the region, and its excess of volume of the intrusive is three orders of magnitude larger than a typical kimberlite. With an intrusive volume of over 1000 km³ the complex is suggested to be a possible supervolcano. This explains the vast areal distribution of the pyroclastics and diamonds. This different hypothesis is discussed, and further research is suggested, mainly of multidisciplinary nature.