Diamonds in Brazil and a proposed model for the origin and distribution of diamonds in the Coromandel region, Minas Gerais, Brazil

Abstract

The geology of diamonds in Brazil is briefly summarized in relation to the known primary source localities and secondary host rocks. Emphasis is placed on the geotectonic control of mineralized primary diamond source rocks and it is shown that the distribution and occurrences in Brazil of diamond-bearing versus nondiamond-bearing kimberlites and/or lamproites are compatible with the geotectonic constraints observed worldwide. In Brazil the distribution of the majority of the diamonds can be correlated with three principal glaciations: the first, in the lower Proterozoic, is designated the Jequitai glaciation; the second, probably in the Cambrian, is represented by the Santa Fe tillites; and the third, in the Carboniferous, is represented by the Itarare Subgroup. These glaciations with the capacity to transport kimberlitic minerals and diamonds intact for extreme distances are considered the principal sources of diamonds in, for example, the Coromandel region of western Minas Gerais. Cretaceous kimberlites and lamproites (of unknown age) in Brazil were intruded along major arcs and lineaments along basin borders and on the Amazonico craton. Upper Cretaceous kimberlites in the Alto Paranaiba are noneconomic in respect to diamonds; to date, the only known economically viable primary diamond sources are Cretaceous kimberlites in the Guapore shield, within a substratum that has remained tectonically and thermally stable for more than 1,500 Ma. It is postulated that a Precambrian diamondiferous kimberlite event occurred in Brazil which intruded principally into the Sao Francisco craton. It is proposed that these are the primary diamond source(s) for the large diamonds (> 100 kt) that are found in the alluvial deposits of the Coromandel region of Minas Gerais.