Diamonds in Brazil and a proposed model for the origin and distribution of diamonds

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 ontrol 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 Jequital glaciation; the second, probably in the Cambrian, is represented by the Santa F $ tillites; and the third, in the Carboniferous, is rep-resented by the Itarar • 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 kim-berlites and lamproites (of unknown age) in Brazil were intruded along major arcs and lin-eaments along basin borders and on the Amaz•nico craton. Upper Cretaceous kimberlites in the Alto Paranalba re noneconomic in respect o diamonds; to date, the only known econom-ically viable primary diamond sources are Cretaceous kimberlites in the Guapor • 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 S5o 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