Primary source of alluvial diamonds from the Santo Antônio do Bonito, Santo Inácio, and Douradinho rivers, Coromandel region, Minas Gerais, Brazil

Abstract

In the midwestern Minas Gerais, Brazil, kimberlite intrusions, particularly kamafugite and alkaline complexes, occur along the NW-SE-oriented Alto Paranaíba structural high. Diamonds in this region were discovered in the Bagagem River and later in the alluvial deposits of the Santo Antônio do Bonito, Santo Inácio and Douradinho rivers. Diamond-bearing kimberlites observed in the region are the primary sources of diamond deposits, as in the case of the Vargem Bonita diggings-in the upper São Francisco River. However, the primary sources of the alluvial diamonds that occur in the Santo Antônio do Bonito, Santo Inácio, and Douradinho rivers have not been clarified. These diamond populations have characteristics common to all three drainage area, where large stones are frequently recovered. Diamond accumulation in the alluvium is due to the erosion and re-concentration of material from basal conglomerate of the Capacete Formation. There is evidence that the sources that fed the conglomerate are local diamond-bearing kimberlites of approximately 90–120 Ma underlying the Capacete Formation, which in an upper unit of the Mata da Corda Group. Recent fieldwork led to the location of a kimberlite intrusion in the Santo Inácio River Basin, southeast of Coromandel. The intrusion fulfills the requirements constituting a primary source of diamonds in the area.