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 Jequital glaciation; the second, probably in the Cambrian, is represented by the Santa F$ tillites; and the third, in the Carboniferous, is represented 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 kimberlites and lamproites (of unknown age) in Brazil were intruded along major arcs and lineaments along basin borders and on the Amaz•nico craton. Upper Cretaceous kimberlites in the Alto Paranalba are noneconomic in respect to diamonds; to date, the only known economically 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.

The roles of primary kimberlitic and secondary Dwyka glacial sources in the development of alluvial and marine diamond deposits in Southern Africa

“…In the central Karoo Basin, Dwyka Group sedimentary rocks Veevers et al (1994b), showing highlands, icetransport vectors and terrestrial-marine margin. Localities of alluvial diamonds derived from Permo-Carboniferous glacial sediments were taken from Tompkins and Gonzaga (1989) for South America, Griffin et al (2001) for Myanmar/Sumatra, and Davies et al (1999) for Australia. Fig.…”

“…Itarar e Subgroup; Tompkins and Gonzaga, 1989), and it is apparent that large volumes of Dwyka sedimentary rocks have been eroded in the inland area, immediately Fig. 1.…”

Evolução Climática Do Gondwana Na Região Centro-Sul Do Brasil E Seus Registros Geológicos Continentais Durante O Mesozóico, Enfatizando O Arco Do Paranaíba, a Borda Nne Da Bacia Do Paraná E a Porção Meridional Da Bacia Sanfrasciscana, No Oeste Do Estado D

“…4 e 5 Outro fator com implicações econômicas e geotectônicas é o fato de que os citados sedimentos basais da Bacia Sanfranciscana conterem diamantes (Hasui & Penalva (1970). Sua origem foi atribuída à incorporação dos diamantes aos conglomerados cretácicos pelo retrabalhamento dos tilitos subjacentes, os quais teriam atuado como agentes concentradores de tais gemas oriundas de longas distâncias (Tompkins & Gonzaga 1989). Tal hipótese descarta uma origem para os diamantes a partir dos kimberlitos cretácicos do Alto Paranaíba, sendo a solução deste problema de fundamental importância ao aprofundamento do conhecimento geológico regional.…”

Crystal forms and surface textures of alluvial diamonds from the Western Region of the Central African Republic

“…The most important production is of gem quality diamonds; industrial diamonds do occur in proportions ranging from 10 to 60% by weight (Applin 1972). (f) The main diamantiferous sedimentary fields of Brazil offer some differences with regard to the quality, the colour, the varieties and the average size of their stones (Tompkins and Gonzaca 1989). For example, in Diamantina, the stones are small (10-12 stones to the carat), and the gemstone percentage is 80%.…”

“…In Romaria, the diamonds are, in the majority, white and rhombododecahedral types; 30% are gemstones. The description of all the known diamantiferous sediments of Brazil and the main characteristics of their diamonds are summarized in the paper of Tompkins and Gonzaca (1989). (g) In the Eastern Region of the CAR, gemstone diamonds comprise 60-80% and rhombododecahedral types occur more often than the octahedral types (Polinard 1932).…”