Important diamond deposits southeast of Coromandel and the local geology have been studied in an attempt to understand what surface source provided the stones. River gravels of Pleistocene to Recent age from this region have supplied most of Brazil's large diamonds over 100 ct. The upper cretaceous Capacete Formation of the Mata da Corda Group, composed of mafic volcanoclastic, pyroclastic and epiclastic material, has been worked locally for diamonds, nevertheless considered non-economic. The authors present results of their study of a deactivated small mine, representing the first report with description and analyses of two gem diamonds washed from this material. Hundreds of kimberlites, discovered in the last half century in the region, are sterile or non-economic. We propose that the surface source of the diamonds is the Capacete "conglomerado". The volume of this material is enormous representing a potential resource for large-scale mining. The authors suggest detailed studies of the volcanic facies of this unit focusing on the genesis, distribution and diamond content. As to the question concerning the origin of these diamondiferous pyroclastic rocks, the authors exclude the kimberlites and point towards the large Serra Negra and Salitre alkaline complexes which are considered the primary source for the pyroclastic units of the Mata da Corda Group. They propose that early eruptive phases of this alkaline complex brought diamonds from a mantle source to the surface, much as happens with traditional kimberlites, to explain the association of such huge carbonatite complexes and diamonds.

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A pesquisa aqui apresentada desenvolve um método para estimar a distância que a ametista percorreu no Ribeirão Brejaúba desua rocha fonte para o seu local de recuperação no sistema fluvial a partir da análise do desgaste. Utilizando amostras que foram coletadasna sua própria origem, em laboratório, com os sedimentos do sistema fluvial simulou-se o desgaste de transporte semelhante ao dosistema fluvial com a utilização do de um tambor rotatório (tumbler). O desgaste foi determinado por uma lupa de mão de 10x e lupabinocular. As curvas de desgaste foram então desenvolvidas em tempo de rotação em relação à perda de peso. Ao comparar o desgastedas amostras a partir do sistema fluvial com as do tumbler, chegou-se à razão de 1 x 10 (distância percorrida no sistema fluvial versus adistância percorrida pela gema no tumbler), respectivamente. Para futura prospecção de ametista em sistemas fluviais com característicassimilares ao da Brejaúba, este método permite a eliminação de muitas estações de amostragem, pois a distância até a fonte pode sermelhor estimada. Representa um método auxiliar de prospecção aluvionar, maximizando o custo/ benefício em relação ao métodoconvencional. Esse método pode ser usado em outros minerais gemológicos como foi realizado para a brasilianita da área de MendesPimentel, em Minas Gerais.Palavras Chaves: desgaste fluvial, ametista, prospecção ABSTRACTFLUVIAL WEAR OF THE AMETHYST FROM BREJAÚBA, MG: AN AUXILIARY METHOD FOR ALLUVIAL PROSPECTING. The research presented here develops a method for estimating the distance that alluvial amethyst has traveled from its source rock to its recovery site inthe fluvial system of the Ribeirão Brejaúba from analysis of the fluvial wear shown on experimental samples. Using samples collected fromthe source region, they were tumbled in the laboratory with sediments from the fluvial system so as to simulate the wear from transport inthat fluvial system. The wear was determined by a hand loupe and 10x binocular loupe. Graphs were then developed showing the time ofrotation versus the weight loss. By comparing the wear of samples from the fluvial system to that in the tumbler a ratio of 1 to 10,respectively, was found. For future prospecting for amethyst in fluvial systems similar to that of Brejaúba, this method permits eliminationof many sampling positions because the distance to the source may be better estimated. This gives an auxiliary method to traditionalfollow-up methods of prospecting maximizing cost/benefits of exploration. This method may be used on other gemstones as has beendemonstrated for brazilianite from the Mendes Pimentel area in Minas Gerais.Keywords: fluvial wear, amethyst, prospecting