RESUMO
A geologia do diamante no Brasil é apresentada neste trabalho, tanto do ponto de vista das fontes primárias, quanto das secundárias (sedimentos). Atenção especial foi dedicada ao controle geotectônico das fontes primárias, que apresentam no Brasil uma distribuição de corpos mineralizados (significativos) e estéreis, plenamente compatível com Os exemplos mundiais. Três eventos glaciais são os principais responsáveis pela distribuição do diamante no Brasil. O primeiro, que ocorreu no Proterozóico Superior, foi a glaciação Jequitaí, o segundo, possivelmente de idade cambriana, é informalmente denominado de glaciação Santa-Fé e o terceiro é a glaciação Carbonífera, representada no Brasil pelo subgrupo Itararé. Estas três glaciações são formas de transporte com capacidade de distribuir diamantes com centenas de quilates, intactos, a longas distâncias. Elas são as principais fontes secundárias de diamantes no Triângulo Mineiro (ou província kimberlítica Alto Paranaíba). Os kimberlits do Cretáceo no Brasil, encontram-se distribuídos Segundo três lineamentos principais: lineamento 125oAZ, lineamento Transbrasiliano e Blumenau. Os kimberlitos do Triângulo Mineiro (província kimberlítica Alto Paranaíba) são do Cretáceo Superior e não apresentam mineralização significativa (estéreis). Os kimberlitos mineralizados no Brasil ocorrem em Mato Grosso, região do escudo do Guaporé, em uma área na qual o último evento termotectônico ocorrido foi o Parguazense (1600-1500 m.a.). A idade desta província (Aripuanã) também é Cretácea. Fontes primárias seguramente ocorreram no Pré-Cambriano, principalmente no Cráton do São Francisco. Estas fontes foram responsáveis pela maioria dos diamantes do Triângulo Mineiro.
ABSTRACT
The following compilation on the geology of diamonds in Brazil represents a synthesis of various aspects of diamond geology in Brazil known by the authors until March, 1987. Consideration is given to both the primary and secondary sources of diamonds, with particular attention paid towards demonstrating that Brasil is not anomalous on a worldwide basis with respect to the occurrence and distribution of mineralized versus non-mineralized kimberlites.
The paper is divided into five chapters: 1) Introduction, 2) Primary sources of diamonds in the world, 3) Primary deposits of diamond in Brasil, 4) Secondary occurrences of diamond in Brasil, and 5) the occurrence of diamonds in the Triângulo Mineiro region. The intent of the second chapter is to briefly introduce the subject of kimberlites, lamproites, upper mantle xenoliths, and diamonds. The summary is brief because each aspect contains hundreds of references, and several books have been written on the subject of kimberlites, nodules, xenoliths, and diamonds. Of particular importance in this chapter is to note the definitions of kimberlite and lamproite. Kimberlite is a difficult rock to study but extensive research in South Africa and elsewhere has shown that diamond does not form part of the definition for either of its host rocks. This is often initially surprising to those who are unfamiliar with the subject, but it stands that kimberlites and lamproites are merely the transporting media for diamonds from the upper mantle to the surface of the earth. In light of this fact coupled with age dating data of diamonds and diamond inclusions, diamonds are considered xenocrysts to their host rocks. The chapter also includes a summary on the various types of diamonds, and the research undertaken related to diamond inclusion studies.
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The second important aspect to note from this chapter is the tectonic control of kimberlites and the occurrence and distribution of mineralized versus non-mineralized primary diamond source rocks worldwide. The principal factor is the empirical observation that all mineralized primary diamond source rocks known until today are intruded into substrates that have remained tectonicaly stable for at least 1,500 my. It is this simple fundamental kimberlite law that is used in the following chapter to explain the economic nature of the known kimberlite-lamproite occurrences in Brazil.
Seven kimberlite-lamproite provinces are thus far recognized in Brazil and occur along three principal lineaments: lineament 125o Az which runs NW-SE from Rondônia to Rio de Janeiro, the transbrasiliano lineament transecting Brazil in a NE-SW direction with a continuation to West Africa, and the shorter NE-SW trending Blumenau lineament in the south of Brazil which continues on to Angola. For the purposes of the third chapter lineament 125o AZ was divided into five segments denominated from SE to NW: segment Brasiliano-SF; segment Brasiliano-CK; segment Brasiliano-DL; segment Parguazense-PA; and segment Rondoniense-RO. Segment-SF has a predominance of syenites and phonolites and no diamonds are known to occur in this segment. Segment-CK is characterized by the occurrence of carbonatites, kimberlites, and lamproite and corresponds to the kimberlite province Alto Paranaiba. Along segment-DL occur abundant ultramafic and alkaline type rocks with two known kimberlite intrusions. All three of these segments are underlain by a substrate effected by the Brasiliano event (450-70( )my) and contain non-mineralized primary source rocks. Mineral chemical data of garnets from segment CK and DL yield off-craton signatures corroborating the tectonic observations. Continueing northwestwards along lineament 125o AZ, segment-PA occurs on the Guaporé shield and contains two kimberlite provinces; the Paranatinga (or Batovi) and Aripuanã (or Juína) provinces. The last tectonic event in this segment is the Parguazense (1,500-1,600 m.y.) and both provices contain mineralized kimbertlite. The final segment, segment RO, is situated on-craton but within a substrate effected by the Rondoniense (1,000-1,300 m.y.). The Pimenta Bueno kimberlite province is situated in this segment but to date no known diamondiferous kimberlites are known to be present.
The Gilbués kimberlites province, situated in the state of Piauí along the Transbrasiliano lineament, also occurs within a substrate effected by the Brasiliano (450-700my) and all the known kimberlites are non-economic in diamond. The Lajes kimberlite province in Santa Catarina state is located within the Lajes alcaline complex. At least three kimberlites are known in this region but all are non-mineralized in diamon as confirmed by mineral chemical data of garnets. The area was also effected by the Brasiliano.
In summary known primary diamondiferous source rocks in Brazil are confined to the Guaporé shield and more specilically to kimberlites intruded into a substrate in whic: the last tectonic event was 1,500my (Parguazense). Diamond are being mined along the São Luiz river in alluvial deposit directly associated with the Aripuanã-Juína Kimberlit province. Diamonds also occur in alluvial deposits throughou Brazil in non-mineralized kimberlite rich provinces such a the Alto Paranaíba in Coromandel. In order to further under stand the origin and distribution of diamonds in Brazil one must next look to the sedimentary record.
Diamonds occur in the sedimentary record in Brazil from the Lower Proterozoic to the Quaternary. In chapter four the geology of the diamond bearing sediments is described. The oldest known diamond bearing sediments are conglomerates of the Lower Proterozoic Vila Nova group in Amapá Diamonds also occur in Middle Proterozoic conglomerates of the Roraima Formation in Roraima. These are the only known diamondiferous sediments in the north of Brazil, and their diamonds are probably linked to ancient gondwana primary sources.
In central Brazil the Middle Proterowic is marked by the occurrence of a shallow epicontinental sea. Diamonds are found in the Araxá group and Araí Group (Arraias Formation), in Goiás state; the Sopa Brumadino formation in Minas Gerais state; the Lavras, and Morro do Chapeu Formations in Bahia; and possibly the Aguapeí Group in Mato Grosso the Santo Onofre Group in Bahia, and the Natividade Group in Goiás. Sediment transport directions of some of the units are from the central portion of the São Francisco craton with reworking in a storm beach environment along the craton margins. This is the first evidence of a Brazilian primary diamond source.
The next rnajor event related to diamond transport and concentration is associated with three glaciations; the Jequitaí in the Upper Proterozoic, the Santa-Fé possibly in the Cambrian, and the Carboniferous glaciation. The Jequitaí glaciation is recorded by glacial sediments of the Jequitaí Formation, Samburá unit, Ibiá Formation, Topázio unit, and Bebedouro Formation in Minas Gerais, Goiás and Bahia states. The ice advance was from the north to the south and is considered the principle mode of transport particularly for some of the larger diamonds, to the Coromandel district, and nearby areas. The Santa-Fé glaciation which is considere possibly Cambrian in age, is described for the first time in this chapter. This glaciation also contributed diamonds to the Minas Gerais region. The Carboniferous glaciation had its origins in southern Africa and the diamond-bearing Aquidauana Formation of the Paraná basin represents glacial outwash plains of a northwestwards moving ice front. This glaciation contributed diamonds to the periphery of the Paraná basin. In the Parnaiba basin diamonds of the Carboniferous Poti Formation are also associated with ice transport from the Chapada Diamantina region.
Subsequent to the Carboniferous there appears to be little evidence from the sedimentary record of major events related to diamond transport. The diamonds from the older units are simply reconcentrated into basal conglomerates of the Lower Cretaceous Areado Formation and Botucatu Formation, the Upper Cretaceous Bauru Formation and Parecis Formation, possible Terciary occurrences in the Amazon, and Quaternary sediments throughout Brazil. The only location where diamonds are known to concentrate along the coast is in Bahia, south of ilheus but these deposits are known to be uneconomic.
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The Coromandel region contains the highest concentration of non-economic kimberlites in Brazil and is also the location of some of the largest diamonds known in Brazil. In chapter five a model is proposed in order to explain the apparent enigma of the Corornandel district. The area also serves as a summary of the previous chapters. The model is simple and relys on the observations and conclusions drawn from the occurrence and distribution of diamonds in other parts of the world.
It is considered that Pre-Cambrian primary diamond sources were intruded into the São Francisco craton. Fluvial transport of diamonds from these primary sources and reconcentration along the craton margins in a storm-beach environment occurred during the Middie Proterozoic. Diamonds from these sedimentary deposits and from the on-craton primary sources were then incorporated into southward moving ice sheets where the diamonds were carried, virtually intact, from the São Francisco craton to the Coromandel/SW-Minas Gerais region. The Jequitaí glaciation is considered the principal mode of transport of the diamonds, particularly the larger stones, to the Coromandel district. The Permo-Carboniferous glaciation in southern Brazil, with its origins in southern África, added a secondary influence to diamond deposition in the Coromandel/SW-Minas Gerais area. Diamonds where then reconcentrated into sediments of the Lower Cretaceous Areado Formation and São Bento Group. These units were cut by hundreds of kimberlite intrusions that are non-economic in diamond, having been intruded into an off-craton substrate. Conglomerates of the Upper Cretaceous Bauru Formation are diamondiferous and contain abundant kimberlitic indicator minerals. The diamonds of this forrnation originated principally from the older diamond-bearing sediments with the kimberlites supplying the indicator minerals and minor quantities of diamond. Quaternary alluvial diamond deposits in the region sustain numerous garimpos, with large stones (maior que 100 cts) still being recovered until today.