Abstract
The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515+/-3 Ma, which may correlate with 1710+/-12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.
Highlights
The origin of the presence of diamonds in Proterozoic sedimentary rocks along the Serra do Espinhaço in Minas Gerais and Bahia States has been controversial since their discovery in the 1790’s and the pioneer papers from Spix and Martius (1828) and Eschwege (1833)
40Ar/39Ar analyses of metasomatic muscovite by Battilani et al (2005) from the Chapada Diamantina region yielded an age of 1515 ± 3 Ma interpreted as the minimum age of intrusion
The mineral composition of the intrusive rocks of Chapada Diamantina is identical to the hematitic phyllites of Minas Gerais, suggesting that these phyllites could be a product of regional greenschist metamorphism of similar hydrothermally altered igneous rocks
Summary
The origin of the presence of diamonds in Proterozoic sedimentary rocks along the Serra do Espinhaço in Minas Gerais and Bahia States has been controversial since their discovery in the 1790’s and the pioneer papers from Spix and Martius (1828) and Eschwege (1833). None of the several suggestions to explain the origin of the gem proved to be conclusive. Other researchers suggested a proximal source located within the confines of the diamondiferous Proterozoic sedimentary basin. Two possibilities have been proposed to explain this proximal source. One states that diamonds bearing kimberlites and lamproites were eroded or buried by thick sedimentary sequences (Almeida-Abreu and Munhóz 1983, Fleischer 1995, 1998). It is almost impossible to prove the first hypothesis because until now kimberlites and lamproites have not been identified. The other possibility indicates as a probable diamond source, a rock mainly composed of mus-
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