The occurrence of microdiamonds in Mesoproterozoic Chapada Diamantina intrusive rocks: Bahia / Brazil

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 fi rst discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.

Geotectonic setting and metallogeny of the northern São Francisco craton, Bahia, Brazil

“…Diamond occurs in conglomerates of the Tombador and Morro do Chapéu Formations, of the Chapada Diamantina Group and alluvial and colluvial deposits originated by weathering and erosion of the conglomerate beds. The conglomerates of Tombador Formation were deposited mainly by braided river systems (Dardenne and Schobbenhaus, 2003) in a time interval older than 1515 Ma (Battilani et al, 2007).…”

Microssonda eletrônica de elétrons U-Th-Pb e La-ICP-MS U-Pb datando de múltiplos eventos hidrotermais e metamórficos registrados em minerais de as albitites uraníferas da Lagoa Real (Brasil

“…1.5 Ga hydrothermal event is probably related to lithospheric mantle activation process that led to extensional episodes in Lagoa Real region. This process is recorded by 1.5 Ga basic rocks (U-Pb by Babinski et al, 1999;Guimarães et al, 2005) and 1.5 Ga muscovite-martite dykes and sills (Ar-Ar by Battilani et al, 2007) crosscutting regional rocks. The ca.…”

Proterozoic first-order sedimentary sequences of the São Francisco craton, eastern Brazil

“…The sag stage did not persist after 1.57 Ga, the age of a second episode of basic magmatism (Babinski et al, 1999;Battilani et al, 2007;Loureiro et al, 2008;Danderfer et al, 2009) recorded by the basal unit of the Espinhaço II sequence that marks the onset of renewed extensional tectonism in the Paramirim aulacogen. Alluvial sandstones, diamond-bearing conglomerates and basic volcanics cut by dikes and sills of the same nature form the base of the Espinhaço II 1st-order sequence in the Chapada Diamantina (Dominguez, 1993;Battilani et al, 2007;Guimarães et al, 2008) (Figs. 7 and 8).…”