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.

The Monte Santo Alkaline Intrusive Suite (MSAIS) is an association syenite foid, nepheline syenite and syenite, which are intruded in metapelites of the Rio do Coco meta-volcanic-sedimentary Sequence, presenting abundant pegmathoid veins cutting all of them. The ages obtained by Shrimp (1051 ± 22 Ma, 1048 ± 11 Ma) are very close those younger age obtained by U-Pb laser ablation (1056 ± 21), beeing interpreted as crystallization age. These dating reveal also that MSAIS rocks were affected by common succession of younger events below 550 Ma ago, responsible by the later rocky bodies of varying composition occurring in the region, including the alkaline pegmatites hosted in the nepheline syenite of the MSAIS.

The Mesoproterozoic Chapada Diamantina Group crops out in the central part of Bahia State, coveringthe São Francisco Craton. It is composed, from the base to the top, of the following formations: Tombador,Caboclo and Morro do Chapéu. The lower contact of the Chapada Diamantina Group is characterized byonlap with the Archean basement or by an unconformity with the Mesoproterozoic rocks of ParaguaçuGroup. The upper contact with the Una Group is unconformable and characterized by a depositional hiatus.A depositional model, a rock classification and a diagenetic evolution of the Morro do Chapéu Formation arehere presented based on stratigraphic profiles, outcrop description and microscopic studies in 45 thin sections.The rocks were deposited by transitional and fluvial systems. They show high mineralogical and texturalmaturities, and can be classified as quartz-arenites. Besides quartz and chert, which usually make up to 95 %of the rock framework, feldspars, micas, heavy minerals as well as mud and carbonate intraclasts also occur.The carbonate intraclasts are frequently silicified. The presence of these intraclasts, which come from theunderlying Caboclo Formation consists an important record of a depositional hiatus between both units.Nine diagenetic events could be recognized in the fluvial sandstones of the Morro do Chapéu Formation:mechanical compactation, mechanical clay infiltration, early hematite cementation, quartz overgrowths,pressure solution, secondary porosity generation, clay authigenesis, late diagenetic phases and telodiageneticprocesses. The transicional sandstones, display, aditionally, four other events: a second quartz overgrowthphase, an incipient dolomitization, antraxolite and gipsite authigenesis.

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