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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.

GLACIOCONTINENTAL FÁCIES (OUTWASH PLAIN) IN THE MACAÚBAS MEGASEQUENCE (SERRA DA ÁGUA FRIA/MG) MEGASEQUENCE AND ITS TECTONIC CONTEXT This paper presents the results of integrated studies on sedimentary fácies, depositional architecture, sedimentary petrology and paleogeography, carried out in sandstones associated with the Jequitaí diamictites. These rocks belong to the base of the Macaúbas Megasequence and crop out in the Serra da Água Fria, in the central-north part of Minas Gerais State. The data allow to interpret the studied deposits as a product of a proglacial outwash plain sustem, which has evolved in the NNE direction. These paleocurrent data, analysed together with data from the western border of the Serra da Agua Fria, indicate the existence of a sindepositional structural high in the Serra da Água Fria area, which, under the action of glaciers and seasonal cycles of melt-water, had supplied sediments (most probably from a source área in the Espinhaço rocks) towards a structural low located to the west. This paleogeography is related to the Early Neoproterozoic rift extensional tectonics responsible for the development of the Macaúbas/Salinas basin. Although many authors had stated that the Jequitaí glaciation was of a continental type with eastward moving glaciers, we interpret that this glaciation as a rift-shoulder related, altitude model. The paleocurrent data achieved in the Serra da Água Fria área show sediment dispersion towards the interior of the São Francisco Craton and from this area towards a NNE direction.

OCCURRENCE OF FERROPYROSMALITE IN THE MINERALIZATED BRECCIAS FROM IGARAPÉ BAHIA Au-Cu-(±REE-U) DEPOSIT, CARAJÁS MINERAL PROVINCEIn the last years, several works report the presence of pyrosmalite mineral series [(Fe,Mn) 8 Si 6 O 15 (OH,C1) 10 ] commonly associated with volcanic exhalative massive sulphide or Fe-Mn metamorphosed deposits. In this paper, we present the inedit occurrence of ferropyrosmalite in the Au-Cu (+REE-U) of Igarapé Bahia deposit, located in the Au-Cu district of the Carajás Mineral Province. We consider the Igarapé Bahia mineralization as being related to the génesis of iron-oxide class deposit, like the Olympic Dam type. Ferropyrosmalite occurs in two different contexts: i) associated with carbonate veins; ii) associated with heterolithic breccias, composed by BIF and mafic metavolcanic fragments imersed in a magnetite, chálcopyrite, bornite, pyrite, carbonates (calcite to siderite), uranium and REE minerais, and gold, -rich matrix. The growth of ferropyrosmalite is probably due to the substitution of iron rich minerais (chlorite, magnetite and siderite), controled by magmatic fluid influx rich in chlorine. The permeability of breccias and the discontinuity of veins favour fluid percolation. The mode of occurrence of ferropyrosmalite and its relation with amphibole (ferro-hornblende-actinolite) indicate metasomatic growth of the former under temperatures in the transition of greenschist/amphibolite fácies. The ferropyrosmalite of the Igarapé Bahia deposit represents an uncommom type of occurrence linked to hydrothermal/magmatic conditions. Keywords: Carajás Mineral Province, Igarapé Bahia deposit, Au-Cu-REE-U mineralization, ferropyrosmalite. RESUMO Nos últimos anos, diversos trabalhos têm reportado a presença de minerais da série da pirosmalita [(Fe,Mn) 8 Si 6 O 15 (OH,CI) 10 ] em depósitos de sulfetos maciços vulcano-exalativos e depósitos de Fe-Mn metamorfizados. Neste trabalho, apresentamos uma descrição inédita da ferropirosmalita no depósito de Au-Cu-(±ETR-U) de Igarapé Bahia, localizado no distrito auro-cuprífero da Província Mineral de Carajás. Admite-se que esse depósito foi gerado por processos similares àqueles envolvidos na génese dos depósitos da classe óxidos de ferro-(Au-Cu-U-ETR) do tipo Olympic Dam. A ferropirosmalita descrita ocorre sob dois contextos: í) associada a veios e vênulas carbonáticas; e ii) associada a brechas heterolíticas compostas por fragmentos de formação ferrífera bandada, metavulcânicas básicas e matriz rica em magnetita, calcopirita, bornita, pirita, siderita, minerais ricos em urânio e elementos terras raras, anfibólio, stilpnomelana, clorita e quartzo. O crescimento da ferropirosmalita teria envolvido a substituição de minerais ricos em ferro (clorita, magnetita e siderita), controlado pela introdução de fluidos magmáticos ricos em cloro. A percolação dos fluidos foi induzida pela permeabilidade elevada das brechas e também pela descontinuidade ao longo das paredes de vênulas carbonáticas. O modo de ocorrência da ferropirosmalita e o seu ...