Abstract The Bonsucesso zinc-lead (Zn-Pb) sulfide deposit is hosted in carbonate rocks from the Vazante Group (transition of the Mesoproterozoic to the Neoproterozoic time), northwest of Minas Gerais State, Brazil. Regional deformation is associated with the thin-skinned tectonics that led to the formation of the foreland zone of the Brasília Orogen during the Neoproterozoic. These orogenic processes are the trigger of Zn-Pb mineralization in many base-metal deposits in the Vazante Group. Here, we describe the ore textures and the structures of the deposit to define its structural controls in deposit scale and formulate hypotheses about its forming process in the context of other Vazante Zn-Pb deposits. Mineralization distribution is controlled by a high-angle fault zone striking NNW and dipping 60° to WSW. Fault-related fractures functioned as pathways for Zn-Pb-rich fluids to be redistributed and crystallize where chemical conditions were adequate. Mineralization was likely formed in an extensional setting in breccia zones controlled by flexurally induced normal faults related to development of the foreland basin of Brasília Orogen. Later, the entire Vazante sequence underwent an inversion process and extensional structures were reactivated or obliterated. Finally, an understanding of local controls may be of good use to target exploration for new orebodies in the district scale.

Abstract Fracturing analysis of low-permeability rocks as reservoir analogs have increased in recent years. The main mechanism involved in the development of secondary porosity in low-permeability, fine-grained limestones of the Irati Formation is fracturing. In these rocks, oil accumulates along fracture planes, vuggy porosity, microfractures, breccias, as well as bedding discontinuities. Joints represent the central element for oil migration and the connection between accumulation sites. Joints are unevenly distributed across the succession of rock types, where carbonate rocks have a much denser array of joints than shales and siltstones. From a mechanical stratigraphy point of view, limestones have a brittle behavior and constitute mechanical units. Ductile shale and siltstone are mechanical interfaces capable of blocking joint propagation. Joints running NW-SE are more effective in trespassing the mechanical interface and are, therefore, more persistent. Joints running NE-SW are less persistent because the ductile behavior of the first two shale beds above the limestone blocks their propagation. The spatial arrangement of regional NW-SE and NE-SW joints promoted reservoir connection, allowing oil migration and accumulation. The joints and oil migration (at least three phases) developed as a consequence of the Gondwana Breakup and are also associated with local pressure gradients.

Abstract The Luminárias Nappe was formed in the agglutination of West Gondwana. A high-pressure metamorphic gradient oblique to the geological contacts is recorded in metapelitic rocks from this Nappe. In the northern portion, the metamorphic peak conditions are at high-pressure lower-amphibolite facies at 580 ± 4°C and ca. 0.9 GPa (Chl + Ky + St + Ms + Qtz + Rt); in the central portion, they are at high-pressure amphibolite facies at 600 ± 15°C and 1.1 ± 0.3 GPa (St + Bt + Grt + Ms + Qtz + Rt); and in the southern portion, they reach the eclogite facies at 630 ± 13°C and 1.4 ± 0.6 GPa (St + Ky + Grt + Ms + Qtz + Rt). Clockwise metamorphic P-T-t paths are registered in the studied rocks, with temperature and pressure increase followed by a strong decompression with retrograde phases as chloritoid (northern portion), chlorite and ilmenite (central portion) and biotite, chlorite and ilmenite (southern portion). U-Th-PbT monazite ages range from 632 ± 4 Ma (southern portion) to 600 ± 8 Ma (northern portion included crystals in garnet and staurolite). The metamorphic age, the high-pressure conditions calculated in this paper and the clockwise metamorphic path indicate that the tectonic evolution of the Luminárias Nappe rocks is tightly associated with the subduction and collision processes of the southern Brasília belt. The overprint of the younger Ribeira belt is interpreted to be responsible for rock pile tilting, thus producing the oblique metamorphic gradient.

ABSTRACT: The C1-Santaluz gold deposit is located in the Itapicuru greenstone belt in Bahia State, Brazil. Metavolcanic and metasedimentary rocks dominate at the base and top, respectively. The ore minerals related to the auriferous mineralization are pyrite, arsenopyrite, sphalerite, chalcopyrite and stibnite. Arsenopyrite is related to, and associated with the presence of gold in quartz veins, a feature which is detected by scanning electron microscope analyses. The deposit is structurally complex, with tree deformational phases being recognized: D n-1 , D n , and D n+1 . Phases D n-1 and D n have a direct relation to the mineralization, and mineralized quartz veins are parallel to the S n foliation (S 0 //S n-1 //S n ). The intersection between the S 0 //S n-1 bedding and the S n foliation generates an intersection lineation parallel to the D n fold axis, plunging to the NW, which has favored an increase in the volume of mineralized bodies.

RESUMO: Após o aparecimento dos metazoários, o início da bioclasticidade e a competição pelo substrato marinho, a saga das esteiras microbiais é marcada por um progressivo declínio. Pontuais aumentos em sua diversidade e na produção de carbonatos durante o Paleozoico em geral estão associados a eventos globais de extinção em massa. Ao longo do Fanerozoico, as colônias microbiais ocuparam nichos ecológicos específicos e ambientes livres de organismos predadores caracterizados por condições ecológicas extremas, como aqueles encontrados em baías hipersalinas, lagos alcalinos e locais sujeitos a hidrotermalismo. Neste trabalho é reportada uma das mais antigas ocorrências de um campo de exudação de metano no registro geológico associado com microbialitos e estruturas sedimentares induzidas por atividade microbial. Nesse sistema, durante a atividade de exalação de metano, instalaram-se condições de extrema anoxia e alta salinidade, proibitivas para a manutenção da vida animal, permitindo, desse modo, o florescimento de colônias microbiais. Por causa das severas imposições do ambiente, os organismos microbiais tornaram-se altamente adaptáveis às condições adversas então vigentes. Nós sustentamos a hipótese de que aumentos na diversidade e na quantidade de colônias microbiais na história da Terra não estão apenas restritos a períodos que sucedem à extinção em massa de organismos predadores de esteiras, mas ocorrem em resposta a condições ambientais localizadas que proporcionam o crescimento de colônias microbiais. De fato, o registro geológico Fanerozoico é pontuado de exemplos semelhantes ao aqui reportado, simulando uma espécie de jogo de esconde-esconde das condições ambientais vigentes no Éon Pré-cambriano.

ABSTRACT: Following the rise of metazoans, the beginning of bioclasticity and substrate competition, the saga of microbial mats was in a fluctuating decline in the end of the Neoproterozoic era. Increases in diversity during the Phanerozoic and punctual upturns in the microbial carbonate production occured after the events of global mass extinctions. Gradually along the Phanerozoic, the microbial colonies occupied isolated niches and grazers-free environments, characterized by physically and/or geochemically stressful conditions, such as those found in saline bays, alkaline lakes and hydrothermal or cold seep vents. Here we report one of the oldest occurrences of a vent camp coupled with cold seepage of methane in the geologic record, associated with well-preserved microbialites and elephant skin structures. During the seep activity, oxygen depletion and high salinity conditions are prohibitive for complex animal life, clearing the way to microbial colonies to flourish. Due to the co-occurrence of high adaptability and low competitiveness of microbial forms, they became highly specialized in stressful conditions. We argue that the sporadic microbial mat upturns in Earth’s history are not restricted to geological periods, following massive death of metazoan species; they also may occur in response to punctual paleoenvironmental conditions that enable microbial colonies to growth. Indeed, the Phanerozoic geological record is punctuated of these examples, in a kind of hide-and-seek game of Precambrian times.