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The 40Ar/39Ar ages (muscovite and biotite) and P-T estimations presented here allow us to reconstruct the Pressure–Temperature–time (P–T–t) path from the metamorphic peak to the generation of the orogenic Au (As, Bi, Ag) mineralization in the Buracão area. The application of software-based pseudosections for bulk phyllite composition allows us to better recognize three main stages in the P-T-t path. Rocks from the study area evolved from (i) syn-D 1a greenschist metamorphic peak (T = 480–550 °C and P < 4.0 kbar) attained at ca. 538–530 Ma during the W-E shortening related to local D 1 deformational stage, which is regionally reported in the external domain of the northern Brasilia Fold Belt. These younger ages, compared to those of mafic and felsic granulites from the northernmost internal domain west of the study area (ca. 610–570 Ma), indicate that the metamorphic peak in the external domain was attained after the internal domain. The (ii) syn-D 1b barren type I quartz vein formation (T = 510–260 °C and P < 4.0 kbar) at ca. 531–525 Ma; and (iii) syn-D 2a ore-bearing type II quartz vein formation and gold mineralization (T = 345–285 °C and P = 2.7–1.6 kbar) at ca. 522–515 Ma suggest that low temperature may represent cooling P-T conditions of the peripheral supracrustal cover. Moreover, the diachronicity between the local metamorphic peak and the gold mineralization (22 M.y.) is consistent with the structural relationship between gold veins and post-metamorphic peak D 2a high deformation zones, which may have acted as fluid corridors through the metasedimentary pile. These features are coherent with the time span observed in several orogenic gold deposits worldwide, especially those associated with high-angle transpressive shear zones developed in the late stages of orogeny. The combination of the Ar–Ar petrochronology with previous published fluid reconstruction and tectonic data suggests metal-rich fluid percolated along regional shear zones through the Traíras Formation under post-metamorphic peak conditions. This regional tectonic and chronology might be useful for geodynamical reconstruction and as a prospection guide in poorly studied orogenic gold of the area. [Display omitted] • 40Ar/39Ar-petrochronology to reconstruct the P–T–t path from the metamorphic peak to orogenic gold deposit. • syn-D 1a metamorphic peak (T = 480–550 °C and P < 4.0 kbar) at ca. 538–530 Ma. • syn-D 1b barren type I vein formation (T = 510–260 °C and P < 4.0 kbar) at ca. 531–525 Ma. • syn-D 2a ore-bearing type II vein formation (T = 345–285 °C and P = 2.7–1.6 kbar) at ca. 522–515 Ma. • Contribution to reconstruct the tectonic and low-temperature evolution of the northern Brasília Fold Belt. [ABSTRACT FROM AUTHOR]

The orogenic pyrite–rich Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is one of several orogenic deposits in the Brasilia Fold Belt (BFB; Central Brazil), exposed to tropical weathering that shows extensive oxidized profiles. The distribution and thickness of these gossan profiles lead to variable gold enrichments and uncertainties to gold prospecting in this region. In order to investigate physicochemical processes related to the evolution of the gossan cover and the supergene gold enrichment, we present mineralogical, textural and geochemical data from each zone of twenty gossan profiles in the Buracão area. These results show that the mineralogy and textures along the gossan profile reflect oxidation, hydrolysis, dissolution and precipitation processes controlled by different Eh–pH conditions, which are controlled by supergene conditions and hypogene ore composition. The geometry, distribution, and thickness of gossan profiles, as well as the style of gold occurrence, reflects textures of the hypogene orebodies, while the weathering has an important role in the gold chemical refining (i.e., increasing ca. 25% in gold fineness) and the distribution of gold grades along the profile. These points allowed us to identify that the leached spongy hematite–rich zone in the upper indigenous profile contains the higher fineness gold grades (i.e., the "bonanza level"), and Bi–minerals such as waylandite and bismutite that may be important pathfinders for high gold grades in these gossan profiles. Our results demonstrate that ore prospection in gossans over Au–As–Bi–Ag deposits should be done focusing on leached zones, different to that traditionally conducted in gossans related to base metal sulfide deposits that have focused on precipitation zones. The similarities between the hypogene mineralization of the Buracão area and many deposits in the BFB and worldwide encourage the application of this approach elsewhere, which improve gold prospecting in weathering profiles over orogenic deposits. • Gossan evolution over orogenic pyrite–rich Au (As, Bi, Ag) mineralization • The role of environment, climate and primary ore composition on the gossan evolution • Mineralogy and textures related to physicochemical processes controlled by contrasting Eh–pH conditions • The nature of supergene gold enrichment and its distribution along the gossan profile • The implication of mineralogy and textures as prospecting guides in weathered profiles [ABSTRACT FROM AUTHOR]

Abstract The mineralization of the Lavras do Sul Mining District and Seival Mine are controlled by brittle structures associated with the emplacement of the volcanogenic rocks and granitoids located in Lavras do Sul region, southernmost Brazil. This magmatism is associated with the evolution of the Camaquã Basin, an Ediacaran basin generated during the post–collisional stage of development of the Dom Feliciano Belt. The regional tectonic process has led to the activation of a large–scale high–angle shear zones and intense fracturing in the brittle to brittle–ductile regime. The relative chronology of structures and stress–field variations were recognized through remote sensing and structural data. The ore–deposits are controlled by extensional fractures and normal fault systems with a N70°W/70–85°NE direction in Lavras do Sul and a N40–60°E/70–88°NW direction in Seival and are disseminated or occur associated with quartz and calcite veins, respectively. We made a comparison of structures going from regional scale to smaller mineralized areas. The structural analysis of remote sensing and geophysical images (aeromagnetometry and aerogamma–spectrometry) integrated with structural data allowed us to reconstruct the paleostress tensor that generated the main fault systems and associated fractures of the host rock, which enabled the hydrothermal fluid circulation and the ore deposition. The integration of structural data such as fault slip planes, slickenlines, shear fractures systems, extensional fractures and kinematic indicators with the stratigraphic data allowed us to reconstruct the main directions of the tectonic efforts associated with the emplacement of the mineralizing magmas. The mineralization is associated with the sinistral N70°W/80–88°NE oriented Palma–Jacques Fault in the Lavras do Sul Mining District as well as the sinistral N50°E/80–88°SE oriented Cabritos–Perau Fault and the sinistral N20°E/80–88°SW oriented Segredo Fault in Seival Mine region. Their maximum compressional stresses are subhorizontal and NW–SE oriented, while a late subhorizontal and NE–SW oriented compressional stress affected only the rocks of the Seival Mine. These two main compressive stresses are associated with the development of a transcurrent (escape) tectonics during the final evolution of the Dom Feliciano Belt, constrained by U Pb zircon geochronology between 600 and 580 Ma. Transcurrent and normal NE–SW oriented faults are more expressive in volcanogenic rocks of the Seival Mine area, while extensional vein–type NW–SE oriented fractures are the most important structures in the Lavras do Sul area. We suggested that N70°W/70–88°NE oriented monzonitic dikes in Lavras do Sul and N50°E/70–88°NW oriented andesitic to trachyandesitic dikes in Seival area are probably the source of the mineralization. Highlights • Brittle structural control in Au Cu Ag mineralizations. • Regional stress associated to faults and fractures ore–bearing. • Compressive stress related to mineralized areas in Camaquã Basin. • Neoproterozoic faults associated to ore deposition. [ABSTRACT FROM AUTHOR]

The Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast and represents the eastern end of the submarine E–W Vitória–Trindade Chain. This is interpreted as the Trindade plume track beneath the South American plate during the Cenozoic. Almeida (1961) recognized five volcanogenic successions at Trindade in decreasing age: the Trindade Complex (TC) and the Desejado (DF), Morro Vermelho (MV), Valado (VF) and Paredão (PF) formations, composed of nephelinitic to phonolitic effusive-pyroclastic deposits, dykes and necks. Here, we present new 40 Ar/ 39 Ar ages and re-evaluation of available 40 K/ 40 Ar* data that, coupled with previous petrological information, allowed us to reconstruct the volcanic history of the island (i.e., 3.9–0.25 Ma) distinguishing near synchronous volcanic episodes and solving several stratigraphic uncertainties reported in the literature. The geochronological dataset show that the nephelinitic–phonolitic volcanism at Trindade was discontinuous throught time, being marked by periods of high volcanic activity and periods of quiescence, which suggests variable melt production and eruption rates. The peak of the volcanic activity occurred between 3.9 and 2.5 Ma (i.e., Lower Pliocene to Lower Pleistocene) that is represented by the TC, which correspond to the largest volume of volcanic deposits preserved in the island. The volcanic activity slows down progressively from 2.5 Ma to cease at ca. 1.6 Ma, period that is represented by the DF. This volcanism was followed by a quiescent period that lasted until the ephemeral nephelinitic volcanism of the MV (no age), VA (no age) and PF (ca. 0.25 Ma). Thus, the volcanic activity in Trindade was ceased completely at ca. 0.25 Ma, event registered in the uppermost volcanic deposits of the PF and that represents the last volcanic activity in the Brazilian territory. [ABSTRACT FROM AUTHOR]