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Graphical abstract Highlights • Link between low–grade metamorphism and deformation on the ore genesis. • Carbonaceous phyllites and carbonate rocks as sources for ore–forming fluids. • P–T–t evolution from metamorphic peak to ore deposition based on fluid and mineral data. • Contrasting f O 2 – f S 2 as a chemical control for ore precipitation by fluid–rock interaction. • The Buracão area in the context of orogenic metasediment–hosted gold deposits. Abstract The orogenic Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is hosted in the metasedimentary Paleo–Mesoproterozoic Traíras Formation, Araí Group, Brasília Fold Belt, Brazil. In order to discuss the P–T–X conditions that generated this mineralization, we present mineralogical, geochemical and fluid inclusion data of veins, gold and wall rocks. Coupled with previous structural data these additional data allow us to link metamorphism and deformation with fluid migration and ore genesis. The integration of fluid inclusion microthermometry with mineral geothermobarometers and structures show that rocks of the study area evolved from: (i) syn–D 1a metamorphic peak (400–560 °C, <6 kbar), through to the (ii) development of syn–D 1b type I barren veins from low salinity H 2 O–NaCl fluids during syn– to late–metamorphic peak; and to the (iii) development of syn–D 2a type II veins with the pyritic ore precipitation from low to high salinity H 2 O–CO 2 –CH 4 –NaCl ± HCO 3 fluids during post–metamorphic peak (345–285 °C, 2.7–1.6 kbar). The high fineness gold nuggets accompanied by As, Bi and Ag within pyrite–arsenopyrite–bismuthinite association in the pyritic ore allows the Buracão area to be classified as "a gold-only orogenic ore deposit". Fluid inclusion compounds and the enrichment of As, Bi, Pb, Cu, Te, Se and Ag in pyrite from the pyritic ore point out to metamorphism of carbonaceous phyllites and carbonate rocks of the Traíras Formation as a source for ore–forming fluids. We suggest that sulfur and gold in these fluids were derived by metamorphism of sedimentary rocks and conversion of pyrite into pyrrhotite in a metamorphic–hydrothermal system. The Au(HS) 2 −–bearing fluids percolated along intrabasinal D 2a regional shear zones through the Traíras Formation towards the Buracão area, where they precipitated type II ore–bearing veins within local structures. The ore precipitation was induced by contrasting f O 2 – f S 2 related to the interaction between the sulfur-bearing fluid and host magnetite–graphite phyllites. The lithological control is represented by a series of reactions such as 2FeO (in silicates/magnetite) + 4Au(HS) 2 − (aq) + 4H+ (aq) ↔ 2FeS 2(s) + 4Au (s) + 4H 2 S (aq) + 2H 2 O (l). The deposits of the Buracão area and other orogenic gold deposits hosted in metasedimentary rocks in the Brasília Fold Belt and worldwide confirm the potential of ore genesis in zones that do not show any relation with (meta)igneous rocks, thus, expanding the exploration possibilities in metasedimentary successions. [ABSTRACT FROM AUTHOR]

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]