No abstract

The Serra do Cavalo Magro orogenic gold deposit is located in the southern Ribeira Belt, Brazil. Gold-quartz veins are hosted by Calymmian (1500-1450 Ma) metasedimentary and metabasic rocks and Ediacaran (610-600 Ma) granitic rocks. Petrological modeling indicates peak metamorphic conditions from 560°C and 7 kbar (chlorite-biotite phyllite) to 625°C and 6.8 kbar (garnet-biotite phyllite) recorded in immediate host rocks. Gold mineralization occurs in extensional and shear quartz veins structurally-controlled by N E-trending second-third order sinistral transcurrent shear zones. NNW-trending, vertical and subhorizontal extensional veins are oriented 55-85°in relation to the fault planes, indicating that the shear zones were severely misoriented for frictional react ivation. Deformation within the shear zones was accommodated by bulging recrystallization of quartz aggregates, while feldspar aggregates from granitic protoliths underwent cataclastic flow or were replaced by sericite-epidote producing phyllonites. Gold precipitated during stages of vein deformation within microfractures in my lonitized shear veins with quartz previously subjected to bulging recrystallization. Ore-fl uids are recorded in coexisting C02-N2, l-hO-NaCl-CaCh and H20-C02-N2-NaCI-CaCh fluid inclusions oflow to moderate salinities (1-18 w1.% NaCl eq.). Microthennometric data indicate entrapment conditions of240-260 °C and 0.4-2.5 kbar , recording hydrostatic to supralithostatic fluid pressure conditions. High-temperature l-hO-NaCl-CaCh-KCl brine inclusions (up to 475 DC, 25-33 wt.% aCl eq.) are considered unrelated with gold mineralization. Strong fluid pressure fluctuations from 0.4 to 1.6 kbar are associated with earthquake cycles and the faultvalve behavior. In this scenario, gold mineralization was associated with fluid immiscibility. There is evidence for restricted fluid mixing, but this process is interpreted as unrelated with gold mineralization. Available geochronological data and structural-petrological relationships with host rocks and shear zones indicate that mineralization was formed within 580-540 Ma in post-peak metamorphic episodes. The hydrothermal ore-fluids were likely produced by devolatilization reactions during prograde metamorphism at deeper levels. Subsequently, the metamorphic-hydrothermal fluids were channeled upwards within the strike-slip shear zones into rocks that have been metamorphosed and devolatilized at earlier times.

Shear zones are zones of localized high strain accommodating differential motion in the lithosphere and impacting the crustal rheology and deformational history of orogenic belts. Although terrane bounding shear zones are widely studied, intraterrane shear zones and their tectonic significance, especially in association with supercontinent assembly, is a largely unexplored topic. The Ribeira Belt (SE Brazil), a Neoproterozoic-Cambrian orogenic belt from West Gondwana, is dissected by a crustal-scale NE-trending transcurrent shear zone system that juxtaposes composite terranes. Despite its extensive coverage and complexity, this shear zone system remains poorly investigated. In this paper, we explore the thermal and deformational regimes, and timing of ductile shearing using a multiscale approach combining structural analysis derived from remote sensing and field-based structural data, microstructures, quantitative structural analysis, and multimineral U–Pb geochronology (zircon, titanite, monazite, and xenotime). Our data, combined with previously published data, indicate a transitional northeastward increase in metamorphic conditions from lower greenschist to granulite facies conditions (from 250–300 to 750–800°C), reflecting the different crustal levels that are exposed. Vorticity and finite strain data indicate a complex strain regime with varied contributions of pure and simple shear and oblate-shape ellipsoids in strike-slip shear zones and prolate-shaped ellipsoids in dip-slip reverse shear zones. The strain set suggests that all shear zones were developed under subsimple shear deformational regimes involving thrusting and folding followed by wrench tectonics. The pure shear component of deformation was accommodated in folded domains between shear zones. Geochronological data suggest intermittent ductile shear zone activations from ca. 900–830 to 530 Ma, partially coeval with at least two major episodes of terrane accretion at 850–760 Ma and 610–585 Ma. The spatial and temporal record of shear zones within the Ribeira Belt indicates that some relate to assembly of the belt and represent either terrane bounding structures (e.g., Itapirapuã shear zone) or intraterrane structures (e.g., Ribeira, Figueira, and Agudos Grandes shear zones), whereas others are terrane bounding, postcollisional shear zones (e.g., Taxaquara shear zone) reactivated in an intracontinental setting (560–535 Ma).