P-T-fluid-deformation regime of the Ediacaran Serra do Cavalo Magro orogenic gold deposit, Ribeira Belt, Brazil

摘要

The Serra do Cavalo Magro orogenic gold deposit is located in the southern Ribeira Belt, Brazil. Gold-quartz veins are hosted in Calymmian (1500-1450 Ma) metasedimentary and metabasic rocks and Ediacaran (610-600 Ma) granitic rocks. Geochronological data available in previous literature and structural-petrological relationships with host rocks and shear zones indicate that the gold mineralization was formed within 580-540 Ma during the late-stage evolution of the Brasiliano-Pan African Orogeny, related with Gondwana supercontinent assembly. Petrological modeling indicates peak metamorphic conditions from 560 degrees C and 7 kbar (chlorite-biotite phyllite) to 625 degrees C and 6.8 kbar (garnet-biotite phyllite) recorded in immediate host rocks. Gold mineralization occurs in quartz shear and extensional veins, structurally controlled by NE-trending, second- and third-order sinistral, transcurrent shear zones. The NNW-trending subvertical and subhorizontal extensional veins are oriented 55-85 degrees in relation to the fault planes, indicating the shear zones were severely misoriented to frictional reactivation. Deformation within the shear zones was accommodated by bulging recrystallization of quartz aggregates, while feldspar aggregates from the granitic protolith underwent cataclastic flow or were replaced by sericite-epidote, producing phyllonites. Gold precipitated during stages of vein deformation within microfractures in shear and extensional veins with quartz previously subjected to bulging recrystallization. Metamorphic ore-bearing fluids are recorded in distinct assemblages of low to moderate salinities (1-18 wt.% NaCleq.) H2O-NaCl-CaCl2 and H2O-CO2-N-2-NaCl-CaCl2 fluid inclusions and CO2-N-2 inclusions. Microthermometric data indicate entrapment conditions of 250-350 degrees C and 0.3-2.3 kbar, recording hydrostatic to supralithostatic fluid pressures in post-peak metamorphic stages. Sharp fluid pressure fluctuations (0.4-1.6 kbar) resulted from earthquake rupture events and the fault-valve behavior induced circulation of fluids from distinct sources with limited partial fluid mixing, favoring the gold transportation and precipitation.