Fluid regimes, fault-valve behavior and formation of gold-quartz veins — The Morro do Ouro Mine, Ribeira Belt, Brazil

Fluid inclusion studies, petrographic analyses and structural analyses for Neoproterozoic auriferous veins from the Morro do Ouro mine, Ribeira Belt, southeastern Brazil, are presented. The deposit is classified as an orogenic gold deposit. The vein-type Au mineralization at the mine is restricted to structurally controlled domains in a low-grade Calymmian (ca. 1500–1450 Ma) metasedimentary sequence that also hosts syngenetic Au mineralization. Two main auriferous quartz vein systems are present: (i) NW-trending subvertical extensional veins and (ii) NE-trending subvertical veins parallel to the axial surface of tight folds. The mineralized veins are adjacent to a major dextral transcurrent fault zone, and their structural relationships indicate that this fault is severely misoriented for frictional reactivation. The NW-trending veins present a fluid inclusion assemblage dominated by CO2N2CH4 inclusions, H2OCO2N2CH4NaClCaCl2 inclusions and H2ONaClCaCl2 inclusions, both with highly variable salinities (0.5 to 52 wt.% NaCl equivalent), while the NE-trending veins are dominated by H2OCO2N2CH4NaClCaCl2 and H2ONaClCaCl2 inclusions also of highly variable salinities (1 to 45 wt.% NaCl equivalent). The entrapment of fluids occurred at temperatures between 225 and 240 °C in the NW-trending veins and at approximately 208 °C in the NE-trending veins, involving processes of fluid immiscibility and mixing between fluids of contrasting compositions. The NW-trending extensional veins were formed predominantly under pressure fluctuating between near-lithostatic and strongly supralithostatic conditions (100–250 MPa) during pre-seismic failure stages. The NE-trending veins precipitated predominantly under subhydrostatic to lithostatic pressure conditions (10–150 MPa), following the discharge of fluids from different reservoirs along the ruptured zone after earthquake rupture stages. The hydrothermal fluids were probably enriched with gold through interactions with the host rocks, and its precipitation is attributed to changes in physicochemical properties due to fluid immiscibility and mixing between fluids of highly contrasting salinities, as a consequence of cyclic fluctuations in the values of fluid pressure and tectonic stresses accompanying episodes of seismogenic fault activity.