Fluid evolution and chemical controls in the Fazenda Maria Preta (FMP) gold deposit, Rio Itapicuru Greenstone Belt, Bahia, Brazil

The Fazenda Maria Preta (FMP) gold deposit is confined to two regional-scale, sinistral-oblique brittle–ductile shear zones which are located in a greenschist facies metamorphosed volcano-sedimentary sequence in the northern sector of the lower Proterozic Rio Itapicuru greenstone belt, NE Brazil. Alteration is pervasive throughout the shear zones and characterized by carbonatization and sericitization of intermediate volcanic, volcaniclastic, and sub-volcanic rocks. The orebodies are mainly shear veins hosted by, or directly associated with, highly deformed carbonaceous volcaniclastic rocks, breccia and stockwork systems or disseminations in felsic and mafic sub-volcanic bodies. Native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite, pyrite, pyrrhotite, sphalerite, and Fe-chlorite in the carbonaceous wallrocks. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy revealed that the vein quartz is largely dominated by primary, pseudosecondary and rarely secondary, populations of CO2–(±CH4±N2) inclusions (type I), whereas primary groups of low salinity (<6 wt.% eq. NaCl) H2O–CO2–(±CH4±N2) inclusions (type II) comprise the dominant inclusion type in only a few veins. The ThCO2 (L–V→L) data indicated a variation in the CO2 density of 1.05 to 0.60 g/cm3 for the type I inclusions and, conversely, a narrower range of 0.76–0.73 g/cm3 for the type II inclusions. The broader variation of the CO2 density for the CO2-rich fluid could have been the result of trapping of the vein fluid under a variable pressure regime, or by its re-equilibration during continuous deformation within the shear zone domains and reduction of the overburden pressure during uplift. The CO2-rich fluid inclusions provide no evidence that they have been the result of H2O loss either during infiltration (e.g., hydration reactions) or after trapping of an original H2O–CO2 fluid. Accordingly, the CO2±(CH4+N2) and the H2O–CO2–(±CH4±N2) fluids are considered as representatives of two distinct fluid regimes, which were active during the gold mineralizing events within the shear zones. Both types of ore fluids are interpreted as part of a deep metamorphic–magmatic hydrothermal system in which (i) a CO2-rich fluid, probably originated in the mantle, was transported to, and released in, higher structural levels mainly by tonalitic–granodioritic and alkaline magmas, whereas (ii) a H2O–CO2 fluid, generated by devolatilization reactions during the regional metamorphism, was either directly channeled into favorable structural sites (i.e., metamorphic fluid), or absorbed and later exsolved by the crystallization of felsic magmas (i.e., magmatic fluid sensu lato). In this context, gold deposition occurred between 320°C and 420°C and 2.1 to 4.4 kb and calculated values of fO2 between 10−26.5 and 10−32.4 bar reveal the relatively reducing nature of the mineralizing fluids. The ore paragenesis constrains fS2 to the range of 10−10.1–10−6.5 bar for these fluids. Under such conditions, the gold was transported mainly as bisulfide complexes by the H2O–CO2 fluid, but in the case of the CO2-rich fluid, the role of thiocomplexes or some other types of ligands is obscured by the lack of high temperature experimental data. Deposition of the metal occurred in response to redox changes, which accompanied fluid–carbon interaction, particularly a decrease in fO2 and fS2 of the fluid. On a microscopic scale, phisisorption and chemisorption processes at the fluid–iron sulfide interface may have further enhanced the deposition of the gold, particularly on the surfaces of precipitating pyrite and arsenopyrite.