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-scal e, sinistral-oblique brittle-ductile shear zones which are located in a gre enschist facies metamorphosed volcano-sedimentary sequence in the northern sector of the lower Proterozic Rio Itapicuru greenstone belt, NE Brazil. Al teration is pervasive throughout the shear zones and characterized by carbo natization and sericitization of intermediate volcanic, volcaniclastic, and sub-volcanic rocks. The orebodies are mainly shear veins hosted by, or dir ectly associated with, highly deformed carbonaceous volcaniclastic rocks, b reccia and stockwork systems or disseminations in felsic and mafic sub-volc anic bodies. Native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite, pyrite, pyrrhotite, sphale rite, and Fe-chlorite in the carbonaceous wallrocks. Fluid inclusion studie s by microthermometry and laser Raman microspectroscopy revealed that the v ein quarts is largely dominated by primary, pseudosecondary and rarely seco ndary, populations of CO2-(+/- CH4 +/- N-2) inclusions (type I), whereas pr imary groups of low salinity(< 6 wt.% eq. NaCl)H2O-CO2-(+/- CH4 +/- N-2) in clusions (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/cm(3) for the type I inclusions and, conversely, a narrower ra nge of 0.76-0.73 g/cm(3) for the type II inclusions. The broader variation of the CO2 density for the CO2-rich fluid could have been the result of tra pping of the vein fluid under a variable pressure regime, or by its re-equi libration during continuous deformation within the shear zone domains and r eduction of the overburden pressure during uplift. The CO2-rich fluid inclu sions provide no evidence that they have bren the result of H2O loss either during infiltration (e.g., hydration reactions) or after trapping of an or iginal H2O-CO2 fluid. Accordingly, the CO2 +/- (CH4 + N-2) and the H2O-CO2- (+/- CH4 +/- N-2) 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 dee p metamorphic-magmatic hydrothermal system in which (i) a CO2-rich fluid, p robably originated in the mantle, was transported to, and released in, high er structural levels mainly by tonalitic-granodioritic and alkaline magmas, whereas (ii) a H2O-CO2 fluid, generated by devolatilization reactions duri ng the regional meramorphism, was either directly channeled into favorable structural sires (i.e., metamorphic fluid), or absorbed and later exsolved by the crystallization of felsic magmas (i.e., magmatic fluid sensu late). In this context, gold deposition occurred between 320 degrees C and 420 deg rees C and 2.1 to 4.4 kb and calculated values of fO(2) between 10(-26.5) a nd 10(-32.4) bar reveal the relatively reducing nature of the mineralizing fluids. The ore paragenesis constrains fS(2) 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 accomp anied fluid-carbon interaction, particularly a decrease in SO, and SS, of t he fluid. On a microscopic scale, phisisorption and chemisorption processes at the fluid-iron sulfide interface may have further enhanced the depositi on of the gold, particularly on the surfaces of precipitating pyrite and ar senopyrite. (C) 1999 Elsevier Science B.V. All rights reserved.