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

摘要

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 dissemination 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 CO_2-(+- CH_4 +- N_2) inclusions (type I), whereas primary groups of low salinity (< 6 wt.% eq. NaCl) H_2O-CO_2-(+- CH_4 +- N_2) inclusions (type II) comprise the dominant inclusion type in only a few veins. The ThCO_2(L-V -> L) data indicated a variation in the CO_2 density of 1.05 to 0.60 g/cm~3 for the type I inclusions and, conversely, a narrower range of 0.76-0.73 g/cm~3 for the type II inclusions. The broader variation of the CO_2 density for the CO_2-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 CO_2-rich fluid inclusions provide no evidence that they have been the result of H_2O loss either during infiltration (e.g., hydration reactions) or after trapping of an original H_2O-CO_2 fluid. Accordingly, the CO_2 +- (CH_4 + N_2) and the H_2O-CO_2-(+- CH_4 +- 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 deep metamorphic-magmatic hydrothermal system in which (i) a CO_2-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 H_2O-CO_2 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 deg C and 420 deg C and 2.1 to 4.4 kb and calculated values of fO_2 between 10~(-26.5) and 10~(-32.4) bar reveal the relatively reducing nature of the mineralizing fluids. The ore paragenesis constraings fS_2 to the range of 10~(-10.1) - 10~(-6.5) bar for these fluids. Under such conditions, the gold was transpoted mainly as bisulfide complexes by the H_2O-CO_2 fluid, but in the case of the CO_2-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 accompanie fluid-carbon interaction, particularly a decrease in fO_2 and fS_2 of the fluid. On a microscopic scale, phisisorption an chemisorption processes at the fluid-iron sulfide interface may have further enhanced the deposition of the gold, particularl on the surfaces of precipitating pyrite and arsenopyrite.