PACROFI VI - Electronic Program

CO2 - Dominated Fluids in Greenschist and Amphibolite Facies Mesothermal Lode - Gold Deposits in Brazil

Roberto P. Xavier; Paulo F. R. Pires; Job de Jesus Batista

Instituto de Geociências - UNICAMP, Caixa Postal 6152, 13.081-970 - Campinas-SP, Brasil

Introduction. It has been well established that low salinity (<6 wt% eq. NaCl) H2O-(10 - 30 mol%) CO2 solutions are generally involved in the formation of Precambrian mesothermal lode-gold deposits hosted by greenschist facies metamorphosed volcano-sedimentary units. The characteristics of the ore-bearing solutions associated with gold deposits in higher-grade metamorphic settings are not well constrained, but fluid inclusion and mineral alteration assemblage investigations in some amphibolite facies deposits have pointed to fluids of the same nature (Groves, 1993). Notwithstanding this unique fluid signature, some lode - gold deposits are dominated by CO2-rich (H2O - deficient) fluids, represented by the widespread occurrence of monophase carbonic inclusions in the auriferous vein quartz (Garba & Akande, 1992; Huizenga, 1995). This is also the case of the Fazenda Maria Preta and Ambrósio gold deposits in the lower Proterozoic (2.2 to 2.0 Ga) Rio Itapicuru greenstone belt (RIGB), north-east Brazil (Xavier, 1991; Pires, 1996). The main scope of this work is to assess possible mechanisms which have generated CO2 - dominated fluids in these deposits and the implications to the precipitation of the gold.

Geological Setting of the Deposits. The Fazenda Maria Preta (FMP) and Ambrósio (AMB) gold deposits are confined to N-S regional scale, left-lateral strike-slip brittle-ductile to ductile shear zones, respectively, in the northern sector of the RIGB. At the FMP deposit, the orebodies include meter-wide quartz veins, sub-parallel to the foliation and hosted by carbonaceous phyllonites, and quartz-ankerite breccia and stockworks in rhyodacitic and dioritic sub-volcanic bodies. Hydrothermal alteration is pervasive throughout the shear zone domains and is chiefly represented by syntectonic carbonatization and sericitization, compatible with greenschist facies P-T conditions. Native gold occurs as free grains, or spatially related to the paragenesis pyrite+ arsenopyrite+pyrrhotite+sphalerite+chlorite, disseminated in the veins and altered wallrocks.

The AMB deposit is located in a narrow sequence of highly deformed sequence of mafic metavolcanics and clastic-chemical metasediments, tightly squeezed between two syn-tectonic granitic batholiths, and metamorphosed to amphibolite facies. The gold mineralization is closely associated with centimeter - wide quartz veins containing disseminations of sulfides (arsenopyrite, pyrite and pyrrhotite), at the contact of amphibolite (mafic volcanics) and garnet-andalusite-biotite schists (aluminous sediments).

Paragenetic and microstructural relationships in both deposits reveal that the gold-bearing veins were emplaced during syn to late deformation events within the shear zones, and broadly synchronous with regional metamorphism and granitic intrusions.

Fluid Inclusion Studies. CO2 - rich inclusions, mostly monophase at room temperature, are the dominant, or virtually the only fluid inclusion type, in the auriferous quartz veins of the greenschist facies FMP and amphibolite facies AMB deposits. In the FMP deposit, vein quartz commonly exhibits incipient recovery and these inclusions form large and pervasive intragranular swarms or planar arrays. In the AMB deposit, quartz veins have undergone intense recovery with quartz recrystallization, in which the CO2-rich inclusions appear restricted to clusters, intergranular trails, or along subgrain boundaries. In both cases, the TmCO2 values for the CO2-rich inclusions vary from -56.8 to -58.6oC and, combined with laser micro-Raman (LRM) analysis, demonstrate that the carbonic phase is composed of pure CO2 to a mixture of CO2 and <= 10 mol% of CH4 + N2. In the case of the FMP deposit, within-grain swarms of CO2-rich inclusions show ThCO2 (L) values ranging from -25 up to +30oC, indicating a large variation in the density of the carbonic phase (1.05 to 0.63 g/cm3 for pure CO2). Single swarms or planar arrays of monophase carbonic inclusions also display a similar broad range of the ThCO2, and yield multiple peak histograms.

The clusters and intergranular trails of CO2-rich inclusions in the quartz veins of the AMB deposit homogenize to the critical state, within the range of -8 to +28oC, with a well defined maxima between 23 and 30oC, equivalent to a variation of 0.85 to 0.57 g/cm3 for the CO2 density.

Low-salinity (0 up to 6 wt%eq. NaCl) H2O-CO2 inclusions were observed only in some gold-bearing veins of the FMP deposit, but seem to be completely absent in the mineralized veins of the AMB deposit. These inclusions occur as large swarms throughout the quartz grains and show considerable scattering of the degree of fill (DF= 0.1 - 0.8), suggesting that trapping may have occurred when the H2O-CO2 fluids were immiscible. Locally, however, the aqueous-carbonic fluids of these inclusions were trapped from an homogenous fluid, inasmuch as the DF of the H2O-CO2 inclusions for some swarms is less variable (DF= 0.45 - 0.70). The composition of their carbonic phase, as indicated by TmCO2 and LRM data, are broadly similar to the CO2-rich inclusions, but their ThCO2 (L) values are much less variable and tightly constrained between +16 to +28oC (0.76 - 0.73 g/cm3 for pure CO2).

Mineralizing Solutions, Gold Deposition and Generation of CO2-Dominated Fluids. The great abundance of CO2-dominated inclusion fluids indicates that the mineralizing solutions may have become saturated with a CO2 (+/-CH4 +/- N2)-rich phase during vein formation. Consequently, these CO2-rich fluids, which dominate most of the investigated gold-bearing veins of the greenschist facies FMP and amphibolite AMB deposits, are not considered as repreentatives of the original bulk composition of the solutions responsible for the transport and deposition of the gold. In order to carry significant amounts of silica and gold, these solutions must have originally been H2O-rich, since the solubility for these two components is limited in CO2-rich fluids. In this context, the bulk composition of H2O-CO2 inclusions in groups showing the least variation in the DF, as defined locally in the gold-bearing veins of the FMP deposit, is probably the closest to the composition the parental homogeneous ore-bearing fluids: low-salinity (< 6wt%eq. NaCl) aqueous solutions, containing 12 to 25 mol% CO2 and <= 5 mol% CH4 + N2.

The evolution of the ore-bearing H2O-CO2 fluids towards the CO2(±CH4 ± N2) saturation and gold deposition, can be envisaged by the following scenario, in the case of the investigated deposits of the RIGB:

(1) Phase separation of the ore-bearing H2O-CO2 fluids during the formation of quartz veins in dilatant zones within the shear zone domains would form H2O-rich and CO2-rich fluid phases. This fluid phase separation would be further enhanced by the enlargement of the solvus of the H2O-CO2 system, due to the addition of CH4 (and possibly N2; Naden & Shepherd, 1989). The incorporation of CH4 could have been the result of the interaction of the fluid with the carbonaceous matter, commonly found dispersed in metasedimentary units in both deposits, via the reactions: 2C + 2H2O = CO2 + CH4 or 2H2O + C = CH4 + O2 (Xavier, 1994a). Episodes of fluid phase separation would also trigger the deposition of the gold (transported as thiocomplexes) and sulfides, as a result of an increase in pH and fO2 of the residual aqueous solution. The P-T regime of these fluids was estimated at 360 -420oC/2.0-4.1kb for the FMP deposit, whereas the ore-related mineral assemblage suggests T > 500oC and similar pressure values for the AMB deposit.

(2) Due to contrasting differences in their wetting properties, the H2O-rich phase would be lost through the migration along quartz grain boundary, leaving behind the non-wetting CO2-rich phase, which would then be trapped as CO2-dominated fluids in inclusions. At a more restricted scale, part of the H2O could have been lost in the formation of OH- - bearing minerals during the metamorphic/hydrothermal process, as well as by its selective incorporation into migrating grain boundaries during dynamic recrystallization of the auriferous quartz veins (e.g. AMB deposit; Bakker & Jansen, 1991; Johnson & Hollister, 1995).

(3) Deformation acting upon the gold-bearing quartz veins during uplift may have induced CO2 density re-equilibration of these CO2-dominated inclusions, which would explain the large scattering of the ThCO2 values observed in both deposits.

Potential CO2 Reservoirs. The d13C composition of fluid inclusion CO2 is available only for the greenschist facies FMP deposit: the d13CCO2 values vary from -6.0o/oo to -10.2o/oo and show a mean at -7.9 +/- 1.3o/oo. This d13C compositional range indicate that the bulk of the carbon of the fluids was not derived from a reduced carbon reservoir (d13C= -23.2o/oo to -30.8o/oo; Xavier,1994b) nor from marine carbonates (d13C=-2o/oo to +4o/oo), but seems to be consistent with a magmatic/mantle source. The contribution of a magmatic/mantle source can be further attested by the occurrence of lamprophyres along the auriferous shear zone of the FMP deposit area.