CARLOS EDUARDO SILVA COELHO
cesc@ufba.br

Gênese dos fluidos nas zonas deformadas e mineralizadas ao no cinturão de rochas Verdes do Rio Itapicuru: jazidas de Fazenda Brasileiro e Fazenda Maria Preta. Uma reconstituição baseada eno estudo das inclusões fluidas em seu contexto microestrutural.

Université d'Orléans
Banca: Jean-Claude Touray, Claire Ramboz, Anne Marie Boullier, Marcel. A. Dardenne, Michel Faure, Alain Prost, Gaston Giuliani
Data: 21/11/94

CARLOS EDUARDO SILVA COELHO
cesc@ufba.br

Genesis of the fluids in the deformed and mineralized zones  of the Rio Itapicuru greenstone belt: Fazenda Brasileiro e Fazenda Maria Preta deposits. A reconstitution based in the fluid inclusions study at its microstructural context.

Universitée d'Orléans
Commitee: Jean-Claude Touray, Claire Ramboz, Anne Marie Boullier, Marcel. A. Dardenne, Michel Faure, Alain Prost, Gaston Giuliani
Date: 21/11/94

We studied the mineralogy and the fluid inclusions from gold-bearing quartz veins and their host rocks of two deposits of the Rio Itapicuru greenstone-belt, in the São Francisco Craton.
The Fazenda Brasileiro mine (FB) (150 tons of gold) is situated in the southern extremity of the belt, where it took a E-W direction. The mineralization hosted by altered basalts (XM facies) seems to be controlled by a sedimentary guide horizon, rich in carbon (GRX facies). Two tectonic events successively affected the region: the D1 event corresponds to a transcurrent dextral ductile shearing event which created the main foliation S1, followed by the D2 event which reflects brittle-ductile northward thrusting. Hydraulic fracturing is one of the processes responsible for the opening of the mineralized veins. The deformation of the quartz vein network and the development of the hydrothermal alteration are sin- to late-D2 event.
Three main lithologic facies were distinguished in the FB mine area according to the petrogenetic study of the quartz veins and their host-rocks: chlorite-magnetite schists (XM facies); organic-rich acidic tuffs (CLXv facies); and sulphide-rich quartz-albite altered rocks (SUF facies), resulting from hydrothermal alteration of the XM facies around mineralized concordant and discordant veins. Four quartz generations were recognized in the FB mine area, each of which were characterized by their specific fabric and fluid(FI)/solid inclusion content (analysed by Raman spectrometry, RS): (i) Generation Q0 groups the cogenetic pre-D1 porphyroclasts from the XM and CLXv facies, mainly the quartz from the CLXv facies with enterolitic texture, rich in carbon particles of semi-graphite type. This type of quartz is the deformed, metamorphosed and gold-bearing equivalent quartz, rich in hydrocarbons, with nodules of chalcedony and barite of the Cretaceous deposit of Munela, hosted in the ophiolitic rocks of Mirdita, Albany (Sinojmeri, 1992). Q0 traps primary vapour-rich FI, sometimes halite-satured, and aqueous FI CO2-CH4-bearing and halite-saturated. They commonly contain abundant 'semi-graphite' particles and trapped complex Ca-Mg-Fe-Mn carbonates with carbone relicts. (ii) Generation Q1 consists of syn-D1 fibers in the pressure shadows of pre-D1 clasts (quartz Qo, apatite, albite, magnetite) and it contains very small vapour-dominant FI and dominant aqueous FI. Q2 and Q2r quartz generations are characteristic of the mineralized quartz veins. (iii) Q2 quartz generation consists of plastically-deformed quartz porphyroclasts in mineralized discordant veins with primary type Lc water-deficient (= 1 mole% H2O) CO2-CH4±N2-bearing FI that commonly contain 'anthracite'-like trapped particles; (iv) Quartz Q2r, mainly developed in the concordant veins, results from the recrystallization of Q2 quartz; it contains both type Lc and Lwc CH4±CO2±N2±H2O FI, with rare anthracitic compounds, either inside (=primary FI) and/or along the Q2r grain boundaries. Additionally, types Lc and Lcw CH4±N2±H2O FI may occur in secondary trails crosscutting Q2r quartz.
The mineralogical and microthermometric studies permitted the establishment of the poliphasic hydrothermal history of the supracrustal rocks of the FB mine area in the Proterozoic: (i) Early hydrothermal seafloor metamorphism affected the pile (stage H1) prior to the D1 event. In the CLXv facies, this induced the maturation of interstratified organic matter to 'semi-graphite'. In the tuffs, long-lived fluid unmixing caused the formation of oxidized brines which in turn caused late hematitization and the precipitation of abundant carbonates (also partly formed after the volatilization of organic matter, as shown by RS). Gold is seen as inframicroscopic particles attached on the semi-graphite from the CLVv facies. The formation of all pre-D1 porphyroclasts in both the XM and CLXv facies (in particular the magnetite in the XM schists) are related to stage H1. The ƒO2 in the XM schists at stage H1 was buffered to ˜QFM values by the Fe2+-bearing wall-rock, and not by the boiled oxidized solutions. (ii) The opening of the vein system occurred during the prograde metamorphism (400°-500°C, P = 2 to 3,5 kb; gradient of 40°C/km during D1?) by the combustion of earlier-formed semi-graphite in order to reabsorb the metastable semi-graphite-hematite association inherited from the H1 stage. The composition of the near water-free C-O-H fluids is compatible with that of fluids trapped on both parts of the ƒO2-upper limit of stability of 'graphite' (ƒO2=2.10-25 bar at 450°C and 3 kbar). In this way, it is demonstred the primary character of the water-free FI trapping, which will evolve in the brittle-ductile deformation during D2 event. Gold was probably introduced in the vein system attached on the semi-graphite and left behind as the carbon compounds volatilized; (iii) the trapping of low compressible fluids as secondary type Lwc FI in the Q2 and Q2r quartz implicates in an increase in the fluid pressure regime compared to the P-T conditions previously inferred, compatible with the thrusting of the Barrocas dome during late D2 deformation. Only the isochores of those low dense Lwc FI may explain the formation of sphene in the SUF facies in prograde P-T conditions: P= 4.5 kbar and T= 450°C. Those high temperatures probably favoured gold solubilization (Gibert et al., 1993). The textural study of SUF facies showed that gold was precipitated during the late-D2 retrograde evolution, when the vein system evolved in the brittle-ductile domain, by the following processes: cooling; reduction of gold-sulphide-complexes in the fluid at the surface and/or fractures of the sulphides (mainly arsenopyrite) and transient boiling (late hematite and sericite).
100 km to the north, the only brittle-ductile progressive D1 deformation event is regionally recognised in the greenstone-belt, responsible for the NS foliation dipping of 60°W with shallowly-dipping stretching lineations plunging predominantly to the north. The gold mineralization of Fazenda Maria Preta (FMP) is hosted in shear zones are which cut rocks of andesitic composition interlayered with lenses of pyroclastics and carbon-rich (anthracite type) sediments. The mineralized quartz veins are mainly hosted in late D1 shear fractures and rarely in extensional fractures. The analysis of the structural indicators (mylonitic foliations, stretching lineations) define a sinistral strike-slip deformation generated by a dominant non-coaxial simple shear component with minor early oblique-reverse motions. The ductile to brittle-ductile deformation is limited. The FI in the quartz contain CO2±CH4+N2 and they are aqueous (type Lwc) or water-deficient (type Lc which may contain 'anthracite'-like particles). Both types of FI may occur in secondary fluid inclusion planes (FIPs) crosscutting vein quartz, parallel to vein and mylonitic foliation directions. Both type Lwc and Lc inclusions are also present at the grain boundaries of recrystallized quartz grains or at sub-grain boundaries of deformed grains. Note that the carbon compounds in metasediments around the veins are also anthracitic. The intersection of isochores representative of cogenetic type Lwc and Lc FI fixes the P-T conditions of vein formation and fracturing at FMP at ~350°C and 2kbar. The absence of thrusting (=D2 event at FB) in this part of the belt explains why in FMP there is no trapping of low compressible type Lwc FI, analogous to those measured at FB. Probably the vein-opening process at FMP was favoured by the combustion process of carbon particles analogous to that of FB. However, the volatilization character at FMP was not as violent as at FB (constant trapping of aqueous Lwc and water-deficient Lc FI during the opening and the deformation of the veins). The abundance of hematite and anthracite coexisting in the carbon levels of the metasedimentary host-rocks around the veins indicates that temperatures at FMP have only transiently reached the blocking temperature of the 'graphite'-fluid, i.e., 400°C and never overpassed it. Smaller economic gold concentrations at FMP than at FB can be explained by combined chemical and mechanical effects of lower temperatures in the former deposit: lower gold solubilization, persistence of metastable association hematite-anthracite, lower permeability of the shear zone caused by limited plastic deformation and quartz recristalization.

CARLOS EDUARDO SILVA COELHO
cesc@ufba.br

Genese des fluides dans les zones deformées et mineralisées enor de la ceinture de roches vertes de Rio Itapicuru (Bresil): gisements de Fazenda Brasileiro e Fazenda Maria Preta. Une reconstitution basée sur l'étude des inclusions fluides dans leur contexte microstructural.

Universitée d'Orléans
Jury: Jean-Claude Touray, Claire Ramboz, Anne Marie Boullier, Marcel. A. Dardenne, Michel Faure, Alain Prost, Gaston Giuliani
Date: 21/11/94

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