Geologia, petrografia e geoquímica do granito Bom Jardim, região de São Félix do Xingu, Província Mineral de Carajás

Abstract

In the Carajás mineral province, located in the Amazonia Central geochronologic province, three important granite suites (Fig 1) were individualized on geologic, petrochemical, geochronological and isotope data in Jamon, Velho Guilherme and Serra dos Carajás (CPRM/DNPM 1999, Teixeira et al. 2002a, Dall´Agnol et al. 2005). These three suites are composed of ~1.89 to 1.86 Ga (Tab. 1) nondeformed stocks and batholiths. The absence of deformation along the massifs, the discordant character of the plutons and the presence of micrographic intergrowths suggest a high level of emplacement. All granites display characteristics of A-type granites and within-plate signature. The plutons of the Jamon suite show oxidized character, while the plutons of the Serra dos Carajás and Velho Guilherme suites are moderately reduced and reduced, respectively. In the domain of the Velho Guilherme suite, São Felix do Xingu region, the massifs Antonio Vicente, Serra da Queimada, Velho Guilherme, Mocambo, Ubim-sul and Benedita are tin (±W)-mineralized granites. The mineralization is related to evolved granitic facies affected by late to postmagmatic alteration or hosted in small bodies of greisens. The granitic rocks of the Velho Guilherme suite are dominantly hololeucocratic to leucocratic, monzogranite to syenogranite with subordinate alkali-feldspar granite. Show low contents of TiO2, Al2O3, CaO, MgO, P2O5, Sr, Ba and Cl and moderate of Hf, U and Zr. Rb, Y, F, Li, Th, Nb, Ta, Ga, and Rb/Sr, Rb/Ba, F/Cl ratios are systematically high. The Bom Jardim granite, inserted into Velho Guilherme suite, is a rounded Sn-W mineralized massif that cover ~ 400 km2, outcrops south of São Felix do Xingu city, and is intrusive in intermediate to felsic volcanic rocks of the Uatumã Group. It is dominantly constituted by isotropic, pink and medium-to coarse grained monzogranite and syenogranite. Chloritized biotite is the main primary mafic phase. Aplite dikes are common cross cutting the granite in different directions. These rocks are affected in different intensities by late- to posmagmatic alteration. Greisenized rocks containing small primary concentrations of cassiterite+wolframite, besides quartz veins with milimetre to centimetre-sized crystals of wolframite+pyrite+fluorite, were identified in pervasively altered cupolas. Magnetic Susceptibility (MS) data showed values of 5,34x10-4 SIv (minimum valor) to 9,18x10-4 SIv (maximum valor), similar to those found in the syenogranitic rocks of the Antonio Vicente pluton. Scanning Electron Microscopy (SEM) studies indicate that the Sn and W mineralization are dominantly associated with syenogranite and greisenized rocks. The zircons of the different facies of the Bom Jardim granite are characteristically enriched in Hf, Y, Th and U, and display low Zr/Hf ratios, such as zircon crystals of other specialized tin-granites of the Amazonian Craton. The obtained results demonstrate that a preliminary evaluation of the potential for Sn and W (±Nb+Ta) in evolved granites may be based on zircon compositional studies and in EDS semi-quantitative analyses obtained with a SEM. Geochemically, the rocks of the Bom Jardim granite have low contents of TiO2, MnO, MgO, CaO e P2O3. The Al2O3 is near of 13%, with RG1 displaying higher values (14%). Na2O e K2O show medium contents, except in the RG. The Fe2O3 rarely is superior to 2.0%, but it is higher in the greisenized rocks 1 (GR1), with ~7% (medium value) and greisenized rocks 2 (GR2), with ~3.7% (medium value), probably resulting from the presence of syderophyllite. Rb and Ba show inverse behavior. Rb increase in the sense of MzG-LMzG-SGRG1-RG2, with average of 294, 711,790, 874 and 1835 ppm, respectively. Ba display, in this same sense, medium contents of 670, 87, 18, 9 and 2 ppm, respectively. Sr shows similar behavior of the Ba, except in the GR, originating an increase in the Rb/Sr ratios in the sense of the more fractionated rocks. Sn and W show enrichment from less evolved to the more evolved rocks, with the GR2 displaying higher concentrations. F and Li are high in all studies facies (1400 to 3000 ppm and 15 to 134 ppm, respectively), but the higher values are found in the GR1 (21000 ppm and 698 ppm). The Rare Earth Elements (REE) show similar fractionated patterns, with negative Eu anomalies increasing from monzogranite to syenogranite rocks. The GR have similar patterns, but the GR1 are more enriched in REE than the GR2. The geochemical diagrams indicate that the fractionated crystallization was the dominant process during the magmatic evolution of the Bom Jardim granite, such as observed in others granitic bodies of the Velho Guilherme suite. Based in the obtained data and in the petrographic, geochemical and geochronological similarity with others granites of the Velho Guilherme suite, the Bom Jardim granite can be included in the context of this important Paleoproterozoic granitic suite.