Metodologia de análise de inclusões fluidas por LA-ICP-MS e estudo de inclusões de líquidos silicáticos aplicados ao depósito de Sn e In do Maciço Granítico Mangabeira - Goiás, GO

Abstract

This dissertation deals with the methodology of LA-ICP-MS analyses of fluid and melt inclusions of the Mangabeira granitic massif, which forms part of the Goiás Tin Province (PEG) in the The northern Goias state. The equipment is composed of an Agilent 7500ce quadrupole ICP-MS and an excimer laser (ArF) 193 nm Lambda Physik Geolas operated with helium. The NIST SRM 610 and the element sodium were selected external and internal standards, respectively. The LA-ICP-MS analyses of fluid inclusions helped to decipher the chemical characterization of mineralizing fluids that deposit. The results show a precision better than those obtained in the literature, and a good efficiency of the instrument to remove polyatomic interferences even for Fe, K and Ca. Primary and secondary fluid inclusions composed of two- and three-phases were analyzed in quartz and topaz coming from three rock types of the Mangabeira deposit which are: (1) Li-siderophyllite granite (g2d), (2) topaz-albite granite (TAG) and (3) topazite. Iron and potassium concentrations in the host quartz arrived at up to 150 and 190 ppm, respectively. Between the other elements present in the host, only iron showed interference in the analysis of fluid inclusions. Sodium showed higher concentrations in the fluids analyzed (3.5 to 5.0%, 0.5 to 6.5%, 0.2 to 2.0% for g2d granite, TAG and topazite, respectively) than in the whole rocks (2-4% for g2d granite and TAG, and 0.5% to topazito). These results suggest that the hydrothermal fluids related to Mesoproterozoic magmatism of the Mangabeira massif are preserved in spite of the Neoproterozoic Brasiliano event that took place in Brasília Belt. Primary fluid inclusions in the TAG contain up to 3,330 ppm of Sn, while the topazite may contain up to 7,850 ppm of In. Both Sn and In are absent in the g2d granite. The results suggest that g2d granite fluids did not contribute to the concentration of tin, but that the hydrothermal fluids derived from the topaz-albite granite were responsible for the formation of the deposit. As for the indium, TAG metasomatism remobilized several elements and concentrated them in topazite. This concentration turn indium into a sub-economic element in the deposit. The Mangabeira tin deposit was generated from a mixture of two distinct fluids, a high temperature and highly saline fluid (325-401°C and 40 to 48.5% NaClequiv., respectively) of igneous origin, and another low temperature and median saline fluid (129-211°C and 0% to 19.5% NaClequiv.) interpreted as meteoric fluid. There is high concentration of arsenic in some massive rocks like the topazite (above 3.5%). This is reflected by several arsenates found in the metasomatic rocks studied. The fact that As partitions preferentially into the vapor phase suggests that arsenic may be a good indicator mineral for the mineralization in the Mangabeira massif. Although topazite occurs spatially associated with topaz-albite granite, the concentrations of Ba, Sr, As, Ta and W fluid inclusions reach the same levels in secondary inclusions of the Li-siderophyllite granite and primary inclusions of the topazite, suggesting that fluids that altered g2d granite also contributed to the formation of the topazite. Three groups of silicate inclusions were identified in three lithologies of the Mangabeira massif which are (1) biotite granite (g1c), (2) g2d granite and (3) topaz-albite granite. In the first and last one, the inclusions are classified as primary and azonal. The large size of all inclusions was probably due to the high water concentration in the melt. The large size might have facilitated the crystallization of these inclusions. The results of microprobe analysis do not necessarily reflect the concentration present in the magmatic fluid inclusions trapped in silicates. Thus, only results with levels of around 70% SiO2 can approach the values expected for magmatic fluid trapped. Heating studies suggest temperatures from 865 to 925°C for homogenization of the melt inclusions of the g1c granite. According to the type of magmatism and behaviour of melt inclusions, they are too high to represent the crystallization temperature of the Mangabeira granite.