Estudo de inclusões fluidas e isótopos de oxigênio e enxofre em veios de quartzo-carbonato-sulfetos do depósito Carvoaria Velha, lineamento aurífero Córrego do Sítio, Quadrilátero Ferrífero, MG, Brasil

Abstract

The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zone, which controls fourteen gold deposits, one of them being the Carvoaria Velha deposit. The mineralization is hosted in Archean rocks of Rio das Velhas greenstone belt, represented by carbonaceous phyllites and schists that exhibit alteration to quartz, carbonate and sericite, besides sulphides and sulphosalts The main gold mineralization styles at the Córrego do Sítio lineament are quartz-carbonate-sulphide±sulphosalt veins of varied distribution. The veins are classified as: V1 veins quartz-ankerite-pyrite-berthierite-gold parallel to the main regional foliation Sn, and usually mineralized in gold; V2 veins quartz-ankerite-pyrite developed at the crenulation cleavage Sn+1, extensional and rarely gold mineralized; V3 veins quartz-ankerite filling Sn+3 fractures, usually free of sulphides and sulphosalts; V4 veins quartz-calcite of restricted occurrence in metamafic dykes and sills; they crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulphides. Fluid inclusion data have identified a H2O-CO2±CH4-NaCl mineralizing fluid that may contain KCl, MgCl2 e FeCl2. The presence of CH4 in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units. Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate ~8.5 eq. wt% NaCl salinity, and minimal trapping temperature of 330±17oC; and ii) a late-stage fluid trapped in recrystallized quartz with lower ~4.6 eq. wt% NaCl salinity, and a minimal trapping temperature of 365±33oC. The pressure is variable from 0.5 to 5.5 kbars. Isotopic values of ä18Ofluid, Ä33S e ä34S suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH4 during fluid-rock carbon interaction may have caused some decrease in O2 of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S-2 for the formation of Fe sulphides and sulphosalts (Apy, Py, Po and Bert etc.), and outcome gold deposition.