TOPÁZIO IMPERIAL DAS MINAS DO VERMELHÃO E JJC, OURO PRETO, MG: ESTUDOS DE INCLUSÕES FLUIDAS E CONSIDERAÇÕES GENÉTICAS

Abstract

Petrographic studies of fluid inclusions of the imperial topaz from Vermelhão and JJC mines, Ouro Preto, Minas Gerais State, permitted to group them in two different types: primary three-phase inclusions, with regular shapes, and pseudosecondary two-phase inclusions, with regular and irregular shapes, both of them with varied orientations. Microthermometric analyses showed that the mineralizing solution was composed by CO₂ -bearing fluids, in which the eutectic temperature suggested the presence of Ca²⁺ , Mg²⁺ , Na⁺ , and, possible, Al³⁺ ions in solution. The minimal trapping temperature and pressure of the primary fluid inclusions, and consequently of the topaz formation in the Vermelhão and JJC mines, determined by the microthermometric results, were 410°C/4,930bar and 412°C/5,240bar, respectively. The pseudosecondary inclusions presented lower T-P values (300°C/2,650bar) than the primary ones. The small differences between primary inclusions in both deposits suggested that the topaz crystals were formed after the hydrothermal fluids at slightly distinct P-T conditions. Based on the reported results and on the genetic models described in the literature, we could propose two hypotheses for the imperial topaz genesis. The first may be related to the uplifting of the deposits, which occurred by thrust faults and folding with west vergence, during the Brasiliano orogeny. The other one would be related to the normal faults (horsts and grabens) occurred during the Cretaceous. Both of them would explain the relatively high pressures obtained, that could also be responsible for the formation of OH-rich topaz, a distinctive feature of the Ouro Preto deposits.