ALEXANDRITA DO PELA EMA, MINAÇU/GO: INCLUSÕES FLUIDAS E CONDIÇÕES GENÉTICAS

Abstract

A primary alexandrite mineralization occurs in metasedimentary rocks of the Serra da Mesa Group (GSM) in the district of Minacu (Goias State), near the Pela Ema River, SE border of the Serra Dourada granite-gneissic dome (GrSD). Among the varieties of chrysoberyl, 10 to 15% are alexandrite of good gemological quality. Most crystals show pseudo-hexagonal habits due to multiple twinnings, with dimensions from 0.4 to 3cm. They contain inclusions of quartz, muscovite, biotite, garnet, monazite and a relatively high number of fluid inclusions (IF). The GSM in the area is represented by biotite-muscovite-quartz schist containing chrysoberyl, garnet, staurolite and kyanite porphyroblasts, fine lenses of feldspatic schist, thin amphibolite layers, pegmatites, exogreisens and quartz veins sometimes with emerald mineralizations. Optical studies of alexandrite crystals revealed the presence of primary and pseudosecundary IF. The microthermometry of primary IF showed that during the formation of the alexandrite aquocarbonic solutions rich in NaCl, CaCl 2 , and probably KCl and AlCl 3 were present, with salinities varying from 18.5 to 21.5 wt % NaCl equiv. and densities from 0.96 to 1.03g/cm 3 . Estimatives of P-T values resulted in the following ranges: 6.1-7.5 kbar at 535-576 o C and 4.4-5.9 kbar at 530-565 o C, depending on the thermodynamic equations used. The conditions for formation of chrysoberyl occurred at medium-grade amphibolite facies, determined by the intersection of the fields generated by the paragenetic associations, for kyanite and staurolite, with the isochores diagrams from the IF analysis. The studies indicate that the formation of the crysoberyl started with the Be liberated from the quartz vein zone with beryl (emerald) and remobilized in the metamorphic fluids, where the precipitation may have been conducted by the presence of Al 3+ from the host rocks.