Chysoberil-Sillimanite association from the Roncadeira pegmatite, Borborema Provinze, Brazil: implications for gemstone exploration

Abstract

The first occurrence of chrysoberyl in the Borborema Pegmatite Province (BPP) in northeastern Brazil is reported from up to 0.5 m large boulders in the dumps covering the Roncadeira pegmatite, worked in the 1940's for Sn- and Ta-ore concentrate. The chrysoberyl occurs in tabular, transparent light yellow crystals, up to two centimeters across. It is associated with quartz and fibrolitic sillimanite along centimeter-thick sigmoidal shear zones crosscutting banded sodic aplite facies of the pegmatite. Along the crystal borders, the chrysoberyl contains commonly small rounded inclusions of quartz and microscopic fibrous radial aggregates of sillimanite. A metamorphic-metasomatic origin of the chrysoberyl by the reaction Beryl + albite + muscovite + H+ = chrysoberyl + quartz + sillimanite + Na + K + H2O,seems to be the most plausible to explain the observed textural relations. Chrysoberyl contains 1.25 to 1.81 wt. % FeOtot. Contents of SiO2 and other, potentially chromophore oxides of Cr, V, and Ti are mostly below limits of detection. A few exceptions for Si and V are below 0.01 wt. % oxide. The average formula along an electron-microprobe line scan from core to rim of a selected Cbl crystal, is: Be0.931 ± 0.017(Al2.028 ± 0.011,Fe0.026 ± 0.002)Σ2.054 ± 0.013 O4 (± 1σ, n = 92). The origin of sillimanite + chrysoberyl + quartz under amphibolite-grade metamorphic conditions (~600 °C, 3.5 to 5 kbar) implies either the existence of a new generation of syn-metamorphic REL-pegmatites in the BPP or a late-tectonic recurrent increase in P-T conditions in the area. Another, less probable, alternative is a metastable formation of chrysoberyl and sillimanite at lower, retrometamorphic P-T conditions. The interaction of similar pegmatites known to occur south and north of Roncadeira with metabasic rock intercalations in the Seridó Formation opens the perspective for the finding of the gem-variety alexandrite in the BPP.