Chrysoberyl-sillimanite association from the Roncadeira pegmatite, Borborema Province, 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 (Cbl) 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 + H 2 O, seems to be the most plausible to explain the observed textural relations. The origin of sillimanite + chrysoberyl + quartz under amphibolite-grade metamorphic conditions (~600 °C, 3.5 to 5 kbar) implies either an 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.

Chrysoberyl and associated beryllium minerals resulting from metamorphic overprinting of the Maršíkov–Schinderhübel III pegmatite, Czech Republic

“…Magmatic chrysoberyl in assemblage with beryl, sillimanite and gahnite was described from the highly-evolved, peraluminous Belvís de Monroy leucogranite, Spain (Merino et al , 2013). However, most occurrences of chrysoberyl consider it to have formed from the breakdown of primary beryl as a result of metamorphic overprinting of the pegmatite e.g: Kolsva, Sweden; Haddam, USA; and Maršíkov, Czech Republic (Franz and Morteani, 1984, 2002; Černý et al , 1992); Kalanga Hill, Zambia (Žáček and Vrána, 2002); Ethel Mary and Virorco, Argentina (Lira and Sfragulla, 2011; Galliski et al , 2012); New York, USA (Lupulescu et al , 2012); Roncadeira, Brazil (Beurlen et al , 2013); and Mt. Begbie, Canada (Dixon et al , 2014).…”

Nigerite Group “Zinconigerite-(6n6s)” Variety in the Roncadeira Pegmatite, Nova Palmeira-County State of Paraíba, Borborema Province, Ne-Brazil

“…This paragenesis was complemented by Beurlen et al (2010Beurlen et al ( , 2013 with the description of chrysoberyl in this pegmatite (the first one in the BPP) in association with sillimanite and presence of "zinconigerite-6N6S" a variety of the nigerite group. Most of the pegmatite lenses of the Roncadeira-Corredor group today are almost completely covered by regolith, waste and soil.…”

“…Fibrous sillimanite, quartz and porhyroblastic chrysoberyl found only in reddish intensively sheared parts of the sample (Figs. 5 and 6) were discussed in detail by Beurlen et al (2013) and allowed to establish the temperature and pressure of its formation at approximately 600 ° C and 3.5 to 5.0 kb. Preliminary EPMA (Electron Probe Micro Analyses) data in that study indicated a Zn-dominant composition of the nigerite group grains, without a detailed description.…”

Fine-grained petalite and spodumene dykes in the Stankuvatske Li-deposit, Ukrainian Shield: products of tectono–metamorphic recrystallisation

“…9d). An origin of chrysoberyl by metamorphic overprint has also been documented in several beryl-bearing granitic pegmatites Morteani, 1984, 2002;Černý et al, 1992;Beurlen, et al, 2013). The possible reaction describing the formation of metamorphic chrysoberyl could be written as: Be 3 Al 2 Si 6 O 18 (beryl) + 4(K, Na)AlSi 3 O 8 (feldspar) + 4H + 3BeAl 2 O 4 (chrysoberyl) + 18SiO 2 (quartz)…”