Non-pegmatitic beryl related to Carboniferous granitic magmatism, Velasco Range, Pampean Province, NW Argentina

Abstract

The specialized leuco-monzogranite of the La Chinchilla Stock is a small Carboniferous stock located in the center of the Velasco Range, Pampean Province, La Rioja, Argentina. It is highly evolved and locally F- and Be-bearing, and has the potential for hosting U mineralization. Three different facies can be identified in the granitoid: border, porphyritic and equigranular facies. In all three facies the main minerals are quartz, microcline, plagioclase, biotite, and muscovite. Accessory minerals present in all facies include fluorite, zircon, and apatite. In addition, monazite, rutile, and uraninite occur as accessory minerals in the equigranular facies. Secondary minerals are muscovite, sericite, kaolinite, and opaque minerals. Secondary uranophane occurs in the equigranular and border facies. In localized areas, the equigranular facies contains small, green idiomorphic crystals of beryl (Be3Al2Si6O18) as accessory mineral. One of these beryl crystals was chemically analyzed for major and minor element contents using an electron microprobe and this information, along with fractional crystallization models and comparison with compositions of non-pegmatitic beryl from the literature, were used to understand the degree of evolution of the granitic melt. The chemical formula of beryl from the La Chinchilla Stock can be written as: C(Na0.014-0.033, K0.001-0.002, Ca0.001-0.004) T(2)(Be2.978-2.987, Li0.016-0.022) O(Al1.889-1.967, Fe0.045-0.103, Mg0.001-0.007, Mn0.001-0.007) T(1)(Si5.994-6.040O18). The alkali contents are low (Na2O<0.18 wt%; K2O<0.02 wt%), while FeOt is dominant among the divalent cations that substitute trivalent aluminum in the octahedral position of the mineral (FeOt/(MgO+MnO)>6; FeOt<1.27 wt%). In a longitudinal geochemical profile, Al enrichment is observed at the border while the highest Na content is found in an internal point. In a transversal geochemical profile, the highest concentration of Al is seen in an internal point while Na remains almost invariable. Ferromagnesian elements vary randomly within the crystal. This indicates compositional changes in the magma for Al, ferromagnesian elements, and Na. The FeOt content of the analyzed beryl is within the compositional range of other disseminated beryl from granitoids but slightly higher than that of beryl from hydrothermal veins and greisens. It contains similar to slightly lower amounts of FeOt, MgO, and Na2O than beryl from medium to little evolved granitic pegmatites. Overall, the composition of beryl in the La Chinchilla Stock is quite similar to that from medium to poorly evolved granitic pegmatites of the nearby Velasco Pegmatite District. The formation of beryl in the La Chinchilla Stock is attributed to precipitation from a F-bearing, highly fractionated, Al- and Si-rich melt saturated in BeO. A fractional crystallization model using Rb and Ba suggests that the beryl-hosting rock crystallized from the parent melt after extreme fractionation and 75% crystallization. The occurrence of beryl as a magmatic accessory mineral in the equigranular facies of the La Chinchilla Stock is indicative of a very high degree of fractionation of the parental magma.

Age and petrogenesis of the beryl-bearing granitic magmatism of the Velasco Pegmatite District, Pampeana Province, NW Argentina

“…The Lower Carboniferous geochemically evolved La Chinchilla granite (Grosse et al , 2009 is a very small body that intruded the Huaco granite. It is characterized by having beryl as an accessory mineral (Sardi et al 2016). The Huaco and the Sanagasta granites are not deformed nor intruded into older metagranitoids and mylonites (Grosse et al 2009), as the contact between both granitoid bodies is transitional.…”

Some Mineral-Geochemistry Aspects of Beryll Belokurichinskii Pluton of Gorny Altay

“…Берилл с идеальной формулой Be 3 Al-2 SI 6 O 18 является источником бериллия, который используется преимущественно в сплавах с другими металлами для получения легковесных и прочных сталей, а также в ядерной промышленности в качестве экрана электромагнитного излучения и высокотехнологичных решениях при создании ракетных двигателей, аппаратов медицинской диагностики [1][2][3][4]. Бериллий относится в России к группе стратегических и дефицитных металлов, запасы которых находятся на грани исчерпания.…”