Pegmatitic Beryl as Indicator of Melt Evolution: Example From the Velasco District, Pampeana Pegmatite Province, Argentina, and Review of Worldwide Occurrences

Abstract

Rare-element class, beryl type, beryl-columbite-phosphate subtype pegmatites of the Velasco district from the Pampeana Pegmatite Province, Argentina, contain varieties of green, yellow, and aquamarine beryl. The major and trace element chemistry and dimensions of the unit cell parameters of beryl from ten pegmatites from the Velasco district were used to identify differences among beryl types, determine chromophore elements, and determine the relative degree of fractionation of the pegmatites. Concentrations of Rb, Li, Cs, and Na in all beryls analyzed are among the lowest measured in pegmatitic beryl of similar colors worldwide. Within and among individual pegmatites, an increase in Li and Cs contents and decrease in Na/Li ratios in the order green beryl → yellow beryl → aquamarine suggest that green beryl formed in the early stages of crystallization of the pegmatites while yellow beryl and aquamarine formed from more evolved fluids in the late stages of crystallization. The chemical variations from green beryl to yellow beryl to aquamarine reflect crystallization during melt fractionation and/or crystallization of Fe-Mn phosphate minerals. Beryl from the El Bolsoncito and El Principio pegmatites, with the highest Li and Cs and some of the lowest Fe + Mg contents in the studied beryl, reflects the highest degree of evolution among the Velasco pegmatites. Within individual pegmatites Cr contents in beryl decrease in the order green beryl → yellow beryl, and green beryl → aquamarine, consistent with Cr being a chromophore element for green beryl. Niobium contents increase from green to yellow beryl, and from green beryl to aquamarine, indicating that Nb tends to concentrate in late forms of beryl. The absence of a systematic change in the size of unit cell parameters a and c with varying compositions and the small size of the c parameter reflect the extremely limited incorporation of Li, Na, and Cs in the mineral. A compilation of beryl compositions worldwide shows the highest degree of fractionation for the Tanco, Koktokay No. 3, Minas Gerais, Kaatiala, Namivo, Bikita, Czech Republic, and Greer Lake pegmatites.

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

“…For comparative purposes, the composition of beryl from the La Chinchilla Stock is plotted along with compositional data for non-pegmatitic beryl and beryl in pegmatites from the Velasco District, which are hosted in the Huaco and Sanagasta granites (Figs. 4, 5;Sardi and Heimann, 2014). A negative correlation is observed between the Al 2 O 3 and FeO t +MgO+MnO (wt%) contents in beryl from various sources (Fig.…”

“…Beryl, with an ideal formula given by Be 3 Al 2 Si 6 O 18 , is a common accessory mineral in granitic pegmatites (Tindle and Breaks, 1998;Charoy, 1999;Černý et al, 2003;Alfonso and Melgarejo, 2008;Thomas et al, 2009Thomas et al, , 2011Uher et al, 2010;Rao et al, 2011;Sardi and Heimann, 2014), especially those of the LCT (Li-Cs-Ta) family (based on the classification of Černý, 1991), occasionally appears associated with chrysoberyl in Be-rich pegmatites (Soman and Nair, 1985;González del Tánago, 1991), as well as in highly evolved granites (Charoy and Noronha, 1996;Charoy, 1999;Merino et al, 2013). It is a source of Be, which is used primarily for alloys with other metals to achieve a lightweight and durable steel, and for nuclear and high technology applications (Barton and Young, 2002).…”

Major and Trace Element Geochemistry of Dayakou Vanadium-Dominant Emerald from Malipo (Yunnan, China): Genetic Model and Geographic Origin Determination

“…The Dayakou deposit was classified as tectonic magmatic-related emerald deposit hosted in meta-sedimentary rocks (Type IB) [10]. Considering the presence of syntectonic intrusions of emerald-bearing pegmatites and quartz veins and the highest concentration of Cs ever reported for emeralds, with average content of 1754 ppm, and with average Li content of 353 ppm, a magmatic origin is preferred for the parental fluids of Dayakou emeralds [1,4,10,14,16,22,51]. In addition, the enrichment of Cs, W, Sn, and As in emeralds and the presence of scheelite, tourmaline in pegmatite veins indicate a supply of alkali and incompatible elements such as Be, Rb, Cs, W, Sn, As, F, and B from the parental fluids.…”

“…With the ideal formula of Be 3 Al 2 (Si 6 O 18 ), emerald crystallizes in a hexagonal structure which consists of six-membered rings (Si 6 O 18 ) 12− bonded by tetrahedrally coordinated Be 2+ (T1 site) and octahedrally coordinated Al 3+ (Y site). Y sites, preferentially occupied by Al 3+ , can accommodate Mg, Mn, Fe, Cr, V, Ti, Sc, Co, and Ni ions, while T1 sites can accommodate Li [1][2][3][4][5]. The six-membered rings filled by Si 4+ (T2 site) are stacked one above the other, forming channels of approximately 5.1 Å in diameter along the c-axis.…”

“…The geochemical fingerprints of the Dayakou emerald are not yet determined.Besides, the trace element geochemistry of emeralds was used for building the genetic model. B, Ga, Ba, Sr, and alkali elements Li, Cs, and Rb are regarded as indicators of parental fluid and the fractionation and evolution for pegmatite [3,4,22,23]. Reference [24] reported the formation conditions of Dayakou emerald, but the crystallization process and the sources of emerald-forming fluids have yet to be determined.In this study, thirty-four single Dayakou emerald crystals are investigated.…”