Compositional Variation of Tourmaline-Group Minerals in the Borborema Pegmatite Province, Northeastern Brazil

Abstract

Various species of the tourmaline-group are common accessory minerals in the Boqueirão, Capoeira and Quintos Li-Be-Ta-Nb-bearing, LCT-family, rare-element-class granitic pegmatites from the Borborema Pegmatite Province (BPP), northeastern Brazil. Tourmaline from the border and wall zones of the pegmatites is enriched in Mg and Fe, crystallizing as Mg-rich members of the dravite-schorl solid-solution series. Tourmaline from the intermediate zone is richer in Fe and Al+Li, crystallizing as schorl. The Fe-Mg-free and Li-Al-rich elbaite is typical of the transition between the intermediate zone and the quartz core, and of replacement pockets. The bright turquoise blue elbaite known as "Paraíba tourmaline" is produced in the Capoeira 2 and Quintos pegmatites and is distinguished by high Cu contents, up to 1.07 wt.% CuO. In other occurrences of the "Paraíba tourmaline" in the BPP, up to 2.37 wt.% CuO were reported. Differences in the geochemical evolution trend of tourmalines among the pegmatite bodies investigated (vacancy at the X site, Fe, Mg, Zn, Li and F contents), suggest that they reached variable degrees of fractionation. This observation agrees with chemical data on white mica, feldspar, garnet and gahnite, and therefore permits the use of tourmaline composition as an indicator of the degree of evolution of the hosting pegmatite. According to these data,"Paraíba tourmaline"-producing pegmatites share the following characteristics: 1) they are the most evolved pegmatites so far known in § Volume 46October 2008 Part 5 the cANADiAN miNeRALogist the BPP; 2) they are hosted by quartzites or metaconglomerates (iron-poor host rocks); 3) they exhibit comb-textured dravite in the border zone (early saturation in tourmaline) and 4) the elbaitic "Paraíba tourmaline" is found in the most evolved parts of the pegmatites.sommAiRe De diverses espèces de tourmaline sont présentes comme accessoires dans les pegmatites granitiques de Boqueirão, Capoeira et Quintos, enrichies en éléments rares (Li-Be-Ta-Nb) et de type LCT, situées dans la province pegmatitique de Borborema (BPP), nord-est du Brésil. Dans les zones de bordure et de paroi des pegmatites, la tourmaline est enrichie en Mg et Fe; les compositions sont des membres magnésiens de la solution solide entre dravite et schorl. Dans la zone intermédaire, la tourmaline est enrichie en Fe et Al + Li, et correspond à la composition de schorl. L'elbaïte dépourvue de Fe-Mg et enrichie en Li-Al est typique de transition entre la zone intermédiaire et le noyau de quartz, et des poches de remplacement. L'elbaïte bleu turquoise connue sous le nom de "tourmaline de Paraíba" se rencontre dans les pegmatites de Capoeira 2 et Quintos, et se distingue par des teneurs élevées en Cu, atteignant jusqu'à 1.07% de CuO (poids). Dans d'autres indices de ce genre d'elbaïte dans la ceinture BPP, on a signalé jusqu'à 2.37% de CuO. Des différences dans les tracés géochimiques évolutifs des tourmalines parmi les massifs de pegmatites étudiés, par exemple dans la proportion de lacunes au site X, et de...

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

“…This is consistent with the fact that these pegmatites contain economic Li and Ta mineralization, in addition to gem-quality minerals such as cuprian (BParaiba^) tourmaline, and that other minerals, such as feldspars and mica, also reflect a high degree of melt evolution (Soares 2004;Soares et al 2007Soares et al , 2008Soares et al , 2012.…”

The incorporation of fluorine in tourmaline: internal crystallographic controls or external environmental influences?

“…Of particular note is the bonded H in the W site, shown as being oriented toward the X site. Bowman (1902), Slivko (1955), Rath & Puchelt (1957), El-Hinnawi & Hofmann (1966), Donnay & Barton (1972), Chaudhry & Howie (1976), Dunn et al (1978), Foord & Mills (1978), Sahama et al (1979), Povondra et al (1985), Jolliff et al (1986), Bloomfield (1997), Deer et al (1997), Lyckberg & Hawthorne (1997), Ahn & Buseck (1998), Federico et al (1998, Aurisicchio et al (1999aAurisicchio et al ( , 1999b, Dyar et al (1999), Keller et al (1999), Morgan & London (1999), Teertstra et al (1999), Dutrow & Henry (2000), Akizuki et al (2001), Ertl et al (2003a), Tindle et al (2003Tindle et al ( , 2005Tindle et al ( , 2006Tindle et al ( , 2008, Bosi et al (2005aBosi et al ( , 2005b, Breaks et al (2008), Soares et al (2008), Ertl et al (2010aErtl et al ( , 2010c 2) Li-poor pegmatites, granitic rocks, and adjacent host-rocks Otroshchenko et al (1971), Neiva (1974), Povondra (1981), Pieczka & Kraczka (1988), Kretz et al (1989), Nmec (1989), Sinclair & Richardson (1992),…”

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