Tourmaline in Petalite-Subtype Granitic Pegmatites: Evidence of Fractionation and Contamination From the Pakeagama Lake and Separation Lake Areas of Northwestern Ontario, Canada

Abstract

Tourmaline in petalite-subtype granitic pegmatites from the Pakeagama Lake and Separation Lake areas of northwestern Ontario provides evidence for the compositional evolution of pegmatite-forming melts through magmatic crystallization and pegmatite melt -host-rock interactions. At the former locality, tourmaline from the pegmatite units is essentially magmatic, whereas at the latter locality, tourmaline in many of the Fe-suite pegmatites displays characteristics (such as elevated Mg, Ti and Ca contents) that indicate significant contamination through the incorporation and digestion of components of amphibolite (mafic metavolcanic units) and banded iron-formation (BIF) host-rocks. The presence of tourmaline in mafic host-rocks indicates that cation migration also involved the introduction of B, Li and Al from pegmatite-generated fluids to form metasomatic haloes. The Li + Mn versus Mg + Ti + Ca diagram is introduced to discriminate tourmaline with a dominantly magmatic signature from tourmaline with a signature influenced by pegmatite -host-rock interaction. The behavior of Ca in tourmaline is also shown to be potentially useful in recognizing late-stage Ca-enrichment in pegmatite-forming melts as a result of Ca-F complexing, but can give similar trends to tourmaline that has suffered Ca-metasomatism as a result of interaction with granitic host-rocks. The tourmaline data suggest that at Separation Rapids, pegmatites mainly crystallized in an open system (i.e., pegmatite -host-rock interaction was common), whereas at Pakeagama Lake, the pegmatite units were predominantly unaffected by interaction with host rocks (i.e., they crystallized in a closed system). This has implications for concentrating rare-elements such as Rb and Cs, which otherwise would be dispersed throughout pegmatite and host rocks (as at Separation Lake). The potential for a buried pollucite deposit associated with the SE pegmatite at Pakeagama Lake has increased as a result of these observations, particularly as Cs is elevated in associated K-feldspar and mica, and is also present in a late pollucite-bearing vein.

Chemical and spectroscopic characterization of tourmalines from the Mata Azul pegmatitic field, Central Brazil

“…% for Ti. The mineral formulae were calculated based on 31 anions (O, OH, F), assuming B = 3 apfu (atoms per formula unit), (OH + F) = 4 apfu and Li = 15 − (T + Z + Y) using the Excel™ worksheet of Tindle et al (2002). Finally, all formulae were classified according to the rules of Henry et al (2011).…”

Li tourmaline from Nagatare, Fukuoka Prefecture, Japan

“…Li contents estimated from this calculation are in good agreement with Li contents calculated from site -scattering refinements, and a consideration of mean bond -lengths shows that both Fe and Mn are in the divalent state (Burns et al, 1994). This calculation has been validated in other studies of tourmaline fractionation trends (e.g., Tindle et al, 2002).…”

“…Chemical formulae were calculated on the basis of 31 anions. The B 2 O 3 , Li 2 O, and H 2 O contents were assumed similarly to previous study (e.g., Burns et al, 1994;Tindle et al, 2002) by stoichiometry as below. B = 3 apfu, a deficiency of Si on T site is filled by Al, Z Al = 6 apfu, Li = 3 -sum of the Y -site cations, and OH + F = 4 apfu, Fe and Mn are divalent.…”

“…The chemical variations of the tourmaline found in Li -Cs -Ta enriched (LCT) (Černý et al, 2005) granitic pegmatites show increasing (Li + Al) and decreasing Fe from the country rock to the core of the pegmatite, reflecting the chemical development of melts (Jolliff et al, 1986). The specific chemical trends of tourmaline in some pegmatite subtypes have also been documented (e.g., Novák and Povondra, 1995;Selway et al, 1999;Tindle et al, 2002;Tindle et al, 2005), but no such studies have been reported for an island arc setting such as Japan. While the Li tourmaline from the Nagatare pegmatite was described as elbaite (e.g., Shibata, 1934), the chemical compositions of the tourmaline was not determined in detail, and no XRD data reported. In this study, tourmaline from the Nagatare pegmatite were classified into five types on the basis of the collected points and macroscopic properties such as color, texture and associated minerals, and the chemical compositions and unit -cell parameters of 29 samples were examined.…”

Secondary blue tourmaline after garnet from elbaite-subtype pegmatites; implications for source and behavior of Ca and Mg in fluids

“…However, late enrichment of Ca in pegmatites, although it was observed on numerous localities worldwide (e.g. Novák et al 1999;Tindle et al 2002Tindle et al , 2005, is still not fully understood. It could be caused by originally Ca-enriched melt (Novák and Gadas 2010;Gadas et al 2012) or the melt contaminated by Ca in early stage of crystallization (postemplacement stage).…”