Quartz chemistry of granitic pegmatites: Implications for classification, genesis and exploration

Axel Müller,William Keyser,William B Simmons,Karen Webber,Michael Wise,Hartmut Beurlen,Idoia Garate-Olave,Encarnación Roda-Robles,Miguel Ángel Galliski

doi:10.1016/j.chemgeo.2021.120507

Axel Müller, William Keyser + Show 7 more

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https://doi.org/10.1016/j.chemgeo.2021.120507

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Journal: Chemical Geology	Publication Date: Aug 27, 2021
Citations: 21	License type: cc-by

Affiliation: Universidade Federal de Pernambuco

Abstract

Quartz from 254 pegmatites representing eight pegmatite fields and provinces worldwide was investigated by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to determine concentrations of trace elements Al, Ti, Li, Ge, B, Be, Rb, Na, K, Ca, P, Ga, Sb, Zn and U. A total of 271 new analyses combined with 535 published LA-ICP-MS quartz chemistry data were evaluated with binary and ternary trace element discrimination plots and principal component analysis (PCA). The classifications applied for discrimination of pegmatite types include the widely applied NYF(Nb-Y-F) - LCT(Li-Cs-Ta) classification and the new RMG (pegmatites derived from residual melts of granite magmatism) - DPA (pegmatites as direct products of anatexis) grouping. Pegmatites of both classifications can be well distinguished via Al-Ti, Al-Li and Al/Ti-Ge/Ti binary trace element plots and the Ti - Al/10 - 10*Ge ternary diagram. PCA applied to Al, Li, Ti, Be, B, Ge and Rb contents in quartz allowed to further distinguish between anatectic DPA-1 (Li-enriched DPA) and granite-pluton-derived RMG-1 (Li-enriched RMG) pegmatites. Some pegmatite fields and provinces (Hagendorf-Pleystein, Oxford County) are distinguishable by region-specific Li, Ge and Al contents. The results imply that the chemistry of pegmatite quartz is mainly controlled by the origin (source rock chemistry) of pegmatite melts and, to a much lesser extent, by the geodynamic setting of the pegmatite fields and provinces. Chemically primitive NYF and DPA-2 type pegmatites contain quartz with the lowest total trace-element contents and lowest internal-pegmatite trace-element variation, making it potentially suitable for high-tech application. Pegmatite quartz containing >30 μgg-1 Li and >100 μgg-1 Al is strongly indicative of economic spodumene/montebrasite mineralization and, thus, serves as a strong Li-mineralization pathfinder mineral. Quartz with >5 μgg-1 B may be a potential indicator for gem-quality tourmaline mineralization.

Highlights

Quartz comprises 20 to 40% of granitic pegmatites, typically dis playing large crystal dimensions, making it a major mineral constituent of these rocks and an important economic resource
Quartz from 254 pegmatites representing eight pegmatite fields and provinces worldwide was investigated by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to determine concentrations of trace elements Al, Ti, Li, Ge, B, Be, Rb, Na, K, Ca, P, Ga, Sb, Zn and U
principal component analysis (PCA) applied to Al, Li, Ti, Be, B, Ge and Rb contents in quartz allowed to further distinguish between anatectic DPA-1 (Li-enriched DPA) and granite-pluton-derived RMG-1 (Lienriched RMG) pegmatites

Summary

Introduction

Quartz comprises 20 to 40% of granitic pegmatites, typically dis playing large crystal dimensions, making it a major mineral constituent of these rocks and an important economic resource. The high resistance of quartz to alteration compared to, for example, feldspars and micas, makes it a reliable mineral to study fractionation and crystallization processes and the nature of melt over protracted geological histories. Quartz is the least studied of the important minerals in pegmatites. Trace elements of rock-forming minerals are considered important petrogenetic indicators for determining the P-T-X conditions of mineral formation, for obtaining information about fluid or melt origin and for discriminating different crystallization environments. Trace elements in lesser concentrations in quartz are Ga, Sn, Sr, Rb, Mn and Sb (Walenczak, 1969; Larsen et al, 2000; Rusk et al, 2011; Monnier et al, 2021). Other elements commonly identified by mass spectrometry in quartz, such as Ca, Mg, Ba, REE, U and Th, are attributable to microinclusions of fluids or other minerals (Gerler, 1990; Blankenburg et al, 1994; Gotze et al, 2004; Gotze, 2009)

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