The Chemistry of Quartz in Granitic Pegmatites of Southern Norway: Petrogenetic and Economic Implications

Abstract

Trace element concentrations in quartz from 188 granitic pegmatites in the Froland and Evje-Iveland pegmatite fields, southern Norway, have been determined to establish exploration targets for high-purity quartz and to gain a better understanding of the genesis of pegmatites hosting these deposits. Both pegmatite fields were formed during the Sveconorwegian (Grenvillian) orogeny (1145-900 Ma) at the western margin of the Fennoscandian Shield.In situ raster analyses within single quartz crystals were undertaken by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS); spot size 75 µm) to assess levels of lattice-bound impurities, rather than mineral and fluid inclusions that are relatively easily removed during high-purity quartz processing. Quartz in the Froland pegmatites has relatively pure and homogeneous compositions containing 46 ± 24 µgg -1 Al, 8 ± 3 µgg -1 Ti, 1.4 ± 0.8 µgg -1 Ge, and 11 ± 7 µgg -1 Li. The Ti-in-quartz geothermobarometer gives an average pegmatite crystallization temperature of 537° ± 39°C. Temperature estimates are highest along the northwestern margin of the pegmatite field (>550°C), whereas the most differentiated pegmatites occur toward the northeast. The area of greatest economic potential for high-purity quartz lies just north of the central part of the field where individual pegmatites contain >1 million metric tons (Mt) quartz with low average trace element contents of 67 ± 11 µgg -1 . From mineral-chemical criteria, and a range of other geologic factors, we propose that pegmatite melts in the Froland field were generated by fluid-present crustal melting at about 1060 Ma, in zones of localized high-strain deformation during progressive thrusting along the Porsgrunn-Kristiansand fault zone.Quartz in the Evje-Iveland pegmatites has more variable compositions with 69 ± 57 µgg -1 Al, 19 ± 11 µgg -1 Ti, 2.3 ± 1.8 µgg -1 Ge, and 7 ± 5 µgg -1 Li. From its Ti content, it crystallized at temperatures of 613° ± 70°C. The regional spatial distribution of Ti-in-quartz temperatures appears irregular mainly due to the scattered distributions of chemical evolved pegmatites with "amazonite"-"cleavelandite" replacement zones, which show crystallization temperatures down to 442°C. Quartz from the Evje-Iveland pegmatites is unlikely to be of current economic interest due to its moderate to high trace element contents, heterogeneous chemistry, and low volume. The Evje-Iveland pegmatites show no apparent genetic link to a granite intrusion; instead they probably formed as a result of partial melting at the depth of their amphibolite country rocks at around 910 Ma. This is related to a regional low-pressure/high-temperature metamorphic event at about 930 to 920 Ma.

Magma-driven, high-grade metamorphism in the Sveconorwegian Province, southwest Norway, during the terminal stages of Fennoscandian Shield evolution

“…To the east of the study area, geographically widespread pegmatites were emplaced as late as ca. 910 Ma (Seydoux-Guillaume et al, 2012), possibly as a result of ongoing mafic underplating (Müller et al, 2015). We interpret these data in terms of the widespread and protracted nature of arc and/or backarc processes (e.g., Currie and Hyndman, 2006).…”

Trace Element Compositions and Defect Structures of High-Purity Quartz from the Southern Ural Region, Russia

“…In particular, high-purity quartz (HPQ), with less than 50 ppm of contaminating trace elements [3,4], is of high economic value, resulting in prices up to 20 times higher than those of low-quality ("common") silica raw materials [5,6]. High-purity quartz is of strategic importance for the high-tech industry, because it is a critical material for the manufacture of crucibles used for single crystal growth of silicon metal (needed for solar panel and micro-chip production), high-temperature lamp tubing, telecommunications, optics, and semiconductor materials.…”

Tools and Workflows for Grassroots Li–Cs–Ta (LCT) Pegmatite Exploration

“…However, pegmatites with a missing apparent parental granite are common [3], and it is suspected that the source granite occurs at depth [30]. On the other hand, studies published in the 2010s on the anatectic origin of granitic pegmatites in Europe and North America demonstrate that there are "thousands of pegmatites without parental granites" [13], i.e., pegmatite fields can be unrelated to a source granite and instead form by partial melting and anatexis of crustal material [4,13,14,[35][36][37][38][39], or energy and melt circulation along deep lithospheric fault zones [40]. In the current literature, there are numerous descriptions of pegmatite sub-classifications, but to date there is no universally accepted model explaining the diverse features and genesis of granitic pegmatites [39].…”

“…This is particularly evident in the pegmatite fields of Mt. Mica [4] and Southern Norway [14,47] where geophysical gravity surveys, radiometric dating, isotopes, whole-rock and mineral chemistry indicate that pegmatites are unrelated to nearby S-type granite plutons. Secondly, there is evidence that sub-solidus hydrothermal alteration, specifically the transition between pure magmatic fractionation and hydrothermal alteration, plays a significant role in localising economically significant mineralisation of Sn, W, Li, Cs, Ta and Nb [7].…”

“…Over the last four decades research into rare metal Li-Cs-Ta (LCT) pegmatite mineralisation has predominantly focused on the understanding of late-stage magmatic fractionation and hydrothermal alteration processes enriching incompatible elements of potential economic interest in felsic peraluminous melts. In particular, the mineralogy and geochemistry of LCT pegmatites have been studied in great detail and are presented in case studies from Canada and the US [1][2][3][4], Ireland [5,6], the European Variscides [7][8][9][10][11][12], the Sveconorwegian orogeny [13,14], Central-East Africa [15][16][17][18] and Australasia [19][20][21][22]. These case studies have resulted in the definition of tectonic, mineralogical and geochemical factors controlling the location of mineralisation and zonation within the roof of peraluminous S-type granite plutons, pegmatitic offshoots into metasedimentary country rock or anatectic melts with no established link to S-type granites.…”