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-1Al, 8 ± 3 μgg-1Ti, 1.4 ± 0.8 μgg-1Ge, and 11 ± 7 μgg-1Li. 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-1Al, 19 ± 11 μgg-1Ti, 2.3 ± 1.8 μgg-1Ge, and 7 ± 5 μgg-1Li. 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.