- © 2015 Society of Economic Geologists.
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.