Abstract
The granitic pegmatites of Tørdal pegmatite field, which are situated in the Vestfold and Telemark county in South Norway, have gained lot of mineralogical and economic interest the last hundred years mainly because of their enrichments of the rare elements Sc, Be, Li and Mo. In this study, Sc- and Be-bearing minerals, comprising mica, beryl, gadolinite-(Y) and bazzite, have been studied from 17 Tørdal pegmatites in order to better understand the distribution of Sc and Be within pegmatite bodies as well as across the entire pegmatite field. The mineral chemistries were determined with EPMA and LA-ICP-MS. In addition, whole rock chemistries of pegmatites and their host rocks, amphibolites of the Nissedal Outlier and the Tørdal granite, were evaluated in order to discuss possible sources of Sc and Be. Detailed geological mapping of the Høydalen area was performed to reveal the spatial distribution, size and shape of pegmatite bodies. The chemical data of investigated micas illustrate a variation of average Sc concentration from 38 to 1576 ppm. At pegmatite scale, micas from the border and wall zones (margin of the pegmatite body) contain higher average Sc concentrations compared to micas from the intermediate and albite replacement zones (inner zones of the pegmatite body). In the Svåheii 2b pegmatite, for example, average Sc compositions range from 1125 ppm in wall zone micas to 38 ppm in albite zone micas. This indicates that the Sc concentration in micas continuously decreases from pegmatite margin towards the core of individual pegmatite bodies, which is accompanied by decreasing K/Rb ratios. Comparison of micas and coexisting garnets from same pegmatite bodies revealed a distribution coefficient of Sc-in-mica : Sc-in-garnet of 0.85 : 1, indicating that slightly more Sc is incorporated in garnet than in co-existing mica. Mapping of Be-minerals shows that gadolinite-(Y) occurs only in the wall zones of pegmatites, while beryl is found in the intermediate and albite replacement zones. The Na, Li, and Cs content of the beryl increases continuously whereas Fe decreases from early crystallized green beryl to late crystalized pink beryl (variety morganite). At regional scale, the distribution of Sc and Be across Tørdal pegmatites shows a general increase of these elements from the SW (Mjeltedalen 2a-Kleppe-Buvatnet area) to the NE (Heftetjern-Høydalen-Skardsfjell area) documented by mica and garnet chemistries and Be-mineral abundance. Whole rock analysis of amphibolitic host rocks (bulk mean 35 ppm Sc; this study) and Tørdal pegmatites (Heftetjern = bulk mean 53 ppm Sc; Rosing-Schow et al., 2020) define both rocks as Sc-enriched compared to the Sc-poor Tørdal granite (bulk mean 2 ppm Sc; Steffensen, 2018). In addition, Li, Ta, Sn, and W are enriched in both amphibolitic host rocks and Tørdal pegmatites compared to average crustal compositions and to the Tørdal granite. These findings supports the most recent model that Tørdal pegmatite melts were formed by partial melting of their amphibolitic host rocks. Traditionally it was suggest that the pegmatite melts are residual melts originating from the adjacent Tørdal granite pluton occurring immediately south of the pegmatite field. The finding that the Sc content in pegmatite melts decreases with fractionation degree and the field relationships observed at the Kleppe quarry indicate that the anatectic melts migrated only about 10 to 500 m to their final location of emplacement and crystallization. Previously it was assumed that the Sc enrichment in the Heftetjern-Høydalen-Skardsfjell area is related to the fractionation when the melts moved for 4 to 5 km the from SW to the NE of pegmatite field. The relative low K2O content of the Sc-rich Heftetjern pegmatite (4.1 wt.% K2O; Rosing-Schow et al., 2020) compared to the high K2O content of the Sc-poor Kleppe quarry (7.4.wt% K2O; Rosing-Schow et al., 2020) indicate that the Heftetjern pegmatite melt was formed by a higher degree of partial melting (about 10 to 15%) compared to the Kleppe quarry (<5%) (Kushiro et al., 1996). This suggests that the Sc enrichment in Tørdal pegmatite melts is related to low but not very low degree of partial melting of Sc-rich amphibolites and not to the fractionation degree. Analysis of these amphibolitic host rocks indicates a high Sc and Be content in amphiboles (bulk mean 56 ppm Sc and 6 ppm Be) compared to biotites (bulk mean 3 ppm Sc and <2 ppm Be). Both minerals are relative rich in F (1.2 wt.% F in biotite and 0.5 wt.% F in amphibole) causing the lowering of the melting temperature of the amphibolites. During partial melting, F formed Sc-F and Be-F complexes enabling transported of Sc and Be by the anatectic pegmatite melt to their place of crystallization, where these elements become incorporated in gadolinite-(Y), mica, garnet, beryl and bazzite during final crystallization. Economic implications of this study are that mica and garnet are the major carriers of Sc, while gadolinite-(Y) and beryl are major carrier of Be, minerals which are abundant in the NE of Tørdal pegmatite field. Scandium is most enriched in garnets and micas from the pegmatite margin and the Sc content of these minerals decreases towards the core of the pegmatites. However, the Sc-enriched pegmatites of the Heftetjern-Høydalen-Skardsfjell area are small in size (500 – 10,000 m3) and bulk pegmatite Sc concentrations are about 50 ppm, which is below of the current cut-off grade of about 300 ppm of active Sc mines, making them economically not profitable.