| dc.description.abstract | The Tørdal pegmatite field, located in the county of Telemark in southern Norway, is known for its enrichment in Sc as well as other rare metals. During the last decade, the interest for Sc has increased, due to its important uses in e.g. the aircraft, automotive and space industries. In light of its economic importance, mineable resources of Sc are of great interest. Garnet, which is a common accessory mineral in the Tørdal pegmatites, naturally incorporates Sc in the B-site of its structure. Besides mica, garnet is a major host for Sc in pegmatites. In addition, various and very rare Sc minerals have been described from the Tørdal pegmatites (bazzite, cascandite, heftetjernite, kristiansenite, oftedalite, scandiobabingtonite, and thortveitite). Therefore, mapping and sampling of garnet-bearing Tørdal pegmatites and their host rocks was performed, with the intent for chemical analysis. The regional distribution of Sc in the garnets from 16 Tørdal pegmatites was determined by electron microprobe analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The chemical data indicate a general Sc increase from the SW (Butvatnet-Grønliheii areas) to the NE (Heftetjern-Høydalen area) in the studied area. Energy Dispersive X-ray Spectroscopy (EDS) demonstrates that only wall zone garnets from the Sc abundant pegmatite locations (Upper Høydalen and Svåheii 2) contain Sc-rich (thortveitite) micro inclusions. The Sc-enrichment is accompanied by an increase in the spessartine component from 30 up to 60 mol.% (up to 0.29 Sc2O3 wt.% and 2197 ppm Sc). However, garnets from both Sc-rich SW (Svåheii 2 location: ~47 mol.% spess) and Sc-poor NE corners (Bratterud and Sjauset locations: ~54 mol.% and ~76 mol.%, respectively) do not apply to the general systematic increase of Sc. Through internal fractionation in the most evolved NYF pegmatites of Høydalen, Sc drops from both the Sc- and almandine-rich wall zone spessartines (max. 1538 ppm Sc and ~50 mol.% spess) to the nearly Sc-absent “cleavelandite”-zone garnets (81-93 mol.% spess). The Sc-rich wall zone garnets from the Heftetjern, Upper Høydalen, and Svåheii 2 locations show a general decrease from the core to the crystal margin (max. 688 ppm Sc difference). In particular, this decrease of the Sc contents in the Heftetjern garnets is correlated with decreasing Mn/(Mn+Fe) ratios and Y and HREE contents as seen from BSE-imaging. Similarly, IBMA-imaging shows Sc-rich cores where Sc may slightly decrease towards the rim of the investigated Sc-rich garnets from the Heftetjern and Svåheii 2 locations. The rest of the analyzed and relatively Sc-poorer garnets from the Tørdal do not exhibit any clear correlation for intracrystalline Sc nor with any other trace elements such as e.g. Y and HREEs. It has long been believed that the Tørdal pegmatites are formed by fractionated melt derived from the Tørdal granite pluton, located in the south of the Tørdal pegmatite field. Scandium was, according to this theory, leached from the mafic supracrustals of the Nissedal outlier into the pegmatitic melts, when the melts moved from the pluton to their final destination of emplacement. This theory was verified by geochemical whole rock analysis, which demonstrates that Sc is much more abundant in amphibolitic host rocks of the outlier (29-30 ppm) than in the Tørdal granite (2 ppm). However, fieldwork evidence proves that the pegmatites were generated by direct and local anatexis by these host rocks. This anatectic origin is supported by recent dating of the pegmatites and the Tørdal granite, which indicate that the Sc-enriched pegmatites Tørdal cannot originate from the ~40 Ma older Tørdal granite. Plausibly, through SW-NE migration of the increasingly fractionated and continuously Sc-enriched anatectic pegmatite melt, Sc was mobilized and transported by ScF ligands, while mostly ending up in the Sc-rich pegmatites of the Heftetjern-Høydalen area. Scandium was in these pegmatites mostly incorporated into late hydrothermal wall zone micas and garnets. Most importantly, the main outcome of this study clearly demonstrates that the Heftetjern-Høydalen area, being most Sc-enriched, proves to be the best potential for future exploitation of Sc. Scandium contents in the garnets were found to be consistent within each pegmatite. Therefore, garnet is a very useful and reliable pathfinder mineral for exploration of Sc mineralization in granitic pegmatite fields. However, during Sc exploration, it is of utmost importance to bear in mind, especially for the most evolved NYF pegmatites, which zone the garnets originate from since Sc decreases drastically through internal fractionation. | eng |