The Geological Survey of Finland (GTK) has been carrying out assessments of undiscovered mineral resources in the Finnish bedrock since 2008. As a part of this process, the lithium resources hosted by undiscovered lithium–caesium–tantalum (LCT) pegmatite deposits were estimated down to the depth of one kilometre in 2018.
Presently, the world’s dominant sources of lithium are continental brines and pegmatites, but hectorite clay deposits and jadarite deposits may become important in the future (British Geological Survey 2016, Jaskula 2017). All the known lithium deposits and occurrences in Finland are hosted by pegmatites, and at least all the well-known lithium deposits are hosted by LCT pegmatites (Alviola 2012, Ahtola et al. 2015). No indications of other types of lithium deposits are known in Finland.
LCT pegmatites are granitic rocks that form small igneous bodies characterised by large crystals and distinctive textures. They are highly enriched in lithium, caesium and tantalum, and this diagnostic suite of elements gives LCT pegmatites their name and distinguishes them from other rare-element pegmatites. LCT pegmatites occur in Cainozoic to Mesoarchaean orogenic belts on all continents.
The best-known LCT pegmatite lithium deposits in Finland are all located within or near the border of the municipality of Kaustinen in central Ostrobothnia. The identified resources in these six deposits at the time of the assessment in 2017 were approximately 46,000 t of lithium. The reported resources have increased since that time and are now approximately 58,000 t of lithium (Keliber Oy 2018).
Well-known LCT pegmatite deposits located outside of Finland have an order of magnitude larger median ore tonnage than the Finnish deposits. However, the global and Finnish deposits do not differ in their median lithium grade. Because of this similarity in the lithium grade, the small number of well-known Finnish deposits, and the probably incomplete delineation of most of the known LCT pegmatite deposits both in Finland and abroad, a grade-tonnage model was constructed using data for 29 deposits in seven countries. This model was used to estimate the metal contents of undiscovered LCT pegmatite deposits in Finland.
Nineteen areas (so-called permissive tracts) where geology allows the existence of LCT pegmatite lithium deposits were delineated in different parts of Finland, covering altogether 22,404 km2. The number of possibly existing but as yet undiscovered LCT pegmatite Li deposits was estimated by GTK experts in a series of workshops for each of the permissive tracts. The expected number (mean estimate) of undiscovered deposits within all the permissive tracts is seven deposits.
Monte Carlo simulation estimates indicate that the undiscovered deposits contain, with a 50% probability, at least 510,000 t of lithium. Over 90% of the estimated undiscovered lithium resources in Finland are located within either the Kaustinen permissive tract or the surrounding larger Järvi-Pohjanmaa tract (Figure 1). The assessment results indicate that at least 90% of the remaining lithium endowment within the uppermost one kilometre of the Finnish bedrock is in poorly explored or entirely unknown deposits.
Detailed results of the assessment have been published in the GTK Bulletin series: Rasilainen K., Eilu P., Ahtola, T., Halkoaho T., Kärkkäinen, N., Kuusela, J., Lintinen, P., Törmänen, T. 2018. Quantitative assessment of undiscovered resources in lithium-caesium-tantalum pegmatite-hosted deposits in Finland. Geological Survey of Finland, Bulletin 406. 172 p.
Ahtola, T., Kuusela, J., Käpyaho, A. & Kontoniemi, O. 2015. Overview of lithium pegmatite exploration in the Kaustinen area in 2003–2012. Geological Survey of Finland, Report of Investigation 220. 28 p. Available at: http://tupa.gtk.fi/julkaisu/tutkimusraportti/tr_220.pdf
Alviola, R. 2012. Distribution of rare element pegmatites in Finland. In: Kukkonen et al. (eds) Lithosphere 2012. Seventh Symposium on the Structure, Composition and Evolution of the Lithosphere in Finland, Espoo, November 6−8, 2012, programme and extended abstracts. University of Helsinki, Institute of Seismology, report S-56. Helsinki: Institute of Seismology, 1−4.
British Geological Survey 2016. Lithium. British Geological Survey, Mineral profiles. 39 p. Available at: http://www.bgs.ac.uk/mineralsuk/statistics/mineralProfiles.html. Last accessed 20 October 2017.
Jaskula, B. W. 2017. Lithium. In: U.S. Geological Survey, Mineral commodity summaries 2017, 100–101.
Keliber Oy 2018. Mineral resources and ore reserves. Web page, available at https://www.keliber.fi/geologia/mineraalivarannot-ja-malmivarat/. Accessed 22 November 2018.
Teksti: Kalevi Rasilainen
Kalevi Rasilainen completed his MSc degree on geology and mineralogy at the University of Helsinki in 1983. The theme of his PhD thesis in 1996 was the orogenic gold mineralisation in the Ilomantsi region. He has worked at GTK since 1985, mostly carrying out research on mineral resources-related subjects. For the last 11 years, he has coordinated a project assessing the undiscovered mineral resources in Finland.