Prospecting

The Estonian crystalline basement consists primarily of orogenic Svecofennian metamorphic and igneous rocks, as well as anorogenic rapakivi granitic intrusions (Puura et al., 1996). The erosional surface of the crystalline basement dips southward 2–4 m/km and is buried under 150 to 800 metres of Neoproterozoic and Paleozoic sedimentary rocks.

The basement consists of two major units: amphibolite facies metamorphic rocks in Northern and granulite facies metamorphic rocks in Southern and Western Estonia (Puura et al., 1996; Soesoo et al., 2004). These units are separated with approximately 30 km wide northwest-oriented Paldiski-Pskov Deformation Zone.

The major units are further divided into six zones based on petrological and geophysical characteristics:

• The Tallinn zone comprises amphibolite-facies gneisses and metasediments (amphibolites, biotite-plagioclase gneisses, quartz-feldspar gneisses, migmatitic gneisses, sulfide-bearing graphite gneisses and magnetite gneisses).
• The Tapa zone is characterized by garnet- and pyroxene-quartzites, Al-rich gneisses (garnet-cordierite-sillimanite), and pyroxene-, amphibole-, and biotite-bearing gneisses with elevated Fe and S content.
• The Alutaguse zone is primarily composed of metasedimentary Al-rich gneisses and biotite-plagioclase gneisses, as well as amphibolite-facies pegmatitic gneisses, amphibolites, and quartz-feldspar gneisses.
• The Jõhvi zone predominantly contains migmatized pyroxene, quartz-feldspar, biotite-plagioclase, amphibole, garnet-cordierite, and magnetite gneisses.
• Western Estonia is characterized mainly by metasedimentary amphibolites, biotite-plagioclase gneisses, and amphibolite- to granulite-facies quartz-feldspar gneisses.
• Southern Estonia is typically characterized by granulite-facies metavolcanic rocks, primarily consisting of amphibole-pyroxenite and biotite-pyroxene gneisses, along with quartz-feldspar gneisses.

Figure 1. Structural zonation and topography of the Precambrian basement of Estonia (Koppelmaa, 2002)

The age of the crystalline basement ranges from 1.9 to 1.54 billion years (Kirs et al., 2009; Soesoo et al., 2006) whereas the oldest rocks are in the Alutaguse zone (1.90–1.88 billion years old). The metamorphic ages of the Southern Estonia granulite-facies metamorphic rocks range from 1.84 to 1.80 billion years. The known shoshonitic Muhu, Virtsu, and Taadikvere plutons have ages around 1.83 billion years (Kirs et al., 2009). The youngest of the crystalline basement are rapakivi plutons belonging to the Fennoscandian rapakivi province, which formed 1.67–1.62 (Märjamaa, Neeme, Ereda, Naissaare, and Taebla) and 1.59–1.54 billion years ago (Riia) (Rämö et al., 1996).

Structurally, the Estonian crystalline basement is part of the Svecofennian complex (Huhma et al., 1991) that consists of a mosaic of orogenic belts and microcontinents (Lahtinen et al., 2005; Bogdanova et al., 2006; 2008; 2015; Korja et al., 2006). The Estonian area of the Fennoscandian shield can be correlated with Southern and Western Finland and Central Sweden (Bogdanova et al., 2015). The magnetite gneisses and sulfide mineralization counterparts in both the Tallinn and Jõhvi zones occur in the Uusimaa zone in Southern Finland. The rocks in Northern Estonia have undergone significant migmatisation, similarly to metamorphic rocks in Southern Finland (Kurhila et al., 2011). Both the Uusimaa and Northern Estonia structural zones resemble the Bergslagen area in Southern Central Sweden, characterized by 1.91–1.89-billion-year-old Fe-quartzites associated with volcanic and subvolcanic rocks and sulfide mineralization with scattered metacarbonates and skarnification. The Alutaguse metaturbidite zone can be correlated with similar sediments in the Novgorod domain, which, in turn, can be correlated structurally with the Botnia domain in Fennoscandia and Southern Central Finland.

Among the known mineralization occurrences in the crystalline basement, the most significant are Mn-rich magnetite gneisses, magnetite-pyrite-pyrrhotite mineralization in Al-rich gneisses with sulfide mineralization in graphite gneisses, and rare earth metal mineralization in anorthosite-rapakivi rocks.

The most extensive and well-studied mineral occurrence is the Mn-rich magnetite and iron-sulphide mineralization in the Jõhvi Magnetic anomaly area. It has been studied at various times since the 1930s, including the latest study with drillholes conducted from 2018 to 2022 (Nirgi et al., 2022). Recent assays of mineralized sections revealed up to 200 ppb of Au that is caused by up to 30 μm inclusions of Pb-Bi-Te-Ag and Ag-Au element associations spread in hairline veins in magnetite-bearing gneisses.

Most promising sulphidic occurrences are related to graphite-containing gneisses and accompanying graphite schists, quartzites and paragenetic rocks of sedimentary and volcanic origin in Tallinn and Alutaguse zones. These rocks contain sulfides (pyrrhotite, pyrite, chalcopyrite, sphalerite, and galena) with Cu contents reaching 3000 ppm and Zn contents exceeding 10,000 ppm. The graphite content ranges up to 15%. In the Uljaste drill cores, 45 km west of Jõhvi, similar vein system and mineral association has been described (Kont, 2022). These findings suggest that these mineralization occurrences are part of a larger system and may be connected with the tectonic evolution of the entire Northeastern Estonian section of the Fennoscandian shield.

The main results of mineral resource exploration related to crystalline rocks have been summarized in the study report by Petersell et al. (1991). Earlier research projects provided high-quality general geological and petrographic descriptions, but element contents (especially trace elements) were mainly determined using less reliable semi-quantitative spectral analysis, with little attention paid to the occurrence of technologically significant metals in the context of today's green technology needs.

As of now, there are no known mineralizations in the crystalline basement rocks of Estonia that have immediate significant economic potential. However, considering the petrological and structural similarities between the crystalline basement of Estonia and that of Southern Finland and Central Sweden, which are known for Zn, Pb, Cu, Fe, and Au mineralization provinces, there is reason to believe that similar types of mineralization may exist in the Estonian crystalline basement.

The history of geological research and mineral potential exploration of the crystalline basement of Estonia dates back to the first half of the last century when the first deep exploration boreholes were drilled into the Jõhvi magnetite gneisses. Systematic geological investigations into crystalline rocks began in the 1960s in Northern Estonia, accompanied by gravity and magnetic field mapping, but were halted in 1991.

Now, we have prepared a roadmap to carry out systematic studies with different scales on the crystalline basement rocks with a main goal to discover mineralisation and assess the mineral potential of known occurrences. The planned activities include structural and geophysical studies together with petrographic and geochemical analysis from archived and fresh drill cores. We have already started to implement the first stages by digitalising and interpreting the historical databases, and collecting new data from legacy core archive.

Estonia's exploration and research efforts for potential mineral resources in the crystalline basement should focus on minerals that have the potential to be found in Fennoscandian ore provinces that are geologically related to Estonia's crystalline basement.

Basing on a report (Eilu et al., 2021) that described the potential distribution of critical raw materials (CRM) and other economically significant mineral resources necessary for the green transition in the Nordic region and considering geological structure of Estonia’s crystalline basement it is justifiable to prioritize exploration and research efforts for the following metals:

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Bogdanova, S., 2008. The East European Craton (Baltica) at 1.6-1.4 Ga: Continuing supercontinent agglomeration or break-up? Conference paper, The 33rd International Geological Congress, Oslo 2008.

Bogdanova, S., 2006. Tectonic zoning of the crystalline crust in the west of the East European Craton: Characterization of the belts and lithotectonic (structural material) complexes. Structure and Dynamics of the Lithosphere of Eastern Europe [in Russian]. 226-232.

Eilu, P., Bjerkgård, T., Franzson, H., Gautneb, H., Häkkinen, T., Jonsson, E., Keiding, J.K., Pokki, J., Raaness, A., Reginiussen, H., Róbertsdóttir, B.G., Rosa, D., Sadeghi, M., Sandstad, J.S., Stendal, H., Þórhallsson, E.R. & Törmänen T., 2021. The Nordic supply potential of critical metals and minerals for a Green Energy Transition. Nordic Innovation Report. https://www.nordicinnovation.org/2021/nordic-supply-potential-critical-metals-and-minerals-green-energy-transition

Huhma, H., Puura, V., Klein, V., Mänttäri, I., 1991. Nd-isotopic evidence for Paleoproterozoic crust in Estonia. Geological Survey of Finland. Special Paper, 12, 67–68.

Kirs, J., Puura, V., Soesoo, A., Klein, V., Konsa, M., Koppelmaa, H., Niin, M., Urtson, K., 2009. The crystalline basement of Estonia: rock complexes of the Palaeoproterozoic Orosirian and Statherian and Mesoproterozoic Calymmian periods, and regional correlations. Estonian Journal of Earth Sciences, 58, 219–228.

Koppelmaa, H., 2002. Eesti kristalse aluskorra geoloogiline kaart. Mõõtkava 1:400 000. Eesti Geoloogiakeskus.

Kont, R. 2022., Jälgelemendid Uljaste sulfiidse mineralisatsiooni ilmingutes. Master’s thesis, University of Tartu, 1-68.

Korja, A., Lahtinen, R., Nironen, M., 2006. The Svecofennian orogen: A collage of microcontinents and island arcs. Geological Society, London, Memoirs, 32, 561-578.

Kurhila, M., Mänttäri, I., Vaasjoki, M., Rämö, O.T., Nironen, M., 2011. U–Pb geochronological constraints of the late Svecofennian leucogranites of southern Finland. Precambrian Research, 190, 1–24.

Lahtinen, R., Korja, A., Nironen, M., 2005. Palaeoproterozoic tectonic evolution of the Fennoscandian Shield. Precambrian Geology of Finland -Key to the Evolution of the Fennoscandian Shield. Developments in Precambrian Geology, 14, 418-532.

Nirgi, S., Maala, L., Kaasik, T., Smyth, D., Wrobel, F., 2022. Jõhvi magnetanomaalia uuringupotentsiaali hindamine (EGF:9552). Geoloogiafond.

Petersell, V., Kivisilla, J., Pukkonen, E., Põldvere, A., Täht, K., (1991). Maagistumise ja mineraalistumise ilmingud Eesti sette- ja kristalse aluskorra kivimites. EGF 4523

Puura, V., Kirsimäe, K., Kivisilla, J., Plado, J., Puura, I., Suuroja, K., 1996. Geochemical anomalies of terrestrial compounds in nonmelted impactites at Kärdla, Estonia. Meteoritics & Planetary Science, 31, A112–A113.

Rämö, O.T., Huhma, H., Kirs, J., 1996. Radiogenic isotopes of the Estonian and Latvian rapakivi suites: new data from the concealed Precambrian of the East European Craton. Precambrian Research, 79, 209–226.

Soesoo, A.; Puura, V.; Kirs, J.; Petersell, V.; Niin, M.; All, T., 2004. Outlines of the Precambrian basement of Estonia. Proceedings of the Estonian Academy of Sciences. Geology, 53, 149−164.

Soesoo, A., Košler, J. & Kuldkepp, R., 2006. Age and geochemical constraints for partial melting of granulites in Estonia. Mineralogy and Petrology, 86, 277–300.