Gold deposits in northern Finland - Arkisto.gsf.fi

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25°E 30°E 66°N 67°N 68°N Kittilä Rovaniemi Sodankylä Kuusamo 0 25 50 km Devonian alkaline igneous rocks Palaeoproterozoic rocks (2.50-1.75 Ga) Volcanic rocks (c. 1.96-1.75 Ga) Supracrustal rocks, predominantly sedimentary rocks (1.96-1.75 Ga) Intrusive rocks, predominantly granitoids (1.96-1.75 Ga) Supracrustal rocks, predominantly mafic to ultramafic volcanic rocks and sedimentary rocks (2.50-1.96 Ga) Intrusive rocks, predominantly mafic and ultramafic (2.50-1.96 Ga) Archaean rocks Intrusive rocks, orthogneiss, migmatitic gneiss (c. 3.20-2.50 Ga and possibly older) Supracrustal rocks (c. 3.20-2.75 Ga and possibly older) Au deposits Figure 7. Gold deposits and occurrences shown by drilling in northern Finland (Eilu & Pankka 2009, Hulkki et al. 2010, Talikka & Eilu 2011, www.mawsonresources.com). In the immediate vicinity of Rompas, only the S and N Rompas and Rajapalot occurrences have been drilled. Geology based on Geological Survey of Finland in-house bedrock data base. 22 Pasi Eilu & Tero Niiranen (ed.)
of Ag, Au, As, CO 2 , K, Rb, S, Sb and Te, 5) Au/ Ag ratios above 1, 6) low-salinity aqueous fluids, 7) any primary rock type within the greenstone belts may act as host rock and 8) most occurrences are in greenschist facies rocks (Eilu et al. 2007, Hulkki & Keinänen 2007, Patison 2007). In several cases, the host rocks have also been albitised and carbonatised before gold mineralisation (Hulkki & Keinänen 2007, Patison 2007). This pre-mineralisation alteration has prepared the ground for mineralisation as described below. Similarities to IOCG mineralisation, as the deposit category was originally defined by Hitzman et al. (1992), occur in a number of gold occurrences. These features include: 1) highsalinity aqueous-carbonic fluids, 2) multi-stage alteration, 3) deposits are enriched in Cu, Co, Ag and/or U in addition to Au, and Au is typically a subordinate commodity with respect to Cu and 4) there are abundant iron oxides in the ore. Especially the Kolari magne tite-dominated deposits contain many of these features (Niiranen et al. 2007). Also many of the gold deposits in the Kuusamo, Peräpohja and central Lapland belts have some of these features (but do not contain iron oxides). However, at least the CLGB Au-Cu ± Ni deposits fulfil practically all the diagnostic features of the orogenic gold category, except the high base metal content and indications of medium-salinity fluids. These exceptional features have been explained by attributing such deposits to the subcategory of ‘anomalous metal association’ as defined by Goldfarb et al. (2001) for similar deposits at, for example, Sabie–Pilgrim’s Rest in South Africa and Tennant Creek, Pine Creek and Telfer in Australia. About 60 epigenetic gold occurrences shown by drilling are presently known from the Central Lapland Greenstone Belt, and 13 occurrences from the Peräpohja schist belt (Fig. 7). Of these, the Suurikuusikko deposit (Kittilä Mine, Table 1) is, by far, the largest deposit, and it is a classic example of a gold-only orogenic deposit hosted by a north-striking shear zone in lower greenschist facies greenstones (Patison 2011). Nearly all occurrences in central Lapland probably belong to the orogenic category in the sense the deposit class is defined by Groves (1993) and Goldfarb et al. (2001). For example, more than 30 deposits and occurrences shown by drilling occur within the Sirkka Thrust Zone and subsidiary faults branching from this crustal-scale, more than 100 km long structural break within the Central Lapland Greenstone Belt in Finland (Eilu et al. 2007, 2012b). Locally, the two most significant controls of mineralisation are structure and rock type; some of the ore bodies are hosted by local dilatational sites and by the locally most competent lithological units, some are at locations with soft and hard rock units in contact. For many lodes, part of the local control is the intersection of two faults or a fault along the boundary between lithological units with contrasting competence (Sorjonen- Ward et al. 2003, Holma & Keinänen 2007, Patison 2007, Saalmann & Niiranen 2010). In addition to structure and the primarily competent lithological units, a further significant factor in locally controlling gold mineralisation is the ground preparation by pre-gold alteration: pervasive albitisation (± carbonatisation) of tuffite and phyllite units and carbonatisation of komatiitic units changed these originally soft rocks into hard and competent units. During the later orogenic stages, these were the locally most competent units, and thus provided the best sites for local dilation, veining and mineralisation (e.g. Grönholm 1999, Saalmann & Niiranen 2010). Fluid compositions (Billström et al. 2010, Niiranen et al. 2012) suggest variable, mixed origins for volatiles and metals with no obvious indications of local sources. These features are present in both the gold-only and the anomalous metal association (typically Au-Cu) subtypes. Obvious IOCG-type deposits have been detected only at Kolari, in westernmost Finnish Lapland in the western margin of the Central Lapland Greenstone Belt (Niiranen et Excursion Guidebook FIN1 23
Page 1 and 2: 12th Biennial SGA Meeting 12-15 Aug
Page 3: Excursion Guidebook FIN1 Gold depos
Page 7 and 8: Contents Programme ................
Page 9 and 10: Saattopora Levi Suurikuusikko (Kitt
Page 11 and 12: Timanide orogen 500 km Murmansk Lap
Page 13 and 14: Metallogenic Areas and Mineral Depo
Page 15 and 16: 24°0'0"E 26°0'0"E 68°0'0"N Suuri
Page 17 and 18: 24°0'0"E 25°0'0"E 26°0'0"E N Rom
Page 19 and 20: Table 2. Selected, potentially sign
Page 21 and 22: stone Belt (CLGB). The CLGC is geol
Page 23: schist to upper amphibolite facies.
Page 27 and 28: 3472000 3476000 3480000 3484000 750
Page 29 and 30: 30% decrease in net volume. Amphibo
Page 31 and 32: 7525300 A’ 3391900 3390500 A 200
Page 33 and 34: eralisation took place at 300-350
Page 35 and 36: comm. 2002). Ore-grade rock was fou
Page 37 and 38: Figure 18. Loading ore. Underground
Page 39 and 40: from progressive alteration of mafi
Page 41 and 42: least 25 km (Fig. 15). The dip of t
Page 43 and 44: Table 4. Alteration minerals presen
Page 45 and 46: tober 15, 2012 (Fig. 28). The prese
Page 47 and 48: 3 395 000 mE 3 400 000 mE 3 405 000
Page 49 and 50: ciated with the gold and uraninite.
Page 51 and 52: Table 6. North Rompas drilling inte
Page 53 and 54: Grönholm, P., 1999: The mesotherma
Page 55 and 56: J.S., Siedlecka, A. & Solli, A., 19
Page 57: Sorjonen-Ward, P., Ojala, V.J. & Ai

Gold deposits in northern Finland - Arkisto.gsf.fi

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