Gold in Southern Finland: Results of GTK studies 1998–2011 Edited by Sari Grönholm and Niilo Kärkkäinen Geological Survey of Finland, Special Paper 52, 115–130, 2012 DISCOVERY AND MINERALOGY OF GOLD OCCURRENCE AT VELKUA, SOUTHWESTERN FINLAND by Niilo Kärkkäinen*, Raimo Lahtinen and Lassi Pakkanen Kärkkäinen, N., Lahtinen, R. & Pakkanen, L. 2012. Discovery and mineralogy of gold occurrence at Velkua, Southwestern Finland. Geological Survey of Finland, Special Paper 52, 115–130, 13 figures and 1 appendix. The first indication of gold in the Velkua area came from a nationwide lithogeo- chemical study by the Geological Survey of Finland. During follow up explora- tion, the gold occurrence was discovered in 2006 from a sulphide-rich outcrop. The Au occurrence is hosted by an amphibolite with intercalations of felsic and intermediate gneisses. Amphibolite is surrounded by migmatitic greywackes and coarse-grained microcline granites. Two major types of mineralizations are iden- tified. Higher grade (1 to 24 ppm Au) ore comprises gold that occurs as native grains associated with silicate minerals. In lower grade (0.2–2 ppm Au) zones, gold is closely associated with sulphides. Larger gold grains are locally zoned, the core being an Ag-Au alloy and the border being pure gold. The Velkua area belongs to southern Svecofennia, which went through two events of deformation and metamorphism at 1.88–1.87 Ga and 1.83–1.80 Ga, re- spectively. Gold in high grade ores occurs as inclusions in metamorphic silicate minerals in amphibolites and gneisses. The gold occurrence is metamorphosed, and is probably older than the younger regional peak metamorphism. Whether it was originally an orogenic Au deposit, epithermal deposit or a mixture of these two remains unresolved. Keywords (GeoRef Thesaurus, AGI): gold ores, mineral exploration, amphibo- lites, gneisses, mineralogy, ore minerals, Proterozoic, Velkua, Naantali, Finland *Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland *E-mail: [email protected] 115 Geological Survey of Finland, Special Paper 52 Niilo Kärkkäinen, Raimo Lahtinen and Lassi Pakkanen INTRODUCTION The Velkua Au occurrence is situated in the ar- 2008). The sampled rock consisted of typical mica chipelago of southwestern Finland (Fig. 1). The gneiss without any sulphide enrichment. Four first indication of gold in this area was an Au- new samples collected from the same location anomalous sample in the nationwide lithogeo- also contained anomalous concentrations of gold chemistry database (Rasilainen et al. 2008). This (16.8–61.9 ppb Au). The analysed samples did not sample formed part of an east–west-oriented line contain high amounts of arsenic (maximum 170 of samples with elevated gold concentrations (> ppm As) or sulphide minerals (maximum 0.11% 5 ppb) (Fig. 2), also including the Paimio and S). During follow-up mapping in the vicinity of Halikko Au mineralized areas (Grönholm et al. the sampling site, a rusty outcrop of arsenopyrite- 2005). Anomalous values seem not to be con- bearing amphibolite was found (Fig. 3). Chemi- trolled by any known geological or geophysical cal analyses from this outcrop indicated grades of factor. These Au anomalous sample points were 1–2 ppm Au, and a heavy mineral concentrate of re-sampled during 2006. a crushed rock sample contained numerous gold The original sample from Velkua contained grains. 23.6 ppb Au (N9208576), which is a substantially No previous observations of gold have been de- higher value than normally found in the nation- scribed (Eilu & Pankka 2010), and no mineral ex- wide geochemical database (Rasilainen et al. ploration in general has been carried out in the re- Figure 1. Location of the Velkua area. Bedrock map of Finland according to Korsman et al. (1997). Basemap © National Land Survey of Finland, licence no 13/MML/12. 116 Geological Survey of Finland, Special Paper 52 Discovery and mineralogy of gold occurrence at Velkua, Southwestern Finland Figure 2. Anomalous Au (>5 ppb) in bedrock in southwestern Finland based on the nationwide lithogeochemical database of Finland (Rasilainen et al. 2005). The geological map is according to Korsman et al. (1997) and green squares indicate gold prospects. Basemap © National Land Survey of Finland, licence no 13/MML/12. gion according to the archives or databases of the Geological Survey of Finland (GTK). Historical iron mines and some occurrences of pyrrhotite in a gneiss-amphibolite zone in the western part of the Velkua area have been reported (Karhunen 2004). Typically, Svecofennian orogenic gold deposits are related to shear zones formed syn- to post- peak metamorphism (Eilu & Pankka 2010, Saal- mann et al. 2009, 2010), and their variable tim- ing reflects the multiphase orogenic evolution of Fennoscandia (Lahtinen et al. 2005). Examples of metamorphosed epithermal Au deposits in- clude Enåsen (Hallberg 1994) and Orivesi (Eilu et al. 2003, Talikka & Mänttäri 2005). Kultanummi Figure 3. The arsenopyrite and pyrrhotite mineralized am- also represents an Au occurrence in high grade phibolite in the discovery outcrop of the Velkua occurrence gneisses formed during pre- or early orogenic (observation JA – 07- 29, x = 6718627 and y = 3209288; com- stages (Grönholm et al. 2005). mon KKJ coordinates) 117 Geological Survey of Finland, Special Paper 52 Niilo Kärkkäinen, Raimo Lahtinen and Lassi Pakkanen The most favourable depth for precipitation of amphibolite to granulite facies metamorphic con- gold from orogenic hydrothermal fluids within ditions occurs at Velkua. crustal scale shear systems is in the transition- The purpose of this article is to describe the al zone from greenschist to amphibolite facies discovery history and the characteristic features (Groves 1993). Svecofennian orogenic gold oc- of the Velkua gold occurrence. The work is a con- currences in the Pohjanmaa, Tampere, Pirkanmaa tinuation of mapping of the gold potential in the and Häme belts are mainly met in bedrock meta- high metamorphic zone of southwestern Finland morphosed under mid-amphibolite facies con- (Rosenberg 2000, Grönholm et al. 2005, Lehto & ditions (Eilu & Pankka 2010). In contrast, high Kärkkäinen 2007). Location and research methods The Velkua gold occurrence is located 30 km west regia leach. Analyses were conducted by Labtium of Turku, and 1 km north of the village of Teersa- Oy. lo in the centre of Velkua archipelago, in the mu- Heavy mineral concentrates from drill core and nicipality of Naantali (Fig. 1). The discovery was outcrop samples were prepared from samples that made in a rocky hill surrounded by small fields were selected based on chemical analyses. Rock and forest. samples were first crushed to fine-grained material Exploration at Velkua started with map- (90% < 1 mm). Gold and ore minerals were con- ping and sampling of the mineralized outcrops. centrated using a spiral separator and by panning Crushed bulk rock samples were chemically ana- the overflowed material several times to obtain lysed and their heavy mineral contents were deter- both the gold-enriched fraction and the sulphide- mined. Later, a ground geophysical survey, bed- rich fraction. Sulphide concentrates and hand- rock mapping, till heavy mineral studies, diamond picked gold grains were mounted in epoxy resin core drilling and mineralogical studies were con- and polished. These were studied by ore micros- ducted. copy and with an SEM-EDS electron microscope Gold was analysed from the outcrop samples and microprobe at the laboratories of GTK (Ap- by ICP-MS using aqua regia leach and from drill pendix 1). The mineralogy of three heavy mineral core samples using the Pb fire assay method. Oth- samples is included in this article. er metals were analysed by ICP-OES from aqua BEDROCK General geology The Svecofennian in southern Finland has been man et al. 2008) in southern Svecofennia. divided into central and southern arc complexes Velkua is situated in the 1.90–1.80-Ga-old ac- (Korsman et al. 1997), also known as southern cretionary arc complex of southern Finland and central Svecofennia, where the latter is char- (Korsman et al. 1997), which comprises arc-type acterized by migmatites with trondhjemite leu- volcanism at 1.90–1.88 Ga with partly coeval plu- cosomes and metamorphism culminated at c. 1.88 tonism at 1.89–1.87 Ga. Velkua is also located Ga (Korsman et al. 1999, Mouri et al. 1999, Rut- within the northern part of a microcline granite land et al. 2004, Lahtinen et al. 2009). In contrast, belt characterized by pre-, syn- and late-tecton- southern Svecofennia went through two events ic granites of the younger metamorphic event. of deformation and metamorphism, at 1.88–1.87 Anorogenic rapakivi granite (1.59–1.57 Ga) is lo- Ga and 1.83–1.80 Ga, the latter characterized cated less than ten kilometres north of the Velkua by granite leucosome in migmatites (Korsman et area. al. 1999, Väisänen et al. 2002, Mouri et al. 2005, The bedrock at Velkua is composed of mica Skyttä et al. 2006, Skyttä & Mänttäri 2008, Pa- gneisses, volcanic rocks and granites, and either junen et al. 2008). Major unconformity between belongs to the western extension of the Häme these two events is seen in the occurrence of lat- belt or is part of the Uusimaa belt. Volcanic eritic paleosols (Lahtinen & Nironen 2010) and ≤ rocks include amphibolites and quartz feldspar 1.87 Ga quartzites and meta-arkoses (e.g., Berg- gneisses with carbonate rock interlayers (Kar- 118 Geological Survey of Finland, Special Paper 52 Discovery and mineralogy of gold occurrence at Velkua, Southwestern Finland hunen 2004), but similar associations are met in growth in the Turku area (Fig. 2) during granulite both belts (Kähkönen 2005). The central part of facies metamorphism, with peak metamorphism the Uusimaa belt comprises arc-type rocks vary- dated at 1824 ± 5 Ma (Väisänen et al. 2002). It ing from an island arc (1.895 Ga) to mature arc is assumed that the supracrustal rocks at Velkua affinity (1.878 Ga) (Väisänen & Mänttäri 2002), have an age of 1.90–1.88 Ga and that they have and ages of 1.89–1.88 Ga have been recorded been affected by both metamorphic peaks, where from the Häme belt (Kähkönen 2005, Saalmann the high grade metamorphism correlates with the et al.