15 the Halikko Kultanummi PROSPECT – a NEW TYPE of Gold Mineralization in the High-Grade Gneiss Terrain of Southwestern Finlan

Home , Halikko

Geological Survey of Finland, Current Research 2003–2004, Edited by Sini Autio. Geological Survey of Finland, Special Paper 38, 15–23 , 2005.

ThE hAlikkO kulTANuMMi PROSPECT – ANEwTyPEOFGOld MiNERAliZATiON iN ThE hiGh-GRAdEGNEiSS TERRAiNOFSOuThwESTERN FiNlANd

by Sari Grönholm 1) , Niilo Kärkkäinen 1) and Jonas Wiik2)

1) Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland 2) Boliden Mineral AB, S-93681 Boliden, Sverige E-mail: [email protected]

Key words (GeoRef Thesaurus, AGI): mineral exploration, gold ores, gneisses, sulfides, hydrothermal alteration, Proterozoic, Kulta - nummi, Halikko, Finland

introduction finally drilled; the results of these investigations are reported below. In this article we consider the gold prospectivity of the highly metamorphosed gneisses and migmatites Geological setting of southwestern Finland, using the Kultanummi occurrence at Halikko as a potentially representative ThebedrockatKultanummiconsistsofupperamphi- example of the type of mineralization to expect in this bolitefaciesmicagneissesandm etavolcanics,intruded terrain. In recent years a number of gold occurrences by granitic veins. The Korveanala-Kaleva prospect, have been found in the region between Paimio and which was studied earlier by GTK, is situated some Halikko in southwestern Finland, largely due to the 10 km west of Kultanummi (Fig. 1). Interpretations efforts of amateur prospectors, who have submitted based on both bedrock maps and airborne magnetic both outcrop samples and mineralized glacial boul- data indicate that both prospects lie within a discrete ders to the Geological Survey of Finland (GTK) for structural domain bounded by shear zones and grani- further evaluation. The first discovery was the Kor- toids (Fig. 3). Regional structural trends tend to be venala-Kaleva occurrence near Paimio but further nearly E-W, with tight folding. Kultanummi is located incentive was given in 2001 when Veli-Matti Koi- within a magnetic anomaly zone that is stronger in vula recovered gold while panning oxidized regolith the southern part. The area immediately to the north material at Kultanummi, near Halikko. Three short of Kultanummi is reminiscent of the structural block holes were drilled at this site, with the best intercept defined at Paimio. The Korvenala-Kaleva prospect at yielding gold grades of 0.5–6 ppm over intervals of Paimio was located by heavy mineral studies follow- 1–6 m. Follow-up exploration during the next field ing discovery of a mineralized boulder, and it is also season delineated a gold-critical sulfide-bearing zone well expressed in till geochemistry and has a distinct over a distance of 600 m, and with a maximum width IPresponse (Rosenberg 2000). Drill cores analyzed of 150 m, to the south of Isorahkaneva mineralized from the from Korvenala-Kaleva show anomalous zone (Figs. 1 and 2). The anomalous area was then gold values over a relatively wide area, with a mean sampled for heavy mineral fractions, and systemati- of 310 ppb for 252 samples. However, in the most cally covered by ground magnetic and IPsurveys, and anomalous intervals, grades are usually 0.1–1 ppm,

15 Geological Survey of Finland, Special Paper 38 Sari Grönholm, Niilo Kärkkäinen and Jonas Wiik

Fig. 1. Locations of the Korvenala-Kaleva and Kultanummi gold occurrences on the bedrock geological map sheets 2021 (Lehijärvi, 1955) and 2022 (Huhma, 1957).

with only a few exceptional intersections of 5.4 ppm quartzo-feldspathic intercalations also occur, with over 1 meter and 1 ppm over 5.45 m (Rosenberg distinctly less biotite, forming discrete aggregates, 2000). Arsenopyrite is also present, but there is the and correspondingly more potassium feldspar than degree of correlation between Au and As is not high in the mica schists. (Rosenberg 2000). Quartz-rich sillimanite gneisses and sillimanite- cordieritegneisses tendtopalegreenishgrayonweath- Geological setting at kultanummi ered surfaces and are somewhat rusty, with complex, tightly folded quartz veins (Fig. 6). These rocks are Mica schists and gneisses intruded by granitic and also readily distinguishable in drill core because of pegmatitic veins predominate at Kultanummi, with theirpalecolourandtexturalheterogeneity.Sillimanite minor intercalations of amphibolites and plagioclase is fibrous, indeed fibrolitic (Fig. 7), while cordierite porphyry. However, the most prospective lithology occurs as pale blebs 1–5 mm in size; microscopic appears to be a relatively quartz-rich gneiss, occur- inspection shows that they are extensively pinitized. ring as discrete units 10–30 m thick within the more In addition to sillimanite and cordierite, the gneisses typical mica gneisses. They typically contain dissemi- consist principally of quartz, plagioclase, potassium nated sulfides and are characterized by aggregates of feldspar andbiotite. Compositionalvariationbetween sillimanite and sporadic cordierite, alternating with layers is defined by variations in the proportions of bands of calc-silicate rock. sillimaniteandcordierite. Accessoryminerals include Mica schists locally display distinct banding, as a muscovite, garnet, tourmaline and a range of sulfides, result of systematic variations in mica abundance as notably chalcopyrite, pyrite, pyrrhotite; arsenopyrite well as concentrations of presumably metamorphic, is also present, and galena has been found in fracture idioblastic magnetite grains (Figs. 4 and 5). Principal fillings. minerals are quartz, plagioclase, potassium feldspar During drilling, a dark green plagioclase porphyry and biotite, with smaller amounts of red garnet, apa- unit about 10 m thick was intersected. The rock is tite, zircon, carbonate and tourmaline. Mica schists foliated, with some silicification and alteration of occasionallygrade intocoarser-grained gneisses hornblende to biotite and also contains angular frag- and may even display a more granitic appearance ments of mafic composition several cm in diameter. orshowextensiveepidotealteration.Occasional Reddishor grey pegmatite dykes, usually less than 5 m

16 Geological Survey of Finland, Special Paper 38 The Halikko Kultanummi prospect – a new type of gold mineralization in the high-grade gneiss terrain of southwestern Finland

Fig. 2. Detailed map of the Kultanummi prospect (Wiik, 2004), showing drill hole locations. Area of map is approximately 8 km 2 . Gold-critical rusty outcrops are indicated with pale brown colour.

Fig. 3. Korvenala-Kaleva (Paimio) and Kultanummi (Halikko) occurrences shown on airborne magnetic image. Tapio Ruotoistenmäki (2004) has indicated inferred tight fold hinges with E-Wtrending axial surfaces by yellow crosses. Yellow lines indicate trends of proposed Au-critical zones, subparallel to fold limbs.

17 Geological Survey of Finland, Special Paper 38 Sari Grönholm, Niilo Kärkkäinen and Jonas Wiik

Fig. 4. Banding in mica schists defined by magnetite. Photo: Sari Grönholm

Fig. 5. Photomicrograph of magnetite-bearing mica schist. Mgt = magnetite, Bt = biotite, Qz = quartz, Pl = plagioclase. Photo: JariVäääätäinen

thick, are common and contain, in addition to quartz, Gold mineralization feldspar and mica, black tourmaline, magnetite and occasionallysillimanite.Medium-grained reddish PyriteisthemosttypicalsulfidephaseatKultanum - granitic dykes are also present, sometimes containing mi,impartinga rustyaspect tooutcrops,andoccurring garnet and magnetite. as disseminations, as solitary grains and aggregates

18 Geological Survey of Finland, Special Paper 38 The Halikko Kultanummi prospect – a new type of gold mineralization in the high-grade gneiss terrain of southwestern Finland

Fig. 6. Rusty sillimanitee-c-cordierordiertie gneiss; light-coloured,red,eelongalongateekknobblnobbly aggregates of bothhssillimaniillimaniteeaandnd cordierite stand out in relief on weathered surfaces. Note also intense folding of reddish feldspathic quartz vein. Photo: Sari Grönholm

Fig. 7. Photomicrograph of sillimanite gneiss. Si =s= sillimaniillimanitee,, Qz=q= quartz.P. Phohoto: Jari Väätäinen

and along joint planes. Disseminated chalcopyrite phase in mineralized rock. Chemical data reveal a and pyrrhotite also occur, and isolated arsenopyrite reasonable correlation between Au and S, but only a grainsorgrainaggregatesarepresent,particularlynear weak relation between Au and As (Fig. 8). quartz vein margins. Galena has also been found in a Gold at Kultanummi appears to be closely associ- few narrow veins. Tourmaline is also a characteristic ated with the relatively silicic, or silicified sillimanite

19 Geological Survey of Finland, Special Paper 38 Sari Grönholm, Niilo Kärkkäinen and Jonas Wiik

Fig. 8. Correlation diagrams for Au and S and Au and As; data from the first three holes drilled at Kultanummi (R379-R381) are not included.

Fig. 9. Variation in Au (left) and S (right) for different rock types at Kultanummi: 1 = mica schist and gneiss, 2 = sillimanite- quartz rock, 3 = sillimanite-cordierite gneiss, 4 = pegmatite, 5 = plagioclase porphyry, 6 = amphibolite, 7 = quartzofeldspathic schist. Lithologies determined from drill core logging.

gneisses, the highest abundances of 6490 ppb and Alteration 2100 ppb having been recorded from sillimanite mica gneiss and sillimanite-cordierite gneiss respectively. Hydrothermalalterationinthegold-anomalouszone However, this correlation is not so obvious from the at Kultanummi is typically evident from increased geochemical data, in that results appear to indicate sulfide abundances (pyrite, pyrrhotite, chalcopyrite that other lithologies can be anomalous with respect and more rarely arsenopyrite), presence of silliman- to Au. It is likely however, that the gold in the more ite-cordierite rocks, silicification and locally from typical mica gneisses is associated principally with abundantmagnetiteinadjacentmicaschists.Silicified quartzofeldspathic veins. Altered lithologies (Groups rock almost invariably contains black, fine-grained 2 and 3 in Fig. 9) also contain relatively high S, com- tourmaline, accompanied by sillimanite, cordierite, pared of the generally low background values in other sulfidesandgold.Carbonatemineralsareoccasionally rock types at Kultanummi. present in Au-enriched rocks. Intensely foliated lay-

20 Geological Survey of Finland, Special Paper 38 The Halikko Kultanummi prospect – a new type of gold mineralization in the high-grade gneiss terrain of southwestern Finland

Fig. 10. Selected major element abundances for various rock types at Kultanummi: 1= mica schist and gneiss, 2= sillimanite-quartz rock and sillimanite-cordierite gneiss, 3= plagioclase porphyry, 4= amphibolite 5= quartzofedspathic schist 6= pegmatite. Rock types defined from drill core logging.

ers also display late metamorphic retrograde mineral throughout the study area. Nevertheless, the Au-min- assemblages, notably biotite replacement of other eralized sillimanite-cordierite gneisses are generally minerals,inparticularhornblende,biotitereplacement only weakly magnetic. Susceptibilities are higher in bychloriteandsericitizationofplagioclase.Withinthe associated mica schist intercalations. Magnetite also zone of hydrothermal alteration, even the plagioclase occurs as large discrete crystals at the margins of the porphyry contains more elevated gold concentrations sillimanite-cordierite gneiss layers, and within cross- (106 ppb over 1 m) compared to values of below 5 ppb cutting granitic and pegmatitic dykes, particularly when it is associated with unaltered amphibolites. withinthehydrothermallyalteredzone.Magnetitecan Major element abundances in altered rocks broadly also be found in the mica schists as isolated idioblastic mirror those of mica schists and mica gneisses; mag- grains, or forming narrow bands. nesium, aluminium, potassium and iron contents in Susceptibilitymeasurementsindicatesomesystem- particular suggest that the sillimanite and cordierite aticrelationshipsbetweenmagnetite,sulfidesandgold, gneisses were originally typical pelitic gneisses in and these correlation may be related to mineralization composition (Fig. 10). process. For example, in drill core from R391 and Magnetiteisacharacteristicaccessorymineral R394, Au and S abundances correlate well (Fig. 11).

21 Geological Survey of Finland, Special Paper 38 Sari Grönholm, Niilo Kärkkäinen and Jonas Wiik

Fig. 11. Variations in gold concentration (red), susceptibility (black) ja sulfur (blue) for selected drill core intervals from Kultanummi. Note the strong positive correlation between Au and S and high susceptibility values, due to the presence of magnetite, at the margins of the mineralized zone.

22 Geological Survey of Finland, Special Paper 38 The Halikko Kultanummi prospect – a new type of gold mineralization in the high-grade gneiss terrain of southwestern Finland

Whilst in general Au and S abundances fall markedly plagioclase porphyry, which has somewhat higher as magnetite content – and hence susceptibility – in- As abundances than at Halikko. At both prospects crease, in some cases (R388), a positive association however, free gold must be present, as deduced from between S abundance and susceptibility is observed, goldgrains inheavymineral separates recoveredfrom related to the presence of pyrrhotite. till and weathered bedrock. Athird Au occurrence in relatively high grade terrain is at Stenmo, near Kemiö, discussion where gold is present in sillimanite gneisses within a metavolcanic sequence (Nordbäck 2003). GTK has been studying the Paimio Korvenala- Kaleva and Halikko Kultanummi gold occurrences Summary sporadically since 1995. The geological environment of these prospects differs from that of the Tampere Recent studies in the relatively high grade meta- and Häme schist belts, which are at lower metamor- morphic terrain of southwestern Finland have deline- phic grade and have been the main focus of GTK ated several gold occurrences, including the Halikko activity for many years (Kärkkäinen et al. 2003). The Kultanummi prospect. Mineralization at Halikko is Korvenala-Kaleva and Halikko prospects both occur associatedwiths ilicicalterationinsillimanite-co rdier - within similar rock types, within a distinct structural ite gneisses and a strong association between silicifi- zone bounded by shear zones and relatively late oro- cation, sulfidation and gold has been recognized. In genic granitoids. The mineralization at the Halikko addition,rockssurroundingthemineralizedzoneshow Kultanummi occurrence is characterized by rusty, a prominent enrichment in magnetite. The alteration strongly foliated sillimanite-cordierite gneisses and zone and associated quartz veins carrying anomalous mica schists. gold values (>10 ppb) are also strongly folded. The ThisAu-criticalassociation ofs illimanite-and distribution of gold is relatively well constrained by cordierite-bearing gneisses is considered to repre- drilling within the alteration zone and correlates well sent hydrothermal alteration accompanying the gold with sulfide abundance. mineralization, and the sillimanite and cordierite is Both the Halikko Kultanummi prospect and the attributed to a younger prograde metamorphic event Paimio Korvenala-Kaleva occurrence previously dis- superimposed on pre-existing sericitic and chlorite covered by GTK attest to the mineralization potential schists. On the basis of currently available data it is of this previously neglected metamorphic terrain in however, difficult to determine whether or not the southwestern Finland. sericitic/chloritic alteration would have been formed within a shear zone developed during early stages of the regional metamorphic event, rather than predating REFERENCES themetamorphism. Geochemicaldata suggest that the huhma, A. 1957. Marttila. Geological Map of Finland1: protoliths to the altered rocks were mica schists and 100 000, Pre-Quaternary Rocks, Sheet 2022. Geological Sur- the unaltered mica gneisses adjacent to the mineral- vey of Finland. ized zone also appear to represent the more strongly kärkkäinen, N. , lehto, T., Tiainen, M., Jokinen T., Nironen recrystallized derivatives of mica schists. M., Peltonen, P. & valli, T. 2003. Etelä- ja Länsi-Suomen kaarikompleksi,kullanjanikkelinetsintävuosina1998 The Kultanummi gold mineralization is character- – 2002. Geological Survey of Finland, unpublished report ized by strong geochemical correlation between gold M19/21,12/2003/1/10. 118 p. + 4 app. andsulfur,therelativelylowAsandaprominentenrich- lehijärvi, M. 1955. Salo. Geological Map ofFinland1:10000000000, ment of magnetite in wall rocks. Sulfidation reactions Pre-QuaternaryRocks,Sheet2021.GeologicalSurveyof Finland. are suggested as a mechanism for gold precipitation, Nordbäck, N. 2003. Malmgeologiska studier över en guldminer- because the highly sulfidized zone and elevated Au alisering i Stenmo, Kimito, SV-Finland. Unpublished master´s abundances occur withina restricted part of thehydro- thesis, Åbo Akademi. 62 p. thermalalterationzone,whilethecordierite-sillimanite Rosenberg, P. 2000. Paimion Korvenaenalanlan alueeellaella vuosina 1996 – 1998 suoritetut kultatutkimukset. Geological Survey of Fin- rocks in the western part of the Kultanummi zone are land, unpublished report M19/2021/2000/1/10. 9 p. + 18 app. less mineralized when compared to the main zone. Ruotoistenmäki, T. 2004. Kultanummi ja Korvenala-Kaleva- The altered rocks and accompanying quartz veins are alueen geofysiikan kartoista, rakennetulkintaa ja niiden asso- also tightly folded, indicated that gold mineralization sioitumisesta kultakriittisiin kohteisiin. Geological Survey of Finland, unpublished report. 4 p. predates at least some of the deformation. wiik, J. 2004. Beskrivning av en guldmineralisering i Kulta- AccordingtoRosenberg(2000),thepredomi- nummi,HalikkoSV-Finland.Unpublishedmaster´sthesis,Åbo nant rock type at the Korvenala-Kaleva prospect is Akademi. 50 p. + 21 app.