Geological Survey of 2021

Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district,

Hannu Huhma, Jukka Kousa and Jouni Luukas

GTK Open File Research Report 8/2021 GEOLOGICAL SURVEY OF FINLAND

Open File Research Report 8/2021

Hannu Huhma, Jukka Kousa and Jouni Luukas

Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Unless otherwise indicated, the figures have been prepared by the author of the publication.

Front cover: Geological map of the Vihanti area showing sample locations. Photo: Jouni Luukas, GTK.

Layout: Elvi Turtiainen Oy

Espoo 2021 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Huhma, H., Kousa, J. & Luukas, J. 2021. Geochronology of the Paleoproterozoic Pyhäsalmi– Vihanti district, central Finland. Geological Survey of Finland, Open File Research Report 8/2021, 31 pages, 31 figures and 5 appendices.

Abundant isotopic data have been obtained from the Pyhäsalmi–Vihanti district since the 1970s. Some old U–Pb results have more recently been confirmed by chemical abrasion TIMS or laser spot analyses. This paper reports on the isotopic data now available on the Pyhäsalmi–Vihanti district, which has been an important mining area. The U–Pb data on zircon include old U–Pb TIMS analyses from nearly 50 samples, together with more recent ICP-MS spot analyses or CA-TIMS data on 15 samples. We also report Pb–Pb TIMS data on ca. 60 galena samples, of which ca. 20 are previously unpublished. The Sm–Nd data produced at GTK consist of more than 100 previously unpublished analyses. These data, together with ca. 250 analyses from published papers, provide the basis for understanding the geological evolution.

The available results confirm that the oldest crust within Svecofennia is ca. 1.93 Ga, which yields highly positive initial €-values displaying juvenile characteristics. The sulphide ores in Pyhäsalmi and Vihanti have a distinct Pb isotopic composition consistent with this ju- venile nature. The bulk of the igneous rocks within the study area yield ages of ca. 1.88 Ga, the youngest U–Pb zircon ages being close to 1.8 Ga.

Appendices are available at https://tupa.gtk.fi/raportti/aineistotallenne/8_2021.zip

Keywords: Finland, Pyhäsalmi-Vihanti, juvenile crust, absolute age, U–Pb, Sm–Nd, Pb–Pb

Hannu Huhma Geological Survey of Finland P.O. Box 96 FI-02151 Espoo, Finland E-mail: [email protected]

Jukka Kousa Puijonkatu 24 FI-70110 Kuopio, Finland E-mail: [email protected]

Jouni Luukas Geological Survey of Finland P.O. Box 1237 FI-70211 Kuopio, Finland E-mail: [email protected]

2 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

CONTENTS

1 INTRODUCTION...... 4

2 ANALYTICAL METHODS...... 4

3 GEOLOGICAL SETTING...... 4

4 PYHÄSALMI AREA, U–PB AGES...... 7 4.1 Venetpalo plutonic suite...... 8 4.2 Mullikkoräme formation...... 9 4.3 Ruotanen formation...... 10

5 VIHANTI AREA, U–PB AGES...... 12

6 HAAPAJÄRVI AREA, U–PB AGES...... 18

7 PIHTIPUDAS– AREAS, U–PB AGES...... 20

8 KAJAANI GRANITE SUITE, U–PB AGES...... 22

9 SM–ND RESULTS FROM THE PYHÄSALMI-VIHANTI DISTRICT AND SURROUNDINGS...... 24

10 PB ISOTOPE RESULTS...... 26

11 CONCLUDING REMARKS...... 28

ACKNOWLEDGEMENTS...... 29

REFERENCES...... 29

3 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

1 INTRODUCTION

Isotope geology has contributed to the under- province displayed a strongly juvenile char- standing of crustal evolution in Finland since acter (initial €Nd +3) compared to reworked the pioneering studies by Kouvo (1958). In early older crustal material within the Karelia prov- studies utilizing the U–Pb zircon TIMS method in ince (€Nd -6). Positive initial Nd epsilon values the GTK laboratory, many granitoid rocks yielded were further confirmed from the 1.93–1.91 Ga ages of ca. 1.88 Ga. However, slightly older gneissic tonalites in several locations of the Savo ages of ca. 1.93 Ga were also obtained from the belt (Lahtinen & Huhma 1997). Pyhäsalmi–Vihanti district (Helovuori 1979). In The Pyhäsalmi–Vihanti district has been one of addition to U–Pb dating, these studies measured the most significant base metal mining areas in Pb–Pb isotopes in whole rocks and sulphides. Finland since the 1950s (Mäki et al. 2015). Much The Pb isotope data on Finnish galenas revealed of the old conventional U–Pb TIMS data from distinct genetic groups, one of these being the Pyhäsalmi have already been published by Kousa “main sulphide ore belt”, including Pyhäsalmi et al. (1994). The aim of the current report is to col- and Vihanti ores (Vaasjoki 1981). The Sm–Nd lect the abundant U–Pb, Sm–Nd and Pb–Pb isotopic isotope studies further emphasized the major data available on the Pyhäsalmi–Vihanti district and difference between roughly coeval felsic rocks surroundings. (Huhma 1986), as some rocks in the Svecofennia

2 ANALYTICAL METHODS

The oldest U–Pb analyses in this report are from report procedures used for TIMS U–Pb and Sm–Nd the 1960s, when mineral decomposition was under- analyses. Images of zircon analysed by laser ICPMS taken using the borax fusion method (Kouvo 1958). are presented in the data tables to the right of the After the early 1970s, the procedure described by analytical results. Plotting of the isotopic data and Krogh (1973) was adopted for multigrain U–Pb age calculations was performed using the Isoplot/ analyses. The methods used for laser ICPMS analy- Ex 3 program (Ludwig 2003). ses follow those in Huhma et al. (2018), who also

3 GEOLOGICAL SETTING

The Paleoproterozoic Pyhäsalmi–Vihanti dis- (1.89−1.87 Ga) in the southwest (Korsman et al. trict (Fig. 1) in central Finland belongs to the 1997). Svecofennia (1.95−1.80 Ga) province (Nironen The Svecofennian supracrustal rocks in the et al. 2016). It covers the northwestern part of the Pyhäsalmi–Vihanti district have been divided into highly tectonized and metamorphosed (– older (1.93–1.92 Ga) and younger (1.89–1.87 Ga) Ladoga) zone between the Archean basement groups. In the recent geological map (Nironen et complexes (3.1–2.6 Ga) in the east and the al. 2016), the older rocks named as the “Northern Svecofennian Central Finland Granitoid Complex Ostrobothnia supergroup” include both the

4 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Pyhäsalmi and Vihanti groups, whereas the younger basalts to potassium-rich rhyolites with a mature group is assigned as the group of the island arc affinity, and in many cases have well- “Central Ostrobothnia supergroup”. The third unit preserved primary structures indicating a subaerial included in the older Svecofennian rocks is the or shallow water depositional environment (Kousa Venetpalo plutonic suite, which is characterized by & Luukas 2004). gneissic tonalites. These may represent subvolcanic The Pyhäsalmi area belongs structurally to a intrusive rocks connected to the felsic volcanic event complex domain where the intercrossing of the of the Pyhäsalmi group. Mainly turbiditic metasedi- NW-trending and the SW-trending shear zones has mentary rocks between the Pyhäsalmi–Vihanti dis- produced crustal-scale tectonic blocks with separate trict and the Archean domain have been defined as metamorphic and lithological characteristics (Kousa the Näläntöjärvi suite, which in the recent compila- & Luukas 2004). The deformation history can be tion has been included in the Savo supersuite of the divided into an early (D1–D2) tectonometamorphic Karelia province (Nironen et al. 2016). stage caused by thrusting of the Svecofennia prov- The Pyhäsalmi group is a bimodal volcanic asso- ince towards the Archean craton and a younger ciation characterized by felsic and mafic members (D3–D4) phase of folding and shearing that pro- (Mäki 1986, Kousa et al. 1994). Quartz porphyries duced the major vertical shear zones of the central and volcanic breccias and their altered varieties are Fennoscandian shield (Kärki et al. 1993). common felsic rock types. In places, mafic volcanic The early stages (D1–D2) caused considerable rocks show well-preserved shallow-water primary crustal thickening and increased metamorphism features such as pillow lavas, with plagioclase, car- and migmatization. Voluminous magmatism dur- bonate or epidote-filled amygdales, pillow breccias ing D2–D3 produced abundant tonalites and ton- and pyroclastic beds. The Pyhäsalmi volcanic rocks alitic migmatites related to high temperature, low are dominantly low- to medium-K rhyolites, tran- pressure metamorphism at 670–800 °C and 5 kb sitional between calc-alkaline and tholeiitic affin- (Korja et al. 1994). Earlier flat-lying structures were ity, and sub-alkaline low- to medium-K tholeiitic refolded into an upright position during D3, but later basalts and basaltic andesites (Kousa et al. 1994). during this stage the deformation style gradually The Vihanti group is dominated by voluminous changed from folding to ductile shearing. This intermediate to felsic volcanic rocks with calc sili- shearing produced largescale dextral SE-trending cate rock interlayers (U−P horizon), which in the strike-slip faults and initiated fragmentation of the Vihanti area are defined as the Vilminko formation crust. The Ruhaperä fault zone and Revonneva shear (Nironen et al. 2016). Primary volcanic structures zone (Fig. 1) are examples of these shear zones. are not well preserved due to locally strong defor- A new set of folds and ductile shear zones was mation and rather high metamorphic conditions. generated during D4, but the most conspicuous In places, volcanic breccia structures are observed structural feature of this stage is the crustal-scale in exploration drill cores. Porphyry textures with sinistral SW-trending Oulujärvi shear zone, which plagioclase and/or quartz are abundant in felsic and transects the Archean craton. The southwestern- intermediate rocks. Subordinate dacitic to rhyolitic most faults of the Oulujärvi shear zone extend to the volcanic, calc-silicate rocks, graphite-bearing tuf- Pyhäsalmi area and have a strong structural influ- faceous schist (black schist) and minor mafic sub- ence on the Pyhäsalmi and Mullikkoräme depos- alkaline basaltic rocks exist as intercalations in the its. The Ruhaperä and Revonneva zones were also intermediate volcanic units. reactivated with significant movement during 4D . The younger volcanic rock units, i.e., the Ylivieska Potassiumrich neosomes and abundant pegmatites group of the Central Ostrobothnia supergroup in the along the shear zones indicate that granitic partial west, include a range of rocks from calc-alkaline melts were generated during the D4 stage.

5 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Fig. 1. Geological map of the Pyhäsalmi–Vihanti district showing sample locations. RuSZ = Ruhaperä shear zone. OSZ = Oulujärvi shear zone.

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4 PYHÄSALMI AREA, U–PB AGES

The main rock units in the Pyhäsalmi area are the the formation covers an area of about 8.4 km2 and Ruotanen formation in the west, the Mullikkoräme consists of felsic (78%) and mafic (22%) volcanic formation in the east, the Venetpalo plutonic suite rocks. and younger syntectonic ca. 1.89–1.87 Ga plutonic The U–Pb zircon dating results published by rocks (Helovuori 1979, Kousa et al. 1994, Mäki Helovuori (1979) and Kousa et al. (1994) include et al. 2015). The Ruotanen formation is a N–S- an age of 1932 ± 12 Ma from the Kettuperä gneiss trending, 9-km-long and 0.5–4-km-wide volcanic (A751) assigned to the Venetpalo plutonic suite, formation with the Pyhäsalmi Cu–Zn mine at its an age of 1921 ± 2 Ma from the Riitavuori rhyolite centre. The formation is bimodal, with approxi- (A1121) in the Mullikkoräme formation and ages of mately 70% felsic and 25% mafic volcanic rocks. 1.87–1.88 Ga from several plutonic rocks in the area. Intermediate volcanic rocks form only 5% of the The new isotope data reported here were obtained area of the formation. The Mullikkoräme formation using (CA-)TIMS, LA-MC-ICPMS and SIMS meth- is a N–S-trending, approximately 12-km-long and ods described by Huhma et al. (2018). The sample 0.5–2-km-wide volcanic belt. The Mullikkoräme Zn sites are indicated in the map (Fig. 2), and the data deposit is in the southernmost part of the forma- are presented in tables in Appendices 1 & 2, which tion, near its eastern margin. The southern part of also contain some previously published data.

Fig. 2. Geological map of the Pyhäsalmi area showing sample locations.

7 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

4.1 Venetpalo plutonic suite

The zircon from the old Kettuperä gneiss sample suggesting an igneous age of 1924 ± 3 Ma, consist- A751 was analysed using the chemical abrasion (CA) ent with the published result (Appendix 1, Fig. 3). TIMS method. The analysis yielded concordant data

data-point error ellipses are 2s 0.40 A751 Kettuperä gneiss

0.36 Intercepts at 1900 190 16 & 1924 3 Ma A751D +4.6 CA*** MSWD = 0.83 n=3 0.32 (C excl.) 1700 U 238 0.28 Pb/ 1500 A751A +4.2 +200 206

0.24 A751B +4.2 1300

0.20 A751C 4.0-4.2 Intercepts at 204±45 & 1924±9 Ma MSWD = 3.4 n=4 0.16 2 3 4 5 6 207Pb/235U Helovuori 1979: 1932±12 Ma

Fig. 3. Concordia plot of U–Pb TIMS data on zircon from the Kettuperä gneiss A751 (A751A-C from Helovuori 1979).

About 20 km north of Kettuperä, another gneiss to the Pyhäsalmi-type felsic volcanic rocks. The sample has been collected from Venetpalo repre- Venetpalo gneiss is mainly composed of plagioclase senting the type locality for the Venetpalo plu- (oligoclase), quartz, biotite and locally hornblende, tonic suite. This sample (A1265-Sarakangas) was and rare secondary microcline. Minor amounts of taken near the eastern contact of the ovoid-shaped epidote, chlorite, titanite, apatite and zircon exist 5-km-broad and 10-km-long north–south trend- as accessory minerals. Zircon grains from the ing, probably flat-lying structure named as the sample are relatively small, subhedral and slightly Venetpalo dome (Fig. 1). The rock type of this dome rounded. The conventional multigrain U–Pb TIMS is gneissose granitoid, having compositions vary- analyses yielded a chord with intercepts at 1917 ± 5 ing from granite to quartz diorite. This rock type is and 403 ± 67 Ma (Appendix 1, Fig. 4). This result is also described as oligoclase gneiss (Hautala 1968). consistent with a CA-TIMS analysis yielding slightly Luukas (2006) has proposed this plagioclase gneiss reversely concordant data with a 207Pb/206Pb age as a felsic subvolcanic intrusion closely related of 1920 ± 2 Ma.

8 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Fig. 4. Concordia plot of U–Pb TIMS data on zircon from the Venetpalo gneissose granite A1265.

4.2 Mullikkoräme formation

Based on three slightly discordant U–Pb zircon analysed using an LA-MC-ICPMS instrument. The analyses, an age of 1921 ± 4 Ma has been pub- new analyses confirm the homogeneity of zircon lished for the Riitavuori rhyolite A1121 from the and provide concordant data with an average Pb/Pb Mullikkoräme formation (Kousa et al. 1994). Zircon age of 1932 ± 10 Ma (Appendix 2, Fig. 5). from the sample was subsequently mounted and

Fig. 5. Concordia plot of U–Pb data on zircon from the Riitavuori rhyolite A1121. LA-MC-ICPMS data are presented as error ellipsoids.

9 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Two rhyolite samples were collected from the The separation yielded both zircon and titanite Mullikkoräme mine for isotope studies. No zircon for U–Pb dating. The zircon grain size is mainly was found in a sample (A1584) from the mine at less than 100 mesh, comprising rather bright and tunnel level (+380 m), near the western edge of pale brown coloured crystals with clear prism faces the “Kharon” ore body. The other sample, A1598, and small bi-pyramid heads. Four analyses from is from drill hole PyO/Mu-116 at a depth of 562.4 m two density fractions have been conducted and the to 598.8 m, near the eastern contact of the deep ore results fit well with a chord yielding intercept ages body named “Siberia”. The rock type is plagioclase- of 1925 ± 4 Ma and 328 ± 52 Ma (Appendix 1, Fig. 6). quartz porphyry, also containing some K-feldspar The U–Pb analysis on titanite yielded a concordia in the ground mass. age of 1846 ± 3 Ma.

Fig. 6. Concordia plot of U–Pb TIMS data on zircon from the Mullikkoräme rhyolite A1598.

4.3 Ruotanen formation

Several felsic rocks from the Ruotanen formation data points, gave an age of 1923 ± 7 Ma (Appendix have been collected over the years, but the recov- 2, Fig. 7). The few values suggesting younger ages ery of zircon has often been poor. However, the have mostly come from analyses hitting the zircon most recent samples, A2141 and A2142 from the surface domain, just before the laser spot passed Pyhäsalmi mine, yielded zircon for dating. Sample through the grain (e.g., analysis 6a-end in Appendix A2142 (drill hole R-2224 888.10) is a plagioclase 2). BSE images of zircon can be found in the ana- quartz porphyry provisionally interpreted as an lytical table. intrusive sill cutting the felsic volcanic rock of the The other plagioclase quartz porphyry sam- Ruotanen formation. The mineral composition of ple, A2141 (drill hole PYS-135 579.50), from the the sample is plagioclase, quartz, and biotite as Pyhäsalmi mine was taken from a hornblende- the main constituents and titanite, apatite, epi- bearing plagioclase quartz porphyry dyke cut- dote, zircon, and pyrite as accessory minerals. The ting Kettuperä quartz-feldspar orthogneiss of the mineral separation of the sample yielded subhedral Venetpalo plutonic suite north of the felsic volcanic zircon, which has been mounted and analysed using Ruotanen formation (Bedrock of Finland – DigiKP). an LA-MC-ICPMS instrument. The U–Pb analyses The main minerals of the sample are plagioclase, yielded concordant results and, excluding a few quartz, hornblende and biotite. Titanite, apatite,

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Fig. 7. Concordia plot of U–Pb LA-MC-ICPMS data on zircon from the Pyhäsalmi porphyry A2142.

epidote and zircon occur as accessory minerals. This Pb/Pb age of 1935 ± 9 Ma (Appendix 2, which also orthogneiss is proposed as subvolcanic intrusive presents zircon images, Fig. 8). As is shown in the rock related to felsic volcanic rocks of the Ruotanen data table, the errors in 207Pb/206Pb during the formation. Abundant subhedral zircon was obtained analytical session were much larger compared to from this sample. Most LA-MC-ICPMS analyses sample A2142 above. yielded concordant results providing an average

Fig. 8. Concordia plot of U–Pb LA-MC-ICPMS data on zircon from the Pyhäsalmi porphyry A2141.

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The third sample, A2140 (drill hole PYS-131B A1583 (Ruotanen, open pit, 13-JPK-97, ca. 430 m 1063.00), from the mine represents a felsic quartz east of the main ore body). However, some titanite porphyric-type volcanic rock of the Ruotanen found in sample A1562 yielded slightly discordant formation. The sample is composed of plagio- U–Pb data suggesting an age of ca. 1.8 Ga (Appendix clase, K-feldspar, quartz and biotite with acces- 1, Fig. A1598). sory epidote, muscovite, garnet, zircon, carbonate, Sample A1597 (Pyhäsalmi mine, R-2139 87.00- pyrrhotite, pyrite and chalcopyrite. According to 98.00) from a granite pegmatite has also been col- field/logging evidence, this rhyolite-X member lected for dating. The underground sample (20 kg) exists on top of the uppermost part of the felsic from drill hole R-2139 in the mine, a few tens of Ruotanen formation and has its own type of sul- metres southeast of the ore body’s southern end at phide mineralization. a depth of 1080 m, is a coarse-grained pink micro- Very few zircon grains were obtained from this cline pegmatite granite. This pegmatite granite sample, and only two of the LA-MC-ICPMS analyses intrudes into the Ruotanen formation and is related are relevant due to low common lead (Appendix 2). to the youngest deformation event in the Pyhäsalmi The rhyolite samples from the Ruotanen for- area. Zircons of the pegmatite sample were muddy mation that did not yield zircon include A1561 and rather dark, especially in the vicinity of fraction (Ruotanen, outcrop 11-JKL-97, ca. 350 m west of +200 mesh. After four hours of abrasion, the result the ore body), A1562 (Ruotanen, open pit, 17-JPK- was still very discordant and further attempts seem 97, ca. 350 m southeast of the main ore body) and to be inefficient (Appendix 1).

5 VIHANTI AREA, U–PB AGES

The older Svecofennian rocks of the Vihanti area The Kokkoneva quartz porphyry sample belong to the Vilminko formation of the Vihanti A1710 (2434/R469/177.35) was collected from a group (Nironen et al. 2016, Mäki et al. 2015). The age 20-m-thick subvolcanic sill occurring between for these rocks was obtained from the Kokkoneva mafic and intermediate volcanic units near the quartz porphyry sill (A1710), which yielded a U–Pb Kokkoneva zinc deposit, ca. 15 km east of the Vihanti zircon upper intercept age of 1922 ± 6 Ma (Kousa et mine (Fig. 9). The main minerals are K-feldspar, al. 2004, Kousa et al. 2013). The U–Pb zircon ages quartz, plagioclase and biotite, and the accessory for the younger Svecofennian volcanic rocks were phases include apatite, carbonate, chlorite, musco- also reported in that paper, which was included in vite and zircon. A small amount of euhedral zircon a volume discussing the geology and ore potential was obtained from the sample. A U–Pb analysis of the Vihanti area (Kousa & Luukas 2004, editors). on chemically abraded zircon yielded concordant These results include an age of 1874 ± 3 Ma for data and, including the published data, an age of the Vilminko rhyolite (A1537) and 1872 ± 3 Ma for 1926 ± 3 Ma is considered the best estimate for the the Korkatti porphyry (A1541), ca. 30 km SE of the rock (Appendix 1, Fig. 10). Vihanti mine (Kousa et al. 2004). Several attempts in the 1980s failed to obtain The U–Pb ages for mostly plutonic rocks in the zircon from the felsic volcanic rocks in the Vihanti Vihanti area have also been reported by Vaasjoki & mine. However, some zircon was subsequently Sakko (1988). Some of the old samples have been found from the Lampinsaari metarhyolite A1865 reanalysed using an updated technique and the collected from a drill core intersecting the felsic vol- results are presented here. The sample sites are canic rocks at the roof of the ore (2434/R586/69.30- indicated in the map (Fig. 9), and all U–Pb data, 70.10). The main minerals of the sample are quartz, together with the old published results, are included plagioclase, cordierite and biotite. The accessory in the tables in Appendix 1, which also present some minerals include antophyllite, chlorite, titanite, isotope diagrams. apatite, carbonate, zircon and sulphides.

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Fig. 9. Geological map of the Vihanti area showing sample locations.

Fig. 10. Concordia plot of U–Pb TIMS data on zircon from the Kokkoneva quartz porphyry A1710 (Analyses

A1710A-D from Kousa et al. 2004).

13 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Fig. 11. Concordia plot of U–Pb SIMS and TIMS data on zircon and monazite from the Lampinsaari rhyolite A1865 (open error ellipses are SIMS data on zircon).

The few euhedral zircon grains obtained from the 1877 ± 5 Ma obtained from the U–P horizon, which sample were analysed at VSEGEI in St Petersburg was considered to reflect the age of regional meta- using SHRIMP II and the analytical methods morphism (Vaasjoki et al. 1980). described by Larionov et al. (2004). The U–Pb data Zircon from two plutonic rocks from the Vihanti and analytical spots in the CL images are presented area has been treated using the chemical abra- in Appendix 2. The data indicate that some analyses sion method by Mattinson (2005). The U–Pb TIMS have low Th/U and relatively high common lead analyses of both samples yielded concordant results yielding large errors. Excluding the low Th/U data, (Appendix 1). The age for the Alpua gabbro (A780) is an average 207Pb/206Pb age of 1898 ± 12 Ma can be now estimated to be 1885 ± 2 Ma instead of 1901 ± 12 Ma calculated. The low Th domains are generally con- (Vaasjoki & Sakko 1988, Fig. 12). The concordant sidered as metamorphic, and here these data give data on the Hirsikangas granodiorite (A781) zircon an age of 1870 ± 13 Ma (Fig. 11). This is supported give an age of 1884 ± 2 Ma (Fig. 13). These precise by the U–Pb TIMS analysis on monazite (Appendix ages suggest that the plutons were older than the 1), which yielded a concordant result at 1874 ± 2 Ma. youngest Svecofennian volcanic rocks in the area These ages are consistent with the U–Pb age of (Fig. 14, Appendix 1) reported by Kousa et al. (2004).

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Fig. 12. Concordia plot of U–Pb TIMS data on zircon from the Alpua gabbro A780 (Analyses A780A-D from Vaasjoki & Sakko 1988).

data-point error ellipses are 2s 0.38 A781 Hirsikangas granodiorite, Vihanti 1950 A781D CA-TIMS 0.34 Concordia Age = 1850 1884 ± 2 Ma A781D +4.2 CA*** 1750

U 0.30 1650 A781A +4.2 238 1550 Pb/

206 0.26 1450

A781B 4.0-4.2 0.22 A781C 4.0-4.2 magn Intercepts at 236±77 & 1882±12 Ma MSWD = 5.8 n=4

0.18 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 207 235 Pb/ U V&S 1988: 1874±13 Ma

Fig. 13. Concordia plot of U–Pb TIMS data on zircon from the Hirsikangas granodiorite A781 (Analyses A781A-C from Vaasjoki & Sakko 1988).

15 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Fig. 14. Concordia plot of U–Pb TIMS data on zircon from the younger Svecofennian volcanic rocks (Kousa et al. 2004).

Some isotope studies have also been conducted from mostly due to an analysis made conducted after HF areas NW–W of Vihanti, but the data have remained leaching. Excluding these data (A68C), the upper unpublished. Two samples from the Vasankari intercept age would be 1873 ± 12 Ma (MSWD = 2.8). gneiss were already collected in the 1960s (Fig. 9). Zircon from another old sample from the Vasankari The homogenous zircon population obtained from gneiss (A110) was analysed in the 1960s using the sample A68 consists of short euhedral and clear borax-fusion method (and a sample weight of grains. The six U–Pb TIMS analyses carried out on 460 mg!). Including these data, all seven analyses zircon yielded discordant data and a chord with from the Vasankari gneiss yielded intercept ages of intercept ages of 1881 ± 14 and 199 ± 64 Ma. The 1880 ± 13 and 199 ± 59 Ma (MSWD = 9, Appendix scatter of the data (MSWD = 9.2) is quite large and 1, Fig. 15).

data-point error ellipses are 2s 1900 0.34 A68 Vasankari gneiss

A68 Intercepts at 1800 0.32 172±48 & 1873 ± 12 Ma MSWD = 2.8 n=5 (HF excl) A68C 4.2-4.6/HF 1700 0.30 A68F +4.6/abr 5 h

U A1196D 4.3-4.5/abr 2 h 1600 A1196A +4.5 238 0.28 A1196C 4.2-4.3/+200 A68B +4.6 Pb/ 1500 A110 Vasankari gneiss, borax 206 0.26

A1196B 4.3-4.5/+200 0.24 A68E +4.2/abr 2 h A1196 Pirttikoski granodiorite 0.22 reference line A68A Total 42±250 & 1885 ± 7 Ma A68D 4.2-4.6 MSWD = 0.4 n=3(/4) 0.20 3.0 3.4 3.8 4.2 4.6 5.0 5.4 207Pb/235U

Fig. 15. Concordia plot of U–Pb TIMS data on zircon from the Vasankari gneiss samples A68 and A110 and the Pirttikoski granodiorite A1196.

16 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Granodioritic gneisses occur in a dome struc- concordant, but obviously yield a range of ages ture NE of the Vasankari gneisses (Fig. 9). Sample from ca. 1.86 Ga to Archean (Appendix 2, Fig. A16). A1196 from Pirttikoski represents these rocks and Twenty-two points cluster at 1879 ± 6 Ma, which has yielded light-coloured simple zircon prisms for can be considered as the age of granite. Six data dating. The four multigrain TIMS U–Pb analyses points give estimates from 1.96 to 2.08 Ga and two yielded discordant and slightly heterogeneous data are Archean (although 17a with high common lead is (Appendix 1, Fig. 15). Including the three analyses badly discordant). Ages of ca. 1.88 Ga were obtained conducted on the heaviest zircon (>4.3 g/cm3), the from both large core domains (20a) and rims (7b, data yield a chord with intercepts at 1885 ± 7 and c) from grains that appear to have older inherited 42 ± 250 Ma. Some caution is thus involved in the cores (7a, Fig. A16). age but likely that the rock does not belong to the A few kilometres south of these granites, a dia- older pre-1.9 Ga Svecofennian group, which is the base dyke (A1542 Kallioniemi) cutting metasedi- case with some other dome structures within the ments has been a target of isotope studies. The Svecofennia province, i.e., A292 (Lahtinen et al. conventional U–Pb TIMS analyses conducted on 2016). a slightly heterogeneous zircon population have Sample A1406 from Mansikkakallio represents yielded scatter data with 207Pb/206Pb ages of granites occurring in Pattijoki. The granite con- 1.89–2.06 Ga (Kousa et al. 2004), suggesting that sists of K-feldspar, quartz, plagioclase and bio- zircon was inherited from country rocks. tite. The zircon grains from the sample are dark Since the earliest rocks within the Svecofennian and the conventional TIMS U–Pb analysis yielded domain are of special interest for understanding the a very discordant result (Kousa et al. 2004). The geological evolution, we would like to point out a CA-TIMS analysis is, again, concordant, with an recent CA-TIMS result from the Niskakoski granodi- age of 1905 ± 2 Ma (Appendix 1, Fig. A16). This age orite A1270 in Kannus (Fig. 1). The U–Pb data yield a appears somewhat high compared to the geological concordant result and suggest an age of 1898 ± 2 Ma setting, and in fact only 5% of zircon was left for (Lahtinen et al. 2016, Appendix 1), which is consist- analysis after the CA-TIMS treatment. Therefore, ent with the age 1896 ± 6 Ma obtained by SIMS for zircon A1406 was mounted on epoxy and analysed sample Kannus 2 from the same location (Williams using LA-MC-ICPMS. Most of the 31 analyses are et al. 2008).

Fig. 16 Concordia plot of U–Pb data on zircon from the Mansikkakallio granite A1406 (LA-MC-ICPMS data are presented as error ellipsoids).

17 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

6 HAAPAJÄRVI AREA, U–PB AGES

The Haapajärvi area is located on the western side cent, short prisms with well-developed crystal faces. of the Ruhaperä shear zone, which appears to be The six multigrain TIMS analyses yielded a chord the western margin of the Pyhäsalmi–Vihanti type suggesting an upper intercept age of 1883 ± 3 Ma, rocks (Fig. 1). The felsic and mafic volcanic rocks which is considered as a good estimate for the age belong to the Kuusaa formation of the Ylivieska of this granitoid (Appendix 1, Fig. A18). group and thus represent the younger Svecofennian Sample A1407 Kettukallio represents the por- supracrustal rocks. Several samples from the area phyritic granites SE of the Kuusaa formation vol- were collected in the 1990s for dating. canic rocks discussed above (Fig. 1). Zircon grains Sample A1267-Kontiomäenrämeet (JPK-88-14) obtained from the sample are pale, clear and euhe- is a metarhyolite from the Kuusaa formation rep- dral. The five U-Pb analyses conducted on zircon resenting calc-alkaline high-K volcanism (Fig. 9). are discordant and define a chord with an upper Zircon grains extracted from the sample are clear, intercept age of 1881 ± 9 Ma, which can be consid- subhedral and short. The six multigrain U–Pb TIMS ered as an age estimate for these granites (Appendix analyses are of good quality, the analyses after air 1, Fig. A19). abrasion being close to concordia. They yield a chord Here, we also report the old U–Pb data obtained suggesting an upper intercept age of 1887 ± 3 Ma, from zircon extracted from a quartz dioritic pebble which is considered as the age of volcanism of the Settijärvi conglomerate (A188). This sedi- (Appendix 1, Fig. A17). The U–Pb data on zircon mentary formation extends for several kilometres from two other felsic rocks in the area support this in the vicinity of the volcanic rocks (Fig. 9). The five age. These are the porphyry A1266 and dyke A1268 U–Pb analyses on zircon yielded a chord suggesting from Kontiomäki (Appendix 1, Fig. A17). an upper intercept age of 1892 ± 9 Ma (Appendix 1, A few kilometres west of these volcanic rocks, Fig. 19). Although there is some scatter in the data a sample (A1269-Hakulinkangas) was collected (MSWD = 4.2), the age can be considered reliable, from a gneissic quartz diorite occurring within mica since one of the analyses is concordant within error. gneisses. Zircon grains in the sample are translu-

Fig. 17. Concordia plot of U–Pb TIMS data on zircon from the Kuusaa formation. Analyses from A1267 are shown as error ellipses.

18 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Fig. 18. Concordia plot of U–Pb TIMS data on zircon from the Hakulinkangas gneiss A1269.

Fig. 19. Concordia plot of U–Pb TIMS data on zircon from the Kettukallio granite A1407 and Settijärvi conglom- erate clast A188.

19 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Fig. 20. Concordia plot of U–Pb TIMS data on zircon and monazite from the Hitura gabbro A758.

The old U–Pb analyses from the Hitura gabbro crystallize from a mafic magma, and some con- (A758) are also included in this volume. In addi- tamination has probably taken place to allow the tion to zircon, some monazite was also obtained formation of monazite. The two analyses on zircon from the coarse-grained gabbro. The U–Pb analysis are discordant, but consistent with the age obtained from monazite yielded concordant data and an age from monazite (Appendix 1, Fig. 20). of 1877 ± 3 Ma. It is unlikely that monazite could

7 PIHTIPUDAS–PIELAVESI AREAS, U–PB AGES

The U–Pb isotope data on zircon from the Pihtipudas K-feldspar, titanite, apatite, carbonate, zircon and area published by Aho (1979) suggested age indica- sulphides. The rock is sheared and considered as a tions of ca. 1.88 Ga for several rock types. However, subvolcanic intrusion closely related to the felsic these results were based on only a small amount of volcanic rocks that host the sulphide ore deposit. discordant data. Subsequently, two further zircon Brownish subhedral zircon was obtained from the U–Pb analyses have been conducted on the porphy- sample. The four conventional multigrain U–Pb ries. One (A308B) is concordant within error, and analyses yielded discordant data and a chord with combining the available data, an age of 1885 ± 5 Ma intercepts at 1897 ± 13 Ma and 362 ± 250 Ma (MSWD can be calculated for the Pihtipudas porphyries = 0.026, Appendix 1, Fig. 22, location in the map (Fig. 21, data in Appendix 1, location in the map in in Appendix 5). A U–Pb TIMS analysis on titanite Appendix 5). yielded concordant data and an age of 1805 ± 3 Ma, About ten kilometres east of the Pihtipudas por- thus like the titanite ages obtained from the phyries, on the eastern side of the Ruhaperä fault Pihtipudas samples (Aho 1979). zone, volcanic rocks are assigned to the Kangasjärvi In this context, we report the result of CA-TIMS formation of the Pyhäsalmi group. No zircon was analysis of the Kirkkosaari gneiss sample A291 found from the two felsic volcanic samples (A1759, from Pielavesi (Fig. map 1). The age of 1916 ± 12 Ma A1760), but the tonalitic gneiss A1757 Mustinvuori reported by Ekdahl (1993) was based on discord- (157-AVP-99) yielded zircon for dating. The main ant data, but now, after chemical abrasion treat- minerals of the rock are plagioclase, quartz, epi- ment, the result is concordant, suggesting an age dote and biotite. The accessory minerals include of 1914 ± 3 Ma (Appendix 1, Fig. 22).

20 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Fig. 21. Concordia plot of U–Pb TIMS data on zircon from the Pihtipudas porphyries A308 and A310.

Fig. 22. Concordia plot of U–Pb TIMS data on zircon from the gneisses in Mustivuori A1757 and Kirkkosaari A291 (Analyses A291E-H from Ekdahl 1993).

21 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Fig. 23. Concordia plot of U–Pb TIMS data on zircon from the Aittojärvi pyroxene granite A1234.

Pyroxene-bearing granites are common in the the sample are euhedral, strongly zoned prisms. The Archean–Proterozoic boundary zone. Sample A1234 four multigrain TIMS analyses yielded a chord with from Aittojärvi in Kiuruvesi represents these rocks an upper intercept age of 1883 ± 3 Ma (Appendix 1, and was also a target of isotope studies a long time Fig. 23). ago (Fig. 1). Abundant zircon grains obtained from

8 KAJAANI GRANITE SUITE, U–PB AGES

Major rock types in the area between the Vihanti is considered as the best estimate for the age of the and belts in the north consist of gran- rock (Fig. 24). These granites represent late mag- ites of the Merikoski granite complex assigned as matic activity close to the NW (Raahe–Ladoga) and Kajaani granite suite in the recent geological map NE (Oulujärvi) trending shear zones. (Nironen et al. 2016). These rocks may provide tools Based on concordant data on monazite, an age of for dating the tectonometamorphic evolution of the 1784 ± 3 Ma was obtained from a granite pegmatite Archean–Proterozoic boundary zone. In addition to further south at Lehtomäki (A1538, Fig. 1, Kousa et the results reported by Kousa et al. (2004), U–Pb al. 2004). The formation of pegmatite has been cor- data available from a few more granitoid samples related with the D4 structures (Luukas et al. 2004). near the Kiiminki belt are included in this compi- The bulk of the rocks in the Merikoski granitoid lation. The first age indication of the granite was complex are granites, but some granodiorites and reported by Honkamo (1989), who published U– quartz diorites also occur. Two samples from these Pb ages of 1815 ± 16 Ma (zircon) and 1793 ± 5 Ma mildly deformed rocks have been collected for dat- (monazite) for the Tyrnävä granite (A343). ing, A1382 Mämmisuo and A1383 Kukkonen (Fig. 1). A leucocratic porphyric granite from (A1539 The zircon grains separated from the quartz diorite Aarrekangas) was one of the samples in Kousa et al. A1382-Mämmisuo are euhedral, transparent sim- (2004, Fig. 1). Discordant U–Pb TIMS data on zircon ple prisms. The four multigrain TIMS analyses are suggested an upper intercept age of 1823 ± 9 Ma, discordant and yielded a rough chord with an upper but concordant analysis on monazite at 1810 ± 3 Ma intercept age of 1807 ± 33 Ma (Appendix 1, Fig. 25).

22 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Abundant zircon was extracted from the analyses is concordant within error and suggests Kukkonen quartz diorite (A1383). The grains are an age of 1799 ± 4 Ma, which is supported by the pale, transparent and euhedral, typical for a mag- other two discordant analyses (Appendix 1, Fig. 25). matic population. One of the three U–Pb TIMS

Fig. 24. Concordia plot of U–Pb TIMS data on zircon and monazite from the granites at Lehtomäki A1538 and Aarrekangas A1539.

data-point error ellipses are 2s 0.36 Merikoski granitoids 0.34 A1383A Kukkonen quartz diorite Concordia age 1850

0.32 1799 ± 4 Ma A1383A +4.5 +200 a6h 1750 A1382A +4.5 a5h U 0.30 A1383B +4.5 +200

238 1650 A1382B +4.5 Pb/ 0.28 A1383C 4.3-4.5 +200 206 1550

0.26 A1382C 4.3-4.5 +200 A1382 Mämmisuo qu dr Intercepts at 0.24 A1382D 4.2-4.3 81±320 & 1807 ± 33 Ma MSWD = 14 n=4 0.22 3.2 3.6 4.0 4.4 4.8 5.2 207Pb/235U Fig. 25. Concordia plot of U–Pb TIMS data on zircon from granitoids A1382 Mämmisuo and A1383 Kukkonen.

23 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

Zircon from two granite samples, A1384 nium (3 ppm) and unradiogenic Pb. The U content in Väärälänperä and A1385 Laukkala (Fig. 1), appears titanite from the other sample, A1387 Pitkäselkä, is turbid and unsuitable for dating. One U–Pb TIMS relatively high. The U–Pb analysis yielded reversely analysis conducted A1384 is badly discordant concordant data, but the 207Pb/206Pb age of 1819 ± (Appendix 1), but consistent with the chord defined 6 Ma should be close to the formation of meta- by the data from the old sample, A343. Further evi- morphic titanite (Appendix 1). The mineral separa- dence of ca. 1.8 Ga granites has been obtained from tion of the sample also yielded zircon that probably the Puolanka area, east of the Merikoski granites originates from the surrounding quartzite, since the (Vaasjoki et al. 2001). skarn layer is only 50 cm thick. The four U–Pb TIMS To constrain the metamorphic evolution, titanite analyses conducted on zircon are discordant and has been extracted from two skarn rocks from the suggest an Archean origin (Appendix 1). Ca. 1.9 Ga Kiiminki Jatulian supracrustals. Abundant titan- titanite was also reported from the Huttukylä diorite ite was obtained from the tremolite skarn A1386 (A905), whereas gabbro A907 has yielded a zircon Lamminselkä. However, the U–Pb TIMS analysis U–Pb age of 1873 ± 7 (Honkamo 1988). was not successful due to the low abundance of ura-

9 SM–ND RESULTS FROM THE PYHÄSALMI-VIHANTI DISTRICT AND SURROUNDINGS

Important information on the origin of crust has the data on mafic volcanic rocks also clearly show been obtained via Sm–Nd analyses on whole rock positive initial values, as was the case with mafic samples since the 1980s (Huhma 1986, Patchett & volcanic rocks further west in the Pohjanmaa belt Kouvo 1986). The Sm–Nd data currently available (Lahtinen et al. 2017). Further south, similar results on the 1.93–1.77 Ga rocks from the 200-km-wide were earlier obtained from the Rautalampi, Joroinen NW–SE zone in the Archaean–Proterozoic boundary and Rantasalmi areas (Huhma 1986, Lahtinen & between the Bothnian Bay and Lake Ladoga consist Huhma 1997, Makkonen & Huhma 2007). of nearly 400 analyses on whole rocks. The pub- Positive initial €Nd values are also characteris- lished data include ca. 90 analyses on felsic igne- tic of the few 1.88 Ga rocks from the Vihanti area. ous rocks, ca. 100 analyses on mafic rocks and ca. In contrast, values close to zero are evident from 60 analyses on metasediments (e.g., Huhma 1986, volcanic rocks from Haapajärvi (Kuusaa forma- Patchett & Kouvo 1986, Lahtinen & Huhma 1997, tion) and Pihtipudas. Significantly lower initial €Nd Lahtinen et al. 2002, 2015, 2016, 2017, Makkonen & values are characteristic of Paleoproterozoic rocks Huhma 2007, Rämö et al. 2001, Woodard & Huhma within the Karelia province (e.g., Huhma 1986). In 2015, Woodard et al. 2016). This paper includes ca. this study, rocks from the Kajaani granite suite are 110 previously unpublished Sm–Nd results, mostly good examples of this group (Fig. 26). The range of from the Pyhäsalmi–Vihanti district, but also from initial €Nd values is also shown in the geological other areas (Appendix 3). map, in which data are split into five groups using The previously unpublished Sm–Nd results colour codes (Fig. 27). It becomes evident that low confirm the positive initial €Nd values of the older values (red) suggesting recycling of crustal mate- Svecofennian crust (Fig. 26) and consist of rocks rial are typical of the Archean areas, whereas most from the Pyhäsalmi, Vihanti, –Kangasjärvi positive values (blue) appear to occur in the zone and Pielavesi areas. In addition to felsic volcanic between the Central Finland granitoid complex and rocks and (Venetpalo plutonic suite) granitoids, the Archean crust.

24 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

A-P boundary zone, igneous rocks

4 Depleted Mantle

CHUR 0

-4

Nd epsilon felsic rocks (n=150): circle: Svecofennian triangle: Karealian diamond: boundary zone -8 filled symbol: publ. here open symbol: publ. before mafic rocks (n=80): + published here x published before -12 1780 1820 1860 1900 1940 Age (Ma) note! ages are based on U-Pb or estimated!

Fig. 26. Initial €Nd values for whole rocks in the Raahe–Ladoga zone.

Fig. 27. Initial €Nd values for 1.94–1.77 Ga whole rocks split into five colour groups: dark blue (highest values), blue, green, yellow and red (lowest values). Circle - felsic igneous rocks, break values -4.9, -1.4, +0.6, +2.1 Star – mafic rocks, break values -2, 0, +2, +3 Triangle – sedimentary rocks, break values €Nd(1900) -8.7, -4.1, -2.3, -1. (Two felsic samples with a low initial €Nd in the Svecofennian domain have anomalously high Sm/Nd ratios and probably suffer from secondary REE fractionation)

25 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

10 PB ISOTOPE RESULTS

Following the early studies, it was realized the lead 1979) and Haveri (Vaasjoki & Huhma 1999). The isotope results for the Svecofennian massive sul- Pb isotope data on 57 galena samples available phide ore deposits provide groups suggesting a dis- from the Pyhäsalmi–Vihanti area and surround- tinct origin. Abundant Pb isotope data on Finnish ings are included in Appendix 4 and presented galenas were published by Vaasjoki (1981), and one in Figure 29. In addition to the published data of the groups was the “main sulphide ore belt”, (Vaasjoki 1981, Vaasjoki & Sakko 1988), previ- including Pyhäsalmi and Vihanti ores (Fig. 28). In ously unpublished results are also included. addition to sulphides, Pb isotopes were measured in Finland. Distinct groups include Outokumpu, from whole rocks. These data provided rough Karelian schists (Hammaslahti), the main sulphide estimates of the ages, but importantly, the ini- ore belt (Pyhäsalmi–Vihanti), the batholith of cen- tial ratios were consistent with the data obtained tral Finland (Pihtipudas), and Svecofennian suprac- from associated galenas, as shown in Figure 30 rustal formations in S Finland (Orijärvi). for Pyhäsalmi (Helovuori 1979), Pihtipudas (Aho

Galena from Finland

15.45

G12 Orijärvi 1600 15.35

1800 15.25 G16 Ritovuori

Pb Pihtipudas 2000 204 15.15 G126 Vihanti Pb/ G25 Pyhäsalmi 207 2200 G183 Hammaslahti 15.05 G30 Outokumpu hvr10-Haveri 14.95 (chalcopyrite)

14.85 14.4 14.6 14.8 15.0 15.2 15.4 15.6 15.8 16.0 206Pb/204Pb

Fig. 28. Representative samples showing the Pb isotope composition of Proterozoic sulphide ores in Finland. Distinct groups include Outokumpu, Karelian schists (Hammaslahti), the main sulphide ore belt (Pyhäsalmi– Vihanti), the batholith of central Finland (Pihtipudas), and Svecofennian supracrustal formations in S Finland (Orijärvi).

26 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Fig. 29. Pb isotope data on galena from the study area.

Svecofennian volcanics

16.6 Pyhäsalmi Age = 1911 79 Ma MSWD = 9.4 n=12

16.2 Pb 204

15.8 Pihtipudas Pb/ Age = 1906 78 Ma

207 0 MSWD = 4.3

Galenas 15.4 G12 Orijärvi Haveri G16 Ritovuori G25 Pyhäsalmi Age = 1954 140 Ma G183 Hammaslahti MSWD = 3.6 G30 Outokumpu 15.0 14 16 18 20 22 24 26 28 206Pb/204Pb

Fig. 30. Lead isotope results from whole rocks and sulphides. Note that initial ratios of whole rock isochrons approach the composition of related sulfide.

27 Geological Survey of Finland, Open File Research Report 8/2021 Hannu Huhma, Jukka Kousa and Jouni Luukas

11 CONCLUDING REMARKS

This paper reports on the abundant isotopic data confirm that the oldest crust within the Svecofennia available for the Pyhäsalmi–Vihanti district in cen- is ca. 1.93 Ga, which is largely juvenile with clearly tral Finland. The U–Pb data on zircon discussed positive initial €-Nd values (Fig. 26). East of the here represent ca. 50 samples, including 22 previ- assumed crustal boundary, much lower initial ously unpublished samples. The ages of many old €-values are typical, showing the influence of older samples have been confirmed utilising the chemi- lithosphere in the genesis of Palaeoproterozoic cal abrasion TIMS technique and a few ages were rocks. The galena samples from the sulphide ores obtained using laser MC-ICPMS. Previously unpub- associated with the juvenile rocks also display a dis- lished isotopic data include more than 100 Sm–Nd tinct Pb isotope composition (Fig. 28). analyses of whole rocks and ca. 20 Pb–Pb analyses Felsic rocks with ages of 1.93–1.91 Ga and highly of galena samples, which together with the abun- positive initial €-Nd values are observed from dant published data provide tools for evaluation of several locations within the Raahe–Ladoga zone, the origin of rocks and sulphide ores. including Vihanti, Pyhäntä (Kontinen & Huhma The age results are summarised in Appendix 5 in prep.), Pyhäsalmi, Pielavesi (Ekdahl 1993), and illustrated in Figure 31. The table also contains Rautalampi (Vaasjoki et al 2003) and Joroinen ages obtained from monazite and titanite, together (Huhma 1986, Vaasjoki & Sakko 1988, Kousa et al. with appropriate references. The Sm–Nd results 2018), and further west from Veteli (Lahtinen et al are summarized in a table in Appendix 3, which 2016). The bulk of the igneous rocks discussed in also contains previously published data from the this report yield ages of ca. 1.88 Ga, the youngest Raahe–Ladoga zone and surroundings. The results U–Pb zircon ages being close to 1.8 Ga.

Fig. 31. U-Pb ages of zircon from the Pyhäsalmi–Vihanti district, also including samples from Pielavesi and Pihtipudas. The ages 1.93–1.92 Ga represent older Svecofennian supracrustal rocks and the Venetpalo plutonic suite. Most magmatic rocks, including the younger Svecofennian supracrustals, have yielded ages of 1.89–1.87 Ga.

28 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

ACKNOWLEDGEMENTS

The personnel of the GTK isotope laboratory over laboratory in St. Petersburg (VSEGEI, SHRIMP/ the years are greatly acknowledged (Olavi Kouvo, Sergei Sergeev) for SIMS analyses. Pekka Tuisku Matti Sakko, Tuula Hokkanen, Marita Niemelä, provided good samples from the Merikoski granite. Matti Vaasjoki, Matti Karhunen, Arto Pulkkinen, We acknowledge discussions with Raimo Lahtinen Lasse Heikkinen, Leena Järvinen, Irmeli Mänttäri, and Asko Kontinen. Hugh O’Brien, Yann Lahaye). We acknowledge the

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30 Geological Survey of Finland, Open File Research Report 8/2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland

Vaasjoki, M. & Huhma, H. 1999. Lead and neodymium Vaasjoki, M., Kärki, A. & Laajoki, K. 2001. Timing of isotopic results from metabasalts of the Haveri For- Paleaeoproterozoic crustal shearing in the central mation, southern Finland: evidence for Palaeoprote- Fennoscandian Shield according to U-Pb data from rozoic enriched mantle. In: Kähkönen, Y. & Lindqvist, associated granitoids, Finland. Bulletin of the Geo- K. (eds) Studies related to the Global Geoscience Tran- logical Society of Finland 73, 87–101. Available at: sects/SVEKA Project in Finland. Bulletin of the Geo- https://doi.org/10.17741/bgsf/73.1-2.007 logical Society of Finland 71 (1), 143–153. Available at: Williams, I. S., Rutland, R. W. R. & Kousa, J. 2008. A https://www.geologinenseura.fi/sites/geologinen- regional 1.92 Ga tectonothermal episode in Ostroboth- seura.fi/files/sgs_bt_071_1_pages_143_153.pdf nia, Finland: Implications for models of Svecofenn- Vaasjoki, M. & Sakko, M. 1988. The evolution of the ian accretion. Precambrian Research 165, 15–36. Raahe-Ladoga zone in Finland: isotopic constraints. DOI: 10.1016/j.precamres.2008.05.004 In: Korsman, K. (ed.) Tectono-metamorphic evolution Woodard, J. & Huhma, H. 2015. Paleoproterozoic mantle of the Raahe-Ladoga zone, eastern Finland. Geologi- enrichment beneath the Fennoscandian Shield: Iso- cal Survey of Finland, Bulletin 343, 7–32. Available topic insight from carbonatites and lamprophyres. at: http://tupa.gtk.fi/julkaisu/bulletin/bt_343_pag- Lithos 236–237, 311–323. es_007_032.pdf Woodard, J., Tuisku, P., Kärki, A., Lahaye, Y., Majka, Vaasjoki, M., Äikäs, O. & Rehtijärvi, P. 1980. The age of J., Huhma, H. & Whitehouse, M. J. 2016. Zircon Mid-Proterozoic phosphatic metasediments in Fin- and monazite geochronology of deformation in the land as indicated by radiometric U-Pb dates. Lithos Pielavesi Shear Zone, Finland: multistage evolution 13, 257–262. of the Archaean-Proterozoic boundary in the Fen- Vaasjoki, M., Huhma, H., Lahtinen, R. & Vestin, J. 2003. noscandian Shield. Journal of the Geological Society. Sources of Svecofennian granitoid rocks in light of ion Available at: http://doi.org/10.1144/jgs2016-020 probe measurements on their zircons. Precambrian Research 121, 251–262.

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