2021 Geochronology of the Paleoproterozoic Pyhäsalmi-Vihanti district, central Finland
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Geological Survey of Finland 2021 Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland 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on ca. 60 galena samples, of which ca. 20 are previously unpublished.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produced at GTK consist of more than 100 previously unpublished analyses.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: hannu.huhma@gmail.com
Jukka Kousa
Puijonkatu 24
FI-70110 Kuopio, Finland
E-mail: jukka.kousa@outlook.com
Jouni Luukas
Geological Survey of Finland
P.O. Box 1237
FI-70211 Kuopio, Finland
E-mail: jouni.luukas@gtk.fi
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–PIELAVESI 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 (Raahe– 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 Ylivieska 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 D4.
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. 6
Geological Survey of Finland, Open File Research Report 8/2021
Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland
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.
7Geological 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
206Pb/238U
0.28
1500 A751A +4.2 +200
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.
8Geological 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.
9Geological 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,
10Geological Survey of Finland, Open File Research Report 8/2021
Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland
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.
11Geological Survey of Finland, Open File Research Report 8/2021
Hannu Huhma, Jukka Kousa and Jouni Luukas
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.
12Geological Survey of Finland, Open File Research Report 8/2021
Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland
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).
13Geological 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).
14Geological Survey of Finland, Open File Research Report 8/2021
Geochronology of the Paleoproterozoic Pyhäsalmi–Vihanti district, central Finland
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
0.30
206Pb/238U
1650
A781A +4.2
1550
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
207Pb/235U
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).
15Geological 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
A1196D 4.3-4.5/abr 2 h
206Pb/238U
1600 A1196A +4.5
0.28 A1196C 4.2-4.3/+200
A68B +4.6
1500 A110 Vasankari gneiss, borax
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.
16Geological 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).
17Geological 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.
18Geological 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.
19Geological 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).
20Geological 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).
21Geological 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 Kiiminki 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 Vaala (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).
22Geological 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
1850
Concordia age
0.32
1799 ± 4 Ma A1383A +4.5 +200 a6h
1750
A1382A +4.5 a5h
206Pb/238U
0.30 A1383B +4.5 +200
1650
A1382B +4.5
0.28 A1383C 4.3-4.5 +200
1550
0.26 A1382C 4.3-4.5 +200
A1382 Mämmisuo qu dr
0.24 Intercepts at
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.
23Geological 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
207
analysis conducted A1384 is badly discordant concordant data, but the Pb/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, Keitele–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.
24Geological 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
Nd epsilon
-4
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)
25Geological 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
15.35
G12 Orijärvi 1600
1800
15.25 G16 Ritovuori
Pihtipudas
207Pb/204Pb
2000
15.15 G126 Vihanti
G25 Pyhäsalmi
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).
26Geological 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
207Pb/204Pb
15.8
Pihtipudas
Age = 1906 78 Ma
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.
27Geological 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.
28Geological 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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