Definition of the Kisko Group for Finstrati

GEOLOGIAN TUTKIMUSKESKUS Alueellinen geotieto Espoo 30.10.2017 57/2017

Definition of the Kisko group for Finstrati

Mikko Nironen and Jouni Luukas

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 2

Sisällysluettelo Abstract

1 KISKO GROUP 4

1.1 Background 4 2 ORIJÄRVI FORMATION 7

2.1 Previous interpretations 7 2.2 Keys to stratigraphy 8 2.3 Interpretation of stratigraphy 8 2.4 Speculation of mineralisation 14 3 VETIO FORMATION 14

4 METSÄKULMA FORMATION 16

5 AHDISTO FORMATION 18

6 SALITTU FORMATION 19

7 REDEFINITION, STRATIGRAPHY AND CORRELATION OF THE KISKO GROUP20

8 REFERENCES 23

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 3

Abstract The rocks in the Orijärvi area, Uusimaa belt, have been studied for their economic potential, and first stratigraphic interpretation was made in the late 1970’s. In the 2000’s the rocks were defined to form the Kisko group, consisting of the formations Orijärvi (lowermost), Kisko, Toija, and Salittu (uppermost). In 2016 the Vetio and Ahdisto formations were added. The formations of the Kisko group were slightly reorganized and renamed for Finstrati, and the formations were divided into members. The division is based mainly on the observations by the Outokumpu Mining Company, added with a brief field trip. Division of the Orijärvi formation in to members was difficult instead of the large amount of field observations available, because of large variation of rock types and alteration which is associated with mineralization. The Kisko group consists of the formations Orijärvi, Vetio, Metsäkulma, Ahdisto, and Salittu. With the exception of the Salittu formation, the formations are restricted to the so called Orijärvi triangle area, bordered by the Kisko and Jyly shear zones. The rocks outside the triangle, including the rocks of the former Toija formation, were merged into the Häme migmatite suite. The Orijärvi formation is correlative to the Kimito suite.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 4

1 KISKO GROUP

1.1 Background

The rocks in the Orijärvi area, in the western part of the Uusimaa belt, were first studied by Eskola (1914). Because the (exhausted) Orijärvi and Aijala Cu-Zn, Metsämonttu Zn-Pb and Iilijärvi Zn-Pb-Ag deposits are located in the Orijärvi area, a vast amount of field bedrock observations by the Outokumpu Mining Company exists in the mine area around the deposits. The Orijärvi area, as understood here, covers the so-called Orijärvi triangle (Ploegsma & Westra 1990), bounded by two shear zones, and extends northeast and southwest of the triangle (Fig. 1). Interpretations of the stratigraphy in the mine area have previously been presented by Colley and Westra in 1987, by Latvalahti in 1979, and by Mäkelä (maiden name Latvalahti) in 1989. Väisänen and Mänttäri (2002) defined the stratigraphy of the Orijärvi area so that the lowermost stratigraphic unit is the Orijärvi formation, overlain by the Kisko and Toija formations, and the Salittu formation is uppermost in stratigraphy. This division was adopted in the Finstrati database of GTK, forming the Kisko group. Nironen et al. (2016) presented internal stratigraphy of the Salittu formation, and they added the Vetio and Ahdisto formations to the Kisko group. According to Skyttä et al. (2006), in the Orijärvi triangle area primary layering and subparallel S1 foliation generally trend E–W, and the rather few top of strata observations indicate younging towards north. Primary and D1 structures are deformed by F2 folding with NE to E– W trending axial plane; S2 is the main foliation in the Orijärvi triangle area. The northern part of the triangle is defined by an upright, gently NE plunging D2 synform, and the southern part by a steeply south-dipping, W plunging D2 antiform. This rather simple structure is consistent with the interpretation by Väisänen and Mänttäri (2002) of stratigraphic younging towards north, at least in the northern part of the Orijärvi triangle.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 5

Figure 1. Formations of the Kisko group as defined by Väisänen and Mänttäri (2002). The Orijärvi triangle is bounded by the Kisko and Jyly shear zones. Present interpretation of shear zones and faults is marked as violet lines.

The present report is an effort to define stratigraphy in the Orijärvi area, based on investigation of previous stratigraphic interpretations, existing outcrop data as well as a short field trip. The observations by the Outokumpu Mining Company as well as outcrop and bedrock maps at the scale 1:10 000 are available as a separate database (hereafter called Oku database) in the GTK bedrock database, and they cover almost the whole of the Orijärvi area. The field observations include information of rock types and possible layering, foliation, and linear structures. The rocks were metamorphosed under amphibolite facies conditions but in the following text the prefix ‘meta’ has been left away.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 6

Figure 2. Locality names and dating sites.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 7

2 ORIJÄRVI FORMATION

2.1 Previous interpretations

According to Colley and Westra (1987), lowermost in stratigraphy of the Kemiö-Orijärvi-Lohja belt are mafic volcanic rocks with frequent pillow structures and with calcareous rocks associated with the pillow lavas. These grade upward into polymict volcanic breccia. Some breccias include felsic clasts in mafic matrix, and banded amphibolites contain alternating mafic and felsic layers, both suggesting contemporaneous felsic and mafic volcanism. Further up there are turbiditic greywackes, and on top are felsic volcanic rocks, mainly tuffs and flow breccias. Syngenetic gabbro-tonalite intrusions and thin gabbroic sheets have intruded into all these units. Latvalahti (1979) and Mäkelä (1989) offered a different interpretation to the southern part of the Orijärvi area (‘Aijala–Orijärvi area’): lowermost is the Orijärvi granodiorite, overlain by picritic volcanic rocks (in the stratigraphy of Väisänen and Mänttäri, 2002, these are part of the Toija formation). The stratigraphy may be divided into lowermost felsic unit, mafic unit, and uppermost sedimentary unit. According to Latvalahti (1979), lowermost in the felsic unit are the mainly pyroclastic rocks (agglomerates and tuffs) with interlayers of mafic- intermediate volcanic rocks, limestone and quartz-banded iron formation. Mäkelä (1989, p. 30-32) set an acid volcaniclastic rock lowermost, beneath the ore deposits. The upper part of the felsic unit starts with thin beds of limestone and chert-banded iron formation; these are overlain by tuffs and epiclastic tuffites rich in biotite, and on top there is a skarn-bearing volcanic conglomerate with clasts of chert and fine-grained felsic volcanic rock. The mafic unit consists of lava rocks in the base (including pillow lava), followed by agglomerates and tuffs; the ejecta are angular and andesitic or dacitic in composition. Eastward the amount of agglomerates decreases and is replaced by tuffites with diopside skarn interlayers. The sedimentary unit consists of lowermost mica schist with cordierite porphyroblasts, overlain by greywacke schist with primary sedimentary structures, phyllite with iron sulfides, a volcanic conglomerate bed, and on top arkosite. According to Latvalahti (1979), at Orijärvi D1 deformation produced isoclinal F1 folding with axes plunging gently east, and D2 resulted in isoclinal F2 folding with subvertical axes. Lineation is mostly associated with F2 folding, and F2 is more intense at Aijala, 10 km WSW of Orijärvi, than at Orijärvi (Fig.2). Mäkelä (1989) discerned three deformation phases, with Da occurring only as schistosity in the fragments of volcanic conglomerates (at Iilijärvi); Fb and Fc correspond to F1 and F2, respectively. At Iilijärvi, 1 km west of the Orijärvi mine, S1/Sb axial plane schistosity parallels the E–W trending layering whereas F2/Fc axial plane schistosity crosscuts the layering at 10–20° angle and fold axis and related lineation plunge 45–70° east.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 8

2.2 Keys to stratigraphy

In addition to 1:10 000 scale bedrock maps, Ulla Latvalahti made in 1977 a map at the scale 1:20 000 (r220_76_77.tif in the Oku database), covering most of the Orijärvi formation. The stratigraphic maps in Latvalahti (1979) and Mäkelä (1989) are generalizations of this map. Latvalahti (1979) interpreted that the rock sequence is a stratigraphic package that has been turned northward into a subvertical position. The interpretations of Väisänen and Mänttäri (2002) and Skyttä et al. (2006) are essentially similar, with younging of rocks towards north. In spite of the large amount of field observation data, evidence for stratigraphy in the Orijärvi formation are few: there are not many top of strata observations, and rock type variation in field observations is large and difficult to correlate. Despite few observations of younging directions the evolution of volcanic and sedimentary strata can give a hint of stratigraphic order. The volcanic evolution and mineralization processes resembles the bimodal Pyhäsalmi-Vihanti system in Northern Ostrobothnia (Mäki et al. 2015). At both localities the volcanic sequence starts with bimodal volcanic stage with contemporaneous subvolcanic intrusions. Later the intensity of volcanism diminish and silisiclastic sediments become more abundant and cover the underlying volcanic strata.

2.3 Interpretation of stratigraphy

The stratigraphic interpretation starts with the assumption that the D1 structures developed as the result of subhorizontal movements that generated weak S1 schistosity (sub)parallel to bedding. The gently plunging F1 fold axes (Latvalahti 1979) suggest thrusting and associated recumbent folding (cf. Skyttä et al. 2005), but repetition of strata was probably only local and stratigraphic younging remained essentially upward. The oldest rocks in the Orijärvi triangle, i.e. the Orijärvi granodiorite (Fig. 2; A0933, 1898 ± 9 Ma, Väisänen et al. 2002) and the rhyolite at Multsilta (A1581, 1895 ± 2 Ma, Väisänen & Mänttäri 2002), are located in the area defined as a D2 antiform by Skyttä et al. (2006). The felsic, intermediate and mafic metavolcanic rocks, with calcareous interlayers, are the oldest rocks, exposed in the erosion level in the core of the D2 antiform, or possibly in the core of a D2-D5 interference dome (Nironen et al. 2016). A brief field study was made during the summer 2017 around the dated rhyolite (A1581) at Sorronniemi (Fig. 2). The mafic rock is fine-grained and generally massive. Clasts, occurring in places in the mafic rock, indicate that it is a volcanic rather than dyke rock. The intermediate rock contains tiny (2-3 mm), abundant subhedral plagioclase phenocrysts in fine-grained groundmass. In outcrop it is difficult to decide whether the rock is volcanic or subvolcanic but the occurrence of this rock in a rather large area suggests volcanic origin. The only contact was found between the mafic volcanic rock and the rhyolite at the site from

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 9

which the sample for dating the rhyolite was taken. Considering the rather steeply dipping foliations around the site of the rhyolite, the rhyolite is interpreted to be cut by a D1 thrust, whereas the main structure is D2 antiform (Fig. 3). The E-W trending fault was delineated to explain the abrupt ending of the rhyolite towards north; the fault may be a D2 slip plane. The rock distribution and structural interpretation suggest that the felsic volcanic rock (rhyolite) is lowermost in stratigraphy and contains interlayers of mafic volcanic rock, carbonate rock (marble) and iron formations. These rocks are overlain by the intermediate volcanic rock.

500 m

A1581

Lake Määrjärvi

3 602 013'

Mafic metavolcanic rock S1 Fault Intermediate metavolcanic rock D1 thrust Felsic metavolcanic rock S2 Carbonate rock F2 fold axis, lineation Gabbro D2 antiform Volcanic clasts F5 axial trace

Figure 3. Structural interpretation of rocks at Sorronniemi, based on the Outokumpu database and field observations (M. Nironen, upper part of the figure). We consider that the felsic and mafic volcanic rocks at Sorronniemi are part of the oldest unit within the Orijärvi formation, here named as the Multsilta member (Fig. 4). The felsic volcanic rocks in the southern part of Lake Määrjärvi, containing iron formations as interlayers (Mäkelä 1989), and the southernmost felsic volcanic rocks at Orjanperä, are tentatively considered to form the eastern extension of the Multsilta member.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 10

Mäkelä (1989) described ghost-like inclusions of amphibolite, as well as ‘inclusions of the rock types found in its surroundings’ in the Orijärvi granodiorite. Moreover, the northern and northwestern contacts of the granodiorite (granodiorite-granite) are gradational (ibid.). These observations are in line with an interpretation that the oldest rocks in the Orijärvi area are volcanic rocks, and that the granodiorite intruded these rocks early, at around 1.90 Ga. The oldest volcanic rocks and the Orijärvi granodiorite are in the core of a D2 antiform (cf. Skyttä et al. 2006), folded openly during D5 to the present form.

Fig. 4. Stratigraphic members at Orijärvi.

The large ‘Orijärvi granitoid’ has previously been considered to consist of a differentiation series from peridotite to granite, and to form a diapirically uplifted anticline batholith (Eskola 1914, Mäkelä 1989). We interpret that the 1.90 Ga granodiorite is restricted to the area between the lakes Orijärvi and Määrjärvi. The northwestern part of the batholith is granitic (Fig. 4). We have named the granodiorite and granite to form the Määrjärvi subvolcanic

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 11

suite, together with another granitic intrusion. However, since only the granodiorite has been dated, we cannot be sure that the granitic phase is of the same age. Most of the ‘Orijärvi granitoid’ is a younger, very heterogeneous unit including gneiss inclusions, with compositional variety from peridotite to granite, and locally with composition and structure of a layered intrusion (Sipilä 1981). The younger unit, with an inferred age of 1.88 Ga, is here named as the Iso-Kisko batholith. A feature in favor of this interpretation is that at Liipola, a NW trending shear zone is crosscut by the northernmost gabbro of the Iso- Kisko batholith. A WNW trending fault is outlined between the gabbro of Iso-Kisko batholith and the Multsilta member. At Kuovila, 11 km southwest of Multsilta, felsic volcanic rocks are crosscut by the intrusive rocks of the Iso-Kisko batholith. A felsic tuff at Kuovila yielded an age of 1891 ± 4 Ma (Skyttä et al. 2005). Since the rock assemblage at Kuovila, with mafic volcanic rocks, iron formations and carbonate rocks (marbles) as interlayers in felsic volcanic rock, is very similar to that at Multsilta, we consider that the Kuovila rocks are part of the Multsilta member. Thus the Multsilta member is a bimodal volcanic unit of 1.90–1.89 Ga age. The next unit in stratigraphy, here named as the Aijala member, is very heterogeneous with respect to rock types. The Oku database includes mica gneiss, amphibolite, mafic volcanic rock, felsic volcanic rock, and various kinds of skarn, carbonate rocks, and iron formation. The most common rock type is mica gneiss whereas in the map by Latvalahti (1979, Fig. 1) the main rock type is acidic volcanic rock. In the maps of Mäkelä (1989, Figs. 1 and 14) the mica gneiss is marked as (andalusite-) cordierite-muscovite gneiss. In the description by Mäkelä the cordierite-bearing mica gneiss contains cordierite and locally also garnet (almandine) as porphyroblasts in a distinctly schistose rock with ghost-like relics of bedding. Based on this description the rock is probably redeposited and may contain weathered material. We include the intermediate volcanic rock at Multsilta, and a volcaniclastic conglomerate (intermediate agglomerate in Oku database) at Kuovila into the Aijala member; both are interpreted to lay conformably upon rocks of the Multsilta member. The Aijala member decreases in thickness to the northeast and is crosscut by the Jyly shear zone. In the west it is deformed by the Kisko shear zone (Fig. 1). At Aijala the southeastern (lower) part of the Aijala member consists of rocks that are named in the Oku database mainly as mica gneiss, and in the northwestern part the rocks are felsic volcanic rocks. The internal stratigraphy of the Aijala member remained unresolved. In the cross-section from Iilijärvi (Mäkelä 1989, Fig. 15) there is a variety of rock types in the area defined as cordierite-muscovite gneiss. At Orjanperä there is much variation in rock types as well, especially carbonate and skarn beds occur at several levels. This may result from D1 thrusting or folding but I could not find out proper repetition of lithology. The Orijärvi and Iilijärvi ore deposits and associated altered rocks (cordierite antophyllite rocks and cordierite-muscovite gneisses) are located within the Aijala member. According to Mäkelä (1989), the cordierite-muscovite gneisses were originally felsic volcanic rocks, the

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 12

cordierite-mica gneisses were felsic-intermediate volcanic rocks, and the cordierite- antophyllite rocks were mafic volcanic rocks. On the other hand, Smith et al. (1992) concluded that the cordierite-antophyllite rocks enclosing the Orijärvi deposit were felsic- intermediate volcanic rocks and the cummingtonite-plagioclase rocks were mafic- intermediate volcanic rocks prior to alteration and subsequent metamorphism. Between the Orijärvi and Iilijärvi deposits the layering is tightly folded, with foliation in the axial plane. This structure is interpreted as D2 synform but generally the only foliation is subvertical, ESE-WNW striking S2. To the east of the Orijärvi deposit there is a N-S trending fault, named as Orijärvi fault by Mäkelä (1989), which appears to include vertical and horizontal (dextral) components. In addition, we interpreted a NW-SE trending fault (with the name Orijärvi mine fault) to define the contact between the Orijärvi granodiorite and the supracrustal rocks, to restrict the occurrence of the altered rocks around the Orijärvi deposit and to continue to the Iilijärvi deposit. Interpretation of the fault is based on aeromagnetic map and bedrock distribution. Mäkelä (1989) described a felsic volcanic rock at Iilijärvi as fine-grained, equigranular or plagioclase-phyric metatuff with lapilli and agglomerate interlayers; the andalusite-cordierite- mucovite gneiss grades into the felsic volcanic rock with the amount of volcanic layers increasing towards north. At Aijala, the felsic volcanic rocks are equigranular or quartz-eyed rock. In the Oku database the rocks are felsic lava or tuff. The thickness of the felsic rocks decrease towards east and may be folded isoclinally. Thus the felsic layer widens to the southwest and probably has continuations further to the west. A quartz porphyry at Iilijärvi was dated at 1869 ± 4 Ma (Fig. 2; A0936, unpublished GTK database). It is unclear whether the dated rock is volcanic or subvolcanic because there are (altered) felsic dyke rocks at Iilijärvi. At Aijala there is a rather homogeneous area of intermediate volcanic rocks northwest of the felsic rocks of the Aijala member. Here the unit is named as the Pyykylä member, and interpreted to overlie the Aijala member. In addition, there are separate areas within rocks of the Aijala member, consisting of mafic-intermediate plagioclase-phyric tuff with tuffite interlayers (Mäkelä 1989). These are here considered to belong to the Pyykylä member, deformed during folding into small areas in the present erosional level. Since fold axes in this area plunge moderately ENE, the folding may be F2. At Iilijärvi, to the north of the felsic volcanic rocks there are thin (tens of meters) layers of carbonate rock and skarn iron formations. Mäkelä (1989) divided these into stratigraphically lower skarn iron formations and carbonate rocks (in felsic volcanic rock), and upper quartz- banded iron formations (in arkosite). Between and above these layers the rock is mica schist, mica gneiss, and felsic to intermediate tuff in the Oku database. The rock is probably volcaniclastic and at least partially redeposited. Here the unit is named as the Hyypiämäki member. Mäkelä (1989, p. 18-19) described these rocks as volcaniclastic–epiclastic metasediments: at the bottom sandy, volcanic-derived layers display graded bedding with top

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 13

to the north. At the top of the Hyypiämäki member there is a 2-5 m thick layer of quartz- banded iron formation, and a volcanic conglomerate with clasts of felsic volcanic rock, chert and occasionally mafic volcanic rock. In the Oku database the rocks in the upper part of the Hyypiämäki member are greywacke and conglomerate. The Hyypiämäki member is crosscut in the west by the Kisko shear zone. We defined the continuation of the Hyypiämäki member across the shear shear zone (at Liipola) by a discontinuous layer of carbonate rock, overlain dominantly by felsic volcanic rocks with interlayers of iron formation. At Aijala, these rocks are to the northwest of the intermediate rocks of the Pyykylä member (Fig. 4). We tentatively consider that the Hyypiänmäki member overlies the Pyykylä member. At Orijärvi to the north of the Hyypiämäki member there is mafic volcanic rock as a rather thick unit (300-400 m in the west, 200 m in the east). In the Oku database the rock is generally named as mafic lava. According to Mäkelä (1989, Fig. 14) there is agglomerate at the southern margin of the unit. At Iilijärvi most of the fragments in the agglomerate are felsic. There are intermediate volcanic rocks to the north of the mafic rock. This mafic-intermediate unit is here named as the Lintunen member. If the stratigraphic younging is towards north as interpreted by Skyttä et al. (2006), the Lintunen member is the uppermost of the members in the Orijärvi formation. In the east this member thins down and is crosscut by the Jyly shear zone; no extension of the member can be found west – southwest of the Kisko shear zone. In addition to supracrustal rocks, there are mafic dyke rocks that are generally conformal with layering, i.e. sills. One such dyke rock is in the western part of the Orijärvi formation (Latvalahti 1979). Since the dyke may be important for mineralization (see section 2.4), we consider it as a separate member, here named as the Puurlammi member. The other dyke rocks have not been described; in the Oku database the rock is generally amphibolite. In Finstrati these rocks are defined as ‘undefined mafic dyke member’. The stratigraphic order within the Orijärvi formation (from lowermost to uppermost) is following:

Multsilta member Felsic-intermediate-mafic volcanic rocks, carbonate rock and BIF interlayers Aijala member Mica gneiss, sericite-cordierite gneiss, sulphide ore Puurlammi member Mafic sill Pyykylä member Intermediate volcanic rock Hyypiämäki member Volcanilasitc-epilastic sedimentary rocks Lintunen member Intermediate volcanic rock undefined mafic dyke member Mafic dyke

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 14

2.4 Speculation of mineralisation

According to Latvalahti (1979), the Orijärvi deposit is stratabound: mineralization occurred after the decline of intense acid pyroclastic volcanism, and was associated with dolomitization, silicification, sericitization, and Mg-Fe metasomatism. What was the reason for alteration, was it fluid circulation as a late effect of volcanism or was there an additional heat source? Such a later heat source could be the granite south of the Orijärvi and Iilijärvi deposits. Dating was made of the granodiorite, and thus the granite may be a subvolcanic rock not directly related to the Orijärvi granodiorite. The mafic sill of the Puurlammi member may have been an overlying cap rock during mineralization, enabling thorough circulation of fluids. Timing of mineralization is unclear but we assume that the dyke intruded before development of the rocks of the Hyypiämäki member. The Aijala and Metsämonttu deposits are located at the boundary of the Aijala and Pyykylä members (see Mäkelä 1989, Fig. 5) Both are hosted by altered rocks. According to the Oku database, steeply NNW or SSE plunging lineations are characteristic of the area around the Aijala and Metsämonttu deposits, and the Aijala deposit is at one of the shear zones of the group collectively called as the Kisko shear zone. These features suggest that although the two deposits were possibly stratabound like the Orijärvi deposit, they were at least strongly modified by shearing. Further northwest, at Liipola, the lineations in the Kisko shear zone plunge moderately ESE, indicating that development of the shear zone was complex (see also Väisänen & Skyttä 2007). The Aijala member, including a very heterogeneous lithology and the mineralizations of the Orijärvi formation, is much different from the overlying Hyypiänmäki member. According to Latvalahti (1979, p. 1038), deposition of tuffites and chemical sediments (of the Hyypiänmäki member) reflect a long and peaceful evolution stage between eruptive volcanic stages. Following the reasoning of Latvalahti (1979), the Hyypiämäki member may represent a small, local basin with fill from surrounding volcanic rocks. This rouses the question whether the Multsilta and Aijala members should constitute a separate formation, different from the overlying sequences.

3 VETIO FORMATION

Vetio formation is a new name (Nironen et al. 2016), given to a sedimentary-dominated unit which according to Latvalahti (1979) represents the uppermost, weathering sedimentary unit in the Aijala–Orijärvi area. At Aijala the rocks are mica gneisses with local iron formations as interlayers. At Orijärvi the lowermost rocks are layered mica gneisses with cordierite porphyroblasts, overlain by greywacke schist with primary sedimentary structures (graded

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 15

bedding, current bedding, slumping), phyllite with iron sulfides, a thin bed of volcanic conglomerate with clasts of felsic volcanic rock, and on top arkosite. In the detrital zircon study of Claesson et al. (1993) one greywacke sample was from Vetio (A934 Orijärvi). Of the 16 studied zircons 14 were 1.93-2.12 Ga in age, and two were Neoarchean. As all these ages are considerably older than the Svecofennian rocks at Orijärvi – and in southern Finland – the greywacke probably represents a wide basin, filled with detritus from a long distance. This was the main reason to consider the weathering sedimentary unit as separate formation. Most of the Vetio formation consists of mica gneiss with amphibolite units, named here as Haukialammi member. The origin of the amphibolite units is unclear: they may be volcanic interlayers, or sills that intruded during emplacement of mafic rocks of the overlying Metsäkulma formation (see below). In Finstrati these rocks are grouped under ‘undefined mafic dyke member’. Mica gneisses occur as a separate unit north of the Kisko village. The area is here interpreted as part of the Haukialammi member, exposed in the core of a D2 antiform. The mica gneisses northwest of Aijala, crosscut by the Perniö granite batholith, are considered part of the Haukialammi member. The greywacke north of the Haukialammi member is here named as Vetio member. The conglomerate bed, described by Latvalahti (1979), is uppermost in the Vetio member. The arkositic rocks to the north of the Vetio member are here named as the Sorvasto member. The Vetio formation thins towards east and is crosscut by the Jyly shear zone; no extensions have been found to the east of the shear zone. In the west the formation is deformed by the Kisko shear zone but the Haukialammi member is exposed also at Aijala (see Latvalahti 1979, Fig. 1). The occurrence of the Haukialammi member as small units within the overlying Metsäkulma formation is probably the result of E-W trending F1 and NE-SW trendind F2 fold interference. A road-cut outcrop at Vetio (23°32’42.2”E, 60°14’33.4”N), belonging to the Vetio greywacke member, has been studied in detail by Skyttä et al. (2006), Pajunen et al. (2008), and Sayab et al. (2014). According to Skyttä et al. (2006), the Vetio outcrop displays andalusite grown during D2 (and1), with S1 seen as inclusion trails. Penetrative, subvertical foliation (S2) deforms and wraps around and1. According to Pajunen et al. (2008) there was a cooling stage between D1 and D2. Sayab et al. (2014) interpreted the foliation that wraps around and1 as S1, and therefore concluded that the first structures at Orijärvi developed as the result of horizontal shortening and vertical stretching.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 16

The internal stratigraphy of the Vetio formation is following:

Haukialammi member mica gneiss (biotite paragneiss) Vetio member greywacke Sorvasto member arkositic rock undefined mafic dyke member mafic dyke

4 METSÄKULMA FORMATION

Väisänen and Mänttäri (2002) named this formation as Kisko formation, but in order to avoid confusion between Kisko group and Kisko formation, we propose the name Metsäkulma formation. The Metsäkulma formation is restricted to the Orijärvi triangle area (Figs. 4 and 5). According to Väisänen and Mänttäri (2002), the formation consists of tuffs with composition varying from mafic to felsic. Local primary sedimentary structures (graded bedding, load casts and erosional bases) indicate a subaqueous environment. The volcanic rocks are intruded by gabbros and plagioclase porphyries that show both concordant and discordant contact relationships to the adjacent supracrustal rocks. According to the Oku database lowest (i.e. southernmost) are felsic volcanic rocks, here named as the Korpi member. The majority of the Kisko formation consists of intermediate volcanic rocks, here named as the Pyysuo member. The amount of outcrops is small in the eastern part. The intermediate volcanic rocks to the east of the Jyly shear zone are probably extensions of the Pyysuo member, although they are at present included in the Häme migmatite suite as intermediate volcanic rocks. According to the Oku database among the intermediate rocks there are mafic and felsic volcanic rocks. However, since some of the mafic bodies may be dyke rock, they are named here as ‘undefined mafic rock member’. The gabbros bodies described by Väisänen and Mänttäri (2002) roughly follow S1 foliation in the volcanic rocks, suggesting emplacement as sill-like bodies. Here they are named here as the ‘Tanner member’. Uppermost (northernmost) in the volcanic pile is a felsic rock defined by Väisänen and Mänttäri (2002) as felsic fragmental flow, and here named as the Vaarainmäki member (Fig. 6). The age of this member is 1878 ± 3 Ma (Fig. 2; A1582, Väisänen & Mänttäri 2002), which is within error limits the same as the 1869 ± 4 Ma quartz porphyry in the Aijala member of the Orijärvi formation. Either the upper part of the formation and the Vetio and Kisko formations were deposited within a few million years, or the dated Iilijärvi rock is a dyke rock that does not yield the age of the Aijala member. We prefer the second alternative and consider that

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 17

the dated dyke rock represents a conduit to felsic volcanism of the overlying Vaarainmäki member. The internal stratigraphy of the Metsäkulma formation is following:

Korpi member felsic volcanic rock Pyysuo member intermediate volcanic rock Tanner member gabbroic sill undefined mafic rock member undefined mafic volcanic rock Vaarainmäki member felsic volcanic rock

Figure 5. Stratigraphic members around Lake Ahdistonjärvi.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 18

5 AHDISTO FORMATION

Ahdisto formation is a new name (Nironen et al. 2016) to a volcanic-sedimentary formation overlying the Metsäkylä formation. The Ahdisto formation is broadest in the west, is probably deformed by a fault of the Kisko shear zone, and bends towards east until it fades away in the Jyly shear zone (Fig. 5). The lowermost unit overlying the Metsäkylä formation, named here as the Ahdistonjärvi member, is found around Lake Ahdistonjärvi (Fig. 2). The rock is layered greywacke, showing in places well-preserved bedding from which top of strata can be determined. Andalusite porphyroblasts, varying in size from 1-2 mm to 15 mm, indicate the effects of weathering although the material probably was originally volcanic. Darkish, fine-grained rocks, varying from rather homogeneous to layered, given a collective name of Kaukmäki member, overlie the Ahdistonjärvi member. The field names of the rock are psammite and semipelite, depending of the degree of layering and the amount of mica. The material of the rocks is probably volcanic because the psammitic rock does not contain porphyroblasts and the larger crysts in the semipelite is biotite. These two types occur in the eastern part of the formation and in the north (Kaukmäki). In the west there are layers containing andalusite porphyroblasts among semipelite, indicating sedimentogeneous interlayers. Overall, both the Kaukmäki and Ahdistonjärvi members consist of volcanic or volcanic-derived material, the amount of weathered material varies, and the boundary between these two members is gradual rather than sharp. The member overlying the Kaukmäki member consists mainly of fine-grained, finely layered rock devoid of porphyroblasts, interpreted as intermediate tuff. There are small bodies of mafic and felsic volcanic rock as well. The member is here named as the Karhumäki member. Two types of igneous or subvolcanic rocks crosscut the volcanic-sedimentary rocks. The older of these is here named as the Kipusti member, containing amphibole phenocrysts that give the weathered surface a pitted appearance. In places there are small (1-2 mm) amphibole phenocrysts in fine-grained groundmass, suggesting that the gabbro is subvolcanic. The other igneous rock is a medium-grained, homogeneous rock with dioritic composition. It appears to be only slightly deformed and appears to crosscut the gabbro, and may be included in the Southern Finland plutonic suite rather than in the Ahdisto formation. The internal stratigraphy of the Ahdisto formation is following:

Ahdistonjärvi member greywacke Kaukmäki member psammite, semipelite Karhumäki member intermediate tuff Kipusti member gabbro

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 19

6 SALITTU FORMATION

The Salittu formation was first described by Väisänen and Mänttäri (2002), and subsequently by Nironen et al. (2016). According to the latter, the formation consists of a mafic and an ultramafic volcanic unit. The mafic unit, here named as the Laukkallio member, consists of a basaltic rock that is generally compositionally banded, with amphibole-rich bands alternating with diopside-rich bands (Nironen 2017). Breccia structure is rather common, and pillow structure occurs locally. The Laukkallio member is overlain by a picritic volcanic unit, here named as the Laari member. It varies from homogeneous to fragmental type that probably represents autobrecciated lava. Weathered surface is brownish green, and clinopyroxene porphyroblasts that are less weathered than the groundmass cause the typical pitted surface of the rock (Nironen 2017). The northernmost part of the Orijärvi triangle is occupied by rocks of the Salittu formation but most of the formation is found northeast of the Orijärvi triangle. Extension of the formation to west of the Kisko shear zone is based on the interpretation of Väisänen and Mänttäri (2002). Few gabbroic bodies occur within ultramafic rocks of the Laari member (Nironen et al. 2016, Fig. 3). The largest one of these, at Aromäki, is synvolcanic (Nironen 2017). Thus these bodies may be considered to belong to the Salittu formation as the Aromäki member. The internal stratigraphy of the Salittu formation is following:

Laukkallio member mafic volcanic rock Laari member ultramafic volcanic rock Aromäki member gabbroic rock

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 20

7 REDEFINITION, STRATIGRAPHY AND CORRELATION OF THE KISKO GROUP

Väisänen and Mänttäri (2002) included the Orijärvi, Kisko, Toija and Salittu formations in the Kisko group. The Toija formation is located west of the Kisko shear zone (Fig. 1). According to Väisänen and Mänttäri (2002), the Toija formation is bimodal, consisting of mafic (in places pillow-structured) and felsic volcanic rocks with intercalations of marble and metapelite; a small ultramafic volcanic unit close to the contact of the Salittu formation may be dyke or lava flow. Based on Oku database, there are also volcanic rocks of intermediate composition in the area of the Toija formation. The geochemical characterisitcs of the mafic volcanic rocks in the Toija formation and Kisko formation (renamed here as Metsäkylä formation) are similar. A felsic volcanic rock of the Toija formation yielded a 1878 ± 4 Ma age (Väisänen & Kirkland 2008), the same age as obtained from the uppermost Vaarainmäki member of the Metsäkylä formation. Väisänen and Mänttäri (2002) could not define the stratigraphic order between the Toija and Kisko (Metsäkylä) formations properly. We consider that the rocks of Toija formation are continuations of the volcanic rocks of the Metsäkylä formation – and possibly also the sedimentary rocks of the Ahdisto formation – to the west across the Kisko shear zone. The rock boundaries are poorly defined because of low observation abundance, and therefore we prefer to merge the rocks into the Häme migmatite suite (Fig. 6). We included the ultramafic volcanic rock of the Toija formation in the Salittu formation. The Kisko group is here redefined to include the following formations in the Orijärvi area (from lowermost to uppermost): Orijärvi formation, Vetio formation, Metsäkulma formation, Ahdisto formation, and Salittu formation. More detailed stratigraphy of the Kisko group is shown in Figure 7. The depositional basement of the group is unknown.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 21

Figure 6. Main lithological and structural units in the Orijärvi area.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 22

u Laari mb Aromäki mb

f a Laukkallio mb

o Karhumäki mb

d f Kaukmäki mb Kipusti mb h Ultrama?c volcanic rock A Ahdistonjärvi mb Ma?c volcanic rock Vaarainmäki mb ä Synvolcanic gabbro

k Pyysuo mb Ma?c subvolcanic rock

m Tanner mb

s Intermediate volcanic rock

t e Korpi mb Felsic volcanic rock M Felsic subvolcanic rock Sorvasto mb o Vetio mb Redeposited volcanic rock

e f Haukia- Sedimentary rock V lammi mb Greywacke Lintunen mb Arkositic wacke Hyypiämäki mb Calcareous rock (marble) Pyykylä mb

m Iron formation

2 i Cu-Zn-Pb mineralzation

v Puurlammi mb r Aijala mb 1 1 = Orijärvi, Iilijärvi

j 2 = Aijala, Metsämonttu

r Orijärvi

O gr Hydrothermal alteration

Multsilta mb Orijärvi grdr

Figure 7. Stratigraphy of the Kisko group. The rocks of the Kisko group are restricted to the Orijärvi triangle area, with the exception of the rocks of the Salittu formation. As interpreted above, the volcanic rocks the Metsäkulma formation may extend east across the Jyly shear zone and west across the Kisko shear zone, but presently these extensions are included in the Häme migmatite suite. The rock association in the Orijärvi formation is much the same as in the Kimito suite that extends from the Kimito island (in the west) to south of Orijärvi, and continues northeast (Fig. 6). Both stratigraphic units are parts of the traditional “leptite zone” of southernmost Finland (see Latvalahti 1979), and although the rocks of the Kimito suite are higher in metamorphic grade than those of the Orijärvi formation, the two stratigraphic units are correlative.

GEOLOGIAN TUTKIMUSKESKUS Definition of Kisko group 23

8 REFERENCES Claesson, S., Huhma, H., Kinny, P.D., Williams, I.S., 1993. Svecofennian detrital zircon ages - implications for the Precambrian evolution of the Baltic Shield. Precambrian Research 64, 109-130. https://www.sciencedirect.com/science/article/pii/0301926893900719 Colley, H., Westra, L., 1987. The volcano-tectonic setting and mineralization of the early Proterozoic Kemiö–Orijärvi–Lohja belt, SW Finland. In: Pharaoh, T.C., Rickard, D. (eds.) Geochemistry and Mineralization of Proterozoic Volcanic Suites. Geological Society, London, Special Publication 33, 95-107. Eskola, P., 1914. On the petrology of the Orijärvi region in south-western Finland. Bulletin de la Commission géologique de Finlande 40, 279 p. http://tupa.gtk.fi/julkaisu/bulletin/bt_040.pdf Latvalahti, U., 1979. Cu-Zn-Pb ores in the Aijala-Orijärvi area, southwest Finland. Economic Geology 74, 1035-1059. Mäkelä, U., 1989. Geological and geochemical environments of Precambrian sulphide deposits in southwestern Finland. Anneles Academiæ Scientiarum Fennicæ, Series A, III. Geologica-Geographica, 151. 102 p. (PhD thesis) Mäki, T., Imana, M., Kousa, J., Luukas, J. The Vihanti – Pyhäsalmi belt. In Maier, W.D., Lahtinen, R., O Brien, (2015), Mineral Deposits of Finland. Elsevier Nironen, M., 2017. The Salittu Formation in southwestern Finland, part II: Picritic-basaltic volcanism in mature arc environment. Bulletin of the Geological Society of Finland 89, 5-19. https://doi.org/10.17741/bgsf/89.1.001 Nironen, M., Mänttäri, I., Väisänen, M., 2016. The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy. Bulletin of the Geological Society of Finland 88, 85- 103. https://doi.org/10.17741/bgsf/88.2.003 Pajunen, M., Airo, M-L., Elminen, T., Mänttäri, I., Niemelä, R., Vaarma, M., Wasenius, P., Wennerström, M. 2008. Tectonic evolution of the Svecofennian crust in southern Finland. In: Pajunen, M. (Ed.) Tectonic evolution of the Svecofennian crust in Finland – a basis for characterizing bedrock technical properties. Geological Survey of Finland, Special Paper 47, 15-160. http://tupa.gtk.fi/julkaisu/specialpaper/sp_047.pdf Ploegsma, M., Westra, L., 1990. The Early Proterozoic Orijärvi triangle (southwest Finland): a key area on the tectonic evolution of the Svecofennides. Precambrian Research 47, 51-69. https://www.sciencedirect.com/science/article/pii/030192689090030T Sipilä, P. 1981. Lounais-Suomen rautamalmeista. Unpublished M.Sci. thesis, Universtiy of Turku, 106 p. (in Finnish) Skyttä, P., Käpyaho, A., Mänttäri, I., 2005. Supracrustal rocks in the Kuovila area, Southern Finland: structural evolution, geochemical characteristics and the age of volcanism. Bulletin

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of the Geological Society of Finland 77, 129-150. http://www.geologinenseura.fi/bulletin/Volume77/Skyttaetal.pdf Skyttä, P., Väisänen, M., Mänttäri, I. 2006. Preservation of Palaeoproterozoic early Svecofennian structures in the Orijärvi area, SW Finland – Evidence for polyphase strain partitioning. Precambrian Research 150, 153-172. https://ac.els- cdn.com/S0301926806001707/1-s2.0-S0301926806001707-main.pdf?_tid=5df9fb14-bdfb- 11e7-b407-00000aab0f01&acdnat=1509427477_2001a4c47cf3e044d17f596fe628a042 Smith, M.S., Dymek, R.F., Schneiderman, J.S., 1992. Implications of trace element geochemistry for the origin of cordierite-orthoamphibolite rocks from Orijärvi, SW Finland. Journal of Geology 100, 545-559. Strand, K., Köykkä, J., Kohonen, J. (eds.) 2010. Guidelines and Procedures for Naming Precambrian Geological Units in Finland. 2010 Edition Stratigraphic Commission of Finland: Precambrian Sub-Commission. Geological Survey of Finland, Guide 55. 41 p. http://tupa.gtk.fi/julkaisu/opas/op_055.pdf Väisänen, M., Mänttäri, I., 2002. 1.90-1.88 Ga arc and back-arc basin in the Orijärvi area, SW Finland. Bulletin of the Geological Society of Finland 74, 185-214. http://www.geologinenseura.fi/bulletin/Volume74/VaisanenManttari.pdf Väisänen, M., Skyttä, P., 2007. Late Svecofennian shear zones in southwestern Finland. GFF 129, 55-64. http://www.tandfonline.com/doi/abs/10.1080/11035890701291055 Väisänen, M., Kirkland, C.L., 2008. U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: Constraints on the age of volcanism and metamorphism. Bulletin of the Geological Society of Finland 80, 73-87. http://www.geologinenseura.fi/bulletin/Volume80/Vaisanen.pdf Väisänen, M., Mänttäri, I., Hölttä, P., 2002. Svecofennian magmatic and metamorphic evolution in southwestern Finland as revealed by U-Pb zircon SIMS geochronology. Precambrian Research 116, 111-127. https://ac.els-cdn.com/S0301926802000190/1-s2.0- S0301926802000190-main.pdf?_tid=834cda8e-bdfc-11e7-a6b5- 00000aab0f6b&acdnat=1509427969_bfdc0a0be3cbc71f7ed9d22b39fa7239