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