Structural and Hydrothermal Event History of the Koli-Kaltimo-Eno Region - a Preliminary Regional Framework

Itä-Suomen yksikkö CM 60/2006/3 8.12.2006 Kuopio

M60/2006/3

Structural and hydrothermal event history of the Koli-Kaltimo-Eno region - a preliminary regional framework

Peter Sorjonen-Ward

FINU1 2006 Karelian structural studies Commercial - in - confidence, Public 1.1.2013 GTK Report CM 60/2006/3

GEOLOGICAL SURVEY OF DOCUMENTATION PAGE

FINLAND Date / Rec. no. 8.12.2006

Authors Type of report Peter Sorjonen-Ward Commercial - in - Confidence Commissioned by COGEMA/GTK Contract 2004/Tender 2006 (K129/53/2004)

Title of report Structural and hydrothermal event history of the Koli-Kaltimo-Eno region – a preliminary regional framework

Abstract A preliminary structural framework and geological event history is outlined, as a basis for considering uranium mobilization, transport and deposition in the Koli-Eno area in eastern Finland.

The study area represents a well-preserved, though tectonically modified and metamorphosed Paleoproterozoic sedimentary sequence overlying a Neoarchean basement comprising granitoids and greenstone belts, some of which appear to be relatively enriched in uranium and thorium. Accordingly, the area may be evaluated in terms of diagenetic to metamorphic mobilization of uranium, as well as primary depositional enrichment. It is necessary to establish whether the lowest formations in the stratigraphical sequence were deposited before or after redox shift in atmosphere and regolith.

Further scenarios may be considered, such as extrapolation of Riphean basins across the study area, with potential for mineralization if the present erosion level is close to that in the Mesoproterozoic. However, current isotopic and paleomagnetic data favour late Svecofennian rather than younger hydrothermal activity. A late Svecofennian event can also be reconciled with the interpreted kinematic framework, involving partitioning of deformation into strike-slip zones subparallel to the craton margin, and associated small scale thrusts and duplex systems.

Keywords Uranium, mineral system, stucture, hydrothermal, granite, paleoregolith, Archean, Svecofennian

Geographical area Finland, North Karelia

Map sheet 4223, 4224, 4232, 4241, 4242, 4313, 4331 Other information

Report serial Archive code Unpublished reports CM 60/2006/3 Total pages Language Price Confidentiality 16 English Public 1.1.2013

Unit and section Project code 401 1901006 Signature/name Signature/name

Peter Sorjonen-Ward

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Contents

1 OUTLINE OF REGIONAL STRUCTURAL FRAMEWORK AND PRINCIPAL GEOLOGICAL EVENTS 1

2 OUTLINE OF EVENTS AND PROCESSES POTENTIALLY RELEVANT FOR MOBILIZATION, TRANSPORT AND PRECIPITATION OF URANIUM IN THE KOLI-ENO AREA 10

3 REFERENCES 14

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1 OUTLINE OF REGIONAL STRUCTURAL FRAMEWORK AND PRINCI- PAL GEOLOGICAL EVENTS The purpose of this study was to provide a structural framework and outline a history of geological and hydrothermal events, to assist in defining a more detailed research and exploration pro- gram. The report therefore provides a preliminary synthesis based on regional geological and geophysical data, checked against outcrop observations in key areas, but no microscopic or ana- lytical studies have been undertaken. For documentation and descriptions of known uranium mineralization in the area, please refer to Äikäs (2000).

21°E 24°E 27°E 30°E

FIRE 1 64°N 64°N Kokkola

FIRE 3a

63°N 63°N

FIRE 3 Kuopio Joensuu

62°N 62°N

61°N 61°N

FIRE 2 100 km Helsinki 27°E 30°E

1.87 Ga ”post-kinematic” granodiorite, tonalite, Mesoproterozoic rapakivi granite quartz diorite; locally olivine and pyroxene 1.89-1.87 Ga porphyritic granodiorite and granite 1.85-1.80 Ga biotite granites, asociated predominate over mafic granitoids with migmatites and granulite metamorphism 1.89-1.87 Ga tonalite and quartz diorite 1.88 Ga pyroxene-bearing, K-feldspar phyric predominate over granite and granodiorite Granodiorite, mostly in granulite facies terrain Figure 1 Location of the Koli-Eno study area in eastern Finland, indicated by rectangle north of Joen- suu. Ochre dotted line defines western limit of exposed Archean rocks. Image superimposes Proterozoic granitoids on grayscale DEM, to indicate the absence of Svecofennian orogenic magmatism in the study area, in contrast to the Itä-Uusimaa study area (to NE of Helsinki). Note also the orthogonal trends of the craton margin and the northern boundary of the late orogenic migmatite and granite zone in southern Finland (adapted from Sorjonen-Ward, 2006).

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The Koli-Eno area is perhaps the most intensively studied area in Finland, not only in terms of uranium mineralization (Piirainen, 1968), but also with respect to lithostratigraphic and sedimen- tological (Gaál, 1964; Piirainen, 1968; Laiti, 1983, 1985; Marmo, 1992; Kohonen and Marmo, 1992; Kohonen, 1995, Pekkarinen et al., 2006). Figure 1 shows the location of the study area, together with the distribution of various groups of Paleoproterozoic granitoids. It is immediately apparent, in contrast to the Uusimaa region for example, that Paleoproterozoic granitoids are absent from this region, and have played no part in uranium mobilization and enrichment. On the other hand, late Archean crustal evolution in eastern Finland shows many parallels with Sve- cofennian orogenic events (Sorjonen-Ward and Luukkonen, 2005), including the intrusion of widespread late orogenic monzogranites with conspicuous radiometric signatures. Therefore, a primary magmatic enrichment of uranium during late Archean time is at least theoretically possible. It is also evident from Figure 2 that the Koli-Eno region is distinctive in terms of Protero- zoic geology, including quartzites and arkoses of the Herajärvi Group and the Koli mafic sill complex, which sills intruded across the unconformity between Archean basement and Paleopro- terozoic cover.

29°15´E 30°00´E Late Archean rock units Felsic volcanic and volcaniclastic rocks Turbiditic metagraywackes Mafic and ultramafic volcanics and sills Nurmes Complex supracrustal gneisses, migmatites, Koli granodiorites and late orogenic monzogranites Biotite tonalitic and hornblende/pyroxene granodioritic plutons; locally granulite facies

PaukkajaPaukkaj a 6980

400 Riutt a Kon Outokumpu Ilomantsi 1200 So t 6950 2000 Jo e n s u u 6000 4000 5200 400 Ju o 3200 Kovero

1200 Kiihtelysvaara 6920

4460 4500 4540 Paleoproterozoic rock units Proterozoic granodiorites and granites, typically with isotopic Höytiäinen Province marine shelf to deep basin evidence for derivation from underlying Archean crust sedimentary and mafic volcanic units (< 2.1 Ga) Turbidites and black schists enclosing Outokumpu assemblage Autochthonous terrigenous quartzites and (khaki), emplaced onto Karelian domain at 1.9 Ga layered mafic sills(2.4-2.2 Ga)

Figure 2. Regional geological context of the Koli-Eno region. Note in particular the 2.2 Ga Koli mafic sill complex (dark brown) and westwards dipping Herajärvi Group, between the Archean basement to the east and Kalevian (2.1-1.9 Ga) turbidites (blue). Location of the FIRE 3 seismic reflection profile is also indicated.

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Whether this spatial association has any significance with respect to uranium transport during weathering and deposition, or redox processes at mafic sill contacts, remains to be confirmed, although these concepts played a role in previous exploration targeting (Piirainen, 1968; Koho- nen and Marmo, 1992). Further work clearly needs to focus on the possible structural controls on mineralization, and in particular, dating of minerals; there is some indication of uraninite precipitation at Riutta (Ristimonttu) coincident with late Svecofennian metamorphism (Michel Cuney, pers. comm. 2006).

Ko li

Paukkaja

RiutRiutt t a

Kiihtelysvaara Huhtilampi

Värtsilä

Jänisjärvi

Figure 3. Total magnetic intensity image combined for eastern Finland and adjacent Russia (data compi- lation from joint project between GTK and Minerals Sp., purchased also by AREVA; this image processed by Jouni Lerssi, GTK Kuopio). Note the tendency for high magnetic response in many Archean granitoids (and also the Puruvesi Granite in the southwestern part of the image). For explanation see text.

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The total magnetic intensity images in Figures 3 and 4 clearly reveal the prominent positive anomalies associated with the Koli sill complex, and analogous intrusions along and immediately east of the national border (Koidanvaara in Figure 4). Intercumulus titanomagnetite is the primary mineral phase responsible for the magnetic signature (Vuollo, 1994; Vuollo and Huhma, 2005). Note however, that these distinctive intrusions are not present south of Riutta. Instead, between Kiihtelysvaara and Värtsilä (and further across the border to Jänisjärvi), the Archean-

Koidanvaara Havukka

Paukkaja

Riutta

Mustavaara

mv Huhtilampi

Värtsilä

Jänisjärvi

Figure 4. Total magnetic intensity image combined for eastern Finland and adjacent Russia (data compi- lation from joint project between GTK and Minerals Sp., purchased also by AREVA; this image processed by Jouni Lerssi, GTK Kuopio). Red dotted line indicates gradual change in magnetic intensity of Archean basement, across transition from potassic monzogranites and migmatites eastwards into more uniform tonalite, quartz diorite and granodiorite. Termination of linear magnetic anomaly at position ‘mv’ marks northern extent of mafic volcanic and carbonate sequence between Kiihtelysvaara and Värt- silä. Note linear loss of magnetic fabric in Proterozoic fracture zones, as for example at Havukka.

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Koli – Kaltimo - Eno Kiihtelysvaara - Värtsilä

Fe-tholeiite dykes 1972±5 Ma Ottola Formation Petäikkö Formation Group Group Annala Formation Viistola Formation Hyypiä Hyypiä Kalkunmäki Formation

Fe-tholeiite dykes 2115±6 Ma, 2113 ±5 Ma Koljola Formation

Puso Formation Haukilampi Formation Jero Formation Särkilampi Formation? Koli sill 2212±30 Ma

Group Koli Formation Group Koli Formation Herajärvi Herajärvi Vesivaara Formation

Pölkkylampi Formation? Urkkavaara Formation Hokkalampi Paleoregolith Group Group Kyykkä Kyykkä Ilvesvaara Formation

Kutsu monzogranite 2638±18 Ma Koitere sanukitoid 2723±22 Ma Huhtilampi monzogranite 2685±22 Ma

Figure 5. Lithostratigraphic scheme for correlating rock units in the Koli - Eno area (based on Kohonen and Marmo, 1992, Vuollo et al., 1992; Vuollo, 1994 and Vuollo and Huhma, 2005) with the formations defined in the Kiihtelysvaara area by Pekkarinen and Lukkarinen (1991). The Hyypiä Group has lithological and probably stratigraphic affinities with rock units assigned to the Ludikovian in the Ladoga area. Uranium mineralizations in the Koli-Eno study area tend to occur within the Vesivaara (Paukka- janvaara), Koli (Hermanninmonttu), and Jero (Ipatti) Formations. The Archean basement granitoid ages are unpublished SHRIMP results (Sorjonen-Ward, Hölttä and Matukov, in prep.). Proterozoic boundary coincides with a narrow intense anomaly, generated by a lithologically diverse sequence including carbonate facies, graphitic schists and mafic volcanics (Pekkarinen and Lukkarinen, 1991). The lithostratigraphic scheme in Figure 5 reinforces this distinction between the Koli-Eno and Kiihtelysvaara-Värtsilä areas. The simplest explanation is that the lower units in the Koli-Eno area, namely the Kyykkä Group, the Hokkalampi Paleoregolith and the Koli and Jero Formations of the Herajärvi Group (Kohonen and Marmo, 1992) are absent, or considerably thinner in the Kiihtelysvaara-Värtsilä section. Conversely, there is a relatively thin but lithologically diverse sequence in the latter area, defined as the Hyypiä Group (Pekkarinen, 1979; Pek- karinen and Lukkarinen, 1991), which is not represented at all in the Koli-Eno region. The key features for correlation are the ages of tholeiitic dykes swarms dated around 2.11 Ga (Pekkarinen and Lukkarinen, 1991, Vuollo et al., 1992; Vuollo and Huhma, 2005). At Kiihtelysvaara, there is evidence that these dykes were feeder conduits for mafic lavas in the Hyypiä Group (Pekkarinen, 1979, Pekkarinen and Lukkarinen, 1991), further constraining the upper age

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GTK Report CM 60/2006/3 6 Commercial - in - confidence, Public 1.1.2013 limit for deposition of the Herajärvi Group – assuming the correlation of the Puso Formation in the Koli area with the Haukilampi formation at Kiihtelysvaara is valid (cf. Kohonen and Marmo, 1992). Otherwise, the ages obtained for the Koli sill complex (Vuollo and Huhma, 2005) indicate that the Jero Formation arkoses were deposited around 2.2 Ga. Vuollo et al. (1992) established that there are actually at least two distinct tholeiitic dykes swarms intruding the Koli-Eno area. The younger group of dykes has been dated at 1972±5 Ma at Paukkajanvaara, but the two dyke generations are generally difficult to distinguish from one another, both chemically and petrophysically. There are indications that the two groups may have somewhat different trends, which may reflect different far-field stress patterns. Interpreta- tions from the Kuhmo district, more than a hundred kilometers north of the study area (Vuollo and Huhma, 2005) and from the area between Eno and the Russian border (Sorjonen-Ward, 1993, 2006) are both consistent with the earlier 2.1 Ga swarm tending to have ESE-WSW trends and the younger 1.95 Ga dykes being more NW-SE in orientation (Figure 6).

SilvevaaraSilvevaara pluton (2757 ±4 Ma) åNdGranodiorite (2750) = -0.4 to –2.1 Koli sill Huhus

Kuittila Tonalite (2746 ±9 Ma) åNd (2750) = +0.4±2.3 Ar Hattu

Porttilammit FZ

Sonkaja Möhkö Tonalite Hömötti FZ

6100 Havukkakallio array 5500 5865 Pr Sarvinki FZ

Otravaara Kuuksenvaara array Ylinen 4950 4150 Marjovaara array 4700 3660 3890 4460 Pogosta pluton (2724 ±5 Ma) åHf (2720) = +0.2±0.9 10 km Kutsu Granite

Figure 6. Interpretation of Proterozoic dykes and fracture zones in the Archean basement to the southeast of the Koli-Eno study area, from Sorjonen-Ward (2006). Arrays are defined on orientation, cross- cutting relationships not generally demonstrated. ESE-trending Kuuksenvaara array is marked in black and is inferred as correlating with 2.1 Ga event. Marjovaara array trending E-W may also be related. NW-trending Havukkakallio array is correlated with 1.97 Ga event. Archean greenstone belts, generally magnetically anomalous (Ylinen, Otravaara, Sonkaja, Huhus, Hattu) also labelled. Continuity of linear magnetic anomalies indicates that strike-slip displacements along Proterozoic fracture zones nowhere attain or exceed a kilometer; the Sarvinki fracture zone may also have a dipslip component disrupting the margin of the Otravaara greenstone belt. Sarvinki, Hömötti and Portilammit fracture zones (white dotted lines) are attributed to late Svecofennian brittle-ductile deformation rather than rift-phase extension.

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The magnetic patterning of the Archean basement in Figures 3 and 4 can be used to define three domains, from west to east. Adjacent to the Proterozoic unconformity, is an arcuate zone extending from Värtsilä towards Riutta, and then trending more northeasterly, characterized by generally positive magnetic anomalies. A more subdued magnetic pattern to the east corresponds to the Hattu schist belt and a suite of non-magnetic tonalite and granodiorite and quartz diorite plutons, while further east again, in adjacent Russian Karelia, there are more prominent magnetic anomalies at least some of which correlate with leucocratic granites. The western zone adjacent to the Archean-Proterozoic boundary is of most interest, with respect to both primary composition and the potential for structurally controlled hydrothermal overprinting. It is difficult to identify evidence of hydrothermal activity in basement and cover rocks associated with rifting and dyke emplacement. However, independent evidence for local fluid flow accompanying Svecofennian thermal overprinting of the Archean basement is provided by authigenic and metamorphic mineral growth (Pajunen and Poutiainen, 1999) and widespread resetting of the K-Ar system in biotite and in some cases hornblende (Kontinen et al., 1992).

Nunnanlahti deformation zone Suhmura thrust system

Sotkuma basement klippe

Jatulian sequence

Ilomantsi Archean domain Höytiäinen phase Kalevian sediments Listric half-graben reactivation Tohmajärvi antiformal stack during inversion and thrusting?

Figure 7. FIRE 3 seismic profile interpretation from Sorjonen-Ward (2006). Note bright non-reflective zone attributed to layer of monzogranites, consistent with surface geology immediately east of Protero- zoic unconformity. Schematic relationship of tectonically juxtaposed rocks units with W-dipping thrust surfaces shown in lower diagram.

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The magnetically anomalous Archean area extending in a NNE direction between Paukkaja and Havukka (Figure 4) is characterized by porphyritic granitoids with sanukitoid affinity (Halla, 2005) and where magnetic they typically contain abundant magnetite and pyroxene, and could be classified as enderbitic. Loss of magnetic character, particularly along NW-trending fracture zones, is reflected mineralogically by hornblende and biotite assemblages, accompanying magnetite destruction, which may be of both late Archean and Svecofennian origin. Microstructural and paleomagnetic studies (Korhonen et al., 2006) provide further evidence for a structurally fo- cused late to post-orogenic Svecofennian hydrothermal event throughout the Archean basement, with localized strongly magnetic domains apparently retaining relict Archean remanence, dis- rupted by Proterozoic fracture zones. Although this scenario differs from conventional models for fluid-rock interaction at buried basin-basement unconformities, it may be of exploration significance. At least preliminary observations indicate that such fracture zones have been the locus of oxidizing fluid flow, with hematite-calcite-chlorite-quartz assemblages. The Svecofennian timing is also significant with respect to conceptual models based on extrapolation of the Riphean basin and hydrothermal systems into the Koli-Eno area (which otherwise seems plausible). Figure 7 illustrates the potential regional fault architecture across the Proterozoic – Archean boundary. The Archean basement granitoids near the Proterozoic unconformity between Paukkajanvaara and Värtsilä have a somewhat different magnetic patterning in Figures 3 and 4, which is probably a consequence of Proterozoic dyke emplacement and subsequent faulting and fluid-rock reaction, as well as primary compositional differences. This region is characterized by medium-

Figure 8. U channel in radiometric data on left and total magnetic intensity on right, data processed by Meri-Liisa Airo, GTK Espoo. Note the antithetic correlation, the anomalous uranium associated with monzogranites in proximity to the Proterozoic unconformity.

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GTK Report CM 60/2006/3 9 Commercial - in - confidence, Public 1.1.2013 to coarse-grained monzogranites, and stromatic migmatites with potassic granite leucosomes, with ages of 2.68 Ga (Huhtilampi) and 2.63 Ga or younger (Kutsu), contrasting with ages of 2.72-2.75 Ga for tonalites and granodiorites further to the north and east. In this respect, Neoar- chean late orogenic crustal evolution in the region is reminiscent of the two-stage crustal evolution in the Svecofennian of the Uusimaa area. Therefore, the potential for primary magmatic- related enrichment of uranium and thorium in latest Archean granites should be investigated. Even if there are no significant concentrations, background enrichment in granitoids near the unconformity may be of relevance to assessing enrichment in overlying sediments, or hydrothermal remobilization during Svecofennian (or younger Riphean) events. Figure 8 indicates a broadly antithetic correlation between uranium radiometric data and magnetic data, in particular the monzogranites between Kiihtelysvaara and Värtsilä. In addition to the established age difference between the strongly magnetic enderbitic tonalites and granodiorites

Havukka

Paukkaja

Ri ut t a

Mustavaara

Figure 9. Uranium channel from airborne radiometric data (yellow and orange colors show higher val- ues), superimposed upon obliquely illuminated total magnetic intensity. Isolated anomaly at Havukka coincides with local occurrence of structurally late reddish monzogranite in proximity to hematite-bearing Proterozoic fault zone.

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to the northeast of Eno and the monzogranites which dominate further south, it is interesting to note that Halla (2005) found the Pb isotope data from the enderbitic rocks to be very depleted in uranium, relating to an ancient extraction event. Is it therefore possible that the monzogranites of the Kiihtelysvaara – Värtsilä area represent complementary magmas resulting from this deeper crustal depletion and melting event? Figure 9 further indicates the spatial association evident between the uranium channel in airborne survey data and structural fabric revealed in total magnetic intensity data, which clearly reflects bedrock fracture systems.

2 OUTLINE OF EVENTS AND PROCESSES POTENTIALLY RELEVANT FOR MOBILIZATION, TRANSPORT AND PRECIPITATION OF URANIUM IN THE KOLI-ENO AREA On the basis of structural framework and geological event history outlined above, we may make the following preliminary summary of potential events and processes that might have promoted uranium mobilization, transport and deposition in the Koli-Eno area in eastern Finland. The study area represents a well-preserved, though tectonically modified and metamorphosed Paleoproterozoic sedimentary sequence overlying a Neoarchean basement comprising granitoids and greenstone belts. Accordingly, the area may be evaluated in terms of diagenetic to metamorphic mobilization of uranium, as well as primary depositional enrichment. The following features are relevant to defining potential primary sources and sedimentary enrichment of uranium: Neoarchean late orogenic monzogranite intrusions occur directly beneath the Paleopro- terozoic unconformity and commonly coincide with regional and outcrop scale radiometric anomalies An extensive paleoregolith, characterized by a high Al/Fe ratio, was developed on Ar- chean basement and the earliest Paleoproterozoic sediments, which include the glacio- genic deposits of the Kyykkä Group. There are no direct age constraints on the duration of this weathering event, but tentative correlation suggests that these events would post- date the 2.44 Ga layered intrusive complexes in northern Fennoscandia. Timing with respect to the transition to an oxygenated atmosphere is therefore equivocal and the potential mobility of uranium in the regolith uncertain Sediments of the fluvial to deltaic Herajärvi Group commence with a basal pebbly con- glomerate facies (Vesivaara Formation), which represents reworking of the underlying regolith and includes sporadic Th-channel anomalies attributed to detrital accumulation of monazite. Essentially stratiform uranium anomalies within the orthoquartzites of the Koli Formation show local secondary enrichment in fracture networks, but may be of detrital origin Evidence exists locally for hematite matrix in the Vesivaaara and Koli formations; this may be of relict depositional, pedogenic or diagenetic origin, although the role of redox reactions during superimposed metamorphic processes needs to be better constrained Texturally immature arkosic wackes, rich in sericite and angular feldspar detritus (Jero Formation) overlie the Koli Formation orthoquartzites. No mafic volcaniclastic deposits or lava flows are recognized, but the Jero and Koli formations were intruded by the dif- ferentiated Koli Fe-tholeiitic sills with U-Pb age constraints of 2170 ± 40 Ma and 2212 ± 30 Ma.

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Tectonic instability, with localized uplift and subsidence associated with sill emplacement may explain the apparent absence of Jero Formation equivalents to the south of the study area. Consequently, the role of local depositional patterns and fault architecture needs to be addressed further, to assess whether the spatial association with known mineralization is causal or merely coincidental The uppermost unit of the Herajärvi Group records a reversion to mature orthoquartzites, which are tentatively correlated with the quartzites overlying Archean basement further south, between Eno and Jänisjärvi. If this interpretation is correct, then the diverse Ludikovian sequences in the Ladoga region, with associated U-P enrichment, and their supposed Kalevian equivalents in Finland, are conversely absent from the Koli-Eno area Kalevian turbidites are nevertheless present in the western part of the study area, although the nature of the contact between the Herajärvi Group and the Kalevian turbidites is obscure and controversial. Until further constraints become available, it is therefore conceivable that the Herajärvi Group could have been buried beneath several kilometers of sediments between 2.1-1.9 Ga, prior to basin inversion and Svecofennian orogenic deformation. It is thus possible to speculate whether diagenetic and anchimetamorphic conditions prevailed in the underlying quartzites and basement for up to 200 million years and what effect this might have had on uranium mobility. The following thermal and tectonic events related to rifting and basin subsidence at the Karelian craton margin might also have potentially caused hydrothermal mobilization and precipitation of uranium: Fluid-rock interaction driven by mafic intrusions, depending on magma and country rock redox state and the effectiveness of primary or structurally enhanced permeability in Herajärvi Group sediments. In practice, the effect of superimposed Svecofennian regional metamorphism makes it difficult to recognize authigenic minerals and diagenetic over- growths The 2.2 Ga Koli sill, which was intruded at two discrete levels above and below the Ar- chean-Proterozoic unconformity, has a strong magnetic response, related to cumulus and intercumulus titanian magnetite, evidently crystallized from a hydrous, Fe-rich low-Al tholeiitic magma. Although the sills locally contain extensively altered patches with epidote, actinolite, hematite and quartz-calcite veins, there is at present no compelling evidence for interaction between magma and oxidizing diagenetic brines, such as the pervasive albitization associated with similar intrusions in the Kuusamo district and Lapland. Two further Fe-tholeiitic dyke swarms intruded the Archean basement and Herajärvi Group sediments, at 2113±5 Ma and 1972 ± 5 Ma respectively. Both of these ages coincide with volcanism and rifting elsewhere along the Karelian craton margin and potential subsidence, burial and thermal and structural perturbations in the Herajärvi Group. The reticulate network of subhorizontal sills and vertical dykes would have thus generated a complex hydrologic architecture, if the sediments had retained primary porosity. During the Svecofennian orogeny, the Koli-Eno study area was metamorphosed at higher tem- peratures and pressures than those normally experienced by sedimentary sequences considered prospective for unconformity-related uranium deposits; the Hokkalampi paleoregolith records kyanite replaced by prograde andalusite, while adjacent Kalevian turbidites may contain garnet- staurolite-andalusite assemblages. Biotite and locally hornblende in Archean basement granitoids also record Svecofennian thermal resetting, indicated by Paleoproterozoic K-Ar ages. Therefore,

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1. Antigorite out /Olivine + talc in 3. Anthophyllite in 2. Prograde talc + carbonate out 4. Enstatite in

Paukkaja Kuopio

4 Ri ut ta Jo e n su u

FIRE 3 2 3 1 FIRE 3A a

A B B A

B A

b c d Figure 10. Regional kinematic framework for interpreting deformation, from about 1.87 Ga (constrained by ages of granitoids associated with shear zones). Brittle-ductile deformation zones in the Koli-Eno area typically display orientations and displacement or shear sense consistent with this scenario. Note also eastwards decrease in metamorphic grade between Kuopio and Joensuu, defined by mineral assemblages in ultramafic rocks of the Outokumpu assemblage (after Säntti et al., 2006). Diagrams b-d are dynamic numerical models simulating deformation (Zhang, 2003). an alternative scenario may be considered, relating to fluid-rock interaction during incipient prograde and late orogenic retrograde conditions, for which the following observations are relevant. The Koli-Eno area represents an orogenic foreland setting with respect to NE-vergent Svecofennian thrusting and later deformation oriented subparallel to the craton margin (Figures 2 and 7). Deformation history is progressive, with a transition from thrusting through strike-slip displacement, accompanied by granite emplacement (Halden, 1982, Koistinen, 1981, Ward, 1987, Ward and Kohonen, 1989; Kohonen et al., 1991, Kohonen, 1995). Although the Herajärvi Group retains stratigraphic coherence and is not signifi- cantly folded, tectonic repetition of the basement-cover interface is demonstrable in the Paukkajanvaara area. More local effects, including strain partitioning in ductile shear zones, intense pressure solution phenomena in the basal conglomerates of the Vesivaara Formation and throughout the Jero Formation arkoses may have been important with respect to fluid production, focussing and compositional buffering.

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Basement rocks also show extensive fracturing with chlorite-sericite-quartz-carbonate alteration and vein-fill assemblages. It is unclear whether these features represent the effects of fracturing during rifting and sill emplacement as well as contractional deformation, but it is likely that strain perturbations resulted from mechanical contrasts across the basement-cover interface. Structural complexities at misoriented contacts between mafic dykes and quartzites may have provided opportunities for fluid focussing, with a recurrent structural pattern comprising N-NNW-trending steeply dipping fault and fracture zones, interacting with more gently dipping reverse sense structures (Figure 10). This geometry may control mineralization at Paukkajanvaara (Figure 11), but is less obvious, though still plausible, at Riutta.

Figure 11. Map view of Paukkajanvaara by Matti Tyni (1961), showing sinistral offset of basement-cover contact along N-S trending fault, and section view below, showing reverse sense component of fault displacement, with mineralization in Vesivaara Formation in hanging wall between fault and NW-trending dolerite dyke.

Wall-rock sulfidation is one potential mechanism for fluid-rock reaction within oxidized magnetite-bearing tholeiitic dykes, consistent with disseminated pyrite and biotite- chlorite alteration and quartz-calcite vein assemblages in late brittle-ductile shear zones. Quartz veins with massive coarse pyrite may represent destabilization during phase sepa- ration. Although gold has not been systematically explored, these vein and alteration sys-

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tems are characteristic of late orogenic shear-hosted gold. Whether the chalcopyrite associated with the numerous, though small historical copper workings within the study area. Further studies are thus needed to assess whether structurally focussed, somewhat reduc- ing orogenic fluids have had any contribution to uranium mobilization or fixation Some late fault zones – notably the Kalliojärvi-Hautajärvi-Kolvananuuro fault zones - have clearly penetrated the basement-cover contact as well as graphite- and sulfide- bearing turbidites to the west of the study area. At present however, there is no information relating to possible lateral migration of fluid along fault zones, and insufficient constraints on architecture at depth, which might permit oxidized fluids of basement origin to interact with reduced cover rock lithologies. A further set of NW-trending fracture zones has been identified - in particular the Ihanto- joki-Havukka fracture zone - associated with magnetite destruction in Archean basement rocks, and pervasive alteration and brecciation with hematite-quartz-epidote-calcite assemblages. These features are all consistent with a Riphean origin, in which case the study area could have been subjected to a further episode of hydrothermal activity associated with faulting and redbed sedimentation, related to the Ladoga-Pasha graben to the southeast and the Muhos graben to the northwest. However, provisional paleomagnetic data and some isotopic constraints favour a late Svecofennian age. Indeed, it is possible to speculate that the Koli-Eno region could have been transiently buried beneath late- to post-Svecofennian foreland basin sediments, correlative with those now preserved in the Lake Onega area. Irrespective of age, the mineral assemblages are suggestive of late gen- eration or infiltration of more oxidizing fluids within the basement.

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Koistinen, T., 1981. Structural evolution of an early Proterozoic strata-bound Cu-Co-Zn deposit, Outokumpu, Finland. Transactions of the Royal Society of Edinburgh, Earth Sciences 72, 115-158. Kontinen, A., Paavola, J. & Lukkarinen, H., 1992. K-Ar ages of hornblende and biotite from Late Archaean rocks of eastern Finland - interpretation and discussion of tectonic implications. Geological Survey of Finland Bulletin 365, 31 pp. Korhonen, F., Sorjonen-Ward, P., Mertanen, S. & Airo, M.-L., 2006. Magnetic signatures of Archean and Proterozoic magmatic and metamorphic events in the Lieksa area, eastern Finland. 27th Nordic Wintermeeting, Oulu, Finland, January 9-12, 2006, Abstract Volume, Bulletin of the Geological Society of Finland, Special Issue 1, 72. Laiti, I., 1983. Eno. Suomen geologinen kartta 1:100 000 : Kallioperäkartta lehti 4242. Geological Map of Finland 1: 100 000 Series, Pre-Quaternary Rocks, Sheet 4242 Eno, Geological Survey of Finland. Laiti, I., 1985. Joensuu. Suomen geologinen kartta 1:100 000 : Kallioperäkartta lehti 4223. Geological Map of Finland 1: 100 000 Series, Pre-Quaternary Rocks, Sheet 4223 Joensuu, Geological Survey of Finland. Marmo, J.S. 1992. The Lower Proterozoic Hokkalampi paleosol in North Karelia, eastern Finland. In: Schidlowski, M. et al. (eds.) Early organic evolution: implications for mineral and energy resources. Berlin: Springer-Verlag, 41-66. Nykänen, O., 1971. Kiihtelysvaara. Suomen geologinen kartta 1:100 000 : Kallioperäkartta lehti 4241. Geological Map of Finland 1: 100 000 Series, Pre-Quaternary Rocks, Sheet 4241 Kiihtelysvaara, Geological Survey of Finland. Pajunen, M. & Poutiainen, M.,1999. Palaeoproterozoic prograde metasomatic-metamorphic overprint zones in Archaean tonalitic gneisses, eastern Finland. In: Kähkönen, Y. & Lindqvist, K., (eds.), Studies related to the Global Geoscience Transects/SVEKA Project in Finland. Bulletin of the Geological Society of Finland, 71, 73-132 Pekkarinen, L. J., 1979. The Karelian formations and their depositional basement in the kiihtelysvaara – Värtsilä area, east Finland. Geological Survey of Finland Bulletin, 263, 141p. Pekkarinen, L. J., Kohonen, J., Vuollo, J. & Äikäs, O., 2006. Kolin kartta-alueen kallioperä. Summary: Pre-Quaternary rocks of the Koli map-sheet area. Geological map of Finland 1 : 100 000. Explanation to the maps of Pre- Quaternary rocks, sheet 4313. 116 p. Pekkarinen, L. J. & Lukkarinen, H., 1991. Paleoproterozoic volcanism in the Kiihtelysvaara- Tohmajärvi district, eastern Finland. Geological Survey of Finland Bulletin, 357, 30 p. Piirainen, T. 1968. Die Petrologie und Uranlagerstätten des Koli–Kaltimogebiets in finnischen Nordkarelien. Bulletin de la Commission géologique de Finlande, 237. 99 p. Säntti, J., Kontinen, A., Sorjonen-Ward, P., Johanson, B. & Pakkanen, L., 2006. Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites in the Paleoproterozoic Outokumpu–Jormua Ophiolite Belt, Eastern Finland. International Geological Review 48, 494-546. Sorjonen-Ward, P., 1993. An overview of structural evolution and lithic units within and intruding the Late Archean Hattu schist belt, Ilomantsi, eastern Finland. Geological Survey of Finland Special Paper 17: 9-102 Sorjonen-Ward, P., 2006. Geological and structural framework and preliminary interpretation of the FIRE 3 and FIRE 3A reflection seismic profiles, central Finland. In: I. T. Kukkonen and R. Lahtinen (editors), Finnish Reflection Experiment FIRE 2001-2005, Geological Survey of Finland, Special Paper 43. Sorjonen-Ward, P. and Luukkonen, E. J., 2005. Archean rocks. Chapter 2 in Lehtinen, M., Nurmi, P. A. & Rämö, O. T. (eds.), The Precambrian Geology of Finland – Key to the

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FinU1 2006 Karelian structures and hydrothermal event history Peter Sorjonen-Ward