Abstract
Jatulian meta-arenites form an extensive part of the Karelian Supergroup (middle to lower Proterozoic) in the Baltic Shield. In North Karelia, eastern Finland, the middle and lower Karelian supracrustal rocks
(Jatulian and Sariolian; 2,000-2,500 Ma) trend northwesterly for 150 km along the margin of the Archean basement in the east. The thickness of this sequence varies from less than 500 m to 2,000 m.
Already in the late 1950's, near Koli in North Karelia small uranium deposits were discovered. Uranium and, in places, thorium occur in the lower Jatulian Quartzite Member as lenses along a horizon that bears spotty pigmentation of hematite. Interbeds of quartz-pebble conglomerate are typical of this horizon. Uranium is also enriched at the contacts of Jatulian diabases where they cut across the uraniferous horizon. Small uranium bodies also lie in the middle Jatulian Arkosite Member. Southeast of Koli, pitchblende veins occur in Prejatulian weathering crust; the host material is derived mainly from Archean schists and gneisses.
Ipatti, a well-exposed occurrence of uranium at Koli (71,000 tons of mineralized rock averaging 0.083 % U) is situated in the Arkosite Member 200-300 m above the hematite-stained horizon. The Ipatti arenites are predominantly pebbly and clayey subarkoses indicating poor sorting and, hence, rapid burial; the sedimentary structures suggest a fluvial-dominated
deltaic environment. Mineralization is restricted to beds of quartzose grit and small-pebbled conglomerate on top of a 5-20 m thick unit of sericite quartzite. The maximum dimensions of the uraniferous body are 6 x 50 x 300 m.
Colloform low-thorium pitchblende forms part of the cement material between the quartz clasts in the mineralized grit. Secondary uranium
minerals envelop the pitchblende mineralization as a network of fracture fillings. Magnetite occurs both as dissemination of subhedral grains and as rare rock pebbles of magnetite - quartz. Systematic lithogeochemical study across the Ipatti deposit and the lower Jatulian Quartzite Member at Koli did not reveal anomalous contents of gold.
The uranium mineralization at Ipatti is suggested to have originated from pore fluids in at least three stages of epigenetic precipitation.
Sedimentary control for the migration of the mineralizing fluids is
displayed by the host unit that is interpreted as a channel-fill structure with relatively better permeability than the wall rocks. The antecedent concentration of magnetite in the host acted as precipitant in deposition of pitchblende. The Prejatulian weathering crust and Archean granitoids are considered to be the most probable sources of uranium.
Sedimentological, mineralogical, and geochemical evidence for syngenetic conglomerate-type mineralization of uranium in the Koli area is negative; on the whole, the uranium occurrences of the area reflect more than one superimposed stages of epigenetic mineralization.
Introduction Purpose of the study
Uranium occurrences in lower Proterozoic (Karelian) quartzites and conglomerates southeast of Koli, eastern Finland, were discovered and explored by Atomienergia Oy and Outokumpu Oy in 1957-63. Outokumpu Oy
continued the work in 1967-72 by further drillings and by extending the exploration to the north and northwest. All occurrences found by the companies proved too small to be economic. The Geological Survey of Finland is presently investigating one of the first discoveries at Riutta
in Eno, about 30 SE of Koli.
Piirainen (1963, 1968) attributed the primary occurrence of uranium in the Koli area to a modified sandstone-type deposition: uranium was syndiagenetically absorbed from groundwaters by a laterally extensive horizon of previous iron hydroxide gel in the host sandstone and conglom-erate. Emplacement of diabasic magma into the sediments caused crystalli-zation of the absorbed uranium to pitchblende and the iron gel to hematite, partly even to magnetite. Subsequent circulation of oxidizing fluids
remobilized part of the uranium and, where reductive agents were available to precipitate pitchblende, produced transverse occurrences along the contacts of cross-cutting diabases, in fracture zones, and in underlying Archean sulphide deposits. Thorium showings discovered in 1970 northwest of Koli and coinciding with the uraniferous hematitic horizon in the quartzite have been interpreted to be of placer origin (Piirainen, 1979).
Uranium in Karelian metasediments at Koli and elsewhere in Finland has been ascribed to the deposits of quartz-pebble conglomerate type
(Bowie, 1970; Pretorius, 1981), largely because of the apparent similarity of the conglomeratic host rocks to, e.g., those in the Elliot Lake - Blind River area. On application of the conglomerate model, however, the evi-dently epigenetic mode of uranium mineralization in the Koli area has been found to be exceptional (IAEA, 1970, p. 368; Boyle, 1982, p. 76).
Ipatti, a well-exposed occurrence of uranium at Koli, was chosen for detailed study in connection with Project III of the IAEA Working Group on Uranium Geology. The purposes of this study were 1) to determine the controls of uranium mineralization at Ipatti and 2) to discuss the Koli area with regard to the scope of Project III: uranium deposits in
Karelian formations
Karelian formations include rocks of early Proterozoic age (1,600-2,500 Ma; Laajoki, 1983) deposited or extruded on the Archean basement in eastern and northern Finland. With corresponding rocks in Soviet Karelia (U.S.S.R.), they form the Karelian Supergroup in the eastern part of the Baltic Shield (Salop, 1983). In Finland their nomenclature and subdivision are under discussion and their sedimentology is being studied (Laajoki, 1983; and, e.g., Marmo and Ojakangas, 1984).
The rocks of the Archean basement in eastern Finland are schists, paragneisses, and orthogneisses, which date back to 2,600-3,000 Ma (Simo-nen, 1980). The orthogneisses are quartz dioritic to granitic in composi-tion while the schists consist mainly of mafic volcanogenic rocks with minor arenitic and pelitic metasediments.
The Karelian formations have been divided into lower, middle, and upper groups called Sariolian, Jatulian, and Kalevian, respectively (Fig. 1;
Meriläinen, 1980). In his recent review, however, Laajoki (1983) avoids presenting a single stratigraphie scheme for the Karelian formations.
The Sariolian Group (2,300-2,500 Ma) consists of discontinuous sedimentary and volcanogenic sequences of different compositions. The arenites and rudites have been interpreted as fluvial sediments (Pekkari-nen, 1979). Glacigenic deposits have recently been described south of Koli (Marmo and Ojakangas, 1984). The Sariolian Group may reach a thick-ness of 400 m; in North Karelia it usually is 100-150 m thick (Pekkarinen,
1979).
Deposition of the Jatulian Group (2,000-2,300 Ma) was preceded by a period of intensive chemical weathering (Meriläinen and Sokolov, 1981).
In eastern Finland this group comprises sequences of fluvial to shallow-water and marine sediments and volcanics, which are up to 1,500 m in thick-ness. The lower Jatulian sandstones may have covered an area of 400,000 km on the basement (Ojakangas, 1965).
Simplified stratigraphy
Figure 1. Schematic Karelian stratigraphy, lithology, and uranium occurrences in the Koli area, eastern Finland, modified from Piirainen (1968) and Meriläinen (1980). Locations of occurrences
1-5 are shown in Fig. 2.
The Kalevian Group (1,900-2,000 Ma) represents a sequence of meta-pelites several thousand meter thick, which in eastern Finland occupies vast areas west of the northwesterly trending Jatulian and Sariolian rocks.
The rocks of the Karelian Supergroup were subjected to deformation, regional metamorphism, and plutonism during the Svecokarelian orogeny 1,800-1,900 Ma ago (Simonen, 1980).
Location and geological setting
The village of Koli which is included to the municipality of Lieksa, is situated in North Karelia about 400 km northeast of Helsinki (Fig. 2).
The top of Koli Hill (347 m above sea level) rises 250 m above nearby Lake Pielinen.