Glacial Geology and Till Geochemistry in Ore Exploration in the Tervola Area, Southern Finnish Lapland
Total Page：16
File Type：pdf, Size：1020Kb
GLACIAL GEOLOGY AND TILL GEOCHEMISTRY IN ORE EXPLORATION IN THE TERVOLA AREA, SOUTHERN FINNISH LAPLAND PERTTI SARALA, VESA PEURANIEMI and RISTO AARIO SARALA. PERTTI, PEURANIEMI, VESA and AARIO, RISTO 1998. Glacial geology and till geochemistry in ore exploration in the Tervola area, southern Finnish Lapland. Bulletin of the Geological Society of Finland 70, Parts 1-2, 19-41. Glacial geology and till geochemistry were studied in an ore exploration project in the Palaeoproterozoic Peräpohja Schist Belt in the Tervola area of southern Finnish Lapland. The starting point for the exploration were numer- ous Au-Cu-rich erratics, the source rock of which was unknown. The mapping of the glacial landforms was based on aerial photo interpretation. The structure, texture, lithology and geochemistry of the till were studied using test pits. Three sieved fractions together with a heavy mineral fraction of the till were analyzed chemically. The glacial landforms of the area include cover and ground moraines, drum- lins, flute ridges and Rogen moraines. The ridges of the Rogen moraine were found to be composed of three different till beds. The surficial parts of the Rogen ridges showed very short transport distance, whilst the lower till was of more distant derivation. Gold and Cu contents in the till are above their regional background levels in every analyzed fraction in the two case study areas of Petäjävaara and Vam- mavaara. High metal contents in till are associated with basic volcanic rocks and their contact zones with quartzite in bedrock. Metal anomalies in till are usually very sharp-contrasted and dilute quickly with increasing distance from the source rock. The most useful indicator elements in till geochemistry, in ad- dition to Au, are Cu, Te and Co. The combination of careful studies of glacial geology and till geochemistry proved to be a powerful means in locating source rock of glacially transported ore boulders in this area. As a result of the explo- ration, Cu-Au mineralization was localized at Petäjävaara. Key words: mineral exploration, geochemical methods, till, stratigraphy, geo- chemistry, gold, copper, pathfinders, heavy minerals, glacial transport, erratics, Tervola, Finland Pertti Sarala, Vesa Peuraniemi and Risto Aario: Institute of Geosciences, Uni- versity of Oulu, PL 3000, 90401 OULU, Finland. E-mail: vesa.peuraniemi@ oulu.fi 20 Pertti Sarala, Vesa Peuraniemi and Risto Aario Fig. 1. The location of the study area in the Peräpohja Schist Belt. INTRODUCTION of knowledge on glacial history and till stratigra- phy in prospecting has been stressed by Campbell Ore exploration has been quite intensive in the area and Schreiner (1989), Hirvas (1989), Hirvas and of the Peräpohja Schist Belt during the last deca- Nenonen (1990), Aario and Peuraniemi (1992), des. Many small mineralizations have been disco- and McClenaghan et al. (1992). vered (e.g. Mikkola 1947, Peuraniemi 1982, Äyräs Delineation of the geochemical provinces in 1991), but only one, a Cu-Au deposit in Kivimaa Finland is possible on the basis of regional till (Rouhunkoski & Isokangas 1974) has been mined geochemical maps (Koljonen 1992, Salminen so far (Fig. 1). Gold exploration in the greenstone 1995, Salminen & Tarvainen 1995). Maps of the and schist belts in Finland has intensified prospec- regional till geochemistry of northern Finland ting also in the Peräpohja Schist Belt. (sampling density 1/30 km2) show that the study Till geochemistry is an effective method in area is a part of a SW-NE trending Cu-anomalous prospecting for metal mineralizations in glaciat- zone (Fig. 2a). Gold, instead, has no anomalous ed areas. Different sampling densities, fractions values in the study area (Fig. 2b). Both maps are and soil horizons have been used in Au explora- based on the work of the so-called Nordkalott tion e.g. by Shilts (1984), Bloom and Steel (1989), Project (B0lviken et al. 1986). A more detailed Gleeson et al. (1989), McClenaghan (1992), Ni- (sampling density 1/4 km2) regional till geochem- chol et al. (1992), Hartikainen and Nurmi (1993), istry (Koivisto 1992) shows several Cu and Au and Sibbick and Fletcher (1993). The importance anomalous samples in the study area (Fig. 3). Glacial geology and till geochemistry in ore exploration in the Tervola area, southern Finnish Lapland 21 Cu Symbol size Cumulative frequency RUSSIA SWEDEN Study area Area of regional geochemical survey 100 km Fig. 2. The distribution of a) Cu and b) Au in till in northern Finland. Sampling density is one sample per 30 km2. Number of samples is 5 400 in Cu map and 1036 in Au map. Modified from the maps of the Nordkalott Project (B0lviken et al. 1986). 22 Pertti Sarala, Vesa Peuraniemi and Risto Aario Fig. 3. The distribution of a) Cu and b) Au in till in the central part of the Peräpohja Schist Belt. Sampling density is one sample per 4 km2. Slightly modified from the map of Koivisto (1992). Numerous erratics (basic volcanic rock and/or Quaternary geology quartzite) containing pyrite, chalcopyrite and Au Quaternary geology in southern Finnish Lapland have been the starting point for this study. The aim has been studied since Okko's (1941, 1944) work has been to decipher the glacial geology of the on grain size properties and lithology of till. The area in order to shed light on the ice transport di- earliest study on till stratigraphy in the area was rections and distances. The ultimate goal has been made by Korpela (1969). He found two different to locate the source rocks of the mineralized er- ice flow phases, the younger one from the west, ratics. Detailed studies were concentrated in the and the older one from the northwest; and between Petäjävaara and Vammavaara areas (Fig. 1); field the respective tills there were sorted sediments and work was done during the years 1992-1995. organic matter suggested to belong to the Peräpoh- jola Interstadial. Hirvas et al. (1977) further stu- GEOLOGICAL SETTING died the till stratigraphy in North and Central Fin- nish Lapland and had also some observations from Bedrock geology southern Lapland. In his till stratigraphy, Hirvas The study area is a part of the Palaeoproterozoic (1990) suggested six different till units numbered Peräpohja Schist Belt (Fig. 1). The schists are I-VI from the youngest to oldest. Units I—III are bordered in the south by the Archaean Pudasjärvi of Weichselian age and the Peräpohjola Intersta- Granite Gneiss Complex and in the north by the dial was interpreted as lying between units II and Palaeoproterozoic Central Lapland Granite Comp- III. Units II and III are accordingly correlated with lex. Layered mafic intrusions occur between the the upper and lower till beds of Korpela (1969). schist belt and granite gneiss complex. According Units IV-VI are older and they have been found to Perttunen (1989, 1991), the supracrustal schist mainly in the former ice divide zone in Central belt contains three volcanic formations separated Lapland, but not in the present study area. The by quartzite-dominated sedimentary formations latest stratigraphical studies have been made by that typically consist of quartzite and dolomite the present authors on the northern side of the vil- beds. Volcanic rocks are composed of tholeiitic lage of Tervola, where the Sihtuuna moraine rid- basalts or tuffitic rocks. Quartzites are usually cut ges were described (Aario et al. 1997). by albite diabases. Other rock types are conglome- The final retreat of the ice sheet from northern rate, phyllite and mica schist. The schist belt has Finland took place at about 9 000 B.P. (Hyyppä been folded along an E-W trending fold axis. 1966, Ignatius et al. 1980). During deglaciation, Glacial geology and till geochemistry in ore exploration in the Tervola area, southern Finnish Lapland 23 26" 00' 66° 25 Ä Kuusijärvhjg&f 'pß Muurola Tervajärvi Hinasjärvi Wer,; i C Latvajarvi ,Pisajärvi Ingmjärvt 1 ((.*- - ' f !Q) Petäjäyaara Suolijärvi Petäjäskoskii Vähäjoki Yli-Ulkujärvi P<J ^Vammavaara ° hl ssauskoski Ground moraine Roads Cover moraine I O I Active ice hummocky moraine Esker Lakes and I £ I Rogen moraine | ^ | Fluting Shore deposits rivers [ I Drumlin j o^j Dead ice hummocky moraine Meltwater channel Fig. 4. Glacial geology of the study area. Youngest ice flow direction has been from west to east. the proglacial Ancylus Ice Lake left its marks on cover moraine. It drapes the bedrock topography the slopes of the highest hills in western and south- (Aario 1977). ern Lapland. The highest shore lines are now seen Rogen moraine is the most dominant landscape on the level of about 215 m a.s.l. (Eronen 1974). type. It was formed during a younger ice flow The shore deposits under the level of 100 m be- from the west. The Rogen areas consist here both long to the Litorina Sea phase. of hummocks and ridges (Fig 5), which commonly An aerial photo interpretation of the study area have a boulder-strewn surface. The dimensions of (Fig. 4) shows that till is the most common surfi- the ridges are variable: the length is 100-500 m, cial deposit occurring as various morainic land- width 50-100 m and height 2-10 m. Three areas forms and their assemblages (Aario et al. 1996, dominated by Rogen moraines have been deline- Sarala et al. 1996). Ground moraine having only ated. The first is at Petäjäskoski, where the main very gently undulating topography occurs mostly area occurs on the west side of the Kemijoki riv- in the western and southern parts of the area. The er and continues to the east as far as the thickness of till is 5-8 m in low-lying areas and Petäjävaara case study area (Fig. 6). The second 1-3 m on higher ground. In places, ground mo- area begins from Suolijärvi and continues as a raine passes laterally into active ice hummocks of narrow chain up to Perunkajärvi.