GEOCHEMICAL COMPARISON OF METAMORPHOSED BLACK SHALES ASSOCIATED WITH THE VIHANTI ZINC DEPOSIT AND PROSPECTS IN WESTERN FINLAND

Geological Survey of Finland, Current Research Edited by Sini Autio. Geological Survey ofFinland, Special Paper 23,5-13, 1997. GEOCHEMICAL COMPARISON OF METAMORPHOSED BLACK SHALES ASSOCIATED WITH THE VIHANTI ZINC DEPOSIT AND PROSPECTS IN WESTERN FINLAND by Kirsti Loukola-Ruskeeniemir), Urpo Kuronen2) and Hilkka Arkimaar) r'Geological Survey of Finland, P.O. Box 96, FIN-02151 ESPOO, FINLAND 2)Outokumpu mining Australia PTY.LTD, l. Floor, Burswood Court, l4l Burswood Road, Burswood, WESTERN AUSTRALIA 6IOO e-mail : kirsti.l-r@ esf.fi Key words (GeoRef Thesaurus, geophycical surveys. Proterozoic. Introduction From the point of view of economic geology, the most interesting occurrence in the Vihanti area is the Vihanti (Lampinsaari) zinc ore deposit. A total of I 406 496 tons of zinc, 128 47 5 tons of copper, 98 489 tons of lead, 1 90 395 kg of silver, and 3 083 kg of gold were mined by the Outokumpu Com- pany during 1954-1992. The deposit was discov- ered by the Geological Survey of Finland after local citizens had sent samples of mineralized gla- cial erratics to the Survey. A magnetic and electric anomaly was encountered in the presumed source area of the boulder train and diamond drilling revealed several parallel mineralized zones. Fur- ther exploration has been carried out in the area ever since, but it has not yet proven successful. The terrain is covered by extensive swamps, and out- crops are rare. Geophysical survey, the search for anomalous boulders and till geochemistry are there- fore key methods in exploration, and based on this infbrmation, several propects have been drilled during the past decades, mainly by the Outokumpu Company. Occurrences have been found, but none of them have been of economic grade. The aim of the present research is to compare chemical and textural characteristics of graphite- bearing rocks encountered in the Vihanti zinc mine with those at the prospects; graphite-bearing rocks might constitute a useful exploration tool in poorly AGI): metasedimentary rocks, black schists, zinc ores, geochemistry, carbon, sulfur, Vihanti, Pattiioki. Finland exposed areas such as Vihanti because they can be located and even classified by geophysical meas- urements (Airo and Loukola-Ruskeeniemi 1991). The research is a joint work where the main respon- sibilities are divided as follows: K.L-R., geochemistry and textures of black schists; U.K., geology and stratigraphy of the prospects; and H.A., geo- phy si cs. Regional geology and geophysics The bedrock of the Vihanti area consists mainly ofgneisses and granitic, granodioritic and gabbroic rocks. The host rock association of the Vihanti LampinsaariZnis enveloped by mica gneisses and consists of dolomite, calc-silicate rocks, meta- volcanics, graphite-bearing schists and cordierite gneisses. Rock associations resembling that of the Lampinsaari deposit have been encountered in several prospects. Rocks in the Vihanti area are Early Proterozoic in age, and have undergone several phases of deformation. Age determinations on whole rock samples from the U- and P-rich rocks of the Lampinsaari rock association indicate that the rocks underwent amphibolite facies metamorphism about I 880 Ma ago (Rehtijärvi et al. 1979). The rocks in the Pattijoki prospect represent a more sediment - dominated environment than rocks in the Vihanti area, and black shales constitute thicker beds that are not as intensely deformed as in the Vihanti area. Ge olo gic al Surv ey of Finland , Current Rese ar ch 1995- 1996 Ediled by Sini Autio. Geo log ic al Surve y of Finland , Sp ec ial Paper 23, 5- 13,1997 . GEOCHEMICAL COMPARISON OF METAMORPHOSED BLACK SHALES ASSOCIATED WITH THE VIHANTI ZINC DEPOSIT AND PROSPECTS IN WESTERN FINLAND by Kirsti Loukola-Ruskeeniemi ' ), Urpo Kuronen 2 ) and Hilkka Arkimaa ' ) " Geologieal Survey of Finland , P.O. Box 96 , FIN-02151 ESPOO , FINLAND 2J Outokumpu mining Au s tralia PTY.L TD, I. Floor , Burswood Court, 141 Burswood Road , Burswood , WESTERN AUSTRALlA 6100 e-mail: kir sti.l-r @gs f.fi Key words ( GeoR ef Th es aurus, AGI): metasedimentar y rocks , blaek sehi sts, zine ores, g eo e hemistry , earbon , s ulfur , ge ophy e ic al s urvey s, Proterozoi e, Vihanti , Pattijoki , Finland Introduction From the point of view of economic geology, the most interesting occurrence in the Vihanti area is the Vihanti (Lampinsaari) zinc ore deposit. A total of I 406496 tons of zinc, 128475 tons of copper, 98489 tons oflead, 190395 kg of silver, and 3 083 kg of gold were mined by the Outokumpu Com- pany during 1954-1992. The deposit was discov- ered by the Geological Survey of Finland after local citizens had sent sam pIes of mineralized gla- cial erratics to the Survey . A magnetic and electric anomaly was encountered in the presumed source area of the boulder train and diamond drilling revealed several parallel mineralized zones. Fur- ther exp l oration has been carried out in the area ever si nce, but it has not yet proven successful . The terrain is covered by exten ive swamps, and out- crops are rare. Geophysical survey, the search for anomalous boulders and till geochemistry are there- fore key methods in exploration , and based on this information, several propects have been drilled during the past decades, mainly by the Outokumpu Company. Occurrences have been found , but none of them have been of economic grade. The aim of the present research is to compare chemical and textural characteristics of graphite- bearing rocks encountered in the Vihanti zinc mine with those at the prospects; graphite-bearing rocks might co nstitute a useful exploration tool in poorly exposed areas such as Vihanti because they can be located and even c1assified by geophysical meas- urements (Airo and Loukola-Ruskeeniemi 1991). The research is ajoint work where the main respon- sibilities are divided as folIows: K.L-R., geochemistry and textures of black schists; U.K ., geology and stratigraphy of the prospects; and H.A., geophysics. Regional geology and geophysics The bedrock of the Vihanti area consists mainly of gneisses and granitic, granodioritic and gabbroic rocks. The host rock association of the Vihanti Lampinsaari Zn is enveloped by mica gneisses and consists of dolomite, calc-silicate rocks, meta- volcanics, graphite-bearing schists and cordierite gneisses. Rock associations resembling that of the Lampinsaari deposit have been encountered in several prospects. Rocks in the Vihanti area are Early Proterozoic in age, and have undergone several phases of deformation. Age determinations on whole rock sampies from the U- and P-rich rocks of the Lampi nsaari rock association indicate that the rocks underwent amphibolite facies metamorphism about 1880 Ma ago (Rehtijärvi et al. 1979). The rocks in the Pattijoki prospect represent a more sediment - dominated environment than rocks in the Vihanti area, and black sha les constitute thicker beds that are not as intensely deformed as in the Vihanti area. 5