Geophysical surveys for strata-bound Outokumpu-type CU-CO-ZN deposits at Kylylahti, Eastern Finland

Abstract

Economically the Outokumpu region in eastern Finland ranks among Finland's most important metallogenic provinces. Exploration for Outokumpu-type Cu-Co-Zn-deposits has been carried out in the region for more than 80 years. To date several such deposits have been discovered in a zone covering an area about 4500 kmý. Three of the deposits were in production and all of them are now mined out. The last mine, Outokumpu, was closed down in 1989. A later stage of exploration, started in 1979, led to the discovery of the Kylylahti deposit in 1984. This triggered the latest phase of exploration in 1991. The present paper describes geophysics used in this discovery and later studies of the surrounding area. The relatively small Kylylahti deposit was successfully detected although at this depth of about 500 m. The most successful methods for locating conductive deposit near surface were Mise-a-la-masse, HLEM and Gefinex 400S (Sampo), an EM frequency sounding system developed by companies in the Outokumpu Group. Even though the Kylylahti deposit is sandwiched between thick, conductive layers of black schists it was eventually intersected with some drill holes guided by the above mentioned geophysical surveys, lithogeochemistry and geological reasoning.