The first nation-wide black shale map in the world: Applications for exploration and regional planning in Finland

Abstract

The map ‘Proterozoic black shale formations and aeromagnetic anomalies in Finland’ was published in the year 2000 in scale 1: 1 million. It is the first black shale map in the world of this type. Black shales rich in graphite and sulfides are encountered in the vicinity of most Proterozoic sulfide deposits in the Fennoscandian Shield, for example the Outokumpu Cu-Co- Zn deposits and the Vihanti Zn-Cu-Pb deposit in Finland and the Pechenga Ni deposits in the Kola peninsula in NW Russia. The Pechenga Ni deposits were discovered by Finnish geologists in 1920 by following the geophysical anomalies caused by the black shales. Airborne magnetic and electromagnetic data were used to assist in locating black shales, since only some 3% of the bedrock in Finland is outcropped. Black shales are good conductors, because they contain graphite and sulfides. In addition, pyrrhotite-bearing black shales cause magnetic anomalies. The geophysical properties of black shales have been studied in graphite exploration projects covering the whole country (Kukkonen, 1984) and in more detailed regional studies in eastern Finland (Airo, Loukola-Ruskeeniemi, 1991, Loukola-Ruskeeniemi et al., 1998, Loukola-Ruskeeniemi, 1999) and western Finland (Niskanen et al., 1992, Loukola-Ruskeeniemi et al., 1997). Magnetic and electromagnetic airborne low-altitude data were available in 1998 over most of the volcano-sedimentary formations in the country for map production and interpretation. Apparent resistivity was calculated from airborne electromagnetic data using the methods described in Pirttijärvi (1995).