Metalliferous black shales - a probable source of mercury in pike in Lake Kolmisoppi, Sotkamo, Finland

LOUKOLA-RUSKEENIEMI, KIRSTI, 1990: Metalliferous black shales — a probable source of mercury in lake Kolmisoppi, Sotkamo, Finland. Bull. Geol. Soc. Finland 62, Part 2, 167—175. Black shale formations 20—400 m thick, deposited 1.96—2.10 Ga ago and rich in carbon, sulphur and metals, are encountered in eastern Finland. They commonly contain some 0.2 ppm mercury, but in certain localities concentrations are high due to hydrothermal addition; in the area of lake Kolmisoppi in Sotkamo there is a Ni-Cu-Zn occurrence in black shales, which contains 1.7 ppm mercury on average and 7.5 ppm maximum. Compared with the background mercury concentrations in pike (Esox lucius L.) in Finnish lakes, which are generally lower than 0.5 ppm, pike in lake Kolmisoppi exhibit an anomalously high concentration of 0.96 ppm. As mercury concentrations in pike in lakes in Sotkamo vary widely, and as metal-rich black shale is one of the most readily weathered rocks in Finland, the anomaly might be due to the bedrock.