Geochemical evidence for the hydrothermal origin of sulphur,base metals and gold in Proterozoic metamorphosed black shales,Kainuu and Outokumpu areas,Finland

Metamorphosed black shale formations 120–150 m thick with median concentrations of 7% non-carbonate carbon and 6–8% sulphur are abundantly met in drill cores in the Kainuu and Outokumpu areas. Carbon isotope ³C values are comparable to the average isotopic composition of non-carbonate carbon in sedimentary rocks. The rare earth element patterns show cerium depletion, indicating a marine origin. Base metal concentrations in black shales are high in the Talvivaara mineralization (ore estimate: 300 Mt with 0.26% Ni, 0.14% Cu and 0.53% Zn) and in the vicinity of ophiolite complexes. In the Kainuu schist belt, the median value for gold is 35 ppb (max 170 ppb) in the western part and 16 ppb (max 180 ppb) in the east. Highest platinum and palladium concentrations encountered are 60–70 ppb. It is shown that in the Kainuu black shales, concentrations of sulphur, base metals and precious metals have been increased by hydrothermal processes, as indicated by positive europium anomalies, elevated mercury concentrations (max 7.5 ppm), high sulphur isotope ³⁴S values and the enrichment of sulphur and base metals relative to carbon. The processes operative during the deposition of these shales find an analogy in recent processes in ocean ridge spreading axes.