Geochemistry, mineral chemistry and pressure–temperature conditions of the Jõhvi magnetite quartzites and magnetite-rich gneisses, NE Estonia

The Jõhvi magnetite quartzites (MagQ) occur as subvertical beds with a complicated structural outline in biotite-garnet-cordierite and pyroxene gneisses which in places also contain high concentrations of iron. Drill core study shows that the complex of MagQ and magnetite-rich gneisses may be up to 100 m thick. The MagQ provide a wide range of chemical composition: SiO2 ranges between 40.3 and 60.1 wt%, Al2O3 between 1.7 and 19.7 wt% and total iron between 15 and 45.2 wt%. This study also revealed unusually high manganese contents of 1–6 wt%. The rare earth element (REE) patterns of MagQ and the surrounding gneisses partly overlap. Cutting granitoids form two different REE patterns. Magnetite occurs as anhedral grains elongated along rock fabric, as rounded inclusions in other minerals or as tiny platelets along grain edges and along cleavage planes of amphibole and biotite. Sulphides are present as pyrite, pyrrhotite and other minor sulphide minerals (chalcopyrite, galena and sphalerite). Analysis of the magnetite grains from drill core J-1 shows that classifying Jõhvi magnetites into a certain deposit type is not unambiguous.

The garnet–biotite geothermometer revealed metamorphic temperatures between 650 and 750 ºC. The garnet–biotite–plagioclase–quartz geobarometer yielded the pressure range of 2.9 to 4.9 kbar. However, having in mind that the entire Jõhvi ore complex may be a result of repeated metasomatic events, which have influenced the primary volcanic-sedimentary sequences, the estimate of primary pressure–temperature conditions might not be a straightforward task. The current understanding of the geological-geochemical correlation hints at geological similarities between the Bergslagen area in Sweden and the Jõhvi Zone in Estonia.