Geochemical and mineralogical properties of sulfide-bearing fine-grained sediments in Finland

The geochemical and mineralogical characteristics of bulk samples and vario us size fractions of sulfide-bearing fine-grained sediments in the Petalax area, western Finland, were studied with ICP-AES (aqua regio digestion and "total" digestion) and X-ray diffraction. The sediments, which are dominate d by particles <0.06 mm (clay and silt size), are composed of phyllosilicat es (micas, kaolinite, chlorite, vermiculite), quartz, feld-spars, amphibole s, organic matter, and secondary S minerals. The chemical analyses show tha t Na, Ca, and Sr are enriched in the silt and sand fractions, that Zr is en riched in the silt fraction, and that the concentrations of all other studi ed metals (Al, Ba, Co, Cr, Cu, Fe, K, La, Mg, Mn, Nb, Ni, Sc, Th, Ti, V, Y, and Zn) increase from the sand to the clay fraction. Sodium, Ca, and Sr oc cur largely in poorly dissolved feldspars, which explains the distribution of these metals in the sediments. The increase in the concentrations of mos t metals in the clay fraction is interpreted to be the result mainly of an increase in phyllosilicates in this size fraction. A geochemical comparison between the sulfide-bearing fine-grained sediments and glacial till shows that the former are enriched in several potentially toxic metals. It is the refore argued that the hydrological and ecological problems associated with the sulfide-bearing sediments are related not only to the production of ac idity in oxidized layers, but also to mobilization and dispersion of toxic metals. Strategies to minimize damage of freshwater systems in areas covere d with sulfide-baring fine-grained sediments are suggested.