Abstract

The Pyhaesalmi deposit is a stratabound massive pyrite-copper-zinc deposit in a Precambrian metamorphic schist belt mainly composed of metavolcanites. The host rocks of the ore are silicic fine-grained metavolcanites, and the stratum of ore is stratigraphically overlain by mafic metavolcanites. In the close environment of the ore the silicic metavolcanites, rhyolitic in composition, and the more mafic rocks have been chemically altered; hence, they are enriched in potassium, iron, and magnesium and depleted in sodium and calcium. The metamorphic derivatives of the altered volcanites are sericite schists and cordierite-anthophyllite rocks. The ore deposit, which is syngenetic with silicic volcanites, was folded during polyphase deformation. The orebody averages 0.85 percent Cu, 2.8 percent Zn, 37 percent S, 33 percent Fe, 0.2 g/ton Au, and 14 g/ton Ag. The main gangue minerals are quartz, barite, and local carbonates. The data presented indicate a fairly constant isotope composition of sulfur with an average value of 34 S of +7.5 per mil for pyrite. Isotopic dating of intrusive and supracrustal rocks yields ages of about 1,900 m.y. for the metavolcanites but a somewhat higher age (1,932 + or - 1.5 m.y.) for the gneissic granite, east of the schist belt. The lead isotope compositions of the galena in the Pyhaesalmi ore and its silicic metavolcanite hosts lie on the same isochron within limits of experimental uncertainty. The lead isotopes applied to a plumbotectonic model yield an age of 1.970 m.y. and indicate a mantle origin.

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