The Kylmäkoski nickel-copper deposit in south-western Finland

Abstract

The Kylmäkoski sulfide Ni-Cu deposit is located in southwestern Finland in the WNW trending Pori-Kylmäkoski nickel belt which contains several other sulfide Ni-Cu occurrences besides Kylmäkoski. The original reserves of the Kylmäkoski ore were ca. 0.5 million tons of ore assaying 0.48 % Cu and 0.55 % Ni. The ore was exploited in 1971—1974. The wedge-shaped host ultramafic body with sinuous contacts against the wall-rock mica gneiss was almost wholly mineralized. The body was bordered in the north by an intersecting quartz diorite. The main rock types are peridotites, pyroxenite and hornblendite, and the differentiation is supposed to be caused by wall-rock contamination. The peridotites include rare orbicular and nodular varieties whose peculiar texture is due to the rapid crystallization of skeletal olivine. The ultramafic body has undergone regional metamorphism and recrystallization. Sulfides exist either as primary disseminated grains or accumulations brought about by gravitative settling or as massive and breccia ores generated by later tectonic events. A nickel-arsenide mineral assemblage abounds as massive veins related to the intersecting quartz diorite dikes. Platinum-group elements are concentrated in the nickel-arsenide mineralization. The lead-lead modal age of the body is 1856 Ma. The emplacement of the ultramafite is suggested to predate folding and regional metamorphism.