A minor Ni-Cu-deposit at Telkkälä, SE-Finland

Abstract

The Telkkälä Ni-Cu-deposit is situated in SE Finland about 16 km NW of the town of Lappeenranta. The mineralization, which is in association with a differentiated mafic pipe-like body, was the object of mining activities of the Outokumpu Co. in 1969 and 1970. During that time 211 331 tons of ore and 122 500 tons of barren rock were hoisted from the open pit. The ore averaged 1.06 % Ni and 0.29 % Cu. The rocks in the mafic body range from the central peridotites to perknites, pyroxenites and gabbros that occur as concentric zones around the ultramafic core. The basic body is enveloped by a garnet-rich and cordierite-bearing gneissose rock that has been interpreted as a product of thermometamorphism due to hot basic magma reacting with the surrounding argillaceous sediments. Disseminated ore with fairly fresh sulphides is mainly restricted to the cummingtonite gabbro. High-grade breccia ores favour peridotites and perknites whereas massive and stringer ores occur in almost every rock type. The massive ores exhibit intensive alterations, the pyrrhotite and pentlandite having largely been replaced by pyrite, marcasite, bravoite and violarite. The elemental distribution and the correlations between the ore-forming components suggest that the mineralization process was controlled by four factors at least. Lead isotope studies on the sulphides show them to be very rich in radiogenic lead, which obviously derives from the adjacent mica gneiss. The lead isochron gives an age of 1 820 Ma for the orebody. This is in excellent agreement with the Svecofennian common lead ages for sulphides.