Lahnaslammen kaivos Sotkamo · Kainuu · Finland

The talc-magnesite deposit at Lahnaslampi, situated south of Nuasjärvi in the municipality of Sotkamo, Kainuu, was identified in the late 1940s. Suomen Malmi undertook deep diamond-drilling at the site in 1956 and conducted test beneficiation in 1957. Preliminary batches of Lahnaslampi talc were delivered to the Kajaani paper mill in 1961, demonstrating the suitability of the product for the paper industry.

Commercial open-cast mining began in 1968 under the operating company Suomen Talkki Oy, which had been founded in Finland in 1967 specifically to exploit a newly developed flotation process for separating magnesite from talc ore. The principal product was talc concentrate used primarily as a filler in printing and fine papers; nickel concentrate was recovered as a by-product. A processing plant was constructed on site to enrich the ore.

Ownership passed through successive changes: in 1984 the business continued under Finnminerals Oy; the operating name became Mondo Minerals in 1998 and subsequently Mondo Minerals B.V. Branch Finland in 2007; and in 2018 the talc business was acquired by Elementis. Throughout these changes the Sotkamo operations remained centred on the Lahnaslampi pit and its associated processing plant.

Over the mine's operational life of more than forty years, the open-cast pit reached a depth of 185 metres. Total ore extracted was close to 18 million tonnes, from which approximately 7 million tonnes of talc and more than 100,000 tonnes of nickel concentrate were recovered. The talc was used at paper mills in Kajaani, Jämsänkoski, and Simpele.

Extraction at Lahnaslammen kaivos ended in October 2010. The increasing depth of the pit had made continued mining progressively more difficult and costly. From that date, the adjacent Punasuon kaivos, opened in 2010, took over as the primary raw material source for the Sotkamo processing plant. Following closure, the Lahnaslampi pit was designated for progressive infilling over a period of approximately 25 years using waste rock from Punasuon kaivos and magnesite sand generated during processing. The uppermost section of the pit is expected eventually to fill with water, creating a new lake on the former mining footprint.