Pyhäsalmi Mine in Central Finland has produced copper-zinc-pyrite ore since 1962. Started form an open pit operation, the mine has deepened down gradually to a depth of 1400 metres from surface. Since the mine startup, the varying local geology and rock mechanical conditions have challenged the mine design in Pyhäsalmi. Innovative, sometimes imaginative, methods for ground control and extraction have been applied. This paper aims to summarize more than five decades of production history and, hopefully, to list some of the lessons learned.
Hit by a local farmer with a shovel, Pyhäsalmi massive sulphide in Central Finland was found in 1958 (Figure 1). After a busy period of drilling, resource evaluation and setting up the necessary infra in the rural township of Pyhäjärvi, a Finnish base metal company Outokumpu Oy started mining operations already in 1962. Today, Pyhäsalmi Mine is the oldest operating metal mine in Finland and one of the deepest underground mines in Europe, where total of 54 Mt of massive copper-zinc bearing massive sulphide has been mined.
Geologically Pyhäsalmi is situated on Svecofennian part of the Fennoscandian shield. The complex deformation history has placed the 1.93 Ga massive sulphide orebody in subvertical isoclinal fold, with alteration rim of talc and mica bearing rocks surrounding the upper part. The deep part between the depths of 1000 and 1400 metres' depth is wrapped by sheared felsic schists, with mafic bands of varying width. The whole geology is overprinted by deep fractures and faults from later brittle deformation. Pegmatite dykes have also intruded in the contact zone of the massive sulphide and wall rock. On surface, the massive sulphide outcrops as N-S trending S-shape. Schistosity in the surrounding altered rocks mainly follows the ore lineation. This feature is present even in the deepest parts where massive sulphide forms a potato-like shape in the tip of the fold. The massive sulphide has mostly been recrystallized to strong coarse-grained pyrite-chalcopyrite-sphalerite. Quaternary weathering oxidized the near- surface part of the ore. However, in general the massive sulphide is less fractured than the wall rock (Kousa et al., 2015).