Ruinous headframe and flooded open-cast pit of the former Metsämonttu zinc mine, a satellite workings of the Aijala copper mine in Kisko, Salo.
The Metsämonttu mine was a zinc-lead mine located in Kisko, in the area now forming part of the city of Salo in Varsinais-Suomi, southwest Finland. Situated approximately 1.5 to 2 kilometres west-southwest of the main Aijala copper mine, Metsämonttu functioned as a satellite operation to that complex, with ore transported from Metsämonttu to the Aijala concentrator for processing. The Metsämonttu deposit was identified in 1946, and the mine operated in two phases: 1952 to 1958, and 1964 to 1974.
Ore was extracted from both an open-cast pit and an underground mine. Total production amounted to approximately 1.51 million tonnes of ore containing zinc, copper, lead, sulphur, silver, and gold. The mine was owned by Outokumpu Oy.
After closure the site was left without formal remediation. A ruinous headframe tower and the remains of a crushing building survive at the site, adjacent to a water-filled open-cast pit. Drainage from the area contains elevated concentrations of zinc and cadmium.
The Metsämonttu ore deposit was identified in 1946, during the wider period of mineral exploration in the Kisko area that also led to the discovery of the Aijala copper ore. The deposit lay approximately 1.5 to 2 kilometres west-southwest of the main Aijala mine complex. Outokumpu Oy, which owned both operations, developed Metsämonttu as a satellite mine, since the ore extracted there required processing at the Aijala concentrator rather than having its own treatment plant.
Mining at Metsämonttu commenced in 1952, initially combining open-cast and underground extraction. The first operational phase ran until 1958, after which the mine was idle. A second phase of production ran from 1964 to 1974. Over both periods combined, approximately 1.51 million tonnes of ore were extracted, with a total mining volume including waste rock of approximately 1.7 million tonnes. The ore grades recorded were 3.34 per cent zinc, 0.28 per cent copper, 0.74 per cent lead, and 14.1 per cent sulphur, with silver at 24.8 grams per tonne and gold at 1.43 grams per tonne. The site had its own electrical substation with a 500 kVA transformer, supplied from the same 35 kilovolt line serving the Aijala complex. Throughout both production phases, extracted ore was transported to the Aijala concentrator for separation and processing.
The mine was closed definitively in 1974. No formal environmental remediation was carried out under the environmental legislation applicable at the time of closure. Post-closure investigations, including a GTK field inspection in May 2016, confirmed that the site retains a waste rock dump to the north of the headframe and crushing building complex. Drainage seeping through the waste rock discharges from a pipe at the base of the building and flows onto the waste rock area, with run-off reaching the Kiskonjoki catchment. Water quality measurements taken during the 2016 survey found mildly acidic to neutral conditions, with elevated zinc and cadmium concentrations attributable to the sulphide-bearing waste rock. GTK concluded that, given the acid-generating potential of portions of the waste rock, a more detailed study of current chemical conditions and drainage pathways would be warranted.
The physical remains on site include the headframe tower, described by visitors as a substantial structure whose concrete elements remained standing while the wooden internal components had collapsed over time, and the adjacent crushing and ore processing building in a ruinous state. The open-cast pit at the site has filled with water. The mine site lies within the Kiskonjoki Natura 2000 conservation area (FI0200083), which was designated to protect the river ecosystem including freshwater pearl mussel (Margaritifera margaritifera), thick-shelled river mussel (Unio crassus), and sea trout (Salmo trutta m. trutta) populations.