The Pyhäsalmi deposit is a copper-zinc volcanogenic massive sulphide deposit of Proterozoic age. The mineralization is hosted by altered felsic and mafic volcanic rocks. The enveloping alteration zone is at least four kilometres long and one kilometer wide at its widest point. Alteration of the felsic volcanic rocks includes sericite and cordierite dominated mineralogy. Cordierite, anthophyllite and garnet dominate in the altered mafic volcanic rocks. The metamorphic grade is upper amphibolite facies.

The upper part of the Pyhäsalmi deposit was mined between 1962 and 2001 and is now depleted. Deep drilling in 1996 led to the discovery of an extension to the deposit below the +1050 meter level. The newer deep deposit is located between the +1050 meter level (from surface) and the +1416 meter level. The inner part of the lens consists of massive pyrite with low copper and zinc values. This core is surrounded by massive chalcopyrite-pyrite and the outer rim consists of massive sphalerite pyrite. The main sulphide minerals are pyrite (65%), chalcopyrite (3%), sphalerite (4%) and pyrrhotite (3%).

The mining method applied is benching with delayed backfill in a primary-secondary or horizontally retreating sequence. Most stopes are mined transversely across the width of the ore body along mining profiles originally designed for the primary-secondary sequence. Stopes in primary profiles, with single draw point and overcut drifts, are 20 to 30 metres high, approximately 30 to 70 metres long and 15 to 20 metres wide. Stopes in secondary profiles are identical to those on primary profiles, except that their width varies between 20 metres and 25 metres. Stope tonnages vary from 30,000 tonnes in the narrow profiles to 150,000 tonnes for the wide profiles. The sequencing method applied varies between different areas of the ore body: the basic sequence is primary-secondary, but some areas are extracted by horizontal retreat because of connecting development or location in sill pillars.

Each stope is designed individually considering the value of ore, anticipated rock mechanical challenges, backfill quality in the surrounding stopes, phyllosilicate zones in massive sulfide and other foreseeable risks.

Extracted stopes are filled using delayed rockfill from wall rock development and a surface backfill quarry. In addition, most stopes are consolidated using hydraulic fill, mixed of mill tailings, lime, fly ash, slag and water.

After single-stage primary crushing in the underground jaw crusher, the ore is hoisted to the screening plant, which is of the vertical type. There is also a jaw crusher for oversize boulders but in practice, little more than screening is done at the screening plant. After screening, the ore is wet ground in three stages: one primary SAG mill, three secondary SAG mills and one tertiary ball mill. The secondary and tertiary mills are in closed circuit with cyclone classifiers. The fineness of grinding is approximately 3% +0.210 millimetres and 65% -0.074 millimetres.

The copper circuit has a regrind ball mill to reduce recirculation flow and sanding in the middling flotation.

After single-stage primary crushing by an underground jaw crusher, ore is hoisted to a fines crushing plant, where it is ground in three stages. This is followed by conventional flotation.

The flotation arrangement is direct, selective flotation consisting of three sequential flotation circuits, one each for copper, zinc and pyrite. Zinc is floated from copper circuit tailings and pyrite from zinc circuit tailings.

The copper and zinc flotation circuits consist of roughing, scavenging, middling and cleaning flotations, with four cleaning stages for copper and three for zinc. In both circuits, scavenger concentrates are pumped to middling flotation, and the final circuit tailings are the combined scavenger and middling flotation tailings. In the zinc circuit, the first stage is open circuit roughing and scavenging, and cleaning tailings are pumped to middling flotation. The copper circuit roughing and scavenging is similar, but the cleaning tailings are pumped back ........