The Garpenberg supracrustal inlier is one of the most economically important inliers in the Bergslagen mining district. Silver and copper have been mined for more than 500 years (Tegengren, 1924) and are still extracted from the Garpenberg mine, yet nowadays as a by- product of Zn and Pb mining together with minor amounts of Au. The supracrustal inlier owes its name to the Garpenberg stratabound limestone-skarn hosted Zn-Pb-Cu-(Ag-Au) deposit, which at the time of writing is the largest known sulphide deposit in Bergslagen.

Proven reserves and mineralization is known to a depth of 1.5 km with new discoveries still being made (Boliden Mineral data). The mineralized system is 7 km long and comprises a number of ore-bodies that mainly occupy a 10-80 m thick altered marble horizon (Allen et al. 2003). Considerable mineralization is also located in the uppermost part of the stratigraphic footwall volcanic rocks, which generally show strong alteration proximal to the massive sulphide orebodies (Allen et al. 2003).

The Garpenberg mine is an underground mine where the ore is mined from between 500 to more than 1,200 metres below ground level. The main mining method is known as sublevel stoping. This means that the ore is mined in layers between two drifts (tunnels), which are driven through the ore body. Other mining methods include cut-and-fill mining, rill mining and residual mining of sill pillars.

In the case of sublevel stoping, holes for the explosives are drilled using vertical drilling rigs. The drill holes can be up to 28 metres in length with a diameter of 76 millimetres. For cut-andfill stoping and drifting, tunnel rigs with two booms are used.

A liquid emulsion explosive is used for blasting. The emulsion is mixed mechanically when charging. During sublevel stoping, a normal round produces approximately 10,000–20,000 tonnes of loose ore.

Loading ore from the sublevel stoping area takes place using remote-controlled LHD loaders. These can either be controlled entirely by an operator or controlled automatically, where loading and unloading are performed remote by an operator a CCTV camera system is used and the transport between is conducted automatically via a local WLAN system. Loading when drifting and cut-and-fill mining is performed with a wheel loader. The ore is transported using trucks to one of the two underground crushing plants. Ore and waste rock are transported by a contractor.

There are two underground crushing plants where the ore is crushed in jaw crushers. The crushing plants are situated 700 metres and 1,087 metres below ground level. After crushing, the ore is hoisted to surface in a shaft. The machinery for the ore hoist is situated above the shaft installed in the headframe. The ore is unloaded into a bin in the headframe and then transported by a belt conveyor to an intermediate ore storage, which can hold approximately two days’ of production.

The ore is transported on belt conveyors from the ore storage to the grinding circuit. Water is added during grinding and the ore is ground in two stages, with autogenous grinding in the primary stage and pebble mill grinding in the second. Autogenous grinding means that the ore grinds itself without the addition of external grinding media. In the pebble mill, pebbles are added that have been extracted from the autogenous mill. The grinded ore is classified using screens and hydrocyclones. The coarse particles, that have not been ground down sufficiently, are returned to the grinding circuit. The grinding process results in a slurry containing water and finely ground ore.

In the flotation process, the ore is concentrated and valuable minerals are separated from the waste rock. The flotation process is a surface-chemical process, where small amounts of chemicals are used to affect the valuable minerals’ surface characteristics, causing them to become hydrophobic. This means that when air is blown into the slurry, air bubbles are attached to the surface of the hydrophobic mineral particles and are transported up to the surface of the flotation cell, where they can be removed.

The mineral concentrates are dewatered using thickeners and air pressure filters. Three mineral concentrates are produced: zinc, lead and copper concentrates. The precious metals are reported primarily in the copper and lead concentrates.

The zinc and lead concentrates are transported by truck to Gävle port for onward ship transport, mainly to our own smelters in Sweden (Rönnskär), Finland (Kokkola) and Norway (Odda). The copper concentrate is trucked and reloaded to railway for onward transport to the Rönnskär smelter in Skelleftehamn.

Waste rock, which is mined and separated in the mine, are used for back filling in the mine. The portion of the finely ground waste rock that is separated in the concentrator is known as tailings. Some of the tailings are mixed with a binding agent and pumped back into the mine for back filling. Surplus tailings are deposited in a pond at the Tailings Management Facility (TMF) from where processed water is recovered.