The Perkoa deposit has been classified as a volcanogenic massive sulphide (VMS) deposit (Billiton, 1998). VMS deposits are lenses and sheets of massive sulphide that form from seafloor hydrothermal systems where metal rich fluids (black smoke) precipitate on (exhalative) or near the seafloor (sub seafloor replacement.).

The Footwall Lens is the thickest (up to 25 m to 30 m) and most continuous of all the lenses. The Footwall Lens typically has higher zinc grades than that of the Hanging Wall Lenses. The Footwall Lens is composed of coarse grained sphalerite and pyrite of approximately the same proportions (around 20% to 25%) as the Hanging Wall Lenses with minor content of pyrrhotite, plus locally valuable galena associated with silver, however, with generally lesser amounts than the Hanging Wall Lenses. A large proportion of barite (up to 15%) is a common constituent of the gangue. In the upper northeastern area of the deposit, the Footwall High Grade Lens has grades in excess of 50% Zn at the contact of the sulphide lens and granodiorite pluton. Average zinc grades in the footwall are more in the region of 9% to 15 % Zn.

The Hanging Wall Lenses are typically thinner than the Footwall Lens and represent more interbedded features of massive, vein type, or disseminated VMS together with tuffaceous material along the same geological horizon. In these lenses, the pyrite and pyrrhotite content is higher than that of the Footwall Lens, while barite content is lower and almost no lead and silver are present. Due to the interbedded and disseminated nature of the Hanging Wall Lenses, they are typically of lower average zinc grade, however, they are found to have some enriched zones in proximity to fault zones and/or the footwall granitic pluton. At depth, the Hanging Wall Lenses thicken up to 15 m in width.

Longhole stoping is being used as the primary extraction method. There are several variations on this mining method employed such as longitudinal and transverse, with both bottom-up and top-down mining sequences. The exact method chosen is dependent on the orebody geometry. Stopes are backfilled either with cemented rock fill (CRF) or waste rock.

Longitudinal bottom-up stoping has been used above 190 level. Due to the orebody width and mine sequencing, stopes between the 190 level and the 280 level are mined using the transverse method with primary and secondary stopes, with some longitudinal retreat mining occurring at the extremities of the orebody. Most of the primary stopes above the 280 level have been mined out with the majority of the remaining ore to be mined as secondary stopes. A recoverable sill pillar is being established on 310 level in order to be able to convert the mining below 310 level to a longitudinal retreat method, which is expected to reduce the amount of development required. As well, a higher percentage of waste backfill instead of CRF can be used with this method, which is expected to lower the mining cost. Bottom-up mining will be utilized between the 340 level and the 430 level and top-down longitudinal retreat will be utilized between the 460 level and the 520 level.

The process plant at Perkoa is a conventional sulphide flotation plant capable of processing 2,000 tpd and is currently operating at approximately 1,900 tpd. The process plant includes crushing, screening, and grinding, followed by zinc flotation and filtering to produce a zinc concentrate. The process plant originally included a lead recovery circuit, however, this circuit is no longer used and has been reconfigured to increase capacity in the zinc circuit due to higher zinc head grades.

The ROM ore is fed through a grizzly to the primary jaw crusher at a rate of approximately 150 tonnes per hour (tph). The product is sent to a secondary cone crusher circuit operating in closed circuit with a screen, splitting at 10 mm. The screen oversize (+10.0 mm) is sent to the cone crushers. Product from the cone crushers is returned to the primary screen. The undersize (-10.0 mm) from the primary screen is sent to the mill feed stockpile.

From the mill feed stockpile, t ........