The mineral deposits at Neves-Corvo are classified as volcano-sedimentary massive sulphide (VMS). They typically occur as lenses of polymetallic (Cu, Zn, Sn, Pb) massive sulphides that formed at or near the seafloor in submarine volcanic environments. They formed from accumulations of the focussed discharges of hot metal-enriched fluids associated with seafloor hydrothermal convection, typically in tectonic areas of active submarine volcanism, including rift spreading centres and island arc subduction zones. The massive sulphide lenses are commonly underlain by sulphide-silicate stockwork vein systems, although the stockwork systems may extend into the hanging-wall strata above the massive sulphide lenses. The immediate host rocks can be either volcanic or sedimentary. The deposits are overlain by a repetition of volcano-sedimentary and flysch units.

VMS deposits readily accommodate strain during regional deformation because of the ductile nature of massive sulphide bodies, and can therefore display much higher degrees of recrystallisation and remobilisation than the surrounding volcanic and sedimentary strata. The tectonic remobilisation may result in duplication of the stratigraphy further localising the sulphide mineralisation.

Six massive sulphide mineralised zones have been defined within the Neves-Corvo Mining Area and comprise Neves, Corvo, Graça, Zambujal, Lombador and Monte Branco. The Semblana massive sulphide zone is located within the Semblana Mining Area and is located 1.3km northeast of Zambujal.

The mineralised zones lie on both flanks of the Rosario-Neves-Corvo anticline. The mineralised zones of Neves, Corvo, Graça, Zambujal and Lombador are connected by thin massive sulphide “bridges” over the crest of the fold and are conformable with the stratigraphy. Within the area of these five main deposits this has resulted in an almost continuous complex volume of mineralised rock showing a large range in both style of mineralisation and geological structure. The mineralised zones are located at depths of 230m to 1,400m below surface.

The mineral deposits occur as concentrations of high-grade copper and/or zinc mineralisation within massive sulphide pyritic lenses, and copper mineralisation within stockwork zones that typically underlie the massive sulphide. Base metal grade distributions within the massive copper/zinc sulphide lenses typically show good internal continuity, but laterally can terminate abruptly in barren pyrite. The massive sulphide deposits are generally very large, regular, continuous and predictable.

The base metal grades are segregated by a strong metal zoning into copper, tin and zinc zones, as well as barren massive pyrite. Three styles of mineralisation have been identified at Neves-Corvo:

- Rubané mineralisation - characterised by thin banded alternations of shales, breccias and massive sulphide or tin mineralisation (found mainly in Corvo but now predominantly mined out);

- Massive sulphide mineralisation; and

- Stockwork (fissural) sulphide mineralisation. Disruptions and tectonic deformation of the lenses and stockworks have been observed related to the three tectonic events that deformed the Neves-Corvo region.

Neves-Corvo has been developed as an underground operation and exploits a number of polymetallic sulphide orebodies. The mine currently hoists approximately 3.5Mt of ore per year via a 5m diameter shaft from the 700m level (underground elevations relate to a datum of 1,000m below sea level with the mine surface elevation at approximately 220mASL, or 1,220m above datum). Ore from the deeper levels is transported to the 700m level via an incline conveyor from the 550 level. Principle access to the mine is via a ramp from surface and the numerous internal ramps serving the various mining areas. Mining methods have been dictated by geology and geotechnical considerations and at the present time, drift and fill as well as bench and fill mining methods are utilised with the fill comprising predominantly paste fill.

Drift-and-fill was the original mining method selected for Neves-Corvo. Although the method has relatively low productivity rates and high unit costs, it was chosen because it is highly flexible and can achieve high recovery rates in high grade orebodies with complex and flat dipping geometry. The initial copper reserves at Neves-Corvo, largely in the Graça and Upper Corvo orebodies, averaged in excess of 8.0%Cu and it was important to select a method that extracted all of this high grade mineralisation.

Drift-and-fill stopes at Neves-Corvo are normally accessed from a footwall ramp with footwall access drives driven along the orebody strike at 20m vertical intervals. Access crosscuts are driven down from the footwall access drives in to the orebody. A horizontal slice is subsequently mined using drifts developed either longitudinally or transversely in sequence. Standard drift dimensions are 5.0m x 5.0m, with the sidewalls often being slashed before backfilling. Following completion of a drift it is tightly backfilled with hydraulic sand fill or Paste fill before the drift alongside is mined. When a complete 5m high orebody slice is mined and filled, the back of the access drive is “slashed” down and mining recommences on the level above. Drift-and-Fill is generally applied to areas of the mine with a mining thickness of less than 10m and has become the prevalent mining method at Neves Corvo, as the thicker parts of the orebodies that are more suitable for bench and fill mining have become depleted.

The bench-and-fill mining method has long been used at Neves-Corvo in areas where the mineralisation is of sufficient thickness and continuity. The method is more productive and has lower operating costs than drift and fill mining. The method is generally applied in areas of the orebodies greater than 20m in vertical thickness.

Bench-and-fill stopes are also accessed from a footwall ramp, with footwall drives driven along strike in waste at 20m vertical intervals. Upper and lower access crosscuts are driven across the orebody to the hangingwall contact.

Primary bench-and-fill stopes have been mined up to 120m long, but in secondary stopes are more typically broken in to 30 to 40m across-dip lengths. The stopes are normally mined in an up-dip primary-secondary sequence. Primary stopes are normally filled with cemented paste fill and then tightly filled with hydraulic sand fill. Secondary stopes are filled with either waste rock or low cement paste fill and then also tight filled with hydraulic sand fill, with the exception being in the Lombador area, where hydraulic fill has not been used.

The process plants use conventional flowsheets consisting of crushing, grinding and flotation and the company is a significant producer of both copper and zinc concentrates. Operations are split between two processing plants, namely the Copper Plant and the Zinc Plant.

Copper Plant:

The installed capacity of the Copper Plant is 2.5Mtpa. To achieve this throughput the plant is manned continuously using a five-shift rotating roster. The operation starts at the coarse ore stockpiles, through pre-screening, crushing, grinding, flotation, filtration to concentrate storage and despatch and includes utilities and tailings management.

The Pre–Screen, installed upstream of the crusher circuit, is designed to remove the fines fraction (<19mm) existing in the run-of-mine ore. This increases the efficiency of the crushing circuit, especially when the ore has a high moisture content. The circuit consists of a Metso TS502 double deck screen, with a nominal capacity o ........