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.

Copper Plant
Crushing
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 of 800tph.

The undersize discharges directly on to the conveyor belt which feeds the fine ore silo. Fine ore can be stored in the silo to feed the primary grinding line, or stockpiled beside the silo to feed the second grinding line installed in 2008.

Screen oversize is reclaimed by the front end loader and fed into the crushing circuit either alone, or mixed with run of mine ore, this is then conveyed to a 60” ‘Superior’ secondary crusher. All ore passes through the crusher to two 20’ x 8’ Allis Chalmers Screens.

Screen undersize at <19mm passes via a conveyor to the fine ore silo. Screen oversize passes to two 60” ‘Hydrocone’ crushers and the crushed product conveyed again to the screens.

Crushing plant throughput averages 350tph and is operated primarily at night to take advantage of cheaper electricity tariffs and to maximise available maintenance time whilst ensuring sufficient feed stock ahead of the grinding section. The silo has a capacity of 2,500t allowing 10 hours of rod mill feed.

Grinding and Regrind
The primary grinding circuit (Line 1) consists of a rod mill (Allis 3.8 x 5.5m with 1000kW) in open circuit and a primary and secondary ball mill (Allis 4.1 x 6.7m rubber lined with 1600kW) in closed circuit with hydrocyclones, Sala 20” for the primary and Sala 10” for secondary. Secondary cyclone overflow, at 80% passing 40 µm passes to the flotation circuit.

The second grinding line (Line 2) is fed by front-end loader from the fine ore stockpile adjacent to the silo. The circuit consists of a Rod Mill (3.0m x 5.6m, with 650kW) in open circuit and a primary ball mill (Allis 4.1 x 5.5m rubber lined with 1200kw) in closed circuit with Sala 20” hydrocyclones. Cyclone overflow, at 80% passing 45µm passes to the Flotation circuit.

Planned milling rate is 260tph (Line 1), and 80tph in the second line. The copper regrind mill (Allis 4.1m x 5.5m rubber lined with 1200kw) works in closed circuit fed by the underflow of 15 to 25 Sala 6” cyclones that can achieve a d80 of 18-25µm.

Zinc Plant
Crushing
Run-of-mine (ROM) ore is dumped from the mine stacker to the ore park. The ore is fed to a prescreen for screening at 20mm. The oversize reports to the primary jaw crusher (750 x 500mm) while the undersize reports directly to one of the fine ore silos. The crushed coarse ore is conveyed to the secondary cone crusher (Standard H400 cone). The conveyor system has a rated capacity of 200tph. The secondary crusher product is conveyed to the tertiary crusher screen with the oversize (+20mm) reporting to the tertiary cone crusher (H400 Shorthead cone) and the undersize reporting to the fine ore bin.

Grinding
The crushed ore is reclaimed from one of three 500t ore silos via three feeders and fed to the rod mill feed conveyor. The grinding circuit consists of a single line consisting of a 3.81m diameter by 4.87m long rod mill (900kW), and two 3.00m diameter by 4.10m long (600kW) primary ball mills and a Vertimill (930kW). The rod mill product is pumped to the ball mill circuit via two feed distributors (one operating and one standby), for distribution to two primary ball mill sumps. The rod mill discharge, together with the ball mill discharge, is pumped to cyclone clusters for classification. The cyclone clusters operate in closed circuit with the ball mills. The cyclone overflow, at a P80 product size of 200µm, is fed to the secondary grinding mill circuit. The cyclone underflow returns to the ball mills.

The primary cyclone overflow together with the secondary grinding mill discharge is pumped to a cyclone cluster. The cyclone overflow, at a P80 product size of 60µm, is fed to the flotation circuit. The cyclone underflow is returned to the Vertimill.

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.

From the primary grinding circuit the slurry passes through two aerator conditioners (2 Dorr-Oliver 38m3 cells) to the flotation circuit, 62 cells of 17m3 and 12 of 38m3, all Dorr Oliver.

Concentrate from the first bank of rougher #1 (3 cells of 17m3) feeds the first cleaner, but can optionally go to the regrinding. The remai ........