The presence of abundant (>10%) hydrothermally precipitated magnetite with associated copper–iron sulfides in the breccias hosting the mineralization suggests a deposit type such as an iron oxide–copper–gold (IOCG). However, there are features of the Boa Esperança deposit that do not match the proposed IOCG deposit type. Among these are the presence of high sulfur mineral assemblage (chalcopyrite–pyrite), rather than the low sulfur copper sulfide mineral assemblage characteristic of the IOCG deposit type (chalcopyrite–bornite–chalcocite), as well as the high quartz content, the absence of pervasive hydrothermal alteration of the host rock, specifically sodic (albite) alteration, and the absence of gold. As such, the Boa Esperança copper deposit is interpreted to be a variant of an IOCG deposit type.

IOCG deposits are classified as separate from other large intrusive-related copper deposits such as porphyry copper deposits and other porphyry metal deposits, primarily due to their substantial accumulations of iron oxide minerals, association with felsic–intermediate type intrusions (Na-Ca rich granitoids), and lack of the complex zonation in alteration mineral assemblies commonly associated with porphyry deposits.

The Boa Esperança deposit displays primary and secondary zoning. The primary zoning corresponds to a distal zone, where pyrite (py) dominates, grading towards copper mineralized zones of pyrite–chalcopyrite (py–cpy), chalcopyrite–pyrite (cpy–py) and chalcopyrite (cpy).

The secondary zoning is a supergene alteration and consists of sub-horizontal and discontinuous lenses of a barren leached zone (LIX), a copper oxide zone (OXI) and a mixed zone (MIX) of oxides and primary copper sulfides. The barren leached zone crops out at the hill top, and is composed of hematite, goethite and clay minerals. Despite the large amount of sulfide boxworks found in this area, the leached zone does not contain copper in economic concentrations. The copper oxide zone is located immediately beneath the leached zone and consists of malachite and copper-bearing clays. Below the copper oxide zone is an area of mixed oxides, carbonates, secondary supergene sulfides (chalcocite and covellite) and primary sulfides (pyrite and chalcopyrite).

The bottommost layer, beneath the mixed oxide zone, is a copper-enriched (ENR) zone consisting of sub-horizontal 5–10mthick lenses extending up to 20–30 m and formed by primary and secondary sulfides. Geochemical associations were identified. Cobalt is concentrated on the surface. However, there is no correlation between copper and cobalt grades in the mineralization. The cobalt is intimately associated with sulfur and iron, suggesting that this element is in the mineral structure of pyrite. Iron is more abundant in the pyrite and chalcopyrite zones, and a higher molybdenum content is found in the chalcopyrite–pyrite mineralization and in the leached zone.

The planned Boa Esperança mine will use conventional open pit mining techniques and diesel mining equipment to mine a total of 203 Mt of material over the LOM. This will consist of 54 Mt of ore and 149 Mt of waste material. Selective mine practices will be applied avoiding dilution. The operation will include normal drilling, blasting loading with 5.2 m3/3.9m3 (waste/ore) backhoe configured excavator and 38 t conventional trucks over an 8 m bench height (double bench of 16 m in fresh rock in interim and final slopes). Mining will be performed on a sub-bench of flitch basis to improve selectivity.

The mine plan was developed by NCL. The plan is focused on a single mine area, mined through consecutive mining phases or pushbacks. The mill throughput is based on an economic throughput assessment study, resulting in an average throughput of 4.0 Mtpa of sulphide ore and a ramp-up period of 12 months that assumes a production rate of 3.2 Mt in the first year of production. Production starts during the second quarter of Year 1 to avoid the rainy season.

The required pre-stripping amounts to 13.2 Mt, and activities have been scheduled over 24 months, starting in Year -2 quarter 2, through Year 1 quarter 1. The mining schedule requires an average mine extraction of 20 Mtpa. The mine movement decreases from Year 10 until the mining operations are completed in Year 12.

The mine is scheduled to work on a seven-days-a-week, three 8-hour shift basis, for 365 days per year. The operation will include normal drilling, blasting, loading with 5.2 m3/3.9 m3 (waste/ore) backhoe configured excavator and 38 t conventional trucks over an 8 m bench height (double bench of 16 m in fresh rock in interim and final slopes). Mining will be performed on a sub-bench or flitch basis. Mining will include supporting functions such as ancillary activities, dewatering, grade control, and equipment maintenance.

Pit shell 31, obtained at revenue factor 0.96, corresponding to a copper price of US$ 2.88/lb, was selected as final pit limit and guide for mine design. Lower inter-ramp angles were considered for initial phases that do not reach the final limit. Angles were reduced by 3° for each zone of these phases by extending the width of the berms. The loading operation is envisaged using 8 m benches with 5.2 m3 excavators for waste and 3.9 m3 for ore (higher density) and double flitching to improve selectivity. Mine design assumed 18 m ramps at a 10% maximum gradient, allowing double traffic for this truck type and enough space for the drainage system and protection at the crest of the ramp. No tapering was considered in the ramp direction going down, allowing access to the berms for cleaning purposes and as escape exits for operational safety.

The pit shell sequence provided by Whittle was used as guide for the mining phase designs, providing interconnection between the phases. In general, double access was considered to reduce traffic congestion with exits to the north for the ore to the plant and to the south for the waste. The final pit is the result of the phase designs.

ROM Ore Handling and Crushing
The crushing facility will be a conventional three-stage crushing circuit that will process ROM ore at a processing rate of 10,959 t/d. ROM ore will be trucked from the open pit and fed directly to the ROM ore bin or stockpiled on the storage pad, which can be reclaimed by a front-end-loader (FEL) for continuous feed. The ROM ore bin will be equipped with a fixed grizzly and will have a 200-t live capacity. Large rocks will be broken using a mobile rock breaker.

The ROM ore from the ROM bin will be withdrawn by the grizzly feeder where the coarse oversize will report directly to a single jaw crusher. The feed material will be crushed by the jaw crusher to reduce feed size P80 from 684 mm to 185 mm. The crushed material from the primary jaw crusher will be combined with the grizzly feeder undersize and will be conveyed to the double-deck secondary screen. Secondary screen oversize from the two decks will be fed directly to a 450-kW secondary cone crusher, while its undersize will report to tertiary crushing.

Two 450-kW tertiary cone crushers will operate in closed circuit with two double-deck tertiary screens. Feed to the two tertiary screens will be a combination of secondary screen undersize, secondary crusher discharge and tertiary crushers discharge. The tertiary screen oversize will report to tertiary crushing feed bins to be withdrawn by vibrating feeders to feed the two tertiary crushers. Each tertiary crushing feed bin will have a live capacity of 74 t or approximately 15 minutes of live storage.

Secondary crushing and tertiary crushing will reduce feed size P80 to 47 mm and 12 mm, respectively.

Undersize from the two tertiary screens will be discharged onto the crushed ore stockpile feed conveyor delivering material to the crushed ore stockpile.

Ball Mill Grinding
The grinding circuit will consist of a ball mill operating in closed circuit with a classifying cyclone cluster. The ball mill circulating load will be a nominal 300% of new feed. The grinding circuit is designed for a product size 80% passing size (P80) of 110 µm where the cyclone overflow pulp density will be 35% (by weight).

The 9.0-kW ball mill will be a single pinion overflow mill with an inside diameter of 6.7 m and an effective grinding length (EGL) of 10.7 m. The ball mill will receive crushed ore and process water will be added at a variable flowrate to achieve the target pulp density. Ball mill discharge will flow through a slotted trommel screen with an aperture size of 12 x 45 mm to remove any trash or broken mill balls, which will then be discharged to a concrete ball mill scats bunker.

Undersize from the ball mill trommel screen will discharge directly into the cyclone feed pump box, where it will be diluted with process water and pumped to the cyclone distribution manifold via a cyclone feed pump. Cyclones will classify the feed slurry to achieve overflow stream of 35% solids (by weight) comprising product sized particles. The cyclone underflow will report back to the ball mill feed chute to be ground in the ball mill.

The cyclone overflow will report to a trash screen via gravity, which will remove trash to a trash bin. Trash screen undersize will be sampled and analysed by an on-stream analyser (OSA) and a particle size indicator (PSI) prior to the rougher flotation circuit.

Provisions will be made for the addition of lime to the ball mill feed chute to adjust the pH of the slurry in the grinding circuit prior to the flotation process.

Steel balls will be used as grinding media with diameter of 50–75 mm by means of hoist and ball kibble. The grinding media will be transferred by FEL from the storage bunker into kibbles via a ball loading chute. A ball loading hoist will be utilized to lift the kibbles and discharge the grinding media into the ball mill ball loading hopper. A ball feeder will also be used to add the grinding media periodically to the ball mill feed chute at a controlled rate.

A feed chute transporter will be used to remove the feed chutes from the ball mill for maintenance and reline tasks. A mill liner handler and hydraulic liner bolt removal tools will be utilized to service the ball mill. These tools will be provided by the ball mill supplier.

Concentrate Regrind
The concentrate regrind circuit will consist of a regrind cyclone cluster in open circuit with the regrind mill. The proposed regrind mill is a HIG1200. The size of the regrind mill is based on a maximum specific energy of 10.4 kWh/t and a design circuit feed rate of 99.3 t/h. The proposed HIG mill will be able to reduce the grind size of the copper rougher concentrate and copper cleaner scavenger concentrate to a P80 of 38 µm.

Concentrate from the copper rougher flotation cells will be combined with the copper cleaner scavenger concentrate in the regrind mill pump box to be pumped to the regrind mill cyclone cluster. Particles finer that the target P80 will report as cyclone overflow whereas coarser particles will report as cyclone underflow and will be fed to the regrind HIG mill. Cyclone overflow will be combined with the regrind mill discharge in a pump box to be pumped to the cleaner–scalper Jameson cell.

Ceramic regrind media will be charged to the HIG mill via the media feed hopper. A media hoist will load grinding media into the media hopper, which will be equipped with two gravity discharge outlets, to enable the addition of media after the mill maintenance, as well as media make-up during operation. In the event of the mill inspections and maintenance, the regrind media will be manually drained into bulk bags.

The Boa Esperança mineral processing plant is designed to treat 4 Mtpa of ore from an open pit mine and will produce copper concentrate over a 12-year mine life. The process plant will operate three shifts per day, 365 d/y with an overall plant availability of 92%. The surface crushing plant will operate at 70% availability or 6,132 h/y. Concentrate and pyrite tailings filtration will operate at 84.4% and 82.8% availability or 7,390 h/y and 7,253 h/y respectively.

The process plant will consist of the following:
• Run-of-mine (ROM) bin;
• Three-stage crushing circuit;
• Crushed ore stockpile with reclaim system;
• Ball mill grinding in closed with hydrocyclones;
• Copper flotation with conventional concentrate regrind and cleaner scalper, and two stages of cleaning;
• Pyrite flotation;
• Copper concentrate thickening and filtration;
• Copper concentrate load-out and storage;
• Pyrite tailings thickening and filtration;