The Bathurst Mining Camp occupies a roughly circular area of approximately 70 kilometres diameter in the Miramichi Highlands of northern New Brunswick. The area boasts some 46 mineral deposits with defined tonnage and another hundred mineral occurrences, all hosted by Cambro-Ordovician rocks that were deposited in an ensialic back-arc basin.

The volcanogenic massive sulphide deposits in the Bathurst Mining Camp formed in a sediment-covered back-arc continental rift during periods when the basin was stratified with a lower anoxic water-column. The basin was subsequently intensely deformed and metamorphosed during multiple collisional events related to east-dipping subduction of the basin. The rocks in the Bathurst Mining Camp are divided into five groups: the Miramichi, Tetagouche, California Lake, Sheephouse Brook, and Fournier groups, which are largely in tectonic contact with one another. The lower part of each group is dominated by felsic volcanic rocks and the upper part by mafic volcanic rocks, which are overlain by carbonaceous shale and pelagic chert. The basalts are both tholeiitic and alkalic and show a progression from enriched, fractionated continental tholeiites to alkali basalts to more primitive, mantle-derived midocean ridge, tholeiitic pillow basalts. Most massive sulphide deposits of the Bathurst Mining Camp are associated with felsic volcanic rocks in each group.

The Caribou deposit is a VMS deposit, is located in the northern part of the Bathurst Mining Camp and occurs in the core of a synformal structure that plunges steeply (80°-85°) to the north. The Caribou deposit is a VMS typical of the Bathurst Mining Camp but is sufficiently distinct from the Brunswick type to warrant a subtype designation (Caribou type) within the Bathurst Mining Camp. Unlike the Brunswick-12 deposit, which is hosted by the Tetagouche Group, the Caribou deposit occurs in the California Lake Group near the base of a felsic volcanic rock sequence that comprises part of the Spruce Lake Formation. The Spruce Lake Formation volcanic rocks are petrologically and geochemically distinct from those of the Tetagouche Group. Furthermore, the Caribou deposit is not associated with the Algoma-type carbonate-oxide-silicate iron formation that overlies and is lateral to the Brunswick-12 and Heath Steele deposits.

Mineralization within the Caribou deposit is composed of seven en échelon lenses striking parallel to the Caribou fold numbered 10 to 80 that are zoned mineralogically and chemically from a copper-rich ventproximal facies (vent complex) near the bottom and western part of each lens, to a lead-zinc-rich vent-distal facies (bedded sulphides) near the top and eastern part of each lens. The zones typically consist of 90% sulphides, mainly pyrite, sphalerite, galena and chalcopyrite. The main gangue minerals are magnetite, siderite, stilpnomelane, quartz and chlorite. Lenses 10, 20, 30, 70, and 80 occur on the north limb of the Caribou fold while lenses 40 and 60 are mostly on the eastern limb of the fold. Individual lenses vary in thickness between a few metres to approximately 30 metres and extend over 1,000 metres along strike on the North Limb. Lenses on the Eastern Limb pinch at a depth of approximately 800 metres below the topographic surface while lenses along the North limb have been drilled to a depth of approximately 1,000 metres and are still open at depth.

Access to the underground mine is via a connected, dual-ramp system, with an existing Main 2460 Portal located on the north side of the deposit, and an existing 2560 Portal located on the east side near the crusher pad, which connects to the main ramp near the top of the 2360 Level.

During past operations ore was skipped to surface using an existing shaft and then conveyed through a conveyor gallery to the Fine Mill-Feed Bins.

Trevali decided to use truck haulage over shaft hoisting.

During 2015, the construction of an underground ramp connection to the existing conveyor portal (approximately 400 m of development) was completed which allowed operational efficiencies by reducing the underground haul distance to a stockpile within approximately 100 m of the Fine Mill-Feed Bins versus the previous approximately 1.5 km surface haul route. This stockpile is the primary feed to the concentrator, and has a capacity of 50,000 tonnes.

All new ramps are designed at a maximum gradient of -15%, with dimensions of 5.0 m wide by 5.0 m high.

Modified Avoca is the main mining method, supplemented by uphole retreat for partial sill pillar recovery. This method uses development waste and surface-stockpiled waste as backfill. Modified Avoca is a longitudinal retreat mining method. Stope sequencing generally follows a retreat along strike from lens extremities or strategic starting points, and retreats to ramp access points.

The stopes are typically excavated 16 m along strike, and to a nominal height of 25 m floor-tofloor. Stope width is normally the same as the lens width. A standard stope will yield approximately 10,600 tonnes of ore, including the development tonnes.

Blast holes are 16 m to 20 m downholes drilled at 76 mm diameter with a 2.0 m burden x 2.0 m spacing pattern. Some drill holes are fanned out where lens width exceeds 5.0 m. On average, the drill factor is 11.0 ore tonnes per metre of drilling.

Blast holes are loaded with either emulsion or ammonium nitrate/fuel oil (ANFO), depending on local water conditions, to an average powder factor of 0.45 kg/t. Slots are opened by drop raising on only the first stope of each retreat mining front. On average, one slot raise will suffice for four stopes. Trevali is planning to convert to use 1.1 m diameter raisebore holes for slots in all stopes.

Production mucking is undertaken by 14 t capacity Load-Haul-Dump (LHD) mobile equipment. Ore mucked from drawpoints is trammed to remuck bays located on each level close to the main ramp, then loaded into 40 t capacity haul trucks. Loaded trucks then travel up the ramp and dump material on the surface crusher pad located adjacent to the mill (Main Feed) conveyor.

When stope mucking is completed, the mined-out stope is tightly filled with waste rock; no cement is used for the backfill material.

The plant feed is delivered as primary crushed ore at nominally 119 mm (4.5 in.) top size by inclined conveyor from the mine to two 2,000 tonne storage bins at the concentrator. Belt feeders discharge each bin individually, controlled by a variable frequency drive (VFD) located on each bin’s discharge conveyor.

The plant feed is delivered to the 6.7 m (22 ft) diameter by 2.1 m (7 ft) long Hardinge primary SAG mill, equipped with a 1,491 kW (2,000 hp) drive motor. A maximum ball charge of 15% is allowed in the mill to reduce the feed material to nominal 650 µm as feed to the secondary ball mill. The discharge from the SAG mill is pumped to a Derrick vibratory screen, with 3.2 mm (0.13 in.) apertures, to classify the mill discharge and recycle the oversize particles back to the SAG mill feed. Derrick screen undersize gravitates to the secondary ball mill cyclone feed pump for secondary classification in a battery of 508 mm (20 in.) diameter Krebs cyclones.

Overflow, at 80% passing nominal 30 µm to 35 µm, reports directly to flotation while the cyclone underflow will recycle to the 4.3 m (14 ft) diameter by 6.7 m (22 ft) long Nordberg secondary ball mill, equipped with a 1,864 kW (2,500 hp) drive motor, for further grinding.

Secondary ball mill discharge combines with the SAG mill product in the cyclone feed pump. Soda ash and sodium cyanide are added to the grinding circuit as pyrite and sphalerite depressants.

LEAD REGRIND
The combined rougher-scavenger concentrate passes to the lead regrind circuit, where it is classified in a battery of 152 mm (6 in.) cyclones to divert the plus 12 µm to 15 µm mineralization to an M1000 Isa regrind mill rated at 500 kW (670 hp). Previous operations required two M1000s in parallel owing to higher head grades. Current conditions have allowed Trevali to utilize the additional ISA mill as a spare

Isa mill discharge combines with the cyclone overflow at nominal 12 µm to 15 µm as feed to the primary cleaning.

The primary cleaner concentrate undergoes secondary regrind in an additional M1000 Isa mill using 100 mm (4 in.) diameter cyclones as classifiers. Product, at nominal 8 µm to -10 µm, passes to secondary cleaning.

ZINC REGRIND
The combined rougher and scavenger concentrate is pumped to a single-stage, 3.0 m (10 ft) diameter by 6.7 m (22 ft) long regrind ball mill, equipped with a 746 kW (1,000 hp) drive motor.

The process plant at Caribou is a conventional milling and sulphide flotation plant with a 3,000 tonne per day nameplate capacity. The process plant includes crushing, screening, grinding, regrinding, and zinc, and lead flotation and filtering circuits to produce zinc, and lead concentrates. Concentrate production is stockpiled onsite prior to shipping and sale to Glencore. The zinc concentrate is transported by rail to Valleyfield, Quebec for further processing while the lead concentrate is trucked to the port at Belledune, New Brunswick, where it is subsequently shipped to designated smelters for processing.

Product from the grinding circuit, at nominal 35% solids by weight and pH 8.2, gravitates to the pre-aeration circuit for depression of the pyrite.

PRE-AERATION
The first two cells in the lead rougher bank are DR500 units which are used to pre-aerate the slurry and tarnish the iron minerals to aid in their depression. There is no flotation in this circuit, as the cells are simply to provide aeration of the pulp.

LEAD ROUGHER-SCAVENGER
Discharge from the pre-aerators passes directly to the lead rougher bank, which consists of five Outokumpu 16 m3 (565 ft3 ) units and three additional DR500 units for recovery of the lead mineralization. The collector (3418A) is used to recover the lead mineralization selectively from the zinc, with Methyl Isobutyl Carbinol (MIBC) being used as the frother. The combined rougher and scavenger concentrate is sent for regrinding, while the scavenger tailings forms the majority of the feed to the zinc circuit.

LEAD REGRIND
The combined rougher-scavenger concentrate passes to the lead regrind circuit. Isa mill discharge combines with the cyclone overflow at nominal 12 µm to 15 µm as feed to the primary cleaning.

The primary cleaner concentrate undergoes secondary regrind in an additional M1000 Isa mill using 100 mm (4 in.) diameter cyclones as classifiers. Product, at nominal 8 µm to -10 µm, passes to secondary cleaning.

LEAD CLEANING
The primary cleaner consists of an eight cell rougher and a six cell scavenger, with all cells being DR300 units. The rougher concentrate passes to secondary regrind, while the scavenger concentrate is recycled back to the head of the primary cleaner. Primary cleaner tailings report directly to the primary cleaner scavenger via gravity.

The lead secondary cleaner consists of six DR300 cells, with total secondary concentrate passing to tertiary cleaning. Secondary cleaner tailings combine with the rougher concentrate as feed to the regrind cyclone ahead of the primary cleaner.

The cleaner scavenger tailings join with the rougher scavenger tailings to make up the feed to the zinc circuit.

LEAD DEWATERING
The final lead concentrate from the fourth cleaner is pumped to a 4.6 m (15 ft) diameter conventional thickener for recovery of the excess water.

ZINC CIRCUIT
Combined lead rougher scavenger and cleaner scavenger tailings will combine as feed to the zinc circuit.

CONDITIONING
A single conditioner is utilized as a reagent addition stage to allow activation and promotion of the zinc mineralization, and to adjust the pH for pyrite depression.

ZINC ROUGHER-SCAVENGER
Overflow from the conditioners gravitates to a combined zinc rougher-scavenger circuit of eighteen cells, with the rougher being a bank of twelve DR300 cells, and the scavenger a bank of six DR500 units.

ZINC REGRIND
The combined rougher and scavenger concentrate is pumped to a single-stage regrind ball mill. All mill feed reports to the cyclone feed box for classification in a battery of 254 mm (10 in.) diameter Krebs cyclones, with a product of nominal 80% passing 14 µm. The cyclone underflow recycles to the regrind mill, maintaining the regrind in closed circuit. The cyclone overflow passes by gravity to the cleaner circuit for upgrading to final concentrate specification.

ZINC CLEANERS
The cleaner circuit consists of a four-stage cleaning plant, with the final three stages of cleaning operating in closed circuit, with the tailings of each cleaning stage reporting to the feed of the previous stage. The primary cleaner operates in open circuit, with the tailings passing directly to the cleaner scavenger feed.

The primary cleaner consists of a single bank of eleven DR500 cells, with total primary cleaner concentrate product passing directly to the secondary cleaner bank of seven DR300 cells. Tailings from the secondary cleaner recycles to the regrind mill, while the total secondary concentrate passes to the tertiary cleaner, a bank of five DR300 cells operating in open circuit. Tailings report to the second cleaner feed pump, while the third cleaner concentrate is delivered to the fourth cleaner of four DR300 cells, operating as the final cleaner bank. Tailings recycle to the third cleaner feed pump, while the product, as final concentrate, will be delivered to the dewatering section.

ZINC DEWATERING
The final zinc concentrate from the fourth cleaner is pumped to a 12 m (40 ft) diameter conventional thickener for recovery of the excess water. The underflow, at approximately 60% solids, is pumped to a holding stock tank ahead of the VPA 1515-33 plate and frame filter. Filtrate from the filter operation is recycled to the zinc thickener. Thickener overflow is pumped to the tailings pond.