The Caribou deposit is a volcanic hosted massive sulphide deposit (VMS) typical of the The Bathurst Mining Camp (BMC).

The Caribou deposit consists of the following units from the base upward:
(1) dark gray to black carbonaceous shale, pale gray phyllite, graywacke, and chloritic schist interbedded with hydrothermally altered pale green felsic volcanic rocks (footwall of the deposit);
(2) stringer sulfides cutting hydrothermally altered sedimentary and felsic volcanic rocks; (3) massive sulfides comprising a vent complex and bedded sulfides;
(4) chloritic schist at the contact between massivesulfides and overlying felsic volcanic rocks; and
(5) interbedded felsic volcanic and sedimentary rocks.

Mineralization within the Caribou deposit is composed of seven lenses that are zoned mineralogically and chemically from a copper-rich vent-proximal 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. Although it is impossible to rule out one large sulphide lens that was dismembered during deformation, the common clustering of modern and ancient deposits in vent fields is consistent with multiple vent sites at Caribou.

The vent complex consists predominantly of pyrite + pyrrhotite + chalcopyrite + magnetite, whereas the bedded sulphides are composed of pyrite + sphalerite + galena + arsenopyrite + tetrahedrite. The massive sulphides are zoned such that copper, bismuth, cobalt, europium/europium*, and copper/ (copper + lead+ zinc) decrease, and zinc, lead, silver, tin, indium, arsenic, antimony, molybdenum, cadmium, mercury, and gallium increase, from the vent complex to the bedded sulphide facies. Compared to bedded sulphides, sphalerite, galena, and pyrite in the vent complex are also iron-, silver-, and arsenic-rich, respectively.

The western sulphide lenses at Caribou are underlain by a sulphide stringer zone that is composed mostly of pyrite + quartz + siderite impregnations and veins with iron chlorite selvages cutting hydrothermally altered felsic volcanic and sedimentary rocks. Because the rocks are highly strained, the sulfide stringer zone appears strata bound and individual sulphide veins occurs sub-parallel to bedding.

The massive sulphides consist of seven en échelon lenses, numbered 10 to 80 around the Caribou fold. The zones 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.

The Caribou Mine has been previously developed and mined by different owners, employing a variety of mining methods. On November 2, 2012, Trevali gained control of Caribou through the acquisition of Maple Minerals Corporation. Trevali commenced underground mining in 2015 and commercial production was declared for the Caribou Mine and processing plant effective July 1, 2016.

Modified Avoca is the main mining method, supplemented by uphole retreat for partial sill pillar recovery. Modified Avoca stopes employ unconsolidated waste rock and surface stockpiled waste rock as backfill. Crown pillar recovery is not currently considered in the mine plan due to a lack of geotechnical information. 45-tonne capacity underground haul trucks transport the mined material through the underground ramp system and out of the mine through the conveyor portal, where surface stockpile pads, crusher and the process plant is located. On February 1, 2017, the Company announced, that as part of the planned transition from contract mining to an owner-operated model, the Company would procure a new underground mining fleet. The Company subsequently committed to an investment of approximately C$20 million to supply and maintain a full fleet of Sandvik supported mining equipment for the Caribou Mine. The fleet of equipment started arriving at site in May 2017. Increased operating efficiencies were seen in the new underground haul trucks and scoop trams of 23% and 4.3%, respectively. In addition, mine dilution has decreased to 8.4 % from 18.2% in the first half of 2017 due to the new cable bolter performance and efficiency.

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 includes crushing, screening, and grinding followed by lead/zinc flotation, and filtering to produce separate zinc and lead concentrates with silver as a by-product.
Concentrates are shipped through the Port of Belledune, located 75 km from the Caribou mill site.

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,long 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.