Summary

Deposit type

• Porphyry

Summary:

Mineralization
The primary hypogene minerals are chalcopyrite, bornite, chalcocite, and minor pyrite. Copper sulphide minerals occur mainly as disseminations, foliaform stringers and as net-textured copper sulphides. The intensity of copper sulphide minerals increases with ductile deformation. The highest-grade mineralization occurs as semi-massive net-textured intergrowths of bornite and chalcopyrite. Typical bornite-chalcopyrite ratios are 3:1, and net-textured bornite is especially abundant in melanosome (mafic sections), where it forms higher grade (1 - 2% Cu) domains. Covellite locally occurs rimming bornite. Both bornite and chalcopyrite are commonly replaced by secondary digenite. Molybdenite locally occurs intergrown with net-textured copper sulfides.

Hessite (a gold telluride), native gold, and electrum occur as inclusions in bornite, accounting for high gold recoveries in copper concentrate. Coarse free gold has also been identified in late chloritic fractures, which may be the result of secondary hydrothermal enrichment. Copper sulphide mineralization is almost always associated with elevated biotite and magnetite.

At the Area 2, Area 118, Copper Keel, and Minto East deposits, mineralization occurs mainly as disseminated and foliaform grains, and the net-textured domains are generally absent. The mineralogical assemblage consists mainly of chalcopyrite-bornite-magnetite and minor pyrite.

The mineralogy of the Minto North deposit differs from the other deposits. At Minto North bornite is dominant over chalcopyrite and occurs as net-textured domains to massive lenses up to 2 m thick. Precious metal grades are elevated, and rare visible gold also occurs.

At the Minto North 2 deposit the dominant copper assemblage is chalcopyrite-bornite-chalcocite. Chalcocite commonly occurs as disseminated or local intergrowths with magnetite.

At the Ridgetop and Copper Keel South deposits, mineralization is subdivided into a near-surface horizon of supergene oxide, and a lower zone of more typical sulphide mineralization. The supergene copper oxide mineralization is characterized by malachite, chrysocolla, and local azurite. Oxidized magnetite and pyrite are also common. The mixture of oxide material with sulphides is commonly referred to as ‘POX’ (partially oxidized material). The lower zone is marked by an assemblage of chalcopyrite, magnetite, minor pyrite, and only minor amounts of bornite. Chalcopyrite occurs as disseminations and foliaform stringers. Magnetite is present as disseminated grains, local stringers and bands up to 0.3 m in thickness.

Copper grades increase progressively northwards from the lower grade material found at the Ridgetop towards the highest-grade material at the Minto North deposit (Mercer and Sagman, 2012). This trend is also observed on a regional scale, indicated by lower grade (chalcopyritedominant) mineralization of the Carmacks Copper deposit, progressively increasing northwestward in grade towards the bornite dominant higher-grade Minto deposits. This change in grade is likely caused by the increasing northward metamorphic gradient responsible for higher copper grades.

Deposit Types
Since discovery of the Carmacks Copper and Minto deposits in the 1970s, several models have been proposed for their genesis, including (1) copper mineralization in digested Triassic volcanic rocks (A. Archer, pers. comm., in Sinclair,1977), (2) metamorphosed red-bed copper (Kirkham, 1974), (3) deformed and metamorphosed porphyry copper-gold (Pearson and Clark, 1979; Tafti, 2005),(4) iron-oxide copper gold (IOCG; Mercer and Sagman, 2012), and (5) a shear-hosted hydrothermal system generated in the ductile root zones of a porphyry system (Hood, 2012).

The most current geologic and geochronologic constraints require that mineralization was an inherited feature of a Late Triassic protolith, which was subsequently metamorphosed in the latest Triassic and texturally modified during subsequent magmatism in the Early Jurassic. For this reason, a syn-metamorphic or syn-intrusion model for mineralized material formation is unsupported. Furthermore, deep emplacement of the Minto pluton cannot be used as a proxy for the emplacement depth of mineralization at the Minto mine, as the emplacement of the Minto pluton postdates mineralization by >10 Ma. Similarly, the oxidation state of the pluton and the widespread presence of alteration hematite (Tafti, 2005; Hood et al., 2008) i irrelevant to the deposit that formed >10 Ma prior to the emplacement of the pluton. Lastly, the structurally controlled distribution of mineralized material is also not a demonstrably primary feature of either of the deposits, as material was melted and remobilized during the emplacement of the Minto pluton. In addition, the intensity and extent of alteration that is common in IOCG deposits is not well-developed at Minto and mineralized zones are not breccia hosted. As such the IOCG deposit model is not considered viable for the Minto deposit.

The recognition that the least deformed and migmatized host rocks at the Carmacks Copper deposit contain low-grade, disseminated Cu as a chalcopyrite-pyrite assemblage hosted in biotite-bearing and K-enriched host rocks is consistent with a porphyry copper deposit model. Hypogene grades from ~0.2 to 1% Cu and ~0.1 to 1 g/t Au at the Carmacks Copper, Minto, and Stu systems are within the range of typical porphyry copper grades globally (e.g., Kesler et al., 1992). The caveat to this is that post processes may have affected grade. Copper to gold ratios of 23,000 to 34,000 are also typical of gold-bearing porphyry copper deposits. Although no intrusive phases related to the pre-metamorphic hydrothermal system are recognized at Carmacks Copper, it is permissible that the population of 217.53 ± 0.16 Ma igneous zircons represents magmatic activity temporally and genetically related to >212.5 ± 1.0 Ma copper mineralization.

Hydrothermal features such as veins, alteration halos, or hydrothermal breccias are not recognized through the overprinting effects of metamorphism, penetrative deformation, and partial melting. However, the general lack of quartz rich domains within metamorphic rocks suggests that quartz-sulphide veins were likely absent from the protolith. It is therefore likely that protolith mineralization was introduced as disseminations or as sulfide dominant veinlets in conjunction with widespread biotite - magnetite alteration. Together, these observations suggest that the Carmacks Copper and Minto deposits each preserve the high-temperature potassic core of a porphyry copper system. Several features listed above are also consistent with alkalic porphyry affinity:
1. Low abundance of pyrite;
2. Association with alkaline intrusions;
3. Low volume or absence of hydrothermal quartz; and
4. Cu-Au metal tenor (compared to Cu-Au - Mo in calc alkalic porphyry systems).

The interpretation of the Carmacks Copper and Minto deposits as metamorphosed porphyry copper systems is further supported by their temporal and lithotectonic affinity with porphyry belts in British Columbia. First, correlation of metavolcanic host rocks at Carmacks Copper with Stikinia arc equivalents in Yukon (Kovacs, 2018) supports a similar tectonic and geodynamic setting to porphyry systems in British Columbia. Second, the ~217 to 213 Ma age of mineralization at Carmacks Copper constrains the system to within the prolific 227 to 178 Ma epoch of porphyry Cu mineralization in the Stikinia and Quesnellia arcs of British Columbia, and broadly coincident with peak productivity in Stikinia (e.g., Schaft Creek ~222 Ma, Galore Creek ~210–205 Ma, Red Chris ~204 Ma; Logan and Mihalynuk, 2014, and references therein).

Mining Methods

• Longhole upper cut retreat

Summary:

As at 2021:
In the past, the operation performed mostly open pit mining using a standard truck and shovel fleet, which was contractor operated. Since the restart of operations in 2019, Minto has undertaken only underground mining, using a ramp access system with longhole upper cut retreat mining with standard rib and pillar support. The longhole stopes are left open when completed. Underground mining is planned for Copper Keel, Minto East, Area 2 Minto North and Ridgetop. Open pit mining is planned for both Ridgetop and 118 deposits.

Underground Mining Methods
All the underground mineralized zones can be described as lenses of foliated metamorphic rocks bounded at their hanging wall and footwall contacts by under formed granodiorite host rock. The metamorphic zones are typically 5 m to 30 m thick. These zones typically dip at 20° to 35°. Typical depths are 200 m to 250 m below surface, with vertical thickness of 5 m to 25 m.

The mineralized zones bifurcate, which means that a mineralized zone can contain a significant amount of waste, or that thinner zones can merge with larger zones. A bifurcating geometry complicates geological modelling and may increase internal dilution. In Copper Keel, the footwall can undulate, and the operation has undertaken drilling to better delineate the footwall contact. The width and dip of mineralized zones are locally variable.

The change in thickness might be as much as an order of magnitude over less than 30m in horizontal distance. At least some of the irregularity in the geometry and thickness of the mineralized zones is due to small-scale and large-scale structural displacements.

This study utilizes the current method of underground extraction of longhole upper cut retreat mining with rib pillars. This method is considered appropriate for the modelled geology.

Mine Design
Historically the M-zone, Area 118, Area 2 and Minto East zones were all mined using a longhole upper cut retreat mining method. The currently active Copper Keel zone is mined in the same manner.

The mining method requires a series of parallel sill drifts to be developed along the strike of the deposit, following the footwall contact. From these 6 m wide and 4.3 m high sill drifts, a top hammer longhole machine drills rings of 3” diameter up-holes into the deposit, drilling to the hanging wall contact. To provide adequate void space for blasted muck when initiating a new stope, a 1.8 m x 1.8 m inverse raise is drilled. The raise is composed of six 6-inch diameter reamed holes, which are left unloaded, surrounded by a pattern of eleven 3½-inch diameter blast holes. Generally, each stope is initiated with one or more rings of blast holes on either side of the inverse raise; subsequent blasts increase the number of rings fired simultaneously to take advantage of the void space in each block.

Production drift centerlines are spaced 20 m apart. From each 6 m wide sill drift, drill holes are fanned out to blast a 14 m wide stope. Rib pillars with 6 m width separate neighboring stopes and support the hanging wall. For the Area 2 zone, production drift centerlines were 20 m apart and stope and pillar widths were varied based on the thickness (stope and pillar height). Historically typical stope widths were 15 m and pillar widths were 5 m.

The mining method does not use backfill. Small quantities of development waste are sometimes placed in completed stopes to reduce waste haulage requirements to surface. For purposes of this study all excavated material is assumed to be hauled to surface.

Underground Access
The main ramp of the Minto Underground extends to the currently active Copper Keel zone. The upper ramp is 5.0 m wide and 5.0 m high; the ramp below the 690 level has been driven at dimensions of 5.0 m wide and 5.5 m high to provide additional clearance between vent ducting and haul trucks. This access is currently used for all mineralized material and waste haulage, personnel/equipment access, and services. It is also used as an exhaust airway. Additional ramps to surface have been proposed for both Copper Keel and Ridgetop. This will assist with both haulage and ventilation of the mine. Minto North will be accessed via a single ramp collared from the Minto North eastern pit wall. Ramps and access development will be driven at a maximum +/- 15% grade.

Stope Layout
The 6.0 m(w) x 4.3 m(h) crosscut drift has typically been designed on the down-dip side of the stope to better manage dilution and increase mining recovery. Longhole stopes have been designed at 14 m wide with a 6 m rib pillar between stopes.

Comminution

Crushers and Mills

Summary:

As at 2021:
Crushing
Currently, at the Minto Mine, crushing is performed through a contract with Kode Contracting. Under the current contract, Minto is responsible for feeding into the contractor’s crushing circuit, which will then crush 4,000 tpd of mineralized material to the target P100 of 19 mm. Prior to feeding into the circuit, Minto Mine employees remove any large pieces of metal from the mineralized material with an excavator.

The contract crushing circuit consists of an ELRUS 3042 Jaw Plant, 2 Cedar Rapids MVP 450 cone crushers, an Elrus 6x20 Screen Plant and the associated conveyor belts to move rock through the circuit. The mineralized material produced by the contract crushing plant is then conveyed by the Minto Mine stacking conveyor to the coarse mineralized material stockpile.

The Minto crushing plant, which is currently not operational, consists of a Westpro,40”X50” jaw crusher powered by a 300 HP motor and an Elrus (Sandvik) S4800 cone crusher powered by a 300 HP motor. This circuit was sized to produce 3,000 tpd of the mineralized material at the original mill feed size of approximately 115 mm.

Grinding
The grinding circuit at the Minto Mine consists of a 16.5-foot diameter by 5-foot-long SemiAutogenous Grinding (SAG) grinding mill powered by a 900 HP motor, two 12.5’ diameter by 10’ long overflow type ball mills, each with a 900 HP motor and two sets of 3 – 15 inch cyclones.

Material is fed to the SAG mill from the SAG feed conveyor, a 36” wide by 137-foot-long belt with a tonnage control system. The SAG mill is operated at a solids density of 77% Solids and uses a mixture of 5” and 3” steel grinding balls to reduce the particle size from a P80 of 12.7 mm to a P80 of 1.7 mm. The SAG Mill discharge slurry is then pumped to the cyclone feed pumpboxes for each of the ball mill circuits.

In the Ball Mill circuit, the SAG discharge slurry is combined with the Ball Mill discharge and water is added to achieve a density of 55% solids. The slurry is then pumped to the cyclones. The cyclone underflow is fed to the ball mill for further grinding at a density of 70% solids. The cyclone overflow from both mills is combined and fed to the rougher flotation circuit.

The grinding circuit produces a flotation circuit feed with particle size P80 of 250 µm and has a solids density of approximately 35% solids.

Processing

• Flotation
• Dewatering

Summary:

As at 2021:
The processing plant at the Minto Mine was constructed in 2006-2007 and commercial production was declared in October 2007, after a four-month long commissioning period. The processing plant operated continuously until it was placed on care and maintenance in October 2018, following discontinuation of mining operations. The operation was acquired by the Pembridge Resources, and the processing plant was recommissioned in October 2019.

The Minto process plant design incorporates standard industry comminution, flotation and dewatering circuits to produce a final copper concentrate product. The plant was designed to achieve an availability of 92% and has demonstrated the capacity to process an average of 4,000 dry metric tpd with a peak operation at 4,400 tpd sustained over a few days.

Flotation
The flotation circuit is comprised of 4 groups of flotation cells: Rougher, Scavenger, Cleaner and Re-cleaner.

Rougher flotation is conducted using three 1,350 ft3 tank cells each powered by a 75 HP motor. The rougher concentrate is pumped to the Cleaner circuit.

The rougher tailing is pumped to the Rougher-Scavenger circuit, a bank of four 500 ft3 flotation cells, each powered by a 40 HP motor. The Rougher-Scavenger concentrate is pumped to the cleaner circuit. The Rougher-Scavenger tailing is the final tailing from the flotation plant and is pumped to the tailings building.

The Cleaner circuit consists of a bank of four 350 ft3 tank cells each powered by a 25 HP motor. The Rougher concentrate, Rougher-Scavenger concentrate, and the Re-Cleaner tailings are combined in the Cleaner circuit feed. The cleaner concentrate is pumped to the Re-cleaner circuit. The Cleaner circuit tailings are gravity fed to the Rougher-scavenger circuit.

The Cleaner concentrate is pumped to a series of six 100 ft3 Re-Cleaner cells, each with a 15 HP motor. The re-cleaner concentrate is gravity fed to the concentrate thickener and dewatering circuit. The Re-Cleaner tailings are pumped to the Cleaner circuit.

Precious Metals Recovery
Currently, precious metals are recovered to the flotation concentrate as a by-product of flotation.

The Minto milling circuit includes a centrifugal “Knelson” concentrator which is currently not operational. The centrifugal concentrator works to concentrate gold by the difference in density between gold and the host rock, typically referred to as gravity separation. When operating, the Knelson concentrator will discharge a gold concentrate which will be pumped to the concentrate thickener.

The Knelson concentrator is located in the circulating load of the ball mill as is typical for this type of recovery device.

Concentrate Dewatering
The concentrate travels by gravity to an Outotec 9.4 m diameter Supaflo high-rate thickener. The concentrate is thickened to approximately 70% solids density.

The concentrate is pumped to a 55 m3 stock tank prior to filtering. The concentrate is pumped from the concentrate stock tank to a Larox 30 m2 ceramic filter with 10 discs for a total filter area of 300 m2. The dewatering rate averages 25-30 t/h with a typical product moisture content of 7 - 8 %.

The filter cake is transported to a concentrate storage shed via a 24” by 38’ long concentrate discharge conveyor powered by a 15 HP motor. The concentrate storage shed has a capacity for storing 18,000 t of flotation concentrate.

The concentrate is loaded into 25 or 50 t trucks and transported to Skagway, Alaska from which it is shipped to market. The road to access site crosses the Yukon river and concentrate transport is halted 2x per year for approximately 6 weeks each while the crossing is switched from summer to winter operation and vice versa.

Water Supply

Summary:

As at 2021:
Water used for mineral processing is provided from the mill pond, tailings thickener, and the water storage dam. After processing is complete, water exits the mill through concentrate, tailings, and the mill pond. Water extracted from the dry-stack tailings facility is sent to the water treatment plant where the water is treated and released into the environment.

Seepage water is pumped back up to the Main Pit where they are discharged into the water stored in the Main Pit.

The current plan is to process excess water through the Water Treatment Plant for several months each spring, summer and fall and then discharge the treated water into Minto Creek.

Inflow to the mine through faults and un-grouted core drill holes is generally sufficient to supply the mine with water for drilling and dust control. The mine currently recirculates water through a network of sumps. There are no issues regarding shortage of service water supply to date.

Water Treatment
Water treatment is required if:
• Water stored in the WSP does not meet water quality limits prescribed in the water licence;
• Flows in Minto Creek are too low to accommodate material discharge volumes from the Mine Site; and/or
• Mine water stored in the MPTMF or the A2PTMF is to be released to Minto Creek and it does not meet water quality limits prescribed in the water licence.

The WSP is intended as a clean water reservoir where discharge compliant water is stored and monitored prior to release to Minto Creek. However, storm events, seepage losses from the mill valley or other events may cause the concentrations of certain water quality parameter to exceed water quality limits. Mine water stored in the MPTMF or the A2PTMF will likely require treatment, and where WTP water meets WQO then it would be released directly (not via WSP).

Defining Mine Water Inventory Target
The mine water inventory target is defined based on the following considerations:
• As a guiding principle, a minimum of 600,000 m3 of free water, or one year of operational water demand, should be stored on site at all times. This water inventory will ensure that the operation has adequate supply of water and that the mine water reservoir has sufficient residence time to allow for proper settling and management of suspended solids;

• Storage capacity to hold a minimum of 1,000,000 m3 of mine water runoff must be available on October 31st of each year (per Water Use Licence condition). This storage capacity is estimated to represent water that would report to the MPTMF or the A2PTMF in a 1 in 200 wet year (approximately 1,400,000 m3 of site-wide runoff minus approximately 400,000 m3 diverted); and

• Climatic variability. The water and load balance model developed for the Minto site will be used to evaluate the mine water inventory targets against a range of precipitation and runoff events (dry and wet years).

The inventory targets are used by water operators as a basis for deciding whether to release additional water from site or to scale back water release. Inventory targets are updated every 6 months or as required.

Commodity Production

Operational metrics

Personnel