The Halfmile and Stratmat projectss are volcanic massive sulphide (VMS) deposits typical of the Bathurst Mining Camp (BMC). The BMC hosts 45 volcanic-sediment hosted massive sulphide deposits and 95 occurrences, including the world-class Brunswick 12 Mine. BMC deposits 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 VMS deposits typically form lenses of polymetallic massive sulphide. Most deposits are zoned vertically and laterally from a high temperature, vent-proximal, copper-polonium bismuth-rich veined and brecciated core to vent-distal zinc-lead-silver-rich hydrothermal sediments. The vent complex is commonly underlain by a highly deformed sulphide stringer zone that extends hundreds of metres beneath deposits and consists of veins and impregnations of sulphides, silicates, and carbonates that cut chloritized and sericitized volcanic and sedimentary rocks.

The sulphide minerals consist of disseminated and massive pyrite-sphalerite-galena and chalcopyrite. The sulphide minerals are fine- to medium-grained, and are coarser than those typically found in deposits of the BMC.

Disseminated mineralization, commonly of economic grade, occurs in the phyllitic sedimentary rocks as well as in the talc layers which locally grade into massive sulphide. A layer of massive pyrite-chalcopyrite, typically less than 1 m thick, occurs locally on the stratigraphic footwall side of the economic mineralization.

Stratigraphic relationships are based on observations of drill and rock exposures. The copper-rich layer may be in contact with, or grade into pyrite-poor massive sphalerite galena ore that locally is up to a few meters thick (grades average 5 to 15% lead and 15 to 35% zinc). Gangue minerals include muscovite, talc, chlorite, quartz, and carbonates. Many of the copper-rich and lead-zinc-rich massive sulphide layers are separated by talcose or phyllitic zones that commonly carry disseminated sulphide mineralization. Calcite is the most common carbonate, but ferroan dolomite and siderite also are present.

At Halfmile, all run-of-mine (RoM) material will be mined by underground mining method. Sublevel open stoping (SLOS) and Sublevel Retreat (SLR) will be the main mining methods planned, supplemented by post pillar cut and fill (PPCF) and mechanized cut and fill (MCF) mining for Upper zone where mineralization dip angle is approximately at 39 degrees. All mining methods will employ either cemented or unconsolidated waste rock as backfill. The mine plan includes 92% of RoM tonnes from SLOS/SLR with down holes (up holes for sill pillar recovery) on 20 m sublevels and cemented or unconsolidated waste rock fill, and 8% of RoM tonnes from PPCF/MCF with unconsolidated waste rock and cemented waste rock (sill level stopes only). There is no permanent sill or rib pillar considered for the SLOS/SLR mining. Limited sill pillars were planned only for the two trial mining opened mining fronts.

The Halfmile portion of the Halfmile-Stratmat integrated project consists of a ramp access underground mine with a mine life of 13 years accessed from an existing portal. The maximum underground production rate is set at 0.91 million tonnes per annum (Mtpa), or 2,600 tonnes per day (t/d) except for the years 2028 and 2029. Additional development has been scheduled prior to these years in order to develop enough stopes to allow the mine to produce at 0.98 Mtpa , or 2,800 t/d for those two years in order to offset the effect of the Stratmat Mine closing. This effort can only be maintained for a short time before the Halfmile Mine production starts to taper off as the currently defined resources are exhausted.

Due to the complex geometry and variability in strike and dip, the development of a tool box of mining methods is recommended in order to maximize extraction of the Halfmile project.

Post Pillar Cut and Fill:

The Upper zone will mainly be extracted using the post pillar cut and fill (PPCF) method, utilized in 2012 for the trial mining. This will be supplemented by mechanized cut and fill method in narrower sections. Levels are nominally on a 30-metre vertical spacing with each level accessing up to five 5-metre high cuts. Backfill in this area will be largely unconsolidated RF, though use of CRF has been proposed for the initial lift of each mining front to allow for sill pillar recovery and to provide better working floors. The two sill pillars established in 2012 will not be recoverable.

Most lifts are designed with 5 m high x 8 m wide rooms with 13 x 5 m barrier pillars either side of the access drift and 6 x 5 m standard pillars.

The introduction of cemented backfill for the Lower and Deep zones presents several opportunities in the Upper zone that are not currently reflected in the mine plan. By filling the initial cut of each stoping block with a good quality cemented backfill, an additional 50–100 kt of mineralized materials could potentially be recovered from the 7 m thick sill pillars. Further geotechnical investigations and modelling would be required.

Mechanized Cut and Fill:

Mechanized cut and fill (MCF) stopes above 5320L are extensions of the PPCF stopes where the Upper zone is less than 10 m horizontal width. One small area of the Lower zone on the 5180L was also designed as MCF.

Transverse and Longitudinal Retreat Longhole:

The Lower and Deep zones will tend to use transverse sublevel longhole open stope mining method and longitudinal sublevel retreat longhole open stope mining method when the mineralization width is less than 10–15 m. A sub-level spacing of 20 m has been selected for these zones. Backfill for the transverse stopes would be CRF for primary stopes and CRF and/or RF for secondary stopes. Backfill for the longitudinal retreat stopes would be a mix of CRF with RF.

A modified Avoca method, could also be utilized in some areas of the Lower and Deep zones, instead of the more typical SLR. Modified Avoca is an alternative of longitudinal retreat mining method.

The PEA is based on the construction of a new concentrator at the Stratmat site to process both the Stratmat mineralization and the Halfmile mineralization which will be trucked to the Stratmat site. The new concentrator will have a capacity of 3,000 t/d and employ conventional differential flotation technology to produce three saleable metal concentrates of zinc, lead, and copper. For both the metallurgical performance and the operating costs of a new concentrator, reference has been made to the current operations of Trevali at the nearby Caribou Mine.

The RoM production will initially be subjected to primary and secondary crushing at the Stratmat mine site to reduce the rock size to a suitable feed for a dense media separation plant (DMS). This operation will reject barren material from the +3 mm size from the mill feed effectively increasing the mill head grade and reducing the overall operating costs of the concentrator. A Barely Autogenous Grinding (BAG) mill will be us ........