The Halfmile and Stratmat 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.

At Stratmat, RoM material will be mined by underground mining, too, similar to Halfmile underground mining except there is no PPCF/MCF mining method employed. RoM materials will be directly hauled by 45-tonne capacity underground trucks to Stratmat DMS plant for waste separation.

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 Stratmat project.

The most of the S1 Deep zone will tend to use transverse Sublevel open stoping (SLOS) supplemented with longitudinal Sublevel Retrea (SLR) open stope mining method when mineralization width is less than 8–12 m. All other zones will be mining using SLR. A small portion of mineralization can be mined using Uppers such as relatively isolated stope and extremity of strike length. A sub-level spacing of 20 m has been selected for these zones. Backfill for the transverse stopes would be cemented rock fill (CRF) for primary stopes and RF and/or CRF 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 Main zone and S1 Shallow zone, instead of the more typical SLR. Modified Avoca is an alternative of longitudinal retreat mining method, which has been successfully implemented in Trevali’s Caribou and Santander operations.

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 used for primary grinding followed by a conventional ball mill for secondary grinding to liberate the minerals to allow flotation to concentrate the metals to saleable concentrates.

Also included in the process flowsheet is the application of Isamills for regrinding which eliminates the use of steel media and the associated difficulties in subsequent cleaner flotation operations.

The grinding and flotation plant equipment has been sized to provide a capacity to treat nominally 3,000 tonnes per day of plant feed with a design factor of 8%. At this production, the annual throughput capability will be 1.12 million tonnes assuming operating at 92% of the time for 350 days per year.

For the this study it has been assumed that primary crushing, secondary crushing and screening will be contracted out. The crushed product will be fed to a covered stockpile with a live content of 4,000 tonnes and the DMS plant will be fed using two apron feeders in a tunnel under the stockpile at 175 tonnes per hour. The dense media process is expected to reject 22% of the mass of RoM material in a reject which will be suitable for producing backfill with the addition of the required amount of cement. As only the +3 mm fraction of the RoM material can be upgraded, the -3 mm fraction will be screened out on a wet screen, thickened and the thickener underflow pumped to the ball mill discharge pump box or to a lagoon if the DMS plant is operating and the main plant is down. DMS upgraded material will be conveyed to a second stockpile with a live content of 3,000 tonnes.

Design feed tonnage to the grinding circuit is 145 tonnes per hour. The BAG mill will operate in closed circuit with a vibrating screen recycling the +5 mm fraction back to the BAG mill. Screen undersize will flow to a secondary ball mill which will operate in closed circuit with 15-inch diameter cyclones and will grind the material to a P80 of 72 microns.

The ground material will be conditioned by aeration in flotation cells followed by Pb/Cu rougher and scavergener flotation with subsequent zinc activation and zinc rougher flotation from the tailings from the lead/copper flotation. Cleaner circuits will utilize Isamills for regrinding in both the lead/copper and zinc circuits. As noted above, this technology provides highly efficient grinding of fine concentrates and also largely eliminates the iron contamination of mineral particle surfaces as a ceramic grinding media is employed.

The lead/copper separation circuit design has been based on that used in the Brunswick mill with the exception of heating the process stream as it may not be practical at this location. To compensate extra retention time in the conditioning circuit has been included in the design. Column cleaning has been specified to provide efficient froth washing of the lead/copper and copper concentrates and recirculation sparging columns will be specified. Zinc rougher and first cleaner flotation will be carried out in conventional cells but, as with lead and copper cleaning, the final stage will be in a recirculation sparging column.