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 us ........