McIlvenna Bay is a VMS deposit, of a type commonly found in Canada in Precambrian through Mesozoic volcano-sedimentary greenstone belts occupying extensional arc environments such as a rifts or calderas. They are typified by synvolcanic accumulations of sulphide minerals in geological environments characterized by submarine volcanic rocks. The associated volcanic rocks are commonly relatively primitive (tholeiitic to transitional), bimodal and submarine in origin (Galley et al., 2006). The spatial relationship of VMS deposits to synvolcanic faults, rhyolite domes or paleotopographic depressions, caldera rims or subvolcanic intrusions suggests that the deposits were closely related to particular and coincident hydrologic, topographic, and geothermal features on the ocean floor (Lydon, 1990).

McIlvenna Bay consists of structurally modified, stratiform, volcanogenic, polymetallic massive sulphide mineralization and associated stringer zone mineralization. The sulphides contain copper and zinc, with low lead and silver and gold values.

McIlvenna Bay has undergone strong deformation and upper greenschist to amphibolite facies metamorphism. The massive sulphide lenses are now attenuated down the plunge to the northwest. Typical aspect ratios of length down-plunge to width exceed 10:1. The extent of remobilization of sulphides within the deposit is uncertain.

McIlvenna Bay comprises five different zones and includes three distinct styles of mineralization. The five different zones identified are the Lens 2, the Upper West, Lens 3, and two separate Copper Stockwork Zones. The three different styles of mineralization are massive sulphides, semi-massive sulphides, and stockwork. Each style is mineralogically and texturally distinct.

The McIlvenna Bay deposit dips to the north at 65° to 70°, although in selected areas it dips vertically. The average strike length of the economic deposit is 600 m with an average thickness of approximately 14-17 m. In areas where mineralized zones run parallel to each other, the distance between the host rock hangingwall and footwall can exceed 50 m.

It has been identified that due to distinctly different metallurgical properties between the different mineralization zones, it is economically beneficial to mine them separately.

JDS selected sub-level long hole (LH) stoping with cemented paste backfill as the principal mining method at McIlvenna Bay due to its high productivity, low cost, appropriate level of selectivity, and successful history of application for deposits of this nature, such as the Hudbay Mineral Inc. nearby 777 operation, which produces 4,330 tpd from their underground operation (Hudbay, Oct 2012).

Drift and fill mining may also be used at McIlvenna Bay for areas of the deposit which fall below an allowable dip for LH stoping.

LH stoping is a semi-selective and productive underground mining method, and well suited for steeply dipping deposits of varying thickness with good continuity and minimal small-scale variability is thickness, dip and strike. It is typically one of the most productive and lower cost mining methods applied across many different styles of mineralization. In the planned LH for McIlvenna, a top and bottom drift delineate the upper and lower boundaries of the stope and a dedicated long hole drilling machine drills blast-holes between the upper and lower sub levels. The drill holes would be loaded with explosives and the stope blasted in vertical slices, with broken material falling to the bottom drift for extraction. Once the stope has begun the blasting phase it cannot be accessed by personnel. For this reason, a tele-remote load haul dump machine (LHD) would be required to remove the blasted material from the stope.

One of the limitations with LH stoping is that the dimensions of the stope should not exceed a long hole drilling machine’s effective range which, for top hammer drill rigs, is generally 30 m. Another limitation with LH stoping is the stopes must remain open long enough to remove the mineralized material and fill with an engineered backfill material (if pillars are not used). These limitations generally restrict level spacing to 30 m or less, and subject stope strike lengths to geotechnical constraint.

Two methods of LH stoping are considered for McIlvenna Bay. Transverse stoping is planned to be the primary method, whereby crosscuts would cut through the stope perpendicularly, and long hole fans would drill off the strike length of the stope.

This method is beneficial for production rates as multiple stopes can be in operation at once on a level. It is also beneficial for mining parallel mineralization zones, as relatively clean separation between the mineralization types is possible with careful fan drilling.

The shortfall of transverse LH mining is that a footwall drift is required outside the mineralized zone for the entire strike length, and crosscuts must be driven long enough to maintain a safe distance between the footwall drift and the zone, which depending on geotechnical constraints can exceed 25 m.

The metallurgical processing selected for the different types of mineralization, CSZ, L2MS and UWMS, were designed to produce copper concentrate, zinc concentrate or a bulk concentrate as final products depending on the type of mineralization fed to the plant.

The 5,000 tpd process plant flowsheet design follows conventional crushing, a semi-autogenous mill with a pebble crushing circuit, a ball mill grinding circuit using cyclones for classification followed by a talc pre-flotation step to remove detrimental talc prior to copper/zinc/bulk flotation. Conventional sequential flotation for the recovery of copper, zinc and bulk concentrates is planned in the flowsheet. Rougher and scavenger flotation cells are designed to be used for zinc, while the copper and bulk circuits would have only rougher cells. Regrinding prior to cleaning is required for all mineral types. Each of the three types of mineralization requires three stages of cleaning following regrind to produce final concentrates grades. Each concentrate produced is designed to have a dedicated concentrate thickener and concentrate filter press. Concentrates ware planned to be shipped via bulk trucks. Thickener overflow from each concentrate thickener would be recycled back into its own flotation circuit to prevent cross reagent contamination.

Tailings from talc pre-float and the zinc flotation circuit is designed to be sent to a common paste backfill system followed by a tailings thickening. The thickener underflow would be sent for disposal in the tailings management facility. Process water recycled from the tailings thickener overflow and recovered water from the tailings management facility is envisioned to supply the required process water for the plant. Fresh water from the lake would be used for gland service and reagent preparation. The 5,000 tpd process plant is designed to consist of the following unit operations and facilities:
- Crushed material (from underground) bins;
- SAG mill incorporating a pebble crushing circuit;
- Ball mill grinding circuit and cyclones for classification;
- Talc pre-flotation;
- Copper/Bulk conditioning tank followed by rougher flotation;
- Copper vertical regrind mill and cyclones for classification;
- Copper three stage cleaner flotation circuit;
- Copper concentrate thickener;
- Copper concentrate filter press;
- Copper concentrate bulk storage and handling;
- Zinc conditioning tank followed by rougher and scavenger flotation;
- Zinc vertical regrind mill and cyclones for classification;
- Zinc three stage cleaner flotation circuit;
- Zinc concentrate thickener;
- Zinc concentrate filter press;
- Zinc concentrate bulk storage and handling;
- Bulk vertical regrind mill and cyclones for classification;
- Bulk three stage cleaner flotation circuit;
- Bulk concentrate thickener;
- Bulk concentrate filter press;
- Bulk concentrate bulk storage and handling;
- Backfill cyclones;
- Tailings thickening;
- Tailings deposition;
- Process water reclamation;
- Reagent preparation facilities; and
- Utilities.