Mineralization in the Keno Hill Silver District comprises carbonate vein hosted polymetallic silver-lead-zinc as described by Boyle (1965), Cathro (2006), Murphy (1997) and Roots (1997). It typically exhibits a succession of hydrothermally precipitated minerals deposited in veins or veinlets as a result of multiple pulses of hydrothermal fluid boiling events, probably related to repeated depressurization due to movement along the host fault structures. As a result, a progressive series of differing mineral depositional stages, assemblages and textures can be identified. To a minor extent, supergene alteration may have further changed the nature of the mineralogy in the veins, although this may have been largely removed due to glacial erosion.

The Bellekeno vein system consists of ten known veins with variable characteristics. Vein material has been extracted from the Ram, Eureka, Tundra, 48, 49, and 50 veins that generally strike 030° to 040°, with dip directions varying 60° southeast to 80° northwest. Recent mechanized mining has focused on the stronger 48 Vein structure, while conventional historical narrower mining focused on the smaller, higher grade vein structures.

There are three main zones within the 48 Vein structure: the Southwest, 99, and East zones, each with distinctive silver to lead ratios, zinc content, and accessory mineral assemblages.

The thickness of the vein ranges between a few cm to upwards of 5.5 m. Post-mineral faulting typically shows intense iron carbonate alteration and local brecciation while the distribution of syn-mineral faulting is observed to have a strong impact on silver grades and mineral textures. Left oblique-normal movement along the 48 Vein structure is estimated from stratigraphic offset to be approximately 35 m.

The mineralized zones appear as discontinuous steeply plunging shoots, hosted within manganese-rich siderite vein structures and may have pervasive secondary limonitic alteration where exposed to groundwater. Minerals of economic interest include very fine-grained silver-bearing sulphosalts associated with galena and sphalerite. Common accessory minerals include pyrite, arsenopyrite, and chalcopyrite while anglesite, cerrussite, smithsonite, malachite and azurite have been occasionally observed.

The mine plan for Keno Hill project comprises mining from four deposits (also referred to as “mines”); Bermingham, Flame & Moth, Bellekeno, and Lucky Queen. The majority of the ore will come from Bermingham and Flame & Moth (over 90%). Two mines will be operating at any given time, with the exception of the initial three month ramp up period of ore from Bellekeno only.

The mine plan is based on conventional mining methods. Based on the orientation, width of the veins, review of historic mining in the District, and geotechnical information, a combination of mechanized overhand cut and fill, and longhole stoping with cemented rock fill have been selected as the appropriate mining methods for all four deposits. The deposits require the use of mining methods that can adequately support the vein and that are flexible and selective while minimizing the direct mining costs. The backfill is planned to be a mixture of waste rock fill and tailings from the dry stack tailings facility (“DSTF”) with cement added as required.

Rehabilitation and capital development continues on schedule at each of the three underground mines slated for initial ore production, with emphasis on the Bellekeno Mine where initial ore will be used for commissioning of the mill in Q4 2020. All major pieces of new underground mine equipment have been delivered to site and are currently operating or are being commissioned. This new fleet includes two (2) CAT R1300 3.5 yard scoops, two (2) CAT AD22 20 tonne haul trucks, two (2) Atlas Copco 282 twin boom jumbos, two (2) MacLean SSB bolters and other support gear. Ore will first be delivered from the Bellekeno Mine where development to ore on the 680 level is already complete. [09/15/2020, PR, p.1]

CRUSHING
The crushing system will be operated for 8 h/d at a process rate of 50 t/h (up to 10 h/d at a process rate of 55 t/h). The ROM ore will be crushed in a two-stage crushing circuit closed with a vibrating screen. The final crushed product size P80 will be minus 10 mm. ROM ore will be dumped by 30 t haulage trucks into either a ROM ore stockpile of directly into the jaw crusher feed hopper with a 47 t capacity. The hopper is equipped with a stationary grizzly with a 450 mm aperture opening to prevent oversize ore entering the downstream jaw crusher.

The feed to the primary crusher, the jaw crusher, is controlled by using a vibrating grizzly feeder. The 508 mm by 914 mm jaw crusher with a 75 kW motor will crush the ROM ore from minus 450 mm to a P80 size of 50 mm with the crusher closed side setting of 70 mm. The primary crushing products, joined by the secondary crushing discharge, will feed the sizing screen to produce the final crushed product. The oversize product from the sizing screen will feed the secondary cone crusher.

The crushed material will be conveyed to a fine ore stockpile with 550 t live capacity. Crushed ore reclaim will be achieved via a draw down pockets located beneath the fine ore stockpile.

GRINDING
Crushed ore will be reclaimed from the stockpile onto the ball mill feed conveyor belt at a nominal rate of 18.1 t per operating hour (25 tpd). The primary grinding circuit is a two-stage ball milling process producing a ground material with a P80 size of 100 to 120 µm. The spillage sump and pump are located near the No.1 mill discharge pumpbox. Spillage will be returned to the ball mill feed hopper.

Ore from the ball mill feed conveyor will discharge into the feed chute of the primary No.1 ball grinding mill which will be a heavy-duty tyre driven type grinding mill 1,800 mm in diameter and 3,600 mm long, with an installed power of 150 kW. The primary mill slurry will discharge into a pumpbox and will then be pumped to join the secondary No.2 ball mill discharge. The combined streams will be pumped to the classifying cyclones. The cyclone underflow will
return as feed to the secondary ball mill, while the cyclone overflow will report to flotation circuit. The secondary No.2 ball mill has the same size and motor as the primary ball mill.

Grinding media as steel balls will be added as required via the ball mill feed conveyor. This conveyor will discharge the grinding media into the ball mill feed hopper. The ball mill feed conveyor belt will be sampled every shift to provide a head assay of the feed to the plant.
The reagents 3418A and zinc sulphate will be added to the ball mill feed to suppress zinc minerals during the lead mineral flotation stage.

The primary and secondary ball mill discharges will be combined in a common pumpbox and will then feed a hydrocyclone pack. Two cyclones will be installed each with a diameter of 250 mm. Operating in a mode of one running and one standby, the cyclone undersize material will return to the secondary grind ball mill by gravity, while the cyclone overflow with a P80 size between 100 and 120 µm will gravity flow to the flotation circuit.

Grinding media as steel balls will be added as required via the ball mill feed conveyor. This conveyor will discharge the grinding media into the ball mill feed hopper. The ball mill feed conveyor belt will be sampled every shift to provide a head assay of the feed to the plant.

The process described in this section represents 400 tpd mill throughput, as the plant will be operated at 400 tpd for the first two years before ramping up to 550 tpd. The values corresponding to the increased throughput for the 550 tpd are shown in parentheses following. Most of the plant modifications to allow it to achieve 550 tpd throughput target will be completed prior to mill commissioning.

SIMPLIFIED PROCESS FLOWSHEET
The mill complex consists of the following process sections and unit operations:
• primary and secondary crushing circuits with a radial stacker belt conveyor to transport the crushed ore to the covered fine ore stockpile;
• draw down pocket reclaiming the crushed ore from the covered fine ore stockpile;
• two-stage grinding in a closed circuit with hydro-cyclones to produce a grinding product P80 size of 120 µm;
• a grinding classification circuit to produce a cyclone overflow to feed the lead rougher and rougher scavenger fl ........