The deposits of the Coeur d’Alene District, including Lucky Friday, are classified by Beaudoin and Sangster (1996) as clastic metasediment-hosted vein silver-lead-zinc deposits. In addition to Coeur d’Alene, the world’s most prolific silver district, this deposit type embraces a number of historical mining localities including the Harz Mountains and Freiberg in Germany, Keno Hill and Kokanee Range in Canada, and Príbram in the Czech Republic. They are typified by the following general characteristics:
- Hosted in thick, monotonous sequences of fine- to medium-grained clastic sedimentary rocks transected by deep-seated regional-scale faulting;
- Sedimentary basins occur in a wide range of tectonic environments, but all have been subject to deformation, intrusion, and regional metamorphism, typically greenschist facies;
- Economic minerals are predominantly galena and sphalerite with minor accessory pyrite and a wide range of sulphosalt minerals including tetrahedrite, pyrargyrite, stephanite, bournonite, acanthite, and native silver;
- Gangue minerals typically comprise siderite and quartz with lesser amounts of dolomite or calcite;
- Comparatively low gold content;
- Temperature of sulphide mineral deposition in the range of 250°C to 300°C;
- Hydrothermal alteration constrained to a few metres of the veins and characterized by sericite, silicification, and pyrite.

The signature for all economic deposits discovered within the Coeur d’Alene District is vein-like morphology hosted within the meta-sediments of the Belt Super Group. In the Lucky Friday Complex, as well as other sub-districts in the Coeur d’Alene District, veins occur as branching fissures that cross-cut or invade the sedimentary bedding or host rocks. Previous studies have indicated the veins are mesothermal origin (Leach, 1982). The vein structures are known to branch, split or bifurcate, forming duplexing and anastomosing geometries. The majority of veins strike west-northwest, are steeply- dipping, elongated down-dip and can have strike lengths over 4,000 ft and dip lengths over 8,000 ft (Hobbs et al., 1965).

The Lucky Friday deposits are fissure-hosted silver-lead-zinc veins typical of the Coeur d’Alene District. Principal vein systems are the Lucky Friday and Gold HunterVeins. Economic mineralization consists of silver-bearing galena and tetrahedrite, with relatively minor amounts of sphalerite and chalcopyrite. These minerals occur in veins, fracture-fillings, and disseminations along with accessory pyrite and a gangue of iron carbonate (siderite), calcite, and quartz. Mineralization is strongly structurally controlled with a significant influence from the competency of the wall rocks. Ore bodies are best developed where faults and fractures intersect more siliceous and competent lithologies, and are less likely to occur in the comparatively incompetent argillites.

Lucky Friday Vein
Wall rock alteration consists of a weak carbonate zonation. Calcite dominates in areas distal to the mineralization that gives way to ankerite as the vein is approached and then is altered to siderite closer to the vein system. The geometry of this alteration depends upon wall rock porosity and permeability. Iron and magnesium in fluids flowing out from the vein systems altered the original calcite. Some host rocks contain dolomite, which compromises the alteration pattern. This alteration can be seen for distances of more than 300 ft from vein systems depending upon bedding orientations to the vein source. Additionally, disseminations of sulfide extend some distance into host stratigraphy. This sulfide material usually consists of galena, sphalerite, and tetrahedrite.

The Lucky Friday Vein has an economic strike length of up to 1,500 ft. The vein is a high-angle south-southeast dipping vein that varies from inches to as much as 20 ft in width. The average varies from four to six ft over the full economic length.

The source for this mineralization is current unclear. The Lucky Friday Vein is connected with the NCF and SCF, which are mineralized locally. These may have been the major structural controls, enabling mineralizing fluids to flow into the host fissure, which eventually became the vein.

The vein consists of both gangue and sulfide mineralization. It contains quartz and siderite with lesser amounts of pyrite and arsenopyrite. Ore minerals include argentiferous galena, sphalerite, and local tetrahedrite.

Mineral textures vary. Gangue minerals are often cataclastic for quartz and siderite with milling evidenced by rounded mineral grains. Quartz and siderite “eyes” are common in Lucky Friday ores. Sulfide textures vary from very fine-grained to coarse-crystalline.

A simplified paragenesis begins with early quartz carbonate, plus or minus sericite and pyrite, followed by sphalerite, and then by tetrahedrite and argentiferous galena.

Gold Hunter Vein System
Gold Hunter historic mining extending from the surface at +4,700 ft MSL elevation to the 4900 level at -1,510 ft MSL elevation demonstrates grade trends and variability. The vein zones are stacked and parallel to sub-parallel with often ill-defined mineralized lenses. The historic surface mining had bulk resource grades of 4.1oz/ton Ag and 3.9% Pb in three main lenses. On the 4050 level at the -675 ft MSL elevation there were five lenses at a bulk grade of 9.1 oz/ton Ag, 4.1% Pb, and 1.3%Zn. At 4900 level (-1,510 MSL) there are ten lenses at a bulk grade of 9.6 oz/ton Ag, 5.4% Pb, and 2.3% Zn.

Mine geologists report that silver, lead, and zinc bulk grades generally increase with depth. Lead and zinc content increases slightly relative to silver.

There are currently 101 definable, parallel veins identified in the Gold Hunter system. These vary in width and grade with the most productive being the 30-Vein. This vein has the greatest value, width, and economic length when compared to the other Gold Hunter veins. The 30-Vein averages more than four ft in width as a composite of closely spaced veins and veinlets. It strikes N83W and dips 80°S to vertically. The economic vein length is approximately 2,300 ft. This vein has yielded a significant percentage of total Lucky Friday Unit production since 1997 and has been largely mined out down to approximately 6000 level. The other “Intermediate” veins have shorter strike lengths and generally narrower widths. The distribution of silver, lead, and zinc varies randomly for each vein. Production from Intermediateveins is in the LOM plan and is anticipated to contribute proportionally more to the overall mill feed as time progresses.

As with Lucky Friday, the source of the Gold Hunter mineralization is not fully understood. The Gold Hunter zone’s downward projection eventually reaches an intersection with the Independence Fault, which hosted the Star Mine mineralization. Lucky Friday geologists consider that this fault may be the source structure for the Gold Hunter mineralization, forming a conduit whereby fluids moved along reactivated axial plane cleavage to form the deposit.

Individual vein constituents vary but a typical vein contains quartz and siderite with lesser amounts of pyrite and barite. Ore minerals include argentiferous galena, sphalerite, and local tetrahedrite. There are also minor amounts of other sulfosalts, including pyrargyrite (ruby silver), bournonite, and boulangerite.

Mineral textures vary. Gangue mineral textures are often cataclastic for siderite and local quartz. Sulfide textures vary from locally coarse crystalline galena to fine-grained steel galena. Very fine-grained sheared galena is observed to be more silver-rich that coarser-grained variants. Sphalerite textures range from medium crystalline to fine-grained and is generally lower in iron content relative to the Lucky Friday Mine sphalerite.

The Lucky Friday operation is a deep, narrow vein, mine which commenced operations in 1942. Operations were on care and maintenance due to a strike from 2017 to January 2020, at which time operations resumed. The operation produces silver contained in silver and zinc concentrates.

Mining methods used at Lucky Friday include underhand closed benching (UCB), and overhand or underhand mechanized cut and fill (LFUF) using mechanized mining equipment. Stopes are back filled with cemented paste fill from the process plant tailings. Mine operations are currently close to 6,500 ft below surface and the mine will continue to below the 8,000 ft level in the current long-range plan.

The Lucky Friday mine operation has been designed and constructed to target two broad vein systems: the Lucky Friday and Gold Hunter Veins. The Lucky Friday Vein was actively mined until 2001, is now inactive, and contains infrastructure critical to the active Gold Hunter area. Mining is underway or planned in the 30, 50, 60, 70, 80, 90, and 110 Veins of the Gold Hunter deposit. The 30 Vein is the largest single vein and the source of the majority of the production.

Access to all underground workings is via the 6,205 ft deep Silver Shaft. The Silver Shaft is near the idle Lucky Friday vein system and 5,000 ft south-southeast of the Gold Hunter system. The 18 ft diameter, concrete lined, circular, two-compartment shaft has a hoisting capacity of 12 tons per skip.

Shaft stations are developed on 200 ft centers beginning on the 4900 level. Broken material reports to the Silver Shaft through level pockets and transfers to the 5370 and 5970 loading pockets.

The No. 2 shaft is the 5,489 ft deep, three compartment (four compartment at lower levels) shaft used for man hoisting, supplies, and ventilation.

Access to Gold Hunter is through the 4900 and 5900 level main haulage levels and 4050 access level. Two interlevel ramps connect the 4900 level and 5900 level and three ramps are being developed between the 5900 level and the 6500 level. Below the 6500 level, only two ramps will continue to the 7500 level. The No. 4 Shaft is a 4,800 ft winze with hoistroom located at the 4760 level and bottom at 8620 level.

The No. 4 Shaft provides access to deep-seated portions of the Gold Hunter vein system. Construction commenced in 2010 and was completed in 2016.

All mine development is completed using conventional drill and blast mining techniques. Ramps and sublevels are driven at nominally 12 ft wide by 14 ft high and drilled using single boom or two boom jumbos. Ramps are typically located in the footwall with swinging-ramp crosscuts driven into the vein to access the ore. Level spacing depends on vein size and geometry, but typically four to five stope cuts are taken from each sublevel.

Today underhand techniques continue to be the primary method used, with overhand employed in areas with no seismic risk.

In 2020, an adaptation to UCB mining was developed by Lucky Friday. This change was made to control the release of mining induced seismic events and to improve seismic exposure thereby improving the safety conditions in the mine. The UCB method is applied in the 30 Vein.

Underhand Cut and Fill (LFUL)
In cut and fill mining, levels are typically spaced at 50 ft vertical centres. The vein is accessed through a single slot drive driven roughly perpendicular to the vein strike. Once the slot drive is driven across the vein an ore drive is driven in both directions along the vein until either backfill from an adjacent stope is encountered or the vein becomes uneconomic.

Cut and fill drives are developed using conventional drill and blast techniques, with single boom jumbos drilling 8 ft rounds. Material is removed with loaders to muck bays and eventually to trucks to report to shaft pockets. Ground support is installed after each round according to standards in the ground control management plan. Each stope round is mapped and sampled by the geology department, and a projection-map is developed from the collected data and used to guide the next cut’s extraction.

In the overhand cut and fill technique the slot drive and first cut commence on the bottom and progress upward, such that equipment and personnel work on top of backfill. Conversely, in the more commonly used underhand technique, mining progresses downwards, such that equipment and personnel work on unbroken rock, and cemented backfill and the previous cut horizon is overhead. Typically, five cuts are taking from a single sublevel.

All cuts are backfilled with cemented paste fill. Prior to paste backfilling a 1.5 ft bed of broken ore material is emplaced to prevent the backfill from being damaged during blasting of the subsequent lift below. Additionally, vertical rebar bolts are placed in the bedded material in a regular pattern such that plates and nuts can be attached to the bolt ends when exposed during the next development sequence. In this manner the backfill exposed overhead is always fully supported.

The geometry and thickness of the vein being mined, as well as equipment being used, dictates the cut-and-fill stope widths. The Intermediate Veins are typically narrow and mined with 1 yd3 loaders and single boom jumbos, which can effectively mine to a minimum mining thickness of 6.5 ft. In thicker veins where 2 yd3 loaders are used the minimum mining thickness is 8.0 ft.

Underhand Closed Bench
The UCB method is an adaptation of underhand cut-and-fill mining developed by Lucky Friday for use in high stress mining environments where seismic events are anticipated. It was first used in 2020 and is in full use in the 30 Vein stopes. Work continues to refine the method.

The UCB method was developed specifically for improvement in the safety of mining personnel in seismically active rock masses. In particular, the method is designed to proactively trigger fault-slip seismicity by blasting at a time of choosing of the mine operations when personnel can be restricted from the affected area. The exact location of faults is generally unknown until they are intercepted in mining, thus presenting a seismic hazard. In typical cut-and-fill mining, unstable slip on these faults can occur at any time as a result of stress redistribution around the small, excavated stope which is slowly and incrementally advanced horizontally beneath the engineered fill as described earlier. In addition to fragmenting the ore in the floor of the stope, the stress wave induced by the blast transmits down through the orebody and surrounding wall rock causing both stress disruption and deformation on any near-stope fault surface that it encounters. If the fault is critically-stressed the dynamic disruption to clamping and shear stress-induced by the stress wave will cause slip to occur with subsequent energy release as a seismic event. The large amount of explosive detonated in the UCB blast (up to 37,000 lb as opposed to 150 lb in typical cut and fill face blast) facilitates triggering of the fault slip at the time of the blast, thus relieving stored energy and allowing resumption of personnel access within a short time thereafter.

The drilling and blasting techniques were changed in 2021 to use three inch diameter blast holes with ring burden on 4.5 ft and four to five holes across the stope. Blasthole design targets a corridor of 10 ft to 11 ft wide for the bench. Hole toe spacing is kept to no wider than four feet and is commonly less than four feet. SLR notes that the stated 4.5 ft spacing and four holes represents a minimum blasting width of 13.5 ft assuming the perimeter holes are located on the stope limit. This configuration may lead to overbreak and extra tonnage beyond the Mineral Reserve limits. The stope is opened with a 27 ft deep burn cut using nine four inch diameter holes in a two foot square and nine three inch diameter blast holes for a five foot square cut.

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The Lucky Friday mill is a conventional silver and zinc flotation concentrator. The mill operates at a nominal 42 stph) and can be operated at rates of up to 54 stph for limited periods. Silver concentrate and zinc concentrate are produced. Concentrates are shipped by highway trucks to the Tecksmelter at Trail, British Columbia, Canada.

The primary unit operations in the Lucky Friday concentrator include:
- Primary jaw crushing;
- Secondary cone crushing;
- Tertiary cone crushing;
- Triple deck screen closing both secondary and tertiary crushing circuits;
- Ball milling;
- Lead flash flotation with concentrate reporting to silver concentrate thickener;
- Hydrocyclone classification;
- Lead rougher and scavenger flotation in conventional cells;
- Lead rougher scavenger concentrate to lead rougher feed;
- Lead rougher scavenger tailings to zinc conditioners;
- Lead rougher concentrate cleaning and recleaning using column flotation cells;
- Lead cleaner scavenger flotation of cleaner tailings in conventional cells followed by column cells;
- Lead cleaner scavenger and cleaner scavenger column tailings to regrind milling;
- Regrind milling closed with hydrocyclones
- Flash flotation, second cleaner column flotation and cleaner scavenger column flotation concentrates to silver concentrate thickener;
- Zinc conditioning in mixed reactors;
- Zinc flash flotation;
- Zinc rougher and scavenger flotation in conventional cells;
- Zinc rougher scavenger tailings to final tailings sump feeding sand plant;
- Zinc rougher scavenger concentrate to zinc conditioning;
- Zinc rougher concentrate cleaning and recleaning using column flotation cells;
- Zinc cleaner scavenger flotation of cleaner tailings in conventional cells followed by column cells;
- Zinc cleaner scavenger and cleaner scavenger column tailings to zinc rougher flotation;
- Flash flotation, second cleaner column flotation and cleaner scavenger column flotation concentrates to silver concentrate thickener;
- Lead and zinc concentrate thickening and filtration and concentrate storage;
- Flotation tailings hydrocyclone classification, thickening and filtration of coarse sand;
- Coarse sand stockpiled and delivered to mine backfill cement plant;
- Sand thickener overflow to final tailings thickener;
- Tailings thickener overflow to water treatment and process water tank;
- Tailings thickener underflow to the tailings storage facility (TSF).

Mine ore discharges from the Silver Shaft skips into two coarse ore bins with a total live capacity of approximately 1,000 tons. Ore is crushed in three stages to 100%passing (P100) 3/8 in. using a primary jaw, secondary cone, and tertiary cone crushers closed by a triple deck vibrating screen.

The ore is ground in a single ball mill and discharges to the flash flotation feed pump box where reagents are added. The slurry is pumped to a flash flotation cell to recover coarse lead and silver from the mill circulating load. Flash cell concentrate is final silver concentrate grade and flash cell underflow is pumped to a cyclone cluster for classification. The cyclone underflow returns to the mill and the cyclone overflow advances to lead rougher flotation.

The lead flotation circuit consists of conventional agitated flotation cells for rougher and rougher scavenger flotation, column flotation cells for lead cleaning, and a combination of conventional and column cells for lead cleaner scavenger flotation. Lead flash flotation, lead second cleaner, and lead cleaner scavenger concentrates report to the silver concentrate thickener. Lead cleaner scavenger tailings are reground and pumped to the lead cleaner scavenger column cell.

Lead rougher scavenger tailings report to the zinc conditioners and then to zinc rougher scavenger flotation. The zinc flotation circuit configuration is similar to the lead circuit, with conventional agitated flotation cells for rougher and rougher scavenger flotation, column flotation cells for zinc cleaning and a combination of conventional and column cells for zinc cleaner scavenger flotation. Zinc second cleaner and zinc cleaner scavenger concentrates report to the zinc concentrate thickener. Zinc rougher scavenger tailings are pumped to the final tailings sump and zinc rougher scavenger concentrate is recycled to the zinc conditioners.

Silver and zinc concentrates are thickened, filtered, and stockpiled in storage bunkers, then loaded into trucks and shipped to a smelter.

The flotation tailings are classified with hydrocyclones. The cyclone underflow slurry reports to the sand thickener and the thickener underflow sands are filtered and transported to the backfill cement plant for underground backfill. Sand thickener overflow is pumped to the tailings thickener. Thickener overflow is pumped to thewater treatment plant for recycle as process water or for discharge and the underflow is pumped to a TSF where the solids settle out of the tailings slurry and clear wateris treated and discharged.

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