Castle Mountain is classified as a low-sulfidation epithermal gold deposit (Scott et al., 2018), a sub-type of the epithermal class of gold and silver deposits (Sillitoe and Hedenquist, 2003). Epithermal gold-silver systems are driven by magmatic activity and form high-level vein, stockwork, disseminated and/or replacement deposits that may be mined by open-pit and/or underground methods. Some deposits also contain substantial resources of Ag, Pb, Zn, Cu and/or Hg.

Structure and associated rock porosity-permeability characteristics are the first-order control on the distribution of gold. Flow-dome breccia margins, phreatic diatremes, fault cataclasite and fractures focused provided conduits for hydrothermal fluids and contain the highest gold grades. Unfractured coherent flow-dome facies, clay altered volcaniclastic facies, and clay altered phreatomagmatic diatremes with low or variable permeability are weakly mineralized due to lower fluid interaction. Lower permeability units that are mineralized have been cut by structures such faults, fractures, or phreatic diatremes that promoted hydrothermal fluid interactions.

Gold is focused along structures and margins of facies contacts. It is believed that sub-vertical structures acted as pathways for magma and are responsible for the emplacement of the felsic volcanic package. These same structures also acted as conduits for gold-bearing hydrothermal fluids. Intersections of the steep structures with more permeable volcanic rocks created an environment for enhanced gold precipitation from hydrothermal fluids, possibly due to processes of boiling and interaction with meteoric water.

Lithologic controls are dependent on the host rock texture. Tuff beds, auto-breccias, and hydrothermal breccias have permeable fragmental textures. Brittle rhyolite flows and intrusive equivalent rocks exhibit intense fracturing and are characterized by cooling joints, vesicular zones, spherulitic vugs, and flow foliations. Gold occurs within secondary silica in all these features. Major fault and fracture systems and intersections of fracture systems provided structural controls for mineralization. In the deposit area, north-northeast-striking, mineralized fracture zones are exposed in outcrop.

The morphology of mineralization follows two patterns. Firstly, gold is enriched along steep to vertical brecciated contacts of flow-domes and phreatomagmatic diatremes. Secondly, gold occurs in broad tabular zones that correlates with the general orientation bedding. The lateral extent of the mineralized bodies centered around fault zones are dictated by the intensity and extent of fracturing and faulting, in addition to the paleo-porosity of the host rocks.

Some faults and fracture zones are not gold-bearing since the structural regimes through the Project were active both pre- and post-mineralization. Gold seems to have precipitated during a single phase within a larger and longer-lived structural and hydrothermal event.

Silicification is commonly associated with gold occurring as pervasive silica flooding and quartz veining. Quartz veins can be vitric and “gel-like” or opaque white-gray opal. Vitric quartz veins typically occur in clusters as sheeted veins or stockwork in zones ranging from 3 to 35 ft (1 to 10 m) wide. Amorphous quartz occurs as discontinuous irregular veins and as open space filling quartz. The strongest silica alteration associated with gold is found along brecciated coherent rhyolite margins; this results in mosaic breccias where angular rhyolite clasts are within a hydrothermal-related silica matrix.

Gold on the Project occurs in oxidized fractures, faults, discontinuous veins, and breccia matrix. Gold mineralization correlates best with the deep red, red-brown and brown iron oxide that can range in color from pink to red-brown. The iron oxide intensity and appearance are commonly controlled by the volcanic facies occurring as discontinuous, fracture-controlled textures in coherent rhyolite facies, as matrix replacement in rhyolite breccias, wispy selvages and clast haloes in volcaniclastic rocks, and pervasive or matrix selective in diatremes. These iron oxide textures can be cut by fracture and vein filling iron oxide that ranges in color from brown-tan to red.

Visible gold is rarely observed in hand specimen and core. In petrographic samples collected near JSLA, visible gold is associated with iron oxide and silica and proximal to illite and adularia alteration (Cline, 2016). Gold deportment studies from Oro Belle by Chudy and Lane (2020) indicate that mineralization is roughly 79% native gold, 17% electrum and 4% silver minerals by frequency of grain count. Quartz may be intergrown with iron oxides/hydroxides, most commonly as hematite, which have formed as oxidation products of former sulfide minerals. There is a low abundance of sulfides observed on the Project. The most common sulfide mineral is pyrite, and varies from nil to 1%, which occurs within clasts and matrix.

Phase 2 mining depends on several things including permits and is expected to commence two years prior to full production. The expanded heap leach and mill are anticipated to reach full production by the start of Year 6, designated as Year 1 for Phase 2.

The mine plan will depend on several activities including permitting and incorporates the following assumptions:
• Phase 1 – initiated in month 1 with contractor mining of backfilled ROM ore from the JSLA open pit to Phase 1 leach pad.
• Phase 2 – initiated in month 36 with JSLA pioneering and access road construction to commence using contractors in month 37. Contractor to develop JSLA East pit to 4,540 ft elevation.
• Staggered mining equipment deliveries month 42 to month 48. Mining rate begins to ramp up as equipment is delivered.
• Ramp up overall mining rate to 73 Mton/y (66 Mt/y) through to Year 9 then expand gradually to 80 Mton/y (73 Mt/y) through to Year 16 after which production begins to drop through Year 19.
• Overall sequence of development in the JSLA, Jumbo, Oro Belle and East Ridge area is clockwise development to final to pit limits in each area to allow for an orderly sequence of backfilling waste as pits are completed.
• Sequence at South Domes is an initial southwest pit with an expansion to the northeast.
• The resource block model was developed on 20 ft (6 m) benches. The mine design was developed using the 20 ft bench height with triple benching to 60 ft (18 m) between design catch benches or berms. Operations are planned for a 30 ft bench height. Sinking rates in the schedule were limited to 300 ft/y or the equivalent of 10 benches/year. Drills, loading units and support equipment appropriate for mining a 30 ft (9 m) bench height have been selected for the mine plan and associated cost estimates.

The open pit mine development plan consists of nine pit development phases expanding to two large open pits. A waste dump will be located along the east property boundary and a second dump will be expanded to the northwest property boundary. A high-grade mill feed stockpile will be located adjacent to the mill primary crusher.

The mine will operate as a conventional diesel-powered truck shovel operation. The typical production cycle will be drilling, blasting, grade control, loading and hauling. Primary loading units will be hydraulic shovels and wheel loaders with support equipment providing development access, road maintenance and equipment servicing capability.

The Phase 1 mine plan continues with contractor mining of JSLA open pit backfill material that is economic to process by ROM leaching. It is proposed to initiate hard rock mining of the JSLA East Pit with contractors in month 37 of the plan. Mining continues in the JSLA Backfill Pit which is completed to bench 4140 during Year 5.

During Years 6 to 8, JSLA East Pit is mined to 3860 bench. In Year, 6 full production is reached for ROM ore production at 50,000 ton/d. JSLA West Pit developed to 3980 bench. Jumbo Pit pioneering commences in Year 8 and by Year 8 has reached 4520 bench. Oro Belle and East Ridge Pits are pioneered as well in Year 8 to the 4800 Bench.

The JSLA Pit is completed in Year 12. Oro Belle Pit reaches 4260 bench in Year 12. East Ridge Phase 6 Pit is completed in Year 12 and East Ridge Phase 7 Pit is down to 4360. The South Domes Phase 1 Pit is pioneered in Year 10 and 11 and by Year 12 is down to 4180 bench. The average annual mining rate during this time is 73.2 Mton. The Northwest and Southeast dumps are filled to capacity during this time interval and backfilling commences in JSLA Pit.

The Oro Belle Pit is completed in Year 15 and East Ridge Phase 7 Pit is completed in Year 16. Pioneering of South Domes Phase 2 commences in Year 15 and by the end of the year Phase 1 is down to 4060 and Phase 2 is down to 3620. Waste backfilling continues in the JSLA and Jumbo Pits.

South Domes Phase 1 is completed during the first half of Year 19 and South Domes Phase 2 is completed later in the year. Overall mining rates may become limited by sinking rate limits of 300 ft/y in Year 18 and Year 19.

Crushing
ROM ore will be dumped onto stockpiles and a front-end loader (FEL) will be used to reclaim ore via a static grizzly with 24 inch openings. Ore will flow into a 20 yd3 dump pocket, while any boulders will be removed for later reduction using a backhoe with a rock breaker attachment. An apron feeder will draw ore from the dump pocket and discharge onto a vibrating grizzly feeder. The vibrating grizzly oversize ore will feed directly into the 49 in x 37 in jaw crusher with an installed power of 175 hp. The minus 4 inch ore will bypass the crusher and feed directly onto the coarse ore transfer conveyor. The primary crushing stage will produce a product P80 of approximately 4 in with a closed side setting (CSS) of 4 in. A magnet will be installed over the coarse ore transfer conveyor to ensure any tramp metal is removed ahead of fine crushing.

Jaw crusher discharge will be combined with the vibrating grizzly undersize on the coarse ore transfer conveyor and will feed an 8 ft x 20 ft double deck inclined screen. The top deck of the secondary screen will have an aperture size of 1 inch and the bottom deck will have an aperture size of ½ in. The plus 1 inch material from the top deck and the plus ½ inch ore from the bottom deck will be conveyed to a surge bin ahead of the secondary cone crusher. The undersize from the bottom deck, minus ½ inch ore, will be the final product that will discharge onto the screen undersize transfer conveyor and ultimately report to fine ore bin via the final crushing product conveyor.

A belt feeder will withdraw ore from the bottom of the surge bin and feed it into a standard cone crusher with an installed power of 500 hp. The secondary crusher will operate in closed circuit and will reduce the ore to produce a product P80 of approximately 0.8 in. Crushed product will be combined with crushed material from the primary crushing stage on the coarse ore transfer conveyor and return to the secondary screen. Between the screen undersize transfer conveyor and the final crushing product conveyor, a diverter gate will be installed to allow for recovery of crushed product material should the bin be temporarily unavailable.

The primary crushing facility will utilize a 24 ft high mechanically stabilized earth retaining structure to build up elevation to allow for proper vertical clearance between equipment within this facility. The retaining wall is backfilled with select structural backfill material while further extension of the pad is achieved using mine waste from Phase 2 mine pre-development activities.

The crushing plant product will be conveyed to a fine ore bin. Two draw points under the bin will provide ore to two reclaim belt feeders located underneath the bin. The reclaim feeders will discharge onto the ball mill feed conveyor.

Grinding
Reclaimed ore from the fine ore stockpile will feed a 16.5 ft x 21 ft long overflow ball mill via the ball mill feed conveyor. The mill will be supplied with rubber liners, a single 3,300 hp wound rotor induction motor with a VFD, a trommel screen, and a retractable feed spout/chute. Pebble lime will be slaked, and milk of lime will be added to the ball mill feed for pH control. An automated feeder will supply grinding media to the ball mill. Ground slurry will overflow from the ball mill onto the trommel screen attached to the discharge end of the mill. The trommel screen oversize will discharge into a trash bin for removal from the system, while the undersize will flow into the cyclone feed pump box.

Slurry from the cyclone feed pump box will be pumped to a cluster of four (2 operating/2 standby) 12 inch cyclones for size classification. Process water will be added to the ball mill feed and the cyclone feed sump to achieve the appropriate pulp density. The coarse cyclone underflow from one operating cyclone will flow by gravity back to the ball mill for additional grinding while the underflow from the second operating cyclone will flow by gravity to the gravity circuit. The fine cyclone overflow, at a final target product P80 of 100 mesh (150 µm), will flow by gravity to a vibrating trash screen ahead of the pre-leach thickener.

For Phase 2, the heap leach pad will be designed to process 18.2 Mton (16.5 Mt) annually at an average life of mine grade of 0.012 oz/ton (0.41 g/t), while the mill will be designed to process 1.3 Mton (1.2 Mt) annually at an average LOM grade of 0.068 oz/ton (2.32 g/t). When considering both Phase 1 and Phase 2, operations will extend to approximately 19 years with an additional estimated three years of heap rinsing as part of reclamation where gold will continue to be leached and recovered.

Heap leaching
ROM ore will be delivered to the leach pad by the mining fleet. Pebble Quicklime (CaO) will be added for pH control of the process from two 100 ton silos. The lime will be metered into a clamshell and dumped into the loaded trucks which will then deliver the ore to the active stacking area.

Phase 2 design considered both use of a single pad to be adjoined with the current Phase 1 pad as well as construction of a separate pad. Both options would use a soluti ........