Gold-antimony-silver-tungsten deposits of the Stibnite Mining District (the District) are not readily categorized based on a single genetic deposit model due to complexities associated with multiple overprinting mineralization events and uncertainties regarding sources of mineralizing hydrothermal fluids. Workers in the early 1970s considered some of the mineralization to be similar in style to deposits in the Yellow Jacket Co-Cu-Au belt farther east and attributed the precious metal mineralization to iron formations associated with what were interpreted as metavolcanics rocks (Jayne, 1977). Cookro (1985) attributed the tungsten to Cretaceous skarns. Cookro et al. (1987) noted isotopic signatures that suggested an igneous or metamorphic origin likely of Late Cretaceous age but also noted the potential for overprinting Tertiary mineralization. Criss et al. (1983; 1991) noted associations between Tertiary intrusions and meteoric dominated epithermal systems including Yellow Pine. Bookstrom et al. (1998) attributed the various metals in the District to a variety of deposit types including distal disseminated gold, Au-Ag and mixed metal veins, simple antimony veins, disseminated antimony, quartz-scheelite veins and breccia deposits, mixed metal skarns and hot springs mercury. Konyshev (2020) noted similarities to the reduced intrusive systems in the Tintina Belt, specially Donlin Creek. Others have noted similarities to Carlin-type systems and reduced intrusion gold deposits (Dail et al., 2015; Dail, 2016; Hofstra et al., 2016) and orogenic gold to antimony-gold bearing Carlin-like systems in China (Dail, 2014; Gillerman et al., 2019b). The complicated paragenesis and prolonged extent of mineralizing events in the area spanning tens of millions of years preclude application of a single genetic model.

Within the Project area, the focus of past exploration for, and development of, Au-Ag-Sb-W-Hg deposits has been from both disseminated deposits extracted using conventional open pit methods and higher grade structurally controlled AuSb, W-Sb, Hg and Au-only deposits extracted using various underground mining methods. Mineralization occurs in numerous locations throughout the District in medium- to coarse-grained, felsic to intermediate intrusive host rocks and typically occurs as disseminated replacement mineralization within structurally prepared dilatant zones or adjacent to district- and regional-scale fault zones. Mineralization also occurs in association with sheeted veins, stockworks, endoskarns, and complex polymictic breccias. In the metamorphosed sedimentary rocks, mineralization occurs in association with dense fracture zones in structurally prepared sites and as stratiform manto-style replacements in reactive carbonate and calcareous siltite and schist units, as well as in cross-cutting breccia veins and dikes and jasperoids (quartz-replaced carbonates).

Intrusive hosted precious metals mineralization typically occurs in structurally prepared zones in association with very fine-grained disseminated arsenical pyrite (FeS2) and, to a lesser extent, arsenopyrite (FeAsS). Base metal sulfides are uncommon. Arsenical pyrite is the primary host for gold mineralization and the vast majority of the gold occurs in solid solution within the sulfide crystal lattice. Arsenopyrite is the only other significant gold-bearing sulfide mineral in the intrusive hosted deposits. Gold rarely occurs as discrete sub-micron particles in pyrite and other sulfides. Base metals are rare and occur at very low concentrations, at or below typical crustal abundance levels. Various oxidized products of the weathering of the primary sulfides are found in the intrusives including goethite, hematite, jarosite, and scorodite, and host precious metal mineralization in the oxidized portions of the deposits.

Antimony mineralization occurs primarily in the form of the mineral stibnite (Sb2S3). Other antimony-bearing phases include miargyrite (AgSbS2), gudmundite (FeSbS), chalcostibite (CuSbS2), tetrahedrite [(Cu, Fe)12Sb4S13], and owyheeite [(Pb)10(Ag)3-8(Sb)11-16(S)28]. There is a weak, but persistent association of volumetrically small base metal mineralization, typically <0.25%, associated with the antimony mineralization and includes rare occurrences of chalcopyrite (CuFeS2), galena (PbS), sphalerite (ZnS) and molybdenite (MoS2). Zones of high-grade, silver-rich mineralization locally occur with antimony and are related to the presence of pyrargyrite (Ag3SbS3), hessite (Ag2Te), and acanthite (Ag2S).

Tungsten mineralization is associated with the mineral scheelite (CaWO4). Observations indicate that some of the tungsten is associated with the stibnite mineralization but may also precede it since stibnite has been found in numerous past studies cementing veins and brecciated scheelite fragments.

Metasediment-hosted mineralization has a similar sulfide suite and geochemistry, but with higher carbonate content in the gangue and a much more diverse suite of late stage minerals. As in the intrusive-hosted mineralization, gold is associated with very fine-grained arsenical pyrite and is tied up in the pyrite lattice.

Yellow Pine Deposit
In the central region of the deposit, between 1,188,200N and 1,189,600N, mineralization is broadly disseminated over a width of 500 feet east of the Hanging Wall fault and west of the post-mineralization Hennessey fault, except where Hennessey fault has offset the western part of the mineralization to the north. Gold and antimony mineralization in the central region of the deposit are bounded to the south by a complex fault network consisting of the C-structure, the Granite fault, and the northwesterly striking Midnight fault. The width of mineralization in Central Yellow Pine ranges from 165 ft to over 650 ft wide, over 1,400 feet of strike length and extends down dip over 1,200 ft.

Mineralization in the northern Homestake area of the Yellow Pine deposit ranges from 80 to 150 ft thick and extends for over 800 ft along strike and down dip. Here, mineralization occurs as a tabular body in the hanging wall of the Hidden fault/Clark Tunnel structure. Gold mineralization also occurs within the metasediments at Homestake, where both disseminated and vein-hosted gold occurs within the upper Calc-Silicate and Middle Marble formations.

Hangar Flats Deposit
Mineralization in the Hangar Flats Deposit is entirely intrusive hosted and is localized in and along the flanks of the MCFZ. The highest grades of gold, silver, and antimony, defined on the basis of drilling and legacy production, occur within sub-vertical, north-plunging, tabular to pipe-like breccia bodies formed at the intersection of the main north-south structural features and shallowly northwest-dipping dilatant splay structures. These mineralized breccia zones range from 16 ft to over 330 ft in true thickness and can be traced several hundred feet down dip. Disseminated replacement style gold mineralization occurs throughout the MCFZ and eastern footwall encompassing the higher-grade tabular breccia zones. Disseminated gold mineralization also occurs as shallowly dipping tabular bodies along the northwest dipping splay structures which pinch out to the east away from the main MCFZ.

West End Deposit
Mineralization in the West End Deposit is structurally and stratigraphically controlled. Within the WEFZ, gold mineralization occurs within silicified breccia zones and as replacement style mineralization where the northeast-dipping calc-silicate and schistose units are sheared and offset by the structure. Outside of the WEFZ, mineralized zones occur as stacked ellipsoidal bodies plunging along the intersection of favorable lithologic units and structural zones and as tabular bodies extending along bedding. Mineralization also occurs as fracture filling within siliciclastic sequences and other less favorable lithologic units. True widths of these bodies range from 50 ft to over

Mining at the Stibnite Gold Project would be accomplished using conventional open pit hard rock mining methods. Mining is planned to deliver 8.05 Mst of ore to the crusher per year (22,050 st/d), with stockpiling by ore type (low antimony sulfide, high antimony sulfide and oxide).

The mine plan developed for the Project incorporates the mining of three primary mineral deposits – Yellow Pine, Hangar Flats, and West End – and re-mining and re-processing of the Historic Tailings.

The Historical Tailings will be trucked to a re-pulping facility adjacent to the tailings eposit and hydraulically transferred to the process plant grinding circuit via a re-pulping facility.

Most of the development rock from the three open pits will be sent to one of five destinations: the TSF embankment, the TSF Buttress, the Yellow Pine open pit backfill, the Hangar Flats open pit backfill, and the West End open pit backfill.

YELLOW PIT PHASE DESIGN
In addition to the nested pit shells produced in the Ultimate Pit Limit Analysis, a suite of directional pit shells was generated for the Yellow Pine deposit to identify potential for mining the main portion of Yellow Pine first and the northern Homestake area last. This phasing sequence allows for accelerated access to high-value ore deep in the central Yellow Pine deposit and provides for a short development rock haul from the Homestake area to the Yellow Pine pit backfill to reduce haulage cost.

HANGAR PLATES PHASE DESIGN
The Hangar Flats pit design consists of a single phase due to its small size and steep topography which requires a topdown mining approach. An internal phase within Hangar Flats would likely result in very narrow bench widths in the northwest highwall causing significantly reduced mining production rates.

WEST END PIT PHASE DESIGN
Four pit phases were designed for the West End pit: (1) Middle Marble limestone mining, (2) Midnight area pit production, (3) South West End pit production, and (4) Main West End pit production. Mining limestone from the Middle Marble geologic unit located in the northeast portion of the West End open pit is required for the lime kiln to produce lime used in ore processing. The Midnight Area phase sequence is primarily driven by when access is available for backfilling this area using development rock produced in the Main West End phase. The South West End phase is accessible via the ROM-to-West End Haul Road and can be mined independent of the Main West End phase. The Main West End phase does not benefit significantly from additional phasing due to the homogeneous nature of the ore body.

HISTORICAL TAILINGS PIT PHASE DESIGN
The 2,687 kt of Historical Tailings will be excavated and hauled by truck to a nearby handling facility where it would be screened, re-pulped, and pumped to the grinding circuit. For mine planning purposes, the Historical Tailings resource is modeled with constant grade and value throughout the deposit. Therefore, phasing the Historical Tailings is not influenced by advancing access to higher value ore but instead by the need to accommodate construction of adjacent facilities and avoid costs associated with double handling of the material. The Historical Tailings are planned to be excavated and processed during the first 4 years of mill operation.

Crushing and Stockpile
Haul trucks with 150-ton capacity are planned to transport mined material to the primary crusher pad for processing. The mill feed is either dumped directly into the primary crusher feed hopper or onto one of four 100,000-ton ROM stockpiles. The stockpiles allow blending of materials to control sulfide or carbonate concentrations. They also provide storage for mined materials that are segregated and accumulated for future processing as feed campaigns through the process plant.

The crushing circuit design is based on a 24-hour per day, 365-day year operation at an average utilization of 75% yielding an instantaneous design throughput of 1,225 stph. ROM material dumped onto a grizzly screen passes into the crusher dump hopper. Stockpiled material is fed to the crusher as needed for blending or campaigning using a front-end loader. The dump hopper is designed with live capacity for one haul truck. A rock breaker at the dump pocket breaks oversize materials, allowing them to pass the grizzly. An apron feeder draws material from the dump hopper to feed a vibrating screen feeder. The oversize feeds the jaw crusher and the undersize passes through to the stockpile feed conveyor. The crusher product joins the undersize on the conveyor for delivery to the coarse ore stockpile.

Belt scales are included in the design to monitor production rate – one on the coarse-ore stockpile feed conveyor and another on the SAG mill feed conveyor.

A concrete and steel structure is designed to support the primary crusher and associated equipment. Concrete piers are designed to support the concrete dump pocket, insulated metal roof and wall panels, and a 20-ton bridge crane. Water sprays would be installed at the crusher dump pocket and at material transfer points to reduce dust emissions.

The stockpile is designed with 12 hours of live capacity (11,023 st) and a total capacity of approximately 40 hours’ worth of production (44,500 st). Four feeders would be provided for material reclaim to the milling circuit. A domeshaped cover supported by a concrete ring structure is designed to reduce dust emissions and for protection from the weather. Twenty-four concrete piers 22 ft 6 inches tall at 15° spacing are designed to support the concrete ring and dome. The dome consists of coated a metal tube framing with metal roof/siding attached to the metal framing.

A concrete reclaim tunnel underneath the stockpile is designed to reclaim ROM material for conveying to the grinding circuit. The reclaim tunnel is designed to be 20 x 20 x 160 ft with two perpendicular 38-ft tunnel segments at the center of the stockpile. Four 12 ft by 6 ft draw holes arrayed around the center of the stockpile provide feed to the SAG mill feed conveyor. The reclaim design contains two of the draw holes along the axis of the tunnel centered 46 feet from the center of the dome and two draw holes perpendicular to the axis of the tunnel centered 36 feet from the center, one on each of the perpendicular tunnel segments. The design permits stockpile material to be drawn from two of the four draw holes at a time to produce 1,021 stph of ore to grinding. Rotating among the draw holes is intended to provide even drawdown and mitigate “rat holing.” Stockpile material flow from the draw holes is controlled by belt feeders at each draw hole that transfer the material to the SAG mill feed conveyor. A dust collector is designed to control dust in the reclaim tunnel.

Grinding
The SAG mill feed conveyor is designed to deliver reclaimed material from the stockpile to the SAG mill at an instantaneous design throughput of 1,021 stph, based on 90% availability. The grinding circuit design includes one SAG mill with a discharge trommel, one pebble crusher, one ball mill and a cyclone cluster. The trommel screen on the SAG mill discharge returns oversize “pebbles” to the SAG mill feed conveyor after crushing.

The trommel screen undersize falls into the cyclone feed pump box where it combines with the ball mill discharge. The combined slurry is then pumped to a hydrocyclone cluster for particle size classification. When historical tailings tons are processed during the early years of operation, the slurry from the tailings repulping plant would also flow into the cyclone feed pump box. Cyclone underflow flows by gravity to the ball mill for additional size reduction by grinding. The cyclones parameters will be set to achieve a target size of 80% passing 75 microns at 33% solids. Cyclone overflow is designed to flow by gravity to the flotation area after passing through a screen to remove trash.

The grinding area is to be enclosed in a steel structure supported on concrete piers with preformed insulated metal roof and wall panels. The grinding area floor is designed to be concrete on grade with containment walls to contain spills. The floor is sloped to a trench that directs spills to a sump that enables pumping back to the cyclone feed pump.
box.

The Stibnite Gold Project process plant has been designed to process both sulfide and oxide mineralized material from three deposits (Hangar Flats, Yellow Pine, and West End) as well as Historic Tailings from former milling operations. The design of the processing facility was developed based on the laboratory testing, to treat an average of 22,046 st/d, 365 days per year for a total of 8.05 million tons per year.

ROM material would be crushed and milled, then flotation and hydrometallurgical operations would be used to recover antimony as a stibnite flotation concentrate (with some silver and minor gold), doré bars containing gold and silver, and small quantities of elemental mercury, collected in flasks, to prevent its potential release into the environment. Historic Tailings would be introduced into the ball mill during the first 3 - 4 years of operation.

The process operations include the following components:

• Crushing Circuit – ROM material would be dumped onto a grizzly screen and into the crusher dump hopper feeding a jaw crusher operating at an average utilization of 75% yielding an instantaneous design-throughput of 1,111 tonnes per hour (t/h).

• Grinding Circuit – The grinding circuit incorporates a single semi-autogenous (SAG) mill, single ball mill design with an average utilization of 90%, yielding an instantaneous design- throughput of 926 t/h. When Historical Tailings are processed during early years of the operation, the slurry from the plant would also flow to the cyclone feed pump box. Cyclone underflow flows by gravity to the ball mill; cyclone overflow, at 33% solids with a target size of 80% passing (P80) 85 microns, would be screened to remove tramp oversize and flow through a feed sample system and on to the antimony or gold rougher flotation circuit, depending on the antimony concentration of the material.

• Flotation Circuit (Antimony and Gold) – The flotation circuit consists of up to two sequential flotation stages to produce two different concentrates; the first stage of the circuit was designed to produce an antimony concentrate when the antimony grade is high enough, or bypassed if not, and the second stage was designed to produce a gold-rich sulfide concentrate. The antimony concentrate will be packaged and sold. The goldrich sulfide concentrate will be stored in three surge tanks.

• Pressure Oxidation Circuit – Concentrate from the surge tanks would be pumped to the autoclave feed tank, which would feed the autoclave. The autoclave is designed to provide 75 minutes of retention time at 220 degrees Celsius (428 degrees Fahrenheit) to oxidize the sulfides and liberate the precious metals. Autoclave discharge would be processed through flash vessels and gas discharge would be condensed and the remaining gas cleaned through a scrubber.

• Oxygen Plant – An oxygen plant producing 607 t/d of gas at 95 percent oxygen and a gauge pressure of 40 bars is planned. The oxygen would be from a vendor-owned oxygen plant located near the autoclave building providing the autoclave with an “over the fence” supply.

• Lime Plant – Limestone quarried from the West End pit would be hauled to an area south of the primary crusher pad. The material would be crushed and screened to feed the limestone grinding mill and the lime kiln. Ground limestone slurry and milk of lime are used to control acid in the autoclave, neutralize solutions and slurries coming out of the POX process, and control pH for leaching.

• Oxidized Sulfide Processing – After pressure oxidation, slurry discharge from the flash vessels would be neutralized and cooled prior leaching. The slurry would then be leached in cyanide solution, followed by a seven-stage pump-cell carbon-in-pulp (CIP) circuit for precious metal recovery from this high-grade stream. The sulfide CIP tailings would be detoxified and discharged to the flotation tailings thickener. Alternatively, the sulfide leach tailings would be combined with flotation tailings when the latter undergoes cyanide leaching, as described in the next bullet point.

• Oxide Carbon-in-Leach and Tailings Detoxification – A future oxide leach circuit is included in the design of the process plant to be running in Year 7 of mill operations. This circuit would recover gold from nonrefractory material in the flotation tailings when the mill is processing transition ore from the West End deposit. This circuit would also directly process oxide material from the West End deposit as a whole-ore leach process, that is, without undergoing flotation.

• Carbon Handling – Loaded carbon from the CIP circuit would be processed through a conventional carbon handling circuit, using the hot pressure-stripping of loaded carbon.

• Gold Room – Precious metals would be recovered from the strip solution by electrowinning.

• Tailings – Neutralized and thickened tailings would be pumped from the process plant to the TSF in a HDPElined carbon steel pipe.

• Process Control Systems – The process plant design includes an integrated process control system.