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Location: 159 km NE from Boise, Idaho, United States
13181 Highway 55PO Box 429DonnellyIdaho, United States83615
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The Stibnite Project is situated along the eastern edge of the Idaho Batholith, on the western edge of the Thunder Mountain caldera complex and within the Central Idaho Mineral Belt.Large, north-south striking, steeply dipping fault structures exhibiting pronounced gouge and multiple stages of brecciation occur in the District and are often associated with east-west and northeast-southwest trending splays and dilatant structures. The Yellow Pine and Hangar Flats deposits are hosted primarily by intrusive phases of the Idaho Batholith along the Meadow Creek Fault Zone (MCFZ). The West End Deposit is hosted primarily by Neoproterozoic to Paleozoic carbonate and siliciclastic metasedimentary rocks of the Stibnite roof pendant along the West End Fault Zone (WEFZ).Mineralization and alteration in the District are associated with multiple hydrothermal alteration events occurring through the Paleocene and early Eocene epochs. 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 vein arrays, breccia veins and dikes, and jasperoids (quartz-replaced carbonates).Main-stage gold mineralization and associated potassic alteration typically occurs in structurally prepared zones in association with very fine-grained disseminated arsenical pyrite (FeS2) and, to a lesser extent, arsenopyrite (FeAsS), with gold almost exclusively in solid solution in these minerals. Antimony-tungsten mineralization is associated with silicification and brecciation resulting in stibnite (Sb2S3) veining and distinctive black matrix breccias within discrete structural zones. A later stage of mineralization crosscutting early disseminated styles and primarily effecting rocks of the Stibnite roof pendant is associated with epithermal quartz-adularia-carbonate veins. Carbonates, primarily iron-magnesium-rich calcite and ankerite, along with potassium-rich illite ("sericite") and to a lesser extent chlorite and smectite clays are common alteration assemblages peripheral to the pervasive potassium feldspar and sericite alteration in the cores of the intrusive hosted deposits or in late structural zones.Yellow Pine DepositMineralization in the Yellow Pine Deposit is structurally controlled and localized by the northerly striking MCFZ and by conjugate splay or cross structures. The deposit shows metal zonation with gold mineralization occurring throughout the deposit footprint, with antimony and tungsten primarily in the central and southern portions of the deposit. Most of the mineralization in the deposit occurs west of the MCFZ and east of the Hidden fault zone. The geometry, width and continuity of precious metals mineralization changes along strike in the deposit in conjunction with a bend in the MCFZ and its intersection with the Hidden fault zone. To the south, gold and antimony mineralization occur within a breccia zone of the MCFZ. The width of mineralization ranges from 80 ft to 165 ft, extends for over 800 ft along strike, and is open at depth in this area.In the central region of the deposit, between 1,188,200N and 1,189,600N, mineralization is broadly disseminated over a width of 500 ft 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. The width of mineralization in the central area of the Yellow Pine deposit ranges from 165 ft to over 650 ft wide, over 1,400 ft 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. Mineralization occurs as a tabular body in the hanging wall of the Hidden fault/Clark Tunnel structure. The tabular zone steepens to the west and is truncated to the west against the East Boundary fault, a gouge zone within the MCFZ. Directly east of the MCFZ gouge, is a silicified fault corridor which is moderately mineralized in the Homestake area. 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 DepositMineralization 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 occur within sub-vertical, north-plunging, tabular to pipelike breccia bodies formed at the intersection of the main north-south structural features and shallowly northwest-dipping dilatant splay structure. 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 in 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. Alteration zonation is similar to that developed in the Yellow Pine deposit, but more tightly constrained to structures.West End DepositMineralization in the West End Deposit is structurally and stratigraphically controlled. Within the WEFZ, gold mineralization occurs within silicified breccia zones, sheeted quartz-adularia vein arrays and as replacement style mineralization situated where the northwest striking, northeast dipping calc-silicate and schistose units intersect the WEFZ. Alteration is dominated by sulfide replacement of iron-bearing mineral phases in favorable metasedimentary rocks and associated with quartz-potassium feldspar replacement and quartz-adularia-carbonate-sulfide veining. These mineralized zones occur as stacked ellipsoidal bodies plunging along the intersection of favorable lithologic units and faults 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 330 ft. Drilling by Perpetua Resources has intersected gold mineralization associated with the WEFZ well below the historic pit bottom as deep as 1,300 ft below the original ground surface - where mineralization was exposed prior to mining. The hanging wall of the WEFZ tends to exhibit relatively more dilatant and dispersed structures relative to the footwall and, therefore, is more significantly mineralized. Open-space-fill quartz veins and silicified breccias are typical within higher-grade zones of mineralization. The degree of oxidation in the West End Deposit is a function of both depth and proximity to faults and fractures. Both pervasive and fracture hosted oxidation is common throughout the deposit to depths of approximately 300 ft below the pre-mining topographic surface. Discrete zones of pervasive oxidation occur below this depth in the vicinity of the WEFZ and subsidiary structures. Oxidation is interpreted to have resulted from both infiltrating precipitation and from deep-seated circulation of meteoric fluids through structural zones.
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 deposit 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 DesignIn 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 DesignThe 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 DesignFour 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 DesignThe 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 Circuit - ROM material would be dumped directly into the primary crusher feed hopper or onto ROM stockpiles and the primary crusher discharge is delivered to the coarse ore stockpile. The coarse ore stockpile provides 12-hour live capacity. 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.• 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 1,021 stph. When Historical Tailings are processed during early years of the operation, the slurry from the tailings repulping plant would also flow into the cyclone underflow tub. Cyclone underflow flows by gravity to the ball mill. The cyclone overflow, at 35% solids with a target size of 80% passing (P80) 85 microns, is screened to remove tramp oversize and flows through a sampler and on to the antimony or gold rougher flotation circuit, depending on the antimony concentration of the material.
The 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 Historical Tailings from former milling operations. The design of the processing facility was developed based on the laboratory testing to treat 1,021 short tons per hour (stph) (excluding historical tailings) through crushing, grinding, flotation, concentrate oxidation, leaching by cyanidation, carbon handling and refining, oxide leaching, gold recovery, and tailings processing operations with a design availability of 90%.Run-of-mine (ROM) materials from the three pits and historical tailings have characteristics that require several process variations. Process variations are as follows:• Sulfide ROM with high antimony concentrations is crushed, ground, and treated in an antimony flotation circuit before sulfide flotation and pressure oxidation (POX) to release refractory gold for cyanide leaching and gold recovery;• Sulfide ROM with low antimony concentrations is sent directly from grinding to sulfide flotation, POX, leaching, and gold recovery;• Oxidized ROM is sent from crushing and grinding directly to a whole-ore cyanide leaching and gold recovery circuit, which is scheduled to be constructed when such ROM is anticipated in the mine plan;• Mixed sulfide and oxide (transition) ROM is handled as low-antimony sulfide ROM except that the flotation tailings are cyanide leached with the same circuit used for oxide material.Process DescriptionThe gold-bearing sulfide concentrate of pyrite and arsenopyrite is processed using pressure oxidation to break down the sulfide crystalline structure to liberate gold and silver to be leached and recovered to doré bars containing gold and silver. Small quantities of elemental mercury are collected in flasks to prevent its potential release into the environment. The design introduces Historical Tailings into the ball mill during the first 3 to 4 years of operation. Tailings from the operation are deposited in a geomembrane-lined tailings storage facility (TSF). The process operations are described as follows:• 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 is designed to produce a gold-rich sulfide concentrate. The antimony flotation circuit includes a regind mill for size reduction. The antimony concentrate will be packaged and sold. The gold-rich sulfide concentrate will be stored in three agitated surge tanks.• Pressure Oxidation Circuit - Sulfide concentrate from the surge tanks is pumped to the autoclave feed tank. The autoclave is designed to provide 75 minutes of retention time at 220º Celsius (428º 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 670 stpd of gas at 95% oxygen and a gauge pressure of 40 bars is planned. The oxygen plant equipment is planned to be purchased from a vendor including installation supervision.• Lime Plant - Limestone quarried from the West End pit is hauled to an area south of the primary crusher pad. The material is crushed and screened to feed the limestone grinding mill. 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 is neutralized and cooled prior to leaching. The slurry is leached in cyanide solution, followed by an eight-stage, pump-cell carbon-in-pulp (CIP) circuit for precious metal recovery from this high-grade stream. The sulfide CIP tailings are detoxified and discharged to the flotation tailings thickener. Alternatively, the sulfide leach tailings are combined with flotation tailings when the latter undergoes cyanide leaching, as described below.• Oxide CIP 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 is designed to recover gold from non-refractory material in the flotation tailings when the mill is processing transition ore from the West End deposit. This circuit also directly processes oxide material from the West End deposit as a whole-ore leach process without undergoing flotation.• Carbon Handling - Loaded carbon from the CIP circuits is processed through a conventional carbon handling circuit, pumping eluant from the strip solution tank through heat exchangers to the bottom of the elution vessel at 45 psig and 293°F and a flow rate of 2 bed volumes per hour.• Gold Room - Precious metals are recovered from the strip solution by electrowinning, mercury retort, and a gold furnace that produces doré bars as a saleable product.• Tailings - Neutralized and thickened tailings are pumped from the process plant to the TSF in an HDPE-lined carbon steel pipe. Water produced by the settling of the tailings solids is reclaimed with barge-mounted pumps and returned to the process water storage tank.• Process Control Systems - The process plant design includes an integrated process control system.The two finished products from the Stibnite Gold Project ore processing facility will be: gold/silver bars, known as doré; and antimony-silver concentrate.
Water supply for the mine, process plant, worker housing facility, and site dust control would be provided by four types of water systems: freshwater (including fire water), potable water, reclaim water, and contact water re-use. Freshwater for the process would be supplied from groundwater resources by a surface intake on the EFSFSR near the south tunnel portal, a water supply well field, and dewatering wells associated with Hangar Flats and Yellow Pine pits. Reclaim water would be pumped back to the process facility from the supernatant water pond in the TSF. A water supply wellfield would be developed for potable water supply to the worker housing facility. Potable water would be filtered and chlorinated before use. Potable water for the office and other mine facilities would be supplied from the potable water tank at the worker housing facility via a holding (head) tank at the same location as the freshwater/fire water tank for the process. Contact water, when available, would be reused for process makeup and for dust control on haul roads, plant roads, backfills, stockpiles, and the TSF Buttress when of suitable quality for direct use.Freshwater / Fire Water Supply Freshwater for process needs would be supplied by an intake on the EFSFSR, a water-supply well field located in the Meadow Creek valley upstream from its confluence with the EFSFSR, and from dewatering of the Yellow Pine and Hangar Flats pits. Surface water from the EFSFSR intake would be pumped to a booster tank adjacent to the Midnight contact water pond, and then to the process plant. The mill water supply wellfield would consist of approximately twelve 14-inch diameter alluvial wells, ranging from 200 to 270 feet in depth. Groundwater pumped from dewatering and supply wells would be collected in equalization tanks (with destination tank depending on well location) and pumped either to the process plant or to the freshwater/firewater head tank located at approximately 6,800 ft amsl along the access road.Potable Water Supply Water for the worker housing facility would be obtained from a separate water supply wellfield located in the EFSFSR valley to its southwest. This water will be filtered and chlorinated for cleaning, cooking, showering, and consumptive use in the worker housing facility. Water from the worker housing facility potable water tank is designed to flow by gravity to a potable water tank at approximately 6,800 ft amsl on the access road to provide gravity flow of potable water to eye wash-safety showers and sinks, showers, and restrooms in the process plant and associated areas.Reclaim Water System Reclaim water pumped from the TSF supernatant water pond would be reused as process water. Water reclaimed from the TSF would be pumped to the reclaim water tank at the plant site. From the TSF to the plant site, the reclaim water pipeline would share a lined secondary containment trench with the tailings pipeline. At the plant site, the reclaim line would diverge and be located in its own secondary containment trench. Water from the reclaim water tank would be distributed to various points of use in the process. During reprocessing of the Bradley Tailings, an additional pipeline would shunt a portion of reclaim water from the main reclaim line to the repulping plant where it would be used to slurry the tailings for pumping to the processing plant.Contact Water Reuse Contact water collected in ponds and in-pit sumps would be pumped via pipeline or directly into trucks to points of reuse or treatment/evaporation. Contact water for reuse in the process plant would report to the process water tank. Contact water for road/mine dust control would be pumped directly to water trucks. Stormwater retained in haul road sediment traps may also be pumped out for use in dust control on those roads or other mine features.
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