Technical Report and Preliminary Feasibility Study: Hasbrouck and Three Hills Gold-Silver Project

This report presents the proposed design for two separate heap-leach gold-silver operations approximately 5 miles apart, with Three Hills constructed first followed by Hasbrouck.

Report context

This technical report and preliminary feasibility study for the Hasbrouck and Three Hills Gold-Silver Project, dated July 15, 2015, describes the proposed recovery methods for two separate facilities. The proposed Three Hills Mine is a 15,000 ton per day run-of-mine (ROM) heap-leach operation with a full recovery plant. The proposed Hasbrouck Mine is a 17,500 ton per day crushed-material heap-leach operation, which will utilize the recovery plant located at the Three Hills site.

Processing route

Three Hills Proposed Mine Recovery Methods

Process description. The proposed Three Hills Mine will be a 15,000 ton per day ROM heap-leach operation. Processing will be by conventional heap-leaching of ROM material stacked on a single-use pad. Gold will be leached from the mineralized material with dilute cyanide solution and recovered from the solution using a carbon adsorption-desorption-recovery (ADR) plant to produce doré bars.

Lime storage and addition. Pebble lime will be required for pH control in the heap-leach at a nominal consumption of 4.0 lbs/ton of material. Lime will be stored in a 150 ton silo (3.6 days capacity) equipped with a variable speed feeder, which meters the lime directly into loaded haul trucks for delivery to the heap-leach. Lime will be added in proportion to the tonnage of material being hauled.

Stacking. Material will be processed in a truck-stacked ROM heap-leach. Material from the mine will be loaded into haul trucks, mixed with lime, and delivered to the heap-leaching facility (HLF) where it will be placed in 30 ft lifts by haul trucks. A dozer will periodically assist in heap construction and rip the heap surface prior to leaching.

Solution application and leaching. Leaching will use a dilute solution of sodium cyanide applied by a system of drip emitters at a nominal application rate of 0.0025 gpm/ft². Drip emitters generate less evaporation than sprays and minimize make-up water requirement. The dilute cyanide leach solution will percolate through the stacked material, dissolve gold, and drain by gravity to a pregnant solution tank. Vertical turbine pumps in the pregnant solution tank will pump solution to the head tank of the carbon columns. The solution will flow by gravity through the carbon in columns to a barren solution tank. High-strength cyanide solution and antiscalant will be added to the barren tank by metering pumps.

Leach pad design and solution collection. The HLF will be a multiple-lift, single-use leach pad designed to accommodate approximately 10 million tons of ROM material. The HLF has been designed with a lining system in accordance with International Cyanide Code requirements. The HLF has been sized using an average stacked material density of 97 lbs/ft³ and a maximum heap height of 150 ft. ROM material will be truck-stacked at an average rate of 15,000 tons per day, 360 days per year. Material will be stacked in approximately 30 ft lifts with benches creating an average overall slope of 3:1 (horizontal to vertical). The HLF will be lined with a composite lining system consisting of a prepared subgrade, a 12 in thick compacted low-permeability soil layer or geosynthetic clay liner (GCL), and an 80 mil high-density polyethylene (HDPE) geomembrane liner. The HLF will be constructed in a single phase providing a total lined leach pad surface area of approximately 3 million square feet.

Geotechnical investigation. A geotechnical investigation was completed at the proposed Three Hills mine in September 2014. Three boreholes and three test pits were excavated within the footprint of the HLF. The site is characterized by shallow bedrock overlain by granular soils typically less than 8 ft thick. Bedrock was a rhyolitic volcanic tuff described as weak to extremely weak rock, slightly to moderately weathered, and relatively unfractured. Slope stability analysis was performed for both static and seismic conditions. Static analysis factors of safety for circular and block failure modes were 1.9 and 1.3, respectively. Pseudostatic conditions during operations gave factors of safety of 1.6 and 1.0 respectively. In an extreme seismic event (2,475 year return event), slope movement up to 24 in may occur, which may result in minor sloughing but not compromise slope integrity. Groundwater was not encountered and is not anticipated to influence design, construction or operation.

Solution storage. The event pond will have a total storage capacity of approximately 7.5 million gallons, based on runoff from the estimated 100-year, 24-hour storm event and anticipated drain-down from a 12-hour power outage. The event pond lining system will consist of two layers of HDPE geomembrane liner sandwiching a geonet layer. The pregnant solution tank is sized for 30 minutes at the nominal primary leach rate of 3,000 gpm (90,000 gallons). The barren solution tank is sized for 30 minutes at the nominal secondary leach rate of 1,500 gpm plus 3,000 gpm barren solution flow (135,000 gallons).

Solution management. The process system is designed as a zero discharge facility. Based on weather data and the HLF water balance, the project will operate in a monthly water deficit under all weather conditions; cyanide neutralization will not be required.

Process water balance. Ecological Resource Consultants, Inc. (ERC) completed a water balance for the Three Hills HLF. The evaluation included development of a stochastic water balance using GoldSim (version 9.60) with a Monte Carlo simulation model using a monthly timestep for 22 months. The water balance tracks inputs (process water and precipitation) and outputs (evaporation and material uptake). Process water is added at an average rate of 0.0025 gpm/ft². A heap loading rate of 15,000 tons per day was used. Make-up water requirements are anticipated to range from approximately 170 gpm at the beginning of operations to nearly 450 gpm during the second summer.

Adsorption-desorption recovery plant. The adsorption facility will consist of a single train of 5 up-flow, open-top carbon columns (CICs) capable of holding 6 tons of carbon each (30 tons total inventory). Pregnant solution will flow at a nominal rate of 3,000 gpm by gravity through the 5 columns in series. Loaded carbon from the first column will be pumped to an acid wash vessel where hydrochloric acid will be circulated to remove calcium carbonate scale. The elution vessel is designed to process up to 3 tons of carbon in a modified Zadra-type desorption cycle (12 to 16 hours per cycle) at 275°F and 70 psig. Pregnant eluent will flow to 2 electrowinning cells operated in parallel, with gold plated onto stainless steel cathodes. The resulting sludge will be filtered in a plate and frame filter press, then processed in an electric mercury retort. Mercury emission requirements have been investigated and Three Hills will meet all current emission guidelines. Carbon thermal regeneration uses a diesel fired kiln at approximately 1,400°F, capable of treating approximately 154 lbs activated carbon per hour.

Refining and smelting. Retorted sludge will be treated in an electric induction smelting furnace with fluxes (silica, borax, niter, soda ash, and possibly fluorspar). Doré bars will be quenched, cleaned, labeled, weighed, and shipped to an off-site refiner.

Reagents delivery, storage and consumption. Annual consumption estimates include: pebble lime 10,950 tons; sodium cyanide (30%) 1,230 tons; activated carbon 23 tons; diesel 111,300 gal; antiscalant 23,400 gal; hydrochloric acid (32%) 34,200 gal; sodium hydroxide (50%) 73 tons; and various fluxes.

Hasbrouck Proposed Mine Recovery Methods

Process description. The proposed Hasbrouck Mine will be a 17,500 ton per day heap-leach operation. Processing will be by conventional heap leaching of crushed material stacked on a single-use pad. Gold and silver will be leached with dilute cyanide solution and recovered using a carbon adsorption-desorption-recovery process. The adsorption equipment will be located at Hasbrouck. Loaded carbon will be trucked at approximately 3 tpd to Three Hills for desorption and precious metals recovery. Carbon will be reactivated at Three Hills and backhauled to Hasbrouck for reuse.

Ore stockpiles. The proposed Hasbrouck Mine will include a ROM stockpile sized to accommodate 70,000 tons and a crushed material stockpile with live capacity of approximately 2,300 tons.

Crushing. ROM ore will be delivered by haul trucks to a primary crusher's dump hopper or to the ROM stockpile. A stationary 20 in grizzly will prevent oversized ore from entering. ROM ore will be delivered at an average rate of 730 dry tph to a vibrating grizzly with 6 in spacing. Oversize material will be crushed using a primary jaw crusher to 100% passing 12 in. Discharge will feed secondary screens. Material greater than 2 in will be sent to secondary cone crushers. Output from cone crushers and screen undersize will feed the HPGR feed bin (950 tons capacity). The HPGR will operate with an adjustable edge splitter recycling approximately 15% of discharge.

Agglomeration. Crushed material will require agglomeration with cement prior to cyanide leaching. Cement will be added at a nominal rate of 5 lbs per ton of crushed material from a 100 ton silo. Crushed material and cement will be fed to a pug mill for blending with barren solution to adjust moisture content between 7% and 13%.

Stacking. The heap will be constructed using a conveyor stacking system including an overland conveyor, ramp transfer conveyors, grasshopper transfer conveyors, an index feed conveyor, a horizontal index conveyor, and a 136 ft long telescoping radial stacking conveyor. The radial stacker can place crushed material in 30 ft lifts. The pad will be stacked from the down slope toe in an upslope direction.

Solution application and leaching. Leaching will use dilute sodium cyanide solution applied by drip emitters at a nominal application rate of 0.0025 gpm/ft². Solution will percolate through the heap, dissolve gold and silver, and drain to a pregnant solution tank. Submersible pumps will pump solution to the head tank of the carbon columns.

Leach pad design. The Hasbrouck HLF will be a multiple-lift, single-use leach pad designed to accommodate approximately 36.1 million tons of crushed material, constructed in two phases. The HLF has been sized using an average stacked material density of 93.6 lbs/ft³ and a maximum heap height of 150 ft. Material will be conveyor-stacked at a nominal rate of 17,500 tons per day in 30 ft lifts. The HLF will be continuously lined with a composite lining system (prepared subgrade, 12 in compacted low-permeability soil layer or GCL, and 80 mil HDPE geomembrane liner). The HLF will be constructed in two phases providing a total lined leach pad surface area of approximately 8.5 million square feet.

Geotechnical investigation. Geotechnical field investigations at the Hasbrouck HLF were not performed during this stage of design and are planned to be completed during the next phase. Surface and subsurface conditions were characterized by surface mapping conducted by Vista Gold Corporation. The current proposed location of the HLF is approximately 1 mile south of Hasbrouck Peak. Slope stability analysis for static conditions indicated factors of safety for circular and block failure modes of 1.9 and 1.6, respectively. Pseudostatic conditions during operations gave factors of safety of 1.5 and 1.3. Groundwater is not anticipated to influence design, construction or operation.

Solution storage. The event pond will have a total storage capacity of approximately 17.7 million gallons. The event pond lining system will consist of two layers of HDPE geomembrane liner separated by a geonet layer. The pregnant solution tank is sized for 30 minutes at the nominal primary leach rate of 3,800 gpm (114,000 gallons). The barren solution tank is sized for 30 minutes at 3,800 gpm (114,000 gallons).

Solution management. The Hasbrouck Mine is designed as a zero discharge facility. Based on weather data and the site water balance, the project will operate in a water deficit under all weather conditions; cyanide neutralization will not be required.

Process water balance. KCA completed a water balance for the Hasbrouck HLF. Process makeup water will be required for all months and for all precipitation conditions analyzed. The HLF will require an average of 256 gal/h of makeup water. Water balances were calculated for average year, extreme wet year, and extreme dry year conditions.

Adsorption. The adsorption facility will consist of a single train of 5 up-flow, open-top CICs capable of holding 7 tons of carbon each (35 tons total inventory). Pregnant solution will flow at a nominal rate of 3,800 gpm through the 5 columns, exiting as barren solution. Loaded carbon will be transferred by truck to Three Hills for desorption, acid washing and regeneration.

Carbon handling. Loaded carbon will be trucked to Three Hills. Assuming a nominal carbon advance rate of 1.7 tons per day, the kiln can reactivate 58% of stripped carbon in each cycle. Regenerated or new carbon will be trucked back to Hasbrouck.

Reagents delivery, storage and consumption. Annual consumption estimates for Hasbrouck include: cement 15,750 tons; sodium cyanide (30%) 2,400 tons; activated carbon 30 tons; diesel 134,

Key reported parameters

Technical qualifications

The source reports design and consumption estimates for a proposed operation; actual performance is not inferred.