This preliminary economic assessment examines the Doby George deposit within the Aura Project in Elko County, Nevada, evaluating conventional open-pit mining and cyanide heap-leaching for gold recovery.
Report context
The Doby George preliminary economic assessment (PEA) technical report, dated with an effective date of January 27, 2025, for mineral resources, presents a study of the Doby George deposit located approximately fifteen miles southwest of Mountain City, Nevada. The report covers mineral processing, metallurgical testing, mining methods, recovery methods, and economic analysis for the Doby George deposit only, not including other deposits within the Aura Project. The study assumes a processing rate of 7,500 tonnes per day over a mine life of approximately 4.2 years following a one-year pre-production period.
Processing route
Crushing and agglomeration
Ore will be mined using conventional open-pit methods and delivered to a run-of-mine stockpile near the primary crusher. A front-end loader will feed material to a dump hopper. The run-of-mine ore will be crushed to a final product size of 80% passing 12.5mm (half-inch) using a three-stage closed-circuit crushing plant operating 7 days per week, 24 hours per day with an estimated availability of 75%.
Crushed product will be stockpiled using a stacking conveyor and reclaimed by vibrating electromechanical feeders. Cement or pebble lime will be added to reclaim material for agglomeration and pH control. Although testwork has indicated that agglomeration with cement is not required, cement will be added during the first lift as a precautionary measure to ensure permeability.
Heap leaching
Ore will be stacked on a single-use, multi-lift leach pad designed to store 12 million tonnes of ore. The pad has been designed with a lining system approved by the state of Nevada. Phase 1 stacking area will accommodate 5.5 million tonnes, with an additional 6.5 million tonnes placed after Phase 2 expansion. Stacking will use retreat stacking uphill from the toe of the heap.
Single-stage leaching with a 140-day leach cycle is assumed, using a drip irrigation system for solution application. Gold-bearing pregnant leach solution will drain by gravity to a pregnant solution pond. Load permeability tests on residual leached 12.7mm material indicated adequate permeability for commercial heap leach stack heights of up to 91 meters (300 feet) without any pretreatment agglomeration.
Carbon adsorption and gold recovery
Pregnant solution will be pumped to a set of carbon-in-columns (CICs) where gold will be removed by activated carbon. The adsorption circuit will consist of three trains of five CICs, each column containing 2 tonnes of carbon. Pregnant solution will flow up through the first column and exit from the top into the next column. When carbon in the first column of a train reaches a loading of 2,500 grams gold per tonne, it will be advanced into acid wash or elution vessel. Each train will be advanced every 3 days, resulting in one strip per day.
The acid wash vessel will treat carbon by circulating dilute hydrochloric acid at pH 2 for several hours to dissolve carbonate scale. Residual acid will be neutralized with caustic before carbon transfer to the elution vessel. Gold on carbon will be stripped with strip solution at high temperature and pressure, with vessel pressure controlled by valve and temperature controlled by a boiler.
Gold will be recovered from strip solution onto cathodes of electrowinning cells as sludge. The sludge will be removed using a high-pressure washer and dried in a filter press. The filter cake will be treated in a retort furnace to remove contained mercury. The dried mercury-free cake will be mixed with fluxes in a furnace before pouring into gold doré bars.
Barren leach solution leaving the CICs will flow to a barren solution sump and be pumped to the heap leach pad for further leaching. Cyanide solution will be injected into barren solution to maintain desired cyanide concentration. An event pond is included to collect contact solution from storm events.
On-site infrastructure
The overall site plan includes an open pit mine, waste rock dumps, mine shop, magazine, crushing plant, heap leach pad and ponds, process plant, and main access road. The crushing plant, leach pad, process ponds, and process plant are generally located on a downhill trend in a north-to-south direction.
Site access is via Maggie Summit Road (County Road 729), a dirt road off State Route 225 eight kilometers south of Mountain City. Internal roads will provide access between process plant, heap leach, crusher, and mine facilities. A network of mine haul roads will be constructed and maintained by the mining contractor.
Site buildings will generally be modular and include an administration building, security building (gatehouse), process office, process maintenance shop, mine maintenance shop, and portable restrooms. The project will be serviced by an existing 14.4/24.9 kV power line owned and operated by NV Energy, with a 24.9 kV spur power line to be constructed to distribute power to process, crushing, and mine facilities.
Water will be supplied from well DG-1 located at elevation 1,880 meters (6,169 feet) near Doby George Creek, pumped uphill to an 820 cubic meter raw water tank at elevation 1,960 meters (6,430 feet). Potable water will be delivered by truck and stored in a HDLPE tank, with sodium hypochlorite used for disinfection.
Key reported parameters
| Parameter | Value | Units | Basis |
|---|---|---|---|
| Processing rate | 7,500 | tpd | Design |
| Crush size (final product) | 80% passing 12.5mm | mm | Design |
| Leach cycle | 140 | days | Design |
| Heap leach stack height (max) | 91 | meters | Design (permeability tested) |
| Gold recovery (overall) | 67 | % | Design assumption |
| Column leach recovery (-50mm feed) | 56.1 to 77.8 (avg 65) | % | Testwork (2022) |
| Column leach recovery (80% -12.7mm feed) | 64 to 81.8 (avg 72) | % | Testwork (2022) |
| Cyanide consumption (12.7mm crush) | <0.4 | kg NaCN/mt ore | Testwork (2022) |
| Hydrated lime consumption | 0.7 to 2.0 | kg/mt ore | Testwork (2022) |
| Heap leach gold recovery (oxidized material) | ~70 | % | Historical testwork |
| Carbon loading target | 2,500 | g Au/t carbon | Design |
| Carbon advance frequency | Every 3 days per train | days | Design |
| Crushing plant availability | 75 | % | Design |
Project website: https://www.westernexploration.com/projects/doby-george/default.aspx
Technical qualifications
This preliminary economic assessment is preliminary in nature and includes inferred mineral resources that are considered too speculative geologically to have the economic considerations applied that would enable them to be classified as mineral reserves. There is no certainty that the preliminary assessment will be realized. The estimated capital and operating costs are considered to have an accuracy of plus or minus 25 percent.
Historical metallurgical testing on oxidized material from Doby George showed heap leach gold recoveries approaching 70 percent for most materials tested, though testing was conducted on relatively fine feeds (3/4 inch or finer). Only limited testwork was conducted on mixed-oxidized and unoxidized mineralization, which reported significantly lower gold recoveries of approximately 35 percent and 10 percent, respectively. The 2022 column testwork, while positive, was conducted primarily on material from the West Ridge Deposit, which contains approximately 82 percent of the 2021 mineral resource.
For the Doby George deposit, most of the drilling pre-dates WEX's involvement, and there is little or no quality assurance/quality control support available for a significant portion of historical drill-hole data. Historical holes with some check analyses and quality assurance/quality control data show that average assay grades in the database may be high by 5 to 10 percent relative to check assays, though check assay samples were prepared several years after original assays.
*Source: Doby George PEA Technical Report, M0047.24003, effective January 27, 2025 (mineral resources), Sections 1.5, 1.7, 1.8, 1.9, 1.10, 1.11, 1.12*
