This report describes the proposed processing facilities and design criteria for the North Bullfrog Project as of the 2020 Preliminary Economic Assessment.
Report context
The North Bullfrog Project report is dated 2020 and presents a Preliminary Economic Assessment for the project, which is located in Nevada. The document includes processing design criteria and recovery methods based on metallurgical test work performed on mineralized material from the Yellowjacket, Sierra Blanca, Mayflower, Jolly Jane, and Savage Valley deposits.
Processing route
Heap Leach/ADR Facility
Mineralization from the Yellowjacket, Sierra Blanca, Mayflower, Jolly Jane, and Savage Valley open pits would be mined and placed onto a heap leach pad. Run of Mine (ROM) mineralized material would be mined at an average rate of approximately 44,700 tonnes per day and placed on a conventional heap leach pad in nominal 9.1 m (30 ft) lifts. Through the use of ultra-high-intensity blasting, the mineralization would be expected to have a P80 of approximately 84 mm (3.3 in).
Quicklime (CaO) would be added to the mineralized material prior to placement on the leach pad for pH control. Barren solution containing dilute sodium cyanide would be applied via drip emitters at an application rate of 6.1 L/h/m2 (0.0025 gpm/ft2). Pregnant solution would be collected in a divided pond where sediment would settle out of the solution before overflowing an internal berm. The pregnant solution pumps would feed a pair of vertical carbon-in-columns (VCIC). A second pair of VCICs would be added to manage the increased barren flow rate in Year 5. Gold and silver extracted from the pregnant solution onto activated carbon would be further processed via the desorption and recovery circuit to produce a final doré product.
The leach pad would be designed as a double-lined facility consisting of a layer of geosynthetic clay liner (GCL) having a hydraulic conductivity less than or equal to 1×10-6 centimeters per second (cm/s) which acts as the secondary liner system. A layer of 80-mil geosynthetic liner made from high density polyethylene (HDPE) would be placed over the GCL to act as the primary liner. A leak detection system would be installed between the GCL and the HDPE liner.
The mixed ROM mineralization and gravity mill tails would be stacked in lifts averaging 9.1 m (30 ft) high to a maximum of 91.4 m (300 ft) at a nominal rate of 47,315 tonnes per day. The leach pad would be constructed in four phases. Phase 1A would have an area of approximately 630k m2 with a capacity to hold 38.5M tonnes. Phase 1B would have an area of approximately 342k m2 with a capacity of 29.6M tonnes. Phase 2 would have a lined area of approximately 953k m2 and a capacity of 92.9M tonnes. Phase 3 expansion would include an additional 605k m2 of lined area with a capacity of 60.5M tonnes.
Dilute cyanide solution would be distributed to the leach pad by three vertical turbine pumps installed in a common barren solution sump. One pump would serve as an installed spare, with two pumps operating to deliver the barren solution at a flow rate of 949 m3/h. During Phase 2 leach pad construction, an additional barren sump and pumps would be added to deliver an additional 1,300 m3/h for a combined barren flow up to 2,249 m3/h.
The solution would be pumped through an 18-inch distribution pipeline around the toe of the leach pad. The solution would be conveyed to each consecutive lift via a series of 12-inch riser pipes. Seven risers would be installed for Phase 1A, with an additional four risers installed to service Phase 1B. Approximately seven risers would be installed for the Phase 2 leach pad expansion, with an additional seven risers for Phase 3.
Blasting, loading, hauling, and placement of the mineralized material on the leach pad would be by an owner mining fleet. The haul trucks would place mineralized material on the lift being constructed. The mineralized material would be truck stacked (end dumped) and then pushed out and cross-ripped by a dozer to eliminate any compaction that occurred during placement.
Gravity Mill Circuit
High grade vein and vein stockwork mineralization from the Yellowjacket deposit would be processed through a mill circuit at a rate of approximately 4,700 tonnes per day. The mineralized material would be run through a two-stage portable crushing circuit and conveyed into a fine crushed product bin at a P80 of 19 mm (¾ in). The fine crushed product bin would feed a rod mill with a target product size of 48 mesh. Slurry classification would be via multi-stage screens. The screen undersize would feed a gravity concentrator circuit. Gravity concentrate would be processed via an intense leach reactor with the pregnant solution processed via a dedicated electrowinning cell for doré production. The gravity tails would be thickened and filtered prior to stockpiling. The filtered tails would be loaded into haul trucks and dumped on the leach pad for residual gold and silver recovery.
Carbon Adsorption and Desorption
Pregnant solution would flow from the heap leach pad into the pregnant solution pond. The solution would then be pumped into two parallel vertical carbon-in-column (VCIC) trains at a nominal flowrate of 863 m3/h. During Phase 2, a second set of VCICs and pregnant solution pumps would be installed to accommodate the necessary increase in flow, operating at a nominal 1,181 m3/hr for a combined pregnant solution flow of 2,044 m3/hr.
Pregnant solution would enter the bottom of each VCIC at a nominal flowrate of 432 m3/hr. Six separate chambers would contain 2.7 tonnes of carbon each for gold and silver adsorption. The design efficiency would be 98.4% for gold and 87.7% for silver.
Loaded carbon would be stripped using a Pressurized ZADRA strip scheme, utilizing 10-12 bed volumes of a circulated 149°C (300°F) solution of a minimum 1.25% caustic (NaOH) and 0.2% sodium cyanide (NaCN) concentration at a minimum pressure of 380 kPa (55 psig). The strip cycle time was estimated at 10 hours per day.
Carbon regeneration would be performed in an electric rotary kiln, heating the carbon to 650 to 750 °C in a slightly oxidizing atmosphere to remove organic fouling.
Electrowinning and Refining
The heap leach electrowinning (EW) cells would be designed to treat 30 to 45 gpm per cell of pregnant strip solution in two cells operated in parallel with each cell containing 34 anodes and 33 cathodes. A dedicated 20 anode/19 cathode EW cell would receive the pregnant solution from the gravity mill intense cyanidation reactor.
The loaded cathodes would be pressure washed, producing sludge that would be pumped to a plate-and-frame filter press. After mercury removal in a retort, the dried sludge would be mixed with fluxes and smelted in an induction furnace to produce doré bullion.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| ROM mineralized material to leach pad | 44,700 | tonnes per day | Design |
| Mill throughput | 4,700 | tonnes per day | Design |
| Total mineralized material to leach pad | 47,300 | tonnes per day | Design |
| LOM mineralized material to leach pad | 221.4 | million tonnes | Design |
| P80 (heap leach) | 84 | mm | Design |
| P80 (mill feed) | 19 | mm | Design |
| Rod mill target product size | 48 | mesh | Design |
| Crusher Work Index (CWi) | 11.5 | kWh/t | Testwork |
| Design bulk density | 1.65 | t/m3 | Testwork |
| AVG ROM mineralized material grade, Au | 0.20 | gpt | Design |
| AVG ROM mineralized material grade, Ag | 0.55 | gpt | Design |
| AVG mill tails grade, Au | 0.80 | gpt | Design |
| AVG mill tails grade, Ag | 9.71 | gpt | Design |
| LOM AVG leach pad grade, Au | 0.23 | gpt | Design |
| LOM AVG leach pad grade, Ag | 1.12 | gpt | Design |
| Heap leach gold recovery – Yellowjacket | 73.2 | % | Testwork |
| Heap leach gold recovery – Sierra Blanca | 73.2 | % | Testwork |
| Heap leach gold recovery – Savage Valley | 74.1 | % | Testwork |
| Heap leach gold recovery – Jolly Jane | 61.7 | % | Testwork |
| Heap leach gold recovery – Mayflower | 76.4 | % | Testwork |
| Heap leach gold recovery – mill tails | 71.8 | % | Testwork |
| Heap leach silver recovery – Yellowjacket | 13.3 | % | Testwork |
| Heap leach silver recovery – mill tails | 59.1 | % | Testwork |
| Lift height | 9.1 | m | Design |
| Maximum heap height | 91.4 | m | Design |
| Solution application rate | 6.1 | L/h/m2 | Design |
| Design barren flow rate, Phase 1 | 949 | m3/h | Design |
| Design barren flow rate, Phases 2-3 | 2,249 | m3/h | Design |
| Average leach cycle, Phase 1 | 60 | days | Design |
| Average leach cycle, Phases 2-3 | 75 | days | Design |
| Solution cyanide concentration | 250 | mg/L | Design |
| Carbon adsorption efficiency, Au | 98.4 | % | Design |
| Carbon adsorption efficiency, Ag | 87.7 | % | Design |
| Heap/ADR cyanide consumption | 0.25 | kg/t mineralized material | Testwork |
| Heap/ADR lime consumption | 1.25 | kg/t mineralized material | Testwork |
| Strip temperature | 149 | °C | Design |
| Strip pressure | 380 | kPa | Design |
Project website: https://www.anglogoldashanti.com/portfolio/americas/united-states-projects/
Technical qualifications
This document is a Preliminary Economic Assessment as defined by National Instrument 43-101. The report states that test work has been performed on the NBP mineralized material by a number of metallurgical laboratories, as described in Section 13, which is not included in this extract. The test work has indicated that the Yellowjacket, Sierra Blanca, Mayflower, Jolly Jane, and Savage Valley mineralized materials are amenable to cyanide leaching for the recovery of gold and silver. The process design criteria and recovery estimates are based on this test work together with design assumptions. The report does not include full flowsheet details, operating data from historical production, or economic analysis within the sections provided. Recovery percentages vary by deposit and by metal, as shown in the table above.
Source: North Bullfrog Project , 2020 Preliminary Economic Assessment, Sections 17 (Recovery Methods, Process Design Basis, Heap Leach and ADR, Heap Leach Pad and Solution Distribution, ROM Truck Stacking, Ponds, Carbon Adsorption, Carbon Handling, Carbon Acid Washing, Carbon Stripping (Elution), Carbon Regeneration, Electrowinning, Refining, Reagents).
Consolidated report summaries
The following former standalone pages about this operation were consolidated here on October 25, 2026. They are retained for source traceability; dates and figures reflect each report’s own effective date and may be superseded by the current information above.
North Bullfrog & Mother Lode Project — 2018 Preliminary Economic Assessment
This report presents the proposed processing routes and metallurgical assumptions for the integrated North Bullfrog and Mother Lode Project as of the 2018 Preliminary Economic Assessment.
Report context
The 2018 Preliminary Economic Assessment for the North Bullfrog and Mother Lode Project describes proposed processing facilities for oxide and sulphide mineralized materials from both properties. The integrated Project includes heap leaching for oxide materials and a mill processing facility for both oxide and sulphide materials. The sulphide mill material and energy balance were modeled with the METSIM™ process simulator.
Processing route
Oxide heap leaching
Oxide materials from both Mother Lode and North Bullfrog properties would be processed in a standard heap leach recovery circuit. Mother Lode heap leach materials would be trucked to the pad at a nominal rate of 13,600 tonnes per day. North Bullfrog heap leach materials would be trucked to the pad at a nominal rate of 61,000 tonnes per day. Mineralized material would be stacked in 9.1 m lifts. Lime would be added to individual truck loads. Leaching would use buried emitters with a weak cyanide solution. Solution application rates would range from 12 l/hr-m² to 6.0 l/hr-m². Pregnant solution from the heap would be collected in a pond and pumped to a Carbon-in-Columns plant.
The Mother Lode CIC plant would treat 680 m³/hr and consist of a single train of five cascading carbon columns. The NBP CIC plant would consist of two parallel trains processing approximately 1,818 m³/hr. Carbon would advance on a weekly basis countercurrent to solution flow. Loaded carbon would be acid washed, stripped, and regenerated. Strip solution would be sent to electrolytic cells for gold recovery in a precipitate sludge, followed by retorting for mercury removal and melting into doré bars.
Mill processing
The MLP mill would process NBP oxide, NBP sulphide, and MLP sulphide materials separately from stockpiles at a rate of 8,200 tpd. The study assumed the mill would process oxide material 50% of the time and sulphide material 50% of the time. The flotation plant would remain idle while oxide material is processed. The pressure oxidation plant would be fed from the flotation plant or stockpiled flotation concentrate and would run continuously.
YellowJacket oxide milling
YellowJacket oxide material would be processed using a mill/gravity/cyanide leach circuit. Material would be primary crushed at 8,200 tpd, followed by primary SAG milling with pebble crushing and secondary ball milling, with gravity separation of cyclone underflow. Gravity tails at a P80-size of 74 µm would be leached in a CIL circuit for 24 hours. CIL tails slurry would be treated in a cyanide detoxification process using Inco/SO₂ air process methods, followed by pumping to a paste tailings thickener. Slurry would be pumped from thickener underflow as a paste into a storage facility. Loaded carbon from the CIL circuit would be acid washed, stripped, and regenerated.
Sulphide milling
Mother Lode and NBP sulphide materials would be delivered from the mines via truck and stockpiled or directly dumped into the primary crusher feed bin. Reclaimed materials would be blended to control sulphide content in feed to the flotation/POX plant. Milling would be performed using a primary SAG mill with recycle pebble cone crushing and a secondary ball mill in closed circuit with cyclones. Cyclone overflow at a P80 of 74 µm would exit the grinding circuit to the flotation circuit.
Sulphide flotation and concentrate handling
Gold and sulphide minerals predominately in pyrite would be concentrated by bulk sulphide flotation in rougher and scavenger flotation cells. Flotation concentrate would be thickened and either pumped to the regrinding mill or filtered and stored in covered storage buildings. Reclaimed concentrate would be ground to P80 10-15 µm at a rate of 410-1066 tpd and stored in agitated tanks for feed to the POX circuit.
Pressure oxidation
Concentrate slurry would be pumped to a pre-acidification tank and combined with acidic overflow solution from the pressure leach autoclave discharge. Pre-acidified slurry would be pumped into the pressure leach autoclave by high pressure piston positive displacement pumps. Sulphide minerals would be oxidized by oxygen at a temperature of 225°C and 3,600 kPa. Oxygen partial pressure would be maintained at 690 kPa. Oxygen at 93% v/v would be supplied from a vacuum swing adsorption oxygen plant. POX slurry would be let-down to atmospheric pressure through flash vessels and retained in brick-lined agitated Hot Cure tanks. Slurry from Hot Cure tanks would flow to a thickener, then through a two-stage CCD wash circuit.
Cyanide leaching and carbon absorption
Oxide mill material and POX neutralized slurry would be pumped to the CIL circuit providing approximately 24 hours of leach time. Activated carbon would be added to CIL tanks, each containing seven tonnes of carbon. Loaded carbon from the first CIL tank would be taken in seven-ton batches for processing in the carbon handling circuit. Slurry exiting the last CIL tank would be screened and flow to the cyanide detoxification circuit.
Tailings detoxification and storage
Material exiting the CIL circuit would be detoxified with an Inco/SO₂ air process and pumped to a paste tailings thickener. Thickener underflow would be pumped to the storage facility. Solution overflow from the paste thickener would be pumped back to the mill and heap leach pad for reuse.
Carbon handling and refinery
Carbon from both the CIC and CIL circuits would be processed in one carbon handling facility located at the mill. Carbon would be batch stripped in seven-ton batches in one of two strip vessels, acid washed in weak hydrochloric acid solution, and pumped to a regeneration kiln for reactivation at 700°C in a reducing atmosphere. The refinery would include electrowinning cells, a mercury retort, flux mixer, and furnace. Pregnant solutions from high intensity leach of gravity concentrate would be combined with pregnant strip solution and processed in electrowinning cells. Cathodic sludge would be retorted for mercury removal, then placed into a furnace with fluxing agents and melted into doré bars.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Mother Lode heap leach throughput | 13,600 | tpd | Proposed design |
| NBP heap leach throughput | 61,000 | tpd | Proposed design |
| Mill throughput | 8,200 | tpd | Proposed design |
| Heap leach lift height | 9.1 | m | Proposed design |
| Solution application rate range | 12 to 6.0 | l/hr-m² | Proposed design |
| Mother Lode CIC plant flow | 680 | m³/hr | Proposed design |
| NBP CIC plant flow | 1,818 | m³/hr | Proposed design |
| CIL leach time | 24 | hours | Proposed design |
| Grind size (P80) | 74 | µm | Proposed design |
| Concentrate regrind size (P80) | 10-15 | µm | Proposed design |
| Concentrate regrind feed rate | 410-1066 | tpd | Proposed design |
| Pressure oxidation temperature | 225 | °C | Proposed design |
| Pressure oxidation pressure | 3,600 | kPa | Proposed design |
| Oxygen partial pressure | 690 | kPa | Proposed design |
| Oxygen purity | 93 | % v/v | Proposed design |
| Carbon batch size (CIL) | 7 | tonnes | Proposed design |
| Carbon batch size (Mother Lode CIC) | 3.0 | tonnes | Proposed design |
| Carbon batch size (NBP CIC) | 3.5 | tonnes | Proposed design |
| Carbon regeneration temperature | 700 | °C | Proposed design |
| Mother Lode oxide heap leach Au recovery | 74 | % | Based on bottle roll tests and historical Daisy mine ROM data |
| NBP oxide heap leach Au recovery (average) | 74 | % | Based on +100 bottle roll tests and 40 column tests |
| NBP oxide heap leach Ag recovery (average) | 6 | % | Assumed range 5.8% to 9.7% |
| Mother Lode sulphide gold dissolution | 80 | % | Weighted for Tip1 and Tjvs materials |
| NBP sulphide Sierra Blanca overall Au recovery (soda ash) | 93 | % | Based on flotation and AAO testwork |
| NBP sulphide Pioneer Formation tuff overall Au recovery (soda ash) | 94 | % | Based on flotation and AAO testwork |
| NBP sulphide Rhyolite Au dissolution (soda ash) | 89 | % | Based on AAO testwork |
| NBP sulphide Dacite Au dissolution (soda ash) | 88 | % | Based on AAO testwork |
| NBP sulphide overall Ag recovery | 57 | % | Weighted: 77% in flotation, 74% in cyanide leach |
| Mother Lode oxide heap leach NaCN consumption | 0.05 | kg/t | Based on bottle roll tests and historical data |
| Mother Lode oxide heap leach CaO consumption | 2 | kg/t | Based on bottle roll tests and historical data |
| Mother Lode sulphide NaCN consumption | 0.2 | kg/t | Estimated |
| Mother Lode sulphide lime consumption | 25 | kg/t | Estimated |
| NBP sulphide average NaCN consumption | 0.2 | kg/ton ore | Based on bottle roll tests |
| NBP sulphide average lime consumption | 0.6 | kg/ton ore | Based on bottle roll tests |
| NBP heap leach makeup water requirement | 2,800 | l/min | Estimated |
| Total process manpower | 97 | personnel | Proposed design |
Technical qualifications
This Preliminary Economic Assessment is based on proposed designs and metallurgical testwork. Gold recovery for Mother Lode oxide heap leach materials is based on bottle roll tests and historical Daisy mine ROM gold production data. NBP disseminated mineralization gold recovery for heap leach materials is based on more than 100 bottle roll tests at varying sizes and 40 column tests. Mother Lode sulphide material gold dissolution is based on testing of Tip1 and Tjvs materials. NBP sulphide gold dissolution assumes that AAO and POX oxidized processes would provide similar sulphide oxidation and gold dissolution, based on correlation observed in AAO tests. Silver recovery for NBP sulphide was based on Sierra Blanca concentrate values back calculated through flotation to a calculated head grade. The report states that the NBP and Mother Lode properties have main electrical power and water located nearby in quantities sufficient for the operation and that process reagents are common commodities utilized in the Nevada gold mining industry. The State of Nevada requires that all processes with temperatures exceeding 79°C be equipped with mercury abatement equipment, and the project would comply with this requirement.
Source: Corvus Gold Inc., Preliminary Economic Assessment – North Bullfrog Project, 2018, Section 17 Recovery Methods.
