Overview
The Three Hills Mine, a pivotal component of the larger Hasbrouck Gold Project, represents a significant development in the historic Tonopah mining district of Esmeralda County, Nevada. Strategically located approximately one mile west of the town of Tonopah, the Three Hills deposit is owned and operated by West Vault Mining Inc. (formerly Hasbrouck Gold Corp). This project is characterized by its dual-phase development strategy, wherein the Three Hills Mine is slated for construction and operation first, providing the initial cash flow and operational infrastructure necessary to support the subsequent development of the larger Hasbrouck Mine located just five miles to the south. This phased approach is a hallmark of modern junior mining strategy, prioritizing lower capital intensity and faster paths to production.
The significance of the Three Hills Mine lies in its streamlined mineral processing circuit. Unlike many contemporary gold operations that require intensive comminution (crushing and grinding), Three Hills is designed as a Run-of-Mine (ROM) heap leach operation. This technical decision is supported by extensive metallurgical testing which indicates that the mineralization, primarily hosted within the Siebert Formation, is highly amenable to direct cyanidation without the need for prior size reduction. This “no-crush” model drastically reduces initial capital expenditure (CAPEX) and ongoing operating costs (OPEX), making it a high-margin prospect even in fluctuating gold price environments. By utilizing a 15,000 ton per day (tpd) processing rate, the project aims to capitalize on the porous, oxidized nature of the orebody to achieve an average operational gold recovery of approximately 79.0%. As the foundational phase of the Hasbrouck Gold Project, Three Hills sets the technical and environmental benchmark for responsible gold production in one of the world’s most prolific mining jurisdictions.
Geologically, the deposit is situated within the Walker Lane structural belt, a region renowned for its epithermal gold-silver deposits. The mineralization at Three Hills is largely hosted within the Tertiary-age Siebert Formation, consisting of tuffaceous sediments and volcanoclastics. The gold is fine-grained and associated with quartz-adularia alteration. Because the deposit has undergone significant oxidation, the pyrite and other sulfide minerals that might otherwise impede gold recovery have been largely destroyed. This natural oxidation is what allows for the high recoveries associated with ROM heap leaching, as the cyanide solution can easily penetrate the rock fabric to contact and dissolve the gold particles.
Key Process Stages
The mineral processing circuit at Three Hills is designed for simplicity, efficiency, and environmental containment. The following stages outline the flow of material from the open pit to the final gold recovery:
- Open-Pit Mining: Conventional drill-and-blast methods are employed to extract ore from the Three Hills deposit. Due to the ROM nature of the processing, the blast patterns are optimized to achieve a fragmentation profile suitable for direct stacking.
- ROM Ore Stacking: Ore is transported via haul trucks directly from the pit to the heap leach pad. Unlike traditional operations, there is no primary, secondary, or tertiary crushing circuit. Ore is stacked in lifts on a single-use leach pad, utilizing “retreat stacking” to minimize compaction from haulage equipment.
- Leaching Circuit: A dilute cyanide solution is applied to the surface of the heap via a network of drip emitters. This method minimizes evaporation and ensures a uniform distribution of the lixiviant. The solution percolates through the ROM ore, dissolving the gold as it migrates toward the bottom of the pad.
- Solution Collection: The “pregnant” leach solution (PLS), now enriched with gold, is collected by a system of perforated pipes located at the base of the heap, above the impermeable liner. This solution is then directed to the pregnant solution pond or directly to the recovery plant.
- Adsorption (ADR) Process: Gold is recovered from the PLS using a conventional carbon adsorption circuit. The solution passes through a series of Carbon-in-Column (CIC) tanks where the gold is adsorbed onto the surface of activated carbon granules.
- Toll Stripping and Refining: Once the carbon is “loaded” with gold, it is removed from the circuit. Under the current development plan, the loaded carbon is transported off-site for toll stripping and refining to produce gold doré bars, further reducing the need for on-site high-temperature processing infrastructure.
- Reagent Management: The barren solution, stripped of gold, is recharged with cyanide and its pH is adjusted (typically with lime) before being pumped back to the heap, creating a closed-loop system.
Critical Data
The following table summarizes the key technical and economic parameters for the Three Hills Mine mineral processing operation based on the latest Pre-Feasibility Study (PFS) data.
| Parameter | Value | Unit |
|---|---|---|
| Processing Rate (ROM) | 15,000 | Tons per day (tpd) |
| Average Gold Recovery | 79.0 | Percent (%) |
| Cut-off Grade | 0.005 | oz Au/ton |
| Mining Method | Open-Pit | N/A |
| Leach Pad Type | Single-use / Multi-lift | N/A |
| Expected Mine Life (Phase 1) | Approx. 2-3 | Years |
| Operating Cost (Total Project) | 10.02 | USD per ton |
| Gold Recovery Method | CIC Adsorption / Toll Stripping | N/A |
| Silver Recovery | Negligible / Not Modeled | N/A |
Technical Details and Sustainability
Metallurgical Amenability and ROM Logic
The decision to utilize a Run-of-Mine (ROM) heap leach process at Three Hills is rooted in the unique metallurgical characteristics of the ore. Metallurgical testing, including column leach tests performed by Kappes, Cassiday and Associates (KCA) and McClelland Laboratories, demonstrated that the gold is remarkably accessible. In many deposits, gold is “locked” within a silica or sulfide matrix, requiring fine grinding to liberate the precious metal. At Three Hills, the gold resides along fractures and within the porous tuffaceous matrix of the Siebert and Fraction formations. This allows the cyanide solution to penetrate the rock even at larger fragments. Testing confirmed that while crushing to 3/4 inch might slightly increase recovery rates, the marginal gain in gold does not offset the substantial capital and operating costs of a crushing plant. By skipping the crushing stage, the project minimizes its energy footprint, reduces dust emissions, and simplifies the overall operational flow.
Hydraulic Conductivity and Heap Height
A critical technical challenge in ROM leaching is maintaining adequate “percolation” or hydraulic conductivity. Because ROM ore contains a wide range of particle sizes—from fine silts to large boulders—there is a risk of “blinding” or compaction, which can cause the solution to pool on the surface or channel through specific paths, leaving large areas of the heap un-leached. To mitigate this, the Three Hills design incorporates strict monitoring of percolation rates. Engineers have planned for the installation of intermediate drains and the potential use of “stair-step” stacking to manage the heap’s structural integrity. Testing has confirmed that the material maintains sufficient permeability up to significant stacking heights, though the project remains prepared to adjust stacking methods if field observations during the early phases of mining suggest compaction issues.
Environmental Engineering and Sustainability
In the modern regulatory landscape, particularly in Nevada, environmental stewardship is inseparable from technical design. The Three Hills Mine utilizes a state-of-the-art composite liner system to protect the local environment. This system typically includes a 60-mil high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) geomembrane placed over a prepared layer of low-permeability soil. A leak detection system is situated beneath this liner to provide real-time monitoring of the pad’s integrity. Furthermore, the use of drip irrigation instead of sprayers significantly reduces the risk of “overspray” and minimizes the loss of water—a precious resource in the high desert of Nevada.
The project’s sustainability profile is also enhanced by its reclamation plan. As a single-use pad, the heap will be rinsed and neutralized at the end of the mine life to ensure that any residual cyanide is destroyed or sequestered according to Nevada Department of Environmental Protection (NDEP) standards. The site will then be re-contoured and re-vegetated to match the surrounding landscape. By leveraging the natural oxidation of the ore and the simple ADR recovery process, Three Hills minimizes the use of heavy machinery and chemical additives, resulting in a lower carbon intensity per ounce of gold produced compared to more complex milling operations.
Future Outlook
The Three Hills Mine serves as the catalyst for the Hasbrouck Gold Project. Its successful implementation will prove the viability of the “Nevada ROM” model for this specific geological corridor. As production moves from Three Hills to the Hasbrouck Mine, the experience gained in managing the Siebert Formation’s unique properties will be invaluable. With a low-cost profile and a high degree of technical readiness, Three Hills is poised to be a contributor to Nevada’s gold output, demonstrating that technical simplicity, when backed by rigorous data, is often the most sustainable path forward in mineral processing.
Source: Three Hills Mine
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
