Overview
The North Inca (Main Zone) represents a pivotal high-grade gold deposit within the broader Colomac Gold Project, situated in the resource-rich Indin Lake Greenstone Belt of the Northwest Territories, Canada. Managed by Nighthawk Gold Corp (a subsidiary of Calibre Mining), the North Inca deposit is part of the Leta Arm Area, a series of satellite deposits that also includes the Diversified and Lexindin zones. Strategically located approximately 200 kilometers north of Yellowknife, the North Inca site has a storied history as a former producer, dating back to exploration efforts in the 1940s. Today, it stands as a cornerstone of a modern, multi-million-ounce gold development plan that leverages centralized mineral processing to maximize economic efficiency and minimize the environmental footprint in a remote subarctic environment.
The project’s significance lies in its “hub-and-spoke” development model, where high-grade material from deposits like North Inca (Main Zone) is blended with the larger, lower-grade tonnage from the Colomac Main deposit. This synergy is essential for optimizing the feed grade to the processing plant, ensuring a robust life-of-mine (LOM) production profile. Geological assessments of the North Inca Main Zone reveal complex quartz-carbonate vein systems hosted within sheared volcanic and metasedimentary rocks, characterized by “free-milling” gold that is highly amenable to conventional gravity and cyanide leaching processes. As the mining industry pivots toward sustainable development in Canada’s North, the North Inca deposit exemplifies the balance between technical excellence, metallurgical innovation, and responsible resource management within the Tłı̨chǫ traditional lands.
Key Process Stages
The mineral processing circuit designed for the Colomac Gold Project, which serves as the central destination for North Inca (Main Zone) ore, utilizes a conventional and robust flowsheet. The design emphasizes high gold recovery through a combination of gravity concentration and carbon-in-pulp (CIP) leaching. The following stages represent the critical path of the mineralized material from Run-of-Mine (ROM) to the production of gold doré:
- Primary and Secondary Crushing: The process begins with a primary jaw crusher to reduce ROM material (max size 800 mm) to a manageable size, followed by a secondary cone crusher operating in a closed circuit with vibrating screens. This stage ensures a consistent feed size (F80 of 36 mm) for the grinding circuit.
- Grinding Circuit (SABC): A Semi-Autogenous Grinding (SAG) mill and a Ball mill, each equipped with powerful 7.0 MW motors, operate in a closed circuit with hydrocyclones. The goal is to achieve a target grind size (P80) of 150 μm, liberating gold particles for subsequent recovery.
- Gravity Concentration: A portion of the ball mill discharge is diverted to a gravity circuit featuring centrifugal concentrators (e.g., Knelson). This stage is critical for North Inca ore, as it contains a significant proportion of coarse, “free” gold, achieving an estimated 27.7% recovery before the leaching stage.
- Intensive Cyanidation and Electrowinning: Gravity concentrates are processed in an intensive leach reactor. The resulting pregnant solution is sent to a dedicated electrowinning cell to produce a high-purity gold sludge for smelting.
- Leach and Carbon-in-Pulp (CIP): The cyclone overflow (fine material) is thickened and fed into a series of four leach tanks followed by six CIP tanks. With a total retention time of 24 hours, this stage utilizes sodium cyanide to dissolve the remaining gold, which is then adsorbed onto activated carbon.
- Elution and Refining: Loaded carbon is stripped of its gold using a Pressure Zadra elution system. The gold is recovered via electrowinning and finally smelted in an induction furnace to produce gold doré bars.
- Tailings Treatment and Cyanide Destruction: Final tailings undergo cyanide destruction using the SO2/Air process to meet strict environmental discharge limits before being thickened and deposited into management facilities or exhausted open pits.
Critical Data
The following table summarizes the key technical parameters and design criteria for the processing of North Inca (Main Zone) ore within the centralized Colomac facility.
| Parameter | Value | Unit |
|---|---|---|
| Design Throughput (Annual) | 6.10 | Mt/a |
| Design Throughput (Daily) | 16,715 | t/d |
| Design Gold Head Grade | 1.80 | g/t Au |
| Grinding Circuit Product Size (P80) | 150 | μm |
| Overall Gold Recovery | 96.3 | % Au |
| Gravity Recovery (Design) | 27.7 | % Au |
| Leach + CIP Residence Time | 24 | Hours |
| SAG/Ball Mill Power (Each) | 7.0 | MW |
| Bond Ball Mill Work Index (BWi) | 16.0 | kWh/t |
| Bond Crusher Work Index (CWi) | 18.7 | kWh/t |
| Bond Abrasion Index (Ai) | 0.47 | g |
| Cyanide Consumption (NaCN) | 0.50 | kg/t |
| Hydrated Lime Consumption | 0.1 | kg/t |
Technical Details and Sustainability
The metallurgical characteristics of the North Inca (Main Zone) are a primary driver for the current process design. Extensive testwork conducted between 2016 and 2019 confirmed that the gold at North Inca is largely “free-milling,” meaning it is not chemically trapped within other minerals and can be recovered using standard physical and chemical methods. This is a significant advantage, as it avoids the need for expensive and environmentally complex treatment processes such as pressure oxidation or bio-leaching. The high Bond Ball Mill Work Index (16.0 kWh/t) and Crusher Work Index (18.7 kWh/t) classify the rock as medium-hard to hard, necessitating the robust 7.0 MW motors installed in the grinding circuit to maintain the high throughput of 16,715 tonnes per day.
A technical highlight of the North Inca circuit is the optimization of the grind-recovery relationship. Metallurgical simulations determined that a P80 of 150 μm provides the ideal balance between energy consumption and gold liberation. While finer grinding could marginally increase recovery, the additional power required for the SAG and ball mills would disproportionately increase the carbon footprint and operating costs (OPEX). By maintaining this grind size and utilizing a high-efficiency gravity circuit, the project achieves a superior overall recovery rate of 96.3%, which is world-class for this type of deposit.
Sustainability and environmental stewardship are integrated into every stage of the technical design. In the remote Northwest Territories, the preservation of the delicate subarctic ecosystem is paramount. The use of the SO2/Air cyanide destruction system ensures that tailings are detoxified to levels well below regulatory requirements before leaving the plant. Furthermore, the project’s plan to utilize exhausted open pits for tailings storage represents a significant innovation in land reclamation. By backfilling pits with thickened tailings, the company reduces the need for new, large-scale tailings dams, thereby minimizing the surface disturbance and reducing long-term environmental liability.
Social sustainability is also a core component of the North Inca development strategy. The project operates within the Tłı̨chǫ traditional territory and is subject to the Tłı̨chǫ Land Claims and Self-Government Agreement. The design includes provisions for local employment, environmental monitoring by Indigenous groups, and a commitment to maintaining the “social license to operate.” Looking forward, the North Inca (Main Zone) is poised to become a flagship example of how modern technology and Indigenous collaboration can unlock significant mineral wealth in a responsible manner. Future exploration in the Leta Arm area may further extend the mine life, providing decades of economic benefit to the region while adhering to the highest standards of technical and environmental performance.
Source: North Inca (Main Zone)
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
