Overview
The North Inca (A Zone) represents a critical high-grade component of the expansive Indin Lake Gold Project, situated approximately 200 kilometers north of Yellowknife in the Northwest Territories, Canada. Managed by Nighthawk Gold Corp (now part of STLLR Gold), the North Inca deposit is part of the strategically significant Leta Arm area, which also encompasses the Diversified, Lexindin, and No. 3 deposits. Historically, this region has been recognized for its prolific gold mineralization within the Archean Indin Lake Greenstone Belt, with exploration dating back to the 1940s when Rayrock Mines initially developed the site. The modern resurgence of the project, culminating in a comprehensive Preliminary Economic Assessment (PEA) in 2023, has positioned North Inca (A Zone) as a primary target for unlocking high-grade underground and open-pit potential that complements the larger Colomac Main sill mineralization.
The North Inca deposit is characterized by shear-hosted quartz-carbonate veins and replacement-style mineralization, typically manifesting in steeply plunging chutes. The “A Zone” specifically refers to the historically developed and highly prospective portions of the North Inca structure, which includes the East and West zones. Recent drilling campaigns have confirmed the presence of exceptionally high-grade gold intercepts, sometimes exceeding 200 g/t Au over several meters, reinforcing the zone’s status as a “sweetener” for the project’s overall mill feed. As part of a multi-million-ounce gold district, the processing of North Inca ore is integrated into a centralized metallurgical framework designed to maximize recovery from free-milling gold across various satellite deposits. The significance of North Inca lies not only in its grade but also in its geological continuity and the relative ease with which its mineralogy responds to conventional processing techniques.
The project’s strategic location in the subarctic requires robust logistical planning and advanced technical engineering to overcome the challenges of remote operations. The integration of the North Inca (A Zone) into the broader Colomac production schedule allows for a versatile mining approach, where high-grade underground material can be blended with lower-grade open-pit ore to optimize the project’s Net Present Value (NPV). This overview sets the stage for a detailed examination of the processing circuit, technical parameters, and the sustainability initiatives that define this world-class gold asset.
Key Process Stages
The mineral processing circuit for the North Inca (A Zone) and the associated Indin Lake deposits is designed as a high-capacity, conventional gold recovery plant. Following extensive metallurgical testing between 2016 and 2019, a flow sheet was developed to handle the free-milling nature of the ore while ensuring high recovery rates. The primary stages include:
- Primary and Secondary Crushing: A two-stage crushing circuit featuring a primary jaw crusher and a secondary cone crusher. This setup reduces Run-of-Mine (ROM) material from a maximum size of 800 mm down to a fine-crushed product suitable for the grinding circuit.
- Grinding and Classification: The crushing product reports to a SAB (SAG/Ball Mill) circuit. It consists of a 7.0 MW SAG mill and a 7.0 MW Ball mill operating in a closed circuit with hydrocyclones to achieve a target grind size (k80) of 150 µm.
- Gravity Concentration: Approximately 100% of the ball mill circulating load is processed through a centrifugal gravity concentrator. This stage is crucial for capturing coarse gold early in the circuit, which is then treated via intensive cyanidation and electrowinning to produce high-purity gold doré.
- Leach and Carbon-in-Pulp (CIP): The cyclone overflow is directed to a series of four leach tanks followed by six CIP tanks. This configuration provides a total residence time of 24 hours, facilitating the dissolution of gold into the cyanide solution and its subsequent adsorption onto activated carbon.
- Elution and Electrowinning: Loaded carbon is treated in twin 8-tonne carbon elution systems (Pressure Zadra) to strip the gold. The resulting pregnant solution is processed via electrowinning and smelting to produce final doré bars.
- Cyanide Destruction: To meet environmental standards, the final tailings slurry undergoes cyanide destruction using the SO2/air process, reducing residual cyanide concentrations to non-toxic levels before discharge.
- Tailings Management: Processed tailings are thickened and then pumped to either a dedicated Management Facility or utilized for pit backfilling, minimizing the project’s surface footprint.
Critical Data
The following table summarizes the technical design criteria and metallurgical performance metrics for the centralized processing facility that services the North Inca (A Zone) and larger Colomac complex.
| Parameter | Value | Unit |
|---|---|---|
| Design Throughput (Annual) | 6.1 | Mt/a |
| Daily Plant Throughput | 16,715 | t/d |
| Design Gold Head Grade | 1.80 | g/t Au |
| Mill Availability | 92 | % |
| Target Grind Size (P80) | 150 | µm |
| SAG Mill Power | 7.0 | MW |
| Ball Mill Power | 7.0 | MW |
| Gravity Gold Recovery | 27.7 | % |
| Overall Gold Recovery | 96.3 | % |
| Leach/CIP Residence Time | 24 | hours |
| Bond Ball Mill Work Index (BWi) | 16.0 | kWh/t |
| Bond Crusher Work Index (CWi) | 18.7 | kWh/t |
| Cyanide Addition Rate | 0.50 | kg/t |
Technical Details and Sustainability
The technical success of the North Inca (A Zone) lies in its exceptionally high metallurgical recovery and the “clean” nature of its ore. Metallurgical testwork has consistently shown that the gold is free-milling, with no significant presence of deleterious elements like arsenic or antimony that often plague other Archean gold deposits. The 96.3% overall recovery rate is remarkably high for the industry, driven largely by the synergy between gravity concentration and the CIL/CIP circuit. By capturing nearly 28% of the gold through gravity early on, the plant reduces the load on the leaching circuit and mitigates the risk of gold “nuggets” escaping into the tailings. The 150 µm grind size was selected as the economic optimum, balancing the energy costs of grinding against the incremental gold recovery achieved at finer sizes.
Sustainability and environmental stewardship are central to the modern development plan for the Indin Lake Project. Given its remote northern location, energy efficiency is a primary concern. The 2023 technical analysis included a renewable energy feasibility study conducted by Green Cat Renewables, which proposed a power configuration where 60% of the project’s energy is supplied by wind and solar power, with the remaining 40% provided by diesel generators. This hybrid approach significantly reduces the carbon footprint and lowers the long-term operating costs associated with diesel transport via winter ice roads. Furthermore, the project employs a SO2/air cyanide destruction system, which is a global industry standard for neutralizing hazardous reagents before tailings deposition.
Water management is another critical technical pillar. The project utilizes a closed-loop system for process water wherever possible, recycling water from the tailings thickener back to the grinding circuit. In a subarctic environment, managing the water balance requires accounting for seasonal snowmelt and permafrost conditions. The use of exhausted open pits for tailings storage is a sustainable alternative to large surface impoundments, as it helps in the reclamation process and reduces the potential for long-term environmental impact. Looking forward, the North Inca (A Zone) remains open at depth and along strike, suggesting that the current processing capacity may eventually be expanded as more high-grade resources are converted to reserves. The integration of advanced automation in the mill and the focus on ESG (Environmental, Social, and Governance) principles ensure that the North Inca (A Zone) will remain a cornerstone of northern Canadian mining for decades to come.
Source: North Inca (A 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.
