Overview
The North Inca and Island Veins are pivotal components of the Island Gold Mine complex, a flagship asset owned and operated by Alamos Gold Inc. Located approximately 83 kilometers northeast of Wawa and just outside the town of Dubreuilville in Ontario, Canada, this operation has emerged as one of the highest-grade and lowest-cost gold mines in North America. The North Inca and Island Veins represent critical mineralization zones within the larger Michipicoten Greenstone Belt, a region renowned for its prolific orogenic gold deposits. These zones are characterized by narrow, high-grade quartz veins that require sophisticated mineral processing techniques to ensure maximum recovery and economic efficiency.
Since the acquisition of the project in 2017, Alamos Gold has undertaken a transformative journey to unlock the value of these deposits. The ongoing Phase 3+ Expansion is a testament to the mine’s potential, aiming to double the current processing capacity and extend the life of mine well into the next decade. The significance of the North Inca and Island Veins lies not only in their grade but in their continuity at depth, which has necessitated the construction of a new 1,373-meter shaft to provide efficient access and ventilation. As the mine transitions to a higher throughput model, the mineral processing circuit is being optimized to handle increased volumes while maintaining the industry-leading recovery rates that have become a hallmark of the Island Gold operation. This project stands as a beacon of modern Canadian mining, combining high-grade geology with cutting-edge engineering and sustainable practices to deliver long-term value for stakeholders and the local community.
Key Process Stages
The mineral processing circuit at the Island Gold facility, which treats ore from the North Inca and Island Veins, is designed for high-efficiency gold extraction using a combination of physical and chemical methods. The flowsheet has evolved through multiple expansion phases to handle the specific metallurgical characteristics of the orogenic quartz-vein ore.
- Crushing and Comminution: The process begins with primary crushing, typically utilizing a jaw crusher to reduce run-of-mine (ROM) ore to a manageable size. This is followed by a secondary crushing stage using a cone crusher in a closed circuit with a vibrating screen, ensuring a consistent feed size for the grinding mills.
- Grinding Circuit: The comminution continues in a two-stage ball milling circuit. The primary and secondary ball mills operate in a closed circuit with hydrocyclones to achieve a target grind size (P80) of approximately 74 to 100 microns, which is essential for liberating the finely disseminated gold.
- Gravity Concentration: A significant portion of the gold in the Island Veins occurs as “free gold.” To capture this, a portion of the cyclone underflow is diverted to a gravity circuit featuring centrifugal concentrators (such as Falcon or Knelson units). The gravity concentrate is then treated in an intensive cyanidation reactor (e.g., an Acacia or Gekko reactor) to recover gold into a pregnant solution.
- Cyanide Leaching and Carbon-in-Leach (CIL): The cyclone overflow, or the “fines,” reports to the leaching circuit. The slurry is conditioned with lime and cyanide in a series of agitated tanks. In the CIL circuit, gold is dissolved and simultaneously adsorbed onto activated carbon, minimizing the risk of “preg-robbing” by any organic matter in the ore.
- Elution and Carbon Regeneration: Loaded carbon is periodically removed from the CIL circuit and sent to the elution (stripping) circuit. Here, gold is stripped from the carbon using a high-temperature, high-pressure caustic solution. The barren carbon is then thermally regenerated in a kiln before being returned to the process.
- Electrowinning and Refining: The gold-rich solution from the intensive leach and elution circuits is passed through electrowinning cells, where gold is deposited onto cathodes. The resulting sludge is dried, mixed with flux, and smelted in an induction furnace to produce high-purity gold doré bars.
- Tailings Management and Cyanide Destruction: Before discharge, the tailings undergo a cyanide destruction process (typically using the SO2/Air or INCO process) to ensure environmental compliance. The treated tailings are then pumped to the Tailings Management Facility (TMF).
Critical Data
The following table outlines the technical parameters and performance metrics of the Island Gold Mill as it supports the extraction of ore from the North Inca and Island Veins zones.
| Parameter | Value | Unit |
|---|---|---|
| Current Mill Throughput | 1,200 | tonnes per day (tpd) |
| Expanded Throughput (Phase 3+) | 2,400 | tonnes per day (tpd) |
| Average Gold Recovery | 96.5 – 98.0 | percent (%) |
| Gravity Recovery Component | 25 – 40 | percent (%) |
| Target Grind Size (P80) | 74 – 106 | microns (µm) |
| Reserve Gold Grade (Avg) | 10.12 | grams per tonne (g/t) |
| Mill Availability | 92 – 95 | percent (%) |
| Reagent: Cyanide Consumption | 0.3 – 0.5 | kg/tonne |
| Mill Power Consumption | 4.5 – 6.0 | Megawatts (MW) |
Technical Details and Sustainability
The technical success of the North Inca and Island Veins project is deeply rooted in its geometallurgical understanding. The ore is primarily composed of quartz veins with minor amounts of pyrite and pyrrhotite. Because the gold is often associated with these sulfides or exists as coarse free gold, the processing plant must be robust enough to handle varying grades without sacrificing recovery. The inclusion of a dedicated gravity circuit is paramount; by recovering up to 40% of the gold through physical separation early in the process, the plant reduces the load on the downstream leaching circuit and lowers overall reagent consumption, which is both an economic and environmental advantage.
One of the most significant technical advancements at the site is the Phase 3+ Expansion project. This expansion is not merely about increasing volume; it is about optimizing the entire logistics chain. The transition from truck-hauling ore via a ramp to a dedicated shaft-hoisting system significantly reduces the diesel consumption and carbon footprint of the operation. The new shaft will allow for the efficient transport of ore from the deeper North Inca zones directly to the surface, adjacent to the expanded mill. This integration minimizes surface handling and maximizes the energy efficiency of the comminution circuit by providing a steady, high-quality feed.
Sustainability is a core pillar of the Island Gold operation. The mine benefits from being connected to the Ontario power grid, which is largely powered by carbon-free nuclear and hydroelectric sources. This provides the mine with one of the lowest greenhouse gas (GHG) emission profiles per ounce of gold produced globally. Furthermore, the mill utilizes advanced water recycling systems, recirculating process water to minimize the intake of fresh water from local watersheds. The tailings management strategy includes rigorous monitoring and the use of the INCO cyanide destruction process, which is recognized as an industry best practice for neutralizing hazardous chemicals before they enter the environment.
Looking ahead, the exploration potential of the North Inca and Island Veins remains vast. With the deposit open at depth and laterally, the processing facility is being designed with modularity in mind, allowing for potential future upgrades as new mineralized zones are identified. The combination of high-grade ore, state-of-the-art processing technology, and a steadfast commitment to environmental stewardship ensures that the North Inca (Island Veins) project will remain a cornerstone of North American gold production for decades to come.
Source: North Inca (Island Veins)
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
