Tailings

Background

Tailings are a common by-product of the mineral recovery process and often take the form of a liquid slurry consisting of finely crushed rock, water, and chemical additives used to facilitate mineral extraction. Following mineral processing, tailings are usually disposed within surface storage facilities (also known as tailings facilities or TFs). In some cases, tailings are used for underground backfill. The safe and responsible management of tailings is essential for protecting the natural environment, local communities, our operations, and workforce.

Tailings Disposal and Construction Methods

Some common methodologies for on-surface disposal of tailings include:

impounding tailings slurry behind dams, where the solids settle and consolidate over time and much of the water is reclaimed and recycled,

storing dewatered tailings in a stack (also known as dry-stack tailings),

co-disposal with waste rock, or

in-pit storage utilizing existing mined-out pits.

The selection of the TF design depends on various parameters, including the geochemical composition of the tailings being stored, geotechnical/foundation considerations, precipitation, seismic activity, community preference, and environmental protection. The most common form of tailings storage across the mining industry is to construct a containment dam (or dams) in conjunction with natural topography to create a tailings facility.

There are several methods of constructing tailings dams. The method for each TF is chosen based on various factors as stated above, with the priority being the safety and security of communities, employees, and the environment. There are three (3) basic raise geometries defined by how the crest of the dam is increased in height relative to the original “starter dam” at the outset of the TF’s development: Upstream, Centerline, and Downstream.

Upstream Construction Method

Upstream construction begins with a starter dam constructed at the ultimate toe of the facility. The tailings are then discharged into the facility where they form a tailings beach. Tailings will naturally segregate so that coarse material settles closest to the starter dam, while liquid and fine material settles farthest away. This tailings beach and the previous dam raise form the foundation for subsequent levels of the dam as it is raised. As such, the crest of the dam moves upstream with each raise.

Centerline Construction Method

In the Centerline construction method, the dam is raised vertically on the centerline of the starter dam. The dam crest therefore remains fixed relative to upstream and downstream directions, as the dam is sequentially raised. Like the upstream method, the tailings are discharged on top of the dam to form a beach behind it. However, the dam only partially uses beached tailings for foundation support.

Downstream Construction Method

Similar to the other construction methods, the Downstream design begins with a starter dam. Tailings are then discharged into the impoundment and as the embankment is raised, each new lift is constructed and supported on top of the downstream slope of the previous raise, so the dam crest moves downstream with each raise. Unlike the Upstream or Centerline construction methods, the dam does not rely upon beached tailings for support.

Other Design Components

Beyond the three construction methods outlined above, other impoundment types or components may be applied. Common to the mining industry are hybrid dams, water retaining dams, and landforms.

Hybrid (or mixed) dams use a combination of the three raise techniques, often due to changes over the mine life (e.g., shortage of dam fill, building to higher design criteria).

Water retaining dams are structures with specifically engineered features (e.g., low permeability liners or core zones) to safely manage stored water and seepage. These are suitable for storing water and tailings slurry in direct contact with the dam wall and are generally constructed to full height at the beginning of the mine life.

Landforms are typically legacy/closed TFs that are no longer impounding water. They may have undergone reclamation and rehabilitation (e.g., flattening and vegetating slopes) such that they no longer resemble their previous state.

Tailings Governance

Structure

A formalised tailings governance structure has been in place at Alamos since 2023. An Independent Tailings Review Board (ITRB), Tailings Management Working Group, and executive Tailings Management Steering Committee have now been in place for several years.

Alamos’ ITRB was established in February 2023 and is comprised of two technical experts whose purpose is to provide an independent opinion on Alamos’ compliance or alignment with requirements described in the following:

Alamos’ Tailings Management Standard (v1.1 – 2024)

The Canadian Dam Association (CDA) Dam Safety Guidelines (2013)

CDA Technical Bulletin: Application of Dam Safety Guidelines to Mining Dams (2019)

The Mining Association of Canada’s (MAC) Guide to the Management of Tailings Facilities (2021)

The Global Tailings Review’s Global Industry Standard on Tailings Management (2020)

In 2024, with the acquisition of the Magino Mine, Alamos inherited a second ITRB responsible for overseeing the Magino TF. This ITRB consists of three technical experts who serve a similar advisory role.

Alamos’ Tailings Management Governance Charter outlines the purpose, composition, responsibilities and authority of the Tailings Management Working Group and the Tailings Management Steering Committee. The Working Group is comprised of the Director of Environmental Sustainability, the Engineer of Record, and each sites’ Responsible Tailings Person (typically the General Manager or Mill Manager).

The group meets at least quarterly and is responsible for: directly supporting the ITRB, the preparation of relevant reports, and adhering to the Alamos Tailings Management Standard and each TF’s Operations, Maintenance & Surveillance Manual.

The Steering Committee is made up of the Chief Operating Officer, the Senior Vice President (SVP) of Technical Services, the SVP of Projects, and the VP of Sustainability and External Affairs. The committee also meets at least quarterly, and is tasked with responsibilities including, but not limited to, reviewing the progress of the Working Group towards meeting company goals and initiatives, reviewing the recommendations of and providing oversight to the ITRB, and providing direction on evolving tailings management requirements, both internal and external.

While ultimate accountability for responsible tailings management remains at the most senior level of our business (with the Chief Operating Officer), the reporting structure begins at the site level. Site leadership works with Alamos’ ITRB, the Engineer of Record, and the Working Group, who then report to the Steering Committee. The Steering Committee reports to the Technical & Sustainability Committee of the Board.

Board of Directors

Executive

Corporate & Site Level

Technical &
Sustainability
Committee

Steering
Committee

ITRB

Engineer of
Record

Working Group

Island Gold
District

Young-
Davidson Mine

Mulatos
District

Lynn Lake
Project

Corporate
Sustainability

Accessible Descriptions for Charts

Internal Frameworks

The Alamos Sustainability Policy outlines our guidelines and commitment to exercise vigilance in all phases of the mining process. This includes the design, construction, operation, and closure of TFs in a manner that protects health, safety, and the environment, and complies with applicable laws and international guidelines. This policy commits the necessary human and financial resources to achieve this, including the provision of necessary training, equipment and systems to manage environmental and sustainability risks.

The Alamos Tailings Management Standard defines the requirements for developing and implementing site-specific tailings management systems, and outlines the minimum expectations, practices and guidelines for the location, design, construction, operation, maintenance, surveillance, and closure of our TFs. This standard provides guidance to our operations in identifying, understanding, and managing TF-related risks. The standard applies to all Company TFs that are planned, in construction, in operation, being closed and closed.

Tailings Management

Overview

Responsible tailings management is a core component of our operations and an important focus of Alamos. We are responsible for the treatment, storage, and management of tailings both during and after mining, and are subject to rigorous regulatory requirements that govern our efforts from design through to closure and reclamation. For our mines that currently produce tailings or plan to produce tailings in the future (the Island Gold District, Young-Davidson, and the Lynn Lake Project)^{1The Mulatos Mine does not produce tailings as it processes gold by heap leaching and carbon adsorption.}, Alamos constructs, maintains, and manages each TF in close collaboration and consultation with government representatives, independent engineering firms, tailings experts, and local stakeholders to mitigate risks. When designing our TFs, we work with independent experts to assess the local environment and potential risks of natural phenomena that can adversely affect the integrity of our facilities, including seismic events, flooding and operational constraints.

We also consider the TF location relative to our operations, local communities, and environmentally sensitive areas. These conditions are incorporated into our TF design and management practices. In addition, we follow recommendations from the Mining Association of Canada’s (MAC) Guide to the Management of Tailings Facilities and the Canadian Dam Association’s (CDA) Guidelines and Technical Bulletins for the design, construction, operation, closure, and post-closure of TFs. TF closure and rehabilitation objectives are factored into decision making throughout the mine life cycle. Closure and rehabilitation plans are developed in accordance with applicable laws and international guidelines, and as such, are periodically updated throughout the mine life and approved through the regulatory process.

We proactively engage with governments, Indigenous communities, regulators, and communities of interest concerning our operations and TFs. Consultation includes the disclosure of potential risks, and information gained through this exercise is factored into decision making and the update of site environmental and sustainability plans.

2025 Tailings Produced (t)

Table 6.1

Scrolling horizontally may be needed to view the full table.

Tailings Type	Young-Davidson	Island Gold District
Surface Tailings	1,142,480	3,346,783
Sub-Surface Tailings	1,563,189	0
Total Tailings	2,705,669	3,346,783

Alamos Tailings Facilities

Table 6.2

Scrolling horizontally may be needed to view the full table.

Mine	Facility	Type	Raising Method	Status
Young-Davidson Mine	Tailings Impoundment Area -7	Slurried tailings	Downstream and Centerline	Care & Maintenance
Young-Davidson Mine	Tailings Impoundment Area -1	Slurried tailings	Downstream / Modified Centerline	In operation
Island Gold District	Island Gold Primary Pond	Slurried tailings	Downstream	In operation
Island Gold District	Magino Tailings Facility	Slurried tailings	Downstream	In operation
Lynn Lake Mine	Tailings Facility	Slurried tailings	Downstream	Pre-construction

Monitoring, Inspections and Audits

Each TF is supported by an Operations, Maintenance & Surveillance (OMS) Manual aligned to MAC’s OMS Guide. Three levels of governance and assurance are required and in place:

Surveillance inspections and monitoring: Routine daily, monthly and quarterly inspections, instrumentation monitoring, and data analysis by qualified persons with sufficient training, experience and understanding of the TF.
Dam Safety Inspections: Annual independent inspections undertaken by a qualified professional engineer, typically the Engineer of Record (EOR) assigned to the TF.
Design and Construction Audits: Periodic independent reviews (Dam Safety Review and inspections by the Independent Tailings Review Board) of the design and construction of the TF to assess the integrity and safety of the TF.

TF monitoring, inspections, and audits are undertaken in accordance with Canadian regulations and guidelines from the Canadian Dam Association. Any findings or non-conformances from independent inspections and audits are reported to mine management, Alamos’ Tailings Steering Committee, and the Technical & Sustainability Committee of the Board when warranted.

Continuous Improvement

As part of our broader sustainability initiatives, Alamos is committed to continuously improving its tailings management practices. This includes maintaining site-specific Emergency Preparedness and Response Plans (EPRPs) and regularly testing EPRP protocols to identify potential issues, assess knowledge gaps, and develop remediation plans.

In 2025, Alamos continued to address gaps identified through a previously completed internal assessment of its tailings management practices against the Global Industry Standard on Tailings Management (GISTM). Key updates included finalising the OMS Manual for the Magino TF. At Island Gold, the Dam Safety Review resumed following the finalisation of the District’s long-term tailings management strategy. To support longer-term closure objectives, the deposition strategy was updated to optimise the remaining months of tailings placement and minimise the re-sloping requirements during reclamation.

During 2025, Alamos completed its third year of review with our original ITRB. As part of this process, design documents for the planned Lynn Lake Tailings Facility were reviewed and feedback was provided to the Engineer of Record in advance of construction, which is expected to commence in 2026. In addition, a detailed site inspection was conducted at Young-Davidson to assess seepage identified at the TIA-7 East Dam in late 2024, and recommendations were provided to support management of the condition.

The Magino Tailings Facility also underwent its annual ITRB inspection in August 2025. Each year, the ITRB visits the facility and presents its findings to site management, providing recommendations based on field observations and a review of documentation, including construction design drawings and reports, Dam Safety Inspections, and recent as-built information. Between annual visits, virtual meetings are held to follow-up on new information and track progress against the ITRB Action Log. In addition to tailings oversight, the Magino ITRB provides governance over the Mine Waste Rock Facility and site-wide water management, including acid rock drainage management and water balance assessments.