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Abstract

Mine Langlois, located approximately 40 km northeast of Lebel-Sur-Quévillon in western Québec, operated as a polymetallic mine that produced zinc, copper, silver, and gold from a Volcanogenic Massive Sulfide (VMS) deposit between 1996 and 2019. Its inactive Tailings Storage Facility (TSF) contains Potentially Acid Generating (PAG) tailings with 10–25 wt. % sulfide, predominantly composed of pyrite. Most tailings were deposited sub-aqueously and are not expected to generate Acid Rock Drainage (ARD). However, in the last decade, sub-aerial “islands” and beaches have formed, exposing approximately 30% of the TSF surface area while the remainder is covered with water. These oxidizing tailings may pose a future ARD risk, potentially impacting the supernatant pond's water quality and its ability to discharge into the nearby Wedding River. This paper describes field investigations completed in 2024 to evaluate whether ARD is currently being produced by the exposed tailings and to assess the long-term risk of ARD impacts on the supernatant pond. The study involved test pitting, geochemical analyses (e.g., Acid-Base Accounting, TIMA mineralogy, porewater chemistry), and pond water quality sampling. Results suggest acidic conditions (pH 4.5) in the older exposed tailings, with circum-neutral pH conditions below 10 cm, while newer tailings showed acidic porewater (pH 2.5–4.0) and depleted Neutralization Potential (NP) up to 20 cm in depth due to dry, well-drained conditions. Tailings porewater was classified as either ARD or Saline Drainage (SD) and characterized by elevated concentrations of SO4, Fe, and Zn at shallow depths, with mobile trace metals (Zn, Mn, Ni, Cd) detected up to 100 cm below the surface. Despite some oxidation of near-surface exposed tailings, the study found that the pond water remained circum-neutral with low trace metal concentrations, indicating a limited impact from this source under current conditions.