We have discovered diffuse warm air vents at the surface of a chrysotile milling waste heap at the Black Lake mine, Thetford Mines, Québec, Canada. The venting areas are inconspicuous, except in winter when the vents form snow-free areas of unfrozen ground, each with a surface area of 1–15 m2. The temperature and chemical composition of the warm air vents have been monitored from March 2009 to July 2010. The temperature of the warm air and ground surface at the venting sites ranged from 6.6 to 20.0 °C, whereas that of the ambient air ranged from –13.2 to 20.0 °C. The difference between atmospheric and vent air temperatures is greater in cold-weather months. The warm air has low CO2 content, but has otherwise normal atmospheric gas composition. Warm air volumetric flow varies from 2.1 to 19.9 L/m2/s in winter, when it contains between <10 and 18 ppm CO2. In summer, the venting areas are more diffuse, with volumetric flow rates ranging from 0.5 to 1.5 L/m2/s, and are less depleted in CO2 (260–370 ppm). Frozen ground is likely focusing airflow in winter compared to summer. We present a conceptual model in which air enters the steep flanks of the chrysotile milling waste heap, into which CO2 reacts with Mg-rich minerals, stripping CO2 from air by exothermic mineral carbonation reactions. Considering the surface area of summer and winter venting areas, flow rates, and concentration of CO2 in warm air vents, we estimate that the Black Lake mine heap passively captures at least 0.6 kt CO2 per year.