Following mine closure, open-pit mines gradually fill with ground and surface water. Due to its thermal inertia, this water maintains a relatively stable temperature year round, making it suitable for heating and cooling buildings. Previous estimates of the geothermal potential of a flooded open-pit mine primarily focused on the water volume alone, often underestimating the total potential by neglecting heat exchanges with the surrounding rock and incoming water. This paper introduces a novel analytical approach based on an improve thermal power balance concept to better estimate the geothermal potential of a flooded open-pit mine. Over a 25-years analysis, it was shown that the host rock can contribute over 15 % of the thermal energy in the water, while water supply can double this energy. The method developed is both quick and reliable, allowing for early stage evaluation of geothermal resources by accounting not only for the mine's water volume but also energy inputs from precipitation, runoff, groundwater recharge and the host rock. The study focuses on the Carey Canadian and King-Beaver open-pits, two closed asbestos mines in southern Quebec (Canada).