Assessing the environmental impacts of underground mines requires that the mine water sources and the geochemical processes that alter their chemical composition be determined. At the Kittilä underground mine, located near the village of Kiistala in Finnish Lapland, we used chemical and water isotope composition to investigate the contribution of surface and deep water to the mine complex and the source of mine water chlorinity. 39 water samples were collected from surface facilities, rivers, groundwater sources, seeps and drill holes. Four types of water were identified based on chemical composition: a surficial Ca-Mg-HCO3-type water with low total dissolved solid (TDS) concentrations represented by river and ground waters; a shallow Ca-SO4-type groundwater represented by seeps, also called 'mine water'; a deep Na-Cl±SO4-type groundwater sampled from drill holes; and a deep high-Cl brine with a high deuterium enrichment, also collected from drill holes. Water samples from ponds and underground pumping stations highlight three different mixing processes between the: surficial meteoric low-TDS Ca-Mg-HCO3-type water and the mine water; mine water and the deep Na-Cl±SO4-type groundwater and, to a lesser extent; surficial Ca-Mg-HCO3-type water and the deep Na-Cl±SO4-type groundwater. In contrast, no evidence of mixing involving the deep high-Cl brines was identified, suggesting that it remains mostly isolated from the other water types. The hydrogen and oxygen isotope composition of the surficial Ca-Mg-HCO3-type water and the deep Na-Cl±SO4-type groundwater, together with chemical evidence of mixing, suggests a possible genetic link between the two endmembers. This is consistent with the presence of the Kiistala shear zone facilitating infiltration by shallow meteoric water into the underlying rock mass and mineralized zone. The negative deuterium excess of the mine water concurrent to sulfate enrichment indicates that it forms from the mixing of surficial Ca-Mg-HCO3-type water and deep Na-Cl±SO4-type groundwater that evolved through evaporation and sulfide oxidation. A mixing ratio of 80% of the surficial Ca-Mg-HCO3-type water and 20% of the deep Na-Cl±SO4-type groundwater best explains the Cl concentration of the mine water. The linear relationship between the sulfate concentrations of the mine waters and its isotopic deviation from the Global Meteoric Water Line suggests a correlation between evaporation and sulfide oxidation at Kittilä, which could represent a new tool for the assessment of water-rock reactions.