Abstract

Indium is considered a critical element worldwide due to its specific function in low-carbon technology and its vulnerability to supply disruption. Although it occurs in a wide range of primary and secondary deposits, there is still a gap in knowledge regarding the geochemistry, mineralogy, and cycling of indium, particularly in mine waste environments. This review examines the current state of knowledge of indium cycling from ore to mine waste deposits and identifies opportunities for its recovery. The highest indium content has been commonly described in Cu-rich parts of polymetallic deposits related to magmatic-hydrothermal sources, in contrast with lower indium content in deposits formed at low temperatures and involving diagenetic processes. Based on a compilation of published data (n = 411), we identified 45 minerals with indium content, including sulphides, silicates, oxides, phosphates, sulphates, sulfosalts, arsenates, carbonates, and hydroxides. Sphalerite is the main In-bearing mineral phase incorporated indium through couple substitution, involving Cu, Ag, Ge, Ga, and Sn. Magmatic-hydrothermal sphalerite tends to be characterised by higher In, Cu, Sn, and Fe, and lower Ge contents and is associated with chalcopyrite, stannite, kesterite, cassiterite, and magnetite, which involve exsolution processes. It is hosted mainly in polymetallic xenothermal/epithermal veins, mantos, skarn and granite-related mineralisation styles associated with Sn systems in preference to those found in Cu systems. Indium accumulates in waters and sediments through oxidative chemical weathering of indium-bearing minerals and by deposition of atmospheric fall-out from Zn smelting. We estimate that indium can be up to 10,000 times more enriched than crustal abundance in solid mine waste, and 55,000 times higher in aqueous mine waste compared to the baseline average concentration that we calculated for natural aqueous environments. Sulphidic waste rock and tailings reported the highest indium content associated with chalcopyrite, sphalerite, pyrrhotite, arsenopyrite and pyrite; as well as significant indium contents in smelting residues associated with sphalerite and franklinite. In aqueous systems, indium can be found as a free and mobile ion (In3+) at pH < 4 and at 25 degrees C. However, at pH > 4, and in the absence of other ligands, indium may form hydroxide complexes, and the formation of solid phases (e.g., dzhalindite), which are believed to play a key role in controlling its transportation. Adsorption and/or coprecipitation mechanisms have been proposed for indium-rich secondary occurrences. Selective flotation of the In-bearing ore minerals phases has been successfully tested. Bioleaching has been identified as a potential environmentally-friendly and efficient option for indium recovery from sulphidic mining waste. For extracting indium from smelting residues, methodologies such as leaching and selective extraction using deep eutectic solvents have been demonstrated. Additionally, new phytomining and solvometallurgy technologies have been performed. In order to enhance our understanding of indium cycling in mine waste systems, further research is necessary. This includes investigating how the occurrence of indium within mineral phases affects its dissolution, the reactions involved in the oxidation/dissolution of In-bearing minerals, mineral chemistry in In-bearing secondary phases, and the role of microorganisms in the cycling or biogeochemical behaviour of indium.