Fate, distribution, and transport dynamics of indium in a polymetallic mine waste environment: An integrated mineralogical characterisation and geochemical modelling study.

The global shift towards clean energy technologies has increased demand for critical minerals such as indium, driving interest in secondary resources like legacy mine sites. Oxidative weathering of indium-bearing waste can generate indium-rich leachates, potentially posing environmental risks but also offering opportunities for recovery. However, indium's geochemical behaviour in mining-impacted and natural waters remains underexplored, with a lack of field-based data on aqueous speciation and complexation. This study investigates the fate of indium through detailed characterisation of waste rock, incorporating Maps Min automated mineralogy, synchrotron-based X-ray Fluorescence Microscopy, Electron Probe Micro-Analysis (EPMA), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), alongside water physicochemistry, and geochemical modelling of indium in field-derived samples. The study focuses on the abandoned Baal Gammon mine in Australia, a recognised indium resource, and > 3 km of its downstream system. Indium release was linked to the weathering of chalcopyrite and sphalerite, enhanced by pyrrhotite. Acid mine drainage waters (pH <3) exhibit the highest indium concentrations (up to 73 μg/L), which correlate positively with copper, zinc, cadmium, and arsenic. Geochemical modelling indicates In, InSO₄, In(SO₄)₂, InF, and InCl dominate under acidic conditions, shifting to hydroxide complexes at neutral pH. Mineralogy, and saturation index (SI) calculations support indium sequestration into secondary phases such as natrojarosite and schwertmannite. Dzhalindite (In(OH)₃) remains undersaturated, but SI increases with pH, suggesting downstream transport and potential bioavailability. These findings enhance understanding of indium behaviour in mine waste systems and surrounding environments. Nevertheless, further research is needed to clarify its biogeochemical cycling and ecological impacts.