Textural, chemical, and microthermometric characteristics of In-bearing sphalerite from the Toyoha Zn-Pb-Ag-Cu-Sn-In vein-type deposit are investigated using petrography, X-ray mapping, electron microprobe analyses, and near-infrared (NIR) microscopy.

The studied ore is rich in zinc and mainly consists of sphalerite. The In-bearing sphalerite formed as euhedral growth bandings after corrosion of In-free sphalerite and replacement of a precursor mineral. The In-bearing sphalerite, which appears black and dark red in thin sections, is zoned with alternating high and low In bands. This zoning is only visible by X-ray mapping. There is a positive correlation between In and Cu, supporting the case for coupled substitution: In3+ + Cu+ <−> 2 Zn2+. The maximum concentrations of In and Cu in sphalerite are 7.03 and 3.65 wt %, respectively. Indium is also commonly correlated with trace amounts of Ag and Sn.

Primary fluid inclusions suggest that the maximum formation temperature of In-bearing sphalerite is 305°C. This indicates that In concentration in Zn-rich ores occurred at lower temperatures than in Cu-rich In-bearing ores (350°–400°C) previously reported at Toyoha.

Textural and chemical characteristics indicate that fluids enriched in In, Cu, Ag, and Sn formed In-bearing sphalerite after corrosion of In-free sphalerite and replacement of a precursor mineral. Combined with previous geological, mineralogical, and experimental studies, we propose that In was transported by high-temperature (≥300°C) fluids enriched in In, Cu, Ag, and Sn. Indium may have been derived by remobilization of the metals from the preexisting Cu-rich ores at depth and/or injection of metal-rich fluid from a new source.

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