Indium enrichment in sphalerite from magmatic-hydrothermal systems: Insights from the Sandaoqiao Pb-Zn-Ag deposit, the North Greater Khingan Range, NE China

The Sandaoqiao Pb-Zn-Ag deposit, located within the Erguna metallogenic belt of the Greater Khingan Range in NE China, contains exceptionally high indium (In) concentrations (up to 6000 ppm in sphalerite). This study provides a comprehensive analysis of In mineralization in the deposit, combining field geology, petrography, precise geochronology, in-situ sulfur isotope data, and trace element microanalysis. Zircon U-Pb ages from pre- and post-ore dikes bracket the mineralization event between 136.0 ± 0.7 Ma and 120.8 ± 0.6 Ma, while U-Pb dating of hydrothermal rutile yields a direct ore-forming age of 120.8 ± 5.8 Ma, contemporaneous with Early Cretaceous granite emplacement. In-situ δ 34 S values of sulfides range from 2.3‰ to 5.6‰, supporting a magmatic-hydrothermal origin. Sphalerite is the primary host mineral for In, with two generations (Sp-I and Sp-II) identified that record a progressive enrichment trend of In. The early Fe-rich Sp-I contains abundant chalcopyrite inclusions and has In concentrations of 70.3–443.3 ppm, whereas the late Sp-II without inclusions, exhibits significantly higher In contents (107.1–6037.1 ppm). Trace element mapping and correlation analysis indicate that In incorporation occurs mainly through the coupled substitution mechanism In 3+ + Cu + ↔ 2Zn 2+ . LA-ICP-MS data and physicochemical modeling suggest that the residence state of Cu in sphalerite, coupled with low temperature, reduced, and acidic hydrothermal conditions, facilitated pronounced In enrichment in Sp-II. These results offer new insights on the behavior of In during sphalerite crystallization in magmatic-hydrothermal systems. It further highlights the Sandaoqiao Pb-Zn(-Ag) deposit as a high potential In resource and identifies the Erguna metallogenic belt as a prospective target for In exploration.