<p>The three super-large coal-hosted germanium (Ge) deposits of Wulantuga (Inner Mongolia, China), Lincang (Yunnan, China), and Spetzugli (Russian Far East) together supply more than 50% of the world’s industrial Ge. Power generation has recently become of minor interest in these mines, compared to the Ge recovery process from fly ash. The mineralization of Ge in coal has been investigated, but Ge sources still lack strong evidence. The Shengli Coalfield in Inner Mongolia provides an opportunity to address this question, as both high-Ge (Wulantuga) and low-Ge coals occur within the same coal horizon. Significant differences exist in both the content and modes of occurrence of pyrite between high-Ge and low-Ge coals in the Shengli Coalfield. High spatial resolution in-situ sulfur and iron isotope data on cell- and fracture-filling pyrite in high-Ge coals range from + 0.6‰ to + 3.0‰ <i>δ</i>³⁴S and − 1.61‰ to + 0.38‰ <i>δ</i>⁵⁶Fe, respectively. These values indicate that both sulfur and iron in pyrite from the high-Ge coal relate to fluids that leached the adjacent granites and were subsequently injected into the peat swamp from the erosional surface of the granite. Whereas pyrite in low-Ge coals shows more positive sulfur (+ 9.7‰ to + 71.9‰) and iron isotopic (-0.07‰ to + 2.14‰) values, consistent with bacterial sulfate reduction, ferric iron reduction, and abiotic sulfidization of iron phases. Ge mineralization occurs only when Ge-rich fluids enter the peat swamp after peat burial but prior to early diagenesis.</p>