The sources and melting mechanism of granites control large-scale indium mineralization in the southwestern Yangtze Block, China

• Clay-rich protoliths with biotite/amphibole are crucial for indium-rich melts. • The lack of biotite/amphibole crystallization promotes indium-rich magmas. • Biotite-dehydration melting is key to indium incorporation into granitic melts. Many studies have focused on the models of indium occurrence, its enrichment and substitution mechanisms in sulfide minerals, and its transport and concentration in ore-forming fluids. However, the influence of magma sources, melting mechanisms, and magmatic evolution on the pre-enrichment of indium in granitic melts has received comparatively little attention. This study evaluates the sources, melting mechanisms, and magmatic evolution of granites associated with indium mineralization, with the objective of identifying the key controls on the indium mineralization. To achieve this, we synthesize petrological, geochemical, and Sr-Nd-Hf-O isotopic characteristics of granites related to indium mineralization in the Gejiu, Bainiuchang, Dulong, and Dachang Sn-polymetallic deposits, situated on the southwestern margin of the Yangtze Block, China. Our results indicate that indium pre-enrichment in granitic melts is jointly controlled by source composition, melting mechanisms, and magmatic evolution. Clay-rich protoliths containing biotite or amphibole appear to be a prerequisite, whereas the absence of biotite- and amphibole-dominated fractional crystallization during magmatic evolution is a critical factor in producing magmas associated with indium-enriched deposits. Besides, biotite-dehydration partial melting is identified as the principal mechanism facilitating indium incorporation into granitic melts. Combined with previous studies on the regional tectonic setting, we propose that the indium mineralization in the southwestern Yangtze Block, China, developed within an extensional tectonic regime, probably influenced by the far-field effects of the Paleo-Pacific plate subduction beneath the South China Block.