Crustal recycling and metamorphic dehydration govern the fertility of granite-associated tin systems

Tin is a critical metal predominantly sourced from granite-associated deposits linked to anatexis of metasedimentary rocks and extreme magmatic differentiation. However, how metamorphism of sedimentary protoliths prior to crustal melting influences granite Sn fertility remains poorly constrained. Here we present an integrated boron (B) and mercury (Hg) isotope study of granites, metasedimentary basement rocks, and ores from the western Yunnan segment of the Southeast Asian Tin Belt. Tin granites and ores display Δ199Hg values of −0.41‰ to +0.06‰ and δ¹¹B values of −19.0‰ to −9.4‰, comparable to those of the basement rocks, indicating a dominant crustal volatile source. Coupled B–Hg isotopes further reveal variable volatile loss during metamorphic dehydration prior to melting, with partial melting of weakly dehydrated, volatile-rich metasedimentary protoliths favoring the generation of Sn-fertile granites. These findings highlight metamorphic preconditioning as a critical control on Sn metallogenesis and have important implications for global exploration of Sn resources. Integrated boron and mercury isotopes indicate that weakly dehydrated metasedimentary sources retain the volatiles needed to generate tin-fertile granites, highlighting metamorphic preconditioning as a key control on tin metallogenesis.