We have conducted Mo isotope analyses of granites and related hydrothermal molybdenite from six Sn-W deposits in the western belt of the Southeast Asian Tin Province (Myanmar and Yunnan, China). Our data show that tin granites have notably heavier Mo isotope compositions (avg. delta Mo-98 = 0.44 parts per thousand) than arc lavas (avg. delta Mo-98 = -0.12 parts per thousand). The lack of systematic variation of Mo isotope composition with SiO2, Differentiation Index, Rb/Sr, and Zr/Hf values of the studied tin granites suggests there was no Mo isotope fractionation caused by fractional crystallization of the reduced magma. We infer that the elevated Mo isotope compositions in tin granites are mainly inherited from reduced sedimentary sources with black shales (avg. delta Mo-98 = 0.44 parts per thousand). We also observed remarkable Mo isotope fractionation during the magmatic-hydrothermal transition. The preferential incorporation of Mo4+ and isotopically light Mo into magmatic sulfides and partitioning of Mo6+ and heavy Mo isotopes into hydrothermal fluids account for significant fluid-melt fractionation of Mo isotopes during fluid exsolution from the reduced melt. The elevated Mo isotope composition in hydrothermal molybdenite from granite-related Sn-W deposits compared to that of porphyry Cu-Mo deposits derives from both the notably heavy Mo isotope signature of the reduced sedimentary sources and fluid-melt fractionation of Mo isotopes during the magmatic-hydrothermal transition. The sequestration of Mo during crystallization of reduced melt and the low Mo precipitation efficiency in the hydrothermal evolution prevents the formation of economic Mo mineralization in reduced magmatic-hydrothermal systems. Combined with published data from different Mo sinks, we propose a Mo cycling and isotope fractionation model for arc and back-arc systems of Andean-type subduction settings.

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