Abstract:The large-scale, high-grade Sn ores and the Cu-Sn metal association in the volcanic massive sulfide (VMS) ores inNeves Corvo are unique among VMS deposits, not only in the Iberian Pyrite Belt but also worldwide. Thus, theexceptional nature of the Neves Corvo deposit calls for unusual metallogenic processes. Previous efforts toconstrain these processes have been hampered by a lack of high precision ages for the mineralization. This hasled to a debate as to whether tin mineralization occurred at the same time as the associated volcanism and VMS-style mineralization, or is the result of later processes events associated with the Hercynian orogeny. We reportin-situ U-Pb dating results of hydrothermal cassiterite and sulfur isotopic compositions of sulfides in the NevesCorvo deposit that place tight constraints on the timing of mineralization and on the origin of the hydrothermalore-forming fluids. The cassiterite samples yield U-Pb ages of the Sn mineralization of 363–366 Ma; identical tothe age of the host volcanic rocks. δ34S values range from ？32.3‰ to +17.4‰ among the different VMS oretypes and are largely consistent with microbiological reduction of seawater sulfate, but with some positivevalues resulting from thermochemical sulfate reduction. In contrast, the Sn-rich ores have a limited range in δ34Sthat clusters near 0‰, which suggests derivation from magmatic fluids. Hence, the unusually high Sn grades atthe Neves Corvo deposit are interpreted to be derived from fluids exsolved from a hidden granitic source es-sentially synchronous with formation of the volcanic rocks and VMS mineralization.