In-situ U-Pb geochronology and sulfur isotopes constrain the metallogenesis of the giant Neves Corvo deposit, Iberian Pyrite Belt

摘要

The large-scale, high-grade Sn ores and the Cu-Sn metal association in the volcanic massive sulfide (VMS) ores in Neves Corvo are unique among VMS deposits, not only in the Iberian Pyrite Belt but also worldwide. Thus, the exceptional nature of the Neves Corvo deposit calls for unusual metallogenic processes. Previous efforts to constrain these processes have been hampered by a lack of high precision ages for the mineralization. This has led 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 report in-situ U-Pb dating results of hydrothermal cassiterite and sulfur isotopic compositions of sulfides in the Neves Corvo deposit that place tight constraints on the timing of mineralization and on the origin of the hydrothermal ore-forming fluids. The cassiterite samples yield U-Pb ages of the Sn mineralization of 363-366 Ma; identical to the age of the host volcanic rocks. delta S-34 values range from -32.3 parts per thousand to + 17.4 parts per thousand among the different VMS ore types and are largely consistent with microbiological reduction of seawater sulfate, but with some positive values resulting from thermochemical sulfate reduction. In contrast, the Sn-rich ores have a limited range in delta S-34 that clusters near 0 parts per thousand, which suggests derivation from magmatic fluids. Hence, the unusually high Sn grades at the Neves Corvo deposit are interpreted to be derived from fluids exsolved from a hidden granitic source essentially synchronous with formation of the volcanic rocks and VMS mineralization.