Tracing back sulfur isotope reequilibration due to contact metamorphism: a case study the Perubar VMS deposit, Central Peru.

The mid-Cretaceous Perubar VMS deposit, Central Peru, affected by contact metamorphism, has been the subject of a sulfur isotope investigation. The measured δ34S values for sulfate-sulfide pairs from the deposit indicate that close to equilibrium isotopic reequilibration occurred during metamorphism. After “filtering” the contact metamorphism effect, sulfur isotope desiquilibrium trends for coexisting sulfate-sulfide pairs were obtained on the δ34S versus Δ34S diagram, indicating a mixing between seawater sulfate and hydrothermal H2S. It suggests that contact metamorphism occurred in a closed system and that the original bulk sulfur isotope composition of the Perubar deposit was mostly preserved. Sulfur reequilibrated only locally, by isotope exchanges between adjacent sulfate and sulfide minerals

Oxidation state inherited from the magma source and implications for mineralization: Late Jurassic to Early Cretaceous granitoids, Central Lhasa subterrane, Tibet

Arc magmas are more oxidized than mid-ocean ridge basalts; however, there is continuing debate as to whether this higher oxidation state is inherited from the source magma or developed during late-stage magmatic differentiation processes. Well-constrained Late Jurassic to Early Cretaceous arc-related intermediate to felsic rocks derived from distinct magma sources provide us with a good opportunity to resolve this enigma. A series of granitoids from the western Central Lhasa subterrane were analyzed for whole-rock magnetic susceptibility, Fe2O3/FeO ratios, and trace elements in zircon. Compared to Late Jurassic samples (1.8 ± 2.0 × 10−4 emu g−1 oe−1, Fe3+/Fetotal = 0.32 ± 0.07, zircon Ce4+/Ce3+* = 15.0 ± 13.4), Early Cretaceous rocks show higher whole-rock magnetic susceptibility (5.8 ± 2.5 × 10−4 emu g−1 oe−1), Fe3+/Fetotal ratios (0.43 ± 0.04), and zircon Ce4+/Ce3+* values (23.9 ± 22.3). In addition, positive correlations among whole-rock magnetic susceptibility, Fe3+/Fetotal ratios, and zircon Ce4+/Ce3+* reveal a slight increase in oxidation state from fO2 = QFM to NNO in the Late Jurassic to fO2 = ∼NNO in the Early Cretaceous. Obvious linear correlation between oxidation indices (whole-rock magnetic susceptibility, zircon Ce4+/Ce3+*) and source signatures (zircon εHf(t), TDMC ages) indicates that the oxidation state was predominantly inherited from the source with only a minor contribution from magmatic differentiation. Thus, the sources for both the Late Jurassic and Early Cretaceous rocks were probably influenced by mantle wedge-derived magma, contributing to the increased fO2. Compared to ore-forming rocks at giant porphyry Cu deposits, the relatively low oxidation state (QFM to NNO) and negative εHf(t) (−16 to 0) of the studied granitoids implies relative infertility. However, this study demonstrates two potential fast and effective indices (fO2 and εHf(t)) to evaluate the fertility of granitoids for porphyry-style mineralization. In an exploration context for the west Central Lhasa subterrane, features indicative of potential fertility might include more oxidized, positive εHf(t), young rocks (<130 Ma)