Zircon and apatite constraints on the key factors controlling porphyry Mo mineralization in the Shijiawan deposit: Implication for Mo metallogeny in the Qinling Orogenic Belt

Abstract

Porphyry Mo deposits are the most important source of global molybdenum reserves. The late Mesozoic granitic magmatism in intra-continental setting within the Qinling Orogenic Belt (QOB) in central China generated numerous Mo-fertile porphyries (total proven Mo resource of 8.43 Mt) together with barren granites. However, the factors controlling porphyry Mo mineralization potential remain equivocal. Here we present whole-rock geochemistry, zircon U-Pb-Hf and trace elements, and apatite compositions from the Shijiawan monzogranite porphyry, biotite granodiorite porphyry, and mineralized K-feldspar granite porphyry. In conjunction with the data from coeval fertile and barren granitoids in the QOB, we evaluate the key factors including the magma source, water content, oxygen fugacity, and volatiles for the porphyry Mo mineralization. The porphyries of the Shijiawan deposit show emplacement ages of 139–136 Ma and I-type granite affinity, with SrNd and Hf isotopes mixing models suggesting a crust-mantle mixed source and involving ∼24 % mantle-derived materials. The mineralized K-feldspar granite porphyries show a higher contribution of mantle-derived materials as indicated by more depleted Hf isotopes and higher REE contents in apatite than those of barren phases. Their higher whole rock La/Yb (39–56) and V/Sc (5.9–12.5), apatite ΣLREE contents, and lower zircon saturation temperatures (average T_Zr = 776 °C) also suggest higher water content (>6 wt%) than barren porphyries. In the QOB, the Late Jurassic to Early Cretaceous Mo-fertile porphyries and barren granites have similar crust-mantle mixed sources, but the porphyries with larger Mo tonnage have higher mantle contribution and higher magma water content leading to efficient fluid exsolution, as recorded by the extremely low Cl content of apatite crystallized in residual melt. The relatively high fluorine (F) content and occurrence of fluorite indicate that F can effectively promote Mo mineralization. In addition, both fertile porphyries and barren granites show consistently high oxygen fugacity (ΔFMQ = 0–2, Ce⁴⁺/Ce³⁺ > 100) and S content (> 715 ppm). The higher mantle contribution, magma water content, and F are probably the key controlling factors for porphyry Mo mineralization in this region. The comparable oxygen fugacity and sulfur content of the fertile and barren intrusions suggest that although these factors are prerequisites for mineralization, but might not be the prime controlling factors of the porphyry Mo potential.