Petrogenesis and redox state of late Mesozoic granites in the Pingmiao deposit: Implications for the W–Cu–Mo mineralization in the Dahutang district

Abstract

The Pingmiao W (Cu) deposit is located in the central part of the newly-discovered giant Dahutang ore-concentrated district, South China. Field relationships and mineral assemblages confirm that the W (Cu) mineralization in the Pingmiao deposit is genetically related to multiphase late Mesozoic granites, which include the porphyritic two-mica granite, biotite granite porphyry, fine-grained muscovite granite, and Li-mica albite granite. Zircon U–Pb dating shows that these granites emplaced at ca. 146–143 Ma, which are coeval with the W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district. The strongly peraluminous (A/CNK > 1.1) feature, negative correlations between Rb and Th and between Rb and P2O5, low Zr + Nb + Ce + Y values (mostly lower than 200), and relatively low Zr saturation temperatures (lower than 800 °C) suggest that these granites are typical S-type granite. These granites show similar Nd isotopes (−8.91 to −4.61), CaO/Na2O (0.2–0.57), and Al2O3/TiO2 (mostly ranging of 50–90) ratios with those W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district, suggesting that they derived from partial melting of metapelites in the Shuangqiaoshan Group (or the Proterozoic metamorphic basement geochemically similar to the Shuangqiaoshan Group), with involving of minor mafic–ultramafic volcanic rocks. Geochemical fractionation trends recorded by whole-rocks and micas permit to distinguish the evolutionary trend from the porphyritic two-mica granite and biotite granite porphyry to the fine-grained muscovite granite and Li-mica albite granite. The apparent rare earth element tetrad effect, high Li and F contents, low K/Rb, Zr/Hf, and Nb/Ta ratios, as well as the zoned features of micas suggest that the melt–fluid interaction occurring during the formation of Li-mica albite granite. The Pingmiao granites have reduced redox state, revealed by their low whole-rock Fe2O3/FeO ratios (mostly <0.5), biotite Fe3+/Fe2+ ratios (ranging of 0.02–0.07), zircon Ce4+/Ce3+ ratios (median value of 18.70), and oxygen fugacity (near or below the FMQ buffer). The data in this study indicate that the late Mesozoic granitoids in the Dahutang district are favorable for W, Cu (<1 Mt), and Mo (<0.3 Mt) mineralization. Notably, the more evolved Li-mica albite granite has potential for W, Sn, Nb and Ta mineralization.