The effect of magma differentiation and degassing on ore metal enrichment during the formation of the world-class Zhuxi W-Cu skarn deposit: Evidence from U-Pb ages, Hf isotopes and trace elements of zircon, and whole-rock geochemistry

Abstract

The world-class Zhuxi W-Cu skarn deposit is located along the southeastern edge of the Yangtze Block, South China. Three types of granite (biotite granite, muscovite granite, and granite porphyry) are spatially related to the deposit. To decipher the W enrichment process in the Zhuxi magmatic system, we systematically study the whole rock major- and trace-element geochemistry of the three types of granites, together with U-Pb dating, Hf isotopes, and trace elements on zircon. All the Zhuxi granites are silica-rich, peraluminous, and sub-alkaline. Their major-elements exhibit a general decreasing trend with increasing SiO2 from 70.6 to 77.6 wt%, except P2O5, which has a positive correlation with SiO2. The Zhuxi granites are enriched in large ion lithophile elements (LILE) with notable negative Ba, Nb, Sr, and Ti anomalies. The total REE contents, (La/Yb)N, and (Eu/Eu*)N show a decreasing trend from the biotite granite, to the muscovite granite, and to the granite porphyry. The Zhuxi granites (εHf(t) = -13.6 to −6.8) were likely derived from partial melting of the Neoproterozoic Shuangqiaoshan Group. Fractional crystallization of feldspar, mica, zircon, and apatite caused the magma differentiation. Gradual cooling of the Zhuxi magmatic system from maximum temperatures of 800 ± 15 °C in the biotite and muscovite granites, and 720 °C in the granitic porphyry to minimum temperatures of 640 ± 20 °C (Ti-in-zircon thermometer) was accompanied by periodic degassing. The corresponding oxygen fugacity (expressed as ΔNNO) decreases from −5.4 to + 3.8 in the biotite granite, to −6.1 to + 2.7 in the muscovite granite, and to −10.5 to −1.8 in the granite porphyry. The most evolved granite porphyry has the highest contents of volatile (H2O, P, F, B, and CH4), silica, and Nb and Ta, and it is most likely responsible for the mineralization of the Zhuxi W-Cu deposit.