Zircon LA-ICP-MS U-Pb age,Ce4+/Ce3+ ratios and the geochemical features of the Machangqing complex associated with copper deposit

The Machangqing complex is composed of early syenitic-monzonitic porphyry and late granite-porphyry. Geochemically, they are similar in major and trace element characteristics, and are characterized by relatively low TiO-2 （{<0.5}） and P-2O-5 （{<0.4}） and high alkali contents （K-2O+Na-2O > 8%）, with K-2O/Na-2O>1, the absence of significant Eu anomaly and pronounced negative anomalies in Nb, Ta, Ti, and enrichment in large-ion lithophile elements and light rare-earth elements. Zircon U-Th-Pb ELA-ICP-MS age determination of the Machangqing complex has revealed two discrete igneous events. The syenitic-monzonitic porphyry has an age of {35.6}± 3 Ma, MSWD={2.80}; the granite-porphyry, {35.0}±{0.2} Ma, MSWD={2.91}. The lifespan of magmatic activities of the Machangqing complex associated with Cu+Mo mineralization is about {0.6} Ma. The late-stage ore-bearing granite-porphyry has higher Ce4+/Ce3+ ratios （264） than the early-stage barren syenitic-monzonitic porphyry （102）. The increase of zircon Ce4+/Ce3+ ratios from the early-to late-stage porphyries indicates that the oxidation state of the magmas associated with copper mineralization increases with time.