Zircon LA-ICP-MS U-Pb age, Ce~(4+)/Ce~(3+) ratios and the geochemical features of the Machangqing complex associated with copper deposit

摘要

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 Ce~(4+)/Ce~(3+) ratios (264) than the early-stage barren syenitic-monzonitic porphyry (102). The increase of zircon Ce~(4+)/Ce~(3+) ratios from the early- to late-stage porphyries indicates that the oxidation state of the magmas associated with copper mineralization increases with time.