Petrogenesis and oxygen fugacity of the Xintian complex associated with the Naozhi-Xintian porphyry-epithermal system in the Yanbian area, NE China

摘要

The Naozhi polymetallic veins classified into the intermediate-sulfidation (IS) type and the 2 km northwest Xintian porphyry Cu-Mo ore occurrence in the Yanbian area, Northeast China, constitute a telescoped porphyry-epithermal system. IS veins are surrounded by Early Jurassic granodiorite stocks (189 +/- 1 Ma; zircon(U-Pb)) and Early Cretaceous andesite lava (128 - 130 Ma; zircon(U-Pb)). They are partially covered by younger pyroclastic rocks dated at 108 +/- 2 Ma (Zircon(U-Pb)). The Xintian occurrence is connected in space and time with an intrusive complex including five sequential phases from gabbro diorite and granodiorite stocks through diorite porphyry dykes to granodiorite porphyry and granite porphyry stocks, with indistinguishable ages within errors (128 +/- 1 Ma to 126 +/- 1 Ma zircon Pb-206/U-238 by laser ablation inductively coupled plasma mass spectrometry). The broad compositional variations (eg., 52.24 - 69.23 wt% SiO2) of the complex and the igneous mafic microgranular enclaves (MMEs) hosted by granodiorite stocks are likely responses to inputs of hot mafic melts into a shallow felsic magma chamber. Based on the trace element contents of zircons, the redox conditions during the crystallization of zircon minerals in equilibrium with the melts are evaluated by calculating the FMQ values. The early mafic phase (gabbro diorite; FMQ 4.60 +/- 1.33) is more oxidized than later phases (granodiorite, diorite porphyry, granodiorite porphyry, and granite porphyry) with FMQ values of 1.70 +/- 1.80 to 2.28 +/- 2.36. The highly oxidizing conditions of associated magmas, as well as high magmatic water contents (>4%) because of ubiquitous biotite and amphibole in this complex, are conducive to escape of primary volatiles from shallow, fertile porphyry magmas, with consequent ore deposition in structurally optimal positions at Naozhi-Xintian.