Mineral and whole-rock chemistry of the Chating porphyry Cu--Au deposit related intrusions in the Middle-Lower Yangtze River Belt, Eastern China: Implications for magma evolution and mineralization

Abstract

The Chating deposit is the largest Cu--Au porphyry deposit in the Xuancheng district of the Middle-Lower Yangtze River Metallogenic Belt. It is associated with three intrusive phases, an amphibole diorite porphyry, an ore-bearing quartz diorite porphyry and a late diorite porphyry. The amphibole diorite porphyry and quartz diorite porphyry are characterized by large ion lithophile element enrichment, depleted high field strength elements, low Yb/Lu and Y/Yb ratios, low εHf(t) (−8.29 to −12.02), εNd(t) (−6.93 to −7.42), and high 87Sr/86Sr(i) (0.705723 to 0.705802), suggesting a source comprised of both subduction-modified mantle- and crust-derived materials. The zircon U--Pb ages of the amphibole diorite porphyry (138.8 ± 3.0 Ma) and quartz diorite porphyry (137.6 ± 3.0 Ma) are consistent with their cross-cutting relationships and within error of the average molybdenite Re--Os age of 136.0 ± 1.3 Ma, suggesting that the Chating porphyry mineralization belongs to the early metallogenic event in the Middle-Lower Yangtze River Metallogenic Belt. Both the amphibole diorite porphyry and quartz diorite porphyry contain high and low Al amphiboles. The high Al amphiboles from the two intrusions show similar ranges of calculated temperatures (842 to 908 °C), pressures (143 to 229 MPa; corresponding to depths of 5.2 to 8.6 km), melt H2O contents (2 to 4 wt%), ΔNNO values (0.12 to 0.99), and Mg# (57 to 65), suggesting coeval crystallization in the same magma chamber. The low Al amphiboles with higher Mg# (68 to 75) formed at lower temperatures (711 to 812 °C) and pressures (49 to 89.3 MPa; corresponding depth of 1.9 to 3.4 km). The core-rim textures and abrupt compositional changes of amphibole and plagioclase from the amphibole diorite porphyry suggest that mafic magma was repeatedly injected into the magma chamber. In contrast, the patchy amphiboles and oscillatory zoned plagioclases in the quartz diorite porphyry may result from continuous fractional crystallization of biotite, amphibole, and plagioclase which would have increased the metal content and fO2 of the residual magma, and resulted in the exsolution of ore-forming fluids.