Geochronology of the Hongniu-Hongshan porphyry and skarn Cu deposit, northwestern Yunnan province, China: Implications for mineralization of the Zhongdian arc

摘要

The Hongniu-Hongshan porphyry and skarn copper deposit is located in the Triassic Zhongdian island arc, northwestern Yunnan province, China. Single-zircon laser ablation inductively coupled plasma mass spectrometry U-Pb dating suggests that the diorite porphyry and the quartz monzonite porphyry in the deposit area formed at 200 Ma and 77 Ma, respectively. A Re-Os isotopic date of molybdenite from the ore is 78.9 Ma, which indicates that in addition to the known Triassic Cu-(Au) porphyry systems, a Late Cretaceous porphyry Cu-Mo mineralization event also exists in the Zhongdian arc. The quartz monzonite porphyry shows characteristics of a magnetite series intrusion, with a high concentration of Al, K, Rb, Ba, and Pb, low amount of Ta, Ti, Y, and Yb, and a high ratio of Sr/Y (average 26.42). The Cretaceous porphyry also shows a strong fractionation between light and heavy rare earth elements (average (La/ Yb)_N 37.9), which is similar to those of the Triassic subduction-related diorite porphyry in the Hongniu-Hongshan deposit and the porphyry hosting the Pulang copper deposit. However, in contrast to the older intrusions, the quartz monzonite porphyry contains higher concentrations of large ion lithophile elements and Co, and lesser Sr and Zr. Therefore, whereas the Triassic porphyry Cu-(Au) mineralization is related to slab subduction slab in an arc setting, the quartz monzonite porphyry in the Hongniu-Hongshan deposit formed by the remelting of the residual oceanic slab combined with contributions from subduction-modified arc lithosphere and continental crust, which provided the metals for the Late Cretaceous mineralization.