Multiphase intrusion at the giant Pulang porphyry Cu-Au deposit in western Yunnan (Southwestern China): comparison between ore-causative and barren intrusions

Abstract

Porphyry Cu-Au deposits (PCDs) have commonly multiphase porphyry intrusions, yet only one (or a few) of the intrusive phase is/are ore-causative. Understanding the criteria to form ore-causative porphyry is important for both ore deposit research and PCD exploration. At the giant Pulang PCD in western Yunnan, there are three generations of porphyries, i.e., (from oldest to youngest) the quartz diorite porphyry (QDP), quartz monzonite porphyry (QMP) and granite porphyry (GP), and only the QMP has major mineralization. Based on ore deposit geology, and zircon U–Pb–Hf–O isotope systematics, we suggested that the QMP is spatially and temporally ore-related. Oxygen fugacity of the QMP and GP magmas is estimated to be higher than that of the QDP. In addition, Hf–O isotope compositions of the QMP and GP are close to those of the average mantle. We suggested that with the continuous Late Triassic subduction of the Garze–Litang ocean basin, the subducted-slab dehydration and the addition of mantle-wedge materials to the ore-forming magmas increased gradually. The younger magmas (e.g., QMP and GP) are thus more oxidized, and have higher water and ore-material contents, giving them higher ore-forming potential. The reason maybe that the late GP is barren may be due to the depletion of ore-forming materials by the main-stage mineralization during the QMP intrusion.