Ore-forming volatiles linked to mafic magma recharge in the Xiongcun porphyry Cu-Au system, southern Tibet: Evidence from zircon petrochronology and apatite inclusions

The formation of porphyry Cu deposits (PCD) is related to deep magmatic systems, which serve as sources for porphyry magmas and metal- and sulfur-bearing mineralizing fluids. However, the interactions among deep magmas, mid- to upper-crustal magma chambers, and shallow porphyritic intrusions that trigger PCD formation remain poorly understood. This study focuses on the Xiongcun porphyry Cu-Au district in the Gangdese belt of the southern Tibetan Plateau, which experienced two distinct mineralization events: an earlier event related to a hornblende quartz diorite porphyry (HQDP), followed by a later event with a quartz diorite porphyry (QDP). The magmatic and volatile evolution linked to the Xiongcun PCD formation is investigated based on zircon textures, in situ zircon trace element and U-Pb isotope data, and the geochemical compositions of zircon-hosted apatite inclusions. Zircon U-Pb geochronology and trace element compositions indicate that the mid- to upper-crustal magma chamber linked to the Xiongcun porphyry system experienced mafic magma recharge at ca. 174.9 ± 0.4 Ma, followed by the emplacement of the HQDP (173.2 ± 0.4 Ma) and the QDP (170.6 ± 0.5 Ma). Variations in zircon trace element concentrations from both porphyries indicate that magmatic evolution was primarily driven by amphibole-dominated fractional crystallization, reflecting a hydrous magmatic system. Zircon-hosted apatite inclusions reveal a pronounced increase in volatile concentrations, particularly sulfur, likely provided by mafic magma recharge rejuvenating the mid- to upper-crustal magma chamber. Compared with the earlier and low-grade HQDP, the younger and more fertile QDP has lower zircon Gd/Yb ratios, and higher Eu/Eu*, (Ce/Nd)/Yb ratios, sulfur concentrations, and apatite XF/XCl ratios. Therefore, the formation of the high-grade Au-rich PCD centered on QDP is attributed to a protracted magmatic evolution in the mid- to upper-crustal magma chamber. During this prolonged period, the QDP likely underwent multiple mafic magma recharges and extensive fractional crystallization, leading to increasing volatile concentration, particularly sulfur, ultimately reaching volatile saturation. This study highlights the critical role of mafic magma recharge in the formation of the Xiongcun PCD, as it supplied essential ore-forming volatiles, especially sulfur, to the mid- to upper-crustal magmatic system.