Volatile evolution of magmas associated with the Bairong deposit, Tibet, and implications for porphyry Cu-Mo mineralization

Abstract

Volatiles are considered to play an important role in porphyry Cu mineralization, and high apatite S and Cl contents are often used to distinguish fertile magma systems from barren ones in the subduction zone. However, magmatic volatile evolution and its implications in post-collisional porphyry deposits remain enigmatic. To address this issue, we conducted zircon U-Pb dating and Hf isotope, and zircon and apatite chemical compositions of multi-stage magmatic rocks from the Bairong post-collisional porphyry Cu-Mo deposit in the Gangdese belt, Tibet. The magmatic rocks included pre-mineralized porphyritic monzogranite and biotite monzogranite, syn-mineralized monzogranite porphyry, and post-mineralized dacite porphyry that formed in 14.1–12.7 Ma, ca. 12.1 Ma, and 11.8–11.6 Ma, respectively. All these intrusions have depleted zircon Hf isotope (εHf(t) = 1.7–8.0, 3.7–9.0, 3.3–9.7 for pre-, syn-, and post-mineralized intrusions, respectively) together with the research results of contemporaneous magmatic rocks in the adjacent area, suggesting that the multi-stage intrusions at Bairong are derived from a cogenetic magma chamber in post-collisional extension setting. All the intrusions have similar zircon CeN/CeN*, EuN/EuN* ratios with average ΔFMQ values of 2.1 – 2.6, together with the high apatite EuN/EuN* ratios and low Ga concentrations, implying that their magmas are all oxidized. These intrusions have zircons (10,000 × EuN/EuN*) /Y all > 1 and Ce/Nd/Y mostly > 0.01, and obtain low average titanium-in-zircon temperatures of 659 to 714 °C, which indicate their magmas are hydrous. Apatite Cl contents decrease, combined with the drop of apatite XCl/XOH and the surge of XF/XCl ratios from pre- to syn-mineralized intrusions, possibly indicating that fluid exsolved from magma at the syn-mineralization stage. The apatite SO3 contents in these different-stage intrusions, however, are almost unchanged. Combined with previous studies, the buffering effect of magmatic anhydrite maintains the stability of magma S content, which together with fluid exsolution leads to the S-Cl decoupling. Consequently, we conclude that the oxidized and hydrous magma with effective fluid exsolution is a critical control on the formation of Bairong porphyry Cu-Mo mineralization. Considering the wide variations of apatite Cl contents and the occurrences of S-Cl decoupling, simple application of high apatite SO3 and Cl contents to distinguish the ore-forming and barren systems in the post-collisional porphyry system should be cautious.