Low-H2O zircons record fluid saturation-exsolution and formation of fertile porphyries in the Yulong mineralization Belt, southeast Tibet

Abstract

The exsolution of aqueous fluids during the medium- to high-temperature stages of magmatism is required for porphyry ore mineralization, but the occurrence and nature of this process remain unclear. Zircon, crystallizes at high temperatures (>550 °C) in granitic magmas and, is a reliable indicator of the magmatic evolution and volatile saturation due to its low hydrogen diffusion rates and resistance to alteration. The Machangqing porphyry Cu–Mo–(Au) (MCQ) deposit in the Yulong mineralization belt, southeast Tibet, is ideal for investigating fluid saturation and exsolution, and its role in mineralization. This study compared the geochemical compositions and H₂O contents of zircons from the MCQ deposit with those from the fertile Yulong and Gangdese porphyry Cu belt porphyries, and barren granites. Although the fertile porphyries have whole-rock Sr/Y and 10,000 × (Eu/Eu*)/Y ratios indicative of H₂O-rich melts, their zircon H₂O contents (MCQ = 480 ppm; Yulong = 473 ppm) are lower than those of the Gangdese batholith (699 ppm). Zircons from MCQ and Yulong porphyries have negative correlations between 10,000 × (Eu/Eu*)/Yb_N and Ce/√(U × Ti) ratios, indicating fluid-saturated mineralizing magmas. In contrast, the barren granite shows no evidence of fluid saturation. These findings indicate that the low zircon H₂O contents in post-collisional porphyry deposits reflect fluid-saturation-induced exsolution, with fluid saturation being essential for mineralization. High zircon crystallization temperatures (550–800 °C) confirm that the magmas reached water saturation and initiated exsolution during medium- to high-temperature stages. Magmas rich in H₂O undergo shallow magma emplacement and degassing, leading to the formation of large porphyry deposits, whereas H₂O-poor systems are less favorable for significant mineralization.