High water contents of magmas and extensive fluid exsolution during the formation of the Yulong porphyry Cu-Mo deposit, eastern Tibet

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The Yulong porphyry Cu-Mo ore district in eastern Tibet features a series of Eocene felsic porphyry intrusions, only one of which is intensively mineralized. In this study, zircon U-Pb ages, and major and trace element compositions of whole rock samples and in-situ accessory minerals (zircon, apatite and titanite) from these various intrusions were analyzed to determine the factors controlling Cu mineralization. New zircon U-Pb dating, together with published zircon U-Pb ages, suggest that these porphyry intrusions were emplaced over a relatively short period of time (43.9 ± 0.6 Ma to 40.9 ± 0.3 Ma), with the mineralized Yulong intrusion emplaced at a late stage of the magmatic activity (41.1 ± 0.3 Ma to 40.9 ± 0.3 Ma). These intrusions have indistinguishable whole-rock major and trace element compositions, but the mineralized Yulong intrusion has higher apatite and titanite La/Yb ratios (average apatite La/Yb = 115.1 ± 28.5, n = 52; average titanite La/Yb = 22.4 ± 4.9, n = 38) than the earlier subeconomic intrusions (average apatite La/Yb = 50.4 ± 12.3, n = 72; average titanite La/Yb = 12.4 ± 3.5, n = 58), suggesting more fractionation of amphibole from the magmas that sourced it. Additionally, a negative relationship between La/Yb and CeN/CeN* ratios in apatite and titanite are interpreted to reflect increasing oxidation states through magma evolution.Indistinguishable zircon EuN/EuN* values and apatite core SO3 contents from the mineralized (zircon EuN/EuN* = 0.69 ± 0.06, n = 25; apatite SO3 = 0.65 ± 0.24 wt%, n = 21) and subeconomic intrusions (zircon EuN/EuN* = 0.62 ± 0.05, n = 64; apatite SO3 = 0.61 ± 0.27 wt%, n = 15) suggest that the entire magmatic suite was relatively oxidized, hydrous and sulfur-rich, and was therefore fertile for ore-formation. However, apatite crystals from the mineralized Yulong intrusion have significantly lower Cl contents (0.08 ± 0.03 wt%, n = 72) and higher F/Cl ratios (47.80 ± 21.97, n = 72) than those from the subeconomic intrusions (Cl = 0.29 ± 0.29 wt%; F/Cl = 12.87 ± 5.16; n = 78). These data may reflect more extensive fluid exsolution from the parental magma chamber at the time of emplacement of the mineralizing Yulong magmas. Sudden and voluminous release of volatiles coeval with emplacement of the Yulong intrusion was triggered by an as yet unidentified process, but possibly including magmatic recharge.