The Yulong belt in eastern Tibet is a typical post-subduction porphyry Cu belt. Large to giant Cu deposits are widely developed in middle to south section of the belt, whereas only two small to medium deposits (Lurige and Narigongma) are currently discovered in the northern section. The petrogenesis and Cu fertility of porphyries from the northern section remain poorly investigated. Here we address these issues by presenting comprehensive whole-rock and mineral analyses on porphyries from these two deposits. The Lurige (∼58 Ma) and Narigongma (∼40 Ma) porphyries have high whole-rock SiO 2 (> 68 wt%) contents and overlapped Sr-Nd-Hf isotopes, which plot in the evolution trend of cospatial Permian Paleo-Tethyan arc magmas, suggesting their derivation from Paleo-Tethyan arc root. The two deposits coincide broadly with the two episodes of regional plateau uplift at ∼ 64–50 Ma and 44–40 Ma, indicating that regional plateau uplift played a first-order role in porphyry Cu mineralization. The Lurige and Narigongma porphyries are both oxidized, hydrous and Cl-rich, as supported by the existence of amphibole phenocrysts, low whole-rock Zr-saturation temperatures (727 ± 14 °C and 719 ± 12 °C, respectively), high Cl contents of zircon-hosted apatites (0.539 ± 0.126 wt% and 0.532 ± 0.152 wt%, respectively) and zircon ΔFMQ values (1.34 ± 0.44 and 1.89 ± 0.57, respectively), and estimated melt Cl (3756 ± 1378 ppm and 3971 ± 1554 ppm, respectively) and H 2 O contents (5.17 ± 1.52 wt% and 4.68 ± 1.86 wt%, respectively). However, the two deposits are distinguished by whole-rock 10000*(Eu N /Eu N *)/Y (536 ± 48 vs. 709 ± 189) and La/Yb (30.2 ± 5.28 vs. 39.9 ± 4.25) ratios, and zircon Eu N /Eu N * (0.28 ± 0.05 vs. 0.47 ± 0.08), (Ce/Nd)/Y (0.029 ± 0.013 vs. 0.086 ± 0.049), 10000*(Eu N /Eu N *)/Y (2.42 ± 0.72 vs. 7.36 ± 2.98), and zircon copper fertility index values (ZCFI, 4.16 ± 0.90 vs. 7.27 ± 1.38). These differences probably indicate more intensive amphibole fractionation and elevated porphyry Cu fertility for the Eocene Narigongma porphyries. We argue that intra-crustal processes may have been critical in modulating the fertility of otherwise cogenetic and oxidized-hydrous magmas. Stronger crustal thickening/uplift during the Eocene, the primary period of uplift in eastern Tibet, may have caused prolonged magma stalling and differentiation in the deeper crust, allowing build-up of larger volume of magmas and more intensive amphibole fractionation, the latter of which may further resulted in higher magmatic oxidation states. These new findings bear important implication for the coupling between Plateau uplift and porphyry Cu mineralization.
Liu et al. (Fri,) studied this question.