Zircon U–Pb dating and Sr–Nd–Pb–Hf isotopes of the ore-associated porphyry at the giant Donggebi Mo deposit, Eastern Tianshan, NW China

Abstract

The giant Donggebi porphyry Mo deposit is located in the Jueluotage metallogenic belt of Eastern Tianshan, Xinjiang, Northwest China. Mo mineralization mainly occurs as numerous veinlets in the altered sandstone wall-rocks, with the development of potassic, phyllic, argillic and propylitic alteration assemblages outward from a buried porphyritic granite stock. Zircon crystals from the buried porphyritic granite yield a weighted average 206Pb/238U age of 236±2.2Ma (MSWD=1.2, 1σ, n=17), slightly older than the molybdenite Re–Os isotope ages of 231–234Ma, suggesting that the Donggebi porphyry Mo deposit was formed in the Triassic, post-collisional tectonism subsequent to termination of the Paleo-Asia Ocean. The samples from porphyritic granite show high contents of SiO2, K2O and Al2O3, low contents of TiO2, MgO and CaO, and peraluminous high-K calc-alkaline to shoshonite affinity, with obvious LREE enrichment and negative Eu anomalies. They have high initial 87Sr/86Sr ratios of 0.70618 to 0.70821, εNd(t) values of 0.60 to 1.62, and TDM2(Nd) ages of 0.88 to 0.96Ga. Their (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values are 17.122–18.577, 15.493–15.574, and 37.887–38.006, respectively. Zircons from the porphyritic granite yield εHf(t) values of −1.58 to 4.82, and TDM2(Hf) ages of 0.96–1.36Ga. These geochemical and isotopic data imply that the Donggebi porphyritic granite originated mainly from partial melting of lower continental crust derived from a depleted mantle. The Donggebi Mo deposit is unique based upon the crustal source for the causative porphyry, the distal position of mineralization, and the previously revealed CO2-rich ore-forming fluids, and thus belongs to the collisional- or Dabie-type porphyry deposits as exemplified by the Qiane'chong and Yaochong deposits in Dabie Shan, the Donggou deposit in Qinling Orogen, China.