The formation of the Suyunhe large porphyry Mo deposit in the West Junggar terrain, NW China: Zircon U–Pb age, geochemistry and Sr–Nd–Hf isotopic results

Abstract

Newly-discovered Suyunhe Mo deposit (0.57Mt Mo metal), located in the Barluk Mountains of the West Junggar terrain, Xinjiang, NW China, is the largest porphyry Mo deposit discovered in Xinjiang so far. The Suyunhe Mo deposit is associated with the Suyunhe intrusive complex which was emplaced into the volcano-sedimentary sequences of the Middle Devonian Barluk Group. The Suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry, granodiorite porphyry, monzonitic granite porphyry, and tonalite porphyry stocks which are cut by the felsic dikes. Dominant Mo mineralization is hosted in the Barluk Group volcano-sedimentary rocks but is minor in the granite, granite porphyry, granodiorite porphyry, and monzonitic granite porphyry of the Suyunhe complex. We report in situ zircon U–Pb ages, whole rock geochemistry, Sr–Nd isotopic and zircon Lu–Hf isotopic data for the ore-bearing intrusive rocks from the Suyunhe deposit.Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the deep granite emplaced at 298.4±1.9Ma–295.3±3Ma and the shallow granodiorite porphyry intruded during 294.7±2.1Ma–293.7±2.3Ma. The Suyunhe granite intrusion, including fine- and medium-grained granite, belongs to the high-K calc-alkaline series. Most of them have low Zr+Nb+Y+Ce (<350ppm) and 10,000Ga/Al (<2.7) values, showing a geochemical affinity to I-type granite. These granites are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE) and Eu, Ba, Sr, Nb, P and Ti, indicating that they have experienced advanced fractional crystallization. They have low initial 87Sr/86Sr ratios (0.70318–0.70380), high whole-rock εNd(t) values (+4.4 to +6.0), high zircon εHf(t) values (+10.8 to +14.6), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (<0.35). These features demonstrate that the ore-bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization. The Suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite, indicating that they share a common magma source and process with the granite. The Suyunhe granodiorite porphyries belong to slightly high-K calc-alkaline series and have LREE enrichment and HREE, Nb and Ti depletion. They have low (87Sr/86Sr)i ratios (0.70421–0.70444), high εNd(t) values (+4.9 to +6.2), high zircon εHf(t) values (+9.6 to +15.2), and young whole-rock Nd and zircon Hf model ages. They also show low Mg# (<0.39) with one reception of 0.48. These features indicate that the granodiorite porphyries have a relatively deeper magma source region of the lower crust and have not experienced as much fractional crystallization. The Suyunhe barren tonalite porphyries show enrichment of LREE and depletion of HREE, Nb and Ti depletion, and have chemical composition (e.g., Mg#=0.41–0.48) different from the Suyunhe granodiorite porphyries, though isotopically they are indistinguishable. This suggests that the parental magma of the barren tonalite porphyries originated from melting of a lithospheric mantle. Therefore, the Suyunhe ore-bearing magmas derived from the juvenile lower crust which has a high potential to bear porphyry Mo deposit and result in the formation of the Suyunhe large porphyry Mo deposit.