Recongnition of A-type granite and its implication for magmatism and mineralization in Tangge skarn-type Cu-polymetallic deposit, Tibet

This paper is the result of mineral exploration engineering.

Objective The mineralization of Tangge skarn-type Cu-polymetallic ore district, located in the western part of the Gangdese volcano-magmatic arc, is closely related to the diagenesis of quartz porphyry in the mining area. There is little research on the quartz porphyry developed in the mining area, which restricts the further understanding of the genesis of the deposit and the guidance of prospecting and exploration.

Methods We firstly reports zircon U-Pb age and Hf isotope, and petrogeochemistry of quartz porphyry in the Tangge ore district.

Results The quartz porphyry is characterized by high silicon (SiO₂=73.97%-76.85%), aluminum (Al₂O₃=13.03%-14.06%), potassium (K₂O=2.2%-4.68%), FeO^T(0.97%-1.80%) contents, and high FeO^T/MgO ratios (3.57-8.22) and A/CNK= values(1.14-4.24). The quartz porphyry have high REE (∑REE=478.59×10^-6-532.71×10^-6) and its chondrite-normalized REE distribution patterns diagram show obvious right-leaning or "gull" type. They are enriched in Rb, Th, U, K and Pb, depleted in Nb, Ta, Ti, P, Ba and Sr. The low contents of Rb (69.64×10^-6-249.00×10^-6, less than 270×10^-6) and high 10000×Ga/Al ratios (2.54-2.71), indicate that the quartz poyphyry are post-collisional and aluminous A-type granite. LA-ICP-MS zircon dating for quartz porphyry yields a weighted mean age of (78.0±0.9) Ma, suggesting that they formed in the Late Cretaceous. Their zircons have positive ε_Hf(t) values (+1.5–+5.3, averages of +3.6), showing a relatively homogeneous Hf isotopic composition. Two-stage Hf model ages (T_2DM=1052-806 Ma, averages of 912 Ma, less than 1.0 Ga), indicate that they were mainly generated by partial melting of the juvenile felsic lower crust.

Conclusions Combined with previous research, we proposed that the Tangge quartz porphyry were formed in the post-collisional extensional tectonic environment, which was resulted from the northward subduction of the Indian contient to the Eurasian continent. Tangge quartz porphyry has relatively high background content of Cu, Pb, Zn and Au, which suggest that genetical relationship between the aluminous A-type granitic magmatism and Tangge skarn-type Cu mineralization. It is inferred that Gangdese Belt at least develops a period of Cu polymetallic mineralization related to Late Cretaceous magmatism during 69-89 Ma, however, Tangge skarn-type Cu polymetallic deposits response to magmatism in the middle segment of the Southern Gangdese Belt during late Yanshanian.