Origin of the Haobugao skarn Fe-Zn polymetallic deposit, Southern Great xing'an range, NE China: Geochronological, geochemical, and Sr-Nd-Pb isotopic constraints

摘要

The Haobugao skarn Fe-Zn polymetallic deposit is located in the Southern Great Xing'an Range, northeastern China, and hosted in the Lower Permian carbonates. Laser ablation inductively coupled plasma mass spectrometer zircon U-Pb dating constrains the crystallization of the granite and feldspar-phyric diorite at 139 +/- 2 Ma and 134 +/- 2 Ma, respectively. According to the geological investigation, the Haobugao ore bodies are cut by the feldspar-phyric diorite emplaced along the post-mineralization fault, indicating that the mineralization occurred between 134 Ma and 139 Ma. Five molybdenite samples from skarn-type ores yield a Re-Os isochron age of 138 +/- 3 Ma, agree with the crystallization age of the granite, indicating the Early Cretaceous magmatism and mineralization events. The enrichment of Zr, Y and Ga in the Haobugao granite suggest its A-type granite affinity. The positive epsilon(Nd(t)) values, young T-DM2 ages and low initial Sr-87/Sr-86 values for the granite potentially reveal significant amount of juvenile material contributing to the parental magma. Based on Sr-Nd isotopic two component mixing model, the Haobugao A-type granite was derived by magma mixing between mantle-derived juvenile component (similar to 80%) and the lower crust component (similar to 20%), also supported by the Pb isotopic compositions. The geochemical and isotopic signatures indicate that the Haobugao granite are derived from a postcollisional extensional setting. A high oxidation state between Ni-NiO and Fe2O3-Fe3O4 buffer could be predicted by the assemblage of quartz-magnetite-titanite-amphibole-biotite in the granite. Hence, sulfur would have been present as sulfates (SO2-4) in such highly oxidized magmas, and the chalcophile elements (Zn, Pb, Cu, Mo) were retained as incompatible elements in the melt, facilitating subsequent mineralization. A compilation of existing data reveals that the skarn Fe-Zn polymetallic mineralization from the Haobugao and other areas along the Southern Great Xing'an Range metallogenic belt took place coevally in the Early Cretaceous and was related to a post-collisional extensional environment. This significantly differs from the typical porphyry-skarn related deposits that are commonly formed under the arc-compressive setting (e.g., the Pacific Rim).