Geochronology and geochemistry of zircon and columbite–tantalite group minerals from the Weilasituo Sn–polymetallic deposit, northeastern China: Implications for the relationship between mineralization and the magmatic–hydrothermal transition

Abstract

The Weilasituo Sn–polymetallic deposit, located in the southern portion of the Great Xing'an Range of Inner Mongolia, China, is a super-large Sn deposit associated with abundant Li, Rb, Nb, Ta, Zn, and Cu resources. Despite that previous studies have characterized the age, hydrothermal evolution, and genetic relationship between the alkali feldspar granite porphyry and associated Sn–polymetallic mineralization, the magmatic–hydrothermal evolution and ore–forming processes remain ambiguous. To address these ambiguities and bridge this knowledge gap, this contribution characterizes the texture, geochemistry, and U–Pb isotope composition of zircon and columbite–tantalite group minerals (CGMs) from amazonitized alkali feldspar granite (AMG) and albitized alkali feldspar granite (ABG) in the Weilasituo deposit. Type-I zircon (Zr1) is inclusion-poor, translucent, and is characterized by low light rare earth element (LREE) contents, high (Sm/La)N ratios (mean = 655.7), and low Th/U (mean = 0.21 < 0.3), Zr/Hf (mean = 5.46), and Y/Ho ratios (mean = 4.42), indicating that it formed during the magmatic–hydrothermal transition. Type-II zircon (Zr2) is inclusion-rich, porous, has a low CL response, and is characterized by low Th/U ratios (0.08–0.32, mean = 0.17), high LREE contents, weakly positive Ce anomalies, and low (Sm/La)N ratios (mean = 20.7), indicating that it formed via hydrothermal alteration of Zr1 (i.e., magmatic–hydrothermal zircon). Type-I CGM (CGM1) generally lacks oscillatory zoning, and has similar and positively correlated Ta/(Nb + Ta) and Mn/(Mn + Fe) values (AMG: R2 = 0.93; ABG: R2 = 0.75), indicative of a magmatic origin. Type-II CGMs (CGM2) occur as rims on CGM1 and are characterized by higher Ta/(Nb + Ta) and lower Nb/Ta ratios than CGM1, indicative of precipitation from a hydrosilicate liquid during the magmatic–hydrothermal transition. Based on the characteristics of CGMs, two stages are evident in the evolution of the Weilasituo granite porphyry: an earlier magmatic differentiation stage and a later magmatic–hydrothermal transition stage. Furthermore, the magmatic–hydrothermal transition stage can be subdivided into the AMG and ABG substages within the porphyry system based on petrological and mineralogical characteristics. The U–Pb ages for zircons and CGMs from the alkali feldspar granite porphyry range from 137.2 ± 2.0 Ma to 129.8 ± 1.4 Ma, without a notable age gap. The similarity in U–Pb ages between CGM1–CGM2 and Zr1–Zr2 indicates that the magmatic stage and the magmatic–hydrothermal transition at Weilasituo were continuous processes that occurred during the Early Cretaceous.