Insights into the formation of Shihuiyao Ta-Nb deposit in southern Great Xing'an Range, NE China: Evidence from chronology and fluid inclusion

Abstract

Shihuiyao is a typical granite-type Ta-Nb deposit in the southern Great Xing'an Range (SGXR), Northeast China. The ore field is comprised of several granitic intrusions that were emplaced into the Early Permian Linxi Formation during the Late Jurassic (ca. 145 ∼ 150 Ma). The main Ta-Nb mineralization (stage 1–2) is found within the leucogranite, with minor identified in the porphyritic granite in the deposit. Four distinct stages in the metallogenic process can be identified: weakly mineralized magmatic stage (stage 1), strongly mineralized magmatic stage (stage 2), post-magmatic hydrothermal stage (stage 3) and low-temperature hydrothermal stage (stage 4). Cassiterite samples collected from stage 2 were dated to be 147 ± 3.5 Ma and 147.7 ± 1.9 Ma, providing evidence for the latest Jurassic Ta-Nb metallogenic event at Shihuiyao. To acquire a more profound comprehension of the properties and behaviors of the fluids, fluid inclusions within quartz, albite, amazonite and fluorite were analyzed among all stages. The results showed that the Shihuiyao fluid inclusions were relatively homogeneous and predominantly comprised liquid-rich inclusions, with occasional occurrences of CO2-type, vapor-rich type, and solid-type inclusions. From stage 1 to stage 4, the temperature of the ore-forming fluids decreased gradually, while the salinity showed increasing during stage 4, which indicates a possible contribution from surrounding strata sourced fluids. Raman analysis of the inclusions at each stage revealed that the vapor components were primarily H2O, CO2, and CH4, with additional N2 and CH4 appearing in the late stage, pointing to mixing between the metallogenic fluids and the surrounding strata source materials. Notably, the simultaneous occurrence of liquid-rich and solid-type inclusions under low pressure conditions (0.6 ∼ 1.5 km) in the early ore-forming stage (stages 1–2) suggests a fluid boiling process. We argue that the fluid boiling and rapid chemical quenching of the granitic melt play a crucial role in changing the physicochemical conditions of the ore-forming fluids, ultimately resulting in the enrichment and precipitation of niobium and tantalum. The fluid mixing and intense water–rock interaction in the late stages (stages 3–4) may have also contributed to the minor mineralization.Drawing upon our investigations utilizing petrographic features and composition analysis of individual fluid inclusions, we have prognosticated on the Sn mineralization potential within the Shihuiyao deposit. Our results indicate the Shihuiyao granites are comparable to the tin granites worldwide. Genesis of cassiterites is linked to both magmatic and hydrothermal processes, exsolution from tin-rich fluids (up to 233 ppm) and interaction with country rocks attribute to the probable economic tin mineralization. Consequently, the Shihuiyao deposit has been noteworthy prospectively for further tin mineralization.