Phenocryst Zonation Constraints on the Formation of the Haliheiba Sn Deposit in the Southern Great Xing’an Range, NE China

Abstract Globally magmatic hydrothermal Sn deposits are typically associated with partial melting of reduced, Sn-enriched metasedimentary protoliths. The newly recognized Southern Great Xing’an Range Sn metallogenic belt in northeastern China, however, is characterized with Sn-mineralized intrusions commonly being A- or I-type granites derived dominantly from juvenile crust. How Sn is enriched in these magmas remains unclear. Haliheiba is a typical Sn deposit in this region that is associated with a biotite granite stock containing mafic microgranular enclaves (MMEs). In this study, we aimed to clarify the role of magmatic processes in the Sn deposit formation. Laser ablation ICP-MS U–Pb dating on magmatic zircons from the biotite granite and MMEs yields identical ages of ~142 Ma, consistent with the regional Early Cretaceous Sn mineralization event. The texture and composition zoning patterns are abundant in zircon, plagioclase, and amphibole, and can be attributed to the mafic and felsic magma mixing within a shallow felsic magma chamber. Amphibole and biotite thermobarometers indicate an estimated depth of ~13 km of the magma chamber, while the deep mafic magma resided at ~26 km with a temperature of ~871 °C. The calculated oxygen fugacities from zircon geochemistry for the biotite granite and the MMEs are FMQ + 0.30 and FMQ + 0.11, respectively, indicative of a relatively reduced condition similar to many other Sn-related intrusions. The amphibole Fe3+/(Fe2+ + Fe3+) molar ratios of the biotite granite and the MMEs range from 0.19 to 0.28, also supporting a relatively low magmatic ƒO2 nature. The reduced magmas could prevent Sn from oxidizing to Sn4+ to partition into Ti- or Fe-bearing minerals, so that Sn remains incompatible during magma crystallization. The high-temperature (~871 °C) mafic magma injection could lead to decomposition of biotite and amphibole from both the previously crystallized Sn-bearing phases and the country rocks, releasing a significant amount of Sn into the mixed melt, which has elevated the Sn contents from ~49 to ~102 ppm in the melt, as revealed by the amphibole Sn concentration and Monte Carlo modeling. The current study highlights the importance of the injected mafic magma, the reduced nature of the parental magmas, and the interaction with Sn-rich country rocks in facilitating the Sn mineralization at Haliheiba, and suggests that these factors may also be critical in the formation of the other Sn deposits in the Great Xing’an Range.