Fengmei Chai

The Bailingshan submarine volcanic-hosted iron deposit in the Eastern Tianshan provides an opportunity to investigate skarn hydrothermal evolution and high-grade Fe mineralization. This study integrates garnet petrography, in situ major- and trace-element analyses, garnet U–Pb geochronology, and amphibole ⁴⁰Ar/³⁹Ar geochronology. Four generations of grossular–andradite garnet (Grt1–Grt4) exhibit systematic textural and compositional variations reflecting the combined effects of crystal-chemical controls, hydrothermal fluid composition, and fluid–rock interaction. U–Pb dating of Grt2 yields 331.8 ± 8.9 Ma, linking the principal skarn formation and Fe mineralization stage to Early Carboniferous volcanism. Amphibole ⁴⁰Ar/³⁹Ar dating yields 302.6 ± 2.4 Ma, constraining a younger, localized hydrothermal overprint with limited Fe addition. Integrated evidence indicates that the Bailingshan deposit formed through a multistage, episodic hydrothermal evolution involving initial skarn formation, the principal Grt2–magnetite stage, subsequent shifts in hydrothermal conditions recorded by Grt3 and Grt4, and a localized late-stage hydrothermal overprint. These results not only constrain the timing of mineralization and subsequent overprinting but also offer valuable exploration implications for high-grade Fe mineralization in the Eastern Tianshan and comparable submarine volcanic-hosted iron districts.

The Bastielieke W-polymetallic deposit, located in the Xinjiang Altay metallogenic belt, records a complex hydrothermal history critical to understanding multi-stage metallogenic processes in the southern Altay. This study integrates in situ U-Pb dating of hydrothermal titanite and zircon with textural and compositional analyses of titanite to reconstruct this history. Three types of hydrothermal titanite, identified from pyroxene skarn (TtnI), epidote skarn (TtnII), and quartz–sulfide ore (TtnIII), display dissolution–reprecipitation textures and systematic compositional variations, indicating distinct fluid compositions and origins. TtnI, TtnII, and TtnIII yield U-Pb ages of 244.7 ± 7.8 Ma, 252.4 ± 5.5 Ma, and 250.6 ± 3.0 Ma, respectively, and hydrothermal zircon from pyroxene skarn yields an age of 249.9 ± 2.1 Ma, constraining the hydrothermal event to the latest Permian to Early Triassic. These ages are interpreted to record the timing of U-Pb system resetting during regional shear–thrust movements. Compositional variations among the three titanite types reveal a two-stage hydrothermal history. The earlier stage involved W–Cu mineralization and protolith titanite precipitation related to magmatic–hydrothermal fluids exsolved from Permian granites. The later stage was driven by regional shear–thrust movements and metamorphic–hydrothermal processes, which reset the titanite U-Pb systems, partially altered TtnI and TtnII, precipitated TtnIII, and remobilized metals. This model links the Bastielieke deposit to multi-stage hydrothermal processes and provides insights into similar metallogenic events along the southern margin of Xinjiang Altay.

The Kalatag mineralization belt is an important metallogenic belt of polymetallic mineral deposits in the northern part of eastern Tianshan, and its age and tectonic setting are still controversial. We identified a set of Devonian volcanic rocks hosted in the Early Palaeozoic package of dominantly marine sediments with a small amount of terrestrial rocks. This study presents petrological, U–Pb geochronology, and geochemical data for the volcanic rocks. The ages of the rhyolite (407.2 ± 1.9 Ma) and basaltic andesite (380.4 ± 2.8 Ma) suggests that the Kalatag belt is a Devonian volcanic succession. These rocks consist mainly of marine calc–alkaline lava, tuff, pyroclastic rocks, and minor terrestrial basaltic andesite. The lavas are characterized by the enrichment of light rare earth elements and strongly depleted in Nb and Ta, typical of island arc magmatic rocks. The volcanic rocks probably originated from the partial melting of the mafic lower crust which was modified by subducted slab-related fluids. During their ascent through the crust, these volcanic rocks underwent variable extents of fractional crystallization (rhyolites) and crustal contamination (basaltic andesites). Combined with the results of previous studies, we suggest that the Devonian rocks formed in an island arc related to the northward subduction of the Northern Tianshan Ocean with a crustal thickness of ~35–40 km.

Eastern Tianshan hosts a number of porphyry Cu deposits. However, these mainly formed in the Jueluotage Belt, in the middle part of Eastern Tianshan. The Tonggou porphyry Cu mineralization is an exception to this, since it is located in the Bogda Orogenic Belt, north of Eastern Tianshan. We obtained new zircon U-Pb ages, whole-rock geochemical data, zircon Hf isotope data, and zircon trace element compositions. LA-ICP-MS zircon U-Pb dating indicates a crystallization age of 302.2–303.0 Ma for the Tonggou mineralized granodiorite (TMG), which suggests that the Tonggou porphyry Cu mineralization formed in the Late Carboniferous period. ε_Hf (t) data (1.8–14.1) for TMG suggests it was sourced from juvenile crustal melts, mixed with some mantle materials. TMG displays low ΣREE, compatible elements (Ba, Sr, Zr, and Hf), Zr/Hf and Nb/Ta ratios, as well as clearly negative Eu anomalies in whole rocks analyses. In addition, TMG is enriched in P, Hf and Th/U ratios in zircon, and has lower crystallization temperatures (734 to 735 °C) than the Daheyan barren granodiorite (DBG) (753 to 802 °C). Whole rock and zircon geochemical analyses show that the TMG was formed by fractional crystallization to a greater extent than the DBG in the Bogda Orogenic Belt. Moreover, zircon grains of the TMG show high Ce⁴⁺/Ce³⁺ ratios (159–286), which are consistent with related values from large porphyry deposits of the Central Asian Orogenic Belt (CAOB). High Ce^4+/Ce³⁺ ratios reflect oxidizing magmas as a result of fractional crystallization, which indicates that the Tonggou deposit has potential to host a large porphyry Cu deposit.