Metallogenic age and fluid evolution of the Baituyingzi W-polymetallic deposit, northern margin of North China Craton: constraints from wolframite U–Pb chronology and scheelite geochemistry

• The Baituyingzi W-polymetallic deposit is the only large W deposit in the northern margin of the North China Craton. • U–Pb wolframite dating yielded 222.2 ± 4.1 Ma, which indicates that the deposit is one of the few large-scale tungsten deposits formed during the Indosinian period in China. • The composition of multiple generations of scheelite reveals that the ore-forming fluids evolved from reducing to oxidizing conditions. The Baituyingzi tungsten-polymetallic deposit, located on the northern margin of the North China Craton, is the region’s only large-scale tungsten deposit. This deposit features two distinct mineralization types: porphyry-style molybdenum (Mo) mineralization and tungsten (W) mineralization within silicified fracture zones. This study focuses on the tungsten mineralization within silicified fracture zones in which wolframite is the primary ore mineral, with subordinate scheelite. Wolframite crystallized during the oxide stage exhibited Fe/Mn molar ratios of 0.04–0.44. U–Pb dating of wolframite yielded a low-intercept Tera-Wasserburg age of 222.2 ± 4.1 Ma. This indicates that the deposit is one of the few large-scale tungsten deposits formed during the Indosinian period in China. The scheelite in the deposit can be classified into four distinct generations, each exhibiting unique geochemical characteristics. Sch-I is notably enriched in Nb and Na but depleted in Sr and Mo. It displays high total rare earth element (REE) contents with negligible light-to-heavy REE fractionation (LREE/HREE = 0.92–1.15) and a weak negative Eu anomaly (δEu = 0.67–0.88). With the evolution of fluids, the Nb and Na contents Sch-Ⅱ and Sch-Ⅲ gradually decrease, while Sr and Mo contents show an overall significant increase. The total REE content decreases to 44.19 ppm–296.24 ppm (Sch-Ⅱ) and 12.29 ppm–115.02 ppm (Sch-Ⅲ), exhibiting a positive Eu anomaly. In Sch-Ⅳ, the REE content further decreases, Sr content also declines, while Mo content increases significantly (Mo/W ratio: 0.48–1.46), with some phases transitioning to powellite (W). During this stage, Mo extensively substitutes for W and incorporates into the scheelite structure. A comprehensive geochemical and geochronological data analysis indicates that the Baituyingzi W polymetallic deposit synchronized with the Indosinian orogeny (222.2 ± 4.1 Ma) following the final closure of the Paleo-Asian Ocean along the NCC-Siberia collision zone. Mineralisation occurred during post-collisional extensional tectonics, with ore-forming fluids evolving from reduced to oxidized conditions during the main metallogenic stage. Late-stage meteoric fluids remobilised Mo from earlier porphyry mineralisation, triggering scheelite dissolution-reprecipitation and subsequent tungstite–molybdenite formation.