Ore genesis and fluid evolution of the Xiaobaishitou skarn W–Mo deposit in East Tianshan, NW China: Insights from the geochemistry of garnet, scheelite, and apatite

• Multiple generations of garnet (Grt IA, Grt IB), scheelite (Sch IB, Sch II, Sch III, Sch V), and apatite (Ap V) were identified. • Garnet U‒Pb dating yielded lower intercept 206 Pb/ 238 U ages of 245.0 ± 3.9 Ma and 246.1 ± 1.5 Ma. • Scheelite precipitation control shifted from pH-dominant (prograde stage) to f O2 -controlled (retrograde and quartz-sulfide stages) in the skarn type-mineralization. • Magmatic fluid replenishment critically sustained ore-fluid f O2 in vein-type mineralization. Xiaobaishitou is a typical skarn W–Mo deposit located in East Tianshan, NW China. Garnet, scheelite, and apatite of the Xiaobaishitou deposit not only crystallized separately during the prograde stage (Grt IA, Grt IB, and Sch IB), the retrograde stage (Sch II), and the quartz–sulfide stage (Sch III) in the skarn-type mineralization, but also were deposited in an extensive vein-type mineralization (Sch V and Ap V). Grt IA is grossular-dominant (Adr 17–23 Grs 72–78 ) and contains positive Eu anomalies, whereas the Grt IB comprises oscillatory-zoned grossular–andradite (Adr 35–52 Grs 36–58 ) that contains weakly negative Eu anomalies, suggesting changes in the fluid pH during the prograde stage. Sch IB is characterized by complex vaguely zoned textures in cathodoluminescence (CL) images, with negative Eu anomalies (0.12–0.58) and light rare earth element (LREE) enrichment, which precipitated in the initial oxidizing ore-forming fluids derived from single granitic magma reservoirs. Sch II and Sch III exhibit homogeneous textures in CL images, with LREE enrichment and significantly positive Eu anomalies. Notably, the variations in Eu anomalies (2.92–7.43 to 14.74–87.35) and Mo contents (7811 to 157 ppm) from Sch II to Sch III reflect the degree of oxygen fugacity from the retrograde stage to the quartz–sulfide stage, which are due to the fluid–rock interactions and the formation of skarn minerals. The changes in the contents of trace elements (e.g., Nb, Ta, and ΣREEs) in Sch V suggest that the vein-type mineralization in Xiaobaishitou deposit formed in an open system. Nb-rich Sch V (up to 1192 ppm) developed as a perfect oscillating belt and precipitated in long-lived weakly reducing environments, which were buffered by repeated replenishment of magmatic fluids. Moreover, Sch V and Ap V exhibit notably heavy rare earth element (HREE) enrichment and elevated 87 Sr/ 86 Sr ratios (0.69472 to 0.72070). In addition, in situ laser ablation–inductively coupled plasma‒mass spectrometry U–Pb dating conducted on two garnet samples produced lower intercept 206 Pb/ 238 U ages of 246.1 ± 1.5 Ma and 245.0 ± 3.9 Ma. Overall, we suggest that the skarn-type mineralization and vein-type mineralization in the Xiaobaishitou W–Mo deposit were derived from single and multiple pulsed flows of Triassic magmatic fluids, respectively