High-precision TIMA identification of apatite and titanite and in-situ U–Pb dating: Constraints on the metallogenic age and genesis of the stratiform orebody in the Xinqiao Cu–S polymetallic deposit

In this study, we used TIMA automated mineralogy to systematically identify apatite (Ap) and titanite (Ttn) in the stratiform orebody, skarn orebody, and ore-related intrusion of the Xinqiao deposit. In situ U–Pb dating of Ap3 and Ttn3 intergrown with pyrite in the stratiform orebody yields ages of 140.0 ± 2.0 Ma and 139.2 ± 1.9 Ma, respectively, constraining the stratiform orebody to the Early Cretaceous (∼140 Ma). Compositional variations of apatite and titanite—increasing F, decreasing Cl, a shift from negative to positive Eu anomaly, decreasing Mo, Sn, and Cu, and increasing V and As—record a continuous evolution of ore-forming fluids from high-temperature, oxidized conditions to low-temperature, reduced conditions. Collectively, the evidence indicates that the skarn and stratiform orebodies in the Xinqiao deposit are products of the same magmatic–hydrothermal system at different spatial locations. This study provides a new approach for genetic studies and geochronological constraints on similar deposits in the Middle–Lower Yangtze River metallogenic belt. • In situ U–Pb dating of apatite and titanite constrains the formation age of the stratiform orebody at Xinqiao to ∼140 Ma, indicating an Early Cretaceous magmatic–hydrothermal origin. • Systematic compositional variations of apatite and titanite from magmatic to hydrothermal stages document the continuous evolution of ore-forming fluids from high-temperature, oxidized to low-temperature, reduced conditions. • The coupling of spatial zoning and compositional evolution of the minerals reveals that the skarn and stratiform orebodies at Xinqiao formed from the same magmatic–hydrothermal system but at different spatial locations. • The integrated workflow combining TIMA automated mineralogy with apatite–titanite geochronology and in situ micro-geochemistry expands the application of TIMA in hydrothermal accessory mineral studies and provides a transferable technical approach for geochronological and genetic studies of hydrothermal deposits. The formation age and genesis of the stratiform orebody in the Xinqiao Cu-S polymetallic deposit in the Middle–Lower Yangtze River metallogenic belt have long been debated. In this study, we used TIMA automated mineralogy, in situ U–Pb dating, and micro-geochemical analyses on apatite and titanite from the causative intrusion, skarn, and stratiform orebody to precisely determine the mineralization age and genetic mechanism of the stratiform orebody. Based on mineralogical and geochemical characteristics, three generations of apatite and titanite are recognized: late magmatic (Ap1/Ttn1), skarn (Ap2/Ttn2), and quartz–sulfide (Ap3/Ttn3). Among these, apatite (Ap3) and titanite (Ttn3) intergrown with pyrite and cut by late carbonate veins are products of the main mineralization stage. LA-ICP-MS in situ U–Pb dating yields ages of 140.0 ± 2.0 Ma for Ap3 and 139.2 ± 1.9 Ma for Ttn3, which are consistent within error, thus constraining the stratiform orebody to the Early Cretaceous. During magmatic–hydrothermal evolution, apatite and titanite show systematic compositional changes: F increases, Cl decreases, Eu anomaly shifts from negative to positive, ore-forming elements such as Mo, Sn, and Cu decrease, and V and As increase. These trends indicate that the ore-forming fluids evolved continuously from high-temperature, oxidized conditions to low-temperature, reduced conditions. Collectively, the evidence suggests that the stratiform and skarn orebodies represent different end-members of the same magmatic–hydrothermal system. This study also indicates that apatite and titanite may be common throughgoing minerals in similar deposits of the Middle–Lower Yangtze River metallogenic belt. The integrated workflow combining TIMA automated mineralogy with apatite–titanite geochronology and in situ micro–geochemistry provides a useful approach for dating such deposits and for unraveling the genetic links between skarn and stratiform orebodies.