Subduction-related magmatism and associated porphyry-skarn Fe-Cu ± Mo mineralization in the Chinese northwestern Tianshan

The Chinese northwestern Tianshan contains widespread Paleozoic granitoids and numerous porphyry-skarn Fe-Cu ± Mo deposits, which formed primarily in association with the subduction of the Junggar Ocean (North Tianshan Ocean). However, the links between the evolution of this ancient subduction zone and regional mineralization, as well as the controls exerted by subduction dynamics on deposit distribution, remain poorly understood. This study integrates field geology, zircon U-Pb dating, Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes to constrain the ages and petrogenesis of ore-related granitoids from representative porphyry-skarn Fe-Cu ± Mo deposits and to examine how subduction dynamics influenced mineralization in the Northern Yili Block of the Chinese northwestern Tianshan. Zircon U-Pb dating, together with existing mineralization ages, indicates that the subduction-related porphyry-skarn mineralization and associated magmatism were concentrated in a narrow time window (366–362 Ma) during the Late Devonian. Subsequently, porphyry-skarn mineralization weakened markedly during the Early Carboniferous, coinciding with the emergence of epithermal Au polymetallic deposits. Geochemically, the ore-related granitoids exhibit typical arc-magma signatures with enriched to depleted Hf-Nd isotopes [εNd(t) = −2.2 to +0.8; εHf(t) = −4.8 to +4.7], combined with widespread mafic microgranular enclaves, indicating that they originated from magma mixing between mantle-derived (~10%–30%) and crust-derived (~70%–90%) components. Integrated geochronological and geochemical data from the Late Devonian–Early Carboniferous magmatic rocks record a tectonic transition from advancing subduction in the Late Devonian to retreating subduction in the Early Carboniferous during the southward subduction of the Junggar Ocean beneath the Northern Yili Block. Notably, porphyry-skarn Fe-Cu polymetallic deposits predominated during advancing subduction, whereas epithermal Au polymetallic deposits formed in a backarc extensional setting during retreating subduction. We interpret that the decline in porphyry-skarn Fe-Cu ± Mo mineralization during subduction was primarily controlled by reduced metasomatism of the mantle wedge following this tectonic switch. Conversely, the Early Carboniferous retreating subduction triggered backarc extension, leading to the opening, subsidence, and burial of the Tulasu volcanic fault basin in the backarc region, which favored the preservation of epithermal Au deposits. Consequently, future exploration for porphyry-skarn systems in this region should focus not only on the periphery of existing deposits in the magmatic arc, but also on the subsurface beneath known epithermal occurrences in the backarc area.