Magmatism and gold mineralization in the Tianmashan skarn deposit, Tongling district, China

Skarn gold deposits constitute an important gold resource in China, and numerous porphyry-skarn Cu-Au and skarn Au systems distributed along the Middle-Lower Yangtze River Metallogenic Belt. However, the genesis of gold-dominant skarn systems in this region remains poorly constrained. The Tianmashan gold deposit, the largest skarn Au deposit in the Tongling district, provides an excellent opportunity to investigate the processes responsible for gold mineralization. This study integrates zircon and garnet U-Pb geochronology, whole-rock geochemistry, zircon Hf isotopes, and sulfide sulfur isotope and trace-element data to constrain the timing, magma source, and ore-forming processes. LA-ICP-MS U-Pb dating of magmatic zircon and hydrothermal garnet yields ages of 137.1 ± 1.0 Ma and 137.1 ± 2.8 Ma, respectively, indicating that magmatism and gold mineralization were coeval during the Early Cretaceous. The quartz diorite porphyry exhibits adakitic geochemical characteristics, indicating derivation from partial melting of a thickened lower crust. Low zircon ε Hf (t) values (−18.9 to −12.0) further suggest involvement of both juvenile and ancient crustal components, with the juvenile lower crust likely serving as an important metal reservoir for the ore-forming magmas. Zircon oxybarometry indicates elevated magmatic oxygen fugacity (ΔFMQ + 0.9 to + 4.3), which was likely enhanced by the input of oxidized mantle-derived magma during crust–mantle magma mixing. Sulfide trace element and sulfur isotope data indicate that the ore-forming fluids were dominantly magmatic-hydrothermal, with a limited contribution from sedimentary-derived fluids during the late-stage mineralization. Gold was primarily sourced from magmatic-hydrothermal fluids, whereas the formation of the interlayer replacement-type orebodies was likely facilitated by mixing between magmatic-hydrothermal fluids and sedimentary-derived fluids. Such fluid mixing may have lowered the oxidation state of the hydrothermal system and promoted gold precipitation.