Regional Metamorphism, Magmatism and Lode Au–Ag Mineralization Controlled by Composite Orogenesis in the Tongbai Orogen, Central China

The Tongbai Orogen, located in the Qinling–Tongbai–Dabie orogenic belt, contains numerous lode Au–Ag deposits, yet the relative roles of Paleozoic orogenic processes and Mesozoic magmatism in their formation remain controversial. In the Weishancheng ore field, Au–Ag mineralization is spatially associated with structures formed during Paleozoic deformation, but available age constraints suggest a much younger mineralization event.

We conducted an integrated study combining in situ U–Pb dating of titanite from metamorphic rocks and xenotime from ore veins, petrography and LA–ICP–MS trace-element analysis of pyrite and marcasite, in situ S–Pb isotopes, and EPMA/TOF-SIMS element mapping. Titanite records two Silurian–Devonian metamorphic events, indicating prolonged Paleozoic tectonometamorphism. Pyrite related to these events locally contains elevated Au, suggesting that early metamorphism was capable of mobilizing gold from the sedimentary sequence and modifying the chemical conditions of the host rocks.

In contrast, xenotime from ore-stage veins yields consistent Early Cretaceous U–Pb ages of ~125 Ma, directly constraining the timing of lode Au–Ag mineralization. Ore sulfides show a systematic chemical evolution from Co–Ni-bearing pyrite to As-rich pyrite with invisible Au–Ag, followed by Ag–Sb-enriched pyrite and late marcasite. Their restricted sulfur isotope compositions and uniform Pb isotopic signatures differ from those of the ore-host strata and regional basement, indicating that the ore-forming fluids were not dominated by metamorphic devolatilization. Instead, the geochemical and geochronological data are consistent with a magmatic–hydrothermal origin related to Early Cretaceous intrusions.

These results suggest that Paleozoic orogenesis primarily established favorable structures and locally redistributed gold, whereas Early Cretaceous magmatism provided the heat and fluids responsible for economically significant Au–Ag mineralization. The Tongbai Orogen therefore represents a case where mineralization reflects the superposition of multiple orogenic processes rather than a single tectonic event.