Genesis of Cu–Mo zoning in a composite porphyry system: Insights from the Late Jurassic Gutian deposit, southeastern China

Porphyry Cu–Mo systems are commonly assembled through repeated magmatic and hydrothermal events, but the spatial and temporal relationships between Cu and Mo enrichment in composite porphyry systems remain difficult to understand. The Late Jurassic Gutian porphyry Cu–Mo deposit in the Southeast Coastal Metallogenic Belt of southeastern China provides an opportunity to evaluate Cu–Mo zoning within a composite porphyry system. Here we integrate drill-core logging, vein paragenesis, grade-contour mapping, binary statistical analysis of Cu–Mo orebody overlap, molybdenite Re–Os dating, and hydrothermal rutile U–Pb geochronology. The results show that Cu and Mo mineralization at Gutian is spatially and vertically zoned, with local Cu–Mo superposition. Binary statistics from four representative exploration sections indicate limited but variable Cu–Mo overlap, with overlapping orebody points accounting for 1.1–4.9 % of valid samples. Molybdenite is mainly associated with quartz ± K-feldspar and epidote–quartz veins formed during K-silicic to early propylitic alteration, whereas chalcopyrite is most abundant in phyllic-stage quartz–sericite–pyrite veinlet systems. Molybdenite Re–Os dating yields a weighted mean age of 162.8 ± 1.0 Ma and an isochron age of 162.4 ± 2.9 Ma for Mo-rich hydrothermal products. Rutile U–Pb dating of rutile closely associated with chalcopyrite yields a lower-intercept age of 157.6 ± 2.6 Ma, which is interpreted to provide a temporal constraint on a Cu-associated hydrothermal product. These ages, together with intrusive relationships and vein crosscutting textures, are consistent with the interpretation that the Gutian system records episodic hydrothermal activity superimposed on the incremental assembly of a Late Jurassic composite intrusive complex. The Cu–Mo zoning is therefore most likely to reflect stage-dependent relative enrichment of Mo and Cu during successive hydrothermal pulses, with local overprinting between mineralized domains. This study highlights the need to interpret molybdenite Re–Os ages within their paragenetic context and to combine metal-specific geochronology with detailed vein and alteration relationships when reconstructing the timing of porphyry Cu–Mo mineralization.