Integrated petrographic, geochemical and geochronological insights into the polyphase evolution of the Paleozoic metamorphic basement in the Elatsite porphyry-copper deposit, Bulgaria

The Palaeozoic metamorphic basement hosting the Upper Cretaceous Elatsite porphyry-copper deposit in the Etropole Mountain consists of a greenschist facies fine-grained silicate to carbonate-silicate sedimentary sequence further affected by contact metamorphic transformations in a tide aureole next to the Upper Carboniferous Vezhen pluton. Field and petrographic observations distinguish alternating phyllite, chlorite-sericite-biotite, chlorite-sericite, and chlorite-sericite-calcite schists with chlorite and biotite porphyroblasts, and local carbonate and sulfide saturation. Their trace-element patterns, including LREE enrichment (La_N/Sm_N = 2.6–4.4), negative Eu anomalies (Eu/Eu* = 0.6–1.0), and low Zr/Sc–Th/Sc ratios indicate a felsic continental provenance with limited sediment recycling and deposition in a continental island-arc setting. Elevated Sr (up to 794 ppm) reflects primary carbonate input, while localized Cr-Ni enrichment (up to ~900 ppm and 440 ppm, respectively) is linked to Fe-oxides and sulfides rather than to a mafic protolith. U-Pb detrital zircon geochronology reveals two major age clusters, Neoproterozoic zircon cores (600–580 Ma) and Carboniferous zircon metamorphic rims (340–330 Ma), together with minor presence of younger Late Ordovician–Early Devonian ages (414–411 Ma). The obtained age of a hydrothermal rutile of ~100 ± 36 Ma records the influence of the Cretaceous hydrothermal activity on these rocks. All these data point to a complex evolution of the studied metamorphic basement including: (1) post-late Neoproterozoic sedimentation which continued up to Early Devonian, (2) mid Carboniferous greenschists facies metamorphism and late Variscan contact metamorphic transformations, and (3) Cretaceous hydrothermal overprint related to the porphyry-copper hydrothermal alteration and mineralization. These features suggest that the metamorphic basement of the Elatsite porphyry-copper deposit acted as a structurally and chemically active constituent, providing pathways for movement and sources of components for the hydrothermal fluids during the Late Cretaceous magmatic-hydrothermal activity.