Geochronology and petrogenesis of copper-bearing tonalitic dykes from the Kohistan Batholith in the Kohistan Island Arc, north Pakistan: Implications for mineralization and metallogenesis

Investigating magma sources, geochemical evolution, and tectonic controls is essential for comprehending petrogenesis and mineralization processes. This study presents new data on zircon and rutile U–Pb geochronology, zircon Hf isotope compositions, and whole-rock and mineral chemistry for the newly identified copper-bearing tonalitic dykes in the Kohistan Batholith (KB), northern Pakistan. These datasets provide insights into the petrogenesis, geochronology, mineralization, and tectonic significance of these dykes within the Kohistan Island Arc (KIA). Zircon U–Pb dating indicates that the tonalitic dykes were emplaced at ∼152 Ma, representing the oldest known intrusive unit within the KB. Mineralogical and geochemical similarities between the dykes and adjacent tonalites suggest a co-magmatic origin, coinciding with the onset of Neo-Tethys subduction. Geochemically, these dykes exhibit exceptionally high SiO 2 content (avg. 79.9 wt%) and strongly positive ɛ Hf (t) values (+15.3 to + 19.3), consistent with highly fractionated magmas derived from a depleted mantle source. Comparison with Rhyolite-MELTS modeling of co-magmatic tonalites and mafic microgranular enclaves from the region further supports this genetic model. Late-stage interstitial textures of copper-bearing phases and zircon trace element compositions suggest that the tonalitic magma was copper-poor (similar to barren granites globally). Apatite grains from ore-bearing dykes exhibit distinct late-stage textures and significant elevated concentrations of Cu, Eu, Ce, Mn, V, and Ga, as well as Cl/F ratios, compared to apatite from barren tonalites. Our rutile U–Pb dating yields ∼52 Ma hydrothermal mineralization age, coinciding with the development of an Andean-type continental margin due to the collision of the KIA with the Eurasian plate. The spatial and temporal association of these dykes with the suture zone, alongside sulfide trace element and sulfur isotopic compositions, implies that Cu-enriched magmatic-hydrothermal fluids originating from a metasomatized slab, infiltrated fractures and induced mineralization within the tonalitic dykes. This study underscores that Andean-type magmatism, rather than earlier Tethyan magmatism, was the dominant driver of Cu deposits formation in the KIA.