Early miocene adakitic porphyries in the Chagai arc, Pakistan: Petrogenesis, tectonic setting and implications for porphyry Cu-Au mineralization

Numerous porphyry Cu-Au deposits were formed in the Chagai arc, western Pakistan, but their origin remains debated. To address this issue, we conducted zircon U-Pb dating, zircon Hf-O and plagioclase Sr isotopic analyses, and geochemical characterization of ore-bearing biotite granitic porphyries (BGP) from the Saindak area in the Chagai arc. These BGP, formed during the early Miocene (21.9–21.7 Ma), exhibit adakite-like geochemical characteristics, including high SiO₂ (60.5–67.9 wt%) and low Y (6.41–9.13 ppm) contents, coupled with high Sr/Y (26.2–116) ratios. The depleted Hf (ε_Hf(t) = 2.42–9.73) and mantle-like O (δ¹⁸O = 4.99–6.06 ‰) isotopic compositions of zircon grains, along with depleted Sr ((⁸⁷Sr/⁸⁶Sr)_i = 0.70297–0.70482) isotopic compositions and high anorthite (An = 56.5–69.4 %) contents of plagioclase crystals, suggest that these BGP rocks were derived from an oceanic slab. Slab melting may cause the exceptionally high Cu (667–3060 ppm, average of 1416 ppm) contents of the studied BGP samples, which facilitates the formation of porphyry Cu-Au deposits.