Genesis of the Kuh-Panj porphyry copper deposit, Kerman, Iran: Constraints from mineralization, geochemistry, fluid inclusion, and zircon U[sbnd]Pb isotope systematics

Abstract

The Miocene Kuh-Panj porphyry copper deposit, with reserve estimated at about 100 Mt. at 0.21 % Cu, is located 120 km SW of Kerman in the southern part of the Urumieh-Dokhtar magmatic arc. The Eocene host rocks consist of tuff, lithic tuff, breccia tuff, and lava flows, ranging in composition from rhyodacite to andesite. A late Miocene granodiorite porphyry stock (8.57 ± 0.49 Ma, U/Pb zircon) intruded into the Eocene sequence. Subvolcanic dykes cut the Eocene rocks and porphyry stock. The Kuh-Panj porphyry system has an extensive and typical hydrothermal alteration zoning pattern, which is comprised of potassic, phyllic, argillic, and propylitic alteration types from inner proximal to outer distal zones. Copper mineralization occurs in potassic and to some extent in phyllic zones. Mineralization is mostly observed as disseminated, stockwork, and vein-veinlets forms in the porphyry stock. Vein paragenesis includes barren quartz, M-type (magnetite+biotite), A1-type (quartz+magnetite+chalcopyrite±chlorite±biotite), A2-type (quartz+chalcopyrite±pyrite), D-type (quartz+pyrite±chalcopyrite), and L-type (galena+sphalerite±chalcopyrite±pyrite) veins. The I-type granitoids with calc-alkaline affinity have enrichments of LILE (Pb, K, U, Sr, and Cs), depletion of HFSE (P, Nb, Ta, Zr, and Ti), and enrichment in LREE displaying geochemical similarity to arc-related magmatic rocks. The high SiO2 (ave. 68.9 wt%), high Al2O3 (ave. 14.5 wt%), low MgO (ave. 1.3 wt%), low P2O5 (< 0.2 wt%), high Sr (ave. 365 ppm), high Sr/Y (ave. 45), relatively high LaN/YbN (ave. 28), relatively high Th (ave. 8 ppm), low Y (ave. 8 ppm), and low Yb (0.5 ppm) contents are similar to those of fertile porphyries and lower crust-derived adakites. The most dominant type of fluid inclusions in quartz from mineralized veins are liquid-vapor-halite-solid (LVHS), liquid-vapor-halite (LVH), liquid-vapor (LV), and vapor-liquid (VL) fluids. Homogenization temperatures of LVHS, LVH, LV, and VL fluids range from 306 to 574 °C, 141–566 °C, 215–422°, and 349–571 °C, respectively. Salinities of LVHS, LVH, LV, and VL fluid inclusions range from 30 to 69, 29.0–65.5, 0.35–22.0, and 0.7–13.0 wt% NaCl eq., respectively. The extreme variations in salinities of fluid inclusions represent the combined effects of varying salinity of the magmatic fluid, aqueous fluid-phase immiscibility (boiling), and mixing with lower salinity meteoric water later in the cooling history of the porphyry system. Normal cooling, water-rock interaction, surface fluid dilution, and boiling are dominant factors that control both the distribution and abundance of mineralization. Zircon from the Kuh-Panj system has high Th/U (0.53 to 0.83), low Dy/Yb (<0.3) ratios, positive Eu/Eu* (1.28–1.85), strong positive Ce/Ce* (1.36 to 197) reflecting high Ce4+/Ce3+ ratios, and enrichment in HREEs relative to LREEs, which indicate the magmatic origin of zircon, oxidized, and hydrous conditions of the primary magma. This magma with adakitic affinities is favorable for producing fertile porphyry systems. Based on these multidisciplinary studies, the Kuh-Panj deposit is classified as a continental arc-related porphyry copper system hosted predominantly at the transition zone between potassic and phyllic alteration zones due to decreasing temperature, boiling, and fluid-rock reaction that all enhance CuFe sulfide saturation.