The Khopik porphyry copper prospect, Lut Block, Eastern Iran: Geology, alteration and mineralization, fluid inclusion, and oxygen isotope studies

The Khopik porphyry copper (Au, Mo) prospect in Eastern Iran is associated with a succession of Middle to Late Eocene I-type, high-K, calc-alkaline to shoshonite, monzonitic to dioritic subvolcanic porphyry stocks emplaced within cogenetic volcanic rocks. Laser-ablation U-Pb zircon ages indicate that the monzonite stocks crystallized over a short time span during the Middle Eocene (39.0 ± 0.8 Ma to 38.2 ± 0.8 Ma) as result of subduction of the Afghan block beneath the Lut block.Porphyry copper mineralization is hosted by the monzonitic intrusions and is associated with a hydrothermal alteration that includes potassic, sericitic-potassic, quartz-sericite-carbonate-pyrite (QSCP), quartz-carbonate-pyrite (QCP), and propylitic zones. Mineralization occurs as disseminated to stockwork styles, and as minor hydrothermal breccias. Some mineralization occurs in fault zones as quartz-sulfide veins telescoped onto the porphyry system. The main ore minerals are chalcopyrite and bornite with minorpyrite and magnetite and the highest Cu (0.01–0.9 wt. %), Au (> 2 ppm), and Mo (< 80 ppm) grades are closely associated with potassic alteration zones.Unidirectional, pre-ore stage solidification texture (UST) represented by comb-quartz layers within the potassic alteration zone formed from a hypersaline brine (57–73 wt. % NaCl equiv.), low density vapor-rich fluids (4–22 wt. % NaCl equiv.) at temperatures of 482 °C to over 600 °C. The isotopic composition of oxygen (δ18Owater = 8.7–8.9 ‰) suggests that the quartz layers crystallized from magmatic-hydrothermal fluids that exsolved in the upper part of the monzonitic intrusions. Potassic alteration formed from high salinity fluids (51–73 wt. %) at temperatures between 432–592 °C, and low salinity vapor-rich solutions with 11–19 wt. % NaCl equiv. Later veinlets in the QSCP zone formed from lower salinity fluids (< 47 wt. % NaCl equiv.) at temperatures between 332°–400 °C. The oxygen isotopic data for the early alteration zones (δ18Owater = 9–9.3 ‰ for potassic and 7.3 ‰ for QSCP) also indicate a magmatic origin for the ore fluids.The widespread presence of Middle Eocene to Lower Oligocene magmatism and mineralization in Eastern Iran suggests the presence of another important porphyry copper belt in addition to the northwest-southeast Urumieh-Dokhtar copper belt of Iran.