Geology and genesis of the Takht-Gonbad porphyry Cu deposit in the Kerman belt, SW Iran

• The Takht-Gonbad Cu deposit was formed in the Oligocene, as constrained by systematic zircon U-Pb (26.7 ± 0.2–26.0 ± 0.4 Ma) and molybdenite Re-Os dating (26.3 ± 0.3 Ma, MSWD = 1.7). • Chalcopyrite intergrows intimately with dull-CL and low-Ti quartz, and were precipitated at the terminal stage of potassic alteration. • The Takht-Gonbad deposit shares many geological features with other porphyry Cu deposit worldwide, and therefore highlights the potential of Oligocene porphyry Cu deposits in the Kerman belt. The Takht-Gonbad Cu deposit in the Kerman porphyry belt in southwest Iran is unique due to its genetic association with Oligocene magmatism, which was previously considered infertile for porphyry Cu mineralization. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating of pre-mineralization volcanics, syn -mineralization granodiorite porphyry, and post-mineralization granodiorite at Takht-Gonbad yield ages of 27.9 ± 0.4–27.3 ± 0.2 Ma, 26.7 ± 0.2–26.0 ± 0.4 Ma, and 25.2 ± 0.2 Ma, respectively. Timing of mineralization is constrained at 26.3 ± 0.3 Ma (MSWD = 1.7) by two molybdenite Re-Os ages, agreeing with U-Pb ages of the syn -mineralization intrusions. Hydrothermal alteration and vein sequences at Takht-Gonbad are grouped into three main stages. The early stage is characterized by potassic alteration, expressed by pervasive secondary biotite and magnetite-dominated veins, quartz-magnetite veins, and quartz-dominated veins. The transitional stage represents the end of potassic alteration. Associated veins include chalcopyrite-dominated microfracture infills and chalcopyrite-quartz centerlines in early-stage veins, and molybdenite-siderite-kaolinite-quartz veins. The late stage is marked by chloritic-sericitic alteration. Associated veins include quartz-pyrite and calcite veins. Detailed petrographic and SEM-CL observations and in-situ LA-ICP-MS trace element analysis demonstrate three major generations of hydrothermal quartz (Q1 to Q3) at Takht-Gonbad. The Q1 typically developed in early-stage veins, and is characterized by granular (Q1a) to anhedral shapes (Q1b), sugary mosaic textures, and blurred (Q1a) to oscillatory (Q1b) CL zonings. It has the brightest luminescence and highest titanium contents (Ti > 30 ppm) among all generations. The Q2 develops mainly in transitional stage veins, and occurs as anhedral overgrowth (Q2a) or euhedral crystals (Q2b-c) along earlier Q1 in fracture networks. It is featured by dull-CL and low Ti contents (0.8–53.1 ppm), and is explained by dissolution-reprecipitation of earlier Q1 under conditions of retrograde silica solubility during fluid cooling. Quartz in the late-stage veins is classified as Q3, which is characterized by dark-CL and absence of luminescence zoning. The Q3 has elevated Ti (13.5–39.6 ppm) contents compared with the Q2, indicating its formation from a new batch of magmatic fluid. Chalcopyrite intergrows intimately with dull-CL, low-Ti Q2 in transition-stage veins, suggesting that they were precipitated at the terminal stage of potassic alteration. Hematite daughter minerals in halite-bearing brine inclusions indicate relatively oxidized nature of the ore-forming fluid, which is in line with extensive magnetite alteration in the deposit. Collectively, this study highlights the potential of Oligocene porphyry Cu deposits in the Kerman belt, although the Cu endowments may be not as large as their Miocene counterparts.