Geological characteristics and geochronology of the Takht-e-Gonbad copper deposit, SE Iran: A variant of porphyry type deposits

Abstract The Takht-e-Gonbad, or simply Takht, copper deposit in the Kerman belt in southern section of the Cenozoic Urumieh-Dokhtar magmatic arc (UDMA) of Iran is spatially associated with late Oligocene shallow granodiorite porphyries intruded into pyroclastic rocks. Hypogene ore minerals include pyrite, chalcopyrite, magnetite and rare molybdenite and bornite, occurring as veinlets and disseminations mostly in the pyroclastic rocks. Hydrothermal alteration is marked by an extensive phyllic assemblage and irregular zones of propylitic and calc-silicate assemblages with no consistent zoning patterns. The various igneous rocks at Takht area are calc-alkaline to shoshonitic, and are distinguished by enrichments in LILEs relative to HFSEs, and LREEs relative to HREEs (14.41 ≥ La N /Yb N  ≥ 2.85). These features suggest a subduction-related setting for the rocks and the associated copper deposit in the area. A late Oligocene age is obtained for the main Takht granodiorite porphyry and the granodiorite batholith (24.2 Ma and 25.0 Ma, respectively, zircon U-Pb). The age of the main Takht porphyry is distinctly older than the middle-late Miocene ages reported for most porphyry Cu deposits occurring to the north of the Takht deposit, including the world-class Sarcheshmeh and Meiduk. Three distinct types of fluid inclusions are identified: F1: L + V, V  h values ranging between 340 °C and >600 °C and salinities from 55 to 68 wt% NaCl eq best represent characteristics of the mineralizing hydrothermal fluids. The δ 34 S values are measured for pyrite and chalcopyrite; the calculated δ 34 S values for ore fluids fall in the range +4.4 to +7.4‰, slightly heavier than those reported from most other porphyry copper deposits (PCDs) elsewhere. The shift towards more positive values could be attributed to contribution of isotopically heavy sulfur from crustal sources or involvement of marine sulfate in the magma source. The δ 18 O SMOW values for quartz veins in the phyllic assemblage define a narrow range, between +7.6 and +8.1‰. The calculated δ 18 O values for ore fluids range between +6.0 and +6.6‰ suggesting a magmatic source for the hydrothermal fluids. In spite of clear similarities with classic PCDs, in terms of ore mineralogy, ore texture/structure, and ore fluid characteristics, the Takht deposit stands distinct from most known PCDs in the Kerman belt in that most mineralization occurs in the pyroclastic host rocks and in close association with phyllic alteration.