Abstract
The Torud–Chah Shirin (TCS) ore district in northern Iran is defined by an NE alignment of Tertiary Ag-Au and base metal-rich epithermal systems, and it is part of the eastern Alborz orogenic belt of Iran. Intermediate- to high-sulfidation mineralization occurs as veins hosted by the Eocene−Oligocene volcanic, subvolcanic and volcaniclastic rocks. The TCS district is characterized by three fault system populations including ∼70°, ∼270°, and ∼340°−trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320°–340° but varies from ∼290° to ∼350°. The ∼N70°−trending faults are parallel to the Anjilow and Torud regional faults in the TCS ore district. Green- to grey-schist and metamorphosed dolomite and limestone are the oldest units (Ordovician–Silurian) in the TCS. Sedimentary rocks were initiated by limestone, dolomite and green shale in the Cretaceous and continued with conglomerate into the Paleocene (Fajan Formation). Magmatism mainly occurred sporadically from the Eocene to Oligocene (ca. 55–24 Ma), with two major episodes between early to middle Eocene (ca. 55 and 37 Ma, EME) and early to late Oligocene (ca. 34 and 24 Ma, ELO). Whole rock geochemical data of EME and ELO rocks of the TCS district shows a range from basalts to rhyolites with low-K calc-alkaline and shoshonitic affinity. Their rare earth elements (REEs) and high field strength elements (HFSE) signatures indicate the occurrence of a supra-subduction zone magmatism and all rocks have been sourced from the same parent melt. Samples from ELO display higher alkali contents compare with EME but have a similar trace element characteristics.Hydrothermal alteration is pervasive in volcanic and subvolcanic rocks but is mainly localized near the veins and ore zones. Alteration assemblages include quartz-sericite (±pyrite ± adularia), quartz-illite (kaolinite ± pyrite), carbonate-quartz and chlorite + calcite + epidote (propylitic). Homogenization temperatures of fluid inclusions in the epithermal prospects is in range of 125–375 °C, and ore-forming fluids were mainly of magmatic-hydrothermal origin (e.g., Gandy and Abolhassani prospects), with some contributions from meteoric water. The epithermal prospects have more or less similar salinities, ranging from 2 to 18 wt% NaCl equiv., with a distinct cluster between 6 and 15 wt% NaCl equiv.Our shreds of evidence suggest that the EME and ELO calc-alkaline volcanic activity in TCS occurs occurred in tension fractures related to orthogonal plate convergence that postdates the NE-trending strike-slip regional faults, and plays an important role in the development of the epithermal prospects. Simultaneous with, or soon after crustal heating related to the magmatism, strike-slip faults movement may also have been critical in the ore-forming process, leading to trans-tension of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.
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