Epithermal systems of the Torud-Chah Shirin district, northern Iran: Ore-fluid evolution and geodynamic setting

摘要

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 similar to 70 degrees, similar to 270 degrees, and similar to 340 degrees - trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320 degrees-340 degrees but varies from similar to 290 degrees to similar to 350 degrees. The similar to N70 degrees - 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 talc-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.