Genesis of the Abbasabad epithermal base metal deposit, NW Iran: Evidences from ore geology, fluid inclusion and O–S isotopes

Abstract

The Abbasabad epithermal Pb–Zn–Cu deposit is located in the Tarom–Hashtjin Metallogenic Belt (THMB), northwest Iran. Mineralization occurs as quartz-base metal sulfide veins and breccias mainly hosted by Eocene volcanic and volcaniclastic rocks of the Karaj Formation. Four mineralization stages are recognized: (1) quartz-chalcopyrite-pyrite veins and breccias, (2) quartz-calcite I-sphalerite-galena ± chalcopyrite ± pyrite veins and breccias, (3) late stage barite veins and veinlets, and (4) barren post-ore calcite II veinlets. The primary metallic minerals, in order of abundance, are galena, sphalerite, chalcopyrite, and pyrite; gangue minerals are mainly quartz, barite, calcite, sericite/illite, and chlorite. The main wall-rock alteration is silicification, intermediate argillic, carbonatization, and propylitic alteration. Microthermometric measurements of primary LV fluid inclusion assemblages in quartz, sphalerite and barite show that the ore-forming fluids belong to the low- to moderate-temperature (170–315 °C), moderate-salinity (3.6–7.1 wt% NaCl equiv.), low-density H2O–NaCl system. Estimated trapping hydrostatic pressures are 60 to 100 bars, corresponding to entrapment depths of 614 to 1022 m below the paleowater table. Oxygen isotopic data (δ18OH2O of + 7.6 to + 0.6‰) indicate that the ore-forming fluid was derived largely from magmatic fluid, with the addition of meteoric water in the later stage. δ34SH2S values of sulfides vary over a narrow range from –17.4‰ to –15.4‰, (averaging − 16.3‰), indicate that sulfur might be derived from a relatively homogeneous magmatic source which oxidized by meteoric water through fluid mixing. The Abbasabad deposit is interpreted as an intermediate-sulfidation epithermal mineralization derived from a mixed source during a phase change in temperature and pressure interpreted as being related to fluid mixing and boiling. Our data suggest that mineralization at Abbasabad and other epithermal deposits in the THMB are related to late Eocene (~40 to 36 Ma) post-collisional magmatism, which is a useful concept for local and regional epithermal base metal exploration targeting.