Geology, geochemistry, fluid inclusions, and H–O–C–S–Pb isotope constraints on the genesis of the Atash-Anbar epithermal gold deposit, Urumieh–Dokhtar magmatic arc, central-northern Iran

Abstract

The newly discovered Atash-Anbar gold deposit, with ∼2 Mt of ore grading 2.13 g/t Au (locally up to 14 g/t), is located in the Buin-Zahra Range, central-northern Iran. Hosted by Middle Eocene (ca. 39.0 Ma) volcanic-subvolcanic rocks, the Au-polymetallic orebodies occur as NW− and NE − trending quartz-sulfide veins that are structurally controlled by NW-trending faults. The Middle Eocene host rocks are characterized by high-K calc-alkaline, peraluminous signatures, and moderately Eu negative anomalies (Eu/Eu* = 0.2–0.6). Four primary paragenetic stages of veining have been recognized: (I) smoky-grey quartz-chalcopyrite stage, (II) grey-white quartz-polysulfide stage, (III) white-pinkish barite-sulfide stage, and (IV) late quartz-carbonate stage. Fluid inclusion investigations coupled with laser Raman analyses indicate that the ore-forming fluids were formed in a NaCl − H2O ± CO2 system with two types of FIs: two phase, liquid-rich inclusions (type I) and two phase, vapor-rich inclusions (type II). The primary coexisting types I and II inclusions are observed in gold-bearing ore-stage II, sharing similar homogenization temperatures in the range of 233–286 °C and 243–281 °C, but contrasting salinity values of 10.7–14.8 and 14.6–15.5 wt% NaCl equivalent, respectively. Fluid inclusion examinations show that fluid phase separation occurred concurrently with gold precipitation during the ore-stage II. The δ18Ofluid values calculated from the δ18OSMOW and δDSMOW values of the inclusions in quartz veins are 4.9–11.1 ‰ and −84.6 ‰ to −68.3 ‰, respectively. δ13CPDB and δ18OSMOW values of Fe-dolomite are −6.5 ‰ to −5.0 ‰ and 3.1 ‰–4.9 ‰, respectively. H–O–C isotope data indicate magmatic origin of initial ore-forming fluids with minor addition of meteoric water through time. The δ34S values (−3.1 ‰ to −1.2 ‰, avg. = −2.0 ‰) of the ore-stage II sulfides suggest that sulfur comes from a homogeneous magmatic source. The Pb isotopic compositions of sulfides (207Pb/204Pb = 15.420–15.525, 206Pb/204Pb = 18.423–18.536, 208Pb/204Pb = 37.135–38.208) indicate that the Pb in the Atash-Anbar gold deposit is a mixture of crust and mantle components. We conclude that (1) the Atash-Anbar is an IS epithermal system that (2) resulted from Eocene subduction of the NeoTethys Ocean plate beneath the central Iran plate and thus (3) ore-forming fluid and its metal components (i.e., Ag, Pb, and Zn) were emanated through crystallization of the final phase of a granite porphyry intrusion.