Fluid inclusion and stable isotope (O and S) constraints on the genesis of the Guydash iron skarn deposit, East Azarbaijan province, Iran

Magnetite mineralization accompanied by minor hematite, pyrite, chalcopyrite, tetrahedrite, tennantite, and goethite, occurs in the Guydash iron skarn deposit in East Azarbaijan province, Iran. Geologically, it is located in the northwestern part of the Sanandaj–Sirjan zone. The skarn was formed by the intrusion of igneous bodies, especially porphyritic diorites, in contact with Middle-Upper Jurassic limestones and lesser Eocene pyroclastics. During skarn formation, four paragenetic stages of mineralization are distinguished: the prograde, retrograde, sulfidic and supergene stages, with magnetite deposited in the retrograde stage. Microthermometric data from fluid inclusions in calcite and quartz showed that the retrograde mineralization stage occurred at low to moderate temperatures (159.7–299.5 °C), a maximum pressure of 95 bar, and a maximum depth of 1 km. The fluids responsible for mineralization in this stage were aqueous and had low to high salinity (2–34 wt% NaCl equivalent). Fluid inclusion data indicate that the mineralizing fluid in the Guydash deposit was derived from a mixture of magmatic, meteoric, basinal, and metamorphic waters. The δ¹⁸O values in magnetite range from + 5.8 to + 10.2‰. The δ¹⁸O values of water in equilibrium with magnetite at an average homogenization temperature of 230 °C were calculated to range from -2.43‰ to 1.97‰. The O isotope values in magnetite revealed that the mineralizing fluids were mainly from magmatic waters. The δ³⁴S values in pyrite from sulfidic stage range from + 10.2 to + 12.6‰, indicating that the sulfur was supplied from seawater sulfate source. Geological, mineralogical, fluid inclusion and isotopic data suggest that the Guydash deposit is a typical calcic-type Fe skarn deposit related to the intrusion of dioritic rocks into the Jurassic limestones.