S-C-O Isotopes, Fluid Inclusion Microthermometry, and the Genesis of Ore Bearing Fluids at Qaleh-Zari Fe-Oxide Cu-Au-Ag Mine, Iran

Qaleh-Zari is a Fe-oxide Cu-Ag-Au vein type deposit located 180 km south of Birjand, in eastern Iran. Host rocks are mainly Tertiary calc-alkaline to K-rich calc-alkaline with transition to shoshonitic andesite and andesitic basalts, but in the central part shale and sandstone of Jurassic age. Andesitic rocks from the western region of Qaleh-Zari were dated to 40.5 ± 2 Ma. Four trends of faults and joints are identified in the mine area. Cu-Ag-Au mineralization is present only in the oldest sets of faults and joints that crosscut the Tertiary and Jurassic units. Three major sub-parallel steep quartz veins are identified. No.1 vein is about 650 m long and No.3 vein is less than 500 m long. No.2 vein is traced for more than 3.5 km horizontally along strike (N40° W) and more than 350 m down dip. Specularite and quartz are the most abundant primary oxides. Chalcopyrite is the only hypogene copper mineral. Silver is present as sulfosalt minerals. Paragenesis: Stage I: specularite, quartz, Fe-chlorite, chalcopyrite and sulfosalts. Specularite deposited first and forms 10 to 25 percent of the vein. Stage II: quartz, chalcopyrite, pyrite, chlorite ± hematite ± sulfosalt minerals. Stage III: quartz ± pyrite ± chalcopyrite. Stage IV: hematite, quartz, and ± calcite. The ores grade typically 2-9% Cu, 100-650 ppm Ag, and 0.5-35 ppm Au. Homogenization temperatures of fluid inclusions associated with Cu, Ag, and Au deposition were varying between 360°C and 240°C. The salinity of the fluid was between 1 to 6 wt % NaCl equiv and the CO2 content was low (less than 1 mole %). The δ34SCDT values of pyrite and chalcopyrite were between 0.4 to 2.2‰, which was consistent with a magmatic-hydrothermal or leached volcanic source. The δ13CPDB values of calcite were between 16.9 to 17.4‰ and the calculated δ13CPDB of the fluid is between −3.8 and −3.0‰. The δ18OSMOW of the carbonates were between 16.96 and 19.40‰. Calculated fluid δ18OSMOW values were 7.4 to 9.8‰, which overlapped the range of magmatic water. The C and O isotopic values of calcite were similar to porphyry copper deposits, possibly indicating magmatic affinities for the ore fluids. Based on the presence of hematite, chalcopyrite, Fe-rich chlorite and locally pyrite, and on the absence of magnetite and pyrrhotite, the ore fluid was very oxidized. Oxygen fugacity was estimated to have been between 10−27 and 10−32, and the fugacity of H2S was less than 10−3.5.