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

Abstract

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 ?34SCDT values of pyrite and chalcopyrite were between 0.4 to 2.2‰, which was consistent with a magmatic-hydrothermal or leached volcanic source. The ?13CPDB values of calcite were between 16.9 to 17.4‰ and the calculated ?13CPDB of the fluid is between ?3.8 and ?3.0‰. The ?18OSMOW of the carbonates were between 16.96 and 19.40‰. Calculated fluid ?18OSMOW 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.