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Susan Ebrahimi, Shahid Beheshti University (Islamic Republic of Iran)
Saeed Alirezaei, Shahid Beheshti University (Islamic Republic of Iran)
Mahmood Mehrpartou, Research Center of Geological Survey of Iran (Islamic Republic of Iran)
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The Sharafabad gold district lies to the west of the E-W trending Cenozoic Alborz-Azarbaijan Magmatic Belt (AAMB) in northern Iran, near the junction with the Cenozoic NW-SE trending Urumieh- Dokhtar Magmatic Belt (UDMB) in western Iran.
The UDMB bears features characteristic of Andean type continental arcs, and is considered to be related to the subduction processes associated with the closure of Neotethys in late Mesozoic-Cenozoic ages. The UDMB hosts many porphyry Cu-Mo, base metal skarn and epithermal precious metal deposits.
The Sharafabad area is characterized by Tertiary silicic to intermediate volcanic and pyroclastic rocks. Mineralization is mainly restricted to silica and silica-carbonate veins and veinlets.
Some 18 mineralized veins have been identified; they vary in length and width from 10-1000 meters and 0.5-10 meters, respectively. The main host rocks are Eocene tuffs and porphyritic andesites. Intermediate argillic alteration dominates. Exploration activities include hundreds of meters of trenches and 14 drill holes.
Pyrite is the main sulfide, associated with subordinate chalcopyrite, sphalerite, galena and covellite. Quartz and carbonates are the main vein materials. Gold occurred as scattered microscopic grains in quartz and pyrite, and along the grain boundaries of base metal sulfides. On the basis of crosscutting relationships, four stages of mineral parageneses can be distinguished:
(I) pre-mineralization, (II) mineralization, (III) post-mineralization and (IV) supergene.
Fluid inclusion data have been obtained from the ore-stage gray quartz and sphalerite, carbonates, and the late clear quartz and amethyst. The data from the ore stage gray quartz indicate homogenization temperatures (Th) of 190- 340 °C and salinities of 1 to 8.7 wt% NaCl equiv. Fluid inclusions in the sphalerite yielded lower Th of 215- 265°C, but higher salinities at 10.8-15.3 wt% NaCl equiv. The late carbonates formed at 160-250 °C from fluids of low salinities, 1.5-3.8 wt% NaCl equiv. Amethyst from the final stages of vein formation, was deposited at 173- 203 °C from fluids containing 3.5 to 9.5 wt% NaCl equiv.
The presence of coexisting vapor and liquid-vapor inclusions suggests the occurrence of boiling during the evolution of the ore fluids. The fluid inclusion data further suggest that the ores were deposited at an average depth of about 370 m below the paleosurface. Sulphur isotope values on the sphalerite, galena and pyrite from 6 ore samples are in the range +1.7‰ to -5.3‰ The relatively narrow range ofδ34S and proximity to 0‰ imply a magmatic source. Carbonates displayδ13C values in the range -2.41‰ to + 2.04‰ suggesting a marine source for the carbon.
Considering the intermediate argillic alteration, association of gold with base metal sulfides, and the moderate salinities, the Sharafabad district can be classified as an intermediate-sulfidation epithermal system.
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