عنوان مقاله [English]
The Sharafabad auriferous epithermal vein system is located in the Alborz-Azarbaijan magmatic assemblage in northwestern Iran. The veins are hosted by Eocene andesitic volcanic and pyroclastic rocks and tuffs. Wall rock alteration includes an inner silicic zone often bordered by argillic and propylitic zones. Mineralization is mainly restricted to silica and silica-carbonate veins and veinlets in faults and fault zones. A total of 18 ore-bearing veins have been identified. The veins vary from 10 -1000 meters in length and 0.5-10 meters in width, respectively. Pyrite is the main sulfide, commonly associated with subordinate chalcopyrite, sphalerite, and galena. Gold occurs as scattered microscopic grains in quartz and pyrite, and along the grain boundaries of the sphalerite, galena and chalcopyrite. Silica occurs as gray quartz, white quartz, clear quartz, opal, chalcedony, and minor amethyst. On the basis of crosscutting relationships and mineral paragenesis, four stages can be distinguished: (I) pre-mineralization, (II) mineralization, (III) post-mineralization and (IV) supergene. The gold and the base metals sulfides occurred in the mineralization stage. Fluid inclusion data have been obtained from the ore-stage gray quartz and sphalerite, carbonates, and the late clear quartz and amethyst. The fluid inclusions from the gray quartz indicate homogenization temperatures of 170-270 ºC and salinities of 1 to 8.7 wt% NaCl equiv. The fluid inclusions in the sphalerite indicated homogenization temperatures of 215-265 ºC, and salinities of 10.8-15.3 wt% NaCl equiv. The carbonates formed at lower temperatures, between 160-250 ºC, from fluids of low salinities, at 1.5-3.8 wt% NaCl equiv. Amethyst from final stages of vein formation, not associated with sulfides and gold was deposited at 173-203 ºC from fluids containing 3.5 to 9.5 wt% NaCl equiv. The coexistence of vapor-dominant and liquid-dominant inclusions in quartz and sphalerite suggests that boiling occurred during the evolution of the ore fluids. The occurrence of boiling is supported by hydrothermal breccias, bladed calcite, and adularia. Fluid inclusion data suggest that ore was deposited at an average depth of about 400 m below the paleosurface. 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.