The genesis of base and precious metals-bearing epithermal veins in the Gharehchay-Kurmolla area, south of Tikmehdash, NW Iran

The Gharehchay-Kurmolla (Gh-Ku) base and precious metals occurrence is located in ~2 km south of Tikmehdash, 75 km south-east of Tabriz, and is a part of Bostanabad-Miyaneh gold-bearing district in the West Alborz-Azarbaidjan structural zone. Mineralization in the study area occurs in quartz veins and veinlets hosted by the Eocene volcanic-pyroclastic units as well as granite. Recognizable alteration zones around the quartz veins and veinlets include silicic, phyllic, intermediate argillic, and propylitic types. The mineralization was developed during three conspicuous stages. In stage 1, minerals such as quartz, pyrite, and chalcopyrite with slight amounts of gold were formed. During stage 2, minerals such as quartz, galena, sphalerite, and gold together with pyrite and chalcopyrite were developed. Stage 3 was concurrent with deposition of quartz accompanied by Mn-oxides and hydroxides (pyrolusite and psilomelane). The major gangue minerals are quartz, adularia, sericite, epidote, chlorite and calcite. Micro-thermometric investigations on primary 2-phase (LV) fluid inclusions in quartz crystals showed that the hydrothermal fluids responsible for mineralization had temperatures and salinities ranging from 215 to 325°C and from 2.6 to 10.4 wt.% NaCl eq., respectively. The oxygen isotopic composition of the fluid (+9.7 to +12.5‰) suggests that the ore-forming solutions had a largely magmatic component. The sulphur isotopic composition of the fluid (–1.5 to –3.4‰) is also indicative of magmatic origin. On the basis of data obtained from micro-thermometric and stable isotope analyses, boiling along with mixing were two important mechanisms involved in the precipitation of ore and gangue minerals in the study area. The geological and geochemical characteristics of the Gh-Ku area indicate that mineralization in this area is of epithermal type with a low-sulphidation style.