Fluid Evolution During Mineralization of Atashkuh Fluorite-Barite (±Sulfide) Deposit, South of Delijan

Introduction More than 30 fluorite occurrences with approximately 1.35 million tons of reserves have been recognized in Iran (Ghorbani, 2013). The Atashkuh fluorite-barite (±sulfide) deposit is one of four occurrences located south of the city of Delijan in Markazi province, about 80 km SE of Arak city. The Atashkuh deposit occurs between the central Iran structural zone on the north and the Sanandaj-Sirjan structural zone on the south. The geology of the area is dominated by folded and faulted Jurassic carbonates and shales (Thiele et al., 1968). The lower Jurassic shale and calcareous sandstone of the Shemshak Formation and the Middle to Upper Jurassic dolomite of the Badamu Formation are the main host rocks for the fluorite veins. In this study, 40 samples from fluorite veins and host rocks were collected, from which 25 thin sections and 8 doubly-polished thin sections were prepared. Micro-thermometric studies were conducted on primary fluid inclusions using the Linkam THM600 heating-freezing stage. In addition, 10 samples were analyzed by XRD. Results Fluid inclusion data indicate that the Atashkuh fluorite-barite (±sulfides) veins were deposited as a result of mixing a primary multi-component Na-K(-Mg-Ca) high-salinity brine (SH type inclusions) with less saline calcium-rich connate water (LVHH type inclusions) and pressure reduction of ore bearing fluids. Fluid inclusions containing halite in high-salinity brine, and hydrohalite in connate water show suggest a high-salinity brine and connate water before mixing. The main mineralization stage was followed by circulation of low temperature meteoric water, responsible for the late stage mineralization. The micro-thermometry results suggest that the main fluorite mineralization occurred at 250 °C and 150 Mpa pressure. Dolomitization and silicification are the main alteration types associated with the Atashkuh mineralization. The occurrence of chlorite, talc, illite and dolomitized host rock all suggest Mg metasomatism associated with the main stage of fluorite mineralization. As a result of these observations, three stages of mineralization were recognized in the Atashkuh area: (1) Mobilization of a primary multicomponent Na-K(-Mg-Ca) high-salinity basinal fluid from the underlying rocks and its migration to the upper horizons. This stage is corresponds with the multi-phase SH inclusions with salinity of 31 wt% NaCl equivalent and 220 to 350 °C temperature. (2) Mixing and dilution of the basinal saline fluid with low-salinity formation or connate water resulted in deposition of fluorite-barite (±sulfide) mineralization and the formation of ferruginous dolomites. This stage contains Ca-Na rich inclusions with hydrohalite (LVHH). (3) Carbonate dissolution and increase in CO2 content of solutions due to oxidation of pyrite, formation of sulfuric acid. This stage is supergene oxidation and deposition of supergene minerals. The dissolution of carbonate rocks by acidified meteoric water produced permeable structures and breccias, by followed by of supergene mineralization in the later stages and formation of cerussite, malachite and goethite. Discussion The Atashkuh fluorite-barite (±sulfide) mineralization is mainly associated with dolomitized and silicified Jurassic carbonates and shales of Shemshak and Badamu Formation. The mineralization is in the form of two veins about 1 km apart. The epigenetic vein mineralization mainly consists of fluorite and barite with subordinate quartz, calcite, dolomite, galena, chalcopyrite, pyrite, hematite, goethite, malachite and cerussite. The main ore textures are replacements, open-space fillings, breccias and veins. The petrographic studies suggest three paragenetic suites: fluorite/quartz/calcite, barite/dolomite/sulfides and hematite/cerussite/goethite as early, middle and late stages. Micro-thermometric measurements were carried out on primary fluid inclusions of fluorite, barite and quartz crystals from both mineralized veins of the Atashkuh deposit. Fluid inclusions are of several types: (1) two-phase, liquid-vapor (LV), two- phase aqueous with hydrohalite (LVHH), multiphase (SH), H2O-CO2 with clathrate (C1) and without clathrate (C2) were recognized and analyzed. The first ice melting temperature (Te) of fluorite, barite and quartz two-phase aqueous (LV) inclusions varies between -22°C and -25°C, representing a H2O ± NaCl ± KCl multiphase solution (Van den Kerkhof and Hein, 2001). The last ice melting temperatures of three samples (Tmice) vary between -4.9°C to -9.7°C, -3.2°C to -7.2°C and -2°C to -4.8°C which indicate salinities of 7.7-13.6, 5.2-10.7 and 3.2-7.5 wt% NaCl equivalent for fluorite, barite and quartz. The final homogenization temperatures (Thtotal) of these inclusions vary between 90 and 205 °C for fluorite, 130 to 270 °C for barite and 110 to 193 °C for quartz. The CO2 melting temperatures (TmCO2) of fluorite and quartz C1 inclusions show ranges of -57.1 to -58.5 °C which suggest the presence of CH4 and/or N2 impurities (Burruss, 1981). The clathrate melting temperature (Tmclath) varies between 4.8 and 8.5 °C representing a salinity of 5.3 to 9.2 and 3 to 6.7 wt% NaCl equivalent for fluorite and quartz. The CO2 homogenization temperature (ThCO2) in these inclusions is 7.4 to 18.8 °C for fluorite and 13.4 to 27.5 °C for quartz. The homogenization temperature (Thtotal) for these inclusions is 170-210 °C for fluorite and 195-280 °C for quartz. References Burruss, R.C., 1981. Analysis of phase equilibria in C–O–H–S fluid inclusions. Mineralogical Association of Canada Short Course, 6(3): 39-74. Ghorbani, M., 2013. The economic geology of Iran, mineral deposits and natural resources. Springer Netherlands, 569 pp. Thiele, O., Alavi, M., Assefi, R., Hushmand-zadeh, A., Seyed-Emami, K. and Zahedi, M., 1968. Explanatory text of the Golpaygan quadrangle map, scale 1:250,000. Geological Survey of Iran. Geological quadrangle E7, 24 pp. Van den Kerkhof, A.M. and Hein, U.F., 2001. Fluid inclusion petrography. Lithos, 55(1-4): 27-47.

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