Geochemistry, fluid inclusion study and genesis of the sediment-hosted Zn-Pb (± Ag ± Cu) deposits of the Tiran basin, NW of Esfahan, Iran

Abstract

The Tiran basin in the southern part of the Malayer-Esfahan metallogenic belt (MEMB), Iran, contains numerous shalehosted Zn-Pb (± Ag ± Cu) sulfide deposits in black limey siltstone and carbonate rocks of the Early Cretaceous sedimentary sequence. This sequence deposited in an extensional back-arc environment between the Sanandaj-Sirjan Zone (SSZ) and Central Iranian microcontinent. The Tiran Basin consists of six sediment-hosted Zn-Pb deposits (Anjireh-Chekab, Vejin-Bala, Vejin-Paein, Sefidkhal, Khaneh-Sormeh and Kope-Motaleb) with estimated total reserves of 10 Mt at 4 % Zn, 2 % Pb, 0.2 % Cu and 20 ppm Ag. Based on geometry of ores, mineralogy, and texture of sulfide mineralization, three different ore facies can be distinguished in these deposits: feeder zone, massive ore, and extensive banded replacement sulfide ore facies. Silicification, dolomitization and sericitization are the main wall-rock alteration styles; alteration intensity increases toward the feeder zones. The high V/ (V+Ni) values (> 0.6) of the host rocks, and presence of organic matter and framboidal pyrite within fine-grained detrital sediments suggest an anoxic event for the host basin. Fluid inclusions in coarse-grained sphalerite, related to massive and banded replacement ore facies, and hydrothermal calcite, dolomite and quartz that are cogenetic with sulfide minerals in the main stage of mineralization, were studied by microthermometry. Based on fluid inclusion assemblages (FIAs), the homogenization temperatures decrease from the feeder zone (172 °C to 284 °C, av. 205.6 °C) adjacent to the synsedimentary normal faults, through the massive ore (157 °C to 270 °C, av. 195 °C) to the banded replacement ore (115 °C to 270 °C, av. 172 °C) facies. This variation is associated with an increase in salinity from 4.4 through 5.1 to 6.3 wt.% NaCl eq. in the same way. This trend is interpreted to represent mixing of ore fluids with saline seawater at the sub-seafloor unconsolidated sediments in the banded replacement ore facies. Fluid inclusion data show that the Tiran shalehosted Zn-Pb deposits are more like the vent-proximal sedimentary-exhalative (SEDEX) and VMS deposits than other sedimenthosted Zn-Pb deposits, such as Mississippi Valley Type (MVT) and Irish-type mineralizations. However, geological data, ore facies and ore forming processes is compatible with a sub-seafloor replacement SDEDX deposit that has been suggested for some large SEDEX or shale-hosted Zn-Pb deposits such as Red Dog and Anarraaq in Alaska and Century deposit in Australia.