Volcanic redbed-type copper mineralization in the Lower Cretaceous volcano-sedimentary sequence of the Keshtmahaki deposit, southern Sanandaj-Sirjan Zone, Iran

摘要

The Keshtmahaki Cu (Ag) deposit, hosted in the Lower Cretaceous lithic tuffs, is located in the southern part of the Sanandaj-Sirjan zone (SSZ), 180 km NE of Shiraz. The deposit consists of stratabound and lenticular orebodies that are distributed with a NW-SE trend for ~35 km from NW to SW of Safashahr in the Fars province. The ore textures are dominated by open space filling, vein-veinlet and replacement with minor disseminated and laminated-like mineralization. The ore consists predominantly of primary chalcocite, bornite, native copper, and chalcopyrite and to some extent secondary chalcocite, digenite, covelite, malachite, azurite, and minor Ag-bearing clausthalite, Ag-bearing selenide and black copper (copper wad). There are two types of alteration including regional propylitic alteration in lavas characterized by epidote, chlorite, calcite, sericite and quartz and ore-related hydro-thermal alteration in the lithic tuff resulting from fluids which generated due to deep burial diagenesis characterized by the assemblage of quartz, calcite, epidote, chlorite, albite, hematite and zeolite. Microthermometric data of the liquid-vapor fluid inclusions in quartz show average homogenization temperatures varying between 129 and 145 °C and salinities between 19.1 and 22.9 wt.% NaCl equivalent, respectively. The mineralization is divided into pre-mineralization, mineralization and post-mineralization stages. The mineralization is accompanied by hydrothermal alterations. Sulfur isotope compositions have a wide range from δ~(34)S = -13.7 to -4.9 ‰, suggesting the presence of a reducing environment resulting from activation of sulfate reducing bacteria. The field observations, ore mineralogy, alteration characteristics, sulfur isotopic compositions and fluid inclusion data suggest the Keshtmahaki deposit may be classified as a volcanic redbed-type copper (Manto-type) deposit.