Geology and genesis of the Siahouki intermediate-sulfidation epithermal Cu-Au-Ag deposit, SE Iran: Constraints from fluid inclusions and S-O-C isotopes

The Siahouki Cu-Au-Ag deposit is situated approximately 50 km northwest of Bam in the southern Urumieh-Dokhtar magmatic arc. Mineralization is hosted by Eocene dacitic and andesitic tuffs that have undergone extensive silicic, argillic, and propylitic alteration. Ore deposition primarily occurs in NW-SE-trending, fault-controlled quartz-carbonate ± sulfide veins. Based on field observations, petrography, and microanalytical data, five hydrothermal stages are defined: (I) early distal argillic-propylitic alteration; (II) the principal Cu-Au-Ag stage; (III) late Au enrichment; (IV) mineralization dominated by calcite; and (V) supergene oxidation and secondary enrichment. Fluid inclusions show homogenization temperatures of 112-331 °C and salinities of 0.9-10.1 wt.% NaCl equiv. Stable isotope constraints confirm a mixed magmatic-meteoric origin for the ore-forming fluids. Sulfur isotope compositions of pyrite, chalcopyrite, and tetrahedrite ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> ³⁴S = −5.4 to +0.5‰) indicate a dominantly magmatic sulfur source. Oxygen isotope values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> ¹⁸O fluid = −1.4 to +1.8‰) calculated from quartz-water equilibrium indicate significant meteoric water mixing. Carbon isotopes ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> ¹³C = −0.8 to +0.0‰) from hydrothermal calcite further suggest limited carbonate wall-rock interaction. Collectively, these data indicate that ore deposition was controlled by episodic boiling and fluid mixing during the transition from magmatic-hydrothermal to meteoric-dominated conditions, defining the Siahouki deposit as an intermediate-sulfidation epithermal system.