Geology and geochemistry of D–O–C isotope systematics of the Qolqoleh gold deposit, Northwestern Iran: Implications for ore genesis

The Qolqoleh gold deposit is located in northwestern part of the Sanandaj–Sirjan metamorphic belt, northwestern Iran. Igneous and sedimentary units exposed in the area have undergone greenschist metamorphism. The area was affected by a NE–SW trending shear zone and subsequent deformation. Two different types of mineralization are distinguished in the Qolqoleh gold deposit based on geological–structural conditions indicated by microtextural analysis: ductile and then brittle. Ore-forming processes are divided into three stages: Early (I), Middle (II) and Late (III), which include quartz–pyrite (I), sulfides and gold (II) and carbonate veinlets (III), respectively. The stage I fluids are characterized by δ¹⁸O = 15.5‰ at 440 ºC, and are thought to be deep-sourced metamorphic waters; the stage III fluids, with δ¹⁸O = 1.6‰, are shallow-sourced meteoric waters; whereas, the stage II fluids, with δ¹⁸O = 13.1‰, are a mixture of deep-sourced metamorphic and shallow-sourced meteoric fluids. Based on comparisons of the D–O–C isotopic systematics, the ore-forming fluids with characteristic high δ¹⁸O and δ¹³C and low δD originated from metamorphic devolatilization of Cretaceous volcano-sedimentary (felsic to mafic metavolcanic rocks–shale–carbonate–carbonaceous chert) sequences, locally rich in organic matter. During late Cretaceous continental collision of the Afro-Arabian continent and the Iranian microcontinent, a crustal slab consisting of felsic to mafic metavolcanic rocks, carbonate, shale and carbonaceous chert was underthrust northwards beneath the central Iranian microcontinent along the Zagros fault. During further contraction, deformation was localized in reverse oblique-slip structures with vergence toward south; shear zones generally follow contacts between more competent and less competent rock units. Metamorphic devolatilization of this underthrust slab is the source of the ore-forming fluids that generated the Au ore belt, which includes the Qolqoleh gold deposit.