A review of tectono-magmatic evolution and gold metallogeny in the inner parts of Zagros orogeny: a tectonic model for the major gold deposits, western Iran

The metamorphic-magmatic Sanandaj-Sirjan Zone (SSZ) and Urumieh-Dokhtar Magmatic Belt (UDMB) are the most important gold metallogenic provinces of Iran, occurring as two NW-SE parallel tectono-magmatic zones in the inner parts of Zagros orogeny, a part of the Alpine-Himalayan orogenic belt. In an orogenic system such as the one which has occurred in Iran, various gold deposits associated with each stage of the Neo-Tethys evolution are expected. Hence, occurrence of a certain deposit type in each stage indicates dominant conditions for the formation of the deposits at that time and conversely, lack of some deposit types points to the change and removal of those conditions. Therefore, identification of mineralized phases and geological settings of their formation in the development of the orogenesis plays an important role in the delineation of very narrow phases of gold mineralization in these regions. Based on tectono-magmatic setting, notable gold deposits in the SSZ and the UDMB could be categorized as: 1) Gold-bearing stratabound deposits such as massive sulfide and sedimentary exhalative deposits in the interval of Triassic-Upper Cretaceous, simultaneous with formation and annihilation of Neo-Tethys, 2) Orogenic and intrusion-related gold deposits during Paleocene-Eocene coincided with influx of magmatic activity as continental arc, asthenosphere upwelling, slab roll-back, formation of metamorphic corecomplex and shear zones, and 3) Gold deposits related to the Miocene magmatic activity including epithermal deposits (high and low sulfidation), porphyry Cu-Mo (Au), porphyry Cu-Au and skarn in collisional and post-collisional settings. Meanwhile, in the SSZ and UDMB, main concentrations of gold mineralization occur in tectono-magmatic phases of Paleocene-Eocene and Miocene. The maximum gold mineralization restricted to the SSZ was formed in depths during Paleocene-Eocene. However, major gold mineralization in the UDMB and SSZ occurred in shallow levels in Miocene. Although similar development conditions after Oligocene (particularly Miocene) justify anticipation of similar mineralization types in the SSZ and the UDMB, different settings of these zones in this time interval cause different types of gold mineralization. Since magmatic arc crust in the UDMB is thicker than the magmatic arc of SSZ, it has more differentiation and contamination resulting in the development of Cu-Mo porphyry and associated epithermal gold deposits. While in the SSZ, extension and shortening are associated with magma emplacement, lack of thick continental crust and presence of metamorphic rocks, resulting in gold mineralization related to sub-deep intrusive (subvolcanic) in the form of sedimentary host and epithermal gold deposit types. In final stages of Neo-Tethys evolution, collision to post-collision regime, identical development of uplifting and erosion didn’t occur all over these zones but it is evident that severe erosion coupled with remittent frequent over-thrusting led to thickening of the SSZ crust. Thus, deepening of mineralization in these zones contributed to its intactness and uplifting was the major factor in their outcropping, promising that there are reserves of notable potential in areas with no outcropping.