Abstract

The Chah Bagh gold deposit, in the Muteh gold district, is located in the central part of the Sanandaj-Sirjan zone. Gold mineralization at Chah Bagh is hosted in deformed felsic schist and metarhyolite rocks which are interbedded with chlorite-actinolite schist. Rock sequences in the area were affected by several phases of intense deformation, which led to generation of D1 through D3 fabrics and structures. The NW-trending (N280-N290) dextral strike-slip shear zone in the district was formed during D2 ductile events. Gold mineralization occurred in ductile and brittle shear zones. Ductile style of gold mineralization occurs as auriferous sulfide-bearing quartz veins and lenses parallel to the milonitic foliation. Brittle style of gold mineralization includes set of gold-bearing, sulfide-rich quartz veins, which are occur within NW-trending normal faults cutting the milonitic foliation. Quartz, feldspar, sericite, epidote, and tourmaline constitute the main non-sulfide alteration phases, whereas the ore minerals in the auriferous quartz zones are pyrite, chalcopyrite, arsenopyrite, and native silver. Iron oxides and hydroxides, kaolinite, azurite, malachite, digenite, and covellite were formed during supergene alteration. Gold is not visible in quartz and altered sulfide margins and is mainly present in sulfide minerals. Mineral enrichment factors in Chah Bagh inclue deformation, alteration, metamorphic fluids and host rocks. Granitic intrusions have no genetic link with mineralization. Four stages of gold enrichment are distinguished in the Chah Bagh gold deposit based on geological-structural conditions indicated by micro-textural analysis and petrofabric studies: (I) During or prior to the Devonian, volcano-sedimentary units were deposited with significant gold concentrations in sedimentary diagenetic pyrite. (II) During the main phase of crustal shortening in the Late Jurassic-Early Tertiary transpressional regime, the original gold-rich Paleozoic strata were progressively deformed (D1-D2) and underwent medium- to high-grade metamorphism. As part of this event, the Chah Bagh ductile shear zone was formed and metamorphic devolatilization provided ore-forming fluids that generated the earliest significant lode gold mineralization along the ductile shear. (III) During Eocene regional uplift and exhumation of the metamorphic rock, deformation changed from ductile to brittle and gold-hosted normal faults were formed in this transitional stage. This extensional structural framework included emplacement of granitic intrusions and additional metamorphic fluid flow, with gold deposited in normal faults adjacent to the earlier ductile features. (IV) During supergene processes, some of the gold was remobilized from the hypogene sulfide minerals. These results show an orogenic model for Chah Bagh gold deposit that is consistent with a dextral transpressional deformation kinematic model along the Sanandaj-Sirjan zone.