The role of deformation and alteration in gold mineralization in the Qolqoleh ductile andbrittle southwest of Saqez shear zones،

Abstract:

Qolqoleh gold deposit is located in northwestern part of Sanandaj-Sirjan zone. Igneous and sedimentary units exposing in the area have undergone greenschist metamorphism. The area was affected by NE-SW trending shear zone and subsequent deformation. Two different types of mineralization were distinguished in the Qolqoleh gold deposit based on geological–structural situation and microtextural analysis: ductile and then brittle. The maximum gold concentration with NW dipping lenticular shape occurred along the 2 km long, 250 meter wide NE-SW trending (N50-60E) ductile shear zone. Ductile deformation occurred as protomylonite، mylonite and ultramylonite. Brittle type mineralization includes quartz-sulfide bearing microfractures, microveins and veinlets in mesoscopic-microscopic scales that are both parallel and cross-cutting with mylonitic foliation.

The rock units hosting the above mentioned shear zones mainly consist of intensely deformed meta-acidic and mafic volcanics and volcano-sedimentary rocks (felsic and mafic metavolcanics and sericite schist). The most important alterations are sericitization, chloritiztion, carbonatization, kaolinitization, silicification and sulfidation. Silicified and sulfide alteration observed in the inner parts of alteration zones, coincide with the ore-bearing zones. The investigation showed spatial and temporal relationship between deformation and alteration. Spatially, this relationship is distinguished by overlapping alteration and deformation zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic units and silicified sulfide–bearing microveins and veinlets coinciding with the inner parts of alteration zones. Important point is that, in the Qolqoleh deposit the presence of large volumes of weakly mineralized mylonite, gold concentrations are slightly higher than background (1 to 20 ppb), with a weakly alteration, and also the presence of some silicified high grade protomylonites (16 ppm) indicating the specific role of brittle deformation in high grades gold mineralization. Mineralogical paragensis consists of pyrite, chalcopyrite, arsenopyrite, pyrhotite, magnetite and sphalerite. It seems that, gold mineralization in the Qalqoleh deposit can be used as a model for prospecting and reconnaissance of gold mineralization in similar districts between Saqez and Sardasht that have similar geological features.

Keywords: Gold mineralization, ductile and brittle shear zones, deformation, mylonitic and ultramylonitic Fabrics, silicified-sulfide alteration، Qolqoleh, Saqez