Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern Iran

Abstract

The Qolqoleh gold deposit is located in the northwestern part of the Sanandai-Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confi ned to a series of steeply dipping ductilebrittle shear zones generated during Late Cretaceous -Tertiary continental collision between the Afro-Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano-sedimentary sequences consisting of felsic to mafi c metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore-controlling structure is NE -SW-trending oblique thrust with vergence toward south ductile -brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal -plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au-bearing highly deformed and altered mylonitic host rocks and cross-cutting Au-and sulfi de-bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz -sulfi de microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartzsericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fl uid to create disseminated and cross-cutting Au-quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicifi cation and sulfi dation zone (major ore body); sericite and carbonate alteration zone; and sericite -chlorite alteration zone that may be taken to imply wallrock interaction with near neutral fl uids (pH 5 -6). Silicifi ed and sulfi de alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicifi ed highly deformed mylonitic and ultramylonitic domains and silicifi ed sulfi de-bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore-forming stages have proved that the Qolqoleh deposit was formed in the compression -extension stage during the Late Cretaceous -Tertiary continental collision in a ductile -brittle shear zone, and is characterized by orogenic gold deposits.

Prospectivity analysis of orogenic gold deposits in Saqez-Sardasht Goldfield, Zagros Orogen, Iran

“…for deposition of gold mineralization in the study area (Aliyari et al, 2007(Aliyari et al, , 2009(Aliyari et al, , 2012Tajeddin, 2011;Almasi et al, 2014Almasi et al, , 2015a. Thus, in order to detect the edges of aeromagnetic anomalies we applied total horizontal derivative (THD) filter (Airo, 2001(Airo, , 2007Bierlein et al, 2006a,b Henson et al., 2010Ferreira et al, 2011;Almasi et al, 2014).…”

“…The study area, which is located in the Saqez-Sardasht orogenic gold zone, a ductile to brittle shear zone (Aliyari et al, 2007(Aliyari et al, , 2009, comprises a regional E-W trending gold belt with seven known gold occurrences (Fig. 1b) (Table 2).…”

“…Gold deposits near and around granitoids have been studied by Groves et al (2000), Knox-Robinson (2000), Nykänen (2008), and Herbert et al (2014). In the study area of this paper, there are granitoids of two periods (Aliyari, 2006;Aliyari et al, 2007Aliyari et al, , 2009Aliyari et al, and 2012Tajeddin, 2011): (1) granitoids, which were emplaced prior to the mineralization, causing an increase in the geothermal gradient and remobilization of large amounts of metamorphogenic fluids; and (2) granitoids that were emplaced earlier than the first group and were metamorphosed during the metamorphism processes (e.g., mylonitic granites). The later granitoids together with the metavolcanic and meta-sedimentary rocks have been suggested as the ore traps.…”

“…In the study area, these units consist of meta-volcano sedimentary units such as schists, amphibolites and mylonitic granites (Aliyari et al, 2007(Aliyari et al, , 2009(Aliyari et al, , 2012Tajeddin, 2011) which have been portrayed in the geological map of the area (Fig. 1c).…”

“…The gold veins and veinlets were then deposited in the upper metamorphosed mafic to felsic volcanic and sedimentary rocks (e.g., greenschist facies). The granitoids played an important "heat engine" role, activating the percolation of gold-bearing fluids (Aliyari, 2006;Aliyari et al, 2007Aliyari et al, , 2009Aliyari et al, , 2012. Mineralogical studies revealed that oxide minerals in mafic meta-volcanic rocks are generally magnetite, titanomagnetite and ilmenite, which make them detectable using aeromagnetic data (Aliyari, 2006).…”