Sagh iron oxide Cu-Ag±Au mineral occurrence, SE of Torbat-e-Heydarieh: evidence of Geology, mineralization, geochemistry and fluid inclusion

Sagh mineral occurrence is located southeast of Torbat-e-Heydarieh, Khorasan Razavi province, and in the eastern part of the Khaf-Kashmar-Bardeskan magmatic belt. The rock units of area are divided into two categories: intrusions (monzonite, monzodiorite, diorite, and syenite) in the southern half, and conglomerate in the northern half. One square kilometer of continuous mineralization may be observed as stockwork, while there are other locations where it has a linear trend and is supported by intrusive rocks. Primary minerals include specularite, chalcopyrite, pyrite, galena, and sulfosalt, and secondary minerals include malachite, goethite, hematite, chalcosite, caveolite, and anglesite. The mineralization textures are vein-veinlet, disseminated, replacement, and cloform, mainly with a strong chloritic-silicified alteration. The average amount of copper is 0.8 with a maximum of more than 3%, the average amount of silver is 24.4 with a maximum of more than 113 ppm, and the average amount of gold is 44 with a maximum of 250 ppb. The average amount of lead is 761 ppm with a maximum of 0.4% and the average amount of zinc is 430 ppm with a maximum of 0.1%. The formation temperature of ore-forming fluid is between 159 and 328 °C and the salinity is between 7.2 and 16.7 wt.% equiv. NaCl. The mixing of magmatic fluids with meteoric waters with low temperatures and salinity was the most important mechanism of mineral formation. Based on the evidence of tectonic setting, lithology, type of alteration, shape, and state of mineralization, and the presence of abundant specularity with copper, silver, and gold anomalies, probably the Sagh area is iron oxide Cu-Ag±Au type. 

The geological settings, hydrothermal alteration, and mineralizing fluid compositions vary among the deposits of “IOCG-type” (Hitzman et al., 1992; Sillitoe, 2003). However, they belong to a family of Cu ±Au deposits that include substantial hydrothermal alkali (Na/Ca/K) alteration and a lot of low-Ti iron oxide (magnetite and/or hematite). According to Williams et al. (2005), these deposits likewise exhibit strong structural constraints and a temporal but not a tight geographical relationship with igneous rocks. They formed in rift or subduction settings (Hitzman, 2002) from the Late Archean to the Pliocene (Groves et al., 2010).
Sagh mineral occurrence is located the southeast of Torbat-e-Heydarieh, Khorasan Razavi province, and in the eastern part of the Khaf-Kashmar-Bardeskan magmatic belt (Fig.1). This belt has a high potential for iron oxide copper-gold type deposits and sometimes skarn and porphyry copper (Karimpour, 2004).
The purpose of this research is geological studies and determine the relationship of intrusions with mineralization, examine the total paragenesis sequence, geochemistry, fluid inclusions studies and finally determine the mineralization model, and the formation of mineral occurrences in the Sagh area, for the first time is done.

To investigate the lithology, alteration, and mineralization of the Sagh area, 61 samples were taken mainly from the intrusions. 32 samples for the thin section and 10 samples for the polished thin section and polished block were selected, prepared, and studied. Then, the geological and alteration-mineralization map with a scale of 1:5000 was prepared in Arc GIS software. Furthermore, for geochemical studies of mineralization zones and veins, 24 samples were taken and sent to the Zarazma laboratory for analysis. Analysis was done by the ICP-OES method. Furthermore, 11 samples were selected for gold analysis with Fire assay, and sent to Zarazma laboratory. Using a cooling and heating system made by Linkam Company, model THM 600, microthermometric tests and salinity determination were performed on 2 wafers of quartz minerals and 31 fluid inclusions at Ferdowsi University of Mashhad.

The rock units of the area are divided into two categories: subvolcanic and plutonic intrusions in the southern half and conglomerate units in the northern half. Intrusive rocks are composed of monzonite, monzodiorite, diorite and syenite. Mineralization can be seen in the form of stockwork in a wide and continuous zone with an area of about one square kilometer, but in some places, it has a linear trend (NE-SW and NW-SE trend) which is hosted by intrusive rocks. Primary minerals include specularite, chalcopyrite, pyrite, galena, and sulfosalt, and secondary minerals include malachite, goethite, hematite, chalcosite, covellite, and anglesite. Vein-veinlet, disseminated, replacement, and cloform mineralization textures are seen, with a dominant chloritic-silicified alteration. The average concentration of copper is 0.8%, with a maximum concentration of more than 3%, silver is 24.4 ppm, with a maximum concentration of more than 113 ppm, and gold is 44 ppb, with a maximum concentration of more than 250 ppb, according to geochemical data. The average amount of lead is 761 ppm with a maximum of 0.4% and the average amount of zinc is 430 ppm with a maximum of 0.1%. Based on fluid inclusions studies, the formation temperature of ore-forming fluid is between 159 and 328 °C, and the salinity is between 7.2 and 16.7 wt.% equiv. NaCl.

Comparing the characteristics of Sagh prospect area with other copper-bearing deposits shows that this area is very similar to iron oxide copper-gold deposits. An empiric definition of IOCG deposits is summarized as having the following five characteristics (Williams et al., 2005): (1) copper, with or without gold, as economic metals, (2) hydrothermal ore styles and strong structural controls, (3) abundant magnetite and/or hematite, (4) Fe oxides with Fe/Ti ratios greater than those in most igneous rocks and bulk crust, and (5) no clear spatial associations with igneous intrusions as, for example, displayed by porphyry and skarn ore deposits.
Sillitoe (2003) proposed a close genetic relationship between IOCG deposits in northern Chile, and dioritic plutons. Mineralization in the Sagh area has a close relationship with monzonitic, monzodiorite, and diorite, which are similar to Kuh-e-Zar, Bahariyeh, Namaq, Fadiheh, Chenar, and other KKBMB deposits (Table 3).
The main alteration related to mineralization in the Sagh is propylitic-silicified, and its propylitic alteration is characterized by chlorite mineral. Extensive chlorite alteration in the Sagh area is similar to Monteverde deposit in Peru (Vila et al., 1998), Mont-del-Aigle in Canada (Simard et al., 2006), Kuh-e-Zar Tarbat Heydarieh (Karimpour et al., 2017), and other IOCG type deposits in the KKBMB belt (Almasi et al., 2015; Taghadosi and Malekzadeh Shafaroudi, 2018; Najmi et al., 2023; Sahebi Khader et al., 2021; Behnamnia et al., 2023) and Qala Zari (Karimpour, 2005) in the Lut block, where the temperature and salinity of ore-fluid are lower than some IOCG type deposits in the world.
Based on the available evidence, the mineral occurrence of the Sagh includes 1) the presence of oxidant intrusions formed in the subduction zone in the KKBMB, 2) mineral paragenesis of specularity, chalcopyrite, pyrite, and galena, 3) structural control of mineralization, 4) copper, silver, gold, and lead geochemical anomaly, 5) chloritic-silicified alteration, which is very compatible with iron oxide copper-silver-gold systems. The location of this area in the KKBMB belt, which has great potential for IOCG deposits, and near other IOCG deposits that have many similarities (Almasi et al., 2015; Karimpour et al., 2017; Sahebi Khader et al., 2021; Najmi et al., 2023), is a confirmation of this claim.
Although monzonitic, monzodiorite, diorite, and syenitic intrusions are the host rock of mineralization and mineralization is controlled by structures and faults, this magmatism can be represented of source rock at deep. The mineral paragenesis of the Sagh and the abundance of specularity with sulphide minerals of copper, lead, and silver, which are associated with quartz and chlorite, show that mineralization generated from a high fO2, Fe-Si rich ore fluid.
The metal originated from an oxidan magmatism from deep, and moved up through faults, joints, and fractures. The mixing of magmatic ore solution with higher temperature and salinity with meteoric water with lower temperature and salinity has finally led to the deposition of sulfides, and the formation of mineralization. Temperature-salinity and alteration evidence show that we are currently in the upper parts of the system, and we need more information. The relevance of this magmatic belt in eastern Iran as a significant metallogenic zone for deposits of copper, gold, and silver is growing as more and more IOCG mineral occurrences are found there.