Intermediate-sulfidation epithermal base and precious metal mineralization in the Qebchaq deposit (NW Qarachaman, East Azerbaijan): Geology, mineralization, and geochemical evidence

Precious and base metal mineralization in the Qebchaq deposit occurred as brecciated quartz-sulfide veins within the Eocene tuff and lava strata, and the Oligocene quartz diorite-gabbro intrusion. Pyrite, chalcopyrite, galena, sphalerite, and gold along with minor realgar, psilomelane, and pyrolusite, are ore minerals; quartz, sericite, chlorite and calcite are gangue minerals. The ore minerals show disseminated, vein-veinlet, brecciated, comb, cockade, colloform, crustiform, plumose, and vug infill textures. Five stages of mineralization can be distinguished at Qebchaq. Stage 1 is represented by silicification of host rocks along with minor disseminated pyrite. Stage 2 is characterized by quartz veins and breccias that contain variable amounts of disseminated pyrite, chalcopyrite, galena, sphalerite ± native gold ± realgar. Stage 3 is marked by quartz-manganese oxides-hydroxides (psilomelane, pyrolusite, braunite) veins and hydrothermal breccia cements. Stage 4 is represented by quartz (calcite-chlorite) vein-veinlets, and stage 5 is characterized by calcite as veinlets and vug infill texture. Wall-rock alterations include silicification, intermediate argillic, carbonate, chlorite and propylitic alteration. Chondrite–normalized trace elements and REE patterns of the mineralized samples and the host rocks are similar and indicate that host rocks are probably involved in mineralization. Characteristics of Qebchaq deposit are comparable with intermediate-sulfidation type of epithermal deposits. 

Qebchaq base and precious metal deposit, 15 km northwest of Qarachaman, is located in the Western Alborz–Azerbaijan zone, northwestern Iran. Several types of deposits are present in this zone including porphyry and skarn Cu-Mo (Au) porphyry deposits, Cu-Mo and Fe skarn deposits, Cu-Mo-Au vein deposits, and epithermal Au deposits (Jamali et al., 2010; Kouhestani et al., 2018). The most important deposit discovered to date within the Western Alborz–Azerbaijan zone is the Sungun porphyry Cu-Mo deposit, which has a defined reserve of 796 Mt at 0.6% Cu (Hezarkhani and Williams-Jones, 1998; Aghazadeh et al., 2015; Simmonds et al., 2017). Other important deposits or occurrences include Haft-Cheshmeh, Sonajil, Ali Javad, Mirkuh-e-Ali Mirza, Astergan, Avan, Anjerd, Mazraeh, Astamal, Pahnavar, Masjed Daghi, Sharafabad, Mivehroud, Nabijan, Zaglig, Aniq, Zaily Darreh, Qara Darreh and Qarachilar (Ebrahimi et al., 2011; Jamali et al., 2010; Mokhtari, 2012; Maghsoudi et al., 2014; Mokhtari et al., 2014; Adeli et al., 2015; Baghban et al., 2015; Baghban et al., 2016; Simmonds and Moazzen, 2015; Kouhestani et al., 2018).
Although geological general characteristics of the location of the Qebchaq deposit have been determined (Asadian et al., 1993), no detailed studies have been conducted on the mineralogy, geochemistry, and genesis of the Qebchaq deposit. In this paper, detailed geology, mineralogy, geochemistry, and alteration styles of the Qebchaq deposit to constrain its ore genesis are investigated. These results may have implications for the regional exploration of epithermal base and precious metal deposits in the Western Alborz–Azerbaijan zone.

Detailed fieldwork has been carried out at different scales in the Qebchaq area. A total of 50 samples were collected from various parts of ore veins and breccias, host volcanic rocks, and intrusions. The samples were prepared for thin (n=8) and polished-thin (n=32) sections in the laboratory at the University of Zanjan, Zanjan, Iran. Thirty nine representative samples from the mineralized veins and breccias, 1 sample from host dacitic rocks, and 1 sample from altered quartz diorite-gabbro intrusion were analyzed for REE, Au, Ag, Cu, Pb, Zn, and other rare elements using Fire Assay and ICP–MS in the Zarazma Analytical Laboratories, Tehran, Iran.

The geological units hosting the Qebchaq deposit are mainly Eocene volcanic and volcaniclastic rocks that have been intruded by Oligocene intrusions. The Eocene sequence includes tuff, andesite, and andesitic basalt, rhyolite, rhyodacite-dacite, and ignimbrite. The intrusive rocks in the Qebchaq area include Oligocene quartz diorite-gabbro and granite-alkali granite. They show porphyritic, microgranular, and granular textures. Mineralization at Qebchaq occurs as the epithermal base and precious metal quartz-sulfide brecciated vein that occupies NE-trending faults in the Eocene volcanic rocks and Oligocene intrusions.  The ore veins are 50 to 1000 m long, from 0.5 to 4 m wide, and generally, dip steeply (65–85°) to the southeast and northwest. Wall-rock alterations developed at the Qebchaq deposit include silicification, intermediate argillic, carbonate, chlorite, and propylitic alteration. The first four types are closely related to mineralization. Five stages of mineralization can be distinguished at Qebchaq. Stage 1 is represented by silicification of host rocks along with minor disseminated pyrite. This stage is usually crosscut by stage 2. Stage 2 (the main ore-stage) is characterized by millimeters to several centimeters wide quartz veins and breccias that contain variable amounts of disseminated pyrite, chalcopyrite, galena, sphalerite ± native gold ± realgar. Clasts of this stage and associated wall-rock alteration have been recognized in the hydrothermal cement of stage 3 breccias. Stage 3 is marked by quartz- manganese oxides-hydroxides (psilomelane, pyrolusite, braunite) veins and breccia cement. It is usually crosscut stage 2 and is cut by stage 4 veinlets. Stage 4 is represented by < 1 mm wide quartz (calcite-chlorite) vein-veinlets. This stage usually crosscuts previous ore stages. No sulfide minerals are recognized in stage 4. Stage 5 is characterized by up to 2 mm wide veinlets or vug infill texture of calcite. Stage 5 calcite veinlets usually crosscut previous ore stages. The ore minerals at Qebchaq have been formed as vein-veinlet and hydrothermal breccia cement, and show disseminated, vein-veinlet, brecciated, comb, cockade, colloform, crustiform, plumose, and vug infill textures. Pyrite, chalcopyrite, galena, sphalerite, native gold, realgar, psilomelane, and pyrolusite are the main ore minerals. Malachite, azurite, smithsonite, cerussite, goethite, secondary pyrolusite, and braunite are supergene minerals. Quartz, sericite, chlorite, and calcite are present in the gangue minerals. 
Comparison of Chondrite–normalized rare elements and REE patterns of host dacitic lavas, fresh and altered quartz diorite-gabbro intrusion, and the mineralized samples at Qebchaq indicate that leaching of some elements from the host rock units may have been involved in mineralization. The data in this study suggest that Qebchaq is an example of intermediate-sulfidation type of epithermal base and precious metal mineralization.