Geology, mineralization, and geochemistry of the Aqjeh Qaleh Pb-Zn-Cu deposit (North of Zanjan): Intermediate-sulfidation epithermal mineralization in the Tarom-Hashtjin metallogenic belt

Document Type : Research Article

Authors

Shima Eyvazi ¹

Hossein Kouhestani ²

Mir Ali Asghar Mokhtari ²

Behnam Mehdikhani ³

¹ M.Sc., Department of Geology, Faculty of Science, University of Zanjan, Zanjan, Iran

² Associate Professor, Department of Geology, Faculty of Science, University of Zanjan, Zanjan, Iran

³ Ph.D., Mining Engineering Organization of Zanjan Province, Zanjan, Iran

10.22067/econg.2026.98540.1172

Abstract

The Aqjeh Qaleh Pb-Zn-Cu deposit is part of the Tarom-Hashtjin metallogenic belt. Mineralization occurs as five quartz-sulfide veins hosted by Eocene volcaniclastic and volcanic units, and is enclosed by 1–5-meter haloes of intermediate argillic alteration. The ore veins have N10-20E/70-85NW trends, and vary between 30 and 150 meters in length, and 0.5 and 2 meters in thickness. In the ore veins, textures and structures of ores include disseminated, veinlets, brecciated, crustiform, cockade, colloform, plumose, and vug infill. Ore assemblages include pyrite, chalcopyrite, galena, sphalerite, cerussite, smithsonite, goethite, malachite, and covellite, and gangue minerals are quartz, calcite, and chlorite. Five stages of mineralization can be recognized at Aqjeh Qaleh, in which Cu and Pb-Zn mineralization occurred during the second and third stages. Hydrothermal alterations around the ore veins mostly consist of silicification, carbonate, intermediate argillic, and propylitic alteration. Geochemical data display negative correlation between Pb, Zn, and Cu, positive correlation between Pb and Ag, and positive correlation between Ag and S, As, and Sb. Similar Chondrite-normalized trace elements and REE patterns of ore samples, pyroxene quartz monzonite porphyry intrusion, fresh and unaltered intermediate crystal tuffs, and basaltic lavas reveal that intrusion and host rocks are possibly involved in mineralization. The Aqjeh Qaleh deposit is an intermediate-sulfidation epithermal deposit.
 
Introduction
Porphyry and epithermal deposits are the main sources of precious (Au, Ag) and base metals (Cu, Pb, Zn) in the world. Among these, epithermal deposits provide about 6% of gold and 16% of silver of the world, and are also the main source for base metals in the world (Simmons et al., 2005; Dilles and John, 2021). Many of the important epithermal deposits are of Tertiary age and are concentrated on the Pacific rim and in the Mediterranean and Carpathian regions of Europe. Older deposits are scattered in the Tethys metallogenic belt and others in volcanic arcs of various ages, with a few of Archean age also found (White and Hedenquist, 1990; Cooke and Simmons, 2000; John, 2001; Sillitoe and Hedenquist, 2005; Simmons et al., 2005).
Iran, as a part of the Alpine-Himalayan orogenic belt, hosts numerous epithermal deposits. These deposits are mostly reported from the Urumieh-Dokhtar magmatic arc, Ahar-Arasbaran zone, Miyaneh-Bostan Abad metallogenic belt, Tarom-Hashtjin metallogenic belt, Torud-Chahshirin ore belt, Binaloud subzone, Torbat-e Heydariyeh-Taknar zone, Central Iran zone, Lut block, and the Bazman-Taftan volcanic arc (Yasami et al., 2025 and references therein). The Aqjeh Qaleh Pb-Zn-Cu deposit, discovered ~55 km north of Zanjan in the Tarom-Hashtjin metallogenic belt. Although exploration activities have been carried out on the quartz-sulfide veins in the area, the genesis of the deposit remains poorly understood. This study presents geological, mineralogical, alteration, and geochemical characteristics of the Aqjeh Qaleh Pb-Zn-Cu deposit to constrain its ore genesis and mineralization type. The outcomes are planned to help regional exploration programs within the Tarom-Hashtjin metallogenic belt and other parts in northwest Iran.
 
Materials and methods
This research includes fieldwork and laboratory analyses. The geological map of the Aqjeh Qaleh area, at a 1:5,000 scale, was prepared during the fieldwork. Additionally, fifty samples from quartz-sulfide mineralized veins, host rocks, and intrusive bodies were collected. Then, thirteen thin and seventeen polished-thin blocks were prepared and studied under transmitted and reflected polarized light microscopes at the mineralogy laboratory of the University of Zanjan, Zanjan, Iran. Moreover, thirteen samples from ore samples and host rocks were analyzed for rare, REE, and base metal contents using ICP–MS technique at Zarazma Analytical Laboratory in Tehran.
 
Results and Discussion
The rock units exposed in the Aqjeh Qaleh area include Eocene volcanic (olivine basalt, andesitic basalt, andesite, rhyodacite) and volcaniclastic (intermediate to acidic crystal to lithic crystal tuff, basic lithic crystal to crystal lithic tuff) sequence, and late Eocene gabbro to pyroxene quartz monzonite porphyry intrusion. Mineralization at Aqjeh Qaleh occurred as five quartz-sulfide ore veins hosted by Eocene strata, and is covered by 1 to 5-meter halos of intermediate argillic alteration. The veins strike N10–20E and dip 70–85° NW and range from 30 to 150 meters in dimension, with widths varying from 0.5 to 2 meters. Hydrothermal alterations comprise silicification, carbonate, intermediate argillic, and propylitic alteration; the first three are closely related to the mineralized veins. Ore minerals consist of pyrite, chalcopyrite, galena, and Fe-poor sphalerite. Gangue minerals are quartz, calcite, and chlorite. Cerussite, smithsonite, goethite, malachite, and covellite are supergene minerals. The ores display different textures and structures, such as disseminated, veinlets, brecciated, crustiform, colloform, cockade, plumose, and vug infillings. Mineralization can be separated into five stages. Stage-1 is characterized by the silicification (fine-grained quartz) of host rocks comprising minor anhedral fine-grained disseminated pyrite. Stage-2 is categorized by quartz-pyrite-chalcopyrite as vein-veinlets and hydrothermal breccia cement. Stage-3 is represented by quartz-galena-Fe poor sphalerite vein-veinlets and breccia cement. Stage-4 includes post-mineralization barren quartz-calcite vein-veinlets. Stage-5 corresponds to supergene processes.
Geochemical data of mineralized samples disclose moderate positive correlation between Pb and Zn, weak negative correlation between Pb, Zn, and Cu, as well as moderate positive correlation between Pb and Ag, and strong positive correlation between Ag and As. Chondrite–normalized trace elements and REE patterns of ore samples, and fresh and unaltered intermediate crystal tuffs, basaltic lavas, and pyroxene quartz monzonite porphyry intrusion, indicate that they are probably involved in mineralization. Characteristics of the Aqjeh Qaleh Pb-Zn-Cu deposit are comparable with the intermediate-sulfidation style of epithermal deposits. The close spatial association between late Eocene granitoid intrusions and epithermal mineralization in the Tarom-Hashtjin metallogenic belt proposes the role of late Eocene magmatic-hydrothermal activities in providing metals and ore-forming hydrothermal fluids for this type of mineralization in NW Iran, which should be considered in exploration programs.

Keywords

Pb-Zn-Cu mineralization

Epithermal

Intermediate-sulfidation

Aqjeh Qaleh

Zanjan

Tarom-Hashtjin metallogenic belt

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