Low-sulfidation epithermal Au mineralization in the Tikmehdash 2 deposit (southeast of Bostan Abad): Constraints on geology, mineralization, geochemistry, and fluid inclusions

Document Type : Research Article

Authors

• Hamid Hashempour ¹
• Hossein Kouhestani ²
• Mir Ali Asghar Mokhtari ²
• Ghafour Alavi 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 Assistant Professor, Department of Earth Sciences, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran

10.22067/econg.2025.1147

Abstract

Tikmehdash 2 gold (Au) deposit is part of the Miyaneh-Bostan Abad metallogenic subzone. Au mineralization occurs in Mn- and Au-bearing quartz veins hosted by Eocene and Miocene volcanic-sedimentary sequences. The ore veins trend northeast-southwest have NE-SW trends and dip approximately 65-80º northwestward. They range from 30 to 500 meters in length, with thicknesses between 0.1 and 3 meters. Au grades vary from less than 1 g/t up to 88 g/t. Pyrite, chalcopyrite, galena, sphalerite (Fe-rich), native gold, arsenopyrite, tennantite-tetrahedrite, psilomelane, and primary pyrolusite are ore assemblages; quartz, calcite, and barite are gangue minerals. Supergene minerals include goethite, malachite, secondary pyrolusite, and braunite. The ores display various textures, such as vein-veinlets, brecciated, comb, cockade, plumose, disseminated, crustiform, colloform, bladed, and vug infills. The mineralization at Tikmehdash 2 can be divided into six stages, with gold occurring predominantly during the second stage. Hydrothermal alterations include silicification, carbonate, intermediate argillic, and propylitic alteration. Geochemical investigations of ore samples show weak positive correlation between base metals and Au, as well as weak to moderate positive correlation between Au and As, Sb, and Ag. Fluid inclusion microthermometry indicates that ore-forming fluids belong to the high- to moderate-temperature (224–292 °C), and low- to moderate-salinity (0.7–13.4 wt.% NaCl equiv.) H₂O–NaCl system. These data suggest that boiling and mixing processes played a key role in the evolution of the ore-forming fluids. The characteristics of the Au mineralization align with those of low-sulfidation epithermal deposits in the Tikmehdash 2 deposit.
 
Introduction
Tikmehdash 2 Au deposit is located in the Western Alborz-Azerbaijan zone (Nabavi, 1976), forming part of the Miyaneh-Bostan Abad metallogenic subzone. This subzone is a prominent regional metallogenic area in northwest Iran, hosting numerous deposits of metallic (Cu, Mo, Pb-Zn, Au, Mn), and non-metallic (pumice and perlite) deposits and occurrences (see Nabatian et al., 2017; Alizadeh et al., 2021; Sohbatloo, 2021; Hassani Soughi et al., 2021; Jafari et al., 2002; Sohbatloo et al., 2023; Hassani Soughi et al., 2023; Barati et al., 2023; Jahangiryar et al., 2023). Located approximately 20 km southwest of Bostan Abad, the Tikmehdash 2 Au deposit, is a relatively recent discovery in this district. Despite the presence of ancient mining activities such as filled trenches and collapsed tunnels on Au-bearing veins, and ongoing exploration of Fe skarn mineralization and Au-bearing veins in the area, the genesis of Tikmehdash 2 Au deposit remains poorly understood. This study investigated the geological, mineralization, alteration, geochemical, and fluid inclusion characteristics of the Tikmehdash 2 Au deposit to constrain its ore genesis and mineralization style. The results are intended to aid regional exploration programs within the Miyaneh-Bostan Abad metallogenic subzone and elsewhere in northwest Iran.
 
 
Materials and methods
This research combines fieldwork and laboratory analyses. During investigations, a geological map, at a 1:5000 scale of the Tikmehdash 2 area was prepared. Fifty samples were collected from various ore veins, host rocks, and intrusive bodies. Among these, eighteen thin and twenty-five polished-thin sections were prepared and examined under transmitted and reflected polarized light microscopes at the mineralogy laboratory of the University of Zanjan, Zanjan, Iran. Additionally, 105 samples from ore veins were analyzed for Au and other metal contents using Fire Assay and ICP–MS techniques at Zarazma and Novin Shimyar Analytical Laboratories in Tehran. Fluid inclusion studies were conducted on four doubly polished thick sections (~150 μm) containing quartz crystals from the second, and fourth ore stages. Microthermometric measurements were performed using a Linkam THMS600 heating–freezing stage attached to an Olympus BX60 microscope in the fluid inclusion laboratory of Payam-e Noor University, Tabriz, Iran.
 
Results and Discussion
The geological units in the Tikmehdash 2 area belong to the Eocene and the Miocene volcano-sedimentary strata that were intruded by early Miocene (Kouhestani, 2025) gabbro-gabbro diorite, and quartz diorite intrusions. The mineralization at the Tikmehdash 2 Au deposit is predominantly hosted in NE–SW to NW–SE trending Mn- and Au-bearing quartz veins, which dip steeply (65–80º) to the northwest. These veins extend from 30 to 500 meters in length, with thicknesses ranging from 0.1 to 3 meters, and Au grades ranging from <1 to 88 g/t. Hydrothermal alterations include silicification, carbonate, intermediate argillic, and propylitic alteration; among these, the first three are intimately associate with the ore veins, whereas propylitic alterations affect the surrounding host rocks. Mineral assemblages include pyrite, chalcopyrite, galena, Fe-rich sphalerite, native gold, arsenopyrite, tennantite-tetrahedrite, psilomelane, and primary pyrolusite. Gangue minerals are quartz, calcite, and barite. Goethite, malachite, secondary pyrolusite, and braunite are supergene minerals. The ores display a variety of textures, such as vein-veinlets, brecciated, comb, cockade, plumose, disseminated, crustiform, colloform, bladed, and vug infillings.
Ore formation can be divided into six stages. Stage 1 is signified by the silicification of host rocks containing minor disseminated pyrite. Stage 2 is considered by Au-bearing quartz veins-veinlets, and hydrothermal breccias with quartz-sulfide (pyrite, chalcopyrite, arsenopyrite) cements with minor barite. Stage 3 is marked by pyrite, galena, sphalerite, and tennantite-tetrahedrite-bearing calcite veins. Stage 4 is marked by veinlets or vug infill textures of manganese ores (pyrolusite and psilomelane) and quartz. Stage 5 is represented by quartz and calcite with vein-veinlets and vug infill textures. Stage 6 corresponds to supergene processes, forming goethite, malachite, secondary pyrolusite, and braunite with vug infill, replacement, and residual textures. Geochemical investigations of ore samples reveal weak positive correlation between base metals (Pb, Zn, and Cu) and Au, as well as weak to moderate positive correlations between Au and As, Sb, and Ag. Notably, silver correlates moderately with lead, while cadmium and zinc show positive relationships, and gold and manganese exhibit weak correlations.
Fluid inclusion microthermometry suggests ore-forming fluids at Tikmehdash 2 Au deposit belong to high- to moderate-temperature (224–292 °C), and low- to moderate-salinity (0.7–13.4 wt.% NaCl equiv.) H₂O–NaCl system. Fluid inclusion data shows that fluid boiling and mixing were key factors in the evolution of the ore-forming fluids at Tikmehdash 2 Au deposit. These data specify that estimated trapping pressure of the fluids varies from 45 to 71 bars, which corresponds to entrapment depths of 460 to 730 m beneath the paleowater table. Characteristics of the Tikmehdash 2 Au deposit are similar to the low-sulfidation type of epithermal deposits. The close spatial association between late Miocene dacite-rhyodacite domes, and epithermal mineralization in the Miyaneh-Bostan Abad metallogenic subzone suggests the role of magmatic-hydrothermal activities during late Miocene magmatism in providing metals and ore-forming hydrothermal fluids for this type of mineralization in NW Iran, which should be considered in exploration programs.

Keywords

• Gold mineralization
• epithermal
• low-sulfidation
• Tikmehdash
• Miyaneh-Bostan Abad subzone

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