The Kahyaz prospect is located approximately 45 km southeast of Ardestan, Esfahan Province, within the central part of the Saveh-Naein-Jiroft magmatic belt. Gold-bearing siliceous veins are hosted by Oligocene andesitic–trachyandesitic volcanic rocks. The main mineralized vein is approximately 2 km long and has a width of 2–5 m, with gold grades ranging from 0.1 to 2.1 g/t. Electrum is the main gold-bearing mineral, whereas base metal sulfides, including pyrite, chalcopyrite, chalcocite, and covellite, occur as minor minerals. Quartz, calcite, and adularia are the dominant gangue minerals. Colloform, crustiform, vein–veinlet, open-space filling, and bladed textures are characteristic features of the Kahyaz. Geochemical data indicate a positive correlation of Au with Ag and Sb, and negative correlations between Au and As, S, and base metals. Hydrothermal alteration is charachterized by silicic, intermediate argillic, and propylitic alteration zones. Fluid inclusion studies indicate that ore-forming fluids were characterized by moderate to high homogenization temperatures (169–278°C) and low to moderate salinities (2.73–10.29 wt% NaCl eq.). These fluids were affected by mixing and boiling processes. The geological, mineralogical, geochemical, and fluid inclusion characteristics of the Kahyaz prospect are consistent with a low-sulfidation epithermal system.

Introduction
The Kahyaz prospect is located 16 km northwest of Neystanak Village, and 45 km southeast of Ardestan City, Esfahan Province, central Iran. The Kahyaz epithermal gold (Au) prospect was discovered in 2018 by Pars-Olang Company. The exploration program, which included geological mapping and diamond drilling (14 drill holes), was recently conducted by Kowsar Mining and Industries Development Investment Company, resulting in the identification of 300,000 tonnes of mineral resources with an average gold grade of 1.1 ppm Au. Unlike the numerous studies conducted on porphyry and epithermal deposits in the northwestern and southeastern parts of the belt, the central segment of the Saveh-Naein-Jiroft magmatic belt has received relatively little attention. The discovery of new mineralized zones, including the area investigated in this study, in the central Saveh-Naein-Jiroft magmatic belt may indicate the presence of significant mineralization that has so far remained unexplored. Therefore, the geological, mineralization, geochemical, hydrothermal alteration, and fluid inclusion data presented in this study provide a basis for developing an exploration model to identify additional mineral occurrences within the central Saveh–Naein–Jiroft magmatic belt.

Materials and methods
Petrographic analyses were performed on approximately 100 thin sections was carried out at Bu-Ali Sina University (Hamedan, Iran) using a ZIESS Axioplan2 transmitted- and reflected light polarizing microscope. Textural imaging of five selected samples was performed using an FEI QEMSCAN® Quanta 650F instrument on polished carbon-coated thin sections at the Faculty of Earth Sciences, University of Geneva, Switzerland. Fifty drill-core samples were analyzed for gold using the fire assay method at Zarrin Felez Kavoshgaran Co. (Najafabad, Iran) and Zarraazma Co. (Tehran, Iran). To determine correlation coefficients, 35 samples were analyzed using ICP-OES at Zarazma Co. For fluid inclusion studies, seven quartz samples associated with gold mineralization were selected from drill cores and surface outcrops. After preparing double-polished sections with a thickness of 300 μm, petrographic and microthermometric analyses were performed at Tarbiat Modares University. Microthermometric measurements were conducted using a Linkam THMS 600 heating–cooling stage equipped with a TP94 temperature controller and an LNP cooling system, mounted on a Zeiss microscope. The analytical system operated over a temperature range of −190 to +600 °C.

Results and Discussion
The Kahyaz mineral prospect, located in northeast Isfahan, Iran, hosts a low-sulfidation epithermal precious-metal system associated with Eocene volcanic and subvolcanic rocks. The host succession consists predominantly of andesite–trachyandesite and rhyolite, which are overlain by Oligocene basaltic andesite and rhyodacite–dacite units. Mineralization is principally confined to a quartz–calcite vein extending for approximately 2 km with a thickness ranging from 2 to 5 m. Quartz is the dominant gangue mineral and the principal host of electrum, whereas calcite and adularia occur as subordinate gangue minerals. Quartz exhibits a wide range of open-space filling textures, including crustiform, comb, colloform, bladed, plumose, cockade, brecciated, and vein-veinlet textures.
The hypogene ore assemblage comprises electrum, acanthite, pyrite, chalcopyrite, and manganite, whereas chalcocite, covellite, and goethite constitute the supergene mineral assemblage. Hydrothermal alteration includes propylitic, intermediate argillic, and silicic–carbonate alterations. Based on mineral assemblages and cross-cutting textural relationships, the hydrothermal evolution of the Kahyaz prospect is divided into three stages: (1) pre-ore alteration, (2) ore stage, and (3) post-ore supergene alteration.
During the pre-ore stage, propylitic and intermediate argillic alteration developed through interaction of near-neutral hydrothermal fluids with the andesitic–trachyandesitic host rocks, producing alteration zonation from argillic adjacent to the veins to propylitic outward. Illite, illite/smectite, montmorillonite, chlorite, epidote, and calcite constitute the principal alteration mineral assemblages. The ore stage was initiated by precipitation of quartz–calcite veins, followed by hydrothermal boiling.
Boiling promoted CO₂ degassing, increased fluid pH, and induced supersaturation of silica and potassium, resulting in the precipitation of quartz and adularia. Simultaneously, the loss of dissolved H₂S destabilized Au–Ag bisulfide complexes, triggering electrum precipitation. Bladed calcite, quartz pseudomorphs after calcite, comb and crustiform textures, together with open-space filling fabrics, provide strong evidence for boiling during mineralization.
The post-ore stage is characterized by supergene oxidation and enrichment, leading to the formation of chalcocite, covellite, goethite, jarosite, gypsum, and zeolite-group minerals. Fluid inclusion studies of vein quartz indicate homogenization temperatures of 169–278 °C and salinities of 2.73–10.29 wt.% NaCl equivalent, suggesting mineralization from low- to moderate-salinity hydrothermal fluids. The coexistence of fluid mixing and boiling is interpreted as the principal mechanism responsible for gold deposition. Geological setting, alteration mineralogy, quartz–adularia gangue assemblages, characteristic open-space filling textures, fluid inclusion data, and the scarcity of base-metal sulfides collectively indicate that the Kahyaz prospect represents a low-sulfidation precious-metal system.