Geology, alteration and mineralization style of the lithocaps in the north-northeastern Chah Musa deposit, part of the Toroud-Chah Shirin magmatic arc (south of Shahrood)

The study area is situated in the northern part of the Central Iranian structural zone, northeast of the Toroud-Chah Shirin magmatic arc. In this area, late Eocene hypabyssal intrusions are emplaced within Eocene volcano-sedimentary sequences. Hydrothermal fluid activity has completely destroyed the original structure and mineralogical composition of these rocks, leading to the formation of significant amounts of fine-grained quartz (indicative of silicic alteration) and kaolinite, along with secondary iron oxides such as hematite and goethite. Mineralization in this area has developed in at least four argillic-silicic zones associated with breccia, vuggy quartz, and veins. The highest occurrences of sulfide mineralization are formed within the silicic veins. In these mineralized zones, vuggy to brecciated silicic rocks—referred to as lithocaps—are present alongside advanced argillic alteration containing pyrophyllite. Surrounding these central features, propylitic alteration is also observed. The primary ore minerals identified include pyrite, chalcopyrite, enargite, magnetite, hematite, and goethite. Results from analyses of trapped two-phase fluid inclusions (L+V) in quartz reveal that homogenization temperatures range from 160 to 362.8 °C, with salinity varying between 2.24 and 9.08 wt.% NaCl eq. Comparative investigation of structural characteristics, texture, mineralogy, alteration features, and properties of ore-forming fluids indicate that the copper-gold vein mineralizations in this area significant similarities to high-sulfidation (HS) epithermal deposits. Introduction The Toroud-Chah Shirin magmatic arc (TCSMA) is situated in the northern part of the Central Iran zone, as shown in the 1:250,000 geological map of Toroud (Houshmandzadeh et al., 1978) and the 1:100,000 geological map of Moalleman (Eshraghi and Jalali, 2006). The magmatic activity in this arc and surrounding regions is attributed to the subduction of the Sabzevar-Daruneh branch of the Neo-Tethys Ocean beneath Central Iran (Yousefi et al., 2017). The studied area contains a substantial volume of volcanic and pyroclastic rocks from the Eocene epoch, as well as numerous subvolcanic intrusions. Previous studies conducted within this magmatic arc have identified a range of epithermal mineralization events characterized by low to moderate sulfide content (e.g., Mehrabi and Ghasemi Siani, 2012; Roohbakhsh et al., 2018; Mahabadi and Fardoust, 2018, Eskandari et al., 2024; Tale Fazel et al., 2019). Geological investigations in the northeastern section of the Toroud-Chah Shirin magmatic arc (coordinates 54°50' to 54°51' E and 35°30' to 35°29' N) reveal a diverse array of alteration types, including propylitic, argillic, and quartz-alunite alterations, with extensive advanced argillic alteration. These investigations have also documented several silica caps and clear evidence of mineralization that has not been addressed in previous studies. Among the limited research conducted in this area, Zadsaleh et al. (2012) identified indicators of argillic alteration (kaolinite and montmorillonite) and advanced argillic alteration (pyrophyllite and alunite) based on X-ray diffraction method. Therefore, this research aims to systematically investigate the mineralogy, existing alteration types and their zoning patterns, geochemistry, microthermometry of fluid inclusions, and structural geological features, ultimately striving to achieve a comprehensive understanding of the genesis of this mineralization zone. The introduction of this mineralization system not only provides critical insights for exploration in the studied area but may also facilitate the identification of other epithermal-porphyry systems in the Toroud-Chah Shirin magmatic arc and other regions of Iran. Material and methods After completing field studies and processing satellite images, sampling was conducted across various silicic-argillic zones. A total of 30 polished thin sections and 20 thin sections were prepared for petrological, mineralogical, and ore microscopy studies. These samples were examined using an Olympus polarizing microscope at Clausthal University of Technology in Germany and Shahrood University of Technology. Additionally, two doubly polished sections from quartz-calcite veins were prepared to investigate the physicochemical properties of the mineralizing fluids. The temperature and salinity of the fluid inclusions were measured in the Economic Geology Laboratory at Shahrood University of Technology using a heating and freezing stage mounted on a polarized microscope with Linkam MDSG600 model. Following these procedures, X-ray diffraction (XRD) was conducted on 11 samples collected from altered sections (silicic and argillic) to identify the composition of clay minerals. The XRD was performed using a diffractometer equipped with copper tubes coated with nickel at Zarazma Laboratories in Tehran and the IELF at Clausthal University of Technology. Furthermore, several rock samples with minimal alteration, altered and mineralized ones were selected and examined at Acme Laboratories in Vancouver, Canada, using ICP-OES and ICP-MS methods to assess both major and trace elements. The analytical precision for major elements is ±1%, while most trace elements have an uncertainty of ±0.05%. Results The mineralized region features several argillic alteration zones and silica caps, located approximately 2 to 7 km north-northwest of the Chah Musa copper deposit (Figures 1 and 2). During field observations, several relatively intact outcrops of andesite were documented; however, many of these rocks have experienced significant silicic and argillic alterations, leading to a complete loss of their original texture and mineralogical composition. Based on field and laboratory evidences, it appears that the mineral assemblages influenced by hydrothermal fluid activity exhibit a distinct zoning pattern. This distribution, identified solely through surface observations due to the absence of exploratory drilling, includes propylitic alteration (chlorite-epidote-albite-carbonate) at the outer margins, moderate argillic alteration (quartz-montmorillonite-smectite-illite-natrojarosite), and advanced argillic alteration (vuggy quartz, quartz-alunite, kaolinite, montmorillonite-illite) towards the center. The silica caps, vuggy quartz structures, and this zoning pattern (Figure 5) are consistent with the alteration patterns of high-sulfidation systems described by White and Hedenquist (1990), indicating the presence of hydrothermal fluids with a pH of less than 2 in this area (Stoffregen, 1987). These acidic fluids have resulted in the complete leaching of elements from the mineral-bearing rocks, forming residual silica in the silica caps or as quartz crystals filling the cavities. Field studies and hand samples collected from the mineralized area under investigation reveal that the andesitic rocks contain silica-sulfide veins with varying thicknesses, ranging from 1 centimeter to 2 meters (Figure 3a). Mineralization is present as veins and veinlets that fill void spaces, as well as through replacement and disseminated occurrences (Figure 3a). The mineralized veins include hypogene sulfide phases such as pyrite, chalcopyrite, enargite, and galena, alongside supergene minerals like rutile, hematite, goethite, and limonite. The gangue minerals consist of variably thick quartz (both banded and vuggy), calcite, and clay minerals (Figure 6). At the surface, the hypogene sulfides have largely oxidized due to supergene processes, leading to their replacement by iron hydroxides, oxides, copper silicates (chrysocolla), copper carbonates (malachite, azurite), hematite, goethite, and limonite. Fluid inclusion studies were conducted to investigate the nature and composition of the fluids involved in the mineralization processes observed in calcite and massive quartz crystals collected from the examined mineralized area. The results of the fluid inclusion study reveal final homogenization temperatures (Th) for the liquid phase ranging from 160 to 362.8 °C, final melting temperatures of ice (Tmice) between 1.3 and 5.9 °C, and salinity levels between 2.24 and 9.8 wt.% NaCl equivalent (Table 2 and Figure 9). These values align with those found in high-sulfidation (HS) epithermal systems (Hedenquist et al., 1998; Jannas et al., 1999). Discussions The studied area is situated in the northeastern section of the Toroud-Chah Shirin magmatic arc and lies within the northern margin of the structural zone of Central Iran. The mineralized veins are hosted in Eocene andesite rock and have experienced alteration due to the intrusion of hydrothermal fluids, leading to advanced silicification, argillic alteration, and propylitic alteration. Indicators such as the presence of enargite (a significant copper mineral in high-sulfidation deposits), vuggy quartz, extensive advanced argillic alteration in a hypogene form, and hypogene alunite (an indicator mineral for high-sulfidation deposits) suggest that the northeast mineralization area of Chah Musa is characteristic of high-sulfidation epithermal deposits. The analysis indicates evidence of advanced argillic and silicic hydrothermal alterations, characterized by a silicified cap or lithocap at the center, with propylitic alteration surrounding it. Notable features in the studied area include vein texture and structure, hydrothermal breccia, vuggy quartz, and the mineralogical composition of the mineralization, which includes enargite and alunite. Mineralization occurs in two stages: hypogene stage (e.g., pyrite, chalcopyrite, enargite, galena, magnetite, and specularite) and supergene stage (e.g., goethite, limonite, malachite, and azurite). Gold concentrations in the analyzed samples are at anomalous levels. Results from the study of trapped two-phase fluid inclusions (L+V) in quartz reveal homogenization temperatures ranging from 160 to 362.8 °C, with salinity levels between 2.24 and 9.08 wt.% NaCl equivalent. Based on comparisons of structural characteristics, texture, mineralogy, alteration features, and ore-forming fluid properties, the copper-gold vein mineralizations in the studied area exhibit significant similarities to high-sulfidation epithermal deposits, which may potentially be associated with porphyry copper-gold-molybdenum deposits at depth.