The Torud-Chah Shirin volcanic-magmatic belt in Semnan province is the result of Tertiary volcanic-magmatic events. Geological, geochemical and mineral surveyors in the Torud-Chah Shirin area have identified numerous base and precious metal deposits, including mines, deposits and mineral works of base metals (copper, lead and zinc) and precious metals (gold and silver), iron, manganese and non-metallic deposits such as turquoise, feldspar, barite, kaolin, zeolite, bentonite, celestite, silica, amethyst, sodium sulfate, gypsum, rock salt and evaporite salts. The Kahavan copper deposit is located 125 km south of Shahroud in Semnan province and is accessible via the Shahroud-Torud and Shahroud-Damghan-Moaleman-Torud asphalt roads. In this study, the Kahovan copper deposit is investigated in terms of geology, mineralogy, geochemistry and generative pattern. In this study, 50 samples of rocks and ores have been selected for petrography, mineralogy and mineral texture studies. 14 rock samples and 4 ore samples have been collected for geochemical studies. All samples were prepared for measuring the amounts of oxides of major elements by alkaline melting method and for measuring the amounts of major, trace and rare earth elements by four-acid mixture method and analyzed using inductively coupled plasma-optical scattering/mass spectrometry (ICP-OES/MS) devices in the ZarAzma laboratory in Tehran. 2 pure chalcocite samples were analyzed in the geochemical laboratory of Mesbah Energy Company using a high-precision isotope mass spectrometer (IRMS) to measure the sulfur isotopic ratio

Tectonically, the Kahavan copper area is located in the northern part of the Central Iran Plateau and in the Torud-Cah Shirin magmatic belt. In the Kahavan area, sedimentary rocks (shale, limestone, sandstone, and marl), pyroclastic rocks, volcanic rocks, and andesite and basaltic dykes are exposed. A major part of the Kahavan area is formed by pyroclastic rocks, which consist of epiclastic tuffs, crystalline tuffs, lithic tuffs, sandy tuffs, tuff sandstones, and tuff shale. Epiclastic tuffs, especially along fault trends, host copper mineralization (chalcocite, malachite, chrysocolla, and azurite). Volcanic rocks are also significantly widespread. Volcanic units, sometimes prismatic in structure, are observed on top of Eocene pyroclastic units and sometimes as interlayers within pyroclastic units. The composition of the volcanic units varies from andesite, trachyandesite, andesite-basalt, basalt, and olivine basalt. In the Kahuan area, the Middle-Late Eocene volcanic-agglomerative-sedimentary complex is cut by trachyandesite and olivine basaltic dykes of Late Eocene to Oligocene age. In the Kahuan area, depending on the chemical composition of the host rocks, the chemistry of the hydrothermal fluids, temperature, and pressure, the alterations include propylitic, argillic, chloritic, zeolitic, silicic, and carbonate types. Chlorite, epidote, calcite, and albite are the characteristic minerals of the propylitic alteration zone, and volcanic and pyroclastic rocks with compositions ranging from andesite-basalt to olivine-basalt have undergone this alteration. Argyllitic alteration is characterized by the formation of clay minerals, especially kaolinite, montmorillonite, and illite, which are usually formed as a result of the action of acidic hydrothermal solutions on alkali feldspars. In this zone, volcanic and pyroclastic rocks with compositions ranging from andesite to trachyandesite have undergone argyllitic alteration. Zeolitic alteration is characterized by the formation of zeolite from primary silicate minerals by the action of low-temperature hydrothermal fluids. Siliceous and carbonate alteration is observed in vein-vein forms, filling of the space between the cutting pieces, filling of pores and silicification of the ground in volcanic-sedimentary rocks. In the Kahavan area, copper mineralization is observed mostly in vein-vein forms, shear and filling of pores in volcanic-sedimentary rocks. In the Kahavan area, vein systems are divided into siliceous, siliceous-sulfide, iron oxide, siliceous-chlorite ± calcite and siliceous-calcite vein types based on their important constituent minerals. In shear mineralization, the empty spaces between the cutting pieces are filled with waste and mineral minerals (quartz, chlorite, calcite, chalcocite and malachite). The material of the sections is andesite, trachyandesite, andesite-basalt and basalt. In the mineralization of the type of filling of empty spaces, waste minerals (quartz, chlorite and calcite) and minerals (chalcocite, chrysocolla, malachite and azurite) are formed in the cavities of volcanic rocks (andesite, andesite-basalt, olivine basalt) and in the space between the pieces in epiclastic tuffs. Based on mineralogical, structural and textural studies, in the Kahuan mineralization system, minerals are formed in two endogenous (hypogene) and exogenous (supergene) stages. In the endogenous stage, sulfide minerals such as pyrite, chalcopyrite and primary chalcocite are formed from hot metal-bearing hydrothermal fluid. Chalcosite is the main mineral in this stage, which is rarely accompanied by scattered and fine grains of pyrite, chalcopyrite and bornite (Fig. 9a, b). In the exogenous stage, due to the activity of atmospheric waters under surface oxidation conditions (oxidative supergene), minerals such as chrysocolla, malachite, azurite, iron oxides and hydroxides (hematite, goethite and limonite) are formed, and in the reduced supergene zone, secondary chalcosite, covellite and rarely pure copper are formed (Fig. p-c). Exogenous minerals are formed as a result of the replacement of primary sulfide minerals such as chalcosite and chalcopyrite (mostly chalcosite). Quartz, chlorite, epidote, zeolite, calcite and gypsum are important waste minerals in the Kahuan mineralization system. Waste and mineral minerals are observed in the form of vein-vein, shear, pore-filling, radial, and replacement textures.

In the chemical classification diagrams, the volcanic rock and dike samples are located in the range of andesite, dacite, trachyandesite, basaltic andesite and basalt. In the magmatic series and tectonic environment determination diagrams, the rocks of the region are of the calc-alkaline type and are located in the range of continental arc basalts related to subduction zones. In the ore samples, the amounts of Cu, Ag, Zn, Pb, and Mo elements have been compared with their crustal abundance. The results of these studies show that the concentrations of copper and silver are on the verge of enrichment in the form of ore concentrations and lead are on the verge of enrichment, while the amounts of other elements are at a low level. In this study, the average concentrations of trace elements and rare earth elements in the volcanic rock and ore samples have been normalized to the composition of the early mantle and chondrites. The geochemical behavior of the elements in these patterns indicates the genetic affinity between volcanic and mineralized rocks. The sulfur isotopic ratio values (δ34S) of -7.6 ppm and -5.4 ppm (average -6.5 ppm) in the chalcocite of the Kahavan copper deposit have been compared with some mantle-type copper deposits in Iran and Chile, and the Kahavan deposit is most similar to the eastern Narbaghi mantle-type copper deposit. The probable origin of the sulfur in chalcocite is from primary pyrites formed in the initial diagenesis stage during the bacterial reduction of seawater sulfate.

Comparison of the geological and mineralogical characteristics of the Kahavan copper (silver) deposit with different types of copper deposits shows that this deposit is most similar to copper deposits with volcanic host rocks. Therefore, some of the important characteristics of this deposit have been compared with types of copper deposits with volcanic host rocks (Michigan-type copper deposits, mantle and volcanic red layer), among which the Kahavan deposit has a great similarity with mantle-type copper deposits. In the Kahavan area, copper mineralization has formed and developed in the following three main stages. Pre-mineralization stage, in this stage, volcanic-sedimentary activities in the form of alternating lava and pyroclastic eruptions (with intermediate to mafic composition) and sedimentary in the subduction-related volcanic arc environment have occurred, and a sequence of volcanic, pyroclastic and sedimentary rocks of Middle-Late Eocene age has been formed. In this stage, gas-rich basaltic lavas, after eruption and rapid cooling, have developed extensive fractures and numerous cavities have appeared in them. At the same time, in the initial diagenesis stage, the activity of sulfate-reducing bacteria has created reducing conditions in the sediments and provided the basis for pyrite deposition in the mineralizing host rocks. The appearance of pyrite in this stage has been a key factor in creating a suitable reducing environment for the formation of copper-bearing minerals in the later stages. Burial and Mild Metamorphism Stage As material deposition continues and the thickness of the volcanic-sedimentary sequence increases, these units undergo a deep burial process. Due to increased temperature and pressure, mild metamorphism occurs and causes the release of copper and other elements from the crystal lattice of silicate minerals such as feldspar, amphibole, pyroxene, and magnetite. This event could indicate changes in water-brine chemistry in the basin and initiates the leaching of copper from the host volcaniclastic rocks. In this process, minerals such as feldspars are altered to secondary minerals such as zeolite, calcite, epidote, sericite, and amphibole, and pyroxenes to chlorite, epidote, biotite, magnetite, hematite, sulfides, and primary magnetite to hematite. The significant volume of volcano-sedimentary sequences, volcanic activity, and deep and semi-deep intrusions (diorite and monzodiorite masses and basaltic and trachyandesitic olivine dikes) in this region can generate high temperatures, which cause the movement of interpore fluids and oxidant brines, and these fluids are enriched in copper due to the high ambient temperature and circulation in volcanic and pyroclastic rocks. The liberated copper is transported in the form of copper ions by high-temperature fluids resulting from burial diagenesis and directed towards zones with high permeability and previous reduction conditions. In these places, geochemical (such as pyrite-bearing rocks, carbonate rocks, etc.) and physical (the presence of impermeable layers such as fault ridges, etc.) barriers cause the instability of copper-bearing complexes, and primary copper sulfide minerals such as chalcopyrite, bornite, and chalcocite form in vein-vein, shear, and cavity-filling forms in host volcanic and pyroclastic rocks. Iron released from pyrite decomposition precipitates in the form of hematite. After the formation of primary copper sulfide ore, the Eocene sedimentary volcanic sequence has been folded and faulted by orogenic and uplift phases. In the exogenous stage, the effect of oxygenated atmospheric waters causes the oxidation of primary copper sulfides. As a result of this process, primary copper-bearing minerals such as chalcocite, chalcopyrite, and bornite are replaced, and malachite, chrysocolla, azurite, hematite, goethite, and limonite are formed in the oxidized supergene, and secondary chalcocite, covellite, and rarely pure copper are formed in the reduced supergene. Oxidation processes not only change the mineralogical composition of the ore, but also cause secondary enrichment of copper in the surface parts of the deposit.