Introduction
The Torud- Chah Shirin belt in Semnan province is the result of Tertiary volcanic-magmatic events. The geological, geochemical and mineral deposit surveys in the Torud- Chah Shirin area have identified numerous base and precious metal deposits, including mines, mineral deposits and ore indications of base (Cu, Zn, and Pb) and precious (Au and Ag) metals, iron, manganese and non-metallic deposits such as turquoise, feldspar, barite, kaolin, zeolite, bentonite, celestite, silica, amethyst, sodium sulfate, gypsum, salt and evaporites. The Kahovan Cu (Ag) deposit is located 125 km south of Shahroud in Semnan province and is accessible by the Shahrood-Torud and Shahrood-Damghan-Moaleman-Torud asphalt roads. In this study, the Kahovan deposit is investigated in terms of geology, mineralogy, geochemistry and genesis.
Materials and Methods
In this study, the number of 50 rock and ore samples have been selected for petrography, mineralography, and mineral texture studies. The number of 14 rock and 4 ore samples have been collected for geochemical studies. All samples were prepared for measuring of major oxides by alkali fusion and for major, trace and rare earth elements by four-acids mixture methods and then analyzed using inductively coupled plasma-optical mission/mass spectrometry (ICP-OES/MS) devices in the Zarazma lab in Tehran. Two 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.
Results and Discussion
Geology: The Kahovan copper area is located in the northern part of the Central Iran Zone and in the Torud- Cah Shirin magmatic belt. In the Kahovan area, sedimentary rocks (shale, limestone, sandstone, and marl), pyroclastic rocks, volcanic rocks, and andesite and basaltic dykes are exposed. A major part of the Kahovan area is covered by pyroclastic rocks, which consist of epiclastic tuffs, crystalline tuffs, lithic tuffs, sandy tuffs, tuffaceous sandstone, and tuffaceous shale. Epiclastic tuffs, especially along fault trends, is hosting copper mineralization (chalcocite, malachite, chrysocolla, and azurite). Volcanic rocks are also significantly widespread. Volcanic units, have occasionally prismatic structure, and 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 Kahovan area, the Middle-Late Eocene volcanic-pyroclastic-sedimentary complex is cut by trachyandesite and olivine basaltic dykes with Late Eocene to Oligocene age.Alteration: In the Kahuan area, depending on the chemical composition of the host rocks, the chemistry of the hydrothermal fluids, temperature, and pressure, the alteration zones 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 andesite-basalt to olivine-basalt compositions have undergone this alteration. Argillic alteration is characterized by the formation of clay minerals, especially kaolinite, montmorillonite, and illite, which are usually formed as a result of acidic hydrothermal solutions on alkali feldspars. In this zone, volcanic and pyroclastic rocks with andesite to trachyandesite compositions have undergone argillic alteration. Zeolitic alteration is characterized by the formation of zeolite from primary silicate minerals by the action of low-temperature hydrothermal fluids. Silicic and carbonate alteration is observed in vein-vein forms, space filling between breccia fragments, pore filling, and silicification of background in volcanic-sedimentary rocks.
Mineralization: In the Kahovan area, copper mineralization is observed mostly in vein-vein forms, breccia, and pore-filling in volcanic-sedimentary rocks. vein-veinlet systems are divided into the silicic, silicic- sulfide, iron oxide, silicic- chlorite ± calcite and silicic-calcite vein types based on their constituent minerals. In breccia form mineralization, the open spaces between the breccia fragments are filled with gangue and ore minerals (e.g., quartz, chlorite, calcite, chalcocite and malachite). The rock type of breccia fragments is andesite, trachyandesite, andesite-basalt and basalt. In the pore-filling type of mineralization, gangue (quartz, chlorite, and calcite) and ore 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 fragments in epiclastic tuffs. Based on mineralogical, structural and textural studies, in the Kahuan mineralization system, minerals are formed in two hypogene and supergene stages. In the hypogene 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 disseminated and fine grains of pyrite, chalcopyrite and bornite. In the supergene stage, due to the activity of meteoric waters under surface oxidation conditions, are formed minerals such as chrysocolla, malachite, azurite, iron oxides and hydroxides (hematite, goethite and limonite), and in the reduced supergene zone are formed secondary chalcocite, covellite and rarely native copper. Supergene 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 gangue minerals in the Kahuan mineralization system. The gangue and ore minerals are observed in vein-veinlets, breccia, pore-filling, radial, and replacement forms.
Geochemistry: In the chemical classification diagrams, the volcanic rocks and dike samples are located in the range of andesite, dacite, trachyandesite, basaltic andesite and basalt. In the magmatic series and tectonic environment discrimination diagrams, all rock samples are calc-alkaline type and located in field of continental arc basalts related to subduction zones. In the ore samples, the Cu, Ag, Zn, Pb, and Mo contents have been compared with their crustal abundance. The results show that the Cu and Ag concentrations are in ore grades, and Pb is enrichment, while the amounts of other elements are low. In this study, the average contents of trace and rare earth elements in the volcanic rocks and ore samples have been normalized to the composition of the primitive mantle and chondrite. The geochemical behavior of the elements in these patterns indicates the genetic affinity between volcanic and mineralized samples. The sulfur isotopic ratio values ​​(δ³⁴S) of -7.6 ‰ and -5.4 ‰ (average -6.5 ‰) in the chalcocite of the Kahovan copper deposit have been compared with some Manto-type copper deposits in Iran and Chile, and the Kahovan deposit is most similar to the eastern Narbaghi ​​Manto-type copper deposit in Iran. The probable source of the sulfur in chalcocite is from primary pyrites formed in the initial diagenesis stage during the bacterial reduction of seawater sulfate.
Ore deposit genesis: Comparison of the geological and mineralogical characteristics of the Kahovan Cu (Ag) deposit with various types of copper deposits shows that this deposit is most similar to the volcanic rock-hosted Cu deposits. Therefore, the main characteristics of this deposit have been compared with the volcanic rock-hosted Cu deposits (Michigan-, manto-, and volcanic red bded-types Cu deposits). Among, the Kahovan deposit has a great similarity with manto-type Cu deposits. In the Kahovan area, copper mineralization has formed during 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 have occurred in the subduction-related volcanic
arc environment, and has been formed a sequence of volcanic, pyroclastic and sedimentary rocks with Middle-Late Eocene age. The gas-rich basaltic lavas, after eruption and rapid cooling, have developed extensive fractures and numerous cavities have appeared in these rocks. 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 condition for pyrite deposition in the 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. In the burial and low-grade 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, low-grade 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 bodies and olivine basalte and trachyandesitic olivine dikes) in this region can generate high temperatures, which cause the movement of pore oxidant brines and fluids, and these fluids are enriched in Cu due to the high temperature of environment and circulation in volcanic and pyroclastic rocks. The released 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 and carbonate rocks, etc.) and physical (the presence of impermeable layers such as fault zone, etc) barriers cause the instability of copper-bearing complexes, and primary copper sulfide minerals such as chalcopyrite, bornite, and chalcocite are formed in vein-veinlets, breccia, and cavity-filling shapes at the host volcanic and pyroclastic rocks. Iron released from pyrite decomposition precipitates as hematite. After the formation of primary copper sulfides, the Eocene sedimentary-volcanic sequence has been folded and faulted by occurrance of orogenic and uplift phases. In the supergene stage, the effect of meteoric 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 by malachite, chrysocolla, azurite, hematite, goethite, and limonite in the oxidized supergene, and secondary chalcocite, covellite, and rarely native 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.
Conclusion
In the Kahavan area, copper mineralization has occurred as stratabound form in the epiclastic unit and andesite to basaltic andesite volcanic rocks. The geochemical characteristics of volcanic rocks and ores indicate their formation in an intra-arc extensional environment related to the subduction zone. The mineralogy of the ore deposit is simple and mostly consists of chalcocite, malachite, and chrysocolla. Vein-vein forms, breccia, and cavities- and amygdales-filling forms are the dominant textures of mineralization. Alterations include propylitic (especially chloritic and epidotic), argillic, zeolitic, silicic, and carbonate zones. Copper is the main and silver is byproduct elements of mineralization. The possible origin of sulfur in copper sulfides is provided from the primary pyrites of the initial diagenesis stage, during the bacterial reduction process of seawater sulfate. Kahavan deposit is most similar to the Manto-type copper deposits with andesite to basaltic andesite host rock.