University of Isfahan Petrological Journal 2228-5210 16 4 2025 12 22 Genesis of the Delkan iron deposit based on the geological, tectonic, petrographic as well as δ18O and δ34S stable isotope studies (SW of Bardeskan, Khorasan Razavi Province) Genesis of the Delkan iron deposit based on the geological, tectonic, petrographic as well as δ18O and δ34S stable isotope studies (SW of Bardeskan, Khorasan Razavi Province) 57 82 30101 10.22108/ijp.2025.146371.1369 FA Masih Kassaeian Ph.D. Student of Economic Geology, Department of Mineral and Water Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran Mehrdad Behzadi Associate Professor, Department of Mineral and Water Geology, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran 0009-0002-5154-3550 Journal Article 2025 08 18 Introduction Delkan iron mine, geologically, located in the south of Bardeskan city. It is geologically located in the eastern part of the Central East Iran Microcontinent. Regarding structural division, the Delkan iron mine lies in the northeast of the Kashmar-Kerman tectonic zone and on the northeastern ridge of Kuh-e-Sarhangi. Some of the most important iron mines in Iran are located in the Kashmar-Kerman structural zone, for example, the Bafgh iron mines with a total reserve of 5 billion tons (Torabian, 2007). Geology of the Area The rock units in the study area are predominantly the metamorphosed units of schist, quartzite, limestone, dolomite, and amphibolite, belonging to Precambrian and the Cambrian units composed of limestone, dolomite, carbonaceous shales, schist, and quartzite. These rock units were subjected to intrusion of a plutonic stock, which gave rise to contact metamorphism halo and iron mineralization during the Silurian. Materials and Methods Following the field investigations, for structural studies, 37 fault planes were structurally sampled, and 284 rock samples were taken from the surface and the archive of drilled cores in the mine for petrology, mineralogy, and mineralization studies. 73 microscopic sections of the samples were studied with an Olympus BX60F5 microscope at the University of Isfahan. Maps of the area were drawn using ArcGIS software. To measure the main oxides, 36 rock samples were taken and after preparation using the peroxide fusion method, were analyzed by the use of the ICP-OES technique. δ18O stable isotope analyses were carried out on 2 magnetite and 2 quartz samples. Also, δ 34 S stable isotope analysis were performed on 2 pyrite samples. All isotope analyses were carried out at the Stable Isotope Research Laboratory of Arak University (SIRL). Mineralization, Alterations, Mineralogy and Mineralography Iron mineralization in the Delkan mine is observed in the two forms: 1) Iron oxide apatite with disseminated and veinlet texture within the monzonite intrusive stock, 2) Massive proximal and distal magnetite without apatite. Alterations observed in the Delkan deposit can be divided into prograde skarn, sodic, calcic, phyllic-silicic, and secondary carbonate alteration in order of occurrence. The intensity and spread of Prograde calc-silicate skarn alteration in the area are extremely limited. Pyrite occurs in two forms, pentagonal and anhedral to cubic. Primary quartz is pentagonal and secondary quartz is anhedral. Metamorphic garnets are red, isotropic, and metasomatic garnets are brown to green, and anhedral to euhedral. Albite is often replaced by other minerals and can be seen as pseudomorphs. Acicular actinolite with fibrous textures, and anhedral to subhedral apatite, euhedral to anhedral magnetite with massive, disseminated, and replacement textures are noticeable. In the central parts of the deposit, magnetite mineralizations are present in massive form without apatite, but at the margins of the intrusive stock, disseminated magnetite mineralized with apatite. Hematite is seen with disseminated, replacement, and martitization textures; in some cases, it is replaced by goethite or limonite. Chalcopyrite is observed in an anhedral shape. Galena and sphalerite mineralizations were also observed in shallow quartz veins of Delkan (Shabani et al., 2015). Fault Patterns and Their Relationship with Iron Mineralization Two main fault distributions are extended in the mining area, including longitudinal faults trending northeast-southwest parallel to the extension of the Kuh-e-Sarhangi and NW-SE trending transverse faults almost perpendicular to the first group. According to studies (Sahandi et al., 2010; Nozaem, 2012), the Kuh-e-Sarhangi and Delkan areas have undergone multiple tectonic regime shifts between transpressional tectonic phases and extensional phases accompanied by volcanism and mineralization. It seems that the Silurian extensional phases in the longitudinal faults of the area under study have played a significant role in creating a suitable space for the intrusion of monzonite stock, which ultimately gave rise to the formation of proximal IOA and massive magnetite mineralization. Transverse faults have also played the role of escape routes for part of the hydrothermal fluid, caused the formation of distal mineralizations. Geochemistry of stable isotopes The isotopic values of δ18O for magnetite samples in ranges from 8.6 and 10‰. According to several people (Einaudi et al., 1981; Bowman, 1998; Meinert et al., 2005), these values indicate a Juvenile origin for the hydrothermal fluid that caused the mineralization of massive magnetites without apatite. The values of δ18O for quartz samples are between 15.6 and 16.2‰. These values indicate isotopic equilibrium between the hydrothermal fluid and the host rock during the gradual cooling processes of the fluid. The isotopic δ34S values for pyrite samples ranging from 20.1 to 20.6‰. According to (Einaudi et al., 1981; Meinert et al., 2005), we consider the studied sulfur sources in the area of study to be non-magmatic and related to isotopic changes in the hydrothermal fluid in equilibrium with marine sulfates and host rocks as well. Discussion and Conclusion Two types of iron mineralizations (skarn and kiruna) occurred in the Delkan mine, but most iron reserves in this deposit share similar characteristics to iron skarn deposits. Mineralization in the Delkan iron deposit has been subjected by several factors, of which the most important are the following: A) The tectonic regime shifts between the transtensional and transcompressional regimes, which played a key role in intrusion and trapping of the intrusive stock, B) The direction and patterns of faults have been effective in determining the intrusion paths for both the plutonic stock and the juvenile hydrothermal fluids, mineralization type and locations of mineralizations, C) Differences in Oxidation state between the intrusive stock and the host rocks (especially the black carbonaceous phyllite layers that are highly reduced) plays a key role in the consumption of dissolved oxygen in the juvenile hydrothermal fluid due to intensity of decarbonation reactions between the fluid and these reducing layers, and as a result, the increase in the CO2 fugacity of the fluid, which will decrease the intensity of calc-silicate alterations (prograde skarn) and also a decrease in the amount of iron skarn mineralizations. Introduction Delkan iron mine, geologically, located in the south of Bardeskan city. It is geologically located in the eastern part of the Central East Iran Microcontinent. Regarding structural division, the Delkan iron mine lies in the northeast of the Kashmar-Kerman tectonic zone and on the northeastern ridge of Kuh-e-Sarhangi. Some of the most important iron mines in Iran are located in the Kashmar-Kerman structural zone, for example, the Bafgh iron mines with a total reserve of 5 billion tons (Torabian, 2007). Geology of the Area The rock units in the study area are predominantly the metamorphosed units of schist, quartzite, limestone, dolomite, and amphibolite, belonging to Precambrian and the Cambrian units composed of limestone, dolomite, carbonaceous shales, schist, and quartzite. These rock units were subjected to intrusion of a plutonic stock, which gave rise to contact metamorphism halo and iron mineralization during the Silurian. Materials and Methods Following the field investigations, for structural studies, 37 fault planes were structurally sampled, and 284 rock samples were taken from the surface and the archive of drilled cores in the mine for petrology, mineralogy, and mineralization studies. 73 microscopic sections of the samples were studied with an Olympus BX60F5 microscope at the University of Isfahan. Maps of the area were drawn using ArcGIS software. To measure the main oxides, 36 rock samples were taken and after preparation using the peroxide fusion method, were analyzed by the use of the ICP-OES technique. δ18O stable isotope analyses were carried out on 2 magnetite and 2 quartz samples. Also, δ 34 S stable isotope analysis were performed on 2 pyrite samples. All isotope analyses were carried out at the Stable Isotope Research Laboratory of Arak University (SIRL). Mineralization, Alterations, Mineralogy and Mineralography Iron mineralization in the Delkan mine is observed in the two forms: 1) Iron oxide apatite with disseminated and veinlet texture within the monzonite intrusive stock, 2) Massive proximal and distal magnetite without apatite. Alterations observed in the Delkan deposit can be divided into prograde skarn, sodic, calcic, phyllic-silicic, and secondary carbonate alteration in order of occurrence. The intensity and spread of Prograde calc-silicate skarn alteration in the area are extremely limited. Pyrite occurs in two forms, pentagonal and anhedral to cubic. Primary quartz is pentagonal and secondary quartz is anhedral. Metamorphic garnets are red, isotropic, and metasomatic garnets are brown to green, and anhedral to euhedral. Albite is often replaced by other minerals and can be seen as pseudomorphs. Acicular actinolite with fibrous textures, and anhedral to subhedral apatite, euhedral to anhedral magnetite with massive, disseminated, and replacement textures are noticeable. In the central parts of the deposit, magnetite mineralizations are present in massive form without apatite, but at the margins of the intrusive stock, disseminated magnetite mineralized with apatite. Hematite is seen with disseminated, replacement, and martitization textures; in some cases, it is replaced by goethite or limonite. Chalcopyrite is observed in an anhedral shape. Galena and sphalerite mineralizations were also observed in shallow quartz veins of Delkan (Shabani et al., 2015). Fault Patterns and Their Relationship with Iron Mineralization Two main fault distributions are extended in the mining area, including longitudinal faults trending northeast-southwest parallel to the extension of the Kuh-e-Sarhangi and NW-SE trending transverse faults almost perpendicular to the first group. According to studies (Sahandi et al., 2010; Nozaem, 2012), the Kuh-e-Sarhangi and Delkan areas have undergone multiple tectonic regime shifts between transpressional tectonic phases and extensional phases accompanied by volcanism and mineralization. It seems that the Silurian extensional phases in the longitudinal faults of the area under study have played a significant role in creating a suitable space for the intrusion of monzonite stock, which ultimately gave rise to the formation of proximal IOA and massive magnetite mineralization. Transverse faults have also played the role of escape routes for part of the hydrothermal fluid, caused the formation of distal mineralizations. Geochemistry of stable isotopes The isotopic values of δ18O for magnetite samples in ranges from 8.6 and 10‰. According to several people (Einaudi et al., 1981; Bowman, 1998; Meinert et al., 2005), these values indicate a Juvenile origin for the hydrothermal fluid that caused the mineralization of massive magnetites without apatite. The values of δ18O for quartz samples are between 15.6 and 16.2‰. These values indicate isotopic equilibrium between the hydrothermal fluid and the host rock during the gradual cooling processes of the fluid. The isotopic δ34S values for pyrite samples ranging from 20.1 to 20.6‰. According to (Einaudi et al., 1981; Meinert et al., 2005), we consider the studied sulfur sources in the area of study to be non-magmatic and related to isotopic changes in the hydrothermal fluid in equilibrium with marine sulfates and host rocks as well. Discussion and Conclusion Two types of iron mineralizations (skarn and kiruna) occurred in the Delkan mine, but most iron reserves in this deposit share similar characteristics to iron skarn deposits. Mineralization in the Delkan iron deposit has been subjected by several factors, of which the most important are the following: A) The tectonic regime shifts between the transtensional and transcompressional regimes, which played a key role in intrusion and trapping of the intrusive stock, B) The direction and patterns of faults have been effective in determining the intrusion paths for both the plutonic stock and the juvenile hydrothermal fluids, mineralization type and locations of mineralizations, C) Differences in Oxidation state between the intrusive stock and the host rocks (especially the black carbonaceous phyllite layers that are highly reduced) plays a key role in the consumption of dissolved oxygen in the juvenile hydrothermal fluid due to intensity of decarbonation reactions between the fluid and these reducing layers, and as a result, the increase in the CO2 fugacity of the fluid, which will decrease the intensity of calc-silicate alterations (prograde skarn) and also a decrease in the amount of iron skarn mineralizations.

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