Magnetite and pyrite chemistry of the Kuh-e-Kapout porphyry copper deposit, southern part of the Urumieh-Dokhtar magmatic arc, north of Bam, Kerman province

Introduction
Porphyry deposits are commonly associated with calc-alkaline to alkaline magmas (Seedorff et al, 2005). Oxidized magmas are essential for efficiently transporting copper, gold, molybdenum, and sulfur from the metasomatized mantle to the upper crust (Richards, 2015). Hydrothermal fluids released from magmas, particularly those of intermediate composition, form a series of magnetite-bearing quartz veins and potassic alteration complexes within and around intrusive rocks (Holliday and Cooke, 2007). Magnetite is an indicator mineral for porphyry deposits (Cooke et al, 2020). The presence of primary magnetite as phenocrysts or groundmass phases indicates the oxidized state of the magmas. The formation of hydrothermal magnetite in porphyry deposits involves the reaction between ferrous iron and water or sulfate, and leads to a decrease in pH and oxygen fugacity in the fluid, which causes pyrite precipitation (Sun et al, 2013). Studies on this mineral found that hydrothermal magnetites have high values of the Mg-Mn factor, in contrast, igneous magnetite can be distinguished by high values of Co, Ni, and V (Nadoll et al, 2012). Some studies focusing on the chemistry of trace elements of magnetite have indicating the factors affecting compositional differences in iron oxide and mineralization (Pisiak et al, 2017). The role of pyrite as an indicator of fluid composition changes in porphyry deposits has also been used (Reich et al, 2013). Therefore, changes in the hydrothermal fluid's physicochemical conditions and the mineral assemblages' thermodynamic stability cause in changes in the trace element content of the ore and gangue sulfides (here, the pyrite). The aim of this study is to investigate the trace element composition of igneous and hydrothermal magnetite and pyrite using electron microprobe analysis (EMPA) in the potassic, propylitic, and phyllic alteration zones and to investigate changes in the formation conditions of these minerals in the Kuh-e-Kapout porphyry copper deposit.
Materials and Methods
In order to petrological, mineralogical and chemical mineral studies, sampling was carried out from the cores of the quartz diorite unit (samples number NT-6-729 and NT-3-583) and microdiorite (sample number NT-4-638).
At this stage, deep magnetite samples were selected from the potassic zone; also, a representative sample for electron microprobe analysis was selected from the surface microdiorite sample in the propylitic zone (NT-S-26). After preparing thin-polished sections of the samples, the samples were analyzed at the University of Leuven, Austria, with an EMPA Superprobe Jeol JXA 8200 electron microscope to determine the chemistry of magnetite. BSE images of the samples were also obtained using the same device. The measured values were calculated as weight percent using the Integrated Jeol Software of the aforementioned device. In order to perform EMPA studies on pyrite samples from quartz + pyrite veins, sample NT-3-195 of the quartz-diorite in phyllic alteration zone was used. Standards of pyrite were used to measuring Cu, Fe, and S. For other elements such as Au, Te, Zn, Ag, Se, As, and Co, the Jeol JXA 8200 device was used. The same device (Jeol JXA 8200) was used to prepare the WDS elemental map of pyrite and magnetite minerals.
Results and Discussion
Based on the data obtained from magnetite analysis, the amounts of Fe, Ti, V, and Al in the samples are higher than other elements, and the amounts of Cu, Mo, Ni, Ca, Co, Pt, Mg, and Pt are often lower than the detection limit. The Fe values range is between 40.88 to 68.92 wt%. The Fe average value (weight percent) in the fertile quartz diorite samples are 67.244, which is approximately similar to other porphyry copper deposits in the Urumieh-Dokhtar magmatic arc in Iran. The highest Al value is 0.318, and the lowest is 0.087 wt%, the highest V value is 2.722 wt%, the lowest is 0.181 wt%, and about Ti, the highest value is 20.66 wt%, and the lowest is 0.060 wt%. The microdiorite sample point the average V values in quartz diorite samples from the Kuh-e-Kapout porphyry copper deposit is 0.229 wt%, which is in good agreement with the V values of other porphyry copper deposits in the Urumieh-Dokhtar magmatic arc. NT-S-26 An4 simultaneously shows the highest titanium content (20.661 wt%) and the lowest iron content (40.880 wt%). Studied pyrite minerals contain a wide range of trace elements in their structure, Cu, Fe, As, S, Zn, and Pb are more important. The average weight percentage of sulfur is 21.54, and iron is 44.46. The average value of arsenic is 0.0193, and the average value of copper is 0.0063. The range of variation in the amounts of iron and sulfur, which are the main components of pyrite, is from 46.07-46.95 and 53.56-54.74 wt%, respectively. The amounts of Co, Os, Ti, and Pt elements are below the detection limit in most points. The amounts of major elements, iron, and sulfur, in the pyrites of the Kuh-e-Kapout copper deposit, are similar to other porphyry copper deposits in the Urumieh-Dokhtar magmatic arc. In the vanadium versus titanium diagram, the magnetite samples of quartz diorite are located in the hydrothermal and magmatic magnetites zones with a tendency towards the magmatic zone, and the microdiorite sample is located at the border of the magmatic magnetite zone. Using the V/Ti-Fe diagram, determined that the quartz-diorite samples reequilibrated, which could be due to the development of the potassic alteration zone and the presence of continuous stages of hydrothermal fluid exsolved during this alteration. The results of the magnetite data plot on the Mg+Al+Si vs. Ti diagram for both magnetite series indicate that no significant interaction between the magmatic fluid and the wall rock occurred during the formation and crystallization of magnetite. Studies shown that the amount of vanadium in magnetite is one of the most important indicators for measuring the oxygen fugacity (fO₂) of magma or hydrothermal fluid during magnetite crystallization (Nadoll et al, 2014; Knipping et al, 2015). At higher fO₂ conditions, the magnetites do not have high levels of vanadium (Canil and Lacourse, 2020). This change is due to the preference of V⁺³ in the magnetite structure over V⁺⁴ and V⁺⁵ in the under reduced conditions. Vanadium levels can also be affected by temperature and the reaction of the hydrothermal fluid and wall rock during magnetite crystallization (Knipping et al, 2015; Zarasvandi et al, 2023b). The petrography of the Kuh-e-Kapout magnetites and the chemistry of this mineral in the potassic zone indicate that mineralization occurred under oxidizing conditions. This observation is reflected in the occurrence of hematite rims in magnetite grains. This rim ty pically indicates magmatism with oxidizing conditions in the magnetite-hematite buffer zone (Liang et al, 2009). On the other hand, the abundance and presence of scattered magnetite grains in the groundmass and copper mineralization veins are significant. Also, the scattered occurrence and paragenesis of anhydrite with magnetite as a sulfate mineral indicate the occurrence of magmatism with high oxygen fugacity. Pyrite is a common mineral in a wide range of hydrothermal deposits. Its deposition can effectively control the segregation of a wide range of economically and environmental importance trace elements (Large et al, 2009). While As is a structurally limited element 
in pyrite, Cu and Au can occur both in solid solution and as micro to nanoscale chalcopyrite and Au (or Au-tellurides) in pyrite. The averages concentration of arsenic in pyrite samples is 0.019%, and the nearly positive correlation between S and As in pyrite samples indicates that arsenic is not substituted sulfur, which may indicate the presence of an oxidizing environment in which arsenic is present as As⁺³. The lack of definitive correlation between Fe and Cu in pyrite samples indicates the lack of Cu⁺² substitution in the pyrite structure at the Fe⁺² site, which suggests that much of the Cu in pyrite is structurally replaces Fe in octahedral sites, which could be due to the presence of As, Sb, and Co in the pyrite structure.
 
Conclusion
In this study, the chemistry of magnetite and pyrite minerals of the Kuh-e-Kapout porphyry copper deposit were studied for the first time. Studies have been done on two series of quartz diorite and microdiorite dike-like intrusions. Studying on these minerals is important to understanding the physicochemical conditions of the deposit formation during the crystallization of magnetite and sulfide minerals. The data obtained from the EPMA showed that the minerals have a good agreement with other porphyry copper deposits of the Urumieh Dokhtar magmatic arc in terms of elemental abundance. The magnetites studied in Kuh-e-Kapout porphyry copper deposit are magmatic type and re-equilibrated and have high temperatures (more than 500 ° C). Studies on magnetite chemistry indicate that in tetms of genesis, Kuh-e-Kapout deposit is located in the porphyry deposit area. A comparison of the two magmatic systems of quartz diorite and microdiorite specifically shows a more fertile quartz diorite magmatism in the potassic zone and an isothermal system with weak mineralization evidence in the propylitic zone for the microdiorite intrusion. In this magmatic-hydrothermal system, the abundant occurrence of anhydrite and martitization of magnetite is evidence of a high fO₂ condition in the magmatic system of the region. The results of study on pyrite mineral indicate the presence of As⁺³ in the pyrite structure in the form of dispersed micro to nanoparticles, which is consistent with the oxidant conditions of mineral formation. The occurrence of copper in pyrite is in the form of replaces Fe in octahedral sites and in the form of chalcopyrite micro particles.