Mineralography, paragenesis sequence, and sulfur isotopic composition of Baba-Ali and Galali iron ore deposits from the Sanandaj-Sirjan zone, northwest Iran

Document Type : Research Article

Authors

• Majid Shirmohammadi ¹
• Ali Asghar Sepahi ²
• Mohammad Maanijou ³
• Ashraf Torkian ⁴
• Mirmohammad Miri ⁵
• Mahsa Tahmasebi ⁶

¹ Ph.D., Exploitation Geology Division, National Iranian South Oilfields Company, Ahvaz, Iran

² Professor, Department of Geology, Faculty of Sciences, Bu-Ali Sina University, Hamedan, Iran; Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

³ Professor, Department of Geology, Faculty of Science, Bu Ali Sina University, Hamedan, Iran

⁴ Associate Professor, Department of Geology, Faculty of Science, Bu Ali Sina University, Hamedan, Iran

⁵ Assistant Professor, Department of Geology, Faculty of Earth Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran

⁶ M.Sc., Department of Geology, Faculty of Science, Bu Ali Sina University, Hamedan, Iran

10.22067/econg.2024.85829.1098

Abstract

The northern part of the Sanandaj-Sirjan zone hosts important iron deposits formed during the Mesozoic magmatism. The Baba-Ali and Galali ore deposits occurred on the border of Hamedan, Kermanshah, and Kurdistan provinces. Ores in these deposits include magnetite together with pyrite, pyrrhotite, chalcopyrite, and hematite. The petrographic studies show at least two generations of magnetite and one generation of pyrite in the Baba-Ali, and one generation of magnetite and pyrite in the Galali ore deposits. The supergene processes replaced the magnetite with hematite, goethite, and limonite in the surface parts of the deposits. Field and mineralographic evidence show that the studied ore deposits are of skarn type. The δ³⁴S values of pyrite from the Baba-Ali and Galali deposits range from +6.8 to +13.3‰ and +6.1 to +7.3‰ respectively, and calculated δ³⁴S_H2S of the mineralizing fluid range from +7.8 to +14.3 ‰ and +7.1 to +8.3‰, respectively. The δ³⁴S values of the Baba-Ali and Galali ore deposits are higher than the magmatic source values for the skarn deposits. Therefore, these positive values indicate the incorporation of sulfur from both magma and host/wall rocks. The igneous bodies of the studied area are mainly composed of mafic to felsic intrusive rocks, which formed as a result of magmatism resulting from the subduction of the Neo-Tethys oceanic crust beneath the Central Iran Microcontinent (CIM) during the Late Jurassic-Early Cretaceous time. Due to the emplacement of dioritic magmas within the carbonate rocks of the region, iron mineralization (magnetite) of the skarn type has occurred with the metamorphism of the host rocks.
 
Introduction
The Sanandaj-Sirjanzone in western Iran contains many iron ore deposits. The Baba-Ali, Galali, Tekyeh-Bala, Chenar, and Khosrow-Abad iron ore deposits are located in the NW Hamedan and SE Songhor provinces (Fig. 1). In the present study mineralography, paragenetic sequence, and field relation of the Baba-Ali and Galali ore deposits are compared and investigated to identify the type of mineralization system. Subsequently, sulfur isotope data for the Baba-Ali and Galali ore deposits are presented and compared to understand the source of sulfur and their probable genetic relationship.
 
Materials and methods
Forty-five thin, thin-polished, and polished sections from the ore deposits were studied. Sixpyrite samples from the Baba-Ali and six pyrite samples from the Galali iron ore deposits were analyzed for their sulfur isotope contents using an isotope ratio mass spectrometer (IRMS, IsoPrime100) at Tsukuba University, Japan. The CDT (Canyon Diablo Troilite) standard sample was used to measure sulfur isotopes.
 
Results
Petrography
The Baba-Ali iron ore deposit is mainly composed of magnetite (two generations). The first-generation magnetite is formed massively and the second-generation is formed as vein. Pyrite is the most abundant sulfide mineral in the Baba-Ali ore deposit which is formed in one generation. Pyrite crystals are often anhedral to subhedral in shape and are usually formed together with magnetite. In some samples, large crystals of pyrite show evidence of crushing resembling the cataclastic fabric. Other sulfide minerals such as pyrrhotite and chalcopyrite are present as anhedral crystals and sometimes have magnetite patches. The mineralography of the Galali ore deposit is similar to the Baba-Ali ore deposit. Magnetite as the main ore mineral occurs in one generation including massively anhedral crystals. There is also one generation of pyrite which mainly includes euhedral and large crystals (up to 2 cm in diameter). Goethite, limonite, and hematite have been formed as a result of supergene processes.
 
Discussion
Paragenetic sequence
The paragenetic sequences of the studied ore deposits are similar (Fig. 8). Field, mineralographic, and paragenetic similarities of the studied ore deposits may indicate their probably similar genesis. The difference is because of the size of the ore deposits; the Baba Ali is the largest one and has the most diverse mineral assemblage.
 
Sulfur isotopes
The δ³⁴Svalues of the pyrite from the Baba-Ali and Galali iron ore deposits are presented in Table 2. Except for two samples (BS-304, BS-303), the δ³⁴S values have a limited range and are similar. The δ³⁴S values of the Baba-Ali and Galali ore deposits range from +6.8 to +13.3‰, and +6.1 to +7.3‰, respectively. According to Ohmoto and Rye (1979), the main composition of sulfur in ore-forming fluids is often in the form of H₂S. Therefore, to measure the Δ³⁴S_H2S of the fluid in equilibrium with the pyrite, the formula proposed by Ohmoto and Rye (1979) was used. The calculated δ³⁴S_H2S values of the ore-forming fluid (T=375 ˚C) in the studied deposits (Baba-Ali and Galali) range from+7.8 to +14.3‰and +7.1 to +8.3‰, respectively. The T=375 ˚C is considered based on the average temperature values of the Baba-Ali formation (Zamanian et al., 2007; Sarjoughian et al., 2020). In the same way, similar temperature conditions (T=375 ˚C) have been assumed for the Galali ore deposit.
The sulfur isotopic values compared to the isotopic ranges in the geological systems (Rollinson, 1993) are shown in Fig. 9. Based on the petrographic evidence and the paragenesis sequence, the pyrites formed in the retrograde stage and at a T<400 ˚C. In such conditions, sulfur should exist as H₂S which is also reported from the Korkora-1 skarn deposit of the Shahrak region (Sepahi et al., 2020). The positive δ³⁴S values indicate the contribution of sulfur from a source richer in δ³⁴S than the magmatic origin. This feature has been reported in many skarn deposits (Ishihara et al., 2002; Peng et al., 2016; Zhang et al., 2017). Mixing of meteoric waters with ore-forming fluids can change the physico-chemical conditions of the fluids. However, this process can affect the formation of ore deposits and their accompanying sulfides. Therefore, it can be concluded that the main source of sulfur in the Baba-Ali and Galali ore deposits was possibly from magma that later increased due to the interaction with sedimentary and metamorphic host/wall rocks.
 
Conclusions

the Baba-Ali iron ore deposit is comprised of two magnetite and one pyrite generation. The Galali ore deposit contains one generation of magnetite and pyrite. Supergene processes have formed hematite, limonite, and goethite.
The enriched δ³⁴S values in the Baba-Ali and Galaliore deposits indicate that the main source of sulfur was from magma. However, interaction with the host (or wall) carbonate or metasedimentary rocks and probably meteoric waters caused the enrichment compared to a purely magmatic origin.
The studied iron ore deposits are of skarn type probably formed by a series of processes including injection of dioritic magma, interaction with the host rocks, change of physicochemical condition of ore-forming fluids, and mixing of the fluids with meteoric waters. Nevertheless, more isotopic data (i.e., Oxygen isotope) are required for confirmation of these issues.

Keywords

• Iron deposit
• Sulfur isotopic
• Paragenesis
• Skarn
• Sanandaj-Sirjan

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