University of Isfahan Petrological Journal 2228-5210 17 1 2026 03 21 Tourmaline geochemistry in the granitoids of the Sanandaj–Sirjan Zone (A case study of Astaneh and Aligudarz granitoids) Tourmaline geochemistry in the granitoids of the Sanandaj–Sirjan Zone (A case study of Astaneh and Aligudarz granitoids) 1 26 30161 10.22108/ijp.2025.147492.1376 FA Farimah Ayati Associate Professor, Department of Geology, Payame Noor University, Tehran, Iran Maryam Ahankoub Associate Professor, Department of Geology, Payame Noor University, Tehran, Iran Journal Article 2025 11 17 Introduction Tourmaline exhibits a broad range of chemical variability (Hawthorne and Henry, 1999). This diverse and complex chemistry reflects a clear genetic link between tourmaline and the environment from which it crystallized. Such a relationship provides a reliable and robust indicator for determining the origin and evolution of hydrothermal systems responsible for ore formation, and serves as an effective guide in exploration for ore deposits (Jiang et al., 1995). The Astaneh and Aligudarz granitoid plutons are situated within the Sanandaj–Sirjan zone. A brief overview of relevant studies carried out around the investigated areas is provided below: Tahmasebi et al. (2009) investigated tourmalinization within the Astaneh granitoid pluton. The relatively flat REE patterns observed in tourmaline nodules reflect mobilization of light rare earth elements due to alteration processes. Esna-Ashari et al. (2012) conducted geochemical and geochronological studies on the Aligudarz granitoid complex. They suggest that these granitoids are analogous to I-type, crust-derived granitoids formed in continental arc settings. U–Pb zircon isotopic data yield a crystallization age of 165 Ma (Jurassic) for the granites. Given the geological significance of this mineral, the objective of the present study is to investigate the petrography and mineralogy of tourmaline, to evaluate its paragenetic relationships, to characterize the geochemical features of the tourmalines, to determine their structural formulas, and to reconstruct the conditions of formation in Aligudarz and Astaneh regions. Regional Geology The Astaneh granitoid pluton, in the Markazi Province, intruded the Jurassic shales and sandstones, which gave rise to contact metamorphism. Biotite granites represent another lithological unit in the region which exhibit minimal alteration and appears to be younger than the surrounding rock units. Geological features of this pluton include quartz–sulfide and quartz–tourmaline veins. Circulating meteoric and magmatic fluids have induced hydrothermal alteration within the intrusive body and resulted in the development of various alteration zones, including phyllic, propylitic, intermediate argillic, and silicic alteration. The Aligudarz granitoid complex formed during the Middle Jurassic along an active continental margin (Esna-Ashari et al., 2012). The lithological composition of this complex varies from quartz diorite to granodiorite and granite. Tourmalinization in these outcrops occurs within alteration halos adjacent to intrusive bodies and is directly related to hydrothermal systems generated by the interaction of the intrusion with pelitic host rocks. Materials and Methods Some standard thin sections were prepared and examined using an OLYMPUS BH2 polarizing microscope. Six tourmaline samples were analyzed using a CAMECA SX-100 electron probe microanalyzer (EPMA) in Iranian Mineral Processing Research Center (Karaj) under operating conditions of 15 kV accelerating voltage and 20 nA beam current. Sixteen analytical spots were measured for each sample, and the results were reported as oxide concentrations of the constituent elements. Petrography Tourmalines in the study area can be categorized into the following types: 1. Nodular tourmalines: This type occurs as discrete nodules developed on the surfaces of granitoid rocks and its formation is attributed to the crystallization of late-stage magmatic fluids filling the fractures of the host rocks (Rozendaal and Bruwer, 1995; Morgan). 2. Layered tourmalines: developed within metamorphosed rocks of the region and are found at the contact zone between the intrusive body and the host rocks. 3. Disseminated tourmalines: appear as fine-grained disseminations within the groundmass of the rocks. Potassic alteration has affected plagioclase crystals, leading to their transformation into tourmaline and orthoclase. The released Na is incorporated into the tourmaline structure, whereas K becomes part of the orthoclase lattice. 4. Pegmatitic Tourmalines: These tourmalines occur within pegmatitic veins located in the contact zone between the intrusive body and the host rocks. 5. And finally patchy (irregular) tourmalines: appear in hand specimens as irregularly shaped grains that vary from coarse- to fine-grained. Chemical Composition Based on the dominant ions occupying the X site, specifically Na⁺(K), Ca, and the degree of X-site vacancy, the analyzed tourmalines fall into alkali tourmaline group. The concentration of K + Na in the X site is significantly higher than that of Ca, and that the X site in the studied tourmalines is partially vacant. The X-site vacancy in the samples ranges from 0.26 to 0.49. To determine tourmaline type, binary diagrams of Na/(Na + Ca) and Fe/(Fe + Mg) were used (Trumbull and Chaussidon, 1999). In these diagrams, samples from the Astaneh area plot mainly within the schorl field, while those from Aligudarz area fall largely within the dravite field, trending towards schorl. Discussion Based on the variations of FeO/(FeO+MgO) relative to MgO, all tourmalines from the Astaneh and a portion of Aligudarz samples represents compositions intermediate between magmatic and hydrothermal origins, indicating their formation during the mixing of magmatic and hydrothermal fluids. In contrast, several tourmalines from the northern Aligudarz region, corresponding to tourmalines formed at greater distances from the intrusive body. As the Figure 11 displays, the tourmalines fall within the compositional field of metapsammites coexisting with an Al-saturated phase. This suggests that the fluids responsible for their formation were aluminum-rich, consistent with derivation from Al-enriched hydrothermal solutions. Conclusion The studied tourmalines occur in several distinct forms, including nodular, layered, disseminated, pegmatitic, and patchy/irregular varieties.The analyzed tourmalines belong to the alkali group, reflecting the dominance of K + Na over Ca in the X site and the presence of a measurable X-site vacancy. The tourmalines under study represent compositions within the schorl–dravite solid-solution series, exhibit chemical zoning, and display Al substitution in the Y site. According to FeO/(FeO+MgO) versus MgO systematics, all samples from the Astaneh region and a portion of those from Aligudarz indicating combined magmatic and hydrothermal influences, with meteoric water involvement during the final stages of formation. Several tourmalines from northern Aligudarz consistent with formation farther from the intrusive body. The primary source of boron is interpreted to be metapelites and metapsammites. Their assimilation into the granitoid magma likely contributed essential components such as B, F, Al, Mg, and Fe, trace amounts of other elements, thereby facilitating tourmaline formation within the investigated regions. Introduction Tourmaline exhibits a broad range of chemical variability (Hawthorne and Henry, 1999). This diverse and complex chemistry reflects a clear genetic link between tourmaline and the environment from which it crystallized. Such a relationship provides a reliable and robust indicator for determining the origin and evolution of hydrothermal systems responsible for ore formation, and serves as an effective guide in exploration for ore deposits (Jiang et al., 1995). The Astaneh and Aligudarz granitoid plutons are situated within the Sanandaj–Sirjan zone. A brief overview of relevant studies carried out around the investigated areas is provided below: Tahmasebi et al. (2009) investigated tourmalinization within the Astaneh granitoid pluton. The relatively flat REE patterns observed in tourmaline nodules reflect mobilization of light rare earth elements due to alteration processes. Esna-Ashari et al. (2012) conducted geochemical and geochronological studies on the Aligudarz granitoid complex. They suggest that these granitoids are analogous to I-type, crust-derived granitoids formed in continental arc settings. U–Pb zircon isotopic data yield a crystallization age of 165 Ma (Jurassic) for the granites. Given the geological significance of this mineral, the objective of the present study is to investigate the petrography and mineralogy of tourmaline, to evaluate its paragenetic relationships, to characterize the geochemical features of the tourmalines, to determine their structural formulas, and to reconstruct the conditions of formation in Aligudarz and Astaneh regions. Regional Geology The Astaneh granitoid pluton, in the Markazi Province, intruded the Jurassic shales and sandstones, which gave rise to contact metamorphism. Biotite granites represent another lithological unit in the region which exhibit minimal alteration and appears to be younger than the surrounding rock units. Geological features of this pluton include quartz–sulfide and quartz–tourmaline veins. Circulating meteoric and magmatic fluids have induced hydrothermal alteration within the intrusive body and resulted in the development of various alteration zones, including phyllic, propylitic, intermediate argillic, and silicic alteration. The Aligudarz granitoid complex formed during the Middle Jurassic along an active continental margin (Esna-Ashari et al., 2012). The lithological composition of this complex varies from quartz diorite to granodiorite and granite. Tourmalinization in these outcrops occurs within alteration halos adjacent to intrusive bodies and is directly related to hydrothermal systems generated by the interaction of the intrusion with pelitic host rocks. Materials and Methods Some standard thin sections were prepared and examined using an OLYMPUS BH2 polarizing microscope. Six tourmaline samples were analyzed using a CAMECA SX-100 electron probe microanalyzer (EPMA) in Iranian Mineral Processing Research Center (Karaj) under operating conditions of 15 kV accelerating voltage and 20 nA beam current. Sixteen analytical spots were measured for each sample, and the results were reported as oxide concentrations of the constituent elements. Petrography Tourmalines in the study area can be categorized into the following types: 1. Nodular tourmalines: This type occurs as discrete nodules developed on the surfaces of granitoid rocks and its formation is attributed to the crystallization of late-stage magmatic fluids filling the fractures of the host rocks (Rozendaal and Bruwer, 1995; Morgan). 2. Layered tourmalines: developed within metamorphosed rocks of the region and are found at the contact zone between the intrusive body and the host rocks. 3. Disseminated tourmalines: appear as fine-grained disseminations within the groundmass of the rocks. Potassic alteration has affected plagioclase crystals, leading to their transformation into tourmaline and orthoclase. The released Na is incorporated into the tourmaline structure, whereas K becomes part of the orthoclase lattice. 4. Pegmatitic Tourmalines: These tourmalines occur within pegmatitic veins located in the contact zone between the intrusive body and the host rocks. 5. And finally patchy (irregular) tourmalines: appear in hand specimens as irregularly shaped grains that vary from coarse- to fine-grained. Chemical Composition Based on the dominant ions occupying the X site, specifically Na⁺(K), Ca, and the degree of X-site vacancy, the analyzed tourmalines fall into alkali tourmaline group. The concentration of K + Na in the X site is significantly higher than that of Ca, and that the X site in the studied tourmalines is partially vacant. The X-site vacancy in the samples ranges from 0.26 to 0.49. To determine tourmaline type, binary diagrams of Na/(Na + Ca) and Fe/(Fe + Mg) were used (Trumbull and Chaussidon, 1999). In these diagrams, samples from the Astaneh area plot mainly within the schorl field, while those from Aligudarz area fall largely within the dravite field, trending towards schorl. Discussion Based on the variations of FeO/(FeO+MgO) relative to MgO, all tourmalines from the Astaneh and a portion of Aligudarz samples represents compositions intermediate between magmatic and hydrothermal origins, indicating their formation during the mixing of magmatic and hydrothermal fluids. In contrast, several tourmalines from the northern Aligudarz region, corresponding to tourmalines formed at greater distances from the intrusive body. As the Figure 11 displays, the tourmalines fall within the compositional field of metapsammites coexisting with an Al-saturated phase. This suggests that the fluids responsible for their formation were aluminum-rich, consistent with derivation from Al-enriched hydrothermal solutions. Conclusion The studied tourmalines occur in several distinct forms, including nodular, layered, disseminated, pegmatitic, and patchy/irregular varieties.The analyzed tourmalines belong to the alkali group, reflecting the dominance of K + Na over Ca in the X site and the presence of a measurable X-site vacancy. The tourmalines under study represent compositions within the schorl–dravite solid-solution series, exhibit chemical zoning, and display Al substitution in the Y site. According to FeO/(FeO+MgO) versus MgO systematics, all samples from the Astaneh region and a portion of those from Aligudarz indicating combined magmatic and hydrothermal influences, with meteoric water involvement during the final stages of formation. Several tourmalines from northern Aligudarz consistent with formation farther from the intrusive body. The primary source of boron is interpreted to be metapelites and metapsammites. Their assimilation into the granitoid magma likely contributed essential components such as B, F, Al, Mg, and Fe, trace amounts of other elements, thereby facilitating tourmaline formation within the investigated regions.

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