Geochemistry and Raman spectroscopic studies of Seh Qaleh agates, NW Birjand (Central Iran)

Document Type : Research Article

Authors

• Mehdi Rezaei-Kahkhaei
• Hadiseh Aghaeighojeh
• Farajollah Fardoost

Department Petrology and Economic Geology, Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran

10.22067/econg.v12i3.79434

Abstract

Introduction
The Seh Qaleh agates, located at 120 km NW Birjand, are parts of Central Iranian (Lut block) (Aghanabati, 2004) with geographic coordinates of 58˚ 00′ to 58˚ 30′ longitudes and 33˚ 00′ to 34˚ 00′ latitudes. The host rocks of the agates are Eocene-Oligocene tuff, andesite and basalt. Silica mineralization in the area has occurred inside the volcanic units in the form of filling cavity and fractures. Here, the agates have very attractive textures such as concentric, flow and dogtooth textures that are accompanied with jasper, amethyst, opal, calcite and gypsum.
Although Seh Qaleh agates are attractive and delightful, with high economical values, there is no scientific research about them. Therefore, their petrography, geochemistry and Raman spectroscopic characteristics are reported in the present paper for the first time.
 
Materials and methods
More than 400 samples of agates have been collected for this research study and five of these samples were in yellow, white, green, red and black colors. Moreover, four of these rocks were selected for major and trace elements analysis by XRF and ICP-MS. The samples were powdered in Tehran University by a tungsten carbide mill and analyzed in the Zarazma Company (Mashhad). XRD analyses and Raman spectroscopic studies on the agate of the Seh Qaleh area were done in Damghan and Shahrood University of Technology, respectively.
 
Petrography and Raman spectroscopy
The combination of different analytical techniques such as polarizing microscope, XRD and Raman spectroscopy provided information about the distribution of silica phases in the Seh Qaleh agates. Polarizing microscopy was used here to distinguish between the chalcedony and quartzine fibrous varieties. Moganite has similar optical properties with chalcedony, whose presence in agate is difficult to reveal. Thus, Raman studies were used to investigate these structural disparities. Raman spectroscopic studies showed that moganite and chalcedony can be distinguished based on their different spectral characteristics (Fig. 4, B, C). The use of a focused laser beam (diameter 1 μm) enabled us to analyze the variations in phase composition in the μm-range.
The measurement of Seh Qaleh agates by Raman spectroscopy provided an overview of the quantitative distribution of moganite in the studied samples, in which the fibrous chalcedonies contain more moganite in comparison with nodule chalcedonies (Fig. 4, B, C). The presence and spatial distribution of different silica phases in the Seh Qaleh agates is a result of the primary crystallization processes such as temperature and chemistry conditions (Götze, 2011). Moreover, the presence of moganite and calcite in the Seh Qaleh, confirmed by petrography and XRD studies suggest that the agates had formed in an arid, alkaline environment.
 
Geochemistry of the agates
Major and trace elements can be incorporated into the agates by substitution of Si by Al, Fe, Na, and Ca and as inclusions or fluid inclusions. The substitution of these elements are limited due to the small number of ions that have similar ionic radii and valence and can substitute for Si⁴⁺ in the crystal structure. The Seh Qaleh agates have 95.78 to 98.9 wt.% SiO₂ with minor amounts of Al₂O₃ (0.01-0.34 wt.%), Fe₂O₃ (0.01-1.07 wt.%), Na₂O (0.11-0.15 wt.%), and CaO (0.01-0.4 wt.%), supplied from alteration of the volcanic host rocks.
The high concentrations of U in some of the agates of the study area (especially ~ 38 ppm in the red one) are surprising and propose the operation of specific processes for mobilization, transport and deposition. These processes caused concentrations of U in quartz and chalcedony that can exceed the concentration of U in the Seh -Qaleh volcanic rocks (Table 1).
Zielinski (1979) observed a parallel accumulation of Si and U and investigated the mobility of U during the alteration of volcanic rocks. Based on the theory that the transport of chemical compounds is mainly realized by diffusion processes in aqueous fluids (e.g. Si as monomeric silicic acid Si(OH)₄), Porter and Weber (1971) inferred for the uranyl ion a complex with monomeric silica UO₂SiO(OH)³⁺. In addition, the presence of calcite, as associated mineral with silica polymorphs and the concentration of Na, K and Ca elements in agates, indicate that volatile chloride compounds might play a role in the alteration of volcanic rocks as well as the mobilization and transport of SiO₂ and other chemical compounds (Götze et al. 2012).
 
Acknowledgements
Thanks to the Shahrood University of Technology for supporting this project under grants provided by the research council.
 
References
Aghanabati, S.A., 2004. Iran Geology. Geological Survey of Iran, Tehran, 400 pp.
Götze, J., 2011. Agate-fascination between legend and science. In: J. Zenz (Editor), Agates III. Bode Verlag GmbH, Lauenstein, Germany, pp. 19–133.
Götze, J., Schrön, W., Möckel, R. and Heide, K., 2012. The role of fluids in the formation of agate. Chemie der Erde, 72(3): 283–286.
Porter, R.A. and Weber Jr, W.J., 1971. The interaction of silicic acid with iron (III) and uranyl ions in dilute aqueous solution. Journal of Inorganic and Nuclear Chemistry, 33(8): 2443-2449. Porter, R.A. and Weber Jr, W.J., 1971. The interaction of silicic acid with iron (III) and uranyl ions in dilute aqueous solution. Journal of Inorganic and Nuclear Chemistry, 33(8): 2443–2449.
Zielinski, R.A., 1979. Uranium mobility during interaction of rhyolitic obsidian, perlite and felsite with alkaline carbonate solution. Chemical Geology, 27(1–2): 47–63.

Keywords

• Uranium
• Geochemistry
• Raman spectroscopy
• Agate
• Seh Qaleh
• South Khorasan
• Central Iran

20.1001.1.20087306.1399.12.3.3.4

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