Origin and Genesis of the Color of Variscite in the Koushk Zn-Pb Deposit, Yazd Province: Evidence from Mineralogy, Geochemistry, and Fluid Inclusions

Document Type : Research Article

Authors

Kiamars Hosseini ¹

Mohammad Yazdi ²

Hossein Azizi ³

¹ Ph.D. student, Department of Minerals and Groundwater Resources, Shahid Beheshti University, Tehran, Iran

² Professor, Department of Minerals and Groundwater Resources, Shahid Beheshti University Tehran, Iran

³ Professor, Department of Mining, Faculty of Engineering, University of Kurdistan, Sanandaj, Iran

10.22067/econg.2026.1173

Abstract

The Koushk Zn-Pb deposit in Central Iran is considered the largest mineral potential for variscite mineralization. Macroscopic and microscopic observations indicated minor variscite mineralization occurring in veinlet-vein form and as disseminations, associated with massive and layered ore zones, as well as extensive mineralization related to zinc- and lead-barren marginal shales, dioritic sills, and late carbonate ± quartz veinlets. Textural features and chemical characteristics indicated that variscite formed through multi-stage phosphatization of Al-rich shales with minor silica. XRD data reveal variscite and strengite as the dominant phases. Other phosphates such as phosphosiderite, metavariscite, and crandallite were also identified, occurring as intergrowth mixtures with variscite. Color variations in the Koushk variscites are attributed to intergrowth with other phosphates, sulfates, or silicates; electronic transitions; crystal field transitions of chromophore elements; structural defects; and the arrangement of water molecules within the variscite structure. The highest intensity Raman peak observed in the spiderweb variscites occurs at approximately 1033 cm⁻¹, whereas in the simple green variscites and soapy variscites (pale green-blue to white), the strongest peak appears at around 1022 cm⁻¹. Three additional peaks with lower intensities are also present, all corresponding to the symmetric stretching vibrations of the phosphate (PO₄) group in minerals of the variscite–metavariscite group. The REE+Y patterns, LREE enrichment, and negative Ce anomaly indicate low-temperature hydrothermal fluids with a composition approaching seawater. Microthermometric studies of fluid inclusions reveal that fluid mixing and temperature decrease played a significant role in variscite mineralization, with formation temperatures ranging from 110 to 340 °C.

Introduction
The Kushk zinc-lead deposit is the most important variscite mineral mass in Central Iran. Variscite a relatively common mineral that forms as a result of phosphate-bearing surface solutions reacting with aluminum-rich rocks (Calas et al., 2005). First described in 1837 from a source close to Variscia, the former name of the Vogtland region in northeastern Germany, its chemical formula was determined in 1896 (Willing et al., 2008). Variscite was used in Neolithic jewelry and is found in many archaeological sites (e.g., Ervedosa, Portugal; Pannecé, France; and Sardinia, Italy; see Calas et al., 2005; Querré et al., 2019; Willing et al., 2008). Variscite is a member of the hydrous aluminum phosphate group, Al(PO₄)·2H2O, while strengite is part of the iron phosphate group, Fe3+(PO₄)·2H2O. Both are orthorhombic phosphate members of the variscite mineral group, while mansfieldite, scorodite, and yanomamite are the arsenate isostructural members of this group. The general chemical formula of variscite group minerals is A(XO₄)·2H₂O, where A = Al, Fe³⁺, In and X = P, As.

Regional Geology
The Koushk Zn–Pb deposit is situated within the upper part of the Lower Cambrian volcano‑sedimentary sequences, in the central part of the Zarigan–Chahmir Basin in Central Iran (Fig. 1). Two major stratigraphic sequences have been recognized in the area (Fig. 2): (1) a mineralized sequence comprising the Upper interval of the Lower Cambrian volcano‑sedimentary sequences at the base, and (2) an overlying volcano‑sedimentary sequence that includes Lower Paleozoic shales, argillaceous limestone, dolomitic units which are hosting Fe-mineralization, as well as the rhyolitic and tuffaceous rocks (Gibbs, 1976). Rhyolitic and dacitic domes are present in the southeastern part of the deposit, adjacent to the mineralized sequence, as well as to the north and northwest of the deposit.

Materials and Methods
In this study, 35 variscite samples were collected from ore-bearing rocks across different geological sections. Petrographic and mineralogical investigations were carried out on 23 thin and polished-thin sections using transmitted and reflected light microscopy at Shahid Beheshti University, Iran. In addition, 7 samples representing different generations of variscite were analyzed by using scanning electron microscopy (SEM) at Aria Electron Optics Company, Iran. To complement the mineralogical data, one variscite sample was analyzed via XRD at Kansaran Binalud Company, and three variscite samples of varying colors were examined using Raman spectroscopy at Shahid Beheshti University. For geochemical studies, a deep-green variscite sample was analyzed at 10 points using a CAMECA SXFive X‑ray Probe Microanalysis (XPMA) (France) at Kansaran Binaloud Company, Iran. Furthermore, 6 samples were analyzed by ICP-MS using a PerkinElmer NexION 300 instrument at Zarazma Laboratory, Iran. 10 samples were analyzed by XRF at Tarbiat Modares University of Iran. Petrographic examination and microthermometric studies of fluid inclusions were performed on 2 quartz and calcite samples associated with variscite mineralization in the marginal zone, using a Linkam THMCG 600 heating–freezing stage at the Microthermometry Laboratory of Tarbiat Modares University, Iran.

Results
Macroscopic and microscopic observations indicate that variscite mineralization occurring as veins, veinlets, and disseminated form associated with the massive and stratiform sulfide ore zone, as well as extensive mineralization related to the marginal zone and microdioritic sills. These variscites occur intergrown with strengite and minor amounts of other minerals belonging to the variscite and metavariscite groups. The color variations in these variscites are attributed to intergrowths with other phosphates, sulfates, or silicates, electronic transitions, crystal field transitions in chromophore elements, structural defects, and the arrangement of water molecules within the variscite structure. These factors may lead to significant fluctuations in both the green hues and the apparently low Cr³⁺ content. In the variscites from the Koushk Zn–Pb deposit, the Cr³⁺ content is low compared to that of similar worldwide variscites. However, our samples containing higher amounts of Cr³⁺ exhibit a more intense and deeper green color. The highest Raman spectral peak intensity show that spiderweb variscites occurs at approximately 1033 cm⁻¹, whereas in the simple green variscites and the soapy variscites (pale greenish-blue to white), the most intense peak occurs at around 1022 cm⁻¹. In addition, three lower-intensity peaks are present, all of which are attributed to the symmetric stretching vibrations of the phosphate (PO₄) group in minerals of the variscite and meta-variscite groups. The Raman spectra of variscite from the Koushk Zn–Pb deposit and Tajikistan show a remarkable overlap. The ΣREE values in the variscite ore are negligible. The REE+Y pattern, characterized by LREE enrichment and a negative Ce anomaly which is similar to that of seawater. Such evidence may indicate the involvement of low-temperature hydrothermal fluids, which is further supported by micro-thermometric studies of fluid inclusions, confirming the corresponding temperature range.

Discussion
In the Koushk deposit, aluminum was derived from the weathering of the host rock, whereas phosphate originated from the leaching of ancient sedimentary phosphorites or phosphorites contemporaneous with the carbonate rocks of the Soltanieh Formation, Ediacaran deposits, or evaporitic and phosphatic crustal units in Central Iran by magmatic and hydrothermal fluids related to the intrusion of granitoid bodies (Hosseini and Rajabzadeh, 2022). The interaction of aluminum-rich and phosphate-bearing hydrothermal fluids resulted in variscite mineralization. In the Koushk Zn–Pb deposit, variscite commonly displays a wide range of green hues, including bluish green, olive green, moss green, and pale bluish white. These color variations are not dependent on depth. The color variation in variscite is, in part, attributed to intergrowth with phosphates, silicates, sulfates, and oxides. SEM–BSE, EDS, and XRD studies confirm this interpretation. On the other hand, color variations in minerals are associated with various mechanisms, such as electronic transitions (Nassau, 1978), crystal field transitions in chromophores (Burns, 1993) such as Cr, Mn, Fe, Co, Ni, Cu, and V, charge transfer between elements (Mattson and Rossman, 1987; Bill and Calas, 1978), as well as color centers caused by structural defects in the crystal lattice and variations in the concentration of Cr³⁺ and V⁴⁺ (Krambrock et al., 2007). Therefore, the green color of variscite is related not only to the presence of combined structural impurities but also to crystal defects and the arrangement of water molecules within the variscite structure, which has led to the formation of different variscite polymorphs. Furthermore, color variations are attributed to metasomatic replacement processes and the development of zoning within the primary microcrystalline and porous minerals, driven by successive pulses of late-stage fluids (Díaz-Acha et al., 2022). Consequently, the color changes gradually along the veins and nodules. Some variscite samples from Koushk area exhibit a progressive color transition from a gray phosphosiderite core to a pale green unit (consisting of phosphosiderite-dominated mixtures with variscite) and, ultimately, to dark green margins dominated by variscite. The diversity in microscopic textures and ore geochemistry points to multistage mineralization and reflects dissolution–reprecipitation processes controlled by the prevailing physicochemical conditions during formation.

Acknowledgements
The authors appreciate Shahid Beheshti University Research Council that supported this work. The Director General and personal of the Koushk Mine Company are acknowledged for their assistance in the field works.

Keywords

Variscite

mineralogy

geochemistry

fluid inclusions

Koushk Zn-Pb Deposit

Central Iran

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