Stable isotope (O, C) and geochemical constraints of mineralization in the Qamishlu lead deposit, Isfahan, Iran

Introduction
Lead–zinc deposits in Iran are distributed across four major structural zones. These tectonic units include: 1) the Sanandaj–Sirjan Zone (SSZ), hosting deposits such as Irankuh (Ghazban et al, 1994), Tiran (Nejadhadad et al, 2018), Dareh Noghreh (Nejadhadad et al, 2023), and Angouran (Boni et al, 2007); 2) the Yazd–Anarak Metallogenic Belt (YAMB) in central Iran, including the world-class Mehdiabad MVT¹ Zn–Pb deposit (Reichert et al, 2008; Nejadhadad et al, 2025) and Nakhlak lead mine (Jazi et al, 2017); 3) the Tabas–Posht-e Badam area, hosting Pb–Zn–Ba deposits such as Ozbak-Kuh (Ehya et al, 2014); and 4) the Central Alborz Range, containing deposits like Duna and Ellika (Zabihitabar et al, 2015). The Qamishlu lead deposit is part of the Isfahan–Malayer Pb–Zn metallogenic belt within the Sanandaj–Sirjan metamorphic zone. In this deposit, mineralization occurs predominantly in lower Cretaceous massive limestone, localized along fault surfaces, shear zones, and contacts between Cretaceous carbonate and shale units. The average Zn/(Zn+Pb) ratio is less than 0.1, classifying the mineralization as Pb-rich, analogous to the Viburnum Trend in the USA and the Ravanj deposits in Iran (Plumlee et al, 1994; Nejadhadad et al, 2016). Silver concentrations in pure galena samples average 660 ppm, corresponding to the 50th–75th percentile range of Pb–Zn deposits. A strong Sb–Ag correlation (r = 0.93), compared to moderate As–Ag (r = 0.66) and Cu–Ag (r = 0.6) correlations, suggests that Ag enrichment is controlled by both lattice-bound silver in galena and sulfosalt-hosted phases, including jordanite and the tetrahedrite–freibergite group. Alteration at Qamishlu primarily comprises host rock dolomitization, silicification, and late-stage open-space-filling calcite. Systematic stable isotope analyses (δ¹⁸O and δ¹³C) of late-stage calcite, pure dolomite, dolomitized and silicified mineralized host rocks, weakly altered non-mineralized rocks, and distal fresh limestone indicate isotopic mixing between hydrothermal fluids depleted in ¹⁸O but enriched in organic carbon and the carbonate host rocks. The strong spatial association of mineralization with NE–SW-trending faults, combined with isotopic variations in altered zones, suggests that these structures acted as primary fluid conduits. Open spaces in shear zones along normal faults, coupled with interaction between ore fluids and carbonate host rocks, modified the physicochemical conditions of the metal-bearing fluids, ultimately resulting in the deposition of epigenetic mineralization.
Materials and Methods
Field Investigation and Sampling Methodology
Comprehensive fieldwork was systematically carried out across the ore-bearing zone and adjacent areas, including: 1) detailed geological mapping, 2) structural analysis of fault systems and fracture networks, and 3) representative sampling of all lithological units. Special emphasis was placed on collecting specimens with varied textural features (e.g., massive, brecciated, and vein-type mineralization) from both mineralized and unaltered rocks.
Laboratory Processing and Analytical Techniques
 
A total of 30 thin and polished sections were prepared from the collected samples. Additionally, 12 hand-picked galena specimens were carefully purified under a binocular microscope and submitted to LabWest Laboratory, Western Australia, for ICP-MS analysis. Samples of fresh limestone, mineralized limestone, pre-ore calcite, late-stage calcite, mineralization-related dolomite, quartz, and silicified host rocks were similarly purified through manual separation under a standard binocular light microscope and subsequently sent to the Cornell Isotope Laboratory (COIL), Cornell University, USA, for carbon and oxygen isotope analyses
Results and Discussion
Geological constraints
The Qamishlu lead deposit is a carbonate-hosted Pb–Zn deposit situated within the Isfahan–Malayer lead–zinc belt, part of the Sanandaj–Sirjan metamorphic zone in Iran (Fig. 1). This study demonstrates that multiple, interacting factors controlled the localization of mineralization. Ore emplacement is influenced by lithological, stratigraphic, and structural controls, which govern fluid flow at both regional and deposit scales, facilitating fluid focusing and the development of open spaces necessary for ore deposition (Nejadhaddad et al, 2023).
The deposit is classified as a vein-type system and formed epigenetically relative to the Cretaceous carbonate host rocks. Limestone, the dominant host lithology, is commonly associated with Mississippi Valley-type (MVT) base-metal sulfide deposits (Leach et al, 2010). In Qamishlu, mineralization occurs within Cretaceous carbonates overlying Jurassic to Cretaceous shale–sandstone sequences (Fig. 2). Shale and carbonate–shale units act as impermeable aquitards within the stratigraphic column, playing a critical role in channeling hydrothermal fluids (Leach et al, 2005).
Structural features, including joints and fractures related to fault activity, significantly enhanced fluid flow and created open spaces for ore deposition. Most mineralization is concentrated within NE–SW-trending fault veins and associated fracture networks, indicating that faulting and brecciation of Upper Cretaceous carbonates were key in generating structural conduits for hydrothermal fluids.
The mineralogy of the Qamishlu deposit is relatively simple. Primary ore minerals, in order of abundance, include galena, pyrite, sphalerite, tetrahedrite, and chalcopyrite. The dominant gangue phases are calcite, barite, dolomite, and quartz. Secondary supergene minerals comprise cerussite, iron oxides (mainly limonite), smithsonite, covellite, and malachite
Galena geochemistry
In the Qamishlu deposit, galena is more abundant than sphalerite, similar to Southeast Missouri lead deposits (Sverjensky, 1986). The deposit is classified as Pb-rich, with a Zn/(Zn+Pb) ratio below 0.1. In addition to Pb and S, silver represents the most economically significant by-product in galena due to its relative abundance and high market value (Zeng et al, 2000). Minor trace elements, including antimony, bismuth, arsenic, zinc, cadmium, selenium, and copper, are also present within galena.
The average Ag content in galena samples from Qamishlu is approximately 660 ppm. A strong correlation is observed between Ag and Sb (r = 0.84), while moderate correlations exist with As (r = 0.66) and Cu (r = 0.6) (Table 2). Silver occurs in galena both as a solid solution and as inclusions of sulfosalt minerals such as jordanite and tetrahedrite (Gregory et al, 2014; Lan et al, 2023).
Stable Isotopes (O, C)
The δ¹⁸O values in altered rocks reflect multiple factors, including the initial δ¹⁸O of the host rock, the isotopic composition of the fluid, the temperature of fluid–rock interaction, and the degree of equilibrium achieved during alteration (Sánchez-España et al, 2003; Bortnikov, 2006; Nejadhadad et al, 2023).
In Qamishlu, δ¹⁸O values in altered carbonates (silicified and dolomitized limestones) are lower than in distal, unaltered carbonate rocks. Unmineralized host rocks display δ¹⁸O values averaging ~+22‰, whereas altered and mineralized rocks show values around +20‰. Secondary alteration minerals—calcite, silica, and dolomite—exhibit δ¹⁸O values of approximately +16‰, +18‰, and +18‰, respectively. This trend indicates a ~6‰ decrease in δ¹⁸O during mineralization, reflecting extensive fluid–host rock interaction. The lowest δ¹⁸O values occur in late-stage calcite, consistent with isotopic exchange between hydrothermal fluids and carbonate host rocks. Such depletion likely reflects high temperatures and prolonged interaction, leading to secondary isotopic equilibrium in alteration minerals formed during mineralization (Schindler et al, 2016; Nejadhadad et al, 2023). Isotopic signatures of carbonate phases, spatial patterns of alteration intensity provide further evidence for focused hydrothermal fluid flow along structurally prepared pathways.The progressive transition from fresh limestone in distal zones to weakly altered, silicified, and finally intensely dolomitized rocks toward the fault-controlled ore zones suggests a thermal and chemical gradient decreasing outward from the fluid conduits.
The δ¹³C (PDB)² values of fresh and weakly altered host rocks average +1‰, typical of Cretaceous marine carbonates (Gilg et al, 2008; Drake et al, 2009). These values progressively decrease in altered samples, fracture-filling dolomites, and silicified rocks, reaching –2‰, with late-stage calcite recording δ¹³C values as low as –3‰. The depletion in heavy carbon isotopes is likely due to biological activity or the presence of organic matter in the mineralizing fluids. Thermal oxidation of organic matter and hydrocarbons during epigenetic carbonate precipitation can produce isotopically lighter carbonate minerals relative to the original host rock (Gilg et al, 2003; Evans and Battles, 2011; Drake et al, 2009).
Conclusion
In the Qamishlu deposit, barite and galena precipitated together, often in alternating sequences. Geological and textural evidence indicates that ore deposition occurred after lithification of the primary carbonate host rocks and following tectonic deformation, suggesting a post-tectonic mineralization event. This behavior is comparable to other epigenetic sedimentary Pb–Zn deposits, such as Mississippi Valley-Type (MVT) systems.
The solubility and precipitation conditions of barite and galena differ significantly. Lead-rich, oxidized fluids under sulfur-deficient reducing conditions can transport substantial amounts of sulfur as dissolved lead–chloride complexes. When sulfur concentration increases, lead is reduced to lead sulfide (galena) and precipitates rapidly (Hanor, 2000).
Alteration in the Qamishlu deposit is characterized by dolomitization of the host rock, silicification, and precipitation of late-stage secondary calcite. Stable isotope analyses (δ¹⁸O and δ¹³C) of carbonate samples indicate isotopic exchange between hydrothermal fluids—depleted in δ¹⁸O but enriched in organic carbon—and the δ¹⁸O-rich carbonate host rocks. The strong spatial association of mineralization with NE–SW-trending faults, along with isotopic variations observed in altered zones, suggests that fault planes served as primary fluid conduits.
The availability of open space, combined with fluid–rock interactions between ore-bearing fluids and carbonate host rocks, modified the physicochemical conditions of the metal-bearing fluids, ultimately leading to the deposition of epigenetic mineralization.