Evaluating the Economic Potential of Listvenitization of Anarak Ophiolite Serpentinites using Mineralogical and Geochemical Evidence

The Anarak ophiolite complex is in northeastern Isfahan, at the northwestern boundary of the Central Iranian Microcontinent. This complex consists of serpentinized mantle peridotites, cumulate gabbros, pillow lavas, mafic-ultramafic dikes, glaucophane-bearing metabasalts, and listvenites, which are associated with metamorphic schists of Chah-Ghorbeh, Morghab, and Patyar, as well as the Lakh marble unit. The serpentinites and listvenites in this region are recognized as host rocks for Cu, Ni, Co, Sb, and Au mineralization. The serpentinites are primarily composed of antigorite, lizardite, chrysotile, chlorite, talc, and chromian spinel, along with pyrite, chalcopyrite, nickeline, and magnetite. Through hydrothermal processes, these have been transformed into an assemblage of magnesite, dolomite, calcite, quartz, fuchsite, and secondary sulfides within the listvenites. This study focuses on the geochemical investigation of Anarak serpentinites and listvenites to analyze their origin and alteration processes. Results from geochemical diagrams indicate that the serpentinites fall within the subducted field with a harzburgitic protolith. The listvenites are predominantly of carbonate, silica-carbonate, and birbirite types, reflecting the influence of Ni-Sb-As-rich hydrothermal processes and metasomatism in their evolution. These findings highlight the economic geological significance of this complex for valuable element mineralization. Introduction Listvenites, resulting from carbonation alteration of ultramafic rocks by CO2-rich hydrothermal fluids, have recently gained considerable scientific interest due to their dual significance in carbon sequestration and economic potential (Beinlich et al., 2020). These rocks form through metasomatic alteration of peridotite protoliths under temperatures of 150 to 250°C and moderate pressures (Klein and Garrido, 2011) in tectonically active zones, particularly along greenstone belts and crustal-scale fault systems (Sieber et al., 2018). Petrologically, listvenites are characterized by an assemblage of magnesite ± dolomite, quartz, and relict Cr-spinel, with variable amounts of serpentine, talc, and accessory sulfides. Their precursor serpentinites exhibit geochemical signatures fundamentally controlled by protolith composition and the tectonic environment of hydration, with three principal genetic types recognized: 1) abbyssal serpentinites formed at mid-ocean ridges, 2) mantle wedge serpentinites developed in subduction interfaces, and 3) subducted serpentinites metamorphosed under high-P/T conditions (Deschamps et al., 2013). The transformation from serpentinite to listvenite occurs through progressive carbonation reactions along structurally controlled fluid pathways, typically involving sequential replacement of serpentine minerals by silica-carbonate assemblages. This process is particularly active in subduction zone environments, transcurrent fault systems, and extensional tectonic regimes. While historically noted for their association with gold mineralization, contemporary studies reveal that listvenites may host diverse mineralization types including Ni-Co-Sb-Cu assemblages, particularly when derived from Cr-rich harzburgitic protoliths. The present investigation focuses on the geochemical evolution of listvenitized serpentinites within the Anarak ophiolite complex (Central Iran), employing whole-rock geochemistry and mineral phase analysis to clarify their alteration pathways, petrogenesis, and role in ore-forming processes. Complementary studies of serpentinite-listvenite transitions in the Anarak area aim to establish genetic relationships between progressive alteration stages and ore-forming processes. Materials and methods Representative serpentinite and listvenite samples were systematically collected from the Anarak ophiolitic complex, with particular focus on the Patyar and Chah-Gorbeh tectonic units, ensuring coverage of both protolith and variably altered lithologies. Detailed petrographic studies utilizing transmitted and reflected light microscopy documented primary mineral relics, alteration paragenesis, and critical microtextural relationships, with special emphasis on pseudomorphic replacement textures and vein networks indicative of progressive carbonation. Whole-rock geochemical analyses were conducted using X-ray fluorescence spectrometry (XRF) for major oxides and inductively coupled plasma-mass spectrometry (ICP-OES) for minor elements at ZarAzma laboratory. Mineralogical characterization was performed using X-ray diffraction (XRD). Result Petrography The Anarak serpentinites, derived from harzburgite/lherzolite protoliths, exhibit characteristic mesh-textured lizardite (after olivine) and bastite pseudomorphs (after orthopyroxene) with relict chromian spinels. Antigorite replacement of lizardite indicates prograde overprinting of initial retrograde serpentinization. Listvenites display two alteration facies: 1) carbonate-dominated and 2) silica-rich, reflecting progressive CO2-metasomatism. Mineralization includes Au (up to 7278 ppb) associated with primary Ni-Co-Sb-Cu sulfides and secondary oxidation products. Serpentinite Geochemistry Geochemical analyses of Anarak-Patyar serpentinites show SiO2 (36–44 wt.%), CaO (0.5–4.5 wt.%), Al2O3(0.5–3.3 wt.%), MgO (31.6–37.3 wt.%), and LOI (11–13.8 wt.%). FeO-MgO plots (7–9 wt.% FeO) indicate uniform alteration intensity. High LOI values correlate with antigorite-dominated serpentinization (∼12% structural water), while MgO depletion reflects both mantle source depletion and magmatic fractionation. The major oxide plots of MgO versus FeO, CaO, and LOI demonstrate compositional consistency with the field of subducted serpentinites (Fig. 5). The Anarak serpentinites, enriched in mobile trace elements such as Cs, U, Pb, and Sr, which are absent in primary peridotitic protoliths, serve as key indicators for fluid–rock interaction processes. While their major-element composition largely reflects the mantle-derived protolith, the distribution of mobile elements has been modified by the nature, composition, and physicochemical conditions of fluids (hydrothermal, seawater, or slab-derived). Enrichment in Ba, Pb, Th, U, La, Nb, and Yb relative to global serpentinites partly results from pre-serpentinization processes and partly from the infiltration of oxidizing and sediment-derived fluids during subduction. The distribution of chalcophile elements such as As, Sb, and Pb highlights the influence of subducted sediments and sediment-derived fluids, and Pb isotope signatures together with As and Sb enrichment suggest two scenarios for secondary serpentinization, in which serpentinites act as both reservoirs and carriers of mobile elements. Geochemical data on sulfides-bearing samples indicate that copper and nickel mineralization in the area are genetically independent, as evidenced by distinct trends in Ni–Co and Cu–Co plots, with Co/Ni ratios directly related to the degree of cobalt mineralization and nickel concentrations. Listvenite Geochemistry Major oxides show wide ranges: SiO2 (14–96 wt.%), MgO (0.07–7.32 wt.%), FeOt (1–19 wt.%), and LOI (0.6–34 wt.%), reflecting carbonation variability. CaO-SiO2 enrichment coupled with MgO loss confirms carbonate (dolomite-magnesite-calcite) formation in type-I listvenites. Patyar listvenites are distinguished by quartz-fuchsite assemblages (elevated K2O-Al2O3). Ternary diagrams position samples above the terrestrial melting line, demonstrating extensive carbonation, with Patyar specimens reaching 90 wt.% SiO2 (birbirite-like). This siliceous-carbonate spectrum correlates with observed hydrothermal alteration intensities. Discussion The results indicate that the Anarak and Patyar serpentinites, formed in a subduction setting, have played a significant role in concentrating and transporting ore-related elements, particularly gold. Serpentinization, involving substantial water addition (11-~14 wt.%) to peridotitic protoliths, facilitated both pervasive alteration of ultrabasic rocks and the mobilization of gold from primary opaque minerals (Fig. 5B). Gold contents (0.02-0.1 g/t) in these serpentinites are several times higher than the global average for upper mantle peridotites (0.001-0.01 g/t), reflecting secondary enrichment through subduction-related hydrothermal processes mediated by CO2-, S-, and As-rich fluids (Buisson and Leblanc, 1987). Possible sources of arsenic and sulfur include adjacent sedimentary–metamorphic host rocks, crystallized products of sulfide–arsenide melts in ultrabasic protoliths, and mantle metasomatism (Leblanc and Billaud, 1982; Buckman and Ashley, 2010). Nickel sulfides and arsenides precipitated during antigorite formation within the subduction channel, increasing As, Sb, Te, Bi, and Pb contents. Listvenites share Cr, Co, and Ni mineralogy with their serpentinite protoliths, indicating compositional inheritance, but deformation and brecciation liberated these elements, with shear zones focusing Cu-, As-, and Sb-rich ore-bearing fluids. Two main stages of listvenite formation are recognized: early carbonate-rich type I formed under CO2-, H2O-, and Ca2+-rich fluids at high pH, and later silica-rich type II formed through Si transfer from serpentine minerals under lower pH hydrothermal conditions, with associated magnetite dissolution and gold mobilization. Listvenites experienced greenschist facies metamorphism (290-340°C, 100-300 MPa), later overprinted by retrograde metamorphism during Tethys closure and post-orogenic uplift (Bagheri and Stampfli, 2008). Their silica–carbonate composition, high shear deformation, and mineralogical evidence point to a genetic link with serpentinite protoliths, with tectonic deformation controlling fluid pathways and creating localized reducing environments favorable for selective ore deposition. Conclusion The integrated geochemical and petrographic investigation of serpentinites and listvenites in the Anarak area reveals a complex alteration history linked to subduction zone processes. Serpentinites formed from subducted mantle peridotites underwent extensive hydration and metasomatism, promoting significant enrichment in economically important elements such as gold. Listvenites represent advanced hydrothermal alteration stages involving carbonation and silicification, which contribute to ore concentration. This study highlights the importance of serpentinite and listvenite alteration in controlling fluid composition, metal mobility, and mineralization in subduction-related settings, emphasizing their role in economic geology exploration frameworks. Further studies integrating isotopic and fluid inclusion data could refine the understanding of fluid sources and pathways in this region.