Early Cretaceous Zn-Pb (Ba±Ag±Cu±Fe±Mn) Deposits of Iran: Irish Type or Mississippi Valley Type? Reply to Nejadhadad et al. Comment on “Rajabi et al. Barite Replacement as a Key Factor in the Genesis of Sediment-Hosted Zn-Pb±Ba and Barite-Sulfide Deposits: Ore Fluids and Isotope (S and Sr) Signatures from Sediment-Hosted Zn-Pb±Ba Deposits of Iran. Minerals 2024, 14, 671”

Abstract

This study critically examines the early Cretaceous carbonate-hosted Zn-Pb (±Ba±Cu) deposits of the Malayer-Esfahan (MEMB) and Yazd-Anarak (YAMB) metallogenic belts in Iran, which have been inaccurately classified as Mississippi Valley type (MVT) deposits by Nejadhadad et al. (2025). Our findings reveal significant differences in mineralogy, fluid inclusion characteristics, and geochemical signatures compared to typical MVT deposits. These deposits are more akin to Irish-type Zn-Pb mineralization and formed in extensional and passive margin environments around the Nain–Baft back-arc basin. The normal faults in this back-arc rift can transform significantly during inversion and compressional tectonics, reactivating to behave as reverse faults and leading to new geological structures and landscapes. Our study highlights barite replacement as a crucial factor in forming sediment-hosted Zn-Pb (±Ba±Cu) and barite-sulfide deposits. Based on textural evidence, fluid inclusion data, and sulfur isotope analyses, we propose that barite plays a fundamental role in controlling subsequent Zn-Pb (±Ba±Cu) mineralization by serving as both a favorable host and a significant sulfur source. Furthermore, diagenetic barite may act as a precursor to diverse types of sediment-hosted Zn-Pb (±Ba±Cu) mineralization, refining genetic models for these deposits. Sulfur isotope analyses of Irish-type deposits show a broad δ³⁴S range (−28‰ to +5‰), indicative of bacterial sulfate reduction (BSR). Nevertheless, more positive δ³⁴S values (+1‰ to +36‰) and textural evidence in shale-hosted massive sulfide (SHMS) deposits suggest a greater role for thermochemical sulfate reduction (TSR) in sulfide mineralization.

1. Introduction

We sincerely thank Nejadhadad et al. [1] for their contribution and the opportunity to respond to their comments. Whilst we appreciate their feedback, we believe that certain aspects of the comments warrant further clarification and revision. Our article [2] examined the role of barite replacement in the formation of some sediment-hosted (SH) Zn-Pb (±Ba±Ag±Cu) and barite-sulfide deposits. Based on textural evidence, fluid inclusion studies, and sulfur isotopic data, we suggest that barite can be a crucial controller of subsequent sulfide mineralization in these deposits, as it can provide an appropriate host and a significant sulfur contribution. Their comment [1] on our work [2] consists of three addenda:

(A)

Barite is often present in minor to trace amounts and is predominantly deposited as a post-sulfide gangue mineral rather than being systematically replaced by sulfides.

(B)

The comment challenges the exclusive role of thermochemical sulfate reduction (TSR) in providing reduced sulfur for Pb-Zn mineralization, highlighting sulfur isotope data that indicate both bacteriogenic and thermochemical reduction mechanisms in sulfide deposition.

(C)

Based on geological parameters and ore-forming factors, the comment emphasizes that most Iranian carbonate-hosted Pb-Zn (±Ba±Ag±Cu) deposits belong to the Mississippi Valley Type (MVT) and focuses on economically significant deposits with post-Early Cretaceous mineralization to refine the understanding of their formation processes.

They have also conducted a selective rather than comprehensive review of geodynamic evolution, controls on mineralization and ore genesis, the ore paragenesis and timing of barite deposition, and sulfur isotope geochemistry to present and substantiate their arguments.

Considering the evidence, we discuss the key geological, textural, mineralogical, and geochemical aspects of the host environment and sediment-hosted Pb-Zn (±Ba±Ag±Cu) deposits of Iran. Before discussing the points raised by Nejadhadad et al. [1], it is essential to emphasize several key aspects of metallogeny studies:

(1)

A thorough understanding of all previous research on ore deposits is crucial in metallogeny studies. All perspectives must be examined to select a model that aligns with the regional tectonic framework, geological characteristics, and geological events.

(2)

A metallogenic model must account for all geological aspects and the spatial as well as temporal distribution of various ore deposits. However, the proposed MVT model [1] lacks this capability.

(3)

Many studies on sediment-hosted Zn-Pb (±Ba±Ag±Cu) ore deposits of Iran have been overlooked and disregarded by Nejadhadad et al. [1]. Researchers [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21] have conducted extensive studies on these deposits, which, if reviewed, could have addressed many of the raised issues. However, for whatever reason, Nejadhadad et al. [1] have neglected to study or cite these works.

(4)

The provided comment [1] lacks proper citations, as many statements have been included in the article without citing the sources. For instance, the introduction of the four metallogenic belts of sediment-hosted Zn-Pb (±Ba±Ag±Cu) deposits of Iran by Rajabi et al. [3,4] has been incorrectly attributed to other individuals, without the proper citation of the original reference. Properly referencing other articles is crucial for attributing sources, maintaining research integrity, acknowledging prior authors’ contributions, and enabling readers to verify information.

(5)

Nejadhadad et al. [1] have failed to adequately consider relevant field observations and integrate previous studies on Zn-Pb mineralization in metallogenic belts, revealing a lack of understanding of Zn-Pb deposit classification. The classification of mineral deposits and metallogeny analysis necessitate extensive field studies, comprehensive geological data from multiple deposits, and detailed examination. Proposing a metallogenic model cannot be performed without a thorough investigation of these deposits.

2. Geodynamics and Metallogeny of Sediment-Hosted Zn-Pb (±Ba±Ag±Cu) Deposits in the SSZ

Rajabi et al. [3,4] reviewed the metallogeny of sediment-hosted Zn-Pb (±Ba±Ag±Cu) and F-rich deposits of Iran and introduced four main metallogenic belts for these deposits, including the Malayer-Esfahan (MEMB), Yazd-Anarak (YAMB), Tabas-Posht e Badam (TPMB), and Central Alborz (Figure 1A,B). Additionally, the metallogeny, distribution, and classification of SH Zn-Pb deposits in all metallogenic belts have been reviewed by Rajabi et al. [8,19] and Rajabi [18].

Nejadhadad et al. [1] pointed out that “the Neo-Tethys and Nain-Baft occurred from the Late Cretaceous to the Miocene. Therefore, after the late Jurassic to the Miocene time, a compressional regime prevailed in Iran’s territory”.

Irish-type deposits are typically associated with extensional tectonic environments, whereas orogenic-related MVT deposits commonly occur in compressional settings. However, Ostendorf et al. [22] noted that some MVT deposits can also form in extensional tectonic environments. This highlights the variability in tectonic settings associated with MVT deposits. In the case of the MEMB, numerous geological studies indicate that sediment-hosted Zn-Pb deposits in the MEMB are associated with extensional environments during the Cretaceous period. Here, we list some evidence that shows the extensional environment of the Nain–Baft basin during the Jurassic to Early Cretaceous.

The closure of the Paleo-Tethys Ocean in the Late Triassic (Carnian) was succeeded by the subduction of the Neo-Tethys oceanic crust beneath the SSZ [23]. This significantly influenced subsequent tectonic and magmatic activity in the region. An extensional regime, possibly associated with slab roll-back, led to the separation of the SSZ, and the Central Iranian Microcontinent in the Late Triassic [23,24], while the formation of the back-arc rift basin continued from the Early Jurassic to the Cretaceous (Figure 1C).

Figure 1. (A) The metallogenic belts and distribution map of sediment-hosted Zn-Pb (±Ba±Ag±Cu) and F-rich deposits (modified from [2,3,20]) on a simplified tectonic map of Iran [25]. (B) Crustal domains of the Western Tethysides (from [8]). (C) A geodynamic model of the Iranian plate in the Cretaceous (modified from [3,8], based on [23,24,26,27]) and Zn-Pb (Ag±Cu±Ba) mineralizations around the Nain–Baft basin. A: Alborz ranges; CIM: Central Iranian Microcontinent; NB: Nain-Baft extensional back-arc basin; Sb: Sabzevar back-arc basin; SC: South Caspian basin; SSZ: Sanandaj-Sirjan zone.

Figure 1. (A) The metallogenic belts and distribution map of sediment-hosted Zn-Pb (±Ba±Ag±Cu) and F-rich deposits (modified from [2,3,20]) on a simplified tectonic map of Iran [25]. (B) Crustal domains of the Western Tethysides (from [8]). (C) A geodynamic model of the Iranian plate in the Cretaceous (modified from [3,8], based on [23,24,26,27]) and Zn-Pb (Ag±Cu±Ba) mineralizations around the Nain–Baft basin. A: Alborz ranges; CIM: Central Iranian Microcontinent; NB: Nain-Baft extensional back-arc basin; Sb: Sabzevar back-arc basin; SC: South Caspian basin; SSZ: Sanandaj-Sirjan zone.

A summary of Iran’s geodynamic and tectonic evolution has been provided in our article [2]. Overall, the points presented by Nejadhadad et al. [1] are relatively consistent with the explanations provided in the article. Nejadhadad et al. [1] do not dispute the back-arc setting of the Nain–Baft basin and just pointed out a compressional regime in the Nain–Baft basin after the Late Jurassic by referring to Mehdipour Ghazi et al. [28]. However, these authors [28] proposed an intra-oceanic subduction and an oceanic back-arc basin in the Nain–Baft suture zone (Figure 2) in the Cretaceous based on the geochemical data of Cr-spinels.

Figure 2. Geodynamic scenarios for the evolution of the Nain–Baft basin from the Permian to Cretaceous ((A) from [29] and (B) from [28]). SSZ: Sanandaj-Sirjan zone, CIM: Central Iranian Microcontinent. Also, see Figure 5 in [24].

Figure 2. Geodynamic scenarios for the evolution of the Nain–Baft basin from the Permian to Cretaceous ((A) from [29] and (B) from [28]). SSZ: Sanandaj-Sirjan zone, CIM: Central Iranian Microcontinent. Also, see Figure 5 in [24].

Nazemei et al. [30] examined the late Jurassic–Early Cretaceous mafic volcanic rocks in the southeastern SSZ. These basalts and basaltic andesites exhibit tholeiitic affinities, are depleted in high-field-strength elements (HFSEs) such as Nb, Ta, and Ti, and are enriched in LREEs and large ion lithophile elements (LILEs). The geochemical signatures suggest an island arc or back-arc basin environment for their formation.

Shomali et al. [31] studied the Varcheh mafic intrusions in the northern SSZ, and their field evidence, petrography, geochemistry, and U-Pb geochronological data indicate that these plutonic rocks, composed mainly of monzogabbro, intruded into Cretaceous sedimentary rocks. Zircon U-Pb dating reveals that these rocks formed at 125–118 Ma in the late Early Cretaceous (Barremian–Aptian). The Varcheh rocks are not typical calc-alkaline rocks; some exhibit alkaline affinity. Negative anomalies in Nb-Ta-Ti and enrichments in some LILEs are consistent with a subduction-zone setting [31]. The magma is thought to have arisen from the partial melting of the subcontinental lithospheric mantle wedge above a subducting slab of oceanic lithosphere [31], and the emplacement of these mafic intrusions was accommodated by dominant dextral strike-slip movement in an arc and back-arc environment experiencing extension during the late Early Cretaceous.

In addition to arc magmatism, the SSZ also exhibits geological evidence of back-arc or extensional tectonic regimes, particularly during the Jurassic and Early Cretaceous [23,24,26,32]. Azizi et al. [33] addressed the controversy surrounding the tectonic setting of Jurassic magmatic rocks in the SSZ. In the Ghorveh area, zircon U-Pb dating indicates that metabasitic rocks crystallized around 145–144 Ma in the Late Jurassic. The Ghalayan metabasites are tholeiites enriched in LREEs, lacking significant Nb, Ta, Pb, Sr, and Ba anomalies, resembling continental intra-plate tholeiitic basalts. Geochemical data indicate an intra-continental (or back-arc) rifting environment for these Jurassic metabasites (Figure 2A). The results show that an extensional tectonic regime dominated SSZ tectonics from the Middle Jurassic to the Cretaceous [33].

Salehi and Tadayon [34] present geological data from Early Cretaceous strata of the SSZ, including petrographic, structural, basin fill evolution, and geochemical analyses. Sandstone provenance analysis indicates the presence of sub-mature litho-quartzose sandstones derived from plutonic, metamorphic, and quartzose sedimentary rocks exposed in the central SSZ. The sediments were likely sourced from an active continental arc in a convergent setting, and carbonate rocks deposited in a back-arc basin [34].

Larvet et al. [35] used numerical modeling to identify the parameters controlling the deformation of the Iranian plate, resulting in localized back-arc extension along the Nain–Baft basin. The Iranian plate has a long convergence history marked by numerous episodes of intraplate deformation due to Late Triassic to Oligocene Neo-Tethys subduction. They suggested that the Cretaceous back-arc environment (the Nain–Baft basin) may have been triggered by changes in internal slab dynamics or regional-scale convergence dynamics [35].

The paleogeographic and paleotectonic reconstructions (Figure 3 and Figure 4), geological mapping [36,37,38,39], and tectonostratigraphic and structural synthesis [40] of the Iranian and Arabian plates suggest the development of a back-arc extensional basin between the SSZ and Central Iran during the Jurassic–Early Cretaceous. This extensional regime is attributed to the subduction of the Neo-Tethyan oceanic lithosphere beneath the SSZ, driving regional tectonic deformation and basin formation.

In addition to tectonic and petrological studies, the investigation of various mineralization events during the Jurassic and Cretaceous can provide valuable insights into the prevailing tectonic environment [18]. The formation of volcanogenic massive sulfide (VMS) deposits (e.g., Jiyan, Mazayjan, and Chahgaz [41,42]), and some shale-hosted massive sulfide deposits (e.g., Ab-Bagh, Hossein-Abad, Gol-e-Zard [7,43,44,45]) within the Jurassic volcano-sedimentary sequences in the Bavanat region, along with the association of the Saleh Peyghambar and Darreh Noghre [46] volcanic sediment-hosted massive sulfide (VSHMS) Zn-Pb [8], and Barika gold-rich and Abdossamadi barite-Cu VMS deposits [47,48,49] with Early Cretaceous volcanic rocks in the MEMB and northwestern SSZ, supports the occurrence of an extensional regime during the Jurassic and Early Cretaceous between the Sanandaj-Sirjan zone and Central Iran.

Figure 3. Paleotectonic maps of the Iranian plate during the Cretaceous and evolution of the Nain–Baft back-arc basin (from [38]; also see Figure 30 in [50]).

Figure 3. Paleotectonic maps of the Iranian plate during the Cretaceous and evolution of the Nain–Baft back-arc basin (from [38]; also see Figure 30 in [50]).

3. Metallogeny and Controls on SH Zn-Pb (±Ba±Ag±Cu) Mineralization and Ore Genesis

Nejadhadad et al. [1] have merely stated that carbonate-hosted Zn-Pb deposits in Cretaceous rocks are of the MVT and that their formation falls within the “global time frame”, without providing logical justification. According to their model, MVT mineralizations in the MEMB are synchronous with thrusting and typically occur primarily along thrust faults. Ore formation is commonly associated with faults, fractures, and dissolution–collapse breccias. To refute a concept or theory, it is essential to clearly articulate logical reasoning. The classification of the mentioned deposits as MVT has been rejected by numerous authors [6,8,10,11,13,15,16,18,19,20,21,43,46,51], each of whom has provided their justifications. Therefore, we invite Nejadhadad et al. [1] to review these studies. In addition, the tectonic evolution and geological characteristics of ore deposits in Iran may differ from those in other parts of the world. A generalized perspective without providing detailed explanations can lead to misinterpretations of the mineralization type and the processes involved in their formation. Therefore, a thorough geological investigation of the deposits, considering the tectonic evolution of the study area, is strongly emphasized. Supplementary Table S1 presents a comparative summary of the diagnostic features of the main types of sediment-hosted Pb-Zn deposits, emphasizing their key differences and similarities.

Figure 4. Paleotectonic maps of Middle to Late Jurassic (A) and Early Cretaceous (B) of the Sanandaj–Sirjan Zone showing magmatic arcs, back-arc basin development (modified from [52]), and the Malayer-Esfahan metallogenic belt (MEMB).

Figure 4. Paleotectonic maps of Middle to Late Jurassic (A) and Early Cretaceous (B) of the Sanandaj–Sirjan Zone showing magmatic arcs, back-arc basin development (modified from [52]), and the Malayer-Esfahan metallogenic belt (MEMB).

Detailed geological investigations of several deposits (e.g., Irankuh [6,11], and Tiran [51] mining districts, Robat [10], Gavankuh, Khanabad [18], Ahangaran [13,53], Shamsabad [15,21], Eastern Haft-Savaran [54], Darrehnoghreh, Saleh Peyghambar [8,46], Kuhkolangeh [55], Lakan, Shamsabad [14,15,21], Sarchal [19], Mehdiabad [9,16,19,20]) suggest that these deposits significantly differ from orogenic-related MVT deposits and are more like Irish-type ore deposits in an extensional environment. In this context, we highlight several aspects that are inconsistent with the model proposed by Nejadhadad et al. [1].

(1)

The SH Zn-Pb (±Ba±Ag±Cu) deposits of the MEMB are distributed across multiple stratigraphic horizons or positions (Figure 5). This distribution underscores the crucial role of the host strata (i.e., the host basin) as the primary controlling parameter in the formation of these deposits, rather than the influence of younger thrust faults [19]. Furthermore, many of these mineralizations are spatially associated with syn-sedimentary normal faults (unlike MVT deposits, e.g., Mehdiabad [9,16,19,20], Mansourabad [56], Farahabad [57], Irankuh [5], and Tiran [19,51] deposits), indicating that their genesis is independent of thrust belt tectonics. It is important to note that some MVT deposits, even in thrust belts and foreland tectonic settings, can be associated with normal faults, as highlighted by Song et al. [58]. This association may complicate the classification of carbonate-hosted Zn-Pb deposits if based solely on structural or tectonic criteria.

(2)

Dolomitization is the prominent alteration in MVT deposits, whereas silicification is typically absent [59,60]. However, silicification represents a major hydrothermal alteration process in the MEMB deposits. Additionally, while MVT mineralizations are typically Cu-poor [59], most MEMB and YAMB deposits (we refer to these as Irish-type deposits) contain abundant chalcopyrite and tetrahedrite and other complex sulphosalts (e.g., Irankuh, Shamsabad, Ahangaran, Mehdiabad, and Sarchal deposits) [5,19,20,21,53] similar to Navan, Silvermines and Tynagh deposits.

(3)

A fundamental issue in classifying sediment-hosted Zn-Pb deposits is the critical understanding of the timing of the mineralizing event. MVT deposits form in lithified rock, significantly later than (on average, between 20 and 50 million years later) and in a different structural setting (orogenic-related compressional environments) from the host rock’s formation. In contrast, Irish-type deposits display a range of mineralization timing, from contemporaneous (syn-sedimentary) to late diagenetic and post-lithification replacement. These deposits develop in a geological setting similar to their host rock, with a minimal time gap (shortly after sedimentation or/and extending up to 20 million years later) and in an extensional environment.

(4)

Ore fluids in MVT deposits are typically basinal brines with salinities ranging from approximately 10 to 30 wt. % NaCl equivalent and ore fluid temperatures between 75 °C and 200 °C [59]. In contrast, fluid inclusion data of the MEMB deposits represent that the homogenization temperatures in sphalerite vary from 115 to 280 °C (high-temperature ore fluids, Figure 6), and the salinities range from 6 to 24 wt. % NaCl equation [2]. These characteristics are inconsistent with orogenic-related MVT ore fluids and instead align more closely with submarine hydrothermal systems formed through replacement processes in Irish-type deposits [2].

Figure 5. A generalized stratigraphic column of the Early Cretaceous sequence of the MEMB and western CIGS transition zone, highlighting the main SH Zn-Pb ore-bearing strata (from [3,8,19]).

Figure 5. A generalized stratigraphic column of the Early Cretaceous sequence of the MEMB and western CIGS transition zone, highlighting the main SH Zn-Pb ore-bearing strata (from [3,8,19]).

Figure 6. A summary of published fluid inclusion homogenization temperature–salinity pairs from MVT (A) and Irish-type (B) ore deposits of Iran compared to other SH Zn-Pb (Ba-Ag) deposits (from [2,19]). A base chart of different fluid sources is from [61]. (C) Temperature and salinity ranges of fluid inclusion data in the sphalerite and barite of Irish-type and MVT deposits of Iran, based on A and B [2]. (D) A comparison of fluid inclusion data in the sphalerite of Irish-type deposits of Iran with similar deposits in the Irish Orefield and Harberton Bridge MVT deposit in Ireland [62].

Figure 6. A summary of published fluid inclusion homogenization temperature–salinity pairs from MVT (A) and Irish-type (B) ore deposits of Iran compared to other SH Zn-Pb (Ba-Ag) deposits (from [2,19]). A base chart of different fluid sources is from [61]. (C) Temperature and salinity ranges of fluid inclusion data in the sphalerite and barite of Irish-type and MVT deposits of Iran, based on A and B [2]. (D) A comparison of fluid inclusion data in the sphalerite of Irish-type deposits of Iran with similar deposits in the Irish Orefield and Harberton Bridge MVT deposit in Ireland [62].

(5)

Detailed mineralogical and textural analyses of the MEMB and YAMB SH Zn-Pb (±Ba±Ag±Cu) deposits reveal two main paragenetic stages of sulfide mineralization, with a third stage identified in some deposits. These paragenetic stages are observed in most deposits, including the Irankuh and Tiran mining districts, as well as the Gavankuh, Robat, Kuhkolangeh, Eastern Haft-Savaran, Khanabad, and Lakan deposits.

(A)

Early fine-grained sulfide deposition: This stage involves the precipitation of minor, fine-grained, disseminated sulfides (occasionally laminated; see Figure 11B–E in [2] and Figure 6 in [19]) and euhedral barite within unconsolidated sediments at or near the seafloor [3,6,8,10,19,21,51,54]. In most deposits, this stage is characterized by the presence of abundant framboidal pyrite [63]. The fine-grained nature of sulfides in the earliest stage of mineralization (Stage 1) suggests rapid crystallization within unconsolidated mud beneath the seafloor, likely triggered by the mixing of ascending metalliferous fluids with seawater [64,65].

(B)

Main sulfide mineralization and sub-seafloor replacement: The primary sulfide mineralization consists of coarse-grained sulfides and the extensive replacement of pre-existing barite, carbonates, and early sulfide laminations/bands by sulfides. This stage also involves hydrothermal minerals such as quartz, dolomite, and siderite, occurring within the host siltstone and/or limestone units.

(C)

Late-stage sulfide mineralization (in some deposits): In certain deposits (e.g., Irankuh and Tiran mining districts), the final stage of mineralization is identified, characterized by coarse-grained sphalerite and galena with minor pyrite, concentrated in reverse fault zones that formed due to later orogenic movements. These faults show evidence of intense deformation affecting both the sulfide minerals and their host rocks.

(6)

Beyond the SH Zn-Pb (±Ba±Ag±Cu) deposits, several unusual Fe-Mn-Pb (±Ba±Cu) deposits occur in the northwestern MEMB, hosted in tuffaceous siltstone, sandstone, dolomitic limestone, and volcanic sequences. Examples include the Ahangaran, Sarchal, Shamsabad, Ghezeldar, and Saki deposits [13,15,19,21,53], which exhibit transitional characteristics between Irish-type and volcanogenic massive sulfide (VMS) deposits [19]. These deposits are distinguished by the presence of Fe-bearing carbonates, primarily ankerite, and siderite, as the most significant hydrothermal minerals, closely associated with barite, pyrite, chalcopyrite, and galena. The occurrence of abundant Fe carbonates in association with barite and sulfide minerals is uncommon in MVT deposits. Instead, this mineralogical assemblage is indicative of sub-seafloor replacement mineralization in an extensional setting, a characteristic feature of sideritic Irish-type Fe-Mn-Pb (±Ba±Cu) ore deposits.

(7)

Fe-rich dolomite and ankerite are among the most prevalent carbonate alterations in the MEMB deposits [6,13,17,53]. This carbonate alteration is consistent with typical hydrothermal alteration processes in submarine, sediment-hosted hydrothermal systems.

(8)

Detailed tectonic analyses and kinematic measurements in the Tiran [51], Irankuh [5,66], Eastern Haft-Savaran [52], Shamsabad [13,21], Ab-Bagh II [45], and Mehdiabad [9,20] deposits reveal that their formation is linked to Early Cretaceous syn-sedimentary normal faults. Some of these faults were later reactivated as reverse faults following the late Cretaceous tectonic event and basin inversion [8,51,66]. Yarmohammadi [67] and Maghfouri et al. [9] documented the presence of debris flows and sedimentary breccias (associated with igneous components in the Tiran area) adjacent to a normal fault at the Vejin-Paein and Mehdiabad deposits (see Figure 8 in [19] and Figure 18 in [47]). Debris flow and sedimentary breccia thickness increase toward the normal faults. The interfingering of debris flows with fine-grained sediments and the pronounced lateral facies and thickness variations strongly indicate the influence of syn-sedimentary faulting [9,68,69] unlike MVT deposits.

(9)

However, the normal faults in back-arc rift are significant features associated primarily with extensional environments and the formation of the Irish-type Zn-Pb deposits in the MEMB; during an inversion or compressional tectonic regime (the collision of the Arabian plate with the Iranian plate in the late Cretaceous–Miocene), the dynamics of these faults can undergo significant changes. The stress regime changes from extensional to compressional due to the collision of tectonic plates and the existing normal faults can become reactivated as reverse or thrust faults. This means the movement will now be opposite to that of the original formation. This reactivation can lead to the formation of new geological structures, such as folds or thrust belts, and can create significant topographic changes in the area. This process is crucial for understanding the evolution of tectonic environments and their associated geological history in the MEMB.

(10)

In addition to the SH Zn-Pb (±Ba±Ag±Cu) and Fe-Mn-Pb (±Ba±Cu) deposits, the region hosts several shale-hosted massive sulfide (SHMS or SEDEX) type deposits within Late Jurassic black shales, siltstones, and sandstones, which are also associated with back-arc extension (e.g., Gol-e-Zard, Hossein-Abad, Ab-Bagh I, and Western Haft-Savaran [7,17,43,45]. Furthermore, numerous volcanogenic massive sulfide (VMS) deposits have been identified within Jurassic sequences (e.g., Bavanat and Chahgaz deposits [41,42]) and Cretaceous rocks (e.g., Barika and Abdolsamadi deposits [47,48,70]) of the SSZ. The coexistence of these ore deposits in the SSZ highlights the complex tectonic and metallogenic evolution of the SSZ in the Jurassic– Early Cretaceous, which cannot be adequately explained by the model proposed by Nejadhadad et al. [1].

(11)

Some of the MEMB SH Zn-Pb (±Ba±Ag) deposits occur concordantly within silicified and dolomitized limestone (see Figure 5 in [19]), specifically at the contact between the early Cretaceous massive orbitolina-bearing limestone and the overlying shale and marl units (e.g., Robat, Emarat, Kuhkolangeh, Lakan, and Muchan deposits, Figure 4). These deposits exhibit tabular morphologies (see Figure 10 in [2] and Figures 5 and 9 in [19]). Despite their stratabound nature, indicated by their alignment with host rock layers and deformation under the same folding regimes due to post-ore compressional tectonism, these deposits formed prior to thrust faulting. Additionally, certain mineralizations, like the Sarchal Fe-Mn-Pb (±Ba±Cu) deposit, display a completely tabular geometry and are hosted within early Cretaceous siltstones and tuffaceous rocks (Figure 5). This further suggests that ore formation predated regional compressional tectonics and was not structurally controlled by thrust fault systems.

4. Ore Paragenesis and Timing of Barite Deposition

Nejadhadad et al. [1], in a very general and imprecise examination of barite generations in the deposits, without studying the presence of diagenetic barite, stated that the carbonate host Pb-Zn mineralization of Iran features sphalerite, galena, and pyrite, with barite forming as a late-stage mineral after sulfides. Barite’s role varies, being crucial in Mehdiabad deposits but less significant in others like Angouran and Ravanj.

We would like to clarify that this interpretation presented by Nejadhadad et al. [1] does not fully capture the complexity of barite mineralization in these deposits. In this study, the Angouran deposit is not examined, as its mineralization time and ore-forming processes differ to those of the deposits in the Malayer-Esfahan metallogenic belt [71,72,73,74]. Furthermore, this deposit was formed within the Cambrian sequence and exhibits distinct metallogenic characteristics and mineralization types compared to the other studied deposits [75,76]. It is worth pointing out that the first and second authors of the original paper (Rajabi an Mahmoodi [2]) have served as consultants for the Angouran mine exploration program (2024–2025). We have conducted extensive research on the structures and genesis of the Angouran deposit, and the findings will be published soon.

A detailed petrographic and mineralogical study of the Irish-type deposits in the MEMB reveals that barite occurs in at least two (or more) distinct generations (see Figures 6–9, 11 and 12 in [2]): (1) an early diagenetic fine-grained phase and (2) a later coarse-grained hydrothermal phase associated with sulfide mineralization [5,6,7,8,10,19,21,53,54,55,63].

Contrary to the assertion that barite is merely a late-stage mineral deposited after sulfides [1], our detailed field and petrographic investigations indicate that barite has undergone multiple stages of formation, replacement, and recrystallization. In several deposits, including Ravanj, Tiran, Ahangaran, Haft-Savaran, Robat, Gavankuh, Kuhkolangeh, Irankuh, and others, the first-generation diagenetic barite appears as fine-grained disseminations within carbonate or as thin laminae parallel to host rock layering, forming before major sulfide deposition. Meanwhile, the second-generation coarse-grained hydrothermal barite occurs as massive replacement zones, zebra-textured veins, and radiating bladed crystals, which are often overprinted by sulfide mineralization.

Moreover, the role of barite varies significantly between different deposits. While it plays a major role in Mehdiabad, it also forms substantial parts of the ore in several other deposits, albeit with a complex paragenesis. The extensive replacement of barite by galena, sphalerite, and pyrite in various deposits, as demonstrated in our petrographic analysis [2], challenges the oversimplified view that barite is a mere late-stage addition [1]. Additionally, the figures presented in our study [2] provide clear petrographic and textural evidence of barite replacement by sulfides. In particular, microscopic images distinctly illustrate the gradual replacement of different generations of barite with sulfide minerals, supporting the multi-stage evolution of these deposits. These observations further reinforce the necessity of a more detailed and precise paragenetic interpretation rather than a generalized late-stage [2] classification of barite.

Given these findings, we emphasize the importance of comprehensive petrographic and textural studies in understanding the complex history of mineralization in these deposits. Our ongoing research continues to refine these models, and we welcome further discussion and petrographic evidence supporting alternative interpretations.

5. Sulfur Isotope Geochemistry

Nejadhadad et al. [1] pointed out that sulfur isotope studies of Zn-Pb deposits in the selected areas reveal a wide range of δ³⁴S values, indicating multiple sulfur sources deposits and diverse reduction mechanisms, and these variations highlight the complex sulfur cycling in MVT deposits. The main flaw in this interpretation is that Nejadhadad et al. [1] initially classified all carbonate-hosted lead–zinc deposits as MVT before analyzing sulfur isotope geochemistry and its sources. Given this approach, no better interpretation could be expected than what has already been presented.

In geochemical studies, due to the influence of various physical and geochemical processes on the fractionation of stable isotopes in sulfide and non-sulfide minerals, several factors must be considered before interpreting isotopic data. These include the geochemical conditions of the depositional environment, temperature, ore-forming fluids, deposit type, and so on. Only after establishing these parameters should the isotopic data be interpreted accordingly. In our study [2], we have analyzed the isotopic data while considering these essential conditions (as possible). This approach allows for a more accurate interpretation of the data, which we have examined in detail.

Irish-type deposits in the MEMB and YAMB (e.g., Ahangaran, Irankuh, Mehdiabad, Farahabad, Mansourabad, Ravanj, and Tiran) exhibit low δ³⁴S values (−25‰ to +5‰), resembling those in the Central Irish Orefield and Alpine-type deposits (Figure 7). This isotopic signature suggests bacterial sulfate reduction (BSR) as a key process, though fluid temperatures (115–280 °C) exceed the threshold for active BSR, indicating possible sulfate reduction in distal environments before mineralization.

SHMS deposits show a broad range of δ³⁴S values (−5‰ to +36‰), with most sulfides exhibiting high positive values (+9.7‰ to +32.7‰). These values suggest thermochemical sulfate reduction (TSR) as the dominant reduction mechanism rather than BSR, given the elevated fluid temperatures (177–250 °C). Barite dissolution and replacement by pyrite, sphalerite, and galena further contribute to the observed δ³⁴S enrichment in the SHMS deposits of Iran. In the Koushk and Chahmir deposits, textural evidence supports barite replacement as a major source of sulfur in diagenetic sulfides.

Excluding Kuh-e-Surmeh, MVT deposits display moderate δ³⁴S values (−4‰ to +16‰), consistent with sulfur derived from seawater sulfate through TSR at ore-forming temperatures of 85–170 °C. Some fluorite–barite MVT deposits, hosted in Middle Triassic carbonate rocks, show highly positive δ³⁴S values, likely resulting from Rayleigh fractionation or evaporative basin sulfate influence. In contrast, the Kuh-e-Surmeh deposit exhibits exceptionally low δ³⁴S values, akin to Cretaceous Irish-type deposits, implying a distinct sulfur source and reduction pathway.

Low δ³⁴S values do not preclude an MVT classification in sediment-hosted Zn-Pb deposits, as some MVT deposits are characterized by negative or low sulfur isotope values, particularly when sulfur reduction occurs distally or prior to mineralization. However, Irish-type deposits typically exhibit low δ³⁴S values due to their association with seawater flows beneath the seafloor. In contrast, orogenic-related MVT deposits generally display higher positive δ³⁴S values, reflecting the dominant process of TSR. In extensional environments, MVT deposits may exhibit sulfur isotope values similar to those of Irish-type deposits. Although there is some overlap, the sulfur isotope signatures of these two deposit types are generally distinct.

Sulfur isotope data provide robust evidence for varying sulfur sources and reduction mechanisms in SH Zn-Pb and barite deposits. The isotopic signatures reflect the interplay of TSR, barite dissolution, and diagenetic processes, underscoring the complex geochemical evolution of these deposits in Iran (Figure 7).

6. Conclusions

Nejadhadad et al. [1] classified all carbonate-hosted Zn-Pb deposits in Iran as orogenic-related MVT. However, these deposits differ significantly in mineralogy, fluid inclusion characteristics, host rocks, sulfide paragenesis, ore textures, and geochemical signatures from typical MVT deposits. The early Cretaceous SH Zn-Pb (±Ba±Ag±Cu) deposits are mainly located around the Nain–Baft and Sabzevar suture zones (Figure 8B), far from the Zagros Thrust Zone (ZTZ), the collisional boundary between the Iranian and Arabian plates (Figure 1A and Figure 8B). If these deposits had formed due to the Arabia-Iran collision in a forearc setting, their extensive distribution in the YAMB and MEMB on both sides of the Malayer-Esfahan super basin would be difficult to explain.

This study reevaluates the classification of Iran’s carbonate-hosted Zn-Pb deposits, challenging their designation as typical MVT deposits. The early Cretaceous Zn-Pb (±Ba±Ag±Cu) deposits of the Malayer-Esfahan and Yazd-Anarak metallogenic belts exhibit distinct geological, tectonostratigraphic setting, geochemical signatures, and mineralogical characteristics that align far more closely with Irish-type deposits rather than the typical compressional environment of MVT systems. Their formation within extensional and passive margin settings (Figure 8C), the influence of syn-sedimentary faulting, and the role of barite replacement processes suggest a more complex metallogenic evolution than previously recognized. Furthermore, sulfur isotope data and the role of barite in sulfur cycling provide critical insights into ore-forming processes, highlighting the need for a refined approach to metallogenic modeling.