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

1. Introduction

Rajabi et al. (2024) [1] argue that barite is a common mineral in Iranian sediment-hosted Pb-Zn deposits [1]. They conclude that barite is a key mineral and usually overprinted by later stages of sulfide mineralization. The authors provide textural and isotopic evidence supporting the model of barite replacement with late-stage sulfide minerals. In their theory, the ore zone is enveloped by barite, and sulfide mineralization is widely dependent on precursor barite deposition through the preparation of the host and sulfur contribution in sediment-hosted Zn-Pb deposits. Therefore, they conclude that thermochemical sulfate reduction (TSR), driven by barite dissolution and replacement, plays the primary role in supplying sulfur for sulfide mineralization.

In this commentary, we examine Rajabi et al.’s (2024) hypothesis concerning economically exploited Pb-Zn deposits in Iran. Our analysis suggests that barite typically occurs as a gangue phase in minor-to-trace quantities. Furthermore, sulfur isotope data indicate that sulfide deposition occurred through either bacteriogenic or thermochemical sulfate reduction mechanisms. Consequently, most of these deposits may not exhibit the pattern of barite replacement with sulfides proposed by Rajabi et al. [1]. Additionally, based on geological parameters and detailed ore-forming factors, most Iranian Pb-Zn deposits are classified as Mississippi Valley-type (MVT) deposits. In this study, we focus specifically on economically significant Pb-Zn deposits to better understand their depositional mechanisms.

2. Controls on Mineralization and Ore Genesis

The prevailing accepted idea regarding economically exploited Pb-Zn deposits in Iran is that of MVT mineralization during the Cretaceous to the Tertiary periods [2,3,4,5,6,7,8]. This is consistent with the convergence of the western margin of North America–Africa–Eurasia and MVT mineralization worldwide [9]. The MVT deposition in Iran is synchronous with thrusting and, afterwards, burial through the thrust faults. Mineralization occurred on the faults, fractures, dissolution collapse breccias, and stratigraphic transitions [2,3,4,5,6,7,8]. Tabular, lens-shaped, and pocket-like orebodies in the MVT deposits of Iran are common, e.g., Mehdiabad [8,10], Angouran [7], Emarat [6], Irankuh [2,11], Ravanj [5], Dareh-Noghre [4], and Tiran mining district [3] Pb-Zn deposits (Figure 1).

Ore Paragenesis and Timing of Barite Deposition

Pb-Zn mineralization with carbonate host rocks in Iran consists of hydrothermal mineral assemblages including sphalerite, galena, and pyrite, as well as minor chalcopyrite and sulfosalts, together with abundant quartz, dolomite, and minor barite [1,2,3,4,5,6,7,8,9,10,11]. For example, barite deposition has played a crucial role in the interval deposition of sulfide ores in the giant Zn-Pb Mehdiabad carbonate-hosted Pb-Zn-Ba deposits [8], Ravanj mine [5], Tiran mining district [3], Angouran marble-hosted Pb-Zn ore deposit [7], Irankuh area [2,11], and Nakhlak mine [12] Pb-Zn deposits (Figure 1, Figure 2 and Figure 3). In the supergiant Mehdiabad deposit, with 630 million tons of Pb-Zn resources, the barite content is approximately 40 million tons (about 6% of the total reserve) ([1], p. 9, Section 4.2). In the Pb-Zn deposits of the Isfahan–Malayer belt, the barite content ranges from 1 to 5 wt.% of the total mineralization, while the associated dolomitization and silicification halos often equal or exceed the volume of ore mineralization [3,4,7,11]. The Irankuh deposit, the main producer of concentrate sulfide ore in Iran, provides a striking example, where the volume of hydrothermal alteration zones (dolomite and silica) significantly surpasses—several fold—the actual ore volume [2,11]. Detailed mineralogical studies of the Emarat deposit have revealed a complete absence of barite mineralization [6]. These observations demonstrate that barite plays a limited quantitative role in Pb-Zn mineralization systems. Rajabi et al. [1]’s interpretation appears primarily based on high-grade barite zones and/or late-stage barite veins that may not represent the main mineralization stages. Therefore, the argument by Rajabi et al. (2024) that barite deposition is a key precursor mineral ([1], pp. 9–17, Section 4.2) for the deposition of sulfides in lead–zinc deposits is incompatible with the available data.

3. Sulfur Isotope Geochemistry

Discussions on the sulfur source of sediment-hosted Pb-Zn ore generally extend over a wide range of δ³⁴ values [9]. Isotopic studies of sulfur in the Pb-Zn deposits of the selected deposits of this study (Figure 4) indicate that sulfur sources are derived from a variety of crustal sources, and sulfur reduction mechanisms are either biogenic or thermochemical (δ³⁴ values ranging from −27‰ to +15‰), which fundamentally contradicts the barite-replacement model proposed by Rajabi et al. ([1], p. 1, abstract and pp. 26–28, Section 7.2). The Mehdiabad deposits of sphalerite and galena have negative values of δ³⁴ ranging from −17.3 to −3.8‰ [8], as well as reduced sulfur generated by earlier BSR processes [8]. In the Ravanj deposit, the δ³⁴S values of sulfide minerals vary from −27‰ to −11‰, suggesting a bacteriogenic sulfate reduction [5]. The δ³⁴S value of sulfide minerals in the Tiran mining district range from −9‰ to +4‰, suggesting the involvement of both bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) during mineralization [3]. In the Emarat deposit, where barite has not been reported to date; the δ³⁴ values for sphalerite and galena range from +5.3‰ to +14.4‰, consistent with the thermochemical sulfate reduction of Cretaceous seawater [6]. The δ³⁴S values of the Angouran deposit (ranging from 3.9‰ to 7.4‰) reflect that the sulfur was sourced by mixing magmatic–basinal brine [13]. In the Irankuh deposit, the δ³⁴S values of sulfide minerals vary from −3.6‰ to −9.6‰, suggesting thermochemical sulfate reduction [2]. Consequently, δ³⁴S isotopic compositions indicate contributions from local sulfur reservoirs and varied sulfate reduction mechanisms (Figure 4).

4. Concluding Remarks

Geological, textural, paragenetic, and sulfur isotopic data from selected Iranian Pb-Zn deposits indicate epigenetic mineralization spatially associated with carbonate sequences. The presented data demonstrate that barite typically occurs as a minor phase in most economically exploited Iranian Pb-Zn deposits. Furthermore, the δ³⁴S values of sulfide minerals exhibit a broad range (−27‰ to +18‰), incompatible with a single sulfur source or reduction mechanism. Notably, the barite-rich Mehdiabad and Ravanj deposits display the most δ³⁴S-depleted signatures (−27‰ to −4‰), characteristic of bacterial sulfate reduction (BSR)-dominated systems. Collectively, our integrated mineralogical, textural, and isotopic evidence fundamentally challenges the barite-replacement model proposed by Rajabi et al. (2024) [1] for sulfide mineralization in Iranian Pb-Zn deposits. Field work and petrographic studies indicate that barite and sulfides precipitated during discrete mineralization episodes, in segregated zones, controlled by systematic variations in fluid chemistry (Figures 6, 7 and 15–17 of Rajabi et al. 2024 [1] and Figure 2 of this study).