Editorial: Stratabound Barite Deposits: Mineralogy, Isotope Geochemistry and Geochronology

This Special Issue comprises five articles, one review paper, and a comment on one of the articles with the author’s reply. The contributions are global in reach, describing barite deposits in Canada, Iran, Ireland, Türkiye, and the United Kingdom. The Guest Editor is grateful to the authors for their contributions which serve well to illustrate the characteristics of stratabound barite deposits worldwide, as well as the variety of methods used to decipher how they were formed. Interpretations of the origin of bedded barite deposits range from entirely syngenetic, i.e., deposited on the seabed from hydrothermal fluids exhaled into the marine environment (the “sedimentary exhalative” or SEDEX model), to entirely epigenetic, i.e., subsurface precipitation within or replacement of pre-existing sediments or rocks by barite. These processes may be elucidated from a combination of the macro-scale context, textural features, and chemical and isotopic characteristics of the barite and associated minerals.

Colin Andrew, in describing the Ballynoe barite deposit in Ireland (contribution 1), remarks on the “abundant and compelling geological and isotopic evidence for early local exhalation” including the presence of a hydrothermal vent fauna in limestone beds immediately below the barite bed. The onset of hydrothermal activity is indicated by faunal flourishment, specifically in crinoids growing in seabed hollows, followed by subsequent mass extinction probably due to metal or sulphide toxicity. The convincing control exerted by paleo-topography on the shape of the orebody, together with other evidence, indicates that the barite formed at or just below the sediment/water interface. Tectonism contemporaneous to mineralization assisted in the rapid burial and early onset of diagenetic recrystallization and stylolitisation of the barite at shallow burial depths. Late crystalline barite (the “cap” barite) formed by the up-dip migration of fluids following the main stage mineralization.

The Aberfeldy (Scotland, UK) bedded barite deposits described by Moles et al. (contribution 2) bear similarities to the Ballynoe deposit despite being older (Neoproterozoic), being hosted by clastic rather than carbonate-dominant sediments and being orogenically deformed and metamorphosed. As with Ballynoe, sharp external and internal boundaries of the Aberfeldy mineralized beds and the general absence of incorporated sediment suggest formation by syn-sedimentary exhalative processes rather than by post-depositional replacement. The Aberfeldy mineralized beds are spatially related to the inferred exhalative centres and to the paleo-topography. Seawater stratification and redox chemistry were major controls on the mineralogy of the hydrothermal precipitates including the mineral chemistry of sphalerite. Moles et al. conclude that the Aberfeldy barite deposits provide an example of the “classic” model of SEDEX mineralization precipitated on the contemporaneous seafloor from the mixing of exhaled metalliferous hydrothermal fluids with seawater of spatially varying redox chemistry, with only minor post-depositional alteration as evidenced by barium carbonate occurrences and barite δ³⁴S and δ¹⁸O isotopic variations.

Chaneil Wallace and co-authors (contribution 3) develop a very different model for the origin of barite and siderite in a carbonate replacement Ag-Pb-Zn-Cu sulphide orebody, namely the past-producing Walton deposit in Nova Scotia, Canada. In terms of age (Carboniferous) and component mineralization (separate barite and sulphide orebodies hosted by limestone), Walton is similar to the Ballynoe Silvermines deposit in Ireland. The Walton barite ore is mainly microcrystalline, locally showing radial and tabular crystal morphologies, and mostly has δ³⁴S values typical of Viséan seawater (~15‰). Heavier and lighter δ³⁴S values occur in coarser tabular barite crystals that show zonation under cathodoluminescence. Wallace et al. attribute barite formation to sulphate-driven anaerobic oxidation of methane derived from organic matter decomposition. A subsequent influx of a heated metalliferous fluid caused the formation of widespread sulphide mineralization and initiated the partial recrystallisation of earlier-formed siderite rock and microcrystalline barite units, accompanied by sulphur isotope fractionations associated with synchronous sulphate reduction and the oxidation of H₂S.

Ebrahim Ansari and co-authors (contribution 4) provide a detailed account of the central Iran Ardakan stratabound barite deposit that has been quarried and mined along a 5 km strike interval aligned with major thrusts and faults. Coarsely crystalline barite and fluorite occur as irregularly shaped masses and veins within Triassic carbonate rocks. The low total REE contents and Ce/La ratios of the analysed barite and fluorite crystals indicate a hydrothermal or terrestrial origin, as opposed to a deep marine origin, for the mineralization. Using barite S and O isotope compositions, the authors infer that the hydrothermal fluids originated from evaporites and/or connate waters of Ediacaran–Lower Cambrian age, or from basinal brines with slightly heavier δ¹⁸O. Fluid inclusion analyses support a basinal brine source with a contribution of meteoric water. The authors conclude that the deposit is epigenetic and formed during tectonic disturbance when upwelling, sulphate-bearing, hot basinal brines encountered cooler, Ba-bearing connate water trapped in the Middle Triassic dolomites and limestones.

The two remaining contributions to the Special Issue are review articles considering not one, but many barite deposits occurring within large regions, namely Türkiye (Zeynep Cansu et al. (contribution 5) and Iran (Rahman Rajabi et al. (contribution 6). Comparisons using multiple criteria enable the authors to classify the deposit types occurring in different tectono-stratigraphic settings.

In their review of Turkish barite occurrences, Cansu et al. (contribution 5) distinguish between the Oligocene magmatism-associated deposits and the volumetrically more important passive margin-hosted deposits that are predominantly Palaeozoic and Triassic. In the magmatism-associated F+Ba+REE+Th deposits of northwestern Türkiye, barite has similar Sr isotope ratios to the carbonatite host rocks with low ratios (0.706‰) suggesting a mantle contribution. Conversely, in the belt of Palaeozoic sediment-hosted deposits straddling the Taurides and Arabian Platform, barite has relatively radiogenic ⁸⁷Sr/⁸⁶Sr values (up to 0.718‰) consistent with Ba and Sr originating from the underlying continental crust. Unique amongst the Turkish barite orebodies, the Triassic limestone-hosted Karakaya deposit overlies a large Zn-Pb orebody and preserves both stratiform and feeder vent complex structures that are typical of vent-proximal, sediment-hosted SEDEX deposits.

Rajabi et al. (contribution 6) categorize barite deposits of Iran, based on orebody morphology and textural features, as sediment-hosted massive sulphide (SHMS), Irish-type, and Mississippi Valley-type (MVT). Fluid inclusion and stable isotope analyses show that basinal brines were the primary fluid source for all the sediment-hosted Zn-Pb-Ba deposits. A secondary source of seawater or dilute, low-temperature fluids was important in forming the Irish-type deposits. These deposits have hydrothermal sulphides with light negative δ³⁴S values suggesting that mineralization involved bacterial sulphate reduction. In contrast, Iranian SHMS deposits have a wide range of δ³⁴S values in sulphides and barite indicating complex fractionation processes likely involving thermochemical sulphate reduction, with barite replacement contributing to positive δ³⁴S values in diagenetic sulphides. Rajabi et al. conclude that early diagenetic barite was a key controller for subsequent Zn-Pb mineralization by providing a suitable host and a significant sulphur contribution.

In a Comment, Mostafa Nejadhadad and co-authors (contribution 7) challenge the barite replacement model for Iranian Zn-Pb-Ba deposits which they regard as entirely Mississippi Valley-type. They note that barite occurs only as a minor phase in the major base metal deposits, and that δ³⁴S values of sulphides exhibit a broad range (−27‰ to +18‰) incompatible with a single sulphur source or reduction mechanism. They conclude that barite and sulphides precipitated during discrete mineralization episodes, in segregated zones, controlled by systematic variations in fluid chemistry. Rajabi et al. (contribution 8) replied that the early Cretaceous Zn-Pb (±Ba±Ag±Cu) deposits of Iran exhibit distinct characteristics that align far more closely with Irish-type deposits within extensional and passive margin settings, rather than the typical compressional environment of MVT systems. In these deposits, the extensive replacement of barite by galena, sphalerite, and pyrite challenges the oversimplified view that barite is a mere late-stage addition.

In summary, the articles and discussions published in this Special Issue reflect the wider literature on stratabound barite deposits in affirming the wide range of environments in which such mineralization has formed, and the range of processes involved. In the 20^th century, it was often assumed that all bedded barite ± sulphide mineralization had originated by sedimentary exhalative processes. In the past 25 years, many researchers have refuted the SEDEX model, instead making a strong case for non-hydrothermal processes forming barite through “cold seep” methane-rich fluid upwelling and diagenetic replacement below the sediment–seawater interface, as described by Wallace et al. (contribution 3). However, this re-interpretation does not imply that all bedded barite occurrences worldwide are similarly replacive in origin. The barite-dominant deposits of Ballynoe and Aberfeldy described here have (to again quote Colin Andrew) “abundant and compelling geological and isotopic evidence for early local exhalation” and models for their formation resemble the original SEDEX concept. Conversely, at Ballynoe and elsewhere in the Irish Carboniferous Orefield, sulphide-dominant stratabound mineralization evidently formed within sediments below the contemporaneous seabed. A similar scenario, in places involving the replacement by sulphides of previously crystallized barite, applies to Irish-type stratabound Zn-Pb sulphide deposits elsewhere in the world such as in Iran (contribution 6 and 8). The “take home message” is that the formation of bedded barite typically involved a spectrum of processes over an extended period of time, from seabed precipitation and/or early diagenesis just below the seawater–sediment interface through to post-lithification hydrothermal alteration and metamorphism.

We look forward to the publication of further detailed geological, geochemical, and geochronological investigations of this intriguing type of mineral deposit.