# A dynamic H2 system with multi-source methane in chromitite-rich ophiolitic settings

Laurent Truche, Frédéric-Victor Donzé, Bardhyl Muceku, Malavika Sivan, Thomas Röckmann, Dan Levy, Manuel Moreira, Corinne Loisy, Adrian Cerepi, Marianne Quéméneur, et al.

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To cite this version:

Laurent Truche, Frédéric-Victor Donzé, Bardhyl Muceku, Malavika Sivan, Thomas Röckmann, et al. A dynamic H2 system with multi-source methane in chromitite-rich ophiolitic settings. Geochimica et Cosmochimica Acta, 2025, 409, pp.281-307. 10.1016/j.gca.2025.09.039. insu-05304144v2

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Submitted on 7 Nov 2025

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# A dynamic H₂ system with multi-source methane in chromitite-rich ophiolitic settings

Laurent Truche<sup>a, *, ID</sup>, Frédéric-Victor Donzé<sup>a</sup>, Bardhyl Muceku<sup>b</sup>, Malavika Sivan<sup>c</sup>, Thomas Röckmann<sup>c, ID</sup>, Dan Levy<sup>d</sup>, Manuel Moreira<sup>e</sup>, Corinne Loisy<sup>f</sup>, Adrian Cerepi<sup>f</sup>, Marianne Quéméneur<sup>g</sup>, Delphine Tisserand<sup>a</sup>, Hugo Dutoit<sup>a, ID</sup>, Ivan Vujević<sup>a</sup>, Yan Yao<sup>a, ID</sup>, Nicolas Lefeuvre<sup>a, ID</sup>, Anais Lavoué<sup>a</sup>, Christophe Monnin<sup>h, ID</sup>, Edmond Goskolli<sup>i, ID</sup>

## ARTICLE INFO

Associate editor: Esther M. Schwarzenbach

**Keywords:**

- Serpentinization
- Natural hydrogen
- Exploration
- Radiocarbon
- Clumped isotopes
- Noble gases

## ABSTRACT

The generation and migration of hydrogen and methane in serpentinizing environments represent fundamental processes with implications for deep carbon cycling, the origin of life, and emerging carbon-free energy resources. Here we present an integrated geochemical study of gas emissions from the Bulqizë chromite mine in Albania, where exceptional underground access to depths exceeding 1000 m enables direct investigation of one of Earth's most intense natural hydrogen systems (~200 tones H₂/year). Through a comprehensive analysis of bulk and clumped isotopes, noble gases, molecular compositions, and structural controls, we report a novel occurrence of radiocarbon-bearing methane (3.76 ± 0.06 pMC; ~26 ka) in any ophiolite globally. This discovery, combined with modern water ages (3H = 3.5 TU; 14C-DIC = 95.8 pMC) yet ancient gas signatures, reveals a dramatically decoupled fluid systems where meteoric water circulates rapidly within the mine while gas migrates slowly from depth. Methane clumped isotopes (Δ13CH3D = 2.52 ± 0.26 ‰; Δ12CH2D2 = 9.95 ± 1.6 ‰) indicate a lack of isotopic equilibrium with H₂ (δD-H₂ = -743.5 ± 1.1 ‰; ΔDD = 255 ± 35 ‰) and suggest formation from a single hydrogen source. They also show positive Δ¹²CH₂D₂ deviations likely due to diffusion or mixing, and point to a microbial origin probability of less than 10 %. High δ¹³C-CH₄ values (-12.3 ‰), complete isotopic reversal in C₂-C₄ alkanes, and Volatile Organic Compounds signatures further support a predominantly abiotic synthesis. Noble gas and nitrogen isotopes reveal mixing between atmospheric (80–85 %) and crustal (15–20 %) components with negligible mantle input, confirming an open system with active recharge. Most remarkably, gas emissions are structurally controlled and exclusively occur along a major fault zone intersecting the chromitite ore body, a spatial relationship observed in chromite mines globally. The presence of Fe-Ni alloys and platinum group elements in chromitite appears to catalyze CO₂ reduction to methane at temperatures significantly lower than required for uncatalyzed abiotic synthesis. This catalytic effect, combined with optimal serpentinization conditions at 3–5 km depth, sustains extraordinary gas fluxes that cannot be explained by conventional models. The radiocarbon signature suggests either ongoing methane formation incorporating traces of modern carbon or reactivation of the system following regional deglaciation ~26,000 years ago.

Our findings transform understanding of ophiolitic gas systems from fossil reservoirs to dynamic features cycling carbon on 10⁴-year rather than 10⁶-year timescales. The identification of chromitite as a key catalyst provides new targets for hydrogen exploration throughout the global ophiolite belt, while the demonstration of rapid carbon cycling establishes ophiolites as active participants in the contemporary carbon cycle. These results

<footnote><sup>a</sup> Université Grenoble Alpes, CNRS, IRD, ISTERRE, UMR 5275, Grenoble, France</footnote>

<footnote><sup>b</sup> Polytechnic University of Tirana, FGJM, Department of Earth Sciences, Tirana, Albania</footnote>

<footnote><sup>c</sup> Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Netherlands (the)</footnote>

<footnote><sup>d</sup> Université de Paris Cité, CNRS, Institut de Physique du Globe de Paris, Paris, France</footnote>

<footnote><sup>e</sup> Université Orléans, CNRS, BRGM, OSUC, ISTO UMR 7327, Orléans, France</footnote>

<footnote><sup>f</sup> Université Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, Pessac, France</footnote>

<footnote><sup>g</sup> Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Marseille, France</footnote>

<footnote><sup>h</sup> Université Toulouse 3, CNRS, IRD, GET, UMR 5563, Toulouse, France</footnote>

<footnote><sup>i</sup> National Agency of Natural Resources, NАНR-AKBN, Tirana, Albania</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: laurent.truche@univ-grenoble-alpes.fr (L. Truche).</footnote>
offer a new paradigm for natural hydrogen generation with immediate relevance to both Earth system science and the urgent search for carbon-free energy resources.

# 1. Introduction

The generation and migration of hydrogen and methane in serpentinizing ophiolites remain poorly understood despite their implications for deep carbon cycling, subsurface habitability, and geochemical processes on Earth. Beyond their geological significance, these ultramafic rock systems have recently emerged as potential sources of natural hydrogen, positioning them at the intersection of fundamental Earth science and the quest for carbon-free energy resources (Smith, 2002; Zgonnik, 2020; Truche et al., 2020; Templeton et al., 2024; Ballentine et al., 2025). The serpentinization reactions that transform olivine and pyroxene into serpentine minerals, magnetite, and molecular hydrogen create some of Earth's most reducing environments, where the boundaries between geochemistry and biology blur, and where processes analogous to those on other planetary bodies may unfold (Sleep et al., 2004; Nealson et al., 2005; Holm et al., 2015).

The transformation of ultramafic rocks through serpentinization generates hydrogen through the oxidation of ferrous iron coupled with the reduction of water, creating conditions thermodynamically favorable for the abiotic synthesis of methane and other organic compounds (McCollom, 2013; Etiope and Oze, 2022; Reeves and Fiebig, 2020). This process, exemplified by the Sabatier reaction where CO₂ is reduced to CH₄ in the presence of H₂ and appropriate catalysts, has profound implications extending far beyond terrestrial energy resources (Etiope and Sherwood Lollar, 2013). The discovery of methane in the Martian atmosphere and evidence for past aqueous alteration of ultramafic rocks on Mars suggest that similar processes may have operated on our neighboring planet (Oze and Sharma, 2005; Tarnas et al., 2018). Hydrogen enriched gases with high H₂/CH₄ ratios were also detected within the vapor plume of Saturn's small icy moon Enceladus, consistent with inorganic H₂ production through the hydration and alteration of ultramafic rocks (Waite et al., 2017, Zandanel et al., 2021). On Earth, serpentinization environments may have provided the chemical disequilibrium necessary for the emergence of life, offering a compelling scenario for the origin of life in the deep subsurface isolated from the harsh conditions of the early Earth's surface (Russell et al., 2010; Schrenk et al., 2013).

Decades of research on hydrogen and methane seeps in ophiolite complexes worldwide have revealed both common patterns and puzzling variations. The hyperalkaline springs of Oman's Semail ophiolite, perhaps the most extensively studied system, discharge waters with pH values exceeding 12 and gas compositions ranging from methane-dominated to hydrogen-rich mixtures (Neal and Stanger, 1983; Kelemen et al., 2011; Boulard et al., 2014; Miller et al., 2016, Vacquand et al., 2018, Leong et al., 2023). In Turkey, the eternal flames of Chimaera have burned for millennia, fed by gas seeps where the isotopic signatures of methane suggest a complex interplay between abiotic and possibly thermogenic sources (Hosgörmez et al., 2008, Etiope et al., 2011; Reeves and Fiebig, 2020). The Zambales ophiolite in the Philippines and the Coast Range ophiolite in California add further complexity to the global picture, with gas compositions and isotopic signatures that challenge simple genetic models (Abrajano et al., 1990; Morrill et al., 2013, Cook et al., 2021). So far, waters and gases were mostly sampled at the surface in bubbling hyperalkaline springs or advective dry gas seeps. Only a few boreholes were drilled for scientific purpose to depths < 400 m, with the Oman drilling project being one of the most advanced programs (Kelemen et al., 2021, Ellison et al., 2021; Nothaft et al., 2021). Sampling deep geological fluids circulating through ophiolites is key to document H₂ behavior as close as possible to its source and eventual reservoirs, but also to avoid mixing and contamination in shallow aquifers and surface runoff streams.

Despite this wealth of observations, fundamental questions persist about the mechanisms governing gas generation, the pathways of migration and accumulation, and the relative contributions of abiotic and biotic processes in these extreme environments. Most critically, it remains unclear whether the gases we observe today result from ongoing generation or the slow release of ancient accumulations. The role of geological structures in controlling gas migration remains poorly constrained, as does the influence of specific minerals and catalysts on methane formation. In hyperalkaline environments hostile to most life forms, the potential contribution of specialized microbial communities adapted to these extreme conditions remains an open question (Canovas et al., 2017; Rempfert et al., 2017; Fones et al., 2019, Quéméneur et al., 2015, 2023).

The Bulqizë ultramafic massif in eastern Albania offers an extraordinary opportunity to address these fundamental questions. As part of the Jurassic Eastern Mirdita ophiolite, one of the most complete ophiolitic sequences in the Mediterranean region (Dilek and Flower, 2003; Dilek et al., 2008), Bulqizë hosts world-class chromite deposits that have been mined for decades, creating an extensive network of underground galleries. This mining infrastructure, reaching depths exceeding 1000 m, provides unprecedented three-dimensional access to active gas emission zones, enabling direct sampling and observation of deep processes typically hidden from view. Since 2016, systematic monitoring has revealed that Bulqizë hosts one of Earth's most intense natural hydrogen systems, where nearly pure H₂ (84 vol%) escapes through well-identified fractures at a minimum flow rate of 200 tons per year (Goskolli et al., 2022; Truche et al., 2024).

Recent investigations at Bulqizë have uncovered features that challenge existing paradigms. The high hydrogen emission rate confined to a narrow tectonic zone in the deepest level of the mine cannot be explained solely by active serpentinization or the release of occluded gases through fluid inclusions decrepitation. Currently, no known continuous process can generate hydrogen rapidly enough to sustain this flux. Instead, geological evidence suggests that hydrogen has accumulated in a faulted reservoir. Although this reservoir is likely limited in size due to the low porosity of peridotite and its confinement within fractures, it demonstrates that hydrogen can be preserved over time, even at temperatures that could support microbial activity such as methanogenesis prone to consume hydrogen (Truche et al., 2024; Yao et al., 2025).

However, the timescales required for hydrogen generation and accumulation in ophiolite settings remain uncertain. The system may be entirely fossil, with hydrogen produced millions of years ago, or it could still be generating hydrogen at low rates today, with slow accumulation in specific zones. In the latter case, the system may be dynamic, where hydrogen generation slightly exceeds leakage or consumption rates.

In this study, we present a comprehensive geochemical and isotopic investigation of the Bulqizë gas system that integrates multiple analytical approaches to construct a coherent model of gas generation and migration. Our analytical approach includes measurements of bulk and clumped isotopes of both hydrogen and methane, providing temperature constraints independent of conventional geothermometry. Noble gas isotope systematics offer insights into fluid sources and migration pathways, while radiocarbon dating of methane and dissolved inorganic carbon provides crucial temporal constraints. Structural mapping relates gas emissions to specific geological features, while comprehensive analysis of major and trace gas components, including volatile organic compounds, reveals unexpected complexity in gas composition.

Through this multi-proxy approach, we aim to address several key questions: i) the sources and formation mechanisms of hydrogen and methane, ii) the relative role of abiotic, thermogenic, and microbial
processes in shaping gas compositions, iii) the residence times and migration pathways of gases, and iv) the influence of chromitite ore bodies and ophiolite structure on gas generation and accumulation. By addressing these questions, we seek not only to understand the Bulqizë system in detail but to develop broader insights into the dynamics of gas generation in ultramafic environments, with implications for both Earth's deep volatile cycles and the potential for ophiolites to serve as natural hydrogen resources in our energy transition.

## 2. Geological setting

### 2.1. Regional tectonic framework

The Bulqizë ultramafic massif (Fig. 1) forms an integral part of the Eastern Mirdita ophiolite belt, which stands as one of the most complete and well-preserved ophiolitic sequences in the Mediterranean region (Dilek and Flower, 2003; Dilek et al., 2008). This remarkable geological feature extends for approximately 150 km along strike through northern Albania, representing a key segment of the larger Dinaric-Hellenic ophiolite belt that traces the suture zone of the former Neotethys Ocean (Robertson and Shallo, 2000; Dilek and Furnes, 2009, Schmid et al., 2020). The emplacement of this ophiolite during the Late Jurassic to Early Cretaceous (160–140 Ma) resulted from complex tectonic interactions involving intra-oceanic subduction, arc magmatism, and the eventual collision with the Adria microplate (Robertson, 2004; Bortolotti et al., 2005).

The Mirdita zone is divided into western and eastern ophiolite belts, distinguished by their contrasted geochemical compositions and crustal architecture (Beccaluva et al., 2005; Saccani et al., 2011). While the western belt exhibits typical mid-ocean ridge basalt characteristics indicative of its origin at a spreading center, the eastern belt, which hosts the Bulqizë massif, displays clear supra-subduction zone affinities with boninitic and island-arc tholeiitic compositions (Pearce et al., 1984; Shallo and Dilek, 2003). This geochemical dichotomy elegantly records the tectonic evolution of the Neotethys from initial seafloor spreading to subduction initiation and subsequent arc development.

### 2.2. Structure and lithostratigraphy of the Bulqizë massif

The Bulqizë ultramafic massif encompasses approximately 370 km² and represents one of the largest coherent mantle sections exposed anywhere in the Mediterranean region (Hoxha, 2007). Despite local disruption from subsequent tectonism, the massif preserves a complete ophiolitic pseudostratigraphy that provides exceptional insights into oceanic lithosphere architecture (Meshi et al., 2010).

The basal mantle sequence, exceeding 3 km in thickness, consists predominantly of variably serpentinized harzburgite comprising 70–80% of the exposed ultramafic rocks, with subordinate lherzolite and numerous dunite lenses accounting for 10–15% of the sequence. The degree of serpentinization shows a systematic variation with depth and structural position, ranging from modest 20–30% alteration in near-surface exposures to pervasive transformation exceeding 80% at depths greater than 1 km, particularly in the vicinity of the major fault zones (Meshi et al., 2010, Yao et al., 2025). This alteration pattern reflects the fundamental control of fracture networks on fluid circulation and water-rock interaction.

Overlying the mantle sequence, a 200–500 m thick transition zone marks the crust-mantle boundary, characterized by interlayered dunite and chromitite pods that record intense melt-rock interaction and melt extraction processes during the ophiolite's magmatic evolution (Xiong et al., 2015). The crustal sequence, where preserved, comprises 2–3 km of layered and isotropic gabbros, sheeted dike complexes, and pillow basalts, though these upper units have suffered extensive erosion and tectonic dismemberment, limiting their exposure (Saccani et al., 2011).

Beneath the ophiolite, a metamorphic sole composed of amphibolites, greenschists, and metasediments provides crucial constraints on

emplacement conditions. These rocks record peak metamorphic conditions of 750–850 °C and 0.8–1.2 GPa, consistent with their formation at the base of hot, young oceanic lithosphere during the initiation of intra-oceanic subduction (Dimo-Lahitte et al., 2001; Tremblay et al., 2015).

### 2.3. Chromitite mineralization

The Bulqizë massif hostsone of the world's premier podiform chromitite deposits, with proven reserves exceeding 20 million tonnes at grades of 35–42% Cr₂O₃ (Beqiraj et al., 2000; Economou-Eliopoulos, 2022; Junge et al., 2021). These economically significant ore bodies occur as concordant to semi-concordant lenses within the critical harzburgite-dunite transition zone, invariably enveloped by dunite halos ranging from 1 to 20 m in thickness (Rollinson, 2008). The scale of individual ore bodies varies dramatically, from meter-scale pods to extensive tabular deposits that exceed 1 km in strike length and reach thicknesses of 100 m (Fig. 1).

The chromitites exhibit diverse textures ranging from massive varieties containing more than 80% chromite to disseminated types with 20–80% chromite content. Their compositional spectrum spans from high-Cr varieties (Cr# = 0.70–0.85) to high-Al types (Cr# = 0.45–0.65), reflecting crystallization from boninitic and tholeiitic melts, respectively (Beccaluva et al., 2005; Xiong et al., 2015, Qiu et al., 2018). Of particular significance for gas generation processes, these chromitites host platinum-group minerals including laurite, erlichmanite, and Os-Ir alloys that occur as minute inclusions within chromite grains and interstitial sulfides, with total platinum-group element contents reaching 200–500 ppb (Beqiraj et al., 2000; Çina et al., 2002; Qiu et al., 2019). Furthermore, Ni and Co concentrations can reach 3200 and 130 ppm, respectively, in the Bulqizë rocks (Xiong et al., 2015), creating additional catalytic sites in CO₂ methanation reactions.

### 2.4. Structural architecture and fault systems

The present-day structural architecture of the Bulqizë massif reflects a complex deformation history spanning initial ocean floor spreading, ophiolite obduction, and subsequent Alpine orogenesis (Hoxha and Boullier, 1995; Meshi et al., 2010). The dominant structural feature trends NNW-SSE, parallel to the regional strike of the ophiolite belt, with subsidiary NE-SW and E-W trending structures creating a complex network of potential fluid pathways.

Three principal fault systems control the massif's architecture and profoundly influence fluid circulation patterns (Kilias et al., 2000, Kilias, 2023). Primary thrust faults oriented N330° and dipping 30-45°E represent structures initially formed during ophiolite emplacement and subsequently reactivated during Alpine compression. These faults juxtapose different lithological units and commonly display evidence of multiple movement episodes. A second set of high-angle normal faults trending N030-050° and dipping 60-80°W formed during post-emplacement extension, segmenting the massif into distinct structural blocks while providing crucial pathways for fluid circulation. The third major system comprises strike-slip transfer faults oriented N070-090° with subvertical dips, which accommodate differential movement between fault blocks and typically exhibit zones of intense fracturing and brecciation. Finally, high angle normal faults strongly modified from the Miocene onwards the pre-existing deformation geometry of the Albanides.

The intersection of these fault systems may create zones of dramatically enhanced permeability that serve as preferential pathways for both groundwater circulation and gas migration. Deep mining operations have provided exceptional three-dimensional exposure of these structures, revealing that gas emissions concentrate at fault intersections and within damage zones extending 5–20 m from major fault planes (Truche et al., 2024). This structural control on gas distribution proves fundamental to understanding the hydrogen system at Bulqizë. Still, the full details regarding fracture generation, drainage, and nature of the
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Simplified geological map of the Bulqizë ultramafic massif with surface projection of the chromite ore deposit and the deep N215-80°W H₂-rich fault zone. Bottom: Geological cross-sections along the A-B and C-D transects. Modified after Qorlaze and Stermasi (1989), Gurabardhi et al. (2003), and Hoxha (2007). Along the A-B cross section, the geometry of the sedimentary lithologies beneath the ophiolite is based on limited constraints and superseded tectonic concepts, warranting a comprehensive reevaluation.</figcaption></figure>
any sealing remain elusive today.

## 2.5. Hydrogeological setting

The hydrogeology of the Bulqizë massif reflects complex interactions between topography, lithology, and structural architecture (Eftimi, 2010). The massif forms a prominent topographic high, with elevations ranging from 300 m in valley floors to over 2,000 m at the summit of Me Dhoksit. This substantial relief drives a gravity-dominated groundwater flow system wherein meteoric water infiltrates at high elevations and circulates through the extensively fractured rock mass before emerging at lower elevations.

Hydrogeological properties vary dramatically with lithology and degree of serpentinization, creating a highly heterogeneous flow system. Fresh harzburgite exhibits low matrix permeability ranging from $10^{-8}$ to $10^{-7}$ m/s, with groundwater flow confined primarily to fracture

networks. In contrast, serpentinized peridotite displays enhanced matrix permeability of $10^{-6}$ to $10^{-5}$ m/s, resulting from volume expansion and microfracturing associated with serpentine mineral formation (Farough et al., 2016; Tutolo et al., 2016; Manning and Ingebritsen, 1999). Most significantly, major fault zones act as high-permeability conduits with values ranging from $10^{-4}$ to $10^{-2}$ m/s, channeling both water and gas flow over vertical distances exceeding 600 m (Sibson, 1996).

## 2.6. Present-day geodynamic setting

The Bulqizë region remains tectonically active, situated within the complex collision zone between the Adriatic microplate and the Eurasian plate (Handy et al., 2015). Contemporary GPS measurements reveal ongoing NE-SW oriented compression occurring at rates of 3–5 mm/yr, which drives continued uplift and maintains the high topographic relief essential for groundwater circulation (Jouanne et al.,

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Schematic 3D view of the Bulqizë underground chromite mine, highlighting the deeper levels, with inset images showing water and gas sampling sites at various depths within the mine. Gas is observed bubbling along gutters and in a pool at Level 19. At Level 17, the drainage network consists of 38 boreholes extending down to Level 21 (170 m deep) through the fault zone. This network is continuously flushed with compressed air to capture, remove, and dilute hydrogen ($H_2$), preventing its uncontrolled entry into the galleries (Truche et al., 2024). The mine entrance is situated at an elevation of 840 m above mean sea level (amsl).</figcaption></figure>
2012). Regional seismicity, characterized by moderate magnitude events (M 4–6) occurring at depths of 10–30 km, reflects active deformation along inherited ophiolite-related structures (Aliaj et al., 2001).

Recent magnetotelluric surveys have provided crucial insights into the deep structure beneath Bulqizë, revealing that the Eastern Mirdita ophiolite extends to depths of 5–6 km, underlain by a distinctive conductive zone interpreted as either serpentinized mantle or fluid-rich sediments (Yao et al., 2025). Temperature measurements from deep boreholes indicate a geothermal gradient of 25–30 °C/km, which places the 90–150 °C temperature window optimal for serpentinization reactions at depths of 3–5 km (Klein et al., 2020). This depth range coincides with the interpreted thickness of the ophiolite, suggesting that active serpentinization may be ongoing at the base of the ophiolitic sequence, providing a potential source for the extraordinary hydrogen flux observed in the mine.

# 3. Materials and methods

## 3.1. Sampling of groundwater and free gas in the Bulqizë mine

Our investigation of the Bulqizë gas system drew upon an extensive sampling program conducted through two field campaigns in 2023 and 2024. This multi-year effort provided comprehensive coverage of all accessible levels within the chromite mine, from Level 5 at +410 m above mean sea level (amsl) to Level 21 at –180 m below mean sea level (bmsl). This represents a vertical span exceeding 600 m through the ophiolite sequence, with Level 5 already situated 400 m below the surface (Fig. 2). The sampling strategy was guided by preliminary hydrogen screening using portable gas detectors (GeoTech®, GA 5000), which allowed real-time identification and mapping of H₂ emission zones in the mine.

Three distinct types of gas emission zones were systematically sampled to capture the full range of degassing phenomena: active gas seeps characterized by visible bubbling through water pools on gallery floors, dry gas vents emanating from fractures and fault zones in gallery walls, and diffuse emissions encountered during drilling operations for mining exploration.

Groundwater and free gas samples were collected within the Bulqizë mine on June 4–5, 2024. Free gases were also sampled during an earlier visit on May 3–4, 2023. Fig. 2 shows the sampling locations for water and gas throughout the mine, with exact depths relative to mean sea level.

The upper levels of the mine are wet, with significant water inflow requiring continuous drainage. In addition, water may enter the mine directly from through the open surface galleries. At Level 5, water samples were collected from three distinct locations: i) a gutter draining water from an abandoned section of the mine known as “Qaf Bulli”, ii) the roof of the excavated chamber housing the engine of Shaft N9, and iii) a pipe located within the same chamber.

In contrast, the deeper levels of the mine (Levels 17 to 21) are notably dry. These galleries are dusty, with no visible water inflow. In the deepest sections, we were unable to collect water directly from boreholes, drillholes, or fractures, and miners confirmed that such water sources are absent. However, water was observed flowing in the main transfer galleries at Levels 17 and 19, within gutters carrying water from both the shaft and the ore zone. While the origin of this water is unclear, it is likely that at least part of it comes from upper levels via Shaft N9, where water is visibly cascading along the walls. In this context, mixing between deep, shallow, and even surface water, at least from Level 5, where the top of the shaft is located, is likely. One water source was also identified near the ore zone at Level 20, although its origin could not be clearly determined.

## 3.2. Sample collection and preservation protocols

Free gases bubbling in a ~ 30 m² drainage pool at Level 19 (–110 m

bmsl) were collected using the water displacement method. Samples were stored in 10- and 120-mL glass vials capped with thick rubber septa and sealed with aluminum collars while submerged to prevent atmospheric contamination. Vials were stored upside-down to position the headspace at the bottom of the bottle.

Additional gas samples were collected in 40 mL Dursan®-coated stainless steel cylinders (Swagelok®, model 304L-HDF2-40-PD, Silco-Tek® coating) vacuumized before sampling for noble gases, volatile organic compounds (VOCs), and methane radiocarbon (¹⁴C) analyses. Cylinders were sealed using a Swagelok® SS-OVM2-BKR valve fitted with an additional protective cap. The classical copper tube method for noble gas sampling was also employed but proved unsuccessful due to air contamination after clamping and storage.

Water samples for measurements of cations, anions, and hydrogen/oxygen isotopes were filtered using 0.20 µm PTFE membranes and stored in 50 mL high-density polyethylene (HDPE) Nalgene bottles. Cation samples were immediately acidified to 0.2 wt% HNO₃. Raw water samples for tritium analysis were collected in 1 L HDPE bottles.

Alkalinity and dissolved sulfides were measured at ISTerre laboratory immediately upon return from the field using samples stored in completely filled 120 mL glass bottles sealed with rubber septa. Samples for dissolved organic carbon (DOC) analysis were filtered at 0.2 µm with PTFE membranes, acidified to 0.3 vol% HCl (analytical grade), and stored in 50 mL glass vials with screw caps featuring PTFE-lined septa. Raw water samples for ¹⁴C and δ¹³C analysis of dissolved inorganic carbon (DIC) were collected in 1 L HDPE bottles with airtight caps and no headspace to exclude atmospheric CO₂.

## 3.3. Analytical methods

### 3.3.1. Water chemistry and isotope analysis

Redox potential, conductivity, temperature and pH were measured in situ with a calibrated field meter (Thermo Scientific®, Orion 5-Star pH/ORP/Conductivity/Temperature Meter). Redox readings (ORP) were corrected for the standard Ag/AgCl-3 M KCl reference electrode (Eh = ORP + 197 mV at 25 °C) and are reported as Eh values.

Stable isotopes in water samples (δ¹⁸O-H₂O and δD-H₂O) were measured at the Hydroisotop® GmbH laboratory (Schweitenkirchen, Germany) using Cavity Ring-Down Spectroscopy (CRDS). Results are reported to the Vienna Standard Mean Ocean Water (VSMOW) standard, with analytical precision of ± 1.5 ‰ for δD and ± 0.15 ‰ for δ¹⁸O (1σ). Tritium concentrations were determined via liquid scintillation counting following electrolytic enrichment and are reported in Tritium Units (TU; 1 TU = 0.119 Bq/L). Results are referenced to the measurement date without applying half-life correction.

Alkalinity was determined via acidimetric titration to a pH of approximately 4.0. Dissolved sulfides (HS⁻ and S²⁻) were measured using a Ag⁺/S⁻¹¹ ion selective electrode (Metler Toledo®). The quantification limit is ~ 0.01 mM with a precision of 3 %.

Anions concentrations (F⁻, Cl⁻, Br⁻, NO₃⁻, NO₂⁻, SO₄²⁻, PO₄³⁻, formate, acetate and oxalate) were analyzed at the ISTerre laboratory using ion chromatography with self-regenerating suppression and conductivity detection, using a Dionex® Integrion HPIC, and an IonPac AS9 HC column, 4x250 mm Analytical. Analytical error was ± 5 % within the 95 % confidence level.

Cations concentrations (Na⁺, K⁺, Ca²⁺, Mg²⁺) were measured at the EPOC laboratory using High-Performance Liquid Chromatography (Dionex® ICS-900), equipped with a CS12A analytical column and an IONPAC® CG12A guard column. Ammonium (NH₄⁺) concentrations were determined spectrophotometrically at the MIO laboratory using the indophenol blue method. Certified reference standards were used to ensure accuracy. Analytical error was ± 5 % within the 95 % confidence level.

Dissolved organic carbon (DOC) concentrations were measured at ISTerre using a Shimadzu® TOC-VCSN analyzer equipped with an ASI-V autosampler, as detailed in Tisserand et al. (2024). Each sample was
analyzed twice, and the mean value was reported when the relative standard deviation (RSD) was below 2%. If the RSD exceeded 2%, a third analysis was performed, and the two closest replicate values were retained. External calibration was performed using a certified DOC standard (100 mg/L potassium hydrogen phthalate, Chemlab®), diluted to match sample concentrations.

The δ¹³C values of dissolved inorganic carbon (δ¹³C-DIC) were determined using an OI Analytical® Aurora 1030 TOC Analyzer with a reduction furnace, water trap, and packed GC column, coupled to either a Thermo® Electron Delta V Plus IRMS via a Conflo III interface or a Finnigan MAT 252 IRMS. Carbon was extracted as CO₂ via wet chemical oxidation, using phosphoric acid for total inorganic carbon (TIC) and sodium persulfate for DOC. Accuracy and reproducibility were ± 1.5% RSD for concentrations and ± 0.5‰ for δ¹³C values. δ¹³C is reported in delta notation relative to the Vienna Pee Dee Belemnite (VPDB) standard, with a 1σ precision better than ± 0.3‰. Radiocarbon measurements of DIC (¹⁴C-DIC) were performed by Hydroisotop® GmbH using accelerator mass-spectrometry and are reported as %-modern with 2σ uncertainties. A value of 100 pMC corresponds to 0.226 Bq/g of carbon.

### 3.3.2. Gas composition and isotopic analyses

The concentrations of He, H₂, O₂, N₂, CO, CH₄, and CO₂ were measured at ISTerre laboratory using a Perkin Elmer® CLARUS 500 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and a 2-m long column (RESTEK®, Shin Carbon ST 80/100) using Ar as carrier gas. The GC was calibrated using several Ar + H₂ + He + CH₄ + CO₂ + N₂ + O₂ mixtures of standard gases of different concentrations injected with a gas-tight syringe of calibrated volume. The estimated analytical error is ± 5% at the 95% confidence level.

The concentrations of ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), and hexane (C₆H₁₄) were measured using a Varian® 3400 gas chromatograph equipped with a flame ionization detector (FID). The hydrocarbons were separated on a GS-Q column (30 m 0.32 mm ID) with helium as carrier gas and the following temperature program: 40 °C hold for 2 min, increase to 220 °C at 10 °C/min hold for 10 min. The detection limits were in the range of 1 ppmv.

Volatile organic compounds (VOCs) were analyzed by SGS® (Villafranca Padovana, Italia), according to the EPA TO-15 protocol. A dedicated gas sample was collected from the bubbling pool at Level 19 using a Dursan®-coated stainless-steel canister that was vacuumized before sampling. The collected sample was processed using cryogenic pre-concentration to enhance sensitivity. Separated VOCs were analyzed using a high-resolution capillary GC column. Detection was performed via mass spectrometry and compounds were identified based on retention times and mass spectra from the NIST library.

The carbon and nitrogen isotopic compositions of C₂ to C₄ alkanes and N₂ were measured at the Hydroisotop® GmbH laboratory using a Thermo® TRACE gas chromatograph coupled via a Thermo® GC/C III interface to a Delta Plus XP gas-source isotope ratio mass spectrometer (IRMS). The carbon isotopic composition is reported as δ¹³C (±0.5‰) relative to the VPDB standard and the nitrogen isotopic composition is reported as δ¹⁵N (±0.3‰) relative to air standard.

### 3.3.3. Clumped isotope measurements of CH₄ and H₂

Clumped isotope analysis of methane was performed at Utrecht University following the method described by Sivan et al. (2024), using a custom-built High Concentration Extraction System. In the initial step, the full gas mixture was cryogenically trapped on silica gel. Methane purification was achieved using two packed gas chromatographic columns: a 5 m long, 1/4" OD 5A molecular sieve column, and a 2 m long, 1/4" OD HayeSep D column, operated at 50 °C with helium as the carrier gas (30 mL/min flow rate). Purified CH₄ was then re-collected on silica gel.

Both the bulk and clumped isotopic composition of the extracted CH₄ was analyzed with a Thermo® 253 Ultra high-resolution IRMS. Given

the long measurement time of several hours (compared to < 1 min for GCMS, including single-substituted isotopologues), isotope measurements with the 253 Ultra achieve exceptionally high internal precision (better that 0.01‰ for δ¹³C and 0.1‰ for δD). However, larger systematic uncertainties arise from sample handling procedures and from conversion to international standards, which dominate the external uncertainty. Taking into account the uncertainty in the laboratory calibration scale of Utrecht University, based on international inter-comparison exercises and the MPI Jena scale, we report the carbon isotopic composition as δ¹³C (±0.1‰) relative to the VPDB standard, and the hydrogen isotopic composition as δD (±1‰) relative to the VSMOW standard. Typical analytical precision was ± 0.3‰ for Δ¹³CH₃D and ± 2‰ for Δ¹²CH₂D₂. Long-term instrument reproducibility was approximately ± 0.3‰ for Δ¹³CH₃D and ± 1.7‰ for Δ¹²CH₂D₂. Δ¹³CH₃D and Δ¹²CH₂D₂ are reported on an empirical reference scale by comparing CH₄ that is isotopically equilibrated at different temperatures to thermodynamic equilibrium isotope distributions (Sivan et al., 2024, Eldridge et al., 2019).

Clumped isotope analysis of H₂ was conducted using the MAT253-Ultra at Caltech (USA) following the method described by Mangenot et al. (2023). Purification of the gas sample was carried out at IPGP (Paris, France), using a cold head maintained at 20 K. We monitor the pressure of the cold trap using a capacitance manometer until the pressure stabilizes, indicating that nominally all the condensable trace constituents of the gas mixture have condensed in the cold trap. Gases remaining in the headspace of the glass vacuum line include H₂, He, Ne and a non-zero but minor vapor pressure of N₂ – all these gases are mass-resolvable from each other on the mass spectrometer, so trace abundance are unlikely to lead to systematic errors in hydrogen isotope measurements. The purified residual H₂ was then collected on activated charcoal cooled with liquid nitrogen. To assess any potential alteration of the isotopic signature due to this purification protocol, an in-house standard was processed using the same method.

### 3.3.4. Radiocarbon dating of methane

Radiocarbon measurements of CH₄ were performed by Beta Analytic®. Methane was isolated from the bulk gas mixture by injecting a known volume and pressure into a stainless-steel loop of a Shimadzu® gas chromatograph equipped with a non-destructive TCD detector. Target gases were then collected into a manifold system. For conversion, a measured volume of purified methane was transferred into a pre-cleaned quartz combustion tube (furnace-baked at 900 °C for 2 h) and sealed at one end. The tube was connected to a vacuum manifold and evacuated to below 0.2 Torr. The methane was cryogenically condensed using liquid nitrogen (–196 °C), then flame-sealed and combusted in a temperature-controlled furnace at 900 °C for 4 h. Post-combustion, the sealed quartz tube was cracked under vacuum. The resulting CO₂ was purified cryogenically using a dry ice/alcohol slurry (~-76 °C) to remove residual moisture. CO₂ was then transferred to a calibrated volume containing a catalyst. In the presence of hydrogen, the CO₂ was reduced to graphite at 550 °C. The ¹⁴C measurements were conducted using an NEC® accelerator mass spectrometer. Values are reported as percent modern carbon (pMC), based on the Libby half-life of 5568 years and normalized to the NIST SRM-4990C standard. All values were corrected for isotopic fractionation. Agreement between expected and measured values is considered acceptable within 2σ (twice the analytical error), accounting for total laboratory uncertainty.

### 3.3.5. Noble gas analyses

⁴He and ²⁰Ne concentrations were measured on a e-Vision 2 (MKS Instruments®) quadrupole mass spectrometer. A 0.21 cm³ aliquot of gas sample from the reservoir was introduced into the purification and separation line. Gas purification was performed with SAES ST-707 getter (pills) at 400 °C, followed by an additional step at room temperature for 30 min. Helium and neon were separated from other noble gases using a charcoal trap cooled to liquid nitrogen temperature for 30 min. The He-
Ne standard consisted of laboratory air, introduced using the same pipette volume of 0.21 cm³. The inlet gas pressure was measured using a MKS Baratron (0–10 torr) to correct for pressure differences between the sample and the standard. ²⁰Ne signals were corrected for ⁴⁰Ar⁺⁺ interference using the measured ⁴⁰Ar⁺ and a double ionization factor of 0.1. Measured ⁴He and ²⁰Ne concentrations are corrected for blank contributions of 0.1 and 4%, respectively.

A separate aliquot was analyzed for helium isotopes using the procedure developed by Núñez-Guerrero et al. (2025) on the 5F5M Noblesse (Nu Instruments®) mass spectrometer at ISTO laboratory (Orléans). Operating in high resolution mode, the Noblesse 5F5M can separate HD⁺ and H₃⁺ from ³He, which is particularly critical for the H₂-rich samples. The measured ⁴He blank was 5.2 10⁻¹⁰ ccSTP, corresponding to less than 0.03% of the measured helium. Helium isotope compositions are reported as R/Ra, where R is the ³He/⁴He ratio of the sample and Ra is the atmospheric ratio (Ra = 1.386 10⁻⁶). The helium standard is a mixture of gas collected in Iceland and has a ³He/⁴He ratio of 21.07 ± 0.15 Ra (calibrated against air).

### 3.3.6. Mineralogical characterization

Salt efflorescences collected from gas emission zones underwent detailed mineralogical analysis to constrain their origin and formation mechanisms. X-ray diffraction patterns were collected on randomly oriented samples using a Bruker® D8 Advance diffractometer with Cu-Kα radiation, with quantitative phase analysis performed by Rietveld refinement using the BGMN computer program (https://www.bgmn.de). To trace the source of chloride, chlorine isotope ratios in halite sample were measured at the Hydroisotop® GmbH laboratory. This required conversion of chloride to methyl chloride through reaction with methyl iodide, followed by isotope ratio mass spectrometry achieving precision of ± 0.06 ‰ for δ³⁷Cl values reported relative to Standard Mean Ocean Chlorine (SMOC).

## 4. Results

### 4.1. Water geochemistry

#### 4.1.1. Depth-dependent evolution of water chemistry

The geochemical composition of waters flowing through the mine galleries exhibits a striking systematic evolution with depth, transforming from weakly alkaline, freshwater waters at shallow levels to hyperalkaline and saline water at depth (Table 1). At Level 5 (+420 m amsl), waters display pH values of 8.5 and electrical conductivity of 100 µS/cm, characteristic of shallow groundwater that has undergone limited water-rock interaction. In contrast, at Level 20 (–145 m bmsl), pH reaches 11.6 and electrical conductivity rises to 5,600 µS/cm, indicating extensive serpentinization reactions and the development of hyperalkaline conditions typical of serpentinizing environments.

Water temperature increases progressively with depth, from 18 °C at Level 5 to 28 °C at Level 20. Notably, water temperatures remain consistently below ambient air temperatures in the corresponding galleries, where air temperature can reach 33 °C at Level 20. The ambient air temperature in the galleries does not reflect the true temperature of the rock, as strong ventilation continuously renews and cools the air.

#### 4.1.2. Major ion concentrations and water types

The major ion chemistry reveals a fundamental shift in water type with depth (Fig. 3). Shallow waters at Level 5 are of the Na-Mg-HCO₃ type, typical of meteoric waters that have undergone limited silicate hydrolysis. Two samples collected in the shaft room at Level 5 show milli molar levels of magnesium as the primary cation, possibly reflecting localized interaction with ultramafic minerals or mixing with Mg-enriched waters.

From Level 17 downward, waters transition to Na-Cl type, with sodium and chloride accounting for more than 90% of the total cationic and anionic charge, respectively. Chloride concentrations increase by

<caption>**Table 1**</caption>

Geochemical composition of water samples collected from Level 5 down to Level 20 in the Bulqizë mine. Concentrations of dissolved species are reported in mmol/L (mM). ‘DL’ indicates the detection limit expressed in µmol/L. ‘n.m.’ denotes ‘not measured’. ‘pMC’ refers to percent Modern Carbon. *Eh values are corrected for the standard Ag/AgCl-3 M KCl reference electrode.

<table><thead><tr><th></th><th>Skanderbeg Fountain<br>Bulqizë valey</th><th>Qaf Bulli<br>Level 5</th><th>Shaft N9 roof<br>Level 5</th><th>Shaft N9 Pipe<br>Level 5</th><th>Ore zone<br>Level 17</th><th>Bubbling pool<br>Level 19</th><th>Pipe<br>Level 20</th><th>Ore zone<br>Level 20</th><th>DL (µmol/L)</th></tr></thead><tbody><tr><td>Depth (m relative to m.s.L.)</td><td>+754</td><td>+420</td><td>+420</td><td>+420</td><td>+10</td><td>-110</td><td>-145</td><td>-140</td><td></td></tr><tr><td>pH</td><td>8.9</td><td>9.9</td><td>8.6</td><td>8.5</td><td>10.6</td><td>11.2</td><td>11.6</td><td>11.1</td><td></td></tr><tr><td>T (°C)</td><td>10.9</td><td>17.6</td><td></td><td></td><td>22.1</td><td>21.5</td><td>25.6</td><td>27.8</td><td></td></tr><tr><td>Eh (mV)*</td><td>355</td><td></td><td></td><td></td><td>163</td><td>51</td><td>44</td><td>34</td><td></td></tr><tr><td>Conductivity (µS/cm)</td><td>134</td><td>106</td><td>266</td><td>266</td><td>1434</td><td>3520</td><td>5670</td><td>4530</td><td></td></tr><tr><td>δ¹⁸O-H₂O (‰ VSMOW)</td><td></td><td>-9.62</td><td></td><td></td><td>-8.98</td><td>-8.97</td><td></td><td></td><td></td></tr><tr><td>δD-H₂O (‰ VSMOW)</td><td></td><td>-62.7</td><td></td><td></td><td>-52.8</td><td>-47.1</td><td></td><td></td><td></td></tr><tr><td>Deuterium excess (‰)</td><td></td><td>14.26</td><td></td><td></td><td>19.04</td><td>24.66</td><td></td><td></td><td></td></tr><tr><td>³H-H₂O (TU)</td><td></td><td>3.5 ± 0.6</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cl⁻ (mmol/L)</td><td></td><td>0.23</td><td>0.22</td><td>0.23</td><td>13</td><td>40</td><td>80.6</td><td>74.6</td><td>0.2</td></tr><tr><td>SO₄²⁻</td><td></td><td>0.02</td><td>0.04</td><td>0.03</td><td>0.30</td><td>0.5</td><td>1.11</td><td>0.66</td><td>0.6</td></tr><tr><td>NO₃⁻</td><td></td><td>0.04</td><td>0.08</td><td>0.08</td><td>0.59</td><td>0.97</td><td>1.78</td><td>0.91</td><td>0.8</td></tr><tr><td>NO₂⁻</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.7</td></tr><tr><td>PO₃⁻</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.8</td></tr><tr><td>F⁻</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.1</td></tr><tr><td>Br⁻</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.01</td><td>0.032</td><td>0.052</td><td>0.042</td><td>0.6</td></tr><tr><td>Formate</td><td></td><td>0.002</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.3</td></tr><tr><td>Acetate</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.3</td></tr><tr><td>Oxalate</td><td></td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.017</td><td>0.8</td></tr><tr><td>Dissolved sulfides</td><td></td><td>&lt;DL</td><td>n.m.</td><td>n.m.</td><td>n.m.</td><td>&lt;DL</td><td>n.m.</td><td>&lt;DL</td><td>10</td></tr><tr><td>Alkalinity</td><td></td><td>0.7</td><td>2.6</td><td>2.2</td><td>13.7</td><td>17.6</td><td>7.7</td><td>17.8</td><td>100</td></tr><tr><td>Na⁺</td><td></td><td>0.4</td><td>0.11</td><td>0.09</td><td>25.3</td><td>57.8</td><td>90.1</td><td>92.1</td><td>0.01</td></tr><tr><td>K⁺</td><td></td><td>0.02</td><td>0.03</td><td>0.03</td><td>0.23</td><td>0.61</td><td>1.19</td><td>0.82</td><td>0.04</td></tr><tr><td>Mg²⁺</td><td></td><td>0.16</td><td>1.4</td><td>1.23</td><td>1.19</td><td>0.59</td><td>0.55</td><td>0.90</td><td>0.01</td></tr><tr><td>Ca²⁺</td><td></td><td>0.17</td><td>0.03</td><td>0.02</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>&lt;DL</td><td>0.01</td></tr><tr><td>NH₄⁺</td><td></td><td>n.m.</td><td>n.m.</td><td>&lt;DL</td><td>n.m.</td><td>0.029</td><td>n.m.</td><td>n.m.</td><td>0.1</td></tr><tr><td>DOC</td><td></td><td>0.06</td><td>0.1</td><td>0.1</td><td>0.07</td><td>0.18</td><td>0.47</td><td>0.31</td><td>2.5</td></tr><tr><td>δ¹³C-DIC (‰ VPDB)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-19</td><td></td></tr><tr><td>¹⁴C-DIC (pMC)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>95.8 ± 0.5</td><td></td></tr><tr><td>δ³⁷Cl<sub>halite</sub> (‰ SMOC)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>-0.56 ± 0.06</td><td></td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Piper diagram illustrating the major ion composition of the sampled waters. The fresh waters from Level 5 are clearly distinguishable from the hyperalkaline waters collected at deeper levels (Levels 17 to 20).</figcaption></figure>

three orders of magnitude from Level 5 to Level 20, reaching 90 mM at the deepest accessible level. The Na/Cl molar ratio ranges from 1.1 to 1.9 in these deep waters, significantly exceeding the seawater ratio of 0.86, and indicating excess sodium.

At Level 20, the total dissolved solids reach 5.4 g/L, approximately 15 % of seawater salinity, yet the ionic ratios differ markedly from marine composition. The Na/K molar ratio ranges from 76 to 110 at Levels 17–20, compared to 47 in seawater. Dissolved sulfate and nitrate concentrations increase with depth, reaching up to 1.11 mM and 1.78 mM, respectively, at Level 20. This pattern is compatible with microbial growth and respiration, utilizing sulfate and nitrate as electron acceptors and hydrogen as the electron donor. In contrast, calcium becomes progressively depleted and is ultimately undetectable at the deepest levels. Ammonium was detected only at low concentration (29 µM) at Level 19.

Bromide appears only in waters from Levels 17–20, yielding Cl/Br molar ratios of 1,250–1,800, substantially higher than the seawater value of 655. The strong positive correlation ($r^2 = 0.95$) between Cl⁻ and Br⁻ concentrations indicates conservative mixing between a saline end-member and dilute meteoric water. Nitrite, phosphate, fluoride and sulfides remain below detection limits in all samples.

### 4.1.3. Carbon system dynamics

Alkalinity increases with depth from < 3 mM at Level 5 to 8–18 mM at deeper levels. The speciation of dissolved inorganic carbon shifts from bicarbonate-dominated at shallow levels to carbonate-dominated at depth, with hydroxide ions significantly contributing to the measured

alkalinity in the hyperalkaline waters. Water from the ore zone at Level 20 contained dissolved inorganic carbon with a carbon isotope value (δ¹³C-DIC) of −19 ‰ VPDB. Radiocarbon analysis of DIC from Level 20 yielded 95.8 ± 0.5 pMC (percent Modern Carbon), unequivocally demonstrating a modern carbon source. Dissolved organic carbon (DOC) concentrations, while remaining relatively low, show a systematic increase from approximately 0.1 mM at Level 5 to 0.5 mM at Level 20. Trace amount of specific carboxylate anions were also detected, including 2 µM formate at Level 5 and 17 µM oxalate at Level 20.

### 4.1.4. Water isotopes and residence time constraints

The stable isotope composition of water samples provides critical constraints on water origin and flow paths. All analyzed samples from Levels 5, 17, and 19 plot along the local meteoric water line (LMWL), with δD values ranging from −63 to −47 ‰ and δ¹⁸O values from −9.6 to −8.9 ‰ VSMOW (Fig. 4a). These values are consistent with the isotopic signature of modern precipitation in northeastern Albania (Eftimi et al., 2009) and show no evidence of isotopic modification through high-temperature water-rock interaction or evaporation, which would shift samples to the right of the LMWL.

Using the isotopic lapse rate established by Eftimi et al. (2002, 2009) for northeastern Albania, we calculate mean recharge altitudes between 900 and 1,400 m amsl (Fig. 4b). This range aligns well with the local topography, where the Bulqizë valley floor sits at 750 m amsl and surrounding peaks reach elevations up to 2,000 m amsl, supporting the interpretation that groundwater derives from precipitation on the upper portions of the ophiolite massif. The lowest part of the massif, near Klos,
<figure><img src="image_8.png" /><figcaption>Fig. 4. (A) δ¹⁸O vs. δD and (B) δ¹⁸O vs. altitude (in m above mean sea level) binary plots of water samples from Level 5, 17 and 20 of the mine compared with the Local Meteoric Water Line (LMWL), δD = 7.29 δ¹⁸O + 13.18 (Eftimi, 2009); and the Global Meteoric Water Line (GMWL), δD = 8 δ¹⁸O + 10. Error bars are within the size of the symbols.</figcaption></figure>

is at about 300 m amsl.

Tritium analysis provides tight constraints on groundwater age. The measured concentration of 3.5 ± 0.6 TU at Level 5 unequivocally indicates modern recharge. In the Balkans region, modern precipitation typically contains 1.6–9 TU annually, with an average of 5.3 TU based on IAEA/WMO Global Network of Isotopes in Precipitation (GNIP) data from neighboring stations in Greece and Croatia (IAEA, 2018). Given the tritium input history in the region, any water recharged before 1952 would contain < 0.6 TU due to radioactive decay. The observed tritium levels therefore constrain the apparent groundwater age at Level 5 to less than 10 years, indicating rapid circulation from the surface to depths exceeding 400 m.

The modern age of deep groundwater is further corroborated by the radiocarbon content of DIC at Level 20 (95.8 ± 0.5 pMC). Following established criteria (Olsson, 1970; Kelemen and Matter, 2008), waters with > 95 % modern ¹⁴C activity are considered to have interacted with the atmosphere within the past few decades. This modern signature, combined with the meteoric water isotope values and measurable tritium, demonstrates that the bulk of water flowing through the mine represents active, rapid circulation of surface-derived water rather than connate fluids or ancient formation waters.

## 4.2. Structural controls on gas emissions and salt precipitation

### 4.2.1. Spatial distribution and structural controls of H₂ degassing

Hydrogen gas emissions within the Bulqizë mine display strong spatial heterogeneity controlled by structural geology. Our comprehensive mapping, based on continuous monitoring data from static sensors, spot measurements of H₂ concentrations in gallery air and drill holes, and information from mine safety personnel, reveals that significant gas discharge is restricted to the deeper levels of the mine (≥Level 17) and concentrated along specific structural features (Figs. 5 and 6).

Background H₂ concentrations in gallery air show a systematic increase with depth: below detection limit (<5 ppmv) at Level 5, 2,400 ppmv at Level 17, 4,000 ppmv at Level 19, and 7,000 ppmv at Level 21. These background levels are strongly reduced by the mine's ventilation system, which dilutes gas concentrations through continuous air circulation. Local H₂ concentrations near active emission points can exceed these background values by orders of magnitude, with measurements up to 40,000 ppmv (4 vol%) recorded in poorly ventilated dead-end galleries.

The primary control on gas emission is a major fault zone oriented N215° and dipping steeply westward at 80°. This structure extends through multiple mine levels and appears to serve as the main conduit for deep gas migration. The fault core, approximately 10 m wide, consists of highly fractured and partially serpentinized rock with abundant

slickensides indicating repeated movement. The damage zone extends 5–20 m on either side of the main fault and is characterized by two dominant sets of secondary structures: (1) N50–59° trending fractures dipping 50–85°W, and (2) N90–99° trending fractures dipping 35–55°E. (Fig. 6). At Level 17, structural mapping identified 41 fracture planes with two primary orientations: N80–89° dipping 45–60°W and N10–19° dipping 40–85°E. The similarity between the N90–99° family at Level 19 and the N80–89° family at Level 17 suggests that they represent the same structural system with slight local variations in orientation. Importantly, gas emissions, as revealed by bubbling observed in gutters adjacent to the galleries and point measurements in shootholes, are not uniformly distributed along these structures but are concentrated at fault intersections and where faults cut through or run adjacent to chromitite ore bodies.

### 4.2.2. Salt efflorescence mineralogy and formation

A striking feature of the gas emission zones is the presence of white salt efflorescences coating fracture surfaces and gallery walls, despite the absence of visible groundwater seepage at these locations. These precipitates occur exclusively in areas of active H₂ degassing and form delicate crystalline crusts up to several millimeters thick (Fig. 5).

X-ray diffraction analysis reveals that halite (NaCl) comprises 85–95 % of the salt deposits, with minor amounts of trona (Na₃H(CO₃)₂·2H₂O, 3–10 %) and trace quantities of thenardite (Na₂SO₄, <2 %). The dominance of halite is consistent with evaporation of Na-Cl type waters, while the presence of trona may indicate either CO₂ uptake by alkaline droplets during evaporation or that the original fluid carried dissolved carbonates, forming sodium bicarbonate that subsequently dehydrates to trona under the warm, dry conditions of the mine galleries.

Chlorine isotope analysis of the halite yielded δ³⁷Cl = -0.56 ± 0.06 ‰ SMOC (Standard Mean Ocean Chloride), indicating slight depletion in ³⁷Cl relative to seawater (defined as 0 ‰). This isotopic signature provides important constraints on the chloride source. The observed value falls within the range typical of seafloor serpentinites (-1.6 to +0.5 ‰; Barnes and Sharp, 2006) and suggests either interaction with paleo-seawater during serpentinization or derivation from a crustal source with similar isotopic composition.

The formation mechanism of these salt deposits appears to involve the capillary migration of minute amounts of saline groundwater along fractures in association with gas flow. As this water reaches the gallery walls, rapid evaporation in the warm, ventilated mine atmosphere precipitates the dissolved salts. The spatial correlation between salt deposits and gas emissions suggests that these fluids share common migration pathways, though the volumetric flow of water is minimal compared to gas discharge.
<figure><img src="image_9.png" /><figcaption>Fig. 5. Map showing gas emissions and salt (halite) efflorescences from the section spanning Levels 18 to 20, as covered by the investigations. Pictures of salt efflorescence along fractures, as well as images of permanent H₂ sensors, are also included.</figcaption></figure>

### 4.3. Free gas composition and isotope systematics

#### 4.3.1. Bulk gas composition and molecular ratios

Free gas actively bubbling through water pools, drainage gutters, and emanating from fractures at Level 19 displays a remarkably uniform composition dominated by hydrogen and methane (Table 2, Fig. 7).

The H₂/CH₄ ratio of 6.4 falls within the range observed in other serpentinization-influenced systems but toward the hydrogen-rich end of the spectrum. The virtual absence of CO₂ is noteworthy given that

CO₂ reduction is often invoked as the carbon source for abiotic methane formation. This suggests either complete consumption of CO₂ in methane-forming reactions or, more plausibly, that all CO₂ remains dissolved in hyperalkaline, pristine water.

Analysis of C₂-C₆ alkanes reveals a distribution dominated by ethane (C₂H₆, 1141 ± 50 ppmv), followed by propane (C₃H₈, 150 ± 10 ppmv), n-butane (n-C₄H₁₀, 180 ± 10 ppmv), and n-pentane (n-C₅H₁₂, 9 ± 1 ppmv). The dry gas index, calculated as CH₄/(C₂+C₃+C₄+C₅), equals 88, substantially higher than typical thermogenic gases (15–30) but
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 6.** (A) Picture of the bubbling pool (commonly referred to as the 'jacuzzi') at Level 19 (–110 m bmsl), with symbols highlighting relationships between the tectonic zone, fracture planes, and gas emissions. (B) Structural characterization of fracture planes measured at Levels 19 and 17 of the Bulqizë mine.</figcaption></figure>
<table><caption>**Table 2**
Molecular and isotopic composition of free gas collected at Level 19 (–110 m bmsl) in Bulqizë Mine. ‘pMC’ refers to percent Modern Carbon.</caption><thead><tr><th>Free gas from L19</th><th>Concentration vol%/ppmv</th><th>δD % VSMOW</th><th>δ<sup>13</sup>C % VPDB</th><th>δ<sup>15</sup>N % VAir</th><th>ΔDD</th><th>Δ<sup>13</sup>CH<sub>3</sub>D</th><th>Δ<sup>12</sup>CH<sub>2</sub>D<sub>2</sub></th><th>pMC<sup>a</sup></th><th>R/Ra <sup>4</sup>He/<sup>20</sup>Ne</th></tr></thead><tbody><tr><td>O<sub>2</sub></td><td><u>&lt;50</u></td><td></td><td></td><td></td><td>255 ± 35</td><td></td><td></td><td></td><td></td></tr><tr><td>H<sub>2</sub></td><td><u>84</u></td><td>-743.5 ± 1.1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>CH<sub>4</sub></td><td><u>13.2</u></td><td>-132 ± 1</td><td>-12.3 ± 0.1</td><td></td><td></td><td>2.52 ± 0.26</td><td>9.95 ± 1.6</td><td>3.76 ± 0.06</td><td></td></tr><tr><td>C<sub>2</sub> (Ethane)</td><td><u>1141</u></td><td></td><td>-15.9 ± 0.5</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>C<sub>3</sub> (Propane)</td><td><u>150</u></td><td></td><td>-21.8 ± 0.5</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>C<sub>4</sub> (i-But.; n-But.)</td><td><u>11; 180</u></td><td></td><td>-25.6 ± 0.5</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>C<sub>5</sub> (i-Pent.; n-Pent.)</td><td><u>&lt;1; 9</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>CO<sub>2</sub></td><td><u>100.9</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>CO</td><td><u>&lt;100</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>N<sub>2</sub></td><td><u>2.7</u></td><td></td><td>-0.2 ± 0.3</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>He</td><td><u>9.3</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.081 ± 0.001</td></tr><tr><td>Ne</td><td><u>1.5</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td><u>6.8</u></td></tr><tr><td>Kr</td><td><u>6.3</u></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 7.** Ternary diagram (H₂-CH₄-N₂) showing the composition of free gas from Bulqizë mine compared to gases from other ophiolite degassing sites worldwide. Free gas samples from Bulqizë are O₂-free, which is not the case for some sample reported in the literature. In those cases, the data were corrected for O₂ dilution.</figcaption></figure>

consistent with high-temperature abiotic synthesis or extensive molecular fractionation during migration (Milkov and Etiope, 2018; Reeves and Fiebig, 2020).

### 4.3.2. Volatile organic compounds

Gas chromatography-mass spectrometry analysis revealed a complex suite of volatile organic compounds beyond the simple alkanes (Table 3). The most abundant VOCs include:
<caption>**Table 3**</caption>

Volatile organic compounds in the free gas phase sampled at Level 19 (–110 m bmsl) in Bulqizë mine.

<table><thead><tr><th>Probable structure</th><th>Concentration (ppmv)</th><th>Concentration (µg/m³)</th></tr></thead><tbody><tr><td>3-Methylpentane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>3-Methylhexane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>n-Hexane</td><td>21.3</td><td>74,200</td></tr><tr><td>n-Heptane</td><td>0.09</td><td>370</td></tr><tr><td>Methylcyclopentane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Dimethylcyclopentane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Methylcyclohexane</td><td>4.7</td><td>18,800</td></tr><tr><td>1-Ethyl-2-methylcyclopentane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Dimethylcyclohexane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Ethylcyclohexane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>1-Ethyl-3-methylcyclohexane</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Cyclohexane</td><td>13.3</td><td>45,000</td></tr><tr><td>Benzene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Toluene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Ethylbenzene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Xylene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Propylbenzene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Ethyltoluene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Trimethylbenzene</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Ethanol</td><td>0.75</td><td>1400</td></tr><tr><td>Methanol</td><td>&lt;DL</td><td>&lt;DL</td></tr><tr><td>Trichloroethylene</td><td>0.08</td><td>420</td></tr><tr><td>Dimethyl Sulfide</td><td>&lt;DL</td><td>&lt;DL</td></tr></tbody></table>

**Cycloalkanes:** Cyclohexane dominates at 13.3 ± 0.1 ppmv, followed by methylcyclohexane at 4.7 ± 0.1 ppmv. These compounds are unusual in gases from serpentinization environments and their presence suggests either a thermogenic contribution or novel synthesis pathways.

**Linear alkanes:** n-hexane (21.3 ± 0.1 ppmv) and n-heptane (0.09 ± 0.01 ppmv) are present in measurable quantities, while longer-chain alkanes (>C7) remain below detection limits.

**Aromatic hydrocarbons:** BTEX compounds (benzene, toluene, ethylbenzene, xylenes) all remain below the detection limit of 10 ppbv, arguing against significant thermogenic or petrogenic contributions.

**Alcohols:** Only ethanol was detected (0.75 ± 0.01 ppmv), with methanol notably absent despite its common formation through CO₂ hydrogenation.

**Chlorinated compounds:** Trichloroethene (0.08 ± 0.01 ppmv) represents the only detected organochlorine compound.

**Sulfur compounds:** No organosulfides (dimethyl sulfide, diethyl sulfide, dimethyl disulfide, carbon disulfide) were detected, consistent with the absence of H₂S and the generally sulfur-poor nature of the system.

This VOCs distribution presents a puzzle: the abundance of cyclohexane and methylcyclohexane suggests a relatively low-temperature origin, as these compounds typically decompose during high-temperature maturation, yet the absence of aromatic markers as BTEX compounds does not point toward a typical thermogenic source.

### 4.3.3. Hydrogen isotope systematics

Hydrogen gas exhibits extreme deuterium depletion with δD = –743 ± 3‰ VSMOW, placing it among the most D-depleted terrestrial hydrogen measured. This value falls within the range reported for other continental serpentinization systems (–700 to –800‰) and reflects the strong isotopic fractionation during H₂ generation from water (Etiope et al., 2011; Walter et al., 2012; D'Alesandro et al., 2018b; Milkov, 2022; Suda et al., 2022; Lévy et al., 2023).

The clumped isotope composition of H₂, expressed as ΔDD = 255 ± 35‰ (2σ), provides measurements not previously available from an ophiolitic setting. Assuming intramolecular isotopic equilibrium, this ΔDD value corresponds to an apparent formation temperature of 0 ± 35°C using the calibration of Popa et al. (2019), and accounting for the associated uncertainty. While the lower bound of this temperature

range is geologically implausible for active hydrogen generation, the upper bound (~35°C) is consistent with current venting temperatures measured in the mine.

The large uncertainty in ΔDD reflects the analytical challenges of measuring clumped isotopes in extremely D-depleted samples, where the abundance of the DD molecule approaches the detection limit. Nevertheless, this measurement provides important constraints on the thermal history of the hydrogen, suggesting either formation at low temperatures or rapid re-equilibration during ascent.

### 4.3.4. Methane isotope systematics

Methane displays a complex isotopic signature that provides crucial insights into its origin and formation conditions. The carbon isotope composition (δ¹³C = –12.3 ± 0.1‰ VPDB) is unusually enriched in ¹³C compared to thermogenic and microbial sources, while the hydrogen isotope composition (δD = –132 ± 1‰ VSMOW) falls within the typical range for abiotic methane in ophiolites (Fig. 8a).

The clumped isotope composition of methane reveals significant isotopic disequilibrium:

$$ \bullet \Delta^{13}CH_3D = 2.52 \pm 0.26 \text{‰} $$

$$ \bullet \Delta^{12}CH_2D_2 = 9.95 \pm 1.6 \text{‰} $$

These values plot distinctly above the thermodynamic equilibrium curve in Δ¹³CH₃D vs. Δ¹²CH₂D₂ space (Fig. 8b), indicating either formation under disequilibrium conditions or post-formation modification. This confirms that the methane has not equilibrated internally, possibly due to kinetic isotope effects during rapid formation, mixing of multiple sources, or fractionation during migration.

Most remarkably, radiocarbon analysis yielded 3.76 ± 0.06 pMC, corresponding to an apparent age of 26,150 ± 130 years BP. This detection of ¹⁴C in ophiolitic methane is unprecedented, since all previous measurements at serpentinization sites have yielded ¹⁴C-free methane (Etiope and Oze, 2022). This non-fossil signature demands either incorporation of young carbon during methane formation or mixing between ancient and recently formed methane.

### 4.3.5. Higher alkane isotope patterns

The carbon isotope compositions of C₂-C₄ alkanes reveal a complete reversal relative to methane (Fig. 8c):

$$ \bullet \text{Ethane (C}_2\text{H}_6): \delta^{13}\text{C} = -15.9 \pm 0.5 \text{‰} $$

$$ \bullet \text{Propane (C}_3\text{H}_8): \delta^{13}\text{C} = -21.8 \pm 0.5 \text{‰} $$

$$ \bullet \text{n-Butane (n-C}_4\text{H}_10): \delta^{13}\text{C} = -25.6 \pm 0.5 \text{‰} $$

This pattern, where δ¹³C becomes progressively more negative with increasing carbon number, contrasts sharply with typical thermogenic gases where δ¹³C increases with chain length.

### 4.3.6. Nitrogen and noble gas systematics

Nitrogen isotope analysis yielded δ¹⁵N = –0.2 ± 0.3‰ (2σ) relative to air, indicating an atmospheric origin for the nitrogen, although we cannot exclude that post genetic processes may fortuitously lead to this signature. The absence of detectable oxygen, rules out air contamination during sampling and suggests that nitrogen was introduced via air-saturated water recharge. In this scenario, O₂ was likely consumed during the early stages of water infiltration by microbial respiration and weathering of ultramafic rocks.

Noble gas isotope ratios provide critical constraints on gas sources and migration pathways (Table 2):

$$ \bullet \text{R/Ra} = 0.081 \pm 0.001 \text{ (where Ra is the } ^{3}\text{He}/^{4}\text{He atmospheric ratio of } 1.386 \times 10^{-6}\text{)} $$

$$ \bullet ^{4}\text{He} = 9.3 \text{ppmv (air has } 5.24 \text{ppmv)} $$
<figure><img src="image_14.png" /><figcaption>Fig. 8. (A) Plot of gaseous methane δ13C vs. δD for the sample from the bubbling pool at Level 19, compared with data from other dry seeps and bubbling hyperalkaline springs in ophiolites worldwide: Oman (Nothaft et al., 2021), The Cedar, Aqua de Ney, and Tablelands (Cook et al., 2021), Cabeço de Vide (Etiope et al., 2013a), Ronda (Ojeda et al., 2023), Elba (Sciarra et al., 2019), Voltri (Boschetti et al., 2013, Etiope et al. 2013b), Orthys (Etiope et al., 2013b), Argolida (D’Alessandro 2018a), Chimaera (Etiope et al., 2011), Kizildag (D’Alessandro et al., 2018b), Happo (Suda et al., 2014), Zambales (Abrajano et al., 1990), Tanjung Api (Subroto et al., 2004), Poison Bay (Lyon et al., 1990); Dun Mountain (Pawson, 2014). For comparison, the serpentinite-hosted Lost City submarine hydrothermal field, characterized by hyperakaline fluids, is also included for comparison (Proskurowski et al., 2008). The typical range where most of the data from thermogenic methane plot is also shown. The commonly referenced microbial and abiotic fields are intentionally omitted, as they are based on purely empirical classifications that are currently debated within the scientific community. Notably, the data from dry seeps cluster within a much narrower range than those from bubbling hyper-alkaline springs. (B) Plot of Δ13CH3D vs. Δ12CH2D2 for gas sample from Level 19 of the Bulqizë Mine compared with available data for other ophiolites worldwide (Young et al., 2017; Blank et al., 2009; Ojeda et al., 2023; Nothaft et al., 2021). The equilibrium curve is shown for reference. Possible qualitative scenarios that could explain the positive Δ12CH2D2 deviations from equilibrium, such as mixing and diffusion, are also illustrated. (C) Plot of δ13C values for the C1 to C4 alkane series from the bubbling pool at Level 19, compared with two other dry seeps from Kizildag ophiolite (D’Alessandro et al., 2018b) and the Chimaera ophiolite (Etiope et al., 2011), along with a typical thermogenic trend (Des Marais et al., 1981; Chung et al., 1988; Milkov et al., 2020). A clear isotopic reversal is observed in the alkane gases from Level 19 in the Bulqizë mine. All error bars are within the size of the symbols.</figcaption></figure>

$$ \bullet \ ^{4}\text{He}/^{20}\text{Ne} = 6.8 \pm 0.1 $$

These compositions plot along a mixing line between atmospheric and crustal end-members in ³He/⁴He vs. ⁴He/²⁰Ne space, with no resolvable mantle contribution (Fig. 9). Three-component mixing calculations using the formulation of Sano and Wakita (1985) yield:

* Atmospheric He: 80–85 %

* Crustal He: 15–20 %

* Mantle He: <1 %

The dominance of atmospheric helium, combined with measurable crustal radiogenic ⁴He, indicates an open system with active recharge of air-saturated water and contribution from deeper crustal sources. The absence of significant mantle helium argues against deep-seated magmatic or mantle-derived fluids contributing to the gas system.

# 5. Discussion

## 5.1. Decoupled hydrological systems: Modern water circulation versus deep gas migration

The comprehensive hydrogeochemical data from Bulqizë reveal a fundamental paradox that challenges conventional understanding of fluid circulation in ophiolites. Supplementary Tables S1 and S2 synthesize the key observations that constrain our understanding of this unique natural hydrogen system. Despite exhibiting extreme chemical evolution characteristic of extensive water-rock interaction and serpentinization, with pH values reaching 11.6 and Na-Cl dominated chemistry at depth, multiple isotopic tracers unequivocally demonstrate that these waters are modern and of meteoric origin. The water isotope compositions (δD = -63 to -47 ‰; δ¹⁸O = -9.7 to -8.9 ‰) plot precisely along the local meteoric water line, showing no evidence of the isotopic shifts expected from high-temperature water-rock interaction
<figure><img src="image_15.png" /></figure>

Fig. 9. $ ^{3}He/$$^{4}He $ vs. $ ^{4}He/$$^{20}Ne $ of free gas sampled from the bubbling pool at Level 19, with mixing lines between air, mantle, and crustal sources (R/Ra = $ ^{3}He/$$^{4}He $ ratio of the sample normalized over the same ratio for the air = 1.386 $ 10$^{-6}$). The H$_{2}$-rich gas from Bulqizë plots along a mixing line between a dominant air component (80–85%) and a crustal component (15–20%), with no detectable mantle contribution. The end member values are: Air $ ^{3}He/$$^{4}He $ = 1 Ra and $ ^{4}He/$$^{20}Ne $ = 0.32, (calculated from Porcelli et al., 2002), Mantle $ ^{3}He/$$^{4}He $ = 8 Ra (Graham, 2002) and $ ^{4}He/$$^{20}Ne $ = 3,500 (Barry et al., 2013); Crustal $ ^{3}He/$$^{4}He $ ranges from an average crustal production value of 0.008 RA (Ballentine and Burnard, 2002) to an often-used upper limit of 0.05 Ra (e.g. Barry et al., 2013). Error bars are within the size of the symbol.

or evaporation (Fig. 4). Calculated recharge altitudes of 900–1,400 m amsl correspond perfectly to the local topography of the Bulqizë massif, where peaks reach 2,000 m elevation.

The modern nature of these waters is further confirmed by tritium concentration of 3.5 ± 0.6 TU at Level 5, which falls well within the range of contemporary precipitation in the Balkans (1.6 to 9 TU). This constrains groundwater residence times to less than 10 years, implying remarkably rapid vertical flow velocities exceeding 60 m/year through the fractured ophiolite. Even more striking, radiocarbon analysis of dissolved inorganic carbon (DIC) at Level 20 yields 95.8 ± 0.5 pMC, confirming recent atmospheric exchange at depths exceeding 600 m below surface and ruling out the possibility that all DIC derives from dissolution of ancient carbonate rocks or deep-seated carbon sources. Notably, these modern signatures persist in the most chemically evolved waters, suggesting that the hyperalkaline chemistry results from recent interaction with readily soluble secondary phases rather than prolonged primary water-rock interaction.

The carbon isotope composition of dissolved inorganic carbon provides crucial insights into carbon sources and cycling. The $ ^{13}C-$DIC value of -19‰ VPDB measured in water from the ore zone at Level 20 indicates that a substantial portion of the DIC originates from the oxidation of organic matter, whether through microbial respiration or abiotic oxidation processes. This interpretation is supported by the stark contrast with local Triassic and Jurassic carbonate rocks, which exhibit $ ^{13}C $ values ranging from -1 to +4‰ (Tanner, 2010; Husinec et al., 2022; Gemignani et al., 2022). The depleted $ ^{13}C $ signature could result from the oxidation of methane, other hydrocarbons, or sedimentary organic matter, all of which would produce isotopically light DIC. Interestingly, DOC concentrations, while remaining relatively low, show a systematic increase from approximately 0.1 mM at Level 5 to 0.5 mM at Level 20, potentially stimulating the activity of heterotrophic microorganisms (Brazelton et al., 2013; Suzuki et al., 2014). A similar trend is observed for other electron acceptors, such as nitrate and sulfate, which also support microbial processes. The detection of trace amount of specific carboxylate anions, 2 µM formate at Level 5 and 17 µM oxalate at Level 20, further indicates microbial activity or organic

matter degradation in these extreme pH conditions (Rempfert et al., 2017).

The evolution from Na-Mg-HCO$_{3}$ type waters at shallow levels to Na-Cl dominated hyperalkaline water with up to 90 mM chloride at depth requires a novel explanation given these modern water ages (Fig. 3). We propose that this chemistry reflects a dynamic process of salt recycling within the mine environment. Salt efflorescences observed exclusively in gas emission zones consist predominantly of halite (85–95%) with a distinctive $ ^{37}Cl $ signature of -0.56‰, consistent with either paleo-seawater interaction or sedimentary brine sources. The spatial correlation between these salt deposits and H$_{2}$ emissions indicates shared migration pathways for gas and minute amounts of saline water. As this pristine saline water reaches gallery walls, rapid evaporation in the warm, ventilated mine atmosphere precipitates the salts. These salts are subsequently re-dissolved by descending meteoric water, creating the observed chemistry. This cyclic enrichment model reconciles modern water ages with evolved chemical signatures while explaining the extreme Cl/Br ratios (1,250–1,800) that significantly exceed seawater values. Additionally, the presence of dissolved carbonate and the modern radiocarbon age of DIC support the hypothesis of direct atmospheric CO$_{2}$ uptake by hyperalkaline water, occurring both during evaporation along the gallery walls (leading to the precipitation of small amount of trona, Na$_{2}$(CO$_{3}$)·NaH(CO$_{3}$)·2 H$_{2}$O) and as water flows through the drainage system. When modern meteoric water percolates from the surface, it can leash and dissolve these salt deposits, and eventually mix with minute amounts of hyperalkaline and saline groundwater, thereby explaining both the chemical and isotopic characteristics of the deeper water samples. Even if the end member of the groundwater migrating at depth could not be directly identified (and may not emanate in the mining system), it is expected that this pristine water must be hyper-alkaline and salty.

The stark contrast between water and gas residence times provides the key to understanding fluid dynamics in this system. While water shows residence times of less than 10 years, methane contains radio-carbon indicating an apparent age of ~ 26,000 years. This five-order-of-magnitude difference in timescales demonstrates a fundamental decoupling of liquid and gas phases during migration. Gas moves as a separate phase through the fracture network, with minimal interaction with rapidly circulating meteoric water. The N215° fault system and its associated damage zone provide high-permeability conduits that preferentially focus buoyant gas flow, while meteoric water enters through open surface galleries, circulates within the broader fracture network in the upper part of the mine, and descends by gravity through shafts and galleries to reach the lower levels. In partially saturated zones, capillary forces may create additional barriers that further segregate gas and water flow paths, explaining the absence of visible water seepage in gas emission zones despite active groundwater circulation elsewhere in the mine. The presence of salt deposits along fractures provides additional evidence for this decoupling, indicating that minute amounts of saline water migrate alongside the gas phase but separately from the dominant meteoric water flow. Given the high gas flux and the absence of visible water inflow, the occurrence of salt deposits associated with active gas discharge suggests a diphasic migration of fluids, involving both minute amount of pristine saline groundwater water and gas phases.

## 5.2. Sources and sinks of chloride in ophiolitic environments

The majority of hyperalkaline waters found in serpentinizing environments on continents (e.g. Oman, Liguria, New Caledonia) have salinities comparable to those of continental waters. The presence of significant chloride concentrations in Bulqizë deep waters raises fundamental questions about salt sources in ultramafic environments. Unlike sedimentary basins where evaporites or formation brines provide obvious chloride reservoirs, ophiolites typically lack such sources, making the origin of salinity enigmatic.

The spatial association between salt efflorescences and gas emission
may suggest a shared migration pathway between a pristine hyper-
alkaline saline groundwater and gas, which is distinct from most of the
water circulating within the galleries (Figs. 5 and 6). The occurrence of
Na-Cl type saline water in ophiolites is uncommon but has been docu-
mented in the Cedars and Aqua de Ney springs (both in Northern Cali-
fornia, USA), as well as in the Troodos ophiolite in Cyprus (Morrill et al.,
2013; Boschetti et al., 2017; Neal and Shand, 2002; Evans et al. 2024).
The origin of the salt, however, remains poorly constrained.

At The Cedars, it has been proposed that chloride, bromide, and
hydrocarbons originate from Late Cretaceous marine shales of the
Franciscan Subduction Complex underlying the peridotite body (Morrill
et al., 2013). The high pH water at Aqua de Ney has a seawater-like
salinity. Based on the δ¹¹B, δ⁷Li, B/Cl and Br/Cl ratios, Boschetti et al.
(2017) proposed that this water has originated from the dehydrating
slab of oceanic crust in the Cascades subduction zone, similarly to the
upwelling fluids of the mud volcanoes in the Marianna forearc (Mottl
et al., 2004). However, these chemical signatures are also compatible
with an evaporitic environment (Cook et al., 2021). In Cyprus, increased
groundwater salinity has been attributed to mixing with a saline end-
member, possibly indicating the presence of relict seawater in the host
rock, as well as the dissolution of evaporite minerals facilitated by
seawater dilution (Neal and Shand, 2002). Alternatively, Evans et al.
(2024) suggest that the strong saline signatures observed in the Troodos
ophiolite derive from inclusions within serpentine minerals that contain
highly soluble salts. These inclusions, characterized by slab-derived
geochemical compositions, have become accessible to groundwater
through tectonic deformation and uplift during the Messinian.

In serpentinites, chloride exists both as a water-soluble phase and as
a structurally bound component within the serpentine lattice. Halite
precipitated along grain boundaries or as inclusions in serpentinite is the
principal source of water-soluble chloride (Barnes and Sharp, 2006).
This chloride in seafloor serpentinites is a potential contributor to sub-
duction zone salinity. Serpentinization consumes water, leading to the
formation of brines that may become trapped in fractures and cavities or
migrate upward, eventually erupting at the seafloor as saline geysers
(Scribano et al., 2017; Manuella et al., 2018).

Seafloor serpentinites typically exhibit chlorine isotope compositions (δ³⁷Cl) ranging from ~ -1.6 to + 0.5 ‰ relative to SMOC (Standard Mean Ocean Chloride) (Barnes and Sharp, 2006; Barnes et al., 2008, 2009; Bonifacie et al., 2008; Boschi et al., 2013). Water-soluble chloride is generally ~ 0.3 ‰ more depleted in ³⁷Cl than its structurally bound counterpart (Sharp and Barnes, 2004; Barnes and Sharp, 2006). Consequently, most seafloor serpentinites are isotopically enriched in ³⁷Cl due to hydration by seawater along faults and fractures, with serpentine preferentially incorporating ³⁷Cl into its structure.

Halite precipitated on the gallery walls at Levels 19 to 21 has a δ³⁷Cl value of -0.56 ‰, suggesting a chloride source slightly depleted in ³⁷Cl compared to seawater (defined as 0 ‰). This isotopic signature can be explained by either paleo-seawater interaction with peridotite or secondary brines from evaporite dissolution. The crustal signature of helium, along with a possible involvement of thermogenic hydrocarbons (see below), suggests that deep fluids, possibly saline, are penetrating the ophiolite from the underlying sedimentary formations. However, to date, there is no evidence for evaporite formations beneath the Bulqizë ophiolite (Robertson and Shallo, 2000; Bortolotti et al., 2005). Triassic evaporites are known from the Peshkopi window, located approximately 30 km northeast of Bulqizë. This NE-trending structure exposes Cretaceous neritic and pelagic limestones and hosts several salt domes (Peshkopi and Mali i Bardhë), with surface areas up to 25 km². Nevertheless, the Peshkopi window is structurally decoupled from the Bulqizë ophiolite by the intervening Pelagonian units. These are characterized by a Paleozoic basement of Ordovician–Devonian rocks, unconformably overlain by a Permo-Triassic clastic sequence that transitions into Triassic and Jurassic neritic-pelagic carbonates (Robertson and Shallo, 2000).

Thus, while the involvement of secondary brines derived from

Triassic evaporites is the most convenient hypothesis to explain the
source of NaCl, sulfates, crustal helium and hydrocarbons, it is also
plausible that the peridotite itself, particularly the serpentinized basal
sections, serves as the salt source. For Bulqizë, we favor a hybrid model
where multiple chloride sources contribute to the observed salinity.
Primary chloride likely reflects seafloor serpentinization prior to sub-
duction, preserved within fluid inclusions and mineral assemblages. This
ancient signature is supplemented by possible inputs from deep sedi-
mentary brines, as suggested by noble gas data. Most importantly, the
association of salt precipitates with active gas vents demonstrates
ongoing evaporative concentration that can amplify initially low salin-
ities through repeated cycles of seepage and evaporation. Detailed
petrographic analyses, coupled with ⁸⁷Sr/⁸⁶Sr analyses of halite pre-
cipitates could definitively test these hypotheses, as Jurassic seawater
(⁸⁷Sr/⁸⁶Sr = 0.7072–0.7076), Triassic evaporites (0.7080–0.7082), and
modern meteoric water (0.7089–0.7092) have distinctly different iso-
topic signatures.

We note that the hyperalkaline signature of runoff waters in mine gutters could potentially result from the alteration of concrete or cementitious materials, which are present along the shaft walls and main transfer galleries (Rinder et al., 2013). However, this hypothesis fails to explain the Na-Cl composition of the water, as well as the absence of calcium and the low sulfate concentrations.

5.3. *Hydrogen generation and migration dynamics*

Free gases bubbling in hyperalkaline springs or emanating from dry
seeps in ophiolites worldwide consistently consists of a mixture of N₂,
CH₄, and H₂, in varying proportions (Fig. 7; Monnin et al., 2021). Other
common gases such as CO₂, H₂S, and helium are sometimes found in
trace amounts, while carbon monoxide (CO) and sulfur dioxide (SO₂)
typically remain undetected. The relative proportions of N₂, CH₄, and H₂
are not random. Notably, gas mixtures composed solely of N₂ and H₂ are
absent. In many cases, the concentrations of H₂ and CH₄ follow a trend
consistent with the stoichiometry of the Sabatier reaction, although
most mixtures are dominated by either methane or nitrogen (Monnin
et al., 2021). Hydrogen-dominated mixtures (>80 % H₂) are rarer and
considered more exotic. Importantly, the gas composition can vary both
spatially and temporally within the same ophiolite body (Monnin et al.,
2021; Ojeda et al., 2023).

The measured H₂ flux of approximately 200 tons per year at Bulqizë ranks among the highest documented from any ophiolite globally, exceeding even larger systems like Oman's Semail ophiolite when normalized to accessible area. This extraordinary flux poses a quantitative challenge: what combination of sources and processes can sustain such intense degassing? Our analysis of gas contents in host rocks reveals H₂ concentrations of only 16 ± 12 µmol/kg and CH₄ at 135 ± 89 µmol/kg (n = 9), indicating that complete degassing of a cubic kilometer of rock would yield merely 100 kg of H₂, insufficient to maintain observed fluxes for even a single year (Truche et al., 2024). This rules out fluid inclusion deprecitation as a primary source.

Contemporary serpentinization presents similar quantitative con-
straints. Based on experimentally determined kinetics compiled by
Leong et al. (2023) and field observations in Oman by Kelemen et al.
(2021), serpentinization rates of 1–10 volume percent per 10,000 years
are expected at temperatures below 150 °C. Considering that an average
peridotite can produce 2–4 kg of H₂ per cubic meter of rock upon
complete oxidation (Osselin et al., 2022), serpentinization of fresh
peridotite could generate up to 2 to 40 tons of H₂ per year from a
reactive volume of one cubic kilometer. Achieving the observed 200 tons
per year would require either unrealistic reaction volumes (>10 km³) or
kinetics far exceeding laboratory measurements. While we cannot rule
out enhanced reaction rates due to catalysis or unique conditions at
Bulqizë, serpentinization alone appears insufficient. The most plausible
explanation involves a small but actively recharging gas reservoir that
buffers the difference between gas generation and discharge rates.
The hydrogen isotope systematics provide additional constraints on generation mechanisms. The extreme deuterium depletion (δD = −743 ‰) falls within the typical range for serpentinization-derived H₂ and reflects the strong kinetic isotope effects during Fe²⁺ oxidation coupled to H₂O reduction. The isotopic composition of H₂ is primarily influenced by equilibrium fractionation and rapid isotopic exchange with H₂O, which implies that δD-H₂ values are not reliable indicators of the gas's source, whether crustal, primordial or mantle-derived (Martín-Monge et al., 2022, Pester et al., 2018; Ricci et al., 2022, Milkov, 2022, Hochscheid et al., 2025). Furthermore, δD-H₂ values cannot effectively distinguish between hydrogen produced by water radiolysis, microbial metabolism, or serpentinization, even if in the present case the geological context cannot be ignored (Truche et al., 2020; Mangenot et al., 2023). However, δD-H₂ values can be used in combination with other isotopic proxies (e.g. ΔDD-H₂, δD-H₂O, δD-CH₄/ethane/propane) to assess equilibrium conditions and infer formation temperatures (Turner et al., 2025, Turner et al., 2021; Mangenot et al., 2023).

In the case of Bulqizë, measurements of H₂ clumped isotopes, assuming isotopic equilibrium, suggest temperatures below 35 °C, consistent with the current venting temperature observed at depth in the mine. Mangenot et al. (2023) have also observed that the DD isotopologues of H₂ from the Rainbow, Ashadze, and Lost City deep-sea hydrothermal vents are in thermodynamic equilibrium with the host vent fluids. Alternatively, equilibrium ΔDD values observed at near-ambient temperatures in natural settings may result from hydrogenotrophic microbial methanogenesis. However, this hypothesis is not conclusively supported, as both the clumped isotopic composition of methane and the presence of higher alkanes are incompatible with this scenario (see discussion below). Assuming isotopic equilibrium between CH₄ and H₂, a temperature of 26 °C is inferred from the δD values of these molecules, which is consistent with the observed venting temperatures (Turner et al., 2021). A similar temperature can be inferred, assuming equilibrium between H₂O(l) and H₂(g) and considering a δD-H₂O value of −47.1 ‰ measured at Level 19 (Hochscheid et al., 2025). Nevertheless, such equilibrium is not guaranteed. In fact, the e-folding timescale for hydrogen isotope exchange between CH₄ and H₂ is estimated to be approximately 1 million years at 200 °C (Sessions et al., 2004), suggesting that equilibration at temperatures below 100 °C would require around 10 million years. Moreover, given that water circulation and gas migration are demonstrably decoupled in the mine, it is not appropriate to infer temperatures from H₂–H₂O or CH₄–H₂O isotopic equilibria, even though the calculations are straightforward (Turner et al., 2021, 2022; Hochscheid et al., 2025).

Structural geology exerts a primary control on the focusing of gas emissions. The apparent association with the N215° fault system and its 10–20 m wide damage zone reflects the critical role of permeability contrasts in creating preferential flow paths. While pristine harzburgite exhibits permeabilities of 10⁻⁸ to 10⁻⁷ m/s, fault damage zones can reach 10⁻⁴ to 10⁻² m/s, a six-order-of-magnitude increase that effectively channels buoyant gas flow (Manning and Hingebritsen, 1999). The gas flux is maximum where faults intersect the chromitite bodies, suggesting that competency contrasts enhance fracturing or that chromitite itself plays a more active role in the gas generation system. The vertical continuity of these structures, extending through the full 5–6 km thickness of the ophiolite, provides pathways capable of tapping gas from the optimal serpentinization window at 3–5 km depth where temperatures can reach 90–150 °C.

5.4. *Methane genesis: Reconciling multiple lines of evidence*

The origin of methane in serpentinizing environments remains a topic of active scientific debate. A central scientific question is whether methane is formed abiotically, using H₂ as a reducing agent in reactions such as the Sabatier or Fischer–Tropsch-type processes (Sherwood et al., 2006; Etiope and Whiticar, 2019; Etiope and Oze, 2022). The origin of methane is commonly inferred from genetic diagrams involving isotopic

signatures of carbon (¹³C/¹²C) and hydrogen (D/H) in methane and higher alkanes. The δ¹³C–CH₄ vs. C₁/(C₂ + C₃) diagram is one of the most frequently used tools for interpreting the origin of hydrocarbon gases (Bernard et al., 1976, 1977; Whiticar, 1999; Milkov and Etiope, 2018). In this approach, genetic domains (abiotic, biogenic, thermogenic, etc.) are empirically defined as data envelopes representing groups of gases with a common origin, inferred from non-isotopic criteria such as geological constraints (see discussion in Reeves and Fiebig, 2020). In regions where these domains overlap, the interpretation of isotopic data becomes ambiguous.

Abiotic gases display a wide range of C₁/(C₂ + C₃) ratios because alkane production depends on the extent of methane polymerization, which in turn is influenced by temperature, H₂/CO₂ ratios, catalyst type, and reaction duration (Sherwood Lollar et al., 2008). Alkane distributions can also be affected by processes such as molecular fractionation during migration or differences in solubility in water. As a result, the C₁/(C₂ + C₃) ratio lacks diagnostic specificity for abiotic methane (Etiope and Oze, 2022). Similarly, the δ¹³C-CH₄ vs. δD-CH₄ isotopic space is shaped not only by the source material and formation pathways but also by mixing with gases from different origins and secondary alteration processes such as oxidation or diffusion (Etiope and Sherwood Lollar, 2013; Xia and Gao, 2022).

Despite these uncertainties, methane in continental ultramafic systems tends to occupy a relatively homogeneous isotopic field in the δ¹³C vs. δD diagram (Fig. 8a), distinct from microbial and thermogenic gases found in sedimentary basins (Etiope and Whiticar, 2019; Etiope and Oze, 2022). Interestingly, the data from dry seeps cluster within a much narrower range than those from bubbling hyperalkaline springs (Monnin et al., 2021). As thermogenic hydrocarbons have been shown to have a ¹³C enrichment with increasing carbon number (Des Marais et al., 1981; Chung et al., 1988), the observed isotope trend reversal (Fig. 8c) is often associated to abiotic gas generation via a polymerization reaction (Sherwood Lollar et al., 2002, 2008; Proskurowski et al., 2008). During such reactions, the lighter isotopes ¹²C–¹²C will bond at a faster rate than the heavier and lighter isotopes ¹²C–¹³C, resulting in a depletion of ¹³C of ethane compared to that of methane (Sherwood Lollar et al., 2008). However, alternative explanations, including mixing or unusual maturation pathways cannot be excluded.

These peculiar isotopic features have often been used to argue for an abiotic origin of methane, sometimes without critically examining the foundational principles of isotopic chemistry. Growing evidence (e.g., Xia and Gao, 2022) now suggests that these genetic approaches, while widespread, rest on weak foundations and may lead to misleading interpretations. Thus, in the case of Bulqizë, it would be premature to infer an abiotic origin for methane based solely on: i) bulk CH₄ isotope ratios (¹³C/¹²C and D/H), ii) the ¹³C depletion in higher alkanes relative to methane, and iii) the extreme gas dryness. Focusing genetic analysis solely on methane and higher alkanes may lead to oversimplification. A comprehensive understanding requires integrating the broader geological context, including microbiological, mineralogical, and independent experimental data. While this study is focused on geochemistry, we have assembled an extensive suite of molecular and isotopic analyses to characterize this aspect of the system as thoroughly as possible, while remaining mindful of the inherent limitations of this approach.

The high δ¹³C value of -12.3 ‰ measured at Bulqizë is unusual compared to the extensive dataset from thermogenic natural gas fields and confirmed methanogenic sources, but may in fact reflect substrate effects or oxidation (Xia and Gao, 2022). In addition, highly mature thermogenic gases can sometimes display inverse isotopic patterns, resulting from isotopic fractionation during thermal cracking in open systems or mixing of multiple thermogenic sources (Fiebig et al., 2019). However, the latter explanation seems unlikely in this case, as the notable presence of cyclohexane and methylcyclohexane points toward a less mature diagenetic origin – these compounds tend to degrade during advanced thermal maturation (Hunt, 1996; Lyons and Plisga, 2011). It would therefore be compelling to explain the observed δ¹³C
value of methane at Bulqizë as the result of mixing between mature thermogenic methane (δ¹³C<sub>thermogenic</sub> = -35 ± 5 ‰, typical of gas from the Dinaric flysch) and an abiotic endmember (δ¹³C<sub>abiotic</sub> = -5 ± 3 ‰, based on values from Oman and Kizildag). Such a model would suggest a thermogenic contribution ranging from 15 % to 30 %.

Still, the composition of volatile organic compounds (VOCs) remains puzzling. It does not clearly indicate a thermogenic source, aromatic markers such as BTEX compounds are absent, nor does it support an abiotic origin, since methanol is undetected and (methyl-)cyclohexane is not a typical product of CO₂ hydrogenation. Although recent meta- genomic studies have revealed microbial potential for VOCs metabolism (mostly alkenes, alcohols, ketones, benzenoids, pyrazines, sulfides, and terpenes), the presence of higher alkanes (totaling 1500 ppmv) and methylated cyclohexanes suggests that a microbial origin is unlikely (Kietäväinen et al., 2025; Purkamo et al., 2016; Nuppunen-Puputti et al., 2018; Hinrichs et al., 2006, Mouttaki et al., 2007).

The use of CH₄ clumped isotopes, may help to overcome part of the limitation of traditional ‘bulk’ isotope analyses. The Δ13CH₃D and Δ12CH₂D₂ values provide a robust diagnostic for equilibrium conditions and offer the valuable advantage to be independent of carbon and hydrogen source isotope ratios (independent of substrate). The CH₄ clumped isotope data from Bulqizë plot slightly above the thermody- namic equilibrium curve, thereby preventing unambiguous assessment of the CH₄ formation temperature. Temperatures inferred from the Δ13CH₃D value alone range from 180–210 °C, relatively high for the geological setting, but not implausible given the ophiolite’s depth (6 km) and geothermal gradient (~25 °C/km). The Δ12CH₂D₂ value suggests somewhat lower formation temperatures, between 110–140 °C. Overall, these data suggest that methane most likely formed, or subse- quently equilibrated, at elevated temperatures (>100 °C). At such con- ditions, methane isotopes approach equilibrium despite a slight positive anomaly in 12CH₂D₂, which is typically associated with high- temperature processes. This implies isotopic exchange either during or after formation, placing a lower bound on methane’s formation tem- perature. While disequilibrium signatures can arise during abiotic syn- thesis, potentially complicating the use of clumped isotopes as a thermometer, the dual clumped isotope patterns observed here could not be produced by low-temperature processes. Diffusion and mixing may also have influenced the signal, but these are more likely to represent secondary modifications rather than primary controls.

At several hydrothermal sites (Von Damm, Lucky Strike, Rainbow, Lost City), Mangenot et al. (2023) observed that Δ¹²CH₂D₂ in CH₄ and ΔDD in H₂ indicate near-complete isotopic re-equilibration at post- formation fluid temperatures. However, this re-equilibration is not seen in Bulqizë. Preferential reordering of doubly deuterated iso- topologues (¹²CH₂D₂ and D₂) at low temperatures (<100 °C) may occur, but may also be coincidental. Interestingly, the δ¹³C-CH₄, δD-CH₄, δD- H₂ and the Δ¹³CH₃D and Δ¹²CH₂D₂ values from H₂ and CH₄ outgassing in Chimaera dry seeps hosted in the Tekirova ophiolite (Turkey) are very similar to Bulqizë, with the only deference that the mass-18 methane isotopologues values from Chimaera are consistent with thermodynamic equilibrium, within analytical uncertainty. Hence, the same discrepancy between inTRA-species vs. inTRA-species temperatures was observed. Such discrepancy was interpreted by Young et al. (2017) to be the result of isotopic disequilibrium between CH₄ and H₂.

The clumped isotope signature of methane from Bulqizë does not align with the typical combinatorial effects expected from reactions involving hydrogen from different sources, which would otherwise yield exclusively negative Δ¹²CH₂D₂ values with large deficit relative to equilibrium (Röckmann et al., 2016; Taenzer et al., 2020; Labidi et al., 2024). Nor does it support unambiguously an abiotic origin of CH₄, based on the limited experimental data available. For example, Young et al. (2017) reported significant ¹²CH₂D₂ deficits in CH₄ produced via the Sabatier reaction using ruthenium as catalyst, possibly due to quantum tunneling effects. Labidi et al. (2024) proposed that near- equilibrium Δ¹³CH₃D, combined with negative Δ¹²CH₂D₂ values,

could indicate abiotic CH₄ formation at low extent of reaction. An important caveat to this conclusion is the potential for bond reordering, which could erase the isotopologue signatures associated with methane formation pathways if the methane resided for a sufficiently long time in a sufficiently high-temperature environment (Labidi et al., 2024). Therefore, conclusions drawn from short-term experiments using CO as a carbon source or employing strong catalysts such as pure ruthenium over limited durations (a few hundred hours) may not be directly transferable to geological settings.

Interestingly, such a positive Δ¹²CH₂D₂ excursion from the equilib- rium curve has also been observed in methane from other ophiolitic settings, such as Voltri (Acquasanta spring) in Italy and, to a lesser extent, Tekirova (Chimaera) in Turkey (Young et al., 2017, Etiope et Oze, 2022). A possible explanation for the disequilibrium Δ¹³CH₃D and Δ¹²CH₂D₂ values is that methane was affected by kinetic processes that raised its δ¹³C and δD values, thus altering the stochastic reference frame for calculating the Δ¹³CH₃D and Δ¹²CH₂D₂ values. One such process is Rayleigh distillation by molecular diffusion. Positive Δ¹²CH₂D₂ dis- placements from the equilibrium curve can be also attributable to either mixing of gases with contrasting δD values, such as for example an equilibrated thermogenic gas at 180 °C and an abiotic gas with either very low Δ¹²CH₂D₂ value or equilibrated at T > 100 °C (Young et al., 2017). If such pure end-members exist, they have yet to be clearly identified or well constrained.

The Bayesian statistical method for identifying methane produced by microbial methanogenesis using both bulk isotope ratios and clumped isotope data proposed by Young et al. (2025) can also be used. Bulk isotope ratios (δ¹³C and δD) are used to define a prior probability, while clumped isotope data (Δ¹³CH₃D and Δ¹²CH₂D₂) define the likelihood function. A beta distribution (“Haldane” prior) ensures the prior is sample-size independent, and Gaussian functions model the likelihood across the isotopic space. Applying this method to the Bulqizë sample would imply that the biosignature probability is below 10 %.

Although factors such as mixing, isotopologue fractionation, desorption, oxidation, kinetic effects, and quantum tunneling signifi- cantly complicate the interpretation of clumped isotope data, several key conclusions can nonetheless be drawn in the present case:

i) temperature derived from interspecies equilibrium (H₂-H₂O and H₂-CH₄) and from H₂ clumped isotopes suggest re-equilibration occurred at or near to the sampling point;

ii) the clumped isotope data indicate that methane formed or equilibrated at elevated temperatures (>100 °C), with isotopic patterns that cannot be explained by low temperature processes and secondarily effected by diffusion or mixing;

iii) CH₄ formation does not result from reactions involving hydrogen from multiple sources;

iv) positive Δ¹²CH₂D₂ deviations from the equilibrium curve, pre- viously observed in other ophiolitic settings, may result from molecular diffusion or the mixing of gases with contrasting δD values; and

iv) the probability that methane is of microbial origin is estimated to be less than 10 %.

While these complexities prevent definitive conclusions regarding the origin of CH₄ in the Bulqizë ophiolite, where temperatures range from 30 °C at Level 19 to approximately 150 °C at 4.5 km depth, they nonetheless impose strong constraints on the system.

Most notably, a non-fossil radiocarbon age (~26,150 years BP) has been measured for methane in an ophiolitic setting. This is an unex- pected result given that previous ¹⁴C–CH₄ measurements at serpentin- ization sites (e.g., Zambales, Ronda, Chimaera, Acquasanta, Happo, and Lost City) have consistently yielded ¹⁴C-free methane (Abrajano et al., 1990; Proskurowski et al., 2008; Etiope et al., 2011, Etiope et Whiticar, 2019; Suda et al., 2022). More recently, Price et al. (2024) reported non- fossil ¹⁴C–CH₄ in the submarine serpentinizing hydrothermal system of Prony Bay (New Caledonia), with apparent ages ranging from 12,000 to 49,500 years, likely attributed to microbial activity. At Bulqizë, the apparent radiocarbon age could result either from mixing between fossil
methane (i.e. carbon dead) and ¹⁴C-bearing methane, or from a single reaction pathway capable of producing methane from atmospheric CO₂ or photosynthetically derived organic carbon within the past 26,000 years. At this stage, the only robust conclusions are that a purely thermogenic origin is excluded, and that the carbon source for methane formation cannot derive solely from the Jurassic carbonate formations surrounding the ophiolite. It would therefore be tentative to conclude that microbial activity is the main source of the methane, or at least a contributor to it (Etiope and Oze, 2022).

Indeed, the non-fossil ¹⁴C signature could support the hypothesis of a microbial origin, either autochthonous or resulting from anthropogenic contamination. Methane generation induced via microbial activity during storage is highly unlikely, as the stainless-steel cylinders used to sample the gas contained no water and no oxidized carbon sources such as CO₂ or CO. Notably, even in samples collected using the water displacement method, where water is present, we observed no changes in both gas composition and methane isotopic signatures over the course of a year. Recent contamination within zones disturbed by mining activities is also improbable. The short residence time of gas near the mine galleries, due to high outgassing rates, and the stability of its composition over nearly a decade both argue against such a scenario. The gas composition remains unchanged since its first detection while drilling the shaft N9 in 2016 until 2025. In addition, the microbial contribution to CH₄ from DIC in the mine water must be less than 4% as deduced from their respective ¹⁴C content (= 3.76/95.8 pMC). Isotopic modeling based on δ¹³C-DIC (−19‰) and carbonate-CH₄ fractionation during microbial methanogenesis from hyperalkaline subsurface fluids (α = 1.028–1.089; Miller et al., 2018) yields δ¹³C₄H₆ values from −46 to −99‰. Mixing 4% of such microbial methane with 96% CH₄ having δ¹³C of −12.3‰ would alter the overall isotopic value by only 1–3‰, a negligible shift. Thus, methanogenesis using DIC from the mine's water as a carbon source can account for only a minor fraction of the observed methane outgassing. It is important to keep in mind that we previously demonstrated that the water flow from the mine is decoupled from the gas flow. An autochthonous microbial source for methane cannot be definitively ruled out, but both the clumped isotopes and VOCs data do not point toward such a scenario. Further investigations, including the analysis of solid organic compounds, biomarkers, and DNA are necessary to assess the potential role of microbial activity.

Thus, we propose that a large part of methane is of abiotic origin, with a notable thermogenic contribution and a likely minor microbial component. Recent experiments underline the possibility that CO₂ methanation occurs below 200 °C through gas-rock interactions in relatively dry conditions (Medina et al., 2000; Jacquemin et al., 2010; Etiope and Ionescu, 2015; Etiope et al., 2019; Ruiz et al., 2021). Dutoit et al. (2025) have shown that CO₂ methanation (H₂ abiotic consumption) can proceed at a faster rate than serpentinization (H₂ generation) under dry conditions when ultramafic rocks from Bulqizë, specifically harzburgite and chromitite, are present. These experiments, conducted at 200 °C and 250 °C, demonstrated significant methanation activity within few days. When extrapolated to lower temperatures using an activation energy of 80 kJ/mol, the reaction remains rapid in term of geologic time scale, with the half-life of CO₂ estimated to range between 30,000 years and 1 million years at temperatures as low as 100 °C. Methane is not the only reaction product; other solid organic compounds such as graphite, bitumen and asphaltenes are also formed. The reaction yield and selectivity depend on the nature of the catalyst, but the H₂/CO₂ ratio is also expected to play a significant role in determining the speciation or organic compounds in dry reaction zones within ophiolitic settings.

At Bulqizë, the conditions appear favorable for efficient abiotic reduction of carbonate by hydrogen. The observed diphasic water-gas migration suggests that parts of the ophiolite may be dry, a prerequisite for this type of reaction. This scenario is further supported by the temperature data from in-situ measurements in the mine (Goskolli, 2022), indicating a possible geothermal gradient of 25–30 °C/km. Additionally,

a recent magnetotelluric survey indicates that the ophiolite is approximately 6 km thick (Yao et al., 2025). At depths ranging from –2500 m to –4000 m, where the rock may be subject of ongoing serpentinization (Yao et al., 2025), these gradients would yield temperatures ranging from 100–160 °C in this zone, which remains sufficient for lower-rate serpentinization reactions (Leong et al., 2023). Moreover, the presence of chromitite provides a variety of catalytic mineral phases, including Ni- and Co-bearing minerals, platinum group elements, and phyllosiliates, that are known to enhance CO₂ methanation (Jacquemin et al., 2010; Ruiz et al., 2021; Pappalardo et al., 2024, Dutoit et al., 2025). Therefore, a residence time of 26 ka is compatible with significant abiotic methane generation.

The measured radiocarbon age of 26,150 years BP may also be interpreted in the context of the regional paleoclimate. Several lines of evidence, such as moraines, glacial cirques, and glacial lakes, indicate that the Bulqizë massif, culminating at 2009 m amsl, was once glaciated. Moraine systems have been dated in neighboring Balkan regions: 15.2 ± 0.8 ka on Mount Pelister in North Macedonia (Ribolini et al., 2018), 14.9 ± 1.1 ka in the Velež Mountains of Bosnia and Herzegovina (Žebre et al., 2019), and 17.3 ± 0.6 ka in Montenegro. Moraines generally form during glacier retreat, and those that have been dated may represent the best-preserved features, corresponding to the most recent stage of deglaciation. The glaciers were most extensive during the early phase of the Last Glacial Maximum (29–25 ka), followed by a retreat between 25–19 ka due to increasing aridity, which led to the formation of rock glaciers in some areas. It is therefore plausible that the hydrological system of Bulqizë was reactivated after ice and permafrost melting at the end of the Last Glacial Maximum, around 25,000 years ago, consistent with the observed radiocarbon age.

In summary, the methane at Bulqizë likely has a complex origin involving multiple processes. A purely microbial or thermogenic source can be excluded based on isotopic, molecular, and radiocarbon evidence. While features such as CH₄/H₂ ratios and δ¹³C values are consistent with abiotic synthesis, disequilibrium clumped isotope signals and the presence of non-fossil ¹⁴C suggest either partial microbial contribution or rapid abiotic formation from a young carbon pool. Current tools (bulk isotope data, molecular ratios, and even clumped isotopes) have limitations and must be interpreted cautiously. Only through integrated analysis can the genetic origin of methane in ultra-mafic settings like Bulqizë be properly constrained.

### 5.5. Evidence for both atmospheric and crustal inputs from nitrogen and noble gases isotopes

Nitrogen is the third most abundant gas at Bulqizë. Nitrogen isotopes point toward an atmospheric origin (δ¹⁵N–N₂ = −0.2‰). Air contamination during sampling can be excluded as no oxygen, even in trace amounts, was detected in the sampled gas. On the other hand, noble gases (He and Ne) isotopes show a clear crustal signature (Fig. 9). The water displacement method used for gas sampling effectively prevents air contamination. Of course, this close-to-air isotope fractionation may be fortuitous as nitrogen may also be affected by post genetic processes like methane. However, considering the non-fossil radiocarbon signature of methane, it is consistent that nitrogen from the atmosphere enters the hydrologic system dissolved in water together with inorganic carbon. Such a conclusion is also in line with the noble gas isotopic composition plotting along a mixing line between the crust and the atmospheric end-members. A mantle contribution in the R/Ra signature, if any, would remain below 1‰.

To date, very few nitrogen isotopic data have been reported from gas seeps in ophiolitic environments. Comparable atmospheric δ¹⁵N–N₂ signatures have been previously observed by Vacquand et al. (2018) and Sano et al. (1993) in hydrogen-rich hyperalkaline springs in Oman (Ain Al-Waddah, Ain Al-Shubaikhah, and Hammam Al-Ali springs) and the Philippines (Mangatarem and Nagasa springs). Vacquand et al. (2018) combined nitrogen and noble gas isotope analyses (He and Ar) to infer
that the nitrogen present in these H₂-rich gas mixtures originates from the atmosphere. It was either transported by an aquifer equilibrated with atmospheric gases or acquired during the upward migration of gas through the aquifer. An alternative source of N₂ could be denitrification, driven by nitrate input from atmospheric deposition (Rempfert et al., 2023). In contrast, nitrogen from the Chimaera dry seeps exhibits δ¹⁵N values ranging from +2.1 to +2.8‰, which have been interpreted as deriving from mature to highly mature organic matter (Hosgörmez et al., 2018).

The relatively high concentrations of ⁴He suggest the occurrence of crustal degassing. Ultramafic rocks are typically depleted in radioactive elements such as uranium and thorium. Although some radiogenic helium may be generated within the ophiolite itself, it is likely that a portion originates from underlying geological formations. Consequently, the presence of crustal helium (up to 20%) may indicate the migration of minor amounts of gases (such as CO₂, N₂, and CH₄) into the ophiolite from deeper sedimentary and crystalline formations. The crustal helium component supports the idea of a fluid input along the sedimentary-ophiolite interface, potentially accompanied by a contribution of thermogenic methane.

## 5.6. The chromitite connection: hydrogen reservoir and abiotic methane

Several lines of evidences support the presence of a deep hydrogen reservoir of unconstrained size, with ongoing or very recent recharge. As previously noted, the magnitude of the gas flow and its confinement within a well-defined deep fractured zone cannot be sustained solely by a known gas generation process that is both sufficiently prolific and rapid to maintain the observed output (Truche et al., 2024). The non-fossil radiocarbon age of methane, along with the atmospheric signatures of helium and nitrogen isotopes, indicates that the hydrologic system is open and much more dynamic than previously thought. Therefore, the reservoir zone may act as a buffer, mediating between a slowly recharging flow and the rapid leakage exacerbated by recent mining operations.

In the Bulqizë mine, the presence of combustible and explosive gases was first documented in 1992 at Level 10 (+220 m amsl), where it was associated with a burning flame (Goskolli, 2022). Since that time, hydrogen has been detected at nearly every subsequent level of the mine. Previous observations (Goskolli, 2022; Truche et al., 2024) along with the present study, indicate that hydrogen emissions are typically encountered at depths exceeding 600 m within a well-defined tectonic zone. Additionally, pressure measurements conducted in 2006 in a geotechnical borehole, drilled from Level 16 (+40 m amsl) down to Level 21 (–180 m bmsl), where gas was flowing at rates of 4.6 to 5.3 m³/min, recorded gas pressures of 18 to 20 bar (Goskolli, 2022). Although these values are sub-hydrostatic given the measurement depth, they may not accurately reflect the true reservoir pressure due to the absence of borehole casing, potential leakage at the wellhead, and substantial loss of charge along the borehole and surrounding rocks. Nonetheless, they provide support for the existence of a pressurized reservoir.

The existence of H₂-rich gas reservoirs in ophiolitic settings, where hydrogen is generated through serpentinization, has been postulated based on the intensity of surface gas seeps (Etiope, 2023, Etiope and Oze, 2022). Numerous continental serpentinization-related seeps and springs release significant amounts of hydrogen and methane. For instance, the H₂–CH₄ burning seep at Chimaera, in the Tekirova ophiolite (Turkey), has been interpreted as the surface expression of a faulted, pressurized subsurface reservoir, based on the intensity and persistence of the advective gas flow (Etiope, 2023). It is estimated that approximately 94 million cubic meters of hydrogen (at standard temperature and pressure) have been released into the atmosphere since the site was first described by Pliny the Elder more than two millennia ago. Other notable examples include:

- Tahtakopru and Kurtbagi sites within the Kizildag ophiolite in Turkey, where hydrogen concentrations reach up to 60 vol%

(D'Alesandro et al., 2018b);

- Los Fuegos Eternos in the Zambales ophiolite, Philippines, with 44 vol% H₂ (Vacquand et al., 2018);

- Poison bay in New Zeeland, with 75 vol% H₂ and many seeps on the bouldery shore and beneath the sea, some of them with single point flow rate up to 10 l/min (Wood, 1972);

- Tanjung Api in Indonesia with > 42 vol% H₂ and gas burning throughout the year along 650 m of coastline and intensively bubbling beneath the sea (Sanjaya et al., 2024);

- Haylayn bubbling hyperalkaline spring, in Oman, with 87 vol% H₂ and a total output of 158 kg of H₂ per year (Leong et al., 2023).

Hydrogen-rich gas has been described in many other ophiolites, but without information about the flux: The Cedars with 50 vol% H₂ (Morrill et al., 2013), Vuçe in Kosovo with 16 vol% H₂ (Lévy et al., 2023), Vaićeva voda in Bosnia and Herzegovina, with up to 48 vol% (Etiope et al., 2017), Mokra Gora in Serbia with 85 vol% H₂ (Randazzo et al., 2021).

The continuous release of hundreds of tons of gas per year must be driven by strong pressure gradients, and such a release is only possible if pressurized gas accumulation exists. Although rare, petroleum accumulations have been documented in serpentinized peridotites within ophiolitic complexes, such as the Havana–Matanzas hydrocarbon system in the *peri-Caribbean* ophiolite belt of Cuba (Rossello and García-Sánchez, 2017). These observations indicate that reservoir properties capable of trapping hydrocarbons may exist in ophiolitic settings. In these systems, the reservoirs are found in fractured peridotites, while undeformed serpentinites function as effective cap rocks.

The spatial association between intense gas emissions and chromitite ore bodies at Bulqizë is not merely coincidental but reflects a fundamental relationship that extends to ophiolitic chromite deposits globally. This pattern, first noted in the Ural Mountain, and now documented across multiple continents, suggests that chromitite plays a crucial role in either generating or focusing gas emissions. Some of the most intriguing measurements were obtained in the former USSR (now Russia and Kazakhstan), in several chromite mines located in the Ural Mountains. Bohdanowicz (1934), reported that gas was flowing over two weeks from a well drilled in 1925 at a depth of 600 m at Nijni Taguil, on the Ural River, for the purpose of investigating a massif of platinum-bearing dunite. The gas composition was as follows: 66.5 vol% H₂; 20.7 vol% N₂, 9.5 vol% CH₄. No helium gas was found. Lidin et al. (1982) indicate that hydrogen-rich gas emissions were observed in the Tagil dunite massif in 1926 and 1933 during exploratory drilling and mining operations. The composition of the gases continuously emitted for 22 days from an exploratory well at a depth of 600 m consisted of 79–81 vol% H₂, 9–11 vol% CH₄, 8–9 vol% N₂, and 5 ppmv He. In 1979, hydrogen jets were observed in the “Molodezhnaya” chromite mine during the excavation of mining galleries at depths ranging from 433 to 600 m, but only within highly serpentinized dunites located within 0–300 m of the mineralized zone, as well as in drill holes intersecting the chromitite ore body. Further fascinating details are provided by Lidin et al. (1982) and Dievirtch et al. (1992). The hydrogen emissions within the mine are not uniformly distributed but are associated with steeply dipping fractured zones (dip angles of 70–80°) with widths up to 40 m. The gas is released as pulsed jets, bubbling through water that covers the mine floor at several locations along a 9-meter stretch. The total flow rate of these pulsed gas emissions was estimated in May 1979 at approximately 20 m³/day, based on a rough and likely underestimated evaluation. The gas is predominantly composed of hydrogen (92–98 vol %), accompanied by nitrogen (5 vol%), methane (1.2 vol%), and higher alkanes (several tens of ppmv). These gas emissions continued for at least five years after their initial discovery, with their composition remaining virtually unchanged. Groundwater also flows through certain fractures within the mine. The total water discharge at the gas emission site reaches 6.3 m³/h. Waters from depths of 450–600 m are of the hyperalkaline sodium-chloride type (pH 11–12). The isotopic composition of hydrogen was also measured: δD-H₂ = –744‰ to –766‰. Hydrogen-rich natural gas emissions have also been observed at
neighboring deposits such as “Almaz-Zhemchuzhina” and “Tsen-
tralnaya”, as well as in other chromite deposits within the region. These
authors highlight the close association of these gas manifestations with
the chromite ore body: Cr₂O₃ content in the mineralized bodies reaches
9–12% at Nijni Taguil and 65% at Khromtau, with an average con-
centration in the host rocks of 0.4–0.5%.

Similarly, the Tišoviţa well, located within the Tišoviţa–Iuţi ophiolite in Romania, encountered pressurized gas within fractured dunite at a depth of approximately 800 m (Baciu and Etiope, 2024; Filipescu and Huma, 1979). A gas sample, likely collected after the eruption, revealed the following composition: 69.1 vol% CH₄, 28.7 vol% H₂, 0.04 vol%, and nearly 1 vol% of higher alkanes. The Tišoviţa–Iuţi body corresponds to the lower part of a typical ophiolitic series (Saccani et al., 2011; Plissart, 2012), starting with mantle peridotites, represented by harz-bugites and dunites with small podiform chromitites, followed by ultramafic cumulates.

These striking similarities of gas emission characteristics between all
these podiform chromite ore deposits strongly suggest a significant
relationship between hydrogen, methane, chromite, dunite and deep-
seated tectonic faults. The dunite that hosts podiform chromite de-
posits may be also more fertile for hydrogen generation due to its higher
olivine content. Furthermore, it has been demonstrated that chromitite
can catalyze the reduction of CO₂ to CH₄ and contains key catalytic el-
ements and mineral phases that facilitate this process (Medina et al.,
2000; Jacquemin et al., 2010; Etiope and Ionescu, 2015; Etiope et al.,
2019; Ruiz et al., 2021; Pappalardo et al., 2024; Dutoit et al., 2025). A
major challenge in interpreting the behavior of hydrogen and methane
from sources to seeps lies in understanding the system's ability to sustain
longstanding continuous gas flow. In this context, the CO₂
hydrogenation-reservoir model, which incorporates the identification of
both source and reservoir rocks, offers a framework analogous to that of
petroleum systems.

However, these factors alone cannot explain why gas emissions are so strongly concentrated at chromitite localities compared to ordinary serpentinized peridotite. The contrasting chemical and physical properties at chromitite-dunite contacts may create gradients that drive focused fluid flow, enhancing local reaction rates. Additionally, the higher permeability often observed in mineralized zones facilitates water access to fresh reaction surfaces. Structural controls related to chromitite bodies further enhance gas accumulation. The extreme competency contrast between massive chromitite and surrounding serpentinized peridotite creates zones of intense fracturing that serve as high-permeability conduits. Chromitite pods may also act as impermeable barriers that create structural traps for buoyantly migrating gas. The intersection of regional faults with these competency contrasts produces the specific structural architecture observed at Bulqizë, where maximum gas flux occurs at fault-chromitite intersections. While mining operations preferentially follow ore zones and could bias observations, the consistency of this association across diverse geographic settings argues for a fundamental genetic relationship.

The implications for hydrogen exploration are significant. Podiform chromitites occur throughout the global ophiolite belt, from the Alps to the Himalayas, typically in dunite envelopes within the mantle- crust transition zone. If the chromitite-gas association proves general, these deposits represent high-priority targets for natural hydrogen explora- tion. The global distribution of podiform chromitites could provide a framework for estimating the hydrogen generation potential of ophio- litic terranes. Understanding the catalytic role of chromitite could also inform strategies for enhanced hydrogen recovery, potentially through engineered introduction of catalytic materials or optimization of water-rock interaction in chromitite-bearing zones.

5.7. *Conceptual model synthesis and broader implications*

Integration of all observations leads to a conceptual model that
captures the essential features of the Bulqizë system while

acknowledging remaining uncertainties. Deep hydrogen generation oc-
curs through ongoing serpentinization at 3–5 km depth, where temper-
atures of 100–150 °C enhance reaction kinetics. The process is enhanced
near chromitite bodies due to both higher olivine contents in dunite
envelopes and promoting effects of chromitite-hosted minerals. The
N215° fault system acts as a natural hydrogen drain, creating focused
discharge from a distributed source region. Its high-permeability dam-
age zone provides a vertical conduit connecting the deep generation
zone to shallow levels accessed by mining.

During ascent along this structural pathway, chromitite bodies
catalyze methane formation through CO₂ hydrogenation. The carbon
sources are multiple: primarily deep carbonates or mantle carbon, with
minor contributions from thermogenic hydrocarbons migrating from
underlying sediments (~15–30%) and trace amounts of modern atmo-
spheric CO₂ or organic carbon (~4%) acquired through limited inter-
action with groundwater. The specific catalytic minerals, Fe-Ni alloys,
platinum group elements, and possibly chromite itself, enable methane
formation at temperatures lower than typically required for abiotic
synthesis. Two-phase migration maintains separation between gas and
water, allowing preservation of deep isotopic signatures.

Mining operations since 2015 have enhanced discharge by providing additional drainage pathways and reducing confining pressure. However, the fundamental system predates human intervention, as evidenced by gas accumulations encountered during earlier mining phases. The current high flux represents tapping of a modest but actively recharging reservoir system, not catastrophic degassing of an ancient fossil accumulation. The ~26 ka radiocarbon age suggests cyclical recharge potentially linked to climatic events, with the current active phase initiated by deglaciation and reactivation of deep groundwater circulation.

This model reconciles apparently contradictory observations: mod-
ern water ages versus ancient carbon in methane, extreme H₂ fluxes
versus limited source volumes, high-temperature formation signatures
versus low-temperature venting, and abiotic molecular patterns versus
¹⁴C detection. Each component is necessary to explain the full dataset,
and removal of any element creates inconsistencies with specific ob-
servations. The model makes testable predictions about gas composition
variations with depth, isotopic evolution during ascent, and distribution
of gas accumulations relative to structural and lithological features.

**6. Conclusion**

The comprehensive investigation of the Bulqizë ophiolite has
revealed a natural hydrogen system of extraordinary complexity and
dynamism, fundamentally challenging our understanding of gas gener-
ation and migration in ultramafic environments. Through the integra-
tion of multiple geochemical and isotopic approaches, including several
analytical techniques applied for the first time to ophiolitic gases, we
have constructed a detailed picture of a system where deep Earth pro-
cesses intersect with near-surface dynamics in unexpected ways.

The detection of radiocarbon in methane from Bulqizë represents a
paradigm shift in our understanding of carbon cycling in ophiolites. This
unprecedented finding of 3.76 ± 0.06 pMC, corresponding to an
apparent age of approximately 26,000 years, demonstrates that these
systems operate on timescales of 10⁴ years rather than the 10⁶-year
scales typically assumed. This suggests that ophiolites may play a more
dynamic role in the global carbon cycle than previously recognized. The
identification of chromitite as a key catalyst for gas generation provides
a new framework for understanding why some ophiolites exhibit intense
gas emissions while others do not. The global distribution of podiform
chromitites in ophiolitic terranes may control the distribution of sig-
nificant natural hydrogen resources.

Our multi-proxy dataset reveals a system characterized by remark-
able structural control, where gas emissions are focused along a major
fault zone (N215°, 80°W) that channels flow from depth. The exclusive
association of intense gas emissions with chromitite ore bodies,
documented here through detailed underground mapping and corroborated by similar observations from chromite mines in the Urals and Romania, establishes a new paradigm for understanding gas generation in ophiolites. The catalytic properties of chromitite-hosted minerals, including Fe-Ni alloys, platinum group elements, and chromite itself, appear to sustain abiotic methane synthesis at temperatures lower than previously thought, expanding the potential for methane generation beyond the narrow window of active serpentinization.

The complete decoupling of gas and water flow paths represents another fundamental insight. While meteoric water circulates rapidly through the fractured ophiolite with residence times of less than 10 years, the gas phase migrates separately, preserving isotopic signatures acquired at depth. This two-phase flow system explains the apparent paradox of hyperalkaline water chemistry coexisting with modern water ages, while the discovery of salt efflorescences in gas emission zones provides direct evidence for the migration of minute amounts of saline fluids alongside the dominant gas phase.

The extraordinary hydrogen flux of approximately 200 tons per year, among the highest documented globally, cannot be explained by conventional models invoking either contemporary serpentinization or degassing of ancient fluid inclusions. Instead, our data support a model of ongoing gas generation at depth, with accumulation in a modest but actively recharging reservoir system. The confluence of optimal temperatures (100–150 °C), abundant reactants, and efficient catalysis near chromitite bodies creates a natural hydrogen factory operating at depths of 3–5 km, with products efficiently extracted through the high-permeability fault network.

The implications of our findings extend well beyond Bulqizë to fundamental questions about Earth's deep volatile cycles and the potential for natural hydrogen as an energy resource. The recognition that chromitite bodies play a crucial catalytic role in gas generation provides a new exploration target, as podiform chromitites occur throughout the global ophiolite belt from the Alps to the Himalayas. The documentation of rapid carbon cycling in these systems suggests that ophiolites may play a more significant role in the global carbon cycle than previously recognized, particularly in regions of active tectonics where fresh ultramafic rock is continuously exposed to circulating fluids.

Looking forward, several critical questions emerge from our work. The true extent and connectivity of the gas reservoir system requires geophysical imaging and deep drilling to resolve. The kinetics of chromitite-catalyzed reactions under realistic conditions need experimental validation across the full range of temperatures observed in natural systems. The potential for microbial processes, while apparently minor based on our isotopic data, deserves investigation using modern molecular biological approaches. The regional extent of similar systems in other Albanian ophiolites or equivalent terranes worldwide needs systematic exploration. Perhaps most importantly, the feasibility of enhancing natural hydrogen production through engineered intervention, whether by optimizing water-rock ratios, introducing additional catalysts, or managing reservoir pressure, merits serious evaluation (Osselin et al., 2022; Templeton et al., 2024).

The Bulqizë ophiolite ultimately emerges from our study not as a geological anomaly but as a natural laboratory that reveals the hidden dynamics of deep volatile cycling in Earth's lithosphere. The complex interplay of serpentinization, catalysis, structural focusing, and multi-phase flow creates a system that is at once unique in its intensity and potentially representative of processes operating in ophiolites worldwide. Sites like Bulqizë offer both immediate practical relevance and profound insights into the fundamental processes that shape our planet's chemistry. The challenge lies in translating our growing understanding of these natural systems into practical strategies for sustainable energy production while preserving their value as windows into Earth's deep volatile cycles.

### CRediT authorship contribution statement

**Laurent Truche:** Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Frédéric-Victor Donzé:** Writing – original draft, Validation, Resources, Investigation, Funding acquisition, Conceptualization. **Bardhyl Muceku:** Writing – review & editing, Resources, Investigation, Conceptualization. **Malavika Sivan:** Writing – review & editing, Resources, Methodology, Data curation. **Thomas Röckmann:** Writing – review & editing, Methodology, Investigation, Data curation. **Dan Levy:** Writing – review & editing, Methodology, Data curation. **Manuel Moreira:** Writing – review & editing, Methodology, Data curation. **Corinne Loisy:** Validation, Data curation. **Adrian Cerepi:** Validation, Resources. **Marianne Quéméneur:** Writing – review & editing, Resources, Investigation, Data curation. **Delphine Tisserand:** Validation, Methodology, Data curation. **Hugo Dutoit:** Visualization, Validation. **Ivan Vujević:** Visualization, Validation. **Yan Yao:** Writing – review & editing, Validation, Investigation. **Nicolas Lefeuvre:** Validation, Resources. **Anais Lavoué:** Validation, Resources. **Christophe Monnin:** Writing – review & editing, Resources. **Edmond Goskolli:** Validation, Resources, Investigation.

### Data availability

Data are available through Zenodo at https://doi.org/10.5281/zenodo.15965338.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

We are grateful to AlbChrome Ltd. mining company and its CEO Dr. Cemil Acar, the miners, engineers Mrs. Bardhyl Raman and Veysel Ozen Karatay, and the director of the Bulqizë mine Mr. Mikel Demçe for their strong support in the field. We thank Esther Schwarzenbach and three anonymous reviewers for their insights and manuscript reviews. We also thank our esteemed colleague Valentin Nivin (KSC RAS, Apatity, Russia) for sharing the two scientific treasures from Lidin et al. (1982) and Dievirsch et al. (1992). This project received funding from the LabEx OSUG@2020 endorsed by the Université Grenoble Alpes in the frame of the project IDEX UGA (Initiative d'Excellence), which is supported by the French National Research Agency in the framework of the "France 2030" program (ANR-15-IDEX-02). Y.Y. and I.V. received funding from the European Union's Horizon Europe research and innovation program under the Marie Skłodowska-Curie Doctoral Network "Safe Underground Hydrogen Storage in Porous Subsurface Reservoirs (SHINE)" (Grant Agreement No. 101006632). Part of this work was financially supported by the Laboratoire ISTERe (cross-cutting themes grant, "BQR"). L.T. acknowledges support from the Institut Universitaire de France. We thank Dr. Gesine Lorenz from Hydroisotop GmbH, and Mégane Tremblay and Casey Ackley from Beta Analytic Inc. for their valuable support. D.L. thanks SLB for funding his postdoctoral project, P. Cartigny, G. Bardoux, and I. Martinez (IPGP) for providing access to the H₂ purification line for gas samples, and J. M. Eiler, N. Lotem and N. Kitchen (Caltech, USA) for access to the facilities used to measure H₂ clumped isotopes.

### Appendix A. Supplementary material

Supplementary Tables S1 and S2 synthesizing the key observations that constrain our understanding of hydrogen system in Bulqizë
ophiolite. Supplementary material to this article can be found online at
https://doi.org/10.1016/j.gca.2025.09.039.

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