Zn-Pb Mineralization Associated with Salt Diapirs in the Basque-Cantabrian Basin, Northern Spain: Geology, Geochemistry, and Genetic Model

Vein and stratabound Zn-Pb sulfides are hosted within siliciclastic rocks and marine carbonates of Cretaceous age and within caprock carbonates at the margins of the Murguía and Orduña saline diapirs in the BasqueCantabrian basin. Organic matter is ubiquitous, and textures indicate a genetic link to sulfide precipitation. Sulfides (pyrite, sphalerite, and galena) precipitated from brines with halogen ratios compatible with halite dissolution. Thermal indicators (fluid inclusion, organic matter, and sulfur isotope data), point to formation temperatures between 150° and 200°C. The d34S values of sphalerite and galena (4.1–15.1‰) suggest a sulfur source related to the reduction of evaporite sulfate (thermochemical sulfate reduction) of Triassic age (15.3–17.4‰). The interaction of carbonpoor, metaland sulfate-bearing hot brines with host rocks activated the cracking of organic matter, triggering sulfide precipitation at a rate controlled by the H2S production. Textural relationships suggest that ore precipitation was related to dolomitization of host rocks (siliciclastic rocks, marine carbonates, and caprock). The d13C and d18O of carbonates range from 3.5 to –20.5‰ and from 16.1 to 28.7‰, respectively, indicative of different carbon sources and host-rock types. Carbonates associated with sulfide mineralization depict d13C/d18O assemblages and 87Sr/86Sr ratios (0.70801–0.71202) resulting from the interaction of a basinal brine with the different host rocks. Galena Pb isotope ratios—206Pb/204Pb from 18.643 to 18.696, 207Pb/204Pb from 15.650 to 15.676, and 208Pb/204Pb from 38.720 to 38.780—point to metal source rocks similar to other Mississippi Valley-type (MVT) deposits of the Basque-Cantabrian basin. Sulfide concentrations around the Murguía and Orduña diapirs are not concomitant with caprock formation but with dolomitization, as in MVT deposits. This is in contrast with the diapir-related deposits of the Gulf Coast and shares characteristics with the diapir-related mineralization in North Africa.


Introduction
The relationship between salt diapirs and hydrocarbons is widely known, and these structures have been targets for oil and gas exploration elsewhere (e.g., Grunau, 1987;Harding and Huuse, 2015). An additional economic interest of some salt diapirs is their association with Zn-Pb sulfide deposits, especially in North Africa and the Gulf Coast (USA). Moreover, studies and field examinations of some Mississippi Valley-type (MVT) deposits in China and in South and North America suggest that the relationship between ores and evaporites or salt tectonic structures is more common than previously recognized (Leach et al., 2010(Leach et al., , 2017Leach, 2014).
According to Kyle and Price (1986), Posey and Kyle (1988), Kyle and Posey (1991), and Kyle and Saunders (1996), in the Gulf Coast diapirs, sulfide precipitation is related to caprock formation. In contrast, in salt diapir-related ores in North Africa (e.g., Tunisia: Bou Grine, Fedj-el-Adoum) the genesis of the deposits is not related to caprock development (Rouvier et al., 1985;Sheppard et al., 1996;Bouhlel et al., 2016), and fracture systems around the diapiric structures may act as channelways for fluids, extending mineralization into the surrounding rocks. Distinguishing between these two models has exploration implications, because sulfide ores would be constrained not only to areas on top of salt domes but also to favorable rocks surrounding the diapirs.
Historically, the Basque-Cantabrian basin (northern Spain) has been an important Zn-Pb mining region. It contains numerous MVT Zn-Pb deposits enclosed within dolomitized carbonates of Lower Cretaceous age, including the worldclass Reocín district (60 Mt; 12% Zn) (Velasco et al., 1994). Small stratabound Zn-Pb deposits also occur related to diapirs of Triassic evaporite successions intruding sediments of Mesozoic and Cenozoic age. Two of these diapirs, Murguía and Orduña (Fig. 1), are known to host sulfide mineralization (sphalerite and galena) replacing intruded Cretaceous sandstones and carbonates and a discontinuous carbonate horizon bordering the diapirs. Ore deposits were actively mined from the mid-19 th to the mid-20 th centuries but had not been studied in detail; however, their potential economic interest was investigated through geologic, geochemical, and drilling surveys by an Outokumpu Spain-Ente Vasco de Energia (EVE) joint venture between 1997 and 1998. As a result, a resource of ~0.5 Mt Zn + Pb was estimated.
dolomitization at the contact between the Triassic evaporites and the host Cretaceous sedimentary pile. Our work is based on a detailed field, mineralogical, petrographic, and geochemical (organic matter, stable, and radiogenic isotope) study on samples from outcrops and drill cores.

Geologic Setting and Diapirism
The Basque-Cantabrian basin is related to rift systems that developed between the European and Iberian plates during the Mesozoic (Salas et al., 2001;Vergès and García Senz, 2001;Taviani and Muñoz, 2012). The Mesozoic stratigraphic record includes sedimentary rocks from the Triassic to the Upper Cretaceous. The Triassic series, deposited during the first rifting stages, is composed of red clastic sediments, platform carbonates, evaporite-rich shales, and ophites. Jurassic rocks typically consist of platform limestones and marls. The intense fault activity during a second rifting event that began in the Late Jurassic led to the deposition of a thick succession of sedimentary rocks, mainly during the Early Cretaceous.
The Aptian and Albian sediments may reach up to 7,000 m in the western and central parts of the Basque-Cantabrian basin (García-Mondéjar et al., 2005). This period was characterized by the deposition of shallow-platform limestones with abundant rudist-rich reefs of the Urgonian facies (Rat, 1959).
In the central and eastern regions of the Basque-Cantabrian basin, the Urgonian facies are mainly terrigenous, whereas reefal carbonates predominate in the west (García-Mondéjar et al., 1996). Between the Albian and Santonian, the Cretaceous sedimentation was accompanied by submarine alkalinetype volcanism (Castañares et al., 2001). Sediments directly overlying the Urgonian facies are commonly siliciclastic (Utrillas and Valmaseda Formations) and were deposited in continental, delta, and talus environments. Upper Cretaceous and Paleogene rocks range from continental clastics to platform limestones to flysch-type marls. The Valmaseda Formation (Late Albian to Early Cenomanian; Pérez García et al., 1997), developed in an area of around 1,500 km 2 at the southern margin of the basin, extending to the Álava, Vizcaya, and Burgos Provinces (Fig. 1). Its thickness reaches 5,000 m (Ente Vasco de la Energia, 1994) with an average total organic carbon of 1%, constituting a potential unconventional gas source (U.S. Energy Information Administration, 2015).
Sediments of Mesozoic and Cenozoic age are occasionally intruded by evaporite diapirs consisting of Upper Triassic rocks (Keuper facies). Geologic information from outcrops, drill cores, and geophysical surveys shows that the Keuper facies is essentially composed of halite together with red shales, anhydrite, dolostones, and dolerites (Nettelton, 1968;Serrano et al., 1989;Serrano and Martínez del Olmo, 1990). The diapirs are aligned northwest-southeast and northeastsouthwest and are concentrated in the regions surrounding the depocenters of the Aptian-Albian basin (García-Mondéjar et al., 1996;Frankovic et al., 2016, and references therein), which in turn are outlined by the reactivation of basement faults. Examples along the northwest-southeast alignment (Las Losas fault; Martín-Chivelet et al., 2002) are the Murguía, Orduña, and Villasana de Mena diapirs (Fig. 1A).
According to Rat (1988), the diapiric structures were active from at least the Late Cretaceous and were deformed by the Alpine orogeny, which modified their initial morphology; however, the geometry and unconformity of the Cretaceous sediments around the Murguía diapir show that its ascent initiated during Early Cretaceous times, particularly in the Albian when the older recognized halokinetic sequences developed around the Murguía and the neighboring Villasana de Mena diapir (Abalos et al., 2003;E. Roca, unpub. report, 2014). Diapir ascent continued at least up to Santonian times, as attested to by younger halokinetic sequences with reefal deposits. The later evolution is constrained by the existence of Campanian and lower Miocene deposits above the Murguía diapir (López-Horgue and Hernandez, 2003;Barrón et al., 2006), indicating that it did not grow during these times but collapsed, probably by salt dissolution. On the other hand, the lack of contractional structures at the diapir flanks as well as the nearly circular cross-sectional shape of the Murguía, Orduña, and Villasana de Mena diapirs indicate that they were not squeezed during the Alpine compression (Fig. 1).

Samples and Methods
Ore deposits and occurrences were studied on samples from outcrops and dumps of abandoned mines and on samples from drill cores of the Altube prospect (Murguía diapir) and Paúl deposit (Orduña diapir) (see Fig. 1). In addition, samples from an outcrop at the apparently barren Villasana de Mena diapir were also studied. Transmitted-and reflected-light optical microscopy and cathodoluminescence were used for the petrographic and mineralogical study. Some mineral phases were identified by X-ray diffraction and electron microprobe analysis.
Organic matter microscopic observations were performed with a Leica DMR XP microscope under white (reflected and transmitted) and ultraviolet light. Random reflectivity measurements (reported in %) were made on polished samples (thin sections and densimetric concentrations), following the procedures of the International Committee for Coal Petrology (1963) and using an oil immersion objective (50×). Rock Eval analyses were carried out on a RE6 pyrolyzer (Vinci Technologies), under standard conditions and operating principles described in Lafargue et al. (1998). Hydrogen index, total organic carbon, and temperatures of the maximum rate of hydrocarbon generation (Tmax) are reported. This study was carried out at the Institut des Sciences de la Terre d'Orléans (ISTO, France).
Sulfides and sulfates were analyzed for their sulfur isotope composition on a Thermo Electron Delta Plus XP isotopic mass ratio spectrometer at Centres Científics i Tecnològics of the University of Barcelona (CCiTUB). After careful separation under a binocular microscope, samples were weighed in a tin capsule, adding V2O5 as an oxidizing catalyst, and then combusted in a Finnigan elemental analyzer (EA) coupled online with the spectrometer. Sulfide results were calibrated with IAEA S-1, IAEA S-2, IAEA S-3, and NBS-123 and sulfates with NBS-127, IAEA SO-5, and IAEA SO-6 international standards. Results are reported as ‰ deviation relative to Vienna-Canyon Diablo Troilite (V-CDT). The analytical precision, based on replicate measurements of the international standards, was ±0.1‰.
Oxygen and carbon isotope analyses were performed on ore-stage, host, and regional carbonates (calcite and dolomite). Individual crystals were carefully handpicked under a binocular microscope and powdered or, alternatively, drilled with a manual microdrill from polished slabs. Carbonates were analyzed at CCiTUB. About 0.05 to 0.08 mg of sample were reacted at 70°C with 100% phosphoric acid in a Thermo Finnigan Kiel Device III, coupled online with a Thermo Finnigan MAT-252. Results were calibrated with the NBS-18 and NBS-19 international reference materials. Carbon isotopes are reported in ‰ deviation relative to Vienna Pee Dee Belemnite (V-PDB) and oxygen isotopes in ‰ deviation relative to Vienna-standard mean ocean water (V-SMOW). Reproducibility, determined by replicate analysis of standards, was better than ±0.02‰ for carbon and ±0.06‰ for oxygen.
The Sr isotope composition of carbonates was analyzed at the Centro de Asistencia a la Investigación (CAI) laboratories of the Universidad Complutense de Madrid (Spain). Fifty to 100 mg of sample powder was dissolved in 2.5 N HCl at 80°C, dried and redissolved using 3M HNO3. The strontium aliquots were extracted using conventional cation exchange procedures, and the 87 Sr/ 86 Sr ratios were analyzed in a TIMS-Phoenix thermal ionization mass spectrometer. The reproducibility (2s) was better than ±0.03%.
The Pb isotope composition of galenas was analyzed at the Servicio General de Geocronología y Geoquímica Isotópica (Universidad del País Vasco, Spain). Pb was purified with micro/electrodepositional techniques and was converted into a nitrate and dissolved in water to be analyzed in a Finnigan-Mat-262 mass spectrometer equipped with eight Faraday collectors, a device to multiply secondary electrons and NBS-981 as a standard. Samples were loaded onto Re filaments in a mixture of silica gel and phosphoric acid. Precision was better than ±0.01% for 206 Pb/ 204 Pb and 208 Pb/ 204 Pb and better than ±0.03% for 207 Pb/ 204 Pb.

Diapiric Alteration (Caprock)
Crestal caprock deposits result from halite dissolution at the top of salt diapirs. They consist of a lithological sequence with an upper calcite zone, an intermediate zone with gypsum, and a lower zone with anhydrite overlying halite (Posey et al., 1987;Posey and Kyle, 1988). Its formation involves fluid circulation, bacterial activity, and organic matter. During crestal caprock development, sour gas deposits can be produced and can act as a source either of sulfate reductants (CH4) and/or of H2S for sulfide precipitation. Anhydrite lateral caprocks are also known to occur as sheaths around the diapirs, although their genesis is still a matter of debate (Jackson and Lewis, 2012).
At the Murguía diapir, brecciated carbonate rocks discontinuously crop out at the contact between the Triassic evaporites and the Cretaceous sedimentary pile (see Fig. 1; e.g., Jugo, Aperregui). These were interpreted either as enclaves of Jurassic-age dolostone dragged during piercement (Ente Vasco de la Energia, 1994) or as false caprock (Ábalos et al., 2003). At Altube, this lithology was intersected by drill cores reaching 280 m below the surface. It consists of a brecciated carbonate horizon (up to 20 m thick) and minor zebra rocks at the contact between the Valmaseda Formation and the diapir. This carbonate horizon, named "carbonate transition zone," consists of a greenish-colored clast-or matrix-supported heterometric breccia, composed of idiomorphic crystals of zoned dolomite ( Fig. 2A), calcite replacing dolomite, phyllosilicates (chlorite, illite-smectite, dickite, nacrite), and minor albite and tourmaline. Bipyramidal millimeter-size quartz crystals with anhydrite inclusions are also present. Pyrite appears disseminated, as idiomorphic to subidiomorphic crystals (up to 1 mm) associated with organic matter and phyllosilicates and platy-like crystals (Fig. 2B) probably related to a replacement of a previously existing mineral (sulfates, marcasite [?]). Tourmalines present green cores and noncolored rims (Fig. 2C). In the Al-Fe-Mg diagram (Henry and Gidotti, 1985), the green cores plot within the metapelite and granitoid field tourmalines, whereas the noncolored overgrowths fall within the metaevaporite field (Fig. 3).
As a whole, the carbonate transition zone can be considered a possible remainder of an original caprock. Interestingly, ore grades have been recognized in four boreholes intersecting this carbonate zone (see Fig. 4). Due to their similar macroscopic textures and carbon isotopic signatures (see "Stable Isotope Data" section), carbonate outcrops at Jugo and Aperregui distributed along the diapiric contact have been attributed to the carbonate transition zone described at Altube.
At Orduña, a brecciated horizon that was crosscut during oil exploration drilling performed from the mid-1950s to 1960s was also interpreted as a caprock by Serrano and Martínez del Olmo (1990). In addition, sour gas pockets were found near the contact between this caprock and the evaporites (Baquedano, 2007).
Sandstone-hosted mineralization consists of Zn-Pb-Fe sulfides replacing microconglomerates, sandstones, and carbonate beds of the Valmaseda Formation. Although at Villasana de Mena diapir, local occurrences of sphalerite, barite, kaolinite, and bitumen were also recognized, the most outstanding example is Altube, located at the northeast margin of the Murguía diapir (see Fig. 1). The following description is based on field observations and the study of nine drill cores, distributed in a 500-× 1,200-m area, reaching 365 m in depth (Fig. 4).
Sandstones of the Valmaseda Formation are made up of monocrystalline quartz (60-95%), feldspars (5-35%), rock fragments, muscovite, tourmaline, zircon, and apatite. These rocks show syntaxial quartz and poikilotopic carbonate cements. When sulfides are present, feldspar components are altered to phyllosilicates. Carbonate beds consist of sandy limestones with abundant Orbitolina and bivalves. Amorphous organic matter and bitumen are present in all these rock units (see "Organic Matter Characterization" section).
Mineralization within the carbonate transition zone at Jugo, Aperregui, and Iturlum consists of veins and replacements (Fig. 2G, H). Ore minerals are the same as in Altube. Barite is abundant at Iturlum and Jugo (Fig. 2D), and albite is present at Aperregui, Iturlum, and Jugo.
A particular occurrence is found at Paúl, located at the southern border of the Orduña diapir (see Fig. 1). It consists of two black shale horizons (45 and 75 cm thick, respectively) with fine-grained disseminated to semimassive pyrite-rich sulfides (up to 3% Zn), interbedded within marls and limestones. They represent the oceanic anoxic event (OAE) that occurred at the Cenomanian-Turonian boundary (Jenkyns, 1980;Wilson and Norris, 2001). Drilling showed that ore horizons are up to 150 m long with a lesser lateral extent. Mineralization consists of pyrite, marcasite, sphalerite with minor galena, dolomite, and calcite. Small veins with similar paragenesis that underlie these disseminations have also been recognized.

Mineralogy and Paragenetic Sequence
After optical microscopy, cathodoluminescence, and microprobe analyses, a similar mineralogy and paragenetic sequence was deduced for all studied deposits (Fig. 5). Barite and replacive dolomite predate sulfides (sphalerite, galena, and pyrite/marcasite), which are contemporaneous to saddle dolomite. A brief description of ore and gangue minerals is given below.  Sphalerite is present as allotriomorphic to idiomorphic crystals ranging in size from <1 mm up to 5 cm. Fe and Cd content is up to 2 and 0.5 wt %, respectively, and Ge, Ga, In, and Hg are below the detection limit (Perona, 2016). From cathodoluminescence, two sphalerite types (I and II), have been distinguished. Sphalerite I is characterized by nonluminescent crystals, whereas sphalerite II shows oscillatory dark, brightyellow cathodoluminescent micron-scale banding. Sphalerite II fills small veinlets or appears as overgrowths around sphalerite I (Fig. 6A, B). Both types are chemically similar, although luminescent bands in sphalerite II have slightly lower Cd and Fe contents. Galena occurs as disseminated crystals or fills cracks in sphalerite (I and II). Pyrite is ubiquitous in all deposits. It is found as idiomorphic (pyritohedra) to subidiomorphic disseminated crystals (up to 1 mm) often associated with organic matter and as a <5-mm discontinuous rim around some sphalerite crystals (Fig. 6C). Pyrite-rich zones in the Valmaseda Formation are not always related to Zn-Pb sulfides.
According to crystal shape and size, extinction type, cathodoluminescence, and macroscopic color, four dolomite types have been distinguished. Not all types appear in the same locality, and a gradual transition between types I, II, and III is observed. Dolomite I is made up of 10-to 25-mm, nonluminescent idiomorphic replacive crystals. Dolomite II corresponds to nonluminescent idiomorphic crystals of 40 to 50 mm. Dolomite III is of the saddle type, related to ore sulfides, with crystal sizes up to 1 cm, showing red luminescence, occasionally with a nonluminescent core. Finally, dolomite IV consists of 500-to 1,000-mm red luminescent saddle-type crystals, found in veinlets that crosscut all previous dolomite types.
Two calcites have been distinguished: ore-related calcite I, characterized by weak to bright homogeneous orange cathodoluminescence with crystal sizes up to 10 cm, and calcite II, a late-phase crosscutting ore and gangue minerals. The latter shows banded bright-orange cathodoluminescence zoning and crystal sizes from 50 mm to 5 mm.
Barite occurs as veins and breccia fillings at Iturlum, Jugo, and Beluntza, always related to an early mineralization stage. At Iturlum it shows a bimodal crystal size distribution with large (1-2 cm) crystals that coexist with millimeter-size crystals. Barite shows weak blue cathodoluminescence and is partially replaced by dolomite II.
Bitumen appears all along the paragenetic sequence and formed after liquid hydrocarbons, as evidenced by fluidal textures (Fig. 6D). Solidification took place before or during the precipitation of the Zn-Pb sulfides, as suggested by fractures and vesicles filled with sulfides, sulfates, and carbonates. Organic matter and sulfides are spatially related (Fig. 6E, F), and degassing textures in bitumen are observed (Fig. 6E, G) (see next section).

Organic Matter Characterization
Organic matter is mainly present as bitumen. In all deposits except Altube, bitumen reflectance (BR0) is isotropic, with mean values ranging from 1.24 (Artomaña) to 3.5% (Aperregui) (Table 1). At Altube, anisotropy is related to mosaic and fluidal textures. In this deposit, nine of the 16 measured samples were isotropic, with BR0 values from 2.76 to 3.22%, equivalent to vitrinite reflectances (R0 eq.) between 2.81 and 3.19%, according to the equation of Schoenherr et al. (2007). Using the equation of Barker and Pawlewicz (1994), which relates R0 and temperature for hydrothermal maturation (T[°C] = [LnR0 + 1.19]/0.00782), a range between 284° and 300°C was obtained. The lowest reflectivity measured is found on samples from the Villasana de Mena diapir (BR0 mean value 0.75%). It is important to note that the dispersion of RB0 values within the same occurrence is high, and therefore the temperature/RB0 correlation is not straightforward.
Total organic carbon, Tmax, and hydrogen index from Rock-Eval are shown in Table 1. In whole-rock samples from Murguía (Altube) and the Villasana de Mena diapirs, total organic carbon varies from 0.1 to 7.0% and Tmax from 430° to 630°C. The highest Tmax values were obtained at Altube, indicating that organic matter is overmatured, in agreement with the anisotropic character of bitumen in some samples (see Table  1). As the bitumen Tmax is within the whole-rock Tmax range, it has also been used as a thermal indicator. The lowest Tmax and highest hydrogen index were found at Villasana de Mena plotting within the catagenesis field, whereas all the other samples, including bitumen from Jugo, Aperregui, and Artomaña, plot within the metagenesis field (Fig. 7).
The less matured samples inferred from Rock-Eval match with those of the lowest BR 0, showing that both thermal indicators are consistent. Large variations in BR0 and Rock-Eval in samples from the Murguía diapir have been measured: bitumen from Altube and Aperregui shows high BR0 and Tmax, whereas at Jugo these indicators are clearly lower (see Table 1).

Stable Isotope Data
Carbon and oxygen isotope analyses were performed on 256 carbonate samples, including Cretaceous host rocks (n = 38), carbonates related to ores (n = 151), and carbonates from the transition zone (n = 67). Results are shown in Table 2 and Figure 8.
The d 13 C and d 18 O values of marine limestones of Cenomanian and Turonian age around the diapirs range from 0.5 to 2.8‰ and from 23.0 to 27.3‰, respectively (n = 11). Interbedded carbonate levels within the Valmaseda Formation (Late Albian to Early Cenomanian) show d 13 C and d 18 O values of -1.7 to 1.6‰ and 18.8 to 26.7‰ (n = 6), respectively, lower than local marine limestones.
At Altube, ore-stage calcite (cal I) from mineralization associated with the Valmaseda Formation has d 13 C values from -4.9 to 2.4‰ and d 18 O values from 17.3 to 20.1‰, respectively (n = 20). Late calcite (cal II) has d 13 C values from -5.4 to 1.8‰ and d 18 O values from 16.6 to 22.5‰ (n = 9). Orestage dolomite has a d 13 C value from -3.2 to 3.5‰ and a d 18 O value from 17.6 to 20.0‰ (n = 10).
Nine galena samples from Altube, Jugo, Montaleón, La Antigua, Beluntza, and Paúl were analyzed for their Pb isotope composition. Results are shown in Table 4 and Figure 11B. 206 Pb/ 204 Pb ratios of galenas range from 18.643 to 18.696; 207 Pb/ 204 Pb ratios range between 15.650 and 15.676, and 208 Pb/ 204 Pb ratios range between 38.720 and 38.780. In the uranogenic diagram, data plot on a Stacey and Kramers growth curve with a m between 9.6 and 9.8, and in the thorogenic diagram, with k values from 3.80 to 3.98.

Mineralogy and paragenesis
Zn-Pb sulfide deposits distributed around the Murguía and Orduña diapirs are located in radial or annular structures (carbonate transition zone) developed during the diapir emplacement. Mineralization shows a replacive character and the common presence of organic matter. Regardless of the locality and host lithology, the paragenetic sequence and textures in all deposits are similar (Fig. 5). Barite and replacive dolomite predate sulfides (sphalerite, galena, and pyrite/marcasite), which are contemporaneous with saddle dolomite. However, contemporaneity between deposits cannot be inferred from these observations. Compared to other Zn-Pb deposits of the Basque-Cantabrian basin (e.g., Reocín; Velasco et al., 2003), the lack of botryoidal textures in sulfides from the studied deposits suggests that precipitation took place at low supersaturation and under slow growth conditions (García-Ruíz and Otálora, 2015).
Although several types of dolomite have been distinguished, they do not necessarily represent separate events, nor did they form contemporaneously. Indeed, the gradual contact between dolomite II and III suggests that they could result from a continuous process, in accordance with the selforganized dynamics dolomitization and ore-related model of Merino and Canals (2011). In this model, dolomite and ores of late pulses may partially replace ores and dolomites that formed in early growth pulses. Successive replacements released CO 3 2-, Mg 2+ , Zn 2+ , and S 2-, which are advected forward, thus promoting reprecipitation of more dolomite and/or sphalerite, therefore generating a larger deposit.

Fluid inclusion, organic matter, and isotope geothermometry
Microthermometric and crush-leach data of fluid inclusions in sphalerite, quartz, and dolomite from the studied deposits were published by Grandia et al. (2003). According to these authors, complex polysaline (CaCl2-NaCl-H2O system) and hydrocarbon-bearing fluids were involved in ore formation, with salinities ranging from 10.9 to 25.5 wt % NaCl equiv. From halogen data, Grandia et al. (2003) Grandia et al. (2003), can be related to postentrapment stretching, trapping a nonhomogeneous fluid, or temperature changes during trapping. Although all three possibilities are plausible, the heterogeneous trapping is favored here. This is supported by the presence of volatile components (CH4) in some fluid inclusions (Grandia et al., 2003) and by the spatial relationship between organic matter and sulfides and the occurrence of degassing textures (see Fig. 6E, F). The local incorporation of different amounts of volatile components into the ore fluid, produced after in situ organic matter maturation, would affect the density of the fluid, resulting in variable Th values.
Sphalerite-galena pairs gave average isotopic equilibrium temperatures of 200 ± 50°C (n = 23; Fig. 10) with 10 of them showing a narrower range (178 ± 7°C). The extreme values of the calculated temperatures could be explained by the replacement of isotopically heterogeneous pyrite by sphalerite and galena ( Fig. 6C; see "Sulfur isotopes" section). We take a value ~175°C to be representative of the isotope's equilibrium temperature. The small difference between isotopic temperatures and fluid inclusion Th values assumed here suggests that mineralization formed close to surface.
In most of the studied deposits, organic matter is overmatured, as shown by the Rock Eval and BR0 results (see Table  1). However, the large differences in these parameters over short distances within the same diapir cannot be explained by simple maturation at a regional scale. Therefore, organic matter thermal indicators point to the existence of local and different thermal anomalies around the diapirs that are probably related to the flow of hydrothermal fluids, a process widely documented to occur around salt domes (e.g., Evans et al., 1991). On the other hand, the mosaic anisotropy textures observed on some bitumen samples can be the result of a rapid heating event (Schoenherr et al., 2007) or of a syn-or postheating deformation (Fink et al., 2016).
Calculated temperatures for the bitumen isotropic samples from Altube (284°-300°C) are higher than those obtained from sulfur isotope geothermometry. Nevertheless, geothermometrical data from bitumen reflectance should be taken with caution, because compared to vitrinite, the different  Table  3). Abbreviations: Brt = barite, Gn = galena, Gy = gypsum, Py = pyrite, Sp = sphalerite, VCDT = Vienna Canyon Diablo Troilite.    textures and origins of organic matter can affect the evolution of physical properties (Suarez et al., 2012, and references therein). Our attempts to find possible relationships between different thermal indicators (bitumen reflectivity, fluid inclusion, and isotope geothermometry) for the anomalous samples were unsuccessful. Regardless the enclosing rock type (siliciclastic rocks, marine carbonates, or carbonate transition zone rocks), the studied peridiapiric deposits are indistinguishable in terms of fluid salinity and temperature of formation. Temperature and depth of formation are within the range reported for MVT deposits (Wilkinson, 2001;Leach et al., 2010) and especially with other diapir-related mineralization Posey et al., 1994;Sheppard et al., 1996;Piqué et al., 2009;Bouhlel et al., 2016).

Sulfur isotopes
Except for some pyrite samples, d 34 S values of ore sulfides in Orduña and Murguía range from 4.1 to 15.1‰ (see Table  3; Fig. 9), pointing to a sedimentary sulfur source, either seawater sulfate or sulfate derived from the dissolution of evaporites. No significant isotopic differences between mineralization types and localities have been found, except for pyrite (see below). The d 34 S values of barite (16.2-24.3‰), although higher than gypsum of upper Triassic age (Keuper facies: 15.3-17.4‰), suggest a similar source, the heavier values corresponding to 34 S enrichments after reduction of sulfate to sulfide.
Concerning the origin of H2S for sulfide precipitation, we can envisage two situations: (1) the generation of H2S reservoirs simultaneous with caprock development and (2) H2S produced by ore fluid-organic matter interaction, unrelated to caprock formation. In terms of the d 34 S of sulfides and sulfates, the scenarios are indistinguishable.
In the first scenario, closed system evaporitic sulfate reduction (thermochemical sulfate reduction [TSR] or bacterial sulfate reduction [BSR]) around the diapirs would produce dispersed H2S-rich reservoirs (sour gas) with d 34 S values similar to the initial sulfates (d 34 S ≈ 16‰), concomitant to caprock development. Metal-bearing basinal brines became sulfate enriched during flow along the diapiric structures. As they encountered H2S reservoirs (chemical traps), sulfate is reduced. This process increases the H2S concentration, generating isotopically light H2S and triggering sulfide precipitation. The presence of small and separate sulfide occurrences in the study area would be consistent with the existence of gas traps irregularly distributed around the diapirs; however, the spatial relationship of sulfides and degassing of organic matter (see below) point to an alternative hypothesis.
In the second scenario, the sulfate-enriched brines encountered organic matter-rich rocks (e.g., Valmaseda Formation), thus inducing cracking and triggering ore precipitation. Cracking of organic matter would produce H2S and/or CH4, starting up the thermochemical reduction of sulfate. A BSR is unlikely considering the sulfide precipitation temperatures found (~175°C; see thermal indicators). The presence of degassing textures within the bitumen and the spatial relationship between sulfides and organic matter (see Fig. 6E, F) support this scenario. Under these conditions, the rate of sulfide precipitation is controlled by the rate of H 2S generation, a slow process during TSR but fast enough to produce small orebodies (Thom and Anderson, 2008), resulting in low nucleation and large-size crystals as observed in sphalerite (slow growth textures). Therefore, the development of the caprock would be unrelated to ore deposition, matching the general MVT deposits model (Leach et al., 2010) and the Merino and Canals (2011) model, where the SO4 contributed by the dolomitizing brine is reduced to sulfide by organic matter present in host carbonates as they are being replaced by dolomite.
At Altube, the isotopic composition of pyrite is highly variable (d 34 S = -41.5 to 48.1‰), although most values are close to the isotopic compositions of galena (d 34 S = 4.1-12 ‰) and sphalerite (d 34 S = 4.6-15.1‰). Extreme d 34 S values correspond to framboidal pyrite produced by BSR, unrelated to ore stages. The d 34 S of remaining pyrite samples depict two populations, right and left skewed with a mode around 10‰, corresponding to disseminations within the Valmaseda Formation and the carbonate transition zone, respectively (see Table 3). This distribution is compatible with two different sulfate reduction processes: BSR related to the caprock development, thereby producing pyrite with low d 34 S values, and TSR related to the mineralization stage, producing pyrite with higher d 34 S values.
Variations of up to 8‰ in d 34 S of sulfides within a single deposit (e.g., Montaleón, Altube) could be explained by changes in T and/or fO 2 conditions during ore deposition or by the mixing of isotopically distinct sulfur sources. Considering a single source, d 34 S variations cannot be related either to fO 2 or T changes, since aFeS in sphalerite is almost constant (Perona, 2016) and the d 34 S range of sulfides in each deposit is similar regardless of the temperature of formation. As sphalerite and galena replace host rock, including former isotopically heterogeneous pyrite, their d 34 S ranges could be explained as due to a mixture between the dominant evaporitic sulfur source and sulfur from 32 S-enriched replaced pyrite. The d 34 S values of sulfides are similar to other MVT deposits of the Basque-Cantabrian basin (Velasco et al., 1994(Velasco et al., , 2003Fernández-Martínez and Velasco, 1996;Piqué et al., 2009), pointing to evaporite sulfate as the main sulfur source, although most of the values are not apparently related to saline diapirs.

C, O, and Sr isotopes in carbonates
Most carbonates in the peridiapiric environment show d 13 C/d 18 O values different from those of Mesozoic marine carbonates of the Basque-Cantabrian basin (Table 2). In a d 13 C/d 18 O plot, three assemblages can be distinguished (Fig. 8) (Posey et al., 1987;Posey and Kyle, 1988;Machel et al., 1995).
Ore Ore-stage carbonates from Montaleón, Aperregui, and Jugo show large d 13 C and small d 18 O variations. The d 13 C vertical distribution from marine to highly negative values can be explained by a progressive increase of 12 C in the fluid from the cracking of organic matter during sulfide precipitation and/ or by the interaction of a low-carbon hydrothermal fluid with a preexisting 13 C-depleted caprock. The high and near-constant d 18 O values of ore-stage carbonates would be the result of precipitation from a fluid with an oxygen isotope composition buffered by the host rock (marine and/or caprock carbonates). The presence of a carbonate-poor fluid agrees with the Merino and Canals (2011)   . Group A shows Central and Eastern districts (Matienzo, Aitzgorri, Troya). Group B shows Western district (Reocín). Curves according to Stacey and Kramers (1975) Machel et al., 1995;Worden et al., 1996).  Fig. 11A; Table 2). This trend is expected when oxygen and strontium isotope compositions of fluids are buffered by diagenetic cements in sandstones and marine carbonates, respectively. This negative correlation has also been reported in other MVT deposits (Fallara and Savard, 1998;Savard and Kontak, 1998), with the slope depending on the lithology of the surrounding rocks. Carbonate transition zone calcites from Altube and Jugo also plot within this trend, supporting the idea that ore fluids overprinted an existing caprock.
Carbon and O isotope compositions of ore-stage carbonates from MVT deposits in the Basque-Cantabrian basin depict homogeneous d 13 C but variable d 18 O values (Velasco et al., 1994(Velasco et al., , 2003Piqué et al., 2009). Since the highest d 18 O values correspond to the lowest precipitation temperatures (Reocin deposit; Grandia et al., 2003;Velasco et al., 2003) and most deposits are carbonate-hosted, d 18 O variations cannot be only explained after lithology buffering and temperature effect are considered.

Pb isotopes in galenas
Lead isotope values of galenas are homogeneous and indicate a crustal source (Table 4) and plot between the two fields, Western and Central-Eastern districts, defined by Velasco et al. ( , 2003 in the Basque-Cantabrian basin (Fig. 11B). According to these authors, these two fields reflect regional differences in the composition of Mesozoic-age underlying rocks derived from erosion of two different older massifs (Asturian and Sierra de la Demanda-Basque Pyrenees). Lead from peridiapiric deposits could be explained either by a mixture of Pb from the two sources defined by  or by a single source from underlying rocks having different Pb isotope compositions. Both hypotheses are plausible, considering the block movements that took place during the opening of the Bay of Biscay involving different types of rocks (Roca et al., 2011).

Conceptual Model and Conclusions
A combination of field and laboratory work including textural, isotopic, and organic matter analyses has provided insights into the genesis of ore deposits related to the Murguía and Orduña salt diapirs in the Basque-Cantabrian basin. The conceptual model and evolution for this type of mineralization is shown in Figure 12.
The structural evolution of the Basque-Cantabrian basin resulted in the development of tectonic lineaments with diapiric structures of Triassic rocks, which concentrated the Mesozoic extensional and the Alpine compressional deformations (Serrano and Martínez del Olmo, 2004). At Murguía, reactivation of basement faults during the Early Cretaceous induced diapirism, producing thickness changes in the siliciclastic Albian-Lower Cenomanian materials (Valmaseda Formation) (Fig. 12A). Diapiric ascent continued up to the Upper Cretaceous, as evidenced by major thickness changes in the sedimentary pile around the diapir. During the halokinesis, a caprock developed. Piercement created permeable structures, enabling the flow of basinal metal-bearing hydrothermal fluids, which produced Zn-Pb deposits and dolomitized the caprock (carbonate transition zone). During the Campanian, reefal structures formed on top of the diapir (Fig. 12B). As the diapir reached the surface, it started to dissolve, the reefal structure collapsed, and continental sediments of lower Miocene age deposited in a small endorheic basin (López-Horgue and Hernández, 2003;Barrón et al., 2006). The present erosion level of the Murguía diapir brought mineralization to the surface (Fig. 12C). The timing of mineral deposition cannot be constrained with precision, but halokinetic evolution of the studied diapirs suggests that ore deposits formed no later than Campanian times.
Mineralization consists of vein and stratabound sulfides (pyrite, sphalerite, and galena) with dolomite and calcite as gangue minerals, hosted by Lower Cretaceous siliciclastic sediments (Valmaseda Formation), and Upper Cretaceous carbonates and carbonates from former caprock structures (carbonate transition zone). Dolomitization and organic matter as solid bitumen are ubiquitous and spatially related to sulfide minerals.
After equilibrating with evaporite minerals (halite, anhydrite), basinal metal-bearing hydrothermal fluids became sulfate rich, increased in salinity, and attained halogen ratios compatible with halite dissolution (Grandia et al., 2003). Sulfide precipitation was triggered after sulfate reduction by organic matter present in the host rocks. The temperature attained during ore deposition (150°-200°C) suggests that TSR was the dominant sulfate reduction process. The d 34 S values of ore-stage pyrite, sphalerite, and galena (4.1-15.1‰) and barite (16.2-24.3‰) are compatible with reduction of Triassic evaporite sulfates from the diapirs (15.3-17.4‰).
Around the diapirs, mineralizing fluids flowed through different rock types, as recorded by the C and O isotope composition of ore-stage carbonates. The d 13 C values of ore-stage carbonates hosted in siliciclastic rocks and marine limestones (~-3 to 4‰) are close to marine carbonate values, indicating that metal-bearing basinal brines were carbon poor and isotopically buffered by the enclosing rocks. The most negative d 13 C values (~-10‰) of ore-stage carbonates hosted in the carbonate transition zone reflect the interaction of the hydrothermal fluid with an isotopically light C source as the preexisting caprock. The original isotopic composition of the caprock is preserved at the Jugo deposit, where carbonates of the carbonate transition zone show high d 18 O values (~25‰) and d 13 C up to -20‰, typical of sulfate-reduction processes by organic matter as described in diapir-related caprock formations (e.g., Prikryl et al., 1988;Kyle and Saunders, 1996). These flow paths are also recorded by the d 18 O values and 87 Sr/ 86 Sr ratios of ore-stage carbonates, ranging from 17 to 20‰ and 0.71202 when hosted in siliciclastic rocks and from 23 to 26‰ and 0.70801 in marine carbonates, respectively. Mineralizing fluids acquired metals from a crustal source with a m between 9.8 and 10, similar to other Zn-Pb deposits of the Basque-Cantabrian basin .
We have shown that peridiapiric mineralization and caprock formation at Murguía and Orduña are not related processes in terms of time and reductant sources, in contrast with that observed in the classic examples of the Gulf Coast. The studied deposits are associated with dolomitization and the presence of organic matter, sharing the main geochemical characteristics of the traditional MVT deposits and being comparable to diapir-related Zn-Pb deposits of North Africa. In the case of the Murguía diapir, the presence of dolomitized ore-bearing remnants (carbonate transition zone) below the Upper Cretaceous and Cenozoic sedimentary rocks in the central part of the diapir cannot be ruled out. This situation contrasts with the evolution of the Orduña diapir, where the caprock has been eroded, cropping out only evaporites of Triassic age in the central part of the diapir. Joaquim Perona obtained a degree in geology at the Universitat de Barcelona (UB), Spain, and a Ph.D. degree in ore deposits at the Universitat Autònoma de Barcelona, Spain. Since 2004 he has been working at the Scientific and Technological Center of the UB as a stable isotope analyst.