Mineralogy, Geochemistry, and Stable Isotopes (C, O, S) of Hot Spring Waters and Associated Travertines near Tamiahua Lagoon, Veracruz, Gulf of Mexico (Mexico)

Abstract

Laminated travertine forms in and around an active hot spring on the west coast of Tamiahua Lagoon, north of the state of Veracruz, Mexico. Fluid chemistry is characterized by discharging slightly acidic pH hot water and gas at a constant flow rate. Moreover, finely interbedded mineralogical products from discharging waters at 70 °C host scattered hydrocarbons. The mineralogy and geochemistry of the travertine formations were characterized to determine their origin. Rock samples were collected and further studied by transmitted light petrography, X-ray diffraction, and EDS-coupled scanning electron microprobe. Identified mineralogy from outcrop samples includes aragonite, gypsum, anhydrite, and elemental sulfur as essential minerals, with calcite, celestine, barite, jarosite, opal, and fluorite as accessory minerals. Isotopic analyses for C and O were determined in carbonates, S isotope ratios on both elemental sulfur and sulfates, whereas measurements for trace elements and lanthanides were performed on carbonates. A suit of brines and condensates from gas samples was collected for H and O isotopic analyses and concentration determinations of the main ions and major and trace elements. Isotopic values of δ¹³C and δ¹⁸O of aragonite are in the range of +1.75‰ to +2.37‰ and −1.70‰ to −0.78‰, respectively. The δ³⁴S isotopic values of native sulfur and sulfates ranged from −4.0‰ to +1.2‰. The isotopic values of δ²H (−5.50‰) and δ¹⁸O (+7.77‰) of hot water samples collected in terraces where aragonite precipitates suggest a mixture between meteoric water and the Gulf of Mexico oil-field related waters. It was concluded that the aragonitic formations near Tamiahua Lagoon are hypogenic and were generated by CO₂ and H₂S emanations of deep origin and by oxidation-reduction reactions that can be linked to surficial bacterial activity.

1. Introduction

Travertines are very active sedimentary continental systems around the world, where above ambient temperature carbonates (calcite and aragonite) chemically precipitate by rapid CO₂ degassing from highly supersaturated calcium carbonate brines. Travertine formation results from a complex interaction between geochemical, physical, and microbial processes [1,2,3,4,5]. Carbonates precipitated from hot spring water, seeps, and fumaroles are classified as thermogenic travertines [6,7,8], while those derived from ambient-temperature water are known as tufas. According to Pentecost (2005) [6] and Pentecost and Viles (1994) [9], those carbonates formed from groundwater loaded with some meteoric component and where atmospheric carbon dioxide is fixed are called meteogene. The carbonic acid formed in this process dissolves the regional deep limestone layers and enriches the groundwater with CO₂ [6]. On the other hand, thermogene travertines are formed by CO₂ [8,10] released by thermal processes within the earth’s crust. When CO₂ is dissolved in groundwater under higher-pressure conditions, it dissolves rocks rich in calcium and rises to the surface as hot springs, often in zones of recent volcanic or hydrothermal activity [6,11,12]. It is worth mentioning that different authors [13] choose to use the acronym CATT (Calcite and Aragonite Travertine and Tufa) to designate these deposits jointly.

The detailed study of travertine formations requires the combined use of different disciplines and analytical methods such as mineralogy, sedimentology, isotope geochemistry, carbonate chemistry kinetics, paleoclimatology, and geomicrobiology ([8,14] and references therein). The most frequent low-temperature travertine morphologies are mounds, pendants, and terraces associated with dams. The laminations are the most common textural lithofacies observed in many of these formations. Their cyclicity, origin, and relationship with biogenic activity are still discussed [8,15,16,17,18,19,20,21,22,23,24,25,26].

Travertines have historically been recognized worldwide as construction material [27,28]; they are also located in sightseeing areas, and their scientific importance is even more relevant. Travertine formation results from complex interactions between the upper crust lithologies, aqueous chemical processes in the sub-surface environment, and microorganism metabolic processes. According to some authors, current continental deposits represent a recent analog of the non-marine stromatolites from the past [29]. In addition, travertine characterization is important to understand the effects of past climate change based on its paleoformation, the survival mechanisms of extremophile microorganisms at high temperatures, and the natural mechanism of CO₂ sequestration [6].

The Gulf of Mexico is a well-known zone characterized for hosting a series of hydrocarbon oil fields at depth. On this area, on the west coast of Tamiahua Lagoon—north of the state of Veracruz and just on the shoreline—laminated travertines currently precipitate near a high-temperature hot spring with active degassing. Although it was previously reported solely as a hydrothermal feature, the present work is the first study to characterize the hydrothermal precipitates in the margin of Tamiahua Lagoon. The present work aims to: (a) determine the mineralogy, fluid chemistry, and classification of the travertine formation; (b) characterize the compositional and isotopic signatures of the different types of water (brines) related to the travertine precipitation; and (c) determine travertine genesis, establishing its relationship with the regional fault system and with the hydrocarbon emanations of the area.

2. Geological Context

The studied area is located in the northern area of the state of Veracruz, east of the Campo del Mamey oil field, on the eastern side of Tamiahua Lagoon, and between the coordinates 21°31′27″ N and 97°36′26″ W (Figure 1).

Non-consolidated detrital sedimentary units and carbonates from the Eocene to the Pleistocene ages outcrop in the study area. Recent lacustrine-type material is recorded, given the constant influence of the Tamiahua Lagoon and the rivers that flow into it, whereas basement units are not superficially exposed (Figure 2).

At the regional scale, Tamiahua Lagoon is located on the Gulf of México coast and belongs to the "Eastern Intraplate Volcanism" sector of the Trans-Mexican Volcanic Belt (TMVB), being the area therefore dominated by a convective-type heat flow [30]. The center and the southeast area of this province are characterized by recent volcanic activity of the Miocene to the Quaternary age. The most important volcanic fields in the zone are San Carlos, Sierra de Tamaulipas, Tlanchinol-Tantima-Alamo, Chiconquiaco-Palma Sola, Anegada High, and Los Tuxtlas Volcanic Field, all of which are associated with an extensional tectonic setting [31], which in turn determines the existence of the main scale tectonic lineaments with regional NW-SE orientation. The second type of lineament is associated with the Otontepec or Tántima mountains, which display 40° to 60° NE-SW direction. Finally, a third younger lineation from NE 80° SW to E-W [32] is recorded, tectonically unrelated to the previous ones.

The travertine formations of Tamiahua Lagoon could indirectly be associated with an anthropogenic event that occurred in July 1908, during the drilling of an oil well called San Diego de la Mar Number 3, which was conducted by the “Pennsylvania Oil Company of Mexico”. When the drilling reached a depth of approximately 556 m, it suddenly went out of control, causing the release of the oil and setting fire to the areas where the hydrocarbon spilled out. Furthermore, due to the reservoir’s internal pressure, a second outlet was generated, causing the collapse of the surface and the formation of a crater denominated “Pozo de dos Bocas”, situated at 2.65 km of the studied area (Figure 1).

The spilled oil flowed to the east into Tamiahua Lagoon, covering an area of 30 km². After the explosion, it left an artificial lagoon with a diameter of ~500 m, where the fire lasted for approximately three years until the oil field was exhausted and the fire extinguished. Currently, this artificial lake is a remnant of the well San Diego de la Mar Number 3, which intermittently emanates crude oil (chapopotera) and sulfurous saltwater [33].

Outcrop Description

The studied travertine formation is on the eastern shoreline of Tamiahua Lagoon, and—according to Pentecost’s [6] classification—it can be described as a small composite autochthonous flat mound with subcircular morphology and an irregular sub-vertical cone (Figure 3).

In 2017, the original cone elevation was less than a meter high and was almost inactive (probably by sealing at depth), but later raised, and lateral emanations on lower cones have been developed. Morphologically, it is characterized by a slight elevation (approx. 2.5 m above sea level) towards the center, where small fumaroles that exhale vapors stand out (Figure 3b). The surficial outcrops of older precipitates are laterally exposed as thin terraces concentrical to the main fumarole and have a diameter of nearly 50 m. The present surface is covered by elemental sulfur, giving a characteristic yellow–green color (Figure 3a,b,e). Small fissure walls were observed surrounding the original cone, and the whole system is crossed by fractures enriched in iron oxides (Figure 3g,h). The carbonate layers are intercalated with very dark ones and related to hydrocarbon emanations (Figure 3c).

The current active spring is situated at sea level to the west of the fossil formation and covers an area of approximately 12,000 m². Additionally, the area is characterized by the presence of mud terraces (0.5 to 1.5 m in height), shallow pools (~10–30 cm), and low-temperature hot waters flow, from which the hydrothermal fluid emanates (Figure 3d,f). The measured surface temperature is ~70 °C. The deposition rate in the terraces is very high and the carbonate accretion remains ongoing.

3. Materials and Methods

Aiming to reveal the origin of the hot spring precipitates, investigations were carried out during two field trip periods corresponding to November 2017 and September 2018.

3.1. Rock System

Forty-seven rock samples from the travertine formation were collected from several sites (Figure 4), cut, and polished to characterize mesofabric textures, after which they were selected as dedicated samples for the transmitted light petrographic analysis (Figure 4). Polished slabs and thin sections were observed with a Zeiss Discovery V8 and a Zeiss Axio Imager A2m, respectively; both are equipped with an AxioCam camera (Carl-Zeiss Micrscopy, Oberkochen, Germany). Microscopic features were described using petrographic and scanning electronic microscope observations (SEM). The SEM operated at low vacuum (as environmental electronic microscope) and this allowed the samples to be visualized without coating them. Nitrogen gas and small pressure variations were used to increase conductivity. Samples were studied using both backscattered electrons (BSE) and semi-quantitative analysis by energy-dispersive X-rays (EDS). The equipment used was a Zeiss model EVO MA10 (Carl-Zeiss Micrscopy, Oberkochen, Germany) from the Laboratory of Electron Microscopy and Nacional de Geoquímica y Microanalysis of LANGEM (Laboratorio Mineralogía, UNAM, Mexico City, Mexico).

For X-Ray diffraction (XRD) analysis, 36 selected samples were finely grounded using an agate mortar and pestle, sieved (<45 µm), and mounted as random fractions using a double-sided aluminum holder to determine their mineralogy. The fine material with banded texture was sampled using a micro-drill with a DREMEL 4000 tungsten carbide bit. XRD measurements were made using a Malvern Panalytical Empyrean diffractometer (Malvern Panalytical, Malvern, UK) at the LANGEM, operating with an accelerating voltage of 45 kV and a filament current of 40 mA, using CoKα radiation and iron filter (samples rich in iron) and CuKα radiation and nickel filter (all the samples). All samples were measured using high-resolution routines over a 2θ angular range of 4–80° at a step scan of 0.003° and an integration time of 40 s by step. Phase identification and Rietveld quantification were made with a PDF-2 database using Highscore v.4.5 software. Relevant data from the diffractometer used in the refinement are shown in

Table 1. Relevant data from X-ray diffractometer EMPYREAN XRD used for the Rietveld refinement.

Geometry	Bragg–Brentano
Goniometer radius	240 mm
Radiation source	CoKα & CuKα
Generator	45 kV, 40 mA
Tube	Fine Focus
Divergence Slit	½° (fixed)
Soller Slits	0.04 rad (incident and diffracted beam)
Incident beam optics	Parallel mirror
Filter	Iron filter & Nickel filter
Detector	PIXcel3D
Step Size	0.002°
Integration Time	40 s

. The refined specimen-dependent parameters were the zero error, displacement error, polynomial fitting for the background with six coefficients, cell parameters, crystallite size, atomic coordinates, and isotropic temperature factors. The GOF values (Goodness of Fitting) show the fit between Rietveld refinement and the experimental profile.

Trace element analyses were obtained from ACTLABS (Activation Laboratories, Ontario, CA, Canada) in four aragonite samples (Lithogeochemistry package 4B2-Res). The samples were melted with a flow of lithium metaborate and tetraborate using an induction furnace. Once this process was completed, digested samples were continuously mixed with a 5% nitric acid solution containing an internal standard until the sample was completely dissolved. Dissolved samples were analyzed with a Perkin Elmer Sciex ELAN 6000, 6100, or 9000 ICP–MS (PerkinElmer, Waltham, MA, USA). Two samples were analyzed with blank and three controls (two before the sample group and one after).

The carbon and oxygen isotopic measurements were determined in six calcium carbonate powders obtained with a DREMEL 4000 tungsten carbide bit. First, each sample was cleaned with acetone to remove surface organic residues. Then, the analyses were carried out (by triplicate) using the procedure described by Révész et al., (2001, 2002) [34,35] using a Gas Bench II coupled to a Thermo Finnigan MAT 253 (Thermo Electron, Bremen, Germany) stable isotope mass spectrometer at LANGEM Stable Isotope Laboratory. An Oztech CO₂ tank of certified isotopic composition (δ¹⁸O_VPDB = −9.78 and δ¹³C_VPDB = −10.99) was used as a working standard. Two types of calcium carbonate internal standards (Sigma and Merck) were included, each with a different isotopic fingerprint. δ¹⁸O_VPDB and δ¹³C_VPDB results of carbonates were normalized using NIST reference materials NBS-19, NBS-18, and LSVEC to the VPDB scale according to the corrections described by Coplen (1988) [36] for δ¹⁸O_VPDB and scale change to δ¹⁸O_VSMOW (Vienna Standard Mean Ocean Water) and Coplen et al., (2006) [37] for δ¹³C_VPDB. A standard deviation of 0.2% was obtained for oxygen and carbon with this technique.

The determination of δ¹³C and δ³⁴S in solid hydrocarbons (chapopote) was carried out in two samples of approximately 2 g each. The analyses were performed at the GeoMark Research laboratory in Houston, Texas, USA, using a Vario ISOTOPE Cube elemental analyzer and a VISION isotope ratio mass spectrometer. Carbon isotopic analyses were run twice each and calibrated using international standards (USGS24 and NBS22) and three internal standards, with an analytical error of ±0.1‰ on the VPDB scale. Sulfur isotopes were measured in triplicate and were calibrated using international standards (IAEA-S-1, IAEA-S-2, and NBS127) with an analytical error of ±0.3‰ and reported on the VCDT scale.

Seven samples of native sulfur and three samples of sulfates (gypsum and anhydrite) were selected to obtain their δ³⁴S isotopic fingerprints; all these samples were previously sieved to <45 microns. The method used was described by Grinenko (1962) [38], using cuprous oxide (Cu₂O) as an oxidizing agent. For the samples to enter the sulfur line, each one was mixed with copper oxide and quartz in an agate mortar, and later the material was introduced into glass tubes with quartz wool at the ends to allow the gas to flow. In the sulfate samples (30 mg), 200 mg of cuprous oxide and 600 mg of quartz were used, whereas only 200 mg of cuprous oxide was added for the elemental sulfur (10 mg). The sulfur and sulfate in the samples were transformed to SO₂ through a furnace, employing oxide-reduction processes at temperatures above 1000 °C (1080 °C for native sulfur and 1150 °C for sulfates). The SO₃ was reduced to SO₂ in a second copper furnace at 750 °C to avoid fractionation, and the water was trapped in an acetone trap cooled with solid CO₂ while the vacuum pumps discarded the gaseous CO₂ through a pentane trap. Final SO₂ removal was performed by passing gas trapped on a cold finger (liquid nitrogen) into a bottle. Subsequently, SO₂ was isotopically analyzed in a SIRA II mass spectrometer at the University of Salamanca, Spain.

3.2. Hydrothermal System near Tamiahua Lagoon

Fluid sampling consisted of acquiring five samples from hydrothermal brines (Figure 4A): (a) thermal water that emanates at constant flow from the aragonite terraces; (b) freshwater from a well close to the studied area; (c) seawater from the coast of Tamiahua Lagoon; (d) water from a small river that flows next to the carbonate formation and into the lagoon; and (e) condensate from the fumarole obtained using a titanium bell. According to the pursued chemical analysis, samples were placed on polyethylene containers that were previously washed. Four bottles of 120 mL (analysis of stable isotopes, trace elements, anions, and cations) and one of 250 mL (analysis of bicarbonates) were filled with brine for each sample. It was difficult to accumulate enough volume of condensate from the fumarole, hence only stable isotopes (H and O) and major ions could be measured in this sample. Type 1 water was used as a blank.

Additionally, a HANNA Instruments Waterproof Tester was used to obtain in situ physicochemical measurements (pH, temperature, conductivity, salinity, and total dissolved solids) of the different waters.

Samples for trace elements and isotopic analysis were filtered using a 0.45 µm cellulose acetate membrane. Major ions, trace, and isotopic analysis samples were filtrated using cellulose acetate membrane. After filtration, the water samples for cation and trace elements determinations were acidified (pH = 2) with ultrapure HNO₃ (70% w/w%). The bicarbonate/carbonate concentration was determined in the field by volumetric acid-base titration with pre-normalized HCl and visual indicators to identify the endpoint.

Using a Thermo brand Dionex ICS-5000 ion chromatography instrument, anions and cations were determined. The eluent used for the anions was sodium hydroxide (NaOH) and methanesulfonic acid for the cations. The calibrating solutions used were High Purity Standard with a stock concentration of 1000 mg/L. As a quality criterion of the standards and sample repetitions, the variation coefficient was less than or equal to 2%.

For the δ1⁸O and δ²H analyses, the containers were filled, avoiding the formation of bubbles. Furthermore, each flask was sealed and stored in a cooler to refrigerate the samples at 5 °C, and later taken to the laboratory for analysis. Stable isotope analyses of δ¹⁸O and δ²H in water samples were carried out using an intracavity laser absorption spectrometer (CRDS) Picarro L2130i with a high precision vaporizer from the Geothermal Fluid Geochemistry Laboratory of the Instituto de Geofísica, UNAM, using international standards (VSMOW2 and GRESP). The analyses performed on the water samples for isotopes were done in triplicate, with reproducibility of ±0.1‰ and ±1.0‰ for δ¹⁸O and δ²H, respectively.

4. Results

4.1. Rock System

4.1.1. Mesofabric

The Tamiahua Lagoon travertines are constituted mainly by abiogenic cyclic crystalline multi-layered crusts. The principal mound is formed by well-bedded and finely laminated compact carbonates, intercalated with very thin layers of native sulfur (yellow–orange color) and sulfates (mainly white). In addition, layers of variable thickness and very dark color are scarce and related to the emanation and deposition of asphaltene-like hydrocarbons (cookie oils). Microbialitic bio-formations were not observed on the fumarole site, although active biogenic processes cannot be ruled out due to small concentrations of algal mats being found closer to the main discharging high-temperature hot spring. Lime-mudstone layers dominate the facies and, bordering the fumaroles, cross-cutting bands (banded palisade carbonate, [39,40,41]) were observed (Figure 3i).

4.1.2. Microfabric

The lamination at different scales (microns to centimeters) is the most noticeable textural feature of the deposits and it can be associated with a periodic change in precipitation rates (Figure 5a–f) or to changes in the dynamics of the hydrological cycle, associated with processes of natural or anthropogenic origin. The layers are plane, undulated, or wavy and, in general, show alternate (two fabrics and colors) or cyclic (more than two fabrics) repetitions [28]. The process of formation of wavy and porous lamination can be observed in the terraces zone, carbonate layers being white or with different hues of blue. The sequences with layers of different thicknesses (heteropachous) are predominant but not exclusive on the site (Figure 5a,b).

The size and distribution of porosity are very heterogeneous. The cavities have greater length than width, are parallel to the bedding, and seem related mainly to the inclusion and emission of gas, causing fenestral type porosity (Figure 5c). Characteristic soft-sediment deformation textures associated with the recurrence of gas bubbles (e.g., [12]) were also frequently observed (Figure 5b,e).

In the petrographic observation, it was possible to verify that the aragonite is the predominant mineral (Figure 6g). Crystalline crusts can be defined as bundles of acicular to dendritic crystals of aragonite (e.g., [18,42,43,44,45,46] and references therein). This mineral occurs as euhedral crystals with a fibrous habit (length to width > 6:1) (Figure 6a,b). These fine needle-shaped crystals of tens of microns in length are rarely found as single crystals and form regular arrays as plume-like fans, and spherulites made up of hundreds of fibrous crystals (Figure 6d,h). Sometimes, in the center of these structures, small particles of other mineral phases are observed; in some cases, the structures are a little more complex and of a branched or dendritic type (Figure 6c). Acicular aragonite crystal growth was observed directly in the water from terraces. The aragonite crystals are oriented perpendicularly to the surface and can be associated exclusively with a rapid and inorganic crystallization process [20,44,47,48,49]. In the water–air interface at the terraces zone and where the hot spring is active, the rapid formation of aragonite paper-thin raft fabrics (also described as ‘hot water ice’) and its continuous accumulation at the bottom was identified [6,27,50,51]. These textures were also recorded in the travertine’s fossil outer part (Figure 6e,f). This texture is not very compact; therefore, sulfate crystals (gypsum, anhydrite, and, more specifically, celestine) have been most frequently identified in the cavities between the aragonite crystals (Figure 7a–c).

In the petrographic study, it was also possible to verify that calcite is a very minor phase (less than 10%); it appears as aggregates of very small anhedral crystals (a few microns) of the micrite type. X-ray diffraction shows that such carbonate has low crystallinity (very wide peaks in the diffractograms), and thus it is not ruled out that its precipitation may be related to some process associated with biological activity. However, no petrographic evidence of this hypothesis could be found.

Sulfur is abundant, unevenly distributed, and occurs as botryoidal aggregates of anhedral crystals, often associated with iron oxides, making it difficult to observe in a transmitted light microscope (Figure 7d). Small layers of native sulfur and hydrocarbons (Figure 6g) are also found between the different aragonite layers. Associated with the presence of sulfur, it is also common to find aggregates of euhedral to subhedral crystals of gypsum and anhydrite (Figure 7b,c). These minerals are mainly found filling the cavities that exist between the aragonite crystals, but it is also common to find them forming thin layers interspersed between the carbonate layers

4.1.3. Mineralogy

The predominant minerals identified by XRD analysis were aragonite, gypsum, anhydrite, and elemental sulfur. Aragonite is mainly found forming regular aggregates of acicular crystals. Gypsum and anhydrite occur mainly as subhedral crystals forming irregular aggregates and filling cavities. Finally, elemental sulfur appears as microcrystalline and botryoidal aggregates associated with iron oxides and sulfates. The accessory minerals, generally filling cavities or in the interior of the main phases, were calcite, celestine, barite, quartz, and opal (Figure 7 and Figure 8 and Table S1 of Supplement Material). In addition, hematite, goethite, and local hydronium jarosite-group minerals have been identified in the small fractures present in the area.

Aragonite, elemental sulfur, gypsum, anhydrite, celestine, opal, fluorite, and goethite were identified using scanning electron microscopy coupled to SEM (Figure 9a–d). It is worth mentioning that some authors (e.g., [26]) have also described the presence of fluorite in travertines of deep origin. Quartz (<5 µm) is scarce and has been identified as detrital origin (Figure 9a). Celestine crystals (~10µm) were found to be associated with aragonite by the bright hue of the image due to the presence of Sr (Figure 9a). Measurement with EDS allowed us to determine the existence of 1–3% of Sr in the aragonite structure. Furthermore, the presence of opal particles—rounded in shape, without crystalline faces, and with a diameter of approximately 60 µm—was also identified (Figure 9c), as well as aggregates of native sulfur of poor crystallinity (Figure 9d).

4.1.4. Geochemistry and Isotopic Composition of the Rock

Table 2. Trace element composition of travertine samples including lanthanide concentrations of four aragonite samples and one native sulfur sample (Tami 2AS) normalized to chondrite-type meteorites [52].

Sample	V	Cr	Co	Ni	Cu	Zn	Ga	Ge	As	Rb	Sr	Y	Zr	Nb
DL	5	20	1	20	10	30	1	0.5	5	1	2	0.5	1	0.2
TAMI 6.2	<5	<20	<1	<20	<10	<30	<1	<0.5	<5	<1	>10000	<0.5	1	<0.2
TAMI 2.2	<5	<20	<1	50	<10	<30	<1	<0.5	<5	<1	9350	<0.5	<1	<0.2
TAMI 2AS	<5	<20	<1	<20	<10	<30	<1	<0.5	<5	<1	2370	<0.5	<1	<0.2
TAMI AC	<5	<20	3	<20	10	<30	<1	<0.5	<5	<1	>10,000	3.5	2	<0.2
TAMI BC	<5	<20	3	40	<10	<30	<1	<0.5	<5	<1	>10,000	<0.5	<1	<0.2
Sample	Mo	Ag	In	Sn	Sb	Cs	Ba	Hf	Ta	W	Tl	Pb	Bi	Th	U
DL	2	0.5	0.1	1	0.2	0.1	3	0.1	0.01	0.5	0.05	5	0.1	0.05	0.01
TAMI 6.2	<2	<0.5	<0.1	<1	<0.2	<0.1	35	<0.1	<0.01	<0.5	<0.05	<5	<0.1	<0.05	<0.01
TAMI 2.2	<2	<0.5	<0.1	<1	<0.2	<0.1	17	<0.1	<0.01	0.8	<0.05	<5	<0.1	<0.05	0.01
TAMI 2AS	<2	<0.5	<0.1	<1	<0.2	<0.1	12	<0.1	<0.01	<0.5	<0.05	<5	<0.1	<0.05	<0.01
TAMI AC	<2	<0.5	<0.1	<1	<0.2	<0.1	70	<0.1	<0.01	<0.5	0.06	<5	<0.1	<0.05	1.33
TAMI BC	<2	<0.5	<0.1	<1	<0.2	<0.1	82	<0.1	0.02	0.5	<0.05	<5	0.1	<0.05	0.02
Sample	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	ΣREE
DL	0.05	0.05	0.01	0.05	0.01	0.005	0.01	0.01	0.01	0.01	0.01	0.005	0.01	0.002
TAMI 6.2	<0.05	<0.05	<0.01	<0.05	<0.01	<0.005	<0.01	<0.01	0.01	<0.01	<0.01	<0.005	<0.01	<0.002	0.01
TAMI 2.2	<0.05	<0.05	<0.01	<0.05	0.01	<0.005	0.01	<0.01	0.01	<0.01	<0.01	<0.005	<0.01	<0.002	0.03
TAMI AS	<0.05	<0.05	<0.01	<0.05	<0.01	<0.005	<0.01	<0.01	<0.01	<0.01	<0.01	<0.005	<0.01	<0.002	0
TAMI AC	2.57	3.51	0.4	1.53	0.3	0.057	0.45	0.07	0.42	0.08	0.25	0.034	0.21	0.035	9.916
TAMI BC	0.09	0.17	0.02	0.1	0.01	0.007	0.01	<0.01	0.01	<0.01	0.01	<0.005	0.01	0.002	0.439

and Figure 10 show the concentrations of trace elements measured by ICP in four Tamiahua Lagoon samples with pure aragonite crystals and one sample of native sulfur. The sample labeled as Tami AC has a dark blue coloration and contains the highest concentration of Y, Zr, U, and of all Rare Earth Elements (REE), while the samples Tami BC and Tami 2.2, of white coloration, have a slightly higher concentration of Ni. All aragonite outcrop samples have a high content of Sr (>1%), whereas the barium content in aragonite samples is low (17–82 ppm) [13].

Regarding the lanthanide content (Table 2 and Figure 10), there is a notable difference between the concentrations normalized with chondrite [52]. The Tami AC sample has the highest abundance of lanthanides (∑REE = 9.9 ppm); while light-colored aragonite samples (Tami 6.2 and Tami 2.2) have the lowest (∑REE < 0.5 ppm).

The δ¹³C and δ¹⁸O values obtained for the six samples of aragonite (

Table 3. Isotopic composition of travertine deposits of Tamiahua Lagoon (carbonates, sulfates, and sulfur) and solid hydrocarbon samples. Cc—calcite, Arg—aragonite, Gy—gypsum.

Carbon and Oxygen Isotopic Composition of Travertines
Sample Number	Sample Code	XRD Mineralogy	δ13CVPDB (‰)	δ18OVPDB (‰)	δ18OVSMOW (‰)
1	Cima 1	31% Ar, 9% Cc, 60% Gy	+2.37	−0.78	+30.11
2	Capa 2	100% Ar	+1.83	−1.39	+29.47
3	Banda 3.1	78% Ar, 28% Gy	+1.93	−1.52	+29.34
4	Banda 3.2	100% Ar	+1.83	−1.60	+29.26
5	Banda 3.3	100% Ar	+2.02	−1.46	+29.40
6	Tami 7	100% Ar	+1.75	−1.70	+29.16
Sulfur Isotopic Composition of Travertines
Sample Number	Sample Code	XRD Mineralogy	δ34S (‰)
1	Tamiahua 2	Gypsum-anhydrite	−4.0
2	Tamiahua 1	Gypsum-anhydrite	−0.6
3	Tamiahua 5	Gypsum-anhydrite	−0.8
4	M 1.2 Cristal	Native sulfur	−0.1
5	Tamiahua 2R Sup	Native sulfur	−3.7
6	M 5	Native sulfur	+1.2
7	M 5.2	Native sulfur	−2.3
8	M6	Native sulfur	−1.5
9	M 8.2 Sup	Native sulfur	−0.6
10	M 13	Native sulfur	+0.3
Sulfur Isotopic Composition of Hydrocarbons
Sample Number	Sample Code	δ13CVPDB (‰)	% Sulfur	δ34S (‰)
1	MX0242	−26.85	2.16	7.64
2	MX0243	−26.44	3.02	7.50

) are similar; the δ¹³C range is of +1.75 to +2.37‰, while for δ¹⁸O it is of +1.70 to −0.78‰.

Table 3 shows the δ³⁴S results for the ten analyzed samples of native sulfur and gypsum. The range of δ³⁴S is of −4.0‰ to +1.2‰. The isotopic values obtained for native sulfur (+1.2‰ to −3.7‰) are very similar but slightly more positive than the sulfur isotopic values obtained for sulfates ( −0.8‰ to −4.0‰).

Table 3 also shows the results for δ¹³C and δ³⁴S for the hydrocarbon asphaltene-like samples (chapopote). Isotopic values do not vary significantly; values between −26.85 and −26.44‰ are obtained for carbon, whereas values from +7.65 to +7.50‰ are acquired for sulfur. The extremely negative carbon values of the hydrocarbon are due to its organic origin, and the range of sulfur isotopic values are compatible with a thermochemical (δ³⁴S: 5–15‰) or bacterial (δ³⁴S: 0–10‰) sulfate reduction genesis [53].

4.2. Water System

A Durov diagram [54] was constructed to visualize the chemistry of major ions and to compare it with pH and total dissolved solids (TDS). The pH of the hydrothermal emanation water in the aragonite terraces is slightly acidic (Figure 11a), approaching neutral, as is the water from the well (meteoric water). In comparison, the water from the river and the lagoon is slightly alkaline (

Table 4. The chemical and physicochemical properties of the water samples collected at different points in the Tamiahua Lagoon area. The isotopic composition of stable elements in the water results is also presented.

Sample	Description	Surficial Temperature (°C)	pH	Electrical Conductivity (mS/cm)	δ18OVSMOW2	δ2HVSMOW2
a	Terraces water	70	6.1	118	+7.94	−5.39
b	Well water	27	6.4	3	−3.82	−18.87
c	Lagoon water	34	8.1	45	+3.73	+23.75
d	River water	32	7.9	0.06	+3.58	+22.49
e	Fumarole water	Not measured	8.4	3	+3.53	−9.54
f	Duplicate of a	70	6.1	Not measured	+7.77	−5.50
g	Duplicate of b	27	6.4	Not measured	−3.76	−18.85

). The temperature recorded in the three-point intake (well, river, and lagoon) is similar, being between 27 °C and 34 °C (the average annual maximum temperature recorded at the climatological station of Majagual in Tamiahua is 29 °C; https://smn.conagua.gob.mx/es/informacion-climatologica-por-estado?estado=ve (accessed 15 June 2022), while the minimum surface temperature of the aragonite terraces was registered as 70 °C, similar to the outer part of the fumaroles in which steam emanates. Table 4 contains the physicochemical data taken and recorded in the field of the water samples, except for the condensate from the fumarole, which was not determined due to the limited amount of water collected.

We determined whether the aragonite is in isotopic equilibrium with the water from which it precipitates. Among the numerous equations of carbonate–water isotopic equilibrium, two equations were chosen. The first (Equation (1) is from a classical source [55], and the second (Equation (2)) is one of the most recent recalibrations [56]. First, the calculation uses a mean value of δ¹⁸O = +7.86 from the two available water measurements (Table 4, analyses a and f). Second, for each carbonate (Table 3), its oxygen value was used (δ¹⁸O = −1.30 to −1.70), except for sample Cima 1, which had much calcite and gypsum besides aragonite. Finally, both oxygen values for each sample (aragonite, water) were used to calculate α = (1000 + δ¹⁸O_aragonite)/(1000 + δ¹⁸O_water), which is then input into the equations, and the temperature was calculated by solving the equations iteratively. The error in T obtained from Equation (2) is from the ± stated in the equation [56].

1000 ln α_{carbonate-water} = 0.9521 (10⁶/T²) + 11.59 (10³/T) − 21.56

(1)

1000 ln α_{aragonite-water} = 17.88 [± 0.13] (10³/T) − 31.14 [± 0.46]

(2)

The temperature calculated using Equation (1) [55] is 65 °C and using Equation (2) [56] is 69 ± 4 °C, both similar to the water temperature measured in aragonite terraces (70 °C). Based on this result, we can consider that the aragonite precipitation occurred not too far from the chemical equilibrium [55,56].

The ionic balance was carried out with the cation and anion concentration from the different Tamiahua Lagoon water samples (

Table 5. The concentration of ions and trace elements from water samples in mg/L. DL—detection limit; QL—quantification limit; NA—not analyzed.

Sample	Al	As		B		Ba		Fe		Cs		Mn	SiO2		Sr		V
Terraces	0.02	0.13		32.27		0.18		<DL		8		<DL	38		73		<DL
River	0.02	≤DL		2.96		0.1		<DL		2.06		0.16	6.6		4.55		<DL
Fumarole	NA	NA		NA		NA		NA		NA		NA	NA		NA		NA
Lagoon	0.42	≤DL		3.11		0.07		0.36		2.88		0.03	6.8		4.61		<DL
Well	0.04	≤DL		0.07		0.08		<DL		1.13		0.5	67.4		0.87		0.01
DL (mg/L)	NA	0.08		0.03		0.003		0.002		NA		0.001	0.15		NA		0.002
Sample	F-		Cl−		SO42−		HCO3−		Na+		Mg2+			Ca2+		K+	Li+
Terraces	58		31,014		1288		730		15,953		1121			410		0.1	20
River	59		17,830		2864		45		9236		1023			452		339	0.03
Fumarole	0.2		907		151		15		634		0.1			40		44	5
Lagoon	104		16,836		2548		45		9762		1080			446		347	0.03
Well	13		864		655		231		366		49			147		15	0.03
DL (mg/L)	0.2		0.3		0.3		NA		0.1		0.1			0.08		0.1	0.03

). The allowable range in ion balance for geothermal systems is ± 10%, therefore the only water sample outside the limit corresponds to the well sample.

The analytical results were plotted on the Durov diagram (Figure 11A), indicating that all water samples can be classified as chloride–sodium water type and are related to subsurface seawater invasion, given their proximity to the shoreline sedimentary basin. The bicarbonates and lithium content are low for almost all the samples, except for the terraces sample, which presents the highest values with 730 mg/L of bicarbonate and 20 mg/L of lithium. The terraces, river, and lagoon samples have similar concentrations in some anions, such as fluorides, chlorides, sulfates, and cations including sodium, magnesium, and calcium. However, the terraces sample contains almost double the chloride (31,013 mg/L) and sodium (15,953 mg/L) content and has half the sulfate content of the other two marginal samples. The lagoon sample with 104 mg/L of fluoride, has almost twice the amount of this element as the other samples. The potassium content is similar for the lagoon and the river samples, which show the highest amount, while the other samples show low concentrations. A ternary Na-K-Mg diagram (Figure 11B) [57,58] was created using the water sample values: these are major elements only for water from terraces, which presents the highest surface temperature, and the fumarole, since it represents the condensate of the vapor and gas that comes from the hydrothermal vent. The condensate sample from the fumarole is almost at the equilibrium line, which corresponds to a temperature of around 200 °C in the reservoir.

In Table 5, it can be observed that the water labeled as terraces sample presents the highest contents of B, Ba, As, Cs, Li, and Sr with values of 32.27, 0.18, 0.13, 8.0, 9.21, and 20 mg/L, respectively. On the other hand, the well sample has the highest value of SiO₂ with 67.4 mg/L. Although it is of near-alkaline conditions, the abundance of silica in well water can be related to the dissolution of silicate minerals at higher temperatures and depths, conditions that do not exist around the lagoon and surficial terraces where we only found carbonates and sulfates.

The analytical results obtained for trace elements indicate low values, even below the instrument’s detection limit, as is the case for As, Fe, Sb, Pb, and V.

The diagram proposed by Arnórsson and Andrésdóttir [59] was replicated for this study in order to represent the relationship between Cl and B in the waters and to infer the origin of these elements either derived from being located in the coastal zone or arising from hydrothermal activity at depth (Figure 12A). Generally, the boron content in groundwater is less than 1 mg/L; concentrations above this may be due to the presence of hydrothermalism [60], evaporative processes, seawater intrusion, evaporite dissolution [60], mineral weathering [60], anthropogenic contamination [61,62], and sorption and desorption processes on mineral surfaces [63,64].

The mineral saturation index was calculated with the software The Geochemist’s Workbench (version 11.0.8; module SpecE8) and the results are plotted in Figure 12B.

Isotopic δ²H values for each type of water are quite different, with negative δ¹⁸O values (−4.0 to −4.1‰) for well water and positive for terrace water (+7.4 to +7.6‰), river water (+3.2%), water from the lagoon (+3.6‰), and fumarole (+3.3‰) (Table 4). The values for the isotopic pairs δ²H and δ¹⁸O were plotted in Figure 13. The duplicates (aragonite water and well water) yielded similar values to the original samples (Samples 1 and 5).

As can be seen in Figure 13, the well water is found above the meteoric water line, while the other water samples tend to be away from this line; this can be attributed to the fact that water from the terraces and the fumarole may be the product of a surficial mixture between meteoric waters and ascending oil-field derived brines. In addition, the water from Tamiahua Lagoon presents anomalous high values of δ²H that can be linked to the high coastal environment evaporation rates.

5. Discussion

5.1. Origin of the Hydrothermal System

All the water samples collected at the different points of Tamiahua Lagoon were classified as chloride–sodium water type, related to seawater or sedimentary brines. When analyzing the ion content in the water, it was found that Tamiahua Lagoon exerts a strong influence on the hydrothermal system, mainly by providing Cl⁻, SO₄^2-, Na⁺, and Mg²⁺ as the most abundant ions. Therefore, if the influence of marine sodium chloride is eliminated, the water from the aragonite terraces could be classified as magnesium sulfate since the sulfate is above 1200 mg/L and the magnesium itself is above 1100 mg/L. According to Herman et al. [65], the B/Cl ratio in the terraces sample (0.00104) is indicative of a deep fossil brine input. The other samples (well, lagoon, and river samples) have a ratio outside the range established for a fossil brine (0.0099–0.00056) but have a strong influence from seawater (Figure 12A). Well water also contains similar ions such as Cl⁻, SO₄²⁻, and Na+, possibly due to the proximity to the coastline.

As the discharge temperature for the terraces sample was around 70 °C, the corresponding temperature at the depth of this sample point and the fumarole was estimated using the Giggenbach diagram. Before the estimation, the existence of the equilibrium state had to be considered to apply the cationic geothermometry. According to Figure 12B, the terraces sample is coherent with the surface temperature, but the fumarole chemistry reveals a temperature of approximately 200 °C (Figure 12B) at depth, which corresponds to the starting stage of catagenesis, the hydrocarbon generation window. An objective of the present work was to determine the origin of the fluids; therefore, the isotopic composition of hydrogen and oxygen obtained from the water of the aragonite platforms was compared with the nearby water sources: well water, water from Tamiahua Lagoon, and water from a small river that flows into the lagoon and is in contact with the aragonitic formation.

The δ²H and δ¹⁸O values of the well water sample indicate that it is dominantly meteoric water, since they are projected above the global meteoric water line (Figure 13). Isotopic values are similar, for example, to those measured for the waters of the distant hot springs of the Los Humeros geothermal field [66], but the latter displays more negative values due to latitude and sea distance differences between both areas. On the other hand, isotopic values of fluids sampled at aragonite terraces (δ²H: −7 to −9‰; δ¹⁸O: +7.4 to +7.6‰) are close to those published for the waters of the oil brines of the Gulf of Mexico [67] but with slightly more negative δ²H. The fumarole sample’s isotopic values are similar to those in the brines, but slightly different from those of the terraces sample; these differences could result from a condensate generating an isotopic fractionation or that the vapors come from a deeper source.

The terraces sample that gives rise to the aragonite terraces has two components; the first is a meteoric component given by well water (sample on meteoric global line) with δ²H at −21‰ and δ¹⁸O at −4.1‰. In contrast, the second component is related to the oil brines of the Gulf of Mexico (average δ²H = −0.18‰ and δ¹⁸O = +4.75‰).

Tamiahua Lagoon presents an anomaly in the δ²H of the lagoon sample being highly enriched in deuterium (δ²H = 21.5‰). These high values have been previously reported in meteoric waters in the Tamiahua Lagoon area, ranging from +17‰ to +35‰; these are observed as an annual trend in the winter seasons, mainly in January and February [68]. Evaporation tends to be significant for the isotopic record only in closed basins of relatively arid regions, where lake waters may be deuterium-enriched by 10–15‰ [69]. Therefore, the high values reported for the waters of Tamiahua Lagoon indicate the presence of an evaporative/hypersaline system. Continuous evaporation of water from Tamiahua Lagoon is an ongoing process and could contribute to an enrichment of the heavy isotopes of the water (²H and ¹⁸O). The increment in δ¹⁸O in basinal brines that comes with a temperature increase is primarily due to its effect on the isotopic and chemical exchange rate between the water and the enclosing rock. Sedimentary rocks have very high δ¹⁸O and the water becomes enriched due to exchange with the surrounding host rocks during rising thermal waters. Furthermore, the loss of H by gases such as H₂S, H₂, or CH₄—associated with hydrocarbon emanations—also provokes deuterium enrichment in water relative to the rocks and precipitates [70]. It is important not to discard the possible influence of deep brines associated with the hydrocarbon layers that were mixed with the surface water in the lagoon area due to the spill that occurred approximately 110 years ago.

5.2. Origin and Classification of Mineral Travertine System

5.2.1. Aragonite Precipitation and Its Typologies

Calcite occurs only in Tamiahua travertines as a minor phase of low crystallinity. This point could be verified in the XRD patterns by the punctual presence of very small and wide peaks in the 2theta positions characteristic of this mineral. The calcite saturation indices for the terrace area show the balance of the fluid with this mineral (Figure 12B). The theories that explain the precipitation and formation of aragonite layers instead of calcite precipitation are diverse [71,72,73]. Three main factors that favor aragonite precipitation have been identified in the water from the Tamiahua aragonite terraces. The first factor is the temperature being greater than 70 °C; when the temperature is higher than 40 °C, aragonite will precipitate regardless of the fluid’s composition [74]. The second factor is the Mg/Ca molar ratio, which is greater than 1:1 [75]; magnesium molar content is greater than calcium molar content in the water. The third and last factor is the content of Sr present in the water and the aragonite structure; the strontium content in aragonite is relatively high since it is found as a major element (>1%). This composition has been observed in aragonite formed in hot springs from non-marine sources such as the travertines of Rapolano Terme, Italy [45], where aragonite incorporated approximately 1% strontium into its structure (Figure 13). The Ca²⁺ ion has a coordination number of nine in the aragonite structure, while calcite has a coordination number of six [76]. The Ca²⁺ distribution in calcite has a cubic packing, unlike the Ca²⁺ in aragonite that has an almost hexagonal packing. Sr²⁺ is slightly larger than Ca²⁺ when forming a bond with CO₃^2-; it tends to form a coordination of nine—greater than that formed with Ca²⁺—that transforms the packing from cubic to hexagonal [77]. When the aragonite nucleates, the structure’s growth will be controlled if the concentration of Sr and the temperature do not change. Aragonite does not precipitate exclusively in the zone of higher temperature, and the saturation indices of calcite and aragonite present parallel trends and are very similar to each other in the different parts of the deposit (Figure 12B). Consequently, the high concentration of some ions (mainly Mg, SO₄^2-, and Sr) can be the key factor that favors, by inhibiting the growth of calcite, aragonite precipitation in the Tamiahua zone.

The most evident micro-factory in the different typologies described for Tamiahua is the fibrous laminations of aragonite, formed by aggregates of acicular crystals. These microfabrics, associated with CO₂ degassing and high carbonate precipitation rate, have been preferentially defined in thermogene travertines and are characterized by presenting fine, sequential, or rhythmic lamination, with low permeability and low, mainly fenestral, porosity [78]. Genetically they are related to the inorganic processes that predominate in the formation of the travertines of the Tamiahua coast (e.g., [43,79,80]).

Another micro-factory found in travertines are the so-called "rafts" structures formed at the water–air interface that later, when sinking, accumulate at the bottom of terraces or pools where the flow of water is very slow [46].

A less frequent micro-factory is the formation of "bushes" or dendrites; they appear as fans of fibrous aragonite with undulating extinction (Figure 6d. Whether the origin of these microfabrics is inorganic or biogenic continues to be discussed by various authors; however, the prevailing conditions (for example, temperature ~70 °C) in the Tamiahua area and the predominance of aragonite allow us to link their origin to a dominantly inorganic process [79]. The porous and less consolidated textures seem to be associated with lower flow conditions [20].

5.2.2. Geochemistry, Isotopic Composition, and Travertine Classification

A factor to consider is that the aragonitic laminations present different colorations, especially white and blue in the light and dark tones. When comparing the rare earth content (lanthanides) of the samples with contrasting coloration (white and dark blue), it was possible to verify a notable difference in the samples of distinct colors. The bluish coloration seems to be related to a higher lanthanide content, mainly in light rare earth (Table 2). The colorless (white) aragonite has the lowest ∑REE (sum of all lanthanides) content (<0.5 ppm), while the ∑REE of dark blue aragonite has higher values (9.9 ppm). The sample Tami BC has a higher abundance in all rare earth than other samples and shows a negative anomaly in Eu (Figure 10); this anomaly is very common in the sediments of the upper continental crust and surface waters [81,82]. The darker bands with the most abundant rare earth content can be related to hydrothermal pulses more enriched in these elements, and it is possible to interpret that the pattern of lanthanides in aragonite is analogous to that of the initial fluids [83].

The stable isotopes of carbon and oxygen in aragonite do not present significant variations, suggesting that the fluid’s chemical composition remains constant throughout the crystallization of this mineral. The carbonate dissolved in the fluid that gives rise to the mineralization seems to originate from the dissolution of Mesozoic marine carbonates of the El Abra formation, as they form the carbonate platforms characteristic of the oil fields of the Tampico-Misantla Province. Nonetheless, we cannot rule out a small carbon component from hydrocarbon generating rocks. Although the high contents of strontium in the fluid also seem to indicate that the dissolution of the Mesozoic carbonates may be the fundamental process in the formation of the carbonates of the travertines, a contribution from other rocks rich in calcium (mainly magmatic) cannot be ruled off. Therefore, in a new sampling campaign, we plan to carry out isotopic analyses of water on a more regional scale to better understand the hydrological cycle—for example, possible processes of mixing and isotopic fractionation due to CO₂ degassing—of the area that, due to the presence of the lagoon, the sea, the deep oil system, and anthropogenic activity, results in a quite complex system.

Oxygen isotopic values are very close to zero and the obtained isotopic temperatures, using Equations (1) and (2), seem to indicate equilibrium precipitation conditions despite evaporation and degassing [84]. Additionally, the carbon isotopes of aragonite suggest that the CO₂ degassing is associated with the decarbonization of limestones. The waters are very hot; therefore, the effect of evaporation is probably the dominant δ¹⁸O controlling factor [85].

The classification of the travertines as thermogene was based on the stable isotopes of carbon from aragonite, as well as the microfabric observed during the petrographic study. Travertines of thermogenic origin are characterized by having more positive values of δ¹³C (enriched in ¹³C, −3 to +8‰) than meteogene [6,79]; thus, they are more consistent with the isotopic values obtained for aragonite (δ¹³C from +1.75‰ to +2.37‰) in Tamiahua. According to different authors [5], the ¹⁸O signature of aragonite is strongly linked to meteoric waters and therefore does not allow to define the origin of the travertines correctly.

According to Teboult et al., (2016) [13], the relation between Ba and Sr allows for discrimination between epigean and hypogean travertines and can help to discriminate between different source rocks. For example, limestones, evaporites, and dolomites exhibit low barium (>100 ppm) and high strontium (>400 ppm) contents [13]. In Tamiahua, the low barium (70 to 82 ppm) and high strontium (>1%) contents of aragonite, and mainly the presence of powerful carbonate layers with levels of evaporites in the subsoil, suggest the fluids’ hypogean nature and point to a source rock constituted mainly by limestones and evaporite [13] (Figure 14).

The El Abra Formation, composed mainly of limestone, contains abundant layers of anhydrite [86], which was thought to be in origin related to the sulfates found interspersed in the layers of carbonates. The thermal springs in the Tamiahua area contain H₂S, giving them the characteristic smell of sulfur. H₂S can likewise be related to hydrocarbons from oil wells [87]. In the presence of hydrocarbons and a reducing environment, bacterial activity consumes H⁺ and can release H₂S into the system [88,89,90]; this reaction (Equation (3)) implies that sulfates can be reduced in the presence of hydrocarbons, producing H₂S.

SO₄²⁻ + 2(CH₂O)n + 2H⁺ → 2nCO₂ + 2H₂O + H₂S

(3)

The isotopic data concluded that the elemental sulfur that covers the carbonate formation is attributed to the mainly inorganic oxidation of the deep-source H₂S [91], probably following Equation (4) and consequently precipitating elemental sulfur and carbonate in one step.

Ca (HCO₃)₂ + 2H₂S → CH₂O + CaCO₃ ↓ + 2S ↓ + 2H₂O

(4)

However, the isotopic data do not allow us to rule out the possibility that there is a deep sulfur component of volcanic origin.

5.3. Geotectonic Context of Formation

The rise of hydrocarbons mixed with the carbonate formations can be attributed to a regional fault and fracture system generated by deformation events during the Cretaceous–Paleogene. The first event occurred due to the magmatism generated during the Laramide deformation, which determined tectonic structures with NW-SE orientation. Later, this was followed by a lateral regime that predominated during the Cenozoic [92]. These dextral transpressive slip fault and fracture systems also had an NW-SE orientation [93], affecting the Chicontepec Formation and originating new migration routes for hydrocarbons. These events share the same NW-SE direction, which is in accordance with the fractures identified at different points of the carbonate formations (N05° W to N17° W).

These old fractures and faults are how fluids rise to the surface, allowing waters from different sources to mix during the ascent. However, it is still unclear why the temperature is ~70 °C in the aragonite terraces, while it is less than 35°C in the other nearby water sources. The fractures operate as fluid conduits, permitting the flow of groundwaters to the surface [6]. Different authors such as Berriguete et al., (2017) [94], have determined that the flow of water and its physicochemical characteristics can be greatly affected by diagenetic modification (dissolution, cementation, recrystallization) of the underlying rocks from the substrate. Unfortunately, in the case of the studied travertines and their location at sea level on the coast of Tamiahua lagoon, it is impossible to observe the travertine deposit roots without perforating the carbonated body. Therefore, it is unattainable to verify whether these diagenetic processes modified the water flow in depth [94].

Where the extensional zone is located, and the conditions in which the fluids rise through faults and fractures with NW-SE orientation, suggest that the heat source may come from a deep zone, which can be linked to a regime dominated by convection in a zone extension [30].

6. Conclusions

Based on the analyses carried out on the collected samples, the representative mineralogy of the hydrothermal formations is constituted by aragonite, sulfur, gypsum, anhydrite, celestine, barite, calcite, quartz, opal, hematite, jarosite, and halite.

Aragonite is the main mineral identified in the hydrothermal formation; it has high crystallinity and a mainly inorganic origin. The precipitation of this carbonate is due to several factors, including a high content of Mg in solution, a high content of Sr (>1%) in the aragonite structure, and a temperature of ~70 °C. On the other hand, the scarce calcite crystals found are of low crystallinity and, in some cases, a biogenic origin is attributed to them.

According to the results obtained for the mineral microfabric, the stable isotopes of carbon and oxygen and the temperature measured at the surface, the aragonitic formations of the Tamiahua coast can be classified as travertines mainly of thermogenic (hypogean) origin.

The dominant microfabric is of fibrous laminations formed by acicular crystals of aragonite and fine rhythmic laminations attributed to an inorganic origin. The dendritic textures found in aragonite indicate rapid precipitation.

Native sulfur seems to be related to the emission of H₂S present in hydrocarbons from oil wells, which generates the precipitation of native sulfur and calcium carbonate under suitable physicochemical conditions.

Based on the isotopic results, the water analyzed in the aragonite terraces must have a mixed origin, including two components: a first one of meteoric origin, represented by well water; and a second component isotopically related to the oil-field waters of the Gulf of Mexico. Deep brines associated with the hydrocarbon layers were mixed with the surface water in the lagoon area due to the spill approximately 110 years ago. These basin waters are, in turn, mixtures of meteoric water and evolved seawaters.

The mixing of meteoric water and brackish water is carried out at depth, where it passes through sedimentary sequences in which there are hydrocarbons typical of warehouse rocks of petroleum systems such as the El Abra Formation; these formations contain interspersed evaporites typical of marine environments. Finally, the fluids are heated at depth and rise by convection through faults and fractures generated by extension events; during this process, there occurs the dissolution of carbonates from the sedimentary units through which they rise, as well as the isotopic exchange of carbon, oxygen, and sulfur between fluids and dissolved components.

Once on the surface, the variations in pH, oxidation, the very limited bacterial activity, and the escape of gases result in the precipitation of different minerals such as elemental sulfur—which oxidizes to sulfates—and aragonite, as well as the release of H₂S and CO₂ gases.