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Geochemical processes in an active shallow submarinehydrothermal system, Bahía Concepción, México:mixing or boiling?Ruth Esther Villanueva-Estrada a , Rosa María Prol-Ledesma a , Augusto Antonio Rodríguez-Díaz b , Carles Canet a , Ignacio S. Torres-Alvarado c & Eduardo González-Partida da Departamento de Recursos Naturales, Instituto de Geofísica, Universidad NacionalAutónoma de México, 04360, México, D.F., Méxicob Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, 67700, Linares,Nuevo León, Méxicoc Centro de Investigación en Energía, Universidad Nacional Autónoma de México, A.P. 34,Temixco, Morelos, 62580, Méxicod Centro de Geociencias, Universidad Nacional Autónoma de México, Juriquilla, Querétaro,76230, MéxicoVersion of record first published: 13 Jun 2011.

To cite this article: Ruth Esther Villanueva-Estrada, Rosa María Prol-Ledesma, Augusto Antonio Rodríguez-Díaz, CarlesCanet, Ignacio S. Torres-Alvarado & Eduardo González-Partida (2012): Geochemical processes in an active shallow submarinehydrothermal system, Bahía Concepción, México: mixing or boiling?, International Geology Review, 54:8, 907-919

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International Geology ReviewVol. 54, No. 8, June 2012, 907–919

Geochemical processes in an active shallow submarine hydrothermal system, Bahía Concepción,México: mixing or boiling?

Ruth Esther Villanueva-Estradaa∗, Rosa María Prol-Ledesmaa, Augusto Antonio Rodríguez-Díazb, Carles Caneta,Ignacio S. Torres-Alvaradoc and Eduardo González-Partidad

aDepartamento de Recursos Naturales, Instituto de Geofísica, Universidad Nacional Autónoma de México, 04360 México D.F., México;bFacultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, 67700 Linares, Nuevo León, México; cCentro deInvestigación en Energía, Universidad Nacional Autónoma de México, A.P. 34, Temixco, Morelos 62580, México; dCentro de

Geociencias, Universidad Nacional Autónoma de México, Juriquilla, Querétaro 76230, México

(Accepted 7 May 2011)

Hydrothermal activity at Bahía Concepción, on the western coast of the Gulf of California, is not linked to present volcanicactivity. This site is a potential energy source; however, geothermal modelling of the system is needed in order to determinethe processes that generated this activity. Two processes might lead to the formation of the secondary fluids that were sampledin this submarine hydrothermal system: the thermal endmember might be undergoing either boiling or mixing with a fluidtrapped within the sedimentary strata.

However, boiling was ruled out as this process would not produce a highly saline fluid within the temperature rangedetermined for the Bahía Concepción system (∼87◦C). We modelled the mixing of a highly saline fluid with a geothermalfluid of meteoric origin. Our results suggest that the composition of the thermal water representing the hot endmember atBahía Concepción consists of 20–30% highly saline fluid and 70–80% aqueous fluid of meteoric origin. The computed fluidhas a chemical composition similar to that determined for the submarine vents. Its derived contents of calcium, chloride,and silica are similar to those we measured for the vent discharge. Our hypothesis of saline water addition is supported bythe mineralizing fluid characteristics determined from fluid inclusion microthermometric study of a fossil intertidal systemthat discharges in the vicinity of the vent areas. This intertidal spring fluid at ∼40◦C, according to the geochemical model,would be supersaturated in opaline silica and calcite, consistent with the mineralogy of the stromatolitic sinter encircling thedischarges.

Keywords: hydrothermal vent; fluid inclusions; calcite; chemical modelling; Baja California

1. Introduction

Hydrothermal fluids originate from different watersources including meteoric, magmatic, and seawater (SW)(Giggenbach 1992). When this water penetrates and cir-culates through the rocks, it may be heated by differentthermal environments and can reach high temperatures. Abroad range of chemical reactions occur when the hot fluidencounters the host rock, such as dissolution or deposi-tion (Arnórsson et al. 2007). Geothermal fluids (GFs) canbe classified as follows (Arnórsson et al. 2007): (a) pri-mary fluids, which are found in the roots of geothermalsystems and can form as a mixture of two or more sourcefluids, such as meteoric water, SW, brines, and magmaticvolatiles; the main types of primary fluids are NaCl watersand acid-sulphate waters (Arnórsson et al. 2007); (b) sec-ondary fluids, whose formation can be the result of severalprocesses at shallower levels, such as effervescence, boil-ing, and further mixing of the primary fluid with shallowgroundwater.

∗Corresponding author. Email: ruth@geofisica.unam.mx

In submarine vents, the thermal endmember (EM) isdefined as the hydrothermal fluid that flows up from thereservoir and is generally deprived of Mg; as the thermalEM is generally mixed with SW, it can be calculated as thezero-Mg component assuming a linear mixing model (VonDamm et al. 1985). This fluid may be considered as the pri-mary fluid according to the definition of Arnórsson et al.(2007). Determination of the characteristics of the thermalfluid EMs and their chemical characteristics is crucial toa full understanding of a hydrothermal system. This ther-mal EM may be a mixture of fluids of different chemicalcomposition that cannot be directly sampled; however, thestable isotope composition of δD and δ18O in water sam-ples from the discharged fluid can be used to infer the originof the fluids (e.g. Vidal et al. 1981; Brombach et al. 2003;Prol-Ledesma et al. 2004; Schwinn et al. 2006).

Processes that take place in the zones of hydrothermalreservoirs that are unreachable by conventional samplinghave been identified through fluid inclusion studies, which

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2011.588496http://www.tandfonline.com

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are considered fundamental for understanding the genesisof fossil and active hydrothermal systems (Roedder 1984;Wilkinson 2001). The microthermometric analysis of fluidinclusions provides information regarding the temperatureand, in some cases, pressure of fluid trapping and the equiv-alent salinity of mineral-depositing fluids (Goldstein andReynolds 1994), and can also provide evidence of boiling(Canet et al. 2011). Fluid inclusions may represent flu-ids trapped during (primary inclusions) or after (secondaryfluid inclusions) crystallization of the host mineral. Ventfluid chemistry combined with fluid inclusion data can beused to estimate the geochemical characteristics of the ther-mal EM (Leach et al. 1991; Stüben et al. 1996; Schwinnet al. 2006), as can numerical methods (Noda and Shimada1993; Todesco 1997; Zhao et al. 2006) and Q-mode factoranalysis (Edmonds and Edmond 1995).

In the 1970s, a variety of geochemical modelling pro-grams were developed for hydro-geochemistry, economicgeology, and geothermal research (Mangold and Tsang1991). The first applications of geochemical modellingfocused on changes in physico-chemical conditions ofhydrothermal fluids during flow and interaction with hostrocks. Hydrothermal activity in mid-ocean ridges has beenmodelled by Janecky and Seyfried (1984), Bowers et al.(1985), and Janecky and Shanks (1988); however, geo-chemical processes in shallow submarine hydrothermalvents have not hitherto been modelled.

The first chemical model proposed for the shallowsubmarine vents at Bahía Concepción (Villanueva et al.2005) was based on the conceptual model reported byProl-Ledesma et al. (2004). These authors proposed thethermal EM composition under the assumption that it didnot contain any magnesium (Prol-Ledesma et al. 2004).The results of that model suggest that fluids discharged bythe submarine vent are composed of about 40% thermalEM and 60% SW, with large variations in the proportions ofSW depending on the sampling site and method. Villanuevaet al. (2005) proposed a geochemical model for the systemthat assumed a mixture of three different fluids to reproducethe discharged fluid. The components considered in thatmodel were primary thermal fluid (whose composition hasbeen described), meteoric water, and SW, but the resultingconcentrations did not agree with the observed Ca, Si, andCl concentrations. Concentrations calculated by this mix-ing model were too low. Therefore, it was suggested thatmixing occurred with a more saline fluid, not with mete-oric water; however, there was no evidence of the presenceof such fluid in the hydrothermal system.

In this article, we use a mixing model to explain geo-chemical processes in the shallow submarine hydrothermalvents in Bahía Concepción. The input data were the chem-ical concentrations of GFs from local active and fossilhydrothermal manifestations. Additionally, we documentthe presence of a saline fluid in previous stages of theintertidal hydrothermal manifestations. The main objective

of the mixing model is to identify the geochemical pro-cesses that produce the primary fluid in the hydrothermalsubmarine system of Bahía Concepción.

2. Geological setting of Bahía Concepción

Bahía Concepción is a bay 40 km long near the village ofMulegé on the central-eastern coast of the Baja CaliforniaPeninsula (Figure 1). The local climate is arid with annualtemperatures between 28◦C and 32◦C (Bustamante-García1999).

The rocks outcropping in Bahía Concepción corre-spond to the volcanic and volcaniclastic Comondú Group(Oligocene to Miocene; Figure 1). Recent marine sedi-mentary rocks, lava flows, volcanic domes, and pyroclasticdeposits (Pliocene to Quaternary) also occur. Basementoutcrops are scarce and are composed of Cretaceous gra-nodiorites (Camprubí et al. 2008).

Two important geothermal fields lie to the north ofBahía Concepción: Las Tres Vírgenes and Cerro Prieto(Figure 1). To the south, there are at least four geother-mal areas (Figure. 1): Valle de San Juan Londó (Prol-Ledesma and Ortega 1988), Saquicismunde (Tovar 1986;Casarrubias and Leal 1993), San Nicolás, and PiedrasRodadas (Lira 1985). Most thermal manifestations in theBaja California Peninsula are associated with NW-trendingstrike–slip faults (Barragán et al. 2001), which are relatedto the formation of pull-apart basins associated with theextensional Gulf Province (Stock et al. 1991).

2.1. Hydrothermal activity in Bahía Concepción

The occurrence of shallow submarine, intertidal, and sub-aerial hydrothermal manifestations in the northern andsouthern part of Bahía Concepción is related to recentextensional tectonics. Hydrothermal activity is constrainedby normal faults (NW–SE) that serve as channels forfluid circulation across the sedimentary and volcanicrocks (Camprubí et al. 2008). There are two types ofhydrothermal manifestations on or near the coast in BahíaConcepción:

(a) Submarine hydrothermal vents: These are dis-tributed discontinuously along a linear trend andconsist of diffuse gas and water flow that takesplace at depths from 5 to 15 m in Mapachitos Inlet(Figure 1). Fluid temperature varies from about50◦C at the sea bottom to 87◦C at a depth of 10 cmwithin the sediments (Prol-Ledesma et al. 2004).Reddish brown to yellowish oxyhydroxides withHg–Mn–Ba coating boulders and cobbles occuraround the main vents (Canet et al. 2005a).

(b) Intertidal hot springs: These occur in MapachitosInlet, La Posada, and Santispac on the western

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Figure 1. Geologic map of Bahía Concepción, Baja California Sur, México, showing the distribution of hydrothermal vents and hotsprings (modified from Camprubí et al. 2008).

coast of Bahía Concepción (Figure 1). Vent tem-peratures range from 40◦C up to 62◦C (Prol-Ledesma et al. 2004). Botryoidal crusts ofmanganese oxides, together with stromatoliticaggregates of opal-A, barite, and calcite, occuraround the main discharges (Canet et al. 2005a).The calculated saturation index shows that the hotspring water is supersaturated with respect to bariteand silica (quartz, chalcedony and amorphous sil-ica), and cooling processes are responsible foropal-A saturation (Canet et al. 2005b).

2.2. Fluid chemistry

Chemical composition of discharged thermal water wasreported by Prol-Ledesma et al. (2004) and indicates a

sodium chloride water type (Table 1). Thermal water isenriched in Ca, Mn, Si, Ba, B, As, Hg, I, Fe, Li, HCO3, andSr in comparison with SW (Prol-Ledesma et al. 2004). Gasdischarged by the submarine vents is composed primar-ily of CO2 (44%), N2 (54%), and methane (2.2%) (Forrestet al. 2005).

Prol-Ledesma et al. (2004) calculated the chemicalcomposition of the thermal EM assuming Mg = 0 (Table1); they then used this thermal EM composition to calculatea reservoir temperature of about 200◦C with geothermome-ters. According to the isotopic and chemical composition,they proposed a conceptual model of the thermal fluid flowpath, assuming that the EM fluid is mostly of meteoric ori-gin and was heated during deep circulation and changedits chemical and isotopic characteristics due to water–rockinteraction. Therefore, the dominant process in the shallow

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Table 1. Measured chemical composition of hydrothermal fluids discharged at the vents in Bahía Concepción, seawater far away fromthe hydrothermal emission, and the calculated thermal endmember (from Prol-Ledesma et al. 2004).

pHTemperature

in situ Na K Ca Mg Cl HCO3 SO4 Si Li B

Measurements on BahíaConcepción submarine

manifestations

BC1 6.0 87 395 12.7 23 36 458 4.9 17.0 3.1 0.3 0.8BC4 6.0 87 415 12.5 19 42 501 4.3 21.2 2.1 0.3 0.7BC6 6.0 87 409 12.5 21 40 494 4.5 20.6 2.4 0.3 0.6

Endmember calculated EM – 244 13.0 45 0.0a 380 10.3 4.1 7.8 0.8 1.5Published data used for present

mixing modelSeawater 7.8 25 460 12.4 11 58 528 1.7 26.6 0.02 0.1 0.4GF 7.7 88 178 19.8 7 0.1 220 0.7 0.2 9.0 2.7 14.3HSF 6.0 50 816 41.4 171 11 1166 1.9 2.6 2.1 2.4 1.0

Notes: Chemical composition of the components used for the geochemical modelling: GF, geothermal fluid (Portugal et al. 2000); HSF, highly saline fluid(Barragán et al. 2001). Units are mmol/kg. The temperature in situ of the endmember (EM) cannot be measured, because it represents a hypothetical fluid.aThe hypothetical concentration of Mg is 0 because this element was used for calculation of the chemical composition of the endmember using a linearregression model (Mg vs. ion) (Prol-Ledesma et al. 2004).

evolution of the GF (as secondary fluid) is SW mixing(Prol-Ledesma et al. 2004).

3. Fluid inclusion microthermometry

Analysed fluid inclusions are related to barite that pre-cipitated during a mineralizing stage in the intertidal hotsprings deposits (Rodríguez-Díaz et al. 2008). It is in thisstage (intertidal hot springs) where the main mineraliza-tion processes occurred. Barite forms tabular crystals in thecore of the NW–SE veins that attain 4 cm in length.

Microthermometric measurements were performed ona Linkam THMSG 600 heating–freezing stage at theCentro de Geociencias (Geosciences Centre), UniversidadNacional Autónoma de México. Reproducibility of ice-melting temperature measurements was ± 0.2◦C below0◦C, and ± 2◦C for homogenization temperatures. Onlyprimary fluid inclusions were studied; fluid inclusionswith microscopic evidence of leakage and necking-downwere not measured. Isolated, clustered, scattered, and ran-domly distributed fluid inclusions are considered primary(Figure 2). A detailed petrographic examination wasdone to confirm microthermometric data validity, sincebarite is a mineral susceptible to stretching or leak-age (Ulrich and Bodnar 1988; Van den Kerhof andHein 2001).

Salinity, expressed as wt.% NaCl equivalent, and fluiddensity of H2O–NaCl of inclusions were calculated withBodnar’s equations (1993). Molality (m) and density prop-erties of fluid inclusions were modelled for the H2O–NaClsystem data according to Bodnar (1983), Roedder (1984),and Zhang and Frantz (1987).

4. Modelling of geochemical mixing

4.1. Conceptual model

Since the earlier simple model (Villanueva et al. 2005)resulted in concentrations of Ca, Cl, and Si that did

Figure 2. Photomicrographs showing representative fluid inclu-sion associations hosted in barite from chalcedony-barite-calciteveins: (A) sample from the external part of vein and (B) innersample. L=liquid, V=vapour.

not agree with concentrations measured in the sam-ples, we assumed the presence of a third component, ahighly saline fluid (HSF) that represents connate water(Figure 3). Hence, we carried out geochemical modellingbased on the hypothesis that the GF is meteoric water

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Figure 3. Conceptual model of the hydrothermal system of Bahía Concepción. The flow path considered in the geochemical modellingis represented including the different components: highly saline fluid (HSF), geothermal fluid (GF) of meteoric origin, and local seawater.Mixing with seawater occurs before the fluid is discharged at 87◦C on the seafloor. Horizontal and vertical scales are approximate.

heated during deep circulation (reservoir GF) up to a tem-perature of 200◦C. This is the calculated equilibrium tem-perature for the thermal EM (Prol-Ledesma et al. 2004).Our hypothesis is that the GF interacts with a HSF proba-bly trapped in the Oligocene marine sediments in El SaltoFormation (Figure 3), since connate water is commonlyhigh in those ions (Hutcheon 2000). Further mixing withseawater is unavoidable for the submarine and subtidalsprings before they discharge to the bottom of the sea orat the coast (Figure 3).

An alternative process may also explain the observedconcentrations. According to this alternative model, the GFof meteoric origin is heated while it is descending untilit reaches a temperature of 200◦C. Then, this hot fluid isinvolved in a boiling process before it is mixed with SWand finally discharged to the sea bottom.

4.2. Input data

The characteristics of the fluid components used in themodel are the following:

Hydrothermal fluids with a chemical composition thatrepresent a GF of meteoric origin and a HSF were selectedto generate the EM composition. Such fluids have been

reported in geothermal systems within the Baja CaliforniaPeninsula (Portugal et al. 2000; Barragán et al. 2001) thatare hosted by rocks similar to those that prevail in the studyarea, that is, GF can be represented by Las Tres Vírgenes(central part of Baja California Peninsula) deep GF, andHSF can be represented by fluid discharged at Riito (north-ern part of Baja California Peninsula) which is a hot, deepwell, near the coast.

4.2.1. Meteoric fluid, modelled on data from Las TresVírgenes zone

The hot GF in the model is represented by the well LV-1from Las Tres Vírgenes geothermal field. This componentwas selected because it represents a fluid with a domi-nance of meteoric water in equilibrium with the rocks fromthe Comondú Formation (Portugal et al. 2000), which arealso the predominant rocks in the present study area. Thereservoir temperature for the geothermal well reported byPortugal et al. (2000) is 259◦C.

The reservoir fluid in the model has a neutral pH(7.5), electrical conductivity of 24.4 mS/cm, and salin-ity of 1.6 wt.% NaCl eq. Salinity was estimated with thepolynomial equation derived from Dauphinee et al. (1980),

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which expresses salinity as a function of temperature andelectrical conductivity. The fluid chemical composition ispresented in Table 1.

4.2.2. Highly saline fluid, modelled on data from Riitozone

The HSF that is assumed to mix with the reservoir fluidrepresents sedimentary waters with 1166 mmol/kg Cl. Thedata (Table 1) selected to represent the HSF in the modelwere reported by Barragán et al. (2001) from a hot, deepwell in Riito, which is also on the northern part of theeast coast of the Baja California Peninsula (Figure 1). Thisfluid is enriched in calcium, sodium, and chloride, and isrelated to highly saline sedimentary waters, which are littleaffected by meteoric water. Calculated reservoir tempera-ture is 192◦C (Barragán et al. 2001). The fluid chemistryat Riito deep wells is typical of chloride-type geother-mal water and electrical conductivity is 122.8 mS/cm(Barragán et al. 2001); salinity was estimated for these sitesusing the polynomial equation derived from Dauphineeet al. (1980), assuming an electrical conductivity meanvalue for the SW about 50 mS/cm (Cox et al. 1970). Thisleads to an estimated salinity value of 10.3 wt.% NaCl eq.for the Riito well.

4.3. Modelling of mixing fluids

The model was developed using the software, TheGeochemist’s Workbench® (release 6.0; RockWare Inc.,Golden, CO). The main assumption of this program isthe existence of chemical equilibrium between dissolvedchemical species and the minerals present in the sys-tem. This equilibrium is governed by constants, whichare calculated from a thermodynamic dataset for a rangeof temperatures (Bethke 1996). Chemical compositionof each component used in the mixing model is pre-sented in Table 1. The activities for dissolved specieswere calculated using the B-Dot model (Bethke 1996)for activity coefficients, bearing in mind that the ionicstrength obtained from the mixing process is only about0.63 m. Calculations used the REACT subroutine, whichdetermines equilibrium states and reaction progress (ξ )when two fluids with different compositions are mixedin different proportions. In order to perform the twofluids mixing calculations, the ‘pick up’ command wasused. The program was run assuming a polythermal path.As part of the modelling process, precipitation of silicaminerals was prevented over the course of the reactionpath.

Calculations were performed as follows:

Mixing path 1 (Figure 4): The first assumption of themodel is that the thermal EM that was proposed

by Prol-Ledesma et al. (2004) is the result of mix-ing of a GF of meteoric origin with a HSF. Tomodel mixing of these two fluids (GF + HSF), firstwe input the composition of HSF as a reactant tofinally titrate it into the GF. Because the model ispolythermal, the temperature varies as a functionof reaction progress. In mixing path 1, temperaturevaries from 200◦C, which corresponds to the esti-mated temperature for the geothermal reservoir ofBahía Concepción (Prol-Ledesma et al. 2004), to192◦C for HSF (Barragán et al. 2001). Differentmixing fractions of GF and HSF yield fluids with dif-ferent chemical compositions and temperature. Wesuggest that this process was responsible for the gen-eration of the thermal EM of the coastal geothermalsystem at Bahía Concepción.

Mixing path 2 (Figure 4): Fluids resulting from mixingpath 1 were then mixed with SW in a polythermalprocess. In the mixing processes, initial temperaturecorresponds to the fluid obtained from mixing path 1and final temperature is 25◦C, as measured for localSW. As the fluid discharged by the submarine ventsis a mixture of EM with SW, we expect the chemistryof the resulting fluid from mixing path 2 to be similarto that of the submarine manifestations.

We used only major ions to evaluate the geochem-ical model results because they are constrained by thethermal EM observed composition (Ca and Si), and Clis included as a conservative element in sodium chloridewaters originated by hydrothermal processes (Arnórssonet al. 2007).

5. Mixing versus boiling process

The process that takes place before the thermal fluid isdischarged in the vents must agree with the observed salin-ity in the fluid inclusion microthermometry. The modelinvestigated whether the same results could be obtained bymixing or boiling. An estimation of the pressure at depthof the GF of Bahía Concepción was made as proposedby Allen et al. (2006). Because there is no informationabout a geothermal gradient in Bahía Concepción or insurrounding geothermal systems such as Tres Vírgenes,we estimated this value using a heat flow value of 240mW/m2 for the Baja California Peninsula calculated byProl-Ledesma et al. (2008) and a thermal conductivityvalue of 2.5 mW/m2 (Clauser and Hegens 1995) bearingin mind that the main rock type in Bahía Concepción isan andesite (Camprubí et al. 2008). The value obtainedfor the geothermal gradient is 96◦C/km, and assumingan equilibrium temperature of 200◦C (Prol-Ledesma et al.2004), the depth at which the EM thermal fluid occurs isapproximately 2 km.

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Figure 4. The two mixing processes included in the model of the hydrothermal system in Bahía Concepción. Mixing path 1 is the mixof a highly saline fluid (HSF) with a geothermal fluid (GF). Mixing path 2 involves the fluid obtained from mixing path 1 and seawater(SW).

Under the assumption that the fluid is involved in aflashing process to 100◦C, we calculated the concentrationof chloride when the thermal EM at 2 km is ascend-ing through faults until it reaches a pressure of 1 bar.The steam fraction was calculated as proposed by Henleyet al. (1984); we did this assuming an isenthalpic pro-cess and a single steam separation from 200◦C to 100 ◦C.Under these conditions, the mass fraction that remains asliquid is 0.8 and the mass fraction in the vapour phaseis 0.2. Assuming the chloride concentration of the ther-mal EM (380 mmol/kg = 13,500 mg/kg, see Table 1)at 200◦C, after this fluid is involved in a boiling pro-cess the chloride concentration should be 17,710 mg/kg,which corresponds to a salinity of 2.9wt.% as NaCleq. Thissalinity is too low compared to that obtained by fluid inclu-sion microthermometry and could not yield the observedchemical composition for the Thermal EM.

6. Results

6.1. Evidence of a highly saline fluid in BahíaConcepción intertidal manifestations

The requirement to include a HSF comes from the highervalues of Ca, Cl, and Si (see Section 4); after severalsampling seasons, no marked changes in the salinity of thedischarged fluids were observed, and previous hydrother-mal stages were studied to determine whether differentsalinities have characterized the hydrothermal fluids inBahía Concepción. The presence of a HSF in the BahíaConcepción hydrothermal system was revealed by themicrothermometry results for barite from the chalcedony-barite-calcite veins occurring in the intertidal zone, near themodern vents.

Fluid inclusion microthermometry in barite crystalsfrom the intertidal hot springs allowed us to determine

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homogenization and ice-melting temperatures. The inter-tidal hot springs are closely related with the submarinemanifestations and with chalcedony-barite-calcite veins,showing the same structural occurrence (NW–SE faults)and the same host rocks (Camprubí et al. 2008). It alsooccurs associated with opal-A and calcite in stromatoliticsinter deposits around the intertidal hot springs.

Microthermometric measurements were performed on59 fluid inclusions, with sizes ranging from 10 to 100 μm,hosted in six barite crystals from the inner and externalparts of veins. Inclusion shapes were typically (a) elon-gate to acicular, oriented parallel to host crystal axes; (b)blocky to rectangular; or (c) flattened or irregular. Mostfluid inclusions had two phases (liquid + vapour) at roomtemperature with a liquid–to–vapour ratio (F = VolumeLiquid phase/Volume Total fluid inclusion) of 0.80–0.95.

Fluid inclusions can be classified as two major typesbased on the phases present in the inclusions (at roomtemperature) and on paragenesis and texture: (a) primarytwo-phase (L + V) inclusions that are localized on acrystallography plane or isolated without any orientationand (b) pseudosecondary and secondary monophasic liq-uid inclusions (L) that are localized in the cracks or formtrails of negative crystal inclusions.

The results of fluid inclusion microthermometry aresummarized in Table 2. Microthermometric data for sam-ples from the external part of the veins represent an earlystage of mineralization and yield homogenization tempera-tures (Th) from 124◦ to 154◦C, with an average of 139◦C.The ice-melting temperature (Tm) ranges from −1.4◦ to−3.3◦C, with an average of −1.9◦C; this corresponds toa salinity range of 2.4–5.4 wt.% NaCl eq. with an averageof 3.2 wt.% NaCl eq. (an average density of 1.01 g/cm3).In the inner part of the vein, Th ranges between 97◦ and144◦C, with an average of 129◦C, and Tm varies from−5.5◦ to −8.0◦C, with an average of −7.1◦C. Calculatedsalinities vary from 8.6 to 11.7 wt.% NaCl eq. with anaverage value of 10.6 wt.% NaCl eq.

The histograms of Th and Tm (Figure 5) show two Th

populations with ranges of 125–130◦C for external vein

parts (early mineralization) and a range between 97 and144◦C for the inner vein and two Tm populations with arange of −1.0 to −2.0◦C for the external vein and −8.0 to−9.0◦C for the inner vein.

The two populations of fluid inclusions are clearlyshown in a homogenization temperature versus salinity plot(Figure 6). A first group, corresponding to the external partof the vein, indicates the presence of a low temperatureand low-salinity fluid (127◦C Th and 2.7 wt.% NaCl eq.,0.9 g/cm3 density). A second group, corresponding to theinner part of the vein, represents a HSF (11.6 wt.% NaCleq.) with temperature between 97◦C and 144◦C (density of∼ 1.1 g/cm3). Consequently, these results indicate the pre-dominance of HSFs during deposition of barite and theircoexistence with lower salinity fluids. The trend observedbetween HSF to low-salinity fluids shows the occur-rence of mixing processes between the two hydrothermalfluids.

6.2. Mixing model

The fluids involved in the model are (a) a GF, (b) a HSF,and (c) local SW composition for the Bahía Concepciónarea. The content of Mg in the resulting fluid is assumed tobe entirely due to mixing with seawater, since it has beenproven that heating removes Mg (Bischoff and Seyfried1978).

Mixing of the three EMs generates fluids with differ-ent compositions, depending on the proportions of eachcomponent. The model results show the variation in thechemistry of the fluids produced for the two modellingstages. A range of mixing proportions (GF of meteoric ori-gin plus HSFs) for the primary fluid and further mixingwith SW are plotted in Figure 7.

Table 3 shows the results for different proportionsof GF and HSF, and further mixing with (10–90%) SW.Comparing the concentrations calculated with the model(Table 3) with those reported by Prol-Ledesma et al. (2004)(Table 1), the best fit for the measured concentrations wasobtained with a mixture of 70–80% of the GF, 30–20%

Table 2. Summary of microthermometry data of fluid inclusions in barite crystals from veins located in the vicinity of the studiedhydrothermal manifestations (intertidal host spring).

Sample nRange of Th

(◦C) Range of Tm (◦C)Salinity

(wt.% NaCl eq.)Fluid inclusion

types

Vein 1 Map-1 (EP) 23 124 − 131(127) −1.6 2.7 L + V, LMap-2 (IP) 16 97 − 138(109) −7.0 to −8.0 (−7.9) 10.5 to 11.7 (11.6) L + V, L

Vein 2 Map-3 (EP) 4 144 − 154(148) −1.4 to −1.9 (−1.6) 2.4–3.2 (2.7) L + V, LMap-4 (IP) 3 136 − 139(138) −6.9 to −7.8 (−7.5) 10.4–11.5 (11.1) L + V, L

Vein 3 Map-5 (EP) 8 139 to 145(142) −2.1 to −3.3 (−2.5) 3.6–5.4 (4.2) L + V, LMap-6 (IP) 5 138 − 144(141) −5.5 to −6.3 (−5.9) 8.6–9.6 (9.1) L + V, L

Notes: n, number data analysed; Th, homogenization temperature; Tm, ice-melting temperature; EP, external part of vein; IP, inner part of vein; L + V, L,liquid and vapour, liquid predominance; L, liquid. Values in parenthesis are average data. MAP-1, -3, and -5 correspond to the external part of the vein.MAP-2, -4, and -6 correspond to the inner part of the vein.

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Figure 5. Distribution of homogenization temperature (Th) and ice-melting temperature (Tm) measured in barite from veins in thevicinity of the intertidal hot spring. Samples were from the external part (EP) or the inner part (IP) of the vein. V1, V2, V3, veins sampled.MAP-1, -3, and -5 correspond to the external part of the vein. MAP-2, -4, and -6 correspond to the inner part of the vein.

Figure 6. Homogenization temperature (◦C) versus salinity(wt.% NaCl eq.) for fluid inclusions in barite from an intertidalhydrothermal manifestation. Two populations of fluid inclusionsare observed: low and high salinity. Th, homogenization temper-ature; EP, external part of vein; IP, inner part of vein; V1, V2, V3,veins sampled. MAP-1, -3, and -5 correspond to the external partof the vein. MAP-2, -4, and -6 correspond to the inner part of thevein.

of the HSF, and further mixing with SW plots along theobserved concentrations of the water discharged by thehydrothermal vents in Bahía Concepción, as they presentdifferent proportions of thermal EM to SW.

7. Discussion

HSF and GF mixing produces a fluid chemically similar tothe EM calculated by Prol-Ledesma et al. (2004). Majorion concentrations that characterize the thermal EM (Caand Si) and SW (Cl) are useful in model evaluation.

Results suggest that the fluid responsible for the forma-tion of the thermal EM of Bahía Concepción is 30–20%HSF and 70–80% hot GF. The predominance of mete-oric water is revealed by the isotopic characteristics of thefluid discharged at the vents, which plot very closely tothe meteoric line (Prol-Ledesma et al. 2004). The modelinput data are consistent with fluid inclusion data, whichprovide evidence that two different hydrothermal fluids areinvolved in barite deposition at veins related to the sub-marine manifestations of Bahía Concepción: a low-salinityfluid (about 127◦C and 2.7 wt.% NaCl eq.) and a HSF(about 97–144◦C and 11.6 wt.% NaCl eq.). In some stagesone of them prevails but in the present stage the modelindicates that a mixing is taking place.

Another possibility for vein formation would be boiling(Figure 6). However, no evidence for boiling, such as brec-cias, bladed calcite, or coexisting liquid-rich and vapour-rich inclusions, has been found. Moreover, the estimatedsalinity (2.9 wt.% NaCl eq.) during a single step steam

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Figure 7. Concentrations of Ca, Cl, and Si versus Mg for the mixing processes in the hydrothermal system in Bahía Concepción.Different tonalities of the circle indicate different mixing proportions as indicated in the legend. The arrows indicate the results of bothmixing path 1 and 2. The chemical composition of the submarine hydrothermal vents (asterisks) agrees with the mixture fluid of 20–30%HSF and 80–70% of a GF. Also are shown the concentrations of the components used in the model: geothermal fluid (GF), highly salinefluid (HSF), and seawater (SW). EM refers to the composition of the thermal endmember calculated by Prol-Ledesma et al. (2004).

separation when the EM GF is involved in a flashing pro-cess from 200◦C to 100◦C (the temperature necessary forboiling) does not reach the high salinity calculated withmicrothermometry data. This supports the mixing modelrather than a boiling process. The fluid is cooled by beingmixed with SW before it is discharged to the seafloor(Table 2).

HSF origin in Bahía Concepción could be watertrapped in the Oligocene marine sedimentary rocks of ElSalto Formation (Figure 3). The presence of two fluidswith different salinities in the fluid inclusions also suggeststhat fluids may have occasionally flowed as discrete pulseswith little mixing between them, as Simmons (1991) pro-posed for a brine reservoir responsible for the formation of

Table 3. Calculated concentration (mmol/kg) of Ca, Cl, and Si in fluids resulting fromthe mixing path 2.

Proportion of mixing Ca Si Cl

90% GF + 10% HSF + SW 21.1 8.1 285.780% GF + 20% HSF + SW 33.2 7.6 356.070% GF + 30% HSF + SW 43.5 7.2 415.160% GF + 40% HSF + SW 52.3 6.9 464.850% GF + 50% HSF + SW 59.8 6.5 508.2

Notes: These concentrations were calculated by the linear equation of the mixing path 2 and representthe ordinate to origin value. We used only major ions in the results that are mostly related to thethermal endmember, as Ca and Si, and Cl is included as a conservative element in the hydrothermalprocesses. GF, geothermal fluid; HSF, highly saline fluid; SW, seawater.

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an epithermal deposit in Fresnillo, México. The processesthat lead to stable stratification of two chemically dis-tinct hydrothermal fluids (HSF and dilute water) in BahíaConcepción could be explained by the finding for Wairakei,New Zealand, that differences in fluid density can lead toa layered double-diffusive convection (Griffiths 1981). Inaddition, separate convection cells can exist in a porousmedium, if heat and chloride ions diffuse at different ratesacross the density interface (Simmons 1991).

A similar process has been reported in Schwarzwald,Southwest Germany, where hydrothermal veins occur;some fluid inclusions from those veins show salinities inthe range of 20–25 wt.% NaCl eq. and indicate that miner-als such as barite, quartz, and calcite precipitate as a resultof the mixing of deep saline brine with surface meteoricwater (Schwinn et al. 2006).

Saturation indices were calculated for the mixing pro-cesses between the primary fluids obtained from the firstmodelling stage (HSF + GF) with different proportions oflocal SW (Figure 8). Amorphous silica is undersaturatedfor temperatures above 89◦C. Below this temperature, thefluid becomes saturated in amorphous silica, in agreementwith the presence of opal deposits around the intertidalthermal springs. Calcite is supersaturated throughout thetemperature range, but supersaturation decreases at temper-atures below 89◦C.

The mixing model presented here agrees with observa-tions regarding calcite deposition in the field; this deposi-tion has not been observed around the submarine manifes-tations of Bahía Concepción where the fluid is dischargedat a temperature of 50◦C at the sea surface, although itoccurs in the intertidal springs as a consequence of CO2

loss (Canet et al. 2005a). Calcite is a mineral with retro-grade solubility and supersaturation occurs if a degassing

process takes place. Also, calcite supersaturation could becaused by mixing of two fluids, saline brine and meteoricwater (Schwinn et al. 2006), which is due to the increase ofthe solution ionic strength (Arnórsson 1978). Alternatively,supersaturation could be the result of pH increase caused bymixing of hydrothermal fluid with SW as Glasby and Notsu(2003) propose to explain calcite deposition in OkinawaTrough, SW of Japan.

The model results show that the saturation index is toolow to cause amorphous silica deposition at a tempera-ture around 89◦C, but it increases at lower temperaturesand supersaturation occurs at temperatures of about 40◦C(Figure 8). Submarine vent temperature is close to 89◦Cat 10 cm below the sea bottom (Prol-Ledesma et al.2004); therefore, silica deposition around the submarinevents is not likely, whereas it is expected at the intertidalhydrothermal manifestations, where measured tempera-tures are slightly over 40◦C and consequently abundantdeposition of opal is reported (Canet et al. 2005b). Someother factors that affect precipitation of non-crystallineforms of silica at low temperatures could be the chemicalreactions between aluminosilicates and abundant biogenicsilica (Bloch and Hutcheon 1992), as occurs at BahíaConcepción where diatoms are abundant around the vents(Canet et al. 2005b).

8. Conclusions

Mixing modelling including the selected fluid componentsallowed us to define the processes that take place in theBahía Concepción hydrothermal system. Characterizationof fluid components in the model is appropriate, as it repro-duces the chemical composition of the fluid discharged.The thermal EM at Bahía Concepción is formed by mixing

Figure 8. Saturation index (SI) of calcite and amorphous silica when a fluid composed of 30% HSF and 70% GF is mixed with differentproportions of local seawater. This plot shows that at temperatures higher than 89◦C, amorphous silica is undersaturated and is onlyslightly calcite saturated. At temperatures lower than 68◦C, the fluid is saturated in amorphous silica and calcite, whose deposition occursin intertidal vents from Bahía Concepción.

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20–30% HSF with 70–80% GF of meteoric origin. Furthermixing with SW takes place within the shallow sediments,before it discharges to the seafloor, in different proportionsfor different samples.

The inclusion of a HSF in the model is based on theequivalent salinity of fluid inclusions from chalcedony-barite-calcite veins.

Mixing is more important than boiling at BahíaConcepción. The salinity value of the fluid involved ina boiling process is too low in comparison with thatdetermined with fluid inclusion microthermometry.

Calculated mineral saturation indices agree withobserved subsaturation of amorphous silica in the deepvents, and calcite and opal deposition in the intertidalsprings.

The analysed chemical composition of the vent fluid,together with the observed mineralogy, supports the pro-posed geochemical model for Bahía Concepción.

AcknowledgementsThis study was sponsored by the projects CONACYT-CB-2008-01-102387 and J51127-F, IMPULSA ‘Desalación de agua de marcon energías removables (Using renewable energies to desalinatesea water)’, and PAPIIT IN107003, IN106907-2, IN115611-3,and IN118009-2. The authors thank the reviewers Ann Grant,Luis Marín Stillman, Francis Edward Bayon, and Maribel Delfínfor their contributions that improved the manuscript.

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