Ore Geology Reviews 72 (2016) 603–611

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Ore Geology Reviews

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Boiling and depth calculations in active and fossil hydrothermal systems:A comparative approach based on fluid inclusion case studies fromMexico

Miguel A. Cruz-Pérez a,b, Carles Canet b,⁎, Sara I. Franco b, Antoni Camprubí c,Eduardo González-Partida d, Abdorrahman Rajabi e

a Facultad de Ingeniería, Universidad Nacional Autónoma de México, Del. Coyoacán, 04510 México, D.F., Mexicob Instituto de Geofísica, Universidad Nacional Autónoma de México, Del. Coyoacán, 04510 México, D.F., Mexicoc Instituto de Geología, Universidad Nacional Autónoma de México, Del. Coyoacán, 04510 México, D.F., Mexicod Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Santiago de Querétaro, Mexicoe Department of Geology, Faculty of Basic Sciences, University of Birjand, Birjand, Iran

⁎ Corresponding author.E-mail address: ccanet@geofisica.unam.mx (C. Canet).

http://dx.doi.org/10.1016/j.oregeorev.2015.08.0160169-1368/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 February 2015Received in revised form 13 August 2015Accepted 19 August 2015Available online 28 August 2015

Keywords:Epithermal depositsHydrothermalFluid inclusionsMicrothermometryPaleo-depth estimationGeothermal exploration

A boiling model that considers the increase of salinity due to the steam loss and uses a combined density of thecoexisting vapor and liquid phases was applied to fluid inclusion data from Los Azufres geothermal zone andfrom anEocene epithermal vein of Taxco. These case studies are taken as examples of active and fossil hydrother-mal systems, respectively. In Los Azufres high temperatures of homogenization (N300 °C) are commonly attainedat depths between 1500 and 2000 m whereas salinity values above 2.0 wt.% NaCl eq. occur within the upper~500 m of the system, suggesting that the geothermal zone is largely affected by boiling. The depths calculatedwith the boiling model are close to real depths, with accuracy greater than 99% for one case; however, consider-ably large error (30%) was obtained to the top of the geothermal system due to enhanced CO2 concentrations.Contrastingly, the depths estimated by plotting microthermometric data on boiling point curves (of constant sa-linity and discarding the effect of vapor on hydrostatic pressure) were systematically shallower than real ones,implying an underestimation of depth of up to ~50%. For the application case of the Taxco epithermal deposit,microthermometric data describe a boiling evolution path in the temperature–salinity space although somevalues deviate from it, thus likely reflecting local mixing with fluids of contrasting salinity. According to ourmodel, boiling occurred from a paleo-depth of 360 m, which corresponds to a current (sampling) depth ofabout 200 m; this level in the hydrothermal system coincides with the boundary between a lower base metalzone and an upper silver-rich zone. These results suggest that the descriptive models for epithermal depositscould be incorrectly calibrated in terms of depth; therefore, they could be revised and corrected by applyingthe boiling model used in this paper.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The upper zone (1–2 km) of active hydrothermal systems is affectedby the boiling of hot upwelling fluids due to decompression, which re-sults in rapid separation of steam from brines and in the partitioningof volatiles (typically CO2 andH2S) into the vapor phase. This phase sep-aration controls the key chemical parameters for mineral supersatura-tion and precipitation, such as pH and fluid composition (cf. Roedder,1984; Henley et al., 1984; Canet et al., 2011). In geothermal systems,boiling determines, inter alia, vapor-to-liquid ratios and discharge en-thalpies (Scott et al., 2014, and references therein), whereas forepithermal deposits boiling is themost importantmechanismof precip-itation, with special regard to precious metals (e.g., Hedenquist andHenley, 1985a; Cole and Drummond, 1986; Skinner, 1997; Camprubí

et al., 2001; André-Mayer et al., 2002; Simmons et al., 2005 and refer-ences therein). Furthermore, the precipitation of these metals hasbeen confirmed in geothermal wells undergoing intense boiling(Brown, 1986; Simmons and Browne, 2000a).

In order to study the role of boiling in the epithermal environment,however, the complex zoning that generally affects the deposits at allscales as a result of successive mineralizing events and changes in thestate of sulfidation needs to be considered (e.g., Einaudi et al., 2003;Dreier, 2005; Camprubí and Albinson, 2007). Vertical ore zoning at thedeposit scale is a feature of many epithermal deposits (André-Mayeret al., 2002 and references therein) and also has been described in geo-thermal systems (Simmons and Browne, 2000b). In many cases ofsouthwest North America (Mexico and western United States) twozones are distinguishable, (a) a deeper zone enriched in base metals(Pb, Zn, Cu) and (b) a shallower zone with precious metals (Ag, Au),usually coinciding the boundary between the two zones with the boil-ing level (e.g., Buchanan, 1981). However, the vertical distribution of

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metal associations can be more complex than assumed since Buchanan(1981). In this regard, epithermal deposits in Mexico also exhibitcharacteristically (I) earlier and deeper polymetallic intermediatesulfidation stages, followed by (II) shallower Ag–Au low sulfidationstages (Camprubí and Albinson, 2007). Such a feature has been mistak-en as a “deep basemetal” and “shallowpreciousmetal” zoningwhen re-searchers did not pay enough attention to the paragenetic sequence ofmineralization.

Another common feature of epithermal deposits are high-gradeore zones—or bonanzas—that are constrained to discrete depth intervals(e.g., Cole and Drummond, 1986; Sillitoe, 1993; Albinson and Rubio,2001; Dreier, 2005), likely corresponding to boiling levels relatedto major feeder channels (Cole and Drummond, 1986; Scott andWatanabe, 1998; Camprubí et al., 2001). Thus, mineralogical and fluidinclusion evidence for boiling can be taken as key exploration criteria(cf. Simmons et al., 2005; Camprubí, 2010; Canet et al., 2011). If fluidinclusions are used for this purpose, microthermometric data mustbe carefully separated according to stages of mineralization (whereapplicable), as feeder channels do not necessarily remain stationaryduring the activity of the hydrothermal system (e.g., Camprubí, 2010;Rowland and Simmons, 2012). The occurrence of major feeder channelsis suggested by temperatures of homogenization (TH) defining domeshaped isotherms, whereas mushroom shapes may suggest lateral out-flow (e.g., Albinson and Rubio, 2001).

Finding evidence for boiling points out not only possible high-gradezones, but also can be used to constrain paleo-depths based on the prin-ciple that vapor pressure equals hydrostatic pressure during boiling(Haas, 1971). In turn, the determination of the paleo-depth of the boil-ing level in fossil hydrothermal systems can be useful in terms ofminer-al exploration and for the model of ore deposit by the following: (a)vertical zoning of the hydrothermal system can be referenced to thepresent topography and, therefore, (b) the level of erosion of the depos-it, relative to the paleo-surface (actually to the paleo-water table posi-tion at the time of hydrothermal activity; cf. Simmons et al., 2005),can be estimated.

The boiling point curves of Haas (1971) were calculated for hydro-static conditions for different—but constant—salinities and are oftenused in estimations of paleo-depth in epithermal deposits (e.g.,Shamanian et al., 2004; Masterman et al., 2005; Orgün et al., 2005;Fard et al., 2006; Richards et al., 2006; Rice et al., 2007; Harris et al.,2009; Cooke et al., 2011; Fornadel et al., 2012; Lesage et al., 2013;Koděra et al., 2014). However, the effect of steam bubbles loweringthe hydrostatic pressure is discarded and, therefore, the paleo-depthscalculated in this way may have been systematically underestimated.Alternatively, Canet et al. (2011) proposed a method for paleo-depthcalculation that takes the increase of salinity due to steam loss duringboiling into consideration, and uses a combined density of thecoexisting vapor and liquid phases. This method was tested in theEarlyMiocene Bolaños Ag–Au–Pb–Zn epithermal deposit, southwesternMexico. In this case, the calculated paleo-depth for the level in whichboiling started resulted very close to the sampling (present) depth,about 440 m, whereas using the curves of Haas (1971) a paleo-depthof ~225 m is obtained; the latter is an unreliable value because it iseven smaller than the sampling depth.

In this paper we present firstly the results of applying the boilingmodel of Canet et al. (2011)—henceforth referred to as the variablesalinity model (or method)—to fluid inclusion data of the Los Azufresgeothermal zone, central-western Mexico. This area is regarded as anexample of an active hydrothermal system in which key parametersas temperature gradients and the conditions of depressurization boilingare well constrained; depths of drill-core samples can be treated in thiscase as real depths, since erosion can be neglected. In addition, weapplied the variable salinity model to an intermediate sulfidationepithermal vein from the Taxco district, southern Mexico, which repre-sents a fossil hydrothermal system that was active in the Eocene; in thiscase, its well-established vertical ore zoning is used to evaluate our

results. Additionally, having assessed the performance of the paleo-depth estimation based on the variable salinity model, a review of fourrepresentative epithermal deposits of Mexico is presented, which callsinto question the depths of mineralization that so far have been as-sumed for this type of deposit.

2. Geological setting

2.1. Los Azufres geothermal field

Los Azufres, in the state ofMichoacán, is a geothermalfield hosted bya silicic volcanic complex lying on the E–W Cuitzeo Graben, at the cen-tral part of the Trans-Mexican Volcanic Belt (TMVB) (Fig. 1A). It is, alongwith Los Humeros in the state of Puebla, one of only two geothermalsites in the TMBV under commercial exploitation. The total installedgenerating capacity of Los Azufres is about 188 MWe, which makes ofit the second most important field in Mexico, having started electricityproduction in 1982 (Gutiérrez-Negrín, 2007).

The volcanic complex of Los Azufres consists of four ignimbrite unitsand many dome complexes that, according to Ferrari et al. (1991) andPradal and Robin (1994) are controlled by a poorly-defined, sub-circular volcanic collapse feature; this caldera-like structure, of about20 km in diameter and a Middle Pleistocene age, contains the geother-mal system. The local pre-caldera basement consists of Oligocene toMiocene andesitic lavas that crop out south of the Cuitzeo Graben(Dobson and Mahood, 1985).

The Los Azufres caldera formed in response to two main periods ofvolcanic activity (Pradal and Robin, 1994). During the oldest one(~1.5–0.8Ma), twomagmatic cycles consisting in the extrusion of acidicmagmas followed by andesites and basalts took place. This episode pro-duced large volumes of ignimbrites and rhyolitic domes and flows. Thelater episode (~0.6 Ma to Late Pleistocene) is responsible for the resur-gent doming affecting the southern part of the volcanic complex, havingproduced rhyolites and dacites, and an up to 40m-thick ignimbrite unitthat yielded a 14C age at ~26,000 (Pradal et al., 1988; Pradal and Robin,1994).

Graben-related, E–W striking, pervasive normal faulting is devel-oped across the Los Azufres area, affecting most of the volcanic se-quence, except for domes younger than 0.3 Ma (Ferrari et al., 1991). Inaddition, NE–SW and N–S striking normal fault arrays can be distin-guished (Campos-Enriquez and Garduño-Monroy, 1995). The E–Wsys-tem controls a great deal of geothermal fluid circulation, as it conferssecondary permeability to the andesitic units (González-Partida et al.,2000).

The hydrothermal system of Los Azufres attains up to 320 °C in theliquid-dominated geothermal reservoir, at a depth of ~3500 m (Birkleet al., 2001; Pinti et al., 2013). The hydrothermal liquid phase is of theNaCl type (e.g., Pandarinath et al., 2008), with Cl ranging between 0.26and 0.34% (Kruger et al., 1985). CO2 represents up to 90% of the non-condensable gases, followed by N2, H2S and H2 (Santoyo et al., 1991),and it increases to the top of the geothermal system due to volatile ex-solution; however, this gas is not observed in fluid inclusions(González-Partida et al., 2000). Geothermal wells in Los Azufres aregrouped into two distinct productive zones, northern and southern,the latter being the more productive as it records higher temperaturesat shallower depths than the northern zone (Torres-Rodríguez et al.,2005).

2.2. Taxco mining district

The Ag–Zn–Pb(–Cu–Au) Taxco mining district, in the state of Guer-rero (Fig. 1B), is one of the oldest in the Americas, with an almost con-tinuous mining activity from colonial times to the present. It containsmore than 50 epithermal veins, mostly of intermediate sulfidation,with associated large replacement mantos, stockworks and breccias(Camprubí et al., 2006a). These deposits formed in the Late Eocene

Fig. 1. Location of the study areas and schematic geologic maps. (A) Los Azufres geothermal field, state of Michoacán, showing the distribution of selected production wells for modeling(after González-Partida et al., 2000); (B) Taxcomining district, Guerrero state, showing the tract of the El Cobre–Babilonia vein system (after Osterman, 1984; Camprubí et al., 2006a). Key:TMVB = Trans-Mexican Volcanic Belt; SMS = Sierra Madre del Sur.

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(Camprubí and Albinson, 2007), linked to the latest stages of arcmagmatism of the Sierra Madre del Sur, which produced severalmajor silicic volcanic centers (e.g., Alaniz-Álvarez et al., 2002;González-Torres et al., 2013).

Exposed in twooutcrops nearby the townof Taxco, the local basementconsists of an assemblage of phyllites, metalavas and metaignimbritesthat are Early Cretaceous in age (Campa-Uranga et al., 2012). Suchmeta-volcano-sedimentary unit, affected by low-grade metamorphism,has been assigned to the Guerrero Composite Terrane (Campa-Urangaet al., 2012), which is a subduction-related complex influenced bymajor translation and rifting during theMesozoic along thewesternmar-gin of Mexico (Campa and Coney, 1983; Centeno-García et al., 2008). AMiddle to Upper Cretaceous sedimentary sequence unconformably over-lies the meta-volcano-sedimentary rocks, being the visible contact be-tween both units mostly structural (Campa-Uranga et al., 2012). Thissedimentary sequence consists of Albian–Cenomanian limestones anddolostones followed by Turonian–Campanian flysch deposits (Aguilera-Franco and Hernández-Romano, 2004). It is in turn unconformably over-lain by Late Eocene continental sedimentary and volcanic rocks. Theformer are conglomerates deposited in pull-apart basins associated toNW- and N-trending strike-slip faults, whereas the later are ignimbritesand dacitic to andesitic lava flows and dikes (Alaniz-Álvarez et al., 2002;Morán-Zenteno et al., 2007; Martiny et al., 2013).

Epithermal veins in Taxco have a general NW–SE strike and are up to3 km long (Fig. 1B), with thicknesses averaging ~2 m (Camprubí et al.,2006a). Also characteristic of these structures is (a) vertical metalzoning, in a typical sequence Cu→ Pb–Zn→ Ag(–Au) from deep to shal-low mine levels (Osterman, 1984; Camprubí et al., 2006a), and (b)crustiform banding. El Cobre–Babilonia is a polymetallic vein thatmeets the above characteristics and that was studied by Camprubí

et al. (2006a). These authors found homogenization temperatures(TH) and final ice melting temperatures (TMi) ranging from 160° to290 °C and from−11.6° to−0.5 °C, respectively (Table 2), and pointedto boiling as themainmechanism for ore deposition based on the occur-rence of liquid- and vapor-rich fluid inclusionswithin the same fluid in-clusion assemblages, and of adularia.

3. Methods

3.1. Application of a boiling model

The variable salinity model for fluid inclusions of Canet et al. (2011)was applied to data in the literature from the Los Azufres geothermalfield and an epithermal vein of the Taxco district, which were taken asexamples of active and fossil hydrothermal systems, respectively. Thismodel was constructed to investigate fluid evolution during boilingand enables the following: (a) to establish boiling paths in the TH–salin-ity (–TMi) space, based on mass and heat balance equations (cf. Henleyet al., 1984), such that fluid inclusion data can be compared with themto confirm or discard boiling, and (b) to calculate paleo-depths of theboiling level, for which the effect of steam bubbles lowering the hydro-static pressure is taken into account; for the later, the starting point arethe equations of Haas (1971, and referenced therein). However, whilefor depth calculations Haas (1971) took constant values of salinity anddiscarded the effect of steam bubbles, Canet et al. (2011) used a com-bined density of the coexisting vapor and liquid phases, for which thevolume of steam per mass unit was approximated by means of theideal gas law (at a given pressure and temperature).

The equations included in the variable salinitymodel consider a sim-ple NaCl–H2O system, whose thermodynamic properties approach

Fig. 2. Salinity–temperature diagramproposed by Canet et al. (2011), showing the feasibleboiling paths (arrows), modeled with the mass and enthalpy balance equations of Henleyet al. (1984). Microthermometric data from quartz (González-Partida et al., 2000) for theLos Azufres geothermal field were plotted by their ice-melting TMi and homogenizationtemperatures to confirm or discard boiling. Blue numbers indicate depth of samplingbelow present surface and help to identify a boiling process, taking into account that thefluid is ascending vertically (i.e., simulated by the well itself). Red numbers indicate theinitial temperature (Ti) value. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Fig. 3. Salinity–temperature diagramproposed by Canet et al. (2011), showing the feasibleboiling paths (arrows), modeled for the epithermal environment using the mass and en-thalpy balance equations of Henley et al. (1984). Microthermometric data from quartz(Camprubí et al., 2006a) for the El Cobre–Babilonia epithermal vein system were plottedby their ice-melting TMi and homogenization temperatures to confirm or discard boiling.Blue numbers indicate the mine level (depth) of sampling below present surface andhelp to identify a boiling process, taking into account that the fluid is ascending vertically.(For interpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

Table 1Summary of microthermometric data of primary fluid inclusions (hosted in quartz) of theLos Azufres geothermal field, Trans-Mexican Volcanic Belt.From González-Partida et al. (2000).

Well Deptha n TH (°C) TM (°C) Salinity(wt.% NaCl eq.)

Min. Aver. Max. Min. Aver. Max.

Az-27A 2000 58 283 309 327 −0.5 −0.8 0.9 1.41700 20 291 321 328 −0.8 −0.8 −0.8 1.41600 16 339 320 394 −0.8 −0.8 −0.8 1.41500 24 311 320 341 −0.9 −1.0 −1.1 1.71100 15 274 278 281 −0.1 −0.7 −0.9 1.2800 44 228 275 295 −0.1 −0.7 −1.7 1.2400 10 196 212 215 −1.2 −1.3 −1.4 2.2

Az-44 4200 35 250 260 276 −0.1 −0.7 −1.5 1.23300 23 302 313 327 −0.4 −0.6 −1.2 1.03200 14 300 302 310 −0.9 −1.0 −2.1 1.73100 19 297 288 303 −0.9 −1.2 −1.6 2.02880 6 293 303 321 −1.1 −1.1 −1.1 1.92400 17 301 320 335 −0.6 −0.8 −1.3 1.41400 46 231 278 284 −1.4 −1.4 −1.4 2.4900 49 204 250 270 −1.2 −1.2 −1.2 2.0

Az-9 2450 23 317 334 347 −0.1 −0.2 −0.3 0.32300 28 312 335 351 −0.1 −0.3 −0.6 0.52127 26 324 329 340 −0.3 −0.4 −0.5 0.71900 25 307 310 311 −0.1 −0.2 −0.3 0.31700 11 282 296 305 −0.8 −0.7 −0.7 1.21300 22 320 322 327 −0.9 −0.9 −0.8 1.41000 11 234 285 295 −0.4 −0.5 −0.8 0.8800 10 223 243 273 −1.5 −1.5 −1.5 2.4

Az-23 1700 11 285 310 315 −0.1 −0.2 −0.2 0.31275 8 251 276 292 −0.1 −0.3 −0.6 0.51100 14 210 264 282 −0.5 −1.1 −1.6 1.91000 32 229 250 261 −0.2 −0.6 −1.1 1.0900 10 220 254 265 −0.6 −0.9 −1.3 1.5800 31 202 253 264 −0.3 −0.8 −1.2 1.4600 9 237 257 261 −0.3 −0.5 −0.7 0.8500 5 221 221 224 −0.3 −1.1 −1.6 1.9400 9 209 220 233 −1.2 −1.5 −1.6 2.5

Az-16 2450 8 328 335 346 −0.3 −0.4 −0.7 0.72100 42 284 305 330 −1 −1.3 −1.8 2.21550 11 268 286 288 −0.6 −1 −2.2 1.71250 80 241 256 265 −0.6 −0.6 −0.6 1.0550 5 220 223 225 −1.2 −1.2 −1.2 2.0400 10 204 208 210 −1.1 −1.1 −1.1 1.9

Location:wells Az-9, Az-27AandAz-44, northernproductive zone;wells Az-16 andAz-23,southern productive zone.Key: TH = temperature of homogenization, TM = temperature of ice melting (freezingpoint depression), n = number of analyzed inclusions, Min. = minimum value; Aver. =arithmetic mean; Max. = maximum value.

a Approximate; in m below ground surface.

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those of natural solutions containing, besides Na, cations such as Caand K. Hence, apparent salinity values, obtained by fluid inclusionmicrothermometry and expressed in terms of weight percent NaClequivalent (wt.% NaCl eq.), can be directly used.

The first step in applying the variable salinity model is to plot fluidinclusion data in a TH–salinity diagram with depicted boiling paths(Figs. 2 and 3). The plotted microthermometric data should be repre-sentative of a set of samples collected at different depths in a mineral-ized structure that presumably acted as a simple conduit throughwhich fluids vertically rose to the surface. For this studywe used the ar-ithmetic mean per sample of TH and of salinity (Tables 1, 2 and 3). Thus,a boiling interval is revealed between two or more samples of consecu-tive decreasing depth that follow a boiling path. The progressive in-crease in apparent salinity that is observed along with a decrease of THand depth reflects the partitioning of non-volatile solutes into the liquidphase during steam loss.

The second step of the modeling leads us to construct thermalprofiles and, consequently, to perform paleo-depth calculations(Figs. 4 and 5); the starting data for this model are the TH and salinityvalues that correspond to the deepest sample of the boiling interval(established in the previous step). Depth is obtained by numerical inte-gration of the aforementioned equations with a program written in

Table 2Summary of microthermometric data of primary fluid inclusions (hosted in quartz) of theEl Cobre–Babilonia Ag–Pb–Zn–Cu epithermal vein, Taxco Mining District, SouthernMexico.From Camprubí et al. (2006a).

Sample Deptha n TH (°C) TM (ice) (°C) Salinity(wt.% NaCl eq.)

Min. Aver. Max. Min. Aver. Max.

S-04 0 28 155 162 170 −3.4 −4.4 −5.9 7.0N1-04 65 30 182 189 199 −7.0 −8.8 −9.6 12.6N2-10 130 40 180 194 201 −6.6 −8.4 −9.9 12.1N3-01 200 30 208 221 230 −7.0 −7.9 −9.1 11.5N5-30 265 25 231 247 258 −8.0 −10.4 −12.2 14.3N7-10 330 48 235 244 260 −4.0 −5.9 −7.1 9.0N9-15 400 45 271 289 301 −4.2 −5.4 −7.5 8.4

Key: TH= temperature of homogenization, TM (ice)= temperature of icemelting (freezingpoint depression), n = number of analyzed inclusions, Min. = minimum value, Aver. =arithmetic mean, Max. = maximum value.

a Inferred (maximum) depth of sampling, in m below present surface.

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FORTRAN code (available in Canet et al., 2011), from the construction ofthe corresponding boiling-point curve. The resulting depth values arerounded to tens of meters. Additionally, we constructed a table and adepth–TH–salinity plot, both available as supplementary files, thatallow a direct estimation of depth from the microthermometric dataof a sample representative of the beginning of boiling (i.e., the deepestlevel of the hydrothermal system in which boiling occurred).

3.2. Fluid inclusion data

For a reliable application of the variable salinity model, fluid inclu-sions must meet the following conditions: (a) be of primary origin (ac-cordingly to the criteria of Roedder, 1984) or belong to fluid inclusionassemblages that are associated with the precipitation of ore minerals(Goldstein, 2003), (b) be undersaturated with respect to halite, havingsalinities below 23.3 wt.% NaCl eq. (i.e., the eutectic composition of theNaCl–H2O system), and (c) record no clathrate formation during freez-ing experiments, thus indicating low CO2 contents (b3.7wt.% accordingto Hedenquist and Henley, 1985b).

All the fluid inclusions used in this study are hosted in quartz. Fluidinclusion data for Los Azufres geothermal field were taken fromGonzález-Partida et al. (2000), and correspond to cores recoveredfrom the production wells Az-9, Az-27A, Az-44 (northern productivezone), Az-16 and Az-23 (southern productive zone) (Table 1). On theother hand, the microthermometric data from the El Cobre–Babiloniapolymetallic vein of the Taxco district were taken from Camprubí et al.(2006a). One sample (S-04) is from the surface and the rest from differ-ent mining levels (increasing depth: N1, N2, N3, N4, N5, N6, N7 and N9;Table 2).

The arithmetic mean per sample (i.e., at different depths) of TH, TMi

and salinity was used in all cases for description, modeling and subse-quent interpretation.

Table 3Paleo-depth estimation of the boiling level for three epithermal deposits of Central Mexico, wiusing the boiling-point curves for brines of constant salinity constructed by Haas (1971). On thprogressive increase of salinity due to the steam loss during boiling (Canet et al., 2011), but, ineffect of vapor bubbles on fluid density and therefore on the hydrostatic pressure were taken i

Epithermal deposit Agea

(Ma)Microthermometry(fluid inclusion data)

Sampling d(m below p

TH Salinityb

(°C) (wt.% NaCl eq.)

San Martín (Querétaro) 27.5 210 1.5 ~2Veta Madre (Guanajuato) 27.4–30.7 276 12.4 ~5La Guitarra (Edo. México) 33.3–32.9 200 4.0 ~1Real de Catorce (San Luis Potosí) 35.7? 275 2.6 ~2

a Source: Camprubí and Albinson (2007).b Average values.

4. Results

4.1. Los Azufres

A statistics summary of fluid inclusion microthermometric data ofthe Los Azufres geothermal field is provided in Table 1.

In the case of well Az-27A (northern productive zone), TH ranges be-tween212° and 321 °C, TMi between−1.3° and−0.7 °C, and salinity be-tween 1.2 and 2.2wt.%NaCl eq. Temperatures above 300 °C are found atdepths greater than 1500 m (below ground surface), with maximumTH values at 1600–1700 m. On the other hand, the highest salinity(2.2 wt.% NaCl eq.) corresponds to the shallowest sample (400 m); atgreater depths salinity is rather constant, between 1.2 and 1.4 wt.%NaCl eq., except for a value of 1.7% NaCl eq. found at 1500 m. In the sa-linity–temperature plot, the distribution of data does not showneither aregular trend of cooling with decreasing depth—as the temperature at2000 m is lower than at 1500 m—or a general tendency of boiling(Fig. 2). However, the increase of salinity at 1500 m suggests that fluidevolution from 1700 m (TH = 321 °C; salinity = 1.4 wt.% NaCl eq.)could be affected by boiling; in contrast, above 800 m, fluid variationscannot be explained in terms of boiling.

For well Az-44 (northern productive zone) microthermometric dataare available down to an ultimate depth of 4200 m. In this case, THranges between 250° and 320 °C, TMi between −1.4° and −0.6 °C,and salinity between 1.0 and 2.4 wt.% NaCl eq. Temperatures above300 °C are found at depths from 2400 to 4200 m, with the maximum(320 °C) at 2400 m. The highest value of salinity (2.4 wt.% NaCl eq.)was found at 1400 m; at depths greater than 2100 m salinity remainsbelow 2.0% NaCl eq. The salinity–temperature plot shows a highly irreg-ular distribution of data, away from a consistent trend of salinity and THvariationwith depth (Fig. 2),which reflects abrupt changes and reversalof the slope of the temperature gradient curve.

In well Az-9 (northern productive zone) TH ranges between 243°and 335 °C, TMi between −1.5° and −0.2 °C, and salinity between 0.3and 2.4 wt.% NaCl eq. Temperatures higher than 300 °C occur at1900 m and increase gradually with depth up to a maximum of~335 °C (at 2300–2450 m). In addition, there is a TH peak (322 °C) at1300m. Thehighest salinity value is found toward the top of the system,at a depth of 800 m. As in the preceding case, the salinity–temperatureplot does not allow to point out a depth interval for which boiling con-trols fluid evolution (Fig. 2).

Based on the distribution of microthermometric data in the salinity–temperature plot, Az-27Awas selected among the three studiedwells ofthe northern productive zone to construct the thermal profiles; subse-quently, the depth for the level at which boiling begins was calculated(Fig. 4). The starting data for obtaining the temperature–depth profilesare TH = 321 °C and salinity = 1.4 wt.% NaCl eq. Two curves were con-structed, one considering a liquid column without suspended vaporbubbles, and the other calculating the effect of steambubbles on the hy-drostatic pressure. The calculated depth is 1460 m considering a liquid

th the approximate sampling depths for comparison. In column (A) depths were inferrede other hand, depths in columns (B) and (C) were calculated taking into consideration a(B) a liquid column without suspended vapor bubbles is considered, whereas in (C) thento account.

epthresent surface)

Paleo-depth calculation(boiling level depth, m)

References

(A) (B) (C)Haas (1971) Canet et al. (2011)

00 200 200 310 Albinson et al. (2001)00 641 640 820 Orozco-Villaseñor (2014)25 175 160 260 Camprubí et al. (2001)50–300 700 690 870 Albinson et al. (2001)

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column, and 1690m considering the effect of suspended bubbles, beingthe last value very close to the sampling (well) depth (1700 m).

For the southern productive zone two wells were investigated: Az-23 and Az-16 (Table 1). In the former TH ranges between 220° and310 °C, TMi between −1.5° and −0.2 °C, and salinity between 0.3 and2.5 wt.% NaCl eq. Microthermometric data are available to a depth of

Fig. 4. Boiling point curves corresponding to a rising hydrothermal fluid whose tempera-ture and salinity conditions evolve from values given in red. Both curves (1) and (2) wereconstructed taking into consideration a progressive increase of salinity due to the steamloss during boiling, but in curve (1) depth was calculated considering a liquid column,without suspended vapor bubbles, whereas in curve (2) the effect of vapor bubbles onflu-id density and on hydrostatic pressure were considered. In contrast, the black curves rep-resent the ones proposed by Haas (1971) with constant salinity (0 wt.% NaCl) anddiscarding the effect of vapor bubbles. Bold dotted red lines represent the real depth ofsampling (under present surface) for each well, according to the initial values (Ti and sa-linity). (For interpretation of the references to color in this figure legend, the reader is re-ferred to the web version of this article.)

only 1700 m, where the only TH value above 300 °C is found; the max-imum and minimum salinity values are reached at the shallowest(400 m) and deepest (1700 m) sampling levels, respectively. The salin-ity–temperature plot shows a general cooling trend regular in relationto depth, although there is a strong grouping (and inversion) of THvalues for the depth interval of 600–1000 m. On the other hand, the in-terval of 1700–1275m fits to a boiling path, suggesting that fluid evolu-tion from 1700 m upwards (TH = 310 °C; salinity = 0.3 wt.% NaCl eq.)could be affected by boiling (Fig. 2).

For well Az-16 (southern productive zone), TH ranges between 208°and 335 °C, TMi between −1.2° and −0.4 °C, and salinity between 0.7and 2.2wt.%NaCl eq. In this case, temperature varieswith depth accord-ing to a constant geothermal gradient of ~6 °C per 100 m of depth. Incontrast, salinity has an irregular variation with depth, so that maxi-mum values (≥2.0 wt.% NaCl eq.) occur at 550 and 2100 m, and mini-mum values (≤1.0 wt.% NaCl eq.) at 1250 and 2450 m. Due to thiscomplex pattern of salinity variation, a trend of boiling can only bepointed out with confidence enough at the shallowest part of the sys-tem (from 550 m; TH = 223 °C; salinity = 2.0 wt.% NaCl eq.).

Thermal profiles and depth calculations in the southern productivezone were done for both wells Az-23 and Az-16 (Fig. 4). In the case ofwell Az-23, the starting data for the boiling model are TH = 310 °Cand salinity = 0.3 wt.% NaCl eq. The calculated depth considering a liq-uid column is 1250 m, and considering the effect of suspended bubblesis 1470 m. For the well Az-16 the data used are TH = 223 °C andsalinity=2.0wt.%NaCl eq.; the calculated depths considering a columnof only liquid and of liquid with suspended bubbles are 260 and 390m,respectively (Fig. 4).

4.2. Taxco mining district

Fluid inclusion microthermometric data of the El Cobre–Babiloniavein, Taxco mining district, is summarized in Table 2. TH varies withdepth from 162 °C at the surface to 289 °C in the deepest level(~400 m below present surface), describing a linear geotherm of~30 °C per 100 m of depth. TMi fluctuates between −10.4° and−4.4 °C, which corresponds to a range of salinity between 7.0 and14.3 wt.% NaCl eq. The variation of salinity with depth is not at all linear,as it peaks twice (salinity ≥12.0 wt.% NaCl eq.), at level 5 (~265 m) andbetween levels 1 and 2 (~65–130 m). The minimum salinity value(7.0 wt.% NaCl eq.) corresponds to the surface sample. Due to the com-plex pattern of salinity variation, which results in a high dispersion ofdata in the salinity–temperature plot (Fig. 3), a trend of boiling canonly be pointed outwith confidence at the shallowest part of the system(from sample N3-01; 200m below present surface). On the other hand,the salinity drop in the surface sample is consistent with a process ofmixing with cool and dilute (probably meteoric) fluids.

The starting data for determining the boiling point curves and for thepaleo-depth calculations are TH = 221 °C and salinity = 11.5 wt.% NaCleq. The depth thus calculated not considering and considering the effectof suspended bubbles on hydrostatic pressure is, respectively, 230 and360 m (Fig. 5).

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5. Discussion

An overall analysis of fluid inclusion data of Los Azufres allows twogeneralizations to be made, although large differences between thetwo productive zones and between contiguous wells can be observed:(a) high temperatures (N300 °C) are commonly attained at depths be-tween 1500 and 2000 m, and (b) salinity above 2.0 wt.% NaCl eq.occur within the upper ~500 m of the system. Taken together, thesepoints suggest that the geothermal zone is largely affected by boiling,which is in good agreement with previous studies (e.g., Iglesias et al.,1985; González-Partida et al., 2000). However, the variation patternsof both parameters with depth are complex; sharp fluctuations in salin-ity are not uncommon, nor are local temperature maxima at relativelyshallow levels (e.g., 322 °C at 1300 m in well Az-9). These are signs ofthe hydrological complexity of a geothermal system thatmay be associ-ated with lateral outflow. Under this scenario, selection of the startingdata for the variable salinity model should be made with special care;it is therefore important to be cautious about interpreting the boilingpoint curves.

In the case study of Los Azufres, thermal profiles were constructedfor three wells and the depths thus obtained are evaluated by compar-ing them by the sampling depth, which in this case can be taken asthe actual depth (Fig. 2). In all cases, calculated depths are closer to ac-tual depths if themodel considers the effect of suspended bubbles, as al-ready stated by Canet et al. (2011); therefore, depths modeled ignoringthis effect will not be discussed further.

For one of the three wells (Az-27A) there is a very good agreementbetween the calculated and the actual depth—1690 and 1700m, respec-tively, with closeness greater than 99%. For well Az-23, calculated andactual depths are 1470 and 1700m, respectively, which implies a differ-ence of ~14%, whereas for well Az-16 the underestimation of themodeldepth (390m)with respect to the actual depth (550m) ismuch greater(~30%). The large error obtained in the latter calculationmight be due to

Fig. 5. Boiling point curves corresponding to a rising hydrothermal fluid whose tempera-ture and salinity conditions evolve from values given in red. Both curves (1) and (2) wereconstructed taking into consideration a progressive increase of salinity due to the steamloss during boiling, but in curve (1) depth was calculated considering a liquid column,without suspended vapor bubbles, whereas in curve (2) the effect of vapor bubbles onfluid density and on the hydrostatic pressure were considered. In contrast, the blackcurve represents the one proposed by Haas (1971) with constant salinity (10 wt.% NaCl)and discarding the effect of vapor bubbles. Bold dotted red line represents the samplingdepth under present surface. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

the effect of CO2, whose concentration increases toward the top of thegeothermal system in Los Azufres (e.g., Santoyo et al., 1991). The occur-rence of CO2 in hydrothermal fluids increases the vapor pressure alongthe liquid–gas curve, thus lowering the boiling point temperature(Hedenquist and Henley, 1985b); therefore, at a given salinity andtemperature the boiling point is reached at greater depths than inthe absence of this gas (e.g., Simmons, 1991). CO2 can also affectmicrothermometry measurements, since the presence of this gas maylead to significant overestimation of apparent salinities (Hedenquistand Henley, 1985b); this can contribute to some extent to the underes-timation of depth (see Table S1 in supplementary files).

The values obtained using the boiling point curves of Haas (1971)were systematically lower than real ones, implying an underestimationof depth of ~15 to 50%. In all three cases examined at Los Azufres, theerror in depth calculations is higher with the curves of Haas (1971)than using the variable salinity model; it is worth mentioning thatonly by means of the latter the closest match between the calculatedand the actual depth was achieved (well Az-27A).

In the case of El Cobre–Babilonia epithermal vein, microthermometricdata describe a general linear trend of negative slope in the TH–salinityspace, but there are two values (N5-30 and S-04) that deviate significant-ly from this tendency (Fig. 3). This linear distribution of data is nearly con-sistentwith a boiling path, and the values that deviate from it likely reflectlocal mixingwith fluids of contrasting salinity. Therefore, the salinity var-iation from the deepest samples (N9-15 and N7-10) to N5-30 cannot beexplained in terms of boiling, but it probably accounts for the episodic in-jection of magmatic (?) brines. The influence of such fluids has alreadybeen noted in epithermal deposits in Mexico (e.g., Albinson et al., 2001;Camprubí et al., 2001, 2006b; Wilkinson et al., 2013). Also, the dramaticdrop of salinity indicated by the surface sample (S-04) reflects a dilutionprocess due to the influence of colder and low-salinity fluids, probablymeteoric, at the top of the hydrothermal system (cf. Hedenquist, 1991;Canet et al., 2011).

The paleo-depth of sample N3-01, which represents a level in theepithermal vein in which boiling started, is ~225 m using the boilingpoint curves of Haas (1971), whereas applying the variable salinitymodel a value ~60% greater, of 360 m, is obtained. The depth of thislevel (measured from the highest point of the current surface) isabout 200m; so, given that the age of the epithermal deposit is Late Eo-cene (36–39 Ma; Camprubí and Albinson, 2007), the depth estimationwithHaas (1971) curveswould imply only ~25mof erosion for a periodof almost 40Myr. This does not seem reasonable since the Taxco regionand the Sierra Madre del Sur have been deeply dissected by erosion,being more plausible an erosion of ~160 m that arises from the paleo-depth of 360 m obtained by the variable salinity model.

The El Cobre–Babilonia epithermal vein exhibits a vertical ore zoningin which Ag grades are higher between a depth of 200 m (mine level ofsample N3-01) and the present surface (Osterman, 1984; Hynes, 1999;Camprubí et al., 2006a). The bottom of the Ag-rich zone coincides withthe depth at which the TH–salinity boiling paths suggest that boilingstarted (Fig. 3). Similarly, in the Apacheta epithermal deposit (Mio-cene), Peru, there is a vertical zoning influenced by boiling, whichmarks the boundary between the base metal- and precious metal-richzones in a brecciated, boiling level (André-Mayer et al., 2002). Thepaleo-depth of this level was estimated of 580 m by André-Mayeret al. (2002) considering the hydrostatic pressure exerted by a liquidcolumn, and the sampling depth (below present surface) is about200m. Given that this calculationwas donewithout taking into accountthe effect of suspended bubbles on hydrostatic pressure, the paleo-depth could be actually significantly greater, so considerably morethan 400 m of the deposit could have been eroded.

According to the foregoing discussions, many paleo-depths assessedfrom fluid inclusion data could have been underestimated, so thedescriptivemodels for epithermal deposits could be incorrectly calibrat-ed in terms of depth. To assess the extent of this inaccuracy, wereviewed paleo-depth estimations for fourMexican epithermal deposits

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that are representative of intermediate to low sulfidation-dominatedstyles (Table 3). As expected, the paleo-depths calculated with the var-iable salinity model are systematically greater than those estimatedfrom the boiling point curves of Haas (1971), so that the differencebetween both varies from ~25 to 50%. In the case of the San Martínvein (Oligocene), the depth estimated using the curves of Haas(1971), of ~200 m, is unreliable because it coincides with the samplingdepth which, if true, would imply no erosion in almost 30 Myr. Paleo-depths calculated with the variable salinity model range from 260 min the La Guitarra deposit, to 870 m in Real de Catorce, so the positionof the boiling level varies greatly from one epithermal deposit to anoth-er. The depth of erosion based on these calculations is roughly consis-tent with the age of the deposits, being ~570 m for the Upper EoceneReal de Catorce deposit, ~130–330 m for the Lower Oligocene LaGuitarra and Veta Madre deposits, and ~110 m for the Upper OligoceneSan Martín deposit.

6. Conclusions

The variable salinity boiling model, which considers the increase insalinity due to steam loss during boiling and the effect of vapor bubbleson the hydrostatic pressure, can be used for determining the depth ofboiling from fluid inclusion data, both in active and fossil hydrothermalsystems. The occurrence of CO2 in hydrothermal fluids, however, maysignificantly decrease the accuracy of depth calculations.

In Los Azufres geothermal zone, the depths calculated in this way ingeneral are close to actual depths, with an accuracy greater than 99% forone case; however, a considerably large error (30%) is obtained to thetop of the geothermal system due to enhanced CO2 concentrations.The values estimated directly from boiling point curves at constant sa-linity, and discarding the effect of vapor bubbles, were systematicallylower than actual ones, which imply underestimations of depth at upto ~50%; the error with such procedure was always higher than byusing the variable salinity method.

In the case of the Eocene El Cobre–Babilonia epithermal vein boilingoccurred from a paleo-depth of 360 m, which corresponds to a current(sampling) depth of about 200m; this corresponds to the boundary be-tween the basemetal- and the silver-rich zones. In addition, the episod-ic entrainment of magmatic brines and meteoric fluids in the deeperpart of the mineralized structure and near the current surface, respec-tively, is suggested by fluid inclusion data that deviate significantlyfrom the general TH–salinity boiling path.

As depth determinations for epithermal deposits are generallyunderestimated, descriptive models for such deposits could be incor-rectly calibrated in terms of depth. Paleo-depths have been traditionallyestimated by plotting microthermometric data on boiling point curvesof constant salinity and discarding the effect of vapor bubbles, butsuch determinations should be revised and corrected. An effective wayto do so can be by applying the variable salinity model presented inthis paper.

Acknowledgments

Fluid inclusion microthermometry analyses were carried out inCentro de Geociencias, UNAM. The first author was supported with agrant from the UNAM's Instituto de Geofísica scholarship program. A.Camprubí wishes to thank CONACYT for economical support throughgrant 155662. We thank F. Pirajno, S. Simmons and two anonymous re-viewers for helpful comments on the manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.oregeorev.2015.08.016.

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