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Chemical Geology

journal homepage: www.elsevier.com/locate/chemgeo

Chemical and isotopic evolution of groundwater through the active Andeanarc of Northern Chile

L.V. Godfreya,⁎, C. Herrerab, C. Gamboab, R. Mathurc

a Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USAbDepartamento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chilec Department of Geology, Juniata College, Huntingdon, PA, USA

A R T I C L E I N F O

Editor: Donald Porcelli

Keywords:Lithium isotopesBoron isotopesStrontium isotopesWater-rock interactionsNorthern ChileVolcanic arc

A B S T R A C T

The role of rock-water reactions, leakage of sedimentary brine, evaporite dissolution and variations in lithologyon the chemical and isotopic evolution of groundwater draining the active arc and Preandean basins in theAntofagasta Region of northern Chile has been investigated using Li, B and Sr isotopes. The studied basin is thatof the upper Loa river and its sub-basins, which is adjacent to the basin occupied by the Salar de Atacama whereLi-rich brines are commercially exploited. Due to the arid climate, water draining this part of the Andes does soprimarily through the subsurface. Water from a thermal source is a common component of water sampled, but itsgeochemical signature evolves through multiple processes including dilution, cold rock-water reactions, influxesof sedimentary brine and evaporite mineral dissolution. These processes lead to a range of Li and B con-centrations in groundwater (0.01–14mg/L and 0.18–20mg/L) and isotope compositions (−1 to +12‰ and −6to +14‰). The effect of rock dissolution on δ7Li and δ11B within aquifers is apparent from their low values, buteven in streams these values only undergo small increases relative to other rivers worldwide. This may be due tothe weathering of ignimbrites producing water with less Al and limited clay formation, the large reservoir ofthermally-sourced Li and B of which only a small component is removed, and because the streams acquiregroundwater through their beds. The wider range in δ11B is a result of variable lithology and dissolution ofevaporite minerals, which have lesser effects on δ7Li.

Our multi-isotope approach highlights the role of lithology and different processes in defining groundwaterchemistry. Isotopic signatures derived from marine sediments are recognizable in some springs even though thistype of rock has nearly no surface exposure due to extensive volcanic rock cover, and are acquired directly or viaincorporation of these sediments into volcanic systems. These high-Cl rocks can weather to produce water with achemical composition like that of sedimentary brine but is distinguishable through δ11B composition.Combination of 87Sr/86Sr, δ7Li and δ11B also indicate that mixed salts which contribute to groundwater drainingthe ignimbrite covered areas of the basin did not form following cold temperature weathering, but from waterwhere thermal contributions dominated solute budgets keeping δ7Li low, and Li/Cl and B/Cl high compared toother fluid reservoirs such as seawater. We propose that these saline systems lay within the confines of one of themultiple late-Miocene-Pliocene calderas following the active stage of volcanism and while heat gradients re-mained high.

1. Introduction

The objective of this study is to determine the source and evolutionof solutes in groundwater draining the volcanically active Andean arc ofnorthern Chile where climate is arid. Weathering of volcanic and se-dimentary-evaporitic rocks contributes the majority of the solute load,while inputs of airborne desert dust only affects the most dilute waters(Risacher et al., 2003; Boschetti et al., 2007; Rissmann et al., 2015;

Herrera et al., 2016). Chemical and isotopic characteristics combinedwith volcanic activity and high heat input means that many ground andsurface waters can be identified as thermal or mix with thermal water(Aravena and Suzuki, 1990; Risacher et al., 2011; Rissmann et al.,2015). The arid climate and dominance of closed basin morphology hasproduced an abundance of salt pans (salars) which contain evaporiticsalts and brine, which can be enriched in lithium and boron. Of theevaporitic salts which have formed in the past, only gypsum is recycled,

https://doi.org/10.1016/j.chemgeo.2019.04.011Received 6 July 2018; Received in revised form 1 April 2019; Accepted 8 April 2019

⁎ Corresponding author.E-mail address: godfrey@marine.rutgers.edu (L.V. Godfrey).

Chemical Geology 518 (2019) 32–44

Available online 15 April 20190009-2541/ © 2019 Elsevier B.V. All rights reserved.

T

suggesting that brine has leaked from these basins into regionalgroundwater flow (Risacher et al., 2003; Rissmann et al., 2015; Herreraet al., 2016). The thick (~1 km) halite fill of the Salar de Atacama(Jordan et al., 2002), suggests this basin is the discharge point of manygroundwater flow paths, and here Li concentrations in the brine canexceed 5000mg/L (Munk et al., 2018). With the increase in globaldemand for lithium, a better understanding of the processes whichoccur in the weathering and transport of solutes is needed.

Lithium and boron are elements whose elevated concentrations ingroundwater have been used as tracers of hydrothermal activity (Ellisand Mahon, 1964; Giggenbach and Soto, 1992; Fouillac and Michard,1981). However, Li and B are also considered to be somewhat con-servative during evaporation leading to high concentrations in sedi-mentary brine (McCaffrey et al., 1987). While these two mechanismsfor raising Li and B concentrations are very different, both can operatein volcanically active areas under dry climate regimes, such as the ac-tive Andean arc of northern Chile. Use of isotopes (δ7Li and δ11B) hasdone much to elucidate their behavior during high temperature water-rock reactions and evaporation. The temperature dependence of isotopefractionation causes the extraction of Li and B from rocks at hightemperature with transfer of the rock isotopic composition to solution(Millot and Négrel, 2007; Millot et al., 2007). While changes in the Liisotope composition of thermal waters was found to be related to re-servoir temperatures based on other chemical geothermometers (Millotand Négrel, 2007; Millot et al., 2007), they found that the isotopicheterogeneity of lithology made temperature a secondary effect fordefining δ11B. At the low temperatures typical of most surface en-vironments, Li and B are incorporated into secondary minerals whichinvolves an increase in solution phase δ7Li and δ11B. The global dis-charge-weighted average riverine flux of Li has a concentration of1.5 μg/L and δ7Li of +23‰ (Huh et al., 1998), while that of B is 10 μg/L with δ11B of +10‰ (Lemarchand et al., 2002).

The influence of evaporation on solution δ11B and δ7Li has been lesswell studied. The assemblage of minerals formed during evaporationcan have a strong influence on the isotope fractionation of the eva-porating water. Precipitation of halite appears to occur without isotopefractionation for both Li and B (Godfrey et al., 2013; Paris et al., 2010),but minerals formed earlier in the evaporation sequence do. During theevaporation of seawater and continental water increases up to 30‰have been reported for δ11B (Vengosh et al., 1992) due to the pre-ferential occupation of 10B in tetrahedral sites and incorporation intocarbonate and certain borates (Palmer and Helvaci, 1997). The role ofthese early precipitating minerals on fractionating Li isotopes has yet tobe rigorously determined. While inorganically formed calcite frac-tionates Li by −2 to −5‰ and aragonite by −11‰ (Marriott et al.,2004) these minerals have low distribution coefficients towards Li andincreases in solution δ7Li are expected to be minimal.

We aim, by using a multi-isotope approach, to distinguish betweenthe different processes that weather rocks of variable lithology and thatcombine to influence groundwater chemistry in this region. These in-clude low temperature silicate weathering, hydrothermal reactions,evaporation and evaporite dissolution (Ericksen, 1981; Risacher et al.,2003, 2011; Risacher and Fritz, 2009; Rissmann et al., 2015).

Our study area is the Western Cordillera of the Andes in theAntofagasta Region of Chile on the eastern edge of the Atacama Desert.The low mean annual precipitation limits dilution by meteoric water sothat the geochemical and isotopic compositions of groundwater andgroundwater fed streams strongly reflect rock-water interactions in thesubsurface. Aridity also limits the development of vegetation, so thatmeteoric water infiltrating from the surface is not made as acidic byrespired CO2 or organic acids, reducing the aggressiveness of meteoricwaters towards silicate weathering. The aridity, persistent for much ofthe Neogene, and particularly severe since the early Holocene, hasproduced many evaporite beds and brine which occupy salt pans(salars). By dissolution or dilution, they contribute substantially to so-lute loads draining the segment of the Andes adjacent to the Atacama

Desert (Risacher et al., 2003). The solute load of surface and ground-water may originate from low temperature weathering and recyclingthrough salars (Risacher and Fritz, 2009) or high temperature weath-ering and volcanic sources (Rissmann et al., 2015). We aim to addressthese different mechanisms using isotope tracers.

2. Geological and hydrological description of the study area

2.1. Geology of Western Cordillera of the Andes

The study area focuses on a pre-Andean basin which occupies thearea between the crest of the Western Cordillera of the Andes and theDomeyko range which forms the Precordillera. In the upland area, theclimate is arid to semi-arid, but at lower elevations to the east it tran-sitions to hyper-arid. Mean annual precipitation varies from<50mmat elevations below 2500masl to> 150mm above 4000masl (Houston,2007); mean annual temperature ranges are> 15 °C and<4 °C at thesame elevations. The basin of interest is that of the upper Loa River,which drains to the ocean. To its south, and formed within virtuallygeologically identical units, is the internally drained Salar de Atacamabasin, the host of lithium rich brine.

The elevation of the magmatically active Western Cordilleraaverages 3800–4500masl and locally reaches 6350masl. The volcanoesunconformably overlie a complex basement consisting of earlyPaleozoic crystalline basement rocks and Mesozoic sedimentary, vol-canic and plutonic rocks. Volcanic rocks consist of andesitic and daciticlavas and voluminous, but compositionally homogenous calc-alkalinedacitic and minor rhyolitic and andesitic ignimbrites which eruptedbetween the late Miocene and mid-Pleistocene to form one of the lar-gest and youngest silicic volcanic fields worldwide. These eruptionsoccurred within the Altiplano-Puna Volcanic Complex (APVC) whichoverlies the Altiplano-Puna Magmatic Body, some 200 km in diameterand 11 km thick (Ward et al., 2014). There are numerous endorheicbasins in the Central Andes filled with alluvial material and at theirtopographic low points, salars.

Immediately to the west of the Western Cordillera of the Andes arethe Preandean basins followed by the Precordillera, the CordilleraDomeyko. The Cordillera Domeyko is a N-S trending basement blockcomprised of fault bounded Paleozoic, Mesozoic and Cenozoic crystal-line rocks and sediments, and the host of Eocene-aged giant copperporphyry deposits. Sedimentation in the Preandean basins over much ofthe Cenozoic has consisted of siliciclastics, evaporites and minor car-bonate units (Dingman, 1967; Blanco and Tomlinson, 2009; Jordanet al., 2007; Muñoz et al., 2002; Tomlinson et al., 2010; Evanstar et al.,2015). Differences between basins reflect their drainage: the Salar deAtacama Basin has been internally drainage since the Paleogene andcontains a thick accumulation of halite, but halite is absent in the UpperLoa groundwater basin which drains to the Pacific (May et al., 1999;Jordan et al., 2007).

Paleozoic sediments and volcanic/plutonic rocks outcrop to thewest of the upper Loa groundwater basin in the Precordillera and in afew localities within the arc itself. In the southern part of the Upper Loabasin are Permo-Triassic plutonic rocks as well as siliciclastic sedi-ments, sometimes intercalated with volcanic rocks of Triassic andCretaceous age (Marinović and Lahsen, 1984). These rocks form part ofthe topographic high which separates the Loa drainage basin and theSalar de Atacama basin (Marinović and Lahsen, 1984). The CretaceousLomas Negras Formation is mostly comprised of siliciclastic sedimentsbut bears carbonate sands with marine fossils (Marinović and Lahsen,1984). Its outcrop is limited to the eastern end of the Turi Basin im-mediately west of the El Tatio geothermal field. East of the westerncrest of the Andes are Early Late Cretaceous continental sediments ofthe Salta Rift System in Argentina and southern Bolivia, separated fromcontemporaneous eastern Chilean depocenters by the Huaytiquina Highbasement massif (Mpodozis et al., 2005). During the Late Cretaceous-early Cenozoic, transgression of the El Molino – Yacoraite Sea over

L.V. Godfrey, et al. Chemical Geology 518 (2019) 32–44

33

northwestern Argentina left limestone and sandstone sediments(Mpodozis et al., 2005). Any connection between the Cretaceous – earlyCenozoic basins of northern Chile and the shallow marine systems tothe east are obscured by volcanism of the western Cordillera. TheLomas Negras Formation, as well as a marine limestone protolith ofxenoliths in the Lascar volcano bordering the Salar de Atacama(Matthews et al., 1996) suggest that these marine sediments may reachas far west as the crest of the active Western Cordillera.

Volcanism in the Western Cordillera has produced stratovolcanoesand massive calderas, the source of extensive and thick ignimbrite unitswhich create the ignimbrite plateau of the central Andes. Orthogonal tothe roughly N-S strike of the Andean crest are lines of volcanoes whichform sub basins filled with volcano-clastic deposits, ignimbrites andtuff, as well as recent salar deposits and mud (Houston, 2007). Thesebasins have surface drainage to the Loa River, whereas east of the Salarde Atacama basin they are small and endorheic. Although topo-graphically separated, these endorheic basins may be hydraulicallyconnected at depth, and leakage from these basins can profoundly in-fluence the geochemistry of groundwater and may obscure geochemicalsignatures of geothermal activity and primary low temperature silicateweathering (Risacher et al., 2003, 2011; Risacher and Fritz, 2009).

The surface consists of gavels and alluvial material where claysminerals can be notably absent (Sun et al., 2018). Where soils exist theyare gypsic in the drier low elevation areas, but older paleosols are calcicVertisols or calcic Argillisols (Rech et al., 2006). All soils containmontmorrilonite and illite in addition to oxides, calcite or gypsum. Thevolcanic units have alteration zones with smectite, illite and chlorite(Maza et al., 2018). Erosion of the volcanic summits has exposed hy-drothermal alteration including kaolinite plus elemental sulfur.

2.2. Groundwater flow

Information regarding groundwater flow has been compiled fromdata published by the largest mining companies and inferred betweenthese points because large parts of the basin aquifers cannot be accessedfor observation. Additional understanding is gained from the hydro-geology of the springs in the Western Cordillera and Precordillera todetermine the geological units though which groundwater flows.Overall, groundwater flow is to the west through fractured volcanicaquifers, unconsolidated sediments and alluvial fans. Streams aregroundwater fed. Within the arc, shallow volcanic aquifers overlie anancient erosion surface which may direct flow. Groundwater flow isalso determined by tectonic structures, which can act as barriers orfocus it. Examples of structure-related barriers are horsts and reversefaults which form wetlands such as the Vegas de Turi on the north sideof the Salado River and parts of the San Pedro basin (Houston, 2007).Folds also provide barriers and redirect flow, for example the N-S ridgesof the Peine Formation (Kuhn, 2002) prevent direct east to west flow ofgroundwater towards the southwest sector of the Salar de Atacama.Within the Calama Basin groundwater flow has been described in detailby Jordan et al. (2015) and is predominantly to the southwest. Wheresedimentary units thin across the ChiuChiu monocline, formed by dis-solution of underlying gypsic beds (Blanco and Tomlinson, 2009), theresulting constriction may limit water from the Salado catchmentflowing directly into the basin aquifers (Jordan et al., 2015).

3. Samples and methods

Samples were collected from springs, groundwater fed rivers andwells (Fig. 1). Sampling employed plastic syringes, peristaltic pumpsand silicone tubing or in-situ submersible pumps. All samples werefiltered to<0.4 μm in the field and stored in new, acid cleaned poly-ethylene bottles. Samples were not acidified. Temperature and pH weremeasured in the field, and alkalinity on the same day by acid titrationafter being stored in glass bottles or by a field checker (Hanna Instru-ments). The distribution of different carbon species and mineral

saturation states were calculated using the GWB software (Bethke,2011) which incorporates major ion concentrations into the calculation.

Cations were measured at Rutgers University. Major cations, Si, Liand Sr were measured using a Varian ICP-OES; Br, B and Rb con-centrations were measured using a Thermo iCAP-Q at RutgersUniversity using a Teflon introduction system with trace HF in thecarrier acid. Concentrations were determined using synthetic standardmixtures and accuracy checked with certified standards and seawater.Chloride and sulfate were measured at Juniata College by ion chro-matography with each reported value representing the mean of 3 dif-ferent measurements (the 2σ variation of the samples is< 3% of thereported value).

Strontium isotopes were measured following chromatographic se-paration using AGW50x8 or Sr-spec resin using either a VG Sector 54TIMS at Cornell University or a ThermoScientific Neptune Plus MC-ICPMS at Rutgers University. Measurement of NBS 987 was includedwith all analyses, and average 0.710245 ± 0.000006 (2σ, n= 40) byTIMS or 0.710270 ± 0.000004 (2σ, n= 13) by MC-ICPMS. Analysesby MC-ICPMS were then corrected based on those values of NBS 987values to match those of the TIMS instrument. Lithium isotopes were allanalyzed at Rutgers University by MC-ICPMS following separationusing two cation exchange (AGW50x12) columns 200–400 mesh (7mLresin bed followed by 0.2 mL resin bed) using 0.5 N HCl. A sample ofseawater was included with all sample batches and separated usingdifferent column pairs to ensure that yields were quantitative. Lithiumsolutions were analyzed in solutions of 20 ppb concentration, which,utilizing a Teledyne Cetac Aridus II and X-type Ni skimmer cone,yielded a signal exceeding 18 V on 7Li with an acid blank of< 150mV.The high sensitivity of the instrument allows small samples to be pro-cessed though ion chromatography which enables column calibrationsto be stable. Standard-sample-standard (L-SVEC) bracketing was used,matching signals to within 5%. A solution of IRMM-016 was included ineach batch of analyses, and the long-term δ7Li average of that, andseawater, were 0.1‰ ± 0.07 (2σ, n= 25) and +30.6‰ ± 0.12 (2σ,n= 18). Measurement of boron isotopes followed separation using themicro-sublimation technique described by Wang et al. (2010). Sampleswere run in duplicate and on the rare occasion (1 in 40 samples) when aduplicate gave δ11B which differed by>0.5‰, the sample was re-analyzed in duplicate. A seawater sample was included in every sub-limation batch. Measurement of δ11B was made with the ThermoNeptune Plus at Rutgers University using a PFA-barrel or PFA micro-concentric spray chamber (ESI) and diluted in a solution of 2% HNO3

and 0.1% HF. The signal on 11B was about 350mV for a 30 ppb solu-tion, the acid blank was< 2.5mV. Sample-standard bracketing wasused with NIST 951a. A sample of Sargasso seawater was included inthe micro-sublimation step with each sample batch, and solutions of theBAM standards AE120 and AE122, as well as an acid blank, were runperiodically throughout each analysis. Standards AE120 and AE122yielded −19.9‰ ± 0.08 (2σ, n= 13); +39.5‰ ± 0.14 (2σ, n= 13),and Sargasso seawater, +40.3‰ ± 0.14 (n= 11).

4. Results

Locations, field measurement, concentration and isotope data arelisted in Table 1.

4.1. pH, major and minor ion concentrations

The pH range is between 6 and 8.5, but most samples are neutral toslightly alkaline, averaging 7.7 ± 0.6. Dissolved inorganic carbon(DIC) is high and varies between 1 and 10meq/L. The range of partialpressures of CO2 with which water is in equilibrium is wider, 10–3.5 to10–0.8 atm. Samples with the highest pCO2 and lowest pH were col-lected from a spring system near the crest of the Altiplano and fromflowing wells and springs at the eastern end of the Calama basin.Important contributions of magmatic CO2 are expected from samples

L.V. Godfrey, et al. Chemical Geology 518 (2019) 32–44

34

located close to the volcanic front but carbonate dissolution by wateracidified by reactions with volcanic gases, such as CO2, SO2 and HCl isanother important way in which CO2 becomes supersaturated relativeto the atmosphere.

The major cation and anion compositions of ground and surfacewater produce two trends on a Piper diagram (Fig. 2, which includesdata from the DGA (Direccíon General de Aguas) for nearby basins;Risacher et al., 1999). End-member (IG) is dominated by (Na+K) and(Cl+ SO4), with Cl being the dominant anion. Geothermal fluids fromEl Tatio are coincident with this point, and data from the Salado andCaspana rivers, and the Chitor spring plot close by. These samples comefrom areas of the drainage basin dominated by ignimbrites. The othertwo end-members (AND and ALT) get 60–70% of their positive chargefrom (Ca+Mg) but differ in their carbonate - sulfate ratio. End-

member ALT has a higher proportion of carbonate than AND. The Silalastream plots closest to end member ‘ALT’, and the Paniri spring to endmember ‘AND’. The samples which plot between ‘IG’ and ‘ALT’ are twoof the wells in the eastern Calama Basin, spring water at Chitor andLinzor 1. Samples which to plot towards ‘AND’ are Loa river water nearthe Mino volcano, Ojo San Pedro, one well in the western Calama Basin,and Baños de Turi. Stratovolcanoes are dominant in the landscapearound these sample locations. Two outlying samples are the spring atthe base of Volcan Chela and Linzor 2. Chela has low HCO3

– and Mg,and Linzor 2 has high HCO3

– and is distinguished from end-memberALT by high Na+K.

Silica concentrations are between 27 and 50mg/L. The sampleobtained from the Tatio geothermal field has a much higher value,94mg/L. Bromide concentrations vary between 0.01mg/L and 12mg/

a

b

Fig. 1. (A) Oblique topographic map of the Loa Basin showing the association of the PreAndean basins occupied by the Upper Loa River and the Salar de Atacama andtheir position relative to the Altiplano and western Cordillera of the Andes. Semi-transparent ovals show the approximate locations of late Miocene – Pliocenecalderas. The inset map shows country borders. (B) Geological map with sample locations.

L.V. Godfrey, et al. Chemical Geology 518 (2019) 32–44

35

Table1

Con

centration

andisotop

eda

ta.U

TMco

ordina

tesWGS8

4-19

K.C

oncentration

unitsaremg/

L.Sr

isotop

eda

tano

rmalized

toNIST98

7=

0.71

024,

Liisotop

eda

tarelative

toLS

VEC

,Bisotop

eda

tarelative

toNIST95

1.

UTM

(E)

UTM

(S)

Elev

ation(m

asl)

pHHCO3

Ca

KMg

Na

Cl

SO4

BrSi

SrRb

87Sr/8

6Sr

Liδ7Li

Bδ1

1B

Upp

erLo

agrou

ndwater

basin

Loaat

VMiño

5353

9976

5865

838

108.20

142

4514

2010

391

186

0.43

35.5

0.05

0.70

636

0.6

5.1

3.7

6.1

Loaat

Taira

5419

6475

8356

532

707.05

351

5125

5631

655

425

21.41

31.8

0.07

0.70

591

0.7

5.3

13.8

10.6

Loaat

SantaBa

rbara

5402

0675

6982

130

708.00

3720

4823

342

335

21.02

36.1

0.90

0.06

0.70

589

0.8

5.9

9.7

9.1

Loaat

Con

chi

5404

9275

6884

530

308.00

4027

8335

661

440

71.04

39.0

1.85

0.09

0.70

606

0.7

7.9

9.3

9.4

Che

la54

3891

7639

944

3880

7.60

59.4

8112

1091

179

101

0.29

29.3

0.78

0.07

0.70

512

0.1

6.4

4.5

7.7

Loaat

Ang

ostura

5271

0775

1640

424

708.02

114

7598

1071

1845

361

0.8

5.38

0.33

0.70

743

4.0

1.5

16.8

−1.0

Salado

water

5453

4775

3013

826

308.20

400

5668

5478

414

9240

1.4

43.9

3.89

0.33

0.70

763

4.2

1.2

11.9

−4.0

ElTa

tio

6016

3875

2987

142

807.06

8414

523

34

2180

3886

403.2

94.3

2.30

0.70

900

20.0

−0.5

88.0

−5.4

Vd.

Barrostream

6016

3175

2575

043

708.22

499

1919

6513

90.01

230

.80.30

0.04

0.70

897

0.0

10.1

0.2

−5.3

Silala

atInacaliri

5966

2275

6389

341

408.50

113

185

823

3911

0.02

527

.50.10

0.02

0.70

816

0.1

8.8

0.3

−3.1

Baño

sde

Turi

5741

3975

4070

430

907.90

401

117

4546

432

733

251

0.12

646

.23.47

0.21

0.70

779

1.3

0.0

3.7

−2.9

Paniri

5751

2675

5063

532

658.20

243

513

1433

135

120.23

839

.70.41

0.00

0.70

716

0.0

1.8

0.4

13.4

Caspa

na57

8953

7533

247

3170

8.20

383.3

4136

4178

912

2931

20.12

627

.31.62

0.17

0.70

791

0.5

4.1

1.7

2.6

Linz

or1

6006

6775

4144

641

057.53

70.4

1011

570

148

290.06

933

.00.10

0.04

0.70

891

0.4

0.6

1.6

−6.1

Linz

or2

""

4105

7.56

346.5

1110

566

101

490.06

34.6

0.08

0.04

0.70

888

0.3

0.7

1.5

−5.6

Chitorspring

5852

0475

2082

237

706.01

603.7

124

1666

803

948

270

0.03

153

.61.19

0.10

0.70

827

0.2

2.5

0.8

−6.9

Chitorstream

""

3740

8.28

550.5

117

1564

815

1160

960.03

449

.91.18

0.10

0.70

826

0.1

3.0

0.8

−7.0

Ang

ostura

spring

5275

7875

1651

824

756.62

557

118

6195

1058

1940

201

0.8

39.3

5.17

0.30

0.70

743

3.7

1.1

14.9

−1.0

Ojo

SanPe

dro

5666

2675

6956

938

007.15

329

8217

5417

118

529

10.15

48.5

0.58

0.07

0.70

783

0.3

2.1

2.9

1.0

PCAR7C

5085

0475

2080

026

406.86

564

112

3035

340

448

143

0.3

40.5

1.27

0.09

0.70

625

1.4

5.1

3.9

−0.9

TB05

5330

6275

3470

526

306.65

567

8322

5821

232

377

43.2

1.10

0.01

0.70

642

0.3

2.8

1.7

Si37

E53

3000

7532

628

2610

6.90

618

184

2057

270

654

770.59

27.6

1.27

0.02

0.70

587

0.7

6.2

2.5

2.2

Precordille

raChu

gChu

g48

4593

7554

130

2170

7.71

77.2

587

234

1919

16,227

18,222

18,312

11.2

0.7

1.68

0.70

822

1.0

10.3

76.1

22.5

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L, the lowest concentrations were measured in samples collected in theTuri Basin and at Chitor, the highest measured at ChugChug. The lowestCl/Br occur in the headwaters of the Loa, and the highest in some of thesprings in the upper Salado basin. The concentration of Li varies be-tween 0.01mg/L and 20mg/L, though it is only El Tatio water whichcontains> 4.2mg/L Li. The concentration of B varies between 0.2 mg/L and 88mg/L, only El Tatio water contains more, 17mg/L B.

4.2. Sr, Li and B Isotopes

87Sr/86Sr varies between 0.705 and 0.709. The samples which havethe lowest 87Sr/86Sr (< 0.706) are upstream of the confluence with theoutlet from San Pedro basin plus one of the flowing wells in the east endof the Calama basin. The samples with the most radiogenic Sr are closeto the crest of the Altiplano: El Tatio, the stream near Volcan de Barro,and Linzor. Groundwater at ChugChug, which drains Upper Jurassicmarine limestone has 87Sr/86Sr of 0.70822, slightly more radiogenicthan the seawater Sr isotope curve for this period, indicating minorinfluence of more radiogenic silicate rocks of the Cordillera Domeyko inweathering processes within its catchment.

δ7Li varies between −0.5‰ and +10.1‰. El Tatio has the lowestδ7Li followed by samples located near it or associated with the SanPedro-Linzor volcanic chain. Samples where δ7Li exceeds +6‰, andwhich tend to have the lowest Li concentration are surface water andgroundwater in the eastern Calama Basin. The range in δ11B is −7.0‰to +13.4‰. The lowest values occur in samples close to the Altiplano,and the highest in the Río Loa upstream of its union with the RíoSalado, and the Paniri spring. The Precordillera spring ChugChug hasδ7Li of +10.3‰, and δ11B of +22.5‰.

5. Discussion

The importance of evaporation in affecting the major elementgeochemistry is clear when brine data for nearby closed basins reportedby Risacher et al. (1999) are included in the Piper diagram (Fig. 2).These data plot between mixing line IG-AND and the axis where

HCO3–+CO3

2– is virtually zero. Brackish and saline water collectedclose to salt pans plot along this axis because carbonates are one of thefirst minerals to form as water evaporates, while SO4

2− and Cl− ac-cumulate. Fluctuating lake levels can allow meteoric and shallowgroundwater to wash over and dissolve sulfates and halite formedearlier, contributing SO4

2− and Cl−. Whether this type of evaporated orsalt-bearing water characterizes all water draining the arc is less clear.Data for groundwater in the MNT (Monturaqui-Negrillar-Tilopozo)aquifer at the south end of the Salar de Atacama collected from wellsbelonging to BHP-Billiton (Rissmann et al., 2015) plot between ANDand ALT, unlike data for nearby surface water reported by Risacheret al. (2003) which trend towards endmember IG. The differences couldreflect different sampling between studies, with a water that has dis-solved evaporites being more predominant in the Risacher et al. (1999)study. However, the role of geothermal water in defining waterchemistry is also unclear because the three geothermal fields within thestudy area are chemically diverse. In the case of El Tatio, its very highCl content could reflect a magmatic brine, or its composition is influ-enced by leaching of evaporite beds (Giggenbach, 1978; Cortecci et al.,2005). In comparison, water is dominated by SO4 at Pampa Apachetaand water from Torta de Tocorpuri is HCO3

– rich (Tassi et al., 2010).Consequently, the contribution of evaporated saline water versus geo-thermal water to groundwater is not resolved.

5.1. A model of water sources based on soluble elements Li, B and Br

Li and B are elements which are enriched in hydrothermal waterand evaporitic brine, and this makes it difficult to assign their enrich-ment to a single process (Risacher et al., 2011). Ternary diagrams of Cl,B and Li have been used to distinguish geothermal water from per-ipheral groundwater, and to assess their maturity (Giggenbach andSoto, 1992; Cortecci et al., 2005), but the presence of Cl from evensmall contributions by inputs of sedimentary brine or salt dissolutionmakes this type of plot very complex. Even on the Cl-SO4

2—tAlk plot(Fig. 2), the field for mature Na-Cl hot springs (Giggenbach, 1988), andthe acidic thermal Tatio brine in particular (Giggenbach, 1978), is

Fig. 2. Piper diagram. Additional samples(Giggenbach, 1978; Risacher et al., 1999; Tassiet al., 2010; Rissmann et al., 2015) plotted forMonturaqui-Negrillar-Tilopozo (MNT) system graytriangles; Tuyaito (TUY), Capur (CPR) and AguaCalientes 3 (AC3), gray hourglass, Laguna Miscanti(MISC) and Laguna Lejía (LL), gray diamond; TortaTocorpuri, Pampa Apacheta Puritama, open graystar, El Tatio, black star; Tatio brine black opencircle.

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consistent with any cold water which might have encountered eva-porite minerals, and halite in particular. Steam heated thermal water iseither enriched in S or C, for example some water at El Tatio, Apachetaand possibly Torta de Tocporpuri, which places these systems close tothe axis linking the SO4 and tAlk end-members.

Lithium concentrations have been noted to increase with watertemperatures, with concentration and Na/Li ratio reflecting the rockreservoir. Lithium concentrations relative to major elements are slightlylower than the rock because Li is taken into secondary quartz whiletemperatures remain above ~200 °C (Giggenbach and Soto, 1992). Aswaters cool further, Li is incorporated into secondary minerals andespecially into clays with octahedral sites (Fouillac and Michard, 1981).Boron is enriched in young geothermal systems by dissolution ofmagmatic vapor (Smith et al., 1987) but is depleted through boiling,adsorption to clay or precipitation of borate minerals (Giggenbach andSoto, 1992; Chong et al., 2000). As a result of these removal processes,mature geothermal systems become Cl dominated and reach neutral pHas they equilibrate with surrounding rock (Giggenbach, 1988). Asgeothermal water migrates away from the heat source it mixes withmeteoric dominated groundwater which reacted with rock at lowtemperatures. In this area of the Andes, groundwater may also bebrackish to saline because surface water has dissolved surface eva-poritic salts before infiltrating or contains a component of brine whichleaked through the bottom of saline lakes (Risacher and Fritz, 2009).These processes can dramatically alter compositions and obscure che-mical indicators of geothermal systems, especially those which mayexist at depth with no surface expression.

Li and B concentrations increase with Cl concentration, but becauseof potential contributions from salts or brine and processes that removeLi and B to clay minerals, there is considerable variation in their Li/Cland B/Cl ratios (Fig. 3A and B). Bromine is a conservative elementwhich has been used to identify salt and brine contributions to water inthe central Andes among other places (Holser, 1966; Risacher et al.,2003). It has also been used with Cl to identify halite-bearing marinesediments as halide sources to the Precordillera porphyries of the Loadistrict (Arcuri and Brimhall, 2003). In water draining the arc, in theupper Loa drainage basin, the variation in Cl/Br (Fig. 3C) is more thandouble than for Cl/Li or Cl/B (a factor of 150 compared to 40 and 60).Like Br, Li is concentrated in the brine relative to halite by a factor of 10(Holser, 1979; Godfrey et al., 2013), so precipitation or dissolution ofhalite should not redistribute Li relative to Br. The difference in therelationships of these three elements with Cl could reflect greatervariability in Cl/Br ratios of rocks, emissions to the atmosphere fromwater specific to Br and not Li or B (Risacher et al., 2006) or additionsof Li and B which reduce their concentration variations relative to Cland Br. Thermal water at El Tatio, Apacheta and Torta de Tocorpurihave high Li and B compared to Br when normalized against Cl, so it ispossible that inputs of thermal water to groundwater reduces the var-iation in Li/Cl and B/Cl, but not Br/Cl. Low Br relative to Li and B whenCl concentrations are high providing evidence that sedimentary brinesare the source of Cl because Br is lost to the atmosphere during eva-poration at the elevations of the Altiplano (Risacher et al., 2006) re-quires further study.

In a geochemically complex environment of the arid western foot-hills of the volcanically active Andes, and similar basins worldwide,there are too many processes that will influence elemental distributions.Even using elements that are often considered soluble and conservative,chemical compositions alone do not distinguish whether groundwatercarries a signature of geothermal systems, or is affected by dissolutionof evaporites, leakage of brine from other basins, or weathering ofmarine sediments. The use of isotopes is explored to distinguish be-tween these different scenarios.

5.2. Isotopic composition of solute sources to surface and groundwater

The western slope of the Andes is dominated by stratovolcanoes

which sit atop widespread thick ignimbrite sheets erupted during thelate Miocene-Pliocene volcanic flare-up in the Central Andes. Theseignimbrites spread westward into the Preandean basins where theyoverlie clastic and evaporitic sediments as well as older volcanic se-quences (Mpodozis et al., 2005; Blanco and Tomlinson, 2009; Jordanet al., 2007, 2015). Interbedded in the older sequences are Mesozoicmarine carbonate and siliciclastic sediments, possible equivalents to theYacoraite Formation of Argentina (Muñoz et al., 2002; Marquillas et al.,2007), and Paleozoic sedimentary sequences (Bahlburg and Bretkreuz,1991). There is limited surface exposure of these sediments east of the

Fig. 3. Lithium, boon and bromide concentrations relative to chloride. The linesindicate elemental:chloride ratios for seawater and mixed salts from Olarozsalar (Garcia pers. comm, 2019) and for Br/Cl, sedimentary halite identified byRisacher et al. (2011) as a source of Cl. Additional thermal water data is fromTassi et al. (2010). Steam heated water has higher B relative to Cl. Li and Bconcentrations are high relative to seawater, which receives large influxes ofnon-thermal water.

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West Fault, but these rocks can directly interact with groundwater or beincorporated into volcanic systems influencing their geochemistry, andultimately groundwater as well.

The rocks of this area are predominantly volcanic. Volcanic rocksand their clastic erosional derivatives and other silicate rocks have alimited δ7Li and δ11B variation and vary within a few per mil of theupper continental crust or MORB (Xiao et al., 2013; Penniston-Dorland,2017). Strontium has greater variability because of the mixing betweencrystalline basement rocks and young volcanic rocks, both in terms ofclastic sediments and magmatic mixing as expressed by ignimbrites(Lucassen et al., 1999; Kay et al., 2010). However, the presence ofmarine rocks will impart greater variability in terms of B and Li isotopesthan to Sr, because seawater is very enriched in 7Li and 11B compared tocrustal rocks (Xiao et al., 2013; Penniston-Dorland, 2017). The end-member sources of Li and B are described below and listed in Table 2.

The two most important types of volcanic rock which are weath-ered, are rhyo-dacite ignimbrites and andesitic lavas of the compositevolcanoes; these are labeled as IG and AND in isotope plots. The87Sr/86Sr values range from 0.7050 to 0.7095 in lavas (Walker, 2011;Godoy et al., 2014) and 0.7083 to 0.7116 for ignimbrites reflectingassimilation of crustal contaminants (Kay et al., 2010; de Silva et al.,2006). The δ11B composition of nearby ignimbrites range from −9.7 to+2.2‰ and average −4.1 ± 3.3‰ (Schmitt et al., 2002). Lavas areslightly more enriched in 11B, and range between −6 and +2.4‰(Rosner et al., 2003). There is little data for δ7Li of rocks in this area.Ignimbrites analyzed to date vary from +1.9 to +4.4‰ (Godfrey,unpubl. data), and we assume that since silicate reservoirs do not ex-hibit large variations, the lavas will be similar.

Mesozoic marine sediments are an important Cl source in thePrecordillera (Arcuri and Brimhall, 2003), and there is evidence thatthese rocks exist under the arc. If we assume that these sediments aremixed carbonate-siliciclastics, their isotope composition will be a mixbetween contemporary seawater and detrital material according tomass balance. Strontium and B are more concentrated in the carbonatecomponent, so we assume that 87Sr/86Sr will be close to that of sea-water at the time of deposition, 0.707–0.7075 (Veizer et al., 1999), andδ11B slightly lower than marine limestone, +9 to +12‰ (Paris et al.,2010). Carbonates are not an important reservoir for Li, so we expectthat δ7Li will only be slightly elevated by seawater from the marinedetrital component of −2 to +5‰ (Chan et al., 2006). A springemanating from Jurassic marine carbonate-bearing sediments inQuebrada Chug Chug was sampled. It's δ7Li and especially δ11B are highfor the overall area (+10.3‰ and +22.5‰), while 87Sr/86Sr is moreradiogenic than contemporary seawater 0.70822, probably due toweathering of radiogenic basement rocks of the Precordillera.

Geothermal water, sedimentary brine and evaporites are importantin defining groundwater chemistry in this region. Isotopic fractionationof Li and B of geothermal water compared to the host rock is minimaldue to the temperature dependence of isotope fractionation duringsecondary mineral formation and rock dissolution being the dominantprocess. Analysis of water at El Tatio by various investigators(Giggenbach, 1978; Cortecci et al., 2005; Tassi et al., 2010) shows itscomposition has been consistent over time, and we use the isotopecompositions determine in this study differences to characterize it. Thecomposition of sedimentary brine which may infiltrate into regionalgroundwater is based on the chemical data of the most saline water

from Risacher et al. (1999) and B and Li isotope data reported bySchmitt et al. (2002) and Lagos (2017). Evaporite minerals which formwithin the salars consist of halite with minor calcite and aragonite,gypsum, sylvite and borates, primarily ulexite, but less common mi-nerals such as uralborite have also been recorded (Risacher et al., 1999;Lagos, 2017; Garcia, pers. Comm., 2019). While there is almost noisotope fractionation of Li or B during halite precipitation (Paris et al.,2010; Godfrey et al., 2013) there is significant fractionation during theprecipitation of earlier forming evaporite minerals. Due to the greaterproportion of B that exists in tetrahedral coordination in the observednatural borate assemblages (Grice et al., 1999) and the partitioning of10B into the borate anion (Oi et al., 1989) these borates' δ11B are lowerthan the solution phase or borates with equal proportions of B(OH)3and B(OH)4− (Vengosh et al., 1995; Palmer and Helvaci, 1997). Ana-lyses of an evaporite assemblage from Tuyaito and Olaroz salars con-firm this expectation, with δ11B of −10.3‰ and −12.4‰ ± 1.0 re-spectively (Lagos, 2017; Garcia, pers. Comm., 2019). Changes in δ7Liduring evaporation are less studied, but preliminary analyses of eva-porating seawater (Godfrey, unpubl. data) as well as inorganicallyformed calcite (Marriott et al., 2004) indicates that δ7Li might be lowerin non-halite evaporite minerals than Li in solution. However, theamount of Li that is incorporated into early forming evaporite mineralsis small, because the δ7Li composition of mixed surface salts in Olaroz iswithin error of the surface co-existing brine (Garcia pers. Comm.,2019).

5.3. The role of lithology on δ11B and δ7Li based on radiogenic Sr

The 87Sr/86Sr of water is consistent with Sr released from volcanicrocks, lavas and ignimbrites, according to their distribution: Sr is moreradiogenic in the southeastern part of the Upper Loa Basin because it iscloser to the ignimbrite plateau of the central Andes where ignimbriteslike the Atana and Toconao have whole rock 87Sr/86Sr above 0.710. Weobserve negative correlations between δ11B, δ7Li and 87Sr/86Sr. Thesamples within the ignimbrite water group overlap the field for ig-nimbrites in both 87Sr/86Sr - δ11B and 87Sr/86Sr - δ7Li space (Fig. 4A andB), but the samples from the Loa group have Sr isotope values con-sistent with the lavas while their δ11B and δ7Li are higher than theexpected andesitic values (Rosner et al., 2003; Deegan et al., 2016;Magna et al., 2006). There is also an offset between surface andgroundwater which is most notable in δ11B. A few samples plot welloutside of these fields, Silala and V Barro's stream have high δ7Li, andPaniri spring has high δ11B.

The Paniri spring has much higher δ11B relative to the other samplesand indicates additional B sources may be present. Modern seawaterhas δ11B around +40‰ and limestone forming today about +21‰.Although seawater δ11B, and hence limestone, have been lower in thepast, the 11B enrichment of marine limestone will always be high re-lative to silicate rocks (Ishikawa and Nakamura, 1993). The role oflimestone weathering is demonstrated by ChugChug spring water. Thedrainage basin of ChugChug in the Cordillera Domeyko includes largevolumes of Jurassic - Cretaceous limestone and its water has δ11B of+22.5‰, well within the range of Mesozoic marine limestone(Joachimski et al., 2005; Paris et al., 2010). ChugChug Sr is moreradiogenic than Mesozoic seawater which can be explained by theweathering of radiogenic silicate rocks in the Cordillera Domeyko. Al-though ChugChug lies on a mixing line between the headwaters of theLoa and marine rain (Chetelat et al., 2005), weathering of marinelimestone releasing B and Sr derived from carbonate with a minorcontribution of radiogenic Sr from the basement is a more plausibleexplanation for groundwater in this particularly dry part of the Desert.Furthermore, Cl in supergene atacamite in multiple copper depositswithin the Atacama is sourced from basement marine Jurassic rocks inthe Domeyko Basin (Arcuri and Brimhall, 2003; Cameron et al., 2007)and not rain. The extent of marine limestone is unknown east of thePrecordillera due to the coverage of volcanic rocks. The Paniri spring

Table 2End-member solute sources. Concentrations in mg/kg.

Na Cl Br Li δ7Li B δ11B

Thermal water 3240 5400 20 40.0 −0.5 140.0 −5.0Sedimentary brine 72,000 134,000 23 68.0 7.4 225.0 6.0Mixed salts 497,000 607,000 5 63.0 2.0 135.0 −10.0Cold dilute water NA 554 1 4.0 NA 13.8 −5.0

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has around 15‰ higher δ11B relative to its Sr isotope compositionbased on the trend described by other samples, and it plots within the B-Sr isotope field for Mesozoic limestone. Limestone could be present atdepth where water might interact with it directly, or decarbonationreactions of limestone could release B(OH)3 which is 11B enriched re-lative to the carbonate itself (Deegan et al., 2016). Limestone-magmainteraction supports similar conclusions of marine rocks underlyingparts of the arc based on rare earth element data from the scoria cone ofEl Rojo III adjacent to Paniri (Godoy et al., 2014), and wollastonitexenolith rare earth element data from Lascar volcano (Matthews et al.,1996).

The Silala and V. de Barros stream plot well above the 87Sr/86Sr -δ7Li array, especially compared to other members of the ignimbritegroup. While they plot close to ChugChug we do not think their δ7Licompositions are directly related to limestone lithology, as high δ11B inPaniri was, because Li is not enriched in limestone although its δ7Li ishigh relative to silicates (Chan et al., 2006). There is no evidence fromδ11B that limestone is important in their catchments and a more likelyexplanation is that secondary minerals have removed 6Li since waterreached the surface.

5.4. The effect of silicate weathering and the formation of clays on δ7Li andδ11B

Silica, B and Li are released from primary minerals during weath-ering. These elements are more soluble at high versus low tempera-tures, and their dissolved concentrations decrease by formation or up-take into secondary minerals (Vigier et al., 2008). When thermal waterscool, Si precipitates as sinters and, with Al, as clays which fractionateaqueous δ7Li and δ11B (Pistiner and Henderson, 2003; Vigier et al.,2008; Meredith et al., 2013; Pogge von Strandmann et al., 2014).

Precipitation of silica sinters during cooling, represented by El Tatioand Baños de Turi, produces little change in δ7Li (Fig. 5) or δ11B (not

shown). Silica and δ7Li then increase in groundwater as residence timeincreases (Wanner et al., 2014). The increase in δ7Li is from 0 to 2.5‰while Si increases from 1 to 2mmol/kg, reaching the highest values atChitor, the only sample, apart from El Tatio where water is in equili-brium or saturated with amorphous silica (Table 1). Over a similarincrease in Si, but with a granitic mineral assemblage, Wanner et al.(2014) predicted an increase of nearly 10‰ in δ7Li. Pogge vonStrandmann et al. (2014) measured an increase of 7‰ in the sandstonehosted Great Artesian Basin, much more than our measured increase of2.5‰. Our small increase in δ7Li may be a consequence of the dom-inance of silica-rich rhyolitic glass of the ignimbrites and lack of Al,limiting the formation secondary minerals (e.g., Declercq et al., 2013),indeed, dissolution rates of andesitic glass is lower than other volcanicaluminosilicates (Rowe and Brantley, 1993; Wolff-Boenisch et al.,2004). We did not measure Al, but as an example, water in the MNTaquifer which runs between the Altiplano and the southeastern end ofthe Salar de Atacama in very similar lithology, has one hundred timeslower Al than in the Great Artesian Aquifer, but over 40 times more Li(Pogge von Strandmann et al., 2014; Rissmann et al., 2015). The largeamount of Li derived from thermal sources undergoes a minor relativereduction in amount, rock continues to release Li and consequently wemeasure only minimal increases in δ7Li.

When groundwater reaches the surface δ7Li can rapidly increasebecause the formation of clays dominates over rock dissolution, and atthe same time Si concentrations decrease through dilution with me-teoric water (Wanner et al., 2014). Water from the Salado plots withgroundwater on the δ7Li-Si plot, while water from the Loa show in-creasing Si and δ7Li. The Salado has been recognized as a stream whichis heavily impacted by water from El Tatio (Romero et al., 2003) anddisplays a reservoir effect. The Río Loa, a gaining stream along thissection (Jordan et al., 2015), shows increases in Si and δ7Li over 50 kmbefore being joined by the Salado, because of groundwater inputs. Ex-change between surface and groundwater also accounts for the highδ7Li of the eastern Calama Basin samples where it has been hypothe-sized that water which enters these aquifers to the west of the Chiu Chiumonocline originates from the Loa River (Jordan et al., 2015), a theorywe can also support from the similarity of the Sr isotope composition.

The incorporation of Li and B into secondary minerals, especiallyclays, increases Na/Li, Na/B, and their isotope composition. For Li,samples which formed the “Loa” group in δ7Li – 87Sr/86Sr space form aroughly linear array which is parallel to a second roughly linear arraydefined by samples from the “Ignimbrite” group (Fig. 6A). Using alogarithmic Na/Li axis, mixing and open system isotope fractionationproduce curves, and Rayleigh fractionation produces lines. For

Fig. 4. The role of lithology on boron (a) and lithium (b) isotopes indicated bytheir correlation with 87Sr/86Sr. Ignimbrite δ11B data Schmitt et al. (2002),lava: Rosner et al. (2003); rain: Chetelat et al. (2005).

Fig. 5. Weathering causes an increase in silica but minimal change in δ7Li ingroundwater. Surface water in diluted by meteoric water causing a decrease insilica with adsorption increasing δ7Li. Groundwater input to the Loa detractsfrom this pattern, causing Si and δ7Li to increase. Cooling of water is indicatedby data for El Tatio and Baños de Turi.

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groundwater, Rayleigh fractionation is not an appropriate model to use,because the reactant, in this case dissolved Li, is constantly replenishedby rock dissolution and the Rayleigh model assumes that the reactant'sisotope or chemical composition changes as the reaction proceeds. Forsurface water, Rayleigh conditions are more likely to be met, unless theaddition of groundwater to streamflow is large and persistent along therivers course.

The applicability of mixing, open system and Rayleigh fractionationmodels in describing the data have been investigated for Li using theend-members from Table 2 following Henchiri et al. (2014). Opensystem fractionation follows δ7Liw= δ7Lii – (1-f). Δ7Lic-w and Rayleighfractionation δ7Liw= δ7Lii – Δ7Lic-w. ln(f), where δ7Liw is the isotopecomposition of the evolving water, δ7Lii its initial isotope composition,Δc-w the isotope separation between clay and water, and f the fraction ofLi remaining in solution. We assume that Na is released from rock ordissolved from mixed salts similarly to Li, but once in solution, it is notremoved to clays while Li is. The initial water composition used for themodel is the thermal endmember, El Tatio. While mixing betweenthermal water and salt or brine produces a line that is close to many ofthe groundwater samples, open system fractionation also gives a goodfit if we assign a value for Δ7Lic-w of−3.5‰. This value is small for coldtemperatures if Li is taken into octahedral sites of Mg-smectite (Vigieret al., 2008) but consistent with either surface adsorption or uptake intokaolinite (Vigier et al., 2010) and is in the range found for some basaltic

systems (Vigier et al., 2009). Kaolinite occurs in the eroded and exposedvolcanic summits, but little is known how common kaolinite is in thesurrounding subsurface, adsorption on the other hand is common-place.The large fractionation effects measured on smectite do not appear toapply here, even though smectite is present. For surface waters, mixingrequires an endmember with elevated δ7Li and high Na/Li. One can-didate is sedimentary brine or the very dilute water draining V. Barro.However, the concave-down curve that describes mixing of such waterwith thermal water does not encompass the surface water compositionswe have and mixing with other endmembers is equally unsatisfactory.In contrast, removal of Li from a thermal water source following aRayleigh model and using the same isotope separation between clayand water of−3.5‰ can explain the data. For most samples, 60 to 95%of the Li must be removed from solution, but these percentages may bereduced if the initial water composition reflects a mixed thermal-coldwater with a higher Na/Li ratio.

In such a geologically complex area, it is surprising that simple re-moval models quite successfully describe solution Li isotope systema-tics. It is possible that this stems from the high concentration of Li inthermal water which makes weathering reactions at low temperaturessomewhat uninfluential. While the only sample which plots near to themixing line between thermal water and salt is Baños de Turi, several ofthe water samples in the ignimbrite group plot near a mixing line be-tween thermal water and sedimentary brine. The sedimentary brineaccounts for 30% of the mixture.

The role silicate weathering and the formation of clays on the Bisotope composition is less apparent (Fig. 6B). Mixing between thermalwater and sedimentary brine, and between thermal water and saltwhich could in part explain groundwater Na/Li - δ7Li does not describemeasured B. Only a few groundwater samples plot close to open systemfractionation with thermal water as the starting composition. Further-more, it requires an isotope separation (Δ11Bc-w) on the order of −10 to−8‰ which is consistent with adsorption of only tetrahedral B (Palmeret al., 1987). Since pH indicates that both B species are present, andthat the trigonal form will be more abundant, such limited fractionationis problematic.

Unlike Li, there is a decrease in the Na/B ratio of the Loa surfacesamples compared to thermal and groundwater which further high-lights the inconsistency of removal processes in controlling B isotopes.A second difference between Li and B is that thermal water had thelowest δ7Li but for B, there are several samples with lower δ11B, andthese plot towards the mixed salt end-member.

5.5. Evaluating the influence of evaporite mineral dissolution using δ11Band Cl/Br

In terms of δ11B and Cl/Br, our samples fall into the two groups,“Loa” and “Ignimbrite”, but the Paniri spring plots with the “Loa” group(Fig. 7). The “Loa” group has δ11B above +6.1‰ and Cl/Br below1400; the “ignimbrite plateau” group has δ11B below +2.6‰ and Cl/Brabove 2400. Based on the data for mixed salts in the region, dissolvingsalts add Cl and 10B-rich B to water, and no Br.

The samples in the ignimbrite group most affected by salt inputs areChitor followed by Linzor (Fig. 7). The location of this salt source is notobvious since the closest salt lake or salar are Laguna Colorada andSalar de Chalviri 40–60 km to the east in Bolivia. However, the Cl/Brratio of El Tatio is greater than subduction related andesite or rhyolite(45–64, 225–8451; Aiuppa et al., 2009, though we caution that thehalide data base for rocks is small) or other geothermal systems, in-cluding Apacheta and Torte de Tocorpuri (Tassi et al., 2010). AlthoughGiggenbach (1978) argued against dissolution of buried evaporites informing the Cl-rich Tatio brine and favored magmatic HCl due to its lowpH, its near unity Cl/Na strongly suggests that halite dissolution is in-volved. Leakage of water into which mixed salt assemblages containingborates have dissolved, and characterized by Chitor, can explain thecomplexity of the mixing relationships involving δ11B and Na/B, and

Fig. 6. The formation of secondary minerals increases δ7Li relative to Na/Li (A)δ11B relative to B/Na (B). Mixing (dashed) lines describe can describe some ofthe groundwater samples when thermal and sedimentary brine is used. Opensystem removal processes describes water with high Na/Li better than mixingbetween thermal and mixed salts, but for high Na/B the opposite holds. Oncewater reaches the surface rock buffering is minimized relative to adsorption tosecondary phases, and Rayleigh fractionation processes can describe surfacewater samples. Samples from the Salar de Atacama (Munk et al., 2018) areincluded, and indicate that before the water evaporates to form Li-rich brine,the inflow water has strong characteristics of a thermal water mix.

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the samples most strongly affected are those of the ignimbrite group.While Caspana plots near the sedimentary brine in terms of δ11B andNa/B it's lower δ11B and δ7Li compared to saline lakes on the Altiplano(Schmitt et al., 2002; Godfrey et al., 2013; Orberger et al., 2015; Lagoset al., 2015) suggest that it is influenced by both evaporite and brinesources.

We can elucidate the source of the salts from 87Sr/86Sr (Fig. 7B). ElTatio and Linzor have the most radiogenic, ignimbrite-like composi-tions, but as salt contributions increase Cl/Br ratios, 87Sr/86Sr increasestoo suggesting that the salt itself has a Sr isotope composition like thatof the ignimbrite. Considering the closed basin morphology of the Al-tiplano plateau which is partly due to the presence of multiple calderas,it is not surprising that these calderas would have hosted saline lakesand salt plans following their eruption and that the lakes would acquirethe Sr isotope signature of the surrounding ignimbrites. The samplesfrom the Loa group are least affected by salt dissolution, though theirproximity to Ascotan makes it possible that windblown material fromthis borate-rich salar washes into the river. Boron precipitates earlierduring evaporation than Li, so Li will be largely unaffected by thisprocess, but we cannot exclude the possibility that the rivers δ11Bmight have been lowered. However, we think wind-blown salts do notcontribute as much material to the river as rock sources.

6. A regional, isotope-driven model

Multi-isotope analyses are used to highlight the role of lithology,high and low temperature weathering, inputs of sedimentary brine andassociated evaporite minerals. The impact of sedimentary brine andevaporite dissolution in the Upper Loa Basin is important in the area

covered by ignimbrite, but far from pervasive, and barely present in theupper reaches of the Loa River. The area most strongly impacted bybrine or evaporites dissolving in the subsurface occurs close to El Tatioand is particularly apparent at Chitor. These salts have 87Sr/86Sr con-sistent with ignimbrite, and Chitor, with low δ7Li indicates that the saltsformed from water that acquired its chemical composition from water-rock reactions at above surface temperatures because higher δ7Li wouldbe expected if temperatures had been low. If the salts are from ancientand large salars now buried under ignimbrites, then they formed duringor shortly after the eruptions of the Miocene-Pliocene ignimbrite flare-up when the geothermal gradient was high (de Silva, 1989; Kay et al.,2010). A modern example is the lake currently occupying the CerroGalan caldera in Argentina where δ7Li is indistinguishable from theCerro Galan ignimbrites (Godfrey et al., 2013). Groundwater flowunder the western rim of the Pastos Grandes caldera has been suggestedas a source of spring water to the east of the Ascotán salar (Keller andSoto, 1998), and westward dipping ignimbrites erupted from thesecalderas (such as the Sifon and Puripicar) may provide additional flowpaths for groundwater to the west. Drainage through the Chitor spring,and other places, is more than probable.

We expect groundwater flowing towards the Salar de Atacama in itsnorthern part is very similar to groundwater in the Upper Loa Basinsince they share a long boundary and their geology is similar. Based onwater from Chitor and Caspana, the δ7Li composition of the San Pedroand Vilama rivers are probably< 5‰, lower than the +5 to +10‰values for water entering the southern end of the Salar through theMNT aquifer (Munk et al., 2018). The isotopic and Na/Li ratios of waterentering the SE sector of the Salar de Atacama are indistinguishablefrom the values for sections of the Loa, rather than rivers draining ig-nimbrite or the El Tatio area, or sedimentary brines. East of the Salar deAtacama is la Pacana caldera, the origin of thick and extensive ignim-brites (de Silva, 1989; Lindsay et al., 2001). Within La Pacana's confinesare numerous salars (Lindsay et al., 2001), which similar groundwaterlevels around its southern end suggest are connected by a single shallowaquifer system connected through fractures in the volcanic rocks(Herrera et al., 2016). The final outflow of this large groundwatersystem is unknown since La Pacana's western rim for much of its lengthacts as a barrier to groundwater flow and retains several lakes whichwould otherwise drain to the salar and hence not exist.

Rissmann et al. (2015) found that water within of the MNT aquiferacquires thermal water from Socompa volcano before it eventuallyreaches the Salar de Atacama. The water in the MNT aquifer could becompatible with a mixture of thermal and sedimentary brine, thoughadditional samples would be needed to test this. Most of the data pre-sented by Munk et al. (2018) are shallow groundwater (or close tosprings), and have δ7Li values suggesting that significant removal of 6Lito clays has occurred, probably within the clay-rich margin of the salar.At the same time, evaporation has allowed halite to precipitate leavingLi in solution and decreasing Na/Li.

In conclusion, groundwater draining the volcanic arc in theAntofagasta region of northern Chile has a composition which isstrongly buffered by rock dissolution because very low rainfall does notproduce enough flow through shallow aquifers or rivers to overprint itssignature. A common component of groundwater is hydrothermalwater. To account for high Cl and Na, dissolution of mixed salts, in-cluding borates, is required, but isotopes indicate even these saltsformed from evaporation of water derived from weathering at tem-peratures higher than air temperatures. Explosive volcanism along thissegment of the arc left a series of calderas in which lakes and salarsformed. Groundwater with particularly high Cl and Na drains fromthem into regional groundwater flow and can be distinguished fromother water when their δ7Li and δ11B compositions are considered.Given the similarity of the geology and volcanic landscape of thewestern side of the Andes, this hydrologic model is applicable to waterentering other pre-Andean basins located next to geologically recentlyactive calderas. Those basins high Li and B content also highlight the

Fig. 7. The presence of salts, including borates, with high Cl/Br and low δ11B,has a strong effect on the B isotope systematics (A), 2-component mixing in-dicated by the dashed line. These salts originate from the weathering of ig-nimbrites followed by evaporation from their 87Sr/86Sr composition (B).

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role of high temperature silicate weathering in forming Li and B richwater.

Acknowledgements

We would like to acknowledge useful discussions with Dr. T.E.Jordan and other members of the Geology Department at UNC inAntofagasta. The reviews of two anonymous reviewers greatly im-proved this contribution. This research was supported by NationalGeographic Society award #800006, NSF OISE-1037929 and EAR-1117496, the Anillo Project ACT1203 of the CONICYT of Chile and by aPh.D. grant from CONICYT-PCHA/Doctorado Nacional, number 2016-21160152 for C. Gamboa.

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