q 2001 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; November 2001; v. 29; no. 11; p. 10591062; 3 figures; 2 tables. 1059

Hydrothermal element fluxes from Copahue, Argentina: Abeehive volcano in turmoil

Johan C. VarekampAndrew P. Ouimette*

Scott W. HermanDepartment of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown,

Connecticut 06459-0139, USAAdriana Bermudez

Daniel DelpinoConsejo Nacional de Investigaciones Cientficas y Tecnicas, National University of Comahue, Neuquen, Argentina

ABSTRACTCopahue volcano erupted altered rock debris, siliceous dust, pyroclastic sulfur, and rare

juvenile fragments between 1992 and 1995, and magmatic eruptions occurred in JulyOctober 2000. Prior to 2000, the Copahue crater lake, acid hot springs, and rivers carriedacid brines with compositions that reflected close to congruent rock dissolution. The ratiobetween rock-forming elements and chloride in the central zone of the volcano-hydro-thermal system has diminished over the past few years, reflecting increased water/rockratios as a result of progressive rock dissolution. Magmatic activity in 2000 provided freshrocks for the acid fluids, resulting in higher ratios between rock-forming elements andchloride in the fluids and enhanced Mg fluxes. The higher Mg fluxes started several weeksprior to the eruption. Model data on the crater lake and river element flux determinationsindicate that Copahue volcano was hollowed out at a rate of about 20 00025 000 m3/yr,but that void space was filled with about equal amounts of silica and liquid elementalsulfur. The extensive rock dissolution has weakened the internal volcanic structure, mak-ing flank collapse a volcanic hazard at Copahue.

Keywords: hydrothermal fluids, volcanic processes, hydrochemistry, limnology, eruptions.

Figure 1. Map of Copahue region. Light gray area at left is steep volcanic cone. CPLcrater lake, CPacid hot springs. Gently sloping foothills are between dashed line andLake Caviahue, which is situated on flat caldera floor. Arrow shows location for river fluxmeasurements.

INTRODUCTIONCopahue is a 2997-m-high composite vol-

cano (Province of Neuquen, lat 37.538S, long71.108W), located at the east side of the SouthVolcanic Zone of the Andes in western Ar-gentina. The basaltic-andesitic products are ofPleistocene to Holocene age (Delpino andBermudez, 1993). The volcano summit wasextensively glaciated in Pleistocene time, anda modern small glacier provides meltwater tothe crater lake. The active cone consists ofsurge and near-vent fallout deposits, whereasthe broad base consists of debris avalanches,lahars, and lava flows (Goss, 2001).

Copahue activity has been reported sincethe eighteenth century. A new eruptive cyclestarted in July 1992; explosions continued in1993, and major eruptions occurred in Decem-ber 1994 and September 1995 (Bermudez andDelpino, 1995; Delpino and Bermudez, 1993).The crater lake explosions ejected hydrother-mally altered rock fragments, siliceous whitedust, copious amounts of green and yellowliquid sulfur (Delpino and Bermudez, 1995),and some basaltic-andesitic juvenile frag-

*Present address: U.S. Geological Survey, Vol-cano Hazards Division, Menlo Park, California94025, USA.

ments. Magmatic eruptions (VEI 12), start-ing with phreatomagmatic events, began inJuly 2000; continuous degassing occurred be-tween eruptive phases. Incandescent bombswere ejected, and dark ash and chilled sulfurfragments covered an area up to 50 km fromthe source (Global Volcano Network, 2000a,

2000b); the larger ash and gas plumes weredetectable up to 250 km from the vent.

An acid crater lake, acid hot springs, and ageothermal field (Martini et al., 1997) are thesurface expressions of an extensive volcanic-magmatic hydrothermal system (Fig. 1),which has been the focus of our investigations

1060 GEOLOGY, November 2001

TABLE 1. CRATER LAKE, ACID HOT SPRING, WHITE RIVER TRIBUTARY, AND LAKE CAVIAHUECOMPOSITIONS IN 1999 AND 2000

Sample Date pH* Cl SO4 Al Fe Mg Ca Si Na K(mo/yr)CPL1 1/99 0.30 7978 55,416 1375 748 279 890 61 361 319CPL2 11/99 0.18 9775 65,983 1421 794 274 1038 98 342 334CP1 1/99 0.32 10,883 65,473 3236 2090 539 871 102 819 791CP2 11/99 0.46 7018 45,438 1910 1067 310 968 98 413 423CP3 7/00 0.30 9781 59,530 5478 3907 2199 1161 NA 1484 723CP3/CP1 1 0.9 0.9 1.7 1.9 4 1.3 1.8 0.9WR1 11/99 5.4 115 115 0.5 BD 22.7 25.1 18.9 10.1 4.3CVL-N1 11/99 2.2 96 473 32.4 20.6 14.7 21.8 13.7 14.1 7.6

Note: Element values for crater lake (CPL), main acid hot spring (CP), White River tributary (WR), and LakeCaviahue (CVL) samples are in parts per million. BDbelow detection, NAnot analyzed.

*If pH , 0.5, calculated by charge balance.

TABLE 2. FLUXES FOR THE CRATER LAKE, ACID HOT SPRINGS, UPPER RIO AGRIO,AND COPAHUE VOLCANO

Location Date (month/year)3/97 3/98 1/99 11/99 7/00

Copahue crater lakeT (8C) 54 ;33 21 33 No lakeCl (ppm) 10 157 8893 7978 9775Brine influx* (m3/s) 0.087 0.01 0.024Energy input* (MW) 45 7 15Net rock removal rate (m3/yr) 7000 480 1150

Copahue springsT (8C) 68 63 83 72 75Cl (ppm) 10 617 10 166 10 883 7018 9781RFE/Cl (molar) 1.0 1.07 0.84 0.79 1.61Water flux (m3/s) 0.031 0.035 0.052 0.063Heat flux (MW) 9 12 16 20

Upper Rio Agrio watershedCl (ppm) 1609 950 178 176Water flux (m3/s) 0.26 0.48 2.25 3.25Mg flux (t/yr) 1310 1510 1990 7800Sulfur flux (kt S/yr) 21 24 25 39Net rock removal rate (m3/yr) 7200 8300 10 900 N.A.So accumulation rate# (m3/yr) 5870 6650 6960 10 900Eq. volcanic SO2 input (t/d) 174 197 206 323

Whole Copahue systemEq. volcanic SO2 input (t/d) 656 245 344So accumulation rate (m3/yr) 22 200 8300 11 600Net rock removal rate (m3/yr) 14 300 8800 12 000Note: N.A.not applicable because rock dissolution is no longer congruent.*Energy steady-state model (Pasternack and Varekamp, 1997; Ouimette, 2000).Taking silica precipitation into account (about 50% of rock mass is SiO2).Assuming that all SO4 is derived from the hot springs.#Using a density of sulfur of 1800 kg/m3.

since 1997 (Herman, 1998; Ouimette, 2000).The crater lake water varied in color fromdark-green to gray; clouds of HCl vaporsexsolved when the water was hot. The mainacid hot spring emerges about 100 m belowthe lake level and feeds an acid river, the Up-per Rio Agrio, which discharges ;12 kmdownslope into Lake Caviahue, a large, acid-ified glacial meltwater lake.

About 300 water samples, collected be-tween March 1997 and August 2000, were an-alyzed at Wesleyan University by inductivelycoupled plasmaatomic-emission spectrosco-py and ion chromatography. The water flux ofthe Rio Agrio was measured repeatedly witha flow meter, and element fluxes were deter-mined from water flux and local river watercompositions.

COPAHUE CRATER LAKE AND ACIDHOT SPRINGS

Copahue crater lake contains an acid brine(Table 1) with temperatures of 21 (January1999) to 54 8C (March 1997). The hot springshad higher temperatures and higher concentra-tions of rock-forming elements (RFEsCa,Na, Mg, K, Al, Fe) than the lake waters (Table1). Chemical and isotopic data show that thecrater lake and hot springs are both fed bydeep hydrothermal fluids, which contain up to70% magmatic brine (Ouimette, 2000). Ashallow reservoir with a temperature of about175 8C probably feeds the crater lake directly,whereas the hot-spring fluids have undergoneadditional water-rock interaction and conduc-tive cooling prior to reaching the spring areas(Ouimette, 2000). Both the lake and the hotspring had high concentrations of toxic ele-

ments; e.g., the spring had 7.9 ppm As, 4.3ppm B, 3.2 ppm Pb, 2.8 ppm Zn, and 1.8ppm Cr in 1997, and the spring dischargeinto the Upper Rio Agrio is environmentallyundesirable.

The Cl concentrations in the crater lake in-creased from 2500 ppm Cl in 1920 to.10 000 ppm Cl in the late 1970s (Casertano,1964; Martini et al., 1997; Herman, 1998;Ouimette, 2000). From 1997 to 2000, the Clconcentrations in the lake initially decreased,but during 1999 the concentrations and lake-water temperatures increased considerably.The compositional variation in the crater lakesamples relates to the magnitude of meteoricand glacial meltwater inputs and to changes inthe rate of volcanic input. The compositionalvariations in the hot-spring fluids result frommixing between meteoric waters and the vol-canic brine at depth.

Power inputs into the lake were calculatedfrom steady-state energy budgets (Pasternackand Varekamp, 1997; Ouimette, 2000) andvaried between 7 and 45 MW (Table 2; inputvariables: lake diameter 125 m, lake temper-atures, and meteorological parameters). Siz-able hydrothermal inputs are needed to keepthe crater lake warm (average ambient tem-perature at the summit is 22 8C) and lakeseepage is an important process in keeping hy-drological balance (Rowe et al., 1995).

Speciation-saturation simulations with theprogram SOLVEQ (Reed and Spycher, 1984)indicate that the crater lake and hot-spring flu-ids are saturated in silica and close to satura-tion with gypsum and anhydrite. The ratiosbetween RFEs in crater lake and hot-springfluids did not vary much during the twentiethcentury and are close to those in average rockof Copahue (Goss, 2001; Fig. 2). The similar-ity of the RFE ratios in average rock and flu-ids and the general lack of secondary mineralsaturation strongly suggest that congruentrock dissolution was the dominant mode ofwater-rock interaction in the Copahue volcano-hydrothermal system. Almost all RFEs arecarried off by the hydrothermal fluids, exceptfor silica, which is highly depleted in the flu-ids relative to all other elements. With the cal-culated water input fluxes from the energymodel, lake cation concentrations, and an av-erage of Copahue rock compositions (Goss,2001), we calculated net rock-removal ratesfor the lake (Table 2), assuming congruent dis-solution with approximately quantitative silicaretention in the hydrothermal system.

The RFE/Cl values and the degree of neu-tralization of the fluids (as used by Varekampet al., 2000) have declined in the crater lakefluids and hot spring during the past few years(Table 2). We hypothesize that this resultedfrom increased water/rock ratios in the hydro-thermal system as a result of rock dissolution

GEOLOGY, November 2001 1061

Figure 2. Concentrations of K and Mg in twentieth century Copa-hue fluids (solid circles) are uniform, and K/Mg ratios are similarto those in average Copahue rocks (small circles, parts per millionvalues divided by 20). July 2000 hot-spring fluids have exception-ally high Mg concentrations, with different K/Mg ratios than 2000magma or older Copahue fluids, suggesting noncongruent dis-solution of newly intruded magma.

Figure 3. Magnesium concentrations in Lake Caviahue as function oftime (month/year). Black symbols are measured Mg concentrationsat .20 m depth; large open circles are average deep-water valuesfor each year; small open circles are shallow-water (,20 m) data.Evolution curves for Mg concentrations over time assume 5 yr resi-dence time for water in lake and start in 1992 at 12 ppm Mg. Bottomcurve represents 1350 t Mg/yr input, whereas other curves showcompositional evolution with stepwise increase in Mg flux as mea-sured in Upper Rio Agrio over time. July 2000 data plot above modelcurve for 2000 t Mg/yr input and represent values that must be as-sociated with much higher Mg input fluxes. Steep line is evolutioncurve for flux of 7800 t Mg/yr as measured in July 2000 during erup-tive period. That flux must have started at least six weeks prior toeruption to create these observed bottom-water Mg concentrations.

as well as that the rock protolith became cov-ered with liquid sulfur and/or cristobalite,slowing water-rock reaction. The July 2000fluids show a dramatic change from this trend(Tables 1 and 2); the RFE/Cl ratios increased,and RFE ratios differ from those in older Co-pahue rocks and in the new magma (Fig. 2).The composition of these fluids resulted fromthe noncongruent dissolution of the newly in-truded magma. Calculations with SOLVEQindicate saturation of alunite, anhydrite, andsilica phases at temperatures .150 8C, whichmay explain why the K and Ca concentrationshave increased less than those of Fe, Mg, andNa (Table 1, CP3/CP1 ratio column).

UPPER RIO AGRIOThe Upper Rio Agrio starts as pure hot-

spring water, which is cooled and diluted withglacial meltwater and tributary inflows fartherdownstream. Many tributaries derive waterfrom thermal springs lower on the slopes ofCopahue, which tend to be much less acidicand have much lower sulfate concentrations(e.g., WR tributary, Table 1). Annual elementand energy fluxes (Table 2) were calculatedfrom water flux measurements taken wherethe river enters Lake Caviahue (Fig. 1); cal-

culated net rock-removal rates range from7000 to 11 000 m3/yr (Table 2).

The rate of elemental sulfur accumulationin the volcano was calculated from the sulfateflux and the stoichiometry of the SO2 dispro-portionation reaction (Kusakabe et al., 2000):3SO2 1 2H2O 1 S8 1 2H1. We22HSO4assume that this is the main disproportionationreaction in the system, given that H2S is vir-tually absent in the lake and hot-spring fluids.This calculation provides for the Upper RioAgrio sulfate flux an approximate sulfur ac-cumulation rate of ;6000 m3/yr (Table 2).The amount of void space generated by therock dissolutionsilica precipitation mecha-nism is of the same magnitude as the sulfuraccumulation rate (compare Rowe et al.,1992a), and rocks in the volcano interior arereplaced by roughly equal amounts of silicaand liquid native sulfur.

LAKE CAVIAHUELake Caviahue is a two-finger glacial lake

with a volume of ;0.47 km3 (Rapacioli,1985; Pedrozo et al., 2001). Water is suppliedlargely by the mineralized Upper Rio Agrioand the Aqua Dulce, a glacial meltwaterstream. The lake drains by an overflow in the

northern arm through the Lower Rio Agrio(Fig. 1). The acidity and moderate concentra-tions of dissolved substances in Lake Cavia-hue (Table 1) are supplied by the Upper RioAgrio, and given the large volume of the lake,the lake-bottom water compositions provide amemory of the element fluxes of the UpperRio Agrio in the past. The average deep-water(.20 m) compositions show a progressionfrom ;13.4 ppm Mg in 1997 to ;14.5 ppmin 1999 and then they jumped to 15.524 ppmMg in July 2000 (Fig. 3).

We modeled the composition of a well-mixed lake as a function of time with the ex-pression Ct 5 Css (1 2 e2t/R) 1 e2t/R Cinit,where Ct is the concentration of a componentat time t, Css is the component concentrationat steady state, Cinit is the initial concentration,t is the time elapsed, and R is the residencetime of the component in the lake at steadystate (Varekamp, 1988). The term Css can berephrased as RFin /V, where Fin is the inputflux of the component and V is the mass ofthe lake. We used a bottom-water residencetime of 5 yr for Lake Caviahue, based on ourwater flux measurements and models, takinginto account that the lake is thermally strati-fied in the Austral summer. The modeled time-

1062 GEOLOGY, November 2001

concentration curves for Mg fluxes of 1350 to1500 to 2000 t Mg/yr (Table 2) cover the av-erages of the deep-water data for 19971999quite well (Fig. 3). Surface-water data showevidence for imperfect source mixing whensampled close to incoming rivers. The muchhigher Mg concentrations in deep lake watersmeasured only 10 d after the onset of the July2000 eruption indicate that the high Mg fluxof the Upper Rio Agrio must have precededthe eruption by some time. Using the July2000 Mg flux (7800 t Mg/yr) and a conser-vative average of 17 ppm Mg for the July2000 deep-water Mg concentration, we derivethat the enhanced Mg flux of the Upper RioAgrio preceded the eruption by at least sixweeks (the lake is well mixed during the aus-tral winter months). The Mg flux thus was anexcellent precursor to the eruption and a tracerfor the rise (and dissolution) of new magmainto the acid hydrothermal system.

CONCLUSIONSCopahue volcano acts as a beehive vol-

cano, accumulating liquid sulfur and silicaphases in void spaces generated by rock dis-solution. Part of that accumulated sulfur andsilica was ejected during the 19922000 erup-tions. The rock dissolution leads to higher wa-ter/rock ratios and therefore declining RFE/Clvalues in the fluids over time. The 2000 intru-sion of fresh magma into the hydrothermalsystem led to increased RFE/Cl values in thefluids, and water compositions became espe-cially enriched in Mg.

The Rio Agrio sulfate flux and associatednative sulfur storage rate can be expressed inequivalent volcanic SO2 inputs into the hydro-thermal system; these ranged from 174 to 323t SO2/d (Table 2). These are typical SO2 emis-sion rates for passively degassing volcanos(Andres, 1998), and Copahue is an examplein which hydrothermal SO2 scrubbing is verysignificant (Symonds et al., 2001).

Integration of the measured river flux dataand the modeled volcanic fluxes into the craterlake provide the following parameters for thewhole Copahue system in November 1999: anenergy flux of ;32 MW, an equivalent sulfurgas input of ;344 t SO2/d, a net rock removalrate of about 12 000 m3/yr, and an elementalsulfur accumulation rate of about 11 600 m3/yr. Estimates of bulk hydrothermal rock alter-ation rates at Poas volcano are of a similarmagnitude (;25 000 m3/yr; Rowe et al.,1995).

The evolution of the crater lake fluids dur-ing the twentieth century suggest a gradualawakening of Copahue, and future activity canbe expected. Monitoring the RFE/Cl in the hotsprings and temperatures in the newly reestab-

lished crater lake (January 2001) will be use-ful in assessing future volcanic activity (e.g.,Rowe et al., 1992b). The water compositiondata from Lake Caviahue serve as a memoryof the magnitude of element fluxes of the Up-per Rio Agrio, and strongly suggest that en-hanced Mg fluxes preceded the 2000 eruptionsby several weeks. We began a weekly moni-toring of the composition of the Upper RioAgrio in February 2001.

The bulk rock dissolution process (20 00025 000 m3/yr) is comparable to drilling a 1-km-deep hole with a diameter of 2.5 m in thevolcano each year, a hole that is then partiallyfilled with equal volumes of silica and liquidsulfur. The weakened structure creates risks offlank collapse (Lopez and Williams, 1993; vanWijk de Vries et al., 2000), especially takinginto account the fracture system that is presentin the east flank of the volcano. Flank collapsewith the snow and ice cap would cause severelahar hazards, and the rounded shape of thevolcano and abundance of lahar deposits onits lower slopes indicate that such collapseshave occurred in the past.

ACKNOWLEDGMENTSThis research was supported by National Science

Foundation grants INT-9704200 and INT-9813912.We appreciate the contributions of Caniche, JaneCoffey, Jelle deBoer, Noah Garrison, Adam Goss,Rob Kreulen, and Danielle Piraino.

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Manuscript received March 12, 2001Revised manuscript received July 9, 2001Manuscript accepted July 27, 2001

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