Hydrogeochemistry and stable isotopes of ground and surface waters from two adjacent closed basins, Atacama Desert, northern Chile

Applied Geochemistry. Vol, 5, pp. 719-734. 1990

0883-2927/90 $3.(~)+ .00 Pergamon Press plc

Printed in Great Britain

Hydrogeochemistry and stable isotopes of ground and surface waters from two adjacent closed basins, Atacama Desert, northern Chile CHARLES N . ALPERS Department of Geological Sciences, McGill University, 3450 University Street, Montrral, Qurbec H3A 2A7, Canada

and DONALD O . WHITTEMORE Kansas Geological Survey, 1930 Constant Avenue, Campus West, The University of Kansas. Lawrence, KS 66047, U.S.A.

(Received 5 January 1990; accepted in revised form 1! June 1990) Abstract--The geochemistry and stable isotopes of groundwaters, surface waters, and precipitation

indicate different sources of some dissolved constituents, but a common source of recharge and other constituents in two adjacent closed basins in the Atacama Desert region of northern Chile (24015 ' 24°45'S). Waters from artesian wells, trenches, and ephemeral streams in the Punta Negra Basin are characterized by concentrations of Na > Ca > Mg and C1 -> SO a, with TDS < 10 g/l. Values of bD and 6180 for Punta Negra Basin waters follow an evaporitic trend typical of closed basin waters in northern Chile and elsewhere. In contrast, ground waters in the Hamburgo Basin, located about 25 km NW of the Punta Negra Basin, have concentrations of Na > Mg -> Ca and SO4 > CI, with TDS also <10 g/I. Aqueous speciation calculations indicate that Hamburgo Basin groundwaters are close to saturation with respect to gypsum. The relatively high SOn and low Ca in Hamburgo Basin waters result from SO4 influx and subsequent gypsum precipitation related to weathering at La Escondida, a large porphyry copper deposit located near to the center of the basin. Deep mine waters from 130 m below the water table at La Escondida also have Na > Mg -> Ca and S O 4 > CI, but with TDS up to 40 g/1. The deep mine waters have pH between 3.2 and 3.9, and are high in dissolved CO 2 (613C = - 4 . 8 % PDB), indicating probable interaction with oxidizing sulfides. The deep mine waters have 6180 values of --1.8%0 compared with values <-3.5%o for other Hamburgo Basin waters; thus the mine waters may represent a mixture of meteoric waters with deeper "metamorphic" waters, which had interacted with rocks and exchanged oxygen isotopes at elevated temperatures. Alternatively, the deep mine waters may represent fossil meteoric waters which evolved isotopically along an evaporative trend starting from values quite depleted in 6180 and 6D relative to either precipitation or shallow groundwaters. High I/Br ratios in the Hamburgo Basin waters and La Escondida mine waters are consistent with regionally high I in surficial deposits in the Atacama Desert region and may represent dissolution of a wind-blown evaporite component. Rain and snow collected during June 1984, indicate systematic b 18O and 6D fractionation with increasing elevation between 3150 and 4180 m a.s.l. (-0.21%o 61So and - 1.7%0 6D per 100 m). Excluding the deep mine waters from La Escondida, the waters from the Hamburgo and Punta Negra Basins have similar 6D and 0180 values and together show a distinct evaporative trend (rD = 5.0 0180 - 20.2). Snowmelt from the central Andes Cordillera to the east is the most likely source of recharge to both basins. Some of the waters in the Hamburgo Basin may have been recharged during late Pleistocene, when the climate was wetter and a lake filled the intervening Punta Negra Basin, as suggested by recent archaeological and geomorphological studies.

INTRODUCTION AND HYDROGEOLOGICAL SETTING THE ATACAMA D e s e r t region is o n e of the driest places o n E a r t h . A l t h o u g h l o n g - t e r m averages are difficult to e s t i m a t e b e c a u s e of the i n f r e q u e n c y of s t o r m events, extensive areas in n o r t h e r n Chile, s o u t h e r n Peru, a n d a d j a c e n t A r g e n t i n a a n d Bolivia receive < 1 cm/a of p r e c i p i t a t i o n (TREWARTHA, 1981). T h e p r e c i p i t a t i o n rate in the A n d e s precordillera varies with latitude a n d e l e v a t i o n ; with increasing elevation, the p r e c i p i t a t i o n rate t e n d s to increase, e.g. f r o m > 1 0 cm/a a b o v e 3000 m a.s.1, to > 2 5 cm/a above 4000m a.s.1, in n o r t h e r n m o s t Chile

(CAVIEDES,1973; STOERTZ a n d ERICKSEN,1974). Because e v a p o r a t i o n far exceeds precipitation, it is not surprising t h a t n u m e r o u s c o n t i n e n t a l basins with internal d r a i n a g e ( " c l o s e d " basins) occur in the A t a cama D e s e r t region, including the Salar de Uyuni, Bolivia (RE'I'rIG et al., 1980) a n d the Salar de A t a cama, Chile (MORAGA-B. et al., 1974; Fig. 1). T h e geological setting a n d mineralogical zoning of these a n d o t h e r A n d e a n salars h a v e b e e n described by STOERTZ a n d ERICKSEN (1974), VILA-G. (1974, 1975), CHONG-D. (1984), ERICKSEN a n d S A L A S - O , RISACHER a n d FRITZ (1990). (1989). H o w e v e r , the origins of solutes a n d of w a t e r in m a n y of the "closed" basins of the A t a c a m a D e s e r t h a v e not b e e n well d o c u m e n t e d . 719

720

C.N. Alpers and D. O. Whittemore 70"

68"

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FIG. 1. Location map of northern Chile, showing principal porphyry copper deposits and physiographic features mentioned in text. Area of Fig. 2 is outlined. For example, it is uncertain whether the dominant mechanism of recharge to these basins is from direct rainfall or by subsurface groundwater flow from areas of higher elevation. Also, it is not known whether the dominant source of moisture to Andean basins is from the Pacific Ocean, mobilized during "El Nifio" Southern Oscillation (ENSO) events (e.g. WYRTKI, 1975), or from the Amazon basin mobilized by "Bolivian winter" storms as suggested by FRITZ et al. (1978). With regard to sources of solutes in groundwaters within "closed" intermontane Andean basins, it is generally not known which of the following processes predominates: (1) the dissolution of previously formed evaporite minerals (e.g. SMITH and DREVER, 1976; JONES et al., 1977); (2) direct weathering of Neogene volcanic rocks (e.g. ERICKSEN, 1961); or (3) simple evaporative concentration of rain water and snow melt. The goal of the present study is to determine the origins of waters and dissolved constituents in two "closed" basins in the Atacama Desert region. The two basins in the study area are: (1) the Punta Negra Basin, located 100-200km south of the Salar de Atacama (Fig. 1); and (2) the Hamburgo Basin,

located about 25 km north-west of the Salar de Punta Negra (Fig. 2). Preliminary results of this study were presented by ALPERS and BARNES (1986). The Salar de Punta Negra is in a graben bordered on the west by the Domeyko Range and on the east by the Andes Cordillera (Figs 1, 2), a physiographic setting equivalent to that of the Salar de Atacama (Fig. 1). The Andes at this latitude (-24°45'S) are within the southernmost portion of the central zone of active volcanism in the South American cordillera, including two of the tallest active volcanoes in South America, Volc~n Llullaillaco (6723 m) and Volcfin Socompa (6031 m) (FRANCmet al., 1985; GARDEWE6 et al., 1984). The Punta Negra and Atacama Basins have probably received sediment and run-off chiefly from the east since the onset of Andean uplift and volcanism at - 2 5 Ma (LAHSEN, 1982; COIRA et al., 1982; BORIC et al., 1987). Prior to early Miocene, the two basins may have received significant amounts of sediment (and recharge) from the Cordillera Domeyko to the west. The Salar de Hamburgo (Fig. 2) is located near the center of the Hamburgo Basin and within 2 km of La Escondida, a large-tonnage, high-grade porphyry copper deposit discovered in 1981 by Minera Utah de Chile, Inc., a joint venture between Utah International, Inc. and Getty Minerals, Inc. Dating by K Ar methods has shown that hydrothermal (hypogene) mineralization at La Escondida occurred during early Oligocene, between 35 and 31 Ma, followed by intense chemical weathering and consequent supergene copper enrichment during early to middle Miocene, between 18.0 and 14.7 Ma (ALPERS and BRIMHALL, 1988). Many wells have been drilled in the Punta Negra and Hamburgo Basins in search of water for drilling, mining, and milling operations at La Escondida. In addition, more than 200 diamond and rotary holes were drilled within the sulfide deposit at La Escondida during 1981-1984 to establish the ore reserves. These drill holes provided unusually good access to the hydrogeochemical system. Static water levels were measured in more than 150 open drill holes in the Hamburgo Basin during 1984, defining an essentially flat piezometric surface at 2989 _+ 3 m a.s.l. (BRIMHALL et al., 1985). In contrast, water levels in the Punta Negra Basin show significant spatial variation, particularly on the eastern side of the basin where subsurface flow from the Andes is indicated (Minera Utah de Chile, unpub, data). Additional access to groundwater was provided by an underground shaft and tunnel within the ore body (Fig. 3), developed at La Escondida during 1983-1984 to extract a bulk sample for metallurgical testing. Water and gas samples collected from these drill holes and mine workings were analyzed chemically and isotopically. These data are used in this paper to deduce the hydrogeological history of the area. Determination of 6D and 6180 was carried out for twenty-four water samples from the Punta Negra and

Hydrogeochemistry of closed basins, Atacama Desert, Chile Hamburgo Basins to assess the origin of groundwaters within the sulfide deposit at La Escondida and in the adjacent Hamburgo and Punta Negra Basins. Two possible mechanisms for the origin of the these waters are." (1) precipitation as rain and snow followed by partial evaporation in situ in hydrologically closed basins; or (2) subsurface flow into the basins from the Andes Cordillera via the Punta Negra Basin. We also consider the possibility that the present waters within the sulfide deposit at La Escondida and the Salar de Hamburgo may represent fossil waters, which could have accumulated in a hydrologic and climatic regime somewhat different from the present hyper-arid setting. We expect that the results of this study and related geochronological and geological investigations (e.g. ALPERS and BRIMHALL, 1988) will help to decipher the coupled tectonic, climatic, and hydrogeologicai evolution during the Neogene and Quaternary of the basins and ranges in selected portions of the Atacama Desert region.

721

WATER TYPES AND SAMPLING METHODS

The water samples analyzed in this study are divided into seven types (Fig. 2, Table 1): (I) rain and snow from a storm on 13/14-06-1984; (II) surface meltwater from snows on Voic~in Llullaillaco; (III) surface waters from open trenches in the Salar de Punta Negra; (IV) artesian wells on the east side of the Punta Negra Basin; (V) downhole samples (grabbed or pumped) from wells in the Salar de Hamburgo; (VI) downhall grab samples from areas of sulfide mineralization at La Escondida; and (VII) underground mine waters emanating from the floor of tunnels within sulfide mineralization at La Escondida. Sample locations are shown on Figs 2 and 3. Water in the underground mine workings at La Escondida (type VII) was sampled at two locations where tunnels opened during 1983-1984 intersected diamond drill holes. The elevation of the water table prior to excavation was 2989 _+ 3 m a.s.l., whereas

69'00'

68"45'

24"1.=

24'3C

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24"45

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V • S. Hamburgo drilr holes Vl • LaEscondicladrilJholes VII • La Escondida mine waters salars

L. I I

0 '

5 J

lOkm

F~6.2. Location map for water and snow samples from the Hamburgo and Punta Negra Basins. See text for description of sample types and sampling methods. Section A-A' shown in Fig. 3.

722

C.N. Alpers and D. O. Whittemore

Elevation (m) 3400 -

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ore

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and underground

the elevation of floor of the tunnel is 2850 + 1 m a.s.1. Mine dewatering created a hydrodynamic potential for water to flow into the tunnel from all sides, including upward from the floor. This resulted in small "fountains" on the tunnel floor where drill holes were intersected. Small excavations, - 2 0 cm in diameter, were made at each of these sites to allow water to accumulate. A rubber hose was used to siphon the water from each pool into 0.5-liter Nalgene® bottles. (The use of brand names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.) Temperature, Eh, pH, and conductivity were measured in

and

the

water

Salar

samples;

de Hamburgo symbols

showing

as in Fig. 2.

situ using portable meters. ORION ~ 399a and 404 meters were used with a Ross combination electrode to measure pH and with an ORION~ 96-78 platinum redox electrode to measure Eh; a YS1~ model 32 conductance meter was used to measure specific conductance and temperature. Samples were filtered using compressed air to force the water through Millipore® membrane filters of 0.1 pm pore size, using a cylindrical, 250ml Millipore® stainless steel filtration cell of 47-mm diameter. Gas and water samples for stable isotope analysis were stored in 125ml flint glass bottles with Polyseal ® caps. Water samples from open drill holes (types V and

Table l. Analytical data for water samples from the Hamburgo and Punta Negra Basins

Sample type*

Index number*

Sample number

Meters below surface

I 1 I I 1 1 11 II III 111 111 111 IV IV V V V V VI VI VI VI VI VII VII VII VII

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 19A 20 21 22 23 23A 24 24A

CAW-13 CAW-15B CAW-16B CAW-17B CAW-18 CAW-19 LLF-1 SANC-1 AGZ-1 QLZ-I QLZ-2 QELSAL-1 ES-10-1 ES-24-1 ES-I-I ES-1-2A RDH-250-1 SC-I-IB DDH-162-2A DDH-114-1B DDH-174-1 RDH-312-2 RDH-312-3 DDH-190-3 DDH-190-2 DDH-223-1 DDH-223-1

0 (1 0 0 0 0 0 1 0 0 0 0 0 0 44.5 44.5 31.6 15 190 144,8 140 145 183 235 235 235 235

Meters below water t a bl e

Elevation (m)

~tSo (%)

6D (%~)

TDS (g/l)

Field pH

------0 0 0 0 0 0 0 0 16.5 16.5 17.7 5.0 78.2 27.7 0.9 10 48 130 130 125 125

3, 150 3,500 3,700 3,950 4,180 3,150 2,959 2, 958 3,700 3,550 3,550 3,540 3,428 3,332 2,972 2, 972 2,971 2,984 2,909 2,957 2,989 2, 980 2, 942 2,850 2, 850 2,850 2, 850

-12.56 -12.99 -14.28 -14.57 -15.02 -13.47 -2.27 -5.14 -6.95 -3.57 -4.14 -2.38 -6.01 -5.16 -5.03 -4.96 -5.00 -4.90 -4.11 --4.90 -4.97 -3.51 -1.79 --1.83 --

-85.8 -87.1 -94.3 -98.1 -104.1 -87.5 -34.5 -44.6 -53.5 -35.6 -34.4 -32.8 -53.5 -49.5 -49.8 -48.4 -49.3 -51.0 -55.6 --55.0 -54.0 -54.1 -53.3 --48.4 --

. <.005 <.005 <.005 <.005 . 9.85 1.07 1.16 4.34 -4.13 1.17 3.05 4.87 4.85 3.97 6.51 8.5 6.88 -11.9 18.7 38.6 38.8 41.7 40.9

.

* S e e text f o r d e s c r i p t i o n o f s a m p l e t y p e s a n d m e t h o d s of collection. t I n d e x n u m b e r s c o r r e s p o n d t o l o c a t i o n s on Figs 2 a n d 3. -- Not determined. < L e s s t h a n d e t e c t i o n limit.

SO4 (mg/1) .

6.3 5.9 5.5 5.3 .

. 8.1 -7.4 8.2 8.4 --7.5 7.2 7.2 -6.7 5.2 4.2 7.2 6.8 7.5 3.9 3.3 3.7 3.9

CI (rag/I)

. . <1 <1 2 <1 <1 <1 <1 <1 . . . 860 4,710 160 370 480 180 1,510 981) . . . 1,38(I 1,030 130 310 360 1,340 2,410 520 2,410 520 2,260 210 2.890 92/) 4,610 518 3,780 510 . . . 6,010 2,190 6,560 4,580 15,100 9,100 15,200 9,340 15,800 10,300 16,000 9,990

Br (mg/I)

I (rag/l)

Li (mg/1)

. -----

-----

-----

-----

-----

. ----. ---0.37 --0.43 -0.47 . --2.6 2.7 2.5 4.9

. ---1.2 --2.1 -0.20

---0.19 --0.31 0.08 0.26

. --10.8 10.5 12.6 12.8

--0.27 0.24 0.66 0.57

Hydrogeochemistry of closed basins, Atacama Desert, Chile VI) were collected with two different grab sampling devices. Relatively shallow samples were taken using a 70-ram diamter plastic tube of approximately l-liter capacity with hollow spherical stoppers at each end, which trap water as the tube is pulled upward. A 5-kg lead weight allowed this device to penetrate up to 15 m below the static water level in open drill holes. For deeper samples, a l-liter steel cylinder with an electronic trap door was lowered with a truckmounted winch. In all cases, samples were discarded until two consecutive samples showed consistent values of Eh, pH, temperature and specific conductance. All downhole and surface water samples were filtered and stored in the manner described above. Splits of samples were preserved for cation determination by acidification to pH < 2 using 50% H N O 3 (reagent grade). Snow samples (type I) were collected within 72 h of the storm on 13/14-06-1984, by which time a crust had developed in the upper - 1 0 c m of snow due to melting and refreezing. Loose, powdery snow was collected from a depth of 20-60cm beneath the surface of the crust. Snow samples were collected in 5-liter plastic bags, then allowed to melt at -20°C. Splits of snowmelt were analyzed for pH, Eh, and conductance, then filtered, acidified, and stored in the same manner as the other water samples.

ANALYTICAL TECHNIQUES

Basins are given in Table 1. The data are plotted in Figs 4-10 and are discussed below.

Major elements As can be seen in the trilinear equivalence diagrams of major dissolved constituents (Fig. 4), the chemistry of the waters in the two basins is quite different. Major element data for waters from the Salar de Imilac (Fig. 2) are also shown in Fig. 4 for comparison, and are similar to the data for the Punta Negra Basin waters. The Salar de Hamburgo waters are relatively high in dissolved SO 4 and low in carbonate alkalinity. Waters from the Punta Negra Basin are characterized by concentrations of Na > Ca > Mg and Cl > SO 4 with TDS < 10g/l. These concentrations are typical for closed basin waters in northern Chile (VILA-G.,1975) and elsewhere (HARDIE and EUGSTER,1970). In contrast, waters from drill holes in the Hamburgo Basin (type VI waters) have concentrations of Na > Mg -> Ca and SO 4 > C1, with TDS also <10g/l. Speciation-saturation analysis using the W A T E Q 4 F program (BALLe t al., 1987) indicates that all of the groundwaters sampled from the Hamburgo Basin were close to saturation with respect to gypsum and undersaturated with respect to other common evaporite minerals, including halite, epsomite, bloedite, thenardite, and mirabilite. There can be little doubt that the proximity of the Salar de Hamburgo to La Escondida, a large porphyry copper deposit, has affected the major element chemistry of waters in the basin. The relatively high SO4 and low Ca in Hamburgo Basin waters have clearly resulted from SO4 influx and subsequent gypsum precipitation related to weathering at La Escondida. Mine waters at La Escondida (type VII) are similar to type VI waters in having Na > Mg >- Ca and SO 4 > C1, but differ in having TDS up to 40 g/l.

Determination of blSo in the water samples was carried out using a Finnigan delta-E mass spectrometer by equilibration with CO 2 over a 24-h period at 25°C, resulting in a precision of +0.05%0. Deuterium was determined by reaction of water with U to yield H2 gas, which was analyzed on a Finnigan 251 mass spectrometer with a resulting precision of +0.5%0 (BARNES, 1984). Both 6180 and 6D are reported relative to SMOW (O'NEIL, 1986). Major dissolved constituents were determined at the Sunnyvale (California) laboratory of BHP-Utah International Minerals, under the supervision of D. Ellsworth, and at the Lawrence Berkeley Laboratory under the supervision of A. White. Aqueous SO4 was determined gravimetrically after precipitation as BaSO 4. Total dissolved solids (TDS) were determined gravimetrically after evaporation. Chloride was determined by titration with mercuric nitrate. Other elements were determined by atomic absorption spectrometry, direct-coupled plasma spectrometry, and ion chromatography using standard methods. Dissolved Br and I were determined at the Kansas Geological Survey. Bromide was measured using a modification of the automated phenol-red method of BASELet al. (1982). Total inorganic I (iodide plus iodate) was determined by an automated method similar to that of FISHMANand FRIEDMAN (1985), based on the catalytic reduction of ceric ions by SO,~ arsenious acid.

RESULTS Results of stable isotopic and chemical analysis of water samples from the Punta Negra and Hamburgo

723

CI

Na + K

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Ca

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oo

o

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CO 3 Mg o "~" • • •

PuntaNegraBasin Salarde Imilao Salarde Hamburgo La Esc0ndidadrill holes La Escondidaminewaters

o

oceanwater

FI~. 4. Trilinear equivalence diagrams for major solutes in Punta Negra, Hamburgo and [milac Basin waters.

724

C.N. Alpers and D. O. Whittemore

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FIG. 5. Values for 01So and 6D of waters in the study area. Symbols as in Fig. 2. Letters and corresponding open diamonds represent average values and postulated source waters (see Table 2 and text).

6D = 7.3 6180 + 7.99) determined by FRrrz et al. (1978) for precipitation near the Salar de Atacama (Fig. 1). Note that the slope of this line is <8.0, the value for the global MWL (CRAIG, 1961). Such a lower slope is relatively common in arid and semiarid regions and is probably due to evaporation effects during precipitation (YURTSEVERand GAT, 1981). For comparison, an average of five values for meteoric waters at E1 Tatio, Chile (G1GGENBACH, 1978) is also shown on Fig. 5 (point I). The geothermal fields of El Tatio are located - 8 0 km east of Chuquicamata (Fig. 1). The "deuterium excess" parameter, defined by DANSGAARD(1964) as d = 6D - 8 6180, relates the isotopic composition of any water sample to the global MWL of CRAZ6(1961). For most precipitation collection stations in the I A E A network, both the mean and weighted average are close to the global average value of d = +10%o (YuRTSEVER and GAT, 1981). The six snow and rain samples from the 13/1406-1984 storm have d values which range from +14.8 to +20.3%o, with a mean of + 17.7%0. Relatively high values of d such as these have been noted in the Middle East and in Australia, and may be the result of evaporation under extreme non-equilibrium conditions into very dry air masses (GAT and CARMI, 1970; YURTSEVERand GAT, 1981). It seems likely that such non-equilibrium evaporative effects would contribute to the observed deuterium excess in precipitation from the hyper-arid Atacama region. Snow from the storm on 13/14-06-1984 was collected at four elevations (3500--4180 m a.s.1.) on the western flanks of Volc~in Llullaillaco (Fig. 2). Rain and snow from this storm were also collected at the Minera Utah de Chile camp (3150 m a.s.l.) near La Escondida (Fig. 2). Linear least squares regressions of ~180 and diD vs elevation (Figs 6A and 6B) show strong inverse correlations (n = 6; r = -0.898 and

Relative to average crustal abundances, porphyry copper deposits represent larger chemical anomalies of S than of metallic elements (HUNT, 1980). Thus, it is likely that much of the SO4 in the Salar de Hamburgo was contributed by weathering at La Escondida, which occurred chiefly during early to middle Miocene (ALPERSand BRIMHALL,1988). The principal primary (hypogene) sulfide minerals at La Escondida are pyrite, chalcopyrite and bornite, whereas the most abundant supergene copper sulfides are chalcocite-, digenite-, and covellite-like phases (ALPERS and BRIMHALL, 1989). The sulfate minerals such as anhydrite and alunite are also major constituents of hypogene alteration assemblages in Andean porphyry copper deposits (e.g. E1 Salvador, Chile; GUSTAFSON and HUNT, 1975). Alunite tends to be stable in the acidic solutions produced during weathering of sulfidic rocks; this mineral formed at La Escondida during middle Miocene as a secondary 4000I-~Jl~ I (supergene) precipitate (ALPERS and BRIMHALL, 1988, 1989). Anhydrite is known to occur at depth as part of the K-silicate alteration assemblage at La Escondida. The relative proportions of sulfide and \ sulfate minerals prior to weathering at La Escondida -¢ are not well known, but the relatively high Mg/Ca g 2000 ratio in Hamburgo Basin waters (types V, VI, and VII) indicates that sulfide oxidation was the most likely source of SO 4.

I

,~1

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I

IB

|

1000

I,

\

\

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Stable isotopes Meteoric waters (type I). The 6D and (~180values for six samples of snow and rain from the storm on 13/14-06-1984 are shown on Figs 5 and 6 and on Table 1. On Fig. 5, these points plot in reasonable agreement with the regional meteoric water line (RMWL:

sea level

-16

[ -12

\\

\ \

I ~1/ -8 -6.4 -4

~5~80 (O/oo)

\A

A -100

-60 ~5D (o/oo)

-33 -20

Fie. 6. Stable isotope for six rain and snow samples from storm on 13/14-06-1984: A. 6Jso vs elevation. B. 6D vs elevation. Point A represents extrapolation of each trend to sea level using linear least squares regression.

Hydrogeochemistry of closed basins, Atacama Desert, Chile - 0 . 9 5 4 , respectively). T h e slopes of these regressions ( T a b l e 2) are in the r a n g e of typical global t r e n d s of f r a c t i o n a t i o n with e l e v a t i o n (1.5-5.0%o 6x80 a n d 15--40%o b D p e r k m elevation; YURTSEVER a n d GAI", 1981). E x t r a p o l a t i o n of t h e s e linear regressions to sea level yields a h y p o t h e t i c a l e s t i m a t e d isotopic c o m p o s i t i o n for sea level precipitation from this s t o r m (point A , Figs 5, 6; T a b l e 2). This particular s t o r m travelled from west to east, resulting in a f r a c t i o n a t i o n t r e n d of lighter isotopes with increasing elevation. If p r e c i p i t a t i o n o c c u r r e d c o n t i n u o u s l y during this s t o r m from the west coast inland, t h e n the isotopic c o m p o s i t i o n of the precipitation at sea level would be a p p r o x i m a t e d by point A. Point A is r e a s o n a b l y close to the m e a n 6180 value of -4%0 d e t e r m i n e d from precipitation received at sea level I A E A stations in this p o r t i o n of the s o u t h - e a s t e r n Pacific region (YuRTSEVER a n d GAT, 1981). This suggests t h a t p r e c i p i t a t i o n from this s t o r m m a y have b e e n typical in isotopic c o m p o s i t i o n for storms derived f r o m the Pacific O c e a n d u r i n g an " E l Nifio" episode.

Waters from the Punta Negra Basin (types lI, II1, and IV). Samples of g r o u n d a n d surface waters from

725

t h e P u n t a N e g r a Basin show a distinct e v a p o r a t i v e t r e n d with respect to b180 a n d 6 D (Fig. 5). A linear least squares regression of 6180 vs 6 D (n = 8) yields the expression 6 D = 4.97 b l S o - 20.19. T h e slope of 4.97 is in good a g r e e m e n t with slopes b e t w e e n 4 a n d 6 o b s e r v e d for e v a p o r a t i v e processes (CRAIg, 1961). E x t r a p o l a t i o n of the a b o v e linear regression back to the R M W L gives an intersection at values of b l S o = --12.09%O a n d d D -- -80.3%0 (point B, Fig. 5), which is a r e p r e s e n t a t i o n of the c o m p o s i t i o n of a p o s t u l a t e d average m e t e o r i c source w a t e r for the P u n t a Negra Basin.

Waters from the Salar de Hamburgo (type V). F o u r w a t e r samples from the Salar de H a m b u r g o show tightly clustered values of 6 D a n d 6180 (Fig. 5, T a b l e 1). Samples ES-I-1 a n d ES-1-2 are p r o b a b l y the most r e p r e s e n t a t i v e of the shallow p o r e waters within the salar because well ES-1, s c r e e n e d f r o m 28 to 61 m d e p t h , h a d b e e n p u m p e d regularly for several years for use as drilling w a t e r during e x p l o r a t i o n and pred e v e l o p m e n t drilling at La Escondida. In contrast, samples SC-I-1 a n d RDH-250-1 are grab samples t a k e n relatively close to the w a t e r table from inactive wells. T h e m e a n isotopic values for the four samples

Table 2. Postulated source waters and linear least squares regressions of stable isotope data (Letters and values correspond to points on Fig. 5) Relevant regressions Point

blSo (%0)

6D (%0)

A

-6.4

-33.0

B C D E F G H

-12.09 -4.97 -14.28 +7.0 -6.45 -7.77 -9.44

-80.3 -49.6 -96.2 -40.0 -57.1 -58.8 -60.9

I

J K

-8.3

-1.81 -21.65

-53

-50.9 -150.0

Expression*

n~

r

blSO = --0.00205(z) - 6.42 bD = -0.0166(z) - 33

6 6

-0.898 -I).954

6D = 4.97 6180 - 20.19 -~D = 5.0 6180 - 24.85 -bD = 1.27 6180 - 48.92 ---

8 4 --6 ---

0.923 ---0.705 ---

--

-OD = 5.0 blSo - 41.80

5

--

2 --

---

*z = Elevation in meters above sea level. +n = Number of samples in regression (where value of r given) or in mean (no value of r). A. Extrapolation of type 1 meterioric water samples (storm of 13/14-06-1984) to sea level (see Fig. 6). B. Extrapolation of Punta Negra Basin Waters (types 11, III, and IV) to RMWL (FRixZ et al., 1978). C. Mean values, type V waters (Salar de Hamburgo). D. Projection of point C to RMWL, assuming evaporation at slope of 5.0. E. Possible "metamorphic" water on mixing line with La Escondida mine waters (types VI and VII). F. Possible common source for Salar de Hamburgo (type V) and La Escondida mine waters (types VI and VII). Regression for type V1 and VII waters only. G. Possible common source for Punta Negra Basin (types II, 111, and IV) and La Escondida mine waters (types VI and VII). H. Possible source on RMWL for La Escondida mine waters (types V1 and VII) assuming mixing with "metamorphic" water (point E). I. Mean values for meteoric water at El Tatio, Chile (GIGGENBACH,1978). J. Mean values for La Escondida deep mine water (type VII). K. Projection of point J to RMWL, assuming evaporation at slope of 5.0.

726

C.N. Alpers and D. O. Whittemore

are 6D = -49.6%0 and 618 O = -4.97%o (point C, Fig. 5). To deduce the origin of this water, we tentatively assume that its isotopic composition was derived exclusively by evaporation at a slope of 5.0 from point D on the R M W L (Fig. 5, Table 2). Other possible explanations for the isotopic composition of waters in the Salar de Hamburgo are discussed below, including recharge from the high Andes during the Pleistocene.

Waters and gases from within the copper sulfide deposit at La Escondida (types VI and VII). The six analyzed water samples from within the sulfide deposit at La Escondida are divided into two types based on sampling location and methods, as discussed above. The type VII samples, from the floor of the underground tunnel, show the highest 6180 values (>-2%o). The other four grab samples from open drill holes (type VI waters) have 6180 values ranging from -3.51 to -4.97%o and a relatively restricted range in 6D ( - 5 4 . 0 to -55.6%0). Taken together, these data indicate an apparent "6180 shift" of the kind observed characteristically in geothermal environments (CRAI6, 1963). This type of shift is generally due to exchange of O in H 2 0 for relatively heavy O (6180 = +5.5 to +10.0%o; TAYLOR, 1974) from "fresh" rock-forming silicates, i.e. feldspars, clays and other layer silicates, quartz, and ferromagnesian minerals. Hydrothermal silicates (e.g. clays, micas and quartz) also generally contain 61So > +3%0 (TAYLOR,1974). Hydrogen isotopes in geothermal waters are generally not affected much during this process because of the relatively low initial concentration of H in rocks; thus, the "6180 shift" is a shift to the right on 6180 vs 6D plots (e.g. Fig. 5). It is extremely unlikely that such oxygen isotope exchange takes place under supergene conditions (15-40°C) because of the sluggish nature of these reactions at low temperature (TAYLOR,1974). Clay minerals with interlayer water, such as montmorillonites (SAvIN and EPSTEIN, 1970) and halloysites (LAWRENCEand TAYLOR,1972), are more susceptible to exchange effects than kaolinite, and thus are an exception. Nevertheless, the possible effects of such clays cannot explain the observed enrichment in 6180 for the La Escondida mine waters. One hypothesis to explain the observed 6180 shift in la Escondida mine waters is that a significant component of the mine water underwent isotopic exchange with magmatic or metamorphic rocks at relatively high temperatures. It is implausible that these temperatures could be related to late effects during cooling of the hydrothermal system associated with hypogene mineralization and alteration, which has been dated by K - A r methods at 35-31 Ma (ALPERS and BRIMHALL,1988). Most chemical weathering probably took place between 18 and 14 Ma (ALPERS and BRIMHALL,1988) and temperatures probably did not exceed 40°C (EMMoNS, 1915). In hypogene and supergene clay minerals from La

Escondida 6180 and 6D indicate that supergene kaolinite near to the "kaolinite line" of LAWRENCE and TAYLOR(1972) probably did not form at temperatures significantly greater than 30°C (BIRD, 1988). Thus, a possible cause for the observed 6180 shift in La Escondida mine waters is mixing of meteoric and "metamorphic" waters. A linear least squares regression for type VI and VII samples (n = 6) yields the relation 6D = 1.27 6180 - 48.92 (r = 0.705). This could represent a mixing line between a metamorphic water such as point E (Fig. 5, Table 2) and a point such as F, G or H on either an evaporation-derived trend line or on the RMWL itself. A second interpretation of the apparent 6180 shift in the deep-mine waters is that these waters represent an evaporated remnant of ancient meteoric waters, recharged at a time when the average precipitation was significantly lighter isotopically than for the present shallower waters in the basin. Assuming a slope during evaporation of 5.0, this would correspond to evaporation of a meteoric water with 6D of - 150%0 (point K, Fig. 5, Table 2). Such depleted meteoric water could have been the dominant type of recharge during a cooler, wetter period, such as portions of the late Pleistocene. A third explanation for the apparent 6180 shift is oxygen exchange between water and carbonate minerals. Oxygen isotope exchange at low temperatures ( < 100°C) has been observed to occur at reasonably rapid rates in certain laboratory experiments (summarized by O'NEIL, 1987) where the dissolution and recrystallization of the carbonate has been an essential part of the isotopic exchange mechanism. No carbonate minerals were observed within the underground mine workings at La Escondida, which is not surprising given the intensity of hypogene and supergene alteration in this portion of the deposit. Calcite is more typically found in the propylitic alteration assemblage associated with the peripheral zones of porphyry copper systems (LOWELL and GUILBERT, 1970; ROSE, 1970; BEANE and TITLEY, 1981). In addition to carbonates related to hydrothermal alteration and mineralization that may occur at depth in the mine area, it is also possible that evaporitic carbonates may occur at depth beneath the Salar de Hamburgo and that such carbonates may have contributed dissolved inorganic carbon (DIC) to the deep mine waters. The composition of the gas found to be degassing from the type VII mine waters is given in Table 3. The gas is nearly pure N 2 with relatively high CO 2 and low 02, Ar and H 2 compared with sea-level air. The 613C of the CO2 in this gas was found to be -4.75%0 (PDB), consistent with a deep-seated (volcanic or "mantle") source (TuRI, 1986). Volcanic CO2 has been proposed as a possible source of DIC in waters within the drainage basin of the Salar de Atacama (FRITZ et al., 1978). Some of these waters have 613C of DIC ranging from - 3 . 6 to -8.5%0 and Pco2 of about 10 -2 bars (1 kPa), similar to the values from La

Hydrogeochemistry of closed basins, Atacama Desert, Chile Escondida mine waters (Table 3). Although the 613C values are by no means definitive, a regional volcanic source for dissolved CO 2 in groundwaters cannot be ruled out in this tectonically active province. The value for 3180 in C O 2 evolved from the type VII mine waters is +39.7%0. This value is close to the value for 6180 in atmospheric CO 2 of +41%o (Table 3) and is also close to the value for 6180 in CO 2 calculated to be in equilibrium with 6180 of the type VII water. Using a fractionation factor for CO2-H20 of a = 1.0412 at 25°C (FRIEDMANand O'NEIL, 1977), the calculated value for 6180 in CO 2 in equilibrium with the type VII mine water is +39.4%0. The temperature of the mine waters at the time of collection was about 15°C. The temperature coefficient of the CO2-H20 fractionation given by TRVESDELL (1974) indicates a difference of 2.1%o between 25 and 15°C. Applying this temperature leads to a calculated value of +41.5%0 for 6180 in CO2 in equilibrium with the type VII mine water, which is still fairly close to the observed value. Deep regional fault structures which could serve as conduits for postulated deep fluids are known to pass through La Escondida. In fact, the intrusive rocks coeval with hypogene mineralization and large-scale hydrothermal circulation in the district were probably localized by two such structures: (1) the Falla Oeste (West Fissure), which strikes roughly N-S throughout much of northern Chile (BAKER and GUILBERT 1987), and (2) the Falla Imilac (Imilac Fault) which strikes NW-SE through the La Escondida area and appears to have localized several other intrusive-volcanic complexes, including Volcfin Llullaillaco. Another possible mechanism for the origin of dis Table 3.Chemical analysis of gas evolved from mine water sample compared with air. (Values in wt % except as indicated) i

Constituent

Gas evolved from mine water (DDH-190-3)*

Air¢

N2 02 Ar H2 He CO2 CH4

97.0 0.91 0.07 <0.005 <0.005 2.03 0.004

78.0 21.0 0.94 0.01 0.004 0.04 --

C2H 6

<0.01

--

Totals

100.0

100.0

613C in CO2 (%0PDB)

-4.75

- 7 + I:~

6180 in CO2 (%0SMOW)

+39.7

+41 _+ l:~

*See text for sample description and methods of collection and analysis. tArter HODGMAr~(1948). :~HOEFS(1980).

727

solved CO 2 in type VII waters is the oxidation of sulfides in the immediate vicinity of the mine, accelerated by mine dewatering and ventilation. This mechanism also explains the relatively low pH (3.33.9) of the type VII mine waters. Acidification from pyrite oxidation would have converted most dissolved HCO~- to H2CO 3, which would then have degassed as CO 2 as the waters flowed into the mine workings. These factors notwithstanding, a deepseated source for the dissolved CO 2 (and/or HCO 3) cannot be ruled out. Carbon dioxide is known to attain anomalously high concentrations in soil gases over oxidizing sulfide deposits (e.g. LOVELL et al., 1983). A limited number of studies which have attempted to document the source of CO2 in this setting have called upon various combinations of oxidizing organic material, dissolved carbonates, and plant respiration products (WALLICKet al., 1981; ALPERSet al., 1990). A soil gas survey at la Escondida (D. BRABEC,unpub. data) showed a substantial CO 2 flux, especially in zones of intense fracturing and brecciation. The soil CO 2 anomalies at La Escondida could be due simply to increased surface area and therefore more oxidizing organic matter (cf. CLIFTON, 1984), or alternatively to increased permeability allowing the escape of deep-seated CO 2. Additional stable isotope analysis of soil gasses above oxidizing sulfide deposits is needed to clarify the origin of CO 2 in these settings.

Trace elements Bromide and total inorganic I were measured in eight water samples from the Salar de Hamburgo and the porphyry copper deposit at La Escondida (types V, VI, and VII waters; Table 1). The Br/C1 and I/C1 ratios were used to help determine the sources of solutes in the different waters. The most distinctive characteristics of these waters are the high I/C1 and the low Br/C! ratios in both type VII mine waters and type V Salar de Hamburgo groundwaters (Table 1). Figure 7 illustrates the unique character of the halide ratios of these waters in comparison with several other types of saline waters and brines. The Br/C1 ratio in Hamburgo Basin waters is appreciably lower than that for most oil-field brines, igneous rocks, volcanic emissions, thermal waters, and mine waters (FUGE, 1974a,b; WHITEet al., 1963), but is within the range of halite solution waters (WHITrEMORE, 1988). The I/C1 ratios for the Hamburgo Basin waters and mine waters are considerably larger than those for most oil field brines and halite solution brines (WHITE et al., 1963, WHITrEMORE, 1988), but are within the range of atmospheric precipitation and of some volcanic rocks and volcanic emissions (FUGE, 1974b,c; FUGE et al., 1986). In general, the salts in the Chilean nitrate deposits have an exceptionally high I/Br ratio ( > 10 by weight) in comparison with values for other salt deposits and

728

C.N. Alpers and D. O. Whittemore

Ix !0 3

Mole Percent I ~ 6 a l

!

~

this

studv

/

\

Na- Ca- CItype / V NaCltype / ' ~ ' ~ . .

7~.

• S. Hamburgo ~ • LaEscondidadrillhole ~ Esc°ndidamine srew ta

MinaLaCompaniaJStripawaters ~ ~ • ~ minewaters / (Nordstromet al.,1989} .~

\

TM

Brxl03

.....

/

,.-_.j,.

,

oceanwater

\

natitesolutionBrines (Whittemore,1988)

v CI

F1G. 7. Trilinear diagram for C1, Br (x 103), and 1( x 103) showing relative molar abundance of halides in water samples from various environments. Mine waters from La Escondida have among the highest I/C1 and I/Br ratios observed in natural waters. Data include: oil field brines (WHITEet al., 1963), halite solution waters (Wm~EMORE,1988), Stripa fluid inclusion leachates and groundwaters (NORDSTROMet al., 1989) and Mina La Compania mine waters (POHL, 1986).

most rocks, ground and surface waters, and atmospheric precipitation (ERICKSEN, 1983, 1986). The I/C1 ratio is -0.007 in the soluble material from Chilean nitrate ores (ERICKSEN, 1986). ERICKSEN (1983) has proposed that the I is most likely concentrated from atmospheric sources, mainly from iodine-rich organic films on the sea surface that are ejected in spray form and from volcanic emanations. The I may be later oxidized to iodate (104) either by photochemical reactions during atmospheric transport or at ground level (ERICKSEN,1983). Much of the Br which could have been similarly derived would probably have been lost in the gaseous state. The highly oxidized iodate and chlorate (C104) forms of I and CI are known to exist as solids in the Chilean nitrate ores (ERICKSEN, 1981, 1983), whereas no analogous bromate (BrO4) salts are known. These observations are consistent with the fact that the perbromate ion eluded sythesis for many years, whereas the perchlorate and periodate ions are relatively stable (HUHEEY, 1972). Salar de Hamburgo waters (type V) and the highTDS La Esconidida mine waters (type VII) fit a mixing curve for Br/C1 vs CI concentrations (Fig. 8). The curve represents the predicted chemistry of waters from conservative mixing between two hypothetical end points. The low-Cl end of the curve has Br/CI values typical for many fresh waters (Wan-rEMORE 1988). This supports the hypothesis that the main source of CI in the high-TDS mine waters (type VII) is from the dissolution of previously formed evaporite salts by meteoric waters, as discussed later. An obvious source of CI is the dissolution of halite in

evaporites accumulated in salar areas, such as the Hamburgo Basin, or blown i n from other basins by aeolian transport. The Li/C1 ratios are anomalously low in type VII mine waters (Table 1), which suggest that "metamorphic" or fossil hydrothermal waters are not the dominant source of C1. Similarly low ratios of Li/CI are found in groundwaters associated with some salt deposits in arid basins (WHITEet al., 1963). Throughout the central Andes of northern Chile and southern Bolivia, the most likely source of Li (and B) to ground and surface waters is thought to be the leaching of volcanic rocks (ERICKSEN, pers. commun., 1990). Dissolution of halite plus minor amounts of I-

1000 500

%

100 50

Q 10 5

1 0.01

I I 0.05 0.1

I 0.5

I 1

I 5

I 10

I

50 100

cI (g/I) FIG. 8. Logarithmic plot of CI vs Br/CI for waters in the Hamburgo Basin, showing possible mixing curve; symbols as in Fig. 2.

Hydrogeochemistry of closed basins, Atacama Desert, Chile bearing minerals could explain the relatively high I/Cl and relatively low Br/C1 and Li/Cl ratios observed in the type VII waters. The waters from drill holes in the mineralized area of La Escondida (type VI) have relatively low C1 concentrations. Data for I and Br are only available for one type VI sample, which indicates an appreciably lower I/C1 ratio and a somewhat higher Br/C1 ratio than for waters from the Salar de Hamburgo (type V) and the high-TDS mine waters (type VII) (Fig. 7). Nevertheless, the Br/C1 ratio for the type VI sample is close to the mixing curve in Fig. 8, so a common origin of Br and Cl is indicated.

DISCUSSION Possible sources o f chemical constituents

Several explanations are possible for the observed chemical and isotopic composition of mine waters and associated gases at La Escondida. There is little doubt that the elevated SO4 in Hamburgo Basin waters is related to weathering of the porphyry copper deposit at La Escondida. Elevated Mg/Ca ratios in Hamburgo Basin waters (Fig. 4) are most likely caused by gypsum precipitation having depleted Ca in response to a large influx of SO 4 from sulfide oxidation. The relatively high CI and I in all water samples from the Hamburgo Basin could have been derived from mixing with an unsampled "reflux" brine (cf. MACUMBER,1990) which may occur at depth. This brine could perhaps coexist with postulated evaporitic halite and carbonate zones at depths >60 m, the maximum depth penetrated by drilling beneath the Salar de Hamburgo (as of 1985). Total sediment depth in the Salar de Hamburgo is - 1 2 0 m, based on a gravity survey (Minera Utah de Chile, Inc., unpub, data). Thus, it is possible that such evaporites exist in the Salar de Hamburgo but have gone undetected due to limited drilling and water sampling. This mechanism may help to explain the salinity of the mine waters, but does not afford a straightforward explanation for the observed shift to larger 61~O values (Fig. 5). One possibility, discussed above, is that a deep brine formed from evaporation of meteoric waters which accumulated at a time in the past when the climate was wetter and/or colder (e.g. point K, Fig. 5). Another possible source of dissolved constituents is the release of saline waters from primary and pseudo-secondary fluid inclusions trapped during hydrothermal alteration and mineralization at La Escondida. Fluid inclusions in recently mined granitic rocks at Stripa, Sweden have been shown to contain I/Br and Br/Ci ratios similar to Stripa groundwaters, and therefore the inclusions have been identified as a possible source of salinity (NORDSTROMet al., 1989). Fluid inclusions trapped during alteration at La Escondida would probably show a magmatic sig-

729

T D S (g/I)

0

i 1

2 [ ~

3

5 i

A /

2

7 i

10 I

20 i

30 I

50 I

//B

.t~/

A/i/

/ /

[]

~ .4 ~ .~ .8 -10

0

1

I

2

I

3

In [ T D S (g/I)]

FIG. 9. Plot of 6ISo vs ln(TDS) (g/l). Symbols as in Fig. 2. Line A represents increase in salinity and 618O expected for equilibrium evaporative fractionation, using a = 1.009 (CRAm et al., 1963). Lines B and C represent linear least squares regressions for data from Punta Negra Basin (open symbols) and Hamburgo Basin (closed symbols), respectively. nature, i.e. 6180 - +5.5%0. Fluid inclusion waters at La Escondida could have been slowly released over millions of years due to post-supergene microfracturing in association with late Miocene to recent tectonic activity in the area. Additional microfracturing related to mining activity could also account for the release of waters from fluid inclusions. Although these mechanisms are attractive in that they would conceivably explain both the elevated I/CI ratios and the 6180 shift in the mine waters, there is a mass balance problem in accounting for the elevated concentrations of C1 and I in the groundwaters as well as the relatively large proportion of 61SO-shifted fluid that would be necessary. Also, the fluid-inclusionleakage hypothesis offers no explanation for the CO2-rich gas in the type VII waters. Theoretical considerations indicate that, under conditions of equilibrium (Rayleigh) fractionation during evaporation, a linear relation should be expected between - I n ( f ) and 6180, where f represents the fraction of remaining liquid (CRAIG et al., 1963). Residual salinity is an inverse linear function off, so a plot of 6180 vs In(salinity) should also show a linear relation (LLOYD, 1966). Plots of 6180 vs ln(TDS) and 6180 vs ln(mso,) (Figs 9, 10) show two distinct populations, corresponding to the Punta Negra Basin and Hamburgo Basin waters. On each plot the samples from the Punta Negra Basin define a generally linear trend (line B), 6180 increasing with higher salinity. The slopes of these linear trends are about one-fourth the slope that would be expected if the increase in salinity were due solely to equilibrium evaporative concentration (line A, Figs 9 and 10). Kinetic effects during evaporation cause departures from the equilibrium liquid-vapor fractionation (CRAIGe t al., 1963; SOFERand GAT, 1975). However, the departures from equilibrium at salinities below that of sea water are usually toward higher isotopic fractionation, and therefore steeper slopes on plots such as Figs 9 and 10. Although it is clear from the trend in Fig. 5 that evaporation has affected the

730

C.N. Alpers and D. O. Whittemore

isotopic composition of Punta Negra waters, it is apparent that evaporative concentration is not sufficient to account for the salinity in these waters. We propose that dissolution of pre-existing evaporite salts and/or subsurface interaction with relatively fresh volcanic rocks have been important mechanisms for the origin of the saline constituents of the Punta Negra and Hamburgo Basin waters. Since middle Miocene (or perhaps earlier), salts have slowly accumulated in salars throughout the Atacama Desert region (STOERTZand ERICKSEN, 1974). Increased rainfall during the glacial/pluvial periods of the Pleistocene would probably have caused higher groundwater levels in the Hamburgo and Punta Negra Basins, forming lakes in these basins. The initial effects of increased runoff and recharge would have been dilution of shallow groundwaters and dissolution of very soluble salts, such as haline, gypsum, and various Na-. Mg- and Ibearing salts which would have accumulated in the salars. After flushing and dilution of saline groundwaters immediately below the salars during the moister periods of the Pleistocene, the salars may not have had time to regenerate the original dissolved salt concentrations. The most readily leachable salts in the basin sediments would have been removed by Pleistocene flushing, and thus the present rate of salt precipitation could be lower than it was immediately prior to the relatively moist periods of the Pleistocene.

Probable sources o f recharge

Stable isotopic and chemical analyses of water samples from the Hamburgo and Punta Negra Basins help to constrain possible scenarios for the hydrogeologic evolution of both basins. In the present setting, the Hamburgo Basin appears to be a hydrologically static, "closed" basin with very little recharge and a nearly constant water level of 2989 + 3 m a.s.l, in the vicinity of the Salar de Hamburgo and the nearby

c SO4 (g/I) 01

0

I

02

I/

05

10

20

50

l

I

l

1

-2

[]

"~- .4

100

200

1"11"" I

/

ZS. / /

/ / / :',

W

A // -8 -10 -8

/'/~l -7

/ .///" I -6

// I

[

I

I

-5

-4

-3

-2

-1

In (m S O 4 )

Fro. 10. 6180 vs ln(mSO4). Symbols as in Fig. 2; lines as in

Fig. 9.

porphyry copper deposit at La Escondida

(BRIMHALL

et al., 1985; ALPERS and BRIMHALL,1989). The Punta

Negra Basin is characterized by relatively steep hydraulic head gradients, particularly on its eastern side, leading down to a base level of 2959 _+ 2 m (Minera Utah de Chile, unpub, data). This strongly suggests subsurface recharge of the Punta Negra Basin by snowmelt from the Andes Cordillera (Fig. 11). FRlTZ et al. (1981) noted that most precipitation in the Andes of northern Chile and Argentina arrives during the "Bolivian winter" (December-March) and probably originates in the Amazon Basin. The relatively complex history of these continentally derived air masses may explain why FRITZ et al. (1978) were unable to recognize characteristic changes in stable isotope composition as a function of elevation in their study near the Salar de Atacama (Fig. 1). Nevertheless, FmTz et al. (1981) did observe a systematic, though poorly defined, decrease in 6180 and 6D of precipitation with increasing elevation in the Pampa del Tamurgal, located - 4 0 0 km north of the Salar de Hamburgo and at comparable elevation. The storm observed on 13/14-06-1984 tracked from west to east and yielded precipitation which showed smaller values of 6180 and 6D to the east and at higher elevations (Fig. 6). This storm evidently originated in the south-eastern Pacific Ocean, and may have been associated with the late stages of the 19821983 El Nifio event (WvRTKI, 1975; BARRIE~TOS and LECOMTE, 1983). Given the similarity of 6180 and 6D in groundwaters from the Hamburgo and Punta Negra Basins (Fig. 5), this type of storm may not be typical of the precipation most commonly leading to recharge in the area. The stable isotopic data for La Escondida mine waters show a shift to higher values of 6180 with increasing salinity, accompanied by little change in 6D. This apparent "6180 shift" resembles that observed in geothermal areas, but it is unlikely that elevated temperatures were a factor in this nearsurface geological environment, based on geochronology and geological observations (ALPERS and BRIMHALL, 1988, 1989). Because oxygen isotope exchange with silicate minerals is extremely sluggish at low temperatures (TAYLOR, 1974), another mechanism must be responsible. We propose that mixing of meteoric waters with "metamorphic" waters, which have interacted with rocks at elevated temperatures at depth, can acocunt for the observed data and can explain the anomalously high CI concentrations ( - 1 0 g/l) in the deep mine waters. Elevated chloride concentrations are rare for acid mine waters, which are usually dominated by aqueous sulfate (EMMoNs, 1915; NORDSTROM, 1977; STOFF~E~EN, 1985; ALPERSet al., 1989). BARNESet al. (1984) stated that "mixing of metamorphic and meteoric waters is suspected if the concentration of chloride is h i g h . . . " . In addition, the 613C content of CO2 in gases collected from the mine workings are consistent

Hydrogeochemistry of closed basins, Atacama Desert, Chile

731

5500 5000

Hamburgo Basin :i Punta Negra Basin ,,

to Volc,~nf Llullaillaco (6751

g g n7

4500

,

La Esc0ndida / 3500

S f " ~..': ./

Salarde !

HamburgoI

4000

mine shaft / .

/

,

m)~,9.

~

.-" Andes

', , ,

/I..-" /

Pleistocene (?) lake level I

_~-"]

(3072m)

/;-"

. ~..~a

/

/

/

/

/

/

/

/

3000 2500

anthropogenic~ ,ow

,',~,~..

~

J

o

."--

~

-

<

9.i

reOOn L-i o

exaggeration P 4uu

12.5x i i 0 5

/ / o 10km

FIG. 11. Northwest-southeast cross-section connecting La Escondida and Volc~n Llullaillaco, showing schematic regional groundwater flow lines and hypothetical level of Pleistocene lake in Punta Negra Basin. Vertical exaggeration = 12.5x.

with a deep-seated ("mantle") source. The "meta- rates than at present (ALPERSand BRIMHALL,1988). morphic" fluids could have been guided to the sur- Thus, in this scenario, subterranean recharge to the Hamburgo Basin from the high Andes could have face by deep regional structures which pass through the La Escondida district. Alternatively, it is also taken place at any time since the middle Miocene possible that the stable isotopes of the type VII mine when the climate was wetter; a late Pleistocene origin waters could be due to evaporation of an ancient for the present waters in the Hamburgo Basin seems meteoric water originally quite depleted in 180 and most likely. D. It is probable that the deep waters now present in the Hamburgo Basin were recharged under SUMMARY AND CONCLUSIONS hydrologic and climatic conditions somewhat different than the present. Recharge to the Hamhurgo The groundwaters in the Hamburgo and Punta Basin by subsurface flow from the Andes is unlikely Negra Basins of northern Chile are chiefly meteoric at present, because such flow would have to pass w a t e r s , having 6180 and 6D values that have been. beneath the Salar de Punta Negra (Fig. 11). Such a modified by evaporation. Recharge to the Punta flow path would have been facilitated in the past if the Negra Basin is dominantly by subsurface flow of lowest water level in the Salar de Punta Negra had snowmelt from the Andes Cordillera to the east. A been at an elevation higher than that of the Salar de small amount of recharge to the Hamburgo Basin Hamburgo. Such conditions may have existed during may occur at present by direct precipitation from late Pleistocene when a large lake (Laguna de Punta infrequent storms. However, the lack of a discernible trend toward lighter values for 6180 and 6D in Negra) is thought to have occupied the Punta Negra Basin (LYNCH, 1986). The presence of such a former groundwaters with increasing elevation in the Hamlake is indicated by travertine deposits, which per- burgo and Punta Negra Basins suggests a common haps represent former shore lines or underwater source of recharge in the two basins. If direct precipispring zones, to the south and east of the Salar de tation is the dominant recharge mechanism in the Punta Negra at elevations ranging from 3200- Hamburgo Basin, then most precipitation probably 3400 m a.s.l. (STOERTZand ERICKSEN,1974; LATTMAN comes from the Amazon Basin via "Bolivian winter" and JONES, pers. commun.). storms rather than from the Pacific Basin such as the The Punta Negra Basin probably began to form storm observed on 13/14-06-1984, which tracked from west to east and fractionted 6180 and 6D with during early to middle Miocene, as uplift of the Andes commenced. Quebradas (incised stream increasing elevation. beds) on the NW flank of Volcdn Llullaillaco cut Dissolution of pre-existing evaporite salts and tilted sedimentary rocks of probable Miocene or weathering of Neogene volcanic rocks are the probyounger age (PERRELL6, pers. commun.), which were able sources for most of the dissolved constituents in shed to the west from the emerging Andes. The Punta Negra Basin ground and surface waters. Disdeeply incised quebradas indicate fluvial erosion solved constituents in the Hamburgo Basin waters, under conditions of higher precipitation and erosion particularly the relatively high concentrations of

732

C.N. Alpers and D. O. Whittemore

SO4, were derived from w e a t h e r i n g of t h e n e a r b y p o r p h y r y c o p p e r deposit at La E s c o n d i d a , which w e a t h e r e d most actively d u r i n g the middle M i o c e n e (ALPERS a n d BRIMHALL, 1988). Relatively high I/Br a n d I/Cl ratios in H a m b u r g o B a s i n g r o u n d waters are consistent with a regional p a t t e r n of I e n r i c h m e n t in surficial deposits (ERICKSEN, 1983, 1986). Mine waters from within the p o r p h y r y c o p p e r deposit at La E s c o n d i d a show a positive shift in 6180 of at least 3%o relative to o t h e r H a m b u r g o B a s i n waters, with only a m i n o r increase in 6D. T h e s e h i g h e r 6180 values m a y r e p r e s e n t a mixing line between m e t e o r i c a n d d e e p - s e a t e d " m e t a m o r p h i c " waters. T h e speculation t h a t some c o m p o n e n t s of the mine waters were derived from substantial d e p t h s is s u p p o r t e d by the unusually high Cl c o n t e n t of the mine waters and by the C O 2-rich gas (613C = -4.75%o) which evolved from these waters. A n a l t e r n a t i v e i n t e r p r e t a t i o n is t h a t the relatively high 6180 values m a y r e p r e s e n t e v a p o r a t i v e concent r a t i o n of m e t e o r i c waters r e c h a r g e d u n d e r w e t t e r a n d cooler climatic conditions, p e r h a p s during late Pleistocene. Acknowledgements--This study would not have been possible without the support and encouragement of the late lvan Barnes, who provided the first author with inspiration and equipment for the 1984 Atacama field season. The authors are grateful to BHP-Utah International Minerals, inc., Getty Minerals, Inc., and Minera Utah de Chile, Inc. which provided logistical support for the field work and permission to publish these results. Many employees of Minera Utah de Chile, Inc. assisted with the sampling program, including B. Sepulveda, S. AbduI-Kader, P. Schipke, R. Jones and K. Krishnamurthi. D. Ellsworth of BHP-Utah International Minerals' laboratory in Sunnyvale, CA assisted with the water chemistry; A. White and A. Yee of the Lawrence Berkeley Laboratory also provided chemical data on water samples. We thank W. Evans, of the U.S. Geological Survey, Menlo Park, for carrying out the gas analyses. Y. Kharaka and T. Presser of the U.S. Geological Survey also provided field equipment. Stable isotope measurements were performed by C. Maley and M. Huebner in the Ivan Barnes Memorial Laboratory. We also would like to thank D. K. Nordstrom for suggesting and facilitating the Br and 1 analyses. The manuscript was improved by the thoughtful reviews of G. E. Ericksen and A. Maest and by the editorial input of Y. K. Kharaka. Editorial handling: Y. K. Kharaka:

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