Earth and Planetary Science Letters 299 (2010) 447–457

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Earth and Planetary Science Letters

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Andean uplift and climate evolution in the southern Atacama Desert deduced fromgeomorphology and supergene alunite-group minerals

Thomas Bissig ⁎,1, Rodrigo RiquelmeDepartamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile

⁎ Corresponding author.E-mail address: tbissig@eos.ubc.ca (T. Bissig).

1 Mineral Deposit Resarch Unit, Department of Earthof British Columbia, 6339 Stores Road, Vancouver, B.C.,

0012-821X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.epsl.2010.09.028

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 February 2010Received in revised form 21 September 2010Accepted 21 September 2010Available online 18 October 2010

Editor: T.M. Harrison

Keywords:Atacama DesertAndesupliftclimate evolutionsupergene aluniteEl SalvadorPotrerillosgeochronologystable isotopesChile

Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Huesoepithermal gold deposit and the Coya porphyry Au prospect located in the Precordillera of Northern Chile(~26 to 26° 30´ Lat. S) have been dated by the 40Ar/39Ar method and analyzed for stable isotopes. These datasupport published geomorphologic and sedimentologic evidence suggesting that the Precordillera in theSouthern Atacama Desert had been uplifted as early as the late Eocene and, thus, significantly prior to theAltiplano which attained its high elevation only in the late Miocene.The oldest supergene alunite from the Damiana exotic deposit at El Salvador was dated at 35.8±1 Ma andyielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of atleast 3000 m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coyahave less negative δD signatures reaching values as high as−23 to−25‰ at El Hueso and El Salvador betweenabout 8.2 and 14 Ma. LateMiocene to Holocene supergene alunite, jarosite and natroalunite ages are restrictedto El Hueso and Coya located near 4000 m above sea level in the Precordillera, roughly 1000 m higher than thepresent elevation of El Salvador. The δD values of samples younger than ~5 Ma vary between−57 and−97‰.The complex evolution of the δD signatures suggests that meteoric waters recorded in supergene alunitegroupminerals were variably affected by evaporation and provides evidence for climate desiccation and onsetof hyper arid conditions in the Central Depression of the southern Atacama Desert after 15 Ma, which agreeswell with published constraints from the Atacama Desert at 23–24° Lat. S. Our data also suggest that wetterclimatic conditions than at present prevailed in the latest Miocene and early Pliocene in the Precordillera.The new and previously published age constraints for El Salvador indicate that supergene mineralization atthe Damiana exotic Cu deposit occurred periodically over 23 Ma in a locally exceptionally stablepaleohydrologic and geomorphologic configuration.

and Ocean Sciences, UniversityV6T 1Z4, Canada.

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1. Introduction

The Andean uplift history, its causes and effects on the climatehave been subject of significant research in recent years (e.g.,Garzione et al., 2008; Lamb and Davis, 2003; Schlunegger et al.,2006). Much of this work has been concentrated in the northern Chileand Altiplano transects (~18–20º Lat. S, Fig. 1). Farías et al. (2005) andVictor et al. (2004) suggest that up to 2600 m of uplift occurred in thelate middle Miocene and was accommodated by high-angle westverging faults in the western Cordillera. Geomorphologic (Garcia andHérail, 2005; Hoke et al. 2007; Schlunegger et al., 2006; Thouret et al.2007) and stable isotope evidence (Garzione et al., 2008) places themajor uplift which gave rise to the present day high elevations of theAltiplano in the late Miocene. The southern Atacama Desert (~ 26–27º

Lat. S, Fig. 1) has received comparatively less recent attention, butavailable evidence indicates that the uplift history was fundamentallydistinct, irrespective of the controversies on the exact timing ofAltiplano uplift. For example, no significant high angle west vergingfaults active during the Miocene have been documented for thesouthern Atacama Desert. In addition, geomorphologic, apatite fissiontrack and sedimentological evidence (Nalpas et al., 2005; Riquelmeet al., 2007) suggest that in the southern Atacama Desert thePrecordillera had attained considerable elevations in the Oligoceneor earlier, which greatly precedes the Altiplano uplift. We hereinassess the uplift and climate evolution in an oblique transect acrossthe Precordillera at 26–26° 30´ Lat. S (Figs. 1, 2) on the basis of the wellestablished geomorphologic framework (Bissig and Riquelme, 2009;Nalpas et al. 2008; Riquelme et al., 2003, 2007, 2008) and eleven new40Ar/39Ar ages and corresponding stable isotope data for supergenealunite group minerals from the El Salvador porphyry Cu district (e.g.,Gustafson et al., 2001), the El Hueso epithermal Au deposit (Marshet al., 1997; Thompson et al., 2004) and the Coya porphyry Auprospect (Rivera et al., 2004), all located in the southern Atacama

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Fig. 1.Map of the western Andean slope of northern Chile. The study region is outlined(Fig. 2). Dotted lines indicate physiographic boundaries from Riquelme et al. (2007).Abbreviations: CC: Coastal Cordillera; CD: Central Depression; PC: Precordillera; PD:Preandean Depression; SP: Salar de Pedernales; WC: Western Cordillera; CB: Calamabasin; CCG: Cordillera Claudio Gay. Ore deposits and prospects relevant for this paperare 1: Chuquicamata, 2: Escondida; 3: El Salvador; 3: Potrerillos/El Hueso/Coya; 5: LaCoipa.

448 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

Desert of Chile (Fig. 2). Our new age constraints complementpublished data for El Hueso and El Salvador (Marsh et al., 1997;Mote et al., 2001, respectively). Our results confirm the notion thatimportant differences exist between the timing of uplift in theAltiplano region and the Southern Atacama Desert and provide newinsights into climate evolution across the Precordillera.

2. Tectonic history and landscape evolution of the southernAtacama Desert

The present day geomorphologic configuration of the fore-arcregion of northern Chile (18–28°S) is dominated by extensivepediplain surfaces which are the result of interaction betweenclimatic and tectonic evolution during the late Cenozoic (e.g. Lamband Davis, 2003; Mortimer, 1973; Rieu, 1975; Riquelme et al., 2003).These relict pediplains have resisted significant modification througherosion for exceptionally long periods of time in some areas (e.g.,Dunai et al., 2005). The tectonic and geomorphologic evolution of thestudied transect at 26–26°30´ S Lat is summarized in the following.

Folding and thrusting in the Precordillera took place during the lateEocene Incaic Orogeny and is evident in the Potrerillos area (NiemeyerandMunizaga, 2008; Tomlinson et al., 1994). This orogenic phase led touplift, exhumation and supergene oxidation of the El Salvador porphyryCu district as early as 36 Ma (see below; Mote et al., 2001; Nalpas et al.,2005). In the Oligocene, following the Incaic orogeny, a deeply inciseddrainage network developed in the Precordillera and valleys formed atthat time were as deep as 2 km below the highest neighbouringsummits, indicating that the Precordillera was already uplifted andreached altitudes of at least 2000 m (Riquelme et al., 2007). Nosignificant movement has been documented on the principal Incaicfaults, which includes the Sierra Castillo fault (Fig. 2) representing thelocal segment of the extensive Domeyko Fault system, since the lateOligocene (Cornejo and Mpodozis, 1996; Niemeyer and Munizaga,2008). At that time, the focus of thrusting shifted east to the westernedge of the Cordillera Occidental (Cordillera Claudio Gay:Mpodozis andClavero, 2002). This shift in the locus of deformation led to the presentday configuration of the internally drained Preandean depressionhosting the Salar de Pedernales (Figs. 1, 2).

The deeply incised Oligocene valleys in the western Precordillerawere filled with continental clastic sediments with a minimum age of16.3 Ma at their base, as constrained by the oldest intercalated tufflayers (Nalpas et al., 2008). Infilling of the incised landscape of thewestern Precordillera was probably accompanied by pedimentformation as represented by the early Miocene Sierra Checo delCobre surface in the Coastal Cordillera (Mortimer, 1973). Low reliefsurfaces are present above El Hueso and La Coya in the easternPrecordillera and are tentatively assigned to the Sierra Checo delCobre surface (Fig. 2; Bissig and Riquelme, 2009). A pediplain with alocal base-level in the Salar de Pedernales incised the Sierra Checo delCobre surface in the early to middle Miocene (Asientos pediplain:Bissig and Riquelme, 2009). Later landscape evolution was largely theresult of slow tilting of the Precordillera and Central Depression thatbegan in the middle Miocene. A relatively low tilting rate resulted inthe middle Miocene alluvial fan backfilling in the Central Depressionand the formation of the Atacama Pediplain in the westernPrecordillera (Riquelme et al., 2007; Sillitoe et al., 1968). The ElSalvador porphyry Cu deposit is situated at the back-scarp of theAtacama Pediplain (Fig. 3). The Atacama Pediplain is composite innature and likely formed over several stages between ~14 and 10 Ma(Bissig and Riquelme, 2009). Minimum age constraints for this surfaceare given by an ignimbrite deposit covering the pediment surfacedated between 9 and 10 Ma (Clark et al., 1967; Cornejo et al., 1993;Riquelme et al., 2007), which is in good agreement with theradiogenic nuclide exposure age of 9 Ma reported by Niishizumiet al. (2005). A change from alluvial fan backfilling to incision of theAsientos and El Salado canyons into the relict Atacama pediplain hasbeen interpreted as being the result of slightly increased tilting rateswhich allowed the transition from a depositional to erosional regime.This led to incision of the Salado in the Central Depression andmoderate (b800 m) uplift of the Precordillera in the Late Miocene(Mortimer, 1973; Riquelme et al., 2007).

3. The use of supergene alunite group minerals

Supergene alunite groupminerals (e.g., alunite, natroalunite, jarosite)are weathering products of porphyry Cu or epithermal deposits and arefound in the leached caps of porphyry Cu deposits (Sillitoe, 2005),commonly in paleospring settings under acidic fluid conditions andupstream fromexotic Cudeposits (Mote et al., 2001). Oxidationof sulfidesin porphyryCudeposits is controlled by thefluctuations of thewater tablewhich in turn depends on tectonic and geomorphologic processes as wellas climate (Sillitoe, 2005). Supergene alunite canbedatedby the 40Ar/39Armethod (Vasconcelos, 1999) and although minor recoil loss of 39Ar mayoccur in some cases reasonable age dates are usually obtained(Vasconcelos and Conroy, 2003). Alunite group minerals can also be

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Fig. 2.Map and cross section of the Precordillera showing principal landscape elements, locations of ore deposits and other features mentioned in the text. A and A' indicate the endpoints of the cross section on the map. Cross section is slightly angled at Potrerillos. Modified from Bissig and Riquelme (2009).

449T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

analyzed for stable oxygen andhydrogen isotopes to potentially constrainthe paleometeoricwater at the time of its formation (Arehart et al., 1992;Rye et al., 1992). Since the isotopic composition of meteoric waterdepends on elevation (e.g., Poage and Chamberlain, 2001), supergenealunite has the potential to record uplift histories (Taylor et al., 1997).However, the isotopic composition of meteoric waters in arid climatesmay also be influenced by evaporation (e.g., Godfrey et al., 2003) and the

relative importance of the latter may be assessed if the tectonic andgeomorphologic framework of a region is independently constrained.

4. Samples and analytical methods

Supergene alunite, natroalunite and jarosite, ranging from pow-dery to porcellaneous, white to slightly greenish to yellowish veins,

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Fig. 3. Environment of exotic mineralization at El Salvador. A) Map of El Salvador andassociated exotic Cu deposits. The principal geomorphologic elements are shown andapproximate locations of sample sites for supergene alunite and copper wad are shown.Age data from Mote et al. (2001) and this study are indicated, the latter in bold letters(see Table 1 for more details). B) Photograph taken from upstream of the Damianaexotic deposit, looking W. The original pediment surface hosting Damiana wasdisturbed by mining.

450 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

were sampled from surface outcrops. The mineralogy was confirmedby X-ray diffraction and no significant contaminating phases (exceptfor some kaolinite in sample STB012A-2) were identified. Both 40Ar/39Ar geochronology and D/H, O and S stable isotope analyses havebeen performed on the same samples. The supergene nature of thealunite was confirmed by S isotope analyses and only samples withδ34 S (CDT) between −1.8 and +3 were considered supergene.Where the alunite was porcellaneous and not powdery, the analyzedmaterial was extracted from specific locations within the hand

specimen, where possible using a micro-drill tool. In these cases thesample for geochronology was extracted first, followed by the samplefor stable isotope analysis. Sample material for XRDwas extracted lastdue to the larger amount required. Most 40Ar/39Ar analyses wereperformed at the Noble Gas Laboratory, Pacific Centre for Isotopic andGeochemical Research (PCIGR), University of British Columbia,Vancouver, BC, Canada, but samples CTB43, CTB46, CTB48 andCTB49 were dated at the 40Ar/39Ar facility at the Geophysical Instituteat the University of Alaska at Fairbanks (UAF). At PCIGR, the sampleswere step-heated at increasing laser powers in the defocused beam ofa 10-W CO2 laser. The flux monitor used was Fish Canyon Tuffsanidine, 28.02 Ma (Renne et al., 1998). For further details onanalytical methods refer to Bissig et al. (2008). At the UAF, an 8 WAr laser was used and the flux monitor was TCR-2 with an age of27.87 Ma (Lanphere and Dalrymple, 2000); the analytical methodsare described in Layer (2000). All ages are reportedwith the analyticalerrors at the 2σ level and represent statistically relevant plateau agesunless indicated otherwise. The reported plateau ages are all withinerror of the corresponding inverse isochron ages. All 40Ar/39Ar dataare included in digital appendices.

The δ34S, δ18OSO4, δD values for alunite were determined at theQueen's University facility for Isotope Research using a methodmodified from Arehart et al. (1992) and Wasserman et al. (1992).Sulfur was extracted online with continuous-flow technology, using aFinnigan MAT 252 isotope-ratio mass spectrometer. Sulfate oxygenwas extracted using the technique of Clayton and Mayeda (1963) andhydrogen was extracted from alunite by pyrolysis. All values arereported in units of per mil (‰), and were corrected using NISTstandards 8556 for sulfur, and 8557 for sulfur and oxygen and NIST8535 for hydrogen. Sulfur is reported relative to Canyon DiabloTroilite (CDT), oxygen and hydrogen relative to Vienna StandardMean Ocean Water (V-SMOW). Analytical precision for both δ34S andδ18OSO4 values is 0.3‰ and for δD 5‰.

5. Episodes of supergene mineralization

5.1. El Salvador

Supergenemineralization atEl Salvador is principally representedbytwo exotic deposits, Damiana and Quebrada Turquesa (Figs. 2, 3). Moteet al. (2001) established an overall age range of 35.4 to 11.1 Ma forsupergene activitymostly on the basis ofMn-oxide ages in the Damianaexotic deposit. In this studywe obtained 6 additional supergene aluniteages (Fig. 4, Table 1) which confirm the overall age range at El Salvador.However, at the outcrop scale, the published ages were not reproduc-ible. At Quebrada Riolita, upstream form the Damiana exotic deposit(Fig. 3, see also Fig. 6 inMote et al., 2001) two alunite samples extractedfrom a horizontal vein were dated (Figs. 3, and 4, Table 1): sampleSTB12A-1 represents homogeneous porcellaneous alunite from thecentral part of the vein and yielded an 40Ar/39Ar age of 14.22±0.16 Ma.Sample STB12A-2 represents alunite completely replacing the feldsparsand groundmass from a rhyolitic wall rock clast within the porcellane-ous vein and was dated at 35.82±0.95 Ma. Both of our new ages areconsiderably older than the 12.89±0.06 to 13.02±0.06 Ma age rangeobtained byMote et al. (2001) from a subhorizontal vein from the sameoutcrop. Two additional sampleswere dated from the brecciated infill ofa steeply dipping fault exposed in the Quebrada Riolita outcrop. Thealunite is porcellaneous and occurs as white to pale yellowishsubangular breccia clasts of less than 1 cm in diameter (SampleSTB12B-1), as well as white to pale greenish alunite groundmass(Sample STB12B-2), which suggests that alunite was emplaced in atleast two stages separated by fault movement. Alunite extracted from aclast was dated at 15.31±0.63 Ma whereas alunite form the ground-mass yielded an age of 13.83±0.23 Ma. Mote et al. (2001) obtainedyounger ages ranging from 13.22±0.12 to 13.61±0.06 Ma from a subvertical vein in the same outcrop.

Fig. 4. 40Ar/39Ar age spectra and inverse isochron diagrams for supergene alunite from El Salvador dated in this study. Samples STB12A-1, 2 and STB12B-1,2 are from QuebradaRiolita, samples STB22 and STB26 were collected upstream from Quebrada Turquesa.

451T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

In the El Salvador district, supergene alunites outcroppingupstream from the Quebrada Turquesa exotic deposit were collected.Sample STB026 from a powdery white alunite vein yielded a plateauage of 16.31±0.12 Ma (Figs. 3, 4); an additional sample (STB022)yielded, in two separate analytical runs, reproducible age spectra withstepwise increasing ages from ~9 to 14 Ma albeit without attaining aplateau. This sample is interpreted as a mixture of two or moregenerations of fine grained alunite. Mote et al. (2001) reported onealunite age of 14.8±0.16 Ma as well as supergeneMn oxide ages from

22.9 to 14.4 Ma for Quebrada Turquesa. The geochronological resultssuggest that exotic mineralization processes at Damiana apparentlyoutlasted those at Quebrada Turquesa.

5.2. El Hueso/Potrerillos

Late Miocene supergene activity at El Hueso led to the precipitationof powdery white alunite within a fracture outcropping on theuppermost bench of the open pit at 3940 m a.s.l. near the pre-mining

Table 1List of new Ar–Ar data. 40Ar/39Ar data.

Sample Mineral Location Coord. UTM;elevation (m)

Plateau age(Ma)

Plateau/39Ar % Inv. isochron(Ma)

Preferredage

Comment

STB12A-1 Alunite ES, Qebr. Riolita 443.038/7096.252; 2700 14.22±0.16 10 of 10 steps 100% of 39Ar 14.31±0.36 14.22±0.16STB12A-2 Alunite ES, Qebr. Riolita 443.038/7096.252; 2700 35.82±0.95 5 of 8 steps, 83% of 39Ar 36.3±1.4 35.82±0.95STB12B-1 Alunite ES, Qebr. Riolita 443.038/7096.252; 2700 15.31±0.63 9 of 9 steps, 100% of 39Ar 14.7±1.5 15.31±0.63STB12B-2 Alunite ES, Qebr. Riolita 443.038/7096.252; 2700 13.83±0.23 9 of 9 steps, 100% of 39Ar 13.64±0.4 13.83±0.23STB22 Alunite ES, Qebr. Turquesa 443.881/7096.874; 2770 N/A N/A N./A 9 to 14 Ma Mix between 2

or more agesSTB26 Alunite ES, Qebr. Turquesa 443.701/7096.691; 2860 16.31±0.12 7 of 9 steps, 81.5 % of 39Ar 15.92±0.31 16.31±0.12HTB04 Alunite El Hueso 460.300/7069.153; 3940 8.19±0.1 7 of 9 steps, 62.8% of 39Ar 8.31±0.39 8.19±0.1CTB43 Alunite Coya, Plateau 461.189/7064.792; 3800 20.09±0.14 3 of 14 steps 83% of 39Ar 20.11±0.4 20.09±0.14CTB46 Jarosite Coya Maya 460.554/7065.736; 3600 N/A N/A 4.29±0.12 4.29±0.12 Excess ArgonCTB48 Natroalunite Coya Maya 460.713/7065.450; 3690 0.07±0.6 9 of 26 steps 76% 39Ar 0.39±1.4 0 Age based on two aliquots,

excess Ar in spectrumCTB49 Natroalunite Coya Maya 460.482/ 7065.289; 3710 N/A N/A 4.83±0.5 4.83±0.5 Age based on two aliquots,

excess Ar in spectrum

452 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

paleosurface. The alunite yielded an age of 8.19±0.1 Ma (Fig. 5,Table 1), which is slightly younger than the youngest supergene aluniteage reported byMarsh et al. (1997). These authors report 40Ar/39Ar agesfor supergene alunite from El Hueso of 26±1.4, 12.0. ± 0.5, 9.6±0.9,and 9.1±0.5 Ma, plus an additional jarosite age of 6.3±0.5 Ma.

5.3. Coya

At Coya, a porphyry Auprospect 4 km to SE fromElHueso (Figs. 2, 6),supergene alunite from a fracture infill collected at 3800 m elevation onthe north edge of a prominent plateau assigned to the Sierra Checosdel Cobre surface (Fig. 6; Bissig and Riquelme, 2009) yielded an ageof 20.09±0.14 Ma (Fig. 5). Three samples collected about 500 m Non a separate hill (Coya Maya, Fig. 6) have also been dated. SampleCTB-46 represents a jarosite veinlet exposed at an elevation of3600 m. No statistically significant plateau age was obtained and theage spectra from two analytical runs reveal possible 39Ar recoil loss(Fig. 5). A pseudo-plateau containing only two analytical fractionsyielded an age around 4.4 Ma, which is within error of the inverseisochron age of 4.29±0.12 Ma obtained from both aliquots (Fig. 5,Table 1). The latter is taken as the preferred age. A similar age wasobtained for sample CTB-49, which, based on the 40Ar/39Ar and XRDanalysis, consists of natroalunite mixed with minamiite (Na,Ca,K)Al3(SO4)2(OH)6). This sample is also from Coya Maya (3690 melevation) and yielded an isochron age of 4.83±0.56 Ma on thebasis of two aliquots, but similar to sample CTB-46, the age spectramay be affected by 39Ar recoil loss (Fig. 5). Thus, neither of thealiquots provides a statistically significant age spectrum, but run 1yielded a pseudo-plateau age of 5.8±0.8 Ma when the errors areincreased to two sigma on the individual heating steps. Due to theevidence for recoil effects we prefer the inverse isochron age. Anadditional sample of natroalunite (CTB-48) was dated from CoyaMaya. Scanning Electron Microscope energy dispersive analysisdetermined the presence of sufficient K for 40Ar/39Ar dating. Thissample, like the other samples from CoyaMaya, exhibits evidence forrecoil effects but two analytical runs yielded an age not significantlydifferent from zero (Fig. 5, Table 1).

6. Stable isotope constraints

The alunites dated in this study have all been analysed for δ34 S,δ18OSO4 and δD isotopic composition (Fig 7; Table 2). The δ34 S valuesserve to confirm the supergene nature of the alunite. δD values ofhydroxyl groups in the alunite directly reflect the meteoric watercompositions at the time of supergene processes, because thehydrogen isotopic fractionation between water and alunite ornatroalunite is minimal at surface temperatures (Bird et al., 1989;

Rye et al., 1992) and the δD of water in equilibrium with alunite iswithin the analytical uncertainty from the latter. δ18O values on thesulphate oxygen in the supergene alunite occupy a wide range due tothe incorporation of oxygen both from the water as well as theatmosphere (Rye et al., 1992).

The late Eocene alunite from Quebrada Riolita yielded a δD value of−73‰ whereas the other alunites from the same location exhibit amarked increase in δD from −61‰ at 15.4 Ma to −50‰ at 13.8 Ma(Fig. 7). Alunites from the headwaters of Quebrada Turquesa exhibitsignificantly higher δD values of −34 to −23‰ at ages younger than16.3 Ma. The δD composition of the 8.2 Ma alunite sample from ElHueso is at −25‰, similar to those from Quebrada Turquesa.

At Coya, the early Miocene alunite has a δD value of −53‰,whereas the early Pliocene natroalunite and jarosite yielded stronglynegative δD values of−88‰ and−97‰ respectively. The most recentsupergene natroalunite has at −57‰ a less negative δD composition.

7. Discussion

7.1. Chronology of supergene oxidation

As documented for an outcrop near the Damiana exotic deposit,ages of supergene alunite vary widely within a single outcrop or vein,indicating that fluids from which these supergene minerals precip-itate exploit the same permeability network periodically overextended periods of time. Although this has been known on aporphyry district scale (Sillitoe, 2005), our data, combined withpublished data (Mote et al., 2001) suggest that this is also the case at alocal scale at the Damaina exotic deposit. Here, both within the exoticdeposit as well as at the corresponding paleo spring setting ages rangefrom about 36 to 13 Ma, indicating that exotic mineralizationprocesses operated periodically over 23 Ma in an individual oreforming system. Thus, the permeability network exploited bysupergene fluids remained active over an extended period of timeand implies that the local geomorphologic configuration has notchanged substantially. Although the pediment hosting Damiana haslikely experienced modifications and was shaped most recentlyduring the formation of the multi-stage Atacama pediplain, erosionwas never substantial enough to strip the gravels down to thesupergene ore.

The timing of the cessation of supergene activity in the CentralDepression and western Precordillera, proposed at ca. 13 Ma (Moteet al., 2001), has been roughly confirmed. The respective youngestsupergene ages of Damiana and Quebrada Turquesa correspond to theinferred relative ages of the pediment surfaces hosting these twoexotic deposits (Figs. 3, 8), indicating a potential link between localpediment formation and exotic mineralization. The cessation of

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Cumulative 39Ar percent

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Cumulative 39Ar percent

36A

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39Ar/40Ar 39Ar/40Ar 39Ar/40Ar

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HTB4 Alunite

HTB4 Alunite CTB43 Alunite

CTB43 Alunite

CTB48, Natroalunite, run 1 CTB49, Natroalunite, run 1

CTB46 Jarosite, run 2 CTB48, Natroalunite, run 2 CTB49, Natroalunite, run 2

CTB49, Natroalunite, 2 runs

8.19 ± 0.1 Ma

20.09 ± 0.14 Ma

8.31 ± 0.39 Ma20.36 ± 0.95 Ma

3 steps at ~5.8 +/- 0.8 Ma

Reference zero age line

4.83 +/- 0.28 Ma

Fig. 5. 40Ar/39Ar age spectra and inverse isochron diagrams for supergene alunite group minerals from El Hueso and Coya dated in this study. Sample HTB04 is from El Hueso, theremainder of samples are from Coya.

453T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

Coya

Coya Maya

a: 20.09 +/- 0.14 Ma

n: ~4.8 Maand zero age

j: ~4.3 Ma

Relict Asientos pediplain

Asientos Canyon

Cordillera Claudio Gay

Fig. 6. View from Cerro El Hueso (see Fig. 2 for location) towards the E showing the Coya prospect with sample locations and supergene alunite (a), natroalunite (n) and jarosite(j) ages. The horizon is represented by the Cordillera Claudio Gay. Gravel covered relics of the Asientos Pediplain are indicated.

454 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

supergene alunite precipitation at El Salvador occurred at a similartime as in porphyry Cu and epithermal districts farther North (e.g.,Arancibia et al., 2006; Bouzari and Clark, 2002; Hartley and Rice, 2005;Sillitoe and McKee, 1996), which, together with other paleoclimaticevidence (Alpers and Brimhall, 1988; Rech et al., 2006) indicatesclimate desiccation in the middle Miocene (Fig. 8).

The periods of most intense supergene activity in the lateOligocene and Middle Miocene originally defined for northern Chileand southern Peru (Sillitoe and McKee, 1996) become more blurry asmore geochronological data become available (Hartley and Rice,2005) and recent studies suggest a continuous period of intensesupergene processes lasting from the late Eocene to the early lateMiocene in Northern Chile (Arancibia et al., 2006). Our results areconsistent with a prolonged history of supergene mineralization forthe El Salvador district.

In the eastern Precordillera at El Hueso and Coya, at elevationsapproximately 1000–1200 m higher than at El Salvador, 40Ar/39Arconstraints, admittedly still limited, indicate that supergene pro-cesses occurred in the late Oligocene and early Miocene as wellas from the late Miocene to early Pliocene and may still be occur-ring at the present day (Fig. 8). Contrasting with El Salvador, super-gene oxidation in the eastern Precordillera appears to have beenlimited throughout the middle Miocene. While the late Oligoceneand early Miocene ages roughly coincide with the incision of theSierra Checos del Cobre and Asientos pediplains (Fig. 8) and super-gene oxidation may have been related to these erosive processes, weinterpret the Late Miocene and younger oxidation to be controlledby uplift to elevations sufficient to capture increased precipitationcombined with the incision of deep canyons into the previous planarlandscape (Bissig and Riquelme, 2009). This would lead todepression of the water table, but increased availability of meteoricwater in the vadose zone, generating conditions favorable for sulfideoxidation.

7.2. δD through time

The Late Eocene meteoric water at El Salvador was at δD=−73‰similar to the present day precipitation at~3500 m a.s.l. whencalculated using the empirical relationship for South America fromPoage and Chamberlain (2001). The estimated elevation for the Late

Eocene would be no more than 500 m lower if the long term oxygenisotopic variations in seawater (Zachos et al., 2001) are considered.Miocene meteoric waters are considerably less deuterium depletedand the least negative δD values of −23 to −34‰ were obtained forsamples between 8.2 and 16.3 Ma from both El Hueso and El Salvador.Early Pliocene waters at Coya were at δD=−88 to −97‰ similar topresent day precipitation around the 3800–4000 m elevation at whichCoya is presently situated (Poage and Chamberlain, 2001). The mostrecent sample yielded a less negative δD value of −57‰. Our datastarkly contrast earlier work (Taylor et al., 1997) which suggestssharply decreasing δD values from ~−15‰ in the Late Oligocene to asmuch as−60‰ in the middle to late Miocene which they interpret asevidence for a marked uplift pulse in the Middle Miocene. Thediscrepancy between the two datasets can probably be explained bythe different scales of the two studies. Taylor et al. (1997) analyzedalunite samples from 20 to 27º S Lat S (see also Sillitoe and McKee,1996) which likely reflect significant along strike variations ingeomorphology, uplift history and climate. North of about 23º Lat. S,there is no Preandean Depression (Fig. 1) and independent evidencesuggests that much of the uplift of the Altiplano has occurred in themiddle or late Miocene (e.g., Gregory-Wodzicki, 2000; Hoke et al.,2007). In the southern Atacama Desert, the Precordillera attainedelevations of at least 2000 m in the early Oligocene (Riquelme et al.,2007) and our stable isotope data suggest a Late Eocene elevation of3000 m a.s.l. or more for the Precordillera near El Salvador. These highelevations may be attributed to intense folding and thrusting(Niemeyer and Munizaga, 2008) and crustal thickening (e.g., Haschkeet al., 2002) affecting the region in the late Eocene.

The increasing δD values throughout the middle Miocene arecontrary to the trend expected for an uplifting mountain range.However, the isotopic composition of meteoric water is not onlycontrolled by orographic effects, but also by evaporation and recyclingof meteoric water (Godfrey et al., 2003). Thus, we interpret the higherthan expected Miocene δD values largely as an effect of evaporation.Bird et al. (1989) and Sillitoe (2005) suggested that high rates ofevaporation are conducive for supergene alunite formation, providingsupport to our interpretation. Thus, the least negative δD valueswould coincide with the most intense evaporation and hyper aridconditions which likely persisted between about 15 and 8 Ma. Thetiming of the onset of hyper-arid conditions is also recorded by a

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er d

omin

ant

Fig 7. Stable isotope composition of supergene alunite group minerals. A) δD vs.δ18Οso4. Note that all alunite samples fall within the large field for supergene alunitebut generally closer to the air dominated than water dominated oxygen isotopecomposition. The jarosite sample plots immediately right of the air dominatedboundary for supergene alunite. Reference field for high altitude precipitation for theChilean Altiplano is from Herrera et al. (2006). B) δD isotopic composition of supergenealunite group minerals through time. The right vertical axis is labeled with theelevations corresponding to the δD values on the left axis. Values were calculated usingthe empirically determined relationship for central and South America (Poage andChamberlain, 2001). Interpreted general climatic trends are indicated (see text fordiscussion).

Table 2Stable isotope data.

Sample Mineral dD d34S d18OSO4 age (Ma)

HTB004 Alunite −25 −1.8 3.7 8.19±0.1STB-022 Alunite −23 1.4 4.8 9 to 14STB-026 Alunite −34 −0.5 6.8 16.31±0.12STB-12A-1 Alunite −54 −0.9 5.3 14.22±0.16STB-12A-2 Alunite −74 0.3 9.7 35.8±1STB-12B-1 Alunite −61 0.1 4.1 15.3±0.6STB-12B-2 Alunite −50 0.0 3.9 13.8±0.2CTB-43 Alunite −53 0.0 10.7 20.1±0.1CTB-46 Jarosite −97 0.8 11.0 4.3±0.1CTB-48 Natroalunite −57 1.1 2.8 0CTB-49 Natroalunite −88 3.0 2.6 4.8±0.6

455T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

marked decrease of sediment accumulation in the Central Depressionin the middle Miocene (Nalpas et al., 2008; Riquelme et al., 2007).Given that the Precordillera attained considerable elevations signif-icantly prior to the middle Miocene hyper arid climate, the uplift ofthemountain range probably does not by itself account for the climatedesiccation in the southern Atacama Desert (see also Lamb and Davis,2003). However, the eastward migration of the deformation into theCordillera Claudio Gay in the late Oligocene (Mpodozis and Clavero,2002) and the formation of the Preandean depression likely enhancedaridification of the Central Depression. We suggest that the wideningrather than simply the uplift of the Andes likely has resulted inincreased rain shadow effects at the western Andean slope.

Somewhat wetter conditions probably dominated the earlyPliocene in the Precordillera when compared to the arid middleMiocene climate. Stable isotope evidence suggests that the Precordil-lera probably had attained elevations similar to the present and thatevaporation effects were limited. This is interpreted as the result ofincreased capture of orographically controlled precipitation at thattime. Moreover, sedimentological evidence in the Calama basin, some400 km farther north (Fig. 1; Hartley and Chong, 2002), indicates thatsemiarid climatic conditions prevailed in the Precordillera andwestern Andes between about 6 and 3 Ma.

The present climate and hydrologic conditions in the easternPrecordillera are potentially still wet enough to permit the formationof supergene alunite groupminerals, but significant evaporation likelyaffects the meteoric waters. Strong evaporation effects have beendocumented for meteoric waters in the internally drained basins ofthe Salar de Hombre Muerto and Salar de Atacama basins (Godfreyet al., 2003).

8. Conclusions

− Geomorphologic and stable isotope evidence strongly suggeststhat the Precordillera in the Southern Atacama Desert has attainedelevations of at least 3000 m a.s.l. already in the early Oligoceneand thus, significantly prior to the major uplift of the Altiplano.

− The climate evolved differently in the western Precordillera andCentral Depression from the eastern Precordillera. The cessation ofsupergene processes at El Salvador around 13 Ma has beenconfirmed and is attributed to climate desiccation, an interpreta-tion also supported by sedimentological and stable isotopeevidence. However, conditions at Coya and El Hueso in the EasternPrecordillera, situated near 4000 m present day elevationremained conducive for at least episodic supergene aluniteformation until the early Pliocene, and possibly up to the presentday. Uplift to elevations near 4000 m a.s.l. have led to increasedcapture of moisture and consequently increased availability ofmeteoric waters.

− The new 40Ar/39Ar age constraints presented herein provideevidence confirming the previously proposed protracted historyof the Damiana exotic Cu deposit and indicate that the localgeomorphologic and hydrologic configuration has remainedrelatively stable over 23 Ma.

Supplementary data to this article can be found online at doi:10.1016/j.epsl.2010.09.028.

Acknowledgements

This study has been funded by Fondo Nacional de DesarrolloCientífico y Tecnológico de Chile (Fondecyt) grant # 11060516. KerryKlassen is thanked for the stable isotope analyses whereas Paul Layerand TomUllrich provided the Ar/Ar analyses. Fritz Schlunegger and ananonymous EPSL reviewer are thanked for their constructive reviews.This is MDRU publication P-264.

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slow tiltingin the fore-arc

moderate tiltingin the fore-arc

Literature This study

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8)

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Fig. 8. Chart integrating landscape chronology, tectonic episodes, ages of supergene minerals and climate. Abbreviations: A1: early stage Atacama pediplain, A2: Main stage Atacamapediplain; A3: late stage Atacama pediplain; AS: Asientos surface; SC: Sierra Checos del Cobre surface. WP: Western Precordillera; EP: Eastern Precordillera. Supergene ages areplotted individually (black bars; bold correspond to this study) or as groups of ages (boxes; number of dates indicated). References as follows: supergene ages from El Hueso: Marshet al. (1997); supergene ages from El Salvador: Mote et al. (2001); Pediment formation: Mortimer (1973), Sillitoe et al. (1968), Bissig and Riquelme (2010); canyon incision:Riquelme et al. (2003, 2007); Gravel deposition: Riquelme et al. (2007); Tectonic episodes: Niemeyer and Munizaga (2008), Mpodozis and Clavero (2002), Riquelme et al. (2003,2007).

456 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

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