Andean Uplift and Climate Evolution in the Southern Atacama Desert Deduced from Geomorphology and Supergene Alunite-Group Minerals

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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 from geomorphology and supergene alunite-group minerals

Thomas Bissig ⁎,1, Rodrigo Riquelme

Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile article info abstract

Article history: Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Hueso Received 5 February 2010 epithermal gold deposit and the Coya porphyry Au prospect located in the Precordillera of Northern Chile Received in revised form 21 September 2010 (~26 to 26° 30´ Lat. S) have been dated by the 40Ar/39Ar method and analyzed for stable isotopes. These data Accepted 21 September 2010 support published geomorphologic and sedimentologic evidence suggesting that the Precordillera in the Available online 18 October 2010 Southern Atacama Desert had been uplifted as early as the late Eocene and, thus, signiﬁcantly prior to the Editor: T.M. Harrison Altiplano 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 and Keywords: yielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of at Atacama Desert least 3000 m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coya Andes have less negative δD signatures reaching values as high as −23 to −25‰ at El Hueso and El Salvador between uplift about 8.2 and 14 Ma. Late Miocene to Holocene supergene alunite, jarosite and natroalunite ages are restricted climate evolution to El Hueso and Coya located near 4000 m above sea level in the Precordillera, roughly 1000 m higher than the supergene alunite present elevation of El Salvador. The δD values of samples younger than ~5 Ma vary between −57 and −97‰. El Salvador The complex evolution of the δD signatures suggests that meteoric waters recorded in supergene alunite Potrerillos geochronology group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset stable isotopes of hyper arid conditions in the Central Depression of the southern Atacama Desert after 15 Ma, which agrees Chile well with published constraints from the Atacama Desert at 23–24° Lat. S. Our data also suggest that wetter climatic 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 at the Damiana exotic Cu deposit occurred periodically over 23 Ma in a locally exceptionally stable paleohydrologic and geomorphologic conﬁguration. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Lat. S, Fig. 1) has received comparatively less recent attention, but available evidence indicates that the uplift history was fundamentally The Andean uplift history, its causes and effects on the climate distinct, irrespective of the controversies on the exact timing of have been subject of significant research in recent years (e.g., Altiplano uplift. For example, no significant high angle west verging Garzione et al., 2008; Lamb and Davis, 2003; Schlunegger et al., faults active during the Miocene have been documented for the 2006). Much of this work has been concentrated in the northern Chile southern Atacama Desert. In addition, geomorphologic, apatite fission and Altiplano transects (~18–20º Lat. S, Fig. 1). Farías et al. (2005) and track and sedimentological evidence (Nalpas et al., 2005; Riquelme Victor et al. (2004) suggest that up to 2600 m of uplift occurred in the et al., 2007) suggest that in the southern Atacama Desert the late middle Miocene and was accommodated by high-angle west Precordillera had attained considerable elevations in the Oligocene verging faults in the western Cordillera. Geomorphologic (Garcia and or earlier, which greatly precedes the Altiplano uplift. We herein Hérail, 2005; Hoke et al. 2007; Schlunegger et al., 2006; Thouret et al. assess the uplift and climate evolution in an oblique transect across 2007) and stable isotope evidence (Garzione et al., 2008) places the the Precordillera at 26–26° 30´ Lat. S (Figs. 1, 2) on the basis of the well major uplift which gave rise to the present day high elevations of the established geomorphologic framework (Bissig and Riquelme, 2009; Altiplano in the late Miocene. The southern Atacama Desert (~ 26–27º Nalpas et al. 2008; Riquelme et al., 2003, 2007, 2008) and eleven new 40Ar/39Ar ages and corresponding stable isotope data for supergene alunite group minerals from the El Salvador porphyry Cu district (e.g., ⁎ Corresponding author. Gustafson et al., 2001), the El Hueso epithermal Au deposit (Marsh E-mail address: [email protected] (T. Bissig). 1 Mineral Deposit Resarch Unit, Department of Earth and Ocean Sciences, University et al., 1997; Thompson et al., 2004) and the Coya porphyry Au of British Columbia, 6339 Stores Road, Vancouver, B.C., V6T 1Z4, Canada. prospect (Rivera et al., 2004), all located in the southern Atacama

Folding and thrusting in the Precordillera took place during the late Eocene Incaic Orogeny and is evident in the Potrerillos area (Niemeyer and Munizaga, 2008; Tomlinson et al., 1994). This orogenic phase led to 70° W 69° W 68° W 18° S uplift, exhumation and supergene oxidation of the El Salvador porphyry Cu district as early as 36 Ma (see below; Mote et al., 2001; Nalpas et al.,

Altiplano ARICA 2005). In the Oligocene, following the Incaic orogeny, a deeply incised drainage network developed in the Precordillera and valleys formed at that time were as deep as 2 km below the highest neighbouring summits, indicating that the Precordillera was already uplifted and

Segment reached altitudes of at least 2000 m (Riquelme et al., 2007). No SouthAmerica signiﬁcant movement has been documented on the principal Incaic IQUIQUE faults, which includes the Sierra Castillo fault (Fig. 2) representing the local segment of the extensive Domeyko Fault system, since the late Oligocene (Cornejo and Mpodozis, 1996; Niemeyer and Munizaga, P 2008). At that time, the focus of thrusting shifted east to the western edge of the Cordillera Occidental (Cordillera Claudio Gay: Mpodozis and Clavero, 2002). This shift in the locus of deformation led to the present O C E A N WC day conﬁguration of the internally drained Preandean depression 1 hosting the Salar de Pedernales (Figs. 1, 2). CALAMA The deeply incised Oligocene valleys in the western Precordillera

TRENCH CB were ﬁlled with continental clastic sediments with a minimum age of CD 23° S 16.3 Ma at their base, as constrained by the oldest intercalated tuff layers (Nalpas et al., 2008). Inﬁlling of the incised landscape of the PD western Precordillera was probably accompanied by pediment ANTOFAGASTA formation as represented by the early Miocene Sierra Checo del Cobre surface in the Coastal Cordillera (Mortimer, 1973). Low relief P A C I F 2 Southern Atacama Desert surfaces are present above El Hueso and La Coya in the eastern CHILE CC Precordillera and are tentatively assigned to the Sierra Checo del

(Puna Segment) Cobre surface (Fig. 2; Bissig and Riquelme, 2009). A pediplain with a

- local base-level in the Salar de Pedernales incised the Sierra Checo del TALTAL Cobre surface in the early to middle Miocene (Asientos pediplain: Bissig and Riquelme, 2009). Later landscape evolution was largely the result of slow tilting of the Precordillera and Central Depression that

PERÚ 3 SP began in the middle Miocene. A relatively low tilting rate resulted in CHAÑARAL 4 the middle Miocene alluvial fan backﬁlling in the Central Depression

5 CCG 27° S and the formation of the Atacama Pediplain in the western COPIAPO Precordillera (Riquelme et al., 2007; Sillitoe et al., 1968). The El 50 0 50 100 km PC Salvador porphyry Cu deposit is situated at the back-scarp of the Atacama Pediplain (Fig. 3). The Atacama Pediplain is composite in nature and likely formed over several stages between ~14 and 10 Ma Fig. 1. Map of the western Andean slope of northern Chile. The study region is outlined (Bissig and Riquelme, 2009). Minimum age constraints for this surface (Fig. 2). Dotted lines indicate physiographic boundaries from Riquelme et al. (2007). Abbreviations: CC: Coastal Cordillera; CD: Central Depression; PC: Precordillera; PD: are given by an ignimbrite deposit covering the pediment surface Preandean Depression; SP: Salar de Pedernales; WC: Western Cordillera; CB: Calama dated between 9 and 10 Ma (Clark et al., 1967; Cornejo et al., 1993; basin; CCG: Cordillera Claudio Gay. Ore deposits and prospects relevant for this paper Riquelme et al., 2007), which is in good agreement with the are 1: Chuquicamata, 2: Escondida; 3: El Salvador; 3: Potrerillos/El Hueso/Coya; 5: La radiogenic nuclide exposure age of 9 Ma reported by Niishizumi Coipa. et al. (2005). A change from alluvial fan backfilling to incision of the Asientos and El Salado canyons into the relict Atacama pediplain has Desert of Chile (Fig. 2). Our new age constraints complement been interpreted as being the result of slightly increased tilting rates published data for El Hueso and El Salvador (Marsh et al., 1997; which allowed the transition from a depositional to erosional regime. Mote et al., 2001, respectively). Our results confirm the notion that This led to incision of the Salado in the Central Depression and important differences exist between the timing of uplift in the moderate (b800 m) uplift of the Precordillera in the Late Miocene Altiplano region and the Southern Atacama Desert and provide new (Mortimer, 1973; Riquelme et al., 2007). insights into climate evolution across the Precordillera. 3. The use of supergene alunite group minerals 2. Tectonic history and landscape evolution of the southern Atacama Desert Supergene alunite group minerals (e.g., alunite, natroalunite, jarosite) are weathering products of porphyry Cu or epithermal deposits and are The present day geomorphologic configuration of the fore-arc foundintheleachedcapsofporphyryCudeposits(Sillitoe, 2005), region of northern Chile (18–28°S) is dominated by extensive commonly in paleospring settings under acidic fluid conditions and pediplain surfaces which are the result of interaction between upstream from exotic Cu deposits (Mote et al., 2001). Oxidation of sulfides climatic and tectonic evolution during the late Cenozoic (e.g. Lamb in porphyry Cu deposits is controlled by the fluctuations of the water table and Davis, 2003; Mortimer, 1973; Rieu, 1975; Riquelme et al., 2003). which in turn depends on tectonic and geomorphologic processes as well These relict pediplains have resisted significant modification through as climate (Sillitoe, 2005). Supergene alunite can be dated by the 40Ar/39Ar erosion for exceptionally long periods of time in some areas (e.g., method (Vasconcelos, 1999) and although minor recoil loss of 39Ar may Dunai et al., 2005). The tectonic and geomorphologic evolution of the occur in some cases reasonable age dates are usually obtained studied transect at 26–26°30´ S Lat is summarized in the following. (Vasconcelos and Conroy, 2003). Alunite group minerals can also be T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457 449

70°30´ 70° 69°30´

C. Doña Inés A

NNotot mmappedapped

QTuQ . q Tu

res q u

a El Salvador 26° 26° DDmaaiamiannaa on

anyanyon

C C o

da ldl o a

Salar de Pedernales El S

AsientosAsien Ca

tos Ca

nyo n

Jerónimo Potrerillos El Hueso 26°15´ 26°15´

C. El Hueso Coya Pediment surfaces Sierra Checos N del Cobre Asientos 10 km Early Atacama Cerros Bravos (> 14?) Atacama >10 Ma Atacama < 10 Ma

Pediment backscarp

Sierra Castillo Fault 26°30´ (approx. trace) A`

Asientos Canyon El Salado Canyon Asientos Canyon Cerros Bravos m a.s.l. A A` 5000 El Hueso Potrerillos Coya

4000 El Salvador Sierra Checos del Cobre Asientos 3000 Early Atacama Atacama pediplain SCF 2000 04020 80 80 Km

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 end points of the cross section on the map. Cross section is slightly angled at Potrerillos. Modiﬁed from Bissig and Riquelme (2009). analyzed for stable oxygen and hydrogen isotopes to potentially constrain relative importance of the latter may be assessed if the tectonic and the paleo meteoric water at the time of its formation (Arehartetal.,1992; geomorphologic framework of a region is independently constrained. Rye et al., 1992). Since the isotopic composition of meteoric water depends on elevation (e.g., Poage and Chamberlain, 2001), supergene 4. Samples and analytical methods alunite has the potential to record uplift histories (Taylor et al., 1997). However, the isotopic composition of meteoric waters in arid climates Supergene alunite, natroalunite and jarosite, ranging from pow- may also be inﬂuenced by evaporation (e.g., Godfrey et al., 2003)andthe dery to porcellaneous, white to slightly greenish to yellowish veins, 450 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

69°37’ W specimen, where possible using a micro-drill tool. In these cases the A sample for geochronology was extracted first, followed by the sample 1km N for stable isotope analysis. Sample material for XRD was extracted last Q. Turquesa due to the larger amount required. Most 40Ar/39Ar analyses were performed at the Noble Gas Laboratory, Pacific Centre for Isotopic and 21.5 Geochemical Research (PCIGR), University of British Columbia, 14.4 14.8 Vancouver, BC, Canada, but samples CTB43, CTB46, CTB48 and 16.3 CTB49 were dated at the 40Ar/39Ar facility at the Geophysical Institute 22.9 at the University of Alaska at Fairbanks (UAF). At PCIGR, the samples 21.4 were step-heated at increasing laser powers in the defocused beam of a 10-W CO laser. The flux monitor used was Fish Canyon Tuff El Salvador 2 sanidine, 28.02 Ma (Renne et al., 1998). For further details on townsite analytical methods refer to Bissig et al. (2008). At the UAF, an 8 W 26°15’ S 13.6, 13.5, 13.2 Ar laser was used and the flux monitor was TCR-2 with an age of 13.0, 12.9, 12.0 27.87 Ma (Lanphere and Dalrymple, 2000); the analytical methods 35.8, 15.3, are described in Layer (2000). All ages are reported with the analytical 14.2, 13.8 Damiana errors at the 2σ level and represent statistically relevant plateau ages 35.4 19.4 unless indicated otherwise. The reported plateau ages are all within 25.3 error of the corresponding inverse isochron ages. All 40Ar/39Ar data 13.9 are included in digital appendices. 11.1 34 18 The δ S, δ OSO4, δD values for alunite were determined at the Queen's University facility for Isotope Research using a method modified from Arehart et al. (1992) and Wasserman et al. (1992). Sulfur was extracted online with continuous-flow technology, using a Finnigan MAT 252 isotope-ratio mass spectrometer. Sulfate oxygen was extracted using the technique of Clayton and Mayeda (1963) and hydrogen was extracted from alunite by pyrolysis. All values are reported in units of per mil (‰), and were corrected using NIST standards 8556 for sulfur, and 8557 for sulfur and oxygen and NIST 8535 for hydrogen. Sulfur is reported relative to Canyon Diablo Alunite age Landscape elements Troilite (CDT), oxygen and hydrogen relative to Vienna Standard (this study in bold) Mean Ocean Water (V-SMOW). Analytical precision for both δ34S and Main Atacama 18 Copper wad age δ OSO4 values is 0.3‰ and for δD 5‰. Early Atacama (>14 Ma) Exotic Cu deposit Inselbergs 5. Episodes of supergene mineralization Primary Cu deposit (El Salvador) 5.1. El Salvador

B Supergene mineralization at El Salvador is principally represented by El Salvador two exotic deposits, Damiana and Quebrada Turquesa (Figs. 2, 3). Mote Town et al. (2001) established an overall age range of 35.4 to 11.1 Ma for supergene activity mostly on the basis of Mn-oxide ages in the Damiana exotic deposit. In this study we obtained 6 additional supergene alunite Damiana ages (Fig. 4, Table 1) which confirm the overall age range at El Salvador. However, at the outcrop scale, the published ages were not reproducible. At Quebrada Riolita, upstream form the Damiana exotic deposit (Fig. 3, see also Fig. 6 in Mote et al., 2001) two alunite samples extracted from a horizontal vein were dated (Figs. 3, and 4, Table 1): sample STB12A-1 represents homogeneous porcellaneous alunite from the Fig. 3. Environment of exotic mineralization at El Salvador. A) Map of El Salvador and central part of the vein and yielded an 40Ar/39Ar age of 14.22±0.16 Ma. associated exotic Cu deposits. The principal geomorphologic elements are shown and approximate locations of sample sites for supergene alunite and copper wad are shown. Sample STB12A-2 represents alunite completely replacing the feldspars Age data from Mote et al. (2001) and this study are indicated, the latter in bold letters and groundmass from a rhyolitic wall rock clast within the porcellane- (see Table 1 for more details). B) Photograph taken from upstream of the Damiana ous vein and was dated at 35.82±0.95 Ma. Both of our new ages are exotic deposit, looking W. The original pediment surface hosting Damiana was considerably older than the 12.89±0.06 to 13.02±0.06 Ma age range disturbed by mining. obtained by Mote et al. (2001) from a subhorizontal vein from the same outcrop. Two additional samples were dated from the brecciated infill of a steeply dipping fault exposed in the Quebrada Riolita outcrop. The were sampled from surface outcrops. The mineralogy was confirmed alunite is porcellaneous and occurs as white to pale yellowish by X-ray diffraction and no significant contaminating phases (except subangular breccia clasts of less than 1 cm in diameter (Sample for some kaolinite in sample STB012A-2) were identified. Both 40Ar/ STB12B-1), as well as white to pale greenish alunite groundmass 39Ar geochronology and D/H, O and S stable isotope analyses have (Sample STB12B-2), which suggests that alunite was emplaced in at been performed on the same samples. The supergene nature of the least two stages separated by fault movement. Alunite extracted from a alunite was confirmed by S isotope analyses and only samples with clast was dated at 15.31±0.63 Ma whereas alunite form the ground- δ34 S (CDT) between −1.8 and +3 were considered supergene. mass yielded an age of 13.83±0.23 Ma. Mote et al. (2001) obtained Where the alunite was porcellaneous and not powdery, the analyzed younger ages ranging from 13.22±0.12 to 13.61±0.06 Ma from a sub material was extracted from specific locations within the hand vertical vein in the same outcrop. T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457 451

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 Quebrada Riolita, samples STB22 and STB26 were collected upstream from Quebrada Turquesa.

In the El Salvador district, supergene alunites outcropping 22.9 to 14.4 Ma for Quebrada Turquesa. The geochronological results upstream from the Quebrada Turquesa exotic deposit were collected. suggest that exotic mineralization processes at Damiana apparently Sample STB026 from a powdery white alunite vein yielded a plateau outlasted those at Quebrada Turquesa. age of 16.31±0.12 Ma (Figs. 3, 4); an additional sample (STB022) yielded, in two separate analytical runs, reproducible age spectra with 5.2. El Hueso/Potrerillos stepwise increasing ages from ~9 to 14 Ma albeit without attaining a plateau. This sample is interpreted as a mixture of two or more Late Miocene supergene activity at El Hueso led to the precipitation generations of ﬁne grained alunite. Mote et al. (2001) reported one of powdery white alunite within a fracture outcropping on the alunite age of 14.8±0.16 Ma as well as supergene Mn oxide ages from uppermost bench of the open pit at 3940 m a.s.l. near the pre-mining 452 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

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

Sample Mineral Location Coord. UTM; Plateau age Plateau/39Ar % Inv. isochron Preferred Comment elevation (m) (Ma) (Ma) age

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±0.16 STB12A-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±0.95 STB12B-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±0.63 STB12B-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±0.23 STB22 Alunite ES, Qebr. Turquesa 443.881/7096.874; 2770 N/A N/A N./A 9 to 14 Ma Mix between 2 or more ages STB26 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±0.12 HTB04 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±0.1 CTB43 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±0.14 CTB46 Jarosite Coya Maya 460.554/7065.736; 3600 N/A N/A 4.29±0.12 4.29±0.12 Excess Argon CTB48 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 spectrum CTB49 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

paleosurface. The alunite yielded an age of 8.19±0.1 Ma (Fig. 5, Rye et al., 1992) and the δD of water in equilibrium with alunite is Table 1), which is slightly younger than the youngest supergene alunite within the analytical uncertainty from the latter. δ18O values on the age reported by Marsh et al. (1997). These authors report 40Ar/39Ar ages sulphate oxygen in the supergene alunite occupy a wide range due to for supergene alunite from El Hueso of 26±1.4, 12.0. ± 0.5, 9.6±0.9, the incorporation of oxygen both from the water as well as the and 9.1±0.5 Ma, plus an additional jarosite age of 6.3±0.5 Ma. atmosphere (Rye et al., 1992). The late Eocene alunite from Quebrada Riolita yielded a δD value of − ‰ 5.3. Coya 73 whereas the other alunites from the same location exhibit a marked increase in δD from −61‰ at 15.4 Ma to −50‰ at 13.8 Ma At Coya, a porphyry Au prospect 4 km to SE from El Hueso (Figs. 2, 6), (Fig. 7). Alunites from the headwaters of Quebrada Turquesa exhibit fi δ − − ‰ supergene alunite from a fracture infill collected at 3800 m elevation on signi cantly higher D values of 34 to 23 at ages younger than δ the north edge of a prominent plateau assigned to the Sierra Checos 16.3 Ma. The D composition of the 8.2 Ma alunite sample from El − ‰ del Cobre surface (Fig. 6; Bissig and Riquelme, 2009) yielded an age Hueso is at 25 , similar to those from Quebrada Turquesa. δ − ‰ of 20.09±0.14 Ma (Fig. 5). Three samples collected about 500 m N At Coya, the early Miocene alunite has a D value of 53 , on a separate hill (Coya Maya, Fig. 6) have also been dated. Sample whereas the early Pliocene natroalunite and jarosite yielded strongly δ − ‰ − ‰ CTB-46 represents a jarosite veinlet exposed at an elevation of negative D values of 88 and 97 respectively. The most recent − ‰ δ 3600 m. No statistically significant plateau age was obtained and the supergene natroalunite has at 57 a less negative D composition. age spectra from two analytical runs reveal possible 39Ar recoil loss (Fig. 5). A pseudo-plateau containing only two analytical fractions 7. Discussion yielded an age around 4.4 Ma, which is within error of the inverse isochron age of 4.29 ±0.12 Ma obtained from both aliquots (Fig. 5, 7.1. Chronology of supergene oxidation Table 1). The latter is taken as the preferred age. A similar age was 40 39 obtained for sample CTB-49, which, based on the Ar/ Ar and XRD As documented for an outcrop near the Damiana exotic deposit, analysis, consists of natroalunite mixed with minamiite (Na,Ca,K)Al3 ages of supergene alunite vary widely within a single outcrop or vein, (SO4)2(OH)6). This sample is also from Coya Maya (3690 m indicating that fluids from which these supergene minerals precip- elevation) and yielded an isochron age of 4.83 ±0.56 Ma on the itate exploit the same permeability network periodically over basis of two aliquots, but similar to sample CTB-46, the age spectra extended periods of time. Although this has been known on a 39 may be affected by Ar recoil loss (Fig. 5). Thus, neither of the porphyry district scale (Sillitoe, 2005), our data, combined with fi aliquots provides a statistically signi cant age spectrum, but run 1 published data (Mote et al., 2001) suggest that this is also the case at a yielded a pseudo-plateau age of 5.8 ± 0.8 Ma when the errors are local scale at the Damaina exotic deposit. Here, both within the exotic increased to two sigma on the individual heating steps. Due to the deposit as well as at the corresponding paleo spring setting ages range evidence for recoil effects we prefer the inverse isochron age. An from about 36 to 13Ma, indicating that exotic mineralization additional sample of natroalunite (CTB-48) was dated from Coya processes operated periodically over 23 Ma in an individual ore Maya. Scanning Electron Microscope energy dispersive analysis forming system. Thus, the permeability network exploited by fi 40 39 determined the presence of suf cient K for Ar/ Ar dating. This supergene fluids remained active over an extended period of time sample, like the other samples from Coya Maya, exhibits evidence for and implies that the local geomorphologic configuration has not fi recoil effects but two analytical runs yielded an age not signi cantly changed substantially. Although the pediment hosting Damiana has different from zero (Fig. 5, Table 1). likely experienced modifications and was shaped most recently during the formation of the multi-stage Atacama pediplain, erosion 6. Stable isotope constraints was never substantial enough to strip the gravels down to the supergene ore. The alunites dated in this study have all been analysed for δ34 S, The timing of the cessation of supergene activity in the Central 18 34 δ OSO4 and δD isotopic composition (Fig 7; Table 2). The δ S values Depression and western Precordillera, proposed at ca. 13 Ma (Mote serve to confirm the supergene nature of the alunite. δD values of et al., 2001), has been roughly confirmed. The respective youngest hydroxyl groups in the alunite directly reflect the meteoric water supergene ages of Damiana and Quebrada Turquesa correspond to the compositions at the time of supergene processes, because the inferred relative ages of the pediment surfaces hosting these two hydrogen isotopic fractionation between water and alunite or exotic deposits (Figs. 3, 8), indicating a potential link between local natroalunite is minimal at surface temperatures (Bird et al., 1989; pediment formation and exotic mineralization. The cessation of T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457 453

12 30 HTB4 Alunite CTB43 Alunite 8.19 ± 0.1 Ma 10 25 20 8 15 20.09 ± 0.14 Ma

Age (Ma) 6 10

4 5

2 0 0 20 40 60 80 100 20 40 60 80 100 Cumulative 39Ar percent Cumulative 39Ar percent

.0030 .004 HTB4 Alunite CTB43 Alunite .0026 .003 .0022 Inverse isochron Inverse Isochron

Ar 8.31 ± 0.39 Ma 20.36 ± 0.95 Ma 40 .002 Ar/

36 .0014 .001 .0010

.0006 0.2 0.3 0.4 0.6 0.7 0.8 0 .002 .006 .008 39Ar/40Ar 39Ar/40Ar

30 25 50 CTB46 Jarosite, run 1 CTB48, Natroalunite, run 1 CTB49, Natroalunite, run 1 25 20 40 20 15 30 15 3 steps at ~5.8 +/- 0.8 Ma 10 20

Age (Ma) 2 steps at ~4.4 Ma 10 10 5 5

0 0 0 2040 60 80 100 2040 60 80 100 20 40 60 80 100 Cumulative 39Ar percent Cumulative 39Ar percent Cumulative 39Ar percent

30 30 50 CTB46 Jarosite, run 2 CTB48, Natroalunite, run 2 CTB49, Natroalunite, run 2 25 24 40 20 18 30 15 12 20 Age (Ma) 10 10 5 6

0 0 0 2040 60 80 100 20 40 60 80 100 20 40 60 80 100 Cumulative 39Ar percent Cumulative 39Ar percent Cumulative 39Ar percent

.004 .004 .004 CTB46 Jarosite, 2 runs CTB48, Natroalunite, 2 runs CTB49, Natroalunite, 2 runs

.003 .003 Reference zero age line .003 Inverse isochron 4.29 +/- 0.12 Ma Inverse Isochron Ar 4.83 +/- 0.28 Ma 40 .002 .002 Ar/ 36 .001 .001 .001 (arrows denote fractions used in age calculation) (calculated age excluding large error fractions) 0 0 0 0 .005 .01 .015 .02 .025 0 .005 .01 .015 .02 .025 0 .001 .002 .003 .004 .005 39Ar/40Ar 39Ar/40Ar 39Ar/40Ar

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, the remainder of samples are from Coya. 454 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

Cordillera Claudio Gay

Relict Asientos pediplain

Coya a: 20.09 +/- 0.14 Ma Asientos Canyon n: ~4.8 Ma and zero age j: ~4.3 Ma Coya Maya

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.

supergene alunite precipitation at El Salvador occurred at a similar Eocene would be no more than 500 m lower if the long term oxygen time as in porphyry Cu and epithermal districts farther North (e.g., isotopic variations in seawater (Zachos et al., 2001) are considered. Arancibia et al., 2006; Bouzari and Clark, 2002; Hartley and Rice, 2005; Miocene meteoric waters are considerably less deuterium depleted Sillitoe and McKee, 1996), which, together with other paleoclimatic and the least negative δD values of −23 to −34‰ were obtained for evidence (Alpers and Brimhall, 1988; Rech et al., 2006) indicates samples between 8.2 and 16.3 Ma from both El Hueso and El Salvador. climate desiccation in the middle Miocene (Fig. 8). Early Pliocene waters at Coya were at δD=−88 to −97‰ similar to The periods of most intense supergene activity in the late present day precipitation around the 3800–4000 m elevation at which Oligocene and Middle Miocene originally defined for northern Chile Coya is presently situated (Poage and Chamberlain, 2001). The most and southern Peru (Sillitoe and McKee, 1996) become more blurry as recent sample yielded a less negative δD value of −57‰. Our data more geochronological data become available (Hartley and Rice, starkly contrast earlier work (Taylor et al., 1997) which suggests 2005) and recent studies suggest a continuous period of intense sharply decreasing δD values from ~−15‰ in the Late Oligocene to as supergene processes lasting from the late Eocene to the early late much as −60‰ in the middle to late Miocene which they interpret as Miocene in Northern Chile (Arancibia et al., 2006). Our results are evidence for a marked uplift pulse in the Middle Miocene. The consistent with a prolonged history of supergene mineralization for discrepancy between the two datasets can probably be explained by the El Salvador district. the different scales of the two studies. Taylor et al. (1997) analyzed In the eastern Precordillera at El Hueso and Coya, at elevations alunite samples from 20 to 27º S Lat S (see also Sillitoe and McKee, approximately 1000–1200 m higher than at El Salvador, 40Ar/39Ar 1996) which likely reflect significant along strike variations in constraints, admittedly still limited, indicate that supergene pro- geomorphology, uplift history and climate. North of about 23º Lat. S, cesses occurred in the late Oligocene and early Miocene as well there is no Preandean Depression (Fig. 1) and independent evidence as from the late Miocene to early Pliocene and may still be occur- suggests that much of the uplift of the Altiplano has occurred in the ring at the present day (Fig. 8). Contrasting with El Salvador, super- middle or late Miocene (e.g., Gregory-Wodzicki, 2000; Hoke et al., gene oxidation in the eastern Precordillera appears to have been 2007). In the southern Atacama Desert, the Precordillera attained limited throughout the middle Miocene. While the late Oligocene elevations of at least 2000 m in the early Oligocene (Riquelme et al., and early Miocene ages roughly coincide with the incision of the 2007) and our stable isotope data suggest a Late Eocene elevation of Sierra Checos del Cobre and Asientos pediplains (Fig. 8) and super- 3000 m a.s.l. or more for the Precordillera near El Salvador. These high gene oxidation may have been related to these erosive processes, we elevations may be attributed to intense folding and thrusting interpret the Late Miocene and younger oxidation to be controlled (Niemeyer and Munizaga, 2008) and crustal thickening (e.g., Haschke by uplift to elevations sufficient to capture increased precipitation et al., 2002) affecting the region in the late Eocene. combined with the incision of deep canyons into the previous planar The increasing δD values throughout the middle Miocene are landscape (Bissig and Riquelme, 2009).Thiswouldleadto contrary to the trend expected for an uplifting mountain range. depression of the water table, but increased availability of meteoric However, the isotopic composition of meteoric water is not only water in the vadose zone, generating conditions favorable for sulfide controlled by orographic effects, but also by evaporation and recycling oxidation. of meteoric water (Godfrey et al., 2003). Thus, we interpret the higher than expected Miocene δD values largely as an effect of evaporation. 7.2. δD through time Bird et al. (1989) and Sillitoe (2005) suggested that high rates of evaporation are conducive for supergene alunite formation, providing The Late Eocene meteoric water at El Salvador was at δD=−73‰ support to our interpretation. Thus, the least negative δD values similar to the present day precipitation at~3500 m a.s.l. when would coincide with the most intense evaporation and hyper arid calculated using the empirical relationship for South America from conditions which likely persisted between about 15 and 8 Ma. The Poage and Chamberlain (2001). The estimated elevation for the Late timing of the onset of hyper-arid conditions is also recorded by a T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457 455

A marked decrease of sediment accumulation in the Central Depression 0 in the middle Miocene (Nalpas et al., 2008; Riquelme et al., 2007). Given that the Precordillera attained considerable elevations signif- -20 icantly prior to the middle Miocene hyper arid climate, the uplift of the mountain range probably does not by itself account for the climate -40 desiccation in the southern Atacama Desert (see also Lamb and Davis, N-Chile Meteoric water line 2003). However, the eastward migration of the deformation into the -60 Cordillera Claudio Gay in the late Oligocene (Mpodozis and Clavero, 2002) and the formation of the Preandean depression likely enhanced -80 Altiplano ﬁ ~4500 m jarosite aridi cation of the Central Depression. We suggest that the widening a.s.l. rather than simply the uplift of the Andes likely has resulted in D (VSMOW) -100 δ increased rain shadow effects at the western Andean slope. -120 Somewhat wetter conditions probably dominated the early supergene alunite Pliocene in the Precordillera when compared to the arid middle sulfate field -140 Miocene climate. Stable isotope evidence suggests that the Precordil- water dominant lera probably had attained elevations similar to the present and that air dominant -160 evaporation effects were limited. This is interpreted as the result of -20 -15 -10 -505 10 15 increased capture of orographically controlled precipitation at that δ18 OSO4(VSMOW) time. Moreover, sedimentological evidence in the Calama basin, some 400 km farther north (Fig. 1; Hartley and Chong, 2002), indicates that B semiarid climatic conditions prevailed in the Precordillera and -20 western Andes between about 6 and 3 Ma. The present climate and hydrologic conditions in the eastern 2500 Precordillera are potentially still wet enough to permit the formation of supergene alunite group minerals, but signiﬁcant evaporation likely -40 affects the meteoric waters. Strong evaporation effects have been 3000 documented for meteoric waters in the internally drained basins of the Salar de Hombre Muerto and Salar de Atacama basins (Godfrey Dessication trend -60 et al., 2003). Orographic effect 3500 D (VSMOW)

δ 8. Conclusions -80 − Geomorphologic and stable isotope evidence strongly suggests 4000 that the Precordillera in the Southern Atacama Desert has attained -100 elevations of at least 3000 m a.s.l. already in the early Oligocene 0 10203040 and thus, significantly prior to the major uplift of the Altiplano. 40Ar/39Ar age (Ma) − The climate evolved differently in the western Precordillera and Central Depression from the eastern Precordillera. The cessation of Symbols supergene processes at El Salvador around 13 Ma has been El Salvador, Damiana Coya confirmed and is attributed to climate desiccation, an interpreta- Other El Salvador El Hueso tion also supported by sedimentological and stable isotope evidence. However, conditions at Coya and El Hueso in the Eastern Fig 7. Stable isotope composition of supergene alunite group minerals. A) δD vs. Precordillera, situated near 4000 m present day elevation δ18Οso4. Note that all alunite samples fall within the large field for supergene alunite but generally closer to the air dominated than water dominated oxygen isotope remained conducive for at least episodic supergene alunite composition. The jarosite sample plots immediately right of the air dominated formation until the early Pliocene, and possibly up to the present fi boundary for supergene alunite. Reference eld for high altitude precipitation for the day. Uplift to elevations near 4000 m a.s.l. have led to increased Chilean Altiplano is from Herrera et al. (2006).B)δD isotopic composition of supergene alunite group minerals through time. The right vertical axis is labeled with the capture of moisture and consequently increased availability of elevations corresponding to the δD values on the left axis. Values were calculated using meteoric waters. the empirically determined relationship for central and South America (Poage and − The new 40Ar/39Ar age constraints presented herein provide Chamberlain, 2001). Interpreted general climatic trends are indicated (see text for evidence confirming the previously proposed protracted history discussion). of the Damiana exotic Cu deposit and indicate that the local geomorphologic and hydrologic configuration has remained relatively stable over 23 Ma. Table 2 Stable isotope data. Supplementary data to this article can be found online at doi: 34 18 Sample Mineral dD d SdOSO4 age (Ma) 10.1016/j.epsl.2010.09.028. HTB004 Alunite −25 −1.8 3.7 8.19±0.1 STB-022 Alunite −23 1.4 4.8 9 to 14 STB-026 Alunite −34 −0.5 6.8 16.31±0.12 Acknowledgements STB-12A-1 Alunite −54 −0.9 5.3 14.22±0.16 STB-12A-2 Alunite −74 0.3 9.7 35.8±1 STB-12B-1 Alunite −61 0.1 4.1 15.3±0.6 This study has been funded by Fondo Nacional de Desarrollo STB-12B-2 Alunite −50 0.0 3.9 13.8±0.2 Científico y Tecnológico de Chile (Fondecyt) grant # 11060516. Kerry CTB-43 Alunite −53 0.0 10.7 20.1±0.1 Klassen is thanked for the stable isotope analyses whereas Paul Layer CTB-46 Jarosite −97 0.8 11.0 4.3±0.1 and Tom Ullrich provided the Ar/Ar analyses. Fritz Schlunegger and an − CTB-48 Natroalunite 57 1.1 2.8 0 anonymous EPSL reviewer are thanked for their constructive reviews. CTB-49 Natroalunite −88 3.0 2.6 4.8±0.6 This is MDRU publication P-264. 456 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457

Climate Supergene ages Landscape Tectonics Literature This study 0 WP EEPP Coya El Hueso Plio- cene 5 moderate tilting hyper arid Canyon Incision in the fore-arc Other, El Salvador Pediment formation Damiana, El Salvador Q. Turquesa, El Salvador Atacama gravel deposition Late

10 Miocene A3 A2 7 slow tilting A1 in the fore-arc 2 15 Middle Miocene

AS Hyper aridity Hyper aridity 20 ? thrusting and

uplift, Cordillera Early

3 Claudio Gay Miocene (Hartley and Chong, 2002)

SC (Alpers and Brimhall, 1988) 25

30 Oligocene ?

35 semi-arid semi-arid

thrusting and folding, Potrerillos Eocene Fold and Thrust belt 40

Fig. 8. Chart integrating landscape chronology, tectonic episodes, ages of supergene minerals and climate. Abbreviations: A1: early stage Atacama pediplain, A2: Main stage Atacama pediplain; A3: late stage Atacama pediplain; AS: Asientos surface; SC: Sierra Checos del Cobre surface. WP: Western Precordillera; EP: Eastern Precordillera. Supergene ages are plotted 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: Marsh et 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).

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