Earth and Planetary Science Letters 299 (2010) 447–457 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl 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 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.09.028 448 T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457 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).