Earth and Planetary Science Letters 299 (2010) 447–457
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
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
a r t i c l e
i n f o
Article history:
Received 5 February 2010
Received in revised form 21 September 2010
Accepted 21 September 2010
Available online 18 October 2010
Editor: T.M. Harrison
Keywords:
Atacama Desert
Andes
uplift
climate evolution
supergene alunite
El Salvador
Potrerillos
geochronology
stable isotopes
Chile
a b s t r a c t
Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Hueso
epithermal 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 data
support published geomorphologic and sedimentologic evidence suggesting that the Precordillera in the
Southern Atacama Desert had been uplifted as early as the late Eocene and, thus, significantly prior to the
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
yielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of at
least 3000 m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coya
have less negative δD signatures reaching values as high as −23 to −25‰ at El Hueso and El Salvador between
about 8.2 and 14 Ma. Late Miocene to Holocene supergene alunite, jarosite and natroalunite ages are restricted
to El Hueso and Coya located near 4000 m above sea level in the Precordillera, roughly 1000 m higher than the
present 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 alunite
group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset
of hyper arid conditions in the Central Depression of the southern Atacama Desert after 15 Ma, which agrees
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
The Andean uplift history, its causes and effects on the climate
have 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 Chile
and Altiplano transects (~ 18–20º Lat. S, Fig. 1). Farías et al. (2005) and
Victor et al. (2004) suggest that up to 2600 m of uplift occurred in the
late middle Miocene and was accommodated by high-angle west
verging faults in the western Cordillera. Geomorphologic (Garcia and
Hérail, 2005; Hoke et al. 2007; Schlunegger et al., 2006; Thouret et al.
2007) and stable isotope evidence (Garzione et al., 2008) places the
major uplift which gave rise to the present day high elevations of the
Altiplano in the late Miocene. The southern Atacama Desert (~ 26–27º
⁎ Corresponding author.
E-mail address: tbissig@eos.ubc.ca (T. Bissig).
1
Mineral Deposit Resarch Unit, Department of Earth and Ocean Sciences, University
of British Columbia, 6339 Stores Road, Vancouver, B.C., V6T 1Z4, Canada.
0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2010.09.028
Lat. S, Fig. 1) has received comparatively less recent attention, but
available evidence indicates that the uplift history was fundamentally
distinct, irrespective of the controversies on the exact timing of
Altiplano uplift. For example, no significant high angle west verging
faults active during the Miocene have been documented for the
southern Atacama Desert. In addition, geomorphologic, apatite fission
track and sedimentological evidence (Nalpas et al., 2005; Riquelme
et al., 2007) suggest that in the southern Atacama Desert the
Precordillera had attained considerable elevations in the Oligocene
or earlier, which greatly precedes the Altiplano uplift. We herein
assess the uplift and climate evolution in an oblique transect across
the Precordillera at 26–26° 30´ Lat. S (Figs. 1, 2) on the basis of the well
established geomorphologic framework (Bissig and Riquelme, 2009;
Nalpas et al. 2008; Riquelme et al., 2003, 2007, 2008) and eleven new
40
Ar/39Ar ages and corresponding stable isotope data for supergene
alunite group minerals from the El Salvador porphyry Cu district (e.g.,
Gustafson et al., 2001), the El Hueso epithermal Au deposit (Marsh
et al., 1997; Thompson et al., 2004) and the Coya porphyry Au
prospect (Rivera et al., 2004), all located in the southern Atacama
448
T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457
69° W
70° W
18° S
Altiplano Segment
ARICA
SouthAmerica
O C E A N
IQUIQUE
P A C I F I C
TRENCH
P
WC
1
CALAMA
CB
CD
23° S
PD
2
-
CC
3 SP
4
5
COPIAPO
0
50
100 km
PC
CCG
PERÚ
CHAÑARAL
Southern Atacama Desert
(Puna Segment)
CHILE
ANTOFAGASTA
TALTAL
50
68° W
27° S
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: Calama
basin; CCG: Cordillera Claudio Gay. Ore deposits and prospects relevant for this paper
are 1: Chuquicamata, 2: Escondida; 3: El Salvador; 3: Potrerillos/El Hueso/Coya; 5: La
Coipa.
Desert of Chile (Fig. 2). Our new age constraints complement
published data for El Hueso and El Salvador (Marsh et al., 1997;
Mote et al., 2001, respectively). Our results confirm the notion that
important differences exist between the timing of uplift in the
Altiplano region and the Southern Atacama Desert and provide new
insights into climate evolution across the Precordillera.
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
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.,
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
reached altitudes of at least 2000 m (Riquelme et al., 2007). No
significant movement has been documented on the principal Incaic
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,
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
day configuration of the internally drained Preandean depression
hosting the Salar de Pedernales (Figs. 1, 2).
The deeply incised Oligocene valleys in the western Precordillera
were filled with continental clastic sediments with a minimum age of
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
western Precordillera was probably accompanied by pediment
formation as represented by the early Miocene Sierra Checo del
Cobre surface in the Coastal Cordillera (Mortimer, 1973). Low relief
surfaces are present above El Hueso and La Coya in the eastern
Precordillera and are tentatively assigned to the Sierra Checo del
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
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
began in the middle Miocene. A relatively low tilting rate resulted in
the middle Miocene alluvial fan backfilling in the Central Depression
and the formation of the Atacama Pediplain in the western
Precordillera (Riquelme et al., 2007; Sillitoe et al., 1968). The El
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
(Bissig and Riquelme, 2009). Minimum age constraints for this surface
are given by an ignimbrite deposit covering the pediment surface
dated between 9 and 10 Ma (Clark et al., 1967; Cornejo et al., 1993;
Riquelme et al., 2007), which is in good agreement with the
radiogenic nuclide exposure age of 9 Ma reported by Niishizumi
et al. (2005). A change from alluvial fan backfilling to incision of the
Asientos and El Salado canyons into the relict Atacama pediplain has
been interpreted as being the result of slightly increased tilting rates
which allowed the transition from a depositional to erosional regime.
This led to incision of the Salado in the Central Depression and
moderate (b800 m) uplift of the Precordillera in the Late Miocene
(Mortimer, 1973; Riquelme et al., 2007).
3. The use of supergene alunite group minerals
2. Tectonic history and landscape evolution of the southern
Atacama Desert
The present day geomorphologic configuration of the fore-arc
region of northern Chile (18–28°S) is dominated by extensive
pediplain surfaces which are the result of interaction between
climatic and tectonic evolution during the late Cenozoic (e.g. Lamb
and Davis, 2003; Mortimer, 1973; Rieu, 1975; Riquelme et al., 2003).
These relict pediplains have resisted significant modification through
erosion for exceptionally long periods of time in some areas (e.g.,
Dunai et al., 2005). The tectonic and geomorphologic evolution of the
studied transect at 26–26°30´ S Lat is summarized in the following.
Supergene alunite group minerals (e.g., alunite, natroalunite, jarosite)
are weathering products of porphyry Cu or epithermal deposits and are
found in the leached caps of porphyry Cu deposits (Sillitoe, 2005),
commonly in paleospring settings under acidic fluid conditions and
upstream from exotic Cu deposits (Mote et al., 2001). Oxidation of sulfides
in porphyry Cu deposits is controlled by the fluctuations of the water table
which in turn depends on tectonic and geomorphologic processes as well
as climate (Sillitoe, 2005). Supergene alunite can be dated by the 40Ar/39Ar
method (Vasconcelos, 1999) and although minor recoil loss of 39Ar may
occur in some cases reasonable age dates are usually obtained
(Vasconcelos and Conroy, 2003). Alunite group minerals can also be
T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457
70°
70°30´
449
69°30´
C. Doña Inés
A
Not mapped
Q.
ue
Salar de Pedernales
rq
Tu
sa
El Salvador
El S
ald
a
o Ca
nyo
Damiana
n
26°
26°
As
ie
nt
os
C
an
yo
n
Jerónimo
Potrerillos
El Hueso
26°15´
26°15´
C. El Hueso
Coya
Pediment surfaces
N
Sierra Checos
del Cobre
Asientos
10 km
Early Atacama
(> 14?)
Cerros Bravos
Atacama >10 Ma
Atacama < 10 Ma
Pediment
backscarp
Sierra Castillo Fault
(approx. trace)
A`
El Salado Canyon
Asientos Canyon
26°30´
Asientos Canyon
Cerros Bravos
m a.s.l. A
A`
El Hueso
Potrerillos Coya
5000
El Salvador
4000
Sierra Checos del Cobre
Asientos
3000
2000
Early Atacama
Atacama pediplain
SCF
0
20
40
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
the paleo meteoric water at the time of its formation (Arehart et al., 1992;
Rye et al., 1992). Since the isotopic composition of meteoric water
depends on elevation (e.g., Poage and Chamberlain, 2001), supergene
alunite has the potential to record uplift histories (Taylor et al., 1997).
However, the isotopic composition of meteoric waters in arid climates
may also be influenced by evaporation (e.g., Godfrey et al., 2003) and the
relative importance of the latter may be assessed if the tectonic and
geomorphologic framework of a region is independently constrained.
4. Samples and analytical methods
Supergene alunite, natroalunite and jarosite, ranging from powdery 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
A
1km
N
Q
.T
ur
qu
21.5
14.4
es
a
14.8
16.3
22.9
21.4
El Salvador
townsite
13.6, 13.5, 13.2
13.0, 12.9, 12.0
35.8, 15.3,
14.2, 13.8
26°15’ S
na
ia
m
Da
35.4
25.3
13.9
11.1
19.4
Landscape elements
Alunite age
(this study in bold)
Main Atacama
Copper wad age
specimen, where possible using a micro-drill tool. In these cases the
sample for geochronology was extracted first, followed by the sample
for stable isotope analysis. Sample material for XRD was extracted last
due to the larger amount required. Most 40Ar/39Ar analyses were
performed at the Noble Gas Laboratory, Pacific Centre for Isotopic and
Geochemical Research (PCIGR), University of British Columbia,
Vancouver, BC, Canada, but samples CTB43, CTB46, CTB48 and
CTB49 were dated at the 40Ar/39Ar facility at the Geophysical Institute
at the University of Alaska at Fairbanks (UAF). At PCIGR, the samples
were step-heated at increasing laser powers in the defocused beam of
a 10-W CO2 laser. The flux monitor used was Fish Canyon Tuff
sanidine, 28.02 Ma (Renne et al., 1998). For further details on
analytical methods refer to Bissig et al. (2008). At the UAF, an 8 W
Ar laser was used and the flux monitor was TCR-2 with an age of
27.87 Ma (Lanphere and Dalrymple, 2000); the analytical methods
are described in Layer (2000). All ages are reported with the analytical
errors at the 2σ level and represent statistically relevant plateau ages
unless indicated otherwise. The reported plateau ages are all within
error of the corresponding inverse isochron ages. All 40Ar/39Ar data
are included in digital appendices.
The δ34S, δ18OSO4, 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
Troilite (CDT), oxygen and hydrogen relative to Vienna Standard
Mean Ocean Water (V-SMOW). Analytical precision for both δ34S and
δ18OSO4 values is 0.3‰ and for δD 5‰.
Early Atacama (>14 Ma)
Exotic Cu deposit
Inselbergs
Primary Cu deposit
(El Salvador)
5. Episodes of supergene mineralization
5.1. El Salvador
B
El Salvador
Town
Damiana
Fig. 3. Environment of exotic mineralization at El Salvador. A) Map of El Salvador and
associated exotic Cu deposits. The principal geomorphologic elements are shown and
approximate 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 Damiana
exotic deposit, looking W. The original pediment surface hosting Damiana was
disturbed by mining.
were sampled from surface outcrops. The mineralogy was confirmed
by X-ray diffraction and no significant contaminating phases (except
for some kaolinite in sample STB012A-2) were identified. Both 40Ar/
39
Ar geochronology and D/H, O and S stable isotope analyses have
been performed on the same samples. The supergene nature of the
alunite was confirmed by S isotope analyses and only samples with
δ34 S (CDT) between −1.8 and + 3 were considered supergene.
Where the alunite was porcellaneous and not powdery, the analyzed
material was extracted from specific locations within the hand
Supergene mineralization at El Salvador is principally represented by
two exotic deposits, Damiana and Quebrada Turquesa (Figs. 2, 3). Mote
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
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
central part of the vein and yielded an 40Ar/39Ar age of 14.22± 0.16 Ma.
Sample STB12A-2 represents alunite completely replacing the feldspars
and groundmass from a rhyolitic wall rock clast within the porcellaneous vein and was dated at 35.82 ± 0.95 Ma. Both of our new ages are
considerably older than the 12.89 ± 0.06 to 13.02 ± 0.06 Ma age range
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
alunite is porcellaneous and occurs as white to pale yellowish
subangular breccia clasts of less than 1 cm in diameter (Sample
STB12B-1), as well as white to pale greenish alunite groundmass
(Sample STB12B-2), which suggests that alunite was emplaced in at
least two stages separated by fault movement. Alunite extracted from a
clast was dated at 15.31 ± 0.63 Ma whereas alunite form the groundmass yielded an age of 13.83 ± 0.23 Ma. Mote et al. (2001) obtained
younger ages ranging from 13.22 ± 0.12 to 13.61± 0.06 Ma from a sub
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
upstream from the Quebrada Turquesa exotic deposit were collected.
Sample STB026 from a powdery white alunite vein yielded a plateau
age of 16.31 ± 0.12 Ma (Figs. 3, 4); an additional sample (STB022)
yielded, in two separate analytical runs, reproducible age spectra with
stepwise increasing ages from ~9 to 14 Ma albeit without attaining a
plateau. This sample is interpreted as a mixture of two or more
generations of fine grained alunite. Mote et al. (2001) reported one
alunite age of 14.8 ± 0.16 Ma as well as supergene Mn oxide ages from
22.9 to 14.4 Ma for Quebrada Turquesa. The geochronological results
suggest that exotic mineralization processes at Damiana apparently
outlasted those at Quebrada Turquesa.
5.2. El Hueso/Potrerillos
Late Miocene supergene activity at El Hueso led to the precipitation
of powdery white alunite within a fracture outcropping on the
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.
40
Ar/39Ar data.
Sample
Mineral
Location
STB12A-1
STB12A-2
STB12B-1
STB12B-2
STB22
Alunite
Alunite
Alunite
Alunite
Alunite
ES, Qebr.
ES, Qebr.
ES, Qebr.
ES, Qebr.
ES, Qebr.
STB26
HTB04
CTB43
CTB46
CTB48
Alunite
Alunite
Alunite
Jarosite
Natroalunite
ES, Qebr. Turquesa
El Hueso
Coya, Plateau
Coya Maya
Coya Maya
CTB49
Natroalunite Coya Maya
Riolita
Riolita
Riolita
Riolita
Turquesa
Coord. UTM;
elevation (m)
Plateau age Plateau/39Ar %
(Ma)
443.038/7096.252; 2700
443.038/7096.252; 2700
443.038/7096.252; 2700
443.038/7096.252; 2700
443.881/7096.874; 2770
14.22±0.16
35.82±0.95
15.31±0.63
13.83±0.23
N/A
10 of 10 steps 100% of 39Ar
5 of 8 steps, 83% of 39Ar
9 of 9 steps, 100% of 39Ar
9 of 9 steps, 100% of 39Ar
N/A
14.31 ± 0.36
36.3 ± 1.4
14.7 ± 1.5
13.64 ± 0.4
N./A
443.701/7096.691;
460.300/7069.153;
461.189/7064.792;
460.554/7065.736;
460.713/7065.450;
16.31±0.12
8.19 ± 0.1
20.09±0.14
N/A
0.07 ± 0.6
7 of 9 steps, 81.5 % of 39Ar
7 of 9 steps, 62.8% of 39Ar
3 of 14 steps 83% of 39Ar
N/A
9 of 26 steps 76% 39Ar
15.92 ± 0.31
8.31 ± 0.39
20.11 ± 0.4
4.29 ± 0.12
0.39 ± 1.4
N/A
4.83 ± 0.5
2860
3940
3800
3600
3690
460.482/ 7065.289; 3710 N/A
paleosurface. The alunite yielded an age of 8.19 ± 0.1 Ma (Fig. 5,
Table 1), which is slightly younger than the youngest supergene alunite
age reported by Marsh et al. (1997). These authors report 40Ar/39Ar ages
for 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 Au prospect 4 km to SE from El Hueso (Figs. 2, 6),
supergene alunite from a fracture infill collected at 3800 m elevation on
the north edge of a prominent plateau assigned to the Sierra Checos
del Cobre surface (Fig. 6; Bissig and Riquelme, 2009) yielded an age
of 20.09 ± 0.14 Ma (Fig. 5). Three samples collected about 500 m N
on a separate hill (Coya Maya, Fig. 6) have also been dated. Sample
CTB-46 represents a jarosite veinlet exposed at an elevation of
3600 m. No statistically significant plateau age was obtained and the
age spectra from two analytical runs reveal possible 39Ar recoil loss
(Fig. 5). A pseudo-plateau containing only two analytical fractions
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,
Table 1). The latter is taken as the preferred age. A similar age was
obtained for sample CTB-49, which, based on the 40Ar/39Ar and XRD
analysis, consists of natroalunite mixed with minamiite (Na,Ca,K)Al3
(SO 4 ) 2 (OH) 6 ). This sample is also from Coya Maya (3690 m
elevation) and yielded an isochron age of 4.83 ± 0.56 Ma on the
basis of two aliquots, but similar to sample CTB-46, the age spectra
may be affected by 39Ar recoil loss (Fig. 5). Thus, neither of the
aliquots provides a statistically significant age spectrum, but run 1
yielded a pseudo-plateau age of 5.8 ± 0.8 Ma when the errors are
increased to two sigma on the individual heating steps. Due to the
evidence for recoil effects we prefer the inverse isochron age. An
additional sample of natroalunite (CTB-48) was dated from Coya
Maya. Scanning Electron Microscope energy dispersive analysis
determined the presence of sufficient K for 40Ar/39Ar dating. This
sample, like the other samples from Coya Maya, exhibits evidence for
recoil effects but two analytical runs yielded an age not significantly
different 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 values
serve to confirm the supergene nature of the alunite. δD values of
hydroxyl groups in the alunite directly reflect the meteoric water
compositions at the time of supergene processes, because the
hydrogen isotopic fractionation between water and alunite or
natroalunite is minimal at surface temperatures (Bird et al., 1989;
Inv. isochron Preferred
(Ma)
age
Comment
14.22 ± 0.16
35.82 ± 0.95
15.31 ± 0.63
13.83 ± 0.23
9 to 14 Ma
Mix between 2
or more ages
16.31 ± 0.12
8.19 ± 0.1
20.09 ± 0.14
4.29 ± 0.12
Excess Argon
0
Age based on two aliquots,
excess Ar in spectrum
4.83 ± 0.5
Age based on two aliquots,
excess Ar in spectrum
Rye et al., 1992) and the δD of water in equilibrium with alunite is
within the analytical uncertainty from the latter. δ18O values on the
sulphate oxygen in the supergene alunite occupy a wide range due to
the incorporation of oxygen both from the water as well as the
atmosphere (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 a
marked increase in δD from −61‰ at 15.4 Ma to −50‰ at 13.8 Ma
(Fig. 7). Alunites from the headwaters of Quebrada Turquesa exhibit
significantly higher δD values of −34 to −23‰ at ages younger than
16.3 Ma. The δD composition of the 8.2 Ma alunite sample from El
Hueso 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 strongly
negative δD values of −88‰ and −97‰ respectively. The most recent
supergene 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 precipitate exploit the same permeability network periodically over
extended periods of time. Although this has been known on a
porphyry district scale (Sillitoe, 2005), our data, combined with
published data (Mote et al., 2001) suggest that this is also the case at a
local scale at the Damaina exotic deposit. Here, both within the exotic
deposit as well as at the corresponding paleo spring setting ages range
from about 36 to 13 Ma, indicating that exotic mineralization
processes operated periodically over 23 Ma in an individual ore
forming system. Thus, the permeability network exploited by
supergene fluids remained active over an extended period of time
and implies that the local geomorphologic configuration has not
changed substantially. Although the pediment hosting Damiana has
likely experienced modifications and was shaped most recently
during the formation of the multi-stage Atacama pediplain, erosion
was never substantial enough to strip the gravels down to the
supergene ore.
The timing of the cessation of supergene activity in the Central
Depression and western Precordillera, proposed at ca. 13 Ma (Mote
et al., 2001), has been roughly confirmed. The respective youngest
supergene ages of Damiana and Quebrada Turquesa correspond to the
inferred relative ages of the pediment surfaces hosting these two
exotic deposits (Figs. 3, 8), indicating a potential link between local
pediment formation and exotic mineralization. The cessation of
T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457
30
12
HTB4 Alunite
10
Age (Ma)
453
CTB43 Alunite
8.19 ± 0.1 Ma
25
20
8
20.09 ± 0.14 Ma
15
6
10
4
2
5
0
20
40
60
80
Cumulative 39Ar percent
100
0
20
40
60
80
Cumulative 39Ar percent
100
.004
.0030
HTB4 Alunite
CTB43 Alunite
36Ar/40Ar
.0026
.003
Inverse isochron
8.31 ± 0.39 Ma
.0022
Inverse Isochron
20.36 ± 0.95 Ma
.002
.0014
.001
.0010
.0006
0.2
0.4
0.3
0.6
0.7
0
0.8
.002
39Ar/40Ar
30
25
CTB46 Jarosite, run 1
Age (Ma)
25
20
50
CTB48, Natroalunite, run 1
20
40
15
30
10
20
5
10
CTB49, Natroalunite, run 1
3 steps at ~5.8 +/- 0.8 Ma
15
2 steps at ~4.4 Ma
10
5
0
20
40
60
Cumulative
39Ar
80
100
0
0
20
percent
40
60
Cumulative
30
39Ar
80
20
100
60
80
100
50
CTB48, Natroalunite, run 2
25
40
Cumulative 39Ar percent
percent
30
CTB46 Jarosite, run 2
Age (Ma)
.008
.006
39Ar/40Ar
CTB49, Natroalunite, run 2
24
40
18
30
12
20
6
10
20
15
10
5
0
20
40
60
Cumulative
.004
39Ar
80
20
.001
40
60
Cumulative
.004
Inverse isochron
4.29 +/- 0.12 Ma
36Ar/40Ar
0
percent
CTB46 Jarosite, 2 runs
.003
100
39Ar
80
0
.004
Reference zero age line
.005
.01
.015
39Ar/40Ar
.02
.025
60
80
100
CTB49, Natroalunite, 2 runs
Inverse Isochron
4.83 +/- 0.28 Ma
.002
.002
.001
.001
(calculated age excluding large error fractions)
0
0
40
.003
(arrows denote fractions used in
age calculation)
0
20
Cumulative 39Ar percent
percent
CTB48, Natroalunite, 2 runs
.003
100
0
.005
.01
.015
39Ar/40Ar
.02
.025
0
0
.001
.002
.003
.004
.005
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
n
s Canyo
Asiento
Coya
a: 20.09 +/- 0.14 Ma
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
time 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 paleoclimatic
evidence (Alpers and Brimhall, 1988; Rech et al., 2006) indicates
climate desiccation in the middle Miocene (Fig. 8).
The periods of most intense supergene activity in the late
Oligocene and Middle Miocene originally defined for northern Chile
and southern Peru (Sillitoe and McKee, 1996) become more blurry as
more geochronological data become available (Hartley and Rice,
2005) and recent studies suggest a continuous period of intense
supergene processes lasting from the late Eocene to the early late
Miocene in Northern Chile (Arancibia et al., 2006). Our results are
consistent with a prolonged history of supergene mineralization for
the El Salvador district.
In the eastern Precordillera at El Hueso and Coya, at elevations
approximately 1000–1200 m higher than at El Salvador, 40Ar/39Ar
constraints, admittedly still limited, indicate that supergene processes occurred in the late Oligocene and early Miocene as well
as from the late Miocene to early Pliocene and may still be occurring at the present day (Fig. 8). Contrasting with El Salvador, supergene oxidation in the eastern Precordillera appears to have been
limited throughout the middle Miocene. While the late Oligocene
and early Miocene ages roughly coincide with the incision of the
Sierra Checos del Cobre and Asientos pediplains (Fig. 8) and supergene oxidation may have been related to these erosive processes, we
interpret the Late Miocene and younger oxidation to be controlled
by uplift to elevations sufficient to capture increased precipitation
combined with the incision of deep canyons into the previous planar
landscape (Bissig and Riquelme, 2009). This would lead to
depression of the water table, but increased availability of meteoric
water in the vadose zone, generating conditions favorable for sulfide
oxidation.
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. when
calculated using the empirical relationship for South America from
Poage and Chamberlain (2001). The estimated elevation for the Late
Eocene would be no more than 500 m lower if the long term oxygen
isotopic variations in seawater (Zachos et al., 2001) are considered.
Miocene meteoric waters are considerably less deuterium depleted
and the least negative δD values of −23 to −34‰ were obtained for
samples 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 to
present day precipitation around the 3800–4000 m elevation at which
Coya is presently situated (Poage and Chamberlain, 2001). The most
recent sample yielded a less negative δD value of −57‰. Our data
starkly contrast earlier work (Taylor et al., 1997) which suggests
sharply decreasing δD values from ~−15‰ in the Late Oligocene to as
much as −60‰ in the middle to late Miocene which they interpret as
evidence for a marked uplift pulse in the Middle Miocene. The
discrepancy between the two datasets can probably be explained by
the different scales of the two studies. Taylor et al. (1997) analyzed
alunite samples from 20 to 27º S Lat S (see also Sillitoe and McKee,
1996) which likely reflect significant along strike variations in
geomorphology, uplift history and climate. North of about 23º Lat. S,
there is no Preandean Depression (Fig. 1) and independent evidence
suggests that much of the uplift of the Altiplano has occurred in the
middle or late Miocene (e.g., Gregory-Wodzicki, 2000; Hoke et al.,
2007). In the southern Atacama Desert, the Precordillera attained
elevations of at least 2000 m in the early Oligocene (Riquelme et al.,
2007) and our stable isotope data suggest a Late Eocene elevation of
3000 m a.s.l. or more for the Precordillera near El Salvador. These high
elevations may be attributed to intense folding and thrusting
(Niemeyer and Munizaga, 2008) and crustal thickening (e.g., Haschke
et al., 2002) affecting the region in the late Eocene.
The increasing δD values throughout the middle Miocene are
contrary to the trend expected for an uplifting mountain range.
However, the isotopic composition of meteoric water is not only
controlled by orographic effects, but also by evaporation and recycling
of meteoric water (Godfrey et al., 2003). Thus, we interpret the higher
than expected Miocene δD values largely as an effect of evaporation.
Bird et al. (1989) and Sillitoe (2005) suggested that high rates of
evaporation are conducive for supergene alunite formation, providing
support to our interpretation. Thus, the least negative δD values
would coincide with the most intense evaporation and hyper arid
conditions which likely persisted between about 15 and 8 Ma. The
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
A
Nwa Chile
te r
M
lin e t e
e
or
ic
0
-20
-60
Altiplano
~4500 m
a.s.l.
-80
-100
jarosite
supergene alunite
sulfate field
wa
te
rd
-120
-140
air dominan
t
om
ina
nt
δD (VSMOW)
-40
-160
-20
-15
-10
0
-5
5
10
15
δ18OSO4(VSMOW)
B
-20
ect
2500
ic eff
raph
3000
Dess
Orog
δD (VSMOW)
-40
-60
icatio
n tren
d
455
marked decrease of sediment accumulation in the Central Depression
in the middle Miocene (Nalpas et al., 2008; Riquelme et al., 2007).
Given that the Precordillera attained considerable elevations significantly prior to the middle Miocene hyper arid climate, the uplift of
the mountain range probably does not by itself account for the climate
desiccation in the southern Atacama Desert (see also Lamb and Davis,
2003). However, the eastward migration of the deformation into the
Cordillera Claudio Gay in the late Oligocene (Mpodozis and Clavero,
2002) and the formation of the Preandean depression likely enhanced
aridification of the Central Depression. We suggest that the widening
rather than simply the uplift of the Andes likely has resulted in
increased rain shadow effects at the western Andean slope.
Somewhat wetter conditions probably dominated the early
Pliocene in the Precordillera when compared to the arid middle
Miocene climate. Stable isotope evidence suggests that the Precordillera probably had attained elevations similar to the present and that
evaporation effects were limited. This is interpreted as the result of
increased capture of orographically controlled precipitation at that
time. Moreover, sedimentological evidence in the Calama basin, some
400 km farther north (Fig. 1; Hartley and Chong, 2002), indicates that
semiarid climatic conditions prevailed in the Precordillera and
western Andes between about 6 and 3 Ma.
The present climate and hydrologic conditions in the eastern
Precordillera are potentially still wet enough to permit the formation
of supergene alunite group minerals, but significant evaporation likely
affects the meteoric waters. Strong evaporation effects have been
documented for meteoric waters in the internally drained basins of
the Salar de Hombre Muerto and Salar de Atacama basins (Godfrey
et al., 2003).
3500
8. Conclusions
-80
4000
-100
0
10
20
40Ar/39Ar
30
40
age (Ma)
Symbols
El Salvador, Damiana
Other El Salvador
Coya
El Hueso
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 alunite
but generally closer to the air dominated than water dominated oxygen isotope
composition. The jarosite sample plots immediately right of the air dominated
boundary for supergene alunite. Reference field for high altitude precipitation for the
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
elevations corresponding to the δD values on the left axis. Values were calculated using
the empirically determined relationship for central and South America (Poage and
Chamberlain, 2001). Interpreted general climatic trends are indicated (see text for
discussion).
Table 2
Stable isotope data.
Sample
Mineral
dD
d34S
d18OSO4
age (Ma)
HTB004
STB-022
STB-026
STB-12A-1
STB-12A-2
STB-12B-1
STB-12B-2
CTB-43
CTB-46
CTB-48
CTB-49
Alunite
Alunite
Alunite
Alunite
Alunite
Alunite
Alunite
Alunite
Jarosite
Natroalunite
Natroalunite
−25
−23
−34
−54
−74
−61
−50
−53
−97
−57
−88
−1.8
1.4
−0.5
−0.9
0.3
0.1
0.0
0.0
0.8
1.1
3.0
3.7
4.8
6.8
5.3
9.7
4.1
3.9
10.7
11.0
2.8
2.6
8.19 ± 0.1
9 to 14
16.31 ± 0.12
14.22 ± 0.16
35.8 ± 1
15.3 ± 0.6
13.8 ± 0.2
20.1 ± 0.1
4.3 ± 0.1
0
4.8 ± 0.6
− Geomorphologic and stable isotope evidence strongly suggests
that the Precordillera in the Southern Atacama Desert has attained
elevations of at least 3000 m a.s.l. already in the early Oligocene
and thus, significantly prior to the major uplift of the Altiplano.
− The climate evolved differently in the western Precordillera and
Central Depression from the eastern Precordillera. The cessation of
supergene processes at El Salvador around 13 Ma has been
confirmed and is attributed to climate desiccation, an interpretation also supported by sedimentological and stable isotope
evidence. However, conditions at Coya and El Hueso in the Eastern
Precordillera, situated near 4000 m present day elevation
remained conducive for at least episodic supergene alunite
formation until the early Pliocene, and possibly up to the present
day. Uplift to elevations near 4000 m a.s.l. have led to increased
capture of moisture and consequently increased availability of
meteoric waters.
− The new 40Ar/39Ar age constraints presented herein provide
evidence confirming the previously proposed protracted history
of the Damiana exotic Cu deposit and indicate that the local
geomorphologic and hydrologic configuration has remained
relatively 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 Desarrollo
Científico y Tecnológico de Chile (Fondecyt) grant # 11060516. Kerry
Klassen is thanked for the stable isotope analyses whereas Paul Layer
and Tom Ullrich provided the Ar/Ar analyses. Fritz Schlunegger and an
anonymous EPSL reviewer are thanked for their constructive reviews.
This is MDRU publication P-264.
T. Bissig, R. Riquelme / Earth and Planetary Science Letters 299 (2010) 447–457
Climate
Literature This study
WP
EP
moderate tilting
in the fore-arc
Pliocene
hyper arid
Atacama gravel deposition
Canyon Incision
Coya
El Hueso
Tectonics
Late
Miocene
10
Other, El Salvador
Q. Turquesa, El Salvador
5
Damiana, El Salvador
0
Landscape
Supergene ages
Pediment formation
456
A3
A2
A1
?
3
Oligocene
?
35
40
thrusting and
folding, Potrerillos
Fold and Thrust
belt
Eocene
30
SC
semi-arid
25
thrusting and
uplift, Cordillera
Claudio Gay
semi-arid
20
Early
Miocene
AS
Hyper aridity
(Hartley and Chong, 2002)
2
Hyper aridity
(Alpers and Brimhall, 1988)
15
slow tilting
in the fore-arc
Middle
Miocene
7
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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