Introduction

Atacamite and its polymorph paratacamite (Cu2Cl(OH)3) are major constituents of supergene oxide zones in many Cu deposits from the Atacama Desert of northern Chile (Sillitoe and McKee 1996; Chávez 2000; Mote et al. 2001; Bouzari and Clark 2002; Hartley and Chong 2002; Dunai et al. 2005; Hartley and Rice 2005; Sillitoe 2005; Rech et al. 2006). Atacamite formation has been traditionally interpreted as a primary product of supergene oxidation (e.g., Chávez 2000), formed after leaching of Cu sulfides by oxygenated meteoric waters that percolated through the deposits. Supergene oxidation of copper porphyry deposits occurred over a long period from 44 to 9 Ma, after which the climate became hyperarid (Arancibia et al. 2006). Atacamite requires saline water for its formation (Hannington 1993), rather than fresh meteoric water. Recognizing this, Arcuri and Brimhall (2003) proposed that atacamite in the Radomiro Tomic deposit, where this is the principal oxide mineral, was formed during supergene oxidation by percolating saline meteoric water. However, atacamite dissolves rapidly or undergoes phase change when exposed to fresh, meteoric water. During the long period prior to the onset of hyperaridity, when oxide zones were exposed to percolating rainwater, any atacamite would have been removed. Moreover, some oxide zones containing atacamite were later covered by piedmont gravels. Stream waters carrying these gravels would have permeated through the oxide zones, removing atacamite. Based on these arguments, Cameron et al. (2007) suggested that atacamite-bearing oxide assemblages are more likely to have formed after the onset of hyperaridity by the replacement of preexisting oxide assemblages, either by earthquake-induced pulses of deep saline waters through the deposits or by meteoric waters which became saline by evaporation at the surface.

When hyperaridity commenced is much debated. Previously, it was thought to have followed the main period of supergene enrichment ending at 14 Ma (Alpers and Brimhall 1988; Sillitoe and McKee 1996). However, recent dating of post-14-Ma supergene events and other evidence (Hartley and Rice 2005; Arancibia et al. 2006) suggest that hyperaridity started as late as 4 Ma. Leybourne and Cameron (2008) provide evidence that atacamite and other secondary Cu minerals may currently be forming by oxidation of hypogene and/or supergene sulfides, suggesting that supergene Cu mineralization is possible even under the hyperarid conditions that dominate the present-day climate of the Atacama Desert.

The aim of this study is to provide further insight into the origin and source of waters that form(ed) atacamite by integrating mineralogical and isotopic data of atacamite with groundwater chemistry data from the Mantos Blancos and Spence Cu deposits in the Atacama Desert of northern Chile. We report salinity data of fluid inclusions trapped in atacamite, and we compare these results with groundwater salinities. In addition, we complement these data with high-resolution transmission electron microscopy (TEM) observations of atacamite samples and 36Cl isotopic data of atacamite from various deposits in the Atacama Desert.

Geologic setting and atacamite occurrence

About 40% of the known Cu resources in the world occur in Chile, and the hyperarid Atacama Desert of northern Chile contains some of the world’s largest Cu deposits (Maksaev et al. 2007). Supergene oxidation and leaching processes have produced important enrichment of porphyry and stratabound (“manto-type”) Cu deposits, and the economic viability of these deposits is typically dependent on the size and quality of the supergene enrichment blanket that overlies the hypogene Cu–sulfide zones (Sillitoe and McKee 1996; Maksaev et al. 2007). Notable occurrences of atacamite (with variable proportions of other oxide minerals) are found in several Cu deposits in northern Chile. The location of atacamite-bearing deposits, including Mantos Blancos and Spence, is shown on Fig. 1, and the geology of these deposits has been recently reviewed by Ramirez et al. (2006) and Cameron et al. (2007). Both Spence and Mantos Blancos are located along the prominent Antofagasta–Calama Lineament (ACL, Fig. 1), a major structural feature that is thought to have controlled deformation and tectonic rotation in the region since Early Cretaceous (Ramirez et al. 2006). The Spence porphyry Cu deposit contains 231 Mt of sulfide ore at 1.13% Cu and 79 Mt of oxide ore (atacamite and other oxides) at 1.18% Cu (Cameron et al. 2007). Porphyry intrusion and hypogene mineralization (in the porphyries and adjacent andesites) at Spence took place during the Paleocene, at around 57 Ma (Rowland and Clark 2001). The Mantos Blancos porphyry-like deposit also lies on the ACL, and the oxidized ore represents ~30% of the total Cu reserve (200 Mt extracted since 1960; Maksaev et al. 2007). The main ore formation has been related to an Upper Jurassic hydrothermal event at ~140 Ma and consists of hydrothermal breccias, disseminations, and stockworks associated with sodic alteration (Ramirez et al. 2006). Mantos Blancos and Spence deposits are characterized by well-developed atacamite-bearing oxide zones and are covered by ~10–100- and 50–100-m-thick gravels and gypsum-rich saline soil, respectively. The current water tables at the Mantos Blancos and Spence deposits are located at depths of ~400 and 50 to 90 m below the surface, respectively.

Fig. 1
figure 1

Map showing the location of various Cu deposits in the Atacama Desert in northern Chile, after Cameron et al. (2007). Black squares and circles represent atacamite-containing porphyry Cu deposits and other Cu deposits, respectively (white squares are other porphyry deposits). Physiographic zones (Coastal Ranges, Central Depression, Precordillera, and High Andes) are depicted in shades of gray, and DFZ and ACL are the Domeyko Fault Zone and the Antofagasta-Calama Lineament (Palacios et al. 2007)

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Atacamite is the major component of oxide zones of many Cu deposits in the Atacama Desert (Fig. 1). It typically occurs as complex, fine-grained cryptocrystalline aggregates forming veinlets, disseminations and/or replacing preexisting Cu minerals as corrosion coatings, although it occasionally occurs as coarser, millimeter-scale crystals. Atacamite has also been reported as a secondary mineral in the weathered portions of massive sulfide deposits on the modern seafloor (Hannington 1993). Hannington (1993) described atacamite in TAG mounds as occurring in patchy colloform masses, as crystalline aggregates that fill open spaces, and as disseminated crystals intergrown with fine-grained Fe-oxy-hydroxides. The crystalline habits of atacamite include prismatic crystals and equant, rhombic prisms. Individual euhedra range in size from a few tens of micrometers to about 2 mm but are typically less than 500 μm in size (Hannington 1993). Atacamite commonly associates with gypsum in the supergene zones of Cu deposits (e.g., at Radomiro Tomic; Cuadra and Rojas et al. 2001) as well as in submarine hydrothermal ores (Glynn et al. 2006). Sedimentary textures described by Glynn et al. (2006) in supergene alteration zones of submarine hydrothermal sulfides indicate that gypsum and atacamite precipitation may be contemporaneous.

Materials and methods

Groundwater samples collected from drill holes at the Mantos Blancos deposit were analyzed for major and minor cations (Na, K, Ca, Mg, Si, Cu, Fe; inductively coupled plasma optical emission spectrometry) and anions (Cl, SO4, NO3; ion chromatography) at Anglo American Laboratories.

Thermometric measurements in primary fluid inclusions trapped in atacamite from the Mantos and Spence deposits were conducted using a Linkam THM-600 stage at the Fluid Inclusions Laboratory, Departamento de Geología, Universidad de Chile. The stage was calibrated with liquid CO2 inclusions, distilled mercury, and pure water inclusions (accuracy of the melting point temperatures is estimated to be ±0.4°C). Only freezing temperature measurements were performed. Heating experiments to determine the homogenization temperature of the inclusions were avoided, considering the fact that temperatures obtained from heating experiments may be considerably higher than the true values, due to stretching of fluid inclusions trapped in soft minerals such as atacamite (Moh’s hardness ~3–3.5). Stretching due to overheating of fluid inclusions in soft minerals (Moh’s hardness <3.5) has been extensively documented by many authors, including Bodnar and Bethke (1984), Ulrich and Bodnar (1988), and Vanko and Bach (2005).

Atacamite in oxide zones from ore deposits typically occurs as very fine-grained aggregates in veins and/or disseminated. None of the traditional optical and microanalytical techniques can image the structure and textures of atacamite aggregates below the micrometer level, nor its mineralogical associations at that scale. To investigate the micrometer-to-nanometer texture of fine atacamite aggregates from the Mantos Blancos and Spence deposits, TEM observations were performed at the Laboratorio de Microscopía Electrónica de Transmisión, Departamento de Geología, Universidad de Chile, Santiago, Chile. TEM samples were cut from polished thin sections and final thinning was performed by ion milling with an Ar beam (4.0 keV) in a Gatan precise ion-polishing system. Details on TEM sample preparation are presented elsewhere (Reich et al. 2005). Observation was carried out using a FEI Tecnai F20 FEG TEM operated at 200 kV equipped with an energy-dispersive spectrometry (EDS) detector. Radiation beam damage was not observed during the analyses. Details on the TEM technique can be found in Williams and Carter (1996).

Because of the extremely low abundance of 36Cl in nature, detection of this isotope is only possible by accelerator mass spectrometry (AMS). Isotope determinations of 36Cl in atacamite and water from recent rain events in the area were conducted using AMS at PrimeLab, Purdue University, USA, following the established extraction and analytical methods reported by Fehn et al. (1992) and Sharma et al. (2000). The detection limit is around 1 × 10−15 for the 36Cl-to-Cl ratio with a typical precision of 5–10% (1σ), which decreases, however, considerably for samples close to the detection limit or where carrier material has been used.

Results

Groundwater salinity

The Mantos Blancos groundwaters span a wide range of salinities (Table 1) and can be subdivided into three main groups based on TDS values: (a) low-salinity waters (TDS < 10,000 mg/L), represented by only one sample (14877); (b) saline waters (10,000 < TDS < 100,000 mg/L, 16 samples); and (c) high-salinity waters (TDS > 100,000 mg/L, 15 samples). There is about two orders of magnitude difference in the Cl− and Na+ abundance of the three types of waters, as low-salinity waters show values of 2,050 mg/L Cl− and 1,570 mg/L Na+, whereas the saline waters and high-salinity waters average 18,376 and 76,251 mg/L Cl− and 17,779 and 46,223 mg/L Na+, respectively. Mantos Blancos groundwaters are mainly Na+–Cl––\({\text{SO}}_4 ^{2 - } \)-type waters, and anions are dominated by Cl−, with all samples showing a strong positive correlation between Na+ + Cl− and TDS (Fig. 2a), over the entire range in TDS. A similar trend of increasing \({\text{SO}}_4 ^{2 - } \) with TDS is observed for low-salinity and saline waters, but this correlation breaks down for high-salinity waters, where a strong decrease in\({\text{SO}}_4 ^{2 - } \) with increasing TDS is observed (Fig. 2b). Similar trends have been reported for groundwaters at the Spence deposit by Leybourne and Cameron (2006).

Fig. 2
figure 2

Plots of total dissolved solids (TDS) versus Na + Cl cations (a) and SO4 anions (b) for groundwaters from Mantos Blancos and Spence deposits. Data from Spence taken from Leybourne and Cameron (2006)

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Table 1 Major and minor ions of groundwaters from Mantos Blancos deposit, northern Chile
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Fluid inclusion salinity

All of the fluid inclusions observed in atacamite from the Spence and Mantos Blancos deposits consist of liquid and vapor (L + V) with no daughter mineral. Most inclusions have diameters of <30 μm, with <30% vapor by volume. Inclusions are scarce and are typically found scattered throughout atacamite crystals, forming clusters (Fig. 3a–d). Due to the scarcity and small size of the inclusions, coupled with the fact that atacamite occurs as polycrystalline aggregates of fine-grained crystals and does not show growth zones under polarized light, the primary or pseudosecondary nature of the inclusions could not be confirmed (secondary inclusions were not observed). The salinity of the fluid inclusions was determined using the formula reported by Bodnar (1993) for the NaCl–H2O system, as no evidence was observed for either liquid CO2 or clathrate formation on freezing point depression measurements. Salinities of 48 fluid inclusions trapped within five atacamite samples from Mantos Blancos (each one representing different veinlets) vary between 6.4 and 10.7 wt.% NaCl equivalent (NaCleq, with an average of 8.4 wt.% NaCleq; Fig. 4; Table 2). In contrast, the salinities of 43 fluid inclusions, determined in seven samples of atacamite-bearing veinlets at Spence, vary between 1.7 and 4.3 wt.% NaCleq, averaging 2.7 wt.% NaCleq (Fig. 4).

Fig. 3
figure 3

Photomicrographs of fluid inclusions in atacamite from a Mantos Blancos sample, taken in plane polarized reflected light (uncrossed polars). Rectangle in a shows magnified area in b. All of the fluid inclusions observed in atacamite from Spence and Mantos Blancos deposits (diameters <30 μm, a–d) consist of liquid and vapor (L + V) with no daughter mineral (c and d). Inclusions are scarce and are usually found scattered throughout atacamite crystals, forming clusters

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Fig. 4
figure 4

Histograms showing the distribution of salinities (in wt.% NaCleq) of fluid inclusions in atacamite from Mantos Blancos and Spence deposits in northern Chile

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Table 2 Fluid inclusion salinity data for atacamite from Mantos Blancos and Spence deposits
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TEM observations

Atacamite occurs as polycrystalline aggregates associated with gypsum in veinlets (Fig. 5a). TEM observations in a selected atacamite–gypsum veinlet sample from the Spence deposit (Fig. 5a) reveal that atacamite is in close association with gypsum at all scales of observation (hand sample to nanoscale, Fig. 5a–e) The presence of atacamite and gypsum at the nanoscale was confirmed using EDS spot analyses. TEM images show dark, opaque, and elongated crystals of atacamite (<10 μm in size) associated with lighter gray, translucent gypsum crystals that show its characteristic cleavage (Fig. 5c). Figure 5d shows a detail of the atacamite–gypsum aggregate microstructure, showing the intergrowth of both minerals, and a layer of gypsum can be seen in the middle of the elongated atacamite grain.

Fig. 5
figure 5

The atacamite–gypsum association from hand-sample scale down to the nanometer range: a atacamite–gypsum veinlet from Spence deposit; b photomicrograph of fine-grained aggregates of atacamite in the veinlet shown in a (reflected light polarizing microscopy); c bright-field TEM images of atacamite aggregates showing a close relation with gypsum at the micron scale. Both minerals were identified by EDS analysis. Rectangle shows location of detail in d; d detail showing the mineralogical association between atacamite and gypsum at the submicrometer range, as seen under TEM observation. Intercalations of gypsum (lighter) in atacamite (darker) can be observed; e magnification of the rectangle area shown in d. At the nanometer scale, atacamite and gypsum are found in close association

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36Cl-to-Cl ratios

In order to gain insight into the origin of the Cl-rich fluids from which atacamite precipitated, the 36Cl contents of nine samples of atacamite from five ore deposits in the Atacama Desert were determined together with two samples from recent rain events (Antofagasta and El Laco). Details of the Michilla (stratabound Cu or Manto-type), Antucoya-Buey Muerto (porphyry Cu), Radomiro Tomic (porphyry Cu), and Mantos de la Luna (manto-type) deposits can be found in Trista-Aguilera et al. (2006), Maksaev et al. (2006, 2007), and Cuadra and Rojas (2001), respectively. This suite of Cu deposits, all characterized by the development of atacamite-bearing oxide zones, covers a geographic range between the Coastal Cordillera near the Pacific Ocean (Mantos Blancos, Mantos de la Luna, Michilla) to the Central Depression (Antucoya-Buey Muerto, Radomiro Tomic and Spence; Fig. 1). Results are reported as the ratio of 36Cl atoms to stable Cl atoms (36Cl to Cl; Table 3). The atacamite samples show low 36Cl-to-Cl ratios, varying from 11 × 10−15 to 28 × 10−15 at at−1. The highest and lowest 36Cl-to-Cl ratios are observed for Mantos Blancos (28 × 10−15) and Antucoya-Buey Muerto (11 × 10−15), respectively. In contrast, the two rainwater samples have 36Cl-to-Cl ratios of 867 and 2,247, respectively, a range to be expected for samples from very arid climates and high altitudes (Bentley et al. 1986).

Table 3 36Cl-to-Cl ratios in atacamite from ore deposits in the Atacama Desert in northern Chile
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Discussion

Solution salinities, gypsum supersaturation, and the formation of atacamite

A striking feature observed in all atacamite samples analyzed in this study is the close association of atacamite with gypsum. As seen in Fig. 5a–e, atacamite–gypsum aggregates can be traced from a hand-sample scale down to the nanometer range, and the textures observed at all scales suggest that atacamite and gypsum are coeval. Contemporaneous precipitation of gypsum and atacamite has also been reported by Glynn et al. (2006) in supergene alteration of submarine hydrothermal sulfides.

Present-day groundwater salinity (TDS) and sulfate (\({\text{SO}}_4 ^{2 - } \)) trends (Fig. 2b) and fluid inclusion salinities in atacamite (Fig. 4) provide additional insights into this mineralogical association. At the Mantos Blancos and Spence deposits, the decrease in \({\text{SO}}_4 ^{2 - } \) of the groundwaters at the highest salinities (Fig. 2b) suggests that \({\text{SO}}_4 ^{2 - } \) concentrations are controlled, in part, by the formation of gypsum (Leybourne and Cameron 2006). Therefore, these highest TDS values reflect the salinities at which gypsum supersaturated from groundwaters, ~100,000 and ~30,000 mg/L for Mantos Blancos and Spence, respectively. This argument is supported by mineral saturation calculations reported by Leybourne and Cameron (2006, 2008). In the cited studies, determination of mineral saturation indices for groundwaters at Spence reveal that the highest salinity waters (west of the deposit) are supersaturated with respect to gypsum and close to (or exceeding) saturation with respect to secondary minerals such as atacamite.

In order to make the salinities of the Na–Cl–SO4-rich groundwaters from Mantos Blancos and Spence (expressed as TDS in milligram per liter) comparable to the salinities of fluid inclusions trapped in atacamite from both deposits (expressed as weight percent NaCleq), TDS values of groundwaters have been converted to weight percent NaCleq according to TDS*[weight percent NaCleq] = (Na[milligram per liter] + Cl[milligram per liter])10−4. When the gypsum-saturation salinities of groundwaters are compared with the fluid inclusion salinity data in atacamite for both deposits in a single diagram (Fig. 6), a striking correlation arises. The average salinities of fluid inclusions in atacamite for Mantos Blancos and Spence deposits (~7–9 and 2–3 wt.% NaCleq, respectively, Fig. 6, vertical lines) are strongly correlated to the salinities at which gypsum precipitates from groundwaters in both deposits (corrected TDS* ~5–9 and 1–3 wt.% NaCleq, respectively, Fig. 6, trend line inflections). Therefore, as a first approximation, this correlation suggests that the range of salinities from which atacamite–gypsum formed is comparable to present-day, highly-saline gypsum- (and atacamite)-saturated groundwaters.

Fig. 6
figure 6

Sulfate content (SO4, milligram per liter) versus salinity (weight percent NaCleq, as calculated from TDS data, see text for detail) of groundwaters from Mantos Blancos and Spence. The salinity at which groundwaters supersaturate with respect to gypsum (inflection points) correlate well with the salinity range of fluid inclusions in atacamite from the same deposits (shown as shaded vertical intervals defined by the average (center line) ± standard deviation of salinities from Table 2)

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A deep, old, and saline source for atacamite formation

Although it has long been recognized that saline waters can form by evaporation of groundwaters of meteoric origin in hyperarid climates, recent studies report evidence of alternative sources for the Cl-rich solutions required to form atacamite. Cameron et al. (2002, 2007) propose that deep saline formation waters (basinal brines), dewatered from sedimentary rocks underlying the Atacama Desert, can be forced to surface by tectonic pumping during earthquakes. This hypothesis is supported by different lines of evidence, including: (a) the development of geochemical anomalies in salt soils over orebodies that are strongly correlated with the distribution of fractures–faults below (Cameron et al. 2002; Palacios et al. 2005); (b) stable isotopic signatures that confirm involvement of seawater-like, deep formation waters–basinal brines that mix to variable degrees with the less saline, regional groundwater flow (e.g., δ37Cl in atacamite at Radomiro Tomic, Arcuri and Brimhall 2003; δD and δ18O in groundwaters at Spence, Leybourne and Cameron 2006).

The first measurements of 36Cl in atacamite, presented here, provide new lines of evidence concerning the origin of the saline waters that formed atacamite (Table 3). Natural 36Cl can be derived from cosmogenic, fissiogenic, and anthropogenic sources (Fehn et al. 1988; Snyder at al. 2003; Fehn and Snyder 2005). The anthropogenic component is related to thermonuclear weapons tests, and considering the fact that atmospheric 36Cl has returned to prebomb ratios since about 1980 (Suter et al. 1987) we will focus on the other two sources. The cosmogenic component is formed by spallation by cosmic rays of Ar in the atmosphere and Ca and K in surface rocks and has been used to determine groundwater ages and exposure ages of surface rocks (Purdy et al. 1996; Phillips et al. 2003). In theory, the subsurface isolation time of water between 5 × 104 and 2 × 106 years can be determined using this method, considering that the half-life of 36Cl is 0.3 Ma (Fehn and Snyder 2005). On the other hand, the fissiogenic component is a result of capture by 35Cl (e.g., in pore waters) of neutrons produced by the decay of U and Th isotopes (e.g., in the host rocks). The buildup of 36Cl is a function of time, concentration of U and Th, and the presence of neutron-absorbing atoms competing with 35Cl. The presence of fissiogenic 36Cl can be used to determine potential sources for fluids (Rao et al. 1996) or the residence time of fluids in a given formation if the neutron flux can be estimated (Fehn et al. 1992; Fehn and Snyder 2005).

Despite the low 36Cl contents measured in atacamite in our samples (36Cl–Cl ~(11–28)×10−15, Table 3), its detection suggests that 36Cl was inherited from the waters from which atacamite formed. Thus, the data can be potentially used as an isotopic tracer for the waters involved in the genesis of atacamite if the main source(s) component(s) for 36Cl can be sorted out and assuming that the 36Cl budget incorporated in atacamite (cosmogenic or fissiogenic in origin) represents the 36Cl budget of the water from which it precipitated (assuming, in this particular case, no fluid–mineral isotope fractionation and/or 36Cl loss or addition after the formation of atacamite).

All atacamite samples show very low 36Cl-to-Cl ratios, close to the AMS detection limit of 1 × 10–15, comparable to previously reported 36Cl/Cl ratios of deep formation waters and old groundwaters. Low 36Cl-to-Cl ratios, interpreted as in secular equilibrium with in situ (fissiogenic) 36Cl, have been reported in ultradeep mine waters at the Witwatersrand Basin in South Africa (Lippmann et al. 2003), deep groundwaters from the Great Artesian Basin in Australia (Lehmann et al. 2003), waters from the German Continental Deep Drill Hole in Northern Bavaria (Fehn and Snyder 2005), deep formation waters from the Fruitland Formation in Colorado and New Mexico (Snyder et al. 2003; Snyder and Fabryka-Martin 2007), deep groundwater from the Northern Sahara (Guendouz and Michelot 2006), and the East Irish Sea Basin (Metcalfe et al. 2007). In all these cases, low 36Cl-to-Cl ratios (<30 × 10−15) reflect subsurface, fissiogenic production of 36Cl in secular equilibrium indicating an age in excess of 1.5 Ma, or five times the half-life of 36Cl. Although the meteoric component cannot be discarded, disentangling the cosmogenic (meteoric) 36Cl signal is typically difficult, in particular for deeper and more saline groundwater, mainly due to subsurface production of 36Cl by neutron capture of 35Cl and transport from adjacent aquifers or pore water reservoirs (Lehmann et al. 2003).

A potential check on the dominant source of chlorine in these deposits is possible by relating the results to the distance from the ocean and altitude. 36Cl-to-Cl ratios in rainwater and resulting surface or groundwaters show a strong dependence on the distance from the ocean because chlorine in the atmosphere is derived predominantly from sea spray. The long residence time of chlorine in the oceans results in marine 36Cl-to-Cl ratios below 0.5 × 10−15 (i.e., below the detection limit of AMS). In the atmosphere, 36Cl-to-Cl ratios increase with altitude because the production rate of 36Cl diminishes as cosmic rays are attenuated passing through the atmosphere. As a result of these processes, 36Cl-to-Cl ratios in surface waters generally increase strongly with distance from the oceans and with altitude (e.g., Bentley et al. 1986), also observed for the two rainwater samples measured for this study. In contrast to these surface water samples, results for the atacamite samples do not show a correlation with distance from the oceans or altitude, suggesting that 36Cl in these deposits is mainly derived not from cosmogenic but from fissiogenic sources.

In order to further evaluate the role of fissiogenic 36Cl as a potential source for 36Cl in atacamite, we related the abundances of U and Th in the host rocks of each ore deposit (reported by Dietrich 1999; Oliveros 2005; Ramírez et al. 2008) to their respective 36Cl-to-Cl ratios in atacamite (Fig. 7). There is a positive correlation between U + Th concentration in the host rocks and 36Cl-to-Cl ratios in atacamite, with the highest 36Cl–Cl values at Mantos Blancos and the lowest at Antucoya and Spence. This trend supports our interpretation of the low 36Cl-to-Cl ratios in atacamite (11–28 × 10–15 at at−1) as representing subsurface production of fissiogenic 36Cl in secular equilibrium with the solutions involved in the atacamite origin (Lehmann et al. 2003; Lippmann et al. 2003). Because atacamite does not contain U or Th itself, the production of 36Cl is no longer supported once chlorine has entered the minerals and the 36Cl-to-Cl ratio starts to decrease with age. The fact that we still found measurable 36Cl in atacamite indicates then that the formation of these minerals was relatively recent. The 36Cl data strongly suggest that the chlorine in the saline waters related to atacamite formation is old in origin (>1.5 Ma) but that the atacamite formation occurred less than 1.5 Ma ago (or 5× half-life of 36Cl).

Fig. 7
figure 7

Variation of 36Cl-to-Cl ratios in atacamite as a function of the U + Th concentration of host rocks, for various deposits in the Atacama Desert in northern Chile. AB = Antucoya-Buey Muerto, SP = Spence, ML = Mantos de la Luna, RT = Radomiro Tomic, SS = Michilla, and MB = Mantos Blancos

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Although more than one mechanism may be involved in formation of atacamite in the hyperarid Atacama Desert, the results support the tectonic pumping model proposed by Cameron et al. (2002, 2007). In the context of this model, saline formation water from the dewatering of the sedimentary basin is carried up through the deposit, modifying the existing oxide assemblage, creating an atacamite-stable assemblage. During earthquakes, the saline water is forced to the surface of the covering gravels, creating saline anomalies plus Cu derived from the deposit. Palacios et al. (2005) reported Cu anomalies in soils above the Mantos Blancos copper deposit. Although the Cu anomalies in surface salt samples (gypsum + anhydrite + halite) over the ore bodies are spiky, their contrast with the background Cu values is remarkable. The same study reports a good correlation between Cu and Na where the concentration of Cu is high and shows that Cu anomalies spatially correlate with faults that crosscut Cu mineralization. The previous observation is confirmed by the presence of salt efflorescences (halite + gypsum) that contain atacamite and chalcanthite at the surface along major active faults near the deposits (e.g., Salar del Carmen fault close to the Mantos Blancos), strongly suggesting that Cu was brought to the surface by saline waters. Copper anomalies from Miocene alluvial gravel soils above the Spence porphyry copper have also been reported by Cameron and Leybourne (2005). The anomalous Cu values developed over the ore deposit show the same spiky response documented by Palacios et al. (2005), and the peaks are observed in zones where the partly salt-cemented gravels are fractured. Enzyme leach and deionized water analyses, which measure water-soluble constituents, evidence Cl and Na anomalies in gravel soils above the Spence deposit and also over fractured zones (Cameron et al. 2002; Cameron and Leybourne 2005). These fracture zones over the deposit are interpreted to represent seismically active basement faults that have propagated through the gravel cover Cameron and Leybourne (2005), suggesting an important relationship between the ascent of saline, deep, and old waters and atacamite formation in hyperarid regions such as the Atacama Desert.

Summary and conclusions

The formation of atacamite in oxide zones in Cu deposits from the Atacama Desert in northern Chile is related to gypsum-saturated saline groundwaters (TDS* > 20,000 mg/L or 2 wt.% NaCleq), as confirmed by fluid inclusions in atacamite and present-day groundwater chemistry. The salinity at which groundwaters at the Spence and Mantos Blancos saturate with respect to gypsum (1–3 and 5–9 wt.% NaCleq, respectively) is in the range of salinities measured in fluid inclusions in atacamite, suggesting a close connection between gypsum saturation and atacamite formation. This genetic link is confirmed by observations at different scales that confirm a close mineralogical association between these two minerals, from sample scale down to the nanometer range. In addition, 36Cl data in atacamite show that the low but detectable 36Cl/Cl ratios measured in atacamite (11–28 × 10–15 atat−1) are probably related to fissiogenic production in the subsurface, suggesting that atacamite in these deposits is produced from chloride in saline waters with ages in excess of 1.5 Ma but that the formation process itself occurred relatively recently (<1.5 Ma).

These results, coupled with previous studies of groundwater chemistry and stable isotope distributions, suggest a deep origin for the saline waters responsible for the formation of atacamite. Our results provide new constraints on the age of atacamite in ore deposits from the Atacama Desert and support the recent notion that supergene enrichment may have occurred more recently (<1.5 Ma) than many models for the climate of northern Chile suggest (e.g., 9–14 Ma, Alpers and Brimhall 1988; 3–6 Ma, Hartley and Rice 2005). The results suggest that formation of atacamite in hyperarid climates such as the Atacama Desert is an ongoing process (e.g., Fig. 8) that has occurred intermittently, since the onset of hyperaridity.

Fig. 8
figure 8

Atacamite in miner boots recovered from a deep gallery (150 m deep) in early 1900s workings around the late Paleocene Sierra Gorda Cu-porphyry deposit (photo courtesy of the Geology Museum of the Department of Geosciences, Universidad Católica del Norte, UCN, Antofagasta, Chile). Atacamite (green) was confirmed by powder X-ray diffraction analysis at UCN

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