Chlorine isotope fractionation recorded in atacamite during supergene copper oxidation

Chemical Geology 525 (2019) 168–176

Contents lists available at ScienceDirect

Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo

Chlorine isotope fractionation recorded in atacamite during supergene copper oxidation

T

Martin Reicha,b, , Jaime D. Barnesc, Daniel O. Breeckerc, Fernando Barraa,b, Catalina Milojevica,b, Dana L. Drewc,1 ⁎

a

Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile Millennium Nucleus for Metal Tracing Along Subduction, FCFM, Universidad de Chile, Santiago, Chile c Department of Geological Sciences, University of Texas, Austin, TX 78712, USA b

ARTICLE INFO

ABSTRACT

Editor: Karen Johannesson

In the Atacama Desert of northern Chile, large amounts of the copper hydroxy-chloride mineral atacamite (Cu2Cl (OH)3) are formed in the supergene oxidation zone of Cu deposits. Since atacamite requires saline water to form and is commonly preserved under hyperarid conditions, it has been proposed as a climate-sensitive mineral marker that can also provide relevant geochemical information regarding halogen (in particular chlorine) fluid sources during supergene Cu oxidation. However, chlorine stable isotope data for atacamite in Cu deposits are scarce and no experimental data for chlorine isotope fractionation between atacamite and water are currently available that could provide constraints on the possible mechanisms of fractionation. In this study we report δ37Cl values of atacamite along a thick (~100 m) and well-developed supergene enrichment profile at the Barreal Seco iron oxide-copper-gold (IOCG) deposit in the Atacama Desert. The δ37Cl values of atacamite along this profile range from +2.1‰ to −0.6‰ (mean = +0.4‰ ± 0.7‰; median = +0.3‰), and show a distinct trend with depth, characterized by higher δ37Cl values towards the top of the profile, with lower values at the bottom. In addition, the chlorine isotope compositions of experimentally synthetized atacamite crystals and coexisting CuCl2 solutions were determined at room temperature (23 °C). The per mil fractionation factor (Δ37Clatacamite-Cl) was determined at +0.75‰, showing that the heavy isotope (37Cl) is preferentially incorporated into atacamite. We used numerical models to test two possible scenarios that might explain the observed δ37Cl profile at Barreal Seco. A first scenario involves lowering of the water table with concomitant precipitation of atacamite, which removes Cl from solution following a Rayleigh fractionation process. This model qualitatively fits our observations but does not explain the full magnitude of the observed δ37Cl shift along the supergene profile. In contrast, a model that simulates Cl- diffusion following the upward injection of a deep brine that overlies fresh groundwater reproduces well the observed δ37Cl data, suggesting that seismic pumping of basinal brines, followed by Cl diffusion, is a feasible mechanism to explain the formation of atacamite at Barreal Seco. These data are a first step towards interpreting δ37Cl profiles of atacamite in Cu deposits in the Atacama Desert and exploring the potential use of stable chlorine isotopes to monitor Cu weathering and enrichment in supergene environments.

Keywords: Chlorine isotopes Atacamite Copper deposits Supergene enrichment Atacama Desert Barreal Seco IOCG

1. Introduction Oxidation and leaching of ore deposits in the weathering environment is a major ore-forming process that may result in a three to fourfold increase in metal grade, specifically for commodities such as Cu, Al, Fe, Ni, U, Au and Zn. This phenomenon, broadly known as supergene enrichment, refers to the secondary, in situ accumulation of metals

as a result of a combination of geological and geochemical processes (Reich, 2018). The highly-enriched supergene zone located in the upper portions of ore deposits forms when recurrent chemical weathering promotes the dissolution, transport of soluble chemical components and precipitation of elements at or near the Earth's surface, creating a chemically stratified weathering profile, which may extend down from a few meters to ~1000 m below the surface (Ague and Brimhall, 1989;

Corresponding author at: Department of Geology and Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. E-mail address: [email protected] (M. Reich). 1 Now at Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. ⁎

https://doi.org/10.1016/j.chemgeo.2019.07.023 Received 25 April 2019; Received in revised form 10 July 2019; Accepted 17 July 2019 Available online 19 July 2019 0009-2541/ © 2019 Elsevier B.V. All rights reserved.

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Sillitoe, 2005; Reich and Vasconcelos, 2015; Reich, 2018). The rates of these reactions are invariably climate-dependent, reflecting ambient temperature, availability of liquid water (i.e., rainfall intensity and seasonality), evapotranspiration rates, and biological and microbiological activity (Reich and Vasconcelos, 2015). In Cu deposits, the near-surface hydrolysis and oxidation of hypogene (primary) sulfur-bearing assemblages leads to a decrease in the pH of descending meteoric waters and the formation of SO42− anions (Reich and Vasconcelos, 2015). Due to the simultaneous breakdown of chalcopyrite (CuFeS2) and bornite (Cu5FeS4), soluble Cu2+ ions that are transported downwards precipitate within the oxidation zone, i.e., above the water table, forming extensive deposits composed of “green oxides” or “copper oxides” (Sillitoe, 2005). The formation of this mineralogically and compositionally complex layer is essential for the economic viability of many Cu deposits, and is composed of Cu-oxide, sulfate, hydroxy-chloride, carbonate and silicate minerals, and native Cu (Reich and Vasconcelos, 2015, and references therein). The copper hydroxy-chloride mineral atacamite (Cu2Cl(OH)3) is an important component of supergene oxide zones of Cu deposits in the Atacama Desert of northern Chile, whereas in similar deposits elsewhere, it is rare (Cameron et al., 2007, 2008, 2010). In northern Chile, atacamite is present in variable proportions with other oxide minerals in several major Cu deposits (Maksaev et al., 2007). The presence of massive amounts of atacamite in the oxide zone from Cu deposits is a result of the climatic evolution of the Atacama Desert. As atacamite requires saline water to form and dissolves rapidly when exposed to meteoric water, hyperarid climate conditions are necessary for its preservation (Cameron et al., 2007, 2008, 2010; Reich et al., 2008, 2009). Hence, atacamite has been proposed as a climate-sensitive mineral marker that can be used to infer past hyperarid conditions and climate changes over time. Furthermore, atacamite is important because it provides relevant chemical and isotopic information to constrain halogen (in particular chlorine) fluid sources during supergene Cu enrichment (Leybourne and Cameron, 2006; Reich et al., 2008, 2009, 2013; Alvarez et al., 2015).

The source of saline solutions from which atacamite formed has been the subject of discussion. Reich et al. (2008) reported that atacamite samples from Cu deposits in the Atacama Desert show very low 36 Cl-to-Cl ratios (11 × 10−15 to 28 × 10−15 at·at−1), comparable to previously reported 36Cl-to-Cl ratios of deep formation waters and old groundwaters, discarding other halogen sources including atmospheric, eolian and sea spray contributions. Potentially, one method to determine the origin of the Cl-rich solutions is by the use of chlorine stable isotopes. Chlorine isotope data are reported in standard per mil notation versus standard mean ocean chloride (SMOC), which is defined as 0‰ (Kaufmann et al., 1984). In general, the δ37Cl values of natural terrestrial materials have a narrow range from ~−2 to ~+2‰, however kinetic fractionation processes (e.g., diffusion, ion filtration, Cl loss via degassing) can result in large variations from ~−8 to ~+20‰ (Barnes and Sharp, 2017 and references therein). The chlorine isotope compositions of some of the major reservoirs are: seawater = 0‰; mantle = −0.2 ± 0.3‰; evaporites (halite) ≈−0.6 to +0.4‰; sedimentary porefluids ≈−8 to 0‰; basinal brines/formation waters ≈−4.0 to +1.5‰; marine sediments ≈−2.5 to 0‰; terrestrial sediments ≈−2 to +2‰ (Barnes and Sharp, 2017 and references therein). Chlorine stable isotope data for atacamite in Cu deposits are scarce, and the few studies available show significant isotopic variability. Eggenkamp and Schuiling (1995) reported a positive δ37Cl value (+5.96‰) for a single atacamite sample from Ravensthorpe, Australia. In addition, Arcuri and Brimhall (2003) reported chlorine isotope compositions for atacamite mineralization at the Radomiro Tomic, Chuquicamata and Mina Sur Cu deposits in northern Chile with δ37Cl values ranging from −0.1 to +0.2‰. The distal atacamite mineralization on the periphery of the aforementioned orebodies has lower δ37Cl values (−3.2 to −0.1‰; Arcuri and Brimhall, 2003). These authors concluded that Upper Jurassic marine sediments with a δ37Cl = −0.8 to +0.5‰ could have been the source of chloride for the oxide zone atacamite mineralization at Radomiro Tomic; whereas chloride derived from lower Jurassic marine sediments (δ37Cl = −2.6 to −0.3‰) is the source for the distal atacamite mineralization. Local meteoric water was disregarded as a chlorine source because of their low Cl concentration and the assumption that atacamite compositions would be more homogeneous given a meteoric source (Arcuri and Brimhall, 2003). In an alternative scenario, regional compression or tectonic activity could have forced deep, halogen-rich solutions upward, where they could mix with groundwater generating the observed mineralization (Arcuri and Brimhall, 2003). Although chlorine isotope data on atacamite are sparse, reported δ37Cl values span a remarkably large range from −3 to +6‰. This large range indicates that supergene oxidation processes can result in a significant fractionation of stable chlorine isotopes. To date, no δ37Cl data are currently available for this mineral within a single Cu deposit or for a supergene oxidation profile that could explain the reported large range of δ37Cl values. Furthermore, the lack of experimental data for chlorine isotope fractionation between atacamite and water impedes a proper evaluation of chlorine fractionation mechanisms. Here, we present a new dataset that integrates δ37Cl data of atacamite with mineralogical observations along a thick (~100 m), well-developed supergene oxidation profile at the Barreal Seco iron oxide-copper-gold (IOCG) deposit in the Atacama Desert of northern Chile. This profile has the advantage of containing abundant atacamite from top to bottom, providing a unique opportunity to investigate δ37Cl variations with depth. In addition, we report the first experimental determinations of chlorine isotope fractionation between atacamite and water. The experimental data coupled with isotope fractionation models are used to provide a feasible explanation for the observed δ37Cl variations at Barreal Seco.

2. Origin of atacamite and saline fluid sources In the Atacama Desert of northern Chile, supergene enrichment of Cu deposits has been strongly coupled to the climatic and tectonic evolution of the region since the early Oligocene (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Mote et al., 2001; Hartley and Chong, 2002; Hartley and Rice, 2005; Vasconcelos et al., 2015; Reich and Bao, 2018). Argon-argon dating of supergene minerals, mainly alunite group minerals, has shown that supergene enrichment occurred over a long period extending from 44 Ma to ~15–13 Ma, followed by the onset of hyper-aridity in the region at ~12–10 Ma (Alpers and Brimhall, 1988; Sillitoe and McKee, 1996; Mote et al., 2001; Bouzari and Clark, 2002; Hartley and Rice, 2005; Arancibia et al., 2006; Reich et al., 2009; Sun et al., 2018; Rech et al., 2019). This resulted in the formation of thick supergene oxidation zones in the upper portions of Cu deposits, as a response to Andean uplift and chemical weathering of hypogene CueFe sulfides (Alpers and Brimhall, 1988). The main stage of supergene oxidation occurred between 21 and 15 Ma and was the result of percolating oxygenated meteoric waters in a semi-arid setting. Leaching in the weathering environment was followed by desiccation, an important factor contributing to the preservation of enriched oxidation zones of ore deposits in the Atacama Desert. In addition, some recent works (Reich et al., 2008, 2009; Palacios et al., 2011), based on U-series geochronology, 36Cl measurements and δ65Cu data, showed that Cu in the oxidation zone was later remobilized by highly saline solutions to form massive amounts of atacamite, even after the onset of hyperaridity at ~12–10 Ma (Sun et al., 2018, Rech et al., 2019). This late Cu oxide enrichment phase was dominated by saline groundwaters and is responsible for the extensive precipitation of atacamite in the Atacama Desert.

3. Geologic background The Barreal Seco iron oxide-copper-gold (IOCG) deposit, formerly 169

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Fig. 1. Left: Shaded relief map and precipitation in the central and southern Andes, showing the extension of the Atacama Desert and the location of the Barreal Seco deposit in northern Chile. Right: Map showing the location of major Mesozoic-age iron oxide-copper gold (IOCG) deposits in the Coastal Range of northern Chile, between Antofagasta and Copiapó. The Trace of the Atacama Fault System (AFS) is shown, as well as main morphotectonic units (1: Coastal Range, 2: Central Depression, 3: Pre-Cordillera, and 4: Western Andean Cordillera). Figures modified from WMO (1975) and Benavides et al. (2008).

solutions with high dissolved silica, either by decreasing pH or increasing the activity of Cu2+ (Newberg, 1967; Shlomovitch et al., 1999; Münchmeyer, 1996; Nelson et al., 2007). On the other hand, atacamite formation requires high chloride activity to form, and near-neutral pH conditions (Hannington, 1993). Therefore, the observed supergene assemblage at Barreal Seco is consistent with observations in other deposits of the Atacama region that point to a sharp increase in chloride concentrations in groundwater during the late supergene stage.

known as Teresa de Colmo, is located 75 km southeast of the town of Taltal in the Atacama Desert of northern Chile (Fig. 1). Total geological resources are estimated at ~70 million tons (Mt) ore at 0.8% Cu, of which 20 million tons are supergene Cu oxide minerals and 50 million tons are sulfides (Hopper and Correa, 2000). The deposit is hosted by andesites and marine sedimentary rocks from the Lower Cretaceous Aeropuerto Formation, which were intruded by dioritic to granodioritic stocks (112 Ma) that form part of the Coastal Batholith. The Barreal Seco IOCG deposit is a multiphase hydrothermal-tectonic breccia body associated with the structurally focused emplacement of a metal and volatile-rich stock during the transition from an extensional to sinistral strike-slip regime related to the Atacama Fault System (Hopper and Correa, 2000). The primary (hypogene) alteration and mineralization is considered synchronous to the intrusion of the diorite stock (ca. 112 Ma). The hypogene copper mineralization at Barreal Seco occurs primarily in the matrix of a sub-vertical hydrothermal breccia system and also as centimeter-scale chalcopyrite veins. Chalcopyrite mineralization is associated with pyrite and abundant specular hematite. Two stages of hypogene mineralization were identified at Barreal Seco (Hopper and Correa, 2000). The first stage is characterized by chalcopyrite that frequently replaces pyrite. This stage was followed by a lower temperature event where specular hematite precipitated along vein margins and around clasts within the breccia bodies. The hypogene zone extends from ~110 m below the surface to at least 500 m depth, and comprises chalcopyrite and pyrite with minor bornite and chalcocite. A thick supergene oxidation profile of about 100 m formed by oxidation of the hypogene ore (Fig. 2). The oxidized zone is composed dominantly of atacamite, malachite, and CueFe oxide minerals with minor chrysocolla and cuprite (Hopper and Correa, 2000). The oxidized zone overlies a flat, sub-horizontal blanket composed of secondary sulfides including chalcocite and covellite (Fig. 2). At Barreal Seco, two supergene events were identified. The first event is represented by oxidation of hypogene sulfides to form chrysocolla as the main Cu oxide mineral phase, followed by a second (and late) supergene stage characterized by the formation of abundant atacamite. Thermodynamic models show that chrysocolla forms from

4. Samples and methods 4.1. Atacamite samples Samples for this study were obtained from a sub-vertical ~120 m drillcore (MLC-18-TW) that cuts the oxidation zone (Fig. 2). A set of 23 representative atacamite-bearing samples were obtained from depths −4.49 m to −89.1 m. Petrographic inspection of the samples was carried out by means of polarizing optical microscopy and scanning electron microscopy (SEM) at the Andean Geothermal Center of Excellence (CEGA), Department of Geology, Universidad de Chile. SEM observations were performed using a FEI Quanta 250 instrument equipped with secondary electron (SE), energy-dispersive X-ray spectrometry (EDS), backscattered electron (BSE) and cathodoluminescence (CL) detectors. Observation was performed using a spot size of 1–3 μm, an accelerating voltage of 10–20 kV, and a working distance of 10 mm. Semi-quantitative EDS analyses were used to constrain major elements in individual mineral phases. The EDS operating conditions were 20 kV, a spot size of 1–3 μm and a working distance of 10–18 mm. The presence of atacamite along the drill core was confirmed using X-ray diffraction (XRD) analyses of powdered samples (Supplementary Fig. S1). The XRD analyses were carried out using a Siemens D-5000 diffractometer in the Physics Department at the Universidad de Chile, Santiago. The untreated powder samples (< 200 mm) were scanned at a rate of 0.6°2θ/min, with a step size of 0.01°, from 0 to 80°2θ, and operating conditions of 40 kV and 30 mA. Accessory minerals were identified using the XPowder12 software. 170

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Fig. 2. SW-NE cross section of the Barreal Seco orebody. The oxide blanket is shown in green and the secondary enrichment sulfide zone in blue (based on Hopper and Correa, 2000). Atacamite-bearing samples used in the study were retrieved from a ~100 m subvertical drill hole that cross-cut the supergene oxide zone developed on top of the deposit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.2. Chlorine stable isotope measurements

synthetic mixture of atacamite and bollatackite as atacamite. The chlorine isotope composition of the synthetic atacamite powder and the excess CuCl2 solution was determined using isotope ratio mass spectrometry at the University of Texas at Austin following the same procedures reported in Section 4.2.

Approximately ~10 to 40 mg of atacamite-bearing samples were coarsely crushed and reacted with 10 mL of 50% nitric acid for ~24 h to dissolve the atacamite. The resulting solution was diluted to 100 mL with ultrapure deionized water. An aliquot of the diluted solution was prepared for chlorine isotope analyses following the methods of Eggenkamp et al. (1994). Chlorine in solution was converted to AgCl via reaction with AgNO3. AgCl was filtered from the sample solution onto a quartz filter, dried overnight, and reacted with excess CH3I under vacuum at 80 °C to produce CH3Cl. The gaseous CH3Cl was purified of excess CH3I on a gas chromatographic column and analyzed in continuous flow mode using a ThermoElectron MAT253 isotope ratio mass spectrometer at the University of Texas in Austin. The long-term reproducibility for δ37Cl values is ± 0.2‰ (1σ), based on the average of three internal seawater standards and an internal rock standard. Samples are reported in delta notation (δ37Cl) relative to Standard Mean Ocean Chloride (SMOC).

5. Results 5.1. Mineral paragenesis Petrographic observations show that the secondary mineral paragenesis at the Barreal Seco deposit is characterized by two consecutive supergene events that remobilized and re-precipitated Cu by oxidation and leaching of the primary (hypogene) assemblage. The first event is represented by chrysocolla as the main supergene Cu mineral phase (Fig. 3A, B), which fills cavities and is texturally associated with Fe oxyhydroxides of variable stoichiometry, goethite and lepidocrocite. Minor pseudomalachite and Mn-rich copper oxides (“copper-pitch”) phases are also present. The second supergene event is characterized by abundant atacamite, which represents the main supergene mineral phase present in the oxidized zone of the deposit (Fig. 3C). Atacamite is late in the paragenesis and often occurs intergrown with gypsum forming mm-to cm-thick veinlets (Fig. 3C, D), and also filling amygdales and forming patina-like corrosion coatings over pre-existing minerals such as chrysocolla, goethite and specular hematite (Fig. 3E–I). Atacamite is found along the whole supergene oxide profile, and no significant variations in textures, modal mineral abundances, or mineral assemblages are observed as a function of depth.

4.3. Chlorine isotope fractionation experiment In order to approximate the chlorine isotope fractionation factor between atacamite and water, atacamite was synthetized following the procedures by Sharkey and Lewin (1971) and Le Roux et al. (2016). Synthetic atacamite crystals were prepared at room temperature (23 °C) using the calcite replacement method, which involved adding 1 g of powdered calcite to 1 L of 0.1 M CuCl2 solution, which was left to stir for 24 h to complete the replacement reaction. The atacamite crystals were washed with 0.5 M Mg(NO3)2 to remove excess Cu2+ and Cl− from the mineral surface. AgNO3 was added to the decanted wash water to check for the presence of Cl− by precipitation of AgCl. The atacamite was rinsed with Mg(NO3)2 until no AgCl precipitated in the wash water. Salts were removed by rinsing with ultrapure deionized water, dried overnight (at 50 °C) and gently crushed into a powder. The atacamite powder was analyzed using a Bruker D8 Advance X-ray diffractometer at the University of Texas at Austin. XRD pattern analysis using the EVA and Topas software and the ICDD PDF-2 Minerals database show the powder to be a roughly 50:50 mixture of atacamite and one of its polymorph bollatackite (Supplementary Fig. S2). For simplicity, throughout the text we will refer to the experimentally produced

5.2. Cl stable isotope composition The chlorine stable isotope data of atacamite samples from the Barreal Seco deposit are shown in Table 1. δ37Cl values of atacamite along the profile range from −0.6‰ to +2.1‰ (mean = +0.4‰ ± 0.7‰; median = +0.3‰). The δ37Cl data show an apparent relationship with depth (Fig. 4). The trend is characterized by higher δ37Cl values towards the top of the profile, with lower values at the bottom. The upper 40 m of the profile are dominated by positive δ37Cl values that range from +0.4‰ to +2.1‰, whereas the lower 60 m are characterized by lower and even negative δ37Cl values (from −0.6‰ to +0.7‰). 171

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Fig. 3. Representative polarized-light micrographs and back-scattered electron (BSE) images of supergene mineral textures from the Barreal Seco IOCG deposit. (A) Reflected-light image showing aggregates of tabular hematite crystals (specularite variety) from the ore breccia. A late gypsum veinlet (grey) is shown on right. (B) BSE image shows chrysocolla filling interstices between tabular hematite crystals. Chrysocollabearing assemblages represent the first supergene oxidation event at Barreal Seco. (C) Transmittedlight image of an atacamite veinlet cross-cutting the host andesite. A detail (red square) of the veinlet is shown in the BSE image in (D). Atacamite is the dominant Cu oxide mineral at Barreal Seco, and represents the second supergene oxidation event. (E) BSE image showing a chrysocolla veinlet partially replaced by atacamite and gypsum. Detail (red square) is shown in (F). (G) Reflected-light image of an amygdale filled with Fe-oxides, chrysocolla and atacamite. Atacamite (green) is late in the paragenesis. (H) BSE image showing the same amygdale. The inset (I) shows atacamite replacing chrysocolla. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Two synthetic atacamite samples duplicated well with δ37Cl values of +0.3‰ and +0.2‰. The excess CuCl2 solution had δ37Cl values of −0.6‰ and −0.4‰. These are splits of the solution from one experiment, not separate synthesis products. Overall, the data are reproducible and indicate that the heavy isotope (37Cl) is preferentially incorporated into the solid phase (atacamite), whereas the light isotope (35Cl) preferentially remains in the solution. The estimated fractionation factor between atacamite and water at room temperature (23 °C) is:

103 ln (atacamite–solution) =

37Cl

=

37Cl

atacamite

37Cl

solution

chlorine in the parental solutions (i.e., basinal brines), the observed > 2.5‰ variation in the δ37Cl value of atacamite with depth at Barreal Seco strongly suggests that a Cl isotope fractionation process was involved. We investigated the shift in atacamite δ37Cl to constrain the process by which atacamite accumulated during supergene oxidation. Below we discuss the two existing models for atacamite formation and how Cl isotopes might have fractionated during the processes described in each model. Finally, we use numerical simulations of these

= +0.25‰

( 0.50‰) = +0.75‰ processes to evaluate which one better explains the observations. Two main models for the enhanced salinity of the groundwaters that form atacamite in Cu deposits from the Atacama Desert have been proposed: (1) Lowering of the water table caused by Andean surface uplift and evaporation of fresh groundwater due the extreme

6. Discussion The δ37Cl data of atacamite at Barreal Seco span a range between −0.6‰ and +2.1‰ (Table 1 and Fig. 4). Although δ37Cl values of atacamite have been previously suggested to reflect the source of the 172

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Table 1 δ37ClSMOC values of atacamite from the MC-18-TW drillcore into the Barreal Seco IOCG deposit. Sample number BS-1 BS-2 BS-3 BS-4 BS-5 BS-6 BS-7 BS-8 BS-9 BS-10 BS-11 BS-12 BS-13 BS-14 BS-15 BS-16 BS-17 BS-18 BS-19 BS-20 BS-21 BS-22 BS-23

Depth (m) 4.5 13.6 13.8 17.1 18.8 27.1 29.1 32.2 34.9 41.9 50.0 54.3 55.2 58.4 61.0 61.4 65.2 70.1 73.3 78.4 86.2 86.8 89.1

Mineral occurrence and assemblage Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite Atacamite

+ chrysocolla (disseminated) + chrysocolla (in veinlets) + chrysocolla (in gypsum veinlets) + chrysocolla (disseminated) + chrysocolla (disseminated) (in gypsum veinlets) (in gypsum veinlets) + chrysocolla (in veinlets within specularite breccia) + chrysocolla (disseminated within specularite) (in gypsum veinlets) veinlet (in veinlets and disseminated) (in gypsum veinlets) (disseminated) (in gypsum veinlets) (in veinlets and disseminated) (in gypsum veinlets) (in gypsum veinlets) (in gypsum veinlet and disseminated) (disseminated within specularite) (in gypsum veinlets) (disseminated) + gypsum + chrysocolla (in veinlets)

δ37ClSMOC (per mil) +2.1 +1.2 +0.4 +1.0 +1.1 +0.9 +1.4 +0.8 +0.6 −0.6, −0.6 +0.7 −0.4 −0.1 −0.2 −0.4, −0.9 +0.4, +0.2 +0.3 −0.6, −0.6 +0.1 −0.2 −0.5 0.0

(*)

(*) Sample did not reproduce (analyzed four times) Replicate analyses of some samples are given. In calculations and figures, average values are used.

suggests that lowering of the water table was relatively continuous in space, although not necessarily in time (Alpers and Brimhall, 1989). More recent studies by Cooper et al. (2016) at Cerro Colorado porphyry Cu, Chile, have quantified this effect by using (UeTh)/He dating of hematite precipitation to track a slow and steady lowering of the water table from ca. 16 Ma to the present day (rate of ~10 m/myr). However, it is not clear how this process, coupled with progressive evaporation due to desiccation, would have impacted the composition of groundwater at the regional scale in Atacama, to form large amounts of atacamite. On the other hand, hydrogeochemical evidence presented by Leybourne and Cameron (2006) and Cameron et al. (2007) showed that the compositional variation of groundwater in Cu deposits from the Atacama region is best explained by fluid mixing with perhaps only a minor role for evaporation. Groundwater resources in the Atacama Desert are frequently stored in aquifers that are part of sedimentary sequences that were deposited during the Neogene and the Quaternary (e.g., alluvial fan deposits) (Magaritz et al., 1990). Hydrogeological evidence from buried Cu deposits in Atacama (e.g. Spence; Leybourne and Cameron, 2006) shows that the water table lies in the gravels overlying the deposits, but locally the water table passes through the deposits. Isotopic, chemical and geological evidence support the hypothesis that a regional primary inflow to the aquifers in the Atacama Desert occurs via groundwater associated with recharge areas located in the higher part of the Andes region (Magaritz et al., 1990; Aravena, 1995; Alvarez et al., 2015). Currently, the water table levels in the Central Depression vary from near-surface in salars and wetlands to 30–90 m (below surface) near the Spence Cu mine (see Alvarez et al., 2015, and references therein). A groundwater/mixing scenario implies a deeper, brine source mixing with the less saline regional groundwater-flow system (Leybourne and Cameron, 2006; Reich et al., 2013; Alvarez et al., 2015, 2016). This is consistent with the 36Cl isotopic data on atacamite assemblages from various Cu deposits in northern Chile showing very low 36 Cl-to-Cl ratios (11 × 10−15 to 28 × 10−15 at·at−1), comparable to previously reported values of deep old groundwaters, i.e., basinal brines (Reich et al., 2008, 2009). Also, the 36Cl value of atacamite is in agreement with 129I data obtained in atacamite-bearing assemblages (Reich et al., 2013; Alvarez et al., 2015), suggesting that focused

Fig. 4. δ37Cl versus depth variation in atacamite from the Barreal Seco IOCG deposit. Delta values are reported in per mil (‰), and sample depth is in meters (m).

desiccation of the Atacama Desert, and (2) Mixing of between fresh groundwater and deep saline waters that were seismically injected upward throughout faults and fractures, a process known as seismic or tectonic pumping (Sibson et al., 1975; Sibson, 2001). Brimhall et al. (1985), Alpers and Brimhall (1988, 1989) and Sillitoe (1990, 2005) have emphasized that efficient supergene Cu enrichment in northern Chile was favored by tectonically induced surface uplift. Continuous or episodic surface uplift is believed to have caused lowering of the water tables at a regional scale, exposing (abruptly or gradually) hypogene sulfide assemblages to oxidative weathering conditions (Sillitoe, 2005). Consistent trends of limonite mineralogy and abundance in vertical profiles at La Escondida porphyry CueMo deposit in northern Chile 173

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

upward migration of saline pore fluids from the Jurassic marine basement may explain this geochemical signature. Considering this evidence, seismic injection of deeper, more saline groundwater along preexisting structures and active faults has been proposed as a viable mechanism to explain the abundance of atacamite in supergene oxidation zones (Cameron et al., 2002, 2008, 2010). This model has also been invoked to explain surface geochemical anomalies for elements that are characteristic of saline, mineralized groundwaters (e.g., Cu, Mo, Re, Se, Te, Cl, I) in soils and gravels above Cu deposits and along structures/faults in the Atacama region (Cameron et al., 2002, 2008, 2010; Leybourne et al., 2013). More recently, Brown et al. (2019a, 2019b) reported similar occurrences of surficial anomalies formed as saline pockets by mineralized groundwater forced to the surface by earthquake-induced pumping through fracture zones at the Atlántida buried porphyry-skarn Cu-Au-(Mo) deposit in the Atacama Region, Chile. These saline pockets were identified by the cited authors at four other sites throughout the Atacama Desert extending as far north as Chuquicamata (Brown et al., 2019a), demonstrating that this process was widespread in the Atacama Desert. In the next two sections, we use the δ37Cl data to test possible scenarios that might explain the observed profile in Fig. 4. In order to evaluate the trend in Cl isotope composition of the atacamite with depth, we modeled the δ37Cl values of atacamite using two different scenarios: (1) uplift and desiccation lower the water table and increases the Cl and Cu concentration in the remaining porewater allowing for the precipitation of atacamite, which removes Cl from solution following a Rayleigh fractionation process (water table lowering model); and (2) upward injection of a brine that overlies fresh groundwater producing a chlorine concentration gradient which in turn induces net downward ionic diffusion (seismic pumping model).

average of numerous experimental and empirical calibrations; see Barnes and Sharp, 2017 and references therein) were used to calculate diffusion coefficients (D) of 35Cl and 37Cl. During each step in our numerical integration, for each of the species 35Cl, 37Cl and Cu, we used the Forward Euler method to calculate changes in concentrations resulting from downward migration of the water table, then used the Reverse Euler method to solve the diffusion equations and finally used the Forward Euler method to calculate change due to atacamite precipitation, including use of our experimentally determined atacamitewater Cl isotope fractionation factor (see Appendix for more details). The resulting trend in atacamite δ37Cl values with depth only qualitatively fits the observations (Fig. 5). The modeled trend has a smaller magnitude decrease with depth than our observations. However, we cannot entirely rule out the effect of water table lowering and resulting atacamite precipitation, but because this model does not explain the full magnitude of the observed δ37Cl shift we explore the seismic pumping model. 6.2. Seismic pumping model: Injection of brine The second possible explanation for atacamite supergene enrichment involves a seismic pumping model in which a pulse of saline fluid is pumped upwards through faults, generating a chlorine concentration gradient in which a saline fluid overlies a more dilute groundwater aquifer (Cameron et al., 2002 and references therein; see also Section 6). In order to test the seismic pumping scenario, a diffusion model was used as a first approximation. In this model, a chlorine concentration gradient was imposed which results in net downward diffusion of chloride and therefore decreasing δ37Cl values with depth. We used the following boundary and initial conditions. The initial conditions were a top fluid layer of brine (36,000 ppm Cl; δ37Cl = 0‰) 10 m thick over 80 m of fresh groundwater (3 ppm Cl; δ37Cl = 0‰) in contact with the ore body at depth. As in the scenario described above, a zero flux was imposed at the top (water table, the depth of the water table in this model was held constant) and bottom boundaries. We used the same diffusion coefficients as in the water table lowering model described above. We solved the diffusion equations under transient state conditions numerically using the Reverse Euler method (see Appendix A2). Atacamite precipitation was not explicitly simulated in this model, rather the δ37Cl values of atacamite were calculated from δ37Cl values of aqueous Cl− at the end of the simulations using the experimentally determined Δ37Clatacamite–Cl of +0.75‰. The simulations were run for 2000 years, 10,000 years, and 20,000 years. After 60,000 years, the magnitude of the down-profile δ37Cl shift is < 0.2‰. Therefore, if the variability of the chlorine isotope composition with depth is due to diffusion, then the atacamite must have formed within 60,000 years of the brine injection event. Fig. 6 shows the results of the numerical simulation. The magnitude change in δ37Cl values of atacamite as a function of depth along the profile can be explained by this model at 20,000 years. This model assumes that atacamite at all depths forms 20,000 years after brine injection. In addition, this model assumes that chlorine is only transported by diffusion (whereas convection resulting from the juxtaposition of dense brine over fresh groundwater would have been likely) and that there is a single pulse of saline fluid injection upwards along faults. Despite its inherent limitations, this model constitutes a first-order approximation to a geological phenomenon which is significantly challenging to simulate numerically (e.g., multiple fluid injections and sequential precipitation of atacamite along the profile, convection, recharge effects, among other factors). Therefore, our model suggests that the injection of saline waters, followed by Cl diffusion, is a feasible mechanism to explain the observed δ37Cl data at Barreal Seco. However, it is important to note that this particular mechanism should have operated on short times scales, i.e., tens of thousands of years. Evidence by Cameron et al. (2002, 2008, 2010), Reich et al. (2008, 2009) and Brown et al. (2019a, 2019b) are consistent with

6.1. Water table lowering model The first model tested here takes into consideration the effect of evaporation of groundwater and resulting lowering of the water table during uplift and extreme desiccation (see Appendix A1). In our numerical representation of this model, desiccation increases the concentrations of chloride and copper in the remaining porewater. In addition, diffusion of chloride and copper is accounted for. When these concentrations are sufficiently high to supersaturate the solution and precipitate atacamite, chloride and copper are removed from the water stoichiometrically according to the following reaction to maintain thermodynamic equilibrium with atacamite:

2Cu2 + + Cl + 3H2 O

Cu2 (OH)3 Cl + 3H+

(1)

Atacamite formation preferentially removes 37Cl from solution (Δ37Clatacamite–Cl = +0.75‰) resulting in progressively lower δ37Cl values of aqueous chloride and subsequently precipitated atacamite as the water table is lowered (Fig. 5). In this model, we used the following parameters and initial conditions. We assumed a starting concentration of Cl and Cu in the porewater of 3 ppm and 10 ppm, respectively, unless otherwise indicated. Zero flux was imposed at the top (water table) and bottom boundaries. We used a solubility product (Ksp) of 107.34 for atacamite (Ball and Nordstrom, 1991). We assumed that the pH is fixed at 6, unless indicated, and that activities equal concentrations. We also assumed that no other minerals precipitate or dissolve, such that the chloride and copper concentrations are controlled solely by evaporation, atacamite precipitation, and diffusion. The water table lowering rate was 10 m/ myr, unless otherwise indicated, based on estimations using (UeTh)/He hematite geochronology by Cooper et al. (2016). The model is not sensitive to the water table lowering rate; nearly identical δ37Cl trends with depth resulted when the water table was lowered at rates between 1 m/myr and 100 m/myr (Fig. 5A). A Cl diffusion coefficient (for Cl diffusion through water) of 2.03 × 10−9 m2/s (PHREEQC database) and a 35Cl/37Cl ratio of diffusion coefficients of 1.0015 (based on the 174

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Fig. 5. Outputs from the water table lowering model. A) δ37Cl values of atacamite versus depth for different rates of water table lowering: 1 m/ 106 yr (black dotted line), 10 m/106 yr (blue dashed-dotted line), and 100 m/106 yr (orange dashed line). The solid green line shows a scenario with no diffusion. Calculations used 3 ppm Cl and 10 ppm Cu as initial groundwater concentrations and a pH of 6. B) δ37Cl values of atacamite versus depth for different initial Cu concentrations in groundwater: 5 ppm (orange dashed line), 10 ppm (blue dashed-dotted line), and 15 ppm (black dotted line). All calculations used 3 ppm Cl, a water table lowering rate of 10 m/106 yr, and pH = 6. C) δ37Cl values of atacamite versus depth for different pH conditions: pH = 6 (blue dasheddotted line), pH = 7 (orange dashed line), and pH = 8 (black dotted line). Calculations used 3 ppm Cl and 10 ppm Cu as initial groundwater concentrations and a water table lowering rate of 10 m/106 yr. The water table was lowered to 100 m and zero meters of erosion are imposed (i.e. atacamite samples are at their original depth below surface) in all three scenarios shown in panels A, B, and C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

this timeframe. If atacamite at Barreal Seco formed by seismic pumping of saline groundwater to surface (e.g., pore waters from marine basement units) and was later preserved, the implication is that this mineral was most likely formed after the onset of hyperaridity in Atacama. Hence, atacamite is most likely younger than the other supergene Cu minerals in the oxide zone at Barreal Seco, consistent with the petrographic observations where atacamite is late in the Cu oxide paragenesis. Finally, this result is in good agreement with 36Cl and U-series disequilibrium series dating of atacamite-gypsum assemblages in Cu deposits from Atacama, which show formation of < 250 ka (e.g., 240 ka at Chuquicamata, 130 ka at Mantos Blancos and Spence, and 80 ka at Mantos de la Luna and Michilla) (Reich et al., 2008, 2009) that are difficult to explain by the water table lowering model. 7. Conclusions We report stable Cl isotope data for atacamite from a well-developed ~100 m deep oxidation profile at the Barreal Seco IOCG deposit in northern Chile, and combine these results with Cl isotope fractionation experiments to elucidate the processes resulting in atacamite formation. The δ37Cl data span a range between −0.6‰ and +2.1‰, and even though the data may reflect the source of the chlorine in the parental solutions (i.e., basinal brines), the reported > 2.5‰ change in the δ37Cl value of atacamite with depth suggests that a Cl isotope fractionating process was involved. Experimental data show that atacamite formation preferentially removes 37Cl from solution (Δ37Clatacamite–Cl = +0.75‰). We used these data to test possible scenarios that might explain the observed profile. Numerical models indicate that mixing between fresh groundwater and deep saline waters that were seismically injected throughout faults and fractures (seismic or tectonic pumping) is a feasible mechanism to explain the observed δ37Cl data at Barreal Seco. However, it is important to note that the effect of water table lowering in atacamite precipitation cannot be ruled out, even though the numerical representation of this model only qualitatively fits the observations. Collectively, our data provide a framework to interpret the δ37Cl profiles of atacamite in Cu deposits, present new constraints on atacamite formation, and help explore the potential use of stable chlorine isotopes to monitor Cu weathering and enrichment in supergene environments. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.chemgeo.2019.07.023.

Fig. 6. Outputs from the seismic pumping (brine injection) model, in which a 10 m thick layer of brine (36,000 ppm Cl; δ37Cl = 0‰) overlays a 80 m thick layer of fresh groundwater (3 ppm Cl; δ37Cl = 0‰) and the Cl is allowed to diffuse. A) Cl concentration (ppm) versus depth after Cl was allowed to diffuse for 2000 years (orange dashed line), 10,000 years (blue dashed and dotted line), and 20,000 years (black dotted line). A) δ37Cl values of atacamite in equilibrium with aqueous chloride after Cl− was allowed to diffuse for 2000 years (orange dashed line), 10,000 years (blue dashed and dotted line), and 20,000 years (black dotted line). Open black circles are the measured δ37Cl values of natural atacamite from the Barreal Seco IOCG deposit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 175

Chemical Geology 525 (2019) 168–176

M. Reich, et al.

Acknowledgements

possible tool for exploration. J. Geochem. Explor. 55, 249–255. Eggenkamp, H.G.M., Middelburg, J.J., Kreulen, R., 1994. Preferential diffusion of 35Cl relative to 37Cl in sediments of Kau Bay, Halmahera, Indonesia. Chem. Geol. 116, 317–325. Hannington, M.D., 1993. The formation of atacamite during weathering of sulfides on the modern seafloor. Can. Mineral. 31, 945–956. Hartley, A.J., Chong, G., 2002. Late Pliocene age for the Atacama Desert: implications for the desertification of western South America. Geology 30, 43–46. Hartley, A.J., Rice, C.M., 2005. Controls on supergene enrichment of porphyry copper deposits in the Central Andes: a review and discussion. Mineral. Deposita 40, 515–525. Hopper, D., Correa, A., 2000. The Panulcillo and Teresa de Colmo copper deposits: two contrasting examples of Fe-Ox Cu-Au mineralization from the Coastal Cordillera of Chile. In: Porter, T.M. (Ed.), Hydrothermal Iron Oxide-copper-gold and Related Deposits: A Global Perspective. 1. PGC Publishing, Adelaide, pp. 177–189. Kaufmann, R., Long, A., Bentley, H., Davis, S., 1984. Natural chlorine isotope variations. Nature 309, 338–340. Le Roux, S.G., Miller, J.A., Dunford, A.J., Clarke, C.E., 2016. The dissolution kinetics of atacamite in the acid range and the stability of atacamite containing soils from Namaqualand, South Africa. Appl. Geochem. 64, 22–29. Leybourne, M.I., Cameron, E.M., 2006. Composition of groundwaters associated with porphyry-Cu deposits, Atacama Desert, Chile: elemental and isotopic constraints on water sources and water– rock reactions. Geochim. Cosmochim. Acta 70, 1616–1635. Leybourne, M.I., Cameron, E.M., Reich, M., Palacios, C., Faure, K., Johannesson, K.H., 2013. Stable isotopic composition of soil calcite (O, C) and gypsum (S) overlying Cu deposits in the Atacama Desert, Chile: implications for mineral exploration, salt sources, and paleoenvironmental reconstruction. Appl. Geochem. 29, 55–72. Magaritz, M., Aravena, R., Peña, H., Suzuki, O., Grilli, A., 1990. Source of ground water in the deserts of northern Chile: evidence of deep circulation of ground water from the Andes. Ground Water 28, 513–517. Maksaev, V., Townley, B., Palacios, C., Camus, F., 2007. Metallic ore deposits. In: Moreno, T., Gibbons, W. (Eds.), The Geology of Chile. The Geological Society, London, pp. 179–199. Mote, T.I., Brimhall, G.H., Tidy-Finch, E., Muller, G., Carrasco, P., 2001. Application of mass-balance modeling of sources, pathways, and sinks of supergene enrichment to exploration and discovery of the Quebrada Turquesa exotic copper orebody, El Salvador district, Chile. Econ. Geol. 96, 367–386. Münchmeyer, C., 1996. Exotic deposits; products of lateral migration of supergene solutions from porphyry copper deposits. Soc. Econ. Geol. Sp. Pub. 5, 43–58. Nelson, M., Kyser, K., Clark, A., Oates, C., 2007. Carbon isotope evidence for microbial involvement in exotic copper silicate mineralization, Huinquintipa and Mina Sur, Northern Chile. Econ. Geol. 102, 1311–1320. Newberg, D.W., 1967. Geochemical implications of chrysocolla-bearing alluvial gravels. Econ. Geol. 62, 932–956. Palacios, C., Rouxel, O., Reich, M., Cameron, E.M., Leybourne, M.I., 2011. Pleistocene rewor-king of copper at the Spence porphyry copper deposit, Atacama Desert, Chile. Mineral. Deposita 46, 1–7. Rech, J., Currie, B.S., Jordan, T.E., Riquelme, R., Lehmann, S.B., Kirk-Lawlor, N.E., Li, S., Gooley, J., 2019. Massive Middle Miocene gypsic paleosols in the Atacama Desert and the formation of the Central Andean rainshadow. Earth Planet. Sci. Lett. 506, 184–194. Reich, M., 2018. Supergene. In: White, W.M. (Ed.), Encyclopedia of Geochemistry. Springer International Publishing AG. https://doi.org/10.1007/978-3-319-39312-4. Reich, M., Bao, H., 2018. Nitrate deposits of Atacama: a marker of hyperaridity. Elements 14, 251–256. Reich, M., Vasconcelos, P., 2015. Geological and economic significance of supergene metal deposits. Elements 11, 305–310. Reich, M., Palacios, C., Parada, M.A., Fehn, U., Cameron, E.M., Leybourne, M.I., Zúñiga, A., 2008. Atacamite formation by deep saline waters in copper deposits from the Atacama Desert, Chile: evidence from fluid inclusions, groundwater geochemistry, TEM, and 36Cl data. Mineral. Deposita 43, 663–675. Reich, M., Palacios, C., Vargas, G., Luo, S., Cameron, E.M., Leybourne, M.I., Parada, M.A., Zuniga, A., You, C.F., 2009. Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert, Chile. Mineral. Deposita 44, 497–504. Reich, M., Snyder, G.T., Alvarez, F., Perez, A., Palacios, C., Vargas, G., Cameron, E.M., Muramatsu, Y., Fehn, U., 2013. Using iodine isotopes to constrain supergene fluid sources in arid regions: Insights from the Chuquicamata oxide blanket. Econ. Geol. 108, 163–171. Sharkey, J.B., Lewin, S.Z., 1971. Conditions governing the formation of atacamite and paratacamite. Am. Mineral. 56, 179–191. Shlomovitch, N., Bar-Matthews, M., Matthews, A., 1999. Sedimentary and epigenetic copper mineral assemblages in the Cambrian Timna Formation, southern Israel. Isr. J. Earth Sci. 48, 195–208. Sibson, R.H., 2001. Seismogenic framework for hydrothermal transport and ore deposition. Rev. Econ. Geol. 14, 25–50. Sibson, R.H., Moore, J.M., Rankin, A.H., 1975. Seismic pumping—a hydrothermal fluid transport mechanism. J. Geol. Soc. 131, 653.659. Sillitoe, R.H., 1990. Copper deposits and Andean evolution. Circum-Pacific Council for Energy and Mineral Resources. Earth Sci. Ser. 11, 285–311. Sillitoe, R.H., 2005. Supergene oxidized and enriched porphyry copper and related deposits. Econ. Geol. 100th Anniv, 723–768. Sillitoe, R.H., McKee, E.H., 1996. Age of supergene oxidation and enrichment in the Chilean porphyry copper province. Econ. Geol. 91, 164–179. Sun, T., Bao, H., Reich, M., Hemming, S.R., 2018. More than ten million years of hyperaridity recorded in the Atacama Gravels. Geochim. Cosmochim. Acta 227, 123–132. Vasconcelos, P., Reich, M., Shuster, D., 2015. The paleoclimatic signatures of supergene metal deposits. Elements 11, 317–322. WMO/Unesco, 1975. Climatic Atlas of South America: Maps of Mean Temperature and Precipitation. vol. I.

Support was provided by Iniciativa Científica Milenio (ICM) grant “Millennium Nucleus for Metal Tracing along Subduction”, and CONICYT funding through FONDECYT grant 1140780 and FONDAP project “Andean Geothermal Center of Excellence” (CEGA 15090013). Access to drill cores from the Barreal Seco deposit and logistical support was kindly provided by Compañía Minera Las Cenizas. J. Cullen and J. Maner are thanked for help with atacamite synthesis reactions and XRD analyses. We also acknowledge editor Karen Johannesson for handling the manuscript, and Hans Eggenkamp and one anonymous reviewer for their helpful comments. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. This paper is LLNL contribution LLNLJRNL-773155. References Ague, J.J., Brimhall, G., 1989. Geochemical modeling of steady state fluid flow and chemical reaction during supergene enrichment of porphyry copper deposits. Econ. Geol. 84, 506–526. Alpers, C.N., Brimhall, G.H., 1988. Middle Miocene climatic change in the Atacama Desert, northern Chile: evidence from supergene mineralization at La Escondida. Geol. Soc. Am. Bull. 100, 1640–1656. Alpers, C.N., Brimhall, G.H., 1989. Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile. Econ. Geol. 84, 229–255. Alvarez, F., Reich, M., Perez-Fodich, A., Snyder, G.T., Muramatsu, Y., Vargas, G., Fehn, U., 2015. Sources, sinks and long-term cycling of iodine in the hyperarid Atacama continental margin. Geochim. Cosmochim. Acta 161, 50–70. Alvarez, F., Reich, M., Snyder, G.T., Perez-Fodich, A., Muramatsu, Y., Daniele, L., Fehn, U., 2016. Iodine budget in surface waters from Atacama: natural and anthropogenic iodine sources revealed by halogen geochemistry and iodine-129 isotopes. Appl. Geochem. 68, 53–63. Arancibia, G., Matthews, S.J., De Arce, C.P., 2006. K-Ar and Ar-40/Ar-39 geochronology of supergene processes in the Atacama Desert, Northern Chile: tectonic and climatic relations. J. Geol. Soc. Lond. 163, 107–118. Aravena, R., 1995. Isotope hydrology and geochemistry of northern Chile groundwaters. Bull. Inst. Fr. Etudes Andin. 24, 495–503. Arcuri, T., Brimhall, G.H., 2003. The chloride source for atacamite mineralization at the Radomiro Tomic porphyry copper deposit, Northern Chile. Econ. Geol. 98, 1667–1681. Ball, J.W., Nordstrom, D.K., 1991. User’s Manual for WATERQ4F With Revised Thermodynamic Data Base and Test Cases for Calculating Speciation of Minor, Trace and Redox Elements in Natural Waters. U.S. Geological Survey Open-file Report. pp. 91–183. Barnes, J.D., Sharp, Z.D., 2017. Chlorine isotope geochemistry. In: Tang, F.Z., Watkins, J., Dauphas, N. (Eds.), Measurements, Theories and Applications of Non-traditional Stable Isotopes. Rev. Mineral. Geochem. 82. pp. 345–378. Benavides, J., Kyser, T.K., Clark, A.H., Stanley, C., Oates, C., 2008. Exploration guidelines for copper-rich iron oxide–copper–gold deposits in the Mantoverde area, northern Chile: the integration of host-rock molar element ratios and oxygen isotope compositions. Geochem. Explor. Environ. Anal. 8, 343–367. Bouzari, F., Clark, A.H., 2002. Anatomy, evolution, and metallogenic significance of the supergene orebody of the Cerro Colorado porphyry copper deposit, I Region, northern Chile. Econ. Geol. 97, 1701–1740. Brimhall, G.H., Alpers, C.N., Cunningham, A.B., 1985. Analysis of supergene oreforming processes and ground-water solute transport using mass balance. Econ. Geol. 80, 1227–1256. Brown, A.E., Winterburn, P.A., Bissig, T., 2019a. Geochemical signature of earthquakeinduced surface flooding by mineralized groundwater over the buried Atlántida Deposit, northern Chile. Geochem. Explor. Environ. Anal. https://doi.org/10.1144/ geochem2018-065. Brown, A.E., Winterburn, P.A., Bissig, T., 2019b. Identification of the expression of earthquake-induced surface flooding by groundwater using detailed regolith mapping at the buried Atlántida Deposit, northern Chile. Geochem. Explor. Environ. Anal. https://doi.org/10.1144/geochem2018-064. Cameron, E.M., Leybourne, M.I., Kelley, D.L., 2002. Exploring for deeply- covered mineral deposits: formation of geochemical anomalies in northern Chile by earthquakeinduced surface flooding of mineralized groundwaters. Geology 30, 1007–1010. Cameron, E.M., Leybourne, M.I., Palacios, C., 2007. Atacamite in the oxide zone of copper deposits in northern Chile: involvement of deep formation waters? Mineral. Deposita 42, 205–218. Cameron, E.M., Leybourne, M.I., Palacios, C., Reich, M., 2008. Geochemical exploration and metallogenic studies, Northern Chile. Geosci. Can. 35, 97–107. Cameron, E.M., Leybourne, M.I., Reich, M., Palacios, C., 2010. Geochemical anomalies in northern Chile as a surface expression of the extended supergene metallogenesis of buried copper deposits. Geochem. Explor. Environ. Anal. 10, 1–14. Cooper, F.J., Adams, B.A., Blundy, J.D., Farley, K.A., McKeon, R.E., Ruggiero, A., 2016. Aridity-induced Miocene canyon incision in the Central Andes. Geology 44, 675–678. Eggenkamp, H.G.M., Schuiling, R.D., 1995. δ37C1 variations in selected minerals: a

176