arcuri and brimhall-2003

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IntroductionTHE RADOMIRO TOMIC porphyry copper deposit is located inthe Atacama Desert of northern Chile approximately 150 kmnortheast of Antofagasta (Fig. 1). The deposit lies 10 kmnorth of the world-famous Chuquicamata porphyry copperdeposit at an elevation of 2,900 m above sea level. TheRadomiro Tomic orebody consists of copper chloride miner-alization distributed in two main, subhorizontal oxide zonesoverlying a chalcocite enrichment blanket (Fig. 2). A detailedproduction history of the mine and the geology are presentedin the recent work by Cuadra and Rojas (2001). Brimhall andTidy (2001) describe the heap-leaching copper extractionmethods used in relationship to the nature of the atacamitemineralization. In these works, detailed descriptions of theoxide zone distribution and mineralization are presented. Thetwo oxide zones are differentiated by the abundance of theprimary ore mineral atacamite, with 40 percent of the totalcopper content in the upper zone and 70 percent in the lowerzone contained in atacamite. The remainder of the copper iscontained in various proportions of copper-bearing smectites,chrysocolla, malachite, and copper wad.

A detailed study of oxide zone copper mineralization wasrecently summarized by Chvez (2000) who found copper

oxide development to be a function of multiple source- andhost-rock parameters, including host-rock mineralogy, pyrite,and copper sulfide abundance and distribution, fracture den-sity and distribution, and the occurrence and stability of the

The Chloride Source for Atacamite Mineralization at the Radomiro Tomic Porphyry Copper Deposit, Northern Chile

TERRY ARCURI

Shell Exploration and Production Company, One Shell Square, 701 Poydras Street, New Orleans, Louisiana 70139-6001

AND GEORGE BRIMHALLDepartment of Earth and Planetary Science, University of California, Berkeley, California 94720-4767

AbstractSupergene mineralization at the Radomiro Tomic porphyry copper deposit involves the copper chloride at-

acamite, Cu4Cl2(OH)6, as the principal ore mineral, distributed in a thick oxide zone. Chloride-rich aqueousfluids were introduced during supergene mineralization causing this atypical variation in copper oxide miner-alogy. The source of this chloride has been investigated using several geochemical techniques, including chlo-rine isotopes and bromide-chloride geochemistry. These methods have proven to be the most useful in evalu-ating the potential chloride sources and their likely influence on copper mineralization. Unmineralized,biotite-bearing, whole-rock protore samples from the Radomiro Tomic deposit have typical igneous values of

vadose zone. In addition, it was noted that the introduction ofchloride and the subsequent increase in salinity of mineraliz-ing fluids could fix copper in the oxide zone by precipitatingcopper chloride minerals such as atacamite.

The oxide zone mineralization at Radomiro Tomic is be-lieved to be the product of supergene enrichment processes(Cuadra and Rojas, 2001). Evidence of downward cumulativeenrichment is seen in the development of a weak enrichmentblanket of secondary copper sulfides below the oxide zone(Fig. 2). Previously formed chalcocite enrichment blanketswere exposed to solutions with high chloride contents duringmultiple episodes of ground-water fluctuation (Arcuri andBrimhall, 1998). Mineralogical observations from drill coresamples suggest that high chlorinity, coupled with the lowtotal sulfide content of the protore, allowed for the precipita-tion of the copper chloride mineral atacamite, Cu4Cl2(OH)6,in the oxide zone rather than the downward migration of cop-per ions to the enrichment blanket (Cuadra and Rojas, 2001).

The major objective of this study was to determine the ori-gin of the chloride involved in the atacamite mineralization atthe Radomiro Tomic deposit. To address this question it wasfirst necessary to determine the possible in situ versus exoticorigins for the mineralizing chloride (e.g., the retention of aninitial magmatic component versus a positive flux of chlorideinto the system). The potential pathways and timing of chlo-ride introduction from possible exotic sources were alsoinvestigated. Understanding the origin of the chloride in

atacamite mineralization offers insight into the local and re-gional geologic processes as well as the climatic conditionsthat existed during the supergene enrichment of theRadomiro Tomic deposit.

Regional geology

The major lithologies of the region are shown in Figure 3,which is based on the mapping of Pardo and Chong (1993;Codelco exploration internal report). The regional geologyconsists of Lower Jurassic through middle Cretaceous marineand nonmarine sedimentary rocks deposited in a Permian-Triassic granitic basement. The sedimentary rocks strikeroughly north-south and are typically west dipping due to re-gional compression and east-west shortening associated withthe Andean uplift (Scheuber et al., 1994). These lithologieswere later intruded by a Cretaceous granodiorite and ulti-mately by a series of Tertiary granitic intrusions. Tertiary vol-canic sediments are present throughout the region and are as-sociated with the emplacement of contemporaneous plutonicbodies. The intrusions are focused at the intersection of twomajor structural features, the West fissure and the northwest-trending Calama lineament (Richards, 2000). The final mag-matic intrusion in this series was the syntectonic emplace-ment of the Chuquicamata porphyry between 36 and 31 Ma(Boric et al., 1990; Ossandn et al., 2001). An overview of theregional geologic evolution has recently been completed inanimation form by Arcuri and Brimhall (2002).

Mote et al. (2001) obtained a K-Ar radiometric age of 17.03 0.03 Ma on copper wad associated with supergene copperoxide mineralization at Chuquicamata. This date is consistentwith the regional supergene ages of 15 to 19 Ma of Sillitoeand McKee (1996). The Radomiro Tomic deposit is locatedon the northern extension of the Chuquicamata porphyry andis believed to have experienced supergene mineralizationcontemporaneous with that of the Chuquicamata orebody.

Sampling and methodology

The distribution of sedimentary rock, mineralized samples,and water samples collected for this study is shown in Figures3 and 4 in relationship to active mines and cities in theCalama region. Sample numbers listed in Figure 4 are pre-sented in Table 1 along with all geochemical and isotopicdata. Samples of sedimentary rocks were selected in order tocharacterize the complete Jurassic stratigraphic section in theCalama region. Both the Upper and Lower Jurassic se-quences were sampled, with most samples collected wellaway from any later intrusions to limit the possibility of post-depositional metamorphic alteration. Lower Jurassic sedi-ments were sampled in an erosional channel named Que-brada Chug chug in the western part of the study area andthe valley of the Rio San Salvador west of Calama. UpperJurassic sedimentary rocks were obtained from eastern Que-brada Chug chug as well as from the anticline northwest ofRadomiro Tomic. Sedimentary rocks collected from the areadefined as Anticline represent the basin margin as well asthe uppermost sedimentary rocks of the Jurassic and are con-formably overlain by Cretaceous volcanic sediments.

Samples of atacamite mineralization were mainly collectedfrom the Chuquicamata porphyry (including the Chuquica-mata and Radomiro Tomic deposits) and the Fortuna Igneous

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FIG. 2. Schematic cross section of the Radomiro Tomic deposit showingthe various mineralogical zones. A distinction is made between the upper andthe lower oxide zone on the basis of the percentage of total copper containedin atacamite. Upper and lower oxide zones have 40 and 70 percent of thetotal copper content present in atacamite, respectively.

Complex. Gravel-hosted samples from Mina Sur and distalsamples from Cerro Quetena were the only mineralizationsamples from other igneous bodies. In the process of fieldmapping, numerous small workings, many with substantialquantities of copper chloride mineralization, were encoun-tered and sampled. These copper chloride samples have a dis-tribution that forms a semicircle around the Chuquicamatadeposit at distances up to 25 km (Fig. 4).

Water samples were collected from all flowing rivers in theregion as well as any large standing bodies of water. Two

samples were collected from active seeps flowing into themine at Radomiro Tomic. A total of eight water samples wascollected and sent to ALS-Chemex commercial laboratories,Reno, Nevada, for chemical and isotopic analyses.

Geochemical analysis of chloride and bromide in all sedi-ment and water samples, as well as oxygen and deuterium iso-tope analyses of water samples, were preformed by ALS-Chemex laboratories with results presented in Table 1.Analysis of chloride and bromide concentrations in atacamitewas preformed on a Cameca 50, electron microprobe at the

ATACAMITE MINERALIZATION, RADOMIRO TOMIC, NORTHERN CHILE 1669

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Chuquicamata Porphyry 35-32 Ma

Fortuna Complex 40-36 Ma

Los Picos Diorite 45-43 Ma

Monte Christo Granodiorite 69-50 Ma

x xx xx Cretaceous Elena

Granodiorite 146-125 Ma

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FIG. 3. Simplified geologic map of basement lithology in the Calama region with sample locations from Figure 4 super-imposed. Figure is based on regional exploration mapping of Pardo and Chong (1993) presented in Codelco Exploration in-ternal report API-9701-3. Cross-section line shows the location of Figure 8, with the arrow indicating the westward projec-tion of this section toward the ocean. Coordinates are in UTM.

University of California, Berkeley. When bromide concentra-tions were below the detection limit of ~100 ppm, sampleswere sent to ALS-Chemex laboratories. Bromide in ata-camite, whole-rock, and sediment samples was analyzed byneutron activation analysis (NAA) with a detection limit of 0.5ppm and a precision of 0.5 ppm. Detection limits for chlo-ride by NAA in sediment and whole-rock samples were 0.01wt percent with a precision of 100 ppm. Water samples wereanalyzed using ion chromatography with detection limits andprecisions of 0.05 0.05 ppm for bromide and 0.1 0.01 ppmfor chloride, respectively.

Chlorine isotope analysis of all sediment and mineralizationsamples was preformed at the University of Arizona with thehelp of Chris Eastoe, following the method described in Eas-toe et al. (1989). In this method, chlorine was extracted fromsediment samples and precipitated as AgCl. Sulfate was re-moved when necessary from all sediment samples. Atacamitesamples were reacted with concentrated sodium hydroxide,extracting the chloride to solution and yielding a black residue.Biotite analysis was performed by leaching the chloride from2 to 3 g of pure biotite with 1-m nitric acid. Chloride-richsolutions were buffered to a pH

the Atacama Desert is a testament to the hyperaridity, whichhas been present since the middle Miocene (Alpers andBrimhall, 1988). Prior to the onset of aridity approximately 15m.y. ago, the region experienced greater precipitation.Ground water derived from this precipitation would havegeochemical and isotopic characteristics which reflect anocean source and concentrations which depend on the dis-tance inland (Duce et al., 1965; Mussi et al., 1998). Chloridein atacamite at Radomiro Tomic might also have been derivedfrom initial magmatic chloride contained in biotite or otherhydrous phases of the porphyritic intrusion. Another possibil-ity is the addition of chloride from a later magmatic event.

Quantity of ChlorideTo investigate the chloride source for the atacamite miner-

alization at Radomiro Tomic, it was first necessary to calculatethe quantity of chloride involved in the total mineralizationand determine if any had been introduced from an exoticsource. To this end, a copper mass-balance calculation wasperformed on assay data for the Radomiro Tomic depositbased on the method developed by Brimhall et al. (1985).Assay data from over 120,000 m of drill core were used in thiscalculation. An assumption of zero lateral copper flux, indica-tive of a closed system during supergene enrichment, wasused for the calculation. Results indicate that approximately197 m has been lost to erosion over the 1- 5-km area of themodeled deposit. The inclusion of this calculated eroded pro-tore material gives a total initial protore volume of 3.98 km3(1 km wide, 5 km long, and 800 m thick.) The quantity ofchloride present in the preoxidation protore at RadomiroTomic prior to erosion and supergene mineralization was es-timated to be approximately 1.32 million metric tons (Mt),based on a protore bulk-rock density of 2,560 kg/m3 and anaverage concentration of 130 ppm chloride (see Table 1).

Current chloride distributions through the Radomiro Tomicdeposit are reflected in the abundance of atacamite in the var-ious mineralogical zones (Fig. 2). Therefore, it is possible toestimate the current chloride budget by using the mineralogi-cal zones together with copper assay data. Cuadra and Rojas(2000) reported that the upper and lower oxide zones (Fig. 2)have 40.2 and 70.3 percent of their total copper contained inatacamite mineralization, respectively, with stoichiometric at-acamite containing 16.6 wt percent chloride. The mixed oxide-sulfide zone is assumed to have approximately 50 percent ofthe total copper contained in atacamite, whereas the mineral-ized gravel and exotic gravel have 40 and 25 percent, respec-tively. From this data it can be calculated that approximately1.3 Mt of chloride has been added to these zones.

The preserved leached zone rock initially had protore con-centrations of chloride, potentially yielding 0.12 Mt of chlo-ride for downward cumulative enrichment into oxide zonemineralization. This quantity was determined assuming com-plete chloride extraction from the 9.4 108 t of existing leachzone material. Similarly, assuming that the 197 m of erodedrock was completely leached prior to its erosion, 0.33 Mt ofchloride would have been available from this rock. Combin-ing the potential sources and sinks, and comparing this to thecurrent chloride distribution, at least 0.9 Mt of chloride hasbeen added to the Radomiro Tomic deposit and fixed in oxidezone atacamite mineralization. This chloride quantity is a

minimum estimation due to the probability of incompletechloride leaching of the leached zone and eroded protore ma-terial. The calculation indicates that a positive flux of chlorideinto the deposit occurred, generating the observed chloridedistribution and requiring an exotic origin for the chloride.

Geochemical TracersElements such as bromide, lithium, and boron have been

used widely to investigate the origin and evolution of evapor-ites and salt deposits (Herrmann et al., 1973; Vila, 1975; Her-rmann, 1980; Wilgus and Holser, 1984; Fisher and Hovorka,1987; Raup and Bodine, 1991; Risacher and Fritz, 1991). Thehalogen-rich nature of the mineralization at Radomiro Tomicallows for the wealth of knowledge developed in the study ofevaporites to be utilized. In addition, the recent advances instable isotope analysis have allowed chlorine sources to betraced through the isotopic composition of the chlorine.Chlorine isotope ratios, 37Cl and 35Cl, have previously beenused to study brines (Kaufmann et al., 1987; Liu et al., 1997;Stiller et al., 1998; Eastoe et al., 2000), evaporite deposits(Eggenkamp et al., 1995; Eastoe and Peryt, 1999), ground-water flow (Desaulniers et al., 1986), sediments (Eggenkampet al., 1994), magmatic systems (Banks et al., 2000), and oxidemineralization (Eastoe et al., 1989; Eggenkamp and Schuil-ing, 1995). Eggenkamp and Schuiling (1995) reported thefirst 37Cl value for atacamite at 5.96 0.17 per mil fromRavensthorpe, Australia. This study represents a detailed ap-plication of the chlorine isotope system to the ore-formingprocess in the supergene environment.

Bromide and chloride geochemistry

Chloride and bromide geochemistry have proved to be use-ful tracers in delineating various natural reservoirs (Wede-pohl, 1974; Gleeson et al., 2001). The major natural reservoirsof chloride and bromide, such as igneous and sedimentaryrocks, seawater, river and rain water, and evaporative brines(Fig. 5A), have characteristic concentration ranges that canbe used to identify chloride and bromide sources.

Bromide substitutes for chloride in halite, and bromide parti-tion coefficients (D) between evaporating seawater and precip-itating halite have been determined experimentally to be about0.14 at 25C and about 0.12 at 40C (Herrmann and Knake,1973), where d = BrH/BrB (d = distribution coefficient, Br =bromide concentration, H = halite, and B = brine). Althoughthe distribution coefficient between atacamite and brine has notbeen determined, the low temperature of the atacamite precip-itation implies a reasonably consistent distribution coefficientfor all samples. Sedimentary rock samples contained measur-able quantities of halite and therefore bromide distributionshould mimic the behavior during halite-brine partitioning.

Evaporating seawater becomes increasingly concentratedin both bromide and chloride until the point of halite satura-tion, at which chloride concentrations drop dramatically withrespect to bromide. The composition of the first halite pre-cipitated from evaporating seawater is shown in Figure 5A asa small open square ([Br] = 65 ppm; Valyashko, 1956). Ashalite continues to precipitate bromide is concentrated in theresidual liquid causing increased concentrations in laterhalite, regularly reaching values over 120 ppm (Hermann etal., 1973; Hermann, 1980).


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TABLE 1. Geochemical and Isotopic Data for Radomiro Tomic and the Surrounding Region

Atacamite mineralizationSample Collection no. Location Br (ppm) Cl1 (ppm) 37Cl (SMOC)

1 14 Radomiro Tomic porphyry 25 160,000 0.12 1002 Radomiro Tomic porphyry 23 160,000 0.04 1501 Radomiro Tomic mineralized gravel 63 160,000 0.65 1503 Radomiro Tomic mineralized gravel 180 160,000 0.66 2030 Mina Quetena 45 160,000 0.88 2101 Mina San Manuel 100 160,000 1.011 2127 Mina La fiesta 20 160,000 1.112 2137 A-16-a 50 160,000 3.113 2151 Mina Irma 48 160,000 1.714 2180 A-19 Bonus 26 160,000 3.215 2191 Mina La Hermosa 34 160,000 1.616 2209 Mina Tres Amigos 35 160,000 0.216 2209 Mina Tres Amigos 35 160,000 0.517 2228 Mina Blanca Julia 37 160,000 1.418 2244 Mina Cissua 32 160,000 0.319 2266 Chris 2 31 160,000 1.319 2266 Chris 2 31 160,000 1.220 2272 N 27 160,000 1.221 2294 TA-17-Aug 62 160,000 0.922 2308 Mina El Nino 83 160,000 1.823 2321 A-20-New 39 160,000 1.624 2334 New #2 28 160,000 2.225 2346 RR 32 160,000 0.525 2360 RR 32 160,000 0.426 2370 Mina Sur 52 160,000 0.127 2370 Mina Sur 48 160,000 0.029 2390 Chuquicamata paleocanal 51 160,000 0.230 2410 Chuquicamata vein 75 160,000 0.130 2410 Chuquicamata vein 75 160,000 0.132 2450 Mantos Blancos 60 160,000 1.7

Radomiro Tomic porphyry samplesR1 62 Radomiro Tomic, biotite from porphyry 2 400 0.5R2 2066 Radomiro Tomic, biotite from porphyry 2 400 2.5R3 487 Radomiro Tomic porphyry 0.2 120 NAR4 3 Radomiro Tomic porphyry 0.5 140 NAR5 539 Radomiro Tomic porphyry 0.2 292 NAR6 733 Radomiro Tomic porphyry 0.4 130 NAR7 553 Radomiro Tomic porphyry 0.3 100 NAR8 457 Radomiro Tomic porphyry 0.2 140 NAR9 505 Radomiro Tomic porphyry 0.3 140 NAR10 613 Radomiro Tomic porphyry 0.4 280 NAR11 685 Radomiro Tomic porphyry 0.2 411 NAR12 709 Radomiro Tomic porphyry 0.5 140 NAR13 2575 Chuquicamata porphyry 0.4 100 NAR14 2580 Chuquicamata porphyry 0.2 93 NAR15 739 Radomiro Tomic porphyry 0.3 150 NAR16 748 Radomiro Tomic porphyry 0.2 160 NAR17 765 Radomiro Tomic porphyry 0.3 150 NAR18 772 Radomiro Tomic porphyry 0.2 170 NAR19 791 Radomiro Tomic porphyry 0.4 260 NAR20 817 Radomiro Tomic porphyry 0.2 308 NAR21 749 Radomiro Tomic porphyry 0.3 225 NAR22 870 Radomiro Tomic porphyry 0.3 190 NA

Sediment samplesSample Collection no. Location Rock type Br (ppm) Cl (ppm) 37Cl SMOC

A1 2047 Quebrada Chug chug Siltstone 23 5,610 NAA2 2048 Quebrada Chug chug Halite 150 606,000* 0.6A3 2049 Quebrada Chug chug Siltstone 21 52,400 0.8A4 2051 Quebrada Chug chug Siltstone 16 34,600 0.5A5 2052 Quebrada Chug chug Siltstone 9 3,010 0.6A6 2053 Quebrada Chug chug Siltstone 11 4,830 0.8A7 2054 Quebrada Chug chug Siltstone 16 11,900 2.5A8 2056 Quebrada Chug chug Siltstone 14 12,100 2.0A9 2057 Quebrada Chug chug Siltstone 15 16,200 1.5

The second major trend shown in Figure 5A tracks thechanges in halogen concentration through the weathering ofgranitic rock. This trend indicates an initial increase in bothbromide and chloride, followed by the preferential loss of chlo-ride (Wedepohl, 1974). The initial increase in halogens is dueto the breakdown of feldspars, whereas the later preferentialloss of chloride is due to the conversion of biotite to variousclay minerals. The end-product of this weathering trend is thegeneration of soils and sediments with bromide and chlorideconcentrations less than the parent material.

Figure 5B shows the bromide and chloride variations in thesamples of the Radomiro Tomic region compared to the nat-ural reservoirs of Figure 5A. Regional atacamite mineraliza-tion (black circles) plot on the line constrained by the stoi-chiometry of atacamite, with variations in bromide contentranging from 23 to 180 ppm. Regional sediment samples(black triangles) have typical to slightly elevated chlorideconcentration with respect to the sedimentary rocks de-scribed by Wedepohl (1974). Bromide data for the Calamaregion sediment samples plot at and above the values of


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B1 2001 Anticline Siltstone 22 1,610 0.5B2 2002 Anticline Limestone 23 100 0.1B3 2004 Anticline Limestone 24 170 0.8B4 2008 Anticline Limestone 24 100 0.1

C1 958 Gravel pit C-14 Evaporite (sulfate) 19 99 NAC2 959 Gravel pit C-6 Evaporite (sulfate) 9 74 NAC3 960 Gravel pit C-10 Evaporite (nitrate) 8 85 NAC4 961 Gravel pit C-18 Evaporite (sulfate) 15 443 NA

D1 982 Traverse Siltstone 12 4,120 NAD2 985 Traverse Siltstone 16 350 NA

E1 2019 Cerro Quetena Sandstone 13 475 NAE2 2025 Cerro Quetena Limestone 24 648 NAE3 2027 Cerro Quetena Siltstone 17 818 NAE4 2030 Cerro Quetena Mudstone 26 2,120 NAE5 2031 Cerro Quetena Mudstone 18 1,660 0.1E6 2033 Cerro Quetena Limestone 15 2420 NA

F1 2038 San Salvador Mudstone 18 15,500 0.8F2 2039 San Salvador Mudstone 16 15,300 NAF3 2040 San Salvador Mudstone 21 27,900 0.3F4 2041 San Salvador Mudstone 15 9,030 NAF5 2042 San Salvador Mudstone 15 7,910 0.5F6 2043 San Salvador Mudstone 19 18,800 2.4F6 2043 San Salvador Mudstone 19 18,800 2.6F7 2044 San Salvador Siltstone 15 10,600 NAF8 2045 San Salvador Siltstone 14 1,440 NA

G1 989 Cerro Chintoraste Halite 98 606,000 1 0.9G2 990 Cerro Chintoraste Sandstone 30 2580 NAG3 991 Cerro Chintoraste Halite 76 606,000 1 0.5G4 992 Cerro Chintoraste Halite 93 606,000 1 1.0G5 993 Cerro Chintoraste Halite 91 606,000 1 0.2G6 994 Cerro Chintoraste Halite 100 606,000 1 0.6G7 995 Cerro Chintoraste Limestone 20 1,320 NAG8 996 Cerro Chintoraste Halite 130 606,000 1 0.8G9 999 Cerro Chintoraste Limestone 8 160 NA

Regional water samplesSample Location Br (ppm) Cl (ppm) 18O SMOW D SMOW

1 Radomiro Tomic mine water 2.15 1930 0.9 352 Radomiro Tomic mine water 1 1910 NA NA3 Salar del Indio 7.23 18800 0.0 364 Salar de Telebre 1.54 1820 4.5 545 Rio Loa, Mirador 1.08 2150 6.9 566 Rio Loa, Chiu chiu 0.43 688 7.6 677 Rio San Salvador 0.67 2300 6.8 638 Quebrada Chug chug 3.76 3130 1.7 39

Abbreviations: NA = not analyzed, SMOC = seawater chloride standard1 Stoichiometric chloride

TABLE 1. (Cont.)

Sediment samplesSample Collection no. Location Rock type Br (ppm) Cl (ppm) 37Cl SMOC

typical sedimentary rock, with nearly half of the samplesdisplaying elevated concentrations. Sedimentary halitesamples plot on the stoichiometric halite line, with all sam-ples showing high concentrations of bromide relative to thefirst halite precipitated from evaporating seawater. RadomiroTomic whole-rock porphyry samples (squares) have bro-mide and chloride concentrations that are consistent with

typical igneous rocks. Biotite separates show slightly in-creased bromide concentrations compared to whole-rockvalues (R1 and R2) but concentrations are still within theaccepted igneous rock range. The high concentrations ofbromide in biotite relative to whole-rock samples is due tothe preferential incorporation of halogens into the hydroxylsites of biotite. Regional water samples in Figure 5 have


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0

1

2

3

4

5

6

Seawater

TypicalSedimentary

Rocks

Sea Spray

Stoichiometric Halite

Stoichiometric Atacamite

Log

Cl (

pp

m)

RainWater

SeawaterEvaporation

Trend

First HalitePrecipitated

from Seawater

River Water

IgneousRocks

DeadSea

Brine

Log Br (ppm)0-1-2 1 2 3 4

0

1

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6Stoichiometric Halite

Stoichiometric Atacamite

Log

Cl (

pp

m)

A6

A

Radomiro Tomic mine water1

Quebrada Chug chug8

Salar del Indio3

Salar de Talabre4

Rio Loa Mirador5

Rio Loa Chiu chiu6

Rio San Salvador7

Radomiro Tomic mine water2

Water Samples

Legend

Atacamite

Sediments

Radomiro Tomic Porphyry

7

6

25

4 1

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R3-R22 R2

A1

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E4

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G7

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F5

E6

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8

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F6

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A4A3

A9

F3

G5G1

G8A2

G3 G4 G6

3A8

B

Sea Spray

GraniteWeathering

Trend

FIG. 5. A. Geochemical plot of bromide vs. chloride content for known natural reservoirs and regional samples. The dif-ferent fields indicate overall natural variations in the lithologies and fluids indicated (Wedepohl, 1974). There has been noattempt to isolate anthropogenic contamination. B. Geochemical plot of bromide vs. chloride content for known naturalreservoirs with samples from this study superimposed. All sample numbers correspond to Table 1.

elevated concentrations of bromide and chloride comparedto typical river and rain water (Wedepohl, 1974). Annualprecipitation rates in the Calama region are very low and itis evident that the chemical signatures of regional watersamples have been affected by evaporation and interactionswith local lithologies.

Chlorine isotopes

Chlorine isotope data for sedimentary rocks and ata-camite mineralization of the Calama region are plotted ver-sus bromide concentrations in Figure 6A-B. All samples arereferenced to the seawater chloride standard (SMOC) and


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Br

(pp

m)

AtacamiteIgneous Biotite

Legend

Sediments

Atacamite Mineralization

Igneous Rocks

Cerro Chintoraste

Lower Jurassic

Upper Jurassic

LowerJurassic

Sediments

IgneousBiotite

CerroChintoraste

Halite

180

200

UpperJurassic

Sediments Seawater

AMarine Halite andResidual Liquidfrom Seawater

Evaporation

37Cl (SMOC)

0

20

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-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Br

(pp

m)

Chuquicamataand Mina Sur

Radomiro TomicOxide Zone

Mineralization

RadomiroTomic

MineralizedGravel

Distal AtacamiteMineralization

B

FIG. 6. A. Geochemical plot of chlorine isotope values (37Cl) vs. bromide content for seawater, sediment, and igneousbiotite samples listed in Table 1. Field of marine halite and residual liquid from evaporation of seawater is from Eggenkampet al. (1995). Sediment samples are grouped into Upper and Lower Jurassic fields. B. Data for atacamite samples from theCalama region superimposed on Figure 6A. Distal atacamite mineralization includes all samples from mines other thanChuquicamata, Radomiro Tomic, and Mina Sur.

seawater bromide concentration ([Br] = 65 ppm; Valyashko,1956). All data presented in Figure 6 are grouped by locationand sample type (Fig. 4) and listed in Table 1.

Chlorine isotope fractionation has been applied in a varietyof situations, however the only experimentally determinedfractionation factors exist between saturated solutions and theevaporite minerals halite (+0.26 0.07), kainite (0.09 0.09), and bischofite (0.06 0.10; Eggenkamp et al.,1995). Eastoe and Guilbert (1992) have reported evidence offractionation between biotite and dissolved chloride at tem-peratures greater than 400C, although no fractionation fac-tors were determined. For this study, equilibration of chlo-ride-bearing fluids with sediments or supergene atacamite isbelieved to have occurred at low temperature (~25C) andtherefore should have caused little fractionation during ata-camite precipitation. Sediment samples have measurablequantities of halite and should have experienced fractiona-tions similar to that of halite. Higher temperatures and largerconcentration gradients have been responsible for previouslydocumented isotopic fractionations.

Igneous biotite samples separated from the unmineralizedprotore of the Radomiro Tomic porphyry from both potassicand sericitic alteration have very low bromide concentrations(~2 ppm Br) and high 37Cl values from 0.5 to 2.5 per mil(Fig. 6A). These data are consistent with high 37Cl values forbiotite samples from other porphyry copper deposits (Eastoeand Guilbert, 1992; Banks et al., 2000).

The sedimentary rock samples shown in Figure 6A repre-sent both the Upper and Lower Jurassic marine sedimentarysequences. Quebrada Chug chug (A1A9) and San Salvador(F1F8) samples represent the Lower Jurassic sedimentaryrocks deposited in a shallow marine back-arc basin. Thesesediments consistently have negative 37Cl values, (0.3 to2.6) and bromide concentrations below 23 ppm. UpperJurassic sedimentary rocks from the Anticline location(B1B4) have slightly higher bromide contents (2224 ppmBr) and 37Cl values near 0.0 per mil (Fig. 6A).

Sedimentary halite samples from Cerro Chintoraste (G1G9) have a wide range of 37Cl values (0.6 to +1.0) andhigh bromide contents (76130 ppm Br; Fig. 6A). These halitesamples are believed to be the product of seawater evapora-tion based on their elevated bromide contents (Fig. 5B). Nor-mal evaporation of seawater will produce a 37Cl fractionationbetween halite (negative 37Cl values) and residual chloride-rich liquids (positive 37Cl values) but only with values in therange of 0.5 to +0.5 per mil (Eggenkamp et al., 1995). Figure6A shows that the experimentally defined region for haliteprecipitated from evaporating seawater as well as the compo-sition of the residual liquid (Eggenkamp et al., 1995).

Atacamite samples from Radomiro Tomic, Chuquicamata,and the surrounding region are plotted in Figure 6B. Theseawater field and evaporation trend from Figure 6A, as wellas the igneous biotite and sedimentary environments, are alsoshown for comparison. The atacamite fields are grouped bydeposit when applicable. The atacamite samples plot in threemain areas: (1) an area of negative 37Cl values, (2) near 0.0per mil, and (3) an area of positive 37Cl values. The sampleswith negative 37Cl values correspond to atacamite mineral-ization which is distal to the Chuquicamata orebody. A ma-jority of the smaller deposits plot in the distal atacamite

mineralization field (Fig. 6B). Atacamite samples from theRadomiro Tomic oxide zone (1, 2), Chuquicamata (29, 30),and Mina Sur (26, 27) have 37Cl values near 0.0 per mil, al-though their bromide concentrations are more variable.Upper Jurassic sedimentary rocks have bromide contents andchlorine isotope values very similar to those observed in theoxide zone mineralization at Radomiro Tomic. Lower Jurassicsediments have negative 37Cl values and low bromide con-centrations, similar to the distal atacamite mineralization(Fig. 6B). The region of positive 37Cl values corresponds tothe atacamite samples contained in the mineralized graveloverlying the oxide zone at Radomiro Tomic.

Vein-filling atacamite from oxide zone mineralization atChuquicamata (30) has higher bromide contents than ata-camite sampled from a gravel-filled erosive channel (paleo-canal) at the south end of the pit (29). The paleocanal ata-camite, in turn, has bromide concentrations similar to thoseobserved in Mina Sur mineralization samples (26, 27). Theoverall impression is that bromide contents and chlorine iso-tope values both decrease with increasing distance fromChuquicamata, moving toward Lower Jurassic sedimentaryrock values. Two exceptions are the distal atacamite samples8 and 22, which have high bromide contents and very nega-tive 37Cl values: 1.0 and 1.8 per mil, respectively. Twoother exceptions are two atacamite samples from the miner-alized gravel of Radomiro Tomic, which display positive 37Clvalues greater than 0.5 per mil and elevated bromide concen-trations. These samples all have elevated bromide contents,similar to seawater-derived sedimentary halite.

Oxygen and hydrogen isotopes

Regional water samples were analyzed for bromide andchloride contents, as well as for their 18O and D isotope val-ues (Table 1). The samples displayed in Figure 7 are identi-fied by their geographic locations. Sample 1 was obtainedfrom the open pit at Radomiro Tomic at the contact betweenbedrock and overlying gravel where water seeped into the pit.Surface water samples are from both modern salars (3 and 4)and flowing rivers (5, 6, and 7). The sample from QuebradaChug chug (8) was obtained from a small, water-filled holedug in the dry stream bottom and reflects local ground water.

Water samples collected from the Radomiro Tomic regiondisplay a distinct trend in 18O and D isotope values (Fig.7). The regional trend for the water samples has a slope of3.65 and, when extrapolated, intersects the regional meteoricwater line of Fritz et al. (1978) at a 18O value of 12.2 and aD value of 80 per mil. This intercept is consistent with thevalue obtained for the Salar de Punta Negra water samples as-sociated with the mineralization at La Escondida (Alpers andWhittmore, 1990). The slopes of these two trend lines areslightly different, but both agree with the expected results forevaporation of meteoric water in a low-humidity, arid envi-ronment (Campbell and Larson, 1998; Gat and Bowser,1991). The shallower slope for the Calama region reflects alower humidity environment than the Salar de Punta Negratrend of Alpers and Whittmore (1990).

DiscussionThe geochemical tracers and isotopic techniques in this

study have been used to investigate indirect and direct


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contributions from seawater, meteoric water, and magmaticsources of chloride for atacamite mineralization at RadomiroTomic. The findings for each possible source are discussed in-dividually below.

Indirect seawater sources

The chloride provenance indicated for oxide zone ata-camite mineralization at Radomiro Tomic is an indirect sea-water source. Upper Jurassic sedimentary rocks have a geo-chemical signature very similar to the atacamitemineralization in the oxide zone, with bromide concentra-tions ranging from 22 to 24 ppm and 37Cl values of 0.8 to+0.5 per mil (Figs. 5B and 6). Lower Jurassic sedimentaryrocks have lower bromide concentrations (219 ppm Br) andvery light chlorine isotope values (37Cl = 0.3 to 2.5;Figs. 5B and 6A).

In general, all of the sedimentary rock samples have ele-vated bromide concentrations compared to typical sedimen-tary rocks (Fig. 5) and negative 37Cl values (Fig. 6A).Thelarge, negative range of sedimentary chlorine isotope values isinterpreted to be the product of diffusion-related fractiona-tion. Previous studies have indicated the preferential diffu-sion of 35Cl relative to 37Cl in the pore water of sediments(Desaulniers et al., 1986; Eggenkamp et al., 1994; Eastoe etal., 2000). Desaulniers et al. (1986) calculated nearly a 3 permil positive shift in chlorine isotope values over ~45 m ofglacial till that was generated in about 15,000 yr. In Chile nu-merous marine regressions were recorded in the Domeykobasin through the Jurassic (Ardill et al., 1998; Arcuri andBrimhall, 2001, 2002). During these intervals, buried marine

waters were overlain by less-saline meteoric ground water.The contrast of salinities between the two fluids accounts forthe differences in the chlorine isotope compositions. Latermarine transgressions buried the fractionated sedimentsunder new Jurassic sediments with a seawater signature. Thissequence of events was repeated throughout the Jurassic andinto the Early Cretaceous at which time the Domeyko basinexperienced its last marine regression (see further discussionbelow).

The copper oxide mineralization at Chuquicamata and ata-camite from the overlying gravel at Radomiro Tomic havesimilar geochemical signatures to halite samples from CerroChintoraste (Fig. 6B). Atacamite mineralization fromChuquicamata has chlorine isotope values near zero per miland bromide concentrations slightly elevated with respect toseawater (75 ppm: Fig. 6B). Radomiro Tomic mineralizedgravels have elevated bromide concentrations (63180 ppm)and positive 37Cl values of 0.6 per mil. Halite samples fromCerro Chintoraste have similar elevated bromine contents(>65 ppm) and 37Cl values between +0.5 and 0.6 per mil(Fig. 6A) and are thought to reflect a marine evaporitesource. The chlorine isotope values for Cerro Chintorastesamples have a much greater range than expected (+0.55 to0.40) for the precipitation of halite from the evaporationof seawater (Eggenkamp et al., 1995; Fig. 6A). One possibleexplanation for the wider range of isotopic values is the possi-bility that Cerro Chintoraste is a salt dome (E. Tidy, pers.commun., 1998; R. Pardo, pers. commun., 1999). Previousstudies of chlorine isotopes in salt dome environments argueagainst this interpretation, with no indication of substantial


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138

4

67

10

0

-10

-20

-30

-40

-50

-60

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-90

-100

-110-16 -14 -12 -10 -8 -6 -4 -2 0 2

18O (SMOW)

Global MeteoricWater Line

D = 8 18O + 10

Regional MeteoricWater Line

D = 7.3 18O + 7.99Fritz et al. 1978

El TatioGeysers

1 Radomiro Tomic Mine Water3 Salar del Indio4 Salar de Telebre5 Rio Loa, Mirador6 Rio Loa, Chiu chiu7 Rio San Salvador8 Quebrada Chug chug

Data from Alpersand Whittmore, 1990This Study

Intercept of Salar de PuntaNegra Trend with Regional

Meteoric Water Line5

X

SMOW

Calama RegionalTrend

D = 3.65 18O - 36.13

D (S

MO

W)

Salar d

e Punt

a

Negra T

rend

(D = 4

.97

18 O - 20

.19)

FIG. 7. Oxygen and deuterium isotope analyses of water samples from the Calama region. Data from the Calama region(black triangles) are compared to samples from the Salar de Punta Negra region of northern Chile (gray squares) compiledby Alpers and Whittmore (1990).

fractionation between the sedimentary halite and the mobi-lized material (Eastoe and Peryt, 1999). Additional field ob-servations at Cerro Chintoraste have revealed massive crystalsof vein-filling halite which cut and offset earlier iron oxideveins. Within the halite veins, multiple generations of thefracture fillings have been recognized. These observations ledto our interpretation that halite-saturated fluids were mobi-lized from buried, evaporite-bearing sedimentary rocks andprecipitated halite in a series of vein-forming events ratherthan from the ductile flow of salt. If incongruent dissolutionof halite from buried Jurassic sedimentary rocks was thesource of this chloride, large isotopic fractionations would beexpected in the precipitating, vein-filling halite crystals (Eas-toe et al., 2000). This could explain the large 37Cl variationsseen between halite samples (Fig. 6A) and also could accountfor the interpreted marine source for Cerro Chintorastehalite based on bromide contents (Fig. 5B). These observa-tions suggest that fluids generated from incongruent dissolu-tion of halite from buried Jurassic evaporites were the mostlikely source of chloride for the halite at Cerro Chintoraste,the atacamite mineralization of the Radomiro Tomic gravel,and the Chuquicamata oxide zone.

Evaporitic brines, such as the Dead Sea brines plotted inFigure 5A, have similar elevated bromide and chloride values,reflecting the evaporative concentration of seawater. Thebrines typically preserve bromide and chloride concentra-tions acquired at the time of burial. This may vary if subse-quent evaporite dissolution concentrates halogens in the flu-ids or if an influx of less-saline fluids causes dilution. This typeof brine could precipitate atacamite, although the resultantbromide concentration of the atacamite would be muchhigher than is seen at Radomiro Tomic.

Direct seawater sources

A paleogeographic reconstruction of the Calama regionshows that it was far inland during the mineralization atRadomiro Tomic (Pindell and Tabbutt, 1994; Arcuri andBrimhall, 2001). The Pacific Ocean had regressed by the EarlyCretaceous, drying out the back-arc basin that had previouslyoccupied much of western Chile. During the emplacement ofthe Radomiro Tomic protore at 32 Ma a marine seawater sourcewould have been over 130 km to the west and over 1,000 mlower in elevation, similar to today. All evidence regarding themobility and halogen content of sea spray indicates that con-centrations decrease with increased distance and elevation in-land (Duce et al., 1965; Mussi et al., 1998), so any major directcontribution from Pacific Ocean sea spray is unlikely.

Meteoric water sources

Local rainwater would have contained less than 10 ppbbromine and approximately 10 ppm chloride at sea level dur-ing the supergene mineralization of the Radomiro Tomic pro-tore (Wedepohl, 1974). At these concentrations, derivation ofbromide and chloride solely from precipitation would requirea volume of 1011 m3 of rainwater to account for the Br and Clin the Radomiro Tomic ore. Precipitation of 2 cm/yr for 1 m.y.could generate the halogen quantities observed in the oxidezone mineralization at Radomiro Tomic if there was completeor nearly complete retention of all bromide and chloride fromthe precipitation. The effects of higher altitude would require

more precipitation due to the decrease in the chloride andbromide content of precipitation with increased elevation(Duce et al., 1965; Mussi et al., 1998).

The bromide and chloride content of meteoric watershould reflect an ocean origin. However, the isotopic compo-sition and bromide concentration of atacamite at RadomiroTomic and Chuquicamata (Figs. 56) does not indicate anysignificant contribution of meteoric origin derived from theocean and introduced through precipitation.

Local river, salar, and ground-water samples have elevatedvalues of bromide and chloride relative to typical river water(Fig. 5B; Wedepohl, 1974). These samples have values moretypical of igneous and sedimentary rocks, indicating that theoverall chemistry of regional waters is strongly influenced bythe rocks with which they interact. Oxygen and deuterium iso-tope analyses of these waters (Fig. 7) indicate a meteoric watersource that has undergone evaporation in a low-humidity, aridenvironment. However, none of the local water samples hascompositions that could have been responsible for the Br andCl in the atacamite. Moreover, if meteoric water could make ameaningful contribution to the Cl concentrations in supergenesystems, then one would expect chloride minerals to be morecommon throughout northern Chile. To date, RadomiroTomic and Chuquicamata are the only deposits that have ap-preciable quantities of atacamite mineralization.

Magmatic sources

A magmatic source of chloride for the atacamite mineral-ization at Radomiro Tomic could have occurred either by re-tention of initial magmatic chloride or by the introduction ofchloride from subsequent local volcanism. The chloride andbromide contents calculated for the entire protore prior toany erosion or supergene enrichment are not sufficient to ex-plain the observed quantities seen in oxide zone atacamitemineralization (an excess of 0.9 Mt of chloride). Chlorine inmagmatic biotite is isotopically heavy at Radomiro Tomic(37Cl = 0.52.5), whereas oxide zone atacamite mineral-ization has values close to 0.0 per mil. Similarly, atacamitemineralization at Chuquicamata and Mina Sur have elevatedbromide contents (3875 ppm) and 37Cl values near 0.0 permil (Fig. 6B), indicating a similar source for the introductionof chloride. Atacamite from the mineralized gravel atRadomiro Tomic has positive isotopic values (37Cl = 0.6;Fig. 7B), indicating the input of isotopically heavy fluids. Theatacamite-bearing gravel of Radomiro Tomic contains erodedcobbles of mineralized, oxide zone porphyry and therefore re-flects a postmineralization deposition. These results indicatethat retention of initial magmatic chloride was not an impor-tant source of chloride for atacamite mineralization, althoughit could have contributed minor quantities.

A detailed paleogeographic reconstruction indicates thatthere has been no major magmatic activity in the study regionsince the protore emplacement during the Tertiary (Arcuri andBrimhall, 2001). This observation effectively rules out the pos-sibility of the addition of chloride from any recent magmatism.

Relationship to Regional Geologic EvolutionThe mineralizing events that produced the oxide orebody at

Radomiro Tomic are closely linked to the regional geologicand hydrologic evolution. An interpretation of the regional


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geologic evolution of Radomiro Tomic and its atacamite min-eralization is shown in Figure 8A-C, which comprises a seriesof east-west cross sections across northern Chile at 22 16 Slatitude (7,538 km UTM) and corresponds to the cross-sec-tion line depicted in Figure 3.

The regional geology was dominated by the Domeyko back-arc basin, which separated the Jurassic coastal volcanic beltfrom Permian-Triassic basement rocks to the east (Fig. 8A).During the Early Jurassic a shallow marine depositional envi-ronment existed in this basin accumulating thick sequences ofsiliciclastic sediments with interbedded carbonates (Prinz etal., 1994; Ardill et al., 1998). The eastern margin of the basinwas marked by a basin-bounding fault, which was repeatedlyactivated to allow continued sediment accumulation (Prinz etal., 1994). At approximately 152 Ma a complete marine re-gression isolated the back-arc basin from the Pacific Oceanleading to the basin-wide deposition of vast quantities of evap-orite minerals as the stranded marine waters evaporated(Prinz et al., 1994; Ardill et al., 1998; Arcuri and Brimhall,2002). These evaporite minerals were later buried under sed-iments deposited in the Upper Jurassic after a marine trans-gression had reconnected the basin with the Pacific Ocean.Cretaceous sediments buried the Jurassic deposits and weresubsequently overlain by Tertiary volcaniclastic rocks.

Major regional uplift of the Incaica phase occurred duringthe Eocene at approximately 40 Ma (Fig. 8A; Scheuber et al.,1994). Compression from the west caused reactivation of the

basin-bounding fault in a reverse motion. A series of intru-sions was focused along the eastern fault (West fissure), usingthis fracture as a conduit. The Los Picos granodiorite was em-placed at 45 to 43 Ma and was subsequently intruded on itseastern margin by the Fortuna Complex at 40 to 36 Ma (Boricet al, 1990). The ultimate intrusion was that of the Chuquica-mata Porphyry, which was emplaced on the eastern side ofthe West fissure at 35 to 32 Ma (Boric et al, 1990; Reynoldset al., 1998; Ossandn et al., 2001). Contemporaneous vol-canic lithologies associated with these intrusions were de-posited over large areas of the Calama region (Fig. 3).

Supergene enrichment (Fig. 8B) of the Chuquicamata andRadomiro Tomic protore began in the middle Miocene at 15 to19 Ma (Sillitoe and McKee, 1996; Mote et al., 2001; Ossandnet al., 2001). Regional uplift coupled with erosion exposedLower Jurassic sediments as well as intrusive bodies to oxidationand weathering. Meteoric water mobilized chloride from saltsin the Jurassic sediments and incorporated it into the supergenemineralization at both Radomiro Tomic and Chuquicamata.Chloride mobilized during this event (Cl I, Fig. 8B) was re-sponsible for the oxide zone atacamite mineralization atRadomiro Tomic. Also during this time, distal atacamite miner-alization on the periphery of Chuquicamata formed at or nearthe contact between sedimentary and igneous lithologies.

Further uplift associated with the Quechua uplift phase at12 Ma (Scheuber et al., 1994; Hartley et al., 2000) releasedhalogens from Jurassic sediments (Cl II, Fig. 8C). This later


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++

+++

+++

+++

v

v

v

v

v

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++x

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V V

VV V

VV

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x

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Cl- ISupergene Enrichment ~18 Ma

B

++

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+

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vv

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v v vv

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Compression

Incaica Uplift ~40 MaA

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Quechua Uplift ~12 MaC

CompressionCl-II

Legend

++

++

Permian, TriassicUndifferentiatedPlutonic Rocks

Lower JurassicSedimentary Rocks

Upper JurassicSedimentary Rocks

CretaceousSediments

vv

v vv

v

vv Tertiary Volcanic Rocks

ChuquicamataPorphyry35-32 MaFortuna Complex40-36 Ma

Los Picos Diorite45-43 Ma

Monte ChristoGranodiorite69-50 Ma

x xx xx Elena Granodiorite146-125 Ma

Middle JurassicEvaporites

RT

RT

VerticalToward ViewerAway from Viewer

Fault Movement Indicators

FIG. 8. A series of schematic east-west geologic cross sections looking north from the Andes to the Pacific Ocean. Thesecross sections correspond to the line depicted in Figure 3. Lithologic patterns and symbols are the same as those presentedin Figure 3. The Radomiro Tomic deposit is schematically located by the white circle and labeled RT. A. During the Incaicauplift phase the Tertiary intrusive series is emplaced along the tectonically active West fissure zone. B. Supergene enrich-ment of Radomiro Tomic and Chuquicamata at ~18 Ma. Jurassic sedimentary rocks are exposed to uplift and erosion re-leasing chloride (Cl I) responsible for the initial atacamite mineralization of the oxide zone at Radomiro Tomic. C. Quechuauplift phase at ~12 Ma, exposes more of the halite-bearing Jurassic sedimentary rocks to erosion (Cl II). The second intro-duction of chloride (Cl II) was responsible for the atacamite mineralization of overlying gravel at Radomiro Tomic.

chloride was responsible for the atacamite mineralization inoverlying gravel at Radomiro Tomic. The chloride fromRadomiro Tomic mineralized gravel has characteristics of ma-rine evaporites and indicates a different chloride source thanthe main mineralization at Radomiro Tomic and Chuquica-mata (Fig. 6).

Regional compression or fault movement could have forcedsolutions rich in halogens from deep within the basin upwardwhere they mixed with ground water to generate the ob-served mineralization. This situation could have occurred ineither the Cl I or Cl II phase and in this alternate scenariothe fluids generated would also retain the halogen signatureof the source rocks.

ConclusionsResults from a chloride mass-balance calculation indicate

that 0.9 Mt of chloride has been added to the RadomiroTomic deposit and fixed in oxide zone atacamite mineraliza-tion. Geochemical and isotopic analyses indicate that thesource of this introduced chloride was in the Jurassic marinesediments. The main oxide zone atacamite mineralization atRadomiro Tomic has the geochemical and isotopic signatureof Upper Jurassic sediments. Chloride was mobilized fromthe sediments during the supergene mineralization of theRadomiro Tomic protore by meteoric fluids when uplift anderosion exposed evaporite-bearing lithologies.

The mineralized gravel at Radomiro Tomic has higher bro-mide concentrations (63180 ppm) and isotopically heavierchlorine (37Cl = 0.6) than oxide zone atacamite, indicatinga sedimentary halite source. The gravel mineralization wasdetermined to have occurred after the main oxide zoneformed. The chloride in this mineralization was derived fromsediments exposed during uplift associated with the Quechuacompressional phase at approximately 12 Ma.

In both cases of chloride introduction (Cl I and Cl II), ex-posed Jurassic marine sediments were leached by meteoricwater generating high chloride and bromide concentrationsin local ground water. Compression might also have gener-ated a pulse of chloride-rich fluid from buried Jurassic saltsthat migrated through the West fissure zone to interact withlocal ground water. In either case, a Jurassic marine signaturewould be preserved and seen in the resultant atacamitemineralization.

Atacamite at Chuquicamata has a sedimentary halite signa-ture (75 ppm Br, 37Cl = +0.1 to 0.1). Atacamite samplesdistal to the Chuquicamata orebody have geochemical char-acteristics similar to Jurassic sediments, in particular lowerbromide concentrations and negative chlorine isotope values.Paleocanal mineralization at Chuquicamata (51 ppm Br, 37Cl= 0.2) along with atacamite from Mina Sur (5248 ppm Br,37Cl = 0.00.1) indicate decreasing bromide contents withincreasing distance from the Chuquicamata orebody butnear-constant chlorine isotope values. The samples fromoxide zone atacamite at Radomiro Tomic also follow thistrend of decreasing bromide with increasing distance fromthe Chuquicamata orebody.

Recent meteoric water has a signature indicative of the arid,low-humidity environment of the modern Atacama Desert(Fig. 7). Water sampled from within the Radomiro Tomicmine plots on the Calama regional trend and represents a

fluid that has undergone a high degree of evaporative con-centration. Isotopic analysis of regional water samples indi-cates that the origin of precipitation in the Calama region issimilar to that from the Salar de Punta Negra (Alpers andWhittmore, 1990). Understanding the source of chloride in-volved in the atacamite mineralization at Radomiro Tomic hasallowed us to determine past climatic and tectonic eventswhich led to the formation of this deposit and affected the en-tire Calama region.

AcknowledgmentsThis study was funded by Codelco Chile. Logistical field

support by Codelco and in particular Patricio Cuadra madecollection of field samples possible. Discussions with En-rique Tidy, Pedro Carrasco, Guillermo Mller, and GerardoBehn in Codelco, Santiago, helped to focus the direction andscope of this work. While at the Radomiro Tomic mine, dis-cussions with Gonzalo Rojas, Victor Araya, GuillermoSalazar, Gabriel Lieva, Xiomara Rubio, Renato Villarroel,Yuri Aravena, Renato Valdes, and Julio Echegaray allowedfor a more complete interpretation to be made of the depositcharacteristics. Support from Reuben Pardo, Jorge Pizara,and the staff of the Codelco Calama exploration office wasgreatly appreciated and aided in determining regional sitesto be investigated. We greatly appreciate the access to theChuquicamata deposit facilitated by Roberto Frraut and toMina Sur by Jorge Vega.

In particular, the authors would like to acknowledge thecontributions of Chris Eastoe at the University of Arizona forhis insights and instructions on the chlorine isotope techniqueand its application to this study. The authors would like topersonally thank him for his generous hospitality. We wouldlike to thank Robert Slade from Maptek for his instruction re-garding the use of the Vulcan software package. Additionally,the insightful reviews of David Banks, Jacob Lowenstern, anda third, anonymous reviewer strengthened the manuscriptand were greatly appreciated. Continued editorial review byMark Hannington helped to produce the final manuscriptand was greatly appreciated.

March 15, 2002; June 17, 2003

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