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Pleistocene Recycling of Copper at a Porphyry System, Atacama Desert, Chile: Cu Isotope Evidence

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Pleistocene Recycling of Copper at a Porphyry System, Atacama Desert, Chile: Cu Isotope Evidence Miner Deposita (2011) 46:1–7 DOI 10.1007/s00126-010-0315-6 LETTER Pleistocene recycling of copper at a porphyry system, Atacama Desert, Chile: Cu isotope evidence Carlos Palacios & Olivier Rouxel & Martin Reich & Eion M. Cameron & Matthew I. Leybourne Received: 24 June 2010 /Accepted: 29 September 2010 /Published online: 16 November 2010 # Springer-Verlag 2010 Abstract We present Cu isotope data of hypogene and -5.72‰ to -6.77‰ (n=17). These data suggest redox supergene minerals from the Late Paleocene Spence Cu-Mo cycling of Cu during supergene enrichment of the Spence porphyry in the Atacama Desert of northern Chile. Cu deposit, characterized by a first stage of supergene Chalcopyrite displays a restricted range of δ65Cu values chalcocite formation from acidic, isotopically-heavy leach within the values reported for primary porphyry Cu sulfides fluids of meteoric origin down-flowing in a semi-arid (+0.28‰ to +0.34‰, n=6). Supergene chalcocite samples climate (44 to ~ 15-9 Ma). Reworking of the initial show heavier and remarkably homogeneous δ65Cu values, supergene copper assemblage, during the Pleistocene, by between +3.91‰ and +3.95‰ (n=6), consistent with rising neutral and chlorine-rich deep formation waters previous models of Cu leaching and enrichment in under well-established hyper-arid climate conditions lead porphyry systems. Secondary Cu minerals from the oxide to the formation of atacamite with extremely fractionated zone show a wider range of composition, varying from Cu compositions. Essentially coeval chrysocolla formed by +1.28‰ and +1.37‰ for chrysocolla (n=6) to very light dissolution of atacamite during short episodes of wetter Cu isotope signatures reported for atacamite between climatic conditions occurring in the latest Pleistocene. Keywords Cu isotopes . Supergene enrichment . Copper Editorial handling: B. Lehmann porphyry. Spence . Atacama Desert . Chile C. Palacios (*) : M. Reich Departamento de Geologia, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile Introduction e-mail: [email protected] In the Cu deposits of the Atacama Desert, northern Chile, O. Rouxel the meteoric supergene enrichment stage covered a long Marine Chemistry and Geochemistry Department, – Woods Hole Oceanographic Institution, period extending from 44 Ma and ending near 14 9Ma Woods Hole, MA 02543, USA with a step-wise onset towards the hyper-aridity in the region (Sillitoe and McKee 1996;Moteetal.2001; O. Rouxel Rowland and Clark 2001; Bouzari and Clark 2002; Hartley IUEM European Institute for Marine Studies, Universitée Européenne de Bretagne, and Rice 2005; Arancibia et al. 2006; Evenstar et al. 2009; Technopole Brest-Iroise, Place N Copernic, Reich et al. 2009a, b). This first stage of supergene 29280 Plouzane, France oxidation peaked between 21and 15 Ma as the result of percolating oxygenated meteoric waters in a semi-arid E. M. Cameron Eion Cameron Geochemical Inc., setting. This was followed by a second stage of supergene 865 Spruce Ridge Road, oxidation where the Cu-hydroxychloride atacamite (Cu2Cl Carp, ON K0A 1L0, Canada (OH)3) was formed from uprising basinal brines and subsequently preserved under hyper-arid conditions for M. I. Leybourne Ocean Exploration, GNS Science, the last 2 Ma (Cameron et al. 2002, 2007; Reich et al. P.O. Box 30-368, Lower Hutt, New Zealand 2008, 2009a,b). 2 Miner Deposita (2011) 46:1–7 Aside from the characteristic occurrences of atacamite in Fe-hydroxides, hematite, and jarosite) of porphyry copper the Atacama Desert, atacamite is rather rare because it systems is commonly characterized by isotopically lighter requires high chloride activities to form and dissolves rapidly signatures (negative δ65Cu), whereas secondary sulfides when exposed to meteoric water (Cameron et al. 2007). (e.g., chalcocite) precipitated fr om the leach waters are Recent studies have shown that the two stable isotopes characterized by more positive, heavier δ65Cu signatures of Cu, 65Cu and 63Cu, can be fractionated during oxidative (Mathur et al. 2009). dissolution and reworking of primary Cu sulfides (Larson et Cu isotope data has been made increasingly available in the al. 2003; Rouxel et al. 2004; Mathur et al. 2005, 2009; last decade for hypogene and supergene Cu minerals, Borrok et al. 2008; Pokrovsky et al. 2008; Asael et al. including sulfides, carbonates, sulfosalts, oxides, and hydrous 2009; Kimball et al. 2009). It has been suggested that Cu silicates (e.g., Larson et al. 2003; Markl et al. 2006; Mathur et isotope systematics may be used to identify the nature and al. 2009). However, no data are yet available for atacamite, extent of supergene enrichment in porphyry Cu systems a major constituent of oxide zones in Cu deposits in the (Mathur et al. 2005, 2009). In particular, both experimental Atacama Desert, with the exception of one isolated, and field studies have revealed that during oxidative geographically unconstrained sample from the Atacama dissolution of primary Cu sulfides (e.g., chalcopyrite), the Desert reported by Gale et al. (1999)showingavery leach fluid becomes enriched in 65Cu, leaving the residue negative Cu isotopic signature (δ65Cu=−7.71‰). isotopically lighter as leaching continues (e.g., Mathur et al. In this study, we report the first geologically con- 2005; Kimball et al. 2009). Likewise, the leach cap (e.g., strained Cu isotope data set of atacamite from the Spence Fig. 1 Location of the Spence copper porphyry. In the right,anE–W enriched sulfides in red, hypogene-enriched zones in blue,and cross section of the Spence orebody at 7,480,500 N is presented. hypogene sulfides in yellow (based on Cameron et al. 2007) Leach cap is shown in brown, oxide blanket in green, supergene- Miner Deposita (2011) 46:1–7 3 porphyry copper deposit (Atacama Desert), along with The analyzed specimens consist of individual minerals that Cu isotope data of chrysocolla, chalcocite, and chalco- were inspected with a petrographic microscope and X-ray pyrite from the same deposit. When coupled with the diffraction. The isotopic compositions of the samples are existing age constraints for supergene enrichment and expressed as a deviation relativeÀÁ to the standard following 65 3 oxidation in the region, we show that the Cu isotope data the equation: d Cu ¼ 10 Â Rspl=Rstd À 1 , where Rspl 65 63 is most consistent with multiple cycles of supergene is the measured Cu/ Cu ratio for the sample and Rstd is leaching and oxidation of Cu that started at 44 Ma and the 65Cu/63Cu ratio of the NIST 976 Cu standard. Copper continued at least until the late Pleistocene. isotope ratios were determined using a multicollector ICPMS (Neptune, Thermo-Electron TM) operated by the Woods Hole Oceanographic Institution and the French Geologic setting and samples Research Institute for Exploitation of the Sea. Isotope analyses of chalcopyrite, atacamite, chrysocolla, The location of the Spence copper porphyry is shown in and chalcocite followed previously published methods. In Fig. 1. The Spence porphyry copper deposit is related to short, the samples were purified by an anion exchange three stocks of dacitic porphyries and tourmaline-bearing chromatography column using a procedure modified from hydrothermal breccias, which were emplaced in andesitic that of Merechal et al. (1999), Chapman et al. (2006), and volcanic rocks of the Upper Cretaceous–Lower Tertiary. Borrok et al. (2007). After a complete digestion step in 40 39 Laser Ar/ Ar plateau ages in magmatic biotites of pre- concentrated HNO3-HCl, the sample was dissolved in 2 ml and post-mineralization dacitic stocks show evidence that 6 N distilled HCl in a closed beaker on a hot plate. A hypogene mineralization (chalcopyrite, pyrite, molybdenite, precise volume of this solution, corresponding to about and lesser amounts of bornite and tennantite) occurred 10 μg Cu, was then purified using anion exchange AG1-X8 between 56 and 57 Ma (Rowland and Clark 2001). resin (Bio-Rad, 200–400 mesh). After elution of matrix Unconsolidated gravel deposits and a leach cap cover an elements, Cu was eluted (and separated from Fe) with oxidation blanket characterized by atacamite, chrysocolla, 30 ml 6 N HCl, collected in a Teflon vial, and evaporated to and minor brochantite, in which the Cu-hydroxychloride is dryness. The residue was re-dissolved in 2–3mlof2% by far the dominating mineral. Underlying the oxide zone is HNO3 and then further diluted to form a 0.1 to 0.5 ppm Cu a well-developed chalcocite- and lesser covellite-bearing solution ready for isotope analysis. Analyses of 65Cu/63Cu supergene enrichment blanket. Supergene alunite and natro- were carried out on a MC-ICPMS operating at low alunite from the leached and enriched zones were dated resolution mode. The samples were introduced into the between 44.4±0.5 and 27.7±5.4 Ma, based on 13 inverse plasma using a double spray quartz spray chamber system isochron 40Ar/39Ar ages (Rowland and Clark 2001). (cyclonic and double pass) and a micro-concentric PFA Following a long interval, there was substantial modifica- nebulizer operating at a flow rate of about 60 μl min−1. tion of the initial supergene assemblage, principally the Instrumental mass bias was corrected for using Zn isotopes formation of atacamite, during the Pleistocene by rising as an internal standard and involves simultaneously basinal brines. Reich et al. (2008) reported fissiogenic 36Cl measurement of a Zn standard solution (SRM 3168a analysis for atacamite indicating a maximum age of 1.5 Ma Standard Solution). In addition, a standard bracketing for its formation. Reich et al. (2009a) further refined this approach, which normalizes the Cu isotope ratio to the dating by measuring U-series disequilibrium ages of average measured composition of a standard (SRM 976) atacamite-gypsum intergrowths in atacamite-bearing was carried out before and after each sample. The internal assemblages, indicating precipitation at 127 ka. precision of the data has 95% confidence levels based on Representative samples from pit walls and drill cores were isotopic deviation of duplicated sample analysis.
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