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Origin of Fluids in Iron Oxide–Copper–Gold Deposits: Constraints 37 87 86 from Δ Cl, Sr/ Sri and Cl/Br

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Origin of Fluids in Iron Oxide–Copper–Gold Deposits: Constraints 37 87 86 from Δ Cl, Sr/ Sri and Cl/Br View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Miner Deposita (2006) 41: 565–573 DOI 10.1007/s00126-006-0082-6 LETTER Massimo Chiaradia . Dave Banks . Robert Cliff . Robert Marschik . Antoine de Haller Origin of fluids in iron oxide–copper–gold deposits: constraints 37 87 86 from δ Cl, Sr/ Sri and Cl/Br Received: 15 May 2006 / Accepted: 22 June 2006 / Published online: 18 August 2006 # Springer-Verlag 2006 Abstract The origin of the hypersaline fluids (magmatic the literature. The heavy chlorine isotope compositions of or basinal brine?), associated with iron oxide (Cu–U–Au– fluids of the investigated IOCG deposits may indicate a REE) deposits, is controversial. We report the first prevailing mantle Cl component in contrast to porphyry chlorine and strontium isotope data combined with Cl/Br copper fluids, an argument also supported by Os isotopes, ratios of fluid inclusions from selected iron oxide–copper– or could result from differential Cl isotope fractionation gold (IOCG) deposits (Candelaria, Raúl–Condestable, processes (e.g. phase separation) in fluids of IOCG and Sossego), a deposit considered to represent a magmatic porphyry Cu deposits. end member of the IOCG class of deposit (Gameleira), and a magnetite–apatite deposit (El Romeral) from South Keywords Chlorine isotopes . Strontium isotopes . America. Our data indicate mixing of a high δ37Cl Iron oxide–copper–gold deposits . Chile . Peru . Brazil magmatic fluid with near 0‰ δ37Cl basinal brines in the Candelaria, Raúl–Condestable, and Sossego IOCG de- posits and leaching of a few weight percent of evaporites Introduction by magmatic-hydrothermal (?) fluids at Gameleira and El Romeral. The Sr isotopic composition of the inclusion Iron oxide–copper–gold (IOCG) deposits are an economic- fluids of Candelaria, Raúl–Condestable, and El Romeral ally important style of hydrothermal mineralization, char- confirms the presence of a non-magmatic fluid component acterised by abundant magnetite and/or hematite, variable in these deposits. The heavy chlorine isotope signatures of amounts of Cu-sulfides, pyrite, gold, and REE, and fluids from the IOCG deposits (Candelaria, Raúl– variably intense and voluminous sodic ± calcic and Condestable, Sossego), reflecting the magmatic-hydro- potassic alteration (e.g. Williams et al. 2005). They are thermal component of these fluids, contrast with the near formed by hypersaline, CO2- and CaCl2-rich, and S-poor 0‰ δ37Cl values of porphyry copper fluids known from fluids at shallow to mid-crustal levels within cratonic or continental margin settings and, although often occurring Editorial handling: B. Lehmann within magmatic rocks, they are not always clearly related to igneous activity (e.g. Pollard 2000; Williams et al. M. Chiaradia . D. Banks . R. Cliff School of Earth and Environment, University of Leeds, 2005). Leeds, LS2 9JT, UK The origin of the hypersaline fluids associated with e-mail: [email protected] IOCG deposits is controversial: either (1) fluids were e-mail: [email protected] exsolved from magmas (e.g. Pollard 2000; Sillitoe 2003; Pollard 2006), or (2) brines were derived from evaporites R. Marschik Department of Earth and Environmental Sciences, and heated by nearby intrusions (e.g. Barton and Johnson Ludwig-Maximilians Universität, 1996; Haynes 2000). Geological, mineralogical, chemical, 80333 München, Germany and fluid inclusion data, as well as stable isotopes of O, H, e-mail: [email protected] and S have been used to support both the magmatic and the M. Chiaradia (*) . A. de Haller evaporitic hypothesis. Department of Mineralogy, University of Geneva, In the present study, we report the first chlorine and Rue des Maraîchers 13, strontium isotope data and Cl/Br ratios of fluid 1205 Geneva, Switzerland inclusions of three IOCG deposits (Candelaria, Raúl– e-mail: [email protected] – Tel.: +41-22-3796634 Condestable, Sossego), a Cu Au deposit considered to Fax: +41-22-3793210 be a magmatic end member of the IOCG class of deposit e-mail: [email protected] (Gameleira; Lindenmayer et al. 2001; Pimentel et al. 566 2003), and a magnetite–apatite deposit (El Romeral) 208Pb/204Pb=38.13–38.40), and δ18O (+6.9 to +8.6 ‰) from South America. Since IOCG deposits constitute a values, which indicate mixed mantle and crustal sources 187 188 variant of the iron oxide (Cu–U–Au–REE) deposit class (Marschik et al. 2003). The Os/ Osi values of that also includes magnetite–apatite (Kiruna-type) de- magmatic magnetite from the plutons range from 0.21 to posits (Hitzman et al. 1992; Hitzman 2000), comparative 0.41, also suggesting mixed mantle and crustal compo- investigations on these types of mineralization can nents in these batholiths (Mathur et al. 2002). Re–Os enhance our understanding of these ore-forming systems molybdenite ages of 114.2±0.6 and 115.2±0.6 Ma and hence improve exploration strategies. represent the time of the main Cu–Au mineralization Chlorine is the main metal ligand in the hypersaline (Mathur et al. 2002). fluids of IOCG deposits and behaves more conservatively The results of isotope and microthermometric studies are during fluid–rock interaction than oxygen and hydrogen compatible with the involvement of both magmatic and due to the precipitation of usually lower amounts of non-magmatic fluids at Candelaria (Rabbia et al. 1996; chlorine, with respect to oxygen and hydrogen, in mineral Marschik et al. 1997, 2000, 2003; Ullrich et al. 2001; 34 phases resulting from the fluid–rock interaction. Therefore, Marschik and Fontboté 2001). Most of the sulfur (δ SCDT chlorine isotope compositions may be a powerful tool to of sulfides mainly between −3.2 and 3.1‰) and metals constrain the origin of brines of IOCG deposits. (e.g. initial 187Os/188Os of the ore of 0.36±0.1) were In principle, chlorine isotopes are able to discriminate derived from magmatic sources (Marschik and Fontboté between heavy, depleted mantle-derived [δ37Cl=3–7‰ 2001; Mathur et al. 2002) during the main ore stage, (Magenheim et al. 1995)whereδ37Cl is expressed with whereas the late ore stage fluids were characterised by 37 34 respect to seawater], and light, crust-derived (δ Cl∼0‰: more oxidised and heavier S-fluids (δ SCDT of sulfides up Eggenkamp et al. 1995;Eastoeetal.2001)chlorine.On to +7.2‰) of probable evaporitic origin (Ullrich et al. the other hand, chlorine isotope compositions of fluids 2001). Fluid inclusions in quartz associated with chalco- associated with arc magmas are poorly constrained (e.g. pyrite from the main ore stage are hypersaline and CO2- Stewart and Spivack 2004). Condensed volcanic gases rich and have homogenization temperatures ranging from from various volcanoes display a range of δ37Cl from −1 328 to ≥396°C. A model in which cooling magmatic- to +4‰ (Pitcairn 2002), whereas magmatic fluids hydrothermal fluids mix with non-magmatic (evaporitic?) associated with porphyry copper deposits at convergent fluids can explain the ore formation at Candelaria (e.g. margins, i.e. the setting of many IOCG deposits, have Ullrich et al. 2001; Marschik and Fontboté 2001; Mathur et δ37Cl values between−0.5 and +0.5‰ (Eastoe and al. 2002). Guilbert 1992). Godon et al. (2000) have measured We have analysed fluid inclusions in quartz from two δ37Cl values of −0.8 to +0.1‰ for hot waters, +0.8 to samples. Sample PC98014, from level N464 in the former +2.0‰ for condensates, and of 0‰ for an andesite in the South Pit, is a quartz + sericitised feldspar vein with traces Lesser Antilles island arc. of magnetite and paragenetically later biotite and chalco- pyrite, hosted in volcanic or volcaniclastic rocks with pervasive biotite–quartz–magnetite alteration. Sample Geology, geochemistry and sampling of the ore deposits PC99147, from a drill hole in the former Candelaria North Pit, is a quartz vein containing minor K–feldspar, We have investigated Cretaceous deposits (Candelaria, traces of magnetite and paragenetically later chalcopyrite– Raúl–Condestable, El Romeral) that occur in the Peruvian pyrite, plus minor chlorite and epidote, hosted by and Chilean Andes in extensional to transpressional pervasively altered (biotite–magnetite) volcanic–volcani- continental arc settings (e.g. Marschik and Fontboté clastic rocks. 2001; Sillitoe 2003), and Late Archean to Early Proterozoic deposits (Gameleira and Sossego) that occur in the Carajás Mineral Province, Brazil, within rocks of either a Raúl–Condestable continental rift (e.g. Olszewski et al. 1989) or a subduc- tion-related arc environment (Dardenne et al. 1988). The Raúl–Condestable IOCG deposit, with estimated cumulative production and reserves exceeding 32 Mt at 1.7% Cu, 0.3 g/t Au, and 6 g/t Ag, is located 90 km south of Candelaria Lima (Peru), approximately 5 km from the Pacific coast. The ore consists of chalcopyrite, pyrite, pyrrhotite, and The Candelaria IOCG deposit (470 Mt at 0.95% Cu, magnetite, and is found as dissemination, pore infill, 0.22 g/t Au, and 3.1 g/t Ag at a cutoff grade of 0.4% Cu; replacement, and veins in amphibolitised rocks belonging Marschik et al. 2000), situated in the Chilean Coastal to a Lower Cretaceous volcano–sedimentary sequence. The Cordillera south of the city of Copiapó, is hosted by copper deposit, dated at 115.2±0.3 Ma (U–Pb on hydro- Early Cretaceous volcanic and volcaniclastic rocks, thermal titanite; de Haller et al. 2006), is located in the core about 1 km east of the sub-alkaline to alkaline, I-type of a dacite–andesite volcanic edifice (116.7± 0.3 Ma, U–Pb plutons of the Chilean Coastal Batholith. The latter have on zircon). Its age is similar, within error, to that of two 87 86 Sr/ Sri (0.7031–0.7032), ɛNd (1.8–2.8), lead isotope tonalite intrusions dated at 114.8±0.4 and 115.1±0.4 Ma (206Pb/204Pb=18.43–18.77, 207Pb/204Pb=15.55–15.60, (U–Pb on zircon, de Haller 2006), respectively, which 567 187 188 occur at the core of the mineralised zone (de Haller Mathur et al.
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