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Título Aquí (Times New Roman 16, Negrita, Centrado) XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S11_037 Strontium and Sulfur Isotope Signatures Related to Chilean Magnetite(-Apatite) Mineralization Marschik, R.1, Akker, B.1, Rieger, A.A.1, Spangenberg, J.E.2, Hölzl, S.3 (1) Economic Geology Research Group, Ludwig-Maximilians Universität, Luisenstrasse 37, 80333 Munich, Germany (2) Institut de Minéralogie et Géochimie, Université de Lausanne, Anthropole, 1015 Lausanne, Switzerland (3) Bayerische Staatssammlung für Paläontologie und Geologie, Richard-Wagner-Str. 10, 80333 Munich, Germany [email protected] Introduction The magnetite(-apatite) deposits of the Chilean Iron Belt occur in the Chilean Coastal Cordillera between latitudes 25°30' and 31°00'S. The genesis of these iron deposits is controversial, as some authors suggest an orthomagmatic origin while others favor hydrothermal genetic models [1]. Magmatic-hydrothermal, non-magmatic hydrothermal, or hybrid (fluid mixing) models are proposed. The magnetite(-apatite) deposits are of particular interest because of their similarities with IOCG deposits, which occur in a similar setting and constitute an important source of copper and gold. In this paper, we discuss new strontium and sulfur isotope data from these iron deposits. Geology of the Chilean Iron Belt Most of the magnetite(-apatite) deposits of the Chilean Iron Belt are hosted in Jurassic or Lower Cretaceous andesitic volcanic or volcaniclastic rocks adjacent to intermediate, predominantly calc-alkaline plutons of the Chilean Coastal Batholith. The igneous rocks formed in a volcanic-arc environment. An associated marine back-arc basin, locally with evaporites, had developed to the east of the arc. The magnetite(-apatite) ores show a close spatial association with the Atacama Fault Zone (AFZ), which is a major wrench-fault system that is related to the oblique subduction of the Pacific plate under the South American continent. The magnetite(-apatite) ores mainly occur as pervasive replacement bodies and veins. The latter are commonly emplaced into splays of the AFZ. The metallic mineralization is characterized by abundant magnetite and subordinately hematite(- 1 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 martite), with minor pyrite ± trace chalcopyrite. The ores are spatially related to Na or Na-Ca alteration, which affects the volcanic host rocks and usually also the marginal portions of adjacent plutonic rocks. Sodic or sodic-calcic alteration is commonly manifested by variable proportions of albite, marialitic scapolite, calcic amphibole, and epidote. Apatite is common in some of the deposits but is relatively scarce in others. The ages of magnetite(-apatite) ores are only poorly constraint. Radiometric ages reported from the Chilean Iron Belt are between 100 to 131 Ma [1]. Geochemical Analysis Strontium isotope analysis was carried out on intensely hydrothermally altered rocks. Strontium concentrations in the analyzed rocks range from 54 to 1379 ppm, whereas Rb contents are up to 73 ppm. Initial 87Sr/86Sr were calculated based on an age of 116 Ma. The Sr isotope signatures are shown relative to relevant Sr reservoirs in Fig. 1 [2,3,4,5,6,7]. Sulfur isotope analysis was conducted on pyrite from magnetite(-apatite) bodies. Sulfur isotope ratios are between -2.2 and +3.6‰ δ34S relative to VCDT. Discussion and conclusions Magmatic and non-magmatic inputs into the magnetite(-apatite) hydrothermal systems have long been suggested, and several studies provide strong evidence for non-magmatic fluid components in these systems. Chlorine and F distribution in apatite from Carmen and Fresia mines led to conclude that Fe-P rich magmatic vapour mix with meteoric fluids that scavenged Ca and Cl from country rocks [8]. Fluid inclusions in apatite from El Romeral have relatively high Cl/Br values and δ37Cl near 0‰. These data are consistent with evaporite-derived hydrothermal fluids or with brines equilibrated with evaporites [9]. However, a magmatic fluid contribution has not been ruled out, because depending on the actual Cl/Br value of the evaporitic source, as little as <5 wt.% of an evaporitic fluid could mask a magmatic hydrothermal component [9]. Based on high Os initial ratios for magnetite from the Chilean Iron Belt, a possible involvement of basinal brines with strong crustal Os signatures has been discussed [10]. Initial 87Sr/86Sr ratios of intensely hydrothermally altered host rocks and S isotope ratios of pyrite from magnetite(-apatite) deposits were determined to further investigate on possible fluid sources. Most of the altered host rocks show a similar range of initial 87Sr/86Sr as the Jurassic - Early Cretaceous magmatic-arc igneous rocks of northern Chile (Fig. 1). One sample from above the Los Colorados ore body, however, is significantly more enriched in radiogenic Sr. Taking into account an Early Cretaceous age for the magnetite(-apatite) mineralization, the Sr isotope signature of a magmatic-hydrothermal fluid should be essentially the same as that of coeval plutons of the Coastal Batholith (i.e. around 0.7030- 0.7036 [2,6]; Fig. 1). Therefore, the range of Sr isotope values derived from the hydrothermally altered rocks could result from mixing of magmatic-hydrothermal fluids 2 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 with non-magmatic aqueous fluids (seawater or evaporitic brines). Sulfur isotope ratios of pyrite span a relatively narrow range around 0 ‰ δ34S, which is consistent with an igneous sulfur source. Evaporite-derived sulfur is significantly more enriched in 34S. Jurassic evaporites in the Aconcagua-Neuquén basin have δ34S from 17.2 to 18.5 [4]. Therefore, sulfur may be contributed directly by magmatic-hydrothermal fluids or leached from the igneous rocks by hydrothermal fluids. In general, the Sr and S isotope data are suggestive of a magmatic-hydrothermal fluid component in the ore-forming systems of the Chilean Iron Belt. These and the previously published data are compatible with a fluid mixing model, which is also proposed for the formation of IOCG deposits [9]. Acknowledgements We tank Leonardo Vergara O., Compañía Minera del Pacífico, and Serjio Ardiles C. and Hernan Gonzales V., Compañía Minera Huasco S.A., for their kind support and Martin Reich for constructive comments. This project was funded by the German Research Foundation (DFG). References [1] Moreno, T., Gibbons, W. (2007) The geology of Chile. The Geological Society of London, 414 p. [2] Berg K, Baumann, A (1985) Plutonic and metasedimentary rocks from the Coastal Range of northern Chile: Rb-Sr and U-Pb isotopic systematics. Earth and Planetary Science Letters, vol. 75, 101-115. [3] Claypool GE, Holser WT, Kaplan IR, Sakai H, Zak I (1980) The age curves of sulphur and oxygen isotopes in marine sulphate and their mutual interpretation. Chemical Geology, vol. 28, 199-260. [4] Lo Forte, G.L., Ortí, F., Rosell, L. (2005) Isotopic chracterization of Jurassic evaporites. Aconcagua-Neuquén basin, Argentina. Geologica Acta, vol. 3, 155-161. [5] McNutt RH, Crocket, JH, Clark AH, Caelles JC, Farrar E, Haynes SJ (1975) Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26°- 29°S. Earth and Planetary Science Letters, vol. 27, 305-313. [6] Marschik, R., Fontignie, D., Chiaradia, M., Voldet, P. (2003) Geochemical and Nd- Sr-Pb-O isotope characteristics of granitoids of the Early Cretaceous Copiapó plutonic complex (27º30’S), Chile. Journal of South American Earth Sciences, vol. 16, 381-398. [7] Veizer J, Ala D, Azmy K, Bruckschen P, Buhl D, Bruhn F, Carden GAF, Diener A, Ebneth S, Godderis Y, Jasper T, Korte C, Pawellek F, Podlaha OG, Strauss H (1999) 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chemical Geology, vol. 161, 59-88. 3 XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 [8] Treloar, P.J., Colley, H. (1996) Variations in F and Cl contents in apatites from magnetite-apatite ores in northern Chile and their ore-genetic implications. Mineralogical Magazine, vol. 60, 285–301. [9] Chiaradia, M., Banks, D., Cliff, R., Marschik, R., de Haller, A. (2006) Origin of fluids 37 87 86 in South American iron oxide-copper-gold deposits: Indications from δ Cl, Sr/ Sri and Cl/Br. Mineralium Deposita, vol. 41, 565-573. [10] Mathur, R., Marschik, R., Ruiz, J., Munizaga, F., Leveille, R.A., Martin, W. (2002) Age of mineralization of the Candelaria iron oxide Cu-Au deposit and the origin of the Chilean Iron Belt based on Re-Os isotopes. Economic Geology, vol. 97, 59-71. Figure 1. Strontium signatures of altered rocks from magnetite(-apatite) deposits and of relevant Sr reservoirs. J. Jurassic; E.C. Early Cretaceous 4.
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