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Geology and Discovery of Escondida Este Porphyry Cu-Mo Deposit, Escondida Cluster District, Northern Chile Alejandro Pizarro (1)*, Marco Ipinza (2), Walter Véliz (1)

(1)Minera Escondida Ltda. Avenida La Minería #501, Antofagasta, Chile. (2)CODELCO NORTE. Calle 3 Poniente 2778, Villa Exótica, Calama, Chile.

*E-Mail: alejandro.aa.pizarro@bhpbilliton.com

Abstract. Escondida Este is the most recent of several large porphyry copper-molybdenum deposits discovered in the supergiant Late Eocene-Early Oligocene Escondida district in northern Chile. Minera Escondida Ltda (57.5% BHP Billiton; 30% Rio Tinto and 12.5% JECO corporation) has been conducting a brownfield exploration program since 2000. Major successes include the discovery of the concealed Pampa Escondida porphyry Cu-Au deposit in late 2006, followed by the discovery of Escondida Este deposit in 2009. Both discoveries resulted from reinterpretation of historical data and changing conceptual and geological models developed in recent years. The Escondida Este target was successfully tested by deep drill holes, resulting in the discovery of two mineralization zones: the northern orebody in early 2009 and the southern one in late 2010. Keywords: Escondida Este, Late Eocene-Early Oligocene, Brownfield Exploration 1. Introduction Escondida Este is located immediately to the east of the Escondida deposit and the two deposits, while distinct in age, overlap with each other at depth (Figure 1). The newly discovered porphyry system, extending 4.5 km north-south by 1 km east-west, is composed of two centers of mineralization that are genetically related to a granodioritic porphyry complex that largely postdate mineralized feldspar porphyries in the Escondida deposit. The northern orebody lies underneath a nearly barren quartz porphyry dome that post-dates Escondida mineralization, whereas the southern mineralization zone is concealed by a sequence of Miocene gravels (Figure 5). 2. Geology Escondida Este porphyry system is hosted by a succession of bimodal volcanic and intrusive rocks of Carboniferous-Permian La Tabla Formation, Paleocene-Eocene andesite of Augusta Victoria Formation and also by the Escondida Intrusive Complex (~38-35 Ma; U-Pb zircon dating) (Figure 2). All these units are intruded by the “Escondida Este Intrusive Complex”, which composed of a pre-mineral phase of dioritic-monzodioritic intrusives (~36

Ma; U-Pb zircon dating), an early-mineral intrusive phase characterized by multiple pulses of granodioritic porphyry stocks (34.5-33.7 Ma; U-Pb zircon dating) and a late-mineral porphyry stock of granodioritic composition (33.3 Ma; U-Pb zircon dating) (Figure 6A).

Figure 1. Location of Escondida Cluster and Escondida Este system. Contour lines represent >0.3 % Cu isopleth.

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Figure 2. A. Andesitic host rock. B. Early granodioritic porphyry of Escondida Este. 3. Alteration The bulk of copper mineralization is paragenetically and spatially related to early gray-green sericite alteration (31.8 Ma; Ar-Ar sericite dating) that is the strongest hydrothermal event within the potassic stage and is centered at the top of the early-mineral porphyry stocks. This alteration event overprints early K-feldspar and biotite assemblages and also the eastern side of the older Escondida mineralization system (Figure 3 and Figure 6B).

Figure 3. A. Green gray sericite with bornite ore. B. Green gray sericite with chalcopyrite ore. 4. Mineralization Disseminated and vein-hosted hypogene copper mineralization, mostly fractionated in the potassic alteration stage, is dominated by chalcopyrite and bornite with minor chalcocite (Figure 4 and Figure 6C). The top of the high grade hypogene mineralization zone is encountered approximately 900 meters below the current surface. Although mineralization at Escondida Este is predominately hypogene, an immature supergene chalcocite blanket was developed in the uppermost part of

the southern orebody, 150 meters beneath current surface. Three pulses of molybdenum mineralization are recognized by spatial and temporal relations between 34 and 33 Ma (Re-Os molybdenite dating), associated in part with early potassic alteration stage and in part with subsequent intrusion of the late-mineral granodioritic porphyry (Figure 6D).

Figure 4. A. Hypogene ore from Escondida Este with Chalcopyrite-Bornite and Magnetite association. B. Polished section with chalcopyrite-bornite exsolution texture. 5. Conclusions

The large size of Escondida Este deposit has been interpreted as a hydrothermal system with multiple porphyry phases, which led to continuous metal contribution over a ~2 Ma period. 6. Acknowledgment The contributions of all past members of the Minera Escondida brownfields exploration team, to the support and recommendations from each of the Escondida partners. Permission to present from Minera Escondida Ltda. and BHP Billiton. We would like to appreciate the assistance and contributions from Rick Preece, Geoff Ballantyne, Walter Véliz and Leonardo Retamal with presentation. 6. References Pizarro, A., Ipinza, M., Zuñiga, G., y Araya, N. (2013). Geología y

Descubrimiento del Pórfido Cu-Mo Escondida Este. Reporte Interno, Gerencia de Exploraciones, Minera Escondida Ltda.

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Figure 5. Show two mineralized center of Escondida Este and U-Pb ages. Red dot represent >0.3 % Cu isopleth plan projection and location of cross section B-B’.

Figure 6. A. Cross section B-B’-Lithology. B. Cross section B-B’-Alteration and C. Cross section B-B’- Cu Mineralization and D. Cross section B-B’-Mo mineralization.