TOMO 2 - Geología Económica

THE RIO GRANDE COPPER-GOLD PORPHYRY DEPOSIT, , . Kevin B. Heather, Vice President Geology, Antares Minerals Inc., Casilla 730, La Serena . [email protected] Javier Robeto, Geologist, Minera Antares Argentina S.A., Los Ciruelos 121-B° Tres Cerritos 4400-Salta, Argentina. [email protected] Christian Hub, Senior Geologist, Minera Antares Argentina S.A., Los Ciruelos 121-B° Tres Cerritos 4400-Salta, Argentina. [email protected] John E. Black, President and CEO, Antares Minerals Inc., 8723 Fairview Oaks Ln. Lone Tree, CO 80124. [email protected] Jorge Kesting, Vice President, Mansfield Minera S.A., Av. Belgrano 1499 (A4400AWM) Salta, Argentina. [email protected] Facundo Huidobro, Geologist, Mansfield Minera S.A., Av. Belgrano 1499 (A4400AWM) Salta, Argentina. [email protected] INTRODUCTION The Rio Grande copper-gold deposit is an emerging, potentially giant porphyry-style deposit located on the southern edge of Salar Arizaro, western Salta Province in northwest Argentina, ap- proximately 260 kilometres west of the city of Salta and 40 kilometres east of the Chilean border (Figure 1). The project is located only 12 km west of the Linderos Au-porphyry and Arizaro Cu porphyry prospects (Minera Mansfield S.A.).

The Rio Grande project is a grass-roots discovery made by Minera Mansfield S.A. (Mansfield) in 1999. In 2000-2001, Teck Corporation (“Teck”) optioned the property and completed 11 diamond drill holes totalling 3220.6 metres. Additional work on the property was recommended; however, Teck terminated its exploration efforts in Argentina in early 2002, and returned the property to Mansfield Minerals Inc. In 2004, Antares Minerals Inc. (“Antares”; ANM-TSX.V) entered into a joint venture whereby it can acquire a 60% interest in the Rio Grande project from Minera Mansfield S.A. by completing payments totalling US$600,000 and work commitments totalling US$4.5 million over a five and a half-year period; plus granting 900,000 shares.

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GENERAL GEOLOGY

The Rio Grande property is a large porphyry copper-gold deposit with mineralization covering a large area measuring at least 4 sq. km. and consisting of disseminated and veinlet-controlled magnetite-chalcopyrite-gold. Primary copper sulphides have been weathered to secondary, black and blue-green copper oxides to depths in excess of 200 m. A thorough review of work previously completed by Teck Corporation (“Teck”) in 2000-2001, as well as new geological and structural mapping by Antares, has identified a series of target areas with potential for higher copper-gold grades within a distinct 2 km diameter ring of IP chargeability and anomalous surface and subsurface copper and gold geochemistry. The 200 m average estimated depth to the top of the IP anomalies suggests that there is potential for an extensive area of oxidized mineralization that may be amenable to SX-EW recovery of copper. The Rio Grande copper-gold prospect is located within a partially eroded, earliest Middle Miocene (16.5 Ma), andesite-dominated volcanic-intrusive complex of high-K, calc-alkaline affinity. The Rio Grande complex intrudes through granitic basement rocks into a continental back-arc basin filled with a thick (>1500m), oxidized sequence of continental red bed sandstones. Sections within this sequence contain sulphate evaporites and possible halite in the upper parts of the section. The Rio Grande complex is localized along the ESE-trending Archibarca lineament; where it is intersected by NNE-trending “East Fissure” parallel structures. Host rocks include a wide variety of dacite to andesite, sub-volcanic, hypabyssal intrusive rocks and dykes. A large area of hydrothermal alteration is centered on the Rio Grande volcanic- intrusive complex. An early, locally intense potassic (K-feldspar type) alteration is overprinted by a weakly to intensely developed Ca-Fe-Na alteration consisting of variable amounts of diopside, magnetite, scapolite, and actinolite. The Ca-Fe-Na alteration is locally transitional to, and overprinted by, a late potassic (Biotite type) alteration. A late, strong supergene oxidation and leaching of the aforementioned rocks is commonly observed; especially within well fractured rocks and fault zones.

264 TOMO 2 - Geología Económica

Figure 1: Landsat TM image of the Rio Grande Cu-Au porphyry deposit. MINERALIZATION TYPES The Rio Grande property potentially hosts four (4) distinct mineralization styles: 1) Cu-Au Oxide (#7, North, Sofia, Discovery) a) Occur in a circular pattern centered on the Rio Grande complex b) Low sulphide c) Low quartz d) Strong fracture control e) Associated with Ca-Fe-Na / Biotite alteration f) Magnetite common g) Black and blue-green copper oxides after chalcopyrite 2) Cu ± Au Deep Sulphide (Porphyry) a) Centered on & possibly 400-600m depth in the middle of the circular pattern centered on the Rio Grande complex

265 XI CONGRESO GEOLOGICO CHILENO

b) Pyrite - Chalcopyrite c) Possibly associated with classic quartz stockwork d) Associated with potassic alteration e) Magnetite common in area above 3) Structurally-controlled Au ± Cu (not shown) a) Associated with Archibarca trend faults and fracture zones b) Associated with limonitic oxidation zones c) After Pyrite (± Chalcopyrite)? d) No obvious quartz veining e) Appears to overprint the Cu-Au style (1) of mineralization 4) Porphyry Au ± Cu (Northeast) a) Associated with diorite intrusions b) Associated with quartz-magnetite veinlets c) Appears to overprint the Cu-Au style (1) of mineralization

Figure 2: Location of Rio Grande mineralized zones and drill holes (2000-2005) on a Quick Bird image. The alteration and mineralization style of the Rio Grande Cu-Au porphyry deposit are not typical of “classic” porphyry systems. Rio Grande has aspects of both the IOCG and high-K to alkalic type porphyry systems.

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