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The Cerro Negro mining district is located in the northwestern margin of the Deseado Massif, 50 km southeast from Perito Moreno city, Santa Cruz province (Patagonia, Argentina). It contains a group of epithermal Au-Ag vein deposits with estimated reserves of 6.7 Moz Au eq., being the most relevant epithermal project in the area and one of the most important recent discoveries in the world. This thesis illuminates the highest grades comprising Eureka West and the Marianas-San Marcos structures at the westernmost part of the district. The geology of the area is characterized by a thick volcanic sequence of Upper Jurassic age, grouped in the Bahía Laura Volcanic Complex, lying on a non outcropping low grade metamorphic rocks with maximum depositional age of 379 ± 4 Ma (Upper Devonian). These rocks are equivalents of the La Modesta Formation (Deseado Massif) and the Eastern Metamorphic Complex (Andean Cordillera of Argentina and Chile). The volcanic sequence initiates with a Lower Section assigned to the Bajo Pobre Formation. It comprises a series of lava flows alternating with ignimbrites of andesitic to dacitic composition, intruded by subvolcanic bodies of the same composition. These rocks are genetically related by a comagmatic trend of intermediate composition, interpreted as the product of a long-lived polygenetic volcanic edifice (>3 Ma). During the last stages of this sequence, and as a consequence of a high geothermal gradient, a large geothermal hydrothermal field (~15 km) developed in the area. Hydrothermal fluids focus on major permeable extensional faults (probably with basement roots) such as Eureka and/or on through secondary order structures (Marianas-San Marcos system) generating shallow Au-Ag epithermal deposits. A broad discordance separates these rocks from the overlying volcanic units, and the latter were grouped in the Upper Section and assigned to the Chon Aike and La Matilde formations. This sequence is characterized by tuffs, ignimbrites and domes mostly of rhyolitic composition, and can be related to a waning stage in the volcanic activity, which turns into more episodic and possible related to one or several monogenetic sources. The mineralization comprises NW, WNW and EW trending structures, with exceptional longitudes, thicknesses and Au-Ag grades. Eureka is the most prominent vein in the district and also in the Deseado Massif, with almost 5kilometers long and up to 20 meters thick. The northwestern portion of this vein is called Eureka West, at which 3 major mineralization phases were recognized: hydrothermal, tectonic and supergene. The first hydrothermal phase is mostly composed of quartz-chalcedony, adularia and Fe-Mn carbonates, arranged in crustiform-colloform banding with a decrease in band thickness and a silica increase toward the center of the banding. Gold appears as electrum (Au40Ag60) in dark ginguro bands related with silver sulfides and sulfosalts along with base metals bearing minerals, ore minerals totalize less than 1% of the vein volume. Calcite veinlets commonly develop late within the vein evolution. The tectonic phase is composed by fault breccias with erratic silver anomalies. A major supergene phase affects this vein down to 400 m below the surface, replacing primary ore minerals by a secondary assemblage, relatively increasing the precious metals contents. The geochemical signature of the vein includes Au, Ag, Pb, Zn, Cu and As, with Ag:Au ratios averaging about 17:1. The hydrothermal alteration assemblage is made of quartz, adularia, illite, smectite and chlorite. Fluid inclusion studies reveal homogenization temperatures of 260oC and salinities of 1-2% NaCl eq., with extensive boiling evidences. Stable isotopes demonstrate a meteoric origin for the hydrothermal fluids, with involvement of a magmatic component at the main mineralization episode. Mineralization ages yielded 156 Ma, overlapping with the youngest ages of the volcanic sequence within the Lower Section. Based on these conclusions, Eureka West is classified as an intermediate to low sulfidation epithermal deposit, shallow equivalent of other vein systems described in the Deseado Massif (e.g. Manantial Espejo and Mina Martha), Sierra Madre in Mexico (e.g. Fresnillo y Veta Madre), and several locations across the world. The Marianas-San Marcos vein system is composed of several quartz (chalcedony)adularia-smectite Au-Ag veins with well-developed colloform-crustiform bandings. Mineralization formed during 3 major hydrothermal episodes and fourth late-stage episode with tectonic features. Ore minerals are related to dark ginguro bands with silver sulfide and sulfosalts and associated base metals minerals. Geochemically, the vein system shows Au, Ag, As, Sb, Zn, Pb, Se and Hg anomalies, with Ag:Au ratios typically <10:1. The hydrothermal alteration minerals evidence an increase of temperatures towards the veins, including quartz, adularia, illite, smectite, illite-smectite and chlorite as mineral indicators. Acid hydrothermal alteration assemblage can be found above the veins, and can be related to the hydrothermal eruption breccia deposits found on top of the vein system. The fluid responsible for mineralization yield homogenization temperatures ranging between 220o and 300oC, and salinities from 1 to 3% NaCl eq., with a strong meteoric signature for the fluids and a possible magmatic source of sulfur and metals. Lately in the paragenetic sequence, steam heated waters collapsed into the vein system, giving raise to widespread calcite veins that dilutes the main mineralization. Based on vein textures, fluid inclusion studies and the identification of several hydrothermal eruption breccia deposits, flashing processes is proposed as the main ore-forming mechanism. This vein system was dated in ~155 Ma, and can also be linked with the youngest andesitic to dacitic magmatism of the Lower Section. The Marianas-San Marcos correspond to a typical shallow low sulfidation epithermal deposits, comparable with Hishikari (Japan); Round Mountain, Sleeper, McLaughlin and Midas (western USA); and Martha Hill-Favona (New Zealand), among others. Based on the close relation with the paleo-watertable, this vein system could also be classified as a hot spring-type epithermal deposit. Finally, the described vein systems are characterized by the preservation of several shallow features of the hydrothermal systems. Hydrothermal eruption breccias deposits, associated with acid alteration, were described as a cover unit at the Marianas veins, with surfaces over 1 km size. These deposits are composed of two main vein-bearing breccia facies, separated by two layers of reworked volcaniclastic material, showing pauses within explosive events. Moreover, shallow features expressed as hot spring travertine deposits developed in fluvio-lacustrine environments are preserved to the southeast in close association with Eureka vein. Cerro Negro represent the best example of high grade Au-Ag deposits of the Geothermal Model proposed for the area, with the preservation of two subtypes (proximal and distal to intrusives), contributing with a variety of unknown mineralogical, geochemical and exploration features to the Deseado Massif metallogenic province. Agradecimientos El autor agradece a las siguientes instituciones, compañías y personas que han colaborado con el presente trabajo de Tesis Doctoral: -A mi director Dr. Diego Guido, por su apoyo incondicional, por sus valiosas sugerencias y por estar permanentemente dispuesto a mis consultas y dudas. -A las empresas Oroplata S.A y Goldcorp Inc. por el apoyo económico y logístico que permitieron la realización de este trabajo de investigación. En especial a Gassaway Brown, Damián Echavarría, Analía Bruna y a todo el equipo geológico y técnico de Cerro Negro. -A mis compañeros del INREMI, a su director Dr. I. B. Abel Schalamuk, y en especial a Sebastián Jovic por haberme acompañado en varias campañas, como también a Gerardo Páez, Remigio Ruiz y Luciano López por su invaluable ayuda a lo largo de estos años. -Al Dr. Joan Carles Melgarejo de la Universidad de Barcelona (España) por su ayuda en la realización de las mediciones con microsonda electrónica. -A Javier Ríos y su grupo de trabajo del CDTN (Belo Horizonte, Brasil), a Pablo Leal y Teresita Montenegro del Departamento de Geología de la UBA, a Daniel Moncada, a Robert Bodnar, y su grupo de trabajo del Laboratorio de Inclusiones Fluidas de la Universidad de Virginia (EEUU) por su asesoramiento y aporte durante los estudios de inclusiones fluidas. -A la María Eugenia Rodríguez por la ayuda en las interpretaciones realizadas mediante ASD. A Pablo García, Daniel Poiré y al Centro de Investigaciones Geológicas (CONICETUNLP) quienes aportaron los análisis de difracción de Rayos X. -A la Lic. Manuela Salazar por su colaboración desinteresada en el estudio de las brechas hidrotermales. Índice Capítulo 1. Introducción 1.1. Objetivo  1 1.2. Antecedentes de exploración y su descubrimiento  5 1.3. Plan de Tesis  7 1.4. Esquema de la tesis  8 Capítulo 2: Geología del oeste del distrito auroargentífero Cerro Negro, Macizo del Deseado, Argentina 2.1. Introducción  10 2.2. Geología regional del Macizo del Deseado  10 2.3. Geología del distrito Cerro Negro  13 2.3.1. Basamento Pre-Jurásico

Caracterización detallada de la mineralización en veta Eureka, y su comparación con otras mineralizaciones del Distrito Cerro Negro, Macizo del Deseado, Santa Cruz, Argentina