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Raga 63.4 Corregida.Qxp Revista de la Asociación Geológica Argentina 63 (4): 665 - 681 (2008) 665 LOS DEPÓSITOS DE ORO Y PLATA VINCULADOS AL MAGMATISMO JURÁSICO DE LA PATAGONIA: REVISIÓN Y PERSPECTIVAS PARA LA EXPLORACIÓN Raúl R. FERNÁNDEZ1,2, Adriana BLESA1,3,a, Pilar MOREIRA1,3, Horacio ECHEVESTE1,2, Karina MYKIETIUK1,3,b, Pablo ANDRADA DE PALOMERA4 y Mario TESSONE1 1 Instituto de Recursos Minerales (INREMI). Facultad de Ciencias Naturales y Museo (UNLP-CICBA) La Plata. Email: [email protected] 2 Comisión de Investigaciones Científicas de la provincia de Buenos Aires (CICBA) 3 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) 4 Fomento Minero de Santa Cruz (FOMICRUZ S.E.). Río Gallegos, Santa Cruz. a Actualmente RTZ Argentina b Actualmente Medanitos SA RESUMEN La importancia del magmatismo jurásico de la Patagonia con relación a mineralizaciones de Au-Ag, ha sido reconocida desde hace menos de 20 años y al presente se definieron recursos por más de 700 t de Au equivalente contenido, en tres depósitos de clase mundial (>100 t AuEq contenido), algunos depósitos menores y numerosos prospectos en exploración. Se alojan en rocas volcánicas y volcano-sedimentarias jurásicas (o más antiguas), silíceas y mesosilíceas generadas en ambientes de arco y retro-arco de las regiones andina y extra-andina. Por sus características mineralógicas, texturales y geoquímicas corresponden a depósitos epitermales asociados principalmente a la paragénesis cuarzo + calcita + adularia + illita que acompañan a los depósitos de sulfuración baja e intermedia. Por su contenido metálico se dividieron en: a) Au-Ag y Ag>Au, b) polimetálicos con Ag-Au o sólo Ag, y c) polimetálicos complejos con Ag-Au. Son depósitos vetiformes, en general con control estructural, con una excepción de fuerte control litológico. Dataciones de minerales hidrotermales y roca de caja indican que la actividad hidrotermal fue posterior al magmatismo principal. Relacionadas a ellos se hallan manifestaciones hidrotermales poco profun- das (sinter y silicificaciones en el paleonivel freático) que pueden indicar que sistemas epitermales se hallan intactos en pro- fundidad. Los fluidos formadores de mineralización fueron soluciones diluidas o de baja salinidad, con temperaturas entre 160 y 330ºC. Los controles más importantes en la formación de estos depósitos se consideran el fracturamiento extensional que afecta a las volcanitas jurásicas y la incorporación de aguas freáticas a los sistemas geotermales. Palabras clave: Oro-Plata, Depósitos epitermales, Jurásico, Exploración, Patagonia. ABSTRACT: Gold and silver deposits related to Jurassic Patagonian volcanism: review and perspectives for exploration. Potential for gold-silver mineralization related to Jurassic magmatism in Patagonia was only recognized 20 years ago. At the present the Au+Ag resour- ces exceed 700 t in three world class deposits (> 100 t Au Eq), several smaller ore deposits and many prospects in exploration stage. The deposits are hosted in silicic and mesosilicic volcanic and volcanic-sedimentary Jurassic rocks (occasionally older) related to arc or back-arc settings in Andean or extra-Andean environments. The ore geology, textures, mineralogy and geoche- mistry of these mineralizations indicate that they are epithermal deposits, and are mainly associated to the quartz + calcite + adularia + illite assemblage of the low and intermediate sulfidation types. Based on their metallic association, the deposits were divided into: a) Au-Ag and Ag>Au, b) polymetallic with Ag-Au or only Ag and, c) complex polymetallic mineralizations with Ag-Au. The deposits form tectonically controlled veins with only one example where strong lithological control dominates. The few hydrothermal minerals and hostrocks dates, suggest that the hydrothermal activity is several millions years younger than the peak magmatism. The sinter, carbonate-lacustrine deposits and silicic lithocaps related to steam-heated waters in the water table suggest regions or tectonic blocks with potential for epithermal systems at depth. The mineralizing fluids are dilute to low-inter- mediate salinity waters with temperatures between 160 to 330ºC. Extensional fracturing developed in the Jurassic volcanics and the influx of meteoric waters in the geothermal systems are considered the main controls in the ore genesis. Keywords: Gold-silver, Epithermal deposits, Jurassic, Exploration, Patagonia.. INTRODUCCIÓN nes sobre los depósitos minerales de Ar- mienza en 1980 su tercera actualización gentina (Angelelli 1950 y Angelelli et al. (Angelelli 1984). En esos años, algunos Luego de las fundamentales contribucio- 1970), el profesor Victorio Angelelli co- de los autores de esta contribución (RRF 666 R.R. FERNÁNDEZ, A. BLESA, P. MOREIRA, H. ECHEVESTE, K. MYKIETIUK, P. ANDRADA DE PALOMERA Y M. TESSONE Figura 1: Evolución de los descu- brimientos de mineralización de Au y Ag en la Patagonia, mostrada como sumatoria de recursos expre- sados en toneladas de oro equiva- lente (AuEq) según la relación eco- nómica Au:Ag = 1:60. Los recursos son aproximados y se basan en información, de distinto tipo, reco- pilada para esta revisión y citada posteriormente. Las curvas de coti- zación de Au y Ag se tomaron de www.kitco.com, febrero 2008. y HE) trabajaban con él en su reciente- mente a los eventos magmáticos ocurri- un importante objetivo regional para la mente creado Instituto de Recursos Mi- dos en el Jurásico Superior-Cretácico In- búsqueda de depósitos de Au-Ag. Las re- nerales de la Facultad de Ciencias Natu- ferior en la cordillera Patagónica que rela- laciones entre los ambientes volcánicos rales y Museo (Universidad Nacional de cionaba con la generación de depósitos jurásicos y las mineralizaciones auríferas La Plata) y atendían sus comentarios polimetálicos, portadores en algunos cas- del Macizo del Deseado fue señalada por acerca de la falta de información sobre la os de Au y Ag. En la región extrandina Fernández y de Barrio (1994), Schalamuk edad de depósitos minerales y por lo este autor incluyó al distrito Los Manan- et al. (1995) y Schalamuk et al. (1997). tanto su asignación con dudas a una de- tiales. Posteriormente, el interés se extiende y terminada época metalogénica. Tomando No sólo eran escasos los datos geocrono- acrecienta en toda la Patagonia, condu- como base las edades de depósitos de la lógicos de mineralizaciones jurásicas, si- ciendo a otros importantes descubri- cordillera andina y la importancia de los no que tampoco se conocían muchas mi- mientos. procesos formadores de mineralizacio- neralizaciones de oro y plata en la región La evolución de los hallazgos, desde el nes en este orógeno, varios autores cita- patagónica. En las actualizaciones sobre tiempo en que se definieron las reservas dos en Angelelli (1984) proponían edades la geología regional argentina no existía de la mina Angela, se esquematiza en la del Cretácico Superior al Terciario para la ninguna información que indicara la pre- Fig. 1. Este gráfico muestra el incremen- mayor parte de las mineralizaciones hi- sencia de estas mineralizaciones en el to de los recursos de oro y plata de la Pa- drotermales de la Patagonia que, si bien Macizo del Deseado, por lo que no figu- tagonia con los sucesivos descubrimien- se alojaban en volcanitas jurásicas, estarí- raba como una provincia metalogénica tos de mineralizaciones alojadas en las an vinculadas con el magmatismo de esas (Angelelli y Fernández Lima 1980, An- volcanitas jurásicas. También puede verse edades. Un solo depósito (polimetálico gelelli 1984). Es a partir del descubri- en la Fig. 1, que esa evolución no está portador de Au y Ag), situado en las cer- miento de mineralizaciones auríferas en necesariamente relacionada con los pre- canías del lago Fontana, fue asignado por el área de cerro Vanguardia (Genini cios de los metales, aunque los recursos Angelelli (1984) al ciclo metalogénico ci- 1988) y fundamentalmente por la defini- pueden aumentar con las cotizaciones mérico (Triásico Superior-Cretácico). ción de sus importantes recursos/reser- más elevadas debido a la incorporación Contemporáneamente, Ramos (1983) vas en 1995 (información impresa pro- de zonas de menor ley (o viceversa). destacaba la importancia metalogénica porcionada por Cerro Vanguardia S.A.), El objetivo de este trabajo es brindar una del ciclo patagonídico (Triásico Superior que el volcanismo jurásico del Macizo del reseña sobre los depósitos de metales a Cretácico Superior). Se refería especial- Deseado comienza a considerarse como preciosos (oro y plata) asociados al mag- Los depósitos de oro y plata vinculados al magmatismo jurásico de la Patagonia... 667 matismo jurásico de la Patagonia, los básicas algo más jóven de ~177 Ma. En de Somún Curá y en la denominada avances en su exploración y los éxitos la porción occidental en cambio, las uni- cuenca de Cañadón Asfalto (Márquez et obtenidos en una región y lapso temporal dades volcánicas dominantes son de al. 1988). En esta última, el prospecto que comenzó a tenerse en cuenta y des- composición andesítica, acompañadas Fortuna está actualmente en etapa de arrollarse hace no más de 20 años. Al- por escasas volcanitas riolíticas y basálti- exploración mediante sondeos. gunas de las conclusiones a las que se cas, que conforman la Formación o Gru- En los años posteriores al 2000 comen- arriba en esta revisión indican sus pers- po Lonco Trapial (u otras unidades equi- zaron a desarrollarse otros prospectos pectivas y podrían ser útiles para futuros valentes); la edad de este volcanismo no que, al menos en la asociación
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