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El Depósito De Magnetita De El Laco (Chile): Evidencias De Una Evolución Magmático

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El Depósito De Magnetita De El Laco (Chile): Evidencias De Una Evolución Magmático macla nº 11. septiembre ‘09 revista de la sociedad española de mineralogía 181 El Depósito de Magnetita de El Laco (Chile): Evidencias de una Evolución Magmático- Hidrotermal / FERNANDO TORNOS (1,*), FRANCISCO VELASCO (2) (1) Instituto Geológico y Minero de España. c/Azafranal 48, 37001 Salamanca (España). (2) Departamento de Petrología y Mineralogía. Universidad del País Vasco. Leoia (España). INTRODUCCIÓN. colada sin dejar restos. La mineralización discordante es texturalmente muy distinta y está El depósito de magnetita de El Laco, con En este resumen se muestran formada por magnetita en grandes más de 500 Mt de magnetita masiva se evidencias de que la mineralización de cristales que se interpretan como localiza en el actual eje magmático de El Laco es compatible con un origen debidos a disyunción columnar. El los Andes a una altura entre 4800 y magmático para la magnetita y que la apatito es mucho más abundante que 5200 msnm. Está relacionado espacial y propia cristalización de ese magma es en el otro estilo de mineralización. cronológicamente con un volcán de responsable de la intensa alteración Aunque forma afloramientos andesita datado en 2.0±0.3 Ma hidrotermal existente. independientes, hay una relación directa (Gardeweg & Ramírez, 1985). La entre estos cuerpos y los estratoides. importancia de este depósito estriba en ASPECTOS GEOLÓGICOS GENERALES. que debido a su carácter sub-actual es La alteración hidrotermal ha afectado a el lugar idóneo para discutir la génesis La mineralización de El Laco está grandes zonas alrededor de El Laco y de los depósitos de magnetita-apatito formada por magnetita masiva en todavía hay una cierta alteración tipo Kiruna y de las mineralizaciones de cuerpos estratoides y discordantes. Los fumarólica. La andesita, principalmente tipo IOCG, de las que se interpreta que cuerpos estratoides se localizan en el la de la base, muestra una alteración forma un término extremo (Hitzman et contacto entre dos de las coladas y con clinopiroxeno, escapolita, cuarzo y al., 1992). Aunque situado a más de están formados por magnetita con muy feldespato K residual remplazada por 200 km del cinturón de depósitos variadas texturas entre las que domina otra dominante con smectita, pirofilita, magnetita-apatito y de tipo IOCG de la magnetita masiva con abundantes caolinita, natroalunita, jarosita, cuarzo, edad cretácica y situado en la costa del vesículas y una potencia de hasta unos yeso, florencita y azufre nativo. Ésta es Pacífico (Sillitoe, 2003), los mecanismos 10 m. En superficie hay estructuras de muy similar a la alteración argilítica genéticos han de ser similares. tipo aa y pahoehoe e incluso en corteza avanzada que se observa en los Actualmente, hay dos modelos para de pan. La base incluye brechas depósitos de oro de alta sulfidización explicar su génesis. La hipótesis caóticas con fragmentos de andesita formados sub-actualmente a altitudes hidrotermal defiende que la alterada hidrotermalmente que aquí se equivalentes. mineralización es fruto del interpretan como brechas basales de remplazamiento hidrotermal de la coladas. En conjunto, estos cuerpos de La geoquímica existente muestra que la andesita encajante y para ello se apoya magnetita guardan grande similitudes magnetita de la zona mineralizada tiene en la existencia de una extensa morfológicas con la lava de volcanes de valores de δ18O entre 3.5 y 5.4‰. Los alteración hidrotermal alrededor del tipo hawaiano. Lo que es muy estudios de inclusiones fluidas (Broman cuerpo de magnetita, la existencia de significativo en estas rocas es la et al., 1999) indican que el piroxeno en fragmentos de andesita alterada presencia de abundantes tubos que la base de la magnetita estratoide tiene hidrotermalmente dentro de la cortan a todo el cuerpo desde la base. inclusiones muy hipersalinas que mineralización, el bajo contenido de Ti Estos tubos están parcialmente rellenos homogeneízan entre 710 y 840ºC en la magnetita y la alta densidad de los de magnetita idiomorfa y clinopiroxeno, mientras que en el apatito éstas son magmas de óxidos de hierro, que a veces con estructura esquelética. En acuosas y con salinidades (0-59 %NaCl imposibilitaría su ascenso a la corteza detalle, la magnetita está cubierta por eq.) y temperaturas de (Rhodes & Oreskes, 1999; Rhodes et al., abundante fluorita y rockbridgeita. Tanto homogeneización (250-350ºC) muy 1999; Sillitoe & Burrows, 2002). El en los tubos como en las vesículas hay variables. modelo magmático (Henriquez & Martin, abundantes minerales tales como 1978; Naslund et al., 2002; Nyström & tridimita, sanidina, natroalunita, LA ANDESITA ENCAJANTE. Henriquez, 1994) se basa en la hematites, apatito, rockbridgeita, thorita existencia de abundantes estructuras y y fluorita. Lateralmente, estos cuerpos La andesita encajante de la texturas indicativas de que la magnetita masivos pasan a rocas clásticas en las mineralización incluye fenocristales de cristalizó a partir de un magma. que se distingue un bandeado grosero y plagioclasa (an39-80), augita y enstatita Además, la estructura de la magnetita abundantes fragmentos de magnetita y en una matriz microcristalina de vidrio y es muy distinta a la de la andesita diadochita que se interpretan como microlitos de plagioclasa. encajante y parece poco probable que bombas volcánicas (Henriquez & Esporádicamente hay fenocristales de se haya remplazado la totalidad de una Naslund 2004). magnesiohastingsita blindada dentro de palabras clave: Magnetita-apatito, Tipo Kiruna, Chile, Inclusiones key words: Magnetite-apatite, Kiruna type, Chile, Melt inclusions, vítreas, Geoquímica, Vulcanismo, Sistema magmático-hidrotermal Geochemistry, Volcanism, Magmatic-hydrothermal system resumen SEM 2009 * corresponding author: [email protected] 182 magnetita. La magnetita rica en Ti es un magma de óxido de hierro inmiscible de Burnham, C.W. (1979): Magmas and mineral muy abundante en estas rocas. la andesita adyacente en una cámara hydrothermal systems. En: Barnes H.L. (ed) magmática profunda produce una Geochemistry of Hydrothermal Ore No hay diferencias petrográficas intensa alteración hidrotermal. La Deposits. Wiley, New York, pp 71-132. Gardeweg, M. & Ramírez, C.F. (1985): Upper geoquímicas apreciables entre las rocas presencia de minerales saturados en Cenozoic Volcanism of Northern Chile: de techo y muro. Los fenocristales agua y de magnetita vesicular sugiere characteristics, age and distribution. Final tienen abundantes inclusiones vítreas. que el sistema estaba saturado en Symposium of Project IGCP 120 En el ortopiroxeno están formadas por fluido. Este magma debía de ser "Magmatic Evolution of the Andes". vidrio, una burbuja de retracción y extremadamente poco viscoso y la Santiago, Chile, pp 93-96. proporciones variables de magnetita elevada proporción de fundentes tales ─, Sparks, R.S.J., Matthews, S.J. (1998). rica en Ti. Son frecuentes los cristales de como el P y el F facilitó su ascenso con Evolution of the Lascar Volcano, Northern anhidrita y apatito. En la plagioclasa la andesita, probablemente en una Chile. Journal Geological Society, 155, 89- 104. estas inclusiones coexisten con otras de pequeña zona extensional durante una Henriquez, F. & Martin, R.F. (1978). Crystal vidrio pardo con texturas de deformación fundamentalmente growth textures in magnetite flows and devitrificación sin magnetita. Los valores transpresiva. Si su comportamiento era feeder dykes, El Laco, Chile. Canadian de δ18O de la andesita varían entre 7.2 y similar al de los magmas silicatados, el Mineralogist, 16, 581-589. 8.3‰. descenso de la presión facilitó la ─ & Naslund, H.R. (2004) El Laco. Field Trip. “ebullición primaria” en el sentido de IAVCEI General Assembly. DISCUSIÓN Y CONCLUSIONES. (Burnham, 1979) y exsolución gradual Hitzman, M.W., Oreskes, N., Einaudi, M.T. de fluidos acuosos fraccionados en el (1992): Geological characteristics and tectonic setting of Proterozoic iron-oxide La presencia de inclusiones vítreas con fundido enriquecido en óxidos de hierro. (Cu-U-Au-REE) deposits. Precambrian diversas proporciones de magnetita La inmiscibilidad por debajo de la solvus Research, 58, 241-287. dentro de los fenocristales de del sistema H2O-NaCl a bajas presiones, Laroque, A.C.L., Stimac, J.A., Keith, J.D., ortopiroxeno sugiere que estos probablemente ampliado por la Huminicki, M.A.E. (2000): Evidence for minerales cristalizaron en presencia de presencia de P y F, sería en forma de open-system behavior in immiscible Fe-S-O una fase inmiscible de óxido de hierro y una muy pequeña proporción de un liquids in silicate magmas: Implications for que ésta se separó durante la fluido hipersalino – del que quedan contributions of metals and sulfur to ore- pocos restos – y una elevada proporción forming fluids. Canadian Mineralogist, 38, cristalización de los fenocristales de 1233-1249. plagioclasa. La discrepancia entre los de vapor atrapado en el apatito. Este Naldrett, A.J. (1969). A portion of the system contenidos en Ti de la magnetita en la vapor, en el que se fraccionan los Fe-S-O between 900 and 1080°C and its andesita (9-11 % TiO2) y en la volátiles, se separaría rápidamente del application to sulfide ore magmas. Journal mineralización (<800 µg/g), uno de los magma y al ascender reaccionaría con Petrology, 10, 171-201. argumentos clave para la hipótesis el agua nival dando lugar a un fluido Naslund, H.R., Henriquez, F., Vivillo, W., hidrotermal, es interpretada como extremadamente ácido y la alteración Dobbs, F.M. (2002): Magmatic iron ores debida al diferente comportamiento del argilítica avanzada que
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