Publication: La alteración hidrotermal asociada al plutón epizonal de Navalcubilla, Sierra del Guadarrama (Sistema Central Español)
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1993
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Sociedad Geológica de España
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Abstract
El plutón de Navalcubilla está formado por granitoides peralumínicos ricos en sílice y pobres en agua de emplazamiento epizonal (P "" 100 MPa) y en relación con estructuras extensionales. En relación espacial con esta intrusión se observan distintas alteraciones hidrotermales a las que se asocian mineralizaciones de W-(Cu-Zn-Sn). Se han reconocido dos etapas de actividad hidrotermal. La primera (291.4 ± 9 Ma), va ligada probablemente a fluidos magmáticos, y produce una alteración feldespática, relleno de cavidades miarolíticas, greisenes de alta temperatura y filones con wolframita. La segunda alteración es probablemente bastante más reciente (267.1 ± 7.4 Ma) y superficial ("" 650 mt.) y da lugar a la formación de greisenes de baja temperatura y reemplazamiento s de la wolframita anterior por scheelita y sulfuros. Este esquema es muy similar al observado en diversos sistemas hidrotermales perigraníticos. El estudio de inclusiones fluidas y consideraciones basadas en el equilibrio fluido-roca muestran que la alteración tardía (Etapa TI) tuvo lugar a bajas presiones de fluido, a lo largo de la curva líquido-vapor y evolucionando el sistema de cerrado a abierto con la
temperatura en descenso. Los fluidos fueron de salinidad baja a moderada «11.5 % peso NaCl eq.) y pertenecientes al sistema H,O-Na-K-Ca-Mg-Cl, con pequeñas cantidades de CO, y CH4• Las inclusiones fluidas y la composición de los filosilicatos muestran que esta alteración tuvo lugar a temperaturas entre 380 y 250°C en un rango intermedio de fugacidades de oxígeno y azufre (fOz = 10-34.5_10-34.5 bars;
fS, = 10-9_10-10 bars), dentro del campo de estabilidad de la pirita. El análisis químico de los fluidos
contenidos en las inclusiones fluidas de los filones y las estimaciones termodinámicas sobre la composición del fluido en el greisen permiten calcular la solubilidad de los metales en el ambiente de transporte y precipitación. La greisenización es una importante trampa geoquímica, en la que los metales precipitan por la reacción fluido-roca, que aumenta el pH ácido original del fluido ("" 4) a valores próximos a la neutralidad (pH "" 5.4). La escasez de casiterita y la ausencia de scheelita primaria se explican como resultado de los contenidos muy bajos de Sn y W en el fluido hidrotermal. La temperatura y la relativamente elevada rrU:Ca del fluido de esta etapa son los factores responsables de la formación de scheelita secundaria a partir de la wolframita. La escasez de arsenopirita es consecuencia del carácter moderamente oxidante del medio.
The Navalcubilla pluton consists of water-poor peraluminous leucogranites emplaced at shallow depths (P,," 100 MPa) along extensional structures. Allied to thi,s pluton several hydrothermal alterations are found, that contain W -(Cu-Zn-Sn) ores. Two events of hydrothermal alteration are present. The first (291.4 ± 9 Ma) is probably linked to magmatic fluids and consists of an alkali feldspar metasomatism, fillings of miarolitic cavities and development of high temperature greisens and veins with wolframite. The second event is more recent (267.1 ± 7.4 Ma) and shallow ("" 650 mt.) and gave rise to low temperature greisens and the replacement of earlier wolframite by scheelite and sulphides. This evolution is similar to that found in other perigranitic hydrothermal systems. Fluid inclusions work and inferences from fluid-rock considerations show that the second alteration event (Stage TI) took place at low fluid pressures along the liquid-vapor curve, the system evolving from closed to open with decreasing temperatures. Fluids were low to medium in salinity «11.5 wt % N aCl equiv.) and can be described by reference to the system H,O-Na-K-Ca-Mg-Cl, with little amounts of CO, and CH4• The alteration temperature, as deduced from fluid inclusions and phyllosilicate composition" was in the range 380-250°C. Calculated oxygen and sulphur fugacities are within the pyrite stability field in agreement with petrographic observations (fOz = 10-30.5_10-34.5 bars; fS, = 10-9_ 10-10 bars). Chemical analysis of fluids contained in fluid inclusions from veins and estimation of the composition of fluid in equilibrium with greisen by thelmodynamic considerations, allow to predict the solubility of metals during transssport and precipitation. Greisenization is an important geochemical trap, with metal s precipitating by fluid-rock interaction. This rises the pH of the original fluid ("" 4) to close-to-neutrality values (pH,," 5.4). The scarcity of cassiterite and the absence of primary scheelite, are explained as resulting from the very low concentrations of Sn and W in the hydrothermal fluid.
The Navalcubilla pluton consists of water-poor peraluminous leucogranites emplaced at shallow depths (P,," 100 MPa) along extensional structures. Allied to thi,s pluton several hydrothermal alterations are found, that contain W -(Cu-Zn-Sn) ores. Two events of hydrothermal alteration are present. The first (291.4 ± 9 Ma) is probably linked to magmatic fluids and consists of an alkali feldspar metasomatism, fillings of miarolitic cavities and development of high temperature greisens and veins with wolframite. The second event is more recent (267.1 ± 7.4 Ma) and shallow ("" 650 mt.) and gave rise to low temperature greisens and the replacement of earlier wolframite by scheelite and sulphides. This evolution is similar to that found in other perigranitic hydrothermal systems. Fluid inclusions work and inferences from fluid-rock considerations show that the second alteration event (Stage TI) took place at low fluid pressures along the liquid-vapor curve, the system evolving from closed to open with decreasing temperatures. Fluids were low to medium in salinity «11.5 wt % N aCl equiv.) and can be described by reference to the system H,O-Na-K-Ca-Mg-Cl, with little amounts of CO, and CH4• The alteration temperature, as deduced from fluid inclusions and phyllosilicate composition" was in the range 380-250°C. Calculated oxygen and sulphur fugacities are within the pyrite stability field in agreement with petrographic observations (fOz = 10-30.5_10-34.5 bars; fS, = 10-9_ 10-10 bars). Chemical analysis of fluids contained in fluid inclusions from veins and estimation of the composition of fluid in equilibrium with greisen by thelmodynamic considerations, allow to predict the solubility of metals during transssport and precipitation. Greisenization is an important geochemical trap, with metal s precipitating by fluid-rock interaction. This rises the pH of the original fluid ("" 4) to close-to-neutrality values (pH,," 5.4). The scarcity of cassiterite and the absence of primary scheelite, are explained as resulting from the very low concentrations of Sn and W in the hydrothermal fluid.