La alteración hidrotermal asociada al plutón epizonal de Navalcubilla, Sierra del Guadarrama (Sistema Central Español)

Abstract

The Navalcubilla pluton consists of water-poor peraluminous leucogranites emplaced at shallow depths (P ≈ 100 MPa) along extensional structures. Allied to this 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 II) 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 % NaCl equiv.) and can be described by reference to the system H2O-Na-K-Ca-Mg-Cl, with little amounts of CO2 and CH4. The alteration temperature, as deduced from fluid inclusions and phyllosilicate composition, was in the range 380-250ºC. Calculated oxygen and sulphur fugacities are within the pyrite stability field in agreement with petrographic observations (fO2 = 10-30.5-10-34.5 bars; fS2 = 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 the1modynamic considerations, allow to predict the solubility of metals during transssport and precipitation. Greisenization is an important geochemical trap, with metals 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 temperature and the relatively high m∑Ca of the fluid during stage II is the responsible for the formation of secondary scheelite after wolframite. The scarcity of arsenopyrite is probably a consequence of the relatively oxidant conditions prevailing during this alternation.