The original mineralogical and geochemical compositions of andesitic volcanic rocks from SE of Spain have been modified by different processes, under hydrothermal environment. In particular, silicification process increases the initial silica concentration of the volcanic rock from 69 % to 97%. One of the characteristics of this alteration is the breakdown of mafics minerals and feldspars with supply of Al, Fe, Mg…etc to the interstitial fluids which can be later introduce as traces in the new quartz formed. The silica rocks created in the process are mainly constituted by quartz with different crystal sizes and textures. The spectral analysis of the cathodoluminiscence (CL) emission in combination with the Scanning Electron microscopy observations show that the igneous quartz crystals of the volcanic rock are characterized by a dominant band at 2,95 eV (420 nm) while the quartz crystals formed by silicification have a dominant band at 2,54 eV (~ 500 nm). Although it is difficult to identify the causes of these spectral differences, it is considered that the 420 nm emission band is due to native defects of the igneous quartz and the ~ 500 nm emission band is due to the substitutional incorporations of impurity ions in the hydrothermal quartz crystals. The SEM-CL imagines show a non uniform luminescent emission. This is interpreted in terms of either microsilicifications affecting the quartz crystals and the groundmass of the volcanic rocks or as consequence of the pseudomorphic replacements by quartz of the primary minerals. The hydrothermal silicification is an early process that increase the amount of silica of the initial andesitic rock making other petrological and geochemical types of volcanic rocks.

Ministerio de Ciencia y Tecnología y Comunidad de Madrid BTE-2002-04017-CO-2-01 MAT-2003-00455 y GR/ MAT 630-04

Peer reviewed