Hydrothermal silica deposits occur in a small fault (dike) as well as in several veins located in the contact between the andesitic volcanic rocks (8.1-8.7M years old) and the biocalcarenites (UpperTortonian-lowerMessinian) of the Agua Amarga area. The silica deposits have different colours (green, blue, greyish, white, red or brown) and luminosity (shiny or matt surfaces).Their mineralogy was determined by X-ray diffraction (XRD), white their textures and microstructures were examined by transmitted light microscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray microanalysis. The XRD analysis indicated that the dike and vein s(which show mineral zonation) are made of opal CT, quartz, sepiolite and dolomite. The outer parts are formed only by sepiolite, the intermediate areas by mixtures of sepiolite and opal CT, and their inner areas sometimes only by quartz. Fe-(hydro)oxides give the green and red colours. Near these hydrothermal deposits, the biocalcarenites show dolomitization and ferrugination. Optic and SEM observations showed the dolomite to be superimposed on the opal CT- sepiolite mixtures and quartz; it is therefore younger. The opal has similar textural characteristics to the sepiolite, and therefore originated from the sepiolite by silicification. It is suggested that this sepiolite is the result of a chemical precipitation brought about by the reaction of hydrothermal solutions with sea water in fractures and voids. A pseudomorphic silicification then occurred on the sepiolite, and an opaline rock with the texture and structural features of the sepiolita was formed. The replacement of sepiolite by opaline minerals released Mg that produced a later formation of dolomite on the opaline chert and also on the biocalcarenites associated with the fractures. The ageing of the opaline minerals formed quartz in the inner parts of the dyke and veins.Ministerio de Ciencia y Tecnología BTE2002-04017-C02-01Peer reviewed