Hydrothermal silica deposits occur in a small fault (dike) as well as in several veins located in the contact between the andesitic volcanic rocks (8.1-8.7 M years old) and the biocalcarenites (Upper Tortonian-lower Messinian) of the Agua Amarga area. The silica deposits have different colours (green, blue, greyish, white, red or brown etc.) and luminosity (shiny or matt surfaces). Their mineralogy was determined by X-ray diffraction (XRD), while their textures and microstructures were examined by transmitted light microscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray microanalysis. The XRD analyses indicated that the dike and veins (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 the 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. Optical 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 textural and structural features of the sepiolite 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