Alteración hidrotermal en yacimientos epitermales desarrollada en rocas ácidas (Macizo del Deseado, Argentina)

Garcia Ezquerra, Nelson
Bauluz Lázaro, Blanca (dir.) ; Subías Peréz, Ignacio (dir.)

Universidad de Zaragoza, CIEN, 2016
Departamento de Ciencias de la Tierra, Área de Cristalografía y Mineralogía

Graduado en Geología

Resumen: The main objectives of the present project are the identification and description of the mineralogy and textural changes generated by hydrothermal alteration on several volcanosedimentary rock samples from the Chon Aike and La Matilde Jurassic formations located at the “El Deseado” mining district (Argentina). In order to get the aforementioned objectives, a series of analytical steps have been taken, which include transmitted-light and reflected-light microscopical studies, x-ray diffraction analyses and the use of a field emission scanning electron microscope. The studied samples are crystalline lapilli tuffs and coarse and fine grained crystalline tuffs. The igneous phases found include potassium feldspar, muscovite and quartz phenocrysts, glass fragments and rock fragments. The matrix of these samples is sometimes it is isotropic, which indicates the presence of glass. These rocks have undergone hydrothermal alteration processes which can be observed mainly on the potassium feldspars, muscovites, glass fragments and the matrix. These phases have been replaced by illite, kaolinite and illite-smectite mixed layers, as well as goethite, Fe-Ti oxides, rutile, quartz and alunite group minerals. The studied samples, taken from 2 different areas of Martha Vein system, show that the distribution of the hydrothermal mineralogy differs depending on its proximity to the vein. Close to the vein, the potassium feldspar phenocrysts have been partially replaced by several clay minerals, including kaolinite, which evidences an argillic alteration type. Far away from the vein, the phenocrysts exhibit a greater degree of alteration, and are characterized by an association of illite, kaolinite and alunite group minerals (alunite-crandallite). This indicates a change towards an advanced argillic alteration type. In the eastern part of Martha Vein (Samples ME-19, ME-28 and ME-49), a silicic alteration type can be observed towards the distal area of the Vein. This alteration results in vuggy quartz and silica sinter (massive quartz). The textural and mineralogical differences found in the studied samples reflect the physico-chemical properties of the fluid which gave rise to the alterations. In the northern area of Martha Vein, the presence of kaolinite and alunite indicates an acid pH (3-5) and temperatures lower than 250ºc. On the other hand, the presence of silica sinters and vuggy quartz in the eastern area of the vein indicates that the pH of the fluids evolved towards lower values (pH<3). The features exhibited by the studied alterations correspond to high sulphidation epithermal deposits.

Tipo de Trabajo Académico: Trabajo Fin de Grado