CLAY MINERALOGY AND CHEMISTRY OF ALTERATION HALOS ASSOCIATED WITH GYPSUM VEIN DEPOSITS FROM MAINIT AREA, MABINI GEOTHERMAL FIELD, CALUMPAN PENINSULA, PHILIPPINES

In Mabini geothermal field, low-sulphidation style of mineralization is encountered at approximately epithermal level to surficial environment as manifested by the presence of alteration halos, open space-filling gypsum deposits and warm grounds. The most impressive thermal manifestations are the altered grounds found in the pre Plio-Pleistocene andesite-basaltic sequence located in Mainit area, South of Mabini. Such altered grounds occur in patches and are partially covered by recent tuff layers. Their mode of formation can be ascribed to the NW and NE trending fault structures that provide permeability for circulating hydrothermal fluid. The observed assemblages of clay minerals in major halos are induced by two fronts of alteration, i. e., argillization and sericitization. They are found to associate intimately with gypsum vein mineralization. Their distribution exhibits a characteristic zonality whose succession in the direction away from the gypsum lode is: kaolinite-illite zone, flanked by an illite-mixed layer illite/smectite (I/S)-smectite zone and a chlorite-mixed layer chlorite/smectite (C/S)-smectite assemblage further away. The formation of a narrow zone of alunite is considered as a post-gypsum process and was probably generated under steam-heated environment condition during the later stage of hydrothermal activity. High layer charge 1M type illite is the most widespread clay species in the entire volume of altered rock while kaolinite persists almost exclusively in the inner zone. Smectite is minor and commonly occurs as high layer charge beidellitic clay in both illite and chlorite-rich zones. Abundant aluminian chlorite is frequently observed on the fringes of major alteration halos and tends to associate appreciably with illite. Mixed-layer minerals in the alteration zone comprise of irregular interstratified illite/smectite and chlorite/smectite with low expandable layers, and regular mixed layer chlorite/smectite similar to tosudite. The mechanism of evolution of the zones is interpreted here as a continuous process resulting from a single hydrothermal episode. This means that the alteration in the zone of strongest leaching next to the gypsum lode and the slight alteration of the outermost zone took place at the same time.