The hydrothermal alteration of continental basaltic flows in northern Parana Basin (Ribeirao Preto, Sao Paulo State, Brazil)

Résumé En

The mineralogical investigation of altered and columnary jointed basaltic rocks of northern Parana Basin (Brasil) indicates that the grey alteration halos observed around columnar joints have been developed during hydrothermal alteration events. Pétrographie evidences show that alteration parageneses are developed following succesive alteration zones from columnar joints towards "fresh" rock. In the weakly altered rocks olivine is replaced by K-nontronite whereas other primary components remain unaltered. In the outer part of the grey alteration halo, both glass and olivine are replaced by celadonite and nontronite. In vicinity of columnar joints the celadonite is associated with saponite. Columnar joints are filled with a zonal arrangement, from edge to center, as follows : (1) celadonite+saponite on wall rock, (2) heulandite, (3) celadonite + saponite, (4) heulandite, (5) opal and calcedony and (6) portlandite. This sequence suggests that the alteration can be genetically divided into two successive alteration processes : (1) water expulsion from basaltic lavas during late cooling stage, (2) isothermal infiltration metasomatic process which affects the rock around veins.