SÍLICA MICROCRISTALINA (TRÍPOLI) EM ROCHAS SEDIMENTARES PERMIANAS DO FLANCO LESTE DA BACIA DO PARANÁ, ESTADO DE SÃO PAULO, BRASIL

Abstract

MICROCRYSTALLINE SiLICA (TRiPOLI) IN PERMIAN SEDIMENTARY ROCKS OF THE EASTERN FLANK OF THE PARANÁ BASÍN, SÃO PAULO STA TE, BRAZIL Microcrystalline silica occurs as tripoli, siliceous concretions and chert with microbial mats in Permian sedimentary rocks at the top of the Tatuí Formation, in the region of the Pitanga Anticline, Paraná Basin, southeastern Brazil. The tripoli is porous and mineralogically simple, composed almost entirely of aggregates of fine-grained crystals of quartz (~ 2 pm). The microcrystalline silica originated by the percolation of hydrothermal fluids along NE and NW faults and fractures probably during the late stages of the Paraná flood magmatism (Early Cretaceous). The introduction of silica and precipitation of quartz were contemporaneous with the dissolution of carbonate rocks. The same siliceous fluids produced a cement of euhedral quartz crystals commonly more than 5 pm long as overgrowths on detrital grains in sandstones. The hydrothermal silicification occurred below shales and siltstones of the Taquaral Formation, which acted as a permeability barrier to the fluids.