Estratigrafia, petrografia e mineralização de potássio em siltitos verdes do Grupo Bambuí na região de São Gotardo, Minas Gerais

Abstract

Green siltstones, traditionally known as verdetes occurs in the São Gotardo region, in western Minas Gerais State, Brazil, and is associated with the Serra da Saudade Formation of the Bambuí Group. Petrographic studies based on thin sections, X ray diffratometry, electron microprobe and scanning electron microscope were carried out to identify the mineralogy of these siltstones. They are silty-clayed sedimentary rocks composed of glauconite (40-80%), potassium feldspar (10-15%), quartz (10-60%), muscovite (5%), and minor quantities of biotite (2%), goethite (<1%), titanium and manganese oxides (<1%), barium phosphate, and rare-earth elements phosphates (<1%). Enriched levels of potash with K2O grades ranging from 8 to 12% are associated with the glauconitic levels. These levels are dark green in color. The glauconite is authigenic and highly mature. The high concentration of this mineral indicates that the green siltstones can be associated with a maximum inundation surface developed in the Bambuí Basin during the Ediacaran Period.