Light on the origin of the verdete siltstone, Bambuí group, central Minas Gerais state, Brazil

Abstract

The Verdete is a variety of a greenish siltstone that occurs occasionally at different stratigraphic levels of the Serra da Saudade Formation, Bambuí Group (Neoproterozoic to Cambrian age). The main rock types related to the Verdete are potassium rich and show the follow mineralogy: quartz, illite/glauconite, orthoclase, albite, ilmenite, fluorapatite and dawsonite. Conventionally, the Verdete has been considered as formed by diagenetic processes in transgressive conditions at calm water environments. However, the present research based on different data considers the Verdete as result of potassic metasomatism in preexisting detritic sedimentary rock (siltstone, fine sandstone and greywacke). The main arguments that support the potassic metasomatic alteration hypothesis are: i) coincidence of the Verdete main occurrences with geophysical anomalies interpreted as alkaline intrusions in depth; ii) presence of dawsonite which is one hydrothermal sodium carbonate; iii) local presence of brecciated facies of phosphorite cross cutting the Verdete facies; iv) lateral discontinuity of the beds and v) microscopic evidence of substitution in coarser facies. The potassic fluids percolated in fracture zone in a northeast trend in the main area of occurrence in central Minas Gerais State (Cedro do Abaeté Town), and besides this region there are also incidence in other areas. The Verdete is considered as a possible source of potassium to be applied to agromineral or industrial supply for fertilization production. In this way, the understanding of its origin must be important to develop new processes or improve the methods to the potassium recover (chemical, biological and thermal routes).