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UPPER PRECAMBRIAN DOLOMITES FROM CRISÓPOLIS (BAHIA): CHEMICAL AND PETROGENIC STUDY

1977; Sociedade Brasileira de Geologia; Volume: 7; Issue: 3 Linguagem: Inglês

10.5327/rbg.v7i3.120

2317-4889

G. P. Sighinolfi, Geraldo da Silva Vilas Boas, Silvio De Queiroz Mattoso, C. Gorgoni,

Paleontology and Stratigraphy of Fossils

The Crisopolis dolomite, Northern Bahia, occurs in a limited vertical sequence of predominantly dark-coloured compact dolomite strata containing minor beds of lighter oolith-rich siliceous dolomite and rare chert partings. This sequence, believed to be of Upper Precambrian age, has undergone low-grade metamorphism. Mineralogically, the rocks consist of dolomite, quartz and talc. Chemical data show an overall abundance of silica with clear signs of metasomatic diffusion in the compact dolomite strata. Sodium content decreases as silica metasomatism increases. Detrital and/or hydrolized elements like Fe, Mn, Ti, Zr and K are present in minor amounts. A model for the genesis of the dolomite proposes: a) deposition of primary CaCO 3 and magnesian calcite from water which is probably a little hypersaline; b) dissolution of primary carbonates at lower pH conditions and reprecipitation of dolomite with extensive sweating out of more soluble CaCO 3 ; c) contemporaneous silica precipitation and its metasomatic diffusion during the diagenetic stage. A paleo-environmental model is proposed. Accordingly, the primary carbonate deposition would have occurred in an intertidal zone and the dissolution-reprecipitation stabilization of carbonates related to the interference of meteoric waters.