Bioformación de sulfuros y carbonatos en ambientes de lago salino (Mioceno, Cuenca de Madrid)implicaciones en la alteración de silicatos.

Abstract

This paper gives new insight in the microbial weathering of phyllosilicates and the subsequent formation of pyrite. These back-feeding processes were coeval with dolomite precipitation, which reinforces the role of the sulphate reducer microbes in the formation of dolomite in earth-surface conditions. Sedimentary sequences composed of selenite gypsum, mudstone, and dolomite microbialites cropping out in the eastern part of the Madrid Basin were deposited in a mudflat - saline-lake system during the Miocene. In some dolomite bed enriched in detrital grains, dolomite crystals occur intergrown with pyrite framboids that in many cases are arranged along the associated phyllosilicate cleavages, which accounts for the mineral disaggregation and, eventually, for the mineral replacement, with retention of the parent structure. These transformations are observed to take place across an irregular weathering front. Pyrite framboids yield a number of features susceptible of being interpreted as microbial remains, including the abundance of organic molecules. In addition, the altered phyllosilicates are significantly depleted in iron, consistent with a preferential microbial colonization of the Fe-bearing minerals and the release of this metal that may be used as micronutrient and/or as electron acceptor. Depleted d34S values in pyrite further suggest that sulphate microbial reduction to sulphide was active within the sediment during the alteration of the silicates.