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 beds enriched in detrital grains, dolomite crystals occur intergrown with framboidal pyrite clusters that in many cases are arranged along the associated phyllosilicate cleavages, which accounts for the mineral disaggregation, and eventually for the mineral replacement. High-resolution FE-SEM investigations across the interface between the pyrite and the phyllosilicate show that a single phyllosilicate is replaced by iron sulphides in different microsites, with retention of some aspects of the parent structure. Pyrite formed in the sediment as texturally complex framboids, coexisting with amorphous to poorly crystalline iron-rich sulphide phases, and include carbonaceous filaments with biogenic isotopic signatures (δ13C = −22.95‰). Microprobe analytical data indicate the weathered phyllosilicates to be significantly depleted in Fe relative to Si and Al. The selective release of Fe has not involved the mineral transformation to secondary phyllosilicates, which suggests a preferential microbial colonization of the Fe-bearing minerals rather than a thermodynamically driven degradation of those minerals. Depleted δ34S values in pyrite further suggest that sulphate microbial reduction to sulphide was active within the sediment during the alteration of the silicates.
This paper gives new insight into the microbial weathering of phyllosilicates and the subsequent formation of pyrite through a sequence of intermediate products enriched in iron. These processes were coeval with the microbially-mediated precipitation of dolomite, which further reinforces the role of the microbes in the formation of the sulphides.