Cretaceous Fe-oxidising microbial mats in injection dykes in the Variscan gold-bearing metamorphic basement of the Tisza Mega-unit (SE Hungary)

Abstract

Large quantities of Cretaceous Fe-oxidising bacteria (FeOB) are documented here for the first time in carbonate injection dykes. These injection dykes occur in fissures of a Palaeozoic metamorphic complex in the Geresd Hills, Hungary, and are made up of microlaminated haematitic carbonate that contain very well preserved helical FeOB stalk fossils similar to recent iron-oxidising Gallionella and Mariprofundus. The presence of filament clusters with semi-parallel textures suggests aerotactic bacterial cell movements. Fractal analysis of the filament clusters showed higher fractal dimension values (Df ~ 1.9) compared to the typical fractal dimension values of the texturally similar, but inorganic Diffusion-Limited Aggregation (DLA) structures (Df ~ 1.7). This finding is interpreted to suggest that the observed filamentous texture is of microbial origin. Cretaceous basaltic intrusions that penetrated the gold-bearing pyritic metamorphic basement launched a low-temperature flow of reductive iron-rich fluids. Well‑oxygenated bottom waters percolated through thin pelagic sediments and picked up ferrous iron from the pyritic metamorphic bedrock and made it available for the Fe-oxidising microbes. Reactivation of a major fault zone (Mecsekalja Zone) resulted in the development of extensional faults in the metamorphic basement beneath the seafloor. Pelagic carbonate sediments with FeOB mats were drawn into these fissures, forming a network of injection dykes. This study shows that fractal dimension analysis and directional analysis may be a helpful method to differentiate FeOB textures from inorganic textures in ancient rocks. The presented results are expected to provide useful analogues to aid the reconstruction of similar palaeoenvironments worldwide.