Correlated chemostratigraphy of Mn-carbonate microbialites (Urkut, Hungary)

The genesis of a suite of Jurassic (Lias) microbialites at the Urkut black shale-hosted manganese carbonate ore body (central Hungary) is described by a two-step microbial formation model that uses mineral chemistry combined with whole-profile (up section) textural context and sulfur isotopic compositions of associated pyrite and barite. Petrogenetic analysis and paleo-environmental reconstructions show that the sedimentary regime of the Oita microbialites mostly behaved as an open system during deposition of black shale under early diagenetic conditions. Sulfur isotopes and other chemostratigraphic indicators, however, reveal that the Mn ore bed horizons reached semi-closed/closed conditions which modulated sedimentation rate and organic matter burial. Barite horizons within Mn-carbonate layers preserve delta S-34(VCDT) values that average +22.2 parts per thousand, with a maximum at +35.2 parts per thousand. Barite formation occurred under semi-closed/closed conditions at diagenesis, and the Ba source is attributable to the decomposition of organic matter derived from plankton and other marine organisms, as well as transformation of biogenic silica. Pyritiferous horizons host equant, framboidal and euhedral morphotypes. The distribution and size of euhedral and framboidal sulfide habits is consistent with later diagenetic sulfate reduction under an oxic water column; more equant types occur at the contact zone of black shale and Mn-carbonate horizons. The microbialites of Urkut bear strong similarities to ore bodies at Molango (Upper Jurassic, Mexico) and Tao Jiang (Middle Ordovician, China). Manganese supply and trace metal contents (Co, Ni, Zn, Cd and As) of the sulfides also point to the effects of distal hydrothermal fluid sources to the system. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.