Oxic and post-oxic chemical changes related to eogenesis and mesogenesis in a Miocene paleolake

Abstract

In the Forez Basin of the French Massif Central, clay-rich alluvial deposits include a series of red- and green-colored sediments and carbonate cement that record evaporation and oxidizing/reducing conditions related to intra-continental climate during the Early to Middle Miocene. The mineralogy, chemistry and relative chronology of authigenic calcite-ankerite, ferroan dolomite, pyrite, and analcite-clinoptilolite in clay-rich sediment enabled reconstruction of a series of processes related to deposition, eogenesis, and mesogenesis. The low-Mg calcite, ankerite, and ferroan-dolomite cement, systematically associated with zeolites (analcite and clinoptilolite), represent eogenetic precipitation associated with oxic to post-oxic water with Ca–Fe–Mg carbonic, then Na–Al–Si(OH)4-rich water. Occurrence of framboidal and cubic pyrites with low-Mg calcite and analcite is related to post-oxic conditions associated with deposition, eogenesis and times of early mesogenesis. Changes in the morphology, size, and chemistry of framboidal and cubic pyrite grains were related to reducing-oxidizing cycles and to the growth of grains. Sulfur isotope measures on framboidal and cubic pyrite suggest that both morphologies are related to bacterial reduction of SO42− to H2S and HS−1. With the exception of some sample depths (0–40 m below surface), similar chemical contents (trace elements [TE], rare earth elements [REE] and platinoids) suggest a similar, constant reservoir of metal and metalloids associated with the clayey sediment and volcanic fragments. Moreover, the TE and REE chemistry of cubic pyrite, in contrast to carbonates, indicates growth of pyrite during eogenesis to mesogenesis stages. The mineralogical and chemical changes are interpreted as reflecting dissolution of iron oxyhydroxides, rather than ankerite and ferroan-dolomite. Chemical elements from dissolution of iron oxyhydroxides would have mixed with hydrogen monosulfide and contributed to the growth of cubic pyrite. The crystal size distributions of pyrite grains suggest a closed system with surface-controlled growth. Assuming those conditions, the growth time of framboidal to cubic pyrite, based on diffusion of HS−1 in the clay-rich sediments, was estimated to have been from several days to a few years, to as much as 500 years (mm-scale pyrite).