Fossilization of Precambrian microfossils in the Volyn pegmatite, Ukraine

The Volyn biota are a distinct and uncommon example of 3D-preservation of ~1.5 Ga old Precambrian fossils, recovered from cavities in pegmatites, which were the habitat for microorganisms in the deep biosphere. The Volyn pegmatite district is associated with the Paleoproterozoic Korosten Pluton, Ukraine. Breccias, formed during collapse of miarolitic cavities, contain decaying OM, which released NH4+, responsible for the late-stage formation of buddingtonite and tobelite-rich muscovite. The age of the fossils can therefore be restricted to the time between the pegmatite formation, at ~1.760 Ga (zircon U-Pb), and the breccia formation at ~ 1.5 Ga (muscovite Ar-Ar).

The organic matter (OM) is characterized as (oxy)kerite. Microanalytical investigations show that fossilization likely occurred during a hydrothermal, post-pegmatitic event, by silicification dominantly in the outermost 1-2 µm of the microfossils. The hydrothermal fluid, derived from the pegmatitic environment, was enriched in SiF4, Al, Ca, Na, K, Cl, and S. The OM shows O enrichment where N and S content is low, indicating simultaneous N and S loss during anaerobic oxidation. Mineralization with Al-silicates starts at the rim of the microfossils, continues in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, Ca-sulfate, Ca-phosphate, Fe-sulfide, and fluorite. As geological environment for growth and fossilization of the microorganisms we assume a geyser system, in which the essential biological components C, N, S, and P were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids.