Fossilization of Precambrian microfossils in the Volyn pegmatite, Ukraine

Abstract

Abstract. We report on Precambrian microfossils from igneous rocks of the Volyn pegmatite district, associated with the Paleoproterozoic Korosten pluton, northwestern Ukraine. The fossils were recovered from meter-sized miarolitic cavities and show a well-preserved 3D morphology, mostly filamentous but with a large variety of types and also in irregular, flaky shapes reminiscent of former biofilms, as well as rare spherical objects. Based on literature data, pyrolysis experiments, and reflected light microscopy results, the organic matter (OM) is characterized as (oxy-)kerite. Further investigations with microscopic techniques, including scanning and transmission electron microscopy, and electron microprobe analysis 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 in which 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, continuing in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, Ca sulfate, Ca phosphate, Fe sulfide, and fluorite. Breccias, formed during collapse of some the miarolitic cavities, contain decaying OM, which released high concentrations of dissolved NH4+, responsible for the late-stage formation of tobelite-rich muscovite and buddingtonite. The age of the fossils can be restricted to the time between the pegmatite formation, at ∼1.760 Ga, and the breccia formation at ∼1.49 Ga. As the geological environment for the growth of the microorganisms and fossilization, we assume a geyser system in which the essential biological components C, N, S, and P for growth of the organisms in the miarolitic cavities were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids. The Volyn occurrence is a distinct and uncommon example of Precambrian fossils, and the results underline the importance of cavities in granitic rocks as a possible habitat for microorganisms preserved in the deep biosphere.

The Volyn biota (Ukraine) – 1.5 Ga old (micro)fossils in 3D-preservation, a spotlight on the ‘boring billion’

“…One such object (Fig. 8) is >1 mm large and from the view in different perspectives is can be seen that it is grown onto mineral substrate; next to the clay minerals fluorite is a characteristic mineral and indicates a high fluorine activity in the fossilizing fluid (Franz et al, 2022a). The base of mineral substrate is followed by an approximately 10 µm thick solid rim with bulbous outgrowths.…”

“…For investigation of the internal structure we used SEM images of broken filaments and other objects, as well as polished sections embedded in epoxy, investigated by BSE images including mapping of element distribution. Data of open-pyrolysis and TEM data (Franz et al, 2022a) had shown that the OM is highly mature, amorphous oxy-kerite. Indications for an outer cell wall are absent, because the outer rim of the fossils is silicified, partly with formation of mineral incrustations.…”

“…The organisms lived in these cavities and provide an example of the Precambrian deep biosphere. Their fossilization conditions included sudden influx of hot hydrothermal waters in the geyser system, where magmatic fluids rich in SiF4 mixed with meteoric waters (Franz et al, 2022a), infiltration of Si-Al into the outermost layer of the fossils, and formation of dominantly clay mineral encrustations. The 1.76 Ga intrusion age of the pegmatites (U-Pb zircon; Shumlyanksyy et al, 2021) provides a maximum age of the fossils; the minimum age of 1.5 Ga is provided by the age of formation of a breccia, which contains degraded OM, opal with OM, buddingtonite which NH4-content was provided by the degraded OM, and muscovite ( 40 Ar-39 Ar https://doi.org/10.5194/egusphere-2022-1116 Preprint.…”

Etch Pits in Heliodor and Green Beryl from the Volyn Pegmatites, Northwest Ukraine: A Diagnostic Feature

“…They were later reinterpreted as fossil cyanobacteria (Gorlenko et al, 2000;Zhmur, 2003) from a geyser-type deposit. Recent investigations (Franz et al, 2022(Franz et al, , 2023 have also identified a number of different fossils, some of them fungi-like. Filamentous fossils and remnants of former biofilms were also identified in etch pits of beryl.…”

Another Fluid Inclusion Type in Pegmatite Quartz: Complex Organic Compounds

“…The hard bitumoid kerite [C 491 H 386 O 87 (S)N] formed under the same conditions [4]. Kerite was found in the center of a pegmatite body and comprises heavy black aggregates of up to 3 kg of abiogenic origin (according to Ginsburg et al, [5], cited in Gorlenko et al Last time, an intense and controversial discussion started on the 1.5-billion-year-old Volyn 'biota' by Franz et al [5] and Head et al [6].…”