Composition and Formation Conditions of Neoproterozoic Phosphorites in the Middle Urals

Abstract

Phosphorite breccias compose a 6-m-thick member in the Vendian terrigenous Kernos Formation in the basin of the Mezhevaya Utka and Sylvitsa rivers (Middle Urals). Phosphorite pebbles and gravelites are accumulations of the fragments of redeposited crusts, originally formed during early diagenesis near the surface of sandy–clayey rocks below the water–sediment boundary. Phosphorite is represented by fluorocarbonate apatite with the following parameters: unit cell a ranging from 0.9359 to 0.9363 nm, spectral mode parameters in Raman spectra (ν₁ FWHM = 1–2 cm^–1 and peak position from 963 to 966 cm^–1), and a band at 1095 cm^–1 in FTIR spectra. The bands at ~1430 cm^–1 and 1453 cm^–1 in the FTIR spectra correspond to substitution of the orthophosphorus group by the carbonate ion (B type). According to the thermal analysis data, the CO₂ content in apatite is 0.04–0.8%, and the admixture of dispersed organic matter as aliphatic compounds is 0.3–0.8%. Apatite is represented by two generations: primary (basal structureless fine-crystalline cement) and secondary (euhedral crystals up to 10 µm in size). Both generations corrode the detrital quartz grains. Relative to the primary apatite, the secondary variety is enriched in P₂O₅, CaO, and F, but depleted in SiO₂, Fe₂O₃, Al₂O₃, MgO, and K₂O. The PAAS-normalized REE distribution in the enriched phosphorites has a smoothed profile with a La/Yb ratio of about 2 and positive Ce and Eu anomalies. The average F/P₂O₅ value (0.09) corresponds to that in typical fluorocarbonate apatites subjected to catagenesis. High ⁸⁷Sr/⁸⁶Sr values (0.7130–0.7253) in detrital phosphorites of the Kernos Formation suggest their deposition in a desalinated wave-dominated shallow-marine paleobasin near a significant inflow of continental water (delta) or catagenetic recrystallization.