Composition and Formation Conditions of Neoproterozoic Phosphorites in the Middle Urals

Phosphorite breccias compose a 6-m member in the Vendian terrigenous Kernos Formation in the basin of the Mezhevaya Utka and Sylvitsa rivers in the Middle Urals. Phosphorite pebbles and gravel are accumulations of fragments of redeposited crusts, originally formed in the early diagenesis near the surface of sandy-argillaceous deposits below the water-sediment boundary. Phosphorite is represented by fluorocarbonate apatite with unit cell parameter a from 0.9359 to 0.9363 nm, spectral mode parameters in Raman spectra (FWHM = 1–2 cm–1 and position from 963 to 966 cm–1), and a band at 1095 cm–1 in IR spectra. The absorption bands at ~1430 and 1453 cm–1 in the IR spectra correspond to the substitution of the orthophosphorus group by the carbonate ion (B-type). According to thermal analysis data, the CO2 content in apatite varies in the range of 0.04–0.8%, and the admixture of dispersed organic matter in the form of aliphatic compounds is 0.3–0.8%. Apatite is represented by two generations: primary in the form of basal structureless fine-crystalline cement and secondary in the composition of euhedral crystals up to 10 µm in size. Both generations corrode detrital quartz grains. Secondary apatite is enriched in P2O5, CaO, and F and contains less SiO2, FeO, Al2O3, MgO, and K2O than primary apatite. The PAAS-normalized REE distribution has a smoothed profile with a La/Yb ratio of about 2 and positive Ce- and Eu anomalies in enrichment phosphorites. The average value of F/P2O5 is 0.09 and corresponds to that in typical fluorocarbonate apatites that have undergone catagenesis. The high values of 87Sr/86Sr (from 0.7130 to 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 waters or catagenetic recrystallization.