Mineral forms of phosphorus in ooidal iron ores of the Marsyaty deposit (Northern Urals)

Horizons enriched in phosphorus are identifed in ooidal ironstones underlying the oxide-carbonate manganese ores of the Marsyaty deposit (Northern Urals). The maximum P2O5 content reaches 6.37 wt. % in a quartz sandstone interbed with ooids consisting of Fe3+ oxyhydroxides and is associated with authigenic fuorapatite. In iron oxyhydroxide ooidal ores, phosphorus is present mainly as apatite, which forms massive and radial zones in both the ooids and the matrix. In siderite ooidal ores, the phosphate minerals include apatite and hydrous Al and Ca silicate-phosphates, possibly a mineral of the crandallite-goyazite series and perhamite. They occur as radial aggregates emphasizing the zonation of carbonate ooids and common in the matrix. Phosphorus is also part of an authigenic rhabdophane-like mineral flling the radial and concentric cracks in iron oxyhydroxide ooids. Some phosphorus is associated with detrital monazite and apatite. Phosphorus for the formation of phosphates was most likely sourced from organic relics, whereas seawater and minerals unstable under sedimentation and diagenesis conditions were the source of cations. The formation of crystalline phosphates is associated with diagenetic processes, during which the organic relics were fermented and replaced by mineral phases, while the minerals metastable in shallow marine basin were decomposed with desorption of elements captured by Fe3+oxyhydroxides from seawater.