Occurrence and Formation of Phosphorus in Late Devonian Phosphate-Bearing Ooidal Ironstones from Western Hubei, China

Ningxiang-type iron deposits are widely distributed in Devonian strata in southern China with huge reserves, but exploitation of this type iron deposits have been severely circumscribed due to their high phosphorus content. Occurrences of phosphorus are theoretical foundations for dephosphorization researches, which should be solved first. Preliminary researches of occurrence, formation and origin of phosphorus in Late Devonian phosphate-bearing ooidal ironstones from Western Hubei in China have been done in this study by means of whole-rock element analysis (wet chemical whole-rock analysis, inductively coupled plasma mass spectrometry), mineral phase analysis (scanning electron microscopy, X-ray diffraction analysis) and in-situ element analysis (electron microprobe analysis, laser ablation inductively coupled plasma mass spectrometry). Phosphorous-bearing minerals are mainly carbonate fluorapatite in ironstone and there are three formations, including prismatic apatite crystal particles (more than 65% particle sizes < 20 μm), apatite intraclasts (very coarse and coarse sand size level particles concentrate in lenticular and banded beddings), and layered gelatinous apatite entangled by hematite in ooids (layer thickness are of 10-50 μm). Apatite crystals recrystallized from pore water, and might have originated from phosphorite in late Sinian strata. Apatite intraclasts were the product of in-situ chemical deposition in seawater, and might have come from land around paleo-ocean. Gelatinous apatite also was in-situ chemical deposition product, but formed at iron deposition position and mixed or mutual parcel with chamosite and formed ooids.