鄂西晚泥盆世含磷鲕状铁矿石中磷的赋存状态与形成

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

广泛分布于我国南方泥盆纪地层的＂宁乡式＂铁矿储量巨大,然而含磷高严重制约了该类型铁矿的开发利用.铁矿石中磷的赋存状态是设计该类型铁矿＂提铁降磷＂方案的理论基础,是开发该铁矿首先要了解的问题.充分利用湿化学全岩分析、电感耦合等离子体质谱分析等全岩元素分析,扫描电子显微镜、X射线衍射等物相分析,电子探针微分析、激光剥蚀电感耦合等离子体质谱分析等微区分析技术,对鄂西晚泥盆世含磷鲕状铁矿石中磷的赋存状态、物质来源与磷矿物形成过程进行了初步探讨.铁矿石中的含磷矿物主要为碳氟磷灰石,分别以短柱状磷灰石晶体颗粒（65%以上粒径小于20μm）、磷灰石内碎屑（粗砂至极粗砂级,集中形成透镜状或带状层理）以及鲕粒中与赤铁矿相互包裹的凝胶状磷灰石（层厚度1050μm）3种形式存在.磷灰石晶体是在孔隙水中重结晶而生成,磷质可能来源于晚震旦世地层的磷块岩;磷灰石内碎屑是古海水体中原位化学沉积的产物,磷质可能来源于古海周边的大陆;鲕粒中凝胶状磷灰石也是原位化学沉积的产物,但与铁质沉积位置相同,并与富铁的鲕绿泥石混合或相互包裹形成鲕粒.

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.