贵州织金磷-稀土矿床磷块岩的矿物组成及胶磷矿中杂质元素成因

Mineral composition of phosphorites and the origin of impurity elements in collophanite in the Zhijin REEs-rich phosphorite deposit,Guizhou Province

海相沉积磷块岩中常伴生较高含量的稀土元素,其中的重稀土元素含量占稀土总量的30%以上,有望成为未来重稀土元素的重要来源。我国磷块岩资源丰富,主要产于扬子克拉通西南缘的云南、贵州、四川等地。贵州织金磷矿床稀土元素含量高、规模大,其中的稀土资源达大型矿床规模,具有重要的资源意义,但对磷块岩的矿物组成及造成胶磷矿中复杂化学组成的原因仍缺乏系统的研究。本文在对贵州织金磷-稀土矿床磷块岩样品详细的岩矿相观察基础上,通过SEM/EDS、XRD、FIB-TEM、LA-ICP-MS分析,查明了磷块岩的矿物组成和胶磷矿的内部结构及其中多种杂质元素的成因。研究结果表明,颗粒结构是磷块岩最主要的结构类型,由磷质颗粒和碳酸盐胶结物组成。磷质颗粒的矿物组成复杂,主要为氟磷灰石和白云石,另含有少量石英、白云母、黄铁矿(有些氧化后成为褐铁矿)、固体沥青、重晶石,胶结物的矿物组成主要为白云石,并含少量石英、黄铁矿、重晶石和微量的锆石、金红石、铀钙石(可能为其他铀矿物氧化而成)。FIB-TEM结果表明,磷质颗粒外层的隐晶质部分(胶磷矿)由纳米级矿物集合体组成,主要为氟磷灰石,并含少量白云石、黄铁矿和有机质,其矿物组成与磷质颗粒的矿物组成相似。胶磷矿中纳米级氟磷灰石、白云石、黄铁矿和有机质的发现为解释胶磷矿中杂质元素的来源提供了依据。固体沥青和铀钙石的发现及有机质中的高的稀土元素含量也为解释磷块岩中U和稀土元素的赋存状态提供了新的证据。

Marine sedimentary phosphorites usually contain high rare earth elements(REEs)content,especially the heavy REEs(HREEs),which account for over 30%of the total REEs.The phosphorites may became another important source for HREEs in the future.Phosphorite deposits are widely distributed in China and most of them are found in Yunnan,Guizhou,Sichuan Provinces,etc.High REEs abundances are confirmed in the Zhijin phos-phorite deposit in Guizhou Province and with total REEs resource over 3 Mt.The collophanite and apatite are the main host phase for both REEs and P,and the collophanite has complex chemical composition.There are still con-troversies for the mineral and chemical composition of collophanite.Based on detailed petrological,mineralogical,SEM/EDS,XRD,FIB-TEM and LA-ICP-MS results of phosphorites in Zhijin deposit,this paper discuss the mineral composition of phosphorites and the origin of the complex chemical composition of collophanite.The results show that the most phosphorites in the deposit show the pelletal texture,which is composed of phosphorous pellets and carbonate cement.The mineral composition of the phosphorous pellet is very complex,including mainly fluorapatite and dolomite,and with minor quartz,dolomite,pyrite(some oxidized into limonite),muscovite,solid bitumen and barite.The mineral composition of the cement is mainly dolomite,with minor quartz,pyrite,barite,and trace amounts of zircon,rutile,and liebigite(possibly formed by oxidation of other uranium minerals).The FIB-TEM results show that the cryptocrystalline part of the outer layer of phosphorus pellet is composed of nanoscale mineral aggregates including mainly nanoscale fluorapatite,and small amount of nanoscale dolomite,pyrite and organic matter.The mineral composition of collophanite is similar to that of phosphorus pellet.The discovery of nanoscale fluorapatite,dolomite,pyrite,and organic matter in phosphorite provides reliable evidence for explaining the chem-ical complexity of collophanite.The discovery of solid bitumen and liebigite in phosphorites,as well as high REEs content in organic matter,also provides new evidence for an alternative occurrence of U and REEs in phosphorites.