河北省宣龙式铁矿的地球化学特征及其地质意义

Geochemical features of Xuanlong type iron ore deposit in Hebei Province and their geological significances

本文报道了宣龙式铁矿的主量元素、微量元素、稀土元素、Fe同位素和Nd同位素的分析结果。结果显示:主量元素的化学成分中Fe2O3T的含量最高,并且具有较高的Al2O3、TiO2和P2O5含量;微量元素含量总体与平均地壳相似,并显示氧化-还原敏感元素V、U、Mo的相对富集;稀土总量高、轻稀土元素亏损重稀土元素富集、无Ce、Eu正异常、Y/Ho比值与地壳平均值相似;同时,铁矿具有较低的εNd值,这些特征表明宣龙式铁矿的成矿物质来源为陆源。相对于标准物质IRMM-014,宣龙式铁矿中赤铁矿的Fe同位素组成均为正值,这与由现代蓝细菌氧化而成的赤铁矿Fe同位素组成应小于或等于0不同,因此,宣龙铁矿是由铁细菌氧化为主的生物粘结矿床。此外,铁矿无Ce的负异常;V、U、Mo的相对富集;Fe同位素比值为正值的这些特征均说明当时铁矿沉淀的环境为低氧状态。

The results of major,trace,rare earth elements,Fe and Nd isotopic compositions of Xuanlong type iron ore deposit are reported.The average bulk compositions of iron ore are characterized by highest total Fe2O3,and relative enrichment in Al2O3,TiO2 and P2O5.The overall contents of trace elements are similar to those of the average crust,but with distinct enrichments of redox-sensitive trace elements（V,U,Mo）.The total rare earth element contents are high,and the Shale-normalized REE patterns display depletion of light REE relative to heavy REE,non-significant positive anomalies of Eu and low Y/Ho values.Isotopically the iron ore samples have negative value of εNd.These characteristics demonstrate that the iron source of Xuanlong type iron ore deosit are from detrital input.Hematites show heavy Fe isotope enrichment relative to IRMM-014,which is different from the negative or zero iron isotopic composition of modern analogues formed by cyanobacteria.Therefore,the iron isotope data of hematite provide evidence for the presence of Fe-oxidizing bacteria.Geochemical features including the lack of Ce negative anomalies,enrichments of V,U,Mo,and the heavy Fe isotope enrichment of hematite samples implies the oxygen fugacity in the depositional environment was relatively low.