白云鄂博铌稀土铁矿的成矿地质条件及矿床成因

Metallogenic Geological Conditions and Genesis of Bayun Obo Iron Niobium REE Deposit

白云鄂博矿区的铁矿和铌稀土矿分属两个成矿期和两种不同的成因。矿区位于华北地台北缘白云鄂博大陆坡的南缘。铁矿是中元古界蓟县系哈拉霍疙特组内的层控受变质铁矿，赋存在宽沟南侧的层状白云岩中，该白云岩及铁矿层是在两条同生断裂所控槽状泻湖中沉积的。除白云岩外，宽沟南北两侧地层均可对比，同属白云鄂博群。铁矿层的原生矿物为以菱铁矿为主的ＦｅＣＯ３－ＭｇＣＯ３系列矿物，受后期侵入岩的热变质作用转变为磁铁矿及赤铁矿。后期另有少量的热液型磁铁矿脉的贯入，使铁矿石品位富化。铌稀土成矿作用与４００～８００Ｍａ侵入的脉状、瘤状碳酸岩有关。矿区已发现该类脉岩４０余条，可分为白云石质及方解石质两类，以前者为主，其ＲＥ２Ｏ３平均含量各为１．８９％及３．４８％。两类碳酸岩的稀土元素特征均为轻稀土高度富集型，铕弱亏损到富集。矿物平衡温度为１７５～６０７℃，平衡压力为１．０３～９．７６ｋｂａｒ，反映其来源较深。碳酸岩体沿断裂侵入到尖山组及其以下层位，造成强烈而广阔的碱交代蚀变带，以霓长石化为主，其强度与稀土矿化强度呈正相关关系。推测矿区在不大的深度内赋存有较大的隐伏碳酸岩侵入体。

Bayun Obo mining district is located on the southern fringe of Bayun Obo continental slope(Pt 2-3 )within the northern margin of North China platform.The iron deposit and the niobium rare earth deposit of Bayun Obo mine belong to two different epochs of mineralization under different conditions.Its iron deposit,which was controlled by Halahuogete Formation of Jixian system (Middle Proterozoic Era),existed in stratiform dolomites to the south of Kuanggou(fault),the dolomites and iron ores were deposited in the trouh lagoon controlled by two contemporaneous faults.Except for dolomites,the strata lied to the south and to the north of Kuangou can be all correlated and belong to the lithostratigraphic sequences of Bayun Obo Group.The primary minerals of iron deposit are a series of FeCO 3 MgCO 3(siderite mainly),which were transformed into magnetite and hematite by late period thermometamorphism of the intrusives,and the grade of iron was improved highly owing to the injection of some hydrothermal magnetite veins.The niobium rare earth mineralization,on the contrary,was concerned with veined and tumorous carbonatite intruded in 400 800 Ma.Over 40 rock veins have been discoved in the mine,which can be classified as dolomite(mainly)and calcite of which average contents of RE 2O 3 are 1 89% and 3 45% respectivly.The characteric of two kinds REE carbonatites is LREE enriched intensivly.The balance temperatures of minerals and from 175℃ to 607℃,and the balance pressures of minerals are from 1 03 kbar to 9 76 kbar,which show that they were originated from the deep.Jianshan Formation and lower Formation were intruded by carbonatites along faults,which led to intense and wide alteration zones of alkali metasomatism (mainly aegirine microcline),the intensity of alteration is positively correlated with that of rare earth mineralization.It is speculated that the larger concealed intrusive body of carbonatites may be not too very deep under Bayun Obo district,and the REE may come partly from the Archaeozoic strata within the middle and lower part of the crust.