Characteristics And Formation Mechanism Of Magnetite In Xuanlong-type Iron Ore Deposit In The Northwestern Hebei Province

The Xuanlong-type iron deposit(XLD)in the northwestern Hebei Province is one of large-scale sedimentary iron deposits in China,and the reserves of iron ore resource have great potential.As a result of the intrusion of the Yanshanian magma,the main ore mineral of the XID was transformed from hematite to magnetite,which increased the value of the deposit.Based on the investigation of the XID and the existing research in literature,this paper summarized the geological characteristics of hematite and magnetite ore bodies exposed in four typical XID in Zengjiagou,Mayukou,Pangjiapu and Chenjiayao.The distribution range of XID hematite and magnetite ore bodies in these area is determined by analyzing the exploration data.The magnetite ore body and hematite ore body generally have a gradual contact,where transitional iron ore can be formed.Four types of iron ore of hematite,maghemite,erythrite and magnetite were collected from the transitional section in the Pangjiabu iron ore district.The color of these ores gradually changes from red to black with the hematite content of these ores gradually decrease and magnetite content gradually increase.The macroscopic oolitic-structure of these ores has no obvious change,but the microstructure changes significantly.The hematites in the oolite are gradually replaced by the magnetites,and the quartz change from intact to broken grains.The combined analysis of iron ore and iron-phase analysis reveal that the magnetite in the XID was mainly transformed from hematite.The drilling data from three ore districts of Maqiankou,Pangjiabao and Chenjiayao,and the iron-phase transformation of magnetite ore in different exploration lines of single ore body indicate that the formation of magnetite in the three ore districts was affected by the heat of magmatic intrusions.The closer to the intrusions,the higher the conversion rate of magnetite(mFe/oFe)in the ore.The heat provided by the magmatic intrusions has a significant control over the conversion rate of magnetite.This thesis believes that the formation process of the XID has nothing to do with the hydrothermal metasomatism.It is formed by the reduction of the originally formed XID and defined as magmatic thermal metamorphic-reformed iron deposit.Magmatic intrusionsprovide the essential heat,and the bioorganic carbon and particulate siderite contained in the magnetite ore provide reductants for the reaction.The formation mechanism is that the hematite ore body was subjected to the heat from the magmatic intrusions.the bioorganic carbon and particulate siderite contained in the magnetite ore act as reductants to form a trace amount of CO,so that the hematite ore body was enclosed in an anoxic environment with high temperature CO,and the hematite was gradually reduced to magnetite.