Research On The Characteristics Of Iron-Bearing Series Of BIF Type Iron Deposits In Gongchangling

BIF type iron deposits are hosted in Archean Anshan group in the middle of An-Ben ore belt which located in the northwest of Jiao-Liao uplift in the northeast margin of North China platform. The iron ore reserve is above 12.5 billion tons which contains about 24.2 percent of the nation's reserve. Gongchangling ore field located in the middle of An-Ben ore belt has high-grade iron ore. BIF is not only the most important iron source in the world, but also the chemical sedimentary formation which is unique in the earth early stage. The research on the characteristic of the iron-bearing series would help deepen our standing towards geological environment and evolution, the whole forming process of BIF and high-grade ore and yields the following result.Overlaying on gneissic granite with the age of 2.99Ga, Gongchangling Archean iron-bearing series formed over sedimentary basement similar to modern acidic continental crust. The major rocks show the right deviation, light REE enrichment and Ce negative anomaly. Those indicate the back-arc basin environment.Iron orebody occurred in iron-bearing series which consist of amphibolites, biotite leptynite, magnetite quartzite and altered rocks. The diversity of the wall rock demonstrates the variety of the sedimentary environment and the change of material supply. Multilayer ore bodies were formed in different ore-forming process. All the ore bodies are similar in Shape and attitude, but change greatly in thickness and scale, which probably caused by ore-forming process of different stage and topgraphic change in different parts of the sedimentary basin. However, the consistency of the ore composition, texture and structure indicates the similarity of the metallogenic material and environment, which may be induced by the time difference of diagenesis and mineralization. High-grade ore bodies occurred almost in magnetite quartzite and they have same ore composition. The phenomenon that magnetite quartzite cut by high-grade ore vein and the residual in the high-grade ore were seen occasionally. Combining the research of the sulfur and oxygen isotope characteristics, the ferrous iron of iron-rich ores was mainly derived from the ferrous of the magnetite quartzite.Magnetite quartzite are characterized by low total REE, enriched Light REE, right deviation, positive Eu anomaly and slight negative Ce anomaly.The result of major elements, trace elements and REE implies that magnetite quartzite is marine chemical sedimentary rocks interfused by very few clay substance or terrigenous clastic materials and formed in submarine hydrothermal environment which is closely related to volcanic activity and that iron and silicon mainly originate from mantle. The major elements of amphibolites are very similar to that of basalt. All samples show the relative concentration Rb, Ba, K and high content of Sc, V, Cr, Co, Ni, Cu. The ratios of incompatible elements and the total REE content are close to that of normal mid-ocean ridge basalts. Combining the result of protolith recovery, the original rock of amphibolites in this area is mid-ocean ridge basalts. Biotite leptynite displays high content of REE, strongly enriched Light REE, right deviation, slight negative Eu anomaly and negative Ce anomaly. According to the result of protolith recovery, its original rock is intermediate and acid dacitic tuff. The altered rocks show the high content of REE, light enriched LREE, right deviation and negative Eu, Ce anomaly. According to the diagram of A-C-FM, its original rock is siltstone.In a word, In Archaean era, Gongchangling area was long in the environment similar to back-arc basin. Constant Volcanic activities provide rich matter for diagenesis and mineralization. Numerous ore-forming process formed many ore layers. The original rocks of iron-bearing series consist of mid-ocean ridge basalts, intermediate and acid dacitic tuff and siltstone.