ARCHEAN BANDED IRON FORMATIONS （BIF） IN EASTERN HEBEI PROVINCE

The Archean metamorphic sequence in eastern Hebei may be divided from older to younger into the Qianxi,Dantazi and Zhuhangzi Groups,with a total thickhess of more than 13,810 m.The Qianxi Group underwent granulite-facies regional metamorphism and different degrees anatectic migmatization Strong and polyphase structural deformation modified it into a complex fold sys- tem.The Dantazi Group and the Zhuzhangzi Group originated from amphibolite-facies regional metamorphism and paligenetic migmatization and was shaped into a linear fold system.The BIF occur widely in the supracrustal metamorphic series and provide an important iron mining base for China. The BIF are composed chiefly of quartz and magnetite,usuall yassociated with several species of silicate minerals.There is evidence of volcan iclastic origin in some of the constituent minerals.The mineral composition shows that the BIF in this district fundamentally belong to oxide facies.The magnetite is present either in minute mineral inclusions in quartz crystals or in minor interstitial crystals or single crystals between the quartz grains, or in coarse-grained crystals.Chemically,the magnetite is nearly pure except for some samples that show a high content of TiO₂ and MgO.In general,the BIF here are closely similar to other Precambrian BIF all over the world.Their TFe content is 28.5±1％in average,but in comparision with the Supreior Lake iron ore they have a much higher TiO₂ content. Most of the BIF are characterized by banded structure; however,a part do not possess such a typical feature but show gneissoid or massive structures.The bandad structure has a two-lamina rhthmic texture,i.e. two lamellae different in colour,mineral composition and grain size occur alternatively.The dark-coloured lamina is dominated by manetite and the lightcoloured lamina by quartz.This phenomenon was the result of the rhythmic change of sedimentary conditions, and the non-banded structure is merely the reworked banded structure. The iron-bearing rocks show a clear cyclicity.Three major volcanic-sedimenary cycles and one imperfect cycle are recognized in the three groups and each of the major cycles is further composed of a series of subcycles.There are 10 main BIF in the whole stratigraphic column.They usually occur at the topmost horizon of volcanic graywackes or clastic sedimentary rocks.The magnitude of the BIF shows sympathetic relationshps to the thickness of the supracrustal rocks in a volcanic-sedimentary cycle,while the latter is in turn controlled by the stability of the sedimentary environment and the quantity of sediments. The petrographical and petrochemical data do not support the submarine character of the sedimentary sequences.On the contrary,they support the volcanic and volcanic-sedimentary character,as they lie on continental crust The initial sedimentary basin in which the Qianxi Group containing BIF developed might be represnted by isolated small depressions interspersed with volcanos, while in the later period those small depressions might join together to form a wide openbaisin.The Dantazi and Zhuzhangzi Groups seemed to be deposited in an aulacogen in a nearly cratonize dhigh-grade metamorphic terrain, with the tectonic settings resembling a back-arc rift depression. In spite of having suffered strong, metamorphic alteration and deformation the BIF still retain some of the original sedimentary features as reflected by the Stratigraphical,mineralogical and geochemical studies. Here the author advocates a detrital origin for part of the iron minerals as they came directly from volcanic ashes.The minute clasts of iron minerals together with ferruginous and siliceous matter from volcanic emanation formed a colloid-macro-dispersive system,and in such a system the periodic changes of the condition of the depositional medium resulted in precipitation of banded iron ores.