| Gongchangling iron deposit is located in the southeast of Liaoyang City and it belongs to Anshan-Benxi mining area. Gongchangling iron deposit is a famous pre-Cambrian sedimentary metamorphic type iron deposit in China. The main ore stratum is Anshan group Cigou formation. The ore bodies occur in the iron bearing rock series which consist of magnetite quartzite, amphibolite, biotite schist, biotite granulite and hornblende schist. The main ore minerals are magnetite and hematite. In addition, there are few amount of pyrite and chalcopyrite. The goethite can be seen in the surface and shallow samples. There is obvious metasomatic relation between magnetite and hematite, mainly for the replacement from magnetite to hematite. Metasomatic etching texture, metasomatic relict texture, metasomatic lattice texture, metasomatic star-texture and metasomatic pseudomorph texture can be found.The mechanisms of replacement process between magnetite and hematite are mainly regarded as traditional "redox reaction" mode and the newly proposed "non-redox reaction" mode. Redox transformation is made up of a particular substance as an oxidizing or reducing agent, it is a process under an appropriate temperature and pressure conditions, then oxidizing Fe2+ in magnetite or reducing one third of Fe3+ in hematite. Non redox transformation is a process without the needs of specific redox conditions, which is a conversion of hematite to magnetite by a simple addition of Fe2+ atoms, or a conversion of magnetite to hematite by the leaching of Fe2+ atoms.With the analysis of electron probe microanalysis(EMPA), it is shown that typomorphic components of magnetite in lean ores and rich ores are very close, compositions change of magnetite has little influence by the metasomatism, the chemical composition of various forms of magnetite are relatively pure. Elements contents such as Al, Mg, Cr in hematite are higher than that of magnetite, part of them are close to the lower limit of hydrothermal metasomatic deposit, which suggests that hematite is transformed by the magnetite through hydrothermal leaching mineralization. Si content in magnetite that do not has transformation is rare, which both in transforming magnetite and hematite with transformed hematite are all have a small amount of increase. It reflects that the transformation process occurs in relative acidic environment. Rich ores of hematite show the characteristic of "silicon iron rich" by hydrothermal leaching process. Through the calculation of residual oxygen method, in magnetite, average content of Fe2O3 is 68.53%, average content of FeO is 30.38%, the average total iron content is 98.91%, in hematite average content of Fe2O3 is 98.68%, some samples contain trace FeO. The results of the Fe2+ and Fe3+ content in the two minerals illustrate that both of them are almost close to the normative mineral, and Iron content of magnetite is higher than that of hematite, which explained that a little part of Fe are taken out through the process of magnetite transforming into hematite.Goethite can be seen in the subsurface samples of Gongchangling iron deposit, it presents in irregular and vein sharp which etching hematite, however ores in deep buried do not have goethite, which shows that goethite forms in surface or the following and appears after the formation process of martite. In addition, there are half automorphic granular pyrites in the deep buried ores, which orientational distribute along the quartz stripe shows the primary sedimentary characteristics. The oxidation rate of pyrite is greater than magnetite; the occurrence of unoxidized pyrite reveals that the transformation from depositional magnetite to hematite is a non-oxidation process.With the methods of scanning electron microscopy back scattering(BSE) analysis, it shows that there is a big difference of pore structure between the surface of magnetite and hematite. To be specific, the surface of non-replaced magnetite does not exist porosity, but pore structure can be obviously observed in the surface of the replaced magnetite and new-generated hematite or martite. Moreover, there are more pores in granular martite than that in tabular hematite. It can conclude that mineral volume decreases after transforming from magnetite to hematite, which according to the characteristics of non-oxidation reaction.Through analyzing geochemical data from previous researchers, it is concluded that Gongchangling iron deposit is formed in the late Archean, and the formation takes place in reducing environment, which does not meet the oxidation conditions that depositing process of hematite requires. Those facts indicates that hematite in this deposit is formed in the later metamorphism hydrothermal leaching process. According to the O, Si, S isotopic characteristics, it can be determined that the metallogenic solution is an acidic reducing fluid, which mixes with oxygen-bearing groundwater in the shallow strata, making it possible to oxidize magnetite to hematite. However, oxygen content is quite low in deep earth burial groundwater, this kind of fluid also can dissolve and take away Fe2+ in the magnetite and then transform to hematite through non-oxidation reaction. |
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