| 其他摘要 | Banded Iron Formations (BIFs) are the principal source of iron for the global steel industry, and account for over 60% of the world's total reserves of iron ore. BIFs are also the special type of sedimentary formation in the early Earth, which has recorded the atmospheric and oceanic conditions at the time of their deposition. Therefore, research on BIFs has great economic value and theoretical significance. Spatially, the BIFs in China are mainly distributed in the North China Craton (NCC). The Anshan-Benxi area in the NCC is the largest iron ore resource base of China, and the ore type is BIFs. The Qidashan and Gongchangling iron deposits in the Anshan-Benxi area are located in the Yingtaoyuan and Gongchangling iron belts, respectively. In this dissertation, we have used integrated mineralogy, mineral chemistry, major and trace geochemistry of the Qidashan and Gongchangling iron deposits to constrain the ore genesis and the relationship between Yingtaoyuan and Gongchangling iron belts. Main conclusions are drawn in this study as follows: (1) Sodic and potassic granites are discovered in the Qidashan iron deposit. The sodic granite is pale, and the main mineral assemblages are plagioclase, quartz, K-feldspar and biotite. The chemical composition of sodic granite exhibits the following characteristics: Na2O content > K2O content; lower content of total REE; lower fractionation between LREE and HREE; low degree of LREE and HREE interior fractionation; and obvious negative Eu anomalies. The potassic granite is flesh pink, and shows the following characteristics: K2O content > Na2O content; higher content of total REE; higher fractionation between LREE and HREE; high degree of LREE interior fractionation; relatively flat HREE distribution; obvious negative Eu anomalies, and the potassic granite should be classified as crust-derived type granite. (2) As for the relationship of the Yingtaoyuan and Gongchangling iron belts, it was argued that they were upper and lower stratigraphic position relationship or the same stratigraphic position. It was discovered that the sodic and potassic granite in the Qidashan iron deposit was analogous to the two-type granite in the Gongchangling No.2 mining area. Combined with previous works, we tend to believe that the Yingtaoyuan and Gongchangling iron belts were the same stratigraphic position, and now their observed dislocations were caused by the Hanling Fault in the Indo-Yanshan Movement. (3) The Anshan group rocks suffered strong metamorphism and deformation, and the original occurrence and fabric were demaged, therefore, the chemical methods like DF discriminant function, the Simonen plot and the A–K plot were used to reconstruct the protolith. The results show that the protoliths of plagioclase amphibolite, schist, leptynite and siliceous rock are basic volcanic rock, pelite or siltstone, intermediate– acid tuff and marine chemical sediments, respectively. (4) The schists of the Anshan Group can be divided into two groups based on the Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) values, but all the Index of Compositional Variability (ICV) values for the schists are >1, indicating that they are compositionally immature and derived from an active tectonic setting. Zr/Sc and Th/Sc ratios illustrate that the metapelites, which display compositional variation trend without significant sediment recycling, might be the first-cycle erosion products. Various discrimination diagrams for sedimentary provenance show that the schists predominantly derived from the mixed felsic and basic sources. (5) Geochemical features of basic volcanic rocks are widely used to discriminate tectonic setting. Geochemical studies of clastic sedimentary rocks also provide information regarding tectonic setting. According to protolith nature, we also discuss the ancient tectonic setting based on the geochemical characteristics of plagioclase amphibolites and schists, and it suggests |
修改评论