Formation Mechanism Of Different Types Of Banded Iron Formations Of China:Constraints From Iron, Silicon, Oxygen And Sulfur Isotopes

A detailed geological survey was performed for different types of BIFs of North China Craton and Yangtze Craton. The Gongchangling iron mine (Algoma type), Yuanjiacun iron mine and Huoqiu iron mine (Superior type), and Neoproterozoic Xinyu iron deposits (Rapitan type) were primarily studied. Fe-Si-O-S isotope, zircon dating, Hf isotope, major element and rare earth element analyses were performed. Based on the results of field observation and experimental analysis, the sources of ore-forming minerals, the contemporary oceanic and atmospheric environments, and the formation mechanism of BIFs of various types were discussed. The following achievements are made:1. Established the procedures for the chemical separation and detection of Fe isotopes. The chemical recovery was as high as 99-102%. The workflow-related blank of Fe was only 0.026ug, far lower than 0.1% of the sample. Under high-resolution mode, the standard-sample-standard mass bias correction method was adopted. The detection precision of δ56Fe reached as high as 0.08‰(2SD), and that of δ57Fe was 0.14‰(2SD).2. BIFs of North China Craton are mainly composed of SiCO2 and iron oxides. The contents of Al2O3 and TiO2 indicative of terrigenous clastics were low. Huoqiu ore mine and Xinyu type iron deposit located in Yangtze Craton are high, indicating the inclusion of a certain amount of terrigenous clastics in the formation process. The REE patterns of different types of BIF after PAAS standardization all show depletion of light rare earth elements and enrichment of heavy rare earth elements, which are characteristic of sea water. The Precambrian BIFs show obvious positive Eu anomaly, positive Y anomaly and positive La anomaly, and there is no Ce anomaly. This is indicative of the contribution of hypothermal fluid. But obvious Eu and Y anomalies are observed in Xinyu type iron deposit, and are combined with negative Ce anomaly. It is the result of the combined action of low-temperature hydrothermal fluid and terrigenous clastics. Y/Ho value and Al2O3 content are in negative correlation in Huoqiu iron mine. Moreover, the total rare earth elements are in positive correlation with Al2O3 content, which also indicates the contribution of terrigenous clastics.3. BIFs of different types share similar silicon and oxygen isotopic compositions. The quartz in BIFs has a relative wide distribution range of δ18O (10.4‰～16.7‰). But for over 70% of the samples, the distribution range is 12‰～16‰. The value lies between that of quartz in igneous rock and the oxygen isotope of marine siliceous rocks formed under normal temperature. The oxygen isotopic composition is similar to that of siliceous rocks with hydrothermal sedimentary genesis.δ30Si values of various types of BIFs are generally low, typically in the range of-2.0%o-0%o, with an average of -0.85%o. This implies the high silicon dissolved in the sea water, and only a small portion of silicon was deposited to form BIF. The Si lost through deposition was constantly replenished, thus keeping a low negative δ30Si value. It is believed that various types of BIF are all associated with seafloor hydrothermal activities.4. All types of BIFs show enrichment of heavy iron isotopes, implying that Fe was not completely oxidized due to the limited oxidation capacity of the sea water at the time of formation. The partial oxidation of Fe in the sea water results in the enrichment of heavy Fe isotopes in BIFs.5. The results of sulphur isotope analysis in BIFs of North China Craton reveal the obvious non-mass-dependent fractionation of sulphur isotopes. The BIFs of Algoma type have negative △33S value, while those of Superior type have positive △33S value, which reflects the relation between BIF type and the distance to volcanic activities.6. U-Pb dating of zircon were performed on the biotite-plagioclase gneiss in Huoqiu iron mine of North China Craton. It is found that Huoqiu iron mine was formed at 2546±12Ma, and later underwent metamorphism at 1885Ma. This is consistent with the peak period of BIF formation of North China Craton. It is the ore-forming events prevalent in Neoarchean.7. Iron-rich ore and iron-poor ore have similar silicon isotopic compositions and iron isotopic compositions. Magnetite and quartz have relative low δ18O value. We suggest that the iron-rich ore could be classified into initial enrichment type, post metamorphic hydrothermal type and hydrothermal alteration type base on their different formation mechanism. Gongchangling iron-rich ore deposit is mainly correlated with the late hydrothermal alteration.8. Compared with the adjacent magnetite band in the same sample, the siliceous band has a higher 830Si value in quartz, while the δ18O value is low. The δ57Fe value in BIF is higher than that in magnetite band. This implies that the siliceous band has an earlier deposition and a higher formation temperature than the magnetite band. The SiO2 deposition divides into two stages. In the first stage, silicon was in oversaturated state in the sea water. The SiO2 deposition was featured by high temperature, fast rate, small fractionation, small proportion of deposition and small deposition of iron. The deposited silicon has a high δ30Si value and a low δ18O value. In the second stage, SiO2 was in saturated state in the sea water. SiO2 deposition was featured by low temperature, slow rate, great fractionation and a slightly increased proportion of deposition. In the meantime, large amounts of iron began to deposit. The deposited silicon has a low 830Si value and a high δ18O value. The adjacent band in the iron deposit sample collected from Yangjiaqiao Formation, Xinyu with a low degree of metamorphism was submitted to Fe isotope analysis. The magnetite band is enriched in light Fe isotopes compared with the quartz band. The results show that in the first stage, few iron was deposited and the fractionation was higher. In addition, the δ57Fe value of the deposited silicon was higher. In the second stage, the oxygen fugacity was increased, and a large amount of iron began to be deposited. The deposited Fe had a lower δ57Fe value. Thus, a new formation mechanism for silicon-iron rhythmic layer is proposed. The eruption of seafloor hydrothermal fluid brings up large quantities of ore-forming minerals such as Fe and Si as well as acidic and reductive gases. After the hydrothermal fluid reached the seafloor, the temperature dropped abruptly, and the silicon concentration reached oversaturated level in the sea water. As a consequence, the silicon was rapidly deposited from the sea water, forming the siliceous band. As the hydrothermal fluid further mixed with the sea water, the temperature declined, while the pH value and Eh value rose. Part of Fe2+ was oxidized to Fe3+, forming the Fe(OH)3 precipitate and finally the iron band. Due to the low oxygen fugacity, the oxidation of Fe2+ took some time. Thus, the iron deposition was later than silicon deposition in time. Each seafloor exhalation was accompanied by the first deposition of Si and the subsequent deposition of Fe, giving rise to the silicon band and the iron band, respectively. A rhythmic layer represents a major seafloor exhalation, and the periodic action of seafloor exhalation leads to BIF.