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Banded Iron Formations Pdf Download HOT!

Majority of the banded iron formations (BIFs) in China are hosted in North China Craton (NCC). The BIFs in NCC are interlayered with metamorphics including amphibolite, and mainly composed of quartz, magnetite, and hematite with minor minerals such as garnet, epidote, chlorite, calcite, biotite, and pyrite. They were formed during from Paleoarchean to the early Paleoproterozoic with a peak at Neoarchean. Wall rock and Fe ore bodies in the BIFs have experienced greenschist to granulite facies metamorphism. Because of the intensive post-ore geologic activities, ores in the BIFs in NCC are characterized as low Fe grade (30.35 wt% of Fe in average) compared to those in the world. Most of the BIFs in China are affiliated into Algoma-type BIF. The Fe ores are composed of SiO2 and Fe2O3 T with minor Al2O3, MgO, CaO, Rb, Sr, Zr, Hf, Cr, and Co. Post Archean Average Shale (PAAS) normalized REY (rare earth elements with Y) patterns in the Fe ores show depletion in light REE (rare earth elements; LREE) compared to heavy REE (HREE) with positive La and Eu anomalies, suggesting that the Fe ores in BIFs were precipitated from the seawater with hydrothermal fluids. Trace elements ratios further suggest that the Fe and Si in the BIFs were derived from volcanic-related submarine hydrothermal fluids. Incorporation of continental-derived clastic materials are insignificant in most of the BIFs, while a few exhibit some incorporations. Ce anomalies in the Fe ore indicate an evolution of redox state during the BIF formation. Tectonic models in NCC and the formation of the BIFs were suggested by the geochemical and geological investigations in this study.

banded iron formations pdf download

It is concluded from the present-day geochemistry of iron, and its probable paleogeochemistry that a primary accumulation of this element cannot arise in a marine environment. The Precambrian banded iron formations are, therefore, suggested to have been formed in a lacustrine or closed-basin environment. Known limnological processes are invoked to provide a mechanism for the common rhythmic banding of iron and silica. The difference between the dominantly nondetrital, low-alumina, banded Early-Middle Precambrian and the dominantly detrital, high-alumina, oölitic Late Precambrian and post-Precambrian iron formations is linked to the effects upon weathering processes of a change in the character of the earth's crust and of the development of biotic systems. Hematite is considered to be the only primary iron mineral in sedimentary rocks; other iron minerals, particularly the ferrous silicates, are regarded as essentially diagenetic (although probably preconsolidation) in origin. Organic matter has an important influence on both weathering and diagenetic processes. 350c69d7ab


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