| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4730275 | Journal of Asian Earth Sciences | 2015 | 19 Pages |
•The G2 and QW iron deposits show different alteration and geochemical features.•Desilicification responsible for the high-grade magnetite ores in the G2.•Iron activation–reprecipitation generated the high-grade magnetite ores in the QW.
The high-grade magnetite ores related to banded iron formations (BIFs) in the Anshan-Benxi area, Liaoning Province in China, have been widely interpreted as the product of replacement of protore by epigenetic hydrothermal fluids. The high-grade iron ore reserves in the mining area II (164 million tons) in the Gongchangling (G2) and Qidashan-Wangjiabuzi (QW) iron deposits (11.45 million tons) are the largest deposits in the Anshan-Benxi area. We present a detailed comparison of the geology, geochemical and stable isotopic compositions of the iron ores in the G2 with those in the QW to constrain the role of desilicification and iron activation–reprecipitation in converting the BIFs to high-grade magnetite ores. These two deposits show marked difference in wall-rock alteration, geochemical features, and oxygen and sulfur isotopic compositions. Wall-rock alteration in the G2 is characterized by garnetization, actinolitization, and chloritization, whereas the QW shows chloritization, biotitization and sericitization. The geochemistry of altered rocks in the G2 is characterized by slight REE fractionation, positive Eu and no significant Ce anomalies, whereas the QW is characterized by high ΣREE contents, strong REE fractionation, and the absence of significant Eu and Ce anomalies. High-grade iron ores in the G2 show similar δ18OV-SMOW values for magnetite, lower δ18OV-SMOW values for quartz and higher δ34SV-CDT values for pyrite when compared to the BIFs, whereas the QW shows lower δ18OV-SMOW values for magnetite, similar δ18OV-SMOW values for quartz and similar δ34SV-CDT values for pyrite. These features indicate that desilicification process by hypogene alkaline-rich hydrothermal fluids were possibly responsible for the formation of high-grade iron ores in the G2 whereas iron activation–reprecipitation process by migmatitic-hydrothermal fluids generated the high-grade iron orebodies in QW.
