In situ LA‐ICP‐MS trace element analysis of magnetite from the late Neoarchean Gongchangling BIFs, NE China: Constraints on the genesis of high‐grade iron ore

Abstract

The Precambrian banded iron formations (BIFs) not only relate to the evolution of life, ocean, and atmosphere but also provide important reserves of iron around the world. The Gongchangling iron ore deposit located in the Anshan‐Benxi area of Liaoning Province, China, is oxide facies Algoma‐type BIFs, and the Gongchangling No.2 mining area is famous for the production of high‐grade iron ore in China. Magnetite is the main ore mineral in the Gongchangling iron ore deposit, and the magnetite mainly exhibits three modes of occurrence: BIFs (without actinolite), actinolite‐bearing BIFs, and high‐grade iron ore. Trace elemental compositions of the magnetite with different occurrences of the Gongchangling iron ore deposit were obtained by laser ablation inductively coupled plasma mass spectrometry to constrain the genesis of the high‐grade iron ore. The magnetite from actinolite‐bearing BIFs shows relatively lower contents of Mg, Al, Mn, and Zn compared to the magnetite from BIFs (without actinolite), suggesting that coexisting minerals have played an important role in the trace element concentration in magnetite. The magnetite from high‐grade iron ore has lower contents of Ti and V and higher contents of Al and Mn than counterpart from BIFs (with/without actinolite), indicating that the high‐grade iron ore may be reformed by high temperature metamorphic hydrothermal fluid. The staurolite‐garnet‐biotite schist is the wall‐rock of high‐grade iron ore, and the garnet‐biotite geothermometry is used to evaluate the metamorphic temperature of 593 ± 17 °C. It is proposed that the metamorphic hydrothermal fluid produced during regional metamorphism reformed BIFs to generate high‐grade iron ore.