Abstract:

The Shalong iron formation located in the Central Tianshan block is one of the newly discovered Neoproterozoic iron formations. Previous studies have reported the deposit characteristics, formation age and tectonic settings of the Shalong iron formation, and discussed the genesis and sedimentary environment of the deposit on the base of the whole-rock element geochemistry and iron isotope analysis of the ore. However, there is little attention paid to micro-geochemical analysis of ore minerals in Shalong iron formation, which restricts the in-depth understanding of the iron mineral formation process and formation environment. This study focuses on the magnetite and hematite of the representative iron ores from the Shalong iron formation. Based on the detailed mineralogical study, in-situ geochemical analysis of magnetite and hematite using femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry (fs-LA-ICP-MS) was carried out, so as to further reveal the formation process and environment of magnetite and hematite. The hematite in the Shalong iron formation is mainly presented as subhedral micro-fine grained tabular texture, and distributed along the layer of rocks, the magnetite is presented as euhedral to subhedral granular. The results of the in-situ geochemical analysis show that magnetite has a high mass fraction of Si, Ca and Mn and a low mass fraction of Ni, Cu and Zn. The mass fraction of V  of magnetites from different samples  are relatively consistent, and the mass fractions of Ti vary greatly. Hematites have a higher mass fraction of Si, Ca, Ti,  Cr and a lower mass fraction of Mn, Ni and Zn than magnetites. The total rare earth elements in magnetite are low (w(ΣREY)=1.49×10-6-51.16×10-6, average 12.15×10-6), showing the characteristics of depletion in light REEs (LREEs) and enrichment in heavy REEs (HREEs), with no observed Eu anomalies and positive La anomalies. Magnetite and hematite in different samples have similar REE distribution patterns as those of the whole ore, indicating that magnetite and hematite in iron formation  are the main carriers of rare earth elements, and the trace element characteristics of magnetite and hematite can provide a new insight for revealing the source of ore-forming materials and redox conditions during the formation of iron formation. Our results, in conjunction with previous studies, suggest that the magnetite in the Shalong iron formation is the product of hematite recrystallization, and rare earth element characteristics of magnetites indicate that the Shalong iron formation has a source of mixed ore-forming materials of low-temperature hydrothermal solution and seawater, while the lack of Ce anomalies and the relatively consistent mass fraction of V and Cr indicate that the iron formation was formed in a relatively anoxic environment, and the changes in oxygen fugacity were small during the formation process.

Key words: magnetite, hematite, trace elements, fs-LA-ICP-MS, iron formation, Central Tianshan block