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Formation of monomineralic Fe-Ti oxide ores in the high-Ti ferrobasaltic system: A case study in Emeishan, China

Subject Area Mineralogy, Petrology and Geochemistry
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 391973822
 
Final Report Year 2024

Final Report Abstract

The project was aimed at understanding the formation of Fe-Ti-V-rich rocks in layered intrusion and was focusing on the example of monomineralic magnetite layers in the Emeishan Large igneous Province (ELIP), China. Iron (Fe) ores hosted in basaltic layered intrusions are one of the major sources for Fe, Ti (titanium) and V (vanadium) metals used in industry. However, the formation of thick massive Fe-Ti oxide layers (usually tens meters thick) in these intrusions is still under debate. Mineralogical and geochemical analyses as well as experimental work at high pressure (100 to 500 MPa) were carried out to clarify following main questions: Which factors do exert a major control on the stability of Fe-Ti oxides during the evolution of the most primitive high-Ti ferrobasalt? - To which extent can interaction between basaltic magmas and limestone (occurring as hostrock) affect the crystallization behavior of Fe- and Ti-bearing minerals? - To which extent can magma replenishment, i.e. mixing between evolved and primitive magmas, affect the conditions at which Fe-Ti-oxides are the only stable solid phases? Our analyses of mineral compositions from selected samples collected along a drill core in the southern part of the Hongge layered intrusion show that a normal upward fractionation trend is not recorded by cumulus clinopyroxene and plagioclase, but that frequent reversals to more primitive compositions are observed. According to the mineral reversals, we divided the entire Hongge intrusion into eleven stratigraphic units which are related to major replenishments of primitive high Ti rich magmas. If monomineralic layers are formed at a magmatic stage only, a broad stability field with the coexistence of only oxides with silicate melt (absence of silicate minerals) needs to exist and should be confirmed by experiments simulating crystallization in magma reservoirs. Experiments were conducted to check if the formation of the massive oxide ores may result from the mixing of water-rich and water-poor magmas, simulating multiple magma replenishment events. This mixing scenario is realistic since we demonstrate that the assimilation of carbonates by basaltic melts would lead to the formation of relatively dry magma reservoirs. The experiments demonstrate that a large stability field with only oxides and silicate melt can be observed at oxidising conditions (∆FMQ+3.3 and ∆FMQ+2), at temperatures above 1050 C, and at relatively high melt water contents (> 2.5 wt% H2O). However, the proportions of oxides that would crystallize are still too low to explain the formation of thick monomineralic magnetite layers by simple crystal fractionation processes. Based on this observation, Fe isotopic analyses were performed on magnetites and ilmenites from monomineralic layers in order to check if iron may be of hydrothermal origin. The results show that a significant part of the Fe-oxides ( ~ 20-30 %) precipitated from hydrothermal fluids at subsolidus conditions. It is proposed that the transport of Fe under hydrothermal conditions has contributed to a significant increase in ore volumes and oxide grades in the Fe-Ti-rich layers.

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