Immiscibility in magmas and iron deposits

A team of researchers, including scientists from the Petrology, Geochemistry et Petrophysics Laboratory – GEOLOGY Research Unit (Faculty of Sciences), has just identified a natural process responsible for the formation of an iron-enriched magmatic liquid at the origin of major deposits. The key lies in the immiscibility of magmas in "Kiruna" type deposits. This discovery has just been published in Nature Communications (1).

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ron is the most exploited metal on Earth. Although metal recycling techniques are constantly evolving, the exploitation of natural resources remains essential to the development of society. Specific processes in magmas, fluids and sediments are necessary to lead to local iron enrichment, and thus to form an exploitable deposit. Before a deposit can be mined, it must be located. And it is where the understanding of metallogenic processes, the study of deposits and their genesis, takes all its importance.

"We were interested in "Kiruna" deposits - or "iron oxide-apatite ore" deposits with reference to their main minerals - explains Bernard Charlier, Research Associate at the F.R.S.-FNRS (the Belgian National Fund for Scientific Research) within the Petrology, Geochemistry and Petrophysics Laboratory (GEOLOGY Research Unit). « This type of deposit in which immiscibility, a very particular process in magmas, could occur, represents an important part of the world's iron resources. » Bernard Charlier and his colleagues have been interested in this process of immiscibility for several years. « Like water and oil don’t mix, some magmas separate into two distinct compositions during their cooling phase. The two immiscible magmas can have very different compositions with one being more enriched in iron." It’s the discovery and study of the importance of this process of immiscibility in magmas by researchers from the University of Liège and the KU Leuven, among others, that was the subject of a scientific article published in Nature Communications(1).

"Until now, the study of natural rocks had not revealed the processes involved in iron enrichment" says Tong Hou, first author of the publication and Marie-Curie post-doctoral researcher. « We studied the problem in the laboratory using an original approach: the experimental petrology. » This approach makes it possible to reconstitute in furnaces and presses the high temperature and pressure conditions that prevail in the magmatic chambers present in the Earth's crust. This method allows us to understand the active mechanisms in the evolution of magmas during their cooling.

The experimental approach and its application to natural systems

The new experiments carried out at temperatures of about 1,000°C and pressures of 1 kbar (i.e. 1,000 times the atmospheric pressure) make it possible to identify the necessary conditions to separate the two magmas, one of which could be at the origin of an iron deposit (Fig 1). The results show the importance of the amount of dissolved water and the degree of oxidation of magma for the creation of deposits. These new data show unequivocally that in natural systems many magmas are capable of reaching appropriate conditions to lead to the immiscibility of two liquids and thus to the source of minerals.

Fig1 : Electron microscope image of an experiment performed at 1020°C and 1 kbar showing the separation of a magmatic liquid enriched in silica and a liquid enriched in iron. Crystals of magnetite and apatite are enclosed in the iron-rich magma.
Fig 2: Schematic illustration of the immiscibility process in magmas leading to the formation of an iron deposit. In black, a liquid highly enriched in iron separates from a magma enriched in silica (yellow). This process can be promoted by degassing the magmatic chamber (volcanic fumaroles at the surface) and by interaction with layers of sedimentary rocks (evaporites, clear horizon on the image). Copyright Mark Garlick.

Kiruna-type deposits exist throughout the World: Sweden, Chile, China, Iran, United States... These ancient rocks were formed in active volcanic environments (Fig 2). Indeed, magmatic chambers within the Earth's crust are often connected on the surface to volcanic systems." The study we conducted also shows the importance of various natural processes such as decompression and degassing, as well as the interaction of magmas with surrounding rocks (evaporites for example), comments Bernard Charlier. The study therefore opens up new perspectives for understanding natural rocks and the different deposits that have their own characteristics. »

Europe has become largely dependent of other continents for its mineral resource supply. A list of critical elements has been drawn up. It includes certain elements present in Kiruna iron deposits: rare earths elements, phosphorus and fluorine. The process of immiscibility is intimately linked to the enrichment in these elements. Understanding the processes behind deposits allows us to develop keys that will be useful for exploring and discovering new resources.

Scientific reference

(1) Hou T, Charlier B, Holtz F, Veksler IV, Thomas R, Zhang Z, Namur O (2018) Immiscible hydrous Fe-Ca-P melt and the origin of iron oxide-apatite ore deposits. Nature Communications.

Contact

GEOLOGY Research Unit I Petrology, Geochemistry and Petrophysics Laboratory

Bernard CHARLIER  I B.Charlier@uliege.be I 04/366 22 50