Summary of the context
The formation of magmatic iron oxide deposit is closely related to the role of volatiles in magmas, especially in low-Ti magmatic systems (Kiruna-type). Volatiles have strong effect on phase equilibria, saturation of Fe-Ti oxides, and liquid immiscibility. They also control the magmatic-hydrothermal transition with implications for the remobilization of some strategic elements (P, Au, Cu, REE) by fluids. Volatiles are thus key components for the relation between Kiruna-type ores and Iron Oxides Copper Gold deposits. This project is an attempt to constrain experimentally the role of volatiles in magmas related to the formation of Kiruna-type iron deposits. We will study the development of liquid and fluid immiscibility, major and trace element partitioning between melts and fluids, and iron isotopes fractionation between minerals, silicate melts and fluids. The study will benefit from unique high pressure experimental and UV-femtosecond laser ablation-ICP-MS analytical facilities developed in Hannover. The objective is to constrain the conditions and the processes responsible for the concentration of iron in low-Ti volatile-rich magmatic systems and to identify the key-factors responsible for the enrichment of elements of economic interest.
Overall objectives of the project
The strong debate on the origin of Kiruna-type and IOCG deposits, the major economical importance of these deposits, and the lack of appropriate experiments motivate this study. Volatiles are key components and the effect on phase equilibria and fluid-driven transport of precious elements should be better understood. This project relies on recent experiments and analytical improvement specifically developed in Hannover, which will enable to tackle the following goals.
1) The first objective of this project is to develop models to understand how andesitic magmas evolved chemically as they cool and crystallize in presence of volatiles. What is the late-stage liquid line of descent of these magmas and how does the Fe oxide concentrate. How do oxygen fugacity and especially volatiles influence this liquid line of descent?
2) The second is to determine how does liquid immiscibility fractionate the major and trace elements partitioning between immiscible conjugates. The composition of the fluids in equilibrium with the melts will also be analyzed and partitioning coefficients of economic elements (mainly Cu, Au, Fe and REE) will be determined.
3) Finally, the third objective is to determine the iron isotope fractionation between immiscible conjugates under variable oxygen fugacity conditions with or without presence of volatiles. The results will provide crucial information to use iron isotopes for the discrimination between magmatic and hydrothermal processes.
Major conclusions: Liquid immiscibility had played a very important role in the formation of low-Ti Kiruna-type iron oxide ores.
