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The role of volatiles in the formation of magmatic iron oxide deposits

Periodic Reporting for period 1 - IRON_DEPOSITS (The role of volatiles in the formation of magmatic iron oxide deposits)

Reporting period: 2015-06-01 to 2017-05-31

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.
In this project, we aim at better understanding the magmatic stages in the formation of F-rich Kiruna-type deposit, Vergenoeg Fe-F deposit in South Africa, and especially at testing the hypothesis of silicate liquid immiscibility between a rhyolite and an iron-rich silicate melt. We assess experimentally whether the bulk composition of the ore body may correspond to an immiscible Fe-rich melt and whether the conjugate Si-rich melt could represent the host rhyolite. We investigated the role of volatiles (F and H2O) on the development of immiscibility and conducted experiments with variable F contents to better understand the formation of fluorite in these magmas. Our new experimental data suggest that the Vergenoeg pipe represents the cumulates of an immiscible Fe-rich which is saturated in fluorite.

We report the results of an experimental study aimed at testing that the Kiruna-type Fe-F Vergenoeg deposit (South Africa) is a product of silicate liquid immiscibility between a rhyolite and an iron-rich silicate melt. We assess experimentally whether the bulk composition of the ore body (17 wt.% SiO2 and 55 wt.% FeOtotal) could correspond to an immiscible Fe-rich melt paired with the host rhyolite. Experimental conditions are 1-2 kbar and 1010°C, with a range of H2O and F contents in the starting silicate melt. Pairs of immiscible liquids occur in samples (Figure 1)in which fluorite appears as the stable liquidus phase, under nominally dry conditions, and redox corresponding to QFM-2 to QFM+1.8. Other crystalline phases include fayalite and magnetite. The Si-rich liquids contain 60.9–73.0 wt. % SiO2, 9.1–12.5 wt. % FeOtotal and 2.4-4.2 wt.% F, and are enriched in Na2O, K2O and Al2O3. The Fe-rich immiscible melts have 41.4–49.5 wt. % SiO2, 20.6–32.5 wt. % FeOtotal and 4.5-6.0 wt.% F, and is enriched in MgO, CaO and TiO2. Immiscibility does not develop in experiments performed under hydrous and oxidized (> QFM+3.2) conditions. These results indicate that the Vergenoeg pipe may have formed from a magma chamber hosting two immiscible silicate melts. Crystallization of abundant liquidus magnetite and fayalite from the Fe-rich melt may have led to the formation of a crystal mush and cumulate rocks in the lower part of the magma chamber. The huge amount of fluorine (174 million tons at 28.1% CaF2) observed in the deposit cannot originate solely from the crystallization of fluorite from the Fe-rich silicate melt (4.8-5.7 wt.% F2O-1). Hence, part of the fluorite ores was probably formed by the activities of hydrothermal fluids.
MSCA Individual Fellowships are considered as an essential contribution to the international cooperation. The obvious impact for future collaborations with young research leaders is another motivation to encourage and assist MSCA fellows. Based on this MSCA Individual Fellowship, Dr. Tong Hou had created strong links between the China University of Geosciences (Beijing) and Institute of Mineralogy in LUH in Hannover, Germany.

1) Dr. Tong Hou invited the Drs. Olivier Namur and Bernard Charlier, who are also MSCA postdocs at Hannover, to China for a joint fieldtrip in February and March, 2016. Extensive field studies had been conducted in Panxi layered intrusions which host the world-class Fe-Ti-V oxide mineralization.
2) Dr. Tong Hou and his supervisor, Prof. Francois Holtz are preparing for a proposal that is going to be submitted to Sino-Germany Centre for Scientific Research (Chinesisch-Deutsche Zentrum für Wissenschaftsförderung (CDZ)) for a joint venture of the Deutsche Forschungsgemeinschaft (DFG) and the National Natural Science Foundation of Chi (NSFC).
3) Dr. Tong Hou invited Francois Holtz to China for a joint fieldtrip in September 2017. Extensive field studies had been conducted in the Emeishan layered intrusion and a common PhD student will star a 4 years research project on these rocks at the University in Hannover in November 2017.
4) Dr. Tong Hou , Prof. Francois Holtz and Roman Botcharnikov (University Mainz) have submitted a common proposal to the German Science Foundation (DFG) and in parallel to the chinese National Natural Science Foundation (involving 2 PhDs, one in China, one in Germany) in April 2017 (the proposal is under review).
Results from experimental studies showing immiscibility of silicate and iron-rich melts