Transport of Lithophile Elements in Magmatic-Hydrothermal Fluids

It has long been recognized that H2O-rich (aqueous) and CO2-rich (carbonic) fluids play a fundamental role in a wide range of geological processes. Of particular importance is the ability of such fluids to selectively transport chemical components, such as metals, from one geological reservoir to another. As a consequence, aqueous and carbonic fluids play a key role in the formation of some of the most economically important ore deposits in the world. Magmatic-hydrothermal ore deposits result from the cooling and phase-separation of volatile-rich magma bodies in the shallow crust. The chemistry of the fluids depends on the nature of the magma from which they exsolved, which in turn is influenced by the tectonic setting of the magmatism. Magmatism associated with destructive plate margins tends to be dominated by aqueous fluids, whereas carbonic fluids are more prevalent in intraplate magmatism. Fluids also contain a variety of anions (e.g. F, Cl, S etc) or anionic complexes (CO3, SO4 etc) which play an important role in metal transport. Despite the universal recognition of the importance of fluids in ore formation, we have surprisingly little understanding of their physical chemistry, which in turn limits our ability to predict how and where ore deposits may form. A large part of our ignorance stems from the experimental difficulties of studying high-temperature aqueous or carbonic fluids. Unlike silicate or carbonate melts, fluids do not quench to a solid at room temperature and pressure, making it difficult to characterise them chemically or physically. We have pioneered a novel experimental approach to this problem, in which a laser is used to drill through the walls of a frozen experimental capsule, directly analysing the frozen fluid, without risk of contamination during sectioning. Coupling the laser to an ICP-MS apparatus means that we can analyse the frozen fluid for a wide variety of trace elements. The coexisting silicate or carbonate melt can be quenched and retrieved from the capsule for subsequent analysis. We can systematically vary the composition of the fluid and its concentration, allowing us to explore the key controls on how metals are complexed in fluids. By looking at the variations in melt-fluid partitioning with fluid composition we can hypothesise about the types of metal-ligand complexes that are present. We cannot, however, directly observe these at pressure and temperature. To do this, we have developed an alternative experimental methodology in which a small droplet of fluid of known composition is held between the flattened tips of two diamonds in a resistance-heated diamond anvil pressure cell. The diamonds are transparent to synchrotron-generated X-rays, meaning that the solution can be studied in situ at elevated pressure and temperature. This approach allows us to evaluate the predictions made on the basis of the partitioning experiments. Finally, we can use computational quantum chemistry (classical and ab initio molecular dynamics) to predict the hydration and complexation of cations in fluids at at elevated pressure and temperature. Recent implementations of a technique call metadyanamics enables us to derive free energies and, hence, equilibrium constants, for the formation of metal complexes from molecular dynamical simulations. In summary, we are approaching the problem of metal transport from three quite different, but complementary directions. In its own right, each approach has limitations; in combination these approaches will enable us to generate a comprehensive picture of aqueous and carbonic fluids under precisely the same physical conditions as ore bodies form. We will begin by studying an important, but relatively simple, class of metals, the alkalis, alkaline earths and rare earths, although our methodology can ultimately be extended to encompass the entire range of economically important metals.

The work proposed here seeks to understand how certain lithophile elements (alkali earths, alkali metals and rare-earths) partition between melts and magmatic-hydrothermal fluids. A solid scientific understanding of hydrothermal fluids is needed if we are to develop new tools for the exploration and discovery of new ore deposits. Existing approaches to modeling magmatic hydrothermal solutions rely heavily on the concept of the fluid-melt partition coefficient for a given metal, D. D is sensitive to a wide range of parameters, including pressure, temperature and fluid composition, especially the availability and abundance of ligand forming anions such as carbonate, sulphate and halides. Without an understanding of how D responds to changes in these parameters it is not possible to generate realistic models of lithophile elements in magmatic-hydrothermal systems. Using the thermodynamic models we will develop, geologists will be able to use reactive transport simulations to understand processes such as metasomatism and alteration, providing potentially useful pathfinders to ore deposits. Understanding how changes in fluid chemistry reflect changes in pressure and temperature will enable us to use fluid inclusions to map the evolution of ore-forming fluids at the scale of an individual deposit. This could greatly lower the cost of assessment and exploitation of existing known deposits. In its broad application to hydrothermal ore deposits in general, therefore, we anticipate that our work will have a long-term economic impact.
However, we also anticipate that some of our results will directly address a key challenge facing our future economy and quality of life: Rare earth elements are of strategic concern as they are essential for several emerging technologies. For example, Nd Pr, Sm and Gd are used for the extremely powerful magnets needed by alternative energy technologies such as wind turbines and hybrid vehicles. Er is essential for long-range fiber optic transmission. Although rare earth elements are quite abundant in the crust, economical deposits are rare. The main source of REE is the Baiyunebo deposit in inner-Mongolia. Other deposits such as Mountain Pass in California are not currently producing. Rare earths are almost entirely mined from the mineral bastinite (LaFCO3) that is associated with carbonatites, although the carbonatite association of the Baiyunebo deposit is unclear.
Recently, the Chinese government placed a temporary (and unexplained) ban on the exports of REE to the US, Europe and Japan. Since China has been providing over 90% of REE's to western manufacturing, this export ban has fueled considerable speculation and uncertainty about the future availability of REE to western markets. There is now renewed interest in the west to redevelop known REE mining operations (such as Mountain Pass, CA) and explore for new deposits.Other lithophile elements targeted in this study also have economic importance. Lithium, for example, is an essential component of batteries.
The work done in this project will directly impact our understanding of how rare-earth and alkali metals are transported in high temperature deposits. An understanding of REE complexation in high PT fluids will enable us to develop models of ore deposit formation and may also suggest new pathfinders (e.g., changes in fluid inclusion composition) to aid in the discovery and assessment of new deposits. The UK mining industry, will be a major beneficiary of this research.