Petrologic tools to understand ore-formation in felsic igneous systems

Mineral deposits of some of the critical metals essential to society are associated with the end stages of crystallization of the felsic magmas produced by melting of Earth's sedimentary crust. A unique attribute of these felsic magmatic systems is their extreme level of chemical fractionation, and it has been postulated that a magmatic vapour phase plays a significant role in this behaviour. Key to determining how these deposits form, and where to find new ones, lies in understanding how crystal-liquid separation, magma redox state and the development of a magmatic vapour phase (MVP) affect element behaviour. Biotite and apatite are common minerals in such magmas, and incorporate a variety of elements that can yield such information. However, meaningful interpretation of element abundances in these minerals requires calibration of crystal/melt partitioning at known conditions of pressure, temperature and oxygen fugacity. Information on the relative melt-vapour partitioning of key fractionated elements, including the geochemical twins Nb and Ta, is scarce, but essential to evaluating the roles of melt and vapour in ore formation. The proposed Discovery Grant projects will generate new data on biotite-melt and biotite-apatite partitioning of a large number of elements, including the critical metals and halogens, which can be used to develop quantitative models of crystallization, assess the timing of MVP formation and provide the basis for new methods of magma oxygen barometry. Additional research will evaluate the role of the MVP on fractionating Nb from Ta, and therefore better understand the origin of the extreme level of chemical fractionation exhibited by these unique magmatic systems. From this, it can be established the ways by which relatively "normal" crustal abundances of critical elements can be concentrated to economic levels and unravel the processes that result in chemical fractionation within Earth's crust.