An Experimental Study to Elucidate PGE, Base Metal Sulfide and Au Fractionation in Mafic Layered Intrusions

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Intellectual merit. Two competing models for formation of platinum-group-element (PGE) deposits associated with mafic layered intrusions have been widely considered: 1) the sulfide-settling 'downers' model; and 2) the aqueous fluid 'uppers' model. Model 1 hypothesizes that PGEs are partitioned from melt into early-formed sulfide phases that settle through the magma to yield significant accumulation of PGE-rich sulfides on the floor of the magma body. This hypothesis receives criticism because the characteristic PGEs, base-metal sulfides (BMS) and Au offsets are inconsistent with sulfide liquid fractionation. Model 2 hypothesizes that an aqueous volatile phase exsolved from the magmas extracts the ore metal as dissolved species. Model 2 is attractive in that it provides a mechanism for the observed metal relative abundances, but has not been tested quantitatively owing to unavailability of high-quality partitioning data for the PGE, BMS and Au at the appropriate pressure-temperature-composition (PTX) conditions attending emplacement, solidification and degassing of mafic intrusions. This project proposes to perform experiments and modeling to quantitatively test these hypotheses. The project will produce data constraining: 1) the partitioning of Pt, Pd, Cu and Au between S-free and S-bearing dolerite melt, aqueous supercritical fluid, +/- sulfide crystals, +/- silicate crystals at magmatic PTX, and 2) the partitioning of Pt, Pd, Cu and Au between S-free and S-bearing dolerite melt, co-existing aqueous vapor and brine, +/- sulfide, +/- silicate crystals at magmatic PTX. Experiments will be performed at 100 MPa and 800-1100 C, the PT range over which most layered intrusions solidify and degas. Oxygen and sulfur fugacities, total salinity and HCl of the aqueous phase(s) will be varied systematically. Element partition coefficiencts and will be measured and used to model the chromatographic separation of Pt, Pd, Cu and Au during solidification of an aqueous fluidsaturated mafic silicate magma using a modified form of the program PALLADIUM in collaboration with Alan Boudreau (Duke U.). The data and modeling will constrain whether aqueous magmatic fluids can or can not scavenge and transport economic quantities of PGE, BMS and Au from crystallizing mafic silicate melts and if precipitation from aqueous fluid can produce PGE, BMS and Au characteristics of natural ore deposits.Broader impacts. The study will advance our understanding of the behavior of PGEs, BMS and Au during the evolution of mafic layered intrusions and improve predicative techniques for the discovery of new PGE deposits. The P.I. will train one graduate student in experimental techniques, analytical chemistry and computational modeling and expose the graduate student to a diverse group of scientists through collaboration with faculty at UNLV, Duke University, ETH, and UCLA. The student will be involved in undergraduate teaching and mentoring and gain exposure through presentations of her/his research at professional meetings. The student will gain invaluable technical writing skills while preparing a dissertation and manuscripts for publication in scientific journals. The P.I. plans to incorporate at least one undergraduate student into this project to perform some of the analytical chemistry tasks, microthermometry and sample preparation. The undergraduate student(s) will be encouraged to take an active role in the research project and complete an undergraduate thesis.