Magmas to Metals: Melt Inclusion Insights into the Formation of Critical Element-Bearing Ore Deposits

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Science Issue and Relevance

Global dependance on critical elements such as rare earth elements, antimony, and tellurium, has increased rapidly with expanding modern technology. Identifying new potential resources of these elements within the U.S. is of paramount importance as the majority of the domestic supply is imported. This project applies innovative melt inclusion and mineralogical techniques to characterize several distinctive magma types occurring together with prodigious, critical rare earth elements (REE) and gold-(antimony-tellurium) ore deposits within the United States. We will characterize the pre-eruptive/pre-emplacement magmatic conditions in several districts. Our goal is to determine the role of magmatism in the genesis of critical element-bearing ore deposits.

Methodology to Address Issue

We will initially focus on iron oxide-apatite/iron oxide-copper-gold-rare earth element (IOA-IOCG-REE) deposits. Each of the targeted districts is currently under investigation by the USGS, providing a solid geologic foundation for the proposed study. To accomplish our goals, we will combine interdisciplinary approaches to characterize the pre-eruptive / pre-emplacement magmatic conditions in each district.

• Determine the geochemical evolution of magmas that occurred together using high-resolution analyses of major element, isotope, volatile, ligand, and metal concentrations specific to each ore deposit.
• Evaluate the physiochemical evolution of the magmatic plumbing system relative to the ore deposit.
• Identify the key magmatic processes and timescales important to sourcing, concentrating, and releasing ore metals into ore-forming fluids.

Districts targeted for study:

• iron-oxide-(copper-cobalt-gold-rare earth element) deposits, St. Francois Mountains, Southeast Missouri
• Jurassic uranium-thorium-rare earth element deposit in Bokan Mountain, Alaska
• Late Eocene calc-alkaline igneous centers in central Nevada
• early Eocene Yellow Pine gold-(anitmony-tungsten-silver-mercury) deposit in Idaho.
• gold-silver-(tellurium) deposits, Neogene Walker Lane, western Nevada

Melt Inclusions Overview

What are melt inclusions?

• Melt inclusions form when magmatic liquids (silicate or sulfide) are trapped within growing crystals, sealing tiny pockets of melt in mineral hosts. Whereas igneous rocks yield time-integrated records of magmatism, melt inclusions preserve "snapshots" of changing physical and chemical conditions as magmas evolve.
• Explaining complex "magma-to-metal" pathways is challenged by a lack of knowledge about metal behavior during the earliest stages of magma genesis and throughout the evolution of magmas as they ascend through the Earth's crust.
• There are complex feedbacks between magma crystallization, recharge, mixing, decompression, liquid immiscibility, degassing, and metal fractionation (e.g., Hedenquist & Lowenstern, 1994; Kamenetsky et al., 2004; Muntean et al., 2011).

⇒ Thus melt inclusions offer a powerful tool to probe multi-stage time slices in the development of magmatic-hydrothermal-ore systems that may be difficult (or impossible) to resolve by other means.

What information can melt inclusions provide?

• Volatile contents, major and trace element concentrations, ligand and metal abundances, and isotopic compositions are all sources of information that can be determined from in-situ melt inclusion analysis (e.g., Audétat & Lowenstern, 2014).
• These measurements enable direct quantification of ore metals in magmas, and combined with analyses of co-magmatic phenocrysts, provide a means to determine physiochemical conditions of melt inclusion entrapment and metal partitioning between liquid, vapor, and mineral phases.

Where can melt inclusion studies be applied?

• While studies of melt inclusions are routinely applied to volcanology problems, they have been seldom used to address magmatic-hydrothermal mineral resource problems and thus present an untapped opportunity to explore new dimensions of such systems.
• Melt inclusions have been used to help understand how porphyry ore deposits evolve (e.g., Mercer et al., 2015; Lerchbaumer & Audétat, 2013; Audétat, 2010; Rapien et al., 2003)

References

• Audétat, A., 2010, Source and Evolution of Molybdenum in the Porphyry Mo(-Nb) Deposit at Cave Peak, Texas: Journal of Petrology, 51(8), 1739–1760.
• Audétat, A., and Lowenstern, J.B., 2014, Melt Inclusions, in H.D. Holland and K.K. Turekian, Treatise on Geochemistry, 2nd Edition, 13, 143–173.
• Hedenquist, J.W., and Lowenstern, J.B., 1994, The role of magmas in the formation of hydrothermal ore deposits: Nature, 370 (6490), 519–527.
• Kamenetsky, V.S., Naumov, V.B., Davidson, P., Van Achterbergh, E., and Ryan, C.G., 2004, Immiscibility between silicate magmas and aqueous fluids: a melt inclusion pursuit into the magmatic-hydrothermal transition in the Omsukchan Granite (NE Russia): Chemical Geology, 210(1), 73–90.
• Lerchbaumer, L., and Audétat, A., 2013, The Metal Content of Silicate Melts and Aqueous Fluids in Subeconomically Mo Mineralized Granites: Implications for Porphyry Mo Genesis: Economic Geology, 108(5), 987–1013.
• Mercer, C.N., Hofstra, A.H., Todorov, T.I., Roberge, J., Burgisser, A., Adams, D.T., and Cosca, M., 2015, Pre-Eruptive Conditions of the Hideaway Park Topaz Rhyolite: Insights into Metal Source and Evolution of Magma Parental to the Henderson Porphyry Molybdenum Deposit, Colorado: Journal of Petrology, 56 (4), 645-679. doi:10.1093/petrology/egv010
• Muntean, J.L., Cline, J.S., Simon, A.C., and Longo, A.A., 2011, Magmatic-hydrothermal origin of Nevada/'s Carlin-type gold deposits: Nature Geoscience, 4(2), 122–127.
• Rapien, M.H., Bodnar, R.J., Simmons, S.F., Szabo, C.S., Wood, C.P., and Sutton, S.R., 2003, Melt inclusion study of the embryonic porphyry copper system at White Island, New Zealand: Society of Economic Geologists Special Publication, 10, 41–60.