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Bokan Mountain Rare Earth Element Deposit, Alaska

Project Chief:

Douglas Stoeser
Phone: 303-236-9817
Email: dstoeser@usgs.gov

Project Objectives

There are three advanced rare earth element operations in the U.S.: Mountain Pass in California and Bear Lodge in Wyoming (both are light rare earth element carbonatite), and Bokan Mountain in Alaska (silicate vein dikes enriched in both light and heavy rare earth elements). Presently, China has a monopoly on the production of the heavy rare earth elements, derived from their clay deposits (laterites) in the southern regions of China. There is no current light rare earth element issue in terms of supply, but there is a world shortage of heavy rare earth elements. The most significant heavy rare earth element deposit in the United States is Bokan Mountain, Alaska. The mineralogically complex deposit is still poorly studied and little detailed reporting is available for it. The goal of this project is to understand the mineralogy, petrogenesis, and geochemistry of the rare earth element deposits at Bokan mountain and the geology of the associated peralkaline intrusion, with an emphasis on the main deposit underlying Dotson Ridge. Overall methodologies planned are microbeam mineralogical studies, whole rock and laser ablation-mass spectrometry geochemistry, fluid inclusions, and uranium/lead geochronology.

Tasks

Mineralogic & Petrologic Studies

Task Contact: Douglas Stoeser

Mineralogically the Dotson vein-dikes are extremely complex (Staatz, 1978; Philpotts and others, 1998; Stoeser and Van Gosen, in progress). To date no adequate description of Dotson vein-dike mineralogy has been published. Major minerals are quartz, albite, microcline, magnetite, hematite, zircon, allanite, and mica. Based on our work, the ore mineralogy is highly variable from sample to sample and consists of light rare earth element carbonates (bastnäsite, parisite, synchysite), phosphates (xenotime, monazite), various yttrium-titanium-heavy rare earth element ± niobium phases (oxides, hydroxides, silicates, and carbonate-silicates including fergusonite, aeschynite/polycrase, yittrocrasite(?), pyrochlore, etc.), thorite and uranothorite. The heavy rare earth elements are contained mainly in the yttrium-titanium-niobium phases, xenotime (when present), and zircon. Other minor minerals and numerous types of late and secondary hydrothermal silicate and carbonate minerals are also present. Objectives of the mineralogical and petrological studies will be to define the mineralogy, mineral paragenesis, and conditions of formation of the rare earth element deposits at Bokan Mountain. One particular goal is determine the mineralogical distribution of the rare earth elements, especially the heavy rare earth elements. The longer term goal will be understand the relationship of the mineralization to the late stage evolution of the Bokan Mountain peralkaline granite system and how the late stage magmatic and hydrothermal fluids formed.

Geochemistry

Task Contact: Philip Verplanck

Peralkaline magmas that evolve by fractional crystallization not only become progressively enriched in rare earth elements (REE) and high field strength elements (HFSE), but also volatiles. During the final stages of crystallization, the melts are a complex mixture of melt, aqueous fluids, and incompatible elements. Previous work evaluating the nature of the mineralization at Bokan Mountain has documented that the complexity of the mineralogy and that the enrichment in REEs and HFSEs is not only due to magmatic activity but was also enhanced by hydrothermal metasomatism (Dostal and others, 2011; Philpotts and others, 1998). Constraining the roles of magmatic and hydrothermal processes on the concentration of REEs in peralkaline (and carbonatitic) systems is an area of topical research. Task objectives are to look at the general geochemical evolution of the Bokan Mountain system including comprehensive high resolution geochemistry determination for whole rock samples, including major and trace elements, REEs, fluorine (F), chlorine (Cl), sulfur (S), carbon dioxide (CO2), water (H2O), and ferrous oxide (FeO).

Fluid Inclusions

Task Contact: Al Hofstra

Philpotts and others (1998) note that fluid inclusions are abundant in the vein-dikes including the occurrence of barite and fluorite with small shrinkage bubbles. The fluid inclusions, however, have not been studied. The presence of primary carbonates, hydroxide-minerals, fluorite, and fluorine-bearing minerals throughout the system indicates a high volatile content in the magma. It is not known if melt inclusions are present, but if present they are likely to be well preserved. Dostal and others (2011) did a reconnaissance study of the fluid inclusions of the intrusion and the I & L veins, but reported that fluid inclusions were poorly developed in many of their samples and results were often anomalous. Although fluid inclusion studies to date have not been very successful, examination of new samples may still yield useful results. The task objectives are to conduct reconnaisance work on fluid and melt inclusions (if any are present).

Geochronology

Task Contact: John Aleinikoff

Although geochronologic data are available for the Bokan Mountain intrusion (Dostal and others, 2011), geochronologic study of the vein-dike system has not been done. Since xenotime, monazite and several generations of zircon are ubiquitous in the Dotson rocks they are the first priority for U-Pb dating. The Bokan Mountain mineralized rocks and the Dotson system in particular contain several generations of zircon as well as xenotime and monazite. Thus multiple mineralogical targets suitable for U/Pb dating are available for Secondary Ion Mass Spectrometry (SIMS)/Sensitive High Resolution Ion Microprobe (SHRIMP) type geochronological investigation. Suitable samples for mineral separation will be examined.

Regional Geology

Task Contact: Brad Van Gosen

Regional geologic information is needed to put the Bokan Mountain deposits and associated peralkaline granite into a genetic context. In particular, the Bokan intrusive is still poorly mapped including the main intrusive phases and how mineralization is distributed within the granite. The structural aspects of the emplacement of the vein-dike systems and associated mineralization as well as the shear-zone related deposits is still poorly understood. It is known that there are multiple generations of veins generated at various stages of evolution of the magmatic fluids and subsequent non-magmatic hydrothermal fluids but these are still relatively unstudied. Understanding and modeling the formation of the Bokan rare earth element - yttrium (Y) - zirconium (Zr) - niobium (Nb) mineralization will require a comprehensive understanding of how the late stage magmatic fluids evolved within the peralkaline granitic intrusion and how these fluids were expelled outwards from the intrusion. The task objectives are the compilation of existing geologic mapping, drill core information, geochemistry, petrography, petrology, geochronology, mineralogic data and literature.


References Cited

Dostal, J., Kontak, D.J., Hanley, J., and Owen, V., 2011, Geological Investigation of Rare Earth Element and Uranium Deposits of the Bokan Mountain Complex, Prince of Wales Island, Southeastern Alaska: Final Technical Report in Fulfillment of the Requirement under U.S. Geological Survey Grant G09PA00039, 122 p. Download from http://minerals.usgs.gov/mrerp/reports.html#2009.

Philpotts, J.A., Taylor, C.D., Tatsumoto, M., and Belkin, H.E., 1998, Petrogenesis of late-stage granites and Y-REE-Zr-Nb-enriched vein dikes of the Bokan Mountain stock, Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Open-File Report 98-459, 71 p. Download from http://pubs.er.usgs.gov/publication/ofr98459.

Staatz, M.H., 1978, I and L uranium and thorium vein system, Bokan Mountain, southern Alaska: Economic Geology, 73 (4), p. 512-523. doi: 10.2113/gsecongeo.73.4.512

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