MINERALOGY AND MINERAL PARAGENESIS OF THE DOTSON RIDGE HREE DEPOSIT, BOKAN MOUNTAIN, PRINCE OF WALES ISLAND, ALASKA

The Dotson Ridge heavy rare-earth (HREE) deposit consists of an approximately 50m wide swarm of parallel veins that extend for two kilometers east-south-east from the margin of the Bokan Mountain peralkaline granite. Individually the veins are typically less than a meter wide but may be up to 2m. The vein system is mineralogically and paragenetically extremely complex. Present work indicates that the original veins were almost completely overprinted by a major younger hydrothermal event that remobilized the REE to form a new suite of minerals. It is unclear whether this hydrothermal event is just a late stage part of the evolution of the vein system or a separate later event.

The veins are composed mainly of quartz and albite with variable amounts of phlogopite, magnetite, hematite, and zircon. The dominate HREE-bearing minerals at Dotson Ridge are fergusonite, various Ti-Nb-Y-oxides (mainly polycrase), pyrochlore, an unidentified Ti-Y-HREE-oxide, an unidentified HREE-Nb-Y-oxide, gadolinite, hingganite, Y-Mg-fluosilicate (magnesiorowlandite?), xenotime, and iimoriite. The dominant LREE minerals are bastnaesite, parisite, synchysite, allanite, monazite, and multiple types of Ca-±Y-LREE-silicates. The proportion of LREE and HREE minerals is highly variable from sample to sample. Other minerals present in more than trace amounts include thorite, fluorite, cassiterite, ilmenite, pyrophanite, ilmenorutile, Ti-oxide (rutile?), Nb-oxide, fluocerite and cerianite. Sulfides are present in trace amounts in most samples and include galena, FeS, sphalerite, and chalcopyrite. Uranium-rich minerals are lacking or in trace amounts. In addition, a very complex hydrothermal alteration suite of REE-Th-U Silica-enriched Ti-, Nb-, Y-, Th-oxides, silicates, carbonates and fluocarbonates is present. A volumetrically small suite of low-temperature hydrothermal minerals is also present that consists of chlorite, zincian-chlorite, calcite, and complex inhomogeneous Fe-Mn-Zn-carbonates.