Unnamed Occurrences (ARDF - CR172), Dora Bay, Cholmondeley Sound, Prince of Wales Island, Ketchikan District, Prince of Wales-Outer Ketchikan Borough, Alaska, USA

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Location: This site is near the center of an area about two miles long and 2,000 feet wide that extends south along the valley from the head of Dora Bay. The site is about 0.6 mile northwest of the center of section 25, T. 77 S., R. 87 E.
Geology: Yttrium- and rare-earth-element (REE)-bearing pegmatites and pegmatitic dikes occur at numerous localities over a distance of about two miles from the the head of Dora Bay to Dora Lake (Barker and Mardock, 1990). The pegmatites and dikes are peripheral to a Jurassic syenite stock at the head of Dora Bay (Brew, 1996). The stock was formerly thought to intrude rocks of the Wales Group of Late Proterozoic and Cambrian age (Eberlein and others, 1983), but recent work indicates that the rocks are part of a previously unidentified Silurian and Ordovician unit of heterogeneous, low-grade metamorphic rocks that are less deformed than those of the Wales Group (S.M. Karl, written communication, 2003). The deposits closest to the stock are coarse-grained pegmatite dikes deposited from late-stage fluids; with increasing distance, they grade into 'vein dikes,' and ultimately into silica-rich veins. The pegmatite dikes are up to 13 feet thick and consist mainly of quartz and albite with minor riebeckite, aegirine, and zircon. They commonly have a halo of pyritic and chloritic alteration. The vein dikes generally are1 to 3 feet thick, have a pegmatitic core, contain banded quartz, and feature hydrothermal sericite, rhodochrosite, galena, and sphalerite. The silica-rich veins are usually less than 1 foot thick and often are extensions of the vein dikes; the veins typically are manganese-oxide-stained and more radioactive that the other dikes. There is a pronounced element zoning in the dikes marked by increase in lithophile elements with increasing distance from the pluton. The REE minerals are primarily thalenite and bastnaesite with lesser monazite, euxenite, and eudialyte (which contains most of the niobium). About half of the rare earths are the heavy REE subgroup. Barker and Mardock (1990) calculated the resources in two occurrences. They estimate that a vein dike about 3 feet thick near Dora Lake contains an inferred resource of about 500,000 tons of material with 442 parts per million (ppm) niobium, 71 ppm uranium, 1,775 ppm yttrium, 1.53 percent zirconium, and 2,816 ppm REE. Another block near the south end of Dora Lake is projected to have a strike length of 4,000 feet and a vertical extent of 2,000 feet. Assuming a thickness of 1.5 to 10 feet, this body has an estimated resource of 1.2 to 8.0 million tons of material that contains 340 ppm niobium, 27 ppm uranium, 1,969 ppm yttrium, 1.08 percent zirconium, and 3,647 ppm REE. Of this, 2.4 million tons is inferred. Hedderly-Smith (1999 [Inventory]) sampled the pegmatites, dikes, and veins around the syenite stock. None that he examined approaches ore grade for yttrium, REE, or zirconium. He describes two principal phases of late-stage mineralization: 1) feldspar-amphibole(-pyroxene) pegmatites; and 2) quartz-feldspar-manganese(-eudialyte) veins. There is also a nepheline syenite phase with bands of eudialyte. Electron probe studies indicate that the yttrium and rare-earth elements are not in the eudialyte but in some other (unspecified) silicate(s).
Workings: None beyond extensive sampling by government and private geologists. (The area was logged in the 1980's and considerable outcrop was exposed or blasted out along the many logging roads.)
Age: Genetically and spatially related to the Jurassic syenite stock at the head of Dora Bay.
Alteration: The deposits closest to the stock are coarse-grained pegmatite dikes deposited from late-stage fluids; with increasing distance, they grade into 'vein dikes,' and ultimately into silica-rich veins. The pegmatite dikes are up to 13 feet thick and consist mainly of quartz and albite with minor riebeckite, aegirine, and zircon. They commonly have a halo of pyritic and chloritic alteration. The vein dikes generally are 1 to 3 feet thick, have a pegmatitic core, contain banded quartz, and feature hydrothermal sericite, rhodochrosite, galena, and sphalerite. The silica-rich veins are usually less than 1 foot thick and often are extensions of the vein dikes; the veins typically are manganese-oxide-stained and more radioactive that the other dikes. There is a pronounced element zoning in the dikes marked by increase in lithophile elements with increasing distance from the pluton.
Reserves: Barker and Mardock (1990) calculated the resources in two occurrences. They estimate that a vein dike about 3 feet thick near Dora Lake contains an inferred resource of about 500,000 tons of material with 442 parts per million (ppm) niobium, 71 ppm uranium, 1,775 ppm yttrium, 1.53 percent zirconium, and 2,816 ppm REE. Another block near the south end of Dora Lake is projected to have a strike length of 4,000 feet and a vertical extent of 2,000 feet. Assuming a thickness of 1.5 to10 feet, this body has an estimated resource of 1.2 to 8.0 million tons of material that contains 340 ppm niobium, 27 ppm uranium, 1,969 ppm yttrium, 1.08 percent zirconium, and 3,647 ppm REE. Of this, 2.4 million tons is inferred. Hedderly-Smith (1999 [Inventory]) sampled the dikes and veins around the syenite pluton; none that he examined approaches ore grade for yttrium, rare earth elements, or zirconium.

Commodities (Major) - Nb, REE, Y
Development Status: None
Deposit Model: REE-bearing pegmatites and dikes peripheral to a syenite pluton.

Mineral List

Aegirine Albite 'Allanite' 'Amphibole Group' 'Bastnäsite'

Calcite 'Chlorite Group' Eudialyte Euxenite-(Y) 'Feldspar Group'

Galena 'Monazite' Muscovite var: Sericite Pyrite 'Pyroxene Group'

Quartz Rhodochrosite Riebeckite Sphalerite Zircon

20 entries listed. 12 valid minerals.

References