Bokan Mountain Research Articles

Jurassic Uranium-Thorium Deposit of Peralkaline Granitic Rocks, Bokan Mountain, Prince of Wales Island, Southeastern Alaska

The Lower Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska hosts a high-grade uranium-thorium deposit with past production. The complex is a circular body (~3 km in diameter) which intruded Paleozoic granitoids as well as metasedimentary and metavolcanic rocks of the Alexander Terrane of the North American Cordillera. This shallow seated intrusion, which is composed of fractionated peralkaline granites, is a zoned body with a dominant core of arfvedsonite granite and a rim of aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. Both arfvedsonite and aegirine granites have overlapping compositions. The rocks have high contents of rare earth elements, Th, U and high field strength elements (Zr, Nb, Hf and Ta) and low contents of Sr, Ba, Eu and Ti, typical of peralkaline granites. The complex hosts structurally controlled rare metal mineralization, which is associated with the late-stage of magma evolution and hydrothermal fluids. Fluorine complexing played a role during the transportation of rare metals in hydrothermal fluids. The U-Th deposit, which occurs at the margin of the aegirine granite zone, includes structurally controlled shear zone-hosted lenses and irregularly shaped pipe-like orebodies. U-Th mineralization is associated with desilicified and albitized granitic rocks and includes mainly uranothorite and uraninite. These minerals mostly form small ovoids in veinlets typically 0.1 to 1 mm wide. The mine produced about 77,000 t of ore at a grade of ~0.76% U3O8 and 3% of ThO2. The parent magma of the pluton was likely derived from a metasomatized lithospheric source (mantle or lower crust), which was enriched by subduction related processes during Paleozoic time.

Origin of peralkaline granites of the Jurassic Bokan Mountain complex (southeastern Alaska) hosting rare metal mineralization

The Jurassic Bokan Mountain complex (BMC), composed of arfvedsonite and/or aegirine-bearing peralkaline A-type granitic rocks, is a circular body about 3 km in diameter located in southeastern Alaska. Like many other highly fractionated granitic bodies, the BMC granites were affected by late magmatic or post-magmatic processes, which, however, did not modify the contents of major elements. The granitic rocks are distinctly enriched in high-field-strength elements (HFSEs), rare earth elements (REEs), Y, Th, and U but depleted in Ba, Sr, and Eu and have high positive ɛNd(T) values. Unlike the variations in the major elements, Sr and Ba, which can be accounted by fractional crystallization, the abundance of REE, Y, HFSE, U, and Th (the elements which are hosted in accessory phases) were modified by F-rich hydrothermal fluids. The BMC hosts significant rare metal mineralization related to the late-stage crystallization history of the complex involving late magmatic and/or post-magmatic fluids. The mineralization includes two types: (1) a U–Th deposit which was exploited at the former Ross-Adams mine and (2) REE and Y mineralization mostly hosted in felsic dikes. Thermodynamic modelling of granites and spatially associated mafic rocks using the programme Rhyolite-MELTS implies that the granites can be derived from the mafic rocks by fractional crystallization. It is suggested that such a process (i.e. derivation of peralkaline granitic magma from the alkali or transitional basaltic magmas derived by partial melting from a lithospheric source metasomatically enriched in rare metals) can be invoked for other peralkaline granitic rocks hosting rare metal deposits.

Geophysical interpretation of U, Th, and rare earth element mineralization of the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeast Alaska

A prospectivity map for rare earth element (REE) mineralization at the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeastern Alaska, was calculated from high-resolution airborne gamma-ray data. The map displays areas with similar radioelement concentrations as those over the Dotson REE-vein-dike system, which is characterized by moderately high %K, eU, and eTh (%K, percent potassium; eU, equivalent parts per million uranium; and eTh, equivalent parts per million thorium). Gamma-ray concentrations of rocks that share a similar range as those over the Dotson zone are inferred to locate high concentrations of REE-bearing minerals. An approximately 1300-m-long prospective tract corresponds to shallowly exposed locations of the Dotson zone. Prospective areas of REE mineralization also occur in continuous swaths along the outer edge of the pluton, over known but undeveloped REE occurrences, and within discrete regions in the older Paleozoic country rocks. Detailed mineralogical examinations of samples from the Dotson zone provide a means to understand the possible causes of the airborne Th and U anomalies and their relation to REE minerals. Thorium is sited primarily in thorite. Uranium also occurs in thorite and in a complex suite of [Formula: see text] oxide minerals, which include fergusonite, polycrase, and aeschynite. These oxides, along with Y-silicates, are the chief heavy REE (HREE)-bearing minerals. Hence, the eU anomalies, in particular, may indicate other occurrences of similar HREE-enrichment. Uranium and Th chemistry along the Dotson zone showed elevated U and total REEs east of the Camp Creek fault, which suggested the potential for increased HREEs based on their association with U-oxide minerals. A uranium prospectivity map, based on signatures present over the Ross-Adams mine area, was characterized by extremely high radioelement values. Known uranium deposits were identified in the U-prospectivity map, but the largest tract occurs over a radioelement-rich granite phase within the pluton that is likely not related to mineralization. Neither mineralization type displays a well-defined airborne magnetic signature.

Corrigendum to The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization

Corrigendum to The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization

Comparison of SEM-EDS and EPMA-WDS Analysis of Rare Earth Element Containing Minerals from Bokan Mountain, Alaska

The Bokan Mountain Complex consists of peralkaline granitic intrusions and an associated vein-dike system that is enriched in LREEs, HREEs, Y, Zr, Nb, Th, and U and is mineralogically and paragenetically extremely complex (Fig. 1) [1,2]. Throughout the course of the work, SEM-EDS and EMP-WDS were employed to identify REE-containing minerals to create a REE budget for the prospect and determine the relative timing of the mineralizing events. To date, over 25 minerals have been identified which contain significant amounts of rare earth elements (REEs) including fergusonite, various Ti-Nb-Y-oxides (mainly polycrase), pyrochlore, gadolinite, hingganite, Y-Mg-fluosilicate (magnesiorowlandite?), xenotime, iimoriite, bastnaesite, parisite, synchysite, allanite, monazite, and multiple types of unidentifed Ti-Y-HREE-silicates, LREE±F±Ca±Na±Al-silicates, and an unidentified HREE-Nb-Y-oxide [2].

The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization

Abstract The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt.%, high iron (FeO (tot) ~ 3–4.5 wt.%) and alkali (8–10 wt.%) concentrations with high FeO(tot)/(FeO(tot) + MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O + K2O) ratio

Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40Ar/39Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: (i) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; (ii) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and (iii) intrusion of crosscutting lamprophyre dikes at >150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ∼30 km north and ∼100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

Yttrium recovery from Bokan Mountain, Alaska

The Bokan Mountain area on Prince of Wales Island, AK, contains vast quantities of heavy rare earth minerals. To lessen the US dependence on foreign sources, the US Bureau of Mines (USBM) investigated yttrium recovery from this ore. By sulfating the ore at 250° C for 1 hr at an H2SO4:ore ratio of 1:1 and then water leaching the sulfated material for 1 hr at 20% solids and 25° C, over 74% of the yttrium, cerium, and lanthanum were solubilized. Solvent extraction (SX), using a synergistic combination of OPAP and DEHPA, selectively loaded 100% of the yttrium. Yttrium was stripped directly as a solid using a 2M, 35° C NaOH liquor. When the OPAP/DEHPA mixture was tested in a continuous SX circuit, yttrium loading and stripping were 100% and 96%, respectively. The organic showed excellent recyclability properties with minimal solvent loss.

Geochemical evolution of the Precambrian Old Rag Granite, Virginia, U.S.A.: testing a UTh exploration model

The Old Rag Granite is one of several recognized units within the Virginia Blue Ridge Complex generally composed of Grenville-age gneisses and granitoid intrusive rocks. Chemical variation in the Old Rag Granite indicates simple orthomagmatic crystallization from the borders to the core of the pluton. Fractional crystallization of plagioclase, oxides, apatite and zircon accounts for much of the variation. Some trace elements, however, show anomalous behavior in the central part of the pluton. Y and Sr (and possibly Ba and Nb) are enriched whereas Rb and total REE are depleted in the central part of the pluton. Late crystallization of monazite could contribute to the observed REE patterns. It could not, however, account for Rb depletion and Sr enrichment, and no clear relationship between REE data and Th/U ratios are observed. The geochemical patterns indicate overall orthomagmatic crystallization overlain by a late-stage hydrothermal event. An exploration model for uraniferous granites, based upon the uranium deposits at Rössing and Bokan Mountain, included the Precambrian basement of central and northern Virginia in a list of proposed target areas. Geological, mineralogical and whole-rock chemical characteristics show a close association between the Old Rag Granite and the Rössing-type deposits. The uranium (mean 6.9 ppm, range 0.1–19.8 ppm) and thorium (mean 43 ppm, range 3.5–114 ppm) values are anomalous. Th/U ratios average 7.5 and are widely variable, indicating a decoupling of uranium from thorium. Neither U nor Th are lognormally distributed within the Old Rag Granite, requiring an explanation beyond simple removal of U by weathering. Rayleigh fractionation modeling shows that both U and Th, although probably distributed originally by closed-system fractionation, have been subsequently mobilized. Uranium can be lost in an oxidized form by secondary processes such as weathering or hydrothermal alteration. Thorium, however, will be affected only by hydrothermal processes. It is concluded that although primary fractionation was orthomagmatic, the Old Rag Granite was affected by late hydrothermal alteration whereby U was lost from the body and Th was redistributed. Three primary avenues for U migration are possible: (1) the rocks of the Saddleback Mountain Intrusive Suite which may have been contemporaneous with the Old Rag Granite; (2) Paleozoic fault zones that cut the Old Rag pluton; and (3) quartz veins, pegmatites, cupolas and roof pendants of the country-rock Nellysford gneiss that have been lost to erosion. The last option is preferred on geological and geochemical grounds. It is possible that the sedimentary uranium deposits of the adjacent Triassic Culpepper Basin were derived from eroded upper levels of the Old Rag Granite and like intrusives of the Virginia Blue Ridge Complex.

Geology and uranium-thorium mineral deposits of the Bokan Mountain Granite Complex, southeastern Alaska

The Bokan Mountain (Kendrick Bay) uranium-thorium deposits are associated with a Late Jurassic peralkaline granite ring-dike complex. The uranium-thorium deposits form pipe-like bodies along contacts, or occur as pods in en echelon northwest-striking shear zones. The host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite-aplite, aegirine granite porphyry, and eight other lesser rock types; the ore is closely associated with an albitized aegirine syenite. The ore consists of thorium and rare earth-rich rock containing uranothorite, uraninite, and generally less than 2% of sulfide species. About 100,000 tons (89,000 t) of ore have been mined at an average grade of about 1% U 3O 8 and nearly 3% ThO 2, mainly from the Ross-Adams pipe. The pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry; it is as much as 24 m across and was mined along strike for over 300 m. The ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length. Wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite, fluorite, calcite, quartz, sericite, and tourmaline. Hematite is present in the outer-distal parts of the ore zones. The minerals produced to date occur within the outer annular zone of the Bokan Granite Complex in which aegirine is present as the major alkali ferro-magnesian mineral. Ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber. The pod-like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events. Subsequent magma crystallization yielded riebeckite granites that were depleted in U and Th. Filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331°C with pressure corrections increasing the temperature of formation to 420°C or higher, depending on dissolved gas content. Sulfur isotopic analyses on pyrite, galena, sphalerite and pyrrhotite indicate disequilibrium, but δ 34S H 2S is estimated at 7.6‰. Carbon and oxygen isotopic analyses on ore stage calcite yield δ 18O H 2O of 6.8 to 8.1‰ and δ 13C ΣC of −4.3 to −7.0‰. The oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium-uranium deposits.

Uranium mineralization in the peralkaline Quanah Granite and related pegmatite-aplite dikes, wichita mountains, oklahoma

The coarse-grained facies of the Cambrian peralkaline Quanah Granite and genetically related aegirine-riebeckite and zircon-bearing pegmatite and aplite dikes show significant enrichment in uranium. Quanah granite is ortho-magmatically enriched in uranium with concentrations ranging from 2.6 to 103.8 (ppm U 3O 8). Uranium mineralization of the pegmatite and aplite dikes occurred during the pegmatitic and autometasomatic stages. The mean U 3O 8 value of the dikes is 153 ppm with a range of 2.68 to 1814.7 ppm. Rim albite is commonly developed in the granite at the grain boundary between microperthite grains or between microperthite and quartz. Aegirine-riebeckite pegmatite dikes are located in the Hale Spring area. These dikes are essentially vertical and strike in both north-south and east-west directions. They are characterized by a coarsely crystalline border which grades inward to a fine-grained, aplitic core. The coarse border of the dikes consists predominantly of quartz, microperthite, and riebeckite. A xenomorphic granular aggregate of quartz, K-feldspar, albite, aegirine-riebeckite, and iron oxide constitutes the interior of the dike. Several of the dikes exhibit pronounced banding due to the abrupt variation in grain size and to alternating riebeckite-rich, aegirine-rich, and feldspar-rich layers. Primary minerals have been subjected to deuteric or hydrothermal alteration. Late albite occurs as replacement masses and small veinlets. Fission track analysis shown the uranium is associated with aegirine, riebeckite-arvedsonite, allanite and zircon. Zircon-bearing pegmatites are located mainly in Charon Garden area. These pegmatites occur both as pod-shaped bodies and as narrow dikes. A large mass of milky quartz containing a few large zircon and altered riebeckite crystals is present in the center of the pegmatite. It grades outward into the coarse feldspar-quartz-riebeckite-zircon pegmatite. Zircon is zoned as a result of differing degrees of metamicitization. The metamict zircon is rich in rare earths, Fe 2O 3, thorium and uranium compared to the fresh zircon. Dike systems show enrichment in Zr, Nb, La, Y and Li while relatively high concentrations of Zr and Nb were observed in granite. In general, uranium and other trace element geochemistry of Quanah granite is similar to that characteristic of peralkaline granite of Bokan Mountain, Alaska and the Liruei complex in Nigeria.

Geology of southern Prince of Wales Island, southeastern Alaska

Geologic mapping on southern Prince of Wales Island combined with U-Pb (zircon) geochronologic, geochemical, and conodont studies yield new information about the early Paleozoic geologic framework and evolution of the southern Alexander terrane. The oldest rocks recognized consist of arc-type(?) meta-volcanic and metasedimentary rocks (Wales metamorphic suite) which were derived from late Proterozoic(?)-Cambrian protoliths. These rocks were deformed, metamorphosed, and uplifted during the Middle Cambrian-Early Ordovician Wales orogeny. Beginning soon after this orogenic event and continuing into Early Silurian time, basaltic to rhyolitic rocks and marine clastic strata (Descon Formation) were deposited, and dioritic to granitic plutons were emplaced. A variety of geochemical data suggest that these Ordovician-Early Silurian rocks formed in a volcanic-arc environment. The middle Silurian-earliest Devonian Klakas orogeny marked the end of arc-type magmatism; it is manifest on southern Prince of Wales Island by regional imbrication on southwest-vergent thrust faults, penetrative brecciation and pervasive hydrothermal alteration of pre-Devonian rocks in some areas, and at least several kilometres of structural uplift. Lower Devonian strata (Karheen Formation) record the waning stages of this event: conglomeratic redbeds low in the section were deposited in topographically rugged, subaerial to shallow-marine environments, whereas strata high in the section were deposited in more distal marine environments. Post-Devonian rocks on southern Prince of Wales Island include the Bokan Mountain Granite (Jurassic), basaltic to dacitic(?) dike swarms, and mid-Cretaceous granodiorite and diorite bodies. Prior to emplacement of the mid-Cretaceous plutons, the Devonian and older rocks were offset along several sets of strike-slip faults and then downdropped along the Keete Inlet fault.

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks: Reply

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks: Reply

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks: Discussion

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks: Discussion

Rb–Sr dating of the Bokan Mountain granite complex and its country rocks

Seven out of 11 whole-rock samples of the Bokan granitic complex give a Rb–Sr isochron date of 151 ± 5 Ma (Late Jurassic) as a minimum age for the complex. This result is discordant with published Early to Middle Jurassic K–Ar and U–Pb dates for the same unique peralkaline rock suite. A Late Jurassic age is tenable if excess radiogenic Ar and a trace of xenocrystic zircon are postulated to explain the previously reported results and the U–Pb data are reinterpreted to give an age between 156 ± 6 and [Formula: see text]. Granitic country rock for the complex is dated by Rb–Sr at 432 ± 19 Ma (Early Silurian), in agreement with published K–Ar dates.

U–Pb geochronology of the Bokan Mountain peralkaline granite, southeastern Alaska

The Bokan Mountain arfvedsonite–aegirine granite is the only peralkaline acid intrusion actually known on the margin of the Canadian Cordillera. It is the end product of a peralkaline magmatic evolution and is characterized by genetically associated concentrations of U and Th. The U–Pb method on zircons has been used to date two samples: one from a barren peralkaline granite and one from a low-grade U–Th mineralized albitite. A typological study has revealed two genetically different populations of zircons in the barren granite. The upper intercept age of 171 ± 5 Ma obtained on a concordia diagram dates the emplacement of the peralkaline granite. Acid wash experiments on zircons have allowed us to remove important quantities of common lead; this point is discussed as well as the abnormally low Pb content of the feldspar extracted from the barren granite.A Jurassic period of magmatic activity must be integrated into the geological history of Prince of Wales Island. Peralkaline magmatism may have occurred when the Alexander Terrane rifted away from its more southerly area of origin.

Favorable tectonic belts for granitic uranium deposits: Pan-Africa and the southern Appalachians

Granitic uranium deposits can be considered on the basis of two ideal models (Rössing and Bokan Mountain) that apparently were formed in different tectonic environments. It is possible to relate these models to various deposits or potential deposits found in Pan-African belts and the southern Appalachians. Considerable evidence suggests that Pan-African belts may be considered a uranium province, although similar evidence has not yet been found in the southern Appalachians. Some granitic rocks in certain tectonic belts of the southern Appalachians, however, hold promise as areas for future uranium exploration. In particular, peralkaline and peraluminous granitic rocks of the Raleigh-Kiokee, Inner Piedmont, and eastern Blue Ridge belts, in addition to those in the Grandfather Mountain Window, appear promising. These belts seem to provide the most promising tectonic environments in which granitic uranium deposits can occur.

I and L uranium and thorium vein system, Bokan Mountain, southeastern Alaska

The I and L veins are noteworthy because they contain significant amounts of associated uranium, thorium, and rare earths--an uncommon combination --and because of the distribution of their rare earths. The one to at least nine steep parallel to subparallel veins that form the main vein system strike northwestward and extend for 1.6 miles (2.6 km) from the east side of Bokan Mountain to the West Arm of Kendrick Bay. Typically the veins are thin, rarely exceeding 5 feet (1.5 m) in thickness, and they are characterized by abrupt changes in thickness. They transect parts of the Upper Triassic-Lower Jurassic Bokan Mountain granite throughout their northwestern extents and Ordovician granitic rocks to the southeast. The I and L system is bounded on the north and south by several well-defined fractures that are clearly visible on aerial photographs. Several small transverse veins, which strike almost at right angles to the main group of veins, are exposed in a small area north of the main vein system.The mineralogy of the veins is complex; uranium, thorium, and the rare earths occur in several minerals from different parts of the veins. A total of 34 minerals has been recognized, although many are sporadically or locally distributed. The principal gangue minerals are quartz and albite. Uranium generally occurs in thorium-bearing uraninite, but in several of the transverse veins it is found in brannerite. Thorite is the principal thorium mineral in the northwest and central part of the vein system, whereas allanite is found in the southeastern part of the system and in the transverse veins. In addition to allanite, the rare earth minerals include bastnaesite, xenotime, monazite, and an un-named mineral. The mineralogy of the I and L veins is similar in many respects to that of thorium-bearing veins in other states.The uranium content of 43 vein samples ranges from 0.005 to 2.8 percent; thorium content of 27 samples ranges from 0.0033 to more than 10 percent; and total rare earth content of 17 samples ranges from 0.023 to more than 10 percent. The distribution of the individual rare earths is unusual: one part of a vein may contain predominantly cerium-group rare earths (for example, bastnaesite), and another part of the same vein yields predominantly yttrium-group rare earths (for example, xenotime). The I and L veins also contain abnormal amounts of beryllium, niobium, zirconium, barium, strontium, tin, lead, zinc, copper, and molybdenum. Particularly interesting are beryllium, niobium, and zirconium. The distribution of beryllium is extremely erratic, and samples with high beryllium content, as much as 0.5 percent beryllium, occur adjacent to those that contained little or no beryllium. Abundance of niobium is greatest in the northwest part of the vein system, where 6 of 10 samples yielded from 0.10 to 0.70 percent. Zirconium is widespread in the veins, with only one out of 17 analyses yielding less than 0.15 percent and two more than 2 percent.

Potassium-Argon and Lead-Alpha Ages of Plutonic Rocks, Bokan Mountain Area, Alaska

Most of the granitic rocks in the Bokan Mountain area, southeastern Alaska, are early Paleozoic (probably Ordovician) judged by potassium-argon and lead-alpha age measurements. The Bokan Mountain Granite, the youngest intrusive unit in the area, belongs to a Mesozoic plutonic episode. These age measurements are the first direct evidence for the emplacement of early Paleozoic granitic intrusive rocks close to the Pacific margin of North America.