Peralkaline Granitic Rocks: History
View Latest Version
Please note this is an old version of this entry, which may differ significantly from the current revision.
Contributor:
Jaroslav Dostal

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.

  • peralkaline granite
  • Alaska
  • uranium
  • thorium

1. Introduction

Peralkaline granitic rocks are relatively rare rock-types, which are commonly enriched in rare elements (rare earth elements (REE), radioactive elements (uranium and thorium]) and high field strength elements (HFSE) such as Nb, Ta, and Zr). This enrichment can be of economic significance and has been a focus of recent exploration [1][2][3]. However, the origin of such mineralization is controversial. It has been ascribed to extensive fractional crystallization (e.g., [4][5][6]), late-stage magmatic-hydrothermal processes or low-temperature external hydrothermal fluids [7][8][9] or a combination of the processes [10][11][12][13]. To contribute to these discussions, this paper investigates the Bokan Mountain complex (BMC), a small peralkaline granitic intrusion in a southeastern part of Alaska, which hosts a high-grade uranium-thorium deposit with past production, as well as clusters of felsic dikes, which contain significant REE mineralization. The dikes occur along the margins or adjacent to the BMC in shear zones. The REE mineralization is distinct and ~1.5 km away from the uranium deposit, although both mineralization types are genetically related to the Bokan intrusion. Like other REE deposits associated with peralkaline felsic rocks, the Bokan REE mineralization is enriched in heavy REE as well as Y relative to total REE, while the Bokan uranium deposit is rich in Th.
Bokan Mountain (lat. 54°55′ N, long. 132°09′ W) is in southeastern Alaska, close to the southern end of Prince of Wales Island (the southernmost major island of Alaska). The complex is an approximately circular Jurassic intrusion (~177 Ma old) about 3 km in diameter, composed of highly fractionated peralkaline granitic rocks. The post-tectonic intrusion is well exposed and displays many geological characteristics typical of extension-related peralkaline granitic complexes worldwide (e.g., [14][15][16][17]).

2. Geologic Setting and Petrography

BMC lies within the Alexander Terrane of the North American Cordillera (Figure 1). The Cordillera consists of allochthonous oceanic and pericratonic terranes that were accreted to the northwestern margin of Laurentia (the North American craton) between late Paleozoic and early Cenozoic time (e.g., [18]). The Alexander terrane is a large outboard allochthonous terrane, which is considered to have an exotic nature with respect to the Laurentian margin (e.g., lack paleontological affinities with North America; [19][20]). The terrane (Figure 1) was accreted to the North American craton by the middle Cretaceous (~115–95 Ma), well after the emplacement of the Bokan complex.
Figure 1. Terrane map of southeastern Alaska and west coast of British Columbia. The map shows the location of Figure 2, the geological map of the Bokan Mountain Complex, southern Prince of Wales Island. The insert shows the location of Figure 1. Sites: 1—Dora Bay pluton; 2—Moffat volcanic rocks.
The basement of southern Prince of Wales Island is composed of the Neoproterozoic to Lower Cambrian Wales Group made up of arc-related volcanic rocks, sedimentary rocks and granitoid intrusions, which were all metamorphosed during the Middle Cambrian to Early Ordovician Wales orogeny. These rocks were unconformably overlain by Ordovician–Early Silurian oceanic arc-related metavolcanic and metasedimentary rocks and intruded by plutons composed mainly of Late Ordovician to Early Silurian quartz monzonites, quartz diorites, diorites and gabbros. The Paleozoic units were metamorphosed during the Klakas orogeny (Middle Silurian to earliest Devonian; [21]). These rocks, which represent an oceanic arc complex [21], host the Bokan stock. Mesozoic rocks are rare in this part of the terrane. Early Jurassic igneous rocks in the Alexander terrane are limited to the BMC, the Dora Bay pluton composed of peralkaline granites and syenite and located about 30 km N of the BMC and bimodal Moffat volcanic rocks 100 km to SE of the BMC (Figure 1), all dated at ~175–185 Ma [22][23].
The Bokan Mountain pluton is a typical anorogenic granitic complex, emplaced at a shallow depth. It produced the contact metamorphic assemblages which included andalusite (chiastolite)-bearing hornfels. Around the intrusion, the Paleozoic country rocks, particularly granitoids, are albitized. A wide (>1 km; [24]) aureole of albitization was produced by infiltration of hydrothermal fluids related to the pluton.
The Bokan granites have variable grain size, ranging from fine to coarse-grained. They are composed typically of quartz and feldspars with 2–10 vol.% of mafic minerals (sodic amphibole-arfvedsonite and sodic clinopyroxene-aegirine in various proportions). Locally, the mafic minerals comprise about 30 vol.% of the rocks. The granites are mainly made up of quartz (~25–45 vol.%), K-feldspar (20–30 vol.%) and albite (30–50 vol.%). The intrusion (Figure 2) is composed of two main zones [25][26]: (1) a core made up of arfvedsonite-bearing granite (porphyry) that forms the main part of the body and (2) an outer ring comprised predominantly of aegirine-bearing granite. The aegirine granite forms a nearly complete circular zone around the core [25][27]. A transition unit between these two zones is composed of aegirine-arfvedsonite granite and is typically about 15 m wide. In addition, there is a discontinuous outermost border zone composed of pegmatitic pods with aegirine and arfvedsonite crystals enclosed in aplitic rocks. Fine-grained aegirine granite, which outcrops in the central part of the stock probably represents the preserved roof of the intrusion [5]. The intrusion is associated with the quartz-feldspatic (felsic) dikes, which occur throughout the intrusions as well as outside, mainly along shear or fault zones. The dikes, composed mainly of quartz, albite and K-feldspar, form elongated lenses, which are steeply dipping and commonly 0.1 to 3 m thick. They include both aplitic and pegmatitic types. Most of them are rich in REE, Nb, Ta, Zr, Pb, Th and U and are closely related to the Bokan intrusion [28][29][30].
Figure 2. Geological sketch map of the Bokan Mountain complex and surrounding area showing the location of the former Ross-Adams mine and mineralization prospects including mineralized zones.
The granitic rocks have diverse texture although two main textures are dominant—primary porphyritic and late protoclastic [5][25][28]. The porphyritic texture contains phenocrysts of subhedral to euhedral quartz that ranges from 5 to 20 mm in diameter and K-feldspar enclosed in a fine-to medium-grained groundmass made up of quartz, albitic feldspar and minor K-feldspar. The protoclastic textures are characterized by broken phenocrysts of quartz and feldspars, bending of feldspar twining and pronounced undulose extinction of quartz crystals. Protoclastic textures resulted from an overpressure of fluids generated during the exsolution of the vapor phase from the magma during late stages of crystallization [5].
K-feldspar occurs as microperthite and cross-hatched microcline while plagioclase (An < 10) forms both a separate phase or an intergrowth or replacement of K-feldspar. Albite pervasively replaces K-feldspar, both in phenocrysts and in the groundmass [28][29]. The groundmass is dominated by albite and quartz with subordinate K-feldspar where the primary assemblage is overprinted by the metasomatic assemblage. All the major minerals have formed during at least two stages, one magmatic and the other related to late-stages (late magmatic to post-magmatic). Albite ranges from magmatic crystals to late- to post-magmatic albitic growth from a fluid phase. The albite is generally euhedral without signs of alteration. Quartz has also crystallized during two stages, initially, as the early megacrysts and then in the matrix and as veinlets. Well-developed early quartz crystals were intruded by the quartz veinlets of the second generation. There are also at least two generations of K-feldspar. Early K-feldspar, now occurring as a cross-hatched microcline, is enclosed/embayed by microperthitic K-feldspar that is the main K-feldspar phase. Around the Ross-Adam deposit, Cuney and Kyser [5] recognized two stages of alteration of the granites. The first stage included albitization of K-feldspar followed by the second stage, where albitization was accompanied by quartz dissolution leading to the generation of secondary syenitic rocks. This rock type is composed primarily of albite and aegirine with accessory fluorite, monazite, zircon and Fe-Ti oxides. The U-Th mineralization is closely associated with the syenitic rock [5][31][32]. The ore zone appears to be a strongly altered syenitic rock.
The sodic amphibole (arfvedsonite) occurs as subhedral prismatic crystals 1–4 mm long, which are distinctly pleochroic (lilac to blue) and interstitial. It also displays at least two-stages of growth. The inclusion-rich amphibole cores (stage 1) are surrounded an inclusion-free overgrowths (stage 2). Amphibole is commonly replaced by a mixture of secondary minerals (frequently hematite and quartz) or by fibrous aegirine and hematite with interstitial quartz and minor fluorite. Aegirine forms a variety of crystal shapes ranging from equant to prismatic or skeletal. Like arfvedsonite, it also shows a two-stage growth history. Philpotts et al. [30] argued that some aegirine grains might be of hydrothermal origin. Pyroxene and amphibole may be spatially associated but in other samples only one or the other is present. Where both minerals are present, the anhedral to euhedral amphibole partly encloses and corrodes the pyroxene but in some cases pyroxene post-dates amphibole. This association suggests overlapping crystallization of these two minerals. Amphibole and pyroxene are poikilitic, enclosing crystals of quartz, microcline and albite. Individual major rock forming minerals display only limited chemical variations [28][30]. Granitic rocks of the BMC host numerous accessory mineral phases. Particularly common are zircon, monazite, xenotime, titanite, fluorite and Fe-Ti oxides. Some of the accessory minerals including monazite could be late/post-magmatic.
The analyzed rocks from a wider vicinity of the Ross-Adams mine were subdivided into four groups—unaltered granites (group 1), albitized granites (group 2), mineralized albitized granites (group 3) and syenites (group 4) although there are transitions among the groups. Group 2 differs from group 1 by distinct albitization. Albite pervasively replaces K-feldspar, both phenocrysts (especially along the fractures and cleavage planes) and grains in the groundmass. The groundmass is dominated by albite and quartz. The accessory mineralogy appears to be the same as in group 1. Group 3 contains notably higher amounts of accessory minerals. In addition to ubiquitous accessory minerals such as zircon, monazite, xenotime, titanite, fluorite and Fe-Ti oxides, more than twenty U-Th-, REE- and HFSE-bearing minerals have been identified at the Bokan complex (e.g., [29][30][33]). Some of these minerals are variably present in mineralized altered granites and may occur in two or more generations. Group 4 includes a syenite, a fine-grained rock, which is likely the result of albitization and desilicification of the rocks in the proximity of the mineralization [5]. Quartz occurs in this rock only in subordinate amounts.

3. History of Mining and Exploration

After the 1955 uranium discovery in shear zones of the BMC by airborne and subsequently ground surveys [25], mining commenced in 1957 mainly due to incentive programs provided by the US Atomic Energy Commission [34][35]. The uranium mine was in operation intermittently between 1957 and 1971 and produced roughly 77,000 metric tons of ore averaging about 0.76 wt.% U3O8 and 3 wt.% ThO2 [33][35][36]. These concentrations are among the highest grades of U and Th ores ever mined in the United States and the Bokan is the only uranium deposit to have been mined in Alaska. The uranium ore was mined initially in a small open pit (50 × 110 × 7 m deep [34][35] located on the southeastern flank of Bokan Mountain at the Ross-Adams site (Figure 3). As the irregular cylindrical-shaped ore body plunges steeply at the end of pit, the deposit was subsequently developed as an underground operation through the establishment of two haulage adits (mine entrances). Ore was shipped by barge from the property to continental U.S. for processing, to fulfill contracts with the U.S. Atomic Energy Commission [1]. The mine stopped operations in 1971, due to depressed prices for uranium, leaving some uranium ore in the ground [36]. Subsequently, several companies carried out exploration in the BMC area during 1971–1981 [31][35][36].
Figure 3. (A) Abandoned Ross-Adams open-pit mine; (B) A southern part of the Ross-Adams pit mine with an adit. Host granites show distinct fracture-cleavages parallel to faults, which controlled emplacement of the deposit.
In addition to the Ross-Adam site, historic exploration has resulted in the delineation of several mineral prospects within or proximal to the BMC, some of which contained significant levels of REE, Y, Zr and Nb, in addition to U and Th [25][33][36]. Warner and Barker [36] predicted important resources of Th, REE, Y, Zr, Nb and Ta in the complex, in addition to potentially commercial resources of uranium remaining in and around the Ross-Adams mine.
After several years of hiatus, new exploration activities started in 2007 and continued to the present. The exploration targets shifted from uranium to REE and particularly focused on a prospect called the Dotson zone, a cluster of felsic dikes more than 2 km long (Figure 2), which lies just outside (within 1 km) of the Bokan stock [29]. The mineral resource estimation for the REE ore at the Dotson zone yielded 4.9 million metric tons of ore containing 0.61 wt.% oxides of REE and Y with a high ratio of heavy REE/total REE (heavy REE + Y/total REE + Y = ~0.6; [37]).

This entry is adapted from the peer-reviewed paper 10.3390/min13081032

References

  1. Long, K.R.; Van Gosen, B.S.; Foley, N.K.; Cordier, D. The Principal Rare Earth Element Deposits of the United States—A Summary of Domestic Deposits and a Global Perspective; Scientific Investigations Report 2010-5220; U.S. Geological Survey: Reston, VA, USA, 2010; 96p.
  2. Verplanck, P.L.; Van Gosen, B.S.; Seal, R.R.; McCafferty, A.E. A Deposit Model for Carbonatite and Alkaline Intrusion-Related Rare Earth Element Deposits; Scientific Investigations Report 2010-5070-J; U.S. Geological Survey: Reston, VA, USA, 2014; 58p.
  3. Dostal, J. Rare metal deposits associated with alkaline/peralkaline igneous rocks. Rev. Econ. Geol. 2016, 18, 33–54.
  4. Cuney, M. Felsic magmatism and uranium deposits. Bull. Soc. Geol. Fr. 2014, 185, 75–92.
  5. Cuney, M.; Kyser, K. Magmatic processes involved in uranium deposit formation. In Geology and Geochemistry of Uranium and Thorium Deposits; Cuney, M., Kyser, K., Eds.; Mineralogical Association of Canada, Short Course Series: Quebec City, QC, Canada, 2015; Volume 46, pp. 99–138.
  6. Boily, M.; William-Jones, A.E. The role of magmatic and hydrothermal processes in the chemical evolution of the Strange Lake plutonic complex, Quebec/Labrador. Contrib. Mineral. Petrol. 1994, 118, 33–47.
  7. Salvi, S.; Williams-Jones, A.E. The role of hydrothermal processes in the granite-hosted Zr, Y, REE deposit at Strange Lake, Quebec/Labrador: Evidence from fluid inclusions. Geochim. Cosmochim. Acta 1990, 54, 2403–2418.
  8. Salvi, S.; Williams-Jones, A.E. The role of hydrothermal processes in concentrating HFSE in the Strange Lake peralkaline complex, northeastern Canada. Geochim. Cosmochim. Acta 1996, 60, 1917–1932.
  9. Linnen, R.L.; Cuney, M. Granite-related rare-element deposits and experimental constraints on Ta–Nb–W–Sn–Zr–Hf mineralization. In Rare-element Geochemistry and Mineral Deposits; Linnen, R.L., Samson, I.M., Eds.; Geological Association of Canada: St. John’s, NL, Canada, 2005; Volume 17, pp. 45–68.
  10. Salvi, S.; Williams-Jones, A.E. Alkaline granite-syenite deposits. In Rare-element Geochemistry and Mineral Deposits; Linnen, R.L., Samson, I.M., Eds.; Geological Association of Canada: St. John’s, NL, Canada, 2005; Volume 17, pp. 315–341.
  11. Gray, T.R.; Hanley, J.J.; Dostal, J.; Guillong, M. Magmatic enrichment of uranium, thorium and rare earth elements in late Paleozoic rhyolites of southern New Brunswick, Canada: Evidence from silicate melt inclusions. Econ. Geol. 2010, 106, 127–143.
  12. Dostal, J. Rare Earth Element Deposits of Alkaline Igneous Rocks. Resources 2017, 6, 34.
  13. Dostal, J.; Gerel, O. Rare Earth Element Deposits in Mongolia. Minerals 2023, 13, 129.
  14. Bowden, P. The geochemistry and mineralization of alkaline ring complexes in Africa (A review). J. Afr. Earth Sci. 1985, 3, 17–39.
  15. Bowden, P.; Black, R.; Martin, R.F.; Ike, E.C.; Kinnaird, J.A.; Batchelor, R.A. Niger-Nigerian alkaline ring complexes: A classic example of African Phanerozoic anorogenic mid-plate magmatism. Geol. Soc. Spec. Publ. 1987, 30, 357–379.
  16. Richardson, D.G.; Birkett, T.C. Peralkaline rock associated rare metals. In The Geology of North America; Eckstrand, O.R., Sinclair, W.D., Thorpe, R.I., Eds.; Geological Society of America: Boulder, CO, USA, 1996; Volume P-1, pp. 523–540.
  17. Bonin, B. A-type granites and related rocks: Evolution of a concept, problems and prospects. Lithos 2007, 97, 1–29.
  18. Colpron, M.; Nelson, J.L.; Murphy, D.C. Northern Cordilleran terranes and their interactions through time. GSA Today 2007, 17, 4–10.
  19. Colpron, M.; Nelson, J.L. A Palaeozoic Northwest Passage: Incursion of Caledonian, Baltican and Siberian terranes into eastern Panthalassa, and the early evolution of the North American Cordillera. In Earth Accretionary Systems in Space and Time; Cawood, P.A., Kröner, A., Eds.; Geological Society of London: London, UK, 2009; Volume 318, pp. 273–307.
  20. Colpron, M.; Nelson, J.L. (Eds.) Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of North America, Canadian and Alaskan Cordillera; Geological Association of Canada: St. John’s, NL, Canada, 2006; Volume 45, 523p.
  21. Gehrels, G.E.; Saleeby, J.B. Geology of southern Prince of Wales Island, southeastern Alaska. Geol. Soc. Am. Bull. 1987, 98, 123–137.
  22. Dostal, J.; Karl, S.M.; Keppie, J.D.; Kontak, D.J.; Shellnutt, J.G. Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): Evidence for Early Jurassic rifting prior to accretion with North America. Can. J. Earth Sci. 2013, 50, 678–691.
  23. Karl, S.M.; Barker, J.C.; Dostal, J. Structural controls on emplacement of Early Jurassic peralkaline granite and rare earth mineralization at Bokan Mountain, southeast Alaska. Geol. Soc. Am. Abstr. Programs 2014, 46, 782.
  24. Barker, J.C.; Van Gosen, B.S. Alaska’s rare earth deposit and resource potential. Min. Eng. 2012, 64, 20–32.
  25. MacKevett, E.M., Jr. Geology and Ore Deposits of the Bokan Mountain Uranium-Thorium Area, Southeastern Alaska; U.S. Goverment Publishing Office: Washington, DC, USA, 1963; Volume 1154, 125p.
  26. Gehrels, G.E. Geologic Map of the Southern Prince of Wales Island, Southeastern Alaska; Map 1-2169, Scale 1:63,000; U.S. Geological Survey: Reston, VA, USA, 1992.
  27. Thompson, T.B.; Pierson, J.R.; Lyttle, T. Petrology and petrogenesis of the Bokan granite complex, southeastern Alaska. Geol. Soc. Am. Bull. 1982, 93, 898–908.
  28. Dostal, J.; Kontak, D.J.; Hanley, J.; Owen, V. Geological Investigation of Rare Earth Element and Uranium Deposits of the Bokan Mountain Complex, Prince of Wales Island, southeastern Alaska; U.S. Geological Survey Mineral Resources External Research Program—Report GO9PA00039; U.S. Geological Survey: Reston, VA, USA, 2011; 122p.
  29. Dostal, J.; Kontak, D.J.; Karl, S.M. The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization. Lithos 2014, 202–203, 395–412.
  30. Philpotts, J.A.; Taylor, C.D.; Tatsumoto, M.; Belkin, H.E. Petrogenesis of Late-Stage Granites and Y-REE-Zr-Nb-Enriched vein Dikes of the Bokan Mountain Stock, Prince of Wales Island, Southeastern Alaska; Open File Report 98-459; U.S. Geological Survey: Reston, VA, USA, 1998; 71p.
  31. Thompson, T.B. Geology and uranium-thorium mineral deposits of the Bokan Mountain granite complex, southeastern Alaska. Ore Geol. Rev. 1988, 3, 193–210.
  32. Thompson, T.B. Uranium, Thorium, and Rare Metal Deposits of Alaska. Econ. Geol. Monogr. 1997, 9, 460–482.
  33. Staatz, M.H. I&L uranium and thorium vein system, Bokan Mountain, southern Alaska. Econ. Geol. 1978, 73, 512–523.
  34. Stephens, F.H. The Kendrick Bay project. West. Min. 1971, 44, 151–158.
  35. Keyser, H.J.; McKenney, J. Geological Report on the Bokan Mountain Property, Prince of Wales Island, Alaska; NI 43-101 Report; Landmark Minerals Inc.: Toronto, ON, Canada, 2007; 48p.
  36. Warner, J.D.; Barker, J.C. Columbium and Rare Earth-Bearing Deposits at Bokan Mountain, Southeast Alaska; Open-File Report 33-89; United States Bureau of Mines: Washington, DC, USA, 1989; 196p.
  37. Bentzen, E.H.; Ghaffari, H.; Galbraith, L.; Hammen, R.F.; Robinson, R.J.; Hafez, S.A.; Annavarapu, S. Bokan Mountain Rare Earth Element Project near Ketchikan, Alaska; Preliminary Economic Assessment—NI 43-101; Tetra Tech.: Vancouver, BC, Canada, 2013; 227p.
More
© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license; additional terms may apply. By using this site, you agree to the Terms and Conditions and Privacy Policy.
This entry is offline, you can click here to edit this entry!
Video Production Service
Feedback