Na-Metasomatism and Uranium Mineralization during a Two-Stage Albitization at Kitongo, Northern Cameroon: Structural and Geochemical Evidence

Arnaud Patrice Kouske, Cheo Emmanuel Suh, Richard Tanwi Ghogomu, Vincent Ngako

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Laboratory of Applied Geology-Metallogeny, Department of Earth Sciences, University of Yaoundé 1, Yaoundé, Cameroon.

DOI: 10.4236/ijg.2012.31028                

Abstract

Mapping and documentation of lithological varieties and their corresponding geochemistry at the Kitongo uranium mineralization were concerned. The Kitongo U occurrence is hosted by granitic rocks that include interleaved sequences of metasedimentary and metavolcanic rocks of the collectively termed Poli Group. U-mineralization and Na-metasomatism are related and structurally controlled. The most promising uraniferous bodies are intimately related to intersections between the ductile ENE-trending faults and the brittle conjugate R' faults postdating the shearing event. The con- centration of uranium at fault intersections rather than along individual faults suggests that these zones that are dilatational in nature were also highly permeable and therefore the hydrothermal fluids ponded there could readily precipitate U therein. A two-stage albitization has altered the foliated granitic host rock and the second albitization that has over-printed the first one is more effective at fault intersections. Whole rock geochemistry was performed by using ICP-MS and ICP-AES respectively for major oxides, trace and REE. The U-bearing rock suite exhibits restricted range in SiO2 concentration (62.89% - 70.91%) and Al2O3 (13.16% - 18.59%) and it is poor in MgO (0.02% - 1.03%), CaO (0.24% - 1.88%) and K2O (0.08% - 5.32%). The mineralized rocks are however comparatively richer in Na2O (4.33% - 10.92%) compared to their barren counterparts. The host granite and associated granodioritic rocks in the area are weakly metaluminous, peralkaline, and are calc-alkaline. They are moderately to strongly fractionated and have tholeiitic and shoshonitic affinities with moderate to high HFSE (high field strength elements) and LILE (large ion lithophile elements) enrichment. The Rb/Sr, Rb/Ba and Sr/Ba ratios are 0.31, 0.14 and 1.48, respectively. U content in the mineralized granite is up to 651 ppm while the non-mineralized rock has only 2.4 ppm U. The REE patterns of the granite show LREE enrichment and strong Eu negative anomalies (Eu/Eu* = 0.03 to 0.48). The main mineralization stage characterized by local U, Na, Pb, Zn, Ga, Hf, Sr, Fe, Al, P and Zr enrichments is related to the second albitization event and could probably be associated in time with the calcite-uranium stage. The identification of fault segments favorable for uranium mineralization in northern Cameroon (Poli area) is important for understanding the genesis of hydrothermal ore deposits within continental strike-slip faults and therefore has great implications for exploration strategies.

Keywords

Uranium; Kitongo; Granite; Albitization; Strike Slip Fault; Cameroon

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	R. G. Skirrow, S. Jaireth, D. L. Huston, E. N. Bastrakov, A. Schofield, S. E. Van der Wielen and A. C. Barnicoat, “Uranium Mineral Systems, Processes, Exploration Criteria and a New Deposit Framework,” Geoscience Australia Record, 2009/20, 2009.
[2]	J. R. Faul, “Emerging Demand from Emerging Markets - A Trader’s Perspective,” NEI, International Uranium Fuel Seminar Savannah, 2010.
[3]	W. Zittle, L. B?lkow and J. Schindler, “Uranium Resources and Nuclear Energy,” Energy Watch Group Report, EWG No.1, 2006. http://www.energywatchgroup.com/fileadmin/global/pdf/EWG_Report_Uranium_3-12-2006ms.pdf
[4]	International Atomic Energy Agency, “Integrated Nuclear Fuel Cycle Information System (iNFCIS),” IAEA, Vienna. http://www-nfcis.iaea.org/
[5]	V. Thoste, “Mineral Exploration in North Cameroon, Region of Poli,” Final Report, Federal Republic of Germany, Number of Project 80.2273.3, 1985.
[6]	International Atomic Energy Agency, “World Distribution of Uranium Deposits (UDEPO) With Uranium Deposit Classification,” IAEA, Vi-enna, IAEA-TECDOC- 1629, 2009, p. 12.
[7]	M. Lassere, “Cameroun Mise en Evidence Radiométrique de Deux Séries d’Embréchites au Sein de la Zone Mobile de l’Afrique Centrale,” 10ème Colloque de Géologie Afr- icaine, Montpellier, 25-27 Avril 1979, p. 59.
[8]	M. Lassere and D. Soba, “Migmatisation d’Age Pan- africain au Sein des Formations Camerounaises App- artenant à la Zone Mobile de l’Afrique Centrale,” Compte Rendu Sommaire Société Géologique de France, Vol. 2, 1979, pp. 64-69.
[9]	B. Bessoles and R. Trompette, “Géologie de l’Afrique, la Cha?ne Panafricaine, Zone Mobile d’Afrique Centrale (partie Sud et zone mobile soudanaise),” Me?moires du Bureau de recherches ge?ologiques et minie?res, Vol. 92, 1980.
[10]	P. Affaton, M. A. Rahaman, R. Trompette and J. Sougy, “The Dahomeyide Orogen, Tectono-thermal Evolution and Relationships with the Volta Basin,” In: R. D. Dallmeyer and J. P. Lécorché, Eds., The West African Orogens and Circum-Atlantic Correlatives, Springer-Verlag, Berlin, 1991, pp. 107-122.
[11]	V. Ngako, P. Affaton and E. Njonfang, “Pan-African Tectonics in the Northern Cameroon, Implication for the History of Western Gondwana,” Gondwana Research, Vol. 14, No. 3, 2008, pp. 509-522. doi:10.1016/j.gr.2008.02.002
[12]	D. Küster and J. P. Liégeois, “Sr, Nd Isotopes and Geochemistry of the Bayuda Desert High-Grade Metamorphic Basement (Sudan), an Early Pan-African Oceanic Convergent Margin, Not the Edge of the East Saharan Ghost Craton?” Precambrian Research, Vol. 109, No. 1-2, 2001, pp. 1-23. doi:10.1016/S0301-9268(00)00147-9
[13]	V. Ngako, “Les Deformations Continentales Panafricaines en Afrique Centrale, Résultat d’un Poin?onnement de Type Himalayen,” Thèse de Doctorat d’Etat, Université de Yaoundé I, Yaoundé, 1999.
[14]	S. F. Toteu, “Geochemical Characterization of the main Petrographical and Structural Units of Northern Camer-oon, Implication for Panafrican Evolution,” Journal of African Earth Sciences, Vol. 10, No. 4, 1990, pp. 615-624. doi:10.1016/0899-5362(90)90028-D
[15]	J. Penaye, A. Kr?ner, S. F. Toteu, W. R. Van Schmus and J. C. Doumnang, “Evolu-tion of the Mayo Kebbi Region as Revealed by Zircon Dating: An Early (ca. 740 Ma) Pan-African Magmatic Arc in South-western Chad,” Journal of African Earth Sciences, Vol. 44, No. 4-5, 2006, pp. 530-542. doi:10.1016/j.jafrearsci.2005.11.018
[16]	S. F. Toteu, J. Penaye, E. Deloule, W. R. Van Schmus and R. Tchameni, “Diachronous Evolution of Volcano- Sedimentary Basins North of the Congo Craton, Insights from U-Pb Ion Microprobe Dat-ing of Zircons from the Poli, Lom and Yaounde′ Groups (Cameroon),” Journal of African Earth Sciences, Vol. 44, No. 4-5, 2006, pp. 428- 442. doi:10.1016/j.jafrearsci.2005.11.011
[17]	W. R. Fitches, A. C. Ajibade, I. G. Egbuniwe, R. W. Holt and J. B. Wright, “Late Proterozoic Schist Belts and Plutonism in NW Nigeria,” Jour-nal of the Geological Society, Vol. 142, No. 2, 1985, p. 319. doi:10.1144/gsjgs.142.2.0319
[18]	R. Caby, “The Pan-African Belt of West Africa from the Saharan Desert, the Gulf of Be-nin,” In: J. P. Schaer and J. Rodgers, Eds, Antonony of Moun-tain Ranges, Princeton Univeristy Press, Princeton, 1987.
[19]	R. Caby, “Les Terrains Précambrien du Bénin, Nigéria et Nord-Est Brésil et les Connections Sud-Atlantiques au Protérozo?que Supérieur,” International Meeting on Pro- terozoic Geology and Tectonics of High-Grade Terrains, Ile-Ife, Nigeria, Program and Lecture Series, 1988.
[20]	S. F. Toteu, A. Michard, J. M. Bertrand and G. Rocci, “U-Pb Dating of Precambrian Rocks from Northern Cam- eroon, Orogenic Evolution and Chronology of the Pan- African Belt of Central Africa,” Precambrian Research, Vol. 37, No. 1, 1987, pp. 71-87. doi:10.1016/0301-9268(87)90040-4
[21]	S. F. Toteu, J. Macaudière, J. M. Bertrand and D. Dautel, “Metamorphism Zircon from North Cameroon, Implications for the Pan-African Evolution of Central Africa,” Geologishe Rundschau, Vol. 79, No. 3, 1990, pp. 777-788. doi:10.1007/BF01879214
[22]	C. Castaing, C. Triboulet, J. L. Feybesse and P. Chèvremont, “Tectonometamorphism Evolution of Ghana, Togo and Benin in the Light of the Pan-African Brasiliano Orogeny,” Tectonophysics, Vol. 218, No. 4, 1993, pp. 323-342. doi:10.1016/0040-1951(93)90322-B
[23]	C. Castaing , J. L. Feybesse, D. Thiéblemont, C. Triboulet and P. Chèvremont, “Palaeogeographical Reconstitutions of the Pan-African/Brasiliano Orogen, Closure of an Oceanic Domain or Intracontinental Convergence between Major Blocks?” Precambrian Research, Vol. 69, 1994, pp. 327-344. doi:10.1016/0301-9268(94)90095-7
[24]	R. Trompette, “Geology of western Gondwana (2000 - 500 Ma), PanAfrican-Brasiliano Aggregation of South America and Africa,” A. A. Balkema Press, Rotterdam, 1994.
[25]	J. L. Poidevin, “La Tectonique Panafricaine à la Bordure Nord du Craton Congolais, l’Orogenèse des ‘Ouban- guides’,”12th Colloque on African Geology, Bruxelles, 1983, p. 75.
[26]	P. Jegouzo, “Evolution Structurale du Sud-ouest Came- roun durant l’Orogénèse Panafricaine, Associations de Tectoniques Cisaillantes et Chevauchante,” Colloque CNRS, Chevauchement et Déformation, Toulouse, 1984, p. 23.
[27]	J. P. Nzenti, P. Barbey, P. Jegouzo and C. Moreau, “Un Nouvel Exemple de Ceinture Granulitique dans une Chaine Proterozo?que de Collision, Les Migmatites Yaoundé au Cameroun,” Comptes Rendus Académie Sciences Paris, Vol. 299, 1984, pp. 1197-1199.
[28]	C. Pin and J. L. Poidevin, “U-Pb Zircon Evidence for a Pan-African Granulite Facies Metamorphism in the Central African Republic. A New Interpretation of the High- Grade Series of the Northern Border of the Congo Craton,” Precambrian Research, Vol. 36, No. 3-4, 1987, pp. 303-312. doi:10.1016/0301-9268(87)90027-1
[29]	S. F. Toteu, W. R. Van Schmus, J. Penaye and A. Michard, “New U-Pb and Sm-Nd Data from North-Central Cameroon and Its Bearing on the Pre-Pan-African History of Central Africa,” Precambrian Research, Vol. 108, No. 1-2, 2001, pp. 45-73. doi:10.1016/S0301-9268(00)00149-2
[30]	Y. H. Poudjom-Djomani, J. M. Nnange, M. Diament, C. J. Ebinger and D. J. Fairhead, “Effective Elastic Thickness and Crustal Thickness Variations in West Central Africa Inferred from Gravity,” Journal of Geophysical Research, Vol. 100, No. B11, 1995, pp. 22047-22070. doi:10.1029/95JB01149
[31]	U. O. Njel, “Paléogéographie d’un Segment de l’Oro- genèse Panafricaine, la Ceinture Volcano-Sédimentaire de Poli (Nord Cameroun),” Compte Rendu de l’ Académie des Sciences, Vol. 30, 1986, pp. 1737-1742.
[32]	Y. Le Fur, “Les Indices de Cuivre du Groupe Volcano Sédimentaire de Poli (Cameroun),”Bulletin du BRGM, Vol. 6, 1971, pp. 79-91.
[33]	S. F. Toteu, J. F. Dumont, J. Bassahak and J. Penaye, “Complexe de Base et Séries Intermediaires Dans la Zone Mobile Panafricaine de Poli au Cameroun,” Comptes Rendus de l’Académies des Sciences, Vol. 299, 1984, pp. 561-564.
[34]	P. Pinna, J. Y. Calvez, A. Abessolo, J. M. Angel, T. Mekoulou-Mekoulou, G. Mananga and Y. Vernhet, “Neo- proterozoic Events in the Tcholliré Area, Pan african Crustal Growth and Geodynamics in Central-Northern Cameroon (Adamawa and North Provinces),” Journal of African Earth Sciences, Vol. 18, No. 4, 1994, pp. 347-353. doi:10.1016/0899-5362(94)90074-4
[35]	J. Penaye, “Pétrologie et Structure des Ensembles Méta- morphiques au Sud Est de Poli (North Cameroon),” Unpublished Doctoral Thesis, University of Nancy (INPL), France, 1988.
[36]	J. Bassahak, “Le Complexe Plutonique de Kogué (poli, nord cameroun). Petrologie-Geochimie-Petrologie Struct- urale; sa Place dans la Chaine Panafricaine au Nord Cam- eroun,” Thèse de Doctorat University Nancy I, 1988.
[37]	R. E. Wilson, “Basic Wrench Tectonic,” American Association of Petroleum Geologists Bulletin, Vol. 57, 1973, pp. 74-96.
[38]	K. G. Cox, J. D. Bell and R. J. Pankhurst, “The Interpretation of Igneous Rocks,” Allen and Unwin, London, 1979.
[39]	A. Peccerillo and S. R. Taylor, “Geochemistry of Eocene Calc-Alkaline Volcanic Rocks from the Kastamonu Area, Northern Turkey,” Contributions to Mineralogy and Petrology, Vol. 58, No. 1, 1976, pp. 63-81. doi:10.1007/BF00384745
[40]	T. N. Irvine and W. R. A Baragar, “A Guide to the Chemical Classification of the Common Volcanic Rocks,” Canadian Journal of Earth Sciences, Vol. 8, No. 5, 1971, pp. 523-548. doi:10.1139/e71-055
[41]	P. D. Maniar and P. M. Piccoli, “Tectonic Discrimination of Granitoids,” Geological Society of America Bulletin, Vol. 101, No. 5, 1989, pp. 635-643. doi:10.1130/0016-7606(1989)101<0635:TDOG>2.3.CO;2
[42]	S. J. Shand, “Eruptive Rocks, Their Genesis, Composition, Classification, and Their Relation to Ore-Deposits with a Chapter on Meteorite,” John Wiley & Sons, New York, 1943.
[43]	H. De La Roche, J. Leterrier, C. P. Grande and M. Marchal, “A Classification of Volcanic and Plutonic Rocks Using R1-R2 Diagrams and Major Element Analyses its Relationships and Current Nomenclature,” Chemical Geology, Vol. 29, No. 1-4, 1980, pp. 183-210. doi:10.1016/0009-2541(80)90020-0
[44]	R. A. Batchelor and P. Bowden, “Petrogenetic Interpretation of Granitoid Rocks Series Using Multicationic Parameters,” Chemical Geology, Vol. 48, No. 1-4, 1985, pp. 43-55. doi:10.1016/0009-2541(85)90034-8
[45]	J. A. Pearce, N. B. W. Harris and A. G. Tindle, “Trace Element Discrimination Diagrams for the Tectonic Inter- pretation of Granitic Rocks,” Journal of Petroleum, Vol. 25, No. 4, 1984, pp. 956-983.
[46]	W. Boynton, “Cosmochemistry of the Rare Earth Elements, Meteorite Studies,” In: P. Henderson, Ed., Rare Earth Element Geochemistry, Elsevier, 1984, pp. 63-114.
[47]	F. J. Flanagan, “1972 Compilation of Data on USGS Standards,” In: F. J. Flanagan, Ed., Descriptions and Analyses of Eight New USGS Rock Standards, USGS Professional Paper 840, 1976, pp. 131-183.
[48]	S. R. Taylor and S. M. Mclennan, “The Continental Crust, Its Composition and Evolution,” Blackwell, Oxford, 1985.
[49]	W. R. Van Schmuss, “Natural Radioactivity of the Crust and Mantle Global Earth Physics—A Handbook on Phy- sical Constants,” American Geophysical Union Reference Shelf, Vol. 1, 1995, pp. 283-291.
[50]	J. C. Soula, “Characteristic and Mode of Emplacement of Gneiss Domes and Plutonic Domes in Central Eastern Pyrenees,” Journal of Structural Geology, Vol. 4, No. 3, 1982, pp. 313-342. doi:10.1016/0191-8141(82)90017-7
[51]	B. W. Chappell and A. J. R. White, “I- and S-Type Granites in the Lachlan Fold Belt,” Transactions Royal Society of Edinburgh Earth Sciences, Vol. 83, 1992, pp. 1-26.
[52]	H.-J. F?rster, G. Tischendorf and R. B. Trumbull, “An Evaluation of the Rb vs. (Y+Nb) Discrimination Diagram Toinfer Tectonic Setting of Silicic Igneous Rocks” Lithos, Vol. 40, No. 2-4, 1997, pp. 261-293. doi:10.1016/S0024-4937(97)00032-7
[53]	R. Emmermann, L. Daieva and J. Schneider, “Petrologic Significance of Rare Earth Distribution in Granites,” Contributions to Mineralogy and Petrology, Vol. 52, No. 4, 1975, pp. 267-283. doi:10.1007/BF00401457
[54]	L. A. Haskin and R. A. Schmitt, “Rare-Earth Distributions” In: P. H. Abelson, Ed., Researches in Geochemistry, John Wiley and Sons, Inc., New York, Vol. 2, 1967, pp. 235-258.
[55]	V. O. Olarewaju, “REE in the Charnockitic and Associated Granitic Rocks of Ado Ekiti-Akure, SW Nigeria,” In: P.O. Oluyide et al., Eds., Precambrian Geology of Nigeria, Geological Survey of Nigeria Publication, Kaduna, 1988, pp. 231-239.
[56]	V. U. Ukaegbu and F. T. Beka, “Rare Earth Elements as Source Indicators of Pan African Granites from Obudu Plateau, Southern Nigeria,” Chinese Journal of Geochemistry, Vol. 27, No. 2, 2008, pp. 130-134. doi:10.1007/s11631-008-0130-2
[57]	G. Zhang, R. Hu, X. Bi, H. Feng, P. Shang and J. Tian, “REE Geochemical Characteristics of the No. 302 Uranium Deposit in Northern Guangdong, South China,” Chinese Journal of Geochemistry, Vol. 26, No. 4, 2006, pp. 425-433. doi:10.1007/s11631-007-0425-8
[58]	R. Banerjee and K. Shivkumar, “Geochemistry and Petrogenesis of Radioactive Palaeoproterazoic Granitoids of Kinwat Inlier, Nanded and Yeotmal Districts Maharashtra,” Journal of the Geological Society of India, Vol. 75, No. 4, 2010, pp. 596-617. doi:10.1007/s12594-010-0054-4
[59]	R. K. O’Nions and R. J. Pankhurst, “Rare-Earth Element Distribution in Archaean Gneisses and Anorthosites, Godthab Area, West Greenland,” Earth and Planetary Science Letters, Vol. 22, 1974, pp. 328-338.
[60]	P. O. Okeke and M. A. Meju, “Chemical Evidence for the Sedimentary Origin of Igarra Supracrustral Rocks in the Southwestern Nigeria Basement Complex,” Nigeria Jour- nal of Mining and Geology, Vol. 22, No. 2, 1985, pp. 97-104.
[61]	R. Balk, “Structural Behavior of Igneous Rocks,” Memoire (SAUS) Geological Society of America, Vol. 1, 1937, pp. 291-302.
[62]	G. Courrioux, “Etude d’une Evolution Magmatique et Structural dans le Contexte d’une Zone de Cisaillement Ductile Active, Exemple du Linéament Granitique Her- cynien de Puentedeume (Gallice, Espagne),” Thèse 3e cycle, University Nancy I, 1984.
[63]	S. C. Paterson and O. T. Tobisch, “Using Pluton Ages to Date Regional Deformation Problems with commonly used Criteria,” Geology, Vol. 16, No. 12, 1989, pp. 1108- 1111. doi:10.1130/0091-7613(1988)016<1108:UPATDR>2.3.CO;2
[64]	J. L. Lagarde, “Granites Tardi Carbonifères et Défor- mation Crustale, l’Exemple de la Meseta Marocaine,” Thèse Docteur ès Science, Rennes, 1987.
[65]	J. L. Lagarde, S. Ait Omar and B. Roddaz, “Structural Characteristics of Plutons Emplaced during Weak Regional Deformation, example from Late Carboniferous Plutons Morocco,” Journal of Structural Geology, Vol. 12, No. 7, 1990, pp. 805-821. doi:10.1016/0191-8141(90)90056-5
[66]	D. Gasquet, “Genèse d’un Pluton Composite Tardi- Hercynien, le Massif de Tichka, Haut Atlas Occidental (Maroc),” Thèse Docteur ès Science, Université Henri Poincaré, 1991.
[67]	J. P. Nzenti, V. Ngako, R. Kambou, J. Bassahak and U. O. Njel, “Structures Régionales de la Cha?ne Panafricaine du Nord-Cameroun,”Comptes Rendus de l’Académie des Sciences, Vol. 315, 1992, pp. 209-215.
[68]	J. Li, M. Zhou, X. Li, Z. Fu and Z. Li, “Structural Control on Uranium Mineralization in South China, Implication for Fluid Flow in Continental Strike Slip Faults,” Science in China, Vol. 45, No. 9, 2002, pp. 851-864.
[69]	J. S. Caine, R. L. Bruhn and C. B. Forster, “Internal Structure, Fault Rocks, and Inferences Regarding Deformation, Fluid Flow, and Mineralization in the Seismogenic Stillwater Normal Fault, DixieValley, Nevada,” Journal of Structural Geology, 2010, Vol. 32, No. 11, pp. 1576-1589. doi:10.1016/j.jsg.2010.03.004
[70]	W. H. Newhouse, “Ore Deposits as Related to Structural Features,” Hafner Publishing Co., New York, London, 1942, p. 280
[71]	S. F. Cox, M. A. Knackstedt and J. Braun, “Principles of Structural Control on Permeability and Fluid Flow in Hydrothermal Systems,” Reviews in Economic Geology, Vol. 14, 2001, pp. 1-24.
[72]	R. H. Sibson, “Seismogenic Framework for Hydrothermal Transport and Ore Deposition,” Reviews in Economic Geology, Vol. 14, 2001, pp. 25-50.
[73]	S. Micklethwaite, “Mechanisms of Faulting and Fermeability Enhancement during Epithermal Mineralization, Cracow Gold?eld, Australia,” Journal of Structural Geology, Vol. 31, No. 3, 2009, pp. 288-300. doi:10.1016/j.jsg.2008.11.016
[74]	N. C. White, M. J. Leake and S. N. McCaughey, “Epithermal Gold Deposits of the Southeastern Pacific,” Jour- nal of Geochemical Exploration, Vol. 54, No. 2, 1995, pp. 87-136. doi:10.1016/0375-6742(95)00027-6
[75]	V. G. Voroshilov, “Anomalous Structures of Geoche- mical Fields of Hydrothermal Gold Deposits, Formation Mechanism Methods of Geometrization, Typical Models, and Forecasting of Ore Mineralization,” Geology of Ore Deposits, Vol. 51, No. 1, 2009, pp. 3-19. doi:10.1134/S1075701509010012
[76]	P. Alexandre, “Miner-alogy and Geochemistry of Sodium Metasomatism-Related Uranium Occurrence of Aricheng South, Guyana,” Mineralium Deposita, Vol. 45, No. 4, 2010, pp. 351-367. doi:10.1007/s00126-010-0278-7
[77]	S. Cinelu and M. Cuney, “Sodic Metasomatism and U–Zr Mineralization, a Model Based on the Kurupung Batholith (Guyana),” Geochim Cosmochim Acta, Vol. 70, No. 18, 2006, A103.
[78]	B. T. Zhang, J. Wu, Z. Qiu and Y. Liu, “On the Relationship between Hydrothermal Alteration and Uranium Enrichement,” Geological Review, Vol. 38, 1990, pp. 238- 245.