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Genetic types of economically workable uranium deposits
J. Nuclear Enagy II. 1956,Vol. 3. pp. 371to 382. PergamonPressLtd., London
GENETIC
TYPES OF ECONOMICALLY URANIUM DEPOSITS* D.
YA.
WORKABLE
SURAZHSKY
Abstract-Natural concentrations of uranium form under the most varied conditions, the range of which embraces the late stages of the true magmatic process, occurrences of metamorphism, the cycle of sedimentation and processes of weathering. Accordingly, uranium deposits are divided into four classes: (1) magmatogenic deposits; (2) sedimentary syngenetic deposits; (3) sedimentarymetamorphic deposits; (4) deposits resulting from weathering. To deposits of the first class belong pegmatites, pegmatoid veins, hydrothermal veins formed through the filling-in of open cavities, and stratified deposits of hydrothermal-metasomatic origin. Deposits of the second class are represented by uranium-bearing marine shales and phosphate rocks. Deposits of the third class include economically workable segregations of uranium in limestones and carbonaceous siliceous schists. To deposits of the fourth class belong stratified deposits in sandstones, conglomerates, sub-bituminous coals and lignites. In this article a general description of uranium deposits is given and questions are considered touching the sources of the ore, the character of structures which govern and localize the ore, the content and physical properties of metalliferous solutions and the conditions for deposition of the metal. URANIUM deposits which are known at the present time to be economically workable may be divided into six large groups by the form of their ore-bodies and by the character of the mineralized and ore-bearing rock. To the first group belong uraniumbearing pegmatites and pegmatoid veins; to the second group quartzose, quartzcarbonate, and fluorite-barytic veins in igneous and strongly metamorphosed sedimentary rocks; to the third group, stratified deposits in strongly metamorphosed sedimentary rock; to the fourth group, uranium-bearing strata in sedimentary rock of marine origin; to the fifth group, stratified deposits in feebly metamorphosed sedimentary rock; and to the sixth group, sheet-like and lenticular seams in normal sedimentary rock of continental facies. This grouping is adopted here chiefly for its convenience in subsequent description. Together with the general characteristics of deposits in the separate groups, characteristics compiled on the basis of published literature, this article will contain a consideration of the chief features of their origin, and, in the first place, of questions touching the sources of the ore, the character of the structures governing and localizing the ore, the content and physical properties of metalliferous solutions and the conditions of precipitation of the metal. Finally, some conclusions will be drawn from the factual material set forth; these will take the form of a scheme of genetic classification.
URANIUM-BEARING
PEGMATITES
AND
PEGMATOID
VEINS
These are ore formations of the most varied form, genetically very close to granitic intrusions. The dominant form is that of slab-shaped mineral bodies; beaded (sic) and lens-shaped veins are often met. Urafium in pegmatites is isolated either in the form of independent minerals (uraninite and the products of its oxidation) or in the form of an isomorphous mixture *
Translated by R. H. PONTING. 371
372
D.
YA.
!hRAZHSKY
in niobate-tantalates and in titanoniobates of varying composition. Sometimes carburan is also encountered. Uraninite and the compound oxides of uranium, niobium, tantalum and titanium have usually adapted themselves to the most widely differing parts of pegmatoid bodies. They are most frequently met in the zones rich in perthite. Moreover, uraninite is closely associated above all with finely-laminated muscovite and albite-oligoclase. Euxenite-polycrase minerals are usually met together with beryl; microlite and pyrochlore with lepidolite and spodumene; samarskite with muscovite; orthite and betafite with bi0tite.d) All these minerals, it appears, represent the products of a fractional crystallization of the pegmatite magma. The magmatic origin of uranium-bearing pegmatites (and of the uranium included in them) is accepted in most cases as being established beyond dispute. The practical importance of pegmatites as sources of uranium minerals is very slight. Out of the many hundreds of pegrnatite veins which go to form separate pegmatite fields, only single ones-at the most some few dozens-may be of interest on account of their uranium content. But even in these the segregations of uranium minerals are of a strictly local kind, forming single pockets, usually of very small extent. Until recently the quantity of uranium minerals obtained annually from these deposits was reckoned in tens of kilogrammes. This was in fact a by-product of the working of pegmatites for mica and feldspar. Quite recently in Canada, in the neighbourhood of Lake Charlebois and in other parts of the province of Saskatchewan, there were discovered areas of rock converted into migmatite, containing uraninite as a sort of component part of the quartz-feldspar veins.@) These deposits are regarded as potentially big sources of poor uranium ores, which are nevertheless in process of substantial enrichment. According to the latest information, an economically workable deposit of uranium-bearing pegmatites has also been revealed in a mountain range in Colorado.(3) Fairly close to the pegmatites are the original uranium-titanium veins, composed chiefly of quartz and uranium titanates, notably davidite [(Fe, Ce, U) (Ti, Fe, V, Cr) (OOH)], ilmenite and rutile. To this type belongs the vein deposit of Radium Hill in South Australia, which is of great economic importance.(*p 5,
QUARTZOSE,
QUARTZ-CARBONATE,
AND
FLUORITE-BARYTIC
VEINS
These veins occur not only in igneous rock (mainly in acid granitoids, acid effusives and in rock of sub-effusive facies), but in strongly metamorphosed sediments. The position of ore fields is indicated in the majority of cases by marked cleavage deformities of varying character and origin: by normal faults, reverse-faults, large zones of shearing, etc., extending usually for some dozens of kilometres. As a rule, none of these fractures carries a uranium mineralization; instead, they appear to be structures controlling the ore but not containing it. Uranium ores are localized usually in the finest crevices of a fissure or split. These crevices feather notable fractures, or distribute themselves in related zones of I shattering. The most productive parts of ore veins usually occur in rock rich in divalent iron or in organic substance, i;e. in amphibolites, in carbonaceous shales, in chloritic and
Genetic types of economically
workable uranium deposits
373
hornblende schists. In cases where the matric is igneous rock, the uranium mineralization is connected with zones of chloritization and talc-development, which formed in the pre-uranium stages of the ore process. By the nature of their mineral paragenesis, deposits of this type are extremely varied. The most important ore formations are: uranium alone, the uranium-nickelcobalt-bismuth-silver (the “j$e element complex”), the uranium polymetallic, and the uranium-molybdenum. Veins of uranium alone have an extremely simple composition. Of the ore minerals, there is found in them, besides pitchblende, only haematite and an inconsiderable amount of the usual sulphides-chalcopyrite, galenite, pyrite, and sometimes grey copper. Gangue minerals are represented for the most part by quartz and carbonate. Quite often associated with them is dark-violet, almost black fluorite. Occasionally in the composition of the vein-filling a part is played by zeolites and copper-yellow barytes. Usually the minerals separate out in the following order: quartz (often incrusting the walls of the vein), pitchblende, rosy dolomite. Uranium-nickel-cobalt-bismuth-silver veins are sharply distinguished from uranium veins by their considerably more elaborate mineral complex, by a predominant orecontent of arsenides and sulpho-arsenides of nickel, cobalt, iron (niccolite smaltitechloanthite, rammelsbergite-safflorite, gersdorffite, glaucodote, liillingite, arsenopyrite), by a notable growth of native elements within them (silver, arsenic, bismuth), and similarly of arsenites and sulpho-antimonites (proustite, stephanite, tetrahedrite, tennantite, pyrargyrite etc.), and finally, by the multiplicity of stages in the ore-forming processes. Typical examples of this formation are the veins of the Great Bear Lake in Canada, in which four successive stages of mineralization are distinguished9 (1) quartzose-pitchblende (quartz, pitchblende with a small quantity of diarsenides of nickel and cobalt); (2) quartz-arsenide (quartz, diarsenides, cobaltite, hematite, native bismuth); (3) carbonate-sulphide (dolomite, sphalerite, galenite, tetrahedrite, freibergite, chalcopyrite, bornite); and (4) carbonate-silver (rhodochrosite, strohmeyerite, jalpaite, argentite, hessite, native silver). In certain deposits, represented by veins of this formation, a distinction is drawn between two mineral zones: an upper-silver, and a lower-nickel-cobalt-bismuth. In this case uranium ores are encountered at varying depths. In the upper horizons of the deposits they are accompanied by ores of silver, in the lower ones by arsenides of nickel and cobalt, by native bismuth etc. ; still ‘lower they appear in the form of monometallic pitchblende veins. Correspondences of this kind permit us to regard the silver and nickel-cobalt ores as products of deposition from one and the same solution, whose content has been changed by fractional distillation. It is evident that pitchblende ores arose as the result of an independent stage in the ore-formation, which preceded the silvercobalt mineralization and became fixed in the same crevices, but at later moments in their exposure. The uranium polymetallic veins are noted for an abundance of simple sulphidespyrite, galenite, sphalerite, chalcopyrite, sometimes with arsenopyrite, bismuthinite and complex lead-bismuth-silver sulpho-salts. The gangue minerals are represented for the most part by quartz of several generations, and also by carbonates, barytes, and fluorite. The last-named sometimes constitutes the main mass of the ores. In the process of mineral-formation one can, as a rule, successfully indicate several stages. In one of the uranium-polymetallic deposits the early stages of
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mineralization are represented by quartz with sulphides; the intermediate stages by quartz, barytes and carbonate; the late stages by carbonate with pitchblende. In other deposits, for instance in Gilpin (Colorado, U.S.A.), the quartz-pitchblende stage precedes the quartz-carbonate sulphide stage.“) The uranium-molybdenum veins along with pitchblende and the products of its oxidation contain a mixture of molybdenite in quantities which give them a practical interest. Of the accessory minerals there are encountered haematite and the usual sulphides : chalco-pyrite, galenite, pyrite, and sometimes tennantite. The gangue mass consists of quartz, carbonate and a small amount of dark fluorite. In these deposits the pitchblende stage of the mineralization is one of the earliest. In every vein-type uranium deposit the genuine pitchblende stage is characterized by collomorphous structures not only of pitchblende, but sometimes of such minerals as carbonates, sulphides, chlorite etc. The typically metacolloid structure of the ores testifies to the marked saturation of the solutions and to the formation of gels, lacking the ability to react chemically with the wall rocks. Therefore the formation of ore bodies proceeds exclusively by way of the filling-in of open cavities. The role of metasomatism is insignificant. The most characteristic type of change which occurs around the ore and affects the rock walls is the so-called “red change,” observed, in particular, in the research carried out on the deposits of the Great Bear Lake(*) and the deposit at Ace in the region of Beaver Lodge Lake. w It affects not only igneous but also metamorphic rock and is dependent upon the haematization of the main rock-forming mineralsquartz, albite, and the carbonates. In places within the zones surrounding the ores there forms a jasper-like rock ((‘jasperoid”), consisting principally of quartz, magnetite, sericite, chlorite and carbonates. The pitchblende-polymetallic veins are frequently accompanied by zones of quartz, chlorite and sericite. Extremely common also is propylitization, as a consequence of which there appears an intensive removal from the rock walls of alkalis, principally sodium. In comparatively rare cases epidotization also is observed. High-temperature changes in the rock walls surrounding the ore-the formation of greisen, muscovite, and tourmaline-are not characteristic of endogene deposits of uranium. At the present time the overwhelming majority of research workers consider that the uranium present in vein deposits has been removed from the magma most probably in the form of the uranyl ion in sulphate or carbonate solutions.oO) Joint research on the filling-in of veins and on changes in rock walls surrounding the ore leads us to think of highly-concentrated solutions of elaborate composition with a high content of volatile components-arsenic, fluorine, boron, carbonic acid etc., bearing, moreover, a considerable amount of silicic acid, and also calcium, magnesium, barium, and trivalent iron. Quite frequently (but not always) bismuth, silver,’cobalt, nickel, zinc, lead and molybdenum unite with these elements. In the associations of minerals the vast majority of vein-type uranium deposits are related to mesothermal deposits, which are formed at temperatures ranging from 175” to 300°C and at depths of from 1200 to 3600 metres.(ll) In the precipitation of primary uranium minerals from solutions the main role is evidently played by oxidizing-reducing reactions of the type 3H,O + 2Fef2 + (U02)t2 -+ Fe,O, + UO” + 6H ,
Genetic types of economicallyworkable uranium deposits
375
leading to the formation simultaneously of uranium-bearing hydrocarbon and hsematite. It is possible similarly that pitchblende from uranyl-sulphate solutions is precipitated by hydrogen sulphide according to the equation H,S + (U 2+6)+2+ 4H = S + U02+4 + 2H,O. As long as ore bodies are formed exclusively by means of the deposition of material in open cavities, great importance attaches to the control exercised by structure. The principal ore-controlling structures are regional fissures; ore-localizing structures are the crevices connected with them and shearing of the second and higher orders. Vein deposits of formations of uranium alone, “the five-element” complex, the uranium-polymetallic and the uranium-molybdenum formations are the principal sources of the raw material of uranium on the earth. Besides this, notable segregations of uranium minerals are met also in many tin-tungsten, copper, gold-ore, and other veins; however, their practical importance is very slight. STRATIFIED
DEPOSITS IN STRONGLY METAMORPHOSED SEDIMENTARY ROCK
As a rule these deposits reveal no marked dependence on strong disjunctives. In the process of their formation the dominant r61e is played by the selective replacement of material of the sedimentary-metamorphic strata; moreover the chief factor which determines the distribution of the ore is the content of the rock walls. Ferruginous chert, carboniferous and silicified dolomitic schists are the most favourable to the deposition of the ore. Three ore formations are recognized: (a) The natural uranium formation, in which pitchblende is the sole economically valuable ore ; (b) the iron-uranium, characterized by a clear preponderance of magnetite and hematite in the content of the ore; (c) the copper-uranium, in which the dominant part is played by copper sulphides in association sometimes with the sulphides of cobalt, nickel, and molybdenum, and also with the native elements (copper, silver) and the minerals of thallium. In all these deposits the uranium mineralization in its main mass is represented by fine and relatively uniform dissemination of the fine grains of the primary uranium minerals; among the last, along with the amorphous, a notable part is played by crystalline modifications of pitchblende. Signs of metasomatism are revealed sharply and unevenly. Usually they appear in the successive processes wherein the rockwalls containing the ore are affected by albitization, carbonatization and silicification. Uranium mineralization begins in the first (alkaline) stage, but for the most part it is linked with the second (carbonate) stages of the metasomatic processes. It is sometimes observed that even in seams or layers of homogeneous content the replacement does not affect the whole mass of the rock, but is localized in certain centres, sometimes distinctly separated from centres where processes of metasomatism have been comparatively slight. The explanation of this is contained in the very mechanism of metasomatic replacement, which may take place only in rock characterized by certain (very small) sizes of pores. It is evident that the zones of such
D. k-A. %JRAZIIsKY
376
capillary porosity along which signs of replacement occur with particular intensity, are, in fact, canals which control the movement of solutions even in an environment which, in chemical and mineralogical content, is completely homogeneous. Judging by the characteristics of the new formation, the transporting and precipitating agents of uranium have been alkaline hydrothermal solutions of relatively weak concentration and therefore extremely mobile. These have reacted vigorously with the’rock walls. Their characteristic is a relatively simple composition and a complete lack of any volatile components, with the exception of water and carbonic acid. The question of the source of uranium in these deposits is a subject of discussion. According to some suggestions uranium is derived from magma as it crystallizes at depth; according to others, the part played by hydrothermal solutions brings about the distribution of the uranium, disseminated in the first place in the surrounding sedimentary rock. As an example of this type of deposit may be instanced the’natural uranium and copper-uranium occurrences of the region of Katherine-Darwin in North Australia.03) URANIUM-BEARING
STRATA IN NORMAL OF MARINE ORIGIN
SEDIMENTARY
ROCK
These are usually undisturbed, or sometimes very slightly disturbed, deposits in marine basins, developed over wide areas. In them uranium does not assume, or hardly ever assumes, independent mineral forms; for the most part it combines with organic matter and with the phosphates of calcium. The principal representatives of deposits of this group are the dark-coloured bituminous shales and phosphate rocks of marine origin. Uranium-bearing shales are characterized by a relatively narrow width in comparison with other formations which have been deposited in the same period of time; by a high content of organic matter (principally in the form of organic residues); and also by the absence or the utterly insignificant occurrence of calcium carbonates. In phosphate rocks the uranium content increases approximately in proportion to the increase in the phosphate content and decreases in varieties rich in calcium carbonate. As is indicated, coarse-grained phosphate formations contain more uranium than do fine-grained; sometimes uranium has enriched to a marked degree the aluminophosphates, which have been formed as a result of the weathering of horizons containing phosphate rocks.(14) At the present time it is usually accepted that the direct source of the uranium in marine shales and phosphate rocks has beenocean waters, and that the uranium precipitated out of these at approximately the same time as the accumulation of the sediment occurred. The deposition of the uranium in shales and phosphate rocks proceeds most quickly by way of the adsorption of this metal by carbonaceous material or by phosphate of calcium. Many peculiarities of uranium-containing shales and phosphate rocks suggest that their formation took place close to the edge of the continental shelf. It is known, in particular, that the richest areas of uranium-containing alum shale in Sweden are the lagoon facies of the Upper Cambrian Sea and that uranium-bearing shales of the state of Wyoming in the U.S.A. were formed under similar conditions. Deposits of uranium-bearing marine shales and phosphate rocks are characterized by extremely large supplies of uranium, but in view of the low metal content in the ore (0401 to 0.01%) they have so far been used to an inconsiderable extent.
Genetic types of economicallyworkable uranium deposits STRATIFIED
377
DEPOSITS IN FEEBLY METAMORPHOSED SEDIMENTARY ROCK
The deposits of this group are connected not only with marine precipitations but also with those in continental basins and, as a rule, they have been adapted to zones of intense folding. They are distinguished from true sedimentary (syngenetic) deposits by their considerably smaller extent, their higher and notably uneven metal content, and also by the isolation of the main uranium mass in the form of separate uranium minerals-uranium black pigments and, to an inconsiderable degree, pitchblende. In the upper oxidation-zones of these deposits there is an occasional growth of uranium vanadates of the type carnotite and tyuyamunite. Extremely important ore-containing rocks are carbonaceous-siliceous shales and organogenic limestones. Distinct signs of metamorphism usually appear in them: the sericitization of argillaceous material, carbonization of the organic matter, growth of such clearly metamorphic formations as porphyroblasts, metacrysts and pseudo-hydrothermal veinlets with ankerite, quartz, pyrite, and chalcopyrite. R. V. GETSEVA (personal communication) as the result of a prolonged study of one of these deposits has come to the conclusion that the formation of ore-bodies is possibly a consequence of the dissemination of syngenetically precipitated uranium in the process of the metamorphism of the strata which contain them; moreover the chief factor contributing to the extraction of uranium from metamorphogenic solutions has been its sorption by organic matter.
STRATIFIED AND LENTICULAR DEPOSITS IN NORMAL SEDIMENTARY ROCK OF CONTINENTAL FACIES These deposits are divided into two sub-groups. One of these is related to rocks of fluvial origin-chiefly sandstones and conglomerates; the other is related to caustobioliths. Deposits of the first group have been most thoroughly studied on the Colorado plateau in the U.S.A. Here the “primary” ores contain uranium in the form of oxides of the lowest valency (pitchblende and the silicates of tetravalent uranium) and vanadium in the form of hydroxides ;(15) there are usually present in them copper- and iron sulphides, as well as small quantities of molybdenum, cobalt, nickel, lead, zinc, selenium, and arsenic. The oxidized ores are characterized by the growth of highvalency compounds of uranium and vanadium, notably carnotite and tyuyamunite. The ore-bodies have a lens-like form, they are orientated to accord with the rockstratification and, as a rule, have adapted themselves to points of segregation of plant residues. According to the observations of American geologists, the localization of ores depends on the water permeability of the rock, the “coefficient of filtration” (i.e. the product of the permeability of a given stratum by its thickness) and on the presence of barriers impervious to water.(l@ Occasionally there is established a distinct coordination between the uranium mineralization and boundaries between rocks with a different water-penetrability, for instance between sandstones and ar&lites. The most productive areas appear to be those within whose boundaries a rapid change of facies at short intervals is observed. The presence in the higher parts of the stratigraphical profile of strata with a high “coefficient of filtration” is favourable, as is the presence of pockets having
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in their content volcanic ash or fragments of other effusive rocks. In many cases, notably in the strata of the Salt Wash formation of the Morrison series, ore-bodies are enclosed in insulated lenses of arkosic sandstones, surrounded by considerably less permeable argillites. In the Triassic deposits of the formation of Shinarump (Arizona) the insulated lenses of conglomerates in cross-bedded sandstones bear similarly high concentrations of uranium. In several important ore regions of the plateau of Colorado the uranium-bearing ore seams are restricted in their extent by channels of ancient streams, cut into the rock of the basement formation. It has been established that fluvial rocks are of the highest interest in connection with the carrying of uranium, for they fill in the winding erosional hollows of irregular shape, which have deep, narrow cross-sections and uneven bottoms. The ore concentrates for the most part at the heads of channels, in the water holes of the bottom, on the flanks and in the bends of ancient streams. The lower parts of channels, overlain with argillitic material, do not favour uranium mineralization. Red-coloured sandstones and argillites in the vicinity of segregations of uranium ores become discoloured and take on a greyish tint.(ls-zo) The difference in the colouring of metalliferous and nonmetalliferous rock is widely used as a guide in prospecting for new uranium deposits in the Plateau. For this purpose special maps are sometimes drawn up, in the form of colour charts. With this same sub-group of deposits we may associate the auriferous conglomerates of Witwatersrand in South Africa, which contain as well as gold a remarkable amount of pyrite, uraninite, thucholite, sericite and chlorite, and also small amounts of sulphides of cobalt, nickel, copper, lead and zinc. f21) There are also the ancient kindred conglomerates of the region of Blind River (Canada), which at the present time are receiving special attention on account of the presence in them of uraninite, thucholite and brannerite to an extent which gives them an economic significance.(22) The question of the origin of uranium ores in sandstones and conglomerates is still subject to spirited controversy. It is necessary to stress the essential difference in the conditions of formation of the uranium mineralization in the conglomerates of Witwatersrand and Blind River on the one hand, and in those of the Colorado Plateau on the other. This is revealed in the disparity of their material content, their morphology etc. Therefore their attachment to the same genetic group is a conventional procedure. However the majority of research workers assume that, in their present form, these ores were deposited after the formation of the surrounding rock, i.e. that the deposit is epigenetic. In rejecting the syngenetic hypothesis which formerly prevailed(23) they adduce, in particular, such facts as the presence of uranium seams in more than 20 stratigraphic horizons in the Colorado Plateau, the age of which varies between Permian and Tertiary; the small chance of uraninite being preserved in alluvial deposits; and the customary association of uranium, deposited as a result of chemical and biochemical processes, with fine-grained shales or phosphate rocks, but not with coarse-grained sandstones. (11) Serious opposition to the syngenetic hypothesis of uranium-ore formation in sandstones was provided by STIEFF,STERN, and MILKEY,(~~) who pointed out that the’ average age of the ores of the Plateau, ascertained by the lead method, was 71 million years, or, approximately half as great as the age of the surrounding rocks. However, even among the supporters of the epigenetic hypothesis of the Plateau
Genetic types of economically
workable uranium deposits
379
ore-formation there is no single opinion in the matter of the sources of the mineralization. Some of them assume that the seams of ore are the result of the activity of ground waters, which have extracted uranium from volcanic tuffs and other rocks, associated with deposits.(253 26) Others consider that the agents of the uranium’s transfer have been hypogene solutions, possibly with a strong dilution of ground waters, and that the sorce of the metal was contained in the magma as it crystallized at a depth.t21) Such a theory appears on the whole rather unlikely, if we remember that the majority of ore-deposits of this type reveal no connexion with igneous rocks and faulting, and that the material content of the ores (in particular the presence of vanadium) and the changes in the ore-containing pockets in the immediate vicinity of the ore are not typical of the hydrothermal process. Uranium-bearing coals contain uranium mostly in the form of uranium black pigments of uranium-organic compounds of uncertain composition. As a rule, the highest uranium content is found in semi-bituminous high-ash coals; the lowest in bituminous coals and anthracites. Coals which yield an economically useful ore of uranium are, as a rule, a very low grade mineral fuel. The origin of uranium in coals appears to be the same as that in sandstones in the Colorado Plateau. It is established that lignite in South Dakota (U.S.A.) contains noticeable concentrations of uranium only where it occurs directly beneath the unconformable tuffaceous formation of White River; (28) the lower beds of lignite may bear mineralization, if they lie in coarse-grained sandstones or in other rocks permeable to water. Besides this, the maximum uranium content is usually observed in the upper part of any given layer. This leads to the conclusion that uranium in coals has been precipitated by ground-waters and that the source of the uranium is the uranium-bearing volcanic rocks of the White River formation. The relatively high uranium content in high-ash coals is explained by the fact that these are more permeable to water than low-ash bituminous coals and anthracites. In the process of deposition of uranium it is evident that the adsorption of this metal by organic materials is of the greatest importance. Experiments have shown that sub-bituminous coal, lignite and peat irreversibly attract more than 98 % uranium from a solution containing about 0.02% of the meta1.(2g) It is assumed that the enrichment of coals by uranium is most probably the result of the deposition of this metal out of solutions of alkaline uranyl-carbonates or uranyl-carbonates of alkaline earths, disintegrating in the presence of acids, attracted from the lignite, with the formation of metal-organic compounds, relatively stable in a zone with low pHL30’ In this way, the epigenetic character of uranium in coals is being established, ofi the whole, with sufficient accuracy. Another assumption-that uranium in coal was originally concentrated by live plants-is unlikely, since even in the most favourable conditions the uranium content in the ash of live growths rarely reaches 0.01% while a much higher content is typical of the ash of uranium-bearing coals and lignites. Among deposits in sedimentary rocks of continental facies, uranium-bearing sandstones and conglomerates have at present great importance. These are; moreover, the chief source of the raw material of uranium in the United States.
rocks
shales
Deposits rcmltiug ikom weathering 1. Seams in sandstones and conglomerates 2. Seamsin subbituminouscoalsandlignites
2. Seams in carbonaceous-siliceous
deposits
deposits
HI. Sedimeatary-metamorpiao&Mc 1. Seams in limestones
2. Marine phosphate
II. Sedimautary (ayngenetic) 1. Marine shales
IV.
I
magma
Crystallizing
of
fOllll
Rocks containing uranium in scattered
Rocks containing uranium in scattered form
Ocean water
Crystallizing magma or scattered mineralization in surrounding rocks
magma
Probable source uranium
Crystallizing
_
Fundamental
magmatic
fusion
of
Ground water, containing alkaline and alkali-earth uranyl-carbonates
Metamorphogenic solutions unascertained composition
Hypogene. highly concentrated sulphate and carbonate solutions of compound content with a high proportion of volatile components Hypogene, feebly concentrated alkaline and alkali-earth solutions of relatively simple content with a low proportion of volatile components
Residual
features
Decomposition of uranylcarbonate complexes in the zone with low pH; sorption by organic matter
Oxidizing-reducing reactions in the presence organic matter
of
01
Oxidizing-reducing reactions in the presence divalent iron
1. Sorption by organic substance 2. Co-deposition with phosphates of calcium
01
Oxidizing-reducing reactions in the presence divalent iron
from
process of deposition
Direct crystallization fusion
Probable uranium
of origin
ECONOMICALLYWORKABLEURANIUMDEPOSTS
Probable character of the metalliferous solution
SCHEMEOFGENETICCLASSIFICATIONOF
2. Hydrothermal deposits: (a) uranium alone, uranium-polymetallic, uranium-nickel-cobaltn bismuth-silver, uranium-molybdenur and other veins, formed in open cavit ies (b) uranium alone, iron-uranium, coppet ruranium and other seams, formed through the metasomatism of rock walls
I. Magmatogenic deposita 1. Pegmatites and pegmatoid veins (a) granitic pegmatites with uraninite and/or compound oxides of uranium, tantalum, niobium, titanium, etc. ; (b) pegmatoid veins with titanates of iron and uranium
Classes and types of deposits
TABLET.-
Zones of optimum porosity
Midseam fissures, folded deformations the higher orders
-
-
Zones of optimum porosity
Fissures of faulting shearing, feathering regional breaks
Not established
of
and the
Structures localizing and controlling the ore
Genetic types of economically GENERAL
workable uranium deposits
381
CONCLUSIONS
The foregoing brief survey shows that natural concentrations of uranium form under the most varied conditions, embracing the late stages of the true magmatic process, occurrences of metamorphism, the sedimentary cycle and processes of weathering. According to these conditions of formation it is expedient to divide uranium deposits by their genetic characteristics into four classes: (1) magmatogenic deposits ; (2) sedimentary syngenetic deposits; (3) sedimentary metamorphogenic deposits; (4) deposits resulting from weathering. To deposits of the first class belong pegmatites and hydrothermal veins (the first and second of the groups described above). It would seem possible also to refer to this class the stratified deposits in strongly metamorphosed sediments (third group) although the question of the source of the uranium contained in them is still open. To deposits of the second class belong uranium-bearing beds of normal sedimentary rock (fourth group). Deposits of the third class include stratified deposits in feebly metamorphosed rock (lifth group). Deposits of the fourth class are represented by stratified and lenticular deposits in normal sedimentary rock of continental facies (sixth group). The general scheme of uranium deposit classification, which follows from the facts set forth above, is summarized in Table 1. REFERENCES 1. PAGE L. R. Econ. Geology 45, 12-34 (1950). 2. MAWDSLEY J. B. Canadian Min. Met. Bull. 482, 366-375 (1952). 3. MCKELVEY V. E. US Geol. Survey Bull. 1030A (1955). 4. PARKIN L. W. and GLADDENK. P. Econ. Geol. 49,815-825 (1954). 5. NININGERR. D. Minerals for Atomic Energy, N. J. D. van Nostrand Company Inc. (1954). 6. MURPHY R. Trans. Canad. Inst. Min. Met. 49, 426435 (1946).
7. U.S. Geological Survey and other sources, in the Proceedings af the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 211 et. seq. United Nations 1956. 8. KIDD D. F. and HAYC~CK M. H. Bull. Geol. Sot. Amer. 46, 879-960 (1935). 9. LANG A. H. Econ. Geol. Ser. No. 16 (1952). 10. MCKELVEY V. E., EVERHARTD. L., and GARREL~R. M. Proceedings of the International Con.ference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 551. United Nations, 1956. 11. LINJXREN W. Mineral Deposits, McGraw-Hill 1933. 12. BAIN G. W. Econ. Geol. 31, No. 5 (1936). 13. SULLIVANC. J., and MATHE~~NR. S. Econ. Geol. 47,751-758 (1952). 14. MCKELVEY V. E. and NELSONJ. M. Econ. Geol. 45, 35-53 (1950). 15. ROGENZWEIGA., GRUNER J. W., and GARDNERL. Econ. Geol. 49,351-361 (1954). 16. JOBIN D. A. Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 317. United Nations 1956. 17. MILLER L; Econ. Geol. 50, 156-169 (1955). 18. MATTERSJ. A. Econ. Geol. 50, 111-126 (1955). 19. ISACH~ENJ. W., MITCHAMJ. W., and WCCJDH. B. Econ. Geol. 50, 127-134 (1955). 20. WRIGHT R. J. Econ. Geol. 50, 127-134 (1955). 21. DAVIDSONC. F. Mining Mag. 88, 73-85 (1953). 22. TRA~LLR. J. Canad. Mining J. 75, 63-68 (1954). 23. FISCHER R. P. Econ. Geol. 32,906951 (1937). 24. STIEFF L. R., STERN T. W., and MILKEY R. G. U.S. Geological Survey Circular No, 271, 1953, quoted in FAUL H. (editor) Nuclear Geology, Wiley, New York 1954. 25. KOEBERLINF. F. Econ. Geol. 33,458-461 (1938).
382
D. YA. SIJRAZHSKY
26. GRUNERJ. W. Mines Mug. 44,53-56 (1954). 21. VINEJ. D. Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 452. United Nations 1956. 28. DENSONN. M. and GILL J. R. ibid. Vol. 6, p. 464. 29. MOORE G. W. Econ. Geol. 49, 652-658 (1954). 30. BREGER J. A., DEUL M., and RUBINSTEINS. Econ. Geol. 50,206-226
(1955).
GENETIC
TYPES OF ECONOMICALLY URANIUM DEPOSITS* D.
YA.
WORKABLE
SURAZHSKY
Abstract-Natural concentrations of uranium form under the most varied conditions, the range of which embraces the late stages of the true magmatic process, occurrences of metamorphism, the cycle of sedimentation and processes of weathering. Accordingly, uranium deposits are divided into four classes: (1) magmatogenic deposits; (2) sedimentary syngenetic deposits; (3) sedimentarymetamorphic deposits; (4) deposits resulting from weathering. To deposits of the first class belong pegmatites, pegmatoid veins, hydrothermal veins formed through the filling-in of open cavities, and stratified deposits of hydrothermal-metasomatic origin. Deposits of the second class are represented by uranium-bearing marine shales and phosphate rocks. Deposits of the third class include economically workable segregations of uranium in limestones and carbonaceous siliceous schists. To deposits of the fourth class belong stratified deposits in sandstones, conglomerates, sub-bituminous coals and lignites. In this article a general description of uranium deposits is given and questions are considered touching the sources of the ore, the character of structures which govern and localize the ore, the content and physical properties of metalliferous solutions and the conditions for deposition of the metal. URANIUM deposits which are known at the present time to be economically workable may be divided into six large groups by the form of their ore-bodies and by the character of the mineralized and ore-bearing rock. To the first group belong uraniumbearing pegmatites and pegmatoid veins; to the second group quartzose, quartzcarbonate, and fluorite-barytic veins in igneous and strongly metamorphosed sedimentary rocks; to the third group, stratified deposits in strongly metamorphosed sedimentary rock; to the fourth group, uranium-bearing strata in sedimentary rock of marine origin; to the fifth group, stratified deposits in feebly metamorphosed sedimentary rock; and to the sixth group, sheet-like and lenticular seams in normal sedimentary rock of continental facies. This grouping is adopted here chiefly for its convenience in subsequent description. Together with the general characteristics of deposits in the separate groups, characteristics compiled on the basis of published literature, this article will contain a consideration of the chief features of their origin, and, in the first place, of questions touching the sources of the ore, the character of the structures governing and localizing the ore, the content and physical properties of metalliferous solutions and the conditions of precipitation of the metal. Finally, some conclusions will be drawn from the factual material set forth; these will take the form of a scheme of genetic classification.
URANIUM-BEARING
PEGMATITES
AND
PEGMATOID
VEINS
These are ore formations of the most varied form, genetically very close to granitic intrusions. The dominant form is that of slab-shaped mineral bodies; beaded (sic) and lens-shaped veins are often met. Urafium in pegmatites is isolated either in the form of independent minerals (uraninite and the products of its oxidation) or in the form of an isomorphous mixture *
Translated by R. H. PONTING. 371
372
D.
YA.
!hRAZHSKY
in niobate-tantalates and in titanoniobates of varying composition. Sometimes carburan is also encountered. Uraninite and the compound oxides of uranium, niobium, tantalum and titanium have usually adapted themselves to the most widely differing parts of pegmatoid bodies. They are most frequently met in the zones rich in perthite. Moreover, uraninite is closely associated above all with finely-laminated muscovite and albite-oligoclase. Euxenite-polycrase minerals are usually met together with beryl; microlite and pyrochlore with lepidolite and spodumene; samarskite with muscovite; orthite and betafite with bi0tite.d) All these minerals, it appears, represent the products of a fractional crystallization of the pegmatite magma. The magmatic origin of uranium-bearing pegmatites (and of the uranium included in them) is accepted in most cases as being established beyond dispute. The practical importance of pegmatites as sources of uranium minerals is very slight. Out of the many hundreds of pegrnatite veins which go to form separate pegmatite fields, only single ones-at the most some few dozens-may be of interest on account of their uranium content. But even in these the segregations of uranium minerals are of a strictly local kind, forming single pockets, usually of very small extent. Until recently the quantity of uranium minerals obtained annually from these deposits was reckoned in tens of kilogrammes. This was in fact a by-product of the working of pegmatites for mica and feldspar. Quite recently in Canada, in the neighbourhood of Lake Charlebois and in other parts of the province of Saskatchewan, there were discovered areas of rock converted into migmatite, containing uraninite as a sort of component part of the quartz-feldspar veins.@) These deposits are regarded as potentially big sources of poor uranium ores, which are nevertheless in process of substantial enrichment. According to the latest information, an economically workable deposit of uranium-bearing pegmatites has also been revealed in a mountain range in Colorado.(3) Fairly close to the pegmatites are the original uranium-titanium veins, composed chiefly of quartz and uranium titanates, notably davidite [(Fe, Ce, U) (Ti, Fe, V, Cr) (OOH)], ilmenite and rutile. To this type belongs the vein deposit of Radium Hill in South Australia, which is of great economic importance.(*p 5,
QUARTZOSE,
QUARTZ-CARBONATE,
AND
FLUORITE-BARYTIC
VEINS
These veins occur not only in igneous rock (mainly in acid granitoids, acid effusives and in rock of sub-effusive facies), but in strongly metamorphosed sediments. The position of ore fields is indicated in the majority of cases by marked cleavage deformities of varying character and origin: by normal faults, reverse-faults, large zones of shearing, etc., extending usually for some dozens of kilometres. As a rule, none of these fractures carries a uranium mineralization; instead, they appear to be structures controlling the ore but not containing it. Uranium ores are localized usually in the finest crevices of a fissure or split. These crevices feather notable fractures, or distribute themselves in related zones of I shattering. The most productive parts of ore veins usually occur in rock rich in divalent iron or in organic substance, i;e. in amphibolites, in carbonaceous shales, in chloritic and
Genetic types of economically
workable uranium deposits
373
hornblende schists. In cases where the matric is igneous rock, the uranium mineralization is connected with zones of chloritization and talc-development, which formed in the pre-uranium stages of the ore process. By the nature of their mineral paragenesis, deposits of this type are extremely varied. The most important ore formations are: uranium alone, the uranium-nickelcobalt-bismuth-silver (the “j$e element complex”), the uranium polymetallic, and the uranium-molybdenum. Veins of uranium alone have an extremely simple composition. Of the ore minerals, there is found in them, besides pitchblende, only haematite and an inconsiderable amount of the usual sulphides-chalcopyrite, galenite, pyrite, and sometimes grey copper. Gangue minerals are represented for the most part by quartz and carbonate. Quite often associated with them is dark-violet, almost black fluorite. Occasionally in the composition of the vein-filling a part is played by zeolites and copper-yellow barytes. Usually the minerals separate out in the following order: quartz (often incrusting the walls of the vein), pitchblende, rosy dolomite. Uranium-nickel-cobalt-bismuth-silver veins are sharply distinguished from uranium veins by their considerably more elaborate mineral complex, by a predominant orecontent of arsenides and sulpho-arsenides of nickel, cobalt, iron (niccolite smaltitechloanthite, rammelsbergite-safflorite, gersdorffite, glaucodote, liillingite, arsenopyrite), by a notable growth of native elements within them (silver, arsenic, bismuth), and similarly of arsenites and sulpho-antimonites (proustite, stephanite, tetrahedrite, tennantite, pyrargyrite etc.), and finally, by the multiplicity of stages in the ore-forming processes. Typical examples of this formation are the veins of the Great Bear Lake in Canada, in which four successive stages of mineralization are distinguished9 (1) quartzose-pitchblende (quartz, pitchblende with a small quantity of diarsenides of nickel and cobalt); (2) quartz-arsenide (quartz, diarsenides, cobaltite, hematite, native bismuth); (3) carbonate-sulphide (dolomite, sphalerite, galenite, tetrahedrite, freibergite, chalcopyrite, bornite); and (4) carbonate-silver (rhodochrosite, strohmeyerite, jalpaite, argentite, hessite, native silver). In certain deposits, represented by veins of this formation, a distinction is drawn between two mineral zones: an upper-silver, and a lower-nickel-cobalt-bismuth. In this case uranium ores are encountered at varying depths. In the upper horizons of the deposits they are accompanied by ores of silver, in the lower ones by arsenides of nickel and cobalt, by native bismuth etc. ; still ‘lower they appear in the form of monometallic pitchblende veins. Correspondences of this kind permit us to regard the silver and nickel-cobalt ores as products of deposition from one and the same solution, whose content has been changed by fractional distillation. It is evident that pitchblende ores arose as the result of an independent stage in the ore-formation, which preceded the silvercobalt mineralization and became fixed in the same crevices, but at later moments in their exposure. The uranium polymetallic veins are noted for an abundance of simple sulphidespyrite, galenite, sphalerite, chalcopyrite, sometimes with arsenopyrite, bismuthinite and complex lead-bismuth-silver sulpho-salts. The gangue minerals are represented for the most part by quartz of several generations, and also by carbonates, barytes, and fluorite. The last-named sometimes constitutes the main mass of the ores. In the process of mineral-formation one can, as a rule, successfully indicate several stages. In one of the uranium-polymetallic deposits the early stages of
314
D. YA.
SURAZHSKY
mineralization are represented by quartz with sulphides; the intermediate stages by quartz, barytes and carbonate; the late stages by carbonate with pitchblende. In other deposits, for instance in Gilpin (Colorado, U.S.A.), the quartz-pitchblende stage precedes the quartz-carbonate sulphide stage.“) The uranium-molybdenum veins along with pitchblende and the products of its oxidation contain a mixture of molybdenite in quantities which give them a practical interest. Of the accessory minerals there are encountered haematite and the usual sulphides : chalco-pyrite, galenite, pyrite, and sometimes tennantite. The gangue mass consists of quartz, carbonate and a small amount of dark fluorite. In these deposits the pitchblende stage of the mineralization is one of the earliest. In every vein-type uranium deposit the genuine pitchblende stage is characterized by collomorphous structures not only of pitchblende, but sometimes of such minerals as carbonates, sulphides, chlorite etc. The typically metacolloid structure of the ores testifies to the marked saturation of the solutions and to the formation of gels, lacking the ability to react chemically with the wall rocks. Therefore the formation of ore bodies proceeds exclusively by way of the filling-in of open cavities. The role of metasomatism is insignificant. The most characteristic type of change which occurs around the ore and affects the rock walls is the so-called “red change,” observed, in particular, in the research carried out on the deposits of the Great Bear Lake(*) and the deposit at Ace in the region of Beaver Lodge Lake. w It affects not only igneous but also metamorphic rock and is dependent upon the haematization of the main rock-forming mineralsquartz, albite, and the carbonates. In places within the zones surrounding the ores there forms a jasper-like rock ((‘jasperoid”), consisting principally of quartz, magnetite, sericite, chlorite and carbonates. The pitchblende-polymetallic veins are frequently accompanied by zones of quartz, chlorite and sericite. Extremely common also is propylitization, as a consequence of which there appears an intensive removal from the rock walls of alkalis, principally sodium. In comparatively rare cases epidotization also is observed. High-temperature changes in the rock walls surrounding the ore-the formation of greisen, muscovite, and tourmaline-are not characteristic of endogene deposits of uranium. At the present time the overwhelming majority of research workers consider that the uranium present in vein deposits has been removed from the magma most probably in the form of the uranyl ion in sulphate or carbonate solutions.oO) Joint research on the filling-in of veins and on changes in rock walls surrounding the ore leads us to think of highly-concentrated solutions of elaborate composition with a high content of volatile components-arsenic, fluorine, boron, carbonic acid etc., bearing, moreover, a considerable amount of silicic acid, and also calcium, magnesium, barium, and trivalent iron. Quite frequently (but not always) bismuth, silver,’cobalt, nickel, zinc, lead and molybdenum unite with these elements. In the associations of minerals the vast majority of vein-type uranium deposits are related to mesothermal deposits, which are formed at temperatures ranging from 175” to 300°C and at depths of from 1200 to 3600 metres.(ll) In the precipitation of primary uranium minerals from solutions the main role is evidently played by oxidizing-reducing reactions of the type 3H,O + 2Fef2 + (U02)t2 -+ Fe,O, + UO” + 6H ,
Genetic types of economicallyworkable uranium deposits
375
leading to the formation simultaneously of uranium-bearing hydrocarbon and hsematite. It is possible similarly that pitchblende from uranyl-sulphate solutions is precipitated by hydrogen sulphide according to the equation H,S + (U 2+6)+2+ 4H = S + U02+4 + 2H,O. As long as ore bodies are formed exclusively by means of the deposition of material in open cavities, great importance attaches to the control exercised by structure. The principal ore-controlling structures are regional fissures; ore-localizing structures are the crevices connected with them and shearing of the second and higher orders. Vein deposits of formations of uranium alone, “the five-element” complex, the uranium-polymetallic and the uranium-molybdenum formations are the principal sources of the raw material of uranium on the earth. Besides this, notable segregations of uranium minerals are met also in many tin-tungsten, copper, gold-ore, and other veins; however, their practical importance is very slight. STRATIFIED
DEPOSITS IN STRONGLY METAMORPHOSED SEDIMENTARY ROCK
As a rule these deposits reveal no marked dependence on strong disjunctives. In the process of their formation the dominant r61e is played by the selective replacement of material of the sedimentary-metamorphic strata; moreover the chief factor which determines the distribution of the ore is the content of the rock walls. Ferruginous chert, carboniferous and silicified dolomitic schists are the most favourable to the deposition of the ore. Three ore formations are recognized: (a) The natural uranium formation, in which pitchblende is the sole economically valuable ore ; (b) the iron-uranium, characterized by a clear preponderance of magnetite and hematite in the content of the ore; (c) the copper-uranium, in which the dominant part is played by copper sulphides in association sometimes with the sulphides of cobalt, nickel, and molybdenum, and also with the native elements (copper, silver) and the minerals of thallium. In all these deposits the uranium mineralization in its main mass is represented by fine and relatively uniform dissemination of the fine grains of the primary uranium minerals; among the last, along with the amorphous, a notable part is played by crystalline modifications of pitchblende. Signs of metasomatism are revealed sharply and unevenly. Usually they appear in the successive processes wherein the rockwalls containing the ore are affected by albitization, carbonatization and silicification. Uranium mineralization begins in the first (alkaline) stage, but for the most part it is linked with the second (carbonate) stages of the metasomatic processes. It is sometimes observed that even in seams or layers of homogeneous content the replacement does not affect the whole mass of the rock, but is localized in certain centres, sometimes distinctly separated from centres where processes of metasomatism have been comparatively slight. The explanation of this is contained in the very mechanism of metasomatic replacement, which may take place only in rock characterized by certain (very small) sizes of pores. It is evident that the zones of such
D. k-A. %JRAZIIsKY
376
capillary porosity along which signs of replacement occur with particular intensity, are, in fact, canals which control the movement of solutions even in an environment which, in chemical and mineralogical content, is completely homogeneous. Judging by the characteristics of the new formation, the transporting and precipitating agents of uranium have been alkaline hydrothermal solutions of relatively weak concentration and therefore extremely mobile. These have reacted vigorously with the’rock walls. Their characteristic is a relatively simple composition and a complete lack of any volatile components, with the exception of water and carbonic acid. The question of the source of uranium in these deposits is a subject of discussion. According to some suggestions uranium is derived from magma as it crystallizes at depth; according to others, the part played by hydrothermal solutions brings about the distribution of the uranium, disseminated in the first place in the surrounding sedimentary rock. As an example of this type of deposit may be instanced the’natural uranium and copper-uranium occurrences of the region of Katherine-Darwin in North Australia.03) URANIUM-BEARING
STRATA IN NORMAL OF MARINE ORIGIN
SEDIMENTARY
ROCK
These are usually undisturbed, or sometimes very slightly disturbed, deposits in marine basins, developed over wide areas. In them uranium does not assume, or hardly ever assumes, independent mineral forms; for the most part it combines with organic matter and with the phosphates of calcium. The principal representatives of deposits of this group are the dark-coloured bituminous shales and phosphate rocks of marine origin. Uranium-bearing shales are characterized by a relatively narrow width in comparison with other formations which have been deposited in the same period of time; by a high content of organic matter (principally in the form of organic residues); and also by the absence or the utterly insignificant occurrence of calcium carbonates. In phosphate rocks the uranium content increases approximately in proportion to the increase in the phosphate content and decreases in varieties rich in calcium carbonate. As is indicated, coarse-grained phosphate formations contain more uranium than do fine-grained; sometimes uranium has enriched to a marked degree the aluminophosphates, which have been formed as a result of the weathering of horizons containing phosphate rocks.(14) At the present time it is usually accepted that the direct source of the uranium in marine shales and phosphate rocks has beenocean waters, and that the uranium precipitated out of these at approximately the same time as the accumulation of the sediment occurred. The deposition of the uranium in shales and phosphate rocks proceeds most quickly by way of the adsorption of this metal by carbonaceous material or by phosphate of calcium. Many peculiarities of uranium-containing shales and phosphate rocks suggest that their formation took place close to the edge of the continental shelf. It is known, in particular, that the richest areas of uranium-containing alum shale in Sweden are the lagoon facies of the Upper Cambrian Sea and that uranium-bearing shales of the state of Wyoming in the U.S.A. were formed under similar conditions. Deposits of uranium-bearing marine shales and phosphate rocks are characterized by extremely large supplies of uranium, but in view of the low metal content in the ore (0401 to 0.01%) they have so far been used to an inconsiderable extent.
Genetic types of economicallyworkable uranium deposits STRATIFIED
377
DEPOSITS IN FEEBLY METAMORPHOSED SEDIMENTARY ROCK
The deposits of this group are connected not only with marine precipitations but also with those in continental basins and, as a rule, they have been adapted to zones of intense folding. They are distinguished from true sedimentary (syngenetic) deposits by their considerably smaller extent, their higher and notably uneven metal content, and also by the isolation of the main uranium mass in the form of separate uranium minerals-uranium black pigments and, to an inconsiderable degree, pitchblende. In the upper oxidation-zones of these deposits there is an occasional growth of uranium vanadates of the type carnotite and tyuyamunite. Extremely important ore-containing rocks are carbonaceous-siliceous shales and organogenic limestones. Distinct signs of metamorphism usually appear in them: the sericitization of argillaceous material, carbonization of the organic matter, growth of such clearly metamorphic formations as porphyroblasts, metacrysts and pseudo-hydrothermal veinlets with ankerite, quartz, pyrite, and chalcopyrite. R. V. GETSEVA (personal communication) as the result of a prolonged study of one of these deposits has come to the conclusion that the formation of ore-bodies is possibly a consequence of the dissemination of syngenetically precipitated uranium in the process of the metamorphism of the strata which contain them; moreover the chief factor contributing to the extraction of uranium from metamorphogenic solutions has been its sorption by organic matter.
STRATIFIED AND LENTICULAR DEPOSITS IN NORMAL SEDIMENTARY ROCK OF CONTINENTAL FACIES These deposits are divided into two sub-groups. One of these is related to rocks of fluvial origin-chiefly sandstones and conglomerates; the other is related to caustobioliths. Deposits of the first group have been most thoroughly studied on the Colorado plateau in the U.S.A. Here the “primary” ores contain uranium in the form of oxides of the lowest valency (pitchblende and the silicates of tetravalent uranium) and vanadium in the form of hydroxides ;(15) there are usually present in them copper- and iron sulphides, as well as small quantities of molybdenum, cobalt, nickel, lead, zinc, selenium, and arsenic. The oxidized ores are characterized by the growth of highvalency compounds of uranium and vanadium, notably carnotite and tyuyamunite. The ore-bodies have a lens-like form, they are orientated to accord with the rockstratification and, as a rule, have adapted themselves to points of segregation of plant residues. According to the observations of American geologists, the localization of ores depends on the water permeability of the rock, the “coefficient of filtration” (i.e. the product of the permeability of a given stratum by its thickness) and on the presence of barriers impervious to water.(l@ Occasionally there is established a distinct coordination between the uranium mineralization and boundaries between rocks with a different water-penetrability, for instance between sandstones and ar&lites. The most productive areas appear to be those within whose boundaries a rapid change of facies at short intervals is observed. The presence in the higher parts of the stratigraphical profile of strata with a high “coefficient of filtration” is favourable, as is the presence of pockets having
318
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SURAZHSKY
in their content volcanic ash or fragments of other effusive rocks. In many cases, notably in the strata of the Salt Wash formation of the Morrison series, ore-bodies are enclosed in insulated lenses of arkosic sandstones, surrounded by considerably less permeable argillites. In the Triassic deposits of the formation of Shinarump (Arizona) the insulated lenses of conglomerates in cross-bedded sandstones bear similarly high concentrations of uranium. In several important ore regions of the plateau of Colorado the uranium-bearing ore seams are restricted in their extent by channels of ancient streams, cut into the rock of the basement formation. It has been established that fluvial rocks are of the highest interest in connection with the carrying of uranium, for they fill in the winding erosional hollows of irregular shape, which have deep, narrow cross-sections and uneven bottoms. The ore concentrates for the most part at the heads of channels, in the water holes of the bottom, on the flanks and in the bends of ancient streams. The lower parts of channels, overlain with argillitic material, do not favour uranium mineralization. Red-coloured sandstones and argillites in the vicinity of segregations of uranium ores become discoloured and take on a greyish tint.(ls-zo) The difference in the colouring of metalliferous and nonmetalliferous rock is widely used as a guide in prospecting for new uranium deposits in the Plateau. For this purpose special maps are sometimes drawn up, in the form of colour charts. With this same sub-group of deposits we may associate the auriferous conglomerates of Witwatersrand in South Africa, which contain as well as gold a remarkable amount of pyrite, uraninite, thucholite, sericite and chlorite, and also small amounts of sulphides of cobalt, nickel, copper, lead and zinc. f21) There are also the ancient kindred conglomerates of the region of Blind River (Canada), which at the present time are receiving special attention on account of the presence in them of uraninite, thucholite and brannerite to an extent which gives them an economic significance.(22) The question of the origin of uranium ores in sandstones and conglomerates is still subject to spirited controversy. It is necessary to stress the essential difference in the conditions of formation of the uranium mineralization in the conglomerates of Witwatersrand and Blind River on the one hand, and in those of the Colorado Plateau on the other. This is revealed in the disparity of their material content, their morphology etc. Therefore their attachment to the same genetic group is a conventional procedure. However the majority of research workers assume that, in their present form, these ores were deposited after the formation of the surrounding rock, i.e. that the deposit is epigenetic. In rejecting the syngenetic hypothesis which formerly prevailed(23) they adduce, in particular, such facts as the presence of uranium seams in more than 20 stratigraphic horizons in the Colorado Plateau, the age of which varies between Permian and Tertiary; the small chance of uraninite being preserved in alluvial deposits; and the customary association of uranium, deposited as a result of chemical and biochemical processes, with fine-grained shales or phosphate rocks, but not with coarse-grained sandstones. (11) Serious opposition to the syngenetic hypothesis of uranium-ore formation in sandstones was provided by STIEFF,STERN, and MILKEY,(~~) who pointed out that the’ average age of the ores of the Plateau, ascertained by the lead method, was 71 million years, or, approximately half as great as the age of the surrounding rocks. However, even among the supporters of the epigenetic hypothesis of the Plateau
Genetic types of economically
workable uranium deposits
379
ore-formation there is no single opinion in the matter of the sources of the mineralization. Some of them assume that the seams of ore are the result of the activity of ground waters, which have extracted uranium from volcanic tuffs and other rocks, associated with deposits.(253 26) Others consider that the agents of the uranium’s transfer have been hypogene solutions, possibly with a strong dilution of ground waters, and that the sorce of the metal was contained in the magma as it crystallized at a depth.t21) Such a theory appears on the whole rather unlikely, if we remember that the majority of ore-deposits of this type reveal no connexion with igneous rocks and faulting, and that the material content of the ores (in particular the presence of vanadium) and the changes in the ore-containing pockets in the immediate vicinity of the ore are not typical of the hydrothermal process. Uranium-bearing coals contain uranium mostly in the form of uranium black pigments of uranium-organic compounds of uncertain composition. As a rule, the highest uranium content is found in semi-bituminous high-ash coals; the lowest in bituminous coals and anthracites. Coals which yield an economically useful ore of uranium are, as a rule, a very low grade mineral fuel. The origin of uranium in coals appears to be the same as that in sandstones in the Colorado Plateau. It is established that lignite in South Dakota (U.S.A.) contains noticeable concentrations of uranium only where it occurs directly beneath the unconformable tuffaceous formation of White River; (28) the lower beds of lignite may bear mineralization, if they lie in coarse-grained sandstones or in other rocks permeable to water. Besides this, the maximum uranium content is usually observed in the upper part of any given layer. This leads to the conclusion that uranium in coals has been precipitated by ground-waters and that the source of the uranium is the uranium-bearing volcanic rocks of the White River formation. The relatively high uranium content in high-ash coals is explained by the fact that these are more permeable to water than low-ash bituminous coals and anthracites. In the process of deposition of uranium it is evident that the adsorption of this metal by organic materials is of the greatest importance. Experiments have shown that sub-bituminous coal, lignite and peat irreversibly attract more than 98 % uranium from a solution containing about 0.02% of the meta1.(2g) It is assumed that the enrichment of coals by uranium is most probably the result of the deposition of this metal out of solutions of alkaline uranyl-carbonates or uranyl-carbonates of alkaline earths, disintegrating in the presence of acids, attracted from the lignite, with the formation of metal-organic compounds, relatively stable in a zone with low pHL30’ In this way, the epigenetic character of uranium in coals is being established, ofi the whole, with sufficient accuracy. Another assumption-that uranium in coal was originally concentrated by live plants-is unlikely, since even in the most favourable conditions the uranium content in the ash of live growths rarely reaches 0.01% while a much higher content is typical of the ash of uranium-bearing coals and lignites. Among deposits in sedimentary rocks of continental facies, uranium-bearing sandstones and conglomerates have at present great importance. These are; moreover, the chief source of the raw material of uranium in the United States.
rocks
shales
Deposits rcmltiug ikom weathering 1. Seams in sandstones and conglomerates 2. Seamsin subbituminouscoalsandlignites
2. Seams in carbonaceous-siliceous
deposits
deposits
HI. Sedimeatary-metamorpiao&Mc 1. Seams in limestones
2. Marine phosphate
II. Sedimautary (ayngenetic) 1. Marine shales
IV.
I
magma
Crystallizing
of
fOllll
Rocks containing uranium in scattered
Rocks containing uranium in scattered form
Ocean water
Crystallizing magma or scattered mineralization in surrounding rocks
magma
Probable source uranium
Crystallizing
_
Fundamental
magmatic
fusion
of
Ground water, containing alkaline and alkali-earth uranyl-carbonates
Metamorphogenic solutions unascertained composition
Hypogene. highly concentrated sulphate and carbonate solutions of compound content with a high proportion of volatile components Hypogene, feebly concentrated alkaline and alkali-earth solutions of relatively simple content with a low proportion of volatile components
Residual
features
Decomposition of uranylcarbonate complexes in the zone with low pH; sorption by organic matter
Oxidizing-reducing reactions in the presence organic matter
of
01
Oxidizing-reducing reactions in the presence divalent iron
1. Sorption by organic substance 2. Co-deposition with phosphates of calcium
01
Oxidizing-reducing reactions in the presence divalent iron
from
process of deposition
Direct crystallization fusion
Probable uranium
of origin
ECONOMICALLYWORKABLEURANIUMDEPOSTS
Probable character of the metalliferous solution
SCHEMEOFGENETICCLASSIFICATIONOF
2. Hydrothermal deposits: (a) uranium alone, uranium-polymetallic, uranium-nickel-cobaltn bismuth-silver, uranium-molybdenur and other veins, formed in open cavit ies (b) uranium alone, iron-uranium, coppet ruranium and other seams, formed through the metasomatism of rock walls
I. Magmatogenic deposita 1. Pegmatites and pegmatoid veins (a) granitic pegmatites with uraninite and/or compound oxides of uranium, tantalum, niobium, titanium, etc. ; (b) pegmatoid veins with titanates of iron and uranium
Classes and types of deposits
TABLET.-
Zones of optimum porosity
Midseam fissures, folded deformations the higher orders
-
-
Zones of optimum porosity
Fissures of faulting shearing, feathering regional breaks
Not established
of
and the
Structures localizing and controlling the ore
Genetic types of economically GENERAL
workable uranium deposits
381
CONCLUSIONS
The foregoing brief survey shows that natural concentrations of uranium form under the most varied conditions, embracing the late stages of the true magmatic process, occurrences of metamorphism, the sedimentary cycle and processes of weathering. According to these conditions of formation it is expedient to divide uranium deposits by their genetic characteristics into four classes: (1) magmatogenic deposits ; (2) sedimentary syngenetic deposits; (3) sedimentary metamorphogenic deposits; (4) deposits resulting from weathering. To deposits of the first class belong pegmatites and hydrothermal veins (the first and second of the groups described above). It would seem possible also to refer to this class the stratified deposits in strongly metamorphosed sediments (third group) although the question of the source of the uranium contained in them is still open. To deposits of the second class belong uranium-bearing beds of normal sedimentary rock (fourth group). Deposits of the third class include stratified deposits in feebly metamorphosed rock (lifth group). Deposits of the fourth class are represented by stratified and lenticular deposits in normal sedimentary rock of continental facies (sixth group). The general scheme of uranium deposit classification, which follows from the facts set forth above, is summarized in Table 1. REFERENCES 1. PAGE L. R. Econ. Geology 45, 12-34 (1950). 2. MAWDSLEY J. B. Canadian Min. Met. Bull. 482, 366-375 (1952). 3. MCKELVEY V. E. US Geol. Survey Bull. 1030A (1955). 4. PARKIN L. W. and GLADDENK. P. Econ. Geol. 49,815-825 (1954). 5. NININGERR. D. Minerals for Atomic Energy, N. J. D. van Nostrand Company Inc. (1954). 6. MURPHY R. Trans. Canad. Inst. Min. Met. 49, 426435 (1946).
7. U.S. Geological Survey and other sources, in the Proceedings af the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 211 et. seq. United Nations 1956. 8. KIDD D. F. and HAYC~CK M. H. Bull. Geol. Sot. Amer. 46, 879-960 (1935). 9. LANG A. H. Econ. Geol. Ser. No. 16 (1952). 10. MCKELVEY V. E., EVERHARTD. L., and GARREL~R. M. Proceedings of the International Con.ference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 551. United Nations, 1956. 11. LINJXREN W. Mineral Deposits, McGraw-Hill 1933. 12. BAIN G. W. Econ. Geol. 31, No. 5 (1936). 13. SULLIVANC. J., and MATHE~~NR. S. Econ. Geol. 47,751-758 (1952). 14. MCKELVEY V. E. and NELSONJ. M. Econ. Geol. 45, 35-53 (1950). 15. ROGENZWEIGA., GRUNER J. W., and GARDNERL. Econ. Geol. 49,351-361 (1954). 16. JOBIN D. A. Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 317. United Nations 1956. 17. MILLER L; Econ. Geol. 50, 156-169 (1955). 18. MATTERSJ. A. Econ. Geol. 50, 111-126 (1955). 19. ISACH~ENJ. W., MITCHAMJ. W., and WCCJDH. B. Econ. Geol. 50, 127-134 (1955). 20. WRIGHT R. J. Econ. Geol. 50, 127-134 (1955). 21. DAVIDSONC. F. Mining Mag. 88, 73-85 (1953). 22. TRA~LLR. J. Canad. Mining J. 75, 63-68 (1954). 23. FISCHER R. P. Econ. Geol. 32,906951 (1937). 24. STIEFF L. R., STERN T. W., and MILKEY R. G. U.S. Geological Survey Circular No, 271, 1953, quoted in FAUL H. (editor) Nuclear Geology, Wiley, New York 1954. 25. KOEBERLINF. F. Econ. Geol. 33,458-461 (1938).
382
D. YA. SIJRAZHSKY
26. GRUNERJ. W. Mines Mug. 44,53-56 (1954). 21. VINEJ. D. Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva 1955, Vol. 6, p. 452. United Nations 1956. 28. DENSONN. M. and GILL J. R. ibid. Vol. 6, p. 464. 29. MOORE G. W. Econ. Geol. 49, 652-658 (1954). 30. BREGER J. A., DEUL M., and RUBINSTEINS. Econ. Geol. 50,206-226
(1955).