by Georgi NAUMOV, Dr. Sc. (Geol. & Mineral.), Vernadsky State Museum of Geology, Russian Academy of Sciences
Uranium is our planet's heaviest natural element. Still, it is active enough in migrating-both on the earth's surface and in the earth's crust (true, with some exceptions here and there). Why is that? And what is the mechanism of the migration process? An answer to these questions may clarify a good deal in our understanding of the travel of heavy elements in the bowels of the earth and also enable an insight into how uranium deposits are formed.
Uranium is rather widespread on the terrestrial surface: the mean content of this chemical element is by a factor of 102 higher than, say, in meteorites. Besides, space studies show that the surface shells of the other terrestrial planets are uranium-rich too. Within the earth shell uranium accounts for 2.5*10^-4 percent of the overall mass of rock. And even though the occurrence of uranium there exceeds that of tungsten and molybdenum, bismuth and gold, it is but very seldom found in large accumulations as essential mineral - for the most part, uranium occurs in association with other, nonuranium natural compounds.
But how do uranium accumulations come into being? Physicists attacked this problem toward the close of the Second World War when developing a nuclear weapon. Yet much earlier, in 1910, Academician Vladimir Vernadsky wrote thus:
"Radioactivity phenomena open to us atomic energy sources millions of times as high as the power sources that have been pictured in the human mind." And he warned, "Man will be in for a great future if
he understands that and does not use his work and his intelligence toward self- destruction."
Now where is uranium to be sought? Relevant research was launched back in the 1940s so as to learn more about the chemical behavior of elements under various natural conditions.
A GEOCHEMIST'S VIEW
Two valent states of uranium, U4+ and U6+, starkly different in their behavior, have been identified in naturally occurring compounds. In the earth shell, where free oxygen is absent, uranium exhibits a low degree of oxidation (U4+) and behaves like thorium and rare earths, i.e. it is poorly dissolved in water at any temperature and readily enters into other minerals in the form of impregnations.
But on the terrestrial surface, where atmospheric oxygen is present, the tetravalent uranium (U4+ is readily oxidized to hexavalent uranium (U6+ and, gaining two oxygen ions, forms a stable cation (a positively charged ion) - uranyl UO2+/2. The latter readily interacts with many anions (negatively charged
ions) to give rise to a large group of uranyl minerals (sulphates, carbonates, vanadates, silicates, etc.) that dissolve in water rather well.
And so, the hexavalent uranium U6+, which migrates readily in water solutions, is predominant on the surface of our planet. As a result, U6+ disperses fast and does not build up large accumulations. In deep-seated strata devoid of oxygen, however, it is reduced and becomes inert, occurring for the most part as pitchblende (uranium-containing mineral tar), which is a U4+ oxide free of thorium and rare earths - impurities with which it is associated in magmatic rock.
How does uranium migrate and how are its commercially usable concentrations formed? A scientific explanation of that has become possible only in the last few decades with the use of the chemistry of complex compounds.
TWO APPROACHES TO ONE PHENOMENON
Pondering over the transport of ore elements in the bowels of the earth, geologists would adopt the orthodox view: the solubility of many compounds increases with temperature. Extrapolating experimental data obtained in the t interval <100C to higher temperatures, they hoped to get solubility values quite acceptable for explaining the transportation phenomena. Uranium was no exception either. According to the established view, U separates from a cooling magmatic focus and, in hot fluoride and chloride solutions, migrates toward the terrestrial surface and is deposited there with the cooling of these solutions.
But direct experiments carried out later did not confirm this conventional model. It turned out that even at higher temperatures the solubility of compounds does not increase, it may even go down, and quite significantly at that. Thus the hopes about the dominant role of temperature in the uranium transport and deposition failed to materialize.
Yet Academician Vernadsky (who proceeded from geochemical data on the absence of thorium minerals in uranium-bearing veins) and his followers took a different approach: uranium is migrating in depth strata in the form of uranyl compounds. This assumption relied on two empirical facts: 1) the stable association of uranium ores with carbonates (way back in 1948 Academician Dmitry Shcherbakov, then heading the uranium-prospecting works in the USSR, called attention to that), and 2) the chemists' data on the possibility of uranyl/carbonate complex ions being formed in water solutions, a factor accounting for a change in the behavior of uranium there (in the 1950s Dr. Vladimir Shcherbina, a pupil of Academician Fersman's, drew attention to this phenomenon). The amassed body of evidence allowed to introduce the uranyl/carbonate complex ions into the theory of uranium ore formation. The many years of strenuous work in this area paid off. Uranium proved a very good model for solving many problems related to the natural transportation of many other heavy elements as well. Besides, by virtue of its radioactivity uranium leaves a "trace" both in space and in time in the form of natural decay products. Accordingly, physicists can monitor its migration pathways.
CONDITIONS FOR THE FORMATION OF URANIUM-BEARING VEINS
Various methods of analysis were used in studying uranium deposits.
Special mention should be made of the study of fluid inclusions that are formed where native crystals show growth defects and that contain microscopic droplets of a mineral-forming solution. When such a solution is cooled within a closed volume, the originally homogeneous fluid turns heterogeneous, or dissimilar - a gaseous phase appears in it; and there may be even three phases - the solution proper, liquid carbon dioxide (CO) and gas-in CO-rich solutions. A repeated heating of the mixture within such a closed volume results in its homogeniza-tion again, while the temperature at which this process is taking place is roughly indicative of the mineral formation temperature minimum. More elaborate studies allow to compute the pressure and concentration of individual components, their redox capability as well as other parameters. Since the dimensions of such fluid inclusions are measured in microns, the latest methods of ultramicroanalysis had to be employed to decode the information contained therein.
Eventually it became possible to obtain an array of conclusive and matching data. The temperature required for the formation of most of the uranium-bearing veins was found to be 150+/-50C; carbonate ions were always present in the solutions which contained no free oxygen; and the concentration of sulphide sulphur was on the verge of equilibrium between iron sulphides and oxides
(FeS2-FeO3). Yet another remarkable thing came to light: the pressure under which an ore-forming fluid is may be far lower than one calculated proceeding from the depth of ore formation. More than that, this pressure shows significant variability in the course of the formation process. So, the pressure is not determined by a local load but depends on the degree of "openness" or "closeness" of a deposit due to the presence or absence of fissures in its structure.
The available evidence thus amassed on specific deposits and even individual ore bodies enabled a transition from qualitative generalities to quantitative computations of concrete models.
ASSOCIATES AND ACCESSORIES
Heavy elements dissolve but poorly in water. Yet natural solutions are not water - they are complex multi-component systems containing sodium, potassium, chlorine, fluorine as well as carbonate sulphate and other ions. Some of them enter into chemical reactions with ions of heavy metals to form more complicated complex ions that behave quite differently than simple ones. Such components, helping heavy metals along intricate migration pathways, are called "associates" ("satellites"), while those traveling together in one and the same solution but not entering into reaction are known as "accessories" ("accompaniments"), for their joint migration is not determined by stable bonds but by the common conditions of transport. That is, by being in the same boat only.
The carbonate ion CO2-/3 happens to be a reliable companion for uranium; this ion forms very tight complexes with the uranyl ion UO2-/2 - far tighter than with the chlorine, fluorine and sulphate ions. The "jacket" thus obtained screens off the U6+ and brings down considerably the redox potential necessary for the reduction of uranium and its precipitation as uraninite (a mineral which is an anhydrous oxide of uranium). In this state U6+ is capable of migrating not only in oxidizing but also in reducing media.
Uranyl/carbonate complexes persist stable also at high temperatures and in plutonic oxygen-free media; yet carbonate ions are more sensitive to temperature changes, acidity of solutions and total concentration of dissolved CO2. The higher the temperature and acidity, the more of carbon dioxide will be needed for uranium migration.
A carbonate medium induces changes in the interaction of uranium with other metals. In it this element is oxidized even by weak oxidants, such as gold, silver, arsenic or mercury. The latter are reduced to their native state - it is not accidental that they are sometimes found in pitchblende ores. Therein was the rational grain for solving the tricky problem of migration of the readily reducible hexavalent uranium in plutonic strata free of oxygen.
CARBON DIOXIDE AS THE CHIEF FACTOR
When we know the conditions and forms of transportation of a chemical element, it will be much easier to identify the causes of its deposition in ore bodies. In the case of uranium we are not dealing with the gradual cooling of magmagene solutions, as believed previously, but with the dramatic and contrasting changes of migration conditions. Rather, the deposition of uranium may be attributed to a change in the CO2 behavior due to two purely geological processes - 1) carbonatization of enclosing rocks, and 2) the de-gassing of CO2-containing solutions. Both factors ultimately affect the activity of free ions CO2 and, consequently, the concentration of carbonate complexes.
The formation of uranium-bearing veins is often accompanied by carbonatization of the enclosing rock. Then the C02-/3 and ions will associate with corresponding cations, a process that shifts the chemical equilibrium toward uranium precipitation. This occurs in keeping with the mass action law - in proportion to the degree (n) of mass quantity in complex ions. Thus for
uranyl bicarbonate (n=2) a 90 percent decrease in the activity of CO will reduce the activity of complex ions to 0.01; and for uranyl tricarbonate (n=3), the decrease will be down to 0.001. For this mechanism to materialize, basic rock (rich in calcium, magnesium, iron) is needed - one capable of binding the carbon dioxide of solutions. This comes into play in the lithological control of mineralization - it all depends on the composition of the rock involved.
In the course of the other process - the degassing of CO solutions-elimination of CO2 into a gaseous phase will likewise shift the carbonate equilibrium toward uranium precipitation, but in that case it is systems of macro- and microfissures (structural control) that will be playing first fiddle. Such kind of picture is observed in many lode deposits of uranium in modified acid (persilicic) rocks acting as a buffer inhibiting the alkalinity rise with the elimination of CO. In this situation rocks that prior to ore formation were subject to hydromicatization or even argillaceous modification find themselves in the most favorable condition.
In real terms both mechanisms are at work, interacting every now and then.
HOW URANIUM DEPOSITS ARE FORMED
Proximate and low-cost radio-metric methods employed to determine uranium content in ores yielded a wealth of data on the background presence of uranium (i.e. beyond deposits) - in fact more than on any other chemical element. Subsequently these techniques were augmented by micromethods that allow to find out how uranium is concentrated in separate mineral grains. The evaluation of these findings by reliable statistical techniques enabled a fundamental conclusion on the local redistribution of uranium in the course of various geological processes preceding and accompanying the formation of its deposits. The latter are usually located in an area of certain deficit of uranium compared with a region where its concentration reaches 25-30 percent of the local background.
Just as intriguing facts came to light in a statistical study of the distribution of uranium in groups of rocks exhibiting different modification levels during various geological processes. Non-modified rocks usually show a symmetry (correct alignment) in the uranium distribution pattern in rock samples; however, in modified samples of rock the concentration of uranium displays sharp variability, though some constant mean value is retained nonetheless. Samples having an enhanced concentration of uranium alternate with those containing a low concentration of this element. This phenomenon, called a sub-background uranium aureole (halo), was even suggested as an uranium show in prospecting. As indicated by surveys, the concentration of uranium is locally redistributed in deposits. Thereby the mean values of uranium content within a deposit decrease somewhat over large areas but then start rising rapidly (near ore bodies).
A microscopy of uranium samples enabled geologists to conclude that with further modification the rock-making minerals that contain the evenly dispersed uranium begin extruding the impurities (Academician Fersman has described the phenomenon as autolysis). As a result, a stationary, immobile impurity (in crystal lattice defects) becomes readily mobile (within the intergranular space) and little by little is drawn into microfissures. The ongoing redistribution of the substance sets the stage for a subsequent migration of uranium and formation of ore bodies. All these modifications could have occurred hundreds of millions of years before the immediate formation of such bodies. Thus the ground was prepared for the presence of uranium ore in definite territories.
To bring together the above evidence into an integral system, or model, we must find a clear-cut answer to the central question: Where has an ore vein (lode) taken useful components from? Now we know: processes antecedent to ore formation that liberated inert uranium and built up a reserve of its potentially mobile forms predetermined the subsequent scenario of events and thus had a key role to play in ore formation. A knowledge of such events is just as important for mineral prospecting as a study of the very process of ore deposition. That is to say, the potential mass of ores in a given deposit directly depends on the scope, intensity and duration of prior events.
But how and where the liberated uranium is concentrated (if concentrated at all) - that depends on its subsequent transportation and deposition.
Now let us consider a pentametal formation deposit that contains, besides uranium, also cobalt, nickel, silver and bismuth (five metals in all). Here uranium present in acid rock associates in ore bodies with cobalt and nickel which are concentrated in basic and ultrabasic rocks, with ultrabasic rock minerals appearing for the most part in the final stages of ore formation. A conspicuous example of this type deposits is represented by Schlema-Alberoda in Europe's Ore Mountains (where prospecting holes have penetrated to a depth of 2 kilometers). Ore bodies there are confined to the contact metamorphism zone of the banded schist mass broken by a large granite massif. Here decarbonatization of sedimentary rock mass around granites took place, with the carbon dioxide of primary carbonates being eliminated, and the calcium, magnesium and iron liberated in the course of a chemical reaction entering into the newly formed alumosilicates (silicic compounds of aluminum). That led to the higher basicity of rocks that were losing silica but gaining magnesium and calcium; a portion of the uranium became redundant. Some of it transferred to a CO2 fluid that, owing to the closed structure, gave rise to a strongly pressurized carbon dioxide/water aureole (its presence is indicated by fluid inclusions in minerals formed in the course of metamorphism). Such was the pattern of preliminaries to ore formation processes.
Subsequently, as rocks cooled somewhat, consolidated and became less plastic, tectonic activity led to ruptures in the earth shell wherefrom fluids started escaping. Thereby basic rock underwent carbonization, with uranium deposited in veins. Since carbonization diminishes rock basicity, part of the cobalt, nickel and silver contained in the basic rock in equilibrium
became redundant in a more acid medium. Moving into a mobile state, this mass concentrated in the veined space. But that occurred later, after the deposition of uranium.
The above processes are interconnected and follow a definite sequential pattern. Metal sources are local here, though different: for uranium these are originally acid rocks, and for cobalt and nickel - basic rocks.
What concerns the uranium/ molybdenum formation (as seen in the example of the Streltsovo group east of Lake Baikal), here degassing had the main role to play. Therefore everything depended on the formation of permeable, "open" zones of low pressure - alleys through which carbon dioxide escaped from solution. Rocks subjected to "acid" modifications prior to ore formation became a natural buffer inhibiting the alkalinity rise of the oriferous solution during its degassing.
Thus the new model of uranium ore formation takes account of both the physicochemical characteristics of uranium and other substances implicated in this process, and the geological development features in particular areas of the earth crust.
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