INTERPRETATION: PHYSICO-CHEMISTRY OF URANIUM CONCENTRATION IN LATERITIC GEOCHEMICAL TERRANES

Uranium anomalies in latérites and gossans can be classified into two groups [4, 13, 14].

1. High-grade anomalies containing 0.1 to 0.2% uranium often related to primary uranium iron sulphide-bearing occurrences.

2. Low-grade anomalies containing up to 0.01 % uranium related to normal surficial lateritic processes, but which could also be related to primary uranium deposits.

There are two main physico-chemical fixation processes that can be invoked to explain the origin of lateritic uranium occurrences, namely co-precipitation and surface adsorption of uranium on poorly crystalline iron hydroxide minerals. Optimally, both processes occur in weakly acid to slightly alkaline conditions.

Co-precipitation processes are commonly associated with uranium anomalies in gossans, although surface adsorption may also play a prominent role in the removal of uranium from solution at the appropriate pH.

Adsorption of uranium during co-precipitation stabilizes it sufficiently to resist further secondary dispersion. Co-precipitation of iron and uranium hydroxides from acidic sulphate solutions by neutralization, has been experimentally investigated by Sharkov et al [14], who found that the hydroxides attach themselves onto basic mineral species such as clays, feldspars, and carbonates, resulting in grades up to 0.1 % uranium. Such high-grade concentrations can occur during weathering of uranium-rich sulphide deposits or during weathering of pyritic rocks in the vicinity of very low-grade uranium source rocks.

In carbonate-rich environments, however, although acid solutions are effectively neutralized, precipitation of uranium may be inhibited by the formation of soluble uranyl carbonate complexes which can migrate and be dispersed over large distances.

Surface adsorption involves sorption and precipitation on active surfaces. True sorption is controlled by pH and occurs when ions are attracted to mineral surfaces having opposite electrical charges. Sorption of positive ions or complexes such as UO2(OH)+ implies negative charges on the surface of the sorbing species, which occurs as the result of an increase in pH. The transition point or zero point of charge (ZPC) between positive and negative charges varies according to the mineral species as shown in Table 2 [15].

Table 2

Values of the zero point of charge (ZPC) of some natural mineral species [15]

Geochemical enrichment factors (GEF) of uranium for natural sorbents at pH's between

The ability of a mineral species to fix or adsorb ions from solution can be measured by the geochemical enrichment factor (GEF). This is the ratio of the amount of uranium adsorbed by the mineral species to the uranium concentration remaining in solution. Examples of maximum enrichment for different minerals are given in Table 3.

Many authors [4, 16, 17, 18, 19, 20, 21 ] consider the sorption of uranyl ions onto iron oxy-hydroxide species as an important process for controlling the distribution of uranium in iron-rich geochemical terranes. Experimental work on natural lateritic material (G. Valence, personal communication, 1983) showed that, for pH of 4.5 to 5.5, uranium fixation is low, with a GEF of 3 x 102, whereas between pH 6 and 7, the GEF increases to about 1.8 x 104. The sorption processes are complex and it is difficult to distinguish true sorption from precipitation on finely divided species of high surface area. In the case of sorption, the ZPC of the mineral species will control the process whereas, in precipitation, the kinetics of the reaction, in addition to the size of precipitating particles, are the main factors involved. Precipitation is very weak in an acid medium (pH 4 to 5.5), but highly active in a neutral or weakly acid medium (pH 6.5 to 7). In soil profiles, uranium tends to concentrate mostly in the transition zone between the upper levels (A horizon or iron crust), characterized by an acid pH, and the deeper horizons, in which pH is neutral to basic. This general geochemical principle is exemplified by the uraniferous lateritic anomalies of Tanzania, where uranium is strictly concentrated in the basic faciès underlying the acid faciès [3].

Based on the foregoing geochemical phenomena, the evolution of uraniferous latérites can be explained. As the lateritization proceeds, a front of soluble uranium will move downwards in the profile, motivated by changes in pH and the crystallization of iron hydroxides and the consequent loss of adsorption capacity. This process of

adsorption and desorption may repeat itself and the uranium front can gradually move to deeper zones or, on a slope, move laterally. The degree of vertical and lateral migration will depend upon the permeability of the host material, distances up to several hundreds of metres having been recorded in Tanzania.

5. CONCLUSION

Fourteen iron-rich uranium anomalies have been described and it appears that they fall into three general classes:

1. Lateritic occurrences formed within soil profiles, either - with an underlying uranium deposit, as in Tanzania, or

- without an underlying uranium deposit as in UpperVolta, Central African Republic, and the Bernardan and Saint-Pierre deposits of France.

2. Gossans associated with underlying primary sulphide deposits, either

- with an underlying uranium deposit, as in Goias (Brazil), Bernardan, Saint-Pierre and Bondons (France), and Bihar in India, or

- without an underlying deposit, as in Parnaiba (Brazil), or - of unknown origin as eastern Mali.

3. UTiknown or undifferentiated types, as western Mali, Senegal, Amazonias (Brazil) and Morocco.

This review illustrates the widespread concentration of uranium in the ferruginous terranes. There appears to be a fairly consistent ratio between the uranium contents of the source rocks and the lateritic deposits, and from the data obtained so far this relationship appears to be generally valid [12].

There are two important mechanisms concerned with the formation of lateritic uranium occurrences, namely, physical adsorption on active surfaces of various iron hydroxy mineral species, and the co-precipitation of iron oxides and uranium. Both processes are dependent on the pH of the solution, which, optimally, should be weakly acid to neutral.

During evaluation of lateritic uranium anomalies, there are several pertinent aspects that must be taken into consideration. Attention should be given to the regional and local geomorphology including the identification and age of the various weathering surfaces, the cycles of erosion and the slopes, which will determine the direction and rate of uranium migration within the ferruginous faciès. Local studies of anomalies should take into account the vertical and horizontal forms of the uranium anomalies and their relationships with the various horizons orfacies of the lateritic soils. Such data may assist in distinguishing lateritic enrichments from reworked uranium anomalies occurring as boulders and gossans.

Geochemical associations help in establishing the differences between the two types. For example, the abundances of sulphur and certain base metals such as vanadium, copper, lead and zinc, may characterize a gossan. High concentrations of nickel, molybdenum, arsenic and selenium are typical of lateritic uranium occurrences.

Using this information, it is possible to determine the nature of the primary uranium source, which could either be a nearby primary uranium deposit, or merely a background source rock with easily leachable uranium. The application of these principles may assist in the interpretation of uranium anomalies in ferruginous materials and the selection of the most favourable targets for further detailed and follow-up surveys.

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