REVIEW OF WORLD LATERITIC URANIUM OCCURRENCES

Many surficial iron-rich uranium occurrences are known throughout the world, but most have not been described in the literature because their significance has not yet been fully understood. Descriptions of several important occurrences follow, but are largely incomplete due to a lack of information.

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Figure 1

World distribution of surftcial iron-rich uranium occurrences Numbers 1 to 14 refer to the localities (a) Early Tertiary or older occurrences

(b) Tertiary to Recent occurrences

(c) Areas of latérite development according to G Pedro [22]

Several occurrences appear to be outside the lateritic areas which can be interpreted as the Recent climatic evolution towards drier environments

3 1 Tanzania

The uranium occurrences in lateritic profiles in Tanzania (1 m Figure 1) are the best examples of such deposits within this geochemical environment [3]. A regionally developed lateritic soil, up to 20 m thick, overlies a uranium sandstone-type deposit within the Karoo Sequence in the Madaba area Certain iron-rich faciès occurring above a stone-line are considered to have formed from lateritization of reworked allogemc material, whereas others are autochthonous lateritic profiles which contain the uranium mineralization

Up to 2 km away from the primary uranium occurrence, secondary uranium anomalies m the lateritic profiles occur in fractures and joints in the weathered sandstones or in roll-like accumulations m the lower part of the profile Uranium is present as U (VI), either sorbed with selenium onto an iron-manganese hydroxide or associated with a nickel-chromium nontronite with small amounts of vanadium In every occurrence, uranium is related to both vertical and horizontal acid/alkaline transition zones, the alkaline conditions being indicated by the formation of nontronite Such deposits clearly show that high-grade secondary ores can be the result of migration and reconcentration of uranium in the lower part of lateritic profiles in zones of abrupt pH changes

a 3 2 Mali

The Tin Azzir secondary uranium anomalies m eastern Mali (2, in Figure 1 ), which occuras elongated iron crusts, have no known primary source They are usually associated with quartz veins in Lower Proterozoic sandstones which unconformably overlie older pehtic units [4] Individual samples contain between 200 and 3 000 ppm uranium, associated with significant amounts of zinc, cobalt, nickel, copper, sulphur, and phosphorus (02 to 0 3 %), no thorium is present The uranium is concentrated mainly in the X-ray amorphous goethitic iron fraction, except where these iron species occur as recent precipitates in small cavities

Disregarding the recent uranium-free iron precipitates, there appears to be a direct relationship between the uranium content and the active surface area of the iron species, usually between 6 and 23 m2/g Mossbauer spectrometnc analyses of this material show that the iron oxides are either amorphous or present as very small crystals of goethite or hematite This indicates that the crystallmity of the iron species is in an early phase of evolution, at the stage where adsorption capacities are attheir maximum The lowerthe crystallmity, the higher is

the uranium content of the iron species. During evolution and crystallization towards hematite, iron species lose their adsorption capacities with a corresponding reduction in uranium content.

On the basis of texture, the iron-rich facies can be interpreted either as a lateritic crust or as a gossan, but the general morphology of the anomaly and the geochemical associations indicate the occurrence to be a gossan.

The lateritic terrane around Kenieko in western Mali (5 in Figure 1) consists of two types of géomorphologie units:

an upper terrace, which appears to be the older regional surface and contains uranium along its margins, and a lower, younger, terrace hosting strings of uraniferous boulders (B. Möge, personal communication, 1983). The anomalies of the lower terrace consist of limonitic boulders containing 70 to more than 1 000 ppm uranium and up to 1.4 phosphorus %. The limonitic boulders occur either on the surface or within the iron crust. In the vicinity of boulders, the iron crust may be weakly enriched in uranium, containing up to 50 ppm compared to a background of about 20 ppm. Despite their morphological characteristics, the uranium anomalies in the lower terrace are considered to have formed as a result of the erosion and redeposition of the anomalous limonitic facies of the upper terrace, similar to occurrences elsewhere, for instance in India [5].

Other anomalies occur on the edge of the upper lateritic terrace; uranium concentrations are lower, reaching a maximum of 500 ppm at a depth of 1 to 2 m, just below the iron crust, and only 30 to 40 ppm at a depth of 8 m.

3.3 Upper Volta

Many weak radioactive anomalies were detected by airborne radiometric surveys in central north Upper Volta (3 in Figure 1) during the 1960's. These anomalies are all on the old lateritic plateau and several that are in the Zorghoo area have been investigated. The highest radioactivity is in a horizon occurring at a depth of 2 to 3 m immediately below the iron crust and is due to a few minute crystals of autunite. No primary concentrations of uranium are known in the underlying Birrimian sedimentary formations.

3.4 Central African Republic

In the 1 960's, systematic uranium surveys in the Central African Republic (4 in Figure 1 ) led to the discovery of several radiometric anomalies and the uranium deposit at Bakouma [6]. The first anomalies studied in detail were located at about 100 km from Bakouma and occur within the iron crust, about 1 m below the surface. The underlying rocks, gneisses and mica schists are rich in allanite containing both thorium and uranium.

3.5 Senegal

Several uraniferous anomalies occur in lateritic profiles of eastern Senegal (6 in Figure 1 ) where iron crusts between 0.5 to 1.2 m thick overlie deep weathering profiles (B. Möge, personal communication, 1983). Recent erosion has destroyed the main part of the iron crust except on small residual hills. The maximum radioactivity is at the base of the iron crust and could be related to primary uranium deposits within the granites.

3.6 Brazil

There are several radiometric anomalies associated with lateritic soils in Brazil [7]. In the State of Parnaiba (7 in Figure 1 ), four occurrences are known in reworked ferruginous facies overlying Lower Devonian and Lower and Upper Carboniferous rocks. They occur as blocks of lateritic crust, in places near pyritic schists. In the State of Amazonias, two occurrences overlie Lower and Upper Devonian rocks (8 in Figure 1 ). In all these cases, the iron-rich facies are composed of goethite, hematite and kaolinite. They can contain up to 1 700 ppm uranium, 1 380 ppm vanadium and over 1 % phosphorus, but with thorium contents of less than 100 ppm. Uranium minerals have not been detected by optical methods. In most places, relationships between these anomalies and primary deposits are unknown.

A peculiar deposit is known in the State of Goias [8] where roll-type uranium occurs in Devonian sandstones. One of the rolls is found in a deep surficial lateritic soil derived by the oxidation and remobilization of underlying uranium mineralization but without significant leaching having taken place.

3.7 France

In France, during the Early Tertiary, lateritic soils, crusts and gossans developed in a humid tropical climate. Most of the lateritic profiles have been eroded and are now recognized as redeposited clastic iron concretions, commonly associated with kaolinitic clays, forming the so-called siderolithic facies.

The uppermost level of the Bernardan deposit [9] (10 in Figure 1 ) is a uranium-rich gossan containing up to 1 to 2 % uranium developed on an episyenitic uranium deposit. Concentrations of 100 to 150 ppm uranium are found in the siderolithic facies for up to 3 km around the deposit, usually where iron concretions are associated with clay minerals such as kaolinite (80 96) and smectite (20 %). The uranium distribution is probably controlled either by post-erosional and depositional remobilization, or by a uranium concentration within the deep clay horizon of an older lateritic profile.

The Oligocène uranium sandstone-type deposit of Saint Pierre [10] (11 in Figure 1 ) presents two types of iron formation. Much of the iron impregnation in the arkosic sandstones is derived from pyrite-rich rocks and may be

compared to minute gossans. Uranium grades of up to 1 % are associated with poorly crystalline iron hydroxides that are also locally enriched in phosphorus and vanadium. Approximately 32 % of the iron is amorphous or contains goethite crystals less than 90 A in diameter; the remaining 68 % consists of goethite with crystals less than 250 A in diameter. The chemical and physical characteristics of the iron are given in Table 1. The strong correlation between the content of uranium and that of poorly crystalline iron hydroxide, which has a high surface capacities, indicates that adsorption phenomena or co-precipitation with iron oxy-hydroxides played dominant roles in the precipitation of the uranium.

Table 1

Composition of an iron concretion and its -40 ju. fraction from the St. Pierre deposit [4]

Whole rock sample

Away from the deposit, the siderolithic iron concretions are probably remnants of lateritic crusts developed on rocks of Hercynian age during the Late Cretaceous or Early Tertiary that were reworked and deposited in sediments of Oligocène age. The concretions contain up to 50 ppm uranium, an enrichment relative to their source rocks.

The Bondon deposit [11] (12 in Figure 1) is a Tertiary-Quaternary gossan developed on a sulphide stockworkand is more than 1.5 m thick. The uranium has been adsorbed onto goethite-hematite and kaolinite surfaces and can attain grades of up to 1 %.

3.8 Morocco

The Hercynian metamorphic and granitic units of the Zaer Massif of Central Morocco (13 in Figure 1) are overlain by remnants of a Cretaceous lateritic profile. The profile is well preserved in the vicinity of uranium occurrences associated with goethite-hematite impregnations that are formed along fractures developed in a sulphide-uranium stockwork similar to the Bondon deposit (M. Eulry, personal communication, 1983). The sulphide- uranium-bearing ferruginous faciès may be classified as lateritic and, locally, gossanous.

3.9 India

According to Subramanyan [5], several occurrences of uraniferous limonitic boulders assisted in the discovery of low-grade primary uranium deposits associated with silicified breccia in the State of Bihar (14 in Figure 1 ). The limonitic boulders, which have been transported more than 35 m, have many large cubic boxworks after pyrite, with a marked peripheral zonation of limonite. Their uranium content can be as high as 1 000 ppm, whereas that of the primary breccia is much lower. It appears that these anomalies consist of transported blocks of a uraniferous gossan derived from a pyrite-rich primary deposit.

4. INTERPRETATION: PHYSICO-CHEMISTRY OF URANIUM CONCENTRATION IN LATERITIC