URANIUM DISTRIBUTION IN SURFICIAL DEPOSITS

Detailed studies on the uranium distribution and especially on uranium-bearing minerals in surficial deposits are relatively few and are quite often of a very general nature. The studies have been based on macroscopic and microscopic observations and X-ray data but chemical analyses of these minerals are generally lacking. These problems are highlighted when one considers the fact that solid solutions could exist, for instance in the francevillite [Ba (VO4)(UO2)2.6H20] and tyuyamunite [Ca(VO4)2(UO2)2-8H20] series in the St Pierre du Cantal deposit in France [10]. There is an absence of trace element data in U(VI) minerals, which is unfortunate because

Table 2

General Pétrographie and Mineralogical Data on Uraniferous Calcretes

Calcite Development of dolomite at the expense of calcite has been proven in Yeelirne

Quartz is often detntal

Generally late in the mineralization process, could preserve carnotite from dissolution Important clays for understanding genesis of of the deposit, could be overlooked if no study of the clay fraction is undertaken Zonahties in clay distribution are general but few studies have made clear distinction between those which are really associated with the mineralization process and those which are contemporaneous with the formation and evolution of the calcrete

Detectable in the calcrete of the chemical delta

F and Sr are often anomalous in these deposits

Always detntal

it is possible to distinguish between hexavalent minerals resulting from the oxidation of a previous U(IV) oxide and a hexavalent uranium mineral resulting from direct precipitation from solution. For example, in the Bois Noirs deposit [11], torbernite is especially rich in some elements such as copper, arsenic, and bismuth which are abundant in the primary sulphide minerals as chalcopynte, lollmgite and bismuthmite. In many descriptions, paragenetic relationships are, m general, lacking with the exception of a few cases such as Yeelirne which has been thoroughly studied. It was pointed out by Carlisle et al [1 2] that the epigenetic nature of carnotite does not prove that carnotite and calcite are contemporaneous, as the mineralization has probably passed through one or more stages of dissolution and recrystallization.

Before proceeding with the description of the major uranium-bearing minerals in surficial deposits, some observations are presented concerning the nature of the association between uranium and other minerals, viz 1) association with organic matter, 2) adsorption on clays, 3) adsorption on Fe-Ti oxyhydroxides, and 4) adsorption or co-precipitation with silica.

1 In some surficial deposits and particularly in bogs and peats, the mmeralogical expression of uranium is unknown, even for uranium concentrations up to 2 000 ppm. For the Canadian occurrences, uranium is loosely bonded to organic matter and clays but the type of bonding is totally unknown [1 3] Relationships of uranium with organic matter are complex and several processes have been involved to explain this association. Organometallic compounds have been discovered in uranium deposits or showings [14, 15].

Recently, Nakashima et al [16] have demonstrated that fixation of uranyl species by lignite and subsequent reduction to urammte can be brought about by an increase m temperature, but the initial fixation process takes place from oxidizing conditions before final reduction.

2. Investigations of the adsorption of uranyl species by clays have been conducted m recent years [1 7,18]. The process seems effective and increases with the sequence kaolinite-ilhte-montmorillomte and the amount of uranium adsorbed is dependent upon the pH of the solution [18]

3. Adsorption of uranium on Fe-oxyhydroxides has been investigated by Michel [19] and Michel et al [20]. In the Samt Pierre du Cantal deposit in France there is a good correlation between iron and uranium as determined from chemical analyses. Michel [19] pointed out that this correlation is manifested by a bonding between phosphate ions and goethite particles in addition to adsorption by particles which have a high specific adsorption surface area defined by the zero point of charge (ZPC) [21 ] The adsorption of uranium in latentic profiles has also been described [20, 21]. In the Vosges area, accumulations of uranium occur in placic horizons of hydromorphic soils rich in lepidocrocite with the uranium probably adsorbed onto amorphous surfaces, and enrichment factors due to adsorption can be up to one hundred times greater than substitution in crystalline lattices [22].

4 Butt et al [23] suggested that some uranium could have been adsorbed or co-precipitated by silica in caprocks over the upper terrace calcretes in the Gascoyne Valley and in silica-rich pans at Nangcarrong and Narloo m Australia This silica is highly fluorescent.

The four uramferous environments described above vary considerably in their economic viability, but are important from the point of view of target selection and prospecting methods In surficial uranium deposits, the greater part of uranium is associated with U(VI) minerals and especially carnotite m the majority of cases, but U(IV) minerals are less abundant and occur in organic-rich environments. Among the most common uranium hexavalent minerals present in surficial deposits are the vanadates, silicates, and phosphates and those minerals mentioned m this volume are briefly described m the following sections

3.1 Uranyl Vanadates

Carnotite [K2(V04)2(U02)2-3H20] is monoclmic and is the most frequently occurring U(VI) mineral in valley-fill and lacustrme/playa deposits. Mann and Deutscher [24] have noted that carnotite could vary m colourfrom bright canary yellow to a dark olive green They interpret this variation to be the result of reduction of vanadium (V) to vanadium (IV). Carnotite is an epigenetic mineral and generally appears as fillings m voids and coatings on fracture surfaces. In the Langer Heinrich deposit in Namibia, carnotite has grown at the expense of calcite [12]

and a second generation of calcite filling is covered with carnotite. In Yeelirne, microcrystallme quartz coatings protect carnotite from subsequent dissolution [12] and the latest Raman microprobe data have shown that this is often the case Some dissolution has, however, taken place as indicated by the occurrence of significant disequilibrium [24, 25].

Tyuyamunite [Ca(U0²)2(V04)2.5-8H²O] is orthorhombic and is sometimes present m certain deposits, for example in the Yamarna occurrence in Australia [26] Scarcity of tyuyamunite is explained as the result of its higher solubility than carnotite m mineralizing waters. Metatyuyamunite, m association with carnotite, is known in calcretes m Tanzania [27] and in surficial deposits in Argentina [28]. Accumulations of tyuyamunite in caves of karst terranes have been described in the Bighorn and Pryor Mountains of Montana and Wyoming [29, 30].

3.2 Uranyl Silicates

Soddyite [(U02)2 (Si04).5H20] has been observed in the Welwitchia uranium occurrence in Namibia [31].

Weeksite [K2(U02}2(Si205)3.4H20] occurs in Tanzania in the Kisalalo River prospect m association with calcite, silica (opal-like material), detntal feldspar, and minor iron and manganese hydroxides [27]. It occurs as small yellow-greenish flakes which coat cavities m massive silica or as irregular bands replacing the fine-grained calcitic groundmass. Weeksite is also known in Afghanistan [32] in association with gypsum and calcite in polymictic Neogene sandstones and in the USSR [33] in carbonate concretions m Quaternary clays underlying gypsiferous clays

Haiweeite [Ca(U0²)2(Si20⁵)3.5H²0], also called ranquilite, is very rare and has only been reported from an occurrence in the USSR in association with weeksite [33]

Uranophane [Ca(U02)2 Si2O7 6H20] is sometimes present as a minor uranium-bearing mineral m surficial deposits Beta-uranophane is present m association with carnotite m Lake Hombolo m Tanzania [27]

3.3 Uranyl Phosphates

Metaankoleite [K2(U02)2(PO4)2 6H2O] or a related uranyl phosphate occurs m a playa near Menzies m Australia [23]. This mineral can be distinguished from carnotite by a brighter yellow colour and is highly fluorescent Torbernite [Cu(U02)2(PO4)2 8-12H20)] and autunite [Ca (U02)2 (P04)2 10-12H2O] have not been described in valley-fill and lacustrme/playa deposits, but they are major phases in altered phosphatic rocks such as in the Bakouma deposit [34]. They are late in the paragenesis, often geodic or fissurai in occurrence and result from remobilisation of uranium from the primary apatite Secondary crandallite can also occur Sodian potassian hydroxonian meta-autunite occurs in the Boomerang Lake occurrence m Australia as disseminations in saline muds[35]. Itsformula is[Na056K02g(H30)oi6]UO2.P04 3 25H2O, and appears as bright greenish yellow micaceous crystallites

Phosphuranylite [Ca(U02)3(P04)2(OH)2 6H20] is present in minor amounts in association with apatite in weathered metasediments, granite, and dolente in the Mile 72 pedogenic uranium deposit in Namibia [4] These minerals formed m situ and are alteration products of urammte, betafite, monazite, and apatite A similar

occurrence, in association with weeksite, has been reported from weathered granitoids in the Gascoyne Province of Australia [23].

3.4 Other uranium (VI) minerals

Schroeckingerite [NaCaslUC^ISO^COs^F.IOI-^O] is highly soluble and frequently occurs in association with gypsum and other members of the tri-carbonate-dioxyuranium (VI) group minerals. In the USSR, it occurs with uranophane, autunite, torbernite, and kasolite [2] and also occurs in Wyoming [30, 37]. An unnamed Pb-V-U hydroxide is present at various levels within the calcrete layer in the Mokobaesi No. 1 prospect in Botswana [38]

and is associated with manganese or carbon. It occurs as bright canary yellow aggregates in the calcrete matrix or as coatings on calcrete nodules.

3.5 Uranium (IV) minerals

Uraninite has been reported to be present in organic-rich diatomaceous earth and peat from some localities in South Africa [4], but chemical or X-ray data are lacking. Traces of reduced uranium occurring below carnotite mineralization is found in Lake Dundas [3]. Uraninite, associated with quartz, gypsum, halite, and an autunite phase, is present in muds from Boomerang Lake in Australia [35]. Electron diffraction data indicate that the uraninite has a cubic fluorite type cell, [a = 5X6 A].

The model presented by this geological situation is important from the point of view that potentially larger reduced uranium deposits could underlie oxidized surficial uranium concentrations.

4. A PROPOSED CLASSIFICATION OF URANIUM-BEARING MINERALS OCCURRING IN