ENVIRONMENTS AND PROCESSES OF PRECIPITATION

The various processes leading to precipitation of uranium in the supergene zone and the surficial environments in which each process is significant are summarized and discussed below under the following headings:

1. Reduction mechanisms.

2. Sorption processes.

3. Dissociation of uranyl complexes.

4. Change in redox states of ore-forming constituents.

5. Evaporation of surface and groundwaters.

6. Change in partial pressure of dissolved carbon dioxide.

7. Precipitation due to changes in pH.

8. Colloidal precipitation.

9. Mixing of two or more surface or groundwaters.

6.1 Reduction Mechanisms

Most known surficial uranium deposits occur in zones of oxidation, but there are a number of surficial reducing environments which are capable of concentrating the element, the most notable being wetland areas (peat bogs, swamps, cienagas, flood plains, etc.), closed anoxic lake and playa basins, and sulphide orebodies. The processes by which the uranyl ionic species are reduced to uranous forms may be one or a combination of (a) direct reduction by organic matter, (b) anaerobic bacterial activity, (c) reduction by gases such as H2S and H2, and (d) redox processes accompanying near-surface oxidation of sulphides. Most of the uranium precipitated in organic bodies, such as peat bogs and swamps, results from cation exchange processes discussed in the following section, but direct reduction by certain organic forms may also be important in some environments [21 ].

Anaerobic bacteria, especially the sulphate-reducing species, are powerful reducing agents for uranium.

Although probably a contributing factor in most organic-reducing environments, bacterial reduction is the dominant precipitating mechanism in closed anoxic alkaline and saline lakes (e.g. the Purple Lake deposit.

Canada, described by Culbert et al. [6], some cienaga deposits, and certain reducing playa lakes having a black ooze layer immediately below the surface. In organic environments, both the decay of organic matter and anaerobic bacterial activity produce gases such as H2S and H2 that are potentially strong reducing agents capable of precipitating uranium. When primary uranium is present, or when uraniferous groundwaters are introduced into the oxidation-reduction environment of a near-surface sulphide orebody, considerable enrichment of uranium may occur in the zone of reduction, an excellent example being the Bondon deposit in France [16].

6.2 Sorption Processes

Due to their large ionic radii and high-charge density, respectively, both the uranyl (U0|+) and uranous (U4+) ions are adsorbed onto various organic materials [22, 23, 24], clays [25, 26], zeolites [25], phosphate minerals [25],

and hydrous oxide precipitates of elements such as iron, manganese, silicon, aluminium, and titanium [15, 25, 27]. In most cases, adsorption is physical and is accomplished by processes of simple ion exchange but, in organic regimes, chemisorption may be a precursor step in the formation of more stable organic-uranium complexes. Decaying organic material in peat bogs, swamps and flood plains may contain 1 000 to 5 000 ppm uranium [6, 12, 28] and occasionally as much as 3 % uranium [28]. Such bodies often receive uranium from one or more groundwater sources and may concentrate the element in one or more horizons; often, however, the element is erratically distributed. The ability of organic material to concentrate uranium is dependent largely on the pH of the environment, the degree of humification, and the uranium and total dissolved salt content of the inflowing surface and groundwaters. Depending on the type and composition of organic matter, the optimum pH for adsorption of uranium is between 4 and 7 [22]. At equilibrium conditions, the amount of uranyl ion adsorbed by organic matter is proportional to the uranium content of the interstitial waters [22].- In the vicinity of the uraniferous Masugnsbyn bog in Sweden, surface and groundwaters contain between 4 and 140 ppb uranium [28]. Spring waters entering the uraniferous bog deposits at Kasmere Lake, Manitoba, Canada contain up to 50 ppb uranium whereas interstitial waters in these bogs contain up to 400 ppb of the element [12]. These findings may be compared to those of Halbach et al. [11] who found that, despite the fact that the weathered zones of the Brockan granite (West Germany) showed uranium losses of up to 80 % (from 14 ppm to 2 ppm) the high rainfall in the area (1 500 mm/yr) resulted in very dilute solutions (0.1 to 0.6 ppb uranium) and very little concentration of uranium (3 to 4 ppm), in the associated bogs. Enrichment factors, represented by the ratio of uranium in the organic matter to its concentration in associated waters, may vary from 10000 to 50000:1 [22, 24]. The ability of organic environments to extract large quantities of uranium from natural waters has been demonstrated by Bowes et al. [29] for a bog deposit on the Sierra Nevada Batholith of California. The uranium contents of spring waters entering this bog were reduced by a factor of ten after flowing through the bog.

Generally, the ability of organic matter to concentrate uranium increases with increasing humification, although all of the other above-mentioned factors will affect this relationship. For the uraniferous peat bogs at Kasmere Lake in Canada, Coker and Dilabio [12] found a good relationship between uranium content and degree of humification, concentrations of up to 5 000 ppm occurring in the most humified bottom layers of the bogs. For the Masugnsbyn bog in Sweden, the relationship between uranium content and degree of humification is less clear, indicating that other factors have a stronger influence on concentration processes [28]. In extremely alkaline environments, where most of the uranium in surface and groundwaters is strongly complexed by di- and tricarbonate ions, the ability of organic materials to concentrate uranium is severely hampered. In such environments, highly reducing conditions (e.g. some cienagas) may be required to obtain significant concentrations.

The ability of clay minerals to adsorb uranium from natural waters is well established [25, 26], the conditions being optimum between pH values of 5 to 7 [26]. Few, if any, significant uranium accumulations occur in pure clay deposits. In most cases, uraniferous clays are associated with organic deposits [6], lake sediments [6], playa sediments [13] or halite deposits [30], in all of which uranium is also present in a variety of other forms.

Uranium may be adsorbed from solution during the precipitation of iron, manganese, silicon, aluminium and titanium hydrous oxides of which ion oxyhydroxides seem to be the most significant accumulators. The uraniferous ochre deposits in Botswana [19] would appear to be the first significant deposit described where uranium-iron coprecipitation predominates. The sorption characteristics of iron oxyhydroxides for uranium have been studied by a number of researchers [15, 27,40].

6.3 Dissociation of Uranyl Complexes

Complexing of uranium by such ions as carbonate, sulphate and phosphate greatly aids in transporting large amounts of the element to potential sites of deposition. Destabilization of these complexes liberates sufficient amounts of uranium to form orebodies in some places. The breakdown of uranyl complexes may be accomplished by reduction mechanisms, pH changes, or a change in partial pressure of gases such as C02 (carbonate decomplexing). All of these mechanisms are discussed in following sections. The dissociation of uranyl carbonate complexes is considered to be important in the formation of calcrete and gypcrete carnotite deposits [1,2].

The dissociation of uranyl sulphate complexes is important to the formation of uranium mineralization in oxidi-zing sulphide environments.

6.4 Change in Redox States of Ore-Forming Constituents

Redox changes are considered by Mann and Deutscher [1] to be dominant mechanisms in the formation of carnotite calcrete-gypcrete deposits. Considerably greater concentrations of vanadium can be carried in solution to a site of deposition if it is in the four-valent state, as VO(OH)+ or HV205, rather than the five-valent state required for carnotite formation. The model has U6+ and V4+ moving to the site of deposition in slightly oxidizing to mildly reducing, neutral to acidic groundwaters, where upwelling causes oxidation of V4+to V5"1" and carnotite is precipitated. Convincing field evidence is presented in the form of Eh-pH measurements in drill holes around the North Lake Way prospect that shows a shallow redox boundary corresponding to the calculated boundary for V*+ and V5+.

6.5 Evaporation of Surface and Groundwaters

The behaviour of uranium during evaporation in arid and semi-arid regions has been examined by a number of researchers [31,32.33,34]. In the absence of precipitating agents such as vanadium and phosphorus, evaporation is not a particularly good mechanism for concentrating uranium in the surficial environment [31, 32, 34]. During evaporation, uranium is concentrated in the final bittern residues of alkaline and saline lakes [31, 32, 33, 34] and only precipitates during the final stages of desiccation. Playa deposits are susceptible to wind ablation and flushing during periods of heavy rainfall [32, 35], and uranium accumulations are, therefore, merely transitory. Uranium will accumulate in reducing (black oozes) and clay-rich horizons in these environments, and even then only in relatively low concentrations, due probably to the presence of complexing ions in the associated waters. Evaporation of upwelling groundwaters in calcrete-gypcrete drainage channels maybe partially responsible for the localization of carnotite deposits due to increased activity of K+, V5* and U6"1"

above the solubility product of carnotite [1, 36]. Evaporation discharge involving capillary rise or evaporative pumping of groundwaters above the groundwater table maybe responsible for the formation of autunite deposits in structural zones of granitic bodies, e.g. Los Gigantes, Argentina [18] and Daybreak, USA [17], as well as accumulations of uranium in pedogenic calcretes, e.g. Mile 72, Namibia [9] and Mokobaesi, Botswana [19].

6.6 Change in Partial Pressure of Carbon Dioxide

Due to strong uranium-carbonate complexation, concentrations in groundwater of uranium, vanadium, potassium, and bicarbonate leached from source rocks may exceed the limits of the solubility of carnotite at a given pH. If these waters become deep-seated, they will also have a greater carbon dioxide content due to increased confining pressures. Such conditions will promote higher bicarbonate concentrations and greater uranium complexation. When these waters re-enter the near-surface environment, the hydrostatic pressure will decrease, carbon dioxide will exsolve from solution, calcium carbonate will precipitate, uranium will be decomplexed and carnotite will precipitate. Such a process has been considered as a possible mechanism forthe formation of calcrete-gypcrete carnotite deposits [36]. A similar process (without vanadium) has been used to explain the formation of uraniferous travertine deposits [37].

6.7 Precipitation Due to pH Changes

The solubilities of many uranyl minerals, especially carnotite and uranyl phosphates (see Figure 1 of Hambleton-Jones[38] for solubility of carnotite vs pH), and the stabilities of all uranyl complexes are pH dependent [15, 36].

It follows, therefore, that when uraniferous surface or groundwaters enter a potential ore-forming environment and undergo a significant pH change, uranium mineralization may form. The formation of ore as the result of pH changes can be accomplished by such processes as : (a) mixing of two or more surface or groundwaters, to form a variety of uranium minerals; (b) loss of carbon dioxide by rising uraniferous groundwaters to form carnotite [36]

and uraniferous travertines [37]; (c) oxidation of sulphide bodies to form a host of uranyl and uranous silicate, phosphate, arsenate and sulphate minerals, and (d) reaction with soluble acids in organic (humic, fulvic), clay-rich environments where the pH may change into a range (i.e. 4 to 7) favourable for adsorption.

6.8 Colloidal Precipitation

Very little is known about the role of colloid formation in the precipitation of uranium mineralization and, with the exception of changes in pH, very little is known of the processes involved. Undoubtedly, one or more of dehydration, deflocculation, peptization or polymerization are responsible for colloid formation. During the weathering of sulphide deposits, uranyl solutions may react with colloidal or gelatinous silicates, phosphates, arsenates, molybdates, vanadates, selenites and tellurites to form a variety of mineral types. Reaction of uranyl solutions with silica gels may lead to significant accumulations of minerals such an uranophane, coffinite and soddyite (e.g. Welwitchia occurrence, Namibia [9]).

6.9 Mixing of Two or More Surface or Groundwaters

Although a number of ore-forming scenarios involving mixing of surface or groundwaters of different compositions can be invoked for the formation of surficial uranium deposits, such processes have been difficult to verify under natural conditions. Mixing of separate uranium- and vanadium-bearing groundwater systems has been proposed by Mann [1, 36] to be a possible contributing mechanism for formation of calcrete-gypcrete carnotite deposits.

There is little doubt that surface and groundwaters do mix in the hydrosphere and, therefore, may be a dominant factor in ore formation within a variety of environments,

7. TECTONICS

The tectonic history and structural framework of a region play an important role in the formation of surficial uranium deposits. Tectonic processes, such as uplift and/or extensional tectonism, prepare the source rock area for both surface and groundwater leaching of ore-forming elements. Because the formation of a surficial uranium deposit requires a stable hydrological regime, a distinct period of tectonic quiescence is also required. For some deposits, the period of ore formation may be as short as 5 000 to 10 000 years [6, 28], whereas for others, such as the uraniferous calcrete-gypcrete deposits, a much longer period of tectonic stability may be required [9].

8. PRESERVATION

Surficial uranigm deposits are formed under specific hydrogeochemical and climatic conditions and, as such, are susceptible to destruction due to changes in the original ore-forming conditions. Tectonic activity and climatic changes are the principle causes of destruction. Pleistocene uplift of parts of southern Africa has led to erosion of many of the uraniferous calcrete-gypcrete deposits [39]. Drastic changes in climate, or even sporadic climatic conditions, such as the periods of heavy rainfall and flooding in Western Australia [35], will greatly affect the stability of deposits. The existence of ancient surficial uranium deposits in their original form is unlikely, since very few geological processes would allow their preservation, although they may be remobilized to form other types of deposits. In older geological terranes, such transformed deposits should be located along unconformities.

9. CONCLUSION

Until recently, geologists have always considered the processes leading to surficial accumulations of uranium as interfering factors in mineral exploration for primary ores. In the last five to ten years, it has become apparent that such processes may yield relatively large economic, or potentially economic, uranium deposits. Knowledge of the mechanisms and environments of formation for this class of deposit is, however, meagre and much more research is required to determine the full potential of the supergene zone for uranium ores.

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