ON THE QUESTION OF THE FORMATION OF SECONDARY DISPERSION HALOS UNDER HUMID-CLIMATE CONDITIONS AND THE POSSIBILITY OF USING THEM IN THE SEARCH FOR URANIUM DEPOSITS

P. S. Dolgushin, V. K. Cherepnin, I. N. Ilyin, V. G. Pakhomov

(Presented by Professor A. M. Kuzmin)

The application of geochemical methods to the search for uranium deposits in mountainous taiga terrain under conditions of a humid climate requires knowledge of the features governing the formation of secondary dispersion halos of uranium and of the elements associated with it—pathfinder elements. As work experience shows, in a number of cases the secondary halos of uranium, arsenic, lead, and molybdenum can be regarded with sufficient confidence as a basis for initiating exploratory drilling aimed at discovering hydrothermal uranium mineralization at depth.

The area in which the investigations were carried out is composed of effusive-sedimentary rocks of the Lower Paleozoic, cut by a granodiorite massif and transgressively overlain by red-colored Devonian deposits. The granodiorites, in turn, are intruded by stock- and dike-like bodies of alaskite granites. Primary ore mineralization is localized mainly in feathering fractures of submeridional faults that are traced along the contact of the massif, and is developed in Cambrian marbles penetrated by dikes of lamprophyres.

The district is characterized by a humid climate (mean annual temperature about \(0^\circ\)). The relief is sufficiently dissected, but is distinguished by gentle forms with intensive development of vegetation cover. Groundwater recharge occurs mainly through infiltration of atmospheric precipitation, which amounts to as much as 800 mm per year. The groundwater regime is closely connected with climatic conditions; the type of regime is flushing. The oxidation zone is developed to depths of 50–80 m, and in places descends along tectonic structures to depths of 120–230 m.

The amount of precipitation and the temperature determine the degree of concentration of solutions circulating in the zone of infiltration and discharge [2, 5, 6, 7] and, thereby, the breadth and degree of development of the oxidation zone. The hydrochemical setting shows that in the present case we are dealing with extremely dilute, weakly mineralized, sufficiently active solutions of hydrocarbonate–calcium composition, with total mineralization up to 300 mg/l. The medium ranges from weakly acidic to weakly alkaline (\(\mathrm{pH}=6.6–7.5\)). Present are: \(\mathrm{CO}_2\)—up to 5 mg/l, \(\mathrm{Ca}\)—up to 70 mg/l, \(\mathrm{Mg}\)—up to 17 mg/l, \(\mathrm{Na}+\mathrm{K}\)—up to 3 mg/l, sulfate ion from 4 to 8 mg/l. In addition, the waters contain: arsenic ion from \(3\cdot10^{-5}\) to \(3\cdot10^{-3}\) g/l; molybdenum from \(5\cdot10^{-5}\) to \(2\cdot10^{-4}\) g/l; lead from \(1\cdot10^{-5}\) g/l; zinc from \(3\cdot10^{-4}\)

up to \(5\cdot 10^{-3}\) g/l; chromium—from \(1\cdot 10^{-5}\) to \(3\cdot 10^{-4}\) g/l; also zirconium, yttrium, sometimes boron.

Below an attempt is made to present the main features of the geochemistry of uranium and its satellite elements—arsenic, molybdenum, and lead—characteristic of the conditions under consideration.

Uranium. The high migrational capacity of uranium in an oxidizing environment and its low capacity in a reducing one is a characteristic feature of uranium that determines its behavior in the hypergene zone.

The only primary uranium mineral in the case described is uraninite, occurring as the finest microscopic segregations in a black siliceous-carbonate aggregate. Hypergene minerals are represented by uranium blacks.

Under conditions of a humid climate, hydrocarbonate-calcium waters with a weakly alkaline or weakly acidic reaction are an extremely favorable medium for the dissolution of primary uranium oxides [1, 8]. The presence of free oxygen and carbon dioxide sharply increases the solubility of the indicated compounds and ensures the transfer of uranium, in the form of various compounds, into solution.

Such compounds will probably be uranyl-carbonate complexes in the form of the ions \([\mathrm{UO}_2(\mathrm{CO}_3)_2(\mathrm{H}_2\mathrm{O})_2]^{2-}\) and \([\mathrm{UO}_2(\mathrm{CO}_3)_3]^{4-}\), since, according to the data of G. B. Naumov, they should be the most stable compounds at pH \(=6.2\)—7.5, with a sharply dominant position of the hydrocarbonate ion (200—300 mg/l), free carbon dioxide (up to 50 mg/l), and an insignificant content of the sulfate ion (4—8 mg/l) [4].

At the same time, under conditions of active water exchange and the wide development of blacks in the ores, uranium may occur in aqueous solutions and in the form of a mechanical suspension. In addition, it should be borne in mind that, with an abundance of moisture and decomposing plant residues, considerable amounts of organic acids may arise, which also promote the dissolution of primary uranium minerals. The complex, readily soluble urano-humate salts formed under such conditions, in the opinion of V. V. Shcherbina [8], are more stable than simple uranyl salts; therefore they possess much greater mobility, facilitating the removal of uranium. In our view, this explains the noticeable weakening of the secondary dispersion halos of uranium in deluvial formations over the outcrops of ore bodies. What has been said is illustrated by the results of numerous analyses of samples from loose formations and bedrock, collected both over the outcrops of ore bodies and directly within the limits of ore structures (Table 1).

The formation of secondary dispersion halos under conditions of a carbonate medium occurs chiefly through processes of adsorption of uranium by iron hydroxides. Apparently, the fact noted (Table 1) of a sharply increased uranium content in intensely limonitized bedrock should also be connected with this. L. B. Kolyadin and D. S. Nikolaev [3] indicate that coprecipitation of microconcentrations of uranium with iron hydrate in the presence of \(\mathrm{CO}_2\) ions will occur at pH \(=2\)—6.6. At higher pH values, formation of a uranyl-carbonate complex takes place. Indeed, the very fact of the occurrence of limonite indicates the existence, during the period of its precipitation, of an acid reaction, and consequently of conditions favorable for the coprecipitation of uranium with it. In the case of prolonged treatment of such limonites by hydrocarbonate-calcium waters, the pH rises to 6.5—7.5; in this case there is no longer accumulation, but a gradual removal of uranium. Evidently, along with the processes of formation of stable urano-humate compounds in solution, under near-surface conditions there is a continuous process of removal of uranium in the form of uranyl-carbonate complexes. Thus,

probably accounts for the reduced uranium content in deluvial formations.

Even when arsenic and molybdenum are present in the waters, uranium is not bound in the form of arsenates and molybdates, since formation of the latter is possible only in an acidic medium and from acidic solutions.

Table 1

At the same time, if free lead is present in ores or enclosing rocks, the first products of oxidation of pitchblende at the initial stage may be lead uranates of the curite and fourmarierite type, formed during hydration of nasturan under alkaline conditions (pH = 7.5—8). However, for the conditions under consideration, an almost neutral medium is characteristic, in which formation of lead uranates is unlikely. At the same time, even if such compounds did arise at the beginning of the oxidation process, under conditions of a leaching water regime and a carbonate medium they would quickly decompose with the formation of stable lead carbonate (cerussite) and a complex uranyl-carbonate anion, in the form of which, as noted above, uranium migrates readily. This evidently is the reason for the absence, apart from sooty black uranium, of any secondary uranium minerals in the hypergenesis zone of the territory described.

The setting for the formation of calcium uranium phosphate (autunite) and copper uranyl-phosphate (torbernite) is also unfavorable, since this requires an alkaline medium and an appreciable amount of phosphorus and copper. Apparently one should not expect the formation of uranium arsenates here either, since the latter, on the contrary, are deposited only under conditions of an acidic medium.

Arsenic. The high arsenic content in products of hydrothermal activity, and also its predominant manifestation in sa-

in native form constitutes a characteristic feature of our primary mineralization.

Field observations show that, under conditions of a weakly alkaline environment and an abundance of weakly mineralized groundwaters, the principal product of alteration of native arsenic is scorodite. The latter, despite its fairly good stability in the supergene zone, is destroyed under prolonged treatment by surface waters, which, in the absence of precipitants, as occurs in our example, will be accompanied by vigorous removal of arsenic. True, in places the presence of appreciable amounts of lead-bearing minerals in the ores or near them leads to the formation of stable lead arsenates of the mimetite or beudantite type.

It is interesting to note that, despite the presence of zinc in the hypogene ores (in the form of sphalerite), zinc arsenates were not encountered in the oxidation zone. This is apparently explained by their instability under supergene conditions and by intense removal in the form of dissociated molecules \(Zn^{2+}\) and \(AsO_4^{2-}\). In this case the arsenic anion forms lead arsenate with lead. This, evidently, explains under our conditions the absence of secondary zinc aureoles and, conversely, the presence of spatially coincident aureoles of lead and arsenic. The behavior of arsenic released during oxidation of arsenopyrite, realgar, and orpiment occurring in the primary ores is practically analogous to that described above.

On the whole, in the geological-hydrochemical setting under consideration, it may be expected that arsenic in the oxidation zone will for the most part be dispersed, that secondary aureoles will be strongly weakened, and that its presence in soils and deluvial formations can be established only by highly sensitive methods.

Elevated concentrations of arsenic, exceeding its average content in rocks by 10 times or more, are noted only in eluvial formations and bedrocks above the outcrops of ore bodies and directly in the ore structures (Table 1). Mineralogically it is represented by lead arsenates.

Thus, under certain conditions, the presence of lead arsenates in the supergene zone may be regarded as an indicator of the presence of uranium ores at depth. Consequently, in prospecting for hydrothermal uranium deposits under humid-climate conditions, the carrying out of work connected with establishing secondary arsenic aureoles should, in our opinion, be considered obligatory.

Molybdenum. In the primary ores, molybdenum-bearing minerals—molybdenite and jordisite—are often widely developed, the latter more commonly being paragenetic with pitchblende. No substantial difference is observed in the behavior of the two minerals during oxidation processes. In either case aqueous-air oxidation may proceed according to the following scheme:

\[ 2MoS_2 + 9O_2 + 2H_2O = 2(MoO_2 \cdot SO_4) + 2H_2SO_4. \]

It is quite probable that, during oxidation of jordisite, molybdic acid—\(H_2MoO_4\)—also forms according to the scheme:

\[ 2MoS_2 + 9O_2 + 6H_2O = 2H_2MoO_4 + 4H_2SO_4. \]

Molybdic acid and the compound \(MoO_2 \cdot SO_4\), under conditions of a weakly acidic or weakly alkaline environment, are sufficiently stable, and, in the absence of reducing agents in the waters, molybdenum will be carried beyond the limits of the deposits in the form of the indicated compounds and dispersed. The presence of molybdenum under these conditions can be recorded only by hydrochemical sampling of groundwater outlets. Its transition into solution does not depend on the degree of sulfate content of the waters or on the concentration of hydrogen sulfides.

ions at pH = 6.2–8.5, but depends mainly on the content of calcium bicarbonate in the water, the presence of which leads to the precipitation of molybdenum from solution in the form of calcium molybdate:

\[ \mathrm{MoO_3 \cdot SO_4 + Ca(HCO_3)_2 \rightarrow CaMoO_4 + H_2SO_4 + 2CO_2} \]

or

\[ \mathrm{H_2MoO_4 + Ca(HCO_3)_2 \rightarrow CaMoO_4 + 2H_2CO_3}. \]

This reaction, in our view the most probable one, leads to a noticeable weakening of the aqueous halos of molybdenum and to the formation of secondary dispersion halos in soils.

In the case of the presence in ores of hypogene lead minerals, which in the oxidation zone give the most stable complexes not only with arsenic but also with molybdenum, one may expect the formation of lead molybdate—wulfenite. Indeed, the development in individual cases of spatially coincident secondary halos of lead and molybdenum over ore bodies confirms the joint occurrence of these elements, possibly in the form of lead molybdates.

The presence in waters of iron or its hydroxide inevitably leads to the formation of ferrimolybdite—a mineral most stable under the conditions of hydrocarbonate-calcium waters at pH = 6.5–7.5. However, under prolonged action of bicarbonate solutions, ferrimolybdite is hydrolyzed with the formation of limonite and molybdic acid according to the scheme:

\[ \mathrm{2(Fe_2O_3 \cdot 3MoO_3 \cdot 7.5H_2O) + H_2O = 2Fe_2O_3 + 9H_2O \cdot 6H_2MoO_4}. \]

At the expense of molybdic acid, as we have already seen, powellite will form, or, under the corresponding conditions, wulfenite.

Consequently, in the case of the occurrence in uranium ores of hypogene molybdenum mineralization in the form of molybdenite or jordisite, in the zone of hypergenesis one should expect the formation of powellite, and in the case of the presence in the ores of a noticeable amount of lead—wulfenite. In the latter case the secondary halos of lead and molybdenum should coincide spatially.

It should be borne in mind that, during the oxidation of molybdenite from quartz veins, quartzites, and skarns, secondary halos of molybdenum may also form; however, as a rule, they will occur without any connection with halos of lead.

Thus, the presence of halos of molybdenum alone cannot be an indicator of primary uranium mineralization, whereas the discovery of the same molybdenum halos together with halos of lead and arsenic makes it possible to suppose the presence at depth of uranium mineralization of the hydrothermal type.

Lead. Galena is a fairly widespread mineral of primary ores. Some amount of it is often paragenetic with uraninite. Under conditions of a leaching regime and prolonged action of hydrocarbonate-calcium waters, the final product of alteration of galena is cerussite, usually occurring in a fine mixture with limonite.

Despite the high stability of cerussite in such an environment, observations have shown some depletion of lead in the deluvial layer at a depth of 0.3–0.5 m (Table 1). Possibly this should be connected with the activity of organic compounds, which impart to lead a certain mobility under conditions of prolonged and energetic leaching.

The presence, in the case described, of arsenic in the waters of hypergenesis, as already indicated, leads to the formation of lead arsenates of the mimetite type \((2\mathrm{PbO} \cdot \mathrm{As_2O_5} \cdot \mathrm{H_2O})\) and beudantite \((2\mathrm{PbO} \cdot 3\mathrm{Fe_2O_3} \cdot \mathrm{As_2O_5} \cdot 6\mathrm{H_2O})\). The formation of these minerals should apparently be connected with the action of arsenic-containing solutions on cerussite.

In the case of the presence of molybdenum in primary ores, along with the development of secondary lead halos, there will occur the formation of molybdenum halos spatially coinciding with them, as was noted above.

It follows from the foregoing that the study of secondary lead halos in searches for hydrothermal uranium deposits is essential. Moreover, special attention should be paid to the relationship of these halos with arsenic and molybdenum halos.

Thus, under conditions of a humid climate, in the presence of a slightly alkaline, neutral, or slightly acidic medium ($\mathrm{pH} = 6.6$–$7.2$), with weakly mineralized calcium-bicarbonate waters in carbonate rocks, one should expect a sufficiently well-developed oxidation zone at the outcrops of uranium ore bodies and the formation, in areas where iron hydroxides develop, of sharply weakened secondary uranium halos, which are nevertheless recorded fairly clearly by modern methods of analysis.

The presence of uranium halos, as well as secondary halos of arsenic and lead, lead and molybdenum spatially coinciding with them, should indicate the possibility of discovering at depth primary uranium ores with molybdenum sulfides (jordisite), arsenic, and galena.

At the same time, the absence of arsenic and molybdenum halos, or of one of these two elements, should be regarded as an indication of the monomineral character of the primary mineralization, possibly with an insignificant admixture of lead, and cannot serve as grounds for terminating uranium prospecting work in such areas.