CERTAIN ASPECTS OF THE MINERALOGY AND GENESIS OF URANIUM-BEARING JASPER-LIKE VEIN QUARTZ

A. D. Nozhkin

(Presented by the Department of Geology and Exploration of Mineral Deposits)

Uranium-bearing jasper-like quartz has been encountered in veinlets cutting marbles and skarns of the near-contact zone of a granite intrusion. Among the skarns, garnet varieties predominate, chiefly grossular varieties; pyroxene and amphibole skarns are of subordinate importance. The average thickness of the veinlets is 1.5–2 cm, with a maximum of 5–7 cm. They are oriented mainly across the banding of the skarns and the bedding of the marbles. Their contacts with the enclosing rocks are sharp.

The main mass of the veinlets is composed entirely of jasper-like quartz, and only some of them contain calcite in the selvages. The quartz here is very distinctive. It has a red, cherry-red color, a cryptocrystalline structure, and is characterized by an overall jasper-like appearance. Its fabric is very dense, externally homogeneous or distinctly colloform. In the latter case, kidney-shaped or botryoidal masses are observed, having a concentrically layered structure in section.

Under the microscope, metacolloidal structures of the mineral fillings of the veinlets are always revealed. The predominant part of the quartz is represented by a fibrous variety—chalcedony, distinguished by negative elongation of the fibers and a low value of the refractive indices \((N_g = 1.542,\ N_p = 1.533)\). Ordinary fine- and microcrystalline quartz is of subordinate importance. Chalcedony, and rarely also microcrystalline quartz, contains an admixture of ore material, the uneven, concentrically zonal character of whose distribution precisely emphasizes, in transmitted light, the colloform structure of the siliceous mass. In crossed nicols, the structural pattern is intensified by the zonal alternation of different morphological varieties of chalcedony, as well as of ordinary quartz.

Especially widespread are concentrically zonal spherulites and hemispherical kidney-shaped formations of chalcedony. In eros-

In this case, the outer zone of them usually has a uniform brown or orange-red color and is represented by amorphous silica. The next zone is most often composed of radial-fibrous chalcedony, colored orange. And, finally, the inner zone of the nodules and spherulites is formed by fibrous transparent chalcedony or fine-crystalline quartz. In some veinlets the concentrically zoned structures are very weakly developed. The main mass of the mineral substance in them consists of an aggregate of rosette-like or fibrous orange chalcedony, among which small nests of fine-grained water-clear quartz are observed (Fig. 1). There are also areas composed of the finest flake-like segregations of amorphous, intensely colored silica.

Fig. 1. Nests of fine-grained quartz in an aggregate of rosette-like and radial-fibrous chalcedony

The red and brown tones in the coloration of the jasper-like quartz are due to the presence in it of an admixture of ore material. In some cases this material is so fine that its individual particles are microscopically indistinguishable, and it is precisely such dispersed material that causes the uniform and usually intense coloration of chalcedony, which is characterized by weakly expressed aggregate polarization. However, most of the ore material is represented by coarser dust-like particles—inclusions in the fibrous or radial-fibrous varieties of chalcedony. Studies have established that oxide iron sharply predominates in the composition of the ore material (Table 1), most likely corresponding to the mineral form of hematite.

Table 1

Content of iron oxides in jasper-like quartz

The uranium content in the quartz veinlets is, as a rule, insignificant—0.02–0.03%. In the usual microscopic study of thin sections, any mineral forms of uranium in most cases are not esta-

is observed. Microradiography shows a predominantly uneven, star-shaped character of the distribution of alpha tracks, corresponding to areas where radial-fibrous and fibrous varieties of chalcedony are developed. In this case their greatest density falls on spherulites and kidney-shaped segregations of chalcedony enriched in dusty particles of ore material. A relatively uniform distribution of alpha tracks is observed only in areas composed of uniformly, densely colored, weakly polarizing chalcedony.

On the basis of these observations one may conclude that a large part of the uranium in jasper-like quartz is present in the form of its own mineral phase, represented, evidently, by dispersed uranium oxides. Indeed, in individual polished sections, in areas corresponding to the maximum concentration of alpha tracks on microradiographs, at magnifications of 600–800× it is possible to distinguish the finest point-like segregations (kidneys) of uraninite. As for the densely colored, weakly polarizing chalcedony, the uranium in it is most likely in a sorbed state.

In this connection, experiments on uranium leaching are of definite interest. It is known that hexavalent uranium is readily extracted from a sample by boiling it in a 5% solution of ammonium carbonate [13]. In the present case, the extraction of uranium from a sample of jasper-like quartz into the indicated solution in a single treatment amounted to about 25–30% of its total quantity, which indicates a rather high content of a readily mobile element. When hydrogen peroxide is added to the ammonium carbonate solution, extraction of uranium from the sample increases to 50–60% owing to oxidation of tetravalent uranium chemically bound in uraninite.

A rather noticeable (up to 0.1%) increase in the uranium content in jasper-like quartz is established in zones where veinlets intersect bituminous marbles, amphibole and andradite skarns. The uranium mineralization here is represented by disseminations of spherulites of uraninite. Consequently, in the present case we definitely have lithological control of uranium mineralization. The factor determining the relative “favorability” of these rocks for the formation of uranium mineralization is the presence in them of components that lower the oxidation–reduction potential (organic matter, divalent iron). These questions are considered in greater detail in the specialized literature [9, 10].

Here we shall note only the fact that quartz veinlets intersecting marbles and light-colored skarns—predominantly grossular in composition and distinguished by a low uranium content—have perfectly sharp contacts and bear no noticeable signs of hydrothermal alteration of the host rocks. By contrast, in bituminous marbles and in amphibole and andradite skarns, in contact with the veinlets, thin zones of hydrothermal alteration are noted in most cases. In amphibole skarns these alterations are expressed in the replacement of hornblende by chlorite. The latter is frequently decomposed with the formation of quartz, carbonate, and hematite. The garnet of andradite skarns is replaced by brown oxide compounds of iron, quartz, and also carbonate.

Dark-gray bituminous marbles prove to be bleached. Narrow (5–10 cm) halos of coloration, caused by a finely dispersed admixture of hematite, are often observed in them. The presence of zones of near-vein alteration testifies to the chemical interaction of solutions with the wall rocks, which were out of equilibrium with respect to them [15]. Under conditions of such active interaction of these two phases, organic matter and divalent iron acted

as factors that substantially lower the oxidation–reduction potential of the solution and promote the reduction of hexavalent uranium, transferring it into the precipitate.

In conclusion, let us note certain genetic features of the uranium-bearing quartz. The wide development of colloform structures is regarded by most researchers [1, 6, 7, 11, 14, 16, etc.] as a consequence of the gel state of the substance. At the same time, many works [4, 5, 8, etc.] have shown the possibility of formation of incrustation and kidney-shaped aggregates by direct crystallization from true solutions. Consequently, at present we cannot speak of the inevitable formation of colloform formations from gels.

Nevertheless, microscopic study of chalcedony and quartz suggests that they most likely formed through the recrystallization of silica gel. This is indicated above all by the sharp predominance in the veins of chalcedony [3] over quartz, the presence of amorphous (glassy) silica segregations distinguished by very weakly expressed polarization properties, and the presence of finely dispersed impurities of iron and uranium oxides. In direct crystallization from true solutions, one would expect the appearance of quartz grains and, for example, hematite plates. The layered concentric-zonal distribution of particles of ore material is more simply explained by diffusion and rhythmic precipitation in a colloidal medium. Many concentrically layered spherulites of chalcedony in section very much resemble Liesegang diffusion rings. Desiccation cracks are highly characteristic of kidney-shaped aggregates.

Silica gel, like ferric oxide hydrate, possesses strong adsorption properties. The insignificant (0.01–0.03%) concentrations of uranium that are observed in most veinlets (with the exception of veinlets crossing rocks favorable in lithological terms) could quite well have been produced by an adsorption process during the period of contact between the precipitated hydrogel of silica and iron oxides and the percolating hydrothermal solutions. The latter, evidently, were very weakly mineralized with respect to uranium. This is further supported by the fact that the enclosing rocks—substantially calcareous rocks (grossular skarns, marbles)—could not have exerted any noticeable influence on the process of uranium deposition, since in essence they did not interact with the fissure solutions. In places where the veinlets crossed “favorable” rocks, the decisive influence on uranium precipitation was exerted by their reducing properties.

The silica particles of spherical or irregular form that were produced probably possessed an amorphous structure in the initial moments. This, at any rate, is indicated by the experimental electron-microscopic studies of Z. Ya. Berestneva and V. A. Kargina [2]. Subsequently, processes of recrystallization proceeded inside the particles; moreover, as the results of experimental studies show, recrystallization of such particles can occur fairly rapidly. According to E. A. Rozhkova and N. P. Solov’ev [12], carbonate spherulites formed from a colloidal precipitate undergo recrystallization after 1.5–2 days. Under natural conditions this process takes place more slowly, and it depends on the rate of temperature decrease [16].

Under conditions of slow cooling, recrystallization proceeds completely, and the primary colloform structures may be entirely destroyed. In this connection, the feature becomes understandable that colloform structures are characteristic in most cases of deposits of shallow facies.

The phenomena of recrystallization of the mineral fillings of the veinlets studied are indicated above all by the combination of the aggregate’s granular structure and relict colloform zoning, which is a reflection of the nonuniformity of the recrystallization process. During recrystallization, material is redistributed; in particular, iron oxides are “driven off” into the peripheral zones of reniform and botryoidal aggregates. At the same time, this redistribution and differentiation of matter occurs chiefly at the very earliest moments of recrystallization, and possibly even earlier, in the gel medium, as is believed, for example, by A. M. Kuz’min [6] and E. A. Radkevich [11].

During recrystallization, naturally, the silica and iron oxides are “purified” of sorbed uranium, which, concentrating in the intergranular space, subsequently creates its own mineral forms. Probably, in the initial moments of recrystallization of the gel, fibrous chalcedony is formed, as well as spherochalcedony. Within aggregates of such chalcedony there were pore solutions saturated with silica, which promoted its recrystallization; as a result, larger individuals of radial-fibrous chalcedony arose, as well as fine-grained quartz (Fig. 1). Recrystallization of chalcedony also occurred later, during the superposition of carbonate mineralization. In areas where jasper-like quartz is cut by carbonate veinlets, fringes of granular quartz are always observed at the boundary between chalcedony and carbonate.

Thus, recrystallization of silica gel into chalcedony and the subsequent processes of its recrystallization into quartz contributed to the regrouping of uranium and the emergence of the finest inclusions of uraninite. As for the intensely colored, weakly polarizing reniform bodies of chalcedony, where the silica is evidently in a state close to amorphous, uranium in them, as shown by microradiographic studies, has not undergone substantial regrouping and is present in a dispersed, possibly sorbed, state.

REFERENCES

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