U-Blacks Mineralization in Sandstone Uranium Deposits

0. INTRODUCTION

Ores of hydrogenic sandstone-hosted uranium deposits in the sedimentary cover are everywhere composed of uranium blacks. In the uranium mineralogy, "blacks" refers to earthy, sooty radioactive mineral formations of black (less often brownish-black) colour. This disperse, sooty uranium mineralization in the form of loose powder occurs in almost all types of uranium deposits. In the sandstone type deposits, U-blacks are of the greatest industrial interest, because they are suitable for in situ leaching (ISL) mining. In the hydrothermal deposits, uranium blacks are formed by weathering of sulfide-containing uranium ores in the lowest zone of activity of hypergenic processes. Blacks' mineralization is often the main and even the only component of commercial uranium deposits. The interest in their mineral composition is caused by the wide spread of blacks and their manifestation in ores of various genetic types. These U-ore formations are difficult to diagnose by traditional methods of mineralogy.

Until recently, the term "uranium black" in the geological literature was usually identified with the only mineral form of uranium "the oxide". Studies conducted in our institute since the 70's under the direction of L. N. Belova (an outstanding specialist in hypergenic mineralogy of uranium) proved the polymineral nature of black uranium-ore mineralization (Doynikova et al., 2003; Belova et al., 1991). It was proved that the so-called "uranium black" can be represented by at least three different uranous (U⁴⁺) mineral forms: oxide (uraninite), silicate (coffinite) and phosphate (ningyoite). They are manifested in various proportions, up to the monomineral formations. The oxide form is the most common. Our many years of black uranium ores studies have shown that the terms "uranium black" or "black" ores are valid only as a morphological characteristic of sooty black radioactive material, and using this term to characterize the mineral composition will be mistakenly (Doynikova et al., 2020; Doynikova, 2012; Belova et al., 1991). Further studies of such uranium-ore materials may possible lead to the discovery of additional U-mineral types, new for them. This article is summing the results of long-term mineralogical observations and presents the characteristic diagnostic features of uranous minerals observed under the electron microscope (EM).

1. METHODS, TECHNIQUES, MATERIALS STUDIED

The mineral composition of uranium ore substance was studied predominantly by using the analytical electron microscopy (AEM), and the radiography has been used as a preview method.

Uranium-ore mineralization of sandstone deposits is a friable sooty black formation so its diagnostics is difficult both for optical and other traditional mineralogical methods (XRD, IR, X-ray spectroscopy, etc.). All these methods analyze the total sample composition (of a finely dispersed phases mixture) and the results received characterize the phase that is dominant in the sample whereas sharply subordinate and X-ray amorphous phases remain overlooked. Taking into account the dispersion of the U-minerals studied and, often, their micrometer size, the transmission electron microscopy (TEM) is the most effective for the analysis of phase composition of such uranium ores. The small content of one of phases in the polymineral mixture or the extreme degree of substance dispersion (beyond the X-ray's reach) does not obstacle this method. The main method for minerals identification, on which all our research results are based is the Selected Area Electron Diffraction (SAED) carried out in the transmission electron microscopes: JEM-2100+IETEM INCA-250, 100 kV; JEM 100C+Kevex-5100. The SAED is the most effective method in the crystallochemical characteristics study of dispersed uranous substance (Doynikova and Sidorenko, 2010). It is optimal for obtaining complex of crystallochemical data sufficient for diagnostics, in combination with elemental analysis of the same substance directly in the electron microscope. In recent years analytical scanning electron microscopy (ASEM) methods using JSM-5610LV and energy-dispersive spectrometry (EDS), and INCA-450 have been used to study sandstone-type ores of the Vitim uranium ore region (Doynikova et al., 2014). The diagnostics of uranium-ore minerals was carried out by complex methods of TEM and SEM (SAED, EDS, BSE-imaging).

The ore samples studied from terrigenous sedimentary rocks are loose material of weakly cemented sandstone or siltstone and their gray and dark-gray colour is due to the occurrence of organic carbon. They were studied in different electron microscopy (EM) preparations: handpicked individual grains, transparent polished sections, and polished sections (anshlifs).

2. RESULTS

U-oxides of various crystallinity degrees are the main components of uranium blacks. Numerous SAED studies confirm that the crystallochemical property of natural uranium oxides belongs to a single mineral species which is uraninite, in agreement with previous results noted by many researchers (Sidorenko, 1978; Kerr, 1951; etc.). The subdivision of simple uranium oxides into pitchblende, nasturane, U-pitch ore and U- blacks, adopted in the mineralogy reflects only their external, morphological character, although by diffraction they all represent one mineral with fluorite type cubic face-centered lattice Fm3m.

Uraninite particles (most common in EM-preparations) are dense lumpy formations without defined outlines, which are often aggregate of even smaller (submicron size) particles (Fig. 1).

Figure  1.  Uraninite. (a) SEM-image of mineral incrustation on a sand grain; (b), (c) particles' TEM-images; (d) EDS composition specter; (e) uranium distribution in quartz grain by f-radiography (left-thin section, right-imaging on mylar film, direct uranium distribution); (f), (g) SAED patterns of different particles: ring (f) and characteristical for electron amorphous uraninite (g). Vitreous uraninite (uran-oxide) with a characteristic conchoidal chipping (h), SEM.

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The diffraction patterns of different particles may be points, rings or completely absent. As structural perfection decreases (due to increased structural disturbances during oxidation), point diffraction reflexes disappear and become circular (Fig. 1f). Reflexes become wider, their number decreases, up to preservation of only one wide halo joining former reflexes 111 and 200 (d=3.2–2.5). U-ores' substance gradually becomes X-ray amorphous and then electron-amorphous. That is, the degree of uraninite crystallization may be so small that it does not allow determining the mineral even with the help of electron diffraction. On the electronogram corresponding to such "non-structural" uraninite only a weak intensity thickening in the area of first two uraninite rings area is fixed (Fig. 1g). Electron amorphous uraninite formations are quite common in natural samples and their diagnostics without microanalyzer is problematic. Then the main diagnostic feature is the elemental composition and the indirect feature is the characteristic particle morphology. In the composition of poorly crystallized uraninite (with diffuse ring reflexes) Si is often present, and its content increases with the decrease of crystallinity degree.

New data on uraninite are as follows: its' mineralization is in form of mineraloid; it is solid mineral gel of uranium-oxide composition, which prevails in ores of the Khokhlovskoye infiltration deposit, South Trans-Uralian (Khalezov, 2009; Volkov et al., 2005; Dymkov et al., 2003). In these ores the uranium-oxide "gel" is a mineraloid (Fig. 1h) in form of phyto-morphoses and impregnations in carbonaceous lignite fragments. The majority of vitreous U-oxides contain water by indirect signs; their composition is close to that of uranium dioxide. Such non-crystalline uranium oxide usually contains insignificant impurities (Al, Si, Ca, Zr, Ti, Si, Fe, S, and P).

In exogenous ores, uranium oxides are much more often associated with the cementing material, or act as cement themselves. Such cementing role of uranium oxides is very typical for sandstone-hosted U-ores. In many sandstone deposits with up to 0.n% U, even the most effective mineralogical methods does not allow to establish individual uranium minerals. The increased R-activity here is associated with the appearance of dark colored quartz grains, and only the f-radiography method shows that the U tracks are confined to such dark quartz grains and fine pebbles, forming the thinnest rims around them (Fig. 1e) with rims' density directly proportional to quartz darkening degree. Upon reaching a certain level of their density, the uranium oxides are fixed by the EM method. Further uranium accumulation leads to formation of an individualized mineral phase in form of uraninite. In this way the U-oxides are formed in the intergranular cement and under favorable accumulation conditions they become cementing material. In such ores, organic residues are more often associated with coffinite.

2.1 Coffinite

U⁴⁺(SiO₄)_1-x(OH)_4x is widespread in the composition of finely dispersed black ores and often acts as a significant part. The coffinite is usually well crystallized in exogenous deposits; in hydrothermal deposits it is usually metamict, x-ray amorphous, due to the radioactive decay of uranium during long mineral age. In exogenous ores, it is often impossible to optically distinguish between coffinite and ningyoite, and cryptocrystalline uraninite. In the majority of exogenous deposits coffinites can be mistakenly diagnosed in polished thin rock sections as ningyoites when only the optical diagnostics is carried out (by coloration, reflective power and internal reflexes). Many sandstone-type deposits are characterized by a high degree of coffinite dispersion; the crystal size varies from 1 to 10 microns. The coffinite in TEM-preparats rarely has characteristic forms which are needle, spindle and oval (Fig. 2). More often, these are lumpy particles, which are aggregates of still smaller formations (Figs. 2c, 2d). The coffinite exhibits high crystallinity degree (Figs. 2e–2h). Ring shaped SAED patterns are typical only for shapeless submicron size aggregates of finer particles. In contrast to uraninite, coffinite has never been electron amorphous in all our samples studied. Coffinite SAED-patterns are usually point-like (monocrystal). Even when coffinite particles have patching form, "eaten" by dissolution as in Fig. 2c, they often retain a point-like diffraction picture.

Figure  2.  Coffinite. (a) SEM-images of incrustation (Cof) on a sand grain; (b) EDS-spectrum of composition; (c)–(e) TEM-images of aggregated particles (c), (d) and microcrystals intergrowth (e); characteristic microparticles' SAED patterns (f)–(h). Our laboratory experiments showed that coffinite microcrystals with admixtures (P, V, Fe, As) are easily leached unlike pure (microcrystals without impurities) ones. Finely dispersed uranium oxides and phosphates are also easily leached. So analysis of finely dispersed material (subjected to oxidation) by traditional X-ray diffraction method alone may lead to erroneous diagnostics: against the background of X-ray amorphous uranium oxides, the real composition of such ores will be defined as a mono-coffinite. Such erroneous conclusions were made in the initial study of Chu-Sarysu deposits ores whose composition was later clarified as a nasturane-coffinite using the ATEM methods.

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In sites where clear positive U-P correlation is observed, there are ore samples with P-containing coffinite (Belova et al., 1980); it is very similar in shape to ningyoite. Such coffinite microcrystals are 0.5–2 µm in size with needle, spindle-like or oval shapes. Sometimes such coffinite is represented by particles with partially corroded form, showing natural "leaching", however it retains the point-like SAED-image indicating on high crystallinity degree. Study of numerous samples allowed speaking about the mineralogical coffinite feature that in exogenous ores it is usually closely related to carbonaceous material.

The coffinite-like uranous phosphosilicate was established in association with the disperse uranium oxide during the study of sandy ores of paleovalley (channel type) uranium deposit Dalmatovo (Doinikova et al., 2009). This mineral phase is predominant in ore intervals studied and it is usually confined to altered feldspar. Microcrystals are slightly elongated, lentil-shaped, a few microns in size, they often form aggregates (Fig. 3), and less often collomorphic formations.

Figure  3.  Uranous phosphosilicate. (a) SEM image of aggregate; (b) EDS-spectrum in marked (x) point of (a), (c) analysis result (O calculated); (d) crystals (white) in an shlif; (e) TEM-image of particle; (f) SAED-pattern.

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At the current stage of research the idealized formula of this phase is presented as (U, Ca)[(Si, P)O₄]₂, where U : Ca≈3 : 1 and Si : P≈3 : 1. The ratio of cationic to anionic component is (U, Ca) : (Si, P)=1 : 2, which is half as much as in coffinite, where U : Si=1 : 1. As non polished samples were studied and the results of analyses were normalized, it is required a further composition study with polished samples and the question of OH-groups or water presence in this phase also remains open. Since quantitative EDS analysis showed a significant (three times) excess of silicon over phosphorus (in atomic units), this phase can be characterized as uranous phosphosilicate (Doynikova et al., 2015, 2014). The morphology of this U⁴⁺-mineral is close to forms of coffinite and ningyoite. In contrast to ningyoite, this U-phosphosilicate contains a stable predominance of U over Ca. The described phase differs from P- containing coffinite, first of all, by the constant presence of calcium, as well as by the stable Si/P ratio. The revealed composition features allow speaking about the individuality of this mineral phase. A preliminary SAED-study of this phase has shown that the diffraction patterns (both circular and point) are characteristic of coffinite (Fig. 3f), which makes it possible to call the mineral coffinite-like.

2.2 Ningyoite

Uranous phosphate CaU⁴⁺(PO₄)₂∙2H₂O has been considered for a long time as mineralogical rarity unlike uraninite and coffinite, which are well known in the uranium mineralogy and widely studied to date. Ningyoite was revealed in samples, visually attributed to uranium blacks. This previously little-known U⁴⁺-phosphate of rhabdophane mineral group was discovered in Japan at the basal-type uranium deposit Ningyo-Toge as the main ore component (Muto et al., 1959). Our previous study using ATEM identified fine-dispersed U-ore material in the sedimentary cover deposits, in the late 1970s we discovered ningyoite on the former USSR territory and in Bulgaria (Belova et al., 1978a); later in the weathering crust of Kosachinoe Deposit, Northern Kazakhstan, as the main ore raw material (Doynikova, 2007). After our findings, ningyoite was found in uranium deposits of Czech Republic (Scharmova and Scharm, 1994; Scharm et al., 1980) and Poland (Kucha and Weiczorek, 1980); later this phosphate was discovered in the infiltration basal-type deposits in Canada (Boyle et al., 1981). To date, the findings of ningyoite are numerous; it is widely present in hypergenesis zone formations. The wide distribution of ningyoite in sandstone-hosted deposits has been shown by us (Doynikova, 2007; Doynikova et al., 2003).

In all hypergenesis region U formations, ningyoite is characterized by clear location on boundary of reduction and oxidation mediums. In uranium blacks ningyoite can participate as admixture (can be significant) or it forms monomineral composition and the association with sulphides and organic residues is very typical. The monomineral formations of this Ca-U-phosphate are loose powders of brownish-black or dark brown (in mix with organic matter) colour. In infiltration/hydrothermal ores, ningyoite is manifested in form of granular masses and nodular crust.

Under optical microscope, the colour of ningyoite is greenish-brown, greenish. In transmission light, the mineral is brownish-green or green. Pleochroism is weak, darker by Ng. The average values of refraction index n=l.64 (Muto et al., 1959); n=1.60–1.70 (Boyle et al., 1981). Microcrystals' elongation is positive, along c axis; extinction is parallel to elongation. There is no fluorescence. In reflected light, under optical ore microscope, ningyoite reflection is lower than that of coffinite. The result of detail researching of crystalline ningyoite under optical microscopes (samples from hydrothermal veins) is as follows: when observed in dark field, it was brown; in reflected light it is almost black among uranium minerals, but among sulfides ningyoite crystals are transparent as indicated by Dymkov et al. (1986) who notes that the primary colour of ningyoite should be in green tones, similar to other phosphates of tetravalent uranium; and mineragraphy study of ningyoite does not exclude possibility of erroneous identify. Optical diagnostics of ningyoite in exogenous ores is difficult because of morphological similarity of microcrystals with coffinite. Previously unknown reflectivity of ningyoite was defined as similar for quartz and it was found in optic microscope on the largest crystals (7–8 μm) in sample from one of Vitim deposits (Doynikova et al., 2014).

All ningyoite detections known to date have been made using EM (Scharmova and Scharm, 1994; Atkin et al., 1983; Boyle et al., 1981; Scharm et al., 1980; Muto et al., 1959). Under electron microscopes, ningyoite microcrystals are of needle-like and spindle-shaped habitus. The largest microcrystals (n∙10 μm) are columnar or narrowly columnar; the crystals are rhombic or hexagonal in cross-section (Scharmova and Scharm, 1994). Characteristic feature of this phosphate is its extremely small size. In all samples studied the crystal size rarely exceeds 1.5–2 microns (Fig. 4).

Figure  4.  Ningyoite. TEM. images: (a)–(c) microcrystals and their intergrowths (b) carbon replicas); (d) typical EDS spectrum of crystals' composition without impurities; (e), (f) typical diagnostic diffraction characteristics: point SAED patterns/with an "internal" ring standard (Au), reflections; index H. hexagonal syngony; (g)–(l) Typical mineralization in rich ores (Khiagda, Koretkonde): crystal crust on sand grains (g), (h), typical EDS spectrum of crystals' composition here (m), large ningyoite crystals (bright) within U-ore mass with a sulfide component (light-gray), thin section (j), composition spectrum (n) and analysis result of these crystals (o); (i), (k) grains of sand from poor ores; (i) enlarged fragment: uranium-Al-Si-mass with impregnation of needle-like submicron ningyoite; (l) mineraloid ningyoite mass (light) among pyrite (dark-gray), SEM.

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Earlier it was established the isomorphic Fe occurrence to ningyoite composition (Belova et al., 1978b) and the wide cationic isomorphism manifestation (U, Ca, Th, REE) in its structure has been showed (Doynikova, 2012). Our ningyoite observations made it possible to obtain its crystallochemical data. By SAED method, ningyoite was proved to belong to the hexagonal syngony; its' hexagonal cell parameters are as follows: a=b=6.86 Å; c=6.38 Å (±0.03); calculated density Dcalc=4.74 (Belova et al., 1985). The characteristic diffraction feature of this phosphate is clearly manifested by all reflexes 000l and sharply increased reflexes' intensity with l=3n (Fig. 4e). The set of SAED-patterns (obtained with microcrystals tilting around c* axis) showed that the symmetry axis of sixth order is an inversion one. Consequently, the ningyoite structure is characterized by one of two possible groups' symmetries Р$ \overline{6} $m2 and Р$ \overline{6} $2m (Doynikova, 2012).

Our last ten years' investigations of basal-type uranium deposits (paleo-valley, paleo-riverbed ones) of Khiagda ore field on Vitim Plateau, Russia, made it possible to yield more detailed information on the main therein U-ore raw material-ningyoite (Doynikova et al., 2020; 2014). This Ca-uranous phosphate is represented here in the form of micron-sized crystalline and mineraloid formations in all types of host rocks (sands, basement granites, overlying basalts).

For the crystalline form of ningyoite the micron size of crystals is the main characteristic (Figs. 4g, 4h). The size of massive crystalline ningyoite accumulations rarely reaches n∙100 μm. The largest ningyoite crystals (< 10 μm) were found in the carbonaceous fragment from of clay interlayer (Fig. 4j). Submicron size ningyoite crystals (elongated shape) were determined in the cementing Al-Si-mass (kaolinite-hydromica material) of uranium-bearing sands (Figs. 4m, 4n) and in the carbonaceous debris. Ningyoite often contains up to 1%–2% Ce, La and less Nd, and less commonly the impurities of Sr, Y, and Zr. It is in the Khiagda ore field ores that was established isomorphic entry to ningyoite composition as Fe and S at the same time (Fig. 4i).

The mineraloid form of ningyoite as solid uranium mineral gel was often observed in the studied samples of paleovalley Vitim deposits. The morphology of such formations (curved/arched) indicates the primary plasticity of mineral material (Fig. 4o); it forms phyto- and pseudomorphoses. But more often the mineralization is represented as formless mineraloid aggregates of slightly differentiated mineral mass (Fig. 4j). The size of these segregations rarely reaches n∙10 μm. Their composition U-P-Ca-Fe-S (often with different contribution of Al and Si) is usually inhomogeneous. Spots with different ratios of ningyoite and pyrite components are represented in the colloform mineral mass without visible phase boundaries. The mineraloid form of the "pure" (without impurities) ningyoite is a rare exception.

2.3 Other Uranous Phosphates

The uranous phosphates such as Lermontovite, Vyacheslavite and Urphoite are almost unknown. They are forming within the border of oxidative and reduction mediums. Mineral paragenesis shows that they were precipitated in reduction conditions. These three minerals are currently of mineralogical rather than industrial interest. Theirs detail mineralogical description gathered in the monograph (Doynikova, 2012). These three uranous phosphates are united at present in a single mineral group of Lermontovite with the common idealized formula U(PO₄)OH·nH₂O, but with different structural characteristics (Belova et al., 1998). These minerals do not luminescence and the refractive indices are quite close with each other. Crystal-chemical data of these minerals are limited by information available to ATEM methods when studying micron size objects from small amounts of material. The data on these minerals provided in (Steciuk et al., 2019; Belova et al., 1996, 1985, 1984; Melkov et al., 1983) is some useful information for researchers of finely dispersed exogenous U-ore mineralization from hypergenesis region.

Coloration of uranous phosphates. Before the Vyacheslavite and Urphoite discovery, only uranyl compounds of green colour (including U⁶⁺-phosphates) were known in the uranium mineralogy. Our observation on synthesis urphoite (room conditions) confirmed the colour characteristic: the obtained material (X-ray-and electron-amorphous) was indistinguishable from typical urphoite by colour. The green colouring was extremely important to "remove" the question of uranium valence of green uranous phosphates. So, we can say that the green colour is characteristic for such tetravalent uranium minerals as phosphates.

Little-known or unknown earlier uranous minerals (rare finds) have been formed in reductive conditions; their location among already formed uranyl minerals demonstrates the creation of such locally non-equilibrium conditions into the space of redox boundary of exogenous processes.

3. DISCUSSION

The obvious conclusion from the above data is that the mineralogical study of uranium ores from sandstone-type deposits requires the usage of analytical methods of electron microscopy. After we know ore body mineral composition, we can solve the questions of genesis of these uranous minerals, and, consequently, find out the conditions for the formation of such uranium ores.

The most important contribution of sandstone-type U- deposits with black ores to the world's uranium mining industry explains the increased interest in finding out the genesis of such ores. The crucial importance of biogenic factor in black ores formation was revealed on author's clarification of the genesis of uranium phosphate mineralization. Organic matter (carbonaceous debris and lignite) helps the enrichment of uranium in sandstone deposits (Zhang et al., 2019). As uranium black ore minerals (oxide, silicate and phosphate forms) were formed in similar environment, this allows us to generalize their formation conditions in all sandstone hosted uranium deposits.

There are several publications related to problems of bioremediation-radionuclides immobilization (including U) in near-surface environment using microbial activity (Sivaswamy et al., 2011; Khizhnyak et al., 2005; Suzuki et al., 2005; Lovley et al., 1991). It was shown on uranium example that the biological deposition of radionuclides is always associated with microbial release of PO₄-ion from microorganism cells (mainly of plant residues) (Suzuki et al., 2005). Plant decomposition is ensuring supply of PO₄-ions to solution and is always accompanied by bacterial reduction of U⁶⁺. Microorganisms decompose the organic residues buried and thus create a sedimentation environment for U⁴⁺. The results of modern researches in the fields of ecology and environmental mineralogy have helped the author to solve the question of the phosphorus source when clarifying the genesis of ningyoite Microbiologists' work on uranium immobilization by natural bacteria (in form of phosphate minerals) has convincingly shown that the source of phosphate ions in aqueous solutions are plant cells, more precisely products of bacterial recycling of various P-containing organic compounds in these cells. Microbiological experiments on bacterial reduction of U (Cerrato et al., 2013; Behrends and Cappellen, 2005; etc.) demonstrate that the U⁴⁺-mineral formation (uraninite, ningyoite) in natural waters saturated with organic matter is caused by the microorganisms' activity. All these facts allow considering the organic material of sediments as a source of phosphorus for ningyoite. Possible contribution of abiogenic dissolution of phosphates is insignificant, due to the lack of dissolution traces on apatite grains in association with ningyoite. In natural conditions this is confirmed by ningyoite ores localization to strata most saturated with organic material due to the presence of lacustrine-bog sediments in hosting formation characteristic for phosphate ningyoite ores.

From data of geomicrobiology and microbiology it follows that the vital activity of joint aerobic-anaerobic bacteria community within the ore-hosting strata provides for U mobility and reducing environment during U black ores formation. As the consequence such bacterial activity is considered by the author as a factor of U-ore accumulation, which indicates the biogenic nature of uranium deposition in sandstones (Doynikova et al., 2020; Doynikova, 2017, 2016).

Sandstone type roll-front U-deposits in supergene systems, where weathering is stimulated by microbial activity, are considered as lateral development of hypergene processes from aerobic weathering to anaerobic enrichment (Zammit et al., 2015). It is one of examples from a number of some already known Fe, Mn, Au, Cu deposits of biogenic genesis (Calas et al., 2015; Erlich and Newman, 2008; Southam and Sanders 2005). It was shown that uranium fixation often takes phosphates form, and less often uraninite (Sivaswamy et al., 2011; Lovley et al., 1991). Biogenic uraninite is already studying in many investigations of both microbiological and ecologic-biochemical orientation (Lezama-Pacheco et al., 2015; Cerrato et al., 2013; Singer, et al., 2009). By our data, black uranium mineralization is formed on the geochemical barrier under reducing conditions into the space of redox boundary of exogenous process.

A number of geological publications in recent years also indicate on mineral-forming role of microorganisms in U-ores formation in sandstone type deposits. A study of uraninite and coffinite from the Xinjiang roll deposit in China has shown that these minerals are precipitated by microorganisms (Min et al., 2005). The biogenic nature of the U⁴⁺-mineralization in sandstone type deposits is indicated by and others geological studies (Zhao et al., 2018; Cai et al., 2007). A study of coffinite from Beverly uranium deposit in South Australia concluded that bacterial reduction may be the primary ore-forming process (Wülser et al., 2011). At this sandstone-hosted (tabular type) uranium deposit the bulk of the mineralization consists of P-containing coffinite and/or uraninite. Probably the presence of phosphorous in this U-ore mineralization is caused by increased phosphate-ions activity due to Beverly ore-sandstone depositions in lacustrine shore environment enriched microbiota. But the amount of phosphate ions was likely insufficient to form ningyoite due to little organic matter.

A review of the literature data on environmental mineralogy and geomicrobiology with the geological studies results allows talking about the decisive role of microorganisms in the formation of uranous ores. In the infiltration sandstone type uranium deposits, the organic residues contained in the host sedimentary strata always are abundant with bacteria and other microorganisms; this organic matter is a nutrient medium for them. The action of aerobic-anaerobic bacteria community leads to the creation of biogeochemical barriers (medium) that reduce uranium. It is this bacteria community who should be evaluated as an important factor/ component of uranous ore formation along with other geochemical characteristics of the environment.

Essentially, the very nature of sedimentary accumulation provides the presence of reductive agents that can create uranium precipitation barriers in the filtration of oxygen-bearing uranium solutions.

4. CONCLUSION

In sandstone U-deposits the uranium-ore mineralization (U-blacks) is mainly represented by three different mineral forms: uraninite (oxide), coffinite (silicate) and ningyoite (phosphate). They are manifested here as in crystalline and in amorphous forms, in various proportions, up to monomineral formations. Oxide forms are the most common ore compositions. Ningyoite ore composition is characteristic for paleovalley basal type uranium deposits.

Sedimentary strata in the hypergenesis zone can be considered as a medium that is always favorable for creating black (sooty) uranium-ore mineralization, if there is a source of water-soluble forms of uranium. Such a source can be either solid crystalline massifs exposed by denudation and are washing by surface waters, or endogenous/hydrothermal uranium deposits that are washing out.

For reliable diagnostics of highly dispersed uranium mineralization in sandstone U-deposits, where X-ray amorphous uranium minerals are present quite often, it is necessary to use the whole complex of analytical EM methods in addition to traditional optical ones (Doynikova, 2021). Today environmental studies require the study of modern age mineral formation and, consequently, the study of small quantities dispersed mineral matter, which is possible only when using a complex of ATEM methods.