ISSN 0016�7029, Geochemistry International, 2013, Vol. 51, No. 10, pp. 831–851. © Pleiades Publishing, Ltd., 2013.Original Russian Text © S.F. Vinokurov, V.Yu. Prokofyev, Yu.M. Dymkov, M.V. Nesterova, 2013, published in Geokhimiya, 2013, Vol. 51, No. 10, pp. 924–946.

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INTRODUCTION

This paper reports the results of a comprehensivemineralogical and geochemical investigation of ura�nium ores and their host rocks from four of the bestexplored paleovalley�type deposits: Dalmatovskoe,Khokhlovskoe, Semizbai, and Malinovskoe, which arelocated in different parts of the West Siberian ore region.The Dalmatovskoe deposit is currently mined by in situleaching, and appropriate testing operations are carriedout in the Khokhlovskoe deposit.

During the past decade, our investigations haveincluded systematic measurements of the compositionand temperature of formation of fluid inclusions in lateminerals (mainly, carbonates), which were definitelyformed after the epigenetic alteration of rocks related tothe development of the zones of ancient soil–bedrockoxidation. It should be emphasized that, according tothe most popular current concepts, the exogenic com�

mercial ores of these uranium deposits were formedowing to the development of ancient oxidation withoutany significant contribution of other processes. In thiscontext, the investigation of fluid inclusions in late min�erals is of special importance, because it provides directevidence for the activity of hydrothermal solutions inthe ore�hosting sequences of these deposits.

The results of thermometric and cryometric investi�gations of fluid inclusions by modern techniques indi�cated similar temperature ranges for the formation oflate carbonates in all the deposits considered. There isalso an evident compositional similarity between thefluid inclusions and the modern formation waters of theKhokhlovskoe deposit; the latter are assigned to thebroad geochemical group of thermal nitrogen–meth�ane waters, often with high CO2 contents, which arecommon in the sedimentary cover of the West Siberianplatform.

Fluid Inclusions in Late Minerals from the Paleovalley�Type Uranium Deposits of the West Siberian Ore Region: Thermochemical

Characteristics and Genetic ApplicationsS. F. Vinokurov, V. Yu. Prokofyev, Yu. M. Dymkov, and M. V. Nesterova

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia

e�mail: aleshin@igem.ruReceived June 21, 2011; accepted January 19, 2012

Abstract—Comprehensive microthermometric investigations revealed similar temperature ranges (280–120°C) for the formation of late carbonates in the Khokhlovskoe, Semizbai, and Malinovskoe deposits of theWest Siberian uranium ore region. A close chemical similarity was definitely established between the solutionsof fluid inclusions and thermal nitrogen–methane waters with elevated CO2 concentrations typical of thisregion in general. It was noted that such CO2�rich mineral waters (Yessentuki no. 4 type) are common in theMesozoic sequences of the Shadrinsk region, where Transuralian uranium deposits occur, and are similar incomposition and temperature to the modern CO2�rich formation waters of the host sequences of theKhokhlovskoe deposit. The mineralogical and geochemical features of newly formed late minerals and ura�nium ores were considered as the most probable reflection of the exfiltration of such thermal solutions intothe host levels. Two late mineral assemblages were distinguished: (1) hematite–calcite and (2) goethite–ber�thierine and goethite–smectite–chlorite with siderite or goethite–kaolinite–illite with siderite; they occurboth in the host sequences and in the underlying basement rocks. The development of the latter assemblagecauses a significant change in rock color (bleaching); it is widespread and was observed in all the deposits. Itwas shown that these altered rocks and uranium ores (especially high�grade) are very similar in mineral andchemical composition to the products of acid leaching and accompanying mineralization, which could berelated to low�temperature argillization. It was suggested that exogenic epigenetic processes of ancient soil–bedrock oxidation contributed certainly to the development of uranium mineralization, and the moderncharacter of the uranium ores and their host rocks is related to a large extent to the influence of hydrothermalCO2�rich solutions related to the neotectonic activation of the region. This resulted in the development oftheir specific mineral and chemical compositions and corresponding technological characteristics. It seemsexpedient to estimate the possible contributions of exogenic and endogenic factors to the formation of theuranium mineralization rather than oppose the roles of these processes of different stages.

Keywords: paleovalley�type deposits, uranium, fluid inclusions, thermal solutions

DOI: 10.1134/S0016702913070069

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In view of the wide occurrence of thermal bicarbon�ate waters in the host sequences, we considered twotypes of late epigenetic alterations of rocks resulting inthe formation of new carbonate aggregates discussedhere. The first type is represented by newly formed cal�cite, often with hematite, in the sandstones of the hostsequences; the second type includes, depending on theinitial mineral and chemical composition of the hostsequence, the bleached zones of the kaolinite–illitecomposition with siderite and goethite in the deposits ofthe Transuralian region and green�colored zones of ber�thierine and chlorite–smectite compositions with sid�erite and goethite in the deposits in the eastern part ofthe region. The late newly formed minerals are wide�spread in the host sequence and in the underlying base�ment rocks and developed sequentially actively decom�posing previous newly formed associations.

METHODS

The late newly formed minerals were examined indetail using a variety of state�of�the�art mineralogicaltechniques, including analytical electron microscopy,X�ray phase analysis, electron microprobe analysis, etc.Geochemical investigations included various analyticalmethods: instrumental neutron activation analysis(INAA), X�ray fluorescence spectroscopy (XRF), andsemiquantitative emission spectral analysis (SESA),and were performed at the laboratories of the Instituteof Geology of Ore Deposits, Petrography, Mineralogy,and Geochemistry, Russian Academy of Sciences.

Microthermometric experiments with fluid inclu�sions were carried out at the mineragraphy sector of theInstitute of Geology of Ore Deposits, Petrography,Mineralogy, and Geochemistry, Russian Academy ofSciences, using a measurement set�up based on aLinkam THMSG�600 heating stage (England), anAmplival microscope (Germany) equipped with a set oflong�distance objectives, including an Olympus 80×objective (Japan), a video recorder, and a computercontrol system. The set�up is designed to perform real�time measurements of the temperatures of phase transi�tions in inclusions from –196 to +600°С, observe themat high magnifications, and make digital photomicro�graphs. The salt composition of solutions was deter�mined from eutectic temperatures [1]. Pressure wasestimated from the intersection of the isochore and iso�therm for heterogeneous fluids [2] or from the differ�ence between the temperatures of gas bubble disappear�ance and halite crystal dissolution for inclusions of sat�urated solutions [3]. The salinity of solutions ininclusions was estimated from ice melting temperatureusing the data of Bodnar and Vityk [4] for the NaCl–H2O system. The concentrations of salts, fluid density,and pressure were estimated using the FLINCOR pro�gram [5].

In order to determine the composition of fluids ininclusions, water extracts from 0.5�g samples of the

0.50–0.25 mm grain size were analyzed by a variety ofmethods, including gas and ion chromatography andICP MS at the Central Research Institute of GeologicalProspecting for Base and Precious Metals (analystYu.V. Vasyuta) using the procedure of [6]. Preliminarily,the amount of water was determined for inclusions fromthe same sample for the calculation of element concen�trations in hydrothermal solution. Carbon dioxide andmethane were also measured, and Cl, K, Na, Ca, Mg,and all other detectable elements were analyzed by ICPMS in the extracted solution.

INVESTIGATION OF FLUID INCLUSIONS

Systematic evidence was obtained during our studyon the temperatures of formation and compositions offluid inclusions in newly formed minerals (mainly car�bonates) from three uranium deposits (Tables 1, 2). Partof these results were published elsewhere [7, 8], but themineralogical and geochemical characteristics of thesamples and thermochemical properties of fluid inclu�sions in them are considered in detail for the first time.

Our study was focused initially on fluid inclusions inthe calcite cement of the carbonized sandstones of theMalinovskoe deposit observed in a number of uranium�bearing intervals among permeable sands. They are typ�ically gray�colored, occasionally with small inclusionsof brownish red hematite and show extensive develop�ment of iron sulfides and occurrence of pyrite–coffiniteaggregates [7]. In addition, fission track radiographyrevealed the persistent occurrence in the calcite cementof finely dispersed uranium concentrations up to a fewhundredths of percent without the formation of ownuranium minerals [9].

According to the data of analytical electron micros�copy, the cement of these sandstones is composed ofcrystalline calcite with a minor manganese impurity (upto 2–4%). Detailed mineralogical examination showedthat the crystalline calcite fills the intergranular spacereplacing in part the primary cement consisting of fine�grained flaky illite–smectite aggregates with chloriteand practically does not affect clastic grains of quartzand feldspar (Fig. 1). In the areas of superimposedgreen alteration products, microdiffraction patternsrevealed intense destruction of the calcite cement andformation of colloform aggregates of iron�rich sep�techlorite berthierine [7].

Sample 100/22 collected from the core of drill holeno. 100 at a depth pf 80.6 m was selected as a represen�tative sample. It is a carbonized sandstone of varyinggrain size containing 0.904% uranium (INAA data).The results of the investigation of fluid inclusions in thissample were published elsewhere [7]. The inclusionscontain solutions and brines of calcium chloride withconcentrations ranging from 11.2 to >30 wt % CaCl2 eq.(Table 1). The homogenization temperature of theseinclusions is 119–230°C.

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 833

The examination of fluid inclusions in the calcitecement of the carbonated sandstones of the Mali�novskoe deposit led us to the following particular con�clusions.

—The temperatures of calcite deposition variedwithin the range 119–230°C, which corresponds to(medium�) low�temperature hydrothermal solutions.

—The main salt component of the CO2�rich solu�tions is CaCl2, the concentration of which was from 11to 28–30 wt %; variable amounts of NaCl and magne�sium, iron, and other cations were always present.

—The most complex composition and significantlyhigher temperatures are characteristic of concentratedsolutions (brines) saturated in NaCl and undersaturatedin CO2.

—Considerable variations in temperature, compo�sition, and salinity within a limited space were probablyrelated to the mixing of hydrothermal solutions withformation waters.

Carbonated rocks (conglomerates, gravelstones, andsandstones) are widespread in the host sequence of theSemizbai deposit. They are most common and abun�dant as lenticular and columnar bodies up to 10–12 mthick in the lower conglomerate level of the Early Juras�sic near the southern wall of the paleovalley, which islocated above a near E–W trending tectonic zone in thegranites of the Paleozoic basement. These carbonatedrocks are usually cemented by coarse�grained calcite inassociation with hematite, which imparts a red or rasp�

berry�red color, although there are also gray�coloredareas of calcite development without hematite [10, 11].The calcite sometimes contains uniformly distributeduranium fission tracks, and fission track radiography

Table 1. Results of thermo� and cryometric investigations of fluid inclusions in late carbonates from the Malinovskoe andSemizbai uranium deposits

Sample Mineral Grain no.

Number of fluid

inclusions

Type of fluidinclusions

Homogeniza�tion tempera�

ture, T°C

Eutectic melting tem�

perature, T°C

Ice melting temperature,

T°C

Salinity, wt % NaCl (CaCl2)

Malinovskoe deposit

100/22 Medium�grained calcite (data of [6])

1 7 Primary, two�phase

141–165 –52/–53 –14.0/–14.8 (17.2–17.8)

2 6 '' 175–194 –55/–56 –7.8/–10.0 (11.2–14.1)

3 5 '' 187–194 –55/–57 –19.4/–21.0 (20.5–21.0)

4 7 '' 119–132 n.d. –11.0/–12.8 (15.0–16.3)

5 5 Primary, three�phase

221–230 –63/–67 –39.8/–42.3 (>30)

Semizbai deposit

15/1617 Coarse�grained calcite

1 7 Primary, two�phase

213 –42 –2.5 4.2

2 5 '' 204 –42 –1.2 2.1

3 4 '' 158 –42 –2.6 4.3

4 3 '' 129 –29 –1.2 2.1

17b/902 Fine�grained siderit

1 2 Primary, two�phase

282 –33 –1.0 1.7

2 2 '' 263 –33 –0.6 1.1

3 3 '' 247 –29 –1.5 2.6

200 µm

Fig. 1. General view of heterogranular carbonated sand�stone; gray grains are quartz, feldspars, and siliceous shaleclasts; light gray areas are carbonate; black areas are Can�ada balsam; and white grains are pyrite.

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indicates a total uranium concentration of up to 0.05–0.07%. Homogenization temperatures of 195–215°Cwere reported for fluid inclusions in this calcite. Itshould be noted that this hematite–calcite assemblage,which is also widespread as a system of veinlets amongthe altered granites of the Paleozoic basement of thepaleovalley, was interpreted as hydrothermal on thebasis of various criteria [10].

Fluid inclusions were studied in a typical core sam�ple (15/1617) taken from drill hole no. 1617 at a depthof 149.5 m. This sample with a background uraniumcontent is a carbonated small�pebble conglomerate of areddish brown color cemented with calcite with numer�ous small fractures filled with coarse�grained calcite.Nineteen primary fluid inclusions in four grains wereinvestigated in a polished thin section of this sample.The fluid inclusions were from 5 to 15 µm across andshowed negative crystal or irregular shapes. All the

inclusions consisted of two phases, solution and gas.Their eutectic melting temperature ranges from –29 to–42°C, which indicates that the composition of solu�tions is dominated by sodium and magnesium chlo�rides. Based on ice melting temperatures from –1.2 to–2.6°C, the salinity of the solutions can be estimated as2.1–4.3 wt % NaCl eq.; the homogenization tempera�ture of the fluid inclusions is from 129 to 213°C(Table 1).

Thus, calcites from the Semizbai deposit showedalmost identical intervals of the homogenization tem�perature of fluid inclusions and significant variations ofthis parameter in individual grains, which may indicate,as was noted above, mixing of hydrothermal solutionswith formation (pore) waters. On the other hand, CO2�rich hydrothermal solutions from the Semizbai deposithave significantly lower salinities and somewhat differ�ent (sodium–magnesium) compositions compared

Table 2. Results of thermo� and cryometric investigations of fluid inclusions in two types of quartz of different generationsfrom the Khokhlovskoe deposit

No. of in�clusion group

Inclu�sion type n

Thom, °C

Teut, °C

Tice melting, °C

Tgas hom. °C

Tgas hydrate melting, °C

Gas compo�sition

Salinity, wt % NaCl eq.

d, g/cm3

P,bar

Quartz 1

1 1, P 3 380 –34 –7.1 – 11.2 10.6 0.70 1920

2 1, P 5 339 –32 –7.1 – 17.7 – 10.6 0.78 1660

3 1, P 4 333 –32 –7.1 – – – 10.6 0.79

4 1, P 4 324 –31 –5.9 – – – 9.1 0.78 1390

5 1, P 3 317 –32 –7.1 – – – 10.6 0.81 1430

6 2, P 5 – – – –93.7 L CH4 – 0.28 1920

7 2, P 3 – – – –92.7 L – CH4 – 0.27 1660

8 2, P 7 – – – –88.8 L – CH4 – 0.26 1390

9 2, P 4 – – – –90.1 L – CH4 – 0.26 1430

10 2, S 3 – – – –81.0 L – CO2–CH4 – n.d. –

11 2, S 5 – – – –79.4 L – CO2–CH4 – n.d. –

12 2, S 4 – – – –78.2 L – CO2–CH4 – n.d. –

13 3, S 3 269 –29 –3.5 – – 5.6 0.82 –

14 3, S 6 239 –27 –3.3 – – 5.3 0.86 –

15 3, S 7 192 –27 –3.5 – – – 5.6 0.92 –

16 3, S 3 135 –25 –1.8 – – – 3.0 0.95 –

17 3, S 4 130 –24 –2.4 – – – 3.9 0.97 –

18 3, S 3 129 –34 –4.1 – – – 6.5 0.98 –

Quartz 2

19 3, P 7 144 –24 –1.9 – – – 3.1 0.95 –

20 3, P 3 142 –31 –2.2 – – – 3.6 0.95 –

21 3, P 3 135 –32 –1.4 – – – 2.3 0.95 –

* Inclusion types: 1, gas–liquid inclusion with a large gas phase; 2, gas inclusion; and 3, two�phase gas–liquid inclusion with a small gasphase. Genetic types of inclusions: P, primary and S, secondary. n is the number of inclusions examined. L denotes homogenization ofdense gases to a liquid phase.

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 835

with the calcium–sodium waters typical of the Mali�novskoe deposit (Table 1).

The host sequences and altered basement granites ofthe Semizbai deposit contain abundant secondary goet�hite–chlorite–smectite aggregates with siderite, whichwere evidently superimposed on the carbonated rocksand were also interpreted as hydrothermal products [10,11]. In our opinion, in addition to the relatively localareas of their intense development, known as dark andbrownish green “tobacco” rocks, they occur extensivelyover the area as green alteration products of the ore�bearing sand sequences of similar mineral and chemicalcompositions, which are well correlated with the newlyformed goethite–chlorite (berthierine) assemblages ofthe Malonovskoe deposit.

Fluid inclusions in siderite were investigated in coresample 17b/902 of typical brownish green tobaccorocks taken from drill hole no. 902 at a depth of 72.0 m.The sample is a coarse�grained sandstone with a crust�like brownish and dark green cement and carbonateinclusions; the rock shows a sub�background uraniumcontent of 13 ppm. Detailed mineralogical examinationshowed that the newly formed green aggregates havefibrous, encrusted, spherulitic, and platy morphologies(Fig. 2), and, according to EDS analysis, they are made

up of iron�rich aluminosilicates, which are most similarin composition to smectites, nontronite and montmo�rillonite [12]. The EDS analysis of carbonate coexistingwith chlorite in the cement of the sandstone showedthat its composition ranges from essentially pure sider�ite to iron�rich ferrodolomite with a total fraction ofcalcium, magnesium, and manganese carbonates of upto 29.3% (Fig. 3).

Seven primary fluid inclusions in three grains wereinvestigated in a polished thin section of this sample.The inclusions were from 1 to 5 µm in size and irregularin shape. All of them consist of two phases, solution andgas. Their eutectic melting temperatures are from –29to –33°С, and the ice melting temperatures are from–0.6 to –1.5°С, which indicate rather low salinities of1.1–2.6 wt % NaCl eq. and sodium and magnesiumchlorides as major solution components (Table 1). Onthe other hand, the highest homogenization tempera�tures (247–282°С) were observed in these inclusions.

Bleached zones of kaolinite–illite composition,often with newly formed siderite and goethite are verycommon in the host sequences of the Transuralian ura�nium deposits (Dalmatovskoe and Khokhlovskoe) [13].However, calcite–hematite assemblages are almostlacking there; they were noted in some publications [14,

50 µm(а) 20 µm

10 µm(s)

C

O

Fe Mg

Al

Si

Ca

Fe

Fe

8

6

2

4

02 4 6

Energy, keV

(d)

(b)

Fig. 2. (a)–(c) Crustlike and spherulitic aggregates of smectite and (d) EDS spectrum of the indicated spot. Semizbai deposit,sample 18/902.

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15] but were never observed during our investigations.Late siderite is abundant as spherulites in altered silt�stone–pelitic rocks and rhombohedral crystals in sand�stones; it was studied by us in detail in the Khokhlo�vskoe deposit.

Unfortunately, our repeated attempts to explorefluid inclusions in siderite from the Dalmatovskoe andKhokhlovskoe uranium deposits by microthermomet�

ric methods have failed. The reason was that late newlyformed carbonates in the host sequence are fine�grained radial spherulites and rhombohedral grains withvery small fluid inclusions (less than 1 µm), which arenot suitable for experiments. Keeping this in mind, weinvestigated fluid inclusions in a quartz–carbonate (sid�erite) veinlet in sample 136/6001 of strongly altered(bleached and carbonated) carbonaceous schist, which

200 µm(а) (b)

(c) (d)

200 µm

20 µm20 µm

20 µm

C

O

FeMgSi

Ca

Ca Mn

Fe

Fe

6

4

2

02 4 6 8 10

Energy, keV(e)

(f)

Fig. 3. Back�scattered electron image of a portion of thin section 17b/902 from the Semizbai deposit. (a) Clastic grains of quartz(dark gray) and feldspar with dissolution features (light gray, mottled) cemented by carbonate (light) with chlorite (gray with roughsurface). (b) Enlarged image of the indicated area. (c)–(e) Character of carbonate grains: (d) relationships of carbonate (light, inthe center) with chlorite (to the left) and quartz (to the right), (e) thin stringers of fine�grained siderite (light) in feldspar, and (f)compositional spectrum of spot 8 (Table 7).

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 837

was taken from drill hole no. 6001 at a depth of 620.9 m,directly underlying the ore�bearing sequence.

The mineralogical examination of sample 136/6001showed that the carbonaceous schist is strongly tecton�ically and metasomatically altered. It shows fine inter�calations of thin (a few centimeters) black and light graylayers. The former are carbon�rich and contain numer�ous slickensides; they have quartz–mica compositionswith variable amounts of feldspars, which is indicatedby high K2O contents of 4.6–6.9 wt % (XRF data). Thelight gray interlayers are composed mainly of quartzwith inclusions of fine�grained kaolinite flakes and cutby numerous microscopic veinlets of siderite, often withkaolinite (Fig. 4). Hence, the carbon�bearing schists areintensely foliated, cataclazed, bleached, kaolinized,and carbonated (sideritized); i.e., they underwent lateepigenetic alterations, which were almost identical tothose observed previously in the host sequence.

The 1–2�cm thick quartz–siderite veinlet containstwo quartz varieties, early and late. The early variety iscoarse�grained milky white quartz, probably of meta�morphic origin, and the late variety is drusy quartz over�growing the early generation and formed probably by itsrecrystallization (Fig. 5). In addition, it was establishedthat drusy quartz intergrows and intimately associateswith rhombohedral siderite aggregates, which arealmost identical in composition and morphology to sid�erite widespread in altered clay and sand deposits,which allows us to use the obtained fluid inclusion datafor their common characterization.

Fluid inclusions were investigated in two genera�tions, early milky white and late druse�like quartz,which develops through the recrystallization of the earlyquartz and is probably cogenetic with siderite. Threetypes of inclusions were observed in the early milkywhite quartz (Fig. 6): (1) primary fluid inclusionsrandomly distributed in the quartz volume and con�sisting of aqueous solution and a large gas bubble(~25–30% of inclusion volume), (2) cogenetic withthem primary gas�dominated inclusions, and (3) sec�ondary two�phase inclusions with a small gas bubble(~10–15 vol %). The late drusy quartz contains pri�mary two�phase inclusions similar in appearance tothe secondary inclusions of type 3 in the early quartz(Fig. 6).

The thermometric and cryometric investigation of89 individual inclusions (Table 2) revealed that primaryinclusions in the early quartz contain aqueous magne�sium and sodium chloride solutions (Teut is from –34 to–31°C). Their ice melting temperatures from –5.9 to–7.0°С indicate concentrations of 9.1–10.6 wt % NaCleq. The complete homogenization of the primary inclu�sions in quartz was attained at temperatures from 380 to315°С (Table 2). Since the formation of these inclu�sions was accompanied by the entrapment of gas�dom�inated inclusions (i.e., the system occurred at the two�phase equilibrium boundary), these temperatures donot require pressure corrections and correspond to theconditions of quartz crystallization. The density of fluidin the inclusions is from 0.70 to 0.81 g/cm3. The gasinclusions cogenetic with the inclusions of chloride

500 µm

Fig. 4. Quartz veinlet (upper part) in strongly altered carbonaceous schist (lower part) with numerous cross�cutting carbonatestringers (indicated by arrows). Polished thin section 136/6001, cross�polarized light.

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(а) 20 µm

(b) 20 µm

Quartz 1

Quartz 2

Fig. 5. Formation of drusy quartz 2 through the recrystallization of early milky white quartz 1 associating with rhombohedral sid�erite aggregates. Polished thin section 136/6001 in (a) plane�polarized and (b) cross�polarized light.

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 839

solutions contain high density methane (d = 0.26–0.28 g/cm3) and homogenized to a liquid phase at tem�peratures from –94 to –89°С. This is consistent withhigh gas hydrate melting temperatures of 17.7–11.2°С,characteristic of methane hydrates (Table 2).

The secondary two�phase fluid inclusions of type 3in the early quartz homogenized to a liquid phase at270–130°С and contain an aqueous solution with asalinity of 6.5–3.0 wt % NaCl eq. The solution in thesefluid inclusions is dominated by sodium chloride(eutectic temperatures from –29 to –24°С), and thefluid density is 0.82–0.98 g/cm3. The secondary gasinclusions of type 2 in the early quartz are filled withdense methane with a significant fraction of carbondioxide (approximately 10 mol %) and homogenized toa liquid phase at temperatures from –78 to –81°С(Table 2).

The primary two�phase fluid inclusions of type 3 inthe late drusy quartz homogenized to a liquid phase at145–135°С and contain an aqueous solution with asalinity of 3.6–2.3 wt % NaCl eq. The solution in thesefluid inclusions is dominated by sodium chloride(eutectic temperatures from –31 to –24°С), and thefluid density is 0.95 g/cm3 (Table 2).

The analysis of thermo� and cryometric data for thefluid inclusions indicates the presence of two quartzgenerations, early and late. The early (metamorpho�

genic) quartz was formed at relatively high temperatures(380– 315°С) from chloride fluids of elevated salinity(9.1–10.6 wt %) and a significant methane fraction inthe gas phase (Table 2). The late drusy quartz overgrow�ing the early coarse�grained quartz and cogenetic withsiderite was formed at relatively low temperatures(145–135°С) from low�concentration (3.6–2.3 wt %)chloride�dominated solutions.

It is noteworthy that the secondary fluid inclusionsin early quartz developing along late dislocations aresimilar in shape to fluid inclusions in the late quartz;these secondary inclusions are characterized by signifi�cant variations in homogenization temperature (270–130°С) and salinity (6.9–3.0 wt %) and significant con�tents of CO2 in the gas phase (Table 2). Given theirintermediate characteristics, these secondary fluidinclusions can be attributed to the influence of CO2�rich solutions of the acid stage directly before the for�mation of the drusy quartz and siderite.

The analysis of fluids in quartz�hosted inclusionsfrom sample 136/6001 showed the prevalence ofsodium and calcium among the cations and bicar�bonate among the anions; in addition it revealed ele�vated bromine and boron concentrations and pro�vided evidence for the contents of many other tracecomponents (Table 3).

10 µm(а) (b) (c)

(d) (e) (f)

10 µm 10 µm

10 µm10 µm10 µm

Fig. 6. Types of fluid inclusions in quartz. (a) Type 1 primary high�temperature inclusion (Thom = 380°C). (b) Type 2 gas inclu�sion. (c) Association of cogenetic inclusions of types 1 and 2. (d) Gas�dominated inclusion at –80°C. (e) Gas�dominated inclu�sion at –100°C. (f) Type 3 low�temperature two�phase inclusion (Thom = 135°C).

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DISCUSSION

First, it should be pointed out that fluid inclusions inlate minerals were investigated in all the main types ofuranium deposits in the Mesozoic paleovalleys of theWest Siberian ore region, because the Khokhlovskoeand Dalmatovskoe deposits are spatially closely associ�ated and considered as almost complete analogues. Inall these deposits, direct evidence was obtained for theexfiltration of CO2�rich thermal solution into the ore�hosting sequences with the formation of two mineralassemblages: (1) hematite–calcite (carbonation) and(2) goethite–chlorite (berthierine) or goethite–smec�tite–chlorite with siderite and goethite–kaolinite–illitewith siderite.

The hematite–calcite assemblage develops exten�sively as carbonation in the host sand–pebble sequencesand veinlet zones in the basement rocks of the Semizbaiand Malinovskoe deposits, but was almost never foundin the Transuralian uranium deposits. Calcite�hostedfluid inclusions showed identical homogenization tem�

perature ranges of 130–215 and 120–230°C, respec�tively, but are different in salt concentration and com�position (Table 1).

For instance, fluids in calcites from the Malinovskoedeposit are typically calcium–sodium chloride brineswith salinities from 11 to more than 30 wt % NaCl eq.and relatively low CO2 contents, which is indirectlysupported, in particular, by the absence of carbonateminerals in three�phase inclusions [7]. Perhaps, this iswhy carbonation occurs locally in the host sequence ofthis deposit, and there is no newly formed siderite in thesubsequent goethite–berthierine assemblage. In con�trast, calcite from the Semizbai deposit trapped CO2�rich waters with lower salinities of 2.1–4.3 wt % NaCleq. and sodium�dominated compositions (Table 1).

The second, later assemblage is superimposed onand replaces carbonated rocks; it is widespread in theore�bearing sequence and forms a system of veinlets inthe Paleozoic basement rocks in all the deposits dis�cussed here. It is significantly variable depending

Table 3. Comparative characterization of the chemical compositions and trace element concentrations in aqueous extractsfrom quartz of a quartz–siderite veinlet in sample 136/6001 determined by ICP MS and modern formation water from anore�bearing sequence [15]

Gas, anion, element

Chemical composition and trace elements

Solutions of fluid inclusions with salinities of 2.3–6.5 g/L and homogenization tem�

peratures of 130–270°C

Formation waters of uranium ore horizon with a total salinity of 3.0–4.6 g/L and a

temperature of 24.3–25.0°C

CO2 Major components, g/L 12.9 1.4–2.2

6.1 2.3–2.5

1.13 0.002–0.025

Cl– 0.22 0.2–0.43

Na 1.19 0.88–0.96

Ca 0.99 0.16–0.22

K 0.16 0.07

Mg 0.02 0.06–0.07

Br Trace components, mg/L 390 5

B 81 0.5–3.5

Ba 30 4.3

Cu 121 0.01–0.14

Fe 14.6 0.7–4.8

As 4.9 <0.01

Sb 5.0 <0.01

Mn 1.0 <0.01

Mo 0.01 <0.01

Co 0.07 <0.01

Cd 0.01 <0.01

Br × 103/Cl 1773 12–25

B/Cl × 104 3682 25–81

The concentrations of trace components in formation waters are given mainly on the basis of the quantitative chemical analysis of watersamples from wells 2–3 in a test area obtained by the methods of atomic emission spectrometry and ion chromatography at the analyti�cal center of the All�Russia Research Institute of Chemical Technology.

3−HCO

24−SO

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 841

mainly on the initial composition of cement in clasticsediments [8]. For instance, in the Semizbai and Mali�novskoe deposits, the primary cement is mainly smec�tite, and this assemblage is represented by green alteredproducts based on goethite, smectite–chlorite and sid�erite in the Semizbai deposit and goethite and sep�techlorite (berthierine) in the Malinovskoe deposit. Inthe deposits of the Transuralian region, kaolinite domi�nates the primary cement of sedimentary sequences,and this assemblage is represented by bleached alteredrocks composed mainly of kaolinite, illite, goethite, andsiderite.

The investigation of siderite�hosted fluid inclusionsfrom the host rocks of the Semizbai deposit and aquartz–siderite veinlet in the hydrothermally alteredcarbonaceous schists of the Paleozoic basement of theKhokhlovskoe deposit revealed overlapping ranges oftheir homogenization temperatures (245–280 and130–270°C, respectively) and similar concentrationsand compositions of gas and salt in the solutions(Tables 1, 2). Consequently, this mineral assemblage isevidently hydrothermal and related to the second stageof CO2�rich water exfiltration into the host paleovalleysequences of these uranium deposits.

In addition, a comparison of the results of thermo�and cryometric investigations, the analysis of waterextracted from quartz of the quartz–siderite veinlet,and hydrogeologic data indicates a close similarity inconcentration and major� and trace�component com�positions between the solutions of fluid inclusions andmodern formation waters from the central part of theKhokhlovskoe deposit (Table 3). For instance, thehydrogeologic sampling and hydrogeochemical loggingof exploratory drill holes [15] showed that the formationwaters of the ore�bearing level are distinctly anomalousin temperature (24.3–25°С), CO2�rich (up to 2.2 g/L),and weakly acidic (pH 5.9–6.2); they show elevatedsalinities (3.0–4.6 g/L), sodium bicarbonate composi�tions with significant calcium (220 mg/L) and hydrogensulfide contents (0.2–4.2 mg/L H2S), slightly reducingEh values from +(150–110) to –(20–40) mV, and per�sistent presence of barium (up to 4.3 mg/L) and boron(0.5–10 mg/L). It should be emphasized that they aresignificantly different from the formation waters of thehost sequences of the Dalmatovskoe and other paleov�alley�type deposits of this uranium ore region. The lat�ter waters are mainly of sodium chloride compositionwith lower total dissolved solids, slightly alkaline(pH 8.5–9.0), and usually reducing (Eh of –170 mV).

The anomalously high boron contents in the solu�tions of fluid inclusions result in a B/Cl × 104 value ofapproximately 3680, which is not characteristic of anytype of formation waters [21]. However, similar andeven higher boron concentrations were documented inthe hydrothermal fluids of magmatic origin [22]. Theanomalously high boron concentrations in the solutionsof fluid inclusions may indicate the presence of a mag�matic component in the fluid producing the mineral�

ization discussed here. The same source could beresponsible for the high bromine concentrations. How�ever, in all other respects, the solutions of fluid inclu�sions are rather similar to the formation waters.

Thus, an evident major� and trace�component sim�ilarity was established between modern formationwaters and hydrothermal solutions from fluid inclusionsin the quartz–siderite veinlet from the carbonaceousschists of the basement, which provides compelling evi�dence for the significant exfiltration of CO2�rich watersinto the host sequences of the Khokhlovskoe uraniumdeposit. The proportions of indicator components

(CO2, , and Ca) and trace elements (Ba and Fe)in them provide a means for estimating the fraction ofhydrothermal solutions in the total volume of formationwaters from the ore�bearing levels of the central part ofthe Khokhlovskoe deposit, which might be from 15 to35% (Table 3).

In this relation, it is very reasonable and logical tosuppose a relict character of the observed thermalanomaly in modern waters, which emerged owing to theinfluence of hydrothermal solutions and have persistedup to the present day. Based on this scenario, we per�formed mathematical modeling of heat transfer in thegiven geologic setting with appropriate approximationsunder the supervision of Prof. V.I. Mal’kovskii. Thisprovided an estimate for the possible time period of thepreservation of such a temperature anomaly under var�ious conditions, which appeared to be no longer than100–200 kyr [16].

Hence, the exfiltration of CO2�rich thermal solu�tions was most likely related to the last stage of the neo�tectonic activation of the Transuralian region. This issupported by some radiological characteristics of high�grade uranium ores, which show a deficit in ionium andradium [13, 17]. According to some authors [18, 19],the morphological structure of this region is controlledby recent tectonic movements, which began in thePliocene and continued in the Holocene. Among them,the most important were large northeast�trending dislo�cations with significant strike�slip components; thezone of their influence comprises the Khokhlovskoeand Dalmatovskoe uranium deposits.

In accordance with hydrochemical characteristics,the solutions of fluid inclusions in late mineral aggre�gates from the deposits considered here and the forma�tion waters of the Khokhlovskoe uranium deposit,which should also be classified as thermal, because theirtemperature is higher than 20°C, are assigned to thebroad geochemical type of thermal methane� and nitro�gen–methane�bearing waters, which are common inthe sedimentary cover of Paleozoic platforms (WestSiberian and others), in neotectonic troughs, and youngmarginal and intracontinental depressions [20, 21].Their distribution is controlled by general hydro�geochemical and gas zoning. In the periphery andupper parts of artesian basins, thermal waters are usuallyfresh or brackish (up to 10 g/L), contain nitrogen

3−HCO

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VINOKUROV et al.

(sometimes with admixtures of CO2 and H2S), and havesodium sulfate–bicarbonate or sodium bicarbonatecompositions, whereas their deep parts may containnitrogen–methane� and methane�bearing waters ofsodium chloride and sodium–calcium chloride com�positions with elevated contents of iodine, bromine,and other trace components. Temperature at the depthof their occurrence ranges from 50 to 200°C, pH is 5–9, and Eh is from –250 to 0 mV [21].

Consequently, this geochemical type of thermalwaters comprises the whole range of our data on thehydrothermal solutions of fluid inclusions in the latemineral assemblages investigated in the Malinovskoe,Semizbai, and Khokhlovskoe uranium deposits. In thisrespect, its formation conditions and physicochemicalproperties deserve a more detailed consideration.According to published data, the main resources ofthermal waters of this geochemical type were formedowing to the infiltration of precipitation and/or seawa�ter retention, and the source of heat was the regionalgeothermal field; additional heat supply by high�enthalpy fluids from the mantle is assumed only forhigh�temperature nitrogen–carbon dioxide and meth�ane vapor–hydrothermal systems [21]. The mainsource of gases, major components, and trace elementswas the country rocks, but some of these componentscould be supplied by infiltration and marine waters.

The processes controlling the chemistry of thermalwaters show some specific features related to the tem�perature factor. One of them is phase transitions occur�ring in the near�surface discharge zones of hydrother�mal systems during decompression at the retention ofhigh temperature, which causes boiling accompaniedby vapor formation and degassing. This leads to signifi�cant changes in the component compositions of thevapor and liquid phases: the condensate shows low�salinity Na–HCO3 or Na–SO4 compositions, and theliquid phase is enriched in salts and trace components,which is sometimes accompanied by their removal fromthe solution [21]. The second feature is an active ther�mal influence on the country rocks, which results in theextensive reworking of their composition with theextraction of gases, major components, and trace ele�ments into the solution. This interaction causes alsoactive dissolution and leaching of dispersed organicmatter from the rocks, which is enhanced by elevatedtemperatures (mild thermolysis at 100–150°C) and sig�nificant contents of dissolved CO2 providing acidicwater properties [21].

It is pertinent to note that the Shadrinsk district ofKurgan oblast, within which the Dalmatovskoe andKhokhlovskoe uranium deposits are located, is situatedin a seismic hazard zone of M = 5 and hosts numerousoccurrence of CO2�rich mineral waters of the Yessen�tuky no. 4 type with a temperature of up to 50°C. Thesemineral waters show sodium–magnesium bicarbon�ate–chloride compositions, a CO2 concentration of~2.2 g/L, and a total salinity of 4–12 g/L. The occur�

rence of such waters was observed in deep fault zonesthrough which CO2 is released, and their developmentafter Talitsa�type sodium chloride waters was reported[23, 24].

The carbon isotope analysis of CO2 dissolved ingroundwater from the Mesozoic sequences of Kurganand Sverdlovsk oblasts showed its enrichment in thelight isotope: δ13C values range from –0.1 to –21.6 forthe whole region and from –9.0 to –12.5 for theShadrinsk district. Based on the obtained carbon iso�tope data, the CO2 of these mineral waters is of organicorigin and is most likely related to the thermal decom�position of organic matter [25].

Based on the physicochemical properties of thermalwaters of this geochemical type briefly outlined above,the most probable scenario of their interaction with theore�bearing Late Jurassic sequences of the deposits canbe proposed. The most reasonable and predictableresult of the influence of such thermal CO2�rich acidsolutions appears to be the extensive development ofacid leaching and accompanying high�temperaturemineral formation, i.e., argillization, which is definedas a metasomatic hydrothermal process occurring at atemperature higher than 50°С and resulting in rockreplacement by clay minerals [26, 27]. In the followingsection, we will consider briefly available mineralogicaland geochemical evidence for the altered rocks and ura�nium ores of the deposits described above, analyze theextent of their similarity with the products of hydrother�mal argillization, and discuss possible genetic applica�tions.

MINERALOGICAL AND GEOCHEMICAL FEATURES OF ALTERED ROCKS AND ORES,

AND GENETIC APPLICATIONS

Many years of research on paleovalley�type uraniumdeposits have revealed various epigenetic alterations ofhost rocks, which, according to various authors, havedifferent spatial relations and time of formation. In ouropinion, they can be grouped in general into three mainmineral and geochemical types, which are shortly char�acterized as follows.

Type I, red alteration of siltstones and clays relatedto ancient soil–sediment oxidation.

Type II, calcitization of sands and sandstones, oftenwith hematite.

Type III, green alteration or lightening (bleaching)of sandy and clayey rocks.

It should be pointed out that the scale of combinedoccurrence of these three types of epigenetic alterationdefines the geological, mineralogical, and geochemicalfeatures of the deposits.

Type I is represented in all the deposits by clayey silt�stone interbeds and lenses with brownish red, raspberryred, and other reddish colors, which are considered asrelics of ancient (Late Jurassic–Early Cretaceous) soil–

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 843

bedrock oxidation of initial gray�colored alluviumdeposits. Their main difference from primary red rocksis the presence of oxidized relicts of diagenetic pyriteand sooty plant remains. This type of host rock alter�ation is most thoroughly characterized in numerouspublications, because most researchers attributed theformation of major uranium mineralization to this pro�cess [10, 11, 13, 14, 28, 29].

It should be added that, according to the availableevidence, red alteration products related to the develop�ment of the zones of ancient soil–bedrock oxidation arevery unevenly distributed in the host rocks of the depos�its. In particular, they are most abundant in the Semi�zbai deposit, common in the Dalmatovskoe andKhokhlovskoe deposits, and weakly manifested in theMalinovskoe deposit as rare thin lenses [7, 8].

The second mineralogical–geochemical type is rep�resented mainly by lenses of carbonated sand–gravelrocks from 0.5 m to several meters thick; their cement iscomposed of almost pure calcite containing usually upto 2% manganese and often associating with hematite.Newly formed calcite partly replaces the smectitecement of rocks and slightly corrodes clastic feldspargrains. Carbonated sandstones are often uranium�bear�ing in the Malinovskoe deposit [7], and the contributionof such rocks to the total resources of the Semizbaideposit is 14% [30]. Numerous EDS analyses of suchhigh�grade ores (more than 0.1% uranium) revealed adistinct tendency for the formation or transformation ofsignificantly iron richer chlorite and smectite comparedwith their clastic varieties in low�grade ores and barrenrocks [6].

The second type includes numerous hematite�bear�ing calcite veinlets of identical compositions wide�spread in the basement granites of the Semizbai deposit[11] and a few similar veinlets in the Paleozoic schists ofthe basement of the Malinovskoe deposit. On the otherhand, it should be noted that the second type of epige�netic alteration is very unevenly manifested in the ura�nium deposits of the West Siberian ore region. Forinstance, it is most extensive in the Semizbai depositand much less common in the Malinovskoe deposit. Inthe deposits of the Transuralian region, this type of rockalteration is very rare and local or is not observed at all.This inference is based on the fact that such alterationswere never observed by us in the host sequences androcks of the Paleozoic basement during many years ofinvestigations. Such alterations were only cursorilymentioned in some publications [13, 14]. In particular,Luk’yanova and Kondrat’eva [14] reported the pres�ence of uranium ores made up of coffinite–pitchblendeaggregates in association with carbonate and hematitein the Uksyan area of the Dalmatovskoe deposit.

The third type includes two main varieties: greenalteration and lightening (bleaching) of rocks. There aresignificant mineralogical differences between themrelated mainly to the pronounced differences betweenthe primary compositions of host sequences in the

southeastern part of the West Siberian platform and theTransuralian region [11]. These differences are relatedto the structure of provenances and manifested, first, insignificant variations in the composition of cement interrigenous rocks. Smectite–illite and kaolinite�domi�nated compositions are typical of the former and latterregions, respectively. This resulted in significant miner�alogical variations in the products of chemically similarepigenetic alterations: goethite–septechlorite (ber�thierine) or goethite–smectite–chlorite with siderite inthe Malinovskoe and Semizbai deposits versus goet�hite–kaolinite–illite with siderite in the Dalmatovskoeand Khokhlovskoe deposits.

Almost all authors noted that this is the most com�mon type of epigenetic alteration of rocks in all thepaleovalley�type deposits studied in the ore region; it issuperimposed on all other alteration products and con�trols in general the modern spatial distribution of ura�nium mineralization. These alterations are most exten�sive in permeable sand–gravel beds and are also signifi�cant in siltstone–clay beds. The altered rocks alwaysshow almost complete absence of diagenetic pyrite, asharp decrease in the amount of dispersed carbon�aceous matter, and significant depletion in iron andsome other elements. It should be emphasized that themineral assemblages of the third type of rock alterationwere observed as veinlets in the altered (argillized) gran�ites and schists of the Paleozoic basement in almost allthe deposits. It seems expedient to consider in moredetail the mineral assemblages of this type occurring indifferent deposits.

In the Malinovskoe deposit, the latest iron�richchlorite with КFe = Fe/(Fe + Mg) of 0.93–0.98 andgoethite microinclusions forms typical tubular, reni�form, and crustal aggregates (Fig. 2). The microdiffrac�tion study of these newly formed assemblages on aJEM�100C electron microscope showed that their unitcell parameters are similar to those of septechlorite ber�thierine [8]. According to a number of authors [30],berthierine was formed at temperatures of 65–130°C. Itis supposed that abundant berthierine precipitated fromweakly acidic thermal solutions at a decrease in theirtemperature and an increase in pH owing to interactionwith the host rocks, which is suggested by the intensedestruction of carbonates by them and the replacementof smectite–illite cement.

The formation of a similar minerals assemblage,goethite–chlorite–smectite with siderite, was observedin the Semizbai uranium deposit [11]. According to ourobservations, fibrous, scaly, crustal, and spheruliticchlorite–smectite aggregates (Fig. 2) are similar incomposition to montmorillonite and, in part, nontron�ite, i.e., smectite with a lower iron content (KFe of 0.70–0.89) compared with berthierine from the Malinovskoedeposit. In addition to the local areas of their intensedevelopment, known as tobacco rocks, the same wasobserved over large areas of extensive green alterationproducts in mainly sandy sequences, which was inter�

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VINOKUROV et al.

preted by some authors [11] as late supergene reducingalterations in the Semizbai uranium deposit.

This suggestion is supported by the similarity in min�eralogy and relationships of the newly formed aggre�gates with hydrothermal assemblages. First of all, notethe formation in them of scaly and platy grains of smec�tite (montmorillonite and nontronite), which controlsthe intensity of green color in sandstones and directlydepends on the content of clay material. Furthermore,both assemblages always contain goethite and so�calledprecipitate hydroxides formed owing to an increase insolution pH, i.e., neutralization of their acidity, and dis�tinct corrosion is observed in the case of overlappingover areas affected by early carbonation (calcitization).

In the Transuralian uranium deposits with kaolinite�dominated primary cement, bleaching is common inthe host rocks. This epigenetic alteration includes theextensive replacement of feldspars and micas by orderedkaolinite, intense transformation of clastic ilmenite toleucoxene, considerable removal of iron and carbon�aceous matter, which is often accompanied by the for�mation of yellow iron hydroxides, and the later forma�tion of dispersed siderite aggregates, sometimes in asso�ciation with pyrite and chlorite [13].

Based on the detailed mineralogical and geochemi�cal investigation of drill core samples from the referenceprofile of the Dalmatovskoe deposit, we established thatthe bleached rocks almost always contain dispersednewly formed rhombohedral crystals of siderite or sid�eroplesite (magnesian siderite) and iron�rich dolomite,which are often accompanied by flaky and fibrouskaolinite–smectite aggregates, whereas the gray�col�ored rocks contain only rare thin lenses with siderite orsideroplesite in the cement of silty sandstones. It shouldbe noted that the compositions of siderite and iron�bearing carbonates are very similar to those of respectivenewly formed minerals from the Semizbai deposit(Table 4).

In the Khokhlovskoe deposit, host rock bleaching isalso widespread and even more intense and mineralog�ically diverse. The altered rocks and uranium ores con�tain numerous newly formed minerals: kaolinite, goet�hite, siderite, iron disulfides, and sulfides, arsenides,and selenides of various elements (Mo, Se, Co, Ni, Zn,etc.); silica precipitation and redeposition were docu�mented in sands [29].

Data from exploratory drill holes along profilesVI�2200 and VI�1200 obtained by the detailed visual,binocular, and microscopic examination of core speci�mens taken over the whole section and the results oftheir combined analysis (XRF, SESA, and INAA)characterized epigenetic alterations in the host rocks.The reconnaissance analysis of these materials revealedall the aforementioned newly formed mineral aggre�gates, the sequence of their formation, relative inten�sity, and character of their evolution.

The most widespread newly formed mineral isspherulitic siderite, which was detected in strongly tec�

tonically disturbed clays and siltstones underlying andoverlying the upper ore�bearing unit (Fig. 7). Zoneswith newly formed siderite are up to 5–7 m thick wereobserved only in clays and siltstones; they are usuallyclosely associated with the aureoles of yellowish browngoethite and reddish brown hydrohematite, in whichso�called precipitation oxides and hydroxides of ironare often observed as minute specks in tectonic seamsand thin stringers along fractures and partings. Suchsiderite occurrences are rarer among gray clayey variet�ies with pyrite. In general, they are usually spatially sep�arated from uranium mineralization, but zoned spheru�litic siderite with outer ankerite zones was occasionallyobserved in the ore zone as individual pealike grains upto 1 mm in size and massive concretions, for instance,in sample 11/6013 (Table 4).

It should be pointed out that, in contrast to the Dal�matovskoe uranium deposit, where only rare sideritespherules of identical composition (often also zoned)were observed among gray silty sandstones (Table 4), thearea studied is characterized by exceptionally wide�spread and much more intense sideritization.

The sandy rock varieties recovered from the drillholes contain all the aforementioned newly formedminerals: iron disulfides of various morphology, flakyand fanlike aggregates of white kaolinite, and greenflakes of chlorite or chlorite–smectite, which are rathercommon and were previously reported from the Dal�matovskoe deposit [12]. However, most interesting isthe extensive development of peculiar secondary sili�ceous aggregates, which were previously interpreted inuranium ores as products of silica redeposition and pre�cipitation in the cement of sands [29].

Such newly formed siliceous gel�like materials,which are readily identified in core samples under a bin�ocular microscope, are 0.0n to 1–2 mm in size andaccount usually for 0.5–1.0% of the total mineral com�position of the sandy rocks. They are composed mainlyof light gray gel�like silica (opal and/or chalcedony) lin�ing intergranular pore space or leached cavities, andtheir central part is often filled with flaky kaolinite orremains empty (Fig. 8). Sometimes, small (up to 2 mm)rounded or elongated geodes were observed; their dis�tinct zoning is emphasized by variations in the texture,composition, and color of minerals. Their outer zoneusually consists of gel�like opal and/or chalcedony oflight gray color, and the central part is filled with whiteflaky and fanlike kaolinite.

It is noteworthy that our preliminary analysis indi�cated rather significant heterogeneity in the distributionof newly formed siliceous aggregates in the sandy rocksof the upper mineralized level. For instance, in drillholes 6012 and 6012a located in the tectonically mostactive segment of profile VI�2200, they are abundantalmost through the whole section of sandy rocks, in theintervals containing various uranium ores and showinganomalous uranium contents. In the other drill holes of

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FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 845

500 µm

Siderite

Fig. 7. Development of spherulitic siderite in the bleached areas of clayey siltstone. Thin section 37�6013, plane�polarized light,magnification 100.

this profile, they are significantly less abundant andwere established mainly in uranium ore�bearing units.

In addition, clay rocks from the lower unit containnumerous flaky illite aggregates with parallel opticalorientations developing along microtextural seams ofdifferent directions (Fig. 9). Based on optical properties(up to red interference color) and composition (in addi�tion to silica and alumina, electron microscopy indi�cated approximately 1.5% potassium), the flaky materi�als are composed of illite and/or kaolinite–illite aggre�gates. These materials form numerous microscopicveinlets of various directions up to a few millimeterslong and 0.05–0.20 mm wide (Fig. 9).

Other late newly formed minerals are definitely gel�like pitchblende, which is typical of high�grade ores andhas a distinct hydrothermal appearance [17], and theuranium–titanium association typical of the ores of theKhokhlovskoe and Dalmatovskoe deposits [32].According to the data of most researchers, the mainuranium minerals of ordinary and low�grade ores in thedeposits discussed here are pitchblende and coffiniteassociating with carbonaceous matter and pyrite, as wellas with sulfides, arsenides, and selenides of Mo, Se, Co,Ni, Zn, and other metals [7, 8, 13, 29, 30]. However, inthe deposits of the Transuralian region, the uranium–titanium association is of special importance; itaccounts for approximately 30% of ordinary and morethan 40% of low�grade ores and is evidently related tothe dominance of ilmenite among the accessory miner�als of host terrigenous sediments [13, 14].

According to Vinokurov and Nesterova [32], theuranium–titanium association is formed most likelyowing to the influence of thermal acidic solutions caus�ing intense iron leaching from ilmenite and its transfor�mation to titanium hydroxides, which are known tohave very high sorption capacity for dissolved uranylcompounds. The sorbed uranium is transformed duringsubsequent reduction to crustlike pitchblende films,which are always observed on the altered grains of clas�tic ilmenite with considerable iron depletion.

200 µm

Fig. 8. Zoned siliceous material in the intergranular spaceof light gray medium�grained sand from an ore�bearingunit. Thin section 2�6012a, plane�polarized light.

846

GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 10 2013

VINOKUROV et al.

Tabl

e 4.

Com

po

siti

on

s o

f si

der

ites

fro

m v

ario

us

dep

osi

ts a

cco

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g to

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of

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pro

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nal

ysis

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plin

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sam

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ckM

easu

re�

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t n

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Con

ten

ts o

f ele

men

ts/c

arbo

nat

es,

%

Not

eS

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e/F

eCO

3M

g/M

gCO

3C

a/C

aCO

3M

n/M

nC

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l of M

g, C

a, a

nd

Mn

an

d th

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carb

onat

es

Sem

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i dep

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dep

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, sam

ple

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902.

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en c

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aine

d sa

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wit

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carb

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t.

t�1

0.6

35.1

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62.

1/7.

56.

4/16

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1/4.

510

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8.4

t�2

0.9

35.8

/76.

12.

2/7.

86.

1/15

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4/3.

09.

7/26

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t�3

0.8

34.6

/73.

62.

5/8.

96.

1/15

.62.

2/4.

710

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9.3

t�4

0.9

35.2

/74.

92.

5/8.

95.

4/13

.82.

4/5.

110

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7.8

t�5

0.9

38.5

/81.

92.

1/7.

52.

4/6.

22.

3/4.

96.

8/18

.6

t�6

1.2

38.6

/82.

11.

7/6.

12.

4/6.

21.

9/4.

06.

0/16

.3

t�7

1.8

38.6

/82.

11.

9/6.

82.

7/6.

92.

0/4.

36.

6/18

.0

Dal

mat

ovsk

oe d

epos

it.

Hol

e 12

50, s

ampl

e 37

/125

0. G

ray

silt

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cem

ent a

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plan

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us.

t�2

0.5

45.5

/96.

80.

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10.

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30.

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31.

8/4.

7

t�3

0.4

42.5

/90.

01.

9/6.

80.

9/2.

30.

8/1.

73.

6/10

.8

t�4

1.0

44.8

/95.

30.

3/1.

10.

5/1.

30.

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GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 10 2013

FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 847

Tabl

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848

GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 10 2013

VINOKUROV et al.

An important specific feature of the paleovalley�typedeposits discussed here is the formation of high�gradeores with more than 1% uranium and more than 1 mthick in some levels, which is a genetic indicator. Thesehigh�grade ores were studied in detail in the Dalma�tovskoe and Khokhlovskoe deposits, where they areespecially abundant. There is no definite lithologicalcontrol, because they are localized both in the dark grayto black carbonaceous fine�grained sandstones and silt�stones of the upper mineralized unit and in the gray silt�stones, coarse�grained sandstones, and pebble–gravelsandstones of the lower mineralized unit. On the otherhand, strong tectonic disturbance was occasionallyobserved in the host rocks manifested in the presence oflayer�parallel and cross�cutting fractures with rareslickensides.

The high�grade ores of the Dalmatovskoe andKhokhlovskoe deposits typically contain large (milli�meter�sized) pitchblende aggregates, among which gel�pitchblende is common; it retains properties of solidi�fied uranium oxide gel, including characteristic struc�tural (fracturing and fluidity) and textural (globularity)features and high uranium contents of 70–80% [15, 17,33]. Relatively large segregations display a phase heter�ogeneity caused by the replacement of dark pitchblendezones with relatively low uranium contents of 55–65%by light�colored varieties with higher uranium contentsof 70–80%. In general, the heterogeneity of the micro�scopic structure may be skeletal, graphical, or incipi�ently spherulitic [17].

A specific mineralogical and geochemical feature ofgel�pitchblende from the Khokhlovskoe deposit is itshigh content of zirconium, which is correlated withuranium. For instance, the light phases of uraniumoxide usually show 3–5% and sometimes up to 8.5%zirconium, whereas the dark varieties are poorer in zir�conium (1–2%). Taking into account the absence ofanalyses with silica contents corresponding to the zir�con composition, the presence of zirconium oxide(baddeleyite) can be supposed. In this context, it is per�tinent to mention the finding of a complex U–Fe–Zr–Ti–S–P gel with 5–6% zirconium in the uranium oresof the Khokhlovskoe deposit [34].

The geochemical characteristics of uranium ores arecontrolled in general by a wide spectrum of accompa�nying elements (Se, Mo, Re, V, As, Sb, and Pb), bothtypical and atypical (As, Sb, and Pb) of exogenic epige�netic mineralization, the set of which and the degree ofenrichment increase with increasing uranium contentand reach maxima in high�grade ores [7]. There is nospatial zoning in Se, U, and Mo, which is characteristicof their exogenic–epigenetic accumulation [13, 28].The geochemical differences of uranium ores from thedeposits discussed here are reduced to variable degreesof anomalous enrichment and correlation of accompa�nying elements with uranium, as well as the absence ofsome of these elements.

For instance, the Malinovskoe deposit shows themost complete set of uranium�accompanying ele�ments, except for Sb, high degrees of Re and Se enrich�ment, and significant correlations of the accompanying

200 µm

Kaolinite–illite

Siderite

Fig. 9. Formation of siderite rhombohedra and flaky kaolinite–illite aggregates with parallel optic orientations along the micro�textural seams in light gray silty clay from the lower aquitard. Thin section 50�6012a, cross�polarized light.

GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 10 2013

FLUID INCLUSIONS IN LATE MINERALS FROM THE PALEOVALLEY�TYPE 849

elements with uranium. In contrast, the Semizbai andDalmatovskoe deposits are characterized by the pres�ence of Sb and absence of Pb among the accompanyingelements, the maximum degree of Mo enrichment inthe uranium ores, and its high correlation with uranium[8]. During the reconnaissance geochemical investiga�tion of uranium ores in the Khokhlovskoe deposit, Mo,Re, Se, and As were detected as persistent accompany�ing elements, whereas anomalous contents of otheraccompanying elements were observed only in a fewsamples of high�grade ores. It is also noteworthy thatthese samples also showed anomalous zirconium con�tents (XRF data), 4–10 times higher than those of ordi�nary ores and the host rocks.

Thus, the obtained data on the mineralogical andchemical characteristics of late newly formed materialsand uranium ores are well consistent with the expectedeffect of CO2�rich thermal mineral waters on ore�host�ing sequences and, consequently, provide compellingevidence for the extensive occurrence of hydrothermalprocesses of acid leaching (argillization) in all the pale�ovalley�type uranium deposits that are related to thestage of neotectonic activation in the region. In ouropinion, the typical indicators of argillization are thefollowing: in altered rocks, the extensive developmentof newly formed iron�rich chlorite, smectite, illite, andkaolinite and the significant formation of iron�rich car�bonates and late iron hydroxides; mainly in uraniumores, the formation of siliceous (opal) materials insands, late sulfides, arsenides, and selenides of iron,uranium�accompanying elements and many other met�als, uranium�bearing altered ilmenite (anatase), andmultiphase gel�pitchblende.

On the other hand, the investigation of many yearsof such paleovalley�type deposits has provided consid�erable material demonstrating that they host numerouszones of ancient soil–bedrock oxidation and supportingthe popular opinion that commercial uranium mineral�ization is related to them [10, 11, 13, 14, 28, 29]. We dobelieve that these different scenarios must not beopposed during the consideration of the genesis of thesedeposits, because they evidently reflect different agestages of the formation of uranium mineralization. Themost important problem seems to be the assessment ofthe role (contribution) of these exogenic and endogenic(hydrothermal) processes in the formation of the mod�ern mineralogical and chemical characteristics of ura�nium ores and their host rocks, because this has a directbearing on the efficiency of the method of in situ leach�ing, i.e., their commercial importance and the inven�tory of exploration guides and criteria.

It should be noted that this genetic aspect is not newand was discussed during the investigation of infiltrationdeposits of the Tien Shan uranium�bearing province,where in situ leaching was developed and used as themain industrial technique for uranium mining [27, 34].On the other hand, the analysis of the relative contribu�tions of exogenic and endogenic factors for such young

hydrogenic uranium deposits is mainly of theoreticalimportance and has no considerable practical signifi�cance, because the modern mineralogical and chemicalcomposition of uranium ores was formed during thefinal stage owing to the exogenic infiltration of forma�tion waters, which resulted in their good leachability byacid working solutions [35].

The situation with ancient paleovalley�type hydro�genic deposits in the West Siberian region is essentiallydifferent. The formation of the modern mineralogicaland chemical features of the uranium ores and theirhost rocks is controlled to a large extent by the influenceof CO2�rich thermal mineral waters of the stage of neo�tectonic activation in the region, which resulted inextensive argillization. The processes of acid leachingresulted in the large�scale development in the ore�bear�ing sequences of various newly formed minerals: iron�rich chlorites (berthierine), chlorite–smectite andsmectite, carbonates showing very high acidic capacity,and resistant uranium minerals, which are especiallycommon in high�grade ores [15, 17]. The establishedmineralogical and chemical features of uranium miner�alization are responsible for the significant deteriora�tion of its technological properties with respect to theapplied classic method of in situ leaching, whichneeded to be adapted by using, in particular, additionalore oxidation [36].

Furthermore, in our opinion, it is expedient toextend the existing comprehensive geological prospect�ing model of paleovalley�type uranium deposits byincluding some relevant neotectonic, hydrochemical,mineralogical, and geochemical criteria and indicators.It is pertinent to point out that the most important andfundamental task is the sophisticated analysis of factualevidence for the occurrence of exogenic and endogenicfactors, their scales, and relations in the deposits ofinterest rather than arguments pro and contra differentgenetic concepts.

CONCLUSIONS

1. Microthermometric experiments revealed closetemperature ranges (280–120°C) for the formation oflate carbonates in the Khokhlovskoe, Semizbai, andMalinovskoe deposits of the West Siberian uranium oreregion. It was shown that the solutions of fluid inclu�sions are chemically similar to thermal nitrogen–meth�ane waters with elevated CO2 contents, which are typi�cal of the whole region.

2. Mineralogical and geochemical features of latenewly formed minerals and uranium ores were consid�ered. Two late mineral assemblages were distinguished:(1) hematite–calcite and (2) goethite–berthierine andgoethite–smectite–chlorite with siderite, which aretypical of the Malinovskoe and Semizbai deposits, orgoethite–kaolinite–illite with siderite characteristic ofthe Transuralian deposits; it was shown that theseassemblages occur both in the host sequences and in the

850

GEOCHEMISTRY INTERNATIONAL Vol. 51 No. 10 2013

VINOKUROV et al.

underlying basement rocks. The latter assemblage isresponsible for the significant change in rock color(bleaching) and is widespread in all the deposits consid�ered.

3. It was shown that, in terms of mineralogical andchemical compositions, these altered rocks and part ofuranium ores approach most closely the products ofacid leaching and accompanying mineralization, whichcould be related to the processes of low�temperatureargillization.

4. It is supposed that exogenic–epigenetic processesof ancient soil–bedrock oxidation undoubtedly partici�pated in the formation of uranium mineralization, andthe modern characteristics of uranium ores and theirhost rocks are controlled to a large extent by the influ�ence of CO2�rich hydrothermal solutions related toneotectonic activation in the region. This defined theirspecific mineralogical and chemical signatures and cor�responding technological properties.

5. It seems expedient to estimate on the basis ofavailable data the possible roles of exogenic andendogenic factors in the formation of uranium mineral�ization and extend the existing exploratory and assess�ment model by appropriate neotectonic, hydro�geochemical, and mineralogical–geochemical criteriaand indicators.

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3. V. B. Naumov, “Possibility of determination of pressureand density of mineral�forming media on the basis ofmineral inclusions,” in Use of Fluid Inclusions during theProspecting and Study of Ore Deposits, Ed. by N. P. Lave�rov (Nedra, Moscow, 1982), pp. 85–94 [in Russian].

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Translated by A. Girnis