THE COMPOSITION AND ORIGIN OF VANADIUM-RICH CLAY … 42/42-4-391.pdf · Clays and Clay Minerals, Vol. 42, No. 4, 391-401, 1994. THE COMPOSITION AND ORIGIN OF VANADIUM-RICH CLAY MINERALS

Clays and Clay Minerals, Vol. 42, No. 4, 391-401, 1994.

THE COMPOSITION AND ORIGIN OF VANADIUM-RICH CLAY MINERALS IN COLORADO PLATEAU JURASSIC SANDSTONES

J. D. MEUNIER*

GS CNRS-CREGU, CREGU, BP23, 54501 Vandoeuvre les Nancy, France

Abstract--The composition and origin of vanadium-bearing clay minerals in the Jurassic (Morrison and Entrada Formations) sandstones of the Colorado Plateau are reassessed using microanalyses (microprobe and scanning electron microscope). The main V-clays are authigenic illite and chlorite of various petrologic habits: clay casts and matrix, pore lining, replacement of detrital grains. The chemical composition of the V-clays is similar in three different localities in the Morrison Formation separated by about 50 kin, suggesting that the V-clays are the result of a large regional event. In both illite and chlorite, A1 and V are inversely correlated, showing that V replaces AI in the octahedral position. The chlorite contains a complex mixture of divalent and trivalent cations that cannot fit within a sudoite structure. A classification of V-micas is proposed that employs V3+/sum of the octahedral cations vs. the sum of the interlayer charges. V-illite and roscoelite from the Colorado Plateau are characteristic of diagenetic/hydrothermal environments. For a given locality the composition of the V-clays does not vary with habit, showing that these minerals formed at thermodynamic equilibrium. Key Words--Chlorite, Diagenegis, Illite, Microprobe, Roseoelite, Sandstone, SEM, Vanadium.

INTRODUCTION

Clay minerals containing a substantial amount of vanadium have been described in various geologic environments: sedimentary (Foster, 1959; Hathaway, 1959; Guven and Hower, 1979; Whitney and Nor- throp, 1986; Parnell, 1988; Forbes, 1989; Hofman, 1990), metamorphic (Snetsinger, 1966; McCormick, 1978; Schade et al., 1986; Oh and Trichet, 1990; Pan and Fleet, 1992), and hydrothermal (Yoshimura and Momoi, 1964; Hofman, 1990; Johan and Povondra, 1987). In sandstones from the Colorado Plateau, vanadium clays are produced by the mineralization processes that form uranium deposits (Northrop and Goldhaber, 1990). They also incorporate many other elements (REE, Th, etc.) that are analogous to fission products found in radioactive waste (Brookins, 1990). The mode of formation of the vanadium clays may, therefore, have interesting and important environ- mental applications.

Significant amounts of vanadium are found in most of the phyllosilicates except the kaolinite family: smectite, illite-smectite, illite, muscovite, chlorite, and chlorite-smectite. The widespread occurrence of V-clays suggests that clays incorporate structural vanadium easily over a large range of geochemical and P/T conditions in nature. In natural waters, vanadium geochemistry is generally controlled by redox processes (Wanty and Goldhaber, 1992); vanadium is trans- ported as V 4+ or V 5+ dissolved forms and is precipi- tated in the reduced form, V 3+. In sediments, vana-

* Present address: Laboratoire de Geosciences de l'Envi- ronment, URA CNRS 132 Case 431, Universit6 d'Aix-Mar- seille III, 13397 Marseille Cedex 20, France.

Copyright �9 1994, The Clay Minerals Society

dium is mostly incorporated into clays during diagenesis (Breit and Wanty, 1991). However, the neoformation of vanadium clays with increasing burial, temperature, and metamorphism is not well understood.

Roscoelite is the best described vanadium clay (Hil- lebrand et aL, 1899; Wells and Brannock, 1946; Hein- rich and Levinson, 1955). Roscoelite has a 2:1 mica structure, with V 3+ as the major octahedral cation (Maylotte et al., 1981; Foster, 1959). The ideal structural formula is KA1V2Si3010(OH)2. Abundant vanadium clay minerals are associated with uranium in tabular-type deposits in Jurassic formations in Utah and Colorado. Foster (1959) and Hathaway (1959) have shown that the vanadium clays in the Colorado Plateau are generally a mixture ofillitic material (hydromicas) and chlorite _+ montmorillonite. Foster (1959) demonstrated that V proxies for A1 in octahedral sites in the hydromicas. Whitney and Northrop (1986) described a vanadium chlorite and a perfectly ordered chlorite/smectite from Henry Basin (Utah). Whitney and Northrop (1986) and Wanty et aL (1990) have found that this vanadium is trivalent and proposed that it occupies mainly the interlayer hydroxide sheets in chlorite. The d(060) value of 1.52 ~ indicates a di- trioctahedral structure. It is assumed that chlorite formed by the transformation of a smectite precursor during ore deposition (Schultz, 1963; Whitney and Northrop, 1986; Wanty et aL, 1990). Except for roscoelite, structural formulas presently available on vanadium clays from the Colorado Plateau (Whitney and Northrop, 1986; Foster, 1959) frequently do not represent pure minerals because of the presence of im- purities or mixtures of clays.

In this study, the chemical composition of the vanadium clays of the Colorado Plateau are examined

391

392 Meunier Clays and Clay Minerals

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: 60 km I I

Location of the study areas in the Colorado Plateau

using microanalyses (scanning electron microscopy and electron microprobe) . Diagenet ic cond i t ions and chemical environment during clay genesis are dis- cussed to constrain conditions of ore formation and incorporation of V in these clay minerals.

TECHNIQUES AND MATERIALS

Vanadium clays from three U-V ore deposits in the Jurassic Morrison Formation (Figure l) were studied: Cottonwood Wash, Utah; Slick Rock District, Colo- rado; Henry Basin, Colorado; and one vanadium deposit in the Upper Jurassic Entrada Sandstone near Placerville, Colorado.

The geology and mineralogy of the uranium-vanadium deposits in the Morrison Formation have been extensively described by Meunier (1989), Breit (1986), and Northrop and Goldhaber (1990). Stratigraphically, the Upper Jurassic Morrison Formation in Colorado and Utah is divided into two main members: the Salt Wash Member, which hosts the ore, and the super- posed volcanic-ash rich Brushy Basin Member. Ore host rocks are lenticular sandstones and mudstones. The sediments were deposited by fluviatile aggrading steams, which carried detritus mainly derived from older sedimentary formations in west-central Arizona and southeastern California. The deposits form black tabular lenses ranging from a few meters to tens of meters long and a few decimeters thick. Wood frag-

ments or logs are generally associated with the ore. Sandstones are moderately to poorly sorted and range from very fine to medium grained. Detrital quartz is the dominant mineral (70-85 vol. %). Other detrital minerals include clays, sedimentary rock fragments (1- 15 vol. %) feldspars (up to 8 vol. %), and minor oxides, accessory minerals, and volcanic fragments. Major cements in the barren rocks include quartz overgrowths and carbonates (mainly calcite). Clay minerals present in the barren zones include illite, illite-smectite, smectite, chlorite, illite-chlorite, and kaolinite (Keller, 1962). In the ore zones, uranium- and vanadium-bearing minerals occur in the interstices of the sand grains or replace wood fragments. They consist of authigenic vanadium clays, vanadium oxides, uraninite, coffinite, and their alteration products (mainly tyuyamunite and carnotite). Vanadium minerals consist mainly of chlorite, illite-smectite (hydromica), and roscoelite in vari- able proportions (Hathaway, 1959).

Roscoelite from Placerville, Colorado, appears to be a good reference for comparison with clays in the Mor- rison Formation because of its slightly different geological setting (eolian sediments) and ore mineralogy (illitic material was the only V-clay mineral detected). The Placerville deposit is described in detail by Fischer et al. (1947).

Polished thin sections containing vanadium clays were analyzed with a conventional SEM (Cambridge) in the back scattered electron (BSE) mode before microprobing. A Camebax electron microprobe was used with the following analytical conditions: acceleration voltage, 15 kV; excitation current, 8 nA; counting time, l0 s per element; ZAF correction procedure, MBX COR of Henoc and Tong (1978). Special attention was taken in order to avoid mixtures ofphyllosilicates. For chlorite, only analyses containing less than 0.5 wt. % total Na20 + CaO + K20 were considered (Foster, 1962; Hillier and Velde, 1991). For illite, the selected analyses are those with no excess of octahedral charge (-< 6.2 in the formula) and no excess of octahedral cations (-<2.2 in the formula).

Microprobing does not discriminate the valence of the elements, so a choice must be made for vanadium between, respectively, V 3+ and V 4. and for iron between Fe E+ and Fe 3+ for the chlorites. The predomi- nance of V 3+ in the Henry Basin V-clays was demonstrated by Wanty and Goldhaber 0985) using a chemical procedure (acid digestion and analysis of a V3+-thiocyanate complex by colorimetry), but no data are available in the other areas. Fe 2+ was stable in the reducting environment imposed during mineralization (Breit, 1986, Wanty et al., 1990) and is likely to have incorporated the clays during mineralization. Fe 2+ and V 3+ are, therefore, the forms that have been chosen for simplification in this microprobe study, because occurrences of V 4+ and Fe 3+ (Wanty et al., 1990) although possible, have never proved to be predominant.

Vol. 42, No. 4, 1994 Vanadium-rich clay minerals 393

Figure 3. SEM micrograph and EDS spectra of V-illitic material from a clay cast.

Figure 2. SEM micrograph of the authigenic V-clay minerals in the interstices of the sand grains: a) flakes with lath ofillite; b) plates of chlorite.

RESULTS

The chemical compositions and structural formulae of vanadium-rich clay minerals are reported in Table 1. They represent an average of selected electron microprobe analyses. Data are given with the standard deviation for statistical representation of the spots. V-chlorite and illite occur in both authigenic and detrital habits. Authigenic and detrital clays may be differentiated using the petrographic criteria of Wilson and Pit tman (1977). In thin section, authigenic V-clays line pores and replace detrital grains and coalified wood fragments. SEM observations and EDS spectra show the presence ofauthigenic flakes with laths ofillite and plates of chlorite (Figure 2) in the sandstone interstices. Clay clasts and dispersed matrix classically recognized as detrital are also V-rich (Figure 3). Because the sed- imentation of vanadium-rich clays is unlikely (Breit and Wanty, 1991), the detrital clays must have been altered into V-rich clays during diagenesis. In most

intergranular cements, chlorite and illite were inti- mately mixed, and many microprobe analyses were rejected because the microprobe spot covered both phyllosilicates. Relatively pure phases were identified with the use of BSE imaging. Because BSE imaging is sensitive to atomic number, illite appears brighter because of the presence of heavier elements (mainly K) and a higher amount of V than in chlorite (Figure 4).

Illites and chlorites from the three study areas from the Morrison Formation are not chemically differentiated for the major elements with confidence intervals of --- 1 standard deviation (1~). For example, values of Si in the chemical formulae of chlorite (Table 1) range from 3.32-3.42 in two samples from Henry Basin, from 3.21-3.40 in three samples from Slick Rock, and from 3.35-3.39 in one sample from Cottonwood Wash.

The negative correlation between vanadium and aluminum (Figure 5a) in illites agrees with previous studies (Foster, 1959), which show that vanadium proxies for a luminum in the octahedral position. Placerville roscoelite is characterized by a lack of Ca and a higher amount of interlayer cations and a lower amount of


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Mg and Fe (Table la). These results s h o w that i l l it ic materials from the Morrison Format ion are chemica l ly different than the roscoelite from Placerville.

In chlorites, v a n a d i u m and a l u m i n u m are inversely correlated (Figure 5b) as in the V-il l i te, showing that v a n a d i u m proxies for a l u m i n u m in octahedral posi -


tion. The number of octahedral cations ranges from 4.88-5.02, which suggests a di/tri-octahedral chlorite named sudoite (Bailey, 1986) that is consistent with X R D detonations. Tetrahedral a luminum mean values in the V-chlorites range from 0.53--0.79, trivalent cations (V 3+ + ViAl) range from 2.57-2.83, and divalent cations (Fe 2§ + Mg 2+ ) range from 2.05-2.45, suggesting that the vanadium di/tri-octahedral chlorite is not composed of a 2:1 dioctahedral layer and a trioctahedral interlayer of the form (V,A1)2(Mg,Fe)3.

DISCUSSION

The illite problem

Nomenclature for V-micas has not been established, and the names V-illite and V-muscovite are often used synonymously. Johan and Povondra (1987) proposed to name V-muscovite micas (or more specific names, i.e., V-phlogopite) with (V~+/(V'A1 + V 3-~ -~ M 2+ ) < 0.5 as opposed to roscoelite (Va+/(vIAI + V 3+ + M 2+) > 0.5. V-Illite is defined as V-muscovite or roscoelite with interlayer sites deficient in K. The AIPEA nomenclature committee (Bailey, 1986) has proposed to fix the min imum and maximum interlayer charge lim- its for illite at 0.5-0.6 and 0.8-0.9, respectively. The vanadium bearing micas, V-illite, roscoelite, and V-muscovite, may therefore be differentiated in a diagram (V3+/sum octahedral cations) vs. the sum of the interlayer charges, assuming that all V is in the trivalent form. The data plotted in Figure 6 show that only one sample from the Morrison Formation and the sample of Placerville are classified as roscoelite, whereas the other samples from the Morrison Formation are Veil- lite. Few studies have dealt with the occurrence of V-micas (illite, muscovite, roscoelite) in various geo- graphic and geologic locations outside the Colorado Plateau. References that contain chemical compositions of V-clays by microprobe are reported in Table 2. V-micas are reported in sedimentary and metamorphic rocks and hydrothermal veins but have never

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been described in sediments shallowly buried (< 1 km) and younger than the Jurassic Morrison Formation. The data o f Johan and Povondra (1987), Forbes (1989), and Hofman (1990) range from V-illite to roscoelite while those of Pan and Fleet (1992) and Schade et aL (1986) are V-muscovite to roscoelite. V-muscovites are detected in metamorphic rocks, illite in diagenetic/hydrothermal environment, and roscoelite in both geological situations.

Table 2. Some occurrences of vanadium-rich clay minerals outside the Colorado Plateau.

L o c a t i o n M i n e r a l o g y G e n e s i s a n d T ' C

Hemlo Gold deposit, Canada, green mica schist, late Archean (Pan and Fleet, 1992)

Permian sandstone, Alps, France (Schade et al., 1986)

Akouta U deposit, Niger, Visean sandstone (Forbes, 1989)

Horni Kalna, Czechoslovakia, Permian sediments (Johan and Povondra, 1987)

U-V deposit, Permian red bed, Ireland (Parnell, 1988)

Permian red bed, Switzerland (Hofman, 1990)

roscoelite, V-muscovite, 2M polytype

V-muscovite 1M polytype

chlorite/smectite/illite/roscoelite

V-illite, roscoelite as end member 2M polytype

V-illite

V-illite, roscoelite

during regional metamorphism (=580~ 4 kbar)

prior to Alpine metamorphism (=200~176

diagenetic/hydrothermal event at 85--175~ (fluid inclusions)

vanadium from hydrothermal event

redox process no P/T data

reduction by organic matter T~ (fluid inclusions) = 80"-140"(2


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octahedral cations) vs. the sum of the interlayer charges.

V-illite and diagenesis

Many studies have demonstrated that the degree o f diagenesis (mainly temperature) plays an important role in the mineralogy of days. Smectite, mixed layer clays (smectite/chlorite, smectite/illite), and chlorite formed during early burial may react or re-equilibrate during advanced diagenesis (higher T and P) to more

stable clays such as illite or chlorite (Velde, 1985; Velde and Medhioub, 1988; Harvey and Browne, 1991; Jah- ren and Aagaard, 1989).

Comparison of the diagenetic and mineralogical fea- tures in the four study areas are given in Table 3. In three out of four areas, max imum temperatures at- tained during diagenesis were assessed using fluid inclusion studies. The max imum homogenizat ion tern-


Table 3. Maximum temperatures during diagenesis and V-clay composition of the study areas.

Location V - c l a y s Temperature (T'C)

Cottonwood Wash Chl/I +__ Sm 70-t00 ~ Slick Rock District Chl/I + Sm 70-1002.3 Tony M Chl/I + Sm 1004 Placerville Roscoelite 100-1205

t Meunier et aL (1987) fluid inclusions; z Morrison and Par- ry (1986) fluid inclusions; ~ Meunier (1989) fluid inclusions; 4 Peterson and Turner-Peterson (1980) liptinite reflectance; 5 Meunier, unpublished fluid inclusion data.

peratures (Th) cluster below 100*C for Slick Rock and Cottonwood Wash deposits, while Th at Placerville are slightly above 100~ The homogenization temperatures are m in imum temperatures of trapping; the correction for pressure that is required to obtain the true temperature of trapping is not known. If calcite pre- cipitated during the maximum of burial (about 3000 m) a maximum of + 10~ should be added to the Th data (Meunier, 1989), which is negligible considering the range of the data. No fluid inclusion studies are available at Henry Basin. The presence of untrans- formed smectite surrounding the ore at Henry Basin (Whitney and Northrop, 1986) suggests that temperature in Henry Basin would have been lower than in the other sites. However, Ca-smectite may persist at higher temperature (Velde, 1985). Leptinite reflectance data suggest that temperatures reached 100~ in the Henry basin (Peterson and Turner-Peterson, 1980), which is in close agreement with the fluid inclusion data of the other sites in the Morrison Formation. The presence of V-illite of similar composition in the three sites studied implies that mineralization took place at a regional scale in similar diagenetic conditions.

Octahedral occupancy o f the V-chlorites

The chemical composition of the chlorite does not fit with the model of Northrop and Goldhaber (1990), who suggest that vanadium occurs exclusively in the interlayer octahedral position with Fe and Mg while the 2:1 layer octahedral positions are occupied by A1 and Fe. The V/A1 substitution may be compared to the value of the 060 spacing of 1.52 ,~ in both chlorite and roscoelite, suggesting similarities in the composition of the 2:1 octahedral sheet.

The X-ray powder diffraction pattern of the V-chlorite (Whitney and Northrop, 1986; Meunier, 1984; Breit, 1986) suggests a structure similar to sudoite (Bai- ley and Lister, 1989) with one dioctahedral 2:1 layer and one trioctahedral interlayer. Sudoite is characterized by a dioctahedral 2:1 layer and a trioctahedral interlayer, with the ideal composition A12(Si~AI)- O~o(OH)z(Mg2A1)(OH)6 and tetrahedral a l u m i n u m varying from 0.4 to 1.1. Evidence for a dioctahedral 2:1 layer and a trioctahedral interlayer is indirect and

based on comparison between observed X-ray and the- oretical peak height (Eggleston and Bailey, 1967). The amount of trivalent cations in the trioctahedral interlayer sheet must equal the tetrahedral a luminum for charge balance considerations if the sum of the divalent cations equals 2 (Bailey and Lister, 1989). This model implies that all V is in the 2:1 layer. Our data (Table 1) show that this structural hypothesis may be valid for only one sample. (SR2), with WAl = 0.71 and X 3§ (trioctahedral sheet) = (V 3+ + WA1) - 2 = 0.7; but it does not explain the octahedral occupancy of the whole data set. A more complex distribution of tri- and divalent ions in the two octahedral layer must, therefore, be evoked in which V substituted to A1 is possible in both types of octahedral layers.

V-chlorite and diagenesis

The V-chlorite at Cottonwood Wash, Slick Rock, and Henry Basin are highly aluminous and of the IIb polytype (Meunier, 1984; Breit, 1986; Whitney and Northrop (1986). The high content of a luminum is consistent with the composition of other diagenetic chlorites (Velde and Medhioub, 1988; Velde et al., 1991) and indicates equilibrium with the sedimentary environment rich in aluminum-bearing minerals.

The IIb polytype generally characterizes the chlorites formed at high temperatures, while diagenetic chlorites are Ia or Ib polytype (Hayes, 1970; Walker, 1989). Whitney and Northrop (1986) were puzzled by the presence of IIb polytype of the V-chlorite because they didn' t find any evidence of elevated temperatures at Henry Basin. The conversion of Ib to IIb needs temperature between 150"--200~ (Velde, 1985), but it may occur at temperatures as low as 50~ (Walker, 1989). Temperatures of fluid inclusions (Table 3, around 100~ and the degree of maturation of organic matter (Landais et al., 1984; Peterson and Turner-Peterson, 1980) are actually in disagreement with the higher temperature range that the l ib polytype would imply. Be- cause IIb polytype has been defined for non-vanadiferous chlorites, the validity of this thermometer for vanadium chlorite remains in doubt.

Hurst and Irwin (1982) show that diagenetic chlorite in sandstones is absent in fresh water environments. In the fluviatile Morrison Formation, the occurrence of chlorite may, therefore, support the saline/fresh- water interface model of Northrop and Gotdhaber (1990). Schultz (1963), Whitney and Northrop ( 1986), and Northrop and Goldhaber (1990) proposed that chlorite formed as incremental addition of interlayer vanadium-magnesium-iron hydroxide to authigenic chlorite/smectite. The formation of chlorite from a smectitic precursor does not take into account the result that, in all the three U-V deposits reported here, chlorite is accompanied by V-illite. Their intimate mixing in the sandstone interstices suggests a different mechanism.


Models o f V-illite and V-chlorite precipitation

The chlorite-illite assemblage is common in the high temperature diagenetic and low temperature metamorphic regimes. Thermodynamic calculations show that illite and chlorite can form in equilibrium (Aa- gaard and Jahren, 1992). The composition of these minerals is modified during increased temperature through a mechanism of dissolution/precipitation reactions (Velde and Medhioub, 1988; Jahren and Aa- gaard, 1989; Jahren and Aagaard, 1992). The fact that the chemical composition of the V illite and chlorite does not depend on the petrographic habits (clay casts, matrix, pore lining, replacement ofdetrital grains) shows that the different reaction paths that lead to the crys- tallization of these minerals took place at or near equilibrium with a single fluid (Velde and Meldhioub, 1988). Although thermodynamic data for V-clays are lacking, aqueous activity diagrams for non-vanadium minerals show that the stability ofillite vs. Mg-chiorite strongly depends on the activity of Mg (Helgeson et al., 1978; Jahren and Aagaard, 1989) while those two minerals can co-exist in a large range of K activity. Therefore, local pore environment may explain the variation in the Mg activity in the mineralizing solutions and the relative abundance of V-chlorite over V-illite. Mag- nesium abundance was probably controlled by saline fluids originated from evaporates below the Salt Wash Member (Goldhaber and Northrop, 1990)

The presence of the smectite precursor suggested by Goldhaber and Northrop (1990) may be evaluated by the present data. The precursor authigenic smectitic clays would have resulted from the devitrification of volcanic ash (Waters and Granger, 1953). In the model of Goldhaber and Northrop (1990), V chlorite formed through a precursor of non-vanadium smectite. Sim- ilarly, the formation of V-illite may be viewed through the vanadium enrichment ofnon-V-smecti te particles. In the V-illitic material studied here, V 3+ abundance does not vary with the layer charge, which suggests that both illite and smectite particles are vanadiferous. Gu- yen and Hower (1979) reported a vanadium smectite with about 20% V in a roll front uranium deposit in Wyoming (Eocene Wasatch Formation). The exact location of vanadium in the structure and its oxidation state were not determined, but the high amount of V suggests that it must occupy the 2:1 layer. Compared to Jurassic samples, the degree of diagenesis of the Wasatch Formation is low and the temperatures never exceeded 60~ based on coal rank (written commu- nication). A similar occurrence would have been possible in the Jurassic rocks before they were transformed during advanced diagenesis. The V-smectite particles may, therefore, represent relics of an earlier event of vanadium enrichment.

Both chlorite and illite may form from smectite pre- cursors through a replacement mechanism (Ahn and

Peacor, 1985; Hower et al., 1976). However, authigenic pore-lining V-illite and V-chlorite cannot be directly explained by a replacement mechanism. Because aluminum is highly insoluble, any V-A1 replacement implies a rapid reconcentration orAl as gel or hydroxides, which when combined with Si, V, K, Fe, and Mg would lead to the formation of authigenic V-clays. Hower et al. (1976) and Boles and Franks (1979) showed that the transformation of smectite into illite releases Mg and Fe, which allows the precipitation of chlorite. The stock of vanadium does not appear a limiting step because of the abundance of vanadium oxides such as montroseite in ore zones.

A quantitative model of clay minerals formation is difficult to sort out due to the complexity of the alteration events (Meunier, 1989; Breit et aL, 1990), but the present hypothesis of diagenetic alteration recon- ciles many contradictory interpretations of the relative age of the mineralization based on petrographic observations. The observed V-clays are the result of late diagenesis (Meunier et al., 1987), but they do not represent the original mineralization that formed earlier in the diagenetic history. Meunier et al. (1990) came to the same conclusion for uranium minerals associated with the V-clays. Meunier et al. (1990) found radiation-damaged rims in detrital quartz not associated with uranium minerals indicating that there was an earlier episode of U precipitation followed by an episode of U leaching before precipitation of V-clays.

At Placerville, the occurrence of roscoelite as the main vanadium clay mineral contrasts with the Mor- rison Formation V-clays. The lack of Mg, Fe, Ca, and Na in the roscoelite and the absence of chlorite suggest a different geochemical environment during ore deposition and/or a different diagenetic history. The different environment was probably less reducing, as suggested by the low U content and the absence of organic matter and iron sulphides.

CONCLUSIONS

The present SEM and microprobe analysis help to better understand the composition of V-clays as a func- tion of the geological environment and the mechanisms that lead to important accumulation in the Jurassic sandstones of the Colorado Plateau. A nomenclature is proposed to differentiate the vanadium-bearing micas V-illite, roscoelite, and V-muscovite based on the interlayer charges and the vanadium content in the octahedral sheets. The V-illite of the Morrison For- mation is typical of sedimentary/hydrothermal rocks while roscoelite is found both in sedimentary (Entrada Formation) and metamorphic rocks. The intimate mixture of authigenic chlorite and illite in various petrographic habits demonstrates that these minerals are the result of advanced diagenesis that took place at or near equilibrium. The composition of the V-clays re- suits from a combination of mineralized fluids and P/T


c o n d i t i o n s t ha t were s imi l a r at a regional scale du r ing d iagenes i s o f the M o r r i s o n F o r m a t i o n .

A C K N O W L E D G M E N T S

T h i s work was f inanced by C R E G U . G. Brei t a n d R. W a n t y k ind ly p r o v i d e d m e wi th the samples f rom Slick Rock, Placervi l le , a n d L i s b o n Valley. I wish to t h a n k J. M. C laude a n d A. K o h l e r for the i r t echnica l a ss i s t ance wi th the m i c r o p r o b e a n d SEM as well as J. J. M o t t e for draf t ing. D i scuss ions wi th C. Par ron , W. C. Ell iott , G. Breit , a n d R. W a n t y p r o v e d helpful . T h e m a n u s c r i p t was great ly i m p r o v e d by the rev iews o f G. W h i t n e y a n d R. Yure t ich .

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