The Significance of Green Sandstones and Illite-Chlorite Mixed-layer Clay-bearing Sandstones of the Athabasca Group in the

Close Lake-McArthur River Area (NTS 74H)

David H Quirt 1

Quirt, D.H. (1999): The significance of green sandstones and illite-chlorite mixed-layer clay-bearing sandstones of the Athabasca Group in the Close Lake-McArthur River area (NTS 74H); in Summary oflnvestigations 1999, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2.

The multi-disciplinary, multi-client Wollaston EAGLE2 Project was undertaken by the Saskatchewan Research Council for Cameco Corporation, Cogema Resources Incorporated, PNC Exploration (Canada) Company Limited, and Uranerz Exploration and Mining Limited (now part of Cameco), to provide additional perspectives on the geology of the southeast Athabasca Basin, the thermotectonic evolution of the sub-Athabasca basement complex, and metallogenetic framework of the unconformity-type uranium deposits of the eastern Athabasca Basin (Annesley et al., 1998).

110 106 60

59

I

! (!) .Athabasca Basin

58

57

56

100 km

One focus of the project was to develop a better understanding of atypical features in sandstones of the Athabasca Group in the Close Lake-McArthur River study area (Figure I). This paper summarizes the modes of occurrence and interpreted origins of (a) green sandstone, and (b) illite-chlorite mixed-layer clay-bearing sandstone.

The study area is underlain by the middle to lower members of the Manitou Falls Formation (MF) of the Athabasca Group (Ramaekers, 1990). From the

102 60

59

58

57

56

subcrop, the members present are:

MFd: fine- to medium-grained, well-sorted, and moderately crossbedded sandstone; common presence of generally whitish clay intraclasts (clay pebbles); rare siltstone horizons.

MFc: fine- to medium-grained, moderately sorted, moderately crossbedded, with numerous coarse-grained to granular beds increasing in number with depth; rare pebbly beds >2 cm thick, most pebble beds being single­pebble thickness; rare whitish clay intraclasts, generally only at the top of the member; rare siltstone horizons.

MFb: broadly similar to MFc but there is an increasing number of pebble (conglomerate) beds >2 cm in thickness with depth; clay intraclasts are not present; siltstone layers and thin pebble beds are common. Four

55 submembers can be distinguished:

Figure 1 - Location map of northern Saskatchewan showing the study area (after Saskatchewan Energy and Mines, 1998).

MFbl: medium- to coarse­grained; commonly pebbly with gravelly

I Saskatchewan Research Council, 15 Innovation Blvd., Saskatoon, SK S7N 2X8.

Saskatchewan Geological Survey 121

pebble conglomerate beds up to 20 cm thick.

MFb2: coarse-grained to pebbly; pebble beds up to 30 cm thick with some cobbles commonly present.

MFb3: coarse-grained, to granular, to pebbly; commonly pebble to cobble conglomeratic.

MFb4: basal cobble conglomerate; minor local sandstone intervals.

1. Green Athabasca Sandstone

Numerous examples of green beds and bands occur in bleached, off-white lower Manitou Falls Formation (MFb) sandstone in the study area. As Close Lake drill hole CL-96 (Figure 2) contains a thick drill core intersection of green sandstone (Figure 3), it was selected for further study. Bleached sandstone in this drill hole, which constitutes more than 50 percent of the sandstone, is attributed to the removal of diagenetic specular hematite. The bleaching often originates as haloes around bedding-parallel fractures in otherwise purplish hematitic sandstone (Quirt, l 998a). The first occurrence of pervasive green colouration is just below the MFc-MFbl contact, at a depth of312.8 m, where greenish sandstone occurs in bleached material

adjacent to purplish hematitic sandstone, and in bands entirely within bleached, off-white intervals. Numerous greenish bands and beds occur in bleached intervals in the underlying 392 m ofMFb sandstone and conglomerate. In the 12.5 m of basal MFb4 pebble to cobble conglomerate, there is relatively little bleaching, but where present, these intervals are greenish. In all locations, the green colouration is preferentially developed in bleached, fine- to medium-grained sandstone, often at contacts with coarser-grained beds which generally remain hematitic pinkish to purplish in colour. Locally, brittle fracture sets contain greenish clay coatings and, in one location, a microfault shows a l cm offset of the bedding and of the green banding. Rarely, greenish bands occurring within bleached sandstone are themselves bleached whitish along subvertical fractures.

Below the sub-Athabasca unconformity, there is approximately 20 m of red, red-green, and green zone paleoweathered microgranite which contains minor chloritic shear zones. Below the microgranite is about 85 m ofpegmatite-bearing pelitic gneiss of which the upper 7 m is a green coloured zone attributed to paleoweathering. Locally, the unweathered pelitic gneiss contains friable chloritic zones typically l O to 40 cm wide, as well as intersections of clay-altered, lime-green, coarse-grained pegmatite.

CL-f~;~tc7Q9 CL-&•*8:_:'fa

CL-67'

CL·10A•CL-11 --C-L--96- CL-66

Petrographically, there are no significant differences between the greenish bleached sandstone and the off-white bleached sandstone. Both varieties lack the interstitial diagenetic specular hematite characteristic of the purplish hematitic sandstone. Nine samples of green sandstone from throughout the drill hole (Table 1) were selected for further study that included mineralogical, lithogeochemical, mineral chemical, and stable isotope geochemical analyses .

(/)

CJ c .c t 0 c

6436000

6416000

~ 6396000

6376000

6356000

450000

Mudjatik Domain

470000

CL·93 CL-92 CL·9f. '

MAC-188~

\/\bllaston Domain

drillhole locations WoUaston -Mudjali.k Domain Boundary

490000 510000 530000 UTM eastings

Figure 2 - Drill hole location map for the Close Lake-McArthur River area, Saskatchewan.

122

a) Mineralogy

Both modal (XRD and PIMA infrared reflectance spectrometry) and nonnative (geochemical) mineralogical data were obtained (Table 1 ). The XRD analyses were performed primarily on clay-size fraction portions of the samples with check analyses done on bulk sample material. The normative data were calculated using the CLA YNORM algorithm of Quirt (l 995b). The matrix clay minerals (kaolin, illite, chlorite) were texturally

Summary of Investigations /999, Volume 2

Table 1 - Modal and normative clay mineral proportions for green sandstone samples from drill hole CL-96.

XRD modes (clay-size fraction)

IDdh de12thl \\lite Chlorite Kaolin 1002/1001 Kubler Index \/ll+KI C\avfraction%

CL-96 328.8 64 0 36 0.68 5.8 0 64

CL-96 409.5 43 0 57 0 44 5.8 0.43

CL-96 427.1 51 0 49 0.46 5.8 0.51

CL-96 460 3sltsst 5 0 95 0.50 36 0.05

CL-96 515 0 35 2 63 0.54 5.3 036

CL-96 601.1 57 0 43 0 35 5.6 0.57

CL-96 646.5 77 0 23 0.41 4.8 0 77

CL-96 698.4 4 96 0 n/a n/a 1.00

CL-96 701.1 21 79 0 n/a 5 8 1.00

600

surface[.Mm____ ________________________ ~

overburden 5no 29.QJn__ sul:icrop · ---------- --------- ---

MFd 400

..1JL2..m___ _____________ - ---·---

300

e ~ 200 0

MFc

_312.2_m. __________________ , ___ ,, ____ ---

312.8 m green bands begin

MFb1

O'ti/ ,' ~--"'-~/ /~~Cj

~I /oflii.l,

2

2

2

1

2

3

3

3

3

1 ui 100

,-3>],J ). i ;;:, 328.8 m ,

1

..

illite ,ff,-~/ r~.Q. $ 4096m~f dickite

---- •1--,-"'"--'-,.a_•===_==_=_<,;~u 427.1 m 'f A51.6Jn________ ____ , ~

-·----\ ', , kaolin

0

I ::r:;-- 460.3 m ,·: kaolinite

MFb_2 ____ _.~ r . "'" 1~;,. i\t......... i., 601.1 m , ·

59D0m

-100 MFb3 illite ·:)-r -~~~2HJ 646.5 i.

__ ... ... :_ unconformity:·~· -~M-f~b4~-· ===Jls:~~·;c': ~~. ~

-200 -123.!l..ro microgranite . 698.4 m ~ - _ -----------<

mixed pelitic gneiss/pegmatite end of hole 786.1 m

CL-96 stratigraphy 0 50 100

clay mineral proportions

Figure 3 - Graphic log for drill hole CL-96; see text for abbreviations.

Saskatchewan Geological Survey

calculated norms whole-rock)

lllite Chlorite Kaolin 1/1\+Kl Clav%

57 2 41 Q.58 5.2

37 2 61 0.38 7.1

54 0 46 0.54 13.6

11 1 88 0.11 226

23 0 77 0.23 7.8

27 1 72 0.27 5.6

50 0 50 0.50 5.6

0 100 0 1 00 11.1

8 92 0 1.00 14.1

characterized in energy dispersive mode on a JEOL JXA8600 electron microprobe at the Department of Geological Sciences, University of Saskatchewan.

The clay matrix of the samples taken from 328.8 to 646.5 m (upper MFb sandstone: MFbl to MFb3) is composed of sub-equal proportions of kaolin and illite . Infrared reflectance spectrometric analyses and heating tests followed by XRD analyses confirm that the kaolin polytype present in the sample from 460.3 mis kaolinite and not dickite, as the kaolinite structure was broken down between 500° and 525°C, rather than in excess of 600°C as required for dickite. The matrix of this silty sandstone sample also contains a relatively low illite proportion. The XRD and infrared spectrometric analyses of sandstone from 409.5 m and 515.0 m suggest that these samples contain dickite. The XRD illite parameters (Kubler Index, 1002/IOOI) do not indicate any differences in illite crystallinity and composition between green kaolinitic sandstone present in this drill hole and background kaolinitic sandstone from drill holes used for regional stratigraphic studies ( cf. Ramaekers, 1980; Hoeve et al., 1981 ). The normative data indicate that the calculated amount of clay in these samples (5 to 22 percent) is higher than

123

typical sandstone from this formation (ca. 3 to 7 percent).

In the kaolinitic sandstone, the interstitial pores are filled with kaolin, with moderate amounts of illite occurring along the pore margins and as isolated flakes within the pores. Trace to minor amounts of goyazite also occur along the pore margins. Kaolin morphology is variable, being coarse grained and blocky (409.5 m: 100 µm x 300 µm grains; likely dickite), finer grained and flaky (<10 µm; likely kaolinite), or fine grained and vermicular (460.3 m: < 10 µm wide by up to 50 µm long; likely kaolinite). Some vermicular kaolin is also coarser grained but is sub-blocky, likely being dickite (515.0 m). Where present in microfaults in this latter sample, the kaolin is no longer vermicular but is semi­massive and squeezed, occurring with fine-grained cataclastic quartz and without illite. Also in this sample, pores are locally filled with kaolin and low­potassium/high-iron illite. These latter two samples both contain trace to minor quantities of detrital biotite, partially altered to a relatively titanium-rich clay consisting of illite+kaolin ( ±rutile).

The modal and normative data indicate that the clay matrix of the two deepest samples ( 698 .4 and 701.1 m; MFb4 sandstone) is composed of chlorite ( di­trioctahedral sudoite) and minor illite. The sudoite comprises a fine-grained, semi-massive, flaky to acicular, pore-filling grain network. Locally, it is vermicular in habit, perhaps after kaolinite.

b) Lithogeochemistry

Selected geochemical data for the green sandstone samples are presented in Table 2. The trace and major element geochemical data suggest that several accessory minerals are present in minor amounts. For example, the elevated Sr, CaO, and P20 5 contents reflect the presence of goyazite (Sr, Ca phosphate) whereas the Zr and Hf contents reflect zircon (Zr silicate). Elevated LREE contents, common in the heavy mineral-(zircon-)bearing samples, typically reflect the presence ofmonazite (LREE phosphate).

Kaolinitic Sandstone

The uppermost seven samples (upper MFb sandstone: MFb I to MFb3) are relatively clay-rich (Al20 3) and, as indicated by the mineralogical data, are kaolin-bearing (low MgO/K.20 and Kz0/Al20 3 ratios). The low K20/ Al20 3 ratios (0.05 to 0.14) suggest that the matrix contains a minor illite proportion. The low Fe20 3c contents are indicative of the bleached, non-hematitic nature of the sandstone. Several samples contain minor amounts of FeO (0.2 to 0.3 percent) which is unusual for Athabasca Group sandstone which typically contains less than 0.1 percent ( cf. Quirt, 1985), particularly in kaolinitic bleached sandstone. The weakly elevated FeO values in these samples suggest that the dominant clay mineral present (kaolin) contains some Fe2+. The most clay mineral-rich kaolinitic material (samples at 427.1 m and 460.3 m) is also slightly enriched in U (2.8 to 3.5 ppm) and Zr (-400 ppm; in zircon).

Table 2-Lithogeochemical data/or greenish coloured sandstone samples from drill hole CL-96 (upper table values in%; lower in ppm).

lddh depth I Si02 TiO, Al2 0 3 Fe2 0 3c FeO Cao MgO MnO K20 Na,O P, O, LOI K,O/ MgO/ LO If Mg Of Al, 0 3 Al,0 3 A l,03 K,O

CL-96 328.8 96.5 0.11 2.08 0.09 0 1 0 01 0 06 0.002 0.30 0.02 0. 03 0.6 0 14 003 0.29 0.20 CL-96 409 5 94 7 020 3.01 0.06 0.3 0.01 0.06 0.003 0. 27 0. 03 0.09 11 0 09 0.02 0.37 0.21

CL-96 427.1 89.7 0.50 5.77 0.56 0.3 0.03 0.1 1 0.002 0.77 0 07 0 15 1 8 0.1 3 0.02 0. 31 0.14

CL-96 460.3 83.0 0.35 11.15 0.33 0.2 0.02 0 07 0 001 0 28 0 12 0.08 4.3 0.03 0.01 0.39 0 25

CL-96 515.0 94 7 0 14 3 46 0 10 0 1 001 0.03 0.002 0.18 0.03 0.04 1.2 0.05 0.01 0 35 0 15

CL-96 601.1 96.2 0.13 2.40 0.04 0.1 0.0 1 0.03 0.002 0.15 0.02 0.02 08 0 06 0 01 0.33 0.17

CL-96 646.5 96.6 0.03 2.28 0.00 0.1 0.01 0. 03 0.002 0.28 0.02 0 02 06 0.12 0. 01 0.26 0.12

CL-96 698.4 92.5 0.09 3.67 0.26 0.1 0.06 1.48 0.002 0.13 0.05 0.06 1 5 0 03 0.40 0.41 11.77

lddh depth] Ba Hf Li Ni Sr Th LI v y Zr sum I.a Ce Nd sum

traces RLE

CL-96 328.8 13 4.6 33 4 150 6 0.3 2 4 144 470.9 10 n 9 49 I

CL-96 409 5 15 46 29 4 608 6 0 .6 s 4 149 979.9 19 41 17 87.4

CL-96 427. 1 50 12.8 79 5 7% 43 3 .) 16 10 )86 1775 0 )6 Ill 49 244.J

CI.-96 460.3 39 13.4 123 4 295 JO 2 .8 8 l.i 42 3 1204.9 40 78 35 175 6

Cl.-% 515.0 18 3.7 20 3 153 10 0 .7 6 4 11(1 477.0 l 'I 4 1 16 86.1

CL-96 601.1 10 4.9 19 3 71 6 0.5 ·' 2 160 392.5 IJ 28 I I 58 8

CL-% 646.5 ') 2.0 20 3 76 2 0.4 6 2 6 7 288.4 11 2 1 9 48. 8

CL-96 698.4 12 4.0 32 21 70 15 5.0 41 3 106 507.4 28 54 25 ll'l_')

lr.L-96 701.1 23 R.1 35 30 193 25 .5.4 27 6 239 117',.2 93 162 64 34 7.6

124 Summary of Investigations I 999. Volume 2

Chloritic Sandstone

The two deepest samples are clay-rich (moderately high Al20 3) and, as indicated by the mineralogical data, are chloritic rather than kaolin bearing (high MgO contents, high Mg0/Al20 3 and MgO/KP ratios, and low KP/AIP3 ratios). These chloritic samples show weakly elevated Ni and V contents and contain about 5 ppm U.

c) Mineral Chemistry

The matrix clay minerals (kaolin, illite, chlorite) in the bleached sandstone were analysed using wavelength dispersive mode on a JEOL JXA8600 electron microprobe at the Department of Geological Sciences, University of Saskatchewan. Summary data from these analyses, as well as equivalent data from background samples from nearby drill holes CB-62, CL-91, CL-93, and EL-09, are listed in Table 3.

Kaolinitic Sandstone

Kaolin from samples of background non-green sandstone is low in impurities, containing only trace amounts of potassium, magnesium, and iron. In contrast, illite from both background sandstone and green sandstone consistently contains minor amounts of magnesium, iron, titanium, and sodium. Kaolin from the green sandstone, however, is distinctly iron-rich relative to that from background sandstone (1.47 percent FeO versus 0.08 percent FeO). Otherwise, the kaolin mineral chemical data are similar except for a slight decrease in the Alp/Si02 ratio with increasing iron content.

Also present in trace quantities in one kaolinitic green sandstone sample (460.3 m) is Fe-chlorite, which is strongly iron-dominant over magnesium. This chlorite has a distinctly different AIP/Si02 ratio than either kaolin or sudoite and may be detrital in origin.

Chloritic Sandstone

As noted earlier, the matrix of the green basal sandstone does not contain kaolin, rather it contains the Al-Mg chlorite sudoite, with trace amounts ofillite. Compositionally, the sudoite is aluminous with a moderate magnesium content, similar to other analysed

Athabasca sudoites (e.g. Percival and Kodama, 1989), although somewhat lower in magnesium.

d) Stable Isotope Geochemistry

Two samples of green sandstone, one kaolinitic (460.3 m) and one chloritic (701.l m), were prepared for hydrogen and oxygen stable isotopic analyses by light crushing followed by concentration of the clay mineral matrix using a centrifugal settling method. The stable isotopic analyses were performed at the University of Saskatchewan.

The isotopic data (Figure 4) from the kaolinitic sandstone fall with in the ranges attributed to diagenetic kaolin (Katzer and Kyser, 1995) with a oD value of -62 and a slightly high value of 13.9 for 6180. The sudoitic clay sample (oD: -77, 6180: 9.7) also returns diagenetic isotopic values. The somewhat lower 6 180 value for the chlorite than for kaolin is consistent with the typical equilibration of chlorite with a fluid of a given composition at a lower 6 180 value than of kaolin.

-1 0 ' I ' T

J -30 • badc:r;JO.Jnd kaoli nitlc sa'ldstone

<> gee, kaolinitic scndstO'le

/\,. geen diloritic basaJ ~cne

• ICML·beOOng s;r,cistane

n kaolinitic red zone paleoNeathered pefitic gneiss .50 i ·-

\ E ~

'" -~ 0

"' -70

D.r .. ,

.go ! ~ • i

-110 r · -r r-,-,-,J I -20 -1 0 0 10 20 30

& 180 (perrril)

Figure 4 - SD and 8180 stable isotope data for clay mineral concentrates, corrected/or quartz content; MWL, meteoric waterline.

Table 3 - Wavelength dispersive electron microprobe average mineral chemical data for clay minerals from green sandstone (values in %).

mineral SiO- TiO AID. Cr, 0 , FeO MaO MnO Cao Na, 0 K, 0 Cl Total AIO,/S10

green: kaolin 43.82 0 01 34.48 0. 01 1.47 0.17 0.02 0.05 004 0.91 0.02 81.01 0.79

green: illite 46.34 0.48 34 76 0.02 1.47 0. 66 0 01 0.04 0.48 8.1 0 0.01 92 37 0.75

a reen Fe-ch lor1te 27.16 0.06 16 77 0.02 38.45 1.33 0 00 0 37 0.07 O OD 0.02 84.25 0 62

lareen sudoite 37.60 0 01 34.50 0.02 1.66 962 0.01 0.13 0. 33 137 0.02 85.26 0.92

bkgnd: kaolin 47.48 0 01 37. 98 0.01 0.08 0 05 0.01 0.04 0. 01 0 14 D 01 85.80 0.80

bkgnd: illite 42 90 0.33 31.51 0.02 1.43 0.99 0.03 0.03 0 32 9 14 0. 01 86.70 0.74

Saskntchewan Geolog ica l Survey 125

e) Discussion

The green colouration of beds and bands in the ~thabasca sandstone is a reflection of two mutually mdependent aspects: (1) chlorite in the basal sandstone, and (2) presence of greenish kaolin in sandstone above the basal chloritic interval.

Kaolinitic Sandstone

The geochemistry of greenish bleached kaolin-bearing sandstone beds and bands suggests that the matrix clay assemblage contains a minor amount of ferrous iron. However, the dominant clay minerals present in these sandstone samples are kaolin (both dickite and kaolinite), which typically does not contain significant amounts of ferrous iron, and lesser illite. The modal kaolin proportion in the clay-size fraction is typically lower than in the bulk sample, indicating that there is a minor clay mineral size fractionation within the sandst?ne matrix. Illite XRD mineralogical parameters and mmeral chemistry are similar to background illite values; the XRD mineralogical parameters of the kaolins are also not unusual.

Both dickite and kaolinite are present in the sandstone and occasionally both occur in a single sample. They can be distinguished from each other texturally, with dickite tending to display coarser-grained, sub­vermicular to blocky grain shapes as opposed to the finer-grained, flaky to vermicular habits of kaolinite. However, the mineral chemistry of the kaolin (both dickite and kaolinite) in green sandstone differs significantly from background kaolin, being distinctly ferrous iron-bearing. In addition, trace amounts of Fe­chlorite occur locally in these samples, as do trace to minor amounts of partially altered detrital biotite. Thus, the green colouration appears to be due to the presence of ferrous iron-bearing kaolin. The ferrous iron may have been derived from local sources such as detrital biotite and Fe-chlorite. The more clay-rich samples show a minor enrichment in uranium (about 3 ppm) and are also zircon-bearing, suggesting that this enrichment may be due to the presence of U-bearing zircon.

Chloritic Sandstone

The zone of basal chloritization is attributed to local basement-sandstone diagenetic fluid interaction (Mg alteration) in the vicinity of the sub-Athabasca unconformity. The source of the magnesium is probably the basement lithologies (cf. Hoeve and Quirt, 1984). Textural features suggest that the result of this magnesium enrichment was the chloritization of the clay mineral matrix by replacement of kaolinite (±illite) with aluminous chlorite (sudoite). The relatively clay-rich sudoitic sandstone is matrix-rich and contains moderately elevated uranium concentrations of about 5 ppm.

126

2. Illite-Chlorite Mixed-layer Clay in the Athabasca Sandstone

In 1992, Fritz Hopfengartner, of Cogema Resources Inc., identified Mg-enriched sandstone occurring near the sandstone-overburden contact (subcrop surface) in a number of drill holes (e.g. CL-0 IB, CL-66 to -73, CL-80: Figure 2) in the Close Lake-McArthur River area (Figure l ). XRD clay mineral analyses revealed the presence of a phase identified as an illite-chlorite mixed-layer (ICML) mineral (illite-sudoite: Quirt, 1992, written comm.). In drill hole CL-80, the ICML phase was accompanied by kaolin (±a discrete illite phase) and occurred only at the top of the drill hole. Down the hole, the ICML+kaolin content decreased rapidly and that of illite increased proportionally until the ICML phase disappeared and the kaolin and illite proporti?ns ~ecame sub-equal. Near the unconformity, the matnx mmeral assemblage present in the sandstone changed from kaolin+illite to illite+sudoite. The association of kaolin and ICML clay in the upper part of the hole suggested that the two clays were related in origin, possibly a result of 'late' kaolinitization due to interaction with meteoric waters. The source of the magnesium enrichment remained unknown.

In addition to the above-noted drill holes, material from several other drill holes containing magnesium­enriched sandstone (CB-24, CB-49, CL- IOA, CL-44, CL-79, MAC-188, RL-49) were examined petrographically, mineralogically, and geochemically. The intervals containing the ICML clay are typically fractured, pervasively bleached, only moderately to poorly indurated, and are limonitic relative to both bleached and hematitic sandstone below (Quirt, l 998a, 1998b).

a) Lithogeochemistry

Selected geochemical data for samples ofICML­bearing sandstone are presented in Table 4. This sandstone is typically weakly illitic (low K20/Al20 3),

contains some chlorite, and is dravite-free, as indicated by low to moderate Mg0/Al20 3 ratio values and low B contents. The geochemical data, however, cannot distinguish between chlorite as mixed-layer illite­chlorite or as discrete chlorite grains. Most samples display moderate to high MgO/KzO ratio values (0.50 to 4.4, most> 1) which suggests that only relatively low proportions of illite are present. Again, however, the geochemical data cannot distinguish between illite as mixed-layer illite-chlorite or as discrete illite grains, except that samples with Mg0/K20 ratio values>> l likely contain little discrete illite. Low trace element concentrations, including U, are typically found in these samples. The greatest contributions to the trace element suite include Zr (in zircon) and the LREE (in monazite and clay minerals). The single sample from drill hole MAC-188 is weakly enriched in U (3.5 ppm).

Summary of investigations I 999. Volume 2

Table 4 - Selected geochemical data for ICML-bearing sandstone samples (upper table values in%; lower in ppm).

Ddh depth Si02 Ti 0, Al,O, Fe,O,t Cao MgO

Cl-10A 159.0 98.0 0.03 0.76 0.70 0.01 0.12

Cl-44 83.0 98.0 0.04 1.15 0.09 0.01 0 21

Cl-79 43.5 98.5 0.03 0.86 0.07 0.01 D 14

Cl-79 84 5 97.4 0.11 1 53 0.08 0.01 019

Cl-79 139 7 98.3 0.04 0 94 0.07 0.01 0.17

Cl-79 172.5 97 6 0.10 1.33 o os 0.01 0.22

CL-79 219.8 94 7 0.26 2.70 0 53 0.02 0.40

CL-80 38.0 95.8 0.02 0.46 o 03 0.01 0.08

CL-BO 100 0 99.2 0 03 0.71 0.22 0 01 0.10

CL-80 123.4 98.5 003 0.84 0.19 0.01 0 10

CL-80 208.9 98.7 0.04 0.85 0.03 0.01 0 09

CL-89 46 8 98.4 0.05 0.86 0.05 0.01 0.17

CL-89 52.6 98 9 0.03 0.61 0 04 0 01 0.13

CL-89 75 .9 98.8 0 03 0.66 0.09 o 01 010

CL-89 117 8 98.3 0.12 0 91 0.04 0.01 0 10

CL-89 160 7 96 6 0.09 2 03 0 10 0.01 0.17

CL-89 163.0 93 7 0.15 3.82 0 13 0.02 0.36

MAC-188 165.9 93.7 0.40 3.68 0 15 0.01 0.20

Ddh denth B Ba Cu Li Ni

CL-10A 159.0 5 8 4 6 7

CL-44 83.0 10 9 9 11 5

CL-79 43.5 2 18 10 4 7

CL-79 84 5 19 19 16 4 7

CL-79 139.7 6 11 3 3 4

CL-79 172.5 13 11 4 7 6

CL-79 219.8 47 18 5 21 9

CL-80 38.0 2 12 4 3 6

CL-80 100 0 7 11 4 4 4

CL-80 123 4 9 9 4 7 5

CL-80 208.9 14 12 13 3 5

CL-89 46.8 4 11 14 6 7

CL-89 52.6 4 8 10 4 6

CL-89 75.9 3 11 4 3 6

CL-89 117 8 5 23 4 5 5

CL-89 160.7 15 12 7 11 6

CL-89 163.0 35 18 9 29 9

MAC-188 165.9 17 21 2 8 8

b) Mineralogy

The modal and normative mineralogical data for selected samples of mixed-layer-bearing sandstone are presented in Table 5. The minor magnesium contents (0.1 to 0.4 percent MgO) result in calculated norms which, incorrectly, indicate a significant amount of chlorite (IO to 57 percent). The XRD modes show, however, that a significant amount of ICML clay is present, together with kaolin and illite. Minor discrete chlorite (sudoite) is in the matrix of sandstone closest to the subcrop surface ( drill holes CL-44, CL-79, CL-80, and CL-89). However, in drill holes CL-80 and

Saskatchewan Geological Survey

MnO K,O Na,o P,05 LOI K,0/ MgO/ LOI/ MgO/ Al O AIO Al 0, KO

0.00 0.12 0 03 0.01 0.2 0.16 0.16 026 1.02

0.00 0.19 0 01 0.01 0.3 0.16 0.18 026 111

0.00 0.03 0 01 0.02 0.3 0.04 0.17 0 35 4 44

0 01 0.12 0.01 0 02 0.5 0.08 0.12 0.33 1.61

0 00 0.09 0.01 0.01 03 0.09 0.18 0.32 1.87

O OD 020 0.01 0.02 0.4 0 15 0 16 0.30 1.06

0. 00 0 53 0.05 0.06 0.7 0 20 0 15 0.26 0.74

0 00 o 02 0.01 0.01 0.1 o 04 0.17 0.22 4.22

0.00 0.06 0 01 0.01 0.3 0.08 0.14 0 42 1.67

O OD 0.06 0.01 0 02 0.3 0.07 0.11 D.36 160

0.00 0.16 0.01 0 02 0 1 0.19 0.10 0.12 0.55

D 00 0.07 0.01 0. 01 03 0 08 0.20 0.35 2.52

0 00 0 05 0 01 0.01 02 0.09 0 21 0.33 2.37

0.00 0.05 0.01 0 01 0.2 0.07 0.15 0.30 1.96

0 00 0.08 0.01 0.04 03 0.09 0.11 0.33 1.26

0.00 0 36 0.03 0.02 0.6 o 18 0 08 0.30 0.47

0.00 0.79 004 0.03 0.9 0.21 0 09 0 24 0.46

0.00 0 53 0.05 0.04 1.2 0 14 0 05 0.33 0.37

Pb u v y Zn Zr sum traces sum REE

4 0.3 8 2 3 42 180.4 33.7

2 0.2 2 2 5 55 223.8 34 8

3 0.3 3 3 5 29 229.7 64 6

3 0.7 5 7 9 213 447.2 56.8

1 0.4 2 3 2 56 194 2 42.3

3 0.4 3 6 3 153 352.9 58 1

5 1.6 12 26 5 392 915.5 154.9

1 0.2 2 3 4 26 164.5 47 6

1 02 2 3 4 41 168.7 36.2

1 02 2 8 4 50 209.6 52.9

3 0.3 2 3 7 71 294 3 54.9

3 0.4 4 3 8 97 280.4 51 4

1 0.3 4 1 5 35 152.4 31 3

1 0.2 3 2 4 41 168.2 37.6

4 1 5 9 6 4 240 6306 80.0

1 0.6 8 6 4 112 346 2 55.1

7 1.2 8 7 6 242 592.8 58 5

6 3.5 17 7 3 242 630 1 124.4

CL-89, the uppermost samples (closest to subcrop) are kaolin-, illite-, and sudoite-bearing and contain no, or only trace amounts of, ICML clay. In these drill holes, there is a gradation downhole from a kaolin+illite+ sudoite assemblage to one ofkaolin+illite+ICML clay. At depth, neither the chlorite nor ICML clay are present.

This ICML clay produces a distinct XRD pattern (Figure 5) characterized by diffraction peaks at 12A, 8.5A, 4.80A, and 3.44A. Peak positions following solvation by ethylene glycol remain unchanged, confirming that this clay is not an illite-smectite

127

Table 5 - Selected mineralogical data for ICML-bearing sandstone samples; C, chlorite, ICML, illite-ch/orite mixed-layer clay; I, illite; and K, kaolinite.

XRD modes calculated norms XRD accessorv minerals (0-6 \ D lllite Chlorite Kaolin Clayfract1on J lllite Chlorite Kaolin Clay% I hematite dravite ICML ICMU chlorite % l l+K

CL-10A 159.( 44 0 56 2 0.4< 53

CL-44 83.( 90 7 3 2 0 91 53

CL-79 43.~ 11 10 79 2 0 1: 9 CL-79 84 5 1 4 95 2 nla 27 CL-79 139.7 11 0 89 1 0.11 31

CL-79 172.~ 59 a 41 2 0.59 52

CL-79 219.E 83 1 16 4 a.a, 67

CL-80 38 C 7 25 67 1 0 1( 10

CL-80 100.( 3 1 96 1 0.0, 29

CL-80 123.~ 3 3 94 2 0.0, 25

CL-80 208.~ 69 2 29 2 0 71 34

CL-89 46.E 8 28 65 2 O. H 24

CL-89 52E 28 32 39 2 0 4, 27 CL-89 75.S 1 3 96 3 n/a 25

CL-89 117.1 11 0 89 2 0.11 32

CL-89 160., 48 0 52 2 0.4E 65

CL-89 163.( 86 0 14 3 0.8E 74

MAC-188 165 ! 73 0 27 3 0.7, 55

(montmorillonite) mixed-layer clay, which displays significant movement of the basal 12A peak upon glycolation. Possible mixed-layer clay minerals which give a similar pattern, both before and after glycol solvation, include sepiolite (ribbon-structured Mg­aluminosilicate ), illite-chlorite, illite-venniculite, and hydrobiotite (muscovite/biotite-venniculite ).

C • chlonte (sudoite) I · illile ICML. illite-chlorite mixed-layer clay K • kaolinite Q · quartz

400

200

CL-79/172.5 m

5 10 15

Q

I ICML

20 Degrees 2-theta

K

25

ll+K' IICK'

39 8 21 08 , 2 0 1 0.21

44 2 32 0 9( 0 0 2 0.68

50 41 2.2 0 1! 0 0 1 0.09 sudoite 34 39 3.9 0.41 0 0 1 0.15 sudoite 48 21 2.5 0.6( 0 0 2 049

41 8 3. 7 0.8 / 0 0 4 1 06

33 0 7.5 1 DC 0 1 2 0.40

49 40 1.2 02( 0 0 1 0.01 sudoite 39 32 1.9 0.4E 0 a 2 0.14

32 43 2.1 0.3i 0 1 2 0.28 sudoite 46 21 1.6 0 6, 0 0 1 0 11 sudoite 55 22 2.3 0 5, a 0 0 0.00 sudo1te

57 16 1.7 0 62 0 0 1 0 05 sudoite 41 34 1 7 0.4: 0 0 1 008 sudoite 31 37 23 0.4E 0 0 2 a 09

18 17 5.4 0.7! 0 0 3 0 35

19 7 10.2 0.91 0 0 4 0.72

11 34 9.2 06: 0 0 1 0.08

Mixed-layer clay mineral modeling was carried out using computer simulation software (NEWMOD: Reynolds, 1980; Reynolds and Reynolds, 1996; WinFit!: Krumm, 1995) to calculate, display, and fit XRD patterns of various combinations of illite/muscovite, chlorite (both sudoite and Fe-Mg chlorite), and vermiculite. The best pattern fits were made by the similar illite-chlorite (sudoite) and illite-

vermiculite calculated patterns using 60 percent illite layers and

Q 40 percent chlorite or vermiculite layers. To determine which of these mixed-layer clays the Athabasca ICML clay most resembles, the samples were heated to 525°C for one hour. Sepiolite, illite-vermiculite, and hydrobiotite structures all collapse and display a basal peak shift from -12A to -lOA after heating, due to dehydroxylation, while illite-chlorite (sudoite) does not. No peak shifts were observed for the Athabasca ICML clay, suggesting that it is composed of

30 35

regularly interstratified (mixed) layers of illite and sudoite.

Figure 5 - X-ray diffraction pattern for ICML clay in illite- and kaolinite-bearing sandstone.

Heating tests followed by XRD analysis confirm that the kaolin in the ICML clay-bearing samples is kaolinite, not dickite, as the kaolinite structure was broken down well before 500°C, rather than in excess of 600°C as required for dickite. The

128 Summary of Investigations I 999, Volume 2

relatively low temperature of dehydroxylation obtained for these samples ( <450° to 475°C) suggests that the kaolinite is less crystalline than typical diagenetic Athabasca kaolinite (see Section la).

c) Mineral Chemistry

As examined under the petrographic microscope and the electron microprobe, the ICML clay is inconspicuous, appearing similar to illitic or illitic+chloritic matrix clay. Much of this clay had been plucked out of the polished thin sections during section preparation, suggesting that it was poorly indurated. Average mineral chemical compositions for the matrix clays are given in Table 6. The illite is comparable to that in samples of background sandstone (Table 3). The kaolinite, however, appears to be intimately intermixed with illite and/or ICML clay as it contains, on average, moderate amounts of potassium and minor amounts of magnesium.

The ICML clay is distinctly magnesian and potassic with only minor amounts of iron (Table 6). The Al20 / Si02 ratio for the ICML clay is -0.71 which is most similar to kaolin (-0.74) and which is higher than illite (0.66) and significantly lower than sudoite (0.92). Removal of either representative illite or representative sudoite portions from the ICML clay chemical composition results in remainder mineral chemical

compositions which do not resemble either illite or sudoite (Table 6). This suggests that the ICML clay is not made up of discrete Angstrom-scale interleaved illite and chlorite, as locally observed in the Kombolgie Formation sandstone ( cf. Quirt, 1995a), but is composed of regularly interstratified (mixed) layers of illite and chlorite (sudoite) composition.

d) Stable Isotope Geochemistry

Several samples of sandstone containing ICML clay, with illite and kaolinite, were analysed for hydrogen and oxygen stable isotopes \Table 7, Figure 4). The oxygen isotope data (e.g. 01 0: 11.8 to 13.1 permil) are generally suggestive of diagenetic illite and kaolinite. Several higher 0180 values obtained (14.4 to 14.8 permil) indicate some re-equilibration with lower temperature fluids. Similarly, the hydrogen data (SD: -77 to -82 permit), which are similar to that obtained for the chloritic basal sandstone sample (Figure 4), are interpreted to reflect peak diagenetic compositions. One sample, which also contains the highest proportion ofICML clay, displays the most re-equilibrated stable isotope data with D depletion (oD: -97 permil) characteristic of re-equilibration with mid-latitude meteoric waters (Kotzer and Kyser, 1995).

Table 6 - Wavelength dispersive electron microprobe mineral d1emistry for clay minerals from JCML clay-bearing sandstone samples; sudoite data is from Table 3 (values in %).

mineral Si02 Ti02 Al,O, Cr,O, FeO MgO MnO Cao Na20 K,O Cl Total Al,O/Si02

illite 46 73 0 10 31.03 0.01 0.76 0.97 002 0 04 0.07 9.70 D.02 8944 0.66 mixed illite- 38. 14 0.53 27.44 0.01 0.66 4 59 0 00 0.18 0.03 2.79 0 03 74.40 0.71 chlorite

kaolin 46.52 0.7 1 34.46 0 01 0.20 0.30 0.02 0.06 0.02 2 07 0.02 84.39 0.74

sudo1te 37.60 0.0 1 34.50 0 02 1.66 9.62 0.01 0.13 0.33 1 37 0.02 85.26 0 92 1-C w/ illite 44.66 0.90 3347 0.02 0.80 7.79 0.00 0.31 0 02 0.00 0.04 88.00 0 75

removed

1-C w/ chlorite 53 76 1 32 30.33 0.00 0.00 0.75 0 00 0 3 1 0.00 5.48 0.05 92.00 0.56 removed

Table 7 - Stable isotope geochemical data/or JCML-bearing sandstone samples; CP, clay intraclast; C, chlorite, JCML, illite­chlorite mixed-layer clay; I, illite; and K, kaolinite.

sample ICML/(ICK) 1/C/K (%) Al20 J oD 0180

CL-IOA/1 59.0 CP 0.21 44/0/56 n/a -77 14.8

CL-79/ I 72.5 1.06 59/0/41 1.33 -97 14.4

CL-79/219.5 0.40 83/1/ 16 2.70 -78 13.1

CL-89/ 160.7 0.35 48/0/52 2.03 -79 12.1

CL-89/ 163.0 0.72 &610114 J.82 -82 l l.8

Saskatchewan Geological Survey 129

e) Discussion

Trace amounts of the ICML clay have been detected in sandstone from several locations (drill holes CB-24, CB-49, CL-OlB, CL-66 to -73, CL-80, CL-79, CL-91, CL-96, MAC-188, and RL-49). Mineralogically, the ICML clay is illite-sudoite, which occurs with illite and relatively poorly crystalline kaolinite (not dickite). Sandstone containing ICML clay is typically clay-rich, bleached (±limonite), moderately friable, often fractured, and contains low concentrations of trace elements. Commonly, ICML clay is found in sandstone near the subcrop surface or associated with fracture zones in the upper part of the sandstone. The illite­sudoitic intervals can be up to 275 to 320+ min thickness (e.g. CL-80, CL-89). More rarely, an illite+kaolinite+chlorite (sudoite) assemblage is present at the sandstone subcrop (e.g. CL-80, CL-89, MAC-188), but this assemblage grades quickly down hole into the ICML+illite+kaolinite assemblage. These features suggest that the sandstone has been leached/altered by a relatively recent post-diagenetic fluid.

A number of factors suggest that the formation of the mixed-layer illite-sudoite clay is a result of late meteoric water interaction with an illite- and sudoite­bearing sandstone (±kaolinite/dickite). These include: (a) the preferential development of illite-sudoite clay in bleached, leached, limonitic sandstone near the subcrop surface, (b) proximity to intervals ofsudoite­bearing sandstone, ( c) the poorly crystalline character of the kaolinite, and (d) some stable isotopic evidence for re-equilibration with late meteoric water.

3. Conclusions

Two varieties of green sandstone have been id~ntified; one variety is kaolinitic (+illite) and the other 1s chloritic (sudoite; ±illite).

The green kaolinitic sandstone contains well­crystallized peak-diage?etic kaolin (kaolinite and/or . dickite). The green honzons are typically developed m relatively fine- to medium-grained sandstone, often at contacts between hematitic coarse-grained beds and bleached lighter-greenish to off-white finer-grained beds. There are no significant petrographic, mineralogical, or stable isotopic differences_betwe~n greenish bleached sandstone and the off-white vanety. Chemically, the green sandstone contains elevated amounts of Fe2+ in kaolin. The ferrous iron was probably incorporated into the kaolin structure during peak-diagenetic recrystallization and likely came from the breakdown of detrital chlorite and biotite, trace amounts of which still remain in the sandstone.

Green chloritic sandstone is often matrix-rich and is commonly restricted to the vicinity of the sub­Athabasca unconformity. Chlorite formation is attributed to local basement-sandstone diagenetic fluid interaction (Mg alteration) in the basal sandstone with

/30

the most likely source of the magnesium being the basement lithologies. Chloritization of the clay mineral matrix occurred through replacement of kaolinite (±illite) by aluminous chlorite (sudoite).

As green sandstone can contain either sudoite or kaolin, depending on distance from the sub-Athabasca unconformity, attempts to determine the dominant matrix clay through simple logging of the sandstone colour should be avoided. Matrix mineral identification should be carried out through mineralogical and/or chemical analyses. Chemically, green chloritic sandstone contains elevated magnesium, nickel, chromium, and uranium contents relative to illitic and kaolinitic sandstone.

Illite-chlorite mixed-layer clay consists of interstratified illite-sudoite. This clay is associated with: (a) the Athabasca sandstone subcrop and/or fracture/fault zones extending to the subcrop, (b) relatively poorly crystalline kaolinite, and (c) bulk sandstone stable isotopic data suggestive of re­equilibration with low-temperature mid-latitude meteoric waters. These associations are consistent with an origin for the illite-sudoite through late (Recent?) meteoric water interaction with, and alteration of, a precursor illite-, sudoite-, and kaolin-bearing mineral assemblage. Sandstone containing illite-sudoite returns erroneous geochemical normative chlorite values due to the magnesium present in this mixed-layer clay mineral.

4. Acknowledgments Financial support for this study was provided by Cameco Corporation, Cogema Resources Incorporated, PNC Exploration (Canada) Company Limited, Uranerz Exploration and Mining Limited (now part ofCamec? Corporation), and the Saskatchewan Research Council. The electron microprobe analyses and the stable isotope analyses were performed at the University of Saskatchewan by Tom Bonli and Dave Pezderic, respectively. Thanks are also given to the two Saskatchewan Geological Survey reviewers who suggested ways to improve the manuscript.

5. References

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Hoeve, J. and Quirt, D.H. (1984): Mineralization and host rock alteration in relation to clay mineral diagenesis and evolution of the Middle­Proterozoic Athabasca Basin, northern Saskatchewan, Canada; Sask. Resear. Counc., Pap. R855-2-A-84.

Summary of Investigations 1999, Volume 2

Hoeve, J., Rawsthom, K., and Quirt, D.H. (1981): Uranium metallogenic studies: Clay mineral stratigraphy and diagenesis in the Athabasca Group; in Summary of Investigations 1981, Saskatchewan Geological Survey, Sask. Dep. Miner. Resour., Misc. Rep. 81-4, p76-89.

Kotzer, T.G. and Kyser, T.K. ( 1995): Petro genesis of the Proterozoic Athabasca Basin, northern Saskatchewan, Canada, and its relation to diagenesis, hydrothermal uranium mineralization and paleohydrology; Chem. Geol., vi 20, p45-89.

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Reynolds, R.C. Jr. (1980): Interstratified clay minerals; in Brindley, G. W. and Brown, G. (eds.), Crystal

Saskatchewan Geological Survey

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131