Structural Geology of the Deilmann Orebody, Key Lake,

S.E. Harvey I

Harvey, S.E. (1999): Structural geology of the Deilmann Orcbody, Key Lake, Saskatchewan; in Summary oflnvestigations 1999, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2.

1. Introduction The Gaertner orebody was approximately 800 m long, 10 to 40 m wide, and as much as 50 m thick; the larger This report summarizes of field work undertaken this Deilmann orebody was about 900 m long, 30 to 50 m past summer at the Deilmann Pit, Key Lake wide and 90 m thick (Ruhnnann, 1987). Both mine, northern Saskatchewan. The work represents the orebodies have been mined out and yielded field component ofan M.Sc. thesis study, aimed at approximately 4.2 million tonnes of ore with an investigating the relationships between lithology, average grade of2. l percent. structure, alteration, and mineralization, with emphasis on possible structural controls on mineralization.

The Key Lake deposit is an unconformity-type 2. General Setting uranium deposit (Hoeve and Sibbald, 1978; Ruzicka, At Key Lake the Athabasca Group comprises up to 1993) at the southeastern edge of the Paleo- to 60 m of non-marine flu vial sandstone and Mesoproterozoic (Figure l ). conglomerate of the Manitou Falls Formation Basement to this part of the basin is supracrustal and (Ramaekers, 1989). Regionally, basement to these granitoid rocks of the western part of the Wollaston sedimentary rocks is characterized by a northeast­ Domain. The consisted of two structural grain, and comprises four major Archean orebodies, the Gaertner, which was discovered in 1975, inliers fl anked by Paleoproterozoic Wollaston Group and the Deilmann, which was discovered along strike sedimentary rocks and Hudsonian intrusive rocks (Ray, to the northeast in 1976 (Ruhnnann, 1987) (Figure 2). LEGEND: MUDJATIK DOMAIN Mainly pejitic gneisses with iron fonnatlon .. Felslc gneisses, CJ mainly plutonic WOLLASTON DOMAIN PM.EOPROTEROZCHC COVER D Late granitoids D Cale-silicate rocks D Cak:areoos meta-arkose Meta-8/kose, calcareous meta­ D arkose, minor psammopelite 1111 Pelltic gneiss, psammopeMte Red beds, including meta­ arkose, fanglomenrate. minor CJ slate and meta-volcanics

ARCHE>,N C()f(TINEHTAI. CRVST O Felsic orthogneisses PETER LAKE DOMAIN r.-:-:i Felsic-mafelsic plutons, t..:...... J dionte, and gabbro

ROTIENSTONE DOMAIN 177,71 Biotite and homble".'dic . ~ gnejsses and trondjhemlte-tonaMte WATHAMAN BATHOLTIH

--Major fault or shear zone ~ Wathaman granite: t....__J massive monzogranite Figure J - Location map ofth e Key Lake Uranium mine in the so11theasrem co111er ofth e Athabasca Basin and the west side ofthe Wollaston Domain (modified from and courtesy of H. Tran).

I Department of Geology, University of Regina, Regina, SK S4S OA2.

JOO Summary of Investigations I 999, Volume 2 Athabasca Group sedimentary rocks. Clay alteration is very common and detailed analysis ha~ yet to _be accomplished, therefore, clay content m the different rock types is, as yet, unresolved.

a) Wollaston Group

Psammopelite

Psammopelite occurs as grey-yellow: (depen~ing on intensity of bleaching), fine- to medmm-gramed, well­ layered and foliated gneissic _rocks that ai:e. commonly interlayered with, and grade mto, psamm1t1c rocks. ---,~--· Most have a strong gneissic texture defined by ,•., I \~ centimetre-scale concordant granitic leucosome. ! • ~ • Variably altered biotite composes IO to 20 percent of :6 5 the rock along with 1 to IO percent rare garnet observed in the less altered areas. Graphite content IT21l Archean fnliers ~ o ~~-195 t2.GJ r 'UVU\..,1 •" Approximate sowhern limit appears to grade from 2 to 5 percent at the base up to a D Aphebkm Wollasfon Gloup .J\ Lakes "" of A1t>:Joosca Group metosedimenby rocks "\...J trace. Highly altered cordierite porphyroblasts are very /•Faults ,,{ Antlfam Axis J' Synform Axis rare. Within fault zones biotite is commonly altered to white mica (sericite or muscovite) probably due to Figure 2- Basement geology ofthe Key Lake area; hydrothermal fluids. This type of biotite alteration was modifiedfromRay (1977) and Ruhrmann (~987) an~ also observed at the Midwest Uranium deposit, near geophysical interpretation from Lehnert-Thiel and Rich (Ayres et al., 1983). (1976, SEM Assessment File 74H-0013); 1, Orchid Lake inlier; 2, unnamed inlier; 3, Zimmer Lake inlier; and 4, McMillan Lakes inlier. Psammite

1977; Ruhrmann, 1987) (Figure 2). These rocks have This unit is present only in minor amounts. It is a buff­ been subjected to five episodes of deformation; upper yellow to grey, medium-grained, weakly foliated amphibolite to lower granulite facies metamorphism quartzofeldspathic rock with less than IO to 15 percent was attained during some of the early events (Ray, mafic minerals. The unit has a thickness of 1977). Both the metamorphosed basement rocks and approximately 20 m and is commonly interlayered with the Athabasca Group have been intruded by later the psammopelitic rocks. In altered zones . diabase dykes that were introduced at approximately distinguishing the two rock types can be difficult. 1350 to 1000 Ma (Armstrong and Ramaekers, 1985), which is close in age to the average K-Ar date for the Graphitic Pelite Mackenzie dykes, dated at 1320 Ma (Fahrig and Wanless, 1963) and more recently dated by Grey- to dark-grey, fine- to medium-grained, graphitic Lecheminant and Heaman (1989) at 1267±2 Ma. petite forms two mappable units and other thin units intercalated in psammitic to psammopelitic The Deilmann orebody is located close to the metasedimentary rocks. The graphite-bearing pelite is northwest side of the Archean Zimmer Lake inlier. thinly layered, strongly foliated to mylonitic and Wollaston Group sedimentary rocks exposed in the pit commonly contains decimetre- to ~etre-scale, . are pelites and psammopelites with variable graphite foliation-para! lei, sheets of pegmatlte that constitute as content, along with lesser psammitic and calc-silicate much as 70 percent of the unit. High-strain fabrics are bearing sedimentary rocks (Figures 3 and 4). These are concentrated in discrete zones ranging from a few to part of the lower sedimentary sequence recognized by several metres wide. Within these, mylonitic fabrics are Tran and Yeo (Tran et al., this volume; Yeo and common as is dismembered neosome. Graphite Savage, this volume; Tran et al., 1998) in the composes IO to 40 percent of the petite and occurs Wollaston Domain. These sediments contain abundant with 5 to 15 percent biotite. Bluish-grey porphyroblasts partial melt phases and are cut by pegmatite of various of cordierite, 2 to 15 mm in diameter, are rarely ages. Various alteration phases superimposed on rocks preserved and form IO percent of the rock. Gamet exposed in the pit makes it hard to determine protoliths porphyroblasts are rare, and where observed are highly especially in the basement rocks. altered.

3. Description of Rock Types in the Graphitic Psammopelite Deilmann Pit Grey- to grey-green, fine- to medium-grained graphitic The following section contains detailed descriptions of psammopelite (Figure 5) commonly contains 2 to 10 the Wollaston Group sedimentary rocks and intrusive percent graphite and locally as much as 15 percent. rocks in the basement along with the overlying

Saskatchewan Geological Survey 101 LEGEND

D Athabasca Sandstone lli!lll Graphitic Psammopelite D Pegmatite • Graphitic Pelite 1111 Cale-silicate-bearing psammite to psammopelite D Psammite 0 100 200 300 interlayered with non-calcareous psammopelites D Psammopelite ---

Figure 3 - Preliminary geological map ofthe Deilmann Pit. Relict garnet porphyroblasts A 9+900N 9+800N 9+ 700N B are also seen in quantities less .------,------soom than 5 percent. Like the ";\ pelitic unit, this one is well •,"..Tl foliated and locally has a \~·\ .... ···\ mylonitic fabric. The ;.\\·.:::-:-.\. graphitic psammopelite has a ::::.;::::-::: --.i. weak to moderate ~-~7~~------cf~ compositional layering . ·-·,--,}. defined by variations in . -.. 2'-:-:~L. .---.- .~:·:~:) biotite. - ·/.- ·_·r.. .. .i-·1 ··~ .. -I- .. -,. . -i--- ·-- ,-- fj_·.~~ .·-·· . . :t·-··1 Cale-silicate-bearing +.l l.--··- Psammite to Psammopelite /:1~-·. ·.-=J 400m SYMBOLS Light-green to pink, medium­ t grained, thinly layered and ------LiLhalog~ Contacts 1! ~t_:_ -~----:_.~.J moderately foliated calc­ .. .. -LEGEND Alhabesca S•ndstone ,.-...... , .,,_ Prot.o-mylorut,c lo myton 111c fabfic E3 Lale cross·cumno pegmatiles ··,. E-W Brittle F•ults 'Ke y lak.o Faull 2:one' ~ silicate-bearing psammite to D Syn- to posl- 01 foliation sub·parallel pegmatit1;1 ...... • (Main E-W revorse fault} f'.x1 CaJc-silicat&bftilring psammite to psammopeljte psammopelite is commonly ~ Graphjtic Psa.rttmope!ile "'° 41 • - Unconformity interlayered with non­ ~ ApproJCirnal1;11 trace of orebody • Graphilic Pelito calcareous psammite to (0.1% cut off) 0Psammile EJ Pummope!ite psammopelite. These rocks typically contain trace to Figure 4 - Schematic cross-section across the east-end of the Deilmann orebody, along approximately IO percent line 12+050E. calc-silicate minera1s (now

102 Summary oflnvestigalions 1999, Volume 2 Group (Ramaekers, 1979). This unit comprises interbedded medium-grained quartz sandstone, conglomerate and minor, thin, discontinuous layers of clay and silt. At the base, conglomerate varies in thickness from centimetres up to 2 m, with clasts ranging in size from l cm to over half a metre in diameter. Up section, clast size decreases along with conglomerate beds thinning and becoming more infrequent. Sandstone beds are commonly cross bedded; some are graded. The b unit of the Manitou Falls Formation was deposited in a braided stream setting (Ramaekers, 1979).

As elsewhere in the Athabasca Basin, diagenetic, dark purple, specular hematite (specularite) is common in the sandstone, except were later fluid flow associated Figure 5 - Medium-grained graphitic psamnwpelite with with faults has removed it. Silicification, which is also melt segregations. Card is 9 cm wide. common, is represented by quartz overgrowths epidote, inferred to have altered from diopside), and 2 (Ruhrmann, 1987). to 15 percent biotite. Locally epidote is concentrated in pods and sweats up to 15 cm wide that parallel the Within the pit area, the competency of Athabasca rocks main foliation. is variable, with very stable, yellow to maroon weathering indurated sandstone as the norm. Locally there are 10 to 60 m wide bleached (white) zones b) Hudsonian Intrusive Rocks where the sandstone is very friable and resembles an unconsolidated sand. These friable bleached zones Early Pegmatite appear to coincide with late faults.

Yellow-white to pink, coarse-grained pegmatite occurs Athabasca Group sedimentary rocks have been dated at as lit-par-lit stringers (centimetre-scale) and sheets 1428 ±30 Ma (Ramaekers, 1979), 1450 ±30 Ma (metre to decametre scale) parallel or sub-parallel to (Armstrong and Ramaekers, 1985), and 1513 ±24 Ma the main S1 foliation of gneissic to mylonitic host (Bell, 1981) using the Rb-Sr method. Blenkinsop and rocks. Regionally, similar pegmatite intrusions are also Bell (1981) came up with similar Rb-Sr dates (i.e. 1450 observed to increase in frequency towards the ±30 Ma) for the Wolverine Point Formation, which basement/cover contact (Tran et al., this volume; Yeo overlies the Manitou Falls Formation (Ramaekers, and Savage, this volume), where they are also 1989). More recently, Cumming and Kristie (1987), associated with high-strain zones. Some pegmatite suggested older ages for Athabasca Group sedimentary sheets are dismembered or boudinaged within the rocks at 1700 to 1650 Ma, based on dating apatite of mylonitic foliation, whereas most appear to intrude it. the Wolverine Point Formation. These relationships suggest that pegmatite intrusion was syn- to post-mylonitization. The pegmatite sheets have also been folded by the northeast-trending D, 4. Structural Geology folds. Annesley et al. (1997) dated similar pegmatitic rocks within the Wollaston Domain that gave an age of Five phases of deformation are distinguished. Primary approximately 1820 to 1803 Ma, which is likely close features are not preserved in the basement rocks, to the age of peak metamorphism. although primary cross bedding has locally been observed at Highrock Lake, IO km southeast of Key Lake (Yeo and Savage, this volume). Late Pegmatite

Pink, coarse-grained pegmatite dykes cross-cut all a) First Deformation (D1) basement units. This type of pegmatite is rarer than the Deformation during 0 resulted in a well-developed earlier phase and shows little or no evidence of 1 penetrative regional foliation (S 1) oriented parallel to deformation. It contains large books (2 to 10 cm) of sub-parallel with the basement/cover contact. The biotite along with large K-feldspar crystals. Late foliation is defined by planar preferred orientation of pegmatite appears to be more alkalic than the earlier the peak metamorphic minerals, along with early pegmatite rocks. anatectic neosome. The foliation is paralleled by transposed compositional layering, and coeval isoclinal

c) Athabasca Group fold (F 1) axial planes (Figure 6). These isoclinal folds resulted in bed repetition both on the outcrop and Manitou Falls Formation (Mfb) regional scale, and may account for repetition of the graphitic pelite unit, with a fold closure outside of the Basement rocks are unconformably overlain by unit b pit. of the Manitou Falls Formation of the Athabasca

Saskatchewan Geological Survey 103 Late D 1 Mylonitization On the regional scale, a strain gradient is apparent as an increase in intensity of the S1 foliation in the Wollaston Group towards the basement/cover contact, with development of discrete mylonitic zones (Figure 7). In the Deilmann pit there are a number of thin ( <2 m), discontinuous high-strain zones, marked by protomylonitic to mylonitic fabrics superimposed on the earlier gneissic fabric. These high-strain zones appear to have focused in the graphitic pelitic and psammopelitic units (Figure 3 ). The mylonite zones are very friable and identifying a stretching lineation has proven difficult, although rare evidence suggests a down-dip stretching lineation.

Figure 6 - F, isoclinal fold.folding original bedding (S ) Associated with this increase in strain towards the 0 basement/cover contact is an apparent increase in within a psammopelitic rock. abundance of yellow-white pegmatite. In the pit area, the early pegmatite is judged to have intruded syn-to post-D1 to D2 based on the fact that the pegmatite sheets are clearly folded by DJ open folds, and may

have been folded during D2• However, given the lack of direct evidence of the latter relationship, a more conservative, though less precise age bracket, is syn- to post-01, pre-D3

b) Second Deformation (02)

0 2 caused refolding of the S1 foliation by very rare, minor, tight to isoclinal west-southwest-trending,

steeply dipping F2 folds. Distinguishing the isoclinal F2 folds from the isoclinal F1 folds is difficult, because the F2 axial planes commonly lie parallel to the main S1 foliation. Locally, a weak axial planar fabric (S2) is observed, most commonly in F fold hinges. Figure 7 - Typical beaded mylonitic fabric within the 2 graphitic pelite to psamnwpelite. Dismembered leucosome exemplifies SJoliation. c) Third Deformation (D3)

D3 resulted in the pervasive northeast-trending structural grain in the Wollaston Domain. Close to open northeast-trending folds are characteristic, both at the outcrop and regional scales. These folds plunge shallowly to moderately towards the northeast, and dip moderately to steeply towards the southeast and less commonly steeply towards the northwest. The folds display "Z" asymmetry, which is consistent with being on the northwest limb of a regional-scale F 3 anticline, cored by the Zimmer Lake inlier. The folds have an associated axial planar foliation (S3 ) best preserved in the hinge zones. These folds refold the earlier composite gneissic to mylonitic fabrics, along with the concordant pegmatite (Figure 8) sheets indicating that shearing and the main pulse of pegmatite intrusion was at least pre-D3. Figure 8 - Open F3 folds refolding a late-DI mylonite fabric that developed in a graphitic psammopelite, and a late D, to d) Fourth Deformation (D4) pre-D3 pegmatite (pegmatite at base ofphoto; proto­ myloniticfabric below hammer). D4 fonned open to very open northwest-trending, subvertical F4 folds. These folds are quite rare and are e) Fifth Deformation (D ) commonly seen as broad warps that refold previous 5 structures. 0 5 formed late brittle to brittle-ductile faults that trend in two general orientations: east-west and northwest­ southeast (Figure 3). Both sets of faults appear to have been reactivated over time. The northwest-trending set

104 Summary of Investigations I 999, Volume 2 of faults are generally discrete, steeply dipping faults moderately to steeply towards the north (Figure I 1). that commonly exhibit a curvilinear shape and appear They are commonly anastomosing and discontinuous. to locally anastamose (Figure 9). They are generally Within the basement, faults are preferentially located in less than 0.5 m wide and are commonly marked by highly strained graphitic units and commonly coincide zones of clay gouge. Whereas faults of this type are with the earlier mylonitic fabric. The most prominent is well developed in the basement rocks, they only locally the 60 to 100 m wide Key Lake Fault Zone centered on penetrate into the Athabasca sandstone. This suggests the graphite units. In this zone there are a number of that the faults formed prior to deposition of the discrete faults including a major fault located near the Athabasca sandstone. Subsequent to deposition of the southeast side (Figure 4). This major fault, which Athabasca sedimentary rocks, they were locally and appears closely related to the mineralization, displays weakly reactivated to produce offsets on the scale of the most vertical displacement and is the most centimetres to a few metres. In the basement rocks reactivated. Associated with this main fault is the these faults show predominantly sinistral oblique­ boudinage of the early white-yellow pegmatitic rocks, offset; in the overlying sandstone, they show mostly and convolute folding within the graphitic pelite. The normal but locally reverse offsets (< l m) of pebbly main fault shows major post-Athabasca reactivation layers. In both the basement and Athabasca rocks, defined by clear offset of the unconformity surface and slickenside striae indicate both dip-slip and strike-slip the overlying sedimentary rocks as well as brecciation components, and may only show latest directions of of the sandstone and block rotation. These east-west­ relative motion (Figure l 0). Within the pit area there is striking faults appear to have a ductile component of little evidence of brecciation associated with these deformation defined by a transposition and steepening faults. of the main foliation towards the faults and isolated ductile dragging of the surrounding foliations. This The east-west-trending faults are the dominant suggests that initial faulting occurred earlier and at structures in the Deilmann pit and are oriented parallel deeper crustal levels than the northwest set. to sub-parallel to the S1 foliation (Figure 3). These faults all strike approximately east-west and dip

Figure 10- Steeply plunging striae developed in a northwest-trending fault within the Athabasca sandstone.

Figure 9 - Northwest-trending discrete fault with common sub-vertical orientation within the Paleoproterowic Figure 11 - Moderately dipping east-west-trending reverse metasedimentary rocks (note: anastomosing nature of fault oriented parallel to S,foliation (fault runs bottom fault; runs bottom right to lop left). right to top left).

Saskatchewan Geological Survey 105 The main east-west faults are north-dipping reverse A paleovalley, or a break in the unconformity slope faults (Figure 12) which show offsets of less than (Figures 13 and 4) occurs above the highly sheared and 10 m. Slickenside striae within these faults trend north­ faulted graphitic units of the Key Lake Fault Zone. The northeast with a moderate plunge suggesting axis of this feature parallels the strike of the highly movement towards the southwest. Unlike the sheared graphitic units. Conglomeratic layers appear to northwest-trending faults, the east-west faults c0ntain onlap the erosion surface, indicating early valley-fill black graphite 'gouge'. within this paleovalley. The break in slope (8.5°) has a relief of approximately 15 m over a horizontal distance of I 00 m. This paleovalley could have played a role in ore formation as it would have exerted some control on fluid flow at or near the unconformity surface. This aspect requires further study. This type of slope break is present at other unconformity-type uranium deposits in Saskatchewan. For example, at the Midwest deposit, Ayres et al. (1983, p37) discuss "a pronounced break in slope of the unconformity [that] is coincident with the pelitic zone and probably reflects an intensive shear zone".

5. Alteration Within the pit there are various types of alteration: retrograde, regolith, several stages of diagenesis, and hydrothermal alteration. Post Trans-Hudsonian retrograde alteration of metamorphic minerals is believed to be minor relative to alteration associated with weathering of Precambrian rocks at the unconformity surface (i.e. paleoregolith) and still later hydrothermal alteration associated with the orebody. As retrograde alteration is minor and strongly overprinted it will not be discussed further. The characterization of alteration within the pit will require detailed geochemical and petrophysical analysis to be undertaken in the future. This discussion on alteration is, therefore, based on field observations and work of others (e.g. Wilson and Kyser, 1987; de Carle, 1986; and Macdonald, 1980) elsewhere in the Athabasca Basin.

a) Paleoregolith Like elsewhere in the Athabasca Basin a paleoregolith Figure 12 - East-west-trending brittle-ductile faults is developed in basement to the sub-Athabasca developed within a graphitic pelile oriented sub-parallel to unconformity and extends down metres to tens of the main S,foliation (Faults at hammer, spray paint and metres in the basement. In the Deilmann pit there is a notebook; note the abundant pegmatitic material associated characteristic v~rtical zonation to the regolith, with a with ,:raphitic pelite). lower green part, an upper red part and a narrow "bleached zone" very close to, or along, the unconformity surface. The green zone is predominantly the result of the chloritization of biotite, cordierite, and garnet, accompanied by illitization of feldspar. In contrast, the alteration assemblage in the overlying red zone comprises predominantly hematite, kaolinite, and quartz. The characteristic red colour is a result of the hematization of Figure 13 - Trace ofthe unconformity surface illustrating a probable paleovalley biotite. The uppermost "bleached (dashed line coincides with unconformity). Approximate trace ofmain reverse fault in zone" is normally composed the Key lake Fault Zone. mostly of kaolinite and quartz.

/06 Summary of Investigations J 999, Volume 2 The thickness of the bleached zone varies from Oto] m overprinted by fault related kaolinitization and and appears to be in sharp but irregular contact with bleaching. Early, extensive illitization is only locally the lower hematitic zone, suggesting that bleaching observed on the margins of the deposit but can be overprints the earlier hematitic alteration and is post­ traced for large distances away from it (Wilson and Athabasca Group (i.e. not regolithic). Furthermore as Kyser, 1987). Overprinting this, and associated with this alteration is closely associated with the fault reactivation, is an inner zone where kaolinite is unconformity and is likely due to fluids circulating the dominant clay mineral within the matrix along it. The timing of this alteration is unknown, but (Ruhnnann, 1987). At and within approximately 60 m when looking at proximal and distal core in the Cigar of the main fault zone is an inner core of intensive Lake area, Halter (1988) observed the disappearance of bleaching. This bleached zone is characterized by alteration similar to this away from the orebody, and extremely friable, white sandstone, apparently void of concluded that it accompanied mineralization. original cement.

Using the Rb-Sr method, Fahrig and Loveridge (1981) determined that the pre-Athabasca regolith began to 6. Discussion develop prior to 1632 ±30 Ma. At Key Lake it is apparent that structure played a b) Diagenetic Alteration critical role in the formation of the uranium deposits. Late- to post-0 1 heterogeneous high strain was Several periods of post-Athabasca diagenetic alteration concentrated in the graphitic units proximal to the have included kaolinitization, illitization, several stages basement/cover contact and formed metre-wide ofhematization, and silicification. Fluid inclusion mylonitic zones parallel to sub-parallel to the S1 fabric. studies have indicated that temperatures reached During late D1 or 0 2 these high-strain zones were approximately 200°C (Fayek and Kyser, 1993) intruded by abundant sub-concordant pegmatite dykes. indicating that diagenesis was high grade. Diagenesis Similar pegmatites elsewhere have dated at 1820 started just after initial deposition of the Athabasca to 1803 Ma (Annesley et al., 1997). The high-strain sediments and continued for the next several million zones, along with pegmatite sheets, were refolded by years (de Carle, 1986). Most notable in the Key Lake northeast-trending open to closed 0 3 folds, which, in area is the common presence of specular hematite the Key Lake area, are commonly Z-asymmetric which is present as dark red to purple features that consistent with regional D3 fold vergence related to the appear to crosscut bedding. Clay alteration has Zimmer Lake inlier. commonly resulted in approximately equal proportions of kaolinite and illite in the sandstone matrix, however, Much younger brittle to brittle-ductile faults occur in near the mineralization, this is overprinted by two major sets. One group is northwest trending and hydrothermal illitization and kaolinitization (Wilson comprises discrete sub-vertical sinistral faults which and Kyser, 1987). appear to have been mostly active prior to deposition of the Athabasca Group with only local post-Athabasca rejuvenation. Within the sandstone these faults are c) Hydrothermal Alteration observed as normal faults with minor reverse In many of the unconformity-type uranium deposits of displacements. Of greater significance to , three main stages of ore formation are mineralization is a second set of more ductile, east­ distinguished. The initial uranium concentration west-trending reverse faults that fonned in early shear occurred at approximately 1350 Ma and was followed zones in the graphitic units. Most important is the 60 to by later remobilization and recrystallization of uranium l 00 m wide Key Lake Fault Zone which developed in at about 900 Ma and 300 Ma (Fayek and Kyser, 1993). sheared graphitic rocks. Near the southeastern side of These three stages of ore formation were accompanied the Key Lake Fault Zone is a major reverse fault which by three major hydrothermal fluid events, and, as such, appears to have controlled ore formation as the main the hydrothermal alteration within the ore deposits can orebody is located where this fault intersects the be quite complex. Distinguishing the different unconformity. Like the northwest-trending set, these alteration events is difficult, and at this point only east-west-trending faults appear to have been general alteration features will be discussed. reactivated one or more times after deposition of the Athabasca Group rocks. Only a few of these faults In the Deilmann pit basement rocks, a zone of have extended into the Athabasca sandstone, and is chloritization and illitization within tens of metres of exemplified by the main reverse fault causing the main fault in the Key Lake Fault Zone and ore zone sandstone brecciation, silicification and intense is recognized. According to Wilson and Kyser (1987), bleaching. the chlorite mainly within the ore zone is Fe-Mg chlorite, whereas peripheral to this the chlorite is a Mg Understanding the relationship between mineralization variety. The accompanying illitization is exemplified and structure has been hindered by the mining of the by the conversion of the 'bleached' zone at the orebody. Analysis of cross-section data, however, unconformity from predominantly kaolinite-quartz to indicates that the orebody coincides with the illite-quartz. intersection of two planes: the southeastern most set of east-west-striking reverse faults and the paleovalley In the Athabasca sandstone, alteration is expressed by developed on the unconformity. Ore shoots, running three major phases with an early illitization being into the basement are contained in the main set of

Saskatchewan Geological Survey 107 reverse faults, suggesting that ore forming fluids Future work includes of characterization of the migrated along this pre-existing structure. structure-mineralization relationship utilizing thin section and core information, along with analysis of the Most models of unconformity-type uranium deposits structural data set collected in the field. Geochemistry postulate that uranium is derived from the Athabasca will be completed in order to further investigate the formation and transported by basinal fluids until relationships between alteration types and the role reacting with reducing basement fluids at, or near, the structure plays in its formation. And finally the use of intersection of the sub-Athabasca unconformity and computer techniques to develop a 3-D model of the major faults (Hoeve and Sibbald, 1978). Perhaps an Deilmann pit geology. alternative source for the uranium is syn-metamorphic uraniferous pegmatite. The genetic relationship between the basal graphitic units and yellow-white pegmatite is interesting in the fact that throughout the 8. Acknowledgments Wollaston Domain uraniferous pegmatite is commonly The author would like to thank Kathy Bethune for located within basal pelitic units (Ray, 1977; Thomas, discussions and guidance in the project, along with 1983). The basal pelitic rocks within the Cree Lake Gary Delaney for initiating the project. Extreme zone contain anomalous U and Th compared to other gratitude goes out to Corporation, Cogema amphibolite-facies biotite-rich rocks (Thomas, 1983). Resources Inc., and PNC Exploration (Canada) Co. These rocks were once organic-rich sedimentary rocks Ltd. for financial and logistical support. Visits from which commonly contain relatively high uranium Kathy Bethune, Bruno Lafrance, Vlad Sopuck, and concentrations, and according to Tilsley ( 1988) the Dan Brisbin aided greatly in the interpretation of the conversion of organic matter to graphite during area. A more regional view was achieved at Hai Tran's metamorphism, is accompanied by the release of and Gary Yeo's camps, and their hospitality is greatly uranium. Upon the conversion to graphite, the crystal appreciated. lattice will not accommodate uranium, and the uranium is expelled. The pegmatite suite that intrudes the basal pelite is found within restricted areas of amphibolite- to 9. References granulite-facies metamorphism. It follows that during high-grade metamorphism, U present within mineral Annesley, l.R., Madore, C., Shi, R., and Krogh, T.E. crystal lattices becomes unstable, and when syn­ (1997): U-Pb geochronology ofthermotectonic metamorphic pegmatite is intruded into these rocks it events in the Wollaston Lake area, Wollaston scavenges and concentrates the U. Domain: A summary of 1994-1996; in Summary of Investigations 1997, Saskatchewan Geological This leads one to speculate as to whether the yellow­ Survey, Sask. Energy Mines, Misc. Rep. 97-4, white pegmatite associated with graphitic pelite at the pl62-173. Key Lake mine once had anomalous concentrations of uranium, which were later removed and concentrated Armstrong, R.L. and Ramaekers, P. (1985): Sr isotopic by hydrothermal fluids. study of Helikian sediment and diabase dykes in the Athabasca Basin, northern Saskatchewan; Can. J. Earth Sci., v22, p399-404. 7. Summary Ayres, D.E., Wray, E.M., Farstad, J., and Ibrahim, H. The Deilmann orebody, like many other uranium (1983): Geology of the Midwest Uranium Deposit; deposits in the Athabasca Basin is located at the in Cameron, E.M. (ed.), Uranium Exploration in unconformity between Wollaston Group graphitic Athabasca Basin, Saskatchewan, Canada, Geo!. metasediments and the Paleoproterozoic- to Surv. Can., Pap. 82-11, p33-40. Mesoproterozoic Athabasca sandstone. Formation of the orebody was influenced by a number of factors that Bell, K. (1981 ): A review of the geochronology of the are intimately related. Perhaps of greatest importance is Precambrian of Saskatchewan, some clues to the presence of the graphitic pelite to psammopelitic uranium mineralizations; Mineral. Mag., v44, units proximal to the Archean basement/cover contact p371-378. where foliation parallel to sub-parallel shearing and mylonitization is concentrated (late 0 1). These highly­ Blenkinsop, J. and Bell, K. (1981 ): Saskatchewan sheared zones controlled location of possible shield geochronology project; in Summary of uraniferous pegmatite sheets (pre- 0 3) , and much later, Investigations, 1981, Saskatchewan Geological brittle-ductile east-west-trending faults (Ds). The Survey, Sask. Miner. Resour., Misc. Rep. 81-4, variably reactivated, east-west-trending faults make up p25. the Key Lake Fault Zone which is dominated by one main reverse fault that appears to have controlled ore Cumming, G.L. and Kristie, D. (1987): Age of location. The main orebody was concentrated at the Athabasca Group, northern ; Geo!. Assoc. intersection of this fault and a sub-Athabasca Group Can., Prog. Abst., vl2, p35. paleovalley, which may have had some control on fluid flow. de Carle, A. (1986): Geology of the Key Lake deposits; in Evans, E.L. (ed.), Uranium deposits of Canada, Can. Inst. Min. Metal!., Spec. Vol. 33, p 170-177.

108 Summary of Investigations /999, Volume 2 Fahrig, W.F. and Loveridge, W.D. (1981): Rb-Sr Thomas, D.J. (1983): Distribution, geological controls isochron age of weathered pre-Athabasca and genesis ofuraniferous pegmatites in the Cree Formation basement gneisses, northern Lake Zone of northern Saskatchewan; unpubl. Saskatchewan; in Current Research, Part C, Geo!. M.Sc. thesis, Univ. Regina, 2 l 3p. Surv. Can., Pap. 81-C, pl-16. Tilsley, J.E. (1988): Genetic considerations relating to Fahrig, W.F. and Wanless, R.K. (1963): Age and some deposits; in Roberts, R.G. and significance of diabase dykes of the Canadian Sheahan, P.A. (eds.), Ore deposit models, Geosci. Shield; Nature, v200, p934-937. Can., Reprint Series 3, p91-102.

Fayek, M. and Kyser, T.K. (1993): Petrography, Tran, H.T., Yeo, G.M., Bradley, S., and Lewry, J.F. chemical ages, stable isotopic compositions, and (1998) Geology of the Daly-Suttle-Middle Foster REE contents of three stages of uranium lakes area, eastern Wollaston Domain (NTS 74A- mineralization from the Athabasca Basin; in 5,-l 1, and-12); in Summary oflnvestigations Summary of Investigations 1993, Saskatchewan 1998, Saskatchewan Geological Survey, Sask. Geological Survey, Sask. Energy Mines, Misc. Energy Mines, Misc. Rep. 98-4, p48-65. Rep 93-4, pl66-173. Wilson, W.R. and Kyser, T.K. (1987) Stable isotope Halter, G. (1988): Zonalite des alterations dans geochemistry of alteration associated with the Key l'environnement des gisments d'uranium associes Lake uranium deposit, Canada; Econ. Geo!., v82, a la discordance du Proterzoique moyen, pl 540-1557. Saskatchewan, Canada; These de 3eme cycle, Universite Louis Pasteur, Strasbourg, France.

Hoeve, J. and Sibbald, T.1.1. (1978): On the genesis of Rabbit Lake and other unconformity-type uranium deposits in northern Saskatchewan, Canada; Econ. Geo!., v73, pl450-1473.

Lecheminant, A.N. and Heaman, L.M. (1989): Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening; Earth Planet. Sci. Lett., v96, p38- 48.

Macdonald, C.C. ( 1980): Mineralogy and geochemistry of a Precambrian regolith in the Athabasca Basin; unpubl. M.Sc. thesis, Univ. Sask., 151 p.

Ramaekers, P. ( 1979): The paleolatitude and paleomagnetic age of the Athabasca Formation, northern Saskatchewan: Discussion; Geol. Surv. Can., Pap. 79-10, pl 17-119.

____ (1989): Geology of the Athabasca Group (Helikian) in Northern Saskatchewan; Sask. Energy Mines, Rep. 195, 49p.

Ray, G.E. (1977): The geology of the Highrock Lake­ Key Lake vicinity; Sask. Dep. Miner. Resour., Rep. 197, 36p.

Ruhrmann, G. ( 1987): The Gaertner uranium ore body at Key Lake (northern Saskatchewan, Canada) after three years of mining: An update of the geology; in Gilboy, C.F. and Vigrass, L.W. (eds.), Economic minerals of Saskatchewan, Sask. GeoI. Soc., Spec. Pub!. 8, pl20-137.

Ruzicka, V. (1993): Unconformity-type uranium deposits; in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M. (eds.), Mineral Deposit Modeling, Geo!. Assoc. Can., Spec. Pap. 40, pl25-149.

Saskatchewan Geological Survey 109