Geology of the Highrock Lake Area, Wollaston Domain (NTS 74H-3 and -4)

G.M Yeo and D.A. Savage I

Yeo, G.M. and Savage, D.A. ( 1999): Geology of the Highrock Lake area, Wollaston Domain (NTS 74H-3 and -4); in Summary of Investigations 1999, Volume 2, Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2.

This report presents the results of I :20 000 scale Along with mapping the Wollaston-Mudjatik transition mappin~, during the summer of 1999, of about zone reported elsewhere (Tran et al., this volume), this 400 km around Highrock Lake in central Wollaston report summarizes results from the final field season of Domain (Figure I). Highrock Lake lies 2 15 km north a three-year project to examine the lithostratigraphy of and 10 km southeast of the Key Lake and structural geology of the Wollaston Domain south uranium mine. of the .

The geology of the area was previously mapped by Ray ( 1977) at I: l 00 000 scale and is also included in 1. General Geology the I :250 000 scale Geikie River compilation bedrock geology map (Ray, 1983). Highrock Lake lies in the west-central part of the Wollaston Domain, a north-northeast-trending belt of Paleoproterozoic sedimentary and Archean granitoid rocks which extends beneath the eastern part of • • • • • • 100· • • • • • • • • • • • 104" • Athabasca Basin (Figure I). Five main lithological subdivisions are distinguished in the Highrock Lake • . • • .. . • 0 50 • _"'4id;.lllf•.t. • •• area (Figure 2): • • •• • • - - - • • • •McCteanL. IOometres 58" • • • • • • • • • • • • • ·" Cl\)ar l. • • I) an Archean basement complex, which includes ...... granite, granite-granodiorite, and amphibolite; ATHABASCA BASIN 2) a Paleoproterozoic lower sedimentary sequence, comprising basal garnet-bearing petite and psammopelite, overlain by cordierite- and sillimanite-bearing psammopelite, magnetite-rich and magnetite-poor psammopelite and psammite, transitional to interbedded psammopelite and arkose; 3) a Paleoproterozoic upper sedimentary sequence of typically calc-silicate-bearing siliciclastic rocks; 4) late syn-tectonic to post-tectonic gran ites and pegmatites; and 5) Mesoproterozoic Athabasca Group sandstone. Five phases of deformation affected the Highrock Lake area. DI resulted in development of the dominant LITHOLOGY regional foliation (SI) and isoclinal folding (FI). 02 P.ieozoic refolded Fl folds to produce type 3 interference

Athaba,ca Group patterns (Ramsay, 1967) locally. Although well­ developed regionally, especially to the west (e.g. Tran Huctsonian fluusives et al. , this volume), F2 and Fl folds generally could Pekioproterozok: sedim•nl5 & volcanics not be distinguished. 03 produced the northeast­

Atchean basement trending, dominant regional F3 folds and a steeply

• Uranium mine dipping S3 axial planar foliation. D4 deformation resulted in very open, northwest-trending F4 folds. 05 Figure 1 - Regional geological sketch map ofsou th em produced late, steeply dipping, faults. Cree Lake Zone showing the location ofareas mapped as part of the Wollaston Project: A) Highrock Lake; The rocks have undergone high-temperature, low­ B) Haultain River; C) Cup-Keller lakes; D) Daly-Middle pressure metamorphism, to upper amphibolite­ Foster lakes,· and E) Burbidge- Upper Foster lakes. granulite facies. There have been at least two phases of

I Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2.

38 Summary of In vestigations 1999, Volume 2 metamorphic mineral growth, broadly coeval with the and ranges from 0.19 to 11.3 x 10·3 SJ units. DIID2 and D3 deformation events. Amphibolite xenoliths are Jess common than in Unit 2. Up to 25 percent pegmatite is found in this unit. At Key Lake, as at most other Athabasca Basin uranium occurrences, uranium is localized in fault zones at or near the unconformity between Unit 2: Granodiorite-Tonalite-Quartz Diorite Mesoproterozoic Athabasca Group sandstones and Paleoproterozoic lower Wollaston Group graphitic Granodioritic to tonalitic rocks are light grey to pinkish sediments (Sibbald, 1985; Ruhnnann, 1987; Ruzicka, grey on fresh surfaces, pale pink to white weathered, 1996, and others). The Highrock Lake area is of fine to medium grained, and generally well foliated. particular geological interest as the closest area of Maj or mineral components are quartz ( 15 to 25 extensive exposure of Wollaston Domain rocks to the percent), plagioclase (3 5 to 65 percent), K-feldspar (10 Key Lake mine. Parts of the Highrock Lake area were to 30 percent), biotite (2 to 5 percent), and magnetite intensely explored during the uranium exploration (trace to 5 percent). Magnetic susceptibility averages 5.7 x 10·3 SI units, but is highly variable, from 0.1 7 to boom of the late 1970s and early 1980s. Work included 3 airborne and ground geophysics, lake sediment and soil 22.7 x 10· SI units. Sheets and xenoliths of geochemical sampling, grid mapping, and both amphibolite, typically boudinaged, are common. Up to overburden and diamond drilling. Only a few 40 percent pegmatite melt may be present, especially showings, all associated with late pegmatite and towards the contact with cover rocks. granite, were di scovered. Exploration EM surveys, along with GSC aeromagnetic data (Figure 3) have Unit 3: Amphibolite been used in interpreting the geology of the Highrock Lake area. Amphibolite is dark greenish grey on fresh surfaces, grey weathered, medium grained, and typically moderately well foliated (Figure 4). Major mineral 2. Rock Descriptions components are quartz (<20 percent), plagioclase (60 to 75 percent), hornblende (25 to 40 percent), and The main lithological units distinguished in the field biotite (0 to 5 percent). Magnetic susceptibility are listed in Table I. Names for igneous rocks fo llow averages 5. 7 x 10·3 SI units, but ranges from 0.25 to the JUG S classification, and elastic metasedimentary 18.9 x 10·3 SI units. Up to JO percent pegmatite occurs rocks are named according to the nomenclature in it. This unit is generally not mappable. suggested by Maxeiner et al. ( 1999), except that "arkose" is used in lieu of"feldspathic psammite". All the rocks in the area, except the Athabasca sandstones, b) Wollaston Group Lower Sedimentary have been metamorphosed. Hence, for simplicity, the Sequence prefix 'meta-' has been omitted from rock names in The lower sedimentary sequence rests unconfonnably this report. Magnetic susceptibility was measured at on the Archean granitoid rocks. Five main most outcrops using an Exploranium KT-9 lithostratigraphic units were recognized in this Kappameter. sequence: garnet psammopelite/pelite, cordierite­ sillimanite psammopelite/pelite, psammopelite/ a) Archean Basement psammite, interbedded psammopelite and arkose, and arkose and calcareous arkose. Two of these units can Granitoid rocks are exposed in three north-northeast­ be further subdivided locally. trending basement inliers in the western Highrock Lake area. From east to west these are the Highrock Lake Granite, the McMillan Lakes Granite, and the Zimmer Unit 4: Garnet Psammopelite and Pelite Lake Granite (Ray, 1983). Krogh and Clark (1987) reported U-Pb zircon ages of 2600 ± 18 Ma and 2652 Garnet-bearing psammopelite/pelite is dark grey on ±20 Ma from the Zimmer Lake Granite. Three fr esh surfaces, pale grey weathering, fi ne grained, and lithologic units were distinguished in the basement well foliated. Most exposures are psammopelite, rather in lie rs: monzogran ite-granodiori te, granodiorite­ than pelite. Maj or mineral components are quartz (50 tonalite-quartz diorite, and amphibolite. percent), feldspar (25 to 40 percent), biotite ( 12 to 25 percent), and garnet (trace to 2 percent). The garnets are typically small, l to 3 mm. Although this unit is Unit l: Monzogranite-Granodiorite graphitic along the margin of the Zimmer Lake Granite, graphite was not observed in the Highrock Granitic rocks are pale pink to pinkish grey on fresh Lake area. Magnetic susceptibility averages 0.22 x 10·3 surfaces, pale pink weathered, medium to coarse SI units, and ranges from 0.14 to 0.3 1 x 10·3 SI units. grained, and typically weakly to moderately fo liated. Pegmatite ranges fro m 4 to 50 percent of the rock. These rocks are composed of quartz ( I 5 to 25 percent), K- fe ldspar (35 to 45 percent), plagioclase (25 to 40 Ex posures of this unit were only observed in five areas. percent), hornblende (3 to 5 percent), biotite (trace to 5 West of Nelson Bay, garnet psammopelite is separated percent), and magnetite (trace to 5 percent). K-feldspar from Unit I biotite granodiorite gneiss on the eastern may occur as variably fl attened porphyroblasts (Figure 1 fl ank of the McMillan Lakes inlier by a covered 4). Magnetic susceptibility averages 6.5 x 10· SI units, interval of about 60 m. On a small island north of Scott

Saskatchewan Geological Survey 39 sr,o-

Uppet Wooaston Sequence

Figure 2- Geological sketch mop ofthe Highrock Lake area.

Airborne EM conductors Ground EM conductors

Figure 3- Geophysical summary mop ofthe Highrock Lake area showing the total field magnetic pattern (Geological Survey ofCanada, 1963a, 1963b) with airhome (INPUT and EM-30) and ground (Max-Min, VLEM, HLEM, and VLF) conductors from assessment reports.

40 Summary ofInvesti gations 1999. Volume 2 Table I - Correlation between map units distinguished in the Highrock Lake area, the Burbidge Lake-Upper Foster Lake area (Tra11 a11d Yeo, 1997) to the southeast, and the Haultain River area (Tran et al., this volume) to the southwest. Mineral abbreviations: Bi, biotite; Co, cordierite; Gr, graphite; Gt, garnet; Hb, hornblende; Mag, magnetite; and Si, sillimanite.

Haultain River area Highrock Lake area Burbidge Lake - Upper Foster (NTS 74B-7 & -8) (NTS 74H-3 & -4) Lake area (NTS 74A-14) ------Upper Sequence ------I 0. Plagioclasite 17. Dolomitic marble 16. Cale-silicate rocks IO . Cale-silicate-bearing arkose 9. Cale-silicate-bearing arkose 15. Cale-silicate-bearing arkose and conglomerate 14. Muscovite schist 12. Conglomerate and arkose ------Lower Sequence ---- 9. Arkose 8. Arkose and cale-silicate- 13. Arkose bearing arkose 8. Bi-Hb-Mag gneiss 7. lnterlayered psammopelite and 11. Banded Bi gneiss arkose 7. Psammite and psammopelite 6. Psammopelite and psammite I 0. Psammopelite 6. Cale-silicate-bearing rocks 5. Gt-Co-Si-Gr petite 5. Co-Si psammopelite and petite 9. Si-Co psammopelite and pelite 4. Gt psammopelite and petite 8. Arkosic psammite to psammopelite 7. Gr pelite 6. Gr-Gt pelite and psammopelite ------Basement Complex ------4. Heterogenous orthogneiss 4. Marginal assemblage 3. Amphibolite 3. Amphibolite 5. Amphibolite 2. Granodiorite to tonalite 2. Granodiorite to tonalite 3. White granite to granodiorite- tonalite I. Quartz monzonite to granite l . Monzogranite to granodiorite 1-2. Granite to granodiorite

Island, garnet petite overlies the concealed northern psammopelite is exposed at two localities in the axes of end of the Highrock Lake Granite. On the east shore of Fl anticlines close to their intersections with late faults. the peninsula between Grest and Nelson bays, garnet psammopelite may overlie a concealed western promontory of the Highrock Lake inlier, or the western Unit 5: Cordierite-Sillimanite Psammopelite and margin of a concealed basement inlier, or lie near the Pelite axis ofan anticline in the core of the Nelson Bay synforrn . North ofHighrock Lake, garnet Cordierite- and sillimanite-bearing psammopelite/pelite is dark grey on fresh surfaces, grey on weathered surfaces, fine grained, and well foliated. About 20 percent of exposures are pelitic. Major mineral components are quartz (40 to 60 percent), feldspar (20 to 50 percent), biotite (IO to 30 percent), sillimanite (1 to 7 percent), cordierite (0 to 5 percent), and magnetite (0 to 5 percent). Variable composition which gives the rock a centimetre-scale banded appearance is probably transposed bedding. Sillimanite occurs in layers and irregular zones as two generations of faserkiesel 5 to IO mm long oriented parallel to SI (Figure 5) or S3. Cordierite occurs locally as overgrowths, but more commonly as aggregates in pegmatite. Magnetic susceptibility averages 6.2 x 10-3 SI units, and ranges from 0.2 to 27.l x 10-3 SI units. Pegmatite averages about 15 percent of the rocks, and ranges up to 70 percent. Locally this unit is migmatitic. Figure 4 - Unit 1 megacrystic monzogranite south of Nelson Bay. The amphibo/ite sheet has been folded by both tight F3 and open F4 folds.

Saskatchewan Geological Survey 41 SI units, and ranges from 0.1 to 12.6 x 10·3 SI units. Magnetic susceptibility for Unit 6b averages 20 x I 0·3 SI units and ranges from 2.7 to 60 x 10·3 SI units. Pegmatite averages 18 percent of the rocks and ranges up to 75 percent in migmatite.

Unit 7: Interbedded Psammopelite and Arkose

Interbedded psammopelite and arkose is transitional between Units 6 and 8. The arkose component is generally predominant and forms much thicker layers than the psammopelite layers. The arkose layers average about 25 cm and range from 3 to I 00 cm thick. Magnetic susceptibility in the arkose layers averages 5.6 x 10·1 SJ units and ranges from 0.05 to 30.9 x 10·1 Figure 5 - Unit 5 magnetite-bearing sillimanite SI units. Pegmatite typically composes about 17 psammopelite east ofBennett Bay. Sillimanitefaserkiesel to percent and locally up to 50 percent of the rocks. the left ofthe scale card have been refolded by the FJ folds. Unit 5 was not observed in direct contact with Unit 4. Unit 8: Arkose and Cale-silicate-bearing Arkose The apparent lack of any transitional lithology suggests that the basal contact of Unit 5 is sharp. Arkose is pink, pale grey, or greenish on fresh surfaces, and weathered is pale pink, white, or greenish grey. It is fine to medium grained, and weakly foliated. Unit 6: Psammopelite and Psammite Major mineral components are quartz (35 to 60 percent), feldspar (30 to 60 percent), biotite (0 to 10 Psammopelite and psammite are dark grey to grey on percent), and magnetite (0 to 5 percent). Although fresh surfaces; weathered grey to light grey, fine subarkose is found locally, no true orthoquartzite was grained, well foliated, and gradational from Unit 5. recognized. Layering, defined by variable composition, About IO percent of exposures are psammite. Major is probably transposed bedding. Layers average 3 8 cm mineral components are quartz (40 to 60 percent), and range from 2 to 400 cm thick. Two gradational feldspar (20 to 55 percent), and biotite (2 to 25 types of arkose can be distinguished. Unit 8a is non­ percent). Variable composition which commonly gives calcareous and Unit 8b is calc-silicate-bearing arkose. the rock a centimetre-scale banded appearance is likely The latter is more common, and contains diopside transposed bedding. Two types of psammopelite can be (trace to 20 percent), hornblende (0 to 15 percent), distinguished. Unit 6a is a magnetite-poor calcite (trace to 5 percent), and ilmenite (0 to 2 psammopelite/psammite and Unit 6b is a magnetite­ percent). Diopside and hornblende grains and bearing psammopelite/psammite. Magnetite in the aggregates are commonly oriented parallel to S3 . latter ranges from l to 5 percent, and typically occurs Magnetic susceptibility is similar for both subunits, as cores of feldspar-rich lenses 5 to 20 mm long, averaging 5.7 x 10-3 SI units and ranging from 0.04 to typically flattened parallel to S3 (Figure 6). Magnetite 22 x I 0·3 SI units in the non-calcareous arkose, and also occurs disseminated throughout the rock. averaging 4.6 x 10·3 Sl units, and ranging from 0.04 to Magnetic susceptibility for Unit 6a averages 1.4 x I 0·3 27.4 x 10-3 SI units in the calc-silicate-bearing arkose. Pegmatite averages 16 percent and ranges up to 70 percent in these rocks. Pegmatite in the calc-silicate­ bearing arkose commonly contains hornblende or diopside. Locally, epidote is found on fracture surfaces.

c) Wollaston Group Upper Sedimentary Sequence The upper sequence comprises arkose and pebble conglomerate (Unit 9) and hornblende plagioclasite (Unit IO). The unconformity recognized at the base of the Janice Lake conglomerate and overlying calc­ silicate-bearing strata in the eastern part of Wollaston Domain (Delaney, 1994; Delaney et al., 1995; Tran and Yeo, 1997; Yeo, 1998; Tran et al., 1998) can be identified at Highrock Lake. The appearance of Figure 6 - Unit 6b psammopelite with magnetite-cored abundant conglomerate in Unit 9 probably marks a felsic lenses oriented parallel to SJ (black pencil) at the east relative sea-level fall (i.e. lowstand conditions). shore ofScott Bay. A relict Sl/S2 foliation is preserve,/ in magnetite-biotite-bearing sweats (light-coloured pencil magnet).

42 Summary of Investigations 1999, Volume 2 Unit 9: Arkose and Pebble Conglomerate ·.. · ''x ·. :,-, . :i;('K The characteristics of the arkose (Unit 9a) in this unit 1/l >, m are similar to those of Unit 8. The conglomerate (Unit (l) ... t :i 9b) is pale grey to greenish grey on fresh surfaces, pale . ·. 'f{ 1/l 0 ~ 0 0 grey to white weathered, and coarse- to very coarse­ a; 2 'C -g £ , ~ grained (grit and pebbles). Locally, clasts are flattened ::i J (/)~(I)- parallel to S3. Pebbles range up to 4 cm long, and ... include quartz, diopside-bearing calc-silicate rock, and psammopelite. The matrix is medium- to very coarse­ grained arkose. Trough cross-beds up to 150 cm in 16 amplitude are commonly preserved. Low-angle planar cross bedding also occurs in some beds. Cross-beds were not measured for paleocurrent analysis because Mj the strata have been repeatedly folded, but they appear to be unimodal at an outcrop scale. Magnetic susceptibility measurements on the conglomerate 3 average 5.6 x 10· SI units and range from 0.05 to 30.9 ·. x 10·3 SI units. .:.. ~.K 15

Primary sedimentary structures are particularly well preserved at the south end of Bennett Bay in the low­ strain core of a southeasterly plunging F3 antiform, which has refolded an Fl syncline. Upward thickening and coarsening in a measured section here indicate progradation (Figure 7). Tabular interbedded sands and muddy sediments (silts?) in the lower part of the ... succession might represent delta foreset deposits; lenticular, trough cross-bedded sands and pebbly sands in the upper part of the succession (Figure 8) may represent delta topset or braided river bar deposits.

Unit 10: Hornblende Plagioclasite

Hornblende plagioclasite is white and green on fresh surfaces, greenish white weathered, coarse- to very coarse-grained, and massive to vaguely layered. Major mineral components are plagioclase (70 to 80 percent) and hornblende (20 to 30 percenti. Magnetic susceptibility averages 0.06 x lo· SI units, and ranges from 0.04 to 0.07 x 10·3 SI units. Plagioclasite is found throughout much of the Wollaston Domain, commonly LEGEND in association with calc-silicate-bearing arkose, or Lithology other calc-si\icate rocks (e.g. Delaney et al. , 1995; Tran and Yeo, 1997; Tran et al., 1998). rt is likely Subarkose derived from an evaporite-rich mud (Weber et al., r: :I 1975; Chandler, 1978; Ray, 1981 ; Appleyard, 1984; . ! and others). Di-bearing l:: subarkose

Hornblende-bearing plagioclasite breccia was only Psammile observed on a small island north of Johnson Island, where it occurs as a dyke cutting well-foliated Unit 6 psammopelite and cross-cut by a late pegmatite dyke. II Psammopelite These relationships indicate that brecciation is late D3.

Structures

i Parallel l .. I lamination I i' . ( Low angle planar Figure 7 - Measured stratigraphic section through Unit 9 cross-bedding rocks at the southwest comer of Bennett Bay. Th e section 7 It records a transition from relatively low-energy marine Trough conditions to high-energy coastal b"r or jluvial conditions. . . i cro ss-bedding

Saskatchewan Geological Survey 43 red on weathered surfaces, fine to very fine sand sized, well-sorted, laminated, and flaggf Magnetic susceptibility averages 0.07 x Io· SI units and ranges from 0.04 to 0.1 x 10 3 SI units. The sandstone is cut by numerous quartz veins up to 2 cm wide, but with no apparent preferred orientation.

Local evidence for pre-Athabasca weathering in Units 1 to 11 (e.g. bleaching, hematization, and epidotization) suggests that Athabasca Group sediments formerly blanketed the Highrock Lake area.

3. Structural Geology Four deformation events are recognized in the Highrock Lake area. Three of them produced Figure 8 - Unit 9 trough cross-bedded arkose and pebbly penetrative structures, while the last involved brittle arkose from the upper part ofthe measured section shown faulting. A fifth event, the D2 event, recognized in Figure 7. Photo by S. Johns. regionally by Lewry and Sibbald ( 1980) and Tran et al. (this volume), probably affected the rocks, but cannot d) Intrusive Rocks be clearly distinguished from Dt.

Unit 11: Pegmatite and Granite a) First and Second Generation Structures (Dl and D2) Pegmatite and granite occur as irregular patches, veins, dykes, and large irregular bodies. They are pink on DI affected all the rocks except Units I I and 12 (and fresh and weathered surfaces, medium to very coarse­ the Unit 10 breccia). DI structures include a well­ grained, and generally non-foliated. Major mineral developed penetrative regional foliation (S l) and components are quartz (25 to 30 percent), K-feldspar transposition of compositional layering (SO) parallel to (35 to 45 percent), and plagioclase (30 to 40 percent). this foliation. S 1 is defined by the preferred Biotite and cordierite are common accessories of crystallographic or dimensional orientations of early pegmatite in a\uminous host rocks, while hornblende is metamorphic minerals, including biotite, sillimanite, common in pegmatite in cak-silicate-bearing strata. K-feldspar, and hornblende (e.g. Figures 5 and 6), Locally, in aluminous host rocks, there is a continuum indicating that these early structures are broadly coeval from small melt lenses and felsic aggregates to larger with high-grade metamorphism. SI also includes the sheets and patches. These observations strongly flattening plane of grain aggregates, early melt veins, suggest that the pegmatite is derived by in situ melting. and conglomerate/breccia clasts. A large-scale, Aplite also occurs as thin sheets, commonly associated isoclinal Fl syncline, whose limbs are defined by with pegmatite. Magnetic susceptibility averages 1.6 x inward-dipping bedding, on which tops can 10·3 SI units and ranges from 0.17 to 3.77 x 10·3 SI consistently be determined from trough cross beds, is units. refolded by a regional F3 anti form south of Bennett Bay. Small-scale, isoclinal F 1 folds were also Two generations of pegmatite are distinguished, both recognized. being present in many outcrops. Early (syn-DI or D2j pegmatite is boudinaged, folded by F3 folds, and locally, a foliation due to quartz flattening is developed. Late (syn- to post-D3) pegmatite is undeformed. Second-generation pegmatite is commonly emplaced along late 03 shears (Figure 9). Pegmatite constitutes 15 to 20 percent of most outcrops, and locally as much as 70 percent. Pegmatite increases towards the contacts between basement and cover rocks, and is more abundant towards the western part of the map area.

e) Athabasca Group

Unit 12: Manitou Falls Formation Sandstone

This unit is restricted to a poorly-exposed outlier north of the H ighrock River and about 8 km south of the Figure 9 - Pegmatite veins emplaced along the sheared margin of the Athabasca Basin. This basal Athabasca limbs of F3 folds developed in Unit 6 magnetite-bearing quartz sandstone is maroon on fresh surfaces, and rusty psammopelite in Bennett Bay.

Summary ofInvestigations I 999. Volume 2 44 01 structures are generally well developed in the the sense of offset can be determined, these faults have basement inlier rocks in the western Highrock Lake sinistral oblique displacement. They appear to offset area (e.g. Figure 4) and they are concordant with the northeast-trending fault set, and hence, are structures in the overlying metasedimentary rocks. No younger. These fault zones may be related to those of evidence of an earlier tectonic fabric could be the Tabbemor fault array described by Davies ( 1996) identified. Near the basement-cover contact, S 1 from western Wollaston Lake. foliation generally increases in intensity.

Locally, F 1 folds are refolded by nearly coaxial tight to I\, isoclinal F2 folds, to produce type 3 interference patterns (Ramsay, 1967). Evidence for this 02 deformation is recognized regionally, especially farther (A) F3 folds west (Tran et al., this volume), but in the Highrock :--, ·. Lake area D 1 and 02 structures cannot generally be distinguished. •...... '/'.. -~ x b) Third Generation Structures (D3) / , ' . ',: .... Early structures have been strongly deformed by the ~ - / y :.. dominant regional deformation event (03). S 1 foliation ,,., ., ~ ». ,• is generally refolded by open to tight, locally isoclinal, ,' ...... ,, ..... F3 folds. F3 axial planes trend northeast and dip _p. •• steeply (Figure I Oa). F3 fold axes are doubly plunging, ~ > both northeast and southwest, likely as a result of F4 refolding (Figure 1Ob). .as ar•• An S3 foliation is locally well developed subparallel to • • the F3 axial planes. S3 includes the preferred • ~ ., crystallographic or dimensional orientations of late .... metamorphic minerals, including biotite, sillimanite, • x diopside, hornblende, and magnetite (e.g. Figure 6), ••• indicating that 03 structures are broadly coeval with a • / second phase of high-grade metamorphism. S3 also includes the flattening plane of grain aggregates and lenses, melt veins (e.g. Figure 9), and conglomerate clasts. On the limbs of F3 folds, the SI foliation is commonly transposed into the S3 plane. Late- or syn-03 ductile shear planes generally trend (B) F4 folds northerly to north-northeasterly (Figure 11). They commonly cross-cut SI, shear off limbs of03 folds (e.g. Figure 9), or pass into minor 03 folds. Pegmatite is common on such shears (e.g. Figure 9). The sinistral shears can be interpreted as Riedel R or O synthetic • I' shears, and the dextral shears as Riedel R' antithetic shears (Wilson, 1982). They indicate an overall sense • of sinistral shear parallel to S3. • c) Fourth Generation Structures (D4) Fourth generation, open folds with subvertical, northwest-trending axial planes have refolded the + earlier structures (Figure 4; I Ob). Locally, an axial ••• planar S4 fracture cleavage is developed. • d) Fifth Generation Structures (DS) • Three sets of subvertical brittle or brittle-ductile faults cross-cutting the earlier structures can be recognized. The dominant regional fault set trends north­ northwesterly (Figure 2). They are identified by: a) prominent topographic linears, b) prominent linears and offsets on the magnetic map (Figure 3), Figure 10 - lower hemisphere equal area stereographic c) disruption of lithological units, and d) local small­ plots ofpoles to axial planes (crosses) and/old axes scale brittle faults (e.g. south of Scott Island). Where (fquares) for (a) FJ minor folds, and (b) F4 minorfoldf.

Saskatchewan Geological Survey 45 bioti~e, and calcite/dolomite. Anatectic pegmatite and Mean plane of granite are common, especially in the more pelitic Mean axial plane dextral shears rocks. At least two generations of metamorphic mineral of F3 folds , growth can be distinguished.

I · .L. a) First Generation Metamorphism (Ml) I ./ o I .- ~. The first phase o~ metamorphic mineral growth was -) I __...... - broadly coeval with DI and D2 deformation as - I / indicated by preferred orientation of biotite ~illimanite /./ cord_ierite, and feldspathic lenses parallel to' SI, and DJ l ./ foldmg of early neosome. Ml sillimanite faserkiesel 1--• flattened parallel to SI are commonly refolded by I ~mall-~cale_ F3 folds (e.g. Figure 5). Sillimanite I 1~c!us1o~s m feldspathic lenses suggest that early . .4 o, s11l1mamte was replaced by K-feldspar. Early cordierite occurs as lenses flattened parallel to SI, typically in ~ .// association with sillimanite. Most commonly however "' ./°/ ~ I cordierite o~curs in deformed early neosome.' Early ' ~.. .· //I I metamorphic K-feldspar occurs as lenticular t>~< aggre~ates tl~ttened parallel to SI, and is commonly . 11 / ~/~ • associated with early pegmatite. 0 • Early pegmatite neosome, ranging from small sweat . patches to s~eets over I m wide, is widespread. Locally 1t forms a high prop?rtion of outcrop, especially towards the west. It 1s commonly parallel to SI Mean plane of boudinaged, and folded by F3 folds. ' sinistral shears Figure I l - lower hemisphere equal area stereographic b) Second Generation Metamorphism (M2) plot ofpoles to D3 si11istral (circles) and dextral (squares) shear planes. The seco~d generation of metamorphism was broadly co~val ~tth D3_ d~formation, as indicated by preferred E~st-west-trending magnetic Iinears in the map area onenta~10n o_f s1lhmanite, magnetite, and feldspathic (Figure 3) probably represent subvertical brittle faults lenses m pel_1tes and p~ammopelites, and diopside and similar to those in the Athabasca Basin. One of these hornblende m calc-s1hcate-bearing rocks parallel to S3. faults cuts through the Deilmann orebody at Key Lake Second generation sillimanite occurs as faserkiesel (Harvey, this volume). Several minor EM conductors parallel to S3. It is typically less strongly flattened than tenn!nate against a ~agnetic high along the north shore MI sillimanite and not refolded by D3 folds. Second ofHighrock Lake (Figure 3), but this feature has no generation cordierite occurs as porphyroblastic geologi_ca_I expression and only a weak topographic aggregates in second generation pegmatite neosome. one: It 1s mterpreted as a horst of magnetite-bearing Game~ found in the basal pelite/psammopelite unit is sedimentary rock. most likely a second generation mineral because it is not flattened. Second generation K-feldspar occurs as A northeast-trending, dextral, brittle fault zone was SJ-parallel _lens?i~ POIJJhyroblasts or aggregates, mapped at the narrows east of Johnson Island. The typically with b1ot1te nms, and commonly with sense of displacement on a northeast-trending mylonite magnetite cores. zone o~ the west shore of Grest Bay could not be detennmed. Subpa~allel, northeast-trending linear Second generation anatectic melt occurs as weakly to features on the reg10nal magnetic map (Figure 3) may undeformed sweats, sheets, or large bodies of be related fau_lts. The Key Lake Fault Zone (Dahlkamp, pegmatite and g ranite. M2 pegmatite sheets are 1978), on which the Gaertner and Deilmann orebodies typically parallel to SJ (e.g. along sheared F3 fold at Key Lake are located, is an example of this type of limbs; Figure 9) or cross-cut S 1 ( e.g. along D3 shear fault. planes). c) Metamorphic Conditions 4. Metamorphism Th_e metamorphic mineral assemblages indicate two Except for late pegmatites and rocks of the Athabasca episodes of upper amphibolite to lower granulite facies Group, all r?cks hay~ undergone high-grade condi~ions. No m~tamorphic isograds were defined, but metamorphism. Pehttc and psammopelitic rocks there 1s a notable mcrease in the proportion of neosome contain biotite, sillimanite, cordierite, garnet, K­ in metased iments west of Highrock Lake. The mineral felds~ar, and magnetite. Cale-silicate-bearing rocks assemblages typical of various rock units probably contain hornblende, diopside, tremolite/actinolite, reflect bulk composition, rather than variable metamorphic grade.

46 Summary of Investigations /999, Volume 2 Abundant anatectic melt, and association of sillimanite 5. Geophysics with high-grade minerals such as cordierite and K­ feldspar, indicate that metamorphic conditions were a) Magnetics above the second sillimanite isograd. Sillimanite probably formed via reaction I (Evans, 1965), or, if Inspection of the regional magnetic map of plagioclase was present, reaction 2 (Evans and Saskatchewan (Slimmon, 1999) shows that the Guidotti, 1966). Wollaston Domain corresponds to an extensive magnetic high, trending from the Manitoba border

Muscovite+Quartz--j,Si1J imanite+K-feldspar+H20 ( 1) north of (between 58° and 59"N) to just north of Swift Current (50° 30'N), while the Mudjatik Muscovite+Quartz+Plagioclasc--j,Sillimanite+K-fcldspar+H,O (2) Domain corresponds to a parallel zone of mixed highs and lows to the west. The Wollaston-Mudjatik Cordierite may have formed by the breakdown of boundary lies along the western margin of a prominent sillimanite and biotite via reactions 3 (Holdaway and magnetic low separating the Wollaston and Mudjatik Lee, 1977), 4 or 5 (Wickham, 1987). If phengitic magnetic zones, except south of the Athabasca Basin muscovite was present it may have formed by reaction where the boundary follows its east side. Most of the 6 (Thompson, 1982). uranium occurrences of eastern Athabasca Basin lie along this magnetic low. Drilling confirms Agarwal's

Biotite+Sillimanite+Quartz~Cordierite+K-feldspar+H20 (3) ( 1965) suggestion that the low beneath the Athabasca Basin is due to magnetite-poor Wollaston paragneisses, Biotite+Sillimanite+Quartz+Albite--j,Cordicrite+K-feldspar+Hp (4) but south of the basin it corresponds to Mudjatik granitoid rocks. Biotitc+Sillimanitc+Quartz+Albite+IJ,O--j,Cordierite+Melt (5) The Highrock Lake area straddles the transition Phcngite+Quartz~Cordicrite+ K-fcl dspar+11,0 ( 6) between the magnetic high and low zones (Figure 3). Pronounced northeasterly-trending magnetic highs Although garnet may be produced under low-grade correspond to the McMillan Lake Granite and the metamorphic conditions (e.g. reaction 7, Hsu, 1968), eastern edge of the Highrock Lake Granite. The latter the idioblastic character of garnet in the basal garnet magnetic high extends northeast, well beyond the pelite/psammopelite unit suggests that it more likely mapped northern end of the Highrock Lake Granite. In formed under higher grade conditions such as by the contrast, the Zimmer Lake Granite is only moderately breakdown of sillimanite and biotite via reaction 8 magnetic, but is outlined by a well-defined magnetic (Grant, 1985). This reaction represents the transition low with a coincident conductor, which marks the from upper amphibolite to granulite facies (Bucher and basal graphitic pelite unit. The McMillan and Highrock Frey, 1994), and peak metamorphic temperature lake magnetic highs suggest sheet-like mafic intrusions conditions in the area. (e.g. diabase dykes), but no evidence for these was observed. Elsewhere in Wollaston Domain, probable Chloritc+ M uscov ite~Garnet+Biotitc+Quartz (7) 03 reverse faults are associated with strong northeasterly-trending magnetic features (e.g. the West Riotite+Sillimanite+Quartz~Garnct+Cordieritc+K-feldspar+Melt(8) Burbidge Lake Thrust: Tran and Yeo, 1997). The magnetic pattern in western Highrock Lake area The metamorphic mineral assemblage of sillimanite suggests a possible "flower structure", with basement and cordierite, associated with both generations of thrust over magnetite-bearing sedimentary rock. anatectic melt, suggests P-T conditions above 680°C and 4 kbar, the intersection point of reaction I with the The total field magnetic pattern broadly corresponds to water-saturated granite melting curve (e.g. Tran et al., the distribution of lithologic map units (Table 2). The 1998, Figure 16). The presence of garnet suggests that Archean basement rocks in the west part of the area temperatures above 700°C were reached during second have moderate magnetic susceptibility values, as do the generation metamorphism. The absence of spinet cordierite-sillimanite psammopelite/pelite (Unit 5) and suggests that peak temperature was below 770°C, the arkose-dominated strata (Units 7, 8, and 9), found minimum temperature for garnet breakdown by mainly in the eastern part of the area. The garnet reaction 9 (Bucher and Frey, 1994). pelite/psammopelite and plagioclasite have very low magnetic susceptibilities, but may be too restricted in Alm andine+ Sill imanite--> Spinel+Quartz (9) distribution to trace on the magnetic map. Magnetite­ poor psammopelite/psammite has low magnetic Hence peak metamorphic conditions of 700° to 770°C susceptibility, and is more abundant in the west, while and 4 to 5 kbar are interpreted to have been reached magnetite-rich psammopelite/psammite has very high during M2. Conditions are similar to those reported magnetic susceptibility and is more abundant in the elsewhere in southern Wollaston Domain (Tran and east. Pegmatite has generally low magnetic Yeo, 1997; Tran et al. , 1998; Tran et al., this volume). susceptibility.

Breaks and offsets in the northeast-trending magnetic structures probably correspond to northwesterly­ trending fault zones. Several of these discontinuities correspond to mapped faults, including the Burr,

Saskatchewan Geological Survey 47 Table 1- Summary of magnetic susceptibility data for map units in the Highrock Lake area. Units of measurement are JU-1 SI units. Map Unit Mean M.S. M.S. Range 11 . Pegmatite and granite 1.6 0.17 - 3.77 10. Plagioclasite 0.06 0.04-0.07 9. Cale-silicate-bearing arkose and conglomerate 5.6 0.05 - 30.9 8. Arkose and calc-silicate-bearing arkose 5.7 0.04 - 27.4 7. Interlayered psammopelite and arkose 5.6 0.05 - 30.9 6a. Magnetite-poor psammopelite and arkose 1.4 0.1 - 12.6 6b. Magnetite-rich psammopelite and arkose 20 2.7 -60 5. Co-Si psammopelite and pelite 6.2 0.2 - 27.1 4. Gt psammope!ite and pelite 0.22 0.14-0.31 3. Amphibolite 5.7 0.25 - 18.9 2. Granodiorite to tonalite 5.7 0.17 - 22.7 1. Granite to granodiorite 6.5 0.19-11.3 Bennett Bay, and Scott Island faults. The northeast­ 'Division Lake', a small lake north of Wilson Bay trending Grest Bay mylonite zone lies along the eastern (UTM easting: 4 77941; northing: 6326 138). Grab edge of the McMillan magnetic high. An east-west­ samples assayed 0.103% and 0.269% U30 8. Assay trending magnetic ridge connects the McMillan and results reported from limonite and uranium oxide­ Highrock magnetic highs. A topographic linear stained pegmatite intersected in DOH CH-I, range up trending through the narrows east of Johnson Island to 0.084% U30 8 (SEM Assessment File 74H03-SW- and coincident with the southern margin of this feature 001 I), but averaged only 0.05 % U30 8 over 9 m (Ray, is interpreted as a fault zone. 1977).

b) Conductors b) Nelson Bay Showing (SMDI #2023) Most of the Highrock Lake area was covered by Denison Mines located a showing in Unit l granite to airborne EM (Input and EM-30) exploration surveys. granodiorite of the Highrock Lake Granite (described Conductors are generally restricted to the western part as fractured arkose and pegmatite, locally pyritic and of the map area (Figure 3). A strong, extensive tourmaline-bearing) east of Nelson Bay (UTM easting: conductor associated with the margin of the Zimmer 461928; northing: 6322667)). Grab samples assayed Lake Granite is due to the graphite-rich basal pelite 0.099% and 0.4 7% UP8 (SEM Assessment File unit. Extensive, northeasterly-trending conductors 74H04-0025), but fo llow-up drilling was not between the Zimmer and McMillan granites are encouraging. possibly due to structural repetition of this basal unit. Although some of the short, northeasterly-trending c) Roberts' Showing (SMDI #2022) conductors in the western part of Highrock Lake may be due to lake sediment, those coincident with A small body of pink to red, weakly foliated, fine to magnetic features are more likely faults. Short, medium grained, biotite granite with sheets and lenses northeast-trending conductors also mark the eastern ofpegmatite lies about 400 m north of the narrows edge of the Highrock magnetic high. Ground EM leading to the "High Rocks Bay" area (UTM easting: conductors trend along its western edge as well. The 475794; northing: 6330355). Hematization and absence of extensive conductors in the central and kaolinization of the granite and epidote along fractures eastern parts of the map area reflects the absence of cutting the granite and pegmatite are evidence of pre­ conductive strata (i.e. graphitic pelites). Athabasca weathering. Yellow uranium oxide stain was also observed on fractures. Noreen Energy reported anomalous radioactivity in the fractures and in 6. Economic Geology fractured pegmatite and a pegmatite grab sample that assayed 2.8% lJ (SEM Assessment File 74H03-0022). From 1969 to 1981 the Highrock Lake area was the focus of considerable uranium exploration by Colt Resources and Noreen Energy Resources Ltd., Denison 7. Discussion Mines Ltd., E & B Exploration Ltd., Getty Minerals Co. Ltd., Maverick Uranium Exploration Inc., and Correlation of the lithologic units recognized in the Uranerz Exploration and Mining Ltd. Three showings Highrock Lake area with units mapped to the were discovered; all are associated with pegmatite. southeast, in the Burbidge Lake-Upper Foster Lake area (Tran and Yeo, 1997), and to the southwest, in the a) 'Division Lake' Showing (SMDI #1128) Haultain River area (Tran et al., this volume), is shown in Table 1. Most units can be correlated across the Colt Resources trenched and drilled a showing in Wollaston Domain. Two important exceptions are the pegmatite and fine- to medium-grained granite west of calc-silicate-bearing rocks (Unit 6 of Tran et al., this

48 Summary of Investigations 1999, Volume 2 volume) overlying and intercalated with the basal both basement and supracrustal rocks near their contact gamet-cordierite-sillimanite-graphite-bearing petite makes it likely that the metasediments are at least unit in the Haultain River area and the Janice Lake locally allochthonous. The upper unconformity is conglomerate (Unit 12 of Tran and Yeo, 1997) in the inferred to lie at the base of the calc-silicate-bearing Burbidge Lake area. arkose and conglomerate (Unit 9), but it may lie within the underlying arkose (Unit 8). A summary The restriction of calcareous rocks low in the interpretation of the Highrock Lake stratigraphy is succession to the western part of the Wollaston shelf shown in Table 3. may be a function of relatively shallow water depth and location closer to the paleo-shoreline. Interbedding of carbonates and mudstones (commonly organic-rich) a) The Lower Sequence: Transgressive and is common in shallow marine platform settings. Highstand Systems Tracts Phanerozoic examples include the Taconic The Lower Sequence can be subdivided into two (Ordovician), Acadian (Devonian), and Alleghanian systems tracts, a basal transgressive systems tract and (Carboniferous-Permian) successions of eastern North an overlying highstand systems tract. America (Poole et al., 1970) and the Yoredale Cycles (Carboniferous) in Britain (Wilson, 1975). In the Acadian Catskill delta, cyclic interbedding of Basal Transgressive Systems Tract mudstones and carbonates is interpreted to reflect short-term sea level fluctuation, with muds The basal transgressive systems tract records an accumulating in deeper water and carbonates in upward fining trend in which ferruginous (garnet) shallow water (Ettensohn, 1985). muddy sediments of Unit 4 (garnet psammopelite and petite) are overlain by more aluminous (clay-rich) The restriction of the Janice Lake alluvial fan deposits muddy sediments of Unit 5 (cordierite-sillimanite (Delaney, 1994; Delaney el al., 1995) to the eastern bearing psammopelite and pelite). The higher iron part of the Wollaston shelf can be explained if they content of the garnet-bearing unit may be due to the were shed off a peripheral bulge, generated by higher dissolved iron content of brackish coastal waters convergence of the western Churchill craton with the compared to normal seawater. The presence of La Ronge volcanic arc, which died out cratonward abundant clay in the protoliths of the basal (Yeo, 1998). transgressive systems tract suggests strong chemical weathering of source rocks in a humid climate (Lewis The stratigraphy of the Highrock Lake area can be and McConchie, 1994; and others). interpreted in terms of low-resolution sequence stratigraphy (Walker, 1992; Christie-Blick and A notable feature of the basal garnet psammopelite/ Driscoll, 1995; and others), as demonstrated in the pelite unit in the Highrock Lake area is the absence of eastern part ofWollaston Domain (Yeo, 1998; Tran et graphite, compared to correlative strata to the southeast al., 1998). Two sequence boundaries can be and west. The basal garnet psammopelite/pelite in the recognized. The lower one is the unconformity Daly Lake area is also graphite poor (Tran et al., between Archean granitoid rocks and overlying 1998). Since graphite is relatively immobile, it is psammopelites and pelites. Note that an in situ probably a primary compositional feature of the basal unconformity was not observed, and increased strain in unit. The main factor in preservation of organic carbon in marine muds is reducing conditions at the seabed. Table 3-Summary interpretation ofthe stratigraphic units mapped in the Highrock Lake area.

Lithology ProtoIi th Systems Climate Tract Upper Sequence I 0. Plagioclasite Sabkha evaporite muds Lowstand Dry 9. Cale-silicate-bearing Partly carbonate-cemented deltaic - fluvial arkose and conglomerate sands and gravels Lower Sequence 8. Arkose Partly carbonate-cemented deltaic - coastal Highstand sands 7. Interlayered psammopelite Proximal shelf sands and muddy sands and psammite 6. Psammopelite and Distal shelf muddy sands Transgressive Wet psammite 5. Cordierite-sillimanite Distal shelf muddy sands and muds psammopelite and petite 4. Gamet psammopelite and Distal shelf sands and muds petite Basement Complex

Saskatchewan Geological Survey 49 Such conditions are most strongly favoured where the outer shelf deposits, while the better sorted sands of oceanic oxygen minimum layer intersects the seabed Units 7 and 8 are more likely shallow water inner shelf (Figure 12). On a shelf this can occur either by and shoreface deposits (Walker and Plint, 1982). upwelling (Demaison and Moore, 1980; Figure l 2c) or Interbedded sands and muddy sands or silts such as rapid transgression (Arthur and Schlanger, 1979; those of the transitional Unit 7 have been interpreted as Figure 12d). Either or both conditions may have been shallow shelf storm deposits (e.g. Quinn et al., 1999). involved in deposition of organic-rich muds at the base The shift from mud-dominated to sand-dominated of the Wollaston succession. Since preservation of sediment in this systems tract indicates an increasing sedimentary carbon is depth dependant, parts of the sediment supply, probably a rising source area. The Wollaston shelf that accumulated little or no organic­ increased feldspar content upward suggests unroofing rich sediment probably lay above the oxygen minimum of plutonic rocks and/or an increasingly arid climate layer. (Frakes, 1979; Lewis and McConachie, 1994). Dry climate conditions are also suggested by carbonate and calc-silicate minerals, the latter probably derived Highstand Systems Tract evaporite minerals (Appleyard, 1984; and others). The overlying strata of the Lower Sequence show an overall upward coarsening or progradational trend b) The Upper Sequence: A Lowstand Systems from muddy sediments of Unit 6 (psammopelite and Tract pelite ), through interbedded muddy and feldspathic Conglomerate interbedded with arkose in Unit 9 sands of Unit 7 (interlayered psammopelite and suggests more rapid progradation and sea level fall (or arkose), to feldspathic sands of Unit 8 (arkose). This is craton uplift). Hence, this unit belongs to a lowstand characteristic of a highstand systems tract. The poorly systems tract. In contrast with the Janice Lake sorted sands and muds of Unit 6 are typical deep water conglomerates to the east, these conglomerates do not have a strongly unconformable basal contact. Unit 9 is (a) Normal Ocean (b} Restricted Anoxic Basin probably correlative with Unit r2, which lies above the (e.g. Indian Ocean) (e.g. Black Sea) 0 Janice Lake Formation in eastern Wollaston Domain (Delaney, 1994; Delaney et al., 1995). Arkosic composition and calc-silicate minerals suggest that arid E 1.0 6 conditions persisted, as suggested by Delaney et al. _ca. ( 1995) for the correlative strata of eastern Wollaston

3.0 A Clastic Source to the West? (d) Marine Transgression The prevalence of conglomerates in Unit 9 and overall upward coarsening of the sedimentary succession 1 0 indicates a rising source area. Although Unit 9 was not 2:,·E ..c recognized to the east in the Upper Foster Lake area, it a. may be present as part of Tran and Yeos' (1997) Unit

50 Summary of Investigations 1999. Volume 2 Yeo ( 1998) suggested that the Janice Lake related to easterly directed subduction beneath the conglomerate and its correlatives in eastern Wollaston western Churchill craton (Hoffman, 1988; Ross et al., Domain were shed off a peripheral bulge, formed 1991, 1995). The TMZ is a composite Andean-type before 1870 Ma, as the La Ronge-Lynn Lake arc and continent-continent-collision orogen formed by the approached the western Churchill craton margin, collision and accretion of the 2.4 to 2.1 Ga Buffalo whereas overlying elastics (e.g. Unit 9) were derived Head Terrane with the western Churchill craton (Ross from the west. There are two potential source areas to et al., 1991, 1995). Plutonic rocks in the TMZ yield U­ the west: the Snowbird Tectonic Zone and the Taltson Pb zircon ages from 1.99 to 1.91 Ga (Berman and Magmatic Zone. Bostock, 1997; Grover et al., 1997). U-Pb monazite ages for granulite facies metamorphism in the southern The Snowbird Tectonic Zone (STZ) is a 3000 km long, TMZ range from 1933 to 1913 Ma (Grover et al., northeast-trending geophysical anomaly coincident 1997). Hence, the TMZ was an active orogen and with the Virgin River Shear Zone to the south of sediment source for most of the interval during which Athabasca Basin and the Striding-Athabasca mylonite Wollaston sediments were deposited (2100 to zone (including Tantato Domain) to the north. 1870 Ma; Figure 13). Hoffman ( 1988) and others interpreted the STZ as an early Proterozoic collisional suture between the Rae and Hearne provinces, but Hanmer et al. ( 1994, 1995) and Hanmer (1997) presented three arguments to indicate that the STZ was an intracontinental structure by late Archean time. Firstly, the Rankin-Ennadai greenstone belt in northern Hearne province was (a) Rift Sequence (Courtenay - Cairns Lake Belt): 2100 Ma Rifl ~ed -men1 ,;~ deposited in a basin which must have closed by ,r--~ ~re1r.:lred conl.n('nT.il Cfl!S\ continental collision by 2.65 Ga. Secondly, 2.63 to + -l- f . -i- -1 'I- + , ·, ! I Ii r- l'- r i-- -1- .,. ·1, + -\ ·1 2.58 Ga granites (including the Zimmer Lake granite), ~ -~ Rae - Hearne Croton ~.- .4 '\' · + ·f. . .,~ -~ Y. -\ , · ){ ~- --:· )(' -+ -~ "- .,..' -~ -\ · )( + )< broadly coeval with the late Archean movement on the O(:e ;,1r1i(; r.;,u:s\ STZ, are scattered throughout Rae and Hearne provinces, rather than forming an obvious magmatic {b) Lower Sequence: 2100 -1880 Ma '•";;X,,,~,;'.'/"'• 19.90. 19!X) M.:iuphf\.lo we!.! Pc1ss. ve rr1arg n (1!}10 -1860Mfli arc. Thirdly, the distribution of Archean Nd model r ~('(l~11taliori \t r f.. .• . "1" ··--y:--, . +· - _ -. . ...; v v ·,, _ ·1· ages, 4.0 to 2.5 Ga in the southwest and 2.9 to 2.5 Ga A-' , ~ -.i- ./" , _ ~,- ..., . -:-x _-~ _-f-...i' I J I T I i I I f ·._1-1 I ~.,. v v v ~ I , ·r ;. x "i- · 1 ~- x~ . I - in the north, is the same in both Rae and Hearne x ,,..._ -, .;... 1 ; · · '" , Posl-nfl !ht.rm;~ suns'(lence !'· J I provinces, suggesting that they formed a united western Churchill Craton for most of the Archean. (c) Upper Sequence: 1880. 1870 Ma

Although the geometry of the Striding-Athabasca mylonite zone makes early Proterozoic strike-slip displacement unlikely, dip-slip movement is not i I - precluded (Hanmer et al., 1994, 1995; Hanmer, 1997). I I (d) Cordilleran Margin: 1870 - 1830 Ma I I Juxtaposition of STZ granulites against amphibolite ror!i!1a rxl !i.ed1rn ent<;. (notprese1" 11ed) ~ _ _. __ ·~'.. '. facies metasediments in Saskatchewan and the • . I ·v V : : v v -.t I greenschist facies Rankin-Ennadai belt farther north • ' ,-1 , / , · r;'~-:+'.~;· ;. .. ' I I I I '. I : ~- ). ~- .!,--- y . I must pre-date 1.85 Ga filling of the Baker Lake Basin, 71 . I I . which straddles the STZ. Lafrance and Sibbald ( 1997) Wr, thilmM B<1 \11~ 1Hh showed that the Grease River Shear Zone, within the ,: 1%5- 1 fs$0 Ma} STZ north of , was remobilized in the I I' early Proterozoic, following emplacement of the ca. 1900 Ma (U-Pb zircon) Robillard Granite (Hanmer et Figure 13 - Regional lectono-stratigraphic history after al. , 1994; Hanmer, 1997). Hanmer et al. (1995) and Lewry et al. (1980), Ray and Wanless (1980), and Yeo Hanmer ( 1997) suggested that the Virgin River shear (1998). (a) Bimodal volcanics and rift sediments in the zone may have been an active dextral transform fault, Compulsion River Belt of eastern Wol/aston Domain show responding to collision of the Buffalo Head Terrane that rifting ofthe eastern margin ofthe Rae-Hearne craton with the western Churchill craton, throughout much of took place ca. 2100 Ma (MacNeil, 1998): (b) sediment, shed the early Proterozoic. This must also have produced westward from the 1990 to 1910 Ma Taltson orogen localized uplift, especially towards its northeastern (Berman and Bostock, 1997; Grover, et al., 1997) and possibly ca. 1900 Ma uplift on the Snowbird Tectonic Zone termination south of Lake Athabasca. (Lafrance and Sibbald, 1997) accu111JJ/ated on the Wollaston passive margin: (c) as the 1910 to 1860 Ma La If the STZ was not an early Proterozoic suture, nor a Ronge volcanic arc (Baldwin et al., 1987) converged with barrier to sediment transport, then uplift along the the craton margin, a peripheral bulge probably formed, western margin of the western Churchill craton could resulting in faulting, major unconformity, and deposition also have provided sediment to large river systems ofalluvial fan deposits (i.e. Janice Lake Formation) along draining onto its eastern margin. There are numerous tl:e eastern margin ofthe shelf, elastics may have ponded to modem and ancient examples of passive margins the west ofthis uplift and restricted basin conditions may receiving sediment from orogenic uplifts on the have developed: and (d) arc-continent collision resulted in creation ofan Andean-type margin by 1865 Ma, with opposite craton margin (Potter, 1977). The Taltson reversal ofthe sense of subduction and emplacement of Magmatic Zone (TMZ) represents such an uplift plutons, including the Wathaman Batholith.

Saskatchewan Geological Survey 51 8. Acknowledgments Dahlkamp, F.J. (1978): Geologic appraisal of the Key Lake U-Ni deposits, northern Saskatchewan; Econ. Jaime Graves and Shannon Johns were cheerful and Geo!., v73, pl430-1449. enthusiastic field assistants. Sean Harvey also helped with the mapping and provided insight into Davies, J.R. ( 1996): Post-collisional tectonics relationships between the Highrock and Key Lake associated with the Tabbemor Fault System, areas. Suraj Ahuja and Robert Campbell (PNC), Jean Wollaston Lake area: Preliminary report; in Claude Rippert, Phillipe Portella, David Beaudemont, Summary of Investigations 1996, Saskatchewan and Ken Wheatley (Cogema), Dan Brisbane, Gerard Geological Survey, Sask. Energy Mines, Misc. Zaluski, Darcy Hirsekom, and Brian Powell (Cameco), Rep. 96-4, pl27-129. Charlie Jefferson (Geological Survey of ), Sean Harvey and Hai Tran (University of Regina), and Gary Delaney, G.D. (1994): Geological setting of sediment­ Delaney (Saskatchewan Energy and Mines) provided hosted copper mineralization in the area southwest stimulating discussions during an end-of-field-season of Janice Lake, Wollaston Domain; in Summary of visit to Highrock Lake. We thank Ken Ashton for Investigations 1994, Sask Energy Mines, Misc. helpful advice on metamorphism and Gary Delaney Rep. 94-4, p53-62. and Charlie Harper for critical reading of this paper. We also thank Trevor Hill ofHighrock Lake Lodge, Delaney, G.D., Maxeiner, R.G., Rawsthome, M.L., and Gary Thompson and the staff of Osprey Wings for Reid, J., Hartlaub, R., and Schwann, P. (1995): their support. We are particularly grateful to John Geological setting of sediment-hosted copper Lewry for asking how geological events farther west mineralization in the Janice Lake area, Wollaston might have affected Wollaston sedimentation. Domain; in Summary oflnvestigations 1995, Sask Energy Mines, Misc. Rep. 95-4, p30-48.

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54 Summary ofInvestigations 1999, Volume 2