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Project 8401A

Bedrock Compilation, Greater Beaverlodge Area {NTS 74N-6 to -11)

by R. Macdonald

Macdonald, R (1984) Bedrock compilation. Greater Beaverlodge area (NTS 74N-6 to · 11 ), in Summary ol lnvestigallons 1984. Saskatchewan Geological Survey: Saskalchewan Energy and Mines. Miscellaneous Report 84-4.

The geology of the greater Beaverlodge area has been surrrnarized on an interim 1:100,000 scale map compiled almost entirely from GSC am SGS mapping by the author and W.L. Slirranon (accompanying package). 'Ihis area is difficult to compile, not only because of the different styles of the numerous geologists who have worked there, but because of the complexity of intrusive, granitizing, anatectic and structural reworking events represented, some of which are not fully understood. In addition to previous work, the author spent two weeks this surrmer checking in the area and is grateful for helpful discussions on the outcrops with S. Roscoe, J. Hoeve, H.E. Hendry, P.K. Mazimhaka and M.W. Thomas, and subsequent discussions with T.I.I. Sibbald. These notes describe some of the features examined this SUITal\er; lithological details are given in the legend on the accompanying map. A canprehensive report is expected to be issued later,

Mylonitized and Mixed Supracrustal Rocks (Unit Xs)

unit XS (this compilation legend) was first mapped in the Black Bay area by Hale (1954a) as "mylonitized rocks". In the adjoining Gulo Lake area (Hale, 1954b) his view on their status had changed, for contiguous rocks in the Gulo Lake area are indicated as "Tazin Group - dominantly quartzitic rocks with inter banded greywacke-type i:-ocks". Scott (1978) referred to the same unit in part of the Gulo Lake area as "mylonite canplex"; his descriptions include a variety of mylonites of stated precursor as well as other (presl.mlably unmylonitized) rock types. Spot checking this surrmer confirmed the extreme variation in this group, ranging from ultramylonites to unsheared metasediments.

An important feature of the greater Beaverlodge area is the broad geographical coincidence between the 'successor basin' cover rocks (Thluicho Lake Group, Martin Group, etc.; see Langford, 1980) and the outcrop of unit Xs. At Ellis Bay, in fact, it is difficult to distinguish between mylonitized supracrustal rocks of the basement and the overlying lower grade Thluicho argillites and metasiltstones.

Presumably similar difficulties were met during the original mapping elsewhere in the compilation area. The problem is complicated by the fact that part of the Thluicho rocks may have been involved in the shearing and !ey'lonitization affecting the basement, It has been speculated that this took place in relatively brittle basement contemporaneously with more plastic deformation in the overlying successor basin rocks, perhaps to some extent in resr;onse to stress interactions between cover and basement (S. Roscoe, pers. corrun.)

Milliken Lake Northwest

Strong shearing and mylonitization are present along the margins of the Black Bay Fault. This shearing is not uniformly distributed, as there are enclaves of relatively unsheared precursor rock, mainly feldspathic and siliceous banded gneisses, quartzites and lesser amphibolite. Siliceous and chloritic gneisses examined farther southeast along the road to Milliken Lake and around the shores of Milliken Lake are similarly heterogeneously sheared and 'ribboned'. A tectonic enclave contains recognizable conglomerate. According to Sibbald (pers. cOITUl\.) similar shearing effects extend to just southeast of Milliken Lake. On this basis the contact between rocks of the Nevins Lake Block and the St. Louis Cataclastic Belt (Themas and Macdonald, 1983, Macdonald, 1983) can now be more firmly drawn almost down the middle of the Crackingstone Peninsula.

Black Bay Straight Belt

Examination of the shore of Black Bay confirmed the presence mainly of banded gneisses (unit FB). At Bushell Inlet the bands range from leucocratic quartzofeldspathic to roafic, and in dimension from several metres wide to millimetre-scale delicate layering. The mafic rocks are the strike extension of Tremblay's (1972) amphibolites (his units 16 and 18), and probably represent original flows or sills. Cale-silicate rocks and quartzite layers also occur. Although dipping subvertically and striking very uniformly in the northeasterly

trend of the Black Bay Straight Belt (Macdonald, 1983), strong shearing in this regional trend is not widely developed and mylonites were almost only observed c lose to the Black Bay Fault. On this basi s an approximate northwestern limit to the St. Louis Cataclastic Belt in the area south of Uranium City may be drawn coincident with the mylonites close to the Black Bay Fault (see tectonic sketch on accompanyi ng map).

White Lake Granite

Starting about 3 km west of Bushell Inlet there is a progressive increase in anatexis towards the White Lake granite exposed on the shore and island east of St. Joseph ' s Point. 'Ihis anatexis corrmences with a coarsening of grain in the more felsic bands, and passes into a distinctive feldspathoblastic rock. Selective anatexis culminates at the northwest corner of Black Bay where the granitic material has mobilized to the extent that the more mafic restit e occurs as rounded and partly resorbed xenoliths. '!he granite here is distirx::tively dark brown or grey.

The White Lake granite is interpreted as a le:,.. to intermediate l evel anatectic intrusive canplex (S-type granite), probably not far removed fran its position of generation. The extent of the migmatitic f ringe, now defined along the Black Bay shore, is uncertain elsewhere , although Hale's (1954b) unit 3 suggests it occurs over much of the St. Joseph's Point west of the granite outcrop.

The 'Unconformity' near Elliot Bay

A new tree-fall exposure was examined about 1 km northeast of Elliot Bay. Pink medium­to coarse-grained granitic rock (possibly Chris tie's unit 5) outcrops largely downhill from, as well as in the lower part of the expos ure. Boulders of similar rock, t ogether with boulders and pebbles of cream c:oloured cherty quartzite, vein quartz and pelitic rock, occur in the upper part of the exposure in a medium- to fine-grained matrix apparently similar in ccmposition to the granite. The clasts are slightly deformed aoo the matrix is locally sheared. Quartzite with local quartz pebble horizons is mainly exposed higher on the hill. Further along strike, conglomerate horizons contain amphibolite pebbles . The matrix in the main expos ure is possibly a granite wash containing fragments in an old regolith overlying a granitic basement. Alternatively, the main exposure may represent a tectonic 'roller skate' breccia.

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'North Shore Plut ons '

'!he granite exposed a t the former Gunnar mine s trongly resembles the Mack intosh Bay and Cameron Island granites i n being coarse grained to megacrystic . Exposures visited br iefly i n the mine locality are variously foliated and gnei ssic, and l ocally intensely sheared in places.

'Sponge Rock ' (Hematized Plbite-Carbonate ~

Zones of reddening associ ated wi th fracturing were encountered along the Black Bay shoreli ne. Strongly developed east-str iking fractures on the McLean Islands are accompanied by thin mylonite zones and 'sponge rock' (hematized albite-carbonate dykes), several centimetres wide, of the type noted elsewhere i n the Beaverlodge area in connection with uranium minerali zation (Hoeve, pers . corran.) .

Spheroidal Weathering

The basement gneisses exposed under the Martin unconformity at the former Lorado mill exhibit excellent spheroidal weathering . Similar weathering i s noticeable at and inmediately northwest of the Gunnar mine, s ignifying proximity to a now stripped off unconformable sequence , possibly the 1'1artin or Athabasca .

Sampling for Age Determination

Samples were taken for U/Pb (zircon) determination at the Kansas Laboratories from the White Lake granite (HUD84- 306) , Mackintosh granite (HUD84-307), Cameron Is land granite (HUD84-308), Foot Bay gneiss at Elusive Lake (HUD84-309), Stephens Lake pluton (HUD84- 310 and 311), Gunnar granite (HUD84-315), and the granite at the Elliot Bay 'unconformity ' exposure . In addition samples were taken fran the Nolan block north of the area: Koster's (1972) Granod iorite I (HUD84-301 to 304) and Granodiorite II (HUD84-305). Results of recent Rb-Sr determinations on the Mackintosh and Cameron Is land granites are given by Bell and Blenkinsop (this volume) .

References

Hale, W.E. (1954a): Black Bay map-area, Saskatchewan (prelim. rep.); Geol. Surv. Can., Pap. 53-15.

Hale, W.E. (1954b): Gulo Lake , Saskatchewan; Geol. Surv. Can. , Pap. 54-6 (map with descr. not es) .

Koster, F, (1971): Geological investigations in the Tazin Lake region, northwest Saskatchewan, Canada; unpubl. Ph . D. thesis ; summary in Ned . Akad . Wetensch., Proc., Ser. B, v74, nol, p63-92.

Langford, F.F. (1980): Successor basins in the Beaverlodge area; in Sl..llffl\ary of Investigations 1980, Sask . Geol . Surv., Misc. Rep. 80-4 , p29 .

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Macdonald, R, (1983) : Geology and reg i onal context of t he Oldman Lake area ; i n Summary of Investigations 1983 , Sask . Geol. Surv., Mi sc . Rep. 83- 4 , pl9 - 23 .

Scott, B.P. (1978) : The geol ogy of an area east of Thluicho Lake , Saskatchewan ; Sask. Dep . Mine r. Resour ., Rep. 167 , Slp .

'111omas , M.W. and ~~cdonald , R. (1983) : Review geology: Prince Lake area; in Surranary of Investi gations 1983 , Sask. Geol. Surv. , Misc. Rep . 83-4, p24- 27.

Tremblay, L. P. (1972) : Geolcxiy of t he Beaverlodge mining area , Saskat chewan; Geol . surv. Can . , ~em. 367.