-10-

Compilation Geology, Lac Area and part of the Wapawekka Area

(NTS 73P and 731) by L. H. Forsythe

The Lac La Ronge region is bounded by longitudes 104° - 106°w and latitudes 550 - 560 N; the Wapawekka No rtheast region by longitud. es 1040 -106 0 Wand latitudes 1 11 54° 51 30 -ss0 N. The limit of the Precambrian outcrop trends east-southeast from Pinehouse Lake t o the town of Lac La Ronge thence from Nipekamew Bay, Lac La Ronge eastward south of Wapawekka Lake.

Since 1909, fourteen geologists of the federal and provincial surveys have worked in the Precambrian of the area. Compilation of this work mainl y utilizes the available geologic field maps and some published maps of the provincial geol o­ gists.

The author spent the summer carrying out spot checks throughout the area to r elate existing map units to the compilation map units. The compilation map units were in places replotted from the available field geologic maps by means of air photos of approximately the same scale (1:63,360), except in the Se ttee Lake East and the Stanley areas where no field geologic maps ar e on hand. In these areas published maps and field maps from adjacent areas are being utilized to interpolate some of the map units together with structural and foliation trends from air photos. The following are some of the main problems investigated:

In the Wapawekka Lake - Lacey Lake vicinity, Padgham's (1966) arkose was found t o be a granitoid to fine-grained lineated quartz biotite feldspar gneiss with biotite comp rising over 10 percent of the rock. This is gradational with conglom­ erate that l ocally contains numerous mafic pebbles and cobbles which, because they appear to be derived from the nearby volcanic-intrusive comp l ex, are probably coeval epi c lastics. Padgham (op. cit. p.43) thought that all the c l asts are dissimilar to rocks in the map-area.

In the Stanley Sheet, Mawdsley and Grout (1951) grouped many different rocks apparently inconsistently into various map units. They did not, moreover, dis tin­ guish between hornblende granodiorite and biotite granodiorite, nor between differ­ ent phases of intrusion. Abundant migmatites occur and much, though non-pervasive, feldspa r porphyroblastesis is seen in the rocks . -11-

In the Settee Lake East area a band of meta-arkose , not picked up by Budding (1955), passes through Jay Smith Lake and is flanked on the north by a band of biotite gneiss. The biotite gneisses are migmatitic and a wide garnetiferous zone occurs in the vicinity of Settee Lake. Hornblende gneiss mapped in the or iginal survey has been re- identified as granodiorite in some localities.

In the Otter Lake a r ea, Padgham's (1960, 1963) acid volcanics were found to comprise metasediments with basic metavolcanics and int rus i ves. The unit includes conglomerates, as well as some possible tuffs and rare agglomerates .

The acid volcanic unit originall y mapped in the Forbes Lake area (Pearson and Froese, 1959) near Esmay Lake includes rock types similar to those found i n the Otter Lake area. Also as at Wapawekka, conglomerat e grades into gr anitoid and a fine grained siltstone-arkose. Local concentration s of count r y r ock pebbles and cobbles occur near contacts, suggesting some coeval sediment ation and volcanism but t he sour ce of rock clasts dissimilar t o local country rocks is not known.

At Campbell Lake in the Black Bear Island Lake East area (Mo rris, 1965) the migmatites ori ginall y mapped have been shown to consist mainly of gr anites with numerous country rock inclusions . Diorite-gabbro and hornblende granodior ite are common xenoliths in later granites and also intrude the country rocks sporadically.

At Sandfly Lake in the Needle Falls area (Money , 1965) the western granite is intrusive into a basic vol canic-intrusive complex, and into some of the biotite gneisses. Metagabbro intrudes amphibolite (basic volcanic rock) and biotite gneiss. The quartzite of the Meyers Lake group appear s to be gradat ional to meta-arkose and biotite gneiss. Relationships with the intrusives were not seen. Quar tz cobbles outcropping in a shear ed conglomerat e on the Churchil l River resemble vein quartz and large microcl i ne porphyr oclasts are abundant . The distribution of the conglom­ er ate suggest a fluvatile origin.

Feldspar porphyrobl astesis increases in the Sandfly Lake area towards the mylonite which bounds the Wollaston Belt on the east. The blastesis appears to be related in part to a lineate prophyroblastic granite that i mpregnates the biotite and hornblende gneisses in the vicinity of the myloni te. The blastesis is also common east of the mylonite , where an abundant younger por phyblastic granite l ocally int rudes lineat e porphyroblastic gr anite similar to that encountered near the mylon­ ite. -12-

References

Budding, A. J., (1955): The geology of the Settee Lake Ar ea (East Half). Dep t. Mineral Resources, Rept . no. 17.

Mawdsl ey, J . B., Grout, F. F., (1951): The Geology of the Stanley Sheet : Saskatchewan Dept. Mineral Resources Rept. no. 4 .

Money , P. L., (1965): The Geology of the area around Needle Falls, Churchill River, comprising the Eulas Lake Area (We st Half), Sandfly Lake Area (East Half) and Black Bear Island Lake Area (West Half): Saskatchewan Dept. Mineral Resources Rept. no. 88.

Morris, A., (1965): The Geology of the Black Bear Island Lake Area (East Half): Saskatchewan Dept. Mineral Resources Rept. no. 86.

Padgham, W. A· , (1960): The Geology of the Otter Lake Area (West Half): Saskatchewan Dept. Mineral Resources Rept. no. 41.

(1963): The Geol ogy of the Otter Lake Area (East Half): Saskatchewan Dept. Mineral Resources Rep t . no. 56.

(1966) : The Geology of the Wapawekka Narrow Area: Saskatchewan Dept . Mineral Resources Rept. no. 87.

Pearson, W. J. Froese, E., (1959): The Geology of the Forbes Lake Area: Saskatchewan Dept. Mineral Resources Rept. no. 34.