282 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



BURNTWOOD RIVER AREA^ 



The Burntwood Eiver area lies about 75 miles north of the Cross Lake 

 area. It is readily reached by a portage route starting at mile 185 on the 

 Hudson Bay Eailway. The area shows a complex of volcanic rocks and 

 gneisses surrounding Pipe Lake, a body of water about 10 miles in length, 

 which drained Manasan Eiver into Burntwood Eiver. The belt of green- 

 stone extends northward to the Burntwood and southward west of Pipe 

 Lake. 



The rocks of the area consist of granite, garnetiferous gneisses, green- 

 stones, and green schists. The greenstone belt is very narrow, forming 

 the islands of Pipe Lake and a discontinuous fringe on either side of the 

 lake. Associated with the more massive greenstone rocks are a few local 

 areas of banded rocks which are believed to be tuifs. The characteristic 

 contact phase where the granite intrudes the volcaiiics is a black, glisten- 

 ing hornblende schist. In the granite areas near the contact with the 

 greenstone belt are a number of small, isolated area^ of hornblende schist 

 representing engulfed masses and roof pendants. 



Immediately northwest of Pipe Lake is a ridge composed of finely 

 banded, dark gray gneiss which has the appearance of an altered sedi- 

 ment. It stands on edge; its position between the greenstone band and 

 the granite suggests that it underlies the volcanic rocks. 



Along the contact with the greenstone belt the granite is gneissoid and 

 sheared. On the east side of the lake the banding is highly contorted. 

 This zone grades out into massive granite. On the west side of the lake 

 the granite along the contact with the greenstone is highly sheared with 

 schistosity developed parallel to the contact. 



The granite and granite-gneiss rocks of the area include a number of 

 types which may possibly represent more than one period of intrusion. 

 Along the Burntwood Eiver a garnetiferous gneiss is very abundantly 

 developed. Though locally banded, its general texture and appearance 

 is quite different from the finely Ijanded garnetiferous gneisses of the 

 other areas, which have been described as altered clastic rocks. It is 

 probable that the presence of the garnets means the absorption of en- 

 gulfed sediments. A number of pegmatite dikes were found in the areas 

 in which garnets were similarly developed. Over much of the area the 

 granite is massive, with but little gneissic structure. 



" p. J. Alcock : Ospwagan Lake-Burntwood River area, northern Manitoba. Geol. 

 Surv. Can., Sum. Rept.. 1920. 



