Preliminary Report, Geology of Lake Assinica Area, Abitibi Territory

RP 550(A) PRELIMINARY REPORT, GEOLOGY OF LAKE ASSINICA AREA, ABITIBI TERRITORY P.R. NO. 550 DEPARTMENT OF NATURAL RESOURCES Honorable GASTON BINETTE PAUL-EMILE AUGER Minister Deputy Minister Geology of LAKE ASSINICA AREA ABITIBI TERRITORY PRELIMINARY REPORT by L.B. Gillett 80° 78° 7fi° 74° 71° I 52° R. T _.—! . -- - 57° ✓ ' RAIE JAMES de Rupen R81, Rupert Houx de o i I I Hannah lm~~ b~ ~ R. de Rupert+' N ~ /~ I ~ -- ~_„_ _ L Lae Mlaau:.:J j ' I \ ~ 1 ♦ i S / ~ \ p L. F.lnns ' r~ 1. 116anct • , ~ I ~ Y ~ ~I _ I .,ili i i " I 1~ .. A.vinurn \ ~ô ^t ~ "t' ~ ~ I• z rI Q s ?U $. B C _.- '- -r - - - ''''''1..!L hR '.raniah: . S0. so-I n i pe°° L. ~. .ga, •• ~sr°~lr~ ~ - ~ - 2L Ma ~~, l ~ L. Grossn 11:"."":4.- ~ L. nu CM/n.°! ~s .1L fhib~Jpnmau Montai 1. 2i~ 1 L. Wnsmnip:~ . ~Y' z l ` A ~r ~ t ' nthe. I / - L. \ïro~ Â, O i ~L!Qu6. lan 1 p~yy I M~e.si i ~ '~ Ba.~4 k d „ ~ ~ L.du11:k - saw. ;1,,, ' L Aeitib [ -" Ita n7 q n p, u lA..B.r .~vn.e Manful e. Mahanie Rk_SF.RVOIR y h I COOIV 45 50 • vo d'Or r LC urn I I l~~..®e. i • 48 48° _ -i---4~- ~JfJ ~ - ' ._ - h ~ . -~—~`_ _ . .1 ~ n,~~ g0= 78° 76° 74° 72° QUEBEC 1966 P.R. NO. 550 P.R. NO. 550 QUEBEC DEPARTMENT OF NATURAL RESOURCES Honorable GASTON BINETTE PAUL-EMILE AUGER Minister Deputy Minister GEOLOGICAL EXPLORATION SERVICE ROBERT BERGERON, Director Geology of LAKE ASSINICA AREA ABITIBI TERRITORY PRELIMINARY REPORT by L.B. Gillett QUEBEC 1966 P.R. NO. 550 Preliminary Report on LAKE ASSINICA AREA Abitibi Territory by L.B. Gillett INTRODUCTION The Lake Assinica area, mapped during the summer of 1964, comprises 381 square miles bounded by latitudes 50°30' and 50°45' and by longitudes 75000' and 75030'. Lake Assinica itself straddles the southern boundary of the area and is approximately 55 miles northwest of Chibougamau. Floatplanes can land on many lakes within the area and were used exclusively as a means of access. The nearest commercial floatplane base is at Caché lake, 6 miles south of Chibougamau. The area can also be reached by canoe from the Chibougamau-Senneterre highway, but all water routes involve a series of portages and are generally impractical. The area lies entirely within the drainage basin of Broadback river. The Broadback (North Broadback) drains the northern part of the area; Assinica river (South Broadback) drains Lake Assinica. Both rivers, together with their associated lakes, provide good canoe routes for travel within the area. Assinica river joins the Broadback, which ultmately flows into James bay, 150 miles west of the junc- tion. - 2 - The major topographic features of the area reflect the underlying geology. Discontinuous ridges mark two major bands of metavolcanic rocks that trend easterly through the central part of the area. These ridges, on the average, rise about 300 feet above the intervening lowland which is on metasedimentary rocks. In general, the gneiss-amphibolite complex north and south of the volcanic-sedimentary zone is low and is covered by drift. Large masses of alaskitic granite or of pegmatite are relatively resistant and account for the hilly topography near the eastern boundary of the area. On the other hand, the porphyritic granite and porphyritic syenite have no distinctive topographic expression. The smaller features of the topography are, in most cases, the direct result of glaciation, whereas eskers, drumlins and low morainic ridges are abundant throughout much of the area. Large outcrops are virtually restricted to hilltops. Small outcrops occur on or near hilltops and along some shorelines. The search for outcrops was conducted along traverse lines spaced at half-mile intervals, but intervening areas of abundant outcrop were also visited. The shorelines of all large lakes, streams and rivers were examined. GENERAL GEOLOGY All bedrock in the area is Precambrian in age. The gneisses and amphibolites exposed in the north and south are tentatively considered to be the oldest rocks present. According to this interpretation, the pre-Broadback Group of metasedimentary rocks lies in a syncline that trends easterly across the middle of the area. Most of the lenticular bodies of metagabbro and metadiorite are probably related to the pre-Broadback Group, though some occur in the Broadback Group. Late- and post-orogenic masses of alaskitic granite, porphyritic granite and granitic pegmatite have invaded the gneisses extensively, but are rare in the volcanic-sedimentary zone. The only large intrusive body in the latter is a porphyritic syenite. Younger tourmaline- and, locally, spodumene-bearing pegmatites are closely associated with this syenite and are probably genetically related to it. Dikes of diabase that cut gneiss near the southwest corner of the area are probably the youngest rocks represented. 3 Table of Formations PLEISTOCENE Moraine, outwash; eskers AND RECENT Gravel; sand; silt Diabase Granitic pegmatite Alaskitic granite Post-Broadback Intrusive Rocks Porphyritic granite Tourmaline-spodumene pegmatite Porphyritic biotite- hornblende syenite PRECAMBRIAN Gabbro and diorite S Conglomerate Broadback Group Graywacke ROCK Black shale Andesite and basalt Pre-Broadback Group Layered rocks (chiefly pyroclastics?) Amphibolite Mesocratic hornblende Pre-Broadback (?) and biotite gneiss METAMORPHIC Leucocratic biotite and hornblende gneiss - 4 - The present map differs chiefly from the older one by Shaw (1942) in that the Broadback Group is now recognized in this area. Shaw's map showed the Broadback rocks only in the adjoining area to the west. PRECAMBRIAN Pre-Broadback(?) Leucocratic Biotite and Hornblende Gneiss Gneisses ranging in composition from granitic to granodioritic comprise the major part of the bedrock in the northern and southern thirds of the area. White feldspar is the major constituent of most of the gneisses, but pinkish feldspar is locally dominant. Colorless quartz typically forms 25-35% of these rocks. Biotite is much more common than hornblende, but the total mafic content of the gneisses rare- ly exceeds 10%. No rocks of undoubted sedimentary origin were recognized, though paragneisses may be common in both the southern and northern gneiss complexes. In the south, the gneissosity is generally out- lined by quartz lenticles and flakes of biotite. Mineral lineations are inconspicuous, but weak horizontal lineations may be detected in some of the gneisses near Lake Assinica and dip lineations are fairly common in the more massive gneisses west of Lake Assinica near Triart (Puzzle) lake. In the north, the gneisses tend to be relatively heterogeneous. Irregular compositional layering is more common and textures are more variable. The ages of the gneiss zones relative to the intervening volcanic-sedimentary zone and relative to one another are conjectural. However, the northern and southern gneisses are tentatively correlated and are assumed to under- lie the volcanic-sedimentary rocks. Contacts between the major zones were not seen. 5 Mesocratic Hornblende and Biotite Gneiss A distinctive type of gneiss crops out in a zone 2,000-5,000 feet wide extending 5 miles southwest from Labeau (Jonigill) lake. It is characterized by a high mafic content which ranges from 15% to 55% and averages about 35%. The more mafic varieties are composed essential- ly of hornblende and white feldspar. With decreasing mafic content, quartz forms up to 25% of the rock and biotite also commonly becomes abundant. The gneisses of this unit are all relatively coarse grained. Gneissic fabric ranges from weak in the mafic varieties to strong in the less mafic varieties. Compositional layering is generally absent. The relationship of these gneisses to the sur- rounding rocks is unknown. Amphibolite Discontinuous layers or lenses of amphibolite occur at a number of localities, within the leucocratic gneisses, but are sufficiently wide or abundant to be mapped as a unit only near the northwestern and southwestern corners of the area. The amphibolite contains about equal amounts of hornblende and plagioclase and is finely grained,weakly foliated and layered. It is thought to be metavolcanic. Near the southeast corner of the area, similar amphibolite forms a contorted layer enclosed by pegmatite and gneissic granite. It may be an inclusion or a relic of ungran- itized wall-rock. Pre-Broadback Group Metamorphosed Layered Rocks A diverse assemblage of layered rocks is inter- bedded with metalavas of the pre-Broadback Group. Much of the layered rock forms bodies that are too small or too discontinuous 6 to be mapped as distinct units, but scattered exposures within the southern zone of pre-Broadback rocks indicate that here the layered series predominates across a band up to 8,500 feet wide. The layers range in thickness from a fraction of an inch to several feet. Some of the fine layering may be the result of metamorphism, but the coarser layers are inter- preted as relic beds. In the mapped band of layered rocks, the most common rock type is a fine-grained schist composed of chlorite, amphibole,, feldspar, quartz and minor garnet. It resembles the graywackes farther north, but generally is greenish rather than gray. Structures suggesting crossbedding occur 2,500 feet south of Bruine (Drizzle) lake. Other rock types in this unit include very fine-grained, pale gray, mica schist; pale gray, cherty argillite; medium-grained meta-arkose; and schis- ose agglomerate or conglomerate. None could be traced for more than a few tens of feet along strike. Minor amounts of layered rocks occur throughout the areas mapped as pre-Broadback metalava. The most common type is similar in composition to the lavas with which it is associated, but is thinly layered owing to differences in content of green minerals. Such rocks are thought to be tuffs, though the layering may be a result of metamorphic segregation.

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