RG 149(A) REGNAULT LAKE AREA, ABITIBI TERRITORY f$0 DEPARTMENT OF NATURAL RESOURCES MINES BRANCH REGNAULT LAKE AREA Abitibi Territory GEOLOGICAL REPORT —149 JEHAN RONDOT 1972 G.R.-149 TABLE OF CONTENTS Page INTRODUCTION 1 Physical geography 2 Orography 2 Hydrography 4 Previous work 4 Field work 8 GENERAL GEOLOGY 9 OPHIOLITIC SERIES (PRE-BROADBACK GROUP) 9 General characteristics and distribution 9 Table of formations 10 Pseudo-graywacke 11 Light grey pseudo-graywacke 11 Grey pseudo-graywacke 12 Carbonate rocks 12 Conclusion 13 White and grey tuffs 13 White tuffs 13 Fine-grained grey tuffs 15 Medium-grained and agglomeratic tuffs 15 Conclusion 16 Lavas and fine-grained, greenish grey rocks 16 Basaltic lavas 17 Andesitic lavas 17 Other rocks 19 Medium-grained, ultrabasic to dioritic rocks 19 Peridotite 19 Gabbro 20 Diorite 23 Conclusion 24 MIGMATITES 26 Transition zone 26 Metamorphism of the transition zone 26 Injections in the transition zone 27 Regional migmatites 28 ACID INTRUSIVE ROCKS 30 Porphyroid granite 30 Hornblende syenite 31 Pluie Lake granite 32 Other small masses of grey granite 34 Grey granite of Laloire lake 35 Grey granite exposed north of Pluie lake 35 Other grey granitic rocks 36 Pink to beige biotite granite 37 Alaskitic pegmatites and aplites 38 YOUNGER DIKES 39 Basic dikes 39 Quartzose dikes 40 CENOZOIC 40 Pleistocene 40 Holocene 41 Page TECTONICS 42 Folding 42 Attitude of foliation 42 Folds 43 Faults and shear zones 43 ECONOMIC GEOLOGY z 45 Copper, lead, zinc, molybdenum sulphides 45 History 45 Mineralized occurrences 45 Spodumene (lithium) 46 GEOCHEMISTRY 49 REFERENCES 52 APPENDIX, 53 ILLUSTRATIONS Map No. 1726 - Geological map at 1 mile to the inch (in pocket) Figure - Spodumene deposit of Moblan lake 47 Plates I - View of Coulombe lake 3 II A- View of La Saigne lake 5 B- Chatillon river 5 III Light grey tuff interbedded with greenish grey rocks 14 IV A- Pillow lavas 18 B- Ophitic texture of d pillow lava 18 V A- Foliated and fractured gabbro 22 B- Ophitic texture of a gabbro 22 VI Gabbro hill and spodumene-bearing pegmatite dikes 48 rabies 1- Mineralogical composition of green rocks 25 2- Elements for calculating the modal content (copper) 50 INTRODUCTION The geological study of this part of the Canadian Shield, 60 miles north-northwest of Chibougamau, was undertaken during the summer of 1963, following successive discoveries of various mineral deposits during the previous five years. The Lake Regnault area is bordered by longitudes 74°45' and 75°00' and latitudes 50°30' and 50°52'30". It covers about 285 square miles. Among the Precambrian rocks, the most interesting for miner- alization are those of the pre-Broadback group which are well represented to the east, where they were studied by D.L. Murphy (1962, 1963), but end abruptly in our area. Migmatites and acid and diabasic intrusive rocks are partly exposed through the thick cover of glacial deposits. This area, which is only accessible by plane or by canoe, is sporadically populated by a few Indian trappers from the Mistassini Post in winter and by a few prospectors during the summer. The rugged climate - lakes only thaw out at the end of May and the temperature goes below the freezing point near the end of August - favours the growth of only a few coniferous and leafy trees. Apart from flies and mosquitoes which abound in early summer and a few hares, the fauna is very poor. Trout were only found in Pluie lake, while pike and doré abound in the larger lakes. -2 - Physical geography Orography The area, located just under 200 miles from James Bay and 300 miles from Quebec City, sits on a flat and relatively low part of the Canadian Shield, the average altitude being 1,500 feet. The relief is that of an elevated peneplane which has been worn by the movement of an- cient continental glaciers. Only a few flattened summits, no more than 2,000 feet high, remain of the peneplain. From such a summit, one can see all of the others almost on the same horizon (Plates I and II-B). The maximum relief of the area is 750 feet, measured between the highest point, a hill situated one mile south of Garlier lake that reaches a height of 2,000 feet, and the lowest point, the Broadback river, with an altitude of 1,250 feet at the northwestern extremity of the area. The altitude of Cachisca lake, south of the area, is also in the neigh- bourhood of 1,250 feet. The plain has an average elevation of 1,400 feet and is dominated by hills only 300 to 400 feet high. These are: 1 - at the northern extremity of the area, a granitic hill rises above the plain. 2 - a migmatite (injection gneiss) hill northeast of La Saigne lake. 3 - a hill of gray granite north of La Loire lake. 4 - a chain of hills made up of pre-Broadback rocks, both west and north of Pluie lake (Plate I). 5 - a chain of hills composed of pre-Broadback rocks, both west and north of Moblan lake (Plate II-A). 6 - a chain of granitic hills between lakes Garlier and Cashisca and, particularly east of Laloire lake. 7 - another group of isolated granitic hills, south of Regnault lake. The plain, formed mostly of morainic deposits, is not without relief. These deposits form mounds (drumlins, kames), a few tens of feet in height, irregularly distributed between the swamps and the low parts of the area, particularly to the north where one such mound, situated east of the northernmost exposure, is more than 100 feet above the surrounding - 3 - Plate I -View of the Coulombe lake at center, Chatillon lake on the right, Chatillon river and Pluie Lake hills on the left, taken from a hill north of Moblan lake. lakes. These deposits also form southwesterly elongated ridges rising to a height of about ten feet. These ridges, separated by straight depressions, are readily visible on the aerial photographs. Small glacial basins were also obserswed, for instance to the west of the southern portion of Regnault lake, where such a circular depression measures more than 50 feet in diameter and close to 25 feet in depth. The eskers add a fancy touch to the landscape with their sharp crests and their steep flanks. They meander, uninterrupted, in a general south-southwest direction. In places measuring 50.feet from crest to base, as at the southern tip of Moleon bay, the eskers may also be reduced to small ridges only a few feet high. They are superimposed on an already existing relief, climbing hills (between Moleon bay and Regnault lake) or crossing lakes (chain of islands in the southern tip of Moleon bay 1) . 1 This body of water is really the southwestern part of Frotet lake. - 4 - Hydrography The striation of the plain by glacial deposits has strongly influenced the paths of rivers and the elongation of lakes, so that it is now difficult to recognize the preglacial hydrographie network. An ancient capture may be noted along the course of the Chatillon river. This river now flows rapidly to the southwest of La Saigne lake, while its former bed is still visible to the southeast of the same lake, running through it (cf. ancient path traced on map). The hydrographie basin is that of the north branch of the Broadback river, except for the southern part of the area where the waters from lakes Oudiette, Cachisca and Laloire supply the south branch of the same river. Previous work In his itineraries of 1885 and 1886, south of Evans lake and some 80 miles west of the area under discussion, R. Bell (1909) crossed the volcano-sedimentary belt which continues almost uninterrupted- ly along the Broadback river up to our area. He describes rocks of this belt near Evans lake as Hur_onian, as opposed to the older Laurentian gneisses, as follows: "The rocks of this exposure which are seen along the Broadback river consist of schists of different hues of green-grey and dark grey, sometimes very disturbed and cut by massive diabase; they also consist of grey felsite and dove grey arkose with bluish grey dolomite". His Huronian also includes a fairly similar belt of rocks which runs southward through the Matagami lake and Chibougamau areas. H.C. Cook (1914) carried oat, in 1912, a reconnaissance geological survey of the Broadback river from Evans lake to our area where he went up part of the north branch of the river. He distinguish- ed two series in the volcano-sedimentary belt: a basic schist complex known as the "Lake Evans Series", overlain by a sedimentary series start- ing with the thick "Broadback Series" conglomerate. In his table of formations, reproduced below, Cooke considers rocks of this belt as being older than the neighbouring rocks which cut through them. - quartz diabase dikes - muscovite granite - biotite and hornblende granites and their gneissic equivalents - "Broadback Series" - "Lake Evans Series" Rocks of the "Lake Evans Series", which he observed south of this lake and on the south branch of the Broadback river, are the oldest around and - 5 - Plate II A - View on La Saigne lake, looking south, with hills of the central portion of the area in the background. B - View on Chatillon river, looking northwest, taken from a hill north of Coulombe lake. - 6 - are similar to those of the Keewatin. "They consist chiefly of dacite por- phyries and basalts, together with small amounts of very basic peridotites. Locally they have been converted into hornblende and chlorite schists; this is true more especially of the finer-grained portions of the flows, as the coarser parts have in general resisted the deforming influences more suc- cessfully.