REPORT No. 33. The Geology. of the BRABANT LAKE AREA. Saskatchewan. by S. J. T. KIRKLAND 1959 DEPARTMENT OF MINERAL RESOURCES

Transcription

1 64-D-4-SW REPORT No. 33 The Geology of the BRABANT LAKE AREA Saskatchewan by S. J. T. KIRKLAND 1959 DEPARTMENT OF MINERAL RESOURCES Geological Sciences Branch Precambrian Geology Division HON. A. C. CAMERON Minister J. T. CAWLEY Deputy Minister PROVINCE OF SASKATCHEWAN REPRINTED 1967

2 CONTENTS INTRODUCTION Location and Accessibility Physiography Previous Work Field Work and Acknowledgments References.... GENERAL GEOLOGY General Statement Table of Formations.... Biotite Gneiss and Veined Biotite Gneiss (1)..... Garnetiferous Biotite Migmatite (2) Hornblende and Cale-Silicate Gneisses (3).... Granitoid Hornblende Gneiss (3a)..... Hornblende Gneisses and Amphibolites of Possible Volcanic Origin (3b) Ultrabasic Intrusive Rocks Hornblende Granodiorite and Quartz Diorite ( 4).... Hybrid Hornblende Granodiorite, Quartz Diorite, Diorite ( 4a)..... Biotite Granodiorite and Porphyritic Biotite Granodiorite (5, Sa) Biotite-Muscovite Granodiorite, Quartz Monzonite, Granite (Sb) Pegmatite (6) Pleistocene and Recent D eposits.... STRUCTURAL GEOLOGY.... Folds Lineation Faul ts.... ECONOMIC GEOLOGY..... General Statement Pyrrhotite-Pyrite Mineralization.... Southwest Comer of the Area East Shore of Wierzycki Lake Southwest Shore of Lindsay Lake.... Narrows Between Brabant Bay and Mcivor Chanm W est Shore of Mel vor Channel.... South End of Mcivor Channel.... East Shore of Mcivor Channel Southwest Shore of Brabant Bay Sphalerite-Chalcopyrite Mineralization...,.... PEG Mineral Claims WEP Mineral Claims.... Conclusions 2 Page

3 INTRODUCTION Location and Accessibility The Brabant Lake Area lies between 56 00' and 56 08' north latitude and between ' and ' west longitude. It forms the extreme southwest part of the Reindeer Lake South topographic map sheet, 64-D, on a scale of one inch to four miles. The centre of the area is 90 miles northeast of La Ronge and 115 miles northwest of Flin Flon. During the summer months the area is most readily reached by float-equipped aircraft which may be chartered at La Ronge or Flin Flon. La Ronge and Flin Flon may be reached by all-weather highways, and both are serviced by regularly scheduled airline flights. Flin Flon is also serviced by railway. Brabant Lake, in the eastern part of the area, forms part of a wellestablished canoe route which extends northeastward from the settlement of Stanley on the Churchill River. The eastern half of the area is readily accessible from Brabant Lake. A canoe route, involving five portages, extends from the southwest arm of Brabant Bay along Chartier River to Wierzycki Lake. From the latter most of the western half of the area may be reached. Physiography The Brabant Lake Area forms part of the upper drainage basin of Wapiskau River, an eastward-flowing tributary of Reindeer River. Within the area the general drainage is eastward into Brabant Lake via Chartier River and Payn Creek. A northeastward-flowing tributary of Chartier River drains the south-central part of the area through Main Lake. Ma:{imum local relief in the area is just east of the northern part of Wicrzycki Lake where rocky hills rise about 250 feet above the lake level. Other prominent topographic highs occur along the southeastern margin of the main granodiorite mass west of Wierzycki Lake and north of Lindsay Lake, and along the zone of hornblendic rocks which trends southwestward fro:m Brabant Bay. Near-vertical cliffs, as much as 75 feet high, are locally associated with the hornblendic rocks. Local relief of 100 to 150 feet occurs along a narrow valley which forms a prominent, eastward-trending lineament through the northern part of Main Lake. Relief of similar magnitude also occurs along the narrow valley that trends northeastward from the north end of this lake. Rounded, rocky hills, as much as 100 feet high, occur in places between Mclvor Channel and the southwest arm of Brabant Bay. A flat plain, underlain by deposits of fine silt and clay, extends westward from Brabant Bay. The general level of this plain is broken locally by small outcrop hills and ridges, and, in places, extensive tracts of swamp and muskeg occur. One arm of the plain extends southwestward to the north end of Wierzycki Lake, and a less well defined arm extends north-northwestward from this lake. A similar, low, flat area occurs just east of Wierzycki Lake, one to one and one-half miles north of the south boundary of the area. West of Wierzycki Lake and east of Brabant Bay silt and clay deposits occur in flat-bottomed valleys between long, narrow outcrop ridges. 3

4 The general distribution of outcrop areas is shown on the geological map accompanying this report. The best rock exposures are in an area of relatively clean burn between Mcivor Channel and Chartier River. Previous Work The Oliver Lake map area, of which the Brabant Lake Area is a part, was mapped geologically by Alcock in 1937 on a scale of one inch to four miles (Alcock, 1939). Similar reconnaissance mapping of the Foster Lake Sheet (East Half), adjoining the Oliver Lake Sheet on the west, was done by McMurchy in 1936 (McMurchy, 1938). The Waddy Lake Area, adjoining the Brabant Lake Area on the north, was mapped geologically by Byers in 1948 on a scale of one inch to one-half mile (Byers, 1949). The Windrum Lake Area, adjoining on the west, was mapped by Miller in 1949 on the same scale (Miller, 1951). The Settee Lake Area (East Half), which lies just south of the Windrum Lake Area:, was mapped by Budding in 1953 on a scale of one inch to one mile (Budding, 1955a). The Planinshek-Brabant Lakes Area, which lies just east of the Settee Lake Area and just south of the Brabant Lake Area, was mapped by Budding in 1954 on a scale of one inch to one mile (Budding, 1955b). Field Work and Acknowledgments The Brabant Lake Area was mapped geologically during the summer of Outcrops along the shores of the major lakes were examined, and intervening areas were investigated by means of pace and compass traverses. Over most of the area traverse spacing was one-quarter to one-half mile, depending on the geological complexity. In the northwestern part of the area, which is underlain mainly by granodiorite, the distance between some adjacent traverses was as much as three-quarters of a mile. Vertical aerial photographs, on a scale of one inch to one-quarter mile, provided mapping control. The location of traverses and the positions of outcrops that could be identified on the photographs were plotted in the field on transparent overlays. The basemap, on which all geological observations were plotted, was prepared from the same aerial photographs. The writer was assisted in the field by P. Stringer, senior assistant, and D. R. Pyke, F. J. Sharpley, and E. E. Zederayko, junior assistants. During the last month of the field season D. R. Pyke carried out the duties of senior assistant in the absence of P. Stringer. References Alcock, F. J. (1939): Oliver Lake, Northern Saskatchewan; Geological Survey, Canada, Map 528A, with descriptive notes. Budding, A. J. (1955a1: The Geology of the Settee Lake Area (East Half); Saskatchewan Department of Mineral Resources, Report No (1955b): Geological Map of Planinshek-Brabant Lakes; Saskatchewan Department of Mineral Resources, preliminary edition with marginal notes.

5 Byers, A. R. (1949) : The Geology of the Waddy Lake Area, Saskatchewan; Saskatchewan Department of Natural Resources and Industrial Development, Precambrian Geology Series, Report No. I (1959): Base Metal Mineralization Associated with Pegmatite, Northern Saskatchewan; paper presented at the annual meeting of the Geological Association of Canada, Toronto, March McMurchy, R. C. (1938): Foster Lake Sheet (East Half), Northern Saskatchewan; Geological Survey, Canada, Map 433A, with descriptive notes. Miller, M. L. (1951): The Geology of the Windrum Lake Area, Saskatchewan; Saskatchewan Department of Natural Resources and Industrial Development, Precambrian Geology Series, Report No. 3. 5

6 GENERAL GEOLOGY General Statement The consolidated rocks in the Brabant Lake Area are of Precambrian age. Unconsolidated Pleistocene and Recent sedimentary deposits, which are most widespread in the northern part of the area, partly mask the bedrock. The oldest rocks recognized are quartzo-feldspathic biotite gneisses which represent original sediments. In the southeastern half of the area the original sediments are represented chiefly by garnetiferous biotite migmatite. Metamorphosed calcareous rocks, represented by calc-silicate and hornblende gneisses and granulites, are in places interlayered with the biotite gneisses and migmatite. H ornblende gneiss that outcrops in a narrow, northeastward-trending zone about three miles northwest of Wierzycki Lake may be in part of volcanic origin. Altered ultrabasic rocks outcrop in two widely separated localities, and appear to be the oldest intrusive rocks In the area. They do not form bodies of mappable size. Hornblende granodiorite and quartz diorite form a large intrusive mass in the extreme northwestern part of the area. Smaller bodies of similar rocks occur in a zone extending northeastward from the southwest corner of the area to Waddy River. Hornblende granodiorite and quartz diorite, which form a narrow, northeastward-trending unit through the northwestern part of Lindsay Lake, appear. to be gradational into biotite granodiorite, and may represent a marginal contaminated zone. Biotite granodiorite, in part porphyritic, forms a large northeastwardtrending mass in the northwestern part of the area. It is younger than the hornblende granodiorite and quartz diorite. Small bodies of biotite granodiorite are abundant throughout the meta-sedimentary gneisses and migmatite in the southeastern half of the area. Biotite-muscovite granite and quartz monzonite form a northeastwardtrending unit which extends from near the southwest comer of the area, through Lindsay Lake, to a point north of Payn Creek. Smaller sill-like bodies of similar rock occur in the meta -sedimentary gneisses west of Wierzycki Lake. In most places the rock is sheared or highly foliated, but locally it is massive and gradational into pegmatite. The age relative to the biotite granodiorite is uncertain. Pink pegmatite, at least in part related to the granite and quartz monzonite, forms numerous small bodies in the biotite gneiss. These pegmatite bodies are mainly sill-like, but locally they show a crosscutting relationship with the gneisses. The pegmatite forms only a few bodies of mappable size but east of Wierzycki Lake it forms as much as 85 per cent of some large outcrop areas. White pegmatite is a major component of the garnetiferous biotite migmatite in the southeastern part of the area. It occurs mainly in lenses and small sill-like bodies that appear to be a result of metamorphic segregation. Although there is some overlap, the general boundary between pink and white pegmatite lies along the contact between the biotite gneisses and garnetiferous biotite migmatite. 6

7 Table of Formations CENOZOIC Recent and Pleistocene Till, sand, gravel, lake clays and silt, peat PRECAMBRIAN Granitic Rocks Unconformity Pegmatite, aplite, granite, quartz monzonite, biotite granodiorite Intermediate Rocks Ultrabasic Rocks Meta-volcanic Rocks? Meta-sedimentary Rocks and Migmatite Intrusive Contact Hornblende granodiorite, quartz diorite, diorite, in part hybrid Meta-pyroxenite, serpentinized ultrabasic rocks Intrusive Contact Hornblende gneiss, amphibolite Biotite gneiss, veined biotite gneiss, garnetiferous biotite migmatite, calcsilicate gneisses and granulites, hornblende gneiss, in part amphibolitic Biotite Gneiss and Veined Biotite Gneiss (1) Biotite gneiss and veined biotite gneiss (1) underly the major part of a zone about two miles wide which extends from the southwest corner of the area northeastward to Waddy River. The biotite gneiss is a fine-grained, medium to dark grey rock with light to medium grey weathered surfaces. Quartz, plagioclase, and biotite are the major constituents, but there is usually also a small content of hornblende. Locally hornblende is as abundant as biotite. In places thin gneissic streaks, usually less than one inch wide, are relatively rich in diopside. Magnetite is a common accessory mineral and fine garnet occurs locally adjacent to the contact with the main mass of granodiorite west of Wierzycki Lake. Sillimanite occurs in several places along minor shears parallel to the gneissosity. The total mafic content of the biotite gneiss is in most places 20 to 30 per cent. Locally, however, especially north of Brabant Bay, the mafic content is 10 per cent or less and the rock h as the general appearance of a meta-arkose. In most of the biotite gneiss the gneissosity is accentuated by concordant beaded stringers and thin lenses of pink, granitic to pegma.titic 7

8 feldspar and quartz, which gives the rock a typical veined gneiss or migmatitic appearance. The stringers and lenses most commonly range from one-tenth inch to several inches in width and are as much as several feet in length. Sill-like bodies of pink granite or pegmatite, one foot to several feet wide, also occur in many outcrops. Porphyroblastic feldspars, one-quarter to one inch in diameter, commonly occur in the thinner stringers. In several outcrops the veining closely follows the gneissosity around the noses of sharp minor folds, but some of the larger pegmatitic bodies in places show a cross-cutting relationship. The granitic and pegmatitic vein material consists mainly of feldspar and quartz, but minor amounts of muscovite, biotite, and hornblende may also be present. In some outcrops the small amount of hornblende in the rock is concentrated along or in the granitic stringers. West of Wierzycki Lake, especially near the southwest corner of the area, black, metallic hematite occurs in the granitic and pegmatitic stringers as a conspicuous but not abundant accessory mineral. Magnetite also occurs locally. The amount of vein material in the veined biotite gneiss varies considerably. West of the southern part of Wierzycki Lake it generally forms 30 per cent of the rock, although locally it may form less than 10 per cent. East of the northern part of Wierzycki Lake and northeastward to Brabant Bay pink granitic and pegmatitic material forms 70 to 85 per cent of most outcrops. Some outcrops consist essentially of pink pegmatite and granite with included remnants of biotite gneiss. The small amount of more arkosic biotite gneiss exposed north of Brabant Bay generally shows less than 10 per cent of vein material. In thin section the biotite gneiss and veined biotite gneiss have a granoblastic texture with quartz and plagioclase forming a simple mosaic. Biotite flakes, which are strongly pleochroic from light straw yellow to dark brown or greenish brown, show pronounced parallel alignment, and when the content is high the texture is lepidoblastic. Quartz occurs in part as elongated grains and in thin stringers parallel to the foliation. A fine gneissosity is discernable in thin section due to alternating layers, as much as two millimeters wide, that are relatively rich in biotite, plagioclase, quartz, and microcline and quartz. In some thin sections fine magnetite grains are concentrated along thin layers. The coarse vein material is relatively rich in microcline and quartz. The average grain size shown in 11 thin sections is about 0.5 millimeter. In the coarse vein material the grain size in thin section is as much as 2.0 millimeters. The mineral composition as shown in 11 thin sections is variable and depends mainly on the amount of vein material present. The more veined specimens are generally richer in microcline and quartz. The compositional range is nine to 36 per cent quartz, 22 to 61 per cent plagioclase, a trace to 29 per cent microcline, and 11 to 38 per cent biotite. Seven sections show one to six per cent hornblende and six sections show a trace to two per cent muscovite. Magnetite, zircon, epidote-zoisite, and apatite are minor constituents, and a trace amount of sphene occurs in the section showing the most hornblende. Penninite and carbonate occur as minor alteration products of biotite and plagioclase. In eight of the thin sections the plagioclase is clacic oligoclase; in the other three it is sodic andesine. A thin section of arkosic biotite gneiss from north of Brabant Bay shows a granoblastic mosaic of plagioclase and quartz having an average 8

9 grain size of about one millimeter. The quartz occurs mainly as elongated grains in streaks parallel to moderately aligned biotite and penninite after biotite. The composition as shown in thin section is 59 per cent quartz, 33 per cent plagioclase, seven per cent penninite after biotite, one per cent biotite, and traces of muscovite, opaques, apatite, and carbonate. The plagioclase is intermediate andesine. The mafic minerals in some outcrops of veined biotite gneiss northeast of Lindsay Lake are highly chloritized. A thin section of such rock shows 14 per cent quartz, 56 per cent highly saussuritized plagioclase, 11 per cent microcline, two per cent biotite, 11 per cent penninite after biotite, one per cent hornblende, two per cent opaques, two per cent carbonate, and one per cent epidote-zoisite. Garnetiferous Biotite Migmatite (2) Garnetiferous biotite migmatite (2) underlies most of the southeastern part of the area. The boundary with veined biotite gneiss ( 1) is in places indistinct, especially east of Wierzycki Lake. In many places, however, the difference between the two rock types is readily apparent and small mappable units of the garnetiferous migmatite occur in the veined biotite gneiss southwest of Brabant Bay and along the west shore of Wierzycki Lake. The garnetiferous biotite migmatite (2) is a medium-to coarse-grained, light grey rock which weathers greyish white. It is characterized by abundant garnet and by stringers, lenses, and sills, one-quarter inch to several inches wide, of white pegmatitic material. This pegmatitic material gives the rock a pronounced migmatitic gneissosity. Augen-like porphyroblasts of plagioclase or plagioclase and quartz, averaging three-quarters of an inch but as much as four inches in diameter, are abundant, and in places the rock is massive porphyroblastic rather than migmatitic. Pegmatite dykes and small bodies of whitish grey granodiorite occur throughout the migmatite. South of Main Lake these small bodies are the most abundant, and in this locality some of them are pinkish in colour. The garnetiferous biotite migmatite consists essentially of quartz, plagioclase, biotite, and garnet. The garnets, light pink in colour, average one-quarter inch but are as much as one inch in diameter. Pale bluish cordierite, in rounded knots one-quarter to one inch in diameter, is a conspicuous accessory mineral in many places, being most common and abundant in a broad zone trending northeastward through the northern part of Main Lake and through the narrows between Brabant Bay and Mcivor Channel. Graphite, in small flakes, is a common accessory mineral, and hornblende and sillimanite occur locally in small amounts. The garnets are generally mos t abundant in the medium-grained parts of the migmatite but in places they are equally abundant in the coarse pegmatite material. Cordierite occurs in both the medium-grained and pegmatitic material but is most abundant in, and in places confined to, the latter. Garnet commonly forms l O per cent but may form as much as 25 per cent of the rock; it is absent in very limited areas of some outcrops. In thin section the garnetiferous biotite migmatite shows blocky porphyroblasts of plagioclase, subhedral to cuhedral garnets, and knots 9

10 of plagioclase and quartz set in a matrix of quartz, plagioclase, and biotite. The porphyroblasts and augen of plagioclase and quartz are as much as five millimeters in maximum dimension; the grain size of the matrix averages about one millimeter. The biotite flakes have a strong preferred orientation although trains of biotite flakes are swirled around the larger porphyroblastic knots. Quartz grains tend to be elongated, and streaks and lenticles of quartz parallel the foliation of the biotite. Considerable movement has taken place within the rock as indicated by bent and broken albite twin lamellae in the plagioclase, curved flakes of biotite, and microscopic shears and crush zones paralleling the gneissosity. The biotite in thin section shows very strong pleochroism from light straw yellow to deep reddish brown. The reddish brown colour of the biotite is a distinctive feature of the garnetiferous migmatite. Garnet and cordierite grains are highly fractured. In the larger grains these fractures are commonly normal to the gneissosity. The garnets for the most part have a seive texture, with quartz and biotite inclusions prevalent. Cordierite in thin section shows considerable alteration to a fine micaceous aggregate. Seven thin sections show 23 to 52 per cent quartz, 26 to 44 per cent plagioclase, 13 to 23 per cent biotite, and three to 13 per cent garnet. Five of the thin sections show a trace to seven per cent microcline, three show a trace to two per cent sillimanite, and two show three and nine per cent cordierite. Zircon and graphite are common accessory minerals, and sphene and apatite each occur in two thin sections. Penninite, epidote-zoisite, and carbonate occur in trace amounts as alteration products. The plagioclase in all thin sections is calcic oligoclase. A thin section of a non-garnetiferous, medium-grained, whitish grey, granodiorite-likc rock, which occurs in garnetiferous biotite migmatite southwest of Main Lake, shows grains of calcic oligoclase and quartz, ranging from 0.7 millimeter to 2.5 millimeters in maximum dimension, set in a matrix of quartz, oligoclase, microcline, and biotite having an average grain size of about 0.6 millimeter. Some of the larger grains of quartz and feldspar are elongated parallel to a moderate foliation of the biotite flakes. The biotite shows the strong pleochroism from pale straw yellow to reddish brown which is typical of the mineral in the garnetiferous biotite migmatite. The mineral composition as shown in the one thin section is 27 per cent quartz, 54 per cent calcic oligoclase, nine per cent microcline, 10 per cent biotite, and traces of magnetite and zircon. A very minor amount of penninite occurs as an alteration product of the biotite. A thin section of non-garnetiferous biotite gneiss that is intimately associated with garnetiferous biotite migmatite shows the same general textural features as the garnetiferous rock. The major difference is that the biotite is pleochroic from pale straw yellow to very dark brown rather than the distinct reddish brown which is typical in the gam etiferous migmatite. The biotite content is also distinctly greater in the nongarnetiferous rock. The thin section shows 25 per cent quartz, 44 per cent calcic oligoclase, 31 per cent biotite, and traces of microcline, apatite, zircon, opaques, epidote-zoisite, sillimanite, and penninite. 10

11 Hornblende and Cale-Silicate Gneisses (3) Hornblende and calc-silicate gneisses (3) form several mappable units in the broad zone of meta-sedimentary gneisses which trends northeastward through the centre of the area. Narrow units of hornblende gneiss also occur in the garnetiferous biotite migmatite. Small unmappable units occur in many places in the veined biotite gneiss and at several localities in the gametiferous migmatite. There are many varieties of hornblende and calc-silicate gneisses due to variations in the relative proportions of certain diagnostic minerals. The various rock types are intimately associated with one another and cannot be differentiated on the scale of mapping used. For descriptive purposes these rocks may be divided into hornblende gneisses and calcsilicate gneisses. The hornblende gneisses are fine to medium grained and black to greenish black in colour. A fine gneissosity due to alternating felsic and mafic streaks is common, but some varieties are more massive and could be called granulites. The most abundant variety of hornblende gneiss consists essentially of plagioclase and hornblende and variable amounts of diopside, quartz, and biotite. In thin sections the rocks show a granoblastic, interlocking texture. The average grain size is about 0.75 millimeter but grains range in size from 0.4 millimeter to three millimeters. The plagioclase, which is intermediate to calcic andesine, occurs in blocky, moderately saussuritized grains. The hornblende is pleochroic from yellowish. green to dark green or brownish green. In sections containing diopside some secondary hornblende, which occurs as an alteration product of the diopside, is pale green and weakly pleochroic. The diopside is colourless and occurs as blocky grains. Eight thin sections show 31 to 72 per cent plagioclase and 24 to 64 per cent hornblende. Diopside forms four to eight per cent of three sections, quartz forms a trace to four per cent of four sections, and biotite forms two per cent of two sections. Minor accessory constituents are opaques, apatite, sphene, and carbonate. One section shows four per cent epidote-zoisite and five per cent sphene. The second most common variety of hornblende gneiss is rich in both hornblende and diopside. In thin section the rock shows a granoblastic texture with an average grain size of about 0.5 millimeter. There is a fine gneissosity due to alternating hornblende-rich and diopside~rich layers. Two thin sections show 32 to 36 per cent hornblende, 19 to 25 per cent diopside, 26 to 29 per cent plagioclase, three per cent opaques, a trace to eight per cent quartz, five to seven per cent epidote-zoisite, and traces of apatite, sphene, and carbonate. One section shows two per cent scapolite. A thin section of a slightly coarser grained variety shows 53 per cent hornblende, 47 per cent diopside, and traces of biotite, opaques, and apatite. Some of the hornblende gneisses are more quartzo-feldspathic and appear to be between biotite gneiss and hornblende gneiss in composition. Two thin sections of such rocks show 21 to 25 per cent quartz, 59 to 60 per cent plagioclase, two to nine per cent biotite, four to six per cent hornblende, two to five per cent diopside, and traces of opaques, apatite, I I

12 and sphene. One of the sections shows three per cent penninite and one per cent epidote-zoisite. Two thin sections of gneiss rich in both hornblende and biotite were studied. These show two to 12 per cent quartz, 23 to 37 per cent plagioclase, 23 per cent biotite, 17 to 52 per cent hornblende, and traces of opaques, apatite, and sphene. One of the sections also shows nine per cent diopside and one per cent carbonate. A thin section of a closely associated amphibolitic rock shows 93 per cent hornblende, three per cent diopside, four per cent plagioclase, and traces of biotite and apatite. The opaque minerals in the hornblende gneisses are, in general order of abundance and frequency of occurrence, pyrrhotite, magnetite, pyrite, graphite, and chalcopyrite. Rocks containing appreciable amounts of the sulphide minerals are described in the section on economic geology. Cale-silicate gneisses and granulites form layers a few inches to several feet wide in the hornblende gneisses. These calcareous rocks are most abundant in the zone of hornblende gneisses lying between the veined biotite gneisses and garnetiferous biotite migmatite, especially in the vicinity of Brabant Bay. Small amounts of calc-silicate rocks also occur in the other units of hornblende gneiss and to a minor extent in the veined biotite gneisses. The calc-silicate gneisses and granulites are medium to coarse grained, light greyish green to greyish white on fresh surfaces, and generally brownish coloured on weathered surfaces. In some rocks there is a pronounced mineral layering but others are more massive and granulitic. The calc-silicate rocks consist of variable amounts of carbonate, diopside, scapolite, and hornblende along with feldspar and quartz. Thin section study shows carbonate-rich, carbonate-scapolite, diopside-scapolite, and tremolite-scapolite varieties. A thin section of carbonate-rich rock shows a granoblastic texture and an average grain size of about 0.5 millimeter. The section shows 45 per cent calcite, 36 per cent highly sericitized plagioclase, 11 per cent microcline, six per cent epidote-zoisite, two per cent sphene, and traces of opaques and apatite. A thin section of a greyish white, impure, crystalline limestone shows 63 per cent calcite, 18 per cent quartz, seven per cent tremolite, four per cent diopside, five per cent graphite, two per cent highly saussuritized plagioclase, one per cent sphene, and a trace of scapolite. Another thin section of similar rock shows 55 per cent calcite, 14 per cent scapolite, 14 per cent quartz, 12 per cent diopside, five per cent tremolite, and traces of apatite and sphene. A thin section of diopside-scapolite gneiss shows 47 per cent scapolite, 35 per cent diopside, 11 per cent calcite, three per cent epidote-zoisite, three per cent sphene, and traces of tremolite and apatite. A thin section of tremolite-scapolite gneiss shows 48 per cent tremolite, 38 per cent scapolite, 13 per cent plagioclase, one per cent diopside, and a trace of sphene. Granitoid Hornblende Gneiss (3a) Granitoid hornblende gneisses (3a) outcrop around the southern part of Wierzycki Lake near the southwest corner of the area. Thin, unmap- 12

13 pable units of veined biotite gneiss are intimately mixed with the granitoid hornblende gneiss in many outcrops, and pegmatite sills and dykes are abundant. The granitoid hornblende gneisses are medium-grained, strongly gneissic rocks. The gneissic layers, ranging from a fraction of an inch to about two inches in thickness, range in colour from light grey to black due to differences in the relative amounts of mafic and felsic minerals. Pink layers of medium-to coarse-grained granitic and pegmatitic material, similar to those in the veined biotite gneiss, are abundant. Thin sections of granitoid hornblende gneiss show a granoblastic texture and an average grain size of about 0.6 millimeter. There is a strong parallel alignment of biotite flakes and a definite mineralogical layering. Elongated grains and lenses of quartz parallel the gneissosity. The biotite is strongly pleochroic from pale straw yellow to medium brown or greenish brown, similar to the biotite in the veined biotite gneiss. The hornblende in general is brighter green than in the other hornblende gneisses and is pleochroic from light green to medium green. Six thin sections show two to 16 per cent quartz, 42 to 65 per cent plagioclase, a trace to 23 per cent microcline, two to 17 per cent biotite, 12 to 19 per cent hornblende, one to four per cent opaques, and traces of apatite, zircon, sphene, epidote, carbonate, and penninite. One section shows four per cent diopside. The plagioclase ranges in composition from calcic oligoclase to sodic andesine. Hornblende Gneisses and Amphibolites of Possible Volcanic Origin (3b) Hornblende gneisses and amphibolites of possible volcanic ongm outcrop in a narrow, northeastward-trending unit in the northwestern part of the area. These rocks are fine to locally medium grained, dark grey to black, and finely gneissic to massive. In several outcrops the rock is dark green in colour and closely resembles volcanic greenstone. The gneissosity in most places is very fine and individual layers are lens-like and discontinuous. Thin lenses, streaks, and small knots of epidote are common. Compared to the other hornblende gneisses in the area these rocks are remarkably uniform in appearance. They consist essentially of hornblende and plagioclase, with hornblende usually forming 60 per cent or more of the rock. In some outcrops the rock is essentially a fine-grained amphibolite. Ultrabasic Intrusive Rocks Ultrabasic intrusive rocks were noted in two widely separated localities. They do not form mappable units in the area. Serpentinized ultrabasic rock is intrusive into hornblende gneiss of possible volcanic origin in an outcrop two miles west of the northwest end of Lindsay Lake. The rock is black to greenish black in colour and consists almost entirely of fine-grained serpentine. Thin streaks and veinlets of pale greyish green asbestiform serpentine cut the greenish black serpentine. These veinlets range from one-eighth to one-quarter inch in thickness and form about five per cent of the rock. Most of the streaks and veinlets consist of coarse slip fibre serpentine, but small veinlets of cross-fibre also occur. 13

14 Coarse-grained, greenish black meta-pyroxenite occurs in an outcrop area of garnetiferous biotite migmatite about one-quarter mile north of the northeast end of the long narrow lake between Wierzycki Lake and Main Lake. The rock consists almost entirely of blocky pyroxene grains partly altered to hornblende. The actual contacts with the migmatite are not exposed but the intrusive appears to be a sill-like body about 25 feet wide and at least 100 feet long. The meta-pyroxenite is cut by white pegmatite dykes similar to those in the migmatite. Hornblende Granodiorite and Quartz Diorite (4) H ornblende granodiorite and quartz diorite ( 4) form a large intrusive body in the northwestern part of the area. Small amounts of similar rock occur west of the southern part of Wierzycki Lake and northwest of Brabant Bay. The hornblende granodiorite and quartz diorite are medium-grained, massive or foliated to finely gneissic rocks. The colour ranges from various shades of grey to pinkish buff. Black hornblende grains, usually forming 15 to 20 per cent of the rock, give a finely mottled or speckled appearance. Near the hornblende gneisses of possible volcanic origin (3b) the hornblende content is highly variable and has a patchy distribution. In thin sections the hornblende granodiorite and quartz diorite show anhedral, interlocking grains of quartz, feldspar, and hornblende. The general grain size is one-half to one millimeter, but quartz and feldspar occur in part in large knots composed of aggregates of finer grains. Biotite, pleochroic from straw yellow to medium greenish brown, occurs in thin streaks and lenses with the flakes showing a moderate parallel alignment. The hornblende, pleochroic from yellowish green to bluish green, is partly replaced by biotite and h as embayed, corroded-looking boundaries with quartz and feldspar. The plagioclase shows medium saussuritization and is replaced in part by interstitial microcline. Three thin sections of the hornblende granodiorite and quartz diorite show 13 to 26 per cent quartz, 44 to 61 per cent plagioclase, a trace to 21 per cent microcline, two to 10 per cent biotite, two to 19 per cent hornblende, and traces of opaques, apatite, and sphene. One section shows a trace of zircon. Two sections show traces and the other shows four per cent of epidote-zoisite. The plagioclase was identified in two thin sections; in one it is calcic oligoclase and in the other intermediate andesine. Near the contact with the main mass of biotite granodiorite (5) numerous thin sills and dykes of pink aplite, pegmatite, and leucocratic, biotite granodiorite cut the hornblende granodiorite and quartz diorite. Hybrid Hornblende Granodiorite, Quartz Diorite, Diorite (4a) H ybrid hornblende granodiorite, quartz diorite, and diorite ( 4a) form a narrow unit along the southeast margin of the main bioti.te granodiorite mass. These rocks are medium grained, dark grey to grey and black mottled, strongly foliated to finely gneissic, and have a highly variable, patchy hornblende content. They are gradational into biotite granodiorite north of Lindsay Lake and appear to represent a zone of hybrid rocks 14

15 developed between the granodiorite mass and hornblende gneisses. A small body of similar rock occurs in hornblende gneiss about two miles west of Brabant Bay. A thin section of hybrid hornblende quartz diorite shows an interlocking mosaic of feldspar, hornblende, and quartz having a grain size of about 1.2 millimeters. The section shows 12 per cent quartz, 51 per cent plagioclase, eight per cent microcline, 24 per cent hornblende, five per cent biotite, and traces of opaques, apatite, sphene, zoisite, and penninite. A thin section of hybrid hornblende diorite shows a simple mosaic of plagioclase and hornblende having an average grain size of about 0.75 millimeter. The section shows 54 per cent intermediate andesine, 41 per cent hornblende, five per cent penninite, and traces of opaques, apatite, and sphene. Biotite Granodiorite and Porphyritic Biotite Granodiorite (5, 5a) Biotite granodiorite (5) and porphyritic biotite granodiorite (Sa) form a northeastward-trending mass about two miles wide in the northwestern part of the area. Small bodies of biotite granodiorite occur also in the meta-sedimentary gneisses and garnetiferous migmatite, especially in the vicinity of Main Lake. The main mass of biotite granodiorite is medium to coarse grained, light greyish buff to pink, and well foliated. The biotite content is generally less than 10 per cent and the quartz content usually more than 30 per cent. Along the northwestern margin of the main granodiorite mass the rock is very leucocratic, coarse grained, and noticeably quartz-rich. A thin section of biotite granodiorite shows an interlocking mosaic of plagioclase and quartz with a small amount of interstitial microcline. The grain size averages about one millimeter, but ranges from 0.5 millimeter to five millimeters. As shown in the one section the rock contains 23 per cent quartz, 67 per cent intermediate oligoclase, seven per cent microcline, three per cent penninite after biotite, and traces of biotite, zircon, sphene, and epidote. The central part of the main biotite granodiorite mass is largely porphyritic (Sa). Pink to buff, blocky phenocrysts of microcline, one-half inch to four inches in maximum dimension, form from less than five per cent to locally as much as 20 per cent of the rock. In some outcrops the phenocrysts are prominent but some large outcrops contain only a few, widely scattered phenocrysts. The porphyritic granodiorite is generally more quartz-rich than the normal biotite granodiorite and the quartz grains tend to be elongated parallel to the foliation. A thin section of porphyritic biotite granodiorite shows blocky grains of plagioclase partly replaced by microcline, and elongated grains of quartz and quartz-rich stringers paralleling the moderately foliated biotite, which is pleochroic from straw yellow to very dark brown. The grain size ranges from 0.5 millimeter to three millimeters. The section shows 51 per cent quartz, 42 per cent intermediate oligoclase, five per cent microcline, two per cent biotite, and traces of muscovite, opaques, zircon, and penninite. Due to the coarse grain size of the microcline phenocrysts the composition as shown by the section does not accurately represent the composition of 15

16 the rock as a whole. The composition given is probably representative of the groundmass but in many places the rock will contain much more microcline. The smaller bodies of biotite granodiorite that occur in the garnetiferous biotite migmatite are mainly light grey to greyish buff, uniformly medium-grained rocks. They are less quartz-rich than the main mass of granodiorite and in places they are finely gneissic. Although slightly cross-cutting in part, these bodies appear to be mainly sill-like. Biotite-Muscovite Granodiorite, Quartz Monzonite, Granite (5b) Leucocratic, biotite-muscovite granodiorite, quartz monzonite, and granite (Sb) form a northeastward-trending mass, averaging about onequarter mile wide, which extends from near the southwest corner of the area, through Lindsay Lake, to a point north of Payn Creek. Smaller sill-like units, many of them unmappable, occur throughout the zone of veined biotite gneiss. The biotite-muscovite granodiorite, quartz monzonite, and granite are for the most part medium-grained, pink or pink and white mottled, strongly foliated rocks. In many places they have a coarse, phacoidal or lens-like, pink and grey gneissic layering which gives them the general appearance of a very highly granitized phase of the veined biotite gneiss. In other places they are massive and grade into coarse pink pegmatite, and are very similar to the vein material in the veined biotite gneiss. Near the southwest corner of the area the narrower units are highly sheared to almost schistose rocks with abundant muscovite or sericite along very closely spaced shear planes. The sheared rocks are in places gradational into highly foliated rocks. In thin section the biotite-muscovite granodiorite, quartz monzonite, and granite show similar textural and structural features. They show anhedral, interlocking grains of quartz, plagioclase, and microcline, and have an average grain size of about 0.75 millimeter. Flakes of muscovite and biotite, the latter pleochroic from straw yellow to medium brown or greenish brown, have strong parallel alignment. Some quartz grains are elongated parallel to the foliation, and thin parallel streaks are relatively rich in quartz and microcline. Microscopic shears also parallel the foliation. Mineralogically, granodiorite, quartz monzonite, and granite are all represented, although all have the same general appearance in outcrop. The compositional range as shown in four thin sections is 18 to 58 per cent quartz, 15 to 53 per cent plagioclase, 17 to 26 per cent microcline, a trace to two per cent muscovite, two to seven per cent biotite, one to two per cent magnetite, and traces of zircon and apatite. One of the sections shows five per cent penninite after biotite. The plagioclase was identified in one thin section as calcic oligoclase. A thin section of pink and grey layered, gneissic rock shows 48 per cent quartz, 42 per cent plagioclase, nine per cent biotite, two per cent muscovite, and traces of magnetite, zircon, and penninite. The age of the biotite-muscovite granodiorite, quartz monzonite, and granite relative to the biotite granodiorite and porphyritic biotite granodiorite is uncertain. They probably represent a younger, potash-rich, 16

17 granitic to pegmatitic phase of the main biotite granodiorite mass. The vein material in the veined biotite gneiss is probably closely related. Pegmatite (6) Pegmatite bodies, in the form of sills, dykes, and small masses, are abundant throughout the area. Most of these bodies are too small to be shown on the present scale of mapping, although in places they form over SO per cent of outcrop areas as indicated on the accompanying map. Pegmatite forms SO to 85 per cent of the outcrop areas along a zone extending northeastward from the southern part of Wierzycki Lake to Brabant Bay. The pegmatite bodies in this zone, and in the rocks northwest of the zone, are generally coarse grained, pink to pink and white mottled, and relatively rich in microcline. They consist essentially of quartz, microcline, and a variable but usually small amount of plagioclase. Biotite and muscovite are common accessory minerals, and very minor amounts of magnetite, black metallic hematite, and tourmaline occur locally. The grain size is most commonly one to three inches but in places is as much as 10 inches. Graphic intergrowths of quartz and microcline occur in the coarsest pegmatites. In the veined biotite gneiss the pink pegmatite occurs mainly as conformable stringers, lenses, and sills, but dykes as much as 10 feet wide are also common. The thin, conformable stringers and lenses commonly have a porphyroblastic appearance and a pinch and swell structure suggestive of replacement origin. Pink pegmatite which forms parts of some outcrops of biotite granodiorite, quartz monzonite, and granite are gradational into these rocks. These larger bodies of pegmatite locally contain inclusions of biotite and hornblende gneisses and the gneissosity of the surrounding rocks is distorted. In such localities the pegmatite was probably intruded into the country rock. Along the northwestern margin of the main biotite granodiorite mass sills and dykes of pink pegmatite and aplite intrude hornblende gneisses and hornblende granodiorite. The spatial relationship suggests that in this locality the pegmatite and aplite are related to the biotite granodiorite. White pegmatite forms an essential part of the garnetiferous biotite migmatite, where it occurs chiefly as thin, beaded stringers, lenses, and sills that appear to be of replacement origin. It also occurs as dykes in the migmatite and in the hornblende and calc-silicate gneisses along the northwestern margin of the migmatite. Dykes of white pegmatite, however, are much less common than dykes of pink pegmatite. The white pegmatites consist essentially of plagioclase and quartz with minor but variable amounts of microcline and biotite. White pegmatites in the garnetiferous biotite migmatite also contain garnet and, in places, cordierite. The grain size, usually one-half to one inch, is generally less than that of the pink pegmatites. The general contact zone between the area containing pink pegmatite and that containing white pegmatite is essentially the same as the contact zone between the veined biotite gneiss and garnetiferous biotite migmatite. There is some overlap, however, between the two types of pegmatite along 17

18 this zone. Some pink pegmatite occurs in the garnetiferous migmatite southwest of Main Lake where it appears to be related to small bodies of granodiorite. White pegmatite dykes occur locally northwest of the general contact zone in hornblende and calc-silicate gneisses. Pegmatite containing green or greyish green feldspar occurs in several localities. The two most important localities are the McKenzie copper-zinc deposit three-quarters of a mile northeast of Brabant Bay, and the WEP copper-zinc prospect just north of the north end of Main Lake. Because of the association with mineralization in these localities the green pegmatites are discussed in a later section on economic geology. Pleistocene and Recent Deposits Lake clays and silt, probably of late Pleistocene age, partly mask the bedrock in a broad zone which extends northeastward from the north end of Wierzycki Lake to a point north of Brabant Bay. Flat expanses of clay and silt having the same general elevation occur at many places along Wierzycki Lake, in the northwestern part of the area, and north of McCuaig Bay in the northeastern part of the area. These deposits give a flat, plain-like topography slightly hummocky in detail. A section of the material about 15 feet thick is exposed at the north end of Wierzycki Lake. This exposure shows unconsolidated, light grey to greyish buff, flat-bedded clay and silt. The individual beds, two to four inches thick, differ only slightly from one another in grain size and colour. Gently sloping sand hills occur one and one-quarter to one and one-half miles northwest of the mouth of Waddy River and also about two and one-half miles north-northeast of the north end of Wierzycki Lake. In the latter locality there is also a small, flat sand plain. A sand hill also occurs at the mouth of Waddy River and steep, deeply eroded, fine sand to silt hills occur just east of Waddy River about two and onehalf miles north-northeast of its mouth. These sand deposits probably represent old beaches or shallow areas in the former lake expanse which is outlined by the clay and silt deposits. The exact outline of this former post-glacial lake is unknown. Within the Brabant Lake Area the main body of this lake was just west of Brabant Bay and northeast of Lindsay Lake. The occurences of flat areas of clay and silt at many places throughout the area suggest that the outline of the lake at its maximum extent was very irregular, with numerous arms and bays extending between outcrop ridges. Pleistocene boulder clay occurs in several localities. Ridges of such material occur near the north boundary of the area north and northeast of Brabant Bay, at several localities in the northwestern part of the area, and across the southern part of the map area between Wierzycki Lake and Brabant Lake. Small amounts of Recent alluvium and peaty swamp deposits are most extensively developed in shallow depressions in the flat clay and silt plain west of Brabant Bay. The most notable accumulation of Recent alluvium is in the western part of Brabant Bay where silt is being discharged into the bay by Payn Creek and Waddy River. 18

19 STRUCTURAL GEOLOGY Folds The main petrographic units in the Brabant Lake Area have a general northeast strike and northwest dip. The gneissosity in the meta-sedimentary gneisses parallels the trend of these units and is probably parallel to original sedimentary bedding. About one and one-half miles north and one-half mile east of the southwest corner of the area, biotite and hornblende gneisses outline the nose of a fold structure which is overturned to the southeast and plunges steeply northwest. Due to the lack of primary sedimentary structures in the gneisses and the general poor rock exposure in this locality the exact nature of this fold is unknown. Throughout the rest of the area there are no major fold structures indicated by the trend of petrographic units or by gneissosity attitudes. Minor folds and crenulations which occur in some outcrops are mainly isoclinal and overturned to the southeast, and generally plunge northwest or north. In the southwestern part of the area some minor folds plunge southwest or south. It is possible that major folds do exist in the area, and if so the attitudes of minor folds suggest that they are isoclinal and overturned to the southeast. Large warps in the general northeast trend of the petrographic units occur in several localities. A pronounced S-shaped fold, plunging steeply northwest, occurs along the general contact zone between veined biotite gneiss and garnetiferous migmatite one to one and one-half miles east of Wierzycki Lake. A similar northwest-plunging, S-shaped fold is outlined by hornblende gneiss and granodiorite units around the northwestern part of Lindsay Lake. The same curvature is reflected by the hornblende gneiss unit two miles northwest of Lindsay Lake. The hornblende and calc-silicate gneiss unit along the contact zone between veined biotite gneiss and garnetiferous migmatite outlines northwest plunging anticlinal and synclinal warps northwest of Main Lake. Other open warps occur along this zone further northeast toward Brabant Bay, and also about three-quarters of a mile northeast of the bay in the vicinity of the McKenzie copper-zinc deposit. A prominent anticlinal warp, plunging northwest, occurs about two and three-quarter miles west of the mouth of Payn Creek, and another warp is indicated by gneissosity attitudes in the few outcrops exposed northwest of Brabant Bay. Linea ti on Linear elements, consisting of elongated mineral grains, mineral streaks, and the axes of minor folds and crenulations, are well developed in the eastern part of the area. These linear elements plunge northwest to north-northwest at 30 to 40 degrees. Near the southeast corner of the area the plunge is locally to the north or northeast and the angle of plunge is slightly more variable. In the western part of the area lineation is not well developed, but where observed the plunge is mainly in a northwesterly direction. Around the southern part of Wierzycki Lake the plunge is locally northeast or 19