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REPORT No. 107 The Geology of the NEMEI LAKE AREA (West Half) Saskatchewan by R. L. JOHNSON 1966 DEPARTMENT OF MINERAL RESOURCES Geological Sciences Branch Precambrian Geology Division HOM. A. C. CAMERON J. T. CAWLEY Minister Deputy Minister PROVINCE OF SASKATCHEWAN J I CONTENTS Page INTRODUCTION ------------- ------------------------------------------------------------- ------ 5 Location and Accessibility ----------------------------------------- --- ------------ 5 Previous Work ---------------------------------------------------------------------------- 5 Field Work and Acknowledgements ___ -·············· ···-· ·· ·········· ·- 5 Physiography and Glaciation ··· ······-···-·---· --·-·---···-------·----· ··-·----· 6 SUMMARY OF THE PRECAMBRIAN GEOLOGY ·····-·····-·····-·····- 6 THE KISSEYNEW GNEISSES ···-········· ········----------------·------··---········· 7 Th~ Garnet-Biotite Gneisses (4) ·-·---------·--------------···--- -· ········-·-·· 7 The Biotite Gneisses (1) -·-········-··--·· ···············-·-··-··---··-------------· The Cale-silicate and Associated Rocks (2 and 3) --··-·----··---· 9 9 THE DIAPIRIC GRANITES (5 and 6) ---·· ············ ·--·· ··-·······--·-··------ 10 STRUCTURE ·-··-------·----------- -- --·- --··········---················· ·· ······-··· ···--········ 13 DISCUSSION -----·--·····--···· ·-- ---·-····· ···············-······················-········-·--- -· 19 ECONOMIC GEOLOGY ···- ·· ······---··------·--------··----················· ··-······-· ·· 21 REFERENCES --·· ··················· -· --·- ·······································--- -·-· -- -----·---- 21 3 ILLUSTRATIONS Geological Map 107A, Nemei Lake (West Half), Saskatchewan ... ........ in pocket Structural Map 107B ..................................................................................... in pocket Figure 1: Typical first folds .................................................................................... 15 Figure 2: Typical second folds ················· ········································· ·· ············· ···· ·· 16 Figure 3: Pi-diagrams for two major second folds ...................... ..................... 17 Figure 4: First fold refolded by second folds ...................................................... 18 Figure 5: Diagram illustrating relationship between metamorphism and deformation .............................................................................................. 20 Table 1: Summary of the diapiric granites .......................................................... 11 Table 2: Summary of the structural history of the area .................................... 20 Plate 1: Refolded folds ............................................................................................ 4 23 INTRODUCTION This report describes the geology of an area of Precambrian gneisses in Northern Saskatchewan. The rocks lie within the Churchill provin'ce of the Canadian Shield and were subjected to the Hudsonian orogeny circa 1700 m. y. ago (Stockwell, 1961). LOCATION AND ACCESSIBILITY ·- The Nemei Lake Area (West Half) (63-M-8-W) comprises about 175 square miles extending from 55° 15' to 55 ° 30' north latitude and from 102 ° 15' to 102 ° 30' west longitude. The centre of the area lies just over 40 miles north-northwest of Flin Flon. The Churchill River Power Company's hydro-electric plant and townsite at Island Falls and the adjacent Indian reservation at Sandy Bay lie abo1:1t one-half mile beyond the northern boundary of the map-she'et. The larger lakes in the area are accessible by float-equipped aircraft that may be chartered from Flin Flon or Jan Lake. A number of canoe routes link the region to Flin Flon, the main ones being firstly via Mason, Kipahigan and Mari Lakes and secondly via Nemei, Kakinagimak and Wildnest Lakes. A well-used winter road links Island Falls to Flin Flon. The area, and in particular the northern half, will become readily accessible on the completion of the road linking Island Falls, Sandy Bay, and Pelican Narrows to the Hanson Lake road south of Jan Lake. PREVIOUS WORK Ii The earliest published description of rocks in the area is that of Mclnnes (1913) who in 1910 travelled along the Churchill River and briefly described the gneisses in the northern part of the maparea. The Pelican Narrows quadrangle (63-M), which includes the present area, was reconnoitred in 1929 and 1930 (Satterley, 1931). A further survey of this area was carried out by F. C. Taylor in 1957 and the results were published in 1958 as (Map 1-1958) with marginal notes on a scale of 4 miles to one inch, by the Geological Survey of Canada. The contiguous 15 minute quadrangles to the east, southeast, south, and southwest have been mapped on a scale of one inch to one mile by the Saskatchewan Department of Mineral Resources: (Pyke, 1965); (Cheesman, 1956); (Pyke, 1961) and (Pyke, 1966). FIELD WORK AND ACKNOWLEDGEMENTS r, Field work was carried out during the summer of 1965. The shorelines of all the accessible lakes were mapped in detail and on land, pace and compass traverses were spaced at roughly one-half mile intervals. Data were plotted on one-half mile to one inch base maps compiled from aerial photographs supplied by the National Air Photographic Library, Ottawa. The important contribution to the field work made by R. Netolitski, Senior assistant, is gratefully acknowledged. M. W. Pyke introduced the party to the area at the beginning of the season. Junior assistants were D. Dixon, E. Evans, and R. Wallace. Mr. Bruce Long, Mining Recorder in Flin Flon, and Mr. 0. C. Christensen and his staff at Island Falls gave practical help in the course of the field season. 5 PHYSIOGRAPHY AND GLACIATION The area has the low relief typical of this part of the shield. In the south the hills seldom rise more than 100 fe et above the nearby lakes, but in the west and especially in the north, as the Churchill River is approached, the relief is greater and hills of the order 500 feet in height are common. Although the area lies 200 miles south of the "approximate southern limit of permafrost" (Glacial map of Canada, Geological Association of Canada, 1958), permanently frozen ground was encountered in excavations for the power plant at Island Falls (Johnston, 1930, p. 34-35). Glacial deposits have been seen only in the northern part of the region that lies within the limits of glacial Lake Agassiz, an arm of which extended westward along the Churchill River (See above-mentioned glacial map). Fluvioglacial sands and gravels are exposed in workings for building material near Island Falls. Varved clays are currently exposed a little way beyond the northern boundary of the map-sheet in the spillway of "A" dam at Island Falls. These clays, which are at least 12 feet thick, rest directly on an ice-polished surface of Precambrian gneisses, and at one locality where the surface is sloping, they have an initial dip of about 15°. The varves are of the order 0.75 mm in thickness and each varve grades upwards from a whitish (when dry) silty base to a grey clay top. An unusual feature of these clays is the development within them of concretions. Typically these are lenticular in cross-section and near circular in plan, though a few have more irregular outlines. Most of them range from two to five centimetres in diameter, and have the thickness at the centre about one-fifth of the diameter. They lie with their planes of symmetry parallel to the bedding. The concretions consist of a brownish-grey porcelaneous rock that in thin section is seen to consist almost entirely of calcite varying in grain size from 0.025 mm down to cryptocrystalline dust. At the base of the clays these nodules frequently come into contact with the underlying gneiss, to which they tend to adhere after the clay has been eroded away. SUMMARY OF THE PRECAMBRIAN GEOLOGY The map-area is occupied almost entirely by the Precambrian Kisseynew complex, a group of highly deformed paragneisses that are older than 1700 m.y. and extend over a wide area of Saskatchewan and adjacent Manitoba, north of Flin Flon (Bruce, 1930; Harrison, 1951). Garnet-biotite gneisses underlie most of the maparea though biotite gneisses occur in the west. The hornblende gneisses and associated amphibolites and calc-silicate rocks, that are an important feature of the region to the south, extend into only the southern part of the present area. The biotite and garnet-biotite gneisses represent fine-grained, predominantly sandy sediments that have been highly deformed and have reached the sillimanite grade of regional metamorphism, with the development of veins of granite and granite pegmatite. In the course of the metamorphism the rocks were deformed twice: firstly into tight, isoclinal folds; secondly, refolded into more open structures. The folds that are readily identifiable on aerial photographs belong to the second set. 6 The emplacement of the large bodies of gneissic granite that are a prominent feature of the region, was associated with the second phase of folding. Except where otherwise stated, terminology in this account follows the usage of the Geological Survey of Canada (Harker, 1965). An exception is the term gneissosity, which, following Moorhouse (1959, p. 409), and in conformity with international usage, is omitted from this report. The term foliation is restricted to textures due to the dimensional or lattice orientation of minerals in igneous or metamorphic rocks. The term drag-fold, although widely used in this part of the shield (see for instance Cheesman (1956), Byers and Dahlstrom (1954), or Pyke (1961) is not employed in this account. "Drag-fold" as originally defined by Leith (1914, p. 106) is a genetic term with a specific meaning: "Minor folds are commonly developed in weak beds by the shearing between two or more competent masses of rock. These folds are conveniently designated drag-folds. The position of their axial planes is controlled by the displacement of the more competent beds adjacent. The term "drag-fold" is -desirable as emphasizing the differential movement between the controlling beds. More. recently the designation "drag-fold" has become synonymous with "asymmetric minor fold" or even "minor fold", regardless of the mechanism by which the folds have been formed. There is a strong case for restricting the term "drag-fold" to folds which have been produced or at least modified (Ramberg, 1963) by the mechanism proposed by Leith. None of the small-scale folds examined in the present area are of this type. In the writer's view the indiscriminate use of the term is positively harmful in that it tends to obscure the fact that in a given area in ah orogenic belt, sets of small-scale folds may be of different ages, and may have been produced by a variety of mechanisms. THE KISSEYNEW GNEISSES THE GARNET-BIOTlTE GNEISSES (4) I This group of rocks occupies about two-thirds of the map-area. The gr\_eisses are essentially psammitic and semipelitic schists that have been isoclinally folded, metamorphosed to the silliinanite grade, and in which generally lit-par-lit granitic veins have developed. There is usually a clear distinction between those parts of the rock which represent a metamorphosed sediment and the granitic material. In the more psammitic horizons the sedimentary component is surprisingly fine grained in view of the high metamorphic grade. Sillimanite occurs over the whole of the map-area, as pale-green or white acicular grains occurring as sheaves or thin films along bedding planes. Cordierite is sporadically developed as pale-blue porphyroblasts as much as 5 mm in diameter. Graphite is another accessory mineral commonly visible in hand specimen. Bedding is seen as thin biotite-rich films, and as alternations of more and less pelitic rock on scales of a few centimetres. In the latter case the boundaries between the two types are frequently gradational, but good graded bedding has not been found. The lack of current and graded bedding may be due to the effects of the metamorphism and deformation, but the uniformly fine grain-size and absence of conglomerate must be an original feature of the rock. 7 In general this rock type is poorly exposed, but the mossy ridges that it underlies have scarp and dip slopes which can readily be recognized on air photographs. Most of trend-lines shown on the structural map are of this nature. In thin section the sedimentary component of the garnet-biotite gneiss has plagioclase and quartz as the predominant minerals. Biotite and orthoclase are important accessories, and in some rocks are present in excess of the quartz. Other accessory minerals include garnet, graphite, sillimanite, apatite, and occasionally cordierite. The grain-size of the quartz and plagioclase varies from 0.25 mm to 1 mm; the dimensional orientation of these minerals is weak. The properties of the minerals in this group of gneisses are as follows: ' Varies from oligoclase (An25) to andesine (An45). The foliation is deflected around the larger plagioclase porphyroblasts. Biotite - Ragged to subhedral tabular grains having c. pale yellow brown and W medium reddish brown. Although most of the grains are oriented parallel to one another, defining the foliation, some grains are oriented at high angles to the foliation and are undeformed. Probably nucleation took place during the first phase of deformation, but growth continued later. Plagioclase - Garnet - In metamorphic rocks the shape and orientation of inclusion trails in porphyroblasts, and in particular in garnets, provides important evidence concerning the relationship between the thermal and kinematic history of the rock (see for instance Zwart (1962) and Rast (1965) ). In the present case the smaller ( < 0.25 mm) garnets are generally free from inclusions except where they ophitically enclose biotite laths. The larger garnets, which may be as much as 8 mm in diameter, frequently contain quartz inclusions. Tn,ese increase in size from the centres outwards though there is usually an inclusion-free zone at the margin. In one or two cases inclusions have a parallel dimensional orientation, forming inclusion trails which are straight, but oriented at a high angle to the foliation outside. Clearly the growth of the garnets postdated the initiation of the foliation, but since the inclusion trails are straight the growth must have taken place under static conditions. The rotation which is indicated by the angle between the inclusion trail and the foliation indicates that the garnet was rotated after growth had ceased, and possibly before or during the growth of the outer clear rim. The significance of these observations will be discussed in a later section. Orthoclase - Where much of this mineral is present it occurs as large irregular poikiloblastic grains. Cordierite occurs as large anhedral grains larger than or comparable in size with the plagioclase. Polysynthetic rather than interpenetration twins are developed, but the mineral can readily be distinguished from the plagioclase by its incipient alteration to a yellow isotropic substance. Graphite occurs as rather irregular shaped tabular grains which frequently bifurcate or are bent. In places they are intergrown with biotite. Sillimanite forms large sheaves of acicular grains, and also occurs as groups of parallel noncontiguous needles in the central parts of cordierite and orthoclase crystals. 8 The granitic component of the garnet-biotite gneiss can be subdivided into two groups which differ in age and composition. The early foliated quartz-pla,gioclase-biotite pegmatites are veins and stringers of pegmatite developed parallel to the bedding; they were folded during the first phase of deformation. The veins vary in size and in continuity. They range from lines of isolated porphyroblasts of plagioclase through lines of lenticular granite patches to continuous veins, commonly several centimetres in thickness. These pegmatites tend to be developed in more pelitic horizons; the matrix immediately outside them is usually more biotite-rich than elsewhere. The plagioclase is oligoclase or andesine, and is similar in composition to that in the enclosing rock. Where the enclosing rocks are graphitic, large flakes of graphite are commonly developed in the pegmatites. The foliation due to the parallel orientation of tabular biotite flakes is normally parallel to the bedding, but at first-fold hinges, the foliation can be clearly seen to cut across the bedding and be orie11ted parallel to the axial planes of the folds. The late unfoliated microcline-bearing pegmatites can be seen cutting first folds and clearly post-date the first phase of deformation. Their relationship to the .s econd phase is uncertain. Large sills are developed in places, particularly in the extreme north of the area, and are well exposed beyond the northern edge of the map-area in the Churchill River near Island Falls. These bodies have sharp but irregular contacts with the enclosing rocks, and in places idioblastic "dents de cheval" of microcline as much as 4 cm across can be seen growing in the enclosing rock. A notable feature is the irregularity in the grain-size from place to place. In places the rock is medium grained, in others coarse grained, and pegmatitic patches of irregular shape and various sizes are developed. Films of biotite and larger inclusions of psammitic and pelitic schist are oriented roughly parallel to the foliation outside. THE BIOTITE GNEISSES (1) These rocks differ from the garnet-biotite gneiss in minor respects only: garnet is usually absent; the amount of biotite present is slightly less; and magnetite is frequently present along bedding plane~. The lower content of biotite is reflected in the generally more massive, often granulitic character (Pyke, 1965 p. 9) and rather paler colour of the rock. In spite of the similarity between the two types little difficulty was found in separating the two groups in the field. THE CALC-SILICATE AND ASSOCIATED ROCKS (2 AND 3) This group comprises a series of compositionally layered calcsilicate rocks and associated, often interbedded amphibolites. While at some localities the amphibolites occur as units ten feet or more in thickness, at others the amphibolite is interbedded with calc-silicate rocks on scales of a centimetre or less. In view of this intimate association, and considering the presence of garnet in most of the amphjpolite there seems little doubt that the amphibolites represent sedimentary rather than igneous rocks. The calc-silicate rocks proper, and the associated amphibolites are included in one unit on the map. Differential weathering of the layered formation produces the slabby outcrop which characterizes this group in the field. In the 9 coarser bands diopside grains can be clearly distinguished in the grey feldspar or scapolite matrix. In addition to the above-described mappable formation calcsilicate rocks occur within the garnet-biotite gneisses as isolated lenticular-shaped nodules. Thin section study confirms that the calc-silicate formation consists largely of diopside-plagioclase rocks though a wide variety of accessory minerals are also present. Scapolite and calcite which are normally present as accessories may in certain bands make up more than 50 per cent of the rock. The planar structure in this formation is largely a compositional effect, little lattice or dimensional orientation of the mineral grains is in evidence. The diopside occurs as subhedral to anhedral grains, whic;h are very pale green in thin section. They have rounded outlines where enclosed in calcit~. The extinction angle Z /\ C varies from 30· -42 °. 0 In rocks. that contain a considerable amount of scapolite two generations of plagioclase can be recognized. The first generation occurs as a mosaic of twinned grains which are of the order of 0.5 mm - 1 mm in diameter and vary in composition from andesine with An45 to labradorite with Ano5. The grains are free of inclusions and though anhedral have fairly regular interfaces with each other and with the scapolite. The scapolite generally forms polygonal grains which have straight mutual interfaces and near 120° triple junctions. They are generally larger than the above-described plagioclase crystals. The second generation of plagioclase occurs as mosaics of small (about 0.25 mm diameter) irregular shaped untwinned grains. Within the mosaics are acicular inclusions of calcite which cross plagioclase grain boundaries. The mosaics form tongues and lobes which project into the scapolite grains. In these areas the calcite needles tend to occur at right angles to the advancing "fronts" of scapolite. The above-described textural relationships indicate that at an earlier stage in the metamorphism of the rock both scapolite and plagi6clase were stable. Later, however, at least the meionite (calcium-rich) component of the scapolite has become unstable and decomposed with the production of anorthite and calcite according to the reaction: Ca4AloSio024COa _ _ _,... 3CaAhSi20s + CaCOa Accessory minerals in the calc-silicate rocks include sphene, actinolite, pyrrhotite, apatite, calcite, quartz, and clinozoisite. The last three minerals are characteristic of carbonate-rich horizons. Sphene makes up 5-10 per cent of the rock in some bands. The amphibolite horizons consist of approximately equal amounts of hornblende and labradorite. The hornblende is pleochroic with X-pale olive green; Y-medium green brown, Z-dark olive green and has the extinction angle Z /\ C circa 20 °. Accessory minerals include quartz, phlogopite, pyrrhotite, and garnet. THE DIAPIRIC GRANITES The diapiric granites are sharply bounded, large bodies of gneissic granite that deflect the bedding and foliation in the enclosing paragneisses. The diapiric granites have been named for descriptive convenience after the nearest large lake and their names 10 are shown on the structural map. A number of distinct textural and compositional types of gneissic granite are developed. The main features of the various granites are summarized in Table 1. The north-northwest trending elongate outcrop of granite situated immediately north of the Wuskwi8tic Granite resembles the Oskatim Granite in texture and composition. It is, however, poorly exposed and will not be discussed further. The Wuskwiatic Granite is more resistant to denudation than the enclosing rocks and tends to outcrop on low rounded hills. The rock, which is grey in colour, shows no compositional layering, but nevertheless has a distinctive texture. The biotite, which is black in hand specimen, is segregated into clots, lenticular in cross:section, and having axes of the order of three, five, and fifteen millimetres in length. These clots are oriented parallel to one another and define both the foliation and the lineation. Towards the margins of the body the foliation is more conspicuous than the lineation and tends to lie parallel to the outer contact of the granite. Towards the centre, however, the foliation is less conspicuous and in many places only a lineation can be seen. These relationships clearly indicate that the orientation of the internal structures of the granite is due to flow on emplacement, rather than to the direct effect of tectonic stress (Balk 1937). ·: TABLE 1 Summary of the location and composition of the diapiric gneissic granites NAME OF DIAPIR LOCATION ROCK TYPES Wuskwiatic Granite Immediately west of centre of map-sheet Coarse-grained homogenous xenolithic granodioritic gneiss (6) N~mei Granite Just east of centre of map-sheet Mainly as above, but mediumgrained and leucocratic gneisses also developed, especially on the southern flank (6 and 6a) Oskatim Granite Southwest of the Wuskwiatic Granite Very distinctive cataclastic texture, varies in composition from granodiorite to adamellite (6) McArthur Granite Extreme southwest corner of map-sheet Two types developed; a fairly homogenous biotite gneiss (6); and a more variable biotite gneiss with included hornblende-bearing rocks (5). Xenoliths are common and usually consist of a biotite-rich rock which often contains much magnetite. They are finer grained than the enclosing granodiorite and tend to be flattened in the plane of the foliation and elongated in the lineation direction . . In thin section the granodioritic gneiss is seen to be composed largely of quartz and oligoclase-andesine (Anao) with potassium feldspar and biotite the most important accessory minerals. Minor accessories include hornblende, apatite, zircon, and magnetite. A curious feature of the plagioclase is the complexity of the twinning; 11 '' ., ) 1 grains showing albite-law twins alone are in a minority. The biotites occur as rather ragged grains which have only a moderately good preferred orientation. They are pleochroic with ' £ -pale brown W -dark brown to black. The hornblende forms very irregular grains up to 2 mm in length, that in some cases ophitically enclose biotite and plagioclase crystals. They are pleochroic with X= pale yellow, Y= dark green brown and Z= medium green. The extinction angle Z /\ C =20 °. Potash feldspar occurs as antiperthitic intergrowths with the plagioclase and also as independent grains. The N emei Granite is the easternmost of the diapiric granites in the map-area. It underlies a group of low hills, but nevertheless the quality of the exposure is poor. Two lithological types are distinguished on the map. In the north a coarse granodioritic gneiss is exposed, which in texture and min~ralogical composition closely resembles the Wuskwiatic Granite described above. In th e Nemei Granite the segregation of biotite into clots is even more marked than at Wuskwiatic Lake. Near the northern margin of the granite there are isolated outcrops of a coarse-grained rock composed of roughly equal amounts of quartz and plagioclase with coarse irregular shaped grains of magnetite as an important accessory. The composition of this rock is not that of a normal igneous rock. It could, however, represent a feldspathized iron-bearing sedimentary rock. The southern part of the Nemei Granite (6a) is a medium- to coarse-grained granodiorite in which biotite, the chief accessory mineral, is not segregated into clots but is distributed evenly throughout the rock. Leucocratic types are also developed in this area. A characteristic feature of the southern part of the Nemei Granite is the wide range of grain size evident in a single thin section. ~he larger quartz and feldspar grains, as much as 2 mm across, are enclosed by rims composing mosaics of quartz and feldspar. Clearly, recrystallization has taken place. The dimensional orientation of both the quartz and the biotite is rather weak. The later mineral is pleochroic with £ pale brown, rn dark brown to black. Microcline is ubiquitous as a minor accessory, but is present in larger amounts in the more leucocratic types. Small amounts of magnetite and apatite are also present. The Oskatim Granitic Gneiss has a distinctive texture and differs from the other granites in the area in containing considerable though variable amounts of potassium feldspar. The rock is pink in colour and has a pronounced foliation. In the most characteristic type, lenticular fragments of pink medium-grained leucocratic gneissic granite, of the order of 5 mm in thickness and several centimetres in diameter, are separated by a fine-grained, more biotite-rich matrix. In places the biotite is segregated into thin continuous laminae along which the rock is fissile. Some layers in the rock contain hor~blende. The strong foliation, and the fact that all the early foliat~d pegmatites have been rotated into the foliation plane indicate that this rock has undergone intense deformation. The most important minerals in the rock are quartz, oligoclase (An23), and microcline. Biotite and hornblende are important accessory minerals in some rocks. Minor amounts of magnetite, apatite, and red garnet (almandine?) are also present. The rock varies 12 ,, in composition from a granodiorite to a quartz monzonite (= adamellite). There is little evidence of cataclasis in thin section; clearly, recrystallization took place contemporaneously with or followed the deformation. The quartz and feldspar grains, which average about 0.5 mm in diameter in the matrix and 1 mm - 2 mm in the fragments, show little dimensional orientation. The microcline is locally replaced by myrmekite. The properties of the biotite and hornblende in this rock are similar to those in the Wuskwiatic gneiss described above. The McArthur Granitic Gneiss has been subdivided into two groups; a smaller area of more homogenous biotite gneiss is well exposed along the northern margin, while a larger area of migmatitic gneiss outcrops extensively on the shores of McArthur and Robbestad Lakes. Both these types are granodioritic in composition. The McArthur gneiss extends southwards into the area mapped by Pyke (1961), where the relative proportions of the two types are reversed, and in the granitic body, as a whole, the migmatitic gneiss type is subordinate. The more homogenous type is a buff-coloured medium-grained rock, which commonly shows a rather diffuse compositional layering on a scale of less than a centimetre, due to variation in the amount of biotite in the rock. In thin section the rock can be seen to consist largely of quartz and albite-oligoclase (An3o) with biotite and potassium feldspar as the chief accessory minerals. Minor accessories are apatite, zircon, muscovite, and magnetite. Neither the quartz nor the feldspar grains show much sign of dimensional orientation. The quartz grains are polygonised (Voll 1960 p. 511). Perthite and antiperthite are rare. The biotite has the same pleochroism as that in the Wuswiatic Granite and in some specimens examined it is partly replaced by chlorite. A few small ragged hornblende grains have X = pale yellow; Y and Z dark green and the extinction angle z I\ c =20 °. The migmatitic subdivision of this granitic body is essentially a biotite gneiss, with conspicuous biotite-rich layers. In addition, however, the rock contains inclusions of amphibolite which may be continuous )ayers several centimetres thick, or they may be broken up into sharply bounded lenses or more irregular schlieren. Less commonly, larger bodies of amphibolite have lit-par-lit granitic veins developed within them. Where hornblende is developed with or in place of the biotite in the enclosing gneiss, amphibolite inclusions are also present. The impression was gained that biotite granite has become contaminated by hornblendic material. Apart from the local abundance of hornblende, and also of apatite and sphene, the migmatitic granites are similar in mineralogy to the more homogenous types. STRUCTURE Two approaches can be made to a study of the structure of an orogenically deformed area. The classical approach involves mapping the various lithologies, determining "the way up" of the succession and then constructing a series of profiles by "looking down" the plunge of the folds (Wegmann 1929, Stockwell, 1951). In areas where the stratigraphy can be established or at least marker horizons mapped, this is an extremely valuable technique, provided 13 one is dealing with a set of homoaxial folds with a reasonably constant direction and amount of plunge. This proviso is an important one. It has for many years been realized that the small-scale structures such as cleavage, small-scale folds and lineations are related in attitude and style to the large-scale structures (Leith 1914). More recently, however, it has become apparent that in many orogenic belts successive sets of small-scale structures are related to successive sets of large-scale structures and that structures of different age may be oriented in various directions (Berthelsen 1960, Ramsay 1962). In these areas early folds became refolded by later structures, producing complex outcrop patterns, and the attitudes of the fold hinges vary considerably from place to place. In these circumstances it is difficult or impossible to draw structural profiles. The modern techniques of structural geology are concerned to a considerable extent with unravelling complex structures produced by successive phases of deformation making full use of evidence from the superimposition of small-scale structures, (Clifford, et al 1957, Sutton, 1960, Ramsay in press). The latter paper provides a valuable summary of the techniques of structural analysis and their application to an area of economic importance. In considering the structure of the present area, the following limitations must be borne in mind: a. The quality of the outcrops is in general poor. Most of the small-scale data have been obtained from a very limited number of clean exposures situated along the power-line and on the shores of some of the larger lakes; b. in the absence of good stratigraphical evidence "trend-lines" taken from aerial-photographs have been used in compiling the accompanying structural map. Although the validity of individual lines drawn on this map may be questioned, the resulting pattern clearly indicates the position, extent, orientation, and style of some of the larger folds in the area. ' Examination of the small-scale structures in the area indicate that two distinct types of fold of different age are developed. In the first folds, bedding and pegmatite veins are deformed into tight isoclinal structures with axial planes parallel to the main metamorphic foliation in the rock. Examples of these first folds are illustrated in Figure 1. On the limbs of the folds the bedding and foliation are parallel, but on the fold hinges the foliation, represented by the parallel orientations of biotite flakes, can be clearly seen to cut the bedding and the folded veins. Clearly, the foliation developed during the first phase of folding. The mineral orientation lineation in the paragneisses is parallel to the hinges of these folds and both these structures plunge to the north over the whole map-area. Large-scale· first folds can be tentatively identified only in the southeast quadrant of the map-area; (see structural map 107B). These fold-traces are drawn solely on evidence from the shape of the contact between the biotite gneiss and the garnet-biotite gneiss on the map. Since the first folds are isoclinal, the trend of their axial traces is given by the trend of the foliation and bedding. In those parts of the map-area where major second folds are not apparent these structures run in a northwest-southeast direction. The large-scale first folds may be expected to plunge to the north parallel to the small-scale folds of the same age. 14 SCALE IN FEET 0 SCALE IN FEET 0 Figure I: First folds outlined by pegmatites (stippled). Note that the foliation (dashed) is parallel to the axial planes of the fold. 15 SCALE IN FEET 0 Figure 2: Second folds outlined by pegmatites in upper diagram and by bedding (which is parallel to the foliation) in the lower. Note that in both cases the foliation (indicated by dashes) is folded. 16 N N ~ axis Figure 3: Pi-diagrams of poles to bedding in the major second synforms. The areas covered by these diagrams are marked on the structural map. The upper diagram refers to the fold lying between the Wuskwiatic and Nemei Granites, and the lower one to the fold immediately east of the McArthur Granite. Both diagrams are plotted on the lower hemisphere of a Schmidt net. 17 0 SCALE IN FEET - ----AXIAL TRACES OF FIRST FOLDS ---AXIAL TRACES OF SECOND FOLDS 0 SCALE IN FEET Figure 4: Refolded folds. The upper example is from the Wet claims which are located about 2 miles east of the eastern boundary of the Area at latitude 55°26'. The lower example is situated near the dock at Island Falls, just beyond the northern margin of the map-area. 18 • The second folds are characterized by the fact that in addition to the bedding and the quartz veins, the metamorphic foliation is folded. They thus post-date the development of the foliation and it follows, therefore, that they post-date the above-described set of folds . In general the second folds are more open structures than the first, though the style varies from place to place and depends on lithology. In Figure 2, in which small-scale folds are illustrated, the lower diagram shows more open folds developed in the massive homogenous b.iotite-gneiss, whereas the upper diagram shows tighter folds developed in "injected" garnet-biotite gneiss in which more and less competent bands alternate. There is no visible second schistosity parallel to the axial plane of these folds . Large-scale second folds with north-northwest trending axial traces are readily recognizable on the aerial-photographs (see structural map). One pair of such structures comprises the synform and complementary antiform lying immediately to the east of the McArthur Granite. These folds, like the small-scale structures of the same age deflect both the bedding and the foliation and are fairly open folds. The direction and angle of plunge of the above mentioned synforms have been calculated by plotting poles to the foliation on diagrams (Sander, 1948) (Fig. (3) ). An outline of this simple method of determining the style and attitude of folds is given by Byers and Dahlstrom (1954) pp. 163-169. From the diagrams it can be seen that both these folds plunge to the north-northeast at shallow angles. No refolded folds have been seen in the map-area, although from the above evidence they must exist on both large and small scales. Small-scale refolded folds have been observed at two localities a little way beyond the boundaries of the present area, (Fig. 4, Plate 1). In ·e ach case, as might be expected, early tight isoclinal structures are refolded by later open folds. Although the north-northwest trending fault marked on the map is not exposed, and does not offset any beds, its existence and position can be inferred with a considerable degree of confidence from the marked topographical lineament along which it is drawn. It probably belongs to the extensive set of north-northwest trending sinistral wrench faults, predominantly of Late Precambrian age that are present in this part of the Shield (Byers, 1962). DISCUSSION One of the most striking features of the area is that the suite of concordant granite and granitic pegmatite veins were folded during the first phase of deformation. Regional metamorphism must, therefore, have reached a high grade before deformation began. Evidence from the inclusion trails in garnets, porphyroblasts, summarized in a previous section indicates that garnets overgrew a pre-existing foliation under static conditions and were subsequently rotated. Since the development of the foliation was associated with the first phase of deformation this would imply that (a) most of the garnet growth took place between the two phases of deformation and (b) these two phases were separated by a static interval. The relationship between the structural and metamorphic history of the area is summarized in Table 2 and Figure (5). 19 TABLE 2 History of the Hudsonian orogenic cycle in the Nemei Lake area l. Deposition of Ki ssey new sediments. (On a basem ent of migm at itic g neiss?) 2. Development of oligoclase-an·desine pegmatites. 3. First phase of deformation ; development of isoclinal fo lds and the regional foliation. 4. Static interval ; growth of garnets Emplacement of 5. Second phase of deformation prod ucing mi crocline-bearing open folds, and rotating ga rnets; pegmatites emplacement of diapiric g ranites. } Evidence on the origin and emplacement of the granites is inconclusive, although a number of interesting . possibilities have emerged. It is difficult to visualize how the intense migmatization and granitization, which was involved in the development of the McArthur gneiss, took place without producing locally more intense migmatization in the immediately enclosing paragneisses. There is a strong possiblity that the McArthur granitic gneiss may represent the basement on which the Kisseynew sediments were deposited. On this hypothesis the McArthur Gneiss is a mantled gneiss dome (Eskola, 1948) produced by the mobilization of the (Archaean?) hasement during the Hudsonian orogeny . ... Q :::, 1- z I (l) <[ I I :IE I __ .,., I / , ... \ I I I \I ', .... _ TIME Figure 5: Diagrammatic representation of the relationship between metamorphism (solid line) and deformation (dashed line) in the area. Another curious feature of the diapiric granites is the marked compositional and textural contrast between the Wuskwiatic and Oskatim Granites, in spite of their close proximity to one another. The strong planar structure in the Oskatim granite and the elongate shape of the body suggests that its ·emplacement may have taken place, or at least been initiated, during the first phase of deformation. This hypothesis is supported by the arcuate outline of the body, which suggests that it was deflected when the Wuskwiatic granitic gneiss was emplaced. The relationship between the second phase of folding and the granite emplacement is problematical. The synform between the Wuskwiatic and Nemei Granites would appear to be a direct result of the emplacement of the granite, similarly the synform east of the McArthur gneiss body has the general form of a rim syncline due 20 ,. f to the local depression of the enclosing rocks as the granite moved upwards (Johnson, in press). In the region to the south (Pyke, 1961), however, northerly trending folds that deflect the foliation and are similar in size to the second folds in the present area, do not appear to be due to the emplacement of granite. A possible solution to this problem is that in the present area embryo second folds localized the emplacement of granites, but the subsequent development of these structures was controlled by the rise of the granites. ECONOMIC GEOLOGY There is little surface evidence for the presence of economic mineral deposits, apart from sand and gravel that have been exploited for construction work at Island Falls. Sulphide mineralization is confined to the sporadic occurrence of pyrrhotite, mainly in the calc-silicate formations. Green copper carbonate stains were noted at the contact between meta-hornblendite and a siliceous gneiss about one mile northeast of Lake Oskatim. REFERENCES: Balk, R. (1937): Structural Behaviour of Igneous Rocks; Geo!. Soc. Amer., Mem. 5, 177 pp. Berthelsen, A. (1960): Geology of Tovqussap Nuna. Meddelser om Gronland, 123 pp. 1-226. Bruce, E. L. (1930): The Kisseynew Gneiss of northern Manitoba and Similar Gneisses Occurring in northern Saskatchewan; Proc. and Trans, Roy. Soc. Canada 24, Sec. IV pp. 119-132. Byers, A. R. and Dahlstrom, C. D. A. (1954): Geology and Mineral Deposits of the Amisk-Wildnest Lakes Area Saskatchewan; Sask. Dept. Mineral Res. Rept. No. 14. Byers, A. R. (1962) : Major Faults in the Western Part of the Canadian Shield with Special Reference to Saskatchewan. In J . S. Stevenson (ed.), The Tectonics of the Canadian Shield: Roy. Soc. Can., Spec. Pub. No. 4, University ot' Toronto Press. Cheesman, R. L. (1956): The Geology of the Mari Lake Area, Northern Saskatchewan; Sask. Dept. Mineral Res. Rept. No. 23. Clifford, P., Fleuty, M., Ramsay, J . G., Sutton, J. and Watson, J . (1957): The Development of Lineation in Complex Fold Systems; Geo!. Mag. 94, pp. 1-25. Eskola, P. E. (1948): The Problem of Mantled Gneiss Domes; Quart. J. Geo!. Soc. Lond. 104, pp. 461-76. Harker, P. (1965) (Editor): Guide for the Preparation of Geological Maps and Reports; Geo!. Surv. Canada. Harrison, J. M. (1951): Precambrian Correlation and Nomenclature, and Problems of the Kisseynew Gneisses, in Manitoba; Geo!. Surv. Can. Bull. 20, 53 pp. Johnson, R. L. (in press): The Western Front of the Mozambique Belt in N. E. Southern Rhodesia. Johnston, W. A. (1930): Frozen Ground in the Glaciated Parts of Northern Canada; Proc. and Trans. Roy. Soc. Canada 24, Section IV, p. 31-40. Leith, C. K. (1914): Structural Geology. New York. Mcinnes, W. (1913): Basins of the Nelson and Churchill Rivers; Geo!. Surv. Canada, Memoir 30. Moorhouse, W. W. (1959): The Study of Rocks in Thin Section; Harper Brothers, New York. 21 Pyke, M. W . (1961): The Geology of the Attitti Lake Area (West Half) Saskatchewan, Sask. Dept. Mineral Res., Rept. No. 54. Pyke, M. W. (1965) : The Geology of the Nemei Lake Area (East Half) Saskatchewan; Sask. Dept. Mineral Res., Rept. No. 97. Pyke, M. W . (1966): The Geology of the Pelican Narrows-Birch Portage Area; Sask. Dept. Mi.neral Res., Rept. No. 93 , Ramberg, H. (1963): Evolution of Drag-folds ; Geo!. Mag. 100, pp. 97-105. Ramsay, J. G. (1962): Interference Patterns Produced by the Superposition of Folds of Similar Type; Jour. Geo!. 70, pp. 466-481. Ramsay, J. G. (in press): Structural Investigations in the Barberton Mountain Land, Eastern Transvaal; Trans. Geo!. Soc. S. Africa. Rast, N. (1965) : Nucleation and Growth of Metamorphic Minerals . In "Controls of Metamorphism" (Ed. W. S. Pitcher and G. W . Flinn, Oliver and Boyd, Edinburgh), pp. 73-102. Sander, B. (1948): Einfuhrung in die Gefugekunde der Geologischen Korper, Vienna and Innsbruck. Satterly, J . (1931): Pelican Narrows Area, Saskatchewan; Geo!. Surv., Canada, Summ. Rept. 1931 Pt. C. Schreyer, W . and Schairer, J. F. (1961): Composition and Structural States of Anhydrous Mg - Cordierites; Jour. Petrology, 3 p. 324. Stockwell, C. H. (1951): The Use of Plunge in the Construction of Cross-sections of Folds; Proc. Geo!. Assoc. Canada 3, pp. 1-25. Stockwell, C. H. (1961): Structural Provinces, Orogenies and Time Classification of Rocks of the Canadian Precambrian Shield: Geo!. Surv. Canada Paper 61-17, pp. 108-11 8. Sutton, J. (1930): Some Structural Problems in the Scottish Highlands; 21st Int. Geo!. Congr. Pt. 19, pp. 371. Voll, G. (1960): New York on Petrofabrics; Liverpool & Manchester Geo!. J . 2, 503-567. Wegmann, 0 . E. (1929): Beispiele Tektonischer Analysen des Graundge-birges in Finnland; Bull. Comm . Geo!. Finl. 87, 98-127. Zwart, H. J . (1962) : On the Determination of Polymetamorphic Mineral Associations, and :ts Application to the Bosost Area (Central Pyrenees): Geo!. Rundschau 52, pp. 38-65. 22 Plate I: Refolded folds (for descriptions and localities see Figure 4). 23 24