} 0N- AN QCCURRENCE OF “QUARTZITE” ‘ {EN THE EOUTHERN COMPLEX NEAR PALMER MARQUEE COUNW, MKHJIGAN f I I } ‘ Wheat: 501' We Dogma 0‘ M. S. WCH‘EGAN STATE UNIVERSE“ Robert A. Vehrs 1959 'r 1% ' Wm; WM“! LN 111 @ 1m W IL “M u L I B R A R Y _ Michigan State g '3 University C‘ (.1 "‘ C; O 'r CCURREUCE OF "QUARTZITE" N THE SOUTHERN GOMILEX NEAR PALMER, MARQUETTE COUNTY, MICHIGAN 1 0y ROBERT A. VEHRS A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in Michigan State University East Lansinv, Michigan t.) 1959 -ii- CONTENTS Page Abstract......................................... 1 Acknowledgments.................................. 2 Introduction..................................... 4 Location and Extent......................... 5 Culture..................................... 5 Previous Investigations..................... 6 General Geology and Stratigraphy................. 9 Lower Precambrian........................... 13 Middle Precambrian.......................... 13 Upper Precambrian........................... 15 Definition of the Problem........................ 16 Field Procedure.................................. 17 Laboratory Procedure............................. 19 Physiography of the Problem Area................. 21 Drainage.................................... 21 Sand Plain Topography....................... 22 Rock Nob Topography......................... 22 Geology of the Problem Area...................... 23 Distribution of the Rocks................... 23 Description of the Rocks.................... 25 Graniteoo.OOIOOOOOOOOOO0.00000000000000 25 Granite and "Quartzite" associations... 32 Typical "Quartzite".................... 41 Deformed "Quartzite"................... 46 Dike Rocks............................. 47 Quartz Veins........................... 49 Migmatite.............................. 51 Granite and Gneiss..................... 52 Structural Inferences....................... 52 Metamorphic Rank............................ 55 Origin of the "Quartzite"........................ ' 55 AnIgmmmZmdo.n.n.n.u.u.u.u.u.u. 56 A Large mass of Vein Quartz................. 60 A True Quartzite............................ 62 SiliCified mffOOOOOOOOOOOOOOOOOOOOOOOOOOOO. 64 Recrystallized Chert........................ 67 Age of the Neta-Novaculite....................... 70 Possible Contemporaneous Rocks.............. 72 Conclusions...................................... 73 Suggestions for Further Studies.................. 73 Azeferences Cited-0...OOOOOOOOOOOOOOOOOOOOO0...... 74 ILLUSTP TIOHS page Figure 1. Location of Problem Area in Horthern :‘iiCEVligallOOOOOOOOOOOOOOOOOOOOOOOOOO0.0... 3 Figure 2. Tepography of the Problem Area.......... 20 Figure 3. Uutcrop Map of the Problem Area......... 24 Figzre 4. Photomicrograph of Specimen 5........... 29 Figure 5. Lhotonicrograph of Specimen 6........... 29 Figure 6. Ihotonicrograph or Specimen 3........... 35 Figure 7. Photomicrograph of Specimen 2........... 35 Figur 8. Shotonicrograph of Specimen 16.......... 4O "igure 9. Photomicrograph of Specimen 20.......... 40 Figure 10. Photomicrograph or Specimen 23.......... 45 Figure 11. Photomicrograph of Specimen 41.......... 4S Fiélrc 12. Photomicrograph of Specimen 15.......... 50 7igure 13. Photograph of Higmatite................. 50 :igure 14. Composite Diacram of Orientation of i.) Quartz C-Axes in Specimen 9............. 54 ABSTRACT An occurrence of "quartzite" and its associated rocks in the Southern Complex near Palmer, Marquette County, Michigan is described. The origin and age of the "quart- zite" are discussed in light of field and laboratory observations. It is concluded that the "quartzite" is a metamor- phosed noveculite of probable pre-Huronian age. ACKNOWLEDGMENTS The writer wishes to express his appreciation to Dr. Justin Zinn for his gUidance and continued interest in this study; to Dr. James Trow and Dr. Harold Stonehouse for their review of the manuscript and many valued suggestions; to Robert C. Reed for his aid and interest in the problem; and to Seth Foresman, David Grenell, and Ben Gunning for their companionship and assistance in the field. What merits this thesis may possess are due in the greater part to these men. -_ ,7_._. , _A , - , l 1 0 r5 JO , ““*“~ LEGEND Superior ' e -- i a W t“ . \’ POIOOIOIC ; \ Houghi‘on 7‘7 1 l g, Huronion Oed'onogon/ O . I I PrinCIpolly Archean ‘ MarqucH‘e - §ESX I P oblem Area \ \ \‘4‘\ f I E:Q\\ ‘ v \ \ c“; \;§\4~\ ‘39“ :4” Efii\k¥\g o ‘ (g I: \ ' c IIIIIOS 01's,,v \ Exam-hm .__-_ ___ _._ —-- ——— —— i— 1 error Dickey (modified) , Index Map 1 RSOW R29W R28W R27W R26W R25W T44N -.._._ __*__ _— i l 9 / ,, , , ; i ' ‘ I 7 / . ‘ 41743»: ’ : //l /// _l____ 3—. 4...-__._‘_..- .. , ~_—- ; “rill - i \\ ‘ I | l / / l Figure I. Location of Problem Area in Northern Michigan INTRODUCTION The Southern Complex of the Upper Peninsula of Michi- gan is composed of Precambrian igneous and metamorphic rocks, predominately granites, gneisses, schists, and basic intrusives. It embraces an area of approximately 1400 square miles south of the marquette synclinorium, bounded by folded Huronian (Animikie)l/rocks to the west and south, and covered by flat-lying Paleozoic sediments to the east. In the summer of 1951, Robert C. Reed, geologist for L. P. Barrett, U. 8. Atomic Energy Commission contractor, noted several outcrops of a white, quartzose rock occurring in the granitic terrane of the Southern Com- plex near Palmer, Marquette County (Reed, 1958). The presence of this rock posed many interesting questions regarding its origin, age, and relation to surrounding rocks. This thesis presents a record of the _ author's field and laboratory study of these questions. l/ The U.S.G.S. has recently discarded the term "Huron- ian", used to describe the Middle Precambrian sequence of Michigan, and has replaced it with "Animikie" (James, 1958, p.33). Although most workers agree that Huronian is no longer acceptable in Michigan, they are far from unanimous in their acceptance of Animikie. The present writer, therefore, has retained the term Huronian. Location and Extent The area studied lies approximately 10 miles south- west of Marquette and 5 miles east-southeast of Palmer. The author examined Sections 1 and 12, T 46 N, R 26 w and Sections 6 and 7, T 46 N, R 25 w; portions of the sur- rounding sections were also visited in the course of the investigation. Mapping was completed in Section 1 and adjacent portions of Sections 6, 7, and 12. A bold linear ridge of quartzose rockg/trends in an easterly direction across Section 1; and scattered, less prominent outcrops of this rock occur elsewhere in the mapped area. Numerous outcrops of granite, gneiss, and basic intrusives occur in the mapped area and surrounding sections. Culture Ready access to the area is provided by county road 553 from the east and state highway 35 from the west. Numerous sand roads traverse the interior; they are easily travelled by automobile. Tracks of the Chicago & ‘g/ This rock will be called "quartzite" throughout most of this report. "Quartzite", as used here, is strictly a convenient field term; it implies a granular, metamor- phosed, quartzose rock, not necessarily derived from sandstone. o. Northwestern R.R., connecting the Marquette range with Escanaba, pass to the east. Fire lanes, generally along section lines, were plowed out in the early 1930's; and, although overgrown to some extent, they are still readily discernible. Some section corners are marked by pins. Groves of pine and balsam and large expanses of open grassland cover most of the level, sandy portions. 'Mixed hardwoods grow in the more rugged, rockier areas. With the exception of a few hunting cabins and small-scale logging operations there is little evidence of human activity. Previous Investigations Development of the Marquette synclinorium into a major mining district, following discovery of iron ore in 1844, required intensive geological investigation. By the turn of the century, limits of the Huronian strata had been defined, and a large amount of detailed and reconnaissance mapping had been done in the district. Subsequent work has been devoted almost entirely to the study of the Huronian and to the search for iron ore. The complex igneous and metamorphic areas bordering the synclinorium on the north and south are unimportant economically, and have been largely neglected. Van Hise and Bayley (1897, p.190), in their classic Monograph 28, describe the Southern Complex as: "...occupied by granites, gneisses, hornblendic and micaceous schists, and greenstone schists, together with the various acid and basic eruptives that intrude them." Van Rise and Leith (1909, p.108-178) give a comprehensive summary of literature on the Michigan Precambrian to 1907. Regarding the Precambrian basement complex both north and south of the Marquette synclinorium, they state, (p.332) that: "these granites and gneisses show a variety of characters and are certainly not all of the same age, although with minor exceptions they antedate the Algonkian rocks." Van Hise and Leith (1011) again give an excellent summary of previous work in the Lake Superior region. Their discussion of the Southern Complex (p.255) is brief and is devoted mainly to a description of the schists which occur infrequently in the Complex. They also describe the Palmer gneiss and suggest that parts of it may be metamorphosed sedimentary rocks. Leith, Lund, and Leith (1936) give a compilation of work done in the Lake Superior region since 1911, but do not add to the description of the Southern Complex. Lamey (1931, 1933, 1934, 1935, 1937) has done considerable work in the Southern Complex, particularly in the northern portion near Republic and Palmer. 1e considers the larger part of the complex to be post- Huronian granite, which he calls Republic granite. He also has studied the Palmer gneiss, and concludes that it is predominantly highly metamorphosed Lower and Middle Huronian rocks. Dickey (1936) disputes Lamey's conclusions regarding the granite of the Southern Complex and states (p.317): "Granites representing three distinct periods of pre-Cambrian intrusions are recognized in the Southern Complex...Two of these are believed to be Archean, and one post-Huronian. The Southern Com- plex is made up dominantly of Archean rocks, and is not...composed for the most part of post-Huronian granite." Later (1938), he discusses a post-Lower, pre-Middle Huronian granite which he calls the Ford River granite. Tyler, Marsden,AGrout, and Thiel (1940) have studied the Lake Superior Precambrian rocks by accessory mineral methods. They recognize two pre-Huronian granites, a Huronian granite, and a post-Huronian granite in the Southern Complex. They conclude that the hyacinth variety of zircon indicates a pre-Huronian age, that malacon zircon indicates later pre-Huronian and Huronian ages, and that "normal" zircon indicates post-Huronian. Azres (1943) considers the Republic granite to be -9- post-Huronian. James (1955) delimits zones of regional metamorphism in the Precambrian rocks of northern Michigan. Later (1953) he establishes that Dickey's Ford River iranite is C"? 1 probably pre-Huronian, not Huronian; ne further recommends the abondonment of the term :epublic granite because of the difficulty in determining relative ages of the granites present in the district. 323d (1958) has examined much of the Marquette district in a search for radioactive deposits, but has not written extensively about the area. L233 (1959) describes the granite and metamorphic rocks occurring in an area south of the Palmer gneiss belt. Sahakian l959) discusses the injection gneiss and granite occurring in the northeastern portion of the Southern Complex. EKEKAL GEOLOGY AND STRATIGRAPHY The most prominent geologic feature in Marquette County is a narrow, westward-plunging synclinorium about 40 miles long; it is composed Of Huronian rocks, locally intruded by basic dikes, sills, and possibly by granite. ~10- Flanhing the synclinorium on the north and south are large masses of igneous and metanorphic rocks, called, respectively, the Northern Complex and the Southern Com— plex. The rocks in these areas are of widely varying ages, but are largely pre-Huronian. A block of Huronian sedimeitary rocks occurs on the southern rim of the synclinorium at Palmer. This locality has the aspect of a trough separate from the synclinorium. Bordering this area on the south is the Palmer gneiss belt. A narrow tongue of Huronian formations extends across the Southern Complex from the Marquette synclin- orium to Republic; and an isolated basin of Huronian age occurs in the Gwinn district, about 13 miles south of Palmer. Flat-lying Upper Cambrian sandstone rests unconform- ably on the Precambrian formations in the eastern portions of the Marquette district. Following is a brief discussion of the rocks recog- nized in the Southern Complex and in adjacent Huronian areas. The reader should be aware that the problem of age relations in the Southern Complex is far from solved. Lacking absolute age determinations, the present writer must accept the sequence prOposed by Dickey as that most applicable to the area discussed in this thesis. Dickey -11- (1930, p.339) prOposes the following: " l. The 01 est rocks visible in the area are Keewatin-type schists, which are classed as Archean. 2. These schists {re intimately intruded...to produce injection gneisses...by...medium-grained gray to pink granite. This granite is believed to be Laurentian... 3. The second period of granitic intrusion is represented by a gray to pink granite of pronounced- ly porphyritic character...this granite porphyry... is considered to be Algoman. 4. The youngest granitic intrusion...is a pink to red granite...£iie-grained to pegmatitic...it is considered to he Killarnean.” Rock Sequence in the Marquette District according to (l239): bleistocene ~- UnconIO'mity Upper Cambrian -- Unconformity Keweenawan -- -12- r. Linn Glacial sediments Sandstone Olivine diahase dikes Erosian interval Post-Huronian Upper Huronian -- Unconformity Middle Huroniar ~- Unconformity Lower Huronian -- Unconfiormity Tend. shaming - - Unconformity Keewatin -- folding and Killarney granite intrusion Upper Michigamme quartzite and slate Bijiki iron formation Lower Kicnigamme slates Clarkscurgh volcazics and intrusives Greenwood iron formation Goodrich conglomerate and quartzite Negaunee iron formation Siamo slate Ajibik quartzite Wewe slate Kona dolomite Mesnard quartzite Al.ouan granite and syenite DJ Sediments and volcanics Laurentian granite gneiss Greenstone, lavas, and volcanic sediments -13- Lower irecamhrian The oldest rocks in the Southern Complex are mainly hornblendic and micaceous Keewatin-tyne schists. {any of these schists resemble metamorphosed volcanic rocks. Several different ages are probably represented. Some Keewatin-type schists have been intruded by a granite (Laurentian?) to form migmatites. A granite porphyry (Algoman?) cuts the mignatites. This pink to grey granite exhibits large phenocrysts of orthoclase, microcline, and microperthite, which often show parallel lineation. This granite is often deformed. The Palmer gneiss is a belt of highly metamorphosed rocks immediately south of the Palmer area. The gneiss in places resembles metamorphosed sediments and elsewhere metamorphosed granite. The gneiss is generally placed in the pre-Huronian; however, Laney (1935) believes it represents metamorphosed lower and middle Huronian rocks. Middle Precambrian Lower Huronian Mesnard quartzite is the basal Huronian formation; it lies with narxed unconformity upon older rocks. Generally it grades upward from a basal conglomerate, through a -1],- quartzite, to a slate. 523$ dolomite overlies the Hesnard. Interstratified with the dolomite are slaty and siliceous layers. Possible algal structures are prominent in some sections of the formation. Eggg slate overlies the Kona. It is derived from pelitic sediments and contains slate, graywacke, and chart. Middle Huronian Ajiblk quartzite rests unconformably upon the lower Huronian formations. A basal conglomerate with inter- stratified slate and graywacke grades upward into a quartzite. Siamo slate overlies the Ajibik. It varies from a quartzitic graywacke, though a massive graywacke, to a fine-grained slate. Negaunee iron formation rests on the Siamo. It consists of jaspilite, ferruginous slates, iron-silicate schists, ferruginous charts, and iron ore. Upper Huronian Goodrich quartzite unconformably overlies the Negaunee. It is conglomerate at the base, but dominantly a quartzite. The remaining Upper Huronian formations are missing in the area discussed. Huronian or Post-Huronian Intruded into the Huronian formations are many basic dikes and sills. They are generally deformed and altered. Eost-Huronian A granite (Killarney?) has intruded and in some places granitized Huronian rocks. Upper Precambrian Keweenawan Brown-weathering, olivine diabase dikes cut all Pre- cambrian formations in the district. Aplites, pegmatites, quartz veins, and basic in- trusives occur throughout the Southern Complex; they may be of many different ages and, for the most part, seem to be related to the various granite intrusions. DEFIKITIOI C? THE PROBLEM The unusual occurrence of a large body of "quartzite" in the midst of the granites and gneisses of the Southern Complex present many interesting problems. Prior to the present investigation, the only available information regarding this occurrence was the approximate location of the outcrops and a verbal description of the rock as highly quartzose. The proximity of this occurrence to the controversial lalmer gneiss and to known Huronian formations immediately suggests the possibility that it is simply an outlier of Huronian quartzite. On the other hand, this location may be fortuitous. Relations between this rock and the neighboring granites and gneisses were not known. The very nature of the "quartzite" was an enigma; it might be a true quartzite, recrystallized chert, vein quartz, an igneous rock, or a silicified facies of the granite or gneiss. Because of the unusual nature of the rock and the lack of previous descriptions of the area, it is the purpose of this study to present: 1. An outcrOp map of the area 2. A petrographic description of the "quartzite" and associated rocks 3. A discussion of the origin and age of the "quartzite". FIELD IRUCEDURE Field work was accomplished mainly during the week- ends of late June and July, 1953. Several days in early September were Spent rechecking field observations. Prior to actual field work, a study was made of all available maps and aerial photographs of the area. A composite field map was made from the U.S.G.S. Sands and Palmer quadrangle topographic maps. The locations of all outcrops visible on aerial photographs were noted on the field map. Several days were spent in a general reconnaissance of the area, prior to detailed mapping. This procedure provided a good picture of the regional geology and served to delimit the outcrop areas of the "quartzite.” The level, open nature of the landscape and the numerous sand roads allowed a very rapid examination of almost all of the outcrops in over four sections. The general reconnaissance disclosed that outcrops of the "quartzite" were restricted essentially to Sec. 1, T 46 N, R 26 w and adjacent portions of Sec. 6 and 7, T 46 N, R 25 W. The SE corner pin of Sec. 1 was located and was used as the origin of a north-south picket line, established along the eastern edge of Sec. 1. East-west traverses 1') - k1 were made at eighth-mile intervals from the picaet line. Pace and compass methods were used in mapping; this was considered sufficient because of the excellant control afforded by the topographic maps, aerial photographs, and well-marked section lines. No abnormal magnetic inter- ference with the Drunton compass was noted. A scale of 200 feet to the inch was used in mapping. Outcrops, topographic features, and specimen locations were plotted directly on large sheets of L inch squared paper, which was carried in a clip-board. Detailed notes were entered in a field book. Representative samples were collected from all outcrops mapped. Oriented samples were taken in several places. LABORATORY PROCEDURE Thin sections of 25 representative samples were prepared; those included a thorough sampling of the f: "quartzite", several samples oi the grenit , and one or a O cross-cutting dike. All thin sections were examined under a petrogrephic microscope, and a standard petrographic analysis was made of all rock types present. Mineral percentages were determined by use of a Leitz integrating stage. A petrOLaaric analysis was made of one of the oriented specimens; a Leitz four-axis universal stage was employed. An X-ray powder diffraction pattern was obtained from one specimen. Photomicrographs of selected specimens were taken. Igneous rocks were named according to the classification of Johannsen (1932). Z .2 .._.o . «6.0.3 50:4. .335. a some in: .35 >sa<¢oo;o» PHYSIOGRAPMY OF THE PROBLEM AREA The area studied is near the contact of the flat- lying Paleozoic sediments of the eastern part of the Upper Peninsula with the igneous and metamorphic Pre- cambrian rocks in the west. Indeed, bedrock in portions of the area xamined has been erroneously designated Paleozoic on available published maps. The character of the bedrock is reflected somewhat in the tepogrephy. Westward, across the Paleozoic-Precambrian contact, a level or gently rollirg landscape gradually gives way to the rugged, rock-nob topography typical of Lake Superior Precambrian localities. Glacial processes have modified the landscape here as in other parts of the Great Lakes region. Dra’nafie Goose Lake Outlet flows southward through the problem area and joins the East Branch of the Escanaba River. Several small creeks feed into Goose Lake Outlet from the west. The outlet is a tightly meandering, shallow stream, roughly 20 feet wide. The neander belt occupies the entire width of the flood plain. Sand F ain Togography J. A remarkably level said plain covers most of the area east of Goose Lake Outlet. Infrequently, hills and nobs of bedrock pierce the blanket of sand. his sand plain is a r-art of the extensive outwash >lain drained by the escanata River system. Dune-like accumulations of sand have Zeen built up against the larger outcrops. The wind hrs sandhlasted the surfaces of outcrops; hence, outcrops of the xerd, resistan’ "quartzite” exhilit POliShed: .1 “acetted surfaces. 30 streams, other than Goose Lake Cutlet, are found on the sand plain; this, no doubt, reflects the porosity of the sand. Some marshy tracts exist, but they are of little consequence. A few small rx"r (\ '\ . 's A ‘ "- r C ‘1"/' v~ 1 1» ra -A lengo cccur neitheast or in» (reeled area. w 7‘ 1r.."" 1“ m 7‘ rr 7““:7 ~\OC-‘. AIOL’ .LOJ.O .Jr(\,,u‘ll‘L A Contrasting with the monotony of the sand plain is the hilly area west of Goose Lake Outlet. Sand is the dominant soil here also; but, rather than blanketting the surface, it fills only valleys and depressions and is generally subordinate to bedrock exposures. Here the bedrock determines topographic expression. Numerous shallow, linear valleys, trending N 45 w and N 65 E, are readily seen on aeria photographs of the locality; they -23.. suggest the develOpment of a strong maSter jointing in the bedrock. Less apparent, yet numerous, valleys trend approximately north-south and west-northwest; hese are also probably expressions of tedrock jointing and/or faulting. The sharply angular course of the East Branch of the Escanaba River appears to be controlled by these features. It is likely that the courses of all of the streams in this area reflect the structure of the bedrock. GEOLOGY OF THE TRGRLEM ARE: Distribution of Rocks Reddish-orange, gneissoid, porphyritic granite occupies the northern part of the mapped area (fig. 3). Apparent intrusive contacts of this granite with "quart- zite" occur at the eastern margin of Section 1. The "quartzite” is exnosed for a distance of half a mile west of here, and in smaller, less prominent outcrops elsewhere in the mapped area. Two basic dikes, of differing age, cut the "quartzite". A broad belt of migmatite, or injection gneiss, lies north, east, and south of the "quartzite” and its associated granite. A complex of granite and granite gneiss lies south of the mapped area. «no.4 289:5 a 30mm in: .35 m at flu 0:6 .. . _ I :¢ ‘. m4. .. t 1 :I .A a r.‘ xx\x -. ”a.” nia.J\\ QC .- o-xx 2 . xx 2 .3 xx ._ ex .. xx 1 \\ s. \ \\\~ ax \\s\ ‘ \\\\—. II \N \ \ n; p... 4- \ \’\‘.I-Io a . \ \ \‘III" III- ... % x x\ III 0’... ll 0! ‘0... . if! x >\\ all“ \‘ I” z...” «N . .. xx : ens : + 111 H. il xx , . : w. y \N r‘ xx 3 . 3. ‘8 r. x x: xx .x xx .- : Z w‘ \\ 0......” . ..: xx . xx \\ xx \\\ xx x \ xx x\ _ . xx \\ _ \x \\ x \\ x. . \\ x a- In \ h . 9. \ a C x \\\ : \“\ : \\\ \ \ - : o \ \AW c 0 xx \,\\ x .. xx \\\ a: 3 V ‘ “\ \\\\ x. ,u xxx \\\ I I. . \ .I .— . o. 1. xx \“x . .L Zia» cu . x. xxx . Zuni» ...... .u .. .. 3... 2 xx \\ .79... .( .............. 8 O x\ \\ x .0. .. x \\ . . x x\ x u xx \\ x . .......,.. : xxx : ...,.._..... 3 ~\ \ \ x 2 \\ \ \ \ . xx \\ x xx x\ . \x “\ x— xx xx \ \x \. .A A... x 5m.» \ xx \ \ \\\\.\\ [1” \xx\\\ \\\\\I\\ I’ll]: \L‘II \‘l‘n‘ ‘\n\.\ [HI] \ ‘ II II I | ‘I II I! ‘ II ‘ ‘ I, I \ \ + I] ll \\\\ Il":|1’|”.”‘ ‘ \ (III 'II‘ \\ I I l \\ filisu b2.532 coczooam notuSO 0:22:92 o.:=oo>oz-o.o2 otcocw Beaumoco 3320-032 3035 2.2.0 -25.. Description of the Rocks A thorough petrographic study of a rock thin section, supplemented by field data, generally yields sufficient information for an accurate icentification of the rock and for reasonable conclusions regarding its origin. In outcrop, the "quartzite”, upon which this study is focussed, xhibits little evidence that would aid in its identification; under the microsc0pe, however, the composition, structure, and seme of the history of this rock are revealed. Associated rocks often yield pertinent information; this is true of the granite, basic dikes, and *4 migmatite. A discussion of the petrography of the "quartzite" and its associated rocks follows. It is hoped that the detailed descriptions of a few representative specimens, coupled with data drawn from the field, will provide the reader with a SUL iciently clear and accurate picture of these rocks in their field settine. x') Specimen 5 In its northernmost outcrop in the mapped area, the granite is coarse and porphyritic. Large, prominent phenocrysts of reddish-orange feldspar occur in a coarse, -26... granular groundmass composed essentially of quartz and feldspar. The phenocrysts range in size from about 1 cm. to over 7 cm. Grain size in the groundmass varies considerably, but rarely exceeds 5 mm. Thin, irregular seams of ferromagnesian minerals impart a poorly defined gneissoid appearance to the rock. Rather intense deform- ation of the rock is indicated by numerous shattered phenocrysts. Quartz veins and aplitic dikes frequently cut the granite. Under the microscope, large masses of potash feldspar appear in a coarse to medium-grained, hypidiomorphic- granular matrix of quartz, albite-oligoclase, some potash feldspar, and small amounts of chlorite. The potash feldspar often exhibits the quadrille twinning characteristic of microcline; frequently, however, no twinning is apparent. Twinning is commoner in the potash feldspar grains of the matrix than in the phenocrysts. No indisputable evidence was found to indicate that the untwinned potash feldspar is orthoclase rather than microcline; therefore, tecause of this uncertainty, all non-plagioclase feldspar is called simply potash feldspar. Perthite occurs, but good cx'nples are rare. Occasionally, small poikilitic masses of twinned plasioclase (93-81 A Ab) occur in the micro‘ -27- cline phCchrySlS, forming natch perthite. The potash feldspar has a very fresh appearance; under plane light the grains are lright, distinct, and only slightly clouded by reddish-frown taolin. Plaeioclase, on the other hand, has a highly altered appearance; every grain is altered in some degree to parag nite. Cloudy masses of sausserite often occur. Cleavage is rarely apparent and grain boundaries are very irregular. All plagioclase exhibits polysyn‘hetic twinning. Allite twinning is very common; COmbination Albite-Carlstad twinning is infrequent; and Pericline twinning occurs rarely. Measurements of extinction angles indicate a composition in the range 56-93% Ah, with 8 % At the most probable; the plagioclase is therefore Alhite-Oligoclase. Quartz occurs in irregular masses filling the spaces between feldspar grains. It contains abundant inclusions, frequently in linear arrangement. Margins of quartz grains are often granulated, and undulatory extinction is almost always present. Secondary quartz occurs as veinlets which often follow fractures that have sliced and offset twinning lamellae of feldSpar grains. Shreds and blades of chlorite occur infrequently. The chlorite appears to be in various stages of alteration from a previous ferromagnesian mineral, probably hiotite, '. .n- 2' w. -..-.~.-. n" 4..- ,1 PlLUObSd Jude Oi LHC Olloldml “litrai rewains. The chlorite is pleocaroic {roe very light yellow-green to darker green; the intensity of pleochroism is hit ‘ 4 1 1 2e anc tne k.) variable. Extinction parallel to the cleava anomalous Berlin llue interference color suggest that the chlorite is the variety renninitc. Numerous rounded grains of ragnetite occur in the chlorite; the magnetite is usually in some stage of alteration to martite. Apatite, in small suh-hedral grains and long slender prisms, is connon throughout the rock and plentiful near chlorite grains. Zircon, probably the malacon variety, occurs in small rounded grains and rare terminated prisms. It generally has a cloudy, altered appearance and is surrounded ly a brown, slightly pleocnroic halo. Spidote has occasionally develOped along chlorite-plagioclase contacts. The following model analysis of the rock indicates l’tnat it is leucogranite as defined by Johannsen (1932). Mode of Specimen S mlprtZOOOOOOOOOOOOOCOOOOOZBOtSD’ID Potash feldspar..........38.2 Albite-Oligoclase........29.6 PtpatiteOOOOO0.0.0.00.0... 0.3 .. urn VWJ"- r(' 4 - Photomicro_ l ‘“‘ '° 3C. 5 showing hypidio- morphic-gr 9 the granite. ~ of peta- ‘ -~ h e 'I V 5 mm. in la“ ' mass is fffigx l‘ ' ;' I 'c ’ f" Ur- +1 '- iner .4.“ . ., nou ”! f w a l PP ; ‘ 1 ‘ undx hav= ‘ "w <_‘__~ _m ’1’ i ’ p ’,.- i ’ 5 Photomicrograph (20X) of Spec. 6, a more gneissose facies of the granite -30- Cllloritc................ 3.2 II,311Ct—iteooooooooooooooo 00]. ZirCOrIOOOOOOOOOOOOO...O. '1‘r. Specimen a Closer to its contact with the "quartzite" the granite loses its coarse, porphyritic appearance and beeches more decidedly gneissose. The thin seams of dark constituents become straighter and more regularly spaced. Grains of feldspar and quartz in the groundmass are of more uniform size, and appear elongate parallel to the banding produced by the dark seams. Although phenocrysts of potash feldspar are still prevalent, they rarely exceed .51mn. in length; the average size of grains in the ground- mass is about men. Under the microsc0pe, the texture is hypidiomorphic- inequigranular. Results of deformation are more pro- nounced than in the specimen previously described. Quartz appears as inequant grains exhibiting strain shadows, undulatory extinction, and granulation. Feldspar grains have frequently been sliced by cross-cutting fractures, and twinning lamellae are frequently dispaced or warped. Potash feldspar retains a fairly fresh appearance, but inclusions of suassurite and masses of paragonite suggest -31- more prevalent perthitic intergrowths of plagioclase. Plagioclase is usually badly altered to paragonite, and is replaced occasionally by subhedral grains of carbonate. Occasional rounded aggregates of epidote occur, but chlorite is less abundant in this specimen. The following modal analysis indicates that the rock is leucogranite. It will be noted that the feldspar content is almost equally divided between alkalic and sodic varieties; hence the rock is close to quartz monzonite in composition. Mode of Specimen 6 Quartz...................29.4% Potash Feldspar..........36.6 Albite-Oligoclase........32.4 Apatite.................. 0.1 Chlorite................. 1.5 Nagnetite................ Tr. ZirCOnOOOOOOOOO0.0.0.0... Tr. Specimen 7 A striking example of a highly deformed portion of the granite occurs in an outcrop near the "quartzite“ contact zone on the eastern margin of Section 1. Thin bands of mashed feldspar and quartz grains lie in a matrix of dark green, chloritic material. The contrast of the dark matrix with the reddish-orange feldspar intens- ifies the cataclastic appearance of the rock. The chloritic material comprises about 40% of the rock, suggesting that much of this material has been introduced. The relation of ’ntroduced chlorite to deformation will be discussed in greater detail later. Granite and "Quartzite" Associations Contacts of the granite with the "quartzite" are highly irregular, and never in the form of broad embay- ments. The granite generally appears to intrude the "quartzite" as small veins and stringers branching out from a larger granite mass. These granite-"quartzite" contacts are sharp and well defined. Many granite intrusions are bordered by chloritic aureoles which impart a green iue to the normally white "quartzite". In addition to the chlorite, an aureole often contains a number of small, blocky masses of reddish-orange feldspar. Tnese "dents de cheval" are in striking contrast with their fine-grained "quartzite" host. Specimens 2, 3, and 4 are representative samples of the "quartzite" and granite from a contact typical of that described above. -33 - In a number of instances, small pods, lenses, and stringers of granite occur as sub-parallel inclusions in the "quartzite”. Contacts of these inclusions with the host rock are diffuse, and the inclusions have no visible connections with a larger parent body; although, in places, the inclusions become so numerous that the result- ing rock is essentially granite. Specimen 29 is a typical example of this migmatitic association. Specimens 3 and 4 - Contact Zone Granite The granite occurring in the contact zones does not have the gneissoid appearance characteristic of those specimens previously described; however, it is similar in most other respects. Under the microsc0pe, the granite shows a fine to medium-grained, hypidiomorphic texture. Many grains have suffered marked cata clasis, resulting in a wide range of grain sizes. Potash feldspar occurs in much larger grains than does plagioclase. Microcline is present, but most of the potash feldspar exhibits no twinning. The plagioclase is albite-oligoclase (86-91% Ab), most commonly exhibiting albite twinning. Lobate patches of quartz appear to corrode both types of feldspar along grain boundaries and cleavage planes. Small islands of -34- very fine-grained quartz, probably remnants of the "quartzite" host, frequently occur. The most distinctive feature of the contact zone granite is the development of two types of chlorite. Penninite occurs sparsely, and is probably an alteration of the original ferromagnesian constituents. The second variety of chlorite (pro- chlorite?) is very plentiful, comprising about 7% of the rock. This mineral is light green in color, faintly pleochroic, and has greater relief than penninite. It forms radiating, fibrous clusters which form patches and continuous vein-like masses. It is a late mineral, cutting all other minerals in the rock, and is probably hydrothermal in origin. Hematite (martite?) accompanies this mineral. With the elimination of the prochlorite (?) and remnants of "quartzite", a modal analysis of specimen 3 demonstrates that the rock is a leucogranite of essentially the same composition as that of the specimens previously described. Mode of Specimen 3 QllartZO.0.0.0.0000000000031030/0 Potash Feldspar..........36.4‘ Albite-Oligoclase........3l.6 -35- Fig. 6 - Photomicrograph (20X) of Spec. 3 showing granite with inclusion of "quartzite". Fig. 7 - Photomicrograph (60X) of Spec. 2 showing "quartzite" with quartz vein and introduced chlorite and feldspar. -36- I‘LpgtLitQOOOOO00.0.0000... Tr. Chlorite (Penninite).... 0.6 xaantiteooooooooooooooo 0.1 Zirc011000.00.00.00.00... Tr. q Specimen 2 - Contact Zone "Quartzite’ "Quartzite" occurring near the granite contact on the eastern margin of Section 1 is a dense, fine-grained, light green, quartzose rock, containing small masses of introduced feldSpar, and cut ty a network of quartz veins. These quartz veins also cut the granite. As mentioned previously, an aureole is frequently developed in the "quartzite” bordering a granite intrusion. Within several feet of the contact, the normally white "quartzite" assumes a green hue, which intensifies toward the contact. Blocky, reddish-orange masses of potash feldspar are scattered throughout the aureole, and occur with greater frequency toward the contact. hese "dents de cheval" often attain a diameter of 1 cm. As seen under the hand lens, they are actually intergrowths of potash feldspar, with lesser quantities of clear quartz and greenish-black chlorite. Furthur examination reveals that they are closely associated with the numerous quartz veins cutting the contact area. Although many of these feldspar masses are enclosed in the quartz veins, the maiority appear to be concentrated just outside the vein boundaries. Microscopically, the "quartzite” is an aggregate of microcrystal line quartz and flakes of chlorite. The clear, unstrained quartz veins, with an average size of 0.02 mm., form a fine, even-textured mosaic. Much smaller flakes of light green, slightly pleochroic chlorite are andomly oriented throughout the quartz mosaic. Many quartz veinlets cut the "quartzite". Vein quartz is readily distinguished from the quartz of the host rock by its coarseness, sutured boundaries, abundant inclusions, and strain shadows. Coarse-grained aggregates of potash feldspar, plagioclase, quartz, and chlorite (the dents de cheval) appear to be closely related to the veinlets. Where they occur as islands in the host rock, the aggre- gates are surrounded by an envelope of coarse quartz. Potash and plagioclase feldspars are both highly altered. Patchy perthitic intergrowths of plagioclase in potash feldspar are common. Quartz appears to invade the potash feldspar as lobate masses along grain boundaries and cleavage traces. Shreds of chlorite surround many feld- spar grains. This chlorite contains many small anhedral magnetite grains. Determination of the plagioclase is difficult because of the lack of sufficient grains in a condition suitable for the required measurements. The arohahly unreliable results oltaiued indicate a composition of 92% AL. The extens'vc alteration in the snecimen suggests that some of the nlaqieclase is probahly secondary. Saecinen 29 - Higmatitic "Quartzitc" At several locations on the eastern margin of the mapped area, abundant strinqers, pods, and lenses of reddish-orange granite occur as sub-parallel inclusions in the "quartzite". he resulting rock has a miqmatitic appearance. Higmatite, as used here, describes rocks of mixed igneous and non-igneous ashect, implying that -ected into the I ‘1 igneous material appears to have been in country rock. The granita inclusions are small, never more than a few inches wide and a foot or so long. Abundant potash feldsoar occurs in a finer-grained groundmass of plagioclase, quartz, and chlorite. Contacts of these inclusions with the ”quartzite" are generally gradational. The "quartzite" is a very fine-grained, dense, wuite rock. Chlorite appears to develop only in the granite inclusions; aureoles do not seem to develop in the bordering "quartzite". At the locality where specimen 29 was collected, the migmatitic "quartzite" -39.. grades into a rock which is so thoroughly saturated with inclusions that it is, for all practical purposes, a granite. Under the microscope, the miqmatitic "quartzite" exhibits a confused jumble of badly altered feldspar, coarse quartz, and chlorite, set in a groundmass of microcrystalline quartz. The feldspar-quartz-chlorite aggregates form vein-like masses and isolated patches. he feldspars are often suthedral, and fracturing and granulation are prevalent. All of the feldspars have altered in some degree to paragonite, and are replaced by chlorite and quartz; however, twinning is still visible. Chlorite contains abundant opaque inclusions. Feldspar grains are usually surrounded by masses of quartz, which often develop a distinctive, radiating comb structure. Much of the coarse-grained quartz occurs in linear, vein- like bodies. The groundmass is an equigranular mosaic of microcrystalline quartz, with an occasional flake of white mica. Grains of the microcrystalline quartz are un- strained, free of inclusions, and of rather uniform size, averaging 0.025 nm. A Note on Granitization The development of "dents de cheval” and the -40- I ‘. Fig. 8 - Photomicrograph (20X) of Spec. 16 showing the equigranular fabric of typical "quartzite". Fig. 9 - Photomicrograph (60X) of Spec. 20 illustrating the mosaic texture of the "quartzite". -41.. uigmatitic character of some of the granite-"quartzite" associations suggests that the "quartzite” has been partially granitizcd. An increase in the effect that the granite produces in the "quartzite" at the contact could terminate in the develOpnent of granite, and the present situation may represent an arrested phase in a process of granitization. It has been demonstrated that granitic material has been introduced into the "quartzite", but the present study has not yielded sufficient data to prOperly evaluate the process by which this was accomplished. Typical "Quartzite" Specimens 16, 18, 20,_23, 24 The varieties of "quartzite" previously discussed actually have very limited occurrence in the mapped area. The bulk of exposed "quartzite" is a white, homogeneous, quartzose rock which shows no evidence of feldspathization. This typical "quartzite" forms the ridge trending across the north half of Section 1 and the small hill in the SW% of the section. In outcrop, the ”quartzite" is a strongly jointed, massive rock, thoroughly cut by a network of quartz veins. Hematite staining frequently imparts a dull red color to the rock. 'he "quartzite" shows no evidence of bedding or foliation. Several zones were noted in which the rock was strongly sheared and brecciated. Throughout most of its exposure, the "quartzite" has a remarkably uniform, fine- grained appearance. Exposed surfaces have acquired a dull polish, which accentuates the sub-conchoidal fracture of the rock. Broken surfaces exhibit a fine, sugary texture. Individual grains are not easily distinguished, even with a handlens. Close examination of a broken surface discloses numerous small, angular cavities lined with a coating of hematite. These vugs are small, the largest rarely exceeding 1 mm. in diameter. They probably mark the occurence of an easily soluble mineral, such as a carbonate, which has since been dissolved. Hematite staining, developed in minute fractures and any available pore space, is always present, often prevalent. Quartz veins vary in size from a fraction of an inch to several feet. Quartz crystals are well developed in the larger veins. Nicroscopically, the rock is a fine-grained aggregate of microerystalline quartz. he crudely polygonal quartz forms an equigranular mosaic fabric. Essentially all of the 500 grains measured in 5 thin sections of typical "quartzite” “all within the 0.025-0.0501nn. size range; grains in the 0.0125~0.025 mm. and 0.050-0.075 mm. ranges -43.. comprise only 3% of the total measured. There is no noticeable variation in grain sizes between specimens. The quartz is usually free of inclusions, although some grains contain dust-like material (hematite?). Individual grains exhibit uniform extinction under crossed; undula- tory extinction and strain shadows do not appear. Linear to strongly lifurcating quartz veinlets are abundant in all specimens studied. Vein quartz is distinctive and easily distinguished from the nicrocrystalline quartz of the groundmass. The grains are large and, as a rule, at least five times larger than the microcrystalline quartz ‘grains. Inclusions are numerous and heterogeneous, with hematite dust the most common. Vein quartz grain boundaries are generally sutured, in contrast with the sharp, straight boundaries of quartz veins in the ground- mass. A rudimentary cockscomb structure is deveIOped in the larger veinlets. Strain shadows are prevalent. Sericite, hematite, and rare chlorite are the only other minerals of significance in the "quartzite". All appear to be secondary. Small sericite flakes occasionally occur in the quartz mosaic; more commonly, the sericite fills thin seams which randomly cut the fabric of the rock. Hematite, in concentrations of dust-like particles and small anhedral grains, occurs in the vein quartz and as a -44- coating in cross-cutting fractures. In several instances, ninch-and-swell ! hematite was observed in long, slender, concentrations, apparently following minute fractures. An interesting, but uncommon, occurence of hematite is as dust-like concentrations outlining a hollow rhombohedral form, superimposed on the quartz mosaic. It is possible that the rhombohedral outlines are relics of a now completely replaced carbonate. This is substantiated by the frequent angular cavities visible in hand specimens. It is difficult to determine whether such a carbonate was an original constituent of the "quartzite" or was later introduced. It may be significant that the outlines appear to have no definite relation to the present vein- lets. Chlorite appears in specimen 13, which was collected from the outcrOp on the SW% of the mapped area. The chlorite occurs in patches of small, light green, slightly pleochroic flakes which have formed along fractures in the rock. In summary, the typical "quartzite" is a fine-grained, equigranular, massive rock containing 97.5 - 99.5% micro- crystalline quartz; 0.5 - 2.0% hematite; trace - 0.5% sericite; and O - 1.5% chlorite. Vein quartz is abundant in all samples. -45.. ' 'ta . . " ; '/ . j. ,. . '\ __ 'v . O ' ' .. ’3‘". 339 ' '1 ti . . «>3. V. . « Fig. 10 - Photomicrograph (lOCX) of Spec. 23 showing modes of occurrence of hematite (black) in the "quartz- ite" (grey). The rhombic outline may represent a relic carbonate crystal. Fig. 11 - Photomicrograph (32X) of Spec. 41 showing the cataclastic nature of the groundmass of the pseudoconglomerate. Deformed "Quartzite" Specimens 9 and 41 - Pseudoconglomerate Intense deformation of portions of the "quartzite" body is indicated by zones of a distinctive, highly brecciated rock. This rock resembles a badly deformed conglomerate and is called, for convenience, pseudo- conglomerate. An excellent example of a deformation zone containing pseudoconglomerate is exposed west of the ridge, in the western portion of the mapped area. A zone of pseudoconglomerate, approximately 5 feet wide, strikes roughly east-west and dips steeply to the north. The zone is flanked on both sides by typical "quartzite". Contacts are gradational and appear to be irregular, both laterally and vertically. Angular to rounded fragments of white "quartzite", in a great variety of shapes and sizes, lie in a greenish, fine-grained matrix. The matrix weathers easily, causing the "quart- zite" fragments to stand out in bold relief. The fragments appear to have a vague E-w lineament. Under the microscope, the rock is seen to be composed of angular to subrounded fragments of "quartzite" strewn about in a fine matrix of granulated quartz and small flakes of chlorite and sericite. The marginally granulated fragments of "quartzite" clearly demonstrate the cata- clastic deformation of the "quartzite". Undulatory extinction has deveIOped in some of the quartz grains of the fragments, but it has developed to a much greater degree in the fine quartz grains of the matrix. The fine quartz grains of the matrix appear to have resulted from the milling and crushing of the rock; they are exceedingly fine-grained, generally smaller than 0.0125 mm. The chlorite occurs in small flakes and radiating, fibrous clusters; it comprises as much as 20% of some of the specimens examined. It is the same chlorite previously described as prochlorite. An X-ray powder diffraction pattern discloses d spacings of 13.9 R, 1.54 X, and 7.08 3, indicating that the mineral is a chlorite. The mineral has probably been introduced, and is prevalent in those portions of the "quartzite" which have suffered deforma- tion. Sericite occurs in thin seams and scattered flakes throughout the matrix. Lamination, which would be expected in the matrix of so deformed a rock, does not appear; it may have been destroyed by the development of the introduced minerals. Dike Rocks Two cross-cutting, basic dikes occur in the "quart- -41“;— zite”. Rock in one of the dikes is schistose and highly altered, but the rock in the other is undeformed and fresh in appearance. The lack of deformation and alteration in the latter suggests that this dike is much younger than the altered dike. Specimen 15 - Meta-diabase The older dike occurs on the small hill in the SW% of Section 1. The rock is deeply weathered, and the location of the dike is now marked by a trench-like depression in the "quartzite". The 7 foot wide dike strikes N 45 w and dips approximately 70° NE. Boundaries of the dike are irregular, no doubt a reflection of the deformation the rock has undergone. The rock is a dull, green-weathering, fine-grained, schistose meta-diabase. Abundant dark grains of ore minerals pepper the fine-grained chloritic matrix of the hand specimen. The rock reveals its altered character especially well under the microscope. Infrequent, large laths of altered feldspar and abundant grains of magnetite and rutile are imbedded in a felty groundmass of chlorite, altered feldspar, quartz, and albite. The large, relic feldspar laths, occasionally 2 mm. long, are now an \O I aggregate of abundant carbonate, sericite, some epidote, and albite (?). The groundmass is composed almost entirely of secondary minerals; the opaque minerals also show signs of alteration. Rounded magnetite grains often alter to martite, and the twinned rutile needles are frequently surrounded by cloudy masses of leucoxene. Specimen 1 - Olivine Diabase The younger dike occurs in the western part of the "quartzite" ridge in Section 1. Contacts of this dike with the "quartzite" are not revealed; but the rock is typical of the brown-weathering, olivine diabase dikes which occur as the latest Precambrian intrusives through- out the Lake Superior region. The rock has a very fresh appearance and shows no evidence of deformation. Quartz Veins Quartz veins are prevalent throughout the mapped area. They range in width from fractions of an inch to ever 3 feet. Generally, the veins are strongly bifurcate, and cut the country rocks randomly; some of the thinner veins in the "quartzite" appear to follow old joint patterns. As a rule, the veins are composed of subhedral to euhedral crystals of clear to milky quartz, always with -50- Photmmicrograph (60X) of Spec. 15, from the meta- diabase dike, showing relic feldspar laths in a groundmass of saussurite, rutile, and martite. . -. -,', . , ;. , ._ ' ‘ , ‘ ~ " “'4‘ K '. . : .'.' - \ - ' - . . .' «pkg,» ‘ 3.1.». ‘_. ‘ . . \ ' ,. - . _ . . n. ‘ d v,» J: 3.: .51. *4. \r :55: .n_‘ - ‘ - ._¢-‘_ . - 'v s‘ .- a3 ‘. \ s - ' ,itw%~;y.h'~v n". ,f to ,. ‘1‘-w"~'-I1. ’_. “a— l'iv .- 4; '~- \ _4¢‘L_‘t _ -- ... ° . ':l‘.'.-,u--&o I "-‘o, ._" -> _, . 0 : ._‘ .' ,. , ‘ "' .P~' . 3‘. - .. '-'. .-» ' . . v - c‘kiiwsa- ,, -~- h.~ ;, u.- . - \ f ‘ , ‘ . . _ . ~ ‘ . . - . . . .: _ _'9 as. ,‘fia; "} L33.“ . 3" .. . ~£\O.\ ‘ 31-31.. . .q “ . ., . -w4~~ “ Fig. 13 - Photograph (0.5x) of typical migmatite showing bands of light quartzo-feldspathic minerals alter- nating with dark ferromagnesian bands. some hematite mineralization. Frequently, the hematite is so common that it coats the quartz, producing beautiful red quartz crystals. Radiating clusters of greenish-black tourmaline crystals occur in one of the veins examined. Quartz veins occurring at the granite contact previously discussed contain feldspar; these veins are probably silexite. Field relations indicate varying ages of quartz veins. Further study will probably disclose several genetic types of quartz veins in the area. Migmatite A broad belt of migmatite lies north, east, and southeast of the ”quartzite“ outcrops. The migmatitc is characterized by alternating bands of ferromagnesian and quartzo-feldspathic minerals, giving the rock a gneissic appearance. The proportion of light and dark constituents is highly variable within short distances. Banding is always steeply dipping, and examples of complex folding are numerous. The reddish-orange porphyritic gneissoid granite is found cutting the migmatite in Section 36. The major constituent of the ferromagnesian bands is medium to coarse-grained, subhedral to euhedral horn- blende; liotite, chlorite, and feldSpar are of minor importance. Plagioeiase feldspar is prominent in the -52- light hands; quartz, potash feldSpar, and some chlorite appear less frequently. Examples of migmatite from Seetion 36 are described by Sahakian (1959); he calls the rock an injection gneiss; and considers it to be an injected Keewatin-type green- stone schist. ‘ Granite and Gneiss A complex of granite and granite gneiss lies south of the mapped area; these rocks were not examined in detail during the present study. Several varieties of rock occur, and field relations are complex. Some of the granite gneiss is probably migmatite. Structural Inferences A detailed study of the structure of the area was not attempted in the present investigation. The regional structure is undoubtedly complex, and local structural elements are seldom sufficiently revealed to permit valid interpretations without detailed study. The following discussion is, for the most part, a compilation of inferences based on widely scattered ohservations. The "quartzite” exhibits no definite foliation, bedding, lithologic variations, or other planar elements that would aid in determining its structural setting. The C linear, east-west aspect of most Oi the "quartzite” outcrops may reflect the strike of the formation. It is interesting to note that, although the migmatites are highly contorted, they occur in a tel: which curves in a gentle arc, enclosing the area of "quartzite" exposures. The "quartzite" may be part of the same sequence as the mignatites. The "quartzite" is strongly jointed; but, with the exception of prominent E-N, vertical joints, no pattern is apparent. A detailed study would probably disclose such a pattern. Stronn master jointing in the district is readily seen on aerial thorogranhs. No direct evidence of faulting is found in the area studied, tut the deferred nature of some of the rocks indicates that faulting may have played a prominent role in the structural development of the area. Faults of considerable magnet- itude occur in the Palmer area. Shear zones deform the "quartzite" at several places. Figure 14 is a diagram of quartz c-axis orientation in the mylonitic groundmass of the rock from one of these zones. The diagram shows a weak preferred orientation of quartz with respect to the '1' plane 0i the shear zone. Lower Hemisphere )3-2-I-0 % COMPOSITE DIAGRAM ORIENTATION GOO QUARTZ C-AXES Spec. 90,!) Figaro '14- ”d ‘r ,- v- . ‘ -‘ ") 1 :w'FhUIPuLC “an n 5‘ All of the rocks examined in the present study Lear evidence of metamorphism. The occurrence of chlorite in the granite, migmatite, and dike rocks, and the fine grain size of quartz in the "quartzite”, indicate low rank metamorphism typical of the chlorite zone. CR GIN OF THE “QUARTZIIE” Lack of diagnostic or distinctive features, such as bedding or relic structures, precludes a ready identifica- tion of the ”quartzite"; and it is necessary to consider a number of alternatives from among known highly quartzose rocks. Field relations suggest that the "quartzite" might be interpreted as 1. An igneous rock 2. A large mass of vein quartz 3. True quartzite 4. Sil'c'fied tuff 5. Recrystallized chart In the following discussion, each interpretation is considered in light of the data derived from the present study, and conclusions are drawn regarding the feasibility of each interpretation. [U1 igneous :hxflc Highly quartzose rocks of igneous aspect, although rare, have been noted at various localities. Jonannsen (1932, p. 4-25) discusses many examples of such rocks. He states (p. 4) that: "they occur in the form of dikes, segregated masses (sometimes quite large), and replacements, but do not form typically plutonic or extrusive bodies." With the exception of greisen, which is simply an altered granite, the quartzose igneous rocks described by Johannsen have the same mode of origin as pegmatites; that is, they are of "aqueo-igneous magmatic origin." Tolman (l931) reviews available literature concerning possible igneous quartz masses. He remarks on the paucity of precise and detailed descriptions of the field relations of supposedly igneous quartz bodies; he lists very few criteria which could be considered definitely diagnostic of igneous quartz masses; yet (p. 297), he appears to favor the idea that there is a complete gradation, with increasing quartz content, from typical igneous rocks, through pegnatites and quartz-rich rocks, to hydrothermal quartz veins. Hany workers diSpute the existence of a truly igneous rock composed almost wholly of quartz. Furnival (1935, ”J K '1 C) L'] V summarizes this view, stating that: "no occurrence of a body of quartz which has undoubtedly formed by the action of magmatic process, that is by direct crystallization from an igneous magma, has been described in the literature." Furnival considers such quartz masses, as well as pegma- tites, hydrothermal in origin. The present writer agrees essentially with Furnival. The terms silexite and esmeralditc, as defined by Johannsen, are used in the succeeding paragraphs with this qualification: they are igneous rocks only in the sense that they were formed by solutions, perhaps relatively viscous, which emanated from a magmatic source. Greisen Association of the "quartzite" with granite suggests that the rock may be a greisen. Johannsen (p. 6) describes greisen as: "quartz-mica rocks which originated along fissures in granite by alteration of some of the minerals of the pre-existing rock." The essential constituents are quartz and mica, with quartz generally in excess. The quartz is similar to that of granite; the mica is generally zinnwaldite. Feldspar, when observed, is greatly altered. Cassiterite often occurs. Greisen exhibits a typically granitic texture. The secondary nature of greisen is usually apparent, and gradational contacts with the pre-existing granite are frequently exhibited. If the "quartzite" were greisen, remnants of the pre-existing granite should occur within the "quartzite" mass; however, nowhere were such features observed. There is no evidence that the "quartzite" is derived from a granite older than the present intrusive. Contacts of the ”quartzite" with the present granite are not typically gradational. Where contacts are gradational, as in the migmatitic "quartzite", granite appears to develop in fractures in the "quartzite"; the situation would be reversed in a greisen; furthermore, the granite is younger than the "quartzite". Finally, the fine-grained, mosaic texture of the rock is not a texture typical of greisen. Silexite Johannsen (p. 11-17) describes silexite as an igneous rock composed essentially of primary quartz, practically free of pneumatolitic minerals such as muscovite and tourmaline, and containing no more than 5% feldspar. He states that the difference between silexite and vein quartz lies in the origin of silexite from juvenile (magmatic) waters. It is very difficult to separate the -59... two types of quartz; "only by tracing the connection in the field from pegmatitic rocks, through 1e to pure quartz chs can the origin (of silexite) be definitely estallished." 5 ”'1 '1 m '23 O O H P. f; m '3 H (.1. 0') " 3 .n '1 Johannsen further describes the rock as exhibiting alliotropic granular texture, approaching the sutured. The quartz is typically igneous, containing many in- clusions which often lie in straight lines. Bodies of silexite are not large; generally, the rock occurs as veins, dikes, or inclusions in granite masses. Miller (1919) describes a typical occurrence of this rock with its aplitic and pegmatitic associates. The masses of silexite which he describes are quite small, being measured in tens of feet. They have the aspect of inclusions in the main granite mass. Gradational contacts the silexite with granitic rocks frequently occur. Although the composition of the "quartzite” is similar to that of siiexite, the rock does not appear to be silexite for a number of reasons. The fine-grained p—J mosaic texture of the "quartzite" does not resemble tie jumbled aggregate O“ irregular grains characterizing the texture of silexite; feldspar is not present as a primary constituent of the ”quartzite”; the quartz does not contain the abundant inclusions so characteristic of the quartz of silexite; n: iredation ot the ”quartaite" with known granitic rocks is apparent; and, finally, the great size of the ”quartzite” masses is not tynical of known silexitc occurredces. Johannsen (3. if) descriles tSueraldiLe as: “those quartz-ruscovitc rocks wnich occur as Ci schistic unas:s of large bodies or as diascnistic dikes, and which may be the result of orizinal diife ntiation or of resoriti ion nrocesscs entauaLine the complete consolidation of the rock.’ to coarse-grained rock composed [ 1 U) 0 "S " 1 [—4 (L (.1 (1 1’) r4 U) (‘3 H 1 :0 1 J essenririly ot quartz and :usc 0"ite. The quartz cna mica ex i11t typicrl igneous CHFFVCCCIS and occur in rarying h“; “I 9“ “I 4 :19 'I.‘"r\'- . a r— 1‘ o ‘7‘ \ nroportions. ine loch oiten occurs as a tender phase 0 *ranite. Ifuch the eae arguments as were used in the case of O greise. and sile>;i tc an he used to show that tne iquartz- ite is not esweraldite. Tue ”quartzite” Simpiy does not exhibit an igneo tIS cha acter. In addition, the "quart7ite" does not contain atundant primary mica, such as one would expect in CSmCreldite. "Quartzite", which is more resistant to erosion than ‘7. , q --.- v\~ ‘1‘,‘ ‘A ‘1’ J. " l"“- ‘I q». :‘ . ' L111) SLlLLOLhztlLAJ LOL1\.S, LOri=Lb L110 LOU"), liLlC-fll ridge xtcnding across Section 1 in the mapped area. The I. strongly linear aspect 0i this occurrence reseulles that oi a large quartz vein. Such a quartz vein would have {1) minimum width of 156 feet and a length of over half a nile. Quartz veins of such magnitude are rare, out no occur. Furnival (1935) describes large quartz veins occupying iault zones at Great Sear Lake, which attain g:1a:-:i:1um widths of up to 1,60 feet and lengths measured in miles. He states (p. 847) that the veins "generally consist of two parts; the main part is composed principally of quartz with minor amounts of vein breccia; this is flanked by stockworks of quartz stringers on one or both sides...Ninerals other than quartz are sparse. They are specularite, bornite, chalcopyrite..." He finds that the quartz of the veins is of two main types: massive and handed. The banded quartz is either milky or transparent and colorless; banded quartz is coarse-grained. The massive quartz is medium to coarse- grained and milky, showing no uniformity in texture. he also notes chalcedonic quartz, which is a matrix for quartz fragments. H] Adams (1920) discusses the microscopic features 0 the varieties of quartz which occur in hydrothermal vein deposits. Most of the varieties he describes exhibit -62- properties, such as coub structure, which are typical of minerals growing in open spaces. It 1 nSiQC from the linear nature of the outcrops, there n deposit. H- is little evidence that the ”quartzite' is a ve ho handing or stockworks, as descrited ly Furnival, are found; and the rather uniform Urain size of the "quartzite" contrasts with the wide grain size variation in the massive quartz at Great tear Lake. Comb structure, coarseness of grain, and other evidence of open-space growth is lacking. No diagnostic hydrothermal minerals J are associated with the quartz. :uartz veins which cut the ”quartzite" exhibit typical properties of vein quartz, and are in marked contrast with the fine, even-grained texture of the "quartzite". A True Quartzite Quartzite, in the strict sense used here, is a metamorphosed sandstone. The pre-existing sandstone has been oartially or completely recrystallized to form a granular metamorphic rock consisting essentially of quartz. The composition of quartzite reflects that of the pre- existing sandstone; thus, the abundant quartz in a quartzite is accompanied by small anounts of feldspar and 1. other detritrl minerals. Cenenting material of the sand- stone may he rc-crystallized in the quartzite, and new minerals may be introduced by solutions. Original structures of the sandstone, such as bedding, cross: bedding, and ripple marks may be preserved in the quartz- ite. Where metcnorphism has not been intense, a quartzite can be treated in the field as a sedimentary rock. With increased metamorphism anc deformation, quartzite gradually loses its sedimentary character, foliction develons, and < gradually becomes a quartz schist then a quartz gneiss. Under the microscone, quartzite generally exhibits a sutured or mosaic texture. The rounded grains of the sandstone l P" ave been squeezed together and enlarged by secondary growth. When a mosaic texture has developed, the original rounded grains may appear as nuclei in the new polygonal grains. Assuming that no large-scale fracturing or granulation occurs, the range in grain size of a quartzite should reflect that of the prior sandstone. Increased deformation causes a development of lenticular grains arranged in sub-parallel fashion, and undulatory extinction and strain shadows hecone prevalent. Intense granulation and nylonitization sharply reduce grain size. Although the ”quartzite" is similar mineralogically to a very pure quartzite, few of its remaining preperties -04.. closely resemble those of a true quartzite. Evidence of prime " seci‘cntcr/ features is la cking in the field and in thin section. Even if such structures had been ‘ A. destroyed ty metamorphism, relic bedding, in the form of foliation, might be eXpected. he "quartzite" has no Visible foliation. Under the microscope, the ”quartzite" exhilsits a fin no, equigranular, mosaic texture; close eAa.ination discloses no evidence of original rounded grains. Virtually all of the grains in the roe k are eelow 0.08 mm. in size; thus, disregarding secondary enlargement, origix al sedimentary grains would have to have been close to silt size. It is inorobahle that a formation as pure, and uniformly textured as the "quartzite" would have developed from very fine-grained clsstic material. The euhedral, unstrained nature of the quartz, and the lack of foliation or recognizable orientation do not suggest the ‘evelOpment of the fine grain size by deformation. Silic ifi ed Tuff It is possible that the "quartzite" and the migmatites are pa rt of he same sequence, although the present study discloses no direct evidence of such a relation. Neverthe- less, the "qua rtzite” occurs in a terrain dominated by the migmatites; field relations demonstrate that the reddish- oranec granite is not genetically related to either rock wh JrOFortion of icrro‘c~:esian constituents and the f? U I rel ict foliation in the mi: ‘3 render it likely that hq—l a character is interred, warranting consideration of t "”*“rt 1"“63” "s “ no"°.i»le sii icifiec "o‘csnic roc“ \1 (- Lad-I. v L Ct DO 1 l V -L (.I L... (\0 Silicified Jol ca ni c rocks occur in the Precambrian of many rezions, notale in Scandinavi . Sederholm (1930) 7‘) ‘ ' '- W J describes the erauation or strongly metamorphicrlep ntit .e into slightly metamorphic hallcflinta. Leptites are uartz-feldSpar rocks derived (‘9 o.‘ .'7 ‘ ‘ .Lll- ‘3ré‘iiitu, bk, 6'. "N /3 "- ' - .. ' ~.- . 4 \‘r I: 1 \ ‘ . w 'I an. 'A "- . "~ grained “CLCMOCPMLC iocu Oi volcanit Leiiwetion, and 13 .‘ \J r‘ 1" A .' v’f: ’ J., ft kQS’VJILt.LLJL4__/« ( SlL.L(4-L.L1.CCL LU“... / “" ’\~ 1 ( 0" — q q.“ ‘1 . w r~ -\ 1“" . ' ’ .1: .-tti,ohu iJJJ, n. JJJ) ULSLUSSuo tne alteration oi fl H; r H U) 0. Y’7 A d]: '1 ) \fi"1 ‘- 10" f'. f“ "1 .“‘ l" ":50 . ‘,~ u-- ti .-e etiiicst cutdgcs in suits is the 1 , _‘_: ..3 - '. '. _.. 'T I ‘ - .-, .f. ' ' #51 uovosiiiom in tiOu oi miniateu silica-ouzi - V b 1‘5 ’- 7‘ fl -\ ~‘ ‘ ‘r ‘ A J" ' . l. ". ’VT‘. \ r‘ "'- ‘1. "‘ (WIC. C-ie _.'VCL.(J_LJ: 9 v3 _..( L.) 5.1 -LCLLLKA L4.()ll -. A‘ “J I 0 t“ ‘ ‘ ‘W 1"‘1-0 3 1 hf- “ \ 7": n“1 .Lvl sets L ALL \ |-/\ ~Ql(,L JA. Lil-L J-.L.L( . LC, c‘ ‘1 :‘ ~rr' I f" __ .__ “' ‘ q. “‘ not us cull ,o a tense ilnu.y roe“ wiicn Q 1 >4 .fi - . 1 a a - a: closely to . .ies rhyoiite, novrcul ~te, ‘: 1 9’ ~ ‘ K . F "‘"\ n1 ‘7 2 “ e ’ f ‘ 4“ 1 - A ~~ 1.5;);‘C1‘, {71.11 SO IIOI'L'H. “114-4.8 15‘ .C- ..’.!.S\..<"i.-eu '-' . .l- -.'-- :-‘ ,x T- #3.!- -o: such fOLhS in the fie d in thin , \‘I‘. ‘v‘ ’ ‘1'“1 “ "' ' . 1‘ “ SCCelOLi, .‘LO‘J O‘7L Or, LL10 X7- cmeC 93‘ 1C C uni 3C: (3r ‘ I], may remain and this ircture tocetu-r wito the composition and form of the associated crystals and their chemical composition will serve to prove their pyrocl.astic or1~ii...Dtv-t ification commonly accompaiies the alteration...Careful study in oriinary light may disclose shards, threads, cusps, vesicles, and the like, of the original glass. he devitrification nroduces a montmorillanite clay...Metamorphism superimposed on other alterations may completely obscure the original nature of the denosit." Pettijohn (1943), in his excellent treatise on Archean sedimentation, describes the alteration of Archean volcanics in the southern Canadia. Shield. {e mentions that, although mineral changes are pronounced, the megascopic textures and structures are well preserved. Adams (1920, p. 642) finds that in silica replacement of rocks ”where there is a variation i: the cnaractcr of the original contituents, the grain of replacing silica may change accordirg to the particular portion of the rock attached. Thus feldspar silicified in a tuff may 5. replaced by coarser grainoc quartz than the groundmass.“ The geologic environment of the ”quartzite" is considered iavorahle for the development of volcanic rocks, and it is possible that the "quartzite" is a silicified turt. Granitic rocks of several ages are present in the -1 .9 ’ L. . , . .. I- . ,., J J- .' ‘ 4, "I , g..‘ ULStrlCL, and may reprssen wag atic sources LO: Nae . -_ u 1 '9 3 f— ? , .. _" J__ V. ‘ I-?J- Sl’lC“. it is ncss'uig, ii i-c “quai-'1tv" uiu LSIFlLLC - _.‘ , - L. -. -, ,: -.' .r , 1. 17" . 1 "I _ , " 4.1‘ “q r. _ 113‘.rCS'~3-3L. Flt? ORLSLLQ; elliJL. {HILL J.¢"\fr'°3., twat ‘.:LC Sa..‘1() cldfi‘lTL V I'.("“ I” "" ‘t‘ J“ ‘T'. ‘-"*r\*'*.‘!" '\ r-n '1: \w.'f:: '3}. 4_'..‘ 1- TJ’? .7;Ll...;. LOi. ‘ C Lac A...L'-;Lc_'.lt.t, c 1-00 SJLLJ-C.4.4.J_CLL L..t: LlltL. " ' ~ 9 : 1 - ‘ 1 "t: A v "x v" -: \ -- . . .- rr: - - . v Q N‘ '— . ConcluSive exigence oi . volcanic origin 0i Lie Jquaitzite" C .0- V .L- is lrching, however. Structures resembling relicts 9 rue I "'~ ‘ ’ " V" -..l ’n ‘ 7 A‘ , - *1. ‘y- '1 - -: '- V . ' ‘r firss, tr s_et tree e Ls, can leer «our-s WQLCJ woule lc ' n ‘ ~q . ‘ -— [I 'T r" i 1‘ 1' I H I‘ ‘u" r‘ \ ~ ‘ 1 V llHC-LtJu ire. C ,ll-ehisrLu, til. Uqf -o’ o‘scrve. ii “ f - f: . \ I" vs -4 v «,4 ’C 1 -A . f. ‘ 0 ‘5 ‘ ' " ‘ . t m‘ ‘ the line, rfllll Laiul_ai italic or tee 'qicreZLte’. int 1' r‘“ fi""1“ 1 "“."'"" ”fir:‘~(‘ "3‘. 7 n .. "t-tfj"n*‘ . “"61 C(‘J'H‘ L-laLr“,\.~l-$.i(_b ‘1 1‘.“ Ly“ \ I.“ ..L. 1“ L‘sb'\l Lea-K. 'LO“ -_ 1A.].C'I4C bllL-rk Lk‘l O l ’,". j - CS. . \fiF "' 1 'I 1 -. 4“ 1"— “ ' ‘ 1 ‘Qiietce Slii'CLen. to nos ro. rc;_c iril structures 81 l, ‘1 . .-‘ ,.\ .«..._' ,l .. r111], A ,. ,1- -,.,..~- . ...._..? 1.--, x .5: J“- , 9 .s , .1- ' 1.. Co 4.51. L... ..f.i .c .'.’. . i“; giver. O .7141. .1. L.) 0.9- 1..ch “ '11.-.:‘re21.te" ~v m" " " ,. f.""‘.~\"’ .—-‘ 1‘“, r. («4'13 ~? ‘I 3.3 V .--f'—'o _-,., :;:o a. 4. WULLLLL AJL ., c. L-.\; 1". L "1-. .L.-.L c- .34. 4.-_C........L.‘.t -L-J- L , S thiilCcht. ‘ I V 1 1 I ' " - . 1‘ , ' 1‘ ‘ ’v * ~ "' ”I ‘ ‘ , ‘ ’ V .' ‘ v ‘ EZLINALI S (at c.11uai-z€, SLN;(1, {1A{- L.J.2. s n31JJl :1'11eax . 3a- ~1