THEEIB LIB R A R Y Michigan State University M PLACE IN RETURN Box to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 2/05 a/CIRWoOuejndd-pjs A FIELD AND PETROGRAPHIC STUDY OF THE RUBY CREEK AREA, MADISON COUNTY , MONTANA BY James E. wray A THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in Michigan State University ' East Lansing, Michigan ACKNOWLEDGMENTS The author wishes to express his sincere appreciation to Drs. J. W. Trow, J. Zinn, and B. T. Sandefur for their cooperation and assistance during the preparation of this paper. Acknowledgment is also made to the staff for their constructive criticisms and suggestions. The writer is grateful to Donald Lindgren and to R. K. Hogberg of the Northern Pacific Railway Geology Division for their aid. The Northern Pacific Railway Company has contributed to this work.through loan of equipment and resources. Recognition is given to James Neal and Rederick K. Mbore for their constructive criticisms of this work. ABSTRACT A FIELD AND PETRDGRAPHIC STUDY OF THE RUBY CREEK AREA, MADISON COUNTY, MONTANA The rocks in the Ruby Creek.Area constitute part of the Cherry Creek series, a Precambrian complex in south- western Montana. The results of field observations are supplemented by a laboratory examination of twenty five thin sections from samples collected in the area. Six different lithologic-types were recognized and described. Four metasedimentary types were recognized: slightly argillaceous quartzite, siliceous dolomitic marble, phyllite, and-banded iron formation. The iron formation is composed of quartz, magnetite, and hematite. It was probably laid down as chert, magnetite, hematite, and stilpnomelane. Two igneous types, extrusive flows and intrusive sills were recognized. The flows were originally basalt and are now altered to a massive rock.composed of low-iron hornblende (uralite). The sills, which are now biotite-hornblende schist, have obtained an apparently higher metamorphic rank than the other rock types in the area. This is thought to be due to their low water content in comparison to the other rock types, which allowed metamorphism to proceed further than in the wetter rocks. ii CONTENTS Page INTRODUCTION Location and Accessibility. . . . . . . . . . . 1 Physical Features and Climate . . . . . . . . . 2 Historyof Area . . . . . . . . . . . . . . . . 3 Geologic Setting . . . . . . . . . . . . . . . 4 Purpose and Scape of Problem . . . . . . . . . 5 Sampling Procedure and Laboratory Methods . . . 5 Nomenclature of Names and Terms . . . . . . . . 6 QUARTZITES . . . . . . . . . . . . . . . . . . . . . 7 Quartzite Bed Three . . . . . . . . . . . . . . 8 Megascopic Description . . . . . . . . . . . 8 Petrographic Description . . . . . . . . . . 8 Quartzite Bed Five . . . . . . . . . . . . . . 12 Megascopic Description . . . . . . . . . . . 12 Petrographic Description . . . . . . . . . . 12 Quartzite Bed Six . . . . . . . . . . . . . . . 15 MEgascopic Description . .A. . . . . . . . . 15 Petrographic Description . . . . . . . . . . 15 DOLOMITIC MARBLES . . . . . . . . . . . . . . . . . 18 Megascopic Description . . . . . . . . . . . . l8 Petrographic Description . . . . . . . . . . . 18 iii PHYLLITES . . . . . . . . . . . Megascopic Description . . Petrographic Description . METABASALT . . . . . . . . . . Megascopic Description . . Petrographic Description . BIOTITE-HORNBLENDE SCHIST . . . MegasCOpic Description . . Petrographic Description . IRON FORMATIONS . . . . . . . . Megascopic Description . . Petrographic Description . VOLCANICS . . . . . . . . . . . ECONOMIC IMPORTANCE OF THE IRON PETROLOGICAL INTERPRETATIONS . Iron Formations . . . . . Matabasalt . . . . . . . . Biotite-hornblende Schist .STRUCTURE . . . . . . . . . . . CONCLUSIONS . . . . . . . . . . BIBLIOGRAPHY . . . . . . . . . iv FORMATIONS Page 21 21 22 24 24 24 26 26 27 30 3O 32 35 38‘ 39 39 42 43 46 47 48 10. 11. '12. LIST OF FIGURES Location map of problem area . . . . . . . Geologic map Of Ruby Creek area . . . . . Generalized Stratigraphic Sequence of the RUbycreEkarea000000000000. Photomicrograph of quartzite showing veins of larger grained quartz . . . . . . . . . Photomicrograph showing a crystal of magnetite formed in mica . . . . . . . . . Photomicrograph Of polysynthetic twinning of oligoclase surrounded by quartz . . . . Photograph of hand specimen of quartzite bed six showing regular green banding . . Photomicrograph of grain of microcline in quartZite O O '. O O O O O O O O O O O O O Photomicrograph showing polysynthetic twinning in dolomite . . . . . . . . . . . Photomicrograph of phyllite showing excellent parallel orientation of biotite Photomicrograph of metabasalt showing matted texture Of the hornblende . . . . . Photomicrograph Of schist showing horn- blende, biotite, feldspar and quartz . . . Photograph of banded iron formation. Boudinage structures in top layer of quartz Page 11 14 15 16 19 23 25 30 31 Figure Page 13. Photomicrograph showing two sizes Of quartz grains. Grains smallest where iron minerals most abundant . . . . . . . . . . . 32 14. Photomicrograph of granule of magnetite- with quartz in the middle . . . . . . . . . . . . 33 15. Photomicrograph showing stilpnomelane and hematite. Lighter colored blades arestilpnomelane....o...........34 LIST OF TABLES Table Page I Analysis of Two Southern Quartzite smples O O O O O O O O O O O O O O O O O O O O 0 11 II Analysis of Two Central Quartzite Samples 0 O O O O O O O O O O O O O O O O O O O O 15 III Analysis of Two Northwestern Quartzite sampleSooooooooooooooeoeoooo17 IV Analysis of Three Dolomite Samples . . . . . . . 20 V Analysis of Two Phyllite Samples . . . . . . . . 23 VI Analysis Of'Metabasalt Samples . . . . . . . . . 26 VII Occurrence Frequency of Twinning TypeSinAndeSjJJeoooooooooooooooo28 VIII Analysis of Three Biotite-hornblende . ' SChiSt samples. 0 O O O. O O 0 O O O O O O O O O O 29 IX. Analysis of Ten Samples From Four Main Iron Formations . . . . . . . . . . . . . . . . . 36 x Potential Recoverable Fe in Ten Samples From Four Main Iron Formations . . . . . . . . . 37 vi INTRODUCTION In the summer of 1957 R. K. Hogberg, assisted by the author, mapped the geology Of the Ruby Creek.Area which is located in the southern half of Madison County, Mbntana, about seventy’miles north of Yellowstone National Park. The author, at that time, felt that a petrographic exam- ination Of the rooks would help to interpret the geological history of the area and to determine the economic importance Of the iron deposits found in the area. This thesis is a petrographic study which may help to supply answers to these problems. Location and.Accessibility The Ruby Creek Area is located in Township 9 N., Range 1 W},‘Madison County, Montana. The area investigated includes sections 3,4,9,10,15 and 16, of this township. From its nearest pOint the area is approximately four miles west by dirt road from Route NO. 1, the main north- south highway through southwestern Mbntana. The nearest railroad is a spur of the Northern Pacific Railway at Norris FIG. I-MAP SHOWING LOCATION OF PROBLEM AREA M ONT ANA MADISON ‘ xCLr (WE—:22; Sea” in miles about 17 miles north. Physical Features and Climate The principal industries of the region are stock raising, lumbering and mining. The region is in the eastern foothills of the Gravelly Mountain Range. The terrain is fairly rugged with altitudes ranging from 5,100 feet to 10,000 feet. maximum local relief in the Ruby Creek.Area is 1,480 feet with a maximum altitude of 7,240 feet. The streams generally drain east toward the Madison River which is the major drainage route Of the region. The region was glaciated but there are very few glacial deposits in evidence; much Of the bedrock is exposed. The region is mostly grassland below 6,000 feet, above which there are extensive pine forests. Summers are mild and dry; most of the rainfall comes in the spring and fall. 'Winters are severe with recorded temperatures as low as -55°F. Snowfall is light. History of Area The area has interested prospectors for many years. The iron formations, which occur here, carry small percentages of gold which was the major Spur to amateur exploration. Test holes and prospect pits are scattered over all six sections. One serious attempt to mine the iron formation for its gold values was made in the mid thirties. An adit was driven into the side Of a hill in the southwestern corner of section 9 and a milling plant was constructed in Ruby Gulch to process the ore. The adit and plant were connected by an aerial tramway. The formation was mined for a short time but only 44 percent of the total assayed gold was recovered in the milling operation and the venture was a financial failure (Hogberg, personal communication). The only mine now operating in the area is the Yellow- stone Talc Mine located in the SE% of section 4. The talc deposits occur as isolated bodies in dolomitic marble. About 5,500 tons of ceramic, cosmetic and lava talc are mined by Open pit methods each year (Perry, 1958). Geologic Setting The Gravelly Range Of central Madison County, Montana, is composed of Paleozoic and Mesozoic strata warped into broad open folds by the Laramide orogeny. The Precambrian metamorphic complex, of which the Ruby Creek beds are a part, is exposed low in the foothills on the east side of this range, by erosion of overlaying Cambrian strata. Eastward, in the Madison River valley, the metamorphic rocks are buried beneath alluvial and lake deposits. The uppermost Precambrian Belt series, so widespread in western MOntana, is absent in this region. NO separate studies of the Ruby Creek rocks have been made previously. They are included in a series called the Cherry Creek, after the second creek north of Ruby Creek (Peale, 1896). These rocks consist of light and dark gray gneisses, mica schists and phyllites, schistose quartzites and dolomitic marbles. The beds of the series are well exposed and they have a general northeasterly strike. The Series is mostly sedimentary in origin, but metamorphism has proceeded far enough so that the original sedimentary minerals have altered to metamorphic minerals;any sedimentary structures which may have been present have disappeared. The beds of the series have been closely compressed by folding and commonly dip more than 60 degrees northwest; in many places the beds are nearly vertical. Bedding in the thesis area itself dips from 30 to 90 degrees, generally to the northwest. Purpose and Scope of Problem Until recently the geology of the area adjacent to Ruby Creek.was relatively unknown. The beds in the area were included in the larger Cherry Creek series with no recognition of their special nature. During the summer of 1957 a detailed field study of the area was done by R. K. Hogberg assisted by the author. The purpose Of this Study is to provide petrographic data to augment the field study and to provide more data on the iron formations as to their possible economic value. Sampling Procedure and Laboratory Methods In order to Obtain samples representative Of the entire section, sampling was done primarily along line A-A' (Fig. 2) which is normal to the strike of the beds. These samples were supplemented by several taken along line B-B' (Fig. 2). Standard petrographic procedures were used in the examination of the thin sections. These sections were cut normal to the bedding or foliation whenever possible. Mineral percentages were Obtained with the aid of a six axis intergrating stage in all slides except those Of the iron fonmations and quartzites. In the latter cases percent- ages were determined by using a grid ocular. Nomenclature of Names and Terms The major lithologic units were given a basic rock or textural name (metabasalt, schist, etc.). In the case of the schist the names of two megascopic minerals were used to modify it (Biotite-hornblende schist). No attempt is made in this paper to give formal names to any of the indi- vidual beds other than those which were used in the field. When it is necessary to refer to one particular'member of the series, the basic name is prefixed by a positional name Cenozoic Precambrian GEO LOGIC MAP OE RUBY CREEK AREA Sec’rions .3,4,9,|O)15& Io TprS Pge lW Madison County, MonIana EXPLANATION SEDIMENTARY ROCKS IGNEOUS ROCKS QT Tvo undifferen’rioicd Quo’rernory exIrusives and TerIiory sedimenis UNCONFORMITY mmb b‘I magnesium marble bosolIs Phy phylHIe sch bio’ri’re-horn blende schis’r T qu quarIzi’re _ , . .. . ' . ., "ye-(c . a ,-, . ‘ _ _..-. .V _ Iron formation § road y sIrike and dip of beds or foliofion fi faul’r _____ con’rocI fl shafi 6Q mlne @ sample locoIi-on Geology by R. K. Hogberg. Samples and sec— tions along AA' and BB' by J. Wray. AI Ir! ° scale in fee? 0 500 I000 2000 3000 7 /< 62° mmb Yellows‘Ione é f / $4" ,. / / y/ b) 536% //‘°’)7/ ,/7 451/ ’/ / ,- ‘ i ply: / h / / ’ of“ / / bi“ p y // [66¢X‘R //‘ / / ,1 GENERALIZED STRATHBRAPHHZ SEQUENCE OF THE RUBY CREEK AREA LEGEND ROCK TYPE PhyIIiIe BioIiIe-hornblende SchisI Iron formOIion QuorIziIe I" DolomiIic marble MeIobasoII feeI 0 500 IOOO 2000 I[JIEIIZZIZZJIIIIIIIIIIII Ver‘IicoI scale anA PRECAMBRIAN such as southwestern, southcentral, northcentral, etc. In the case of the quartzites, it was necessary to deviate from this scheme because Of the greater number of beds. The quartzite beds are numbered consecutively from the southeastern most bed to the northwestern most bed (see Fig. 2). The word, trace, as used in the analysis tables means less than 1.0 percent. Mmeers of the same lithologic unit are described separately only if petrographic examination showed signifi- cant differences in their compositions. QUARTZITES Six beds of quartzite crop out in the area. For expedience the beds are numbered cansecutively from south- east to northwest. Beds one and two are by far the thickest. Bed one laas an average thickness of 400 feet and bed two averages 2150 feet. The four remaining beds, to the northwest, do riot exceed fifty feet in thickness. The three beds which are crossed by line ArA' (see Fig. 2) were sampled for petrographic examination. They are beds three, five and six. Quartzite Bed.Three Bed three has a maximum thickness Of twenty feet and a minimum thickness of ten feet. Average thickness is about fifteen feet. It is fine grained and‘slightly foliated. At both its contacts it is graded into the phyllite which is stratigraphically above and below it. In general it is buff in color but in places it has a green tint. Where it is badly weathered it assumes a dark brown shade. Petrographic‘Description Quartz is the major constituent of the rock. It is generally equigranular, but at irregular intervals there are veins Of larger quartz grains that roughly parallel the foliation Of the rock. These grains have an average diameter of 0.5mm while the average for the rest Of the rock is 0.1mm. The quartz in the veins is usually free from the particles of mica and opaque iron minerals which are characteristically found in the other quartz. Figure 3 shows one of these veins. . ' \ Q 0 L- _.~ — ;~-—-—-—-———— -c-v— -s.. .‘g.’ -. _ - ""' “Cr" "" Fig. 3 Photomicrograph of Quartzite showing veins of larger-grained quartz. Crossed nicols, x37 diameters. All of the quartz is recrystallized. None of the original boundaries of the sand grains can be discerned. A few of the quartz grains exhibit a type Of extinction which appears to start from the center of the grain and proceed toward the edges. The author suggests that this may be due to addition of material around the edges Of the grain. The second most abundant constituent of the rock is mica, of which about 95 percent is green muscovite and 5 percent biotite. The green muscovite is probably a chromium variety known as Fuchsite, but it will be referred to as green muscovite in this paper. It is the cause of the green tints Observed in the hand specimen. The mica occurs around and in the quartz and has a general orientation that conforms to the megaSCOpic foliation direction of the rock. The blades of mica average 0.05mm in length, and have a maximum length of 0.15mm. Minor amounts of hematite and magnetite are scattered through the rock. Both iron minerals appear to be closely associated with the mica. They apparently are alteration products resulting from the oxidation of the mica. The magnetite sometimes shows crystal form but in most cases it is anhedral. Red hematite frequently is pseudomorphic after a mineral, sections of which are triangles, squares and rhombs. These are described by Rogers and Kerr (1942) as common cross sections of magnetite crystals, which seems to indicate the original mineral was magnetite. Several different mineral types.can be recognized as inclusions in the quartz. The average diameter of these 10 inclusions is 0.004mm and the maximum diameter is 0.01mm. One was tentatively identified as zircon but their extremely small size precluded any further identifications. A______~______ A ~# g- 4- L“ g 4 ‘ v «we -_. ‘w- ' Ir c ”w --.-- E “"‘ r - Fig. 4 Photomicrograph showing a crystal Of magnetite formed in mica. Plain light, x37 diameters. TABLE I Analysis of Two Samples From Quartzite Bed Three Mineral Sample 1 Sample 2 (Percents) Quartz 90 82 Muscovite 9 l6 Biotite 1 l Magnetite trace trace Hematite trace trace Limonite trace trace Zircon trace trace 11 Sample 1 was taken five feet from the bottom of the bed and sample 2 five feet from the top.‘ Quartzite Bed Five ‘Megascopic Description Bed five averages twenty five to thirty feet in thickness. At its bottom it is dark due to a larger percentage of biotite. The color changes to greenish brown near the center Of the bed and remains the charac- teristic color throughout the rest of the rOCk. Faint layers, outlined by concentrations Of green muscovite, can be Observed in the lighter portions of the bed and may represent relic bedding. Petrographic Description Angular anhedral quartz is the most abundant mineral .in the rock. The grains reach a maximum diameter of 0.6mm and average 0.1mm. There is no apparent dimensional orientation Of the quartz grains. .A few (5%) show undulatory extinction and there are scattered grains which exhibit a 12 mushrooming extinction similar to that described in bed three. Fine-bladed muscovite, average blade length 0.5mm, and slightly larger biotite, average length of blade 0.9mm, make up the bulk Of the remainder Of the rock. The two minerals are closely intermixed with one another. They are oriented parallel to the green layers noted in the mega- sc0pic description Of the rock. Fragments Of oligoclase and orthoclase appear among the quartz grains. Oligoclase comprises better than eighty percent of the total feldspar. The grains are fresh and care must be taken to avoid.mistaking them for the quartz. Maxi- mum diameter Of these grains is 0.1mm and they average 0.6mm. Carlsbad and albite twins and combinations of the two occur in about thirty percent Of the grains. Small amounts of magnetite and hematite have developed with the biotite in a few places in the rock. The magnetite is subhedral, exhibiting crystal faces at the edges of some Of the larger grains. The hematite is of the soft red I variety and is anhedral, Irregular shaped particles of carbonate occur at isolated points in the rock.. It was identified as calcite 13 because of its clear color, anhedral character and lack of twin lamellae parallel to the short diagonal of the grains. The grains vary in size but they do not exceed 0.15mm. A few minute grains of zircon, maximum size 0.01mm, constitute the only heavy mineral Observed. 0.: “. 0 I _4 \ Fig. 5 Photomicrograph showing polysyn- thetic twinning in Oligoclase surrounded by quartz. Crossed nicols, x166 diameters. l4 TABLE II Analysis of Two Samples From Quartzite Bed Five Mineral Sample 10 Sample 11 (Percents) Quartz 74 85 Muscovite 14 14 Biotite 10 2 Calcite 2 trace Oligoclase trace trace Orthoclase trace trace Magnetite trace trace Hematite trace trace Zircon trace trace Sample 10 was collected ten feet from the bottom and sample 11 ten feet from the top of the bed.. Quartzite Bed Six Megasc0pic Description Quartzite six averages thirty feet in thickness, grades from dark gray near the edges to greenish white in the center and has green banding similar to that in bed three. The green layers are straight and parallel to one another with only the distance between them varying. 15 In many parts of the rock red hematite stain cuts randomly across the bedding. The rock is medium-grained and has a granular texture. Megascopically the rock consists Of quartz and mica with subordinate iron oxides. AL; db)" bEig. 6 Photograph of hand speCimenOf"' quartzite bed six showing the regular green banding (black in picture). Petrographic Description Under crossed nicols the quartz appears as a mosaic Of angular grains. Their maximum diameter is 0.6mm and they average 0.1mm. About fifteen percent Of the grains show undulatory extinction; usually they are the larger ones 0 15 Two types of mica are present, biotite and green muscovite. The micas have a general orientation parallel to the green banding noted in the megascopic description. The muscovite is generally smaller than the biotite. Four different types of feldspar can be distinguished in the thin sections. Orthoclase and Oligioclase are the most abundant feldspars with only occasional pieces of microcline and perthite being present. Figure 7 shows a piece of microcline with its typical grid twinning. The feldspar grains vary in diameter from 0.3mm to 0.05mm and average 0.15mm. L... a. 4‘ 4 Fig. 7 Photomicrograph Of grain Of micro- cline in quartzite. Crossed nicols, x166 diameters. 16 Iron oxides in the form Of magnetite and hematite are present at scattered Spots in the rock. They are almost always associated with the miCas. The hematite appears only with the biotite, but the magnetite occurs with both the biotite and muscovite. Two minerals occur as minute inclusions in the quartz. One has high relief and is dull white under crossed nicols. It was tentatively identified as zircon. The other is light green, slightly pleochroic and exhibits white to second order red interference colors under crossed nicols. It may be epidote. Neither is euhedral. TABLE III ' Analysis of Two Samples From Quartzite Bed Six Mineral Sample 14 Sample 15 (Percents) Quartz 82 76 ‘Muscovite 13 6 Biotite . 2 16 Oligoclase 2 trace _ Orthoclase 1 trace Microcline trace trace Perthite trace trace Magnetite trace 2 Hematite trace trace l7 Sample 14 was taken ten feet from the bottom of the bed and sample 15 was taken five feet fromthe top. DOLOMITIC MARBLES Megascopic Description Dolomitic marble oCcurs at four places in the strata of the area. The three southeastern occurrences are thin beds not exceeding twenty feet in thickness. A massive ridge in the northwest corner of section four marks the other occurrence. The rock is a light brownish- gray on a fresh surface. It is medium grained and has a granular texture. The megasCOpic minerals are carbonate and quartz. Petrographic Description Dolomite comprises seventy five to ninety percent Of the rock. The grains are usually anhedral and vary in diameter from 0.1mm to 1.0mm. Banding, caused by alternating 18 zones of coarse and fine grains, can be Observed in the thin sections of the rock. The long dimensions of the dolomite particles usually parallel these bands. In some parts of the rock the banding becomes very irregular, but it is always distinguishable. Approximately twenty five percent of the dolomite grains exhibit polysynthetic twinning. In many instances it is very well developed.. _‘ r' . -r —- r-OC‘ I I | I I I I Fig. 8 PhOtomicrograph showing polysyn- thetic twinning in dolomite. Crossed nicols, x37 diameters. Quartz makes up most of the remainder of the rock. Grains of quartz are scattered throughout, with the greatest concentrations occurring in veins which parallel 19 s 41-. the bands in the dolomite. These bands may represent relic bedding. The average diameter of the quartz grains in the veins is 0.4mm. Those scattered elsewhere are smaller. Undulatory extinction is rare and is usually perceptible only in larger grains. Limonite is present as a stain on some of the grains of dolomite. A few long thin blades of muscovite are scattered among the dolomite and quartz grains. TABLE IV Analysis Of Three Dolomite Samples IMineral Sample 3 Sample 23 Sample 20 Dolomite 95 _ 75 97 Quartz 5 25 3 Limonite trace trace -- Muscovite trace trace trace (percents) Samples 3 and 23 were taken approximately one mile apart , along the strike of the southern most bed (See Fig. 2). Sample 20 is from the large dolomitic marble ridge in the northwest corner of the area. 20 PHYLLITE MegaSCOpic Description The phyllite is the most abundant rock in the area. All the other rock units, with one exception, have phyllite directly above and below them. Samples from two beds with a wide vertical separation were used for the petrographic examination in order that they be as representative as possible of all the phyllites. The foliation is the most striking megascopic feature of the rock. The foliation planes are very regular and parallel the top and bottom of the beds. Microscopic flakes of mica give a sheen to the cleavage surfaces of the rock. In some places, the surfaces are spotted with limonite which appears to have formed in small pits left by crystals of some Other mineral. The rock is dark gray on both fresh and weathered surfaces. It has a very consistent texture and color at all Of its outcrops. Petrographic Description Biotite is the most obvious component of the phyllites. It has a Stringy appearance due to the extremely small size and close packing of the mica blades. The blades are orientated 21 parallel to the foliation of the rock. This orientation is perfect enough to cause all the biotite to go to extinction simultaneously, when the nicols are crossed. The average length of the biotite blades is 0.1mm and their maximum length is 0.3mm. Quartz is the only other important mineral in the rock. It is relatively equigranular, ranging from 0.3mm to 0.05mm in diameter. Average diameter of the grains is 0.1mm. Many quartz grains have a long direction which parallels the lineation Of the mica. Inclusions in the quartz are rare and if present are very small. Very few of the grains show strained extinction features. The quartz shows a slight tendency to concentrate into bands. In places the biotite appears to be closely associated with the magnetite. same of this magnetite is subhedral, but the greater portion is massive. The masses are small, usually not exceeding 0.1mm in diameter. Porphyroblasts of limonite, pseudomorphic after a mineral with octahedron crystal form, are scattered through Ithe'rock. These are the cause of the brown spots mentioned in the megascopic description. 22 I Fig. 9 Photomicrograph of phyllite showing excellent parallel orientation of biotite. The white between biotite is quartz. In the lower right corner there is a porphyroblast of limonite, pseudomorphic after an unknown mineral. Plain light, x37 diameters. TABLE V Analysis Of Two Phyllite Samples Mineral Sample 7 Sample 18 (percent) Biotite 81 86 Quartz 19 14 _Limonite trace trace Magnetite trace -- ‘ Both samples were collected approthately twenty feet from the bottom Of their respective beds. 23 METABASALT ‘Megascopic Description Metabasalt occurs as tabular bodies, 50 to 1,000 feet in thickness, which conform to the bedding of the metasedhments. It is a fine grained rock, generally massive but with some foliated zones. Surfaces are light green when fresh and gray-green when weathered. Petrographic Description The primary constituent is a light green, slightly pleochroic, bladed mineral. The blades are arranged in a crisscrossed manner, giving the fabric a matted appear- ance. Optically the mineral has an extinction angle of ten to nineteen degrees, 2V equals fifty five to sixty degrees and it is biaxial negative. On the basis of these characteristics and.X-ray analysis the mineral was identi- fied as a low iron variety of hornblende (uralite). The blade length Of the hornblende averages 0.05mm and varies from 0.01mm to 0.1mm. 24 Isolated quartz grains, averaging 0.05mm in diameter, are scattered between the hornblende. The grains are fresh appearing and colorless in plain light. ‘Wedge shaped grains of plagioclase can be Observed interstitial to the hornblende. These grains are very small and often are difficult to distinguish from the quartz. On the basis of refractive indices the plagioclase was identified as Oligoclase or albite. Q o . — -—._.-—V - I _ _ __-._L—.‘_ - I I l | _4 Fig. 10 _Photomicrograph Of metabasalt showing the matted texture Of the horn- blende. Plain light, x166 diameters. 25 TABLE VI Analysis Of Metabasalt Sample Mineral Sample 19 (Percents) Hornblende 93 Quartz 4 Plagioclase 2 Sample 19 was taken approximately fifty feet from the contact with the underlaying bed of biotite-hornblende schist. BIOTITE-HORNBLENDE SCHIST Megasc0pic Description Two beds of biotite-hornblende schist, 150 to 500 feet thick, crOp out in the area. They are exposed for distances of 6,000 to 10,000 feet and are separated by a vertical distance of approximately 2,700 feet. The southern bed has 4 iron formation below it and phyllite above it. The north- western bed has phyllite below and metabasalt above it. 26 Contacts with the phyllite are vague, with the two rocks grading into each other. In places the schist appears to grade into phyllite along the strike of individual beds. The contacts with the other rock types are somewhat sharper. The predominating megascopic minerals are biotite and horn- blende. They give the rock a gray to black color on a fresh surface. weathered surfaces have a slight added brownish tint. Petrographic Description Hornblende is the most abundant mineral. It is green to blue-green and always strongly pleochroic. The grains Of hornblende have a maximum length of 0.9mm, a minimum of 0.02mm and average 0.5mm. Two types of feldspar were recognized, orthoclase and andesine (Ab 60-An 40). About 35 percent of the orthoclase shows Carlsbad twinning. Three different twinning habits can be observed in the andesine., Carlsbad and albite twinning are the most frequent. Pericline twinning and combinations of albite and Carlsbad twinning are also present but much less commonly than the first two. Table VII shows the 27 frequency with which each type occurs in three different samples. The feldSpars are of the same order Of size as the hornblende. Quartz is present in all thin sections as equi- dimensional angular grains. In many places it forms "semi-veins" parallel to the foliation. The grain diameter seldom exceeds 0.4mm or is less than 0.01mm. It averages 0.05mm. Only a few Of the larger grains have undulatory extinction. TABLE VII Frequency of Twinning Types in Andesine Twin Habit Sample 6 Sample 24 Sample 25 , NO. Percent No. Percent NO. Percent of of of Of of of Grains Total Grains Total Grains Total Alibite 5 14.29 14 17.05 8 15.09 Carlsbad 6 17.14 27 32.92 12 22.64 Alibite-Carlsbad combinations‘ 4 11.42 10 12.26 5 9.44 Pericline - --- 2 2.43 2 3.77 . Uhtwinned 20 57.14 29 35.45 26 49.05 Totals 35 99.99 82 100.01» 53 99.99 28 Fine bladed biotite, average length 0.1mm, is scattered through the rock. It is characterized by medium-brown color and strong pleochroimm. There are minor amounts Of small anhedral magnetite grains preSent which appear to have formed through the oxidation of the iron silicates. TABLE VIII Analysis Of Three Biotite-hornblende Schist Samples Mineral Sample 6 Sample 24 Sample 25 (Percents) Hornblende 28 32 39 Quartz 29 9 21 Orthoclase 6 9 6 Andesine 15 36 16 Biotite 17 12 13 Calcite 4 -- - 2 Magnetite l trace 2 Leucoxene -- 2 trace Sample 6 is from the southeastern bed. Samples 24 and 25 are from the northwestern bed. In samples 24 and 25 there is a gray mineral that seems to be associated only with the biotite. It is gray in both plain and polarized light and white in reflected light. ‘With a high magnificatibn a very fine aggregate of euhedral grains 29 is perceptible in parts of it; however, it generally appears to have a massive amorphous character. It is probably leucoxene. Calcite is found in two Of the samples as small irregularly shaped grains. ‘2 Fig. 11 Photomicrograph Of schist showing hornblende, biotite, feldSpar and quartz. Plain light, x37 diameters. IRON FORMATIONS MegasCOpic Description Four major and several minor iron formations horizons crop out in the area. They are exposed for distances of 1,000 to 10,000 feet, and vary in thickness from 50 to 30 850 feet. Magnetic and non-magnetic zones, or interbeds occur in the iron formations. Thin beds of phyllite one to five feet thick are also found interbedded with the iron formation. Alternating layers of pure quartz and quartz mixed with iron oxides give the rock a banded appearance. The bands vary in thickness from 1.0mm to 1.0cm and average 5.0mm. Locally the bands are irregular due to micro-folding resulting in boudinage and similar I Structures 0 r , Fig. 127‘EEotograph ongifided iron formation. “Note boudinage structures fin top layer of quartz. The beds vary primarily in their relative iron contents. In some places bands of nearly pure iron oxides alternate 31 c with the layers of purer quartz and at other places the rock becomes practically all quartz. Petrographic Description The banding, Observed micrOSCOpically is still very prominent in thin section. Nearly equigranular quartz grains form a mosaic pattern in the siliceous layers. The quartz grains vary in diameter from 0.3mm to 0.02mm and average 0.06mm. The sizes of the quartz grains vary inversely With the amount Of iron minerals present. In the ferruginous bands the average grain size is only 0.2mm. Fig. 13 Photomicrograph showing two sizes Of quartz grains. Grains are smallest where the iron minerals are most abundant. Crossed nicols,x37 diameters. Undulatory extinction is generally perceptible in the larger grains. Magnetite is usually the second most abundant mineral. It occurs in anhedral to subhedral grains and in solid masses. In three instances the magnetite was Observed in granular form similar to that described by Gruner (1946); magnetite forming a ring with quartz in the center. Fig: 14 Photomicrograph of granule of magnetite with quartz in the middle. Plain light, x37 diameters. The individual magnetite grains vary from 0.009mm to 0.9mm and average 0.1mm in diameter. In a few places the magnetite appears to be altering to hematite. 33 Hematite is usually the only other iron oxide present. It always is the red variety and occurs as platy, fibrous and massive forms. The majority is massive. Badly weathered portions of the rock contain appreciable amounts of limonite, however it is rare in fresh samples. FIgt—15"wPhotomiéiograph"§howing”'”‘_“’” stilpnomelane and hematite. Lighter colored blades are stilpnomelane. Plain light, x37 diameters. An iron mica occurs as yellowish to red blades in the ferruginous bands. It appears to be altering to red hematite. Many blades are nearly concealed by the hematite which has formed from them. The mineral was 34 identified as stilpnomelane because of its Optical. prOperties: gold to red Color; medium to strong pleochroism; uniaxial-negative figure; parallel extinction; and indicies higher than Canada balsam. VOLCANICS Vesicular rhyolitic flows, Tertiary in age, are scattered over the area. They lay unconformably on the Precambrian rocks. These were Very badly weathered and samples suitable for petrographic examination were unobtainable. 35 com OUQHU OUGHH OOQHU OOGHU OOQHU OOOHU onwaoawd OOGHU OUQHU OOQHU OOQHU OOmHU mamamaoamawum OONHU ON on donates: NH mumooou Ou anOHmmwo th> on oasoz ua Canada: a“ carom uoc mm3 ocmamaonmafium onu :« om OSH .oadamm sumo a“ on amuou Ono oumaaoamo on own: mm3 mooaxo sou“ man a“ Oh can ease w.¢a m.m~ w.w~ o.mH unmoumm mwmuo>< ¢.¢ H.nN o.n ¢.¢N mm mo nooouom NH ca oamfimm cuosuuoz «N am ma NH mamamm Hmuuaou suuoz w m mamamm Hmuuomo nusom m a mamamm aumauaom maowumEHOh GOHH :Hmz_u50h Ono scum moaaamm owe a« on manmuo>oomm Hmfiusouom N uqmdfi 37 ECONOMIC IMPORTANCE OF THE IRON FORMATIONS The iron formations probably could not be mined economically for several reasons, which are; l. The average Fe content Of the beds is low and variable as shown in Table X 2. The presence of both magnetic and non-magnetic iron oxides would complicate the problem of recovery. Probably only the magnetite could be easily recovered. This would cut down the amount Of iron obtained from each ton mined. 3. The attitude of the beds is such that open pit Operations could not be carried on easily, thus further increasing the cost of mining. 4. The tonnage of the beds is too small to allow large scale operations. The iron formations are estimated to have twenty million tons Of minable iron I formation (Hogberg, 1958). 38 PETROLOGICAL INTERPRETATIONS It seems probable that the quartzites, dolomites, phyllites and iron formations originated primarily as sediments. Muds were deposited in the Precambrian basin until a thick argillaceous section accumulated. Periodic fluctuations of the basin's depth produced siliceous and limey phases that are now represented by quartzites and dolomites. The iron formations were deposited during several of these fluctuations. Following the deposition of the beds, they were folded and subjected to regional metamorphism. Mineral suites in the four lithologic types listed above give them a metamorphic rank equivalent to the biotite zone of the Greenschist facies. Iron Formations The original form of the iron formations is debatable and a number of modes of origin can be suggested for them. 1. The beds may have originated as magnetite sands alternating with silica sands. (Hogberg, 1958). This hypothesis is difficult to support, however, because the 39 quartz is relatively free Of the heavmeinerals which would accompany a clastic sediment. Further, it is difficult to imagine a source capable of providing enough clastic magnetite for the time that would be necessary to form the beds. 2. The beds could have been laid down as cherty iron carbonate and iron silicate with minor amounts of hematite and magnetite, in a manner similar to that prOposed by Van Hise and Leith (1911) for the Lake Superior iron formations. There is little evidence to support this hypothesis however, because no siderite or greenalite remnants were Observed in the thin sections.” Three granules which originally may have been carbonate OOlites were observed in one thin section, but they could well have other origins. 3. The beds may have been deposited as chert and magnetite with minor amounts of hematite. The majority Of the authorities subscribe to the idea of colloidial chert as the primary source of silica for siliceous iron formations. The quartz is Of the grain size (average diameter 0.06mm) which would be expected for recrystal- lized chert in the biotite zone(H.L. James, 1954). 40 He lists diameter values of 0.05mm to 0.10 mm as characteristic of recrystallized chert in the biotite zone. Primary deposition Of magnetite and hematite is generally viewed with less favor but has its advocates. 'Wolff (1917) believed that the bulk of the iron oxides in the Mesabi iron formations are still in the same chemical state as when they were laid dOwn. Recently, K. Huber (1958) showed that, while the theoretical conditions under which magnetite and hematite may be precipitated together are rather restricted, the actual sedimentary conditions may be less limited. Magnetite is the most important of the iron oxides present. In thin sections it appears to be anhedral to subhedral. Polished sections show it to have slightly more crystalline form than can be seen Otherwise. It was probably deposited in massive form and the subhedral character is the result of metamorphism. The hematite is usually red and closely associated with the magnetite and stilpnomelane. Much Of it is probably an oxidation product Of the two. 41 Stilpnomelane occurs in most Of the slides in minor amounts. It was most likely a primary constituent of the iron formation formed at the time of deposition. Geologists are not generally agreed upon the origin of stilpnomelane. Gruner (1946) believes it is an original mineral formed from a colloidal gel. White (1954) generally concurs with Gruner. Tyler (1949) and James (1954), however, regard stilpnomelane as a product of low-grade metamorphism. Yoder (1957) believes that stilpnomelane is a metamorphic mineral that can occur in several metamorphic zones depending upon the H 0 content of the bulk composition 2 Of the rock. The author of this thesis favors hypothesis 3 for the deposition of the quartz (chert), magnetite, hematite, and stilpnomelane of the Ruby Creek iron formations. Metabasalt Igneous activity during the period of deposition introduced basaltic rock into the sedimentary sequence. The uniform composition of these rocks wherever found suggests that they were originally sills intruded simultaneously. There are, however, several things which 42 seem to indicate they were extrusive flows instead of intrusive sills. First, the adjoining beds have been only slightly or not at all metamorphosed at the contacts with the igneous rock. Secondly the texture Of the rock is generally finer than would be expected in intrusive bodies so thick (maximum width 1000). Third, small beds or lenses of sedimentary material occur in the igneous rock and may represent sedimentary material deposited when the time between two successive flows was short. The strongest evidence favors flow origin, although there is room for doubt. Following its extrusion (or injection) the basalt was metamorphosed to a rock composed primarily of low iron hornblende (uralite). The hornblende is very similar to the "actinolitic" hornblende that James (1954) describes as commonly occurring in the biotite zones. Biotite-hornblende Schist When considering the area from the standpoint of metamorphic rank, the biotite-hornblende schist appears anomalous. The schistose texture and mineral suite point to a position in the amphibolite facies rather than the 43 greenschist facies which is characteristic of the other rocks in the area. Several hypotheses can be conceived to explain the presence of this rock in the area. 1. The schists may have been faulted into their present position after metamorphism. This hypothesis seems unlikely because the indistinct contacts of the schist beds are quite different from fault contacts. Also the fault pattern needed to explain the present position of the beds would be extremely complex. 2. If the metabasalts were originally sills, they may, upon intrusion, have altered the phyllite to a hornfels, which was later metamorphosed to a schist. The schist beds appear to be intimately associated with the phyllites at places and could be genetically related. However, it is difficult to explain why the intrusives would act upon one bed of phyllite and not another. In addition, microcline would probably be expected in a metamorphosed or metasomatically altered rock, rather than the orthoclase which occurs in this rock (Turner and Verhoogen, 1951, p.448). 3. The schists may have been sedimentary (approx- imately graywackes) rocks, which were altered by rising hydrothermal solutions to their present forms. The source Of the solutions in unknown, but they could have been the result of igneous activity during the Precambrian folding Of the beds. It seems possible that the graywackes were more porous than the other rock in the sequence, thus allowing the solutions to flow more readily through them than the other rocks. This might have produced a rock type with an apparently anomalous metamorphic rank for the area. However, hydrothermal action generally produces a different mineral suite than that which is found in the schist. 4. The schists may have been intrusive igneous rock originally. If so, it would have been similar in composition to a granodiorite or dacite, approximately 227 in Johannsen's (1939) classification. The twinning in the andesine (Ab60-An40) is slightly less frequent and simpler than that which would be expected in igneous rock according to Turner (1951). However, this does not rule out an igneous ancestry for the plagioclase in the schist. An igneous intrusive (sill) would have a coarser texture which Offers an explanation for the relative 45 coarseness of the schist's texture. Further, a fairly basic intrusive rock, such as the schist would have been, would be quite low in water content. This would let the metamorphic reactions proceed further than in the wetter rocks comprising the rest of the sequence, thus producing an apparently anomalous rock type (Yoder, 1957) The author favors this latter hypothesis. STRUCTURE Folding was the major deformational activity in the area. Only minor faulting took place. The strata in the area comprise one limb Of a fold. The beds are probably not overturned because they generally conform to the beds of the Cherry Creek series which are upright.. The repetition of the same lithologic types is apparently due to cyclic deposition because the individual beds have distinct characteristics of their own. Iron formations similar to the Ruby Creek beds crap out three miles south of the thesis area and may be part of the same structure as the Ruby Creek beds; however, insufficient information is available at this exposure to be sure of the correlation. 46 CONCLUSIONS Field and laboratory study of the Ruby Creek strata has indicated several hypotheses as to their geologic ancestry. None of the hypotheses is conclusive enough for the author to endorse completely. The rock in the area is of three origins: sedi- mentary, igneous extrusive and igneous intrusive. .All have been subjected to deformation and regional metamorphism. Cyclic deposition caused the various lithologic types to be repeated at different stratigraphic levels. Basaltic flows were included in the sequence during deposition. One rock type, the biotite-hornblende schist, is Of apparently a higher metamorphic rank.than the other rocks. It was probably intruded into the sequence as sills, which because of different bulk composition and physical state (e.g.porosity and included water) reacted differently to the metamorphic force, producing minerals characteristic of a higher rank.of metamorphism. The iron formations were probably laid down as chert, iron oxide, and stilpnomelane, all of which are now recrystallized. It would probably not be possible to mine the iron formations economically. 47 BIBLIOGRAPHY Gruner, J.W. (1924), ContributiOns to the geology of the ' Mesabi Range, Minn. Geol. Surv., Bul. 19, pp 27- 28. (1946), Mineralogy and geology Of the Mesabi Range, Office of the Commissioner Of the Iron Range Resources and Rehabilitation, St. Paul, Minn., p 34. Hogberg, R.K. (1958), Personal communication. Huber, N.K. (1958), The environmental control of sedimentary iron minerals, Econ. Geol., vol 53, pp 123-140. James, H.L. (1954), Zones Of regional metamorphism in the Precambrian of Northern Michigan, G.S.A. bul., ‘vol. 66, pp 1455-1488. Johannsen, A. (1931), A descriptive petrography of the igneous rocks, Univ. Of’ChicagO Press, Chicago, 111., vol. 1, pp 155-59 Peale, A. C. (1896), Three Forks Folio, Mbntana, U. S. Geol. Surv., folio, 24, p 2. Perry, A. S. (1948), Talc, graphite, vermiculite and asbestos in Mbntana, Mentana School of Mines, Butte, MOntana, mem. 27, pp 7- 10. Rogers, A.F. and Kerr, P.F. (1942), Optical Mineralogy, McGraw-Hill, New‘York, New YofiE, pp 200-1. Turner, F.J. (1951), Observations on twinninggof plagioclase in metamorphic rocks, Am. Mineralogist, vol. 36, pp 581- 9. Turner, F. J. and Verhoogen, J. (1951), _gneous and metamorphic petrology, MCGraw-Hill, New'York., New'York, pp 466- 9. 48 Wolff, J.F. (1917), Recentpgeologic developments of the Mesabi Iron Range, Minnesota, Lake Sup. Min. InSto, Pp 245-70 White, D.A. (1954), The stratigraphy_and structure of the Mesabi Range, Minnesota, Minn. GeOl. Surv., bull 38, p 35. Yoder, H.S., Jr. (1957), Isograd problems in metamorphosed iron-rich sediments, Institute on Lake Superior Geology, Mich. State Univ. (1954), "Role Of water in metamorphism", Crust of the earth, G.S.A. special paper 62, pp 505-24. 49 $8.3. ":1 {for r!“ '- . ‘ 1‘" ,...,,r LEI/I In! ' " \‘f ‘IE ‘I no.” a... ami-