Th? ·:-;EOLOGY AN[. OF TH3 I.RONWOOD IRON-FORMATION, GOGE3IC RANGE WISCOT'JSIN A 'fii2SIS SUBMITTED TO TPE FACULTY OF T¥.£ GRADUATE SCHOOL OF 'I'liE UNIVERSITY CF MINKE3UTA BY DENNIS PATRICK LAYBOURN IN PARTIAL FULFILLMENT OF THE RFQUIREMENTS lOR THE D:t:GREE OF MASTER OF SCIENCE .JANUARY 16 I 1979 ABSTRACT The Ironwood Iron-.fo::c-mati.Jn, of the Go gebic Range of nort'.1.ern 'disconsin and Mi chig.?.n, c:x:tends 80 miles in a generally east-northea3terly direction from near Mineral Wisconsin to Lake Michigan. The iron-formation is considered part of the Group of the Marque tte Range Supergroup. cally , the iron-formation has been divided into five members on the basis of the predominance of cherts or ferrugin ous slates in the stratigraphic section. Total thickness of the Ironwood Formation in Wi sconsin seems to show a progressive thickening from around 500 feet near the Wisconsin-Mi chigan border to about·650 f eet near its westernmost extent. The westernmost 20 of +i-,J,;,e iron-.!... or:-aa ....:,ion . nal s •oeen con ......"ac-c ' me ;:;amorp' h ose d b'.Y tne • Mi neral Lake Intrusive, a Middle Keweenawan gabbroic body. data of present study, four metamorphic zones may be distinguished in the Ironwood Formation by changes in m.ineralogy alo!1g strike length of the iron.- formation toward the intrusive contact. Zone 1 extends to within about 7500 feet (2JOO m.) of the intrusive contact. It is recognized by the pre- s2nce of low-grade metamorphic assemblages containing iron-bearing phyllosilicates (minnesotaite, stilpnome- , hl . ' . rl . , . ' ' 1 ( • ' J..ane, c .. _or1 te) 2.ri-.... iron- oe'.lring ca.roona pnases sia.s - rite, dolomite-ankerite). Primary textural features, such as granules and colites, are generally well pre- served, although they have been slightly modified by recrystallization and replacement. Zone 2 is by a moderate grade of illetamorphism and the development of grunerite, talc, and garnet. Iron-bearing carbonates an d iron-beari ng phyllosili2at2s are absent. The degree of metamo rphic recrvstallization has from Zone 1, such that features ara to totally ob- literated. ii Zone 1 occurs about 4500 feet ( 1- J70•1,-, ·•'•).1 -·::::-;;--·_,_ V- 1,,1 ......h -'-' ......J_llt:;:...., "r"" 0 o _,..u-....,unt-.::;0-'.- ,·.,,n,j.. __i·::: ._.,_,-.;, ,:::;__ ·r. .;.:i.... .,,-,, ·c·- ,.,...;r:: ..... "''.."... d hi.gh-.5ra(le ass ernJ)l3.gi?S ,..., t::.,ne ( 'l'Y'Q l'' \ c '1 ; ·-on \ "Y'n .... 11·:::. I .. -i· ....\,;::;, \ hornblendet actinolite and trsmolita. The has usually undergone a complete recry stallization with primsry t 1:;xtural t ·:;1turss being so:11pJ.2t ely e:--as'2d. Zone 4 occurs about 100 meters of tha in- trusive contact and is ch2racterized by the presence of pigeonite and fayalite. Pressures of are believed to be in excess of J kb. Temper-a"'cures attained in Zone 1 were probably in the 150-JSJC. Maximum temperatures of Zone 2 c ould not h :3.'1e 2. bout the upper limit of talc in rocks. Maxi- mum t2mperatures ·along the intrusive conta ct sculd not have greatly exceeded about 700 C., the spproximate decomposition temperature of grunerita . of the is believed to largely isochemical and the result of a progressive dehvdration and decarbonation of the iron- forma tion as 0 ""' ..... _ c '·\JJ.·n+ u 3.,,+'-',.,, i·("Ou 1"1e"'r..--...i._ ..... d • Qvifa'"'"'__ '""' o"-'" J. _ :....... ..... ._ v _.._ic: internally buffered with in th2 co:--ifines of various quartz- ferrous silicate-magnetite buffer asse!·nolages. Evidence from Zone 4, however, suggests the decomposition of mag- netite in response to the external reducing influence of the intrusive contact. iii TABLE OF CONTENTS Page ABSTRACT •••••••••• .. .. .. .. ... .. ... i TABLE OF CONTENTS. ......................... • 11.l LIST OF ILLUSTRATIONS. ...................... • •• v INTRODUCTION ••••••• . .. .. .. .. .. .• 1 ,., ACKNOWLEDGEMENTS ••• ............... ... ) LOCATION AND PHYSIOGRAPHY •• . .. .. .. .. GENERAL GEOLOGY •• . .. .. .. • • 8 PHEVIOUS WORK ••• . .. .. .. .. .. •• 27 STUDY METHODS ••• .. .. .. .. '51 STRATIGRAPHY AND STRUCTURE OF ·rHE I RO NW OOD ?ORMA TI ON •••••• I • I ••••••• I ••••• 53 Development of Stratigraphy ••••••.• 53 General Features of the Iron- formation Members ••.••••••••••• 61 Structure of the Ironwood Formation •••••.•••••••••••••••• 70 FETRCGRA.?HY OF 11.'HE IRCNWOOD FORMATION •••••••• 74 •.••••••••••••••••••••• 74 Sedimentary Textures of Iron- .formation ..•••••••.•••••••••••••• 7 5 of Zone 1 •••••••••••••• 77 Textures ••••• , ••••••••••.••••• 77 Mineralog-.1 •••••••••••••••••••• 84 Petrcgraphy of Zone 2 ••.•.•••.••.• 108 Textures ••••••••••••••••••••• 1 08 Mineralogy ••••••••••••••••••• 109 Petrography of Zone J ••••••••••••• 125 Textures ••••••••••••.•.•••••• 125 Mineralogy ••••.•••••••••••••• 126 Petrography of Zone 4 .•.......•..• 140 Textures ••••••••••••.•••••••• 141 Mineralogy •• •••••••••••••••• 14J Summary of Zonal Mineralogy ••••••• 155 ZONAL METAMORPHISM OF THE BIWABIK . FORMATION - A COMPARISON ••••••••••••••• 161 iv Table of Contents (con't) Page METAMORPHISM OF THE IRONWOOD FOHMATION .••••.. 168 Metamorphism of Zone 1 Iron·-formation ••• 168 Metamorphism of Zone 2 •.........•....•.• 174 Metamorphism of Zone J •...•...••.•.••..• 176 Talc and Tremolite in the Ironwood Formation .......................... 180 Origin of Talc •••••••••••••••••• 180 Origin of Tremolite ••••••••••••• 183 Metamorphism of Zone 4 ••••••..• • •••••••• 186 Pyroxenes •••••.••••••••.•••••.•• 188 Fayali te ............. " ........... 1 94 Maene ti te •••••••••••••••.•.•.••• 196 Gruneri te •••••• R • ••••••••••••••• 199 Conditions of rnetamorphism •••••• 200 The Role of Volatiles in the Metamorphism of the Ironwood Formation ••••••• , •••••.•••••••••• 201 Zone 1 and Zone 2 Assemblages ••• 202 Zone 2 and Zone 3 Assemblages ••• 203 SUMMARY AND CONCLUSIONS ••...•.•••••.••.•••••• 212 . REFERENCES CITED • ...........•...•...••..•.••• 217 APPEI'IDIX A -- SAMPLE LOCATIONS, STRATIGRAPHIC SECTIONS AND LISTINGS OF ASSEMBLAGES FOR THE IRONWOOD IRON-FORMATION, WISC 0 NS IN • . • . • . • • • •. A-1 v LIST OF ILLUSTRATIONS ·TABLES Page 1. CORRELATION CHART FOR T¥j!; MARQUETTE RANGE SUPERGROUP •.......•....•....... 1? 2. SUTviMARY OF FEATURES CHARACTERIZING DIFFERENT TYPES OF' IRON-FORMATION ••.• 41 3. SUIVINIARY OF CONTACT l'IT.ETAMORPHIC EFFECTS NEAR MELI3N, WISCONSIN •.•.•••.•..•••. 48 4. MINERALOGY SUMMARY OF THE FOUR N'iE'l'AMORPHIC ZONES OF THE IRONWOOD FORi\'l..ATION ...........••.••..•.....••. 157 FIGURES 1. LOCATION AND EXTENT OF THE IRONWOOD IRON-FORNiliTION ..•.•....••.•.•..•••.••• .5 2. REGIONAL GEOLOGIC MAP •.•...•••..••......•• 10 3. RADIOMETRIC AGE DATES OF SOME ROCKS IN NORTHCENTRAL WISCONSIN ....•....•••• 23 4. FENCE DIAGRAM ILLUSTRATING THICKNESS VARIATIONS OF THE VARIOUS MEMBERS OF THE IRONWOOD FOffiliATION ......•..... 69 5. APPROXIMATE COMPOSITION OF IRON- CARBONATE IN TERMS OF CaC03-MgC03-FeC03 ••••••••••••••••• 97 6. APPROXIMATE COMPOSITION OF IRON- FORlV!ATION CARBONATE IN TERMS OF 03 ............................• 98 7. AEROMAGNETIC MAP OF THE GOGEBIC RANGE IN WISCONSIN .................. 124 8. LOCATIONS OF lf!ETAMORPHIC ZONES IN THE IRONWOOD IRON-FORMATION .....•••• 158 9. DIAGRAM SHOWING THE MINERALOGY OF THE IRONWOOD IRON-FORMATION AS A FUNCTION OF DISTANCE FROM THE KEWEENAWAN INTRUSIVE CONTACTS •....•• 159 10. METHOD OF CALCULATING THE TRUE DISTANCE SEPARATING THE IRONWOOD FORMATION FROM KEWEENAWAN Il'ITRUSIV"'ES •••• , ...................... 160 vi List of Illustrations (con't) FIGURE Page 11. GEOLOGIC MAP OF PART OF TJ-f.E MESABI RANGE, SHOWING LOCATIONS OF TJiE FOUR METAMORPHIC ZONES DISTIN- GUISHED BY FRENCH ( 1969 ) ..••••...•• 16 6 12. DIAGRAM SHOWING THE MINERALOGY OF THE BIWABIK FORMATION AS A FUNCTION OF DISTANCE FROM THE DULUTH GABBRO CONTACT .... , .........•....... 167 13. SCHEMA'ric DIAGRAM FOR THE MOST COfJ!MON ASSEMBLAGES IN ZONE 1 IN THE FeO-Fez03-Alz03-Si02- H20-C02 ca: . ••• ·• •••••••••••••••••••••••• 171 14. SCHEMATIC ISOBARIC LOG f o -T DIAGRAM ILLUSTRATING CHANGE FROM MINNESOTAITE TO TALC WITH INCREASING OXYGEN FUGACITY •......... 182 15. EXPERIMENTALLY DETERMINED PP".ASE RELATIONS IN SILICEOUS CARBONATES FOR Pf=2kb .• 185 16. EQUILIBRIUM RELATIONSHIPS PROPOSED TO EXIST BETWEEN PROTOENSTATITE, ORTHOPYROXENE, PIGEONITE AND FAYALITE PLUS QUARTZ COEXISTING WITH CA-PYROXENE FOR RELATIVELY LOW PRESSURE GEOLOGIC CONDITIONS •••• 187 17. IDEALIZED SEQUENCE OF CHANGES DURING THE COOLING FROM INITIAL PIGEONITE GRAINS TO ORTHOPYRO- XENE GRAINS CONTAINING TWO SETS OF CA-PYROXENE LAMELLAE •.•.•••.••.•• 191 18. SCHEMATIC TEMPERATURE-COMPOSITION SECTION THROUGH THE PYROXENE QUADRILATERAL ILLUSTRATING THE CRYSTALLIZATION AND COOLING HISTORY OF ZONE 4 IRON-FOPJ11IATION CONTAINING INVERTED PIGEONITE •• , •.• 192 19A. ISOBARIC, ISOTHERMAL CHEMICAL POTENTIAL DIAGRAM, ){ O-)fC02 FOR THE EQUILIBRIUM AMONG QUARTZ, MINNESOTAITE, GRUNERITE AND SIDERITE IN THE SYSTEM FeO-Si02(-H20-COz) ••.•••.••• 204 vii List of Illustrations (con't) FIGURE Page 19A. SC:P.El\!IATIC T-XC DIAGRAM FOR TP3 02 SYSTEM FeO-Si02-(H20-C02) DEPICTING EQUILIBRIUM RELATIONS AMONG QUARTZ, MINNESOTAITE, GRUNERITE AND SIDERITE •••••••••••• 204 20. ISOBARIC, ISOTHERM.AL C¥.EMICAL ASSEMBLAGES EXPRESSED IN THE SYSTEM Si02-FeO-Ca0-(H20-C02) •••••• 207 21. SCHEMATIC T-Xco2 DIAGRAM FOR EQUILIBRIUIVI MINERAL ASSEJ.VI- BLAGES STABLE IN ZONE 2 AND ZONE J IRONWOOD FORMATION FOR THE SYSTEM Si02-FeO-CaO-(H20-C02) •. 209 PLATES 1. TYPICAL WAVY BEDDED FERRUGINOUS CHERTS OF THE IRONWOOD FORMATION •..•• 55 2. WAVY BEDDED CHERTY IRON-FORlf!ATION