UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY PAUL K. SIMS, DIRECTOR BULLETIN 45 Progressive Contact Metamorphism of the Biwabik Iron-formation, Mesabi Range, Minnesota BY BEVAN M. FRENCH MINNEAPOLIS 1968 THE UNIVERSITY OF MINNESOTA PRESS © Copyright 1968 by the University of Minnesota. All rights reserved. PRINTED IN THE UNITED STATES OF AMERICA AT THE LUND PRESS, INC., MINNEAPOLIS I~O Library of Congress Catalog Card Number: A 68-·7000 PUBLISHED IN GREAT BRITAIN, INDIA, AND PAKISTAN BY THE OXFORD UNIVERSITY PRESS, LONDON, BOMBAY, AND KARACHI, AND IN CANADA BY THE COPP CLARK PUBLISHING CO. LIMITED, TORONTO FOREWORD The recent, spectacular growth of the taconite industry, and the expan­ sion of taconite operations from the eastern to the central and western parts of the Mesabi range, emphasizes the importance of knowledge of the geology of the range. The earliest taconite plants were established in the Eastern Mesabi district, in areas in which the Biwabik Iron-formation was metamorphosed by the Duluth Gabbro Complex; most of the recent plants are in the Main and Western Mesabi districts, in areas of "unal­ tered" iron-formation. This report describes the changes in mineralogy and texture from "un­ altered" taconite in the Main Mesabi district to highly metamorphosed taconite in the Eastern Mesabi district. It describes not only the silicate minerals, but also the opaque iron oxides, carbonate minerals, and carbon­ aceous material. Knowledge of the mineralogic changes is extremely im­ portant to the practical problems related to beneficiating characteristics of the magnetic taconites. The report is modified from a Ph.D. thesis submitted to the Graduate School at Johns Hopkins University by Bevan M. French. P. K. SIMS v ACKNOWLEDGMENTS The research was undertaken as part of the writer's doctoral disserta­ tion in geology at the Johns Hopkins University (French, 1964a). The dissertation was supervised by H. P. Eugster of the Department of Geol­ ogy; his encouragement and advice were consistently helpful. The writer is also indebted to David R. Wones for a thorough critical reading of the manuscript in its early stages of preparation. H. L. James originally suggested the suitability of the Mesabi range for a detailed study of progressively metamorphosed iron-formation. The ac­ tual study would have been impossible without the generous and whole­ hearted cooperation given to the writer by the many companies mining the Mesabi range, especially the Oliver Iron Mining Division of the United States Steel Corporation, Pickands Mather & Company, the Reserve Min­ ing Company, the Hanna Mining Company, and the Meriden Iron Com­ pany. The assistance given by individuals cannot be acknowledged in full, but particular thanks are due to H. V. Wuerch and Henry Bakkila of the Oliver Iron Mining Division, to D. R. Croswell of the Erie Mining Com­ pany, to James Emanuelson of the Reserve Mining Company, and to C. A. Beckman of the Hanna Mining Company. In addition, the writer has benefited greatly from discussions with J. N. Gundersen, P. K. Sims, S. S. Goldich, J. W. Gruner, and R. L. Blake. Financial support for the dissertation was supplied by a National Sci­ ence Foundation fellowship for the academic year 1960-61 and by fellow­ ships and research assistantships from Johns Hopkins University for the academic years 1961-62 and 1962-63. A grant from the Penrose Fund of the Geological Society of America for field work on the Mesabi range dur­ ing the summer of 1962 is gratefully acknowledged. Vll CONTENTS FOREWORD v ACKNOWLEDGMENTS VII ABSTRACT ..................................................... XV INTRODUCTION ................................................ 3 NATURE AND ORIGIN OF SEDIMENTARY IRON-FORMATION. .. .. 4 General Discussion .......................................... 4 Earlier Studies of Metamorphosed Iron-formation ...... .. 5 GEOLOGY AND METAMORPHISM OF BIWABIK IRON-FORMATION. .. 7 General Geology of the Mesabi Range . .. 7 Mineralogy and Textures of the Biwabik Iron-formation. .. 10 Metamorphism of the Biwabik Iron-formation .................. 11 METHODS OF STUDY ................... '" ............... , .. , ... 14 Subdivision of the Mesabi Range ................. .. 14 Sampling ................................................... 16 Determination of Mineralogy ................................. 16 PROGRESSIVE METAMORPHISM OF THE BIWABIK IRON-FORMATION AND RELATED R.OCKS ......................................... " 19 Unaltered Taconite (Zone 1) ................................. 19 Introduction .............................................. 19 Cherty Taconite. .. 19 Introduction ........................................... 19 Primary Mineralogy and Textures . .. 19 Secondary Minerals and Textures . .. 21 Silicates ................................................ 22 Carbonates ............................................. 22 Other Minerals . .. 24 IX x METAMORPHISM OF THE BIWABIK IRON-FORMATION Summary .............................................. 26 Slaty Taconite and the Intermediate Slate (Zones 1 and 2) ..... 27 Introduction ............................................ '27 Mineralogy of Slaty Taconite . 27 Mineralogy of the Intermediate Slate . .. 29 Summary .................................... 30 Transitional Taconite (Zone '2) ........................... 31 Cherty Taconite .......................................... 31 Moderately Metamorphosed Taconite (Zone 3) ................. 34 Cherty Taconite .......................................... 34 Introduction ............................................ 34 Formation of Grunerite .................................. 36 Other Minerals .,. .. .... 36 Slaty Taconite ............................................ 38 The Intermediate Slate ................................ 39 Highly Metamorphosed Taconite (Zone 4) ................. 40 Introduction .............................................. 40 Mineralogy ............................................... 40 Veins and Small Pegmatite Bodies. 41 Metamorphism of Hematite-Bearing Units of the Biwabik Iron-formation ................................ 43 Introduction .............................................. 43 Metamorphism ........................................... 44 Metamorphism of the Pokegama Formation .................. '. 45 Introduction ............................................. 45 Unaltered Pokegama Formation (Zone 1) .................... 45 Metamorphosed Pokegama Formation ....................... 46 Metamorphism of the Virginia Formation . 47 MINERALOGICAL CHANGES DURING PROGRESSIVE METAMORPHISM OF THE BIWABIK IRON-FORMATION ............................... 48 General Discussion .......................................... 48 Mineralogy and Composition of the Carbonates ................. 51 Methods of Study ......................................... 51 Experimental Results ......... .. 52 Composition of Cummingtonite-Grunerite Amphiboles ........... 64 CONTENTS XI Compositions of Coexisting Grunerite and Carbonates ........... 67 Changes in the Organic Material during Metamorphism . .. 69 CHARACTER OF PROGRESSIVE METAMORPHISM OF THE BIWABIK IRON-FORMATION ............................... 73 Nature of the Biwabik Iron-formation before Intrusion of the Duluth Gabbro Complex . .. 73 Nature of Metamorphism of the Biwabik Iron-formation ... 74 Mineral Assemblages and Isograds in the Metamorphosed Biwabik Iron-formation ................................. 76 Mineral Assemblages in Cherty and Slaty Taconite . .. 76 Mineralogical Isograds in the Biwabik Iron-formation .......... 81 CONDITIONS DURING lVIETAMORPHISM .................. ...... 85 General Discussion .............................. 85 Formation of Wollastonite. 86 Evidence from the Iron Carbonates . 87 Significance of Fayalite in Metamorphosed Iron-formation ........ 87 SUMMARY AND CONCLUSIONS. .. .... 90 REFERENCES 97 INDEX ........................................................ 101 LIST OF FIGURES 1. Geologic map of part of the Mesabi range, Minnesota 8 2. Generalized stratigraphic section of the Biwabik Iron-formation.. 12 3. Generalized geologic map of part of the Mesabi range, showing lo­ cation of samples and boundaries of the four metamorphic zones distinguished in this study . .. 15 4. Greenalite granule taconite from unaltered Biwabik Iron-forma- tion (Zone 1) ............................................. 20 5. Greenalite granule taconite from unaltered Biwabik Iron-forma- tion (Zone 1) ............................................. 20 XII METAMORPHISM OF THE BIWABIK IRON-FORMATION 6. Stilpnomelane granule taconite from unaltered Biwabik Iron-for- mation (Zone 1) ........................................... 21 7. Intense replacement of greenalite granule taconite by minneso- taite in unaltered Biwabik Iron-formation (Zone 1) ............ 2:3 8. Strong replacement of granule taconite by carbonate in unaltered Biwabik Iron-formation (Zone 1) ............................ 2:3 9. Ankerite-rich cherty taconite from unaltered Biwabik Iron-forma- tion (Zone 1) ............................................. 25 10. Strong replacement of silicate granule taconite by coarse-grained ankerite in unaltered Biwabik Iron-formation (Zone 1) ........ 25 11. Strong replacement of silicate granule taconite by magnetite in unaltered Biwabik Iron-formation (Zone 1) .................. 26 12. Finely laminated slaty taconite composed of alternating carbonate­ and silicate-rich bands, in unaltered Biwabik Iron-formation (Zone 1) ....................................................... 28 IS.