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Geologic Setting and Descriptive Geochemistry of Archean Supracrustal and Hypabyssal Rocks, Soudan-Bigfork Area, Northern Minnes

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Geologic Setting and Descriptive Geochemistry of Archean Supracrustal and Hypabyssal Rocks, Soudan-Bigfork Area, Northern Minnes MINNESOTA GEOLOGICAL SURVEY D.L. Southwick, Director GEOLOGIC SETTING AND DESCRIPTIVE GEOCHEMISTRY OF ARCHEAN SUPRACRUSTAL AND HYPABYSSAL ROCKS, SOUDAN-BIGFORK AREA, NORTHERN MINNESOTA: IMPLICATIONS FOR METALLIC MINERAL EXPLORATION D.L. Southwick, T.J. Boerboom and M.A. Jirsa Report of Investigations 51 ISSN 0076-9177 UNIVERSITY OF MINNESOTA Saint Paul-1998 GEOLOGIC SETTING AND DESCRIPTIVE GEOCHEMISTRY OF ARCHEAN SUPRACRUSTAL AND HYPABYSSAL ROCKS, SOUDAN-BIGFORK AREA, NORTHERN MINNESOTA: IMPLICATIONS FOR METALLIC MINERAL EXPLORATION Minnesota Geological Survey 2642 University Avenue West Saint Paul, Minnesota 55114-1057 Telephone: 612-627-4780 Fax: 612-627-4778 E-mail address:[email protected] Web site: http://geolab.geo.umn.edu/mgs ©1998 by the Board of Regents of the University of Minnesota All rights reserved. ISSN 0076-9177 The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. CONTENTS Page Abstract ......................................................................................................................................................................... 1 Introduction ................................................................................................................................................................... 1 Context and purpose of this study ................................................................................................................................. 3 Geology of the Soudan-Bigfork area ............................................................................................................................. 4 Regional geologic relations .................................................................................................................................... 4 Structure and stratigraphy of the Soudan belt ........................................................................................................ 4 Structure and stratigraphy of the Newton belt ..................................................................................................... 10 Sedimentary environments in the Abitibi subprovince: Possible Minnesota analogs ............. , ............................ 10 Intrusive rocks within the supracrustal successions ............................................................................................. 11 Hypabyssal, subvolcanic intrusions: diabase and gabbro ............................................................................. 11 Hypabyssal, subvolcanic intrusions: quartz-plagioclase porphyry ............................................................... 12 Lamprophyre dikes and sills ......................................................................................................................... 12 Geochemistry.................................................................................................................... ............................ ............... 13 Organization of the geochemical data .................................................................................................................. 13 Geochemistry of the mafic volcanic rocks ........................................................................................................... 14 Basaltic sequences in the Soudan belt ........................................................................................................... 14 Basaltic sequences in the Newton belt .......................................................................................................... 14 Discussion of the mafic volcanic rocks ......................................................................................................... 16 Geochemistry of the volcaniclastic and sedimentary rocks of dacitic provenance, Soudan and Newton belts ... 20 Geochemical indications of a heterolithic sediment source ................................................................................. 21 Geochemistry of iron-formation, Soudan and Newton belts ................................................................................ 26 Geochemistry of hypabyssal interior intrusions ................................................................................................... 28 Mafic subvolcanic intrusions: the Deer Lake Complex ................................................................................ 28 Felsic subvolcanic intrusions: quartz-plagioclase porphyry ......................................................................... 31 Lamprophyres ............................................................................................................................................... 33 Inferred tectonic setting of the Soudan-Bigfork area ........................................................................................... 35 Inferences pertinent to mineral exploration ................................................................................................................. 36 Acknowledgments ....................................................................................................................................................... 37 References cited ........................................................................................................................................................... 38 Appendix ..................................................................................................................................................................... 43 FIGURES Figure l. Approximate location of the Soudan-Bigfork area in the Superior Province of the Canadian Shield ...... 2 2. Bedrock geology of part of the western Vermilion district, northern Minnesota ...................................... 5 3. Simplified bedrock geology of the Soudan-Bigfork area .......................................................................... 7 4. AFriM diagram for basaltic rocks, lower volcanic unit, Soudan belL ................................................... 15 5. Immobile element plot for basaltic rocks, lower volcanic unit, Soudan belt .......................................... 15 6. Spider diagram of mean trace and rare earth element abundance for basaltic rocks, lower volcanic unit, Soudan belt ............................................................. 15 7. AFtiM diagram for basaltic rocks, upper volcanic unit, Soudan belt.. .................................................... 16 8. Immobile element plot for basaltic rocks, upper volcanic unit, Soudan belt .......................................... 16 9. Spider diagram of mean trace and rare earth element abundance for basaltic rocks, upper volcanic unit, Soudan belt ............................................................. 16 iii FIGURES continued. Page Figure 10. AFfiM diagram for basaltic rocks interstratified with felsic wacke and metasedimentary rocks, Soudan belt ................................................................................ 17 11. Immobile element plot for basaltic rocks interstratified with felsic wacke and metasedimentary rocks, Soudan belt ......................................................................... 17 12. Spider diagram of mean trace and rare earth element abundance in basaltic rocks interstratified with felsic wacke and metasedimentary rocks, Soudan belt. ...... .................. ..... ........... ....... .... 17 13. AFfiM diagram for basaltic rocks in the Thistledew Lake, Joy Lake, Wilson Lake and Cook sequences of the Newton belt .......................................................................... 18 14. Immobile element plot for basaltic rocks in the Thistledew Lake, Joy Lake, Wilson Lake and Cook sequences of the Newton belt ..................................................... 18 15. Spider diagram of mean trace and rare earth element abundance in basaltic rocks of the Thistledew Lake, Joy Lake, Wilson Lake and Cook sequences of the Newton belt.. ...... 18 16. AFTiM diagram for basaltic rocks in the Deer Lake sequence of the Newton belt ................................. 19 17. Immobile element plot for basaltic rocks in the Deer Lake sequence of the Newton belt .... ... ............ .... 19 18. Spider diagram of mean trace and rare earth element abundance in basaltic rocks in the Deer Lake sequence of the Newton belL ......................................................................... 19 19. Ti02 vs. Si02 plot for metasedimentary rocks, Soudan-Bigfork area ...................................................... 21 20. Nap - FeO(t) - MgO plot for metasedimentary rocks, Soudan-Bigfork area ......................................... 21 2IA. Ratio diagram of elemental abundances in graywackes of the Soudan belt.. ........................................... 23 2IB. Ratio diagram of elemental abundances in graywackes of the Newton belt ............................................ 23 22A. Photomicrograph of andesite clast, Lake Vermilion Formation ............................................................... 24 22B. Field photograph, iron-formation clasts in felsic breccia ......................................................................... 24 23. Histogram of CIA values for metasedimentary rocks of the Soudan-Bigfork area .................................. 25 24A. Triangular plot of Soudan-Bigfork graywacke analyses as
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