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Archean Geology of the Northern Block of the Ishpeming Greenstone Belt, Marquette County, Michigan

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Archean Geology of the Northern Block of the Ishpeming Greenstone Belt, Marquette County, Michigan Archean Geology of the Northern Block of the Ishpeming Greenstone Belt, Marquette County, Michigan U.S. GEOLOGICAL SURVEY BULLETIN 1904-F AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the cur­ rent-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Sur­ vey publications released prior to the current year are listed in the most recent annual "Price and Availability List" Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated month­ ly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. Order U.S. Geological Survey publications by mail or over the counter from the offices given below. BY MAIL OVER THE COUNTER Books Books Professional Papers, Bulletins, Water-Supply Papers, Techniques of Water-Resources Investigations, Circulars, publications of general in­ Books of the U.S. Geological Survey are available over the terest (such as leaflets, pamphlets, booklets), single copies of Earthquakes counter at the following Geological Survey Public Inquiries Offices, all & Volcanoes, Preliminary Determination of Epicenters, and some mis­ of which are authorized agents of the Superintendent of Documents: cellaneous reports, including some of the foregoing series that have gone out of print at the Superintendent of Documents, are obtainable by mail from WASHINGTON, D.C.-Main Interior Bldg., 2600 corridor, 18th and CSts.,NW. U.S. Geological Survey, Books and Open-File Reports DENVER, Colorado-Federal Bldg., Rm. 169, 1961 Stout St. Federal Center, Box 25425 LOS ANGELES, California-Federal Eldg., Rm. 7638,300 N. Denver, CO 80225 Los Angeles St. MENLO PARK, California-Bldg. 3 (Stop 533), Rm. 3128, Subscriptions to periodicals (Earthquakes & Volcanoes and 345MiddlefieldRd. Preliminary Determination of Epicenters) can be obtained ONLY from RESTON, Virginia-503 National Center, Rm. 1C402,12201 the Sunrise Valley Dr. SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105,125 Superintendent of Documents South State St. Government Printing Office SAN FRANCISCO, California-Customhouse, Rm. 504,555 Washington, D.C. 20402 Battery St. SPOKANE, Washington-U.S. Courthouse, Rm. 678, West (Check or money order must be payable to Superintendent of Docu­ 920 Riverside Ave.. ments.) ANCHORAGE, Alaska~Rm. 101,4230 University Dr. ANCHORAGE, Alaska-Federal Bldg, Rm. E-146, 701 C St. Maps For maps, address mail orders to U.S. Geological Survey, Map Distribution Maps Federal Center, Box 25286 Maps may be purchased over the counter at the U.S. Geologi­ Denver, CO 80225 cal Survey offices where books are sold (all addresses in above list) and at the following Geological Survey offices: Residents of Alaska may order maps from ROLLA, Missouri-1400 Independence Rd. Alaska Distribution Section, U.S. Geological Survey, DENVER, Colorado-Map Distribution, Bldg. 810, Federal New Federal Building - Box 12 Center 101 Twelfth Ave., Fairbanks, AK 99701 FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave. Chapter F Archean Geology of the Northern Block of the Ishpeming Greenstone Belt, Marquette County, Michigan By R.C JOHNSON and T.J. BORNHORST U.S. GEOLOGICAL SURVEY BULLETIN 1904 CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION P.K. SIMS and LM.H. CARTER, Editors U.S. DEPARTMENT OF THE INTERIOR MANUEL LUJAN, JR., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Johnson, R.C. Archean geology of the northern block of the Ishpeming Greenstone Belt, Marquette County, Michigan / by R.C. Johnson and T.J. Bornhorst. p. cm. (Contributions to Precambrian geology of Lake Superior region ; ch. F) (U.S. Geological Survey bulletin ; 1904-F) Includes bibliographical references. Supt. of Docs, no.: I 19.3:1904-F 1. Geology, Stratigraphic Archaean. 2. Rocks, Igneous Michigan Marquette County. 3. Geology Michigan Marquette County. I. Bornhorst, Theodore J. II. Title. III. Title: Ishpeming Greenstone Belt, Marquette County, Michigan. IV. Series. V. Series: U.S. Geological Survey bulletin ; 1904-F. QE75.B9 no. 1904-F [QE653.3] 55.3 s dc20 [551.7'12'0977496] 90-23107 CIP CONTENTS Abstract Fl Introduction Fl Present investigation F2 Previous work F2 Acknowledgments F4 Geologic setting F4 Archean rocks F5 Lighthouse Point Basalt F5 Distribution F5 Thickness F5 Description F6 Nash Creek glomerophyric basalt unit F6 Reany Lake pyroclastic unit F6 Fire Center mine iron-formation unit F9 Hills Lakes pyroclastic unit F10 Metamorphism F10 Alteration Fll Gabbro F12 Gneissic granitoid rocks F12 Rhyolite intrusive rocks F12 Granodiorite near Rocking Chair Lakes F13 Proterozoic rocks F13 Structure F14 Bedding, foliations, and lineations F14 Folds F14 Faults F15 Shear zones F16 Dead River shear zone F16 Willow Creek shear zone F17 Little Garlic shear zone F17 Geologic history F17 Economic geology F18 Base metal occurrence and iron-formation F18 Gold prospects F18 Base metal-silver prospects F18 References cited F19 FIGURES 1. Geologic sketch map showing location of the Ishpeming greenstone belt F2 2. Geologic sketch map of the Ishpeming greenstone belt F3 3. Stratigraphic section showing the Lighthouse Point Basalt and its interflow units F5 4. Jensen cation plots of the Lighthouse Point Basalt and selected interflow units F8 Contents III 5-7. Geologic sketch maps showing: 5. Location of major folds in the northern block of the Ishpeming greenstone belt F9 6. Hills Lakes area F10 7. Distribution of greenschist and amphibolite metamorphic facies in the Lighthouse Point Basalt Fll 8. Diagrams of modes of gneissic granitoid rocks and granodiorite near Rocking Chair Lakes F13 9. Sketch map showing structures in the east end of the northern block F15 10. Stereographic projections of structures in the northern block F16 11. Schematic cross sections showing inferred structure in the northern block F17 TABLES 1. Michigan Geological Survey reports by Michigan Technological University students that have contributed to this report F4 2. Means and standard deviations of chemical analyses of rock units in the northern block of the Ishpeming greenstone belt F7 IV Contents CONTRIBUTIONS TO PRECAMBRIAN GEOLOGY OF LAKE SUPERIOR REGION Archean Geology of the Northern Block of the Ishpeming Greenstone Belt, Marquette County, Michigan By R.C. Johnson 1 and T.J. Bornhorst1 Abstract bodies of gabbro that are chemically similar to the basalts and probably were intruded shortly after eruption of the Lighthouse The northern block of the Ishpeming greenstone belt Point Basalt. Gneissic granitoid rocks that bound the green­ occupies 150 square kilometers in north-central Marquette stone belt to the north and east are granite to tonalite in County, Michigan. This block, as well as the entire belt, is composition. They intruded the greenstone belt during D^ and composed of Archean volcanic and volcaniclastic rocks and is metamorphosed the adjacent basalt to amphibolite grade. bounded by Archean gneissic granitoid and Early Proterozoic Quartz and quartz-feldspar porphyritic rhyolite dikes intrude sedimentary rocks. During Archean time, rocks of the belt the Lighthouse Point Basalt in and adjacent to shear zones. The were metamorphosed to greenschist and amphibolite facies granodiorite near Rocking Chair Lakes intrudes the hinge of a and were subjected to two deformational events. Recumbent major fold within the Lighthouse Point Basalt, and is folding (D,) was followed by isoclinal to open folding about interpreted as being comagmatic with the rhyolite intrusive east-west axes (D2). rocks. The rhyolite and granodiorite were probably emplaced The northern block of the belt is dominated by the during D2 deformation. Lighthouse Point Basalt, as revised and redefined herein. The Cold mineralization occurred in sheared, carbonate- Lighthouse Point Basalt is composed of tholeiitic pillowed altered, and quartz-veined rocks. Anomalous gold commonly basalt flows with minor massive flows and subordinate pyro- is present along the brecciated margins of rhyolite dikes. The clastic and sedimentary rocks; it has a thickness of approxi­ gold mineralization is interpreted as being associated with the mately 7,100 meters. The formation contains four informal second (D2) Archean deformational event. Base metal and interflow units, from oldest to youngest: Nash Creek glomero- silver mineralization occurred in quartz-carbonate veins phyric basalt unit, Reany Lake pyroclastic unit, Fire Center hosted by brecciated fault zones. This mineralization is para- mine iron-formation unit, and the Hills Lakes pyroclastic unit. genetically
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