FINISH LOGGING OF N DULUTH COMPLEX DRILL CORE (And a Reinterpretation of the Geology at the Mesaba (Babbitt) deposit) R By R Mark J. Severson and I Steven A. Hauck 4800E VII OUI d e ? BT4 VI t BT1 a l t e I l K r u S ? V I ? V V a F U BT4 ? ? o O ? n Grid North d ? a ? r BT4 IV n ? ? ? a G BT1 BT1 s 400' o BT1 n t i e y t v i BT1 c i m i BT1 BT1 s l x e o f r p e t a I o l T t 3600E VIII SKI VI VI V V Po ck et VII BT1 BT4 BT4 Pic VI BTLI? BT4 V ? ? ? R BT1 ± PICRITE BT1 BT1 10139 I Troctolitic Rocks Undivided BT1 Roman numerals denote units in the Partridge River intrusion BT-series units denote units in the Bathtub intrusion Persistent Augite Troctolite to Olivine Gabbro at the A-4 B-3 top of Unit IV - Partridge River intrusion Drill Fan Drill Fan 2400E sss sss Unit III - Partridge River intrusion ? BT1 BT4 VI? VII OUI "Pocket Picrite" ? VI ? ? BT4 BT4 ? Bathtub Layered Interval ? V ? BT1 ± Picrite IV t l u BT1 a Mixed Norite and Hornfels BT1 I F x I a "The Hidden Rise" - hornfels and norite common 2 I m a n Virginia Formation - Bedded Po unit n I i M Virginia Formation (footwall) h 400 feet t u Biwabik Iron Formation o 0 S 0 September, 2008 Technical Report NRRI/TR-2008/17 8 Funded by the Minerals Coordinating Committee of the Minnesota Department of Natural Resources, Permanent / University Trust Fund and the Natural Resources Research Institute Project Nos. 1663-187-6574, 1896-783-1224, 1663-187-1600 1 Natural Resources Research Institute University of Minnesota, Duluth 7 5013 Miller Trunk Highway Duluth, MN 55811-1442 This publication is accessible from the home page of the Economic Geology Group of the Center for Applied Research and Technology Development at the Natural Resources Research Institute, University of Minnesota, Duluth (http://www.nrri.umn.edu/egg) as a PDF file readable with Adobe Acrobat 6.0. Date of release: October 2008 Cover Photo Caption Block diagram of the major igneous units in the eastern end of the Mesaba deposit. Recommended Citation Severson, M.J. and S.A. Hauck, 2008, Finish Logging of Duluth Complex Drill Core (And a Reinterpretation of the Geology at the Mesaba (Babbitt) Deposit), University of Minnesota Duluth, Natural Resources Research Institute, Technical Report NRRI/TR-2008/17, 68 p. + 94 plates. Natural Resources Research Institute University of Minnesota, Duluth 5013 Miller Trunk Highway Duluth, MN 55811-1442 Telephone: 218-720-4272 Fax: 218-720-4329 e-mail: [email protected] Web site: http://www.nrri.umn.edu/egg ©2008 by the Regents of the University of Minnesota All rights reserved. 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. ABSTRACT This project was undertaken with the objective to finish logging all drill holes from the basal contact zone of the Duluth Complex. Logging of Duluth Complex holes by Natural Resources Research Institute (NRRI) personnel began in 1989, when Severson and Hauck (1990) defined the igneous stratigraphy for most of the Partridge River intrusion (PRI). During the ensuing years the NRRI logged a total of 955 holes and defined igneous stratigraphic sections for several more intrusions of the Duluth Complex. As of 2005, a remainder of over 220 holes had yet to be logged. At the end of this project, 295 holes, which include some recently-drilled holes, were logged with about 20 holes still to be logged from the far eastern end of the Mesaba deposit. Lithologic logs for most of the holes that have been logged since 1989 are now available on the NRRI Geology Group’s website at www.nrri.umn.edu/egg/. The vast majority of holes that were logged for this project were from the Mesaba (Babbitt) Cu-Ni±PGE deposit, and thus, this report deals mostly with that deposit. A result of logging a large number of holes at the Mesaba deposit indicates that most of the deposit does not exhibit a stratigraphic package that has been recognized within the nearby Partridge River intrusion. This suggests that most of the deposit is situated within another sub-intrusion, informally called the Bathtub intrusion (BTI). The BTI appears to have been fed by a vent in the Grano Fault area on the east side of the Mesaba deposit. Forty-two cross-sections from the Mesaba deposit, showing the geology in over 450 surface holes, are presented in this report. Another 26 cross-sections, showing the geology in 219 underground holes, are also presented for the Local Boy ore zone of the Mesaba deposit. All of these cross-sections are utilized to define the igneous stratigraphy of the BTI and adjacent PRI at the deposit. All publically-available drill holes have now been logged from the Dunka Pit Cu-Ni deposit located in the South Kawishiwi intrusion (SKI). Nineteen cross-sections through the deposit are presented in this report. These cross-sections show the geology, potential Cu-Ni ore zones in the holes, and the down dip extent of potential mineable zones of the Biwabik Iron Formation at depth. Additional areas in the SKI where holes were logged for this project include the Maturi, Spruce Road, and Nokomis deposits. Cross-sections and hung stratigraphic sections are presented, and they show the geology intersected in these newly-logged holes relative to previously-logged holes. Drill holes from two Oxide-bearing Ultramafic Intrusions (OUI) were also logged for this investigation. These logs include ten holes from the Longnose deposit and ten holes from the Water Hen deposit. Six cross-sections through the Longnose deposit are presented in this report. In summary, the holes logged in this investigation have added greatly to our understanding of the geology of basal portions of the Duluth Complex. In some cases, the previously defined igneous stratigraphic sections for the various intrusions have held up remarkably well as additional holes are drilled and logged. Of course, there are always some exceptions to the rule. In other cases, e.g., the Mesaba deposit, as more holes were logged and/or drilled, the igneous stratigraphy had to be modified in order to explain differences in a group of holes that were situated in the BTI versus the nearby PRI. This change serves as an example that definition of igneous units, and modes of mineralization, in the Duluth Complex is an iterative process and has to be continuously refined as more data, in the form of new drill holes, are generated. i TABLE OF CONTENTS ABSTRACT..................................................................................................................................... i TABLE OF CONTENTS................................................................................................................ ii LIST OF FIGURES ........................................................................................................................ v LIST OF APPENDICES................................................................................................................ vi LIST OF PLATES ........................................................................................................................ vii INTRODUCTION .......................................................................................................................... 1 BACKGROUND ........................................................................................................................ 1 GEOLOGIC SETTING .............................................................................................................. 1 METHODOLOGY OF LOGGING DRILL CORE.................................................................... 3 DRILL HOLE LITHOLOGIC LOGS......................................................................................... 5 CROSS-SECTIONS ................................................................................................................... 5 DONATED DRILL CORE......................................................................................................... 6 ACKNOWLEDGMENTS .......................................................................................................... 6 IGNEOUS STRATIGRAPHY OF THE PARTRIDGE RIVER INTRUSION.............................. 7 INTRODUCTION ...................................................................................................................... 7 NORTHMET DEPOSIT (DUNKA ROAD DEPOSIT) ............................................................. 7 WETLEGS AND WYMAN CREEK DEPOSITS...................................................................... 9 SOUTHERN PORTION OF THE MESABA DEPOSIT (BABBITT DEPOSIT)..................... 9 GEOLOGY OF THE MESABA DEPOSIT AND IGNEOUS ..................................................... 12 STRATIGRPAHY OF THE BATHTUB INTRUSION............................................................... 12 INTRODUCTION .................................................................................................................... 12 COMMENTS REGARDING THE CROSS-SECTIONS OF THE MESABA DEPOSIT...... 13 FOOTWALL ROCKS .............................................................................................................. 15 Biwabik Iron Formation........................................................................................................ 15 BIF Submember A – Chert and Marble............................................................................ 18 BIF Submember