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
