The Minnamax Cu-Ni deposit (also called the Babbitt deposit) is situated within what has
been informally referred to as the Partridge River intrusion (or Partridge River Troctolite) of the
Duluth Complex (1.1 Ga), northeastern Minnesota. The deposit has been subdivided into five
contiguous ore zones; the Local Boy area and Bathtub area are two ore zones described in this
report. Within the deposit are a wide variety of troctolitic, ultramafic, and footwall rock types, and
hornfelsed inclusions (both footwall and hanging wall). Many specific rock types are correlative
between drill holes and can be grossly categorized into seven sub-horizontal troctolitic units, three
types of hornfelsed inclusions, and a late cross-cutting pegmatitic phase. Also present are
correlative units within the footwall rocks. All rock units were identified by detailed relogging of
61 surface drill holes (117,605 feet of core) and are portrayed on nine cross-sections that extend
through various portions of the Minnamax deposit.
Severson and Hauck (1990) described the stratigraphy of the troctolitic rocks of the Partridge
River Troctolite to the west of the Minnamax deposit; the stratigraphy is referred to as the Partridge
River Troctolite Series (PRTS). Most of the PRTS rock units defined at the Dunka Road Cu-Ni
deposit (located to the immediate SW of Minnamax) by Severson and Hauck (1990) are present at
Minnamax. However, the overall picture at Minnamax is more complicated than Dunka Road due
to rock type changes that are manifested by: 1) pinch-out and reappearance of specific marker bed
units; 2) down-strike gradational changes of ultramafic horizons; 3) extremely limited areal extent
of some ultramafic horizons; and 4) gradational changes in the troctolitic rock types between drill
holes. In some areas a particular marker horizon may "disappear" laterally and then reappear at the
same stratigraphic level in another group of drill holes. In spite of these local difficulties, a gross
stratigraphy of seven subhorizontal igneous units is present at Minnamax and consists of (from
bottom to top): Unit I - heterogeneous, sulfide-bearing augite troctolite and troctolite with abundant
metasedimentary inclusions; Unit II - homogeneous troctolite with a basal picrite horizon (Unit II
is present only in the SW portion of the Minnamax deposit); Unit III - mottled textured anorthositic
troctolite to troctolite with characteristic olivine oikocrysts (Unit III is present mainly in the SW
portion of Minnamax and is enveloped by Unit I to the NE); Unit IV - mixed homogeneous troctolite
and augite troctolite (augite troctolite is at the top of Unit IV in localized areas) with a semipersistent
basal ultramafic horizon termed the "± picrite"; Unit V - homogeneous anorthositic
troctolite that exhibits a gradational contact with Unit IV; and Units VI and VII - homogeneous
troctolites with persistent basal ultramafic horizons. More abundant and thicker ultramafic horizons
are present in Units VI and VII in the Bathtub area of the Minnamax deposit. Specific marker
horizons utilized in drill hole correlations include: Unit III, "± picrite," "pocket picrite," top of Unit
IV (augite troctolite), and the ultramafic base of Units VI and VII.
The troctolitic stratigraphy is cut by pegmatitic orthopyroxenite and peridotite bodies that
are referred to as OUI - Oxide-bearing Ultramafic Intrusions. Pegmatitic hybrid hornblendite and
granophyre also cut the stratigraphy and are often related to the OUI bodies. Rusty chlorine-rich
drops may commonly coat the core of the ultramafic horizons and OUI bodies.
Several enigmatic hornfelsed inclusions are present in Units VI and VII at Minnamax. These
are grouped in two categories that include: 1) CC-type inclusions that are similar to outcrops of the
Colvin Creek hornfels; and 2) "pic"-type inclusions that are similar to nearby outcrops of basalt
inclusions. Both inclusion types are similar in that they contain fine-grained plagioclase-filled
ovoids or wisps that may represent vesicles, and they exhibit the same chemical signature.
However, they exhibit a different mineralogy (the CC-type inclusions are oxide-rich). Their
stratigraphic position in the troctolitic rocks suggests that they are probably hanging wall material
(North Shore Volcanic Group). While these two inclusion types are readily correlative between drill
holes, the nature of their different mineralogy remains unknown.
Another enigmatic rock type is present within the lower portion of the Virginia Formation
footwall rocks. The rock is unique in that it contains hornblende ± olivine and locally grades into
serpentinized picrite with hornblende. It is generally concordant with the overall bedding of the
Virginia Formation and is referred to as the sill(?) unit. Whole rock geochemistry indicates that this
unit locally exhibits: high Cl contents that are similar to Cl values of ultramafic horizons in the
troctolitic rocks; MG numbers that are more primitive than the ultramafic horizons; and high Cr
contents that are much higher than anything sampled in the overlying troctolitic section. If the unit
was a sill, it now exhibits gradational contacts with the metasedimentary rocks and is characterized
by a granoblastic texture with superimposed euhedral hornblende. These data may indicate that the
sill was intruded before, and hornfelsed during, emplacement of the majority of the Partridge River
The original report contained a 5.25-inch floppy disk labeled "Appendix A MNMXRK.WK1." The file listed on the label was extracted from the disk in September 2019. Since the software used to create that file (Lotus 1-2-3 spreadsheet) is old and difficult to find, staff converted the file to a more accessible format, a comma separated values (.csv) file. Several columns (H, CC, CD, CE, and CF) in the .wk1 file contained formulas which were not converted to .csv (though the resulting values were), so the file was also converted to Microsoft Excel 2016 (.xlsx), which retained the formulas. All three versions (.wk1, .csv, and .xlsx) are attached to this record.
Natural Resources Research Institute, University of Minnesota, Duluth, 5013 Miller Trunk Highway, Duluth, Minnesota 55811; Funded by Minnesota Technology, Incorporated (Formerly the Greater Minnesota Corporation)
Severson, Mark J.
Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part I: Geology.
University of Minnesota Duluth.
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.