Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota
1994-12
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Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota
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1994-12
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University of Minnesota Duluth
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Technical Report
Abstract
The Middle Proterozoic (1,099 Ma) intrusive Duluth Complex contains numerous smaller
sub-intrusions that collectively comprise the Complex. Two of these sub-intrusions are informally
known as the South Kawishiwi intrusion (SKI) and Partridge River intrusion (PRI). A correlative
igneous stratigraphy has been documented in the PRI by Severson and Hauck (1991) and Severson
(1991). In this investigation, detailed relogging of drill holes within the SKI (136 drill holes totalling
214,461 feet of core) also defines an intrusion-wide stratigraphic sequence along a 19-mile strike
length that is referred to as the South Kawishiwi Troctolite Series (SKTS). The stratigraphic
sequences of the SKI and the PRI are completely dissimilar. At least 17 correlative subhorizontal
igneous units are defined within the SKTS; however, they are not equally present in all areas of the
SKI. The SKTS units, from the bottom to the top (roughly), are referred to as: BAN = Bottom
Augite troctolite and Norite; BH = Basal Heterogeneous troctolites (sulfide-bearing); U3 =
Ultramafic Three (sulfide-bearing); PEG = Pegmatitic Unit of Foose (1984); U2 = Ultramafic Two
(sulfide-bearing); U1 = Ultramafic One (sulfide-bearing); AT-T = homogeneous Anorthositic
Troctolite to Troctolite; UW = Updip Wedge (sulfide-bearing); LOW AGT = homogeneous Lower
Augite Troctolite zone; MAIN AGT = homogeneous Main Augite Troctolite zone; AT&T =
homogeneous Anorthositic Troctolite and Troctolite; AT(T) = homogeneous Anorthositic Troctolite
with lesser amounts of Troctolite; AN-G Group = intermixed Anorthositic and Gabbroic rocks;
UPPER GABBRO = oxide-bearing gabbroic rocks; "INCL" = large shallow-dipping inclusion of
magnetic basalt(?); UPPER PEG = Upper Pegmatitic zone; and T-AGT = Troctolite to Augite
Troctolite (the latter five units are restricted to a small area referred to as the Highway 1 Corridor
area). The lowest units of the SKTS are the most varied with respect to textures, rock types, and
sulfide content. They are very unevenly distributed along the strike length of the SKI in a
"compartmentalized" fashion suggesting a complicated intrusive history. The lowest units were
emplaced early into several restricted magma chambers via repeated and close-spaced magmatic
pulses. The effects of contamination from assimilated and devolatized country rocks were the most
pronounced in these units. Three ultramafic-bearing packages (U1, U2, and U3 units) are also
present within the lower portion of the SKTS. These units are characterized by alternating layers
of troctolitic and ultramafic (olivine-rich) rock. The ultramafic-bearing units represent periods of
rapid and continuous magma injection that crystallized more primitive ultramafic layers before
mixing with the resident magma. The U3 Unit is the most unique in that it contains several massive
oxide (magnetite-rich) pods along its strike length. An empirical relationship between the U3 Unit
and the Biwabik Iron-formation (BIF) suggests that the massive oxides were derived from intruded
and assimilated BIF. The U3 Unit also contains the majority of high Platinum Group Elements
(PGE) values sampled to date in the SKI.
In contrast, the upper SKTS units reflect an entirely different intrusive history. Each unit
is characterized by monotonous sequences of texturally homogeneous and sulfide-free rocks.
Gradational contacts are present between each of the units. In contrast to the lower SKTS units,
the upper SKTS units are generally distributed throughout the SKI. Ultramafic members are
restricted to only two thin horizons referred to as High Picrite #1 and #2. All of these features are
indicative of a quiescent and open magmatic system. Thus, the upper SKTS units appear to have
been emplaced as widely-spaced pulses into a progressively developed, single magma chamber with
little interaction with the country rocks.
An entirely different package of rocks is present in six extremely deep drill holes that were
drilled within the Highway 1 Corridor (H1C). The H1C represents a large inclusion of older
Anorthositic Series rocks. It appears that the H1C rocks were underplated by the younger intruding
SKI magmas.
Several drill holes within the PRI were also relogged to better understand the nature and
location of the contact between the SKI and PRI. One important feature noted is that as the contact
between the two intrusions is approached, the upper units of the PRI become heterogeneous and
indistinguishable from each other. The same heterogeneity is not evident in the adjacent SKI. The
presence of the "heterogenous zone" within the PRI adjacent to the SKI suggests that the PRI was
intruded before the SKI. Also present near the PRI/SKI contact zone is a major north-trending fault
(down to the east). Associated with this fault are voluminous amounts of steeply-inclined lenses
of late granitic/felsic material that cross-cut the PRI stratigraphic section. This fault is named the
Grano Fault because the late lenses consist of varied granitic to pyroxenitic material. Though the
Grano Fault trends through both the PRI and SKI, the late granitic lenses are not particularly
common within the SKI. This fact also suggests that the PRI is older than the SKI. Offset units
within the footwall rocks suggests that movement along the Grano Fault was initiated before
emplacement of the PRI.
All geochemical data pertaining to previously sampled SKI drill core is compiled and
correlated with the SKTS units. An additional 80 geochemical samples collected from SKTS units
in this investigation are added to this database. The grouping of the SKTS units on the geochemical
plots supports the geologic correlations of this investigation. Some of SKTS units also show
geochemical overlap with footwall units and indicate the effects of contamination of the magma due
to assimilation of the country rocks during intrusion.
PGE analyses conducted on a multitude of rock types and igneous units within the SKTS
indicate that the U3 Unit, and to a lesser extent the PEG Unit, show the most promise of hosting
a PGE deposit. The PGE origin model of Boudreau and McCallum (1992) is invoked to explain why
anomalous PGEs are common to the U3 Unit. The Boudreau and McCallum model envisions the
upward migration of chlorine-rich, late, magmatic fluids that were capable of transporting PGEs
and concentrating them at stratigraphic traps. However, a straightforward application of the model
does not explain why significantly higher PGEs are restricted to certain areas, e.g., the Birch Lake
deposit. A variation of the "Boudreau and McCallum model" is proposed to explain this difference.
This revised model is similar except that upward-moving, Cl-rich, PGE-pregnant hydrothermal
solutions are envisioned to have been concentrated in fault zones. When fluids associated with
fault zones encountered a proper stratigraphic trap (ultramafic horizon), more PGEs were
deposited relative to areas outside of the fault zones. An intersection of the proper stratigraphic
trap (U3 Unit with massive oxides, sulfides and high Cr contents) and the proper channelway to
concentrate the PGE-pregnant Cl-rich solutions (Birch Lake Fault) reasonably explains the
significant PGE values in the Birch Lake deposit. The Birch Lake Fault is defined by a zone wherein
drill holes commonly encountered either: massive sulfide mineralization within the footwall
granitic rocks; and/or "voluminous" amounts of late granitic/felsic lenses that cut the troctolitic
rocks of the SKI.
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Natural Resources Research Institute, University of Minnesota, Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811; Funded by the Minerals Coordinating Committee Through the Minerals Diversification Plan Project No. 562201
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Severson, Mark J. (1994). Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/188520.
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