Download Field, Petrographic, and Mineralization Characteristics of Mafic

PRECAMBRIAN RESEARCH CENTER PROFESSIONAL WORKSHOP SERIES
Field, Petrographic, and Mineralization Characteristics of Mafic Layered
Intrusions
October 4-10, 2009
University of Minnesota Duluth
FIELD TRIP 2
CU-NI-PGE MINERALIZATION ALONG THE
BASAL CONTACT OF THE DULUTH COMPLEX
IN DRILL CORE
Mark Severson
Economic Geology Group
Natural Resources Research Institute
University of Minnesota - Duluth
Disseminated Sulfide Mineralization
Large resources of low-grade copper-nickel sulfide ore that locally contain anomalous
PGE concentrations are well documented by drilling in the basal zones of the Partridge River
intrusion (PRI), South Kawishiwi intrusion (SKI), and the more recently recognized Bathtub
intrusion (BTI). At least nine subeconomic deposits (Figure 2-1) have been delineated in the
basal 100 to 300 meters of all three intrusions. The mineralization consists predominantly of
disseminated sulfides that collectively constitute over 4.4 billion tons of material averaging
0.66% Cu and 0.20% Ni (Listerud and Meineke, 1977). Overall, the copper to nickel ratio
averages 3.3:1 (see Table 2.2 in Chapter 2, RI 58); however, there are wide variations in the
Cu:Ni ratio from one Cu-Ni deposit to another deposit and also internally within each of the
deposits (the systematic distribution of this Cu:Ni variation has not been studied). PGE
concentrations average about 10 ppm Pt+Pd (recalculated to 100% sulfide), but may range as
high as 50 ppm Pt+Pd (recalculated to 100% sulfide) in associated stratabound zones, such as at
the NorthMet, Nokomis, and Birch Lake deposits.
The disseminated sulfide deposits are hosted by taxitic troctolitic to gabbroic rocks that
contain abundant inclusions of footwall rock types. Within the Partridge River intrusion, the
basal unit - Unit I of Severson and Hauck (1990; Figure 2-2) hosts the vast majority of the
disseminated sulfides. Similarly, mineralization within the South Kawishiwi intrusion is confined
to the bottom-most units (Figure 2-3) that include the following units: BH (basal heterogeneous);
BAN (basal augite troctolite and norite); three ultramafic units (U1, U2, and U3); and UW (updip
wedge). The disseminated sulfide minerals (dominantly pyrrhotite, chalcopyrite, cubanite, and
pentlandite) occur as interstitial grains that make up between trace amounts and 10% of the rock
by volume (visual estimation). Pyrrhotite is generally the dominant sulfide, especially closer to
the basal contact.
Although this mineralization type is categorized as being present within the basal portions of the
intrusions, it is important to stress that the rock units do not always contain sulfides throughout
their entire vertical section. Mineralized zones are typically extremely erratic in their spatial
extent and ore grades. Zones that are barren of sulfides commonly “interfinger” with
25
mineralized zones in a random pattern. This erratic pattern of mineralization, in part, mirrors the
lithologic heterogeneity of the basal units. The only exception to this random mineralization
pattern is the Maturi deposit, its downdip extension (Maturi Extension; Peterson, 2001), and the
Nokomis deposit where the uppermost portions of the BH Unit consistently exhibits copper
values in excess of 1.0% that gradually decrease with depth toward the basal contact. The change
to more consistent mineralization at Maturi and Nokomis may be related to an overall thinning of
the BH Unit and thus the sulfides are more restricted to a specific horizon. Peterson (2002) has
recently classified the deposits of the South Kawishiwi intrusion as either “open style” (random
low grade mineralization over thick intervals, e.g., the Spruce Road and Dunka Pit deposits) or
“closed style” (Maturi-like mineralization).
Figure 2-1. Location of copper-nickel sulfide deposits, Fe-Ti±V deposits, and other exploration
areas along the western base of the Duluth Complex; published reserves/resources are
subject to change depending on numerous factors.
26
Marginal Zone of the Partridge River intrusion
15 miles (24 km)
NorthMet
(Dunka Road)
Mesaba
(Babbitt)
VIII
OUI
VII
VII
Melatroctolite/Peridotite
}
Longnose
Longear
Section 17
OUI
V
IV
II
V
Wetlegs
Layered
Interval
I
FOOTWALL ROCKS
V
?
Melatroctolite
III
I
III
BT4
Bathtub
Layered
Interval
(BTLI)
?
I
V
IV
Melatroctolite
?
II
melatroctolite
I
V
gradational
IV
III
II
OUI
V
V
IV
III
II
I
gra
da
tio
na
l
IV
IV
VI
VI
Melatroctolite/Peridotite
?
?
l
na
tio
da
gra
VI
VI
V &/or VI
HETEROGENEOUS
ZONE
III
± Picrite
"The
Hidden
Rise"
I
I
BTLI
III
I
III
BT1
"The
Hidden
Rise"
Basal ic
af
Ultram
Local Boy
massive
sulfide
VIRGINIA FM
VIRGINIA FM
VIRGINIA FM
SOUTH KAWISHIWI INTRUSION
VII
VII
eastern
Tiger Boy
Local Boy
Bathtub
and western
Tiger Boy
Grano Fault Zone
Southern Edge
VIII
HETEROGENEOUS
Wetlegs
HETEROGENEOUS
Wyman Creek
BIWABIK IF
Figure 2-2. Generalized stratigraphy of the marginal zone of the Partridge River intrusion and
Bathtub intrusion (from Severson and Hauck, 2008). Stratigraphic relationships for the
area between the Wyman Creek deposit and the Water Hen deposit are poorly understood
and are not portrayed.
SW
NE
Marginal Zone of the South Kawishiwi intrusion
19 MILES
(Severson, 1994)
Nokomis and
Maturi Extension
BIRCH LAKE
AT & T
grad
VIRG FM
U2
N
BA
BIF
U2
U3
U3
GIANTS RANGE
BIF
AT-T
BAN
GIANTS RANGE
BH
BH
BH
GIANTS RANGE
PEG
PEG
U3
U3
BH
BAN
BAN
GIANTS RANGE
BH
U1
U1
U2
PEG
PEG
?
BH
LOW AGT
U2
PEG
?
AN-G
GROUP
(thins down dip)
AT-T
AT-T
SHALLOW DRILL HOLES
(FROM INCO LOGS)
MAIN AGT
gradational
U1
U3
AT & T
AT & T
MAIN
AGT
Het Zone
gr
ad
BH
BH
BAN
UPPER GABBRO
SPRUCE ROAD
MAIN
AGT
U1
AT-T
"INCL"
High Picrite 2
gradational
UW
UPPER PEG
AT & T
MAIN
AGT
AT (T)
gradational
T-AGT
High Picrite 2
AT & T
AT (T)
High Picrite 1
MATURI
AT & T
DUNKA PIT
SPRUCE ROAD
DEEP HOLES
AT (T)
?
?
?
?
BH
PEG
BAN
GIANTS RANGE
U1
U2
U2
PEG
BH
BAN
GIANTS RANGE
BH
U3
BIF
U3
BIF
?
BH
GIANTS RANGE
Figure 2-3. Generalized stratigraphy of the marginal zone of the South Kawishiwi intrusion
(modified from Severson, 1994; taken from RI-58 figure 6.12).
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Contrasting with this heterogeneity of rock types and mineralization, some internal PGEbearing sulfide zones within the lower units of both intrusions exhibit a stratabound relationship
to the igneous stratigraphic section. Examples include NorthMet and Birch Lake (see discussion
below).
Basal Massive Sulfide Mineralization
In a few localized areas along the basal zones of the SKI, PRI and BTI, semi-massive to
massive sulfide mineralization is present at the basal contact. In most cases, the massive to semimassive sulfide is proximal to either sulfide-rich footwall rocks or structures such as
faults/conduits and pre-Complex folds. Massive sulfide zones that are spatially related to sulfiderich footwall rocks are intersected in scattered drill holes in the Mesaba, Serpentine, and Dunka
Pit deposits (Severson and others (1994), Zanko and others (1994), and Severson (1994),
respectively). All of these massive sulfides are pyrrhotite-rich (with generally <2% Cu) and are
present at, or slightly above, the basal contact. In all cases, a pyrrhotite-rich member of the
footwall Virginia Formation (BDD PO unit) is located at the basal contact and is situated up-dip
of the massive sulfide occurrences. This relationship suggests that the BDD PO unit acted as a
local sulfur source that generated a Cu-poor, sulfide-rich melt that was concentrated downdip,
along the basal contact, via gravity settling. An example of this relationship is portrayed in
Figure 2-4 for the Serpentine deposit.
Figure 2-4. Distribution of semi-massive to massive sulfide zones in drill holes, relative to the
Bedded Pyrrhotite member within the Virginia Formation, at the Serpentine deposit (after
Zanko and others, 1994; taken from RI-58, Figure 8.3).
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The massive sulfide occurrence in the Local Boy ore zone of the Mesaba deposit (Fig. 2-1)
is clearly structurally controlled. At this locality, the massive sulfide zones are Cu-rich (generally
5-25% Cu) and are situated along the axis of an anticline defined by the footwall rock units. The
highest PGE values (11 ppm Pd and 8 ppm Pt) yet found within the Duluth Complex are
associated with these structurally-controlled Cu-rich massive sulfides. The massive sulfides are
almost exclusively hosted by the Virginia Formation, present as both inclusions above the basal
contact and in the footwall rocks below the basal contact, while interfingering intrusive rocks are
relatively barren of massive sulfide. These relationships, plus sulfide textures that are indicative
Figure 2-5. Potential distribution of semi-massive to massive sulfide types, relative to the Grano
fault and Local Boy anticlinal axis, at the Local Boy ore zone of the Mesaba deposit
(modified from Severson and Hauck, 2008and included in RI-58 as Figure 8.4).
29
of structural preparation, suggest that the massive sulfides were "injected" into the footwall rocks.
Ripley (1986) and Severson and Barnes (1991) propose that an immiscible sulfide melt, formed
in an auxiliary magma chamber at depth, was injected into structurally prepared zones in the
footwall rocks along the anticline to form the Local Boy ores. Late movement of Cl-rich fluids,
along the axis of the anticline, further redistributed and concentrated the PGEs (Severson and
Barnes, 1991). Recent studies (Severson and Hauck, 2008) indicate that there is an overall
increase in the Cu-PGE content of the massive sulfide in an east-to-west direction (Fig. 2-5); this
is perhaps the result of fractional crystallization of immiscible sulfide melt as it migrated into the
footwall rocks. In this scenario, the north-south trending Grano fault is inferred to be a potential
feeder zone.
Structurally-controlled veins of massive sulfide, and disseminated sulfides, are locally
present within the granitic footwall rocks immediately beneath the SKI that occur in scattered
holes that outline two northeast-trending belts (Fig. 2-6). The linearity of the belts suggests they
are fault controlled. One of these belts crudely aligns with the Birch Lake fault zone which trends
through the Birch Lake PGE prospect. In one scenario, the massive sulfide veins were formed by
the downward expulsion of a basal immiscible sulfide melt into fractured and faulted footwall
rocks (Bonnichsen and others, 1980; Severson, 1994). Another scenario for the formation of the
footwall mineralization is depicted in Figure 2-7 (from a talk by Dean Peterson posted on the
Duluth Metals website at: www.duluthmetals.com )
Figure 2-6. Linear distribution of massive
sulfide, disseminated sulfide, and
copper-rich veins in the Giants Range
granitic footwall rocks beneath the
South Kawishiwi intrusion (after
Severson, 1994; taken from RI-58,
Figure 8.5); Nokomis deposit not
shown.
30
Figure 2-7: Integrated Nokomis ore deposit model showing the distribution of Cu-Ni-PGE
mineralization types relative to magma channels cut into the footwall rocks (from
presentation by Dean Peterson posted on the Duluth Metals website at
www.duluthmetals.com.
At the Water Hen area (Fig. 2-1), orthopyroxenite dikelets (structurally controlled?) within
fine-grained intrusive rocks above the basal contact contain up to >3 ppm combined Pd+Pt+Au
(Morton and Hauck, 1987). Although these occurrences are certainly not massive sulfide, most
of the orthopyroxenites are sulfide-enriched and chalcopyrite is the dominant sulfide. Severson
(1995) compares the orthopyroxenite dikelets at Water Hen to the high-grade footwall veins at the
Strathcona mine in the Sudbury Complex, Canada. In the Skibo area (Fig. 2-1), there are massive
sulfide veins, with maximum values of 11.23% Cu and 6.42% Ni that are associated with
troctolitic rocks near the basal contact (Severson, 1995).
The occurrence of local massive sulfide veins near and below the basal contact of the Duluth
Complex is an indication that larger, potentially economic footwall massive sulfide deposits may
yet be found – especially in close proximity to magma conduits. In the Sudbury Complex,
pooling of a monosulfide solid solution (mss) melt at the basal contact appears to be an important
prerequisite to the injection of fractionated sulfide melts (Naldrett, 1997). Until recently,
exploration drilling for basal mineralization rarely penetrated far into footwall rocks beneath the
Duluth Complex and therefore publically-available data are lacking on the petrochemical
attributes of footwall massive sulfides. Peterson (1997) compiled all of the available Cu-Ni data
for the lower 500' of the SKI and PRI to evaluate whether Cu-rich sulfide melts were generated
by fractionation of mss. He defined some interesting target areas that have recently been under
investigation by Duluth Metals.
31
Stratabound PGE Mineralization
PGE-enriched zones in the Duluth Complex, and related intrusions, can be classified into
two categories - stratabound and stratiform. The term “stratabound” refers to PGE-enriched
horizons that are restricted to a general stratigraphic unit; however, the PGE-enriched zones are
often transgressive relative to the enclosing stratigraphy in that they are associated with a wide
variety of internal rock types that characterize the stratigraphic unit. The term, “stratiform”
denotes specific PGE-enriched horizons that are sheet-like in form, concordant, and strictly
coextensive with laterally persistent igneous layers. Stratabound PGE horizons are located within
the lower portions of the intrusions where they are intimately associated with the Cu-Ni sulfide
mineralization and with one or more ultramafic layers that indicate magma recharge events. The
stratiform PGE horizons differ from the stratabound ones in that they are consistently sulfidepoor and tend to occur at mid-levels of well-differentiated intrusions. The origin of the two types
also differ in that stratabound PGE horizons appear to be related to magma mixing and
hydrothermal remobilization, whereas stratiform PGE horizons tend to form by the saturation of
magmas in sulfide brought on by orthomagmatic processes of fractional crystallization,
decompression due to magma venting, cumulus phase changes, as well as magma recharge.
Stratabound PGE-enriched horizons, with low to moderate sulfide concentrations (0.051.0 wt.% S), are commonly associated with ultramafic layers in the NorthMet, Nokomis, Mesaba,
Wetlegs, and Birch Lake deposits (Fig. 2-1). Elevated Cu and PGE concentrations at the
NorthMet deposit, and to a lesser extent the Mesaba and Wetlegs deposits, occur at the extreme
top of Unit I immediately beneath a laterally persistent ultramafic layer. At NorthMet, this
stratabound horizon (red horizon of Geerts, 1991, 1994) averages about 10 meters thick and
contains an average of 1.0 ppm Pd+Pt. Recent work by Theriault and others (1997, 2000)
suggests that the sulfur was largely derived from the mafic magma and that this stratabound
horizon was formed as a result of magma mixing. Two similar stratabound PGE-enriched
horizons, related to laterally discontinuous ultramafic layers, are occur toward the middle of Unit
I at NorthMet (orange and yellow horizons of Geerts, 1991; 1994). To the west of NorthMet, the
stratabound PGE horizon at the top of Unit I is also present at the Wetlegs and Wyman Creek
deposits. There however, the overall Pd content in this horizon exhibits a definite decrease in an
east-to-west direction suggesting that as the magma was intruded it became progressively
impoverished with respect to PGE (Severson and Hauck, 1997; Theriault and others, 1997).
PGE-enriched stratabound horizons associated with ultramafic layers situated well above the
basal contact also occur at the NorthMet and Wetlegs deposits (Geerts 1991, 1994; Severson and
Hauck, 1997). Both of these occurrences span across severals igneous units that include Units III
through VII. Further to the south in the Water Hen area, one drill hole intersected a layered
ultramafic package that contains a 20 cm thick semi-massive oxide (chromium titanomagnetite)
layer assaying 780 ppb Pt. This occurrence is also positioned well above the basal contact. A
similar semi-massive oxide occurrence, associated with ultramafic layers, with anomalous PGE
was also cut by a drill hole in the Fish Lake area (Fig. 2-1) located near the base of the Duluth
layered series (Sassani, 1992; Severson, 1995).
Another example of a PGE stratabound horizon is at the Birch Lake PGE prospect within the
SKI. There, PGE contents as high as 9 ppm Pd+Pt, and Cr2O3 contents locally as high as 10
wt.%, are associated with a wide variety of rock types within the U3 Unit. The U3 Unit consists
of alternating troctolitic and ultramafic layers in which variably sulfide-mineralized zones and
discontinuous pods of Cr-bearing massive oxide both occur (Severson, 1994). The massive oxide
pods are interpreted, based on empirical relationships, to have been produced by assimilation and
partial melting of the footwall Biwabik Iron Formation. This oxide-rich partial melt may have
initially acted as a trap that concentrated Cr and Ti, and through further assimilation and
contamination of the magma, may have led to precipitation of PGE. However, because the
ultramafic layers of the U3 Unit are interpreted to record new influxes of more primitive magma
32
(Severson, 1994), magma mixing may have played a more significant role in PGE mineralization.
Stable and radiogenic isotope data suggest that both magmatic and footwall contamination
processes were active at Birch Lake (Hauck and others, 1997a).
Figure 2-8. Schematic diagram showing the possible role that the Birch Lake fault may have
played in funneling upward-moving Cl-rich solutions and the resultant reconcentration of
significant magmatic PGE within the U3 Unit at the Birch Lake PGE area (after
Severson, 1994; taken from RI-58, Figure 8.8).
Furthermore, the presence of Cl-rich drops on the surface of drill core from the Birch Lake
area suggests that a hydrothermal model of concentrating the PGE could also be invoked. A
model involving ascending Cl-rich hydrothermal fluids, depicted in Figure 2-8 and similar to a
model proposed by Boudreau and McCallum (1992), may have remobilized and further
concentrated the PGE along the northeast-trending Birch Lake fault (Severson, 1994). A possible
explanation, depicted in Figure 2-8, is that the Birch Lake area represents an area where there was
a local increase in the amount of upward-moving Cl-rich solutions that were concentrated, or
funneled, along the Birch Lake fault. Late granophyre bodies, and footwall massive sulfide veins
(Fig. 2-6), situated preferentially along the fault zone imply that the fault was repeatedly
reactivated during emplacement of the SKI (Severson, 1994; Hauck and others, 1997b).
Another explanation is that the Birch Lake Fault may have initially served as a subsidiary
feeder zone to the South Kawishiwi intrusion (Hauck et al., in prep.). According to their model,
uncontaminated PGE-enriched magmas were vented in the Birch Lake area. Upon mixing with
the resident magma, which was contaminated due to interaction with the Biwabik Iron Formation,
PGEs were deposited in zones of turbulent mixing. As the magma migrated away from the vent
area (up-dip?) it became progressively impoverished with respect to PGEs.
Dahlberg (1987) suggested that PGE-mineralizing fluids at the Birch Lake area were
structurally controlled not by the Birch Lake fault but by inferred NW-SE crosscutting structures
that are evident in the regional gravity and aeromagnetic data. In summary, the mineralization
style in the U3 Unit at the Birch Lake area is still poorly understood and appears to be related to a
magmatic process, coupled with footwall contamination, that was later modified by hydrothermal
activity along either a NE- and/or NW-trending fault zone. Exploratory drilling is ongoing at
Birch Lake and concepts pertaining to the origin of the PGE are constantly changing as new
information is collected.
33
Oxide Ultramafic Intrusions (OUI)
Exploratory drilling for Cu-Ni mineralization encountered several OUIs, that later were
evaluated for their Fe-Ti±V potential. Many of the OUIs are expressed as aeromagnetic highs,
commonly with an associated electromagnetic conductor, and thus they were initially drilled in
search of conductive sulfide mineralization. At least thirteen OUIs have been intersected in drill
holes along the basal contact of the Duluth Complex (many are shown in Fig. 2-1).
The OUIs are plugs or pipe-like bodies that commonly have irregular apophyses. They
intrude troctolitic rocks of the Partridge River, Western Margin, and Boulder Lake intrusions.
The Water Hen OUI appears to be rootless as defined by detailed drilling (see Fig. 8-14,
RI-58); the three-dimensional configurations of the other OUIs are unknown due to insufficient
drilling (Hauck and others, 1997b). In general, the OUIs are spatially arranged along linear
trends suggesting that structural control was important to their genesis. All of the OUIs are crosscutting with the exception of the OUI at Boulder Creek (Fig. 2-1) which is roughly stratabound
with a cross-cutting feeder zone. Rock types include coarse-grained to pegmatitic clinopyroxenite, dunite, peridotite, melatroctolite, and minor melagabbro; all rock types are oxide-bearing.
Some OUIs exhibit a crude zonation from an olivine-rich core (dunite, peridotite, melatroctolite)
to an outer clinopyroxenite margin, whereas others consist of only one dominant rock type.
Oxide content is variable in the OUIs and ranges from 15% in disseminated zones to 100%
in localized massive oxide zones. OUIs that contain thick intervals of massive oxide include
Longnose (up to 30 meters thick (Linscheid, 1991)) and Section 34 (up to 40 m thick (Severson,
1995)). Titanomagnetite is dominant in some of the OUIs whereas ilmenite is dominant in other
OUIs. Sulfide minerals (predominantly pyrrhotite) are ubiquitous in all the OUIs and range from
trace amounts to 5% in disseminated zones to >70% in localized net-textured and massive sulfide
zones, as at Water Hen, Boulder Lake South, and Fish Lake (Fig. 2-1).
Core Logging Exercise
Drill Core Display
A single drill hole (number 05-420C) from the NorthMet deposit is on display for you to
log and observe all of the typical units of the Partridge River intrusion. This hole is collared in
the base of Unit VII and intersects all of the PRI units including the main marker unit – Unit III,
as well as, the footwall Virginia Formation and Biwabik Iron Formation. This hole also intersects
“typical” disseminated ore zones including:
• Unit I with a stratabound PGE-bearing horizon at the top and decreasing Cu-grades with
depth; and
• The Cu- and PGE-enriched “Magenta Zone” which uniquely (and mysteriously) crosscuts the stratigraphy (present in Units VI and V in this particular hole).
Note that many of the features displayed in the drill core are not visible at the present erosional
surface due to the easily weathered nature of the sulfide-bearing ores. Thus, the availability of
core from the NorthMet deposit is fortuitous and PolyMet Mining Corp. is gratefully
acknowledged for allowing the core to be displayed.
Participants will log this drill hole as if it were the tenth core hole put down in an area
where the igneous stratigraphic section is well known from historical holes. Each of the units of
the PRI is briefly described below in a “type section” to be utilized during core logging.
Representative drill core pieces, from a skeletonized hole, will also be on display for comparison
purposes. Assay results for this hole are included and participants should note the amount and
type of sulfides present in the core and compare their observations to the assay values.
34
Note that actual definition of the PRI igneous units is largely dependant on recognizing
Unit III as a major marker interval and then counting upward and downward appropriately from
this unit in order to define each of the other units.
Most of the logging campaigns along the western margin of the Duluth Complex drill
have used the classification system of Phinney (1972), as depicted in Figure 2-9 (for good or
bad), and the visually estimated proportions of plagioclase, olivine, and augite present in the drill
core.
PLAGIOCLASE
Anorthosite
A, An
90
Troctolitic Anorthosite
Gabbroic Anorthosite
TA, TAn
GA, GAn
80
Anorthositic Gabbro
Anorthositic Troctolite
AG, AnG
AT, AnT
70
T
Agt
e
vin
bro
Gab
Au
Tro gite
cto
lite
Oli
Troctolite
50
Gabbro
G, Gab
OG
Melatroctolite
Melagabbro
Mt
MelaGab
(Picrite)
Pic
20
Feldspathic Peridotite
Feldspathic Pyroxenite
FP, Feld Per
10
D
Pyroxenite
Peridotie
Dunite
OLIVINE
Pyrox
P, Per
CLINOPYROXENE
Rock Classification Scheme (after Phinney, 1972)
Figure 2-9: Rock classification scheme (after Phinney, 1972).
Unit VII
Unit VII consists of a texturally-homogeneous medium-grained troctolite, or a
plagioclase-olivine cumulate with minor amounts of ophitic to subophitic augite. This unit is
visibly very similar to the underlying Unit VI relative to the modal percentages of minerals
present, grain size, and an overall homogeneous texture. However, these two units are separated
by a serpentinized ultramafic layer, or series of close-spaced ultramafic layers (melatroctolite and
peridotite), that mark the base of Unit VII. Unit VII is known to locally contain elevated PGEs
associated with the Magenta Horizon in various drill holes in the NorthMet and Wetlegs deposits
(but not in this particular hole).
Unit VI
Unit VI also consists of a texturally-homogeneous medium-grained troctolite (as
described above). In turn, this unit also contains a basal serpentinized ultramafic layer
(melatroctolite to peridotite), or several closely-spaced layers. The ultramafic layer that defines
the base of Unit VI in this particular hole is thin and could easily be overlooked. Cu-PGEenriched mineralization associated with the Magenta Horizon is present within Unit VI of this
hole.
35
Unit V
A texturally-homogeneous, medium- to coarse-grained, anorthositic troctolite
characterizes Unit V. This unit is generally more plagioclase-rich and coarser-grained than
overlying units VI and VII but exceptions are always present. The bottom contact of Unit V is
gradational into Unit IV, and the top contact is sharp against an ultramafic horizon that marks the
base of overlying Unit VI. The Cu-PGE-enriched Magenta Horizon crosses the contact and is
present in Unit V as well as Unit VI.
Unit IV
Unit IV is characterized by thick intervals of texturally-homogeneous troctolite and/or
augite troctolite. In the nearby Mesaba deposit, Unit IV grades upward into a persistent zone of
augite-rich augite troctolite and olivine gabbro. At its base, Unit IV has a semi-persistent
ultramafic horizon that contains one or more melatroctolite and/or peridotite layers. Unit IV
generally exhibits a highly gradational upper contact with Unit V - in some areas, the contact
between Units IV and V is difficult to determine and is chosen arbitrarily.
The base of Unit IV is generally based on three criteria, of which, all or only one are
present in any particular drill hole. These criteria include: 1) presence of a pegmatitic olivine
gabbro interval that varies from a few inches to a few feet thick; 2) presence of a semi-massive
oxide band, or wisps, less than one foot thick that is characterized by small chromium
titanomagnetite dots (<0.5 mm) that are poikilitically enclosed in plagioclase and/or olivine); and
3) appearance of a semi-persistent ultramafic horizon.
Unit III
Unit III is the most distinctive "marker bed" of the PRI at the NorthMet deposit. This
unit is fine-grained (2-3 mm plagioclase laths) and is characterized by troctolitic anorthosite and
anorthositic troctolite that locally grade to troctolite and augite troctolite. In all instances, the
rock consistently grades to plagioclase-rich and olivine-rich patches that give the drill core an
overall mottled texture. This mottled appearance is due to very coarse (up to 3.0 cm) olivine
oikocrysts that are irregularly distributed throughout the rock. The mottled-texture and finegrained nature make Unit III unique relative to all the other units of the PRI. It is readily
recognizable in drill core and easily correlated between drill holes. In some holes Unit III
contains internal zones wherein the olivine is cumulus rather than oikocrystic. These zones are
present as both gradational zones and as cross-cutting zones. A thick hornfelsed inclusion of the
Virginia Formation is present in approximately the middle of Unit III in this particular hole.
Unit II
Unit II is characterized by sulfide-poor, texturally-homogeneous, medium-grained
troctolite that locally grades to augite troctolite and anorthositic troctolite. Minor hornfels
inclusions and sulfide-bearing zones are present in some drill holes. Toward its base, Unit II
grades into a persistent ultramafic horizon characterized by melatroctolite, peridotite, and/or
olivine-rich (40%-50% olivine) troctolite. At its base, the ultramafic horizon is in sharp
subhorizontal contact with the underlying Unit I.
Unit I
The lowest troctolitic unit of the PRI consists of intermixed troctolite and augite troctolite
that locally grade to olivine gabbro. Most of the unit is sulfide-bearing. Augite troctolite is
generally the dominant rock type in the bottom half of Unit I, but it is also locally common in the
top half of the unit. Unique to Unit I are extreme variations in modal mineral percentage and
average grain size; both change rapidly over zones from a few feet to tens of feet thick. Due to
this heterogeneous texture, numerous internal contacts divide Unit I into several subunits that
36
cannot be correlated from drill hole to drill hole. Thus Unit I is a mixture of various troctolitic
subunits that are probably related to continuous magma replenishment.
Hornfels inclusions of Virginia Formation are most commonly present within Unit I. The
inclusions vary from one inch to over 275 feet thick. Rock types are the same as those present in
the footwall rocks beneath the basal contact. Near the basal contact and surrounding hornfels
inclusions, the intrusive rocks of Unit I have undergone sufficient silica contamination, and norite
and gabbronorite are often the dominant rock type.
The thickness of Unit I is variable and ranges 300-600 feet thick. The position of the top
contact of Unit I in the PRI (both NorthMet and Mesaba deposits) is generally based on four
criteria, of which, all or only one are present in any particular drill hole. These criteria include:
1) decrease in sulfide content upwards into sulfide-free rocks of Unit II; 2) presence of a PGEenriched horizon at the top of this unit (commonly contains Pd values in excess of 1 ppm and Cu
values in excess of 0.50%); 3) presence of semi-persistent large hornfels inclusion of Virginia
Formation (present mostly along the south-eastern edge of the Mesaba deposit in the Local Boy
area); and 4) appearance of a semi-persistent ultramafic horizon (base of Unit II). However, in
some instances the top of Unit I is not as apparent and the contact "pick" is based on where the
known top is in surrounding drill holes.
Virginia Formation
The Virginia Formation is a thick sequence of argillite, siltstone, and graywacke at the
top of the Animikie Group. In close proximity to the Duluth Complex, the Virginia Formation is
described as being a hornfels that, as defined by Turner (1968), is a nonfoliated rock composed of
a mosaic of equidimensional grains. In reality, many of the “hornfels” textures exhibited by the
metamorphosed Virginia Formation do not meet this criterion, but as the term has been widely
used in descriptions of the Virginia Formation in the vicinity of the Complex, it has been retained.
Mineral assemblages in the hornfels, in both the footwall and in inclusions within the Complex,
consist of varying mixtures of cordierite, quartz, K-spar, and biotite with lesser amounts of
chlorite, muscovite, plagioclase, orthopyroxene, and minor graphite and sulfides.
Furthermore, as the grade of metamorphism and associated deformation progressively
increase towards the Complex several metamorphic textures are superimposed on the original
sedimentary package. The effects of partial melting are profound and portions of the hornfelsed
Virginia Formation no longer even remotely resemble a sedimentary rock. Severson et al. (1994)
subdivided the hornfelsed Virginia Formation, in both the footwall and in inclusions within the
Duluth Complex, into at least five informal units based largely on metamorphic attributes, which
are each related to varying degrees of partial melting. These members, and a pre-Duluth
Complex sill, are described below and are schematically portrayed in Figure 2-10.
Cordieritic hornfels
Directly beneath the basal contact of the Duluth Complex, the adjacent Virginia
Formation typically consists of massive/nonfoliated, cordierite-rich hornfels that display a bluishgray color in drill core. The rock is generally fine-grained, granoblastic, and biotite-poor (due to
loss of water into the Complex) and locally may contain porphyroblastic and/or poikiloblastic
cordierite. Original bedding planes are preserved in some localities, but mostly the bedding
planes have been obliterated by contact metamorphism.
37
Duluth Complex
Du
luth
Hornfels Inclusion
Co
mp
lex
Virginia Fm - Cordieritic Unit
Virginia Fm - Rxtal Unit
Virginia Fm - Disrupt Unit
Virginia Fm - BDD PO/Graphitic argillite
Virginia Fm - Bedded
Cal-silicate/delaminated BIF
VirgSill - Massive Gray Unit
VirgSill - coarse-grained interior
BIF - Submember A (marble/chert)
BIF S
ubme
mber
BIF S
D
ubme
mb e r
s E th
rough
W
BIF - Submember B (diopside/chert)
BIF - Submember C (thin-bedded)
BifSill
Figure 2-10: Schematic diagram showing the general relationships of the metamorphosed
footwall rocks beneath the Duluth Complex at the NorthMet, Mesaba, Wetlegs, Wyman
Creek, and Serpentine deposits.
Recrystallized unit (RXTAL)
Beneath the cordieritic “capping” the next metamorphic variant of the Virginia Formation
nearest to the Duluth Complex is a rock that is referred to as the RXTAL unit. This rock type is
characterized by fine- to medium-grained cordierite, plagioclase, biotite, quartz, and K-spar with
lesser amounts of Opx and opaques. Bedding planes of the original argillaceous rocks are
obliterated and what remains is a massive recrystallized rock with decussate biotite that contains
enclaves (blocks and folded boudins) of more structurally competent calc-silicate hornfels and
thin-bedded siltstone. Also, variably sized patches, or relicts, of the DISRUPT unit (see below)
and the cordieritic unit are commonly found scattered throughout the recrystallized unit.
Disrupted unit (DISRUPT)
With increased distance from the Complex, the RXTAL unit progressively grades into
the DISRUPT unit which is a thin-bedded rock that is visibly deformed and underwent less
degrees of partial melting. Textures that characterize the DISRUPT unit are bedding planes that
are extremely chaotic and random in orientation due to pervasive small-scale folding, faulting,
and brecciation. Superimposed on this chaotic pattern are abundant zones of leucocratic partial
melts that are also chaotic and folded. The rock consists of varying amounts of quartz, cordierite,
K-spar, biotite, plagioclase, and muscovite with luecosome veins and patches containing quartz,
K-spar (microperthite), plagioclase, and muscovite.
Graphitic argillite and Bedded Pyrrhotite (BDD PO) units
Carbonaceous argillite of the Virginia Formation is commonly preserved as either the BDD PO
unit, or graphitic argillite, in close proximity to the Duluth Complex. This rock commonly
contains over 5% disseminated pyrrhotite and/or extremely thin-bedded pyrrhotite laminae
(hairline-thick), and variable amounts of graphite, staurolite(?) and sillimanite. Wherever the unit
contains conspicuous and regularly-spaced laminae of pyrrhotite (0.5-3.0 mm thick at 1-20 mm
spacings) it is informally referred to as the bedded pyrrhotite unit (BDD PO unit). Partial melting
of the graphitic argillite and BDD PO is evidenced by the common occurrence of white to pink
38
“granitic” veins, wisps and patches. Bedding in both rock types is more than often highly
contorted.
Calc-silicate hornfels
Commonly present within the RXTAL unit are discontinuous beds and disjointed pods
(boudins?) of white- to light gray-colored calc-silicate hornfels. These pods themselves show a
wide variation in mineralogy consisting of: diopside, wollastonite, plagioclase/anorthite, grossular
garnet, idocrase, and sphene.
VirgSill
The VirgSill is generally present in the bottom 0.5-130 feet of the Virginia Formation but
is not present over much of the NorthMet deposit (or in the hole being examined). When present
the VirgSill ranges from a few inches thick to 197 feet thick. Identification of the VirgSill in drill
core is hampered by the fine-grained granoblastic texture of the sill that makes it difficult to
distinguish from the enclosing hornfelsed Virginia Formation rocks; both were metamorphosed
by the Duluth Complex. Although no easy task, there are subtle characteristic features that help
in the recognition of the VirgSill in drill core that have been described in detail by Severson and
Hauck, 2008.
Biwabik Iron Formation
The Biwabik Iron Formation (BIF) on the nearby Mesabi Range has typically
been subdivided into four informal lithostratigraphic members (Wolff, 1917) that are, from the
bottom up: Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. At the eastern end of the
Mesabi Range, the Biwabik Iron Formation is further divided by the mining industry into at least
22 informal submembers (Gundersen and Schwartz, 1962). The top three submembers are
commonly drilled beneath the Cu-Ni deposits and are described below (see also Figure 2-10)..
BIF Submember A – Chert and Marble
At the top of the Upper Slaty member is an interbedded chert and marble unit that is
usually 3-6 feet-thick. Both the chert and marble layers are white, fine- to medium-grained, and
commonly alternate in 0.5-3.0 foot thick bands. Rarely and very locally, the marble beds contain
fine-grained magnetite. Andradite garnet is also locally present.
BIF Submember B – Chert and Diopside
The B submember of the BIF is typically 13-20 feet-thick and consists of interbedded
white chert beds and moderate-green, diopside-rich beds. Bedding planes between the two
varieties are extremely irregular, forming bands that vary from 1 inch to over 1 foot thick. White
marble beds may be locally present within the top few feet of submember B. The basal contact
with submember C varies from sharp to gradational.
BIF Submember C – Thin-bedded Taconite
This unit is the uppermost submember of the BIF that contains appreciable amounts of
magnetite, though it is not processed for its iron content in the taconite mines. Submember C is
characterized by thin-bedded (<2 mm), green, black, and gray beds of iron silicates, magnetite,
and chert, respectively. Gundersen and Schwartz (1962) report that the green silicate beds consist
chiefly of ferrohypersthene with lesser amounts of hedenbergite, fayalite, and cummingtonite;
hisingerite veinlets are locally present.
39
Assay Results for Hole 05-420C
40
41
42
43
44
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