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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). 27 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). 28 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 References Bonnichsen, B., Fukui, L.M., and Chang, L.L.Y., 1980, Geologic setting, mineralogy, and geochemistry of magmatic sulfides, South Kawishiwi Intrusion, Duluth Complex, Minnesota: Proceedings, IAGOD Symposium, 5th Stuttgart, Germany, p. 545-565. 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(with plates) Theriault, R.D., Barnes, S.-J., and Severson, M.J., 1997, The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu-Ni-platinum-group element deposit, Duluth Complex, Minnesota: Canadian Journal of Earth Science, v. 34, p. 375-389. Theriault, R.D., Barnes, S.-J., and Severson, M.J., 2000, Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: Economic Geology, v. 95, p. 929-943. Turner, F.J., 1968, Metamorphic Petrology – mineralogical and field aspects: McGraw-Hill Book Company, New York, 403 p. Wolff, J. F., 1919, Recent geologic developments on the Mesabi range, Minnesota: American Institute of Mining and Metallurgical Engineers Transactions, v. 56, p. 142-169. Zanko, L.M., Severson, M.J., and Ripley, E.M., 1994, Geology and mineralization of the Serpentine copper-nickel deposit, Duluth Complex, Minnesota. 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