Browsing by Subject "Oxide-bearing Ultramafic Intrusions"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Geology of the Southern Portion of the Duluth Complex
    (University of Minnesota Duluth, 1995-12) Severson, Mark J
    The Duluth Complex (Middle Proterozoic - 1,099 Ga) is a large intrusive body that contains numerous smaller intrusions that collectively comprise the Complex. Recent work has shown that igneous stratigraphic sections can be delineated within these intrusions through detailed relogging of drill core, e.g., for the Partridge River intrusion (Severson and Hauck, 1990; Severson, 1991) and the South Kawishiwi intrusion (Severson, 1994). This report pertains to the igneous geology of the South Complex area. More than 140 drill holes are located in the "South Complex" area. Most of these holes are relogged (112 holes, 88,000 feet of core) and are correlated into several troctolitic to gabbroic stratigraphic units for several specific areas in the South Complex that have abundant drill holes. While each individually drilled area exhibits good correlative units, these correlative units do not extend into an adjacent drilled area that is located only a few miles distant. This lack of large-scale continuity suggests: 1) the South Complex study area constitutes an area that actually includes several smaller intrusive bodies; 2) drilling is not detailed enough to delineate large-scale correlative units; 3) because most of the drill holes are located close to the basal contact, the effects of contamination to the magma, via assimilation of footwall rocks, hampers large-scale correlations; or 4) combinations of the above. Most of the holes within the South Complex were drilled during exploration for Cu-Ni sulfide mineralization. Only weak sulfide mineralization is present in these drill holes. However, many of the holes intersect small plug-like bodies of Oxide-bearing Ultramafic Intrusions (OUIs) that are intrusive into the troctolitic rocks of the Complex. The OUIs are characterized by coarsegrained to pegmatitic clinopyroxenite, picrite, peridotite, and dunite. Oxide content in the OUI varies from disseminated (15%-20%) to thick massive oxide zones. Ilmenite is the dominant oxide in some OUIs; whereas, titanomagnetite is dominant in others. In almost all instances, the OUIs are spatially arranged along linear trends, suggesting that structural control was important to their genesis. At some localities (northern end of the South Complex), an empirical link between ironformation assimilation near the basal contact and OUI formation is apparent. This relationship suggests that the OUIs were initially formed at depth followed by upward injection of OUI material along fault zones. However, other OUI (southern end of the South Complex) are situated within, or immediately below, layered oxide-rich gabbroic rocks, suggesting that the OUIs formed from a differentiated iron-rich melt that drained down into the cumulate pile along fault zones. These two different OUI groups (north and south) also show some corresponding differences in chemistry. The north OUIs are characterized by relatively higher chromium contents and the south OUIs have relatively higher vanadium contents. All of the OUIs contain titanium mineralization and some sulfide mineralization. A model of origin for the OUIs involving metasomatic replacement of preexisting igneous rock is not considered to be plausible. Also present within the South Complex area are fine-grained granular rocks that are hornfelsed inclusions of basalt and troctolitic-gabbroic-noritic rocks. One of these inclusions, referred to as the FN Unit, is only observed in drill holes in the southern half of the South Complex area. The unit exhibits vesicle-like features in drill core and has often been referred to as a hornfelsed basalt. However, several features argue against a basalt protolith for the FN Unit. These features include the presence of abundant footwall hornfels inclusions within the unit, common gradations into medium-grained intrusive rock, and a "rind-like" overall pattern of the unit at the basal contact at Water Hen. These characteristics suggest that the FN Unit represents an earlier pulse of magma (chilled?) into the footwall rocks that was later hornfelsed by subsequent intrusions of the Complex. The Bear Lake Inclusion, present in numerous outcrops and one drill hole, probably represents a large inclusion of magnetic basalt. The inclusion is a massive rock with no distinct volcanic features, but is similar to magnetic basalt inclusions described elsewhere in the Complex (Colvin Creek Inclusion, and "INCL" unit within the South Kawishiwi intrusion; Severson and Hauck, 1990; Severson, 1994; Patelke, 1996). The Bear Lake Inclusion is over 500 feet thick and dips gently to the southeast. It is located well into the interior of the Complex and is not related to the basal contact (as is the FN Unit). Geochemical plots are constructed for many of the igneous units of the South Complex area. These plots are not particularly instructive in discriminating between the units because many of the spider profiles are fairly similar, and in the X-Y plots only a few units cluster within distinct fields. However, some conclusions can still be drawn from these data. First, similarities in geochemistry indicate that some units of the nearby Partridge River intrusion are present as far south as Water Hen. Second, the FN Unit is chemically similar to both troctolitic to gabbroic rocks, even in the same drill hole. This relationship supports an earlier intrusive protolith rather than a basalt protolith. Third, the north and south OUI can be separated into two groups based on similarities in spider diagram profiles. However, the profiles for the north OUI show similar profiles that alternate with geographic location. The reason for this "leap frog" alternation in profiles is unknown at this time, but may be related to more than one OUI-forming event along a fault zone. Last, rocks of the Bear Lake Inclusion are chemically similar to rocks of the Colvin Creek inclusion (Severson & Hauck, 1990; Patelke, 1996) and the "INCL" unit of the South Kawishiwi intrusion (Severson, 1994); all of which have been inferred to be magnetic basalts. A sample of a semi-massive oxide horizon (0.8 ft. thick), associated with subhorizontal, ultramafic layers (picrite, peridotite, etc.) near the Water Hen area (drill hole SL-19A) has been found to contain anomalous PGE and chromium values (Pt = 737-786 ppb, Pd = 63-106 ppb, Cr = 46,000 ppm). This semi-massive oxide horizon is similar in many respects to PGE- and Cr-enriched semi-massive to massive oxide horizons located elsewhere within the Duluth Complex (Birch Lake and Fish Lake areas). The data suggest that the PGE in SL-19A are magmatic and have not been redistributed by hydrothermal fluids, as has been suggested for other areas within the Complex. Additional targets of vein-like PGE-enriched Cu-Ni ore are also present in the Skibo and Water Hen areas. These targets could potentially have formed via fractional crystallization of a sulfide melt in a vein-like setting.
  • Thumbnail Image
    Item
    Geology, Geochemistry, and Stratigraphy of a Portion of the Partridge River Intrusion
    (University of Minnesota Duluth, 1990-03) Severson, Mark J; Hauck, Steven A
    Detailed relogging of drill holes (83 holes totalling 100,630 feet of core) and reconnaissance mapping have delineated three major rock groups within a portion (T.58-59 N., R.13-14 W.) of the Partridge River intrusion (PRI), Duluth Complex, Northeastern Minnesota. These have been informally designated as the Partridge River Troctolitic Series (PRTS), Partridge River Gabbro Complex (PRGC) and Oxide-bearing Ultramafic Intrusions (OUI). The PRTS consists of at least eight major igneous units which are correlatable in drill holes over an indicated eleven mile strike length extending (NE to SW) from the Dunka Road Cu-Ni deposit to the Wyman Creek Cu-Ni deposit. From the base up, these units are characterized by: Unit I - sulfide-bearing augite troctolite with minor picrite to peridotite layers; Unit II - troctolite and augite troctolite, with abundant picrite to peridotite layers (Wetlegs Cu-Ni area) and/or minor sulfide-bearing zones; Unit III - mottled textured anorthositic troctolite exhibiting a highly irregular olivine oikocryst distribution; Unit IV -augite troctolite with a picritic base and grading upwards into Unit V; Unit V - coarse-grained anorthositic troctolite; Unit VI - augite troctolite to anorthositic troctolite with a picritic base; and Unit VII - augite troctolite with a well-bedded peridotite-picrite base. Field mapping suggests that an eighth unit (Unit VIII) and possibly additional units are present above Unit VII. Unit VIII consists of troctolite to anorthositic troctolite with a well-bedded peridotite base. Most of the upper units (III-VIII) represent single cooling units in that they are floored by a bedded ultramafic member; whereas, other units (I and II) near the footwall exhibit an overall heterogeneous nature and contain abundant internal members reflecting continuous magma replenishment. Some of the units also exhibit downcutting relationships and lateral "facies" changes along strike indicating a complex intrusive history. Structural studies of the basal contact of the Partridge River intrusion have indicated more structure than previously recognized. Structure contour maps of the footwall rocks at the basal contact of the Duluth Complex and on the top of the Biwabik Iron-Formation, and isopach maps of the Virginia Formation beneath the PRI indicate that pre-existing folds in the basement rocks at both Minnamax and Dunka Road exerted a strong control over the form of the base of the intrusion. Cross-sections illustrating the internal "stratigraphy" indicate that in both the Dunka Road and Wetlegs areas, numerous NE-trending normal faults parallel to the Mid-continent Rift are present. These faults support the halfgraben model (Weiblen and Morey, 1980) which envisions a step-and-riser geometry at the base of the Duluth Complex due to extensional tectonics. However, most of the faults delineated show corresponding offsets in both the troctolitic and footwall rocks and are, thus, not true half-graben faults as envisioned in the model. The only exception is within the Wetlegs area where a NE-trending fault exhibits substantial offset in the footwall rocks, but no offset is present in the overlying troctolite rocks. An inferred window of Biwabik Iron- Formation is in direct contact with the PRI along this fault. Three late-stage Oxide-bearing Ultramafic Intrusions (OUI) are also located along this zone that suggests they may be genetically related to areas where massive iron-formation assimilation has occurred. The OUIs are later pegmatitic intrusives consisting of dunite, peridotite, clinopyroxenite, and lesser picrite and melagabbro; all are oxide-bearing (> 10%) and contain semi-massive to massive oxide horizons. These bodies are intrusive into the PRTS and include the Longnose, Longear, Section 17, Wyman Creek, and Skibo Fe-Ti prospects. The PRGC is situated at the southeastern portion of the investigated area and consists dominantly of oxide-bearing gabbroic and troctolitic rocks; both locally exhibit excellent modal bedding, which may be related to magmatic density currents. The Colvin Creek "Gabbro" (CCG) is part of the PRGC and was originally interpreted to be a hornfelsed basalt. However, reconnaissance mapping indicated that similar fine-grained CCG-type "gabbro" is present within the coarse-grained rocks of the Powerline Gabbro and vice versa. Because the Powerline Gabbro is located near the CCG, the two bodies may be intricately related. Within the Colvin Creek "Gabbro" are several unusual sedimentary-like structures that are not indicative of typical North Shore Volcanic basalts. However, textures resembling vesicles/amygdules are locally present. The unusual sedimentary-like structures suggest a magmatic density current origin but the exact origin of these textures is enigmatic. Also within the Colvin Creek "Gabbro" is a mile-long 1,000 foot-thick belt of cross-bedded rocks. Several internal features of these cross-bedded rocks, e.g., lack of rock fragments, no quartz, are not indicative of typical interflow sandstones and their relationship to the surrounding rocks suggests they may have also been deposited by magmatic density currents. The unmineralized portions of all the units were sampled (155 samples) in order to establish background geochemical levels and lithogeochemical signatures for each unit and to investigate possible origins for the different units. Background Pd, Pt, and Au values in the major rock groups average 10 ppb, 20 ppb, and 5 ppb, respectively. However, slightly elevated background values are associated with Unit II (15 ppb, 24 ppb, and 9 ppb, respectively), and the OUI rock group (15 ppb, 24 ppb, and 17 ppb respectively). In the course of sampling unmineralized rock (<1% sulfides), five anomalous samples (>200 ppb combined Pd and Pt) were revealed with a maximum of 910 ppb. The OUI units are the most geochemically unique in that they have elevated background values for TiO2, V, Cr, Co, Cu, Cd, C, Be, Sc, Sb, Pb, Te, Au, and W relative to the other igneous units. Geochemical data support the various rock units identified during relogging of the PRI. Units I and II exhibit a markedly different geochemical signature when compared to the other PRTS units. One interpretation of this difference is that magma contamination due to assimilation of footwall material was important in their genesis. All rock units of the PRGC have the same geochemical signature and, in turn, this geochemical signature is similar to the geochemical signature for the lower half of Unit I. The OUI units exhibit a markedly different geochemical signature when compared to all the other PRI units.