Geology of the Southern Portion of the Duluth Complex

Abstract

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.