Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota

Abstract

The Middle Proterozoic (1,099 Ma) intrusive Duluth Complex contains numerous smaller sub-intrusions that collectively comprise the Complex. Two of these sub-intrusions are informally known as the South Kawishiwi intrusion (SKI) and Partridge River intrusion (PRI). A correlative igneous stratigraphy has been documented in the PRI by Severson and Hauck (1991) and Severson (1991). In this investigation, detailed relogging of drill holes within the SKI (136 drill holes totalling 214,461 feet of core) also defines an intrusion-wide stratigraphic sequence along a 19-mile strike length that is referred to as the South Kawishiwi Troctolite Series (SKTS). The stratigraphic sequences of the SKI and the PRI are completely dissimilar. At least 17 correlative subhorizontal igneous units are defined within the SKTS; however, they are not equally present in all areas of the SKI. The SKTS units, from the bottom to the top (roughly), are referred to as: BAN = Bottom Augite troctolite and Norite; BH = Basal Heterogeneous troctolites (sulfide-bearing); U3 = Ultramafic Three (sulfide-bearing); PEG = Pegmatitic Unit of Foose (1984); U2 = Ultramafic Two (sulfide-bearing); U1 = Ultramafic One (sulfide-bearing); AT-T = homogeneous Anorthositic Troctolite to Troctolite; UW = Updip Wedge (sulfide-bearing); LOW AGT = homogeneous Lower Augite Troctolite zone; MAIN AGT = homogeneous Main Augite Troctolite zone; AT&T = homogeneous Anorthositic Troctolite and Troctolite; AT(T) = homogeneous Anorthositic Troctolite with lesser amounts of Troctolite; AN-G Group = intermixed Anorthositic and Gabbroic rocks; UPPER GABBRO = oxide-bearing gabbroic rocks; "INCL" = large shallow-dipping inclusion of magnetic basalt(?); UPPER PEG = Upper Pegmatitic zone; and T-AGT = Troctolite to Augite Troctolite (the latter five units are restricted to a small area referred to as the Highway 1 Corridor area).

The lowest units of the SKTS are the most varied with respect to textures, rock types, and sulfide content. They are very unevenly distributed along the strike length of the SKI in a "compartmentalized" fashion suggesting a complicated intrusive history. The lowest units were emplaced early into several restricted magma chambers via repeated and close-spaced magmatic pulses. The effects of contamination from assimilated and devolatized country rocks were the most pronounced in these units. Three ultramafic-bearing packages (U1, U2, and U3 units) are also present within the lower portion of the SKTS. These units are characterized by alternating layers of troctolitic and ultramafic (olivine-rich) rock. The ultramafic-bearing units represent periods of rapid and continuous magma injection that crystallized more primitive ultramafic layers before mixing with the resident magma. The U3 Unit is the most unique in that it contains several massive oxide (magnetite-rich) pods along its strike length. An empirical relationship between the U3 Unit and the Biwabik Iron-formation (BIF) suggests that the massive oxides were derived from intruded and assimilated BIF. The U3 Unit also contains the majority of high Platinum Group Elements (PGE) values sampled to date in the SKI.

In contrast, the upper SKTS units reflect an entirely different intrusive history. Each unit is characterized by monotonous sequences of texturally homogeneous and sulfide-free rocks. Gradational contacts are present between each of the units. In contrast to the lower SKTS units, the upper SKTS units are generally distributed throughout the SKI. Ultramafic members are restricted to only two thin horizons referred to as High Picrite #1 and #2. All of these features are indicative of a quiescent and open magmatic system. Thus, the upper SKTS units appear to have been emplaced as widely-spaced pulses into a progressively developed, single magma chamber with little interaction with the country rocks.

An entirely different package of rocks is present in six extremely deep drill holes that were drilled within the Highway 1 Corridor (H1C). The H1C represents a large inclusion of older Anorthositic Series rocks. It appears that the H1C rocks were underplated by the younger intruding SKI magmas.

Several drill holes within the PRI were also relogged to better understand the nature and location of the contact between the SKI and PRI. One important feature noted is that as the contact between the two intrusions is approached, the upper units of the PRI become heterogeneous and indistinguishable from each other. The same heterogeneity is not evident in the adjacent SKI. The presence of the "heterogenous zone" within the PRI adjacent to the SKI suggests that the PRI was intruded before the SKI. Also present near the PRI/SKI contact zone is a major north-trending fault (down to the east). Associated with this fault are voluminous amounts of steeply-inclined lenses of late granitic/felsic material that cross-cut the PRI stratigraphic section. This fault is named the Grano Fault because the late lenses consist of varied granitic to pyroxenitic material. Though the Grano Fault trends through both the PRI and SKI, the late granitic lenses are not particularly common within the SKI. This fact also suggests that the PRI is older than the SKI. Offset units within the footwall rocks suggests that movement along the Grano Fault was initiated before emplacement of the PRI.

All geochemical data pertaining to previously sampled SKI drill core is compiled and correlated with the SKTS units. An additional 80 geochemical samples collected from SKTS units in this investigation are added to this database. The grouping of the SKTS units on the geochemical plots supports the geologic correlations of this investigation. Some of SKTS units also show geochemical overlap with footwall units and indicate the effects of contamination of the magma due to assimilation of the country rocks during intrusion.

PGE analyses conducted on a multitude of rock types and igneous units within the SKTS indicate that the U3 Unit, and to a lesser extent the PEG Unit, show the most promise of hosting a PGE deposit. The PGE origin model of Boudreau and McCallum (1992) is invoked to explain why anomalous PGEs are common to the U3 Unit. The Boudreau and McCallum model envisions the upward migration of chlorine-rich, late, magmatic fluids that were capable of transporting PGEs and concentrating them at stratigraphic traps. However, a straightforward application of the model does not explain why significantly higher PGEs are restricted to certain areas, e.g., the Birch Lake deposit. A variation of the "Boudreau and McCallum model" is proposed to explain this difference. This revised model is similar except that upward-moving, Cl-rich, PGE-pregnant hydrothermal solutions are envisioned to have been concentrated in fault zones. When fluids associated with fault zones encountered a proper stratigraphic trap (ultramafic horizon), more PGEs were deposited relative to areas outside of the fault zones. An intersection of the proper stratigraphic trap (U3 Unit with massive oxides, sulfides and high Cr contents) and the proper channelway to concentrate the PGE-pregnant Cl-rich solutions (Birch Lake Fault) reasonably explains the significant PGE values in the Birch Lake deposit. The Birch Lake Fault is defined by a zone wherein drill holes commonly encountered either: massive sulfide mineralization within the footwall granitic rocks; and/or "voluminous" amounts of late granitic/felsic lenses that cut the troctolitic rocks of the SKI.