Geology, Geochemistry, and Stratigraphy of a Portion of the Partridge River Intrusion

Abstract

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.