Igneous Stratigraphy and Mineralization in the Basal Portion of the Partridge River Intrusion, Duluth Complex, Allen Quadrangle, Minnesota

Abstract

Geologic mapping of the igneous rocks of the Partridge River intrusion (PRI) within the Allen Quadrangle was conducted during the 1996-1997 period by Severson and Miller (1997). In addition to the surface geology, drill holes were logged in order to gain a better understanding of the third-dimensional geology and Cu-Ni mineralization of the quadrangle. Drill hole correlations of igneous units, along with cross-sections and a discussion of Cu-Ni±PGE enriched zones, are presented in this report. Detailed logging of drill core from over 400 drill holes indicates that there are at least seven major igneous units (Units I through VII) within the basal 3,000 feet of the PRI. These units, starting with Unit I at the base, are described in Severson and Hauck (1990) and are reiterated herein for drill holes within the Allen Quadrangle. Also present within the quadrangle and described in this report are late plug-like bodies of Oxide-bearing Ultramafic Intrusions (OUI), a Keweenawan sill within the Biwabik Iron-Formation (K-Sill), and a small Keweenawan(?) intrusive within the Archean rocks near a major fault zone. Establishment of an igneous stratigraphy for the PRI provides a framework that can be used to determine the nature and location of fault offsets between drill holes. The tenor of Cu-Ni mineralization is often locally increased along some of these faults. Fault zones, with or without increased mineralization, that are discussed in this report include faults in the Wetlegs Cu-Ni deposit, faults in the Wyman Creek Cu-Ni deposit, the Siphon Fault, and the Donora Fault. Many of these fault zones are depicted on the cross-sections that accompany this report. Enrichments in Platinum Group Elements (PGE) occur at several stratabound horizons within the Allen Quadrangle (using an arbitrary cutoff of >90 ppb Pd). All of these PGE-enriched horizons are present in sulfide-bearing troctolitic rocks that are positioned immediately below, and in some cases within, laterally continuous ultramafic layers. However, the overall Pd content in some of the horizons shows a definite decrease in an east-to-west direction. This trend suggests that as the magma of the PRI was intruded, in an east-to-west direction(?), it became progressively impoverished with respect to PGEs.