Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part I: Geology

Abstract

The Minnamax Cu-Ni deposit (also called the Babbitt deposit) is situated within what has been informally referred to as the Partridge River intrusion (or Partridge River Troctolite) of the Duluth Complex (1.1 Ga), northeastern Minnesota. The deposit has been subdivided into five contiguous ore zones; the Local Boy area and Bathtub area are two ore zones described in this report. Within the deposit are a wide variety of troctolitic, ultramafic, and footwall rock types, and hornfelsed inclusions (both footwall and hanging wall). Many specific rock types are correlative between drill holes and can be grossly categorized into seven sub-horizontal troctolitic units, three types of hornfelsed inclusions, and a late cross-cutting pegmatitic phase. Also present are correlative units within the footwall rocks. All rock units were identified by detailed relogging of 61 surface drill holes (117,605 feet of core) and are portrayed on nine cross-sections that extend through various portions of the Minnamax deposit. Severson and Hauck (1990) described the stratigraphy of the troctolitic rocks of the Partridge River Troctolite to the west of the Minnamax deposit; the stratigraphy is referred to as the Partridge River Troctolite Series (PRTS). Most of the PRTS rock units defined at the Dunka Road Cu-Ni deposit (located to the immediate SW of Minnamax) by Severson and Hauck (1990) are present at Minnamax. However, the overall picture at Minnamax is more complicated than Dunka Road due to rock type changes that are manifested by: 1) pinch-out and reappearance of specific marker bed units; 2) down-strike gradational changes of ultramafic horizons; 3) extremely limited areal extent of some ultramafic horizons; and 4) gradational changes in the troctolitic rock types between drill holes. In some areas a particular marker horizon may "disappear" laterally and then reappear at the same stratigraphic level in another group of drill holes. In spite of these local difficulties, a gross stratigraphy of seven subhorizontal igneous units is present at Minnamax and consists of (from bottom to top): Unit I - heterogeneous, sulfide-bearing augite troctolite and troctolite with abundant metasedimentary inclusions; Unit II - homogeneous troctolite with a basal picrite horizon (Unit II is present only in the SW portion of the Minnamax deposit); Unit III - mottled textured anorthositic troctolite to troctolite with characteristic olivine oikocrysts (Unit III is present mainly in the SW portion of Minnamax and is enveloped by Unit I to the NE); Unit IV - mixed homogeneous troctolite and augite troctolite (augite troctolite is at the top of Unit IV in localized areas) with a semipersistent basal ultramafic horizon termed the "± picrite"; Unit V - homogeneous anorthositic troctolite that exhibits a gradational contact with Unit IV; and Units VI and VII - homogeneous troctolites with persistent basal ultramafic horizons. More abundant and thicker ultramafic horizons are present in Units VI and VII in the Bathtub area of the Minnamax deposit. Specific marker horizons utilized in drill hole correlations include: Unit III, "± picrite," "pocket picrite," top of Unit IV (augite troctolite), and the ultramafic base of Units VI and VII. The troctolitic stratigraphy is cut by pegmatitic orthopyroxenite and peridotite bodies that are referred to as OUI - Oxide-bearing Ultramafic Intrusions. Pegmatitic hybrid hornblendite and granophyre also cut the stratigraphy and are often related to the OUI bodies. Rusty chlorine-rich drops may commonly coat the core of the ultramafic horizons and OUI bodies. Several enigmatic hornfelsed inclusions are present in Units VI and VII at Minnamax. These are grouped in two categories that include: 1) CC-type inclusions that are similar to outcrops of the Colvin Creek hornfels; and 2) "pic"-type inclusions that are similar to nearby outcrops of basalt inclusions. Both inclusion types are similar in that they contain fine-grained plagioclase-filled ovoids or wisps that may represent vesicles, and they exhibit the same chemical signature. However, they exhibit a different mineralogy (the CC-type inclusions are oxide-rich). Their stratigraphic position in the troctolitic rocks suggests that they are probably hanging wall material (North Shore Volcanic Group). While these two inclusion types are readily correlative between drill holes, the nature of their different mineralogy remains unknown. Another enigmatic rock type is present within the lower portion of the Virginia Formation footwall rocks. The rock is unique in that it contains hornblende ± olivine and locally grades into serpentinized picrite with hornblende. It is generally concordant with the overall bedding of the Virginia Formation and is referred to as the sill(?) unit. Whole rock geochemistry indicates that this unit locally exhibits: high Cl contents that are similar to Cl values of ultramafic horizons in the troctolitic rocks; MG numbers that are more primitive than the ultramafic horizons; and high Cr contents that are much higher than anything sampled in the overlying troctolitic section. If the unit was a sill, it now exhibits gradational contacts with the metasedimentary rocks and is characterized by a granoblastic texture with superimposed euhedral hornblende. These data may indicate that the sill was intruded before, and hornfelsed during, emplacement of the majority of the Partridge River Troctolite Series.