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Item Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part I: Geology(University of Minnesota Duluth, 1991-06) Severson, Mark JThe 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.Item Geology, Mineralization, and Geostatistics of the Minnamax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota: Part II: Mineralization and Geostatistics(University of Minnesota Duluth, 1991-06) Severson, Mark J; Barnes, Randal JThe Minnamax/Babbitt Cu-Ni deposit, located within the Partridge River Troctolite Series (PRTS) of the Duluth Complex, northeastern Minnesota, contains both troctolite-hosted disseminated ore and footwall-hosted massive sulfide ore. This report pertains to the massive sulfide ore zone, which is restricted to a small portion of the deposit, and is referred to as the Local Boy area. Studies conducted in the Local Boy area include: 1) detailed geologic relogging of drill core; 2) sulfide petrography and microprobe analysis; 3) assaying for Pt, Pd, Au, and Ag in the high-grade Cu ore zones; and 4) geostatistical analysis of the Cu-Ni ore (plus PGEs and precious metals). Detailed relogging of 76 underground drill holes, along with pertinent surface drill holes, has been completed within the Local Boy area (from drifts B, C, and D). The data indicates the highly undulatory nature of the basal contact of the Duluth Complex with the footwall Virginia Formation. Intrusive rocks of the Duluth Complex (Unit I of the PRTS) consist of augite troctolite, troctolite, and norite. All exhibit gradational contacts with each other, and all may occur at any stratigraphic position relative to the undulatory basal contact. However, norite is the most common rock type adjacent to sedimentary hornfels inclusions and at the basal contact due to contamination of the magma. The spatial configuration of the intrusive rocks indicates that Unit I was intruded as multiple pulses along bedding planes of the Virginia Formation. The Virginia Formation hosts the majority of the massive sulfide ores that are present within hornfels inclusions positioned above the basal contact, and within the footwall rocks at and below the basal contact. Massive sulfide ore is not as common within the intrusive rocks, and when present, is generally associated with, or in close proximity to, hornfels inclusions. Ore/host rock textures are extremely varied, but all are indicative of structural control in the footwall rocks. Overall, the massive sulfide ores are spatially distributed in a spotty manner in an east-west (EW) direction that corresponds to a major EW-trending anticline present within the footwall rocks. All these factors suggest that an immiscible sulfide melt was injected into structurally prepared footwall rocks along the anticlinal axis in a "vein-like" setting. At some later period, the footwall-hosted massive sulfide ore zone was re-intruded by multiple sills (which collectively make up a portion of Unit I) along bedding planes of the Virginia Formation. The end result is a disjointed zone of mineralized inclusions and mineralized footwall rocks separated by "barren" intrusive rocks. Sulfide textures indicate that the sulfides formed by cooling of a monosulfide solid solution (MSS) followed by limited replacement at very low temperatures. Minerals contained within the sulfide ore are dominantly pyrrhotite, chalcopyrite, cubanite, and pentlandite. Locally present are maucherite, sphalerite, bornite, talnakhite, mackinawite, and an unknown Cu-sulfide ("Cp"). Also present in minor amounts are native silver (primary and secondary), parkerite, chalcocite, covellite, godlevskite, violarite, magnetite, and zincian hercynite. Although no discrete PGE minerals were identified, analytical results of the high-grade (>1% Cu) massive sulfide ore confirms the presence of several anomalous PGE values. These spot values are mainly confined to an EW-trending zone that also roughly corresponds to the EW-trending anticline. Maximum values obtained within the Local Boy massive sulfide ores include: Pd = 11,100 ppb; Pt = 8,300 ppb; Au = 10,900 ppb; and Ag = 34 ppm. Native silver (primary) was found within several maucherite grains in this investigation, and PGE mineral inclusions have previously been found in maucherite (Ryan and Weiblen, 1984). Generally, the drill holes that contain the anomalous PGE values also contain the native silverbearing maucherite; whereas, homogeneous maucherite is more characteristic of drill holes with little to no anomalous PGE values. This suggests that PGEs were scavenged from the sulfide melt by early-formed maucherite, and thus the PGEs are related to a primary (magmatic) process. However, a hydrothermal origin for the PGEs is also indicated. Anomalous PGE values are commonly associated with Cl-drop encrusted massive sulfide drill core. The spatial distribution of the Cl-drop encrusted drill core also coincides with the EW-trending anticline. Presence of the Cldrops indicates that the rocks of the Local Boy area were invaded by Cl-bearing solutions that may have been capable of transporting and concentrating PGEs. Therefore, both primary/magmatic (sulfides injected into a "vein-like" setting) and later secondary/hydrothermal processes appear to have been factors in controlling PGE distribution in the Local Boy area. However, it is difficult to separate the primary and secondary processes. This is due to the coincidence of several features within the EW-trending zone, which include: 1) anticline in the footwall rocks; 2) overall massive sulfide spatial distribution; 3) spatial distribution of anomalous PGE values; and 4) spatial distribution of Cl-drop encrusted core. Reactivation of structures that controlled the initial "vein-like" massive sulfide distribution could have been responsible for channeling later hydrothermal solutions. Geostatistical analysis of the underground drill holes (Drifts B, C, and D), and pertinent surface drill holes, yields five main conclusions: 1) the top of the Biwabik Iron-formation (BIF) is a critical datum, with the higher grade Cu-material located between 100 and 400 feet above the BIF (mainly within the Virginia Formation near the basal contact); 2) inter-variable correlations between Cu and Ni are high, indicating that selective mining of Cu and Ni is physically possible; but, selection on ore grade Cu and Ni will not necessarily capture all the ore grade PGEs and other precious metals; 3) the available drilling gives a spacial range of geologic influence of about 150 feet; 4) potentially economic ore reserves do exist in the Local Boy area; and 5) the property is under-valued due to the inclusion of many "barren" (unassayed) intervals into the compositing process. A coarse block model, and in situ geologic reserves, are presented for the Local Boy area.