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

Abstract

The 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.