Geology, Stratigraphy, and Mineralization of the Dunka Road Cu-Ni Prospect, Northeastern Minnesota

Abstract

The Dunka Road Cu-Ni prospect is located within what is informally known as the Partridge River Intrusion (T. 60 W., R. 13 W.), which is part of the 1.1 b.y. (Keweenawan) Duluth Complex. Seven major lithologic units, along with several internal ultramafic subunits, have been identified and are correlatable over the prospect. The ultramafic subunits (layers of picrite to dunite) exhibit relative uniform thicknesses and are present at the same relative stratigraphic position within the major lithologic units. The major lithologic units, delineated by Severson and Hauck (1990) and Geerts et al., 1990, are defined as upward from the basal contact as follows: Unit I, a fine- to coarsegrained sulfide-bearing anorthositic troctolite to pyroxene troctolite (450 ft. thick) with associated ultramafic subunits I(a) and I(b); Unit II, a medium- to coarse-grained troctolite to pyroxene troctolite (200 ft. thick) with a basal ultramafic subunit II(a); Unit III, a fine-grained, mottled textured troctolitic anorthosite to anorthositic troctolite (250 ft. thick); Unit IV, a coarse-grained pyroxene troctolite to anorthositic troctolite (300 ft. thick); Unit V, a coarse-grained anorthositic troctolite (300 ft. thick); Unit VI, a fine- to coarse-grained troctolitic anorthosite to troctolite (400 ft. thick) with basal ultramafic subunit VI(a); and Unit VII, a coarse-grained troctolitic anorthosite to anorthositic troctolite (400+ ft. thick) with basal ultramafic subunit VII(a). Most sulfide mineralization occurs within Unit I. The sulfide mineralization is both interstitial and widespread, but variable in modal percentage (rare to 5%), continuity, and thickness (few inches to tens of feet). Sulfide mineralization is generally related with proximity to: hornfels inclusions; the basal contact with the footwall Virginia Formation; and some of the internal ultramafic subunits within Unit I. Primary sulfide mineralization includes chalcopyrite, pyrrhotite, cubanite, and pentlandite. Minor amounts of bornite, pyrite, sphalerite, galena, talnakhite, mackinawite, and/or valleriite along with both native copper and gold have also been identified. Pt+Pd values range from 100 to >2400 ppb over 5 and 10 foot intervals, and occur as isolated values or within stratigraphic horizons within Unit I. Several Cu/PGE-enriched horizons (using a 0.5% Cu and >800 Pt+Pd cut-off) have been identified and occur laterally throughout the prospect. Intersected by 76 drill holes, the most continuous horizon (RED Horizon) is found directly beneath ultramafic subunit II(a), within the uppermost portion of Unit I. This horizon ranges from 5 to 100 feet thick (average 33 ft.) and contains average values of 0.6% Cu and 1000 ppb Pt+Pd. Two other horizons (ORANGE Horizon and YELLOW Horizon) occur beneath the RED Horizon and are intersected by 67 and 48 drill holes, respectively. These are less continuous horizons that range from 5 to 140 feet thick (average 35 ft.) and contain average values of 0.6% Cu and 750 ppb Pt+Pd. Only one PGE-enriched horizon has been identified outside of Unit I. This horizon (MAGENTA Horizon) occurs in Unit VI, and is located approximately 150 feet beneath ultramafic subunit VII(a). Although it has been identified in only six drill holes to date, it ranges from 6 to 40 feet thick (average 25 ft.) and contains average values of 0.7% Cu and 1500 ppb Pt+Pd. The predominant host rock for these Cu/PGE-enriched horizons is coarse-grained anorthositic troctolite, which may exhibit some subtle fracturing associated with minor alteration. The alteration assemblage within these mineralized zones is serpentine, uralite, and saussurite. This type of alteration assemblage has also been observed throughout the entire prospect, but is not always associated with mineralization. Although the majority of sulfide mineralization is believed to be primary, mineralized zones that are intersected by fractured/altered zones can contain secondary sulfides and textures, suggesting local enrichment. The majority of the sulfide is coarsegrained (5 mm) and commonly rimmed by secondary red-brown biotite. Ilmenite occurs in two habits within these zones, as euhedral to subhedral laths interstitial to silicate crystals, and as "blebiii like" shiny black droplets within the sulfides. This second ilmenite habit has only been identified in sulfide-bearing zones that are enriched in Pd and/or Pt. A total of 16 samples (13 mineralized, 3 unmineralized) were analyzed for PGEs. Results of the PGE scans indicate that the original magma contained all of the dissolved sulfides and PGEs upon reaching the final site of the intrusion. Therefore, the mineralized horizons contain values of PGEs similar to that of the mantle. No PGE-enriched reefs are found at Dunka Road, most probably due to low concentrations of PGEs in the original mantle melt. The geochemistry of samples taken from the Dunka Road prospect support this theory.