The Dunka Road Cu-Ni deposit is within the Partridge River Intrusion (T. 60 W.,
R. 13 W.), which is part of the Duluth Complex, and is approximately 1.1 b.y.
(Keweenawan) in age. Relogging of 46 drill holes at the Dunka Road Cu-Ni deposit
identified four major lithologic units and several internal ultramafic subunits that can be
correlated over two miles. The ultramafic subunits (layers of picrite to peridotite) exhibit
relative uniform thicknesses and are present at the same relative elevation within the
major lithologic units. The major lithologic units, the same as delineated by Severson
and Hauck (1990), and upward from the basal contact are: Unit I, a fine- to coursegrained
a sulfide-bearing troctolite to pyroxene troctolite (450 ft. thick) with associated
ultramafic layers I(a), I(b), and I(c); Unit II, a medium- to coarse-grained troctolite to
pyroxene troctolite (200 ft. thick) with a basal ultramafic layer II(a); Unit III, a finegrained,
mottled textured troctolitic anorthosite to anorthositic troctolite (150 ft. thick) with
one minor ultramafic layer III(a); and Unit IV, a coarse-grained troctolite/pyroxene
troctolite to anorthositic troctolite with associated ultramafic layers IV(a) and IV(b).
Most sulfide mineralization occurs within Unit I. Within Unit I the sulfide
mineralization is both widespread but variable in modal percentage (rare to 5%),
continuity and thickness (few inches to tens of feet). Sulfide mineralization is somewhat
related with proximity to: hornfels inclusions, the basal contact with the footwall Virginia
Formation, and some of the internal ultramafic layers within Unit I. Precious metal
mineralization (Pd+Pt+Au) is associated with fracturing and alteration of the host rocks.
The alteration assemblage is chlorite, bleached plagioclase, serpentine and uralite.
Pd+Pt values range from 100 to >2400 ppb over 10 foot intervals. These intervals
can occur independently as 10 to 50 foot zones, or as part of a larger correlatable
occurrence/horizon. Two mineralized subareas within the Dunka Road deposit are: 1)
an area which is peripheral to a highly anomalous Pd occurrence (reported by Morton
and Hauck, 1987; 1989) herein termed the "southwest area", and 2) the "northeast area"
which contains several drill holes that have near surface intercepts of >1% Cu. There
are four somewhat large mineralized occurrences within the study area that carry >300
ppb combined total Pt+Pd+Au. These mineralized zones appear to be stratigraphically
controlled by the ultramafic subunits within Unit I. Three of the four correlatable zones
are found within the southwest area, and range from 40 to 130 feet thick. High Pd
values within these zones range from 10 to 20 feet thick with values of 800 to 1650 ppb
Pd. In the northeast area, the fourth mineralized zone appears continuously throughout
Unit I. This zone ranges from 120 to 300 feet thick. High Pd values within this zone
range from 10 to 40 feet thick with values of 800 to 1500 ppb Pd. Many 5 to 30 foot
intersections of >1 ppm Pd+Pt+Au occur throughout the mineral deposit.
Geostatistical analysis based on 72 vertical holes and 12 angle holes suggests:
1) the base of the complex is a critical datum with the higher grade intercepts located
between 100 and 400 feet above the base; 2) high inter-element correlations support
local redistribution/concentration of primary mineralization by a secondary hydrothermal
process and thus, polymetallic mining selectivity is possible; 3) the available drilling
gives a spacial range of geologic influence at 400 foot centers, but sufficient angle
drilling is not available to assess the potential of high grade, steeply dipping mineralized
zones; 4) additional vertical in-fill drilling will almost certainly not discover any additional
quantity of ore within the volume of rock studied; but 5) additional angle drilling to assess
the potential of high grade, steeply dipping, mineralized zones would benefit a more
complete geostatistical analysis.
The files attached to this record include the report (GMIN-TR-1989-16.pdf) and the Plates 1-7 mentioned in the report. Appendix C on the disk in the back pocket has yet to be located.
Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Dept. Civil and Min. Engineering, University of Minnesota, 500 Pillsbury Drive S.E., Minneapolis, Minnesota 55455; Funded by the Greater Minnesota Corporation
Monson Geerts, Stephen D; Barnes, Randal J; Hauck, Steven A.
Geology and Mineralization in the Dunka Road Copper-Nickel Mineral Deposit, St. Louis County, Minnesota.
University of Minnesota Duluth.
Retrieved from the University of Minnesota Digital Conservancy,
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