Large resources of Cu-Ni sulfides are found in troctolitic and
gabbroic rocks at the base of the Duluth Complex in St. Louis and Lake
Counties of northeastern Minnesota. Analysis of unpublished mining
company data shows that there is a substantial reserve of PGE, Au and Ag
associated with these sulfides. Weighted averages for combined Pt and Pd
values vary as follows: 105 ppb in Water Hen, 278 ppb in Dunka Pit, 378
ppb in Minnamax, 570 ppb in Maturi, 651 ppb in Spruce Road to a high of
1259 ppb in Dunka Road. Au values vary from a low of 63 ppb in the Water
Hen to a high of 137 ppb in the Spruce Road. Ag values vary from 1.22 ppm
in Dunka Road to 3.8 ppm in the Minnamax deposit. Because recovery of PGE
in copper-nickel flotation concentrates is very poor (usually less than
50%), these values add less than $5.00 to the ore.
Even though these PGE and Au values are associated with the Cu-Ni
sulfides, it appears that absolute values cannot be correlated with Cu, Ni
and/or s contents. If sulfide values are below 0.2 wt %, then there are
no appreciable PGE values. This is true for all deposits. However, if
Pt+Pd/S is plotted against Cu/S, all sanples with high PGE contents appear
to be related to samples with high Cu/S contents. Ag values, on the other
hand, show a good correlation with absolute Cu content: r=+O.75 for all
deposits and r=+0.86 for Minnamax data.
The largest data base comes from the Minnamax deposit where metal
values are further separated into Basal and Cloud zones. Basal zone
sulfides are those that occur in the lowest 300 feet of the Duluth
Complex. Cloud zone sulfides occur several hundred feet above the base of
the Complex. In general, Basal zone sulfides consist of both massive and
disseminated types, whereas Cloud zone sulfides are disseminated. At
Minnamax, the weighted average sulfur content is 0.38% in the Cloud zone
versus 2.78% in the Basal zone. The corresponding combined Pt and Pd
values are, respectively, 192 and 396 ppb. Even though the absolute
content in the Cloud zone is less, there is a higher metal to sulfur ratio
than in the Basal zone, indicating an enrichment in PGE. This is also
true for Cu and Ni contents. Ag contents, on the other hand, do not show
this relationship. They are related to the absolute Cu content of the ore
at Minnamax.
Detailed studies of two anomalous samples, one from Water Hen and the
other from Dunka Road, have identified some interesting minerals. PGE
bearing minerals were only identified at Dunka Road. At Water Hen the
following minerals were identified by using a reflecting microscope as
well as a scanning electron microscope equipped with an EDS system:
bornite, chalcopyrite, pentlandite (Ni rich), maucherite, sphalerite (pure
ZnS) as inclusions in bornite, native Ag as a cross-cutting veinlet in
maucherite, niccolite, parkerite (Ni3Bi2s2) , native Bi, and
tentatively tetradymite (Bi2Te2s). Previous work by U.S. Steel
identified the following minerals in the anomalous zone at Dunka Road:
pyrrhotite, chalcopyrite, pentlandite, violarite, froodite (PdBi2),
michenerite (PdTeBi), native Gold (Au,Ag), native Bi, and an unknown
mineral composed of Pd, Sb and Bi. Textures within both of the samples
indicate that pentlandite is being replaced by chalcopyrite and bornite at
Water Hen and by violarite, chalcopyrite and the Au and Pd minerals at
Dunka Road. These minerals appear to have been concentrated by later
secondary copper rich fluids and are not part of the initial formation of
Cu-Ni sulfides.Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Geology Department, University of Minnesota Duluth, Duluth MN 5581
