PGE, Au, and Ag Contents of Cu-Ni Sulfides found at the Base of the Duluth Complex, Northeastern Minnesota

Abstract

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 Analysis of unpublished mining company data shows that there is a substantial reserve of PGE, Au and Ag associated with these sulfides.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.