METAL CONCENTRATIONS IN COVER OVERLYING DULUTH COMPLEX NI-CU-PGE DEPOSITS, NE MINNESOTA

The U.S. Geological Survey initiated a project in 2015 to evaluate geochemical exploration methods for glacially-covered Ni-Cu-PGE deposits in the Duluth Complex in the northern Midcontintent Rift. Initial efforts included interpretation of an unpublished 1976 soil geochemical dataset collected over the Mesaba deposit by the Minnesota Department of Natural Resources and AMAX Exploration Inc. The survey was set up on 800-foot grid spacing. Where possible, B horizon soils were collected and peat samples were collected in topographic depressions. Results of 492 samples show a median Ni concentration of 74 ppm and median Cu concentration of 110 ppm. Peat samples clearly show enrichment in Ni (180 ppm) and Cu (440 ppm).

In a first round of field work, samples of unconsolidated material overlying the Spruce Road, Wyman Creek, and Skibo deposits in the Duluth Complex, along with background areas, were collected to determine effective sampling methods and to characterize the geochemical signature from the deposits in the overlying cover. Peat and sediment samples were collected with depth in conjunction with groundwater samples and were analyzed for major and trace element chemistry. Results show enrichment of median concentrations for Ni (262 ppm), Cu (439 ppm), Pd (15 ppb), and Pt (8.5 ppb) in Spruce Road samples (n=13) relative to background sites (median Ni= 45 ppm, Cu=63 ppm, Pd 2.5 ppb, Pt 5 ppb, n=12). Samples from Wyman Creek (n= 4) and Skibo (n=11) are not elevated in these elements when compared to background sites. Relative to sediment, peat samples are elevated in Ni, Cu, Pd, and Pt for Spruce Road and background sites. Peat samples from Wyman Creek and Skibo are similar to background peat samples. Both the 1976 survey over the Mesaba deposit and our preliminary data from Spruce Road, Wyman Creek, and Skibo indicate that Ni and Cu anomalies exist in the unconsolidated cover over Ni-Cu-PGE deposits, with the most substantial anomalies occurring in peat. It remains unclear to what degree these anomalies are caused by underlying mineralized bedrock versus mineralized till within the cover itself. Future sampling will attempt to address this uncertainty along with other processes-based questions related metal accumulation in peats.