DNR Report 318: 1997; Mineral Potential Study Greenstone Belt Boulder Tracing Twp. 60-65N R11-27W Ely Bigfork Area

Summary Extracted from Report 318:

SUMMARY  The Ely-Bigfork area of northern Minnesota is an Archean granite greenstone belt. Low cost sampling of glacial clasts and pebble sampling in gravel pits have revealed several areas of high potential for lode gold and volcanogenic massive sulfide deposits. Sample sites were carefully searched for mineralized rock and evidence of alteration. Mineralized samples were analyzed using a reconnaissance analytical package with these results (gold values from 6 to 3,598 ppb, arsenic values 10 to 1,900 ppm, zinc 100 to 2,899 ppm, copper 100 to 14,944 ppm and lead 20 to 248 ppm, nickel 133 to 3,126 ppm and barium 600 to 1,980 ppm). These values combined with observed alteration and favorable geology mapped seventeen mineralized clast areas (MCAs).



In the eastern part of the area two important MCAs for gold and VMS mineralization correlate very well with high pebble counts for quartzite, metasediments and felsic volcanics. There are also favorable lithologies; iron formation, metasediments, mafic and felsic volcanics with fault contacts. MCAs form distinctive southwest trending boulder trains from the Rainy Lobe glaciation. In the central part of the area MCAs also correlate very well with favorable lithologic units and structural features. In most of this area the glacial Koochiching Lobe from the northwest covers the Rainy Lobe and there is less distinctive development of boulder trains. In the western part of the area there are strong analytical anomalies, with less explicit coincidence with structural features and bedrock lithology. In the western Koochiching Lobe glacial deposits there is some development of boulder trains, but they are not as well defined as they are to the east.

The location of the source of mineralized samples is indicated by: 1. Development of mineralized-altered boulder dispersal trains. 2. The amount of mineralized rock at the sample site which decreases with distal dispersion and denudation. and 3. Study of megascopically determined lithology frequency distribution for randomly collected samples of 100 pebbles from each site. The pebble lithologies are grouped to match lithologic units so that they can be compared with mapped bedrock lithology (Morey 1996). Irrespective of the nature of glacial cover, i.e. Rainy Lobe in the East or Koochiching Lobe in the central and western parts of the area, there is good correlation found between the pebble frequency distributions and bedrock geology. This indicates that although there has been increased dispersion and denudation in the western one-third of the study area mineralized rock samples are still relatively close to their bedrock source. In the far west irregular frequency distribution of carbonate pebbles from the Koochiching lobe suggests reworking of a local as yet unmapped source.


View or download report

Report 318: Report 318: 1997; Mineral Potential Study, Greenstone Belt Boulder Tracing, Twp. 60-65N R11-27W Ely Bigfork Area
(71 pages, 8.5 x 11 inches, 6.01 MB)
Authors: T. Lawler and D. Riihiluoma

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Download GIS data

The following zip file contains data for 165 samples extracted from 144 sample sites described in the Minnesota Department of Natural Resources (MnDNR), Division of Land and Minerals Report, “MINERAL POTENTIAL STUDY: Greenstone Belt Boulder Tracing: Ely – Bigfork Area, Northern Minnesota: Report 318,” published in 1997. Each folder in the zip file contains data from the report in a different format. The data is presented in its original “Paradox Data Base files”, a Microsoft Access database, dBase IV files, and ArcGIS File Geodatabase. 


Download or view site photos

The following zip file lists and shows photos taken of each of the 144 sites discussed and sampled in the report. 


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