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Surficial Geology of Wisconsin



The Bend massive sulfide deposit, located in the Medford District of the Chequamegon National Forest, is a small, metal-rich sulfide body hosted by Early Proterozoic Penokean volcanics. The mineralized horizon subcrop beneath 100-120 feet of glacial cover, and consists of massive pyrite with varying amounts of chalcopyrite, tetrahedrite-tenantite, bornite, arsenopyrite, chalcocite, and rare gold-silver tellurides. Bedrock and ore-grade mineralization has been well characterized by analyses of drill core provided to the USGS by Sharpe Energy and Resources and the Jump River Joint Venture. The suite of sulfide minerals results in unusual geochemistry, with high values of arsenic, bismuth, mercury, tellurium, thallium, and antimony concentrated in the ore.

In July 1999, five drillholes were completed through the unconsolidated glacial cover, averaging about 100 feet thick, on a transect across the Bend deposit. Nearly continuous core was recovered from the surficial material and up to a few feet of the underlying bedrock. Samples representing the entire section are being analyzed by the USGS to give a two-dimensional representation of element dispersal from the unmineralized bedrock. The Wisconsin Geological and Natural History Survey (WGNHS) are doing physical characterization of the glacial material. The unusual suite of elements concentrated in the deposit should define a unique pattern of natural dispersal of elements both by physical transport of glacial material and by chemical transport in groundwater.

The core allows development of a detailed description of the glacial history of the immediate area. Cooperative research with staff from the WGNHS who have recently mapped the glacial geology of the region has produced a detailed three-dimensional model of the glacial deposits in the region that will form a framework for interpretation of the geochemical results. Of particular interest is the presence of two superimposed till sheets with distinctly different age, physical character, and source regions, as well as layers of sand and gravel and organic-rich lake sediments interspersed with the till sheets. The complexity of the glacial geology section permits testing the degree of physical and chemical dispersal of elements under a variety of subsurface conditions.


image of USGS and Forest Service Personnel using Rotosonic drill rig USGS and Forest Service personnel await core samples from rotosonic drilling in the Medford District.

Image of John Attig of WGHNS John Attig of the WGHNS logs core from rotosonic drilling. Core was 3" in diameter and was recovered in 10 foot sections, extracted from the drill stem into plastic bags.

Preliminary results from geochemical analyses of Quaternary materials from the five rotosonic core was reported at the recent 46th Institute on Lake Superior Geology held in Thunder Bay, Ontario, in May 2000 (Woodruff and others, 2000).

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