Geochemical Signatures of Covered Mineral Deposits in the Northern Midcontinent Completed

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Science Issue and Relevance

The northern midcontinent of the U.S. has a rich mineral endowment, and mining is expected to expand in the future. However, recent glaciation and substantial transported regolith (layer of loose, heterogeneous superficial material covering solid rock) depths (often >10 m) complicate prospecting efforts, including the use of traditional geochemical methods. In the past 15 years, research aimed at identifying effective geochemical exploration methods in areas of thick cover has expanded considerably. Promising approaches include identification of:

1. Anomalies in soil chemistry and soil gas associated with a "reduced column" directly overlying the deposit
2. Anomalies in groundwater chemistry
3. Deposit indicator minerals within tills down-ice of the deposit
4. Anomalies in regional geochemical datasets

Though compelling, application of approaches (1) and (2) have been limited, and specific processes controlling the physical, electrochemical, advective, and diffusive dispersion of chemical species upward through overlying cover are only partially understood. Research has focused mainly on sites in Canada, Australia, northern Europe, and South America, with few published studies from the U.S.

Methodology to Address Issue

Our primary objective is to evaluate the potential of geochemical prospecting techniques that have shown promise in other covered terranes for mineral exploration in the northern midcontinent of the U.S. Novel components will be added to these methods with the objective of method advancement and improving our understanding of processes controlling the transmission of unique geochemical signatures from buried mineral deposits to groundwater, soils, and the overlying near-surface environment in glaciated areas.

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Regional Geochemical Dataset Aanalysis: Regional geochemical mapping has proven useful for exploration in covered terranes in Canada, Austalia, and northern Europe. Previous regional geochemical surveys have show high correlation between soil chemistry and bedrock geology, even in highly developed areas. Previous regional water geochemical surveys have focused on surface water. Groundwater chemistry data could be useful in the northern midcontinent where sedimentary cover can be deep and wells are plentiful. Abundandant geochemical data are also available for the northern midcontinent, largely collected by USGS, but these have not been interpreted on a regional scale.

We will assemble existing environmental geochemical data for the northern midcontinent from the U.S. Soil Geochemical Database, National Geochemical Database (NGDB), National Uranium Resource Evaluation database (NURE), and National Water Information System (NWIS). We will integrate this data with existing geologic maps, USGS geophysical data, and data available from other state and federal agencies, and will evaluate data coverages for characteristics associated with known deposits and districts. This preliminary analysis will help guide study site selection. Data coverages will be evaluated for anomalies and correlations in areas outside of known districts/deposits that could be associated with unknown areas of mineralization, incorporating new knowledge gained from field sampling.

Solid Media, Soil Gas, and Groundwater — Sampling and Analysis: Research aimed at identifying effective geochemical exploration methods in areas of thick cover has expanded considerably in the past 15 years. Three approaches that show promise include (1) identification of soil and soil gas anomalies associated with a "reduced column", (2) identification of anomalies in groundwater chemistry, and (3) identification of high metals in wetland peats. Though promising, application of these methods has been limited, published research on sites in the northern midcontinent is limited, and specific processes controlling the dispersion of chemical species upward through overlying cover are only partially understood.

Objectives are to collect soil, wetland peat, soil gas, and groundwater samples in the vicinity of known, characterized buried deposits and analyze them to test for the presence of unique geochemical signatures relative to adjacent background areas. The Duluth Complex Ni-Cu-PGE deposit area in northeastern Minnesota has been selected as a study site. Improved understanding of the processes controlling the transmission of unique geochemical signatures from buried deposits to groundwater and the overlying near-surface environment will assist in the evaluation of potential geochemical prospecting techniques in the northern midcontinent region.

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