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Overview

Elk Creek deposit vicinity map
Complete Bouguer gravity data in the vicinity of the Elk Creek deposit. Data are from the Pan-American Center for Earth and Environmental Studies.

Why are we doing this research?

The Elk Creek carbonatite is a natural laboratory of unique significance to the USGS: it hosts deposits of critical minerals including the largest known niobium deposit in the U.S., light rare earths, titanium, and scandium. This project has access to high-quality airborne gravity gradient (a measure of subsurface density) and magnetic data, and approximately 22 kilometers of public domain core spread out over 100 boreholes available for study. The deposit is also entirely buried under approximately 200 meters of sedimentary rocks, meaning that geophysical methods, integrated with borehole constraints, are critical for rigorous study.

What are this project's objectives?

Our objectives are to:

  • Advance USGS developments in gravity gradiometry and inversion/interpretation,
  • Improve the existing geophysical model of the carbonatite,
  • Serve as a proof of concept for the application of developed techniques to the development and improvement of geophysical models of carbonatite systems.

This project combines previously-acquired geophysical data and physical property information with ground-based gravity gradient data (to be collected). Newly developed interpretation algorithms are currently being applied (and improved) to maximize the information content from the rich variety of datasets available. Ultimately, we will produce a 3D lithologic/geophysical property model of the Elk Creek region.

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Project Activities

data flowchart
Data acquisition and interpretation flow chart to produce a lithologic model. Shaded cells indicate new data (green) and new products (light purple).

Gravity Gradient and Magnetic Data Modeling and Lithologic Inversion

The compact nature and near-surface location of the carbonatite, coupled with strong density contrasts, make it an ideal target for gravity gradient investigations. High-quality airborne magnetic and gravity gradient data were acquired by NioCorp Developments Ltd. and shared with the USGS; however the company employs no geoscientists and lacks in-house capability for interpretation. The data have been jointly interpreted previously using match filtering and 2D modeling; however no 3D model of density (or magnetic susceptibility) has been developed from the borehole and airborne data.

The acquisition of a land gravity gradiometer system and subsequent data collection at Elk Creek will be the first public example of ground-based gradient gradient data. New software is being developed for joint interpretation of gravity gradient and magnetic data. The joint interpretation of geophysical data, when combined with borehole physical property data and other geologic information, can demonstrate lithologic differences.

gravity gradient inversion result
Example isosurface of a gravity gradient inversion result; units are g/cc density contrast.
preliminary lithologic differentiation result
Example of a "cluster", a preliminary lithologic differentiation result; units are g/cc density contrast.

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