Airborne Geophysics for Rare Earth Element Deposits (AGREED) Completed

Sources/Usage: Public Domain. Visit Media to see details.

Science Issue and Relevance

Over two dozen rare earth element (REE) deposits have been identified both in the conterminous U.S. (Long and others, 2010) and in Alaska (Barker and Van Gosen, 2010). The majority of larger domestic deposits are hosted in carbonatite or peralkaline intrusions and related pegmatites and veins, which typically make excellent geophysical targets because of their distinct gamma ray, density, and magnetic properties. As such, airborne geophysical surveying is routinely acquired by industry to map the extent of surface and subsurface mineralization related to these deposits. In many cases, geophysical surveying has figured prominently in the initial discovery of REE deposits.

Thus far, no airborne geophysical data have been collected by the USGS at a resolution sufficient for studying the ore genesis, setting, or characterization of REE deposits. Only coarsely spaced airborne magnetic data and ground gravity data exist over these deposits. However, high resolution data over domestic REE deposits are available from the mining industry. Innovation in geophysical methods and model development, expansion of geophysical skills, and keeping current on technological advancements over deposits of strategic importance requires access to modern geophysical data.

Sources/Usage: Public Domain.

Project Objectives

The overall project objective was to coordinate airborne geophysical efforts over domestic REE deposits and create a petrophysical library that will contribute to and complement geophysical model development and characterization of rare earth deposits. Our project:

• Worked with mining industry to obtain high resolution airborne geophysical data over rare earth deposits,
• Created a petrophysical library from surface and core samples that describe the range of petrophysical properties of host rocks, structures, and mineralized systems, with an emphasis on REE concentrations and occurrences,
• Interpreted and modeled the data using modern techniques, many developed in-house.

USGS acquired mining industry data over three advanced REE deposits with active exploration efforts. Initial focus areas were the Bokan Mountain deposit in southeastern Alaska (data acquired from UCore Rare Metals and Precision GeoSurveys, Inc.), Elk Creek deposit in Nebraska (data acquired from NioCorp, formerly Quantum Rare Earth), and the Bear Lodge deposit in Wyoming (data acquired from Rare Element Resources).

High Resolution Geophysical Survey

A high resolution magnetic and gravity gradiometry survey was conducted of the Pea Ridge iron oxide copper-gold rare earth element (IOCG-REE) deposit in southeast Missouri. This survey will be flown by the USGS.

What Constitutes a High Resolution Geophysical Survey?

• Designed to resolve:
  • low signal
  • information between lines
• Line Spacing / Flight Height: < 2 (Ideal = 1)
• Fly close to the ground with narrow line spacing

All industry surveys were flown at low altitudes (80 to 100 m above ground) with 100 to 200 m flight line spacing. Both fixed wing and helicopters were used.

Low Resolution vs. High Resolution: Scientific benefits of working with high resolution geophysical data are numerous, not the least of which is the wealth of information provided in the detail. The images below show the "before" and "after" of USGS magnetic survey with 1600 m line spacing, compared to the magnetic survey flown by UCore over Bokan Mtn with 100 m line spacing. The resolution provided in the UCore survey allows for detailed mapping of the host rocks, controlling structures, and mineralized system. The UCore data provides more than 10 times the resolution of the older USGS survey.

Sources/Usage: Public Domain.

Sources/Usage: Public Domain.

Multiple Sensors Collecting Data Concurrently: Another obvious benefit working with high resolution survey data is that all the industry surveys had multiple sensors on board collecting data simultaneously. For example, both the Bear Lodge and Bokan surveys include magnetic and gamma-ray data. The gamma-ray data is especially useful over exposed REE deposits as we can begin to understand the relationship between the radioelements (uranium [U], thorium [Th], and potassium [K]) and the concentrations of rare earth elements (REE) within the host and mineralized rocks. For the USGS Bokan study, our geophysicist is working closely with our mineralogist to understand the REE mineralogy as it relates to the uranium, thorium, and potassium minerals. Thru this process, connections are made that allow interpretation of the airborne geophysical data in the context of rare earth deposit setting and mineralization.

Bokan Mountain Deposit, Alaska

Bokan Mountain, located at the southern end of Prince of Wales Island, southeast Alaska, has been the site of past uranium mining and most recently is the focus for exploration of rare earth elements. Bokan Mountain is a peralkaline granite complex with associated dike vein systems that are enriched in a number of heavy rare earth elements (HREE).

An airborne geophysical survey, flown by Precision GeoSurveys and provided by UCore Rare Metals (http://ucore.com/), was recently flown over the Bokan deposit. The survey collected high resolution magnetic and gamma-ray spectrometry data along flight lines spaced 100-m apart. Bokan Mountain objectives are to:

• develop an understanding of the magnetic and radioelement properties of rare earth ore and ore hosts;
• analyze the relation between the rare earth element mineralogy and geophysical properties; and
• develop a regional geophysical characterization of peralkaline intrusion and possible geologic controls on mineralization.

Sources/Usage: Public Domain.

Elk Creek Deposit, Nebraska

The Elk Creek carbonatite, located in southeastern Nebraska, is host to niobium (Nb)-REE minerals and covered by approximately 200 m of sedimentary cover. An airborne geophysical survey that includes gravity gradiometry and magnetic data was recently flown and provides insight into the geophysical expression of a buried carbonatite. These data have been made available to the USGS through a non-disclosure agreement with NioCorp Developments Ltd. (formerly Quantum Rare Earth Developments Corporation), which allows USGS full access to digital flight line data and to publish figures, charts, maps, and interpretations of the data. Elk Creek objectives were to:

• develop an understanding of the magnetic and density properties of the Nb-rare earth ore and ore hosts at Elk Creek;
• develop models of the carbonatite through integration of physical property data collected on drill core and analyses of high resolution geophysical data; and
• develop a model that encompasses geophysical characterization of the Elk Creek carbonatite and possible geologic controls on mineralization.

Sources/Usage: Public Domain.

Bear Lodge Deposit, Wyoming

An airborne geophysical survey, provided by Rare Element Resources (http://www.rareelementresources.com/s/Home.asp), was recently flown over the Bear Lodge carbonatite deposit. The survey collected high resolution magnetic and gamma-ray spectrometry data along flight lines spaced 100-m apart. The map below shows the magnetic anomaly field over the Bear Lodge igneous complex, which hosts the rare earth–bearing carbonatite dikes within the Bull Hill area. Bear Lodge objectives were to:

• develop an understanding of the magnetic and radioelement properties of rare earth ore and ore hosts at Bear Lodge;
• analyze the relation between the rare earth element mineralogy and geophysical properties; and
• develop a regional geophysical characterization of alkaline intrusion, carbonatitic dikes and possible geologic controls on mineralization.

Sources/Usage: Public Domain.

Sources/Usage: Public Domain.

References Cited

Barker, J.C., and Van Gosen, B.S., 2012, Alaska’s rare earth deposit and resource potential: Mining Engineering, v. 64, Issue 1, p. 20-32, http://me.smenet.org/abstract.cfm?preview=1&articleID=2502&page=20.

Long, K.R., Van Gosen, B.S., Foley, N.K., and Cordier, D., 2010, The principal rare earth elements deposits of the United States—A summary of domestic deposits and a global perspective: U.S. Geological Survey Scientific Investigations Report 2010-5220, 96 p., https://doi.org/10.3133/sir20105220.

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