Geologic Diagnostic: An “MRI” for the Earth

by Jake Alexander, Andrew Rupke, and Stephanie E. Mills

Critical minerals, as defined by the U.S. Geological Survey (USGS), refers to commodities that are important to the country’s economy or national defense, but have a supply chain that faces potential risk for various reasons. The most recent critical mineral list generated by the USGS includes 50 commodities such as lithium, rare earth elements (REEs), beryllium, vanadium, and aluminum, to name just a few. In many cases, the United States produces little or none of these important minerals. In 2019, the USGS kicked off the Earth Mapping Resources Initiative (Earth MRI) with the goal of identifying new domestic critical mineral deposits. This initiative facilitates the collection of the basic geological information necessary for mineral exploration, such as modern geological mapping, geophysical surveys, lidar data, and geochemical data. The USGS recognized that in many areas of the country these basic data are unavailable, outdated, or incomplete and slows the exploration efforts of companies seeking to identify new deposits. Another key part of the USGS’s strategy is to tap into the mineral resource expertise of state geological surveys across the country. Over the past several years, the USGS and many state surveys, including the Utah Geological Survey (UGS), have been collaborating to identify the most important areas in each state that have or may have critical mineral deposits. To further investigate a few of these promising areas and deposits, the UGS has been working on several Earth MRI projects, two of which are highlighted below.

Desert Skarn

A photo of a mineralized skarn example, which is dark grey and white mottled in appearance.

An example of mineralized skarn from drill core at the West Desert site. Core sample is approximately 2.5″ in diameter. Photo courtesy of Kayla Smith (UGS).

One of these Earth MRI projects is studying the critical minerals indium and zinc at the West Desert zinc-copper-indium skarn deposit in Juab County at the southern end of the Great Salt Lake Desert. The importance of indium in modern society is wide-ranging as it is commonly used in touch-screen technology. Indium is chemically similar to its neighbor on the periodic table, tin, and the compound indium-tin oxide is an electrically conductive, optically transparent coating used in televisions, smartphones, and laptops that enables us to interact with our daily devices, order food at a restaurant kiosk, and navigate with ease on our vehicle’s GPS system. The West Desert skarn is the only established domestic resource of indium, and the deposit could meet the domestic indium demand for several years if it was produced today.

At the West Desert site, indium and zinc are identified in the porphyry-related skarn zones, and historical working of the area produced minor amounts of lead, silver, zinc, and gold from 1890 to 1953. Geologically, indium and zinc are commonly related since indium is found in the zinc ore mineral sphalerite, and as a result most indium is produced as a by-product of zinc mining. The mineralization at West Desert is the result of skarn alteration caused by Eocene-age (about 40 million years ago) magmatism. A skarn is formed when fluids, transporting metals in solution such as indium, leave hot igneous rock and move through the existing host rock. At West Desert, the fluids altered shales interbedded within massive limestone and dolomite rocks of the Ordovician-age (about 460 million years ago) Wah Wah Limestone and Kanosh Shale, and members of the Cambrian-age (about 520 million years ago) Orr Formation. To understand why and how critical minerals are concentrated in this deposit, we are collecting samples for geochemical analysis which will allow comparison of altered and unaltered rocks exposed at the surface above the deposit.

Additionally, the West Desert deposit is not exposed at the surface, and our investigation provides an opportunity to understand the subtle expression of this mineralizing system at a distance from the deeper resource. This “blind deposit” approach to our geochemical sampling program will provide data that future exploration geologists can use to identify deposits elsewhere that are not exposed at the earth’s surface. We are also collecting high-resolution photogrammetry of key areas via small Unmanned Aerial Systems. Our drones will be flown low to the ground since this field site is extremely close to the southern edge of the U.S. Department of Defense Utah Testing and Training Range. We aim to integrate these two datasets (geochemistry and high-resolution photography) to document and evaluate the surficial alteration in three dimensions.

Phosphoria Formation

A map of the northern half of Utah, with locations of Earth MRI project sites.

The UGS’s Earth MRI project sites.

A second Earth MRI project will seek to determine if significant concentrations of critical minerals are hosted in the Permian-age Phosphoria Formation, which was deposited over 250 million years ago. The Phosphoria Formation is a current source of phosphate in both Utah and Idaho, and several million tons of phosphate rock is mined from the formation each year near Vernal, Utah. Lesser amounts of phosphate rock are produced near Diamond Fork in Utah County. The phosphate rock from these mines is primarily used in manufacturing phosphate-bearing fertilizers. The marine conditions that were favorable for depositing phosphate minerals such as fluorapatite are also known to concentrate and deposit anomalous amounts of certain critical minerals such as REEs, vanadium, and fluorine (or fluorspar), all of which the U.S. currently imports. REEs are used in many high-tech devices such as smartphones, electric vehicles, and flat-screen televisions; vanadium is a key component in producing certain types of strong steel alloys; and fluorine is important for producing refrigerants and aluminum, to name a few applications of these critical minerals. Some potential may exist to produce these minerals as by-products from active or future phosphate mines. For example, some by-product fluorosilicic acid (a fluorine compound that offsets the need for fluorspar) is already being produced from domestic phosphate rock. However, any potential is mostly speculative at this point because we have little information on the actual critical mineral content of phosphate rock in Utah.

Backed by funding from Earth MRI, the UGS is working cooperatively with the Idaho, Montana, and Wyoming Geological Surveys to characterize the geochemistry of the Phosphoria Formation and fill this data gap. Each geological survey is selecting sites in their respective states to measure, describe, and sample the Phosphoria Formation where it crops out or from drill core. The samples collected from the formation will be analyzed for a long list of elements that will include REEs, vanadium, and fluorine. These data will provide some basis for assessing the potential for future production of critical minerals from Utah’s phosphate rock. This project presents a great opportunity for the UGS to use federal funding to study and analyze Phosphoria Formation drill cores that were donated in recent years to the Utah Core Research Center. Currently, we are selecting other strategic sites in Utah where information on the Phosphoria Formation, particularly geochemistry, would be beneficial.

As the Utah Geological Survey takes advantage of current interest and funding in critical mineral research, we hope to provide timely information on the state’s critical mineral resources to support responsible and sustainable management of those assets into the future.

Jake Alexander

is a geologist in the Energy & Minerals Program who joined the UGS in 2022. He has a B.S. degree in geology from Texas A&M University–Corpus Christi and an M.S. degree in geology from the University of Tennessee. Jake investigates Utah’s metal and industrial mineral potential across the state and currently focuses on critical minerals.

Andrew Rupke

is a senior geologist in the Energy & Minerals Program who joined the UGS in 2010. He has a B.S. degree in geology from Calvin College in Grand Rapids, Michigan, and an M.S. degree in geology from the University of Utah. Andrew’s work and research at the UGS focuses on Utah’s diverse industrial mineral resources including potash, phosphate, salt, high-calcium limestone, aggregate, gypsum, and others.

Stephanie Mills

is a senior geologist in the Energy & Minerals Program who joined the UGS in 2019. She has a B.S. Hons degree from the University of Texas at Austin and a Ph.D. from Monash University. Stephanie is an economic geologist with over a decade of experience researching and working in minerals exploration across the globe. Her specialization is in magmatic-hydrothermal systems with a focus on porphyry, skarn, epithermal, and Carlin-style deposits.