Tellurium in Igneous-related Epithermal Precious Metal Deposits in Colorado and New Mexico

Science Center Objects

The project aims to improve our understanding of the causes of tellurium enrichment in epithermal precious metal deposits, and strengthen our ability to assess the Nation's tellurium deposits. Tellurium is used in solar panel technologies and is considered a critical mineral. Epithermal deposits of this type represent a prospective future source of tellurium.

scientists in field looking at rocks

USGS geologists and collaborator from New Mexico Bureau of Geology and Mineral Resources examine rocks in the Jicarilla Mountains, New Mexico.

(Credit: Karen Duttweiler Kelley, U.S. Geological Survey. Public domain.)

Science Issue and Relevance

Tellurium, which is used in metal alloys, semiconductors, and in the solar energy field, is recovered as a primary resource from only two locations in the world, and currently about 85% of global production of tellurium comes as a byproduct of copper mining through a process called electrolytic refining. There is currently a shift in the copper production mode (to increase copper recovery) from electrolytic refining to solvent extraction-electrowinning (SX-EW) and recycling, which do not produce tellurium as a byproduct. It is imperative that we understand the enrichment processes of potential alternative sources of tellurium, such as epithermal gold deposits. From a national security viewpoint, it is a great benefit that there are known domestic occurrences of these deposits.

Tellurium in Colorado and New Mexico Deposits: It is commonly assumed that most epithermal gold deposits associated with alkaline igneous rocks (deficient in silica with respect to alkalis) are enriched in tellurium, and within Colorado and New Mexico, many deposits are indeed tellurium-rich. However, many known alkalic-related deposits contain low tellurium concentrations. The contrast between tellurium-rich and tellurium-poor alkaline-related epithermal gold deposits may be attributed to differences in underlying basement compositions (and indirectly, magma composition), although this is not well documented. In general, the causes of enrichment remain unclear. The crust beneath Colorado is anomalously thick, and radiogenic isotopic evidence suggests that the composition of underlying basement compositions (and indirectly magma composition) may have contributed to the enrichment of tellurium in Colorado deposits compared to similar, but tellurium-poor systems in New Mexico. In general, the causes of enrichment remain unclear and is the rationale for the current study.

Image shows a Fluorite-Tellurite Sample

High-grade mineralized rock from Cripple Creek, CO containing purple fluorite and telluride minerals (in vugs). Image credit: USGS. 

(Public domain.)

Methdology to Address Issue

The project objective is to determine the factors responsible for tellurium enrichment in alkaline rock-related epithermal gold deposits in Colorado and New Mexico, and develop techniques for assessing areas of potential for these deposits. The initial focus of our research will be on the tellurium-rich deposits at Cripple Creek, Colorado and one tellurium-poor system in New Mexico (Jicarilla Mountains). Previous research at Cripple Creek has provided a foundation of abundant existing data. The project has three primary areas of study:

  • Geology, geochemistry, mineralogy, and geochronology - We will analyze unmineralized basement and magmatic rocks from Cripple Creek, Colorado, and unmineralized and mineralized samples from the Jicarilla Mountains, New Mexico for tellurium and other elements, conduct age-determinations, and analyze mineral assemblages.
     
  • Processes of enrichment - We will use integrated analytical techniques to determine the chemical parameters that allow for tellurium enrichment, transport, and deposition and will analyze melt and fluid inclusions to determine conditions prior to and during mineralization.
     
  • Exploration methods - We will use geophysical data sets to determine the geophysical expression of ore-related igneous rocks and use this information to assess additional resources under cover and link observed geophysical anomalies to physical property measurements and geochemical analyses.

This project is a field and laboratory based investigation on tellurium resources in epithermal gold deposits that is designed to answer these questions:

  • Is parental magma composition a controlling factor?
  • What are the processes of tellurium transport in magmatic-hydrothermal fluids?
  • Are there specific zones within a district where tellurium is found?
  • Can we fingerprint tellurium-bearing systems from those that are tellurium-poor using geophysical parameters as proxies for basement or magma composition (e.g., physical or magnetic properties)?