Written by Jarrod Burges, EPS Graduate Student
The electrification of America has driven the need for critical minerals — copper and rare earth elements (REEs) chief among them. Deposits like Mountain Pass, California (REEs) or Morenci and Bisbee, Arizona (copper) are the household names. However, there's a whole generation of mines that closed decades ago and never got a second look. Many copper deposits may host secondary mineralization of primary REE minerals (e.g., Bastnäsite, Monazite, Allanite) that contain REEs in their chemical structure.
Why Old Mines Deserve a Second Look
Between the late 1800s and the 1970s, many historic mines across the Southwest were shut down. As bigger deposits were discovered and higher-grade ore was depleted these historic mines were no longer economically profitable. On top of that, almost none of these sites were ever analyzed for REEs since there was no analog-age use for them. While some mines were analyzed for a single REE, they were solely used for geochemical interpretation rather than economic use.
Fast forward to today: the minimum ore grade worth extracting has dropped substantially. Advancements in mining engineering have made lower grade rock economically viable. Furthermore, mine tailings called “gangue” that was "waste" fifty years ago can be profitable now. The cutting edge of economic geology and mining engineering research is investigating the viability of gangue deposits and closed “depleted” mines.
Arizona's Copper, and a Surprising Gap in the Story
Arizona produces the vast majority of the United States' copper supply, yet the details of how that ore actually got emplaced across the Southwest remain surprisingly underexplored. That's the gap my collaborators and I are digging into, quite literally.
Through a collaboration with my undergraduate alma mater, California State University, Fullerton, we're reanalyzing historic copper mines in the Whipple-
Buckskin-Rawhide Mountains of southwestern Arizona. This range straddles San Bernardino County, CA, and La Paz County, AZ, split by the Colorado River near Lake Havasu and Parker Dam. Our first round of fieldwork centered on the Mammon, Three Story, and Eagle Nest Mines in the Buckskin Mountains.
What makes this area geologically interesting is its structure: kilometer-scale, low-angle detachment faulting — a structural style that's fairly rare among copper deposits and sets this region apart from the more "typical" porphyry systems most people picture when they think of Arizona copper.
The Team Behind the Fieldwork
This past Memorial Day weekend, I headed into the field with:
- Dr. Patrick Phelps (Assistant Professor, CSUF)
- Nick Burdy (M.S. Student, CSUF)
- Sadie Durning (Ph.D. Student, Texas Tech)
What started as an informal side project between Dr. Phelps and me has grown quickly. It's now the foundation of Nick's Master's thesis, and we've since brought on additional collaborators from Occidental College and the Arizona Geological Survey. Altogether, the team spans economic geologists, igneous petrologists, structural geologists, and computational geologists — a mix that feels necessary given how unconventional the emplacement setting is here.
What's Next
This summer, we’re organizing and prepping samples for laboratory analysis. We plan to create thin sections and mineral separate for laser ablation mass spectrometry. Furthermore, certain mineral ore samples will be separated and analyzed at Rutgers in the Morrison Lab X-ray Diffractometer instrument. Our goal is to acquire preliminary data for the next tentative field session in December, when temperatures are not 95 F like this last time.