RMS-AAPG: “Critical Minerals from Sea Bug Corpses”

**Offered as part of the 2026 Annual RMS-AAPG Annual Conference

Field Trip Leaders: Prof. Adrian Van Rythoven (MBMG) and Prof. Marc Hendrix (University of Montana)

Field Trip Fee: $50 for professionals; $20 for students

Field Trip departs from and returns to Montana Tech campus.

Field Trip Highlights: This field trip shows correlations between trace element concentrations and depositional environments.  The Permian age Phosphoria Formation, found in Montana, Idaho, Utah and Wyoming, was deposited during marine transgressive-recessive cycles in a foreland basin during the assembly of the Pangean continent.  The Retort and Meade Park members contain high phosphate and hydrocarbon content of the formation.  At the depositional margins of the Retort depocenter, very well-sorted ore grade peloidal phosphorite is present.  Elevated levels of cadmium, chromium, copper, fluorine, molybdenum, nickel, rare earth elements (REE), selenium, uranium, vanadium, and zinc were also found in peloidal phosphorite.  The exceptionally high concentrations of metals have been interpreted to reflect fundamental transitions from anoxic to oxic marine conditions.

Phosphoria Formation was historically mined in Montana for phosphorous until the 1990s.  In the 1950s and 60s, the formation was also evaluated for vanadium and uranium. With recent economic and political pressures for domestic rare earth elements (REE), the Phosphoria has received another look as a potential resource. REE mineralization in the Phosphoria has been noted historically, but with recent economic and political pressures for domestic REE production, a deeper understanding of how the depositional environment impacts the location and quantities of REE mineralization has become a priority.

This field trip consists of a trip to the former Edgar Mine, a source of peloidal phosphorite, located south of Drummond.  At the mine site, samples from the Retort member obtained from other abandoned or reclaimed mines from southwestern Montana will be provided for viewing, along with their assay results and description of their lithologies.  This information is to demonstrate that lithologies and assay results can be used as a method in determining whether economic REE production is realistic.  The samples used in this trip are discussed in the Montana Bureau of Mines and Geology Special Publication 124.

Questions? Contact 2026 RMS-AAPG field trip co-chairs Mary Bitney and Jake Thacker (rmsaapgmeeting@gmail.com)