OF-26-08 Geochemical and Petrogenetic Characteristics of REE-Bearing Carbonatites, Pegmatites and Syenites from the Wet Mountains, South-Central Colorado

This publication explores the geochemical and petrogenetic characteristics of certain rock types enriched in rare earth elements (“REEs”). The study area for this project is the Wet Mountains region in south-central Colorado, and this analysis was conducted in concert with 1:50,000-scale geologic mapping of the area, available here:

OF-23-07 Digital Geologic Map of the Wet Mountains Area and Associated Geochemical Data, Custer, Fremont and Huerfano Counties, Colorado

Included with this report are geochemical analysis results (Excel spreadsheets) and accompanying GIS files. Digital ZIP download. OF-26-08D

From the abstract:

Carbonatites are the primary REE host rocks and are characterized by elevated light REEs (LREE: La, Ce, Pr, Nd, Sm, Eu, and Gd), although significant heavy REE (HREE: Tb, Dy, Ho, Er, Tm, Yb, Lu, plus Y) enrichment is also present in some samples. These carbonatites commonly contain high concentrations of Th, Mn, Zr, and Ba. Selected mafic and felsic plutonic rocks and pegmatites within the study area also contain elevated REE concentrations, particularly HREE‑rich patterns. These plutonic rocks are characterized by high Mn, Ba, and Th, along with anomalous Cr, Ni, Pb, and Zn.

The new geologic and geochemical dataset supports interpretations derived from the recent U.S. Geological Survey (USGS) airborne magnetic and radiometric survey over the Wet Mountains and constrains the nature of surface and potential subsurface REE‑bearing anomalies. A subset of three carbonatite samples contains “ore‑grade” LREE concentrations comparable to those in major REE carbonatite deposits such as Elk Creek, Bear Lodge, and Mountain Pass. In addition, some carbonatites and REE‑bearing syenites and pegmatites have REE distributions broadly similar to the HREE‑enriched Round Top rhyolite deposit in Texas, suggesting multiple mineralization styles and exploration targets within the district.