CRITICAL MINERALS IN BEACH PLACER DEPOSITS OF THE FOX HILLS SANDSTONE, ELBERT COUNTY, COLORADO

Beach placers deposited along paleoshorelines of the Fox Hills Sandstone in Elbert County, Colorado, contain heavy minerals typically associated with this deposit type. These marine beach placers occur along the eastern margin of the Denver Basin and were deposited during the Late Cretaceous along the western edge of the retreating Western Interior Seaway. Preliminary investigations in the 1960s and expanded exploration in the 1990s determined that a portion of these deposits contain potential critical mineral resources including titanium and zirconium. These previous investigations also reported the presence of REE-bearing minerals.

Analysis of limited surface outcrops indicates that the heavy minerals occur within laminae and thin beds associated with very fine to medium-grained, light-grey, friable to moderately cemented sandstone. Historical exploration reported subsurface beach placers along linear northwest trends over a distance of 20 km. The upper Fox Hills Sandstone in this area includes forebeach, backbeach, and washover deposits. The heavy minerals are most concentrated in the backbeach environment where occasional evidence of high energy washover episodes are indicated by rip-up clasts of older heavy mineral beds which occur along sharp undulating scour surfaces. Detrital zircon analysis indicates a maximum age for the heavy mineral deposits of 74 Ma which is correlative with the regression of the Late Cretaceous seaway in this area, but slightly older than the B. Clinolobatus ammonite zone locally correlated just below the Fox Hills Sandstone.

Preliminary mineralogical analysis of the heavy mineral suite using SEM-based automated mineralogy reported garnet, ilmenite, rutile, titanite, zircon, tourmaline, allanite, monazite, and xenotime. Individual laminae can contain up to 50% heavy minerals with 34% titanium-bearing minerals (ilmenite, rutile, and titanite), 18% garnet, 5.4% zircon, and 1.8% allanite. Handheld XRF screening of samples with heavy mineral-rich laminae detected an average of 6.8% Ti and 1.3% Zr. Laboratory analysis by XRF and ICP-MS detected an average of 8.2% Ti, 1.3% Zr, and total REEs ranging from 637 to 4,909 ppm with an average of 3,326 ppm. Chondrite-normalized REE plots indicate enrichment of the light REEs.