EXPLORATION FOR RARE EARTH ELEMENTS (REE) IN HEAVY MINERAL, BEACH-PLACER DEPOSITS IN THE SAN JUAN BASIN, NEW MEXICO

The San Juan Basin contains many heavy mineral, beach-placer sandstone deposits in Cretaceous Gallup, Point Lookout, and Pictured Cliffs Sandstones. These deposits are accumulations of heavy, resistant minerals that form on the upper regions of beaches or in long-shore bars in a marginal-marine environment. They form by mechanical concentration (i.e. settling) of heavy minerals by waves, currents, and winds. Most of these minerals have a high specific gravity >4. Modern examples of beach-placer sandstone deposits are found along the Atlantic Coast, US, SE Australia, and Andhra Pradesh, India, where they are mined for Ti, Zr, and locally, monazite. Other potential commodities include Nb, Cr, Th, and REE. These elements are becoming increasingly more important to industry, because they are used in the manufacture of devices our society uses every day.

The majority of the NM deposits are in olive green to brown to black, moderately to well cemented, medium- to fine-grained sandstones composed mostly of quartz, some lithic fragments, feldspar, and heavy minerals that were deposited in a beach environment. Cementation is mostly Fe oxide and silica; modern deposits are unconsolidated. Local high concentrations of Ti, Fe, Cr, Nb, Th, U, Zr, Sc, and REE are found in in these deposits. The REE plots exhibit light-REE chondrite-normalized enriched patterns, typically with negative Eu anomalies. Heavy minerals include ilmenite, leucoxene, rutile, zircon, monazite, xenotime, tourmaline, chromite, garnet, and gold. The largest exposed deposit in NM is at Sanostee, which is in the Gallup Sandstone. The Apache Mesa and Standing Rock deposits are in the Point Lookout Sandstone. Drilling at Apache Mesa indicates a potential resource of 120,564 metric tons of ore with grades of 3% TiO₂, 2,187 ppm Zr, and 522 ppm TREE. However, more detailed mapping, drilling and sampling are required of the other deposits to adequately determine their resource potential.

It is unlikely that any of the beach-placer sandstone deposits in the San Juan Basin will be mined in the near future because of small tonnage, low grades, high Fe content, and distance to processing plants and markets. However, as the demand for these elements increases because of increased prices and short supplies, the dollar value per ton of ore rises, enhancing deposit economics.