Fluid inclusion evidence for the temperature and composition of ore fluids in the Lemhi Pass and diamond Creek Ree-Th districts, Idaho-Montana

Abstract

The Lemhi Pass district is the largest thorium (Th) and 5th largest rare earth element (REE) occurrence in the U.S., straddling the continental divide along the central Idaho-Montana border. The Diamond Creek district, thought to be genetically related to Lemhi Pass, is the 19th largest known REE occurrence in the U.S., has substantially less thorium, and is located about 28 km northwest of the Lemhi Pass district. Thorium min-eralization in both districts occurs predominately as thorite (ThSiO4), and REE mineralization occurs predominantly as monazite ([Ce,La,Nd,Th]PO4). The Th-REE mineralization is hydrothermal and occurs in veins filling faults, fractures, and shear zones.