Here is a pdf with info on deposits in the USA
Idaho and Colorado are specified as primary areas in the pdf.

"Wet Mountains Area, Colorado
This thorium district, which is located in Fremont and
Custer Counties of south-central Colorado, may be comparable
in thorium resources to the Lemhi Pass district of Idaho-Mon-
tana (table 2). The thorium deposits occur over an area of about
60 km north-to-south by 24 km west-to-east. Thorium occurs
in veins, syenite dikes, fracture zones, and carbonatite dikes
(Armbrustmacher, 1988) associated with three Cambrian-age
alkaline complexes (Olson and others, 1977) that intruded the
surrounding Precambrian terrane. These alkaline complexes are
(1) the McClure Mountain Complex (Shawe and Parker, 1967;
Armbrustmacher, 1984), (2) the Gem Park Complex (Parker
and Sharp, 1970), and (3) the complex at Democrat Creek
(Armbrustmacher, 1984). The thorium-mineralized veins and
fracture zones, which are distal to the three alkaline intrusive
complexes, have the highest economic potential for thorium
resources. The USGS estimated that the vein and fracture zone
deposits of the Wet Mountains area contain reserves of 58,200
t of ThO2 and probable potential resources of 145,600 t of
ThO2; the seven largest carbonatite dikes in the district contain
additional reserves of 119 t of ThO2 and probable potential
resources of 683 t of ThO2 (Armbrustmacher, 1988).
"

https://pubs.usgs.gov/circ/1336/pdf/C1336.pdf