The most common source of thorium is the rare earth phosphate mineral, monazite.
From the forbes.com
Thorium has been extracted chiefly from monazite through a complex multi-stage process.
From the en.wikipedia.org
World monazite resources are estimated to be about 12 million tons, two-thirds of which are in India.
From the forbes.com
A few grains of monazite are also found in the black streaks.
From the en.wikipedia.org
The phosphate is, in this case, contained within magmatic apatite, monazite or other rare-earth phosphates.
From the en.wikipedia.org
Kerala's coastal belt of Karunagappally is known for high background radiation from thorium-containing monazite sand.
From the en.wikipedia.org
Some of the country's beach sands contain monazite that yield thorium, which find application in fuels for nuclear reactors.
From the online.wsj.com
Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
From the en.wikipedia.org
The residual rare earth phosphate mineral monazite, left over from the original bauxite, could be treated with alkali to yield other rare earth hydroxides.
From the newscientist.com
More examples
A reddish-brown mineral containing rare earth metals; an important source of thorium and cerium
Any of a range of reddish-brown minerals that are mixed phosphates of the lighter rare earth elements lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium and yttrium
A rare earth phosphate mineral ((Ce,La, Y, Th)PO4) mineral which is common both as minor mineral component in granites and is also common in mineralised lodes in south-west England . Nematodes. ...
A phosphate of the cerium metals (cerium, didymium, lanthanum) and other rare-earth metals. Monazite-bearing alluviums have been mined for their thorium content (by dredging) in Idaho and elsewhere.