Answer
Only tiny amounts of plutonium occur naturally in U-238 deposits.
Work Step by Step
When U-238 captures a stray neutron, it turns into U-239, then becomes Np-239, then transforms into Pu-239. Plutonium has a relatively short half-life compared to U and Th, so much of it has decayed.
This is discussed on page 643, in the Check Point.
