Can plutonium be found in nature?

Some transuranic elements have only been created in the laboratory. But trace amounts of at least four plutonium isotopes (plutonium-238, -239, -240, and -244) can be found in nature. Small traces of plutonium-239, a few parts per trillion, and its decay products are naturally found in some concentrated ores of uranium, such as the natural nuclear fission reactor in Oklo, Gabon. The ratio of plutonium-239 to uranium at the Cigar Lake Mine uranium deposit ranges from 2.4×10^-12 to 44×10^-12. Even smaller amounts of primordial plutonium-244 occur naturally due to its relatively long half-life of about 80 million years. These trace amounts of plutonium-239 originate in the following fashion: on rare occasions, uranium-238 undergoes spontaneous fission, and in the process, the nucleus emits one or two free neutrons with some kinetic energy. When one of these neutrons strikes the nucleus of another uranium-238 atom, it is absorbed by the atom, which becomes uranium-239. With a relatively short half-life, uranium-239 decays into plutonium-239.