When fluids supersaturated with silica and other minerals penetrate the remains of organisms with abundant pore systems such as trees, the mineral precipitates within the pores and the original material (at least most of it) eventually rots or dissolves away. This leaves behind a mineralized version of the original organism.
The best-known example of permineralization is petrified wood, which is abundant at Florissant. About 34 million years ago, a volcanic mudflow called a lahar killed and buried a forest of redwoods and other trees. Eventually the portion of the trees above the mudflow rotted away, leaving the encased trunks buried. The wood was slowly replaced with silica from supersaturated ground water. Detailed study has shown what possibly could be original plant material left behind in some of the stumps, but for the most part they are completely replaced with silica. The wood is preserved so well that scientists can study the structure under a microscope, down to the individual cells.
Fossil bones and teeth of mammals, birds, and fish are also preserved by permineralization in many cases.