Geology question about minerals and fossil?

Quartz and calcite are very common. There are examples of pyritized trilobites in New York state (triarthurus eatoni in particular) and brachiopods as well. But I have also seen fossils that have been replaced by copper, turquoise, chrysocolla and even opal. Elemental uranium would be just about impossible and especially elemental iron. Elemental iron is a very rare substance as it is usually found combined with other elements.