The base metal ranges from copper to various alloys. Some older cpus even have steel substrates.
Intel 486's and Pentium I's are typically copper pins, with a thin nickel overlay then gold plated. The Intel MMX cpus families (fiber cpus with legs) are usually kovar. The other miscellaneous cpu families like Motorola's , TI's, LSI fiber cpus, etc vary with type.
The best source of CPU package construction data can usually be found on the manufacturers website.
Steve