Gold, silver, and platinum are relatively rare in the universe. For platinum, the estimated abundance is about 5 x 10^-7% (about the 47th most common element), while silver and gold each make up about 6 x 10^-8% (around #69-70) of the atoms in the universe.

http://www.periodictable.com/Properties/A/UniverseAbundance.v.log.html

Since gold, silver, and platinum are all heavier elements than iron (actually, nickel-56, which is radioactive and decays to produce iron-56), they cannot form through normal stellar nucleosynthesis. Instead, heavier elements are formed in supernova explosions, where the temperature and pressure far exceed those at the core of even the most massive main-sequence star. Supernovae are still occurring, and the minimum mass for a core-collapse supernova is about 8 solar masses, so any star with a mass greater than this can be expected to produce gold, silver, and platinum as it reaches the end of its life (this includes main-sequence stars with spectral types from O through early B).

http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html
http://astronomyonline.org/Stars/HighMassEvolution.asp
http://www.world-builders.org/lessons/less/les1/StarTables.html

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