Source : Webster's Revised Unabridged Dictionary (1913)

Placer \Pla"cer\, n.
   One who places or sets. --Spenser.

Placer \Plac"er\, n. [Sp.]
   A deposit of earth, sand, or gravel, containing valuable
   mineral in particles, especially by the side of a river, or
   in the bed of a mountain torrent. [U.S.]

Source : WordNet®

placer
     n : an alluvial deposit that contains particles of some valuable
         mineral