The Collaborative International Dictionary of English v.0.48:

Placer \Pla"cer\, n.
   One who places or sets. --Spenser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

placer
    n 1: an alluvial deposit that contains particles of some
         valuable mineral