a waterborne or glacial deposit of gravel or sand containing heavy ore minerals, as gold or cassiterite, which have been eroded from their original bedrock and concentrated as small particles that can be washed out
See placer in American Heritage Dictionary 4
(plăsˈər)
noun
A glacial or alluvial deposit of sand or gravel containing eroded particles of valuable minerals.
A place where a placer deposit is washed to extract its mineral content.