Metal concentrated by water and gravity in river gravels rather than locked in hard rock.
A surficial mineral deposit formed by the concentration of heavy minerals through weathering and sedimentary processes. Placer gold deposits in streams and rivers were the target of historic gold rushes and can still be mined economically.
Placer gold is free, coarse and recoverable by gravity alone, which makes it cheap to work and historically the first thing found in any district. Its real significance to an explorer is as a pointer to the hard-rock source upstream.