Placer Deposits

Probably the simplest of the syngenetic deposits is the placer. If a mineral is chemically stable and physically resistant, it can eroded from its primary hard-rock occurrence and transported to river channels, deltas or other sedimentary environments where it can be deposited in a sedimentary bed. Gold, diamonds and other gemstones are found in placer deposits because the can withstand most geological underground and surface processes, remaining intact after most other mineral constituents of a rock have been altered or oxidized. The deposit can remain unconsolidated or can be buried and ultimately turned into a sedimentary rock. The gold-bearing conglomerates of South Africa and the uranium-bearing conglomerates of Ontario are widely thought to be old gravel placers that have solidified into rocks.

Placer diamonds have been found in India, Brazil, Venezuela, the Democratic Republic of Congo and several other African countries. In recent years, the search for placer diamond deposits has even been extended to the ocean floor, notably off the southwestern cost of Africa. But just as all placer gold has its origins in bedrock of one type or another, placers of gem minerals count certain types of bedrock as a source.