Sample of a marine placer gold deposit

A placer deposit is a naturally occurring phenomenon in which detrital or residual material containing valuable minerals is accumulated through weathering of the source rocks and mechanically deposition either on land or in large water bodies such as lakes or oceans. Primary mineral elements such as gold, silver, and diamonds are examples of placer  materials. Cronan (1980) classifies placer minerals into two broad groups: namely, heavy mineral placers and light heavy mineral placers, based on the specific gravity of the minerals. Minerals such as cassiterite, platinum, and gold are high specific gravity minerals with specific gravities ranging from 6.8 to 21.



The light heavy minerals are rutile, ilmenite, zircon, monazite, and diamond, with
specific gravities ranging from 3.5 to 5.3. In general, there are five requirements needed to form placer deposits.

➤ Source rocks containing valuable minerals (light heavy and heavy minerals)
➤ Weathering of the source rocks to release the valuable minerals
➤ Transportation of the valuable minerals released by the weathering process and subsequent redistribution by natural processes
➤ Concentration of transported material on the basis of size, shape, and density either on the land or in large water bodies such as lakes or the sea
➤ Preservation of concentrated material and protection from natural redistribution or re-transportation through the ambient environment.

Cronan (1980) identified the important economic near-shore marine placer deposits known at that time. The significant marine/beach placer deposits are diamonds found along the west coast of South Africa and Namibia; gold and cassiterite deposits in Egypt; ilmenite, rutile, and zircon deposits in southern India and Sri Lanka; cassiterite deposits along the coasts of Indonesia, Malaysia, and Thailand; rutile, zircon, and cassiterite deposits along the Tasmanian coast; gold, ilmenite, and rutile deposits along the South Island coast of New Zealand; and gold deposits along the Alaskan coast.

by P. Saravanakumar†Z*, G.J. Brown†, and G. van Eck*

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