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diamond deposit types

The Two Types of Diamond Deposits

Geologic processes create two basic types of diamond deposits, referred to as primary and secondary sources. Primary sources are the kimberlite and lamproite pipes that raise diamonds from Earth's mantle, where they originate. Secondary sources, created by erosion, include such deposits as surface scatterings around a pipe, concentrations in river channels, and fluxes from rivers moved by wave action along ocean coasts, past and present. Mining of these deposits depends upon sufficient concentration and quality of diamonds.

The diagram here shows the trail of diamonds left by geological processes.

The primary deposits, or diamond pipes, are the vertical portion. The flared top of the pipes can yield substantial quantities of diamonds, but following the narrowing pipe downward eventually becomes unprofitable. Note how erosion of the landscape moves surface minerals -- including the diamonds -- from the pipes down hills, streams, and rivers to their ultimate destination, the ocean.

Because diamonds are dense they concentrate at the bottom of active zones of moving sand and gravel. These secondary deposits are eluvial Ð above a pipe, colluvial -- adjacent to a pipe, alluvial Ð stream and river transported, and marine -- along beaches that can wind up onshore or offshore with changing sea level. Secondary deposits may be found far from active means of transport, in the fossilized channels of now-vanished rivers or under fossil beaches.