Alluvial placer deposits

Although estimates of their abundances vary considerably, Pd and Pt (approximately 0.015 and 0.01 ppm respectively) are much rarer than Ni. They are generally associated with the other platinum metals and occur either native in placer (i.e. alluvial) deposits or as sulfides or arsenides in Ni, Cu and Fe sulfide ores. Until the 1820s all platinum metals came from South America, but in 1819 the first of a series of rich placer deposits which were to make Russia the chief source of the metals for the next century, was discovered in the Urals. More recently however, the copper-nickel ores in South Africa and Russia (where the Noril sk-Talnakh deposits are well inside the Arctic Circle) have become the major sources, supplemented by supplies from Sudbury. 

Ore types where gravity preconcentration is used include Alaskan-type deposits, alluvial and fossil placer deposits. 

Free metallic gold is found in veins of rocks and in ores of other metals. Alluvial gold (placer deposits) is found in the sand and in the gravel at the bottom of streams where it has been deposited as a result of the movement of water over eons. 

A placer deposit is a glacial deposit or alluvial deposit (made by a river, lake, or arm of the sea), as of sand or gravel, containing gold or other valuable material. 

These deposits are formed in the natural process of weathering, transportation, and concentration at the site of heavy minerals originating from some primary source rock. Commercially, beach placers are the most important along with alluvial streams aeolian deposits are insignificant. 

Tin-mining methods depend on the character of the deposit. Primary deposits are embedded in undeiground granitic rock and recovery methods are complex. The more important secondary deposits are in the form of an alluvial mud in the stream beds and placers and the recovery is simpler than lode mining. Cassiterite is recovered from alluvial deposits by dredging, hydraulicking where a head of water permits it, jets and gravel pumps on level... 

The abundance of tin in the Earth s surface is about 2 ppm, significantly less than that of zinc (94 ppm), copper (63 ppm), or lead (12 ppm). The most important ore is cassiterite, Sn02, which occurs as placer (alluvial) deposits. The breakdown of the current production of tin by area is shown in Figure 1.1.1. About 75% of the world s production comes from China and South East Asia, and about 18% from South America, but the annual figures are sensitive to political, social, and economic factors. ... 

Placer Alluvial, marine, or glacial deposit containing particles of valuable minerals, especially gold. 

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