Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 125

Fig. 5.4 Photos of alluvial
diamonds (Source: www.
diamondfields.com )
Fig. 5.5 Techniques used to extract diamonds from offshore deposits
5.2.4 Other Placers: Tin, Platinum, Thorium-Uranium
The primary source of tin is cassiterite (SnO 2 ) a magmatic mineral that crystallizes
in an unusual type of granite. The so-called “tin granites” have a rather restricted
distribution, both in space and geological time; they are very rare in the Precam-
brian and become more abundant in later epochs. The best known examples are on
the Malayan Peninsula, an endowment that has made Malaysia the world's greatest
producer of the this mineral. About half the deposits are in the granites themselves
and the other half are in placers in rivers, beach sands and offshore deposits.
Much the same story applies to the platinum-group elements. The primary
source is in ultramafic rocks such as those of the Bushveld Complex (Chap. 3)
but in certain regions, nuggets of PGE alloys released by weathering and erosion of
ultramafic rocks have produced viable placer deposits. In some cases the sources
are ore deposits in their own right, as in the ultramafic complexes of Siberia; in
other cases ophiolites with no commercial ore concentrations have yielded the ore
minerals, as in New Caledonia.
Thorium and uranium are hosted in uraninite in the Witwatersrand ore bodies
which, as discussed above, may be of alluvial origin, and in monazite in beach
sands. The beaches flanking some parts of India contain high concentrations of
monazite and the country is currently developing a new type of nuclear reactor that
uses Th as a fuel.