Classification, Distribution and Uses of Ores and Ore Deposits - Metals and Society: An Introduction to Economic Geology - page 27

others. What distinguishes the two lists is the minor importance of metamorphism
in the list of important ore-forming process and, in its place, a major class compris-
ing deposits that are linked to hydrothermal fluids. When the operation of the Earth
as a whole is considered, the circulation of hot aqueous fluids through the crust is
normally mentioned only as an agent that alters the composition or texture of
primary magmatic or sedimentary rocks; the same process, however, lies at the
origin of a vast range of important ore deposits and has created well over half of all
ore bodies that are known to exist.
Consider now the first category in Table 2.6 , magmatic deposits. We note that
many types of large and important deposits are found in mafic-ultramafic rocks and
only a few less important types in evolved, felsic or silicic, rocks. Many ore
deposits are indeed hosted by granites, but according to modern ideas of ore
genesis, such deposits generally result from the precipitation of ore minerals from
aqueous fluids and not from the granitic magma itself. The type of ore mineral in
magmatic deposits is directly linked to the composition of the host rock. In mafic-
ultramafic hosts we find deposits of Ni, Cr and platinum-group elements, all of
which partition strongly into minerals that crystallize early in normal magmatic
differentiation. Ores in felsic rocks, by contrast, are confined to elements that
concentrate in evolved magmatic liquids. Some of these are present in late-
crystallizing phases such as ilmenite, which contains Ti, and cassiterite, the ore of
Sn; others enter the water-rich fluid that separates from the silicate liquid, to be
redeposited in pegmatites or in hydrothermal ore bodies. Pegmatites are important
sources of rare but increasingly important metals such as Li and Be.
Some metals are restricted to a single type of ore-forming process, the best
example being Cr, which, with virtually no exceptions, is mined as chromite,
a magmatic oxide that accumulates during the crystallization of mafic or ultramafic
magmas. Some very minor placer (sedimentary) chromite deposits have been
mined, but hydrothermal deposits are unknown. More than 98% of Al is mined as
bauxite, a lateritic soil that forms in hot and humid climates; but the same metal is
also extracted in a Russian mine in nepheline syenite, a magmatic rock. Most
metals, however, occur in deposits of diverse origins. Alloys of the platinum-
group elements and cassiterite, an oxide of tin, are cited as type examples of
magmatic ores, but when the magmatic host rocks are exposed to erosion at the
surface, the same minerals may become re-concentrated by fluvial processes to
form sedimentary placer deposits. Metals such as copper and gold are represented
in almost every class of deposit in the list. The distribution of ore metals and the
types of process that create their deposits are discussed in more detail in following
chapters.
2.3 Global Distribution of Ore Deposits
Ore deposits are not distributed uniformly across the globe. Vast tracts of land are
devoid of viable deposits while others constitute what is known as a 'metal province',
a region containing an unusually high proportion of deposits of one or several different