Geology Reference
In-Depth Information
types. Notable examples include the string of enormous copper deposits along the
American Cordillera from Alaska to Chile, the clusters of lead-zinc deposits in
limestones of the central USA, and the tin-bearing granites of SE Asia. For both
geological and economic reasons it is important to have some knowledge of this
distribution. From a geological point of view, the distribution provides important clues
to the ore-forming process; from an economic point of view, the irregular distribution
strongly influences metal prices and global trade, and lies at the heart of many of the
alliances and conflicts that govern relationships between countries around the world.
In plate tectonic classifications of ore deposits, the emphasis is quite naturally
placed on the tectonic setting in which the deposit occurs, but many deposits
develop in sedimentary settings or as a result of superficial weathering; in such
cases geomorphology, surface relief and modern or past climate exert an additional
important influence of the localization of the deposits. All these factors are
discussed briefly in the following section and are then elaborated upon in
subsequent chapters.
2.3.1 Geological Factors
The global distribution of ore deposits is illustrated in the series of maps of major
deposits that are compiled in Fig.
2.1
. We have selected only a few major
commodities that serve to illustrate the basic principles that govern the distribution
of ores; more detailed and exhaustive information is found in standard texts and on
the internet, as listed at the end of the chapter.
We will start with copper, an industrial metal that is used in every country and is
mined in all parts of the world (Fig.
2.1a
). A large proportion of the resources of this
metal is tied up in a single type of deposit, the so-called “porphyry copper” or
simply “porphyry” deposits (Chap. 4). These deposits are directly associated with
subduction zones and thus are found in island arcs and convergent margins. This is
the reason for the string of deposits that extends not only along the entire west coast
of North and South America, but also throughout the islands of the southwest
Pacific. Large deposits of the same type are also found in accreted island
arcs that have been incorporated into continental collision zones, as in the
Alpine-Carpathian-Himalayan belt. Another major class of copper deposits form
within mature sedimentary rocks in intracratonic basins, as in the deposits of the
central African “copper belt”. Copper also occurs in deposits associated with
volcanic rocks, the VMS deposits of Fig.
2.1b
, and in deposits associated with
shales, the SEDEX deposits in Fig.
2.1b
. The metal is also mined in magmatic
intrusions, most of which form in intracratonic settings (Fig.
2.1c
). Two important
examples formed in very different ways. The Norilsk deposits in northern Siberia
are associated with a large igneous province and those of the Sudbury area in
Canada formed during crystallization of a melt sheet created by the impact of
a large meteorite. Another unusual example of a copper ore body is the Palabora
Search WWH ::
Custom Search