Geology Reference
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the higher cost of mining its lower grade ore, making deposit 6 the most
attractive.
*Were the deposit much larger and on the more hospitable west coast of
Greenland, it might be viable. The Black Angel Pb-Zn deposit, located on a
precipitous cliff on the margin of a fjord near Maamorilik, was mined
successfully from 1973 to 1990 and it closed only because of falling metal
prices (Pb and Zn followed a trend similar to that of copper shown as shown
in Fig. 1.2 ). In 2006 the retreat of a coastal glacier revealed another very large
and rich Zn-Pb-Ag deposit which is currently being mined - a silver lining on
the cloud of global warming?
1.5 Factors that Influence Whether a Deposit Can Be Mined
1.5.1 Tenor and Tonnage
Some idea of the relationship between grade, tonnage and viability of an ore deposit
was given in Box 1.4. For a deposit to be mineable it must contain more than a given
concentration of the valuable commodity, and more than a given tonnage of this
commodity. As shown very diagrammatically in Fig. 1.5a , deposits tend to be
distributed along a trend from an extremely small and rich deposit- a single crystal
of copper is the extreme example - to another deposit that is very large but with
much lower grade - the entire Earth. Most deposits that are both big, close to the
surface and high-grade have been mined out and what remains are small rich
deposits and much bigger low-grade deposits in more remote regions or at greater
depth in the crust.
Figure 1.5 b shows in an equally crude way the relationship between grade and
price (this relationship is explored in far more detail in a later section). Some metals
Fig. 1.5 (a) Sketch showing variation in the grade and size of ore deposits; (b) the relationship
between grade and price of selected metals
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