is an economic entity, not a physical one. It is defined by the presence
of an economically valuable concentration of some material.
However, times and
markets change. Today's waste may be tomorrow's gold mine (or perhaps
mine - exactly what happened to the waste from one region's gold mines). Ores
too dilute to be mined may become valuable as technology changes. For example,
there are huge copper mines which recover copper from ore of only 3% copper, not
feasible a hundred years ago.
Note that some ores are mined primarily for one material (such as lead), and
another (such as silver) is recovered as a bonus. In this way, the mineral
Galena (Lead Sulfide) is
the leading ore for
ORES are typically not isolated - that is, many different ores are
often co-located within a regional "ore body". Ore bodies are
formed through a combination of geology (such as ocean bottoms, volcanoes, or
subducting tectonic plates) and chemistry (due to a natural concentration of
some collection of elements, plus a chemical environment such as an oxidation
zone). Many different ores - for many different metals - are thus found in
One such example is a large body of magma deep within the
Earth. As it slowly cools over thousands of years, minerals begin to crystallize
as the temperature falls below their melting points. If these crystals have a
higher or lower density than the magma containing them, they will settle down or
float up, resulting in layers concentrating those minerals. Also, the "left
over" minerals will be concentrated in the last sections to solidify.
Another example is a drying
lake or sea. Precipitation (rain) dissolves minerals over a wide region (usually
the most soluble salts first), and rivers transport these chemicals into a body
of water. But when conditions are dry, the water will evaporate faster than it
can be replenished, resulting in concentrations of salts, carbonates, and other
chemicals. Here, too, the layers tend to isolate specific minerals, as the
chemicals with lower solubility tend to precipitate first, and those with the
greatest solubility (such as halite) are last (and thus on top).
A somewhat similar circumstance often occurs where an intrusion of magma
provides a heat source for groundwater. Under the high pressures and
temperatures deep within the Earth, water may dissolve many compounds we
normally think of as insoluble, such as quartz. As the water flows out to the
surface, it very gradually cools, the pressure falls, and chemical compounds
precipitate (or crystallize) at different points, resulting in the concentration
of different compounds along the water's path.
The quartz deposits in Hot Springs, Arkansas are a good example.
A related process
is called supergene
enrichment. Supergene enrichment occurs when certain metals are leached out
of slightly soluble minerals by hydrothermal fluids. These metals are then
redeposited as a different mineral in concentrated pockets. These pockets are
usually more rich in ore metals than the original material, thus the process is
aptly termed an enrichment. Chlorargyrite
is an example of a supergene
At some point, an ore body is brought to the surface, where erosion exposes
it, and water and oxygen potentially induce still more changes to the chemistry. Plate tectonics
can also plunge these rocks deep within the Earth, where intense heat and
pressure metamorphize the rocks, often inducing
the formation of additional minerals. These cycles can repeat many times,
resulting in an evolution of mineral species.
An ore for one metal (such as copper) may consist of many different minerals,
while an ore for another metal may consist of only (or at least primarily) a
single mineral, in some cases the pure metal (such as