A body of rock, a vein within it, or a
deposit of sediment, worth mining for the
economically valuable mineral it contains.
The term is usually applied to sources of
metals. Hydrothermal ore deposits are formed
from fluids such as saline water passing
through fissures in the host rock at an
elevated temperature. Examples are the
`porphyry copper' deposits of Chile and
Bolivia, the submarine copper-zinc-iron
sulphide deposits recently discovered on the
East Pacific Rise, and the limestone
lead-zinc deposits that occur in the southern
USA and in the Pennines of Britain. Other
ores are concentrated by igneous processes,
causing the ore metals to become segregated
from a magma, for example, the chromite and
platinum-metal-rich bands within the
Bushveld, South Africa. Erosion and
transportation in rivers of material from an
existing rock source can lead to further
concentration of heavy minerals in a deposit,
for example, Malaysian tin deposits.
Weathering of rocks in situ can result in
residual metal-rich soils, such as the
nickel-bearing laterites of New Caledonia.