The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 165

TABLE 4.1
(Continued)

Metal Ore Mineral Composition Percent Metal

Antimony Stibnite Sb 2 S 3 71

Bismoth Bismuthianite Bi 2 S 3 81

Cobalt Smaltite CoAs 2 28

Cobaltite CoAsS 35

Mercury Cinnabar HgS 86

Molybdenum Molybdenite MoS 2 60

Wulfenite PbMoO 4 39

Tungsten Wolframite (Fe Mn)WO 4 76

Source:
Lacy (2006)

isolated from the surface environment. Natural forces of oxidation and erosion will even-
tually release metals that have not been extracted by mining, into the environment. While
benei cial to life at low concentrations, many metals at increased concentrations become
detrimental (see also Chapter Eleven).

How Mineral Deposits are Formed

For a mineral deposit to be formed, some process or combination of processes must pro-
duce localized enrichment of one or more elements. Minerals can become concentrated in
a number of ways. Concentration of elements or minerals may result from (Robb 2005):
●

the l ow of hot magmatic l uids through fractures in crustal rock producing hydro-
thermal mineral deposits;

●

igneous processes (the term igneous means 'origin by i re', derived from the Latin
word ignis meaning i re) within a magma body producing magmatic mineral deposits;

●

metamorphic banding or re-crystallization producing metamorphic mineral deposits;

●

precipitation from lake water or seawater producing sedimentary mineral deposits;

●

hydraulic forces such as waves or currents in l owing surface water producing placer
deposits;

●

weathering processes including lateritization, producing both residual and enriched
mineral deposits.

A mineral deposit is always
a geological anomaly with
an unusual concentration of
formerly diffuse metals.

A mineral deposit is always a geological anomaly with an unusual concentration of for-
merly diffuse metals. A pure concentration of elements or minerals is rare and miner-
als are commonly interspersed with non-metallic rock, which are referred to as gangue
minerals. Gangue minerals are of no commercial value. Familiar minerals that commonly
occur as gangue are silicates, such as quartz, feldspar, and mica, and the carbonate min-
erals, calcite and dolomite. The mixture of minerals and gangue constitute the mineral
deposit that is generally enclosed in surrounding host rock of no economic value ( Figure
4.2 ). The transition from the zone of unusually high concentration of elements to host rock
with average concentration may be sharply dei ned but is more often gradual.