Geochemistry and Ore Deposits
Analytical chemistry plays a key role in our
continuing quest to understand how ore deposits formand in the practical
exploration for ore deposits. If you pick up an ordinary rock that
builds thecrust of the Earth and determine its chemical composition, for
every billion atoms, 1 to 10,000atoms will be metallic elements such as
gold, silver, platinum, mercury, copper, cobalt, nickel,chromium, lead,
zinc, molybdenum, tin, and tungsten.
Natural processes in the Earth s crust have the
remarkable ability to concentrate and purify certain rare metallic
elements to form unusualdeposits of minerals that contain 1,000 to
10,000 times the amounts found in ordinary rocks.
With today's modern mining and extraction technology, it has become
possible to mine verylow-grade deposits. For example, gold can be
economically recovered from rocks that contain lessthan one tenth of an
ounce of gold per ton of rock. But gold continues to be expensive
because ofthe cost in locating the deposit, mining the rock, and
extracting the small amount of gold in each tonof rock. All of the
inorganic raw materials used to manufacture the products of
todaystechnological society have to be either mined or recycled.
Almost every process that takes place in the Earth s crust, whether from
the action of molten rock,heat and pressure at depth, hot springs or
steam, running water, weather, or biological activity cancontribute to
the formation of an ore deposit.
Geologists use the principles of chemistry
to try tounderstand how these processes scavenge elements from ordinary
rock, transport them, andconcentrate them to form an ore deposit.
Geologists have developed models that describe the physical
characteristics and chemical composition of each ore deposit type and
how they relate tothe geologic environment in which they form similar to
the way biologists describe how an organismfits into a particular
environmental niche. In Australia and many other parts of the world,
almost all of the rich ore deposits exposed atthe surface have already
been discovered. Most of the ore yet to be found is not visible to the
human eye. Therefore, geologists have had to improve their understanding
and develop moresophisticated ways to detect where ore deposits can
How Geologists Detect Ore
Two main approaches are used to detect deposits hidden below the
surface. One uses the ore-deposit model, and the other is based on the
detection of a dispersion halo that extends forsome distance from the
The following analogy shows how geologists use ore-deposit models .
If all but the tip of the tail ofan elephant was buried by a landslide,
a biologist could recognize from the skin, hair, and shape ofthe
appendage that the tail belonged to a mammal. With advanced testing of
tissue samples, abiologist could prove that the tail belongs to an
elephant and could easily predict that the bodyshould be buried about 1
meter below the tip of the tail.
Most ore-deposit models are not as advanced as biologists models for
elephants, but a few arenearly so.
Several copper and molybdenum porphyry deposits, located as deep as 1500
metres below the surface, have been discovered based on small surface
exposures measuring a few metres across. These exposures were of breccia
pipes (vertical pipe-shaped bodies of pulverizedrock), which are known
to extend hundreds of metres above the main body of porphyry deposits.
Because not all porphyries contain deposits of economic metals,
geologists can collect and analyzefield samples to determine what metals
the porphry will contain, and if it is worth drilling.
Mineral scavengers provide a clue
There is another way of detecting the trace elements carried from a
deposit by ground water.
Ground water is drawn upward by evaporation at the surface. During this
upward migration, traceelements in the water are affixed to minerals in
the overburden. The affixation, or bonding, mayrange from weak to very
strong. The strength of this bonding depends on the chemical nature
ofboth the trace element and the host mineral. The differences in bond
strength is comparable to thedifference between the weak electrostatic
attraction that holds an inflated balloon to a wall and anail driven
into a stud.
Minerals that are capable of scavenging trace elements from ground water
with increasing bondstrength include hydrated aluminum silicates
(clays), secondary carbonates, amorphous(noncrystalline) oxides of
manganese, and the amorphous and crystalline oxides of iron.
Traceelements scavenged by these minerals are removed by treating
samples of overburden withchemicals that react selectively with each
mineral phase. Sequential selective extractions are used torelease trace
elements from the host minerals in the order of increasing bond strength
such as claysfirst and crystalline iron oxides last.
The principal advantage of selective extractions is that they facilitate
the distinction of elements thathave migrated from other sources from
those normally present in the overburden. Thus thepresence of a gold
deposit in Western Australia may well be indicated by the occurrence of
gold, or itsassociated elements, arsenic and antimony, in a specific
mineral phase in the overburden.