In order to more readily study mineral deposits and explore for them more
effectively, it is helpful to first subdivide them into categories.
This subdivision, or classification, can be based on a number of criteria,
such as minerals or metals contained, the shape or size of the deposit,
host rocks (the rocks which enclose or contain the deposit) or the genesis
of the deposit (the geological processes which combined to form the
Since there is considerable debate among geologists as to the exact mode
of formation (genesis) of most mineral deposits, this is not a good
It is best to stick to features we can all agree on, namely, the physical
description of the deposit. We soon see that, even though no two mineral
deposits are exactly alike, most of them fall into one or another of a
small number of categories.
We also see that each of these categories coincides with a generally
accepted hypothesis as to how the mineral deposits formed. In other words,
although we started out with a physically descriptive classification, we
end up with a classification which also coincides with what we perceive to
be unique genetic processes.
It is therefore useful to define a small number of terms used in the
classification which have a genetic connotation:
Hydrothermal volcanogenic deposits
Hot water or hydrothermal solutions have actually been observed forming
mineral deposits, for example, the "black smokers" on the sea floor. The
ore constituents, such as Cu, Pb, Au or other metals are dissolved in a
hot aqueous solution along with other deposit constituents such as Si, S
These elements are deposited to form the ore and gangue minerals in
response to a change in the solution, very often a sharp decrease in
temperature. an example of this process would be if you dissolved as much
table salt as possible in boiling water. If you then cool the solution in
the fridge, much of the salt will precipitate or come out of solution.
Some mineral deposits, particularly those containing Ni, Cr and Pt, form
by the separation of the metal sulphide or oxides in the molten form,
within an igneous melt before it crystallizes. These are known as magmatic
They occur within theigneous rock from which they were derived, such as a
gabbro. The ore metals concentrated as liquid in much the same manner as
metals are purified in a smelter or blast furnace. The heavier metal-rich
liquids sink and concentrate at the base of the intrusive body, while
lighter silicate liquid and crystals tend to rise, the same asthe slag in
a blast furnace.
A syngenetic mineral deposit is a deposit which formed at the same time as
the rocks that enclose it. Magmatic deposits are syngenetic in that the
ore minerals crystallize from the same liquid that produces the silicate
minerals which form the bulk of the intrusive - they crystallize more or
less simultaneously as the melt cools.
Deposits which form on the earth's surface in the form of a sedimentary
layer are also syngenetic. The rocks which they lie upon were deposited
just prior to the mineralizing event, while the overlying rocks were
deposited just after - all three layers being deposited at essentially the
same time in terms of the geological time frame.
If a mineral deposit formed much later than the rocks which enclose it, it
is said to be epigenetic.
An example is a vein. The first step in the formation of a vein is the
fracturing or breaking of rock along a fault zone, at a depth ranging from
surface to several kilometers below surface. The rock must be solid
(lithified) and brittle, creating open spaces when it breaks. Hydrothermal
solutions pass along the fault zone and deposit or precipitate the ore and
gangue minerals within the open spaces. Thus, the vein is necessarily
younger than the rocks that contain it.
Since we are fairly certain which deposits are syngenetic and which are
epigenetic (although there will always be some degree of uncertainty and
overlap), it is convenient to begin the classification with this
discrimination. Beyond this, the various categories are based on their
physical description, including size and shape. A third level of
subdivision is usually based on the metals contained. Here, then, is the
Large, low grade deposits usually associated with a porphyritic intrusive
B. Cu (-Au)
C. Mo (-W)
Mineral deposits formed by replacement of limestone by ore and
calc-silicateminerals, usually adjacent to a felsic or granitic intrusive
A. W-Cu (-Zn, -Mo)
B. Zn-Pb-Ag (-Cu, -W)
C. Cu (-Fe, -Au, -Ag, - Mo)
D. Fe (-Cu, - Au)
E. Sn (-Cu, -W, -Zn)
F. Au (-As, -Cu)
Fracture filling deposits which often have great lateral and/or depth
extent but which are usually very narrow.
A. Hypothermal - Cu (-Au)
B. Mesothermal - Cu-Pb-Zn-Ag-Au
C. Epithermal - Au-Ag (-Hg)
Named for the region where they were first described, these deposits
formed within porous carbonate rocks (limestone reefs or caves). They are
Pb-Zn deposits with low Ag values.
Volcanic Massive Sulphide (VMS)
These deposits formed as massive (over 60% sulphide) lens-like
accumulations on or near the sea floor in association with volcanic
A. Felsic volcanic hosted - Cu-Pb-Zn-Ag-Au
B. Mafic volcanic hosted - Cu (-Zn, -Au)
C. Mixed volcanic/sedimentary - Cu-Zn (-Au)
Sedimentary Massive Sulphide (Sedex)
These are formed by hydrothermal emanations on or near the sea floor in
association with the deposition of sedimentary rocks.
Magmatic- layered mafic intrusion
During the crystallization of a magma, usually mafic or ultramafic, heavy,
metal-rich liquids settle and accumulate at specific sites, often at the
base, within the intrusion.
A. PGM (Platinum group metals)
C. Ni-Cu (-PGM)
Formed within sediments by the concentration of heavy resistant minerals
(Au diamond, cassiterite) by stream or wave action.
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