Geology Reference
In-Depth Information
2.2 Classifications of Ore Deposits
There are some parallels between the schemes used to classify ores and those used
to classify ore deposits. Again in older texts, deposits are classified according to the
type of product they produce; copper deposits, gold deposits, energy sources
(uranium and coal), and so on. This type of classification finds some application
in a purely economic context but is not employed here.
Through the twentieth century many classifications were based on the types of
rocks that host the ore deposit or on the geometry of the deposit and its relation to
the host rocks. An example is given in Table 2.4 . Deposits in granites were
distinguished from those in sedimentary rocks; vein-like deposits were distin-
guished from layers conformable with the stratification of the host rock; massive
ores were distinguished from disseminated ores. A popular classification developed
by Lindgren, an influential American economic geologist, distinguished deposits
that formed at different levels in the crust (Table 2.4 ). The terms “epizone”,
“mesozone” and “catazone”, for deposits at shallow, intermediate, and deep levels
in the crust, are still employed today. A further distinction was made between
“syngenetic” deposits, which formed together with and as part of the host rock, and
“epigenetic” deposits, which formed through introduction of ore minerals into
already consolidated rocks.
The development in the latter part of the twentieth century of the theory of plate
tectonics spawned a swarm of classifications based on tectonic settings. As shown
in Table 2.5 , deposits in ocean basins were distinguished from those in convergent
margins or intracratonic settings, and so on. This type of classification is still used,
Table 2.4 Lindgren's classification of ore deposits (Modified from Lindgren 1933 and Evans
1993 )
Depth
Temperature
( C)
Occurrence
Metals
Telethermal Near surface
100
In sedimentary rocks or lava
flows; open fractures, cavities,
joints. No replacement
phenomena
Pb, Zn, Cd, Ge
Epithermal
Near surface
to 1.5 km
50-200
In sedimentary or igneous rocks;
often in fault systems; simple
veins or pipes and stockworks;
little replacement phenomena
Pb, Zn, Au, Ag,
Hg, Sb, Cu,
Se, Bi, U
Mesothermal 1.2-4.5 km
200-300
Generally in or near intrusive
igneous rocks; associated with
regional faults; extensive
replacement deposits or
fracture fillings; tabular
bodies, stockworks, pipes
Au, Ag, Cu, As,
Pb, Zn, Ni,
Co, W, Mo,
U etc
Hypothermal 3-15 km
300-600
In or near deep-seated felsic
plutonic rocks in deeply
eroded areas. Fracture-filling
and replacement bodies;
tabular or irregular shapes
Au, Sn, Mo, W,
Cu, Pb, Zn,
As
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