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Geological criteria for ore prospecting.pptx
1. Geological criteria for ore
prospecting
Prepared By
Dr. Masroor Alam
Deparment of Earth Sciences
Karakoram International University Pakistan
2. where to look?
In the search for mineral deposits it is impossible to examine in detail
every square km of the area or country by, for example, drilling. This
would be too expensive, time-consuming and in most cases pointless.
An area where the required mineral resources can be expected to
occur is therefore delimited using prospecting criteria, that is,
geological features which directly or indirectly suggest the presence
of a given deposit.
These criteria should be distinguished from indications of
mineralization — prospects — which directly show the presence of
ore (ore fragments, outcrops of mineral deposits). In some cases the
criteria (e.g. hydrogeochemical) are equivalent to the indications.
Stratigraphy, the petrography of sedimentary, igneous and
metamorphic rocks, tectonics, structural geology and the facies
concept, geochemistry, geomorphology, hydrogeology, geophysics,
history and mining geology.
4. 1. Stratigraphic criteria
If a mineral material is known to occur in a certain
palaeontologically or otherwise defined stratigraphic horizon, the
first task of prospecting is to determine the surface occurrence
and extent of this horizon by detailed mapping.
Prospecting sedimentary deposits in a given area demands
knowledge not only of world-wide stratigraphical criteria of the
highest order, but also of local stratigraphical and mineralization
peculiarities.
Stratigraphical criteria are important in the search for
sedimentary deposits and hypogene deposits that are associated
with beds of lithologically favourable sediments
Throughout the world, deposits of coal, sedimentary copper ore,
uranium, lead and zinc, pyrite, sulphur, phosphates and bauxite,
sedimentary iron and manganese ores, placers, clays,
carbonates, vanadium and salts are restricted to several definite
Stratigraphical horizons
5.
6. In general, sedimentary deposits originate in periods of waning
orogenic movements and at the beginning of transgressions.
Geosynclinal coal deposits predominated in the Palaeozoic,
transitional deposit types have come to the fore since the Mesozoic,
and platform deposits were dominant in the Cenozoic.
The Precambrian Fe-quartzites formed in oceans of lower salinity
and a higher CO2, that is with a lower pH, than modern oceans. As a
result, trivalent iron migrated farther from the shore and was
deposited with siliceous rocks. From the Precambrian to the
Devonian, submarine exhalative iron ores were formed, which are
unknown in younger formations.
Bog ores have increased in importance since the Silurian and
lateritic iron ores since the Jurassic. Industrial deposits of micas
and of regional metamorphic graphite occur only in Precambrian
rocks. Typically tin, tungsten, mercury and antimony deposits are
associated with Mesozoic and Cenozoic intrusions.
7. 2. Lithological criteria
The composition of sedimentary ores is always closely connected
genetically with the lithology of the wall rocks.
The potential occurrence of an ore can be inferred from the
lithological character of the surrounding sediments.
The study of the palaeochmatic conditions of an area or formation is
also important, since the association of deposits formed under an
arid climate differs appreciably from that produced under humid
conditions.
Prospecting for manganese illustrates clearly the significance of
palaeogeographical and facies conditions for the development of
sedimentary deposits.
Sedimentary manganese ores can originate from the destruction of
rocks in the neighbourhood of a lagoon or shelf even if their Mn
content is only one order higher than the clarke content.
8.
9. In prospecting for endogenic ore deposits, magmatogenic criteria and the
lithology of rocks permeated by ore-bearing mother intrusions are of
primary importance.
Three properties of rocks make them favourable for mineralization:
1. permeability (sandstones, conglomerates, porous lavas, technically
disturbed rocks);
2. chemical reactivity (on reaction with hydrothermal solutions,
precipitation of ore minerals is induced; e.g. carbonate rocks);
3. 3.brittleness (igneous rocks, quartzites and dolomites contrast with
limestones and shales
The presence of carbonate rocks close to acid and intermediate
intrusions is extremely favourable for the genesis of polymetallic,
copper, tin, tungsten, molybdenum, antimony and other deposits of
contact-metasomatic (skarn) and hydrothermal-metasomatic types
(e.g. crystalline magnesite)
10. The lithology of rocks is of special importance for deposits of the
placer type. In depressions of river beds, produced by the selective
erosion of lithologically variable rocks, the natural washing of the
alluvium results in the accumulation of heavy minerals and the
formation of placers
11. 3. Structural criteria
The structure of the earth's crust is often a controlling factor in the
formation of ore deposits.
Numerous types of metallic and non-metallic deposits of endogenic
origin, for example, are confined to folded areas or, more precisely, to
the magmatic bodies intruded into them.
Minerals deposits usually originate in the last orogenic cycle within
an area.
The origin of metallogenic zones can be explained in terms of plate
tectonics.
According to this, there are 7 large lithospheric plates on the Earth —
the North and South American, Eurasian (including pre-Alpine
Europe), Pacific, Australian, African and Antractic — plus the small
Nazca plate.
The metallogenic zones have developed along the margins of these
12.
13.
14.
15. Other metallogenic provinces occur in the oceanic or continental
segments of the plates
The study of plate tectonics is therefore of primary importance for
determining the principal structural criteria that effect
prospecting.
Subduction
Divergence
Transform
Island arc formation
The spreading of the ocean floor from the mid-ocean ridges and
their consumption in the trenches of the Peru—Chile or Izu-Bonin
type is very important in the genesis of ore deposits.
The moving oceanic crust functions as a conveyor belt which
continuously supplies the "melting pot" of the Wadati—Benioff
zone, beneath marginal fold mountains of the Andean type or
beneath an island arc, with pelagic sediments enriched in metals
from the upper mantle.
16. The examples mentioned above indicate that ore deposits often originate
above subduction zones.
Why not in north Pakistan?
Why giant deposits in southern Tibet?
An ore belt associated with a fault may be a separate metallogenic province
or part thereof (a district or partial district). The fault acted as a channel of
ascent for hydrothermal solutions and the deposits themselves occur along
the minor conjugate faults in the neighborhood.
17.
18. During the folding of rocks whose beds are not smooth enough to glide along
each other, cavities open in the anticlinal and synclinal bends and these may
be later filled with ore.
The ore solutions become concentrated in the anticlines and dispersed in the
synclines. Statistical data show that most epigenetic deposits in folded areas
occur in anticlines.
In the limbs of folds, especially if these are overturned, suitable openings for
the deposition of ore minerals arise as a result of crushing and small
movements between rocks of different competency.
Flexure folding also provides openings for mineralization
22. Ore bodies in open fractures take the form of short irregular veins, usually
joined into groups and are often branched.
The bodies occur in zones under tensile stress, bends of the strata, transvers
fractures of dykes (ladder veins) and in the cooling cracks of intrusions.
