Occurrence of Orebodies -
Morphology and Relationship with Host Rocks
of occurrence of an ore deposit is important from the mining point of view.
ore deposits are roughly tabular, most others occur in a variety of forms.
replacement deposits knowledge of the form, as also of the internal
structure and its relationship with the enclosing rock is important.
the point of view of form & structure, ore deposits may be classified
into two broad groups:
– which have formed at the same time as the rock in which they occur.
They are sometimes part of the succession like an iron-rich
sedimentary horizon. They have formed by the same process and at the same time as the
enclosing rock, and
- which are believed to have come into being after the host rock in
which they occur, e.g. a vein or a dyke.
They have been introduced into pre-existing country rock after its
of Syngenetic Ore Deposits:
Magmatic Segregation Deposits:
are either connected by transition with the host rock or are contained
within them. They are generally
irregular, roughly spherical, more often tabular or lenticular in shape. The width and thickness ranges from a few inches to a few
hundred feet, and the length may exceed one mile.
Examples - Chromite deposits in peridotites, titanic iron ore in
Sedimentary Deposits: are
part of the stratigraphic sequence. They
are generally of tabular, sheet-like or flat lenticular form.
Are horizontal if not disturbed, and are frequently folded and
faulted. Beds containing
metallic ore are <20 ft. thick, coal beds may be more than a hundred feet
thick, and rock-salt, gypsum and anhydrite beds may be several hundred feet
Morphology of Epigenetic Deposits:
These have various forms.
But among those that follow fissures (the fissure veins), the tabular or
sheet-like forms are the most common. Deposits
in the zone of weathering are extremely irregular and of limited extent.
Deposits in Limestones:
Form depends upon bedding, fissuring, or contact with other rocks.
Replacement deposits in limestones are extremely irregular.
They seldom form large deposits.
Deposits in Metamorphic Rocks:
In highly deformed metamorphic rocks, the ore deposits occur in lenticular
form. The deposits are
characterized by the successive overlapping of a number of lenses.
of Concordant Orebodies:
orebodies in sedimentary rocks are very important producers of many different
metals. They are particularly
important for basemetals and iron. The
orebodies may be an integral part of the stratigraphic sequence (eg Phanerozoic
ironstones) or epigenetic infillings of pore spaces or replacement orebodies or
syngenetic ores formed due to exhalation of mineralizing solutions at the
sediment-water interface. They
usually show a considerable development in two dimensions (parallel to bedding)
and are limited in the third dimension. Such
deposits are referred to as stratiform. Stratabound ore deposits are any type of
orebodies, concordant or discordant, which are restricted to a particular
stratigraphic horizon. Concordant
orebodies are found in different kinds of sedimentary rocks.
Limestones: Basemetal sulfide deposits
are often found in limestones. Increased
permeability due to dolomitization or fracturing, coupled with higher
reactivity and solubility makes them potential horizons for mineralization.
Sometimes mineralization may occur as syngenetic stratiform orebodies.
argillites, mudstones, slates are important rocks for concordant orebodies eg
Kupferschiefer of Germany (Upper Permian) is a copper bearing shale.
Also the Sullivan orebody of British Columbia occurs in Permian
Arenaceous rocks: Feldspathic
sandstones (often altered), and alluvial gravels are hosts to concordant ore
deposits. Recent and fossil
placers of gold, uranium and thorium are important in rudaceous rocks.
Vesicular filling deposits, massive sulfide and volcanic exhalative
deposits commonly occur with extrusive igneous rocks, while platinum, chromite,
magnetite, nickel and titanium deposits occur as concordant orebodies in
Most concordant ore deposits in metamorphic rocks are sedimentary in
origin. They represent
metamorphosed equivalents of deposits that originated as either sedimentary of
of Discordant Orebodies:
Discordant orebodies may be
subdivided into those that have an approximately regular shape and those which
are highly irregular in their outline.
Regularly shaped orebodies are of two types – tabular and tubular.
orebodies:These are extensive in two dimensions but have a restricted third
dimension. To this category belong
the veins and lodes. Veins are
considered to have resulted mainly from the filling of open spaces, whilst the
formation of lodes was due to extensive replacement of preexisting rock.
Since such a genetic distinction is often ambiguous, all tabular
orebodies are generally referred to as veins.
are often inclined, and in such cases one can speak of hanging walls and foot
walls. They frequently pinch and
swell as they follow up or down a stratigraphic sequence. Often only swells are workable.
An initial fracture in rocks changes attitude as it crosses lithologies
of different physical properties. When
movement occurs to produce a normal fault, the less steeply dipping sections are
held against each other while the more steeply dipping sections are dilated.
When movement is reversed, the less steeply dipping sections are dilated.
are relatively short in two dimensions but extensive in the third.
When vertical to sub-vertical, they are called pipes or chimneys. When horizontal or sub-horizontal they are called mantos.
In Eastern Australia, there are hundreds of pipes in and close to
granitic intrusions. Pipes may be
of various types and origins – infillings of mineralized breccias in volcanic
pipes are quite common e.g. copper bearing breccia pipes of Messina, S. Africa.
Mantos and pipes may be branched and the branches may be cross-connected.
Mantos and pipes are often found in association, the pipes frequently
acting as feeders to the mantos.
Irregularly shaped orebodies are of two types – disseminated deposits
and irregular replacement deposits.
deposits: In these deposits, the
ore minerals are dispersed throughout the body of the host rock e.g. diamonds in
kimberlite. In other deposits, the
disseminations may be mainly along close-spaced veinlets cutting the host rock
and forming an intercalated network called a stockwork.
This kind of mineralization generally fades outwards into sub-economic
mineralization and the boundary of the orebody are the assay limits.
They are therefore irregular in form and may cut across geological
boundaries. The overall shapes are
cylindrical or cap-like.
occur commonly in acid to intermediate plutonic igneous intrusions, but they may
cut across the contact into country rocks.
Most of the world’s copper and molybdenum and some tin, silver, mercury
and uranium comes from such deposits.
Many ore deposits have formed by replacement of preexisting rocks,
particularly carbonate-rich sediments. The
replacement often occurs at the contact with medium to large igneous intrusions
(contact metamorphic or pyrometasomatic deposits).
These deposits are extremely irregular in shape, tongyes of ore may
project along any available planar structure – bedding, joint, fault etc.
These orebodies are characterized by the development of calc-silicate
minerals such as diopside, wollastonite, andradite garnet and actinolite.
For this reason they are also called skarn orebodies.
principal materials produced from pyrometasomatic deposits are iron, copper,
tungsten, graphite, zinc, lead molybdenum, tin and uranium.
Relations of Veins
are tabular or sheet-like masses occupying fracture sets.
are usually developed in fracture systems and therefore show regularities in
their orientation and mutual relationships.
The vein walls may be either sharp or gradational.
Outcrops of veins depend upon their mineralogy, surface conditions and
the characteristics of the enclosing rocks.
Depending on the relation of a number of veins with each other and with
the country rock, they have been classified into a number of types:
Veins in relation to each other:
1. Veinlets or Seams: when the
fissures are very small.
2. Composite Veins: veins
irregularly connected and spread over a considerable distance.
3. Vein System: a number of
adjacent parallel veins.
4. Hammock Structure: when
veins follow two or more sets of fractures intersecting at an acute angle.
5. Linked Veins: a number of
adjacent parallel veins connected by a diagonal vein.
6. Sheeted Zone or Shear Zone:
veins following fractures which are closely spaced and parallel.
The width of a sheeted zone is < 50 feet.
7. Stockworks: Irregular
fractures in various directions along which mineralization has spread.
8. Ladder Veins: Deposits
filling short transverse fissures, sometimes occurring in association with
9. Lenticular Veins: confined
to schists. Sometimes formed
due to deformation
of older deposits.
10. Gash Veins: deposits
filling non-persistent openings
Veins in Relation to Country Rocks:
1. Bed Veins: those that follow
bedding on sedimentary rocks.
2. Cross Veins: Veins crossing
3. Chambered Veins: pinching
and swelling of veins due to differences in lithology of the country rock.
4. Horses: Large masses of
country rock included in the vein material.
5. Contact Veins: when a vein
follows a contact between two lithologies.