Mode of Occurrence of Orebodies -
Morphology and Relationship with Host Rocks

 

  • Mode of occurrence of an ore deposit is important from the mining point of view.

  •  Sedimentary ore deposits are roughly tabular, most others occur in a variety of forms.

  •  In replacement deposits knowledge of the form, as also of the internal structure and its relationship with the enclosing rock is important.

  •  Form the point of view of form & structure, ore deposits may be classified into two broad groups:

  1. Syngenetic deposits – 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

  2. Epigenetic deposits - 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 formation.

Morphology of Syngenetic Ore Deposits:

  1. 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 anorthosites.

  2. 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 thick.

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.

  1. 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.

  2. 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.

Morphology of Concordant Orebodies:

1.      Concordant 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.

Argillaceous rocks:  Shales, 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 argillites.

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.

            Igneous 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 plutonic rocks.

            Metamorphic rocks:  Most concordant ore deposits in metamorphic rocks are sedimentary in origin.  They represent metamorphosed equivalents of deposits that originated as either sedimentary of igneous deposits.

Morphology 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.

1.      Regularly shaped orebodies:  Regularly shaped orebodies are of two types – tabular and tubular.

a)      Tabular 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.

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.

b)      These 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.

2.      Irregularly shaped orebodies:  Irregularly shaped orebodies are of two types – disseminated deposits and irregular replacement deposits.

a)      Disseminated 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.

Stockworks 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.

b)      Irregular replacement 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.

The principal materials produced from pyrometasomatic deposits are iron, copper, tungsten, graphite, zinc, lead molybdenum, tin and uranium.

 

Spatial Relations of Veins

Veins are tabular or sheet-like masses occupying fracture sets.

They 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.

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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 dykes.

 

 

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 the bedding.

 

 

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.

 

Notes & Handouts

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