1

ENGINEERING GEOLOGY (MINING)

SUBJECT CODE: GLGE2A2

FORMATION OF MINERAL DEPOSITS

COURSE NOTES

TYPES OF ORE DEPOSITS

 2

1.1. INTRODUCTION

Just as geologists have named and classified the various minerals and rocks of the earth’s crust,
so have they attempted to classify ore deposits. Together with classification goes an attempt to
understand the origin of these ore deposits. Our concern here is not with ore formation as it can
be a complicated subject, requiring a good grounding in many aspects of geology. For the small-
scale miner a knowledge of the characteristics and attitude of the ore body inasmuch as it affects
the methods of prospecting and particularly mining (exploitation) is sufficient.
Economic mineral deposits (“ore deposits”) are concentrations of one or more useful substances
that are usually sparsely distributed in the earth’s crust. The size, shape and nature of ore
deposits affect the workable grade. Large, low grade deposits which occur at the surface can be
worked by cheap open pit methods, whilst thin, tabular vein deposits will necessitate more
expensive underground methods of extraction. As far as shape is concerned, orebodies of
regular shape can generally be mined more cheaply than those of irregular shape, particularly
when they include barren zones. For an open pit mine the shape and attitude of an orebody will
also determine how much waste has to be removed during mining. The waste will often include
overburden (waste rock above the orebody) as well as waste rock around and in the orebody,
which has to be cut back to maintain a safe overall slope to the sides of the pit.

The way in which a mineral deposit forms is often the controlling influence on the size, shape
and nature of the orebody. An understanding of the type and way in which the ore deposit forms
will thus allow deductions to be made regarding the nature of the orebody, and hence provide
guidelines for the optimal exploitation of the deposit.

1.2. TERMS ASSOCIATED WITH MINERAL DEPOSITS

An ore mineral is a naturally occurring metal or mineral from which one or more metals can be
extracted economically. Economic minerals include both ore minerals and industrial minerals
such as asbestos, clays, refractories, etc. Of some 2 000 known minerals only about 200 are
classed as economic minerals. Ore minerals occur as native metal such as gold or silver, or as
combinations of metals with sulphur, oxygen, arsenic, silicon or other elements.

Gangue minerals are the associated non-metallic minerals of a deposit which are considered
worthless at the time of exploitation but may include some metallic minerals such as pyrite,
which are usually discarded. Such minerals may, however, be utilised in various ways. For
instance, pyrite is sometimes recovered for its sulphur content; fluorite is used as a flux, quartz
as an abrasive, etc.

Ore is the material mined and consists of an aggregate of ore minerals and gangue. To be ore,
the material must contain payable quantities of metals. Although an ore may contain a single
metal, many contain several recoverable metals and minor quantities of additional metal
sometimes constitute valuable by-products. The actual amount of metal present in the ore is
known as the tenor of the ore. The payable tenor of ores varies greatly. A gold ore may have a
tenor as low as 0,0005%, while the tenor of an iron ore may have to be 60% to be payable.
 3

Before discussing the nature of orebodies we must learn some of the terms used in describing
them. If an orebody viewed in plan is longer in one direction than the other we can designate
this long dimension as its strike. (See fig. 1) The inclination of the orebody perpendicular to the
strike will be its dip and the longest dimension of the orebody its axis. The plunge of the axis is
the angle at which the axis dips from the horizontal, measured in the vertical plane that contains
the axis.

Figure 1. Idealised diagram showing dip and strike. (Storrar, 1977)

 4

FIGURE 1.1: Diagram illustrating some terms used in describing orebodies. (Evans, 1987)

Orebodies that cut across the strata or layering of the host rocks are said to be discordant.
Where the orebody is generally parallel to, or part of the enclosing rocks, it is said to be
concordant. Deposits that are an integral part of the stratigraphic sequence (i.e. they are one of
the strata layers and formed at the same time as the enclosing strata), or are replacement bodies
where one of the strata layers has been replaced, are referred to as stratiform deposits. This
term must not be confused with stratabound, which refers to any type of orebody – concordant
or discordant, that is restricted to a particular part of the stratigraphic sequence.

CONCORDANT OREBODIES DISCORDANT OREBODIES

2. THE CLASSIFICATION OF MINERAL DEPOSITS

 5

It is possible to classify orebodies in the same way as we divide up igneous intrusions according
to whether they are discordant or concordant with the lithological banding or layering (often
bedding) in the enclosing rocks. The discordant orebodies can be subdivided into those that have
an approximately regular shape and those which are thoroughly irregular in their outlines.
Concordant orebodies are subdivided largely on the basis of the enclosing rock type.

2.1 DISCORDANT OREBODIES

2.1.1. Regularly Shaped Bodies

These orebodies are laterally extensive in two dimensions, but have a restricted development in
their third dimension. In this class we have veins and ‘lodes’. Veins are often inclined, and in
such cases, as with faults, have a hangingwall and footwall. Veins are usually developed in
fracture systems and may show regularity in their orientation, but they frequently pinch and
swell out as they are followed up or down a stratigraphic sequence. This can create difficulties
during both exploration and mining because often it is only the “swells” that are workable.

Figure 2. Diagram showing two intersecting fractures (c), occupied by a quartz vein, which is enlarged at the intersection of the
two fractures (d). The country rock strata into which the quartz vein has intruded is shown by a and b. (Park, 1927)
 6

Figure 3. Section showing pinch and swell in a vein. The vein (shown in black) is cutting across alternating shale and sandstone
(stippled). Note how the vein becomes thinner when cutting across the thicker sandstone units. (Park, 1927)

The infilling of veins may consist of one mineral but more usually it consists of an intergrowth
of ore and gangue minerals. The boundaries of vein orebodies may be the vein walls or they
may be assay boundaries within the veins.

Figure 4. North – south section across composite vein showing ore (a) and gangue (b). Country rock shale into which vein is
intruded is shown at c. A small offshoot vein cutting through the shale is shown at d. (Park, 1927)

These discontinuous, steep, discordant tabular orebodies usually require a mining method in
which a degree of selective mining is possible and best suited to cut-and-fill or shrinkage
stoping.

Tubular orebodies

Tubular orebodies are relatively short in two dimensions but extensive in the third (usually along
the axis). When vertical or sub-vertical they are called pipes or chimneys and when sub-
horizontal they are known as ‘mantos’. The exact mode of formation of these pipes is not clear
but they often occur in and close to granite intrusions and are probably hydrothermal fluid
conduits. Most have quartz fillings and some are mineralized with bismuth, molybdenum,
 7

tungsten and tin. Infillings of mineralized breccia are particularly common – such as the copper-
bearing breccia pipes of Messina.

Figure 5. Model of a complex pipe-like ore body. (Kreiter, 1968)

These discontinuous, steep, discordant tubular orebodies also require a mining method in which
selective mining is possible. They are best suited to mining with methods such as cut-and-fill,
shrinkage, or sublevel stoping with small stopes.

2.1.2. Irregularly Shaped Bodies

Disseminated deposits

In disseminated deposits the ore minerals are scattered throughout the body of the host rock in
the same way as accessory minerals are disseminated through an igneous rock (in fact the ore
minerals are often accessory minerals). A good example is diamonds in Kimberlite.
Mineralization of this type generally fades gradually outwards into sub-economic mineralization
and the boundaries of the orebody are assay limits or the limit of the host rock itself. They are,
therefore, often irregular in form and cut across geological boundaries with an overall shape that
is cylindrical or cap-like. Disseminated deposits produce most of the world’s copper and
molybdenum (porphyry copper deposits and the Palabora deposit) and are also of major
importance in the production of diamonds.
 8

Figure 6. Cross-section showing kimberlite pipe intrusive through flat-lying sedimentary rocks and lavas. Besides being a good
example of a pipe, kimberlite is also an ore body in which the mineralization (diamonds) is disseminated throughout it. (Hamilton
& Cooke, 1945)

Disseminated deposits are not conducive to selective mining and low cost - high volume mining
methods are necessary. They are thus usually exploited by open-pit mining methods or block
caving, sublevel caving and/or sublevel open stoping.

Irregular replacement deposits

Many ore deposits have formed by the replacement of pre-existing rocks, particularly carbonate-
rich sediments. These replacement processes often occur at high temperatures at the contact with
medium- to large igneous intrusions, and have therefore been called ‘contact metamorphic’ or
skarn deposits. The orebodies are extremely irregular in shape, and tongues of ore may project
along any available planar structure such as bedding, joints, faults, etc. The principal materials
produced from skarn deposits are: iron, copper, tungsten, graphite, zinc, lead, molybdenum, tin,
uranium and talc. The lead-zinc deposits at Pering in the north-western Cape are thought to be
deposits of this type.
 9

Figure 7. Skarn deposit formed at the boundary between sedimentary rocks and an intrusive igneous body. (Pearl, 1973)

2.1 CONCORDANT OREBODIES

2.1.1. Sedimentary Host Rocks

Concordant orebodies in sediments are very important producers of many different metals. They
are of course concordant with the bedding and may be an integral part of the stratigraphic
sequence and are thus referred to as ‘stratiform’. The deposits have a considerable development
in two dimensions - parallel to the bedding, and a limited development perpendicular to it.
Concordant, stratiform deposits of different origin occur in many different types of sedimentary
rocks, a few examples are shown below.
 10

Figure 8. Stratiform ore deposit (2) formed along boundary between limestone (1) and overlying lava (3), which in turn is
overlain by a sandy shale grading upwards into a sandstone. (Kreiter, 1968)

 Lead-zinc sulphide deposits in hydrothermal replacement of reactive limestone layers

 Lead-zinc with barite volcanogenic exhalative sulphide deposits in marine shales
 Rutile and zircon placer concentrations in recent and modern beach sands
 Gold in alluvial placer concentrations with modern and ancient conglomerates
 Sedimentary iron and manganese oxide deposits
 Accumulations of organic material forming coal seams in sandstone/shale sequences.

Because these concordant, tabular deposits are stratabound and usually stratiform, they often
have a low angle of dip (unless there has been major tectonic movement) and mining methods
require direct access to the stope or working face for the removal of broken rock. These deposits
are thus best suited to exploitation by longwall (breast-stoping) or room-and-pillar mining.
Where folding and tectonic movement has resulted in steeply-dipping, tabular deposits they can
be successfully mined by shrinkage stoping, cut-and-fill or sub-level open stoping.

2.1.2. Igneous Host Rocks

Volcanic hosts

The most important deposit types in volcanic rocks are the volcanogenic massive sulphide
orebodies and the volcanogenic exhalative sulphide deposits. They are generally stratiform
bodies, lenticular to sheet-like in shape, developed at the interfaces between volcanic units or at
volcanic - sedimentary rock interfaces.

As with the concordant orebodies hosted in sedimentary rocks, the choice of mining method is
largely governed by the dip of the orebodies. Steeply dipping orebodies can be mined by cut-
and-fill, shrinkage or sub-level open stoping whereas the more flat-lying orebodies will require
longwall, room-and-pillar or similar mining methods.

Plutonic hosts
 11

Many plutonic igneous intrusions, particularly some basic intrusions, possess rhythmic layering
of alternating bands of mafic and felsic minerals developed as a result of magmatic
differentiation. Sometimes minerals of economic interest such as chromite, magnetite and
ilmenite may form discrete, mineable seams within such layered complexes. These seams are
naturally stratiform and may extend over many kilometers (eg. chromite seams in the Bushveld
Complex). Nickel and copper sulphide deposits formed by the sinking of an immiscible sulphide
liquid to the bottom of the magma chamber may also form sheet-like or lens-like orebodies
conformable with the overlying silicate rock. From the base upwards, massive sulphide gives
way through disseminated sulphides in a silicate gangue to barren igneous rock.

Figure 9. North-south section through part of the Bushveld Complex. This is a vast plutonic body in which magmatic
differentiation has settled out layers of valuable minerals in the form of stratiform ore bodies. From left to right are shown the
chromite seams, overlain by the Merensky Reef (platinum) and on the extreme right the magnetite layer which contains
vanadium, iron and titanium. (Hamilton & Cooke, 1945)

The orebodies in plutonic, igneous rocks are invariably fairly flat lying (shallow-dipping),
tabular deposits and mining requires access to the stope face to remove broken rock. Mining will
require longwall, room-and-pillar or similar mining methods.

Figure 10. East-west section through part of the Bushveld Complex showing a nickel-bearing pipe (dunite) intrusive into the
stratiform Lower Chromite layer. (Hamilton & Cooke, 1945)
 12

2.1.3 Metamorphic Host Rocks

Apart from some deposits of metamorphic origin such as the irregular replacement deposits
already described, and mineral deposits generated in the contact metamorphic aureoles of major
igneous intrusions, eg. Andalusite in Pretoria Group shales around the Bushveld Complex,
metamorphic rocks are important for the metamorphosed equivalents of deposits that originated
in sedimentary and igneous rocks and which have been discussed above. The lead-zinc
orebodies of Black Mountain near Aggenys are typical volcanogenic exhalative sulphide
deposits but have undergone extensive regional metamorphism and are now hosted in
metamorphic rocks.

2.1.4. Residual Deposits

Residual deposits are formed by the removal of non-ore material from rock in which an initial
but uneconomic concentration of minerals is present, leaving behind material that the natural
processes have upgraded to form an orebody. The weathering of feldspathic rocks (granites,
syenites) to produce important kaolin deposits is a typical residual deposit.

By the way they are formed these are naturally deposits that form at, or close to surface, and are
mined by quarrying or open pit mining.

Figure 11. A succession of dolomite (c) has undergone chemical weathering to produce a residual deposit of manganiferous earth
(b) called wad. Overlying the wad at (a) is a transported soil. (Gunther, 1912)