MN 277 Economic and Exploration Geology

UNIVERSITY OF MINES AND

TECHNOLOGY
TARKWA

FACULTY OF MINERAL RESOURCES TECHNOLOGY

(DEPARTMENT OF GEOLOGICAL ENGINEERING)

ECONOMIC AND EXPLORATION

GEOLOGY
LECTURE NOTES
[Document subtitle]
ECONOMIC AND EXPLORATION
GEOLOGY (MN 277)

f. Majeed
fmajeed@umat.edu.gh

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 MN 277 Economic and Exploration Geology

CHAPTER ONE

INTRODUCTION

Usually mineral resources can be grouped into about 8 classes, according to their operational
association and usage. They include
1. Precious metals - Au, Ag,
2. Non-precious metals e.g. Cu, Zn, Pb, Sn and Al
3. Iron or ferro-alloy metal - Fe, Mn, Ni, Cr
4. Minor metals and associated non-metals eg. Mg, Cd, Sb, As, Beryl, Bismuth, Hg.
5. Fissionable metals - Ur, Radium, Thorium
6. Diamond and other gemstones e.g. pearls, beryl, topaz, sapphires
7. Industrial minerals – limestone, salt, bauxite.
8. Mineral fuels - hydrocarbon

Important factors in the economic recovery of minerals

The mineral development process i.e. the series of steps that are required to search for
discover, develop and exploit a given mineral commodity can be summarised into 7 phases.
Mineral Exploration with the objective to discover an ore body.
1. Feasibility Study: to improve the commercial viability of the deposit
2. Mine development: to establish the complete infrastructure for successful exploitation of
the deposit
3. Mining: Extraction of the ore from the ground
4. Mineral processing (ore dressing): Milling of the ore, separation of ore minerals from
gangue, separation of the ore minerals into concentrates, e.g. copper, nickel, or zinc
concentrate; separation and refinement of industrial minerals products
5. Smelting: recovering metals from the mineral concentrates
6. Refining: Purifying the metal
7. Marketing: transporting the product to the buyer, e.g. customer smelters, and
manufacturers.

Evaluation of a potential ore body.

The following important considerations which are critical for the evaluation of any potential
ore body are briefly explained.
1. Ore grade: This refers to the concentration of the metal or mineral of interest in the ore
body. The grade of an ore body is normally expressed as a percentage or in parts per
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million (ppm). The method of determining the ore grade is assaying. The lowest minimum
grade of ore that can be mined is referred to as the cut-off grade. It is dependent on a
number of economic and political factors.
2. Head grade: The grade of ore material delivered to the mill, often lower than measured
or geological ore grade to dilution during mining.
3. The grade of industrial minerals: The grade of industrial minerals is normally as critical
as that for metals deposits. The important factors in assessing the usefulness of industrial
minerals (non-metallic) deposits include chemical and physical properties, and many
types of these deposits are used in bulk.
4. By-products: In some ore deposits more than one important metal might occur and both
could be sold. Many examples of this exist. Pb, Zn and Cu polymetallic deposits are
common. In Ghana gold in the Birimian usually has Hg (silver) as a useful by-product.
5. Commodity Price: The price of the metal or mineral to be marketed is an important
factor. The could be influenced by several factors (demand and supply conditions)
6. The Mineralogical form: The properties of a mineral usually governs the ease with which

available technology can be employed to extract and refine some metals and this may
affect the cut-off-grade. In Ghana gold occurs in several mineralogical forms e.g. as
sulphides, oxidised ore, in matrix of conglomerates etc. This affects the processing of the
ore too. Industrial minerals present different challenges. For instance, silica sand deposit
to used for high quality glass making requires Fe2 O3 content to be less than 0.033%. In
some cases the iron may be removed by scrubbing or by acid leaching.

The importance of economic geology

Rapid increases in the world population mean much greater demand for minerals and
mineral products. This means more and more mineral resources are being used to provide
these products. As more and more mineral are discovered and used increasing efforts would
have to be made to discover the remaining material. This means that for such ore is
becoming more complex as such ore are being sought under cover and at greater and greater
depths. In order to obtain supplies in the future, new geological, geochemical, geophysical
exploration ideas and techniques must be developed and applied. Similarly mining,
recovery and recycling techniques will have to be improved. It is these innovation that
ensures that thitherto low grade material of near- surface is now economic thanks to
evolution in processing technologies. The increasing depletion of ore reserves means that
the successful economic geologists who has critical “exploration thinking”, requiring
imagination and ingenuity as well as a thorough knowledge of structural geology,
stratigraphy, petrology and mineralogy and how fluids migrate underground. The

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successful economic geologist should be able to apply geochemistry, geophysics,
interpretation of field and laboratory data, and to bring mathematical and computer
modelling skills to enhance the interpretation.

Under what conditions and as a result of what processes are ores formed? What factors
lead to the concentration of particular mineral ores one environment and not another? What
causes the localisation of ores? These questions can only be effectively answered through
the study of the structure and genesis of known mineral occurrences and then explore
geologically favourable analogous areas. While is true that no two deposits can be exactly
alike, they may share enough unifying characteristics that they may be grouped into
exploration-genetic sets and can therefore be successfully explored and discovered.

DEFINITIONS
The following terms are frequently used in economic geology but may also be applicable
in any field that deals with mineral exploitation.
Economic geology may be defined as:
The science of locating and processing ores;
The study of minerals in connection with their utility and possible profitable extraction;
The practical application of geologic theories to mining (mining geology)
Economic geology stands between mineralogy and geology.
A deposit is a natural accumulation of useful materials in the earth's crust that can be mined
with economic profit.
An ore is the sum total of ore minerals, gangue minerals and country rock which constitute
the material worked for the purpose of extracting a metal from the ore mineral. Technically,
for a material to be called ore, the economic factor must be taken into account and it must
be possible to extract metals profitably from the ore.
The test of yielding a metal or metals at a profit seems to be the only feasible one to employ.
Note also that several similar definitions can be found but ALL emphasize that:
a. it is a material from which we extract a metal;
b. this operation must be a profit-making one.
An ore mineral is one from which a useful metal may be extracted.
Ore body: Economically mineable aggregates of ore minerals.

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 MN 277 Economic and Exploration Geology
Industrial Mineral: Any rock, mineral or other naturally-occurring substance of economic
value, exclusive of metallic ores, mineral fuels and gemstones. They are, therefore, minerals
where either the mineral itself e.g. asbestos, barite, or some other compound derived from
the mineral has an industrial application (end use). They include rocks such as granite, sand,
gravel, and limestone as well as the more valuable minerals with specific chemical or
physical properties e.g. fluorite, phosphate, kaolinite and perlite.
Protore: Mineral material in which an initial but uneconomic concentration of metals has
occurred that may by further natural processes be upgraded to the level of ore.
Gangue minerals are the associated nomnetallic materials of a deposit. The term is also
loosely used for the waste material from the process of separation and concentration of ores.
It should be noted that the gangue minerals of one orebody may be the ore mineral of another
e.g. pyrite may be separated when it occurs with lead and zinc and is referred to as gangue
while in other places pyrite is worked profitably. Some non-metallic minerals are valuable
eg. barite and fluorite.
A country rock surrounds the deposit, contains little or no ore and is composed of rock
forming minerals.
Grade: The concentration of a metal in an ore body; usually expressed in percentage or ppm
(ppb). Grades vary from orebody to orebody, and clearly, the lower the grade the greater the
tonnage required to provide an economic deposit.
Metal groups: It is traditional in the mining industry to divide metals into groups with
special names:
a. Precious metals; gold, silver, platinum group;
b. Non-ferrous metals: copper, lead, zinc, tin (base metals), aluminium;
c. Iron and ferro-alloy metals: Fe, Mn, Ni, Cr, Mo, W, V, Co:
d. Minor metals and related non-metals: Sb, As, Be, Bi, Cd, Mg, Hg, Se, Ta, Te, Ti, Zr, etc
e. Fissionable metals: U, Th, etc.
Two groups of mineral deposits may be established:
Syngenetic and Epigenetic depending on their time relations to the rocks enclosing or
associated with them.

Syngenetic deposits are formed at the same time as the associated rocks. One example is the
type of ore deposit called magmatic segregation which arises by the collection together of
useful minerals during the Orthomagmatic stage (stage in the crystallization of an igneous
mass during which the main mass of silicates crystallize) of consolidation of magma i.e.,
segregation of chromite in ultrabasic igneous rocks.

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Epigenetic deposits are formed later than the enclosing or associated rocks. Some of these
deposits have filled or opened fissures in the country rocks and such bodies of ore are called
veins and lodes. I n others, the ore is deposited in the interstices of the country rocks and
then form impregnations or some replace the country rock.

A mineral deposit may be concentrated in two environments.

Endogenetic (primary) deposits result from processes taking place or originating within the
crust such as solidification of a magma or the operations of metamorphism.
Exogenetic (secondary, supergene) deposits are due to surface processes such as deposition
in a salt lake or sorting by river action.
Stratabound deposit: It is a mineral deposit confined to a single stratigraphic unit (in a
broader sense confined or related to a single stratigraphic unit).
Stratiform deposit: It is a special type of stratabound deposits in which the mineral aggregate
or ore constitutes, is strictly in coexistence with, one or more sedimentary , metamorphic or
igneous layers.
Hydrothermal deposit: a deposit pertaining to hot water, to the action of hot water and the
products of this action.
Pneumatolytic deposit: a deposit formed by volatile components or gaseous emanations
derived from solidifying magma.

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CHAPTER TWO

MINERAL DEPOSIT OR MINE

A mineral deposit is an abnormal concentration of minerals within the earth's crust. The
factors which determine whether a mineral deposit will be mined are mainly political and
economic.

I. Political Factors
Governments are increasingly playing very important roles in determining the direction of
mining. There are a number of reasons why ore bodies are mined for political rather than
economic reasons.

a) Many developing countries depend upon the export of mineral commodities as one of their
principal sources of foreign exchange, and this aspect may take precedence over profitability.

b) Some governments are prepared to subsidize unprofitable mining operations in order to

alleviate the problems of unemployment.

c) Whilst the general trend has been for governments to press for increased mining, they may
also play an inhibiting role eg. mining prohibited in National Parks. Mining of uranium is a
sensitive national issue in many countries.

d) Taxation is a further example of government control on mining which influences the style
of deposit which may be mined. e.g., in Western Australia, gold mining is exempt from
taxation so there is a frenzy of exploration in that state. Most commonly taxation has a
discouraging effect on exploration and mine development.

2. Economic factors.

The first stage of a mining project is to define the size and grade of the deposit. It is normal
practice to quote reserves as either proven, probable and possible or alternatively as
measured, indicated and inferred. If the measured reserves are sufficient to sustain a
mining operation over a reasonable period of time (about 20yrs), the next stage is to carry
out a feasibility study to determine the economic viability of the deposit.

a) Metal Price. Commodity prices are one of the key factors in determining the viability of
a minerai deposit. The main problem is that the lead time between the feasibility study on an
ore deposit and initial production is normally 4 to 7 years and can be much longer. It is
extremely difficult to predict how metal prices will vary during this time.

b) Cut-off grade is the minimum metal content necessary to maintain production costs and

sales income in balance and it therefore represents the breakeven point for a mine. Once the
cut-off grade has been determined the size and average grade of the ore deposit can be
calculated.

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Changes in cut-off can have dramatic effect on the size of the ore reserve e.g., Gaspe copper
Mine Ltd a skarn porphyry Cu deposit in eastern Canada reported to have a reserve of 51.6
x 1020 tonnes at 1.08% Cu in 1970. The following year the cut-off grade reduced and
published reserves increased to 263 x 1020 tonnes at 0.59% Cu.
i) Open pit or underground methods of extraction; the selected of open pit or underground
methods will determine the cut-off grade because open pit mining has lower production costs.
ii) By-product metals; The presence of by-product metals play an important role in the
economics of mining and may determine the cut-off grade. It is evident that those operations
without significant by-products must have higher grade metals in order to be economically
viable.
3. Mining Method During feasibility study, a decision must be made about the mining
method to use. It is important that during the initial drilling programme thought is given to
the geotechnical properties of the rocks in addition to considerations of size and grade. The
key factors influencing the choice of mining method are:
I. Size and shape of orebody,
II. Type and thickness of overburden or rock cover
III. Presence of aquifers overlying the ore body
IV. Strength of the ore body and surrounding rocks
V. Tendency of the mineral to oxidise immediately after mining
VI. Temperature gradient and the resulting demand on ventilation (open pit preferred here).
4. Metallurgical factors During feasibility study, it is necessary to investigate the proportion
of valuable minerals which can be obtained from the ore. Initial studies are concerned with
ore mineralogy which includes an assessment of the ore minerals which are present and their
relationships with other minerals.
Hereafter, it is necessary to determine how much of the mineral can be recovered before a
decision can be made about economic viability. The processing of an ore involves two main
stages; comminution and seperation. Comminution involves breaking down of the ore to a
sufficiently small size for the ore minerals to be liberated. The problem is to define the
optimum point when the maximum amount of ore mineral has been released with minimal
energy. Separation involves utilization of physical property such as density or chemical
property such as solubility to divide the ore minerals from their associated gangue

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CHAPTER THREE

MAGMAS AND MINERAL DEPOSITS

It is generally considered that most metalliferous deposits and many non-metallic deposits result
from igneous activity. Thus valuable materials of most mineral deposits whether endogenetic or
exogenetic have directly or indirectly come from magmas.

It is therefore very important for a proper understanding of the origin and nature of mineral
deposits to consider some features of magmas and their crystallization.

Magmas are high temperature mutual solutions of silicates, silica, metallic oxides and dissolved
volatile substances (H2O, CO2, S, Cl, F, B, etc.) with their temperatures ranging from 6000C (for
rhyolite or silicic or acid magmas) to 16000C (for basaltic or basic magmas).

Magmas are believed to be temporary features within the crust. At depth the rock temperature,
due to the internal heat of the earth, is above its melting point but the enormous pressure induced
by overlying rock load prevents melting. When pressure is relieved by faulting, buckling etc,
melting would follow and magma results.

The natural magmatic melt is a very hot mixture of metallic oxides in silicate or aluminosilicate
binding and represents also a complex system in which constituents of low and high volatility
are kept in mutual solution. If the system is located at a certain depth in the earth under pressure,
it may be in equilibrium but each change in pressure sets off a reaction within the solution phase.
When the temperature of the magma falls below individual saturation points, crystallization will
begin.

The most insoluble substances crystallize first (and these, in general, are the accessory minerals:
apatite, zircon, rutile, magnetite, chromite, etc.) and in general the order of crystallization is one
of decreasing basicity.

With the subtraction of the more basic minerals from the magma, the residual becomes
progressively more silicic (granitic residual magmas are rich in silica, alkalis and water while
basic magmas may be rich in iron ),

Volatile substances or mineralisers, such as F, Br, Cl, along with Sn concentrate in silicic rest
magmas and may be tapped off to form pegmatite dykes rich in rare minerals.

With progressing crystallization the final aqueous extracts gather, metals that originally were
sparsely contained in the magma along with the rare elements, the rare earths (REE's), Cl, Br, F,
H, S, As, etc.

These liquors become expelled upon final crystallization and constitute the magmatic solutions
that give rise to most economic mineral deposits. Consequently, they are a part of the magma of
particular interest to the Economic Geologist.

Reaction Series.

A definite sequence of reactions has been determined during crystallization of magma called
Bowen's Reaction Series.

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Certain minerals once formed, may continue to react with the enclosing liquid magma with the
result that their composition is continually being modified and new mineral or solid solutions
result.
The reaction principle indicates that olivine may crystallize early but by reaction with the
remaining liquid it changes to pyroxene which should precede hornblende and that basic
plagioclase should precede intermediate or acid plagioclase. Also a temperature range of
consolidation will result. The reaction principle operates where fractional crystallization occurs.
Differentiation

When the magma rises to higher zones within the earth's crust, a separation of material takes
place during crystallization which is called magmatic differentiation.
This differentiation results in the formation of various igneous rocks on the one hand and
different mineral and ore deposits on the other.

The causes of differentiation are manifold:

They are chiefly the gravitational field of the earth and the pressure and temperature gradients
prevailing during the uprise of the magma.
The crystals that are precipitated, depending on their densities, will either sink or rise and the
order is given by the Bowen's reaction series.
This sequence of precipitation demonstrates very strikingly the segregation of elements
during differentiation.

We can recognize:
a) Primary Crystallisation stage: with minerals rich in Mg, Fe, Ni, Cu, Ti, Cr, Pt.
b) Main Crystallisation stage: producing most igneous rocks composed of Ca, Al, alkalies
and Si.
c) Residual Crystallisation stage: showing enrichment of Si, O, S, Cl, F and most metals.
Ore deposits are formed mainly during the primary and the residual phases of crystallisation.
Besides crystallisation or magmatic differentiation, there exists differentiation in the liquid
state also called liquid immiscibility.
This occurs mainly where basic magmas contain large quantities of sulphides. At high
temperatures, the sulphides are at first dissolved in the silicate melt. With decreasing
temperature, the sulphides form droplets increasing in size and sink down by gravity within the
magma.
BOWEN'S REACTION SERIES

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Fig. 1 The Bowen's Reaction Series

As a result of crystallisation and differentiation, associations of minerals are formed that yield
many kinds of igneous rocks. The texture is determined chiefly by the rate of cooling and also
by the amount of mineralisers present during consolidation.

Slower cooling rate gives opportunity for a large crystals to grow and results in coarse or
granular texture. If cooling is rapid, the crystals are very small and texture is aphanitic.
Very rapid crystallisation results in the formation of glass as in the case of some lavas.
Interrupted crystallisation may give a porphyrytic texture consisting of large crystals
(phenocrysts) in finer grained matrix.

With progressing crystallisation, the late residual liquid is made up principally of low melting
silicates and considerable water, along with other low melting compounds and volatiles and a
relative concentration of many of the substances that enter into mineral deposits of igneous
origin.
This is an aqueous stage, a transition between a strictly igneous stage and a hydrothermal stage,
leaning more to the igneous and referred to as Pegmatitic stage.

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The pegmatites are important containers of many non-metallic industrial minerals and some
metallic ones such as tantalite, columbite, and cassiterite. A rise of magma to levels of lesser
pressures brings about the separation of the gaseous phase and bubbles rise to the highest part
of the magma chamber.
High temperature gaseous action is called pneumatolytic and is defined as pertaining to
processes of results effected by gases evolved from igneous magmas or entrained with gases
of that origin.
Pneumatolytic processes are capable of effecting important metamorphic and metasomatic
results in portions of the igneous mass and in the contact zone by adding or subtracting
ingredients and bringing about recrystallization.
Certain types of ore deposits such as contact metasomatic deposits have been considered to be
due to gaseous action. It is universally agreed that most mineral deposits of igneous affiliations
result from hot waters of magmatic derivation (hydrothermal solutions) in consequence of
crystallization and differentiation.
Most economic geologists believe that metallizing hydrothermal solutions are the residue of
pegmatitic injections left after the pegmatitic constituents have crystallized during the last stage
of differentiation in the magma chamber. This liquid most probably constitutes the chief ore
forming fluid.

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CHAPTER FOUR

MAJOR THEORIES OF ORE GENESIS (Group)

It is important to note that several processes are responsible for the formation of ore
deposits. These processes could be put under two main headings:
1. Origin due to internal processes;
2. Origin due to surface processes.

PROCESSES OF FORMATION OF MINERAL DEPOSITS

Mineral deposits are formed by diverse processes and more than one process may enter
into the formation of an individual deposit. Their formation is complex and no two are
alike.
They differ in mineralogy, texture, content, shape, size and other features.
Among the agents that enter into the formation of mineral deposits, water plays a dominant
role in the form of water vapour, magmatic water, cold meteoric water, ocean, lake. or
river water.
Other agents are temperature, magmas, gases, vapours, solids in solution the atmosphere,
organisms and country rock.

Mineral deposits are susceptible to field classification according to the processes that
operate to form them. The mode of occurrence of one representative may indicate
favourable places to search for similar unknown deposits.
Different processes may operate to produce distinct types of deposits of the same metal
e.g. deposits of iron ore may be formed by magmatic, contact metasomatic, replacement,
sedimentary and/or supergene processes. (An iron deposit formed by sedimentary
processes may be expected to exhibit the characteristics of a sedimentary rock: lateral
continuity, thinness, general uniformity of composition. A hydrothermal replacement
deposit would by laterally restricted, of smaller tonnage, and of irregular shape.

CLASSIFICATION OF MINERAL [ORE] DEPOSITS. (class ppt)

The study of ore deposits requires the examination of great numbers and types of mineral
districts. Grouping together deposits with similar characteristics facilitates description and
permits generalizations concerning genesis and ore localization.

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A classification must be as simple as possible so that it can be used in the field and during
mapping. Various types of ores grade into each other and their genetic boundaries cannot
be precisely defined; so such a classification must be flexible.
Whereas past attempts at classification have emphasized form, texture, and the mineral
content and associations of ore deposits, more modem classifications are developed around
theories of genesis and environment of deposition.
No universally acceptable classification has been proposed and at present three systems
are in common use.

i. Niggli's classification
ii. Schneiderhohn's ore association classification.
iii. Lindgren's classification and
iv. Meyer classification
Modern studies have made available a great deal of information and necessitate revision and
modernization of the earlier classifications. Niggli (1929) grouped the epigenetic ores into:
 volcanic or near surface, and
 plutonic or deep seated deposits.

The plutonic deposits are divided into:

 hydrothermal, pegmatitic,
 pneumatolytic, and orthomagmatic subgroups;
depending upon whether the ores form from liquids or gases or as direct crystallization
products within the magma. This system categorizes deposits on the basis of their
genesis and mineralogy.
Fundamentally, this classification differs little from Lindgren's and most criticism applied
to Niggli's classification can also be applied to Lindgren's.
Schneiderhohn (1941) classified ore deposits according to:
i) nature of the ore fluid:
ii) the mineral association (represent metal associations in ore forming fluids.
iii) a distinction between deep surface and near surface deposits.
iv) the type of deposition, host or gangue,
The significant category is the second group. Although the schemes of Schneiderhohn and
Lindgren have fundamental similarities, they differ in emphasis.

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A weakness to Schneiderhohn's classification is that a deposit that does not fit one of the
given ore mineral associations or its subdivisions is categorized by formulating a new
group or subdivision.

Lindgren introduced his classification in 1913 and is considered by Americans as the best
for use in the field. The temperature and pressure designations in Lindgren's scheme are
criticized because of the uncertainty of exactly what is meant by "high", "medium" and
"low" when applied to temperature and pressure.

STAGES IN ORE FORMATION

Four stages are recognized in the formation of ore deposits:

I. the source and character of the ore bearing fluids,
II. the source of the ore constituents and how they are obtained in solution,
III. the migration of the ore bearing fluids,
IV. the manner of deposition.
For closer investigation, the ore bearing fluids are divided into four categories.
i) magmas and magmatic fluids,
ii) meteoric waters,
iii) connate waters,
iv) fluids associated with metamorphic processes.

Magmas and their fluids have already been discussed in some detail
Meteoric water is one which comes from the atmosphere and is especially important in
supergene processes, It sinks into the earth and gradually assumes the temperature of the
enclosing rocks. It is believed that meteoric water contains the dominant crustal elements
such as Na, Ca, Mg, and the sulphate and carbonate radicals.

Water trapped in the sediments at the time they were deposited is known as connate (fossil)
water and is widely observed in oil field exploration (as the salty edge waters of many oil
accumulations). Connate waters have little direct relationship to ore bearing fluids, except
where the containing strata are undergoing metamorphism.

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Under favourable conditions, connate and meteoric waters enclosed in rocks buried below
the surface of the earth may be set in motion and made chemically reactive by heat and
pressure accompanying magmatic intrusions or regional metamorphism. These are the so-
called metamorphic waters believed to be active ore carriers.

MIGRATION OF THE ORE BEARING FLUIDS.

Ore genesis should be, and is generally explained by migration of large amounts of ore
bearing fluids, which can be magmas, aqueous solutions, or gases. In general, migration is
controlled by structures that determine the avenues of permeability.
Migration of magma:
The movement of magma to areas of deposition has been discussed already.
They generally move upward toward areas of lower pressure.
They contain gases under pressure and any release in pressure will allow these gases to
expand and move upward.

Tectonic stresses may squeeze the magma or fractions of the magmatic differentiates into
overlying rocks or force its way between rock layers by actually breaking rocks apart.
Some magmas are thought to move by means of stoping, a process whereby magma works
its way upward by engulfing blocks of overlying rock.
The stoped blocks sink into the magma chamber and are assimilated at depth.

DEPOSITION OF THE ORES

Whether an ore deposit is a product of igneous, sedimentary, metamorphic or weathering
processes, its localization is generally the result of many factors.
One factor or a series of factors may account for the ore constituent, another for its
deposition; still another for the size of the deposit.
Some ores are deposited by gravity eg. an early formed chromite crystals may settle out of
a residual magma.
Other ores are deposited because of chemical changes, such as a change in pH, that results
from reactions between the ore bearing solution and the host rocks.
A drop in temperature, pressure or velocity of the transporting medium or the admixture
of a second solution may also bring about chemical reactions to cause ore deposition.
Localized deposition may depend upon the permeability, structure, brittleness or chemistry
of the rocks.

Ground preparation

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Many epigenetic deposits are restricted to areas that have undergone a favourable pre-
metallization change. Such a change may make the country rock more receptive or more
reactive to the ore bearing solutions. Accordingly, the process is known as ground
preparation.
Syngenetic deposits are formed at the same time as the host rock and therefore do not
require any ground preparation.
Ground preparation may take place in several ways.
Any process that:
o increases permeability,
o causes a favourable chemical change, or
o induces brittleness in the rocks, may localize deposition from the ore bearing fluids.
Hence the type of ground preparation depends upon both the country rock and the
preparing agent (heat, fluids, tectonics or a combination of the three).
Examples:
Silicification, dolomitization and recrystallization are common examples of ground
preparation.
Dolomitization tends to make rocks brittle; subsequent movement may shatter the
dolomitized rock and produce a clean permeable breccia that can serve as a porous
receptacle for ore deposition.

Much ground preparation is chemical; perhaps the most common reaction is the addition
and rearrangement of silica in the form of either SiO2 or silicates (e.g. one form of
jasparoid, a cryptocrystalline silica that is transported in hydrothermal fluids and replaces
country rock) which are fractured and shattered.
Permeability and brittleness are increased by simple crystallization of the country rock
common near intrusive masses.

The formation of skarn is another type of ground preparation. Skarn is composed mainly
of lime-rich silicates produced by the introduction of silica, AI, Fe, Mg; into Ca and Mg
rich carbonates.

Alteration and gangue

Country rocks bordering ore deposits of hydrothermal origin are generally altered by hot
fluids that have passed through them and with which the ores may be associated.

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If the wall rocks are unstable in the presence of early, ground preparing hydrothermal fluids
or ore forming solutions, they will undergo physical and chemical changes to reach
equilibrium under the prevailing conditions. The resulting alteration may be very subtle (
e.g. the hydration of selected ferromagnesian minerals) or it may be very complete (e.g.
silicification of limestone).
The alteration may range from simple recrystallization to the addition, removal or
rearrangement of chemical components.

It may take place in advance of emplacement of the ore minerals or during the final stages
of the hydrothermal activities.
The nature of the alteration products depends upon:
 the character of the original rock,
 the character of the invading fluid, which defines such factors as
o Eh,
o pH,
o vapour pressure, and
o the degree of hydrolysis.

 the temperature and pressure at which the reactions took place.

Wall rock alteration may bring about:

o recrystallization,
o changes in permeability, and
o changes in colour.
 Carbonate rocks are characteristically recrystallized along the border of a vein or near
an igneous contact.
 The recrystallized rock is generally more permeable than the unaltered rock suggesting
that some ores may be deposited following an advance wave of recrystallization.
 Conversely argillization may reduce the permeability of a rock, leaving the orebody
enclosed within a relatively impermeable shell.
 Colour changes include
o bleaching,
o darkening, and
o aureoles of various colours.
Different kinds of ore deposits may be typified by certain alteration minerals and different
degrees of alteration.
Due to their depth of burial, the wall rocks around deep seated ores are relatively
impermeable and usually as hot as the ore bearing fluids.
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Therefore unless there is a strong contrast between the ore fluids and the country rocks,
the alteration zone is likely to be thin and inconspicuous.
Conversely, hot solutions that invade cool, shallow permeable rocks typically produce
prominent, widespread alteration halos because the country rocks are far out of equilibrium
with the fluids.

Wall rock alteration is a valuable exploration tool:

the alteration halos around many deposits are widespread and much easier to locate
than the ore bodies.
Gangue consists of all non-economic minerals in an ore deposit.
A mineral considered gangue in one deposit may be regarded as an ore mineral in another.
Gangue minerals are principally silicates and carbonates with subordinate oxides, fluorides
and sulphates.
Knowledge of a common origin for the ore and gangue minerals can be important for
prospecting:
o if the gangue was produced by the same mechanism as the ore, its presence may be used
as guide to ore bearing structures or potentially favourable environments.
o The temperatures of the ore deposition and the composition of the ore forming fluids are
estimated by examining the gangue and recognizing the paragenesis of the ore and
gangue.

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CHAPTER FIVE

FORMATION OF MINERAL DEPOSITS

Processes given rise to formation of Mineral deposits

The various processes that have given rise to mineral deposits are:
a) Magmatic. concentration,
b) Sublimation,
c) Contact metasomatism,
d) Evaporation,
e) Hydrothermal processes,
f) Metamorphism,
g) Residual & mechanical concentration,
h) Sedimentation,
i) Oxidation and supergene enrichment.

MAGMATIC CONCENTRATION
Magmatic ore deposits are characterized by their close relationship with intermediate or
deep-seated intrusive igneous rocks.

They are igneous rocks whose composition is of particular value to man.

They constitute either the whole igneous mass, part of it or form offset bodies.

They are magmatic products that crystallize from magmas.

They are also called magmatic segregation, magmatic injection or igneous syngenetic
deposits.
Magmatic mineral deposits are classified into:

 Early magmatic and

 late magmatic deposits.
a) Early magmatic deposits
They have resulted from straight magmatic processes; those that have been termed
orthomagmatic. . These deposits have been formed by
 Simple crystallization;
 Segregation of early formed crystals;
 Injection of materials concentrated elsewhere by differentiation.

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i) Simple Crystallization or dissemination of a deep-seated magma in-situ will yield a

granular igneous rock in which early-formed crystals may be disseminated in it throughout.
 If such crystals are valuable and abundant, the result is a magmatic mineral deposit and
the individual crystals mayor may not be phenocrysts.
 The diamond pipes in South Africa are examples. The diamonds are sparsely
disseminated throughout kimberlite rock and the whole kimberlite pipe is the mineral
deposit and the diamonds are phenocrysts.
 The resulting deposits of this class have the shape of the intrusive dyke, pipe or small
stock-like mass.

ii) Segregation.
 Early magmatic segregations are early concentrations of valuable constituents of the
magma that have taken place as a result of gravitative crystallization differentiation.
 Such constituents as chromite may crystallize early and become segregated in bodies of
sufficient size and richness to constitute economic deposits.
 The segregation may take place by the sinking of heavy, early-formed crystals to the
lower part of the magma chamber or by marginal accumulation.
 The mineral deposits are generally lenticular and of relatively small size.
 Commonly, they are disconnected pod-shaped lenses, stringers and bunches. Less
commonly they form layers in the host rock.
 Example is the chromite deposits in the Bushveld complex, Transvaal.

iii) Injection
The ore minerals have been concentrated presumably by crystallization differentiation.
They are earlier than or contemporaneous with associated igneous minerals.
They have not remained at the place of original accumulation, however, but have been
injected into the host rock or into surrounding rocks. The structural relations of the deposits
to the enclosing rock structures, include rock fragments, or occur as dykes, or other intrusive
bodies in foreign rocks. They even metamorphose the wall rocks.
Example is the largest magnetite deposit in the world at Kiruna, Sweden.

b) Late Magmatic Deposits

 Late magmatic deposits are bodies of pyrogenic minerals that have crystallized towards the
close of the magmatic period.
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 They are the consolidated parts of the igneous fraction that remained after the crystallization
of the early formed rock silicates and have cut across them, embay them, and react yielding
reaction rims.
 The deposits formed in this category are dominantly associates of basic igneous rocks and
have resulted from crystallization, differentiation, gravitative accumulation of a heavy
residual liquid, liquid immiscibility and other modes of differentiation. This group now
includes most magmatic deposits.

i) Residual liquid segregation

 In certain types of basic magmas, the residual magma may become enriched in iron and
titanium.
 This is true of magmas that yield anorthosite where the plagioclase crystallizes first and
iron oxides with or without pyroxene crystallize last.
 This residual liquid may drain out or otherwise segregate from the crystal interstices within
the magma and there crystallize as the last pyrogenic minerals if valuable.
 The host rocks are commonly anorthosite, norite, gabbro, etc.
 Examples of such mineral deposits formed are titaniferous magnetite bands of the
Bushveld complex in South Africa.

ii) Residual Liquid Injection

In this process, the iron-rich residual liquid accumulates as described above and under
conditions of concomitant disturbance such as accompanies igneous intrusion:
i) may be squirted out toward places of less pressure in the overlying consolidated portions
of the mother rock or into enclosing rocks.
ii) if liquid accumulation had not occurred, interstitial iron-rich residual liquid may be filter
pressed out to form late magmatic injections.
The resulting ore bodies may be irregularly shaped masses, sills, or dykes and generally
transect the primary structure of the host rocks or crosscut the invaded rocks.
iii) Immiscible Liquid Segregation.

 Iron-nickel-copper sulphides are soluble only up to 7% in basic magmas and upon cooling they
may in part separate out as immiscible drops, which accumulate at the bottom of the magma
chamber to form liquid sulphide segregations which crystallize after the silicates have.

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 The accumulated sulphides can be thought of as an enrichment in sulphide of the lowest part of
the magma, which upon consolidation gives rise to a basic igneous rock with 10- 20% sulphur
content.
 This may be sufficient to constitute a valuable ore deposit. The deposits consist of pyrrhotite,
chalcopyirte, nickel, copper ores with accompanying platinum gold, silver, and other elements
are confined to basic igneous rocks of gabbro family.
 An cxample is the Insizwa NiCuS deposits of South Africa.

iii) Immiscible liquid Injection.

If the sulphide-rich fraction accumulated in the manner described above should be subjected to
disturbance before consolidation, it might be squirted out towards places of less pressure, such
as sheared and brecciated areas along the margins of the consolidated mother rock or in the
enclosing rocks.
There, it will consolidate to form immiscible liquid injections which give unmistakable evidence
of late magmatic age.

They intrude older rocks and enclose brecciated fragments of the host and foreign rock.
The deposits are irregular or dyke-like in form.
E.g., Vlackfontein mine of South Africa.

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CHAPTER SIX

CONTACT METASOMATIC (PYROMETASOMATIC) DEPOSITS.

Pyrometasomatic deposits are formed by metasomatic changes in rock, principally limestone, at
or near intrusive contacts under the influence of magmatic emanations.

Thus the changes are caused by heat and by fluids emanating from or activated by intrusives.

Characteristics of the pyrometasomatic zone depend upon the nature of both the intruded rock
and the emanations activated by the intrusive.
Some resistant rocks (quartzites) may be unchanged, even at the contact, others (carbonates) may
be altered up to several kilometers from the pluton.
Two types of alteration are recognized:
i) recrystallization or rearrangement of the constituents already in the rocks, and
ii) addition of materials.
Most pyrometasomatic aureoles show both.

The most striking metasomatic aureoles are developed in the carbonate rocks.
Skarns or tactites are developed where pyrometasomatism has occurred in impure limestones
(or dolomites) and has resulted in the formation of calc-silicate rocks of complex mineralogy in
the aureole.
The emanations may carry the constituents of mineral deposits that replace the invaded rock to
form metallic and non-metallic deposits distributed spasmodically within the contact aureole.

Not all magmatic intrusions yield mineral deposits.

For their formation, certain types of magma are essential.

Those magmas that yield mineral deposits are mostly silicic ones of intermediate composition,
such as quartz monzonite, monzonite, and granodiorite or quartz diorite.
The reason why contact metasomatic deposits are produced from silicic rather than from basic
intrusives is probably because the silicic material has a high content of water, whereas the basic
is relatively dry.
Water in magma is known to be the chief collector and transporter of metals. Contact
metasomatism takes place at temperatures from 400oC to 800oC, perhaps even higher (1000oC).
Usually more material is added to the host rock than is removed from it.

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The depth of intrusion is an important factor in the formation of contact metasomatic deposits
because deposits are found only with rocks of granular groundmass which indicates relatively
slow cooling at considerable depth. The actual depth of consolidation of granular intrusives is
imperfectly known but most of them crystallized at depths greater than 1500m.

Most contact metasomatic deposits are associated with stocks, batholiths, and intrusive bodies of
similar size (rarely associate with laccoliths and large sills but absent from small sills and dykes).

Intrusive bodies whose flanks (limbs) dip gently produce wider zones of contact metasomatism
than those with steeply dipping flanks.

It is also true for those whose roofs are irregular in form.

The deposits which result from contact metasomatism are notably irregular in outline consisting
of several disconnected bodies, and are comparatively small-sized with dimensions 40-100m and
contains a few tens to a few hundreds of thousands of tonnes.
The deposits are a distinctive class characterized by unusual assemblages of ore and gangue
minerals.
They are vexatious deposits to exploit because of their small size, capricious distributions within
the contact aureole and the abrupt terminations.
The textures of the ores are commonly coarse containing large crystals.

The outstanding feature of the mineralogy is the distinctive assemblage of gangue minerals
characteristic of high temperature formation e.g., hedenbergite, tremolite, actinolite, wollastonite,
forsterite, anorthite, albite, zoisite, epidote etc.

 The ore minerals consist of oxides, native metals, sulphides, arsenides and sulphosalts.
 Oxides represented by magnetite, ilmenite, hematite, corundum and spinels. Magnetite is
particularly abundant.
 Graphite, gold and platinum represent native metals but the last two are rare. In addition
scheelite and wolframite (tungsten ore mineral) occur.

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HYDROTHERMAL DEPOSITS
Magmatic differentiation gives rise to an end product of magmatic fluids in which
there may be concentrated the metals originally present in the magma. These
hydrothermal fluids carry out the metals in chemical form from the consolidating
intrusive to the site of metal deposition and are considered to be the major factor in
the formation of epigenetic mineral deposits. (Evidence; deposition of gold, silver,
copper, lead, zinc takes place presently from the hot waters of steamboat Springs,
Nevada).
As the term hydrothermal implies, they are hot waters that probably range from 500oC
down to 50oC.

The essentials for the formation of hydrothermal deposits are:

 available mineralizing solutions capable of dissolving and transporting mineral
matter;
 available openings in rocks through which the solutions may be channelled;
 available sites for the deposition of the mineral content;
 Chemical reactions that result in deposition; and,
 sufficient concentrations of deposited mineral matter to constitute ,workable
deposits.

 In their journey through the rocks, the hydrothermal solutions may lose their
mineral content by deposition in the various kinds of openings in the rocks, to
form cavity filling deposits or by metasomatic replacement of the rocks to form
replacement deposits.

 The filling of openings by precipitation may at the same time be accompanied

by some replacement of the walls of the openings. Thus there may be a gradation
between these two types of mineral deposits.

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 In general, replacement dominates under the conditions of high temperatures and

pressures nearer the intrusive where hypothermal deposits are found, and cavity
filling dominates under the conditions of low temperatures and pressures where
epithermal deposits are formed; both are characteristic of the mesothermal zone.
The movement of hydrothermal solutions from source to sink (site of deposition) is
dependent largely upon available openings-in the rocks. This is fundamental to the
formation of epigenetic deposits.
The various types of openings in rocks that may serve as receptacles for ore or permit
the movement of solutions through the rocks may be classified as follows;

a) Original cavities.
i) pore spaces,
ii) crystal lattice,
iii) vesicles or blowholes
iv) java drain channels,
v) cooling cracks,
vi) bedding planes,
vii) Igneous breccia cavities,

b) Induced cavities
i) fissures with or without faulting
ii) sheer-zone cavities
iii) cavities due to folding and warping which are; saddle reefs, pitches and flats, and
anticlinal and synclinal cracking and slumping.
iv) volcanic pipes
v) tectonic breccias,
vi) collapse breccias
vii) solution caves and
viii) rock alteration openings.

A. CAVITY FILLING

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More types of mineral deposits are formed by cavity filling than by any other process
and fissure veins are by far the most common and important and is usually
accompanied by replacement.

Process of cavity filling:

 Cavities may be permeated by mineralizing solutions and if the prevailing
physicochemical conditions induce precipitation, then crystal will form.
 These will grow as a result of spontaneous nucleation within the solution or more
commonly, by nucleation on the enclosing surface.
 This leads to precipitation and outward growth of the first formed minerals on
vein walls.
 If the solutions change in composition, there may be a change in mineralogy and
crusts of minerals of different composition may give the vein filling a banded
appearance.
 This is called crustiform banding or crustification.
 Its development in some veins demonstrates that mineralizing solutions may
change in composition with time and shows the order in which the minerals were
precipitated. This order is called paragenetic sequence.

Commonly, the filling may not be complete and open vugs remain in the center.
These vugs usually contain treasure houses of beautiful and rare crystals.

RESULTING MINERAL DEPOSITS

Deposits resulting from cavity filling may be grouped as below;

i) fissure veins ii) shear zone deposits,
iii) stockworks iv) saddle reefs,
v) ladder veins vi) pitches and flats, folds, cracks,
vii) porespace fillings viii) vesicular fillings,
ix) breccia filling deposits, volcanic, collapse and tectonic.
x) solution cavity fillings, cave, channels, gash veins.

Fissure veins.

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A fissure vein is a continuous tabular opening in rocks, generally of considerable length and
depth, and is formed by compressional, tensile or torsional forces operating on rocks and
may or may not be accompanied by faulting.

A fissure vein occupies one or more fissures and results from the formation of the fissure
separated by a long interval of time usually.

There are varieties of fissure veins namely; simple, composite, linked, sheeted, dilated,
chambered, each of which may be massive or crustified.

A simple fissure vein occupies a single fissure whose walls are relatively straight and
parallel.
Where the walls are irregular and brecciated, it is called a chambered vein.

Dilational or lenticular veins are fat lenses in schists. Generally, several occur together, like
a string of sausages or may be disconnected en echelon lenses.

Fissure veins tend to change in both fissuring and filling when they pass from one rock
formation into another. This is due chiefly to the different physical behaviour of the different
rocks towards stresses but the chemical composition of rocks also influences mineral
deposition.

Some of world's most famed deposits, both ancient and recent are fissure veins.
Included among them are some of the deepest and richest mines of the world.

Although they do not contain the enormous tonnages of some magmatic, sedimentary or
replacement deposits, vast treasures of gold and silver have been won from them.
Also mined are Cu, Pb, Zn, Sn, W, Hg, and fluospar.

An example is the Ashanti Goldfields.

ii) Shear-zone cavities

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These deposits result where fractures, instead of being concentrated in one or two single
breaks, are expressed in innumerable closely spaced and more or less parallel, discontinuous
surfaces of deep seated rupture and crushing.
Faulting is generally present.
The thin sheet-like openings, mostly of infinitesimal size make excellent channel ways
for solutions.

Because of the minute openings, only minor space deposition can occur but the large
specific surface available makes shear zones favourable localizers of replacement deposits
and has given rise to many large valuable ore deposits.
E.g. Ashanti Goldfields.
iii) Stockworks
A stockwork is an interlacing network of small ore bearing veinlets traversing a mass of
rock.
The individual veinlets rarely exceed a few centimeters in width and some few tens of
centimeters in length, and are spaced a few centimeters apart.

The entire rock mass is mined resulting in a low grade but large tonnage deposit.
In general, the veinlets consist of open space fillings and are formed by crackling upon
cooling of the upper and marginal parts of intrusive igneous rocks or irregular fissures
produced by tensional or torsional forces ( e.g., fault movement downward along a curved
fissure produces crackling where the hanging wall moves over humps on the footwall.

iv) Saddle reefs (Folding and Warping).

Ore receptacles are formed when alternating beds of competent and incompetent rocks (such
as quartzite and slate) are closely folded.
When these receptacles are filled by ore, they resemble the cross-section of a saddle, hence
the name saddle reef.

An example is the Bendigo gold mine in Australia which has yielded $300 million in gold.

The saddles are repeated bed after bed down the axial plane and may be vertical or inclined.

v) Pitches and Flats Fold cracks.

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Under load, slumping or gentle synclinal folding brittle sedimentary beds give rise to a series
of connected tension cracks or openings collectively known as pitches and flats. Gentle open
folding also forms anticlinal tension cracks at the crests of anticlines or

synclines. These openings serve as receptacles for ore.

vi)Breccia filling deposits.
The haphazard arrangement of the angular rock fragments in breccias gives rise to numerous
openings that permit the entry of solutions and mineral deposits, forming breccia-filling
deposits which may result from volcanism, collapse or shattering.

Volcanic pipes form when explosive volcanic activity bores pipelike openings, the material
blown out may fall back or be washed back into the opening, forming angular breccia with
spaces between fragments. These form excellent conduits for mineralizing solutions from
which cavity filling or replacement deposits may occur.
vii) Pore-space fillings.
Pore spaces are interstitial openings between grains capable of absorbing water.
They make rocks permeable and serve as containers for ores, petroleum, gas and water .
viii) Vesicular fillings.
Vessicular fillings or blowholes are openings produced by expanding vapours typical of the
upper part of many basaltic lavas. They serve as channel ways for mineralizing solutions.
An example is the Lake Superior region where vesicles in basalts are filled with native
copper and have given rise to some of the greatest copper deposits of the world that have
been followed down to 3000m.

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CHAPTER SEVEN

SEDIMENTATION
This process, distinct from evaporation has resulted in the formation of common
sedimentary rocks and valuable mineral deposits such as iron manganese, copper
phosphate, coal, carbonates, cement rocks, clay etc.
These substances are exceptional varieties of sedimentary rocks that are valued
because of their physical and chemical properties. They are composed of inorganic
and organic materials and their source, like that of any sedimentary rock is from other
rocks that have undergone disintegration, the ultimate source being igneous rocks.
Some of the materials such as oxygen and carbon dioxide are obtained from the
atmosphere, and a few derived from former deposits.

The formation of sedimentary deposits involves an adequate source of materials, the

gathering of the materials by solution or other processes, the transportation of the
materials to the site of accumulation (if that is necessary) and the deposition of the
materials in the sedimentary basin. Subsequent compaction, chemical alteration or
other changes may take place.
Source of materials.
The materials that enter into sedimentary mineral deposits have been derived chiefly
from the weathering and oxidation of former mineral deposits such as iron,
manganese and copper. (Others have passed through an intermediate organic stage).
The earth's crust contains on the average 5.05% iron (in the USA, up to a depth of
300m, there are about 275,000 billion tons of
iron of which only 0.01% is concentrated into commercial deposits). Iron comes from
the weathering of iron bearing minerals of igneous rocks such as hornblende,
pyroxene or mica and from iron bearing minerals of sedimentary and metamorphic
rocks. Manganese of sedimentary deposits has been derived chiefly from the
weathering of manganese bearing minerals in rocks (and to a minor extent from
former sedimentary concentrations and epigenetic lode deposits). Over 200 minerals
contain manganese as an essential element and manganese makes up 0.09% of the
earth's crust.

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The source of sedimentary phosphate is phosphorus bearing rock minerals and apatite
is the most common. Carbonate deposits such as industrial limestone, dolomite and
magnesite are derived from the sea or saline waters.

Solution and transportation.

During weathering, a large part of the constituents of economic sedimentary deposits
go into solution (this is true of iron, manganese phosphates, carbonates, copper etc).
The chief solvents are carbonated water, humic and other organic acids and sulphate
solutions.

a) Carbonated waters are very effective solvents of Iron, manganese and phosphorus.
Where iron is present in the ferrous state, its solution offers no difficulty, since in that
form it is unstable and soluble. But in the ferric state it is almost insoluble in most
surface waters and has to change in to the ferrous state first. This is aided by organic
matter.

b) Humic and other organic acids derived from decomposing vegetation are
considered effective
solvents. The hydroxyl acids dissolve large quantities of iron so is the weak organic
acids which are considered the most effective of all natural solvents.

c) Sulphate solutions are effective solvents of iron and manganese but they are rarely
abundant enough to effect large scale solution and transportation. The oxidation of
pyrite yields sulphuric acid and ferric sulphate.

Deposition
The materials that form economic sedimentary beds are deposited mechanically,
chemically or biochemically. The manner of deposition depends on the nature of the
solvent and the place of deposition eg. in the sea or in a swampy basin. The conditions
under which deposition occurs determines largely the mineralogical composition of
the resulting deposits, their size, purity, distribution both areal and stratigraphic.
Sedimentary iron and manganese ores are deposited in both fresh and salt water,
swamps, marshes, lakes, lagoons and in the ocean. Phosphate and sulphur form under
marine conditions.

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The solvents are bicarbonate, sulphate and organic solutions. Iron and manganese
may be deposited from these solutions by loss of CO2 oxidation and hydrolysis,
plants, bacteria, replacement of sea bottom shells, reaction with CaCO3 and reaction
with alkalies.

Products of deposition.
Iron is commonly precipitated as ferrous carbonate (siderite), hydrous ferric oxide,
goethite, (limonite), ferric oxide (hematite) and minor basic ferric salts. Manganese
is deposited largely as oxides and the carbonate may be deposited in the absence of
air and in the presence of excess CO2.
Depositional separation of Mn and Fe.
In the formation of manganese ore, manganese separates from iron during deposition.
In the case of chemical precipitation from carbonate solutions, the separation is due
to the fact that MnCO3 is more stable in solution than FeCO3, hence it is carried
further and is thus separated from iron. Similarly, the ferric oxides are less soluble
than MnO2 and so are deposited first. MnO2 is the most stable Mn compound and is
the earliest formed and the principal Mn ore.
Sir Kitson states that the manganese deposits in Ghana are due to residual
accumulation through tropical weathering of manganiferous bedrock that removed
silica and alumina and redeposited the manganese near the surface. Junner however
believes that the manganese deposits were ancient high grade manganese ores slightly
enriched by meteoric waters.

RESIDUAL AND MECHANICAL CONCENTRATION.

Weathering Processes.
Mechanical disintegration by itself is confined largely to arid regions where
pronounced annual and diurnal temperature changes occur, or to very cold regions
where surface chemical changes proceed slowly and frost action is vigorous much of
the time.

Chemical weathering is most active in warm humid regions where rainfall supplies
moisture and nourishes plant life that yields humic and organic acids to assist
chemical activity.

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Mechanical disintegration does not create new minerals, it merely frees minerals
already formed. It also facilitates chemical decomposition by reducing the particle
size and creating greater specific surface available for chemical attack.

Chemical weathering, however creates new minerals of which some remain stable
under surface conditions. The agents of decomposition that operate at the surface are
water, oxygen, carbon dioxide, heat, acids, alkalis vegetable and animal life and some
products of the decomposition of the rocks themselves.

Obviously, the effects of weathering vary with the nature of the rock and with the
climate. The products of weathering may give rise to valuable residues which by
concentration through residual accumulation form residual concentration deposits or
by concentration by means of

water or air to form mechanical concentrations or placer deposits.

a) Residual Concentration.
This results in the accumulation of valuable minerals when undesired constituents of
rocks or mineral deposits are removed during weathering. The concentration is due
largely to a decrease in volume affected almost entirely by surficial chemical
weathering.
(l)The first requirement for residual concentration of economic mineral deposits is
the presence of rocks or lodes containing valuable minerals of which the undesirable
substances are soluble and the desired substances are generally insoluble under
surface conditions. Secondly, climatic conditions must favour chemical decay. The
relief must not be too great, or the valuable residue will be washed away as rapidly
as formed finally, long continued crustal stability is essential in order that residues
may accumulate in quantity and that the deposits may not be destroyed by erosion.

There are two modes through which residual mineral deposit are formed. First is the
accumulation of a pre-existing mineral that has not changed during the process. For
example a limestone formation free from other impurities except minor iron oxide
will slowly be dissolved leaving the insoluble iron oxide as residue which may be
sufficient in thickness and grade to constitute a workable deposit.

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(2) Secondly, the valuable mineral first comes into existence as a result of weathering
processes and then persists and accumulates. For example the feldspar of a syenite
decomposes upon weathering to form bauxite which persists at the surface while the
other constituents of the syenite are removed. Both temperate and tropical climatic
conditions yield residual concentrations although tropical and subtropical conditions
favour the formation of more kinds of deposits.

Valuable deposits of iron ore, manganese, bauxite, clays, nickel, phosphate, kyanite, barite,
tin, gold etc are formed this way. The basic processes of weathering and concentration
apply to each but in as much as source materials, chemical changes and other details of
formation differ for each substance, their cycles are different.
b) Mechanical Concentration
Mechanical concentration is the natural gravity separation of heavy from light minerals by
means of moving water or air by which the heavier minerals become concentrated into
deposits called placer deposits. It involves two stages;
i) The freeing by weathering of the stable minerals from their matrix and

ii) their concentration which can occur only if the valuable minerals possess high specific
gravity, chemical resistance to weathering and durability.
Placer minerals which have these properties are gold, platinum, tinstone, magnetite
chromite, rutile, gemstone etc.
The minerals which make up placer deposits may be derived from commercial and non-
commercial lode deposits (gold veins and veinlets with quartz stringers), sparsely
disseminated ore minerals (platinum disseminated in basic intrusives), rock forming
minerals (magnetite, monazite, zircon etc.) and former placer deposits.

Principles Involved.
The placer minerals are released from their matrix by weathering and they are washed
slowly downslope to the nearest stream or to the seashore. Moving streams sweep away
the lighter matrix and the heavier placer minerals sink to the bottom or are moved
downstream relatively shorter distances.

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The operation of mechanical concentration depends chiefly on differences in specific gravity,

size and shape of particles as affected by the velocity of a moving fluid (in a body of water,
heavier minerals sink more rapidly than lighter ones of the same size. A1so, the settling of two
spheres of the same weight but different sizes in water; the smaller one because of less friction
sinks more rapidly. Thirdly the shape affects settling; the spherical one would sink faster than
the platy disc shaped one). Added to this is the velocity of the moving water. The ability of
moving water or air to transport a solid depends on its velocity and varies as the square of the
velocity. These factors operate together to separate the light and fine minerals from coarse and
heavy ones and with long continued action, placer minerals may eventually become sufficiently
concentrated to constitute workable deposits.

An essential for any mechanical concentration is that a continuous supply of placer minerals be
made available for concentration and this is favoured. in regions of deep weathering and
topographic relief. When weathering yields debris on a hill slope, the heavier particles move
downslope more slowly than the lighter ones giving a concentration called eluvial placers found
below the outcrop. Most important eluvial deposits are gold and tin.

During water transportation, concentration may o;cur in streams giving stream placers or
alluvial deposits or on beaches giving beach placers. Stream placers are the most important type
of placers and yield the greatest quantity of placer gold, tinstone, platinum and gemstones.

Places of accumulation.

Due to the continued movement of the water, separation takes place by specific gravity. The
heavy minerals moving downward to the bottom of the river (the lower sluggish reaches of
streams are not favourable sites for placer accumulations and neither are the upper headwaters
because of the limited supply of source materials). Intensive enrichments are found where the
flow velocity is decreased this may occur due to the following;
1. Rock banking. Where a moving stream is intercepted by a relatively hard rock which is
resistant to weathering, it may bank the stream, the placers are deposited behind the bank. Also
deposition takes place when the streams cross highly inclined or vertically layered rocks such as
slates, schists or alternating hard and soft beds, the harder layer projects upward while the softer
ones are cut away. This forms natural riffles which are excellent traps for placer minerals and
may give rise to exceptionally rich -streaks or bonanzas.

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 MN 277 Economic and Exploration Geology

2. Where materials are delivered by a swift tributary into a slower master stream, they accumulate
under diminished velocity as a pay streak down the convex bank below the mouth of the ore
bearing tributary.

3. If the stream or river crosses a fall, waterfall, the pools (pot holes) after the fall trap the heavy
placers to form deposits,

4. The deposition and formation of pay streaks take place at the convex banks of rapidly flowing
meandering streams, if the stream crosses a mineralized lode, it supplies the placer minerals. The
pay streak will be spread across the stream channel on the downstream side of the lode. (The
early placer miners followed the trail of rich pay streaks or indications downstream to where
they ceased, and then they looked for the mother lode nearby. Valuable deposits were formed in
this way).
Eolian Placer Formation. Wind instead of water may act as the agent of concentration and give
rise to placer deposits. These are usually formed in arid regions eg. In Australian deserts, the
disintegration of gold quartz lodes led to the blowing away of the light materials. The heavy gold
particles freed from their matrix remained behind. The continuation of this process finally
resulted in patches of surface accumulation of placer gold in debris of sand and wind won
pebbles.

OXIDATION SUPERGENE ENRICHMENT

When ore deposits become exposed by erosion, they are weathered along with the enclosing
rocks. The surface waters oxidize many ore minerals and yield solvents that dissolve other
mineral.
An ore deposit thus becomes oxidized and generally leached of many of its valuable materials
down to the ground water table or to a depth where oxidation cannot take place.
The oxidized part is called the zone of oxidation. The effects of oxidation may however extend
far below the zone of oxidation. As the leaching solutions trickle downward, they may lose a
part or all of their metallic content within the zone of oxidation and give rise to oxidized ore
deposits. If the down trickling solutions penetrate the water table, their metallic content may be
precipitated in the form of secondary sulphides to give rise to a zone of secondary or supergene
sulphide enrichment. The lower unaffected part of the deposit is called the primary or hypogene
zone. This zonal arrangement is characteristic of many mineral deposits that have undergone

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 MN 277 Economic and Exploration Geology

long continued weathering.

In places the supergene sulphide zone may be absent and in rare cases the oxidized zone is
shallow or lacking (as in glaciated areas) as in regions undergoing rapid erosion. Special
conditions of time, climate physiographic development and amenable ores are necessary to yield
oxidized and enriched supergene ores. The effect of oxidation renders barren the upper parts of
many deposits or change ore minerals into more usable or less usable forms, or even to make
rich bonanzas.

The effect of supergene enrichment has added much where there was previously little; leaner
parts of veins have been made rich, unworkable protore (material containing ore minerals in too
low a concentration for economic working but may be workable ore where secondary enrichment
has occured) has been enriched to ore grade. Many copper deposits have come into existence
due to the process of supergene sulphide enrichment.

Supergene, oxidation and enrichment go hand in hand. Without oxidation, there can be no supply
of solvents from which minerals may be precipitated in the zones of oxidation or of supergene
sulphides. The processes are in three stages;

i) Oxidation and solution in the zone of oxidation

ii) Deposition in the zone of oxidation,
iii) Supergene sulphide deposition.

I. OXIDATION AND SOLUTION IN ZONE OF OXIDATION

The effect of oxidation on mineral deposits is profound. The minerals are altered to new
compounds that require metallurgical treatment for their extraction quite unlike those for
unoxidized materials. The metallic substances are leached and the texture of the deposit
obscured. Water with dissolved oxygen is the most powerful oxidizing reagent, though CO2 also
plays an important role (so are chlorides, iodides, and bromides).

These substances react with certain minerals to yield strong solvents such as ferric sulphate and
sulphuric acid. Sulphuric acid in turn reacting with sodium chloride yields hydrochloric acid
which with iron yields the strongly oxidizing ferric chloride. There are two main chemical
changes within the zone of oxidation.

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 MN 277 Economic and Exploration Geology

a) the oxidation, solution and removal of valuable minerals

b) the transformation in situ of metallic minerals into oxidized compounds.
Most metallic mineral deposits contain pyrite or pyrrhotite. This mineral under attack readily
yields sulphur to form iron sulphate and H2SO4 ferric hydroxide. Ferric sulphate is continuously
regenerated from pyrite, chalcopyrite and other sulphides. The ferric hydroxide changes over to
hematite and goethite and forms limonite which characterizes all oxidized zones.
Most of the sulphates formed are readily soluble and slowly trickle downward through the
deposit until proper conditions are met to cause deposition of the metallic content.
If pyrite is absent from deposits undergoing oxidation, only minor amounts of the solvents are
formed, little solution occurs, and the sulphides tend to be converted in situ into oxidized
compounds the hypogene sulphides are not enriched.
Gossans are signboards that point to what lies beneath the surface. Most ore deposits except in
glaciated regions are capped by gossans, hence the finding of one may point to the discovery of
buried wealth. Non-commercial mineral bodies however also yield gossans. Limonite is
universally formed during oxidation of iron bearing sulphides and persists in the oxidized zone
and imparts to the gossan and capping, its diagnostic colour.

Factors controlling and limiting oxidation

Oxidation cannot proceed forever and certain factors cause its cessation and others such as the
water table, rate of erosion, climate, time, rock structure and burial affect or regulate oxidation.
a) Water table. The position of water table is a control of oxidation. Above it oxidation can take
place freely; below it no free oxygen is ordinarily available for oxidation. Hence it normally
constitutes the downward limit of the zone of oxidation. It is common observation that at the
water table, oxidized minerals give way to sulphides. This is the normal condition. However in
many cases there is a slight interpenetration of the oxidized and sulphide zones due to the
oscillation of the water table.

b) Rate of erosion. The rate of erosion and the ensuing lowering of the surface and of the water
table may be so rapid that oxidation cannot keep pace with it. Any former oxidized zone would
be removed by erosion and little or no oxidation would be evident on the surface.
b) Climate. The important factors of climate that affect oxidation are temperature and
rainfall both of which go with time. High temperature clearly accelerates oxidation
and low temperature retards it. Oxidation is therefore favoured by warm much more
than by cool climates. It is also

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 MN 277 Economic and Exploration Geology

c) probable however that oxidation proceeds fastest in warm humid climates with
evenly distributed rainfall and inhibited in regions of perennial ground frost.
d) Time. Studies indicate that thorough and deep oxidation is relatively slow and requires
considerable time geologically speaking. It is the order of length of the Miocene to Pliocene and
many zones oxidized of copper have been formed during the Tertiary (between 5 million
years).
e) Rocks. Both physical and chemical properties of enclosing rocks affect oxidation. It proceeds

fastest in porous or brittle rocks that fracture or crackle easily. Consequently permeable
sandstone, sericitized and silicified igneous rocks and brittle schists favour oxidation. Hard,
massive, dense unfractured rocks such as quartzite, clay and shale impede oxidation.

CHAPTER EIGHT

ORE DEPOSITION IN THE ZONE OF OXIDATION.

Oxidized ore deposits may result within the zone of oxidation if the down trickling oxidzing
solutions encounter precipitants above the water table. The economic minerals redeposited
within the zone of oxidation are chiefly native metals, carbonates, silicates and oxides. the
metals
of greatest commercial importance are copper, lead, zinc, silver, vanadium, and less importantly,
manganese, iron and cobalt. For most parts, the ores are deposited in the lower part of the zone
of oxidation.

Methods of Precipitation.
Various processes acting singly or together operate to bring about precipitation of the metals in
the zone of oxidation.
a) Evaporation and saturation. By simple saturation due to evaporation, particularly in arid
climates, metallic compounds are deposited as veinlets and blebs. Such compounds are chiefly
sulphates and the chief ore minerals are various copper sulphates, other sulphates of Fe, Cu, Mg,
Na, K also occur e.g. Chiquicamata, in Chile is the largest copper deposit in the world.
b) Oxidation and Hydration

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 MN 277 Economic and Exploration Geology

Oxidation and hydration yield oxides and hydrous oxides of iron, manganese, copper and other
materials. The formation of ferric hydroxide and finally goethite is an example already
mentioned.
c) Reactions between Solutions
The sulphate solutions formed in the zone of oxidation may encounter various solutions that
bring about precipitation within the oxidised zone.

i) Chloride solutions -Sodium Chloride; a widespread constituent of the soil and ground water
of arid regions reacts with sulphate and other compounds to form chlorides of silver, copper,
lead and other metals.
e.g. Ag2 SO4 + 2Na = 2Ag + Na2 SO4
ii) Bicarbonate solutions react with sulphates and sulphate solutions to form carbonates of
metals. Thus reactions of copper sulphate yields the common basic carbonates of copper, azurite
and malachite
ZnSO4 ZnCO3 ( smithsonite )

PbSO4 PbCO3 (cerussite)

OXIDATION AND SUPERGENE ENRICHMENT

When ore deposits become exposed by erosion, they are weathered along with the enclosing
rocks. The surface waters oxidise many ore minerals and yield solvents that dissolve other
minerals. An ore deposit thus becomes oxidised and generally leached of many of its valuable
materials down to the ground water table or to a depth where oxidation cannot take place. The
oxidised part is called the zone of oxidation. The effects of oxidation may however extend far
below the ward. They may lose a part or all of their metallic content within the zone of oxidation
and give rise to oxidized ore deposits.

If the down trickling solutions penetrate the water table, their metallic content may be
precipitated in the form of secondary sulphides to give rise to a zone of secondary or supergene
sulphide enrichment. The lower unaffected part of the deposit is called the primary or hypogene
zone. This zonal arrangement is characteristic of many mineral deposits that have undergone
long continued weathering.
In places, the supergene sulphide zone may be absent and in rare cases the oxidised zone is
shallow or lacking (as in glaciated areas and in regions undergoing rapid erosion).

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 MN 277 Economic and Exploration Geology

Special conditions of time, climate, physiographic development and amenable ores are
necessary to yield oxidised and enriched supergene ores. The effect of oxidation renders barren
the upper parts of many deposits or change ore minerals into more usable or less usable forms,
or even to make rich bonanzas. The effect of supergene enrichment has added much where there
were previously little, leaner parts of veins have been made rich, unworkable protore has been
enriched to ore grade. Many copper deposits have come into existence due to the process of
supergene sulphide enrichment.
Supergene, oxidation and enrichment go hand in hand. Without oxidation, there can be no
supply of solvents from which minerals may later be precipitated in the zones of oxidation or of
supergene sulphides.
The processes are in three stages;
a) Oxidation and solution in the zone of oxidation
b) Deposition in the zone of oxidation
c) Supergene sulphide deposition.

a) Oxidation and Solution in Zone of Oxidation

The effect of oxidation on mineral deposits is profound. The minerals are altered to new
compounds that require metallurgical treatment for their extraction quite unlike those for
unoxidized materials. The metallic substances are leached and the texture of the deposit
obscured.
Water with dissolved oxygen is the most powerful oxidizing re-agent, though CO2 also plays an
important role (so are chlorides, iodides and bronrides). These substances react with certain
minerals to yield strong solvents such as ferric sulphate, and sulphuric acid. Sulphuric acid in
turn reaching with Sodium Chloride yields hydrochloric acid which with iron yields the strongly
oxidizing ferric chloride.

There are 2 main chemical changes within the zone of oxidation.

i) The oxidation, solution and removal of valuable minerals
ii) The transformation in situ of metallic minerals into oxidized compounds.

Most metallic minerals deposits contain pyrite and pyrrhotite. This mineral under attack readily
yields sulphur to form iron sulphate and H2SO4 and Ferric hydroxide. Ferric sulphate is
continuously regenerated from pyrite, chalcopyrite and other sulphides. The ferric hydroxide
changes over to hematite and goethite and forms limonite which characterises all oxidized

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zones.
1. Pyrite
FeS2 + 7O + H2O = FeSO4 + H2SO4
2FeSO4 + H2SO4 + O = Fe2(SO4)3 + H2O
6FeSO4 + 3O + 3H20 = 2Fe(SO4)3 + 2Fe(OH)3
Fe2(SO4)3 + 6H2O = 2Fe(OH)3 + 3H2SO4
Fe2(SO4)3 + FeS2 = 3FeSO4 + 2S.
2. Chalcopyrite
Cu FeS2 + 2Fe2(SO4)3 = CuSO4 + 5FeSO4 + 2S
3. Chalcocite
Cu2S + Fe2(SO4)3 = CuSO4 + 2FeSO4 + CuS
4. Covellite
CuS + Fe2(SO4)3 = CuSO4 + 2FeSO4 + S
5. Sphalerite
ZnS + Fe2(SO4)3 + 4H2O = ZnSO4+8FeSO4+4H2SO4
6. Galena
PbS+Fe2(SO4)3 + H2O+3O = PbSO4+2FeSO4 + H2SO4
7. Silver
2Ag + Fe2(SO4)3 = Ag2SO4 + 2FeSO4
Most of the sulphates formed are readily soluble and slowly trickle downward through the
deposit until proper conditions are met to cause deposition of the metallic content. If pyrite is
absent from deposits undergoing oxidation, only minor amounts of the solvents are formed, little
solution occurs, and the sulphides tend to be converted insitu into oxidized compounds, the hypo
gene sulphides are not enriched.
Gossans are sign boards that point to what lies beneath the surface. Most ore deposits except in
glaciated regions are capped by gossans, hence the finding of ore may point to the discovery of
buried wealth. Non commercial mineral bodies however also yield gossans. Limonite is
universally formed during oxidation of iron-bearing sulphides and persists in the oxidized zone
and imparts to the gossan and capping its diagnostic colour .

d) Ore deposition in the zone of Oxidation

Oxidized ore deposits may result within the zone of oxidation if the down-trickling oxidizing
solutions encounter precipitants above the water table. The economic minerals redeposited
within the zone of oxidation are chiefly native metals, carbonates, silicates and oxides. The

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 MN 277 Economic and Exploration Geology

metals of greatest commercial importance are copper, zinc, lead, silver, vanadium and uranium
with less important veins being manganese, iron and cobalt. For most part, the ores are deposited
in the lower part of the zone of oxidation.
c) Supergene Sulphide Enrichment
The metals in solution that escape capture in the oxidised zone trickle down to where there is
no available oxygen, generally the water table, and there undergo deposition as secondary
sulphides. The metals removed from above are thus added to those existing below, thereby
enriching the upper part of the sulphide zone. This forms the zone of secondary enrichment or
the supergene sulphide zone which is underlain by the primary or hypo gene zone. Progressive
erosion permits deeper oxidation and after a time the supergene sulphides themselves become
oxidized and their metal content is then transformed to the downward progressing enrichment
zone. The primary ore may thus be enriched to as much as ten times its original metal content.
Favourable conditions must exist for supergene sulphide enrichment to take place. The process
is of greatest importance with copper and silver deposits

Requirements for Supergene Enrichment

i) Oxidation of ore deposits may occur without attendant sulphide enrichment, but enrichment
cannot take place without accompanying oxidation. Therefore oxidation is the primary requisite
and the factors that have been shown to favour oxidation must be present.
ii) Suitable Hypogene Minerals
The deposit must contain primary minerals upon which oxidation yield the necessary solvents.
Sulphides are essential, and deposits lacking them rarely contain supergene sulphide zones. The
primary ore also must contain metals that can undergo supergene enrichment. Lead and Zinc for
example do not ordinarily yield supergene sulphides; Copper and Silver do.

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CHAPTER NINE

THE MINERAL FUELS

Petroleum is composed of many compounds of carbon and hydrogen, with minor oxygen,
nitrogen and a little sulphur. The numerous members of the different CH series each have
different properties, at normal temperatures some are gases, some are liquids and some are solid
waxes. Since the proportion of these varies in different oils, no two oils are alike in their
properties or constituents. A crude oil may contain CH members that give the oil a high gasoline
content, or certain ones may be absent and the oil will contain little or no lubricants. Thus, oils
may be referred to as paraffin base, or asphaltic and naphthene, or mixed base. In general,
paraffin-base oils are light and yield good lubricants, and asphaltic-base oils are heavy, unsuited
for good lubricants, and may be usable only for fuel oil.
Geologic and Geographic Distribution of Petroleum
The home of petroleum is in sedimentary rocks; only rarely has it moved out of this environment
into adjacent igneous rocks. The containing rocks of commercial pools are sands, sandstones,
conglomerates, porous limestones and dolomites, and rarely fissured shale the enclosing rocks
are almost invariably marine strata or associated fresh water beds. Consequently, areas of
igneous Precambrian and metamorphic rocks can be eliminated.

Oil is found in rocks of all ages from Upper Cambrian to Pliocene, but TERTIARY beds the
world over are the most prolific.

Origin of Oil and Gas

It is now universally believed that oil and gas are of organic origin and form from low forms of
marine or brackish -water life. It is held that organic material buried in marine muds underwent
changes to produce natural hydrocarbons, and these subsequently moved into porous reservoir
rocks and accumulated to form commercial oil pools.

Source Materials and Source Places

The slow, oxygen-free decomposition of the remains of plant and animal organisms is
considered to be the source of petroleum hydrocarbons. The higher forms of life that grow and
live on the land are unimportant, but the lower planktonic organisms, such as diatoms and algae,
that thrive abundantly near the surface of the sea, are now considered the most probable and
important source materials. The organic remains accumulate in the bottom muds of lagoons or
in depressions on the floor of shallow seas arid there become incorporated in accumulating
sediments.

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Conversion to Petroleum
The bacteria that thrive in the upper mud of the sea floor are thought to change the organic
matter into the mother material of oil by removing oxygen and nitrogen and producing other
changes. It is known that the composition of average plankton is 24% protein, 3% fat, and 73%
carbohydrates. It is not the fats, cellulose and simple proteins that yield the oil but rather, the
complete proteins and carbohydrates. With deeper burial, bacterial action presumably ceases,
and then other as yet unknown, chemical changes occur to yield oil before consolidation. Further
post consolidation changes probably take place during migration. Pressure probably exerts some
influence but temperatures could not have been high because certain compounds could not exist
at 140 to 300 C. Later changes to liquid hydrocarbons may have been aided by polymerization
and methylation. The liquid hydrocarbons formed are believed to be capable of dissolving other
organic substances such as pigments, waxes, and fatty acids. During migration, other organic
compounds might also be dissolved, thus continually changing the composition of the
petroleum, and perhaps giving rise to the differences in oils. This phase of the subject is far from
being completely known. During the conversion of organic matter into petroleum, natural gas
is also formed. This is dominantly methane.

Formation of an Oil Pool

An oil pool is not a subterranean pond but an accumulation of petroleum in rock pores.
Dispersed droplets generated in the source muds do not constitutes an oil pool. The conditions
necessary for this are:
a) migration and accumulation
b) suitable resevoir and caprocks
c) suitable traps
d) retention

a) Migration and Accumulation of Oil

Since oil pools are found chiefly in sandstones, it follows that the dispersed droplets of oil
generated within the source muds must have migrated out of the source rocks into the sands.
The forces that cause migration are;
i) compaction of the muds
ii) capillarity
iii) buoyancy:
iv) gravity
v) currents

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 MN 277 Economic and Exploration Geology

i) Compaction
Source muds may contain up to 80% water. As covering sediments are deposited, the
accumulating weight gradually compacts the lower beds and the enclosed fluids are squeezed
out into places of less pressure, such as pore spaces in sands. The fluids may move up, or down
or even laterally. It is estimated that compaction results in removal of half the water when burial
has reached 300m and 85% at 1200m. Compaction is seemingly the most important factor
causing migration of oil.

ii) Capillarity
If oil-wet shales are in contact with water-wet sandstones, the water, owing to its higher surface
tension, will move from the coarse sandstone pores into the fine, capillary, shale pores and
displace the oil there into adjacent sandstones.

iii) Buoyancy
Oil being lighter than water tends to rise upon a water surface. Free gas similarly rises above
the oil. This "secondary migration" takes place within resevoir rocks. It is most effective where
the pore spaces are large and when large volumes of fluids are involved. If the host beds are
horizontal, the oil will tend to rise to the top of permeable beds and no pronounced accumulation
may occur. If inclined, the oil tends to migrate up-dip and accumulation into oil pools may occur

iv) Gravity
When water is present, the differences in specific gravity between oil and water give rise to
buoyancy, but where water is absent, gravity causes oil to migrate down-dip until arrested by
impervious beds, this gives rise to synclinal accumulation.

v) Currents
Currents of subsurface waters flush oil along with them and accelerate oil migration. Such
currents may be caused by compaction or artesian water circulation. The latter may be a very
effective means of bringing about large-scale migration over large areas, e.g. Rocky Mountain
Province.

vi) Accumulation

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 MN 277 Economic and Exploration Geology

Migration of oil generally leads to accumulation, which is the collection of oil droplets into
pools. Oil may migrate without accumulating, or it may accumulate into non-commercial
bodies, but concentrated accumulation is essential to produce commercial oil pools, and this in
turn is dependent upon requisite reservoir rocks and traps.

b) Reservoir and Caprocks

i) Reservoir Rocks
Accumulation takes place only in porous and permeable rocks. A rock like clay has high
porosity but small permeability and therefore, is not a suitable resevoir rock. The most suitable
resevoir rocks are sands; and sandstones. Others are porous or cavernous limestone and
dolomite, rarely fissured shale arid rarely jointed igneous rocks. (The total yield of oil from
rocks other than sandstone is relatively small though).
In sandstone the cementing material decreases the porosity from 12% to 25%; an average
porosity of many oil sandstones is 17%.
The greater the porosity, the greater the amount of oil that a resevoir rock can contain; and the
larger the pore size, the greater the amount of oil it will yield, since with small grains and small
pores, more oil clings to the rock grains and is not recovered. The percentage of oil recovered
from resevoir rocks is surprisingly small. For example unconsolidated sands of California with
25% to 40% porosity, yield only 10% to 30% of the contained oil.

ii) Caprocks
A confining impervious caprock is necessary to retain oil in a resevoir rock. Shale and clay are
the most common caprocks, but dense limestone and dolomite and gypsum also serve, and even
well-cemented, fine grained sandstone or shaly sandstones are effective. Good caprocks form
effective seals to underlying oil and.gas for long periods of geologic time.

c) Traps or "Structures"
In inclined sedimentary beds, the up-dip migration of oil will continue until it escapes at the
surface or is arrested in some trap where it accumulates to form an oil pool. If the strata are
folded into an anticline, up-dip migration ceases when the oil reaches the top of the arch, where
it accumulates to form an anticlinal oil pool. This is the commonest trap. There are many other
kinds of oil and gas traps and these may be divided into structural and stratigraphic traps as
follows:

1. Structural Traps

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 MN 277 Economic and Exploration Geology

i) Anticline vi) Terraces

ii) Dome vii) Syncline
iii) Monocline viii) Fissures
iv) Faults ix) Igneous intrusions
v) Salt domes

2. Stratigraphic Traps
i) Unconforrnities vi) Overlaps
ii) Ancient shore lines vii) Reflected buried hills
iii) Sandstone leuses viii) Buried coral reefs
iv) Shoestring sands ix) Up-dip porosity diminution
v) Up-dip wedging of sands

1. Structural Traps

Folds
Up-folds, giving rise to anticlines and domes, have been -the source of the greater part. of oil so
far produced. An elongated anticline is an ideal petroleum trap since it extended flanks facilitate
migration, its arch permits accumulation, and its crest or center generally contains domelike
areas in which oil collects from all sides and is retained beneath the caprock.

Folds with gentle dips offer larger areas for accumulation of oil: generally the dips are less than
30 but may reach as much as 60 .In asymmetric folds, the gentle-dipping limb often contains
the most oil.

In ideal arrangement in an anticline, free gas, if present, is at the top, oil lies beneath, and the
water below. Commonly however, in deep wells where pressure is high, gas is contained within
the oil and is liberated only upon drilling. An individual productive sand may range from a few
meters to a few hundred, or even to a thousand meters or more in thickness. Also, many
productive sands may lie below each other within the same thickness. They may be separated
by
large intervening thicknesses of barren beds, or be essentially continuous, as at Long Beach,
California, where drills intersected productive horizons between depths of 900m and 2l00m.

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 MN 277 Economic and Exploration Geology

Anticlinal oil structures have been formed by compressive force, deposition of beds over ancient
hills, and by differential compaction where shales have undergone greater compaction than
sands. Anticlines and domes form important oil traps in practically all the large oil fields of the
world.
Syclines, in a few cases, serve as oil traps where water is absent, and monoclines and terraces
constitute important oil structures in several places.

Faults
Faulting has given rise to many important oil pools. In inclined beds an impervious shale may
be faulted against the up-dip continuation of an oil sand, causing an effective seal and permitting
oil accumulation beneath it. Rarely, a thick fault gouge may serve as an effective upward seal
to an oil sand. Open faults may permits the oil to escape to upper beds or to the surface.

Salt domes
Surface expression of salt domes may be absent or they may be indicated by low mounds or
depressions. They are dome-shaped, pipe -shaped, or even mushroom-shaped. The plug consists
of salt and anhydrite, anhydrite or gypsum generally constitutes a caprock 150-200m thick. The
salt plug may be several hundred meters thick eg. more than 1800m. No drill has yet penetrated
to their bottoms. The flanks are bordered by upturned strata, which form the resevoir rocks of
associated oil pools. They give the impression that they have been driven forcibly upward into
the overlying strata, bending them upward to make oil traps. Salt domes are found in the Gulf
Coast of Texas and Louisiana and Mexico and Rumania.

2. Stratigrahic Traps
Stratigraphic traps are those formed by conditions of sedimentation in which lateral and vertical
variations in thickness, texture and porosity of beds result. These may result from intercalation
of beds, interruptions to sedimentation, and sites of deposition with respect to shore lines. A
sandstone bed may wedge out or undergo decrease in porosity and thus make a suitable trap.

Unconformities
Produce an important class of oil reservoirs. They mark tilted strata, overlays, variable porosity
and folding. Underlying tilted sand beds may be sealed at the unconformity by overlying
unconformities. The underlying beds are commonly deeply weathered and therefore porons and

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 MN 277 Economic and Exploration Geology

beds, and accumulation can take place. Angular unconformities are more effective than parallel
constitute oil containers.

Shoestring Sands
They are long, narrow bodies of sand enclosed within shaly beds. These sands may
have been deposited in offshore bars by tidal currents in lagoons or by meandering streams.
Many small pools have been found in them in the Mid-Continent field.

Shore-line Sands formed on low-lying submerged coastal plains, tapering in shore and
extending deeper offshore may be covered by clays and form suitable oil containers. This is
spoken of as "lensing out" of sands. lt generally gives rise to small pools, but very valuable.
Buried hills and structures have accounted for several important fields in Kansas etc. Here
projecting granite ridges received a mantle of sediments, thin on top and thicker on the flanks,
simulating anticlines, and giving rise to overlap. These stratigraphic features made excellent oil
traps.
Other stratigraphic traps caused by porosity variation are sandstone lenses enclosed in shales,
shore-line wedges of sand that become fine grained and impervious offshore, sandstone overlaps
against impervious beds, up-dip seals by asphalt and buried coral reef rocks.
i) Pinching sandstone or wedge
ii) Sandstone lens
iii) Sandstone grading into shale,

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CHAPTER TEN

GEOPHYSICAL EXPLORATION

 It is an assortment of methods utilizing physical principles to enable prospectors "see"

underground.
 It is based on the fact that rocks and minerals of different types may display large differences
in certain physical properties such that there must be a physical contrast between the rocks
and the mineral(s) being sought.
 Geophysical prospecting involves the systematic measurement of physical properties of
rocks and minerals.
 These measurements are made in a search for unusual variations in these properties known
as anomalies.
 Although these properties are fairly simple to understand, their measurement and
interpretation is often quite complex.

The physical properties are:

1. ELASTICITY : which determines the speed of seismic waves;
2. DENSITY: which affects the strength of Earth's gravity field;
3. MAGNETIZATION: which affects the strength of the Earth's magnetic field;
4. ELECTRICAL CHARACTERISTICS
5. NATURAL RADIO ACTIVITY LEVELS

These properties form the basis of the five principal methods of geophysical exploration:
1. Seismic reflection and refraction method
2. Gravity method
3. Magnetic method
4. Electrical & Electromagnetic method
5. Radiometric method

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 MN 277 Economic and Exploration Geology

Table 1: Modified from Kearey (2002)

 Surveys could be airborne (for recce) but generally are ground surveys (for more detailed
work) or both (for extended, exploration programs, but there is need to weigh individual
advantages );
 The methods are primarily employed at the surface, but occasionally in underground
workings
 Some have also been adapted for use in boreholes for supplementary data for other
geological and geophysical studies
 Information is interpreted in relation to patterns in geology or evaluated in respect to,
known or supposed relationships between rock type: structure, stratigraphic sequence and
ore mineralization,

Field applications

Oil & Gas: Seismic reflection (most widely used), gravity, seismic refraction, magnetics, in
that order.
Mineral exploration: magnetics, electrical & electromagnetics, radiometric.
 Table 1 further exemplifies the applicability of the various methods.
 By using a combination of these methods, a number of alternative interpretations can be
deduced.

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 MN 277 Economic and Exploration Geology

 For example, a magnetic anomaly appearing in the same position and with approximately
the same shape as that of a strong electrical conductor may indicate a body of pyrrhotite or
mixed pyrite and magnetite rather than a conductive zone of graphitic schist.
 If the conductor were not magnetic but were dense enough to cause a gravity "high", it could
be a body of pyrite rather than pyrrhotite or magnetite.
 Some of the methods have direct application, e.g.,
o radiometric prospecting for uranium ore,
o magnetics for iron ore, and
o electrical for base-metals,

 Most are considered to be a part of exploration in virgin areas.

 Geophysical prospecting has, however, provided many discoveries in older mining districts

 Deep, concealed ore bodies in productive districts are tempting geophysical targets because
new ideas and new techniques can be more easily applied in searching for ore bodies with
relatively well-known characteristics.
 Magnetic, electrical, electromagnetic, and radioactive methods are most popular in mineral
exploration.
 Seismic and gravity methods are used to some extent, but not nearly as much as in petroleum
exploration.

Planning geophysical work

Before embarking on any geophysical exploration programme, the prospector should collect all
the information available about the area he intends to explore. This includes:
 topographical,
 mineralogical, and
 geological information

 The literature often indicates which of the geophysical methods is most likely to produce
successful results in a particular area or in a particular geological setting.
 It should be remembered that a good guide to ore is ore, i.e. it is reasonable to search for
an ore body in an area where other orebodies have already been discovered or in areas
where geologic conditions are similar to areas of known ore deposits.

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 Occasionally, geological conditions will work against the use of the geophysical methods;
for instance, an area which contains a large amount of graphitic schist not associated with
the ore will produce anomalies by methods which measure conductivity.
 There may be ore deposits in the area, but their presence would be "masked" by the
anomalous readings of the schist even though the ore might also produce an anomaly.
 Geophysical exploration needs careful planning and coordination since other professionals
are involved.
 One needs to have a conceptual model of the orebody on geology, i.e. contrast, depths, etc.
 There are great limitations in cost (may be very expensive), time and scheduling
 Procedure needs to be well spelt out e.g. size of the area, degree, of the detail needed,
orientation of survey lines, type of coverage needed (partial or complete), sensitivity,
accuracy, kind of terrain involved seasonal characteristics.

Reconnaissance exploration

 Just as there are two stages in mapping, exploration also involves two stages. Further to the
discussions covered under mapping, it is important also to note the following:
 Recce exploration is the highest risk stage in any exploration programme and must be
viewed as a screening process.
 It covers stages leading to the selection of an area for ground work, i.e. land acquisition;
 Field work begins early and should incorporate orientation visits to key sites of known
mineralisation;
 Recce exploration involves several key players (the private sector, i.e. individuals,
companies and state organisations).

Methods in recce exploration

 It relies on wide ranging traverses

 Stream courses are main traverse lines in thickly vegetated areas; geologic mapping is
prime methods
 Exploration is often identified with geomorphology
 Tracing of floats, boulders in glaciated areas, panning of heavy minerals from alluvium are
typically employed.

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 Pitting, trenching, some drilling (to provide subsurface stratigraphic, lithologic, alteration,
geochemical and structural information), certain geophysical and geochemical methods
may be effectively employed
 Logistics are very simple (temporary camp sites, etc) but may be heavily dependent on
daily
 weather and climate:
 All-season trucks are preferable in view of weather conditions and geomorphology

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CHAPTER ELEVEN

GEOCHEMICAL EXPLORATION

Definitions
i) The determination of the relative abundance of the elements in the earth;
ii) The study of the distribution and migration of the elements in the various parts of the
earth with the object of discovering the principles governing this distribution.
Exploration geochemistry is the use of geochemistry in (geological) exploration. It is now an
accepted part of nearly all exploration programs.
 Geochemical prospecting for minerals includes any method of mineral exploration based on
systematic measurements of one or more chemical properties of a naturally-occurring
material.
 The chemical property measured is most commonly the trace content of some element or
group of elements.
 The naturally occurring material may be:
o rock,
o soil
o gossan,
o glacial debris,
o vegetation,
o stream or lake sediment,
o water or
o vapour

 The purpose of the measurements is the discovery of abnormal patterns or geochemical

anomalies, related to mineralization:

Geochemical methods of exploration form an integral part of the various exploration methods.
The primary objective of which is to find clues that will help in locating hidden.
The basis of geochemical exploration is the knowledge that an envelope of primary
mineralization is likely to occur around a mineral deposit and that a secondary dispersal pattern
of geochemical elements is often created during the weathering and erosion of the deposit.

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The primary envelope and the secondary pattern form geochemical anomalies which, if
pronounced enough, result in larger guides to mineralization than would be provided by the
economic mineral deposit itself.

The primary envelope, called aureole or halo, is an expression of the alteration and zoning
conditions. The secondary dispersal pattern or halo contains remnants of ore mineralization
that may be recognizable in rock, soil, sediment, and water samples taken at distances from the
source. The use of secondary dispersal patterns is exemplified in the tracing of floats to its
source and panning soil or alluvium for heavy minerals in the resistate.

GEOCHEMICAL PRINCIPLES AND GEOCHEMICAL ENVIRONMENT

The geochemical environment as defined by pressure, temperature, and the availability of the
most abundant chemical components, determines the stability of minerals and fluid phases at
any given point. There are two subdivisions of this environment:
DEEP-SEATED ENV'T: This extends downwards from the lowest levels reached by circulating
surface water to the deepest level at which rocks can be formed.
Magmatic and metamorphic processes predominate in this zone. It is an environment of high
temperature and pressure, restricted circulation of fluids, and relatively low free-oxygen
content.
SURFICIAL ENV'T: This is the environment of weathering, erosion and sedimentation at the
surface of the earth. It is characterized by low temperatures, nearly constant low pressure; free
movement of solutions, and abundant free oxygen, water, and carbon dioxide.
The movement of earth materials from one environment to another can be conveniently
visualized in terms of a closed cycle. The cycle is highly simplified.
The major geochemical cycle embraces both the deep-seated processes of metamorphism and
igneous differentiation, and the surficial processes of weathering, erosion, transportation, and
sedimentation.

GEOCHEMICAL DISPERSION
 A given small mass of earth material passes through major transformations in the
geochemical cycle
 It tends to be redistributed fractionated and mixed with other masses of material
 The process by which atoms and particles move to new locations and geochemical
environments is called geochemical dispersion

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 Geochemical dispersion occurs in dynamic systems in which earth materials are undergoing
changes in chemical environment, T, P, mechanical strain, or other physical conditions.
 Dispersion may be either deep-seated or surficial, according to the geochemical environment
in which it occurs, and primary or secondary , according to whether it occurs during the
formation of the ore deposit or during a later stage
 Primary dispersion occurs in deep- seated environment and includes all processes leading
to the emplacement of elements during the formation of an ore deposit, no matter how the
ore body was formed.
 In deep-seated dispersion, the channel-ways and sites of re-deposition are generally the
fissures and inter-granular openings of deep-seated rocks.
 Secondary dispersion, however occurs in the surficial environment and applies to the
redistribution of the primary patterns by any later process, usually in the surface
environment, where patterns are formed in the fissures and joints of near-surface rocks, in
the unconsolidated overburden, in streams, lakes, vegetation and even in the open air.

TYPES OF GEOCHEMICAL SURVEYS

Geochemical surveys are classified as

a) Orientation,
b) Recce,
c) Detailed, according to their purpose and scale.

Geochemical surveys may also be classified according to the material sampled in the survey,
i.e. soil, vegetation; sediment, water, rock, or vapour.
Some of these media are more effective or efficient in recce surveys (e.g. stream
sediment/water) while others are more suitable in detailed surveys (e.g. soil sampling)

Orientation surveys

 It provides the technical information on which to base the complete survey.

 That is, it is a preliminary investigation designed to bring out information for planning an
optimum routine survey.
 It should establish the type and amount of sample and the method of sample collection, all
for the lowest price.

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 Such surveys are essential in terrain where there has been little or no prior experience with
geochemical exploration methods; it is not so vital in terrains with previous geochemical
work.
 If possible, preliminary experiments should be undertaken in the vicinity of known deposits
that have not been disturbed or contaminated by human activity, so that the natural
geochemical pattern can be observed.

Recce survey

 The purpose is to search a relatively large area for indications of ore.

 The spacing and type of samples are chosen to detect but not necessarily outline favourable
provinces, districts, or ore-bodies with as few samples or at as low a cost as possible.
 It is aimed at determining the mineral possibilities of a relatively large area, eliminate the
barren ground, and draw attention to local areas of interest.

Detailed surveys

 Purpose is to outline mineralized ground and to pinpoint the mineralized source with the
greatest possible precision, preparatory to physical exploration by trenching, drilling, or
underground work.
 A relatively close sampling spacing, normally between 1 m -100 m, is usually required to
localize bed rock source.
 In terms of cost, the application is mostly restricted to limited areas of particular interest,
selected on the basis of their geochemical, geophysical, and geological information.
 To be most useful, the anomalies should be well defined and should occur in close proximity
to the ore.

It must be noted that the most popular recce method is stream sediment surveying while soil
sampling is the most popular method for detailed investigation.
Rock sampling is popular where rock outcrops are abundant.
Vapour, vegetation, and water sampling are used to advantage in special situations.

METHODS AND APPLICATION

STREAM-SEDIMENT SAMPLING
 Stream sediments are the natural composite of all the material upstream from a sample site.

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 It is important and very effective in recce work.

 Sample density varies; one each for 100 - 200 km2 area, 2 -3 sample sites per km along main
stream and at points on tributaries just above junctions, 50 -100 m per sample along the
stream for detailed work.
 Samples are easier to collect and easier to process than soils; 80 mesh samples are collected
from an area near the middle of streams.
 Cultural noise is an irritating problem because human kind's trash ends up in streams.
 Stream banks are not usually sampled in recce since they are older, more complex and are
likely to have been affected by slumping and hydromorphic anomalies. They, however, have
high proportions of fines and are easier to sample.
 Where enough heavy resistate material can be found, stream-sediments are sometimes
panned and concentrates collected for analysis.
 Stream-sediment geochemistry is unsatisfactory in the disorganized drainage of glaciated
terrain.
 In lake sediments, stream sediment geochemistry provide a good representation of
anomalous metal concentrations in their immediate catchment area.

RECCE GEOCHEM. SURVEY OF A DRAINAGE SYSTEM.

ANALYTICAL METHODS
Several methods are available, eg. AAS, XRF, QMS, SEM, Radiometry.
The elements to be determined and the analytical procedure should be decided from the
orientation survey.
 The choice of methods should properly be suited to the problem at hand:
 the procedure must be sensitive enough to detect elements present in small quantities;
 it must be reliable enough so chances of missing an important anomaly are negligible
 it must be economical enough that very large numbers of samples can be processed;
 there must be simplicity of techniques so analyses could be entrusted to relatively untrained
personnel;
 equipment must be portable so analytical laboratory can, if desirable, be set up near field
operations.
 In the analytical procedure, the following are necessary:
 sample must be treated to prepare it for transport, storage, and subsequent steps in the
analytical procedure;
 sample must be partially or completely decomposed, so element to be determined is released

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 in a form that can be easily manipulated;

 there must be separation of the element from other constituents that might interfere with
later measurements;
 the quantity of the element present must be estimated.

SOIL SAMPLING

Sampling and analysis of residual soils is one of the most widely used methods.
 It is applicable in areas of deep residual cover and sparse outcrops, where other exploration
methods are either too expensive or are technically ineffective.
 The B horizon, the zone of mineral accumulation, is most commonly sampled.
 Soil surveys are generally made on a grid pattern; sample locations are:
300 -1500 m apart in recce surveys;
15 - 60 m apart in follow-up surveys.
 Holes for soil sampling are dug by a mattock, post-hole digger or mechanical trencher ("
sma11 back hoe").
 20 -100 gm samples are taken and sieved to -80 (-0.17mm) mesh before analysis.

Hypothetical soil profile showing principal horizons.

Soil profile development is the result of vertical (upward/downward movement of material in
solution and suspension, accompanied by a complex series of chemical reactions, many of which
are organic in origin. Water is the essential medium in which this transfer and reconstitution takes
place. The distribution of metals may vary markedly with major changes down the profile. It is,
therefore, important to distinguish these (master) horizons before sampling.
Soil profiles vary in make-up within wide limits according to their genetic and geographic
environment. But most well developed profiles can be divided into 4 principal horizons: A, B,
C, and R.

Rock sampling
 Most flexible method in terms of a sampling site: outcrops, mines, drill core;
 Samples are: 500g for fine-grained rocks; up to 2 kg for coarse grained rocks
 Sampling is more time consuming than in sampling soil and stream;
 Most useful in detailed exploration for individual orebodies and ore shoots and in recce
for productive igneous bodies.

Advantages

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 Data can be directly related to primary aureoles in detailed sampling.

 The geologic context is direct; structure, rock type, mineralization, alteration can be noted
when sample is taken.
 Contamination by extraneous material is unlikely as it is for soil and stream sediment
surveys,

Disadvantages

 Sampling sites are quite rare; exposures do not generally occur exactly where they should
be.
 The scope of rock samples is relatively narrow, geochemical variations are generally weak,
and the effect of inherent rock type is strong.
 A rock sample represents conditions only where the sample was taken.
 Where visible mineralization is exposed, sample will obviously be high grade and not
necessarily representative of a wall-rock aureole.
 Samples can only be tested in the laboratory.

SAMPLING

Definition
Sampling ordinarily denotes something that has been physically removed from its natural
location to be tested (in the laboratory). That is, it is a process of taking small proportions of a
deposit that shall be a representative of the whole.
The amount and kind of sampling depend on the type of the deposit and the degree of its
development (whether in prospecting, exploration phase, or partly/fully developed mine.
It can be done at a surface (outcrop), from shafts, drifts, raises, or cross-cuts in 'the ore body

General principle
An ore body is a mixture of minerals in proportions that vary in different parts of the mass.
As a consequence, the proportion of contained metals also varies from place to place. Therefore
a single sample taken in a particular place would not contain the same proportion of metals as
does the ore body as a whole except by a highly improbable coincidence.
The probable error, which would be very large if only one sample were taken, decreases with
the number of samples, but it never disappears completely unless the samples are so numerous
and so large that their aggregate is equal to the orebody itself, in which case the ore body would

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be completely used up in the process of sampling. Sampling is carried out in conformity with
geological principles.

The aims are to compute the content, quality, quantity, and to know the structure of the ore for
treatment purposes. That is to obtain enough materials for accurate ore-grade calculations and
metal1urgicai testing as well as delineation of the ore body.
Accuracy of sampling depends not only on the number of samples but also on the proper
distribution of them throughout the ore body; it would be wrong to sample either the rich parts
or the lean parts ("eye -catching" sampling). Therefore, it is important to select the places in
such a way that all parts of the ore body will be represented.
Accuracy also depends on the volume of influence assigned to each sample and the statistical
stability of the system.

Sampling techniques
Drilling : Gives us the core or broken material and/or sludge
Excavation: Includes pitting, trenching,
Face sampling Typically chip and/or channel sampling
Bulk sampling Includes grab sampling.
Automatic sampling: This involves sampling from mill treatment at intervals
Efficient sampling depends upon available information. These include:
 knowledge of the relevant geological feature (structure, position, form);
 mineralogical composition;
 fractures and other physical features;
 contact between the H/W and F/W and any other contact between the ore and non-payable
material.

Drilling
 Drill cores and cuttings furnish most of the samples used to evaluate prospects.
 They also furnish most of the samples for testing and extending ore reserves at operating
mines.
 Cuttings from blast holes are often sampled for grade control and detail mine planning.
 A to N size core (30- 35 mm diameter) commonly preferred to less expensive E -size core
 (21 mm).
 Larger core sizes preferred in irregularly mineralized and low-grade deposits.
 The core is split lengthwise and half of the core is for assay.

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 The accurate sampling of blast holes is difficult since sampling is not their main purpose.
 Samples are generally collected in small scoopfuls from a pile of cuttings at the collar of the
hole. A more thorough job is sometimes done by recovering all the dry cuttings in a cyclone
collector or all the wet cuttings in special funnels or drill fittings. The entire volume from
each selected interval can then be carefully mixed and sampled.

Channel sampling
 It is the most conventional method.
 It consists of cutting continuous channel or grooves across the face/strike of vein,
mineralized bed down the wall of a pit, along the bottom or side of a trench and collecting
the resulting chips, fragments and dust from each channel to makeup the sample.
 Channel should be of constant width and depth for true representation, i.e. for uniformity of
sampling.
 One may have to clean the face or exposure before sampling to remove dust, slime and
soluble salts.
 For irregular ore, there is the need for close-spacing but for regular or uniform ore, great
sampling intervals are okay.

Chip sampling
 Consists of a regular grid from the working face, walls or roof of a mine.
 It is used for hard and uniform ore where channeling is not possible.
 Samples obtained by taking or breaking small equal pieces from points uniformly distributed
over the breast or face level. Series of chips of rock is taken either in a continuous line across
the exposure.
 The method is less laborious than channel sampling but rather precise and has higher
productivity in some mining districts and results need to be cross checked with channel
sampling.
 Unrepresentative chip sampling is a result of sampling from "eye-catching" or interesting
outcrops.

Bulk sampling
 This represents large samples taken mainly for mineral dressing, metallurgical testing, grade
confirmation, etc.
 It may be carried out to increase the certainty of information gained by other methods.
 Sample size is arbitrary but must not be less than 1ton.

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 Samples are obtained through shafts intersecting ore bodies, cross-cuts and drives.
 Sample is then treated in pilot plants.
 Bulk sampling for exploration or final feasibility study is costly.

Muck sample
 This is a grab-sample of the muck pile taken after blasting instead of a channel sample of a
face.
 This is usually done by crudely picking up pieces of rock of convenient size.
 Theoretically, it is quite unreliable but it may give correct results in some ores whose values
are evenly distributed or are independent of breaking qualities.

Grab sample
 This is a sample taken at any place.
 It is not truly representative and gives a rough indication.
 It is often applied to operating mines in order to control mill head grade.

Pitting & trenching

 Both are done by excavation for a certain depth over a certain area.
 Pitting is used for surficial deposits (in dry grounds).
 Sampling can be either in slots over the full height or at certain intervals (e.g. every 50 cm)
or at some pre-determined intervals or at every horizon.
 Trenches typically are useful to trace outcrops of an ore body or orientation of structures.
 They provide close visual examination of the material.

Grade and tonnage calculation

The grade of ore throughout a portion of a mine is estimated by [statistically] averaging together
the assay returns of the samples that have been taken.
 To estimate mining grade it is necessary to find composite values for the samples taken
within designated widths of channels or lengths of drill cores. If the individual samples are all
of the same length and of approximately the same density, as in most samples taken from large
deposits, an arithmetic average can be used.
 In complex ore bodies and in zoned veins, however, data from samples of various lengths
in a channel or drill core are averaged by the formula:
Average assay, G = L Σ (Widths x assays)
Σ widths.
The following example illustrates the point:

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Sample # Width (ft) Assay (%Cu) (Width x Assay)

1 3.2 6.2 19.84
2 6.4 7.3 46.72
3 5.3 8.5 45.05
4 2.1 6.4 13.44
TOTAL 17.0 (a) 125.05 (b)
AVERAGE 4.25 7 .35 (b/a)
A refinement in averaging assays is to weight the sample data for specific gravity (S. G. as
well as for length, thus:
Average assay, G = Σ (S. G. x W x A)
Σ (S. G. x W)
Where channel samples are cut at unequal intervals, some will have to represent greater lengths
of vein than others. Each channel must then be weighted by an interval of influence as well as
the width in order to obtain an area of influence (W x I). A customary procedure is to assign an
interval of influence measuring half the distance to each of the neighbouring channels. In this
procedure, the vein is assumed continuous. The formula is

Average grade, G = Σ (I x W x A)
Σ (I x W),

I = Interval, W = Width, A = Assay

The average width would be: Σ (Width x Interval)
Σ (Interval)
If a sample has been subdivided, assays of the fractions are first averaged together, weighting
each for its own width in order to get the value for the full channel. Then the values of the
individual channels are averaged together (as indicated above). These hold for samples spaced
at equal intervals.
The averaging procedures described are conventional, and in a sense, old-fashioned.
Statistical and computerize procedures, e.g. variogram function of geostatistics, are more widely
used for large ore bodies where there are 1000 of samples.
You will go into more details in Mineral Resource Estimation course.

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Some basic definitions

Indicator element: In geochemical surveys, an element measured in order to detect an ore
body is called an indicator element. It may either be the element itself or one of the major
elements in the body being sought.
Pathfinder element: This is an element associated with the ore body but which is more easily
detected or more widely dispersed than the indicator element(s).
Background: This is the normal abundance (average concentration) of an element in
unmineralised earth materials. E.g. Au = O.003ppm, Ag = 0.05 ppm, As = 2ppm. Values may
be expressed as a range since distribution of materials is rarely uniform.
Anomaly: This represents any abnormal chemical patterns related to mineralization, i.e. when
the actual concentration of the element is extra high or low, compared with the background.
Geochemical mobility: This is the ease with which an element may be dispersed relative to
the matrix of other materials surrounding it. In some environments mobility depends on the
mechanical properties of the mobile phase, for instance, on factors such as the viscosity of
magmas and solutions, or the size, shape, and density of clastic grains in a flowing stream.

Reading List
 Ore geology and Industrial minerals; An introduction by Anthony Evans, Third
Edition. Blackwell Scientific publications.
 An Introduction to Economic geology and Its Environmental Impacts. By
Anthony m. Evans. Blackwell Scientific publications
 Introduction to mineral exploration, edited by Gilbert Park
 Ore deposit geology by Park and MacDonald
 Economic geology: 75th Anniversary volume, 1905 –1980. By the Economic
Geology publishing Company.
 Economic geology monograph 6: The geology of gold deposits: the perspectives
in 1988. By the Economic Geology publishing Company.
 Introduction to Geochemistry. By Konrad b. Krauskropf &Dennis k. Bird.
McGraw-Hill, Inc.
 Studies of mineral deposits by V. M. Grigoriev and G.F. YakovievMagmatic
Sulfide Deposits: Geology, Geochemistry and Exploration (Hardcover) by
Anthony J. Naldrett
 Journal of Economic Geology
 Journal of Exploration geochemistry
 Mineral Deposita (Journal )
 Any other good text on economic geology

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