DEPARTMENT OF GEOLOGY, FOURAH BAY COLLEGE, UNIVERSITY OF SIERRA LEONE

MODULE: EXPLORATION AND MINING GEOLOGY COURSE

BY MR. SANTIGIE KEKUDA SESAY
(MSc. Engineering in Mineral Resources Prospecting and Exploration)

COURSE OUTLINE
Section A - Mineral Exploration
1. Geological Prospecting and Exploration
2. Geochemical Prospecting and Exploration
3. Geophysical Prospecting and Exploration
Section B - Mining Geology
1. Geological and Geotechnical terms
2. Surface Mining Methods
3. Grade Control, Waste dumps and Tailings dam
4. Drilling
5. Geological Database Concepts
6. Ore Body Modeling
7. Block Modeling
8. Mineral Resources Classification

SECTION A - MINERAL EXPLORATION

1. Geological Prospecting and Exploration

Understanding of the geology of an ore deposit and its general geologic setting is necessary at every step in
prospecting exploration, and development. The principal method of portrayal of this information is using
geologic maps and cross sections, which are constantly reworked and updated as work progresses and new
information becomes available. Geologic maps and sections are fundamental in exploration planning,
correlation and evaluation of preliminary results, and in reporting to management.

Criteria for Ore Recognition

The geologic features of importance in mineral exploration vary considerably from one ore type to another,
and what might be of importance in one ore type may be of minor significance in another. However, there

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are a few criteria for the recognition of ore that are almost always considered, regardless of ore type, and a
brief listing and discussion will serve to illustrate the methods of the geologist in exploration work. Some
criteria for ore recognition are:
(a) Igneous rock affiliation
(b) Host rock association
(c) Wall rock alteration
(d) Age of mineralization
(e) Gangue mineral association
(f) Trace metal association
(g) Structural controls
(h) Physiographic expression
(i) Weathering effects
(j) Ore mineralogy

(a) Igneous rock affiliation - many ore deposits are associated with or contained within certain kinds of
igneous rocks. For example, chromite ores are always found in a special kind of iron-rich rock. Some types
of tungsten mineralization are always found associated with certain granitic rocks.

(b) Host rock association - certain kinds of wall rock act as host to specific ore types. For example, ancient
reef deposits, like the modern coral reefs of the South Pacific, are interlayered within marine formations
such as limestone. Fossil reefs are an important locus for a variety of important precious and base metal
deposits.

(c) Wall rock alteration - the mineralizing fluids that deposit ores sometimes permeate outward into the
enclosing host rock, causing subtle changes in a ring-shaped contact zone (aureole) around the ore body
(Figure 1). For example, limestone surrounding certain silver-lead ores is recrystallized to dolomite,
coarsening the texture of the rock slightly, and making it visibly lighter in color. The aureole of wall rock
alteration is quite useful in mineral exploration, for it is much larger than the ore deposit itself, and usually is
subtle enough to have escaped notice of the early prospectors and miners.

(d) Age of mineralization - some ore deposits occur only in rocks of a definite age. For example, much of
the world’s potash is Permian in age (280 to 225 million years), and the bedded barite deposits of the West
are largely restricted to formations of Silurian and Devonian age (430 to 345 million years). Many such
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simple age relationships are only now becoming generally recognized, and the concept will be helpful in the
mineral evaluation of many regions.

Figure 1. Wall rock alteration as a guide to ore.

(e) Gangue mineral association - Many ore types have distinctive gangue mineral associations (undesired
minerals associated with the ore) that can be of use in mineral exploration. For example, two major regional
ore belts, the Mother Lode gold and the Foothills base metal zones of California come together and mingle
northwest of Yosemite National Park. Prospectors quickly learned that the appearance of barite in float or in
the prospect pan was good evidence that the mineralization was of the base metal type, not gold.

(f) Trace metal associations - Many kinds of ore deposits have distinct combinations of minute amounts
of metal found in association with the principal ore metal, helping to distinguish one ore type from another.
For example, the copper deposit containing nickel and cobalt is of entirely different character than a
copper-molybdenum association.

(g) Structural controls - the analysis of structural control of ore is usually of prime importance in planning
exploration, development, and production. On a regional scale, ore deposits may be found in elongated rows
of individual ore occurrences or clusters of occurrences which are referred to as mineral belts or mineral
lineaments. Along these trends, above average potential for ore exists. On a more restricted scale, the ore
types of a given district may occur along a single fault or beneath a thrust plane, focusing attention upon an
une xplored block of ground. Such relationships may become apparent only after the most painstaking
detailed geologic mapping.

(h) Physiographic expression - Individual ore deposits, and sometimes entire mining districts, are
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commonly altered, mineralized, and weathered so that the rock matrix consists essentially of chemically
unstable or soft, easily weathered minerals and rocks (Figure 2). Erosion cuts into such zones, and the
resulting depressions are often filled with gravel and lava flows and are usually densely overgrown with
vegetation, all but concealing evidence of mineralization. The recognition of mineralization fringe effects,
and the lateral projection of such indications beneath cover, is an approach used by many explorationists.

Figure 2. Physiographic expression of ore.

(i) Weathering effects - many of the mineral deposits currently of interest consist of relatively small
specks of valuable mineral scattered through a worthless rock matrix. The ore minerals themselves are often
chemically unstable under the weathering conditions at and near the surface. The ore minerals of copper,
silver, and uranium, for example, rarely survive intense weathering and are decomposed so that some or all
the metal is flushed from the outcrop in aqueous solution (ground water). This near-surface zone of leaching
and flushing is called the leached capping, and it may contain none of the ore minerals characteristic of the
un-weathered ore deposit below. The recognition of leached cap rock has been a very successful tool of the
modern exploration geologist, because the various stable oxides, sulfates, and carbonates of metals most
often remaining in outcrop are extremely difficult to recognize and were easily missed by earlier explorers.

(j) Ore mineralogy—in some instances the mineralogy of the ore itself may be important. For example,
aluminum is one of the most abundant elements in the earth’s crust, yet only bauxite (a relatively rare
mixture of aluminum hydroxides) has been mined as an ore of aluminum.

Most geologists have a checklist of ore criteria they think important for each ore type of interest. They might
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refer to the total picture of all criteria considered together, as a “conceptual model” of that type of ore
occurrence. They may also have definite ideas about the size, shape, and grade to be expected of this
hypothetical ore deposit. Obviously, a conceptual model can be of great help in planning exploration, during
mapping and drilling, and in all phases of the evaluation of results, if the risks inherent in any generalization
are kept in mind. The use of a formal conceptual model is often found to improve communications with
management and to facilitate discussions between explorationists, such as those between geologist and
geophysicist.

2. Geochemical Prospecting and Exploration

Introduction

Many mineral deposits are not exposed at the earth's surface. They may either be concealed by thick soil
cover, or they may lie buried beneath layers of rock. More complex techniques based on geochemistry,
geophysics, and geobotany are needed and can be very helpful in finding these deposits. Most of these
techniques require specialized training and, in some instances, expensive equipment.

Geochemical prospecting is based on systematic measurement of one or more of the chemical properties of
rock, soil, glacial debris, stream sediment, water, or plants. The chemical property most commonly measured
is the content of a key “trace” element. The purpose is to discover zones in the soils or rocks that contain
comparatively high concentrations of elements that will guide the prospector to a hidden deposit. Such
concentrations of indicator elements in rocks or soils constitute a geochemical “anomaly”. The actual
amount of the key element in a sample may be very small and yet constitute an anomaly if it is high relative
to the surrounding area. For example, if most samples of soil were found to contain about 0.00001 percent
(0.1 part per million) silver, but a few contained as much as 0.0001 percent (1 part per million), the few high”
samples would be geochemical anomalies. Plots of analytical results on a map may indicate zones to be
explored further.

Geochemical anomalies are classified as primary or secondary. Primary anomalies result from outward
dispersion of elements from mineral-forming solutions. The “high” concentrations of metals surround the
deposit and the dispersion of metals laterally or vertically along fractures may result in a leakage “halo” that
extends hundreds of feet away from the deposit. Halos of this type are especially useful in prospecting
because they may be hundreds of times larger than the deposit they surround and hence are easier to locate.
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Secondary anomalies result from dispersion of elements by weathering. Some primary minerals, such as
cassiterite, are resistant to chemical weathering and are transported by the streams as fragmental material.
Other minerals may be dissolved and the metals may be either redeposit locally or carried away in solution
in ground and surface waters. Some of the metal in solution may be taken up by plants and trees and can be
concentrated in the living tissue. A great many studies have been made of the metal content of residual soils
over sulfide deposits, and in general the distribution of anomalous amounts of metal in the soil has been
found to correspond closely with the greatest concentration of metals in the underlying rock.

All the products of weathering in a drainage basin funnel goes through the system of streams and rivers that
flow out of the area. The weathering products are partly in the form of chemicals in solution and partly in the
form of sediments. Either or both can be sampled and tested, and their composition will reflect the chemical
nature of the rocks in the drainage basin. The presence of an ore deposit in a drainage basin may thus be
detected by analyzing for metals removed by weathering and either incorporated with the stream alluvium or
carried in solution. The deposit can be located within the basin by sampling water and sediment from each
successive tributary and determining which contain higher than normal amounts of metals. This procedure
narrows the search to favorable areas.

Contamination of surficial material is an ever-present hazard in geochemical surveys. The most common
sources of contamination are materials derived from mine workings. Such materials may become scattered
widely over the ground, and the ore minerals in them may oxidize and go into solution, contaminating the
soil, stream sediment, and water, and masking natural anomalies. Similarly, smelter fumes, windblown flue
dust, and metallic objects introduced to the natural environment by man may also contaminate the soils and
rocks.

Typical stages in Geochemical Exploration Program

A full-scale geochemical prospecting program for metals would include the following stages:

(a) Preliminary evaluation of areas, selected based on the available geological data, by sampling and testing
intrusive, metamorphic, and sedimentary rocks and by noting the presence of mineralized zones. In this way,
a metallogenetic province can be identified.
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(b) Primary reconnaissance and orientation surveys, based on sampling major drainage basins, using water,
stream sediment, lake sediment, and heavy-mineral surveys.
(c) Secondary reconnaissance surveys based on detailed testing of drainage basins containing anomalous
values. Poorly drained areas can be tested by widely spaced sampling of soil and ground waters.
(d) Follow-up surveys along dispersion trains or fans to determine the cutoff points and the extent of
dispersion patterns. These surveys are normally a combination of stream sediment, heavy mineral, water,
and soil testing, but biogeochemical surveys may also be useful. Priority for follow-up surveys should be
based on the presence of favorable rocks and geological structures, favorable geophysical indications, and
intensity of the geochemical anomaly.
(e) Detailed surveys carried out in the vicinity of the suspected metalliferous source by soil or vegetation
sampling at closely spaced intervals. Interpretation of the results at this stage generally suggests sites for
trenching, sinking of shallow shaft, or drilling to locate the precise source of the body giving rise to the
geochemical anomaly.

General Principles used in Geochemical Prospecting and Exploration

Mineral deposits represent anomalous concentrations of specific elements, usually within a relatively
confined volume of the Earth's crust. Most mineral deposits include a central zone, or core, in which the
valuable elements or minerals are concentrated, often in percentage quantities, to a degree sufficient to
permit economic exploitation. The valuable elements surrounding this core generally decrease in
concentration until they reach levels, measured in parts per million (ppm) or parts per billion (ppb), which
appreciably exceeds the normal background level of the enclosing rocks. This zone surrounding the core
deposit is known as a primary halo or anomaly, and it represents the distribution patterns of elements which
formed because of primary dispersion. In general, primary anomaly is formed at, or near, the same time as
the central ore body.

However, mineral deposits at, or near, the surface are subjected to chemical and physical agencies of
weathering. Many of the ore minerals undergo decomposition or disintegration, and their chemical
constituents become dispersed into weathering debris, soils, ground water, and plant tissue. Further
dispersion, often over considerable distances, may ensue due to the agencies of glaciers or stream and river
systems. Abnormal chemical concentrations in weathering products are known as secondary dispersion halos
or anomalies and are more widespread. Geochemical prospectors used dispersion halos (both primary and
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secondary) as a guide to mineral prospecting and exploration. Hence, we have two types of geochemical
environments for prospecting and exploration of ore deposit; primary and secondary geochemical
environments.

Sampling techniques in Geochemical Exploration

It is essential that adequate samples be taken that are representative of the material sampled at any given
locality. Representivity is therefore a paramount importance in geochemical sampling. These materials may
be rocks, waters, gases, soils, stream sediments, or vegetation. Wherever possible, sampling should be
confined to one type of material for any survey, such as one rock type in litho geochemical surveys, one
horizon in soil surveys, or one plant organ and plant species in biogeochemical surveys. If, through necessity
such as landscape variations, diverse materials must be sampled, correlative factors must be applied to the
results before the geochemical patterns can be effectively interpreted.

Sample spacing may be in a regular basis. Depending on the type of deposit, sample spacing may be 50 by
50 meters apart or 100 by 100 meters. Iron ore deposit with a large horizontal extent may have a sample
spacing of 100 by 100 meters. Pb-Zn deposit may have a sample spacing of 50 by 50 meters apart. This
means, samples will be collected at every regular 50 meters interval. However, as in sample spacing, cost
must be an important consideration in choice of routine analytical procedures. Most geochemical analytical
procedures are designed to detect elements at low concentrations such as parts per million or parts per
billion. Techniques used for the analysis of geochemical sampling media are varied; some are intended for
use in mobile field laboratories, and some are intended primarily for well-equipped permanent laboratories
such as those operated by commercial firms or government agencies.

Sample preparation is another technique to be considered. After collecting samples in the field, they need to
be prepared before sending them to the laboratory for chemical analysis. Here, field standards, duplicates
and blanks are included in the sampling to test the accuracy of the laboratory results. In preparing field
samples care must be taken to prevent contamination. Selection of analytical procedures for a geochemical
prospecting survey is one object of the initial orientation survey. Samples collected during the orientation
survey are often subjected to multi element analysis which will show the techniques most likely to yield
maximum contrast between anomaly and background, and most likely to detect the target mineral deposit at
the greatest distance. In many cases the sample decomposition procedure that tends to yield the most
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prominent and extensive anomalies is one which releases only a portion of the metal present in the sample.
Similarly, anomaly recognition is often facilitated by several elements being determined, rather than by just
one or two of the most immediate economic significance.

Some of the basic tools needed to collect field samples are: a hand trowel, shovel, sample bag, geologic
hammer and chisel etc.

Field Analysis of Samples

Analytical techniques for on-site sampling of water, soil, stream sediment, and lake sediment are usually
based on colorimetry; they utilize a specific selective organic reagent, such as dithizone, which produces a
characteristic color in the presence of a given metal in solution. The reaction of dithizone with zinc, which
produces a range of blue, violet, and red shades, is commonly utilized in field geochemical prospecting.
Field analytical techniques generally determine loosely bonded, or exchangeable, metal ions, which are
extracted by a citrate, acetate, or dilute acid. Analytical methods used in geochemical prospecting must be
sensitive enough to determine minute amounts of key elements, accurate enough to show small differences
in concentration, fast enough to permit large numbers of samples to be analyzed in a day, and inexpensive.
Wet chemical techniques are usually confined to rapid colorimetric procedures that require a minimum of
equipment and reagents. Instrument techniques (such as emission spectrographic and X-ray fluorescence
techniques) require expensive equipment and trained personnel, but usually yield a lower cost per
determination if thousands of samples must be analyzed.

Laboratory Analysis of Samples

When it is desirable to determine the total content of a given metal in a sample, analysis is generally
performed in a laboratory. A wide range of techniques may be used, but the most common are emission
spectrography and atomic absorption spectrophotometry. The latter technique is particularly suitable for
routine geochemical analysis since it permits a large number of samples to be analyzed relatively quickly
and inexpensively to an adequate standard of precision. Sample digestion prior to atomic absorption analysis
frequently involves a hot mineral acid or mixture of acids; one of the most common digestion agents is a
mixture of nitric and perchloric acids. Other geochemical analytical procedures include x-ray fluorescence,
spectrography, chromatography, and polarography.
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Geochemical Data Processing

Statistical analysis, particularly when large volumes of data are generated by major multi-element surveys,
commonly employs three categories of procedures:
 two-dimensional representations of geochemical data involving automatic plotting and contouring
procedures,
 univariate statistics where populations, background ranges, and anomaly thresholds are determined for
single elements,
 multivariate statistics where correlations between, and associations of, several elements are sought and
distinguished.

Geochemical data processing is necessary and erroneous or unrealistic sample values will be easily detected
and eradicated. A scatter plot of metal values against sample spacing is an example of two-dimensional
representations of geochemical data. The curve produce after the plot gives a reflection of the distribution of
the metal within the study area. The target here is to identify the geochemical anomaly and it is said to fall
within the highs (peaks) and lows (depressions) of the curve. The highest peak of a curve may depict an
anomaly. After identifying regions with higher metal values, detail surveys can be done in those areas to
identify the deposit.

Primary Geochemical Environment

This is more applicable in most igneous host rocks. These rocks have been formed by a differentiation
process. This starts with a parent magma, often of basaltic composition, and as cooling proceeds, early
crystallizing minerals are separated to form cumulate rocks. The remaining melt, having lost elements that
are concentrated in the early minerals, becomes changed in composition. Through such a process the magma
may passes through a differentiation series, such as gabbro-diorite-granodiorite-granite-pegmatite, in which
each point has a distinctive composition of both major and minor chemical elements.

In general, as differentiation of magmatic rocks proceeds, there is an overall decrease in iron, magnesium,
calcium, and titanium in the rocks combined with an increase in silicon, aluminum, sodium, and potassium.
Trace-element concentrations also change considerably with this sequence of rock type and major element
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alteration. The occurrence of trace elements in rock-forming minerals is generally controlled by a
combination of ionic size, valency, and type of chemical bond. Some elements, however, including lithium
(Li), boron (B), beryllium (Be), niobium (Nb), tantalum (Ta), tin (Sn), uranium (U), thorium (Th), tungsten
(W), zircon (Zr), and the rare earths, do not enter rock-forming silicate minerals to any significant degree
during magmatic crystallization. These elements tend to be concentrated in residual aqueous fluids together
with compounds such as hydrogen fluoride (HF), hydrogen chloride (HCl), and carbon dioxide (CO2), and
remain in solution until the final stages of magma crystallization, exemplifying primary mobility. They
eventually may become concentrated in pegmatites, and may play an important role in hydrothermal
alteration and ore formation.

Primary dispersion halos vary greatly in size and shape as a result of the numerous physical and chemical
variables that affect fluid movements in rocks. Some halos can be detected at distances of hundreds of
meters from their related ore bodies; others are no more than a few centimeters in width. Some of the factors
controlling the development of primary halos are presence or absence of fractures in the host rock, porosity
and permeability of the host rock, tendency of mineralizing fluids to react chemically with the host rock, and
volatility of the ore elements.

The main types of primary halos are syngenetic and epigenetic. Syngenetic primary halos are formed
essentially contemporaneously with the enclosing rocks. Epigenetic primary halos are formed after the host
rock has solidified, and result from the introduction of mineralizing solutions to open spaces such as
fractures. Epigenetic primary halos may cover large areal extents, as when a rock mass is pervasively altered
by hydrothermal solutions, or they may be restricted to well-defined channel ways through which
mineralizing fluids have followed open spaces such as fractures or solution channels. They are frequently
developed in rocks overlying an ore body and can be of great value in prospecting.

Volatile elements, such as mercury and radon, offer the greatest possibilities for successful ore discovery
utilizing leakage dispersion, because they move farther than other elements. Many elements occur in
hydrothermal mineralizing solutions, and some may be more mobile than others. In some cases, however,
the element yielding the most extensive primary dispersion halo is not that of greatest economic significance
in the ore body, even though it is closely associated geochemically. Such an element is referred to as a
pathfinder, and is of value in prospecting because its halo is broader than that of the element of primary
interest or because it may be detected more readily by conventional analytical procedures. Arsenic is
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frequently used as a pathfinder element in geochemical exploration for gold. Radon is frequently used as a
pathfinder element in uranium exploration.

Surface rock chip sampling can be used to detect leakage primary anomalies that reflect concealed metal
mineralization. Below are diagrams (Figure 1, 2 and 3) showing the application of geochemistry in
Zn-exploration. Figure 1 is a cross section showing use of surface rock chip sampling to detect leakage
anomalies that reflect concealed zinc mineralization. Horizontal distance is 1 km, vertical distance, 200 m.
Ore bodies have formed in a porous limestone bed capped by an impermeable shale; some mineralizing fluid
has escaped upward along preexisting faults.

Secondary Geochemical Environment

Rocks and minerals that are stable in the primary environment are frequently unstable in the secondary; they
undergo disintegration and modification through a variety of chemical and physical processes that are
known collectively as weathering. Trace elements of ore bodies and their associated primary halos are
frequently released by weathering processes to soils, overburden, and vegetation, with consequent
generation of secondary halos. Some chemical constituents of ore bodies may be widely dispersed through
the agency of ground waters or surface stream systems; analysis of spring and stream waters, or stream
sediments, may indicate the presence of a mineral body from a considerable distance. Some volatile
constituents of ore bodies may migrate through soil gas into the atmosphere where, again, they can be
detected and serve as clues to the source. An adequate interpretation of geochemical data in the secondary
environment requires that the various processes of weathering be distinguished.

There are two primary types of weathering, chemical and physical;

 Chemical weathering requires abundant water, oxygen, and carbon dioxide; it involves the breakdown,
by chemical means, of rocks and minerals with consequent release of their contained trace elements to
the environment. Chemical weathering predominates in tropical environments, but is also important in
temperate regions.
 Physical weathering comprises all processes of rock disintegration which do not involve chemical
changes; it is most common in extremely cold or arid regions where frost shattering and sand blasting
occur. The relative influences of chemical and physical weathering depend on the nature of the landscape
in question.
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Secondary dispersion halos, or anomalies, are sometimes referred to as dispersion trains. The shape and
extent of secondary dispersion trains depend on a host of factors, of which topography and ground-water
movement are perhaps most important. Ground waters frequently dissolve some of the constituents of
mineralized bodies and may transport these for considerable distances before eventually emerging in springs
or streams. Further dispersion may ensue in stream sediments when soil or weathering debris that has
anomalous metal content becomes incorporated through erosion in stream sediment. Analysis of the fine
sand and silt of stream sediment can be a particularly effective method for detection of mineralized bodies
within the area drained by the stream.

Geochemical mobility of elements in the secondary environment depends on certain inherent properties of
the elements in question and the main features of the landscape. In this context, important properties of
elements are electronic configuration, ionic potential, stability relations with variation in acidity and
oxidation potential (pH and Eh), tendency to form complexes with organic matter, and tendency to be
co-precipitated or absorbed with iron or manganese hydroxides. Features of the landscape that can either
significantly enhance or restrict geochemical mobility include topography, soil type, pH and Eh of ground
water, presence or absence of decaying vegetation in soils, calcium carbonate precipitates in soils, and iron
and manganese hydroxide precipitates in stream channels.

Figure 1. Surface-drainage channels of a Pb-Zn ore body with streams having traces of Pb-Zn mineralization.
The mineralization is found upstream.

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Stream Sediments

Surveys of sediments and heavy minerals are carried out to determine the migration path of dispersed
elements and minerals along the surface-drainage channels of an area (Figure 1). Samples are collected from
the fresh sediment in the bottoms of streams and also from old sediment on the terraces and floodplains. For
chemically dispersed elements, the fine fraction (minus 80 mesh) is generally used for analysis; for
mechanically dispersed heavy minerals, a coarser fraction is panned from the sediment. Sampling points are
located at intervals along the length of the drainage system (Figure 2 below), and the results of the chemical
analyses of the stream sediment are plotted on a map of drainage. An increase in the metal content of the
stream sediment upstream may indicate approach to a mineralized zone (Figure 1. Pb-Zn ore body in bed
rock). The heavy mineral fraction may be examined microscopically for ore minerals or accompanying
gangue minerals or the fraction may be analysed for ore elements or indicator elements. Both results are
plotted on the drainage map and interpreted like the stream sediment data.

Figure 2. Drainage map showing Stream sediment prospecting with sampling points located at intervals
along the length of the drainage system.

Certain geological conditions may require that all horizons of the soil be sampled. Frequently, sampling of
the organic horizon is effective in some areas, whereas in some regions other horizons are more rewarding.
In some places, deep sampling of a horizon by drilling is the only satisfactory method for soil and glacial till

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surveys. Heavy and resistant mineral surveys of soil, till, and weathered debris have become increasingly
useful. In these surveys the geological materials are panned, and the heavy and resistant minerals obtained.
They are then examined microscopically for ore minerals or analysed for ore and indicator elements; the
results are plotted on maps. Heavy and resistant mineral maps, prepared on the same grid as those for soil
surveys, provide valuable ancillary data and often aid in the interpretation of the elemental dispersion
patterns. In soil analyses, the fine fraction (minus 80 mesh) is generally analysed for the chemically
dispersed elements, whereas for heavy and resistant minerals a coarser fraction is used. In glaciated terrains,
as well as in certain other terrains, heavy and light clast (mineral fragments, stones, and boulders) tracing
has proved effective in the discovery of certain types of mineral deposits. Examples are quartz boulders or
fragments as indicators of gold-quartz veins, and galena boulders or fragments as indicators of lead-zinc
deposits. Surveys of this type are generally carried out on grids, the abundance of light or heavy clasts being
visually noted and plotted at each sampling point or wherever they occur along the grid lines. In other
surveys, large samples of the till or overburden are obtained at each sampling point, and the light and heavy
clast indicators are counted in the light and heavy concentrates obtained from the samples. When all data
from clast surveys are plotted, fans or trains are commonly outlined whose apexes or starting points often
mark the sites of underlying mineralization.

Figure 3. Plot of a drainage map and interpreted like the stream sediment data.

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Geochemical prospecting surveys include surveys where the sample medium is stream or lake sediment,
stream, spring or lake waters, and detrital heavy minerals from stream sediment.

Rock Geochemical Surveys

Most reconnaissance surveys are carried out on a grid or on traverses of an area, with samples taken of all
available rock outcrops or at some specific interval. One or several rock types may be selected for sampling
and analysed for various elements. The distribution of the volatiles such as chlorine (Cl), fluorine (F), water
(H2O), sulfur (S), and CO2 in intrusives with associated mineralization has received some attention as an
indicator.

Geochemical maps are compiled from the analyses and contours of equal elemental values are drawn. These
are then interpreted, often by using statistical methods, in the light of the geological and geochemical
parameters. Under favourable conditions, mineralized zones or belts may be outlined in which more detailed
work can be concentrated. If the survey is executed over a large expanse of territory, geochemical provinces
may be outlined.

Figure 4. Use of surface rock chip sampling to detect leakage anomalies that reflect concealed zinc
mineralization. Ore bodies have formed in a porous limestone bed capped by impermeable shale; some
mineralizing fluid has escaped upward along pre-existing faults.

Detailed rock geochemical surveys are generally carried out on a local basis and are aimed at the discovery
or definition of primary ore leakage halos associated with mineral deposits. Chip samples of rocks on a
definite grid are used in some cases, and samples from drill cores in others. All the analytical data are plotted
on plans and sections, and compared with the geological situation. The metal values in ppm are shown as
contour values (Figure 5).

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Figure 5. Results of the chemical analyses of the surface rock chip sampling in the form contours.

Frequently, primary halos can be discovered by this method, and these and leakage halos can be traced to the
focus of mineralization if sufficient work is done. From Figure 6 above, the anomaly is clearly identified at
the peak of the curve. The relative location corresponding to this peak will be traced along the X-Y section
line.

Figure 6. X-Y section line drawn at Figure 5 after geochemical exploration to find anomaly of Zn within the
terrain.

In detailed drilling and development work, the use of ratios obtained from analyses of rocks along traverses
or diamond-drill holes can frequently be employed to estimate proximity to mineralized zones. Particularly
useful ratios include potassium oxide/sodium oxide (K2O/Na2O), silicon dioxide/carbon dioxide (SiO2/CO2),
and SiO2/total volatiles; the volatiles commonly include H2O, CO2, S, arsenic (As) etc. Many types of
mineral deposits are characterized by a consistent increase in the ratio K2O/Na2O, which is essentially a
manifestation of increasing potassic alteration. Similarly, several mineral deposits, particularly those
enriched in gold and silver, are marked by a consistent decrease in the ratio SiO2/CO2 as ore is approached.
The ratio SiO2/total volatiles exhibits considerable variation among the various types of mineral deposits; in
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most cases, skarns excepted, there is a consistent decrease in the ratio as mineralization is approached.

Geochemical Anomalies in Residual Cover

In many residual soils, the diagnostic primary and secondary minerals, if present, are too fine grained for
easy identification. Chemical analysis of residual material, however, may bring out latent images inherited
from the parent rock that are not obvious from casual observation. Little use has been made of chemical
analysis of soils in geologic mapping, although experimental work shows that determination of the nickel
content can be helpful in identifying soils derived from serpentine (Stevens and Lakin, 1949). In prospecting,
soil analysis for traces of metals has become the most widely used and successful of the geochemical
methods of mineral exploration. A great many field surveys of the metal content of residual soils over
sulfide deposits have been made, and with no important exceptions the dispersion pattern of metals in the
soils was found to correspond closely with the distribution of metals in the underlying rock and hence
probably with that in the rock from which the soil was derived.

Much of the work, however, has been sponsored by private mining companies as part of their exploration
program, where commercial considerations prevent the release of information to the public. The absolute
amount of metal in the soil as compared with that in the parent rock or ore depends on the mobility of the
metal, or the ease with which it is removed in solution. Thus, although an ore deposit may contain equal
concentrations of lead and zinc, the derived soil may contain many times as much lead as zinc. The lead is
relatively immobile and is retained, whereas the zinc tends to be leached out. In general, it is found that lead,
tin, arsenic and antimony are immobile and are held in the residual soil; zinc and cobalt tend to be
impoverished; copper, tungsten, and molybdenum appear to be intermediate.

Although the major part of the mobile metals dissolved from the weathering products of a vein is entirely
removed, a significant part may be transported in solution only for short distances and then reprecipitated
locally. The effect is a lateral and downhill spreading of the dispersion pattern outward from the bedrock
source. Zinc anomalies hundreds of feet wide have been observed in residual soil associated with veins
measuring only tens of feet in width. The lead anomalies over the same veins were not only much narrower
but corresponded much more closely with the location of the vein. Over such a vein, samples spaced at 100
or 200 feet intervals would have been adequate for locating the zinc anomaly, whereas as 50-foot spacing
would have been necessary for the lead anomaly. It has been the writer's experience, therefore, that where
18
complex ores containing two or more metals are sought, greatest economy is achieved when reconnaissance
geochemical soil surveying is based on the more mobile metal and detailed work is based on the less mobile
one.

The homogeneity of a metal pattern in residual soils is apparently also related to mobility. The
characteristically homogeneous patterns formed by mobile metals such as zinc indicate that original local
and erratic variations in the distribution in soils of the mobile metals tend to be smoothed by solution in rich
spots and local redeposition in lean spots. With immobile metals, solution is inhibited and the original
spottiness of distribution is preserved. Thus, Tikhomirov and Miller (1946) report that the molybdenum
pattern over the Kounrad molybdenite deposit is less erratic than the tin patters commonly found over
Cassiterite veins. The relative homogeneity of a geochemical anomaly is an important consideration in
determining the most efficient spacing of samples in practical survey work.

Geochemical anomalies in residual cover may be distorted by down slope movement of the soil. The result
is an asymmetrical curve in which the metal content fails off less rapidly on the downhill side than in the
Uphill side, as illustrated in the diagrams presented by Huff (1952). Asymmetry of this kind can also be
caused by the action of metal-rich solutions depositing metal in the soil on the downslope side, although the
net effect is the same whatever the cause. In extreme cases of asymmetry owing to down slope movement,
geochemical anomalies may be detected by sampling along the foot of a slope, hundreds of feet below the
outcrop of the vein. Riddell (1954) describes reconnaissance exploration work in an area of high relief by
systematic sampling of soils barely above the modern stream terraces.

Patterns in. residual soil may also be distorted by compaction slumping. At the Friend's Station deposit in
eastern Tennessee, an average of 50 feet of residual clay overlies a gently dipping limestone sequence. The
clay overburden was estimated to represent the weathering product of three times its volume of parent rock.
The resulting flattening of the dip of the gently dipping ore caused the geochemical soil anomaly to come to
the surface several hundred feet on the footwall side of the sub outcrop of the ore (Hawkes and Lakin,
1949).

Gossans

A gossan is the mass of residual limonitic material that remains behind after removal of the soluble products
19
of weathering of a sulfide-bearing deposit. Being residual, gossans can be traced downward through
successively less weathered zones into the unweathered primary sulfide minerals. Historically, gossans have
been one of the best guides to the prospector in areas of residual cover and deep weathering.

At the surface, blocks of gossan material may be dislodged from the main mass and scattered over the
immediate vicinity by frost action, slumping, and slope creep. If the fragments are sufficiently durable, they
may on occasion be transported for substantial distances by glaciation or stream action. Studies of the
dispersion pattern of limonitic fragments, particularly in alluvial material, have led many prospectors to the
parent gossan mass and the hidden sulfide deposit beneath.
The prerequisite for the formation of a gossan, in addition to deep weathering and an oxidizing environment,
is the presence in the bedrock of iron as sulfide or carbonate to provide the raw material for the formation of
limonite. The common gossan-making minerals are pyrite, marcasite, pyrrhotite, copper-iron sulfides,
arsenopyrite, siderite, and ankerite. In general, the more iron is present in the primary ore, the more
pronounced will be the gossan. The occurrence of economically valuable metals in the ore is only indirectly
related to gossan formation, in that ore minerals are commonly associated with the gossan-making iron
sulfides and carbonates. Thus not all gossans indicate base-metal sulfides, and many base-metal sulfide
deposits do not weather to form gossans.

The mineralogy of gossans is limited to species that are stable in contact with acid sulfate solutions. The
dominant minerals are limonite, quartz, and secondary silica. The overwhelming abundance of limonite is
responsible for the conspicuous red-brown to black color of gossans, a feature difficult to overlook even in a
casual reconnaissance of an arid terrain. Depending on the parent material and the maturity of the gossan,
barite, gypsum, secondary metalliferous minerals, and clay minerals resulting from the breakdown of the
country rock may also be present. Secondary green copper minerals in a gossan may be a useful indication
of the presence of copper in the primary ore.

The structure and texture of gossans, to a greater or less extent, reflect the original characteristics of the
parent material. The primary ore minerals during weathering may form minute, tabular veinlets of silica or
limonite along crystal directions. These systems of veinlets may remain behind after the original mineral has
been destroyed, leaving a box work pattern that is characteristic of the primary mineral. Distinctive box
works indicative of most of the common sulfide minerals have been identified and described by Blanchard
and Boswell (McKinstry, 1948, pp. 268-276), and have been used widely in appraising leached outcrops in
20
terms of the commercial grade of the primary sulfide deposit. The chemical composition of gossans has
received far less attention than their mineralogy and texture. What little evidence 'has been reported to date,
however, points to the conclusion that the content of traces of the ore metals in the limonite of gossans can
be used to indicate the relative abundance of those metals in the primary deposit.

Geobotanical Survey

Geobotanical surveys are carried out by mapping the distribution of indicator plant species, or plant disease
symptoms diagnostic of high-metal-bearing soils, such as chlorosis and dwarfing. Where suitable indicator
plants are present, this method is rapid and inexpensive, but unfortunately such plants are seldom consistent
in distribution from one area to another. Geobotanical techniques require careful preliminary orientation
surveys.

Plants have been successfully used as aids in mineral prospecting, and under certain conditions may assist in
locating buried mineral deposits. So many factors are involved, however, that it is not always possible to
predict conditions under which plants will be of practical assistance. Many plants, by means of their
extensive root systems and the absorptive ability of their roots, effectively sample many of the elements that
are within reach of the roots and transfer these elements to the branches, stems, and leaves, which can be
chemically analyzed. Thus, under ideal conditions, the plant has sampled the underlying soil or rock in its
root zone to depths of as much as 50 feet. The advantages to the prospector of being able to sample plants
and thus obtain information about the metals that occur at considerable depth are at once obvious, although
problems in interpreting this information may render this method of prospecting impractical under many
field conditions.

Animal Media

Some biogeochemical surveys of a research nature have been conducted by utilizing various animals as the
sampling media. The animals used have been fish (livers), mollusks (soft parts), and insects (whole
organisms). The results of these surveys show that these animals commonly reflect the presence of
mineralization in regions in which they occur by containing higher than normal amounts of various elements.
Dogs can locate mineral deposits by sniffing out boulders of ore occurring in the dispersion trains and fans
of sulfide deposits. They can be trained to become quite sensitive to sulfur dioxide (SO2) and other gases
21
associated with oxidizing sulfides.

3. Geophysical Prospecting and Exploration

Introduction

Geophysical prospecting combines the sciences of physics and geology to locate ore deposits. Familiar
examples of geophysical prospecting include the use of geiger counters for detecting radioactive uranium
deposits and magnetic surveys to find iron deposits. Five major geophysical methods; magnetic, gravimetric
electrical, radiometric, and seismic, are successfully utilized in mineral exploration. Some of these methods
require complex and costly instruments and highly trained operators. Others, however, are relatively simple
and inexpensive. Among these are the magnetic and radiometric methods described below.

Magnetic Methods

The magnetic method is a very popular and inexpensive approach for near-surface metal detection. The
principal of operation is quite simple. When a ferrous material is placed within the Earth's magnetic field, it
develops an induced magnetic field. The induced field is superimposed on the Earth's field at that location
creating a magnetic anomaly. Detection depends on the amount of magnetic material present and its distance
from the sensor. The anomalies are typically presented on colour contour maps.

Magnetic prospecting is based on the fact that some minerals, such as magnetite, are themselves natural
magnets. The needle in a compass held near a magnetite-rich rock behaves erratically because the earth's
magnetic field is distorted by the local magnetic field. Minerals such as ilmenite (iron-titanium oxide),
hematite (iron oxide), and pyrrhotite (iron sulfide), are weakly to moderately magnetic, and their effects can
be recorded by sensitive magnetic instruments. The common unit of measure for the strength of a magnetic
field is the gamma. The strength of the earth's magnetic field in the United States ranges from a low of about
50,000 gammas in Texas to a high of 60,000 gammas in Minnesota.

Instruments such as the magnetometer and the dip needle are used directly to detect large anomalies over
magnetic iron-ore bodies. The magnetic readings over weakly magnetic host rocks may depart from local
average or “background” values by 10 to 500 gammas, but over magnetic iron-formation the readings may
depart from background by 100 to 100,000 gammas. The magnetometer and dip needle can also be used to
22
trace concealed rock formations that have magnetic properties differing from those of adjacent formations.
For example, the prospector may know that copper is associated with an igneous rock such as quartz
monzonite. If, as is often true, the quartz monzonite is more magnetic than the surrounding rocks, the
magnetometer can be used to detect it beneath soil, talus, or other cover. Similarly, the “black sand” of
placer deposits commonly contains grains of magnetite or ilmenite which affect the magnetometer and the
dip needle. These instruments, therefore, may be used indirectly in the search for gold or ''heavy minerals”
that are in the “black sand.”

The dip needle is a pivoted magnetized needle enclosed in such a way that the case can be held vertically
and the needle can rotate in a vertical plane. For use, the needle is set in a horizontal position by adjusting a
counterweight that is attached to one arm of the needle. If no disturbing magnetic masses are present, the
needle will remain in a horizontal position, but if a magnetic mass is present, the needle will be “pulled”
away from the horizontal and thus will dip at varying angles, depending on the magnetic intensity of the
disturbing mass and the orientation of the needle with respect to the magnetic field. In general, the more
highly magnetic the rock mass, the steeper will be the angle of dip of the needle. The dip needle has been
used chiefly in prospecting for iron deposits. Magnetometers are more elaborate than dip needles and are of
several different types. Schmidt-type magnetometers, or magnetic balances, are essentially carefully
constructed dip needles or compasses, in which the angle of dip (for measuring the vertical component of the
magnetic field) or the angle of declination (for measuring the horizontal component) is calibrated so that a
reading in gamma can be obtained. Such magnetometers are used primarily to determine differences in
magnetic field rather than total field. They require tripod mounting, careful levelling, and careful handling in
operation because of their delicate construction. Under ideal conditions they can be read with an accuracy of
a few gammas.

Other types of magnetometers, generally simpler to operate than Schmidt-type magnetic balances, include
the fluxgate, torsion, and nuclear precession magnetometers. Principles and methods of operation of these
instruments are described in modern textbooks. Many commercial models of magnetometers are available in
a range of prices and the less expensive ones can be read with an accuracy of 20 to 100 gammas. Those in
the intermediate range can be read with an accuracy of 5 to 20 gammas, and the most expensive ones can be
read with an accuracy of 1 to 5 gammas.

Magnetic surveys may be conducted either along a series of lines or in a grid pattern. The spacing of stations
23
is determined by the size of the area being prospected and the type of deposit being sought. Stations spaced
10 to 20 feet apart may be required to locate small magnetic anomalies associated with weakly or
moderately magnetic rocks, but stations spaced 100 feet or more apart may suffice if strongly magnetic
rocks are suspected in a large area. Power lines, rails, automobiles, and other large metallic objects should be
avoided in any type of magnetometer survey because they create strong local magnetic fields that mask the
anomalies inherent in the rocks.

Basic Magnetic Survey Theory

If two magnetic poles of strength m1 and m2 are separated by a distance r, a force, F, exists between them. If
the poles are of the same polarity, the force will push the poles apart, and if they are of opposite polarity, the
force is attractive and will draw the poles together. The equation for F is the following:

where μ is the magnetic permeability of the medium separating the poles; m1 and m2 are pole strengths and r
the distance between them.

Magnetic Units

The magnetic flux lines between two poles per unit area, is the flux density B (and is measured in weber/m2
= Tesla). B, which is also called the “magnetic induction”, is a vector quantity. The unit of Tesla is too large
to be practical in geophysical work, so a sub-unit called a nanotesla (1 nT = 10-9 T) is used instead, where 1
nT is numerically equivalent to 1 gamma in c.g.s. units (1 nT is equivalent to 10-5 gauss).

The magnetic field can also be defined in terms of a force field which is produced by electric currents. This
magnetizing field strength H is defined, following Biot-Savart’s Law, as being the field strength at the centre
of a loop of wire of radius r through which a current I is flowing such that H = I/2r. Consequently, the units
of the magnetizing field strength H are amperes per metre (A/m). The ratio of the flux density B to the
magnetizing field strength H is a constant called the absolute magnetic permeability (μ).

The Earth’s Magnetic Field

24
The geomagnetic field at or near the surface of the Earth originates largely from within and around the
Earth’s core. The geomagnetic field can be described in terms of the declination, D, inclination, I, and the
total force vector F. The vertical component of the magnetic intensity of the Earth’s magnetic field varies
with latitude, from a minimum of around 30,000 nT at the magnetic equator to 60,000 nT at the magnetic
poles.

Magnetic Surveying

Local variations, or anomalies, in the Earth’s magnetic field are the result of disturbances caused mostly by
variations in concentrations of ferromagnetic material in the vicinity of the magnetometer’s sensor. Magnetic
data can be acquired in two configurations:
1) A rectangular grid pattern, 2) Along a traverse
Grid data consists of readings taken at the nodes of a rectangular grid; traverse data is acquired at fixed
intervals along a line. Each configuration has its advantages and disadvantages, which are dependent upon
variables such as the site conditions, size and orientation of the target, and financial resources.

In both traverse and grid configurations, the station spacing, or distance between magnetic readings, is
important. “Single-point” or erroneous anomalies are more easily recognized on surveys that utilize small
station spacing. Ground magnetic measurements are usually made with portable instruments at regular
intervals along more or less straight and parallel lines that cover the survey area. Often the interval between
measurement locations (stations) along the lines is less than the spacing between lines. It is important to
establish a local base station in an area away from suspected magnetic targets or magnetic noise and where
the local field gradient is relatively flat. The base-station memory magnetometer, when used, is set up every
day prior to the collection of the magnetic data. Ideally the base station is placed at least 100 m from any
large metal objects or travelled roads and at least 500 m from any power lines when feasible. The base
station location must be very well described in the field book, as others may have to later locate it based on
the written description.

There are certain limitations in the magnetic method. One limitation is the problem of “cultural noise” in
certain areas. Man-made structures that are constructed using ferrous material, such as steel, have a
detrimental effect on the quality of the data. Features to be avoided include steel structures, power lines,
25
metal fences, steel reinforced concrete, surface metal, pipelines and underground utilities. When these
features cannot be avoided, their locations should be noted in a field notebook and on the site map.

The incorporation of computers and non-volatile memory in magnetometers has greatly increased their ease
of use and data handling capability. The instruments typically will keep track of position; prompt for inputs,
and internally store the data for an entire day of work. Downloading the information to a personal computer
is straightforward, and plots of the day's work can be prepared each night. To make accurate anomaly maps,
temporal changes in the Earth's field during the period of the survey must be considered. Normal changes
during a day, sometimes called diurnal drift, are a few tens of nT, but changes of hundreds or thousands of
nT may occur over a few hours during magnetic storms. During severe magnetic storms, which occur
infrequently, magnetic surveys should not be made. The correction for diurnal drift can be made by
repeating measurements of a base station at frequent intervals. The measurements at field stations are then
corrected for temporal variations by assuming a linear change of the field between repeat base station
readings. Continuously recording magnetometers can also be used at fixed base sites to monitor the temporal
changes. If time is accurately recorded at both the base site and field location, the field data can be corrected
by subtraction of the variations at the base site.

Some Quality Control and Quality Assurance (QC/QA) procedures require that several field-type stations be
occupied at the start and end of each day's work. This procedure indicates that the instrument is operating
consistently. Where it is important to be able to reproduce the actual measurements on a site exactly (such as
in certain forensic matters), an additional procedure is required. The value of the magnetic field at the base
station must be asserted (usually a value close to its reading on the first day) and each day's data corrected
for the difference between the asserted value and the base value read at the beginning of the day. As the base
may vary by 10 to 25 nT or more from day to day, this correction ensures that another person using the same
base station and the same asserted value will get the same readings at a field point to within the accuracy of
the instrument. This procedure is always a good technique but is often neglected by persons interested in
only very large anomalies (> 500 nT, etc.).

Intense fields from man-made electromagnetic sources can be a problem in magnetic surveys. Most
magnetometers are designed to operate in fairly intense 60-Hz and radio frequency fields. However,
extremely low frequency fields caused by equipment using direct current or the switching of large
alternating currents can be a problem. Pipelines carrying direct current for cathodes protection can be
26
particularly troublesome. Although some modern ground magnetometers have a sensitivity of 0.1 nT,
sources of cultural and geologic noise usually prevent full use of this sensitivity in ground measurements.

The magnetometer is operated by a single person. However, grid layout, surveying, or the buddy system
may require the use of another technician. If two magnetometers are available, production is usually doubled
as the ordinary operation of the instrument itself is straightforward.

Distortion

Steel and other ferrous metals in the vicinity of a magnetometer can distort the data. Large belt buckles, etc.,
must be removed when operating the unit. A compass should be more than 3 m away from the magnetometer
when measuring the field. A final test is to immobilize the magnetometer and take readings while the
operator moves around the sensor. If the readings do not change by more than 1 or 2 nT, the operator is
"magnetically clean." Zippers, watches, eyeglass frames, boot grommets, room keys, and mechanical pencils
can all contain steel or iron. On very precise surveys, the operator effect must be held at under 1 nT.

Data recording methods will vary with the purpose of the survey and the amount of noise present. Methods
include taking three readings and averaging the results, taking three readings within a meter of the station
and either recording each or recording the average. Some magnetometers can apply either of these methods
and even do the averaging internally. An experienced field geophysicist will specify which technique is
required for a given survey. In either case, the time of the reading is also recorded unless the magnetometer
stores the readings and times internally.

Sheet-metal barns, power lines, and other potentially magnetic objects will occasionally be encountered
during a magnetic survey. When taking a magnetic reading in the vicinity of such items, describe the
interfering object and note the distance from it to the magnetic station in your field book. Items to be
recorded in the field book for magnetics include:
a) Station location, including locations of lines with respect to permanent landmarks or surveyed points;
b) Magnetic field and/or gradient reading;
c) Time;
d) Nearby sources of potential interference.

27
The experienced magnetic operator will be alert for the possible occurrence of the following:
a) Excessive gradients may be beyond the magnetometer's ability to make a stable measurement. Modern
magnetometers give a quality factor for the reading. Multiple measurements at a station, minor
adjustments of the station location and other adjustments of technique may be necessary to produce
repeatable, representative data.
b) Nearby metal objects may cause interference. Some items, such as automobiles, are obvious, but some
subtle interference will be recognized only by the imaginative and observant magnetic operator. Old
buried curbs and foundations, buried cans and bottles, power lines, fences, and other hidden factors can
greatly affect magnetic readings.

Data Reduction and Interpretation

The data should be corrected for diurnal variations, if necessary. If the diurnal does not vary more than
approximately 15 to 20 gammas over a one-hour period, correction may not be necessary. However, this
variation must be approximately linear over time and should not show any extreme fluctuations. The global
magnetic field is calculated through a previous established model (IGRF-International Geomagnetic
Reference), and obtained analytically with the help of field observations. Since the global magnetic field is
variable, these maps are generated every 5 years.

There are filters used for highlighting the contrast of anomalies; these are:
 Derivatives of different order or gradients
 Upward or downward continuation regarding the anomaly
 Band pass or high pass filters
 Pole reduction

After all corrections have been made, magnetic survey data are usually displayed as individual profiles or as
contour maps. Identification of anomalies caused by cultural features, such as railroads, pipelines, and
bridges is commonly made using field observations and maps showing such features.

Presentation of Results

The final results are presented in profile and contour map form. Profiles are usually presented in a
28
north-south orientation, although this is not mandatory. The orientation of the traverses must be indicated on
the plots. A listing of the magnetic data, including the diurnal monitor or looping data should be included in
the report. The report must also contain information pertinent to the instrumentation, field operations, and
data reduction and interpretation techniques used in the investigation.

Radiometric Methods

Naturally occurring radioactive elements such as uranium or thorium break down or decay to other elements
or isotopes by emission of subatomic particles. Gamma rays (similar to X-rays but of higher frequency),
alpha particles (nuclei of helium atoms), and beta particles (electrons) are the most common particles
emitted during this process. The portable Geiger and scintillation counters, which detect differences in the
intensity of radioactivity, have been widely and effectively used in prospecting for uranium and thorium
deposits in recent years. These instruments are sensitive to very small differences in amounts of radioactive
elements in rocks, but they do not tell what element produces the radioactivity. These distinctions can be
made by chemical analysis of a sample of the radioactive rock.

The Geiger counter is a tube filled with a gas such as helium, argon, or krypton. A high-voltage wire extends
into the central part of the tube. When gamma radiation or beta particles pass into the tube from a
radioactive source, some of the rays collide with gas molecules and produce electrically charged particles
which are then attracted to the central wire and produce electrical pulses. The electrical pulses can be
translated into dial readings of counts per minute.

Scintillometers utilize crystals of certain compounds, such as sodium iodide, which emit flashes of light
when struck by radiation. A photoelectric cell “sees” the flash of light or scintillation and electronically
counts the numbers of flashes per unit of time. This can be transmitted to a dial reading in counts per minute.
Scintillometers are more sensitive than geiger counters. In using radiation counters in the field, the most
common procedure is to walk over the terrain while listening to the counts on earphones or watching the dial
of the counter.

Radioactive deposits may produce readings that are 10 or 100 times as great as “background.” However, if
the deposits are covered by even a few tens of inches of overburden, the radiation cannot be detected. In
using a portable counter, one should be cautious about interpreting the information until it is verified by
29
adequate sampling and chemical analysis.

Gravity Method

Gravity surveying measures variations in the Earth’s gravitational field caused by differences in the density
of sub-surface rocks. Measurements of gravity provide information about densities of rocks underground.
There is a wide range in density among rock types, and therefore geologists can make inferences about the
distribution of strata. In our geothermal fields, we are attempting to map subsurface faults. Because faults
commonly juxtapose rocks of differing densities, the gravity method is an excellent exploration choice. The
equipment used for measuring the variation of the earth gravimetric field is the “gravity meter” or
gravimeter. The primary goal of studying detailed gravity data is to provide a better understanding of the
subsurface geology.

Basic Gravity Survey Theory

The basis on which the gravity method rests on is explained in two laws derived by Newton, namely his
Universal Law of Gravitation, and his Second Law of Motion;

The Universal Law of Gravitation, states that the force of attraction between two bodies of known mass is
directly proportional to the product of the two masses and inversely proportional to the square of the
distance between their centres of mass. Consequently, the greater the distance separating the centres of mass,
the smaller the force of attraction between them.

That is;

The second law is called Newton’s law of motion. This law states that a force (F) is equal to mass (m) times

30
acceleration. If the acceleration is in a vertical direction, then it is due to gravity (g). In theoretical form
Newton’s Second law of motion states that:
Force (F) = mass (m) x acceleration (g)
or
F=Mxg
The first and second equations (1 and 2) can be combined to obtain another simple relationship (3):

This shows that the magnitude of acceleration due to gravity on Earth (g) is directly proportional to the mass
(M) of the Earth and inversely proportional to the square of the Earth’s radius (R). Theoretically,
acceleration due to gravity should be constant over the Earth. In reality, gravity varies from place to place
because the earth has the shape of a flattened sphere, rotates, and has an irregular surface topography and
variable mass distribution. The normal value of g at the Earth’s surface is 980 cm/s2. In honour of Galileo,
the c.g.s. unit of acceleration due to gravity (1 cm/s2) is Gal. Modern gravity meters (gravimeters) can
measure 9 extremely small variations in acceleration due to gravity, typically 1 part in 109. The sensitivity of
modern instruments is about ten parts per million. Such small numbers have resulted in sub-units being used
such as the following:
milliGal (1 mGal = 10-3 Gal);
MicroGal (1 µGal = 10-6 Gal); and
1 gravity unit = 1g.u. = 0.1 mGal [10 gu = 1mGal]

Measurements of Gravity

There are two kinds of gravity meters. An absolute gravimeter measures the actual value of g by measuring
the speed of a falling mass using a laser beam. Although this meter achieves precisions of 0.01 to 0.001
mGal, they are expensive, heavy, and bulky. A second type of gravity meter measures relative changes in g
between two locations. Lateral density changes in the subsurface cause a change in the force of gravity at the
surface. The intensity of the force of gravity due to a buried mass difference (concentration or void) is
superimposed on the larger force of gravity due to the total mass of the earth. Thus, the two components of

31
gravity forces are measured at the Earth's surface: first, a general and relatively uniform component due to
the total earth, and second, a component of much smaller size that varies due to lateral density changes (the
gravity anomaly). By very precise measurements of gravity and by careful correction for variations in the
larger component due to the whole Earth, a gravity survey can sometimes detect natural or man-made voids,
variations in the depth to bedrock, and geologic structures of engineering interest.

The interpretation of a gravity survey is limited by ambiguity and the assumption of homogeneity. A
distribution of small masses at a shallow depth can produce the same effect as a large mass at depth.
External control of the density contrast or the specific geometry is required to resolve ambiguity questions.
This external control may be in the form of geologic plausibility, drill-hole information, or measured
densities. Gravity measurements, even at a single location, change with time due to Earth tides, meter drift,
and tares. Time variations can be dealt with by good field procedure. After elevation and position surveying,
the actual measurement of the gravity readings is often accomplished by one person in areas where solo
work is allowed.

The value of acceleration due to gravity varies over the surface of the Earth for a number or reasons, one of
which is the Earth’s shape. As the polar radius (6,357km) is 21 km shorter than the equatorial radius (6,378
km) the points at the poles are closer to the Earth’s centre of mass and, therefore, the value of gravity at the
poles is greater than that at the equator. There in another aspect, as the Earth rotates once per sidereal day
around its north-south axis, there is a centrifugal acceleration acting which is greatest where the rotational
velocity is largest, namely at the equator (1,674 km/h; 1,047 miles/h) and decrease to zero at the poles.
Gravity surveying is sensitive to variations in rock density, so an appreciation of the factors that affect
density will aid the interpretation of gravity data.

Reduction of Data

Gravimeters do not give direct measurements of gravity. Rather, a meter reading is taken which is then
multiplied by an instrumental calibration factor to produce a value of observed gravity (gobs). The correction
process is known as gravity data reduction or reduction to the geoid. The various corrections that can be
applied are the following;

Instrument Drift - Gravimeter readings change (drift) with time as a result of elastic creep in the springs,
32
producing an apparent change in gravity at a given stations. The instrumental drift can be determined simply
by repeating measurements at the same stations at different times of the day, typically every 1 – 2 hours.

Earth’s Tides - Just as the water in the oceans responds to gravitational pull of the Moon, and to a lesser
extent of the Sun, so too does the solid earth. Earth’s tides give rise to a change in gravity of up to three g.u.
within a minimum period of about 12 hours. Repeated measurements at the same stations permit estimation
of the necessary correction for tidal effects over short intervals, in addition to determination of the
instrumental drift for a gravimeter.

Observed Gravity (gobs) - Gravity readings observed at each gravity station after corrections have been
applied for instrument drift and earth tides.

Latitude Correction (gn) - Correction subtracted from gobs that accounts for Earth's elliptical shape and
rotation. The gravity value that would be observed if Earth were a perfect (no geologic or topographic
complexities), rotating ellipsoid is referred to as the normal gravity. See below for Latitude correction
equation:

Where lat = latitude

Free-air Corrected Gravity (gfa) - The free-air correction accounts for gravity variations caused by
elevation differences in the observation locations. The form of the free-air gravity anomaly, gfa , is given by:

Where h is the elevation (in m) at which the gravity station is above the datum (typically sea level).

Bouguer Slab Corrected Gravity (gb) - The Bouguer correction is a first-order correction to account for the
excess mass underlying observation points located at elevations higher than the elevation datum (sea level or
the geoid). Conversely, it accounts for a mass deficiency at observation points located below the elevation
datum. The form of the Bouguer gravity anomaly, gb, is given by:

33
Where r is the average density of the rocks underlying the survey area.

Terrain Corrected Bouguer Gravity (gt) - The terrain correction accounts for variations in the observed
gravitational acceleration caused by variations in topography near each observation point. Because of the
assumptions made during the Bouguer Slab correction, the terrain correction is positive regardless of
whether the local topography consists of a mountain or a valley. The form of the Terrain corrected, Bouguer
gravity anomaly, gt , is given by:
gt = gobs - gn + 0.3086h - 0.04193r h + TC (mGal)
Where TC is the value of the computed terrain correction.

Assuming these corrections have accurately accounted for the variations in gravitational acceleration they
were intended to account for, any remaining variations in the gravitational acceleration associated with the
terrain corrected Bouguer gravity can be assumed to be caused by a geologic structure. Once the basic
latitude, free-air, Bouguer and terrain corrections are made, an important step in the analysis remains. This
step, called regional-residual separation, is one of the most critical. In most surveys and in particular those
engineering applications in which very small anomalies are of greatest interest, there are gravity anomaly
trends of many sizes. The larger sized anomalies will tend to behave as regional variations, and the desired
smaller magnitude local anomalies will be superimposed on them.

Bouguer Anomaly

The main end-product of gravity data reduction is the Bouguer anomaly, which should correlate only with
lateral variations in density of the upper crust and which is of most interest to applied geophysicist and
geologists. The Bouguer anomaly is the difference between the observed gravity value (gobs), adjusted by the
algebraic sum of all the necessary corrections, and that of a base station (gbase). The variation of the Bouguer
anomaly should reflect the lateral variation in density such that a high-density feature in a lower-density
medium should give rise to a positive Bouguer anomaly. Conversely, a low-density feature in a
higher-density medium should result in a negative Bouguer anomaly.

34
 SECTION B - MINING GEOLOGY
GEOLOGICAL AND GEOTECHNICAL TERMS

Batter slope - The sections of rock mass between catch berm within pit walls - usually excavated to a
specific inclination/angle from the horizontal.

Bedding planes - Planes of weakness in the rock that usually occur at the interface of parallel beds or
lamina of material within the rock mass.

Buttress - A body of material either left un-mined or placed against a section of the pit wall to prevent
continued movement or propagation of wall failure.

Cable bolts - One or more steel reinforcing strands placed in a hole drilled in rock, with cement or other
grout pumped into the hole over the full length of the cable.

Catch berm - The width of lateral ground (bench) separating successive batter slopes. The purpose of the
catch berm is to both reduce the overall angle of the pit walls, and to catch any loose material or local scale
rock mass failures, thus reducing the risk of injury to the workforce at the base of the pit.

Catch fence - A fence constructed either vertically or at an angle to the vertical at the required off-set
distance from the toe of a slope. The purpose of the catch fence is to catch any loose material falling from
overlying blocky ground, thus reducing the risk to the workforce at the base of the pit walls.

Controlled drilling and blasting - The art of minimising rock damage during blasting. It requires the
accurate drilling and placement and initiation of appropriate explosive charges in the perimeter holes to
achieve efficient rock breakage with least damage to the remaining rock around an excavation.

Dip - The angle a plane or stratum is inclined from the horizontal. It is measure perpendicular to the strike.

Discontinuity - A plane of weakness in the rock mass (of comparatively low tensile strength that separates
blocks of rock from the general rock mass.

Dowel - An un-tensioned rock bolt, anchored by full column or point anchor grouting, generally with a face
plate in contact with the rock surface.

Dykes - solidified magma intrusion forming planar body cutting across the beds of adjacent rocks.

Earthquake - Groups of elastic waves propagating within the earth that cause local shaking/trembling of the
35
ground. The seismic energy radiated during earthquakes is caused most commonly by sudden fault slip,
volcanic activity or other sudden stress changes in the Earth's crust.

Elastic - The early stage of rock movement (strain) resulting from an applied stress which does not give
permanent deformation of the rock - where the rock mass returns to its original shape or state when the
applied stress is removed.

Fault - A naturally occurring plane or zone of weakness in the rock along which there has been movement.
The amount of movement can vary widely.

Fault displacement - Is the amount of horizontal component of movement of the two rock masses due to
faulting. The fault displacement can be scaled directly off plan or measured with a tape.

Fill - Waste sand or rock, uncemented or cemented in any way, used either for support, to fill stope voids
underground, or to provide a working platform or floor.

Foliation - Alignment of minerals into parallel layers; can form planes of weakness/discontinuities in rocks.

Geology - The scientific study of the Earth, the rocks of which it is composed and the changes which it has
undergone or is undergoing.

Geological structure - A general term that describes the arrangement of rock formations. Also refers to the
folds, joints, faults, foliation, schistosity, bedding planes and other planes of weakness in rock.

Geotechnical engineering - The application of engineering geology, structural geology, hydrogeology, soil
mechanics, rock mechanics and mining seismology to the practical solution of ground control challenges.

Ground control - The ability to predict and influence the behaviour of rocks in a mining environment,
having due regard for the safety of the workforce and the required serviceability and design life of the mine.

Hazard - A set of circumstances which may cause harmful consequences.

Induced stress - The stress that is due to the presence of an excavation. The level induced stress developed
depends on the level of the in-situ stress and the shape and size of the excavation.

In-situ stress - The stress or pressure that exists within the rock mass before any mining has altered the
stress field.

36
Instability - Condition resulting from failure of the intact rock material or geological structure in the rock
mass.

Joint - A naturally occurring plane of weakness or break in the rock (generally aligned sub vertical or
transverse to bedding), along which there has been no visible movement parallel to the plane.

Kinematic analysis - Considers the ability or freedom of objects to move under the forces of gravity alone,
without reference to the forces involved.

Loose (rock) - Rock that visually has potential to become detached and fall. In critical areas, loose rocks
must be scaled to make the workplace safe.

Plane of weakness - A naturally occurring crack or break in the rock mass along which movement can
occur.

Plastic - The deformation of rock under applied stress once the elastic limit is exceeded. Plastic deformation
results in a permanent change in the shape of the rock mass.

Ravelling - The gradual failure of the rock mass by rock blocks falling/sliding from pit walls usually under
the action of gravity, blast vibrations or deterioration of rock mass strength. A gradual failure process that
may go unnoticed. The term unravelling is also used to mean the same thing.

Strike: The bearing of a horizontal line in a plane or a joint.

SURFACE MINING METHODS

Four mechanical mining methods are compared here, they are: open-pit, quarrying, open-cast and augering.
Surface mining can be used when the ore is close to the earth’s surface and is an older and more productive
method than underground mining.

Open-pit mines differ depending on the nature of the material removed. Shallow mines from which gravel
and sand are extracted are generally referred to as quarries; deeper, long mines from which coal is removed
are known as strip mines. Overburden (i.e. waste) must be removed before extraction can begin. Haul roads
wind up through the mine from the bottom of the pit to the surface. Extraction occurs from benches in the pit
mining. If prospectors deem the extraction of material in an open pit to be viable, they must determine both
the pit design and the plan of operations. Pit design relies on preliminary analysis consisting of (1) an ore
body model in which the deposit is fragmented into a grid of blocks, each of which consists of a volume of
37
material and the corresponding mineral properties; (2) the value of each block, which is determined by
comparing market prices for ore with extraction and processing costs; and (3) a geometric model of the
deposit. Blocks are used as spatial reference points. Solving the pit design problem yields the final pit
boundary, i.e., the ultimate pit limit, while balancing the ore-to-waste (stripping) ratio with the cumulative
value in the pit boundaries.

Open Pit Mining

Common pit terminology is shown in Figure 1.

Figure 1. Common open pit terminology.

Open-cast (Strip) Mining

Strip mining is the term mainly applied to the mining of near surface coal seams. However, other mineral
deposits having low cohesive strengths also can be mined by this method. Most stripping involves bedded
sedimentary formations. Blasting may or may not be required depending upon the type of overburden. Coal
seams mined by stripping range from approximately one meter to 10 m or more. Thicker and multiple seam
deposits are usually mined by benching. With current equipment, overburden to coal ratios of as high as 30:1
and with depths up to 50 metres have been stripped where overburden conditions have been favourable.
High stripping ratios depend partly on mining cost and equipment efficiencies which vary widely between
operations and in most instances substantially control mineable coal and overburden stripping ratio.A feature
38
of strip mining is that where the bedded deposit is relatively flat, or gently dipping then the stripping ratio
remains relatively constant. The strip ratio is best ascertained for bedded deposits in terms of volume for
waste and tonnage for ore.

Stripping ratio = Volume of overburden removed/Tonne of ore mined

Strip mining is usually accomplished by removing the overburden and coal from a strip across one
dimension of the deposit. A parallel strip is then excavated in the opposite direction and the overburden or
waste rock is placed into the strip previously mined. The cycle is repeated.

Dragline Stripping

Stripping equipment is usually large and may discharge the waste material direct to the previously mined
strip. Draglines used for this duty have bucket capacities from 5 to 100 cubic metres. In strip mining,
maintenance of the sidewall is not as critical as for multiple bench pit operations. However, high spoil piles
do present slope failure problems. Bucket wheel and bucket chain excavators are also used for coal mining
and may use terraces.

Stripping shovels have been used in the United States of America but are not common elsewhere.

Quarrying

Quarrying is the term used to describe the surface mining of rock, such as marble, granite, limestone and
slate, that is valuable for either its mechanical or chemical properties. In this type of mining the deposit
usually is either massive, bedded or lenticular, and is suitable for bench mining. Most quarries are in
sedimentary rock (such as limestone). However, metamorphic rock (marble) and igneous (granite) rocks are
mined. There are two basic types of quarries: dimension stone and broken stone (i.e. aggregate and chemical
limestone).

Aqueous Extraction Methods

Placer Mining

Placer mining is effected by the concentration of minerals from detrital materials by selective settling in
running water. A prime requirement is that the material be in or near water and on the ground surface.

39
Panning

Panning, is a form of placer mining and traditional mining that extracts gold from a placer deposit using a
pan. Panning was used in traditional prospecting for gold mining in placer deposits where water was
plentiful and ore (e.g. gold, silver, precious stones) was concentrated in layers or pockets. Panning can only
be used when the ore or valuable mineral is heavier than the gangue (waste rock), and where production is
very limited. Useful as sampling method, panning is used for prospecting and exploration purposes in
tracing placer deposits to the vein or ore source.

Sluicing

Sluicing used in early gold production days has been replaced largely by higher production methods. Water
and a trough-shaped box (sluice box) are used to separate the ore and waste. Ground slope is necessary for
the water to carry material through the sluice box.

Hydraulic Mining

This involves larger placer deposits that generally contain gravel and boulders. Large quantities of water
under pressure are directed through pipes and nozzles (giants) which are used to disintegrate the deposit. The
system may involve a ground sluice where face material is washed through the sluice box. Alternatively, the
sand, gravel and valuable mineral is picked up by a gravel pump and pumped to a sluice or separating plant.
Handling of solids in suspension in pipes is referred to as hydraulic (pipeline) transport. Face heights may
vary from 5 to 20m though heights up to 50 m have been handled using remote controlled monitors.
Bedrock slope needs to be greater than 2 % and up to 5 % for coarse material. Production is limited by
availability of water, including adequate head (pressure), deposit thickness, boulder size and bedrock slope.

Dredging

Dredging is the underwater excavation of a placer deposit of detritus type rock material. For this approach
the deposit is usually low grade, and large in area and thickness. Dredging may be in old river beds or in
active river courses. Dredging may also be done off-shore under suitable conditions. Where the bedrock is
hard and flat, where bottom loss is minimal or where the bottom can be dredged, then recovery rates can be
40
high. Dredges used are fundamentally of two types:
(a) Bucket Ladder Dredge - This type consists of a ladder like frame to which is attached an endless chain
with buckets. Basically, it is a continuous large volume digging machine usually incorporating gravity
concentration facilities, e.g. jigs. A tail stacker allows waste discharge. The dredge is a floating plant
mounted on a barge-like pontoon.

(b) Suction Cutter Dredges - This type of dredge is basically a floating pontoon with a pump mounted on
board which excavates the material by suction and transports it to shore based or floating concentrating
plant. The suction pipe may be equipped with a cutter head for improved excavation of material. Beach
sand deposits of rutile, ilmenite, and zircon are frequently handled by this method.

The bucket ladder dredge can be used in water depths of 4 to 30 metres and has been used for tin mining to
depths of 48 metres. Suction dredges operate in pond depths of up to 9 metres; with greater depths, suction
pump lift must be assisted by jetting the intake. Problems associated with dredging are having sufficient
water in the pond to float the dredge and having sufficient clear water to beneficiate the material being dug.
Since dredging is usually on a large scale, tailings disposal and restoration of the land and water are of major
concern.

GRADE CONTROL

Grade control is to attempt to delineate the boundary between ore and waste. The distinction between ore
and waste is often based on a grade cut-off. This is the case especially in ore bodies where there is no
distinct change in geology between "ore" and "waste" making a visual delineation very difficult. Grade
Control geologist should be able to distinguish ore from waste looking at the physical properties of the
material in the field. In this circumstance a combination of local knowledge and grade estimation provides
the solution. The grade control module provides a grade estimation technique e.g. Arithmetic Mean
(weighted average grade).

Classification of Material Type in Grade Control

Material type here is referred to as ore materials. Ore materials can be classified to as high grade, medium
grade, low grade and waste. In order to delineate the boundary between ore and waste, material type
41
distinction is needed. The classification rests on the amount or quantity of the commodity (Fe, Zn, Pb, Au
etc.) in interest. For iron ore, if the amount of iron in the material ranges from 0 - 15 % Fe, it is considered to
be waste material (considering a cut-off grade of 15 percent Fe is assigned). The Fe % for low grade,
medium grade and high grade could be specific to the deposit under consideration.

However, colour codes are also assigned to these material types. Waste materials have been assigned a blue
colour, low grade is yellow, medium grade is green and high grade material is red. This colour code is
internationally accepted for all material types, irrespective of the commodity but the cut-off grade varies
from one mine to another and from one commodity to another. Pb-Zn ores may have a relatively low cut-off
grade (maybe 2 ppm to 3 ppm).

To gain the maximum cost benefits from an operation, it is necessary, when the ore grade varies, to pay
special attention to the allocation of values to the various blocks of ground. Material (ore) should be blended
(mixed) in the feed to the plant in such a way as to use as much of the lower grade material as possible. In
general, with small quarry workings, such as ballast quarries for the railways and road-metal quarries and
concrete aggregate quarries the deposit is tested and proved satisfactory at the outset of operations and then
no further grade testing is required, except size grading of product. The deposit is then mined out
completely.

However, in metal mining operations we are constrained by minimum grade limit considerations set either
by the plant, or the market, or both. The grade of material sent to the primary crusher is referred to as the
head grade. In most cases it is not in our best interest to force, or to try to obtain, as high a head grade as
possible.

Ore Blend and Blending Ratio

A blending example:
Assuming that our deposit comprises three distinct areas as shown.

42
If we send only Block A to the mill we have only 1,000,000 tonne running 65% metal. If the ore is worth $1
0/tonne; Then, the deposit is worth $10,000,000. If on the other hand, the market will take a product running
(say) 62% metal, we can blend in some of Block B to increase the tonnage and lower the grade. Adding all
of Block B would give us 2,000,000 tonne at 60% metal which is too low.
The quantity we can blend in may be determined in the following way:

Some basic rules that may apply in blending are;

1. Blocks from Section A always go to the crusher
2. Blocks from Section B may go to the crusher but must not exceed 3/7 th of the tonnage from Section
A on any one run.
3. Blocks from Section C never go to the crusher.

Under certain conditions, rules do sometimes get broken.

Determination of Block Values

There are four sources of information;

(1) Initial grid or scout drilling - This only outlines the ore deposit and the holes are often too far apart to
give accurate block values.
(2) In-fill drilling - This simply means, drilling in-between existing drill holes. This is usually carried out
on a pre-determined pattern to allow accurate block grade calculation with the minimum amount of
drilling. To be of any value, in-fill drilling should be at least a year ahead of production.
(3) Blast-hole or grade-control drilling - This will give very accurate indications as you are then down to
very close spacing between holes. The information is ideal for determining day to day digging but is

43
 usually available too late for planning purposes.
(4) Trenches in soft oxide zones - The use of cutting or ripper equipment such as ditch witch dozer or
backhoe.

For medium and longer-term mine plans, block value estimation will be done by the planning engineers. For
day to day digging you inspect the individual blast hole values and divide the shot into grade areas. The
value for the area is taken as the arithmetic average of the values in each area. Trial and experimentation,
including reconciliation from the processing plant samples, may be required to improve block estimation
methods.

How to Calculate Weighted Average Grade

The procedure to calculate the average (mean) value of a borehole intersection of an iron mineralization (see
Figure 2). The data of Figure 2 is also given in Table 1.

Figure 2. Diamond drilled borehole.

44
Table 1. Data from diamond drill bore hole of Figure 2 above.

This weighted average grades are used in the mineral resource estimation. For iron ore deposit, the resource
estimation is done in percentages but not ppm. Pb-Zn ores are reported in ppm.

WASTE DUMPS SITTING AND LAYOUT

The design of the waste dump should accommodate progressive rehabilitation to ensure a minimum area of
disturbance at any one time and to establish final rehabilitation at the earliest opportunity. Alternative uses
for part of the material, such as in landfill or road construction, may also be possible. The area of land
required for waste dumping is an important consideration as it is essential to ensure that sufficient land is
available. Adjoining leases may have to be pegged or purchased to accommodate all the material moved.
Obtaining extra ground may seem expensive however it is usually much cheaper than rehandling overburden
from on top of an ore body at a later date. The following basic objectives for waste dump design need to be
considered in the planning phase so that the parameters of final design can be considered early in the life of
45
the operation:

Where possible, waste rock should be returned to previously excavated areas. (e.g. if open-pit design permits,
or more than one open-pit is proposed). The height, area and profile of the waste rock dump to be
compatible with area of land available, the general topography of the area and the existing vegetation and
land use in the area. All completed surfaces of the waste dump should be stable and can resist long term
erosion. Previously stockpiled subsoil and topsoil should be spread on all completed surfaces where
practicable and re-vegetated with suitable vegetation. The design and construction of the waste dumps
should be such that the final out slopes do not exceed 20° from the horizontal. Drainage should be
constructed to handle heavy rainfall events.

In meeting these objectives, it is essential that consideration is given to the aesthetics of the constructed
waste dump. The long distance perspective of the shape and colour of the dump in relation to the
surrounding landscape needs to be assessed from the main access ways and viewing points of the site. At
closer range, the view of the dump area should provide the viewer with an impression that the area has been
rehabilitated to both blend with the natural land form and that the area supports a stable vegetative cover
similar to the surrounding area. These factors should be established as long term objectives and planned
from the beginning of the operation.

TAILINGS DAM SITTING AND LAYOUT

The tailings dam is referred to as Tailings Storage Facility (TSF). All new tailings dams in Western Australia
are now required to be specifically assessed and approved by the State Mining Engineer and often also
require "Works Approval" under Part V of the Environmental Protection Act. The basic environmental
objectives in any tailings disposal system are quite simple. The method of disposal should be;
 Non-polluting, while the system is operational and following de-commissioning.
 The tailings disposal structure must have long term stability from both engineering and an erosion view
point, and should be maintenance free.
 The design and construction of the final landform produced should be compatible with the surrounding
landscape.

There are many ways in which these objectives can be achieved. The vast majority, however, requires
46
conscious decisions to be made at all stages from initial site selection through design, construction and
operation. There are far too many cases of tailings disposal systems being established without adequate
consideration of the long-term objectives in the early stages, resulting in very costly rehabilitation exercise
at or after the close of operations.

Site selection is one of the most important aspects of successful tailings disposal planning. Many
environmental factors must be taken into consideration and the way in which they will interact with the
design process will depend upon the major climatic factors affecting the site. For example in arid
environments which typically have large excesses of evaporation over rainfall. Irregular storm rainfall
events can be of considerable intensity and therefore must be taken into consideration in the design and site
selection process. The designer is often faced with a decision on whether to place the tailings disposal
structure in a valley or on the interfluves. While valley sites have been traditionally used for tailings disposal
and often have a number of attractions such as being low in the landscape and requiring minimal civil works
to contain a large tailings volume, there are also a number of disadvantages which must be considered.
These include adverse geotechnical conditions resulting from alluvial valley deposits and problems caused
by the catchment area of the valley system upstream from the disposal site. In the goldfields of Western
Australia much of the portable water supplies are associated with alluvials in the typically broad, flat valley
systems. The valleys also contain more diverse ecosystems based with better quality and quantity of
vegetation and include the more productive soil types.

DRILLING
Auger Drilling

Auger drilling is a method of drilling holes by cutting or gouging with the chiselled tip of a rotating drill bit.
The drill stem is shaped like a helical (spiral) screw and is driven rotationally into the ground. The rotational
penetration of the drill bit produces drill cuttings that are lifted to the surface by the helical edges or flights
of the rotating drill stem. Auger drilling can produce boreholes quickly and efficiently, although the rate of
penetration depends on the type of formation being drilled. Water is commonly used to hydrate dry holes to
improve penetration and help lift cuttings. Auger drilling can reach depths of around 20 m depending on the
material being drilled and size of the rig and stem. Rotation is slow, rarely exceeding 30 rpm, but involves
high torque. The rods are typically 1.5 or 3 m long, and require care when handling due to their weight and
sharp flight edges.
47
Auger drilling is commonly used to take samples in soft unconsolidated ground during the reconnaissance
stage of mineral exploration, often for environmental and geotechnical sampling, but is also used in
construction and mining. Augers are available in various sizes. For reconnaissance exploration projects,
small augers mounted on trucks are often used, whereas large augers are used for construction drilling (e.g.
sinking foundation piles) and bucket augers for bulk sampling.

Percussion Rotary Air-Blast (RAB) Drilling

RAB drilling employs a blade bit, tricone rotary bit (roller bit) or downhole hammer to penetrate rock. The
drill bit, which is usually tungsten tipped, is mounted at the end of a hollow drill pipe through which
compressed air (or air mixed with foam or water) is pumped down the hole during drilling. Cuttings are
returned to the surface by upward flow under pressure between the drill pipe and the wall of the hole. RAB
drilling can achieve depths of more than 50 m but can be ineffective below the water table, where the
annulus between the drill pipe and the surrounding formation can become clogged with debris, precluding
removal of drill cuttings from the hole. This can be overcome by the use of stabilisers (also known as
reamers), which are lengths of tubular steel attached to the drill string to ream or line the hole to prevent
formation water and debris entering the hole. Stabilisers commonly have tungsten-buttons on their walls to
break down the cuttings rising from the borehole. RAB drilling is used primarily for first-pass mineral
exploration and development, where reasonable quality samples can be readily obtained. It is also used for
water bore drilling and blasthole
drilling in mines.

Most RAB drilling is carried out by light-weight rigs with relatively small compressors i.e. around 600 cubic
feet per minute (cfm) and 250 pound per square inch (psi). The rods are relatively light and have a small
diameter (typically less than 60 mm).

Air Core Drilling

Air-core drilling employs hardened steel or tungsten blades to bore a hole into unconsolidated ground. The
drill bit generally has three blades. Drill rods are hollow and are fitted with an inner tube within the outer
barrel, similar to the rods used for reverse circulation drilling (described below). Drill cuttings are recovered
48
by injection of compressed air into the annulus between the inner tube and the inside wall of the drill rod,
and are lifted to the surface by upward air low through the inner tube. Samples are then passed through a
sample hose into a cyclone where they are collected in buckets or bags.

Air-core drilling uses small compressors (around 600 cfm at 25 psi) to drill through the weathered layer of
loose soil, sand and rock fragments (regolith) covering bedrock and is unsuitable for drilling into fresh rock.
Air-core drills may achieve slightly greater depths and better sampling quality than does RAB drilling.
Rotation speeds are typically 50 to 100 rpm. For mineral exploration, air core drilling is generally preferred
over RAB drilling as it provides cleaner samples. Cuttings are removed inside the drill rods and are less
prone to contamination than those obtained by auger or un-stabilised RAB drilling, where cuttings may be
contaminated as they are brought to the surface between the outside of the drill rod and the walls of the hole.
Air-core drill rigs usually require a sizable support vehicle, normally a truck, to carry diesel, water and other
supplies needed for rig maintenance.

Reverse Circulation (RC) Drilling

Reverse Circulation drilling is similar to air-core drilling in that the drill cuttings are returned to the surface
via an inner tube inside the drill rods. However, RC rigs commonly have a much greater capacity and are
designed to handle much larger downhole equipment, with rods that are typically 6 m long and weigh about
200 kg. Penetration is achieved by a pneumatic (air filled) reciprocating piston known as a downhole
hammer (DHH), which drives a drill bit (typically 115 to 150 mm in diameter) with round protruding
tungsten-carbide buttons that can cut hard rock. RC drill rods rotate at speeds of 30 to 50 rpm. Before
commencing deeper drilling, a collar (PVC or metal piping) is installed at the surface to prevent
unconsolidated material collapsing into the hole. Collars may extend to 30 m depth, depending on the
stability of the surface formations. Where bedrock is exposed at the surface, collars may not be required.

Circulation is achieved by pumping air down the drill rods between the outer and inner tubes, with the air
returning up the inner tube and lifting cuttings to the surface. At the surface, the cuttings are directed through
a hose into a cyclone for collection and bagging. RC drilling generally produces dry cuttings, as the large
rig-mounted compressors (around 1,000 cfm at 500 psi) that produce reverse circulation also pump air ahead
of the advancing drill bit, thus drying the rock. Space is at a premium on the rig deck and a booster
compressor may be required, so compressors are usually mounted on an auxiliary vehicle. Several such
49
stand-alone compressors (each providing 900 to 1150 cfm at 300 to 350 psi may be used, in which case all
are connected to the rig by high-pressure hoses via a multi-valve manifold. Although RC drilling is
air-powered, water may be injected when collaring a new hole, to reduce dust, and to assist in lifting cuttings
to the surface. In the latter case, an additive known as super foam is mixed with water and pumped down the
rod string. This mixture makes sandy cuttings adhere to each other and increases sample recovery.

RC drill rigs are considerably larger than RAB or air-core rigs and typically reach depths of 300 to 500 m,
although it can be more. This method is slower and more expensive than RAB or air core drilling, but
achieves better penetration in harder rocks. It is less expensive than diamond coring (described below) and is
thus the preferred method for most preliminary mineral exploration work. However, due to the size and
weight of the rigs, good roads to all sites are essential. RC rigs are usually accompanied by a support vehicle
and an auxiliary vehicle, normally trucks. The support vehicle carries diesel, water and maintenance supplies;
the auxiliary vehicle usually carries an auxiliary compressor and booster compressor.

Diamond Core Drilling

Diamond core drilling differs from other drilling methods used in mineral exploration in that a solid core of
rock (generally 27 to 85 mm in diameter, but can be up to 200 mm), rather than cuttings, is extracted from
depth. The diamond drill bit comprises a short steel shank (rod) with a cutting head using natural (“surface
set”) or man-made (“impregnated”) diamonds as the cutting medium. In softer sedimentary formations (e.g.
during geotechnical investigations or coal exploration), other cutting elements may be used, such as
tungsten-carbide and polycrystalline diamond compacts (PCD). Impregnated bits using man-made diamond
compacts are preferred for hard-rock applications because they can drill a wider range of formations. Water
is the usual circulation fluid used to remove the cuttings and cool the drill bit. As the drill bit advances, a
cylindrical core of rock progressively fills a double-tube core barrel immediately above the drill bit.

Core samples are periodically recovered by lowering a cable with an overshot down the drill string,
attaching it to the top of the inner tube (inner barrel) of the core barrel, and winching it to the surface. The
inner barrel is fitted with a core lifter mechanism to prevent core from dropping out during recovery. While
the core sections are being removed from the inner tube and placed in core trays, a replacement inner tube is
lowered into the hole so that drilling can recommence. This is referred to as the wire line system.

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For drilling through coal, clay-bearing, or fractured rock, triple-tube core barrels are sometimes used to
improve the quality of the core recovered. For this method, a stainless steel split tube is fitted inside the
inner tube of the double-tube system. At the surface, the split tube can be opened to reveal the core in
virtually undisturbed condition.

Multipurpose drill rigs that can operate in either an RC or diamond core drilling role are sometimes used,
particularly in isolated locations. These rigs can commence drilling in RC mode (known as pre-collaring)
and change to diamond coring when the target depth is reached, thus overcoming the need for a second rig.
Rotation speeds during diamond core drilling can vary from 150 to 400 rpm for surface set bits, to more than
1,500 rpm using impregnated bits. Compared with RC rods, diamond drill rods are relatively light. An
unusual feature of diamond coring is that the annulus (i.e. distance between the rod and wall of the hole) is
typically only 3 to 4 mm.

Drilling depths greater than 2,500 m can be achieved with diamond core drilling. Penetration is much slower
than other drilling methods because of the hardness of the rocks usually encountered and the time involved
in retrieving core at depth. Under average conditions, the rig can produce 30 to 40 m of core per shift, with
samples having a very high integrity. The typical drilling operation comprises a truck-mounted rig, support
truck to carry items such as the rods, casing, fuel and water, and a 4-wheel drive field vehicle. Heliportable
rigs allow good samples to be collected from almost any location.

The Role of a Geologist in Exploration Drilling

1. He/she prepares the proposed drill plan

2. He/she makes the drilling budget prior the commencement of drilling
3. He/she supervises the entire drilling work
4. He/she decides when to stop drilling at a particular drill hole location
5. He/she decides the next drill hole location
6. He/she constructs drill sections
7. He/she should be able to construct a 3D-solid model of the orebody and calculate a reserve estimation
after drilling.

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The Role of a Geotechnician in a Diamond Drilling Program

1. He/she is responsible for transporting core boxes

2. He/she is responsible for writing the information on the core boxes (depth from, depth to, end of hole
etc.)
3. He/she is responsible for drill core cutting

The Information Written on the Drill Core Box

1. Hole Identification (ID) example, DD-001

2. Depth From (in meters)
3. Depth To ( in meters)
4. End of hole (maximum depth of a hole, in meters)
5. Drill hole core box number, example, Tray 1.
6. Arrow showing the orientation of the cores, see Figure 3 below.

Figure 3. Drill core box with diamond drill cores, depth from and depth to and arrow showing the orientation
of the drill cores.

Site Preparation in Exploration Drilling

The exploration area should be surveyed and hazardous ground conditions delineated (e.g. old workings) or
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addressed (e.g. previously capped drill holes). Overhead power lines or overhanging vegetation,
underground services an d other obstructions should be clearly identified. The camp is located to reduce
exposure to natural hazards. The maintenance and storage facilities and eating and ablution (washing
facilities) amenities are located to reduce exposure to drilling hazards and to ensure hygienic conditions are
maintained. Tracks to the camp and work sites should be made suitable for drilling, support and emergency
vehicle access. There should be a traffic management system for the camp and work sites, including
designated parking areas and escape routes. Safety and warning signs at the camp and work sites are should
be clear, legible and appropriately located.

Drill Pad Set-up

1. The prepared ground or constructed pad is level and stable.

2. The drill site is clear of natural hazards.
3. Sumps have been constructed to contain all drilling fluids and are barricaded to prevent inadvertent
access.
4. Walkways around the drill site are clear of obstacles.
5. Where required, edge protection is in place for the drill site.
6. The drill rig and service vehicles are positioned and set-up to minimize exposure of personnel to drilling
hazards.
7. Access routes to the drill site are clearly marked.

Supervision in Exploration Drilling

Senior drillers and supervisors are responsible for the quantity and quality of the output of others and
contribute to the development of technical solutions to non-routine problems. Supervision of drilling
operations includes managing assigned work areas and crews, communicating regularly with others,
diagnosing and solving routine and non-routine problems, controlling work programs to ensure objectives
are met, and maintaining operating records.

Senior drillers and supervisors ensure a safe workplace by:

1. confirming that workers are trained and competent for the task undertaken
2. providing clear work instructions
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3. inspecting and monitoring workplace conditions
4. continuously evaluating worker performance and correcting unsafe acts
5. reporting and rectifying hazards
6. assuring implementation of the company’s safety systems
7. demanding compliance with safety rules and procedures.

An effective supervisor spends most of their time in the workplace engaged with the workforce conducting
meaningful observations, consultation and interventions. Moving into a supervisory or team leader role
involves the application of a range of new skills. Much of the additional responsibility comes down to
managing people, and to do this successfully requires a comprehensive range of workplace communication
skills.

Hazards in Exploration Drilling

Serious and fatal injuries have resulted from entanglement or entrapment of personnel working close to
rotating and moving parts on drill rigs. People have also received eye and other injuries after being struck by
projectiles ejected during maintenance or repair work, such as bit sharpening. Contact with equipment such
as grinders and chainsaws can lead to abrasive, friction or cutting injuries.

Electrical hazards exist in almost all workplaces. Contact with electricity can be lethal or result in serious
and permanent injuries. Use of wet electrical equipment after rain is a common cause of electric shock. Fires
caused by electrical faults can injure people and damage or destroy equipment.

Other hazards maybe due to manual tasks undertaken during drilling operations include physical work such
as lifting, lowering, pushing, pulling, carrying, moving, holding or restraining anything. They also include
work with repetitive actions (e.g. hammering), sustained postures (e.g. operating plant) and concurrent
exposure to vibration (e.g. driving a truck).

Risk Controls in Exploration Drilling

1. Conduct an audit of drilling operations to ensure that all practical measures have been taken to control
risk associated with rotating and moving parts.
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2. Conduct a risk assessment to identify priorities for replacing, modifying or installing guarding.
3. Develop an action plan with due dates and responsibilities for the replacement, modification or
installation of guarding.
4. Do not rotate drill rods without guarding in place or a safe system of work (e.g. isolation protocols,
cut-out or interlock devices).
5. A competent person should regularly review drilling operations to ensure the adequacy of guarding and
systems of work.
6. Install at least two emergency stops for the drill rig and test them regularly. One should be located at the
rig control panel and at least one other at a location that is easily accessible during operation.
7. Electrical work must be undertaken by licensed electricians.
8. Electrical equipment and installations must be correctly isolated before maintenance or repair work
commences.
9. Use safety switches and residual current devices (RCDs). (An RCD, or residual current device, is a
life-saving device which is designed to prevent you from getting a fatal electric shock if you touch
something live, such as a bare wire.)
10. Use voltage reducing devices (VRDs) on welding equipment.
11. Implement electrical inspection, testing and tagging programs to ensure electrical equipment and safety
devices are maintained in good working order.
12. Electrical equipment must be rated for the expected working conditions.
13. Plan the site to allow for the appropriate placement of electrical equipment, cables and cords.

GEOLOGICAL DATABASE CONCEPTS

Geological data comes from the information we get from drill holes or exploration work. Drill hole data is
the starting point of all mining projects and constitutes the basis on which feasibility studies and ore reserve
estimations are done. A geological database consists of a number of tables (excel sheets), each of which
contains different kind of data. Each table contains a number of fields with many records or information in
the data fields. There are mainly four separate excel tables in the database. These are; COLLAR table,
SURVEY table, ASSAY table and GEOLOGY table.

The COLLAR excel sheet will have the following information: hole-id (or hole identification), x-coordinate,
y-coordinate, z-coordinate, maximum depth and hole path, see Figure 4 below. The information stored in the
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collar table describes the location of the drill hole collar, the maximum depth of the hole and whether a
linear or curved hole trace is to be calculated when retrieving the hole.

The SURVEY table stores the drill hole survey information used to calculate the drill hole trace coordinates.
Mandatory fields include: hole-id, x-coordinate, y-coordinate, z-coordinate, down hole survey depth, dip and
azimuth of the hole. For a vertical hole, which has not been surveyed, the depth would be the same as the
max_ depth field in the collar table, the dip would be -90 and the azimuth would be zero.

Figure 4. An example of a COLLAR sheet for a drill hole geological database.

The ASSAY table should contain the following: hole-id, assay sample depth-from, assay sample depth-to,
and metal or assay value. It can be 20 percent Fe for an iron ore deposit or 12 ppm for Pb-Zn ores.

The GEOLOGY table contains the following: hole-id, rock sample depth-from, rock sample depth-to, and
geologic description or rock type.

After filling all the information in the four excel sheets, the data is now stored as a geological database in the
form of .ddb (it means drillhole database). Based on this database (.ddb) file created, ore reserve estimations
are done. Computer softwares like Gemcom, Surpac, Datamine etc. can be used to estimate ore reserves.

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 OREBODY MODELING
Methodology

Generally, the first step in any modelling project is to produce a set of drill-hole sections from the drill-hole
database. Such sections are first used to check the data in the file and second to draw interpreted geological
limits for the different ore or rock types. The next step is to divide those geological zones into blocks and
have estimates of tonnage and grade(s) in each block. The shape of blocks and the estimation method vary
with the type of deposit being modelled.
In vein-type deposits recognized by drill holes on cross-sections at regular intervals, we typically do a
conventional sectional modeling with user-defined and irregular blocks around each mineralized intercept on
the sections. With interactive programs like SECTCAD, the complete section modeling of a deposit with
moderately complex geology from drill holes on about 15 sections can be completed in less than 2 days.
Vein type or tabular deposits can also be modeled with the polygon method. In this case, the blocks are the
polygons of influence automatically generated by programs like POLYCAD around mineralized intercepts.

Disseminated mineralization is often modeled with small cubic or rectangular blocks on a 3D regular grid.
Grade in each block is interpolated by distance weighting methods such as inverse distance. With programs
like those of the Surpac package, it is possible to estimate and map thousands of blocks in just a few hours.
More sophisticated distance weighting methods involve geostatistics.

Drill Hole Sectioning and 3D-Solid Modelling of Ore Body

3D-Modeling Concepts

The most common use of 3D-modeling is to define the boundary of an ore body. An important property for
any ore body model is the ability to extract horizontal slices and so give you plan view (Figure 5) outlines of
the ore body. Until now, the conventional method of doing this has been to use a sectional model - a set of
parallel vertical sections with polygonal outlines defining the outline of the ore body at each section. The
outlines are manually interpreted by a geologist using grade and structural information obtained from drill
holes, and the geologist's interpretation of other geological features in the vicinity.

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Figure 5. Drill hole plan (showing the plan view outlines of the proposed ore body to be modelled).

However, when the drill holes are displayed vertically, you will see a vertical drill hole section like in the
Figure 6 below.

Figure 6. Vertical drill hole section with the outline of the ore body at section-01 .

The higher metal values (coloured) representing ore along the vertical drill holes are demarcated by creating
a string (line joining a succession of points) forming ore envelop, see Figure 7 above. This is repeated for all
the drill hole sections (section-01, 02, 03 and section-04). Hence we will end up having four vertical drill
hole sections with corresponding ore envelops. Digitization is the process by which ore envelops are formed.

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Figure 7. Four vertical sections created after digitization to form ore envelops.

Interpolation is completed and a 3D-solid ore body model is created, see Figure 7 to 10.

Figure 8. Interpolation (joining one section string of the ore envelop to another) between ore envelops to
form a solid model of an ore body.

Figure 9. 3D-Solid ore body model.

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Figure 10. 3D-Solid ore body model having red coloration.

This can be done with the aid of computer packages like Gemcom, Surpac or Datamine. A 3D-Model
defines a solid object or a void. Typical examples are ore bodies, stopes (step-like working in a mine) and
development drives. A 3D-model is generally used to define an object in a manner such that the triangles
completely enclose the object. This allows for the extraction of closed outlines on a plane of intersection or
the calculation of volumes inside the object. Creation of the 3D-Model triangles is essentially interactive,
although this is automated as much as possible.

Block Modelling

This is just the fragmentation of the 3D-solid model of the ore body crested earlier into regular grid blocks
with values assigned to them. The Block Model is a form of spatially-referenced database that provides a
means for modelling properties of a user-defined volume by amalgamating data and objects into a common
space. Each block assumes values for each of the properties to be modelled. Information contained in the
Block Model may be retrieved as text reports or string files, or may be accessed interactively in the Graphics
module where colour coded representations of the model may be viewed and individual blocks edited.

This model is first created by setting a range model that will just fit the 3D solid model of the ore body, see
Figure 11. Hence, the ore body (with metal values in it) itself is included in the block modelling process. In
the range model, you set range coordinate values of x, y and z, that will cover the entire 3D-solid model of
the ore body to be fragmented, see Figure 12.

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Figure 11. Range block model.
After setting the range block coordinates, you now set sizes to this range block. For example, you can set
sizes of 8 m by 8 m by 8 m for x, y, z planes. This initial block sizes are called the maximum block sizes.
After this, you again set values for smaller block sizes. The smaller block sizes can be 4 m by 4 m by 4 m
for x, y, and z planes, see Figure 12.

Figure 12. Block model with both big and small block sizes within the range model.

Finally, you set a constant specific gravity to the block model for reserve estimation purpose. The model is
now ready to be created. The computer software will automatically produce a 3D-block model of the deposit.
This is shown in Figure 11. Simple techniques such as Nearest Neighbour interpolation and Direct
Assignment through to more complex techniques such as Inverse Distance and Kriging using Variogram
estimators are used here to fill the model with values.

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Filling the Model with Values

Individual blocks within the model must, at some point, have values assigned to the various attribute fields
which exist in the model. Several methods are available to aid in completing this task. These methods
include simple techniques such as Nearest Neighbour interpolation and Direct Assignment through to more
complex techniques such as Inverse Distance and Kriging using Variogram estimators. The Block Model is
used here to estimate volume, tonnes, total and average metal content of one or more elements of interest.

Sub-blocking

This is the process of successively dividing a block into smaller blocks where the dimensions of each of the
sub-blocks is half that of the parent block. Sub-blocking is an automatic feature of the Block Model and it
occurs as required to permit the model to more effectively represent the various constraints which are
applied during modelling. Note that sub-blocking will stop once the minimum block size of the model has
been reached. Sub Blocking has two types, and the type you use is dependent on the type of deposit you are
trying to model.

Standard Sub Blocking

Standard sub blocking simply divides the parent block in half in all three dimensions. This creates 8 child
blocks (always). This method of sub blocking is used widely when your deposit does not need smaller
blocks in one (or two) directions.

Variable Sub Blocking

Variable sub blocking allows you to stop sub blocking in one or two directions, while still progressing in the
other directions. For example, if you have a user block size of 8 m x 8 m x 8 m, standard sub blocking will
allow minimum block sizes of 4 m x 4 m x 4 m, 2 m x 2 m x 2 m and 1 m x 1 m x 1 m (etc.). However,
using variable sub blocking, it is possible to have minimum block sizes of 4 m x 4 m x 2 m, 4 m x 4 m x 1m,
or even 4 m x 2 m x 1 m. This method allows you to get much finer resolution in one direction, without
having to create potentially large numbers of blocks in the other two directions. This method of sub blocking
is particularly useful when modelling thin-seam deposits, as you can effectively model the "thin" direction,

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while still having fairly large blocks in the other two directions. This saves a lot of memory by creating a
smaller number of blocks, but still manages to model the resource very well.

MINERAL RESOURCES CLASSIFICATION

Mineral resource classification is the classification of mineral deposits based on their geologic certainty and
economic value. The Australasian Code for reporting of exploration results, mineral resources and ore
reserves ('the JORC Code') is a professional code of practice that sets minimum standards for public
reporting of minerals exploration results, mineral resources and ore reserves, see Figure 13 below. Public
reports prepared in accordance with the JORC Code are reports prepared for the purpose of informing
investors or potential investors and their advisors. They include, but are not limited to, annual and quarterly
company reports, press releases, information memoranda, technical papers, website postings and public
presentations of exploration results, mineral resources and ore reserves estimates.

Figure 13. JORC style standards for reporting of Mineral Resources.

Resource - A concentration of naturally occurring solid, liquid, or gaseous material in or on the Earth’s
crust in such form and amount that economic extraction of a commodity from the concentration is currently
or potentially feasible.

Original Resource - The amount of a resource before production.

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Identified Resources - Resources whose location, grade, quality, and quantity are known or estimated from
specific geologic evidence. Identified resources include economic, marginally economic, and sub-economic
components. To reflect varying degrees of geologic certainty, these economic divisions can be subdivided
into measured, indicated, and inferred.

Measured - Quantity is computed from dimensions revealed in outcrops, trenches, workings, or drill holes;
grade and/or quality are computed from the results of detailed sampling. The sites for inspection, sampling,
and measurements are spaced so closely and the geologic character is so well defined that size, shape, depth,
and mineral content of the resource are well established.

Indicated - Quantity and grade and/or quality are computed from information like that used for measured
resources, but the sites for inspection, sampling, measurement are farther apart or are otherwise less
adequately spaced. The degree of assurance, although lower than that for measured resources, is high
enough to assume continuity between points of observation.

Demonstrated - A term for the sum of measured plus indicated.

Inferred - Estimates are based on an assumed continuity beyond measured and/or indicated resources, for
which there is geologic evidence. Inferred resources may or may not be supported by samples or
measurements.

Reserves - That part of the reserve base which could be economically extracted or produced at the time of
determination. The term reserves need not signify that extraction facilities are in place and operative.
Reserves include only recoverable materials; thus, terms such as “extractable reserves” and “recoverable
reserves” are redundant and are not a part of this classification system.

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