Transcript VOLCANIC GEOLOGY

GEOLOGICAL AND GEOCHEMICAL
EXPLORATION
Dr. Ahmed Ali Madani
Associate Professor
Tel. (off.): 64324
E-mail:[email protected]
[email protected]
Overview of Exploration Geology
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Exploration geology is the process and science of
locating valuable mineral or petroleum deposits, ie,
those which have commercial value.
The term “prospecting” is almost synonymous with
the term “exploration”.
Mineral deposits of commercial value are called “ore
bodies” (compared to commercially viable deposits of
oil which are called “oil fields”).
This course will be focused largely on mineral
exploration, although many of the same techniques
are used in petroleum exploration.
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The initial signs of potentially significant
mineralization are called “prospects”.
Through the exploration process, the prospect is
investigated to acquire more and more detailed
information.
The goal is to prove the existence of an ore body (or
oil field in the case of petroleum exploration) which
can be mined (or “developed”).
The exploration process typically occurs in stages,
with early stages focusing on gathering surface data
(which is easier to acquire), and later stages focusing
on gathering subsurface data, including drilling data
and detailed geophysical survey data.
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Determining the value of an ore body (or
“deposit”) requires determining two main
features: 1) “tonnage” (or volume), and
2) “grade” (or concentration).
The volume is determined by using drill data to
outline the deposit in the subsurface, and by using a
geometric models to calculate the volume. If the ore
body is exposed at the surface, then the dimensions
of length and width can be gathered at the surface,
possibly with the aid of some trenching or blasting
methods. However, most of the volume which must
be defined is typically located at depth and requires
the use of extensive drilling or underground
excavation methods to define. The volume is difficult
to delineate because ore deposits often have
irregular shapes.
The “grade” is the average concentration
determined from numerous assays of drill
samples. The grade can vary considerably
within different parts of the same ore body.
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Development usually consists of extensive, close-spaced drilling
which outlines the geometry of the deposit in great detail. The
development stage will also conduct extensive testing, with
some preliminary metallurgical testing, to precisely determine
grade of the deposit and the “recovery” (the amount of metal
possible to extract, compared to the total amount of metal
present in the ore body). The final stage before actual mining
or extraction is called “feasibility”. During this stage, the actual
mining or extraction method is proposed, taking into
consideration all of the economic variables which effect the
bottom-line profit (commodity price, milling cost, transportation
cost, labor cost, etc...). At this stage, a decision is made
whether to mine the deposit from the surface (called “open-pit
mining”), or to mine the deposit by tunneling (called
“underground mining”).
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Mineral seldom occur at the surface and are seldom obvious.
Most often they are buried, sometimes at considerable depth.
Since they are not visible we must detect their presence
indirectly and extrapolate between points where data is known.
Many different techniques can be used to detect an ore
body. This class will discuss the more important techniques in
some detail; others are only briefly mentioned.
The most important techniques used in exploration
geology include geological field methods, geochemical
sampling methods, and geophysical methods. Exploration
conducted from the surface is far less expensive than drilling or
underground excavation, so thorough surface exploration
usually precedes either of these activities.
The Exploration Process
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Exploration for a mineral deposit is usually conducted in a
step-wise fashion which progresses through stages, each of
which moves closer to making a valuation of the ore body. Geological
reconnaissance and surface geochemical sampling prevail in
the earliest stage. Simultaneously or afterwards, geophysical
surveys are typically conducted. Following surface
exploration, the project moves into the drilling stage. Drilling
may begin with a small number of exploratory drill holes on select
targets. After this drilling stage, extensive, close-spaced drilling
(called “development drilling”) is conducted. Finally, pending good
results, “reserve drilling” is conducted, which is the type of drilling
which makes the final assessment of the deposit before actual mining
begins. Generally, some amount of drilling will continue throughout the
life of the mine, as further definition is required and new information is
obtained and used to refine the deposit model.
Exploration Methods
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If bedrock is exposed anywhere at or around a prospect, then
surface bedrock mapping is an essential beginning step for an
exploration program. This would include mapping and sampling
(field geologic methods). This work focuses on identifying
and mapping outcrops, describing mineralization and alteration,
measuring structural features (geometry), and making geologic
cross sections.
Geochemical methods involve the collection and geochemical
analysis of geological materials, including rocks, soils and
stream sediments. The results mapping and sampling may
suggest patterns indicating the direction where an ore deposit
could be present underground or at the surface. Geophysical
methods focus on measuring physical characteristics (such as
magnetism, density or conductivity) of rocks at or near the
earth’s surface. The measured values are then used to compare
with the values and models of known ore deposits.
EXPLORATION GEOLOGY TERMS
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Ore: the rock material or minerals which are mined for a profit.
Ore Minerals: the specific minerals within the ore which contain the metals to be
recovered.
Gangue Minerals: the minerals having no commercial value, they just happen to be
mixed up with the ore minerals.
Prospect: potential ore deposit, based on preliminary exploration.
Mine: Excavation for the extraction of mineral deposits, either at the surface (open pit
mine) or below (underground mine).
Orebody or Ore Deposit: naturally occurring materials from which a mineral or minerals
of economic value can be recovered at a reasonable profit.
Mineral Deposit: similar to an ore deposit, but is implied to be subeconomic or
incompletely evaluated at present.
Mineral Occurrence: anomalous concentration of minerals, but is uneconomic at present.
Grade: this means the concentration of the substance of interest, usually stated in terms
of weight per unit volume.
Cut-off Grade: the lower limit of concentration acceptable for making a profit when
mining.
Host Rock: the rock lithology (type) which contains the ore. May or may not comprise
ore.
Country Rocks: the rocks of no commercial value surrounding the host rocks and/or the
ore.
Anomalous: above or below the range of values considered to be normal.
SAMPLING AND CALCULATION
OF TONNAGE AND GRADE
Geochemical Sampling Methods
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Geochemical sampling methods are methods which involve collecting and
analyzing various types of geological materials (such as soils, stream sediments
and rocks) or certain biological materials (such as plants). Historically these
methods have been some of the most productive in of any methods used in
mineral exploration. Sometimes mineralization can be extremely subtle, if not
impossible to recognize, in hand specimen. Without the use of geochemical
sampling methods, many known ore deposits would probably not have been
discovered.
After discovery, geochemical sampling plays a key role in the delineation of
mineralization. For example, geochemical sampling of soils is often employed to
outline the general distribution of mineralization at shallow depths where
outcrops of bedrock are minimal or nonexistent. The procedure involves
collection of materials in the field, laboratory (or field) analysis of the
geochemistry of the materials, plotting of the geochemical values on maps, and
interpretation of the results. The materials may be analyzed for any number of
elements. Which elements are chosen for analysis depends on budget, the
geology of the area, and the commodity which is being sought after. Often
there are specific elements or suites of elements which are known to be
associated with specific types of mineralization. Therefore it is possible to
evaluate the potential for the existence of certain types of mineralization by
evaluating which elements are associated in a given area.
Rock Sampling
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Rock sampling reveals the true potential of an area for containing a mineral
deposit. An anomaly in a rock sample from bedrock has had no effects of
secondary dispersion, so the location of the sample is the location of the source. A
rock sample anomaly will provide much more valuable information about the
location of the mineral deposit because its source is within the mineralizing system,
ie, it helps delineate the zone of primary dispersion. However, this applies only to
rock samples collected from bedrock. Rock samples of float (rock material
suspended in colluvium with no indication of proximity to the bedrock source),
talus, glacial material, etc... give no indication of location of the source, so even if
they are highly mineralized, they are of limited value. Rubble (rock material
suspended in colluvium and due to consistency or other information suggests
proximity to the bedrock source) in some cases may be worthwhile to sample.
Several different types of rock samples are collected for mineral exploration.
Most importantly, rock samples are collected to determine the concentration of
metals, including both the major and trace metals. This type of sample is most
commonly referred to as a “geochem” sample. Trace metal values are often useful
as “pathfinders”, which means they are closely associated with the metal of
interest and may occur within a halo surrounding the mineralization of interest.
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Rock geochem samples are collected in different
manners depending on the goal of the sampling.
The principle types include:
Grab Samples: A grab sample is a sample of rock
material from a confined area (< 1 foot across).
It can be a single piece of rock.
These are the most common types of samples
collected. If it is not specified otherwise, one usually
assumes that is the sample type.
The sample usually consists of a single piece of rock,
or chunks (large piece), which are representative of
a specific type of rock or mineralization.
 Composite Samples: A composite sample consists
of small chips of uniform rock material collected over
a large area (generally > 5 feet across).
 These are the ideal “representative” samples.
 The procedure is to collect small pieces of rock over a
large area (usually at least 10 feet across) and to
make the sample as homogenous as possible.
 A composite sample might be collected to determine
the background values of trace elements in a
particular type of rock, or to determine if ore grade
mineralization is present over a large area.
 High Grade Samples: A high grade sample consists of
selective pieces of the most highly mineralized material, in
which an effort is made to exclude less mineralized material.
 Consequently, a high grade sample is generally not
representative of the overall mineralization type.
 A high grade sample might be collected to get an idea what the
best possible values are, or to provide material for certain types
of trace element analyses.
 If a such a selective sample does not return good results, then it
is unlikely that valuable mineralization is present.
 When a high grade sample is collected it is important to note
that it is a high grade sample so its values will not be
misinterpreted as representing the “average” values.
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Chip Channel Samples: A chip channel sample consists of small chips of rock
collected over a specified interval.
The objective is to obtain the most representative sample possible for the
specified sample interval.
Most of the time chip channel samples are collected in succession along a
sample line which is laid out in advance using a tape.
This provides a great deal of information about the width and other aspects of
the geometry of a mineralized zone. Often the chip channel samples are
collected along the floors or walls of trenches or adits. When chip channel
sampling along walls, sometimes a piece of canvas or plastic is laid out for the
material to fall on so as to avoid contamination and make the collection easier.
The freshest material possible is sampled, preferably chipping directly from
bedrock. Sample intervals are set at a specified width, usually ranging from 1 to
20 feet. For example, in a five foot interval, at the end of the first foot, 20 % of
the sample bag should be filled, at the end of the second foot the bag should be
filled to 40 %, etc... Due to the method of sampling, chip channel samples tend
to be rather large (up to 20 pounds for a five foot interval).
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Several other types of rock samples are sometimes
collected to help interpret the history of mineralization in
an area, to better understand the relationships between
different ore minerals, or to determine more detailed
geochemistry.
These types of samples are often collected to evaluate
the mineralization in a regional context, or to compare
the mineralization with models which might apply to a
given situation. Although they can might be costly, the
information they provide can be invaluable.
Some of these sample types include:
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Whole Rock Major Oxide Samples: Whole rock major oxide samples are
most often collected to study the whole rock geochemistry of plutonic and
volcanic rocks.
The sample must be completely fresh, unweathered, and unoxidized.
If necessary the weathered rind must be removed by chipping or by using a
rock saw.
Samples must also be unaltered by hydrothermal alteration (this adds new
components and removes others, such that it will no longer represent the parent
magma composition).
The sample is analyzed for the principle oxides, including, SiO2, Al2O3, CaO,
Fe2O3, FeO, K2O, MgO, MnO, Na2O, P2O5, TiO2. Usually at least 98 % of the
rock is made up of minerals comprised of some combination of these
components. Not uncommonly igneous rocks contain up to 1 % water. This
water is lost when the rock is oxidized in the furnace (referred to as LOI or “loss
on ignition”).
Major oxide analyses are used to classify igneous rocks based on their chemical
composition. These can be used to compare intrusions within a district or to
use in regional studies by comparing the analyses with those for known models.
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Age Date Samples: Age date samples are used to determine the age of the
rocks.
There are several methods, including 40Ar/39Ar, U/Pb, K/Ar, Rb/Sr, and Carbon
14.
They are all based on the half life theory, which states that certain isotopes of
certain elements decay to radioactive daughter products at a specific rate, called
a decay constant. Knowing the constant, the amount of parent and daughter
product material in the sample is measured and then used to calculate the age
of the rock.
The 40Ar/39Ar method can provide reliable age dates up to several hundred
million years. Argon gas forms by decay of potassium and gets locked in the
crystal lattice.
The U/Pb method is also quite reliable, and can be used to date rocks up to
billions of years old. Older rocks have longer histories, and during those longer
histories more events can occur which cause problems. For example,
metamorphism and tectonic activity. These can cause opening of the crystal
lattice of the mineral being dated, and loss of the daughter product material,
causing erroneous results. Typically these effects cause the methods to yield
ages which appear to be younger than the actual age of the rock. Minerals can
also obtain overgrowths during remelting events, causing excess parent material
to be present, also making the rock appear younger. Ar-Ar and U-Pb age dates
can be obtained can be obtained from very small amounts of material. The
procedure involves separating the grains of one mineral type to be dated. Ar-Ar
age dates are usually obtained on minerals such as mica or hornblende. U-Pb
age dates are usually obtained on zircon or other accessory minerals which are
known to contain small amounts of uranium.
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Petrographic Samples: Petrographic samples are collected to
conduct thin section petrographic analysis of the rock, which is the
identification and evaluation of the minerals comprising the rock by
using a microscope equipped with both plane and polarized light.
A thin section is made of the rock, which is a paper thin slice of the
rock mounted on a glass slide. Different minerals have different optical
properties when the plane light or polarized light is transmitted through
the thin section. Textural relationships also become apparent, which
provides information about the order of crystallization (or
paragenesis).
The proceedure is to cut a flat side and use special epoxy to glue the
piece of rock called a plug, to the glass slide. Thin a special trim saw
cuts off the part opposite the glass. Then the rock wafer is polished
with special grinders to achieve the desired thickness. The thickness
must be very precise to compare the optical properties with known
standards.
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Fluid Inclusion: Fluid inclusion samples are typically samples of
quartz (others include fluorite, sphalerite or tourmaline). The samples
are prepared similar to a thin section, and examined using a special
microscope equipped with a heating stage.
The inclusions can contain solid, liquid or gas, or any combination of
these. The inclusions are formed when they are trapped on the
surface as a new layer of the mineral crystallizes. As the mineral cools
down, the phases separate. The sample is heated gradually while
being examined under the special microscope to find the temperature
at which the gas or solid crystal in the fluid inclusion will goes back into
solution. This provides valuable information about the temperature
and pressure of formation of the ore forming fluids.
Polish Section: to look at reflected light properties of ore minerals;
ie, sulfide and oxide minerals.
Microprobe: highly sophisticated method to determine mineral
compositions and textures using electron beams.
SAMPLE TYPE
When the Assay category is chosen, the sample type must be identified using one of
the following:
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BULK Bulk - a large volume sample collected from one or more sites for assay or
metallurgical testing. It includes limited sampling or mining in initial production
stages for plant site and operations testing.
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CHIP Chip - a large number of small chips or specimens collected over a specific
area.
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CHNL Channel - a sample of all material collected from a channel of specific
dimensions across a sample site.
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DIAD Drill Core - a split or other type of drill core sample.
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GRAB Grab - a single sample normally selected to represent either high or low
grade material.
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ROCK Rock - this may be a chip, channel or grab sample which has been
analyzed by standard geochemical techniques rather than assay techniques.
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TRNC Trench - a sample taken from a trench.
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****Unknown - This may only be used when the data is important and needs to
be included but the sample type is not known.
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Exploration Project Planning
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The extensive effort, high costs, and short
field season require a great deal of planning
for an exploration project to be
successful. Details pertaining (relate) to the
logistics of transportation, field camps,
geological surveys, field equipment,
communications, and emergency procedures
are some of the more important aspects
which must be considered.
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Transportation
Many field projects in remote areas require the use of transportation by fixed-wing aircraft or helicopter,
which are the most expensive forms of transportation. Fortunately there are numerous short, but
sometimes crude, airstrips around the state, particularly in the known mining districts. There are also
many airstrips on private land, which might be used if permission can be obtained. Other areas may have
airstrips built for hunting and fishing access, but if these are maintained by private individuals, permission
should also be obtained, even if they are located on public land. The right type of airplanes equipped with
the right kind of landing gear (large tires) can utilize gravel bars along some of the major
rivers. Floatplanes can access the larger rivers as well as lakes in some areas. Rates for air travel by small
fixed-wing aircraft range up to a few hundred dollars per hour.
Helicopters are the ultimate transportation method for remote areas, but are also much more expensive,
typically ranging from $500 to $800 per hour depending on the type of helicopter used. The most
commonly used types are the Bell Jet Ranger and the Hughes 500, but several others are also available
and suitable for remote work. The Hughes 500 has a reputation for ability to land in very tight spots due
to the greater height and shorter span of the rotors. The big advantage to the use of a helicopter is the
small landing area needed, which means they can be used to mobe gear and personnel to camps in very
remote locations. The helicopter can be used to drop off geologists at the beginning of the day at
locations high on ridges, which would otherwise take many long hours of uphill hiking to access. Then the
geologist can design their daily reconnaissance traverses to cover a much larger area and obtain many
more samples.
Various types of boats can also be used for transportation in remote settings. Airboats are particularly
advantageous in shallow, inland river settings because of the minimal water depth needed.
If a project is fortunate enough to be on a road or trail system, 4-wheelers, or even 4-wheel drive vehicles,
may be used. The use of these vehicles can provide great cost savings when considering the larger area
which is made accessible.
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Field Equipment
Numerous equipment items are necessary to conduct geological field work. Table 15 – 1 is a partial list of
equipment items. Obviously each different type of work activity requires a different selection of workrelated equipment. For example, claim staking requires different equipment than geologic mapping, and
stream-sediment sampling requires different equipment than soil sampling. It is the responsibility of the
field geologist or assistant to make sure they depart for the field with everything they need to conduct in
an efficient manner the work they set out to do. It is also their responsibility to make sure they have the
items necessary to ensure their safety and comfort. This means they need to carry an adequate food and
water supply. If the logistics call for a helicopter pick up, they should plan for the possibility that weather
or mechanical problems may prevent the helicopter from picking them up when and where planned. They
may need to carry a tool such as a brush ax or saw to create a landing zone (or “LZ”) for the helicopter.
Personal Comfort & SafetyWork-RelatedGood raingear (jacket & pants)Large pack w/ good support
systemWarm hat & glovesRock hammerWork glovesSmall shovelWater supplySample bagsFood
supplyWaterproof markerGood leather boots (rocky areas)PencilGood rubber boots (wet areas)Field
notebookWarm jacketMineral I.D. kitFirst aid kitHand lensToilet paperBrunton compass &/or Silva Ranger
compassGun & ammunitionHandheld GPSBear sprayHip chain and threadCowbell &/or whistleField maps &
navigation mapsSun hatBrush ax or macheteBug dopeColored pencilsMosquito headnetPlastic garbage bag
(for wet samples)Signal mirrorFluorescent spraySunglassesTapeWaterproof matches or lighterPick
axRescue blanketHandheld radio w/ extra batteryPocket knifeExtra AA batteries for GPSParachute cordField
vestWater filterGold pan Sample tags
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Field Communications
For many field work projects, the ability to communicate in the field dramatically affects the efficiency of
the operation and the safety of the workers. Communications which are important include person-toperson (or person-to-base camp), person-to-aircraft, and camp-to-town. Equipment includes the handheld
radio, cell phones, Irridium satellite phones, regular satellite phones, and single sideband radios.
The handheld radio is the most common equipment used for person-to-person, person-to-base camp, and
person-to-aircraft communication. Some handhelds transmit for up to tens of miles, but the limiting factor
is that they only transmit and receive line-of-sight. This means one cannot communicate through
obstructions (usually topographic features) in the line-of-sight path. Better line-of-sight is often gained by
climbing to higher elevations. A handheld radio can only communicate with other handheld radios if they
are all on the same frequency. The handheld radio uses a rechargeable battery which usually only lasts a
couple days. It is important to always carry an extra radio battery.
Cell phones, due to their small size, low cost, and extended range, are becoming increasingly popular for
camp-to-town communications. Areas covered by cell phone networks are constantly becoming more
widespread as more and more repeater stations are constructed and antennae systems become more
powerful. Cell phones still require line-of-sight to the antennae or repeater, so this is a limitation in many
instances.
Satellite phones are the most desirable means of camp-to-town communications because of the
dependability, size, and the fact that they can be used in extremely remote areas. Two factors prevent
their widespread use, including 1) cost (usually $3 to $5 per minute), and 2) reception is only as good as
the satellite view (or the clearness of the path between the satellite phone and the satellites). Due to the
fact that the satellites are constantly moving, and never rise much more than about 10 degrees above the
horizon, the reception at a given location will vary greatly over the course of the day. There is also a
safety consideration, because satellite phones transmit using microwave radiation, which is harmful to
anyone in the path of the radiation. Caution is imperative !! There are several different satellite networks
orbiting the earth, including both public and military networks. The Irridium network is owned by the U.S.
military, but is also used for public communications. A Canadian company called Global Star owns another
network used by the public.
Single sideband (or SBX) radios were the standard means of camp-to-town communications prior to the
age of cell and satellite phones. SBX radios transmit low frequency radio waves (1.5 – 6.0 Mhz) which can
travel extremely long distances. This is because the radio waves bounce back and forth between the
ionosphere and the earth. However, this only applies to fair weather conditions. The advantage of using
the SBX radio is the low cost, long range, and the fact they can be used in valleys and at lower elevations
successfully. Disadvantages of the SBX are that they wave transmission is drastically affected by solar
activity, and in some situations by the presence of high power electrical transmission lines. They also
require setting up a fairly elaborate antennae system.
Classes of Ore Reserves:
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Paradoxically enough, no one can be sure how much ore there
is in a mine until it has been mined out; therefore, at best, ore
reserve figures are estimates rather than certainties. The
tonnage of ore that is exposed on all sides by workings can be
calculated with reasonable accuracy, but the tonnage that exists
beyond or below any workings can be estimated only by making
certain assumptions. It is, therefore, conventional to divide the
ore reserve into categories based on the degree of assurance of
its existence. Of several lassifications that have been proposed,
all based on the same principle, the oldest and probably the
most widely used divides the ore reserve into three classes as
follows:
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1. Positive Ore or Ore Blocked Out. Ore exposed
and sampled on four sides, i.e., by levels above and
below and by raises or winzes at the ends of the
block. This definition applies to veins; for wide ore
bodies the workings must be supplemented by
crosscuts.
2. Probable Ore : Ore exposed and sampled either
on two or no three sides.
3. Possible Ore (geologist’s ore) : Ore exposed
on only one side, its other dimensions being a matter
of reasonable projection. Some engineers use an
arbitrary extension of 50 to 100 feet. Others assume
extension for half the exposed dimension.
Although these definitions are relatively rigid, they fail to specify one important
factor - the distance between the workings that expose the ore. This factor is pertinent
because there is always a chance that somewhere within the block there may be a barren
patch, and this chance is greater as the distance between exposures is greater. Therefore,
in order that ore may be considered Proved or Blocked Out, the workings in which
sampling has been done should not be more than some specified distance apart; yet no
arbitrary standard can be set up, because different types of ore vary in their regularity and
dependability. In a spotty erratic ore body the spacing must be closer than would be
permissible in a large uniform ore body. Recognizing this, Hoover says, “In a general
way a fair rule in gold quartz veins below influence of alteration is that no point in
the block shall be over fifty feet from the points sampled. In limestone or andesite
replacements, as by gold or lead or copper, the radius must be less. In defined lead
and copper lodes, or in large lenticular bodies such as the Tennessee copper mines,
the radius may often be considerably greater, - say one hundred feet. In gold
deposits of such extraordinary regularity of values as the Witwatersrand Bankets, it
can well be two hundred or two hundred and fifty feet.”
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Although ore of an erratic nature needs to be blocked out on
four sides, as called for in the conventional definition of positive
ore, a uniform ore body whose structure is well understood
might be counted on with reasonable confidence if it were
exposed on only two sides. Hoover, therefore, proposed
categories based on more flexible definitions which allow some
leeway to the judgment of the individual:
Proved ore : Ore where there is practical no risk of failure of
continuity.
Probable Ore : Ore where there is some risk yet warrantable
justification for assumption of continuity
Prospective Ore : Ore which cannot be included as “Proved”
or “Possible”, nor definitely known or stated in any terms of
tonnage.
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Another set of terms, which allow rather wide latitude to the individual, has
been adopted by the U.S. Geological Survey and the U. S. Bureau of Mines.
Instead of Proved, Probable, and Prospective, these Bureaus use
Measured, Indicated, and Inferred, defined as follows:
Measured ore is ore for which tonnage of computed from dimensions revealed
in outcrops, trenches, workings, and drill holes, and for which the grade is
computed from the results of detailed sampling, and measurements are so
closely spaced, and the geologic character is defined so well, that the size,
shape, and mineral content are well established. The computed tonnage and
grade are judged to be accurate within limits which are stated, and no such limit
is judged to differ from the computed tonnage or grade by more than 20 per
cent.
Indicated ore is ore for which tonnage and grade are computed partly from
specific measurements, samples, or production data, and partly from projection
for a reasonable distance on geologic evidence. The sites available for
inspection, measurement, and sampling are too widely or otherwise
inappropriately spaced to outline the ore completely or to establish its grade
throughout.
Inferred ore is ore for which quantitative estimates are based largely on broad
knowledge of the geologic character of the deposit and for which there are few,
if any samples or measurements. The estimates are based on an assumed
continuity or repetition for which there is geologic evidence; this evidence may
include comparison with deposits of similar type. Bodies that are completely
concealed may be included if there is specific geologic evidence of their
presence. Estimates of inferred ore should include a statement of the special
limits within which the inferred ore may lie.
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Proven (PV): Ore reserves are stated in terms of mineable tonnes and
grades in which the identified substance has been defined using
sufficient metallurgical, mine method, geoscientific, infrastructure,
operating and capital cost data. Other applicable reserve adjectives
may include measured recoverable, diluted, mineable, ore, or in situ.
Probable (PB): Ore reserves are stated in terms of mineable tonnes
and grades where sufficient information is available about the
thickness, grade, grade distribution, mineable shape and extent of the
deposit. Continuity of mineralization should be clearly established.
Other applicable reserve adjectives may include measured geological,
drill indicated, or indicated.
Possible (PS): Ore reserves are stated in terms of mineable tonnes
and grades computed on the basis of limited geoscientific data, but
with a reasonable understanding of the distribution and correlation of
the substance in relation to this data. Other applicable reserve
adjectives may include inferred, geological, mineral inventory, or
potential.
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Measured (MG): Sufficient information is available about the thickness, grade,
distribution, mineable shape and extent of the deposit to give defined grade and
tonnage figures. Continuity of mineralization should be clearly established. Other
applicable resource adjectives may include proven, measured recoverable,
diluted, mineable, or in situ.
Indicated (IN): Tonnage and grade are computed partly from detailed
sampling procedures and partly from projection for a measurable distance,
based on geoscientific data. Sampling procedures are too widely spaced to
ensure continuity but close enough to give a reasonable indication of continuity.
Other applicable resource adjectives may include probable, measured geological,
or drill indicated.
Inferred (IF): An estimate of tonnage and grade computed from geoscientific
data or other sampling procedures, but before testing and sampling information
is sufficient to allow a more reliable and systematic estimation. Other applicable
resource adjectives may include possible, geological, mineral inventory, or
potentiaL
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OTHER: These are to be used only if the data cannot be categorized as Reserves
or Resources.
Combined (CB): This designation is used when an inventory figure is reported to
be a combination of categories (e.g.) PV + PB (Proven and Probable) reserves or
MG + IF (Measured and Inferred) resources. It can be applied to both the
Reserve and Resource categories.
Unclassified (UN): This designation indicates that the criteria for qualifying the
inventory figures are not available. The Unclassified category can be applied to
both the Reserve and Resource categories. For example, a tonnage figure is
given with grades of commodities, but the category is not stated.
Assay/Analysis (BA): Samples of one or more of the various sample types listed
below have been collected and analyzed. This category is reserved for deposits
which have no reported inventory figures. The value quoted should normally be
representative of a group of samples and is not necessarily the assay containing
the highest values. If available the sample size should be identified in the
comment field. The 'SAMPLE TYPE' must be identified when using this category.
Unknown (**): This designation indicates that not enough information is
available to determine the category
GUIDES OF ORE DEPOSITS
Geochemical Guides
Proximity to an ore body is indicated in some
instances by the presence of metallic ions in
rocks, soil or groundwater. Even though the
element in question may be present in traces
so small as to be detectable only by delicate
chemical tests, a map showing its distribution
may disclose target rings surrounding an ore
body.
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Groundwater as a Guide
Groundwater in a mineralized region, especially where
sulphides are undergoing oxidation, contains metals
and sulphates in amount ranging from traces to so
much that the water is undrinkable. If metals are
present in the water, they are likely to be absorbed
by the limonite or by the manganese dioxide
associated with it, and show up as traces on analysis.
Such metals may include Cu, Zn, Pb, Ni, Co, Mo, W,
Sb, and Bi.
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Geobotanical and Biochemical Guides
The possibility of using vegetation as a guide to ore depends;
firstly (and probably least in importance), on the suggestion
that metals and other elements may modify the appearance of
foliage; secondly, on the fact that certain elements play a role in
determining what species of plants which are able or unable to
grow in a given place; and then, on the well-established
bservation that certain plants can take up and concentrate
elements selectively from soil solutions.
Some species of plants are poisoned by certain elements in the
soil, while others, if they do not actually thrive on the same
substances, are at least able to tolerate them and thus grow
more abundance where competition is lacking.
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2 Hypogene Zoning as a Guide
All of the foregoing mineralogical variations might be regarded as aspects of
hypogene zoning, but zoning in the stricter sense – the progressive change in
mineralization along channel ways from source to surface or outward from a
central axis – is serviceable in a somewhat different way. It finds its chief
usefulness in the epithermal and the shallower of the mesothermal deposits,
where noticeable changes may take place either laterally within the limits of a
single company’s holding or in depth within the limits accessible to mining.
“At horizons above the top of the ore zone the vein fracture is often a mere slip
…. Sparse quartz starts to come in with depth, usually at a narrow stringer
along the slip. The quartz increases rapidly with depth and the top of the ore
zone lies not far below the top of the quartz. Base sulphides are sparse here …..
Base sulphides increase with depth and reach a maximum at the heart of zone.
Fragments of wall rock cemented by vein matter become abundant at this
horizon; many are completely replaced by silica and sulphides. Here the vein
attains its maximum width and this width usually continues to the lowest
explored horizon.”
STRATIGRAPHIC AND LITHOLOGIC GUIDES
If ore occurs exclusively in a given sedimentary bed, the bed
constitutes an ideal stratigraphic guide. Less perfect, but still
serviceable as a guide is a bed or group of beds which contains
most of the ore bodies even though other stratigraphic horizons
may not be entirely barren. If the containing rock is not a
sedimentary formation but an intrusive body or a volcanic flow, the
same principles are applicable so far as ore search is concerned,
but since in such cases the guide cannot properly be called
stratigraphic, the term lithologic is more appropriate. The ore may
be syngenetic (an original part of the body of rock) or it may be
epigenetic (introduced into the rock)
1 In Syngenetic Deposits
If the ore is an original part of a body of rock, the rock itself will serve as a guide;
that is the ore will be found within the particular rock formation and will be absent
outside it. The location is most precise in layered rocks especially sediments but it is
definite enough to be useful even in homogeneous igneous rocks
If the ore consists of a bed in a sedimentary formation one need only know the
stratigraphic sequence and the structure of the beds in order to predict where the
outcrop will be found or at what depth the ore will be at any given place. For
this purpose astructure contour map is the most convenient device for depicting
the shape of the ore bed and projecting its position.
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Syngenetic deposits of igneous origin are usually less regular than sedimentary
beds. However in some thick sills and lopoliths, the rock constituents have a very
regularstratiform arrangement.
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2 In Epigenetic deposits
Ore that has been introduced into rocks may show strain
partiality to certain formations whether the ore follows fractures
or replaces formations bodily. Replacement ore bodies differ
from most sedimentary (syngenetic) deposits in that not all of
the favourable stratum is ore; replacement within the bed is
often controlled by someadditional loci which may consist of fold
axes.
The rocks most receptive to gold seem to be those which
contain chloride or otherminerals of similar composition,
although chlorite in the immediate vicinity of the ore is often
altered to sericite. There are more gold deposits in chloritic
slates and phyllites and in basic to intermediate igneous rocks
than in quartzites, rhyolites or limestones.
Structural Controls on Mineralization
Nearly all hydrothermal deposits exhibit some degree of structural control on
mineralization. Structures (fractures, faults or folds) which form prior to a
mineralizing event are referred to as “pre-mineral” (Figure 10 – 6). Geologists are
keenly interested in pre-mineral structures because these structures influence the
localization of ore by hydrothermal fluids utilizing these pathways. By mapping
these structures and projecting the geometry in the subsurface, new ore deposits
may be discovered. Structures which form after a mineralizing event, and hence
may be responsible for offset or removal of mineralized zones, are referred to as
“post-mineral”. In some cases the formation of structures and mineralization appear
to be nearly synchronous (Figure 10 – 7). In these situations, shearing was
probably ongoing during the mineralization event. This is evidenced by ore minerals
localized along a fault plane which are deformed.
Fractures and fault zones provide excellent pathways for hydrothermal fluids to
circulate through. Open-space filling has long been recognized as the primary
method of vein formation. The formation of breccia and gouge due to the grinding
action of the rocks adjacent to the fault plane increases the ‘structural porosity’,
which in turn increases the permeability. Under certain conditions, breccia or gouge
may itself provide the host for mineralization. Intersections of structural features
often are better locations to prospect for mineralization, especially where the
structures are high angle. It is thought that the intersection of high angle structures
provides pathways for fluids from deep sources to move closer to the surface.
Figure 10 – 6. Fracture systems in rocks overlying an igneous
intrusion. A & B: radial fractures above a circular intrusion. C &
D: longitudinal fractures above an elliptical intrusion (from
Emmons, 1937).
Zoned Vein Deposits
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Zoned vein deposits are deposits which form along fractures and faults as open-space
fillings or replacements. They are generally polymetallic. Many have been mined for
copper, lead and zinc, although substantial gold and silver credits occur locally. These
deposits generally fall in the category of low tonnage, high grade types of deposits. There
are two broad categories: 1) vein deposits associated with porphyry base metal deposits,
and 2) vein deposits not associated with porphyry base metal deposits.
Zoned vein deposits which are associated with porphyry base metal deposits appear to form
at lower temperatures during a later mineralization event. These veins are characterized by
a strong sense of zoning from high temperature minerals in proximal (closer to the pluton)
portions of the veins, to low temperature minerals in distal (far away) portions of the
veins. Proximal portions of the veins are copper-rich and contain sulfide minerals with high
metal:sulfur ratios. Distal portions of the same veins are lead-zinc-rich and contain sulfide
minerals with lower metal:sulfur ratios. At Butte, Montana, alteration halos adjacent to the
veins change dramatically along the length of the vein and with increasing distance from
the central porphyry copper-molybdenum deposit (Figure 8 – 2). Proximal portions of the
veins are characterized by advanced argillic alteration adjacent to the vein which is
superceded outwards by sericitic alteration. Distal portions of the veins are characterized
by propylitic alteration adjacent to the vein which gives rise to fresh unaltered rock further
away from the vein. Zoned vein deposits which are not associated with porphyry base
metal deposits are characterized by having moderate, more uniform temperatures over a
larger area. Zoning in these types of vein deposits is usually a function changes in the
fugacity of sulfur along the length of the vein.
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Figure 8 – 2. Example of proximal and distal zoning of base metal
vein deposit of the type associated with porphyry copper/molybdenum
deposits.
ROCK IDENTIFICATION
To classify a rock, three things must be considered: 1) origin, 2) composition, and
3) texture.
Rock Origin
The first step to identify a rock is to try to categorize the rock into one of the three
main types or groups of rocks. These include igneous, sedimentary or metamorphic
types. The only rocks which do not fall into one of these categories are
meteorites. Igneous, sedimentary and metamorphic rock types are distinguished by
the processes which form them.
Igneous rocks: form by crystallization of a melt (molten rock material).
Subcategories:
Plutonic: formed at significant depth below the surface.
Volcanic: formed at or near the surface.
Sedimentary rocks: form by the compaction small or large grains or fragments of
pre-existing rocks, or by the precipitation of mineral matter from a body of water,
such as an ocean, lake or stream.
Metamorphic rocks: formed from pre-existing igneous, sedimentary or metamorphic
rocks by subjecting them to heat and/or pressure and/or migrating fluids, causing
the original mineral assemblage of the rock to change to a new assemblage of
minerals.
The origin is not always obvious, but sufficient training will enable recognition of
certain features which point to the most likely origin. Examples include the common
presence of bedding or layering in sedimentary rocks, and the presence of mineral
foliations or lineations in metamorphic rocks. One must also consider the geologic
environment where the rock is found. For example, in a young volcanic terrane one
is less likely to find sedimentary or metamorphic rocks. When the origin is
completely unobvious, the composition and texture must be relied upon to make the
best guess.
Rock Composition
The rock composition is found by determining which minerals make up the rock. By
definition, a rock is a solid mass or compound consisting of at least two minerals
(although there are some exceptions when a rock may consist entirely of one
mineral).
The minerals comprising the rock can be identified using common field testing
methods for individual minerals, particularly where the texture is sufficiently coarsegrained enough to distinguish the individual minerals with the naked eye or a hand
lens. Where the grain size of the minerals comprising the rock are too fine-grained
to recognize discrete minerals, “petrographic” methods (those using a microscope)
can be used for reliable identification in many cases.
Petrographic methods involve the use of a microscope to examine the optical
properties of discrete minerals magnified through the microscope lens. Properties
include the behavior of refracted, reflected and transmitted light either through a
thin wafer slice of the rock (called a thin section), or of a sample plug (for reflected
light). The light source is adjusted to provide light which polarized in one or two
directions. Different minerals have characteristic optical properties, which can be
used with tables of optical mineral properties to identify the mineral.
Rock Texture
The texture of a rock is defined by observing two criteria: 1) grain sizes, 2) grain shapes.
Grain Size: the average size of the mineral grains. The size scale used for sedimentary,
igneous and metamorphic rocks are different (Figure 1).
Grain Shape: the general shape of the mineral grains (crystal faces evident, or crystals are
rounded).
Examples of the size classifications for each of the three major rock types include:
FINE-GRAINED > > > > > > > > > > > > > > > > COARSE-GRAINED
Sedimentary:
Shale
Siltstone
Sandstone
Wacke
Conglomerate
Metamorphic:
Slate
Phyllite
Schist
Gneiss
Igneous:
Rhyolite
Granite
GEOLOGIC PRINCIPLES
One of the main goals of mineral exploration is to predict the geometry and
relationships of different rock types under the surface where they can’t be seen
either below the surface or beyond the immediate exposures. This is essential to
know in order to plan a mine. Much effort and a variety of techniques are used to
analyze the timing or “geologic history” of the area (see “Geologic Time”
below). There are three main principles, or “laws”, which are used in field geological
studies to guide in determining the relative timing of events.
Law of Cross-cutting Relationship;
Law of Cross-cutting Relationship;
The “Law of Cross-cutting Relations” is a principle which is useful
to employ in igneous provinces. It states that invading rocks are
younger than those invaded. For example, an igneous dike
invading a sedimentary or metamorphic rock. Another example is
a situation where there are multiple intrusions are found; the
sequence of igneous events can be sorted out by observing which
intrusions cut which other intrusions. The sequence might give
an indication of a particular differentiation pattern of the
magma. The same law applies to veining relationships: younger
veins cut across older vein sets (Figure 2). Often times where
there are gold-bearing quartz veins there are also other veins
which are barren, and may have a different orientation due to
different structural conditions during formation.
Figure 2. Vein crosscutting relations. Vein A is cut by
Vein B. Vein C cuts both A and B, so it is youngest.
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Law of Superposition
The “Law of Superposition” is a law which applies to
sedimentary rocks. It states that where undisturbed, layered,
sedimentary rocks occur, younger rocks will be situated on top
(above) older rocks. The same law can apply to layered
volcanic flows, where the ages of the succeeding layers going
up section will be relatively younger than the lower part of the
section. This law is also one which is employed to determine
age relationships of different rock units. In mineral exploration,
a situation where this principle could be employed would be to
project the underground geometry of a mineralized or
petroleum enriched formation.
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Figure 3. Slightly deformed sedimentary rocks (Eagle Bluff, Alaska).
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Principle of Uniformitarianism
The “Principle of Uniformitarianism” states that the earth is a
result of natural forces which are presently active and have
persisted over the course of geologic time. Rocks form most
often as a result of slow, gradual developments resulting from
various geologic processes. Catastrophic events do occur and
contribute to the overall development and history of rocks, but
these events are less frequent and contribute to only a small
percentage of the net effect of natural forces in general. This
principle has been used to study the history of ancient volcanic
rocks by observing present day volcanic activity. For example, a
certain type of massive sulfide deposit has been documented
along an active sea floor rift. This knowledge can be used to
better understand a certain type of Copper-Lead-Zinc ore
deposits, called “volcanogenic massive sulfide depsits”, or
“VMS”.
GEOLOGIC TIME
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When evaluating the ages of rocks we speak of two types of terms of ages
called “absolute age” and “relative age”. “Absolute age” is measured in years,
and depends on having some type of time scale to measure against, typically by
using a highly technical chemical dating method. “Relative age” simply means
placing one geologic event or feature in context with another in a timing
sequence.
Absolute Age During the early 1900’s, shortly after the discovery of
radioactivity, it was discovered that radioactive decay involves the
transformation of radioactive atoms into completely different elements. Each
radioactive substance disintegrates at its own rate and forms a unique set of
daughter products (elements). The rate of decay is generally very slow. For
example, uranium changes into lead at a rate such that half of the original
amount will be converted to lead after a period of 4,500 million years. Half of
the remaining uranium will convert to lead in another 4,500 million years, and so
on. Therefore the “half life” of uranium is 4,500 million years. By measuring
the ratio of unchanged uranium to lead in a sample, and knowing the rate of
decay, we can calculate the length of time the sample has been disintegrating,
or in other words, the age of the rock. Besides the Uranium-Lead method,
several other radiometric techniques are available, including Carbon 14 and
Rubidium-Strontium.
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Relative Age
Where different rocks are in physical contact and observable, the relative ages
of the rocks can often be determined evaluating superposition and cross-cutting
relationships. Rocks comprising the upper strata are younger than rocks
comprising the lower strata. Rocks formed from an intruding magma are
younger than the rocks they intrude. Inclusions within an igneous rock are older
than the magma which formed the matrix.
When different rocks are in close proximity but their actual contacts are not
visible, a geologic map and cross-section can be made which illustrate the
geometric relationships of the rocks, and allows the determination of relative
age.
Difficulty is encountered when attempting to correlate rocks which are not in
direct contact or even close proximity. Fortunately geologists have worked out
the evolutionary succession of fossil forms. It was found that sedimentary rocks
containing fossils could easily be placed in a successive sequence with respect to
time by identifying the fossil assemblages present.
The natural outgrowth of this effort was to begin comparing rocks from all parts
of the globe. Fossils could now be used to attach relative ages to a wide variety
of different sedimentary rock types. They have been used to construct what is
referred to as the “Geologic Time Scale”, which is a chronology of the earth’s
history largely based on the fossil record.
Since the oldest rocks and the oldest fossils are the ones most likely to become
obliterated due to age, we have much more fossil data available for younger
rocks, and hence these contain the smallest subdivisions of time. The Paleozoic
Era was when invertebrates and simple vertebrates (fish, amphibians and
primitive reptiles) were the dominant life forms. The Mesozoic Era was when
reptiles, including the dinosaurs, ruled. The Cenezoic Era is best characterized
as the time when mammals became dominant.
INTRODUCTION TO MAPS
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Maps are one of the most important media used to communicate information in
exploration geology.
Maps are a two dimensional representation of the surface of the earth and its
features.
Maps are a kind of shorthand language media with two main purposes: 1) to convey
detailed information about a specific area, and 2) to indicate the position of the
specific area relative to other parts of the earth.
The first objective is accomplished by recording information in graphic form, either
directly from field observation or indirectly from air photographs or a wide variety of
other sources. The second objective is accomplished by showing reference marks (or
a coordinate system), or by showing a small scale location map with well known
landmarks.
A coordinate system is nothing more than a graphical means of locating any point
on the map, with two coordinates for each point giving positions with respect to the X
axis and Y axis.
Most maps have more than just a map area – they often have lots of other
information that is given in the space around the main map area.
A complete map generally has several main components. In addition to the main
map area, a complete map will usually include the following information in various
positions adjacent to the main map area: 1) title, 2) author(s), 3) date, 4) scale, 5)
indication of true and magnetic north, and 5) coordinates or reference
points. Additionally, almost all geologic maps, as well as geophysical and
geochemical maps, contain an “explanation”. The explanation is where the code for
reading the map is provided. This may include the colors, symbols and all other
abbreviations used on the map.
INTRODUCTION TO MAPS
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Many types of maps are used in exploration geology. Topographic maps
are the most widely used maps. These depict the surface morphology by
showing lines of equal elevation (or “contour lines”). The most basic and
essential type of map used by geologists is the geologic map. A geologic
map shows rock types (or “lithologies”) and their geometry. Geologic maps
are very often constructed on a topographic base map.
Other types of maps which are used in conjunction with geologic maps
include geophysical maps and geochemical maps. Geophysical maps show
readings of magnetism, gravity, electrical conductivity, radioactivity, or other
physical properties of rocks in an area. Geochemical maps, likewise, show
geochemical values of samples collected in an area. These may be
samples of soil, rock, stream sediments or water. There may be numerous
values or readings from an area, so typically a “derivative map” will be
created from these maps which summarizes the information or otherwise
depicts the data in a fashion such that it can be more quickly
evaluated. Typically this is done by designing a map which delineates or
emphasizes the anomalous (outside normal) readings or values. One way
these derivative maps can highlight anomalous values is by contouring the
data similar to the way elevations are used to create topographic
contours. This method clusters data points with similar high values and
shows the gradient towards lower values just in the way hills and valleys
show up on a topographic map. The other method of creating a derivative
map is to create a “thematic map”. A thematic map uses colors or symbols
to “code” the values on the map.
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COORDINATE SYSTEMS
There are many, many types of coordinate systems used for maps, but relatively
few are in common usage in exploration geology. These include latitudelongitude, UTM, metes and bounds and local grids. As stated, the map is a two
dimensional representation of an irregular surface forming a portion of a sphere
of the earth (also called a geoid). Problems arise when trying to fit a flat piece
of paper onto a rounded object. The result is a flat map which contains
distortion, particularly in the corner areas. This distortion is accommodated by
using a “projection”, which is a mathematical or geometric means of minimizing
the problem.
Latitude-longitude has historically been the most frequently used coordinate
system for both navigation purposes as well as for conducting exploration
geology. In this system the coordinates consist of degrees, minutes and
seconds. The latitude, which represents the Y value, is the angular distance
north of the equator, which ranges from 0 degrees at the equator to 90 degrees
at the poles. The longitude, which represents the X value, is the angular
distance westward from the 0 degree meridian, also known as the prime
meridian.
The UTM (Universal Transverse Mercator) coordinate system is rapidly becoming
the coordinate system of choice in creating maps for exploration geology. The
major advantage to this system is that it is based on the metric system, using
meters (or kilometers) for distance units. This greatly simplifies mathematical
calculations concerning scale and distance measuring. The UTM system is
based on a series of geographic zones, each containing a rectangular grid. The
Y value of the grid system is referred to as the Northing and increases towards
the north. The X value of the grid system is referred to as the Easting and
increases towards the east.
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NAVIGATION AND GPS
Accurate navigation is essential to conducting many types of geological investigations. The primary
activities often involve sampling or data collecting on a specified grid or other location system. For detailed
sampling, past work has relied on the compass, although handheld GPS instruments have become
standard surveying equipment since about 1995 in Alaska.
“Bearing” means direction. Bearing can be noted in two main ways. The “quadrant” method indicates the
bearing in terms of the number of degrees from a cardinal direction (N, S, E or W). For example, N30E
indicates a bearing of 30 degrees east of north. The second system is called “azimuth”. The azimuth
system refers to the number of degrees around a complete 360 degree circle. For example, an azimuth of
300 indicates a bearing of 60 degrees west of due north. The azimuth system is becoming the most
common for navigation purposes during exploration activities.
Reconnaissance surveying is often employed during geochemical sampling on grids. This is accomplished
using a compass in conjunction with some type of distance measuring device. The ones most commonly
used are the hipchain and the tape. The hipchain lets out a thread, which is wound around a counting
device and allows distance measurements to be viewed. Tapes are made of a few different materials, but
are manipulated the same way, which is to lay the tape, which has marked distances, out along the length
of surface to be sampled. Hip chains are used mostly for reconnaissance work where the terrain is rough
and less precision is required. Tapes are used for detailed sampling, for example, along a trench floor.
The two main types of compasses in use today are the Brunton and the Silva Rangefinder (or
comparable). The Brunton compass is more expensive, but more accurate than the Silva. The Brunton is
calibrated to the nearest degree, while the Silva is to the nearest two degrees. The Brunton compass uses
a bubble level type inclinometer, which is more reliable than the pendelum type used in the Silva. The
compass must be set to the correct declination of the area being explored. This is given on standard one
inch equals one mile USGS topographic maps for the area. However, where magnetic anomalies exist, the
declination must be adjusted for local variations. This can be done by locating a survey line in the area
with a known bearing. For example, many section lines, especially near population centers are brushed
when they are surveyed.
GPS (global positioning system) is currently an integral part of any navigation purposes. Handheld units
have become very portable and quite reliable in many instances. GPS’s can be used in two main
ways. First, location coordinates can be pre-entered into the unit, so the unit can be used to guide the
explorationist to a pre-determined point, perhaps obtained from a map. The second way GPS’s are used in
the field is to “mark” or automatically record a waypoint while in the field, and then plot the location on a
map. GIS (geographic information system) software can then be used to plot the point on a map. Two of
the most popular GIS programs are MapInfo and Arcview.
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GEOLOGIC MAPS
Geologic maps are central to almost any geological exploration projects. First,
all previous geologic maps and data for an area needs to be sought after. Once
the previous geologic maps have been assessed, there may be need for
additional geologic mapping to be completed at a smaller scale to show more
detail. Geologic maps may be created at different scales to show different levels
of detail. For example, a reconnaissance geologic map will generally have less
detail than an underground mine map. When trench or underground mapping
requires the illustration of great detail, so must be made at a larger size.
Rocks can be exposed at the surface in three main ways. They can be present
in “outcrop”, which is a direct observation of bedrock. They can be present in
the form of “rubble”, which is loose rock having no obvious connection with
bedrock. Rubble is generally pretty consistent, and thus may frequently be used
to represent bedrock. “Float” is defined as loose rock material which has no
obvious origin. Float generally is less consistent, ie, there is more variability in
composition. The type of rock exposure observed in the field should be noted
as outcrop, rubble or float. The map should eventually document what type of
rock exposure is being used to provide the basis for the interpretation of the
geology shown on the map. Outcrop maps are more reliable to predict the
subsurface geology.
There are several different types of outcrop geologic maps commonly made at
an early stage in the exploration of a prospect or area. The decision as to which
lithologies to show is a matter of mapper’s opinion. Each lithology can be made
into a separate map unit, or lithologies can be combined into one map unit. The
amount of detail needs to fit the map scale chosen, such that it will fit within the
map units and be legible. Within each outcrop, the various contacts between
differing map units and structural features are shown.
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GEOLOGIC MAPPING METHODS
The aim of geologic mapping is to create a map which summarizes the geologic data gathered in
the field. Every place that an observation is made, a sample is gathered, or any type of data
collection takes place, it is positioned on the map at the appropriate X – Y
coordinates. Conventionally, reconnaissance geologic maps are created with true north toward the
top edge of the map. The map can be small scale and show much detail, or be large scale and
generalized. At each point, sometimes called a “station”, two essential pieces of information need
to be recorded, including the lithology and the geometry (or structure), which are defined using
color, shading, patterning, and symbology Generally the key to the graphics are shown in an
“explanation” near one edge of the map. The information shown graphically on the map is
generally also recorded in writing in a field notebook.
As each contact between lithologies is traced on the map, the type of contact needs to be
defined. The possible types of contacts including different types of sedimentary contacts,
intrusive contacts, and fault contacts. Sedimentary contacts may be either normal, which is called
a “conformable” contact, or show an erosional surface as the contact, which is called an
“unconformable” contact. Intrusive contacts are often sharp, but can be gradational over a large
zone. This could be illustrated graphically using dashed or stipple lines.
The structure data which should be recorded include the geometry of the bedding in the case of
sedimentary or volcanic rocks. It would include the foliation in the case of a metamorphic rock. In
some cases, layering within plutonic igneous rocks can also be measured. Jointing in igneous
rocks can also be an important type of structural data to collect. Where faults are present, the
surface must also be measured for its orientation. Fault traces on maps are often shown as heavy,
dashed or squiqqly lines. There may be lineations, such as streaks on fault surfaces or alignment
of elongate minerals, which can be measured if they are present at the location. These are shown
graphically as a small arrow in the direction of the lineation. As mentioned, it is important to not
only show the information graphically on the map.
The geometry of many types of planar features are shown using the “strike and dip” symbol. The
strike is the bearing of a horizontal line in the plane of the feature. It is measured with a compass
and plotted on the map. The direction of inclination of the same plane is called the “dip”, and is
measured, using an inclinometer, in a direction perpendicular to the strike. The inclination
direction is shown by the small mark on the side of the strike line, and the measurement is placed
next to it.
The methodology of determining lithology and structure for map units is the same for
reconniassance, trench or underground mapping. However, the normal convention of north at the
top edge of the map is not always the case for trench or underground maps, or any other type of
geologic map where a lot of detail is desired.
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FIELD DATA COLLECTION
Field data collection, done in conjunction with field mapping, is
frequently done in one of two ways. The first way is to record
information chronologically in a field notebook. The notebook
represents a daily log of the field activities which were
completed. Each day should begin with a header consisting of
the date. Then it is customary to summarize the general
location. Then a systematic list of stations, observations,
sample numbers, etc... should follow. The second method of
collecting field data is to use a standard data collection form
which is designed for the project. This method requires a
separate form for each station or sample location.
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Hydrothermal Alteration
Rock alteration simply means changing the mineralogy of the rock. The old
minerals grow are replaced by new ones because there has been a change in the
conditions. These changes could be changes in temperature, pressure, or chemical
conditions or any combination of these. Hydrothermal alteration is a change in the
mineralogy as a result of interaction of the rock with hot water fluids, called
“hydrothermal fluids”. The fluids carry metals in solution, either from a nearby
igneous source, or from leaching out of some nearby rocks. Hydrothermal
alteration is a common phenomena in a wide variety of geologic environments,
including fault zones and explosive volcanic features.
Hydrothermal fluids cause hydrothermal alteration of rocks by passing hot water
fluids through the rocks and changing their composition by adding or removing or
redistributing components. Temperatures can range from weakly elevated to
boiling. Fluid composition is extremely variable. They may contain various types
of gases, salts (briney fluids), water, and metals. The metals are carried as
different complexes, thought to involve sulfur and chlorine.
Sources of hydrothermal fluids are not well understood, however, there are three
main possibilities that exist. One source can be the magmatic rocks themselves,
which exsolve water (called “juvenile” water) during the final stages of cooling. In
metamorphic terranes a potential source of the fluids is dehydration reactions
which take place during the metamorphic event. With increasing temperature of
metamorphism, early, low temperature, hydrous minerals recrystallize into new,
higher temperature, anhydrous minerals. The excess water circulates through the
surrounding rocks and may scavenge and transport metals to sites where they can
be precipitated as ore minerals. Near surface groundwater is another source of
water (called “meteoric” water). Evidence from some ore deposits suggests
meteoric waters may mix with juvenile or metamorphic waters during late stages
of mineralization.
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Hydrothermal fluids in plutonic settings are thought to circulate along a large
scale convective path. It would be analogous to a pot of boiling water: hottest
water rises fastest directly above the heat source, and at the surface changes
flow direction to horizontal, and finally downwards along the sides of the pot. In
a similar manner, hydrothermal fluids circulate upward and outward from an
igneous intrusion at depth. Porous and permeable host rocks (those containing
lots of interconnected pore spaces) allow this to happen more readily, for
example, in a coarse-grained sandstone. Some types of rocks, like shale or slate,
are extremely impermeable. A layer of shale can cause damming or ponding of
the hydrothermal fluids, which can lead to a concentration of mineralization
behind the impermeable barrier. Fluid migration can be also facilitated by the
presence of lots of thin layers .
Hydrothermal fluids also circulate along fractures and faults. A which has a welldeveloped fracture system may serve as an excellent host rock. Veins form
where the fluids flow through larger, open space fractures and precipitate
mineralization along the walls of the fracture, eventually filling it
completely. Fault zones are excellent places for fluids to circulate and precipitate
mineralization. Faulting may develop breccia and gouge, which is often a good
candidate for replacement style mineralization. The form of mineralization and
alteration associated with faults is highly variable, and may include massive to
fine-grained, networks of veinlets, and occasionally vuggy textures in some
breccias.
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Alteration Zoning
Although mineral zoning patterns are not
uncommonly developed around ore deposits,
they are not always present or obvious. The
patterns can be caused by changes in
temperature, fluid chemistry or gas
content. The change in parameters over time,
such as decreasing temperature of the fluids,
can cause overprinting of lower temperature
minerals by higher temperature
minerals. Structural deformation, such as
when a rock shattering or faulting event
affects the host rocks, can cause more
complexity.
Alteration zoning can occur in many different
geometric forms, ranging from concentric
shells, to linear forms, to irregular and
complex.
Porphyry copper deposits are characterized by
concentric shell-shaped zones of alteration,
which overlap to some extent Figure 8 – 1
A. The core area contains “potassic” alteration
in the form of potassium feldspar and
biotite. Further outward is a zone of “phyllic”
alteration consisting of the assemblage quartzsericite-pyrite. The outermost zone, called
“propylitic”, is characterized by the assemblage
quartz-chlorite-carbonate and locally
containing epidote, albite or adularia.
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Epithermal deposits
associated with major
structures (faults or
fractures) have linear zones
which parallel the
structure. The mineralogy
is highly variable, as is the
geometry. One example of
alteration zoning
associated with a volcanic
vent is shown in Figure 8 –
1 B. This example indicates
an inner zone of
silicification forms within a
central breccia formation,
and an outer zone of
propylitic alteration lies
adjacent. Sericite is a
common alteration mineral
formed in zones along fault
structures or fault zones in
low to moderate
temperature settings.
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Alteration Mapping
Alteration can be mapped graphically using patterns or colors in much
the same way that lithologic units are mapped. The primary
characteristics to note are the alteration mineralogy, style and
intensity. The mineral assemblages can be coded using patterns or
colors. The style of alteration refers to the form, which could be
disseminated or massive or anything in between. Another form of
alteration is “veinlet-controlled”, which indicates that alteration is
restricted to narrow zones adjacent to veinlets. The intensity of
alteration refers to how well-developed the alteration is. It could be
incipient mineral growth due to weak development, or it may be
pervasive throughout the rock, indicating strong development.
Mapping alteration can be used to predict mineralization. In theory,
this is done by comparing the results of alteration mapping with known
alteration zoning patterns for known mineral deposits. In practice
however, the process is seldom so simple because every mineral
deposit has some uniqueness to its alteration zoning.
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Alteration Types
There are as many alteration types as there minerals. The following types are the most commonly
described types:
Propylitic: (Chlorite, Epidote, Actinolite) Propylitic alteration turns rocks green, because the new
minerals formed are green. These minerals include chlorite, actinolite and epidote. They usually form
from the decomposition of Fe-Mg-bearing minerals, such as biotite, amphibole or pyroxene, although they
can also replace feldspar. Propylitic alteration occurs at relatively low temperatures. Propylitic alteration
will generally form in a distal setting relative to other alteration types.
Sericitic: (Sericite) Sericitic alteration alters the rock to the mineral sericite, which is a very finegrained white mica. It typically forms by the decomposition of feldspars, so it replaces feldspar. In the
field, its presence in a rock can be detected by the softness of the rock, as it is easily scratchable. It also
has a rather greasy feel (when present in abundance), and its color is white, yellowish, golden brown or
greenish. Sericitic alteration implies low pH (acidic) conditions.
Alteration consisting of sericite + quartz is called “phyllic” alteration. Phyllic alteration associated with
porphyry copper deposits may contain appreciable quantities of fine-grained, disseminated pyrite which is
directly associated with the alteration event.
Potassic: (Biotite, K-feldspar, Adularia) Potassic alteration is a relatively high temperature type of
alteration which results from potassium enrichment. This style of alteration can form before complete
crystallization of a magma, as evidenced by the typically sinuous, and rather discontinuous vein
patterns. Potassic alteration can occur in deeper plutonic environments, where orthoclase will be formed,
or in shallow, volcanic environments where adularia is formed.
Albitic: (Albite) Albitic alteration forms albite, or sodic plagioclase. Its presence is usually an indication
of Na enrichment. This type of alteration is also a relatively high temperature type of alteration. The
white mica paragonite (Na-rich) is also formed sometimes.
Silicification: (Quartz) Silicification is the addition of secondary silica (SiO2). Silicification is one of the
most common types of alteration, and it occurs in many different styles. One of the most common styles is
called “silica flooding”, which results form replacement of the rock with microcrystalline quartz
(chalcedony). Greater porosity of a rock will facilitate this process. Another common style of silicification
is the formation of close-spaced fractures in a network, or “stockworks”, which are filled with quartz. Silica
flooding and/or stockworks are sometimes present in the wallrock along the margins of quartz
veins. Silicification can occur over a wide range of temperatures.
Silication: (Silicate Minerals +/- Quartz) Silication is a general term for the addition of silica by
forming any type of silicate mineral. These are commonly formed in association with quartz. Examples
include the formation of biotite or garnet or tourmaline. Silication can occur over a wide range of
temperatures. The classic example is the replacement of limestone (calcium carbonate) by silicate minerals
forming a “skarn”, which usually form at the contact of igneous intrusions.
A special subset of silication is a style of alteration called “greisenization”. This is the formation of a type
of rock called “greisen”, which is a rock containing parallel veins of quartz + muscovite + other minerals
(often tourmaline). The parallel veins are formed in the roof zone of a pluton and/or in the adjacent
country rocks (if fractures are open). With intense veining, some wallrocks can become completely
replaced by new minerals similar to the ones forming the veins.
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Carbonatization: (Carbonate Minerals) Carbonitization is a general term for the addition of any type
of carbonate mineral. The most common are calcite, ankerite, and dolomite. Carbonatization is also
usually associated with the addition of other minerals, some of which include talc, chlorite, sericite and
albite. Carbonate alteration can form zonal patterns around ore deposits with more iron-rich types
occurring proximal to the deposit.
Alunitic: (Alunite) Alunitic alteration is closely associated with certain hot springs
environments. Alunite is a potassium aluminum sulfate mineral which tends to form massive ledges in
some areas. The presence of alunite suggests high SO4 gas contents were present, which is thought to
result from the oxidation of sulfide minerals.
Argillic: (Clay Minerals) Argillic alteration is that which introduces any one of a wide variety of clay
minerals, including kaolinite, smectite and illite. Argillic alteration is generally a low temperature event,
and some may occur in atmospheric conditions. The earliest signs of argillic alteration includes the
bleaching out of feldspars.
A special subcategory of argillic alteration is “advanced argillic”. This consists of kaolinite + quartz +
hematite + limonite. feldspars leached and altered to sericite. The presence of this assemblage suggests
low pH (highly acidic) conditions. At higher temperatures, the mineral pyrophyllite (white mica) forms in
place of kaolinite.
Zeolitic: (Zeolite Minerals) Zeolitic alteration is often associated with volcanic environments, but it can
occur at considerable distances from these. In volcanic environments, the zeolite minerals replace the
glass matrix. Zeolite minerals are low temperature minerals, so they are generally formed during the
waning stages of volcanic activity, in near-surface environments.
Serpentinization and Talc Alteration: (Serpentine, Talc) Serpentinization forms serpentine, which
recognized softness, waxy, greenish appearance, and often massive habit. This type of alteration is only
common when the host rocks are mafic to ultramafic in composition. These types of rocks have relatively
higher iron and magnesium contents. Serpentine is a relatively low temperature mineral. Talc is very
similar to the mineral serpentine, but its appearance is slightly different (pale to white). Talc alteration
indicates a higher concentration of magnesium was available during crystallization.
Oxidation: (Oxide Minerals) Oxidation is simply the formation of any type of oxide mineral. The most
common ones to form are hematite and limonite (iron oxides), but many different types can form,
depending on the metals which are present. Sulfide minerals often weather easily because they are
susceptible to oxidation and replacement by iron oxides. Oxides form most easily in the surface or near
surface environment, where oxygen from the atmosphere is more readily available. The temperature
range for oxidation is variable. It can occur at surface or atmospheric conditions, or it can occur as a result
of having low to moderate fluid temperatures.
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Placer Deposits
Placer deposits, or simply “placers”, are accumulations
of valuable minerals concentrated in overburden, in
stream sediments or in beach materials by natural
processes. The minerals are freed from solid rock by
mechanical and chemical weathering, and then transported
usually by water or wind action to the final resting place. Most
of the placer deposits being mined today are Cenezoic or
younger and occur in unconsolidated materials. However, some
ancient placers, or “paleo-placers”, are found in sedimentary
rocks as old as Precambrian in age. In fact, some paleo-placers
which are eroded become the source of present day placer
deposits.
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Placer Minerals
Two types of minerals form placers: 1) minerals which are more resistant to
chemical and mechanical erosion (called “resistate” minerals), and 2) minerals
which have high specific gravities (called “heavy minerals”). There are three
categories of resistate minerals, including those which are relatively inert (nonreactive), those which are maleable (tend to bend rather than break), and those
which have greater hardness :
Inert Minerals:
Inert Oxide Minerals:
Cassiterite (tin oxide)
Chromite (chromium oxide)
Rutile (titanium oxide)
Magnetite (iron oxide)
Ilmenite (iron titanium oxide)
Inert Silicate Minerals:
Wolframite
Zircon
Maleable Minerals:
Native Metals:
Gold
Platinum
Bismuth
Hard Minerals:
Diamond
Corundum
Garnet
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Examples of heavy minerals include native metals, sulfide minerals (including
pyrite and galena), magnetite and scheelite. The high density of these minerals
enables them to be concentrated because they are less easily mobilized by
water currents. As a result, less dense mineral grains surrounding them are
“winnowed” (washed away) with ease leaving the heavy minerals to lag
behind. Winnowing also occurs as a result of wave action on beaches, and even
as a result of wind action. The minerals most likely to accumulate in placers are
those which are both resistates and heavy. This is the reason gold and
magnetite (often called “black sand”) are the most common minerals to
accumulate in placers.
Other factors influence the ability of a mineral to become concentrated in a
placer, such as the settling rate. The settling rate is a function of the grain size,
grains shape, specific gravity, surface roughness and electrostatic
charge. Larger grains, because they are heavier, settle faster than small
ones. Thin, flat grains (such as gold “flakes”) tend to catch currents and be
whisked away more easily than rounded grains (such as gold
“nuggets”). Surface roughness causes greater friction, inhibiting ease of
movement. Some mineral grains are known to carry electrostatic charges which
cause them to stick to other grains the way a balloon rubbed against your shirt
will stick to your hand.
Placer gold occurs in many shapes and sizes. Larger pieces (generally > 10
mesh) are called nuggets, and smaller, flat pieces are called flakes. “Colors” are
the tiny pieces (generally < 0.001 oz) which are found by panning or
sluicing. Placer gold is not pure, but instead is a mixture of gold and other
native metals (usually silver, copper or bismuth). The purity of placer gold is
referred to as the “fineness”, which is essentially the volume percent of gold
stated in parts gold per 1000 millileters. As gold particles travel further
downstream, the metal impurities are leached out, causing an increase in gold
fineness downstream. The texture of gold is also an indication of distance of
downstream transport. Rough, angular texture is generally considered to be an
indication of close proximity to the source.
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Placer minerals also accumulate in alluvial fan deposits, which are fan-shaped
areas of unconsolidated, unsorted stream sediments at the mouths of major
stream drainages. This is due to the sudden decrease in the stream gradient
and consequent decrease in stream velocity and turbulence. The maximum
amount of winnowing occurs in the middle portion of the fan, called the “midfan facies”, hence this is where the largest accumulations of placer minerals
occur in the fans. Ancient fan deposits which have been buried and lithified are
the source of some very rich placer gold deposits in an area of South Africa
called the Witwatersrand District.
Most of the concentration of heavy minerals occurs during flooding, when
current velocity and winnowing are at a maximum. Each time a flood occurs,
heavy minerals which were once randomly scattered within the sediments end
up resting on the new stream bed created by the scouring action of the flood
(Figure 11 – 2). Periodically, large scale floods scour the stream bed completely
down to bedrock, resulting in deposition and accumulation of the heavy minerals
on bedrock. This is why the richest pay streaks of placer gold and other heavy
minerals is usually found on or very near bedrock.
Figure 11 – 2. Profile of a stream showing how placer deposits
form by the action of floods, and how major floods cause most
placer deposits to accumulate on bedrock (after Faulkner, 1986).
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Coarse-grained gold and other heavy minerals are usually found
associated with coarse sediments, such as pebbles, cobbles, boulders
and coarse sands. The coarser sediments, due to size and weight,
drop out of suspension in the same hydrologic environments that
deposit the heavy minerals. Similarly, fine-grained heavy minerals are
usually found associated with deposits of sand or silt.
Wave action on beaches winnows away light minerals and leaves heavy
minerals to lag behind, similar to the manner in which water currents
operate in the stream environment. As each wave retreats it washes
light minerals back towards the the ocean or lake. As a result, heavy
minerals, along with larger pebbles and cobbles, lag behind. These
“lag deposits” form a linear band which is generally parallel to the
shoreline. Regional subsidence can result in these deposits becoming
submerged beneath the water, forming offshore placer
deposits. Regional uplift can result in these deposits occurring on high
benches further inland, which represent ancient shorelines.
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General principles
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 exceed the normal background level of the enclosing rocks. These zones or halos afford means
by which mineral deposits can be detected and traced; they are the geochemical anomalies being sought
by all geochemical prospectors.
The zone surrounding the core deposit is known as a primary halo or anomaly, and it represents the
distribution patterns of elements which formed as a result of primary dispersion. 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.
Abnormal chemical concentrations in weathering products are known as secondary dispersion halos or
anomalies and are more widespread. They 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 groundwater
movement are perhaps most important. Groundwaters 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 arid silt of stream sediment can be a particularly effective method for detection of mineralized
bodies within the area drained by the stream.
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Survey design
The degree of success of a geochemical survey in a mineral exploration program is often a reflection of the
amount of care taken with initial planning and survey design. This phase of activity is often referred to as
an orientation survey; its practical importance cannot be overstressed.
When a geochemical prospecting survey is contemplated, four basic considerations must be addressed: the
nature of the mineral deposits being sought; the geochemical properties of the elements likely to be
present in the target mineral deposit; geological factors likely to cause variations in geochemical
background; and environmental, or landscape, factors likely to influence the geochemical expression of the
target mineral deposit. Elucidation of these factors in an orientation survey will permit design of a
geochemical prospecting survey that is most likely to prove effective under the prevailing conditions. See
also Prospecting.
Geochemical prospecting surveys fall into two broad categories, strategic or tactical, which may be further
subdivided according to the material sampled. Strategic surveys imply coverage of a large area (generally
several thousands of square kilometers) where the primary objective is to identify districts of enhanced
mineral potential; tactical surveys comprise the more detailed follow-up to strategic reconnaissance.
Typically the area covered by a tactical survey is divided into discrete areas of high mineral potential within
the general anomalous district.
Soil and glacial till surveys have been used extensively in geochemical prospecting and have resulted in the
discovery of a number of ore bodies. Generally, such surveys are of a detailed nature and are run over a
closely spaced grid.
Biogeochemical surveys are of two types. One type utilizes the trace-element content of plants to outline
dispersion halos, trains, and fans related to mineralization; the other uses specific plants or the deleterious
effects of an excess of elements in soils on plants as indicators of mineralization. The latter type of survey
is often referred to as a geobotanical survey.
Rock geochemical surveys are reconnaissance surveys 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 analyzed for various elements. Geochemical maps are compiled from the
analyses, and contours of equal elemental values are drawn. These are then interpreted, often by using
statistical methods. Under favorable 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.
Isotopic surveys are applicable to elements which exist in two or more isotopic forms. They employ the
ratios between isotopes such as 204Pb, 206Pb, 207Pb, 208Pb, or 32S and 34S to “fingerprint” or indicate
certain types of mineral deposits which may share a common origin. Isotopic ratios may also be used to
determine the ages of minerals or given rock types and may, thus, assist in elucidating questions of ore
formation.
Geochemistry applied to hydrocarbon exploration differs from that in the search for metallic mineral
deposits; the former chiefly involves detection and study of organic substances found during drilling; the
latter, detection and study of inorganic substances at the surface. Once hydrocarbon accumulations have
been discovered, their classification into geochemical families is important. The final stages of detailed
exploration may involve complex multivariate computer-aided modeling of all available geological,
geochemical, geophysical, and hydrological data—to determine the ultimate hydrocarbon potential of a
given basin
Dispersion Halos
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Dispersion is the process of dispersing
elements outward from a source. A
dispersion halo is a zone around a
mineral deposit where the metal values
are less than those of the deposit but
significantly higher than background
values found in the country rocks
around the deposit. Geochemical
sampling and testing can be used to
outline the “dispersion halo”.
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Primary Dispersion Halos: Primary dispersion refers to
dispersion which occurs in rocks at or near the time of
formation of a mineral deposit. It is usually the result of
“hydrothermal” (hot aqueous) fluids which are responsible for
creating the deposit. Fluid movements in rocks are so variable
that the halo formed by primary dispersion may or may not
reflect the shape of the ore deposit itself. The extent of the
primary dispersion halo can range from inches to hundreds of
feet. The extent of the primary halo is dependent on very
dependent on the nature of the rock. Extremely porous or
highly fractured rocks usually develop more extensive primary
dispersion halos.
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Secondary Dispersion Halos: Secondary dispersion
refers to dispersion which occurs in the secondary
environment (soils, stream sediments or plants) long
after the formation of a mineral deposit. This type of
dispersion is usually the result of mechanical and/or
chemical weathering. Mechanical weathering is
caused primarily by breakage due to freezing and
thawing. Chemical weathering is caused by chemical
reactions between minerals and groundwater
resulting in chemical decomposition of
minerals. Chemical decomposition can also be
caused by bacterial action.
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The dominant means of chemical breakdown of
minerals in the near surface environment is
oxidation. Oxidation has dramatic effects on the
behavior of iron and sulfur, which happen to key
elements in many types of ore deposits. After
decomposition, the elements from the minerals are
released into groundwater or surface water, which
carries the elements outward. Halos caused by
secondary dispersion are usually much more
widespread than those caused by primary
dispersion. For this reason, sampling of soils, stream
sediments or plants can detect the presence of a
mineral deposit from a much further distance.
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Groundwater and surface waters migrate and transport metallic
ions away from ore deposits. Weathering, oxidation and water
migration also produce and transport iron and manganese ions,
which are paritcularly abundant in and around ore
deposits. Iron and manganese ions tend to precipitate easily
once they leave acidic water conditions around a weathered ore
deposit and come into contact with normal pH water
conditions. They precipitate as hydroxides forming solid
particles which are abundant in soils and silt size stream
sediments. These hydroxides are negatively charged, and
behave like magnets to metallic cations in solution, causing
them to be precipitated also. This process, called adsorption,
leads to small accumulations of metallic ions in soils and stream
sediments (Figure 12 – 1).
Figure 12 – 1. Dispersion of metallic ions in soils near ore body
(SME Mining & Engineering Handbook).
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Dispersion results in the transport of metallic
ions away from a source. Some of these ions
are precisely the ones sought after, and
others are called “pathfinder” metals or
elements. Pathfinder elements are those
which are closely associated with the metal of
interest. High values of pathfinder elements
may be more significant because they have
better mobility, resulting in greater
dispersion. For example, arsenic and bismuth
are good pathfinders for gold.
Stream Sediment Sampling Surveys
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Stream sediment surveys are very useful for mineral exploration because of greater dispersion in the
stream environment. Greater dispersion means greater ability to detect an ore body from a greater
distance. A drainage basin is an area with a network of streams like the branches of a tree: smaller
streams join together leading into larger and larger streams. It is assumed that the values will decrease
downstream from the source, so following the “path” of increasing values upstream. may lead to
mineralization (Figure 12 – 2).
Mechanical erosion leads to the breakdown of host rocks containing ore minerals. Consequently, tiny
grains of the minerals occur in the suspended load of the stream. Turbulence of the water keeps the
particles in suspension. Turbulence is greatest in steeper areas where the stream water flows
faster. Downstream where the topography is gentler the stream waters move slower, thereby decreasing
turbulence. This causes the suspended load to drop out, resulting in deposition of the mineral grains in
the stream sediments. Heavy minerals, like ore minerals, tend to drop out first because less turbulence is
needed to keep them in suspension.
Studies have shown that the preferred material to collect for a stream sediment sample is the –100 mesh
size fraction, which corresponds with silt size. About ½ to 1 cup of this size material is sufficient in most
cases. If gravel or organic material is mixed with the silt, then a larger sample needs to be
collected. Steep areas may lack the hydrologic conditions which allow silt and fine grained sediments to
settle, which can make sample collection very difficult. The downstream sides of large boulders are
sometimes the best place to look in these areas. Moss growing on boulders within the stream can act as a
filter, trapping finer grained sediments, and can be collected to provide samples from these more difficult
areas. The material needs to be collected from the active stream channel, not dried up side channels.
A single sample taken at the mouth of a large drainage basin may be a good way to quickly evaluate
potential of a large area, but it provides little detail of the location of a source of mineralization. By
sampling the entire stream network of an area, the location of mineralization can be narrowed down
considerably. This can be done by collecting samples at close spacings (approximately ¼-mile spacing is
common) and by sampling both sides of every stream fork. In this manner, if an anomaly occurs on one
side and not on the other, only the fork with the anomaly needs to be considered in locating the
source. The trail of anomalies forms a path upstream towards the source. Generally the values will
increase upstream towards the source and reach a maximum value in close proximity to the source, and
then drop to background values further upstream from the source.
Another type of survey which relies on collection of alluvium is the “pan concentrate” survey. In a pan
concentrate survey, coarse materials (generally pebble-sized) are collected and screened to ¼ inch or
smaller and placed in a gold pan. The screened material is then panned using a standardized method,
down to a volume size of approximately ½ cup. This will be further processed in a laboratory setting and
then analyzed. Pan concentrate samples give an indication of the types of heavy minerals present in an
area. Due to inherent inconsistencies in sample collection and panning methods, results from these
surveys are difficult to evaluate statistically. To help remedy this problem, special methods are sometimes
employed in the field which use screening and collection of specified volume of material, and minimize or
eliminate the use of actual panning of the materials (ie, concentration of heavy minerals).
Figure 12 – 2. Stream sediment anomaly pattern (SME Mining &
Engineering Handbook).
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Soil Sampling Surveys
Soils are the product of weathering of bedrock, decomposition of organic material at the surface, and
deposition of other materials which have been transported. Generally speaking the soils tend to form
certain layers called “horizons”. The lowermost horizon consists largely of decomposed bedrock and is
called the “C” horizon. The uppermost horizon, called the “A” horizon, is variable in composition. In
vegetated areas the “A” horizon consists largely of organic material. The “B” horizon is between the “A”
and “C” horizons, and is essentially a mixed zone. Dispersion is generally greatest in the “A” and “B”
horizons. For this reason, soil samples collected from the “B” horizon can detect a mineral deposit from a
greater distance. In arctic regions, the “B” horizon tends to be poorly developed (if present at all). It is
best to collect soil samples from the “C” horizon in these regions.
Soil surveys are typically situated to investigate target areas outlined by previous geophysical survey or
stream sediment surveys, or they may be positioned to cover certain structural features or rock units which
are known. Generally close spacing (< 500 feet) is needed to detect subsurface mineralization, because
large spacings may miss the target. The pattern which usually emerges is one which shows highest values
directly over the ore, and a broad area surrounding these with highly elevated values corresponding to
alteration in the host rocks adjacent to the main ore zone (Figure 12 - 3).
The strategy most often employed is to collect samples at set line or grid spacings. The tighter the
spacing, the more likely it will be to locate a soil anomaly over a buried ore deposit. A grid survey has a
big advantage over a line survey because the anomalies which are discovered may form a trend indicating
the trend of the buried mineralization. An anomaly discovered along a line survey gives no indication of
trend, and usually must be followed up with a grid survey.
Geostatistics
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Geostatistics is the use of statistics to evaluate geochemical data. Numerous samples of different types of
rocks and other materials comprising the earth’s crust have been analyzed. As a result, the average
abundance of trace elements in these materials is fairly well established. The average value for a specified
rock is called the “background” value. We are interested in values which are much greater than average or
“anomalous” because these values may indicate the presence of an ore body. A cutoff value,
or “threshold value”, is the value above which all values are considered anomalous. The threshold value
can be selected arbitrarily by simply viewing the data, or it can be selected by statistical
methods. Geologists endeavor to select which values of a data set are truly significant and therefore
worthy of follow-up geochemical sampling or other types of exploration. Most element concentrations in
geological materials follow a lognormal distribution. This is demonstrated by plotting of histograms which
show a skewed distribution of values towards either the high or low values. Plotting the log values instead
of the real values yields a typical “bell-shaped” distribution. Plotting the of geochemical values using
geostatistical methods helps define the following types of values:
Threshold Value: the value chosen above which values are considered anomalous.
Anomalous Values: any value above the selected threshold value.
Background Values: “normal” values for the given environment; majority of values are background values.
Threshold values can be selected in several different ways.
Arbitrary threshold – find the highest value, find the median value (the value at which half of the samples
have higher values and half of the samples have lower values), and select a value in between, but closer to
the highest value.
Cumulative frequency diagrams – line up values in by rank; determine class intervals; determine frequency
percent and cumulative frequency percent; plot a graph with class intervals on the X axis and
concentration on the Y axis using log probability paper. Then specify the percentile to use as the threshold
value. This often selected at the 97.5 percentile value (second standard deviation), however, lower cutoffs
may be selected to highlight a greater number of anomalous values. This method also highlights the
presence of different “populations” of values which may be related to different geologic features or rock
types.
The evaluation of results depends largely of the type of samples being studied. For stream sediment, pan
concentrate, and in some cases soil samples, the procedure is often to plot all the values on a map,
determine an arbitrary or statistical threshold and highlight the anomalous values. This will suffice to look
for general mineralization trends.
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For soil sample grids: 1) contour the data; look for trends 2)
make a thematic map which color codes the samples according
to specified class intervals; look for patterns and trends.
One method is to assign a color code system or use symbols for
specified ranges of values. This type of map is called a
“thematic” map (Figure 12 – 4). The advantage of thematic
maps is that they are simple to make and provide the reader
with a quick understanding of the distribution of anomalies in an
area. Another method is to create a “geochemical contour”
map (Figure 12 – 5). Here the values are contoured: lines of
equal value (called isopleths) are extrapolated between every
data point and the adjacent points. This type of map
accentuates possible mineralization trends but is much more
tedious to construct.
Figure 12 – 4. Thematic geochemistry map showing highest
values in red and lowest values in blue.
Figure 12 – 5. Geochemical contour map showing highest values
in red and lowest values in gray.
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Trench/Adit Mapping
Trench or adit mapping is the process of creating a geologic map, which shows the geology of the floor
and walls of the trench or adit. Adit mapping emphasizes mapping of the walls more than the floor
because the floor is often poorly exposed due to the presence of a layer of debree which results from
blasting and mucking. Trench mapping emphasizes floor mapping because: 1) the floor is usually scraped
as clean as possible with a dozer or backhoe, and 2) because floor mapping shows a “map view”. Trench
or adit mapping always involves setting up a base line using a tape. Footage or meter marks are then
painted or flagged and labeled. The base line and footage marks are then drawn to scale on the map page
to facilitate mapping. Often the same base line is used to accomplish a chip channel sampling program.
One approach is to first draw the outline of the floor, which will be oriented with respect to true north and
drawn to scale. The geology of the floor is then mapped just as an ordinary geologic map is made. The
corner of the trench or adit matches the edges of the strip showing the geology. This is the “map view”
(looking straight down) of the geology of the floor. The edges of the “strip map” represent the two bottom
corners of the trench. The walls of the trench or adit are mapped adjacent to the strip map such that the
right wall is mapped as if looking at the vertical on the right, and the left wall is mapped as if looking at
the vertical wall on the left. These can be labeled to indicate they represent the geology of the walls, even
though it is usually obvious. This gives a 3-D perspective of the geology, which greatly facilitates the
interpretation of the geometry of features. For example in determining the dip of layers, faults, joints,
etc... on the floor of the trench, it is useful to show where the feature trends as it intersects the adjacent
walls. Structural measurements can be put directly on the map, in notation form next to the appropriate
footage mark.
Another simpler approach used to make mapping more rapid is to sketch the floor outline at a standard,
average width and not worry about the exact width. The outline is drawn parallel to the edge of the map
sheet without regard to actual geographic orientation. The azimuth of the axis of the trench or adit floor is
carefully measured and noted on the map. If the trench or adit contains bends, then the new orientation
is noted at the appropriate footage mark on the map.
The alteration style can be added to one side or the other of the map if desired. The alteration can be
mapped using colors, patterns or other designators, in the same way the rock types are mapped.
Figure 13 – 1. Example of Trench 5 map oriented to
true north.
Figure 13 – 2. Example of Trench 5 map with trench
axis parallel with map page edges.