Exploration Techniques

DISCOVERING MINERAL DEPOSITS

The process of area selection depends at least in part on a knowledge of the geology and of the known mineral deposits. The first step in exploration is to gather as much information already available about the area as possible and free. Sources may include:

Data from the local geological surveys (this may include geophysical data and photos).
Core samples for previous exploration in the area.
Historical information about the area (perhaps past gold rushes, old tailings dame or old stopes?)
Models for ore deposits
A walk over by a geologist – looking for gossans, weathered ore, quartz veins etc.
Sacred sites - Ore Outcrops.
The photo shows us that the core is cut in half. This means this is likely the ore zone and the Geo's have sent it off to be assayed. Exploration companies store this away for future reference. This means that they can go back and relook at the geology or they can retest the ore zone for another mineral.

STEP ONE - AREA SELECTION, DATA GATHERING and EVALUATION (free information and narrowing of targets)

Area selection depends on the geology associated with different types of ore minerals.

Some areas are considered more likely to contain various minerals deposits than others.
- - Rock assemblages (terranes) formed along the edges of crustal plates are often high in minerals.
  - Terranes form along the edges of crustal plates during the Precambrian.
  - There are locations that have a history of mining. Deposits easily found the surface (free gold) may have be mined out already, but deeper deposits may remain hidden. Many recent mines have been found this way.
Much of this work is done before entering the field. Many governments produce regional geological and geophysical maps that outline the geology.
Exploration geologist may have access to previous drilling and mapping programs. In some cases you can go out to the hole and you will see sample piles that can easily be relogged and resampled.
Once an exploration lease is obtained the exploration/mining company must be seen to be exploring this area. They have have a certain amount of time to do so.

Data gathering first uses techniques that merely indicate the possible presence of a deposit. Modern technology has dramatically increased the size, accuracy and detail with which mining geologist can work with.

Modern technology (which will be in much more detail below) involves;
- - Remote sensing e.g. satellite imagery, conventional and infra-red aerial photography and airborne geophysical surveys.
These techniques have virtually have no environmental impacts but it also helps us get into areas that can't be reached by road etc.
When results are promising from remote sensing we move to the ground surveys which involves;
- - Geological mapping.
  - Geochemistry.
  - Geophysics.
This is done to justify bringing RC/AC rigs and possibly diamond rigs.

Evaluation of the gathered data is assessed and exploration geologist will then decide whether to proceed. Not all projects are successful - of every 1000 exploration programs only around 10 will reach the deep drilling stage and only one will reach the development stage leading to an economic ore deposit.

There are not many virgin deposits found these days, many new deposits are developed from along strike of an already known deposit.
Many re-evaluation of old drill programs are leading mining companies to mine entirely different minerals. Example of this in WA is the COSMOS nickel mine outside of Leinster. This area was first analysed for gold and the initial prospector didn't realise that he was sitting on a very large deposit of nickel.

Examples of Data Collected from Government Departments

STEP TWO - REMOTE SENSING

Remote Sensing

In the data gathering stage, remote sensing methods like satellite imagery and various types of aerial photography, are used to locate possible mineral deposit.

Electromagnetic radiation transmits images of the target area.
- - Aerial photographs are created from aircrafts sensors.
  - Spectrographic images are retrieved from satellites.
This information is publicly available but may require sophisticated computer enhancement to reveal potential exploration sites.
Photography makes use of the visible spectrum which helps map:
- - Topography (mineral rich areas are often found along metamorphic contacts zones from igneous intrusions, fold belts and fault/shear zones).
  - Drainage patterns.
  - Extinct volcanoes.
  - Major folds.
If the results of remote sensing ground surveys may be conducted.

Spectroscopy

Charlotte

Measurement of spectra produced when matter interacts with electromagnetic radiation. Today's spectrometers can acquire data in such fine details that individual absorption features can be identified and spatially mapped. This produces colour images of the landscape.

Commonly used for mapping mineral distribution. Its used for lithology malling (?) and for the exploration for gold, base metals, oil, gas, and geothermal energy.

Satellite Imagery

Charlotte

Provides broad overview of an area for comparatively little cost. However heavily vegetates or tropical areas make it hard to see rocks, so arid or sparsely vegetated areas provide more information.

Different types of images suit different exploration needs. some include; infrared images, colour enhanced images and reverse band images that highlight regolith/rock outcrop boundaries.

STEP THREE - GEOPHYSICAL SURVEYS

Depending on the resources being explored for geophysical surveys may be a next step. These airborne surveys can give further detail for an area. Is used in mineral and energy exploration.

Gravity Survey （付成亮）

Description: Gravity meters detect differences in the acceleration (pull) of gravity of different materials within the Earth. With the difference between the masses and density, different materials can be determined. (Measures density differences in rocks, to determine the type of rock).

Measure variations in gravity
Density variations of rocks
Gravimeter
Blue – low density
Red – high density

Use: used in structural mapping and gold, manganese and oil and gas exploration e.g. in hydrothermal gold deposits the magnetite in the host rocks is often changed to sulfide. This causes a reduction in the magnetic response, so magnetic lows may be possible targets.

Applications: Airborne, ground and downhole survey.

Magnetic Survey （Shijie）

Measurement of the Earths total magnetic field and local magnetic gradients highlights contracts in magnetic susceptibilities of materials. Will give a magnetic signature of rocks in an area. E.g. strong magnetic response of ultramafic rocks which may be a nickel host.

Variations in Earth’s magnetic field.

- - Magnetometer
    - - Blue – low intensity
      - Red – high intensity

Magnetics are the most.....?

Airborne, ground and downhole (survey)?

It is often used in with petroleum exploration.

Radiometric survey (Ethan)

Description: measures the relative proportions of natural radioactive elements like potassium, uranium and thorium in rocks. This technique may assist with identification of large granite bodies or mineral sands.

radiometric surveys measure natural gamma radiation from uranium, thorium and potassium in rock and soil.

Use: detect radioactive mineral and in geological mapping.

Applications: airborne, ground and downholes

Electromagnetic Induction (EM)

Description: Rapid + cost effective method. Uses induction to measure electrical conductivity of subsurface.

system is suspended above ground to avoid direct contact. Ability of most rocks to conduct electrical current varies with its porosity + salinity of any interstitial fluid (fluid within pore spaces).

Uses: geotechnical investigations (such as locating underground caves and mine working) and for nickel, copper, oil + gas and groundwater exploration

also useful for delineating areas off contamination within soils + rock.

Applications: Airbourne, ground, downhole.

Airborne EM surveys are more expensive than airborne gravity/magnetic surveys, so a relatively small area is covered by this technique.

Geological Mapping

Description: Once the remote sensing has been collected GEOs will go out and map. We will log the different geological units and contacts. We will make sure to take dips and strikes of all major structures e.g. faults, folds, jointing etc and also the extent of weathering/oxidation. We will then take this back to the office and add the extra information to a main map. We will then create x-sections. Most of this is computer generated today as it helps overlay any extra information like when we drill RC or Diamond holes.

Seismic Methods

There are two types;

Seismic Reflection.
Seismic Refraction.

Commonly used for oil and gas exploration as it provides better resolution of small features.
Mapping subsurface materials (properties).
Sound waves are reflected off underground rock formation are captured by recording sensors.
Requires expert analysis

STEP FOUR - SAMPLING

Identification of the minerals present in the landscape and in particular the occurrence of ore mineralisation or indications that it may lie nearby, therefore geologist take hand samples from the field to have analysed. Some companies even have their own portable ERF machines that can assay results in the field.
This is only applicable to mineralisation samples because hydrocarbon accumulations are well below the surface that can only be reached through drilling.
There are several methods for sampling but it is determined by the type commodity being explored.

Geobotanical

Vegetation Sampling

How it works

Plants absorb elements from the soil and ground water.

Uses in Prospecting

Roots, stems and leaves can be analysed for the presence of minerals.

Plant Morphology

How it works

Unusual patterns of plant growth or mutations in some plants.

Uses in Prospecting

Susceptible indicator plants that grow in potential regiosn can be analysed.

Distribution Patterns

How it works

Some plants will only grow in the presence of some minerals.

Uses in Prospecting

A distinct change in vegetation can indicate a change in rocks beneath the surface.

Geochemical Sampling

Geochemical Soil Sampling " Geochemical Haloes"

How it works

The concentration of ore and gangue minerals decreases away from the ore body.

Uses in Prospecting

Often in a concentric pattern hundreds of metres across. it allows the location of the central ore body to be predicted.

From the textbook

There are two major zones of supergene gold dispersion developed in the weathering profiles of the Yilgarn Block.
This sampling method is looking for the upper zone of supergene gold dispersion within the laterite.
A shallow hole is dug and about 200 cm down into the dirt. It is sieved to stop any bias sampling.
Sampling happens in a wide grid before narrowing it down if an area of interest is found.
The sample locations are brief description of the sample site are recorded.
This exploration technique has been used very effectively for gold exploration throughout the Goldfields especially in areas that are deeply weathered and outcrop is poor.

Stream Sediment Sampling also Lake Sampling

How it works

Traps in streams are analysed for the desirable mineral and these are followed to the source.

Uses in Prospecting

Gold, tin, rutile and diamonds are followed to their source rocks.

From the textbook

The aim for this sampling method is to recover heavy, or dense mineral grains which may indicate nearby mineralisation.
These transported minerals can be traced back to the source by following the river branches back.
This method is mainly applied in hilly terrains with shallow weathering profiles.
Samples are submitted for low level analysis to either parts per million (ppm) or parts per billion (ppb) detection levels.

Weathered Outcrops

How it works

Mineral outcrops or the haloes exposed at the surface can be altered by remote sensing or ground inspection.

Uses in Prospecting

Weathered sulfide ores; quartz reefs

Lakes and Ground Water

How it works

Elements dissolved in ground water and sampled in streams, lakes, wells and springs.

Uses in Prospecting

Hinger concentrations than normal of metal ions can indicate the presence of mineral deposits.

INNOVATIVE SAMPLING METHODS FOR EXPLORATION CASE STUDIES - CSIRO

Zinc Isotopes in Mineral Exploration

Gold-Coated Fungi

Discovering Gold in Gum Trees

Ultrafine: Soil Sampling for Gold Exploration

STEP FIVE - DRILLING

The selection of a suitable drilling method is governed by such factors as the nature and depth of the deposit, location and access factors assoicated witht the drill site and the cost.

Drilling (Reverse Circulation (RC)

Rotary-percussion (RC) drilling used compressed air to rotate the drill bit (the hammer), and the sample is obtained is in the form of chips.
Diller's foam and PVC pipe is used to secure at the top of the RC hole. This so it doesn't collapse in on itself.
The compressed air also blows the chips samples produced to the surface, where they're collected.
The driller offsider puts the sample through the splitter which will have the calico sample bag with the hole number and the meter sample, this is what can be sent away to be sampled. About 75% of is kept in green bags for the the geologists to come back to if they need to resample or relog etc.
Once the drilling program is finished and the company are certain they have have everything they need, fieldies will be sent out to limited the environmental impact of the drilling.
- - They will plug the holes with cement plugs to stop any animals from falling in but also allow access for any future diamond drilling.
  - All rubbish is collected.
  - Green bags are empty and cleaned to be reused if possible.

Diamond Drilling - Emily

Core (or diamond) drilling name comes from the hammer (known as the bit) this has diamonds incrusted on to its face, this allows it to be sharp enough to cut rock. This method of drilling also requires the use of water to help with the rotating bit. As the rotating rods passed down through the bed rock the driller will bring this back to the surface. The driller will send down a red marker that hits the bottom side of the core, this so GEO's and fieldies know which way the core was facing and we can use this to take structural measurements. Drillers will also use downhole survey which takes a picture of the angle of the hole. Drill holes are never straight.

Drilling for Hydrocarbons

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