Surface and Subsurface Methods of Mineral Exploration

 

The major motive in exploring the earth’s surface and its interior is scientific curiosity or the desire to understand better the nature of the Earth. Another key motive is the prospect of economic profit. Improved standards of living have increased the demand for water, fuel, and other materials, creating economic incentives. Scientific knowledge has often been a by-product of profit-motivated exploration.  In equal measure however, significant economic benefits have resulted from the quest for scientific knowledge.

Many surface and subsurface exploratory projects are undertaken with the aim of locating:

a)      oil and gas accumulations and coal beds

b)      concentrations of commercially important metallic minerals (ores of iron, copper, and uranium)

c)      deposits of nom-metallic minerals (building materials - sand, gravel, etc.)

d)      recoverable groundwater

e)      various rock types at different depths for engineering planning

f)        geothermal reserves; and

g)      archaeological features.

Historically, exploration of the Earth's interior was confined to the near surface, and this was largely a matter of following downward those discoveries made at the surface. Most present-day scientific knowledge of the subject has been obtained through geophysical research conducted since World War II, and the deep Earth remains a major frontier in the late 20th century.  With conventional approaches and tools, only a very limited portion of the subsurface regions of the Earth can be studied. Investigators can drill into only the uppermost crust, and the high cost of drilling severely limits the number of holes that can be dug. The deepest borehole so far drilled extends only to a depth of about 10 kilometres (6 miles). Because direct exploration is so restricted, investigators are forced to rely extensively on geophysical measurements (see below Methodology and instrumentation). In recent years data returned by Earth satellites have led to several notable discoveries, as, for example, drainage patterns in the Thar region, which are relics of a period when this region was not arid.

The main methods of mineral exploration and their applicability (whether surface or subsurface are listed hereunder:

METHOD

APPLICABILITY

Geological Mapping (regional and large-scale)

Surface (sometimes in underground mines also)

Remote sensing and air-photo interpretation

Surface

Geophysical methods

Surface

a)      Gravity

Surface

b)      Magnetic

Surface and subsurface

c)      Electrical and Electromagnetic

Surface and subsurface

d)      Seismic

Surface and subsurface

e)      Radiometric

Surface and subsurface

Geochemical

Surface and subsurface

Drilling

Subsurface

 

Note: Of the above listed methods,  the remote sensing and geophysical methods are discussed in detail in later sections of this unit.  This discussion is therefore restricted to geological, geochemical and drilling methods.

Geological Methods:

Geological maps provide exploration agencies or companies with regional geological and geophysical information so that target areas that are consided to have a better prospect in terms of mineral deposits may be identified. The cost of undertaking geological surveys, many of which will not prove to be prospective, is high. Geological surveys provide exploration and mining companies with pre-competitive geoscientific data that is designed to encourage the company to undertake further exploration.

Geological methods rely on the identification of rocks and minerals and an understanding of the environment in which they formed. These surveys aim to find what rock types occur at or close to the surface and how these rock types are related to each other i.e. their boundaries, ages, and structure.  Based on known "environments for mineralisation" or models for mineralisation, regional geological surveys can be used to define smaller areas in which more detailed studies can be undertaken.  A geological survey can be  undertaken using a number of methods depending on the size of a region and the amount of information that is required.

Geochemical Methods

Geochemical methods involve the measurement of the chemistry of the rock, soil, stream sediments or plants to determine abnormal chemical patterns, which may point to areas of mineralisation.  When a mineral deposit forms, the concentration of the ore "metals" and a number of other elements in the surrounding rocks is usually higher than normal. These patterns are known as primary chemical halos.

When a mineral deposit is exposed to surface processes, such as weathering and erosion, these elements become further distributed in the soil, groundwater, stream sediments or plants and this pattern is called a secondary chemical halo. Secondary halos aid in the search for deposits as they normally cover a greater area and therefore the chance of a chemical survey selecting a sample from these areas is greater than from a primary halo area.

Different elements have different "mobility" in the environment which is based on

a)      their readiness to dissolve in water

b)      their density

c)      their ability to form compounds with other elements and

d)      the acidity (pH) of the environment.

Subsequently, the secondary halo may not contain the "metal" for which a geochemical survey is searching but other "marker" elements.

Drilling:

Drilling is used to obtain very detailed information about rock types, mineral content, rock fabric and the relationships between rock layers close to the surface and at depth.  Drilling is only used in areas that have been selected as "targets" from geological, geophysical and/or geochemical methods.  Four drilling methods are widely used, each depending on the type of information required and/or the rock types being drilled.

Air-rotary drilling: this is a quick and economical method of producing a sample.  Compressed air is forced down the drill pipe and, as it returns to the surface, it carries with it rock chips made by the rotating bit.  This method is used in soft rock materials to about 25m deep, or to drill through the top layer of decomposed rock and soil to get a fresh rock sample from the bedrock below.

Air-percussion drilling: this method is used for penetrating hard rock types to a depth of around 300m. Rather than a rotating bit, a hammer bit is forced through the rock by compressed air. As in air-rotary drilling, rock chips are brought to the surface by the returning air.

Rotary Drill Bit

Diamond Core Bit

 Mud-rotary drilling: this method is used for drilling through soft rocks, sand and clay layers especially in the search for coal, oil or gas. A special mix of clay and water is forced down the drill hole turning a rotary bit, with rock chips returning in the "mud" slurry. This method is used for holes up to 3km deep and can be done from ships or  offshore platforms especially in the search for oil and/or gas. A person who collects the chips and identifies the rock type and other information is known as a "mud-logger".

Diamond core drilling: this method involves a pipe encrusted in industrial diamonds being used to drill through rock layers, with a "core" of rock being left in the centre of the pipe. This core is recovered and gives information not only about the rock types, but also about the relationships between the rock layers in detail.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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