Mineral Deposits and Exploration
A solid naturally-occurring compound having a definite chemical composition
Examples:
quartz - SiO
2(an oxide)
hematite - Fe
2O
3(another oxide) covelite - CuS (a sulphide)
What is a mineral?
• Ore is an aggregate of minerals and contains valuable and gangue minerals.
• Mineral beneficiation involves separation of gangue minerals from ore.
What is an Ore
An occurrence of minerals or metals in sufficiently high concentration to be profitable to mine and process using current technology and under current economic conditions.
A zone that has concentration of minerals of economic interest.
What is an ore deposit
Ore forming processes
1. Syngenetic:
A deposit formed at te same time as the rock in which occur.
Ex. Banded Iron Formation 2. Epigenetic:
A deposit introduced into the host rocks i.e. form after the formation of host rock.
Ex. Most hydro thermal deposits are epigenetic
type
3. Hydrothermal Processes:
• As magma cools, more abundant metals (silicon, aluminum) deposit first . Solidification of magma releases water - a hydrothermal solution.
• Minerals precipitate from hydrothermal solution and deposit in cracks or veins in rock.
• Hydrothermal processes are the source-transport-
trap mechanism.
Concentration of minerals caused by high temperatures and pressures near intrusions Examples:
Lead-zinc deposits in southeast B.C.
Diamonds Garnets
Metamorphic Ore Deposits
Hydrothermal and
Metamorphic Ore Deposits
Intrusion
Hydrothermal solutions entering veins in rocks Geyser or hot spring
Ore deposit
Alteration of rocks by heat and pressure zoning
Deposition of dense, resistant minerals in streams, lakes etc (Alluvial Deposits),
e.g. Placer gold
Precipitation of minerals from ancient oceans (Evaporite Deposits), e.g. Potash and salt deposits
Accumulation, burial and petrification of vegetation, e.g. Coal Deposits.
Sedimentary Ore Deposits
Mineral Exploration
• Exploration is the process and science of locating valuable mineral deposits, i.e. those which have commercial value.
• Exploration is commonly the longest and riskiest
"stage". The exploration process can take up to several years for each project.
• Exploration activity is largely conducted by geologists and geophysicists, assisted by field samplers, drillers, and laboratory personnel (for chemical assays).
What is Exploration
Exploration Methods
§ Geophysical surveys
§ Geological mapping
§ Geochemical surveys
§ Remote sensing
Geophysics
Ø Geophysics is the study of earth by making use of established principles of physics.
Ø this is done by measuring certain physical properties and interpreting them mainly in terms of subsurface geology.
i.e. Rock type, ore deposits, structure, and ground water conditions.
Branches of Geophysics
Based on the aim of investigations, geophysics has two branches 1. General Geophysics (pure side of the subject)
2. Exploration Geophysics (Applied side of the subject) 1. General Geophysics
Geodesy, seismology, earthquake precursory studies etc.
2. Exploration geophysics (Applied Geophysics)
It involves measurement of physical properties like gravity, magnetic, elastic, electrical properties of the rock of the earth crust for investigation of mineral deposits.
Ø It detects differences or anomalies present within the earth crust.
These anomalies are Density
Magnetism
Electrical resistivity
Elasticity/Liquefaction
Ø Aim is to make specific investigations of economic importance
Like, exploration of Oil and gas
Ore deposits Ground water
Ø And solving certain problem of engineering geology.
Geophysical investigation techniques can be broadly grouped into three categories:
(1) Airborne methods
(2) Surface/ground methods, and (3) Borehole methods
Techniques of Geophysical Investigation
§ Prospecting by gravitational method is the technique of measuring the gravitational field at the earth’s surface and the data thus obtained is utilized to predict the nature of subsurface and structures.
§ In this method the natural field of earth’s gravitation is used.
Physical Property
• Density of the material is the controlling physical property.
Principle
• In gravimetric method, the nature of distribution of gravity g on the surface is analyzed. The gravity is influenced positively if the causative body is heavier, larger and occurs at a shallow depth.
§ Measurements of gravity provide information about densities of rocks underground.
1. Gravity Method
• The gravimeter, used in relative gravity measurement is a mass loaded spring.
• If the subsurface has a relatively heavier body, the gravity pull is more there (+g) and the spring extends becoming longer.
• If the subsurface has relatively a lighter body there the gravity pull is less (-g) and the spring contracts and become shorter.
• Thus in particular region, if subsurface bodies (such as ore deposits, coal seam and salt domes ) whose densities are different from the surrounding rocks the gravity field deviates from the normal value.
• From these deviations, it is possible to locate the inhomogeneous bodies in the subsurface
• Positive gravity anomalies are associated with shallow high density bodies.
• whereas gravity lows are associated with shallow low density bodies.
Fig. Illustrations showing the relative surface variation of Earth's gravitational acceleration over geologic structures
Fig. Illustrations showing the relative surface variation of Earth's gravitational
• Thus, deposits of high-density chromite, hematite, and barite yield gravity highs.
• whereas deposits of low-density halite, weathered kimberlite,
and diatomaceous earth yield gravity lows.
Ø Like gravity methods, these investigations also take advantage of natural magnetic field associated with the earth and its relation to subsurface geology.
Controlling property
The main controlling physical property in magnetic method is magnetic susceptibility.
Principle
• The magnetic methods are based on the fact that the magnetic bodies present in the earth’s surface contribute to the magnetic field of the earth.
• In general, when the magnetic field of the earth or one of its components is measured on the surface, bodies possessing magnetic moments different from those of the surrounding rocks contribute to the deviations in the measured quantities. From the magnetic anomalies, it is possible to locate anomalous objects.
2. Magnetic Methods
• The different parameters measured during magnetic investigations are
• total magnetic field (intensity and direction), and
• different space components
Fig: A magnetic reading profile across a dyke
Ø Among the different geophysical methods electrical method are numerous and more versatile.
Ø All electrical methods are widely used in the exploration work connected with metalliferous deposits, prospecting for ground water, and engineering geology investigations.
Controlling Properties
Electrical methods essentially involve measurement of;
1. electrical conductivity or resistivity of material of the earth 2. electro-chemical activity, and
3. dielectric constant
In general electrical methods are employed for relatively shallow subsurface exploration. Depth of 300m – 450m.
3. Electrical Methods
Electrical Methods Principle
Electric methods are based on the fact that the subsurface formation, structures, ore deposits, etc. possess different electrical properties. These differences are investigated suitably and exploited to draw the necessary conclusion.
The ability of a rock unit to conduct an electrical current depends primarily on the following factors.
1. Degree of fracturing, and the percentage of the fractures filled with ground water.
2. The amount of open spaces between the particles
3. The degree of interconnection between those open spaces
4. porosity and permeability, and the concentration of dissolved solids (salts) in that water.
Electrical Properties of Earth Materials
Important categories of electrical methods are classified in to the following groups
1. Resistivity method (electrical resistivity)
2. Self potential method or spontaneous polarization (natural electrochemical activity)
3. Electromagnetic method (for example GPR)
(electrical conductivity, magnetic permeability, dielectric constant) 4. Induced polarization method
(induced electrochemical activity)
Using the electrical methods, one may measure potentials, current, and electromagnetic fields that occur naturally or are introduced artificially in the earth.
Controlling Properties
Elastic property differences in rocks is the controlling property.
Instrument Principle
• Seismic method of study is based on the principle that subsurface rock formations bear different elastic properties.
• Because of this, the velocities of propagation of seismic waves through the subsurface layers of earth, suffer reflection or critical reflection arrive at the surface of the earth where they are detected by geophones.
• From the time taken by the waves to travel through the subsurface formation and from the seismic wave velocities of the media. It is possible to determine the depth of various elastic boundaries.
• With the help of geophones fixed at suitable intervals on the ground, the different seismic waves reaching the surface are recorded.
• and from the times of their arrival, time –distance curves are plotted. The direct waves are the first to reach the geophones placed between point and the distance beyond the point is called the critical distance.
4. Seismic Methods
Depending upon whether reflected waves or refracted waves are used in the investigation, there are two types of methods, namely,
1. Seismic reflection method and 2. Seismic refraction method.
Ø Seismic reflection studies are effective for depths more than 100m but are not suitable for shallow exploration
Ø Refraction methods are employed for investigating depths from close to the surface to several kilometer deep. These methods are also followed for the investigation of deeper crust under seismic studies.
Ø Shallow seismic refraction have found effective application in investigating the suitability of foundation sites for civil engineering structures
