What is mining ? “Mining is the process of excavating minerals of economics value from the earth’s crUSt for benefit of mankind”. Here we can assume that the earth’s crust, the outer surface of the earth, including the oceans, lakes and rivers, extends to depths of 30 to 50 km or so. For mining operations one should have a working knowledge of geology.
The word Geology means science of the earth and deals with the nature and origin of the rocks that constitute the earth. A person interested in the extraction of minerals from the earth is, however, concerned with the thin surface of the rocks which make up the earth’s crust for a depth of a maximum of 5 km so that geology enables him to locate and to decide the sites most economic for mining or quarrying.
In geology the terms mineral and rock have precise but different meanings. A mineral is a homogeneous and naturally occuring substance having definite physical properties and a composition that may be expressed by a chemical formula. The chemical composition of a mineral, as found in the earth, may be the same as that of an artificially prepared chemical compound in the laboratory but the physical characteristics may differ; e.g. lead sulphide, PbS, is generally available in the laboratory as an amorphous powder. It has quite different physical properties from galena Pbs, the name of the naturally occuring mineral, often in the crystalline form. Some few minerals occur as single elements,e.g native gold, silver, graphite, but most minerals are composed of two or more elements in chemical composition, e.g. quartz (SiO2), hematite (Fe2o3), etc. Most rock-forming minerals are oxides, chlorides, sulphides, carbonates, sulphates or silicates. A rock may be composed of one mineral only, but is usually a mechanical mixture or aggregate of two or more minerals. For example, granite is a rock composed essentially of three separate minerals; quartz, felspar and mica. Whereas minerals can be considered as aggregates of chemical elements, so rocks are really aggregates of minerals. They contain minerals in varying proportions and they have no definite chemical composition.
Bed rock is any rock lying in the position in which it was formed; it is therefore not broken up.
Country rock of an orebody is that rock which is predominat in the area and which contains the orebody. The country rock forms the footwall and the hanging wall.
A seam is a mineral deposit limited by two, more or less parallel planes, a shape which is typical of sedimentary rocks. The term is generally used for coal, e.g. a coal seam.When excavating a useful mineral, the uneconomic rock or mineral assdciated With It
Which has to be excavated and discarded is called rejection, dirt or waste in coal mining practice and gangue in metal mining practice.
An ore is a rock which contains minerai and which can be used for economical extraction of metal after processing to separate mineral from gangue. Ores usually occur in veins or lodes
If an ore, when subjected to metallurgical protesses, yields only one metal, it is called a straight ore.
A mineral deposit is a rock or minerai that is of economic value and repays it extraction from the earth.
Avem (or lode) shown in Fig. 1.1. is a crack in the earth’s crust filled with mineral. This filling can occur by precipitation of the mineral from the mineral-rieh water or by the cooling of the magma [iiiing the crack or by the separation of the mineral from vapours and gases nsmg
up the crack. Veins, like seams, have a strike, a dip and a thickness but for the same vein, all these are usually quite variable.
A number of nearly parallel veins constitute ‘compound (lodes. Orebody is the part of a vein that carries the ore. Generally all parts of a vein are not
ore. ‘
Note that :
the country rock contains the vein;
the vein contains the orebody;
the orcbody contains the ore;
the ore contains the minerals; .
the mineral contains a metal or metals.
MINERALS
Minerals possess definite physical properties by virtue of which they can'be distinguished from one another. The most important physical properties are as follows : '
Colour : Some minerals possess a characteristic colour, e.g. Galena, magnetite, olivine, etc; but in some others the colour is variable, e.g. quartz.
Specific gravity : Most roek-forming minerals have a Specific gravity between 2 and 4.
Lustre : 'The lustre may be metallie:(like galena or iron pyrites), pearly (like talc) or
silky.
Taste and smell : Rock salt, alum and some other minerals can be recognised by their taste. ‘ '
Streak; A few minerals when drawn over paper or over an unglazed procelain plate, leave a coloured mark known as the streak; for example, graphite gives a black streak; hematite leaves a cherry red streak.
Crystalline . A crystal is geometrical solid bounded by smooth plain surfaces called faces and capable of increasing in size by the deposit of fresh material on the outside 01 these surfaces. The faces in a crystal show a definite geometrical pattern and the angles between the faces are constant; for example, quartz crystallises in the hexagonal system, while inica or muscovite erystallises in what. is called the monoclinic system, and rock salt, in the cubic system. The crystallisation may take place by : (a) deposition from solution (b) slow cooling from the molten state, or (e) direct change from a vapour to a solid.
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Cleavage : Many crystals have tendency to split along one or more direction parallel to an actual or possible crystal face. This splitting gives plane surfaces known as cleavage planes. For example, mica cleaves in one direction only; galena (lead sulphide) cleaves in three planes at right angles, forming perfect cubes. ‘ '
Fracture : When a crystal breaks independently of the cleavage plane, it is said to fracture. The propert y is prominent in minerals with poor cleavage.
Hardness : 'This term gives the relative ease with which minerals can be scratched. In practice hardness is measured by reference to a set of minerals given below so arranged that the first member can be scratched by all the others. the second by all except the first, and so on.
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