Homogenous rock, sedimentary rocks, in-situ ore, magnetic logging,

Electromagnetic surveys, reverse circulation drilling.

1. Rapidly cooled lava flows harden into massive, ….

2. The value of … … can be calculated by applying market prices to metal contents, and deducting costs of treatment and transportation of concentrates, together with the smelter’s fee.

3. … … inside drill holes is also used, to obtain information for directing holes in core drilling programmes.

4. … … … is carried out with standard percussion rockdrills, using a special technique were the flushing media is introduced through a casing at the hole collar

5. … … measure variations in the Earth’s magnetic field, caused by magnetic properties of subsurface formations.

 

4) Quote the sentences in which these words and word combinations are used. Translate these sentences into Ukrainian:

Saleable product, direct mining costs, supply – demand situations, ocular inspection, economic life, the pull of rock masses, sound velocity, the open-center IDB, return rates and payback period, circular or elliptical in the profile.

 

5.Give answers to the following questions:

1) What is ore?

2) What does fluctuation of metal prices depend on?

3) What are geophysical methods used to explore the bedrock based on?

4) What types of drilling are used in mine development?

5) What infrastructure is required for a typical underground mine?

6. Tell what you know about:

1) about basic types of the rocks of the Earth’s crust

2) ore and ore bodies

3) prospecting and exploration

4) exploration drilling

5) underground mining

6) mine development.

 

Basic Aspects of Geology for Underground Mining

Importance of Geology

A thorough understanding of the geology of a mineral deposit is fundamental to its successful exploitation, and this is especi­ally important for underground workings. Once a mining method is chosen, a major variance in the geology may make it difficult to change the approach to mining, compared to more flexible open­cast work.

Let us review some of the important basic aspects of geology that may affect decisions about mining methods

 

 

The Earth's Crust

 

Tne earth's crust consists of a variety of rocks, formed under different circumstances, and with a wide variety of properties. Rocks usually consist of one or more minerals, ranging from single chemical elements to complex compounds. There are known to be more than 3,000 different minerals.

Of the 155 known elements, some of which do not occur naturally, oxygen is by far the most common, making up about 50% of the earth's crust by weight. Silicon forms about 25%, and the other common elements, such as aluminium, iron, calcium, sodium, potassium, magnesium and titanium, build up the total to 99% of the earth's crust.

Silicon, aluminium and oxygen occur in the commonest minerals such as. quartz, feldspar and mica, which form part of a large group known as silicates, being compounds of silicic acid and other elements. Amphiboles and pyroxenes contain aluminium, potassium and iron. Some of the Earth's commonest rocks, granite and gneiss, are composed of silicates.

Oxygen also occurs commonly in combination with metallic elements, which are often important sources for mining purposes. These compounds can form part of oxidic ores, such as the iron ores magnetite and hematite.

Sulphur also readily combines with metallic elements to form sulphide ores, including galena, sphalerite, molybdenite and arsenopyrite.

Other large mineral groups impor­tant in mining include: halogenides, such as fluorite and halite; carbonates, such as calcite, dolomite and mala­chite; sulphates, such as barite; tungstates, such as scheelite; and phosphates, such as apatite.

Rarely, some elements can occur naturally without combination. The important ones are the metals gold, silver and copper, plus carbon as dia­monds and graphite.

 

Minerals

 

In some circumstances, the properties of individual minerals can be important to the means of mining, and will certainly be important for the means of extraction of the required materials to be exploited. More often, however, minerals will be mixed with others to form the various types of rocks, and the properties will be combined to form both homogenous and heterogeneous structures.

Feldspar accounts for almost 50% of the mineral composition of the Earth's crust. Next come the pyroxene and amphibole minerals, closely fol­lowed by quartz and mica. These min­erals all make up about 90% of the composition of the Earth's crust.

Minerals have a wide variety of properties that can be important in their usefulness to Man, to the best way to mine or tunnel through them, or both. Some of these important char­acteristics, which are also important for correct mineral identification in the field before chemical analysis, are hardness, density, colour, streak, lustre, fracture, cleavage and crys­talline form. The particle size, and the extent to which the mineral is hydrated or otherwise mixed with water, can be very important to the behaviour of the rock structure when excavated.

Mineral hardness is commonly graded according to the Moh 10-point scale shown in the table below.

. The density of light-coloured minerals is usually below 3.0. Exceptions are barite or heavy spar (barium sulphate - BaSO4 - density 4.5), scheelite (calcium tungstate -CaWO4 - density 6.0) and cerussite (lead carbonate - PbCO4 - density 6.5).

Dark coloured minerals with some iron and silicate have densities between 3.0 and 4.0. Metallic ore minerals have densities over 4.0. Gold has a very high density of 19.3. Minerals with tungsten, osmium and iridium are normally even denser.

Streak is the colour of the mineral powder produced when a mineral is scratched or rubbed against unglazed, white porcelain, and may be different from the colour of the mineral mass. Fracture is the surface characteristic produced by breaking of a piece of the mineral, but not following a crystallographically defined plane. Fracture is usually uneven in one direction or another.

Cleavage denotes the properties of a crystal whereby it allows itself to be split along flat surfaces parallel with certain formed, or otherwise crystallographically defined, surfaces. Both fracture and cleavage can be important to the structure of rocks containing substantial amounts of the minerals concerned

 

 

Properties of Rocks

 

Rocks, normally comprising a mixture of minerals, not only combine the properties of these minerals, but also exhibit properties resulting from the way in which the rocks have been formed, or perhaps subsequently altered by heat, pressure and other forces in the earth's crust. It is comparatively rare to find rocks forming a homogeneous mass, and they can exhibit hard-to-predict discontinuities such as faults, perhaps filled with crushed material, and major jointing and bedding unconformities. These discontinuities can be important in mining, not only for the structural security of the mine, and for gaining access to mineral deposits, but also as paths for fluids in the earth's crust which cause mineral concentrations.

In order for mining to be economic, the required minerals have to be present in sufficient concentration to be worth extracting, and within rock structures that can be excavated safely and economically. As regards mine development and production employing drilling, there must be a correct appraisal of the rock concerned. This will affect forecast drill penetration rate, hole quality, and drill-steel costs, as examples.

One must distinguish between microscopic and macroscopic properties, to determine overall rock characteristics. As a rock is composed of grains of various minerals, the microscopic properties include mineral composition, grain size, the form and distribution of the grain, and whether the grains are loose or cemented together. Together, these factors develop important properties of the rock such as hardness, abrasiveness, compressive strength and density. In turn, these rock properties determine the penetration rate that can be achieved, and how heavy the tool wear will be. In some circumstances, certain mineral characteristics will be particularly important to the means of excavation. Many salts, for example, are particularly elastic, and can absorb the shocks of blasting without a second free face being cut, thereby directly influencing mining method. The drillability of a rock depends on among other things, the hardness of its constituent minerals, and on the grain size and crystal form, if any. Quartz is one of the commonest minerals in rocks. Since quartz is a very hard material, a high quartz content in rock can make the rock very hard to drill, and will certainly cause heavy wear, particularly on drillbits.This is known as abrasion.

Conversely, a rock with a high content of calcite can be comparatively easy to drill, and cause little wear on drillbits.

As regards crystal form, minerals with high symmetry, such as cubic galena, are easier to drill than minerals with low symmetry, such as amphiboles and pyroxenes.

A coarse-grained structure is easier to drill, and causes less wear of the drill string than a fine-grained struc­ture. Consequently, rocks with essen­tially the same mineral content may be very different in terms of drillability. For example, quartzite can be fine­grained (size 0.5-1.0 mm) or dense (grain size 0.05 mm). A granite may be coarse-grained (size >5 mm), medium-grained (1-5 mm) or fine­grained (0.5-1.0 mm).

A rock can also be classified in terms of its structure. If the mineral grains are mixed in a homogeneous mass, the rock is termed massive, as with most granite. In mixed rocks, the grains tend to be segregated in layers, whether due to sedimentary formation or metamorphic action from heat and/or pressure. Thus, the origin of a rock is also important, although rocks of different origin may have similar structural properties such as layering.

The three classes of rock origin are: Igneous or magmatic: Formed from solidified lava (at or near the surface) or magma (underground). Sedimentary: Formed by the deposi­tion of reduced material from other rocks, organic remains or by chemical precipitation (salts etc.). Metamorphic: Formed by the transfor­mation of igneous or sedimentary rocks, in most cases by an increase in pressure and heat.

Igneous Rocks

 

Igneous rocks are formed when magma solidifies, whether deep in the Earth's crust (plutonic rock), as it rises to the surface (in dykes cutting across other rock or sills following bedding planes), or on the surface (volcanic, as lava or ash). The most important min­eral constituents are quartz and sili­cates of various types, but mainly feldspars. Plutonic rocks solidify slowly, and are therefore coarse­-grained, whilst volcanic rocks solidify comparatively quickly and become fine-grained, sometimes even forming glass.

Depending on where the magma solidifies, the rock is given different names, even if its chemical composi­tion is the same. A further subdivision of rock types depends on the silica content, with rocks of high silica content being termed acidic, and those with lower amounts of silica termed basic. The proportion of silica content can deter­mine the behaviour of the magma and lava, and hence the structures it can produce.

 

Sedimentary Rocks

 

Sedimentary rocks are formed by the deposition of material, by mechanical or chemical action, and its consolida­tion under the pressure of overburden. This generally increases the hardness of the rock with age, depending on its mineral composition. Most commonly sedimentary rocks are formed by the breaking down of another rock by mechanical action such as weathering or abrasion, its transportation by a medium such as flowing water or air, and subsequent deposition, usually in still water. Thus, the original rock will partially determine the characteristics of the sedimentary rock. Weathering or erosion may proceed at different rates, as will the transportation, affect­ed by the climate at the time and the nature of the original rock. These will also affect the nature of the rock even­tually formed, as will the conditions of deposition. Special cases of sedimen­tary rock include those formed by chemical deposition, such as salts and limestones, and organic material such as coral and shell limestones and coals, while others will be a com­bination, such as tar sands and oil shales.

Another set of special cases is glacial deposits, in which deposition is generally haphazard, depending on ice movements.

Several distinct layers can often be observed in a sedimentary formation, although these may be uneven, according to the conditions of deposi­tion. The layers can be tilted and folded by subsequent ground move­ments. Sedimentary rocks make up a very heterogeneous family, with widely varying characteristics.

 

Metamorphic Rocks

The effects of chemical action, increased pressure due to ground movement, and/or temperature of a rock formation, can sometimes be suf­ficiently great to cause a transforma­tion in the internal structure and/or mineral composition of the original rock. This is called metamorphism. For example, pressure and temperature may increase under the influence of up-welling magma, or because the strata have sunk deeper into the Earth's crust. This will result in the recrystallization of the minerals, or the formation of new minerals. A charac­teristic of metamorphic rocks is that they are formed without complete remelting, or else they would be termed igneous. The metamorphic action often makes the rocks harder and denser, and more difficult to drill. However, many metamorphic zones, particularly formed in the 'contact zones' adjacent to igneous intrusions, are important sources of valuable min­erals, such as those concentrated by deposition from hydrothermal solutions in veins.

As metamorphism is a secondary process, it may not be clear whether a sedimentary rock has, for example, become metamorphic, depending on the degree of extra pressure and temperature to which it has been subjected. The mineral composition and structure would probably give the best clue.

Due to the nature of their formation, metamorphic zones will probably be associated with increased faulting and structural disorder, making the planning of mine development, and efficient drilling, more difficult.