Text 5. Prospecting, exploration and sampling

Geological prospecting is the first step in finding the ore. It is initiated by a study of geological maps and reports, if such exist. Work is directed to regions where the geological conditions are favourable for mineralization. In search for primary deposits, intrusive igneous bodies should be sought, as they may be a source of mineralizing solutions. Features that show the presence of mineralization, such as fissures, faults, shear zones or drag folds, should then be looked for and followed. If limestones are intruded by or lie near intrusives or are cut by fissures, they should receive careful attention. If deposits are known in the area, the mineral habit of the region can then be utilizaed. Oxidized areas are sought to see if there are any croppings that can tell a story of the character and abundance of pre-existing sulfides.

Geophysical exploration may sometimes accompany the geological prospecting. At present, man’s inventiveness culminated in the development of scientific instruments and methods whose foundation is physics and mathematics. The chief methods used for exploration of oil and metals are as follows: magnetic, electrical, electromagnetic, gravimetric, seismic, radiometric.

In connection with exploration boring is sometimes used for the follownig reasons: for location of minerals covered by soil, rock or water; to search for extensions on strike or dip of known orebodies; to search for parallel orebodies; for location of faults and waterbearing strata; for detailed exploration of orebodies for estimating tonnages and values. Boring from the surface is best suited to large deposits of fairly uniform grade such as masses or beds dipping less than 50°. Boring is not always the cheapest method of exploration, and may not furnish all the desired information. In such cases trenching, test pitting, shaft sinking or drifting is used.

Trenches are confined to shallow soil; the economic limit of depth is about 6-7 ft. Test pits are used for alluvium too deep for trenches. They are applicable for depths to 100 ft; yet for depthhs over 30 to 50 ft. and in water-bearing ground test pits may be less suitable than boring methods.

Accurancy of sampling depends largely upon the character of the material drilled. Diamond drills give accurate samples in hard rock where core rocovery is complete. Accurate samples are also obtainable by churn drills in fine-grained unconsolidated material where casing piping can be driven ahead of the drill bit or sand pump. Soft, broken formations which cave in the hole are most difficult to sample. Cores are more satisfactory than churn-drill sludge for determining geological details.

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NOTES:

• fault - сброс, сдвиг;

• shear zone - зона узкого ступенчатого сброса, зона разрыва;

• drag fold - вторичная складка;

• dip - падение залежи, уклон;

• boring - бурение (скважины);

• trenching - рытье котлована;

• test pitting - разведочные работы в карьере;

• shaft sinking - проходка ствола;

• drifting - проведение штрека;

• churn drill sludge - шлам (грязь) ударно-канатного бурения.