Geology

DRILL holes make it possible to investigate blocks of ground that by any other means would be accessible only at much greater expense, if at all. In some investigations, drill holes are intended merely to secure geological information—the position of a contact, the attitude of a formation, or the sequence in a stratigraphic column. In others they are designed to determine the presence or absence of veins or other guides to ore. In still others, drill holes are used to take samples of the ore and to provide all the information that is required for an estimate of tonnage and grade.

Less closely woven into the operating management of the industry than 'the "company geologist" but nevertheless highly important in mining development are the officials of the geological surveys of national, state, and provincial governments.

For underground geologic mapping, a "work sheet" or "field sheet" is traced from the base map. A satisfactory material is a good grade of oiled tracing paper. As an alternate a semi-transparent typewriter paper of 100% rag bond may be used. Tracing cloth is entirely un-suitable for this purpose as it does not take pencil line well and a little water ruins it. Blueprints do not show pencil lines and white prints are soft when they are moist.

In recent years a great deal has been said and written about strategic minerals. Particularly when almost all useful minerals became strategic, governments realized the importance of accurate and up-to-the-minute information as to where metals and minerals occur, how rapidly they can be made available, and how long the ultimate supplies will last. In time of war such information is of critical importance, not only with regard to supplies accessible to ourselves and our allies but also with regard to sources available to the enemy.

If it is not always easy to deduce the orientation of the stress that caused fracturing, it is even more difficult to be sure of the type of loading that set up the stress. Whether loading of a rock-mass consists of compression, tension, or a shearing couple, the fracture pattern within the limits of a mine might be virtually the same, although study of the pattern throughout a wider region might indicate the correct interpretation.

A rock will fail along any plane in which the stress exceeds the strength. Planes of weakness being, by definition, loci of low strength, very little stress is required to cause failure along them. Unless such a plane happens to coincide with the plane of maximum or minimum normal stress, it will experience shearing stress and become a plane of slip. It may be opened or kept closed depending on whether the normal stress on it is compressional or tensional.

Mapping underground geology is a blend of art and measurement. The emphasis is strongly on measurement, yet a natural ability to reproduce details accurately by sketching is a useful asset. Some individuals are better endowed in this respect than others, but no one will fail to improve with practice. (For examples of underground geologic mapping, see Plates I, II, and III, following p. 26.)

For simplicity of description a direct-current field has been assumed; actually alternating current is more commonly used for reasons which will appear later. The methods just described, in which current is introduced directly into the ground are known as galvanic methods. In-stead, the ground may be energized by setting up induced currents in it.

Modern churn drill equipment is made up in two principal sizes, both of which are portable and embody the same mechanical design.  The smaller size is used for shallow drilling, commonly in placer deposits. Depths of 500 feet are attained without difficulty in a moderately soft rock similar to altered granite, and 600 or 700 feet can be reached under favorable conditions. This type of drill is also employed for drilling blast holes in quarry and pit mining. The larger size "swings" a weight of tools up to 6000 pounds and normally drills to depths of 1000 or 1200 feet.

In some districts, at least, competent rocks are more hospitable hosts to ore than incompetent ones, and surely this is what would be expected from their mode of failure in fracturing. "Competent" as the term is used here, refers to rocks that are relatively strong but, when they do fail, break as though they were brittle material. "Incompetent" refers to rocks which are weak and have a tendency to deform plastically or by flow. Under most conditions, quartzites, conglomerates, and fresh igneous rocks are competent.

In contemplating the manner in which high values occur throughout the ore, we may recognize what might be called the "texture" of the variations in grade. In a "fine-textured" distribution, the high assays which some of the samples yield are attributable to small spots of rich material. Recutting the sample or taking additional samples on both sides of it will in general fail to duplicate the high assay but will not disprove the existence of rich spots.

In recent years the technique of correlating sedimentary formations by means of their "heavy accessories" has made rapid progress. The method consists of crushing or otherwise disintegrating the rock and concentrating the scanty amounts of such minerals as garnet, ilmenite, magnetite, and zircon by settling in heavy liquids or by hydraulic classification. Any given stratigraphic horizon is likely to carry these min¬erals in roughly constant proportion and with constant size and shape of grain, a fact which can be used in recognizing the horizon wherever it is met in the district.

The primary cobalt minerals, chiefly arsenides, sulpharsenides, and sulphides, break down in the zone of oxidation. The sulphate is soluble, but the pink arsenate, erythrite (Co3As2O8H2O) is stable and constitutes "cobalt bloom," a conspicuous guide to cobalt. A good share of the world's supply of the metal comes from oxidized ores in the Belgian Congo, in which the cobalt is said to occur as heterogenite, a black hydrous oxide, and in asbolite, a cobalt-manganese oxide, both of in-definite composition.

The usual methods of calculating the average of a series of samples, as used in the foregoing examples, embody the assumption that from each channel to the next the grade of ore changes at a uniform rate or, what amounts to the same thing so far as numerical results are con¬cerned, that each assay represents the value of the ore for an interval extending halfway to the next sample on each side.

Rock analyses are often used for attacking petrological problems and for studying alteration. The material to be analyzed should be a sample and not a specimen, and since these two terms are sometimes confused it may be well here to explain the difference. A specimen, according to the Oxford Dictionary, is an "individual or part taken as an example of a class or whole." A specimen of rock or ore may be taken because it looks like the rest of the material around it, but it is not necessarily representative of the exact chemical composition of the mass.