Ore-deposits. Any mineral which is obtained by mining, and which contains a workable proportion of a metal, is called by miners an ore. Ores are met with in various forms and positions in the earth's crust. Sometimes they are found in gravel, sand, and other alluvial deposits. Examples of this class are afforded by the placers of California (see GOLD), the now exhausted tin-stream works of Cornwall, and the bog iron ores of various localities. In other cases the ores occur disseminated through igneous and sedimentary rocks. Tin ore and magnetic iron ore are frequently met with in this form. When the whole rock is permeated with mineral matter, accumulated in minute veins, the deposit is termed a stock-work. Examples of such deposits of tin ore occur at Carclase and other places in Cornwall, and at Altenberg in Saxony. Again, the ores may occur in detached masses. Such, for instance, are the red haematite deposits of Ulverston in Lancashire, the brown haematite of the Forest of Dean, the iron mountains of Gellivara and Taberg in Sweden, and of Missouri. The ores may occur in regular parallel beds (see MINING) or seams interpolated between rocks of sedimentary origin, as in the case of the ironstone of the coal-measures, and in that of the cupiferous shale of Mansfeld in Prussian Saxony, a seam not more than 5 inches thick which has been worked without interruption since the 12th century. Lastly, ores are met with in tabular masses, known as mineral veins or lodes, differing in character from the enclosing rocks. The simplest classification of ore-deposits is that based on their form, into two divisions: (1) tabular deposits, a class subdivided into (a) beds, whether interstratified or superficial, and (b) lodes; and (2) non-tabular deposits, or masses.
A lode is usually defined as a repository of mineral matter which fills more or less completely a former fissure. Though this definition is undoubtedly true in most cases, deposits are occasionally met with in which the rock at the sides of the fissure, having been so altered as to render it worth working, should be considered as part of the lode. These exceptional cases are included in a more general definition propounded by Dr C. Le Neve Foster, who regards lodes as tabular deposits of mineral, which have been formed subsequently to the rocks by which they are surrounded. Lodes are very variable in thickness, from a mere film up to 150 feet or more. Their longitudinal extent is equally variable. The great Mother Lode of California has been traced for a distance of 70 miles.
In tabular deposits, whether beds or lodes, two dimensions predominate, and the third or smallest dimension, the perpendicular distance between the two bounding planes, is termed the thickness. The adjacent rock on both sides of these two planes is termed the country; the portion on which the deposit lies is the foot-wall, and that covering it is the hanging-wall. With beds or seams, these are known as the floor or roof respectively. The strike of a deposit is the angle formed with the meridian by the direction of a horizontal line drawn in the middle plane, and its dip is the inclination downwards measured in degrees from the horizontal. As the dip of lodes is usually considerable, it is sometimes measured from the vertical, and is then termed underlie or hade. The portion of a mineral deposit occurring at the surface is known as the outerop, basset, or (in the United States) apex. The contents of lodes vary, some parts containing worthless vein-matter or gangue, others being filled with ore. The productive portions are termed courses, bunches, shoots (U.S. chutes), or pipes of ore. Cross-consoles are veins with a direction nearly at right angles to the chief lodes of any particular mining district. Experience shows that the productiveness of lodes is affected by intersection with other veins, by the nature of the adjacent rock, and by changes of dip or of strike.
The origin of mineral veins is a much debated subject which has long occupied the attention of geologists. All the theories which have at various times been brought forward assume in the first place that a fissure has been formed in the earth's crust. This fissure has, it is thought, been filled up by mechanical action causing the attrition of the sides, by sublimation, by injection of molten or plastic material from below, as in the case of dykes of eruptive rock, or, lastly, by depositions from solution, coming from above, from below, or from the sides. The last mentioned, known as the lateral secretion theory, has received great support by the researches of Prof. F. Sandberger. These researches have shown the presence of the common heavy metals in rocks belonging to every geological period. Copper, tin, lead, zinc, cobalt, and nickel have been detected in silicates (mica, augite, and olivine), occurring as component minerals of the commonest rocks. Prof. Sandberger therefore concludes that these metals have been dissolved out and deposited in fissures.
The subject of mineral deposits is systematically treated in J. A. Phillips' Treatise on Ore-deposits (Lond. 1884), in which a full bibliography of the subject will be found. The reduction of ores is discussed under METALLURGY, and under the names of the several metals.