THE FORM OF ORE DEPOSITS Ore deposits occur in a multitude of different forms, but for convenience these may be reduced to two types—namely the tabular or and the Tabular deposits are either beds forming a part of the general stratification of the country, or veins or lodes that have been formed by the filling of fissures, or the replacement of their adja cent "country." As a rule, the veins cut across the stratification; more rarely they conform with the strike and dip of the formation in which they occur and are then termed bedded veins. Where a number of parallel fissures are irregularly connected the system is known as a composite vein or lode. The term linked veins is used in a similar sense. A system of closely spaced and parallel veins is spoken of as a sheeted zone. Gash veins are those that do not extend beyond a given bed of the formation in which they occur, usually limestone.
Although veins have been defined as tabular deposits, the paral lelism of their bounding surfaces is only true in a very general sense, for unless they are fissure-fillings they have more often than not only one well-defined wall, and in some cases none at all. In mining, however, it is usual to speak of the hanging wall and the foot wall of a vein to distinguish the limits of the workable deposit whether they are well-defined or not.
Where the filling of the fissure has been followed by differential movement, one or both of the walls are usually smooth and polished (slickensides or and between the actual vein-filling and the wall of the country rock there is often a thin selvage of comminuted rock or clay known as flucan.
Where the fissuring was accompanied by dislocation, as is mostly the case, the veins have a lenticular character along both the strike and dip. This lenticular character is due to the differential move ment of curved or warped surfaces. Between the lenses the fissure is often so constricted that no ore occurs at all. When the axes of the lenses lie in one plane the lenses are connected by stringers; when they lie en echelon they are usually disconnected.
Not every part of a vein is equally mineralized : the pay-ore tends to localization in certain definite portions of the vein sepa rated by low-grade material or even by barren gangue. The richer portions are termed If they have a tendency towards the horizontal, they are distinguished as or zons; if, on the other hand, they tend towards the vertical, they are known as columnar or chimneys. But the term ore shoot is not restricted to the occurrence of high-grade material in a low grade matrix; it is also applied to the development along a fissure of lenticular bodies of pay-ore, separated by barren country rock. It has been proposed to call this latter class of ore-shoot shoots of occurrence, and to use the term shoots of variation for the former. Shoots of variation are usually distinguished from
shoots of occurrence by a gradual diminution of the values near the boundaries of the ore-shoot.
deposits vary greatly in shape and size. They may be large irregular masses or small pockets of solid ore, or they may be stockworks, i.e., reticulated masses in which a multitude of small veins traverse a portion of the country rock. Of exceptional character are chimneys or pipes, which are ore-bodies having a rudely circular or elliptical cross-section with considerable vertical extent. are a special type of cavity-filling. Those of Bendigo in Victoria, Australia, have resulted from the folding of the Ordovician strata into a series of anticlines and synclines. The deposition of quartz in the openings formed at the crest of the anticline below a sandstone and above a shale bed has given rise to auriferous ore-bodies. Since the sandstone and shale beds alternate repeatedly the saddle-reefs occur, one above the other as disconnected ore-bodies, in what is known to the miners as the "centre country." Thus the New Chum and Victoria mine had 30 superimposed saddle-reefs, down to a depth of 2,3ooft. The deepest mines are working at about 4,000ft.
BIBLIOGRAPHY.—Elie de Beaumont, "Sur les emanations volcaniques et metalliferes" (Bull. de la Soc. Geol. de France, 1847, p. ; W. H. Emmons, "Relations of Metalliferous Lode Systems to Igneous Intrusions" (Amer. Inst. of Min. and Met., 1916) ; J. W. Finch, "The Circulation of Underground Aqueous Solutions and the Deposition of Lode Ores" (Proc. Col. Sci. Soc., vol. vii. 1904, p. 193) ; F. Pokp* "Genesis of Ore-Deposits" (Trans. Amer. Inst. Ming. Eng., vols. 23 and 24, 1893, reprinted as a book, 2nd ed. 1902) ; J. W. Gregory, "Magmatic Ores" (Trans. Faraday Soc. vol. 20, 192g, p.
containing a useful bibliography and followed by a discussion) ; F. H. Hatch, Historical Summary of Theories of Ore Genesis (Presi dential address to the Inst. of Min. and Met. 1914) ; W. Lindgren, Mineral Deposits (New York, 1920) ; R. H. Rasta11, The Geology of the Metalliferous Deposits (1923) ; E. Suess, "Ueber heisse Quellen," Verh. Ges. deutsch. Naturf. und Aertze (Carlsbad, 1902) ; Types of Ore Deposits (San Francisco, 1911), containing chapters by H. Foster Bain, E. R. Buckley, S. F. Emmons, W. H. Emmons, F. H. Hatch, 0. H. Hershey, J. D. Irving, J. F. Kemp, A. C. Lane, C. K. Leith, R. A. F. Penrose, Jr., T. A. Rickard and C. H. Smyth, Jr.; J. E. Spurr, The Ore Magmas (New York, 1923) ; J. H. L. Vogt, "Beitrage zur genet ische Klassifikation der durch magmatische Differentiation-prozesse und durch Pneumatolyse entstandenen Erzvorkommen" (Zeit. f. Prakt. Geol., 1894, ; P. Niggli, Versuch einer natiirlichen Klassifikation der im weiteren Sinne magmatischen Erzlagerstiitten (1925).