ing this silver-bearing stratum in Michigan little or no work has been done upon it in this state. Silver also occurs in the lead ores of southwestern Wisconsin, but in minute quantities only. According to the Geological Reports of the state: "there is no justification for fostering an expectation of rich results, or for incurring an expense beyond what the satisfaction of knowing, or the possibilities, rather than the probabilities, may warrant.1 Wyoming. Granite and gneiss constitute the formations of the central portion of some of the larger mountain ranges. The Cretaceous and Tertiary rocks form the plains and plateaus. The National Park of the northwest is largely volcanic, while in the northeastern portion of the state is a part of the Black Hills. The precious metals are found in both sedimentary and igneous rocks and usually in quartz-veins. The Grand Encampment and Saratoga Districts. - In 1899 the principal mines of this region were: the Rambler or Doane, Rudefeha, Chatterton-Kurtz, Haskins, Bohemian, Charter Oak, Meta, Alma, Puzzler, Spring Creek, Badger, Cox, Evans, etc., all of which were largely in the prospect stage. However, some ore has been mined and assays made on a sample from the Cox mine are said to show 48.8 per cent copper, $17.16 in gold and $3.87 in silver per ton.2 Weston County. This district is situated about 65 miles from Deadwood, South Dakota. The gold occurs in portions of a coal seam, silver is also present but in smaller amounts. Pyrite is associated with the gold, certain specimens contain as much as 3 pennyweight of gold per ton of coke, although the average is between 1 and 2 pennyweight per ton.3 Sweetwater County. Gold and silver occur in quartz-veins traversing granites, gneisses and schists. The mining industry of this state is handicapped by lack of capital, transportation facilities and severe climatic conditions-capital and railroads will come in time and with the consequent development the disadvantage of a rigorous climate will be largely alleviated. To supplement the foregoing discussion a table on the occurrence and mineralogical association of ores has been prepared, see tables following Chapter VII. (Appendix of Tables.) 1 T. A. I. M. E., Vol. 8, p. 488; Geol. Wis. Vol. 1, 1873–79, p. 661; Ibid., Vol. 2, p. 27; Vol. 3, pp. 201, 206, 358, and 669; Vol. 4, pp. 382–383, and Eng. and Min. Jour., Vol. 74, p. 248. 2 Mines and Minerals, Vol. 20, p. 28. Permanence in Depth. The occurrence of workable ores in depth is a subject, the discussion of which is of peculiar interest to the mining engineer and mining geologist. Much is known regarding the genesis of ore-deposits of moderate depth, but as yet our knowledge of the occurrence of ores at greater depth is very meagre and most uncertain and unreliable at best, and is based upon only a comparatively few disconnected and isolated cases. However, we may hope that in time the system, if such exists, of the occurrence of metalliferous minerals in depth will be revealed under the intelligent and painstaking search of those whose business it is to direct the mining industry of the world. It is not unlikely that facts already known, or such fortified by information subsequently obtained, will ultimately be correlated and crystallized into an orderly system of occurrence in a manner similar to the law of vadose or ground-water and permanent water-levels, and the oxidized and unoxidized zones, together with the zone of secondary enrichment. Although the depth to which fissures may extend and remain open sufficiently long to permit the formation of bodies of mineral matter is largely conjectural, yet it has been estimated with some degree of certainty. According to the old German miners the "ewige Teufe, the extreme depth at which fissures may remain open in the most resistant rocks, is 10,000 meters.1 Professor Van Hise's "zone of flowage" begins at a depth of 5,000 to 12,000 meters. Thirty thousand feet then for rocks such as granite and 10,000 feet for softer rocks as shales may be considered the probable depth to which veins remain open and, if other conditions do not prevent, the depth to which mining operations may be carried. The origin of fissures is probably largely due to the intrusions of igneous rocks, such as granite, porphyry, etc., and we may therefore expect to find the widest, deepest and richest veins flanking mountain ranges and in general in the centers of greatest disturbance. The common occurrence of mineral-bearing veins in or adjacent to areas of igneous rocks is generally recognized at the present time, as has been pointed out, in the case of gold deposits, by Lindgren.3 In contra-distinction to the association of metalliferous mineral veins with igneous rocks are the fissures occurring at a distance from 1 Mining Magazine, Vol. 10, p. 91. The limit set by Professor Heim, at which fissures or open spaces cannot exist, is 16,000 feet. U. S. G. S., 17 Ann. Rept., Pt. 2, p. 162. 2 T. A. I. M. E., New York Meeting, Apr., 1907, p. 502. 'T. A. I. M. E., Vol. 33, p. 790, 1903. mountain masses as in plains and valleys, which are usually barren or only slightly metalliferous. As a rule veins contain ores of workable value to comparatively shallow depth only, and therefore mines located thereon are soon exhausted. However, care should be taken that too broad and sweeping generalizations are not made without due consideration as to the nature of the deposit in question, as well as that of the neighboring country-rock and the geological age of both. It may be that erosion has removed the greater part of a vein in its vertical dimensions leaving only the root as it were as the part with which we have to do. Facts obtained and conclusions drawn from such fragmentary evidence are obviously unreliable. Further, other veins protected from erosion by an overflow of igneous material and therefore left intact may represent an unbroken record throughout their vertical length. But in this case the record, although unbroken, is incomplete owing to the elimination of the after effects of the secondary action of percolating waters, having been effectually sealed by the lava cap, and conclusions drawn therefrom may be as illy applicable to ore-deposits in general as in the former instance. The processes of oxidation and leaching of the sulphides of the primary deposits in the upper portion of veins may disguise the true nature of the deposit, and, as has been previously stated, not until the permanent water-level has been reached and passed can the true character of the deposit be determined. Such, indeed, was the case at Leadville, Colorado; Bingham, Utah; Ely, Nevada; and in many other localities both in the United States and abroad one of the most prominent cases outside of the States is that of the Mount Morgan mine, Australia, where a gold mine has proven to be a great copper mine. In other cases the metallic-content of the oxidized portions of a vein may be wholly wanting, owing either to scant primary occurrence of the sulphides or to their more or less complete removal. And as has been remarked, "In such cases the discovery of the subterranean treasures is purely fortuitous " Butte is a case in point.1 The natural concentration often effected in 'the oxidized portions of a vein, together with the metallic state of the values, may render those portions sufficiently valuable to warrant mining, while the unaltered portions with the widely disseminated values combined with sulphide minerals may be unworkable. This condition of 1 Eng. and Min. Jour., Vol. 84, p. 1068. 2 U. S: G. S., 17 Ann. Rept., Pt. 2, p. 161. affairs may exist in narrow and wide veins alike, but under similar conditions of dissemination it may be said that a vein of considerable width can be worked more profitably than a narrow one the AlaskaTreadwell and Homestake mines may be cited as instances where low-grade deposits are worked with profit, which condition of affairs is rendered possible only through a large output. It is claimed that certain veins in California with a width of only three feet have been worked very profitably at or near the surface, which, with depths of 600 to 700 feet, have proven unprofitable to work owing to the scattering of values and that, too, when the width has increased to 40, 60 and even up to 120 feet.1 Sir Frederick McCoy speaking of the probable occurrence of gold with depth says: "I believe, as a rule, the upper portion of gold veins is richer than the lower, but that there is no limit to the depth at which traces of gold may be found."2 Uniform dissemination of values in vein-rock, both vertically and horizontally, is the exception rather than the rule, both primary and secondary occurrences favoring unequal and irregular distribution, and owing to the irregularities it is extremely difficult to determine when the limits of an ore-body have been reached. However, oreshoots, bonanzas, etc., although broken in their continuity, often show a pronounced tendency to maintain more or less vertical, rarely horizontal, lines, which condition, when once established with certainty, is of considerable importance in the development and operation of a mining property. Regarding the influence of width on the comparative richness of veins no definite conclusions can be stated, except in special instances; both wide and narrow veins have produced extremely rich ores, and in fact the same general locality may contain such occurrences. Probably the narrow veins of Grass Valley, California, and Colorado may be classed among the most productive in the United States. 3 The following specific instances are cited illustrative of the actual conditions existing in some of the most prominent mines and districts of the States: In the gold belt of the Sierra Nevadas, Lindgren states that it is "an incontestable fact that many small veins close up in depth." In Amador County, California, at the Kennedy mine, 1 Min. and Sci. Press, Vol. 13, p. 50. 2 Ibid., Vol. 78, p. 258. 3 U. S. G. S., 17 Ann. Rept., Pt. 2, p. 162, 1895-96, and T. A. I. M. E., Genesis of Ore-Deposits, 1901, p. 290. after working in medium and low-grade ores a barren zone was encountered at a depth of 400 to 500 feet, but on proceeding to a depth of 900 feet a body of high-grade ore was struck which extended to a depth of 1000 feet more and yielded several millions of dollars worth of ore. Later another body of high-grade ore was encountered at a depth of 2200 feet, which so encouraged the management of the Argonaut, a neighboring property, that a deep shaft was sunk to a depth of 1000 feet when a valuable discovery was made.1 The depth of a number of the prominent mines on the Mother lode, California, especially in Amador and Calaveras counties, are approximately as follows: 2 In 1906 the Kennedy, Central Eureka, Oneida and Gwin mines were stoping on or below the 2700, 2000, 1900 and 2000-foot levels, which condition of affairs shows that free-gold ores do occur in depth, although the bonanzas are of less frequent occurrence in the lower than in the upper levels. Hopes are entertained that paying values may be found at much greater depths, which cannot, however, probably exceed 5,000 feet, owing to the increasing temperature of the workings. It has been estimated that fully 3000 feet of the veins have been removed by erosion, which, if so, would indicate a depth of some 6000 feet as that of deposition of gold ores in this locality. In Calaveras County, the Utica mine showed a decided improvement in value of ore with depth and the same has been the case with other properties. However, other important mines of the Mother lode have not been so fortunate and, although developed to a depth of 1200 to 1500 feet, the results have been much less satisfactory than at a more moderate depth of 800 or 900 feet. Furthermore, the cases given were located at the most favorable portions of the surface. Speaking of the 3 1 Proceedings Royal Society of New South Wales, Vol. 9, p. 75, 1875. 2 Min. and Sci. Press, Vol. 92, p. 41. Min. and Sci. Press, Vol. 76, p. 106. |