There are large deposits in the arid portions of the globe where water for working is not obtainable. To meet such conditions, numerous inventions continue to be placed upon the market. These devices are all planned in such a way as to use very little or no water. If water is required at all, the machines are expected to use it repeatedly. The machines are built to effect the segregation of the precious contents gravitationally, electrostatically, pneumatically, and by amalgamation with mercury. It is too early to say how successful such devices will prove in commercial operations. Because some of them have not "made good" does not mean that genius will not yet cope with the situation; and we look into the future to see large operations efficiently and economically conducted by dry placer machinery. There are now no authentic figures obtainable upon this question of dry placering costs.
[VIII
OPEN MINING.]
Some mention has been already made of open mining. The greatest development of this sort of mining has come about since the application of the modern steam shovel to the excavation of ore. This practice was an American innovation and it is being adopted throughout the world wherever natural conditions will warrant.
Within the past few years, immense bodies of iron ore have been discovered in northern Minnesota and the adoption of these immense, mechanically operated shovels has worked such economies in the mining of this kind of ore that entirely new cost figures have been established and tonnages are being produced which, a few years ago, would have seemed unbelievable. There are about a dozen mines of this "open pit" type that have each produced over a million tons of ore per year in a season that must cease with the close of navigation on the Great Lakes. One mine has shipped over three million tons a season.
At the Utah Copper Company's mine in Bingham Cañon, Utah, a great deposit of low grade, copper-bearing eruptive rock is being handled upon a steep mountain-side by this same scheme. This ore averages a little less than two per cent. in copper, but so economical is the handling of it in such vast amounts that a neat profit is made above all mining, transportation and milling charges. When the red metal sells at thirteen cents per pound, the gross value of this ore is about $5.20 per ton. This mine has maintained an output of ten thousand tons or more per day over long periods.
A famous gold mine in Queensland, Australia—the Mount Morgan—is also being worked by steam shovel methods. The deposit is here in the form of a small mountain and the operations are gradually razing this landmark to the level of the surrounding plains.
The mining of low-grade gold ores by open-pit methods has taken hold in America, and an example of the practice may be found at the Wasp No. 2 mine in the Black Hills. According to published accounts of the operations of this company, all of the costs of mining and treating the ore amount to only $1.02 per ton. The ore body is a bed of quartzite lying nearly flat, and averaging in the neighborhood of only $2.50 per ton in gold, the only mineral of value. The recovery of this metal is at the rate of between 75 and 80 per cent. efficiency, or about $2 from each ton. The net profit is therefore close to one dollar per ton. This very modern scheme of mining has been made possible through the recent advances made in the cyanidation of ore, and it is going to pave the way for many more such mining plants.
Steam Shovels and Churn Drills, Copper Flat, Ely, Nevada.
The Nevada Consolidated Copper Company has conducted vast mining operations "in the open" at Ely, Nevada, by the use of 95-ton shovels having a capacity of two and one-half cubic yards per dip. One shovel has handled as high as 2,800 cubic yards (the equivalent of about 5,500 tons) in nine hours; but this must be recognized as an exceptional run, and cannot be taken as an average. The ore has a thickness of about 200 feet and covers many acres. As in the majority of such properties, there is here a large amount of "overburden" to be removed and disposed of before the ore can be excavated. This process of uncovering the ore body by the removal of the overburden is called "stripping." The cost per ton of ore mined is said to average 55 cents.