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.
In an open mine there must be maintained a system of continually changing tracks placed upon grades (sometimes rather steep) and with sharp curves. With multiple switches, numbers of small locomotives are kept busy pulling and pushing up and down the tracks with their strings of loaded cars and replacing the “loads” with “empties.” When such operations are upon a mountain-side, a very beautiful panoramic view may be had from the opposite side of the gulch.
Generally, the ore material is disintegrated to some extent. In some cases, it will actually crumble down before the advance of a steam shovel. In other mines, it is necessary to drill large holes which are loaded and blasted.
It is becoming more and more important for the active mining man to post himself upon the methods and economies of this latter-day mining practice. The development of this open or surface mining has introduced entirely new economic ideas. With no costs for timbering of mine passages, for ventilation, or for hoisting, and with a very material decrease in manual labor per ton mined, immense masses of rocks are now really ore, although a few years ago they were nothing but lean, country rock.
In consequence of the success attained by the pioneers in this kind of mining, there has been created a demand for properties possessing large deposits of low grade ore that is workable on this intensive scale. Copper properties have been holding a prominent place recently and stockbrokers carry regular lists of “Porphyries,” this nickname having been coined to cover the companies operating in the low grade porphyry ores of the Western United States. Not all of these porphyry companies will use surface mining methods. Some companies in the Globe District of Arizona have started extensive underground schemes for mining large tonnages very cheaply by “caving” methods.