But with other kinds of ore, as, for example, copper, the best grades are found, not close to the surface but some two hundred or more feet down. The explanation is that the minerals of copper are considerably more soluble than the ordinary gangues and therefore the weathering and oxidation that takes place in the upper horizons of ore bodies will dissolve out the cupriferous compounds and thus deplete the superficial ore. But, by the flowing of the copper solutions to a lower zone, there occur certain reactions that reprecipitate the salts of copper upon compounds of the metal already formed and we have instances of the phenomenon known as "secondary enrichment."
Concentrator Division, Washoe Reduction Works of the Anaconda Copper Mining Co.,
Anaconda, Montana.
Largest Copper Works in the World.
It was this very process that effected the changes in the character of the ore in the famous Anaconda Mine, previously mentioned (page 44). The locator's discovery was upon an outcrop rich in silver. Probably the original compounds of the vein were of both silver and copper. The silver was more stable against dissolution than was the copper, with the result that the base metal was removed more rapidly and completely than was the precious metal. The upper portion of the vein was therefore left rich in silver, and low in copper. But, as depth of mining increased, there was found a gradual diminution of the silver content with a simultaneous increase in the copper. The mines of Butte have become known as copper mines, and the wonderful records they have made are ample testimony to the fact that the change in the prevailing metallic values has not wrought serious havoc in the mining industry of the district.
Regarding the probability of veins persisting to great depths, there is this thought suggested by J. E. Spurr: "Owing to the pressure exerted by gravity, it is doubtless more difficult for a fissure to stay open in depth than near the surface. The tendency is to press the sides together. At a certain depth, it is probably the case that the pressure and the plasticity resulting from this, together with the increase in heat, makes it impossible for fissures, fractures or any openings to exist."
There are still many persons who are reluctant to let go of the cherished notion about the improvement of ores with depth. But there is no economy in deceiving one's self, and the wise thing to do is to accept the truths as they are daily proven. It may be worth while to again refer to the wonderful Camp Bird Mine. This mine was discovered in its true worth years after it had been abandoned by early prospectors because it lacked showy, base-metal minerals. However, since its true merit has been recognized, it has maintained large and remarkably rich annual outputs. As values were beginning to show a material decrease, about five years ago, an experienced mining engineer of recognized standing was engaged to give advice concerning the future exploitation of the property. After exhaustive investigation of the ground, and in the face of adverse opinions, he recommended the discontinuance of further development in depth. At the same time, however, he advised the exploitation of the ground laterally or along the strike of the very persistent vein. His advice was followed and the company's stockholders had reason to be advocates of the new theory; for a very reasonable amount of horizontal development work opened up vast stores of rich gold ore.
And yet, notwithstanding this disquieting feature that seems to apply to mining, there is comfort to be found in the consideration of the exceptional cases. Every man may hope that when he locates a new mine he is taking possession of a property that will have as extensive ore bodies as those that have been proven to exist in the lead-silver mines of Laurium, Greece, the quicksilver mines of Spain, or the copper and tin mines of Cornwall. These mines are in lodes which have persisted and have been mineralized to comparatively great depths, so that their bottoms have not been reached.
There is a modern idea that has taken root in the minds of mining men of the last generation to the effect that the mines with rich ore are not necessarily the ones with big profits. There are many men looking for investments in mines whose contents are of low grade but in large bodies readily worked. If a mine with rich ore can be found and the ore abounds in such liberal amounts as to warrant the inauguration of a company with the essential working equipment, such a proposition will naturally not be turned down. However, the faith of some men is placed in those mines that may be operated upon very large scales for long periods even if the profit per ton be very small. With a large plant, the unit of expense, i.e., the cost of mining per ton, is less than with a small mine. With the assurance of regular outputs of ore of a reasonably uniform grade, the milling equipment can be planned to handle a mine's product to the greatest advantage. The Alaska-Treadwell Mine, on Douglas Island, is an instance of a splendid property that has been continuously operated for about a third of a century. The ore is low grade in gold but immense dividends have been declared because the ore body, a tremendous mass of eruptive rock, has lain in such a position that the owners found it possible to excavate the stuff, to a great extent, by open-pit methods, although not by using steam shovels. The ore is treated in a vast mill contiguous to the mine.
The Homestake, another gold mine, has an ore body quite dissimilar geologically from, but of dimensions approximating those of, the Treadwell. It is a great body of mineralized, crushed shales, standing steeply in the shape of a lode and carrying about $3.75 per ton. It has been followed down considerably over one thousand feet and although the grade has dropped somewhat with depth, there are known to still be millions of tons in reserve. According to estimates, the mine has enough positive ore in reserve to keep the mill running at the rate of 4,000 tons per day for several years even if no more ore were to be opened up. This ore nets but 53 cents per ton above all mining and milling expenses; but a little arithmetic will show that this mine is worth twice as much as the mine that is producing, with more or less regularity, an average daily output of, say, forty tons of high grade ore upon which there is a net gain of $25 per ton, a figure that is rather high for the average of so-called "high grade" mines.
We must, therefore, decide that it is always wise to think twice before condemning a mine because its grade of product is low. It is only recently and by virtue of marked improvements in metallurgical processes that many bodies of mineral have become "ore." Hence it is but natural that many of the older miners fail to grasp the possibilities that lie in such deposits.