Page:Popular Science Monthly Volume 72.djvu/542

 of the unaltered ore below. In many instances the ore discovered from the outcroppings is gold ore. And gold mills are often erected and operated for years upon such ore, without a suspicion arising that extensive bodies of copper or lead sulphides occur at greater depths. Such was indeed the history of Leadville, Colorado; of Bingham, Utah; of Ely, Nevada, and of Mount Morgan, Australia. The latter is one of the world's greatest gold mines; yet it is now producing copper from its lower levels; and developments have proved it to be a great copper mine. Immense low-grade deposits of copper ore are found below the gossan at Ely and at Bingham, although it is doubtful whether the most experienced geologist or keenest observer of mineralization phenomena would in either place have felt justified in predicting the existence of the wealth below.

In other localities the metal values have either all been removed, or else the primary sulphide ore was too poor in gold to leave oxidized ores of value. In such cases the discovery of the subterranean treasures is purely fortuitous. Butte may be considered the most conspicuous example of this class. The outcrops of its copper veins contain the merest traces of that metal; and there is seldom enough silver or gold in them to justify mining even under the low costs obtaining there to-day. The zone of oxidation is generally from one hundred to two hundred feet deep; and if it had not been for the presence of another system of veins carrying silver, veins of different age and origin, but closely associated geographically, this greatest of copper camps might not yet have been discovered. It was in the search for silver ore that copper ore was discovered here, and one can not help wondering how many more camps equal to Butte may be undiscovered and unsuspected where no outcropping silver or gold mines attract the prospector, and reward the efforts of the miner. Here is surely an important and unexplored field for the geologist. The study of oxidized vein phenomena may yield results thoroughly satisfactory from both material and scientific points of view.

Below the zone of oxidation the chemical reactions which take place between the descending acid solutions and the unoxidized ores result in the formation of more and richer sulphides, down at least to the level of the lower limit of free circulation, and as far as surface waters penetrate. And as erosion of the surface is continually bringing deeper and deeper sulphides within the reach of oxidizing and dissolving surface waters the operation is in constant progress, and these lower-lying ores become more and more enriched until in some cases are formed bonanzas of world renown, and almost inestimable value. It is a fact of much significance that such bonanzas are generally limited to depths where descending waters may have penetrated at one time or another. Indeed the very channels through which the enriching