A large proportion of all copper deposits are of such a deep-seated character that at no time can large reserves be positively developed, even when they exist. On the other hand, all over the world the mines with large known reserves are horizontal deposits, lying near the surface, because only in such occurrences is it possible to block out easily and cheaply big tonnages of ore. There are in the United States six very important deposits of this type, the so-called “porphyries.” These are:
Table 40.—“Porphyry” Copper Mines in the United States
| Mine | Reserves (tons) | Years of life at present production |
|---|---|---|
| Utah Copper Co. | 200,000,000 | 31 |
| Ray Consolidated Copper Co. | 90,000,000 | 30 |
| Chino Copper Co. | 80,000,000 | 27 |
| Inspiration Consolidated | 120,000,000 | 20 |
| Nevada Consolidated | 80,000,000 | 20 |
| Miami Copper Co. | 50,000,000 | 20 |
| Total | 620,000,000 | 26 |
These mines in 1917 produced 31.5 per cent. of the total United States output.
The New Cornelia mine is of similar type and has already developed 75,000,000 tons, but as it is new its 1917 output was small. The Arizona Copper Co., Ltd., is also of this type, as are certain new developments in the Phelps-Dodge properties.
These “porphyry” deposits occur in or near intrusive igneous rocks of various ages. Fully one-third of the United States production is now and will continue to be obtained from such deposits. On the average, about 1 per cent. copper is recovered from the ore.
Distinct from the shallow and horizontal-lying disseminated ores or “porphyry coppers,” are the deep mines. The two oldest and most important deep-mine districts in the United States are Butte, Montana, and the Keweenaw Peninsula, Michigan. The mines of Butte work steeply dipping veins. It cannot be considered that over five years of ore reserves are known, and probably not over 2¹⁄₂ years of reserves are actually blocked out on three sides. However, there are no indications of early exhaustion, as the veins are profitable at more than 3,000 feet, the greatest depth to which mining has yet progressed. The deposits of northern Michigan are in pre-Cambrian rocks. They have been important producers of copper for over 50 years, and several mines have reached a vertical depth of more than 5,000 feet. Certainly not over five years of ore reserves are fully developed, but there are no signs of early exhaustion. These two districts, Butte and Michigan, now produce about 30 per cent. of the total United States output, a smaller proportion than before the development of the “porphyries.” The average copper content of the Michigan deposits ranges from 0.5 per cent. to 2 per cent. and of the Butte deposits 2.5 to 5 per cent.
Certain ore deposits (usually massive but irregular) which are situated mainly in Arizona constitute the third important general class. These deposits produce about one-quarter of the total United States output. Bisbee, Jerome and Globe (Arizona), and Kennecott (Alaska), are the main localities. Owing to the irregular nature of the deposits and the distance from the surface at which the ore is found, large developed reserves cannot be blocked out in advance. Such reserves are assumed to be five years.
Apart from the three classes of deposits described above are many smaller deposits, of which the most distinct class are the pyritic bodies, notably those of Tennessee and California. Such deposits are often profitable even when of low grade, because the sulphur as well as the copper is recovered. Reserves in such deposits are large: equal, say, to ten years’ life. Deposits of this class are important in Spain, Norway and in part in Japan. Mines producing copper as a by-product should also be grouped here.
From the above outline the table below has been compiled: