New York Paper - Mining Methods of the Arizona Copper Co.

The mines of the Arizona Copper Co. are situated in the Morenci-Metcalf copper district in southeastern Arizona. This copper-bearing district covers a triangular mountainous area of about 3 square miles, rising abruptly from the gravel plateau of the surrounding country. In this area, an immense intrusion of porphyry lias displaced, shattered, and engulfed the shales, limestones, and quartzites formerly bedded on the basal granite. Outcrops of oxide ore were discovered in 1873, and these surface ores were found to connect in depth with large bodies of high-grade oxide ores in the limestones and shales. The mining of these high-grade ores continued until about 1893, when their rapid exhaustion led to the discovery and exploitation of the low-grade sulphide ores in the adjoining porphyry. Methods of concentration were then devised and the low-grade ores finally rendered profitable. The Arizona Copper Co. and the Detroit Copper Mining Co., operating in the same district, were the pioneers in the successful development of the low-grade "porphyry" properties of to-day. The largest mines of the Arizona Copper Co. are the Humboldt and Clay at Morenci, and the Coronado, Metcalf, and King at Metcalf. These mines produce about 4,000 tons daily of concentrating ore containing from 2.5 to 2.8 per cent. copper. The topography of the country is very rugged and mining operations are conducted principally through adit levels. A few of the mines are opened by shafts, the deepest being 1,100 ft., in the Coronado mine. Ore Occurrence The low-grade sulphide ore occurs as: (1) disseminated deposits in the main stock of quartz porphyry; (2) lode deposits in fault fissures in granite. The high-grade oxide ores were found as tabular deposits in limestone and shale close to contacts with porphyry dikes. Low-grade oxide ore formed the outcrop of some of the sulphide orebodies; the majority of the outcrops, however, were barren.