New Mexico Geological Society Annual Spring Meeting — Abstracts

Economic benefits derived from porphyry copper deposits in southwest New Mexico, southeast Arizona, and· northern Sonora, Mexico have become a major impact in the social and economic lives of these regions in the 20th Century. In 1993, Arizona accounted for over 64% of U.S. copper production, producing 1,277,312 short tons of contained copper, with New Mexico producing 255,072 short tons. A "porphyry copper deposit" is an economic occurrence of veinlet-controlled and disseminated copper mineralization in rock, typically a monzonite, diorite, or granite porphyry, that has been altered and subsequently enriched by hypogene and/or supergene processes, and is amenable to low cost, bulk mining methods. In the southwest U.S, as many as fifty major porphyry-style occurrences are known from a large northwest-trending "belt" defined on the northwest and southeast by Mineral Park, Arizona and La Caridad, Mexico, respectively, and on the west and east by Ajo, Arizona andI Copper Flat, New Mexico, respectively. Many of the deposits formed during the Late Cretaceous-Early Tertiary Laramide Orogeny, a time of intrusive and tectonic activity spanning approximately 55 to 75 million years ago. Patterns can be seen in the distributions of ages: the oldest deposits (greater than 70 m.y.a), such as Copper Flat, New Mexico, and Mineral Park, Arizona, occur on the northeast and western fringes of the belt, the mid-age deposits (60-70 m.y.a), such as Christmas and Silver Bell, Arizona, in the central band of the belt, and the youngest deposits (55-60 m.y.a.), such as La Caridad, Mexico and Tyrone, New Mexico, in the southeast corner of the belt. The only major pre-Laramide deposit, the Jurassic Bisbee, Arizona porphyry, also occurs in this area. A consistency is found in the rocks hosting the deposits, most of which occur in Laramide intrusions. Typical lithologic varieties are the graniteI porphyry at Pinto Valley (Castle Dome), Arizona and the quartz monzonite porphyry at Morenci, Arizona. Many of these plutons invaded Precambrian metamorphic rocks, such as the Pinal Schist (Globe-Miami district), and Paleozoic carbonates. These carbonates, metasomatized by the Laramide intrusions, may constitute porphyryrelated skarn deposits, economically important chiefly because porphyries with carbonate wallrocks typically have higher hypogene copper grades than those without. Skarn hosted mineralization has been an important constituent of the ore-body at Santa Rita, New Mexico, which produced an average grade of 0.94% Cu in its first fifty years of production. In addition, the interest in, nonporphyry related skarns of the southwest U.S. has been considerable, as with the Pinos Altos, New Mexico and Bismark, Mexico properties. Further exploration in the southwest U.S. and adjacent Mexico may center on these porphyry and non-porphyry related high-grade skarns.