Porphyry Copper Deposits of the American Cordillera

Porphyry Copper Deposits of the American Cordillera Frances Wahl Pierce and John G. Bolm, Editors Arizona Geological Society Digest 20 1995 Geological Summary and Perspective of Porphyry Copper Deposits in Southwestern North America SPENCER R. TITLEY Un iversity of Arizona, Tu cson, Arizona ABSTRACT enci, and Bisbee. Under the impetus of material requirements fo r prosecution of the war, the search for copper ores by 1942 had Porphyry copper deposits in the American southwest focused almost solely on discovery of new disseminated orebodies comprise a distinctive and important metal resource. Their and resulted in identification of 25 porphyry copper deposits by exploration and exploitation during the past hundred years 1980. Among the first recognized as having porphyry copper was accompanied by significant advances in geological knowl potential by mid-century were the disseminated ores at Silver Bell, edge and development of increasingly efficient mining and described by Richard and Courtright (1954). Much of the early his extraction methods. Cause and effect are indistinguishable be tory of these deposits has been recorded from economic and engi cause the necessity for increasing mining efficiency resulted in neering perspectives by Parsons (1933, 1957). enhancement of extractive methods and because geological In the early I 990s mining was proceeding at a dozen depos study of the ores resulted in recognition of broadly applicable its in Arizona that had survived the economic downturn of the geological and exploration concepts. The early view of linear 1980s: Twin Buttes, Sierrita-Esperanza, San Xavier-Mission, tectonic-based regional controls was supplanted by integra San Manuel-Kalamazoo, Ray, Morenci, Pinto Valley and Inspi tion of concepts of plate movements. Early views of the envi ration, and Bagdad. Leaching of copper was taking place at ronment of fo rmation of hydrothermal ores have been given Mineral Park, Lake Shore, and Silver Bell. Chino and Tyrone rigor or modified by the results of studies based on modern were operating in New Mexico, and Cananea and La Caridad methods of analysis and application of sophisticated chemical were producing copper in Mexico. At this writing, ten more de principles. Deposits have been sites of basic studies of hydro posits have been explored but are undeveloped or unmined. thermal systems, the results of which have been applicable The porphyry copper deposits of the region have been not across a wide spectrum of ore deposits. Most importantly, in only sites of recovery of significant mineral wealth but also sites fo rmation and methods developed in the study of ores and of detailed and extensive studies of process geology and develop their mining have served as bases from which the search for ment of mining and beneficiation technology. Geological studies deposits in many regions of earth has proceeded successfully of mining districts of the southwest reported mostly by geologists during the past half century. of the U.S. Geological Survey during the early part of the century developed a basis of regional stratigraphy and structural geology INTRODUCTION that is used to this day with only minor modifications. Basic con cepts of ore occurrence and classification developed from studies The American Southwest, comprising the southwestern half of ores at Bisbee, Ray, Miami, and Morenci have been refined of Arizona and contiguous parts of New Mexico and Mexico continuously since the beginning of the century with heightened ("the region") is the site of an extraordinary endowment of por study during the decades of the 60s, 70s, and 80s. Mines and dis phyry copper deposits. Most deposits are in southeastern Ari tricts of the region have been sites of development and testing of zona where historic and recent copper production amounts to now widely used exploration technology that includes both geo about two-thirds of newly mined copper in the United States. physical (Brant, 1966) and geochemical (Bloom, 1966; Chaffee, Significant quantities of by-product molybdenum, silver, gold, 1982a, 1982b) methods. Arizona deposits have been sites of de and platinum are also produced. velopment of innovative technology in mineral extraction during This paper presents a brief history of the technological evo the past half century and have served as field laboratories for de lution of southwest porphyry copper deposits as bulk low-grade velopment of methods and machinery. deposits and the evolution of knowledge of their settings, habits, and genesis over the past century. This summary sets forth im Production and Grade portant parameters that may be contrasted and compared with other porphyry systems in other regions. The writer has selected A tabulation of grade and tonnage of porphyry copper ores significant characteristics described in an extensive literature of the region has been published by Gilmour (1982) and sum developed during the past half century to serve as fundamental marized by Titley and Anthony (1989) and Titley (1994). Re points from which to consider the environment and processes of serves in 1989 have been listed by Beard (1989). Production formation of porphyry copper deposits as viewed in the 1990s. figures summarized for 45 porphyry copper deposits in the United States by Titley and Beane (1981) are 15.8 billion tons of Background copper ore production and reserves with a weighted copper grade of 0.68 percent. Fourteen deposits that reported molybde Following Daniel Jackling's innovations in mass ffilnmg at num had a weighted grade of 0.0325 percent molybdenum. Bingham, Utah, during early years of the century, mines in the southwest gradually evolved from high-grade, mostly underground Literature and the Growth of Knowledge operations to bulk low-grade, mostly open-pit operations. These districts and mines included Miami, Inspiration, Ray, Chino, Mor- Literature of the first half of the century dealt largely with 6 GEOLOGICAL SUMMARY AND PERSPECTIVE OF PORPHYRY COPPER DEPOSITS IN SOUTHWESTERN NORTH AMERICA 7 field observations and descriptions of known deposits. Much of tive and associated ore districts with major lineaments and zones this was in Monographs and Professional Papers of the U.S. of rifting. Even at this early time Schmitt invoked potential con Geological Survey, as Transactions of the American Institute of tinental drift as a mechanism to explain continuation and offset Mining and Metallurgical Engineering, and in serial publications of his major structures and proposed effects related to the influ such as Economic Geology. Viewed from the perspective of the ence of the East Pacific Rise. Using large numbers of potas evolution of ideas, papers of the 1940s were the first important sium-argon radiometric dates, Jerome and Cook (1967) carried reports treating alteration, mineral zoning, rock descriptions, and the notion of occurrence of districts in belts to a degree of so regional settings of these large systems. Collections of current phistication and examined porphyry associations with character reports focusing on the region and treating both deposits and istics of crustal geology. The idea that regional structures control topical infonnation have been published by Titley and Hicks (1966) the location of porphyry copper districts has withstood the ad and Titley (1982a). Papers treating general properties of the de vent of more recent notions concerning the evolutionary history posits of the region as contrasted with deposits of other areas of continental margins based on plate motions. Conflictingideas have been published by Jerome and Cook (1967), Lowell (1974), of the importance of flaws in the crustal fabric of North America Hollister (1978), Titley and Beane (1981), and Titley (1992, to district localization have been expressed by Lowell (1974) 1994). Comparable treatment of porphyry deposits in western and Sillitoe (1975). Canada has been published by Sutherland Brown (1976). A surge of scientific interest has been marked by a compa Areal and Structural Geology rable increase in the flux of papers treating different properties of porphyry copper systems. This increase began in the early Porphyry deposits of the region lie above the North Ameri 1960s, and the numbers of publications reached a peak during can craton in Arizona, New Mexico, and contiguous Mexico. the late 1970s and early 1980s. The period was witness to appli The geology of this region has been described in relevant parts cation of increasingly widely available techniques of measure by various workers including Anderson (1966, 1968) and Titley ment and applications and testing of theory. Many concepts of (198 1, 1982b, 1992). The craton basement of the region com hydrothermal ore genesis were refined. Porphyry systems were prises Proterozoic sedimentary and volcanic rocks at varying studied as examples of epicrustal intrusion-centered ores sys levels of metamorphism and batholithic scale plutons represen tems, of the evolution of hydrothennal processes in time and tative of at least three episodes of Proterozoic igneous activity. space, of the evolution of petrogenetic systems, of metal and al Most of the region's porphyry copper systems lie within or above teration zoning, and of supergene enrichment and weathering a 1.68 to 1.72 Ga basement dominated by clastic metasedimen processes. Many fundamental and now widely accepted notions tary rocks and included plutons; three deposits in northwestern of hydrothermal ore formation evolved from these studies.

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