Field trip to Chile, January 3-14, 1999
Students from the combined Society of Economic Geology Student Chapters of the University of Utah and the NMIMT, with representatives of mineral exploration companies, took part in an 11-day trip to visit ore depositsof northern Chile, January 3-14, 1999. The trip was supported by exploration company contributions, host mines, and a grant from the Society of Economic Geologists Foundation.
Chile ranks number one in copper production, providing nearly 20% of the world's copper; hence, our focus was on the red metal. mine visits started at the Paleocene-age Cerro Colorado porphyry copper deposit near Iquique. Supergene mineralization at Cerro Colorado is characterized by a chrysocolla-chalcocite assemblage and reserves of more than 200 million tonnes (Mt) of ore grading 1.01% Cu. A sulfide resource approximately twice as large with a grade of about 0.5% Cu represents potential future ore. This mine-for-leach operation produces slightly more than 60,000 t/yr of fine copper.
The early Tertiary-age Quebrada Blanca porphyry system, at an elevation of 4,200 m, was our next stop. This mine, part of the Collahuasi district, produces 75,000 t/yr copper from a heap leach operation. Initial reserves were more than 150 Mt of ore at 1.1% Cu as chalcocite derived from a pyrite + chalcopyrite protolith.
The giant Chuquicamata mine provided a chance for participants to see the structural complications associated with emplacement of one of the world's largest metal repositories; total contained copper metal at Chuquicamata is greater than 87 Mt, and the mine produces Mo, Re, Au, Ag, Se, As, and U, in addition to more than 650,000 t of copper per year. The more than 800-km-long Domeyko fault system apparently has served as a synmineral and postmineral structure, along and adjacent to which many porphyry-style ore deposits are located.
In addition to the complex of intrusions at Chuquicamata and Radomiro Tomic, the El Abra porphyry system and adjacent Maria Vein system appear to be part of the development of this metallotect feature. Our visit to El Abra gave participants rhe opportunity to see how the weathering of a silicate-altered intrusive complex results in the in situ formation of copper oxides from a low pyrite, chalcopyrite + bornite protore. El Abra produces about 220,000 t/yr of copper cathodes, derived from the world's largest hep leach operation. Crossing the central Atacama Desert and the historic nitrate fields, now being remined to make Chile the world's leading producer of iodine, we visited the newly commissioned Lomas Bayas mine and the adjacent Fortuna de Cobre prospect, both owned by Boliden. At Lomas Bayas, heap and dump leach operations produce copper from a mixed copper oxide assemblage of brochantite, antlerite, atacamite, and chalcanthite, hosted by a Paleocene-age intrusion and associated breccia bodies. The Fortuna de Cobre prospect is also of Paleocene age, and represents an oxidized pyritic protolith consisting of copper sulfates, iron sulfates, and relic pyrite +/- chalcopyrite. Andesites and rhyolites host the Triassic(?) Mantos Blancos Cu-Ag deposit with mineralization consisting of digenite, chalcocite, bornite, chalcopyrite, and idaite ores, whereas pyrite and chalcopyrite constitute low grade mineralization adjacent to the high grade orebodies. The mine produces about 85,000 t/yr copper, as cathode and in concentrate. The former series of multiple open pits is being turned into a single "super pit" to accomodate production of leachable copper ores.
A quick visit to some prospects in the coastal cordillera near Antofagasta showed that copper oxides are developed in many of the andesitic volcanic rocks of Mesozoic age, probably as a result of low temperature metamorphic remobilization of copper from those same andesites.
Our final mine visit was to the La Escondida district, where we saw the well-developed chalcocite-brochantite enrichment zone at Zaldivar. Producing 120,000 t/yr of copper cathode, this leach operation is adjacent to the La Escondida and Escondida Norte properties, and is located astride the Domeyko fault system. Total contained copper in the La Escondida district now amounts to more than 65 Mt, and will probably increase as exploration continues.
We are very grateful to all our supporters for the financial and logistical assistance they provided to make this trip possible for students from the United States, Papua New Guinea, and the Dominican Republic. In addition, the opportunity for students to learn from mineral exploration professionals was invaluable. We encourage other student chapters to take advantage of the support provided by the Society of Economic Geologists, and we acknowledge the continued motivation of SEG leadership to promote student field trips in association with economic geology professionals. The trip was led by Dr. William X. Chavez, Jr. of NMIMT and Dr. Erich U. Petersen of the University of Utah.
The road to Calama offers excellent exposures for studying the regional geology of Northern Chile. Jurassic metasedimentary rocks are exposed along one segment of the Falla Oeste (West Fault) system. Many copper porphyry systems in northern Chile, such as Zaldivar and Mantos Blancos are associated with this NE-trending regional fault.
Mineralization at Zaldivar consists of a leach cap over sulfide ore. The leach cap in places is 200 meters thick. Its copper grade varies between 0.3 and 0.5%, and the principal minerals are brochanthite and chrysocola. An unusual mineral, neotocite (a Cu-rich Fe-Mn oxide) is also present. The sulfide mineralization occurs as disseminations and veinlets. Chalcopyrite, bornite, and molybdenite are principal ore minerals. Alteration types at Zaldivar are typical of copper porphyry systems.The Atacama plateau, which rises above the Atacama desert, has a mean elevation of about 4000 meters (13000 feet). The Atacama region has been called "the driest desert in the world". It receives fewer than 15 cm (6 inches) of rain per year.
Mineralization at Mantos Blancos covers 30 square kilometers, and the mine consists of five open pits. The oxide ores (average 1.4% Cu) are 75% atacamite and 25% chrysocola. The sulfide ores (average 1.86% Cu) are bornite, chalcocite and chalcopyrite. Mantos Blancos is not a typical porphyry copper deposit. It exhibits Na and Fe enrichment in addition to silicification. Albite and specular hematite are typical alteration minerals.
The fieldtrip was attended by the students of joint New Mexico Tech-Univ. of Utah SEG student chapter and led by Dr. William X. Chavez, Jr.(New Mexico Tech) and Dr. Erich Petersen (Univ. of Utah).