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Geology and ore deposits of Atacocha district, departmento de Pasco, Peru
Geologic investigations in American Republics, geology and ore deposits, Atacocha district, Peru
Geological Survey Bulletin;n° 975 - E
The Atacocha district is in central Peru between the Río Tingo and Río Huallaga north of Cerro de Pasco. Roads from Cerro de Pasco extend along both the Tingo and Huallaga, and all mines are accessible by road. The district lies on a ridge extending north from the high central plateau of Peru and is characterized by steep topography with more than 900 meters of relief. The district and the three producing lead-zinc mines in it were studied in 1952-53 by geologists of the Instituto Geológico del Perú and the U. S. Geological Survey. Bedded rocks range in age from pre-Permian to Tertiary(?). The pre-Permian rocks are exposed only on the edges of the district. The total thickness of Permian and younger rocks is estimated at 2,420 to 3,930 meters, but no complete section is exposed owing to folding and faulting. The Mesozoic section ranges from 1,400 to 2,900 meters in thickness. The Pucará formation of Late Triassic to Early Jurassic age makes up the lower two thirds of the section, and is composed of limestone with some interbedded shale. Cretaceous rocks overlie the limestone and consist of quartz sandstone overlain by basalt flows. Cretaceous limestone is found at the top of the section in two horizons separated by basalt flows. The youngest consolidated rocks are conglomerates of probable Early Tertiary age. The sedimentary rocks are dated largely by lithologic correlation; fossils were found only in the Pucara formation. Dacite of probable Tertiary age intrudes the bedded rocks. Dikes and sills are numerous and four small stocks were found. All but one stock and most of the dikes and sills are near the Atacocha fault, which may have formed a favorable zone for intrusion. The intrusives are most abundant between Milpo and Atacocha. The Atacocha fault divides the district into two structural areas. East of the fault the principal structural features are simple folds. To the west of the fault, folding is more complex and the rocks are cut by a series of northwesterly-striking faults, and a fault parallel to the Atacocha fault. The Atacocha fault, a reverse fault, dips steeply eastward, the east side having moved up and to the north relative to the west side. The history of the area between Permian and Late Cretaceous time was one of marine and continental deposition interrupted by three and perhaps four periods of uplift and erosion. No strong folding took place during this interval as the sedimentary contracts are either conformable or disconformable. Volcanic activity occurred during the Permian and again in the Cretaceous. The Andean orogeny began in Late Cretaceous time and continued through much of the Tertiary. The rocks were folded, faulted, and intruded by dacite. Subsequent to the intrusions, lead and zinc sulfides were deposited from hydrothermal solutions, possibly as a late phase of intrusive activity. The ore deposits of the district are concentrated in three areas, Milpo, Machcán, and Atacocha. Lead and zinc are the principal metals obtained, and silver is an important byproduct. More than 90,000 tons of lead and 65,000 tons of zinc have been produced since 1940, nearly all from the Atacocha mine. Galena and sphalerite are the principal ore minerals. They occur with small amounts of jamesonite and tetrahedrite-tennantite in a gangue of pyrite, calcite, clay minerals, quartz, rhodochrosite, and fluorite. Cerussite is an important lead mineral at Machcán where most of the workings are in the oxide zone. Three stages of mineralization are recognized at Atacocha: a quartz-pyrite stage, a sulfide stage, and a realgar-orpiment stage. The deposits are classified as leptothermal to mesothermal. Both replacement ore bodies and veins are found in the district. Replacement bodies are the principal source of ore at Machcán and Atacocha but are not important at Milpo. The veins at Machcán are formed by replacement, but those at Atacocha and Milpo seem to be formed by fissure filling. The most important features in localizing the ore deposits appear to have been the porphyry intrusions, the Atacocha fault, and the contact of the Mitu and Pucará formations. Nearly all the ore is found in limestone of the Pucará formation or in the overlying sandstones of the Goyllarisquisga formation. Rocks of the Mitu group and rocks younger than the Goyllarisquisga formation seem to have been unfavorable for ore deposition.
United States Department of the Interior, Geological Survey
Johnson, R. F.; Lewis, R. W. & Abele, G. (1955) - Geology and ore deposits of the Atacocha district, departamento de Pasco, Peru. United States Department of the Interior, Geological Survey, Bulletin, 975-E, pp. 337-388
Preparado en cooperación con el Instituto Nacional de Investigación y Fomento Mineros, Ministerio de Fomento y Obras Publicas del Perú bajo el auspicio de Foreign Operations Administration, United States Department of State.

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