Atacocha, El Porvenir
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Super Porphyry Cu and Au|
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The Atacocha and El Porvenir Cordilleran polymetallic epithermal Zn-Pb-Cu-Ag deposits are located at ~4000 m elevation, ~190 km NE of Lima, and 14.5 and 10 km NNE to NE of Cerro de Pasco respectively, in the Department of Pasco, Central Peru. The two deposits are ~2 km apart and are connected by underground drives (#Location: Atacocha - 10° 35' 3"S, 76° 12' 37"W; El Porvenir - 10° 36' 37"S, 76° 12' 34"W).
The presence of mineralisation in the Atacocha district has been known since colonial times due to its proximity to Cerro de Pasco deposit, whose rich silver-bearing oxide ores were exploited by the Spaniards. However, as oxidised ores are relatively scarce in the Atacocha district, and the sulfides were not
amenable to treatment by the priinitive recovery rnethods then in use, production prior to the 20th century was probably insignificant.
Figures are only available from 1940 and show a production of nearly one million tonnes of ore, almost all from the Atacocha mine, which yielded >90 000 t of Pb, 65 000 t of Zn and 225 t of Ag from 1940 to 1953. The mines are operated (in 2016) by Compañía Minera Milpo, a Peruvian company that was formed in 1949 to exploit El Porvenir, and since 2010 has become a subsidiary of the Votorantim Group of Brazil. Modern exploration began at Atacocha around 1910, although successful exploitation did not commence until 1936 when the Compañía Minera Atacocha was formed, which was subsequently acquired by Compañía Minera Milpo in 2008.
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
The sequence within the immediate Atacocha-El Porvenir district is composed of Late Permian to Cretaceous sedimentary and volcano-sedimentary rocks and is 2400 to 3900 in thickness. The sequence in the district is as follows, from the base (after Johnson et al., 1955; Johnson, 1955; Bendezú and Fontboté, 2002):
Devonian or older
• Excelsior formation - grey phyllite and quartzite.
Upper Permian to Lower Triassic
• Mitu Group which occur as anticlinal inliers to the east and west of the deposits, and comprises >600 m of red sandstone and shale with minor amounts of conglomerate, and locally abundant mafic volcanic flows.
Late Triassic-Early Jurassic
• Pucará Group - 1000 to >2100 m of limestones, dolostones and lesser interbedded shales, which disconformably overlie the Mitu Group. Grey to black limestone is the predominant lithology, with minor black shale. Chert nodules and lenses are common in the limestone. Individual beds range in thickness from 0.1 to to 1 m.
• Goyllarisquisga Group - 100 to >150 m of cross bedded white quartz sandstone with a discontinuous conglomerate at base and red beds towards the top. This unit disconformably overlies the Pucará Group. In the Atacocha mine area, it consists of bedded chert and chert-breccia at the base, overlain by dark shale and fine grained sandstone containing charred wood fragments, overlain in turn, by white cross-bedded quartz sandstone that makes up the bulk of the unit. In the Atacocha-El Porvenir district it occurs on both limbs of a gently south-plunging, north-south trending syncline
• Basalt flows - that is 0 to 150 m thick, with minor amounts of red shale and sandstone that conformably overlies the Goyllarisquisga Group and locally contains white quartz sandstone near the centre of the section. These basalts form the core of the syncline mentioned above.
• Chicrín limestone - that is 80 to 120 m thick and comprises a conformably overlying succession of massive and thin bedded, brown, unfossiliferous limestone.
Regionally, additional Cretaceous basalts and limestones overlie the sequence above.
• Conglomerate unit that is up to 450 m thick, and is composed of red weathering conglomerate with chert and limestone pebbles and cobbles, increasing in coarseness upward, with some interbedded sandstone, shale and limestone.
• Intrusive Rocks - dykes, sills and small stocks of dacitic intrusive rocks are common in the central part of the Atacocha-El Porvenir district. All but one stock and most of the dykes and sills are near the Atacocha fault which follows the Pucará Group to Goyllarisquisga Group contact on the eastern limb of the main syncline. The two largest intrusive bodies lie near Atachocha on either side of the Atacocha fault. The western intrusion is more irregular, with a 1.5 km long curved outcrop that is 100 to 200 m wide. It is greenish grey to dark grey in colour, with up to 5 mm phenocrysts of plagioclase and quartz, together with smaller hornblende and biotite, enclosed in a fine-grained groundmass of quartz and plagioclase, possibly with some orthoclase. The stock to the east of the fault is lenticular, with maximum dimensions of 1 km x 150 m, elongated along the strike of the limestone of the enclosing Pucará Group. It is strongly weathered and appears similar in composition to the intrusion to the west, although it probably contains more quartz. Near El Porvenir, sections of a light coloured dacite stock are exposed, containing phenocrysts of andesine, hornblende, biotite and pyrite, in a groundrnass of quartz, feldspar and carbonate. Other small stock are also exposed locally, as are dykes and sills, most commonly adjacent to the larger intrusives but also sporadically over an area of several square kilometres.
The sedimentary rocks and lavas have been metamorphosed for variable distances from their contacts with the intrusive rocks, and have been altered depending on their lithology and the size of the intrusive. The metamorphosed rocks have been modified by the addition of silica, iron and sulphur. The most intense metamorphism/alteration appears to be in the limestone beds of the Pucará Group west of the lenticular porphyry intrusion southeast of Atacocha, where they are completely silicified and impregnated with pyrite for a maximum distance of 180 m from the western margin of the porphyry.
The Atacocha fault divides the district into two structural domains and has a displacement of as much as 2000 m. To the east, the principal structural features are simple folds. To the west, 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 is a reverse structure, dipping steeply eastward, the east side having moved up and to the north relative to the west side.
Within the Atacocha mine, the most important structure is the reverse No. 1 fault that dips 45 to 60° E, and on which Pucará limestone has been thrust over quartz sandstone of the Goyllarisquisga formation. At depth, it steepens and joins a steep westward-dipping fault, and may be pre-ore, although veins rarely cross the structure.
Within the Atacocha mine, rocks of the Pucará and Goyllarisquisga groups are intruded by an irregular dacite body and by several dykes and sills, also of of dacite. Both folding and faulting of these rocks are important ore controls at Atacocha. The major structures comprise an asymmetric, south plunging syncline in the sandstones, west of the mine, and a cross fold in the limestone directly above the mine workings. This latter structure seems to have caused by drag along the Atacocha fault. Ore in the Atacocha mine is found in veins and replacement bodies near the axis of the fold. Replacement bodies are the principal source of ore at Atacocha but are not important at El Porvenir. The veins at Atacocha and Milpo seem to be formed by fissure filling.
Galena and sphalerite are the principal ore minerals, accompanied by 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 in the oxide zone to the north. Three stages of mineralisation are recognised at Atacocha: i). quartz-pyrite; ii). sulphide; and iii). realgar-orpiment.
The most significant features in localising mineralisation appear to have been the porphyry intrusions, the Atacocha fault, and the contact of the Mitu and Pucará groups. Virtually all the ore is hosted in limestone of the Pucará Group or in the overlying sandstones of the Goyllarisquisga Group, whilst rocks of the Mitu group and those younger than the Goyllarisquisga Group seem to have been unfavourable for ore deposition.
Large irregular replacement bodies at Atacocha occur in a 50 to 60 m thick section of the Pucará limestone above the No. 1 fault, and have provided much of the ore at the Atacocha mine. The ore bodies were formed by incomplete replacement of limestone by sulphides and clay minerals, and vary widely in size. One of the largest, had a down plunge length of ~250 m, a breadth of 40 to 70 m, and a thickness of 4 to 10 m. The replacement ore bodies seem to be related to some of the larger veins, which may have served as feeders.
As at Atacocha, the sedimentary rocks are primarily limestone of the Pucará Group and quartz sandstone of the Goyllarisquisga Group, both striking at ~340 to 345°. The limestones dip steeply or vertically, whereas the sandstone dips 10 to 20°W., except near the Atacocha fault where the
dip steepens to 55°. The sequence has been extensively intruded by a stock and a series of dykes and sills of dacite up to 40 m thick. The limestones are cut by a series of transverse faults and by several bedding-plane faults. The transverse faults are grouped into a NE-trending set to the north, and a NW-trending set further south. The NE-trending set strikes from 60 to 80° and dip at 75°N, with most lying to the east of a >500 m long bedding-plane fault. The NW-trending faults strike from 315 to 280° and dip 65 to 80°N. All faults or veins are confined to the area east of the Atacocha fault. Both sets cut the intrusives and offset contacts by 5 to 15 m and are probably conjugate fractures. Ore was deposited mainly in open spaces and is restricted to the NE-striking fractures, with one exception, the San Carlos vein, which is the largest in the area, with a width of from 0.8 to 3 m. Most other veins vary from 0.1 to 1 m in width.
Mineralisation is simple fissure filling with very minor replacement. The principal ore minerals are galena and sphalerite, together with minor jamesonite and tetrahedrite, and occurwith in a gangue of calcite and pyrite. Some quartz, fluorite and rhodochrosite also occur with the gangue minerals. Angular sulphide fragments in faults and ground-up sulphides in gouge indicate post-mineralisation movement.
Reserves and Resources
Published ore reserves and mineral resources at 31 December, 2014 (Milpo, 2015) were:
Proved + probable reserve - 6.839 Mt @ 2.79% Zn, 1.29% Pb, 0.30% Cu, 59.4 g/t Ag;
Measured + indicated resource - 3.392 Mt @ 3.95% Zn, 1.55% Pb, 0.34% Cu, 68.7 g/t Ag;
Inferred resource - 12.869 Mt @ 3.67% Zn, 1.50% Pb, 0.43% Cu, 67.8 g/t Ag;
Reserves + resources - 23.101 Mt @ 3.45% Zn, 1.44% Pb, 0.38% Cu, 65.3 g/t Ag.
Proved + probable reserve - 21.649 Mt @ 3.56% Zn, 0.61% Pb, 0.25% Cu, 42.3 g/t Ag;
Measured + indicated resource - 9.948 Mt @ 4.53% Zn, 1.04% Pb, 0.24% Cu, 66.9 g/t Ag;
Inferred resource - 19.700 Mt @ 3.78% Zn, 0.89% Pb, 0.31% Cu, 63.1 g/t Ag;
Reserves + resources - 51.298 Mt @ 3.83% Zn, 0.80% Pb, 0.27% Cu, 55.0 g/t Ag.
Note: Little recent geological information has been encountered on these deposits, and what us inserted her has been largely derived from "Johnson, R.F., Lewis, R.W. and Abele, G., 1955 - Geology and ore deposits of the Atacocha District, Departamento de Pasco, Peru; Geological Investigations of the American Republics; USGS Bulletin 975-E, 56p.
The most recent source geological information used to prepare this summary was dated: 1955.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Johnson, R.F., 1955 - Geology of the Atacocha mine, Department of Pasco, Peru: in Econ. Geol. v.50, pp. 249-270.|
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