|
COPYRIGHT 1997 The Mineralogical, Inc.
MOROCOCHA DISTRICT
Yauli Province Junin Department
LOCATION
The Morococha district is at about 4400 meters elevation and is located roughly 17 km east-northeast of Casapalca and about 25 km west-southwest of La Oroya.
The Morococha district is a spectacular region of high glacier-clad peaks and lakes close to the crest of the western Andean range. It lies in a nearly east-west trending valley with its outlet to the east. Three lakes occupy the valley, at elevations ranging from 4,350 to 4,600 meters. The Quechua name Morococha means "painted lake," because of the brilliant coloring of the altered rocks surrounding the area (McLaughlin, 1945).
HISTORY
Deposits around Morococha were worked for a time by the Portuguese prior to 1634. Mining was again active in 1760, since in that year applications were submitted to the viceroy for the right to construct metallurgical plants. In 1915 the Cerro de Pasco Corporation bought most of the mining concessions from C. R. Pflucker, who had been the owner for a number of years. As of 1970 the district had seven major mines, three of which were active, and had some 560 km of tunnels, of which about 80 km were being used (Purser, 1971). In 1984 the new Huacracocha mine was placed in operation. In 1989 the Morococha complex suffered a devastating attack by "Shining Path" guerrillas, who destroyed the compressed air plant and other surface installations. In 1990 the mines were again operating, at 63% of capacity (Cavanagh, 1993?).
GEOLOGY
Morococha is considered to be part of the Central Peru metallogenic sub-province, which includes the Cerro de Pasco District, the Casapalca District, and the nearby area of Yauli. Much of the geologic and orebody data presented here are abstracted from Nagell (1960) and Petersen (1965).
The stratigraphy consists of Permian Catalina volcanics and the Jurassic Potosi Formation limestone. The main geologic feature of Morococha is a gently northwest-plunging anticline with intrusions cutting the anticline on the southwest flank. A large portion of the principal mine area is occupied by limestone and dolomite of the Pucara Formation, locally known as the Potosi Formation (Peterson, 1965).
Two main phases of igneous activity have been identified. The first is the Anticona quartz diorite, which bounds the western portion of the district. The second is the younger cross-cutting quartz-monzonite porphyry of the Morococha Series. Alteration by the Morococha intrusions is extensive and pronounced, especially around the San Francisco stock, and has created the colorful alteration assemblage characteristic of the area. Contact metamorphism extends as much as 1.6 km from the intrusives.
Nagell (1960) divided the district into eastern and western portions based on the orebody types. Vein structures from the dominant orebodies in the eastern part of the district, whereas pipe and manto (blanket-like) orebodies predominate in the western part of the district. Mineralization occurs over approximately 50 square kilometers.
The Catalina volcanics, host for the ore deposits, and the San Francisco stock are the principal rock types in the eastern part of the Morococha district. The veins are open-space fillings with well-developed euhedral crystals of enargite, tennantite-tetrahedrite, chalcopyrite, sphalerite, galena, pyrite, quartz and barite in open rugs. The ore minerals generally break cleanly from sharply defined vein walls, allowing easy collecting of specimens.
Alteration products of the Potosi Formation and the Gertrudis stock are the major rock types in the western part of the Morococha district. The pipe and manto replacement orebodies are of primary, importance. Open space-filling textures, such as vugs lined with crystals of sulfide minerals, are abundantly distributed in the manto and replacement bodies.
MINERALS
Both arsenic and antimony are present in the ores, and their proportion varies throughout the district, yielding a range of tennantite-tetrahedrite compositions. Regional zoning is distinct, with the center in the area of the San Francisco and Gertrudis stocks, where quartz-molybdenite veins occur. Enargite occurs mainly within, and immediately adjacent to, the intrusive rocks and is commonly associated with tennantite-tetrahedrite, which is found throughout the district. In the central or core zone, chalcopyrite is common, sphalerite rare, and galena practically absent, with quartz and pyrite the main gangue minerals. The predominant ore minerals in the intermediate zone are tennantite-tetrahedrite, sphalerite and chalcopyrite; galena is common and rhodochrosite, ankerite, and calcite are the gangue mineral assemblage, which may be locally abundant at the expense of quartz and pyrite. The dominant mineral assemblage at the margins of the district is sphalerite, galena, calcite and Ag-bearing tetrahedrite. Generally the arsenic content of the ores decreases away from the central part of the district.
Next to pyrite, sphalerite is the most widespread and plentiful sulfide in the district. Where enargite is plentiful, sphalerite is rare or absent.
Pyrite is abundant both in cubic and pyritohedral habits. For example in the Ombla orebody, vugs in the central part of the orebody are typically lined with pyritohedral crystals, while rugs near the margins of the orebody are lined with cubic crystals. The cubes and pyritohedrons are up to 15 cm in diameter, and are accompanied by well-formed quartz crystals. Quartz is common in the Ombla orebody vugs and, near the margins of the orebody, sphalerite occurs on or with the pyrite, or it occurs as the dominant mineral in the vugs, occasionally associated with some galena. The upper part of the Ombla manto is especially vuggy.
Bornite is found in small amounts throughout the intermediate zones of Morococha district orebodies. Barite is sparsely distributed in the central part of the district, but has a tendency to be more abundant at the fringes. Pyrrhotite and arsenopyrite are rare and have been found at only one locality in the Morococha mine. Bournonite occurs associated with aikinite and tennantite-tetrahedrite.
In the copper-lead-zinc-silver veins that cut the Catalina volcanics, quartz, pyrite and sphalerite are the most abundant minerals and were deposited early in the mineralization phase. They are followed by smaller amounts of chalcopyrite and enargite which are intergrown with tennantite-tetrahedrite. Dolomite, ankerite, siderite, calcite and rhodochrosite were the last to form. The association galena-tennantite-tetrahedrite-rhodochrosite has not been observed with enargite or bornite. In the orebodies which occur in the altered limestones, pyrite is the dominant gangue mineral, and quartz is subordinate, Fluorite is reported to occur along the edges of some of the orebodies.
Common mineral specimen associations are chalcopyrite-pyrite, tennantite-tetrahedrite-chalcopyrite-sphalerite, and sphalerite-galena; these associations commonly occur on a pyrite matrix and are associated with variable amounts of enargite and quartz. Chalcopyrite frequently was deposited on dark brown sphalerite, Much less common is the red sphalerite (in transmitted light) in rugs in chalcopyrite or other minerals, with tiny chalcopyrite crystals perched on the sphalerite.
The results of leaching by later fluids are observed in several veins of the district. The first mineral leached is barite, then galena, sphalerite, pyrite, tennantite-tetrahedrite, and finally the carbonates, in that order. Quartz does not seem to be attacked.
The generalized paragenetic sequence is: hematite, magnetite; quartz, molybdenite; pyrite; brown sphalerite, (arsenopyrite); enargite; bornite, chalcopyrite, tennantite-tetrahedrite; galena, carbonates; barite; red sphalerite, galena.
Today Morococha is not an abundant source of specimens. Occasional specimens seen for sale are usually (but not always) recycled from pre-existing collections.
Beginning in 1991, a large amount of rhodochrosite, in bright pink solid coatings of rhombs on matrix came on the market, These rhodochrosites are attributed to the Manuelita mine, "near Morococha." These same specimens have also been labeled as having been collected from Casapalca, and from the "Santa Rita" mine, "near Morococha" or "near Casapalca." These particular rhodochrosite specimens are discussed in more detail in the section on the Santa Rita mine, Morococha district.
Enargite [Cu.sub.3]As[S.sub.4]
Abundant specimens of enargite were produced from Morococha at one time, but never in the quality associated with other enargite specimen-producing districts, at least not within the last 30 years.
Pyrite Fe[S.sub.2]
Once abundant, pyrite specimens from Morococha are now rarely seen. Crystals are usually pyritohedrons in sizes up to several centimeters, Good typical specimens from here are about 6 x 7 cm, and are composed of bright pyritohedrons about l cm across partly coated with small white drusy quartz crystals. Pyrite, partly coating stout gypsum crystals, was recovered in abundance in the early 1970's; the appearance of these particular specimens on the world market signaled the start of serious interest by mineral dealers in the Peruvian mineral market.
Sphalerite (Zn,Fe)S
Sphalerite has been collected in red-brown translucent crystals to at least 1 cm on pyrite.
Tennantite-tetrahedrite [(Cu,Fe).sub.12][As.sub.4][S.sub.13] - [(Cu,Fe).sub.12][Sb.sub.4][S.sub.13]
Tennantite-tetrahedrite can occur on enargite and on other minerals. Crystals are black and are usually of simple tetrahedral habit. Common associated minerals include pyrite in pyritohedrons, rhombohedral calcite and white gypsum.
Vivianite [Mathematical Expression Omitted]
Dan Belsher (personal communication, 1992) reports that vivianite in little sprays of attractive bladed crystals has been found at Morococha. The specimens look totally different from the large, stout crystals collected from Bolivia.
Table 12. Minerals reported from the Morococha district.
Common or Abundant
Anhydrite Barite Bornite Calcite Chalcocite Chalcopyrite Covellite (*)Enargite Fluorite Galena Gypsum Magnetite Molybdenite (*)Pyrite (*)Quartz Rhodochrosite Rhodonite Scheelite Sphalerite (*)Tetrahedrite/tennantite Wolframite
Rare or Locally Abundant
Aikinite Alabandite Ankerite Arsenopyrite Bournonite Djurleite Dolomite Emplectite Famatinite Greenockite Gypsum Luzonite Marcasite Matildite Native arsenic Proustite Pyrrhotite (*)Siderite Stromeyerite (*)Vivianite
* Collector-quality specimens
COLQUI DISTRICT
Huarochiri Province Lima Department
LOCATION
The Colqui district is situated at the headwaters of the Santa Eulalia River and river valley 75 km northeast of Lima and 27 km northwest of Casapalca. Elevations of the workings range from 4,200 to 5,000 meters. The district measures about 16 km north-to-south and about the same dimensions east-to-west. The district has also been referred to as the "Huampar" district on the Mining Map of Peru (Boggio, 1985), and as the "Venturosa" district (Boggio, 1985, p. 18). The name Colqui comes from the Quechua word for "silver."
HISTORY
The district has been intermittently...
Read the full article for free courtesy of your local library.
|
Bookmark this article
Print this article
Link to this article
Email this article
Digg It!
Add to del.icio.us
RSS
