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Huaron Group.

THE HUARON MINES

Cerro de Pasco Province Pasco Development

LOCATION

Huaron lies 40 km south-southwest of Cerro de Pasco, on the road from Canta, near Huayllay. The district is about 14-15 km due west of the northwest shore of Lake Junin, and lies next to Lake Huaron. The mines are located on the east flank of the Western Cordillera of the Andes, at elevations ranging from 4,300 to 4,800 meters: the district is small, measuring about 3 km by 4 km [ILLUSTRATION FOR FIGURE 81 OMITTED].

HISTORY

Pasco Department was originally part of Junin Department; they were divided after 1919 into the current configuration. Initially the Huaron area was known as the Huancavelica District of Junin Department (Miller and Singewald, 1919). This led to confusion as to precise mineral locations. The mine was referred to as the San Jose mine in the 1920's, and is now considered to be in the San Jose de Huayllay district. The Compagnie des Mines de Huaron has been active at Huaron since 1912, and in continuous production since 1914. Ownership of the mines was partially sold off in 1987 to several other companies. Peak output was reached in the 1970's, but fell off in the 1980's as ore grades declined and costs increased. The low price of silver hurt the company also, as it has so many Peruvian mining companies. In 1991 the Compania Minera Huaron S.A., as the company is titled, filed for temporary closure and reduction of personnel. As of April 1991, ore production was 40% of normal (Cavanagh, 1993?). Production today is predominantly lead and zinc (approximately 96%), with lesser copper and minor silver.

GEOLOGY

The Cretaceous Machay Group limestones and Tertiary Pocobamba (Casapalca Red Beds) continental sedimentary rocks are the dominant sedimentary rocks in the area. The Huaron anticline is the dominant structural feature of the area. The Machay Group limestones and Pocobamba sedimentary rocks are strongly folded, and are intruded by quartz monzonites and quartz monzonite dikes, with accompanying fracturing. This fracturing was followed by alteration and mineral deposition by hydrothermal fluids. Following the intrusion of the dikes, the sedimentary rocks were further compressed and fractured, and the fractures were subsequently mineralized by hydrothermal fluids. These fractures frequently attain considerable width, and persist to great depth. The dikes have undergone extensive hydrothermal alteration, typified by sericitization, kaolinization and pyritization. Post-mineralization Huayllay pyroclastic volcanics have mantled the region.

The area has been glaciated, and surface oxidation is minimal. The ore is described by Petersen (19653 as similar to that found at Morococha. The deposits are composed of veins, shoots and mantos (blanket like orebodies); general zoning occurs from the center of the district outward in an asymmetrical pattern.

Huaron is described as a complex copper-lead-zinc-silver deposit; distinct zonation exists from the center zone where copper ores predominate, to an outer lead-rich zone. Five major east-west-striking lead-zinc ore veins are distributed over 3 or 4 km, along with two northeast striking copper-ore veins. Ore-bearing veins vary from a few dozen centimeters to 10 meters wide, and may extend, in the larger veins, over 1,800 meters horizontally. Ore veins have been traced to depths exceeding 500 meters.

Three major periods of mineralization occurred in the fracture zones. The initial mineralization consists of relatively high-temperature minerals deposited in the following order: milky quartz, pyrite, enargite, and tetrahedrite. Enargite dominates the mineralization in the central part of the district, while tetrahedrite dominates the outer part of the enargite zone. Reopening of the fractures caused the initial mineralization to be brecciated, and the breccia was subsequently cemented together by the next, second-period generation of medium-temperature minerals: milky quartz, brown sphalerite, and galena. Crystals of large size were deposited during this period, usually on a botryoidal gangue of siderite, dolomite and rhodochrosite. A final, third period re-fracturing, followed by a rapid deposition of hydrothermal minerals, resulted initially in the formation of minerals with colloform and botryoidal textures. This rapid deposition continued with fine-grained crystallization and continuous late precipitation of the carbonates, beginning with siderite and gradually changing to dolomite, rhodochrosite, and calcite. As a final pulse during this late-stage deposition, barite, pale to reddish amber-colored sphalerite, galena, tetrahedrite, polybasite and chalcopyrite were deposited.

Most of the ore being mined today is from veins in the second stage lead-zinc mineralization zones. There are about 20 mines in the Huaron district; in the south and middle part of the district are the Andalucia, Restauradora, Cometa, Elena, Yanamina, Travieso, Alianza, and Yanacreston mines; in the north part of the district are the Shiusha, Mechita, and Patrick mines; and in the western part of the district are the Fastidiosa, San Narcisco, and Constancia mines. The Alimon mine, a source of specimens, is not mentioned in the literature.

MINERALS

Through the early 1990's Huaron continued to furnish to the mineral market a large percentage of mixed-sulfide mineral specimens, many of which are mislabeled as coming from other Peruvian areas.

The Alimon mine is a small mine contributing the bulk of the Huaron specimens. Most of the specimens are from just one or two levels within the mine. It became a major supplier in about 1985 of "textbook-perfect" chalcopyrite, sphalerite, pyrite and other attractive minerals. These minerals are often nestled in groups of needle-shaped quartz prisms, some of which show delicate Japan-law twins.

Ankerite Ca([Fe.sup.+2],Mg,Mn)[(C[O.sub.3]).sub.2]

Ankerite occurs as a relatively common minor accessory mineral, usually as small (about 1-mm), beige to pinkish colored rhombic crystals. Some curving of the crystal faces results in saddle-shaped crystals. Visually distinguishing between ankerite and dolomite is difficult.

Barite BaS[O.sub.4]

Crystals are zoned (white in the center, clear and colorless on the edges) and are simple flattened tablets about 1 cm on edge.

Chalcopyrite CuFe[S.sub.2]

Chalcopyrite usually occurs as disphenoids, giving the crystals a pseudo-tetrahedral crystal shape. Twinning is common. Chalcopyrite crystals are frequently coated by later, often microscopic, chalcopyrite, sphalerite, tetrahedrite, marcasite (?) and enargite (?). When not coated by these other minerals, chalcopyrite crystals can be sharp and clean, with a lustrous dark golden yellow color containing just a hint of greenish yellow overtone. When coated by later-growth chalcopyrite, the chalcopyrite crystals are a dull, dark golden yellow in color. Tarnished crystals are not uncommon. Pyrite crystals are often seen embedded in the chalcopyrite crystals like raisins in a cookie. Coatings of gray tetrahedrite on chalcopyrite usually have a preferred orientation; on twinned crystals this is readily apparent, with each twin having a different reflectivity due to the reflection of light off the microscopic tetrahedrite crystal faces. Chalcopyrite crystals often exceed 5 cm in size, and aggregates of crystals can be considerably larger. In many specimens the chalcopyrite crystals are scattered on and amongst the usually white to gray prismatic quartz crystals that are typical of the locality. Others are interspersed with complex black sphalerite crystals.

Dolomite CaMg[(C[O.sub.3]).sub.2]

Dolomite occurs as beige to pinkish colored saddle-shaped crystals that make attractive specimens. Dolomite also occurs as replacements after calcite. These dolomite pseudomorphs after rhombobedral calcite are a pale beige-pink in color with a pearly luster. Quartz is the most common associated mineral.

Galena PbS

Galena from the Alimon mine usually occurs as lustrous, dark gray cubes having a slight bluish tint, and is commonly associated with chalcopyrite. Some of the galena cubes are corroded, giving them a skeletal appearance. The only distinctive feature of the non-corroded galena is a tendency toward wavy, ripple-like lines crossing the cube faces; these lines resemble wavy strand lines on a beach. The crystals routinely average 2.5 cm in size.

Pyrite Fe[S.sub.2]

Pyrite is a very common associated mineral on specimens from Huaron. The crystals are usually very bright, with the cube form dominant. The cubes are often slightly modified by pyritohedral and octahedral forms that add extra, albeit small, faces to the edges and corners. Cube faces are generally striated to some degree, whereas the modifying faces are usually mirror-bright and nonstriated. Cubes commonly range in size from 1 to 2 cm, with exceptional crystals reaching 5 cm in size. Quartz is commonly associated with the pyrite. An attractive and common specimen combination from Huaron is long, tapered quartz crystals with lustrous pyrite cubes nestled among them.

Quartz Si[O.sub.2]

The quartz from the Alimon mine makes very attractive specimens, particularly when combined with other minerals. The quartz is distinctive, with a large aspect ratio (1:8 approximately), and is commonly slightly tapered, giving crystals a quill-like appearance. The crystals are generally oriented perpendicular to the matrix and occur frequently in divergent sprays. They vary in color from white to colorless and are usually transparent, or nearly so. Prism faces vary from smooth, with a glassy to brilliant luster and very fine striations, to ribbed or bumpy, with a dull to satiny luster. The basal areas of quartz dusters usually have many smaller, long, thin, hairlike quartz crystals growing randomly around the larger ones. The initial impression one gets of quartz specimens from Huaron is of white to colorless, porcupine-like aggregates of long, thin, tapered crystals projecting out from the matrix, frequently with a divergent habit. Japan-law twinning is occasionally present, particularly in the denser clusters of small crystals that grow around the bases of the larger crystals. Japan-law twins to at least 5 cm in size have been collected.

An additional relatively common attribute of Huaron quartz is the presence of dark, blood-red inclusions of hematite. This feature can help distinguish some Huaron quartz from other Peruvian localities. The hematite occurs as minute flakes, shreds and freckle-like stains, almost always with a distinctive hematite-red color. The concentrations of hematite inclusions vary from moderately heavy and obvious to the naked eye, to infrequent microscopic shreds that are only observed under magnification. The hematite inclusions generally occur on the same side of all the quartz crystals in any given group. Bright cubic pyrite crystals are commonly implanted among the quartz crystals, creating aesthetic specimens. Sphalerite is frequently the major matrix mineral for the quartz.

Sphalerite (Zn,Fe)S

Sphalerite is thus far ubiquitous in the specimens marketed from Huaron. It is usually present as blackish brown crystals with a satiny to glassy luster. Crystal sizes to 5 cm are common. The most abundant crystal forms are the tetrahedron, octahedron and tristetrahedron. These forms are combined in crystals with a flat, triangular octahedron face that is distinctive. Triangular growth lines are common on crystals, as well as striations from repeated twinning of the crystals. Sphalerite is frequently the matrix mineral for the other mineral species,

Sphalerite occurs in two generations of crystals. The first, and most abundant generation is blackish brown in reflected light, with an amber to brownish red color in transmitted light. The less common, second-generation sphalerite crystals usually occur as tetrahedral to warty-looking rounded crystals that rarely exceed a few millimeters in size, on pyrite and chalcopyrite. These second-generation sphalerites are usually lemon-yellow to pale amber in transmitted light, with a silvery reflectance. Spectacular clusters of sphalerite with associated pyrite, quartz and chalcopyrite dominate the specimen supply coming from Huaron.

Table 11. Minerals reported from the Huaron mines.

Ankerite Barite Calcite (*)halcopyrite (*)Dolomite Enargite (*)Galena Polybasite (*)Pyrite (*)Quartz (*)Rhodochrosite (*)Sphalerite Tetrahedrite

* Collector-quality specimens

THE CARHUACAYAN DISTRICT

Carhuacayan Province Junin Department

LOCATION

The Carhuacayan district and mines are located approximately 25 km south-southwest of the Huaron district and about 55 km northwest of La Oroya. Information on the district is limited.

The Carhuacayan deposit is related to a quartz-andesite stock intruded into limestone. Cobbing et al. (1981) briefly describe the district: "Complex lead-zinc-silver-copper mineralization occurs in radiating veins within the stock, in replacement bodies within megaxenoliths of limestone and in contact-metasomatised tectonic breccias surrounding the intrusion."

MINERALS

Specimens of galena with pyrite and sphalerite have been produced in limited amounts during the recent past but the mines seem to have ceased producing specimens around 1984. Specimens from Carhuacayan are frequently mislabeled as to locality.

Bournonite PbCuSb[S.sub.3]

Bournonite occurs as shiny, 1 to 2-cm, barrel-shaped, metallic black crystals. They are usually untwinned and striated.

Pyrite Fe[S.sub.2]

Pyrite occurs at Carhuacayan in fine pyritohedral and modified pyritohedral crystals to at least 5 cm in diameter. They are very difficult to distinguish from the pyrite of other Peruvian locations such as Huanzala. A large find was made in the early 1980's; the pyrites occurred as "floater" groups that were encased in a whitish clay, and were collected from a clay-filled vein in the mine. This clay coating was easily cleaned off. The pyrite crystals are generally modified pyritohedrons, with one crystal attached to another in a habit somewhat reminiscent of that of the Spanish pyrites. Carhuacayan pyrites are generally near perfect in form, with a brilliant luster; the crystal faces may have small pits, which is an aid in distinguishing the locality.

Sphalerite (Zn,Fe)S

A significant find of sphalerite in good specimens was made in early 1983. The splendent crystals are a very dark blackish brown, frequently twinned on the Spinel law, up to 4 cm in size, and associated with cream to tan-colored, stalactitic, botryoidal dolomite.
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Title Annotation:mining district in Peru
Publication:The Mineralogical Record
Date:Jul 1, 1997
Words:2203
Previous Article:The Cerro de Pasco Group.
Next Article:Morococha-Casapalca Group.
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