Printer Friendly

Morococha-Casapalca Group.

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 worked since Spanish colonial times. Between 1924 and 1929 the Colqui vein was mined for silver. In 1958 the Colqui mines were acquired and operated by the Compania Minera Huampar, S.A. Operations switched to the Finlandia vein in 1958. The Finlandia mine produced mostly lead and zinc, however, several ore shoots along the vein contained high-grade silver-gold ore. Mines within the district include the Aurelio, Colqui, Condor Pasa, Felicidad, Finlandia, Pio Pio, and Venturosa.

GEOLOGY

Petersen and Diaz (1972) and Kamilli and Ohmoto (1977) described the district and the following geologic data are abstracted from their papers.

The Colqui district lies within the Tertiary volcanic belt of Peru. In the district, the volcanic rocks are composed of andesite, andesite tuffs and basalts. A number of stocks are found in the district. The volcanic rocks cannot be directly correlated with other volcanic rock sequences such as those at Casapalca. The volcanic rock has been extensively altered, particularly near the ore veins. The tuffs have undergone the most intense alteration.

The Colqui district exhibits a mineralogical zoning that is centered in the area of the Cobre vein; this is a small chalcopyrite-rich vein situated northeast of the Colqui mine. Westward from the Cobre vein the mineralization grades from copper to lead-zinc, then silver, and finally mercury. The Finlandia, San Juan-Lourdes and Vermouth veins are located between the Colqui mine and the Cobre vein. These veins are principally sphalerite-bearing and galena-bearing.

In the western part of the district, the Colqui and Maria Teresa veins have been mined principally for silver. The Colqui mine and vein has argentiferous tetrahedrite as its principal ore mineral, and it also contains sphalerite, galena, rhodochrosite, realgar and quartz. The Maria Teresa vein contains silver-bearing minerals with abundant pyrite and some stibnite (Petersen and Diaz, 1972). At Pariamina, in the southwestern margin of the district are several small fractures which contain cinnabar. To the east of the Cobre vein is the Pio-Pio vein. This vein contains galena, pyrite, tetrahedrite and sphalerite, and appears to be transitional between the lead-zinc and the silver zones. The silver-gold ore shoots of the Finlandia vein are an obvious exception to this zoning pattern.

MINERALS

Specimens from the Colqui district are frequently labeled as coming from the "Huampar" mine; this is the name of the company that operates the mines. The Colqui district produced attractive barite for a short time. These include honey-colored crystals about 2.5 cm in size, and also delicate white crystals that are slightly smaller in size. Tetrahedrite occurred as large, bright crystals on a microcrystalline drusy quartz and pyrite matrix. The Colqui district has also produced a little amethyst quartz. The mines are currently idle.

Table 13. Minerals reported from the Colqui district.

Common or Abundant

(*)Barite Chalcopyrite Galena Marcasite Pyrite (*)Quartz Rhodochrosite Rhodonite Sphalerite (*)Tetrahedrite

Locally Abundant or Rare

Acanthite Bornite Cinnabar Famatinite Native gold Pearceite Polybasite ProustitePyrargyrite Realgar Native silver Stibnite

* Collector-quality specimens

THE SAN CRISTOBAL DISTRICT

Yauli Province Junin Department

LOCATION

The San Cristobal district was once considered part of the Yauli district; it is located a few kilometers south-southeast of Morococha. The Yauli district proper occupies the Yauli Valley, about 10 km north of the San Cristobal area. The principal San Cristobal mines are the San Cristobal, Huaripampa, Carahuacra, Santa Rita and Andaychagua. The San Cristobal mine, however, is actually considered to be the southernmost mine of the Yauli district. The Carahuacra mine, located in the south-central part of the original Yauli district, lies midway between Yauli and San Cristobal. The San Cristobal mine is 19 km southeast from Morococha, just south of Yauli [ILLUSTRATION FOR FIGURE 88 OMITTED].

HISTORY

The San Cristobal mine has been worked since the early 1900's. The camp and principal adit lie at an elevation of about 4,700 meters. Most of the mining activity has taken place between that level and the surface outcrops at 5,000 meters. Glaciers occupy the valleys high above the camp.

During the early decades, small-scale mining for silver was carried out at shallow depths, below the vein outcrops, near the snow line. Small-scale mining was also done during this same time period in the Carahuacra section, which is the northwestward extension of the San Cristobal district. In 1929, the Cerro de Pasco Corporation began exploitation of the principal vein, known as the Siberia. The mines, as of 1991, were owned by Centromin.

GEOLOGY

The principal structure in the district is the northwest-trending Chumpe anticline. Monzonite porphyry stocks and dikes intrude Excelsior phyllite and overlying Catalina volcanics in the core of the Chumpe anticline. The two most important intrusions are the Carahuacra and Chumpe stocks. The Carahuacra stock crops out 2 km north of San Cristobal. The Chumpe stock crops out near the San Cristobal vein, but is not well mapped as it is partially covered by a glacier. The Carahuacra mine replacement body manto extends for 3 km in the Potosi (Pucara) limestone along the western flank of the Chumpe anticline. The dominant minerals in this deposit are sphalerite and pyrite with a gangue of quartz and manganiferous siderite.

Ore occurs in veins that cut the Excelsior phyllite and the Catalina volcanics, as well as in breccias and replacement mantos along the contact between Catalina volcanics and Pucara limestone. The major vein at San Cristobal is 1 to 8 meters wide. Sphalerite and pyrite are the most abundant minerals, followed by galena, chalcopyrite, tetrahedrite-tennantite, wolframite (hubnerite), calcite, siderite, quartz and marcasite. Hematite, acanthite, pyrargyrite and barite occur rarely. To the northwest, the adjacent Carahuacra mine has been reported to contain native silver and proustite in its upper levels.

The general sequence of crystallization shows a strong zonation from the center of the Chumpe anticline outward. The sequence of deposition is pyrite, wolframite, quartz, chalcopyrite, sphalerite, galena, barite, and carbonates. At any given location, only a few of these minerals are present, and their relative concentrations vary from one area to another. In general the San Cristobal ores are higher in arsenic than antimony, whereas the Carahuacra ores are higher in antimony than arsenic.

San Cristobal has two major veins, the Main vein and the Siberia 1 vein. As of 1984, the Main vein and its branches have been mined for a horizontal distance of 2.5 km and to a depth of 500 meters.

The mineralization is subdivided into three stages. The first was a tungsten stage, consisting of pyrite, wolframite and quartz with trace amounts of sericite and augelite. The second stage was the base-metal phase, consisting of pyrite, chalcopyrite, sphalerite, galena and barite. The third and final stage was mainly carbonate with small amounts of sphalerite and galena.

In the first stage, pyrite, wolframite and quartz occur together; pyrite was deposited first, followed by wolframite, and then by quartz. The wolframite occurs in crystals up to 8 cm across, which are commonly twinned. The majority of the chalcopyrite mineralization occurred in the second stage, and was followed by the deposition of sphalerite and galena. Pyrite occurs as euhedral crystals on or in the chalcopyrite. This suggests that it was deposited shortly before and through the chalcopyrite crystallization phase. Although uncommon, barite is found in crystals up to several centimeters long, which are usually associated with the base metal phase (second stage).

MINERALS

Barite BaS[O.sub.4]

A small number of barite specimens, in large plates with bladed pale blue-gray crystals to over 2.5 cm in size, were collected in the late 1980's and into the early 1990's. Many of these specimens are labeled as having originated from San Cristobal, but may be from other Peruvian localities.

Calcite CaC[O.sub.3]

Calcite is associated with the sulfosalts.

Pyrargyrite [Ag.sub.3]Sb[S.sub.3]

Pyrargyrite occurs as dark red-black crystals that are up to at least 2 cm across.

Pyrite Fe[S.sub.2]

Pyrite occurs as large, shiny, pyritohedral to cubic crystals perched on slightly stubby quartz crystals. San Cristobal is best known for its shiny pyrite crystals that are "outlined" or rimmed with quartz crystals. These pyrite crystals are distinctive; the cubes are commonly modified by other forms which causes a rounding of the cube edges. Very typical of the San Cristobal pyrite-quartz combination is that many of the pyrite crystals show severe fracturing. These fractures are filled with crystallized white drusy quartz. This "healing" of the fractures with quartz is a striking and possibly unique aspect. Pyrites from San Cristobal also typically show parallel growth on crystal faces, in "step-up" formation, and this helps distinguish them from other Peruvian locations. These pyrite specimens can be very attractive, and very large; the best crystals are brilliant pyritohedra exceeding 6 cm across. Although eclipsed by the Huanzala specimens (by sheer volume if nothing else), the San Cristobal crystals still rank as some of Peru's finest pyrite.

Quartz Si[O.sub.2]

White to colorless quartz crystals, with pyritohedrons of pyrite resting on the quartz, are distinctive for San Cristobal. The crystals have a good glassy luster and typical quartz terminations. The quartz has a moderate aspect ratio, and there is nothing definitive about the form or habit to distinguish it from other Peruvian localities such as Pachapaqui.

Siderite FeC[O.sub.3]

Siderite, although not a glamorous mineral, does occur in fair-quality mineral specimens for the species. The crystals are usually dull to satiny in luster, in tan-colored discoids. Crystals exceed 1 cm across in the better specimens. From 1978 to 1980 siderite was commonly collected at the mine as large botryoidal masses up to several kg in weight. When cut open these siderites display a concentric pattern similar to that of Argentine rhodochrosite. Siderite is also a common matrix mineral for the superb pyrites found at San Cristobal.

Silver Ag

Fine specimens of coarse, steel-wool-like to crystalline aggregates of native silver on fine-grained galena and other sulfides have been recovered. Native silver also occurs with pyrargyrite and other sulfosalts in this same wiry habit.

Stephanite [Ag.sub.5]Sb[S.sub.4]

A few specimens of stephanite have appeared on the market since 1977.

Wolframite-Ferberite (Mn,Fe)W[O.sub.4] - [Fe.sup.+2]W[O.sub.4]

San Cristobal has produced very good, distinctive wolframite-ferberite. Crystals are usually twinned, thin, broad-bladed and spear-shaped, pseudo-hexagonal, black in color, strongly striated and standing on edge. Most specimens have been dissolved out of quartz with hydrofluoric acid (HF), the net result being that no quality quartz exists on these specimens. Many, in fact, have a quartz matrix that is badly etched and corroded. The best wolframite-ferberite specimens have individual crystal blades that exceed 2.5 cm in diameter, and are at most a few millimeters thick.

Wolframite-ferberite also occurs in stubby crystals associated with pyrite; these are totally different in appearance from the bladed crystals. Although unlike the bladed habit, these stubby crystals are also completely different in appearance from hubnerite-wolframite-ferberite crystals collected from Mundo Nuevo, Pasto Bueno, and Julcani, which are the other major hubnerite-wolframite-ferberite sources in Peru. Chemically the wolframite-ferberite from San Cristobal ranges in composition from 27 to 94% FeW[O.sub.4]. Many of these specimens were originally mislabeled as from Casapalca, probably due to the proximity of the two localities.

Table 14. Minerals reported from the San Cristobal district.

Common or Abundant

Chalcopyrite Galena (*)Pyrite (*)Quartz (*)Siderite Sphalerite (*)Wolframite (Hubnerite)

Uncommon or Locally Abundant

(*)Barite (*)Native Silver (*)Pyrargyrite (*)Stephanite

* Collector-quality specimens

THE CASAPALCA DISTRICT

Huarochiri Province Lima Department

LOCATION

The town of Casapalca and Centromin's Casapalca mines are about a four-hour drive east from Lima. The town is at an elevation of 4,200 meters: mining in Casapalca centers on the Cerro Casapalca (later named Cerro Carlos Francisco). a mountain rising to over 5,200 meters east of town.

A second Casapalca mine was established by Compania Minera Casapalca S.A., in 1987. The mine is at 4,700 meters and is only 7 km from Centromin's original Casapalca mine. Operations at this new underground mine began in 1987, and by January 1991 it was producing 400 metric tons per day of copper-lead-zinc-silver ore.

HISTORY

Early mining for silver started on the high outcrops where scattered deposits of rich ore were found. The Backus and Johnston Company began systematic exploration and development of the area in the late 1800's, and were subsequently bought out by the Cerro de Pasco Corporation. In 1919 the most important producer was the Carlos Francisco mine, followed by the Aguas Calientes, Carmen, San Antonio, and Chuquichuccho mines. As of 1991, Casapalca was owned by Peru's Centromin: the mine was operating at 61% of capacity at that time, a reflection of the low prices for silver during the past several years.

GEOLOGY

Epithermal silver ores which contain acanthite, proustite, pyrargyrite and miargyrite were found with pyrite in a siliceous and pyritic gangue. These silver-rich ores were the original targets of the early development work, and existed only in the highest part of the veins; they did not last long. Deeper exploration of the mineralized zones located silver-hearing veins which contain pyrite, galena, sphalerite, chalcopyrite, tetrahedrite-tennantite and bournonite in a gangue composed of quartz, calcite and rhodochrosite.

The stratigraphy of the district is as follows: limestone of the Upper Cretaceous Machay Formation is overlain by shale, sandstone, redbeds and conglomerates of the early Tertiary Casapalca Formation: the Casapalca Formation is, in rum, overlain by andesitic tuffs and flows of the Tertiary Carlos Francisco Formation. Intrusive rocks in the Casapalca area consist of diorite and syenite of unknown age. Some of the intrusions predate the mineralization, and none of the intrusions appear to be directly related to the mineralization found at Casapalca. However, skarn zones encountered at depth in the Cretaceous Jumasha Formation limestones (which do not crop out), coupled with hot water encountered at depth in the mines, strongly suggests a nearby heat source driving the mineralization.

The principal veins of the district are part of a northeast-striking fracture zone which is about 5 km long. Most of this fracture zone has been explored for potential ore. Wu and Peterson (1977) describe the fracture system of the veins at Casapalca as follows:

One significant feature of the fracture system is the continuity of open spaces along the veins. This is especially demonstrated by the 241 and 242 veins where a single fracture with an open space up to 2 cm can often be traced for over 100 meters.

According to Rye and Sawkins (1974) the fracture zone has been divided into two main ore-bearing sections, the Aguas Caliente in the southwest portion of the deposit, and the Carlos Francisco in the northeast portion of the deposit.

Casapalca ores show a gradual lateral zonation. The paragenesis of different major veins can be correlated, and four stages of mineralization, without obvious time breaks, have been identified.

Three main zones of mineralization have been defined which are based on alteration, metal content, and mineral type (Centromin, 1977), as modified by Wu and Petersen (1977): Zone I is intensely silicified, with no carbonates in the central part and pure calcite at the edges of the zone. Arsenopyrite and hubnerite occur in this zone. Pyrite is predominantly cubic rather than pyritohedral. Sphalerite is abundant, chalcopyrite is common, galena is minor, and tetrahedrite is rare and As-rich. Zone H contains abundant carbonate and sericite, and the sphalerite is usually dark in appearance due to chalcopyrite inclusions. Sphalerite, galena and tetrahedrite are the dominant ore minerals, chalcopyrite is rare, and pyrite occurs as pyritohedrons. Bournonite is also present. The tetrahedrite in Zone II is Sb-rich. Zone II has a subzone where chalcopyrite is relatively common in association with pyrite. Zone III (peripheral zone) has dolomite, siderite and rhodochrosite as the dominant carbonates, and they are all very common. Sericite is ubiquitous and common. Bournonite, geocronite, stibnite, orpiment and realgar are typical of this zone. Tetrahedrite is usually associated with galena, and sphalerite is much less common. Pyrite is predominantly pyritohedral.

Wu and Petersen (1977) have divided the mineralization into four stages based on temperature of deposition and the type of mineralization. Stage I is the zinc-lead stage, Stage H is the copper stage, Stage III is the copper-silver stage, and Stage IV is the gangue stage (quartz and carbonates). The beginning of chalcopyrite crystallization was later than that of sphalerite, galena and pyrite within stage I, and tetrahedrite deposition corresponds mainly to a single well-defined event within stage III. Minerals formed in stage I are abundant in most of the vein system and often continued to crystallize within stages II and III. Chalcopyrite and tetrahedrite characterize stages II and III, respectively. Stage IV is rather distinct in most of the veins. Carbonate deposition in this stage may be divided into two periods: calcite I; pearly-white carbonates (calcite and/or dolomite); and calcite II. Large rounded balls of dolomite crystals with a very pale pinkish tint, resting on the sulfides, are one of the habits that the pearly white carbonates stage takes.

The general order of crystallization in the veins appears to have been quartz, manganoan calcite, rhodochrosite, quartz (II), pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, quartz (III), chalcopyrite (II), jamesonite and bournonite, and pyrite (II), sometimes followed by a late crusting of thin layers of quartz, calcite and pyrite. Chalcopyrite is common in the deeper workings of the central or Consuelo vein located in the Carlos Francisco section. Sparse chalcopyrite occurs throughout the ore zone and has been observed in small lenses at surface outcrops. The Aguas Calientes section contains more calcite and rhodochrosite, commonly with tetrahedrite, whereas the Carlos Francisco section contains more quartz and tends to have more vugs. The dominant sulfide is sphalerite with lesser quantities of chalcopyrite, galena, tetrahedrite and pyrite.

The most abundant and widespread vein minerals are sphalerite, galena and pyrite. Still widespread, but of lesser importance, are quartz, chalcopyrite, tetrahedrite and the carbonates. Bournonite can be fairly common in places, but is always present in small quantities relative to the other ore minerals. Late-stage realgar and orpiment are reported from the upper levels, and are recorded as common in the peripheral zone where they occur with stibnite and Pb-Ag sulfosalts (McKinstry, 1927; McKinstry and Noble, 1932). Arsenopyrite, geocronite and hubnerite have also been reported, but are quite rare.

Quartz and calcite are commonly associated in the majority of the veins. Sphalerite can be light or dark in color; the dark-colored sphalerite contains numerous blebs of chalcopyrite, while the light-colored sphalerite is free of them. The tetrahedrite is Sb-rich and As-poor. According to McKinstry and Noble (1932), the tetrahedrite crystals are commonly replaced by chalcopyrite in ores from the Aguas Calientes section. This usually occurs on the outer margins of the crystals. Specimens of tetrahedrite from this area are frequently coated by a golden layer of chalcopyrite or are variably replaced by chalcopyrite. McKinstry and Noble (1932) also mention that bournonite is a common associate of the tetrahedrite, and is usually found in a band of irregular width which lies between the galena and the tetrahedrite. There is also some late-stage bournonite which grew as independent crystals interspersed with the other minerals.

Hypogene leaching was recognized by McKinstry and Noble (1932) in veins from both the Carlos Francisco and the Aguas Calientes sections. Galena and sphalerite are usually etched, whereas the late-stage sulfides are not. Barite is the first mineral to be leached, then the sulfides (in the order tetrahedrite-galena-sphalerite, pyrite, and chalcopyrite), and finally the carbonates with deposition of new sulfides terminating the process.

MINERALS

McKinstry (1927) described Casapalca as a great specimen-producing source, particularly for bournonite. Vugs were reported as very common; he described them as follows:

Vugs are first lined with sphalerite crystals, some of which are a centimeter or more in diameter. Upon the sphalerite is bournonite and around the latter crystals, and frequently upon them, quartz prisms attached by their bases form radiating groups. Perched upon all of these minerals are tufts of calcite scalenohedra. Galena and tetrahedrite are usually not well crystallized; occasionally galena crystals are intergrown with the bournonite as though the two minerals had started crystallizing from different centers and interfered with each other during growth. Tetrahedrite is commonly pitted as though by etching and is sometimes covered by a film of chalcopyrite. The pyrite, where it has had an opportunity to crystallize in open vugs, is usually in quite perfect pyritohedrons. Where replacing wallrock it is usually in cubes, while octahedrons of pyrite are unknown at this locality. Bournonite ... crystals are splendent and some are nearly a centimeter in diameter. Usually the basal and prism forms dominate. In portions of the veins, pink cleavable rhodochrosite is abundant; in others the carbonates take the form of manganiferous calcite and dolomite. Rhodonite occurs sparingly in grains up to 1 cm.

McKinstry in a later article (McKinstry and Noble, 1932) describes the Carlos Francisco vein:

Some of the open cavities in the Carlos Francisco mine are a foot or more wide and measure ten feet or more lengthwise. They are lined with quartz, calcite, sphalerite and bournonite, all well crystallized. . . . In some rugs the early sulfides, galena and sphalerite, are pitted and etched, but the tetrahedrite and bournonite are fresh. Occasional pyritohedrons of a late generation of pyrite rest on the sphalerite. Vugs from the Aguas Caliente mine, showing similar etching of galena and sphalerite, contain tetrahedrite crystals entirely coated by chalcopyrite. Bournonite in these specimens is unetched but bears a dusting of minute pyrite crystals. Tiny quartz crystals rest on the coated tetrahedrite but not on the bournonite; in fact one bournonite crystal clearly grew around one of the quartz crystals.

Specimens collected from Casapalca today are essentially identical to those described by McKinstry over 60 years ago. Beautiful examples of tetrahedrite, bournonite and mixed sulfide specimens are still occasionally available on the mineral market. Casapalca has been notable for its lack of enargite, a definite distinction when one tries to identify mineral specimens from here. However, in 1992 some enargite supposedly from Casapalca appeared on the market.

The main Casapalca mine has furnished good to excellent specimens of quartz, bournonite, galena, sphalerite, pyrite, calcite, manganoan calcite and tetrahedrite. The tetrahedrites occur in crystals to over 8 cm in size; they are some of Peru's largest and, it can be argued, some of Peru's best. Most of the minerals seen for sale today are from the main Casapalca mine.

Anhydrite CaS[O.sub.4]

A few specimens of massive purple anhydrite with some accompanying pyrite were found in the 1970's.

Barite BaS[O.sub.4]

Barite is an uncommon mineral in collector-quality specimens from Casapalca. It tends to be colorless, clear to white, and in thick, distinctly bladed, crystals. Quartz casts after bladed barite are often found; the quartz forms drusy coatings over the barite, which has in some cases been leached away.

Bournonite PbCuSb[S.sub.3]

Bournonite is a relatively common associated mineral at Casapalca, usually crystallized on galena, and less commonly on sphalerite and tetrahedrite. It rarely occurs by itself as quality specimens. Crystals are often only partially formed and are usually bright in luster, commonly with a bluish tint. Forms are complex to relatively simple barrel-shapes with distinct striations. Flattened, more discoidal-looking crystals also occur, and these can be difficult to differentiate from the more complex forms of tetrahedrite-tennantite.

Calcite CaC[O.sub.3]

Calcite, when present, can be one of the more distinctive minerals that help to differentiate Casapalca specimens from those of other Peruvian localities. It typically occurs as opaque white to cream-colored, rather narrow scalenohedrons that frequently occur in little bundles and sprays. It is generally one of the last minerals to crystallize when in this habit, and this, in combination with the distinctive quartz crystals from Casapalca, is most helpful in identifying specimens. In addition, this habit of calcite fluoresces a bright orange under shortwave ultraviolet and an even brighter intense orange with a hint of pink under longwave ultraviolet. This colorful fluorescence is distinctive and can be a real aid in identifying Casapalca specimens when this type of calcite is present.

Calcite also occurs in flattened rhombohedral discoids of the type referred to as "nail-head" calcite, as well as being present in the typical rhombohedral form. One type of calcite occurrence is the small (up to 5 mm) white crystals sprinkled randomly on quartz crystal plates. This gives the quartz the appearance of having been sprinkled with "rice grains." Calcite also occurs as coatings on quartz crystal plates, with the calcite occurring preferentially on one side only of each crystal, or sometimes on two or three surfaces; this makes it look as if "snow" has fallen on the quartz specimens. It is also common for specimens that have had a heavy coating of calcite over the other minerals to have had this calcite dissolved off in acid by the piriteros.

Manganoan calcite crystals generally have a bladed habit, with skewed saddle-shaped rhombs as the common form. Other habits are also present, including long thin bundle-like masses of crystals with a stretched scalenohedral habit. Crystals up to 3 cm in size have been recovered on matrix specimens that are over 25 cm across. These crystals have an attractive pale pink color.

Although they are not common, some of the finest Peruvian manganoan calcite specimens have come from Casapalca. From 1978 to 1984 manganoan calcite was relatively abundant, predominantly as scalenohedrons. Some specimens have crystals as large as 10 cm in size on matrix associated with quartz and sphalerite. Although the scalenohedrons are generally not lustrous they still make attractive specimens. The combination of pink manganoan calcite scalenohedrons and brown, twinned sphalerite, both on quartz crystals, is a most attractive association and is quite typical of Casapalca.

Chalcopyrite CuFe[S.sub.2]

Chalcopyrite is a common mineral at Casapalca, particularly as coatings on, and as a replacement of, tetrahedrite. It also occurs as excellent euhedral to poorly-formed crystals and crystal masses having an intense deep yellow color and bright satiny or dull luster. The best specimens, which often occur with sphalerite, have lustrous, golden yellow sphenoidal crystals which are commonly twinned. The individual crystals often exceed 4.5 cm in size, and occur in groups over 9 cm across.

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

The most attractive dolomite at Casapalca is from the "pearly-white carbonate stage," where it is found as large globular, ball-shaped aggregates of tiny, cream to pale pink, rhombohedral crystals. These ball-shaped aggregates can be many centimeters across. Dolomite is also found as small to microscopic-size rhombic and saddle-shaped rhombic crystals. The microscopic crystals are among the last minerals to crystallize. The dolomite does not fluoresce.

Enargite [Cu.sub.3]As[S.sub.4]

First reported from Casapalca in 1992, enargite in various-sized plates up to about 25 cm across has been offered for sale by the piriteros, who insist the location is Casapalca. These specimens may be from the nearby "new" Casapalca mine mentioned above.

The matrix for the specimens is quartz, in white to transparent, non-tapered crystals which come in various lengths up to about 5 cm long. Small enargite crystals are scattered on the quartz. The quartz, in turn, is coated by chalky, beige to cream-colored dolomite or calcite; these carbonates occur as small, crude rhombic crystals. Implanted on the carbonate is more enargite in single crystals (frequently doubly terminated) and small crystal groups up to about 3.5 cm. Some enargite crystals appear to be coated with micro-drusy enargite, and others appear to be coated with microcrystalline pyrite. The enargite crystals are not lustrous.

Galena PbS

Galena occurs most often at Casapalca as cubes and cuboctahedrons; in some cases this combination is more octahedral than cubic. Crystals can be corroded, with a satiny luster, or very bright and mirror-like in their reflectance. Bright, lustrous crystals are uncommon. Some galena crystals have a melted appearance that tends to obscure the original form, and these "melted" crystals may have a bluish, somewhat iridescent tarnish or a satiny luster. Galena is frequently found intimately associated with bournonite, with the galena acting as the host mineral on which the bournonite has crystallized.

Geocronite [Pb.sub.14][(Sb,As).sub.6][S.sub.23]

Geocronite is a very rare mineral at Casapalca in specimen quality. A few small specimens having twinned, steel-gray, 1-cm crystals were recovered in 1981.

Gypsum CaS[O.sub.4][center dot]2[H.sub.2]O

Dan Belsher (personal communication, 1992) reports that gypsum "occurs in crystals up to 18 inches long, similar to those found at Naica, Mexico."

Muscovite K[Al.sub.2]([Si.sub.3]Al)[O.sub.10][(OH,F).sub.2]

Muscovite (sericitic) is common as a minor late-stage associated mineral. It is usually present as near-microscopic, wispy, flaky, delicate masses which possess a white to cream color.

Pyrite Fe[S.sub.2]

Although pyrite is usually associated with many other minerals at Casapalca, it generally does not occur as large groups of solid pyrite like those so common at Quiruvilca and Huanzala. Pyrite, as bright, unstriated to lightly striated pyritohedrons and cubes of small size, is a common minor associate with other sulfides and sulfosalts. Some of the best pyrite specimens from Casapalca are pyritohedrons over 2.5 cm across. These pyrite crystals are commonly associated with lustrous quartz crystals; this combination makes for aesthetic crystal groups. Pyrite is often the matrix mineral on which the other minerals are deposited; it is not usually found in cubes, and not in exceptional specimens when it is. Of the good crystals the cubic pyrite specimens, due to their rarity at Casapalca, tend to be prized by collectors specializing in Peruvian mineral specimens. Cubic pyrite crystals rarely exceed 5 mm in diameter. Octahedral pyrite does not occur at Casapalca.

Quartz Si[O.sub.2]

Quartz is usually found in association with the other minerals from Casapalca. Quartz has had a long period of deposition at Casapalca, so it is not uncommon to find it as the matrix (often with pyrite) for the various sulfides and sulfosalts or to find it as one of the last minerals to crystallize on the other minerals. On some specimens quartz shows two generations of crystallization: the first generation is a drusy coating of small crystals, with no distinctive individuals; the second generation is less drusy-like, with more individuality to the crystals and with larger crystal sizes. However, this demarcation can be obscure. Early-formed (first generation) crystals appear to be more milky white in color, usually with a satiny luster, while later (second generation) crystals vary in color from translucent milky white to bright, gemmy, clear, colorless crystals.

It is common for quartz from Casapalca to have a satiny luster on the prism faces and a bright-glassy luster on the pinacoid faces. Crystals are frequently opaque or translucent at the base and near water-clear at the termination. As seen when viewed vertically, these quartz crystals tend to have a strong triangular rather than hexagonal development due to the dominant growth of alternating prism faces. Casapalca quartz also has a tendency toward sceptering of the crystals. Sometimes this sceptering is very pronounced and thus, beautiful quartz specimens can result. Casapalca quartz occasionally may show a tapering habit, but this is uncommon. Those tapered quartz crystals form individuals with a wide center which tapers toward the top and the bottom of the crystal in what is known as a "Tessin" habit. Generally these crystals are somewhat frosty in luster, with bright terminal faces, and they occur in groups or small clusters of crystals that are up to several cm in size.

Casapalca quartz is usually not striated on the prism faces, but instead has a series of small vertical growth marks which give a parallel growth appearance to the prism faces. The quartz from Casapalca is usually distinctive enough that it helps to identify Casapalca specimens. When quartz and calcite occur together, they form a characteristic combination of color, habit and form which distinguishes Casapalca specimens from those of other Peruvian localities.

Sphalerite (Zn,Fe)S

Sphalerite is commonly found on specimens from Casapalca. The luster varies from very bright to dull; most sphalerite specimens collected recently have bright faces. It is frequently transparent to translucent with the transparent crystals often quite gemmy. Sphalerite in transmitted light varies from pale straw-yellow, through yellow, to a deep reddish amber (almost ruby-red), and dark brown. Crystals in reflected light are silvery to blackish brown in color, and sometimes exhibit a bright, dark bluish black tarnish or a golden-yellow chalcopyrite film that looks like golden spray-paint. Crystal size is variable, and usually parallels that of the other cogenetic minerals, rarely exceeding 5 cm. Rye and Sawkins (1974), however, report that sphalerite crystals in rugs in the Carlos Francisco section exceed 10 cm in size. Crystal forms are often complex with a tendency toward modified octahedrons or positive and negative tetrahedron giving the crystals an octahedral appearance. Spinel-law twins are not common. Casapalca sphalerite generally is not striated; this fact differentiates it from both Huanzala and Quiruvilca sphalerites. The best sphalerite specimens from Casapalca are the gemmy dark amber to reddish crystals over 3 cm in size associated with bright colorless quartz crystals and bright golden pyrite and chalcopyrite.

Tetrahedrite [(Cu,Fe).sub.12][Sb.sub.4][S.sub.13]

Tetrahedrite is a common mineral in specimens from Casapalca. Crystals can be very large for the species, to more than 8 cm on edge, and range in habit from simple tetrahedrons to the more complex habits usually associated with tennantite. These more complex habits may reflect an increase in arsenic relative to antimony within the structure of the mineral. Luster varies from dull (in corroded crystals) to brilliantly metallic. Some crystals have an iridescent tarnish, although they still maintain a high luster.

The majority of tetrahedrite crystals from Casapalca are simple tetrahedrons with minor modifications by other forms and are satiny to brilliant in luster. Chalcopyrite is common as both a coating on and a replacement of the tetrahedrite. Commonly associated are quartz and sphalerite. It is interesting to note that on some specimens with well-developed {111} faces and small {211} faces, chalcopyrite preferentially coats only the octahedral faces; this leaves a bright shiny silvery-black reflectance on the other faces contrasted by a deep golden yellow reflectance on the octahedron faces. It is not uncommon to find microscopic pyrite crystals on the tetrahedrite crystals.

Most of the good tetrahedrite in recent times came out in the very early 1970's. Since 1980 Casapalca tetrahedrite has virtually disappeared from the specimen market, except as recycled specimens.

Table 15. Minerals reported from the Casapalca district.

Abundant

(*)Calcite (*)Manganoan calcite (*)Chalcopyrite (*)Dolomite (*)Galena (*)Pyrite (*)Quartz (*)Rhodochrosite (*)Sphalerite (*)Tetrahedrite

Locally abundant or rare

Acanthite (*)Anhydrite Arsenopyrite (*)Barite Bornite Boulangerite (*)Bournonite Diaphorite (*)Geocronite (*)Gypsum Hubnerite Jamesonite Miargyrite Orpiment Polybasite Pyrargyrite Realgar Rhodonite Stibnite

* Collector-quality specimens

THE SANTA RITA MINE, MOROCOCHA DISTRICT

Yauli Province Lima Department

LOCATION

The Santa Rita Pb-Zn-Ag-Cu mine is located a few km east of and above Casapalca, high in the Ticlio pass at 4,800 meters elevation. The entrance to the mine is right in the pass. Although a part of the Morococha district, it is treated here as a separate mine. The Santa Rita and nearby Huacracocha mines were producing approximately 600 metric tons per day of ore in 1991. The Santa Rita mine has been providing fine specimens of galena, sphalerite, rhodochrosite and quartz in fine scepter crystals, since about 1987. Nothing, however, has been published on its geology.

MINERALS

Many fine Peruvian rhodochrosite specimens were offered for sale in 1991. Most of these consisted of simple, lustrous, rhombic, deep pink crystals over 2.5 cm in size. There is some confusion about whether these specimens originated from the Santa Rita locality: the Manuelita mine, Morococha district, also produced fine rhodochrosite specimens, at about the same time as the Santa Rita mine. Some of the specimens were listed as "near Morococha" by one of us (RHC) at the 1991 Tucson mineral show. Actually these rhodochrosites were mostly from the Manuelita mine, which is east of the Ticlio pass and down near Morococha, along the road to the La Oroya smelter complex. Labels on the rhodochrosite specimens gave their various locations as Santa Rita mine, Casapalca; Santa Rita mine, near Morococha; Casapalca; and occasionally San Cristobal, Yauli district, because the Santa Rita mine has also been considered as part of the Yauli district, which is not too far from Casapalca. A few rhodochrosite specimens were also labeled as from the Manuelita mine. Very little has been published about the ore deposits and minerals of the Manuelita mine.

Distinguishing between rhodochrosite specimens from the Santa Rita mine and the Manuelita mine is very difficult. Rhodochrosite from the Santa Rita mine tends to be on, or associated with, quartz; the rhodochrosite crystal size is up to at least 2.5 cm on edge; and the rhombohedrons are frequently curved on the larger specimens. These can look similar to the rhodochrosite collected from Silverton, Colorado. The Manuelita mine rhodochrosite, on the other hand, is generally in smaller crystals, may be attached directly to the rock matrix without the quartz crystal substrate, and tends to form drusy crusts. Centromin's Casapalca mine has not produced any rhodochrosite, so it can be eliminated as the source.

The confusion is partly due to the way the mines are laid out. A piritero going out to get specimens in this area will normally first stop at Casapalca, then go on to the Santa Rita mine, then go over the top of the pass, and then down the other side to the Manuelita mine. Quite often the piritero will acquire a few or a lot of specimens from each location. Whichever location has the largest amount of material from a particular trip will usually end up carrying the label for the entire amount. These specimens are usually then offered for sale as all from the same mine. To further confuse things, along with these particular rhodochrosite specimens were a number of similar rhodochrosite specimens from Uchucchacua. These occur in both the scalenohedral and rhombohedral forms, came out in the same time frame, and some of these are mislabeled as Casapalca or Santa Rita. A further complication with this locality is the large number of "Santa Rita" mines in the big mining districts of Peru; it is a very common mine name.

Rhodochrosites from Uchucchacua are, for the most part, distinctly different and are discussed in detail in the section dealing with the Uchucchacua district.

In 1992, along with the rhodochrosite specimens, many interesting specimens of drusy quartz were brought out from the Santa Rita mine. Some of these quartz specimens are colored pink by underlying rhodochrosite. At times the quartz is dusted with a late-stage coating of dark, botryoidal pyrite which makes for attractive specimens.

Sphalerite from Santa Rita is frequently gemmy, with a bright luster and a yellowish green color. Crystals may exceed 2 cm on the better specimens. Microcrystalline, pale pink rhodochrosite is the matrix on some specimens.

THE PACOCOCHA DISTRICT

Huarochiri Province Lima Department

LOCATION

The Pacococha district is located 4 km southwest of Millotingo, on the banks of Pacococha Lake about 20 km nearly due southwest of Casapalca, and about 35 km southwest of Yauli. Elevations range between 4,600 and 5,200 meters; the district covers an area of about 4 x 6 km. Pacococha is one of the highest mining camps in the world, with some adits located at 5,100 meters (over 16,500 feet) elevation. Pacococha is situated among the mining districts of Viso-Aruri, Millotingo, Pucacorral and Chanape. Millotingo is considered to be part of the Pacococha district by Ly and Arce (1965). Petersen (1962) and Ly and Arce (1965) briefly describe the district, and much of the data given here is abstracted from their reports.

HISTORY

Scattered workings suggest some mining was going on during the Colonial period.. Modern development began in 1950 with the building of a 27-km road from the Central Highway to the district. The main mine in the 1970's was the Purisima, followed in order of size by the San Alejandro, Incataycuna, San David, Reserva, Carolina, Colquechaca, Santiago Mayor, and Santiago Menor. As of the mid-1970's, approximately 42,000 meters of drifts had been completed on 24 veins in the various mines. In late 1991, unless further reserves were discovered, the life for the district's mines was estimated at four years (Cavanagh, 1993?). The formal name for the company mining the district is the Sindicato Minero Pacococha S.A.

GEOLOGY

The predominant rock types at Pacococha are Cenozoic andesitic volcanic flows which have a composite thickness of 700-1000 meters. These flows are interlayered with tuffs and rhyolites. The volcanic rocks have been intruded by several fine-grained diabase or diorite porphyry stocks, which have pyritized, silicified, kaolinized, and otherwise altered adjacent volcanics.

Mineralization is by fracture filling, with the larger veins having lengths of 500 to 1000 meters and thicknesses of 1 to 1.5 meters. Outcrops of these veins are composed of quartz with a boxwork structure caused by weathered-out sulfides. Veins are discontinuous and new reserves must be continuously sought.

MINERALS

Vein mineralization is predominantly chalcopyrite, galena, dark sphalerite, and quartz, with lesser pyrite. Tetrahedrite, silver sulfosalts, acanthite, marcasite, fluorite, barite, calcite and rhodochrosite are minor constituents. Pyrite is usually present as stringers in the veins and is also disseminated in the wall of the veins as small cubes. High-grade veins have almost no quartz. Quartz is usually found in the sub-economic portions of the veins or where the veins narrow and pinch-out.

Some veins are located in the upper parts of the diabase stocks; these have a more complex mineralogy than do the veins in the tuffs and andesites. Typical minerals in these upper portion veins are pyrite, sphalerite, chalcopyrite, arsenopyrite, galena, bornite, pyrargyrite, tetrahedrite, trace gold, calcite, and quartz.

The Pacococha district is often confused with the Castrovirreyna district because of the presence of Pacococha Lake and an adjacent settlement, informally called Pacococha, within the Castrovirreyna district. Specimens from mines near this lake are frequently labeled as from Pacococha.

Calcite CaC[O.sub.3]

Calcite is found at Pacococha in long, colorless crystals showing both the hexagonal prism and the scalenohedral forms. Calcite is often found with rhombohedral dolomite.

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

Dolomite occurs as colorless to translucent white rhombohedra implanted on other minerals.

Marcasite Fe[S.sub.2]

Marcasite occurs on flat plates, as bright six-sided crystals. These specimens are attractive but not outstanding. Marcasite from Pacococha has often been mislabeled as from Casapalca.

Pyrite Fe[S.sub.2]

Pyrite is usually the matrix material at Pacococha, but is also rarely found in specimens with crude pyritohedrons and striated cubes.

Quartz Si[O.sub.2]

Quartz occurs as colorless, non-tapered crystals with a moderate aspect ratio. Quartz is also present as small crystals and drusy coatings.

Sphalerite (Zn,Fe)S

Sphalerite at Pacococha is the very dark, nearly black variety. This is an aid in differentiating specimens from this locality from sphalerite specimens originating from the Castrovirreyna district, which are lighter in color.

Table 16. Minerals reported from the Pacococha district.

Common or Abundant

Chalcopyrite (*)Dolomite Galena Pyrite Quartz Sphalerite

Rare or Locally Abundant

Acanthite Arsenopyrite Barite Bornite (*)Calcite Fluorite (*)Marcasite Orpiment Pyrargyrite Rhodochrosite Tetrahedrite

* Collector-quality specimens

THE MILLOTINGO MINE

Huarochini Province Lima Department

LOCATION

The Millotingo mine is located a few kilometers southwest of Casapalca, midway between the Pacococha mines and the Casapalca mines. The Pacococha Cu-Pb-Zn district is a short distance to the southwest. Millotingo is considered by Ly and Arce (1965) to be part of the Pacococha district.

HISTORY

In production since about 1958, the Millotingo mine is a rich but small silver mine whose ore is largely composed of silver-bearing sulfosalts. In recent years ore grades have dropped substantially, and since 1987 the company has been operating at a loss. In March of 1991 the phone lines to the mine were out, workers were being paid late, and morale was low. The mine was expected to be shut down before the end of 1991, as the mine could not operate at a profit unless the price of silver rose to over $5.21/ounce (Cavanagh, 1993?). Owned primarily by the Zacarias family, the company's formal name is Compania Minera Millotingo S.A.

GEOLOGY

The main veins at the Millotingo mine have a northeast strike, and in the past have averaged 30 to 40 ounces of silver per ton. Current ore grades in the veins are averaging 9 ounces/ton silver. The two main veins run into the southwest side of a very steep quebrada (valley) which, in turn, runs northwest into the Rimac Gorge. Workings follow these southwest-trending veins and also extend under the quebrada to the northeast. The veins are nearly vertical with silicified andesite as the wall rocks.

MINERALS

The minerals mined are mainly proustite and pyrargyrite. Pyrargyrite has been collected in crystal sizes to over 3 cm. Good mineral specimens from here occasionally appear on the market. It is common for specimens from the Millotingo mine to be labeled as collected from Pacococha district, as this district is only a few km away.

Proustite [As.sub.3]As[S.sub.3]

A very few proustite microcrystals were found during the 1980's, and a few specimens were found in 1981 amid one or two small lots of other minerals. The proustite is mostly in microcrystals 3 to 4 mm long, with beautifully formed scalenohedra, a bright luster and a transparent red colon A few specimens were collected that have microcrystals of proustite nestled in a quartz crystal matrix, associated with orange microcrystalline orpiment crystals.

Pyrargyrite [Ag.sub.3]Sb[S.sub.3]

Pyrargyrite is rare in good crystals at Millotingo. It usually occurs as 4-mm or less sized crystals with a prismatic habit and a deep red color. These specimens can make excellent micromounts. Associated minerals can include quartz, chalcopyrite, orpiment and other sulfides.

Pyrargyrite has been collected from Millotingo in fairly good-sized crystals for the species, but the quality is not as good as those collected from San Genaro in the Castrovirreyna district. Crystals exceed 5 cm in size, but are satiny or etched-looking, and do not possess the desirable bright luster. In 1980-1981, a number of specimens came out with large crystals, up to 3 cm long, on matrix. They are well-formed, with typical pyrargyrite habit and good dark red color, but with a dull luster.

Table 17. Minerals reported from the Millotingo mine.

Chalcopyrite Miargyrite Orpiment (*)Proustite (*)Pyrargyrite Quartz

* Collector-quality specimens

THE YAURICOCHA DISTRICT

Yauyos Province Lima Department

LOCATION

The Yauricocha district is situated about midway between Julcani and Casapalca, 16 km west of the divide between Lima Department and Junin Department and a short distance west of the small town of Huachucmillo. Three glacially carved lakes, Yauricocha, Onacocha, and Acococha, lie southeast of the mines. The Spanish colonialists first worked the district for gold and silver to a depth of 250 meters. Today the district produces copper, lead, zinc and silver [ILLUSTRATION FOR FIGURE 96 OMITTED].

GEOLOGY

Much of the geologic data presented here are from Petersen (1965). The Yauricocha orebodies are closely associated with a granodiorite stock located on the limb of a major anticline of Cretaceous Machay Limestone to the west and a syncline of Tertiary Casapalca Red Beds to the east. Two granodiorite stocks crop out in the mine area, and several others have been encountered in underground workings. The contacts between the stocks and the country rock are sharp. These stocks generally exhibit a cylindrical shape that extends over considerable vertical distances (at least 300 meters in one case).

The Casapalca Red Beds show bleaching for a considerable distance from the contacts with the stocks. Alteration of the red beds has caused hematite to turn to pyrite, sandstones to quartzite, and shales to hornfels. The Machay Limestone, in the immediate area of the intrusions, is altered to thin zones of skarn. Farther away from the contacts with the intrusions, it is bleached or recrystallized.

There are two types of orebodies being mined: one is copper-rich and the other is lead-zinc rich. The copper orebodies occur only in altered rock, whereas the lead-zinc orebodies may be found in nearly unaltered limestone. The copper orebodies, which sustained production through the 1960's, are located within pipe-like pyrite-rich bodies close to the granodiorite stocks. Most copper orebodies are elongated in an east-west or north-northwest direction.

Pyrite composes 60-80% of the sulfide orebodies. In general, cubes formed first and pyritohedrons and octahedrons formed later. The pyritohedrons and octahedrons are more abundant toward the centers of the orebodies.

In the copper-rich orebodies, the most important Cu-bearing mineral is enargite, which is commonly associated with quartz and pyrite. Small amounts of late-formed chalcopyrite, bornite and covellite may occur on the enargite in vugs. Empty casts with the morphology of barite or gypsum crystals are relatively common in the massive enargite ores.

Chalcopyrite is the second most abundant Cu-bearing mineral. Bornite is generally associated with chalcopyrite and also with enargite, but less frequently. Digenite, djurleite, chalcocite, covellite, idaite and tennantite-tetrahedrite occur mainly in the bornite-rich ores. Bornite has been replaced by chalcopyrite, covellite, digenite, chalcocite and sphalerite.

The edges of enargite-bearing orebodies are characterized by the presence of tennantite, which may be replaced by enargite. Tennantite is also found in small amounts within massive chalcopyrite ores. Bismuthinite, stannite (?), emplectite and gallite (?) occur mainly with the enargite-pyrite ore. Realgar and orpiment occur in very small amounts on the margins of the upper level orebodies.

In the lead-zinc rich orebodies, chalcopyrite is the Cu-bearing mineral usually associated with galena and sphalerite. Tennantite-tetrahedrite is a less common associated mineral in these orebodies. Most sphalerite is the dark, Fe-bearing variety, except for late-generation sphalerite, which is lighter in color. Fluorite and arsenopyrite are occasionally found in the lead-zinc ores.

Bournonite is not common in the lead-zinc orebodies and, where found, is usually associated with galena, it may be partially replaced or overlayered by tetrahedrite-tennantite. Enargite rarely replaces bournonite. In the Purisima Concepcion West section of the lead-zinc orebody, plagionite occurs with galena, pyrite and arsenopyrite. In the San Jose de Alis prospect, north of Yauricocha, geocronite occurs intergrown with galena.

Barite is uncommon and, like quartz, appears to have crystallized earlier than the sulfides. Hydrothermal solutions depositing either copper or lead-zinc appear to have leached the early-formed barite. A late generation of calcite is found associated with quartz and sphalerite, peripheral to the main orebodies. Quartz is ubiquitous throughout the orebodies and seems to have crystallized during several pulses of mineralization. Thomson (1960) has identified four generations of quartz deposition.

Oxidation and supergene enrichment extend from the surface to the deepest levels in some orebodies, whereas other orebodies display almost no oxidation or supergene enrichment. Supergene sulfides include covellite, chalcocite and digenite.

MINERALS

Although Yauricocha has good potential for producing mineral specimens, very few have been recovered for the mineral market. Trips to the locality by Peruvian dealers in the 1980's were generally fruitless, other than the acquisition of some low-luster, poor-quality pyrite.

Azurite [Cu.sub.3][(C[O.sub.3]).sub.2][(OH).sub.2]

Although not mentioned in the literature, azurite has been found as tiny crystals on dull, slightly corroded pyrite, and as below.

Malachite [Cu.sub.2](C[O.sub.3])[(OH).sub.2]

Yauricocha is one of the few mineral localities in Peru that actually has an oxide zone. Although malachite is not officially reported from Yauricocha, it is present in the oxide zone. The malachite in the best specimens, which are now rare, occurs as botryoidal aggregates, sometimes with minute azurite crystals nestled in the crevices. A specimen acquired by one of us (TS) in the late 1970's had been used as an ash-tray by one of the mining engineers before it was rescued, cleaned and sold. It is reminiscent of malachite from Bisbee, Arizona, and measures about 15 cm across.

Table 18. Minerals reported from the Yauricocha district.

Common or Abundant

Bornite Chalcopyrite Enargite Galena Pyrite Quartz Sphalerite Tennantite-tetrahedrite

Uncommon or Locally Abundant

Arsenopyrite Azurite Barite Bismuthinite Bournonite Calcite Chalcocite Covellite Digenite Djurleite Emplectite Fluorite Geocronite Hematite Idaite (*)Malachite Orpiment Plagionite Polybasite Realgar Siderite

* Collector-quality specimens

RELATED ARTICLE: REMINISCENCES ON A TRIP TO SAN CRISTOBAL, 1974

The road up to San Cristobal led past the tungsten concentrator at the Mar Tunnel. We were given an extensive tour of the concentrator and told more about the concentration of tungsten ores than we ever wanted to know. In back of the compound against a hill was an old fashioned cement portal, inscribed "The Kingsmill Tunnel," which had a real ripping small river coming out of a cement channel in its floor. I asked what it was. "That is the drainage for the lower level of Morococha," they said. I protested, "But that's about 60 miles away." I was told something like "No, in a straight line it is only about 23 miles." Mr. Kingsmill was one of the former directors of the Cerro Corporation before Peru nationalized the mines, and this amazing, mostly forgotten and now taken for granted engineering feat was named after him.

San Cristobal, like many other high mining camps, is ringed with yet higher mountains, their slopes dotted with mine dumps and tunnels. Its production was mostly wolframite and other tungsten minerals. It occasionally produced fabulous pyrite and quartz specimens of a distinctive nature, and sometimes small blocky wolframite crystals associated with pyrite.

We arrived just after a snowstorm and the camp was paved with three inches of partially frozen slush covering a layer of mud; just the thing to make walking around a real misery. The pavilions where the miners lived also had three inches of partially frozen slush on their corrugated iron roofs, which overhung the wails by about a foot. The slush was melting fast and brisk rivulets of ice water were cascading off the roofs. Every time we would knock on a door we tried to avoid getting a pint of ice water down the back of our necks; we were not always successful.

Mineral specimens were scarce, but two or three of the miners had quartz specimens that consisted of beautiful, thin, 2.5-cm butterfly bladed twins of some black mineral that was thinly coated with tiny quartz crystals. I thought, correctly as it turned out, that I might be able to etch them off with hydrofluoric acid (HF) and produce spectacular specimens. This mineral turned out to be almost pure iron tungstate (ferberite). Some of the specimens we got later, however, were more heavily covered with quartz crystals, and we discovered that, when the ferberite was exposed by etching with HF, they were etched and not-well formed. We only got about two dozen of these specimens, and wondered how many more had been ground into dust when run through the mill.

RHC
COPYRIGHT 1997 The Mineralogical, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1997 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:mining district in Peru
Publication:The Mineralogical Record
Date:Jul 1, 1997
Words:10814
Previous Article:Huaron Group.
Next Article:Huancavelica Group.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters