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COPYRIGHT 1997 The Mineralogical, Inc.
THE RAURA DISTRICT
Cajatambo Province Lima Department
LOCATION
The Raura district is a polymetallic deposit of veins and replacement orebodies located northeast of Lima and northwest of Cerro de Pasco. It is about 15 km due north of the town of Oyon, along the Maranon River. Raura is situated in a spectacular setting: it is at the source of the Amazon River and is surrounded by rugged peaks, with glaciers perched above the mines.
HISTORY
Raura is one of the more important copper-lead-zinc-silver mining districts in Peru. It was certainly worked in the last century and probably in the 17th and 18th centuries as well. Old workings are situated at over 5,000 meters, and are accessed by dizzying llama paths along the cliff faces. One wonders how the Spaniards discovered these rugged, remote ore deposits. The modern development of the mines began on a small scale in 1890. The high-grade ore was transported on llamas to a smelter at Quinchas, some 20 km away. Production was intermittent until the mines were acquired in 1945 by the Cerro de Pasco Corporation. The current company, Compania Minera Raura S.A., was founded about 1960, when all of the Raura mines were consolidated under one company. As of 1991, about 25% of the annual production was coming from two open pit mines operating in the Nino Perdido and Tajo Grande orebodies.
GEOLOGY
The Raura district was briefly described by Miller and Singewald (1919), Porturas (1954), and Purser (1971); their data are summarized below. The geology of the Raura district is shown in Figure 35.
The main sedimentary unit in the Raura area is the Cretaceous Machay Limestone, a non-fossiliferous, pale gray limestone that occurs on both sides of the Raura depression. Known mineralization covers about 24 square kilometers. The veins are associated with a diorite porphyry intrusion to the southwest, which apparently erupted as dacite flows to the northeast. Several small stocks also occur in the mining district (Purser, 1971).
The Raura depression contains several orebodies. The Catuvo orebody occurs along the contact of an intrusive stock, and is predominantly a Pb-Zn producer. Another orebody is located in an area of colorful oxide zones near Lake Ninacocha, which is the official source of the Amazon River. The ore-bearing mineral in this area is Ag-rich galena. The Esperanza orebody is north of the Catuvo orebody. A potential orebody, according to Purser (1971), is the Toramina, situated in limestones northeast of Raura. He reports that there are nice garnets along the contacts in this limestone.
The Esperanza and Restauradora vein systems host the main orebodies, with several smaller mineralized fractures adding to the reserves. Individual ore-bearing veins are usually less than 300 meters in length. Ore minerals of the Esperanza orebody and veins are tetrahedrite, galena and sphalerite, with pyrite, quartz and manganoan calcite as the major gangue minerals. The tetrahedrite shows a characteristic red streak. The main Esperanza vein averages 80 cm in width in its richest portions. Other veins have an east-west strike and are about 1 meter wide. The ore veins are banded, with a central zone of quartz, then tetrahedrite, sphalerite, and galena, followed successively by calcite, and then dolomite. The ore is Cu-rich and Ag-rich, and occurs in shoots.
The order of crystallization for the metallic minerals is (first) pyrite, sphalerite, chalcopyrite, tetrahedrite, galena, and (last) sulfoarsenides. Quartz crystallized up until the period of galena crystallization. Carbonates crystallized last.
MINERALS
Minerals offered for sale at Raura in recent years include: fine transparent gypsum in stout, water-clear classic-shaped crystals; cubic pyrite in crystals reminiscent of those from Spain; stibnite in very fine acicular to bladed crystals with a satiny luster and, less commonly, a bright metallic luster; rhodochrosite: and green fluorite with chalcopyrite. Raura is known for the similarity of its specimens to those from Romania, especially the dull, stout stibnites and the pale pink drusy rhodochrosites.
Barite BaS[O.sub.4]
Barite occurs at Raura in bladed crystals and crystal aggregates that may reach several centimeters across. The color is white to gray and beige. These can make high-quality display specimens.
Calcite CaC[O.sub.3]
In the early 1980's calcite was common as white, rhombohedral crystals in groups and also in a peculiar "witches hat" crystal habit with a cream-white color. These unusual calcites occur as thumbnail-to miniature-size specimens.
Gypsum CaS[O.sub.4][center dot]2[H.sub.2]O
Gypsum occurs in crystals over a meter in length. According to Dan Belsher (personal communication, 1992), Peruvian collector/dealers broke the gypsum crystals into thirds in order to transport them to Lima in their backpacks, where they could sell the specimens. In 1980 and 1981 a few lots of gypsum, with crystals up to 10 cm long appeared on the market. These gypsum crystals are associated with massive pink manganoan calcite; most of the gypsum crystals are damaged.
Pyrite Fe[S.sub.2]
Pyrite is more common at Raura as pyritohedrons than as cubes, and it usually does not occur as quality specimens. It is a common accessory mineral with the other sulfides.
Rhodochrosite MnC[O.sub.3]
In the mid 1970's literally tons of rhodochrosite were produced from this locality. Apparently very little has been forthcoming since. The rhodochrosite occurs as pale pink to bright pink semicrystalline masses and as good crystals with a rhombic form up to about 5 mm in size. Galena is a common association, usually as the matrix mineral. Quartz is also frequently associated, both as matrix and as crystals on the rhodochrosite. The associated quartz is usually present in small needle-like transparent crystals or in drusy aggregates. Needle-like microcrystals of quartz are also common on the rhodochrosite crystals.
Raura has never produced the outstanding rhodochrosite specimens that some other Peruvian localities have. A help in distinguishing Raura rhodochrosite from those of other Peruvian localities is the distinctive presence of coarse cleavages of galena on the back, or underlying matrix, of the specimens. Rhodochrosite from Raura also looks very similar to that collected from Silverton, Colorado, especially in regards to color, form, habit and drusy quartz association. However, the galena from Silverton is much finer grained than that from Raura.
Seligmanire PbCuAs[S.sub.3]
Seligmanite is a rare mineral collected at Raura around 1990. it has only been sight-identified, and to our knowledge has not been verified otherwise. It occurs on flat plates of quartz; the quartz, in turn, is partly coated by a microcrystalline layer of manganoan calcite. Seligmanite is scattered around the specimens as small black crystals up to 5 mm in length.
Stibnite [Sb.sub.2][S.sub.3]
Raura has produced very nice prismatic stibnite blades which have a satiny metallic luster and lead gray color. Terminations have a somewhat rounded look, although they are essentially flat. Crystals range to about 7 cm in length, and fine groups to over 25 cm across have been recovered. These groupings are not abundant and are rarely seen for sale. The main finds of this material were collected in the early 1980's. Less commonly stibnite occurs in sharp, bright, metallic crystals which look very similar to the stibnite from Romania: these crystals tend to be considerably smaller than the satiny blades. Stibnite may be associated with barite and pyrite, and many specimens were coated with calcite that was etched off. Some of the first lots of specimens that came out were ruined; piriteros, attempting to remove the calcite with too strong a solution of HCl, badly damaged the stibnite.
Table 3. Minerals reported from the Raura district.
Common or Abundant
(*)Barite (*)Calcite (*)Chalcopyrite Dolomite (*)Fluorite Galena (*)Gypsum (*)Manganoan calcite (*)Pyrite (*)Rhodochrosite Sphalerite (*)Stibnite (*)Tetrahdrite
Rare or Locally Abundant
Gratonite(?) Realgar Scheelite (*)Seligmanite
* Collector-quality specimens
THE PACHAPAQUI DISTRICT
Bolognesi Province Ancash Department
LOCATION
The Pachapaqui district is at the headwaters of the Rio Pativilca, in the northeast corner of Bolognesi Province. It is 20 km southwest of the Huallanca district, where the Huanzala mine is located, and is about three to four hours' drive south from Huanzala. The town of Pachapaqui is at the south edge of the district, 22 km north of the town of Chiquian.
The area has been extensively glaciated, and the quebradas (valleys) are generally U-shaped, some of them still having active glaciers near their heads. The mining district occupies an area 18 km long by 9 km wide. Elevations for the various mine workings vary from about 3,800 to over 4,500 meters. The area around Pachapaqui is one of the more starkly beautiful areas of Peru, with snow-covered craggy peaks and large glaciers dominating the scenery in many areas. Some of the mines have been worked since Spanish Colonial times, and very old prospect pits, trenches, shafts and open stopes are scattered throughout the district.
HISTORY
The Spanish worked the mines by hand-drilling holes in the rock, then filling them with water and allowing it to freeze overnight; the expansive force of the water freezing would then break up the rock (Cavanagh, 1993?). The name Pachapaqui is Quechua for "broken earth," probably in reference to the intensely folded, twisted and broken rock strata that are visible in the local road cuts.
Pachapaqui is a polymetallic mining district with varied types of ore deposits. Although some mining on a small scale was done as far back as the early 1900's, modern development first began in 1949, when the Cia. Minera Argenta Bolognesi, a family-run enterprise, installed a 30 tons-per-day concentrator. The oldest mines are the Vetilla or Santa Teresita, Arabia, Patria, and Esperanza. The major mines in the district in the 1950's included the Patria, Esperanza, Otito, Sinchi Roca, San Antonio, Abandonada, and San Judas Tadeo. In 1965 the Moro and Vetilla mines were opened on each side of the mountain, Cerro Potosi. Shortly thereafter the Riqueza mine was opened. More recent mines, established by the mid-1970's, include the Pucarrajo, Pozo Rico, Patria-Ishanca, and Ishanca.
In February 1971, Jorge Ganoza, owner of Minera Pachapaqui S.A., bought the Pachapaqui mines for $4,000. It was a good deal: by late 1990 the mines had produced over $75 million worth of ore. Prior to 1989 Pachapaqui was primarily a silver mining area, but in that year the company decided to develop the lead and zinc ores, which bad largely been ignored up to that time. This change resulted in a tripling of the ore production from Pachapaqui by late 1990.
The mines are scattered around the area, mostly north and northeast of Pachapaqui. Some...
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