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The Pasto Bueno Group.

THE PASTO BUENO DISTRICT

Pallasca Province Ancash Department

LOCATION

The Pasto Bueno district is located in the northern-most extension of the Cordillera Blanca in the north-central Andes, just west of the Continental Divide, approximately 90 km east of the Pacific Coast. Elevations vary from approximately 3,300 to about 5,000 meters. The Pasto Bueno mining camp has not been inhabited for many years. The main settlement for mining in the area these days is at Consuzo, with smaller camps at Region de Maria Ofelia, Region de Huaura, Region de Huayllapon, and Mundo Nuevo. The Huayllapon mine area is located on the north side of the Consuzo quartz monzonite stock, 2.5 km north-northeast of the camp at Pasto Bueno [ILLUSTRATION FOR FIGURE 23 OMITTED].

HISTORY

In 1910 wolframite was discovered at Pasto Bueno. The Peruvian mining company Fermin Malago Santolalla e Hijos was established to mine the deposits in 1934, and, as of 1991 was still the owner. On February 14, 1990, "Shining Path" guerrillas blew up the mine's hydroelectric plant, causing a cutback in production. In 1991 the mines were producing a total of about 50 tons per day.

GEOLOGY

Pasto Bueno is a vein-type tungsten deposit. Mineralization occurs in steeply dipping quartz veins associated with the late Tertiary quartz monzonite Consuzo stock. Sedimentary rocks in the area comprise part of the Jurassic and Cretaceous sequence of the Chicama Formation shale and the Goyllarisquizga Group shales, quartzites and limestones of northern and central Peru. Landis and Rye (1974) discuss the detailed geology, and the reader is referred to their work for further detail. Much of the following geologic data on the Pasto Bueno area are abstracted from their paper.

Although not clearly defined, a zonal arrangement of mineralization from the quartz-monzonite stock in the center to the edge of the mineralized areas is as follows: molybdenite-pyrite; wolframite-tetrahedrite-chalcopyrite-pyrite; galena-sphalerite-tennantite; and last, fluorite and carbonates. No single vein has all these assemblages, so the sequence has been determined through observations of a number of veins in the various workings.

Mineralization is classified as greisen, vein, and vug. The vug period is a late phase of vein deposition. Open rugs are common, but only minerals characteristic of final rug crystallization typify the rug period. "Phantom" quartz crystals are common in the veins; these are indicative of breaks in quartz deposition.

The greisen assemblage is typified by coarse-grained, fractured fluorite in varying shades of blue through green, purple and brown. Associated minor minerals occurring as coatings in fractures, include tourmaline, topaz, apatite, titanite, rutile, stolzite, arsenopyrite, bornite, chalcopyrite and molybdenite (in no particular order).

The first major mineralizing episode resulted in molybdenite-pyrite deposition. The second major mineralizing episode resulted in tungsten mineralization, with two depositional periods, which were followed by the sulfide assemblages. The first tungsten period produced thick, stubby brown blades of hubnerite in quartz. The second tungsten period resulted in deposition of black, prismatic, euhedral hubnerite crystals in vugs. Some are a deep blood-red color in transmitted light.

Minor quantities of enargite occur in early-formed sulfide veins, associated with tetrahedrite, pyrite and galena: these minerals apparently crystallized after the tungsten period. Early-formed tetrahedrite is Sb-rich and gradually grades to As-rich tennantite, which is typical of late-stage mineralization. Botryoidal native arsenic is found rarely as an accessory mineral to the late sulfide vein assemblage.

The third major mineralizing episode was the crystallization of galena and sphalerite. Galena preceded sphalerite and continued to be deposited during the sphalerite crystallization. Early-formed sphalerite is dark brown; later-formed sphalerite is pale, transparent green; and the latest-formed is very dark opaque brown. These color differences apparently reflect changes in Fe content of the hydrothermal fluids. Sericitic mica, in the third depositional sequence, was deposited as a late-stage hydrothermal mineral in veins, filling cracks and cleavage planes in hubnerite, earliest late-stage galena, and in fluorite. Sericitic mica was also deposited at the base of open-rugs, as coarse plates to 6 mm in diameter, before the period of major fluorite deposition.

The fourth major mineralizing episode is characterized by fluorite crystallization in large amounts, as a late-stage vein filling. The fluorite is color-zoned, the earliest-formed phase alternating between green and colorless, then a colorless phase, then a brief period of purple, and finally a major period of green. Scheelite is a rare accessory mineral occurring as non-fluorescent yellow to purple crystals in vugs. Rhodochrosite and other carbonates also occur as late crystallizations in the vugs, and are usually found occurring on quartz.

The ore-producing veins are near vertical, quartz-rich systems that are found on either side of the upper intrusive contact of a quartz monzonite stock that has penetrated an older sequence of shale and quartzite. Principal vein minerals of interest are wolframite (usually hubnerite), tetrahedrite-tennantite, sphalerite, galena, quartz, fluorite, pyrite and rhodochrosite. Native arsenic and enargite occur, but are uncommon. Purple apatite, scheelite and molybdenite are rare.

The two veins that have produced the most collector-quality specimens are the Chabuca and Santa Isabel veins. Vugs as large as 80 cm across are common in the Chabuca vein. Minerals reported from these vug occurrences are quartz, hubnerite, ferberite, pyrite, galena, sphalerite, tetrahedrite, fluorite, rhodochrosite and rarely native arsenic, tungstite, calcite, dolomite and scheelite. The centers of these veins have the gemmiest quartz, and also tend to have more translucent hubnerite. Brown sphalerite occurs sparingly with the hubnerite in these centers.

La Magistral is located 10 km southwest of Pasto Bueno. The Magistral deposit is a contact-metamorphic skarn assemblage which contains scheelite, chalcopyrite and other minerals in a limestone adjacent to a hornblende-rich granodiorite. La Magistral produced good specimens of arsenopyrite with quartz, sphalerite and hubnerite, all of which have been collected within the past ten years.

The Mundo Nuevo-Tamboras area is about 30 km northwest of the Huayllapon mine in the district of Cachicadan, Santiago de Chuco Province, Department of La Libertad. The camp of Tamboras is about 500 meters north of Mundo Nuevo. The entire area around Mundo Nuevo is criss-crossed by quartz veins penetrating the black shale and quartzite country rock. Vein walls are frequently slickensided; the slickensides are thought to be the result of post-mineralization movement (Boit, 1955). The veins are sometimes vuggy, with white quartz grown normal to the vein walls. Hubnerite, tetrahedrite and pyrite dominate, with minor amounts of arsenopyrite, sphalerite, chalcopyrite, bornite, stibnite, polybasite, bournonite, corellite, galena, gold and rarely pyrrhotite. Siderite can also be present. No single quartz vein has all of these minerals in it, although the three dominant minerals are present in the great majority of them. Many mines and prospect pits are scattered throughout the district, most small in size, but some open stopes may exceed 100 meters in length. Tabular wolframite, As-rich tetrahedrite, and Fe-rich sphalerite came from the principal vein, the Veta Pueblo, which was being mined in 1974. The La Victoria and Tambora veins, which are no longer actively mined, have similar mineralogy to the Veta Pueblo vein. Wolframite-hubnerite specimens from the La Victoria mine, about 1 km south of Mundo Nuevo, were available until 1974 when the mine closed. These occur as black, bright-lustered, thin, bladed crystals with quartz, and as stout bladed crystals similar to those from Pasto Bueno.

Most minerals currently seen for sale from this area are from the Huayllapon mine at Pasto Bueno.

MINERALS

Augelite (?) [Al.sub.2](P[O.sub.4])(O[H.sub.3])

Augelite (?) in pale blue to green crystals about 1 cm in size has been found in Peru, although there is some doubt as to the authenticity of Pasto Bueno as the specific locality.

Arsenic As

Native arsenic occurs in superb botryoidal aggregates up to 10 cm across which are commonly penetrated by clear quartz crystals. One specimen of this type is now in the Los Angeles County Museum of Natural History. Although uncommon, some hubnerite crystals have 1 to 3-mm spheroids of native arsenic implanted on the prism faces.

Fluorapatite (?) [Ca.sub.5][(P[O.sub.4]).sub.3]F

Apatite in blue-green crystals up to 2.5 cm long have been found at Pasto Bueno. During the early 1980's a very few lilac to purple apatite crystals up to 6 mm in length were collected by one of us (TS). These are thumbnail-size specimens, associated with quartz and rhodochrosite.

Fluorite Ca[F.sub.2]

Fluorite usually occurs as pale green crystals, often in association with quartz. Crystals up to several centimeters across have been found. Fluorite occurs rarely as fine octahedrons to 8 cm or more in size on and associated with hubnerite and quartz. Overall, most fluorite from Pasto Bueno is of mediocre quality and not lustrous.

Hubnerite-Ferberite MnW[O.sub.4]-[Fe.sup.+2]W[O.sub.4]

The hubnerite from the Huayllapon mine at Pasto Bueno is without doubt the world's best. Crystals up to 25 cm in length have been collected, usually in association with attractive, lustrous quartz crystals. Smaller hubnerite crystals can be a translucent deep cherry-red color in transmitted light, and black in reflected light. Crystals in the 5-10 cm size range are relatively common. They are usually prismatic with flat terminations; parallel growth of crystals, rather than individual crystals, is quite common. They vary from smooth and glassy prisms to somewhat abraded-looking prisms. Carbonates may originally have occurred on the crystals, but in specimens offered for sale these have usually been etched away. Some fine specimens of hubnerite are associated with tetrahedrite and fluorite, but these are rare.

In 1985 hubnerite in good crystals was collected from Mundo Nuevo, just northwest of Pasto Bueno. This particular lot of specimens is distinctly different from those collected at Pasto Bueno. The crystals are usually short and stout blades, commonly twinned, with a translucent deep red color toward the edges when backlit, and with a black opaque band vertically oriented within the crystal. Terminations are usually wedge-shaped rather than fiat, as is common for hubnerite from Pasto Bueno. Rarely are the crystals from Mundo Nuevo as bright as the ones from Pasto Bueno.

Pyrite Fe[S.sub.2]

Pyrite is common as bright pyritohedrons, usually associated with quartz and sometimes with fluorite. Pyrite crystals are frequently striated to a moderate degree. It also occurs as lustrous, slightly modified cubes. Pyrite crystals rarely exceed 2.5 cm in diameter. It has never been an abundant mineral at Pasto Bueno.

Quartz Si[O.sub.2]

Quartz at Pasto Bueno is one of the more common minerals. The crystals are attractive, with good luster, and can reach over 40 cm in length. They are frequently transparent in the terminations, with the prisms either translucent white or near colorless. Many of the larger quartz groups have "nests" of needle-like crystals radiating out from the base of the larger crystals, reminiscent of the quartz from Huaron, but on a much larger scale. Japan-law twins have been collected, and Dauphine twinning is common. Some of the larger clear quartz crystals have within them invisible phantoms that fluoresce a pale green color under shortwave ultraviolet light (Dan Belsher, personal communication, 1992). Good specimens of smoky quartz crystals have been collected rarely from this area, sometimes with pale green fluorite on the quartz (Dan Belsher, personal communication, 1992).

Quartz from Mundo Nuevo occurs in stout, prismatic, white to clear crystals, sometimes in Japan-law twins to over 10 cm in size. The average size for the quartz crystals from Mundo Nuevo is about 6 cm in length.

Rhodochrosite MnC[O.sub.3]

The Huayllapon mine has produced rhodochrosite of exceptional quality, some of which is considered among the best in the world. It comes in several shades of pink to near red, including a deep rose-pink similar to that of rhodochrosite from Alma, Colorado. Some of the more spectacular specimens include brilliant, lustrous, translucent to transparent, zoned, raspberry-red rhombohedrons nestled in attractive groups of quartz crystals. Bancroft (1984, p. 181-184) shows several of these. Crystals of rhodochrosite up to 16 cm in size have been collected (Bancroft, 1984). Some of the cherry-red hubnerite crystals have rhombic rhodochrosite crystals implanted on them, but the best of these "just escaped greatness" because of damage. Altogether there were probably not more than two dozen really outstanding rhodochrosite specimens produced from Pasto Bueno.

Scheelite CaW[O.sub.4]

Scheelite occurs in grayish white to purplish gray pseudo-octahedral crystals. Their average size is 5-6 mm and the largest are about 1 cm.

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

Tetrahedrite has been found rarely as classic tetrahedral crystals, 2-9 cm across, on white to colorless, long prismatic quartz crystals: sometimes this assemblage produces very attractive groups. Tetrahedrite is black with a silvery reflectance. It has a luster that varies from satiny to mirror-bright. Crystal forms are generally simple tetrahedrons, but sometimes range to more complex tetrahedral forms. One of the more rare and unusual occurrences from Pasto Bueno is tetrahedrite on hubnerite. An unusual and very fine specimen of tetrahedrite in the collection of one of us (RHC) is a perfect 2.2-cm tetrahedron perched on the termination of a 7-cm-long hubnerite crystal blade.

Table 2. Minerals reported from the Pasto Bueno district.

Common or Abundant

(*)Fluorite (*)Hubnerite-Ferberite (*)Pyrite (*)Quartz (*)Sphalerite (*)Tetrahedrite (*)Rhodochrosite

Rare or Locally Abundant

Arsenic Arsenopyrite (*)Augelite Bornite Chalcopyrite Enargite (*)Fluorapatite Molybdenite (*)Scheelite Stolzite Topaz Tungstite (*)Vivianite Zinnwaldite

* Collector-quality specimens

RELATED ARTICLE: REMINISCENCES ON AN EARLY TRIP TO PASTO BUENO, 1976

The road from Quiruvilca to Pasto Bueno is brutal. Although the distance is something less than 100 km we were told to allow at least five hours for the trip. If the weather had been bad, it would have taken much longer. In a country of remote mines, Pasto Bueno is considered truly remote. In several places we had to stop and engage in road-building to get through, and in places if our tires had slipped, our Volkswagen "bug" would have been left balancing on the high center with our tires dangling in air with nothing ahead of us but hours of road building. There were times when you would just stop and look at the road ahead and think "Am I really supposed to try to drive over that? If I make it over that, will it get worse further on, and if we must rum around, can I drive back over this thing?" The road would snake sharply down into a canyon, and we would struggle back out on a series of ragged zigzags, to cross yet another ridge. Sometimes the drop-off was so great that we thought we could fall a thousand feet before the first bounce. In many places we could not even see the bottom of the canyon. On the edge of the road we passed a number of little crosses put there in memory of those poor unfortunates who missed the road. We met not a single vehicle or person on the road and were told that it was used perhaps only twice a week. At last we rounded a curve and there, clinging to the wall of the steep canyon below us, was Pasto Bueno.

Mines and prospect holes dotted the mountainsides. Down we twisted through a series of jagged switch-backs that eventually brought us to the camp. There was only one main street down to the mining offices at the bottom of the camp. Down we wound past the workers' houses, past the concentrating mill, to the mine offices at the bottom of the camp. Houses and mine buildings were jammed against the mountain on one side, or hugged the road on the other, some with their posteriors hanging over the canyon. In such camps the "high rent" district is always at the bottom of the camp because there is more oxygen to breathe.

We were indeed fortunate to have the permission of management to stay in company housing; and since the mine manager was not there, we were given the use of his house. It seemed to us the lap of luxury with actual hot water and our meals prepared for us. They even repaired minor damage on our Volkswagen and gave us gas for our return trip. We were also able to arrange an actual visit to the Huayllapon mine.

The road to the upper camp where the Huayllapon mine is located, is so steep and so high that our poor little "bug" could only make the grade carrying the driver, assisted by a little push from a breathless passenger, but we made it. We entered the mine through the lower adit and after several hundred yards we climbed a series of ladders to reach the working face. They were mining a nearly vertical quartz vein about 2 meters across. This vein carried hubnerite and other metallic sulfides near its center, which frequently opened into pockets filled with brilliant, transparent, prismatic quartz crystals, with an occasional slender crystal of blood-red hubnerite pointing into the pocket. The mine itself was a rather small (60 miners), primitive operation that did not even use mucking plates to make things easier for the miners.

We were only able to collect one specimen of poor quality quartz and tetrahedrite from one pocket. Before returning to the lower camp we managed to break free of our management guides and spent some time buying specimens from the miners. One specimen that I particularly remember is a single terminated crystal about 1 x 3 x 8 cm that glowed red when you held it up to the sun. It cost about 60 cents. We bought a lot of specimens that day, and that night our dreams were of caverns full of arm's-length red hubnerites and basketball-size rhodochrosites.

After an exhaustive tour of the mill and concentrator, we spent the remaining two days wandering around the camp and socializing with the geologists and engineers, all the while buying and trading for specimens. We were particularly in the market for rhodochrosite because another American, Thomas Nagin, had preceded us to the camp a few months earlier and had come away with a fabulous rhodochrosite specimen that was reportedly later sold for many thousands of dollars. We found only scraps, but we did find the young geologist who had sold Thomas the specimen. I asked him if he felt that he was paid enough for the specimen and he said he didn't think so because later someone else came to the camp and bought a much inferior specimen for 15 dollars.

From another miner in the camp Jorge bought a 30 x 30-cm quartz specimen of beautiful, sharp, lustrous crystals up to 20 cm in length for a dollar. For another dollar I bought a fabulous cluster of quartz and hubnerite that I speculated might bring as much as $2,500. It turned out that the miner had constructed this specimen from six separate quartz and hubnerite specimens using candle wax, which matched exactly the color of the quartz. In this case the fabrication was not done to deceive, but merely as a diversion to pass the time. From another miner I bought a specimen of hubnerite and quartz more than 30 cm in diameter, with one terminated hubnerite crystal measuring about 25 cm long. This specimen is now in the collection of Bryn Mawr College near Philadelphia. At the time I remember thinking "If this is what we can buy on one trip, what has and will the mine produce in years to come?" As it turned out, however, that was the largest hubnerite I ever saw from Pasto Bueno. Those were just the highlights and we came away with our Volkswagen "bug" so full of specimens that we wondered how we were going to squeeze in the rest of the specimens we had stored back in Quiruvilca.

Soon after our trip an enterprising dealer offered the miners a year's salary for another big rhodochrosite and the cost of specimens in the camp escalated 10 to 100 times. In later years the company doctor became the source of many of the good rhodochrosite specimens that came from Pasto Bueno. The most common association with the rhodochrosite was quartz, but a few were associated with pale green fluorite crystals and some with red hubnerite. We never saw a good undamaged combination specimen of red hubnerite and rhodochrosite. In total, I do not think more than one or two dozen fine to superb rhodochrosite specimens were produced. The mine has been operating since World War II: who knows how many fine specimens were destroyed during the mining process or ground through the mill.

In recent years the management of the mine has taken a dim view of people visiting the camp to gather specimens, and has at times controlled access to the camp. This has had little apparent effect on the flow of specimens from the mines. Mineral specimens from Pasto Bueno during the last few years have been intermittently available in Lima and are noticeably inferior to those produced during the 1970's. We are not aware of any really fine specimens of hubnerite or rhodochrosite coming from the district since about 1986. RHC
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Title Annotation:mining district in Peru
Publication:The Mineralogical Record
Date:Jul 1, 1997
Words:3507
Previous Article:The Quiruvilca District.
Next Article:The Raura-Huanzala Group.
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