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The Las Chispas mine: Arizpe, Sonora, Mexico.

In the first decade of the 20th century, the Las Chispas mine near Arizpe in Sonora produced some of Mexico's largest and best specimens of polybasite crystals, large clusters of "poker chip" stephanite crystals, fine acanthite crystal clusters and a few very fine pyrargyrite specimens. Most of these were saved through the enlightened efforts of mine manager Edward L. Dufourcq (1870-1919).

INTRODUCTION

In 1904, Edward L. Dufourcq, a graduate of the Columbia School of Mines, took over the management of the Las Chispas mine near Arizpe (Arispe) in Sonora, Mexico, a modest mining property owned by the Minas Pedrazzini Gold and Silver Mining Company. Soon he began shipping some extraordinary silver mineral specimens back to New York, including the largest and best polybasite specimens known, large clusters of "poker chip" stephanite crystals, fine acanthite specimens and a few very fine pyrargyrite specimens. These were sent to various institutions, but mainly to his alma mater. Dufourcq described some of the material he sent to New York in an article published in 1910 in The Engineering and Mining Journal:
  The crystallized specimens of the silver minerals are especially
  noteworthy ... What is probably the largest single specimen of
  stephanite in the world was presented by Mr. Pedrazzini to the
  Egleston Collection at the Columbia School of Mines, where there
  are also a number of other specimens of polybasite and stephanite,
  as well as a remarkable specimen representing the transition of an
  argentite crystal into cerargyrite and a fine embolite. The American
  Museum of National History in New York also has, from this mine (the
  Las Chispas), what is probably the largest mass of polybasite
  crystals ever taken out in one piece. This originally weighed over
  65 lb., but was broken into two parts during the time it was in
  transit from Sonora to New York (Dufourcq, 1910).


Today many museums and private collections have spectacular examples of Arizpe silver minerals. It is probable that the Las Chispas mine produced no more than 20 million ounces of silver, but because Dufourcq took special care to preserve a suite of mineral specimens, Las Chispas must be considered a classic mineral locality.

HISTORY

Early in the 17th century, Jesuit priests began to enter the rugged basin-and-range desert country that is modern-day Sonora. The Jesuits' intent was to convert the indigenous populations to Christianity, but their missionary settlements also opened the region for Spanish miners and ranchers. In 1640, soldiers under the command of Pedro de Perra discovered a system of veins rich in silver in the Valley of the Rio Sonora, near the future town of Arizpe (West, 1993). Although the exact location of this discovery is lost to recorded history, the descriptions are consistent with the future Las Chispas mine and adjacent Bavicanora mine. There was no documentation of organized exploitation of this original Sonoran silver discovery, but there was extensive exploration of the surrounding area. Jesuit missionary Jeronimo de la Canal established the town of Arizpe in 1646. The name "Arizpe" is a word from the language of the Opatas (the indigenous inhabitants of the Rio Sonora Valley) meaning "place of the fierce ant." The original church, Nuestra Senora de la Asuncion Temple, constructed in the year of the town's founding, is still standing today. Arizpe became a major administrative center in 1776 when it was proclaimed the capital of the Provincias Internal de Occidente, which included Sonora, Durango, Chihuahua, Arizona, California and Sinaloa. Arizpe fell from grace after the Mexican Revolution, and today is a town of approximately 2,000 inhabitants.

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In 1657, rich silver ore was discovered about 40 km south of the original Arizpe discovery, at a site called San Juan Bautista, in the mountains between the Rio Sonora and the Moctezuma Valley. San Juan Bautista became one of the largest silver producers in Sonora during the latter 17th century, and was made the administrative capital of the region. Colonial records give an idea of the mining wealth: in the first two years of operation, miners produced more than 4,000 pounds of refined silver from shallow workings. Almost certainly the ore was chlorargyrite and some minor native silver. By 1680, silver mining had all but ceased at San Juan Bautista, and the miners had moved on to other strikes at Nacozari, Bacanuchi and, most importantly. Alamos. The story of the San Juan Bautista is typical of the early mines of Sonora: subsistence mining, shallow workings, little record of the mine's passing, and few tangible remains except for abandoned mine buildings and ore-processing patios.

After the first 50 years of exploration and colonization, the mountainous regions of Sonora entered a long period of economic decline that was marked by nearly continuous confrontation with Apache and Seri nomads. The first documented Apache raid on a Spanish outpost occurred in 1680, and the raids continued for the next 200 years, forcing the abandonment of outlying communities and mines. The Spanish government organized several military expeditions against the raiding tribes. The expeditions were primarily funded by the mining enterprises and wealthy ranchers, but the peace won by these expeditions was always short-lived until a combined Mexican and American military force finally defeated Geronimo in 1880.

With the Indian threat removed, a renaissance in Sonoran mining took place. Political stability came with the dictatorship of Porfirio Diaz, and during his reign (1877-1911) Mexico welcomed investment in the nation's mining industry. Foreign companies explored extensively in Sonora and reopened many of the old abandoned mines (called antiguas). Around 1880 the Las Chispas mine is first mentioned in the records. Russell (1908) states: "During the early eighties the Santa Maria Mining Company was operating a small gold-silver prospect a few miles south of Arizpe in the northern part of Sonora, Mexico." The mining operation suffered from poor management and apparently lost considerable ore to highgraders. Eventually the Santa Maria Mining Company went bankrupt, and left the close-out of its business to the company clerk, an Italian-Swiss named John Pedrazzini. Pedrazzini acquired the property in lieu of money owed him by the company, and he set about raising capital to reopen the mine. In 1893 the Pedrazzini Corporation provided a special exhibit in The World's Columbian Exposition in Chicago that is reported to have contained some spectacular specimens of silver minerals (Lejeune, 1908).

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As Pedrazzini raised capital he began extensive exploration work which included a tunnel driven into the hill about 125 meters. This adit intersected the Las Chispas vein at a depth of about 185 meters below the surface outcrop, and the real production history of the Las Chispas (also called the Pedrazzini) mine began. In 1904, E. L. Dufourcq was hired as the general manager and consulting engineer; shortly thereafter, Pedrazzini incorporated his holding into the Pedrazzini Gold and Silver Mining Company. Dufourcq rapidly developed the mine, and built a modern 20-stamp mill and cyanide processing plant. By early 1910 the Pedrazzini Company employed approximately 600 men, almost 500 working in the Las Chispas mine (Dinsmore, 1911). For the years 1907-1911 the Las Chispas mine produced an average of 1.5 million ounces of silver and 10,000 to 12,000 ounces of gold.

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Even during the most productive times, ore theft at the Las Chispas mine was a problem. Russell (1908) relates a tale illustrating the extremes to which miners and speculators would go in order to get rich quick: a Cananea ore-purchasing company attempted to secretly purchase high-grade Las Chispas ore, and ended up with tons of specular hematite from a nearby mine.

Between 1911 and 1921 the Las Chispas mine suffered the ravages of the Mexican civil war. The war interrupted transportation, and in general caused considerable labor unrest. In 1917 the mine was confiscated by the local government, which highgraded the mine and mill and expropriated much of the mine infrastructure. The local government also renegotiated a labor contract with the miners which was quite unfavorable to the operation of a profitable mine. Eventually the mine was returned to the Pedrazzini Company, but it took several years of developmental work before the mine could produce ore again. Around 1918 a large flood destroyed the pump station for the mill along the Rio Sonora, and thereafter the mill was limited to using water pumped from within the mine (Montijo, 1920).

In 1921 the Corporation Miniere du Mexique was formed in Paris with a 25 million franc capitalization with the intent of purchasing the Pedrazzini workings. Through a Mexican subsidiary, the Corporacion Minera de Mexico, S.A., the French took over the mine, remodeled the power plant and began a profitable eight-year run at the Las Chispas mine. In 1924 the company struck a bonanza vein, and the next year it paid a 25 % dividend (Berstein, 1964). By 1930, however, the mine had ceased organized operation.

In the 1960s the mine was visited by a number of mineral collectors who recovered some fine stephanite specimens. The late Arizona mineral collector Joseph Urban told a story of his visit to the mine: he had thought that it was abandoned, and so was very surprised to find that people were actually living in the mine openings. He sketched in the dirt a hexagonal shape to indicate what he was looking for; one of the local men understood, went into the mine and came out with a handful of hexagonal polybasite crystals (Wayne Thompson, personal communication).

In the 1970s the mill and mine tailings were hauled away for concentration, and most of the old mine buildings were removed.

GEOLOGY

Arizpe is located in eastern Sonora, in the physiographic province known as La Serrana. The area is composed of north-south-trending fault-bounded mountain blocks separated by south-flowing river drainages. La Serrana forms the southernmost extension of the Basin and Range Province. The first geologic map of the area was published in 1888 by Jose Guadalupe Aguilera Serrano, who described the region as a "gigantic staircase" between the Sierra Madre Occidental in the east and the lowlands in the west bordering the Gulf of California. The mountain ranges are largely composed of Laramide-age flows and tuffs of intermediate composition, which were intruded by a series of subvolcanic stocks (Valencia-Moreno el al, 2007). These stocks are the source of mineralization exploited in the Las Chispas and surrounding mines.

[FIGURE 4 OMITTED]

The Las Chispas vein is one of a series of northwest-southeast trending fault-fissure mineralized zones. Other major veins include the Guillermo Tell, the Carmen, and the Bavicanora (located about 1 km east of the Las Chispas). All of these veins have produced significant quantities of silver. The Bavicanora is thought by some to have been the source of the original silver discovery in 1647 (Box, 1869), and is credited with producing at least 80 million ounces of silver (Woodbridge, 1911).

The Las Chispas vein is hosted by a felsic porphyry and overlying rhyolite. The vein is primarily quartz with metallic mineralization consisting of pyrite, silver sulfides and sulfosaits, and occasionally chalcopyrite. Silver mineralization within the vein shows strong secondary enrichment; ore minerals grade from chlorargyrite and silver at the top of the vein to pyrargyrite at depth. In the wider sections of the vein, open vugs several feet across were common, and this is where the large crystals were found. Russell (1908) wrote:
  The ore occurrence is chiefly remarkable for the extreme richness
  and beauty of the high-grade chispas *, or crystals of silver, from
  which the most fully developed property--Las Chispas--takes its name.
  This ore occurs in boulders, stringers and lenses in a series of 14
  nearly parallel fissure veins in the rhyolite. The vein filling is a
  rhyolite breccia carrying the [silver] values in the cracks and
  seams, which vary from the merest crack to 2 feet or even 3 feet in
  thickness. The stringers of ore extend so far [into the walls] as to
  render it necessary to carry the stopes from 12 feet to 18 feet in
  width for considerable distances.


The vein system is very antimony-rich, and arsenic appears to be largely absent. Figure 4 from Montijo (1920) is a stylized cross-section through the vein, showing the ore concentrated in irregularly spaced pockets.

The prevalence of bonanza-type pockets along the veins in the Las Chispas mine makes it difficult to infer the quality of the ore that was shipped to the mill. Outside the pockets the vein rarely exceeded 10 ounces of silver per ton, but within the pockets the ore averages more than 2,000 ounces per ton (Dinsmore, 1911). The individual pockets have distinctive ore mineralogy, but the bulk of the mineralization consists of acanthite, polybasite and stephanite. It is very common for acanthite and silver specimens to have a matrix of polybasite. The only other mineral of interest to collectors is fluorite. Many of the polybasite specimens have an overgrowth of pale-colored fluorite, which is very distinctive and diagnostic for the locality.

MINERALS

The annual report of the American Museum of Natural History records many donations from Dufourcq. For example, in 1907 it is noted that Dufourcq gave "8 specimens of Argentite (all crystallized and one very unusual); 2 specimens of Polybasite with wire Silver; 8 specimens of Stephanite (crystallized), from Las Chispas mine, Arizpe, Sonora, Mexico." These specimens are exemplars of some truly magnificent silver sulfides and sulfosalts from Arizpe, and are among the finest known from anywhere in the world.

Acanthite [Ag.sub.2]S

Acanthite is the most common of the ore minerals at Las Chispas, and the best examples rank among the world's finest for the species. At Arizpe acanthite comes in two different habits: blocky cubes to 4 cm, and elongated skeletal prisms. The blocky crystals often have a drusy coating of pyrite, and commonly appear to be parasitic growths on polybasite. The elongated prisms are unique to Las Chispas, and typically form clusters of crystals. The finest example known is a specimen that passed from Pedrazzini to Eduard Schmitter (to Miguel Romero.

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Fluorite Ca[F.sub.2]

Many of the polybasite specimens from Arizpe are partially covered with small, highly modified fluorite crystals. Desautels (1960) described the fluorite, noting that a unique feature of Las Chispas fluorite is that each crystal shows seven forms of the hexoctahedral class. Figure 10 shows a typical Las Chispas crystal. Forms present are the cube (a), octahedron (o), dodecahedron (d), tetrahexahedron (e) {012}, trapezohedron (m) {113}, trisoctahedron (q) {133}, and hexoctahedron (t) {124}. The faces of {012} and {111} are always etched. The cube, dodecahedron and trapezohedron are about equally dominant, and the other forms are of lesser importance.

Polybasite ([Ag.sub.8]Cu[S.sub.4]][[(AgCu).sub.6][(Sb, As).sub.2][S.sub.7]

Polybasite is found in the Las Chispas mine as short pseudo-hexagonal prisms and as hexagonal tablets. Some of the prisms reach 12 cm across. Many of the polybasite specimens from Arizpe are associated with small, complex, nearly clear, crystals of fluorite which coat the edges of the polybasite crystals. The largest of the Arizpe polybasite crystals have thick tabular shapes, although perhaps the finest known specimen is a cluster of paper-thin crystals up to 8 cm across. This specimen was a gift from the Pedrazzini family to Dr. Eduardo Schmitter, Mexico's dean of mineralogy. Schmitter stored the fragile specimen in a large coffee can filled with powdered soap. In the 1950s Dr. Miguel Romero Sanchez worked in Schmitter's lab as an undergraduate at the Universidad Nacional Auloma de Mexico (UNAM), and Schmitter inspired a love of minerals in Romero. In the early 1970s, when Romero began to build the world's finest collection of Mexican minerals, Schmitter gave the polybasite to him.

Desautels (1960) described a polybasite specimen in the Smithsonian collection {no. R7867) weighing 37 kg (82 pounds): it consists of an open network of splendent crystals associated with complex fluorite crystals and crystals of pyrite, quartz and chalcopyrite.

Pearceite has been reported from the Las Chispas mine, but every specimen labeled "pearceite" which has been X-rayed by the author has proven to be poiybasite. Hall (1967) included an analysis that showed the Arizpe polybasite to be dominated by antimony.

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

Pyrargyrite from Arizpe is uncommon in collections. However, there are a few truly outstanding examples with fine translucent red crystals to 5 cm which must be considered mineral masterpieces. Within the Las Chispas vein system the pyrargyrite was found in large pods devoid of other silver minerals.

Silver Ag

Native silver is widely reported from Arizpe, although examples are rare in museums. The silver typically occurs in bundles of wires associated with acanthite (the wires grow out of the acanthite). The wire bundles reach 7 cm in length and 3 cm across. An old Arizpe miner interviewed by Wayne Thompson in 1972 described seeing a walk-in vug festooned with stalactitic arborescent wire silver studded with dark crystals of silver sulfides.

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

Stephanite is one of the signature minerals from Arizpe. The best examples are extremely sharp pseudo-hexagonal "poker chips" that reach 5 cm across. Most stephanite crystals from the Las Chispas mine are short, thick, pseudo-hexagonal prisms formed by a combination of the prism and pinacoid, exceptionally to 12 cm across. This habit shows radial striations corresponding to twinning. A second, less common habit consists of elongated pseudo-hexagonal prisms, occasionally reaching 5 cm in length. Ford (1908) speculated that the two habits are related to different periods of formation. The pseudo-hexagonal prisms typically have a brilliant luster whereas the large "poker chips" are a dull black.

CONCLUSIONS

The production history of the Las Chispas mine is mixed, since it was interrupted by revolution, strikes and seizure. The total silver production of the mine probably did not exceed 20 million ounces, but it must be considered one of the "classic" localities owing to its production of fine mineral specimens. The number of specimens now in museums and private collections is the legacy of a very enlightened mine manger, Edward L. Dufourcq. * Although Dufourcq struggled with constant highgrading and ore theft, he recognized that the specimens of acanthite, stephanite, polybasite and pyrargyrite were remarkable and worthy of preservation.

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REFERENCES

BERNSTEIN, M. (1964) The Mexican Mining Industry 1890-1950. The State University of New York.

BOX, M.J. (1869) Capt. James Box's Adventures and Explorations in New and Old Mexico.

DAHLGREN, C. B. (1883) Historic Mines of Mexico: A Review. Printed for the author, New York, 220 p.

DESAUTELS, P. (1960) Occurrence of multi-form fluorite from Mexico. American Mineralogist, 45, 884-886.

DINSMORE, C. (1911) Pedrazzini Gold and Silver Co., Sonora. Mining World, 35, 1129-1131.

DUFOURCQ E. (1910) Minas Pedrazzini operations near Arizpe, Sonora. The Engineering and Mining Journal, 90, 1105-1106.

FORD, W. (1908a) Stephanite crystals from Arizpe, Sonora. American Journal of Science, 25, 244-248.

FORD, W. E. (1908b) Stephanitkrystalle von Arizpe, Sonora, Mexiko. Zeitschrift fur Kristallographie und Mineralogie, 45, 321-325.

HALL, H. T. (1987) The pearceite and polybasite series. American Mineralogist, 52, 1311-1321.

MARQUIS, A. N. & Co. (1943) Who Was Who in America: A Component. Volume of Who's Who in American History. Vol. I, Chicago, p. 344.

MONTIJO, Jr., F. (1920) The Las Chispas mine, in Sonora, Mexico. Mining and Scientific Press, 120, 58-60.

RUSSELL, B. (1908) Las Chispas mines, Sonora, Mexico. The Engineering and Mining Journal, 86. 1006-1007.

UNGEMACH, M. H. (1910) Contribution a la mineralogie du Mex-ique. Bulletin Societe Francaise de Mineralogie, 33, 394-395.

VALENCIA-MORENO, M., RUIZ, J., BARTON, M. D., PATCH-ETT, P. J., ZURCHER, L., HODKINSON, D. G., and ROLDAN-QUINTANA, J. (2001) A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the north American Precambrian basement. Geological Society of America, Bulletin, 113, 1409-1422.

WEST, R. C. (1993) Sonora: Its Geographical Personality. University of Texas Press, Austin.

WOODBRIDGE, D. E. (1911) The Carmen Consolidated Copper Property. Mining World, February 18.

He worked for various American mining companies, mostly in Central America and Mexico, serving as mine superintendant at San Miguel de Mezquital, Mexico (1894-1896); at Sierra Mojada, Mexico (1896-1898); as manager of the Andes mine in Chimbote, Peru (1901); as general superintendant of the Montezuma Lead Company in Santa Barbara, Mexico (1901-1904); and as assistant general manager of the Tezeutlan copper mine in Mexico. He was waylaid and killed in Mexico by Zapatista revolutionaries on April 15, 1919 (Marquis, 1943; Fort Worth Star-Telegram, July 17, 1919).

* Literally "The Sparkles," miners' slang for "The Crystals."

* Edward Leonce Dufourcq was born in New York on August 6, 1870, and graduated from the Columbia School of Mines in 1892.

Terry C. Wallace

5040 Hermosura

Los Alamos, New Mexico 87544
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Title Annotation:Famous Mineral Localifies:
Author:Wallace, Terry C.
Publication:The Mineralogical Record
Geographic Code:1MEX
Date:Nov 1, 2008
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