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Famous mineral localities: San Pedro Corralitos, Chihuahua, Mexico.

Yellow mimetite crystals from Namibia and China are considered to be the finest examples of the species, but attractive botryoidal specimens are so different in appearance that they tend to be judged separately. The best examples of bright yellow botryoidal mimetite from the 1968 San Pedro Corralitos find are treasured for their own unique beauty, and are considered to be the best of their type.

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LOCATION

The San Pedro Corralitos district is located in the Nuevo Casas Grandes municipio in northern Chihuahua, Mexico. In the old days it was reached by the Rio Grande, Sierra Madre & Pacific Railroad to San Pedro Station, 118 miles from Juarez, Mexico, across the Rio Grande from El Paso, Texas. Today the mines are easily reached by a well-maintained dirt road. Altitude in the mine area is about 5200 feet, and the climate is dry and semi-arid. The mines lie in a clump of low hills, the highest of which is San Pedro Peak, rising about 1000 feet above the surrounding plain.

MINE WORKINGS

Of most interest to the mineral collector are the Congreso-Leon workings, wherein the fabulous mimetite pocket of 1968 was discovered. These workings are extensive and include several impressively large stopes measuring up to 100 X 150 feet, some of which are open to the surface. The orebodies contained oxidized lead ore. Most of the known ore chimneys in the area are located in the Congreso-Leon mine, confined to an area of about 400 X 800 feet. The workings consist of 14 levels 40 feet apart and extending to a depth of about 420 meters (1365 feet). These are accessed by two main vertical shafts (the Congreso shaft to the 5th level, and the Laird shaft to below the 8th level) and an inclined shaft. Level 12 and below are flooded (as of Alvarez, 1970).

The Leon Group of mines includes the Leon, Congreso and San Celestino mines (not yet interconnected in 1907) in the southern part of the property. Orebodies are irregularly shaped and carry gold, silver and lead in the gouge (which varies from almost pure quartz to half quartz and half iron oxides). These orebodies occur in contact zones between limestone and porphyritic felsic intrusive rocks, although there is little contact metamorphism present; the zone appears to have afforded channels for ore-bearing solutions. Ores are oxidized to a depth of 335 feet below the present water table. The best and largest orebodies have been found where dikes intersect with more massive intrusive bodies. Orebodies in the Congreso mine occur over a 1000-foot extent as measured along the contact zones. The main orebodies occur in one particular bed of fossiliferous limestone along several intrusive contacts

Shortly after the turn of the 19th century, two 16-inch drill holes were sunk for the purpose of pumping water in order to lower the water table in the mines. On the average, a pumping rate of 1000 gallons per minute lowered the water table by 3 feet per month. Rogers (1907) wrote: "At a level where extensive cavities exist the water level will lower slowly, whereas where there are few cavities the water level will be lowered rapidly. The large cavities, of course, are indicative of large bodies of [oxide] ore, so that there is likely to be compensation for slow progress."

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The Porvenir mine opens on some iron oxide outcrops about 1500 feet north of the Congreso mine. A few small bodies of high-grade oxidized and sulfide lead and copper ore were found in these outcrops.

The Candelaria Group of mines includes the San Pedro, Durita, Candelaria and San Librado mines (along with a number of other small workings). They are situated in low hills to the north of San Pedro Peak, about 4 miles from San Pedro Station. The country rock is impure limestones and shales interspersed with extrusive trachyte bodies and trap dikes, accompanied by mineralized fissure veins generally less than 18 inches thick. The San Librado vein is apparently an extension of the San Pedro vein, thrown southward by a fault.

The San Pedro vein was mined extensively from the surface on down, over a 700-foot distance along the outcrop. The first level adit (782 feet long) intersects the vein at about 45[degrees]. Drifting north and south on this level for over 700 feet encountered ore. Near the point where the adit meets the orebody a shaft from the surface extends to a depth of 242 feet, from which level an inclined shaft connects to the surface.

The San Nicholas Group includes the San Nicholas, the Cobriza and the '99 mine, all situated west of San Pedro Peak, probably on the same vein. The geology is similar to that of the other veins; in places the "pay streak" of the vein was 1 to 2 feet thick.

The San Benigno Group includes the San Benigno, La Cortada, Los Luises and La Llanura mines, all situated a short distance north of the San Nicholas mine. Only the San Benigno produced much ore before 1907, consisting of sulfides with a little gold.

HISTORY

Mining History

The first "modern" ore production at the Congreso mine took place in 1885 when the mine was acquired by E. D. Morgan and Associates of New York, who formed the Candelaria Mining Company, although there were certainly earlier, more primitive operations on the site. The Candelaria end of the property was worked first, and it was not until after 1895 that serious attention was paid to the Congreso-Leon end, except for the "Old Leon" workings which date to the earliest days of the camp. In 1907 the Candelaria Mining Company controlled a total of 266 claims of 1 hectare each, surrounded by a blanket location covering a total area of 1157 hectares (2858 acres) within the San Pedro Mining District.

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After 1900 work was carried out at the Congreso mine and also at the Old Leon mine. As the ore zone was followed south, the New Congreso (or Laird) shaft was sunk and became the principal working shaft. By 1909 the Candelaria Mining. Company was shipping 90 carloads a month of carbonate ore from the Congreso mine (mainly from the Bonanza and 49 chimneys). By 1910 seven orebodies had been discovered, all of them ore-bearing down to the water table, and giving evidence of continuing ore mineralization farther downward. These included the Bonanza, Morgan, Dwight, Reeves, Pilota, Avalos, Espinoza, Garcia and Garcia Extension bodies, some of which joined at depth to form one chimney.

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Work in the area generally ceased by 1913, except for very minor surface work by small lessees and the occasional prospecting work by various mining companies. Because of the inflow of ground water, mining was never able to proceed below the 400-foot level, despite various pumping operations aimed at ameliorating the water problem. At the time of the shut-down two large steam pumps were about to be installed, but they were instead abandoned on the site and never used.

In 1926 a lease contract was negotiated with the Penoles Mining Company, under which there would be a 50-50 division of profits. Mining operations resumed in 1927 under the supervision of Penoles. A small orebody was opened first but the grade proved too low to profitably ship. Another orebody (the Dwight) was discovered, extending from the 4th to the 6th level, and it proved to be of higher grade. Exploratory drilling revealed other orebodies (named the Bonanza, Espinoza, and Morgan orebodies). Nevertheless, the company was unable to develop a profitable orebody before the option deadline, and all mining was again shut down in April 1928. Minor amounts of ore were shipped in 1936-1944 by the Hemly and Blackwell mining companies.

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Further exploration was evaluated by Wisser (1946) for the Eagle Pitcher Mining Company, headquartered in Joplin, Missouri. The Bonanza chimney was thought to persist to at least 30 meters below the 8th level. Eleven other chimneys discovered by that date had already been mined to or near the water table, but were never explored at depth, and another 10 chimneys had produced appreciable ore. Geologic mapping indicated that the chances were good for the extension of the ore chimneys for several hundred meters below the lowest workings. Apparently explorations continued through 1949, drilling from lower levels that had been dewatered with great difficulty and heavy pumping.

Eagle Pitcher tried to operate the mine in the early 1950's but had great difficulty with water control, and the irregularity of the orebodies rendered exploratory drilling inherently inconclusive (Alvarez, 1970). McCarthy (1953) reported that at that time the lowest working level was the 850-foot level, and that a shaft had been sunk from that level to 1050 feet. Exploratory drilling had cut the Bonanza orebody at a depth of 1400 feet below the collar and, remarkably, the ore was still found to be oxidized at that great depth. The mine had shown a profit of 349,000 pesos for 1952, but was reduced to break-even and was predicted to soon show a loss. Operations were suspended in 1953 (Villasenor, 1963).

In 1961 the Candelaria Mining Company merged with the San Luis Mining Company under the name of Minas San Luis S.A., which became Luismin in 1962. In 1991 the property was still held by Minas Luismin S.A. de C.V., which had become Mexico's thirdlargest mining company. Nine claims covered the old productive part of the district, but most were under option to Minas Luismin (Megaw, 1991). Wheaton River Minerals, Ltd. acquired Minas Luismin in 2002.

During its productive years the San Pedro Corralitos district yielded a total of about 1 million tons of oxide ore grading 7% lead, 7% zinc, 1.5% copper, 219 grams of silver per ton, and from 1 to 6 grams of gold per ton.

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The 1968 Mimetite Discovery

In 1968 mineral dealer Benny Fenn decided to do some exploratory work in the deep underground workings of the Congreso-Leon mine, workings that had lain abandoned and had been slowly deteriorating since the 1950's or earlier. The deepest workings there reach 1,400 feet but the lower 200 feet of the mine were flooded at the time. Furthermore the timbering in the main inclined shaft had been burned out at some time in the distant past, a hindrance that would have stopped most people.

Benny, however, lowered himself 900 feet down the incline on ropes. He noticed much evidence of mimetite mineralization showing all along the incline between the 8th and 9th levels. Leaving the incline on the 9th level, he ultimately found a showing of some mediocre mimetite in a wall of the old San Pedro mine section (originally an independent mine which was later connected with and absorbed into the larger Congreso-Leon mine). He began digging into the wall, following a narrow vein; the mimetite got better and better until the vein suddenly opened up into a huge room about 40 feet long, 20 feet wide and 30 feet tall between the 8th and 9th levels, completely lined with the most beautiful botryoidal yellow mimetite anyone had ever seen!

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Being a skilled professional at mineral collecting, Benny was able to extract over two tons of fine specimens with very little or no damage. He began wrapping and packing them carefully, then had to find a way to get them back up to the surface. Ultimately he had a hoist built over a nearby vertical shaft (probably the Laird shaft) to raise specimens so that he could continue to carefully clean out the room for the next three months until he had completely stripped the walls and had collected all that was to be found.

Naturally the mineral world was excited to see such a large number of exquisite specimens of beautiful mimetite, and the prices were such that even collectors of limited means could obtain at least a small example. It was several years before mineral dealers and rock shops across the country had sold most of the find, and during that time window (1969 to ca. 1975) it was possible to obtain superb specimens in any size at very reasonable prices. The quantity was such that even today fine specimens occasionally appear on the market; at the 2004 Denver Show several dealers (including The Arkenstone and Kristalle) had a total of perhaps three flats of fine old specimens available.

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GEOLOGY

The country rock in the district consists of Cretaceous thinbedded limestones and shales that have been injected with felsic eruptive rocks, the largest mass of which is the granodiorite laccolith forming San Pedro Peak near the center of the property. Later more mafic diorites and hornblende andesite porphyries intersect the sedimentary rocks and the earlier eruptive rocks. These intrusions were probably contemporaneous with fissuring which facilitated the emplacement of ore veins north and west of San Pedro Peak. The contact zones between eruptive rocks and limestones in the southern part of the district include some important carbonate-replacement lead-copper-zinc-gold-silver orebodies, especially those of the Leon Group.

Mineralization occurs in two general zones: one at San Pedro Corralitos proper on the southeast side of San Pedro Peak (principally the Congreso-Leon mine), and the other to the northwest of the peak at Candelaria.

Large vertical chimneys containing oxidized sulfide ores occur along the hangingwall contacts of major northwest-trending dikes, and feed horizontal mantos. The mines lie along the axis of a plunging southeast-trending anticline. Andesite porphyry bodies intruded along southeast-trending vertical fractures oriented parallel to the vertical axis of this anticline. Cross-fractures also developed and were intruded by andesite porphyries and later lamprophyres. Mineralization took place along the hanging wall of dikes located within flexures. Extensive recrystallization and marbleization took place around the orebodies, and dikes near orebodies have undergone chalky sericitic alteration. Fluorite dikes occur in southwest-trending fissures but their relationship to ore mineralization is not clear.

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Mineralization is zoned concentrically about the San Pedro Peak laccolith. The mineralized zone is high in copper near the intrusion and becomes more lead-zinc rich with increasing distance from the peak (Megaw, 1991). Skarns about the intrusion consist of finegrained garnet and diopside with pyrite, chalcopyrite and arsenopyrite. Dikes found within 30 feet of a mineralized orebody are highly fractured and sometimes coated by iron oxide.

A small, unnamed prospect about 2 miles north of the Congreso-Leon mine produced a pocket of superb, gemmy-yellow wulfenite crystals in the early 1970's; it is sometimes labeled as the San Pedro mine, though this is probably incorrect.

MINERALS

The Congreso-Leon mine and others in the district were mined out at a time when there was little awareness of the value of mineral specimens. Considering the large empty stopes, the number of orebodies, the depth of oxidation of the Pb-Zn-Cu-Ag sulfides, and the normal mineralogical results of weathering such ore veins in an arid climate, we may well suspect that a vast treasure trove of collectible secondary minerals was crushed and smelted during the years of operation. What is left to us now is the result of the explorations of one man, Benny Fenn, without whom almost nothing of the exciting specimen mineralogy of this remarkable occurrence would be known. Appreciative as we are, we must understand that what has been salvaged is no doubt only a negligibly tiny fraction of what originally existed there. Consequently the descriptions of the collectible species given below are rather brief.

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Mimetite P[b.sub.5](As[O.sub.4])[.sub.3]Cl

Mimetite from the 1968 Congreso-Leon find (8th-9th level) is generally a bright lemon-yellow in the best specimens, grading only slightly to a duller, grayer yellow in some specimens and a greenish yellow in others. A few rare examples are a bright redorange, but never in large coverage on the matrix. The habit is almost universally botryoidal, in attractive specimens up to about 10 inches. The luster, at its best, is a bright, almost sparkling sheen reminiscent of the appearance of the well-known Kelly-mine smithsonite. Inferior specimens have a flatter, less lustrous appearance and a grayer yellow color. The high luster (composed of microscopic crystal points covering the botryoidal surface) is highly prone to showing bruises, nicks, scratches and rubs, so much so that truly pristine examples are especially highly prized.

Elsewhere in the mine, where mimetite occurs with wulfenite, it forms crusts of gemmy, pale yellow microcrystals and botryoidal blebs of orange to red-orange color.

Wulfenite PbMo[O.sub.4]

Wulfenite is comparatively rare at the mine, and has been collected mainly from the 5th level. Crystals are generally small, usually under a quarter inch, but Benny Fenn recovered a very thin crystal measuring almost 1.6 inches, on matrix (illustrated in Panczner's Minerals of Mexico but since broken). Smaller crystals may be thicker and highly lustrous and transparent. The color is pale to bright lemon-yellow (commonly) to yellow-orange (rarely), and the habit thin tabular. All crystals show edges beveled by the tetragonal pyramid, and some also show second-order pyramid faces which, when approaching equal dominance, result in octagonal tabular plates. Crystals that are more than paper thin are extremely rare and highly valued.

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Other Minerals

Benny Fenn reports finding attractive blue barite in transparent crystals to 1 inch on the 4th level, and also blue botryoidal smithsonite and arborescent cave growths of calcite and aragonite elsewhere in the mine. A diagram of the mine workings prepared by Wisser (1944) shows a large cavernous area on the second level, atop two ore pipes, which is labeled "calcite cave" and outlined by a spiky delineation suggesting that the cavern is lined with calcite crystals. Unfortunately no photos or further descriptions survive, but the cavern is probably still there, undisturbed, after all these years, and is well above the water table.

CONCLUSIONS

The San Pedro Corralitos mines probably hold great potential for specimen production in the future, inasmuch as they are entirely in the oxide zone and have, for the most part, not been explored extensively by mineral collectors. The orebodies above the water table have been mostly stripped out, along with most of their treasures in crystallized minerals, and yet Benny Fenn, in one visit, was able to locate a substantial side-cavern that lacked what the miners would have called ore but was nevertheless rich in specimens. A vast area of underground workings above the water table remains to be explored and prospected for mineral specimens.

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In addition, many of the known orebodies extend for hundreds of feet below the water table, where they are still intact and unmined; exploratory drilling indicates that the oxide zone (i.e. the zone containing well-crystallized secondary minerals) also continues for hundreds of feet below the water table, throughout the unmined portions of the orebodies. This represents an enormous potential reserve of specimens that perhaps someday may be mined, if and when technological advances sufficiently lower the cost of water pumping. The same may be said for many famous oxide mines in the Desert Southwest and adjacent Mexico, from the Rowley mine in Arizona to the Los Lamentos mine in Chihuahua.
Table 1. Vein minerals found on the dumps of the Congreso-Leon and
Gachupin mines.

Vein Minerals

Adamite        Goethite
Arsenopyrite   Gold
Austinite      Hemimorphite
Barite         Malachite
Beudantite     Mimetite
Calcite        Smithsonite
Cerussite      Sphalerite
Chalcopyrite   Plumbojarosite
Chlorargyrite  Pyrite
Chrysocolla    Rosasite
Conichalcite   Tetrahedrite
Fluorite       Willemite
Galena         Wulfenite


ACKNOWLEDGMENTS

My special thanks to Peter Megaw for assistance with unpublished company reports in his files, for location photography and for reviewing the manuscript; and to Benny Fenn for reviewing the manuscript and sharing his experiences.

BIBLIOGRAPHY

ALVAREZ, A. (1970) Leon y Congreso Prospecto. Unpublished reconnaissance report for Contratistas Tormex, S.A.

CASTANEDO, J. (1927) El distrito minero de corralitos, del estado de Chihuahua. Boletin Minero, September, p. 162-167.

DINSMORE, C. A. (1909) The Candelaria Mining District of Mexico. Mining World, July 3, 1909, p. 50.

HOWBERT, V. D. (1925) Property of the Candelaria Mining Company, San Pedro, Chihuahua. Unpublished internal report, Compania Minera de Penoles, S.A., 15 p.

HOWBERT, V. D. (1928) Summary of Penoles operations at San Pedro Unit. Unpublished internal report, Compania Minera de Penoles, S.A., 23 p.

KREIGER, P. (1935) Primary silver mineralization at Sabinal, Chihuahua. Economic Geology, 30, 242-259.

McCARTHY, J. C. (1953) Memorandum on San Pedro mine, Corralitos, Chihuahua. Unpublished internal report, Compania Minera de Penoles, S.A., 2 p.

MEGAW, P. (1991) Field report on San Pedro Corralitos, Chihuaua. Unpublished report for Joe Ruetz by International Mineral Development and Exploration, dated October 8, 1991, 11 p.

MEGAW, P. (1992) San Pedro Corralitos Preliminary Evaluation. Unpublished report for Joe Ruetz by International Mineral Development and Exploration, dated December 21, 1992, 11 p.

PANCZNER, W. D. (1987) Minerals of Mexico. Van Nostrand-Reinhold, New York, p. 274, 396.

ROGERS, A. H. (1907) Report on the property of the Candelaria Mining Company. Unpublished report, 23 p.

VILLSENOR, L. (1951) Problema de bombeo de las minas Leon y Congreso de San Pedro Corralitos, Chihuahua, Mexico, en operacion por Minas de Durango, S.A., compania subsidiaria de Eagle Pitcher Mining and Smelting Company. Memorias de la Primera Convencion Interamericana de Recursos Minerales. Mexico, D.F., p. 278-287.

VILLASENOR, L. (1963) Letter to Sr. P. Sanchez Mejorada, Metalurgica Mexicana Penoles, dated May 8, 1963.

WILSON, W. E., and BARTSCH, J. (2004) Mineral Masterpieces of the World. Abrams Books, New York, p. 220.

WISSER, E. (1944) Report on Leon-Congreso lead-gold mine, San Pedro, Chihuahua (August 28, 1944). Unpublished report, 28 p.

WISSER, E. (1945) Interim report Leon-Congreso mine. San Pedro, Chihuahua (July 30, 1945). Unpublished report, 41 p.

WISSER, E. (1946) Exploration program proposed for the Congreso mine. San Pedro Corralitos, Chihuahua. Unpublished report, 52 p.

WISSER, E. (1948) Congreso mine, San Pedro Corralitos, Chihuahua--results of past exploration; present prospects (December 20, 1948). Unpublished report, 32 p.

Wendell E. Wilson

The Mineralogical Record

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Date:Nov 1, 2004
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