Canadian Rockhound: Mineral Collecting in Santa Eulalia, Mapimi, and Cerro del Mercado, Mexico

We have traveled to Mexico for many years and we have enjoyed the great variety of minerals found there. The minerals from Mexico are well known to collectors around the world for their unusual and intrinsic beauty. Since a detailed survey on Mexico's minerals would be rather lengthy in an online magazine, this article will report on recent happenings and briefly introduce the novice collector to three well-known and important localities, Santa Eulalia, Mapimi, and Cerro del Mercado. The first locality is in the state of Chihuahua, while the last two are situated in Durango state. Collectors who wish to read more about Mexico's minerals, mineral localities and mining history should consult the Mineralogical Record and other journals.

Santa Eulalia

Santa Eulalia, an old and more or less constant producer, is one of my favorites. The city served as the first state capital, but was eventually abandoned as the seat of government because of lack of water. Moreover, the lack of water inhibited the milling of the ore. As a result, the government, mill and smelter moved closer to the river, about 15 kilometers away.

The mines of Santa Eulalia are closed now, except for the mine at San Antonio El Grande which is in the east camp about 6 miles from the west camp and about a half hour by road. The recent finds at the San Antonio mine have turned up great creedite, some very nice arsenopyrites, nice fluorites of blue, purple and green, and of course, the ever present hemimorphite (Figures 1 and 2). Hemimorphite is always interesting and often spectacular from this area.

Many mineral labels only mention Chihuahua as the locality, but neglect to mention Santa Eulalia. Other names for the district that may appear on labels include Aquiles Serdan or Francisco Portillo.

Figure 1. Hemimorphite with mimetite and goethite on siderite (?), from Francisco Portillo. Size: 14 cm long, 5 cm wide and 3 cm high.

Figure 2. Rainbow hemimorphite. This specimen is from the old mine called Mina Vieja. Size: about 4 cm tall, by 2 x 2 cm at the base.

Mapimi

The state of Durango, located south of Chihuahua, is home to two famous localities, Mapimi and Cerro del Mercado, which I like to visit. Mapimi is the most spectacular of the two, by far.

On entering Mapimi, one sees the sign welcoming you to the town and directing you to the "Puente Colgante", or the hanging bridge. This bridge was built by the Roebling Company and saved miners an arduous journey down a canyon and back up to haul the ore to the road head. The bridge connects the mail shaft of the Ojuela mine and the "town" side of the canyon. The canyon is about 200 meters deep and the bridge spans about 400 meters. It is great to stand there and watch it sway gently in the breeze. Yes, I have walked across it.

The mines at Mapimi, over 400 years old, have produced thousands of tons of metal and countless specimens. Minerals from these mines include durangite, adamite, aurichalcite (see Figure 3), conicalcite, which are well known to collectors around the world. There are many more that could be listed here. I enjoy the tremendous variety of minerals found at Mapimi.

Figure 3. This aurichalcite was found in the depths of the Ojuela mine and is one of the prettiest minerals from this area. It is accompanied by calcite and limonite. Size is approximately 8 cm long, 4 cm wide and 3 cm thick. The limonite is rather chalky and flakes off easily.

The mines have been inactive since the 1950s and some of the lowest levels are now flooded. However, they are still worked for specimens. The mine is warm, humid and very dangerous. Steep declines drop off from the tunnel floor and surprise you if you are not careful. Ladders are often original equipment and not too inspiring of one's confidence. There are places where dark chasms are crossed by logs that one hugs tightly while crossing. It is an interesting place, eerie and full of stories. The spirits of many miners undoubtedly roam the miles of passages. Natural caverns are often encountered in the mining. The veins here, as many in Mexico, are hosted in limestone.

Near the mines, on the other side of the mountain, is a spectacular natural cavern. This cavern was going to be developed by the government of Mexico. However, like many things in Mexico, it just never got done. There is, however, a marvelous spiral staircase down from the surface, replacing the old rope system.

Cerro del Mercado

Cerro del Mercado used to sit outside of Durango by a kilometer or two. Now the city has marched right up the flanks of the hill to the very edge of the mine. Originally explored by a gentleman from Spain, of course, named Mercado, it was thought to have gold. The "Iron Had" or gossan was an indicator of some heavy mineralization and it also indicated gold in some areas. The explorers soon realized the mine only contained iron, to their disappointment.

The mine, however, is of interest to collectors, because of the presence of apatite crystals (Figure 4). The largest apatite crystal I saw from this locality was approximately 10 cm across and 15 cm high. The interiors of these crystals were very heavily fractured, but the external surfaces were not marred. The fractures come from internal stresses and from some of the high velocity shots of explosives. On our last trip we hit a pocket containing several thousand crystals. We worked for several hours and barely made a significant dent in the pocket. Among the other minerals found at this locality are martite, common opal, apatite, asbestos and chalcedony.

Apatite
Figure 4. Crystals of apatite are found nestled in a bed of chalcedony and common opal, with the whole thing resting on crystals of martite.

Scenes from Mexico

The area around Tepatate, San Luis Potosi resembles the Topaz Mountain area of Utah. It appears to be the same rock and the topaz seems to be almost identical. Don Antonio, 75 years old, is the only active miner of the topaz here (Figure 5). The light patch on the hill is one of the digs. Many of the children, like little Antonio (Figure 6), are casual collectors from the streambeds and flats. There is also tin found here. Tin mines are found a few miles from here and the children find nodules of tin ore in the streams around the area. It looks like slightly pinkish round lumps of inordinate weight for its size.

Figure 5. Don Antonio.

Child
Figure 6. Children casually collect stones from streambeds and flats.

Another place we visited was the old church of Guadalcazar (Figure 7), which was built in the late 1500s. This old town was a center of mining for almost two hundred years. The Spaniards extracted gold, silver, lead, iron pyrite and fluorite. The fluorite from the local mountains is beautiful. It comes in pale shades of blue, pink, champagne, green, reddish pink, peach and clear. It appears to be as pale ice chunks when lying damp in the bottom of a bucket. Faceted it is beautiful and looks like diamonds without the flash of fire.

Figure 7. The old church of Guadalcazar.

Figure 8. Amethyst quartz from the Mina Perigrina, Guanajuato, Guanajuato. Size: 35 cm long, 15 cm wide and 9 cm thick at the base.

This article may not be copied, distributed or reprinted in any form without the author's permission. To contact the author, please use the e-mail address provided. If you are unable to contact the author, please contact the Canadian Rockhound. Authorized reprints must acknowledge the author and the Canadian Rockhound.