Acting on rumors that geodes had been found on Rancho La Prieta in the early 1980s, Sr. Javier Villalobos Seyffert obtained a prospecting permit from the Mexican government in March of 1997 and visited the ranch to either confirm or deny the rumors. By the end of that month, Javier had located a small pit, geodes on the ground, and the makings of major deposit. In March of 2003, Jeff Smith accompanied Javier to the site and had the opportunity to begin documenting the geology, mineralogy and extent of The San Marcos geode deposit.
Since that time, other geode deposits have been discovered in Mexico, but none have rivaled the quality of Las Choyas until now. The San Marcos geodes contain quartz in the varieties of amethyst, smoky and colorless with beautiful agate rinds. The other primary mineral is calcite which occurs in a variety of interesting forms. The San Marcos geodes also host a variety of micro minerals.
During the return trip from my first excursion to Las Choyas in November 2001, (as we passed through the city of Villa Ahumada,) Hector Carrillo Jr. pointed to a distant mountain about 22 miles to the northeast. He said, “You know, smoky and amethyst quartz have been found in geodes on Rancho La Prieta, and the claimholder is a longtime friend of ours.” I filled the information away for future reference. The following November 2002, as we again passed through Villa Ahumada on the return trip from Las Choyas, I asked Hectors, Jr. about the status of this new geode deposit. He said that Gem Center USA of El Paso will work an agreement with the claimholder to distribute any mined geodes. However, production mining had yet to begin and mine run was still being determined. He said that the deposit appeared to have the potential to be a major find in quality and quantity. At that point, Hector Jr. asked me if I would like to conduct a geologic survey of the deposit and characterize the mineralogy. I of course said yes. Hector made the appropriate arrangements and in March of 2003, myself and friend Rodd Seifarth traveled to El Paso. Hector escorted to us to meet the claimholder, Javier Villalobos Seyffert, and we traveled together to the San Marcos geode deposit to begin the survey.
In the early 1980s, Sr. Socorro, a resident of Villa Ahumada, found several geodes on the ground while working on Rancho La Prieta, with no mining papers filed, Sr. Socorro hired several locals and went in search of a treasure. He went back to the location where he found his first sample and his men hand-dug a 20 foot deep pit. Once the pit was dug, a horizontal tunnel was excavated to mine the geodes. However, shortly after the horizontal tunnel was created, there was a cave-in and two miners were killed. This ended Sr. Socorro’s brief mining career and the pit was abandoned.
In 1995, Sr. Javier Villalobos Seyffert, a Ciudad Juarez resident, heard rumors that geodes had been discovered somewhere on Rancho La Prieta many years before. In early 1997, Javier applied for and was granted a prospecting permit from the Mexican government; this document gave Javier several weeks to freely access Rancho La Prieta to search for Socorro’s geodes. He found Socorro’s pit, began exploring the area, and realized that there was the potential for a significant find. Early in March of 1977, Javier escorted Sr. Lorenzo S. Doral Razcon, commissioner of the mining Office from Chihuahua City to Rancho La Prieta and the location where he found the geodes. This began the arduous process of filling for a mining claim. There were two reasons for showing the Commissioner the site, 1) to convince him that there was a potential mineral deposit, and, once convinced, 2) establish the boundaries of the proposed claim by collecting on-site coordinate data. The commissioner was convinced and used a Global Position Unit to determine the PUNTO DE PARTIDA DEFINITIVO COORDENADAS (The Certified Definitive Coordinate Point). From this point, exact the boundaries of the claim were established resulting in a surface area of 1,000,000 square or 100 Hectors. The field data collected was taken back to the mining office in Chihuahua City where it was referenced on the El Cuarenta 1:50,000 topographic map of the area. The map was then approved by the Ministry Of Trade and Industry – Secretaria de Comercio y Fomento Industrial (SECOFI) and signed by Commissioner Dozal of the Agency of Mining in Chihuahua City. It became an official document on March 19, 1997. This document contained the vital information required by the General Director of Mines in Mexico City. It was forwarded there where it was used as the basis for final approval to grant Javier his claim.
Following the site visit with Commissioner Dozal, Javier made several personal trips to Mexico City to help expedite the review process. It took from March of 1997 to April 17, 1998 to be granted the claim with a temporary status. The duration of this temporary status is six years (April 17, 1998 to April 16, 2004). By midnight of April 16, 2004, Javier must provide production records to the General Director of Mines in Mexico City to demonstrate he has viable mine. If his records demonstrate the mines viability, a permanent status will be bestowed upon the “La Winnie” claim for 100 years.
During the time between Javier obtaining his prospecting permit and the issuance of the temporary claim, Sr. Alfonso Caballero L.’s son, Luis Raul Caballero O. to secure right of way to his claim and get a key to the gate. At first Luis Raul was quite reluctant, even when Javier provided all the proper documentation that gave him the legal right of way. Finally, Javier reminded that Mexican mining laws give a claimholder an option to request an army escort to assist in gaining access to a claim if necessary. Luis conceded and gave Javier a key to the gate.
In May of 1998 (one month after receiving his temporary permit), Javier hired men from Villa Ahumada, rented a bulldozer and a front end loader and worked the mine for two months. A high wall was created that was 5 meters high and 200 meters long. This exposed a geode bearing layer of volcanic rock that was essentially at the surface. Horizontal test tunnels, 1.5 meters high, 1.5 meters wide and 2 meters deep were excavated into the high wall. During this time, approximately 10,000 pounds of mine run geodes were collected. In September of 1998, Javier began cutting and polishing his first lot of geodes. Of the original 10,000 pounds, 1,500 pounds of polished halves and 5,000 pounds of rough (unopened) geodes were sold at the 1999 Tucson Show. The site has remained idle since this initial flurry of activity.
Javier estimates that total costs to obtain the claim and do the initial excavation were approximately $20,000 US Dollars, which included his travel costs back and forth to Mexico, filling and processing fees and initial labor costs. Javier named his claim “La Winnie” in honor of his pet Chihuahua dog Winnie.
Javier and the Carrillo family of Gem Center USA have been friends for many years. After Javier obtained his claim, he plans to make a joint venture with the Carrillos. Since Gem Center USA has the infrastructure to market large quantities of geodes. Gem Center USA is now prepared to market the geodes under the trade name “San Marcos” Jeannette Carrillo named the geodes in honor of the youngest of her sons, Marcos.
The San Marcos geode deposit occurs in rocks within the Chihuahua Tectonic Belt. This belt is northwest fold and thrust fault zone that extends throughout eastern Chihuahua, Mexico. Physio-graphically, the geode deposit occurs within the Mesa Central Province (Clark and de La Fuenta, 1978). This province encompasses the eastern half of Chihuahua and is characterized by wide basins separated by isolated ranges. It roughly correlates to the Basin and Range province of the western United States.
Known Precambrian rocks: that occurs in the area beneath the San Marcos geode deposit include granite gneisses which were encountered at a depth of 4,810 meters in a Pemex well (Myotes No.1) drilled approximately 50 kilometers northwest of the geode deposit (Thompson, et. al., 1978). The Precambrian basement is un-conformably overlain by Paleozoic and Mesozoic rocks that are approximately 12,000 meters were encountered by Pemex exploratory well (Banco de Lucero) located off the western flank of Sierra banco de Lucero approximately 45 kilometers southwest of the San Marcos geode deposit. This is the closet known deep well to the site. The well was drilled to a depth of 5,000 meters and bottomed in Jurasic strata, The Jurasic strata are overlain by 2,700 meters of the Cretaceous las Vigas formation composed of red beds conformably overlain by 2,300 meters of the Cuchillo Formation composed of limestone. The Cretaceous strata are overlain by approximately 1,900 meters of Cenozoic rocks. The Tertiary formations encountered at Pemex well Banco de Lucero include 1,600 meters of volcanic covered by 300 meters of basin fill sediments (Nandigam, 2000).
The Cretaceous rocks in the area were eroded and deformed by Laramide Orogenic events prior to the outflow of Tertiary volcanics. The Tertiary rocks at San Marcos geode deposit record magmatic evolution during the subduction of the Farallon plate beneath the North American plate during Late Cretaceous to Mid-Tertiary. During this time, there was a change in the tectonic setting from northeast-directed compression during Larimide time to Basin and Range extension (Henry et. Al., 1991), McDowell and Mauger, 1994).
The San Marcos geode deposit occours approximately 2 km. off the southwestern end of Sierra de Presidio, approximately 52 kilometers northeast of Villa Ahumada. In 1983, the Mexican Government geologically mapped this area on the Nuevo Casas Grandes map 1:250,000 scale. The geologic mapping indicates that the San Marcos geode deposit is underlain by Cretaceous limestone that strikes northwest/southeast and dips to the northeast. The surface geology around San Marcos is comprised of Tertiary volcanics, Tertiary conglomerates and Quaternary alluvium. There are also large areas of the surface covered by a thin veneer of eolian sediments. The Tertiary volcanic rocks at San Marcos were deposited unconformably on the Creatceous limestone. Within a two square kilometer area around the geode deposit, the Nuevo Casas Grandes geologic map shows Tertiary rhyolite and basalt showing through the alluvium and eolian veneer.
The hammer and pick axe symbol on the Nuevo Casas Grandes geologic map indicate the location of the San Marcos geode deposit. The thickness the volcanic rock that contains the geodes at San Marcos is not yet known. The nearest subsurface data on volcanic thickness was obtained from drilling data ten kilometers to the southeast of San Marcos. In the early 1990’s, the Mariana carbonative was explored by Peñoles Mining Co. During the exploration, reverse circulatory drill holes were made. In two exploratory holes, (BCM-05 and BCM-06) the contact depth from the surface through the rhyolite tuff to the Cretaceous limestone was 180 meters and 123 meters respectively (Nandigam,2000).
The ground surface elevation at San Marcos is approximately 1,450 meters above sea level. The area is essentially flat with a slight rise in elevation from west to east. The initial excavation at the site created a high wall with an east-west orientation and exposed a zone of ash flow tuff. On top of the high wall and for several hundred meters beyond, the volcanic rock is covered with eolian sediment. When facing the cut, the rock exposed in the western end of the high wall is composed of a hard, maroon colored, densely welded ash flow tuff containing empty cavities ranging from two to ten centimeters in diameters. Moving from west to east along the high wall, the maroon tuff pinches out at the surface. At the mid-point along the high wall, there is a zone of altered tuff that is light red and has a chunky texture. Ased on the orientation of the high wall, the altered zone is beneath the densely welded maroon zone at the midpoint along the high wall. The altered zone then angles upward to the east, eventually cropping out with pick axes. This is the location where Javier had his men construct the initial test tunnels and mine geodes to determine mine run.
The ash flow tuff at San Marcos appears to be a single cooling unit. Once the unit was deposited, open spaces were preserved during the de-gassing of the lava. These open spaces, known as amygdules, are where the geode formed. Geodes were only observed within the altered portion of the tuff.
After the ash flow unit was deposited and the cavities were preserved, hot magmatic solutions probably migrated upward and altered a portion of the ash flow unit providing silica as the source for primary mineralization in the geodes. Five kilometers southeast of the geode deposit, a large tertiary granite pluton exists that also created the Mariana carbonitities. This intrusion may have been the cause and source for the upwardly migrating hydrothermal solutions that altered the tuff and mineralization in most cases is 1) chalcedony 2) mega-quartz and 3) a host of micro-minerals that also appear to have formed in several stages.
Since San Marcos geode deposit is a new find and has not been previously characterized, a systematic study of the mineralogy of the geodes was undertaken. In early April 2003, Javier shipped 187 pounds of geodes from Juarez to Pittsburgh. The 187 pounds consisted of 235 geodes approximately two and half inches in diameter. Of the 235 geodes that were split open, there were 50 that were hollow or semi-hollow which contained crystals that could be studied. The 50 specimens were then numbered and ready for analysis.
These 50 specimens were examined by hands lens to select promising candidates for micro-mineral identification. Those selected were examined by stereomicroscope and by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray detector (EDS) for qualitative elemental analysis. The SEM was operated in secondary electron mode. Complete geodes halves were studied uncoated when possible. When sample charging degraded image quality to too great a degree, samples were sputter coated with palladium. For some geodes, small specimens were removed and mounted for more detailed investigation. When identification was in question and sufficient sample was available, powder X-ray diffraction was utilized for verification of mineral species. The minerals identified to date are listed below in alphabetical order.
Calcite is the other major mineral found in the San Marcos geodes in addition to quartz. It is present in approximately one quarter of the hollow geodes and can cover up to about one quarter of the internal surface. The most common form is as clusters of small, less than 1 mm, clear colorless equant rhombohedral crystals or as clusters of scalenohedral crystals. Both forms are included with an unidentified brown mineral resulting in a brown to light tan apparent coloration. In several geodes, the rhombohedra were considerably larger, up to about 5 mm, still having brown cores.
The appeared to have two generations of calcite crystallization: The first generation appears to have crystallized approximately simultaneously with the primary quartz. The second generation of growth produced colorless, scattered, stubby scalenohedral crystals, typically less than about 1 mm. in length or, more rarely, colorless simple rhombohedral crystals, Figure 65. Even more uncommon were yellow, rhombohedral calcite crystals which appeared not to be colored by inclusions. No twinning was noted in any of the calcites.
Celestine was identified in a single San Marcos geode as bladed crystals, Figures 47, 48 and 49, associated with yellow calcite on an unidentified brown mineral. The celestine was found in the same geode as the etched gypsum.
Fluorite was observed in a single geode as very small, about 25 microns, colorless, perfect cubic crystals, Figure 50. The fluorite crystallized on another unidentified, orange-brown mineral with associated secondary quartz.
Goethite was observed in many of the geodes in two basic habits; flat, bladed crystals, often found in radiating or sheath-like groups with chisel-like terminations and as globular aggregates, Figures 51. The two morphologies were often found together in the same geode, In the sequence of mineral deposition in the San Marcos geodes, the goethite in both morphologies appears to have formed towards the end of the primary quartz crystallization and continued after primary quartz crystallization had ceased, Both morphologies were seen as inclusions below the quartz crystal surfaces by optical microscopy. Also, many cases of quartz growth interference by goethite were observed, Figure 52.
Globular goethite appears to have grown as fibrous crystals radiating from a single nucleation site. Depending on the specific growth conditions, many variations of the globular structure resulted from hemispheres so smooth that, even by SEM, no surface features could be resolved to radiating, rod-like structures. Several examples are shown in figures 53.
Gypsum was found in one geode as an etched, transparent mass about 15 mm. in largest dimension, Figure 54.
Plate-like hematite crystal aggregates were observed in several geodes.
Hollandite was observed as aggregates of needles, often showing hexagonal symmetry, Figures 55, 56 and 57. Its identification was confirmed by the detection of significant level of barium by EDS. Hollandite was identified in two geodes.
The primary quartz mineral found in San Marcos geodes is quartz. The smoky variety is most common. Amethyst and colorless occur less frequently and the amethyst often tends toward a smoky tint. Colorless quartz often appears dark due to transmission of the of underlying minerals.
Quartz also occurs as secondary mineral crystallizing over the primary quartz causing a white crust. SEM examination of this coating showed that in some cases the secondary coating is oriented with the crystal lattice of the primary quartz, Figure 58 and in some cases not. Secondary quartz is also observed (by SEM) crystallized on other secondary minerals such as goethite, Figure 59, and calcite. Many of these quartz crystals are lying on a (100) face and are doubly terminated. Secondary quartz was also observed as “stacks” of “Kerkimer diamond-like” colorless quartz crystals on primary quartz or calcite. These stacks could be up to several millimeters high.
Evidence of tertiary deposition was also observed. For example, one geode was found, Figure 60, containing calcite crystals over primary quartz, Secondary quartz had crystallized on the calcite and this secondary quartz had yet another generation of oriented quartz crystallization.
Inclusions in the primary quartz are common. Included minerals observed were goethite, ramsdellite, hematite (?) and hollandite (?), in order of occurrence. In several instances, calcite crystals could be discerned lying underneath the primary quartz which was adjacent to calcite clusters. Rarely, goethite was as seen as an inclusion in secondary quartz crystals.
Ramsdellite, Figure 61, was identified in several geodes but was not as common as goethite. It generally consisted of radiating black crystal aggregates to about mm. in length. The crystal terminations were generally more pointed than those of goethite. A coating of manganese mineral, probably ramsdellite, was also detected on several goethite blades.
Todorokite was found in several San Marcos geodes as silver-gray fiberous mats up to 4 mm. in diameter, Figure 63. Mineral fibers were also observed associated with other micro-minerals in several, Figure 64. These fibers were also probably todorokite.
The minerals found in the San Marcos geodes to date represent examples of those often found in Northern Mexican geodes (Finkleman, et., 1972, 1974) and, also, some minerals not previously reported as geode micro-minerals. The source of some of these “new” minerals, celestine and fluorite as well as the secondary calcite, is possibly the underlying limestone since these minerals are often associated with that type of geology. Late stage hydrothermal activity could have transported the constituents of these minerals into the volcanic tuff above where re-crystallization occurred within the geodes. Further study of the San Marcos geode micro-minerals as well as minerals occurring in the volcanic layer should result in a clearer picture of the geological processes that produce this interesting mineralogy. In addition, since this is the first investigation of the minerals occurring in the San Marcos geodes examined were obtained from a limited area of the deposit, continuing investigation should yield even more minerals.
The next best adventure to discovering your own geode deposit is having the opportunity to be the first one to study and characterize a new deposit discovered by somebody else. The San Marcos geode deposit has the potential to be famous. The minerals found in the geodes are is unique and many are of high quality. The site, in its embryonic stage, has much to reveal when production mining begins. Javier is waiting for approval of this application for a low interest loan from the government to obtain the necessary capital needed to proceed. This deposit may be one square kilometer or it might twenty; only time will tell. As one views the vastness of the Chihuahua desert as seen in the site photos, it is inspiring to stop and ponder the billions of geodes which may yet be discovered.
Meeting Javier Villalobos Seyffert was a pleasure; aside from currently being a mineral prospector, Over the last several years, he had also run a milk processing plant in Delicias, Chih. And owned a trucking company in Juarez, Chih. A true entrepreneur and enthusiastic Collector of Agates and Geodes It has been a very interesting to learn the process of obtaining a mining claim in Mexico in the 21st century and discover them.
I particularly wish to thank Hector Carrillo Jr. of Gem Center USA for coordinating the site visit and interview with Javier Villalobos Seyffert. A very gracious thanks goes to Javier for opening up his home and geode deposit. Thanks to John Harper at the Topographic and Geologic Survey, PADCNR, for his technical expertise and to Dr. Charles Shultz for his analysis of field specimens of volcanic rock. Thanks to my traveling partner Rodd Seifarth for his photographic and video work at the site. To Bret Howard, thanks very much for co-authoring this manuscript and for the excellent mineral identification work. For critical review of the manuscript. thanks to Sue Smith, and Judy Neelan.
