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Abstract

Silver and gold mineralization was discovered in the late 1600s in the epithermal sulfide-rich veins of Mineral de Pozos Mining District, northeast Guanajuato state, Mexico. The main exploitation period of this mining district was between 1888 and 1922, with sporadic activities until 1942. Exploitation of the deposit is estimated at 1,200,000 tons of ore with average of 202 g/ton silver, and 11.83 g/ton Au. Mineral waste materials (more than 1 million tons) are scattered along the area on the main creeks and in the ancient processing plants (Haciendas de Beneficio). In this field guide, we present brief descriptions of the mineralization, the geology of the area, some of the ancient processing plants, and the potential dispersion of metals derived from the mine tailings into the environment (soil, sediments, and groundwater). Despite the relatively high concentrations of As and Pb in groundwater (0.011–0.090 mg/l As and 0.025–0.035 mg/l Pb), we consider that these values represent natural background values rather than contamination derived from anthropogenic input. However, we consider that Zn is the only metal potentially derived from mining activities being released to the environment. The historical processing plants offer a very interesting perspective of the more than a century old mining activities.

General Geology and Mineralization of the Area

The purpose of this field guide is to present a brief description of the general geology, mineralization, and environmental aspects of the historical mining district of Mineral de Pozos, east-central Mexico. Its focus is on the mine waste material of the former mining sites 5-Señores, El Triangulo, and Santa Brigida. We will analyze and discuss the potential for background values (natural values) of heavy metals in the natural environment versus potential anthropogenic input (metals derived from man's activities).

The historical mining district of Mineral de Pozos is located 280 km northwest of Mexico City (Fig. 1), some 80 km north of city of Querétaro, on a flat area in a low mountainous zone on the east margin of Laguna Seca (Independence Aquifer or La Laja River sub-basin), an agricultural district where groundwater is the most important source of water (Mahlknecht, 2004).

Figure 1.

General location of the Mineral de Pozos Mining District, and some of the localities of mine tailings. Large arrows indicate general groundwater flow. (After Carrillo-Chávez et al, 2006.)

Figure 1.

General location of the Mineral de Pozos Mining District, and some of the localities of mine tailings. Large arrows indicate general groundwater flow. (After Carrillo-Chávez et al, 2006.)

The Independence Aquifer is an alluvial aquifer that receives its recharge from the mountainous region (along its margins) due to higher precipitation on higher elevations (average precipitation on the zone is less than 400 mm/yr of rain). The mountains surrounding Mineral de Pozos are dominated by Upper Jurassic and Lower Cretaceous sedimentary rocks (which include pure limestone, calcareous shale, shale, sandstone, and argillaceous limestone). These rocks are partially covered by Tertiary rhyolitic and basaltic volcanic rocks (Fig. 2). The Jurassic rocks include calcareous shale and sandstone. Most of the epithermal veins are hosted by these Jurassic strata. This sequence is intensely deformed with highly complex folding. The Lower Cretaceous rocks in the area are formed by pure limestone and impure limestone (clayey limestone). The pure limestone has been assigned to the Doctor Formation (Albian), which is a characteristic Lower Cretaceous formation in all central-east Mexico. This El Doctor Formation is typically formed by a sequence of fossiliferous, thick-bedded limestone. The basinal equivalent includes the Tamaulipas Inferior formation (well-stratified limestone with sporadic dark-chert thins horizons). No mineralization has been found hosted in the Cretaceous rocks. The Tertiary volcanic rocks can be divided into two general sequences: (1) rhyolites and dacites; and (2) andesites and olivine basalts. The rhyolite and dacite outcrops are scattered in the area mostly to the northeast. The andesite crops out mostly to the southwest of the area. Locally, the andesite presents porphyry texture with feldspar crystals. Finally, the basaltic rock crops out to the north and northeast of the area. This rock is mostly olivine basalt with some void-filling zeolite. Most of the area is covered by Quaternary alluvial deposits composed of gravel, coarse and medium-sized sand, and some clay horizons. The alluvial deposits eventually form the material through which water infiltrates, recharging the alluvial aquifer, the source of the groundwater for agricultural, industrial, and municipal use in the region.

Figure 2.

General geologic map of the Mineral de Pozos Mining District. Numbered stars indicate the field trip stops.

Figure 2.

General geologic map of the Mineral de Pozos Mining District. Numbered stars indicate the field trip stops.

The mineralization at Mineral de Pozos occurs as vein systems, with general strike of N40°–60°W and near vertical dip. The veins have been grouped in three subsystems: (1) Pozos argentiferous mineralization; (2) Santa Brigida base metals and mercury; and (3) Carmen southeast system of argentiferous mineralization. The general mineralogy of the epithermal veins is quartz and calcite with a vertical zonation consisting of Cu-Pb-Zn sulfides at the base (chalcopyrite, galena, and sphalerite; not yet exploited), and an upper zone of (Au-Ag with some Hg; exploitation zone) (Mapes et al., 1972). The main exploitation period of this mining district was between 1888 and 1922, with sporadic activities until 1942. Exploitation of the deposit is estimated at 1,200,000 tons of ore with an average of 202 g/ton silver, and 11.83 g/ton Au. Mineral waste materials (more than 1 million tons) are scattered along the area on the main creeks and in the ancient processing plants (Haciendas de Beneficio). Table 1 shows a chronological outline of the exploitation stages of the Mineral de Pozos Mining District. Most of the exploitation has been focused on silver and gold. Eventually, some operations were directed to the exploitation of tin, mercury, and barite.

Table 1.

The Most Important Stages in the History and Exploitation of the Mineral De Pozos Mining District

PeriodCharacteristics
1600s–1767Rudimentary surface works by Jesuits Missionaries on Santa Brigida veins (gold and silver).
1844First survey and exploitation for mercury.
1871Señores Torres and Cobos started the first legal exploitation in Santa Brigida.
1888–1891Main organized exploitation by Cinco Señores Corporation.
1892–1897Bonanza stage using the “patio” method.
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated a cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1902–1913Exploitation mainly by Cinco Señores and Angustias-Dolores Corporations.
1913–1922Cease of activities due to civil war.
1922–1927The Angustias-Dolores Corporation resumes operations. By 1927, the ore reserves decline.
1934–1937Lucero Corporation reworks mostly low-grade piles and some high-grade ore left by previous operations.
1937–1942Sporadic efforts to resume operations, but all of them fail.
PeriodCharacteristics
1600s–1767Rudimentary surface works by Jesuits Missionaries on Santa Brigida veins (gold and silver).
1844First survey and exploitation for mercury.
1871Señores Torres and Cobos started the first legal exploitation in Santa Brigida.
1888–1891Main organized exploitation by Cinco Señores Corporation.
1892–1897Bonanza stage using the “patio” method.
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated a cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1902–1913Exploitation mainly by Cinco Señores and Angustias-Dolores Corporations.
1913–1922Cease of activities due to civil war.
1922–1927The Angustias-Dolores Corporation resumes operations. By 1927, the ore reserves decline.
1934–1937Lucero Corporation reworks mostly low-grade piles and some high-grade ore left by previous operations.
1937–1942Sporadic efforts to resume operations, but all of them fail.

General Environmental Aspects on Mine Deposits

Since early in the history of mankind, exploitation of ore deposits has played a key role in the development of any society. However, the alteration of natural metal geochemistry cycles and the release of heavy metals into the environment have brought a negative consequence of the exploitation of ore deposits (Gray et al, 1994; Gray, 1997). Historically, when mining operations were no longer economically advantageous, the mine sites were abandoned along with all the mine waste material (mine tailings, low-grade piles, and sterile rock) without any consideration for the environment (surface water, groundwater, and soil; Plumlee, 1999; King, 1995). In Mexico, the number of abandoned mine-waste deposit sites is unknown (Carrillo-Chávez et al, 2003). It is very important to focus efforts on researching the environmental aspects of mining sites (historical, inactive, and in exploitation). Our survival depends both on the exploitation of mineral resources and the care and protection of our precious natural environment.

Mineral de Pozos Mining District is a good example of the hundreds to thousands of abandoned mining sites in all the territory of Mexico (and worldwide), where mining operations yield millions of tons of mine waste material with relatively high concentrations of potentially toxic heavy metals and metalloids. With this field guide (and the corresponding field trip) we hope to awaken the environmental conscience of young and old geo-scientists and people working in the government at all levels in order to increase the study of the environmental geochemistry of mining sites and the corresponding actions that reduce the potential of toxic metals entering the environment (water and soil).

Stop 1. HACIENDA 5 SEÑORES AND GENERAL OVERVIEW OF THE MINING DISTRICT

From Juriquilla, Querétaro, we will take Mexico Highway 57 northbound. At the 81 km mark, we will leave the pavement to take a dirt road eastbound toward Mineral de Pozos. After 7 km, we will stop at the 5 Señores historical mineral processing plant (21°13′39.73″ N; 100°31′0.82″ W). We will see the shaft of more than 80 m deep (pozo or tiro) where the mineral was extracted from the underground operations. We also will see the ruins of the mineral extraction facilities. Some of these facilities still have mine tailings. One of the cyanide “heap leaching pools(?)” (Fig. 3), still half filled with mine tailings, has the following concentrations: Pb = 100 mg/kg; Cd = 1.5 mg/kg; As = 110 mg/kg; Cu = 100 mg/kg. Carrillo and collaborators (2006) ran leaching column experiments during 25 weeks on this material, with the following results for some heavy metals and As: Pb = 0.080 mg/l; Cd = 0.06 μg/l; As = 0.02 μg/l; and Cu = 0.02 μg/l (all averages). To the west of the ruins, it is possible to see some of the scattered mine tailings. To the west and across the road it is also possible to see some of the low-grade material piles and another small processing plant (Hacienda de Beneficio). From here, and to the east, the Mineral de Pozos Mining District extends for several kilometers. The town of Mineral the Pozos is just 2 km to the west.

Figure 3.

A pool (cyanide heap leaching?) still half filled with mine waste material (mine tailings). See text for concentrations of some heavy metals and arsenic in this material.

Figure 3.

A pool (cyanide heap leaching?) still half filled with mine waste material (mine tailings). See text for concentrations of some heavy metals and arsenic in this material.

Stop 2. Hacienda El Triangulo And Creek With Jurassic Exposures

Just 700 m east of Hacienda 5 Señores, we will stop to see the Hacienda El Triangulo (21°13′21.59″ N; 100°30′44.12″ W). This ancient processing plant is one of the best preserved in the mining district. We will see the “old shaft” (pozo), the “patio,” the crushing facilities, and the leaching pools and mine tailings dump site. We will take a short hike along the creek where the tailings were dumped to see part of the Jurassic sequence that hosts the epithermal veins. The average heavy metal and As content of the sedimentary rocks is as follows: (1) sandstone Pb = 8 mg/kg, Cd = 0.01 mg/kg, As = 40 mg/kg, and Zn = 30 mg/kg; and (2) shale Pb = 80 mg/kg, Cd = 0.06 mg/kg, As = 8 mg/kg, and Zn = 12 mg/kg. The scattered tailings along the creek have a similar concentration of Pb, Cd, As, and Zn to the tailings in the leaching pool at the 5 Señores location.

Stop 3. Ancient Mines School of Mineral De Pozos

Just on the edge of town is an interesting construction that once hosted the mines school of Mineral de Pozos (21°13′27.06″ N, 100°30′1.89″ W). By the second half of the 1800s, new discoveries and increased production brought crowds to the locality. With good economic prospects, explorers and investors came from France, Spain, Italy, England, and the United States. Likewise, workers arrived from other important mining centers in Mexico such as Guanajuato, Zacatecas, Hidalgo, Guerrero, and San Luis Potosí. With this bonanza of taxes and contributions, the local government made great progress in education and social benefits. Mineral de Pozos occupied fourth place among the 46 municipalities of the state of Guanajuato (Mineral de Pozos is no longer a municipality). One example of this progress is the Model Mining School (Fig. 4), considered at the time one of the best of Mexico. During the government of Porfirio Diaz (1867–1910), the population reached its maximum of 80,000 inhabitants. During the early 1920s, Mineral de Pozos began its decline from which it never recovered. The Cristeros war of Mexico during 1926, along with the Great Depression and the plunge of metal prices in international markets, contributed to placing the last nail in the coffin of Pozos. The massive exodus moved most of the population to other economic centers. Most of the mineral processing facilities were dismantled and sold to other mining districts. It is estimated that by the 1950s, fewer than 200 people lived in the place. In 1980, the federal government declared Mineral de Pozos a “National Historical Monument.” Today, artists, intellectuals, newcomers, local population, and lovers of this place have the goal to rescue this singular and historical place.

Figure 4.

Part of the “Model Mining School,” once one of the best mining and geology schools in Mexico.

Figure 4.

Part of the “Model Mining School,” once one of the best mining and geology schools in Mexico.

Stop 4. Hacienda Santa Brigida and the Tres Hornos Mineral Processing Plant

Santa Brigida mineral processing facilities lies some 3 km northeast of town (21°13′50.46″ N, 100°27′48.10″ W). In this locality there is the Tres Hornos ruins, the first mineral smelter in the area constructed by the “Jesuits” by the late 1600s (Fig. 5). Just 200 m southwest of Tres Hornos is a very interesting mineral processing facility. This processing plant consists of a long, narrow building (150 × 7 m) with alternating inner walls that are half the width of the building. Very likely, this construction functioned as an “oxidation chamber” during the burning of the ore (oxidation of the sulfide minerals is needed in order to release the metals from the sulfide-bond) before the leaching process and gold and silver extraction. Next to this building there are still some scattered remains of the smelting process (scoria) with very high concentrations of metals (40 g/kg of Pb; 5 mg/kg of Cd; 50 mg/kg of As; and 1 g/kg of Zn). All around this mineral processing plant, fine-grain mine waste material (mine tailings) is dispersed and exposed to weathering. The tailings have the following heavy metal and As content: 100 mg/kg of Pb; 5 mg/kg Cd; 100 mg/kg of As; and 80 mg/kg Zn. Shallow groundwater, not far from this locality, has the following heavy metal and As content: 0.01 mg/l Pb; >0.01 mg/l of Cd; from 0.01 to 0.1 mg/l As; and 0.13 mg/l Zn. Figure 6 shows some heavy metal and As content in different material, from sedimentary rocks and sediments to mine tailings. The last bar to the left indicates the average concentration of the element in the earth crust.

Figure 5.

Los Tres Hornos ruins, one of the oldest mineral processing facilities in America (mid-1600s).

Figure 5.

Los Tres Hornos ruins, one of the oldest mineral processing facilities in America (mid-1600s).

Figure 6.

Heavy metal and Arsenic content in rocks, sediments, and mine waste material in the Mineral de Pozos area. Shale and sandstone host the mineralization (Stop 2), scoria is a byproduct of old smelting operations (Stop 4), creek sediments are from west of town (Stop 2), tailings 1 are from the 5 Señores plant (Stop 1), and tailings 2 are from Santa Brigida Locality (Stop 4). Average of crust is from Cox (1995). (After Carrillo-Chávez et al., 2006.)

Figure 6.

Heavy metal and Arsenic content in rocks, sediments, and mine waste material in the Mineral de Pozos area. Shale and sandstone host the mineralization (Stop 2), scoria is a byproduct of old smelting operations (Stop 4), creek sediments are from west of town (Stop 2), tailings 1 are from the 5 Señores plant (Stop 1), and tailings 2 are from Santa Brigida Locality (Stop 4). Average of crust is from Cox (1995). (After Carrillo-Chávez et al., 2006.)

Conclusions

The old mining town of Mineral de Pozos not only teaches us a nice history lesson, but also gives an interesting message on the need for detailed environmental geochemistry evaluations of historical, inactive, and modern mining sites. Mine waste material contains high concentrations of metals and metalloids, and several geochemical factors (dissolution rates, adsorption processes, soil-water interactions) control whether these leach into the environment. Once in the groundwater system, the metals and metalloids also are subject to dilution and further absorption and rock-water interactions, thus decreasing their concentrations in groundwater. In rare cases, the relatively high metal and metalloid concentrations in soil and groundwater in mining areas are due to natural processes (natural weathering of the ore deposit and alteration aureoles). But, the extraction and dispersion of mine waste material enhance the distribution and concentrations of metal and metalloids in the environment. While touring places like Mineral the Pozos, we can look back in history into its glorious “bonanza” past, but we also should look into the future and balance the exploitation of ore deposits with environmental geochemistry evaluations for a clean environment for present and future generations.

References Cited

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Tritlla
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Camprubi
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Cox
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Coolbaugh
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Plumlee
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Atkinson
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Acknowledgments

This work was supported by grant no. UNAM-PAPIIT “IN-112311.” The authors wish to thank the enthusiastic assistance of Carolina Muñoz-Torres and Fabian González-Quijas in laboratory work.

Figures & Tables

Figure 1.

General location of the Mineral de Pozos Mining District, and some of the localities of mine tailings. Large arrows indicate general groundwater flow. (After Carrillo-Chávez et al, 2006.)

Figure 1.

General location of the Mineral de Pozos Mining District, and some of the localities of mine tailings. Large arrows indicate general groundwater flow. (After Carrillo-Chávez et al, 2006.)

Figure 2.

General geologic map of the Mineral de Pozos Mining District. Numbered stars indicate the field trip stops.

Figure 2.

General geologic map of the Mineral de Pozos Mining District. Numbered stars indicate the field trip stops.

Figure 3.

A pool (cyanide heap leaching?) still half filled with mine waste material (mine tailings). See text for concentrations of some heavy metals and arsenic in this material.

Figure 3.

A pool (cyanide heap leaching?) still half filled with mine waste material (mine tailings). See text for concentrations of some heavy metals and arsenic in this material.

Figure 4.

Part of the “Model Mining School,” once one of the best mining and geology schools in Mexico.

Figure 4.

Part of the “Model Mining School,” once one of the best mining and geology schools in Mexico.

Figure 5.

Los Tres Hornos ruins, one of the oldest mineral processing facilities in America (mid-1600s).

Figure 5.

Los Tres Hornos ruins, one of the oldest mineral processing facilities in America (mid-1600s).

Figure 6.

Heavy metal and Arsenic content in rocks, sediments, and mine waste material in the Mineral de Pozos area. Shale and sandstone host the mineralization (Stop 2), scoria is a byproduct of old smelting operations (Stop 4), creek sediments are from west of town (Stop 2), tailings 1 are from the 5 Señores plant (Stop 1), and tailings 2 are from Santa Brigida Locality (Stop 4). Average of crust is from Cox (1995). (After Carrillo-Chávez et al., 2006.)

Figure 6.

Heavy metal and Arsenic content in rocks, sediments, and mine waste material in the Mineral de Pozos area. Shale and sandstone host the mineralization (Stop 2), scoria is a byproduct of old smelting operations (Stop 4), creek sediments are from west of town (Stop 2), tailings 1 are from the 5 Señores plant (Stop 1), and tailings 2 are from Santa Brigida Locality (Stop 4). Average of crust is from Cox (1995). (After Carrillo-Chávez et al., 2006.)

Table 1.

The Most Important Stages in the History and Exploitation of the Mineral De Pozos Mining District

PeriodCharacteristics
1600s–1767Rudimentary surface works by Jesuits Missionaries on Santa Brigida veins (gold and silver).
1844First survey and exploitation for mercury.
1871Señores Torres and Cobos started the first legal exploitation in Santa Brigida.
1888–1891Main organized exploitation by Cinco Señores Corporation.
1892–1897Bonanza stage using the “patio” method.
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated a cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1902–1913Exploitation mainly by Cinco Señores and Angustias-Dolores Corporations.
1913–1922Cease of activities due to civil war.
1922–1927The Angustias-Dolores Corporation resumes operations. By 1927, the ore reserves decline.
1934–1937Lucero Corporation reworks mostly low-grade piles and some high-grade ore left by previous operations.
1937–1942Sporadic efforts to resume operations, but all of them fail.
PeriodCharacteristics
1600s–1767Rudimentary surface works by Jesuits Missionaries on Santa Brigida veins (gold and silver).
1844First survey and exploitation for mercury.
1871Señores Torres and Cobos started the first legal exploitation in Santa Brigida.
1888–1891Main organized exploitation by Cinco Señores Corporation.
1892–1897Bonanza stage using the “patio” method.
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1897–1902Decline in operations after the high-grade and easy-access ore is depleted.
1902The Angustias Corporation initiated a cyanide heap leaching pool facilities for low-grade ore (San Luis de La Paz).
1902–1913Exploitation mainly by Cinco Señores and Angustias-Dolores Corporations.
1913–1922Cease of activities due to civil war.
1922–1927The Angustias-Dolores Corporation resumes operations. By 1927, the ore reserves decline.
1934–1937Lucero Corporation reworks mostly low-grade piles and some high-grade ore left by previous operations.
1937–1942Sporadic efforts to resume operations, but all of them fail.

Contents

GeoRef

References

References Cited

Carrillo-Chávez
,
A.
Morton-Bermea
,
O.
Gonzalez-Partida
,
E.
Rivas-Solorzano
,
H.
Oesler
,
G.
Garcia-Meza
,
V.
Hernandez
,
E.
Morales
,
P.
Cienfuegos
,
E.
,
2003
,
Environmental Geochemistry of the Guanajuato Mining District, Mexico
:
Ore Geology Reviews
 , v.
23
, p.
277
297
,
doi:10.1016/S0169-1368(03)00039-8
.
Carrillo-Chávez
,
A.
Gonzalez-Partida
,
E.
Morton-Bermea
,
O.
Levresse
,
G.
Soto
,
P.
Tritlla
,
J.
Camprubi
,
A.
,
2006
,
Heavy metal distribution in rocks, sediments, mine tailings, leaching experiments, and groundwater from the Mineral de Pozos historical mining site, north-central Mexico
:
International Geology Review
 , v.
48
, p.
466
478
,
doi:10.2747/0020-6814.48.5.466
.
Cox
,
P.A.
,
1995
,
The elements on earth
 :
New York, New York
,
Oxford University Press
,
287
p.
Gray
,
J.E.
Coolbaugh
,
M.F.
Plumlee
,
G.S.
Atkinson
,
W.W.
,
1994
,
Environmental geology of the Summitville mine, Colorado
:
Economic Geology and the Bulletin of the Society of Economic Geologists
 , v.
89
, p.
2006
2014
,
doi:10.2113/gsecongeo.89.8.2006
.
Gray
,
N.F.
,
1997
,
Environmental impact of acid mine drainage: A management problem
:
Environmental Geology
 , v.
30
, no.
1–2
, p.
62
71
.
King
,
T.V.
,
1995
,
Environmental considerations of active and abandoned mine lands
:
Lessons from Summitville, Colorado: U.S. Geological Survey Bulletin
 
2220
.
Mahlknecht
,
J.
Schneider
,
J.F.
Broder
,
J.M.
Navarro de Leon
,
I.
Ber-nasconi
,
S.
,
2004
,
Groundwater recharge in a sedimentary basin in semiarid Mexico
:
Hydrogeology Journal
 , v.
12
, p.
511
530
,
doi:10.1007/s10040-004-0332-6
.
Mapes
,
E.V.
Echegoyen
,
J.S.
Esquivel
,
E.E.
,
1971
,
Distrito minero de Pozos, Guanajuato: Mexico, D.F.
 ,
Reporte Interno del Consejo de Recursos Minerales
,
Mexico
.
Plumlee
,
G.S.
,
1999
,
The environmental geology of mineral deposits
, in
Plum-lee
,
G.S.
Logsdon
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M.J.
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