THE IMPACT OF MINING IN THE REGIONAL ECOSYSTEM: … · THE IMPACT OF MINING IN THE REGIONAL...

De Re Metallica, 15, 2010 pp. 21-34© Sociedad Española para la Defensa del Patrimonio Geológico y MineroISSN: 1577-9033

THE IMPACT OF MINING IN THE REGIONAL ECOSYSTEM: THE MINING DISTRICT OF EL ORO AND TLALPUJAHUA, MEXICO

Pedro Corona Chávez1, José Alfredo Uribe Salas2, Neftalí Razo Pérez1, Mónica Martínez Medina3, Roberto Maldonado Villanueva4, Yann Rene Ramos Arroyo5 and Jasinto Robles Camacho6

1 Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, México. [email protected], [email protected]

2 Facultad de Historia, Universidad Michoacana de San Nicolás de Hidalgo, México. [email protected]

3 Posgrado en Geociencias y Planificación del Territorio, Universidad Michoacana de San Nicolás de Hidalgo, México.

4 Facultad de Ingeniería, Universidad Nacional Autónoma de México.5 Facultad de Ingeniería en Geomática e Hidráulica, Universidad de Guanajuato, México.

6 Instituto Nacional de Antropología e Historia, Centro Regional Michoacán, México.

RESUMEN

El presente artículo se fundamenta en una base de datos estadístico-geográficos, documentos y mapas históri-cos, así como procedimientos acuciosos dedicados a recopilar, organizar y difundir la información acerca delambiente y los recursos naturales del Distrito Minero El Oro-Tlalpujauhua (DMOT). Con esta plataforma de conoci-miento, se valora la zona minera explotada en forma paroxismal a través de los siglos, la cual ha dado riqueza, hacreado un pueblo y ha dejado la devastación en su entorno. Se adopta un enfoque holístico que integra el análisishistórico y espacial del DMOT. Se discuten tres variables explicativas: a) Mayor impacto ambiental en épocas deextracción industrial coincidentes con el desarrollo tecnológico: 1905-1945; b) Variaciones de eficiencia en laobtención de valores de Pb, Ag, Au, Zn, entre Tlalpujahua y El Oro, circunscrito a las tecnologías utilizadas y apli-cación de métodos metalúrgicos por cianuración; c) Pérdida de volúmenes de residuos mineros hasta en 27% en rela-ción con el volumen histórico original y causado por procesos de remoción de masa por erosión acelerada y trans-porte fluvial.

PALABRAS CLAVE: Minería, componentes mineralógicos, impacto ambiental, desarrollo sostenible, Distrito MineroEl Oro-Tlalpujauhua, México.

ABSTRACT

This article is based on a statistical and geographical database, historical maps and documents and their reli-able processing, in order to organize and issue information about the environment and natural resources of the Min-ing District El Oro and Tlalpujauhua (MDOT). On this platform of knowledge, the exploited paroxysmal miningthrough the centuries has given wealth and caused the growing up of towns as well as it has left devastation in theirenvironment. A holistic approach was applied, integrating the historical and geospatial analysis of the MDOT. Threeissue lines are discussed: a) Greatest environmental impact of industrial extraction times coinciding with techno-logical development: 1905-1945; b ) Efficiency variations to get values of Pb, Ag, Au, Zn, between El Oro andTlalpujahua which were constrained to the different technologies during the application of cyanidation metallur-gical methods c) From the historical and recent mapping we point out a relative losing volumes of tailings up 27%which were displaced along the rivers by high erosion rates.

KEY WORDS: Mining, mineral components, environmental impact, sustainable development, Mining District El Oroand Tlalpujahua, Mexico.

De Re Metallica 15 julio–diciembre 2010 2ª época 21

De Re Metallica 15 julio–diciembre 2010 2ª época

INTRODUCTION AND PROBLEM STATEMENT

The mining areas around the world are characterizedby removing large volumes of rock, from which general-ly gives concentrations <5% of the substances of interest(Helgen and Moore, 1996). The remaining volume is suc-cessively rejected directly on the environment. The tail-ings are the result of industrial material processing andare known as a dump, or tailings. The mining wastes arepotentially toxic depending on the geological conditionsof deposits and the metallurgical method used but, thetailings cause always an extent environmental prob-lems: i) changes in landscape, and land use and water;ii) negative impact on the dynamic processes of a hydro-logical deposit, and iii) distribution of potentially toxicelements (PTE), such as mercury, copper sulfate, sulfu-ric and cyanide acid.

Industrial mining in Mexico has been developed since1550 and consequently there are abundant mining dis-tricts associated with several billion tons of waste min-ing scattered around the country. Environmental impactstudies in Mexico have been realized in the last 10 years(Monroy et al., 2002; Talavera et al., 2003, RamosArroyo et al., 2004, 2006; Romero et al., 2008 and ref-erences in). In the most of these studies have prevailedthe application of a common methodological geochem-istry approach, either from its mineral components ofprimary surface water or acid mine drainage and insome cases the impact on the health of the population.The results of these studies have shown that any miningdistrict reveals an own and singular stratigraphicsequence of mine tailings which are related to differentcharacteristics and evolution of pre-, during and post-mining (and eventually abandonment). Consequentlythe diagnosis of environmental impact depends on: i)granulometry and mineralogical composition (mineraland rock fragments), ii) their spatial relationships of theenvironmental basin, and iii) its management (recy-cling, neglect, abandonment) in relation to the historicvillage linked to the development of mine and tailingsdeposits.

The Mining District of El Oro and Tlalpujahua (MDOT)has been characterized by a wealth of gold associatedwith other precious metals like silver, lead and zinc (Flo-res, 1920; Ostroumov and Corona Chávez, 1999; De laTeja Segura, 2000; Bustamante Garcia, 2007). Mineralshave been extracted for more than five centuries relat-ed to concomitant development periods, expansion and,in some cases a desolation in their villages. As an evi-dence of huge economic activity in the MDOT, there aremany mining waste dumps, or tailings that have beenabandoned for more than 60 years (Uribe Salas, 2008and references in).

This paper contains a brief historical review of MDOTwith emphasis on the generation of the work-extractionand mining wastes and their close relationship with min-erals extraction methods used. From this historicaldatabase, and based on a systematic study of mapping(Corona Chávez y Uribe Salas, 2009) and the relationshipwith the geochemical behavior of the tailings of MDOT

(Corona Chavez et al., 2010), we evaluate some alter-native treatment and waste management considering,first of all the economic applications (metallic and/ornon-metallic), but in the other hand, the environmentalimpact if will make eventual economical secondaryextraction.

BACKGROUND GEOLOGICAL-MINING DISTRICT OFEL ORO AND TLALPUJAHUA

The MDOT is located in the boundaries of the statesof Michoacan and Mexico and is part of the hydrologicalbasin “Lerma Santiago River”, region 12 (Figure 1).From the geological point of view the MDOT is locatedwithin of Miocene-Pliocene Trans-Mexican Volcanic Belt(Morales Gámez and Corona Chávez, 2006 and refer-ences in). The geological unit that contains the MDOTmineral deposits is the basement rocks of Jurassic-EarlyCretaceous (Centeno García et al., 2003).

Although the MDOT is recognized essentially as agold-deposit, the mineralization in this district is con-sidered as a part of the big-silver metallogenic province(Ostroumov and Corona Chávez, 1999; Albinson et al.,2001). The mineralized structures are a hydrothermalvein system with a predominant NW-SE direction, whichtends to have a tabular form with a thickness up to 0.5to 20-33 m with lengths up to 3.5 km. Au-Ag mineralsare essentially hipogenic sulphides and sulfosalts associ-ated with a gangue of calcite and quartz.

The importance of the MDOT has been well knownsince pre-colonial and colonial times, however its firstgeological description refers to one of the oldest geolog-ical and mining charts of Mexico (Burkart, 1827), whichhas been systematically updated by other geological andmining descriptions (Flores, 1920; Elvir Aceituno, 1955;Bustamante Garcia, 2007 and references in).

Even though it is usually considered that in Mexicothe extraction of metals started since pre-Hispanictimes, the mining industrial works were beginning relat-ed to the colonial era, and the MDOT was known as the“Real de Minas of Tlalpujahua”. During this time, aseries of exposed veins were exploited (San Rafael, Des-cubridora, Chihuahua, Virgenes, Borda, Coronas), theseveins were located in the heart of the modern cities ofEl Oro and Tlalpujahua (see Fig. 1). Over five centuries(XVI-XX), large, medium and small companies have suc-ceeded in creating a culture linked to the work ofextraction and mineral processing of silver and gold.

Although the working never stopped, the story of thepassed centuries recorded three periods of high activity(Fig. 2, Table 1), i) the first in the 1820s when theBritish committed capital and technology in the rehabil-itation of mines, which were destroyed or abandonedduring the revolution of independence; ii) the second,seventy years later begins during the Diaz regime andpost-revolutionary period (1876-1911, 1914-1937), whenthe richest gold veins were discovered under the “CerroSomera” (Verde, Negra, Nueva), and which wereexploited intensively; iii) the third, and final, covering

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the years of 1938-1959, when the Mining Cooperative“Las Dos Estrellas” in El Oro and Tlalpujahua and theCommission of Mining Development started operations(Uribe Salas, 2008; 2009). As a result of mining activityat different stages were born the mining towns ofTlalpujahua (XVI century) and El Oro (XVIII century), dis-tant 6 miles apart, both including the construction ofmineral separation plant and benefit- casting, as well asmany roads, shafts or pits to access mineral depositsand, a real mountains of dumps and mine tailings whichhave caused an evident environment impact (see Fig.1).

METALLURGICAL METHODS AND THEGENERATION OF MINING WASTE AND TAILINGS INTHE MDOT

During the colonial period and until the late nine-teenth century, the method applied in the MDOT wasthe one known as patio or amalgamation, which wascharacterized by the use of salt and mercury. Residuesor tailings at this time were deposited in the surround-ed areas of the mines and so called Benefit Farms

(Haciendas de Beneficio), which required a rotation sys-tem for animals and water flow of the rivers near them.However, the discharge of mining waste related thismethod was carried out scattered and in relatively smallvolumes, so they were not really accumulated and maynow appear only as remnants related to a natural distur-bance caused by the intense weathering of MDOT rainyregion.

The cyanide method was applied in the MDOT since1898, however, in the early years it was only used as atesting system and according to some sources, thismethod was applied in an industrial form since 1902,with high yields of nearly 99% gold and 56% of silver perton, a fact that, they tried the reprocessing of the “old”tailings produced by the method of patio (Uribe Salas,2008).

The first wastes generated at this stage, were notaccumulated in a specific area, they were thrown intothe River Tlalpujahua, but in a short time an environ-mental damage to water and soil were observed aroundthe rivers, the farmers complained about it, and as aresult in 1902, the Hacienda de Guadalupe was estab-lished as the first tailing dam in the MDOT. The first tail-ings pond was built in the traditional way of wood and


Figure 1. Location of the Mining District of El Oro and Tlalpujahua (MDOT). Note the distribution of mineralized structures in continuous line, which presents anorientation from northwest to southeast and mining works: tunnels or shafts and vertical shots or hole-works.


masonry and, it is currently the main accumulation oftailings of Tlalpujahua.

During the successive years, the tailings were pro-duced by two mills (Cedros 1, 2) at Tlalpujahua. At ElOro, the waste was deposited in a similar way in maincanyon so called Tiro Mexico. It is worth to note thatthese tailings dams are still well identified because theyare located inside or few meters from the boundaries ofboth towns (Fig. 3).

During the years 1904 to 1905, the electric current andthe railway line were installed in the MDOT. Such event ingeneral favored the communication and, consequently itwas a large increasing in underground mining work anda modern industrial cyanidation plant was installed, andduring 1920 the works had reached nearly 20 km long by1920 (Uribe Salas 2006, Table 1, and Fig. 2). In conse-quence, the extraction and processing of metals became

much more intense, resulting on the exponential rise ofthe precious metals recovered (Fig. 4) as well as the gen-eration of huge volumes of waste material that were rap-idly accumulated in the dams tailings (Fig. 4).

As shown in Table 1 and Figures 2 and 4, the exponen-tial increasing of production of metals and mining wastegeneration in the MDOT tailings were produced duringthe period of 1904-1938, specifically during the activityof the french mining company “Las Dos Estrellas”. How-ever, the story would change radically due to a tragicaccident in the tailings pond or “slats” on May 27, 1937;accident which killed over 300 people. This fact precip-itated the exodus of members of the French company,to leave it to the “Cooperativa Minera y Obrera DosEstrellas” since 1938.

Although the “Cooperativa” carried out the innova-tive flotation method, which allowed obtaining a better

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Figure 2. Historical record of the material processed (ton) by the Mining Company “Dos Estrellas” and “Cooperativa Minera”Las Dos Estrellas” and the relation-ships with the metallurgical extraction processes used (Source: Uribe Salas, 2008).

Figure 3. Photographs of the actual distribution of MDOT tailings dams. Tiro Mexico, El Oro (left) and Cedros, Tlalpujahua (right).


1901 22435 61.47 2 22373.51902 52722 144.44 4.7 1 52577.61903 67305 184.40 6 1.5 67120.61904 75157 205.91 6.7 2 74951.11905 89740 245.86 8 2 89494.11906 131133 359.27 11.69 2.321 130773.71907 325309 891.26 29 3.227 324417.71908 432000 1183.56 28.12 3.075 430816.41909 341111 934.55 35 5 340176.41910 424198 1162.19 47.68 6.53 423035.81911 479723 1314.31 62.63 6.775 478408.71912 505000 1383.56 63 6 503616.41913 673053 1843.98 60 4 671209.01914 245664 673.05 21.9 1 244990.91915 191820 525.53 17.1 4 191294.51916 229959 630.02 20.5 4.3 229329.01917 463655 1270.29 41.33 5.2 462384.71918 482354 1321.52 43 5 481032.51919 509770 1396.63 45.44 4 508373.41920 504789 1382.98 45 3.3 503406.01921 503533 1379.54 44.88 3 502153.51922 473313 1296.75 46.67 2.75 472016.31923 493857 1353.03 41.97 2.699 492504.01924 533772 1462.39 35.43 2.548 495796.01925 665316 1822.78 37.02 2.805 625083.01926 632056 1731.66 45.63 2.096 584333.01927 752198 2060.82 51.13 2.351 698715.01928 693460 1899.89 55.85 2.328 635408.01929 693257 1899.33 64.46 1.914 626882.01930 676962 1854.69 76.42 2.034 598503.01931 593835 1626.95 46.45 1.856 545532.01932 786936 2155.99 38.38 1.812 588811.01933 821645 2251.08 31.70 1.585 590527.01934 829663 2273.05 40.11 2.416 778180.01935 777759 2130.85 46.34 2.138 729284.01936 717755 1966.45 40 2 715788.51937 628129 1720.90 28.45 1.258 626408.11938 589275 1614.45 - - 587660.51939 761431 2086.11 - - 759344.91940 772083 2115.30 - - 769967.71941 758148 2077.12 - - 756070.91942 759625 2081.16 - 1.5 757543.81943 744709 2040.30 - - 742668.71944 614713 1684.15 - - 613028.91945 379183 1038.86 - - 378144.11946 266073 728.97 2.68 228 265344.01947 178750 489.73 - - 178260.3Total 23344333 63957.08 1372.39 337.32 22343740.4

YearVolumeExtracted

(ton)Mineral

processedSilver* Gold*

Voluemmine tailings

Table 1. Mineralized material extracted from the mines and mining waste after the operations from 1924 to 1935 profit. Source: Statistical Yearbook of miningfor the year 1936..., pp. 10-29; Mining Company “Las Dos Estrellas”, in El Oro and Tlalpujahua, SA Annual Report 1938; Carlos Herrejón, Tlalpujahua ..., pp. 150-156; Fernando Foglio Miramontes, Economic Geography..., p. 248.


metallurgical recovery and less pollution, it was notenough to withstand the turbulent years of its short life(1937-1946). As we can perceive in Figures 2 and 4, thedramatic drop in their output ranges, which had severalcauses: i) higher costs for operation at greater depths;ii) reduction of Au-values in the ore veins; iii) theincrease in the value of production inputs, and iv) aslight depression in the international market for goldmetals. These different factors associated with otherproblems related to the internal organization unfortu-nately caused the definitive later closing in 1959, leav-ing behind an irregular settlement between the minersand an abandoned structure of historical works in theMDOT.

THE MAPPING TAILINGS OF THE MDOT

Through the consultation of the historical archives ofthe Museo Tecnológico del Siglo XIX, Dos Estrellas, aswell as literature review of the Servicio Geológico Mex-icano1 we were able to find the historical location ofmining waste dumps (Fig. 5). Although field testing isrelatively simple, as tailings dams are always on theedge of the towns of El Oro and Tlalpujahua, we per-formed a systematic mapping using the common criteriaused to any “geological unit”: aerial photo interpreta-tion and field work. The integration and comparison wasassisted by GIS (Martínez Medina et al., 2009).

The current distribution of mining waste of MDOT isshowed in Figure 6. There are four dumps (stars) and sixhistoric tailings dams: El Carmen, Cedros and Dos Estrel-las at Tlalpujahua; CONALEP1, 2 and Tiro Mexico at ElOro. Table 2 shows the ratios of areas for these wastemapped. The largest area is “Cedros” with > 60% ofwaste from the Tlalpujahua tailings followed by the“CONALEP” in El Oro with 18.29%. It is worth note that

the tailing dam El Carmen, which covers an area of22164.02 (3.56%) is related to the sludge that over-flowed on May 27, 1937, the wastes reached the placewhere the village church stood.

Based on the comparative between the historic car-tography and mapping, we can notice a clear reductionof area and volume. This reduction is partly due to theoverflow of 1937 which dispersed the residues, but themain reduction which we have estimated are up >27-35%(Martínez Medina, 2009; Corona Chávez et al., 2010),and it can be related to the high rain precipitation andhigh rate erosion processes of the Tlalpujahua and ElOro regions.

PHYSICAL, MINERALOGICAL AND GEOCHEMICALCHARACTERISTICS OF THE TAILINGS OF MDOT

Based on the field work and sampling we have alsoobtained the physical properties of the tailings and suc-cessively we carried out chemical-mineralogical analysis(Corona Chávez et al., 2010). For sampling we haveused the method by Ramos Arroyo et al. (2004, 2006),which considers vertical, horizontal changes and areas

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Figure 4. Gold values obtained in the Company Dos Estrellas from 1900 to 1950. Source: Mexican Year Book, 1908, 1909-1910, Statistical Yearbook of Mining, 1923,1925, 1928, 1930, 1934, 1937, Eduardo Flores Clair, et al., Mining statistics ..., pp. 16-147; Genaro González Reyna, mining Wealth …, pp. 85-136.

1 For historical reports of the Council of Mineral Resources and Mining Development Commission:http://www.coremisgm.gob.mx/

Tailings Dam Area (m2) Hectare %Los Cedros 394,152.81 39.42 63.28

Dos Estrellas 14,866.47 1.49 2.39

Tiro México 77,682.10 7.77 12.47

CONALEP 113,898.82 11.39 18.29

El Carmen 22,164.02 2.22 3.56

Total 622,764.22 62.29 100.00

Table 2. Percentages area of the tailings dams surface mapped (Martínez-Medi-na, 2009).


Figure 5. General mining drawing of Cooperativa Minera “Las Dos Estrellas” in El Oro and Tlalpujahua 1943. In grey the tailings dams. Source: Historical MapArchive. Museo Tecnologico del Siglo XIX, Dos Estrellas.

Figure 6. Map showing the current distribution of the dumps (stars) and the MDOT tailings dams. Note the irregular shape of the landscapecaused by weathering, erosion volumes. Note also the tailings area at “El Carmen” to the east-north east of Tlalpujahua which is related tothe “strips” from the accident of May 27, 1937.


of potential accumulation of leaching processes. On theother hand, it was also considered the sample positioninto the hydrological basin of the MDOT (Martínez Med-ina et al., 2009), as well as the relative neighbor andrelationship with human settlements and other physicalelements of the basin (e.g. soil, vegetation). The datarefer to 12 sampling stations in the field (8 Tlalpujahuaand 4 at El Oro), where textural differences and physi-cal properties were identified (texture, pH, conductivi-ty, temperature and carbonate) and its successivelygranulometric, mineralogical and chemistry character-ized (Corona Chavez et al., 2010).

Physical and textural analysis

Table 3 presents data from three representative sta-tions of MDOT, where it is observed that the pH variesfrom neutral to slightly alkaline (7.8 to 8.46) and anelectrical conductivity that is somewhat variable, pre-dominantly> 800 moh/cm, which indicates a scarce andvariable concentration of metals. The texture or fine-ness of each of the horizons at the selected sites wasclassified in situ (Siebe et al., 1996). Texture is impor-tant for some industrial processes such as abrasiveprocess in the production of ceramic or the quantities ofclay or particle size influence the final product. Most ofthe tailings have silt to clay textures. This texture wasverified by direct measurement of grain size with aCoulter Lasser equipment (Fig. 7) in which we can seethat the tailings of MDOT tend to be silt (<80%) with sig-nificant variations of clay 7 - 10%) and fine sand (7-38%).

Mineralogy and geochemistry

Based on a modal study of their percentages (CoronaChavez et al., 2010, Table 4), tailings from MDOT havein order of abundance considerable amounts of quartz(> 42%), clay (9-19%) and calcite (11-12%).The percent-ages of opaque minerals are very constant> 2-3%. Otherminor components like feldspar, lithic and mafic miner-als are variables depending on their levels. Consideringtheir economic importance, the texture and composi-tion of opaque minerals were characterized (MaldonadoVillanueva, 2008). The phases that have been describedare pyrite (FeS2), argentite (Ag2S), galena (PbS),goethite [FeO (OH)], ilmenite (FeTiO3), magnetite(Fe3O4), hematite (Fe2O3), arsenopyrite (FeAsS), chal-copyrite (FeCuS2), pyrrhotite [Fe (1-x) S (0-.0.17)] andothers. Although usually opaque minerals are relativelyisolated grains or clasts, they also can be occluded intothe quartz and probably as sub-microscopic grains asso-ciated with a matrix of calcite (CaCO3).

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Muestra thick T °C pH Ce CaCO3

E3H1-Tlalpujahua 2.0 27.1 8.1 100.0 60.0

E3H2-Tlalpujahua 15.0 26.7 8.4 110.0 60.0

E3H3-Tlalpujahua 15.0 26.4 8.4 90.0 60.0

E3H4-Tlalpujahua 48.0 28.5 5.9 2090.0 40.0

E3H5-Tlalpujahua 30.0 29.0 5.1 570.0 40.0

E3H6-Tlalpujahua 20.0 31.9 6.9 330.0 40.0

E4H2-El Carmen 41.0 22.4 8.5 110.0 60.0

E4H3-El Carmen 129.0 23.6 8.4 110.0 60.0

E7H1-El Oro 1.0 19.6 7.9 563.0 60.0

E7H2-El Oro 1.0 19.8 8.3 750.0 60.0

E7H3-El Oro 10.0 19.9 8.5 197.0 60.0

E7H4-El Oro 11.0 19.6 8.0 378.0 0.0

E7H5-El Oro 8.0 18.8 4.5 895.0 0.0

E7H6-El Oro 10.0 19.0 6.8 342.0 20.0

E7H7-El Oro 20.0 19.4 7.1 240.0 20.0

ME07m-El Oro 0.0 19.8 8.6 8180.0 60.0

ME07c-El Oro 0.0 20.2 7.3 469.0 60.0

Table 3. Physical properties of representative field measurements of MDOT tail-ings. E3-Cedros Tlalpujahua, E4-Barrio El Carmen, E7-Tiro Mexico El Oro and MEisolated samples. Units of thick in m and Ce in (S.m-1).

Figure 7. Ternary clay-silt-sand grain size of the 65 samples from tailings ofMDOT. Measured by laser spectroscopy (Lasser-Coulter).

From the geochemical point of view tailings sampleswere analyzed for major elements by the method of X-ray fluorescence (XRF) and additionally Trace elements,lanthanides and potentially economic elements wereanalyzed using a mass spectrometer with inductivelycoupled plasma ICP-MS (see table in Corona et al.,

2010). In order of abundance silica have a very widerange of values between 56 and 92%, aluminum from 5to 13%, iron of 3-5%, calcium 2.5 to 5% potassium of 1-2%. It should be noted that all samples have values thatare potentially compensable Au 0.6 to 4.4 gr / ton andAg ranging from 1.8 to 178.3 g/ton (Fig. 8).


1 2.3 3.0 --- 71.3 20.6 1.6 --- 1.0

2 88.6 --- 9.0 1.3 --- --- --- 1.0

3 40.3 40.7 2.9 14.0 --- --- 0.5 2.0

4 52.1 37.1 8.1 1.5 --- --- --- 1.0

5 71.0 18.0 8.0 0.5 --- --- --- 3.0

7 70.0 26.0 1.4 --- --- --- 0.1 2.4

8 62.4 35.2 1.7 --- --- --- --- 0.6

9 67.0 24.0 3.0 3.0 --- --- --- 3.0

10 90.0 4.0 4.0 --- --- --- --- 2.0

11 75.5 3.0 2.5 18.0 --- --- --- 1.0

12 78.0 16.0 1.0 3.0 --- --- --- 2.0

14 79.0 10.0 9.0 --- --- --- --- 2.0

Point

Opaques (%wt)

quartz clay Calcite Andesite Tuffs ChloritesSericite

Hematite andMagnetite

Amphibole andPyroxenes

Transparents (%wt)

Table 4. Modal percentages of the transparent and opaque minerals in the tailings of MDOT. (After Maldonado-Villanueva, 2008; Corona Chávez et al., 2010)

Figure 8. Photographs of thin sections (left) and polished surfaces (right) from the tailings of MDOT. Notices at the top opaque minerals are isolated and thoseoccluded in quartz grains (after Maldonado-Villanueva, 2008).


The potentially toxic elements (PTE), the samplesshow values ranging from: 3.0 to 83.9 ppm for As, 7.4 to808.6 ppm Cu from 16.5 to 317.5 ppm Pb and 63.8 to548.2 ppm of Zn (Figure 9). These data are close andsome cases exceeding the values established by the offi-cial Mexican standards. Nevertheless, it is important to

point out that their behavior in relation to water chem-istry anomalies (Nieto Monroy, 2007), where apparentlyare dominated by relatively neutralized water andtherefore the values of the PTE of the tailings do notseem generate Acid Drainage.

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Figure 9. Binary diagrams of major elements and trace elements, including metal values from MDOT tailings.

DISCUSSION

The historical metallurgical processes in the MDOTand their economic potential resources

Although MDOT has been pointed out as a paradig-matic mining regions with advanced industrial and min-ing technology, indeed minerals were usually sent as ametallurgical concentrates to the north of the country,specifically at ASARCO Company, in order to obtain themaximum values of gold and silver. This caused, in part,that the economic values varied each month, each year,depending on the economic demands of business too.The tailings, consequently also have variations in vol-ume and sometimes in residual metal values. Despitethe efficient separation of gold and silver the produc-tion reached very high yields of recovering > 80-90% onaverage > 800 g/ton Ag and > 2.9 g/t Au, as seen in thisstudy (Figures 9 and 10), the tailings of MDOT are alwayseconomic values related to remnants of precious metalson:> 1.3 g/t Au,> 36 g/t Ag,> 53 g/t Pb and > 137 g/tZn. Note that the residual values of Pb and Zn suggestprobably a lack of interest on these metals and a prior-ity for the profit of gold and silver which marked thesite.These economic metallic values in the tailings werenot completely unknown to pioneer miners, to the con-trary, following the technological advances in the chem-ical metallurgical, the tailings were studied as a poten-

tial secondary resource in theMDOT since the decade of thefifties. Firstly tailings were studiedby the Cooperativa Minera “Las DosEstrellas” and, successively by theComisión de Fomento Minero, butthere was not any exploitation it.Other studies were carried out inthe last years of twenty century bySan Luis Min Company, which wasthe latter mining company in theMDOT. However, as we shall seelater, there were a lot of metallur-gical problems that inhibited theimplementation of a strategy to beexploited.

In order to evaluate an econom-ic potential of any natural materi-al, it most be verified the volumeavailable, the homogeneity and ofcourse the distribution of metallicvalues. In relation to the volumesand summing up we can say thatthe historical production of theslats or tailings reached > 22 mil-lion tons deposited outside theboundaries between people and

plants of the MDOT. This large volume of mining wastewas generated essentially in four decades using thecyanide method and only a minor percentage (<8 years)using the flotation method. However, through the map-ping and analysis of the landforms is plausible that thecurrent volumes do not correspond to the original vol-umes and suggest that in areas of the tailings there wasa process of removing about >20% (3-4.5, 000 ton) bynatural processes of erosion. On the other hand, unfor-tunately the economic values are random, variable andalthough the average is high, there may be sterile formany samples. This fact causes a relative uncertainty inany economic deposit.

From the metallurgical point of view, different stud-ies were carried out in the tailings of MDOT. The Coop-erativa Dos Estrellas and recently even private compa-nies in the eighties and nineties have conducted numer-ous metallurgical testing by different methods: flota-tion, heap leaching, cyanidation with different types ofgrinding and calcinations, bioleaching and chloride.However, the outcome was never completely satisfacto-ry2 in technical terms, some problems arising from costessentially a finer grind, the separation of ore minerals,including quartz, and that the weighting was consideredthat existing volumes definitely never guarantee recov-ery installation of a new treatment plant neither a pay-ment for transportation.


Figure 10. Concentration ranging of the metallic elements analyzed by ICP-MS of the representative samplesfrom MDOT tailings (after Corona-Chávez et al., 2010).

2 It is useful to note some of the conclusions of an internal report from the University of San Luis Potosi ... “from the previousstudies the difficulties arising mainly ... it is considered that these values are associated with carbonaceous material andembedded in the particles that constitute the quartz matrix ... this suggests roasting and leaching first ... and another optionis to grind to 100% below 325 mesh and floating”.


Economic potential as an industrial material ofMDOT-tailings

An additional advantage of the tailings can be seenfrom the perspective of a nonmetallic material forindustrial use. There are some examples of a secondaryapplication for industrial use for some materials(Ulmanu et al., 2003; Meseguer et al., 2008) and thesecondary recovery of tailings in the world (Mitchell etal., 2004) and Mexico (Vite et al., 2007). The expecta-tions of the tailings of MDOT as a nonmetallic materialhas been well documented (Razo et al., in prep) and itis believed that it can have multiple applications: pro-duction of refractory pieces, paints, floor tiles, treessubstrates, light abrasives and glass crafts. However, itis necessary to be careful in the homogeneity of compo-sition and in some cases in any release of gas from thedecomposition of metal sulfides and carbonates thatwould affect certain processes. In some cases, depend-ing on its application (zeolites, light aggregates, pave-ment, etc ...), adding some components to optimize therespective application should be considered.

Current conditions and environmental impact ofMDOT

The environmental impact caused by mining wastegenerally refers to the geochemical anomaly in thewater, soil, and landscape, which are directly associat-ed with a possible affectation directly or indirectly onhuman health. In relation to water resources chemicalanalysis of potentially toxic elements (e.g. As, Pb, Cu,Zn, Mn) were carried out to determine the potentialmobility of heavy metals from the tailings into therivers. The values in the solid tailings vary from> 53gr/ton Pb;> 137 gr/ton Zn,> 54 gr/ton of Cu and> 25gr/ton of As. These values seem high, but do not neces-sarily represent an alarm and do not imply that they aretoxic; It is depending on the specifical bioavailabilityand its possible transport (in this case water) to humansettlements. The water quality of rivers surrounding themine tailings (Nieto Monroy, 2007; Ramos Arroyo et al.,2010) shows that even though if a chemical changecaused by construction and mining waste registered, theoverall water levels in the MDOT tend to remain neutralwater. However, sometimes there are “streams” associ-ated with the water outlets from the mine tunnelswhich present anomalies of arsenic. Therefore, it isworth noting that studies should be conducted in detailabout the mobility of heavy metals in water or soil sys-tems. Finally, there are records relating to the impact ofthe tailings spilled into the Barrio del Carmen. In thethirty years following the mining disaster, farmers in theregion demanded that scientific studies were conductedon the impact of the tailings spill, as the toxic sub-

stances contained in them “was the cause of death ofthe cattle in the region, without apparent cause, thatdrank from that water (lakes and rivers), they alsoclaimed having noticed a negative effect on their cropsfor the use of water contaminated with cyanide. But thebiggest concern for the residents who were exposed totoxic substances was the death of relatives.”3

In relation to the landscape, it is possible to see acomplete transformation of primary vegetation. Howev-er, although no specific studies have been performed bymeans of comparative photographs of the forties andtoday, we see the regeneration of cedar forests and pinevegetation, probably related to good biodisponibilityand a limited land use or human intervention to refor-est (Corona Chavez et al., 2010).

THE MINING AND HISTORICAL HERITAGE OFMDOT: THE ALTERNATIVE

The mineralogical and geochemical characteristics ofthe MDOT-tailings suggest a chance for secondary eco-nomic approach either as a metallic material (Au, Ag,Pb, Zn) or as non-metallic or industrial material. How-ever, the balance between economic vs. environmentimpact in both cases is strong against and the resultshave been questioned by specialists (Razo Perez et al.,in prep.). Specifically, any new operation would involveremoving the material and would cause a “second envi-ronment impact” to an area that has been naturallyregenerated in the last fifty years.

The most significant outcome from the environmen-tal impact studies in a mining district such as the MDOTis to understand the relevance of a scientific study onthe “environment” of cultural heritage, as a historicspace. The landscape was transformed with crops andmining works scattered around, there is no vegetationor wildlife. There is a biosphere that seeks to recoverfrom the wounds caused by the removal of a presumedand opulent richness of the past.

It is just in this landscape where we understand thebirth of two cities of similar and different stories atonce: El Oro and Tlalpujahua, both populations of thou-sands of inhabitants. Environmental impact has clearlybeen caused. People know that there was a wealth; butit is doubtful that the people knows the risks inheritedfrom the natural geochemical anomaly that represents amineral deposit and subsequently the potential forma-tion of acid waters, which can be generated by miningwaste by sulfur tunnels now exposed and expressed asmining “springs”.

An alternative is to assess the mining district of ElOro and Tlalpujahua as a historic and cultural heritage4

from which to reassess not only the available naturalresources and industrial application, but also to extend

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3 Archive of the Agrarian Reform. Claims Tlalpujahua. Morelia, 1946-1949.4 See the definition of UNESCO. Cultural Heritage is the set of all goods, materials (tangible) or immaterial (intangible) which,

by its own value, should be considered of great interest for the permanence of identity and culture of a group. It is the her-itage of the past itself, with people whom live today and we pass on to future generations.

its meaning to the anthropological sense. The culturalelements associated with spatial and/or environmentalrelationships become any mining district to cultural her-itage and be aware it as a geological park, these newattributes could be a new alternative to inhabitants ofMDOT.

If we consider the MDOT as potential geo-heritage wewill have a wide opportunity to assess the historical her-itage as a natural resource and potentially economic forvisitors. However, it is clear that any vision and percep-tion of an historical heritage requires new backgroundknowledge, and this the only way to make possible asustainable use of the mining district heritage. Ofcourse this is only possible from an interdisciplinary andholistic vision of the “Mining Eco-System”, which con-sists of a variety of elements interrelated by variousphenomena and processes, both social and natural.

ACKNOWLEDGEMENTS

This paper is a contribution of the fideicomiso CONA-CYT-Gobierno del Estado de Michoacán, FOMIX-2005-CO1-014. We thank the field support of the Museo Tec-nológico Mina Dos Estrellas.

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