Among the different economic activities in Mexico, mining stands as one of the most traditional of them, practiced since pre-Hispanic times (^{Pollard, 1987}; ^{Maldonado and Rehren, 2009}). It is also a regional expansion source since the Colony and plays a critical modernization and expansion role, contributing 2.05% of the country's gross domestic product (^{Secretaría de Economía, 2024}), by supplying raw materials to virtually all sorts of industries (e.g., construction, metallurgical, steel, chemistry, electronics).

• ^{Pollard, 1987}
  The Political Economy of Prehispanic Tarascan Metallurgy

  American Antiquity, 1987
  The Political Economy of Prehispanic Tarascan Metallurgy
• ^{Maldonado and Rehren, 2009}
  Early copper smelting at Itziparátzico, Mexico

  Journal of Archaeological Science, 2009
  Early copper smelting at Itziparátzico, Mexico
• ^{Secretaría de Economía, 2024}
  Minería, 2024
  Secretaría de Economía, 2024, Minería (en línea): México, Gobierno de México, disponible en < Secretaría de Economía, 2024, Minería (en línea): México, Gobierno de México, disponible en https://www.gob.mx/se/acciones-y-programas/mineria >, consultado 22 de febrero de 2024.

Due to its long mining history of more than 450 years (^{Corona-Chávez et al., 2010}), Taxco de Alarcón rises as one of the most important Mexican mining towns. It is located in the Guerrero State's northern area (Figure 1). The Taxco Mining District (TMD) is situated south of Taxco de Alarcón. Throughout the colonial period (1521-1810), Taxco mines produced huge silver and gold amounts to be considered among America's richest mines (^{Talavera-Mendoza et al., 2005}). During this period, 5.6 Mt of tailings were generated and deposited in two valleys near the Cacalotenango River (^{Dótor-Almazán et al., 2017}).

• ^{Corona-Chávez et al., 2010}
  The impact of mining in the regional ecosystem: the Mining District of El Oro and Tlalpujahua, Mexico

  De Re Metallica, 2010
  The impact of mining in the regional ecosystem: the Mining District of El Oro and Tlalpujahua, Mexico
• ^{Talavera-Mendoza et al., 2005}
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

  Geofísica Internacional, 2005
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact
• ^{Dótor-Almazán et al., 2017}
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

  Chemical Geology, 2017
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

Its extensive mining history has produced seven mining waste deposits in the region (El Fraile I, La Concha, El Solar, Guerrero I, Guerrero II, Los Jales, and San Antonio), encompassing more than 55 million tons (^{Dótor-Almazán et al., 2017}).

• ^{Dótor-Almazán et al., 2017}
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

  Chemical Geology, 2017
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

The El Fraile I impoundment is an abandoned tailings dam (470 m × 372 m × 60 m) located 5 km SW of Taxco, showing a high oxidation degree and a yellow-reddish color. It was formed from the exploitation of Ag-Cu-Pb-Zn ores during 1940-1970 (^{Talavera-Mendoza et al., 2016}).

• ^{Talavera-Mendoza et al., 2016}
  Water-rock-tailings interactions and sources of sulfur and metals in the subtropical mining region of Taxco, Guerrero (southern Mexico): a multi-isotopic approach

  Applied Geochemistry, 2016
  Water-rock-tailings interactions and sources of sulfur and metals in the subtropical mining region of Taxco, Guerrero (southern Mexico): a multi-isotopic approach

However, this activity also has some negative impacts on the environment and health (^{Armienta et al., 2003}; ^{Ramos-Arroyo et al., 2004}; ^{Talavera-Mendoza et al., 2005}), which frequently represent a severe problem for mining districts. Such waste accumulations have great polluting potential due to their high potentially toxic element (PTE) concentrations, as reported in various studies like ^{Armienta et al. (2003)}, ^{Talavera-Mendoza et al. (2005)}, ^{Talavera-Mendoza et al. (2006)}, ^{Méndez y Armienta (2012)}. For this reason, the study of the distribution and concentration of potentially toxic elements contained in mining wastes (tailings) is currently an issue of significant relevance (^{Talavera-Mendoza et al., 2006}).

• ^{Armienta et al., 2003}
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico

  Bulletin of Environmental Contamination and Toxicology, 2003
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico
• ^{Ramos-Arroyo et al., 2004}
  Características geológicas y mineralógicas e historia de extracción del Distrito de Guanajuato, México. Posibles escenarios geoquímicos para los residuos mineros

  Revista Mexicana de Ciencias Geológicas, 2004
  Características geológicas y mineralógicas e historia de extracción del Distrito de Guanajuato, México. Posibles escenarios geoquímicos para los residuos mineros
• ^{Talavera-Mendoza et al., 2005}
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

  Geofísica Internacional, 2005
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact
• ^{Armienta et al. (2003)}
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico

  Bulletin of Environmental Contamination and Toxicology, 2003
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico
• ^{Talavera-Mendoza et al. (2005)}
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

  Geofísica Internacional, 2005
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact
• ^{Talavera-Mendoza et al. (2006)}
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico

  Environmental Geochemistry and Health, 2006
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico
• ^{Méndez y Armienta (2012)}
  Distribución de Fe, Zn, Pb, Cu, Cd y As originada por residuos mineros y aguas residuales en un transecto del Río Taxco en Guerrero, México

  Revista Mexicana de Ciencias Geológicas, 2012
  Distribución de Fe, Zn, Pb, Cu, Cd y As originada por residuos mineros y aguas residuales en un transecto del Río Taxco en Guerrero, México
• ^{Talavera-Mendoza et al., 2006}
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico

  Environmental Geochemistry and Health, 2006
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico

Although different investigations: ^{Armienta et al. (2003)}, ^{Talavera-Mendoza et al. (2005)}, ^{Dótor-Almazán et al. (2017)}, have been carried out on mine tailings in Taxco, waste-dump physical characteristics (color, oxidation state, etc.) on the one hand, and a limited number of samples per waste-dump (10, between 5 - 15, and 2, respectively) on the other hand, seems to have guided the sampling strategy, rather than a systematic and an exhaustive one. The low number of samples per waste dump taken could be justified due to standard methodologies used normally require ultra-pure facilities, high-purity reagents, and long processing time, making them generally expensive. Therefore, an alternative economic and rapid procedure allowing reliable PTE concentration estimation would be desirable; this would allow economic resources, time, and effort to be directed toward areas with the most significant impact or interest.

• ^{Armienta et al. (2003)}
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico

  Bulletin of Environmental Contamination and Toxicology, 2003
  Geochemistry of Metals from Mine Tailings in Taxco, Mexico
• ^{Talavera-Mendoza et al. (2005)}
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

  Geofísica Internacional, 2005
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact
• ^{Dótor-Almazán et al. (2017)}
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

  Chemical Geology, 2017
  Geochemical behavior of Cu and sulfur isotopes in the tropical mining region of Taxco, Guerrero (southern Mexico)

Different investigations have shown the correlation between magnetic susceptibility and heavy metal content (^{Petrovsky et al., 1998}; ^{Petrovsky et al., 2001}; ^{Ďurža, 1999}; ^{Shu et al., 2001}). This correlation has been attributed to the incorporation of heavy metal elements into the lattice structure of the ferrimagnetics during combustion, or alternatively to their adsorption onto the surface of the ferrimagnetics present in the environments (^{Petrovsky et al., 1998}).

• ^{Petrovsky et al., 1998}
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study

  Physics and Chemistry of the Earth, 1998
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study
• ^{Petrovsky et al., 2001}
  Magnetic properties of alluvial soils contaminated with lead, zinc and cadmium

  Journal of Applied Geophysics, 2001
  Magnetic properties of alluvial soils contaminated with lead, zinc and cadmium
• ^{Ďurža, 1999}
  Heavy metals contamination and magnetic susceptibility in soils around metallurgical plant

  Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 1999
  Heavy metals contamination and magnetic susceptibility in soils around metallurgical plant
• ^{Shu et al., 2001}
  Determining the sources of atmospheric particles in Shanghai, China, from magnetic and geochemical properties

  Atmospheric Environment, 2001
  Determining the sources of atmospheric particles in Shanghai, China, from magnetic and geochemical properties
• ^{Petrovsky et al., 1998}
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study

  Physics and Chemistry of the Earth, 1998
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study

Even though magnetic methods are now broadly applied in most areas of environmental investigation as a proxy for heavy metal pollution in industrial (e.g.,^{Petrovsky et al., 1998}; ^{Petrovsky et al., 2001}; ^{Ďurža, 1999}) and urban regions (e.g., ^{Shu et al., 2001}; ^{Aguilera et al., 2020}), mining wastes, nevertheless, have hardly been investigated by magnetic methods (^{Matasova et al., 2005}).

• ^{Petrovsky et al., 1998}
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study

  Physics and Chemistry of the Earth, 1998
  Correlation between magnetic parameters and chemical composition of lake sediments from Northern Bohemia- preliminary study
• ^{Petrovsky et al., 2001}
  Magnetic properties of alluvial soils contaminated with lead, zinc and cadmium

  Journal of Applied Geophysics, 2001
  Magnetic properties of alluvial soils contaminated with lead, zinc and cadmium
• ^{Ďurža, 1999}
  Heavy metals contamination and magnetic susceptibility in soils around metallurgical plant

  Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 1999
  Heavy metals contamination and magnetic susceptibility in soils around metallurgical plant
• ^{Shu et al., 2001}
  Determining the sources of atmospheric particles in Shanghai, China, from magnetic and geochemical properties

  Atmospheric Environment, 2001
  Determining the sources of atmospheric particles in Shanghai, China, from magnetic and geochemical properties
• ^{Aguilera et al., 2020}
  Spatial distribution of magnetic material in urban road dust classified by land use and type of road in San Luis Potosí, Mexico

  Air Quality, Atmosphere & Health, 2020
  Spatial distribution of magnetic material in urban road dust classified by land use and type of road in San Luis Potosí, Mexico
• ^{Matasova et al., 2005}
  The use of magnetic methods in an environmental study of áreas polluted with non-magnetic wastes of the mining industry (Salair region, Western Siberia, Russia)

  Geochemistry: Exploration, Environment, Analysis, 2005
  The use of magnetic methods in an environmental study of áreas polluted with non-magnetic wastes of the mining industry (Salair region, Western Siberia, Russia)

In the case of Mexican mining wastes, magnetic methods have only been systematically employed in a metallurgical area in the San Luis Potosí State, Mexico (^{Pérez et al., 2014}), and in two western Mexico mining districts: Tlalpujahua-El Oro (^{Morales et al., 2016}), and Angangueo (^{Hernández-Bernal et al., 2016}).

• ^{Pérez et al., 2014}
  Magnetic susceptibility and electrical conductivity as a proxy for evaluating soil contaminated with arsenic, cadmium and lead in a metallurgical area in the San Luis Potosi State, Mexico

  Environmental Earth Sciences, 2014
  Magnetic susceptibility and electrical conductivity as a proxy for evaluating soil contaminated with arsenic, cadmium and lead in a metallurgical area in the San Luis Potosi State, Mexico
• ^{Morales et al., 2016}
  Further evidence for magnetic susceptibility as a proxy for the evaluation of heavy metals in mining wastes: case study of Tlalpujahua and El Oro Mining Districts

  Environmental Earth Sciences, 2016
  Further evidence for magnetic susceptibility as a proxy for the evaluation of heavy metals in mining wastes: case study of Tlalpujahua and El Oro Mining Districts
• ^{Hernández-Bernal et al., 2016}
  Combined rock-magnetic and geochemical characterization of Angangueo mining district, central Mexico

  Environmental Earth Sciences, 2016
  Combined rock-magnetic and geochemical characterization of Angangueo mining district, central Mexico

In this study, the results of a methodical and comprehensive rock-magnetic and geochemical vertical survey of the El Fraile I mining-waste deposits (the oldest of the region) are presented, emphasizing their variations with the dam's depth. To achieve this aim, magnetic, physicochemical, and geochemical methods were applied.

Twenty-six tailing samples from the El Fraile I dam were collected during a field campaign at the TMD. A vertical (from top to dam's base) sampling was carried out, visually identifying the different horizons. At each horizon, approximately half a kilogram was collected and deposited into previously labeled and georeferenced polyethylene bags. Figure 2 shows the sampling points at the El Fraile I dam. The samples were kept for two weeks in a dry room away from direct sunlight and wind to avoid possible microorganism growth and cross-contamination at the laboratory facilities.

Mineralogy identification was carried out by microscopic observation of the samples spread on a petri dish. Sample preparation consisted of manually crushing ~250 g of the sample up to a particle size of clay using an agate mortar.

Half of the material of each bag was manually crushed using an agate mortar and sieved using a #10 plastic mesh (nominal aperture of 2 mm). Standard paleomagnetic acrylic cubes (8 cm³) were filled with the milled samples for magnetic analysis.

An assortment of standard magnetic techniques was employed to magnetically characterize the samples under study:

(i) Magnetic susceptibility measurements at low (κ_LF) and high (κ_HF) frequency using a Bartington MS2 susceptibilimeter. From these low- and high-frequency results, the percent frequency-dependent magnetic susceptibility was determined (^{Dearing et al., 1996}).

• ^{Dearing et al., 1996}
  Frequency-dependent susceptibility measurements of environmental materials

  Geophysical Journal International, 1996
  Frequency-dependent susceptibility measurements of environmental materials

(ii) Induction of a laboratory anhysteretic magnetization (ARM) through the superposition of a 100 mT AC magnetic field over a 50 μT DC magnetic field using an AGICO LDA 3 demagnetizes (iii) Stepwise acquisition of isothermal remanent magnetization (IRM) curves up to 1 T using an ASC Scientific IM-10 pulse magnetizer. Magnetization acquired at this last step was considered as saturation isothermal magnetization (SIRM). (vi) Backfield magnetization induction at 300 mT (IRM_{-0.3 T}) for the S_-300 ratio estimation employing the following formula (^{Thompson and Oldfield, 1986}):

• ^{Thompson and Oldfield, 1986}
  Environmental Magnetism, 1986
  Thompson, R., Oldfield, F., 1986, Environmental Magnetism: London: Allen & Unwin, 237 p. http://dx.doi.org/10.1007/978-94-011-8036-8.

(v) Acquisition of magnetization vs temperature (M-T) curves using a variable field translation balance (VFTB) up to 600°C, at a heating/cooling rate of 20°C/min. In all cases, the remanent magnetizations were measured using an Agico JR-6 spinner magnetometer.

Pressed-pellet samples were prepared for geochemical analysis by mixing and homogenizing within the same agate mortar 3 g of the milled material with 0.5 g of wax-C micro powder (Hoechst). A 3-mm-diameter stainless-steel die and an Atlas (Specac) hydraulic press were employed for pressing up to 20 tons for two minutes the material deposited into the die.

The samples' chemical composition (both major and trace elements) was obtained through energy dispersive (ED) X-ray fluorescence using a Xenemetrix 50-kV-tube X-Calibur ED-XRF spectrometer.

Five grams of the milled material were dissolved into 50 ml of distilled water and stirred for half an hour for physicochemical analyses. An HI 2020 multi-parametric kit was used for the physical-chemical properties: pH and electrical conductivity (EC) determination.

All analyses were carried out at the Laboratorio Universitario de Geofísica Ambiental (LUGA) facilities, Instituto de Geofísica Unidad Michoacán.

The El Fraile I dam's surface shows a series of contrasting and intercalated red, ochre-reddish, green, ochre-greenish, and green-yellowish layers (Figure 3). This sequence presents an alternation of partially well-consolidated sand layers of coarse, medium, and fine grain sizes. Table 1 summarizes the textural and mineralogical characteristics of the analyzed samples.

IRM acquisition curves show that (in most cases) low fields between 200-300 mT can produce IRM values close to 90 % of saturation (Figure 4a, 4b), pointing to low coercivity minerals as remanence carriers. In some other cases, IRM acquisition curves display a slight increase at higher fields, possibly related to a small contribution of a high coercivity mineral, likely hematite (Figure 4c).

From the M-T curves, it is evident that magnetite or low Ti titanomagnetite (Tc ~ 575°C) is the main responsible for the magnetization. In the specific case of sample Fr-2-I (Figure 5b), a notorious Hopkinson peak is observed, which indicates the magnetic carrier is pure magnetite. Also observed are intermediate temperature (Tc < 500°C) magnetic phases, which are not present at the cooling branch (Figure 5a, 5c). Quite irreversible M-T curves are obtained in almost all samples analyzed, which suggests the formation of new magnetic mineralogy.

Magnetic susceptibility shows an average value of 1 × 10^-3 through the entire vertical profile (~ 50-m-depth), with a moderate peak value at 8.9 m, and other three more pronounced peaks at intermediate depths of 25.4 m, 30.6 m, and 32.3 m (Figure 6a). The aforementioned peak values are associated with reddish-ochre color layers.

ARM acquisition curves show a similar trend to that followed by κ, except in the intervals from 10 to 22 and 35 to 40 m depth (Figure 6b). ARM/κ_LF curves show a marked peak at 16.1 m depth, which is not present at the ARM plot (Figure 6c). A general fining of the magnetic grain-size will show an increase in this ratio (^{Vigliotti et al., 2008}).

• ^{Vigliotti et al., 2008}
  Palaeomagnetic and rock magnetic analysis of Holocene deposits from the Adriatic Sea: detecting and dating short-term fluctuations in sediment supply

  The Holocene, 2008
  Palaeomagnetic and rock magnetic analysis of Holocene deposits from the Adriatic Sea: detecting and dating short-term fluctuations in sediment supply

A very similar tendency to that of κ is followed by the SIRM plot (Figure 6d). The above-mentioned (moderate and more pronounced) peak values observed in the κ plot are also present in the SIRM log, but not in the SIRM/κ_LF plot, indicating a concentration- rather than a composition-variation origin of these out-of-trend values. On the contrary, two noticeable peaks at the intervals 10.0 to 18.5 and 34.5 to 39.9 m depth are seen in the SIRM/κ_LF plot (Figure 6e), which suggests a composition- rather than a concentration-variation origin of these out-of-trend values. These peak values could also be interpreted as variations in the grain size of the magnetite (^{Thompson et al., 1980}; ^{Evans and Heller, 2003}).

• ^{Thompson et al., 1980}
  Environmental Applications of Magnetic Measurements

  Science, 1980
  Environmental Applications of Magnetic Measurements
• ^{Evans and Heller, 2003}
  Environmental magnetism - Principles and Applications of Enviromagnetics, 2003
  Environmental magnetism - Principles and Applications of Enviromagnetics

κ_FD % shows more considerable variations, up to 7% (Figure 7b), especially at the upper part of the dam. These high κ_FD % values suggest a significant contribution of (fine particle-size) superparamagnetic ferrimagnetic minerals -in good agreement with the stratigraphic column-, especially at the dam's upper part (12.2-18.5 m-depth), and with the peak value (16.1 m depth) present in the ARM/κ_LF plot. The middle part is characterized by close to zero values.

Large variations (0.92-0.48) of the S_-300 ratio are observed at the upper part of the El Fraile I dam (Figure 7c), particularly at the interval 12.2-18.5 m-depth, suggesting variable and significant hematite contribution. The middle part shows a more consistent behavior with an average value of 0.92%, except at 31.8 m depth. Worth noting is that maxima in the κ_FD % plot are associated with minima at the corresponding S_-300 plot.

The El Fraile I presents an average acid pH of 3.3 through the entire vertical profile, with two markedly maximum values at 12.7 and 39.7 m-depth (Figure 8b). Figure 5a is again reproduced for comparison purposes.

While pH is the measurement of a specific ion (i.e., hydrogen) within a sample, EC is a non-specific measurement of the concentration of both positively and negatively charged ions within it. As shown in Figure 8c, EC shows a mirrored trend to that of pH. This opposite behavior could be likely explained by considering that the presence of any hydrogen ions present in a substance will affect the pH level and, most probably, also influence conductivity levels. Nevertheless, hydrogen ions make up only a small part of the ion concentration measured by a conductivity meter.

Heavy metal elemental composition variations correlate, in general, with that of κ_LF; peak/trough values in half of the elements correspond to κ_LF peak/trough values, except for V, Ni, and Zn. Those of Pb, Ni, and Zn show significant variations. On the contrary, within the lowermost κ stable trend (34.5-50.9 m-depth), all elemental compositions show a uniformly increasing or decreasing trend, except Ni, which remains almost constant (Figure 9b-g). Worth noting is the high total Fe values, between 10 and 20 wt./%, throughout the entire vertical profile.

Following ^{Talavera-Mendoza et al. (2005)}, metal concentrations of tailings are compared against regional background concentrations (RBC) of crop soils to estimate enrichment ratios of tailings relative to regional pristine soils. Through this comparison (Table 2), it is evident that the El Fraile I tailings are highly enriched in all reported elements -mainly in Pb and Zn-, except in V A summary of the elemental composition and the corresponding statistical description of κ_LF and PTEs concentration in the El Fraile I tailing samples is presented in Table 2.

• ^{Talavera-Mendoza et al. (2005)}
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

  Geofísica Internacional, 2005
  Mineralogy and geochemistry of sulfide-bearing tailings from silver mines in the Taxco, Mexico area to evaluate their potential environmental impact

• ^{SEMARNAT (2007)}
  Norma Oficial Mexicana que establece los criterios para determinar las concentraciones de remediación de suelos contaminados por arsénico, bario, berilio, cadmio, cromo hexavalente, mercurio, níquel, plata, plomo, selenio, talio y/o vanadio (NOM-147-SEMARNAT/SSA1-2004)

  Diario Oficial de la Federación, 2007
  Norma Oficial Mexicana que establece los criterios para determinar las concentraciones de remediación de suelos contaminados por arsénico, bario, berilio, cadmio, cromo hexavalente, mercurio, níquel, plata, plomo, selenio, talio y/o vanadio (NOM-147-SEMARNAT/SSA1-2004)

Both low- and high-coercivity minerals are present in the El Fraile I dam as remanence carriers, as shown by the IRM curves (Figure 4).

As evidenced from the M-T curves, magnetite, or low Ti titanomagnetite is the main responsible for the magnetization, although intermediate temperature magnetic phases are also present. Quite irreversible M-T curves are obtained in almost all samples analyzed, which suggests the formation of new magnetic mineralogy (Figure 5).

A general fining of the magnetic grain size at the first third part (top up to 20 m depth) is evidenced by the ARM/κ_LF plot (Figure 6c).

Magnetic susceptibility (and SIRM, as well) vertical survey stands out different layers of variable ferrimagnetic mineral concentration or composition, in good agreement with the stratigraphic column. However, the concentration-independence character of the SIRM/κ_LF points to grain size or mineralogical differences rather than concentration variations as the cause of the observed trends (Figure 6d).

As in the case of the ARM/κ_LF plot, a general fining of the magnetic grain size is evidenced by the κ_FD % plot and the S_-300 ratio values for the same section (first upper part of the sequence) (Figure 6e).

The results above mentioned highlight the usefulness of the magnetic methods as a proxy for stratigraphy and mineral concentration of a depositional sequence.

The old dam's long evolution period could be suggested by the variable and significantly low value of the S_-300 ratio (high oxidation degree of the magnetic mineralogy) and the mostly acid pH character throughout the whole dam.

As noted above, peak values in half of the elements correspond to κ_LF peak values throughout the whole deposit, although at the lowermost κ stable trend (34.5-50.9 m-depth), all elemental compositions show a uniformly increasing or decreasing trend.

Because of the scarce number of rock-magnetic investigations on this kind of anthropogenic origin systems, no regional background concentration (RBC) for κ is available for this investigation, precluding the estimation of an average enrichment factor (AEF). Nonetheless, considering the complex metal speciation within the layers (sulfides and/or sulfosalts), an estimation of an AEF would be likely nonsense.

As seen in Table 2, Pb, Cu, and Zn are the elements with a major variability within the profile (range of 4818.7, 224.3, and 2581.1, respectively). Pb and Zn are the elements with a significantly average enrichment factor (AEF).

From Table 3, the strong positive correlation between κ_LF and SIRM (cc = 0.9425) suggests either that a single magnetic mineral is responsible for carrying the SIRM, causing the κ_LF in the samples, or that the dominant minerals were in constant proportions throughout the different layers (^{Thompson et al., 1975}).

• ^{Thompson et al., 1975}
  Magnetic susceptibility of lake sediments

  Limnology and Oceanography, 1975
  Thompson, R., Battarbee, R., O'Sullivan, EE., Oldfield, F., 1975, Magnetic susceptibility of lake sediments. Limnology and Oceanography, 20(5), 687-698.

The general fining of the magnetic grain size observed in Figure 6c is corroborated by the strong positive correlation coefficient (cc = 0.6205) between κ_FD % and ARM/κ_LF.

As expected, a strong negative correlation exists between pH and EC.

Also observed from Table 3 is a moderate negative correlation (cc = -0.4635) between k_fd% and Fe. As mentioned by ^{Kanu et al. (2017)}, "The significant negative correlation between Fe and k _fd % further confirmed that Fe is of anthropogenic origin." As well, from Table 3, k_LF ARM, and SIRM could be used as a proxy for V, according to the negative correlation coefficients obtained (-0.3743, -0.4722, and -0.3223, respectively). Similarly, most magnetic parameters seem to be good enough proxies for the evaluation of Ni, Cu, and Zn. A strong direct correlation between Pb and Cu contents is evident from the results presented in the matrix correlation (Table 3). Because of its long evolution period, health hazards are mainly due to the high Pb, Cu, and Zn concentrations.

• ^{Kanu et al. (2017)}
  Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

  International Journal of Environmental Science and Technology, 2017
  Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

As mentioned in the introduction, the El Fraile I impoundment is located 5 km SW of Taxco de Alarcón, and one could be tempted to think that the pollution problem is confined to the dam's surroundings. However, there is an adjacent problem directly linked to particulate matter (PM), the material accumulated in the dams; street dust (SD) pollution. SD is mainly composed of particles derived from anthropic processes (^{Dearing et al., 1996}). As stated in ^{Morales et al. (2020)}, "SD consists of a heterogeneous assortment of materials; some originated from natural processes...", but also from "Anthropic activities..., which is later released to the environment.".

• ^{Dearing et al., 1996}
  Frequency-dependent susceptibility measurements of environmental materials

  Geophysical Journal International, 1996
  Frequency-dependent susceptibility measurements of environmental materials
• ^{Morales et al. (2020)}
  Heavy metal content estimation in the Mexico City Street dust: an inter-method comparison and Pb levels assessment during the last decade

  SN Applied Sciences, 2020
  Heavy metal content estimation in the Mexico City Street dust: an inter-method comparison and Pb levels assessment during the last decade

Fine particles of anthropic origin, particulate matter 10 pm in diameter or less (the so-called PM10 particles), are hosted by the SD, which contain magnetic minerals and traces of heavy metals (Cr, Cu, Pb, Zn, etc.) (^{Bautista et al., 2018}). Due to their particular size and shape, PM10 particles can be inhaled by humans, which is why SD is considered a significant source of breathable air pollution (^{Amato et al., 2014}).

• ^{Bautista et al., 2018}
  Evidence of small ferrimagnetic concentrations in mice (Mus musculus) livers and kidneys exposed to the urban dust: a reconnaissance study

  Geofísica Internacional, 2018
  Evidence of small ferrimagnetic concentrations in mice (Mus musculus) livers and kidneys exposed to the urban dust: a reconnaissance study
• ^{Amato et al., 2014}
  Urban air quality: the challenge of traffic non-exhaust emissions

  Journal of Hazardous Materials, 2014
  Urban air quality: the challenge of traffic non-exhaust emissions

The results presented agree with those of previous studies. However, the systematic sampling distribution employed in this investigation enables us to obtain a broader view of the concentration and distribution within these deposits.

Considering that tailings of abandoned mining districts in Mexico are nowadays occupied and serve as settlements for various inhabitant groups (^{Hernández-Bernal et al., 2016}), the study of the distribution and concentration of PTEs contained in mining wastes is currently an issue of significant relevance, as properly noted by ^{Talavera-Mendoza et al. (2006)}. Unfortunately, as properly mentioned by ^{Morales et al. (2023)}, "...waste impoundments are, at least in Mexico, unattended entities..." and "...neither a government nor a particular company would invest time, effort, and economic resources to investigate the dam's internal structure". This fact highlights the usefulness of magnetic susceptibility, and associated magnetic parameters, as a proxy for the investigation of waste dams.

• ^{Hernández-Bernal et al., 2016}
  Combined rock-magnetic and geochemical characterization of Angangueo mining district, central Mexico

  Environmental Earth Sciences, 2016
  Combined rock-magnetic and geochemical characterization of Angangueo mining district, central Mexico
• ^{Talavera-Mendoza et al. (2006)}
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico

  Environmental Geochemistry and Health, 2006
  Geochemistry of leachates from the El Fraile I sulfide tailings piles in Taxco, Guerrero, southern Mexico
• ^{Morales et al. (2023)}
  Magnetic Susceptibility Prospecting and Geochemical Characterization of Taxco's Mining Waste Dam Guerrero I (Mexico)

  Quaternary, 2023
  Magnetic Susceptibility Prospecting and Geochemical Characterization of Taxco's Mining Waste Dam Guerrero I (Mexico)

Simple (and non-destructive) sample preparation, and fast rock-magnetic and elemental concentration estimations, together with a suitable systematic sampling distribution, make XRF measurements an advantageous proxy method for a quick and cost-effective evaluation of contamination by heavy metals.

List of abbreviations

IRM: isothermal remanent magnetization.

IRM_-0.3T: backfield magnetization at 300 mT.

PTEs: potentially toxic elements.

SIRM: saturation isothermal remanent magnetization.

TMD: Taxco Mining District.

Wt./%: weight percent.

κ_FD%: percent frequency-dependent magnetic susceptibility.

Κ_HF: magnetic susceptibility measurements at high frequency.

Κ_LF: magnetic susceptibility measurements at low frequency.

EC: electrical conductivity.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Contributions of authors

MS and JM contributed to conceptualization. NP and JM contributed to the methodology. JM carried out the fieldwork. JM and NP performed formal analysis. JM contributed to writing-original draft preparation. All authors contributed to writing-review, and editing. MS acknowledges the support of UNAM-PAPIIT Project IA102413.

Financing

No funding was received.

Conflicts of interest

The author(s) declare(s) that they have no competing interests.

Handling editor

Claudia Gogorza