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Environmental geochemistry of the Guanajuato

Mining District, Mexico

A. Carrillo-Chaveza,*, O. Morton-Bermeab, E. Gonzalez-Partidaa, H. Rivas-Solorzano c,G. Oeslerd, V. Garca-Mezae, E. Hernandezb, P. Morales f, E. Cienfuegosf

aCentro de Geociencias, Univ. Nacional Autonoma de Mexico, Campus Juriquilla, Postal 1-742, Queretaro, QRO 76230, Mexicob

Inst. de Geofsica. Univ. Nacional Autonoma de Me xico, MexicocPos-grado en Ciencias de la Tierra. Univ. Nacional Autonoma de Me xico, Mexico

dDepartment of Geology, University of Wyoming, USAeFacultad de Qumica, Univ. Nacional Autonoma de Me xico, Mexico

f Inst. de Geologa, Univ. Nacional Autonoma de Me xico, Mexico

Received 22 April 2002; received in revised form 8 October 2002; accepted 1 May 2003

Abstract

The Guanajuato Mining District, once one of the major silver producers in the world, has been exploited for silver and

gold from low-sulfidation quartz- and calcite-rich epithermal veins since 1548. Currently, there are some 150 million

tonnes of low-grade ore piles and mine-waste material (mostly tailings) piles, covering a surface area of 15 to 20 km2

scattered in a 100-km2 region around the city of Guanajuato. Most of the historic tailings piles were not deposited as

formal tailings impoundments. They were deposited as simple valley-filling piles without concern for environmental issues.

Most of those historical tailings piles are without any vegetation cover and undergo strong eolian and hydrologic erosion,

besides the natural leaching during the rainy season (which can bring strong thunderstorms and flash flows). There is

public concern about possible contamination of the local aquifer with heavy metals (Fe, Mn, Zn, As and Se) derived from

the mining activities.

Experimental and field data from this research provide strong geochemical evidence that most of the mine-waste materials

derived from the exploitation of the epithermal veins of the region have very low potential for generation of acid mine

drainage due to the high carbonate/sulfide ratio (12:1), and very low potential for leaching of heavy metals into the

groundwater system. Furthermore, geochemical evidence (experimental and modeled) indicates that natural processes, like

metal adsorption onto Fe-oxy-hydroxides surfaces, control the mobility of dissolved metals. Stable isotope data from surface

water, groundwater wells (150-m depth) and mine-water (300- to 500-m depth) define an evaporation line (dD=5.93

d18O = 13.04), indicating some deep infiltration through a highly anisotropic aquifer with both evaporated water (from the

surface reservoirs) and meteoric water (not evaporated). Zinc concentrations in groundwater (0.03 to 0.5 ppm) of the alluvial

aquifer, some 15 km from the mineralized zone, are generally higher than Zn concentrations in experimental tailings

leachates that average less than 0.1 ppm. Groundwater travel time from the mineralized area to the alluvial valley is

calculated to range from 50 to several hundred years. Thus, although there has been enough time for Zn sourced from the

0169-1368/$ - see front matterD 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0169-1368(03)00039-8

* Corresponding author.

E-mail address: [email protected] (A. Carrillo-Chavez).

URL: http://www.geociencias.unam.mx.

www.elsevier.com/locate/oregeorev

Ore Geology Reviews 23 (2003) 277297


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tailings to reach the valley, Zn concentrations in valley groundwater could be due to natural dissolution processes in the deep

portions of the epithermal veins.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Environmental geochemistry; Mine tailings; Groundwater; Heavy metals; Guanajuato; Mexico

1. Introduction

Since early in the history of mankind, metallic ore

deposits have played an important role in the tech-

nologic and economic development of the human

society. The highly industrialized contemporaneous

society requires an increase in metallic mineral

supplies. Thus, it is a priority to increase our activ-

ities of exploration and exploitation of these ore

deposits (Kesler, 1994). However, along with this

priority in exploration comes another basic need of

our society: the preservation of a clean environment

(airwatersoil) (Berner and Berner, 1996; Fyfe,

1998; Hill, 1997; Keller, 1992; Kesler, 1994; Plant

et al., 2001). Regretfully, in most of historical cases

(and some modern cases too), the exploitation of ore

deposits has been accompanied for a negative impact

to the environment (mainly to surface and ground-

water) (Gray et al., 1994; Guogh, 1993; King, 1995).It is a fundamental task of all of us to find a balance

between exploration and exploitation of metallic ore

deposits with the preservation of a clean environment

for future generations (Hill, 1997; Nriagu, 1990;

Plant et al., 2001). Perhaps the greatest challenge

facing humankind is how to support the growing

population demands, without negatively affecting of

our most basic life-support ecosystems (Fyfe, 1998).

To solve these problems, there is a need for team-

work integrating all the expertise from the natural

sciences and the engineering and economic sciences.It is also clear that the activities needed for equili-

brated development of ore deposits and the environ-

ment are of multidisciplinary character. However,

punctual actions, like environmental geochemistry

assessments, are basic for future decision-making

activities.

Environmental geochemistry is a relatively new

branch of the natural sciences (geology and chemis-

try) that deals with the analysis of the geologic and

chemical processes that introduce and control the

concentrations of elements or substances potentially

contaminant to the environment (Plant et al., 2001).

The distribution of most of these elements or sub-

stances could be normal in the environment, but when

their concentrations are above certain levels, they

could become toxic to the ecosystem. The main

environmental problems related to ore deposits are

acid mine drainage resulting from the oxidation of

sulfides (mostly pyrite) and hydrolysis of metal

oxides, and high concentrations of heavy metals on

water draining from mining sites (Pb, Zn, Cu, Cd, Cr,

Fe, Mn, As, Se, Sb, etc.).

It is also true that natural processes like the

weathering of metallic ore deposits (mostly sul-

fides), weathering of metal-rich rocks, volcanic

eruptions and mineralized hot springs could produce

naturally acid rock drainage and anomalous amounts

of heavy metals in the environment. But, in most

cases, the activities of the mining industry locallyincrease these processes of acid drainage and liber-

ation of heavy metals to the environment (Gray,

1997; Larocque and Rasmussen, 1998; Smith et al.,

1994).

However, during the last two decades, real prob-

lems of high concentration of heavy metals in ground-

water around mining areas have been recognized

(Alpers et al., 1994; Belkin and Spark, 1993; Bowell,

1994; Carrillo and Drever, 1997; Cebrian et al., 1994;

Chavez and Walder, 1995). Since the second half of

the 1980s and during all the 1990s the environmentalresearch on the impact of mining activities has

increased considerably. Some of this research has

been focused on the effects of acid mine drainage

and high concentrations of heavy metals on surface

and groundwater and soil (Alpers et al., 1994; Bow-

ell, 1994; King, 1995; Haneberg et al., 1993; Fyfe,

1998; Larocque and Rasmussen, 1998; Rosner, 1998).

Other works have focused on the mineralogical and

chemical characterization of metallic ore deposits and

their respective wastes (Carrillo and Drever, 1997;

A. Carrillo-Chavez et al. / Ore Geology Reviews 23 (2003) 277297278


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Chavez and Walder, 1995; Smith et al., 1994; Plumlee

and Severson, 1994). Another research approach has

focused on the characterization of mineral surfaces in

the natural environment and their effects on dissolvedheavy metals (Carrillo and Drever, 1998; Duddley et

al., 1988; Koretsky, 2000; Wuolo, 1986). Other

researches have determined the geochemical baselines

to distinguish between natural and anthropogenic

sources of metals in the environment (Matschullat et

al., 2000; Plant et al., 2001). The use of stable and

radiogenic isotopes also has found a solid place in the

environmental geosciences (Ghomshei and Allen,

2000; Toner et al., 2003). Additional environmental

geochemical research concerning ore deposits

involves geochemical and mathematical modeling,

biogeochemistry, molecular environmental geochem-

istry, laboratory experiments and review on sources,

behavior and distribution of individual elements in the

near-surface environment (Banwart and Malmstrom,

2001; Bener et al., 2000; Blowes et al., 1998; Brown

et al., 2000; Eary, 1999; Fein et al., 2001; ODay,

1999; Smedley and Kinniburgh, 2002; Tempel et al.,

2000). All this geochemical information can be com-

bined with hydrogeological data in order to predict

and to model the behavior of heavy metals in ground-

water (geometry of plumes, velocity, residence time,

etc.) (Drever, 1997; Mazor, 1991; Stumm and Mor-gan, 1996).

The objectives of environmental geochemistry

studies of ore deposits are to provide answers to the

following questions:

What are, and where are, the potential sources of

potential contaminant metal located, and how

much material is exposed? How much metal is being introduced to the

environment and what is its residence time, and

flux-in and flux-out rates? What are the geochemical processes controlling the

concentration and distribution of the heavy metals

in the natural environment (groundwater-soil)? What is the distribution of the heavy metals in the

field (potential contamination plumes)?

In the United States alone, between 100,000 and

500,000 mine sites are abandoned or inactive (King,

1995). In Mexico, the number of mining sites is

unknown, but there are estimates of 10,000 to

50,000 abandoned or inactive mine sites. Currently,

in the United States there are more than 50 US

Environmental Agency (US-EPA) Superfund restora-

tion sites related to non-fuel mining activity (King,1995). Currently in Mexico, there are relatively few

environmental studies on mining sites, and no federal

program for restoration of sites exists. One of the

goals of this research is to present pioneering geo-

chemical data on mine tailings and groundwater at a

major mining site in Mexico and to try to motivate the

development of regular environmental geochemical

evaluations in Mexico with the long-term goal of

building a census of mining sites, as a base for future

restoration programs.

In this paper, we present analytical and experimen-

tal geochemical data from several materials of the

Guanajuato mining district (sediments, mine tailings,

natural water, and leachates from column and humid-

ity cell experiments). The purpose of this research is:

(a) to determine, both experimentally and in the field,

the potential for generation of acid mine drainage

and leaching of heavy metals into the aqueous

environment (surface and groundwater) of some

of the historical mine waste materials;

(b) to consider possible natural processes taking place

below the surface to explain the chemical effectson the groundwater of both the natural material

(ore deposits) and the anthropogenic altered

material (mostly tailings);

(c) to compare experimental and field observations

with analytical data from the groundwater;

(d) to determine processes of the hydrogeochemical

evolution, and

(e) to quantify the natural background values and

anthropogenic pollution in the region.

2. Site description

The Guanajuato Mining District, some 350 km

westnorthwest of Mexico City, is located in the core

of the Guanajuato Mountain Range (Fig. 1). This

mining district represents the central zone of a region-

al polymetallic mineralized belt that runs from Taxco

(south central Mexico) to Guanajuato (central Mex-

ico), and to Sta. Barbara, Chihuahua (north-central

Mexico). This regional mineralized belt is related to

A. Carrillo-Chavez et al. / Ore Geology Reviews 23 (2003) 277297 279


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subduction processes during the Middle Tertiary and

extensional tectonic stress defined by the NW SE

orientation of the mineralized veins (Randall et al.,

1994). The geology of the region consists of a

Cretaceous volcano-sedimentary sequence covered

by mafic igneous rocks and intruded by Tertiary

granodiorite batholithic rocks. The older rocks have

been partially covered by volcanic rocks related to a

caldera center, with which the mineralized vein sys-

tems are associated (Randall, 1990; Randall et al.,

1994; Martinez-Reyes, 1992).

Buchanan (1980) described three mineralizing

stages in epithermal veins with abundant quartz,

calcite and adularia. The mineral zoning allowed

Buchanan (1980) to define a mineralization model

and the distribution of an upper zone (precious

metals: Au and Ag), an intermediate zone (transition),

and a deeper zone (base metal: Cu, Zn, Pb). Gold and

Fig. 1. Regional location of the Guanajuato District in central Mexico and location of four historical tailings sites around the city of Guanajuato

(La Luz, Valenciana, San Nicolas and Noria Alta).



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silver occur in veins and stockworks (Veta Madre

Mother lode, Veta La Luz and Veta La Sierra

systems). The gangue minerals are quartz, calcite

and adularia. The hydrothermal wall-rock alterationconsists dominantly of propylitic assemblages and

minor argillic + phyllic assemblages. These alteration

assemblages are overprinted by silicification. This ore

deposit is characteristic of hydrothermal low sulfida-

tion deposits. The mineralized district has been used

as a model of epithermal mineralization due to its

primary mineral alteration assemblages (Buchanan,

1980). In Guanajuato, minerals of tellurium and

selenium associated with gold are not rare. The most

abundant sulfide minerals are: pyrite, marcasite,

acanthite, argentite, polybasite, pearceite, sphalerite,

galena, chalcopyrite, pyrargyrite, tetrahedrite, arseno-

pyrite, pyrrhotite and aguilarite (Querol et al., 1991).

There are two main hypothesis for the source of the

metals in the Guanajuato deposits: (1) magmatic

origin and (2) leaching and remobilization from the

Mesozoic volcano-sedimentary rocks (Taylor, 1972;

Gross, 1975; Mango et al., 1991). The debate con-

tinues, but both parties conclude that the origin of the

sulfur for the metal sulfides is unclear.

Whatever the ultimate source for metals and sulfur

at the mineralized district, one thing is certain;

historically, Guanajuato district has been one of themajor silver producers of the world. The first dis-

covery of silver ore was in 1548 (Wandke and

Martinez, 1928). Veta Madre ( + 20 km in length

and F 8 m wide) was discovered in 1550, and after

451 years of production, this mother lode is still

producing. Since 1550, several other epithermal vein

systems have been intensely exploited for Au, Ag, Pb

and Cu. Currently, three mining companies are active

and are extracting a total of about 300 kg of gold and

50 t of silver monthly. Total metal production of the

Guanajuato mining district is more than 33,000 t ofsilver and 170 t of gold (Cardenas, 1992). The

average grade for gold is 4.5 g/t, and for silver is

260 to 450 g/t. Based on this total production, we

estimate that there are more than 150 million tonnes

of waste material scattered in the mining district.

These historic tailings are dispersed around Guana-

juato city. Some tailings areas have been covered and

used as a flat foundation for urban developments.

Other piles are partially covered by vegetation, and

yet others have been partially eroded and remobi-

lized. The modern tailings impoundments of the

mining companies are environmentally controlled

and fulfill the current environmental law. We will

focus our analysis and discussion on the historictailings.

2.1. Mineral La Luz tailings pile

These tailings are located approximately 8 km

northwest of the city of Guanajuato. This mine-waste

material is derived from the La Luz and other mines

in the La Luz vein system (westernmost mineraliza-

tion in Guanajuato). This locality is not within the

Guanajuato city catchment area, but has environmen-

tal importance because this material is among the

oldest in the region, and was of the first to be treated

with cyanide heap leaching (Martin, 1906). These

tailings are located on the side of one hill covering an

area of some 0.03 km2 with an average thickness of

15 m. The estimated volume of tailings is 1 million

tonnes. The material is quite uniform in texture

(sandy) and highly unconsolidated. The tailings pile

is highly eroded, which has transported approximate-

ly 40% of the material downstream into a creek

draining towards the Silao-Leon aquifer, located west

of the mining district.

2.2. Monte San Nicolas tailings

The Monte San Nicolas mine and tailings pile is

located north of the city of Guanajuato and within

the Sierra Vein system. This mine operated since the

late 1700s and until the 1920s. The tailings pile

consists of three different layers deposited on the

southeast side of one steep hill. The combined

volume of tailings is about 5 million tonnes. The

oldest and thickest layer is at the bottom, and it is

topped by two thinner and younger layers (Oelsner,2001). These tailings are heavily eroded, with ap-

proximately 25% of the original material removed

during a heavy rainy season. It is believed that

during a major storm event (some 40 years ago,

based on some anthropogenic material found in the

deposit), the tailings piles were seriously eroded and

a canyon was cut to a depth of 25 m in the tailings

pile. Most of the eroded tailings material was depos-

ited on the alluvial plain of Presa de Mata, a water

reservoir 3 km downstream from the San Nicolas



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area. In the alluvial plain, a more or less continuous

layer of tailings-rich material of 10-cm thickness can

be observed during the dry season (low water ca-

pacity periods of the reservoir). The water from thisreservoir is mostly used for industrial processes

(mineral extraction). But for some years, at the peak

of the dry season (AprilMay), water in the reservoir

is used for Guanajuato municipal use.

2.3. Valenciana Mine tailings

The Valenciana Mine is perhaps the most famous of

all the Guanajuato mines. The mine started exploitation

in the earliest 1600s, with several bonanzas in the

1700s and 1800s (Garca, 1999). The minetailings were

accumulated on a steep hill along a creek just north of

the city of Guanajuato. There are two tailings impound-

ments located sequentially on the hillside. Their com-

bined volume is around 20 million tonnes of material

covering an area of some 0.5 km2. The mine waste

accumulated in these piles came from several mines

(Valenciana, Rayas, San Vicente), but mostly from the

main Mother Lode (Veta Madre). These tailings piles

have a thickness of 10 to 15 m, and the material is an

inter-layering of sandy and clay horizons. During the

rainy season, a shallow pool forms in the surface of the

upper tailings impoundment. The pool could remainduring several months, indicating the relatively low

vertical permeability of the clay horizons.

2.4. Noria Alta tailings

These tailings are located on the west side of the

city of Guanajuato. During the last 15 to 20 years, the

urban developments have some constructions over the

tailings. This mine-waste material comes mostly from

the mining of the Mother Lode. These deposits are

highly dispersed in an area of more than 0.3 km

2

, witha total volume between 5 and 7 million tonnes. The

material is fairly uniform with a sandy texture and is

highly unconsolidated. Locally, some oxidation hori-

zons can be observed in this material.

The Guanajuato mining area is within a mountain-

ous region with local relief of 400 to 500 m with

respect to the regional alluvial valley. Within the

mountains there are some water reservoirs for use as

municipal water along with some groundwater wells.

A high concentration of Zn (0.5 mg/l) has been

detected in groundwater samples from the wells that

supply the city of Guanajuato (some 15 km west of the

mining and mineralized zone). The wells are located in

the same regional basin (Leon-Silao aquifer). Thegeneral public view is that the mining activities (most-

ly the mine tailings) have contributed to the pollution

of the regional aquifer with heavy metals.

3. Sampling and laboratory methods

3.1. Sampling and analytical methods

Samples of tailings material, sediments, surface

water, groundwater and water from the interior of

some mines were taken during a 3-year period from

1998 to 2001. Bulk tailings samples were taken from

historical tailings piles for total digestion and trace

metal analyses. Also, samples were taken for humid-

ity cell test (HCT) and column experiments. The

bulk solid samples for chemical analyses and HCT

were taken in double-sealed plastic bags. The sam-

ples for column experiments were taken in-situ with

25-cm diameter and 1-m long high-density PVC

pipes. Chemical analyses were done with microwave

digestion (method EPA 3051) and multi-elemental

ICP-MS in duplicate at the Analytical Laboratory ofthe Institute of Geophysics, UNAM, and the Geo-

chemical Laboratory of the University of Wyoming.

Natural water samples were taken from the wells

(140-m depth average) that supply the city of Guana-

juato, from three different reservoirs (Soledad, Esper-

anza and Purisima) in the region (surface water), and

from runoff water from tailings piles after heavy rains

(natural leaching in the San Nicolas area). All water

samples were filtered in the field (0.45 Am). Samples

for cation analysis were acidified (concentrated HNO3

to pH = 2), whereas non-acidified samples were col-lected for anion and stable isotope analysis. In the

field, pH, electric conductivity and total alkalinity

were measured. Hydrochemical modeling was done

with PHREEQC (Parkhurst and Appelo, 1999) for

saturation index and advanced modeling.

3.2. Kinetic and static tests

Humidity cell testing is one form of kinetic test.

This testing provides data on the rates of metal leach-



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ing, acid generation and acid neutralization for a

selected sample. In turn, these rates provide important

information needed to predict drainage chemistry

(Morin and Hutt, 1997). The humidity cell experi-ments (HC) used in this work are plastic containers

with tight-fitting lids. Each cell is about 7-cm high and

has about 50-cm2 cross-sectional area. An air inlet

feeds into the center of the top and a drain fitting is

located in the bottom of each cell. Seven individual

cells were connected to a regulated source of com-

pressed air (f 5 l/min) using 1.3-cm diameter Tygon

tubing. Humidified air was generated using a 20-l glass

carboy that was half-filled with deionized water.

Compressed air was fed directly to the HC (dry-air

cycle for three days) or pumped to the carboy first and

then routed to the HC (humidified-air cycle for 4

days). At the end of the humidified-air cycle, the

sample (200 g of sample) was treated with 200 ml of

double-deionized water passed through a Milli-Q

system and allowed to soak for 1 h (1:1 solid solution

ratio). The leachates were collected using glass

beakers, filtered through 0.45-Am Micro-pore mem-

branes and divided into two subsamples. One was used

for alkalinity titration and anion analysis by anion

chromatography (AC), and the second was acidified to

pH 2 for cation analysis using ICP. The leachates were

stored overnight at 4 jC before analysis. All theglassware and plasticware were cleaned with deter-

gent, 0.12 M HCl, and rinsed with deionized water.

The humidity cell test is a kinetic method that tries

to simulate the weathering conditions of the tailings

piles in the field, especially heavy metal leaching to

the environment (groundwater and/or soil) and acid-

producing and acid-consuming processes that occur in

the natural environment (Sobbek et al., 1978). At the

Guanajuato quartz and calcite-rich epithermal veins

and corresponding tailings material, far more impor-

tant than the acidity generation and consuming pro-cesses is the amount of heavy metals that are being

released to the environment.

The most commonly used static test is known as

acid base accounting (ABA). The ABA test was

originally designed to evaluate acid-producing capa-

bility of coal-mine waters. It is now used to evaluate

both coal- and metal-mine wastes. ABA measures the

balance between the acid-producing potential (AP)

and acid-neutralizing potential (NP) of each mine

(White et al., 1999). For the present research work,

three samples from the tailings piles were used for the

ABA test.

3.3. Column experiments

The samples for the column experiments were

taken directly from the tailings piles with 25-cm

diameter and 1-m-long PVC pipes. The pipes were

hammered all the way down into the tailings material,

and then a hole around the pipe was dug to extract the

pipe and avoid loss of material. The columns were

transported to the laboratory where a valve was

installed at the bottom of the column in order to

collect the weekly leachates.

3.4. Adsorption experiments

These experiments were carried out using plastic

containers as batch reactors with 20 ml of double-

deionized MILLI-Q water. Background solutions

with ionic strengths of 0.1 and 0.01 M NaNO3 were

used for different experiments. Different amounts of

natural material from the Mata basin alluvial plain

were used to get total solid-phase iron concentrations

between 0.01 and 0.001 M. The material collected,

and used for these experiments, is considered to be

representative of the alluvial material of the shallowaquifer of the region, which receives the surface

groundwater and holds the main interaction between

solid-solution (surface complexation reactions). Total

metal (Pb, Zn, Cu, Cd, As) concentrations used

ranged between 3 10 5 and 7 10 5 M. Metalsolutions were prepared from stock standard solution.

HCl or NaOH was added to solutions to adjust the

pH. The experimental solutions were equilibrated for

24 h in the batch reactor with continuous shaking to

maintain a constant solid/liquid ratio. After the equil-

ibration, the final pH was measured and the suspen-sion was centrifuged for 10 min at 10,000 rpm; 10 ml

of the supernatant was filtered through a 0.45-Am

membrane, transferred to a vial and stored in the dark

at 4 jC overnight for total Pb, Zn, Cu, Cd and As

analysis by atomic absorption spectrometry (AA).

3.5. Surface titration

Four surface titration experiments were performed

on the natural aquifer material of the San Nicolas area



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in order to compare the resulting Point of Zero Charge

(PZC) with that of the iron oxy-hydroxides (Parks and

deBruyn, 1962). The setup for the surface titration

was the following: a 200-ml plastic bottle with 20-mldouble-deionized water and a magnetic stirrer was

used as a batch reactor. Nitrogen gas was bubbled

through Tygon tubing to the solution to exclude CO2from the system. A background electrolyte of NaNO3at different ionic strengths was used. About 0.04 g of

sediment material was used, which corresponds to a

Fe solid phase concentration of f 2 10 2 M.NaOH (0.00125 M) and HCl (0.01 M) solutions were

added to raise or lower the pH at constant intervals of

time (5 min); pH readings were made after the 5-min

equilibration time.

4. Results and discussion

4.1. Mineralogy and geochemistry of tailings

Mineralization at the Guanajuato District occurs

mostly in vein structures and minor stockworks. The

mineralogy of the epithermal veins is dominated by

quartz, calcite and adularia (Gross, 1975, Petruk and

Owens, 1974). Ore minerals (silver and gold) are

distributed in bands alternating with gangue minerals(Table 1). Both Gross (1975) and Buchanan (1980)

recognize three mineral assemblages in the mineral-

ization of Guanajuato: (a) upper (precious metals),

middle (transition zone) and (c) lower (base

metals zone). Since early studies (Wandke and Mar-

tinez, 1928), it has been recognized that pyrite occurs

in less than 1% of the ore. Base metals are relatively

low, but in general Cu, Zn and Pb increase in

concentration with depth. These low sulfidation

deposits are considered to have a relatively low acid

mine drainage potential due to their high ratio ofcarbonates/sulfides.

X-ray diffraction data indicate that the tailings

from the different piles consist mostly of quartz,

feldspar, calcite, chlorite, kaolin and pyrite (Ramos,

1991). Quartz represents 60% of the total; feldspar

follows in concentration. Calcite is in 10% and pyrite

in less than 1%. This calcite/pyrite ratio in the tailings

material (and also in the mineralized veins) indicates

that any acidity generated by the oxidation of metal

sulfides (mostly pyrite) would be neutralized by the

calcite and feldspar. Acid base accounting static

analysis (ABA) was performed on several tailings

samples. The results of the ratio (NP/AP) of neutral-

ization potential (NP) and acidity potential (AP) fromthe ABA tests range between 1.3 and 5.7, which

indicate the high neutralization capacity of the mate-

rial (White et al., 1999).

Fig. 2 shows the content of some trace elements in

the different tailings materials. Zinc content averages

400 mg/kg (ppm), Pb = 100 mg/kg, As = 20 mg/kg

and Se < 5 mg/kg. The samples were taken from the

surface of the tailings piles and up to a depth of 1 m,

except from San Nicolas area where a profile sam-

pling was taken. However, in the field, it is possible to

observe dramatic variation in coloration (profile insome trenches) of some tailings. The upper part of the

profiles is highly oxidized (0.25- to 1-m depth). It has

been documented that in the upper part of such

tailings (unsaturated zone) highly complex geochem-

ical processes take place (dissolution, precipitation of

secondary tertiary phases, redox, adsorption, etc.;

Alpers and Blowes, 1994; Jambor, 1994).

Other trace elements are in relatively very low

concentrations. It can be observed that despite the

different location, the trace element content is essen-

Table 1

List of minerals reported in the low sulfidation epithermal vein

mineralization of the Guanajuato District

Gold-

bearing

minerals

Silver-

bearing

minerals

Sulfide

minerals

Silicates Carbonates

Native

gold

Native

silver

Pyrite (+) Quartz (+) Calcite (+)

Electrum Acanthie

(+)

Chalcopyrite Adularia Dolomite

Aguilarite

(+)

Marcasite Amethyst Siderite

Polybasite

(+)

Sphalerite Chlorite Rhodocrosite

Pyrargyrite Galena Montmorillonite Ankerite

Naumanite Pearceite Nontronite

Tetrahedrite Chalcedony

Arsenopyrite ApophyllitePyrrhotite Datolite

Laumonite

Stilbite

Saponite

(Petruk and Owens, 1974; Oelsner, 2001). (+) Indicates the most

abundant minerals in each group.



9/21

tially the same. The uniformity in the vein mineralogy

and the mixing of different ores during the extraction

and mineral processing operations could well explain

this similarity in the different tailings.

4.2. Geochemistry of natural water

Fig. 3 shows the trace elements concentration in

water samples from: (a) Valenciana tailings pile pond

(surface water accumulated during the rainy season-

highly evaporated); (b) a water spring from La Luz

zone; (c) La Purisima water reservoir (dam); and (d)

groundwater from municipal water wells. The con-

centration of the different trace elements is generally

low, but the groundwater (samples from municipal

water wells) is higher in Zn (ranging from 0.03 to 0.5

ppm) than in other localities. Water is iron-rich in theValenciana pond (up to 2.2 ppm), but Fe concentration

decreases towards the groundwater (less than 0.05

ppm). Saturation index calculation using the code

PHREEQC indicates that ferrihydrite, being saturated

in these waters, is precipitated as mineral surface

coatings, thus creating a potential adsorbing substrate.

Water from a spring at La Luz area is relatively higher

in arsenic (0.01 ppm) than any other locality. All the

other samples are very low in arsenic and selenium

( < 0.01 ppm).

Major ion chemistry is dominated by bicarbonate,

sulfate and calcium, which very likely indicate dy-

namic near-surface processes such as dissolution of

calcite, dissolution of metallic sulfides, and dissolu-

tion-precipitation of sulfates. It is true that from the

chemical data available, it is not possible to distin-guish between sulfate derived from dissolution of

sulfides or sulfates, nor Ca from other sources (alu-

minosilicates reactions). But the X-ray diffraction data

help to confirm that the carbonates, sulfates and minor

sulfides would mostly control the major elements

hydrochemistry. Sulfate precipitation can be observed

on the surface of some of the tailings impoundments

(e.g. Valenciana) as a gypsum coating due to

evaporation of rainwater and precipitation of gypsum.

During spring 2001, some other water samples

were taken from water reservoirs (dams), water wells(groundwater) and mine-water (interior of Valenciana

and Torres mines). The major ions geochemistry is

shown in Fig. 4. The surface water from the reservoirs

(Soledad, Mata and Esperanza) presents the lowest

total dissolved ions (TDS: ranging from 140 to 210

ppm), and plots mostly in the central part and lower

section of the Piper diagram. The evaporation arrow

in Fig. 4 indicates very likely processes in the surface

water. The groundwater (wells 3, 12 and 14) has TDS

ranging from 470 to 560 ppm, very similar to mine-

Fig. 2. Content of trace elements in different tailings material from the Guanajuato area. Even though the locations are far apart, the general

concentrations of trace metals are very similar, reflecting the uniform mineralogy of the processed ore.



10/21

water from Las Torres mine. The groundwater and

mine-water plot on the central part of the Piper

diagram with a slightly increases in TDS (with respect

to surface water), indicating the deep infiltrationaccompanied of dissolution processes. The highest

TDS is for mine-water from Valenciana mine (range

between 800 to 4500 ppm). This water plots in the

upper corner of the Piper diagram and indicates

gypsum dissolution-dominated processes. The frac-

ture pattern in the area and the similarity between

surface water at Valenciana ponds and Valenciana

mine-water suggest a possible vertical flow from the

surface as a very possible mechanism of groundwater

evolution.

Minor element chemistry indicates low amounts ofFe, Ni, As, Se and Pb in most of the samples ( < 0.01

ppm). Groundwater (from water wells) is relatively

high in Zn and Cu (up to 1.5 and 0.3 ppm, respec-

tively). This fact could represent zinc derived from the

water pipes in the well or natural Zn in groundwater.

The Zn amount has been constantly high in most of

the groundwater samples for a 3-year period.

Fig. 3. Trace element content of natural water from different location of the Guanajuato area. Iron is the most abundant element. However, Zn

concentrations increase in the groundwater (D).



11/21

4.3. Isotopic behavior of the water samples

Samples for stable isotopes analysis (dD and d18O)

were taken from a variety of environments: mine-water

from Valenciana and Torres mines; three municipal

water wells (regional groundwater; well nos. 3, 12 and

14); and three water reservoirs (Soledad, Esperanza

and Mata dams). From Esperanza and Mata reservoirs,

water samples were taken from the surface and

depths of 2, 5 and 10 m below the surface. The

results are shown in Fig. 5. In the Torres mine-water,

the values range from d18O = 10.7xto 11.00x

and dD = 78.5xto 74.5x, whereas at the

Esperanza reservoir the values range fromd

18

O = 7.6xto 8.7xand dD = 56.5xto 62.9x.The isotopic data at the Esperanza reservoir do not

seem to reflect any evolution or change with depth.

The other samples lie between these two extremes.

According to Craig and Gordon (1965) and Kendall

and Caldwell (2000), the surface evaporation produces

isotopic fractioning, where the remaining water

becomes heavier, while the rain becomes lighter.

On dry climates, with low vapor content in the

environment, the isotope fractionation processes are

Fig. 3 (continued).



12/21

controlled mostly by evaporation. On the other hand,

in wet environments with high vapor saturation

(close to 100%) the vapor tends to be less fractionated

(Gat and Gonfiantini, 1981; Gat, 1996). It is consid-

ered that the Torres mine environment is close to 100%

of vapor saturation (deep inside the mine), and could

well be considered an isotopic closed environment.

On the other hand, the Esperanza reservoir is an

isotopic open system (allowing high evaporation)

with a much higher volume of water being evaporated.

The kinetic isotopic fractionation defines a slope for

the local evaporation curve of dD=5.93xd18O =

13.04x, determined by the water vapor content in the

environment.

Fig. 4. Piper diagram for water samples from groundwater (wells 3, 12 and 14), surface water reservoirs (dams: Esperanza, Soledad, Mata),

mine-water (Valenciana and Torres). The water general types fall between bicarbonate- and sulfate-rich type.



13/21

The groundwater flow in the mountainous region

(Valenciana, Torres, Esperanza and Soledad) is along

highly anisotropic, fractured igneous and metamorphic

rocks. It is assumed that the infiltration at Torres mine

is along a high hydraulic conductivity media that doesnot allow isotopic fractionation (the closed system

scenario), whereas at Valenciana the infiltration is slow

at the surface, allowing isotopic fractionation and

dissolution of gypsum (high SO4 and Ca content). At

the alluvial valley (groundwater wells) the water is a

mix of quick and slow infiltrated water that plots along

the evaporation line.

4.4. Geochemistry of leachates from humidity cell and

column experiments

Fig. 6 shows the trace element content in leachates

from humidity cell test (HCT) experiments for 10

weeks (after 10 weeks, the general trend of all metals

was to decrease). The filling material for the HCT

was taken from the tailings impoundment of Valenci-

ana (new and old; Fig. 6A and B, respectively),

tailings pile from La Luz (Fig. 6C) and tailings pile

at San Nicolas (Fig. 6D). Trace element content of

these solid materials is shown in Fig. 2 and discussed

in Section 4.1. The key objectives of the humidity

cell tests are: (a) long-term stable reaction under

kinetic conditions and (b) depletion times for acid-

generating, acid-neutralizing and meta-leaching min-

erals (Morin and Hutt, 1997). It is very true that

humidity cell data are not easy to interpret, but also itis true that these data, when proper handled, give

good estimates of heavy metal leaching potential and

acid mine drainage. Several efforts have been made

to estimates rates of weathering and to scale labora-

tory tests to predict field rates of weathering (Frostad

et al., 2000a,b). The details on the topic of evaluating

weathering rates are beyond the scope of this work,

and the humidity cell test data are used here for a

general comparison with the ABA test and the metal

concentration in surface and groundwater in the study

area. As in most tailings environments, the flowconditions at the different tailings piles in Guanajuato

are unsaturated. This fact implies that the water/solid

ratios would typically be less than 1:7 or 1:8 by mass

in contact (approximately 20% to 25% porosity, and

grain density of 2.6 g/cm3). Under these weathering

conditions, the general content of trace elements in

the leachates is quite low ( < 0.1 ppm). In general, all

the mine-waste materials decrease their leachate con-

centrations after 3 or 4 weeks, with sporadic spikes of

Zn, As and Se.

Fig. 5. dD vs. d18O plot for some water in the Guanajuato district. Samples were taken from a variety of environments: mine-water from

Valenciana and Torres mines; three municipal water wells (regional groundwater; well nos. 3, 12 and 14); and three water reservoirs (Soledad,

Esperanza and Mata dams); from Esperanza and Mata dams, water samples were taken from the surface and depths of 2, 5 and 10 m below the

surface. Esperanza and Torres water are the end-members of the fractionation processes.



14/21

The leachates from Valenciana new present the

lowest concentrations of metals from all the material

tested ( < 0.1 ppm for Cu, As, Se and Pb; < 0.001 ppm

Cd). Zinc concentrations present 2 weeks of relativelyhigh concentration (0.8 ppm), very likely derived from

some Zn within secondary or tertiary phases. The

leachates from Valenciana2 (old material) have a

general trend to decrease in concentrations of metals,

but with some peaks of Zn. Interestingly, the As

values have a high during week 10, indicating some

potential for arsenic release. Leachates from San

Nicolas present the same general trends of decreasing

concentration with time and some highs on Zn. Las

Luz leachates present the highest concentration of As,

but still this is low ( < 0.1 ppm).

The geochemical and mineralogical analyses, along

with results of the humidity cell test results, indicate

that there is a low potential for release of arsenic, zinc

and other heavy metals to the environment. The main

acidity is generated in the leachates by the oxidation of

metallic sulfide (Smith et al., 1994):

FeS2 3:

5O2 H2O Fe2

2SO2

4 2H

However, the near-neutral pH of the leachates

indicates that the acid-consuming reactions surpass

the acid-producing reactions, which is compatible

with the results of the ABA test (pH of oxidized

tailings between 7 and 9.5; Fig. 7). Other possible

minor acid-generating processes in the area are the

hydrolysis of Fe3 + and Al3 +:

Fe3 3H2O FeOH3 H

Al3 3H2O AlOH3 H

However, in order to have Fe3 + and Al3 + in the

system, the solution must be acidic. Even though

some oxide coatings can be observed in the tail-

Fig. 6. Trace elements in leachates of the humidity cell experiments (10 weeks). The concentrations of metal in the leachates from these

experiments are relatively low. The general trend is to decrease with time.



15/21

ings, the contribution to acidity of these reactions is

considered to be negligible.

On the other hand, the acidity generated by sulfide

oxidation would be neutralized by the following major

acid-consuming processes:

CaCO3 2H Ca2 H2O CO2

dissolution of calcite

KAlSi3

O8

H

7H

2O

K

3H

4SiO

4Al

OH3

dissolution of feldspar

The kinetics of the weathering of feldspar is very

slow compared with the rest of the reactions, so its

contribution to the consuming acidity process is

neglected. An acidity-budget calculation shows that,

at pH values between 3 and 5, for every mole of FeS2dissolved, 2 mol H+ is released. On the other hand, for

every mole of calcite dissolved, 2 mol H+ is consumed.

A simple estimation on the amount of sulfides, calcite

and oxide coatings present in the tailings indicates

that the acid-consuming processes surpass the acid-

generating processes by a factor of 12.

Zinc is a minor element in the system. X-ray

diffraction data do not show any zinc mineral phases.

But the chemical analyses indicate an average of 400

mg/kg Zn in the tailings material (see Section 4.1).

Zinc could be present in different secondary and/or

tertiary solid phases, besides the primary mineral

sphalerite. Even though a minor phase in the system,Zn is important because groundwater increases its

content with respect to the leaching experiments.

But the low Zn content of the leachates indicates

that: (a) a small amount of Zn phases is present in the

tailings material; and/or (b) a slow dissolution rate of

the Zn phases present, so there is insufficient time for

dissolution.

The concentration of trace elements in the leach-

ates is relatively low when compared with leachates

from other type of sulfide-rich ore deposits. Fiklin

Fig. 6 (continued).



16/21

diagrams (sum of Zn, Cu, Cd, Pb, Co and Ni vs. pH)

of the leachates show the low concentrations and

near-neutral pH characteristic of quartzcalcite vein

ore deposits (Plumlee, 1999). Column experiments

described in Section 3 indicate that the results are

similar to the HCT leachates for all the material

tested.

4.5. Natural leaching from the tailings

Water samples at San Nicolas area were taken

directly from the base of the tailings piles, from a

creek flowing directly from the tailings pile, and

from a creek formed by the mixing of upstream,

unpolluted waters, and the creek from the tailings, as

it flowed to Mata reservoir. The sampling was done

after several heavy rainstorms (natural leaching). The

samples were taken at the beginning and the peak of

the rainy season (April August). Fig. 8 shows the

analytical data from this water samples. It was

determined that water downstream of the tailings pile does not contain significant concentrations of

heavy metals. All the samples have a similar com-

position in major and minor elements, with Fe and

Mn being the trace elements with the highest con-

centration (1 to 2 ppm). Manganese is ubiquitous in

most of mineralized terrains. But also, it is known

that Mn balls were used to crush the ore, so it is

assumed that Mn in sediments and leachates could

be derived from both natural and anthropogenic

sources. Iron seems to be derived from a natural

source, probably iron sulfide. The variation between

the two sampling seasons is very small and only Fe

and Mn show significant concentrations in both

sampling.

Major ion chemistry indicates that dissolution of

calcite and minor sulfides and gypsum (gypsum is

present mostly as hardpans on the tailings impound-

ments and it is not rare to find gypsum in fractures

in tunnels in the mines) controls the hydrochemistry

of the region. Modeling with PHREEQC shows thatthe stream water is never saturated with respect to

gypsum or calcite. Total dissolved solids is higher

in samples near the tailings pile, whereas pH is

neutral and remains between 7.1 and 8.3 in all the

samplings.

The low concentrations in trace metals can be

attributed to several factors: low concentration of

metal in solid tailings; not enough time for dissolution;

metal fixed to insoluble phases; pH of the water near

neutral, which is high enough for most metals to be

insoluble. Iron is high enough to precipitate ferrihy-drite (PHREEQC modeling), which in turn becomes a

good adsorbing surface for many metals.

4.6. Adsorption of heavy metals by natural material

Fig. 9 shows the results of the different adsorp-

tion experiments. Fig. 9A shows the percent

adsorbed by the natural material vs. pH on Cd.

The topology of the curves is appropriate for cation

adsorption (higher adsorption at high pH and lower

Fig. 7. Diagram indicating the pH variations during the humidity cell experiments. The pH remains near neutral due to the high acid-consuming

capacity of the material in the tailings (high ratio calcite/sulfides; see text for details).



17/21

adsorption at low pH). The material is relatively

rich in Fe2O3 content (f 10%). The high adsorp-

tion (f 100%) even at pH 7 is probably due to the

highest solid/solution ratio used (total Fe = 0.01 M).

This experiment was in NaNO3 0.1 M background

electrolyte. Fig. 9B shows the adsorption of Pb by

the natural material. A slight decrease is observed.

This experiment used the same amount of solid

total Fe phase (0.01 M) as the first set of experi-

ments for Cd. Fig. 9C and D shows the sameexperiment but for Cu and Zn, but a lower ionic

strength was used (I= 0.01). The four experiments

look very similar even though they were conducted

in solutions with different ionic strengths.

Arsenic adsorption experiments were also con-

ducted (not shown in Fig. 9), but in this case, the

highest percent of adsorption obtained was about also

100%, but the lowest adsorption dropped to near 60%.

This is probably due to the presence of arsenite

(H3AsO3) besides arsenate (H3AsO4) in the solution.

It has been shown that arsenite is less adsorbed onto

iron oxy-hydroxide surfaces than arsenate under sim-ilar conditions (Dzombak and Morel, 1990), but the

natural environment in Guanajuato is highly oxidized,

Fig. 9. Experimental results from the adsorption experiments of Cd, Cu, Pb and Zn onto natural material from the Guanajuato mining area. The

natural material is relatively rich in iron and has very high capacity for adsorbing dissolved metals. Experiments were carried out with a NaNO3 background electrolyte (I= 0.1 and I= 0.01). See text for details.

Fig. 8. Trace metals content in natural leachates. The samples were taken directly at the base of the tailings piles after heavy rain to assure

natural leaching from the tailings. The concentrations are lower than the experiments of the humidity cell, indicating quick infiltration with no

time for dissolution of metal phases contained in the tailings material.



18/21

so it is assumed that most of the arsenic must be

arsenate (higher adsorption).

Even though these experiments have relatively few

points, the results show the expected topology forcations and for total As (arsenate + arsenite) adsorption

curves and show consistency in the maximum and

minimum adsorption ends. A final experiment was the

surface titration of the natural aquifer material. A blank

titration was used to correct suspension titrations for

supernatant effects (Machesjy and Anderson, 1986).

The results for the surface titration indicate that iron

oxy-hydroxides are the most likely adsorbing surface

in the environment. However, the relatively abundance

of calcite and clays indicates that some of the adsorb-

ing processes are controlled by the surface complex-

ation reactions.

In summary, experiments for adsorption of differ-

ent metals onto natural sediments of the Mata basin

alluvial plain indicate the high adsorbing capacity of

the sediments at near-neutral pH. These adsorbing

processes, along with the low metal concentration of

heavy metals in the natural leachates, explain the very

low concentrations in the natural waters.

5. Discussion and conclusions

The chemistry and mineralogy of the different

tailings piles at Guanajuato reflect the vein mineral-

ogy of the mining district. The tailings material is

composed mostly of quartz, feldspar and calcite.

Pyrite is the most common sulfide, but represents

less than 0.1%, and the base metals are present in

very low concentrations. The combination of all the

analytical and experimental results indicates that the

mine-waste material has very low potential for gen-

eration of acidity (AMD) and leaching of heavy

metals to the environment. The ratio of carbonatesto sulfides is 12:1, indicating that acidity generated

by sulfides oxidation would be buffered by dissolu-

tion of calcite. The humidity cell experiments, even

though they increase the amount of water and O2through the material by a factor of 8 with respect to

the natural environment of the tailings material,

indicate very low potential for leaching metals. Nat-

ural leaching is also very low for most trace metals,

and the chemistry of the natural waters is controlled

by dissolution of calcite and gypsum. Previous local

environmental studies conducted by Blumberg (1992)

and Oelsner (2001) in Las Torres mine and San

Nicolas area concluded that acidity and leaching of

heavy metals are not problems of concern. Further-more, the work by Rivas-Solorzanao (2001) demon-

strated that the natural sediments in the Mata basin

alluvial plain have a high capacity for adsorbing the

low metal concentrations of Zn, Pb, Cu, Cd and As in

solution. However, trace elements chemistry of

groundwater some 15 to 20 km southwest of the

mining area and tailings piles indicate relatively high

Zn concentrations (up to 1 ppm). This high Zn

area is located on a continuous groundwater flow path

from the mineralized area. But so far, there is not data

on the groundwater velocity in the system. The

possibility of a long travel time (longer than 200

years, which is the average time for the most impor-

tant tailings piles in the area) for the groundwater

from the mineralized area towards the high Zn area

was explored with a rough calculation for the velocity

of the groundwater (Table 2). This calculation could

be improved with field data for different water-wells,

which currently are not available. Because the lowest

and highest values of hydraulic conductivity (K) and

porosity (n) used in the calculation (Table 2) are the

extreme cases, a good estimate for the total time for

the groundwater to reach the high Zn area wouldbe in the range of 50 to several hundred years. These

velocities indicate that there has been enough time for

Zn to reach the area for the groundwater wells. But, if

not high Zn comes from the tailings piles, the

possibility that Zn concentrations in groundwater

are due to natural processes still remains. So, the

key issue remaining is to explain Zn concentrations in

Table 2

Estimated travel time for groundwater flowing from the mineralizedarea in the Guanajuato mountains (Mines zone) towards the alluvial

valley where the groundwater wells are located (Valley zone)

Mines zone, dh/ds = 0.02 Valley zone, dh/ds = 0.01 Total time

K (m/s) n

(%)

Time

(year)

K (m/s) n

(%)

Time

(year)

(year)

1e08 2 27,059 1e 06 15 19,422 46,4811e06 4 541 1e 04 25 324 8651e04 10 14 1e 02 40 5 19

K= hydraulic conductivity; dh/ds = hydraulic head gradient; n

(%) = percent porosity.



19/21

Guanajuato groundwater despite the low leaching of

Zn from tailings.

Isotopic data (dD and d18O) of water samples from

different environments in the area indicate infiltration

of meteoric water into a highly anisotropic, fracturedmedia in igneous and metamorphic rocks before reach-

ing the granular aquifer in the alluvial valley. The

meteoric water that infiltrates can be evaporated (high-

ly fractionated) as is the water from the Esperanza

dam, or quickly infiltrated (non-fractionated) as the

case of the Torres mine-water. This variation in time of

infiltration could cause dissolution of Zn sulfides at

depth. Buchanans (1980) mineralization model pre-

dicts an increase in base metals at depth in the district.

So, it is reasonable to assume that Zn sulfides increase

below the current mining operations. The Zn concen-trations in groundwater from municipal wells thus can

be interpreted as being derived from Zn-rich natural

sources rather than from Zn-poor mine-waste material.

Fig. 10 shows an schematic representation of the main

environmental geochemistry processes taking place in

the Guanajuato mining district. Infiltration of meteoric

water through the Zn-poor tailings dissolves some As

and Se phases. The surface runoff is relatively poor in

trace elements (base metals). Surface complexation of

Fe oxy-hydroxides, calcite and clays and base metal

(including As) adsorption onto these adsorbing surfa-

ces control the mobility of trace elements in the near-

surface environment. However, slow and deep infil-

tration of water into a relatively Zn-rich environment

leads to dissolution of Zn phases and consequentincreases of Zn in groundwater extracted in municipal

water wells.

Acknowledgements

This work was supported by grant no. 27821-T

from CONACyT-Mexico. The authors wish to thank

the detailed and helpful reviews by Drs. Mark J.

Logsdon and Richard Kyle. The authors also thank the

enthusiastic assistance of Jorge Servin and CarolinaMunoz in the field and laboratory works, and Eloisa

Dominguez-Mariani for her skillful hand in drawing.

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