Geochemical distribution of arsenicin waters, sediments and weatheredgold mineralized rocks from IronQuadrangle, BrazilR.P. Borba Æ B.R. Figueiredo Æ J. Matschullat

Abstract The Iron Quadrangle has been the sceneryof the most important gold production in Brazil. It isestimated that during the three centuries of goldmining in the Iron Quadrangle, at least 390,000 t ofarsenic was discharged into the drainage system. Thisstudy presents geochemical data for the three riverbasins in the region, with focus on surface water andstream sediment monitoring. Samples of primary andoxidized sulfide ores as well as of tailings andgroundwater from the major gold mines were alsostudied. The highest As concentrations in water andstream sediments occur in the vicinity of miningareas. In surface water, up to 300 lg As/l were foundwhereas the As contents in stream sediments were inthe range of 20 to 4,000 mg/kg. The As3+/As5+

concentration ratios obtained for some water samplesrange from 1.10)1 to 4.10)2. The As mobilityassociated with ore-deposit weathering could betraced in some closed gold mines by observation of in-situ pyrite and arsenopyrite oxida-tion, precipitationof scorodite and gippsite, As adsorption ontogoethite, and final liberation of As into undergroundand surface waters. This process is likely to producelarge volumes of mine effluents containing total Asand trivalent As up to 1,960 and 60 lg/l, respectively.River sediments and tailings pile samples weresubmitted to a leaching procedure showing maximal

arsenic release from 1 to 4% of the original total Asin the samples. There are potential risks for Ashazards in some areas induced by, for instance, thedispersion of old tailings by flooding, occupation ofpoisoned soils for settlements, and occasional con-sumption of contaminated surface and groundwater.

Keywords Arsenic Æ Water Æ Sediment Æ Goldmining Æ Brazil

Introduction

Since the end of the 17th century, the Iron Quadrangle inthe State of Minas Gerais, Brazil (Fig. 1a), has been themost important gold producing region in Brazil. Gold wasinitially produced from alluvial placers and oxidationzones and in modern times from primary ores. Total goldproduction must have exceeded 1,300 t in all history. Inthe world-famous Morro Velho mine alone, around 600 tgold has been produced since 1834. In this ore deposit aswell as in the other Iron Quadrangle gold deposits, the oreis rich in As, with As/Au ratios ranging from 300 to 3,000(Vial 1988; Abreu and others 1988). Since the tailingsproduced in the past were mostly discharged into ornearby the drainage systems, the amount of arsenic whichwas released to the environment must have exceeded390,000 t in the whole region.In the southeastern portion of the Sao Francisco Craton,the Iron Quadrangle encompasses four main lithostrati-graphic units (Dorr 1969): (1) the granite-gneissicmetamorphic complexes; (2) the Archean greenstone beltsequence, known as the Rio das Velhas Supergroup; (3)the Paleoproterozoic metasedimentary and metavolcanicrocks of the Minas Supergroup; and (4) the metasedi-mentary rocks of the Itacolomi Group. The main golddeposits are found along shear zones which cut thebasic volcanic rocks and banded iron formations of theNova Lima Group, in the lower Rio das Velhas Super-group. Examples of those are the Morro Velho, Rapososand Cuiaba gold deposits (Nova Lima district) and theSao Bento ore deposit (Santa Barbara district) which

Original article

DOI 10.1007/s00254-002-0733-6 Environmental Geology (2003) 44:39–52 39

Received: 19 August 2002 / Accepted: 22 October 2002Published online: 17 December 2002ª Springer-Verlag 2002

R.P. Borba (&) Æ B.R. FigueiredoInstitute of Geosciences, State University of Campinas,P.O. Box 6152, 13083-970 Campinas, Sao Paulo, BrazilE-mail: borba@ige.unicamp.brTel.: +55-19-32564273Fax: +55-19-32891562

J. MatschullatTechnical University of Freiberg, Brennhausgasse 14,09599, Freiberg, Germany

contain appreciable amounts of pyrite, pyrrhotite andarsenopyrite mainly associated with banded ironformations (Vieira 1988, 1991; Ribeiro-Rodrigues 1998).

Another situation is exemplified by the Passagem deMariana gold deposit (Ouro Preto-Mariana district),close to the contact between the rocks of the Nova LimaGroup and those of the Minas Supergroup. This oredeposit contains major amounts of arsenopyrite, eitherin quartz and carbonate veins or disseminated inbanded iron formation, tourmalinite, phyllite and

40 Environmental Geology (2003) 44:39–52

Fig. 1a The Iron Quadrangle and study areas localization map. b Maps ofthe Velhas River basin, c the Carmo River basin, and d the ConcecaoRiver basin, with sampling sites indicated

Original article

quartzite (Vial 1988; Oliveira 1998). A number of ancientgold mines can be found in the Ouro Preto-Marianadistrict, among these the Chico Rei, Scliar and Lajes oredeposits where gold mineralization looks like that ofthe Passagem mine, although in the former the arseno-pyrite-bearing quartz veins without carbonate are moreimportant and frequent than in the latter (Cavalcanti1999).Only a few geochemical studies, focusing on arsenic, werecarried out so far in the Iron Quadrangle. Among theseprevious studies, most of them specifically designed formineral prospecting, is the excellent stream sedimentgeochemical survey sponsored by the Brazilian ResearchCompany of Mineral Resources in the late 1970s (Oliveiraand others 1979). More recently, some information on Asdistribution in the region has been made available forenvironmental evaluation (Eleuterio 1997; Rawlins andothers 1997; Borba and others 2000) and for humanbiomonitoring (Matschullat and others 2000) in certainplaces.The paper aims to contribute to a better understandingof the geochemical distribution of As in the mostimportant Brazilian gold province. The three fluvialbasins are of interest because of their proximity to themost important gold mining districts: the Velhas Riverbasin within the limits of the Nova Lima district (Fig.1b), the Carmo River basin (Ouro Preto-Mariana district,Fig. 1c), and the Conceicao River basin (Santa Barbaradistrict, Fig. 1d). The potential risks of environmentaland human As contamination in the Iron Quadrangle arediscussed by comparing the analytical data with theobserved geochemical processes which are responsible forAs mobility in surface and near-surface environments intropical regions.

Materials and methods

Surface water and stream sediments were collected at thepreviously selected sites during two different samplingcampaigns carried out in April 1998 (summer) and in July1999 (winter). In addition, samples of primary and oxi-dized ores as well as of the tailings and groundwater werestudied to assess the environmental risks at specificpollution point sources.At every sampling station, along the main rivers andsome tributaries, membrane filtered (0.45 lm) andnon-filtered water samples were collected and thephysico-chemical parameters for water, pH, Eh, electricalconductivity, dissolved oxygen (DO), turbidity andtemperature were measured in situ. The total alkalinity ofthe bicarbonate of the unfiltered water was determined bytitration using 0.1 N sulfuric acid and bromocresolgreen indicator. Filtered water samples were analyzed byICP-OES (cations) and ion chromatography (anions),whereas the dissolved total As as well the As3+ concen-trations in filtered water were determined by AAS with acoupled hydride generator (HG-AAS). The requiredspeciation method was improved by one of the authors

(R.P.B.) based on the analytical procedure suggested byShraim and others (1999). The As3+ determination wasperformed using the HG-AAS method and consisted inmodifying the concentrations of HCl and NaBH4 solu-tions in such a way that only As3+ present in the watersample reacted with the solutions to produce arsine(AsH3).The sediment samples were dried naturally and passedthrough nylon sieves to separate the granulometricfractions below 63 lm which were then analyzed byX-ray diffraction for mineral determination, and byX-ray fluorescence spectrometry for their major-, minor-and trace-element contents. A number of samplescollected from recent and old tailings were also analyzedby these methods. At Unicamp’s laboratory the lowestdetection limits for trace-element analysis by XRF lie inthe interval of 1–8 mg/kg. A variant of the ‘‘toxicitycharacteristic leaching procedure’’ (TCLP) by U.S. EPA(Anon 1986) was applied to estimate the As bioavail-ability in river sediments and tailings. The methodadaptation consisted in taking a lesser amount of sample(5 g of <63 lm material) for weak acid attack,maintaining, however, the recommended solid/solutionrelation as well as the reaction time. Extraction solutionswere analyzed by HG-AAS and the lowest detection limitwas 1 lg/l.Finally, a selected number of thin and polished sections ofprimary gold-sulfide ores and oxidized products wereexamined under the petrographic microscope and thescanning electronic microscope (SEM) with coupled EDSdetector in order to characterize the mode of occurrence ofAs in these materials as well as to access the meaningfulmineral reactions associated with the weathering of the oredeposits.

Results

The composition of surface water and stream sedimentsfor each river basin is presented in this section. For somelocalities, the chemical compositions of groundwater havealso been determined and discussed. In addition, infor-mation about the As speciation in water and on the bio-availability of As in stream sediments is given. Finally, theAs release into aqueous media in the Ouro Preto-Marianaarea is described in terms of in-situ arsenopyrite andpyrite oxidation phenomena assessed by petrographicand SEM studies.

River water and sedimentsThe hydrogeochemistry and the chemical composition ofriver sediments in the Iron Quadrangle are presentedbelow for the three main river basins: (1) the segment ofthe Velhas River under the influence of the Morro Velho,Raposos and Cuiaba gold mines, and of the Queiros orebeneficiation and smelter complex; (2) the Carmo Riverwhich has been affected in the past by the activities of thePassagem gold mine and of the old mine works (e.g.,Chico Rei, Scliar, Quartzito and Lajes) in the Ouro Preto

Environmental Geology (2003) 44:39–52 41

Original article

area; (3) the Conceicao River which has been influencedby the Sao Bento and Corrego do Sıtio gold mines and, toa lesser extent, by the surveys at the Brumal mineralprospect.

The Velhas River basinIn the Velhas River basin (Fig. 1b), water and sedimentsamples were collected at stations along the Velhas River(ERV1 to 6), and at the tributaries Cardoso and Queiroscreeks (EGA1 and ERQ1) which run from the Morro Velhomine and from the Queiros beneficiation and smelterplant, respectively, towards the Velhas River. Anotherstation was at the Sabara River which drains the Cuiabamining area and also runs towards the Velhas River. Thechemical composition of water and sediments arepresented in Tables 1 and 2.At the time of sampling, the river waters were circum-neutral and oxidized, showing a minimum pH of 6.3(EGA1) and Eh values varying between 85 (EGA1) and230 mV (ERV1). The conductivity ranged from 65 to 150lS/cm in the Velhas River and the Cardoso Creek butreached 2,140 lS/cm in the Queiros Creek (ERQ1).A general feature is the predominance of HCO3

) in relationto SO4

2) and of Ca and Mg in comparison with othercations. This can be explained by the occurrence ofextensive carbonate alteration associated with the golddeposits, which has prevented the generation of acid minedrainage in the area. An exception is represented by theQueiros Creek where the concentrations of SO4

2) andCa exceed many times those of other anions and cationsdue to the influence of runoff waters from the Queirostailings ponds.The heavy-metal concentrations in water are low. OnlyAs exceeds its threshold value of 50 lg/l as established bythe Brazilian environmental regulations (CONAMA 1986),and this only at the Cardoso (349 lg/l) and Queiroscreeks (112 lg/l; Fig. 2a).The stream sediment samples exhibited elevated concen-trations for As and some other trace metals. Arsenicconcentrations exceeding 1,000 mg/kg were obtained insamples ERV3, EGA1 and ERQ1 (Fig. 2b) whereas Pb, Znand Cu reached maximum concentrations of 35 mg/kg(EGA1), 167 mg/kg (ESAB1) and 126 mg/kg (ERQ1),respectively. The fluvial sediments collected closer to theMorro Velho mine area showed elevated contents of Fe, Sand base metals, probably due to the influence of the Galotailings pile (Fig. 1b), an old As-rich waste deposit at themargin of the Cardoso Creek. Two samples of this exposedmaterial yielded As concentrations exceeding 5,000 mg/kgas well as 590–1,070 mg/kg Cu, 120–145 mg/kg Zn, and50–60 mg/kg Pb. The occurrence of goethite, ironhydroxide, gypsum, epsomite, hydroniumjarosite andarsenolite was indicated in those samples by XRDdeterminations.Thus, the following main As and metal point sources forwater and stream sediment were identified: (1) the oldtailings piles of the Morro Velho and the Raposos minesat the Cardoso Creek and the Velhas River, and (2) theQueiros Creek which drains the Queiros mining wasteponds also in the Nova Lima district. On the other hand,

42 Environmental Geology (2003) 44:39–52

Tab

le1

Wat

erch

emic

alco

mp

osi

tio

no

fth

eV

elh

asri

ver

bas

in(c

on

cen

trat

ion

sin

mg/

l,ex

cep

tfo

rA

sin

o¥g

/l;

ww

inte

r,s

sum

mer

,d

.l.d

etec

tio

nli

mit

s)

Stat

ion

sD

.l.E

GA

1E

RQ

1E

RV

1E

RV

2E

RV

3E

RV

4E

RV

5E

RV

6E

SAB

1

WS

WS

WS

WS

WW

WW

W

pH

6.3

7.5

6.7

7.6

6.6

7.9

6.6

7.9

6.2

7.2

6.4

6.7

7E

h(m

V)

8511

511

218

519

023

019

012

621

021

421

222

517

7C

on

d(l

S/cm

)18

415

12,

140

1,40

012

490

8370

6513

011

612

898

T(�

C)

1820

2021

1918

.818

.318

.518

19.2

19.5

2019

.7C

a0.

0513

1138

125

614

98

67

157

149

Mg

0.3

65

4645

43

33

35

25

4N

a0.

18

770

384

43

32

43

35

K0.

12

310

81

11

0.7

0.6

10.

80.

81

HC

O3

7433

1215

3518

3517

3131

3931

35C

l0.

12

82

516

12

15

71

22

SO4

0.2

2616

1,04

675

863

168

73

61

2820

NO

30.

21

29

1312

11

11

71

82

F0.

005

<d

.l.0.

20.

11

<d

.l.0.

10.

10.

1<

d.l.

<d

.l.1

<d

.l.<

d.l.

PO

40.

050.

10.

20.

1<

d.l.

0.1

<d

.l.0.

1<

d.l.

0.1

0.1

10.

10.

1M

n0.

001

0.4

0.4

0.9

1.8

0.2

0.1

0.1

0.1

0.1

0.2

0.1

0.2

<d

.l.F

e0.

006

0.5

0.7

0.1

<d

.l.0.

10.

20.

10.

20.

1<

d.l.

<d

.l.0.

10.

2A

sT

ota

l0.

834

916

011

239

6412

5013

334

34<

d.l.

4

Original article

the highest concentrations of As and metals in water werefound in the dry season (winter), whereas the highestmetal concentrations in sediments were favored in thesummer probably by the erosion of poisoned soils andtailings and the reworking of stream sediments byflooding.

The Carmo River basinThe water at the Carmo River (Fig. 1c) yielded pH valuesin the interval 7–8 and Eh ranging from 170 to 240 mV.Electric conductivity in the water varied from 117 to 570lS/cm, the major electrolytes being Ca, Na, HCO3

) andSO4

2) (Table 3).Water samples collected in the interior of the Passagemgold mine (EPM1) and from a lateral mine adit wereslightly more alkaline, with pH in the interval 7–8.3, Eh of128–220 mV, and conductivity oscillating between 240–280 lS/cm at EPM1 and 82–430 lS/cm at the adit. Thesemine waters are rich in Ca and Mg, and HCO3

) and SO42)

are the predominant anions. In the mines in Ouro Preto(Chico Rei, Scliar, Quartzito and Lajes), the water yields apH slightly more acidic in the interval 5.5 to 7.3, Ehvarying from 130 to 215 mV and a low electric conduc-tivity oscillating from 15 to 70 lS/cm.The river waters are extraordinarily enriched in Na,probably due to seepage discharge, and display two groupsof samples, one carbonate rich and another with equiva-lent concentrations of HCO3

) and SO42), whereas in natural

groundwater a predominance of Ca and Mg and homo-geneous compositions for HCO3

) and SO42) were found.

The contents of Cu, Pb and Zn in filtered surface water andmine groundwater were lower than the concentrationsestablished by CONAMA (1986) for non-treated drinkingwater, whereas the As concentration in surface waterreached a maximum of 43 lg/l along the Carmo River(Fig. 2c).Running water in the interior of gold mines were alsosampled. In the Passagem mine (EPM1) and in the lateraladit, elevated As concentrations as high as 2,980 lg/l werefound whereas in the Ouro Preto mines the As contentsvaried from 12 to 260 lg/l. However, some water samplesfrom the Passagem mine yielded extremely low As con-centrations (4 to 20 lg/l) because they have been collectedat certain sites, very rich in carbonate espeleothemes,where the descending water did not percolate the miner-alized rocks. By contrast, the highest As concentration(2,980 lg/l) was found in water running from a mineral-ized vein. Between these two extremes are the As contentsin water from a lake (around 1,800 lg/l) which representsthe water-table level in the mine and from the adit (800 lg/l)which runs directly into the Carmo River (Fig. 2c).The stream sediments along the Carmo River showedextremely high As contents, reaching 4,700 mg/kg in thevicinity of the Passagem gold mine (ERC2 in Figs. 1c, 2d).The highest concentrations for other metals, as shown inTable 4, are 104 mg/kg Cu, 36 mg/kg Pb and 151 mg/kg Znalso at station ERC2.The highest metal concentrations were found in samplescollected in the winter when the dilution effect isless pronounced. Although the volume of running

Environmental Geology (2003) 44:39–52 43

Tab

le2

Ch

emic

alco

mp

osi

tio

no

fV

elh

asb

asin

sed

imen

ts(u

nit

s:o

xid

esin

%,

elem

ents

inm

g/k

g;w

win

ter,

ssu

mm

er,

d.l.

det

ecti

on

lim

its)

Sta-

tio

ns

D.l.

ER

V3

EG

A1

ER

V2

ER

Q1

ER

V1

ER

V5

ER

V6

ESA

B1

ER

V4

WS

WS

WS

WS

WS

WS

WW

SW

S

SiO

244

5055

4954

4766

6251

4346

5541

5958

4244

TiO

21.

10.

90.

81

11

0.7

0.6

11

10.

81

11

11

Al 2

O3

2623

1322

2218

1411

1926

2219

2012

1726

26F

e 2O

318

2016

2022

2010

917

2018

1622

1624

1919

MgO

0.8

1.2

11

0.8

0.8

0.8

1.2

0.7

0.9

0.8

1.1

0.9

0.7

0.8

0.8

0.9

CaO

0.2

0.2

0.6

0.4

0.2

0.3

0.3

0.3

0.2

0.2

0.3

0.5

0.3

0.3

0.4

0.3

0.2

Na 2

O0.

10.

20.

20.

10.

10.

20.

20.

20.

10.

20.

10.

20.

20.

20.

10.

10.

1K

2O

42

22

23

21

34

42

32

24

3P

2O

50.

20.

21

0.1

0.1

0.3

0.2

0.05

0.2

00.

20.

20.

50.

30.

10.

20.

2T

ota

l94

.597

.490

.995

.910

1.7

91.2

9584

.993

94.1

93.4

94.2

89.8

91.1

101.

694

.294

.5A

s1

341,

143

2,11

32,

830

8119

41,

064

1,26

410

368

121

190

360

6447

133

111

Cr

225

030

836

664

024

828

539

762

524

029

923

942

130

826

530

827

434

1C

u1

6663

116

7557

8412

669

5271

7082

9554

5374

72M

n3,

272

2,28

084

82,

261

1,84

42,

168

736

954

2,61

658

32,

920

1,62

02,

600

896

1,52

23,

176

4,39

1P

b2

1932

3521

1922

259

1531

1921

2318

1421

32S

840

31,

077

4,15

976

226

684

21,

290

768

711

403

385

847

1,23

21,

034

365

497

342

Zn

199

101

122

9890

131

6351

8312

611

210

016

416

710

811

312

5

Original article

groundwater in the interior of the Passagem mine isalso influenced by seasonal variation, no related signi-ficant change in As concentration was detected in thegroundwater.

The running waters from the Ouro Preto mines and fromthe Passagem mine are As point sources to the CarmoRiver and, of these, the adit of the Passagem mine is themost noticeable in the area.At the sampling stations closer to the Passagem mine, theriver sediments yielded the highest metal concentrationsin the winter, which suggests that deposition of fine-grained material is being favored by the low water flow at

44 Environmental Geology (2003) 44:39–52

Fig. 2a–fAs concentrations in filtered water and stream sediments from theVelhas River basin (a, b), the Carmo River basin (c, d) and theConceicao River basin (e, f)

Original article

Environmental Geology (2003) 44:39–52 45

Tab

le3

Wat

erch

emic

alco

mp

osi

tio

no

fth

eC

arm

ori

ver

bas

insu

rfac

ew

ater

and

gro

un

dw

ater

(co

nce

ntr

atio

ns

inm

g/l,

exce

pt

for

As

inlg

/l;

ww

inte

r,s

sum

mer

,d

.l.d

etec

tio

nli

mit

s,n

.d.

no

td

eter

min

ed)

Stat

ion

sfo

rsu

rfac

ew

ater

Stat

ion

sfo

rgr

ou

nd

wat

era

Pas

sage

mM

ine

Ou

roP

reto

min

es

ER

C1

ER

C3

ER

C4

ER

C5

ER

C6

ER

C7

EP

M1

EP

M2

EP

M3

EP

M4

EP

M5

EO

P1

EO

P2

EO

P3

EO

P4

WS

WS

WS

WS

WS

WS

WS

WS

WW

WW

WW

W

pH

7.4

7.8

7.5

7.6

6.9

7.4

7.8

7.4

7.4

7.3

7.2

7.4

7.7

7.7

7.7

87.

27.

47.

97.

36.

965.

566

Eh

(mV

)20

518

622

818

917

217

317

320

221

219

424

019

418

912

818

015

816

018

017

023

028

131

513

0

Co

nd

(lS/

cm)

570

362

n.d

.28

728

417

328

319

323

111

722

715

137

243

024

025

318

037

149

070

1751

15

T (�C

)20

.324

1923

.221

24.5

22.9

29.4

18.5

24.9

24.4

26.7

20.6

2219

23.1

21.0

20.0

1919

.015

.315

18

Ca

1512

2911

108

58

97

87

4140

2422

2147

516

23

2M

g4

413

44

31

33

33

322

2314

1214

3032

41

21

Na

7549

5036

2320

2318

3016

3515

24

62

0.5

34

0.5

0.6

20.

2K

33

42

22

12

22

22

310

33

11

61

0.4

10.

2H

CO

366

4763

3447

3739

2835

2335

3812

190

9455

7423

438

112

84

12C

l4

75

49

38

35

11<

d.l.

311

55

21

43

31

41

SO4

179

816

6492

3541

3159

301

2752

7042

3338

1010

192

101

NO

34

98

56

23

35

41

44

93

21

0.3

0.1

32

51

F0.

30.

4<

d.l.

0.4

<d

.l.0.

3<

d.l.

0.2

<d

.l.0.

2<

d.l.

0.3

0.1

0.3

<d

.l.0.

20.

10.

1<

d.l.

<d

.l.<

d.l.

<d

.l.<

d.l.

PO

40.

10.

30.

40.

10.

10.

10.

10.

10.

1<

d.l.

0.2

<d

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1<

d.l.

0.1

<d

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d.l.

<d

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d.l.

0.2

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d.l.

<d

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n0.

80.

46

0.6

1.2

0.3

0.1

0.6

0.4

0.2

0.3

0.1

<d

.l.<

d.l.

<d

.l.<

d.l.

0.01

0.01

<d

.l.0.

2<

d.l.

<d

.l.<

d.l.

Fe

0.1

0.1

<0.

10.

10.

20.

1<

0.1

0.1

0.1

0.1

0.1

0.1

<0.

1<

0.1

<0.

1<

0.1

0.1

0.1

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.l.0.

1<

d.l.

<d

.l.<

d.l.

As T

o-

tal

127

2226

4332

3917

2418

2311

830

757

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01,

932

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04

2026

024

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As3

+–

–2.

5–

––

––

––

––

35–

59–

861

–12

––

–A

s5+

––

19.5

––

––

––

––

–79

5–

1,70

1–

2,89

43

–24

8–

––

aSt

atio

ns:

EP

M1,

adit

;E

PM

2,la

ke;

EP

M3,

wat

erfr

om

min

eral

ized

vein

;E

PM

4,w

ater

fro

mm

ine

roo

f;E

PM

5,w

ater

fro

mm

ine

wal

l;E

OP

1,C

hic

oR

eim

ine;

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P2,

Qu

artz

ito

min

e;E

OP

3,Sc

liar

min

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OP

4,L

ajes

min

e

Original article

that season. The very high As concentrations in thesesediments point to the occurrence of sulfide phases amongthe clay and oxihydroxide particulates and, to a certainextent, to the occurrence of alluvial gold which is re-sponsible for the activity of "garimpeiros" still observed inthis period of the year at that river segment.

The Conceicao River basinThe surface water in the Conceicao River basin (stationsEBR1 to 8) yielded pH values in the range 5.7 to 7.5, Eh of130–280 mV, and electric conductivity between 21 and 112lS/cm (Table 5). At station EBR9 in the Carrapato Creekwhich drains the Sao Bento mining area, a more acidicwater was found with pH 4, Eh 280 mV and a conductivityof 220 lS/cm. In the upper segment of the Caraca River,which runs through the Caraca ecological park (stationECAR1), the low pH (4.6) and conductivity (10 lS/cm)may be related to the contribution of organic acidsproduced by the rich vegetation of the area as suggestedelsewhere by Drever (1997), and to the low buffer capacityof the quartzite bedrock.In the Brumal area, groundwater was sampled at five siteswhere the water is collected from springs or wells fordomestic use (stations EBR10 to 14). These waters havepH, Eh and conductivity values in the intervals 4–7,130–270 mV, and 6–67 lS/cm, respectively.For the whole region, total As concentrations in river waterand groundwater were low and do not exceed 8 lg/l insurface and 3 lg/l in subsurface waters. The only exceptionwas the water sampled at the Carrapato Creek (stationEBR9) which contained 74 lg/l As (Fig. 2e).Ca and Mg, present in equal proportions, are the majorcations in surface and groundwater with the exception oftwo groundwater samples in which Na is also an importantcomponent. Concerning the anions, the HCO3

) is theprincipal component in groundwater whereas in surfacewater there are samples collected closer to the mine’srunoff (EBR3, 6, 7, 9) which are sulfate rich, and othersamples collected far from the mine’s effluents which arecarbonate rich (Table 5). The highest metal concentrationsin groundwater are found in samples collected in thesummer whereas in surface water the cationic concentra-tions increase in wintertime in the presence of smallervolumes of water.Stream sediments of the Conceicao River yielded maxi-mum concentrations for As, Cu, Pb and Zn of 119, 99, 25and 104 mg/kg, respectively (Table 6). The highest Ascontents (136–153 mg/kg) were found in sediment sam-ples collected from one tributary of the Conceicao River inthe gold prospecting area of the Brumal district (Fig. 2f).In general, the highest As contents in stream sedimentswere found in samples collected in wintertime, whichsuggests predominance of fine particulate depositionphenomena.In the Conceicao River basin, the point As sources, such asthe exposed soils at the Brumal prospect, the Sao Bentotailings dam runoff and the Carrapato Creek, are not asprominent as those are in the other studied basins. Thismust be due to the fact that mining activities which startedin the 1860s were less intense than those in the Nova Lima

46 Environmental Geology (2003) 44:39–52

Tab

le4

Ch

emic

alco

mp

osi

tio

no

fC

arm

ob

asin

sed

imen

ts(u

nit

s:o

xid

esin

%,

elem

ents

inm

g/k

g;n

.d.

no

td

eter

min

ed,

ww

inte

r,s

sum

mer

)

Stat

ion

sE

RC

1E

RC

2E

RC

3E

RC

4E

RC

5E

RC

7E

RC

6

WS

WS

WS

WS

WS

WS

WS

SiO

228

6232

6334

7442

2828

1431

4533

38T

iO2

1.6

0.7

1.5

0.8

1.5

0.5

1.4

0.7

1.5

0.6

1.1

11

1A

l 2O

313

317

412

418

330

112

211

2F

e 2O

328

3130

3034

1623

6522

8347

4945

56M

gO1

0.2

10.

21

0.5

10.

31

0.2

10.

21

0.2

CaO

0.5

0.2

0.6

0.2

10.

40.

40.

10.

30.

10.

30.

10.

30.

1N

a 2O

0.04

0.1

0.04

0.1

0.2

0.1

0.2

0.1

0.04

00.

30.

10.

30.

1K

2O

20.

42.

10.

51.

30.

42.

70.

43

0.1

1.7

0.3

1.7

0.2

P2O

50.

70.

12

0.1

0.5

0.1

0.4

0.1

0.5

0.1

0.2

0.1

0.2

0.1

To

tal

74.8

99.5

87.9

99.7

8799

.789

.599

.484

.999

.794

.898

.794

.699

.4

As

161

105

4,70

951

812

4785

856

842

086

255

835

545

729

136

8C

r12

972

185

105

262

107

213

118

231

9223

091

145

102

Cu

5731

105

3067

2759

3849

3651

2842

36M

n5,

600

3,50

03,

864

2,52

05,

216

2,55

63,

992

2,46

83

2,46

83,

304

1,62

13,

672

1,47

0P

b6

1036

813

815

64

615

512

6S

2,58

6n

.d.

2,80

3n

.d.

3,42

880

01,

997

800

1,93

180

054

040

097

540

0Z

n13

140

151

4469

3163

3963

3847

3544

37

Original article

Environmental Geology (2003) 44:39–52 47

Tab

le5

Wat

erch

emic

alco

mp

osi

tio

no

fth

eC

on

ceic

aori

ver

bas

in(c

on

cen

trat

ion

sin

mg/

l,ex

cep

tfo

rA

sin

lg/l

;n

.d.

no

td

eter

min

ed,

d.l.

det

ecti

on

lim

its,

ww

inte

r,s

sum

mer

)

Stat

ion

saE

BR

1E

BR

2E

BR

3E

BR

4E

BR

6E

BR

7E

BR

8E

BR

9E

BR

10E

BR

11E

BR

12E

BR

13E

BR

14

WS

WS

WS

WW

SW

SW

WW

WW

WW

pH

67

5.7

76.

67.

56

67.

56

75.

84

4.8

46.

15

6.5

Eh

(mV

)13

0n

.d.

245

n.d

.24

5n

.d.

230

251

n.d

.24

5n

.d.

280

280

230

250

210

270

255

Co

nd

(lS/

cm)

4312

1510

6811

221

6566

8967

2022

013

1041

667

T(�

C)

1522

1823

1924

1919

2216

2319

1718

1918

1821

Ca

42

11

66

26

38

22

160.

80.

23

0.2

6M

g3

11

0.4

32

23

15

12

41

15

0.4

4N

a2

10.

20.

31

0.4

0.3

10.

21

0.2

0.2

50.

40.

51

0.3

2K

0.1

0.1

0.2

0.4

0.2

0.2

0.1

0.2

0.1

0.3

0.1

0.1

100.

30.

30.

10.

20.

2H

CO

335

n.d

.20

n.d

.20

n.d

.20

20n

.d.

20n

.d.

204

168

398

51C

l0.

10.

50.

30.

30.

40.

20.

20.

30.

30.

41

0.2

3.1

0.3

0.2

0.1

0.2

0.2

SO4

10.

30.

60.

325

201

2313

3310

195

0.4

0.2

0.5

0.3

2N

O3

0.3

n.d

.1

0.2

10.

30.

41

n.d

.1

0.5

0.4

332

0.5

10.

51

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d.l.

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d.l.

<d

.l.<

d.l.

<d

.l.<

d.l.

PO

40.

1n

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0.1

n.d

.0.

1n

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0.2

0.1

n.d

.0.

1n

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0.1

0.1

0.1

0.1

0.1

0.1

0.1

Mn

<d

.l.<

d.l.

0.1

0.1

0.1

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d.l.

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d.l.

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e0.

20.

20.

20.

20.

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10.

10.

10.

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d.l.

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d.l.

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d.l.

As T

ota

l5

33

24

83

88

73

274

11

22

3

aSt

atio

ns

EB

R1

–E

BR

9,su

rfac

ew

ater

;st

atio

ns

EB

R10

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BR

14,

gro

un

dw

ater

Original article

and Passagem-Ouro Preto districts where majorenvironmental effects can be noticed.

Arsenic speciation and bioavailabilityIn some water samples from the Carmo River basin,collected in the Passagem and Chico Rei mines as well asfrom the Carmo River, the As inorganic species weredetermined by a highly reproducible analytical procedure.In these samples significant concentrations of As3+ specieswere detected, with the concentration ratios (As3+)/(As5+)ranging from 1.10)1 to 4.10)2.The As3+ concentrations found in those samples wereunexpected because, at equilibrium conditions(Vink1996), the As5+ oxianions (H2AsO4

) and HAsO42)) are ex-

pected to be dominant at the prevailing pH and Eh con-ditions of the surface and groundwater samples from theIron Quadrangle (Fig. 3). Using a chemical speciationmodel for the chemical compositions of water (PHREEQC,version 2.3, and data bank wateqf4.dat from Parkhurst andAppelo 1999), not only the predominance of the As5+

species was theoretically confirmed as the concentrationratios (As3+)/(As5+) could be calculated in the interval2.10)7 to 1.10)12. These As inorganic species ratios are atleast 6 orders of magnitude lower than those obtained bythe analytical procedure. However, it is suggested that thepresence of As3+ in surface waters of the Velhas andConceicao basins must be less significant than in theCarmo River water.Some lixiviation tests were developed for a number ofsediment samples. In all basins the most common mineralsfound in the stream sediment samples were quartz,Fe-oxihydroxides (mainly hematite and goethite) andkaolinite, as well as minor illite and other minerals notdetected by X-ray diffractometry analyses (Table 7).Three samples collected from the Velhas River (EGA1,ERV6 and ERV3) with variable metal contents (Table 2)were preliminarily tested for As by the sequential extrac-tion method developed by Terashima and Taniguchi

(1998), which includes the analysis of the exchangeable,EDTA (carbonate), organic, oxidic, sulfidic, and residual(silicate) fractions. A similar trend of As release was

48 Environmental Geology (2003) 44:39–52

Table 6Chemical composition of Conceicao basin sediments (units: oxides in %, elements in mg/kg; n.d. not determined, w winter, s summer)

Stations EBR2 EBR4 EBR3 EBR6 EBR1 EBR7

W S W W S W S W S W S

SiO2 48 53 44 42 n.d. 44 n.d. 45 54 35 38TiO2 0.6 1 0.6 0.6 n.d. 0.7 n.d. 2 1 1 0.4Al2O3 17 15 6 7 n.d. 8 n.d. 17 13 13 13Fe2O3 12 11 36 41 n.d. 35 n.d. 16 11 41 59MgO 6 7 0.4 0.4 n.d. 0.5 n.d. 2 5 1 0.4CaO 0.3 0.3 0.1 0.1 n.d. 0.1 n.d. 0.2 0.2 0.1 0.1Na2O 0.3 n.d. 0.3 0.4 n.d. 0.2 n.d. 0.1 0.2 0.4 n.d.K2O 0.6 1 1 1 n.d. 1 n.d. 1 1 2 1P2O5 0.1 0.1 0.1 0.1 n.d. 0.1 n.d. 0.2 0.1 0.2 0.1

Total 85.1 88.5 88.6 93.4 0 89.6 0 83.7 85.7 94.1 112As 54.7 85 29.5 73.2 30 68.1 42 136.5 153 119.6 47Cr 1,152 1,153 181 160 40 163 51 530 1,120 321 98Cu 40 47 32 87 41 87 48 73 43 100 47Mn 656 822 2,800 2,344 n.d. 3,912 n.d. 928 2,851 3,384 2,628Pb 25 12 12 11 7 12 11 18 12 17 n.d.S 220 345 185 220 n.d. 262 n.d. 707 364 433 343Zn 104 73 39 46 27 45 35 125 73 66 48

Fig. 3The As–O–H system as a function of pH and Eh, according to Vink(1996) for As=10)6 m at 25 �C and 1 bar. The field indicates theIron Quadrangle waters plot in relation to the stability fields ofAs-hydroxianions. The symbols are the samples subjected to Asspeciation tests

Original article

obtained for all samples, indicating that most of the As waspreferentially bonded to oxihydroxide phases. The Asexchange fraction was less than 4% of total As content inthe samples, which may represent a maximum concen-tration of 84 mg/kg for the As-richest sediment. A variantof the extraction method TCLP (toxicity characteristicleaching procedure) developed by the U.S. EPA (Anon1986) was applied to one sample of mining waste collectedfrom the Galo tailings pile (EGA1S, Fig. 1b), one from theQueiros creek (ERQ1), and three sediment samples fromthe Carmo River (ERC2, ERC5, ERC6). The As releaseduring the weak acid attack reached 1 to 4% (2 to196 mg/kg) of the original total As content of the samples.These results are quite similar to those obtained in thesequential extraction experiments.

Arsenic mobility related to ore-depositweathering

Arsenic occurs in the Iron Quadrangle primary gold oresmainly in arsenopyrite and arseniferous pyrite (Table 7).Ore petrographical studies from the Morro Velho, Rapososand Cuiaba gold mines (Fig. 1b) reveal the presence ofmajor amounts of pyrite and, to a lesser degree, ofarsenopyrite and pyrrhotite. Arsenic is present as animpurity in these pyrite-rich ores with concentrations upto 2 wt%. The auriferous ores of the Sao Bento minecontain significant amounts of arsenopyrite which also isthe major phase in sulfide ores of the Passagem and ChicoRei mines.In all those ore deposits, both sulfides and arsenidesundergo oxidation during weathering. This is responsiblefor the partial release of As into the environment as dis-solved arsenate and arsenite species, and it explains theelevated As concentrations found in some water samples(EPM1 and EOP1 in the Passagem-Ouro Preto mines),while some As is precipitated in newly formed minerals oradsorbed onto other phases. This latter phenomenon wasstudied in several samples collected from the oxidationzones of the ore deposits using an EDS microanalysercoupled to a scanning electronic microscope. At least twoAs-retention mechanisms were identified. At the Passagemand Ouro Preto mines, the As is retained through pre-cipitation of secondary phases as scorodite (FeAsO4Æ2H2O)and goethite, the latter containing up to 8 wt% As (Table7). On the other hand, in a more advanced stage ofweathering, mixtures of iron oxides and hydroxidesincluding remains of magnetite and hematite are formed.In a sample collected from the oxidation zone of the

Raposos gold deposit, the Fe-rich material yielded Asabove 10,000 mg/kg, well above the detection limit of theequipment (SEM-EDS).

Discussion and conclusions

In Archean greenstone belts hosting meso-thermal golddeposits, the enrichment of the rocks with As and otherelements such as Ag, Sb, B and, to a lesser extent, Cu, Pb,and Zn is a common geochemical feature, as observed inthe Rio das Velhas Supergroup. In the Iron Quadrangle,As contents up to 4,800 mg/kg were found in golddeposits hosted by iron formations (BIF) of the NovaLima Group, exceeding to about 40 times the average Asconcentration in non-mineralized iron formations(Raposo and Ladeira 1995). These rocks generate soilsenriched in arsenic and other metals and give rise tonatural geochemical anomalies similar to those found, forinstance, in Ghana (Africa) and in the Barents region(northern Europe) where natural As anomalies wererevealed in soils in the vicinity of gold mineralizations(Bowell 1994; Reimann and others 1998). In the case ofsome places in the Iron Quadrangle, soil contaminationwas enhanced by anthropogenic activities. In the NovaLima district, As concentrations exceeding 2,000 mg/kgwere determined by Rawlins and others (1997) in theGalo Hill soil where an industrial plant has processed thelocal mining waste for As2O3 production until its closurein 1976. There are also indications about As oxide pro-duction in the Passagem mine until the first decades ofthe last century. An attempt to integrate the sedimentgeochemical data generated in this study with resultsobtained in previous studies, particularly to the CPRMgeochemical survey reported by Oliveira and others(1979), is shown in Fig. 4. The As concentrations instream sediments have arbitrarily been grouped in fourclasses to clarify the widespread distribution of As insediments collected even in upland regions, mainlydominated by metasedimentary rocks of the MinasSupergroup where large gold deposits are not known. Itis clearly indicated that the Iron Quadrangle is the largestgeochemical As anomaly so far described in Brazil. Thehighest As concentrations in sediments are found in thevicinity of the largest secular gold mines of the NovaLima, Passagem-Ouro Preto and Santa Barbara districtswhich have been the focus of the present study.

Environmental Geology (2003) 44:39–52 49

Table 7Minerals related with As occurrence

Gold mineralized rocks Weathered mineralized rocks Sediments Tailings

Arsenopyritea, pyriteb,carbonates (calcite,dolomite), quartz

Scoroditea, goethiteb,Fe-oxihydroxidesb,gypsum

Quartz, Fe-oxihydroxides(mainly hematite andgoethite)b, kaolinite,illite, sulfides (?)a

Arsenolitea, goethiteb,iron hydroxideb,gypsum, epsomite,hydroniumjarosite

aAs mineralsbMinerals which have As as impurity or adsorbed/sorbed (see text for details)

Original article

Currently, the main sources of As, local and regional, areanthropogenic in origin and related to environmentalheritage (contaminated soil, old tailings piles, old mines)as well as to the presence of As-enriched sediments in thewater courses.Natural As release to the environment in the IronQuadrangle also occurs from weathering of ore deposits.Particularly, the weathering of arsenopyrite and pyrite isbeing followed by formation of new As-bearing minerals,As adsorption onto surfaces of other minerals or solubi-lization in aqueous media, as observed in the Passagemand other mines. The oxidation products formed atarsenopyrite surface include As3+ and As5+ hydroxianionswhich can be readily leached by diluted solution (Nesbittand others 1995). Below pH 4 and oxidizing conditions,precipitation of scorodite from As5+ and Fe3+ oxianions

reaction may occur (Dove and Rimdstidt 1985), whereas atpH>4 the incongruent solubilization of scorodite is likelyto take place, yielding goethite, Fe-oxihydroxide andH2AsO4

) as products (Dove and Rimdstidt 1985; Krauseand Ettel 1988; Robins 1987). The amount of As adsorbed/sorbed to goethite can be as high as 8%, as previouslydescribed in the oxidation zone of the Chico Rei oredeposit.It is well known that the oxidation of sulfides generatesacid drainage in many mining areas. Due to the extensivecarbonate alteration in the ore deposits of this study, theseacid solutions have been buffered effectively. In thePassagem and other gold mines in the Ouro Preto region,the pH values are above 7.5. At these conditions, a releaseof adsorbed As oxianions is likely to occur, consideringthat the characteristic zero point of charge (pHzpc) is 8 for

50 Environmental Geology (2003) 44:39–52

Fig. 4The As geochemistry map of theIron Quadrangle based on streamsediment analyses, according toOliveira and others (1979), Eleuterio(1997), and this study

Original article

Fe-oxihydroxides (Dzombak and Morel 1990) and 6–7 forgoethite (Smith 1999).It is expected that the oxidation of arsenopyrite promotesthe release of As3+ and As5+ hydroxianions into solutionwhereas part of As5+ is retained by precipitation of ironarsenate (scorodite), as demonstrated at the Passagem andChico Rei mines, or by adsorption onto or co-preciptationwith Fe-Mn oxihydroxides, as observed in the oxidationzone of the Raposos, Passagem and Chico Rei mines.Finally, the carbonate-induced pH increase of minedrainage favors a solubilization of scorodite as well as theAs desorption from goethite which may contribute to Asrelease to the underground and surface waters.The old and present tailings are As reservoirs and pointrelease sources for As in the Iron Quadrangle. The Galotailings piles disposed along the Cardoso Creek (NovaLima district) contain arsenolite, As-bearing Fe-oxihy-droxides and, probably, amorphic Fe-arsenate, along withCa and Mg sulfates, gibbsite and epsomite, like the mineralassemblage found in the Passagem mine oxidation zone.Similarly, percolating solutions with pH>7 promote thesolubilization of adsorbed As and of arsenolite. The latteroccurs at any pH–Eh conditions, whenever in contact withsolutions in which the activity of H3AsO3 is less than10)0.8 m (Vink 1996). Solubilization of As combined withthe erosion of the tailings pile give rise to the highestknown As concentrations found in water and sedimentsamples in the Velhas River basin.Despite the high As contents in stream sediments and theoccurrence of sulfur in samples from the Carmo River, thepresence of As minerals could not be detected by X-raydiffractometry. Adsorption of As onto surfaces of goethite,hematite and kaolinite grains commonly found in the sed-iments is the most likely mode of As occurrence. However,since the zero point charges (pHpzc) for these minerals are6–7, 4–7 and pHpzc>4.6, respectively, As adorption ontogoethite and hematite is favored by the current pH condi-tions (6–8) of surface water in the Iron Quadrangle. Thispreference for oxihydroxides phases has been confirmed bythe sequential extraction and leaching experiments reportedabove. The bioavailability of As in sediments, i.e., the weaklyadsorbed As in the sediment fraction <63 lm, was estimatedto be less than 4% of the total As content, which can be ofgreat concern where some As-rich sediments are found,especially in the vicinity of mines.The Carmo River receives the As-rich water which per-colates the Passagem and other gold deposits in the OuroPreto area. It seems, however, that the volume of minegroundwater reaching the main river can not explain thehigh As contents found in the surface water. Additionalrelease of As from the sediments into the water is likely tobe another relevant process in both the Carmo and Velhasrivers where great amounts of mining waste have beendischarged in the past.The elevated As concentration in groundwater in themining areas may be due to oscillation of the watertable and exposition of sulfide ore to oxidation. Partialsolubilization of As recalls here the geogenic processdescribed by Schreiber and others (2000) for the aquiferhosted by sulfide-bearing sandstone in Wisconsin, USA.

An estimation of As3+/As5+ ratios in water is of particularimportance because the toxicity of As3+ is known to bemany orders of magnitude greater than that of As5+. Thecharacterization of arsenic species is also mandatory forthe success of water treatment, since As removal fromwater is dependent of the As oxidation state (Kim andNriagu 2000; USEPA 2000). As5+ hydroxianions are par-tially consumed during the formation of scorodite andother sulfo- arsenates. Additionally, As5+ hydroxianionsare more easily adorbed onto goethite and clay mineralssurfaces than As3+ hydroxianions (Bowell 1994; Ladeiraand Ciminelli 2000). This may lead to a partial As3+

enrichment via percolating water at the Passagem andChico Rei mines even under incompatible pH–Ehconditions. Hence, the As3+/As5+ ratios found for thesegroundwaters and for surface waters of the Carmo River(in the range of 1.10)1 to 4.10)2) have been shown to bemuch higher than those calculated by the PHREEQCgeochemical model (10)7 to 10)12). The contrasting resultsbetween experimental and theoretic As3+/As5+ values canbe attributed to (1) the slow kinetics of redox transfor-mations involving As in natural waters (Daus and others2000), i.e., chemical equilibrium was not achieved in thesewaters; and (2) the selective retention of arsenatehydroxianions at solid phases in relation to As3+, asexplained above. Since the PHREEQC program works withsolutions at equilibrium, the observed discrepancy is notsurprising but at the same time suggests that the use ofmodeling programs to environmental approach has to bedone with care, and needs to be supported by analyticalwork.Summarizing the main conclusions arising from thisstudy, it is worth recalling that the Iron Quadranglecomprises an extensive natural anomaly of arsenic relatedto the occurrence of mesothermal gold deposits hosted inthe Rio das Velhas greenstone belt. Weathering of theAs-rich rocks and of ore deposits brings about the releaseof this element into soils, sediments, groundwater andsurface drainages. It is estimated that during the threecenturies of gold mining production in the Iron Quad-rangle, at least 390,000 t of arsenic was discharged into thedrainage system.The main recommendation arising from this study is that itis necessary to carry out an inventory of the point-Asrelease sources in the region to give guidance to the localpopulation about potential hazards of certain habits and toimplement low-cost programs of environmental mitigationand countermeasures. Since the toxic effects of arsenic cantake many years to become apparent and the conventionaltreatment of water is not designed for removal of dissolvedarsenic, a broader geochemical monitoring of water qual-ity in the region must be carried out, including the de-termination of arsenic species dissolved in water and thearsenic bioavailability in soils and sediments.

Acknowledgments The present study was carried out with thefinancial support of the Brazilian agency Fapesp (doctoralscholarship 97/14446-6 and research grant 96/7839-9). Theauthors are grateful to the staff of the Analytical GeochemistryLaboratory and of the SEM Multiusers Laboratory at Unicamp

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(Fapesp 95/6401-7). This project has also benefited from theanalytical data provided by the geochemical laboratories of theBritish Geological Survey (Keyworth, Nottingham, U.K.) and ofHeidelberg University, Germany.

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