ARTICLE

W–Au skarns in the Neo-Proterozoic Seridó Mobile Belt,Borborema Province in northeastern Brazil: an overviewwith emphasis on the Bonfim deposit

João Adauto Souza Neto & Jean Michel Legrand &

Marcel Volfinger & Marie-Lola Pascal & Philippe Sonnet

Received: 27 October 2005 /Accepted: 12 June 2007 /Published online: 28 July 2007# Springer-Verlag 2007

Abstract The Seridó Mobile Belt (SMB) is located in theBorborema Province in northeastern Brazil and consists ofa gneiss basement (Archean to Paleo-Proterozoic), a meta-sedimentary sequence (marble, quartzites, and schists), andthe Brasiliano igneous suite (both of Neo-Proterozoic age).In this region, skarns occur within marble and at themarble–schist contact in the metasedimentary sequence.Most of the skarn deposits have been discovered in theearly 1940s, and since then, they have been exploited fortungsten and locally gold. Recently, the discovery of goldin the Bonfim tungsten skarn has resulted in a betterunderstanding of the skarn mineralization in this region.The main characteristics of the SMB skarns are that they

are dominantly oxidized tungsten skarns, with the exceptionof the Itajubatiba and Bonfim gold-bearing skarns, whichare reduced based on pyrrhotite as the dominant sulfide,garnet with high almandine and spessartine component, andelevated gold contents. In the Bonfim deposit, pressureestimates indicate that the skarns formed at 10- to 15-kmdepth. The mineralized skarns present the prograde stage withalmandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monaziteas accessory minerals. The retrograde stage is characterizedby alkali feldspar, clinozoisite–zoisite–sericite, calcite, andquartz. Scheelite occurs in four ore-shoots distributed withinthe marble and at the marble–schist contact. The main orebody is 5–120 cm wide and contains an average of 4.8-wt.% WO3, which occurs in the basal marble–schistcontact. Fold hinges appear to control the location ofhigh-grade scheelite. The late-stage gold mineralizationcontains bismite (Bi2O3), fluorine-bearing bismite, nativebismuth, bismuthinite (Bi2S3), and joseite [Bi4(Te,S)3],and also chlorite, epidote, prehnite, chalcopyrite, andsphalerite. This gold–bismuth–tellurium mineralizationexhibits a typical late character and occurs as a blackfine-grained mineral assemblage controlled by conjugatebrittle-ductile faults (and extensional fractures) that cross-cut not only the banding in prograde skarn but also theretrograde alkali feldspar and clinozoisite–zoisite–sericiteassemblage. The Au–Bi–Te-bearing minerals are inter-grown with retrograde epidote, prehnite, chlorite, chalco-pyrite, and sphalerite, indicating that gold mineralizationat Bonfim is linked to a late-stage skarn event. Thepolymetallic nature of the Bonfim deposit can be used asan important guide for the exploration of this type of skarndeposit in the Borborema Province, which potentiallycontains significant new, undiscovered gold and polyme-tallic deposits.

Miner Deposita (2008) 43:185–205DOI 10.1007/s00126-007-0155-1

Editorial handling: S. Hagemann

J. A. Souza Neto (*)Departamento de Geologia, Universidade Federal de Pernambuco,Av. Acadêmico Hélio Ramos, s/n, Cidade Universitária,CEP 50.740-530 Recife, Brazile-mail: adauto@ufpe.br

J. M. LegrandDepartamento de Geologia,Universidade Federal do Rio Grande do Norte,Campus Universitário,Lagoa Nova,CEP 59.072-970 Natal, Brazil

M. Volfinger :M.-L. PascalCentre de Recherches sur la Synthèse et Chimie des Mineraux(CRSCM), Centre National de la Recherche Scientifique (CNRS),1A rue de la Férollerie,45071 Orléans Cedex 2, France

P. SonnetDépartement des Sciences du Milieu et de l’Aménagement duTerritoire, Université Catholique de Louvain,2 Place Croix du Sud, Boîte 10,1348 Louvain-la-Neuve, Belgium

Ph. Sonnet

Keywords Tungsten and gold skarns . Bonfim .

Borborema Province . SeridóMobile Belt . Brazil

Introduction

Skarn deposits host a large variety of metals (Fe, Cu, Au,Mo, W, Pb–Zn, Sn) and represent an important depositclass. These deposits are subdivided according to theireconomic metal content (Einaudi et al. 1981; Einaudi andBurt 1982; Meinert 1989). The different ore types and theirparticular geochemical associations reflect the tectonicsetting and the nature of the parental igneous rock (fluid),the composition of protoliths, and the temperature–pressure(depth) of ore formation (Einaudi et al. 1981; Einaudi andBurt 1982; Kwak 1987; Meinert 1989, 1992; Zharikov1991; Lentz 1998).

In northeastern Brazil, mineralized skarns have beenknown since at least the 1940s, and some of them havebeen mined since then (Johnston and Vasconcellos 1945;Maranhão 1970; Lima et al. 1980; Salim 1993; Souza Netoet al. 1999). Their economic interest stems from theircontents of W–Mo and gold. Numerous occurrences(almost 700, according to Lima et al. 1980) have been

found within an area of about 20,000 km2 known as thetungsten province of Seridó (Fig. 1). Most of the depositshave been exploited for W–Mo (e.g., Brejuí, Bodó, Bonfim,Bonito, Malhada Limpa, and Quixaba mines; Fig. 1), and onehas been exploited for gold only (Itajubatiba mine; Fig. 1).In the early 1990s, gold was discovered in the tailings of theabandoned Bonfim W–Mo mine. Until recently, it is the onlydeposit in this Brazilian region where W–Mo and gold havebeen found together. In the context of metals produced,most deposits in the Seridó Mobile Belt (SMB) can beclassified as tungsten skarns, with the exception of theBonfim tungsten–gold skarn and Itajubatiba gold skarn.

With regard to tungsten skarns, their main characteristicfeatures have been pointed out as being reduced and formeddeeper (Kwak 1987) than the gold skarns, which are formedat relatively shallow depth. Gold skarns are oxidized andusually occur associated with copper porphyry deposits,and can contain Cu and Fe as associated metals (Einaudi etal. 1981; Einaudi and Burt 1982; Meinert 1989, 1992;Lentz 1998; Gray et al. 1995; Eilu et al. 2003). However,the recognition of economic gold mineralization in reducedtungsten skarns was only recently observed by Newberry(1998). In the last years, important gold occurrences intungsten skarns have been studied worldwide, for example,

Fig. 1 Geologic setting of the structural province of Borborema, northeastern Brazil (adapted from Archanjo 1993). The location of SMB,tungsten province of Seridó, principal skarn deposits, and gold-quartz vein deposits are also shown

186 Miner Deposita (2008) 43:185–205

in the Nevoria deposit in Western Australia. Nevoria is areduced skarn deposit that formed at midcrustal level (10–15 km), and contrasts with the copper–gold and gold-richiron–copper skarns that formed at shallower depths (2–5km; Mueller 1997; Mueller et al. 2004). In some tungstenskarns, e.g., the Emerald tungsten camp in the CanadianCordillera (Ray et al. 1995; Ray and Dawson 1998), itseems uncertain whether the gold was contemporaneouswith the skarn or represents a later epithermal overprint.

This review of W–Au skarns in the SMB contributes tothe understanding of gold mineralization in tungsten skarns,summarizes the main geological aspects of the SMB and itsore deposits, as well as the detailed geology and petrogra-phy of the Bonfim tungsten–gold skarn deposit. The mainpurposes of the petrographic study at Bonfim skarns wereto: (1) provide a detailed description of the complex oremineralization discovered in these skarns and (2) establishthe chemical composition of ore minerals using modernmicroanalytical techniques such as particle-induced X-rayemission (PIXE), which is able to detect trace elements inminerals, and electron microprobe (EMP). The mineralchemistry results obtained on ore minerals are used toelucidate the composition of the ore-forming fluids and toestablish their geochemical association, providing funda-mental information for understanding the ore genesis andproviding some criteria that can be used in the exploitationof the Bonfim deposit and exploration in the region.

Geology of the SMB

The SMB represents the northern segment of the Borbor-ema Province (Almeida et al. 1981) and is located north ofthe Patos lineament (Fig. 1). In the SMB, continental-scaleNE-trending strike-slip shear zones dominate the tectonicfeatures (Fig. 1). These shear zones with associated foldsand thrusts were developed during transpressional tectonicsthat were active during the Brasiliano/Pan-African orogenyaround 600 Ma (Archanjo and Bouchez 1991; Caby et al.1991; Corsini et al. 1991, 1992, 1996; Vauchez et al. 1995).In the SMB, rocks are grouped (from the oldest to theyoungest) into the gneiss basement, Seridó Group meta-sedimentary rocks, and Brasiliano igneous suite.

Gneiss basement (2.2–2.0 Ga)

The basement contains migmatitic orthogneisses of tonaliticto granitic composition and minor supracrustal rocks. Theyhave Archean to Paleo-Proterozoic ages that vary from 2.2to 2.0 Ga (Hackspacher et al. 1990; Legrand et al. 1991b;Souza et al. 1993; Dantas et al. 2004), but older ages of 3.5Ga were obtained for some blocks located in the eastern-most SMB basement (Dantas et al. 2004; Fig. 1).

Seridó Group metasedimentary rocks (650 Ma)

The metasedimentary sequence is composed of typicalsupracrustal rocks, consisting from bottom to top, calc-silicate paragneisses and marble (Jucurutu Formation),quartzites and metaconglomerates (Equador Formation),and mica schists (Seridó Formation). Uranium–Pb and Sm–Nd dating (zircon and whole rock, respectively) inmetasedimentary and metatuffaceous rocks of the SeridóGroup have determined that its deposition probablyoccurred ca. 610–650 Ma (Van Schmus et al. 2003).

Brasiliano igneous suite (610–530 Ma)

Neo-Proterozoic to Cambrian igneous rocks are widelydistributed in this Brazilian region (Fig. 1). Most of themare spatially linked to the Brasiliano/Pan-African shearzones, and some intrusions exhibit asymmetric sigmoidshapes. Petrostructural and magnetic susceptibility studiescarried out in several of these intrusions support theirsyntectonic character (Archanjo 1993; Archanjo et al. 1994,1999; Lima et al. 2000). Porphyritic and fine-grainedgranites are the dominant rocks in these igneous bodies,with minor equigranular granites, fine- to medium-grainedaplites and pegmatites (see Table 1 for classification ofintrusive rocks related to the main skarn deposits of theSMB). These granitoid rocks are syn- to late-kinematic(Brasiliano orogeny) and present ages between 610 Ma(U–Pb method on zircon; Jardim de Sá 1994; Jardim de Sáet al. 1999) and 555 Ma (U–Pb method on zircon; Legrandet al. 1991a) throughout the SMB. The pegmatite bodies,which constitute late phases of the Brasiliano/Pan-Africanmagmatic activity, are constrained by U–Pb uraninite agesof 480 and 510 Ma (Florencio 1948 and Marble 1949,respectively; both in Ebert 1970) and a 40Ar/39Ar biotitemean age of 523.4±1.1 Ma for six biotite samples fromBe–Ta–Li pegmatites of the SMB (Araújo et al. 2005). Thislate age is considered by the authors to represent the firstincrements of late reactivation of the Brasiliano strike-slipshear zones. Recently, U–Pb analyses of columbite fromsix heterogeneous mineralized pegmatites indicate for-mation ages between 509.5±2.9 Ma and 514.9±1.1 Ma(Baumgartner et al. 2006).

Regional and contact metamorphism

The tectono-metamorphic evolution of the SMB is largelydominated by the widespread Brasiliano orogeny (ca. 600Ma) that affected all geological units and promoted partialfusion (migmatites) and large (high-angle) transcurrentshear zones (Caby 1989; Archanjo and Bouchez 1991;Caby et al. 1991, 1995; Jardim de Sá 1994; Brito Neves

Miner Deposita (2008) 43:185–205 187

Tab

le1

The

characteristicsof

theprincipalskarndepo

sitsin

theSMB

Principal

mine(small

adjacent

mines),metals

Ton

nage,

grade

Hostrocks

Skarn

morph

ology

Intrusive

rocks

Alteratio

nin

intrusive

Skarn

mineralog

yOre

minerals

Formation

cond

ition

scReferences

Progradea

Retrogradeb

Early

stage

Late

stage

Brejuí(Barra

Verde,Boca

deLage,

Zangarelhas,

Carnaubinha,

Juazeirinh

o,

Sacodo

s

Veados)

W

(Mo,

Fe,

Cu)

11Mt(5.5

Mt

remaining

)

0.5–1%

WO3

Neo-

Proterozoic

marble,

gneiss,

pegm

atite

Stratiform;

vugg

y;

occasion

al

vein

skarn,

retrog

rade

alteratio

n

crosscuttin

g

579±4to

555±5Mabt

granite

batholith

;

pegm

atite;

aplitedikes

End

oskarn

Skarn

marble/

gneiss/pegmatite:

px(H

d 30–60Jo

1–8),

pl(A

n 85–92;

An 4

0–47

in

amphiboleskarn),

grt(A

d 19–49A

l+

Sp2–7G

r 51–78),

amp(hbl),ttn

,

qtz,mc

20–30%

(?)Skarn

marble/gneiss/

pegm

atite:am

p

(act),mei,ves,

act,ph

l,ep,prh,

czo(later

than

ep),rds,cbz,

stb,

ser,chl,brt,fl

Sch (early

fine-

grained;

late

coarse-

grainedin

porous

skarns,

richest

ones),

mo,

py,

cp,bn

,

fb,pw

,

wf,mgt,

bi,bm

t,

po(?)

P2–3kb

Tp

450–55

0°C

(mineral

equilib

ria)

T r20

0–45

0°C

<15

eq.wt.%

NaC

l

(mineral

equilib

riaand

fluid

inclusions)

Salim

(199

3);

Maranhão

(197

0);

Maranhãoet

al.(1986);

Barbo

saet

al.

(196

9);Goñi

andPicot

(196

5);

Beurlen

and

Borges

(199

0);

Gou

veia

(197

7);

Cavalcante

Neto(198

6);

Legrand

etal.

(199

1a);

Jardim

deSá

(199

4);

Jardim

deSáet

al.(1986)

Bodó

(Riachão,

Umbuzeiro,

Isidoro,

Queim

adas,

Cafuca)

W

±9Mt

0.2–6

(average

=2)

%WO3

Neo-

Proterozoic

marble,

gneiss

Stratiform;

lens-shaped;

pockets;

vugg

y;

occasion

al

vein

skarn,

retrog

rade

alteratio

n

crosscuttin

g

Neo-

Proterozoic

equigranular

btgranite

(stocks);

pegm

atite;

aplitic

granites

End

oskarn;

ep,mo,

py

px,grt,am

p,wo,

ttn,<fld

10–40%

(?)ep,

ves,am

p(?)

sch,

mo,

py,cp,bn

Costa

(199

5);

Legrand

etal.

(199

4);Roy

(196

6);

Zaniniand

Santos

(197

9);Lim

a

etal.(198

0);

Melo(196

1);

Silv

a(197

1)

Bonfim

(Sulista,

Queiroz,

CatoléII,

Gupiara,

Pedra

Preta-

>0.3Mt

0.5–9

(average

=

4.8)

%

WO30.5–

1.5tAu

Neo-

Proterozoic

marble,

schist

Stratiform;

retrog

rade

alteratio

n

crosscuttin

g

andas

impregnatio

n

Neo-

Proterozoic

btgranite;

pegm

atite

Non

e

observed

Skarn

inmarble:

px(H

d 11–48Jo

1–3),

amp(tr-mhb),ttn

,

ap,wo,

<ol;

Skarn

inschist:

px(H

d 17–53Jo

1–4),

50–70%

Skarn

in

marble:

amp(tr,

mhb

);Skarn

in

schist:kfd,

czo-

zo,ser,bt,am

p

(act-m

hb;ath-

mgt,po

,

cp,py,

sch,

mo,

<<apy

bi,bm

t,

jo,cp,

sp,go

ld

P2.5–4kb

Tp

500–58

0°CT r

400–50

0°C

XCO20.05–

0.3(m

ineral

equilib

ria)

4.8

Sou

zaNeto

(199

9);Lim

a

(198

2);Salim

(197

9);Farias

etal.(197

3);

Potyra

188 Miner Deposita (2008) 43:185–205

Tab

le1

(contin

ued)

Principal

mine(small

adjacent

mines),metals

Tonn

age,

grade

Hostrocks

Skarn

morpholog

y

Intrusive

rocks

Alteratio

nin

intrusive

Skarn

mineralogy

Ore

minerals

Formation

conditionsc

References

Progradea

Retrogradeb

Early

stage

Late

stage

Mulun

gu,

Matinha)W

(Au,

Bi,Te)

2–6(upto

60)pp

m

Auup

to26

,000

ppm

Biup

to50

0

ppm

Te

concordant

totherock

band

ing

pl(A

n 85–99),

grt(A

d 3–5Al+

Sp68–69Gr 21–22),

amp(act-m

hb),

ttn,ap,aln,

zrn,

mnz

cum

in

phlogopite-

bearingschist),

Fe-rich

ep,prh,

chl,ves,scp,

ms

eq.wt.%

NaC

l+CH4

(N2)fluids

in

qtz-sulfides

andcal-

sulfides

veins

intheskarns

(fluid

inclusions)

(1978);

Barros

(1964);

Feitosa

(1964)

Bon

ito

(Pindo

ba-

Mazagão)W

>4Mt

0.5–0.8%

WO3

Neo-

Proterozoic

marbleand

gneiss

Stratiform

579±7Ma

hbl(?)

granod

iorite;

pegm

atite;

aplite

Non

e

repo

rted

px,grt,am

p>40

%(?),ep,>>fl

sch,

mo,

py,cpy,

bn,<gn

Santos

(1968);Lim

a

etal.(198

0);

Brito

Neves

etal.(200

3)

Malhada

Lim

pa-

Tim

baúb

a

W,Mo

5.5Mt

0.3–0.5%

WO30.4–

0.7%

Mo

Neo-

Proterozoic

gneiss,

marble

Stratiform;

lens-shaped;

pockets;

vugg

y;

filling

fractures

Neo-

Proterozoic

bt(?)

granite;

pegm

atite

Non

e

repo

rted

pl,px

,grt,ap,qtz

(?),ttn

,toz(?)

>10

%(?),scp,

ep,

ser,chl,ves,am

p

sch, >>mo,

pw,py,

cp,bn

Roy

(196

4);

Santoset

al.

(1972);Lim

a

etal.(198

0)

Quixaba

(Quixeré,

Malhada

Vermelha,

Cacim

bas)W

>2.5Mt

0.5-1.4%

WO3

Neo-

Proterozoic

marbleand

calc-silicate

gneiss

Stratiform;

lens-shaped

Neo-

Proterozoic

hbl

granod

iorite,

btgranite,

bt-amp-px

alkalisyenite

Sch

(upp

er

aplitic

partof

granites,and

aplitic

veins)

grt,px

,scp,

pl,

aln,

ap,ttn

>20

%(?),hb

l,

act,tr,ep,czo,

prh,

ab,chl,ves,

zeo,

rdn,

fl

sch,

mo,

py,cp,

bn,mgt,

gn,sp

Andritzky

(1972);

And

ritzky

andBusch

(1975);

Cassedane

et

al.(1972);

Saldanh

a

(1963);

Santosand

Brito

Neves

(1984)

Itajubatiba

Au(Fe,

Cu)

<1Mt

0.5–2(up

to6.3)

ppm

Au

Neo-

Proterozoic

marble,

meta-

tonalite,

meta-

syenogranite

Stratiform;

lens-shaped;

vugg

y;

occasion

al

vein

skarn,

retrograde

alteratio

n

crosscuttin

g

573±45

Mabt

syenogranite;

abundant

pegm

atites

px-pl-am

p

andam

p-qtz

endo

skarns;

qtzveins;

widespread

carbonatation;

largeap

and

bt

Skarn

inmarble:

px(H

d 5–75Jo

0–3),

amp(act-tr-mhb

;

<fac-fhb-ts,prg-

ed),ttn

,ap,ol;

Skarn

in

metaton

alite:px

(Hd 2

6–61Jo

1–2),pl

(An 8

7–96),grt

20–30%

Skarn

in

marble:

amp,

hu-

chu,

srp,

cch;

Skarn

in

metaton

alite/

metasyeno

granite:

amp,

ser,ep,bt

mgt

po,py,

cp,

<<apy,

gold

P2.5–4kb

Tp

500–

580°CT r

400–

500°C

XCO20.05–

0.3(m

ineral

equilib

ria)

4.8–7.8eq.

wt.%

NaC

l+

N2(upto

18

Sou

zaNeto

(1995,

1999);

Linsand

Scheid

(1981);

Rebou

ças

(1985);Brito

Neves

etal.

(2003);

Miner Deposita (2008) 43:185–205 189

Tab

le1

(contin

ued)

Principal

mine(small

adjacent

mines),metals

Ton

nage,

grade

Hostrocks

Skarn

morph

ology

Intrusive

rocks

Alteratio

nin

intrusive

Skarn

mineralogy

Ore

minerals

Formation

condition

scReferences

Progradea

Retrogradeb

Early

stage

Late

stage

(Ad 4

–13Al+Sp

69–78

Gr 2–25),am

p

(act-m

hb-fhb-fts;

<fprg-hs,gru),

ttn,ap;Skarnin

metasyenogranite:

px(Hd 1

5–61Jo

1–2),

pl(An 8

7–96;

An 2

0–30

orscp),

grt(Ad 4

6–57Al+

Sp4–5G

r 34–46),

amp(act-m

hb),

ttn,ap,aln

mol%

CH4)

fluids

inqtz-

cal-sulfides

andqtz-cal-

mgt

veinsin

theskarns

(fluid

inclusions)

Galindo

and

Sá(200

0)

Mineralabbreviatio

ns(according

toKretz19

83andSpear

1993):ab

albite,a

ctactin

olite,a

lnallanite,a

mpam

phibole,ap

apatite,a

pyarseno

pyrite,a

thanthop

hyllite,b

ibismuth,

bmtb

ismuthinite,

bnbo

rnite,brtbarite,bt

biotite,calcalcite,cbzchabazite,cchclinochlore,

chlchlorite,chuclinoh

umite,cp

chalcopy

rite,cum

cummington

ite,czoclinozoisite,czo-zo

clinozoisite/zoisite,ed

edenite,epepidote,facferro-actin

olite,fbferberite,flfluo

rite,fhb

ferro-ho

rnblende,fld

feldspar,fprgferro-pargasite,ftsferro-tschermakite,g

ngalena,g

rtgarnet,g

rugrun

erite,h

blho

rnblende,h

shastingsite,hu

humite,jo

joseite,kfdalkalifeldspar,mcmicrocline,

mei

meion

ite,mgt

magnetite,

mhb

magnesioh

ornb

lend

e,mnz

mon

azite,momolyb

denite,msmuscovite,ol

olivine,

phl

phlogo

pite,pl

plagioclase,po

pyrrho

tite,prgpargasite,prhprehnite,p

ypy

rite,pw

powellite,px

pyroxene,qtzqu

artz,rdnrhod

onite,rdsrhod

ochrosite,schscheelite,scpscapolite,sersericite,sp

sphalerite,srpserpentin

e,stbstilb

ite,toztopaz,

trtrem

olite,tstschermakite,ttn

titanite,vesvesuvianite,wfwolfram

ite,wowollaston

ite,zeozeolites,zrnzircon

aPyrox

enecompo

sitio

nsexpressedas

molepercenth

edenbergite

(Hd)

andjohann

senite(Jo);rem

aind

erisdiop

side.P

lagioclase

compo

sitio

nsexpressedas

molepercentano

rtite

(An);rem

aind

eris

albite+orthoclase.Garnetcompo

sitio

nsexpressedas

molepercentandradite

(Ad),almandine

(Al)+spessartine(Sp),andgrossularite

(Gr).Amph

ibolecompo

sitio

nsexpressedas

nomenclatureof

theam

phibolegrou

pminerals.

bExtentof

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190 Miner Deposita (2008) 43:185–205

et al. 2000). In addition, the gneiss basement also records apre-Brasiliano tectono-metamorphic event around 2.0 Ga(Transamazônico/Eburnian orogeny), which developedmigmatites associated with low-angle structures (thrust-type tectonics) and minor magmatic activity (Jardim de Sá1994; Hackspacher et al. 1995; Dantas et al. 2004).

The conditions of peak metamorphism accompanyingthe main tectonic event (Brasiliano orogeny) in the SMBhave been first established at 550–600°C and 3–4 kbars,based on a geothermobarometric study in the central part ofthis belt (Gama and Albuquerque 1985; Lima 1986). Mostrecently, these peak conditions were more precisely estimat-ed to be 560–630°C and 2.7–4.5 kbars, using differentgeothermobarometers applied to the different metamorphiczones (biotite–garnet, cordierite–andalusite, and sillimanite–muscovite) observed in the schists-containing shear-zone-hosted gold-bearing quartz veins of the São Franciscodeposit (Luiz-Silva 1995), approximately 50 km east ofthe Currais Novos town, and of the São Fernando-Caicódistrict (Luiz-Silva 2000), about 100 km southwest of theCurrais Novos (Fig. 1).

Combining different geothermobarometers with40Ar/39Ar geothermochronometry on mineral assemblagesoccurring in the contact metamorphic aureole of theBrasiliano Acari granite (batholith close to the Brejuí skarndeposit in Fig. 1), a thermal peak of 630°C and 3.5 kbarsduring 579–555 Ma (U–Pb) was established, followed by acooling to 300–400°C around 481 Ma (Ar–Ar; Cunha deSouza 1996). These data can be used to indicate a deeplevel (10- to 15-km depth) of pluton emplacement andskarn formation, and subsequent uplift.

Seridó Belt ore deposits

The SMB shows a characteristic presence of mineraldeposits, with a significant variety of mineral resources.Most of these deposits were discovered at the beginning ofthe 1940s and have been explored since then. The mainores exploited are those found in skarn deposits (W–Au),pegmatites (Ta–Nb), and in quartz veins (Au).

Most of the skarn deposits appear to be related in spaceand time to plutons of the Brasiliano igneous suite (610–530 Ma). Their occurrence is close to these plutons, butsome skarns are distal in character (e.g., Malhada Limpaand Quixaba mines). For these latter skarns, shear zonesappear to be the main conduit of fluid flow responsible fortheir formation. In most cases, the skarns are stratacontrolled and occur within marble beds, or at themarble–schist (or gneiss) contact, but locally, skarns alsooccur within gneiss and schist, or within the related igneousrocks (endoskarn; e.g., Bodó and Itajubatiba skarns).

The Ta–Nb pegmatites are related to the igneous rocks ofthe Brasiliano suite, and some of them crosscut massiveposttectonic skarns (e.g., Brejuí, Bodó, and Itajubatibamines—Barbosa et al. 1969; Zanini and Santos 1979;Souza Neto 1999). Most pegmatites intrude the schists(Seridó Formation) and also the underlying quartzites of theEquador Formation. These pegmatites can be homogeneousor heterogeneous in type; the latter ones present a largervariety of minerals and with economical potential (Johnstonand Vasconcellos 1945; Da Silva 1993). The homogeneouspegmatites are relatively older and intruded along litholog-ical and structural discontinuities (e.g., foliation), whereasthe heterogeneous pegmatites were emplaced along tensiongashes and other dilational structures (Araújo et al. 2001).

In the SMB, gold–quartz veins mainly occur within micaschists of the Seridó Formation and are typical shear-zone-controlled mineralizations, located along kilometric-scaleshear zones (Luiz-Silva 1995, 2000; Araújo et al. 2002).Locally, these mineralizations show a close relationshipwith Brasiliano plutons, which are also shear zonecontrolled. The gold–quartz veins are interpreted to havebeen formed during the Cambro-Ordovician ductile-brittlereactivation of Neo-Proterozoic strike-slip shear zones(Araújo et al. 2002, 2003) during two episodes: 510–520Ma and 500–506 Ma (40Ar/39Ar on biotite and muscovitegenetically related to the gold mineralization; Araújo et al.2005).

The principal aspects of these three categories of mineraldeposits of the SMB (W–Au skarns, Ta–Nb pegmatites, andgold–quartz veins) are described below. These descriptionsare based on the reviews by Santos and Brito Neves (1984),Delgado et al. (1994), and Beurlen (1995).

W–Au skarns

There are almost 700 occurrences of tungsten skarns in theSMB (Santos 1973; Lima et al. 1980) but only one gold–tungsten skarn and one gold skarn. Since the discovery oftungsten deposits in SMB at the beginning of 1940s until1992, it was estimated that about 60,000 t of WO3

concentrate was produced in this Brazilian region (Beurlen1995). During the 1980s, the low-cost wolframite productionin China promoted a significant decrease in internationaltungsten price, and consequently led to the progressivecollapse of the scheelite exploitation in SMB by the mid1990s. Until 2006, scheelite has been rudimentarily exploitedonly in the Bodó mine, with a small production (between 30and 200 t/year of scheelite concentrate), and in the Brejuímine, where there is a minor seasonal exploitation activity.The major skarns in SMB are Brejuí, Bodó, Bonfim, Bonito,Malhada Limpa, Quixaba, and Itajubatiba (Fig. 1; Table 1).Except the Itajubatiba skarn that contains gold and Bonfimskarn that hosts both tungsten and gold, all the other deposits

Miner Deposita (2008) 43:185–205 191

are tungsten skarns. Particularly the Malhada Limpa skarnscontain extensive molybdenite (e.g., Timbaúba mine; Santoset al. 1972; Lima et al. 1980).

The Itajubatiba (Au) and Bonfim (W–Au) skarns sharesimilar characteristics such as host rocks, skarn morpholo-gy, intrusive rocks, prograde and retrograde mineralogy(with some small differences mainly in retrograde miner-als), and ore minerals (Table 1) with other skarns in theSMB. At Brejuí, pyrite appears as the predominant sulfide,whereas pyrrhotite is the main sulfide in the Bonfim andItajubatiba skarns. Only viewed differences appear in garnetcompositions (Table 2; Fig. 2a) that are more almandine-spessartine rich in the Bonfim (W–Au) and Itajubatiba (Au)

skarns than in the Brejuí (W) skarn, whereas pyroxenecompositions are similar (Table 2; Fig. 2b) in these threeskarns. These characteristics suggest that Bonfim andItajubatiba could be considered reduced skarns, whereasBrejuí appears to be an oxidized tungsten skarn, accordingto Einaudi et al. (1981). Gold is clearly part of the late-stageskarns in Bonfim, as it fills brittle-ductile faults andfractures crosscutting both the prograde and retrogradeskarns. At Itajubatiba, sulfides and probably gold aremainly shear and fracture controlled. Bismuth and telluriumminerals are associated with Fe–Cu sulfides and gold atBonfim, but the Itajubatiba skarns are characterized by Fe–Cu sulfides only.

Table 2 Average microprobe analyses of skarn minerals from the Bonfim W–Au deposit

Garnet Pyroxene Amphibole Plagioclase Clinozoisite Epidote

Host rock, numberof analyses

Schist n=3 Marble n=9 Schist n=14 Marble n=3 Schist n=6 Schist n=7 Schist n=3 Schist n=1

SiO2 (wt.%) 37.14 53.78 52.05 56.68 48.90 43.84 40.15 38.92TiO2 0.19 0.02 0.04 0.11 0.44 0.02 0.00 0.09Al2O3 20.32 0.31 0.32 2.13 6.86 35.65 32.55 27.88Cr2O3 0.14 0.02 0.07 0.01 0.18 0.01 0.02 0.23FeO 25.89 9.74 12.84 5.74 17.90 0.13 0.77 6.92MnO 5.88 0.83 0.84 0.24 0.63 0.01 0.10 0.25MgO 1.22 12.30 10.12 20.23 10.70 0.02 0.03 0.01CaO 9.20 25.05 23.95 13.45 12.43 19.95 24.79 23.92Na2O 0.01 0.08 0.08 0.22 0.49 0.32 0.16 0.00K2O 0.00 0.01 0.00 0.17 0.62 0.01 0.02 0.00Totala 100.00 102.14 100.29 98.98 99.16 99.95 98.60 98.22Si 5.959 1.985 1.985 7.800 7.158 2.032 3.024 3.054Ti 0.023 0.001 0.001 0.012 0.049 0.001 0.000 0.005Al 3.843 0.014 0.015 0.346 1.184 1.948 2.890 2.579Cr 0.023 0.001 0.003 0.002 0.027 0.000 0.001 0.019Fe3+ 0.176 0.000 0.016 0.000 0.000 0.049 0.454Fe2+ 3.298 0.301 0.393 0.660 2.192 0.005Mn 0.800 0.026 0.027 0.028 0.078 0.000 0.006 0.016Mg 0.292 0.677 0.575 4.148 2.335 0.001 0.003 0.001Ca 1.582 0.991 0.979 1.983 1.950 0.991 2.001 2.011Na 0.004 0.006 0.006 0.058 0.139 0.029 0.023 0.000K 0.001 0.000 0.000 0.029 0.116 0.001 0.002 0.000Total cations 16.000 4.000 4.000 15.066 15.229 5.008 8.000 8.141Oxygens 24 6 6 23 23 8 12.5 12.5End member percentagesAl 55.186Gr 21.520Sp 13.378Di 67.337 56.713Hd 30.079 40.609An 97.1

The EMP analyses were carried out at the Centre d’Analyse par Microsonde pour les Sciences de la Terre (CAMST) of the Université Catholiquede Louvain, Belgium, using a CAMECA SX-50 microprobe. Elements were determined by energy-dispersive analysis at acceleration tension of15 kV, a current intensity of 20 nA, and a live counting time of 10 s (44 s for F); the size of the analytical spot was 2.5×1.6 μm; ferric iron ingarnet, pyroxene, and epidotes (all iron calculated as Fe3+ ) is reallocated following Droop (1987); amphibole formula is the average of minimumand maximum ferric iron estimation according to Schumacher (1997).a Pertains to rows 1 to 10

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Another mineral of economic interest is fluorite, whichappears in tungsten skarns in the SMB and has beenexploited in the Brejuí and Quixaba mines and surround-ings (Beurlen 1995).

Ta–Nb pegmatites

More than 700 mineralized pegmatites occur in an area ofabout 15,000 km2 in the SMB, named Borborema Pegma-tite Province. They are of economic importance because oftheir tantalite–columbite, beryl, cassiterite, and kaolincontents. Lepidolite, spodumene, amblygonite, feldspars,muscovite, and gems (mainly aquamarines and blue andgreen tourmalines) are also extracted (Santos and BritoNeves 1984; Da Silva 1993; Beurlen 1995; Da Silva et al.1995). These granitic rocks contain sulfides (chalcopyrite,bornite, chalcocite, and molybdenite), native bismuth,bismutite, apatite, a large variety of phosphates, and raremonazite (Santos and Brito Neves 1984). The production oftantalite concentrates was irregular since the Second WorldWar and varied between 2 and 100 t/year (Beurlen 1995).

In the SMB, the Isidoro-Riachão mine (neighboringBodó mine; Fig. 1 and Table 1) contains scheelite, whichoccurs disseminated or in pockets with pyrite and molyb-denite in an ore body that is 700 m long and crosscuts thegneiss of the Jucurutu Formation and the basement (Melo1961; Santos and Brito Neves 1984). The other twoscheelite occurrences in pegmatites crosscut the basementgneiss about 120 km west of the town of Currais Novos(Fig. 1) and contain scheelite, molybdenite, and subordinateamounts of bismuthinite, powellite, pyrite, chalcopyrite andcalcite, whereas the other occurrence displays only scheeliteand molybdenite (Lima et al. 1980).

Gold–quartz vein deposits

Gold–quartz vein mineralization in the SMB is hosted bymedium- to high-grade amphibolite facies rocks (mica schistsof the Seridó Formation, biotite gneiss of the JucurutuFormation, and basement gneiss). The gold–quartz veinsoccur particularly in medium- to high-angle shear zones(Brasiliano orogeny) and are essentially composed of quartz,although some veins also contain significant garnet and/or sillimanite. The schists within the garnet zone alteredto cordierite–andalusite zone, and sillimanite–muscovitezones, which hosts the gold-bearing veins, whereas the biotitegneiss altered to gold-bearing mica schists (biotite, muscovite,garnet, staurolite, sillimanite±andalusite), quartzites(muscovite±sillimanite), and calc-silicate gneisses (biotite,Ca–Fe–amphibole, garnet). The basement gneiss altered togold-bearing muscovite (±sillimanite) schists, sillimanitequartzite, and calc-silicate gneisses (Luiz-Silva 2000).

The wall-rock alteration zones related to gold–quartzveins contain biotite and muscovite (Luiz-Silva 2000;Araújo et al. 2005), sillimanite (Luiz-Silva 2000), andtourmaline (Beurlen 1995). Based on crosscutting relation-ship between ductile and late-stage ductile-brittle fabrics, a523-Ma pegmatite swarm along these fabrics, and low-temperature retrograde alteration in the mylonite zonessuggest that gold deposition of gold–quartz veins in theSMB postdates the high-temperature deformation andoccurred during late Brasiliano ductile-brittle reactivationof strike-slip shear zones (Araújo et al. 2002, 2003).

In the gold-bearing environments, native gold occurs inits free state, or disseminated within the host rocks, but alsoassociated with Fe±Cu±Pb sulfides or Fe–Ti oxides (Luiz-Silva 2000) and locally arsenopyrite and sphalerite have

Fig. 2 Compositional diagramsfor garnets (a) and pyroxenes(b) from Brejuí, Bonfim, andItajubatiba deposits. Number inparentheses corresponds to thequantity of analyses. Gr Gros-sular, Sp+Al sperssartine+almandine, Ad andradite, Didiopside, Jo johannsenite, Hdhedenbergite

Miner Deposita (2008) 43:185–205 193

also been reported in the mineralized veins (Beurlen 1995).The geochemical association of Au±Cu±Bi±As±Pb iscommonly found in the gold–quartz veins of the SMB(Luiz-Silva 2000).

North of the Bonito (Santos 1968) and between theBrejuí and Quixaba tungsten mines (Roy 1966; Ferreira1967; Lima et al. 1980; Fig. 1), scheelite-bearing quartzveins are also reported. The Bonito mine is considered avein field, with quartz veins containing biotite, chlorite,besides pyrite, bismuthinite and stibnite. About 25 kmnorthwest from the town of Lajes (Fig. 1), tungsten-bearingquartz veins are related to massive ferberite, both located inthe basement gneiss. These veins also contain minoramounts of garnet, biotite, plagioclase, as well as hematitewithin fractures and scheelite, which forms small coatingson fractures that crosscut the ferberite crystals (Mello 1970;Santos and Brito Neves 1984). Barite and fluorite are otherminerals of economic interest found in quartz veins thatcrosscut, or as stratiform lenses within, quartzites andgneisses of the Equador and Jucurutu Formations, respec-tively (Beurlen 1995). Fluorite is also reported in quartzveins that crosscut marble of the Jucurutu Formation.

Present reserves at the two main gold–quartz veindeposits in the SMB are estimated to be about 4 t of goldwith a range of grades between 2.5 and 6.6 ppm Au, locallyin the supergene profile grades of up to 100 ppm arereported. At the São Francisco deposit, 50 km east ofCurrais Novos (Fig. 1), a recovery of 1–3 ppm from theprimary ore in the host rock and 0.3–0.5 ppm from thetailings is reported by Ferran (1988). The other golddeposits located around 100 km west of Currais Novos(Fig. 1), in the São Fernando-Caicó district, have inferredreserves of 1.03 Mt of ore with an average grade of 3.2 ppmof Au (Luiz-Silva 2000). It is estimated that between 1.0and 2.2 t of gold have already been extracted from thesedeposits (Luiz-Silva, personal communication). During theearly 1990s, about 35 kg of gold was extracted per monthin the São Francisco deposit (Carvalho 1990) only.

Geology of the Bonfim mine area

The Bonfim tungsten–gold skarn deposit is situated in thestate of Rio Grande do Norte, northeastern Brazil, about 27km southeast of Lajes (Fig. 1). This deposit has a proventotal reserve of 97 tonnes of scheelite and an ore gradeaverage of 4.8-wt.% WO3 (Farias et al. 1973; Table 1).Scheelite exploitation occurred between 1969 and 1977 butwas interrupted by a series of gallery collapses (Lima Netoet al. 1985). In the early 1990s, gold was discovered in thetailings of the abandoned mine, and from this material,about 0.1 t of gold has been extracted. In mineralizedskarns, the ore grade varies between 2 and 6 ppm of Au but

Fig. 3 Geological map of the Bonfim W–Au–Bi–Te deposit (simpli-fied from Souza Neto 1999)

194 Miner Deposita (2008) 43:185–205

can be up to 60 ppm. Gold anomalies of up to 100 ppmhave been detected in soil profiles (Docegeo 1996).

Metamorphic rocks

In the Bonfim area, the basement consists of gneiss andaugen gneiss (Fig. 3). The gneiss consists of gray, medium-to coarse-grained rock composed of biotite, quartz, alkalifeldspar, plagioclase, garnet, chlorite, apatite, zircon,magnetite, and minor hematite and ilmenite. They alsocontain amphibolite lenses (1.0–1.5 m wide) that arecomposed of hornblende, plagioclase, pyroxene, titanite,and allanite. The augen gneiss consists of alkali feldspar,plagioclase, quartz, biotite, opaque minerals, chlorite,titanite, epidote, zircon, and apatite. It is medium- tocoarse-grained and displays a porphyritic texture (Carvalho1990; Barbalho 1991; Carneiro Filho 1994). In both typesof gneisses, chlorite is likely a retrograde metamorphicmineral, as well as the epidote in the augen gneiss.

The Seridó Group metasedimentary rocks occur in theBonfim area in three dominant lithologies: marble, quartz-ite, and schist, which belong to the Jucurutu, Equador, andSeridó Formations, respectively. Based on drill coreobservations, marble forms 15- to 20-m-thick layers withinthe schist (Fig. 4), which could reflect either an originallyinterbedded sedimentary feature or the result of a faultedsliver within schist. The fine- to medium-grained marblecontains calcite (minor dolomite), phlogopite, apatite, and

quartz and is white with brown, 2- to 5-mm-widephlogopite-rich layers. At the Bonfim mine site, the marblecan also show dark gray and orange colors.

The medium- to coarse-grained quartzite occurs aselongated white to light gray lenses within up to 100-m-wide marble and schist (Fig. 3) and is essentially composedof quartz, phlogopite, and muscovite and locally fuchsite.

In the eastern portion of the Bonfim area, biotite- andanthophyllite–phlogopite-bearing schists constitute the maingeological unit and occur at the sheared contact of thebasement migmatites (Fig. 3). The biotite-bearing schists arefine-grained and composed of plagioclase, quartz, zircon,apatite, and ilmenite (±garnet, cordierite, staurolite, anda-lusite, sillimanite, graphite, allanite). Sillimanite is presentin sheared bands, whereas garnet, cordierite, staurolite, andandalusite appear in 0.1- to 4-m-wide bands, parallel to theprincipal foliation. These bands appear every 10 to 20 m,and locally, they can be monomineralic. Late chlorite andmuscovite usually overgrow andalusite, staurolite, and cor-dierite porphyroblasts. The schists also exhibit calc-silicatelenses consisting of amphibole–plagioclase that are up to 60cm long and 20 cm wide.

The anthophyllite–phlogopite-bearing schists containgarnet and minor tourmaline, zircon, monazite, epidote,and apatite. These minerals are largely replaced by whitemica, chloritoid, and quartz. Fuchsite replaces phlogopiteand occurs as up to 2-cm-long elongated lenses. The schistscontain anomalous high MgO (15.5 wt.%) and transition

Fig. 4 Geologic section T5 through the workings of the Bonfim mine (simplified from Lima Neto 1975)

Miner Deposita (2008) 43:185–205 195

metals (574-ppm Cr, 237-ppm Ni, 51-ppm Co, and 198-ppm V) when compared to the biotite-bearing schists atBonfim and the other metapelitic schists in the SMB. Asimilar geochemical signature (25.5-wt.% MgO, 1,545-ppmCr, 1,728-ppm Ni, and 93-ppm Co) has also been observedin anthophyllite-bearing schists of the Bonfim area byCarvalho (1990), who interpreted them as komatiite-bearingmetaigneous rocks.

Structure

The dominant structural features in the Bonfim region areNE-trending km-scale shear zones. They have a penetrativemylonitic foliation that strikes about N30E and dip either tothe NW or SE at 40–88°. A stretching lineation associatedwith this foliation plunges 31° toward N18E. Transpositionand folding (mainly of isoclinal and intrafolial types) of apreexistent planar surface are associated with the shearmovement. The most conspicuous structure at Bonfim,however, is an about 3-km-wide antiform, with the corecomposed of granite (Fig. 3).

The main faults in the Bonfim area are high-angle faultsand have an average strike of N86E and dip subvertically.These faults are abundant at the mine site where they showtypical brittle-ductile features (slightly plastic displace-ments at their rim resulting in asymmetrical patterns) anda dominant sinistral movement. Several subsidiary high-angle faults (and fractures) strike about N15E and N70Wand dip subvertically. All of these faults contain up to 3-cm-wide vein fillings made-up of quartz, calcite, quartz–amphibole, and gold that crosscut the skarns.

Granite and pegmatite

The granite stock in the Bonfim deposit is correlated withthe regional Brasiliano igneous suite. This igneous rockintrudes the Seridó Group metasedimentary rocks (Fig. 3).It is pink with a fine-grained texture and composed ofquartz, alkali feldspar, biotite, plagioclase, and minor whitemica, apatite, zircon, rutile (needle-shaped inclusions inbiotite), and primary magnetite, which is mostly altered tohematite. No evidence of metasomatic alteration or endo-skarn, as well as no deformation or a thermal aureole hasbeen observed in this igneous rock.

Many pegmatite bodies intrude the schists and thegranite. The pegmatites contain significant columbite–tantalite and beryl, and some of these bodies have beenmined, mainly for Ta–Nb, for example in the northern partof the area (Fig. 3). Late E–W diabase dikes crosscut thebasement, schists, and skarns at Bonfim. These dikes arecorrelated to the well-documented regional Mesozoic(120–140 Ma) Rio Ceará-Mirim magmatism (Gomes et al.1981).

Petrography of the Bonfim W–Au skarns

Late in the tectono-metamorphic history of the Bonfimregion, a metasomatic event transformed marble and schistinto skarn. The timing of skarn formation is mainlyconstrained by the irregular contacts between the skarnsand the country rocks. These contacts also crosscut themain structural fabric of the region. The skarn bodies occuralong the main structural direction and constitute massivebodies but lack a foliation. Skarns show banding structurethat is attributed to metasomatic zoning, which overprintsthe foliation of country rocks.

A maximum age for these skarns can be estimated basedon the age of the granitoid rocks dated between 555 and480 Ma. This age interval corresponds to the lower agereported of the igneous rocks (maximum value), which areconsidered to be the source of fluids involved in the skarnformation, and the lower age obtained for pegmatite bodies(minimum value) that crosscut some skarns in the Brejuí(Barbosa et al. 1969; Salim 1993) and Itajubatiba deposits(Souza Neto 1999). There is a good correlation of the agesestimated for the skarns (510–520 Ma) and the gold–quartzveins in schists (500–506 Ma based on 40Ar/39Ar onmuscovite and biotite; Araújo et al. 2005).

Whole rock geochemical analyses of Bonfim skarnsrevealed high bismuth (475- to >2,000-ppm Bi) andtellurium contents (up to 510-ppm Te), besides Au and W(Souza Neto 1999). Recently, data from explorationcompanies indicate that Au and Bi can reach 106 and26,000 ppm, respectively.

At Bonfim, most skarn horizons occur in the easternflank of the antiform, and they appear to be the lateralequivalent of larger marble horizons, which are dominant atthe western flank of this structure. The two larger skarnsclose to the granite intrusion are barren, whereas theeasternmost smaller skarns are mineralized (Fig. 3). Thebarren skarns are essentially composed of diopside,whereas the mineralized skarns exhibit a more variablemineralogy. The difference in the skarn mineralogy iscontrolled by the skarn protolith, and it appears that thediopside-rich skarn replaces marble only, whereas the skarnformed at the schist–marble contact presents a highermineralogical diversity. Therefore, two types of skarnshave been recognized at the Bonfim deposit: skarnsreplacing marble and skarns replacing schist, which weredeveloped at the schist–marble contact. The metasomaticzones (from the original rock towards the inner skarn)relating to each skarn type can be summarized as: (1) skarnsreplacing marble: phlogopite marble/wollastonite (olivine)marble/tremolite-magnesio-hornblende skarn/diopsideskarn/tremolite-magnesio-hornblende skarn/wollastonite(olivine) marble/phlogopite marble; and (2) skarns replacingbiotite schist: schists/actinolite-magnesio-hornblende-bearing

196 Miner Deposita (2008) 43:185–205

schists/actinolite-magnesio-hornblende skarn/diopside–anorthite skarn/almandine skarn/diopside–anorthite skarn/actinolite-magnesio-hornblende skarn/actinolite-magnesio-hornblende-bearing schists/phlogopite marble. Most skarntypes are calcic, but some magnesian skarns occur in marblethat are marked by olivine, tremolite, and retrograde talc. Theextent of the hydrothermal alteration in the country rock issignificant, as an estimate of the proximity of the skarns isused as regional exploration guide for the search formineralized skarns (Souza Neto et al. 1997).

Skarns replacing marble

These skarns are up to 10 m thick in outcrop, with green-colored and medium- to coarse-grained granoblastic rocks.They show a massive structure and irregular-shapedcontacts with pockets of remnant marble. Prograde skarnminerals include diopside, tremolite-magnesio-hornblende,and minor titanite, apatite, and allanite. Titanite oftencontains an ilmenite core and rutile inclusions. The mainretrograde minerals observed are talc, serpentine, chlorite,recrystallized calcite, and sericite in marble horizons.

The proximity of the skarns replacing marble is indicatedby marble that contains chlorite (with rutile inclusions),± muscovite±allanite±recrystallized calcite with multipleopaque inclusions. The marble in contact with skarns alsocontains wollastonite and olivine (partly serpentinized), withonly locally minor phlogopite lenses. Up to 0.3 mm largeilmenite is associated with phlogopite. At the marble–skarncontact, sericite also occurs associated with phlogopite.

Skarns replacing biotite schist

These skarns occur as elongated beds that are 5–10 mwide inoutcrops. They commonly show a banded structure and aremedium- to coarse-grained granoblastic rocks. In drill core,these skarns occur as 1- to 5-m-thick lenses, which arepreferentially localized at the schist–marble contact. Theskarn–schist contact is irregular in shape, whereas the contactbetween metasomatic zones is gradational. These zonesappear either as alternating (e.g., garnet–amphibole andpyroxene–amphibole) or mixed bands. The typical mineral-ized skarn in the Bonfim mine exhibits alternating plagio-clase and amphibole–pyroxene bands. Some biotite lenses(up to 5 cm long) occur as remnants within these skarns.

Almandine, diopside, anorthite, and actinolite-magnesio-hornblende are the prograde minerals observed in theseskarns. Multiple round biotite inclusions are observed inactinolite-magnesio-hornblende. Accessory prograde min-erals are titanite, apatite, allanite, zircon, and monazite.Titanite and allanite are in equilibrium, with titanitetypically containing an ilmenite core. Zircon and monazitepreferentially occur as inclusions in amphibole.

The EMP analyses of prograde garnet, pyroxene,amphibole, and plagioclase are shown in the Table 2.

Scheelite, molybdenite, magnetite, and ilmenite are themain ore minerals in the prograde skarn. At the Bonfimmine, scheelite occurs in four mineralized skarn bodiesdistributed within the marble and at the marble–schistcontact. The main mineralized body occurs at the basalmarble–schist contact, with the highest amount of scheelitecontrolled by fold hinges (Potyra 1978; Salim 1979; Lima1982). The main ore body is 5–120 cm thick (average 40cm) and has an average ore grade of 4.8-wt.% WO3.Scheelite occurs in parallel zones within the skarns, and itsanhedral crystals reach up to 5 mm in size.

Scheelite contains inclusions of pyroxene, titanite, apa-tite, chalcopyrite, and molybdenite and is intensively frac-tured (Farias et al. 1973). In the main ore shoots, garnetand vesuvianite are absent (Lima 1982). Scheelite at theBonfim deposit also occurs in ultramafic rocks, whichcontain serpentine, chlorite, and talc, and are crosscut bythin asbestos veins. Scheelite occurs either as dissemi-nated porphyroblasts (Fig. 5a) or as coarse-grained aggre-gates (Santos and Brito Neves 1984). The relationshipbetween these ultramafic rocks and the skarns is stillunknown.

Molybdenite occurs associated with scheelite and alsowithin alkali feldspar veins, which could be related toremobilization of the prograde sulfide. Ilmenite preferen-tially occurs in skarns and close to the skarn–schist contact.It generally exhibits a rutile core and is enclosed withinalmandine. Late hematite, replacing magnetite, and goethite(product of supergene alteration of ankerite) are alsoobserved in these skarns.

PIXE analyses (Table 3) revealed that the main traceelements found in prograde ilmenite and titanite are asfollows (grouped by mineral, in ppm): ilmenite contains Nb(585) and titanite contains F (2,660), Y (1,350), Nb (660),and Zr (138).

The retrograde stage of skarn formation has thefollowing successive mineral assemblages: (1) alkali feld-spar, common in skarns formed at the marble–schistcontact; feldspar content increases toward the schist,probably due to the aluminous composition of the latter;(2) clinozoisite–zoisite–sericite assemblage, probablyformed by replacement of prograde anorthite, as suggestedby the occurrence of clinozoisite–zoisite zones in spatialcontinuity with incipient-altered anorthite, and sericite(calcite) pseudomorphs preserving relicts of anorthitetwinning; and (3) calcite-rich assemblage composed ofrecrystallized calcite and remnants of chloritized biotite.Quartz, calcite, and quartz–calcite veins (up to 5 cm wide)crosscut these skarns, and chlorite-filled microfracturescrosscut prograde diopside. Locally, ankerite-filled micro-fractures also relate to this stage.

Miner Deposita (2008) 43:185–205 197

Fig. 5 Prograde scheelite skarn, outer altered rocks, and retrograde gold-bearing skarn from the Bonfim W–Au deposit, northeastern Brazil. aPrograde diopside–anorthite skarn (gray green) with aggregates ofeuhedral scheelite (Sch) aligned parallel to banding. Late-stage gold–sulfide replacement (Au, black) follows fractures and cuts across thebands. Sample kindly provided by E. J. Santos. b Outer anthophyllitealteration in schist, anthophyllite (Ath) in phlogopite-rich schist (Phl), incontact with clinozoisite–zoisite (Czo–Zo) partly replaced by white mica(Wm). Drill hole FD-1, 27.8 m, plane-polarized light. c Outer biotitealteration in schist, vein of biotite (Bt) and cummingtonite (Cum) cutsacross prograde garnet (Grt) and actinolite (Act) skarn. Outcrop 10, plane-polarized light. d Retrograde gold-bearing skarn, gold–sulfide replace-ment (black) following fractures crosscutting retrograde clinozoisite–zoisite–sericite (pink) and prograde diopside–anorthite (green) bands. The

lens cover is 6 cm across. Sample from mine dumps. e Retrograde gold-bearing skarn, concentric bands of prehnite (Prh) and epidote (Ep) incontact with gold–sulfide aggregates (opaque, see Fig. 5f). Sample Bi-1A,from mine dumps, crossed-polarized light. f Retrograde gold-bearingskarn, contact relationships between chalcopyrite (Ccp), bismuthinite(Bmt), native bismuth (Bi), and sphalerite (Sp). Sample Bi-1A, from minedumps, reflected light in air. g Retrograde gold-bearing skarn, joseite (Js)containing lamellas and blebs of native bismuth (Bi). Numbers correspondto microprobe analyses (Table 4), spots 9 and 10 are in joseite and spots11 and 12 in bismuth. Sample named BONFIM, from mine dumps,reflected light in air. h Native gold (15-wt.% Ag, Table 4) disseminated inretrograde epidote–prehnite skarn crosscutting prograde diopside–anorthite (Di–An) bands. Sample named BONFIM, from mine dumps,reflected light in air

198 Miner Deposita (2008) 43:185–205

The EMP analysis of retrograde clinozoisite–zoisite isshown in Table 2. PIXE analyses (Table 3) revealed that themain trace elements found in clinozoisite–zoisite andsericite are as follows (grouped by mineral, in ppm):clinozoisite–zoisite contains F (9,490), Sr (350), In (330),Y (200), and Pb (165); sericite contains Cu (915), Rb (825),Zr (137), Ga (75), Pb (60), and Sr (45).

High F content (up to 9,490 ppm) has been found intitanite and clinozoisite–zoisite (Table 3), which indicatesfluorine-rich metasomatic fluids during the formation ofprograde and retrograde skarns at Bonfim (Souza Neto etal. 1998). Similar fluorine-rich compositions are usuallyreported in rare-metal-bearing skarn systems (Newberry1998; Aksyuk 2000).

Outer alteration in schists

Both the biotite- and the anthophyllite–phlogopite-bearingschists in the vicinity of skarns show some features thatindicate proximity to the skarns. The prograde actinolite-magnesio-hornblende assemblage replaces biotite andphlogopite and is the dominant metasomatic replacementobserved in both schist types. This reaction is typical forschists in the several meters wide alteration halo around theskarns. The resulting actinolite-magnesio-hornblende-

bearing schists show a characteristic banded texture withactinolite-magnesio-hornblende bands interlayered withbiotite or anthophyllite–phlogopite bands.

Biotite-rich lenses (about 5 cm long) are preserved withinactinolite-magnesio-hornblende-rich bands. The actinolite-magnesio-hornblende-bearing schists also contain stronglyaltered plagioclase and chloritized garnet. Minor mineralsinclude ilmenite (rutile), apatite, tourmaline, allanite, andtitanite. Large apatite, tourmaline, and titanite are up to 1.0–1.5 mm in diameter. Titanite often contains an ilmenite coreand occurs aligned parallel to the banding. Quartz bands(about 3 cm wide) or pockets and alkali feldspar–calciteveinlets (up to 2 mm wide) are also present within theactinolite-magnesio-hornblende-rich bands of the schists.

At Bonfim, the skarn has an outer alteration halo ofactinolite-magnesio-hornblende in biotite-bearing schist,whereas the anthophyllite–cummingtonite alteration is pref-erentially developed in the phlogopite-bearing schist. In thelatter, anthophyllite occurs either associated with clinozois-ite–zoisite as prismatic-shaped pseudomorphs or parallel tothe foliation of the rock (Fig. 5b). Late biotite–cumming-tonite veins crosscut the skarns (Fig. 5c), and cummington-ite alteration also occurs widespread parallel to the bandingof the skarns. White marble adjacent to the skarns formed atthe marble–schist contact and contains clear calcite.

Table 3 PIXE analyses of minerals in prograde diopside skarn (first two minerals), retrograde clinozoisite–zoisite–sericite skarn, and retrogradeepidote–prehnite–sulfide skarn (next three minerals), and from a postskarn vein (carbonate) of the Bonfim W–Au deposit

Sample FD-8/11 FD-8/11 B-97/1 FD-8/11 FD-8/11 B-18/1Host rock Schist Schist Schist (?) Schist Schist SchistMineral Ilmenite Titanite Clinozoisite Sericite Epidote Carbonate

As (ppm) <30Cu 915 497 140Pb 165 60 160Zn <30Ba <250Rb 825Sr 350 45 85 405In <120 330 <90 150 <65W <75Ga 75 23Ge <30Nb 585 660Y 1,350 200 150Zr 138 137F 2,660 9,490 <120

The PIXE analyses were carried out using a proton miniprobe at the CERI laboratory of the Centre National de la Recherche Scientifique (CNRS)in Orléans, France. Analytical conditions were a proton beam of 2.5 MeV, a spot of 1,500 μm2 , and a current of 0.7–0.8 nA (see Maxwell et al.1988 and Volfinger et al. 1997 for analytical details). The sample B-97/1 is a retrograde clinozoisite–zoisite–sericite skarn from the mine site, B-18/1 is a diopside skarn from outcrop 18, and FD-8/11 shows clinozoisite–zoisite–sericite and epidote–prehnite–sulfide skarns and carbonate veinfrom the drill hole FD-8, 70.4 m. See Fig. 3 for mine, outcrop, and drill hole locations. The symbol “<” present in some element amounts meansthat the respective element was detected but could not be quantified because its quantity was close to the detection limit of the analyses. Values inblank refer to elements not detected.

Miner Deposita (2008) 43:185–205 199

Tab

le4

Microprob

eanalyses

ofgo

ldandbism

uthmineralsin

retrog

rade

epidote–prehnite–sulfide

skarn(representativesamplenamed

BONFIM

,from

theminedu

mps;see

Fig.3

forlocatio

nand

Figs.5g

andh)

from

theBon

fim

W–A

udepo

sit

Mineral

Gold

Bismite

Bismite

Bismite

F-bismite

F-bismite

F-bismite

Bi

Bi

Bi

Bi

Bte

Bte

Bte

Joseite

Analysis

124

25+26

2718

1923

45

611

+12

37

89+10

Si(w

t.%)

na0.15

1.01

0.11

0.22

0.29

0.69

nana

nana

nana

nana

Fe

na0.79

1.21

0.68

0.86

0.72

0.88

nana

nana

nana

nana

Mn

na0.04

0.03

0.00

0.05

0.05

0.07

nana

nana

nana

nana

Mg

na0.03

0.39

0.00

0.06

0.06

0.26

nana

nana

nana

nana

Au

84.05

nana

nana

nana

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Ag

14.94

nana

nana

nana

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Bi

0.41

88.26

85.91

85.88

87.98

88.12

86.78

100.89

99.87

100.27

100.28

80.21

80.58

80.72

81.79

Te

0.16

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

10.86

Onc

10.57

11.63

10.18

10.66

10.71

11.20

ncnc

ncnc

ncnc

ncnc

S0.00

nana

nana

nana

0.00

0.00

0.00

0.00

18.22

18.25

18.29

6.27

Fna

0.00

0.00

0.00

1.00

0.88

0.51

nana

nana

nana

nana

Totala

99.55

99.85

100.18

96.85

100.83

100.82

100.38

100.89

99.87

100.27

100.28

98.43

98.83

99.01

98.92

Si(at.%

)nc

0.02

50.14

60.01

80.01

20.01

50.03

5nc

ncnc

ncnc

ncnc

ncFe

nc0.06

40.09

00.05

70.02

30.01

90.02

2nc

ncnc

ncnc

ncnc

ncMn

nc0.00

40.00

20.00

00.00

10.00

10.00

2nc

ncnc

ncnc

ncnc

ncMg

nc0.00

50.06

50.00

00.00

40.00

40.01

5nc

ncnc

ncnc

ncnc

ncAu

0.75

1nc

ncnc

ncnc

nc0.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

0Ag

0.24

4nc

ncnc

ncnc

nc0.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

0Bi

0.00

41.91

81.70

11.93

80.63

20.63

00.59

31.00

01.00

01.00

01.00

02.02

62.03

22.03

14.00

0Te

0.00

20.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

00.00

01.90

2O

nc3.00

03.00

03.00

01.00

01.00

01.00

0nc

ncnc

ncnc

ncnc

ncS

0.00

0nc

ncnc

ncnc

nc0.00

00.00

00.00

00.00

03.00

03.00

03.00

01.09

8F

nc0.00

00.00

00.00

00.07

90.06

90.03

8nc

ncnc

ncnc

ncnc

ncTotalb

1.00

05.01

65.00

45.01

31.75

11.73

91.70

71.00

01.00

01.00

01.00

05.02

65.03

25.03

17.00

0Oxy

gens

33

31

11

Sulph

urs

33

3Tellurium

+sulphu

r3

The

operatingcond

ition

swerethesameas

theanalyses

ofskarnminerals(see

Table2),exceptfor

thecurrent(50

nA)andtheliv

ecoun

tingtim

e(20sforeach

elem

ent;24

sforAg)

used.O

xygen

was

calculated

consideringallelementsin

theox

ideform

,exceptF

.Analyses25

+26

,11+12

,and

9+10

correspo

ndto

theaverageof

twospotson

asamegrain.

The

analyses

11and12

correspo

ndto

anexsolutio

nwith

inthegrainanalyzed

in9+10

(see

inFig.5g

).F-bismite

Fluorine-bearingbism

ite,Bibism

uth,

Bte

bism

uthinite,na

notanalyzed,nc

notcalculated.

aPertainsto

rows1to

11bPertainsto

rows13

to23

200 Miner Deposita (2008) 43:185–205

Late-stage gold mineralization

The age intervals of 510–520 Ma and 500–506 Ma(40Ar/39Ar on hydrothermal muscovite and biotite; Araújoet al. 2005) obtained for the structurally emplaced gold–quartz veins hosted in mica schists are considered a likelyage for the gold mineralization in the Bonfim skarns.

At Bonfim, gold occurs as free grains disseminatedwithin black, fine-grained minerals that fill thin conjugatebrittle-ductile faults and extensional fractures, which cross-cut both the diopside–anorthite prograde and alkali feldsparand clinozoisite–zoisite–sericite retrograde skarns (Fig. 5d).These black minerals also occur as an impregnation in theskarns (Almeida 1994; Fig. 5d). Calcite- and quartz-filledmicroveins crosscut both the prograde skarn and goldmineralization. The gold assemblage exhibits relatively rareminerals, such as bismite (Bi2O3) and fluorine-bearingbismite (75–80 vol%), native bismuth (5–10 vol%),bismuthinite (Bi2S3) and joseite [Bi4(Te,S)3], both 5–10vol%, and minor (<5 vol%) chlorite, epidote, prehnite,chalcopyrite, sphalerite, and gold (Fig. 5e–h).

Gold shows a typical pallid color (up to 14.9 wt.% ofAg; Table 4), and its crystals vary from <5 μm to up to 60μm and are commonly 15 μm in diameter, occurring eitheras individual grains (Fig. 5h) or in contact with nativebismuth, bismuthinite, and joseite. Bismite contains vari-able amounts of tellurium (up to 7.7 wt.%) and appearsto be supergene in origin, probably replacing bismuthtellurides.

Epidote and prehnite occur mainly as concentric bandsaround gold–sulfide aggregates (Fig. 5e). Epidote alsooccurs as round-shaped aggregates and exhibits a pyrrho-tite–chalcopyrite core, or occurs as euhedral pseudomorphspreserving irregular-shaped garnet and filling fracturesassociated with chlorite.

The EMP analyses of gold and bismuth minerals areshown in the Table 4. PIXE analyses (Table 3) revealed thatthe main trace elements found in late-stage epidoteassociated with gold and carbonate in late crosscuttingveins across the skarns are as follows (grouped by mineral,in ppm): epidote contains Cu (497), In (150), Sr (85), andGa (23); carbonate contains Sr (405), Pb (160), Y (150),and Cu (140). Indium appears to be enriched in epidote andclinozoisite–zoisite.

In the Bonfim area, a sulfide-rich retrograde assemblageis found in the epidote skarn replacing prograde garnetclose to the schist contact. Relatively large amounts ofsulfides usually occur in the skarns when they are close tothe schist, but also in surrounding schist. No metal gradesof interest have been reported. Sulfides appear eitherdisseminated or within late quartz–calcite veins crosscuttingprograde skarn. They often contain an epidote aureole thatgenerates a typical dark-green halo. High amounts of quartzand muscovite also occur close to the schist, as well ascrosscutting chlorite-rich bands (up to 8 mm wide) withassociated quartz and quartz–sulfide veins. Sulfides arepresent disseminated within the chlorite bands and aremainly pyrrhotite, chalcopyrite, pyrite, and minor arseno-

Table 5 The main characteristics of the Brejuí, Bonfim, and Itajubatiba skarn deposits

Skarn deposits Brejuí Bonfim ItajubatibaType W W–Au AuSub-type Oxidized Moderately reduced Reduced

Predominant host rock of the ore skarn Marble–gneiss contact Marble–schist contact Marble–meta-tonalitecontact

Predominant sulfide assemblage Pyrite+magnetite Pyrrhotite±pyrite Pyrrhotite+pyritePyroxene composition Hd30–60 Hd11–53 Hd5–75Dominant garnet endmembers Grossular+andradite Almandine+grossular Almandine+grossularW (%) 0.0089–1.861 (1) 0.5–1.0 (2) 0.0021–0.1201 (3) 0.5–9.0, average

4.8 (4)<0.002 (3)

Mo (ppm) 12–722, powellite andmolybdenite (1)

1–2 (3) 104 (3)

W/Mo 7–102 15–21, up to 600 >5Au (ppm) Not found 2–6, up to 60 0.5–2, up to 6.3F (ppm) Not available <9,490a <5,840b

Gold skarn Retrograde Prograde (?)Main calc-silicate minerals associated withthe gold

Epidote, prehnite Amphibole, pyroxene,garnet

References: 1 Salim (1993), 2 Maranhão (1970), 3 Souza Neto (1999), 4 Farias et al. (1973)a Clinozoisite/zoisiteb Titanite (PIXE analysis)

Miner Deposita (2008) 43:185–205 201

pyrite. Pyrrhotite contains chalcopyrite blebs and lamellaeand occurs in the core of the pyrite grains.

At the schist–skarn contact, the chlorite-rich band of theschist overprints the actinolite-magnesio-hornblende skarnand preserves inclusions of this skarn mineral, indicating thatchlorite in the schist was developed post or synchronous withthe formation of the actinolite-magnesio-hornblende skarn.

Discussion

In the SMB, the main tungsten skarns (Table 1) containspyrite as the predominant sulfide, which suggests anoxidized skarn according to the classification proposed byEinaudi et al. (1981). The Bonfim tungsten–gold andItajubatiba gold skarns are exceptions, where pyrrhotitedominates (Tables 1 and 5), indicating that they are reducedskarns.

When comparing the Brejuí (W), Bonfim (W–Au), andItajubatiba (Au) skarns, which at present are the onlydeposits studied in detail, the former is basically distin-guishable from the other two by containing pyrite as thepredominant sulfide, garnet with low almandine andspessartine component, and elevated Mo contents (powel-lite and molybdenite), and no gold (Table 5). The Bonfimand Itajubatiba gold-bearing skarns share some character-istics, with pyrrhotite as the predominant sulfide, garnetwith elevated almandine and spessartine component, andrelatively elevated gold contents (Table 5). Furthermore,rare pyrite in microfractures crosscuts magnetite crystals.The fluorine contents (Table 5) estimated for the Bonfimand Itajubatiba skarns are within the concentration range oftungsten skarns (1,500–55,000 ppm) and contrast withfluorine abundances observed in tin skarns (7,400–185,000ppm), as proposed by Newberry (1998).

In summary, the Bonfim tungsten–gold skarn andItajubatiba gold skarn are reduced skarns; the latter appearsto be slightly more reduced than the former, based on itsrelatively higher Fe–pyroxene component (up to 75 mol%Hd) and almandine and spessartine component of garnet(up to 78 mol%; Tables 1 and 5). At Itajubatiba, skarnsreplacing metasyenogranite (barren) are distinct from theother mineralized skarns and exhibit relatively low alman-dine and spessartine component of garnet (4–5 mol%; Table1). Thus, it seems that the occurrence of gold in the SMBskarns is controlled by their reduced character.

The Bonfim tungsten skarn deposit appears to sharesome characteristic features with the Nevoria gold deposit,an Archean skarn localized in the Yilgarn Craton ofWestern Australia (Mueller 1997; Mueller et al. 2004); forexample: (1) deep crustal setting (10- to 15-km depth) atpressures of 2–4 kbars; (2) Au–Bi–Te element association;and (3) anthophyllite as a distal alteration mineral.

Conclusions

The SMB hosts important tungsten mineralization, as wellas a gold skarn (Itajubatiba) and a recently discoveredtungsten–gold skarn (Bonfim). Although there is a lack ofdetailed studies, except for the Itajubatiba, Bonfim, andBrejuí skarns, the main characteristics of the SMB skarnsindicate that they are dominantly oxidized tungsten skarns.On the other hand, the Itajubatiba and Bonfim gold-bearingskarns exhibit reduced features, such as pyrrhotite as thedominant sulfide, garnet with high almandine and spessar-tine component, and elevated gold contents.

The petrographic and mineral chemistry studies revealeda Au–Bi–Te-rich ore assemblage at Bonfim, which occursas fine-grained black material within conjugate brittle-ductile faults and extensional fractures. The results pre-sented here provide new information about the polymetallicpotential (e.g., Bi and Te) of the Bonfim deposit, which wasknown only for gold and tungsten so far. PIXE data revealthat fluorine (up to 9,490-ppm F) is present in progradetitanite and retrograde clinozoisite–zoisite, which suggests asignificant F activity in the skarn-forming fluids. EMPanalysis showed the existence of fluorine-bearing bismite(0.5- to 1.0-wt.% F) in the ore, which indicates that such F-rich fluids have also participated in the late-stage goldmineralization.

Amphibole-bearing bands in the schist, wollastonite orolivine in the marble, and white-colored recrystallizedmarble are all located in the outer zones of the skarnsand, therefore, may be used as an effective exploration toolfor skarn deposits in the SMB. The SMB not only has avast potential for skarns but also for new gold andpolymetallic deposits.

Acknowledgment The authors express gratitude to Edilton J. Santosand Hartmut Beurlen for the historical data about the skarns of theSeridó region, to Wanilson Luiz-Silva for providing unpublished dataabout gold–quartz veins of the Seridó, to Gaston Giuliani for bismuth-related bibliography, to Jacques Wautier and Marco Bravin formicroprobe analysis and thin section preparation, to Fabriciano L.Neto (Tomaz Salustino company) for access to drill cores, to thedirector and staff of Docegeo Company, to Harrizon L. Almeida(Ecudor company), to Eurico Pereira (Bonfim farm owner) for accessto the mine and private documents, to Adriana B. Garlipp for drawingfigures, and to Hartmut Beurlen and Germano M. Júnior for thereview of the text. The authors also wish to thank the Brazilian Funds(FINEP/PADCT II, Research Project Mass and Fluid Transference inthe Continental Crust) and the Belgian Funds (Fonds National de laRecherche Scientifique, and France–Belgium cooperation of theCommunauté Française de Belgique-Tournesol Project DernièresPhases Magmatiques) for financial support for field work andanalyses. The first author is also grateful to the Brazilian NationalResearch Council, CNPq, for a doctoral grant (process 200.871/95-1).Constructive criticism by Andreas G. Mueller, Craig Hart, SteffenHagemann, and Bernd Lehmann helped to significantly improve themanuscript.

202 Miner Deposita (2008) 43:185–205

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