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IntroductionPlatiniferous mineralization of economic importance occurs

with magmatic sulfides in layered mafic-ultramafic intrusionsand generally has ore grades of a few grams per metric ton(g/t) of Pt plus Pd (e.g., Cawthorn, 2005). Except for alluvialdeposits of platiniferous minerals mechanically derived frommafic-ultramafic sources, bonanza concentrations of Pt andPd with exceptional grades of up to 104 g/t are occasionally

reported from low-temperature oxidizing environments (e.g.,Cabral et al., 2002a). Given that prospecting for Pt-Pd de-posits is commonly targeted at mafic-ultramafic hosts, little isknown about the distribution of platiniferous deposits in theoxidizing milieux and how they formed. Recently, severalstudies have shown that palladiferous gold mineralization as-sociated with oxidizing systems is more widespread than rec-ognized previously (Shepherd et al., 2005; El Ghorfi et al.,2006; Chapman et al., 2009).

In Minas Gerais, Brazil, a major gold rush was engenderedby “ouro preto,” a variety of gold that is coated with dark Feoxide and rare O-bearing Pd phases (e.g., Jedwab and

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THE PLATINIFEROUS Au-Pd BELT OF MINAS GERAIS, BRAZIL, AND GENESIS OF ITS BOTRYOIDAL Pt-Pd AGGREGATES

ALEXANDRE R. CABRAL,1,2,†,* BERND LEHMANN,2 MIGUEL TUPINAMBÁ,3 SANDRA SCHLOSSER,4ROGERIO KWITKO-RIBEIRO,5 AND FRANCISCO R. DE ABREU6

1 Department of Geology: Exploration Geology, Rhodes University, P.O. Box 94, Grahamstown, 6140, South Africa2 Mineral Resources, Technische Universität Clausthal, Adolph-Roemer-Str. 2A, D-38678 Clausthal-Zellerfeld, Germany

3 Tektos-Geotectonic Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua S. Francisco Xavier 524 s. A4016, 20550-050 Rio Janeiro-RJ, Brazil

4 Curt-Engelhorn-Zentrum Archäometrie, D6, 3, D-68159 Mannheim, Germany5 Centro de Desenvolvimento Mineral, Companhia Vale do Rio Doce, Rodovia BR 262/km 296,

Caixa Postal 09, 33030-970 Santa Luzia-MG, Brazil6 Votorantim Metais, Praça Ramos de Azevedo 206, 01037-010 São Paulo-SP, Brazil

AbstractWe delineate an approximately 240-km-long, north-trending Au-Pd-Pt belt in Minas Gerais, Brazil, which

hosts hydrothermal Pd-Pt−bearing gold mineralization in its southern part (Quadrilátero Ferrífero and Itabiradistrict) and alluvial Pd-Pt-Au mineralization in its northern part (southern Serra do Espinhaço). The hydro -thermal Pd-Pt−bearing gold mineralization, locally known as jacutinga, occurs as late-orogenic quartz-hematite-(talc-kaolinite) veins, which truncate the Brasiliano regional tectonic foliation of the itabiritic hostrocks of the Paleoproterozoic Itabira Iron Formation. The alluvial Pd-Pt-Au mineralization occurs as botryoidalPd-Pt nuggets and delicate palladiferous gold aggregates in clastic and organic-rich valley infill on quartzite andhematitic rocks of the Paleo- to Mesoproterozoic Espinhaço Supergroup. The Au-Pd-Pt belt follows the regional trend of major thrust faults related to the ~0.6 Ga Brasiliano orogeny, and the hydrothermal jacutingamineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum. Secondarybotryoidal and arborescent Pt-Pd aggegates and primary (detrital) Au-Pd grains are peculiar to the alluvial pla-tiniferous deposits of the northern part of the belt in the Serra do Espinhaço. We suggest that their source wasjacutinga-style vein mineralization, as deduced from abundant detrital hematite and rutile, and the discoveryof detrital hongshiite, a Pt-Cu alloy that forms bonanza pockets within jacutinga at Itabira. Reconnaissancelaser ablation-inductively coupled plasma-mass spectrometry analyses for trace metals in the botryoidal Pt-Pdgrains show that they have elevated Tl and Se contents of 0.9 to 354 and 54 to 2,700 ppm, respectively. Theseelements correlate positively, with a Tl/Se ratio of about 0.08, a value that contrasts with Tl/Se >1 for hongshi-ite from the hydrothermal jacutinga system at Itabira. The low and roughly constant Tl/Se ratios of the alluvialsamples are close to the average value of 0.12 for fluvial waters. We suggest that the botryoidal Pt-Pd aggre-gates grew in situ by electrochemical metal accretion from dilute solutions carrying Pt and Pd from the disso-lution of detrital Pt-Pd minerals from jacutinga.

The connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend ofabout 240 km for Pd-Pt−bearing gold mineralization reaching from Ouro Preto to Diamantina. The very highoxidation state required for efficient low-temperature aqueous mobilization of Pt, Pd, and Au was apparentlycontrolled by the quartzitic and hematitic rocks in the Serra do Espinhaço, but not in the southern belt wheregreenstone rocks buffered fluids to moderate pH and lower oxidation states.

† Corresponding author: email, A.Cabral@ru.ac.za*Current address: Department of Geology: Exploration Geology, Rhodes

University, P.O. Box 94, Grahamstown 6140, South Africa.

©2009 Society of Economic Geologists, Inc.Economic Geology, v. 104, pp. 1265–1276

Submitted: September 7, 2009Accepted: December 1, 2009

Cassedanne, 1998; Cabral et al., 2003). The rush for ouropreto resulted in several mineralogical and geological de-scriptions; some of the earliest studies indicated the presenceof Pt and/or Pd (e.g., Wollaston, 1809; Mawe, 1812; Es-chwege, 1832; Lampadius and Plattner, 1833; Johnson andLampadius, 1837; Henwood, 1871; Hussak, 1904). In some ofthese studies, evidence is given for the growth of botryoidalPt-Pd aggregates within alluvia. Such aggregates, possibly re-covered from Córrego Bom Sucesso, comprised the samplematerial from which Wollaston (1809) identified native Pdfor the first time (Hussak, 1904; Atencio, 2000) and, alsolikely, from which the element Pd was discovered in 1803(Cassedanne and Alves, 1992; Cassedanne et al., 1996).

Revision of critical works, particularly of the 19th- and early20th-century literature, and recent field work have enabledthe outline of platiniferous alluvia and ouro preto-bearing de-posits in an approximately 240-km-long, north-trending, pla-tiniferous Au-Pd belt, in Minas Gerais. Herein, this belt is de-fined for the first time. The purpose of this communication isto delineate this platiniferous Au-Pd belt based on the geo-graphical distribution of the unusual ore mineralogy. We alsoreport reconnaissance laser ablation-inductively coupledplasma-mass spectrometry analyses of peculiar botryoidal Pt-Pd-(Hg) aggregates found in the northern segment of the pla-tiniferous Au-Pd belt, in an attempt to understand their gen-esis and their exclusive occurrence in a quartzite domain. Themineral belt, in conjunction with the mineralogical observa-tions presented here, has implications for the prospecting of“unconventional deposits” of Au, Pd, and Pt.

Geologic Setting of the Platiniferous Au-Pd beltPlatiniferous minerals and palladiferous gold deposits

occur along a north-trending zone from Ouro Preto towardDiamantina (Fig. 1). In the Quadrilátero Ferrífero andItabira district, Pt and Pd are spatially associated with ouropreto-bearing quartz-hematite-(talc-kaolinite) veins. Theseveins are locally known as “jacutinga” (e.g., Henwood, 1871;Hussak, 1904). They crosscut itabiritic host rocks of the CauêFormation (e.g., Cabral, 2006; Galbiatti et al., 2007), which isa banded iron-formation unit that grades upward intodolomitic rocks of the Gandarela Formation. The Cauê For-mation and the Gandarela Formation form the Paleoprotero-zoic Itabira Group, which overlies clastic metasedimentaryrocks of the Caraça Group, the basal sequence of the MinasSupergroup (e.g., Dorr, 1969; Alkmim and Marshak, 1998).

Uranium-Pb dates on detrital zircon from the lowermost unitof the Caraça Group, the Moeda Formation, define a maxi-mum depositional age of 2.58 Ga (Hartmann et al., 2006).The Moeda Formation unconformably rests on the ArcheanRio das Velhas Supergroup.

Jacutinga is characterized by palladiferous gold with vari-able contents of Ag, Cu, and Hg, together with Pd and Ptminerals (e.g., Henwood, 1871; Hussak, 1904; Clark et al.,1974; Varajão et al., 2000; Kwitko et al., 2002; Cabral, 2006).Platinum is mostly an accessory element in Pd minerals in jacutinga in the Quadrilátero Ferrífero (e.g., Cabral, 2006;Galbiatti et al., 2009). In the Itabira district, a northwesternextension of the Quadrilátero Ferrífero, jacutinga is compar-atively more enriched in Pt. Itabira has Pt-rich aggregatesthat attain a few millimeters across, of which a spectacular ex-ample is the Pt-Cu alloy hongshiite (Kwitko et al., 2002).

Jacutinga commonly occurs within iron ore deposits locatedin the eastern part of the Quadrilátero Ferrífero (Fig. 1).There, itabirite and hematite orebodies exhibit a prominentfoliation that results from the planar arrangement ofhematite. This tectonic fabric that developed under ductileconditions is thought to have formed during the ~0.6 GaBrasiliano fold-and-thrust tectonics (e.g., Chemale et al.,1994; Alkmim and Marshak, 1998). Gongo Soco, once a fa-mous underground gold mine, nowadays an opencast iron oremine, is hitherto the best described jacutinga deposit of theQuadrilátero Ferrífero (e.g., Henwood, 1871; Hussak, 1904;Cabral, 2006). Jacutinga has been studied in two of the Itabirairon ore deposits, Cauê and Conceição, by several workers,including Hussak (1904), Olivo et al. (1995), Cabral (2006),and Galbiatti et al. (2007). In the Itabira and Gongo Soco de-posits, jacutinga manifests itself as predominantly brittle veinsthat truncate the ductile hematite foliation (Cabral, 2006;Galbiatti et al., 2007), a fact that implies an age related to thebrittle stage of the Brasiliano orogenic cycle. Although radio-metric ages for jacutinga in both localities are not well con-strained (vide Lüders et al., 2005, for a discussion on theavailable age data), U-Pb isotope systematic suggests a Brasil-iano age.

The approximate trend of the jacutinga deposits in theQuadrilátero Ferrífero and Itabira can be followed northwardinto the domains of Serra do Espinhaço, where the sametrend is expressed by platiniferous alluvia. Taken together, ja-cutinga and platiniferous alluvia define a north-trending beltthat parallels the west-verging Brasiliano thrust belt from

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FIG. 1. Geologic overview of the Quadrilátero Ferrífero and the southern Serra do Espinhaço in Minas Gerais, Brazil(CODEMIG, 2003), showing the location of representative examples of a variety of auriferous veins with characteristic Pd-Pt signature (jacutinga) and Au-Pd-Pt−bearing alluvia with a historical mining record. Together, the vein and alluvial depositsdelineate a north-trending, platiniferous Au-Pd belt that parallels the trace of major thrust faults, which developed duringthe ~0.6 Ga Brasiliano orogeny. The Brasiliano orogeny generated the pervasive tectonic foliation that is observed insupracrustal rocks. The jacutinga-style Au-Pd-Pt veins consist of specular hematite, with variable amounts of quartz and, sub-ordinately, talc, kaolinite, and Mn oxide, and crosscut the tectonic foliation of the host itabirite. Platiniferous alluvia that con-tain coarse-grained, botryoidal aggregates of Pt-Pd- (Hg) and Au-Pd intermetallic compounds are restricted to the northernpart of the belt, i.e., the quartzite-rich domains of the southern Serra do Espinhaço. The northernmost occurrence of hy-drothermal Pd-bearing Au mineralization is at Diamantina, where quartz-hematite-kaolinite-rutile veins truncate the tec-tonic foliation of the hematitic phyllite. All hydrothermal precious-metal systems of this belt are dominated by hematite andlack sulfide minerals, indicating a high oxidation state. The crosscutting relationship between the veins and the Brasiliano-related tectonic foliation, and their high oxidation state suggest that the veins represent an oxidized variant of the orogeniclode gold deposit spectrum.

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Neoproterozoic terrains,minor Cenozoic deposits

Paleo-/Mesoproterozoic Espinhaço Sg.Quartzite, metaconglomerate, phyllite,hematitic phyllite

PaleoproterozoicSerra da Serpentina Gr.

Ip = Itapanhoacanga Fm.Phyllite (pl), itabirite (black), schist

Itabirite, phyllite, schist

pl

Paleoproterozoic Minas Sg.Itabira Group (dark grey)Itabirite, dolomite

Archean terrains

Town

Palladiferous gold veins (Au-Pd-Pt)-type depositsjacutinga

Thrust fault(~0.6 Ga Brasiliano orogeny)

Fault

Platiniferous alluvia (Pt-Pd-Au)(di = detrital diamond)

Argentiferous gold with minor Pdin quartz-hematite–kaolinite-rutile veins

50 km

Ip

pl

Ouro Preto

Belo Horizonte

Diamantina

44°00’W 43°00’W

20°00’S

19°00’S

Serro

Córrego Bom Sucesso

Candonga

Rio do Peixe

Faz. Limeira

Morro do Pilar

C. Picão

Mil Oitavas

Guanhães

Cauê

Conceição

Timbopeba

Maquiné

Cata PretaÁgua QuentePitangui

Cocais

Brucutu

Gongo Soco

Conceição doMato Dentro

C. Lages

20°00’S

Itabira

Faz. Salvador

44°00’W

43°00’W18°00’S

di

di

QuadriláteroFerrífero

Brasilia

Brazil

Ouro Preto to Diamantina (Fig. 1). This thrust belt delineatesthe border zone between the São Francisco craton to the westand the Brasiliano Araçuaí orogenic belt to the east (e.g., Alk-mim and Marshak, 1998; Uhlein et al., 1998).

The Serra do Espinhaço (Eschwege, 1822; Derby, 1906) isa mountain ridge that runs northward for about 1,200 kmfrom the adjoining Quadrilátero Ferrífero. The southern seg-ment of the Serra do Espinhaço (Fig. 1) consists essentially ofsiliciclastic metasedimentary rocks in a succession of morethan 2,000 m in thickness, the Espinhaço Supergroup (e.g.,Pflug, 1967; Martins-Neto et al., 2001; Almeida-Abreu andRenger, 2007). This siliciclastic succession was deposited be-tween ca. 1.73 and 1.50 Ga in an intracratonic rift basin,which was intruded by tholeiitic dikes at ~1.1 to 0.9 Ga (e.g.,Rosset et al., 2007). The dikes show a penetrative tectonic fo-liation that is parallel to the regional foliation of the Espin-haço Supergroup (e.g., Uhlein and Quemeneur, 2000). Westof the Serra do Espinhaço, on the São Francisco craton, sed-imentation of Neoproterozoic rocks (Bambuí Group) tookplace during the Brasiliano assembly of Gondwana (e.g., Uh-lein et al., 1998; Martins-Neto et al., 2001). The Brasilianoevent thrust Espinhaço strata over Neoproterozoic rocks anddeveloped the regional tectonic foliation observed in the Es-pinhaço Supergroup (e.g., Alkmim and Marshak, 1998; Uh-lein et al., 1998).

Platiniferous deposits, mostly alluvial, are distributed in theeastern flank of the Serra do Espinhaço (Hussak, 1904; Frey-berg, 1934; Guimarães, 1959) along an approximately 90-km-long, north-trending belt (Fig. 1). The platiniferous alluviahave a close spatial association with quartzite and/or con-glomerate of the Espinhaço Supergroup, particularly theSopa-Brumadinho Formation, and with itabiritic rocks. Spec-ular hematite and, subordinately, coarse-grained rutile, areabundant in heavy mineral concentrates from platiniferousgravels. Platinum aggregates have a variety of unusual forms,such as botryoidal, arborescent, and concentric (Hussak,1904; Guimarães, 1959) and, besides Pt, consist of Pd and Hg(Barbosa, 1962; Stumpfl, 1974; Cassedanne et al., 1996; Fleetet al., 2002). Palladiferous gold is also found in the platinifer-ous alluvia. The best studied platiniferous deposit of thesouthern Serra do Espinhaço is Córrego Bom Sucesso (Hus-sak, 1904; Cassedanne and Cassedanne, 1974; Cassedanne etal., 1996; Fleet et al., 2002; Cabral et al. 2007), where ar-borescent palladiferous gold was recovered from hematite-rich platiniferous alluvium (Cabral et al., 2008b). This findinglinks the alluvial Au-Pd-Pt metal association with the ja-cutinga of the Quadrilátero Ferrífero (Fig. 1).

Locally, hematite-quartz veins truncate the tectonic folia-tion of hematitic phyllite above the alluvial platiniferous de-posit in Córrego Bom Sucesso. In the Fazenda Limeira area(Fig. 1; Belezkij, 1959; Guimarães, 1959), quartzite-hostedhematite-quartz veins occur in the vicinity of alluvia contain-ing botryoidal Pt-Pd-(Hg) aggregates. These field relationsindicate that quartz-hematite veins are not only younger thanthe Brasiliano regional tectonic fabric, but are also a potentialsource of Pt, Pd, and Hg.

Native gold from Mil Oitavas in Diamantina is argentifer-ous (electrum), but has a distinctive Pd component (~0.5 wt% Pd; A. R. Cabral and F. R. de Abreu, unpub. data). The de-posit consists of rutile-hematite−rich, kaolinite-quartz veins

that crosscut the Brasiliano regional tectonic foliation of ahematitic phyllite in the São João da Chapada Formation ofthe Espinhaço Supergroup (Abreu, 1991). Mil Oitavas is hereregarded as the northernmost limit, so far known, of the pla-tiniferous Au-Pd belt of Minas Gerais (Fig. 1).

Analytical MethodsPlatiniferous aggregates from the Bom Sucesso alluvium

were selected from a heavy mineral concentrate utilizing abinocular microscope. Backscattered-electron (BSE) imagingwas performed with a Philips XL30 scanning electron micro-scope, equipped with an Oxford Inca Suite 4.07 ATW2 en-ergy-dispersive spectrometer (EDS) as a detector, at Com-panhia Vale do Rio Doce, Minas Gerais, Brazil. Laserablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of botryoidal aggregates are presented inTable 1. Selected platiniferous aggregates were embedded inresin and polished for reflected light microscopy and chemi-cal analysis by the LA-ICP-MS technique at Curt-Engelhorn-Zentrum Archäometrie, Mannheim, Germany. The analyseswere carried out using a solid-state Nd:YAG laser operating at213 nm (Microprobe II Laser Ablation System with later integrated LUV213 Laser, New Wave Research, USA) cou-pled to an XSeriesII quadrupole ICP-MS (Thermo ElectronCorporation) with Collision Cell Technology (CCT). Detailsare given in Tables A1 and A2.

Depending on the microstructure of the grains, either spotor line ablation was done. Where the polished section ex-posed a concentric zonation, line ablation mode was applied,either following the zonation or crosscutting it. For grainswith unclear microstructure, spot ablation mode was used. Inalmost all cases the ablation signal recorded from differentspots or along a line showed well-defined regions with differ-ent concentrations for the elements measured. For this rea-son, the signal was split up into several parts that were sepa-rately calculated.

Data reduction was performed with the proprietary Plas-malab software (Thermo), applying external drift correctionand multiple calibration blocks, as well as Cu as an internalreference standard. For Cd, In, and Te a correction factor hadto be used due to interferences. The results were then ex-ported to Microsoft Excel, normalized to 100 percent, andthen corrected for the relative error of the elements reportedin the NA reference materials.

Gold reference materials (NA1 and NA2), produced byNorddeutsche Affinerie AG, Hamburg, Germany, and ana-lyzed in the LAC (Laboratory of Inorganic Chemistry), ETHZürich (Swiss Federal Institute of Technology Zurich), Swit -zerland, were used for matrix-matched solid calibration. Forquantification, liquid calibration and matrix-matched solidcalibration using FAU7 (NIST 8053, 8054, 8055) and FAU 10(NIST 8062, 8063, 8064) were performed. The estimatederror is 5 to 10 percent for most elements. Details on the cal-ibration methods are described in Kovacs et al. (2009).

Mineralogical Evidence for the Platiniferous Au-Pd BeltJacutinga in the Quadrilátero Ferrífero and Itabira district

and platiniferous alluvia in the Serra do Espinhaço have min-eralogical similarities that corroborate the geologic evidencefor the platiniferous Au-Pd belt of Minas Gerais.

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TAB

LE

1. L

A-I

CP-

MS

Ana

lyse

s on

Bot

ryoi

dal P

t-Pd

-Hg

Agg

rega

tes

from

Cór

rego

Bom

Suc

esso

, Min

as G

erai

s

Gra

in n

o.Sp

ot n

o.Pt

Pd

Hg

Au

TiV

Mn

Fe

Cu

As

SeR

hA

gC

dSb

TeT

l

13A

a, 2

0 s

19.5

54.0

23.9

1.8

<2<8

<2<4

<4<6

108

276

84<1

2<5

113

23A

a, 1

3 s

46.2

44.1

8.7

0.7

<2<8

<2<4

<4<6

330

123

36<1

1<5

843

3Ab,

17

s38

.842

.817

.01.

0<2

<8<2

<4<4

<625

71

3507

<11

797

43A

b, 1

5 s

66.4

31.2

2.1

0.3

<2<8

<2<4

<4<6

365

158

9<1

<1<5

345

3Ac,

14

s52

.538

.08.

60.

7<2

<8<2

<4<4

<626

11

2245

<11

757

63A

c, 1

7 s

69.6

30.0

0.2

0.1

<2<8

<2<4

<4<6

411

1<1

82<1

1<5

267

3Ad,

40

s68

.930

.60.

30.

137

<8<2

1411

<640

31

<182

<11

<520

81B

a, 2

0 s

73.4

26.2

<0.1

0.2

<12

11

391

361

213

1<0

.11

114

91B

b, 2

5 s

66.2

32.7

0.4

0.3

62

111

501

1044

215

7<0

.11

363

101B

b, 1

8 s

54.8

41.3

3.0

0.5

82

116

114

212

402

214

11

211

111

1Bc,

15

s49

.248

.31.

20.

933

23

<131

52

1363

119

0<0

.12

314

412

1Bc,

13

s69

.029

.20.

20.

23

2<1

111

111

692

141

<0.1

<12

5513

2Ba,

19

s62

.735

.50.

20.

435

8112

366

232

226

973

281

<0.1

13

8414

2Ba,

12

s62

.136

.40.

50.

451

35

120

352

1753

216

0<0

.11

262

152B

b, 1

4 s

55.9

36.7

5.8

0.8

<12

11

891

119

133

1510

<0.1

14

231

162B

b, 0

9 s

75.8

20.6

2.8

0.3

12

<11

144

111

815

492

<0.1

12

9217

2Bc,

18

s75

.623

.00.

80.

3<1

21

250

197

08

214

<0.1

13

7718

2Bc,

12

s56

.839

.42.

80.

3<1

11

317

121

803

663

11

335

419

3Ba,

10

s60

.632

.56.

50.

223

21

222

129

21

226

<0.1

12

2920

3Ba,

06

s60

.529

.77.

90.

292

8461

558

209

428

51

826

210

2921

3Ba,

08

s59

.130

.05.

70.

261

6648

2635

524

142

305

192

11

529

223B

b, 1

8 s

49.3

45.1

2.7

0.7

1173

744

494

419

426

362

705

52

727

423

3Bb,

14

s71

.627

.10.

70.

341

74

142

91

704

119

20.

41

266

243B

c, 1

1 s

79.6

19.7

0.2

0.3

171

<111

181

669

241

02

<13

4125

3Bc,

07

s55

.142

.31.

60.

412

31

5513

219

412

352

0.5

14

157

263B

c, 1

8 s

57.7

39.6

1.8

0.4

142

128

104

114

752

399

0.3

13

135

271C

a, 1

1 s

79.4

18.2

1.8

0.3

41

14

526

171

618

531

<0.1

<12

9328

1Ca,

04

s89

.010

.00.

50.

21

21

<155

5<1

591

2348

32

<12

3729

1Ca,

10

s60

.231

.27.

60.

5<1

11

229

01

1335

1110

874

14

225

301C

b, 1

2 s

61.0

32.0

5.1

0.4

41

12

132

113

388

911

11

218

431

1Cb,

09

s93

.35.

70.

40.

32

11

381

0<1

444

1840

3<0

.1<1

117

321C

c, 0

7 s

79.8

16.3

3.2

0.5

12

11

439

<155

617

422

<0.1

<11

8933

1Cc,

12

s93

.95.

6<0

.10.

3<1

11

277

8<1

262

1731

71

<11

534

1Cc,

10

s83

.416

.2<0

.10.

3<1

<11

118

91

418

319

8<0

.1<1

211

352C

a, 0

7 s

69.5

28.4

0.8

0.3

9246

483

492

75

152

168

<0.1

21

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59

Quadrilátero Ferrífero and Itabira

Palladiferous gold in jacutinga is associated with a variety ofPd-As-Sb phases, such as arsenopalladinite, isomertieite, andmertieite-II (Clark et al., 1974; Cabri et al., 1977; Cabral etal., 2002b). Platinum may occur as (1) a minor component inPd-As-Sb phases (Cabral et al., 2002b; Galbiatti et al., 2009)and Pd selenides (Davis et al., 1977; Cabral et al., 2002c;Cabral, 2006), and/or (2) thin patches of Se-Hg−bearing Pd-Pt along the margins of, and within, Pd-O alteration halos onisomertieite (Cabral and Lehmann, 2003).

However, it is in the Itabira district where Pt conspicuouslyoccurs as separate minerals: (1) sudovikovite, PtSe2, found asinclusions in hongshiite (Kwitko et al., 2002) and specularhematite (Cabral et al., 2002c); (2) a Pt-Hg selenide in coarse-grained aggregates of atheneite [(Pd,Hg)As3], potarite(PdHg), and specular hematite (Cabral et al., 2008a); (3)hongshiite, which may host atheneite, an unnamed CuSe-likephase, barite, and kaolinite (Kwitko et al., 2002; Cabral,2006); and (4) platiniferous tetraauricupride [(Au,Pt)Cu], in-tergrown with hongshiite (Kwitko et al., 2002).

Hongshiite is a notable Pt mineral because it occurs ascoarse-grained aggregates in bonanza pockets within ja-cutinga. The hongshiite from Itabira is Hg-bearing PtCu,with Hg contents relatively increased in Cu-depleted, Pt-en-riched alteration patches (Cabral et al., 2008a).

Southern Serra do Espinhaço

Córrego Bom Sucesso is here considered as the referencelocality for the platiniferous alluvia in the southern Serra doEspinhaço. Heavy mineral concentrates in hematitic gravelscontain Pt-Pd-(Hg) aggregates in a variety of delicate botry-oidal and arborescent forms (Fig. 2a-d), as recorded else-where (e.g., Hussak, 1904; Cassedane and Alves, 1992; Fleetet al., 2002; Cabral et al., 2006). However, two new aggregatetypes are recognized herein: detrital, angular palladiferousgold that is intergrown with specular hematite (Fig. 2e); anddetrital, angular hongshiite (Fig. 2f). Locally, a thin Ti oxidecoating may partially cover botryoidal Pt-Pd-(Hg) aggregates(Fig. 2c-d) and hongshiite (Fig. 2f).

LA-ICP-MS analyses show a wide variation in the majorcomponents: ~2 to 97 wt percent Pt, ~2 to 62 wt percent Pd,~0 to 32 wt percent Hg, and ~0 to 21 wt percent Au. Iron,which locally attains a maximum of 3.6 wt percent (anal. 21,Table 1), does not bear any significant correlation with theother major components. Instead, its maximum value corre-sponds to the lowest analytical total, suggesting that Fe isbound to an element that was not measured, perhaps oxygen,possibly as hematite (or a goethite-like mineral) within the Pt-Pd-Hg aggregates. The presence of hematite within them hashitherto gone unidentified, but it matches the widespread oc-currence of hematite in the gravels. The microanalyses alsoindicate elevated Ti contents, up to 1.2 wt percent, likely de-rived from occasional Ti oxide coating around the Pt-Pd-(Hg)grains (Fig. 2c, d, f; Cassedanne et al., 1996; Cabral et al.,2006).

These botryoidal aggregates also contain up to 2,900 ppmCu, whereas contents of other base metals are low: Co(<0.1–4.3 ppm), Ni (<1.3–21 ppm), Zn (<0.5–11 ppm), andPb (0.3–29 ppm). Selenium and Tl appear to be of particular

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541A

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significance. Selenium values vary from 54 to 2,700 ppm andconfirm the results of an earlier electron-microprobe study(Cabral et al., 2006). Thallium has contents extending from0.9 to 354 ppm. The values of Se versus Tl exhibit a positivelinear trend, whose angular coefficient, the Tl/Se ratio, is 0.08(Fig. 3). Spearman’s correlation coefficient for Se versus Tl is0.66, which is >99 percent significance level. The individualTl/Se ratios are in the range between 0.02 and 1.05. Thisrange differs from that measured by LA-ICP-MS on onehongshiite grain from Itabira, 1.6<Tl/Se<10.5. The Itabira

hongshiite represents the jacutinga-type hydrothermal sys-tem that could have been the metal source for alluvial pallad-iferous gold (Fig. 2e; Cabral et al., 2008b) and Pd-Pt miner-als (Fig. 2f) in the Córrego Bom Sucesso.

DiscussionThe platiniferous Au-Pd belt follows the same north-south

trend of the trace of major thrust faults (Fig. 1), which impliesa genetic connection between them. The thrust faults mark thetransition zone between the Brasiliano cratonic sedimentary

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a b

c d

e f

FIG. 2. BSE images of alluvial aggregates recovered from Córrego Bom Sucesso. a. Coralloidal aggregate of botryoidalPt-Pd grains. This aggregate is remarkably similar to Wollaston’s original specimens, from which the element Pd was discov-ered (Cassedanne and Alves, 1992, fig. 3). b. Arborescent aggregate of Pt-Pd grains. c. Botryoidal aggregate of Pt-Pd, whichis partially coated with a Ti oxide film (gray). d. Threefold arrangement of tubular aggregates of botryoidal Pt-Pd, unevenlycovered with Ti oxide (gray). e. Detrital palladiferous gold (white) intergrown with titaniferous hematite (gray). f. Detritalhongshiite (PtCu, light gray) with Ti oxide coating (dark gray).

cover to the west (Bambuí Group) and the Brasiliano Araçuaímobile belt to the east (e.g., Alkmim and Marshak, 1998; Uh-lein et al., 1998). Along the extent of the platiniferous Au-Pdbelt, hematite-rich veins locally crosscut the tectonic foliationgenerated by the Brasiliano orogenic event. A Brasiliano-re-lated age for jacutinga deposits in the Quadrilátero Ferríferohas been favored by a number of authors (Guimarães, 1970;Varajão et al., 2000; Lüders et al., 2005; Cabral, 2006; Galbi-atti et al., 2009), but controversy exists in Itabira, where Olivoet al. (1995, 1996) consider that the formation of the jacutingamineralization was related to an older event, the Transama-zonian orogeny. This suggestion is based on an errorchron Pb-Pb age of 1.83 ± 0.10 Ga, with a mean square of weighted de-viates (MSWD) of 7.55, for quartz, gold, and mixedgenerations of hematite; and on the assumption of parallelismbetween jacutinga and the ductile tectonic foliation of thehost itabirite. However, as pointed out by Lüders et al.(2005), four samples from Olivo et al. (1996) that have themost radiogenic Pb fall below their 1.83 Ga referenceisochron, indicating that the Pb isotope systematic of the sam-ples was disturbed during a younger event. In addition, Gal-biatti et al. (2007) demonstrated that the Itabira jacutinga oc-curs as brittle veins that truncate the ductile itabirite foliation.Uranium-Pb data for vein hematite (jacutinga) scatter about alinear trend, the slope of which corresponds to an age of ca.630 Ma, suggesting a Brasiliano age (Lüders et al., 2005). Al-though this age has no strict geo chronological meaning, alate- to post-Brasiliano age is supported by the geologic evi-dence of the crosscutting relations (Cabral, 2006; Galbiatti etal., 2007). We consider the jacutinga to be late orogenic withrespect to the ductile tectonism of its immediate hostitabiritic rocks. In this regard, jacutinga can be classified as avariant of the orogenic gold deposit spectrum (e.g., Groves etal., 1998), but it differs from other orogenic deposits by con-taining abundant hematite and being deficient in sulfides,particularly pyrite and pyrrhotite.

Genesis of the Córrego Bom Sucesso Pt-Pd-Hg aggregates

Several lines of evidence point to a connection between ja-cutinga and platiniferous alluvia in the southern Serra do Es-pinhaço. This evidence includes the following: (1) their geo-graphic distribution, which delineates a continuous belt withthe jacutinga deposits to the south (Fig. 1); (2) the existenceof hematite-quartz veins close to the alluvia, about 200 m away;(3) detrital hematite and palladiferous gold intergrown withhematite (Fig. 2e), as well as the presence of detrital hong-shiite (Fig. 2f), in the alluvia; and (4) the Au-Fe-Hg-Pd-Ptmetal association with a seleniferous signature. The LA-ICP-MS data further point to the Se signature in the botryoidal Pt-Pd-Hg aggregates, which have an enrichment factor between102 and 104 compared to the average Se content of the uppercontinental crust, i.e., 0.09 ppm (Rudnick and Gao, 2003).

Selenium is a characteristic element of the Brazilian ouropreto-bearing mineralization (Cabral and Lehmann, 2007),where it occurs in jacutinga as selenide minerals and as Se-Hg−bearing Pd-Pt patches in spatial association with Pd-O al-teration halos on isomertieite (Pd11Sb2As2), in which Se andHg could not be detected by the electron-microprobe tech-nique (Cabral and Lehmann, 2003). In the oxidation processof isomertieite, here taken as representative of a number ofhydrothermal platiniferous minerals, As and Sb were leachedwhereas Se and Hg were externally introduced into a Pd-Ophase. In places, isomertieite was completely replaced by thePd-O phase. Locally, the Pd-O phase broke down to Se-Hg−bearing Pd-Pt patches and, ultimately, into native Pd.The first is compositionally similar to the Pt-Pd-(Hg) aggre-gates in the Córrego Bom Sucesso. The second may inheritbotryoidal shapes from the transient Pd-O phase (Cabral,2006, fig. 24). Another process by which a primary Pt mineralalters to a composition that is analogous to the Pt-Pd-(Hg)grains in the Córrego Bom Sucesso is nonoxidative metalleaching. The alteration of Hg-bearing hongshiite fromItabira in friable jacutinga involved leaching of Cu, which re-sulted in the formation of native Pt with some Hg (Cabral etal., 2008a).

The Córrego Bom Sucesso alluvium and the contained de-trital platiniferous minerals were likely derived from jacutinga(Fig. 2e-f). It is important to note that the above-mentionedalteration features were described from friable jacutinga,which was buffered at high oxidation conditions. Althoughthe low-temperature alteration of primary Pd-Pt mineralsmay ultimately give rise to compositions that approach thoseof the alluvial Pt-Pd-(Hg) grains from Córrego Bom Sucesso,it does not explain the diversity of botryoidal shapes and thegrowth zoning of variable compositions (e.g., Fleet et al.,2002; Cabral et al., 2006). These characteristics are suggestiveof precipitation from an aqueous solution.

Thallium is a good indicator of solution-mineral interactionat low temperatures, since it is commonly incorporated intoprecipitates as a trace element from aqueous solutions (e.g.,McGoldrick et al., 1979), and Se is a redox-sensitive element.Therefore, Tl/Se ratios in low-temperature precipitatesshould reflect the solution/precipitate ratio and redox condi-tions. One grain of hongshiite from Itabira has Tl/Se ratiosabove unity, up to 10.5, and reflects the chemistry of the hy-drothermal jacutinga system. By contrast, Tl/Se ratios for the

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FIG. 3. Selenium vs. thallium diagram from LA-ICP-MS analyses on botry-oidal Pt-Pd-Hg aggregates from Córrego Bom Sucesso (Table 1). The linearregression has an angular coefficient which corresponds to a Tl/Se ratio of0.08.

Córrego Bom Sucesso Pt-Pd-Hg grains are mostly belowunity. The linear regression for the Se versus Tl data from thealluvial Pt-Pd-Hg grains gives an angular coefficient (0.08,Fig. 3) that is compatible with the variation of the averageTl/Se ratio for fluvial waters (Fig. 4). The linear regressionroughly intercepts the origin of the Se versus Tl plot, sug-gesting that both elements were derived from aqueous solu-tion in a solution-dominated system that was open relative toSe and Tl; and the alluvial Pt-Pd-Hg aggregates inherited theTl/Se ratio of the solution. We support Hussak’s (1904) origi-nal idea that the botryoidal Pt-Pd-Hg grains grew in situwithin the alluvium. We interpret that the alluvium was satu-rated with water that had a Tl/Se ratio close to the averagevalue for fluvial waters.

Our interpretation implies that jacutinga-derived Pd-Ptminerals were dissolved within the alluvium and precipitatedas botryoidal aggregates. If our assumption is correct, the so-lutions within the alluvium were highly diluted because thosesaturated with respect to metallic Pd-Pt should not maintainthe average Tl/Se ratio of fluvial waters. In this scenario, elec-trochemical reactions are a suitable mechanism of metal pre-cipitation in dilute solutions. Petrographical evidence forelectrochemical reactions is found at the contact of a core ofPd-Hg-Au intermetallic compounds, which become diffuseover a zone of sharp boundaries, with an overgrowth of pal-ladiferous Pt (Cabral et al., 2009, fig. 1b). In this examplefrom Córrego Bom Sucesso, the Pd-Hg-Au core and contactzone, and the Pt-rich overgrowth, are poorly crystallinephases. Precipitation of the Pt-rich overgrowth from dilutesolutions on the Pd-Hg-Au core is thought to have establishedan electrochemical gradient that induced diffusion of metalsthrough a narrow zone within the noncrystalline substratum(Cabral et al., 2009). We point out that a similar overgrowthof Pt on Pd-Hg phases was recorded from the Morro do Pilardeposit (Stumpfl, 1974), indicating that the electrochemicalaccretion of Pt within alluvia has been operative for at least~80 km south of Córrego Bom Sucesso (Fig. 1).

Further evidence in favor of electrochemical metal deposi-tion is the thin Ti oxide coating on the Pt-rich aggregates (Fig.

2c, d, f). In industrial processes, nanocrystalline Ti dioxidefilms are manufactured by electrodeposition of dissolved Tifrom aqueous solutions using Pt as a counter electrode (e.g.,Natarajan and Nogami, 1996). Electrochemical accretion ofaqueous Ti as Ti oxide onto the Pt-rich aggregates is the onlygeologically realistic process in an alluvial system open to nat-ural waters. Direct precipitation of Ti oxide by saturation ofnatural waters flowing through the alluvium is unlikely due tothe very low solubility of Ti.

In the platiniferous Au-Pd belt (Fig. 1), the distribution ofcoarse-grained botryoidal aggregates of Pt-Pd-Hg in alluvialdeposits appears to be restricted to the southern Serra do Es-pinhaço, where the alluvial deposits mostly rest on quartziticrocks of the Espinhaço Supergroup. The chemically inert na-ture of quartz sandstones is probably a precondition to thetransport of soluble Pd and Pt by oxidizing, dilute aqueous so-lutions, which could not overcome the redox-buffering capac-ity of country rocks dominated by basalt and greenschist(Wood and Normand, 2008). It is likely that the regional ge-ology of the platiniferous Au-Pd belt played a major role inthe alluvial accretion of Pt and Pd derived from jacutinga-style mineralization: in the Quadrilátero Ferrífero, wheregreenstones predominate, secondary Pt-Pd-Hg aggregateshave not been described as coarse-grained aggregates fromalluvia, but have only been observed as microscopic grains(up to a few tens of micrometers) in friable jacutinga.

ConclusionsPalladiferous gold mineralization in jacutinga in the Quad -

rilátero Ferrífero and Itabira, and alluvial platiniferous depositsin the southern Serra do Espinhaço define a north-trendingbelt between the towns of Ouro Preto and Diamantina. Theroughly linear distribution of the deposits is controlled bymajor thrust faults developed during the ~0.6 Ga Brasilianoorogeny. On a local scale, jacutinga truncates the ductile tec-tonic foliation of the host itabiritic rock. Jacutinga is regardedas an oxidized variant of the orogenic gold category.

Córrego Bom Sucesso, the best studied platiniferous de-posit of the northern Pt-rich part of the belt, displays thebotryoidal aggregates of Pt-Pd-Hg that characterize the allu-vial Pt deposits of the southern Serra do Espinhaço. The oc-currence of detrital grains of minerals that are typical of ja-cutinga, such as hematite, palladiferous gold, and hongshiite,connects the Córrego Bom Sucesso alluvium with a hy-drothermal jacutinga source. The botryoidal Pt-Pd-Hg aggre-gates possibly formed within the alluvium, as suggested byTl/Se ratios that agree with the average value for fluvial waters.

The platiniferous Au-Pd belt is a prospective domain for ja-cutinga-type mineralization for an approximate length of 240km. In particular, the growth of alluvial Pt-Pd aggregates byaccretion of Pt and Pd dissolved from detrital jacutinga min-erals is favored by the abundance of chemically inertquartzite in the southern Serra do Espinhaço. Contrary toconventional approaches, prospecting for Pd-Pt−bearingmineralization in the belt should target metasedimentary do-mains that contain quartzite and hematitic rocks.

AcknowledgmentsThis study is part of a research project financed by Deutsche

Forschungsgemeinschaft (DFG project LE 578/29-1). The

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0.01 0.1 1 10 100

Tl/Se ratios

Bom Sucesso Pt Pd Hg− −

Itabira hongshiite (PtCu)

Fluvialwaters

FIG. 4. Comparison of Tl/Se ratios between botryoidal Pt-Pd-Hg aggre-gates from Córrego Bom Sucesso and hongshiite from Itabira. The hong-shiite represents the vein system (jacutinga) that likely provided hy-drothermal platiniferous minerals to the alluvium. The variation range ofaverage Tl/Se ratio for fluvial waters falls within the Tl/Se ratios for the al-luvial Pt-Pd-Hg aggregates. Average values for dissolved Tl and Se in flu-vial waters are from Rehkämper and Nielsen (2004), and Measures andBurton (1978), respectively.

authors express their gratitude to Geraldo Pereira de Limafor working several months in the Bom Sucesso garimpo ontheir request to obtain platiniferous concentrates for thisstudy. Early travels to the Serro region were supported by theRudolf-Vogel-Preis of Technische Universität Clausthal toARC. Reviews by Paul Spry, Bruce Eglington, and MarilenaMoroni, as well as the editorial handling of Larry Meinert,substantially improved the original manuscript and are grate-fully acknowledged.

REFERENCESAbreu, F.R. de, 1991, Estudo das mineralizações auríferas filonianas da re-

gião da cidade de Diamantina/MG: Unpublished M.Sc. thesis, Campinas,Brazil, Instituto de Geociências, Universidade Estadual de Campinas, 85 p.

Alkmim, F.F., and Marshak, S., 1998, Transamazonian orogeny in the south-ern São Francisco craton region, Minas Gerais, Brazil: Evidence for Paleo-proterozoic collision and collapse in the Quadrilátero Ferrífero: Precam-brian Research, v. 90, p. 29–58.

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TABLE A1. Operating Conditions for ICP-MS and Laser Ablation System

ICP-MS Thermo XseriesII

Nebulizer gas flow 0.69–0.70 L min−1 ArAuxiliary gas flow 0.70 L min−1 ArPlasma/cool gas flow 13.0 L min−1 ArCollision gas 6.0 mL min−1 Ar H2-He mixture

(8% H2, rest He)RF power 1,200 WLens setting L1 -1200; L2 -85.5; L3 -79.2; D1 -51.8;

D2 -140; DA -65.1 VBiases Slightly KED (kinetical energy

discrimination): Pole bias −12.9, Hexapole bias −13.0

Detector mode DualDwell time 10 msScan mode Peak jumpCarrier gas flow 1.1 L min-1 HeLaser ablation system Microprobe II with LUV213, 213 nmPulse length 4 nsLaser fluence or pulse energy 24−30 Jcm-2

TABLE A2. Laser Operating Conditions for Line and Spot Ablation Mode

Spots Small spots Lines

Spot size 50 µm 15 µm 25 µmDuration 50 s 50 s (15–30 s)Length 300–400 µmEnergy 100% 100% 100%Ablation Frequency 4 Hz 4 Hz 4 Hz

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