Journal of Geochemical Exploration 110 (2011) 260–277

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Journal of Geochemical Exploration

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Geology, mineral chemistry and tourmaline B isotopes of the Córrego Bom Sucessoarea, southern Serra do Espinhaço, Minas Gerais, Brazil: Implications for Au–Pd–Ptexploration in quartzitic terrain

Alexandre Raphael Cabral a,b,⁎, Bernd Lehmann b, Miguel Tupinambá c,Michael Wiedenbeck d, Michael Brauns e

a Department of Geology, Exploration Geology, Rhodes University, PO Box 94, Grahamstown, 6140, South Africab Mineral Resources, Technical University of Clausthal, Adolph-Roemer-Str. 2A, D-38678 Clausthal-Zellerfeld, Germanyc 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, Brazild Helmholtz Centre Potsdam, German Research Centre for Geosciences, Telegrafenberg C128, D-14473 Potsdam, Germanye Curt Engelhorn Centre for Archaeometry, D6, 3, D-68159 Mannheim, Germany

⁎ Corresponding author at: Mineral Resources, TechAdolph-Roemer-Str. 2A, D-38678 Clausthal-Zellerfeld, G

E-mail address: alexandre.cabral@tu-clausthal.de (A

0375-6742/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.gexplo.2011.06.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 March 2011Accepted 16 June 2011Available online 25 June 2011

Keywords:Au–Pd–PtPlacer goldBoron isotopesTitaniferous hematiteMinas GeraisBrazil

The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil,are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often compriseAu–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline andtitaniferous hematite (up to ~6 wt.% TiO2). The latter minerals connect the alluvial mineralisation to therutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvialgravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasiveB metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongsto the alkali group, with Mg/(Mg+Fe) and X/(X+Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29,respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~1 to−10.4‰ δ11B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and thewidespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, underoxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggestepisodic vein emplacement over a temperature range from around 500 °C to ~350 °C. Cross-cuttingrelationships and episodic vein emplacement indicate a late-Brasiliano age.

nical University of Clausthal,ermany..R. Cabral).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Although hematite-associated Au–Pd–Pt mineralisation has beendescribed historically (e.g. Henwood, 1871; Hussak, 1904; Lampadiusand Plattner, 1833), sometimes with insignificant gold (e.g. Armitageet al., 2007; Wagner, 1929), several studies have recently shown thatit is more widespread than generally recognised in oxidising systems(e.g. Chapman et al., 2000, 2009; Dill, 2008; El Ghorfi et al., 2006;Gunn and Styles, 2002; Moroni et al., 2001; Şener et al., 2002;Shepherd et al., 2005; Stanley et al., 2002). In Minas Gerais, Brazil, Au–Pd–Pt mineralisation occurs as specular hematite-rich veins of pre-dominantly brittle character in the Palaeoproterozoic itabirite unit ofthe Quadrilátero Ferrífero, i.e. the Cauê Formation (e.g. Galbiatti et al.,2007; Lüders et al., 2005). The loci of this vein-style Au–Pd–Pt mine-

ralisation define a roughly N–S-trending belt, which is delineated bylineaments of thrusts related to the assembly of West Gondwanaduring the ~0.6-Ga Brasiliano orogenic cycle (Fig. 1; Cabral et al.,2009a; and references therein).

In the northern part of the platiniferous Au–Pd belt ofMinas Gerais,the rift-related Palaeo-Mesoproterozoic siliciclastic metasedimentarysequence of the southern Serra do Espinhaço displays a remarkablespatial association between tourmalinite, specular hematite and rutilein the quartzite country rocks with alluvial concentrations of specularhematite, rutile, tourmaline and Au–Pd–Pt intermetallic compounds.Here we describe this association at Córrego Bom Sucesso, an alluvialdeposit that possibly provided some of the sample material fromwhich the mineral native palladium was first discovered (Cassedanneand Alves, 1992; Hussak, 1906; Wollaston, 1809). Alluvial diamondsand preciousmetals have beenmanuallymined for over two centuriesat Córrego Bom Sucesso. Despite its historical importance and eco-nomic potential for primary Au–Pd–Pt mineralisation, recent workshave largely focussed on the exotic mineral assemblage of the Córrego

Fig. 1. The platiniferous Au–Pd belt of Minas Gerais, Brazil (Cabral et al., 2009a, and references therein), encompassing the Quadrilátero Ferrífero and the southern Serra doEspinhaço. The belt is delineated by representative examples of a variety of auriferous veins with characteristic Pd–Pt signature (jacutinga) and platiniferous alluvia. Their locationdefines a N-trending belt that parallels the trace of major thrust faults developed during the ~0.6-Ga Brasiliano orogenic event.

261A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

Bom Sucesso alluvial deposit (Cabral et al., 2008, 2009b; Cassedanneet al., 1996; Fleet et al., 2002), and the petrography of its surroundingrocks has yet to be properly described. In this contribution we do-

cument the petrography of tourmalinite and hematite-bearing rocks,themineral chemistry of tourmaline, hematite and rutile, aswell as theB-isotopic composition of tourmaline. From these results we are able

Fig. 2. Lithostratigraphical scheme for the basal units of the Espinhaço Supergroup in Diamantina (e.g. Martins-Neto, 1996; Pflug, 1967) and those exposed in the Bom Sucesso area(Fig. 3). Some authors consider the Bandeirinha Formation as part of the underlying Rio Paraúnas Supergroup (vide Section 2.2). We follow Horn et al. (1996) and Morteani et al.(2001), who proposed that the hematite–tourmaline–phosphate concentrations at the base of the Bandeirinha Formation, near Diamantina, distinguish the lowermost EspinhaçoSupergroup from the subjacent Rio Paraúnas Supergroup.

262 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

to provide a geological explanation for (1) the sulfide-free, hematite-rich Au–Pd–Pt mineralisation at Córrego Bom Sucesso, and (2) thewidespread occurrence of tourmaline and hematite in the siliciclasticmetasedimentary rocks of the basal units of the Espinhaço Supergroupin the southern Serra do Espinhaço.

2. Geological background and the alluvial deposit

2.1. Geological background

Along an approximately 90-km-long, S–N-trending belt theCórrego Bom Sucesso (córrego=stream) is one of a number ofplatiniferous alluvial deposits girding the eastern flank of the southernSerra do Espinhaço (Fig. 1; Hussak, 1904; Guimarães, 1959; vonFreyberg, 1934). The southern Serra do Espinhaço (Derby, 1906; vonEschwege, 1822) is a mountain range sustained by siliciclastic meta-sedimentary rocks of the Espinhaço Supergroup (Almeida-Abreu andRenger, 2007; Martins-Neto, 2000; Pflug, 1967; and referencestherein). The sedimentary rocks of this supergroup accumulatedalong a N–S-trending intracratonic rift basin, which attained morethan 2000 m thickness. Recent U–Pb ages of igneous and detritalzircon grains have revealed a regional unconformity over thelowermost section of the basin (Chemale et al., 2010): ca. 1.7-Gasiliciclastic and volcanic rocks of the Bandeirinha and São João daChapada formations are overlain by the Sopa-Brumadinho Formation(Fig. 2), for which themaximumdepositional age is 1.18 Ga. The Sopa-Brumadinho Formation includes fan-deltaic and fluvial quartzite anddiamantiferous metaconglomerate, interbedded with lacustrine peli-tic rocks (e.g. Martins-Neto, 1996). Marine sedimentation is expressedby a schist belt that delimits the eastern border of the southern Serrado Espinhaço. The schist belt partly comprises banded iron formation,quartzite and mica–quartz schist of the Itapanhoacanga Formation(e.g. Almeida-Abreu and Renger, 2007; Knauer and Grossi-Sad, 1997)

Fig. 3. Geological map of Córrego Bom Sucesso area (geology by A.R. Cabral andM. TupinambFormation, the garimpo is situated adjacent to a major thrust fault related to the ~0.6-Ga BFormation.

(Fig. 2). The contact between the Sopa-Brumadinho and Itapanhoa-canga formations is locally preserved from the tectonic overprint andsuggests lateral facies transition (Almeida-Abreu and Renger, 2007;Hoppe, 1980; Paternoster, 1980). Both formations have intercalationsof hematitic phyllite (e.g. Knauer and Grossi-Sad, 1997), a fine-grainedmuscovite–hematite rock that has been interpreted as a metamor-phosed palaeosol derived from eruptive rocks (Knauer and Schrank,1993; cf. Derby, 1900).

At ca. 1.1–0.9 Ga, tholeiitic dykes intruded the Espinhaço basin(e.g. Rosset et al., 2007). Metamorphism of the Espinhaço sedimentaryrocks took place in Neoproterozoic times, when the Brasiliano/Pan-African tectonics thrust Espinhaço strata westward over Neoproter-ozoic rocks deposited on the São Francisco craton, generating theregional tectonic foliation observed in the Espinhaço Supergroup (e.g.Alkmim et al., 2006; Alkmim and Marshak, 1998; Uhlein et al., 1998).The schist belt encompassing the Itapanhoacanga Formation is part ofthe Espinhaço fold-and-thrust belt, which fringes the eastern edge ofthe São Francisco craton and marks the western limit of the Araçuaíbelt, developed during the assembly of West Gondwana.

2.2. The Córrego Bom Sucesso alluvial deposit

The alluvial deposit rests on quartzitic bedrock of the Sopa-Brumadinho Formation, in the vicinity of a major thrust fault thattectonically juxtaposed rocks of the Itapanhoacanga Formation(Fig. 3). Quartzitic rocks of the Sopa-Brumadinho Formation areintruded by metamafic rocks of the Pedro Lessa Suite, which com-prises 1.1–0.9-Ga tholeiitic dykes. The metamafic rocks locally exhibittectonic foliation that broadly parallels the orientation of the E–W-verging thrust fault. The platiniferous alluvium,which has beenminedfor gold and diamonds, comprises gravels between the bedrock and alandslide deposit of quartzite boulders, fallen from a cliff (Fig. 4; videalso Cabral et al., 2008). The boulders and the cliff wall are composed

á on a topographical basemap from IBGE). Resting on quartzite of the Sopa-Brumadinhorasiliano orogeny. The thrust fault tectonically juxtaposed rocks of the Itapanhoacanga

263A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

Fig. 4. Photographs of the Córrego Bom Sucesso alluvial deposit. (a) Overview picturelooking N, showing quartzite boulders fallen from a cliff (arrow). (b) Ancient workingsfor diamonds and precious metals below the boulders. The platiniferous pebble layer issporadically mined between the boulders and the quartzitic bedrock of the Sopa-Brumadinho Formation. The deposit is manually mined by a few garimpeiros. One ofthem, Geraldo, poses as scale.

264 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

of quartzite with greyish layers that lend a banded appearance to thequartzite. The platiniferous gravels contain abundant hematite,particularly vein hematite, and, subordinately, vein rutile and otherheavy minerals (Cassedanne et al., 1996). The vein hematite and veinrutile are characteristically subhedral and coarse grained, up to ~1 cmin length. In the garimpo area, hematite is a rock-forming mineral inphyllite, in tourmalinite and in quartzite where it is locally con-centrated in greyish patches and bands. On top of the cliff, there areoutcrops, dispersed on the densely vegetated plateau, of hematiticphyllite, quartzite with tourmalinite layers and hematite–quartzveins. Although primary Au–Pd–Pt mineralisation on top of the cliffhas not been found, the abundance of vein hematite, the presence ofdetrital Pd–Pt-bearing gold with hematite inclusions and arborescentcrystals of palladiferous gold, all in the platiniferous gravels, link thealluvial deposit to a proximal source of hematite-rich, Au–Pd–Pt-bearing hydrothermal veins (Cabral et al., 2009a).

2.3. Regional distribution of tourmaline and hematite

Tourmaline is not only present in the area surrounding the alluvialdeposit (Córrego Bom Sucesso, Fig. 3), but it is also regionally dis-tributed. In the Serro region, tourmaline is widespread in quartziticrocks of the Sopa-Brumadinho Formation, where it occurs as a tracecomponent in fine-grained quartzite and, more abundantly, in quartz-phyllite varieties (Knauer and Grossi-Sad, 1997). However, tourma-line concentrations in the Córrego Bom Sucesso area are mostly foundin mica-poor quartzite.

Regionally, the quartzitic rocks of the Sopa-Brumadinho Formationhave variable amounts of hematite. Tourmaline is a minor constituentof hematitic phyllite (Herrgesell, 1984; Knauer and Grossi-Sad, 1997).This rock occurs in the São João da Chapada, the Sopa-Brumadinhoand the Itapanhoacanga formations. The former, which is not exposedin the Bom Sucesso area, constitutes the basal unit of the EspinhaçoSupergroup in the Serro region.

Beyond the Serro region, in the Diamantina quadrangle, tourma-line is ubiquitous in siliciclastic rocks and hematitic phyllite at thebase of the Espinhaço Supergroup (Fogaça, 1997): (1) up to 5% inquartzite; (2) as an accessory mineral in the arenaceous matrix ofmetaconglomerate; (3) from ~3 to 10% in quartz–muscovite phyllite.In the hematitic phyllite tourmaline forms radial aggregates ofprismatic crystals and bands associated withminerals loosely referredto as ‘opaques’ (Fogaça, 1997).

Some authors have considered the Bandeirinha Formation to bethe lowermost unit of the Espinhaço Supergroup (e.g. Almeida-Abreu,1996; Horn et al., 1996; Martins-Neto, 2000), whereas other re-searchers have allocated it immediately below the Espinhaço strata(e.g. Almeida-Abreu and Renger, 2007; Fogaça, 1997; Knauer, 2007).The Bandeirinha Formation, mostly of quartzite and micaceousquartzite, has in its basal part concentrations of Al-phosphate mi-nerals and tourmaline in hematite–muscovite–kyanite quartzite. Thishematite–tourmaline–phosphate association has been used to iden-tify the contact between the Espinhaço Supergroup and its underlyingrocks (Horn et al., 1996; Morteani et al., 2001).

This brief review of the occurrence of tourmaline and hematite inrocks of the basal portion of the Espinhaço Supergroup stronglysuggests a spatial relationship between the two minerals at theregional scale of the southern Serra do Espinhaço, although no studieshave as yet targeted the tourmaline–hematite mineral assemblage.

3. Methods

3.1. Whole-rock analysis

Major elements in whole-rock samples, pulverised in an agatemill,were determined by X-ray fluorescence spectrometry (XRF) at theFederal Institute for Geosciences and RawMaterials (BGR), Hannover,Germany. Selected samples had Au, Pd and Pt determined by fireassay–inductively coupled plasma–mass spectrometry (FA–ICP–MS)at Activation Laboratories Ltd. (Actlabs), Ancaster, Ontario. A fewsamples were additionally analysed for Au using instrumentalneutron activation analysis at Actlabs, as well as for organic C by theinfrared technique.

3.2. Electron-microprobe analysis

Electron-microprobe analyses on hematite, tourmaline and apatitewere performed with a Cameca SX100 at the Technical University ofClausthal, Germany. All elements had their Kα X-ray emission linesmeasured at 15 kV and 15 nA, with a beam size of 2 μm. Referencematerials used were: (1) K-feldspar (Al), TiO2 (Ti), Cr2O3 (Cr), MnSiO3

(Mn), Fe2O3 (Fe) for hematite; (2) apatite (F), kaersutite (Na, Mg, Al,Si, Ca, Ti and Fe) and MnSiO3 (Mn) for tourmaline; (3) F-apatite (F, Pand Ca), diopside (Mg) and Cl-apatite (Cl).

Table 1Whole-rock chemical analyses for major elements and precious metals, Córrego Bom Sucesso, Minas Gerais, Brazil.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

SiO2 (wt.%) 92.94 92.60 95.86 92.09 98.10 97.34 50.24 48.09 62.82 80.02 60.34 35.81 33.86 32.00 33.64 39.97 47.69 48.16 14.50 33.43TiO2 0.14 0.03 0.07 0.90 0.18 0.04 0.49 0.51 0.29 0.58 1.55 4.70 5.63 6.20 5.13 4.42 1.70 2.54 2.53 4.10Al2O3 1.93 1.06 0.23 0.25 0.18 0.27 24.05 24.87 13.74 0.14 0.71 22.96 20.92 23.38 22.86 26.75 16.39 13.47 12.84 22.80Fe2O3 2.04 1.50 3.42 5.90 0.71 1.95 7.17 5.77 11.79 18.90 36.38 22.76 26.36 26.50 25.77 13.70 11.62 16.03 58.14 23.58MnO 0.01 0.01 b0.01 0.01 b0.01 b0.01 0.05 0.21 0.12 0.07 0.08 0.06 0.05 0.04 0.06 0.05 0.18 0.22 0.02 0.25MgO 0.20 0.12 0.02 0.03 0.02 0.01 4.95 6.56 3.04 0.02 0.01 1.43 1.45 0.37 0.95 1.30 5.79 5.64 b0.01 0.14CaO 0.68 2.16 0.03 0.03 0.03 0.03 0.07 0.20 0.15 0.03 0.02 0.03 0.03 0.09 0.05 0.03 10.20 8.60 0.01 0.08Na2O 0.02 0.01 b0.01 b0.01 b0.01 b0.01 1.80 1.97 1.02 b0.01 b0.01 0.25 0.13 0.33 0.29 0.31 2.78 2.13 b0.01 b0.01K2O 0.48 0.27 0.06 0.04 0.03 0.06 0.02 0.05 0.32 0.01 0.02 7.95 7.62 7.16 7.36 8.96 0.09 0.20 0.01 0.04P2O5 0.69 1.68 0.02 0.02 0.03 0.01 0.02 0.02 0.09 0.02 0.02 0.05 0.09 0.22 0.06 0.11 0.22 0.02 0.68 0.29Cl 0.01 0.02 0.01 0.02 0.01 b0.01 0.01 0.01 b0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 b0.01 0.01F b0.05 0.06 b0.05 b0.05 b0.05 b0.05 b0.05 0.12 b0.05 b0.05 0.45 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05LOI 0.75 0.43 0.25 0.63 0.63 0.21 2.52 2.67 1.47 0.19 0.53 3.62 3.42 3.32 3.44 4.01 3.06 2.58 11.03 15.04–O≡F 0.03 0.05 0.19Total 99.90 99.92 99.97 99.91 99.92 99.92 91.39 90.99 94.84 99.98 99.93 99.63 99.56 99.62 99.61 99.61 99.72 99.61 99.77 99.75C-Org. 0.20 n.a. n.a. n.a. 0.40 n.a. 0.23 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.Au (ng/g) 10 n.a. n.a. b2a 6 n.a. 10 9 3 7 2 n.a. 6a n.a. b2a 2a 6 17 4 7Pt 1.5 n.a. n.a. n.a. 1.2 n.a. 5.1 6.1 8.1 9.7 4.1 n.a. n.a. n.a. n.a. n.a. 2.5 11.9 5.3 4.0Pd 1.0 n.a. n.a. n.a. 1.7 n.a. 5.1 3.5 0.5 1.5 0.8 n.a. n.a. n.a. n.a. n.a. 2.5 5.7 3.6 2.6

1–2: Apatite-bearing grey quartzite.3–6: Grey quartzite.7–9: Tourmalinite in quartzite.10–11: Hematite–quartz vein.12–16: Hematitic phyllite; Column 16 is a rutile–hematite pocket within hematitic phyllite.17–18: Metamafic rock.19: Lateritised metamafic rock.20: Red soil over metamafic rock.n.a., not analysed.

a Determined by INAA.

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3.3. LA–ICP–MS analysis

Laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS) was carried out at the University of Erlangen with aUP193FX New Wave Research Laser and an Agilent 7500i ICP–MS.Rutile and quartz were measured with 25-μm and 50-μm spots,respectively, at 20 Hz. Analytical conditions were as follows: plasmapower, 1330 W; carrier gas I (He), 0.65 L/min; carrier gas II (Ar),1.05 L/min; plasma gas (Ar), 14.9 L/min; auxiliary gas (Ar), 0.9 L/min.Measurements were calibrated using NIST 610 (Pearce et al., 1997).Repeatability and accuracy were determined on NIST 612 and forrutile additionally on an in-house rutile reference material (R3,

Fig. 5. Whole-rock plot of (a) Al2O3 vs. TiO2 and (b) total Fe as Fe2O3 vs. TiO2 contents fcorrelation displayed in (a), but in (b) the slight positive correlation suggests that Ti is notrepository of whole-rock Ti.

Carsten Münker, University of Münster). Data were evaluated withGLITTER (van Achterbergh et al., 2000).

The LA–ICP–MS facility of the Curt Engelhorn Centre for Archaeo-metry, Mannheim, provided trace-element analyses on unpolishedgold grains. The analyses were carried out under wet conditions usinga solid-state Nd:YAG laser operating at 213 nm (Microprobe II LaserAblation System with later integrated LUV213 Laser, New WaveResearch, USA), coupled to an XSeriesII quadrupole ICP–MS (ThermoElectron Corporation) with Collision Cell Technology (CCT). Fordetails see Appendix 1. Gold grains were ablated with spots andlines (Appendix 2). Gold and Ag were measured in high-resolutionmode to avoid extremely high signals. Data were evaluated with

rom quartzite, tourmalinite and quartz–hematite vein (Table 1). There is no positivebound to clastic aluminosilicate components. Instead, epigenetic hematite is the major

Fig. 6. Fe2O3 vs. TiO2 plot of hematite from different mineral assemblages (Table 2).Note that high TiO2 contents were found in hematite enclosed in Pd–Pt-bearing gold.

Table 2Electron-microprobe analyses of hematite, Córrego Bom Sucesso, Minas Gerais, Brazil.

(wt.%) Al2O3 TiO2 Cr2O3 MnO Fe2O3 Total

Type 2 0.21 1.75 b0.10 b0.06 97.74 99.700.19 1.63 b0.10 b0.06 98.49 100.310.16 1.94 b0.10 b0.06 98.36 100.460.11 1.29 b0.10 b0.06 98.89 100.290.18 1.82 b0.10 b0.06 98.84 100.840.17 1.82 b0.10 b0.06 98.47 100.460.20 1.75 b0.10 b0.06 98.87 100.820.11 1.72 b0.10 b0.06 98.22 100.050.18 1.76 b0.10 b0.06 98.53 100.47

Type 3 0.11 1.35 b0.10 b0.06 99.46 100.920.09 1.41 b0.10 b0.06 99.20 100.710.16 1.48 b0.10 b0.06 98.21 99.860.14 1.41 b0.10 b0.06 98.19 99.750.21 1.49 b0.10 b0.06 98.87 100.560.11 1.74 b0.10 b0.06 98.98 100.820.14 1.50 b0.10 b0.06 99.10 100.740.13 1.65 b0.10 b0.06 97.73 99.500.10 1.34 b0.10 b0.06 99.06 100.500.07 1.45 b0.10 b0.06 98.69 100.210.09 6.16 b0.10 0.09 93.16 99.510.08 6.77 b0.10 0.09 93.43 100.360.09 6.16 b0.10 0.13 93.99 100.370.06 6.35 b0.10 b0.09 93.97 100.38

b0.05 5.85 b0.10 0.09 94.47 100.410.06 6.17 b0.10 b0.09 94.53 100.760.09 4.07 b0.10 0.09 96.27 100.530.07 3.90 b0.10 b0.09 96.32 100.290.05 4.02 b0.10 b0.09 95.39 99.460.11 3.49 b0.10 b0.09 95.56 99.160.05 3.70 b0.10 b0.09 95.89 99.630.12 3.95 b0.10 0.12 95.94 100.130.07 6.07 b0.10 b0.09 94.80 100.940.11 4.39 b0.10 b0.09 95.58 100.070.10 3.55 b0.10 b0.09 95.79 99.440.08 4.80 b0.10 b0.09 95.53 100.410.06 2.17 b0.10 0.11 98.49 100.830.08 2.11 b0.10 b0.09 97.07 99.260.06 2.24 b0.10 b0.09 97.97 100.270.12 2.17 b0.10 b0.09 98.37 100.660.07 2.15 b0.10 b0.09 98.43 100.650.13 1.87 b0.10 b0.09 98.21 100.210.07 2.18 b0.10 0.09 98.32 100.670.10 2.10 b0.10 b0.09 98.56 100.760.09 2.18 b0.10 b0.09 98.56 100.83

Type 4 0.11 2.98 b0.10 b0.09 97.85 100.940.07 2.66 b0.10 0.09 97.34 100.150.08 3.78 b0.10 b0.09 96.86 100.710.06 2.86 b0.10 b0.09 97.89 100.800.08 2.57 b0.10 b0.09 98.22 100.860.09 2.73 b0.10 b0.09 97.39 100.200.09 2.64 b0.10 b0.09 97.52 100.250.07 3.65 b0.10 b0.09 96.77 100.490.08 4.79 b0.10 0.10 94.87 99.830.05 5.02 b0.10 0.09 95.09 100.25

Type 5 b0.05 2.81 b0.07 0.11 97.03 99.95b0.05 2.72 b0.07 0.14 97.42 100.280.08 2.78 b0.07 0.07 96.13 99.050.06 2.99 b0.07 0.04 97.54 100.630.05 3.03 b0.07 0.08 97.20 100.36

b0.05 2.70 b0.07 0.08 96.90 99.680.05 2.63 b0.07 0.04 96.86 99.590.06 2.71 b0.07 0.11 96.53 99.39

Type 6 0.09 6.35 b0.07 0.11 92.76 99.320.07 6.22 b0.07 0.09 93.55 99.930.06 6.25 b0.07 0.08 92.78 99.170.05 6.03 b0.07 0.09 93.17 99.330.10 6.41 b0.07 0.12 93.02 99.650.05 6.15 b0.07 0.11 92.83 99.14

266 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

Plasmalab software (Thermo), taking into account an external driftcorrection, multiple calibration blocks and a 169Tm internal standard.For 111Cd, 115In and 125Te a correction factor was applied due tointerferences. The Plasmalab results were then exported to MicrosoftExcel, normalised to 100% and corrected for the relative error of theelements reported in the gold referencematerials (NA1 and NA2). Thereference materials and calibration methods are detailed in Kovacs etal. (2009).

3.4. SIMS analysis

Secondary ion mass spectrometry (SIMS) determined 11B/10Bratios on tourmaline using a Cameca ims 6f SIMS instrument at theHelmholtz Centre Potsdam (Wiedenbeck et al., 2004). SIMS measure-ments on Au-coated thin sections employed a nominally 12.5-kV,800-pA, 16O− primary beam in conjunction with a 10.0-kV secondaryion extraction potential. The primary beamwas focused to about 5-μmdiameter at the sample surface. A mass resolving power of M/dM=1200 resolved the 11B+ mass station from the nearby 1H10B+ peak.Each analysis was preceded by a 180-s preburn in order to remove theAu coat and to establish equilibrium sputtering conditions at the pointof analysis. Individual analyses had typical measurement uncer-tainties of around 0.4‰ (1 s). The instrumental mass fractionation(IMF) was calibrated on a suite of four natural tourmaline referencematerials (Dyar et al., 2001; Gonfiantini et al., 2003; Tonarini et al.,2003). Based on the repeatability of the IMF determination definedby our four reference materials (N=62 over eight days), which isthe dominant source of uncertainty in these measurements, the SIMSB-isotope results are reliable to ±1.8‰ (1 s) level. This uncertaintyestimate contains both any bias component, which might be presentin the form of matrix effect variations due to major element chem-istry, and the propagation of the uncertainties in the wet chemicaldetermination of the compositions of the four reference tourmalines.

4. Results

4.1. Whole-rock major elements and precious metals

Results of chemical analysis of twenty samples of metasedimentaryand metamafic rocks, as well as samples of hydrothermal quartz veins,from the Bom Sucesso area are presented in Table 1. The grey quartzitehas 92–98 wt.% SiO2 and up to about 6 wt.% Fe2O3 (total Fe). The Fe

component is essentially epigenetic hydrothermal hematite, i.e. non-detrital (see Section 4.2). Apatite-bearing grey quartzite is readilyidentifiedby its contents of CaOandP2O5,whicharehigher than thoseof

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other metasiliciclastic rocks. Quartzite-hosted tourmalinite is charac-terised by its contents of Na2O (~1–2 wt.%) and MgO (~3–7 wt.%), incontrast with virtually Na2O-free, MgO-poor quartzite. Boron was notdetermined, but the difference from the total of 100% suggests about 5–9 wt.% B2O3, which corresponds to themicroscopically identifiedmodalcontents of about 50–90% tourmaline. Contrary to othermetasiliciclasticrocks, TiO2 is not positively correlatedwithAl2O3 (Fig. 5a), as expected ifTiwere immobile, i.e. bound to detrital aluminosilicates in the quartziticrocks. Whole-rock contents of TiO2 in quartzite show a positive linearcorrelation with Fe2O3 content, together with the hematite–quartz veinsamples (Fig. 5b). This trend suggests that the whole-rock Fe2O3 andTiO2 are controlled by an epigenetic Fe–Ti mineral phase, such astitaniferous hematite (see Section 4.2). Taken separately, the tourma-linite samples exhibit a positive trend for Al2O3 vs. TiO2 (Fig. 5a), but anegative trend for Fe2O3 vs. TiO2 (Fig. 5b). Such trends imply that a Tiphase, such as rutile, should occur in the tourmalinite (see Section 4.2).

Hematitic phyllite has a peculiar K2O-rich ferro-aluminous com-position. It is more enriched in TiO2 (~4–6 wt.%) and total Fe (~23–27 wt.% Fe2O3) than the metamorphosed mafic rocks of the PedroLessa Suite (1.7–2.5 wt.% TiO2, ~12–16 wt.% Fe2O3), and has very highpotassium contents of about 7–9 wt.% K2O. A red soil, whichdeveloped from metamafic rock, has contents of SiO2, TiO2, Al2O3

and Fe2O3 that are similar to those of the hematitic phyllite. Such asimilarity led some authors to propose that the hematitic phylliterepresents a palaeosol derived from an eruptive mafic alkaline rock(Knauer and Schrank, 1993; cf. Derby, 1900).

All rocks analysed for Au, Pd and Pt were found to be barren forthese elements. The samples of metamafic rocks of the Pedro LessaSuite do not show any significant enrichment in Pt and Pd, the maxi-mum values of which are about 12 and 6 ng/g, respectively. However,Cassedanne et al. (1996) reported higher contents of Pt (108 ng/g)and Pd (28 ng/g) in one sample of weatheredmetamafic rock from theBom Sucesso area.

4.2. Petrography and mineral chemistry

Several mineral assemblages containing hematite and/or tourma-line can be distinguished in rocks of the Sopa-Brumadinho Formationand intergrown with gold in alluvial aggregates. All samples comefrom the Bom Sucesso garimpo and its proximity. The mineral as-semblages can be grouped into six types (relative mineral abundanceincreasing from left to right):

1. Rutile–tourmaline in quartzite;2. Apatite–hematite in quartzite;3. Rutile–hematite–tourmaline in quartzite and tourmalinite;4. Rutile–hematite pocket in hematitic phyllite;5. Rutile–hematite in quartz vein;6. Tourmaline–hematite in alluvial gold.

Hematite is titaniferous in the type-2 and type-3 assemblages(Fig. 6, Table 2). The hematite from the type-2 assemblage has TiO2 inthe range of 1.3 to 1.9 wt.%, whereas that from the type-3 assemblagehas a broader compositional range, between 1.3 and 6.8 wt.% TiO2.Tighter values, between 6.0 and 6.4 wt.% TiO2, are found in hematitefrom the type-6 mineral assemblage.

The type-1 mineral assemblage occurs in grey quartzite. Tourma-line imparts the greyish tint to the rock and, together with muscovite,defines a penetrative tectonic foliation (Fig. 7a). Tourmaline aggre-gates in shear bands have concentrations of rutile (Fig. 7b), which iscoarser in grain size than any detrital rutile of the quartzite matrix.The tourmaline from the type-1 assemblage is the most Fe-enrichedvariety of all tourmaline assemblages of Córrego Bom Sucesso (Fig. 8),as expected because of the absence of hematite.

The type-2mineral assemblage represents, in part, the grey patchesand layers that give a banded appearance to the quartzite found atCórrego Bom Sucesso garimpo and its delimiting cliff. Hematite is

commonly in contactwith apatite and xenotime (Fig. 7c). The apatite isrich in F (Table 3). The xenotime–F-apatite–hematite assemblagecross-cuts, and locally replaces, matrix quartz (Fig. 7c). The lattercomprises grains of detrital quartz that recrystallised to a mosaic ofplanar grain boundaries meeting in triple points at 120°.

The type-3 mineral assemblage constitutes part of the grey patchesand layers of the quartzite exposed at Córrego Bom Sucesso garimpo.These patches aremostly defined by aggregates of tourmaline (Fig. 7d). Ina similar manner as shown in Fig. 7c, the tourmaline patches truncate therecrystallised fabric of detrital quartz. Rutile and rarely observed Thphosphate are found in tourmaline as inclusions generally not exceeding5 μm across. Tourmalinite is a compact, fine-grained dark rock thatcontrasts with its white quartzitic host rock. In tourmalinite, tourmalinedisplays pseudomorphic replacement of recrystallised detrital quartz(Fig. 7e). The pseudomorphs of tourmaline host abundant relics of quartzand a multitude of fine-grained hematite, which is denoted by intenseinternal reflections in reflected light (Fig. 7f). Coarser hematite occursinterstitially. The type-3 assemblage tourmaline is compositionallydivided into two groups: one group is dispersed parallel to the “oxy-dravite”–povondraite join; the other is less dispersed, clustering close tothe type-1 assemblage tourmaline (Fig. 8).

The type-4 mineral assemblage is made up of specular hematiteintergrownwith rutile which forms pockets that truncate the tectonicfoliation of hematitic phyllite (Fig. 9a and b). The vein hematite hasTiO2 contents varying from 2.6 to 5.0 wt.% (Table 2). Muscovite lathscoexist with the rutile–hematite pocket infill.

The type-5mineral assemblage is characterisedby specularhematitein vein quartz. The sample material represents a hematite–quartz veinlocated on top of the cliff, immediately above the Córrego Bom Sucessogarimpo. The hematite–quartz vein is situated along the contactbetween quartzite and hematitic phyllite. Its specular hematite hasabout 3 wt.% TiO2 (Table 2). The vein hematite occurs as laths on coarse-grained martite, i.e. hematite after magnetite (Fig. 9c and d). Rutile issparsely distributed as inclusions in martite and specular hematite. Therutile inclusions are finer grained than those from the type-4 mineralassemblage. Muscovite forms aggregates with specular hematite.

The type-6mineral assemblage has consistently high contents of Tiin hematite (6.0–6.4 wt.% TiO2, Table 2, Fig. 6). This titaniferoushematite manifests itself as laths and aggregates within, and on thesurface of, alluvial gold recovered from the Córrego Bom Sucessogarimpo. Tourmaline is a constituent of the hematite aggregateswithinthis type of alluvial gold (Fig. 10a and b), which has Pd- (Fig. 10c) andPt-rich patches (Fig. 10d). Compositionally, the type-6 assemblagetourmaline clusters close to the intersection between the schorl–dravite join and the “oxy-dravite”–povondraite join. The gold-hostedtourmaline follows, as is the case with the type-3 assemblage, the“oxy-dravite”–povondraite join (Fig. 8).

Microanalyses of tourmaline are presented in Table 4. All types oftourmaline contain some F, between ~600 and ~3700 μg/g. Thetourmaline is aluminous (AlN6 atoms per formula unit) and belongsto the alkali group (Hawthorne and Henry, 1999), with Mg/(Mg+Fe)and X/(X+Na) ratios in the range from 0.5 to 0.7 and 0.18 to 0.29,respectively, where X represents vacancies in the X site. Using theMg/(Mg+Fe) vs. X/(X+Na) plot of Henry et al. (2002), all types oftourmaline fall in the dravite field.

4.3. Other alluvial gold populations

The type-6 mineral assemblage establishes a link between alluvialPd–Pt-bearing gold and the epigenetic overprint of titaniferoushematite and tourmaline on quartzite. However, different populationsof gold occur in the Córrego Bom Sucesso alluvium. Three other po-pulations of gold, without hematite aggregates on the surface of gold(type 6), can be distinguished in the alluvium based on morphologicalfeatures. They reflect alluvial transport from proximal to distal sources:(1) elongated crystals with angular to slightly rounded edges (Fig. 11);

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(2) platelets with rounded edges; and (3) spherical grains. The elon-gated gold comes from a very proximal source area and has highcontents of Pd (~4–18 wt.%), with Pd/Ag ratios between about 4 and

3700, and a Pt–Ti signature, i.e. ~40–400 μg/g Pt and ~130–5900 μg/g Ti(Table 5). The rounded platelets contain variable amounts of Ag, from0.04 to ~12 wt.%, with a few grains with some Pd, but no clear Pt

Fig. 8. Upper portion of an Al–Fe–Mg ternary diagram for tourmaline from Córrego BomSucesso, Minas Gerais. Magnesium-rich tourmaline, ~2.0 atoms of Mg per formula unit,plots near the “oxy-dravite”–povondraite join, which is characteristic of meta-evaporitic tourmaline (Henry et al., 2008). The type-3 tourmaline contains titaniferoushematite. Note that high contents of Ti in hematite are within detrital palladiferousgold, which has the most proximal tourmaline to the intersection between the schorl–dravite and “oxy-dravite”–povondraite joins. Iron-rich tourmaline without hematiterepresents the furthermost deviation of the “oxy-dravite”–povondraite join.

Table 3Electron-microprobe analyses of quartzite-hosted apatite, Córrego Bom Sucesso, MinasGerais, Brazil.

1 2 3 4 5

CaO (wt.%) 55.99 56.41 56.29 56.29 54.82P2O5 43.42 43.49 43.76 43.65 42.56F 2.87 2.87 2.86 2.87 2.94Cl b0.03 b0.03 b0.03 b0.03 b0.03

102.27 102.77 102.91 102.82 100.32−O≡F 1.21 1.21 1.20 1.21 1.24Total 101.07 101.57 101.71 101.61 99.09

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signature. The spherical grains are depleted in Ag compared to therounded platelets. Both platy and spherical types have Pd/Ag ratioslower than about b0.1.

4.4. Rutile mineral chemistry and Zr-in-rutile geothermometry

Rutile from the type-4 mineral assemblage has Zr contents thatgive Zr-in-rutile temperatures of ~470–490 °C (Fig. 12, Table 6). Thetemperatures were calculated using the equation of Tomkins et al.(2007) and a pressure estimate of 3.4 kbar (Morteani et al., 2001).

Alluvial rutile, commonly found in the platiniferous alluvium, hasZr-in-rutile temperatures, between about 460 and 500 °C (Fig. 12),comparable to those from the type-4 mineral assemblage. Despitetheir similar temperatures, the alluvial rutile shows very different Nb/

Fig. 7. Petrography of quartzitic rocks of the Córrego Bom Sucesso area. (a) Transmitted(undistinguishable from quartz in white) and tourmaline (shades of green). (b) Detail of theassemblage (rutile–tourmaline in quartzite). (c) Backscattered-electron (BSE) image of a grcross-cut detrital quartz with recrystallisation fabrics (rectilinear grain boundaries and gra(d) Transmitted-light photomicrograph of a grey patch in white quartzite. The grey patch ispoor variety of the type-3 mineral assemblage. The tourmaline-rich patch truncates the rehematite–tourmaline mineral assemblage (type 3). Rutile is a minor component and mostlyquartz in tourmaline. The relics of quartz and the cross-cut relationships between quartz antourmaline. Note that tourmaline appears in a shade of grey that is lighter than quartz, whicinclined nicols to show bright red internal reflections of abundant microscopic inclusions o

Ta ratios from the rutile in hematitic phyllite-hosted hematite pockets(type 4). For comparison, data from rutile recovered from auriferoushematite-rich pockets near Diamantina (A.R. Cabral, unpublisheddata) are plotted in Fig. 13. Both the occurrences of vein rutile, nearDiamantina and at Córrego Bom Sucesso (Fig. 1), consistently clusteraround Nb/Ta ratios of ~15 and Fe contents of about 0.6 wt.%. Incontrast, the alluvial rutile from Córrego Bom Sucesso has a broadvariation of Nb/Ta ratios, between 11 and 63, and Fe contentsspanning from 0.6 to 2.7 wt.% (Fig. 13). Estimation of the trivalent Fecontent using the equation of Droop (1987) suggests that all Fe is inthe trivalent state. It is pertinent to point out that Au was detected inboth alluvial rutile and the type-4 assemblage rutile, amounting to afew hundred ng/g.

Only a couple of rutile crystals from quartz–hematite veins (type5) were large enough for LA–ICP–MS measurements, which gavetemperatures of ~340–390 °C (Table 6). This temperature interval ofrutile crystallisation in quartz–hematite veins is lower than that of thetype 4 and alluvial rutile.

4.5. Ti-in-quartz geothermometry

Ti-in-quartz temperatures, calculated using the equation of Warkand Watson (2006), suggest that quartz from hematite-rich veins(type 5) crystallised between ~390 and 440 °C (Table 6). Thistemperature interval overlaps the few Zr-in-rutile temperatures ofthe type-5 assemblage rutile (Fig. 12).

4.6. Boron-isotopic composition of tourmaline

The B-isotopic compositions of tourmaline from the type-1 andtype-3 mineral assemblages show a δ11B range from 0.9 to −10.4‰,withmodal values between−5 and−3‰ δ11B (Fig. 14, Table 4). Mostvalues of the type-3 tourmaline, typified by tourmalinite withtourmaline replacing recrystallised detrital quartz in quartzite, arein the negative range. The δ11B values of tourmaline from CórregoBom Sucesso match those from other B reservoirs, such as magmaticand non-marine evaporitic reservoirs, but are distinct from marineevaporite (Fig. 14).

5. Discussion

Tourmaline fabrics that are relevant for this section, as well as theirregional tectonic setting, are summarised in Fig. 15. Petrographicevidence indicates that tourmaline pseudomorphically replacedrecrystallised detrital quartz, thereby generating quartzite-hostedtourmalinite (Fig. 7e), where tourmaline is coeval with interstitialtitaniferous hematite. This tourmaline also contains abundant in-clusions of Ti-bearing hematite (Fig. 7f). Important implications canbe drawn from these observations: (1) as tourmaline replacementtook place essentially on detrital quartz in quartzite with very lowalumina content, Na–B-rich fluids must have carried significantcontents of Al; (2) such fluids also had Ti and Fe in solution toaccount for the abundance of titaniferous hematite; (3) fluidpercolation must have taken place after the recrystallisation ofdetrital quartz, i.e. after the peak of regional metamorphism.Experimental data show that aqueous Al–B complexes can dominate

-light photomicrograph of grey quartzite with tectonic foliation given by muscovitearea denoted in (a) under reflected light to point out rutile crystals of the type-1 mineraley patch in white quartzite. Apatite–hematite aggregates (type-2 mineral assemblage)in contacts at 120°). Xenotime (bright white) is intergrown with the type-2 hematite.an aggregate of tourmaline (shades of green) with rutile (opaque). This is a hematite-crystallised matrix of detrital quartz. (e) Reflected-light photomicrograph of a typicaloccurs as inclusions (not distinguishable) in tourmaline. Yellow arrows point to relics ofd tourmaline indicate pseudomorphic replacement of recrystallised detrital quartz byh looks clean (no microinclusions of hematite). (f) The same area depicted in (e) underf hematite in tourmaline. Bright white internal reflections come from quartz.

Fig. 9. Reflected-light photomicrographs of vein hematite. All assemblages consist of Ti-bearing hematite (vide Table 2 for microanalyses). (a) The type-4mineral assemblage, of specularhematite and rutile, occurs as pockets truncating the tectonic foliation of hematitic phyllite. (b) Chaotic distribution of specular hematite in (a), i.e. no tectonically induced planar fabric,suggesting a late-orogenic overprint by Ti-bearing fluids. (c) The type-5 mineral assemblage, represented by quartz–hematite veins. (d) Oil-immersion photomicrograph of the type-5specular hematitewith inclusions of rutile. The specularhematite is grownovermartite, i.e.magnetite pseudomorphically replacedbyhematite.Note the serrated contact betweenmartiteand specular hematite. The overgrowth indicates that a later hydrothermal overprint oxidisedmagnetite to hematite (martite) and precipitated specular hematite from Ti-bearing fluids.

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Al speciation in B-rich brines during the formation of tourmaline-bearing mineral assemblages (Tagirov et al., 2004). High concentra-tions of ligands such as Cl and F are required to efficiently transport Tiin solution (Rapp et al., 2010). Destabilisation of aqueous Ti–F speciesby precipitation of F-bearing minerals, such as apatite and tourmaline,would lead to Ti precipitation not only in the form of rutile (Rapp etal., 2010), but also as titaniferous hematite in contact with F-apatite(Fig. 7c) and F-bearing tourmaline (Fig. 7e). We therefore suggest thattitaniferous hematite reflects precipitation from oxidised brines.

The hematite inclusions in palladiferous gold (Fig. 10a), which hasPt-rich patches (Fig. 10d), show consistently high contents of Ti(~6 wt.% TiO2). On the other hand, the hematite found as foliation-truncating pockets in quartzite and hematitic phyllite has generallylower Ti contents. These observations indicate that fluids from whichhigh-Ti hematite precipitated were also capable of transportingprecious metals in solution. Gold, Pd and Pt are expected to becomesoluble in Cl-rich fluids because of the strong metal–chloridecomplexing at high-oxidation state (e.g. Wood, 2002). High-oxidationstate is also indicated by some features of the Au–Pd–Pt mineralisa-tion: the absence of sulfide minerals, and the very unusual occurrenceof both Pt–Pd-rich gold (Fig. 10) and palladiferous gold with high Pd/Ag ratios (Fig. 11) — see Section 5.1.

The gold-hosted tourmaline (type 6) and some of the tourmalinefrom tourmalinite (type 3) define a compositional trend along the“oxy-dravite”–povondraite join (Fig. 8). This join is simply used as areference in the triangular Al–Fe–Mg diagram (Fig. 8) and does notnecessarily imply the actual existence of “oxy-dravite”. Tourmalinecompositions along this join are characteristic of B-rich brines of

evaporitic origin (Henry et al., 2008). However, our data showconsiderable deviation from this meta-evaporitic trend. The tourma-line from the type-1 mineral assemblage in grey quartzite, in whichrutile and tourmaline coexist (Fig. 7a and b), marks the farthestcompositional array from the “oxy-dravite”–povondraite join (Fig. 8).The lack of hematite in the type-1 mineral assemblage contrasts withthe abundance of hematite in the type-3 and type-6 mineralassemblages. The absence of hematite most likely reflects a differentfluid, less charged with Fe, or more diluted with respect to Clconcentrations required for Fe–Cl complexing (e.g. Wood andSamson, 1998). In this regard, the deviation from the meta-evaporitictrend in Fig. 8 could be explained by the involvement of chemicallydistinct, possibly progressively diluted, fluids.

The origin of B-rich brines can be constrained by the B-isotopicsignature of tourmaline. The range of δ11B values, from−10.4 to 0.9‰,is not compatible with evaporitic brines derived from seawater, ratherthis range is indicative of a non-marine reservoir (e.g. Barth, 1993;Palmer and Swihart, 1996). The B-isotope data would also beconsistent with a granitic boron source. However, felsic igneousrocks are unknown in the study area. All tourmaline- and/or hematite-bearing mineral assemblages at Córrego Bom Sucesso occur in theSopa-Brumadinho Formation, where the siliciclastic metasedimentsrecord deposition in lacustrine fan-deltaic environments (e.g.Martins-Neto, 1996). Metamorphism-induced dewatering from lacustrineevaporitic sedimentary rocks in the basal part of the EspinhaçoSupergroup would have generated B–Na-rich brines during theBrasiliano orogenic event. Elsewhere, near Diamantina, such fluidsresulted in concentrations of Al-phosphateminerals and tourmaline in

Fig. 10. BSE images of detrital gold recovered from the Córrego Bom Sucesso alluvium. (a) Black inclusions comprise titaniferous hematite (~6 wt.% TiO2, Table 2) and tourmaline.(b) X-ray mapping for Na as evidence for the presence of tourmaline near (c) Pd- and (d) Pt-rich patches in gold.

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hematite–muscovite–kyanite quartzite along the contact between theBandeirinha Formation and underlying rocks (Horn et al., 1996;Morteani et al., 2001). Not surprisingly, the hematite spatiallyassociated with the Al-phosphate and borate minerals is titaniferous(Morteani et al., 2001). Overall, the mobility of otherwise immobile Ti,as observed in epigenetic rutile and titaniferous hematite coexistingwith F-bearing minerals, is consonant with brines (cf. Rapp et al.,2010). A non-marine evaporitic reservoir better accounts for theabundance of tourmaline at Córrego Bom Sucesso and the presence ofexotic lithologies with Al-phosphate and borate minerals nearDiamantina, compared to marine evaporites (e.g. Eugster, 1980).

Further evidence for a non-marine evaporitic reservoir in thesouthern Serra do Espinhaço has been provided by the fluid-inclusionchemistry of specular hematite from quartz–hematite veins inConceição do Mato Dentro (Fig. 1). The hematite-trapped fluidinclusions are saline (10–12 wt.% NaCl equivalent) and have Licontents of 100 μg/g (de Lima et al., 2009). Such a high content of Liis characteristic of non-marine salt-pan waters and associated boratedeposits (e.g. Helvaci et al., 2004). The moderate salinity of the fluidinclusions is far below saturated high-temperature brines andsuggests dilution by another (metamorphic or meteoric?) fluid.

5.1. Gold composition and brines

Gold precipitated from oxidised brines, such as those responsiblefor the quartzite-hosted tourmalinite with titaniferous hematite,should have a diagnostic chemical composition. The extremelyelevated Pd/Ag ratios (Table 5) found in the alluvial gold (Fig. 11)are rather unusual in nature. Other natural examples, such asBrownstone in Devon (U.K.), suggest that changes in gold Pd/Agratios are potentially controlled by small reductions in the oxygen

fugacity of oxidised chloride-rich brines (Chapman et al., 2009). Highconcentrations of soluble Pd and its fractionation from aqueous Ag arerestricted to saline fluids within the stability field of hematite. Suchoxygenated brines overlap the stability field of cerargyrite (AgCl),where Ag solubility is independent of pH and oxidation state, leadingto separation of Ag from Pd (Gammons et al., 1993). The palladiferousgold recovered from the Córrego Bom Sucesso alluvial deposit lacksevidence of long-distance transport and implies a proximal sourcearea adjacent to the alluvial sediments (Cabral et al., 2008). Thecomposition of palladiferous gold, with a distinctive Pt–Ti signature, isthus consistent with the local geology, i.e. country rocks withabundant hematite and lacking sulfide minerals.

Importantly for prospecting purposes, it should be mentioned thatthe alluvial palladiferous gold from Córrego Bom Sucesso has two in-situ analogues: the Serra Pelada Au–Pd–Pt deposit in northern Brazil(e.g. Cabral et al., 2002; Moroni et al., 2001) and the jacutinga-styledeposits in the southern segment of the platiniferous Au–Pd belt ofMinas Gerais (Fig. 1). In both cases, major gold rushes wereengendered by their bonanza concentrations of palladiferous gold.

5.2. Zirconium-in-rutile thermometry and rutile provenance

The Zr-in-rutile temperatures of alluvial rutile and vein rutile fromCórrego Bom Sucesso are consistent with that obtained by quartz–hematite O-isotope thermometry, at about 440 °C, from mineralassemblages of Al-phosphates and borosilicates near Diamantina(Morteani et al., 2001). This comparison is based on the widespreadoccurrence of borosilicate–hematite assemblages in the above-mentioned localities. Stabilityfields of Al-phosphates andborosilicatesindicate minimum pressure conditions of 3.4 kbar near Diamantina(Morteani et al., 2001), in line with the occurrence of kyanite.

Table 4Average composition of tourmaline and SIMS δ11B values of single-spot measurements, Córrego Bom Sucesso, Minas Gerais, Brazil.

Type 1 Type 3 Type 6

N 11 (1s) 11 (1s) 9 (1s) 6 (1s) 9 (1s) 9 (1s) 9 (1s) 5 (1s)

B2O3a (wt.%) 10.60 0.08 10.66 0.04 10.62 0.08 10.68 0.07 10.47 0.12 10.55 0.06 10.73 0.10 10.44 0.09

SiO2 34.82 0.45 35.41 0.42 35.35 0.69 35.34 0.41 35.01 1.43 34.65 0.50 35.10 0.44 34.10 0.44Al2O3 34.75 0.35 33.94 0.64 33.06 0.59 34.22 0.55 31.67 0.99 33.42 0.61 34.82 0.95 32.04 0.28TiO2 0.12 0.05 0.21 0.09 0.16 0.06 0.15 0.11 0.18 0.10 0.22 0.16 0.19 0.18 0.34 0.04FeO(total) 8.62 0.64 7.85 0.53 6.37 0.29 7.68 0.68 8.62 0.53 8.41 0.64 5.36 0.75 7.20 0.11MnO 0.05 0.01 0.06 0.01 0.31 0.04 0.09 0.01 0.26 0.02 0.08 0.01 0.07 0.01 0.07 0.02MgO 5.67 0.37 6.61 0.19 7.82 0.34 6.67 0.28 7.18 0.21 6.56 0.20 7.93 0.19 7.98 0.05CaO 0.11 0.08 0.05 0.01 0.19 0.26 0.07 0.03 0.08 0.02 0.05 0.02 0.15 0.03 0.39 0.01Na2O 2.18 0.17 2.51 0.16 2.49 0.14 2.23 0.18 2.42 0.20 2.45 0.11 2.21 0.30 2.07 0.17K2O b0.04 b0.04 b0.04 b0.04 b0.04 b0.04 b0.04 b0.04F 0.26 0.04 0.12 0.02 0.19 0.04 0.25 0.05 0.17 0.04 0.17 0.03 0.24 0.08 0.32 0.03Total 97.17 0.70 97.42 0.32 96.57 0.55 97.38 0.52 96.08 1.02 96.55 0.45 96.80 0.56 94.93 0.72−O≡F 0.11 0.05 0.08 0.11 0.07 0.07 0.10 0.13Total 97.06 97.37 96.49 97.28 96.01 96.48 96.70 94.80T+Z+Y=15Si 5.704 0.037 5.771 0.080 5.784 0.134 5.748 0.038 5.796 0.199 5.707 0.063 5.683 0.040 5.678 0.035Al(T) 0.296 0.037 0.229 0.080 0.216 0.087 0.252 0.038 0.204 0.109 0.293 0.063 0.317 0.040 0.322 0.035Al(Z) 6.000 6.000 6.000 6.000 5.975 0.072 6.000 6.000 5.965 0.025Al(Y) 0.413 0.045 0.289 0.068 0.159 0.073 0.307 0.100 0.000 0.054 0.194 0.075 0.327 0.131Mg 1.385 0.083 1.606 0.045 1.906 0.075 1.618 0.066 1.773 0.049 1.610 0.043 1.915 0.037 1.981 0.015Fe(total) 1.181 0.093 1.070 0.074 0.872 0.039 1.045 0.096 1.193 0.082 1.159 0.093 0.725 0.105 1.002 0.011Mn 0.007 0.003 0.009 0.005 0.043 0.006 0.012 0.001 0.036 0.003 0.011 0.004 0.010 0.003 0.010 0.002Ti 0.014 0.009 0.026 0.013 0.020 0.008 0.018 0.014 0.023 0.013 0.027 0.020 0.023 0.020 0.043 0.005Ca 0.018 0.009 0.009 0.005 0.034 0.047 0.013 0.007 0.015 0.004 0.009 0.004 0.026 0.006 0.069 0.002Na 0.692 0.052 0.792 0.049 0.790 0.047 0.703 0.055 0.778 0.066 0.782 0.038 0.693 0.099 0.669 0.057(‰) δ11B (1s) δ11B (1s) δ11B (1s) δ11B (1s) δ11B (1s) δ11B (1s) δ11B (1s)

−0.1 0.37 −4.1 0.48 −6.6 0.41 −5.8 0.37 −6.1 0.40 0.9 0.40 −7.2 0.31−1.4 0.32 −8.4 0.44 −1.5 0.37 −3.6 0.42 −10.1 0.34 −2.7 0.44 −5.2 0.380.1 0.37 −3.9 0.43 −1.8 0.34 −2.8 0.41 −10.4 0.36 0.4 0.34 −7.0 0.32

−3.1 0.40 −4.1 0.40 −3.6 0.44 −2.4 0.33 −6.5 0.35−5.6 0.39 −9.6 0.34 −4.3 0.38 −4.7 0.35 −9.5 0.35

−4.6 0.37 −3.3 0.42 −7.2 0.36−8.6 0.38 −3.3 0.39 −7.7 0.41−1.5 0.41

Atomic proportions on the basis of 15 cations exclusive of Na, Ca and K.a Calculated for three cations of B.

272 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

Despite their similar Zr-in-rutile temperatures, the alluvial rutileand vein rutile from Córrego Bom Sucesso could not have beenderived from the same source, i.e. hydrothermal pockets in hematiticphyllite. The alluvial rutile exhibits broad variation of Nb/Ta ratios andFe contents, which are clearly in contrast to the narrow range found inthe hydrothermal rutile of the type-4 mineral assemblage fromhematitic phyllite (Fig. 13). This Nb/Ta clustering may be indicative ofan inheritance of the host-rock Nb/Ta ratio (e.g. Meinhold, 2010). Theprotolith of the host rock, i.e. hematitic phyllite, is elusive due tostrong metasomatic and tectonic overprint, but most authors agreethat this lithology was originally an eruptive rock (e.g. Derby, 1899;Dussin, 2000; Herrgesell, 1984). More important, however, is thechemically inert nature of the quartzitic domains of the southernSerra do Espinhaço, through which non-marine evaporitic brinescarrying soluble Ti could move, whereas the hematitic phyllitebehaved as impermeable layers. In the latter, whole-rock compositionwould have buffered the Nb/Ta ratios of hydrothermal rutile, whereasthe rutile precipitated in the quartzitic domains would reflect Ti-bearing brines with different Nb/Ta ratios and oxidation states(Fig. 13). It is thus suggested that the alluvial rutile comes from ahydrothermal system within the quartzitic domains.

Evidence for a large-scale hydrothermal system, involving thesiliciclastic metasedimentary sequence containing impermeable hemati-tic phyllite, is that both the type-4 rutile and the alluvial rutileoccasionally have a few hundred ng/g of Au.While rutile is the dominantcarrier of Nb and Ta (Meinhold, 2010, and references therein), Au is notpreferentially incorporated in rutile precipitated from hydrothermalfluids. Consequently, the hydrothermal rutile recovered from the CórregoBom Sucesso alluvium attests to auriferous fluids during crystal growth.

Its mineral chemistry, with high contents of Fe (N0.5 wt.%) and variableNb/Ta ratios, most likely records a highly oxidising, chemicallyheterogeneous hydrothermal system.

5.3. Relative timing of brine percolation

The siliciclastic metasediments of the southern Serra do Espinhaçorecord the metamorphic and tectonic overprint of the ~0.6-GaBrasiliano event (e.g. Alkmim et al., 2006; Uhlein et al., 1998).Petrographic and field observations indicate that: (1) tourmaline,apatite and titaniferous hematite cross-cut the static recrystallisationfabrics of detrital quartz in quartzite; (2) tourmaline and muscovitedefine the tectonic foliation in grey quartzite; (3) the type-4 and type-5 mineral assemblages truncate the tectonic foliation of the host rock.All these pieces of evidence point to a relative timing for the brinepercolation between the static metamorphism and late-Brasilianoorogenesis. Regionally, Córrego Bom Sucesso belongs to the platini-ferous Au–Pd belt of Minas Gerais, the N–S trend of which iscontrolled by the trace of thrust faults generated during the Brasilianoorogeny (Fig. 1; Cabral et al., 2009a).

Alluvial aggregates of palladiferous gold (Cabral et al., 2008) andPt–Pd-bearing gold (Fig. 10, this study; Cassedanne et al., 1996) fromthe Córrego Bom Sucesso alluvium indicate that their source rock washematite–quartz vein material. As suggested by Fig. 6, Au–Pd–Ptmineralisation occurs in veins that have distinctively Ti-enrichedhematite compared to those of the type-4 and type-5 assemblagehematite. These hematite assemblages are further distinguished fromeach other by their formational temperatures: the type 4 had its rutilecrystallised around 500 °C, while rutile and quartz of the type-5

Fig. 11. Secondary-electron (a–c) and backscattered-electron (d) images of elongated crystals of gold. The crystals exhibit angular to slightly rounded edges, indicating a veryproximal source area. All grains are palladiferous and have very high Pd/Ag ratios, between 4 and 3700 (Table 5).

Table 5LA–ICP–MS measurements on alluvial gold, Córrego Bom Sucesso, Minas Gerais, Brazil.

Ti Ni Cu Zn As Pd Ag Te Pt Au

μg/g μg/g μg/g μg/g μg/g μg/g wt.% μg/g μg/g wt.%

1 134 59 33 12 b5 132989 b0.01 b10 169 86.652 516 71 39 16 b5 103207 b0.01 b10 41 89.603 629 46 4627 10 b5 45064 0.08 b10 86 94.874 132 34 52 8 b5 179976 0.01 b10 712 81.895 5851 30 26 7 b5 43110 1.16 b10 395 93.896 b5 53 28 b10 b5 11 0.31 b10 19 99.617 b11 49 36 b10 b5 b5 2.36 b10 b5 97.608 b12 61 39 b10 b5 b5 0.99 b10 b5 98.999a b14 60 34 b10 b5 b5 0.89 b10 b5 99.069b b15 54 29 b10 b5 b5 0.60 b10 b5 99.379c b16 53 27 b10 b5 b5 0.10 b10 8 99.8810a b18 65 405 b10 b5 6 12.34 35 b5 87.6010b b19 47 85 b10 b5 690 1.97 b10 b5 97.9411 b20 44 918 b10 b5 1364 5.42 15 13 94.2412 b22 37 18 b10 b5 b5 0.04 b10 b5 99.9513a b6 53 37 b10 7 175 0.14 b10 329 99.1913b b7 53 34 b10 b5 b5 0.29 b10 6 99.4214 b8 29 22 b10 12 b5 0.13 b10 b5 99.5015 b9 40 23 b10 b5 b5 1.01 b10 b5 98.9416 b10 44 27 b10 b5 21 0.22 b10 b5 99.6017 b13 55 36 b10 6 5 0.06 16 b5 99.6918 b17 67 38 b10 5 222 0.17 b10 15 99.2519 b21 45 26 b10 b5 b5 0.61 b10 6 99.29

Elements sought for, but were not detected, are as follows (detection limits in μg/g): Cr(b7), Co (b5), Se (b10), Ru (b5), Rh (b5), Cd (b5), Sn (b10), Sb (b5, excepting lines 13a,18 and 19, with 6, 7 and 9 μg/g, respectively), Os (b5), Ir (b5), Tl (b5) and Bi (b5).Line measurements:1–5: angular to slightly rounded aggregates of palladiferous gold (1–4 shown inFig. 11a–d, respectively).6–12: rounded platelets.13–19: spherical grains.

273A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

assemblage formed at temperatures about 400 °C (Fig. 12). Such atemperature gradient within a distance of ~2 km implies episodic veinemplacement under variable temperature regimes. Episodic emplace-ment of barren and mineralised veins in the context of the late-Brasiliano orogenesis can be understood within the spectrum of theorogenic gold-lode deposit category (Groves et al., 1998).

Fig. 12. Histogram of Zr-in-rutile and Ti-in-quartz temperatures, applying equations ofTomkins et al. (2007) and Wark and Watson (2006), respectively (Table 6). The twotemperature clusters suggest episodic vein emplacement at Córrego Bom Sucesso. VideDiscussion for clarification.

Table 6LA–ICP–MS measurements on rutile and quartz, Córrego Bom Sucesso, Minas Gerais, Brazil.

Si (μg/g) Sc Fe Y Zr Nb Ta Nb/Ta Au T(°C)a

Alluvial rutileGrain 1 458 8.11 6699 0.035 30.1 85.3 1.35 63 b0.02 472

613 6.89 6724 0.041 29.7 94.8 1.77 54 b0.02 471770 8.31 6065 0.052 29.2 84.0 1.50 56 b0.02 470706 6.31 16561 0.050 35.7 212 7.40 29 b0.02 481554 8.19 10966 0.039 36.7 187 5.24 36 b0.03 483

Grain 2 375 9.42 5994 0.053 38.9 365 31.7 12 b0.03 486606 7.94 6159 0.100 39.2 322 29.5 11 b0.03 487

Grain 3 554 5.91 16519 0.054 22.0 1316 90.8 14 0.04 455666 12.0 8268 0.029 22.7 441 12.6 35 b0.03 457570 12.7 9713 0.039 22.8 411 8.68 47 0.04 457

Grain 5 705 7.68 14613 0.049 36.7 2447 98.5 25 0.05 483555 7.85 15625 0.030 39.2 2015 61.0 33 b0.03 487563 6.82 12352 0.042 34.1 2843 157 18 0.04 479

Grain 6 800 12.3 16019 0.057 44.3 316 18.9 17 0.025 493803 13.6 18740 0.043 44.8 240 15.5 15 0.036 494457 15.4 18031 0.039 48.7 246 16.1 15 0.040 499

Grain 7 495 3.12 7949 0.236 40.3 84.6 3.04 28 b0.03 488583 1.41 27407 0.289 38.9 431 23.8 18 b0.03 486378 1.28 7747 0.728 32.3 168 2.93 57 b0.03 476

Grain 8 525 5.68 6789 0.053 31.1 248 8.82 28 b0.02 474446 5.87 7048 0.037 30.4 303 7.61 40 b0.02 473513 22.3 7342 0.453 26.3 3255 228 14 0.039 465

Type 4 b731 11.3 6202 0.265 38.1 575 48.8 12 b0.08 485b673 10.8 6081 0.146 28.1 434 35.1 12 b0.07 468b605 8.70 6734 0.029 43.7 786 54.4 14 b0.07 493b612 9.93 5576 0.149 26.2 464 41.3 11 b0.07 4653056 8.98 5599 0.164 30.1 562 48.6 12 0.099 472697 5.17 6049 0.144 28.3 705 65.9 11 b0.06 469583 7.33 5164 0.098 38.9 609 55.5 11 b0.05 486

Type 5 388 14.9 6559 4.55 1.41 6.18 0.649 10 b0.02 341534 12.1 6272 3.91 4.91 32.9 2.08 16 b0.02 390

Type-5 Ti-in-quartz temperaturesTi (μg/g) 1.50 1.11 1.29 1.65 0.95 1.36 1.34 1.18 1.81T(°C)b 410 394 402 415 386 405 404 397 420Ti (μg/g) 1.36 1.37 2.62 1.38 1.88 2.00 1.52 2.32 2.49T(°C)b 405 405 441 405 422 426 411 434 438

a Calculated with the equation of Tomkins et al. (2007), using a pressure estimate of 0.34 GPa (Morteani et al., 2001).

274 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

5.4. Titaniferous hematite and prospecting for Au–Pd–Pt mineralisation

In the southern part of the platiniferous Au–Pd belt (i.e.Quadrilátero Ferrífero; Fig. 1), the Au–Pd–Pt mineralisation occurs

b Calculated with the equation of Wark and Watson (2006).

Fig. 13.Distribution of Nb/Ta vs. Fe in rutile from Córrego Bom Sucesso (Table 6). Valuesfor hematitic phyllite-hosted vein rutile, i.e. type-4 mineral assemblage, fromDiamatina are plotted for comparison (A.R. Cabral, unpublished data). The clusteringof the type-4 rutile possibly reflects hydrothermal systems buffered by a relativelyimpermeable host rock, i.e. hematitic phyllite. The dispersion of alluvial rutile likelyrepresents derivation from hydrothermal milieux that were not chemically buffered byhost rocks such as quartzite.

ig. 14. Histogram of tourmaline δ11B values from Córrego Bom Sucesso (Table 4) andariation of δ11B in some reservoirs (Barth, 1993). The abundance of tourmalinite atórrego Bom Sucesso and its range of negative δ11B values, together with the widespreadccurrence of tourmaline in lacustrine to fan-deltaic metasedimentary sequences of the

FvCo

southern Serra do Espinhaço, favour a non-marine evaporitic reservoir of B.

Fig. 15. (a) Schematic cross-section showing the relative location of Diamantina and Córrego Bom Sucesso in the southern Serra do Espinhaço. The fold-and-thrust arrangement ofthe metasiliciclastic rocks of the Espinhaço Supergroup resulted from the amalgamation of West Gondwana during the Neoproterozoic Brasiliano orogenic event (e.g. Alkmim et al.,2006; Uhlein et al., 1998). (b) Schematic representation of tourmaline-bearing assemblages found in the Córrego Bom Sucesso area. The mineral assemblages exhibit a variety offabrics that indicate syntectonic to late-tectonic mineral formation. The tourmaline from the type-3 mineral assemblage cross-cuts the tectonic foliation developed during theBrasiliano orogenic event. A late-tectonic timing for the mineral assemblage in alluvial gold (type 6) is given by the random orientation of hematite (Hem) and tourmaline (Tur).Qz = quartz; Ms = muscovite; Rt = rutile.

275A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

as cross-cutting veins of specular hematite and quartz (Cabral et al.,2009a, and references therein). Vein hematite from the mineralisedveins can be distinguished from the country-rock itabirite andhematite ore by higher contents of Ti and/or Cr (Cabral et al., 2006).At the Timbopeba iron-ore mine (Fig. 1), the hematite ore itself hasspecular hematite-hosted fluid inclusions of highly saline H2O–NaClfluids (N30 wt.% NaCl equivalent) with some Ti in solution (Rosièreand Rios, 2006). At the Gongo Soco mine and in the Itabira district(Fig. 1), vein hematite-hosted fluid inclusions contain dissolvedsulfate and halogen ratios that are consistent with fluids evolved froman evaporitic source (Lüders et al., 2005).

The recognition that titaniferous hematite is genetically related tometa-evaporitic tourmalinite at Córrego Bom Sucesso is significant fortwo reasons: (1) it corroborates the large-scale dimensions of the belt

by reconciling its southern and northern segments with fluids relatedto an evaporitic reservoir; (2) it indicates that titaniferous hematitecan be used as a prospecting guide for Au–Pd–Pt mineralisation. Thepresence of Ti in hydrothermal hematite is thus considereddiagnostic of meta-evaporitic brines. However, not all titaniferoushematite is spatially associated with brines enriched with preciousmetals.

Regarding titaniferous hematite in hydrothermal Au–Pd–Ptmineralisation worldwide, little information on the chemical compo-sition of hematite is available. Examples of Au–Pd–Pt mineralisationare known from selenide vein-type deposits (e.g. Tilkerode, Harz,Germany; Tischendorf, 1959), for which temperatures around 100 °Care suggested (e.g. Simon et al., 1997). An on-going re-examination ofthe Tilkerode hematite has characterised it as titaniferous (A. R.

Pre-ablation Ablation

Line Spot Line Spot

Duration (20–40 s) b3 s (100–200 s) 50 sLength 300–600 μm – 300–600 μm –

Diameter 75 μm 75 μm 50 μm 50 μmEnergy 30% 40% 45% 100%

276 A.R. Cabral et al. / Journal of Geochemical Exploration 110 (2011) 260–277

Cabral, unpublished data). Therefore, titaniferous hematite as apotential prospecting guide for Au–Pd–Pt mineralisation can beapplied to low-temperature hydrothermal systems.

6. Conclusions

Brines were instrumental in the formation of the hematite-rich,sulfide-free Au–Pd–Pt mineralisation in the southern Serra doEspinhaço, Minas Gerais, Brazil. Large-scale brine percolation resultedin regionally extensive hematite and tourmaline formation. Quartzite-hosted tourmalinite replaced detrital quartz showing recrystallisationfabrics. Boron-isotope values of tourmaline and its chemical compo-sition support a non-marine origin for the B, in accordancewith brinesderived from the lacustrine and fan-deltaic settings of the hostmetasedimentary clastic sequence. Such brines also explain thepalladiferous gold with extremely high Pd/Ag ratios, i.e. highconcentrations of aqueous Pd and segregation of Ag from Pd underhighly oxidising conditions. Tourmalinite ‘flooding’ was concomitantwith the deposition of titaniferous hematite and rutile. The presenceof Ti in hematite derives from the ability of brines to transport Ti insolution. Detrital Pd–Pt-bearing gold from the Córrego Bom Sucessoalluvium has tourmaline inclusions and abundant hematite withconsistently high contents of Ti. The hematite associated with the Au–Pd–Pt mineralisation most likely comes from hematite–quartz veinsdeposited from brines that were capable of carrying much aqueous Ti.Titaniferous hematite and rutile can thus be used as prospectingguides for the platiniferous Au–Pd mineralisation.

Acknowledgements

Early fieldwork at Córrego Bom Sucesso was financially supportedby a Rudolf-Vogel Prize of TU Clausthal to ARC, who later benefitedfrom a DFG research project (no. LE 578/29-1). Our fieldwork wasfacilitated by the support of local people, garimpeiros and peasants.Guido Meinhold kindly reviewed an early version of the manuscript.Rob Chapman and Alfonso Pesquera are gratefully acknowledged fortheir in-depth review of the manuscript. Agustin Martin-Izardcarefully handled the manuscript.

Appendix 1. Operating conditions of the LA–ICP–MS system usedfor trace-element analyses on unpolished gold grains.

Instrument Thermo XSeriesII

Nebulizergas flow

0.8–0.9 L min−1 Ar

Auxiliarygas flow

0.7 L min−1 Ar

Plasma/coolgas flow

13.0 L min−1 Ar

Collision gas 6.0 mL min−1 Ar H2–He mixture (8% H2, rest He)RF power 1200 WLens setting L1 -1200; L2 -85.5; L3 -79.2; D1 -51.8; D2 -140; DA −65.1 VBiases Slightly KED (Kinetical Energy Discrimination):

Plot Bias −12.9. Hexapole Bias −13.0Detector mode DualDwell time 10 msScan mode Peak jumpCarrier gas flow 0.8 L min−1 HeLaser ablationsystem

Microprobe II with LUV213, 213 nm

Pulse length 4 nsAblationfrequency

4 Hz

Spot size 50 μmLaser fluence orpulse energy

24–30 J cm−2

Appendix 2. Laser-operating conditions for line- and spot-ablation modes used for trace-element analyses on unpolishedgold grains.

Alkmim, F.F., Marshak, S., 1998. Transamazonian orogeny in the southern São Franciscocraton region, Minas Gerais, Brazil: evidence for Paleoproterozoic collision and

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