1-s2.0-S0024493714003776-main.pdf

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XenotimeUPb dating

in tepollema

inerals

trate U, but exclude Pb on crystallising, making them amenable to precise altered, or even completely dissolved, during subsequent uidrock in-

whole grains in a wideGerdes and Zeh, 2009;, 1975; Mezger andZeh et al., 2010). Con-of zircon andxenotime

Lithos 212-215 (2015) 415427

Contents lists available at ScienceDirect

Lith

.e l1992; Spear and Pyle, 2002). Zircon grains, for example, can grow inmelts during high-grade metamorphic and magmatic events (Blacket al., 1986; Vavra et al., 1999), in aqueous uids during amphibolite-

ages requires detailed information about the relative and absolutetiming of zircon and xenotime formation, in a textural context.

In arenaceous rocks, detrital zircon grains commonly form a minorHowever, interpretation of age data may be complicated because zirconand xenotime in single rocks can form in more than one geologicalevent (Black et al., 1986; Gerdes and Zeh, 2009), and can be affected bydifferent styles of alteration after their formation (Geisler et al., 2001,2003a, 2003b, 2007; Gerdes and Zeh, 2009; Hoskin, 2005; Pidgeon,

loss of radiogenic Pb from local domains or fromrange of geological settings (Geisler et al., 2001;Hay and Dempster, 2009; Krogh and DavisKrogstad, 1997; Pidgeon, 1992; Vavra et al., 1999;sidering such complexities, proper interpretationdating. Both minerals have high-closure temperatures for the UThPbsystem, N900 C for zircon (Cherniak and Watson, 2001; Lee et al.,1997), and similar (N900 C) for xenotime (Cherniak, 2006; Liu et al.,2011); hence, their ages are likely to record the time zircon and xenotimecrystallised during magmatic, metamorphic and hydrothermal events.

teraction, and newly re-precipitated undermetamorphic conditions be-tween 250 and 650 C (Dempster et al., 2004;Hay andDempster, 2009;Rasmussen, 2005; Wilke et al., 2012; Zeh et al., 2010). Alteration, eitherrelated to leaching ofmetamict zircon domains, local recrystallisation, orcoupled dissolutionreprecipitation processes, can lead to partial or totalfacies metamorphism (Zeh et al., 2010; Zeh anding low-grade metamorphic hydrothermal e

Corresponding author.E-mail address: [email protected] (A.R

http://dx.doi.org/10.1016/j.lithos.2014.10.0110024-4937/ 2014 Elsevier B.V. All rights reserved.processes, as well as fore. Both minerals concen-

in sedimentary rocks (Cabella et al., 2001; Cabral et al., 2012a; Suzukiand Adachi, 1991). Furthermore, zircon and xenotime can be partiallymatic, metamorphic and even hydrothermalthe characterisation of sedimentary provenancGold-lode depositPassagem de MarianaMinas Gerais

1. Introduction

Zircon and xenotime are valuable mzircon-age resetting during the Brasiliano orogeny at ca. 500Ma. This interpretation is supported by UPb datingof euhedral xenotime immediately adjacent to altered zircon within the same tourmaline pocket. The xenotimegrains give a Concordia age of 496.3 2.0 Ma, which is identical to that determined for monazite of a quartzhematite vein-type deposit (i.e., jacutinga lode) in the region (Itabira), another important mineralisation styleof gold. The occurrence of relatively abundant inherited detrital zircon, but absence of rock fragments in the tour-maline pocket investigated here, implies that detrital material was completely replaced by tourmaline. Thegraphite overprint on the altered detrital zircon attests to a reducing uid, which was likely formed by uidrock interaction with carbonaceous phyllite of the Batatal Formation, the host rock of the Passagem lode.

2014 Elsevier B.V. All rights reserved.

for UPb dating of mag-

2004; Hoskin and Schaltegger, 2003; Yeats et al., 1996). Xenotimeforms across a range of metamorphic conditions (Heinrich et al., 1997;Pyle et al., 2001), and also occurs as an authigenic and detrital mineralKeywords:Zircon

detrital zircon of the footwall quartzite of the N2.65-Ga-old Moeda Formation. Discordant analyses point toAvailable online 29 October 2014 inductively coupled plasmaDetrital zircon without detritus: a result ointeraction during the gold-lode formationGerais, Brazil

Alexandre Raphael Cabral a,, Armin Zeh b

a Mineral Deposits, Technische Universitt Clausthal, Adolph-Roemer-Strasse 2A, D-38678 Claub Institute of Geosciences, Johann-Wolfgang-Goethe Universitt, Altenhferallee 1, D-60438 Fr

a b s t r a c ta r t i c l e i n f o

Article history:Received 31 July 2014Accepted 20 October 2014

Zircon and xenotime occurMariana, a world-class gold dexhibiting porous domains

j ourna l homepage: wwwGerdes, 2014), and dur-vents (Dempster et al.,

. Cabral).96-Ma-old uidrockf Passagem, Minas

l-Zellerfeld, Germanyurt am Main, Germany

ourmaline-rich hydrothermal pockets in the auriferous lode of Passagem desit. Zircon grains show pristine oscillatory zoning, but many of them are altered,d with graphite. UraniumPb dating of zircon, using in-situ laser ablationss spectrometry, yields ages between 3.2 and 2.65 Ga, which match those for

os

sev ie r .com/ locate / l i thoscomponent (mostly b1 vol.%) compared to the abundance of detritalquartz (N95 vol.%). These relationships, however, can be obscured byinteraction of such rocks with hydrothermal uids, as demonstratedin this study. In this context, the following questions arise: can detritalzircon survive where detrital quartz and other clastic material are

quantitatively dissolved and/or replaced during pervasive uidrockinteraction in a hydrothermal quartz lode? Can zircon and xenotime ina such setting still provide information about their provenance, or dothey reect the timing of hydrothermal lode formation only? Can zirconand xenotime grains provide information that is useful to understand

416 A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427physico-chemical processes during the formation of hydrothermalgold-lode deposits?

The present study addresses such questions by considering thegold-lode deposit of Passagem de Mariana, Quadriltero Ferrfero (QF)of Minas Gerais, Brazil. Passagem represents a historically importantworld-class gold-lode deposit with a long-lasting, but still unresolved,debate on its genesis. Ladeira (1988) categorically dened Passagemas an enigmatic deposit. Genetic classications for Passagem vary froman originally intrusion-related deposit (Hussak, 1898; cf. Derby, 1911),through a syngenetic model (Cavalcanti and Xavier, 2006; Fleischerand Routhier, 1973), to a number of epigenetic models (Cabral andKoglin, 2012; Chauvet et al., 2001; Garda et al., 2010; Guimares,1970; Vial et al., 2007). A peculiarity of the Passagem lode is the abun-dance of tourmaline, a feature that has traditionally been referred toas carvoeira (von Eschwege, 1832). Passagem can be regarded as thetype locality of carvoeira-style mineralisation in Minas Gerais. Twodifferent timings for the carvoeira-style mineralisation of Passagemhave been proposed: one Minas, also referred to as Transamazonian(2.12.0 Ga, Schrank and Machado, 1996; Vial et al., 2007), the otherBrasiliano (~0.6 Ga, Chauvet et al., 1994, 2001).

Uncertainty about the genesis of the Passagem lode and its gold (atleast 60 metric tonnes produced from the end of the 17th century until1954 [Vial et al., 2007]) stems in part from the lack of a robust age forthe Passagem quartztourmaline vein deposit. In order to close thisknowledge gap, we present new results of in-situ UPb dating carriedout on well characterised zircon and xenotime grains found in atourmaline-rich pocket of the Passagem lode. Our combined petrographi-cal and geochronological data sets not only shed new light on the timingof formation of the Passagem lode, but also provide detailed informationabout the behaviour of zircon and xenotime during hydrothermalprocesses that led to the formation of carvoeira-style mineralisation.

2. Geological setting and Passagem de Mariana

The gold-lode deposit of Passagem de Mariana is located inthe Mariana anticline, at the southernmost end of the platiniferousAuPd belt of Minas Gerais, which is delineated by thrust faults(Fig. 1). The limbs of the Mariana anticline are thrust faults that de-formed metasedimentary rocks during the ~0.6-Ga Brasiliano orogeny,resulting in a prominent tectonic foliation (Chauvet et al., 1994). Alongthe anticline limbs ore deposits are zonally distributed following aredox gradient along the ground, from sulphate- and hematite-bearingzones to a sulphide-rich zone (Cabral et al., 2013b). Passagem is withinthe sulphide zone, immediately below an itabirite unit (Vial et al., 2007).

Itabirite is the characteristic rock of the Cau Itabirite of the ItabiraGroup,Minas Supergroup (Dorr, 1969). The term itabirite, originally de-scribed by von Eschwege (1822) and widely used in Brazil, denesmetamorphosed banded iron formation containing hematite and/ormagnetite. The itabirite unit, referred to as the Itabira iron formation(Harder and Chamberlin, 1915), was deposited at 2.65 Ga (Fig. 2;Cabral et al., 2012b). The itabiritic rocks grade upwards to dolomiticrocks of the Gandarela Formation, the upper unit of the Itabira Group(Dorr, 1969). The auriferous sulphidetourmalinecarbonatequartzlodes of Passagem are located beneath the Itabira Group, in a thrustzone encompassing rocks of the Caraa Group, mostly of quartzite

Fig. 1. Location of the Passagem gold-lode deposit in the southern part of the platiniferousAuPd belt of Minas Gerais (Cabral et al., 2009, and references therein). The belt followsthe roughly NS-trending trace of the ~0.6-Ga Brasiliano thrust faults, along whichlode deposits and platiniferous alluvia occur. Abbreviations: C. = Crrego (stream);

Faz. = Fazenda (farm); Fm. = Formation; Gr. = Group; Sg. = Supergroup.

(i.e., the Moeda Formation at the base), followed by pelitic rocks(i.e., the Batatal Formation), locally carbonaceous, that are transitionalto the Cau Itabirite. Detrital zircon ages for the Moeda Formation atPassagem are between 3097 44 and 2606 47 Ma (Machado et al.,1996). Quartzitic rocks of the Moeda Formation and pelitic rocks ofthe Batatal Formation are locally interngered (where not obliteratedby tectonic overprint). The Caraa Groupunconformably overlies green-stone rocks of the Archaean Rio das Velhas Supergroup (Dorr, 1969).The supracrustal sequences of the Rio das Velhas Supergroup and theMinas Supergroup represent structural keels that are delimited bygneissic domes (Marshak et al., 1992).

Regional metamorphism in the Mariana anticline is within thetremoliteanthophyllite zone of Pires (1995), corresponding to upper-greenschist- and lower-amphibolite-facies conditions for the countryrocks of the Passagem deposit (Vial et al., 2007). At Passagem, Chauvetet al. (2001) estimated pressures between ~7 and 10 kbar, andtemperatures around 500 C. We note that Hussak (1898) foundandalusite totally replaced by muscovite. The nding of earlierandalusite at Passagem is at odds with the aforementioned pressureestimates, attesting to the superposition of two metamorphic events:one is the Minas orogeny, which produced andalusite elsewhere in theQuadriltero Ferrfero in response to the ascent of gneissic domes(Marshak et al., 1992); the other is the Brasiliano orogeny, to whichthe metamorphic conditions estimated for Passagem are attributed(Chauvet et al., 2001).

The Passagem gold-lode deposit occurs within a tabular thrust-faultzone involving rocks of the Caraa Group underneath the CauItabirite (Chauvet et al., 2001; Vial et al., 2007), in agreement with theitabirite hanging wall and quartzite footwall depicted in earlier works(Ferrand, 1894; Hussak, 1898). The country rocks exhibit prominenttectonic foliation of ductile character. Graphitic phyllite (Fleischer andRouthier, 1973), also known as graphitesericite phyllite (Vial et al.,

thrust-fault zone. Within this zone, the lode deposit consists ofvoluminous quartz, carbonate and tourmaline-rich pockets. Tourmalineconcentrations that are parallel to the foliation are termed tourmalinite;it occurs along the contact between quartzcarbonate lodes and thewallrock, dening a salband or selvedge, or as boudins in banded dolo-mite (Fig. 3A). Tourmaline pockets refer to tourmaline concentrationswithin the quartzcarbonate lodes (Fig. 3A). The lodes truncate the tec-tonic foliation of the country rocks, which is ascribed to the Brasilianoorogeny (Chauvet et al., 1994).

Themost abundant oremineral is arsenopyrite; some pyrrhotite andpyrite are present. Gold and a AuBiSbTe mineral assemblage arecommonly observed within microfractures cross-cutting the sulphideminerals (Chauvet et al., 2001; Hussak, 1898; Oberthr and Weiser,2008; Vial et al., 2007). It is important to point out that the richestgold contents were found along the contact with hematite-bearingitabirite and within graphitesericite phyllite (Vial et al., 2007).

3. Samples and methods

3.1. Samples

Three samples rich in tourmaline were collected in the Fundoorebody at level 120 (see Fig. 6 of Vial et al., 2007, for geological crosssections of the Fundo orebody). The footwall of the Fundo orebodyat level 120 is quartzite of the Moeda Formation. The quartzcarbonatelode at level 120 encloses tourmaline pockets, some 0.3 m in thickness.The hangingwall is banded dolomite of the Batatal Formation (Fig. 3A).Imbrications of sericite phyllite occur along the thrust-fault zone inwhich the lodes are located (Fig. 3B).

Sample L120a is a tourmaline pocket with interstitial calcite(Fig. 3C). Sample L120b comprises vein quartz and tourmaline-bearingsericite phyllite, a fragment of the wallrock. The contact between the

of thski e

417A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 4154272007), typies the Batatal Formation of the Caraa Group in the tabular

Fig. 2. Schematic lithostratigraphic column inwhich the CaraaGroup and the ItabiraGroupet al. (2012b) on the Cau Itabirite (UPb zircon age from a metavolcanic layer) and Babinlikely reects changes in the Pb abundance during diagenesis.vein quartz and the phyllite is marked by a 12-mm-thick selvedge

eMinas Supergroup are emphasised (Cabral et al., 2013b). Age constraints are fromCabralt al. (1995) on the Gandarela Formation. The 2.42-Ga age is a whole-rock PbPb age that

418 A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427of coarse-grained tourmaline, along which arsenopyrite and pyriteoccur in the phyllite. Sample L120c is a tourmalinequartz pocket.UraniumThPb-isotope analyses were carried out on a thin section ofsample L120a. This hand-specimen sample is representative of thetourmaline-rich pockets that are commonly found in the quartzcarbonate lodes of Passagem (Chauvet et al., 2001; Fleischer andRouthier, 1973; Hussak, 1898; Vial et al., 2007).

Fig. 3. A. Schematic representation of main features of auriferous tourmalinecarbonatequartz lode at level 120, Passagem deMarianamine. The lode is located between quartziteof the Moeda Formation and banded dolomite of the Batatal Formation. B. Photograph,looking300, of a quartz lode rich in tourmaline (Tur) pockets at level 120. Sericite phylliteof the Batatal Formation occurs here as wallrock. C. Reected-light photomicrograph of atourmaline pocket from level 120 (sample L120a). Tourmaline (Tur) crystals, withoutany preferential orientation, have their interstices lled with calcite (Cal). Arrow pointsto a zircon grain (Zrn; see Fig. 4A for a BSE image).3.2. Methods

3.2.1. Petrography and geochemistryPolished thin sectionswere instrumental for recognising zircon- and

phosphate-likeminerals under both transmitted and reected light. Theminerals were conrmed as zircon and xenotime by means of energy-dispersive spectrometry (EDS) using a Cameca SX100 electron micro-probe at the Technische Universitt Clausthal. Backscattered-electron(BSE) images of zircon and xenotime were obtained in high-gainmode in order to identify growth zoning.

Part of the slab remaining aftermaking the thin sectionwasmanual-ly ground in an agatemortar. The rock powderwas submitted for chem-ical analysis to Acme Analytical Laboratories (Vancouver) Ltd., Canada.Trace elements were determined by inductively coupled plasmamassspectrometry (ICPMS), following Li-metaborate/tetraborate fusionand nitric-acid digestion. The same fusiondigestion procedure wasintegrated with ICPoptical emission spectrometry (ICPOES) formajor oxides and Sc. Total C and S were obtained by Leco. Loss onignition (LOI)was quantied bymass difference after ignition at 1000 C.

3.2.2. In-situ analyses for UThPb isotopesAnalyses for U, Th and Pb isotopes in zircon and xenotime were per-

formed with a ThermoScientic Element 2, sector eld (SF)ICPMS,coupled to a Resolution M-50 (Resonetics) 193-nm ArF Excimer lasersystem (ComPexPro 102F, Coherent) at the Goethe-Universitt Frank-furt. Measurements were resolved over time, in peak-jumping, pulse-counting analogue mode over 465 mass scans, with 19-s sampleablation preceded by 21-s background measurement. Laser-spot sizewas 15 m for xenotime, 15 to 23 m for zircon, 33 m for the zircon ref-erence materials: GJ1 zircon (primary reference), Pleovice zircon andOG1 zircon; and 15 m for the referencemonazite Moacir. No xenotimereference material was available. The sample surface, a polished thinsection, was cleaned before each analysis by three pre-ablation pulses.Ablation was carried out in a He stream of 0.63 l min1, which wasmixed immediately after the ablation cell with 0.012 l min1 N2 and0.85 l min1 Ar prior to introduction into the Ar plasma of the SFICPMS. All gaseswere virtually atN99.999%purity andnohomogeniserwas used during gas mixing to prevent signal smoothing. Signals weretuned for maximum sensitivity for Pb and U whilst keeping oxide pro-duction, monitored as 254UO/238U, below 0.6%. The sensitivity achievedwas about 9300 cps/g g1 for 238U with 33-m spot size, at 5.5 Hz andlaser energy of approximately 3 Jcm2. The two-volume ablation cell(Laurin Technic, Australia) of theM-50 enables detection and sequentialsampling of heterogeneous grains (e.g., growth zones) during time-resolved data acquisition, due to its quick response time of b1 s (timeuntil maximum signal strength was achieved) and wash-out (b99.9%of previous signal) time of about 2 s. With a depth penetration of~0.7 m s1 and integration time of 0.46 s (4 mass scans = 0.46 s=1integration), any signicant variation of 207Pb/206Pb and 238U/206Pb inthe m scale is detectable. Raw data were corrected ofine for back-ground signal, common Pb, laser-induced elemental fractionation,instrumental mass discrimination and time-dependent elemental frac-tionation of Pb/U, using an in-houseMS Excel spreadsheet programme(Gerdes and Zeh, 2006, 2009). The common-Pb correction used theinterference- and background-corrected 204Pb signal and a model Pbcomposition (Stacey and Kramers, 1975). The 204Pb content for eachratio was estimated by subtracting the averagemass-204 signal, obtain-ed during the 21-s baseline acquisition,whichmostly results from 204Hgin the carrier gas (about 220 cps), from the mass-204 signal of the re-spective ratio. For the sample material, the calculated contents of com-mon 206Pbwere generally b0.3% of the total 206Pb, but in very rare casesup to 3.8%. Laser-induced elemental fractionation and instrumentalmass discrimination were corrected by normalisation to the referencezircon GJ-1 (Jackson et al., 2004). Prior to this normalisation, the inter-elemental fractionation (206Pb*/238U) during the 19-s ablation of

the sample material was corrected for each individual analysis. The

correction was done by applying a linear regression through allmeasured, common-Pb-corrected ratios, excluding the outliers (2standard deviation; 2), and using the intercept with the y-axis asthe initial ratio. The total offset of the measured drift-corrected206Pb*/238U ratio from the true IDTIMS value (0.0983 0.0004; iso-tope dilutionthermal ionisationmass spectrometry, IDTIMS, obtainedat the Goethe-Universitt Frankfurt) of the reference GJ-1 grain wasabout 15% (during the analytical session), with approximately 3% driftover the day.

Reported uncertainties (2) of the 206Pb/238U ratiowere propagatedby quadratic addition of the external reproducibility (2) that wasobtained from the reference zircon GJ-1 (see Appendix 1) and thewithin-run precision of each analysis (2 SE; standard error). In thecase of 207Pb/206Pb, we used a 207Pb-signal-dependent uncertaintypropagation (see Gerdes and Zeh, 2009). The 207Pb/235U ratio is derivedfrom thenormalised and error-propagated 207Pb/206Pb* and 206Pb*/238Uratios, assuming a natural abundance ratio 238U/235U of 137.88, and

the uncertainty derived by quadratic addition of the propagateduncertainties of both ratios. The accuracy of the method was veriedby analyses of reference zircon Pleovice (338.2 1.7 Ma) and OG1(3469.8 8.9 Ma), and the reference monazite Moacir (506.3 2.5 Ma) see Appendix 1. These ages are, within error, identical tothe quoted TIMS values of 337.13 0.37 Ma (Slma et al., 2008),3465.4 0.6 Ma (Stern et al., 2009), and 504.3 0.2 Ma (Gasquetet al., 2010), respectively. The data were plotted using the softwareISOPLOT (Ludwig, 2001).

4. Results

4.1. Petrography

Zircon and xenotime, illustrated in Figs. 4 and 5, are frequentlyobserved in the tourmaline-rich pocket sampled from a sulphidetourmalinequartz vein at Passagem. Indeed, both minerals, which

normB. Zocallmin

419A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427Fig. 4. BSE images of tourmaline pocket of the Passagem lode. The imageswere taken underin zircon (Zrn) and xenotime (Xtm). A. Zircon between tourmaline (Tur) and calcite (Cal).Altered, sponge-like zircon grain with pores lled with graphite (Gr). Note that graphite lmatrix. F. Euhedral xenotime grains with growth zoning contain inclusions of a Th-silicateal (left column) and high-gain conditions (right column); the latter shows growth zoningircon grain (indicated in A) with an inherited core and an irregular new overgrowth. CD.y replaces tourmaline. E. Grains of zircon and xenotime (rectangle) in tourmalinecalciteeral (bright spots).

420 A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427occur well distributed within the rock, had already been identied byHussak (1898). The tourmaline-rich pocket is made up essentially oftourmaline and interstitial calcite. The calcite also occurs as veinletscross-cutting coarse-grained tourmaline (sample L120a).

Zircon grains are mostly found in the interstices of coarse-grainedtourmaline (Fig. 4A). They have elongations between 10 and 110 m.Many zircon grains are rounded and contain primary domains withrelic oscillatory zoning in BSE images (Figs. 4B, 5A). A few primary grainsshow irregular overgrowths (Fig. 4B). Other grains are surrounded and/ortransected by porous domains, resulting in sponge-like aggregates(Fig. 4D). Some of the porous domains seem to have coalesced to formfractures that truncate the oscillatory zoning of the host zircon (Fig. 5A).The pores are commonly lled with graphite (Fig. 4C), which makessome grains appear black under the transmitted-light microscope. Somesulphide (pyrite) is locally intergrown with graphite (Fig. 5F).

Xenotime is generally euhedral and most grains occur in intersticeslled with calcite (Fig. 4E). Rarely, xenotime is in direct contact with

Fig. 5.A. High-gain BSE image of zircon in tourmaline-rich pocket of the Passagem lode. The gromode) of euhedral xenotime with UTh-silicate inclusion (arrowed), in contact with altered zigrains of tourmaline. CF. X-raymappingof B for ZrL (C), YL (D), CK (E) andSK (F). Note this spatially associated.graphite-bearing zircon (Fig. 5B). In contrast to zircon, xenotime re-mains impervious to graphite (neither inclusions in, nor encrustationson, xenotime) (Fig. 5B). Some xenotime grains show angular growthzoning and tiny inclusions (b3 m across) of a Th-silicate mineral(either thorite or cofnite) (Fig. 4F). A few larger crystals of xenotimeenclose tourmaline grains (Fig. 5B).

No grains of detrital quartz were found in sample L120a despite thepresence of quartzite of theMoeda Formation as the immediate footwallof the lode at level 120 (Fundo orebody). Some relics of detritalmaterial might be rounded quartz and K-aluminosilicate inclusions intourmaline from sample L120b (Fig. 6A, B), also containing zirconwith relic oscillatory zoning (Fig. 6C).

4.2. Whole-rock chemical analyses

The contents of Al2O3 and Na2O are between 19.7 and 26.8 wt.%, and~1.0 wt.%, respectively, reecting the large amounts of tourmaline, the

wth zoning of zircon is truncated by graphite-lled pores and cracks. B. BSE image (normalrcon and graphite (black) in tourmalinecalcite matrix. The xenotime crystal encloses twoat graphite is concentrated only in the vicinity of the altered zircon, some sulphide (pyrite)

main aluminosilicate of the lode samples (samples 120a, 120b and120c,Table 1). The SiO2 contents (34.864.1 wt.%) inversely vary with Al2O3;the highest SiO2 value being the sample with quartz as the preponder-ant mineral. The carbonate component is expressed in the contents ofCaO (1.69.3 wt.%), loss on ignition (LOI, 3.59.4 wt.%) and total C(0.42.0 wt.%). The K2O amounts are small (0.020.08 wt.%), withNa2O/K2O ratios between 13 and 68.

The relative abundance of zircon over xenotime is in good agree-ment with the high Zr content of 296 ppm, compared to 20 ppm Y, ob-tained from the whole rock of sample L120a (Table 1). The tourmalinepockets have ratios of Zr/Hf = 3641 and Y/Ho = 2535, which arewithin the charge-and-radius-controlled (CHARAC) eld of Bau

reference shale PAASthat is, Post-Archaean Australian Shale (Fig. 7):a light-REE pattern sharply falling from La to Sm, and a relatively atpattern from Eu to Lu. Sample L120c shows an abrupt rising from Tm

tourmaline pockets that occur within the Passagem lode. The zircongrains have Th/U ratios between 0.39 and 3.0, which are typical of mag-

Fig. 6. BSE images of sample L120b, illustrating inclusions of rounded quartz (Qz) in tourmaline (A), and K-aluminosilicatematerial (K-Al.Si) within tourmaline (B) in the vicinity of zirconwith relic oscillatory zoning (C, high-gain image).

421A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427(1996). The CHARAC eld indicates that the ZrHf and YHo pairs didnot fractionate, having retained their chondritic ratios. Chondrite-normalised rare-earth-element (REE) patterns are similar to the

Table 1Whole-rock chemical analyses of tourmaline pockets from the Passagem lode.

L120a L120b L120c L120a L120b L120c

SiO2 (wt.%) 34.84 48.36 64.10 Pb 0.5 1.1 0.7Al2O3 26.82 24.08 19.69 Rb 0.9 b0.1 2.1Fe2O3 5.40 10.60 3.95 Sb 0.9 16 1.1MgO 6.23 5.68 4.29 Se b0.5 0.8 b0.5CaO 9.30 1.55 2.76 Sc 16 15 12Na2O 1.34 1.35 1.00 Sn 15 11 12K2O 0.03 0.02 0.08 Sr 193 192 117TiO2 0.52 0.59 0.37 Ta b0.1 0.4 b0.1

P2O5 b0.01 0.11 0.09 Th 9.4 10 1.1MnO 0.11 0.02 0.04 Tl b0.1 b0.1 b0.1Cr2O3 0.044 0.025 0.048 U 2.2 2.1 1.1LOI 9.4 7.5 3.5 V 155 153 114Sum 94.03 99.89 99.92 W 0.7 4.8 1.5Total C 1.97 0.43 0.51 Zn b1 2 b1Total S 0.13 1.53 0.06 Zr 296 98 157Ag (ppm) b0.1 0.1 0.1 Y 20.3 14.1 4.6As 1523 N10,000 1144Au (ppb) 24 1917 3 La 16.3 20.9 2.8Ba (ppm) 3 2 9 Ce 27.8 38.3 4.2Bi b0.1 1.5 b0.1 Pr 3.17 4.09 0.48Be b1 1 b1 Nd 11.6 15.0 2.0Cd b0.1 b0.1 b0.1 Sm 2.02 2.55 0.38Co 1.7 46 1.5 Eu 0.68 0.76 0.20Cs b0.1 b0.1 b0.1 Gd 2.27 2.78 0.63Cu 5.5 16 114 Tb 0.40 0.43 0.11Ga 32 26 21 Dy 2.73 2.60 0.72Hf 8.1 2.4 4.2 Ho 0.69 0.56 0.13Hg 0.04 0.03 b0.01 Er 2.22 1.33 0.45Mo 1.6 1.0 2.7 Tm 0.31 0.21 0.06Nb b0.1 2.1 0.3 Yb 2.08 1.34 0.62Ni 8.0 114 6.1 Lu 0.32 0.20 0.11matic zircon (Hoskin and Schaltegger, 2003). Xenotimehas similar Th/Uratios (between 0.11 and 1.55), but its U contents are higher(434013026 ppm) than those of zircon (1602944 ppm). Seven spotanalyses on zircon yielded concordant UPb ages of 3.18, 2.86, 2.80and 2.67 Ga (level of concordance between 98102%), whereas allother analyses are discordant (Fig. 8A, B). Most of the discordantto Lu. No obvious Eu anomaly is observed. Fig. 7 also shows the Batatalphyllite from guas Claras, an iron-ore deposit near Belo Horizonte,about 70 km NW of Passagem (Spier et al., 2007). The overall patternof the reference Batatal phyllite resembles those of the tourmalinepockets. It is worth mentioning that one sample (L120b), which is richin arsenopyrite, has 1.9 ppm Au and 1.5 ppm Bi.

4.3. UraniumPb ages

During this study 52 UPb spot analyses were carried out on 39 zir-con grains, and 14 spot analyses on 8 xenotime grains (Table 2), all donein situ on a thin section of sample L120a, which is representative of theFig. 7. Chondrite-normalised REEY patterns for tourmaline pockets from the Passagemlode. Reference shale, PAAS (Post-Archaean Australian Shale), is shown for comparison.Sample L120b (grey) is rich in arsenopyrite and has 2 ppm Au. Samples L120a andL120c are barren and poor in arsenopyrite. Values for C1 and PAAS are respectively fromMcDonough and Sun (1995), and McLennan (1989); those for the Batatal phyllite arefrom Spier et al. (2007).

Table 2Results of in-situ analyses for UThPb by LASFICPMS of zircon and xenotime from a tourmaline pocket of the Passagem lode (sample L120a).

Grainnumber

Analysisnumber

207Pba

(cps)Ub

(ppm)Pbb

(ppm)Thb

U

206Pbcc

(%)

206Pbd238U

2(%)

207Pbd235U

2(%)

207Pbd206Pb

2(%)

rhoe 206Pb238U

2(Ma)

207Pb235U

2(Ma)

207Pb206Pb

2(Ma)

Concf

(%)

Zirconzr1 a08 31,627 1624 300 0.90 0.26 0.1647 2.1 3.409 2.6 0.1501 1.6 0.80 983 19 1506 21 2347 27 42zr2 a09 4800 1125 120 1.55 b.d. 0.1008 2.5 1.311 3.5 0.0943 2.5 0.70 619 15 850 21 1515 48 41zr3 a10 35,024 894 230 0.69 0.01 0.2136 2.3 4.783 2.7 0.1624 1.5 0.84 1248 26 1782 23 2481 25 50zr4 a12 22,991 384 200 0.76 0.61 0.4634 2.0 11.35 2.6 0.1777 1.6 0.80 2455 42 2553 24 2631 26 93zr5 a14 20,107 1104 350 1.15 0.42 0.2938 2.2 6.630 2.8 0.1637 1.7 0.79 1660 33 2063 25 2494 29 67zr6c a15 54,673 1013 400 0.88 0.01 0.3464 1.6 8.216 1.8 0.1720 0.8 0.88 1917 26 2255 16 2578 14 74zr7 a16 35,986 555 290 0.82 0.16 0.4346 1.5 10.45 1.8 0.1744 1.0 0.83 2326 30 2476 17 2600 17 89zr8 a17 37,811 809 220 1.00 0.20 0.2191 1.8 5.240 2.2 0.1734 1.3 0.81 1277 20 1859 19 2591 21 49zr9 a18 77,717 2850 760 0.88 1.90 0.2290 2.1 4.652 2.7 0.1474 1.7 0.77 1329 25 1759 23 2316 29 57zr10near xno101 a19 66,082 2104 620 1.11 1.29 0.2348 3.7 4.697 3.9 0.1451 1.1 0.96 1359 46 1767 33 2289 18 59zr11 a20 9878 926 85 1.06 1.16 0.0829 2.9 1.421 12.3 0.1244 11.9 0.23 513 14 898 76 2020 211 25zr12 a23 20,638 214 140 0.72 b.d. 0.5319 1.8 15.33 2.5 0.2090 1.8 0.72 2749 41 2836 24 2898 28 95zr12 a24 35,911 160 100 0.54 0.16 0.5529 1.8 15.53 2.1 0.2038 1.1 0.87 2837 42 2849 20 2857 17 99zr12 a25 60,575 515 200 0.50 0.15 0.3281 1.4 8.578 1.6 0.1896 0.8 0.88 1829 23 2294 15 2739 13 67zr13 a26 39,935 541 280 0.60 0.09 0.4406 1.6 10.92 1.8 0.1797 1.0 0.85 2353 31 2516 17 2650 16 89zr14 a41 47,878 2051 480 1.50 0.82 0.1824 1.8 4.126 2.2 0.1640 1.3 0.81 1080 18 1659 19 2498 22 43zr14 a42 42,173 1394 340 1.26 1.11 0.1784 1.9 3.766 2.2 0.1531 1.2 0.84 1058 18 1585 18 2381 21 44zr15 a43 55,378 725 310 0.39 0.07 0.3780 1.8 10.26 2.2 0.1969 1.2 0.83 2067 32 2459 20 2801 20 74zr16 a47 66,779 958 250 1.11 0.26 0.2244 1.9 5.559 2.1 0.1797 0.9 0.91 1305 23 1910 18 2650 15 49zr17 a48 45,095 1885 330 1.38 0.29 0.1500 1.9 3.338 2.1 0.1614 1.0 0.89 901 16 1490 17 2471 17 36zr17 a49 36,370 1579 270 1.18 2.70 0.1451 2.4 3.053 3.6 0.1527 2.7 0.65 873 19 1421 28 2376 47 37zr18 (graphite rim) a50 18,593 2944 190 0.70 3.82 0.0584 2.4 0.5919 6.8 0.0735 6.3 0.35 366 8 472 26 1028 128 36zr19 a51 45,376 1323 390 1.35 0.24 0.2716 3.0 6.676 3.3 0.1783 1.4 0.91 1549 42 2069 30 2637 23 59zr20 a52 30,013 393 230 1.07 0.28 0.4633 1.9 11.27 2.3 0.1764 1.3 0.82 2454 38 2545 21 2619 22 94zr20 a53 21,533 283 150 0.87 0.34 0.4330 3.2 10.46 3.7 0.1752 1.9 0.86 2319 62 2476 35 2608 31 89zr21 a54 34,634 2614 600 1.83 1.03 0.1962 1.7 3.999 3.0 0.1478 2.5 0.56 1155 18 1634 25 2321 43 50zr22 a55 23,975 260 170 1.95 0.15 0.4226 2.3 12.04 2.7 0.2066 1.3 0.87 2272 44 2607 25 2879 21 79zr23 a56 57,971 1106 440 0.53 0.70 0.3529 1.7 7.531 1.9 0.1548 0.9 0.88 1948 29 2177 17 2399 15 81zr24core a57 82,460 622 470 0.56 0.05 0.6366 1.4 21.88 1.5 0.2492 0.7 0.90 3176 35 3178 15 3180 11 100zr24core a58 77,340 812 340 0.48 0.27 0.3371 2.2 11.07 2.3 0.2383 0.8 0.94 1873 35 2529 22 3108 13 60zr24rim a59 22,461 1800 370 1.30 0.25 0.1992 3.1 4.005 4.4 0.1458 3.1 0.71 1171 33 1635 36 2297 53 51zr25 a61 30,192 746 250 1.12 0.04 0.2752 5.6 6.553 5.9 0.1727 1.6 0.96 1567 79 2053 53 2584 26 61zr25 a62 22,631 245 170 1.32 0.21 0.5143 1.8 13.04 2.5 0.1840 1.8 0.71 2675 40 2683 24 2689 29 99zr26 a63 30,888 1928 290 0.82 0.51 0.1392 2.3 2.763 2.7 0.1439 1.5 0.85 840 18 1346 20 2275 25 37zr27 a64 26,227 341 210 1.36 0.09 0.4947 1.6 12.43 2.1 0.1822 1.3 0.79 2591 35 2637 20 2673 21 97zr27 a65 30,593 350 230 1.14 0.58 0.5164 2.0 13.13 2.2 0.1844 1.0 0.89 2684 44 2689 21 2693 17 100zr27 a66 23,410 262 180 1.22 0.36 0.5138 1.9 12.89 2.3 0.1819 1.2 0.84 2673 42 2671 22 2670 20 100zr28 a68 57,581 858 590 1.47 0.13 0.5366 3.3 14.56 4.0 0.1968 2.2 0.83 2769 76 2787 39 2800 36 99zr29 a81 38,395 2883 290 1.07 1.22 0.0857 1.9 1.660 2.7 0.1406 1.8 0.73 530 10 994 17 2234 31 24zr30 a82 39,480 2045 180 0.53 0.26 0.0759 5.0 1.676 5.2 0.1601 1.4 0.96 472 23 999 34 2457 24 19zr31 a84 53,443 1131 440 0.66 0.34 0.3292 1.9 7.133 2.2 0.1571 1.1 0.87 1835 30 2128 20 2425 19 76zr32 a85 34,596 413 240 0.59 b.d. 0.5129 1.8 12.81 2.0 0.1812 0.9 0.89 2669 38 2666 19 2664 15 100zr33core a86 26,594 481 260 2.31 b.d. 0.3481 1.8 8.179 2.2 0.1704 1.3 0.82 1925 30 2251 20 2562 22 75zr33rim a87 31,968 740 400 3.03 0.23 0.3479 2.0 7.834 2.4 0.1633 1.4 0.81 1924 33 2212 22 2491 24 77zr34 a88 8464 635 93 0.86 b.d. 0.1356 2.7 2.528 3.2 0.1352 1.6 0.86 820 21 1280 23 2167 28 38zr35core a89 56,442 816 450 0.72 b.d. 0.4628 2.1 10.64 2.2 0.1668 0.7 0.95 2452 43 2493 21 2526 12 97zr35rim a90 42,950 850 440 1.07 b.d. 0.4654 3.0 10.73 3.3 0.1672 1.3 0.91 2463 62 2500 31 2529 23 97zr36 a91 33,860 1238 570 1.50 2.14 0.4087 3.0 14.39 4.0 0.2554 2.7 0.75 2209 57 2776 39 3218 42 69zr37 a92 42,451 2477 670 0.93 b.d. 0.2556 3.5 5.272 4.6 0.1496 3.1 0.75 1467 46 1864 40 2341 53 63zr38 a93 37,950 881 290 1.29 0.24 0.2690 3.3 5.911 3.4 0.1593 1.0 0.95 1536 45 1963 30 2449 17 63zr38 a94 37,956 2287 260 0.87 b.d. 0.0939 6.1 2.152 6.1 0.1662 0.8 0.99 579 34 1166 43 2520 14 23zr39 a95 21,500 180 120 1.13 0.31 0.5310 1.7 17.20 2.2 0.2350 1.4 0.76 2746 37 2946 21 3086 23 89

Xenotimexno1 a96 28,339 7510 580 0.33 0.12 0.0784 1.3 0.6188 1.5 0.0573 0.7 0.88 486 6 489 6 502 16 97xno2 a97 19,585 5045 450 1.05 0.41 0.0766 1.4 0.6050 1.7 0.0573 1.0 0.81 476 6 480 7 503 22 94xno3 a98 19,994 4450 440 1.55 0.41 0.0811 1.5 0.6402 2.5 0.0573 2.0 0.60 503 7 502 10 502 44 100xno4 a99 39,581 13,026 1000 0.28 0.11 0.0791 1.5 0.6232 1.9 0.0572 1.2 0.77 491 7 492 7 498 27 99xno4 a100 21,196 5749 440 0.28 0.03 0.0793 1.4 0.6281 1.8 0.0575 1.2 0.77 492 7 495 7 510 26 96xno5 a101 14,661 4341 330 0.28 0.11 0.0797 1.9 0.6244 3.4 0.0569 2.9 0.55 494 9 493 14 486 64 102xno6 a102 23,355 5688 430 0.17 0.12 0.0796 1.5 0.6271 2.0 0.0571 1.4 0.73 494 7 494 8 497 30 99xno6 a103 14,849 3713 300 0.45 0.60 0.0797 1.5 0.6277 4.1 0.0571 3.9 0.36 495 7 495 16 495 85 100xno6 a104 38,310 7165 570 0.27 0.03 0.0809 1.8 0.6392 2.0 0.0573 0.9 0.89 501 9 502 8 504 20 100xno6 a105 25,954 6978 540 0.22 0.06 0.0806 1.5 0.6345 1.8 0.0571 1.1 0.80 500 7 499 7 495 24 101xno7 a106 23,225 6073 460 0.11 b.d. 0.0807 1.3 0.6365 1.9 0.0572 1.4 0.68 500 6 500 8 501 31 100xno7 a107 25,519 6730 540 0.34 0.29 0.0807 1.4 0.6363 1.9 0.0572 1.2 0.77 500 7 500 7 500 26 100xno8 a111 15,461 4560 400 0.96 0.27 0.0782 1.5 0.6181 2.1 0.0573 1.5 0.70 486 7 489 8 503 33 97xno8 a112 34,607 8651 770 0.73 1.58 0.0807 1.5 0.6419 3.1 0.0577 2.7 0.48 500 7 503 12 518 59 97

422 A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427

analyses plot along and below a reference line, having an upper-inter-cept age of 2670 Ma, and a lower-intercept age of 500 Ma (Fig. 8B, lineIVB). This line respectively joins the volcanism during the depositionof the Itabira iron formation and the Brasiliano orogeny. Only a fewanal-yses plot above this line. Two of them (grain zrc35), at 97%-concordancelevel, gave 207Pb/206Pb ages of about 2530 Ma (Fig. 8B, Table 2), whichrepresent some resetting, possibly during the Minas orogeny. Twelveout of the 14 analyses of xenotime resulted in a Concordia age of496.3 2.0 Ma, which is identical (within analytical uncertainties)to an upper-intercept-207Pb/206Pb age of 500.7 9.3 Ma including

zircon-age resetting at ~500Ma is very likely, but Pb loss from zircon

Notes to Table 2:a Within-run background-corrected mean 207Pb signal in cps (counts per second).b Uranium and Pb content and Th/U ratio were calculated relative to GJ-1 reference zircon.c Percentage of common Pb on 206Pb. b.d. = below detection limitd Corrected for background,within-run Pb/U fractionation (in case of 206Pb/238U) and commonP

GJ-1 (IDTIMS value/measured value); 207Pb/235U calculated using 207Pb/206Pb/(238U/206Pb*1/137e Rho is the 206Pb/238U/207Pb/235U-error-correlation coefcient.f Degree of concordance = 206Pb/238U age/207Pb/206Pb age 100

Fig. 8. A. Probabilitydensity diagram showing concordant ages for zircon from a tourma-line pocket of the Passagem lode (sample L120a). BC. Concordia diagramswith results ofUPb dating for zircon (B) and xenotime (C) from sample L120a. In B, Discordia-likelines are reference lines for zircon grains that formed during volcanism coevalwith the de-position of the Itabira iron formation, but affected by Pb loss during the Minas andBrasiliano events.

423A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427could also have happened prior to the xenotime formation. A Minas

busing themodel Pb composition of Stacey andKramers (1975), subsequently normalised to.88)all analyses (Fig. 8C).

5. Discussion

5.1. Timing of zircon and xenotime formation

Results of UPb-age dating indicate that all zircon grains inthe investigated tourmaline-rich pocket are inherited from ametasedimentary precursor rock, most likely quartzite of the MoedaFormation and/or phyllite of the Batatal Formation (see below).This conclusion is supported by the fact: (1) that theMoeda quartziteat the footwall of the sampling site of this study (Fundo orebody,level 120) contains abundant detrital zircon grains, the ages ofwhich are between 3097 44 and 2606 47 Ma (Machado et al.,1996); (2) that detrital grains of zircon with ages of 3.2, 2.85, and2.67 Ga, as found in our sample, have been reported from Moedametasediments elsewhere in the Quadriltero Ferrfero (Hartmannet al., 2006; Koglin et al., 2014).

The zircon grains show evidence of dissolution. The zircon dissolu-tion is recorded by sponge-like zircon crystals, having pores lled withgraphite (Figs. 4C, 5BF), as well as by uid-widened fracturestransecting zircon crystals that partly preserve oscillatory zoning(Fig. 5A). Limited new zircon re-precipitation is revealed by a few irreg-ular zircon overgrowths (Fig. 4B), but they were too thin to be dated bythe LAICPMS technique. The observed overgrowth microstructuresare similar to those described for zircon in greenschist-facies Scottishslate (Hay and Dempster, 2009), for zircon in staurolite schist fromthe Shackleton Range, Antarctica (Zeh et al., 2010), and for zirconfrom tourmaline-bearing metavolcanic rock within itabirite of theItabira iron formation (Cabral et al., 2012b).

In order to evaluate the absolute timing of detrital zircon leachingand xenotime formation, one should consider that xenotime in sampleL120a is generally euhedral, and that some grains occur in direct contactwith sponge-like zircon (Fig. 5B). This could indicate that xenotimewasstable and formed during zircon alteration, perhaps by taking up the Yreleased from zircon, in a way similar to that suggested by Hay andDempster (2009) for the formation of epitaxial xenotime overgrowthsaround porous zircon in Scottish slate rocks. However, no zirconwith a xenotime overgrowth has been observed in the presentstudy. The UPb results reveal that all xenotime grains formed at496.3 2.0 Ma, whereas many zircon grains underwent signicantPb loss at ~500 Ma (Fig. 8B, C). This link between xenotime age and

424 A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427(Transamazonian) overprint (Pb loss) might indeed have affectedsome detrital grains of zircon in sample L120a (i.e., all grains thatplot above the reference line IVB in Fig. 8B, explaining the 2530-Ma age). The more signicant Pb-loss event, however, took place at~500 Ma.

The Concordia age obtained for the xenotime grains is slightly olderthan the ArAr age of 485 4 Ma for vein muscovite at Passagem(Chauvet et al., 2001), which is hitherto the only well documentedage for Passagem. The 485-Ma age should represent cooling, followingthe formation of the xenotime at 496 Ma. Schrank and Machado(1996) reported upper-intercept ages between 1.8 and 2.1 Ga for mon-azite concentrates from Passagem, ages that are broadly compatiblewith the Minas orogeny. Because it is not possible to retrieve the dataobtained by Schrank and Machado (not given in their extendedabstract), their monazite ages are not further considered here. In sum-mary, thin-section observations and geochronological data indicatethat the auriferous lode of Passagem formed during a late stage of theBrasiliano event at 496.3 2.0 Ma. This interpretation is in agreementwith the cross-cutting relationships in which the veins truncate theprominent tectonic foliation generated during the Brasiliano orogeny(Chauvet et al., 1994).

5.2. Protolith nature and uidrock interaction

If the zircon grains were inherited from the Moeda quartzitic rocks(see above), where is the quartz detritus?

Two explanations to account for the lack of detrital quartz canbe proposed or, possibly, a combination of both. One explanation isthat detrital quartz from quartzitic rocks of the Moeda Formationwas completely dissolved and/or replaced in the hydrothermal vein,leaving zircon as residual grains. The other explanation is that theprotolithwas a quartz-poor pelitic rock such as phyllite from the BatatalFormation.

Some rounded quartz and K-aluminosilicate occur as inclusionsin tourmaline of sample L120b (Fig. 6A, B). The rounded quartzgrains could represent relics that survived the coupled (quartzite)dissolution(tourmaline) precipitation process. On the other hand, therelic K-aluminosilicate could indicate that the precursor rockwas a peliticrock. Therefore, these pieces of evidence cannot distinguish between aquartzite protolith and a shale-like protolith.

The graphite in the pores of leached sponge-like grains of zircon(Figs. 4C, 5BF) gives support for uid interactionwith carbonaceousphyllite of the Batatal Formation, or even for uid sourced directlyfrom the Batatal Formation (cf. Fleischer and Routhier, 1973). In ad-dition, the similarity between the C1-normalised REE patterns forthe tourmaline pockets and those of the reference shale PAAS, andthe Batatal phyllite (Fig. 7), suggests that uids interacted withshale-like rocks of the Batatal Formation. The crustal signature ofthe shale is also reected in the Co/Ni ratios of tourmaline andcoexisting arsenopyrite (Cabral and Koglin, 2012), as well as in theratios Zr/Hf (3641) and Y/Ho (2535) for the tourmaline pockets(this study, samples L120a, b and c), which fall in the CHARAC eldof Bau (1996). This eld indicates that the ZrHf and YHo pairsretained their chondritic ratios, that is, they were not decoupledduring the hydrothermal processes that led to the formation of thePassagem lode.

It is pertinent to point out that a pelitic protolith for thetourmaline-rich pockets in the Passagem lode does not excludequartz replacement from a quartzite protolith. In fact, replacementof quartz by tourmaline to form tourmalinite layers in host quartzitein Minas Gerais has been recorded at Crrego Bom Sucesso, about200 km north of Passagem (Fig. 1; see Cabral et al., 2011). Althoughthe protolith nature of the tourmaline-rich pockets of the sampleset cannot precisely be determined, intense tourmalinisation led topartial dissolution of detrital zircon. In this regard, the recognition

of abundant detrital zircon in hydrothermal quartz veins elsewherecan reveal a new facet of uidrock interaction (i.e., uid capableof obliterating otherwise unrecognised detrital material). In thiscontext, it is important to note that detrital zircon can effectively bedissolved and transported in ZrNaSiH2O-bearing hydrothermal so-lutions, where Zr forms NaZr-silicate-hydroxide complexes (Wilkeet al., 2012). That this can happen is well documented in Zeh et al.(2010). However, Zr solubility and, therefore, zircon dissolution areconsiderably suppressed if Al is added to the system ZrNaSiH2O, asshown by the experiments of Wilke et al. (2012). By taking this into ac-count, the preservation of abundant detrital zircon in sample L120a isperhaps directly related to the tourmalinisation process, which requiresuids containing relatively high concentrations of soluble Al, in additionto Na, Si and H2O.

5.3. Gold rush to Minas Gerais

The importance of the gold rush to the highlands of the region thatbecame known as Minas Gerais is symbolised by Ouro Preto, aUNESCO world-heritage site. Two major components of the alluvialgold that triggered the gold rush were derived from the carvoeira-style mineralisation, the type locality of which is Passagem, and thejacutinga-style mineralisation, where the so-called ouro preto (blackgold) occurs (Galbiatti et al., 2009; Hussak, 1904; von Eschwege,1832). Jacutinga is a local denomination for AuPdPt-bearing veins ofabundant specular hematite, hosted in itabiritic rocks (Cabral et al.,2009; Hussak, 1904). The only robust UPb age for the jacutinga-stylemineralisation, based on monazite recovered at Itabira (Fig. 1; Cabralet al., in press), is 496 2 Ma, which is the same age as determinedhere for xenotime at Passagem. Therefore, the regional mineralisingevent responsible for the gold-rush gold took place in the Late Cambri-an. Our xenotime age is indistinguishable within error from the age ob-tained by direct dating of gold, 515 55 Ma, from a quartzhematite-lode deposit in Diamantina (Cabral et al., 2013a), in the northernmostpart of the approximately 240-km-longbelt betweenOuro Preto andDi-amantina (Fig. 1). The common age for quartzsulphide lodes(Passagem) and quartzhematite lodes (Itabira) corroborates the ideaof a common origin for uids that interacted with different rocks at dif-ferent redox conditions (Boiron et al., 1999; Cabral et al., 2013b).

6. Conclusion

For the rst time, the age of the Passagem gold-lode deposit, aworld-class and historically important deposit of controversial origin,is precisely determined at 496.3 2.0 Ma by UPb dating of veinxenotime. In contrast, all zircon grains within the hydrothermal veinare of detrital origin (N2.65 Ga), having only been altered partially at~500 Ma. The abundance of detrital zircon, with ages similar to thosefrom the Moeda metasediments, and the lack of detrital material indi-cate hitherto unrecognised aspects of the Passagem system: (1) hydro-thermal uids were capable of obliterating detrital material (quartziteof the Moeda Formation and/or phyllite of the Batatal Formation), leav-ing abundant detrital zircon as residuum; (2) the zircon grains provideonly limited information about the timing of the hydrothermal over-print; (3) graphite precipitation during hydrothermal alteration of zir-con attests to uidrock interaction with a carbonaceous rock, likelyphyllite of the Batatal Formation; (4) xenotime is useful to place a pre-cise age constraint on the timing of the hydrothermal overprint.

Acknowledgements

ARC gratefully acknowledges the Deutsche Forschungsgemeinschaft(DFG, project CA 737/1-1) for nancing his stay at the TechnischeUniversitt Clausthal, Germany. Reviews by F. Corfu and an anonymousreferee considerably improved the manuscript and are greatly appreci-ated. The manuscript also beneted from further reviewing by F. Corfu

and meticulous comments by A. J. R. White.

(cps) (ppm) (ppm) (%) (%)bd

(%) (%) (Ma) (Ma) (Ma) (%)U 238U 235U 206Pb 238U 235U 206Pb

k o eb r eo .a , u o s f n -

m 2 l /b r l

U P

R s n o r y

425A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427Babinski, M., Chemale Jr., F., Van Schmus,W.R., 1995. The Pb/Pb age of theMinas Supergroupcarbonate rocks, Quadriltero Ferrfero, Brazil. Precambrian Research 72, 235245.

Bau, M., 1996. Controls on the fractionation of isovalent trace elements in magmatic andaqueous systems: evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect. Contribu-

of a 3,930-Ma-old granulite from Mount Sones, Enderby Land, Antarctica. Contribu-tions to Mineralogy and Petrology 94, 427437.

Boiron, M.-C., Moissette, A., Cathelineau, M., Banks, D., Monnin, C., Dubessy, J., 1999.Detailed determination of palaeouid chemistry: an integrated study of sulphatevolatile-rich brines and aquo-carbonic uids in quartz veins from Ouro Finoeferencestions to Mineralogy and Petrology 123, 323333.Black, L.P(Bra., Williamzil). Chemi, I.S., Cocal Geompstology 15, W., 14, 179986. Fou192.r zircon ages fr m one ock: the historf Degree of concordance = 206Pb/238 age/207 b/206Pb age 100e Rho is the 206Pb/238U/207P /235U-er or-corre ation coefcient.

alised to GJ-1 (IDTIMS value/measured value); 207Pb/ 35U calcu ated using 207Pb/206Pb/(238U 206Pb 1/137.88)

d Corrected for b ckground within-r n Pb/U fractionation (in case of 206Pb/238U) and comm n Pb using the model Pb compo ition o Stacey and Kramers (1975), subseque tly norc Percentage of c mmon Pb on 206Pb b.d. = below detection limitb Uranium and P content and Th/U atio wer calculated relative to GJ-1 reference zircon.a Within-run bac ground-c rrected mean 207Pb signal in cps (counts per s cond).2 (%) 1.66 5.53 5.79

2 (abs.) 0.00135 0.0353 0.00328 8 22 128

mean (n = 8) 0.08175 0.6380 0.05660 507 501 475

a125 6800 1682 2600 74.72 1.47 0.08121 1.4 0.6545 2.9 0.05845 2.5 0.49 503 7 511 12 547 56 92

a124 0.08167 0.6451 0.05729 506 505 503

a1236851 1680 2600 74.63 0.40

0.080941.4

0.63734.1

0.057113.9 0.33

5027

50117

49686 101a78

6885 1685 2600 75.73 0.360.08138

1.50.6484

4.80.05778

4.6 0.31 7 19 102 101a77 6789 1691 2600 74.19 0.54 0.08301 1.4 0.6556 5.0 0.05728 4.8 0.29 514 7 512 20 502 106 1026766 1832 2600 68.36 0.68 1.3 4.2 4.0 0.31 504 6 507 17 522 87 97a76 6925 1738 2600 73.30 0.58 0.08244 1.4 0.6240 5.2 0.05490 5.0 0.27 511 7 492 21 408 112 125

a38 6777 1736 2600 72.01 0.53 0.08152 1.4 0.6359 4.4 0.05657 4.2 0.31 505 7 500 18 475 93 106

a37 6899 1690 2700 76.07 0.86 0.08179 1.3 0.6028 5.6 0.05345 5.5 0.24 507 7 479 22 348 124 146

Reference monazite Moacir2 (%) 2.25 2.35 0.48

2 (abs.) 0.0158 0.68 0.0014 60 23 7

Mean (n = 7) 0.7040 29.08 0.2996 3436 3456 3468

a07 196,154 262 240 1.04 0.26 0.6854 1.6 28.01 1.7 0.2964 0.5 0.96 3365 43 3419 17 3451 7 97

a122 0.7077 29.29 0.3002 3450 3463 3471

a121180,498 992 970 1.31 0.14

0.69471.4

28.611.4

0.29870.4 0.97

340137

344014

34635 99a75

144,787 813 760 1.12 0.750.7079

1.329.29

1.40.3001

0.4 0.96 36 14 6 98a74 147,837 167 170 1.55 0.12 0.7079 1.4 29.13 1.6 0.2984 0.6 0.93 3451 39 3458 15 3462 9 100146,787 164 160 1.18 0.27 1.3 1.4 0.4 0.96 3451 35 3463 13 3471 6 99a36 95,645 110 97 0.72 0.19 0.6947 1.4 28.74 1.5 0.3000 0.5 0.94 3400 37 3445 15 3470 8 98

a35 104,671 121 110 0.67 0.23 0.6995 1.4 28.92 1.4 0.2998 0.4 0.96 3419 37 3451 14 3469 6 99

a06 106,789 122 110 0.65 0.01 0.7158 1.4 29.58 1.4 0.2997 0.3 0.97 3480 37 3473 14 3469 5 100

Reference zircon OG-12 (%) 0.84 1.04 0.52

2 (abs.) 0.00045 0.0041 0.0003 3 3 12

Mean (n = 8) 0.05385 0.3962 0.0534 338 339 344

a120 8802 3397 170 0.13 0.19 0.05350 1.4 0.3934 2.0 0.05333 1.4 0.73 336 5 337 6 343 31 98

a119 0.05378 0.3961 0.05342 338 339 347

a739056 3511 180 0.13 0.17

0.054001.5

0.39662.3

0.053271.8 0.65

3395

3397

34040 97a72

8480 711 36 0.12 0.200.05425

1.40.3989

2.10.05333

1.5 0.67 5 6 35 100a34 8888 722 37 0.13 0.13 0.05398 1.4 0.3993 2.2 0.05364 1.7 0.66 339 5 341 6 356 37 956802 522 27 0.13 0.21 1.6 4.3 4.0 0.37 341 5 341 13 343 91 99a33 8412 696 35 0.13 0.05 0.05372 1.5 0.3958 2.0 0.05343 1.3 0.76 337 5 339 6 347 29 97

a05 8991 742 38 0.13 0.07 0.05372 1.5 0.3943 2.1 0.05324 1.4 0.72 337 5 338 6 339 32 100

a04 8614 700 36 0.14 0.02 0.05385 1.5 0.3951 1.9 0.05321 1.2 0.76 338 5 338 6 338 28 100

Reference zircon Pleovice2 (%) 1.23 1.90 1.58

2 (abs.) 0.00121 0.0155 0.0009 7 9 34

Mean (n = 12) 0.09820 0.8139 0.0601 604 605 607

GJ3-12 7242 291 27 0.03 0.06 0.09881 1.6 0.8186 2.0 0.06009 1.3 0.78 607 9 607 9 607 27 100

GJ3-11 0.09743 0.8207 0.06109 599 608 642

GJ3-107318 283 26 0.03 0.06

0.097471.4

0.81001.9

0.060281.3 0.75

6008

6029

61427 93GJ3-9

7010 284 26 0.03 0.070.09873

1.40.8205

1.90.06027

1.3 0.75 8 9 28 98Reference zircon GJ-1GJ3-1 6839 283 26 0.03 0.10 0.09806 1.6 0.8010 2.1 0.05924 1.3 0.79 603 9 597 9 576 28 105GJ3-2 7143 286 26 0.03 0.36 0.09798 1.7 0.8023 4.3 0.05939 3.9 0.39 603 10 598 19 582 85 104GJ3-3 7377 305 28 0.03 0.10 0.09720 1.4 0.8071 1.9 0.06022 1.3 0.74 598 8 601 9 611 28 98GJ3-4 6648 273 25 0.03 0.07 0.09846 1.7 0.8138 2.2 0.05995 1.4 0.76 605 10 605 10 602 31 101GJ3-5 6548 266 24 0.03 0.06 0.09914 1.5 0.8255 2.2 0.06039 1.6 0.71 609 9 611 10 617 33 99GJ3-6 6534 263 24 0.03 0.08 0.09827 1.5 0.8139 2.1 0.06007 1.5 0.69 604 9 605 10 606 33 100GJ3-7 8185 267 25 0.03 1.51 0.09819 1.7 0.8123 2.9 0.05999 2.4 0.59 604 10 604 13 603 51 100GJ3-8 6794 276 25 0.03 0.07 0.09867 1.6 0.8208 2.1 0.06033 1.4 0.75 607 9 608 10 616 30 99

6845 283 26 0.03 0.09 1.5 2.0 1.4 0.73 607 8 608 9 613 29 99Appendix 1. Results of UPb dating of reference materials

Grain number 207Pba Ub Pbb Thb 206Pbcc 206Pbd 2 207P 2 207Pbd 2 rhoe 20Pb 2 207Pb 2 207Pb 2 conc.f

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427A.R. Cabral, A. Zeh / Lithos 212-215 (2015) 415427

Detrital zircon without detritus: a result of 496-Ma-old fluidrock interaction during the gold-lode formation of Passag...1. Introduction2. Geological setting and Passagem de Mariana3. Samples and methods3.1. Samples3.2. Methods3.2.1. Petrography and geochemistry3.2.2. In-situ analyses for UThPb isotopes

4. Results4.1. Petrography4.2. Whole-rock chemical analyses4.3. UraniumPb ages

5. Discussion5.1. Timing of zircon and xenotime formation5.2. Protolith nature and fluidrock interaction5.3. Gold rush to Minas Gerais

6. ConclusionAcknowledgementsAppendix1. Results of UPb dating of reference materialsReferences