Biondi et al 2012 - The Paleoproterozoic Aripuanã Zn-Pb-Ag (Au, Cu).pdf

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IntroductionARIPUANÃ is a polymetallic Zn-Pb-Ag (Cu, Au) massive sulfidedeposit, located in the Rondônia-Juruena Province, in thesouthern part of the Amazon craton (Fig. 1). Discovered in1998, it is a property owned by Votorantim Metais andKarmin Exploration Corporation that announced in 2011 thediscovery of resources totaling 11.6 Mt of mineralized rocksgrading 6.29% Zn, 2.25% Pb, 0.07% Cu, 65 g Ag/ton and 0.25g Au/ton. In 2003, Karmin Exploration Inc. announced 27.7Mt of indicated resources with a 6.44% Zn equivalent plus 5.8Mt of possible resources with a 6.52% Zn equivalent in thedeposit.

In this study, we present the results of a regional geologicmapping of the area around the Aripuanã deposit as well as adetailed mapping of the deposit itself. We also provide a de-tailed description of the lithologies and facies observed at the

scale of the deposit including mineralogical, textural, isotopic,and geochemical data. The present study extends the avail-able published data about this deposit and allows for inter-pretation of the geologic setting of the deposit and its parage-nesis. Aripuanã is an unusual volcanogenic submarine depositin the sense that it contains a high content of hydrothermalcarbonate rocks, which hosts Zn, Pb, and Ag (Cu, Au) massivesulfide mineralization, related to a sequence of felsic igneousrocks.

Previous StudiesNeder (2002) and Neder et al. (2002) dated dacites (U-Pb),

which are located in the southern part of the deposit, at 1.762± 6 Ma, and granite, located in the north, at 1.755 ± 6 Ma.They suggested that the Aripuanã deposit, then called Expe-dito, consists of various lenses, veins, and pipes with pyrite,pyrrhotite, sphalerite, galena, chalcopyrite, and arsenopyrite,locally replaced by magnetite, found in unmetamorphosedrocks. The mineralized bodies were discordant bodies of thevolcanic-hosted massive sulfide type.

The Paleoproterozoic Aripuanã Zn-Pb-Ag (Au, Cu) Volcanogenic Massive Sulfide Deposit, Mato Grosso, Brazil: Geology, Geochemistry of Alteration,

Carbon and Oxygen Isotope Modeling, and Implications for Genesis*

JOÃO CARLOS BIONDI,1,† ROBERTO VENTURA SANTOS,2 AND LEONARDO FADEL CURY1

1 Departamento de Geologia, Universidade Federal do Paraná (UFPR), C.P. 19.001, 81531-980 Brazil2 Laboratory Isótopos Estáveis, Universidade de Brasília (UnB) 70910-900 Brazil

AbstractAripuanã is a Paleoproterozoic (1.76−1.75 Ga), stratiform, volcanogenic Zn-Pb-Ag-(Au-Cu) massive sulfide

deposit, has an estimated resource of 11.6 million metric tons (Mt) of ore, and is located on the south-south-west border of the Amazon craton (Brazil). Aripuanã resides in a caldera composed of strongly fractionated,felsic metavolcanic rocks, transitional between calc-alkaline and tholeiitic compositions. The entire region wasmetamorphosed into lower greenschist facies (1.68−1.63 Ga) and the deposit was subsequently thermallymetamorphosed and metasomatized (1.56−1.53 Ga).

The mineralized bodies are lenticular and elongated. They are located at the base of a sequence of turbidites,which overlie dacitic and rhyolitic ignimbrites. The rocks of the footwall are silicified, sericitized, and chlori-tized, and the hanging-wall rocks are carbonate rhythmites and turbidites, hydrothermal marbles, carbonatebreccias, and breccia-like rocks, carbonate + talc + tremolite + chlorite rock, talc-, tremolite-, and biotite-richrocks, and carbonate- and fluorite-bearing cherts.

The deposit was formed during at least four submarine-exhalative hydrothermal cycles in which siliceousmarbles and marls represent the initial phase of each cycle, followed by the coprecipitation of pyrite +pyrrhotite + sphalerite + argentiferous galenas ± chalcopyrite, which were disseminated in clay-carbonate brec-cias or formed massive lenses. This phase was followed by fluoritic cherts and argillites, which were followedby carbonate and (fluorite?) rhythmites, and turbidites to close the cycle.

The variation in total rare earth element (REE) contents and in the Eu and Ce anomalies indicates that theoriginal sediments were mixtures of hydrothermal carbonate and smectitic clay and were deposited below a redoxcline. Isotopic analysis identified δ18OSMOW values between 8 and 13‰ and δ13CPDB values between −7and −1‰. Modeling isotopic data showed that mineralizing fluids were distinct based on the proportion ofmagmatic fluid that mixed with the seawater. Carbonate rocks and sulfide minerals precipitated while hydro-thermal activity and volcanism evolved from an exhalative to an exhalative-explosive character with tempera-tures increasing between 100° and 200°C.

The regional metamorphism of the deposit formed carbonate + talc + tremolite + chlorite rocks, tremolite-,biotite-, and talc-rich rocks; it recrystallized the minerals, foliated and folded all of the rocks, and caused limited decarbonation at the sites where carbonate rocks contained silica and smectite. However, the originalisotopic signatures were maintained. The thermal metamorphism erased the metamorphic foliation, and themetasomatism generated veinlets of chlorite with sulfides.

† Corresponding author: e-mail, [email protected]*A digital supplement to this paper is available at http://economicgeology.

org/ and at http://econgeol.geoscienceworld.org/.

©2013 Society of Economic Geologists, Inc.Economic Geology, v. 108, pp. 781–811

Submitted: March 8, 2011Accepted: May 18, 2012

Dexheimer Leite et al. (2005) mapped and sampled thearea around the Aripuanã deposit and described four drillholes located in the northern part of the deposit. Thesamples were characterized by petrography, bulk geochem-istry, including rare earth elements, whole-rock Sm-Nd iso-topes, 204Pb/206Pb of massive sulfides, and carbon and oxygenisotope analysis in carbonate minerals. Zircons from a sampleof tuff of the hanging wall were dated by the 206Pb-207Pb zir-con evaporation method, resulting in an age of 1768 ± 28 Ma(Dexheimer Leite et al., 2005), similar to that obtained byNeder (2002). They concluded that the deposit is volcano -genic, comprises volcanic and felsic volcaniclastic rocks, witha lower zone of stringer ore and an upper zone of massive orewith disseminated sulfides. Fluid inclusions indicated that

mineralizing fluids had compositions that could be modeledby the system H2O-MgCl2-FeCl2-NaCl-(KCl) with high CH4

content, salinity between 12 and 25 wt % NaCl equiv, andhomogenization temperatures between 100° and 250°C;however, these fluids were not considered to be those thatoriginally precipitated the sulfides of the orebodies butshould reflect later stages of hydrothermal or metamorphicalteration (Dexheimer Leite et al. 2005). According to theseauthors, however, the deposit may have been affected byhigher temperatures (380° and 510°C) as indicated by tem-peratures calculated based on chlorite geothermometry.They further argued that δ34S values between −0.7 and+3.3‰ for sulfides suggest that the primary mineralizing fluids were formed by the mixture of evolved and heated

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MESOPROTEROZOICCaiabis Group

Arinos Formation

Dardanelos Formation

Canamã Alkaline rocks(Rb/Sr, 1216 ± 30 Ma)Aripuanã Granite (U-Pb 1537±7 Maand Pb-Pb 1546 ± 5 Ma)

Rio Vermelho Granite

Rio do Sangue Suite

PALEOPROTEROZOICZé do Torno granite (U-Pb, 1755± 5 Ma)

Roosevelt Group (U-Pb, 1762 ± 6to 1740 ±8 Ma)

São Pedro Suite (U-Pb, 1784± 6 Ma)

Vitória Suite (U-Pb, 1785±6 Ma)

Shear zonesand faults

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FIG. 1. Summary of the geology of the Aripuanã region 1:250,000 (Albuquerque and Oliveira, 2007a). The Aripuanã de-posit is located in the western part of the Aripuanã main caldera (Fig. 5), whose approximate limits are designated in the top-left corner of this figure. Note the location of Aripuanã, in the Mato Grosso State, in the southern part of the Amazonas cra-ton (Rondônia-Juruena Province).

seawater and magmatic fluid with sulfur from the leached ofvolcanic rocks.

Sampling and Analytical MethodsSamples for this study include 96 outcrop samples collected

during the geologic mapping and 186 samples collected from62 drill holes of the mineralized region. Selected sampleswere ground and crushed in tungsten carbide mills and thenquartered and separated into aliquots of 200 g for analysis ofmajor and trace elements, including REEs, Zn, Pb, Cu, Ag,and Au, at Acme Analytical Laboratories Ltd., Vancouver,Canada. Trace element and REE abundances were determinedby inductively coupled plasma mass spectrometry (ICP-MS),whereas major elements were determined by inductively cou-pled plasma emission spectroscopy (ICP-OES). Fluorine wasdetermined by ion specific electrode, and Zn, Pb, Cu, and Agwere determined by atomic absorption (AA). Gold contentwas measured by fire assay and by AA, and the results are re-ported in Appendix 1. Twenty-three additional analyses werecompiled from the work of Dexheimer Leite et al. (2005).

For determination of the δ13C and δ18O values, 89 carbon-ate samples were collected (each sample weighted 0.7−2.3 g)and analyzed for major elements by ICP-AES at ACME.These samples were obtained by drilling carbonate crystalsfrom drill cores made with a TREMEL microgrinder powertool. For oxygen and carbon isotopes an aliquot of approxi-mately 300 ug of each sample was placed in glass vials thatwere subsequently submitted to an He flush at 72°C. Afterflushing, the aliquots were reacted with concentrated phos-phoric acid, and the CO2 released was analyzed for carbonand oxygen isotopes in a Delta V Advantage connected to aGas Bench II apparatus from the Geochronos Laboratory ofthe University of Brasília. Analyses of NBS 18 during the period of this study yield an average value of −5.1‰ forδ13CVPDB and +23.1‰ for δ18OVPDB.

Regional Geologic and Tectonic SettingsThe Aripuanã gold deposit is located in the Rondônia-

Juruena province, within the tectonic context of the RooseveltGroup (Santos et al., 2000, 2008; Rizzotto et al., 2004). Santoset al. (2008) dated 81 zircons from six samples collected in theRondônia-Juruena province, using the U-Pb by SHRIMP.Among these ages, 54 were concordant and are shown in Figure2. These ages, together with geologic mapping and previouslypublished data, were used to define the geotectonic environ-ment of the Aripuanã region.

The deposit was formed between 1762 and 1755 Ma (Fig.2). During the Quatro Cachoeiras orogeny (1,689−1,632 Ma),the entire sequence was folded, foliated, and metamorphosedto middle greenschist facies. After a geologic quiescence pe-riod of about 70 m.y., the Rondônia-Juruena province was in-truded by the Aripuanã granites, dated at 1542 ± 2 Ma (Riz-zotto et al., 2002), which thermally metamorphosed andmetasomatized rocks related to the deposit. Santos et al.(2008) also reported ages of 1370 to 1320 Ma, which wereconsidered to be metamorphic ages related to the deforma-tional regimes of the Grenville orogenic cycle (Ectasian Pe-riod) in South America, also known as the Candeias orogenyof Sunsás orogen (Fig. 2). Evidence of this orogeny has notbeen identified in rocks of the deposit.

During the Mesoproterozoic, the Rio Vermelho porphyriticsyenogranites and the Rio do Sangue granite were emplaced(Fig. 1), followed by the Aripuanã (or Rio Branco) porphyriticgranite and, finally, by syenites, microsyenites, quartz syenites,and trachytes of the alkaline Canamã Complex (1216 Ma).Later, during an extensional regime, the Dardanelos basinwas opened (1.4−1.2 Ga) and filled with polymictic conglom-erates and arkosic rocks. These rocks were crosscut by dikesand sills of basic rocks from the Arinos Formation (K-Ar =1416−1225 Ma; Silva et al., 1980).

In the area vertical to subvertical, transcurrent brittle struc-tures oriented WNW-ESE are dominant. This sinistral shear-ing coalesced during the continental collision that caused theQuatro Cachoeiras orogeny.

Geochemical characteristics, values of Y + Nb close to 50and Rb above 130 (App. 1), place the granites of the Aripuanãregion between the VAG (volcanic arc granite), synCOLG(syncollisional), and WPG (within-plate) domains, indicatingthat the magmatism was postcollisional (Pearce et al., 1984)and rift related with high K, calc-alkaline rocks predominant.The Aripuanã deposit formed in a back-arc region of the Jamari and Alto Jauru island arc. Since the continental colli-sion that caused the Quatro Cachoeiras orogeny, the Aripuanãcaldera has rested in an accretionary prism, between two paleocontinents.

The limits of the main Aripuanã caldera are shown in thetop-left corner of Figure 1, which summarizes the known ge-ology of the Aripuanã region. The main Aripuanã caldera is anelliptical structure (57 × 28 km) with the major axis orientedat N 80 E (Fig. 3A), within which four subcalderas were iden-tified (Fig. 3A, B). In the interior of the main caldera,metarhyolites, rhyodacites, and grayish ignimbrites are pre-dominant, among which occur rhyolitic domes of varied di-mensions. The subcalderas are structurally depressed regionsthat have metacherts, metaturbidites, and metatuffs in thecentral region, and metarhyolites, metadacites, and metaig -nimbrites at the base and on the edges. All rocks are foldedand metamorphosed, with folds with axial-plane foliation.

ARIPUANÃ Zn-Pb-Ag (Au, Cu) VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, MATO GROSSO, BRAZIL 783

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DEPOSIT RELATED ROCKS

Zé do Torno meta-granites1755 6Ma+_

Hanging wall meta-dacite1762 6Ma+_

Aripuanã granites1542 2Ma+_

FIG. 2. Distribution of 54 U-Pb SHRIMP radiometric ages from sampleswith ages that fit the concordia curve, collected by Santos et al. (2008) inRondônia-Juruena province, in the Amazon craton.

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Dardanelos formation.K-Ar = 1.4 e 1.2 Ga and maximum age 207Pb-206Pb = 1.7 Ga.

Zé do Torno graniteZircon U-Pb SHRIMP = 1755±5 Ma

Rio Branco graniteU-Pb SHRIMP = 1537±7 Ma e 207Pb-206Pb = 1546±5 Zircon

Central microgranite

Red, porphyritic rhyolites

Lithic meta-tuff, meta-welded tuffs andrhyolites of domes and volcanic chimneys (red)Gray meta-ignimbrites, meta-tuffs,meta-rhyolites and dacites

Gray-vesicular meta-rhyolites interbeddedwith bedded meta-chert and metacinerites (volcanic cinders)

Red meta-dacites and meta-rhyolitesdated zircon Meta-sandstone and conglomerates Granites with

U-Pb e 207Pb-206Pb = 1,80-1,74 Ga

U-Pb and 207Pb-206Pb = 1801-1757 Ma

Caldera undated acid volcanic rocks

Xingu Complex

o l roR oseve t G up

C ia is ro p a b G u

Granites, charnockites and meta-sediments

Green schist metamorphism U-Pb SHRIMP = 1689 - 1632 Ma

Volcanic and volcanosedimentary units thatcontain the Aripuanã deposit (Fig. 6)

MESOPROTEROZOIC

PALEOPROTEROZOIC

PALEOPROTEROZOIC (Cont.)

ARCHEAN

Inferred calderalimits

LEGEND

Faults

Rivers

Aripuanãtown

80 Fault strikeand dip

Au Au panninglocation

FIG. 3. The Aripuanã caldera: (A). GDEM (Global Digital Elevation Model) image of the ASTER satellite (source:www.wist.echo.nasa.gov). (B). Simplified geologic map. Note the location of Figure 4.

The volcanic rocks of the Aripuanã caldera have Zr/Y ratiosbetween 2 and 20 and Yb/La ratios between 1 and 25, whichindicates that they represent a transitional series betweencalc-alkaline and tholeiitic. The Zé do Torno granites are metaluminous to peraluminous, subalkaline with an alkalinetendency. The Rio Vermelho and Aripuanã granites are calc-alkaline to alkaline, high K, and can be classified as postcolli-sional or anorogenic. According to Albuquerque and Oliveira(2007a, b), the Arinos gabbros (Fig. 1) are tholeiitic, havehigh Mg, and are not very differentiated.

When the subcalderas collapsed, they were bounded bycurvilinear normal faults with concentric dips on the order of40° to 50°. After having been metamorphosed, folded, andsheared from an E-W direction, the entire region was in-truded by the Aripuanã granite that released fluids that locallymineralized the concentric faults bordering the east sub-caldera. Later, the entire region was covered by sandy andarkosic sediments of the Dardanelos and Arinos Formations.During the extensional phase that opened the Caiabis basin,E-W shear zones were reactivated as normal faults, causingthe fragmentation of the Aripuanã deposit into the followingthree blocks (Fig. 4): AREX to the NW, where a single min-eralized body is known; AMBREX in the center, with at leastfour mineralized bodies; and Babaçu and Massaranduba tothe SE, where there are two mineralized bodies. In the strati-graphic sections of the central part of the deposit, from thetop down to the base, the stratigraphy is as follow described:

1. UVS(A): Volcanosedimentary unit with 400 to 430 m ofa massive and laminated metatuff layers, with a crystallinelithic tuff intercalated with metarhyolites.

2. USV(B): A 20- to 160-m-thick unit of metaturbidites andmetarhythmites that are carbonated in the mineralized loca-tions, with meter-scale lenses of metachert that are interlay-ered with epiclastic metassediments.

3. UV: Volcanic unit with a 60- to 80-m-thick, grayish massive layer of metadacites, deposited on at least 40 m ofmetaignimbrite rhyolites.

In contrast to the Aripuanã mineralized USV(B) unit, car-bonate rocks were never found during regional and calderamapping.

Rocks related to mineralization

The mineralized zone of the Aripuanã deposit stretches fornearly 5 km on the western border of the eastern subcaldera(Figs. 3, 4). The stratiform mineralized bodies are located atthe base of the USV(B) unit (Fig. 4). Metamorphism (Figs.5E, F, 6B) did not obliterate the primary subaquatic struc-tures in rocks of the main caldera (Fig. 5A-C), which is filledwith volcanoclastic sediments and red porphyritic rhyolitescharacterized by partly resorbed quartz phenocrysts (Fig.6A). Volcanoclastic rocks, grayish metadacites, and metarhyo-lites occur in the interior of the subcalderas.

The deposit is hosted by carbonate metaturbidites andmetarhythmites (CTRh) with typical subaqueous sedimentarystructures, such as the very fine, alternated layers of carbon-ate and silica (Fig. 5B), with or without fluorite, in convolutedfolds (Fig. 5C), and flame structures delineated by crystals ofsphalerite and pyrite (Fig. 5A). These structures indicate a

clastic-chemical sedimentary subaqueous origin for the ma-jority of rocks that host the deposit as well as for those directlyrelated to mineralized bodies. The mineralized bodies arehosted by marbles and carbonate breccia with carbonate,tremolite, talc, and chlorite (CTTC), fluorite tremolite-rich-(TRR), fluorite talc-rich- (TAR), biotite-rich- (BR), and chlo-rite-rich rocks (CR). All these rocks are unfoliated, contactmetamorphic rocks with characteristics that identify them ge-netically as recrystallized clastic-chemical sedimentary rocks.

Laminated metaturbidites and metarhythmites overlay thedeposit. These low-grade metaturbidites (Fig. 5E) were de-rived from ash-sized pyroclastic rock fragments and volcanictuffs that were transported as epiclastic sediments. Theserocks are characterized by an alternation of millimetric layersof clastic sediments and carbonate because of their depositionby turbiditic currents during the precipitation of carbonates(Fig. 5B). Carbonate rhythmites (Fig. 5E) differ geneticallyfrom turbidites because their clastic bands are derived fromvolcanic ash falls sedimented also while carbonate precipi-tated. Locally they are interstratified with massive lens of epi-clastic metasediments and cherts that locally have isolatedcarbonate blebs (Fig. 5N). These blebs may coalesce intolarger blebs that were deposited parallel to the local stratifi-cation, thus forming a carbonate layer that can be interbededwith epiclastic sediments or chert.

The ore zone was affected mainly by a pervasive silicifica-tion process that extends for more than 5 km west of the deposit. Besides silicification, other important hydrothermalalteration assemblages are observed mainly below the miner-alized bodies. In order of importance, these alteration typesinclude sericitization, chloritization, and pyritization. Therocks of the footwall contrast in their composition and in thepresence of metamorphic foliation with the rocks of the de-posit, which are generally devoid of foliation (Figs. 5G-M,6D-L) and have poikilitic porphyroblasts of biotite (Fig. 6C).

The mineralized bodies and the associated rocks have alarge amount of carbonate. Due to intense metasomatism,there are facies changes within 50 m from the drill cores,making facies correlation difficult within this scale. Metaso-matism may have driven recrystallization because there is nometamorphic orientation of silicates (Fig. 6F-I) and carbon-ates are zoned (Fig. 6D). Among the recrystallized rocks,there are late metasomatic veinlets of massive chlorite withpyrrhotite, chalcopyrite, and magnetite (Fig. 5Q) that extendfrom the basal volcanic unit UV(A) to the stratigraphicallyhigher mineralized body.

Close to the mineralized bodies, carbonate-rich rocks(CTTC or marbles) are important and they grade laterally tocarbonate-chert followed by massive chert and/or argilliticchert. Locally, CTRh rocks have black bands composed of mi-crocrystalline pyrite and magnetite that are always serrated byregional metamorphic foliation (Fig. 6B).

Massive and banded marbles with thicknesses of up to 6 moccur in the core of the mineralized zones, generally envelopedbeside or below the orebodies. These are white colored rocks(Fig. 5G, App. 1), generally with greenish clusters of talc ±chlorite ± tremolite. Carbonate breccias, usually related tomineralization, are composed of angular marble fragmentswith a matrix of chlorite, tremolite, talc, microcrystalline car-bonate, sphalerite (Fig. 5P), galena, pyrite and rare pyrrhotite,


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Gray, silicified meta-ignimbrites,rhyolitic and dacitic meta-tuffs andwelded meta-tuffs

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Volcanic unit: Gray, silicified andsericitized meta-rhyolites andmeta-dacite with meta-tuff lenses

Volcanosedimentary unit: silicified and sericitizedmeta-tuff, massive and banded silicified andsericitized epiclastic metasedimentswith meta-ignimbrite and meta-rhyolite lenseson top, and silicified and carbonatizedmeta-turbidites and metachert at base

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Silicicified and sericitized bandedmeta-turbidites and epiclasticmetasediments

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RECENTGossan with pyrite - chalcopyrite - pyrrhotite - galena - sphalerite

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Felsic volcanicsFelsic volcanics

A-B section C-D section

Felsic volcanics

meters

Volcaniclasticmeta-turbiditesand rhythmites

FIG. 4. Geologic map of the Aripuanã deposit (see location in Fig. 3). Due to weathering, there are only a few outcropswhere there is mineralization, and the information for this area comes mainly from the descriptions of 61 drill holes. In thelower part of the map, there is a longitudinal section of the main orebodies, and the locations of the A-B cross section (AREX,Fig. 7) and the B-C cross section (AMBREX, Fig. 8) are shown. Locations where gold was panned are also indicated.

chalcopyrite, and magnetite. These breccias occur in theouter parts of the marbles where they are mixed with CTTCrocks.

Pseudofragmental CTTC rocks are composed of heteroge-neous mixtures of carbonate-tremolite-talc-chlorite, in placeswith fluorite, biotite, and spheroid structures (Fig. 5K). Theyare also commonly mineralized and occur enveloping otherbreccias, marbles, and mineralized rocks. The main mineral-ogy of these rocks varies from being silicate (Fig. 5 I) to car-bonate (Fig. 5D) dominated.

Twisted-fragment carbonate breccias (Fig. 5J) and brecciaswith carbonate rhombohedra, which have cloudy core andtransparent edges (Figs. 5L, 6G), occur associated with CTTCrocks and massive marble. The matrix of these breccias iscomposed of microscopic crystals of chlorite (Fig. 6G), car-bonate, talc, and tremolite (Fig. 6F) and, less commonly, ofeuhedral porphyroblasts biotite and radial talc.

Some rocks contain carbonate spheroids, 0.2 to 12.0 mm indiameter (Fig. 5K), and rhombohedral carbonate crystals(Fig. 5L). The carbonate spheroids are rare structures hosted


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2

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(402

,0) 5

-1

5F

27 (

52,

5)

FP

A-

0 (

435

R1

16,

)

FAR

-14

(147

,00)

P

FP

AR

-12

(190

,00)

F-2

47,

904

(6

)

FP

R-2

(31

0)A

0,0

11 cm

F-2

4 (4

6690

),

sph

FR

6 2

,6P

A -

0(

615)

FP

AR

-06

4310

)(

4,

FP

A-0

(765

,60)

R8

Carbonate flakes

7F

PA

R-0

9 (2

8,70

)

PR

14(2

58

FA

-

0,0)

a e iSph l r te

Ga enal

Sp al riteh e

FP

AR

-05

(296

,80)

2(

5F

-7

55,

70)

FP

AR

-09

(27

,0)

72

sph

+ p

ys

+

phpy

A B C D E F G H

I J K

L M N OA

(F

PR

-04

389,

70)

TP

YR

IE

sph

FA

R-

9 (

190

)P

04

,0

eSphalerit

S haleritp e

PR

3,

0F

PA

-11

(30

6)

e

mta

som

atic

or

chl

itite

Vol

cano

nic

gequ

artz

-chl

orite

s

chis

t

Q R

FP

AR

-09

(424

,70)

Recrystallizedetasomaticm

massi ev sphalerite

SMeta omati bio ite and

emol trt

c

te

s

i

FIG. 5. Photographs of cut and polished drill cores. On the left side of each photo, the text reports the drill hole identifi-cation and the distance to the collar. In all of the figures, the length of the horizontal bar is 1 cm. (A). Mineralized epiclasticmetasediment with flame structures delineated by fragments of sphalerite and pyrite. (B). Laminated carbonate metatur-bidite. (C). Convolute folds of metachert. (D). Carbonate-rich CTTC rock. (E) Carbonate metarhythmites. (F). Axial-planemetamorphic foliation in layered epiclastic metasediment. (G). Hydrothermal micritic marble with talc + chlorite nodules.(H). Carbonate breccia with a chlorite matrix. (I). CTTC rock with little carbonate. (J). Breccia with twisted carbonate frag-ments in a biotite matrix. (K). Pseudofragmental hydrothermal rock with zoned, concentric, and carbonate spheroids. (L).Pseudofragmental hydrothermal rock with rhombohedral carbonates in a biotite matrix. (M). Fluoritic chert with clusters oftremolite and chlorite. (N). Massive epiclastic metasediment with carbonate blebs. (O). Massive ore with microcrystallinesphalerite and galena. (P). Breccia with carbonate fragments cemented by sphalerite. (Q). Breccia with fragments of meta-morphosed silicified and chloritized pyroclastic rock cemented by unmetamorphosed massive chlorite. (R). Thermally meta-morphosed talc-tremolite-rich rock (TRR). (S). Ore with pockets of recrystallized sphalerite in biotite-tremolite-rich rock.

in TRR and CTTC rocks, which are composed of carbonate(70−85%), chlorite (5−20%), talc (1−4%), and tremolite (1−2%) and at least 2% pyrite + sphalerite. They are zoned andmade of radially crystallized carbonate layers separated bynarrow bands of microcrystalline chlorite.

TRR and TAR rocks consist of millimeter- to centimeter-long crystals of tremolite and radial talc aggregates (Figs. 5I,

6F, H) mixed with carbonate and biotite. They are always associated marbles or carbonate breccias (App. 2) and aremineralized (Fig. 6I).

Fluorite-bearing cherts with nodules of tremolite-talc andcarbonate (Fig. 5M) are characterized by irregular centimeter-width banding and are composed of quartz (+feldspar) (50−70%), microcrystalline fluorite (0.1−10%), carbonate (1−15%),

788 BIONDI ET AL.

0361-0128/98/000/000-00 $6.00 788

AARI-26

A BFPAR-09 (144,60)

B CFPAR-15 (444,00)

C

D

FPAR-09 (359,70)

D

FPAR-09 (248,40)

EF-50 (625,25)

F

FPAR-04 (389,70)G

FPAR-06 (182,50)H

FPAR-09 (370,75)

sph

sph

trm

I

FPAR-09 (342,70)

gal

gal

sph

po

JL

FPAR-09 (331,05)

gal

popo

sph sph

py

sph

cpy

K

ccpycpy

sph

py

sph

FEX-12 (161,05)

L

trm

biocbtcbt

cbt

clt

cbt

cbt

cbt

cbtclt

cbt

qz

glass

cbt

trm

bio

qz+sct+fel

FIG. 6. Photomicrographs of thin sections of rocks, obtained with plane-polarized transmitted light. In each photographthe text reports the drill hole and the distance to the collar. (A). Partly resorbed phenocrysts of quartz and of feldspar in redrhyolite. (B). Millimeter-scale detail of microcrystalline-pyrite band folded and foliated by regional metamorphic foliation.(C). Porphyroblast of contact-metamorphic biotite in silicified and sericitized rhyolite. (D). Rhombohedra of recrystallizedcarbonate in a breccia with twisted carbonate fragments with a massive chlorite matrix. (E). Microcrystals of pyrite alignedwith the axis of a carbonate fragment from a breccia with twisted fragments. (F). Rhombohedral carbonate in unmetamor-phosed talc-tremolite matrix. (G). Rhombohedra of carbonate in a matrix of metasomatic massive chlorite. (H). Centimeter-scale detail of crystallized tremolite (trm) in a biotite medium (bio) and in a carbonate matrix (cbt). (I). Tremolite cementedby sphalerite. Photomicrographs of polished sections of mineralized rocks, obtained with reflected light. (J). Metasomaticmineralization with talc and tremolite, with sphalerite (sph), magnetite and pyrrhotite (po) cemented by galena (gal). (K).Clusters of pyrite (py), associated with pyrrhotite (po), sphalerite (sph), and galena (gal) in a carbonate breccia with a tremo-lite matrix. (L). Sphalerite (sph) with punctual contents (blebs) of chalcopyrite (ccpy) (chalcopyrite disease) next to euhedralcrystals of metamorphic pyrite.

chlorite (5−30%), and talc + tremolite (2−10%; App. 2). Theyoccur associated with BR rocks and are placed stratigraphi-cally on the top of the mineralized zones and below the car-bonate turbidites (Figs. 7, 8). The presence of four horizons

of fluorite-bearing chert in the AMBREX area, each one onthe top of a mineralized zone (Fig. 8), indicates that the de-posit was formed by at least four distinct sedimentary exhalative episodes. BR rocks with quartz, carbonate, and


0361-0128/98/000/000-00 $6.00 789

Tremolitic sulfide ore with10-80% pyrite, 5-30% pyrrhotite,2-5% magnetite, 0-5% chalcopyriteand 0-5% sphalerite + galena (>Ag)

Tremolite-carbonate sulfide orewith 5-40% pyrite, 5-85% sphalerite,2-10% galena, 0-2% chalcopyrite

(Biotite) - talc tremolite-rich rockwith carbonate, 5-15% pyrrhotite,5-10% sphalerite, 0-2% magnetite

Metamorphosed quartz-feldsparturbidites and rhythmites, quartz-feldspar volcaniclastics and chert

Acid volcanics - Metamorphosedaltered dacite, rhyolite and tuffs with quartz and sericite

Chloritic (tremolite, biotite) sulfideore with 10-80% pyrite, 2-10% ccpy,2-10% magnetite, 0-5% pyrrhotiteand 0 - 2% sphalerite

Massive pyrite

Metamorphosed chert andcarbonate turbidite and rhythmite

Brecciated fault zone

Least altered metamorphosed dacitesand ignimbrites with quartz, feldsparand sericite

Moderately altered metamorphosed dacitesand rhyolitic ignimbrite with quartz, feldspar,sericite, biotite porphyroblasts and pyrite

Pyrrhotite, pyrite, chalcopyrite,sphalerite (magnetite) stringerzone, with quartz, sericite and chlorite

Twisted carbonate breccia with tremoliteand talc groundmass, 0-5% pyrite,0-2% sphalerite, 0-2% magnetite

Chlorite-talc marble with 0-5% pyrite,0-2% sphalerite and 0-2% pyrrhotiteChloritite with phenoblasts of biotite,

(sphalerite, chalcopyrite)

Carbonate breccias and mixedcarbonate - talc - tremolite - chlorite(CTTC) rock with 0-10% sphalerite,0-5% galena and 0-3% chalcopyrite

Fluoritic chert and meta-cineritewith tremolite, talc and carbonatenodules LEGEND

Drillhole identification, collar and traceCircle - Collar in the section zone

?

FEX-45FEX-37 FEX-25 FEX-13

0

10S0

W

20S

W0 0

W30

S

+100

+200

NW ARIPUANÃ AR-05 100m

-200

-100-100

SL

FEX-13(75,50)

FEX-13(86,70)

A B

??

??

+100

+200

-200

-100-100

SL

-300 -300

FEX-45

FIG. 7. Simplified section AR-5 of the mineralized AREX region in the NW part of the deposit. Note the presence of amineralized zone with three lenses of ore hosted among carbonate rocks and topped by a horizon of fluoritic chert (the thick-ness of this horizon has been magnified to make it visible in the scale of the figure).

790 BIONDI ET AL.

0361-0128/98/000/000-00 $6.00 790

Tremolitic sulfide ore with10-80% pyrite, 5-30% pyrrhotite,2-5% magnetite, 0-5% chalcopyriteand 0-5% sphalerite + galena (>Ag)

Tremolite-carbonate sulfide orewith 5-40% pyrite, 5-85% sphalerite,2-10% galena, 0-2% chalcopyrite

(Biotite) - talc tremolite-rich rockwith carbonate, 5-15% pyrrhotite,5-10% sphalerite, 0-2% magnetite

Metamorphosed quartz-feldsparturbidites and rhythmites, quartz-feldspar volcaniclastics and chert

Acid volcanics - Metamorphosedaltered dacite, rhyolite and tuffs with quartz and sericite

Chloritic (tremolite, biotite) sulfideore with 10-80% pyrite, 2-10% ccpy,2-10% magnetite, 0-5% pyrrhotiteand 0 - 2% sphalerite

Massive pyrite

Metamorphosed chert andcarbonate turbidite and rhythmite

Brecciated fault zone

Least altered metamorphosed dacitesand ignimbrites with quartz, feldsparand sericite

Moderately altered metamorphosed dacitesand rhyolitic ignimbrite with quartz, feldspar,sericite, biotite porphyroblasts and pyrite

Pyrrhotite, pyrite, chalcopyrite,sphalerite (magnetite) stringerzone, with quartz, sericite and chlorite

Twisted carbonate breccia with tremoliteand talc groundmass, 0-5% pyrite,0-2% sphalerite, 0-2% magnetite

Chlorite-talc marble with 0-5% pyrite,0-2% sphalerite and 0-2% pyrrhotiteChloritite with phenoblasts of biotite,

(sphalerite, chalcopyrite)

Carbonate breccias and mixedcarbonate - talc - tremolite - chlorite(CTTC) rock with 0-10% sphalerite,0-5% galena and 0-3% chalcopyrite

Fluoritic chert and meta-cineritewith tremolite, talc and carbonatenodules LEGEND

Drillhole identification, collar and traceCircle - Collar in the section zoneDouble arrow - Collar behind or in front ofthe section zone

FEX-45

FIG. 8. Simplified AM-30 section of the mineralized AMBREX region, located in the central part of the deposit. Samelegend as Figure 7. Note the presence of four mineralized zones, emphasizing zone 2, which contains various massive ore-bodies with different compositions. The FPAR-72 drill hole, which is the only one that sectioned mineralized zone 4, is lo-cated around at 100 m to the southeast of this section and was projected horizontally to show the position of zone 4 in rela-tionship to the other mineralized zones.

050N

100N 50N 0

FPAR-06

F-24

F-49

FPAR-05

F-17

FPAR-60

FPAR-70

FPAR-36

-200

-150

-100

-50

SL

+50

+100

+150

-150

-200

-100

-50

SL

+50

+100

+150

100N 50S 100S 150S

50S 100S 150S

FPAR

-86

F-34

FPAR-72

?

??

??

?

??

??

?

?

??

?

?

Mineralized zone n 2

?

?

?

?

?

?

?PF A

R-90

PAR

-1

F1

Gossan Hill

?

C D




tremolite are uncommon and the fluorine contents of 0.2 to0.9% (App. 2) are due mostly to substitution into biotite,rather than to the presence of fluorite.

Sulfide mineralization

The highest concentration of sulfides (more than 60 vol %)occurs associated with the CTTC rocks, TRR and TAR rocksand carbonate breccias. Mineralized rocks are composed ofvarying proportions of sphalerite, galena, pyrite, pyrrhotite,and chalcopyrite and minor fluorite. Sulfides were crystal-lized in irregular and curvilinear shapes (Fig. 6J, K), suggest-ing sedimentary coprecipitation. Pyrite is the most commonsulfide, followed by sphalerite, galena, pyrrhotite, and chal-copyrite. Sphalerite is dark, reddish brown (Figs. 5P, S, 6I)and is generally homogeneous. Zoned sphalerite crystals,however, are also found and are characterized by inclusions ofpyrrhotite, galena, and chalcopyrite and by substitutions bychalcopyrite (chalcopyrite disease, Fig. 6L), which are com-mon features among Kuroko-type deposits (Barton andBethke, 1987; Eldridge et al., 1988; Bortnikov et al., 1991).Chalcopyrite generally occurs in association with pyrrhotiteor on the edges of sphalerite crystals, which suggests reequi-librium due to metamorphism, as described by Craig (1983)for Appalachian metamorphosed Kuroko-type deposits. Mag-netite occurs with pyrrhotite and chalcopyrite, as well as inchlorite- and chlorite-tremolitite-rich rocks.

The mineralized zone has a pronounced vertical zonationthat has been related to different volcanogenic hydrothermalepisodes. The base of the deposit is dominated by Fe (Cu,Zn)-rich minerals including sphalerite (0−2%), pyrite (10−80%), magnetite (2−10%), pyrrhotite (1−5%), and chalcopy-rite (0−2%). In the middle portion of the deposit (50−100 mabove the base), Fe-Cu-Zn minerals such as sphalerite (1−5%), pyrite (10−80%), pyrrhotite (5−30%), magnetite (2−5%),and chalcopyrite (2−5%) dominate. In the upper portion mi-crocrystalline ores of Zn-Pb-Fe, along with sphalerite (5−85%),pyrite (5−40%), galena (2−10%), and chalcopyrite (1−2%),mixed with recrystallized tremolite microscopic crystals aremost important. Massive and disseminated sulfides (pyrite ±chalcopyrite ± sphalerite) that occur within normal faults andfractures were formed during the collapse of the subcalderas.They are interpreted as late metasomatic remobilization ofsulfides, such as those in the Gossan Hill fault (Fig. 8), amongothers.

Morphology and composition of the mineralized bodies

The only mineralized zone of the AREX area consists ofone lens of Zn-Pb-Fe massive sulfide on the top and twolenses of Zn-Pb-Fe massive sulfide at the base. This ore zoneis located on the southeastern flank of a syncline that is paral-lel to a N 45−55 W/50−60 NE fault (Fig. 7) filled with brec-ciated quartz, with sericite and pyrite (Fig. 7, section A-B, Fig.4). Below the mineralized zone, there are quartz-chlorite-sericite metadacites and rhyolitic metaignimbrites with dis-seminated pyrite, chalcopyrite, and sphalerite oriented parallelto the metamorphic foliation. They are cut by veinlets of mas-sive chlorite (biotite) rock. Below, metadacites and metaign-imbrites of the footwall grade into quartz-feldspar metadaciteswith sericite and porphyroblasts of biotite and pyrite, andthen, into metadacites without pyrite, with little silicification.

The main mineralized bodies are located in four zones in thecenter of the deposit (Fig. 8, section C-D, Fig. 4) in the AM-BREX area. They have greater thicknesses above marbles orcarbonate breccias and are all capped by fluorite-bearing chert.The substrate of the mineralized bodies immediately below themineralized zone 1 is the same as that of the AREX area. Min-eralized zones 1, 2, and 4 (Fig. 8) in the Gossan Hill region areseparated from zone 3 by the same fault described in the AREXzone and are filled at depth by massive pyrite, chalcopyrite,and sphalerite, and by massive pyrite close to the surface.

In mineralized zone 1, Fe (Cu, Zn)-type ore is hosted inmarble, in carbonate breccias, and in CTTC rocks. It is strat-iform, with a minimum strike length of 400 m, width between50 and 80 m, and thickness between 2 to 40 m. Zone 2 con-tains Fe (Cu, Zn)-type ore at the stratigraphic base, Fe-Cu-Zn-type ore in middle, and Zn-Pb-Fe-type ore at the top, inthree lenticular mineralized bodies. The two lenses of the oreat the base are overlapping CTTC rocks and marbles thatcontain pockets of breccias with twisted fragments of carbon-ate. They are stratiform, have thicknesses of 2 to 22 m, widthsbetween 80 and 200 m, and lengths of at least 600 m. The lenswith Fe (Cu, Zn) gradually changes in composition in the di-rection of the stratigraphic top into an Fe-Cu-Zn-type ore.The other lens of Fe (Cu, Zn)-type ore, located laterally andat the greatest depth, are covered by chlorite-rich rock withporphyroblasts of biotite followed by CTRh and BR rocksoverlain by metaturbidites and metarhythmites that are inter-calated with metachert.

Zone 3 is above and next to the fault, in the same structuralposition as the AREX mineralized zone. The largest mineral-ized body of this zone, at nearly 50 m in width, 2 to 8 m inthickness, and at least 200 m in length, consists of Fe-Cu-Zn-type ore with a matrix of chlorite, biotite, and talc. Zone 4 wasintercepted by only one drill hole (FPAR-72, Fig. 8) in whicha Zn-Pb-Fe-type ore was described as overlapping CTTCrocks and carbonate breccias and covered by two horizons offluoritic chert separated by a tremolitic zone.

There is not enough information to define the geometryand the dimensions of the Babaçu and Massaranduba miner-alized zones, but the alteration and mineralization featuresare similar to those of the AREX and AMBREX areas.

Geochemistry of Ores and Related Rocks

Geochemistry of the major elements

Elements related to hydrothermal rocks: The volcanic rockshave SiO2 varying between 60 and 82% and CaO + MgO lessthan 6%, increasing toward the mineralized bodies (Fig. 9).For instance, SiO2 decreases and CaO + MgO increases inthe following order: volcanic rocks → CTRh → BR, TAR, andTRR rocks → CTTC rocks → marbles. This variation in com-position reflects two independent but simultaneous processes:(1) silicification and sericitization of the volcanic and volcani-clastic rocks caused by the initial hydrothermal alteration,which increasingly affects the rocks as they get closer to themineralized bodies and/or the hydrothermal conduits (vents);and (2) the increasing abundance of hydrothermal carbonatesapproaching the mineralized bodies and/or vents, whichmixed and reacted with clays and volcanic ash-size pyroclasticrock fragments, derived from ash and volcanic tuffs.


0361-0128/98/000/000-00 $6.00 791

Distribution of the ore elements: Table 1 presents the averagegrades and the standard deviations of the SiO2, MgO, CaO,Ctotal, Na2O, K2O, Zn, Pb, Cu, Ag, Au, ΣREE, Eu, Sr, Al2O3

and As, for the regional rocks and mineralized rocks related tothe deposit. These were calculated for all lithology types shownin Appendix 1. The average grades of the rock types presentedin Table 1 were plotted in Figure 10. The data in this figurewere grouped according to lithologies and their relationshipwith ore zones: host rock, outside the mineralized zone, andthe mineralized zone. The average SiO 2 content of the foot-wall (Fig. 10A, lithology No. 4 = 75.62%) and of the hangingwall rocks (No. 5 = 75.40%) are slightly higher than that of thecaldera (No. 2 = 70.70%). Moreover, the SiO2 concentrationdecreases toward the mineralized body and then increases andexceeds that of CaO + MgO in the mineralized rocks (Fig.10A, B). The variation in the amount of total carbon indicatesthat, in the marbles and in the CTTC rocks, the CaO and theMgO are mainly contained in carbonate, and the carbonatecontents of the TRR-, TAR-, and BR-rich rocks are low.

Figure 10B and C show that the trends of Zn, Pb, Cu, Ag,and Au have a good correlation on a decameter scale, but not

as good on the scale of the sample (meters). This correlationis better for the trends corresponding to Zn, Pb, Ag, and Authan for Cu. In general, in the ore zones Zn (4−30%) presentsthe higher concentration, followed by Pb (4−20%), and thenby Cu (0.1−1.0%). The amounts of Cu exceed those of Zn andPb only in the horizons within chlorite rocks, in which thecontents of Ag (50−670 g/t) are close to 100 times greaterthan those of Au (0.05−1.0 g/t). The correlation coefficientsR2 between Zn and Pb vary between 0.38 and 0.68 in theAMBREX area and between 0.80 and 0.91 in the AREX area.For Zn and Cu, they vary from 0.001 to 0.09 in the AMBREXarea and from 0.002 to 0.19 in the AREX area.

The only lithologies in which the content of Cu is greaterthan that of Zn and Pb are the volcanic rocks of the footwall(Fig. 10B, lithology 4), in which the concentrations of theseelements are lower than 100 ppm. The facies that generallyare mineralized are the CTTC rocks, the TRR-, the TAR-,secondarily the carbonate breccias and, more rarely, the BR-rocks. Among the facies that comprise the mineralized bod-ies, the marbles are not mineralized, and their contents of Zn,Pb, and Cu are generally less than 100 ppm. Feldspar is

792 BIONDI ET AL.

0361-0128/98/000/000-00 $6.00 792

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80

CaO

+ M

gO

(%)

SiO2 (%)

1

100

Marble

CTTC rocks

Regional volcanic

Footwall volcanic

Hanging wall volcanic

Carbonatic turbidite, rhythmite and chert

Fluoritic carbonate chert

Fluoritic tremolite-rich rock

Biotite-rich rock

Massive and silicic marble and carbonatic chert

Biotite rock

1

100

Tremolite rock

1

100

Cb Turbidite

1

100

Footwall volcanics

-

100

1

Hanging wall tuff

CTTC rock

Carbonateturbiditesand chert

Fluoriticcarbonatechert

Fluoritic-tremolitite-rich rock

Fluoritic talc-rich rock

Biotite-rich rock

Massive-silicicmarble andcarbonate chert

+ Carbonate

+ Clay and ash size epiclastic sediments

Eu

Eu

Eu

Eu

Eu

Eu

Eu

Eu

Eu


A

-

1

100 I

H

F - Chert

Fluoritic meta-turbidites and meta-ritmites1

100 G

F

E

1

100

Talc rock

D

CTTC rocks

1

100

CTTCC

B

J

FIG. 9. (A). Relationship of the average contents of SiO2 vs. CaO + MgO for rocks of Aripuanã (crosses indicate ± 2σ).(B-J). Normalized REE diagrams of the Aripuanã deposit rocks. Contents of REEs normalized with the composition ofNakamura (1974) chondrite. Each rock type is related to its REE distribution pattern. Cb = carbonate, F = fluoritic.

almost absent in marbles, CTTC, TRR-, TAR-, and BR-rocks,thus explaining the low concentration of Na2O + K2O (Fig.11A) and Al2O3 (Fig. 10D) in these rocks.

The contents of fluorine (Fig. 10B) vary similarly to thoseof silica and independently from the Ctotal and MgO + CaOcontents, thus suggesting that the precipitation of fluorite andsilica were controlled by a similar process, which did not control carbonate precipitation. The marbles have the lowest average fluorine values and the concentration of this elementincreases with Zn, Pb, Cu, Ag, and Au contents, but they donot proportionally, suggesting that these constituents may haveprecipitated during a single event but by different processes.

Figure 10C shows that the amounts of Ag, Au, and As in-crease toward the mineralized bodies. The contents of Zn,Pb, Cu, Ag, and Au are greater in the lithologies with higherCaO + MgO and lower Na2O + K2O values, but these para-meters vary independently of the total contents of C, CaO +MgO, and Na2O + K2O, which indicate that the crystallizationof the ore minerals occurred associated with carbonate pre-cipitation and feldspar destruction.

Immobile substances associated with mineralization

High field strength elements (HFSEs): Elements such as Al,Ti, and HFSEs (e.g. Zr, Nb, and Y) are generally immobileduring metamorphism and hydrothermal alteration, whichallow them to be used to identify the chemical compositionsof original rocks (Gresens, 1967; Finlow-Bates and Stumpfl,1981; Grant, 1986; MacLean and Barrett, 1993; Winchesterand Floyd, 1997).

The diagrams of MacLean and Barrett (1993) allow rocks tobe identified from volcanic episodes in which there were dis-tinct magmatic differentiation (Fig. 12A), or alteration trendsin each chemically distinct rock unit (Fig. 12B-D). In Figure12A, the volcanic rocks of the footwall and of the hanging wallare grouped according to the slope of their trend lines, whichis consistent with their distinct stratigraphic positions andwith the fact that they may correspond to two different vol-canic episodes. The CTRh and CTTC rocks, which host themineralized bodies, and the recrystallized rocks (TRR, TAR,BR, and CR) are well correlated (R2 = 0.94) and indicate a


0361-0128/98/000/000-00 $6.00 793

TABLE 1. Average Concentrations of Elements and Substances from Lithology Related to the Aripuanã Deposit

Lithologyno.1 1 2 3 4 5 6 7 8 9 10 11 12

Number of samples 6 6 7 10 8 7 4 4 5 4 3 5

% SiO2 76.76 70.7 73.01 75.62 75.4 65.49 53.61 1.46 17.77 53.91 39.76 41.55s.d. 4.71 3.15 1.27 1.52 6.57 9.48 10.9 0.9 7.13 2.75 4.4 6.83% Al2O3 11.58 14.03 13.48 12.48 11.71 11.76 14.50 0.04 1.77 2.83 0.85 7.63s.d.% MgO 0.44 0.51 0.39 2.59 1.5 1.57 6.49 22.07 13.02 19.64 17.51 17.69s.d. 0.28 0.2 0.14 0.99 1.2 1.12 3.15 7.25 4.81 2.31 1.47 7.24% CaO 0.83 1.53 1.35 0.01 0.25 5.01 10.14 23.28 28.97 11.6 1.59 7.9s.d. 0.64 2.21 0.3 0.01 0.13 4.77 2.27 13.4 7.51 0.51 0.39 <LLD% Na2O 2.39 3.61 3.88 0.08 2.29 1.54 1.32 0.02 0.04 0.12 0.03 0.07s.d. 1.19 2.09 0.95 0.02 2.14 1.69 0.75 0.01 0.02 0.11 0.01 1.48% K2O 4.58 5.58 4.14 3.6 4.2 3.69 1.82 <LLD <LLD 0.34 0.48 2.87s.d. 1.01 2.36 1.14 0.83 1.49 2.12 1.05 <LLD <LLD 0.62 0.5 <LLD% Total C 0.04 0.07 0.02 0.02 0.07 1.14 0.86 12.88 7.46 0.43 0.05 0.95s.d. 0.02 0.07 0 0.01 0.04 0.95 0.4 0.81 2.62 0.29 0.04 1.34ppm Zn 29 10 19 12 46 260 296 31 51328 1431 33607 40017s.d. 32 5 11 11 55 313 438 15 79899 1863 8219 94ppm Pb 3 2 9 2 33 478 102 22 19163 235 4505 109s.d. 2 1 5 1 58 643 109 37 37893 205 4498 66ppm Cu 3 2 4 31 12 43 27 5 720 4 43 43s.d. 1 1 3 46 27 44 28 10 1512 1 31 1ppm Ag <LLD <LLD <LLD <LLD 0.2 1.9 1.6 0.3 46.9 0.9 9.5 0.6s.d. <LLD <LLD <LLD <LLD 0.2 0.3 0.1 <LLD 79.7 0.7 7.3 0.6ppm Au <LLD <LLD 2 <LLD <LLD 14 6 4 44 3 17 2s.d. 0.3 <LLD 4.8 <LLD <LLD 1 4.8 <LLD 45.1 3.4 9.5 29.5ppm As 1 20 1 1 61 83 14 29 49 11 18 78s.d. 0.4 51.7 0.7 0.1 169.3 136.9 17 38.3 40.6 20.7 28.3 140.9ppm F 722 881 231 1031 (3170) (5110) 5660s.d. 564 299 197 317 3121ppm REE 15.4 15.3 21.9 12.0 10.2 16.4 14.5 1.0 4.2 4.3 1.6 6.6s.d. 26.4 21.8 36.1 18.5 19.2 26.5 23.9 1.5 6.8 6.2 2.9 10.1ppm Eu 0.99 1.2 1.4 0.3 0.7 1.2 1.5 1.0 1.0 1.2 0.4 1.2ppm Sr 98.7 46.2 128.5 2.3 16.4 35.4 108.3 52.0 56.0 8.8 1.4 9.1

Notes: LLD = lower limit of detection, s.d. = standard deviation, (XXXX) = only one sample analyzed1 Lithology numbers: 1 = regional red metarhyodacite and metadacite, 2 = gray caldera metarhyolites, 3 = caldera granites, 4 = footwall least altered gray

rhyolitic ignimbrite, 5 = hanging-wall gray silicified metamorphosed rhyolite and dacite, 6 = metaturbidites, meta-rhythmites and metacherts, 7 = carbon-ate metachert, metaturbidite, and metarhythmites, 8 = massive marble, silicic marble, and carbonate chert, 9 = CTTC rock, 10 = tremolite-rich rock, 11 =fluoritic talc-rich rock, 12 = Fluoritic biotite-rich rock

line of independent evolution, suggesting either that theyhave a magmatic affinity that is different from those of thevolcanic rocks, or that they were generated by independentprocesses.

The relationships between incompatible and compatible elements (i.e., Zr vs. TiO2), and compatible and compatibleelements (i.e., Al2O3 vs. TiO2) produce trends that separatechemically distinct rock units and are useful for identifyinghomogeneous rock units that may be on the interior or thatcross the hydrothermal alteration zones (MacLean and Bar-rett, 1993). In Figure 12B, C, and D, the alteration lines ofthe metarhyolites, metadacites, and metatuffs of the hangingwall and of the footwall are different from those of the mar-bles (Fig. 12C), the CTTC rocks, and the TRR-, TAR-, BR-,and CR-rocks (Fig. 12B-D), which indicate that they were altered by distinct processes.

Rare earth elements (REEs): Normalization to chondriticvalues (Fig. 9B-J) is the most appropriate for discussing the

REE contents of the volcanic rocks and for showing that therocks of the footwall and of the hanging wall (Figs. 9I and J)have similar REE distribution patterns. In these profiles, thelight rare earth elements (LREEs) were enriched relative tothe heavy ones (HREEs). There is a small variation of HREEs,and concentrations are 20 to 30 times greater than those ofthe chondrites. There are also prominent negative anomaliesof Eu, probably due to plagioclase removal.

From turbidites and rhythmites to marble, as the propor-tions of carbonate and/or clastic increases, there is a gradualdecrease in REE content, from 10 to 100 times greater thanchondrites in turbidites and rhythmites, until 1 to 10 timesgreater in marbles. Additionally, there is a gradual change ofthe Eu anomaly from turbidites and rhythmites (and volcanicrocks), with strong negative Eu anomaly (Fig. 9H-J), to themarbles, with strong positive Eu anomaly (Fig. 9B).

Therefore, volcanic rocks of the footwall exhibit a negativeEu anomaly but, above the footwall, the Eu anomaly became

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1

2

3

4

5

6

7

8

9

10

11

12

Hanging wall volcanics

Regional volcanics

Caldera volcanics

Footwall volcanics

Carbonatic meta-turbidites and meta-rhythmitesFluoritic carbonatemeta-cherts, meta-turbidites and rhythmites

Marbles

CTTC rock

Fluoritic tremolitite-rich rock


Fluoritic biotite-richrock

Caldera granites

LITHOTYPE - NUMBER

% S O2iM % gO + %CaO

10 x %N 2O+%K2O)( a C10 x %total

(ppb)Au As (ppm)

(ppAg m)Eu average (ppm)

eSr av rage (ppm)Al2O3 average (%)F (ppm)

Pb (ppm)C p )u ( pm

Z p )n ( pm ΣREE average (ppm)

1

2

3

4

5

6

7

8

9

10

11

12

Outside mineralized zone

Hosting rocks

1 10 102

10-1

103

10-2

D

Mineralized zone

1

2

3

4

5

6

7

8

9

10

11

12

1 10 102

104

105

103


Hosting rocks

B

Mineralized zone

60 100 140200

Hosting rocks


A

Mineralized zone

1

2

3

4

5

6

7

8

9

10

11

12


Hosting rocks

C

Mineralized zone

1 10 102

10-1

FIG. 10. Variation in the contents of: (A). SiO2, MgO, CaO, total C, Na2O, K2O. (B). Zn, Pb, Cu, F. (C). Ag, Au, As. (D).ΣREE, Eu, Sr, and Al2O3 of the regional, mineralized, and hostess rocks related to the Aripuanã deposit.

more positive reflecting more carbonate content in the rocks,although the Eu concentrations, next to 1 ppm, are constants inall rocks, and only the footwall volcanic and TAR have low con-centrations of Eu. The same behavior is shown in Figure 10D,in which the trends of ΣREE and Al2O3 contents are almostparallel, for rocks outside and inside the deposit, which corrob-orates the direct relationship between the REE and rock clas-tic contents. The trends of Eu and Sr contents are also almost

parallel, both in the mineralized zone and among the hostrocks, suggesting that Eu substitutes for Ca in carbonate cells.

All rocks exhibit a positive Ce anomaly based on seawaterREE normalization (Fig. 11A-D), suggesting that the total con-tents of REEs in the carbonate rocks reflect a seawater source.

Geochemistry of fluorine

Microcrystalline fluorite associated with microcrystallinequartz and carbonate occurs disseminated in the TRR, BR,and TAR rocks, metacherts and epiclastic metasediments.The highest fluorine contents, on the order of 2,000 to 8,000ppm (Figs. 10B, 13), occur in BR, TAR, and TRR rocks, and,above all, in metacherts, which contain locally up to 10% flu-orite (Fig. 5M). With the exception of marbles, fluorine con-tent seems to increase with carbonate (Ctotal) content (Fig.13), suggesting a relationship between the crystallization offluorine and carbonate.

Isotopic Geochemistry of the Carbonate Rocks

Composition of the carbonates

Table 2 presents the chemical compositions and the δ13Cand δ18O values of the carbonates divided on the basis of min-eral composition and rock texture. The carbonates are pre-dominantly ferroan and ankeritic dolomite, with 42 to 55 mol% of siderite and magnesite (Fig. 14). Some of the silicic mar-bles, carbonate cherts, massive epiclastic rocks with carbonateblebs, and mineralized rocks have calcite with less than 12%siderite + magnesite. All of the facies of the carbonate rockshave dolomite and calcite, with the exception of some translu-cent and massive marbles, which are dolomitic or ankeritic,and the carbonate blebs, which are composed only of calcite.

δ13CPDB and δ18OSMOW values of the carbonates

The δ18OSMOW values, between 8 and 13‰, and the δ13CPDB

values, between +1 and −7‰ (Fig. 15A), of the rocks andores (Table 2) overlap the published domains of the VHMSdeposits (Huston, 1999) and magmatic carbonates (Ohmoto,1986, 1996). These compositions are restricted to the part ofthe VHMS field outside of the field of the hydrothermal car-bonates of the Besshi-type deposits (Peter and Scott, 1999),and are distinct from the field of the Cambrian marine car-bonates (Vezier and Hoefs, 1976). The field occupied by theAripuanã carbonates (Fig. 15B) is superimposed among thefields for Thalanga (Hermann and Hill, 2001), Rosebery andHercules (Khin Zaw and Large, 1990), Horne (MacLean andHoy, 1991), Buchans (Kowalick et al., 1981), and Mount Mor-gan (Golding et al., 1993), whereas the carbonates of WestBergslagen (Groot and Sheppard, 1988), Mount Chalmers(Huston, 1999), Afterthought (Eastoe and Nelson, 1988),Franklin Marble and Sterling Hill (Peck et al., 2009) have iso-topic compositions close to those of Aripuanã. All the abovecited deposits have been interpreted as related to submarineenvironments.

Relationships among the facies of the carbonate rocks and their δ13C and δ18O values

Various facies of carbonate rock were identified and theirδ13C and δ18O values are presented in Table 2. The CTRh areclastic-chemical rocks with values of δ13CPDB less than −4.6‰,


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0.01

0.1

1

10

100

Carbonate meta-turbidite and meta-rhythmite

Fluoritic meta-turbidites and meta-rhythmites

0.01

0.1

1

10

100

1000

Massive marble

CTTC rocks

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

1000

La Sm Gd Dy Er YbCe Nd Eu

A

D

C

B

Fluoritic tremolite-rich rockFluoritic talc-rich rock

Fluoritic biotite-rich rockChlorite-rich rock

1000S

ampl

e/S

eaw

ater

Sam

ple/

Sea

wat

erS

ampl

e/S

eaw

ater

Sam

ple/

Sea

wat

er

+

Carb

onate

+ C

last

ic

FIG. 11. Normalized REE diagrams of the Aripuanã deposit rocks. Con-tents of REEs were normalized by the average composition of seawater, asper Elderfield and Greaves (1982). Note that carbonate-rich rocks haveΣREE similar to seawater and that all rocks have Ce positive anomalies.

which places their carbonates in high-temperature domain(cf. Fig. 16A with 15A). In contrast, banded and massive mar-bles facies have similar ferroan dolomite compositions andisotopic values between −2 and zero (Fig. 16A). The veinletsand twisted fragments of carbonate breccias are also com-posed of ferroan dolomite and their δ18OSMOW and δ13CPDB

values coincide with the values of the marble, rocks withwhich it is always associated (Fig. 16A), which suggests thatmarble and breccias formed simultaneously, in low-tempera-ture conditions (Fig. 15A). Therefore, if the δ18OSMOW andδ13CPDB values of the carbonate breccias (Fig. 16B) overlapthose from the marbles (Fig. 16A, F), and the lowest δ13CPDB

values of the breccias (Fig. 16B) are equal to the highest val-ues of the carbonate turbidites (Fig. 16A) and cherts (Fig.16D), than these values, and the stratigraphy sequence ob-served in Figure 8, suggests that the marbles, carbonate brec-cias, chert, and carbonate turbidites were formed sequen-tially, in this order, in similar environments, while the

temperature of the hydrothermal fluid increased and the δ13Cvalues of these rocks changed from −5‰ to zero (Fig. 15A).TRR and CR rocks also have the δ13CPDB values between −7and −5, in the domain of magmatic carbonate, but these rocksare metamorphic, recrystallized 200 Ma later (Fig. 2), and donot belong to volcanosedimentary sequence.

Herrmann and Hill (2001) analyzed rhombohedral carbon-ates from Thalanga that had δ13CPDB and δ18OSMOW valuesthat differ from those of Aripuanã (Fig. 16B), but the carbon-ate spheroids of Aripuanã are similar to those of Thalanga, inthat they were described as spheroids and were considered tobe of hydrothermal origin, formed by the same process thatgenerated the rhombohedral carbonate crystals (Herrmannand Hill, 2001). Only two spheroids were analyzed, and theyhave δ18OSMOW and δ13CPDB values that are different fromthose of the Thalanga deposit (Fig. 16C). The CTTC rocksare the most common carbonate rocks of the Aripuanã de-posit and similar rocks form lenses that are associated with

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A

0

5

10

15

20

25

0 50 100 150 200 250 300 350

Nb

(pp

m)

Zr (ppm)

= 0

R.94

2

0R = .562

R = 0.0362

R = 0.64

2

R = 0.90

2

B

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400

TiO

2 (%

)

Zr (ppm)

Marbles

Hanging-wall gray meta-rhyolite, meta-dacite and meta-tuffs

Footwall gray meta-ignimbrites and meta-tuffs

Meta-turbidites and meta-rhythmites

Fluoritic meta-turbidites and meta-rhythmites

CTTC rock



Fluoritic biotite-rich rock

Chlorite-rich rock

R 0.9

=8

2

R = 0.92 2

0

5

10

15

20

0 100 200 300 400

Al 2O

3 (%

)

Zr (ppm)

C

R = 0.87 2

R = 0

.58

2

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20

TiO

2 (%

)

Al2O3 (%)

D

FIG. 12. Mass ratio diagrams of immobile elements of rocks of the Aripuanã deposit. (A). Zr vs. Nb. (B). Zr vs. TiO2. (C).Zr vs. Al2O3. (D). Al2O3 vs. TiO2. The regression lines and the corresponding correlation coefficients were fitted for themetaignimbrites and metatuffs located below the mineralized bodies (dotted), for the volcanic rocks that host the mineral-ized bodies (dashed) and for the marbles, CTTC rocks and recrystallized rocks (continuous line). Arrows indicate directionof trend evolution with mass loss during alteration.

the mineralized body west of Thalanga (Herrmann and Hill,2001). They are composed of mixtures of ferroan dolomite(Fig. 14), chlorite, tremolite, talc and, less commonly, biotite.

The greatest accumulations of mineralization and thelargest horizons of massive mineralization occur in rocks withtalc, chlorite, ankerite, and ferroan ankerite (Fig. 14). Thecarbonates of the Zn-Pb-Fe and those of the Fe-Cu-Zn andFe (Cu, Zn) mineralization types define two fields (Fig. 16E)that overlap those of all of the other rocks (Fig. 16F), exceptfor the quartz-feldspar massive epiclastic metasedimentaryrocks with carbonate blebs. The fields of the δ18OSMOW andδ13CPDB values from the Thalanga ore carbonates do not coin-cide with those of the Aripuanã mineralized rocks (Fig. 16E).Finally, values of δ18O greater than 11‰ distinguish carbon-ate blebs of massive epiclastic rocks from other clastic-chem-ical facies (Fig. 16A).

Isotopic Modeling

Carbonation

Aripuanã carbonates are characterized by a larger variationof δ13C (−6.5 to +1.0‰) when compared to δ18O values(8.4−11.5‰), which explains the elongated data distribu-tion depicted in Figure 15A and B. The exception is a smallgroup of samples that have δ18O values above 11.5‰ (Table2, Fig. 17B). The larger variation of δ13C values of Aripuanãcarbonates is striking because carbon mineral-fluid isotope


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CTTC rock

Meta-turbidites, meta-rhythmites and meta-chert

Fluoritic meta-chert, meta-turbidite and meta-rhythmite-

Massive and silicic marbles and carbonaceous chert



Fluoritic biotite-rich rock

1

10

1

Tota

l C (m

oles

)

F (moles)10-4 10-3 10-2

10-3

10-2

10-1

10-1

FIG. 13. Diagram of moles of fluorine vs. moles of total C (= total car-bonate in the rocks) of the fluorite-bearing rocks.

Massive-milky marbleTransparent marbleBanded marbleSilicic marbleMilky-carbonate rhombohedronTransparent rhombohedronArgillic-carbonate chertCarbonate flakeMilky-carbonate glomeruleTwisted silicic-carbonate fragmentCarbonate zone - Dexheimer et al. 2005Ore zone - Dexheimer et al. 2005Tremolite zone - Dexheimer et al. 2005

MgCO3

Ferroanankerite

FerroandolomiteAnkerite

Sideroplesite Pistomesite Breunnerite

Calcite

Mg:Fe<1 Mg:Fe>4

CaCO3

Calcite

FeCO3+MnCO3

Siderite+Rhodochrosite Magnesite

Dolomite(Ca, Mg)(CO3)2

Ankerite(Ca, Mg)(CO3)2

FerroandolomiteAnkerite

FIG. 14. Composition of the carbonate rocks ofthe Aripuanã deposit. Also shown are the contentsanalyzed by Dexheimer Leite et al. (2005).

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TABLE 2. Carbon and Oxygen Isotope Data of Carbonates from the Aripuanã Zn, Pb, Ag (Cu, Ag, Au) Deposit

Drill hole ID-depth (m) 18OPDB 18OSMOW 13CPDB (Ca,Mg,Fe,Mn)CO3

1. Clastic-chemical fácies

1.1. Carbonate-quartz-feldspar metaturbidites and meta-rhythmite

F-27-555.70 −21.11 9.15 −4.57F-27-570.40 −21.07 9.19 −4.38FPAR-09-144.60 −21.12 9.14 −5.51F-55-549.15 −21.06 9.2 −5.78 (Ca50Mg12FeMn38)CO3

FPAR-05-104.50 −20.82 9.45 −5.66 (Ca68Mg7FeMn25)CO3

FPAR-11-156.60 −20.43 9.85 −5.58

1.2. Quartz-feldspar massive epiclastic metasedimentary rock with carbonate flakes

F-43-105.60 −19.13 11.19 −2.81F-43-105.60 −18.86 11.47 −3.11F-43-105.40 −18.99 11.33 −2.35FPAR-68-82.70 −18.98 11.34 −2.62FPAR-68-213.60 (A) −19.47 10.84 −3.47 (Ca88Mg5FeMn7)CO3

FPAR-68-213.60 (B) −19.27 11.04 −2.94FPAR-68-213.60 (C) −19.52 10.79 −3.16

2. Chemical facies

2.1. Banded marble

FPAR-14-231.90 (A) −20.61 9.66 −1.09 (Ca55Mg39FeMn6)CO3

FPAR-14-231.90 (B) −20.56 9.71 −2.37FPAR-49-76.65 (A) −20.2 10.09 −1.36 (Ca54Mg41FeMn5)CO3

F-50-625.25 (A) −20.63 9.64 −1.35 (Ca55Mg40FeMn5)CO3

F-50-625.25 (B) −20.63 9.64 -0.96F-24-466.90 (A) −18.78 11.55 −1.18 (Ca54Mg42FeMn4)CO3

FAREX-35-184.45 (A) −20.34 9.94 −1.83 (Ca53Mg30FeMn17)CO3



2.2. Massive marble and massive carbonate chert

F-21-375.70 −20.55 9.72 -0.09FAREX-35-184.45 (B) −20.2 10.09 −4.66 (Ca51Mg37FeMn12)CO3

FPAR-49-76.65 (B) −20.08 10.21 −2.08 (Ca54Mg39FeMn7)CO3

FPAR-11-172.70 −21.8 8.44 −4.55 (Ca91Mg1FeMn8)CO3

F-21-371.30 −20.39 9.89 -0.89FPAR-06-282.50 (A) −20.32 9.96 −3.19 (Ca56Mg32FeMn12)CO3


3. Breccia facies

3.1. Breccia with twisted carbonate fragments

F-21-366.50 −20.41 9.87 −1.06FPAR-68-213.60 −20.05 10.24 -0.96FPAR-09-242.85 (B) −20.17 10.12 −1.5 (Ca48Mg37FeMn15)CO3

FPAR-09-242.85 (D) −20.22 10.06 −1.25 (Ca52Mg36FeMn12)CO3

FPAR-09-242.85 (E) −20.24 10.04 −1.92FPAR-09-248.40 −20.06 10.23 -0.11FPAR-09-359.70 (B) −20.21 10.08 −1.47 (Ca46Mg40FeMn14)CO3

FPAR-09-359.70 (C) −19.99 10.3 −1.86FPAR-09-362.80 (A) −19.89 10.41 −2.21 (Ca49Mg41FeMn10)CO3

FPAR-09-362.80 (B) −19.68 10.62 −1.06FPAR-09-362.80 (C) −20.07 10.22 −1.41 (Ca52Mg41FeMn7)CO3

FPAR-09-362.80 (D) −19.61 10.69 −1.43 (Ca52Mg38FeMn10)CO3

FPAR-09-362.80 (E) −20.09 10.2 −2.35FPAR-11-327.40 −19.86 10.44 −1.73FPAR-20-31.00 −20.19 10.1 −1.92FPAR-06-349.50 −20.05 10.24 -0.96

3.2. Mosaic breccia with carbonate-chlorite-tremolite

FPAR-04-389.70 −20.37 9.91 −1.24FPAR-06-350.40 −20.11 10.18 -0.82FPAR-70-335.50 −20.35 9.93 -0.62


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3.3. Breccia with rhombohedral carbonate-chlorite (tremolite)

F-50-625.25 (C) −21.01 9.25 −4.06 (Ca53Mg34FeMn13)CO3

F-50-625.25 (D) −20.85 9.42 −3.35FPAR-17-224.40 (A) −21.07 9.19 −5.11 (Ca57Mg31FeMn12)CO3


FPAR-17-224.40 (C) −21 9.26 −4.83F-24-466.90 (B) −19.75 10.55 −3.78 (Ca22Mg58FeMn20)CO3

F-24-466.90 (C) −19.34 10.97 −2.76 (Ca3Mg69FeMn28)CO3

4. Breccia-like facies

4.1. Mixed carbonate-tremolite- talc-chlorite (biotite) rock (CTTC rock)

F-24-402.50 −20.35 9.93 −2.13F-27-542.60 −20.36 9.92 −2.01FAREX-35-184.45 (C) −20.41 9.87 −3.85 (Ca50Mg34FeMn16)CO3

FPAR-09-274.25 −20.18 10.11 −1.61F-24-466.90 (D) −19.25 11.06 −2.76

4.2. Cauliflower-like clumps carbonate spheroids with chlorite (tremolite + talc) sphalerite + pyrite) matrix

FPAR-06-261.65 −20.88 9.38 −3.27FPAR-06-261.60 −20.75 9.52 −1.95

5. Talc-tremolite (carbonate, fluorite) facies

F-27-512.55 −21.08 9.18 −6.5F-27-535.95 −20.57 9.7 −5.09

6. Quartz-fluorite rock with tremolite-talc-carbonate nodules

FPAR-14-300.40 −20.18 10.11 −5.21FPAR-70-297.55 −21.01 9.25 −4.98F-54 - 277.70 −21.47 8.78 −5.03 (Ca89Mg6FeMn5)CO3

FPAR-06 - 432.25 −20.74 9.53 −5.05

7. Biotite breccia with quartz, feldspar, and carbonate

F-55-528.70 −20.86 9.41 −5.6FPAR-06-182.50 (A) −20.84 9.43 −5.64FPAR-06-182.50 (B) −20.41 9.87 −5.42

8. Ore facies

8.1. Carbonate-fragment breccia with massive sphalerite (+ pyrite + chalcopyrite)

FAREX-13-86.70 −17.59 12.78 −1.02FPAR-09-419.00 −18.16 12.19 −1.43

8.2. Breccia with twisted carbonate fragments and massive sphalerite (galena, chalcopyrite) (+ pyrite + chalcopyrite)

FPAR-09-242.85 (C) −20.36 9.92 −3.69 (Ca48Mg33FeMn19)CO3

FPAR-11-301.40 −20.06 10.23 −1.58FPAR-11-334.80 −19.82 10.48 −1.91FPAR-09-362.80 (E) −19.68 10.62 −1.06

8.3. Mixed chlorite-tremolite-talc-carbonate rock (CTTC) with sphalerite + galena

F-49 424.90 −19.92 10.38 −2.64FAREX-13-75.50 −19.92 10.37 −2.71FPAR-06-282.50 (A) −20.32 9.96 −3.19 (Ca56Mg32FeMn12)CO3


FPAR-14-250.80 −20.08 10.21 −3.87FAREX-35-185.20 −20.4 9.88 −4.11

8.4. Mixed chlorite-tremolite-talc-carbonate rock with pyrrhotite + chalcopyrite (+ magnetite)

FPAR-09-236.70 −20.2 10.09 −2.28FPAR-09-331.05 −20.38 9.9 −3.57F-49-381.90 −20.86 9.41 −5.14FPAR-06-348.10 −20.49 9.79 −2.77

8.5. Biotitite with tremolite + carbonate and magnetite

FPAR-06-160.90 −19.96 10.33 −5.95F-27-519.50 −21.65 8.59 −6.39

TABLE 2. (Cont.)

Drill hole ID-depth (m) 18OPDB 18OSMOW 13CPDB (Ca,Mg,Fe,Mn)CO3

fractionation is much less dependent on temperature thanthat of oxygen (O’Neil et al., 1969). In contrast, carbon iso-topic fractionation between calcite and CO3

– is nearly constantat 1.5‰ with temperature between 25° and 100°C (Mook etal., 1974), whereas the carbon isotope fractionation betweencalcite and CO2 varies between +3.8 and −2.9‰ for temper-atures ranging from 100° to 300°C, respectively (Chacko et al.1991).

The carbon and oxygen isotope composition of Aripuanãcontrasts with other carbonate-bearing hydrothermal systemsin which there is a much larger variation of oxygen isotopesrelative to carbon (Pineau et al., 1973; Clayton and Steiner,1975; Muehlenbachs and Clayton, 1976; Gregory and Taylor,1981; Ito and Clayton, 1983; Zheng and Hoefs, 1993; Santosand Clayton, 1995). The larger δ18O variations have been re-lated to fluid-rock interactions combined with the strong de-pendence of mineral-fluid oxygen isotope fractionation ontemperature.

The carbon and oxygen isotope compositions of hydrother-mally formed carbonates are controlled mainly by fluid composition and temperature. In most instances, fluids arecomposed mostly of H2O and variable proportions of CO2,meaning that oxygen is usually much more abundant thancarbon. The limited variation of the oxygen isotope composi-tion of Aripuanã carbonates suggests a narrow temperaturerange in the environment of the hydrothermal system. Thisnarrow temperature range hardly explains the isotopic varia-tion in carbon, because the calcite-HCO3

– fractionation factoris not very dependent on temperature. Thus, it is likely thatthe δ13C values of the Aripuanã carbonates are related to the

mixture of fluids with different carbon isotope composition,as for example, a fluid of magmatic origin (δ13C ≈ −5‰) andanother of marine origin (δ13C ≈ 0‰).

To better understand the precipitation mechanisms of theAripuanã carbonates, the isotopic composition of these car-bonates may be modeled under different conditions.

1. The carbonates were precipitated at temperatures be-tween 100° and 500°C in isotopic equilibrium with an infiniteamount of a CO2-enriched aqueous solution (a high fluid/rockratio) derived from an acidic magma (δ18O = +6‰ and δ13C= −5.5‰; Fig. 18). This model predicts that the isotopic com-position of the fluid phase depends on the CO2/H2O ratio aswell as on the temperature of the magma. Furthermore, theassumption of an infinite amount of fluid implies a rock/fluidratio close to zero; that is, the isotopic composition of the pre-cipitated carbonate depends only on the composition of thefluid and the temperature.

2. The carbonates were formed from a carbon dioxide andwater fluid that was outgassed following a Rayleigh process.In this kind of process, as pressure is released, an infinitesimalamount of fluid escapes, affecting the isotopic composition ofthe remaining fluid. Besides temperature, fluid composition(CO2/H2O ratio) and the different degassing rate of CO2 andH2O play a major role in the isotopic composition of the car-bonate being precipitated. Figure 19 presents the results oftwo scenarios. One (in solid line) models the composition ofcalcite being precipitated from a fluid with a CO2:H2O ratioof 2:3 in which the CO2 is released four times faster than theH2O. It shows that, for temperatures below 177°C, in which

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Aripuanã (Brazil) - This work Aripuanã (Brazil) - Dexheimer et al. (2005)

-8

-7

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0

1

0 5 10 15 20 25

Mt Morgan

Bergslagen

SouthHercules

Mt Chalmers

AfterthoughtARIPUANÃ

THALANGA

B

δ18OSMOW (‰)

Franklin marble

Sterling Hill

25-8

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0 5 10 15 20

VHMS Acidvolcanogenichydrothermal carbonate

Magmaticcarbonate

δ18OSMOW (‰)

δ13C

PD

B (‰

)

A

Cambrian marinecarbonate

Besshi maficvolcanogenichydrothermal carbonate

δ13C

PD

B (‰

)

Buchans

SouthHercules

FIG. 15. (A). δ12CPDB and δ18OSMOW values of the carbonates of Aripuanã compared with the domains of carbonates of theVHMS- and Besshi-type deposits, magmatic carbonates, and Cambrian marine carbonates. (B). δ13CPDB vs. δ18OSMOW do-mains of the carbonates of Aripuanã and Thalanga compared to those of VMS deposits that contain abundant carbonate.


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Carbonate meta-turbidite and rhythmiteQuartz-feldspar massive epiclastic sediment withcarbonate flake

Banded marble

Massive marble and massive carbonate chert

Fig. A - Clastic-chemical carbonate facies

Fig. A - Chemical carbonate facies

Breccia with rhombohedra carbonate

Mosaic carbonate - chlorite breccia

Carbonate breccia with twisted fragments

Thalanga - Rhombohedra carbonate-chlorite breccia

Fig. B - Volcanogenic breccia carbonate

Fig. C - CTTC and breccia-like facies

CTTC - Mixed chlorite-talc-tremolite-(biotite)carbonate rockCauliflower-like clumps carbonate spheroids

Thalanga-Cauliflower-like clumps carbonate spheroids

Fluoritic meta-chert with tremolite + talc + carbonatenodules

Talc-fluorite tremolite-rich rock with carbonate

Fluoritic biotite-rich rock with tremolite,carbonate (quartz, feldspar)

Fig. D - Fluoritic tremolite-, and biotite-rich rocks

Fig. E - Mineralized faciesCarbonate Fe-Cu-Zn-rich facies with py+po+cpy+mgt+sph

Carbonate Zn-Pb-Fe-rich facies with py+ sph+gal (+cpy)

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1

8 9 10 11 12 13C

Thalanga spheroids

CTTC and breccia- like facies

CTTC Mixedchlorite-tremolite talc-carbonate rock

Aripuanãspheroids

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8 9 10 11 12 13

Fluoritic tremolite-,and biotite-rich rocks

DTremolite-rich

rock 490-530°C

Fluoritic chert

Biotite-rich rock

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Clastic-chemical and chemical facies

Banded and massivemarble and chert

Carbonate turbidites

Epiclatic sediments with carbonate flakes

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B

Thalanga rhombohedral carbonates

Breccia facies

Twisted fragment and mosaic breccias

Aripuanã rhombohedralcarbonate andspheroids

8 9 10 11 12 13-7

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Thalanga ore

Mineralized facies

E

Carbonate Fe-Cu-Zn-rich facies with py+po+cpy+mgt+sph

Carbonate Zn-Pb-Fe- rich facies with py+ sph+gal (+cpy)

18δ O (‰)SMOW

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31 δC

(‰)

PDB

13 δC

(‰)

PDB

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F

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Tremoliterock (490-530 C)o

CTTC rock (440-490 C)o

Fluoritic chert andbiotite rock(350-450 C)o

VENT A

VENT B130 Co

177 Co

200 Co

FIG. 16. Diagrams of δ13CPDB vs. δ18OSMOW from carbonates of rocks in the Aripuanã deposit. (A). Clastic-chemical andchemical facies. (B). Carbonate breccias. (C). CTTC-pseudo fragmental carbonate rocks. (D). Fluoritic TRR, BR, and CTRhrocks. (E). Carbonate ores. (F). This figure shows the deposition sequence of the hydrothermal carbonate rocks of Aripuanã,deduced on the basis of field observations. The carbonate facies precipitated sequentially from the top to the bottom of thefigure. The δ13C and δ18O isotope fields of the carbonate facies (A-E) were superimposed on the isotopic models of carbon-ation (Fig. 17A, B) and decarbonation (Fig. 21B). The star has the same meaning as in Figure 21. The temperatures werecalculated (Dexheimer et al., 2005) using the chlorite geothermometer (Cathelineau, 1988; Zang and Fyfe, 1995).

the carbon isotope fractionation between calcite and CO2 ispositive (Chacko et al., 1991), the isotopic composition of thecarbonate being formed increases as the fluid outgasses. Sim-ilar results are observed for a fluid with a CO2:H2O ratio of1:4, in dashed lines, except that the carbonate isotopic com-position curves present a gentler slope.

3. The carbonates were precipitated in isotopic equilib-rium with an infinite amount of HCO-enriched aqueous fluid(a high fluid/rock ratio) with δ18O equal to 0‰, that is, withan isotopic composition of oxygen similar to that of seawater(Fig. 17), as suggested by the concentrations of REE of theoriginally chemical sedimentary rocks (marbles and CTTCrocks; Figure 12D). In contrast, the carbon isotope composi-tion of the aqueous fluid would be related to the mixture ofcarbon derived from two main sources: (1) the first, related toa higher temperature source, that would have δ13C valuessimilar to magmatic fluids (≈−5‰) and (2) another, related toa low-temperature source, that would have δ13C values closeto that of the sea (≈0‰). Similarly to condition (1), it is as-sumed an infinite amount of water thus implying a rock/fluidratio close to zero.

Decarbonation

Carbonate devolatilization under metamorphic conditionshas been widely discussed in studies that address skarn for-mation (e.g., Valley, 1986; Oliveira and Santos, 2003; Timón etal., 2007; Santos et al., 2009). During metamorphic processesthat release CO2 produced by carbonate and silicate reac-tions, the isotopic composition of the residual carbonate be-comes progressively more negative because under these tem-perature conditions the heavy carbon and oxygen isotopesfractionate to CO2 (Chacko et al., 1991). In spite of not hav-ing been a major process in Aripuanã, carbonate devolatiliza-tion may have affected samples from Aripuanã that containtremolite (CTTC rocks), because metamorphic reactions mayhave also produced CO2 during carbonate consumption. Inorder to better understand this process, we have modeled theisotopic evolution of calcite with initial δ13C of −2‰ and δ18Oof +10.3‰ that reacts with silicate minerals to produce tremo-lite and CO2 (Fig. 20). The model assumes that the devolatiliza-tion of CO2 follows a Rayleigh law and that the carbonate (its

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B177 C 130 C

200 C

2

0

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Marinecarbon

Magmatic carbon

Line

of m

ixin

g

A

18δ O (‰)SMOW

13 δ (‰

)C

PD

B

FIG. 17. Diagram modeling the isotopic composition of carbonates pre-cipitated in isotopic equilibrium with an infinite amount of HCO3

–-enrichedaqueous fluid (a high fluid/rock ratio), in which the oxygen isotope composi-tion is controlled mostly by seawater (≈0‰), whereas the carbon isotopecomposition is controlled by a mixture magmatic fluids (≈−5‰) and seawa-ter (≈0‰). In contrast to the model depicted here, the isotopic compositionof the precipitated carbonates depends both on temperature and on thesource of C.

100° 100° 100° 100° 100°

2:1 1:1 1:2 1:5 1:10

250° 250° 250° 250 250°

500°

0 2 4 6 8 10 12 14 16 18 20

2

0

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CCCCC

C CC C C

C

Increasing CO2/H2O

18δ O (‰)SMOW

13 δ (‰

)C

PDB

FIG. 18. Diagram modeling the δ13C and δ18O composition of carbonates precipitated at temperatures between 100° and500°C in isotopic equilibrium with an infinite amount of a CO2-enriched aqueous solution (a high fluid/rock ratio) derivedfrom an acidic magma (δ18O = 6‰ and δ13C = −5.5‰). Because we assume an infinite amount of fluids, the isotopic com-position of the precipitated carbonates depends only on temperature. The filled squares correspond to the isotopic compo-sitions of the Aripuanã samples in this figure and in Figures 19 and 20.

initial isotopic composition is represented by the star) is keptin isotopic equilibrium with the associated silicate minerals,which means that the total amount of carbon in the system isrestricted to the carbonate, while that of oxygen includes boththe carbonate and silicate minerals.

As shown in Figure 20, the isotopic composition of theresidual carbonate becomes progressively more negative be-cause of the positive isotope fractionation factor between CO2

and calcite at temperatures above ≈270°C. Curves A and Bcorrespond to two different proportions of carbonate to sili-cate minerals in the rock, ultimately reflecting the O/C ratioof the reactants. The numbers next to the curves representthe fraction of remaining oxygen and carbon of the reactantscarbonate plus silicates. For instance, the values of 0.8 and0.1 indicate, respectively, that around 20% of the oxygen and90% of the carbon were transformed into CO2. Because theisotopic composition of oxygen is buffered by the presence ofsilicates, it does not change as fast as that of carbon. Based onthe modeling, we argue that CO2 devolatilization related tometamorphism was not a major process in Aripuanã, even forrocks like CTTC and TRR.

Discussion

Carbonate in hydrothermal systems and seawater as a fluid source

Rocks from Aripuanã are dominated by felsic volcanic andplutonic rocks, and volcaniclastic sediments with low iron and

very low calcium and magnesium contents. Carbonate rockshave not been described in the area before, in spite of beinga major product of hydrothermal alteration. Ferroan dolomiteand ankerite ± calcite are present in almost all rocks, varyingfrom 98 to 5% in the following order: marble, CTTC, CTRh,TRR, TAR rocks and fluoritic carbonate, and BR chert (Fig.14). According to Mueller (2009), carbonates with composi-tions such as those shown in Figure 14 indicate that Aripuanãis similar to distal or medial deposits formed in volcaniccalderas, similar to the rock sequence dominated by felsicrocks, in the Normetal caldera (Abitibi, Canada).

Published literature on volcanogenic massive sulfide de-posits shows that volcanogenic deposits with large quantitiesof hydrothermal carbonate are uncommon, even if this min-eral is usually found in minor amounts in most deposits; forexample, Rosebery (Cambrian, Tasmania; Solomon and Wal-she, 1979; Huston and Large, 1988; Khin Zaw and Large,1990; Large, 1992; Orth and Hill, 1994); Mattabi (Archean,Sturgeon Lake region, Superior Province, Canada; Franklinet al.,1975); the Kuroku-type deposits (Afterthought-Ingotregion, Permian, East Shasta District, California, UnitedStates; Eastoe and Nelson, 1988); the various VHMS depositsof the Hokuroku district (Japan; Shikazono et al., 1998);Lewis Pond (Silurian, New South Wales, Australia; Agnew etal., 2005); Hellyer (Cambrian, Mount Read, Tasmania;


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+2

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8 10 12 14

100 100CO2:H2O = 2:3

CO2:H2O = 1:4

150

177

200

177

10%

100%

δ13C

BPD

(‰)

CO2:H2O = 1:4

2H

2

C

O:

O=

2:3

C

125 C

C

125 C

C

150 C

177 C177 C

C200 C

18δ O (‰)SMOW

FIG. 19. Diagram showing the isotopic composition of carbonates thatwere formed from a CO2 -H2O fluid that outgases following a Rayleighprocess. The two groups of curves represent different scenarios: one, in full,models a fluid with a CO2:H2O ratio of 2:3 in which the CO2 is released 4times faster than the H2O; and another, dashed, models a fluid with CO2:H2Oratio of 1:4 that is outgassed under the same conditions. The numbers closeto the curves represent the temperature, while the numbers within the blackrectangles represent the amount of remaining fluid along each curve. Iso-topic fractionation factors between calcite and CO2 were obtained fromChacko et al. (1991) and for calcite and water from O’Neil et al. (1969).

+2

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8 10 12 14

0.7:0.25

0.8:0.5

0.7:0.75

0.7:2.25

0.8:0.10

0.9:0.5

A

B

18δ O (‰)SMOW

13 δ (‰

)C

PDB

FIG. 20. Evolution of the isotopic composition of a rock composed of car-bonate and silicate that is subjected to metamorphism at 500°C. The curvesmodel a Rayleigh devolatilization of CO2 as tremolite is formed as a result ofa reaction between carbonates and silicates. The initial isotopic compositionof the carbonates is represented by the star (initial δ13C = −2% and initialδ18O = 10.3%). The composition of the residual carbonate evolved accordingto curves A and B, which refers to rocks with O/C ratios of 2.5/1 and 5/1, re-spectively. The numbers next to the points on the curves indicate the pro-portion of oxygen and carbon remaining in the reactants carbonate and sili-cates. The open squares represent samples with tremolite.

Bradley, 1997, in Herrmann and Hill, 2001; Gemmel andLarge, 1992); Mount Morgan (Devonian, Queensland, Aus-tralia; Ulrich et al., 2003); Mount Chalmers (Large and Both,1980); South Hercules (Cambrian, Australia; Large and Both,1980; Kin Zaw and Large, 1992; Large, 1992); the MiddleValley area, northern part of the Juan de Fuca ridge, wherethere are active hydrothermal systems that may be analogousto the ancient submarine hydrothermal systems in whichAripuanã carbonate was precipitated (Goodfellow et al., 1993;Baker et al., 1994; Fruh-Green et al., 1994); Horne (Archean,Quebec, Canada; MacLean and Hoy,1991) and Buchans (Or-dovician-Silurian, Newfoundland, Canada; Kowalick etal.,1981); the Bergslagen district (Paleoproterozoic, Sweden;Vivallo, 1985; Allen et al., 1996), and Thalanga (Cambrian-Or-dovician, North Queensland, Australia; Paulick et al., 2001;Herrmann and Hill, 2001). All of these are volcanogenic de-posits have carbonate, and their geology is comparable toAripuanã. Besides Aripuanã, major amounts of carbonaterock and of carbonate-talc-tremolite-chlorite paragenesishave been described in volcanogenic deposits only in Tha-langa. The main similarity among these deposits is that thecarbonate was precipitated near hydrothermal vents when thehydrothermal fluid mixed with seawater. In some other de-posits in which carbonates are present (e.g. Rosebery,Hellyer, and South Hercules), there is also a genetic relation-ship between the carbonates and the sulfide ore.

Field and petrographic studies indicate that the deposit ofAripuanã was formed in submarine back-arc volcanic calderalocated to the northeast of an ocean basin. Their characteris-tics are comparable to VHMS deposits formed on the oceanfloor, being locally composed of dacitic lavas and rhyolitic ig-nimbrites. The subcalderas, including the ore deposit, werefilled with cherts, turbidites, and rhythmites, subaqueous sed-iments with preserved sedimentary structures, such as up-ward-coarsening beds, laminated carbonate rocks, convolutefolds, flame structures delineated by fragments of sphaleriteand pyrite (Fig. 5), and banded marbles. The Aripuanã min-eralized rocks and mineralized bodies are hosted by theserocks which, below the stringer zones, grade into quartz-feldspar metadacites, and then, into metadacites, that do nothave carbonates. The CTTC rocks, spheroids, and carbonaterhombohedra from Aripuanã are also similar to carbonatelenses associated with the submarine mineralized body westof Thalanga (Herrmann and Hill, 2001).

The Aripuanã granite crops out about 5 km from the de-posit and no drill hole within the Aripuanã area intersectedgranite. The granite has low concentration of iron, magne-sium, and calcium and does not have any evidence of hydro-thermal alteration. If carbonates were crystallized by metaso-matism caused by an unknown granite body emplaced bellowthe deposit, carbonate alteration should be found also inrocks located between granite and the deposit, includingstringer zone and volcanic rocks below it, but this does nothappens.

Immobile elements (Fig. 12A) indicate that the volcanicrocks of the hanging wall and footwall of Aripuanã are part ofdifferent magmatic differentiation series. Moreover, the al-teration trend of hydrothermal rocks (from the marbles to thefluoritic metaturbidites) does not fit with trends of volcanicrocks, indicating that volcanic and hydrothermal rocks are not

genetically correlated and that volcanic rocks were not theprecursors of the hydrothermally altered rocks (in the senseof MacLean and Barret, 1993).

The trend defined by hydrothermal rocks is composed oftwo groups (Fig. 11B-D): (1) marbles, CTTC, TRR, TAR, BR,and CR rocks that are placed between the origin and the mid-dle of the trend and; (2) CTRh rocks that are placed betweenthe middle to the end of the trend. These figures show thatCTRh, BR, CR, CTTC, and TAR rocks and marble fit to a sin-gle alteration trend, with a correlation coefficient (R2) be-tween 0.87 (Fig. 12B) and 0.98 (Fig. 12C). According to thecriteria of MacLean and Barrett (1993), metaturbidite andmetarhythmite could be altered (= precursors) to BR, CR,CTTC, TAR rocks and marble, in this order, by hydrothermalactivity which due to the gain of mass.

The problems with this model are (1) as discussed above,neither the Aripuanã granite nor the footwall dacites and rhy-olitic ignimbrites could have been the main magnesium, iron,and calcium source for this alteration; and (2) for hydrother-mal carbonate to replace silicates, the Al2O3 of the silicatesmust be leached by hydrothermal fluids. Taking into accountthe lack of evidence for both these processes, only seawaterremains as possible source for carbonate cations.

The distribution of data seen in Figure 12 may be explainedif the line that extends from the marbles up to the carbonatemetaturbidites was formed by hydrothermal carbonate de-posited successively with decreasing quantities of carbonateand increasing quantities of clay minerals with a high contentof immobile elements (Al, Ti, and Zr). In this case, the mar-bles would originally have been hydrothermal precipitates devoid of, or with very little, clastic contribution. The CTTCrocks would have been mixtures of carbonates and clays, andthe BR and CR rocks would originally have been lenses com-posed essentially of smectite. Similarly, the TAR rocks wouldhave been derived from clastic-chemical siliceous-dolomiticsediments devoid of, or with, minor argillic contribution,whereas the TRR rocks would have been derived from Si-Ca-Fe-Mg-rich sediments (dolomitic marl). Contrary to the in-terpretation of a metasomatic alteration of a precursor (thecarbonate metaturbidites and metarhythmites), the modelproposed here suggests that the rocks of Aripuanã are mixingproducts of volcaniclastic and hydrothermal carbonate sedi-ments generated in various episodes of clastic-chemical sedi-mentation. This interpretation explains the morphology of thelithologic units and the sedimentary subaqueous structures de-scribed in the mineralized region and the entire deposit. Evi-dence that carbonate precipitated from seawater is also shownby isotopic, fluid inclusions, and REE geochemical studies.

The most common mechanism of carbonate precipitationin hydrothermal systems involves heating a solution at lowsalinities and/or temperatures, accomplished by mixing a coldwith a hot solution; degassing and boil CO2 from a solution inwhich HCO3

– is dominant over H2CO3, and increasing the pH(Rimstidt, 1997). Because CO2 plus H2O dissociate to H+ andHCO3

–, removing CO2 from solution raises the solution’s pH.Raising the pH causes HCO3

– to dissociate to H+ and CO32–

and the increased activity of carbonate causes carbonate min-eral precipitation. The δ18OSMOW and the δ13CPDB values ofthe rocks and ores (Fig. 15) overlap the domains of the manyVHMS deposits. Isotopic modeling has shown that the δ13C

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values of the Aripuanã carbonates may be related to a mixtureof magmatic (δ13C ≈ −5‰) and seawater fluids (δ13C ≈ 0‰).

The Eifuku volcano, located in the Marianas arc, in a placecalled Champagne, emits bubbles of liquid CO2, with approx-imately 98% CO2 and 1% of H2S (Lupton et al., 2006). Sakaiet al. (1990) reported the existence of a similar volcano situ-ated in the central part of the Okinawa trough. In this case theexhaled fluid is made up of 86% CO2, 3% H2S, and 11% ofother gases, mainly CH4 + H2. In the Eifuku volcano theCO2/H2O of the fluid is approximately 1:7, which is less thanthe 1:2 ratio of the curve of our model that best fits theAripuanã data (Fig. 18). Thus, the fluids vented from theAripuanã volcano may not have been formed solely by fluidsexhaled from an acid magma. Lupton et al. (2006) and Sakaiet al. (1990) do not report which minerals precipitate near thevolcanoes they have studied. In both cases, the temperaturesare low, below 10°C, and there is a layer of CO2 hydrates thatmakes the sampling of sediments around the vents difficult.In contrast to these studies, Khin Zaw and Large (1992) re-ported carbonate precipitates from CO2-rich fluids at SouthHercules (Tasmania). They reported carbonate precipitateswith amounts and features similar to those observed inAripuanã, including carbonate blebs and spheroids, as well ascomparable δ13C and δ18O values. Khin Zaw and Large pro-posed a model of mixing of fluids similar to that presented inFigure 17, and did not make any comment about theCO2/H2O ratio of South Hercules mineralizing fluid. Theyconcluded that “The presence of extensive carbonate alter-ation…indicates the ore fluids …resulted from mixing (ofmagmatic fluid) with seawater in the porous volcaniclasticsubsea floor.” Thus, based on the proposed carbonation mod-els, it is clear that a simple precipitation of carbonates in iso-topic equilibrium with CO2-enriched aqueous solution de-rived from magmatic fluids does not reproduce the observedvalues in Aripuanã.

Similarly, a model involving degassing of CO2-H2O fluidsmay fit part of the data, but seems not to be the major processthat controls the isotopic composition of Aripuanã carbon-ates. On the other hand, a model based on a mixture of fluidsfits the data much better, specifically for thermal gradientsbetween 177° and 200°, and 130° and 200°C. These thermalgradients would be related to temperature differences withinthe volcanic structure and imply that the carbonates wereformed within temperature intervals ranging from 30° to70°C. Thus, it is suggested that the Aripuanã carbonates wereformed from a mixture of fluids with distinct isotopic ratios ofcarbon, at temperatures between 130° and 200°C (Fig. 17A,B) and, as shown in the decarbonation modeling (Fig. 21), itis likely that the tremolites of the CTTC, TRR, and TAR rockscrystallized under the conditions of model B. Based on δ34Svalues of sulfides from Aripuanã, Dexheimer Leite et al.(2005) also concluded that the primary mineralizing fluidswere a mixture of evolved heated seawater and magmaticfluid with sulfur from the leached of volcanic rocks.

The positive Ce anomaly observed in all rocks normalizedto average seawater (Fig. 11) may be related to loss of dis-solved Ce due to sedimentation on the ocean floor, below theredoxcline (Planavsky et al., 2010). The depletion of LREEsobserved in the majority of samples from the Aripuanã min-eralized region could be related to leaching of reducing fluids

under high temperatures (MacLean, 1988; MacLean andHoy, 1991; Liaghat and MacLean, 1995), as observed for themarbles (Fig. 9B), CTTC (Fig. 9C), TRR and TAR (Fig. 9D,E), and BR rocks (Fig. 9F). Distributions patterns withLREE enrichment and positive anomalies of Eu (Fig. 9E)have been observed in vent fluids of the East Pacific Rise(Michard et al., 1983, Michard and Albarède, 1986), the Mid-Atlantic Ridge (Campbell et al., 1988), and in rocks located inhydrothermal discharges zones in which the temperature isup to 350°C. According to Barrett et al. (1991), the enrich-ment of LREEs could be related to primary precipitation ofsilica and carbonate from a fluid undergoing temperature de-crease and oxidation due to mixing with seawater. The distri-bution patterns of REEs and the relationships among thetrends of the contents of ore elements and Na2O + K2O indi-cate the same process.

As observed for chondrite normalized rocks (Fig. 9), thepositive Eu anomalies of carbonate rocks reinforce the pres-ence of hydrothermal fluids in the crystallization process ofthe carbonates (Elderfield and Greaves, 1982; Elderfield,1988). Furthermore, the total contents of REEs in the carbon-ate rocks are equal to the average composition of seawater(Figs. 9D, 11), suggest that the composition of the hydro-thermal fluid was dominated by seawater, from which car-bonate crystallized.

Fluorite in rocks of the Aripuanã deposit

There are no reports in the literature of VHMS base metaldeposits of documented submarine exhalative hydrothermalfluorite. Occurrences of fluorite are common among exhala-tive submarine deposits such as Rosebery (Khin Zaw et al.,1999), Henty and Mount Julia (Callaghan, 2001), but they areusually associated with magmatic and late metasomatic eventsfollowing volcanogenic mineralization.

In the Aripuanã deposit, cherts with nodules of tremolite-talc and carbonate and up to 10% of microcrystalline fluoriteoccur in BR rocks on the top of the hydrothermal rocks piles(Figs. 7, 8), and disseminated in TRR, BR, and TAR rocks.They have convoluted folds (Fig. 5C), which are typical struc-tures of subaqueous environments.

Seawater heating experiments show that fluorine is a par-ticularly reactive element and an important component in dif-ferent minerals over a wide range of temperatures (Seyfriedand Ding, 1995). Reactions of this sort occur during seawaterrecharge of subsea-floor hydrothermal systems, similar to thatinterpreted for Aripuanã. Seyfried and Ding (1995) haveshown that carbonate minerals precipitate from seawater between 150° and 225°C, under conditions where fluorineremoval occurs. It has also been long recognized that carbon-ate minerals can incorporate fluorine (Kitano and Okimura,1973; Rude and Aller, 1989, 1991). Furthermore, dissolved Mgand magnesite formation also contributes to fluorine removal(Ohde and Kitano, 1980), although the exact mechanism ofexchange between F– and CO3

2– still needs to be clarified(Seyfried and Ding, 1995). Data from Seyfried and Ding (1995)also shows that dissolved fluorine partition into solids at tem-peratures below 200°C, even if the fluid contains as little flu-orine as seawater. These conditions, in which the precipita-tion of carbonate and incorporation of fluorine occurs, aresimilar to those estimated in Aripuanã by carbon and oxygen


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A

Sea level

Dacites and rhyolitic ignimbrites

Sea level

Smectitic-silty mood

Stringer

Sea level

Marble

A

F

EN S

N S

N S

1.76 - 1.75 Ga 1.76 - 1.75 Ga

1.76 - 1.75 Ga

1,69 - 1,63 GaRegional folding andmetamorphic foliation 1.69 - 1.63 Ga

Sea levelC

Turbidite flowsand carbonate sedimentation

Ash fall

1.76 - 1.75 Ga

Sea levelD1.76 - 1.75 Ga

D

Gossan HillG

Aripuanãgranite (?)

ulton

Fa z

e

1.56 - 1.53 GaContact metamorphism and metasomatism

Redoxcline

RedoxclineB Sulfide and carbonatecrystallization and sedimentation within ocean floor mud

Redoxcline

Redoxcline

FIG. 21. Interpretation of the sequence of events that generated the Aripuanã deposit. (A). The beginning of the hydro-thermal exhalation, within the silty-smectite mud layer that covers dacites and rhyolitic ignimbrites of the submerged floorof a volcanic caldera, and simultaneous sedimentation of carbonate and ash-size pyroclastic rock fragments, forming a clas-tic-chemical sediment. (B). The first hydrothermal volcanogenic cycle, with the precipitation of massive and stringer miner-alized bodies. (C). Explosive volcanism and emission of turbidite flows and clouds of ash and tephra, simultaneously to thesedimentation of hydrothermal carbonate, forming laminated carbonate-silty turbidites and rhythmites. (D). Burial of themineralized bodies and the structures from the first cycle. (E). Second hydrothermal volcanogenic cycle and formation ofnew mineralized zones that formed lenses of mineralized rocks among the clastic-chemical carbonate turbidites and rhyth-mites. (F). Regional metamorphism at the low green-schist facies, rocks recrystallization, and deformation of the rocks andstructures. (G). Intrusion of granites, contact-metamorphism, and metasomatism of the rocks and mineralized bodies. Clas-tic-chemical carbonate rocks were transformed at CTTC rocks.

isotopes modeling (Fig. 17). Except for the marbles, Figure13 shows that the fluorine content of hydrothermal rocksfrom Aripuanã is associated with carbonate precipitation,similar to the model proposed by Seyfried and Ding (1995).The marbles from Aripuanã have the lowest average fluorinecontent probably because of the high pH conditions, whereasblack smokers with elevated fluorine concentrations, as thatof Manus Basin (Craddock et. al., 2010), develop in low pHenvironments.

The presence in the AMBREX area of four horizons of flu-oritic chert in distinct stratigraphic positions (Fig. 8) suggeststhat the deposit was formed by at least four distinct sedimen-tary exhalative episodes. The mixing of seawater and hydro-thermal fluorine-bearing fluids explains the precipitation offluorite in that stratigraphic position. On the other hand, iffluorine had been introduced by metasomatic process alone,it is not clear what may have triggered the four episodes offluorite precipitation. Because of the positive correlation be-tween fluorine and silica concentrations (Fig. 10B), the sameprocess may have controlled the precipitation of fluorite andmicrocrystalline quartz. Fluorite metacherts have the lowestδ13C values (Fig. 16D), and the hydrothermal carbonates ofthe CTRh rocks (Fig. 16A) fall within the same isotopic do-mains of the carbonates of the fluoritic metacherts, BR, andTRR rocks (Fig. 16D), thus suggesting that all these rockswere originally hydrothermal clastic-chemical sediments.

In spite of the correlation between fluorite and carbonate,there is no relationship between these minerals and the con-tents of Zn, Pb, Cu, Ag, and Au (Fig. 10B). This implies thatfluorine and these metals probably had different origins andthe precipitation of these elements may have been controlledby other processes, distinct from that which caused the co-precipitation of Zn, Pb, Cu, Ag, and Au.

Genetic ModelMixing of ash-size pyroclastic rock fragments, hydrother-

mal carbonate, and mud may explain the chemical and min-eralogical variations of Aripuanã rocks, including their REEcontents (Fig. 9). This scenario could have also caused thepositive Eu anomalies to vary according to the amount ofhydrothermal carbonate crystallized within the mud of thesea floor.

The positive correlations of Zn, Pb, Cu, Ag, and Au indicatethe coprecipitation of sphalerite, argentiferous galena, chal-copyrite, and gold during mineralization. In the JapaneseKuroko-type deposits (Ohmoto, 1983, 1996), the coprecipita-tion of metals occurs in submarine environments in which thetemperature is less than 250°C. This temperature decreasesrapidly due to the mixture of hydrothermal fluid with coldseawater, similar to the isotopic modeling of δ13C and δ18O ofcarbonate precipitation of Aripuanã.

Similar to carbonates, sulfides also precipitated in theocean-floor mud mixed with carbonate in various proportions.They formed mineralized sediments with different sulfide,carbonate, and clay concentrations, as observed in mineral-ized CTTC. In the Aripuanã deposit there is a metal zonation(Fig. 8) in which Fe-Cu sulfides (+ magnetite) are formed atthe base, followed by Fe-Cu (Zn-Pb) sulfides (+magnetite),and by Zn-Pb (Fe, Cu) sulfides at the top, similar to classiczonation observed in VHMS deposits. Zinc-, Pb-, and Ag-bear-ing sulfides precipitated from hydrothermal fluids at temper-ature lower than 280°C (Sato, 1973; Large, 1977; Hanningtonet al., 1999). Because the Aripuanã mineralized bodies are es-sentially composed of pyrite, sphalerite, and argentiferousgalena, it is likely that the deposition occurred at relativelylow temperatures (between 150° and 250°C), which is consis-tent with the temperatures modeled in Figure 17A and B.

The high correlation coefficients observed among metalcontents in samples of the AREX area suggest that there wasa single mineralization episode in this area and that the origi-nal metals distribution was not significantly modified, noteven by late thermal-metamorphism and/or metasomatism.The low correlations observed in the AMBREX area must bethe consequence of several episodes of mineralization (Fig.8), such as the “zone refining” process in which the most re-cent hydrothermal episode partially remobilized the metalsdeposited in the previous one. This could also explain the lackof a correlation, on a meter scale, between Cu and other moremobile metals as Zn and Pb. The preservation of the originaldistributions of metals in the AREX area indicates that thethermal metamorphism and metasomatism only caused therecrystallization of minerals.

The majority of the modeled carbonates underwent meta-morphism with tremolite and chlorite in a situation similar to


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Footwall dacite and rhyolitic ignimbrite

Altered quartz - sericite - chlorite rock

Stringer fractured dacite with sulfides

Twisted fragment carbonate breccia inside marble

Smectitic-silty mud

Carbonate veins and fragments with smectitic-silty mud matrixFluoritic biotite-rich rock and fluoritic meta-chert with tremolite + chlorite nodules

Fluoritic tremolite-, talc-tremolite-, and talc-rich rock

Carbonate meta-rhythmite and meta-turbidite

Footwall meta-dacite and meta-rhyolite

Aripuanã granite

Mineralized rock with py + sph + gal (cpy)Mineralized rock with py + po + mgt + cpy (sph,gal)Mineralized rock with py + cpy + mgt + po

Hanging wall rhyolitic and dacitic meta-rhythmite and meta-turbidite

Explosive rhyolitic and dacitic breccia

Massive chlorite veins with biotite porphyroblasts, pyrrhotite and chalcopyrite

VOLCANOGENIC SEQUENCE

Fluoritic-silicic and argillic mud

Carbonate rhythmite and turbidite

METAMORPHIC/METASOMATIC SEQUENCE

CTTC rock - Mixed carbonate-talc-chlorite-tremolite rock

FIG. 21. (Cont.)

that of Thalanga (Herrmann and Hill, 2001; Paulick et al.,2001). According to Slaughter et al. (1975) and Winkler (1976),this assemblage is produced in metamorphic conditions bythe reaction:

8 silica + 5 dolomite + H2O → 1 tremolite + 3 calcite + 7 CO2,

which occurs at temperatures close to 450°C. This is not con-sistent with the modeled crystallization of carbonate, but it isconsistent with the crystallization temperatures of chlorites ofCTTC rocks (between 380° and 530°C) calculated by Dex-heimer Leite et al. (2005) by means of chlorite geothermom-etry (Cathelineau, 1988; Zang and Fyfe, 1995). The isotopicmodels of carbonation (Fig. 17) and decarbonation (Fig. 21)allow for the consideration that the two processes occurred inAripuanã. Carbonation occurred at temperatures between130° and 200°C during the VHMS hydrothermal activity thatprecipitates carbonate along smectitic clay from the mud ofthe sea floor. Decarbonation probably occurred during the re-gional metamorphism at temperatures close to 500°C (Fig.21B) and crystallized tremolite only in the hydrothermalmarls, where there was carbonate and smectite, transformingthem into chlorite-(talc)-tremolite-carbonate shales.

The quantities of tremolite, talc, and chlorite formed duringthe regional metamorphism was controlled by the presence ofsmectite (= silica) and carbonate in the original sediments,and the carbonate must have kept the original volcanogenicisotopic signature because: (1) metamorphic reequilibrationwould have had to occur at temperatures close to 900°C,which is, as at Thalanga (Herrmann and Hill, 2001), unrealis-tic for the location of the deposit; (2) the preservation of pri-mary structures of carbonate-rich rocks, such as the rhombo-hedra of carbonate, the spheroids, and the twisted carbonatefragments, suggesting that the residual carbonate was not re-crystallized. If this is correct, the original δ18O and δ13C val-ues of the carbonates would have been preserved, thus beingconsistent with precipitation from a hydrothermal fluid simi-lar to seawater (δ18O ≈ 0‰) at temperatures between 130°and 200°C, which agrees with the model proposed here; and(c) as seen in Figure 16A and D, despite the coincidence amongthe isotopic δ18OSMOW and δ13CPDB values of TRR, CR, andCTRh, the CTRh rocks always have metamorphic foliation,which is lacking in the TRR and CR rocks, indicating that theregional and the contact metamorphism did not change theδ18OSMOW and δ13CPDB values of original carbonate.

The δ13C and δ18O fields of different carbonate facies areassembled in Figure 16F, overlying the isotopic models ofcarbonation (Fig. 17A, B) and decarbonation (Fig. 21B). Withthe information contained in Figure 16F and knowledge ofthe sequence of the deposition of the rocks of the deposit,based on the facies deposition sequence (Figs. 7, 8), a seriesof cartoons were designed aiming to summarize the interpre-tation of the sequence of events that generated the Aripuanãdeposit (Fig. 22).

The submarine exhalative hydrothermal activity began at1.76 to 1.75 Ga along a fracture on the floor of the mainAripuanã caldera below the redoxcline. The exhalation formedcarbonate precipitates at temperatures close to 180°C (Figs.16F, vent A, 17, 22A), together with ash-size pyroclastic rockfragments and clays mixed to the sea-floor mud (Fig. 22A)and also silicifying, sericitizing, and chloritizing dacites and

rhyolitic ignimbrites (Fig. 22B). A vent system developed pre-cipitating massive sulfides (py + cpy + mt ± sph) and carbon-ates over the center of hydrothermal calcareous bodies. Talusbreccias and brecciated rocks, composed of fragments of cal-careous rocks with rhombohedra and carbonate spheroids,were formed in the upper and lateral parts of the hydrother-mal vents. During the waning stage of the exhalative cycle,carbonate cherts were precipitated, which were locally cov-ered by carbonate epiclastic sediments (Fig. 22B).

The volcanism of the caldera gained energy and increasedin temperature and evolved from exhalative to explosive (Fig.22C), while the fractures still exhaled carbonic fluid. Thehydrothermal system was covered by turbidites and rhyth-mites interlayered with sedimented hydrothermal carbonateand, after the end of the first exhalation cycle, by epiclasticturbidites only (Fig. 22D). If fluorite was formed from ventfluids, it precipitated together with the carbonate cherts atthe end of the exhalation cycle.

The volcanism was reactivated, initiating a second exhalativecycle, which repeated the sequence of events of the first cycle(Fig. 22E). In this second cycle, fluids from type A vents wereagain emitted, which precipitated Fe (Cu, Zn), then fluidsfrom type B vents, which were less isolated from the seawater(Figs. 16F, vent B, 17), precipitated less iron sulfides, with Fe-Cu-Zn (py + cpy + po ± sph), followed by Zn-Pb-Fe sulfides(sph + gal + py ± cpy ± po) and carbonate, while massive epi-clastic sediments with blebs of carbonate also precipitated. Atleast two other hydrothermal cycles occurred, generatingmineralized zones 3 and 4 (Fig. 8), until the extinction of thehydrothermal system and the diagenesis of the sediments.

During the Quatro Cachoeiras orogeny (1.68−1.63 Ga), theregion was folded and metamorphosed at greenschist-faciesconditions (Fig. 22F) and later (1.56−1.53 Ga) it was ther-mally metamorphosed during the intrusion of Aripuanã gran-ite (Fig. 22G). In this last event, magmatic fluids related togranite emplacement mineralized faults with massive pyriteand with py + po + mgt ± sph ± cpy ± gal and formed mas-sive chlorite veins with biotite, pyrrhotite, and chalcopyrite. Iffluorite is metasomatic it crystallized from fluids emitted byAripuanã granite at this phase.

ConclusionsThe Aripuanã deposit is a 1.76 to 1.75 Ma Zn, Pb, and Ag

(Cu, Au) volcanic- and sediment-hosted massive sulfide(VHMS) deposit located on the edge of a caldera. Carbonatesare important minerals in Aripuanã and were formed by mix-ing CO2-rich vent fluids, at temperatures greater than 300°C,with cold seawater, resulting in a mixed solution with temper-atures between 130° and 200°C. Hydrothermal carbonateprecipitated below the redoxcline, at the same time that ash-size pyroclastic rock fragments from ash falls and fragmentsfrom turbidites were deposited. Mixing, in various propor-tions, of these sediments and ocean-floor mud produced themain rock facies described in Aripuanã (CTTC, TTR, TAR,BR, and CR rocks).

The mineralization is essentially composed of pyrite, pyrr -ho tite, sphalerite, and argentiferous galena, with minor chal-copyrite and magnetite. These minerals form lenses of massivesulfides and zones of semimassive, as well as disseminatedsulfide concentrations with varied quantities of carbonate,

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chlorite, tremolite, talc, and biotite. The ore deposit has fourmineralized zones located at discrete stratigraphically posi-tions. Above the sulfide lenses, closing each exhalative cycle,silica-carbonate sediments precipitated along with smectite,followed by carbonate rhythmites and turbidites.

Rock compositions at present are directly related to origi-nal sediments. Diagenesis transformed the mixture of thehydrothermal carbonate and silica into marbles, siliceousmarbles, and carbonate cherts; the clastic-chemical mixtureof carbonate and clay into marls and breccia-like carbonaterocks with a smectitic clay matrix; the silica-(fluoritic?)-car-bonate sediments into carbonate and (fluoritic?) chert; theprecipitates with sulfides and carbonate into argillic-carbon-ate sulfide ore and the mudstones into shales and argillites.

Between 1.69 and 1.63 Ga the entire region was metamor-phosed at lower greenschist grade. The marbles were recrys-tallized, whereas the marls and carbonate breccia rocks witha smectite-clay matrix were transformed into carbonate brec-cias with a quartz-chlorite-tremolite matrix. The carbonateand chert became marble and metacherts. The lenses withsulfide were recrystallized. The shales recrystallized intoquartz-feldspar-chlorite schists, and the rhythmites and tur-bidites recrystallized into metaturbidites and metarhythmites.

Between 1.56 and 1.53 Ga, intrusion of granites contactmetamorphosed and metasomatized the deposit and hostrocks, obliterating the regional metamorphic foliation. Siliceousmarbles were transformed into talc-tremolite- and talc-richrocks with small quantities of carbonate and biotite. The car-bonate breccias with quartz-chlorite-carbonate matrices weretransformed into breccia-like (pseudofragmental) rocks withcarbonate, tremolite, talc, and chlorite (CTTC rocks). Theargillites were transformed into biotite rocks; the mineraliza-tion was recrystallized a second time, and the other rocks re-crystallized without changing their mineral composition.Metasomatic fluids filled the fractures with massive chloritewith porphyroblasts of biotite, pyrrhotite and chalcopyrite(+fluorite?).

Carbonates and quartz of the mineralized rocks are in dis-equilibrium with the recrystallized rocks with talc, tremolite,carbonate, chlorite, and biotite, which crystallized between350° and 530°C. However, the preservation of the primaryhydrothermal textures in the carbonates and sulfides suggeststhat recrystallization was restricted to the silica-carbonate rocksand that the original isotopic signatures were maintained.

Field and experimental evidence points to the possibility offluorite chert to be a sedimentary hydrothermal rock, butthere is no direct evidence to confirm this hypothesis andeliminate the genesis of fluorite by metasomatism related toAripuanã granite.

AcknowledgmentsThe authors are grateful to Economic Geology reviewers.

We are particularly indebted to Robert Seal, Associate Editorof Economic Geology, for discussions, advice, and multiplecareful revisions of the text.

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