Mineral chemistry of ore and hydrothermal alteration at the Sossego iron oxide–copper–gold deposit, Carajás Mineral Province, Brazil

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Mineral chemistry of ore and hydrothermal alteration at the Sossego iron oxide–copper–gold deposit, Carajás Mineral Province, Brazil
  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Mineral chemistry of ore and hydrothermal alteration at the Sossego ironoxide – copper – gold deposit, Carajás Mineral Province, Brazil Lena Virgínia Soares Monteiro a, ⁎ , Roberto Perez Xavier a , Murray W. Hitzman b , Caetano Juliani c ,Carlos Roberto de Souza Filho a , Emerson de R. Carvalho a a Instituto de Geociências, Universidade Estadual de Campinas, R. João Pandiá Calógeras, 51, CEP 13083-970, Campinas, SP, Brazil b Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States c Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, CEP 05508-080, São Paulo, SP, Brazil A B S T R A C TA R T I C L E I N F O  Article history: Received 10 January 2007Received in revised form 13 October 2007Accepted 31 January 2008Available online 3 March 2008 Keywords: Sossego depositIron oxide – copper – gold depositCarajás Mineral ProvinceBrazilMineral chemistryGeothermobarometry The Sossego iron oxide – copper – gold deposit in the Carajás Mineral Province comprises two major orebodies,Sequeirinho and Sossego. Sodic alteration (albite – hematite) and sodic – calcic alteration zones represented byalbite, ferro-edenite/hastingsite (up to 3.8 wt.% Cl), actinolite/magnesiohornblende, magnetite, titanite, epidote,and calcite are predominant at Sequeirinho. Magnetite bodies with envelopes of apatite-rich actinolitite wereformed with the sodic – calcic event at high temperatures (~500 °C at 1.4 kbar). In the Sossego orebody, potassicalteration with orthoclase and Cl-rich biotite (up to 3.1 wt.%) and chloritization are the main alteration types.Mineralized breccias in both orebodies have coarse-grained zoned actinolite/ferro-actinolite, Cl-apatite, andmagnetite within the matrix. Sul fi des occur in equilibrium with a paragenetically late calcite – quartz – chlorite – epidote – allanite assemblage. The Al IV  contents of the chlorite indicate crystallization at temperatures below300°C.Chalcopyriteoccursassociatedwithpyrite(upto2.3wt.%Coand0.2wt.%Ni),nativegold(upto14.9wt.%Ag), siegenite, millerite, vaesite, Pd-melonite, and hessite.Dilution and cooling of the hot metalliferous  fl uid ( N 500 °C) by mixing with meteoric  fl uids may have been themain mechanisms responsible for the deposition of metals transported as metal chloride complexes in bothorebodies of the Sossego deposit.© 2008 Elsevier B.V. All rights reserved. 1. Introduction The Carajás Mineral Province, located at the southeast part of theAmazon Craton in Pará State, Brazil, contains the world's largestknown concentration of large-tonnage iron oxide – copper – gold(IOCG) deposits, such as Sossego (245 Mt at 1.1 wt.% Cu, 0.28 g/tAu; Lancaster et al., 2000), Salobo (789 Mt at 0.96 wt.% Cu, 0.52 g/tAu, 55 g/t Ag; Souza and Vieira 2000), Cristalino (500 Mt at 1.0 wt.%Cu; 0.3 g/t Au, Huhn et al., 1999), Igarapé Bahia/Alemão (219 Mt at1.4 wt.% Cu, 0.86 g/t Au; Tallarico et al., 2005), Gameleira (100 Mt at0.7 wt.% Cu; Rigon et al., 2000), and Alvo 118 (70 Mt at 1.0 wt.% Cu,0.3 g/t Au; Rigon et al., 2000).The Sossego mine operated by the Companhia Vale do Rio Doce(CVRD) was the fi rst major IOCG deposit to go into production in Brazilin 2004. This deposit is distinctive because it appears to contain hydro-thermal alteration zones similar tothose formed at a range of depthsinIOCG hydrothermal systems worldwide (Monteiro et al., 2008). TheSossego deposit shares a number of similarities with the other CarajásIOCG deposits, including the occurrence of chlorine-bearing mineralphases and strong enrichment in rare earth elements, Co, Ni, Pd and U(Guimarães, 1987; Zang and Fyfe, 1995; Tavaza et al., 1999; Huhnet al., 1999; Lindenmayer et al., 2001; Dreher, 2004). Cobalt and Nienrichments, and their association with Cl-rich silicates, have beenalso recognized as important characteristics of IOCG deposits world-wide (Mazdab et al.,1999; Mazdab and Barton, 2001; Sillitoe, 2003).However, these characteristics are related to the most controversialaspect of the genesis of this deposit class, namely the source of   fl uidsand metals.Distinct sources for salinityand metals in IOCG deposits have beenconsidered in dominantly magmatic-hydrothermal systems, suchas the Cloncurry district, Australia (Rotherham et al., 1998), and theCandelária,PuntadelCobre,MantoVerde,andCerroNegroIOCGdepositsin Chile and Peru (Sillitoe, 2003), in hybrid systems with magmatic-hydrothermal and non-magmatic components (e.g., Emmie Bluff,Australia; Gow et al.,1994; Olympic Dam, Australia; Haynes et al.,1995), andinsystemsinwhichamagmatic fl uidcomponentislikelyabsent(e.g.,WerneckeMountains,Yukon,Canada;Huntetal.,2005).Indeed,avarietyofgeological processesseemtoin fl uencetheformationof IOCGsystems,explaining the diversity of these systems (Hitzman, 2000).Systematic studies on the variations of physicochemical para-meters (temperature, pressure, redox changes) related to evolution Ore Geology Reviews 34 (2008) 317 – 336 ⁎  Corresponding author. Tel.: +55 19 35214575; fax: +55 19 32891097. E-mail addresses:  lena@ige.unicamp.br (L.V.S. Monteiro), xavier@ige.unicamp.br(R.P. Xavier), mhitzman@mines.edu (M.W. Hitzman), cjuliani@usp.br (C. Juliani), beto@ige.unicamp.br (C.R. de Souza Filho), emersonr@ige.unicamp.br (E.R. Carvalho). 0169-1368/$  –  see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.oregeorev.2008.01.003 Contents lists available at ScienceDirect Ore Geology Reviews  journal homepage: www.elsevier.com/locate/oregeorev  Author's personal copy of the Carajás IOCG deposits are of fundamental importance for thecharacterization of the processes responsible for the development of the Carajás giant hydrothermal system(s) and ore deposition.In the Sossego IOCG deposit, detailed petrographic studies per-mitted outline a consistent paragenetic sequence and the spatialzoning of alteration and mineralization (Monteiro et al., 2008). Thesestudies enable the characterization of mineral chemistry variationsduring the hydrothermal system evolution. Additionally, this studyallows comparison to available information on other IOCG deposits inthe Carajás Mineral Province. 2. Geological setting of the Carajás Mineral Province The Carajás Mineral Province, located in the southern part of theAmazon Craton, is divided into two tectonic blocks, the southern RioMaria greenstone terrain (Huhn et al., 1988), and the northernItacaiúnas Shear Belt (Araújo et al., 1988). The oldest units in theprovince occur in the southern block and encompass the Arco VerdeTonalite (2.97 to2.90 Ga; Pimentel and Machado,1994) and the 2.9 GaAndorinhas Supergroup greenstone belt sequences (Docegeo, 1988;Araújoetal.,1988;Faracoetal.,1996).WithinthenorthernblockoftheCarajásMineralProvince(Fig.1),theArcheanbasementisrepresentedby the Xingu Complex (~2.8 Ga, Machado et al., 1991), composed of tonalitic to trondhjemitic gneiss and migmatites, and by the PiumComplex (~3.0 Ga, Rodrigues et al.,1992; Pidgeon et al., 2000).The basement rocks are overlain by the Carajás Basin, whichcomprises the metavolcano-sedimentary sequences of the 2.73 – 2.76 Ga Itacaiúnas Supergroup (Wirth et al., 1986; Machado et al.,1991; Macambira et al., 1996; Trendall et al., 1998; Galarza andMacambira, 2002a,b; Pimentel et al., 2003; Tallarico et al., 2005). Thissupergroup encompasses the Igarapé Salobo, Igarapé Pojuca, GrãoPará, and Igarapé Bahia groups (Wirth et al., 1986; Docegeo, 1988),which were variably affected by deformation and metamorphismduringtheArchean(2.7to2.5Ga;DardenneandSchobbenhaus,2001;Galarza and Macambira, 2002a).The Igarapé Salobo Group, the host to the Salobo IOCG deposit,consists of paragneiss, amphibolites, quartzites, meta-arkoses andironformations,whereastheIgarapéPojucaGroup(2732±3Ma,U – Pb Fig.1.  Simpli fi ed geological map of the Carajás Mineral Province (Docegeo, 1988; Dardenne and Schobbenhaus, 2001).318  L.V.S. Monteiro et al. / Ore Geology Reviews 34 (2008) 317  –  336  Author's personal copy zircon; Machado et al., 1991) contains greenschist to amphibolitefacies rocks, represented by basic metavolcanic rocks, pelitic schistsand iron formations. The latter hosts both the Pojuca Cu – Zn andGameleira IOCG deposits (Galarza and Macambira, 2002b). The GrãoParáGroupcomprisesgreenschistfaciesmetamorphicunits,includingmetabasalts, felsic metavolcanic rocks, and iron formations thatcontain outstanding iron deposits. Greenschist facies metavolcanic,metapyroclastic and metasedimentary rocks, including iron forma-tions, de fi ne the Igarapé Bahia Group. This hosts the Igarapé Bahia/Alemão IOCG deposit.TheItacaiúnasSupergroupisoverlainbyanextensivesuccessionof Archean platform to  fl uvial sandstones and siltstones, known as theÁguas Claras Formation (Nogueira,1985; Araújo et al.,1988) or the RioFresco Group (Docegeo, 1988). This unit covers large areas in theCarajás Mineral Province where it hosts the Azul Mn, the Águas ClarasAu, and the Serra Pelada/Serra Leste Au – PGE deposits.The Itacaiúnas metavolcano-sedimentary sequence is crosscut by2.76 – 2.74 Ga syntectonic alkaline granites (Estrela Granite Complex,Plaquê Suite, Planalto and Serra do Rabo; Barros et al., 2001;Dall'Agnol et al., 1997), which produced a tectono-thermal aureolein the metavolcano-sedimentary envelope and wider scale contactmetamorphiceffects(BarrosandBarbey,1998).OtherArcheanintrusionscomprise the Luanga ma fi c – ultrama fi c complex (2763±6 Ma, Machadoet al.,1991) and 2.65 to 2.70 Ga gabbro dikes and sills that intercept theÁguas Claras Formation (Dias et al.,1996; Mougeot et al.,1996).Granitoid intrusions coeval with the Carajás and Cinzento trans-current fault systems (2581 to 2519 Ma; Pinheiro and Holdsworth,1997), such as the Old Salobo Granite (2573±2 Ma, U – Pb zircon,Machado et al.,1991) and the Itacaiúnas Granite (2560±37 Ma, Pb – Pbzircon, Souza et al.,1996) also occur in the Province.Paleoproterozoic magmatism, represented by within-plate A-type,alkaline to subalkaline granites (~1.88 Ga Serra dos Carajás, Cigano,Cigano,Pojuca,YoungSalobo,Musa,Jamon, Seringa,VelhoGuilherme,andBreves granites;Dall'Agnolet al.,1994,1999; Tallaricoet al.,2004)is widespread in the province. Younger alkali-rich leucogranite dikeswith a U – Pb SHRIMP age of 1583+9/ − 7 Ma (Pimentel et al., 2003) arealso described in the area of the Gameleira deposit.The province was also affected by other magmatic eventsrepresentedbytheBorrachudo,SantaInêsandComplexoLagoGrandegabbros (Villas and Santos, 2001) and late diabase dikes, whose agesare uncertain. 3. The Sossego iron oxide – copper  – gold deposit The Sossego deposit (Lancaster et al., 2000) in the Carajás MineralProvinceislocatedalonga regional WNW – ESE-strikingshearzonethatde fi nes the southern contact between metavolcano-sedimentary unitsof the Itacaiúnas Supergroup and tonalitic to trondhjemitic gneissesandmigmatites of theXinguComplex. This shearzoneis crosscutbyN-and NW-striking faults and by a dextral system of transcurrent E – W toNE – SW-striking subvertical dipping faults. The latter appears todelineate the mineralized zones at the Sossego deposit (Morais andAlkmim, 2005).IntheSossegoarea,graniteandgranophyricgranitearecrosscutbygabbroandlatedaciteporphyrydikes.Alltheseintrusiverocks,whoseexact age of emplacement has not been determined, cut the Xingucomplex basement and Itacaiúnas metavolcanic rocks, and are formWNW – ESE-trending bodies, concordant with the regional structures.TheyhavebeenintenselyalteredbytheSossegohydrothermalsystem.LateNW-oriented,unaltereddiabasedikescrosscutshearzones,faultsand all intrusive units.The Sossego deposit comprises two major groups of orebodies(Fig. 2) with distinct styles and intensities of hydrothermal alteration(Carvalho et al., 2005; Monteiro et al., 2005; Villas et al., 2005).This variability likely re fl ects both different host rocks and differentpaleostructural levels (Monteiro et al., 2008). The Sequeirinhoorebodyrepresentsthebulkofresources,with85%of theorereserves.All of the orebodies occur in the hanging wall of major E – W to NE – SW-trending, high angle faults. Rocks in the footwall of the faults areintensely mylonitized metavolcanic rocks that display biotite – tour-maline – marialitic scapolite – (hastingsite) alteration near the faultcontact.Although the type and intensity of alteration and mineralizationvaries among the different orebodies at Sossego, a consistent para-genetic sequence of alteration and mineralization can be discerned inthe system. This paragenetic sequence is mainly represented by earlysodic and sodic – calcic alteration accompanied by magnetite±apatiteformation (predominant in the Sequeirinho orebody) and potassicalteration, chloritization, calcic, and hydrolytic alteration that predomi-nates within the Sossego orebody (Fig. 3).  3.1. Sequeirinho orebody The Sequeirinho orebody (Fig. 2) is hosted by granite, gabbro, andfelsic metavolcanic rocks that contain minor lenses of metamor-phosed ultrama fi c rocks. These host rocks were strongly affected byboth early sodic (chessboard albite – hematite) and fracture-controlledto pervasive sodic – calcic alteration (amphibole – albite – magnetite – calcite – epidote – quartz – titanite – allanite – thorianite). Towards thecontact between the gabbro bodies and the granite, both are replacedby sodic – calcic alteration assemblage with ferro-edenite/hastingsite(Fig. 4A, B). This assemblage is overprinted by an actinolitge-bearing Fig. 2.  Simpli fi ed cross-section of the Sequeirinho and Sossego orebodies (CVRD).319 L.V.S. Monteiro et al. / Ore Geology Reviews 34 (2008) 317  –  336  Author's personal copy Fig. 4.  Back-scatteredelectronimagesshowing:(A)replacementofigneousclinopyroxenebyhastingsite/ferro-edenite – magnetiteingabbrofromtheSequeirinhoorebody.Actinolitecutsprevioushastingsite/ferro-edenite;(B)thorianiteassociatedwithactinolite,quartz,andtitanite.Thisassemblagereplaceshastingsite/ferro-edenite+magnetiteinalteredgabbro;(C)Ti-bearingphasesassociatedwithmagnetiteandchalcopyriteinmineralizedbrecciaatSossego;(D)zonedallanitecrystalswiththorianiteinclusionsassociatedwithchalcopyriteandpyriteintheorebrecciafromSequeirinho; (E)uraniniteassociatedwithKfeldspar (potassic alteration). Thelattercutsactinolite(sodic – calcicalteration); (F)cassiteriteinclusionin chalcopyrite(Sossegoore).Abbreviations: Act= actinolite;Cc= calcite; Cpx= clinopyroxene; Cpy = chalcopyrite;Hast = hastingsite/ferro-edenite; K feld = potassiumfeldspar; Mt=magnetite; Py = pyrite; Qtz = quartz. Fig. 3.  Schematic pro fi le of the Sequeirinho and Sossego orebodies showing distribution of hydrothermal alteration zones (modi fi ed from Monteiro et al., 2008).320  L.V.S. Monteiro et al. / Ore Geology Reviews 34 (2008) 317  –  336
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