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Geological Society, London, Special Publications doi: 10.1144/SP402.2 p121-145. 2014, v.402;Geological Society, London, Special Publications Evandro L. Klein deposition sources, and mechanisms of Au transport and Brazil: a review of the physico-chemical properties, Ore fluids of orogenic gold deposits of the Gurupi Belt, service Email alerting new articles cite this article to receive free e-mail alerts whenhereclick request Permission part of this article to seek permission to re-use all orhereclick Subscribe Collection London, Special Publications or the Lyell to subscribe to Geological Society,hereclick Notes © The Geological Society of London 2014 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://www.lyellcollection.org/cgi/alerts http://www.geolsoc.org.uk/permissions http://www.lyellcollection.org/site/subscriptions http://sp.lyellcollection.org/ http://sp.lyellcollection.org/ Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a review of the physico-chemical properties, sources, and mechanisms of Au transport and deposition EVANDRO L. KLEIN CPRM/Geological Survey of Brazil. Av. Dr. Freitas, 3645, Belém, State of Pará, Brazil, CEP: 66095-110 (e-mail: evandro.klein@cprm.gov.br) Abstract: The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern margin of the Palaeoproterozoic São Luı́s-West Africa Craton. Orogenic gold deposits of this belt are hosted in Palaeoproterozoic (2160–2147 Ma) metavolcano-sedimentary and calc-alkaline granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny (2240–2080 Ma). These host rock assemblages were tectonically and isotopically reworked during the Neoproterozoic and represent the reworked margin of the craton, that is, the external domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host rocks at the deposit site. Ore deposition occurred at about 300–370 8C and up to 3 kbars in response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatiza- tion, CO2 removal) and local fluid mixing and oxidation. The Gurupi Belt is a Neoproterozoic mobile belt developed at the southwestern margin of the São Luı́s-West African Craton (Fig. 1) during the Brasiliano-Pan African cycle of orogenesis (see Klein & Moura 2008 for a review). Gold is the main known mineral resource in this belt, and the metal has intermittently been mined by artisanal miners since the 17th century. There is no official historical record on the artisanal production, but unofficial data indicate over 16 t of gold has been extracted from Chega Tudo, Serrinha, and Cacho- eira. To date, more than 120 gold occurrences are reported, in addition to more developed deposits (Klein & Lopes 2011). The known gold resources of four deposits are equal to about 120 t Au (Table 1), which will be mined in open pit. The opening of the first organized, industrial operation is planned to occur at Cipoeiro. In the last decade efforts have been made to improve the understanding of the geological evol- ution of the Gurupi Belt (Ribeiro 2002; Klein et al. 2005b; Teixeira et al. 2007; Klein & Lopes 2009, 2011; Palheta et al. 2009) and of individual deposits (Torresini 2000; Ribeiro 2002; Yamaguti & Villas 2003; Klein et al. 2005a, 2006, 2007, 2008b). Results provided constraints on: (1) the nature and age of the host rocks; (2) the nature of the hosting structures; (3) the composition of the ore fluids; and (4) the T–P conditions of ore depo- sition. An integration of these studies is presented in this review, which attempts to link geological processes documented at the provincial scale with those documented in individual deposits. Special attention will be given to the circulation of fluids that have led to the formation of the deposits, espe- cially the evaluation of the mechanisms of gold transport and deposition. Other parameters that are critical for the development of genetic models have not yet been determined. Such parameters are mostly dependent on the absolute age of gold depo- sition, and also include the tectonic setting of ore formation. A brief discussion of these issues is addressed throughout the review. Geological overview São Luı́s cratonic fragment Considering that a significant part of the Gurupi Belt is made of reworked units of the São Luı́s cratonic fragment (Fig. 1), the geology and evol- ution of this cratonic fragment is briefly outlined here (based on Klein et al. 2008a, 2009, 2012; Palheta et al. 2009). Three main stratigraphic units crop out in the cratonic area (Fig. 1a). The oldest rocks known in the São Luı́s cratonic fragment belong to the Aurizona Group, which is an island arc-related metavolcano-sedimentary sequence of 2240 + 5 Ma that was intruded by shallow grano- phyric rocks at 2214 + 3 Ma and by the juvenile, From: Garofalo, P. S. & Ridley, J. R. (eds) 2014. Gold-Transporting Hydrothermal Fluids in the Earth’s Crust. Geological Society, London, Special Publications, 402, 121–145. First published online March 19, 2014, http://dx.doi.org/10.1144/SP402.2 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 1. (a) Location of the São Luı́s cratonic fragment and Gurupi Belt (hatched). For the sake of scale, only the major rock units of the São Luı́s cratonic fragment are considered here. (b) Simplified geological map of the Gurupi Belt showing the main gold deposits and showings. E. L. KLEIN122 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ subduction-related, metaluminous to slightly pera- luminous calc-alkaline granitoids developed in an island arc between 2168 and 2147 Ma. These gran- itoids form batholiths and stocks that are included in the Tromaı́ Intrusive Suite, which comprise mainly tonalites and granodiorites and subordi- nated quartz-diorite, monzogranite, and syenogra- nite. Minor units, which do not appear in the scale used in Figure 1a, include andesites, dacites, felsic tuffs, and subordinately basic volcanic rocks depos- ited over the Aurizona Group and the Tromaı́ Suite at 2164–2160 Ma. These volcanic associations have metaluminous to peraluminous, high-K calc- alkaline to tholeiitic signature and have been inter- preted to have formed in mature or transitional arc with minor back-arc component and active continental margin, respectively. Peraluminous, collision-type granites of c. 2100 Ma belong to the Tracuateua Intrusive Suite, crop out in the western portion of the cratonic area and are covered by Pha- nerozoic sequences. Late- to post-orogenic small plutons of highly evolved/shoshonitic granite of 2056–2076 Ma and are the youngest rocks known so far in the São Luı́s cratonic fragment. Based on rock association, geochronology, and on geochemical and Nd isotope signatures, these Palaeoproterozoic associations of the São Luı́s cra- tonic fragment are interpreted as a section of the Rhyacian orogen, which records an accretionary stage at 2240–2150 Ma and a collisional stage at c. 2100 Ma (Klein et al. 2008a, 2009, 2012). This scenario correlates with what is described for similar successions of the Eburnean-Birimian ter- ranes of the West-African Craton (Klein & Moura 2008 and referencestherein). Gurupi Belt The Neoproterozoic Gurupi Belt developed at the south-southwestern margin of the São Luı́s cratonic fragment (Fig. 1b). It consists of igneous, sedimen- tary, and metamorphic rock units of distinct origin recording a long geological history from 2695 to 549 Ma, comprising Archaean and Palaeoprotero- zoic basement units, the reworked margin of the São Luı́s cratonic fragment, pre-orogenic and oro- genic intrusions, and sedimentary basins of equiv- ocal origin. (Klein et al. 2005b, 2012; Palheta et al. 2009; Klein & Lopes 2011). These rock units are mostly NW-SE trending successions that form the general tectonic orientation in the belt. To date, the oldest rocks found in the belt belong to the Igarapé Grande Metatonalite dated at 2594 + 3 Ma (Klein et al. 2005b), which represents the vestiges of a poorly defined Archean block that is part of the belt basement (Fig. 1b). Most of the basement is composed of Palaeoproterozoic units (Fig. 1b) that have been interpreted as being part of active continental arc and collision settings (Klein et al. 2012). The largest basement unit com- prises the foliated to banded orthogneisses of the Itapeva Complex of 2167 + 3 Ma , with TDM ages of 2.22–2.31 Ga and positive 1Nd values (Klein et al. 2005b). The complex also includes lim- ited remnants of paragneisses, amphibolite-facies schists, and basic-ultrabasic rocks (Klein & Lopes 2011). The weakly peraluminous, high-K, calc- alkaline biotite-bearing Cantão Granodiorite of 2163 + 4 Ma, with TDM model ages of 2.21– 2.92 Ga and 1Nd values of +2.7 to 2 7.1 (Palheta et al. 2009), and the 2142 + 9 Ma-old, weakly per- aluminous, biotite- and muscovite-bearing Jonasa Granodiorite, with TDM model ages of 2.14– 2.40 Ga, and 1Nd values of+3.9 – 0.2, are probably related to a continental arc setting (Klein et al. 2012). The Jonasa Granodiorite underwent meta- morphism and deformation at 525 + 20 Ma (Klein & Moura 2003). Strongly-peraluminous crust- derived granitic magmatism took place between 2116 and 2079 Ma (Klein et al. 2012 and references therein) and is represented by several plutons of biotite- and muscovite-bearing granites and leuco- granites (Fig. 1). These granites show TDM model ages of 2.19–2.62 Ga and 1Nd values ranging from +3.9 to 20.2 (Palheta et al. 2009; Klein et al. 2012), in addition to chemical differences that have been interpreted to result from mixtures of variable amounts of sedimentary and igne- ous sources of Archaean to Palaeoproterozoic ages in a collisional setting (Klein et al. 2012). Table 1. Ore reserves of gold deposits from the Gurupi Belt Deposit Indicated (Mt) Grade (g/t) Inferred (Mt) Grade (g/t) Au (t) Ref Cachoeira 12.5 1.11 0.5 1.27 20.68 1 Cipoeiro 49.4 1.17 6.8 1.10 65.30 2 Chega Tudo 20.7 1.00 12.0 0.98 32.51 2 Mina Nova-S nd nd 0.8* 2.30 1.88 3 nd: not determined. *Unconstrained resources. Key to references: 1: Nakai-Lajoie and Clow (2011), 2: Machado (2011), 3: Pastana (1995). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 123 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ The 2100 + 21 Ma-old metaluminous to slightly peralkaline granites, quartz-monzonites, and quartz- syenites of the Anelis Intrusive Suite show charac- teristics of highly evolved potassic to shoshonitic post-collisional rocks (Klein et al. 2012). This suite is spatially associated with the post-orogenic, 2085 + 4 Ma-old (Palheta et al. 2009), weakly per- aluminous, high-K, calc-alkaline, biotite-bearing Timbozal Monzogranite (Fig. 1). Both suites show Nd isotope signature indicating mixed mantle and crustal sources (Klein et al. 2012). The reworked margin of the São Luı́s cratonic fragment includes plutonic and supracrustal rocks (Fig. 1). The plutonic rocks are the batholiths of the juvenile calc-alkaline granitoids of the Tromaı́ Intrusive Suite, with remnants of coeval amphibo- lite, both showing variable degrees of deformation within the Gurupi Belt, and gabbroic intrusions of supposed Palaeoproterozoic age (Ubinzal Gabbro) (Klein et al. 2005b; Klein & Lopes 2011). The pre- dominant supracrustal sequence is the 2160 Ma old metavolcano-sedimentary Chega Tudo Formation, with TDM ages of 2.20–2.34 Ga and positive 1Nd values (Klein & Moura 2001), seconded by the sili- ciclastic Igarapé de Areia Formation, which is younger than 2110 Ma (Teixeira et al. 2007) and resembles the Tarkwa sedimentary sequence of Ghana, West Africa (Klein & Lopes 2009). Vestiges of the Rhyacian metamorphism and deformation (and collision?) are inferred from the presence of the peraluminous leucogranites, some of them having low Th/U zircon ages of 2098– 2092 Ma (Klein et al. 2005b, 2012). Neoproterozoic units (Fig. 1) comprise very low- grade to greenschist-facies metasedimentary rocks younger than 1140 Ma (Gurupi Group), probably deposited in a passive margin. They also include a pre-orogenic amphibolite-facies, undersaturated alkaline intrusion of 732 + 7 Ma (Boca Nova Nepheline Syenite); and the orogenic, metamor- phosed Caramujinho Microtonalite of 624 + 16 Ma, with crustal signature, and the peraluminous, two-mica granite (Ney Peixoto Granite) of 549 + 4 Ma (Klein et al. 2005b, 2012; Palheta et al. 2009; Klein & Lopes 2011). The Neoproterozoic metamorphism is estimated to have occurred between 624 and 549 Ma (Klein & Lopes 2011). Geology of the gold deposits Structural setting The orogenic gold deposits of the Gurupi Belt are mostly, if not all, hosted in structures related to the Tentugal shear zone (Fig. 1). This is a sinistral strike-slip sinistral shear zone that consists of a .100 km-long and 15- to 30 km-wide corridor of shear zones striking dominantly N408W developed under ductile and ductile-brittle conditions at the greenschist facies (Costa et al. 1988; Ribeiro 2002; Klein & Lopes 2011). The ductile deformation imparted a schistose and/or mylonitic fabric to the rocks, depending on their composition, rheology, and position in relation to the deformation zones. Most of the strain was accommodated by the less competent metavolcano-sedimentary rocks of the Chega Tudo Formation, and to a lesser extent by the coarse-grained granitoids of the Tromaı́ Intru- sive Suite and gneisses and coarse-grained schists of the Itapeva Complex. Ribeiro (2002) described the transposition of the N408W-striking structures by north-NW–south-SE-trending ductile structures, along with the formation of a north–south-striking fault system and associated splays (e.g. Cipoeiro region, Fig. 2) that have subsequently been reacti- vated and displaced by small-scale thrusts and east–west-oriented strike-slip faults. In the northwestern portion of the belt, where the Cachoeira deposit is located, the Chega Tudo For- mation displays a north-south-trending foliation that dips at high angles to the SW, with subhorizon- tal stretching lineations, indicating a transpressional sinistral strike-slip regime. Brittle–ductile, north– south-trending strike-slip shear zones dip at high angles to the west and were reactivated as brittle faults that produced brecciated zones and were sub- sequently displaced by east/west- and WNW/ESE- trending strike-slip faults (Klein et al. 2005a), in accordance with what was described by Ribeiro (2002) for the southeastern portion of the belt. The ore bodies hosted in supracrustal sequences of the Chega Tudo Formation are mostly parallel to the regional structure of the host rocks (Fig. 3), but can also be discordant. At the deposit scale, they are confined to smaller-scale brittle-ductile structures. Accordingly, the ore bodies are discontinuous, sub- vertical, stretched, and mostly have cylindrical to lens shapes. At Cipoeiro, deformation is concentrated in dis- crete shear zones that cut the hosting tonalites. The least deformed tonalites show preserved igneous textures, whereas the more deformed and hydrother- mally altered types show strongly modified texture and mineralogy, with the original rocks transformed in fine-grainedmylonites. The ore bodies have irregular shapes and are hosted in tonalites near the contact with metasandstones of the Igarapé de Areia Formation. Stockwork veining and breccia- tion have been observed at Mina Nova Sul and Pipira and may represent a shallower, brittle style of mineralization. Host rocks Most of the deposits and prospects of the Gurupi Belt are hosted in rocks of the 2160 Ma-old E. L. KLEIN124 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ metavolcano-sedimentary sequence of the Chega Tudo Formation. A smaller number are hosted by the calc-alkaline granitoids of the Tromaı́ Intrusive Suite (2160–2147 Ma), by the Ubinzal Gabbro, and by the conglomerates and sandstones of the Igarapé de Areia Formation (Figs 1 & 2, Table 2). The metavolcano-sedimentary sequence occurs as subparallel elongated layers from a few tens to hundreds of meters in thickness. The rocks are vari- ably foliated and exhibit subvertical schistosity and/or mylonitic fabric that trend according to the regional and local tectonic grain of the Gurupi Belt, that is, mostly NW–SE. The supracrustal rocks underwent metamorphism under greenschist to lower amphibolite facies conditions (Yamaguti & Villas 2003). At Cachoeira, mafic and intermediate rocks of the Chega Tudo Formation are the main hosts for the ore bodies. Klein et al. (2005a) described tuff, basalt, andesite, chlorite schist with intercalated ultramafic rocks (chlorite-talc schist), microdiorite, and carbonaceous phylite as the main host rocks (Figs 3a & 4a, b). The volcanic rocks show aphyric to porphyritic texture. Nakai-Lajoie & Clow (2011) also described sericite schist, dacite, and silexite and characterized the tuffs as being ash, lapilli, and clastic tuffs. To the east, the Chega Tudo Formation makes a tectonic contact with a weakly mineralized metasedimentary unit that Klein & Lopes (2009) assigned to the Igarapé de Areia Formation. The sequence shows a psamitic unit, composed of arkose (Fig. 4b), greywacke, and possible tuffs, and a pelitic unit with intercalated siltstones and car- bonaceous phyllites. The arkose is a medium- to coarse-grained rock with preserved primary cross bedding and has intercalations of conglomerate levels and magnetite bands. At Chega Tudo mineralization took place especially in a metavolcanic domain that parallels the metasedimentary domain (Fig. 3b). The meta- volcanic rocks belong to the Chega Tudo Formation, whereas the metasedimentary domain has rocks of the Chega Tudo and Igarapé de Areia Formations. The metavolcanic domain comprises dacite por- phyry as the predominant host rock, and subordi- nately rhyolite, andesite, and volcaniclastic rocks (Torresini 2000; Ribeiro 2002; Klein et al. 2008b). The host rocks show variable degrees of tectonic deformation (Fig. 4c–f). In the Serrinha and Montes Áureos deposits (Figs 2 & 3c, d), the mineralization occurred predo- minantly in metasedimentary rocks (Ribeiro 2002; Yamaguti & Villas 2003) of the Chega Tudo For- mation (Klein et al. 2006). These are magnetite- bearing quartz-sericite schist, chlorite schist, and fine- to medium-grained dark-coloured carbon- aceous schists (Fig. 4g) with interlayered tuffs and felsic to intermediate metavolcanic rocks. The host rocks for the gold mineralization at Cipoeiro comprise variably altered and sheared calc-alkaline tonalites of the Tromaı́ Intrusive Suite (Fig. 4h). The tonalites occur in tectonic Fig. 2. Detailed geological map of the central portion of the Gurupi Belt showing the location of four of the main gold deposits discussed in the paper. Modified after Ribeiro (2002) and Klein et al. (2006). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 125 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Table 2. Geological attributes from selected gold deposits and prospects of the Gurupi Belt Deposit Cachoeira Chega Tudo Serrinha Montes Áureos Pretos Cipoeiro Mina Nova-S Pipira Host unit Chega Tudo Fm. Igarapé de Areia Fm. Chega Tudo Fm. Igarapé de Areia Fm Chega Tudo Fm. Chega Tudo Fm. Chega Tudo Fm. Tromaı́ Intrusive Suite nd Chega Tudo Fm. Host rock Mafic and felsic (meta)volcanic rocks, tuffs, C-phyllite, metasandstone Dacite porphyry (rhyolite, andesite, vulcanoclástica rocks), metasandstone C-schist Pelitic and C-schists, rhyolite, dacite C-schist Tonalite Altered gabbro(?) tonalite(?) Tuffs, C-pyllite, Age of the host rock (Ma) 2160 2160 ,2100 2160 2160 nd 2148 2148 nd Host structure North–South shear zone NW–SE shear zone NW–SE shear zone NW–SE shear zone NW–SE shear zone North–South splays NW–SE shear zone NNE–SSW faults Structural style Veins, disseminations Veins, disseminations Veins, disseminations Veins Veins, disseminations Veins, disseminations Stockwork, brecciated veins Stockwork Hydrothermal mineralogy Qtz, Dol, Ser, Chl Qtz, Cal, Ser, Chl Qtz, Cal, Dol Qtz Qtz Qtz, Cal, Ab, Chl, Phe Qtz Qtz, Cal Ore mineralogy Py, Apy (Gn, Sp, Ccp) Py, Apy, Ccp (Gn, Sp) Py Apy, Py (Ccp) nd Py nd Py Elemental association Au, Ag, As, Bi Au, Bi, As, Sb, Te nd nd nd nd nd nd Au grade (g/t) 1.7–4.4 0.98 nd nd nd 1.51 2.30–2.37 nd Reserves (t Au) 20 26.6 nd nd nd 60.4 1.88 nd References 1, 2 3 4 5 6 7 6, 8 6 Key to references. 1: Klein et al. (2005a), 2: KLein & Lopes (2009), 3: Klein et al. (2008b), 4: Klein et al. (2006), 5: Yamaguti & Villas (2003), 6: this work, 7: Klein et al. (2007), 8: Pastana (1995). Mineral abbreviations: Qtz, quartz; Cal, calcite; Dol, dolomite; Chl, chlorite; Ser, sericite; Ep, epidote; Ab, albite; Phe, phengite; Py, pirite; Apy, arsenopyrite; Sp, sphalerite; Ccp, chalcopyrite; Gn, galena (minerals in brackets are subordinate). Other abbreviations: nd – not determined or unavailable, Fm: formation. E . L . K L E IN 1 2 6 by guest on A ugust 7, 2014 http://sp.lyellcollection.org/ D ow nloaded from http://sp.lyellcollection.org/ contact with a metasedimentary sequence (Figs 2 & 3e) composed of magnetite-rich metasandstone and metapelite with lenses of metaconglomerate (Torresini 2000) ascribed to the Igarapé de Areia Formation (Klein & Lopes 2009, 2011). Other prospects have been documented only par- tially; therefore, only qualitative models can be pre- sented here. The Ubinzal prospect shows gold mineralization hosted in basic rocks of the Ubinzal Gabbro unit that occurs within the Chega Tudo For- mation, but without clear contact relationships (Klein & Lopes 2011). The basic rocks are predomi- nantly coarse-grained (Fig. 5a), locally porphyritic, actinolite-bearing gabbros, and subordinately mon- zodiorite, monzogabbro, diorite, quartz monzodior- ite and, possibly, pyroxenite and hornblendite. Under investigation are prospects associated with the siliciclastic sequence of the Igarapé de Areia Fig. 3. Cross sections of gold deposits from the Gurupi Belt. (a) Cachoeira (Klein et al. 2005a). (b) Chega Tudo (Ribeiro 2002). (c) Serrinha (Klein et al. 2006). (d) Montes Áureos (Yamaguti & Villas 2003). (e) Cipoeiro (Torresini 2000). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 127 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 4. Host rocks of gold deposits of the Gurupi belt. (a) Cachoeira: mafic metavolcanic rock crosscut by quartz veins (white). (b) Cachoeira: sulphidised felsic metavolcanic rock crosscut by a quartz vein (left) and drill core sample of a sandstone from the Igarapé de Areia Formation crosscut by quartz veinlets (right). (c) Chega Tudo – drill core sample of an undeformed and unaltered dacite porphyry. (d) Chega Tudo: same dacite of ‘C’ showing transition between E. L. KLEIN128 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Formation (Fig. 1). Epigenetic gold mineralization has been detected at least in the Cachoeira deposit and in the Sequeiro prospect (Fig. 5b), and hydro- thermalalteration is observed at Pico 20, Firmino, and Boa Esperança (Fig. 5c). In addition, the silici- clastic formation hosts palaeoplacer gold, which is not a subject of this review. Mineralization styles and gold siting Two similar mineralization styles have been described in all studied deposits of the Gurupi Belt, and both include quartz-sulphide veins (Bet- tencourt et al. 1991; Ribeiro 2002; Yamaguti & Villas 2003; Klein et al. 2005a, 2006, 2007, 2008b; Machado 2011; Nakai-Lajoie & Clow 2011). Differences reside mainly in the number and dimen- sions of veins, in the volume of hydrothermally altered wall rock (Fig. 6), in the form of gold occur- rence, and, at least in part, in the timing of vein deposition. Style (1) is composed of relatively thick (0.5– 5.0 m) tabular to S-shaped quartz veins (Fig. 6a) and vein sets with variable length (20–200 m along strike). The quartz veins show some visible gold (Fig. 7), centimetre-wide alteration envelopes, and have low (1–3 wt%) sulphide contents. These veins are predominantly concordant with the regional structure of the Gurupi Belt, but locally may present variable angle relationships with respect to the foliated host rocks. Furthermore, the veins are interpreted to have been emplaced early in the structural-hydrothermal history of the deposits (e.g. Bettencourt et al. 1991; Klein et al. 2005a, 2007), based on mesoscopic and microscopic defor- mation evidence, such as the laminated charac- teristic of some veins, the undulose extinction, deformation lamellae, aggregates of quartz sub- grains, and the growing of hydrothermal minerals in pressure shadows of sulphide crystals. In places, however, cross cutting relationships among the two styles are equivocal. In this style, gold is predo- minantly of the free-milling type. Style (2) comprises thin, up to a few centimetres- thick, Au-quartz-carbonate-sulphide veinlets and Au-sulphide stringers and dissemination in larger volumes of altered host rocks within sets of closely- spaced shear zones (Fig. 6b). The veinlets may be both concordant and discordant with respect to the hosting structure, and the stringers and disseminations are in general associated with small fractures (Fig. 4). The veinlets also underwent ductile and brittle deformation, as suggested by changes in the thickness of the veinlets, ubiquitous presence of undulose extinction of quartz, forma- tion of subgrains, recrystallization, and fracturing. In addition to free-milling gold, the second style shows several modes of gold siting (Fig. 8): (i) deposited in microfractures cutting quartz crystals, quartz-carbonate veins and veinlets and sulphide (pyrite, arsenopyrite) grains; (ii) in pyrite-arseno- pyrite and quartz-arsenopyrite intergranular con- tacts; (iii) as inclusions of single particles or composed Au-arsenopyrite (+sphalerite) grains included in pyrite; and (iv) as trace-element, espe- cially in pyrite and arsenopyrite. Bismuth, As, Sb, and Te are also present as trace-elements in sulphides. Possibly, a third style of mineralization is pre- sent in the region. In recent field work, structurally controlled stockwork veining and brecciation have been observed in open pits in two prospects (Mina Nova Sul and especially Pipira; Table 2). This style might represent shallow portions of the gold system and will be the subject of future investigation. Hydrothermal alteration As a whole, the gold deposits of the Gurupi Belt share similar aspects regarding the hydrothermal alteration in terms of mineralogical composition, style, and intensity. In addition to gold, the hydro- thermal assemblages are in general composed of variable amounts of quartz, chlorite, white mica, carbonate minerals, minor albite, and sulphide minerals. Ilmenite and rutile are very subordinate phases. Variations may occur at the scale of deposit and are in general related to the intensity of deformation and veining, and, to a minor extent, to the type of host rock. Around the thicker veins of Style 1, the hydro- thermal haloes tend to attain a few tens of centi- metres. At Cachoeira, Nakai-Lajoie & Clow (2011) described moderate to intense wall rock alteration in the vicinity of quartz-vein systems and faults that have been interpreted to be the feeders of the hydrothermal system. In these haloes, albite, carbonate, pyrite, and arsenopyrite are the predominant phases. The quartz vein of the Fig. 4. (Continued) undeformed and deformed zone with stringers of pyrite (arrows). (e) Chega Tudo: drill core samples with concordant quartz and discordant quartz-calcite veinlets hosted in deformed rhyodacite and dacite (from Klein et al. 2008b). (f) Chega Tudo: Mylonite showing a bleached sericitic zone with stringer of pyrite (arrow). (g) Serrinha: carbonaceous schists with veinlets and spots of carbonate (arrows) (from Klein et al. 2006). (h) Cipoeiro: drill core samples of thick quartz vein (left), altered but texturally preserved tonalite (right), and variably foliated and veined (arrows) altered tonalites (three drill cores in the middle). Foliation is outlined by the white dashed line (from Klein et al. 2007). Mineral abbreviations: Cal, calcite; Py, pyrite; Qtz, quartz. GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 129 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 5. Host rocks of gold prospects. Drill core samples are 3 cm-wide. (a) Ubinzal: coarse-grained, altered and veined gabbro. (b) Sequeiro: quartz vein and pyrite (Py) aggregates hosted in coarse-grained sandstone of the Igarapé de Areia Formation. (c) Boa Esperança: quartz veinlets (top) and veins (middle) and hematitic (Hem) alteration (bottom) in sandstones from the Igarapé de Areia Formation. E. L. KLEIN130 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Mina Velha ore body of the Chega Tudo deposit is surrounded by a cm-wide halo of white mica. At Serrinha, only the silicification (quartz vein) is visible (Fig. 7). In Style 2, individual alteration zones range from a few meters to several tens of meters in thickness. Together, these zones form multiple pods that attain several hundreds of meters in thickness (Klein et al. 2005a; Machado 2011). Possibly, multiple generations of veining and fracturing rendered the host rocks highly permeable, allowing high fluid flow and a better development of hydrothermal alteration (e.g. Ribeiro 2002; Machado 2011; Nakai- Lajoie & Clow 2011), which characterizes Style 2 mineralization. Furthermore, a number of stages of deformation, alteration, and gold precipitation have probably occurred in each deposit, but in many cases the establishment of the sequence of these stages has been hampered by deformation and the overlapping of alteration phases (Klein et al. 2007). At Cachoeira, Klein et al. (2005a) described four alteration stages, which here are summarised into three stages: (i) milky and smoky quartz (+pyrite, arsenopyrite) veins, that in this paper correspond to Style 1; (ii) quartz-dolomite-albite-pyrite-arseno- pyrite (+sericite-chlorite) around small fractures, in veinlets or pervasively substituting the host rocks (Style 2); and (iii) post-mineralization calcite veinlets (Fig. 9a–e). The Mandiocal target studied by Klein et al. (2008b) corresponds to the North Zone of the Chega Tudo deposit. Mineralization in this target consists of millimetre- to centimetre-thick quartz (+calcite, +sulphide) veinlets and enclosing altered host rocks. In addition to gold, the hydrother- mal assemblage is composed of variable propor- tions of quartz, chlorite, white mica, calcite, and sulphide minerals (pyrite and minor chalcopyrite, sphalerite, and galena). Locally, the hydrothermal alteration shows an asymmetric zoning at the centi- metre scale with quartz veinlets surrounded by prox- imal calcite (+pyrite), an intermediate chlorite-rich zone composed of chlorite, quartz, calcite, white mica, and pyrite, and a distal zone composed of whitemica, quartz, pyrite, and minor calcite (Fig. 9f). There is increase in the calcite contents and decrease in the white mica content toward the veinlet (Klein et al. 2008b). In the Main Zone of Chega Tudo, Machado (2011) described two alteration zones: (i) a quartz-sericite-pyrite zone, that contains the gold mineralization; and (ii) a chlorite-carbonate-epidote zone that straddles the ore bodies and hosting shear zones limits. In both the Mandiocal/North and Main zones, gold-bearing veinlets and Au-pyrite stringers are located mostly within the foliation planes of the highly strained host rocks (Figs 4d, f & 9g), but they also cut across the foliation (Fig. 4e) (Klein et al. 2008b; Machado 2011). In the Serrinha deposit only two ,11 m-thick mineralized zones have been accessed in a previous study (Klein et al. 2006). Probably because of the Fig. 6. Sketch of the styles of gold mineralization in the Gurupi Belt. (a) Thick quartz veins and sets of veins and their hydrothermal haloes. This style generally shows free-milling gold. (b) Sheared and hydrothermally-altered host rocks, with thin gold-bearing veinlets and disseminated stringers of auriferous pyrite. This style shows predominantly refractory gold. GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 131 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ composition of the host rock, a graphite-bearing schist, and of a relatively small volume of hydro- thermal fluids, hydrothermal alteration in these zones is limited to silicification, in the form of Au-quartz veins, to quartz-carbonate-sulphide vein- ing (Fig. 9h) and to precipitation of spots of carbon- ates (Fig. 4g). Calcite and ankerite are the carbonate mineral in the upper mineralized zone, whilst calcite and dolomite occur in the lower zone. Pyrite is the main sulphide mineral, followed by very small crys- tals and inclusions of arsenopyrite and galena. Outside the ore zones, pyrrhotite and arsenopyrite predominate over pyrite. So far, Cipoeiro is the only important deposit hosted in tonalites of the Tromaı́ Intrusive Suite instead of in metavolcano-sedimentary rocks of the Chega Tudo Formation. Notwithstanding, both host units present similar mineralization and hydro- thermal styles. According to Klein et al. (2007), in the disseminated style, the host shear zones are up to a few meters wide and the hydrothermal altera- tion associated with gold mineralization produced an assemblage consisting of quartz, chlorite, white mica, calcite, albite, and pyrite. This alteration assemblage overprinted the primary magmatic mineralogy. Locally, bleached haloes occur at the margins of quartz-pyrite veinlets and stringers of pyrite in sheared zones are common (Fig. 10). Microprobe analyses (Klein et al. 2007) indicate that the white mica belongs to the phengitic series, and that the chlorite polytypes are ripidolite and picnochlorite, with Fe/(Fe + Mg) ranging from 0.38 to 0.47. Klein & Lopes (2011) observed that the tonalites of the Tromaı́ Suı́te have greenish colours far away from Cipoeiro, which is ascribed to moderate regional chlorite and saussurite altera- tion of mafic minerals and plagioclase, respectively. Machado (2011) reported similar alteration assem- blage, but stressed that gold is also predominantly associated with the quartz-sericite-pyrite altera- tion, as described for Chega Tudo. On the other hand, silicification and the overall alteration of the host tonalite are more intense and more widespread than in Chega Tudo. This likely results from a larger number of closely-spaced discrete shear zones cutting across the tonalites. Timing of gold mineralization The growth of sericite and chlorite within the milo- nitic foliation, the growth of chlorite and quartz in the pyrite pressure shadows, and the deposition of gold within fractures indicate that hydrothermal alteration is syntectonic to late tectonic. At Cacho- eira, the evidence for carbonate alteration overprint- ing the metamorphic minerals and foliation show that hydrothermal alteration is post-metamorphic (Fig. 9d). Ongoing radiogenic isotope dating, however, shows conflicting results. For instance, a well- defined isochron constructed with the Pb isotope compositions of gold particles from the Mina Velha target (Chega Tudo deposit) revealed an Archaean model age, whereas the host rock is 2160 Ma old. The 40Ar/39Ar ratios of hydrothermal white mica from the Chega Tudo deposit gave a minimum age of c. 800 Ma, which is older than the pre-orogenic nepheline syenite that approxi- mately defines the opening of the orogenic basin Fig. 7. Style 1 quartz vein from Serrinha outlined by dashed lines, with visible gold (arrows; from Klein et al. 2006). E. L. KLEIN132 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ that became the locus for the development of the Gurupi Belt (Klein et al. 2005b). The white mica 40Ar/39Ar ratios also show overprinting by a younger event at 600–623 Ma, which is consistent with the estimated timing of the Neoproterozoic metamorphism in the Gurupi Belt. At Cachoeira, apparently coexisting pyrite and arsenopyrite grains from ore bodies hosted in metavolcanic rocks of the Chega Tudo Formation gave Palaeoproterozoic (�2.0 Ga) and Neoproterozoic (�0.84–0.59 Ga) model ages, respectively. In turn, the epigenetic mineralization that took place in rocks of the Igarapé de Areia Formation must be younger than 2110 Ma. Therefore, this major issue needs further con- straints. The hypotheses envisaged so far include: (1) mineralization occurred in the Palaeoprotero- zoic, and remobilization may have taken place in Fig. 8. (a) Irregular gold particles in a quartz-carbonate vein (Serrinha; Klein et al. 2006). (b) Gold and chalcopyrite grains in a fracture of a large pyrite crystal (Chega Tudo; Klein et al. 2008b). (c) Gold in sharp contact with large arsenopyrite crystal (Cachoeira). (d) Rounded inclusion of gold (arrow) in a pyrite crystal (Cachoeira). (e) Composite gold-arsenopyrite-sphalerite inclusion in pyrite (Cachoeira). Mineral abbreviations: Apy, arsenopyrite; cb, carbonate; Cpy, chalcopyrite; Py, pyrite; Qtz, quartz; Sp: sphalerite. GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 133 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 9. (a) Quartz vein of Style 1; crosscutting fractures are filled with quartz-calcite veinlets (arrows) (Cachoeira deposit; from Klein et al. 2005a). (b) Veinlet and (c) pervasive albite-dolomite-pyrite alteration at Cachoeira (Klein et al. 2005a). (d) Dolomite alteration over chlorite schist (Cachoeira deposit; Klein et al. 2005a). (e) Post-mineralization E. L. KLEIN134 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ the Neoproterozoic, with Pb isotopes indicating both Archaean and Palaeoproterozoic sources, in this case, different deposits might have been affected to different extents; and (2) mineralization occurred in the Neoproterozoic from Archaean to Neoproterozoic Pb (and Au) sources. The first hypothesis is preferred here, based on the following constraints and interpretations: (1) All the deposits and showings (.120) are hosted in Palaeoproterozoic rocks (c. 2100– 2160 Ma) that represent the reworked margin of the São Luı́s cratonic fragment (Klein et al. 2005b, 2012; Klein & Lopes 2011). (2) The predominant Neoproterozoic sequence of the Gurupi Belt, that is, the Gurupi Group, likely represents a passive margin association (Klein & Lopes 2011; Klein et al. 2012), which is an inappropriate tectonic setting for orogenic gold, although the inversion of the tectonism from passive to active margin has likely occurred during the Neoproterozoic orogeny. (3) The metamorphic nature of the ore fluid. Metamorphic fluids are produced by devolati- lization and dehydration reactions occurring during the progressive metamorphism of vol- canicand sedimentary sequences deposited in oceanic and accretionary settings (Phillips & Powell 1993; Kerrick & Caldera 1998; Yardley & Graham 2002). This is in line with the orogenic evolution proposed for Palaeo- proterozoic units of the São Luı́s cratonic fragment and reworked/basement portions of the Gurupi Belt (Klein et al. 2008a; Klein & Lopes 2011). (4) The commonly observed close association of gold deposits, granitoids, and thermal events (e.g. Phillips & Powell 1993) is envisaged only for the Palaeoproterozoic of the Gurupi Belt. Fig. 10. (a) Drill core of the host tonalite of the Cipoeiro deposit showing variation in grain size and mineralogy related to deformation and alteration. From left to right, the undeformed tonalite pass to a finer-grained rock with the same chlorite-saussurite alteration; bleached halo in the margin of a quartz-pyrite veinlet and stringer of pyrite crosscutting the tonalite. (b) Photomicrograph of the chloritised and saussuritised portion of the tonalite. (c) Photomicrograph of the contact zone between the undeformed and sheared tonalite. Chl, chlorite; Hbl, hornblende; Pl, plagioclase; Py, pyrite. Fig. 9. (Continued) calcite veinlet (white) crosscutting a fine-grained basalt (light grey) showing a concordant stringer of very fine-grained pyrite-arsenopyrite (dark grey) (Cachoeira deposit; Klein et al. 2005a). (f) Polished slab from a mineralized intersection of the Mandiocal target (Chega Tudo deposit) showing zoned alteration consisting of a quartz veinlet surrounded by a proximal calcite-rich zone, intermediate chlorite-rich zone, and distal sericitic zone (Klein et al. 2008b). (g) Deformed quartz veinlet (white) from the Mandiocal target (Chega Tudo deposit), with stringers of auriferous pyrite and seams of sericite (arrows) defining the foliation (Klein et al. 2008b). (h) Quartz-calcite veinlet at Serrinha (Klein et al. 2006). Mineral abbreviations: Chl, chlorite; Py, pyrite; Cal, calcite; Dol, dolomite; Qtz, quartz; Ser, sericite; Ab, albite. GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 135 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Table 3. Physico-chemical properties of fluids from selected orogenic gold deposits of the Gurupi Belt Deposit n Cachoeira n Chega Tudo n Serrinha n Montes Áureos n Cipoeiro Fluid composition nd CO2–H2O–NaCl 200 (A) CO2–CH4–H2O– NaCl; (B) CO2–H2O–NaCl 170 CO2–CH4–N2– H2O–NaCl 130 CO2–CH4–H2O– NaCl-MgCl/FeCl2 nd nd XCO2 (mol %) nd nd 200 (A) 12–22; (B) 11–13 170 18–33 nd nd nd nd XCH4 (mol%) nd nd 200 (A) ,6; (B) 0 170 ,2 130 ,6 nd nd XN2 (mol %) nd nd 200 (A) 0; (B) 0 170 ,4 nd nd nd nd wt % NaCl equiv. (average) nd nd 200 (A) 5.8; (B) 1.6–2.5 170 4.5 130 2.8 nd nd bulk density (g/cm3) nd nd 200 (A) 0.83–0.96; (B) 0.93–0.96 170 0.89–0.94 130 0.69–0.89 nd nd T (8C) ,20 330–370 200 (A) 340–370; (B) 330–340 170 310–335 130 296–302 70 305–319 P (MPa) nd nd nd nd 170 130–300 130 130–280 nd nd log f O2 nd nd 200 (A) 228.7 to 230.5; (B) 230.5 to 231.5 170 228.5 to 231.0 nd nd nd nd d18OH2O (‰) 23 +6.2 to +12.4 10 (A) +7.9 to +9.4; (B) +5.3 to +7.2 7 +6.2 to +8.4 nd nd 11 +2.4 to +5.7 dDH2O (‰) 3 216 to 228 7 (A) 229 to 237; (B) 212 to 230 3 219 to 280 nd nd 5 220 to 243 d13C carbonate 12 211.5 to 213.6 1 (A) nd; (B) 23.8 6 214.2 to 215.7 nd nd 3 22.1 to 24.2 d13CCO2 (‰) nd 29.5 to 212.7 2 (A) 26.9; (B) 224.1 1 217.6 nd nd 1 210.7 d34S sulphide (‰) 8 +1.9 to +6.4 3 (A) nd; (B) + 0.1 to +0.8 2 22.6 to 27.9 nd nd 2 +1.1 to +1.7 References 1, 6 2 3 4 5 Key to references: 1: Klein et al. (2005a), 2: Klein et al. (2008b), 3: Klein et al. (2006), 4: Yamaguti & Villas (2003), 5: Klein et al. (2007), 6: Klein (unpublished data). nd: not determined/not available; A–B: two types of fluids at Chega Tudo. E . L . K L E IN 1 3 6 by guest on A ugust 7, 2014 http://sp.lyellcollection.org/ D ow nloaded from http://sp.lyellcollection.org/ Nature and sources of ore fluid components Composition of ore fluids Detailed fluid inclusion studies have been un- dertaken at Montes Áureos (Yamaguti & Villas 2003), Serrinha (Klein et al. 2006), and in two targets (mineralization styles) of the Chega Tudo deposit (Klein et al. 2008b), a summary of which is given in Table 3. These studies show that the entrapped vein fluids of the Gurupi Belt deposits are of the aqueous-carbonic (H2O–CO2–NaCl), carbonic (CO2), and aqueous (H2O–NaCl) types (Fig. 11). The aqueous fluid is absent at Serrinha. At room temperature the carbonic fluid is made of single-phase inclusions, while the aqueous and aqueous-carbonic fluids are made by two-phase (liquid-vapor, LV) inclusions with variable phase ratios. Three-phase inclusions are made of an aqueous liquid, a carbonic liquid, and a vapor phase and are exclusive of aqueous-carbonic fluid. The CO2-bearing fluid inclusions form clusters of randomly distributed (primary) inclusions in the inner part of quartz crystals, or form large, three- dimensional trails (Fig. 11). Some aqueous inclu- sions have the same distribution and coexist with the CO2-bearing inclusions (Montes Áureos and Chega Tudo). Most of the aqueous inclusions, however, form secondary trails and have homogen- ization temperatures well below the range of temp- eratures showed by the H2O-CO2-NaCl inclusions (Table 3). The petrographic and microthermometric characteristics of the fluid inclusions described for the deposits of the Gurupi Belt, include (in one or more deposits) contemporaneous entrapment of different fluid types, which is reflected in the coex- istence of carbonic, aqueous-carbonic, and subordi- nate aqueous fluid inclusions, large variation in the phase ratios and in the temperature of homo- genization of the CO2-bearing phase (i.e. density variation), homogenization of the coexisting inclu- sions both to the liquid and vapour phases over the same range of temperatures, and partitioning of salts into the aqueous-rich phase (only observed at Chega Tudo). These characteristics satisfied at least some of the criteria used by Ramboz et al. (1982) to identify fluid immiscibility. In addi- tion, density variations are at least in part due to fluctuating pressure conditions, as indicated by microstructural evidence (coexistence of and/or alternating ductile and brittle features in the same vein). Fig. 11. Fluid inclusion assemblages in gold deposits of the Gurupi Belt. (a) Chega Tudo: clusters of aqueous- carbonic fluid inclusions (black arrows) and aqueous inclusions (white arrow) in quartz. (b) Random and fracture-hosted aqueous inclusions (Klein et al. 2008b). (c, d) Serrinha: carbonic and aqueous-carbonic fluid inclusions with variable CO2/H2O ratios (Klein et al. 2006). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 137 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ The mineralizing fluid is interpreted to be of aqueous-carbonic nature and some lines of evidence outlined previously in this paper give support to the above interpretation: (1) the fluid inclusion studies were performed in (sometimes visible) gold-bearing quartz veins and quartz-carbonate- sulphide veinlets (Chega Tudo, Serrinha, Montes Áureos); (2) the aqueous-carbonic fluid is associ- ated with the highest gold grades in Chega Tudo; and (3) the aqueous-carbonic fluid is the only fluid present at Serrinha. The ore fluid shows subordinate contents of CH4 and N2 in the carbonic phase and local presence of MgCl2 and/or FeCl2 in the aqueous phase (Table 3). The CO2 contents range between 11 and 22 mol% at Chega Tudo, and is quite higher (18–33 mol%) at Serrinha. No information is avail- able for the other deposits. The fluids have low salinity, averaging 1.6–5.8 wt% NaCl equivalent in different deposits (Yamaguti & Villas 2003; Klein et al. 2006, 2008b). Only locally, at Montes Áureos, fluids with salinity up to 15 wt% NaCl equivalent have been identified (Yamaguti & Villas 2003). The aqueous fluid inclusions present in sev- eral depositshave usually been interpreted not to be related to the mineralizing event, but to later infiltration events, partly related to the regional post-orogenic uplift. This has been based on the lower homogenization temperatures presented by the aqueous inclusions when compared to the CO2-bearing inclusions, on crosscutting relation- ships, and on compositional differences. Calculated fluid properties (T–P–fO2–pH) Fluid inclusion homogenization temperatures, stable isotope equilibrium, and the chlorite and arsenopyrite geothermometers indicate formation temperatures between 296 8C and 370 8C, and pres- sures between 130 and 300 MPa for the Gurupi Belt deposits (Fig. 12a, Table 3). These values are in keeping with T–P estimations in deposits elsewhere that have similar hydrothermal assemblages and structural setting (e.g. McCuaig & Kerrich 1998; Robert & Poulsen 2001). Oxygen fugacities have been reported for the Chega Tudo and Serrinha deposits and calculated for the range of XCO2 of the ore fluids and the range of estimated temperatures and pressures (Klein et al. 2006, 2008b). This approach yielded log f O2 values between 228.5 and 231.5. These values plot within the pyrite and magnetite stabil- ity fields, above the CO2-CH4 buffer, and below the SO2/H2S buffer (Fig. 12b), defining relatively reduced conditions for the ore fluids. These con- ditions are also consistent with the measured d34S values of sulphide minerals (mainly pyrite), the large predominance of CO2 over CH4 in fluid inclusions, the frequent coexistence of pyrite and chlorite in the alteration assemblage, and the absence of oxidized minerals, such as hematite Fig. 12. (a) P–T diagram showing the estimated conditions for gold mineralization in deposits of the Gurupi Belt (references in the text). The dashed line is the solvus for the CO2–H2O–NaCl system (Bowers & Helgeson 1983). The shaded box limits the metamorphic conditions for the Chega Tudo Formation (Yamaguti & Villas 2003). (b) Solubility of gold as a function of temperature and oxygen fugacity for the Chega Tudo (light grey) and Serrinha (dark grey) deposits. Dashed lines are for bisulphide complex Au(HS)2 2, dotted-dashed lines are for chloride complex AuCl2 2. The heavy solid lines represent the limits of the stability fields of Fe-oxides and sulphides, and the dotted lines represent gas buffers (adapted from Romberger 1990 and Ohmoto & Goldhaber 1997). Mineral abbreviations: hem, hematite; mag, magnetite; py, pyrite; po, pyrrhotite. E. L. KLEIN138 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ and sulphates, in the alteration assemblage (Kerrich 1987; Romberger 1990; Hayashi & Ohmoto 1991; Ohmoto & Goldhaber 1997). Considering the simi- larities between the studied deposits, it is likely that similar conditions apply also for Cachoeira, Montes Áureos and Cipoeiro. The absence of K-feldspar and the presence of white mica and minor albite as alteration minerals, and the stability of carbonate minerals imply near- neutral to slightly alkaline conditions for the miner- alizing fluids, with pH values between 5.0 and 6.2 (Romberger 1990; Mikucki & Ridley 1993). Sources of ore fluid components Stable isotope studies in silicate, carbonate, and sulphide minerals and in water and CO2 from aqueous-carbonic inclusion fluids have been used to constrain potential sources for the ore fluids (Klein et al. 2005a, 2006, 2007, 2008b). Oxygen and hydrogen isotope compositions have been calculated for the estimated temperature of ore formation in each deposit (Table 3 and Fig. 13). The results gave d18O values between +5.3‰ and +12.4‰ and dD values ranging from 212‰ to 237‰ in deposits hosted by metavolcano-sedi- mentary rocks. These values consistently indicate a metamorphic source for the water present in inclusion fluids and in equilibrium with hydrother- mal minerals. Lower fluid d18O values of +2.4 to +5.7‰ were found in the tonalite-hosted Cipoeiro deposit (Fig. 13 and Table 3). Although compatible with a metamorphic source, it is uncertain if the lower limit of d18O values also implies a limited contribution of meteoric water. However, an accom- panying lowering in the dD values would be expected in the case of influence of meteoric water, which is not the case. Also enigmatic is one strongly negative dD value of 280‰ found in water from aqueous-carbonic inclusions of one sample of Serrinha, and that falls in the magmatic range. Klein et al. (2006) considered H2 diffusion into inclusion cavities induced by deformation as the best explanation for this low value. Limited d13C values of CO2 present in fluid inclusions vary from 26.9‰ to 224.1‰ and car- bonaceous schists and phyllites that are common in the stratigraphy of the host metavolcano- sedimentary Chega Tudo Formation have d13C values of 223.5‰ to 229.7‰ (Table 3). The highest values presented by inclusion fluids likely represent a mantle-derived carbon source. The lowest values are clearly of organic origin. These have been interpreted as a contribution from the Fig. 13. Oxygen and hydrogen isotope compositions of ore fluids of gold deposits from the Gurupi Belt. The fields of metamorphic and magmatic waters are from Sheppard (1986). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 139 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 14. Interpreted evolution of the São Luı́s Cratonic Fragment and Gurupi Belt in the Palaeoproterozoic (not to scale). (a) Accretion phase: intra-oceanic arc construction and reworking, subduction, and calc-alkaline magmatism. (b) Collision phase: accretion of the Rhyacian juvenile terranes to an Archaean block, crust thickening, metamorphism, E. L. KLEIN140 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ carbonaceous rocks, whereas the intermediate val- ues are probable mixtures of deep-seated carbon with organic carbon at the site of ore deposition (Klein et al. 2005a, 2006, 2008b). Sulphur isotope composition of sulphide min- erals show positive d34S values between +0.1‰ and +6.4‰ at Cachoeira, Chega Tudo, and Cipo- eiro. Only Serrinha shows relatively low d34S values of 22.6‰ to 27.9‰ (Table 3). These posi- tive values are interpreted as reflecting both mag- matic and mantle sources, or an average crustal sulphur composition. The magmatic input could be related either to magmatic fluids or to the dissolu- tion of primary magmatic sulphides (Ohmoto & Rye 1979; Lambert et al. 1984). The negative val- ues of Serrinha may be attributed to oxidation of the fluids and could be produced by a few mechan- isms, such as the interaction of hydrothermal fluids with Fe2+-bearing silicates of the host rocks, immiscible separation of reduced gases, and the presence of an originally oxidized magmatic fluid (McCuaig & Kerrich 1998 and references therein). As discussed by Klein et al. (2006), the fluid immiscibility and removal of CO2 from the fluid trapped in fluid inclusions and to form carbon- ate minerals may have led to oxidation of the residual fluid at Serrinha and might be responsible for the negative d34S values. Mechanisms of gold transport and deposition Gold transport and deposition in hydrothermal systems is governed by fluid composition, tempera- ture, pressure, pH, oxygen and sulphur fugacity, and type and amount of dissolved sulphur and other species (Mikucki & Ridley 1993; Mikucki 1998 and many others). Depending on these parameters, hydroxide, chloride, and sulphide complexes may be responsible for gold solubilization and tran- sport (Loucks & Mavrogenes 1999; Stefánsson & Seward 2003, 2004; Pokrovski & Dubrovinsky 2011, and references therein). At the physico- chemical conditions discussed in the previous section, it is likely that H2S (alternatively, HS 2) was the predominant sulphur species in the fluid (Fig. 12b) and that Au(HS)2 2 was probably the domi- nant gold-transporting complex (Shenberger& Barnes 1989; Hayashi & Ohmoto 1991; Benning & Seward 1996). The physico-chemical conditions are also consistent with the high solubility of gold and copper as bisulphide complexes (Fig. 12b). In regard to gold deposition, complex, some- times conflicting and competing, processes such as fluid immiscibility under fluctuating pressure con- ditions, mixing of fluids, and fluid-rock reactions may occur at a scale of a single deposit. In the gold deposits of the Gurupi Belt, extensive fluid immiscibility (phase separation, generally induced and/or accompanied by pressure fluctuations) and local fluid mixing have been documented (Yama- guti & Villas 2003; Klein et al. 2006, 2008b). The first process is considered to have played a major role in gold deposition. In addition, several lines of evidence indicate fluid-rock reactions, which may have contributed to the destabilization of the gold transporting complexes at the depositional sites, through changes in the pH and in the oxygen fugacities and sulphur and carbonate activities. These include: (1) The presence of a large volume of altered host rocks (the deposits extend for more than 1 km in length). Although water/rock ratios have not been quantified, the alteration reactions occurred in fluid-dominated sys- tems and under nearly isothermal condi- tions, as inferred by small variation in the fluid isotope compositions in each deposit (Table 3). (2) Sulphidation of the host rocks, or desulphida- tion of the ore fluid by reaction between sulphur and iron-rich minerals, such as mag- netite, early pyrite and chlorite, produc- ing predominantly pyrite, and being the main responsible for the Au-pyrite paragenesis. This could be produced by the reaction Au(HS)2 2 + FeO ¼ Au + FeS2 + H2O sugge- sted for Montes Áureos (Yamaguti & Villas 2003). (3) The presence of gold in fractures of quartz and pyrite (Fig. 8). (4) Reaction between the fluid and carbon- bearing host rocks, as depicted by the low d13C values of fluid inclusion CO2, at least in Chega Tudo and Serrinha (Table 3). (5) Removal of CO2 from solution by dissociation (CO2 + H2O ¼ H+ + HCO32) to precipitate carbonate minerals (e.g. Rimstidt 1997) or trapped in fluid inclusions. (6) Local oxidation of the ore fluid (Serrinha). Simple cooling is not considered to have been an effective process for gold precipi- tation in the Gurupi Belt (Yamaguti & Villas 2003; Klein et al. 2005a, 2006, 2007, 2008b). Fig. 14. (Continued) crustal melting, peraluminous magmatism, and ore formation. (c) Detail of the present-day external portion of the Gurupi Belt with the location of selected gold deposits and possible fluid and solute sources and paths (modified from Klein et al. 2005b, 2008b). GOLD DEPOSITS OF THE GURUPI BELT, BRAZIL 141 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Tectonic setting and integrated fluid-structural model The definition of the tectonic setting in which the orogenic gold deposits of the Gurupi Belt formed is highly dependent on their absolute timing of for- mation. The host rocks formed in an island arc or back-arc tectonic setting at about 2168–2147 Ma during a Rhyacian orogenic event (Fig. 14a). This terrane represents today the reworked southern margin of the São Luı́s cratonic fragment, that is, it represents the external domain of the Neoprotero- zoic Brasiliano-Pan African orogen. The main host structure, the Tentugal Shear Zone, was certainly active during the Neoproterozoic as a strike-slip structure and was probably active in the Rhyacian orogenic event as part of a compressive regime (Klein & Lopes 2011). Although I cannot discard a Neoproterozoic metallogenic event, the Rhyacian scenario is the preferred one, for the reasons discussed above. An integrated fluid-structural model for gold mineralization in the Gurupi Belt operating in Palaeoproterozoic times (Figs 14 & 15) is based on the data outlined above and includes the nature of the hosting rocks and structures, the hydrothermal assemblage, the structural style of mineralization, hosting structures, wall rock alteration, mineral paragenesis, the fluid inclusion properties, and the stable isotope data. Considering the similarities between Chega Tudo and the other deposits of the Gurupi Belt, the model may also account, in general terms, for the gold genesis in this belt, which is valid for both Palaeoproterozoic and Neo- proterozoic times. The key elements of this model are: (1) a deeply sourced, low-salinity H2O–CO2– NaCl – bearing fluid of metamorphic origin migrated through active structures related to the Tentugal shear zone under fluctuating pressure con- ditions; (2) hydrothermal alteration of the enclos- ing rocks and vein precipitation; (3) the local enrichment of the hydrothermal fluid in CH4 in response to its interaction with carbon-bearing rocks of the Chega Tudo Formation; and (4) a sudden drop in the fluid pressure, probably related to a fault-valve behaviour within the hosting struc- tures, causing fluid immiscibility and forcing the fluid back into the shear zone and accompanying mixing with the original deeply sourced H2O– CO2–NaCl fluid and, in places, with an aqueous fluid of disputable origin (metamorphic and/or meteoric) or exchanged with metamorphic rocks. Mixing, however, has only been documented at Chega Tudo (Klein et al. 2008b). The model is Fig. 15. Schematic (not to scale) illustration modified from Klein et al. (2008b) with the interpretation of possible mechanisms for the migration of the fluids responsible for the deposition of orogenic gold in the Chega Tudo deposit. The interpretation is broadly valid for the other gold deposits. (a) Pre-fault failure with upward and outward migration of a deeply sourced fluid, wall-rock alteration vein formation, and deposition of free-gold. (b) Post-fault failure with continuation of the upward migration of the deeply sourced fluid, opening of spaces, and backward migration of the reacted fluid, with vein formation and precipitation of refractory-gold. Abbreviations: FI, fluid inclusions; cc, carbonate. E. L. KLEIN142 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ consistent with what is described elsewhere for shear zone-hosted gold deposits (e.g. Cox et al. 1995; McCuaig & Kerrich 1998; Sibson 2001). Concluding remarks The epigenetic gold deposits of the Gurupi Belt form a coherent group that share similar charac- teristics, in terms of geological attributes and ore genesis, with orogenic gold deposits found in Palaeoproterozoic metamorphic terranes of South America (e.g. Rio Itapicuru Greenstone Belt (Xavier & Foster 1999); Ipitinga Auriferous District (Klein & Fuzikawa 2010 and references therein); West-African Craton (e.g. Birrimian terranes, Berge 2011 and references therein); and elsewhere (see reviews in Groves et al. 2003; Goldfarb et al. 2005)). The timing of gold mineralization in these terranes is relatively well constrained, placing the formation of gold deposits at about 2100– 2000 Ma, at the end of the long orogenesis. In con- trast, the absolute timing of gold deposition in the Gurupi Belt has not yet been determined. This major issue, which is critical for the establishment of genetic models, stems from the poly-orogenic evolution of the Gurupi Belt, that is, the host rocks of the ore bodies formed during the Rhyacian oro- genesis that built up the São Luı́s cratonic fragment, but these rocks, at least in part, were affected by Neoproterozoic events related to the evolution of the Gurupi Belt. Tectonic reworking, isotope reset- ting, and the lack of isotopic equilibrium in places where petrographic evidence indicates equilibrium are widespread features. Furthermore, the different styles of gold mineral- ization found in all deposits, that is, free-milling gold in veins, and refractory and free gold in altered host rocks could indicate different mechan- isms for gold precipitation and/or differencesin the extent of fluid-wall rock interaction, but could also indicate two separate mineralizing events with distinct ore fluids. The field and research work done so far have not been able to solve this problem, and the model presented here implies fun- damentally that the gold deposits of the Gurupi Belt formed in the Palaeoproterozoic from Archaean to younger source rocks but were remobilized later on, up to the Neoproterozoic. The author thanks P. Garofalo for the invitation to write this paper and for insightful comments on the manuscript. The paper is a contribution to the projects ‘Metalogenia do Cinturão Gurupi’ (CPRM/Geological Survey of Brazil) and ‘Geocronologia e modelamento isotópico de depósitos aurı́feros do Cráton São Luı́s e Cinturão Gurupi: a busca da relação metalogênese do ouro com a evolução crustal’ (CNPq, process 306723/2009-3) and to GEOCIAM – Instituto Nacional de Ciência e Tecnologia de Geociências da Amazônia. J. Kolb and an anonymous reviewer are acknowledged for their comments and suggestions that helped to improve the manuscript. References Benning, L. G. & Seward, T. M. 1996. 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