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Granite-ore deposit relationships in Central Brazil
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Journal of South American Earth Sciences, Vol. 11, No. 5, pp. 427±438, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98 $ - see front matter S0895-9811(98)00026-1
Granite-ore deposit relationships in Central Brazil NILSON F. BOTELHO* and MAÂRCIA A. MOURA Instituto de GeocieÃncias, Universidade de BrasõÂ lia, 70910-900 BrasõÂ lia-DF, Brasil Abstract Ð Ore deposits related to granitic rocks in Central Brazil are associated with dierent Paleo- to Mesoproterozoic granite types. Sn and Au are the economic ore elements with important indium concentrations in some tin deposits. Tin mineralization is related to an A-type granite province, in the State of GoiaÂs, composed of two distinct granite groups of 1.77 Ga and 1.58 Ga ages. The most important tin deposits are closely related to the younger group, hosted in or near to Li-mica2topaz leucogranites. Despite the chemical dierences, both groups are enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. Their Nb/Ta>1 and their high F/Li ratios allow their classi®cation as a NYF fertile granite association. Recent studies reveal indium concentrations that might be recoverable as by-product of tin. Besides indium minerals and In-bearing phases, cassiterite is the most important indium carrier (0.2±0.4% In). Primary gold mineralization is hosted in oxidized I-type calc-alkaline plutons in the northern Mato Grosso State, with characteristics either of volcanic arc or post-collisional granites. Gold occurs in small high grade vein type deposits or is disseminated in widespread hydrothermal zones with alteration such as sericitization, feldspathization and pyritization. d34S values between +1.3 and +3.5- of associated sul®des are typical of magmatic deposits. The association of gold with oxidized I-type granites and the style of hydrothermal alteration are analogous to those associations present in world-class porphyry-style deposits. Although the tectono-magmagtic setting of these granites is not well understood, such an association constitutes an important target in the search for gold in the northern region of the State of Mato Grosso. # 1998 Elsevier Science Ltd. All rights reserved
In the State of GoiaÂs and Tocantins, the so-called GoiaÂs tin province (GTP) comprises a wide region related to the BrasõÂ lia fold belt terranes, of Brasiliano age (Marini and Botelho, 1986). The most important tin granites are Mesoproterozoic A-type granites which are similar in age and type to others described from the Amazonian Craton (Dall'Agnol et al., 1993). In the State of Bahia, minor tin deposits are associated with meta-rhyolites of the Espinhac° o Supergroup and are similar in age and chemistry to the GTP granites. Both GTP granites and the Espinhac° o meta-rhyolites, host the recently discovered indium occurrences.
INTRODUCTION In many studies of mineral deposits in the world, broad associations of granite with ore deposits are recognizable, such as those of tin with fractionated granites and of porphyry Cu±Au deposits with oxidized intermediate magmas. The correlation of these associations with speci®c granite types e.g. porphyries with oxidized calc-alkaline I type granites and Sn deposits with reduced I-, S- and A-type granites, has long been a focus of attention in studies of mineral deposits and mineralizing processes (Sillitoe, 1979; Burnham and Ohmoto, 1980; Plimer, 1980; Ishihara, 1981; Lehmann, 1990; Blevin and Chappell, 1992).
Farther west, in the northern Mato Grasso gold province, many gold deposits are related to Paleo- to Mesoproterozoic oxidized I-type granites (Botelho et al., 1997). Many other gold deposits are spatially related to granitic rocks, in GoiaÂs (Richardson et al., 1986; Arantes et al., 1991; Kuyumjian, 1991) and Mato Grasso (Veiga, 1988) states, but their genetic relationship with granitic magmatism is a controversial issue.
In this discussion, we characterize the granite metallogeny in Central Brazil, describing the granite types, their relationships to ore deposits and the main mineralizing processes, and present an outline of the metallogenetic potential of the main regions containing granite-related mineralization. GENERAL GRANITE METALLOGENY OF CENTRAL BRAZIL
GOIAS TIN PROVINCE
Ore deposits related to granitic rocks in Central Brazil are associated with dierent Paleo- to Mesoproterozoic granite types (Fig. 1). Tin and gold are the economic ore-elements in these granites. More recently, important indium concentrations have been described in some tin deposits.
The GoiaÂs tin province (GTP) comprises a wide region with many tin deposits related to dierent granite types of Mesoproterozoic age (Marini and Botelho, 1986). However, recent geochronological data (Pimentel et al., 1997) suggest that some deposits in the southern part of GoiaÂs State in the Impameri region may be related to Neoproterozoic granites. The northern portion of the GTP, the most important, is
* Corresponding author. E-mail: [email protected]. 427
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Fig. 1. Geological sketch maps for regions containing granite-related ore deposits in central Brazil. A: Northern Mato Grosso gold province (modi®ed from Silva et al., 1974); B: GoiaÂs tin province (modi®ed from Marini and Botelho, 1986).
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Fig. 2. Geological map of the Rio ParanaÄ Subprovince (RPS). Distribution of A-type tin massifs; 1: Serra do Mendes; 2: Mangabeira; 3: Mocambo; 4: Pedra Branca; 5: Sucuri; 6: Soledade.
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separated into two subprovinces, Rio Tocantins (RTS) in the west and Rio ParanaÄ (RPS) in the east (Figs 1 and 2). All important tin deposits are related to the RTS and RPS, comprising an A-type granite province composed of two granite groups of dierent ages, 1.77 and 1.57±1.58 Ga (Pimentel et al., 1991; Rossi et al., 1992). The early tin granites, named g1, outcrop only in the RPS, are weakly anomalous in tin and gave rise to only minor cassiterite occurrences. The most important deposits are closely related to the youngest group, named g2, outcropping in both subprovinces and comprising the true tin mineralized granites. Granites of g1 type are potassic and show an alkaline anity while g2 granites are metaluminous to peraluminous and have lower K/Na ratios and higher Li, Rb, Sn and Ta contents compared to the g1 suite (Fig. 3 and 4). Despite the chemical dierences, both g1 and g2 granites may be considered enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. REE patterns for less evolved biotite granites are highly fractionated ((La/Yb)N=20). Later leucogranites, however, display nearly ¯at REE patterns. Besides the late Ta enrichment in g2 facies (Fig. 3), the high concentration of Nb, Y and F, and the high Nb/Ta and F/Li ratios allows the classi®cation of g1 and g2 as a NYF fertile granite pegmatite association of Cerny (1991). The initial characterization of g1 and g2 groups in the RPS (Botelho, 1992), their suggested relationship with RTS granites (Botelho et al., 1993), based mainly on mineral composition (biotite, apatite) and TiO2/MgO, Th/ Ta and Ta/Nb ratios plus additional geochemical data available in the literature (Bilal, 1991; Bilal et al., 1997; Pinto-Coelho, 1996) indicate that these element
Fig. 4. Ta and Sn distribution for all granites of the GTP grouped as g1 and g2, showing important Sn and Ta enrichment in g2 types. Values represent magmatic concentrations of Ta and Sn.
ratios are an important tool in the separation of g1 and g2 granites (Fig. 3). The most evolved tin granites in the g2 group are Li-siderophyllite- to zinnwaldite-bearing leucogranite and albite±topaz leucogranite, outcropping mainly in the RPS, in the massifs of Mangabeira and Pedra Branca. In the RTS, very evolved granites analogous to g2 types were identi®ed only in the Serra Branca Massif (Pinto-Coelho, 1996). In both subprovinces, these granites have the highest magmatic tin concentrations (Fig. 4). Primary and alluvial deposits have been mined as garimpos and small mines since 1960 and have produced around 15 kt of Sn concentrates (ocial data). Major mines are currently closed and the ocial measured reserves for the whole province were estimated as 27 kt/Sn content (Bettencourt, 1997). These reserves refer only to recoverable cassiterite ten years ago, but the tin potential of the GTP is greater considering non-prospected areas in Serra Branca (SRT) and Pedra Branca (SRP) massifs. The recent discovered indium concentration in some deposits and the indium anomalies in cassiterites from many deposits bring new possibilities for the exploration of GoiaÂs tin deposits and, at the least, the RPS may be considered as an In-bearing tin subprovince. Table 1 shows the main features of the GoiaÂs Tin Province (GTP).
Fig. 3. Behavior of Ta and Nb within the RPS g1 and g2 groups of Botelho 1992, compared with the younger granites from the RTS. RPS granites: 1: g1 (Botelho, 1992); 2: Sucuri (Bilal, 1991); 3: g2 (Botelho, 1992). RTS granites: 4: Serra Dourada (Bilal, 1991; Bilal et al., 1997); 5: Serra da Mesa; 6: Serra Branca (Pinto-Coelho, 1996).
Indium-bearing tin deposits The GTP comprises around ten deposits which range in size from minor showings to small/middleclass deposits (<15 kt Sn). Other elements that may be present in subeconomic concentrations are F, Be,
Granite-ore deposits in Central Brazil Table 1. Main features of tin deposits related to the studied A-type granites Subprovince Granite group Age Main mineralized massifs Mineralizations Indium-bearing deposits
RTS g2 1.58 Ga Serra Dourada and Serra Branca Sn, REE, Be, gemstones Unknown
RPS g1 and g2 1.58±1.77 Ga Pedra Branca, Mangabeira, Mocambo Sn, In
Passa-e-Fica; Pedra Branca Greisenized cupolas, Greisenized cupolas, Tin deposit albitites greisenized country environment rocks Topaz±albite granite Li-siderophyllite (Serra Branca massif ) leucogranite, zinnwaldite±topaz± albite leucogranite (Pedra Branca and Most evolved granites Mangabeira massifs) Tin ocial measured 5kt/Sn 22kt/Sn reserves
Ta, In and REE. However, some of these elements could be recovered as by-product of tin. REE minerals, monazite and xenotime, are widespread in placer deposits of the GTP, with important concentrations in the Serra Dourada massif (RTS). In the RPS, xenotime-bearing greisens are described in the Mocambo massif and apatites from Pedra Branca and Mocambo massifs are enriched in REE (5±15% LREE + Y). Moreover, HREE-rich (1±5%) zircons are concentrated in ®ne fractions in the Pedra Branca (RPS) alluvial deposits (Botelho, 1992). Gemstones may also be important and are related to pegmatites and exogranitic alteration halos, mainly in the RTS (ArauÂjo and Alves, 1979; Marini and Botelho, 1986). Tin deposits are related to several types of environment: (i) greisenized cupolas and fractures, as in Serra Branca (TS), Pedra Branca and Mangabeira (PS) massifs; (ii) albitized zones, as in Serra Dourada (TS) and Sucuri (PS) massifs; (iii) greisenized granite±mylonite country rocks, as in Mocambo (PS) and (iv) extensive pegmatite zones, as in Monte Alegre (PS), related to a dierent granite type (S-type?), hosted by older schist sequences.
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centrations that might be recoverable as by-product of tin. The most important indium-bearing deposit is hosted in the Mangabeira massif, where In minerals are associated with an In-rich cassiterite. In-rich sphalerite, stannite and cassiterite are also found in the Pedra Branca and Sucuri massifs. Cassiterite from RPS tin deposits is at present the important In-bearing phase that may have economic signi®cance because of its indium content (0.2±0.4%In) and volume (Fig. 5). The importance of the indium enrichment in the Mesoproterozoic stanniferous acid magmatism in Central Brazil is enriched by the high indium concentrations found in the wood-tin cassiterite from Espinhac° o meta-rhyolites of the Paramirim region, State of Bahia. These rocks are analogous in age and type to the Ara|¨ Group acid metavolcanics counterparts of the GoiaÂs tin granites. The highest indium content found at Paramirim reached 2% in cassiterite related to phengitic veins in the rhyolite, while the mean value for all analyzed grains is 0.33% In (Fig. 5). The Paramirim cassiterite has the highest indium contents found in tin deposits, exceeding those given in the literature (1.23%) (Oga, 1997; Oga and Botelho, 1997). Despite a high indium content, the small size of the Paramirim tin deposits indicates that they may have little economic signi®cance. The Mangabeira indium±tin prospect Indium minerals were described for the ®rst time in Brazil from the Mangabeira massif, associated with the tin ore in the Passa-e-Fica region (Botelho and Roger, 1990). The main granite in this massif is a pink, porphyritic to equigranular, biotite granite, related to the g1 group, followed by Li-siderophyllite granite and minor zinnwaldite±topaz±albite leucogranite that represents the late mineralized tin granites (g2) (Fig. 6). Tin mineralization is mainly hosted in two types of greisen veins: Li-phengite 2 quartz greisen and Li-phengite 2 zinnwaldite 2 topaz + quartz
The main mineralization processes are related to ¯uorine-rich hydrothermal alteration, giving rise to parageneses with quartz, ¯uorite, topaz, phengite, siderophyllite and minor zinnwaldite in the deposits hosted in the A-type granite massifs. All deposits in the GTP have characteristically low lithium contents and high F/Li ratios. Indium concentrations Cassiterite is historically the only recoverable product in the GTP deposits. However, recent studies (Botelho and Roger, 1990; Botelho, 1992; Moura, 1993; Moura and Botelho, 1994) revealed indium con-
Fig. 5. Indium concentration in cassiterites from the GTP and Espinhac° o meta-rhyolites in Bahia. PA: Paramirim meta-rhyolites; PB: Pedra Branca; MA: Mangabeira.
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Fig. 7. Indium concentration in greisens from the Passa-eFica tin deposit, Mangabeira massif. 1: Li-phengite-bearing greisen; 2: topaz-quartz greisen; 3: albitized granite.
Fig. 6. Geological map of the mineralized zone of the Mangabeira massif. Country rocks: GGC: mylonites and gneiss of the Granite-gneiss Complex; GRA: Aluminous granite. Mangabeira massif: g1c: equigranular to porphyritic pink biotite granite; LGR: Li-siderophyllite-bearing leucogranite; g2 d: medium-grained equigranular pink granite (often greisenized and albitized); GAT: zinnwaldite-bearing topazalbite granite; MGZ: main greisenized zone; In: Indium-rich Li-mica-topaz-quartz greisen; GRE: greisen veins.
greisen. Cassiterite and minor wolframite are the main ore minerals. Indium concentrations were described in zinnwaldite±topaz±quartz greisen, related to the albite±topaz leucogranite, and in an albitized Li-siderophyllite granite, always related to a cassiterite±sul®de association. In-minerals are roquesite (CuInS), dzhalindite (In(OH)3) and yanomamite (InAsO42H2O), a new indium mineral described from the Mangabeira deposit (Botelho et al., 1994). The associated sul®des are loÈllingite, arsenopyrite, sphalerite, chalcopyrite and minor digenite, covellite, chalcocite, bornite, tennantite, stannite, roquesite and bismuth. In some greisen veins, yanomamite and an In-rich scorodite [(Fe,In)AsO42H2O] occur in the same proportions as cassiterite. Indium content in greisen rocks from the main greisenized zone of the Mangabeira massif may reach 1% in the yanomamite-bearing greisen and it is directly related to the tin enrichment. However, there is no important indium enrichment in tin-bearing phengite± quartz greisen (Fig. 7). Chronological and textural relationships between In-minerals and the main minerals in the cassiterite sul®de veins indicate that indium concentration is probably related to a hydrothermal event which gave
rise to the cassiterite ore in the quartz±topaz veins (Botelho and Roger, 1990; Moura and Botelho, 1994). The occurrence of In-minerals and of an In-rich scorodite (up to 6% In) in the Mangabeira tin deposit raises the question of possible economic interest. Moreover, the associated cassiterite that is the most voluminous ore mineral has signi®cant In concentration (up to 0.4%) (Fig. 5) and may represent the best prospect for In.
NORTHERN MATO GROSSO GOLD PROVINCE Primary gold mineralization is widespread along the southern border of the Cachimbo Graben, in the northern Mato Grosso gold province (MGP). Deposits hosted in granitic rocks are distributed in the Archaean/Paleoproterozoic Xingu complex terranes and are hosted in oxidized I-type calc-alkaline and oxidized alkaline granites. This gold province has many similarities to the TapajoÂs granite related gold province, north of Cachimbo graben (Fig. 1) (Coutinho et al., 1997). The genetic link between several deposits and the associated granite in the MGP is poorly understood and we present here only the deposits studied where this association is not controversial. The main prospects are in the MatupaÂ-Peixoto de Azevedo region, in the eastern portion of the area, and Alta FlorestaParanaõÂ ta, in the west (Abreu Filho et al., 1992; Paes de Barros, 1994) (Fig. 8, Table 2). Despite some prospecting programs developed by mining companies since 1985, gold has been produced from alluvial and hardrock workings only by garimpeiros and small mines. The cumulative production, since 1979 is around 200 tonnes gold (Miranda, 1997). Because of this production, despite the absence of true mines, the
Fig. 8. Geological map of the Alta Floresta-Peixoto de Azevedo region (MGP), showing the distribution of the most important primary gold deposits and occurrences.
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Table 2. Main characteristics of gold deposits in the MatupaÂÐPeixoto de Azevedo and ParanaõÂ taÐAlta Floresta regions, related to or hosted in I-type oxidized granites Gold deposit Serrinha (1) (MatupaÂ) ParaõÂ ba (2) (Peixoto de Azevedo) EreÂdio-Melado (3) (Peixoto de Azevedo) Olerindo (4) (MatupaÂ) Waldemar (5) (ParanaõÂ ta)
Host rock Monzogranite, quartz monzonite Granodiorite, tonalite, amphibolite Granodiorite, tonalite Rhyolite, porphyry, andesite Monzogranite, quartz monzonite
Wall-rock alteration K-silicate, albitization, sericitization K-silicate, propylitization, silici®cation Chloritization, sericitization, silici®cation Propylitization, silici®cation K-silicate, silici®cation
Sul®de association in the ore py2 cp2 sl2gn 2po2Ag-sulpho py2 cp2 bmt2bi 2po2sl2 m o2 gn2thd py2 cp py2 cp py2 cp2 bmt2bi 2tdm2sl2 gn
(1): Localization in Fig. 8. (py: pyrite; cp: chalcopyrite; sl: sphalerite; gn: galene; po: pyrhotite; mo: molibdenite; bmt: bismuthinite; bi: bismuth; thd: tetrahedrite; tdm: tetradymite; sulpho: sulphosalts)
exploited gold prospects are here considered as gold deposits. Oxidized I-type calc-alkaline granites The petrographic and geochemical features of the calc-alkaline granites in the MGP discriminate them as oxidized I-type granites (Botelho et al., 1997; Moura et al., 1997a). The oxidation state of silicic magmas ranges over several orders of magnitude of fO2 and may be a re¯ection of their source-region that controls the magmatic Fe3+/Fe2+ ratio (Burnham and Ohmoto, 1980; Carmichael, 1991). The MGP granites contain hornblende, biotite, titanite, magnetite, Fe3+-rich ilmenite, pyrite and pink-coloured Kfeldspar and are similar to oxidized I-type granites described in the Paleozoic fold belts of eastern Australia (Chappell et al., 1988; Blevin and Chappell, 1992). The oxidation state of magmas is a fundamental feature in the mineralizing potential of granitic rocks and Cu±Au mineralizations are related to the most oxidized types (Burnham and Ohmoto, 1980; Candela, 1992). MGP calc-alkaline granites contain high (2.6) MgO/TiO2 ratios, intermediate Zr values (100±249 ppm) and low Nb (15 ppm) and Y (9 ppm) concentrations. REE patterns are strongly fractionated (La/ Yb = 30) with weak Eu anomalies (Eu/Eu* = 0.35).
Fig. 9. Al(tot) Mg diagram (Nachit et al., 1985) for biotites from regional Xingu granite-gneiss and MGP granites. Biotite compositions, except those from alkaline granites, plot in the ®eld of calc-alkaline series.
Biotite compositions on the Mg Al diagram (Nachit et al., 1985) plot in the calc-alkaline ®eld (Fig. 9). On the discriminant diagrams of Pearce et al. (1984), the granites plot in the VAG ®eld, near the SYB and WPG ®elds (Fig. 10), interpreted as a characteristic of post-collisional granites (Harris et al., 1986) (Fig. 11a and b).
Oxidized alkaline granites Alkaline granites are widespread in the MGP region, distributed as irregular bodies in the Xingu complex or as rounded bodies in the volcanic terranes of the Iriri Formation. In regional mapping (IBGE, 1990), these granites are all grouped in the anorogenic Teles Pires-type Suite (Figs 1 and 8). However, despite chemical dierences in relation to the calc-alkaline granites, such as lower MgO/TiO2 ratio and higher Zr and Y contents, preliminary geochemical data indicate that the alkaline massifs of GuarantaÄ and Terra Nova do Norte may be also characterized as volcanic-arc granites (Botelho et al., 1997) (Fig. 10). This group of granites comprises subsolvus to hypersolvus facies containing richterite, magnetite and a biotite, which is characteristic of alkaline granites (Fig. 9).
Fig. 10. Tectonic setting of MGP granites on the discriminant diagram of Pearce et al. (1984). VAG: volcanic; ORG: ocean ridge; syn-COLG: syn-collision; WPG: withinplate. Samples plot in VAG ®eld near WPG and syn-COLG ®elds boundaries, like late to post-collision granites.
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Peixoto de Azevedo prospect along with several smaller deposits. The main gold-bearing granite in this area, the 1872 Ma old Matupa Monzogranite, is related to endogranitic and exogranitic deposits. This massif is a homogeneous, undeformed, equigranular to porphyritic monzogranite, with geochemical characteristics either of volcanic arc granites or of post-collisional granites (Figs 10 and 11) generated in the presence of an oceanic lithosphere (Moura et al., 1997a). The Matupa massif is the type-area for the oxidized calc-alkaline granites of the northern Mato Grosso gold province. Small high grade quartz vein-type deposits occur at the outer margins of the Matupa monzogranite, hosted in andesite±rhyolite volcanic rocks or related to shear zones in hydrothermally-altered granodiorite and tonalite (Paes de Barros, 1994; Siqueira and Leite, 1997, examples 2, 3 and 4 in Table 2). The link between this high-grade gold mineralization and the Matupa Monzogranite remains speculative. The granite-related type-deposit in the MGP, named the Serrinha gold deposit, is located at the northern border of the Matupa monzogranite and contains many hydrothermally-altered zones with disseminated gold concentrations. Serrinha deposit
Fig. 11. Matupa granite samples in Harris et al. (1986) diagrams for acid intrusive rocks. (a): volcanic-arc (VA), withinplate (WP), syn-collision (II) and late to post-collision (III) tectonic settings; (b) group I (volcanic-arc), group II (syn-collision) and group III (late to post-collision) granites. In both diagrams samples plot in late to post-collision granite ®elds.
Many occurrences and small deposits of gold are related to the Teles Pires suite. These are always controlled by regional lineaments and/or shear zones. At the abandoned Novo Planeta mine, important gold placer deposits were related to supergene enrichment in lateritic pro®les in the outer margins of altered and sheared biotite granite. Primary gold mineralization is very rare in the prospect, but there is a probable link between placer gold and an earlier gold concentration related to a magmatic stage, followed by metasomatism, shearing and supergene processes that gave rise to the deposit (Veiga, 1988). Despite these ®eld relationships in the Novo Planeta mine and elsewhere, the generic link between gold mineralization and Teles Pires-type granites remains a controversial issue. Gold deposits Gold occurs in quartz veins and disseminated in widespread hydrothermal zones, in the MatupaÂ-
The Serrinha gold deposit is spatially related to the Matupa granitic massif areas that have been subjected to pervasive hydrothermal alteration. The ®rst metasomatic event was a widespread microclinization. The microclinized granite was then altered by sodic metasomatism when pure albite was formed, mainly replacing K-feldspar. Locally, the extreme sodic metasomatism led to the development of ®brous aegirine. Pervasive chloritic and serictic alteration is observed in the deposit. Chlorite occurs either ®lling fractures of microcline, albite and quartz, together with sericite, quartz and calcite, or as masses, commonly with sericite. Pyritization was the last important hydrothermal phase in the deposit. Pyrite is generally related to hydrothermal magnetite and rutile. At the north of the deposit's core, secondary epidote masses were developed over the granite, possibly representing an outer epidote aureole. The general order of hydrothermal alteration in Serrinha was Ksilicate, albitic, chloritic, sericitic and pyritic alteration phases. Many of the Serrinha gold deposit features, such as granite type and hydrothermal alterations, are analogous to those described in the TapajoÂs Province, related to Maloquinha-type granite (Coutinho et al., 1997). The gold mineralization occurs in the most intense hydrothermally altered zones. Gold is either disseminated or ®lls fractures in pyrite related to potassic, chloritic, sericitic and albitic alteration. Other sul®des
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NILSON F. BOTELHO and MARCIA A. MOURA the GoiaÂs tin province (GTP) and the northern Mato Grosso gold province (MGP).
Fig. 12. d34S in pyrite from Serrinha deposits compared to granite-related mineralization.
are not common, but minor chalcopyrite, sphalerite, galena and pyrrhotite do occur as inclusions in pyrite. Pyrite d34S values for the Serrinha deposit lie between +1.3 and +3.5-, pointing to a homogeneous sulfur source (Fig. 12) (Moura et al., 1997b). Sul®de d34S values near 0- indicate a deep source of sulfur and the related ore deposits are interpreted to have formed from magmatic ¯uids (Ohmoto and Rye, 1979). In the Serrinha deposit, d34S values together with petrologic data are consistent with a magmatic origin for the gold mineralization, probably related to the same ¯uids that caused hydrothermal alteration of the Matupa granite, since d34S values and alterations are similar to those described in some porphyry-type deposits (Eastoe, 1983; Gallagher et al., 1992; Ren et al., 1995; Weihed and Fallick, 1994) and graniterelated gold deposits (Sarkar et al., 1996) (Fig. 12). The spatial association between the Matupa monzogranite and the Serrinha deposit, hydrothermal alteration types related to the gold mineralization, presence of hydrothermal magnetite in association with pyrite, sulfur isotope data and the petrologic data for the Matupa monzogranite in the Serrinha deposit are also characteristic of porphyry copper±molybdenum and copper±gold deposits (Sillitoe, 1997). Nevertheless, the Serrinha deposit has low levels of Cu and Mo, which preclude classi®cation with the base-metal-rich porphyries. Sillitoe (1979) predicted the existence of copper-poor porphyry gold deposits and Sinclair (1982) has classi®ed the Matachewan syenite-hosted gold deposits as porphyry gold deposits. Vila et al. (1991) described a porphyry gold deposit in Marte, Chile and the recently developed Fort Knox deposit (Bakke, 1995) is a further example. The characteristics presented for the Matupa monzogranite and the Serrinha Gold Deposit allow its classi®cation as a porphyry gold deposit.
SUMMARY AND CONCLUSIONS In the central region of Brazil, granite-related ore deposits are part of two important mineral provinces:
The most important tin deposits are hosted in Mesoproterozoic A-type granites distributed in the northernmost of the GTP, of two ages: g1 (1.78 Ga) and g2 (1.58 Ga). Despite tin anomalies in both groups, only the youngest granites gave rise to important tin deposits. This could be explained by the fact that the most evolved granites are related to the g2 group that produced at the end of its evolution facies such as topaz-bearing leucogranites. In the Rio ParanaÄ subprovince (RPS) tin deposits are closely related to the massifs where topaz±albite leucogranite and/or Li-siderophyllite leucogranite outcrop. However, in deposits hosted in g2 granites of the Serra Dourada massif in the Rio Tocantins subprovince (RTS), very evolved leucogranites are unknown. Indium anomalies are widespread within the tin deposits situated in the RPS with an important concentration in the Passa-e-Fica deposit in the Mangabeira massif. Indium is found as arsenate, sul®des and hydroxides and in minor concentrations mainly in sphalerite and cassiterite. Sphalerite is the richest indium carrier (up to 6% In) but, because of its volume, cassiterite, with up to 0.4% In in the Mangabeira massif and elsewhere is the most important source and may have economic signi®cance. The occurrence of In brings new possibilities for reopening closed mines and allows the RPS to be classi®ed as an In-bearing tin subprovince. In the MGP, the common association of gold with oxidized I-type granites is characteristic. The Matupa granite represents the type granite-gold association in this province and its age, 1.87 Ga, and chemical features are an important indicator for gold prospecting in the region, since preliminary mapping indicates a wide distribution of this type of granite in the northern region of Mato Grosso State. Oxidized calc-alkaline granites from the MGP and, perhaps their distribution and hydrothermal alteration, are quite similar to the Maloquinha type described from the TapajoÂs Province, Para State. The genetic link between gold and alkaline granites in the MGP is controversial and speculative. However, some of these granites are also oxidized types and could represent a target for the gold prospecting in the region. The association of porphyry-type gold deposits with less fractionated oxidized granite magmas is similar to the association present in the MGP, mainly in the Peixoto de Azevedo region. Nevertheless, the tectonomagmatic setting of the MGP granites is poorly understood and a better knowledge of this is very important to establish the metallogenetic potential for the region inasmuch as, for example, known worldclass porphyry Cu±Au deposits are directly related to a subduction environment. So, additional regional
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mapping and detailed studies are needed before proposing a complete metallogenetic model for the Mato Grosso gold province.
Botelho, N.F., Moura, M.A., Souza, M.T. and Antunes, J.A. (1997) Petrologia e potencial metalogeneÂtico de granitos da regiaÄo de Peixoto de AzevedoÐAlta Floresta, Mato Grasso. Anais, VI SimpoÂsio de Geologia do Centro-Oeste, CuiabaÂ, 40±42.
Acknowledgements Ð Financial support for research in the MGP and GTP was provided through the Brazilian Research CouncilÐ CNPq, PADCT-FINEP projects and DNPM. We are also grateful to WCM, METAMAT, DNPM-Cuiaba and Brumadinho Group for ®eld assistance. Dr Timothy Liverton is thanked for the comments and review of the manuscript.
Burnham, C.W. and Ohmoto, H. (1980) Late-state processes of felsic magmatism. In Granitic magmatism and related mineralization, eds. Ishihara, S. and Takenouchi, S. Mining Geology, Special Issue, 8, 1±11.
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Moura, M.A., Botelho, N.F. and Iyer, S.S. (1997b) Relac° aÄo entre a mineralizac° aÄo aurõÂfera de Serrinha e o Granito Matupa (MT, Brasil) com base em isoÂtopos de enxofre. Anais, VI Congresso Brasileiro de Geoquõ mica, Salvador, pp. 442±446. Moura, M.A., Botelho, N.F. and Rossi, P. (1997a) Geology, lithogeochemistry and age of the Matupa granitic massif in the Serrinha gold deposit, State of Mato Grosso, central Brazil. Extended abstracts, II International Symposium on Granites and Associated Mineralizations. Salvador, eds. Ferreira, V.P. and Sial, A.N., pp. 133±134. Murao, S., Masaroni, F. and Uchida, A.C. (1991) Geology of indium depositsÐa review. Mining Geology 41(1), 1±13. Nachit, H., Raza®mahefa, N., Stussi, J-M. and Carron, J-P. (1985) Composition chimique des biotites et typologie magmatique des granitoõÈ des. C.R. Acad. Sc. Paris 301(SeÂrie II11), 813±818. Oga, D.P. (1997) Estudo quõ mico-mineraloÂgico das ocorreÃncias de estanho-madeira portadoras de õ ndio, associadas aÁs rochas metavulcaÃnicas da Formac° aÄo Rio dos RemeÂdios (Supergrupo Espinhac° o) na regiaÄo de Paramirim-Bahia. Unpublished M.Sci. thesis. UnB, p. 120. Oga, D.P. and Botelho, N.F. (1997) In-bearing wood tin cassiterite from metarhyolites of Paramirim, State of Bahia, Brazil. Extended abstracts, II International Symposium on Granites and Associated Mineralizations. Salvador, eds. Ferreira V.P. and Sial A.N., pp. 71±73. Ohmoto, H. and Rye, R.O. (1979) Isotopes of Sulfur and Carbon. In Geochemistry of hydrothermal ore deposits, ed. Barnes, H.L., pp. 509±567. Wiley, New York. Paes de Barros, A.J. (1994) Contribuic° aÄo aÁ geologia e controle das mineralizac° oÄes aurõ feras da regiaÄo de Peixoto de Azevedo. Unpublished M.Sci. thesis. USP, p. 161. Pearce, J.A., Harris, N.B.W. and Tindle, A.G. (1984) Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Geology 25, 956±983. Pimentel, M.M., Heaman, L., Fuck, R.A. and Marini, O.J. (1991) U-Pb zircon chronology of Precambrian tin-bearing continentaltype acid magmatism in central Brazil. Precambrian Research 52, 321±335. Pimentel, M.M., Fuck, R.A. and Botelho, N.F. (1997) Granites and the tectonic evolution of the Neoproterozoic Brasõ lia belt, central Brazil. Extended abstracts, II International Symposium on Granites and Associated Mineralizations. Salvador, eds. Ferreira, V.P. and Sial, A.N., pp. 228±230. Pinto-Coelho, C.V. (1996) Evolution magmatique et hydrothermale du massif granitique de Serra BrancaÐeÂtat de GoiaÂsÐBreÂsil: de®nitions des processus d'alteÂration tardi/post-magmatiques en liaison avec les mineÂralisations en Sn, Be et F. Unpublished Dr. thesis. Institut National Polytechnique de Lorraine, France, p. 228.
Plimer, I.R. (1980) Exhalative Sn and W deposits associated with ma®c volcanism as precursors to Sn and W deposits associated with granites. Mineralium Deposita 15, 275±289. Ren, S.K., Walshe, J.L., Patterson, R.G., Both, R.A. and Andrew, A. (1995) Magmatic and hydrothermal history of the porphyrystyle deposits of the Ardlethan tin ®eld, New South Wales, Australia. Economic Geology 90, 1620±1645. Richardson, S.V., Kesler, S.E., Essene, E.J. and Jones, L.M. (1986) Origin and geochemistry of the Chapada Cu-Au deposit, GoiaÂs, Brazil: a metamorphosed wall-rock porphyry copper deposit. Economic Geology 81, 1884±1898. Rossi, P., Andrade, G.F. and Cocherie, A. (1992) The 1,58 G.A. type granite of Serra da Mesa (GO): an example of ``NYF'' fertile granite pegmatite. Boletim de Resumos Expandidos, XXXVII Congresso Brasileiro de Geologia, SaÄo Paulo 1, 389±390. Sarkar, S.C., Kabiraj, S., Bhattacharya, S. and Pal, A.B. (1996) Nature, origin and evolution of the granitoidhosted early Proterozoic copper-molybdenum mineralization at Malanijkhand, Central India. Mineralium Deposita 31, 419±431. Sillitoe, R.H. (1979) Some thoughts on gold-rich porphyry copper deposits. Mineralium Deposita 14, 161±174. Sillitoe, R.H. (1997) Characteristics and controls of the largest porphyry copper-gold and epithermal gold deposits in the circumpaci®c region. Australian Journal of Earth Sciences 44, 373±388. Silva, G.H., Leal, J.W.L., Salum, O.A.L., Dall'agnol, R. and Basei, M.A.S. (1974) Esboc° o geoloÂgico de parte de Folha SC.21 Juruena. Anais, XXVIII Congresso Brasileiro de Geologia, Porto Alegre 4, 309±320. Sinclair, W.D. (1982) Gold deposits of the Matachewan area, Ontario. The Canadian Institute of Mining and Metallurgy Special Volume 24, 83±93. Siqueira, A.J.B. and Leite, J.A.D. (1997) A Mina ``FilaÄo do Paraõ ba'': um sistema de veios de quartzo aurõ feros associados a zonas de cisalhamento do Precambriano no norte de Mato Grosso. Anais, VI SimpoÂsio de Geologia do Centro-Oeste, Cuiaba 52. Veiga, A.T.C. (1988) Mina de ouro de Novo PlanetaÐAlta ¯oresta, Mato Grosso. In Principais DepoÂsitos Minerais do Brasil, eds. Schobbenhaus, C. and Coelho, C.E.S., pp. 569±574. DNPM, Brasõ lia, Brasil. Vila, T., Sillitoe, R.H., Betzhold, J. and Viteri, A.E. (1991) The porphyry gold deposit at Marte, Northern Chile. Economic Geology 86, 1271±1286. Weihed, P. and Fallick, A.E. (1994) A stable isotope study of Paleoproterozoic Tallberg porphyry-type deposit, northern Sweden. Mineralium Deposita 29(2), 128±138.
Journal of South American Earth Sciences, Vol. 11, No. 5, pp. 427±438, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98 $ - see front matter S0895-9811(98)00026-1
Granite-ore deposit relationships in Central Brazil NILSON F. BOTELHO* and MAÂRCIA A. MOURA Instituto de GeocieÃncias, Universidade de BrasõÂ lia, 70910-900 BrasõÂ lia-DF, Brasil Abstract Ð Ore deposits related to granitic rocks in Central Brazil are associated with dierent Paleo- to Mesoproterozoic granite types. Sn and Au are the economic ore elements with important indium concentrations in some tin deposits. Tin mineralization is related to an A-type granite province, in the State of GoiaÂs, composed of two distinct granite groups of 1.77 Ga and 1.58 Ga ages. The most important tin deposits are closely related to the younger group, hosted in or near to Li-mica2topaz leucogranites. Despite the chemical dierences, both groups are enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. Their Nb/Ta>1 and their high F/Li ratios allow their classi®cation as a NYF fertile granite association. Recent studies reveal indium concentrations that might be recoverable as by-product of tin. Besides indium minerals and In-bearing phases, cassiterite is the most important indium carrier (0.2±0.4% In). Primary gold mineralization is hosted in oxidized I-type calc-alkaline plutons in the northern Mato Grosso State, with characteristics either of volcanic arc or post-collisional granites. Gold occurs in small high grade vein type deposits or is disseminated in widespread hydrothermal zones with alteration such as sericitization, feldspathization and pyritization. d34S values between +1.3 and +3.5- of associated sul®des are typical of magmatic deposits. The association of gold with oxidized I-type granites and the style of hydrothermal alteration are analogous to those associations present in world-class porphyry-style deposits. Although the tectono-magmagtic setting of these granites is not well understood, such an association constitutes an important target in the search for gold in the northern region of the State of Mato Grosso. # 1998 Elsevier Science Ltd. All rights reserved
In the State of GoiaÂs and Tocantins, the so-called GoiaÂs tin province (GTP) comprises a wide region related to the BrasõÂ lia fold belt terranes, of Brasiliano age (Marini and Botelho, 1986). The most important tin granites are Mesoproterozoic A-type granites which are similar in age and type to others described from the Amazonian Craton (Dall'Agnol et al., 1993). In the State of Bahia, minor tin deposits are associated with meta-rhyolites of the Espinhac° o Supergroup and are similar in age and chemistry to the GTP granites. Both GTP granites and the Espinhac° o meta-rhyolites, host the recently discovered indium occurrences.
INTRODUCTION In many studies of mineral deposits in the world, broad associations of granite with ore deposits are recognizable, such as those of tin with fractionated granites and of porphyry Cu±Au deposits with oxidized intermediate magmas. The correlation of these associations with speci®c granite types e.g. porphyries with oxidized calc-alkaline I type granites and Sn deposits with reduced I-, S- and A-type granites, has long been a focus of attention in studies of mineral deposits and mineralizing processes (Sillitoe, 1979; Burnham and Ohmoto, 1980; Plimer, 1980; Ishihara, 1981; Lehmann, 1990; Blevin and Chappell, 1992).
Farther west, in the northern Mato Grasso gold province, many gold deposits are related to Paleo- to Mesoproterozoic oxidized I-type granites (Botelho et al., 1997). Many other gold deposits are spatially related to granitic rocks, in GoiaÂs (Richardson et al., 1986; Arantes et al., 1991; Kuyumjian, 1991) and Mato Grasso (Veiga, 1988) states, but their genetic relationship with granitic magmatism is a controversial issue.
In this discussion, we characterize the granite metallogeny in Central Brazil, describing the granite types, their relationships to ore deposits and the main mineralizing processes, and present an outline of the metallogenetic potential of the main regions containing granite-related mineralization. GENERAL GRANITE METALLOGENY OF CENTRAL BRAZIL
GOIAS TIN PROVINCE
Ore deposits related to granitic rocks in Central Brazil are associated with dierent Paleo- to Mesoproterozoic granite types (Fig. 1). Tin and gold are the economic ore-elements in these granites. More recently, important indium concentrations have been described in some tin deposits.
The GoiaÂs tin province (GTP) comprises a wide region with many tin deposits related to dierent granite types of Mesoproterozoic age (Marini and Botelho, 1986). However, recent geochronological data (Pimentel et al., 1997) suggest that some deposits in the southern part of GoiaÂs State in the Impameri region may be related to Neoproterozoic granites. The northern portion of the GTP, the most important, is
* Corresponding author. E-mail: [email protected]. 427
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Fig. 1. Geological sketch maps for regions containing granite-related ore deposits in central Brazil. A: Northern Mato Grosso gold province (modi®ed from Silva et al., 1974); B: GoiaÂs tin province (modi®ed from Marini and Botelho, 1986).
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429
Fig. 2. Geological map of the Rio ParanaÄ Subprovince (RPS). Distribution of A-type tin massifs; 1: Serra do Mendes; 2: Mangabeira; 3: Mocambo; 4: Pedra Branca; 5: Sucuri; 6: Soledade.
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separated into two subprovinces, Rio Tocantins (RTS) in the west and Rio ParanaÄ (RPS) in the east (Figs 1 and 2). All important tin deposits are related to the RTS and RPS, comprising an A-type granite province composed of two granite groups of dierent ages, 1.77 and 1.57±1.58 Ga (Pimentel et al., 1991; Rossi et al., 1992). The early tin granites, named g1, outcrop only in the RPS, are weakly anomalous in tin and gave rise to only minor cassiterite occurrences. The most important deposits are closely related to the youngest group, named g2, outcropping in both subprovinces and comprising the true tin mineralized granites. Granites of g1 type are potassic and show an alkaline anity while g2 granites are metaluminous to peraluminous and have lower K/Na ratios and higher Li, Rb, Sn and Ta contents compared to the g1 suite (Fig. 3 and 4). Despite the chemical dierences, both g1 and g2 granites may be considered enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. REE patterns for less evolved biotite granites are highly fractionated ((La/Yb)N=20). Later leucogranites, however, display nearly ¯at REE patterns. Besides the late Ta enrichment in g2 facies (Fig. 3), the high concentration of Nb, Y and F, and the high Nb/Ta and F/Li ratios allows the classi®cation of g1 and g2 as a NYF fertile granite pegmatite association of Cerny (1991). The initial characterization of g1 and g2 groups in the RPS (Botelho, 1992), their suggested relationship with RTS granites (Botelho et al., 1993), based mainly on mineral composition (biotite, apatite) and TiO2/MgO, Th/ Ta and Ta/Nb ratios plus additional geochemical data available in the literature (Bilal, 1991; Bilal et al., 1997; Pinto-Coelho, 1996) indicate that these element
Fig. 4. Ta and Sn distribution for all granites of the GTP grouped as g1 and g2, showing important Sn and Ta enrichment in g2 types. Values represent magmatic concentrations of Ta and Sn.
ratios are an important tool in the separation of g1 and g2 granites (Fig. 3). The most evolved tin granites in the g2 group are Li-siderophyllite- to zinnwaldite-bearing leucogranite and albite±topaz leucogranite, outcropping mainly in the RPS, in the massifs of Mangabeira and Pedra Branca. In the RTS, very evolved granites analogous to g2 types were identi®ed only in the Serra Branca Massif (Pinto-Coelho, 1996). In both subprovinces, these granites have the highest magmatic tin concentrations (Fig. 4). Primary and alluvial deposits have been mined as garimpos and small mines since 1960 and have produced around 15 kt of Sn concentrates (ocial data). Major mines are currently closed and the ocial measured reserves for the whole province were estimated as 27 kt/Sn content (Bettencourt, 1997). These reserves refer only to recoverable cassiterite ten years ago, but the tin potential of the GTP is greater considering non-prospected areas in Serra Branca (SRT) and Pedra Branca (SRP) massifs. The recent discovered indium concentration in some deposits and the indium anomalies in cassiterites from many deposits bring new possibilities for the exploration of GoiaÂs tin deposits and, at the least, the RPS may be considered as an In-bearing tin subprovince. Table 1 shows the main features of the GoiaÂs Tin Province (GTP).
Fig. 3. Behavior of Ta and Nb within the RPS g1 and g2 groups of Botelho 1992, compared with the younger granites from the RTS. RPS granites: 1: g1 (Botelho, 1992); 2: Sucuri (Bilal, 1991); 3: g2 (Botelho, 1992). RTS granites: 4: Serra Dourada (Bilal, 1991; Bilal et al., 1997); 5: Serra da Mesa; 6: Serra Branca (Pinto-Coelho, 1996).
Indium-bearing tin deposits The GTP comprises around ten deposits which range in size from minor showings to small/middleclass deposits (<15 kt Sn). Other elements that may be present in subeconomic concentrations are F, Be,
Granite-ore deposits in Central Brazil Table 1. Main features of tin deposits related to the studied A-type granites Subprovince Granite group Age Main mineralized massifs Mineralizations Indium-bearing deposits
RTS g2 1.58 Ga Serra Dourada and Serra Branca Sn, REE, Be, gemstones Unknown
RPS g1 and g2 1.58±1.77 Ga Pedra Branca, Mangabeira, Mocambo Sn, In
Passa-e-Fica; Pedra Branca Greisenized cupolas, Greisenized cupolas, Tin deposit albitites greisenized country environment rocks Topaz±albite granite Li-siderophyllite (Serra Branca massif ) leucogranite, zinnwaldite±topaz± albite leucogranite (Pedra Branca and Most evolved granites Mangabeira massifs) Tin ocial measured 5kt/Sn 22kt/Sn reserves
Ta, In and REE. However, some of these elements could be recovered as by-product of tin. REE minerals, monazite and xenotime, are widespread in placer deposits of the GTP, with important concentrations in the Serra Dourada massif (RTS). In the RPS, xenotime-bearing greisens are described in the Mocambo massif and apatites from Pedra Branca and Mocambo massifs are enriched in REE (5±15% LREE + Y). Moreover, HREE-rich (1±5%) zircons are concentrated in ®ne fractions in the Pedra Branca (RPS) alluvial deposits (Botelho, 1992). Gemstones may also be important and are related to pegmatites and exogranitic alteration halos, mainly in the RTS (ArauÂjo and Alves, 1979; Marini and Botelho, 1986). Tin deposits are related to several types of environment: (i) greisenized cupolas and fractures, as in Serra Branca (TS), Pedra Branca and Mangabeira (PS) massifs; (ii) albitized zones, as in Serra Dourada (TS) and Sucuri (PS) massifs; (iii) greisenized granite±mylonite country rocks, as in Mocambo (PS) and (iv) extensive pegmatite zones, as in Monte Alegre (PS), related to a dierent granite type (S-type?), hosted by older schist sequences.
431
centrations that might be recoverable as by-product of tin. The most important indium-bearing deposit is hosted in the Mangabeira massif, where In minerals are associated with an In-rich cassiterite. In-rich sphalerite, stannite and cassiterite are also found in the Pedra Branca and Sucuri massifs. Cassiterite from RPS tin deposits is at present the important In-bearing phase that may have economic signi®cance because of its indium content (0.2±0.4%In) and volume (Fig. 5). The importance of the indium enrichment in the Mesoproterozoic stanniferous acid magmatism in Central Brazil is enriched by the high indium concentrations found in the wood-tin cassiterite from Espinhac° o meta-rhyolites of the Paramirim region, State of Bahia. These rocks are analogous in age and type to the Ara|¨ Group acid metavolcanics counterparts of the GoiaÂs tin granites. The highest indium content found at Paramirim reached 2% in cassiterite related to phengitic veins in the rhyolite, while the mean value for all analyzed grains is 0.33% In (Fig. 5). The Paramirim cassiterite has the highest indium contents found in tin deposits, exceeding those given in the literature (1.23%) (Oga, 1997; Oga and Botelho, 1997). Despite a high indium content, the small size of the Paramirim tin deposits indicates that they may have little economic signi®cance. The Mangabeira indium±tin prospect Indium minerals were described for the ®rst time in Brazil from the Mangabeira massif, associated with the tin ore in the Passa-e-Fica region (Botelho and Roger, 1990). The main granite in this massif is a pink, porphyritic to equigranular, biotite granite, related to the g1 group, followed by Li-siderophyllite granite and minor zinnwaldite±topaz±albite leucogranite that represents the late mineralized tin granites (g2) (Fig. 6). Tin mineralization is mainly hosted in two types of greisen veins: Li-phengite 2 quartz greisen and Li-phengite 2 zinnwaldite 2 topaz + quartz
The main mineralization processes are related to ¯uorine-rich hydrothermal alteration, giving rise to parageneses with quartz, ¯uorite, topaz, phengite, siderophyllite and minor zinnwaldite in the deposits hosted in the A-type granite massifs. All deposits in the GTP have characteristically low lithium contents and high F/Li ratios. Indium concentrations Cassiterite is historically the only recoverable product in the GTP deposits. However, recent studies (Botelho and Roger, 1990; Botelho, 1992; Moura, 1993; Moura and Botelho, 1994) revealed indium con-
Fig. 5. Indium concentration in cassiterites from the GTP and Espinhac° o meta-rhyolites in Bahia. PA: Paramirim meta-rhyolites; PB: Pedra Branca; MA: Mangabeira.
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Fig. 7. Indium concentration in greisens from the Passa-eFica tin deposit, Mangabeira massif. 1: Li-phengite-bearing greisen; 2: topaz-quartz greisen; 3: albitized granite.
Fig. 6. Geological map of the mineralized zone of the Mangabeira massif. Country rocks: GGC: mylonites and gneiss of the Granite-gneiss Complex; GRA: Aluminous granite. Mangabeira massif: g1c: equigranular to porphyritic pink biotite granite; LGR: Li-siderophyllite-bearing leucogranite; g2 d: medium-grained equigranular pink granite (often greisenized and albitized); GAT: zinnwaldite-bearing topazalbite granite; MGZ: main greisenized zone; In: Indium-rich Li-mica-topaz-quartz greisen; GRE: greisen veins.
greisen. Cassiterite and minor wolframite are the main ore minerals. Indium concentrations were described in zinnwaldite±topaz±quartz greisen, related to the albite±topaz leucogranite, and in an albitized Li-siderophyllite granite, always related to a cassiterite±sul®de association. In-minerals are roquesite (CuInS), dzhalindite (In(OH)3) and yanomamite (InAsO42H2O), a new indium mineral described from the Mangabeira deposit (Botelho et al., 1994). The associated sul®des are loÈllingite, arsenopyrite, sphalerite, chalcopyrite and minor digenite, covellite, chalcocite, bornite, tennantite, stannite, roquesite and bismuth. In some greisen veins, yanomamite and an In-rich scorodite [(Fe,In)AsO42H2O] occur in the same proportions as cassiterite. Indium content in greisen rocks from the main greisenized zone of the Mangabeira massif may reach 1% in the yanomamite-bearing greisen and it is directly related to the tin enrichment. However, there is no important indium enrichment in tin-bearing phengite± quartz greisen (Fig. 7). Chronological and textural relationships between In-minerals and the main minerals in the cassiterite sul®de veins indicate that indium concentration is probably related to a hydrothermal event which gave
rise to the cassiterite ore in the quartz±topaz veins (Botelho and Roger, 1990; Moura and Botelho, 1994). The occurrence of In-minerals and of an In-rich scorodite (up to 6% In) in the Mangabeira tin deposit raises the question of possible economic interest. Moreover, the associated cassiterite that is the most voluminous ore mineral has signi®cant In concentration (up to 0.4%) (Fig. 5) and may represent the best prospect for In.
NORTHERN MATO GROSSO GOLD PROVINCE Primary gold mineralization is widespread along the southern border of the Cachimbo Graben, in the northern Mato Grosso gold province (MGP). Deposits hosted in granitic rocks are distributed in the Archaean/Paleoproterozoic Xingu complex terranes and are hosted in oxidized I-type calc-alkaline and oxidized alkaline granites. This gold province has many similarities to the TapajoÂs granite related gold province, north of Cachimbo graben (Fig. 1) (Coutinho et al., 1997). The genetic link between several deposits and the associated granite in the MGP is poorly understood and we present here only the deposits studied where this association is not controversial. The main prospects are in the MatupaÂ-Peixoto de Azevedo region, in the eastern portion of the area, and Alta FlorestaParanaõÂ ta, in the west (Abreu Filho et al., 1992; Paes de Barros, 1994) (Fig. 8, Table 2). Despite some prospecting programs developed by mining companies since 1985, gold has been produced from alluvial and hardrock workings only by garimpeiros and small mines. The cumulative production, since 1979 is around 200 tonnes gold (Miranda, 1997). Because of this production, despite the absence of true mines, the
Fig. 8. Geological map of the Alta Floresta-Peixoto de Azevedo region (MGP), showing the distribution of the most important primary gold deposits and occurrences.
Granite-ore deposits in Central Brazil 433
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Table 2. Main characteristics of gold deposits in the MatupaÂÐPeixoto de Azevedo and ParanaõÂ taÐAlta Floresta regions, related to or hosted in I-type oxidized granites Gold deposit Serrinha (1) (MatupaÂ) ParaõÂ ba (2) (Peixoto de Azevedo) EreÂdio-Melado (3) (Peixoto de Azevedo) Olerindo (4) (MatupaÂ) Waldemar (5) (ParanaõÂ ta)
Host rock Monzogranite, quartz monzonite Granodiorite, tonalite, amphibolite Granodiorite, tonalite Rhyolite, porphyry, andesite Monzogranite, quartz monzonite
Wall-rock alteration K-silicate, albitization, sericitization K-silicate, propylitization, silici®cation Chloritization, sericitization, silici®cation Propylitization, silici®cation K-silicate, silici®cation
Sul®de association in the ore py2 cp2 sl2gn 2po2Ag-sulpho py2 cp2 bmt2bi 2po2sl2 m o2 gn2thd py2 cp py2 cp py2 cp2 bmt2bi 2tdm2sl2 gn
(1): Localization in Fig. 8. (py: pyrite; cp: chalcopyrite; sl: sphalerite; gn: galene; po: pyrhotite; mo: molibdenite; bmt: bismuthinite; bi: bismuth; thd: tetrahedrite; tdm: tetradymite; sulpho: sulphosalts)
exploited gold prospects are here considered as gold deposits. Oxidized I-type calc-alkaline granites The petrographic and geochemical features of the calc-alkaline granites in the MGP discriminate them as oxidized I-type granites (Botelho et al., 1997; Moura et al., 1997a). The oxidation state of silicic magmas ranges over several orders of magnitude of fO2 and may be a re¯ection of their source-region that controls the magmatic Fe3+/Fe2+ ratio (Burnham and Ohmoto, 1980; Carmichael, 1991). The MGP granites contain hornblende, biotite, titanite, magnetite, Fe3+-rich ilmenite, pyrite and pink-coloured Kfeldspar and are similar to oxidized I-type granites described in the Paleozoic fold belts of eastern Australia (Chappell et al., 1988; Blevin and Chappell, 1992). The oxidation state of magmas is a fundamental feature in the mineralizing potential of granitic rocks and Cu±Au mineralizations are related to the most oxidized types (Burnham and Ohmoto, 1980; Candela, 1992). MGP calc-alkaline granites contain high (2.6) MgO/TiO2 ratios, intermediate Zr values (100±249 ppm) and low Nb (15 ppm) and Y (9 ppm) concentrations. REE patterns are strongly fractionated (La/ Yb = 30) with weak Eu anomalies (Eu/Eu* = 0.35).
Fig. 9. Al(tot) Mg diagram (Nachit et al., 1985) for biotites from regional Xingu granite-gneiss and MGP granites. Biotite compositions, except those from alkaline granites, plot in the ®eld of calc-alkaline series.
Biotite compositions on the Mg Al diagram (Nachit et al., 1985) plot in the calc-alkaline ®eld (Fig. 9). On the discriminant diagrams of Pearce et al. (1984), the granites plot in the VAG ®eld, near the SYB and WPG ®elds (Fig. 10), interpreted as a characteristic of post-collisional granites (Harris et al., 1986) (Fig. 11a and b).
Oxidized alkaline granites Alkaline granites are widespread in the MGP region, distributed as irregular bodies in the Xingu complex or as rounded bodies in the volcanic terranes of the Iriri Formation. In regional mapping (IBGE, 1990), these granites are all grouped in the anorogenic Teles Pires-type Suite (Figs 1 and 8). However, despite chemical dierences in relation to the calc-alkaline granites, such as lower MgO/TiO2 ratio and higher Zr and Y contents, preliminary geochemical data indicate that the alkaline massifs of GuarantaÄ and Terra Nova do Norte may be also characterized as volcanic-arc granites (Botelho et al., 1997) (Fig. 10). This group of granites comprises subsolvus to hypersolvus facies containing richterite, magnetite and a biotite, which is characteristic of alkaline granites (Fig. 9).
Fig. 10. Tectonic setting of MGP granites on the discriminant diagram of Pearce et al. (1984). VAG: volcanic; ORG: ocean ridge; syn-COLG: syn-collision; WPG: withinplate. Samples plot in VAG ®eld near WPG and syn-COLG ®elds boundaries, like late to post-collision granites.
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Peixoto de Azevedo prospect along with several smaller deposits. The main gold-bearing granite in this area, the 1872 Ma old Matupa Monzogranite, is related to endogranitic and exogranitic deposits. This massif is a homogeneous, undeformed, equigranular to porphyritic monzogranite, with geochemical characteristics either of volcanic arc granites or of post-collisional granites (Figs 10 and 11) generated in the presence of an oceanic lithosphere (Moura et al., 1997a). The Matupa massif is the type-area for the oxidized calc-alkaline granites of the northern Mato Grosso gold province. Small high grade quartz vein-type deposits occur at the outer margins of the Matupa monzogranite, hosted in andesite±rhyolite volcanic rocks or related to shear zones in hydrothermally-altered granodiorite and tonalite (Paes de Barros, 1994; Siqueira and Leite, 1997, examples 2, 3 and 4 in Table 2). The link between this high-grade gold mineralization and the Matupa Monzogranite remains speculative. The granite-related type-deposit in the MGP, named the Serrinha gold deposit, is located at the northern border of the Matupa monzogranite and contains many hydrothermally-altered zones with disseminated gold concentrations. Serrinha deposit
Fig. 11. Matupa granite samples in Harris et al. (1986) diagrams for acid intrusive rocks. (a): volcanic-arc (VA), withinplate (WP), syn-collision (II) and late to post-collision (III) tectonic settings; (b) group I (volcanic-arc), group II (syn-collision) and group III (late to post-collision) granites. In both diagrams samples plot in late to post-collision granite ®elds.
Many occurrences and small deposits of gold are related to the Teles Pires suite. These are always controlled by regional lineaments and/or shear zones. At the abandoned Novo Planeta mine, important gold placer deposits were related to supergene enrichment in lateritic pro®les in the outer margins of altered and sheared biotite granite. Primary gold mineralization is very rare in the prospect, but there is a probable link between placer gold and an earlier gold concentration related to a magmatic stage, followed by metasomatism, shearing and supergene processes that gave rise to the deposit (Veiga, 1988). Despite these ®eld relationships in the Novo Planeta mine and elsewhere, the generic link between gold mineralization and Teles Pires-type granites remains a controversial issue. Gold deposits Gold occurs in quartz veins and disseminated in widespread hydrothermal zones, in the MatupaÂ-
The Serrinha gold deposit is spatially related to the Matupa granitic massif areas that have been subjected to pervasive hydrothermal alteration. The ®rst metasomatic event was a widespread microclinization. The microclinized granite was then altered by sodic metasomatism when pure albite was formed, mainly replacing K-feldspar. Locally, the extreme sodic metasomatism led to the development of ®brous aegirine. Pervasive chloritic and serictic alteration is observed in the deposit. Chlorite occurs either ®lling fractures of microcline, albite and quartz, together with sericite, quartz and calcite, or as masses, commonly with sericite. Pyritization was the last important hydrothermal phase in the deposit. Pyrite is generally related to hydrothermal magnetite and rutile. At the north of the deposit's core, secondary epidote masses were developed over the granite, possibly representing an outer epidote aureole. The general order of hydrothermal alteration in Serrinha was Ksilicate, albitic, chloritic, sericitic and pyritic alteration phases. Many of the Serrinha gold deposit features, such as granite type and hydrothermal alterations, are analogous to those described in the TapajoÂs Province, related to Maloquinha-type granite (Coutinho et al., 1997). The gold mineralization occurs in the most intense hydrothermally altered zones. Gold is either disseminated or ®lls fractures in pyrite related to potassic, chloritic, sericitic and albitic alteration. Other sul®des
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NILSON F. BOTELHO and MARCIA A. MOURA the GoiaÂs tin province (GTP) and the northern Mato Grosso gold province (MGP).
Fig. 12. d34S in pyrite from Serrinha deposits compared to granite-related mineralization.
are not common, but minor chalcopyrite, sphalerite, galena and pyrrhotite do occur as inclusions in pyrite. Pyrite d34S values for the Serrinha deposit lie between +1.3 and +3.5-, pointing to a homogeneous sulfur source (Fig. 12) (Moura et al., 1997b). Sul®de d34S values near 0- indicate a deep source of sulfur and the related ore deposits are interpreted to have formed from magmatic ¯uids (Ohmoto and Rye, 1979). In the Serrinha deposit, d34S values together with petrologic data are consistent with a magmatic origin for the gold mineralization, probably related to the same ¯uids that caused hydrothermal alteration of the Matupa granite, since d34S values and alterations are similar to those described in some porphyry-type deposits (Eastoe, 1983; Gallagher et al., 1992; Ren et al., 1995; Weihed and Fallick, 1994) and graniterelated gold deposits (Sarkar et al., 1996) (Fig. 12). The spatial association between the Matupa monzogranite and the Serrinha deposit, hydrothermal alteration types related to the gold mineralization, presence of hydrothermal magnetite in association with pyrite, sulfur isotope data and the petrologic data for the Matupa monzogranite in the Serrinha deposit are also characteristic of porphyry copper±molybdenum and copper±gold deposits (Sillitoe, 1997). Nevertheless, the Serrinha deposit has low levels of Cu and Mo, which preclude classi®cation with the base-metal-rich porphyries. Sillitoe (1979) predicted the existence of copper-poor porphyry gold deposits and Sinclair (1982) has classi®ed the Matachewan syenite-hosted gold deposits as porphyry gold deposits. Vila et al. (1991) described a porphyry gold deposit in Marte, Chile and the recently developed Fort Knox deposit (Bakke, 1995) is a further example. The characteristics presented for the Matupa monzogranite and the Serrinha Gold Deposit allow its classi®cation as a porphyry gold deposit.
SUMMARY AND CONCLUSIONS In the central region of Brazil, granite-related ore deposits are part of two important mineral provinces:
The most important tin deposits are hosted in Mesoproterozoic A-type granites distributed in the northernmost of the GTP, of two ages: g1 (1.78 Ga) and g2 (1.58 Ga). Despite tin anomalies in both groups, only the youngest granites gave rise to important tin deposits. This could be explained by the fact that the most evolved granites are related to the g2 group that produced at the end of its evolution facies such as topaz-bearing leucogranites. In the Rio ParanaÄ subprovince (RPS) tin deposits are closely related to the massifs where topaz±albite leucogranite and/or Li-siderophyllite leucogranite outcrop. However, in deposits hosted in g2 granites of the Serra Dourada massif in the Rio Tocantins subprovince (RTS), very evolved leucogranites are unknown. Indium anomalies are widespread within the tin deposits situated in the RPS with an important concentration in the Passa-e-Fica deposit in the Mangabeira massif. Indium is found as arsenate, sul®des and hydroxides and in minor concentrations mainly in sphalerite and cassiterite. Sphalerite is the richest indium carrier (up to 6% In) but, because of its volume, cassiterite, with up to 0.4% In in the Mangabeira massif and elsewhere is the most important source and may have economic signi®cance. The occurrence of In brings new possibilities for reopening closed mines and allows the RPS to be classi®ed as an In-bearing tin subprovince. In the MGP, the common association of gold with oxidized I-type granites is characteristic. The Matupa granite represents the type granite-gold association in this province and its age, 1.87 Ga, and chemical features are an important indicator for gold prospecting in the region, since preliminary mapping indicates a wide distribution of this type of granite in the northern region of Mato Grosso State. Oxidized calc-alkaline granites from the MGP and, perhaps their distribution and hydrothermal alteration, are quite similar to the Maloquinha type described from the TapajoÂs Province, Para State. The genetic link between gold and alkaline granites in the MGP is controversial and speculative. However, some of these granites are also oxidized types and could represent a target for the gold prospecting in the region. The association of porphyry-type gold deposits with less fractionated oxidized granite magmas is similar to the association present in the MGP, mainly in the Peixoto de Azevedo region. Nevertheless, the tectonomagmatic setting of the MGP granites is poorly understood and a better knowledge of this is very important to establish the metallogenetic potential for the region inasmuch as, for example, known worldclass porphyry Cu±Au deposits are directly related to a subduction environment. So, additional regional
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mapping and detailed studies are needed before proposing a complete metallogenetic model for the Mato Grosso gold province.
Botelho, N.F., Moura, M.A., Souza, M.T. and Antunes, J.A. (1997) Petrologia e potencial metalogeneÂtico de granitos da regiaÄo de Peixoto de AzevedoÐAlta Floresta, Mato Grasso. Anais, VI SimpoÂsio de Geologia do Centro-Oeste, CuiabaÂ, 40±42.
Acknowledgements Ð Financial support for research in the MGP and GTP was provided through the Brazilian Research CouncilÐ CNPq, PADCT-FINEP projects and DNPM. We are also grateful to WCM, METAMAT, DNPM-Cuiaba and Brumadinho Group for ®eld assistance. Dr Timothy Liverton is thanked for the comments and review of the manuscript.
Burnham, C.W. and Ohmoto, H. (1980) Late-state processes of felsic magmatism. In Granitic magmatism and related mineralization, eds. Ishihara, S. and Takenouchi, S. Mining Geology, Special Issue, 8, 1±11.
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