W Au skarns in the Neo-Proterozoic Seridó Mobile Belt, Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit

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1 Miner Deposita (2008) 43: DOI /s ARTICLE W Au skarns in the Seridó Mobile Belt, Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit João Adauto Souza Neto & Jean Michel Legrand & Marcel Volfinger & Marie-Lola Pascal & Philippe Sonnet Received: 27 October 2005 /Accepted: 12 June 2007 /Published online: 28 July 2007 # Springer-Verlag 2007 Abstract The Seridó Mobile Belt (SMB) is located in the Borborema Province in northeastern Brazil and consists of a gneiss basement (Archean to Paleo-), a metasedimentary sequence (marble, quartzites, and schists), and the Brasiliano igneous suite (both of age). In this region, skarns occur within marble and at the marble schist contact in the metasedimentary sequence. Most of the skarn deposits have been discovered in the early 1940s, and since then, they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they Editorial handling: S. Hagemann J. A. Souza Neto (*) Departamento de Geologia, Universidade Federal de Pernambuco, Av. Acadêmico Hélio Ramos, s/n, Cidade Universitária, CEP Recife, Brazil adauto@ufpe.br J. M. Legrand Departamento de Geologia, Universidade Federal do Rio Grande do Norte, Campus Universitário, Lagoa Nova, CEP Natal, Brazil M. Volfinger : M.-L. Pascal Centre de Recherches sur la Synthèse et Chimie des Mineraux (CRSCM), Centre National de la Recherche Scientifique (CNRS), 1A rue de la Férollerie, Orléans Cedex 2, France P. Ph. Sonnet Département des Sciences du Milieu et de l Aménagement du Territoire, Université Catholique de Louvain, 2 Place Croix du Sud, Boîte 10, 1348 Louvain-la-Neuve, Belgium are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns, which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10- to 15-km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesiohornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite zoisite sericite, calcite, and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble schist contact. The main ore body is cm wide and contains an average of 4.8- wt.% WO 3, which occurs in the basal marble schist contact. Fold hinges appear to control the location of high-grade scheelite. The late-stage gold mineralization contains bismite (Bi 2 O 3 ), fluorine-bearing bismite, native bismuth, bismuthinite (Bi 2 S 3 ), and joseite [Bi 4 (Te,S) 3 ], and also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold bismuth tellurium mineralization exhibits a typical late character and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that crosscut not only the banding in prograde skarn but also the retrograde alkali feldspar and clinozoisite zoisite sericite assemblage. The Au Bi Te-bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits.

2 186 Miner Deposita (2008) 43: Keywords Tungsten and gold skarns. Bonfim. Borborema Province. Seridó Mobile Belt. Brazil Introduction Skarn deposits host a large variety of metals (Fe, Cu, Au, Mo, W, Pb Zn, Sn) and represent an important deposit class. These deposits are subdivided according to their economic metal content (Einaudi et al. 1981; Einaudi and Burt 1982; Meinert 1989). The different ore types and their particular geochemical associations reflect the tectonic setting and the nature of the parental igneous rock (fluid), the composition of protoliths, and the temperature pressure (depth) of ore formation (Einaudi et al. 1981; Einaudi and Burt 1982; Kwak 1987; Meinert 1989, 1992; Zharikov 1991; Lentz 1998). In northeastern Brazil, mineralized skarns have been known since at least the 1940s, and some of them have been mined since then (Johnston and Vasconcellos 1945; Maranhão 1970; Lima et al. 1980; Salim 1993; Souza Neto et al. 1999). Their economic interest stems from their contents of W Mo and gold. Numerous occurrences (almost 700, according to Lima et al. 1980) have been found within an area of about 20,000 km 2 known as the tungsten province of Seridó (Fig. 1). Most of the deposits have been exploited for W Mo (e.g., Brejuí, Bodó, Bonfim, Bonito, Malhada Limpa, and Quixaba mines; Fig. 1), and one has been exploited for gold only (Itajubatiba mine; Fig. 1). In the early 1990s, gold was discovered in the tailings of the abandoned Bonfim W Mo mine. Until recently, it is the only deposit in this Brazilian region where W Mo and gold have been found together. In the context of metals produced, most deposits in the Seridó Mobile Belt (SMB) can be classified as tungsten skarns, with the exception of the Bonfim tungsten gold skarn and Itajubatiba gold skarn. With regard to tungsten skarns, their main characteristic features have been pointed out as being reduced and formed deeper (Kwak 1987) than the gold skarns, which are formed at relatively shallow depth. Gold skarns are oxidized and usually occur associated with copper porphyry deposits, and can contain Cu and Fe as associated metals (Einaudi et al. 1981; Einaudi and Burt 1982; Meinert 1989, 1992; Lentz 1998; Gray et al. 1995; Eilu et al. 2003). However, the recognition of economic gold mineralization in reduced tungsten skarns was only recently observed by Newberry (1998). In the last years, important gold occurrences in tungsten skarns have been studied worldwide, for example, Fig. 1 Geologic setting of the structural province of Borborema, northeastern Brazil (adapted from Archanjo 1993). The location of SMB, tungsten province of Seridó, principal skarn deposits, and gold-quartz vein deposits are also shown

3 Miner Deposita (2008) 43: in the Nevoria deposit in Western Australia. Nevoria is a reduced skarn deposit that formed at midcrustal level (10 15 km), and contrasts with the copper gold and gold-rich iron copper skarns that formed at shallower depths (2 5 km; Mueller 1997; Mueller et al. 2004). In some tungsten skarns, e.g., the Emerald tungsten camp in the Canadian Cordillera (Ray et al. 1995; Ray and Dawson 1998), it seems uncertain whether the gold was contemporaneous with the skarn or represents a later epithermal overprint. This review of W Au skarns in the SMB contributes to the understanding of gold mineralization in tungsten skarns, summarizes the main geological aspects of the SMB and its ore deposits, as well as the detailed geology and petrography of the Bonfim tungsten gold skarn deposit. The main purposes of the petrographic study at Bonfim skarns were to: (1) provide a detailed description of the complex ore mineralization discovered in these skarns and (2) establish the chemical composition of ore minerals using modern microanalytical techniques such as particle-induced X-ray emission (PIXE), which is able to detect trace elements in minerals, and electron microprobe (EMP). The mineral chemistry results obtained on ore minerals are used to elucidate the composition of the ore-forming fluids and to establish their geochemical association, providing fundamental information for understanding the ore genesis and providing some criteria that can be used in the exploitation of the Bonfim deposit and exploration in the region. Geology of the SMB The SMB represents the northern segment of the Borborema Province (Almeida et al. 1981) and is located north of the Patos lineament (Fig. 1). In the SMB, continental-scale NE-trending strike-slip shear zones dominate the tectonic features (Fig. 1). These shear zones with associated folds and thrusts were developed during transpressional tectonics that were active during the Brasiliano/Pan-African orogeny around 600 Ma (Archanjo and Bouchez 1991; Caby et al. 1991; Corsini et al. 1991, 1992, 1996; Vauchez et al. 1995). In the SMB, rocks are grouped (from the oldest to the youngest) into the gneiss basement, Seridó Group metasedimentary rocks, and Brasiliano igneous suite. Gneiss basement ( Ga) The basement contains migmatitic orthogneisses of tonalitic to granitic composition and minor supracrustal rocks. They have Archean to Paleo- ages that vary from 2.2 to 2.0 Ga (Hackspacher et al. 1990; Legrand et al. 1991b; Souza et al. 1993; Dantas et al. 2004), but older ages of 3.5 Ga were obtained for some blocks located in the easternmost SMB basement (Dantas et al. 2004; Fig. 1). Seridó Group metasedimentary rocks (650 Ma) The metasedimentary sequence is composed of typical supracrustal rocks, consisting from bottom to top, calcsilicate paragneisses and marble (Jucurutu Formation), quartzites and metaconglomerates (Equador Formation), and mica schists (Seridó Formation). Uranium Pb and Sm Nd dating (zircon and whole rock, respectively) in metasedimentary and metatuffaceous rocks of the Seridó Group have determined that its deposition probably occurred ca Ma (Van Schmus et al. 2003). Brasiliano igneous suite ( Ma) to Cambrian igneous rocks are widely distributed in this Brazilian region (Fig. 1). Most of them are spatially linked to the Brasiliano/Pan-African shear zones, and some intrusions exhibit asymmetric sigmoid shapes. Petrostructural and magnetic susceptibility studies carried out in several of these intrusions support their syntectonic character (Archanjo 1993; Archanjo et al. 1994, 1999; Lima et al. 2000). Porphyritic and fine-grained granites are the dominant rocks in these igneous bodies, with minor equigranular granites, fine- to medium-grained aplites and pegmatites (see Table 1 for classification of intrusive rocks related to the main skarn deposits of the SMB). These granitoid rocks are syn- to late-kinematic (Brasiliano orogeny) and present ages between 610 Ma (U Pb method on zircon; Jardim de Sá 1994; Jardim de Sá et al. 1999) and 555 Ma (U Pb method on zircon; Legrand et al. 1991a) throughout the SMB. The pegmatite bodies, which constitute late phases of the Brasiliano/Pan-African magmatic activity, are constrained by U Pb uraninite ages of 480 and 510 Ma (Florencio 1948 and Marble 1949, respectively; both in Ebert 1970) and a 40 Ar/ 39 Ar biotite mean age of 523.4±1.1 Ma for six biotite samples from Be Ta Li pegmatites of the SMB (Araújo et al. 2005). This late age is considered by the authors to represent the first increments of late reactivation of the Brasiliano strike-slip shear zones. Recently, U Pb analyses of columbite from six heterogeneous mineralized pegmatites indicate formation ages between 509.5±2.9 Ma and 514.9±1.1 Ma (Baumgartner et al. 2006). Regional and contact metamorphism The tectono-metamorphic evolution of the SMB is largely dominated by the widespread Brasiliano orogeny (ca. 600 Ma) that affected all geological units and promoted partial fusion (migmatites) and large (high-angle) transcurrent shear zones (Caby 1989; Archanjo and Bouchez 1991; Caby et al. 1991, 1995; Jardim de Sá 1994; Brito Neves

4 188 Miner Deposita (2008) 43: Table 1 The characteristics of the principal skarn deposits in the SMB Principal mine (small adjacent mines), metals Tonnage, grade Host rocks Skarn morphology Intrusive rocks Brejuí (Barra Verde, Boca de Lage, Zangarelhas, Carnaubinha, Juazeirinho, Saco dos Veados) W (Mo, Fe, Cu) 11 Mt (5.5 Mt remaining) 0.5 1% WO 3 marble, gneiss, pegmatite Stratiform; vuggy; occasional vein skarn, retrograde alteration crosscutting 579±4 to 555±5 Ma bt granite batholith; pegmatite; aplite dikes Bodó (Riachão, Umbuzeiro, Isidoro, Queimadas, Cafuca) W ±9 Mt (average=2) %WO3 marble, gneiss Stratiform; lens-shaped; pockets; vuggy; occasional vein skarn, retrograde alteration crosscutting equigranular bt granite (stocks); pegmatite; aplitic granites >0.3 Mt (average= 4.8) % WO t Au Bonfim (Sulista, Queiroz, Catolé II, Gupiara, Pedra Preta- marble, schist Stratiform; retrograde alteration crosscutting and as impregnation bt granite; pegmatite Alteration in intrusive Skarn mineralogy Ore minerals Formation References conditions c Prograde a Retrograde b Early stage Late stage Endoskarn Skarn marble/ gneiss/pegmatite: px (Hd30 60Jo1 8), pl (An ; An in amphibole skarn), grt (Ad Al+ Sp2 7Gr51 78), amp (hbl), ttn, qtz, mc Endoskarn; ep, mo, py None observed px, grt, amp, wo, ttn, <fld Skarn in marble: px (Hd Jo 1 3 ), amp (tr-mhb), ttn, ap, wo, <ol; Skarn in schist: px (Hd Jo 1 4 ), 20 30% (?) Skarn marble/gneiss/ pegmatite: amp (act), mei, ves, act, phl, ep, prh, czo (later than ep), rds, cbz, stb, ser, chl, brt, fl 10 40% (?) ep, ves, amp (?) Sch (early finegrained; late coarsegrained in porous skarns, richest ones), mo, py, cp, bn, fb, pw, wf, mgt, bi, bmt, po (?) sch, mo, py, cp, bn % Skarn in marble: amp (tr, mhb); Skarn in schist: kfd, czozo, ser, bt, amp (act-mhb; athmgt, po, cp, py, sch, mo, <<apy bi, bmt, jo, cp, sp, gold P2 3 kbt p C (mineral equilibria) T r C <15 eq. wt.% NaCl (mineral equilibria and fluid inclusions) P kbt p C T r C XCO (mineral equilibria) 4.8 Salim (1993); Maranhão (1970); Maranhão et al. (1986); Barbosa et al. (1969); Goñi and Picot (1965); Beurlen and Borges (1990); Gouveia (1977); Cavalcante Neto (1986); Legrand et al. (1991a); Jardim de Sá (1994); Jardim de Sá et al. (1986) Costa (1995); Legrand et al. (1994); Roy (1966); Zanini and Santos (1979); Lima et al. (1980); Melo (1961); Silva (1971) Souza Neto (1999); Lima (1982); Salim (1979); Farias et al. (1973); Potyra

5 Miner Deposita (2008) 43: Table 1 (continued) Principal mine (small adjacent mines), metals Tonnage, grade Mulungu, Matinha) W (Au, Bi, Te) 2 6 (up to 60) ppm Au up to 26,000 ppm Bi up to 500 ppm Te Bonito (Pindoba- Mazagão) W >4 Mt % WO3 Malhada Limpa- Timbaúba W, Mo 5.5 Mt % WO % Mo Quixaba (Quixeré, Malhada Vermelha, Cacimbas) W >2.5 Mt % WO3 Itajubatiba Au (Fe, Cu) <1 Mt (up to 6.3) ppm Au Host rocks Skarn morphology Intrusive rocks concordant to the rock banding marble and gneiss gneiss, marble marble and calc-silicate gneiss Stratiform 579±7 Ma hbl (?) granodiorite; pegmatite; aplite Stratiform; lens-shaped; pockets; vuggy; filling fractures Stratiform; lens-shaped bt (?) granite; pegmatite hbl granodiorite, bt granite, bt-amp-px alkali syenite marble, metatonalite, metasyenogranite Stratiform; lens-shaped; vuggy; occasional vein skarn, retrograde alteration crosscutting 573±45 Ma bt syenogranite; abundant pegmatites Alteration in intrusive None reported None reported Sch (upper aplitic part of granites, and aplitic veins) px-pl-amp and amp-qtz endoskarns; qtz veins; widespread carbonatation; large ap and bt Skarn mineralogy Ore minerals Formation References conditions c Prograde a Retrograde b Early stage Late stage pl (An85 99), grt (Ad 3 5 Al+ Sp Gr ), amp (act-mhb), ttn, ap, aln, zrn, mnz cum in phlogopitebearing schist), Fe-rich ep, prh, chl, ves, scp, ms px, grt, amp >40% (?), ep, >> fl sch, mo, py, cpy, bn, <gn pl, px, grt, ap, qtz (?), ttn, toz (?) >10% (?), scp, ep, ser, chl, ves, amp sch, >>mo, pw, py, cp, bn eq. wt.% NaCl+CH 4 (N 2 ) fluids in qtz-sulfides and calsulfides veins in the skarns (fluid inclusions) (1978); Barros (1964); Feitosa (1964) Santos (1968); Lima et al. (1980); Brito Neves et al. (2003) Roy (1964); Santos et al. (1972); Lima et al. (1980) grt, px, scp, pl, aln, ap, ttn Skarn in marble: px (Hd5 75Jo0 3), amp (act-tr-mhb; <fac-fhb-ts, prged), ttn, ap, ol; Skarn in metatonalite: px (Hd26 61Jo1 2), pl (An ), grt >20% (?), hbl, act, tr, ep, czo, prh, ab, chl, ves, zeo, rdn, fl 20 30% Skarn in marble: amp, huchu, srp, cch; Skarn in metatonalite/ metasyenogranite: amp, ser, ep, bt sch, mo, py, cp, bn, mgt, gn, sp mgt po, py, cp, << apy, gold P kbtp C Tr C X CO (mineral equilibria) eq. wt.% NaCl+ N 2 (up to 18 Andritzky (1972); Andritzky and Busch (1975); Cassedane et al. (1972); Saldanha (1963); Santos and Brito Neves (1984) Souza Neto (1995, 1999); Lins and Scheid (1981); Rebouças (1985); Brito Neves et al. (2003);

6 190 Miner Deposita (2008) 43: Table 1 (continued) Principal mine (small adjacent mines), metals Tonnage, grade Host rocks Skarn morphology Intrusive rocks Alteration in intrusive Skarn mineralogy Ore minerals Formation References conditions c Prograde a Retrograde b Early stage Late stage (Ad4 13Al+Sp69 78 Gr 2 25 ), amp (act-mhb-fhb-fts; < fprg-hs, gru), ttn, ap; Skarn in metasyenogranite: px (Hd15 61Jo1 2), pl (An ; An or scp), grt (Ad Al+ Sp4 5Gr34 46), amp (act-mhb), ttn, ap, aln mol% CH4) fluids in qtzcal-sulfides and qtz-calmgt veins in the skarns (fluid inclusions) Galindo and Sá (2000) Mineral abbreviations (according to Kretz 1983 and Spear 1993): ab albite, act actinolite, aln allanite, amp amphibole, ap apatite, apy arsenopyrite, ath anthophyllite, bi bismuth, bmt bismuthinite, bn bornite, brt barite, bt biotite, cal calcite, cbz chabazite, cch clinochlore, chl chlorite, chu clinohumite, cp chalcopyrite, cum cummingtonite, czo clinozoisite, czo-zo clinozoisite/zoisite, ed edenite, ep epidote, fac ferro-actinolite, fb ferberite, fl fluorite, fhb ferro-hornblende, fld feldspar, fprg ferro-pargasite, fts ferro-tschermakite, gn galena, grt garnet, gru grunerite, hbl hornblende, hs hastingsite, hu humite, jo joseite, kfd alkali feldspar, mc microcline, mei meionite, mgt magnetite, mhb magnesiohornblende, mnz monazite, mo molybdenite, ms muscovite, ol olivine, phl phlogopite, pl plagioclase, po pyrrhotite, prg pargasite, prh prehnite, py pyrite, pw powellite, px pyroxene, qtz quartz, rdn rhodonite, rds rhodochrosite, sch scheelite, scp scapolite, ser sericite, sp sphalerite, srp serpentine, stb stilbite, toz topaz, tr tremolite, ts tschermakite, ttn titanite, ves vesuvianite, wf wolframite, wo wollastonite, zeo zeolites, zrn zircon a Pyroxene compositions expressed as mole percent hedenbergite (Hd) and johannsenite (Jo); remainder is diopside. Plagioclase compositions expressed as mole percent anortite (An); remainder is albite+orthoclase. Garnet compositions expressed as mole percent andradite (Ad), almandine (Al)+spessartine (Sp), and grossularite (Gr). Amphibole compositions expressed as nomenclature of the amphibole group minerals. b Extent of retrograde alteration expressed as approximate percent of total volume of skarn; all retrograde assemblages include quartz and calcite. c Tp =temperature of prograde skarn formation; T r =temperature of retrograde alteration.

7 Miner Deposita (2008) 43: et al. 2000). In addition, the gneiss basement also records a pre-brasiliano tectono-metamorphic event around 2.0 Ga (Transamazônico/Eburnian orogeny), which developed migmatites associated with low-angle structures (thrusttype tectonics) and minor magmatic activity (Jardim de Sá 1994; Hackspacher et al. 1995; Dantas et al. 2004). The conditions of peak metamorphism accompanying the main tectonic event (Brasiliano orogeny) in the SMB have been first established at C and 3 4 kbars, based on a geothermobarometric study in the central part of this belt (Gama and Albuquerque 1985; Lima 1986). Most recently, these peak conditions were more precisely estimated to be C and kbars, using different geothermobarometers applied to the different metamorphic zones (biotite garnet, cordierite andalusite, and sillimanite muscovite) observed in the schists-containing shear-zonehosted gold-bearing quartz veins of the São Francisco deposit (Luiz-Silva 1995), approximately 50 km east of the Currais Novos town, and of the São Fernando-Caicó district (Luiz-Silva 2000), about 100 km southwest of the Currais Novos (Fig. 1). Combining different geothermobarometers with 40 Ar/ 39 Ar geothermochronometry on mineral assemblages occurring in the contact metamorphic aureole of the Brasiliano Acari granite (batholith close to the Brejuí skarn deposit in Fig. 1), a thermal peak of 630 C and 3.5 kbars during Ma (U Pb) was established, followed by a cooling to C around 481 Ma (Ar Ar; Cunha de Souza 1996). These data can be used to indicate a deep level (10- to 15-km depth) of pluton emplacement and skarn formation, and subsequent uplift. Seridó Belt ore deposits The SMB shows a characteristic presence of mineral deposits, with a significant variety of mineral resources. Most of these deposits were discovered at the beginning of the 1940s and have been explored since then. The main ores exploited are those found in skarn deposits (W Au), pegmatites (Ta Nb), and in quartz veins (Au). Most of the skarn deposits appear to be related in space and time to plutons of the Brasiliano igneous suite ( Ma). Their occurrence is close to these plutons, but some skarns are distal in character (e.g., Malhada Limpa and Quixaba mines). For these latter skarns, shear zones appear to be the main conduit of fluid flow responsible for their formation. In most cases, the skarns are strata controlled and occur within marble beds, or at the marble schist (or gneiss) contact, but locally, skarns also occur within gneiss and schist, or within the related igneous rocks (endoskarn; e.g., Bodó and Itajubatiba skarns). The Ta Nb pegmatites are related to the igneous rocks of the Brasiliano suite, and some of them crosscut massive posttectonic skarns (e.g., Brejuí, Bodó, and Itajubatiba mines Barbosa et al. 1969; Zanini and Santos 1979; Souza Neto 1999). Most pegmatites intrude the schists (Seridó Formation) and also the underlying quartzites of the Equador Formation. These pegmatites can be homogeneous or heterogeneous in type; the latter ones present a larger variety of minerals and with economical potential (Johnston and Vasconcellos 1945; Da Silva 1993). The homogeneous pegmatites are relatively older and intruded along lithological and structural discontinuities (e.g., foliation), whereas the heterogeneous pegmatites were emplaced along tension gashes and other dilational structures (Araújo et al. 2001). In the SMB, gold quartz veins mainly occur within mica schists of the Seridó Formation and are typical shear-zonecontrolled mineralizations, located along kilometric-scale shear zones (Luiz-Silva 1995, 2000; Araújo et al. 2002). Locally, these mineralizations show a close relationship with Brasiliano plutons, which are also shear zone controlled. The gold quartz veins are interpreted to have been formed during the Cambro-Ordovician ductile-brittle reactivation of strike-slip shear zones (Araújo et al. 2002, 2003) during two episodes: Ma and Ma ( 40 Ar/ 39 Ar on biotite and muscovite genetically related to the gold mineralization; Araújo et al. 2005). The principal aspects of these three categories of mineral deposits of the SMB (W Au skarns, Ta Nb pegmatites, and gold quartz veins) are described below. These descriptions are based on the reviews by Santos and Brito Neves (1984), Delgado et al. (1994), and Beurlen (1995). W Au skarns There are almost 700 occurrences of tungsten skarns in the SMB (Santos 1973; Lima et al. 1980) but only one gold tungsten skarn and one gold skarn. Since the discovery of tungsten deposits in SMB at the beginning of 1940s until 1992, it was estimated that about 60,000 t of WO 3 concentrate was produced in this Brazilian region (Beurlen 1995). During the 1980s, the low-cost wolframite production in China promoted a significant decrease in international tungsten price, and consequently led to the progressive collapse of the scheelite exploitation in SMB by the mid 1990s. Until 2006, scheelite has been rudimentarily exploited only in the Bodó mine, with a small production (between 30 and 200 t/year of scheelite concentrate), and in the Brejuí mine, where there is a minor seasonal exploitation activity. The major skarns in SMB are Brejuí, Bodó, Bonfim, Bonito, Malhada Limpa, Quixaba, and Itajubatiba (Fig. 1; Table 1). Except the Itajubatiba skarn that contains gold and Bonfim skarn that hosts both tungsten and gold, all the other deposits

8 192 Miner Deposita (2008) 43: Table 2 Average microprobe analyses of skarn minerals from the Bonfim W Au deposit Garnet Pyroxene Amphibole Plagioclase Clinozoisite Epidote Host rock, number of analyses Schist n=3 Marble n=9 Schist n=14 Marble n=3 Schist n=6 Schist n=7 Schist n=3 Schist n=1 SiO 2 (wt.%) TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O K 2 O Total a Si Ti Al Cr Fe Fe Mn Mg Ca Na K Total cations Oxygens End member percentages Al Gr Sp Di Hd An 97.1 The EMP analyses were carried out at the Centre d Analyse par Microsonde pour les Sciences de la Terre (CAMST) of the Université Catholique de Louvain, Belgium, using a CAMECA SX-50 microprobe. Elements were determined by energy-dispersive analysis at acceleration tension of 15 kv, a current intensity of 20 na, and a live counting time of 10 s (44 s for F); the size of the analytical spot was μm; ferric iron in garnet, pyroxene, and epidotes (all iron calculated as Fe 3+ ) is reallocated following Droop (1987); amphibole formula is the average of minimum and maximum ferric iron estimation according to Schumacher (1997). a Pertains to rows 1 to 10 are tungsten skarns. Particularly the Malhada Limpa skarns contain extensive molybdenite (e.g., Timbaúba mine; Santos et al. 1972; Lima et al. 1980). The Itajubatiba (Au) and Bonfim (W Au) skarns share similar characteristics such as host rocks, skarn morphology, intrusive rocks, prograde and retrograde mineralogy (with some small differences mainly in retrograde minerals), and ore minerals (Table 1) with other skarns in the SMB. At Brejuí, pyrite appears as the predominant sulfide, whereas pyrrhotite is the main sulfide in the Bonfim and Itajubatiba skarns. Only viewed differences appear in garnet compositions (Table 2; Fig. 2a) that are more almandinespessartine rich in the Bonfim (W Au) and Itajubatiba (Au) skarns than in the Brejuí (W) skarn, whereas pyroxene compositions are similar (Table 2; Fig. 2b) in these three skarns. These characteristics suggest that Bonfim and Itajubatiba could be considered reduced skarns, whereas Brejuí appears to be an oxidized tungsten skarn, according to Einaudi et al. (1981). Gold is clearly part of the late-stage skarns in Bonfim, as it fills brittle-ductile faults and fractures crosscutting both the prograde and retrograde skarns. At Itajubatiba, sulfides and probably gold are mainly shear and fracture controlled. Bismuth and tellurium minerals are associated with Fe Cu sulfides and gold at Bonfim, but the Itajubatiba skarns are characterized by Fe Cu sulfides only.

9 Miner Deposita (2008) 43: Fig. 2 Compositional diagrams for garnets (a) and pyroxenes (b) from Brejuí, Bonfim, and Itajubatiba deposits. Number in parentheses corresponds to the quantity of analyses. Gr Grossular, Sp+Al sperssartine+ almandine, Ad andradite, Di diopside, Jo johannsenite, Hd hedenbergite Another mineral of economic interest is fluorite, which appears in tungsten skarns in the SMB and has been exploited in the Brejuí and Quixaba mines and surroundings (Beurlen 1995). Ta Nb pegmatites More than 700 mineralized pegmatites occur in an area of about 15,000 km 2 in the SMB, named Borborema Pegmatite Province. They are of economic importance because of their tantalite columbite, beryl, cassiterite, and kaolin contents. Lepidolite, spodumene, amblygonite, feldspars, muscovite, and gems (mainly aquamarines and blue and green tourmalines) are also extracted (Santos and Brito Neves 1984; Da Silva 1993; Beurlen 1995; Da Silva et al. 1995). These granitic rocks contain sulfides (chalcopyrite, bornite, chalcocite, and molybdenite), native bismuth, bismutite, apatite, a large variety of phosphates, and rare monazite (Santos and Brito Neves 1984). The production of tantalite concentrates was irregular since the Second World War and varied between 2 and 100 t/year (Beurlen 1995). In the SMB, the Isidoro-Riachão mine (neighboring Bodó mine; Fig. 1 and Table 1) contains scheelite, which occurs disseminated or in pockets with pyrite and molybdenite in an ore body that is 700 m long and crosscuts the gneiss of the Jucurutu Formation and the basement (Melo 1961; Santos and Brito Neves 1984). The other two scheelite occurrences in pegmatites crosscut the basement gneiss about 120 km west of the town of Currais Novos (Fig. 1) and contain scheelite, molybdenite, and subordinate amounts of bismuthinite, powellite, pyrite, chalcopyrite and calcite, whereas the other occurrence displays only scheelite and molybdenite (Lima et al. 1980). Gold quartz vein deposits Gold quartz vein mineralization in the SMB is hosted by medium- to high-grade amphibolite facies rocks (mica schists of the Seridó Formation, biotite gneiss of the Jucurutu Formation, and basement gneiss). The gold quartz veins occur particularly in medium- to high-angle shear zones (Brasiliano orogeny) and are essentially composed of quartz, although some veins also contain significant garnet and/ or sillimanite. The schists within the garnet zone altered to cordierite andalusite zone, and sillimanite muscovite zones, which hosts the gold-bearing veins, whereas the biotite gneiss altered to gold-bearing mica schists (biotite, muscovite, garnet, staurolite, sillimanite±andalusite), quartzites (muscovite±sillimanite), and calc-silicate gneisses (biotite, Ca Fe amphibole, garnet). The basement gneiss altered to gold-bearing muscovite (±sillimanite) schists, sillimanite quartzite, and calc-silicate gneisses (Luiz-Silva 2000). The wall-rock alteration zones related to gold quartz veins contain biotite and muscovite (Luiz-Silva 2000; Araújo et al. 2005), sillimanite (Luiz-Silva 2000), and tourmaline (Beurlen 1995). Based on crosscutting relationship between ductile and late-stage ductile-brittle fabrics, a 523-Ma pegmatite swarm along these fabrics, and lowtemperature retrograde alteration in the mylonite zones suggest that gold deposition of gold quartz veins in the SMB postdates the high-temperature deformation and occurred during late Brasiliano ductile-brittle reactivation of strike-slip shear zones (Araújo et al. 2002, 2003). In the gold-bearing environments, native gold occurs in its free state, or disseminated within the host rocks, but also associated with Fe±Cu±Pb sulfides or Fe Ti oxides (Luiz- Silva 2000) and locally arsenopyrite and sphalerite have

10 194 Miner Deposita (2008) 43: also been reported in the mineralized veins (Beurlen 1995). The geochemical association of Au±Cu±Bi±As±Pb is commonly found in the gold quartz veins of the SMB (Luiz-Silva 2000). North of the Bonito (Santos 1968) and between the Brejuí and Quixaba tungsten mines (Roy 1966; Ferreira 1967; Lima et al. 1980; Fig. 1), scheelite-bearing quartz veins are also reported. The Bonito mine is considered a vein field, with quartz veins containing biotite, chlorite, besides pyrite, bismuthinite and stibnite. About 25 km northwest from the town of Lajes (Fig. 1), tungsten-bearing quartz veins are related to massive ferberite, both located in the basement gneiss. These veins also contain minor amounts of garnet, biotite, plagioclase, as well as hematite within fractures and scheelite, which forms small coatings on fractures that crosscut the ferberite crystals (Mello 1970; Santos and Brito Neves 1984). Barite and fluorite are other minerals of economic interest found in quartz veins that crosscut, or as stratiform lenses within, quartzites and gneisses of the Equador and Jucurutu Formations, respectively (Beurlen 1995). Fluorite is also reported in quartz veins that crosscut marble of the Jucurutu Formation. Present reserves at the two main gold quartz vein deposits in the SMB are estimated to be about 4 t of gold with a range of grades between 2.5 and 6.6 ppm Au, locally in the supergene profile grades of up to 100 ppm are reported. At the São Francisco deposit, 50 km east of Currais Novos (Fig. 1), a recovery of 1 3 ppm from the primary ore in the host rock and ppm from the tailings is reported by Ferran (1988). The other gold deposits located around 100 km west of Currais Novos (Fig. 1), in the São Fernando-Caicó district, have inferred reserves of 1.03 Mt of ore with an average grade of 3.2 ppm of Au (Luiz-Silva 2000). It is estimated that between 1.0 and 2.2 t of gold have already been extracted from these deposits (Luiz-Silva, personal communication). During the early 1990s, about 35 kg of gold was extracted per month in the São Francisco deposit (Carvalho 1990) only. Geology of the Bonfim mine area The Bonfim tungsten gold skarn deposit is situated in the state of Rio Grande do Norte, northeastern Brazil, about 27 km southeast of Lajes (Fig. 1). This deposit has a proven total reserve of 97 tonnes of scheelite and an ore grade average of 4.8-wt.% WO 3 (Farias et al. 1973; Table 1). Scheelite exploitation occurred between 1969 and 1977 but was interrupted by a series of gallery collapses (Lima Neto et al. 1985). In the early 1990s, gold was discovered in the tailings of the abandoned mine, and from this material, about 0.1 t of gold has been extracted. In mineralized skarns, the ore grade varies between 2 and 6 ppm of Au but Fig. 3 Geological map of the Bonfim W Au Bi Te deposit (simplified from Souza Neto 1999)

11 Miner Deposita (2008) 43: Fig. 4 Geologic section T5 through the workings of the Bonfim mine (simplified from Lima Neto 1975) can be up to 60 ppm. Gold anomalies of up to 100 ppm have been detected in soil profiles (Docegeo 1996). Metamorphic rocks In the Bonfim area, the basement consists of gneiss and augen gneiss (Fig. 3). The gneiss consists of gray, mediumto coarse-grained rock composed of biotite, quartz, alkali feldspar, plagioclase, garnet, chlorite, apatite, zircon, magnetite, and minor hematite and ilmenite. They also contain amphibolite lenses ( m wide) that are composed of hornblende, plagioclase, pyroxene, titanite, and allanite. The augen gneiss consists of alkali feldspar, plagioclase, quartz, biotite, opaque minerals, chlorite, titanite, epidote, zircon, and apatite. It is medium- to coarse-grained and displays a porphyritic texture (Carvalho 1990; Barbalho 1991; Carneiro Filho 1994). In both types of gneisses, chlorite is likely a retrograde metamorphic mineral, as well as the epidote in the augen gneiss. The Seridó Group metasedimentary rocks occur in the Bonfim area in three dominant lithologies: marble, quartzite, and schist, which belong to the Jucurutu, Equador, and Seridó Formations, respectively. Based on drill core observations, marble forms 15- to 20-m-thick layers within the schist (Fig. 4), which could reflect either an originally interbedded sedimentary feature or the result of a faulted sliver within schist. The fine- to medium-grained marble contains calcite (minor dolomite), phlogopite, apatite, and quartz and is white with brown, 2- to 5-mm-wide phlogopite-rich layers. At the Bonfim mine site, the marble can also show dark gray and orange colors. The medium- to coarse-grained quartzite occurs as elongated white to light gray lenses within up to 100-mwide marble and schist (Fig. 3) and is essentially composed of quartz, phlogopite, and muscovite and locally fuchsite. In the eastern portion of the Bonfim area, biotite- and anthophyllite phlogopite-bearing schists constitute the main geological unit and occur at the sheared contact of the basement migmatites (Fig. 3). The biotite-bearing schists are fine-grained and composed of plagioclase, quartz, zircon, apatite, and ilmenite (±garnet, cordierite, staurolite, andalusite, sillimanite, graphite, allanite). Sillimanite is present in sheared bands, whereas garnet, cordierite, staurolite, and andalusite appear in 0.1- to 4-m-wide bands, parallel to the principal foliation. These bands appear every 10 to 20 m, and locally, they can be monomineralic. Late chlorite and muscovite usually overgrow andalusite, staurolite, and cordierite porphyroblasts. The schists also exhibit calc-silicate lenses consisting of amphibole plagioclase that are up to 60 cm long and 20 cm wide. The anthophyllite phlogopite-bearing schists contain garnet and minor tourmaline, zircon, monazite, epidote, and apatite. These minerals are largely replaced by white mica, chloritoid, and quartz. Fuchsite replaces phlogopite and occurs as up to 2-cm-long elongated lenses. The schists contain anomalous high MgO (15.5 wt.%) and transition

12 196 Miner Deposita (2008) 43: metals (574-ppm Cr, 237-ppm Ni, 51-ppm Co, and 198- ppm V) when compared to the biotite-bearing schists at Bonfim and the other metapelitic schists in the SMB. A similar geochemical signature (25.5-wt.% MgO, 1,545-ppm Cr, 1,728-ppm Ni, and 93-ppm Co) has also been observed in anthophyllite-bearing schists of the Bonfim area by Carvalho (1990), who interpreted them as komatiite-bearing metaigneous rocks. Structure The dominant structural features in the Bonfim region are NE-trending km-scale shear zones. They have a penetrative mylonitic foliation that strikes about N30E and dip either to the NW or SE at A stretching lineation associated with this foliation plunges 31 toward N18E. Transposition and folding (mainly of isoclinal and intrafolial types) of a preexistent planar surface are associated with the shear movement. The most conspicuous structure at Bonfim, however, is an about 3-km-wide antiform, with the core composed of granite (Fig. 3). The main faults in the Bonfim area are high-angle faults and have an average strike of N86E and dip subvertically. These faults are abundant at the mine site where they show typical brittle-ductile features (slightly plastic displacements at their rim resulting in asymmetrical patterns) and a dominant sinistral movement. Several subsidiary highangle faults (and fractures) strike about N15E and N70W and dip subvertically. All of these faults contain up to 3-cmwide vein fillings made-up of quartz, calcite, quartz amphibole, and gold that crosscut the skarns. Granite and pegmatite The granite stock in the Bonfim deposit is correlated with the regional Brasiliano igneous suite. This igneous rock intrudes the Seridó Group metasedimentary rocks (Fig. 3). It is pink with a fine-grained texture and composed of quartz, alkali feldspar, biotite, plagioclase, and minor white mica, apatite, zircon, rutile (needle-shaped inclusions in biotite), and primary magnetite, which is mostly altered to hematite. No evidence of metasomatic alteration or endoskarn, as well as no deformation or a thermal aureole has been observed in this igneous rock. Many pegmatite bodies intrude the schists and the granite. The pegmatites contain significant columbite tantalite and beryl, and some of these bodies have been mined, mainly for Ta Nb, for example in the northern part of the area (Fig. 3). Late E W diabase dikes crosscut the basement, schists, and skarns at Bonfim. These dikes are correlated to the well-documented regional Mesozoic ( Ma) Rio Ceará-Mirim magmatism (Gomes et al. 1981). Petrography of the Bonfim W Au skarns Late in the tectono-metamorphic history of the Bonfim region, a metasomatic event transformed marble and schist into skarn. The timing of skarn formation is mainly constrained by the irregular contacts between the skarns and the country rocks. These contacts also crosscut the main structural fabric of the region. The skarn bodies occur along the main structural direction and constitute massive bodies but lack a foliation. Skarns show banding structure that is attributed to metasomatic zoning, which overprints the foliation of country rocks. A maximum age for these skarns can be estimated based on the age of the granitoid rocks dated between 555 and 480 Ma. This age interval corresponds to the lower age reported of the igneous rocks (maximum value), which are considered to be the source of fluids involved in the skarn formation, and the lower age obtained for pegmatite bodies (minimum value) that crosscut some skarns in the Brejuí (Barbosa et al. 1969; Salim 1993) and Itajubatiba deposits (Souza Neto 1999). There is a good correlation of the ages estimated for the skarns ( Ma) and the gold quartz veins in schists ( Ma based on 40 Ar/ 39 Ar on muscovite and biotite; Araújo et al. 2005). Whole rock geochemical analyses of Bonfim skarns revealed high bismuth (475- to >2,000-ppm Bi) and tellurium contents (up to 510-ppm Te), besides Au and W (Souza Neto 1999). Recently, data from exploration companies indicate that Au and Bi can reach 106 and 26,000 ppm, respectively. At Bonfim, most skarn horizons occur in the eastern flank of the antiform, and they appear to be the lateral equivalent of larger marble horizons, which are dominant at the western flank of this structure. The two larger skarns close to the granite intrusion are barren, whereas the easternmost smaller skarns are mineralized (Fig. 3). The barren skarns are essentially composed of diopside, whereas the mineralized skarns exhibit a more variable mineralogy. The difference in the skarn mineralogy is controlled by the skarn protolith, and it appears that the diopside-rich skarn replaces marble only, whereas the skarn formed at the schist marble contact presents a higher mineralogical diversity. Therefore, two types of skarns have been recognized at the Bonfim deposit: skarns replacing marble and skarns replacing schist, which were developed at the schist marble contact. The metasomatic zones (from the original rock towards the inner skarn) relating to each skarn type can be summarized as: (1) skarns replacing marble: phlogopite marble/wollastonite (olivine) marble/tremolite-magnesio-hornblende skarn/diopside skarn/tremolite-magnesio-hornblende skarn/wollastonite (olivine) marble/phlogopite marble; and (2) skarns replacing biotite schist: schists/actinolite-magnesio-hornblende-bearing

13 Miner Deposita (2008) 43: schists/actinolite-magnesio-hornblende skarn/diopside anorthite skarn/almandine skarn/diopside anorthite skarn/ actinolite-magnesio-hornblende skarn/actinolite-magnesiohornblende-bearing schists/phlogopite marble. Most skarn types are calcic, but some magnesian skarns occur in marble that are marked by olivine, tremolite, and retrograde talc. The extent of the hydrothermal alteration in the country rock is significant, as an estimate of the proximity of the skarns is used as regional exploration guide for the search for mineralized skarns (Souza Neto et al. 1997). Skarns replacing marble These skarns are up to 10 m thick in outcrop, with greencolored and medium- to coarse-grained granoblastic rocks. They show a massive structure and irregular-shaped contacts with pockets of remnant marble. Prograde skarn minerals include diopside, tremolite-magnesio-hornblende, and minor titanite, apatite, and allanite. Titanite often contains an ilmenite core and rutile inclusions. The main retrograde minerals observed are talc, serpentine, chlorite, recrystallized calcite, and sericite in marble horizons. The proximity of the skarns replacing marble is indicated by marble that contains chlorite (with rutile inclusions), ± muscovite±allanite±recrystallized calcite with multiple opaque inclusions. The marble in contact with skarns also contains wollastonite and olivine (partly serpentinized), with only locally minor phlogopite lenses. Up to 0.3 mm large ilmenite is associated with phlogopite. At the marble skarn contact, sericite also occurs associated with phlogopite. Skarns replacing biotite schist These skarns occur as elongated beds that are 5 10 m wide in outcrops. They commonly show a banded structure and are medium- to coarse-grained granoblastic rocks. In drill core, these skarns occur as 1- to 5-m-thick lenses, which are preferentially localized at the schist marble contact. The skarn schist contact is irregular in shape, whereas the contact between metasomatic zones is gradational. These zones appear either as alternating (e.g., garnet amphibole and pyroxene amphibole) or mixed bands. The typical mineralized skarn in the Bonfim mine exhibits alternating plagioclase and amphibole pyroxene bands. Some biotite lenses (up to 5 cm long) occur as remnants within these skarns. Almandine, diopside, anorthite, and actinolite-magnesiohornblende are the prograde minerals observed in these skarns. Multiple round biotite inclusions are observed in actinolite-magnesio-hornblende. Accessory prograde minerals are titanite, apatite, allanite, zircon, and monazite. Titanite and allanite are in equilibrium, with titanite typically containing an ilmenite core. Zircon and monazite preferentially occur as inclusions in amphibole. The EMP analyses of prograde garnet, pyroxene, amphibole, and plagioclase are shown in the Table 2. Scheelite, molybdenite, magnetite, and ilmenite are the main ore minerals in the prograde skarn. At the Bonfim mine, scheelite occurs in four mineralized skarn bodies distributed within the marble and at the marble schist contact. The main mineralized body occurs at the basal marble schist contact, with the highest amount of scheelite controlled by fold hinges (Potyra 1978; Salim 1979; Lima 1982). The main ore body is cm thick (average 40 cm) and has an average ore grade of 4.8-wt.% WO 3. Scheelite occurs in parallel zones within the skarns, and its anhedral crystals reach up to 5 mm in size. Scheelite contains inclusions of pyroxene, titanite, apatite, chalcopyrite, and molybdenite and is intensively fractured (Farias et al. 1973). In the main ore shoots, garnet and vesuvianite are absent (Lima 1982). Scheelite at the Bonfim deposit also occurs in ultramafic rocks, which contain serpentine, chlorite, and talc, and are crosscut by thin asbestos veins. Scheelite occurs either as disseminated porphyroblasts (Fig. 5a) or as coarse-grained aggregates (Santos and Brito Neves 1984). The relationship between these ultramafic rocks and the skarns is still unknown. Molybdenite occurs associated with scheelite and also within alkali feldspar veins, which could be related to remobilization of the prograde sulfide. Ilmenite preferentially occurs in skarns and close to the skarn schist contact. It generally exhibits a rutile core and is enclosed within almandine. Late hematite, replacing magnetite, and goethite (product of supergene alteration of ankerite) are also observed in these skarns. PIXE analyses (Table 3) revealed that the main trace elements found in prograde ilmenite and titanite are as follows (grouped by mineral, in ppm): ilmenite contains Nb (585) and titanite contains F (2,660), Y (1,350), Nb (660), and Zr (138). The retrograde stage of skarn formation has the following successive mineral assemblages: (1) alkali feldspar, common in skarns formed at the marble schist contact; feldspar content increases toward the schist, probably due to the aluminous composition of the latter; (2) clinozoisite zoisite sericite assemblage, probably formed by replacement of prograde anorthite, as suggested by the occurrence of clinozoisite zoisite zones in spatial continuity with incipient-altered anorthite, and sericite (calcite) pseudomorphs preserving relicts of anorthite twinning; and (3) calcite-rich assemblage composed of recrystallized calcite and remnants of chloritized biotite. Quartz, calcite, and quartz calcite veins (up to 5 cm wide) crosscut these skarns, and chlorite-filled microfractures crosscut prograde diopside. Locally, ankerite-filled microfractures also relate to this stage.

14 198 Miner Deposita (2008) 43: Fig. 5 Prograde scheelite skarn, outer altered rocks, and retrograde goldbearing skarn from the Bonfim W Au deposit, northeastern Brazil. a Prograde diopside anorthite skarn (gray green) with aggregates of euhedral scheelite (Sch) aligned parallel to banding. Late-stage gold sulfide replacement (Au, black) follows fractures and cuts across the bands. Sample kindly provided by E. J. Santos. b Outer anthophyllite alteration in schist, anthophyllite (Ath) in phlogopite-rich schist (Phl), in contact with clinozoisite zoisite (Czo Zo) partly replaced by white mica (Wm). Drill hole FD-1, 27.8 m, plane-polarized light. c Outer biotite alteration in schist, vein of biotite (Bt) and cummingtonite (Cum) cuts across prograde garnet (Grt) and actinolite (Act) skarn. Outcrop 10, planepolarized light. d Retrograde gold-bearing skarn, gold sulfide replacement (black) following fractures crosscutting retrograde clinozoisite zoisite sericite (pink) and prograde diopside anorthite (green) bands. The lens cover is 6 cm across. Sample from mine dumps. e Retrograde goldbearing skarn, concentric bands of prehnite (Prh) and epidote (Ep) in contact with gold sulfide aggregates (opaque, see Fig. 5f). Sample Bi-1A, from mine dumps, crossed-polarized light. f Retrograde gold-bearing skarn, contact relationships between chalcopyrite (Ccp), bismuthinite (Bmt), native bismuth (Bi), and sphalerite (Sp). Sample Bi-1A, from mine dumps, reflected light in air. g Retrograde gold-bearing skarn, joseite (Js) containing lamellas and blebs of native bismuth (Bi). Numbers correspond to microprobe analyses (Table 4), spots 9 and 10 are in joseite and spots 11 and 12 in bismuth. Sample named BONFIM, from mine dumps, reflected light in air. h Native gold (15-wt.% Ag, Table 4) disseminated in retrograde epidote prehnite skarn crosscutting prograde diopside anorthite (Di An) bands. Sample named BONFIM, from mine dumps, reflected light in air

15 Miner Deposita (2008) 43: Table 3 PIXE analyses of minerals in prograde diopside skarn (first two minerals), retrograde clinozoisite zoisite sericite skarn, and retrograde epidote prehnite sulfide skarn (next three minerals), and from a postskarn vein (carbonate) of the Bonfim W Au deposit Sample FD-8/11 FD-8/11 B-97/1 FD-8/11 FD-8/11 B-18/1 Host rock Schist Schist Schist (?) Schist Schist Schist Mineral Ilmenite Titanite Clinozoisite Sericite Epidote Carbonate As (ppm) <30 Cu Pb Zn <30 Ba <250 Rb 825 Sr In < < <65 W <75 Ga Ge <30 Nb Y 1, Zr F 2,660 9,490 <120 The PIXE analyses were carried out using a proton miniprobe at the CERI laboratory of the Centre National de la Recherche Scientifique (CNRS) in Orléans, France. Analytical conditions were a proton beam of 2.5 MeV, a spot of 1,500 μm 2, and a current of na (see Maxwell et al and Volfinger et al for analytical details). The sample B-97/1 is a retrograde clinozoisite zoisite sericite skarn from the mine site, B- 18/1 is a diopside skarn from outcrop 18, and FD-8/11 shows clinozoisite zoisite sericite and epidote prehnite sulfide skarns and carbonate vein from the drill hole FD-8, 70.4 m. See Fig. 3 for mine, outcrop, and drill hole locations. The symbol < present in some element amounts means that the respective element was detected but could not be quantified because its quantity was close to the detection limit of the analyses. Values in blank refer to elements not detected. The EMP analysis of retrograde clinozoisite zoisite is shown in Table 2. PIXE analyses (Table 3) revealed that the main trace elements found in clinozoisite zoisite and sericite are as follows (grouped by mineral, in ppm): clinozoisite zoisite contains F (9,490), Sr (350), In (330), Y (200), and Pb (165); sericite contains Cu (915), Rb (825), Zr (137), Ga (75), Pb (60), and Sr (45). High F content (up to 9,490 ppm) has been found in titanite and clinozoisite zoisite (Table 3), which indicates fluorine-rich metasomatic fluids during the formation of prograde and retrograde skarns at Bonfim (Souza Neto et al. 1998). Similar fluorine-rich compositions are usually reported in rare-metal-bearing skarn systems (Newberry 1998; Aksyuk 2000). Outer alteration in schists Both the biotite- and the anthophyllite phlogopite-bearing schists in the vicinity of skarns show some features that indicate proximity to the skarns. The prograde actinolitemagnesio-hornblende assemblage replaces biotite and phlogopite and is the dominant metasomatic replacement observed in both schist types. This reaction is typical for schists in the several meters wide alteration halo around the skarns. The resulting actinolite-magnesio-hornblendebearing schists show a characteristic banded texture with actinolite-magnesio-hornblende bands interlayered with biotite or anthophyllite phlogopite bands. Biotite-rich lenses (about 5 cm long) are preserved within actinolite-magnesio-hornblende-rich bands. The actinolitemagnesio-hornblende-bearing schists also contain strongly altered plagioclase and chloritized garnet. Minor minerals include ilmenite (rutile), apatite, tourmaline, allanite, and titanite. Large apatite, tourmaline, and titanite are up to mm in diameter. Titanite often contains an ilmenite core and occurs aligned parallel to the banding. Quartz bands (about 3 cm wide) or pockets and alkali feldspar calcite veinlets (up to 2 mm wide) are also present within the actinolite-magnesio-hornblende-rich bands of the schists. At Bonfim, the skarn has an outer alteration halo of actinolite-magnesio-hornblende in biotite-bearing schist, whereas the anthophyllite cummingtonite alteration is preferentially developed in the phlogopite-bearing schist. In the latter, anthophyllite occurs either associated with clinozoisite zoisite as prismatic-shaped pseudomorphs or parallel to the foliation of the rock (Fig. 5b). Late biotite cummingtonite veins crosscut the skarns (Fig. 5c), and cummingtonite alteration also occurs widespread parallel to the banding of the skarns. White marble adjacent to the skarns formed at the marble schist contact and contains clear calcite.

16 200 Miner Deposita (2008) 43: Table 4 Microprobe analyses of gold and bismuth minerals in retrograde epidote prehnite sulfide skarn (representative sample named BONFIM, from the mine dumps; see Fig. 3 for location and Figs. 5g and h) from the Bonfim W Au deposit Mineral Gold Bismite Bismite Bismite F-bismite F-bismite F-bismite Bi Bi Bi Bi Bte Bte Bte Joseite Analysis Si (wt.%) na na na na na na na na na Fe na na na na na na na na na Mn na na na na na na na na na Mg na na na na na na na na na Au na na na na na na Ag na na na na na na Bi Te O nc nc nc nc nc nc nc nc nc S 0.00 na na na na na na F na na na na na na na na na Total a Si (at.%) nc nc nc nc nc nc nc nc nc Fe nc nc nc nc nc nc nc nc nc Mn nc nc nc nc nc nc nc nc nc Mg nc nc nc nc nc nc nc nc nc Au nc nc nc nc nc nc Ag nc nc nc nc nc nc Bi Te O nc nc nc nc nc nc nc nc nc S nc nc nc nc nc nc F nc nc nc nc nc nc nc nc nc Total b Oxygens Sulphurs Tellurium+sulphur 3 The operating conditions were the same as the analyses of skarn minerals (see Table 2), except for the current (50 na) and the live counting time (20 s for each element; 24 s for Ag) used. Oxygen was calculated considering all elements in the oxide form, except F. Analyses 25+26, 11+12, and 9+10 correspond to the average of two spots on a same grain. The analyses 11 and 12 correspond to an exsolution within the grain analyzed in 9+10 (see in Fig. 5g). F-bismite Fluorine-bearing bismite, Bi bismuth, Bte bismuthinite, na not analyzed, nc not calculated. a Pertains to rows 1 to 11 b Pertains to rows 13 to 23

17 Miner Deposita (2008) 43: Late-stage gold mineralization The age intervals of Ma and Ma ( 40 Ar/ 39 Ar on hydrothermal muscovite and biotite; Araújo et al. 2005) obtained for the structurally emplaced gold quartz veins hosted in mica schists are considered a likely age for the gold mineralization in the Bonfim skarns. At Bonfim, gold occurs as free grains disseminated within black, fine-grained minerals that fill thin conjugate brittle-ductile faults and extensional fractures, which crosscut both the diopside anorthite prograde and alkali feldspar and clinozoisite zoisite sericite retrograde skarns (Fig. 5d). These black minerals also occur as an impregnation in the skarns (Almeida 1994; Fig. 5d). Calcite- and quartz-filled microveins crosscut both the prograde skarn and gold mineralization. The gold assemblage exhibits relatively rare minerals, such as bismite (Bi 2 O 3 ) and fluorine-bearing bismite (75 80 vol%), native bismuth (5 10 vol%), bismuthinite (Bi 2 S 3 ) and joseite [Bi 4 (Te,S) 3 ], both 5 10 vol%, and minor (<5 vol%) chlorite, epidote, prehnite, chalcopyrite, sphalerite, and gold (Fig. 5e h). Gold shows a typical pallid color (up to 14.9 wt.% of Ag; Table 4), and its crystals vary from <5 μm toupto60 μm and are commonly 15 μm in diameter, occurring either as individual grains (Fig. 5h) or in contact with native bismuth, bismuthinite, and joseite. Bismite contains variable amounts of tellurium (up to 7.7 wt.%) and appears to be supergene in origin, probably replacing bismuth tellurides. Epidote and prehnite occur mainly as concentric bands around gold sulfide aggregates (Fig. 5e). Epidote also occurs as round-shaped aggregates and exhibits a pyrrhotite chalcopyrite core, or occurs as euhedral pseudomorphs preserving irregular-shaped garnet and filling fractures associated with chlorite. The EMP analyses of gold and bismuth minerals are shown in the Table 4. PIXE analyses (Table 3) revealed that the main trace elements found in late-stage epidote associated with gold and carbonate in late crosscutting veins across the skarns are as follows (grouped by mineral, in ppm): epidote contains Cu (497), In (150), Sr (85), and Ga (23); carbonate contains Sr (405), Pb (160), Y (150), and Cu (140). Indium appears to be enriched in epidote and clinozoisite zoisite. In the Bonfim area, a sulfide-rich retrograde assemblage is found in the epidote skarn replacing prograde garnet close to the schist contact. Relatively large amounts of sulfides usually occur in the skarns when they are close to the schist, but also in surrounding schist. No metal grades of interest have been reported. Sulfides appear either disseminated or within late quartz calcite veins crosscutting prograde skarn. They often contain an epidote aureole that generates a typical dark-green halo. High amounts of quartz and muscovite also occur close to the schist, as well as crosscutting chlorite-rich bands (up to 8 mm wide) with associated quartz and quartz sulfide veins. Sulfides are present disseminated within the chlorite bands and are mainly pyrrhotite, chalcopyrite, pyrite, and minor arseno- Table 5 The main characteristics of the Brejuí, Bonfim, and Itajubatiba skarn deposits Skarn deposits Brejuí Bonfim Itajubatiba Type W W Au Au Sub-type Oxidized Moderately reduced Reduced Predominant host rock of the ore skarn Marble gneiss contact Marble schist contact Marble meta-tonalite contact Predominant sulfide assemblage Pyrite+magnetite Pyrrhotite±pyrite Pyrrhotite+pyrite Pyroxene composition Hd Hd Hd 5 75 Dominant garnet endmembers Grossular+andradite Almandine+grossular Almandine+grossular W (%) (1) (2) (3) , average <0.002 (3) 4.8 (4) Mo (ppm) , powellite and 1 2 (3) 104 (3) molybdenite (1) W/Mo , up to 600 >5 Au (ppm) Not found 2 6, up to , up to 6.3 F (ppm) Not available <9,490 a <5,840 b Gold skarn Retrograde Prograde (?) Main calc-silicate minerals associated with the gold Epidote, prehnite References: 1 Salim (1993), 2 Maranhão (1970), 3 Souza Neto (1999), 4 Farias et al. (1973) a Clinozoisite/zoisite b Titanite (PIXE analysis) Amphibole, pyroxene, garnet

18 202 Miner Deposita (2008) 43: pyrite. Pyrrhotite contains chalcopyrite blebs and lamellae and occurs in the core of the pyrite grains. At the schist skarn contact, the chlorite-rich band of the schist overprints the actinolite-magnesio-hornblende skarn and preserves inclusions of this skarn mineral, indicating that chlorite in the schist was developed post or synchronous with the formation of the actinolite-magnesio-hornblende skarn. Discussion In the SMB, the main tungsten skarns (Table 1) contains pyrite as the predominant sulfide, which suggests an oxidized skarn according to the classification proposed by Einaudi et al. (1981). The Bonfim tungsten gold and Itajubatiba gold skarns are exceptions, where pyrrhotite dominates (Tables 1 and 5), indicating that they are reduced skarns. When comparing the Brejuí (W), Bonfim (W Au), and Itajubatiba (Au) skarns, which at present are the only deposits studied in detail, the former is basically distinguishable from the other two by containing pyrite as the predominant sulfide, garnet with low almandine and spessartine component, and elevated Mo contents (powellite and molybdenite), and no gold (Table 5). The Bonfim and Itajubatiba gold-bearing skarns share some characteristics, with pyrrhotite as the predominant sulfide, garnet with elevated almandine and spessartine component, and relatively elevated gold contents (Table 5). Furthermore, rare pyrite in microfractures crosscuts magnetite crystals. The fluorine contents (Table 5) estimated for the Bonfim and Itajubatiba skarns are within the concentration range of tungsten skarns (1,500 55,000 ppm) and contrast with fluorine abundances observed in tin skarns (7, ,000 ppm), as proposed by Newberry (1998). In summary, the Bonfim tungsten gold skarn and Itajubatiba gold skarn are reduced skarns; the latter appears to be slightly more reduced than the former, based on its relatively higher Fe pyroxene component (up to 75 mol% Hd) and almandine and spessartine component of garnet (up to 78 mol%; Tables 1 and 5). At Itajubatiba, skarns replacing metasyenogranite (barren) are distinct from the other mineralized skarns and exhibit relatively low almandine and spessartine component of garnet (4 5 mol%; Table 1). Thus, it seems that the occurrence of gold in the SMB skarns is controlled by their reduced character. The Bonfim tungsten skarn deposit appears to share some characteristic features with the Nevoria gold deposit, an Archean skarn localized in the Yilgarn Craton of Western Australia (Mueller 1997; Mueller et al. 2004); for example: (1) deep crustal setting (10- to 15-km depth) at pressures of 2 4 kbars; (2) Au Bi Te element association; and (3) anthophyllite as a distal alteration mineral. Conclusions The SMB hosts important tungsten mineralization, as well as a gold skarn (Itajubatiba) and a recently discovered tungsten gold skarn (Bonfim). Although there is a lack of detailed studies, except for the Itajubatiba, Bonfim, and Brejuí skarns, the main characteristics of the SMB skarns indicate that they are dominantly oxidized tungsten skarns. On the other hand, the Itajubatiba and Bonfim gold-bearing skarns exhibit reduced features, such as pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. The petrographic and mineral chemistry studies revealed a Au Bi Te-rich ore assemblage at Bonfim, which occurs as fine-grained black material within conjugate brittleductile faults and extensional fractures. The results presented here provide new information about the polymetallic potential (e.g., Bi and Te) of the Bonfim deposit, which was known only for gold and tungsten so far. PIXE data reveal that fluorine (up to 9,490-ppm F) is present in prograde titanite and retrograde clinozoisite zoisite, which suggests a significant F activity in the skarn-forming fluids. EMP analysis showed the existence of fluorine-bearing bismite (0.5- to 1.0-wt.% F) in the ore, which indicates that such F- rich fluids have also participated in the late-stage gold mineralization. Amphibole-bearing bands in the schist, wollastonite or olivine in the marble, and white-colored recrystallized marble are all located in the outer zones of the skarns and, therefore, may be used as an effective exploration tool for skarn deposits in the SMB. The SMB not only has a vast potential for skarns but also for new gold and polymetallic deposits. Acknowledgment The authors express gratitude to Edilton J. Santos and Hartmut Beurlen for the historical data about the skarns of the Seridó region, to Wanilson Luiz-Silva for providing unpublished data about gold quartz veins of the Seridó, to Gaston Giuliani for bismuthrelated bibliography, to Jacques Wautier and Marco Bravin for microprobe analysis and thin section preparation, to Fabriciano L. Neto (Tomaz Salustino company) for access to drill cores, to the director and staff of Docegeo Company, to Harrizon L. Almeida (Ecudor company), to Eurico Pereira (Bonfim farm owner) for access to the mine and private documents, to Adriana B. Garlipp for drawing figures, and to Hartmut Beurlen and Germano M. Júnior for the review of the text. The authors also wish to thank the Brazilian Funds (FINEP/PADCT II, Research Project Mass and Fluid Transference in the Continental Crust) and the Belgian Funds (Fonds National de la Recherche Scientifique, and France Belgium cooperation of the Communauté Française de Belgique-Tournesol Project Dernières Phases Magmatiques) for financial support for field work and analyses. The first author is also grateful to the Brazilian National Research Council, CNPq, for a doctoral grant (process /95-1). Constructive criticism by Andreas G. Mueller, Craig Hart, Steffen Hagemann, and Bernd Lehmann helped to significantly improve the manuscript.

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