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Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil
Journal of South American Earth Sciences 18 (2005) 293–304 www.elsevier.com/locate/jsames
Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil M. Babinskia,*, L.V.S. Monteirob, A.H. Fetterc, J.S. Bettencourta, T.F. Oliveirad a
Centro de Pesquisas Geocronolo´gicas, Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Rua do Lago, 562, Sa˜o Paulo, SP CEP 05508-080, Brazil b Instituto de Geocieˆncias, Universidade Estadual de Campinas, UNICAMP, Rua Joa˜o Pandia´ Calo´geras, 51, Campinas, SP CEP 13083-970, Brazil c Instituto de Geocieˆncias e Cieˆncias Exatas, Universidade Estadual Paulista, UNESP, Av. 24 A, 1515, Rio Claro, SP CEP 13506-900, Brazil d Companhia Mineira de Metais, Caixa Postal 3, Vazante, MG CEP 38780-000, Brazil Received 1 August 2003; accepted 1 November 2004
Abstract The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U–Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedimentary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Pb compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonatehosted Zn–Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante–Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic. q 2005 Elsevier Ltd. All rights reserved. Keywords: Isotope geochemistry; Mafic dikes; U–Pb geochronology; Vazante Group; Zn–Pb mineralization
1. Introduction The Vazante Group (Dardenne et al., 1998), which occurs in the northwestern part of Minas Gerais, hosts the most important carbonate-hosted nonsulfide (Vazante) and sulfide (Morro Agudo) zinc deposits known in Brazil. It also represents one of the metasedimentary units of the southern segment of the Brası´lia fold belt.
* Corresponding author. Fax: C55 11 3091 3993. E-mail addresses: [email protected] (M. Babinski), [email protected] (L.V.S. Monteiro), [email protected] (A.H. Fetter), [email protected] (J.S. Bettencourt), [email protected] (T.F. Oliveira). 0895-9811/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.11.010
The tectonic setting of this unit is controversial. Recent studies indicate that the Vazante Group could represent sedimentation in a rapidly subsiding basin in the Brası´lia fold belt’s initial thrust fronts (Dardenne, 2000) in a setting similar to that of the Bambuı´ Group or correspond to the top of the Meso-Neoproterozoic passive margin sequence, represented mainly by the Paranoa´ Group (Pimentel et al., 2001). The Vazante Group is composed of a thick sedimentary sequence whose depositional age remains disputable. The available geochronological data indicate that it was metamorphosed during the Brasiliano orogeny. Recently, metamorphosed mafic dikes (here, metabasites) intrusive in the metasedimentary sequence were found in drill cores in
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the Vazante underground mine. The metabasites are tectonically imbricated with hydraulic breccias in the Vazante shear zone, which represents the main structural control of hypogene nonsulfide mineralization, and are affected by hydrothermal alteration processes. Thus, their geologic relationship is promising for constraining the minimum age of the sediment deposition and the hydrothermal processes linked to the hypogene nonsulfide zinc mineralization from Vazante. This article presents U–Pb, Pb–Pb, and Sm–Nd isotopic data along with petrological, geochemical, and C, O, and Sr isotopic studies on the mafic dikes and the metasedimentary rocks of the Vazante Group. It also reports the isotopic signatures of the metalliferous fluid responsible for the hydrothermal alteration that affected the metabasites and its relationship with the hydrothermal system responsible for the zinc mineralization hosted by the Vazante Group.
2. Geological setting The Vazante-Paracatu region is located in the eastern part of the Brası´lia fold belt (Almeida, 1967), which extends for more than 1000 km over a width of 300 km along the western margin of the Sa˜o Francisco craton (Fig. 1). The fold belt represents an unstable crustal block whose final structural differentiation resulted from the closure of a wide
oceanic basin during the Neoproterozoic Brasiliano orogeny (w600 Ma) (Pimentel et al., 2001). The Brası´lia fold belt displays rock sequences thrust to the east with increasing deformation and metamorphism to the west (Marini et al., 1981), which reflect the vergence of the fold belt with respect to the Sa˜o Francisco craton (Dardenne, 2000). The Vazante Group (Dardenne et al., 1998) represents one of the metasedimentary units of the southern segment of the Brası´lia fold belt involved in a complex imbricate system of nappes and thrusts, with the main compressive trend from SW to NE. It is affected by sinistral transcurrent shear zones with SE transport (Dardenne, 2000), as well as low greenschist facies metamorphism. It displays a pervasive cleavage (S 1) related to regional folding, which is overprinted by S2 and S3 cleavages related to local D1 isoclinal and D2 open folds, respectively (Monteiro, 1997). The Vazante Group is divided into the following seven formations (Fig. 2) from base to top: Santo Antoˆnio do Bonito, Rocinha, Lagamar, Serra do Garrote, Serra do Poc¸o Verde, Morro do Calca´rio, and Lapa. Metapelitic units with phosphorite occurrences compose the basal Retiro and Rocinha Formations (Dardenne et al., 1998; Dardenne, 2000). The Lagamar Formation represents a metapsamopelitic and carbonatic unit with basal metaconglomerates, dolomitic breccia, dark gray limestone, and stromatolitic bioherm with columnar stromatolites of
Fig. 1. Geological map of the southern part of the Brası´lia fold belt (Dardenne, 2000).
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295
Fig. 2. Lithostratigraphic column of the Vazante Group (Dardenne et al., 1998; Dardenne, 2000).
the Conophyton/Jacutophyton type (Cloud and Dardenne, 1973; Moeri, 1972). The Serra do Garrote Formation consists of a thick sequence of gray slate, locally carbonaceous and containing pyrite, with quartzite layers (Madalosso and Valle, 1978). The Serra do Poc¸o Verde Formation is made up of gray to pink algal-laminated dolomite, gray to green slates, sericite phyllite, dark gray dolomite with birds’ eyes, marls, and pyrite-bearing carbonaceous shale. Stromatolitic bioherm facies, breccias, dolarenite, and subordinate carbonaceous shale compose the Morro do Calca´rio Formation. These formations are the dominantly dolomitic sequences (Fig. 2) that host the Zn–(Pb) deposits. The dolomitic sequence is overlain by the Lapa Formation, with black carbonaceous slate and phyllite (Serra do Velosinho member), and by phyllite, carbonatebearing metasiltstone, dolomite, and quartzite lenses of the Serra da Lapa member (Madalosso and Valle, 1978). The Canastra Group (1.2–0.9 Ga; Pimentel et al., 2001) metasediments, which comprise the chlorite-rich calc-phyllite of the Landim Formation, and carbonaceous phyllites and quartzites of the Paracatu Formation overthrust the rocks of
the Vazante Group as a consequence of the Brasiliano collisional event (w630 Ma; Dardenne, 2000). 2.1. Mafic rocks Minor magmatism associated with the Vazante Group is represented by small bodies of metabasic rocks, identified mainly at the Vazante underground zinc mine (Fig. 3). In this deposit, the metabasites (Fig. 3) occur in the Vazante shear zone, tectonically imbricated with hydraulic breccias, hydrothermally altered rocks, and nonsulfide zinc ore composed mainly of willemite (Zn2SiO4). These mafic lithotypes were described initially as diabase dikes of Cretaceous age (Rigobello et al., 1988). However, the presence of a metamorphic mineral assemblage (Fig. 4) and mylonitic fabric, not recognized in the Brazilian Cretaceous rocks, indicates that the unit was affected by the regional Brasiliano deformation event (Monteiro, 1997). The metabasites and ore bodies are offset by normal and reverse faults and cut by late hydrothermal veins, which likely caused complex relationships between the ore bodies and the host sequence.
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Fig. 3. Cross-section of the Vazante ore zone showing the spatial relationships among metabasites, the host sequence, willemitic ore, and the Vazante shear zone (Monteiro, 1997; Monteiro et al., 1999).
2.2. Zinc mineralization hosted by the Vazante Group The Zn–(Pb) deposits contained within the Vazante Group metasedimentary sequence predominantly are faultor shear zone-controlled and show distinct ore mineralogy (sulfide- or silicate-bearing) and mineralization styles (synto tardidiagenetic and epigenetic), which are associated with long-lived hydrothermal systems related to the diagenesis and deformation of the Vazante Group during the Neoproterozoic (Monteiro, 2002; Monteiro and Bettencourt, 2001; Monteiro et al., 2003, submitted). The Vazante deposit is a main example of a hypogene nonsulfide zinc deposit, as defined by Hitzman et al. (2003). The zinc mineralization is epigenetic, linked to the Vazante shear zone development (N50/60NW), which was subsequent to the low greenschist facies metamorphism of the Vazante Group. In the shear zone, the hydrothermal alteration is largely fracture controlled and produced a complex zone of net-veined breccia filled by dolomite, ankerite, siderite, jasper, hematite, and chlorite (Monteiro, 1997; Monteiro et al., 1999). The ore bodies display pod morphology and are tectonically imbricated with small metabasite bodies, brecciated metadolomites, and slates (Monteiro et al., 1999).
The Vazante main ore is made of willemite (Zn2SiO4), dolomite, quartz, siderite, hematite, barite, franklinite, zincite, smithsonite, apatite, magnetite, and Zn-chlorite (Monteiro, 1997; Monteiro et al., 1999). Small sulfide bodies composed mainly of Cd-rich sphalerite and galena occur tectonically imbricated with the willemitic ore and the intensely hydrothermalized host rocks in the Vazante shear Vazante Shear Zone development Minerals
Green schist facies metamorphism (Sn)
Sn+1 (ductile-brittle structures)
Sn+2 (ductile-brittle to brittle structures)
Chloritization
Carbonatization Serpentinization Fe-oxides formation
Actinolite Chlorite Sericite Biotite Quartz Epidote Clinozoisite Apatite Carbonates Talc Chrysotile Titanite Rutile Leucoxene Hematite Lepidocrocite Chlor
itization Sericitization Epidotization
Fig. 4. Paragenetic associations of the metabasites (Monteiro, 1997).
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zone. Sphalerite is coprecipited with willemite but commonly replaced by willemite due to fluctuations in the fO2/fS2 conditions. Subsequent sulfide veinlets also cut the willemitic ore (Monteiro, 1997; Monteiro et al., 1999). Other carbonate-hosted Zn-(Pb) deposits in the Vazante Group, such as Morro Agudo, Ambro´sia, and Fagundes, are composed mainly of sphalerite, galena, pyrite, marcasite, dolomite, and quartz. The mineralization styles vary from predominantly syndiagenetic (Morro Agudo) to late diagenetic related to open-space filling (Fagundes) to epigenetic (Ambro´sia) (Monteiro, 2002; Monteiro et al., 2001, 2003, submitted). Mobilization of preexisting ore is common in all sulfide-rich deposits (Monteiro, 2002). 2.3. Previous geochronological constraints The age of the Vazante Group is controversial. Stromatolites indicate relative age intervals of 1650– 950 Ma (Conophyton Cylindricus Maslov, Moeri, 1972) and 1350–950 Ma (Conophyton metula Kirichenko, Cloud and Dardenne, 1973), a possible correlation with the 1200– 900 Ma Paranoa´ Group (Dardenne et al., 1976), which has been related to a passive margin setting. The diamictite units at the base of the Vazante Group, similar to those at the base of the Bambuı´ Group (w740 Ma; Babinski and Kaufman, 2003), also suggest a correlation between these groups (Dardenne, 2000). An Rb–Sr whole-rock isochron for shales from the Vazante Group yields an age of 600G50 Ma (Amaral and Kawashita, 1967), which may represent the time of the closure of the isotopic system during the Brasiliano metamorphic event. Lead isotope analyses of galena collected from different locations within the Vazante and Morro Agudo deposits yield Pb model ages ranging from 930 to 600 Ma (Amaral, 1968; Cassedane and Lasserre, 1969; Iyer et al., 1992; 1993; Freitas-Silva and Dardenne, 1997; Misi et al., 1997; Cunha et al., 2001, 2003), which have been considered the time of mineralization or remobilization. These ages were calculated using different models (Cumming and Richards, 1975; Stacey and Kramers, 1975; cf. Holmes, 1946; Houtermans, 1946), which may influence the large range of age values. Older Pb model ages (1000–1200 Ma) for galena from the Morro Agudo deposit estimated from the plumbotectonic model (Zartman and Doe, 1981) have been interpreted as the age that lead was isolated from its source or of syndiagenetic mineralization (Freitas-Silva and Dardenne, 1997). According to Azmy et al. (2001), d13C, d34S, and 87 Sr/86Sr signatures for the Vazante Group are consistent with an early Neoproterozoic (Sturtian) age, but the Precambrian baseline for isotope stratigraphy is poorly known, and additional studies are required to confirm this assertion. Sm–Nd analyses carried out on metasediments from the Vazante Group indicate a uniform distribution of depleted mantle model ages (TDM) between 1.7 and 2.1 Ga, which, according to Pimentel et al. (2001), reflect
297
the overlap of sources associated with the Paranoa´ (TDMZ 2.0–2.3 Ga) and Bambuı´ (TDMZ1.4–1.9 Ga) Groups. This idea is compatible with the intermediate stratigraphic position of the Vazante Group between the Paranoa´ passive margin and the Bambuı´ foreland sequences (Pimentel et al., 2001). Thus, the Vazante Group may correspond to the top of the passive margin sequence (Pimentel et al., 2001) or represent sedimentation in a rapidly subsiding basin in the Brası´lia fold belt initial thrust fronts (Dardenne, 2000).
3. Analytical procedures All analytical procedures—except the U–Pb chemical preparation analyses conducted at the Isotope Lab of Sa˜o Paulo State University (UNESP)—were carried out at the Center of Geochronological Research of the University of Sa˜o Paulo. U–Pb isotopic analyses were carried out on zircon and titanite grains separated from metabasites collected in two different cores. The samples were crushed and sieved, and the !100 mesh heavy mineral fraction was concentrated. Sample VZ-1 yielded a reasonable quantity of small, pink, clear zircon grains. Another sample (VZ-2) had a very low yield of zircon (also small, pink, clear crystals); however, a great amount of titanite was recovered from it. The titanite grains are creamy brown in color, have a sugary texture, and are opaque. Mineral fractions were prepared and dissolved, and Pb and U were purified using procedures modified after Krogh (1973, 1982) and Parrish (1987). All samples were spiked with a 205Pb–235U tracer solution prior to dissolution. Isotopic ratios were measured using a Finnigan MAT 262 multicollector mass spectrometer equipped with an ioncounting system. Both Pb and U isotopic compositions were analyzed on single Re filaments using silica gel and phosphoric acid and corrected for average mass discrimination by a 0.12G0.05% per atomic mass unit factor for the multicollector mode and a 0.18G0.05% per atomic mass unit factor for the single-collector mode (based on replicate analyses of common Pb standard SRM 981). In the case of titanite fractions, U and Pb contents were determined for one sample; in the other fractions, only Pb isotopic ratios were measured because of their dominant common Pb contents. Uranium fractionation was monitored by replicate analyses of the standard SRM U-500. Uncertainties in U/Pb ratios due to uncertainties in fractionation and mass spectrometry were approximately G0.5% (2s); in some cases, weak signals caused uncertainties up to 2% (2s). Radiogenic Pb isotopes were calculated by correcting for blank Pb and nonradiogenic original Pb, in line with Stacey and Kramers’s (1975) model Pb, for the approximate age of the sample. Uncertainties in radiogenic Pb ratios are typically G0.1% unless the sample has an unusually low 206 Pb/204Pb ratio, in which case uncertainties in the common Pb correction may be greater. Decay constants and isotopic
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ratios used in the age calculations are those listed by Steiger and Ja¨ger (1977). Total procedure blanks are 30 pg for lead and 1 pg for uranium. The U–Pb data were regressed using the ISOPLOT/EX program (Ludwig, 1999). The uncertainty in the concordia intercept age is given at the 2-sigma level. Seven metabasite samples recovered from two drill cores were powdered and digested in acids. Ion exchange resin was used for primary separation of the rare earth elements, followed by a secondary HDEHP-coated Teflon powder column for Sm and Nd separation. The isotope ratios were measured on a Finnigan MAT 262 mass spectrometer, and the quoted errors are given at the 2-sigma level. Sm–Nd
model ages were calculated using the depleted mantle model (TDM) of DePaolo (1981). Sr isotopic analyses were performed on dolomite samples dissolved with HCl 0.1 N. Strontium was isolated by standard ion exchange techniques, and isotopic compositions were determined on a VG 354 thermal ionization mass spectrometer. Multiple NBS 987 standards (nZ28) analyzed during and prior to this work yielded an average value of 0.710241G8 (2s). Carbon and oxygen isotope ratios were obtained after reacting the powdered dolomites with 100% H3PO4 at 25 8C for 1–3 days (McCrea, 1950). The released CO2 was
Fig. 5. (a) Bleached dolomite showing S-C structures associated with the Vazante shear zone. (b) Hydrothermally altered metabasite with chrysotile, hematite, and dolomite veins. (c) Least altered metabasite. (d) Tectonic contact between metabasite and brecciated dolostone from the Vazante zinc mine. (e) Metabasite showing remains of subophitic igneous texture. Plagioclase is sericitized, and pyroxene is replaced by amphibole and chlorite (transmitted light, photo widthZ 3.2 mm). (f) Mylonitized metabasite cut by carbonate veins (transmitted light; photo widthZ3.2 mm). (g) Igneous ilmenite replaced by hydrothermal titanite (SEM image).
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
Mass balance calculations (Monteiro, 1997) indicate that the mylonitized metabasite displays a mass increase (3%) relative to the least mylonitized metabasite (Fig. 6a). This mass increase is related mainly to MgO (w60%), CO2 (w60%), Pb (w174%), and Zn (w212%) enrichments. Relative loss of Fe2O3 (w33%), Ba, and Sr (w60%) also are observed (Fig. 6a). In contrast, the host bleached dolostones exhibit a mass decrease of 6% relative to the least altered dolostone (Fig. 6b). The zinc ore, in tectonic contact with metabasic rocks, shows a different mineral association characterized by Zn-rich chlorite, hematite, talc, and apatite associated with willemite. Within the shear zone, the hydrothermal alteration is mainly fracture controlled, but the host dolostones are affected by pervasive alteration characterized by color alteration from gray or pinkish to red, which is associated with silicification, recrystallization, and replacement by dolomite, siderite, jasper, hematite, and chlorite. Along the contact with the metabasites, however, the dolostones are bleached and display a distinct mineral assemblage compared with those of the hydrothermalized dolostones, which are composed of dolomite, chrysotile, chlorite, and quartz and thereby indicate metasomatic alteration. These rocks also present great differences in their isotopic compositions (d18OVSMOWZC21.4 to C16.7‰, d13CPDBZK0.2 to K1.5‰) relative to unaltered dolomites (d18OVSMOW ZC26.2 to C27.2‰, d13CPDBZK1.0 to C2.7‰). The calculated 87Sr/86Sr initial ratios (tZ750 Ma) of the bleached rocks (87Sr/86SrZ0.7327) are more radiogenic than the unaltered ( 87 Sr/ 86SrZ0.7064–0.7081) and hydrothermalized (87Sr/86SrZ0.7130–0.7160) dolomites (Monteiro, 2002). The calcite of the bleached metadolomite has the most distinct d13CPDB value (K10.3‰) compared with the hydrothermal carbonates in the fault zone (d13CPDBZK5.9 to C1.7‰) (Monteiro et al., 1999).
analyzed by an Europa Geo 20–20 mass spectrometer. The CO2 oxygen isotopic compositions were corrected to calcite and dolomite by applying, respectively, the fractionation factors of 1.01025 and 1.01111. The uncertainties of the isotope measurements were 0.1‰ (2s) during the period of analyses.
4. Results and discussion 4.1. Petrography and hydrothermal alteration of the mafic dikes The metabasites exhibit relicts of subophitic igneous texture and remnants of igneous minerals, such as plagioclase, pyroxene, and ilmenite, replaced by a typical low greenschist facies assemblage composed of actinolite, clinozoisite, epidote, chlorite, talc, sericite, quartz, calcite, rutile, leucoxene, and apatite. Sericite, clinozoisite, chlorite, and carbonates replace plagioclase. Pyroxene exhibits strong chloritization, whereas ilmenite, initially zoned, is replaced by leucoxene and rutile. Biotite remnants indicate early potassic alteration, possibly preceding the mylonitization of this rock (Figs. 4 and 5). In addition to the low greenschist facies metamorphism, mylonitization and hydrothermal activity related to fluid flow within the Vazante shear zone also affected the metabasites. Chlorite (penninite), apatite, and leucoxene formation, related to S–C structures, overprints the potassic alteration. Late dolomitization associated with the formation of talc, chrysotile, lepidocrocite, and hematite within ductile brittle and brittle structures accompanies the total destruction of igneous textures and minerals (Fig. 4). Titanite occurs as fine-grained crystals on the chlorite vein borders and along S–C structures, thus representing a hydrothermal phase associated with mylonitization.
(b) 180
45
Zn
Zn
SiO2 Mylonitized and bleached dolostone
Mylonitized and hydrothermalized metabasite
(a)
299
30
Pb MgO Al2O3 15 LOI Y Fe2O3 Zr Cr Co Cu Sr CaO 0 0
10
20
CA=0.97 C˚ Mass increase of 3% wt.
120
CA=1.06 C˚ Mass loss of 6% wt.
60
CO2
LOI CaO
Ba SiO2
0 30
40
Least-altered metabasite
50
0
MgO 20
40
60
Least-altered dolostone
Fig. 6. Isocon diagrams (Grant, 1986). (a) Mass balance between least altered metabasite and mylonitized metabasite. Oxides in wt.%, and elements in 0.05 ppm. (b) Mass balance between least altered dolostone and bleached dolostone in contact with metabasite. Oxides in wt.%, and elements in 0.02 ppm (Monteiro, 1997).
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4.2. Relationships between the metabasites and zinc ore The metabasic rocks occur mainly within the Vazante shear zone, juxtaposed with dolomites and slates of the Vazante Group. Therefore, recognition of the relationship between the metabasites and the host sequence is difficult, but the strong metasomatic alteration of the bleached dolostone in contact with the metabasites—as evidenced by its distinct aspect, mineralogy, and isotopic signatures compared with hydrothermalized dolostones in the shear zone—may be related to alteration due to the dike emplacement. Metasomatic processes also affected the metabasites and nonsulfide zinc ore, which indicates that strong fluid-rock interactions took place along the tectonic contact zones. Mass balance calculations and stable isotopic studies (Monteiro, 1997; Monteiro et al., 1999) indicate that the metasomatic processes are related to Zn- and Pb-rich fluid circulation in the Vazante shear zone. Thus, metabasites were affected by hydrothermal processes similar to those responsible for the epigenetic nonsulfide zinc ore formation, which is coeval with the Vazante shear zone development (Monteiro, 1997; Monteiro et al., 1999). The low d 13C value of calcite in the bleached metadolomite could indicate a local carbon source, as represented by the metabasites. However, the predominance of positive d13C values in gangue carbonates of the zinc ore may reflect the minimal importance of this carbon source for mineralization processes. 4.3. Geochronology and isotope geochemistry of the mafic dikes Sm–Nd isotopic analyses were carried out on seven whole-rock samples from two mafic dikes (Table 1). The TDM model ages are 1.15–1.21 Ga, and the 3Nd(0) varies from C0.16 to K0.41. Because the Sm/Nd ratios are very fractionated (0.16), the TDM ages are overestimated and represent the maximum crystallization age of the dikes. In addition, crustal contamination during magma ascent, which could increase the TDM ages, cannot be ruled out. Five small zircon fractions with different morphologies (from euhedral to rounded grains) of sample VZ-1 were analyzed, and all are discordant (Table 2, Fig. 7).
The zircon fractions yield 207Pb/206Pb minimum ages of 2096–2398 Ma; the oldest age was determined for euhedral acicular zircon. The single fraction of euhedral zircon recovered from sample VZ-2 yielded a 207Pb/206Pb minimum age of 2092 Ma with a large error (110 Ma). However, this age is similar to ages obtained from two fractions of sample VZ-1 (Table 2, Fig. 7). The U–Pb ages determined for zircon grains recovered from the metabasites are too old and thereby indicate that zircon crystals were assimilated by the magma from the basement rocks. Although the results show some scatter on the U–Pb diagram, fractions define a discordia that yields an upper intercept age of 2100G25 Ma, which indicates the presence of a Paleoproterozoic basement for the Vazante Group. Furthermore, according to the older 207Pb/206Pb ages (2.2–2.4 Ga) determined through some fractions, the magma may have passed through crust composed of rocks older than 2.1 Ga. Another possibility is that the magma assimilated detrital zircon from younger metasedimentary rocks whose source rocks were Paleoproterozoic. Seven single-crystal fractions of titanite separated from a metamorphosed mafic dike (VZ-2) were analyzed for Pb isotopic compositions and U and Pb concentrations. Surprisingly, all the samples contained virtually all common Pb, as shown by the high Pb (878 ppm) and low U (1.2 ppm) concentrations determined from one sample (Table 3; Fig. 8a,b). The isotopic ratios are uniform and do not present a large variation: 206Pb/204Pb values range from 17.68 to 17.85, 207Pb/204Pb from 15.66 to 15.76, and 208Pb/204Pb from 37.07 to 37.34. Despite the large errors (Table 3), these isotopic ratios plot well above the Pb evolution curve proposed by the two-stage model of Stacey and Kramers (1975), which represents the average crustal Pb and has a m2 (238U/204Pb) value of 9.74 and k2 (232Th/238U) value of 3.78 (Fig. 8a,b). Our Pb isotope results fit better on an evolution curve with a m2 value of 10.4 (Fig. 8a) and k2 value of 3.58 (Fig. 8b), which suggests that the hydrothermal fluids originated from a source with a U/Pb ratio higher than the average crustal value. 207Pb–206Pb model ages (Table 3) determined through Stacey and Kramers’s (1975) evolution model range from 780 to 870 Ma. Although Pb–Pb model ages are questionable (Faure, 1986), they could indicate the time when the Pb-rich fluids separated from
Table 1 Sm–Nd isotopic data for mafic dikes from the Vazante Group Sample
Sm (ppm)
Nd (ppm)
147
Sm/144Nd
143
Nd/144Nd
Error (2s)
TDM (Ga)
3(0)
VZ-1a VZ-1b VZ-1c VZ-3a VZ-3b VZ-3c VZ-3d
5.596 5.399 5.548 4.355 4.215 4.406 4.080
21.15 20.37 20.77 16.45 15.999 16.78 15.65
0.1600 0.1603 0.1615 0.1601 0.1593 0.1588 0.1576
0.512625 0.512617 0.512646 0.512632 0.512631 0.512620 0.512617
0.000015 0.000013 0.000022 0.000014 0.000016 0.000011 0.000007
1.19 1.21 1.16 1.17 1.15 1.17 1.16
K0.25 K0.41 C0.16 K0.12 K0.13 K0.35 K0.41
13 33 15 3 10 110
0.42
2250 2150 2050 1950
0.34
1850 1750
2010 2080 2050 1854 1962 1974
206Pb/238U
2157 2398 2225 2097 2096 2092
301
Samples VZ-1 & VZ-2
0.38
0.30 1650
1870 1774 1880 1644 1836 1863
206Pb*/ 238U
Ages (Ma)
207Pb*/ 235U
207Pb*/ 206Pb*
Error 2s
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
0.26 3.5
Model 1 Solution on 4 points Lower intercept: 0 ± 30 Ma Upper intercept: 2100 ± 25 Ma MSWD = 26, P = 0.000
1550
4.5
5.5
6.5
7.5
8.5
0.74 1.94 0.87 0.19 0.55 6.13 0.134454 0.154658 0.139845 0.129950 0.129881 0.129521
Fig. 7. U-Pb concordia diagram of zircon from metabasites of the Vazante Group. Analyses represented by filled ellipses were not used for the regression. The analysis with the larger uncertainty is from sample VZ-2; all the other points were obtained from zircon fractions separated from sample VZ-1.
Zircon morphology: R, rounded; A, acicular (5:1); Eu, euhedral; SR, slightly rounded; and An, anhedral.
0.006 0.002 0.008 0.060 0.016 0.002 VZ-1 R VZ-1 An VZ-1 Eu VZ-1 SR VZ-1 An VZ-2 Eu
255.1 263.0 153.9 220.5 138.3 95.0
129.6 162.8 76.1 106.7 64.4 50.4
135.6 69.6 146.0 116.7 180.3 75.3
6.23688 6.75605 6.52770 5.20649 5.90223 5.98387
1.34 3.48 1.58 0.62 1.05 10.1
0.336428 0.316824 0.338545 0.290576 0.329587 0.335073
1.07 2.76 1.27 0.58 0.861 7.67
2s (%) 206Pb*/ 238U 2s (%)
206Pb/ 204Pb (obs.) Pb* (ppm) U (ppm) Size (mg) Sample fraction
Table 2 U–Pb zircon data for mafic dikes from the Vazante Group
207Pb*/ 235U
Radiogenic ratios
2s (%)
207Pb*/ 206Pb*
207Pb/235U
the source. If so, the maximum age for the mineralizing event would be 870 Ma. However, it is worth noting that, because our Pb data do not fulfill all the conditions of Stacey and Kramers’s Pb model (i.e. the data plot above the Pb growth curve), the ages presented here must be considered with caution. The Pb isotope compositions of titanite plot on the same field defined by the Pb isotopic ratios reported for galena from the Morro Agudo, carbonate-hosted Zn–Pb deposit and minor sulfide ore bodies from the Vazante deposit (Misi et al., 1999; Cunha et al., 2001, 2003). Thus, the hydrothermal metalliferous fluids responsible for the sulfide mineralization exhibit isotopic signatures similar to those that affected the metabasites (Fig. 8). The presence of common Pb in the titanite, with its unusually low U (1.2 ppm) and high Pb (878 ppm) concentrations (U/PbZ 0.001), confirms petrographic observations that the mineral is hydrothermal and coeval with the timing of the Pb-rich fluid percolation in the shear zone. Furthermore, comparison of the Pb isotopic compositions from the hydrothermal mineral phases, such as titanite (Vazante mine) and galena (Morro Agudo and Vazante mines), suggests similarities between the metalliferous fluids that affected the metabasic dikes and the fluids responsible for the galena formation. This finding reinforces that a similar lead source was responsible for the genesis of both nonsulfide and sulfide zinc deposits in the Vazante-Paracatu district. Furthermore, it may reflect the periodic expulsion of metalliferous fluids related to a long-lived hydrothermal system or the hydrothermal/metamorphic mobilization of common lead from syn- to late-diagenetic Zn–Pb mineralization in the district and its subsequent deposition into the epigenetic nonsulfide Zn mineralization.
– – – – – – 1.2 – – – – – – 878 Isotopic compositions corrected for mass fractionation factor of 0.12 amu-1. Errors are 2 sigma (%). a 207 Pb/206Pb model ages determined according to Stacey and Kramers’s (1975) two-stage Pb evolution model.
0.612 0.377 0.815 0.017 0.649 0.114 0.060 37.073 37.252 37.297 37.246 37.341 37.142 37.221 0.612 0.369 0.823 0.015 0.640 0.109 0.060 15.659 15.708 15.738 15.725 15.759 15.693 15.723 0.608 0.376 0.820 0.016 0.641 0.123 0.060 17.726 17.806 17.811 17.780 17.846 17.758 17.777 VZ-2a VZ-2b VZ-2c VZ-2d VZ-2e VZ-2f VZ-2g
U (ppm) Pb (ppm) Error Pb/204Pb 207
Error Pb/204Pb 207
Error Pb/204Pb 206
Sample
Table 3 Pb isotope data determined for titanite from mafic dikes from the Vazante Group
780 810 857 857 868 818 855
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T (Ma)a
302
Fig. 8. (a) 207Pb/204Pb versus 206Pb/204Pb diagram and (b) 208Pb/204Pb versus 206Pb/204Pb diagram showing isotopic compositions determined for titanite from the metabasites from the Vazante Group. Ellipses represent 2-sigma errors. On the right side of (a), Pb isotope data of galena from the Vazante-Paracatu area (Field I, galena from N ore-body from Morro Agudo district; Field II, galena from other ore bodies from Morro Agudo and Vazante districts; Field III, galena from Ambro´sia and Fagundes districts; Cunha et al., 2003) are presented for comparison.
5. Conclusions The U–Pb investigation of zircon of the mafic dikes (metabasites) that cut the metasedimentary rocks of the Vazante Group and are affected by hydrothermal processes yields Paleoproterozoic ages (2.1–2.4 Ga), which suggests that the zircon crystals are xenocrystic grains. These zircon grains probably were assimilated by the magma during ascent through the continental crust; thus, the basement of the Vazante Group is either Paleoproterozoic or contains metasedimentary rocks formed by 2.1 Ga or older detrital sources. Mass balance calculations indicate that the metabasites were affected by hydrothermal processes responsible for the relative Zn and Pb enrichments and that these metasomatic processes are linked to Zn- and Pb-rich fluid circulation in the Vazante shear zone. Metasomatic processes also affected the dolostones and nonsulfide zinc ore in contact with
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
metabasites, which provides evidence that strong fluid-rock interactions took place along the tectonic contact zones. These processes resulted in higher 18O-shifts and more radiogenic Sr signatures in bleached dolostones compared with those of hydrothermalized dolostones located in the shear zone. The similar Pb isotopic compositions of hydrothermal titanite from the metabasites and galena from the Vazante deposit suggest that the metamorphic mafic rocks were affected by epigenetic-hydrothermal processes comparable to those responsible for the Vazante nonsulfide zinc mineralization that is closely linked to the Vazante shear zone development. The titanite Pb isotopic signature also is similar to those reported for galena from other zinc sulfide deposits hosted by the Vazante Group, such as the diagenetic–epigenetic Morro Agudo deposit. This similarity may imply a common source of lead for both epigenetic nonsulfide and diagenetic–epigenetic sulfide zinc deposits in the Vazante-Paracatu district. It also could be related to the metamorphic/hydrothermal remobilization of preexisting Pb or deep-seated, basin-derived metalliferous fluids associated with a long-lived hydrothermal system related to the diagenesis and deformation of the Vazante Group during the Neoproterozoic. Acknowledgements We are grateful to the Companhia Mineira de Metais for continuous support and hospitality at the mine. This research was partially supported by Pronex funds (Project 167/96) and FAPESP grants (Proc. 98/0412-3, 96/3941-3, and 95/4652-2). MB and JSB thank CNPq for a Research Fellowship. We thank S.R.F. Vlach for microprobe analysis on titanite and V.T. Martins for preparing the Pb diagrams. We also thank Massimo Chiaradia (University of Leeds, U.K.) and an anonymous reviewer whose thorough reviews helped improve the article. This study is a contribution to IGCP 450-Proterozoic Sediment-Hosted Base Metal Deposits of Western Gondwana. References Almeida, F.F.M., 1967. Origem e evoluc¸a˜o da Plataforma Brasileira. Rio de Janeiro, Bol. DNPM 243, p. 36. Amaral, G., 1968. Geologia e depo´sitos de mine´rio na regia˜o de Vazante, Estado de Minas Gerais. Doctoral Thesis, Universidade de Sa˜o Paulo, p. 133. Amaral, G., Kawashita, K. 1967. Determinac¸a˜o da idade do Grupo Bambuı´ pelo me´todo Rb/Sr. Congresso Brasileiro Geologia 21, Anais, SBG. pp. 214–217. Azmy, K., Veizer, J., Misi, A., Oliveira, T.F., Sanches, A.L., Dardenne, M.A., 2001. Dolomitization and isotope stratigraphy of the Vazante Formation, Sa˜o Francisco Basin, Brazil. Precambrian Research 112, 303–329. Babinski, M., Kaufman, A.J., 2003. First direct dating of a Neoproterozoic post-glacial cap carbonate. South American Symposium on Isotope Geology 4, Salvador, Brazil. Short Papers 1, 321–323.
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Cassedane, J., Lasserre, M., 1969. Ana´lise isoto´pica pelo me´todo do chumbo em algumas galenas brasileiras: descric¸a˜o do me´todo utilizado. Mining and Metallurgy XLIX (293), 215–224. Cloud, P.E., Dardenne, M.A., 1973. Proterozoic age of the Bambuı´ Group in Brazil. Geological Society of America Bulletin 84, 1673–1676. Cumming, G.L., Richards, J.R., 1975. Ore lead isotope ratios in a continuously changing Earth. Earth and Planetary Science Letters 28, 155–171. Cunha, I.A., Misi, A., Babinski, M., 2001. Lead isotope signatures of galenas from Morro Agudo Pb-Zn deposit, Minas Gerais, Brazil, in: Misi, A., Teixeira, J.B. (Eds.), Proterozoic Base Metal of Africa and South America. CNPq and UNESCO/IUGS, Belo Horizonte, pp. 45–47. Cunha, I.A., Babinski, M., Misi, A. 2003. Pb isotopic constraints on the mineralization from the Vazante Group. Minas Gerais, Brazil. South American Symposium on Isotope Geology 4, Salvador, Brazil. Short papers 2. pp. 727–730. Dardenne, M.A., 2000. The Brası´lia fold belt, in: Cordani, U.G., Thomaz Filho, A., Campos, D.A. (Eds.), Tectonic Evolution of South America. 31 International Geological Congress, Rio de Janeiro, pp. 231–263. Dardenne, M.A., Faria, A., Andrade, G.F., 1976. Occurrence de stromatolithes collumnaires dans le Groupe Bambuı´ (Goia´s, Bre´sil). Anais da Academia Brasileira de Cieˆncias 48 (3), 555–566. Dardenne, M.A., Freitas-Silva, F.H., Souza, J.C.F., Campos, J.E.G. 1998. Evoluc¸a˜o tectono-sedimentar do Grupo Vazante no contexto da Faixa de Dobramentos Brası´lia. Congresso Brasileiro Geologia 40, Belo Horizonte. Resumos, SBG, 26. DePaolo, D.J., 1981. A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. Journal of Geophysical Research 86, 10470–10488. Faure, G., 1986. Principles of Isotope Geology, Second ed. Wiley, New York. Freitas-Silva, F.H., Dardenne, M.A. 1997. Pb/Pb isotopic patterns of galenas from Morro do Ouro (Paracatu Formation). Morro Agudo/Vazante (Vazante Formation) and Bambuı´ Group deposits. SouthAmerican Symposium on Isotope Geology, 1, Extended Abstracts. pp. 118–120. Grant, J.A., 1986. The isocon diagram, a simple solution to Gresens’ equation for metasomatic alteration. Economic Geology 81, 1976– 1982. Hitzman, M.W., Reynolds, N.A., Sangster, D.F., Allen, C.R., Carman, C.E., 2003. Classification, genesis, and exploration guides for nonsulfides zinc deposits. Economic Geology 98, 685–714. Holmes, A., 1946. An estimate of the age of the earth. Nature 157, 680–684. Houtermans, F.G., 1946. Die isotopen-ha¨ufigkeiten im natu¨rlichen Blei und das Alter des Urans. Naturwissenschaften 33, 185–187. Iyer, S.S., Hoefs, J., Krouse, H.R., 1992. Sulfur and lead isotope geochemistry of galenas from Bambuı´ Group, Minas Gerais, Brazil: implications on ore genesis. Economic Geology 87, 437–443. Iyer, S.S., Krouse, H.P., Babinski, M., 1993. Isotope investigations on carbonate rocks hosted lead-zinc deposits from Bambuı´ Group, Minas Gerais, Brazil: implications for ore genesis and prospect evaluation. Simpo´sio do Cra´ton do Sa˜o Francisco 2, 338–339. Krogh, T.E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimimica et Cosmochimica Acta 37, 485–494. Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochimica et Cosmochimica Acta 46, 637–649. Ludwig, K.R., 1999. ISOPLOT/EX (A plotting and regression program for radiogenic-isotope data), version 2.05. Berkeley Geochronological Center, Special Publication 1A, p. 51. Madalosso, A., Valle, C.R.O., 1978. Considerac¸o˜es sobre a estratigrafia e sedimentologia do Grupo Bambuı´ na Regia˜o de Paracatu-Morro Agudo (MG). Congresso Brasileiro de Geologia 30, Anais, SBG 2, 622–631.
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Marini, O.J., Fuck, R.A., Danni, J.C., Dardenne, M.A., 1981. A evoluc¸a˜o geotectoˆnica da Faixa Brası´lia e de seu embasamento. Simpo´sio sobre o Cra´ton do Sa˜o Francisco e suas Faixas Marginais, Anais, CPM/SBG 1981;, 100–115. McCrea, J.M., 1950. On the isotope chemistry of carbonates and paleotemperature scale. Journal of Chemical Physics 18, 849–857. Misi, A., Tassinari, C.C.G., Iyer, S.S., 1997. New isotope data from the Proterozoic lead-zinc (Ag) sediment-hosted sulfide deposits of Brazil: implications for their metallogenic evolution. South-American Symposium on Isotope Geology 1, 201–203. Misi, A., Iyer, S.S., Tassinari, C.C.G., Kyle, R., Coelho, C.E.S., Franca Rocha, W.J.S.F., Gomes, A.S.R., Cunha, I.A., Carvalho, I.G., 1999. Geological and isotopic constraints on the metallogenic evolution of the Proterozoic sediment-hosted Pb-Zn (Ag) deposits of Brazil. Gondwana Research 2 (1), 47–65. Moeri, E., 1972. On a columnar stromatolite in the Precambrian Bambuı´ Group of central Brazil. Eclogae Geologische Helvetica 65, 185– 195. Monteiro, L.V.S., 1997. Contribuic¸a˜o a` geˆnese das mineralizac¸o˜es de zinco da Mina de Vazante, MG. Master Thesis, Universidade de Sa˜o Paulo, p. 159. Monteiro, L.V.S., 2002. Modelamento metalogene´tico dos depo´sitos de zinco de Vazante, Fagundes e Ambro´sia, associados ao Grupo Vazante, Minas Gerais. Doctoral Thesis, Universidade de Sa˜o Paulo, p. 317. Monteiro, L.V.S., Bettencourt, J.S., 2001. Genesis of the Vazante, Ambro´sia and Fagundes Zn-(Pb) deposits (Minas Gerais, Brazil): Geologic and stable isotopic constraints. In: Misi, A., Teixeira, J.B.G. (Org.), Proterozoic Base Metal Deposits of Africa and South America, CNPq/UNESCO/IUGS, pp. 79–81. Monteiro, L.V.S., Bettencourt, J.S., Spiro, B., Grac¸a, R., Oliveira, T.F., 1999. The Vazante zinc mine, MG, Brazil: Constraints on fluid
evolution and willemitic mineralization. Exploration Mining Geology 8, 21–42. Monteiro, L.V.S., Bettencourt, J.S., Bello, R.M.S., Juliani, C., Oliveira, T.F., 2003. Fluid inclusions and stable isotopic constraints on the genesis of the non-sulfide and sulfide zinc deposits in the VazanteParacatu belt, Brazil. In: Cailteux, J.L.H. (Org.), Proterozoic SedimentHosted Base Metal Deposits of Western Gondwana, UNESCO/IUGS, pp. 163–167. Monteiro, L.V.S, Bettencourt, J.S., Juliani, C., Oliveira, T.F. Minor and trace-element contents in sulfides from Vazante, Ambro´sia and Fagundes Neoproterozoic carbonate-hosted Zn-(Pb) deposits, Minas Gerais, Brazil: implications for ore genesis. Ore Geology Reviews. Submitted for Publication. Parrish, R.R., 1987. An improved micro-capsule for zircon dissolution in U–Pb geochronology. Chemical Geology 66, 99–102. Pimentel, M.M., Dardenne, M.A., Fuck, R.A., Viana, M.G., Junges, S.L., Fischel, D.P., Seer, H.J., Dantas, E.L., 2001. Nd isotopes and the provenance of detrital sediments of the Neoproterozoic Brası´lia belt, central Brazil. Journal of South American Earth Sciences 14, 571–585. Rigobello, A.E., Branquinho, J.A., Dantas, M.G.S., Oliveira, T.F., Neves Filho, W., 1988. Mina de Zinco de Vazante. In: Schobbenhaus, C., Coelho, C.E.S. (Coords.), Principais Depo´sitos Minerais do Brasil. DNPM 3, pp. 101–110. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207–221. Steiger, R.H., Ja¨ger, E., 1977. Subcomission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362. Zartman, R.E., Doe, B.R., 1981. Plumbotectonics-the model. Tectonophysics 75, 135–162.
Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil M. Babinskia,*, L.V.S. Monteirob, A.H. Fetterc, J.S. Bettencourta, T.F. Oliveirad a
Centro de Pesquisas Geocronolo´gicas, Instituto de Geocieˆncias, Universidade de Sa˜o Paulo, Rua do Lago, 562, Sa˜o Paulo, SP CEP 05508-080, Brazil b Instituto de Geocieˆncias, Universidade Estadual de Campinas, UNICAMP, Rua Joa˜o Pandia´ Calo´geras, 51, Campinas, SP CEP 13083-970, Brazil c Instituto de Geocieˆncias e Cieˆncias Exatas, Universidade Estadual Paulista, UNESP, Av. 24 A, 1515, Rio Claro, SP CEP 13506-900, Brazil d Companhia Mineira de Metais, Caixa Postal 3, Vazante, MG CEP 38780-000, Brazil Received 1 August 2003; accepted 1 November 2004
Abstract The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U–Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedimentary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Pb compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonatehosted Zn–Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante–Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic. q 2005 Elsevier Ltd. All rights reserved. Keywords: Isotope geochemistry; Mafic dikes; U–Pb geochronology; Vazante Group; Zn–Pb mineralization
1. Introduction The Vazante Group (Dardenne et al., 1998), which occurs in the northwestern part of Minas Gerais, hosts the most important carbonate-hosted nonsulfide (Vazante) and sulfide (Morro Agudo) zinc deposits known in Brazil. It also represents one of the metasedimentary units of the southern segment of the Brası´lia fold belt.
* Corresponding author. Fax: C55 11 3091 3993. E-mail addresses: [email protected] (M. Babinski), [email protected] (L.V.S. Monteiro), [email protected] (A.H. Fetter), [email protected] (J.S. Bettencourt), [email protected] (T.F. Oliveira). 0895-9811/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.11.010
The tectonic setting of this unit is controversial. Recent studies indicate that the Vazante Group could represent sedimentation in a rapidly subsiding basin in the Brası´lia fold belt’s initial thrust fronts (Dardenne, 2000) in a setting similar to that of the Bambuı´ Group or correspond to the top of the Meso-Neoproterozoic passive margin sequence, represented mainly by the Paranoa´ Group (Pimentel et al., 2001). The Vazante Group is composed of a thick sedimentary sequence whose depositional age remains disputable. The available geochronological data indicate that it was metamorphosed during the Brasiliano orogeny. Recently, metamorphosed mafic dikes (here, metabasites) intrusive in the metasedimentary sequence were found in drill cores in
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the Vazante underground mine. The metabasites are tectonically imbricated with hydraulic breccias in the Vazante shear zone, which represents the main structural control of hypogene nonsulfide mineralization, and are affected by hydrothermal alteration processes. Thus, their geologic relationship is promising for constraining the minimum age of the sediment deposition and the hydrothermal processes linked to the hypogene nonsulfide zinc mineralization from Vazante. This article presents U–Pb, Pb–Pb, and Sm–Nd isotopic data along with petrological, geochemical, and C, O, and Sr isotopic studies on the mafic dikes and the metasedimentary rocks of the Vazante Group. It also reports the isotopic signatures of the metalliferous fluid responsible for the hydrothermal alteration that affected the metabasites and its relationship with the hydrothermal system responsible for the zinc mineralization hosted by the Vazante Group.
2. Geological setting The Vazante-Paracatu region is located in the eastern part of the Brası´lia fold belt (Almeida, 1967), which extends for more than 1000 km over a width of 300 km along the western margin of the Sa˜o Francisco craton (Fig. 1). The fold belt represents an unstable crustal block whose final structural differentiation resulted from the closure of a wide
oceanic basin during the Neoproterozoic Brasiliano orogeny (w600 Ma) (Pimentel et al., 2001). The Brası´lia fold belt displays rock sequences thrust to the east with increasing deformation and metamorphism to the west (Marini et al., 1981), which reflect the vergence of the fold belt with respect to the Sa˜o Francisco craton (Dardenne, 2000). The Vazante Group (Dardenne et al., 1998) represents one of the metasedimentary units of the southern segment of the Brası´lia fold belt involved in a complex imbricate system of nappes and thrusts, with the main compressive trend from SW to NE. It is affected by sinistral transcurrent shear zones with SE transport (Dardenne, 2000), as well as low greenschist facies metamorphism. It displays a pervasive cleavage (S 1) related to regional folding, which is overprinted by S2 and S3 cleavages related to local D1 isoclinal and D2 open folds, respectively (Monteiro, 1997). The Vazante Group is divided into the following seven formations (Fig. 2) from base to top: Santo Antoˆnio do Bonito, Rocinha, Lagamar, Serra do Garrote, Serra do Poc¸o Verde, Morro do Calca´rio, and Lapa. Metapelitic units with phosphorite occurrences compose the basal Retiro and Rocinha Formations (Dardenne et al., 1998; Dardenne, 2000). The Lagamar Formation represents a metapsamopelitic and carbonatic unit with basal metaconglomerates, dolomitic breccia, dark gray limestone, and stromatolitic bioherm with columnar stromatolites of
Fig. 1. Geological map of the southern part of the Brası´lia fold belt (Dardenne, 2000).
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295
Fig. 2. Lithostratigraphic column of the Vazante Group (Dardenne et al., 1998; Dardenne, 2000).
the Conophyton/Jacutophyton type (Cloud and Dardenne, 1973; Moeri, 1972). The Serra do Garrote Formation consists of a thick sequence of gray slate, locally carbonaceous and containing pyrite, with quartzite layers (Madalosso and Valle, 1978). The Serra do Poc¸o Verde Formation is made up of gray to pink algal-laminated dolomite, gray to green slates, sericite phyllite, dark gray dolomite with birds’ eyes, marls, and pyrite-bearing carbonaceous shale. Stromatolitic bioherm facies, breccias, dolarenite, and subordinate carbonaceous shale compose the Morro do Calca´rio Formation. These formations are the dominantly dolomitic sequences (Fig. 2) that host the Zn–(Pb) deposits. The dolomitic sequence is overlain by the Lapa Formation, with black carbonaceous slate and phyllite (Serra do Velosinho member), and by phyllite, carbonatebearing metasiltstone, dolomite, and quartzite lenses of the Serra da Lapa member (Madalosso and Valle, 1978). The Canastra Group (1.2–0.9 Ga; Pimentel et al., 2001) metasediments, which comprise the chlorite-rich calc-phyllite of the Landim Formation, and carbonaceous phyllites and quartzites of the Paracatu Formation overthrust the rocks of
the Vazante Group as a consequence of the Brasiliano collisional event (w630 Ma; Dardenne, 2000). 2.1. Mafic rocks Minor magmatism associated with the Vazante Group is represented by small bodies of metabasic rocks, identified mainly at the Vazante underground zinc mine (Fig. 3). In this deposit, the metabasites (Fig. 3) occur in the Vazante shear zone, tectonically imbricated with hydraulic breccias, hydrothermally altered rocks, and nonsulfide zinc ore composed mainly of willemite (Zn2SiO4). These mafic lithotypes were described initially as diabase dikes of Cretaceous age (Rigobello et al., 1988). However, the presence of a metamorphic mineral assemblage (Fig. 4) and mylonitic fabric, not recognized in the Brazilian Cretaceous rocks, indicates that the unit was affected by the regional Brasiliano deformation event (Monteiro, 1997). The metabasites and ore bodies are offset by normal and reverse faults and cut by late hydrothermal veins, which likely caused complex relationships between the ore bodies and the host sequence.
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Fig. 3. Cross-section of the Vazante ore zone showing the spatial relationships among metabasites, the host sequence, willemitic ore, and the Vazante shear zone (Monteiro, 1997; Monteiro et al., 1999).
2.2. Zinc mineralization hosted by the Vazante Group The Zn–(Pb) deposits contained within the Vazante Group metasedimentary sequence predominantly are faultor shear zone-controlled and show distinct ore mineralogy (sulfide- or silicate-bearing) and mineralization styles (synto tardidiagenetic and epigenetic), which are associated with long-lived hydrothermal systems related to the diagenesis and deformation of the Vazante Group during the Neoproterozoic (Monteiro, 2002; Monteiro and Bettencourt, 2001; Monteiro et al., 2003, submitted). The Vazante deposit is a main example of a hypogene nonsulfide zinc deposit, as defined by Hitzman et al. (2003). The zinc mineralization is epigenetic, linked to the Vazante shear zone development (N50/60NW), which was subsequent to the low greenschist facies metamorphism of the Vazante Group. In the shear zone, the hydrothermal alteration is largely fracture controlled and produced a complex zone of net-veined breccia filled by dolomite, ankerite, siderite, jasper, hematite, and chlorite (Monteiro, 1997; Monteiro et al., 1999). The ore bodies display pod morphology and are tectonically imbricated with small metabasite bodies, brecciated metadolomites, and slates (Monteiro et al., 1999).
The Vazante main ore is made of willemite (Zn2SiO4), dolomite, quartz, siderite, hematite, barite, franklinite, zincite, smithsonite, apatite, magnetite, and Zn-chlorite (Monteiro, 1997; Monteiro et al., 1999). Small sulfide bodies composed mainly of Cd-rich sphalerite and galena occur tectonically imbricated with the willemitic ore and the intensely hydrothermalized host rocks in the Vazante shear Vazante Shear Zone development Minerals
Green schist facies metamorphism (Sn)
Sn+1 (ductile-brittle structures)
Sn+2 (ductile-brittle to brittle structures)
Chloritization
Carbonatization Serpentinization Fe-oxides formation
Actinolite Chlorite Sericite Biotite Quartz Epidote Clinozoisite Apatite Carbonates Talc Chrysotile Titanite Rutile Leucoxene Hematite Lepidocrocite Chlor
itization Sericitization Epidotization
Fig. 4. Paragenetic associations of the metabasites (Monteiro, 1997).
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
zone. Sphalerite is coprecipited with willemite but commonly replaced by willemite due to fluctuations in the fO2/fS2 conditions. Subsequent sulfide veinlets also cut the willemitic ore (Monteiro, 1997; Monteiro et al., 1999). Other carbonate-hosted Zn-(Pb) deposits in the Vazante Group, such as Morro Agudo, Ambro´sia, and Fagundes, are composed mainly of sphalerite, galena, pyrite, marcasite, dolomite, and quartz. The mineralization styles vary from predominantly syndiagenetic (Morro Agudo) to late diagenetic related to open-space filling (Fagundes) to epigenetic (Ambro´sia) (Monteiro, 2002; Monteiro et al., 2001, 2003, submitted). Mobilization of preexisting ore is common in all sulfide-rich deposits (Monteiro, 2002). 2.3. Previous geochronological constraints The age of the Vazante Group is controversial. Stromatolites indicate relative age intervals of 1650– 950 Ma (Conophyton Cylindricus Maslov, Moeri, 1972) and 1350–950 Ma (Conophyton metula Kirichenko, Cloud and Dardenne, 1973), a possible correlation with the 1200– 900 Ma Paranoa´ Group (Dardenne et al., 1976), which has been related to a passive margin setting. The diamictite units at the base of the Vazante Group, similar to those at the base of the Bambuı´ Group (w740 Ma; Babinski and Kaufman, 2003), also suggest a correlation between these groups (Dardenne, 2000). An Rb–Sr whole-rock isochron for shales from the Vazante Group yields an age of 600G50 Ma (Amaral and Kawashita, 1967), which may represent the time of the closure of the isotopic system during the Brasiliano metamorphic event. Lead isotope analyses of galena collected from different locations within the Vazante and Morro Agudo deposits yield Pb model ages ranging from 930 to 600 Ma (Amaral, 1968; Cassedane and Lasserre, 1969; Iyer et al., 1992; 1993; Freitas-Silva and Dardenne, 1997; Misi et al., 1997; Cunha et al., 2001, 2003), which have been considered the time of mineralization or remobilization. These ages were calculated using different models (Cumming and Richards, 1975; Stacey and Kramers, 1975; cf. Holmes, 1946; Houtermans, 1946), which may influence the large range of age values. Older Pb model ages (1000–1200 Ma) for galena from the Morro Agudo deposit estimated from the plumbotectonic model (Zartman and Doe, 1981) have been interpreted as the age that lead was isolated from its source or of syndiagenetic mineralization (Freitas-Silva and Dardenne, 1997). According to Azmy et al. (2001), d13C, d34S, and 87 Sr/86Sr signatures for the Vazante Group are consistent with an early Neoproterozoic (Sturtian) age, but the Precambrian baseline for isotope stratigraphy is poorly known, and additional studies are required to confirm this assertion. Sm–Nd analyses carried out on metasediments from the Vazante Group indicate a uniform distribution of depleted mantle model ages (TDM) between 1.7 and 2.1 Ga, which, according to Pimentel et al. (2001), reflect
297
the overlap of sources associated with the Paranoa´ (TDMZ 2.0–2.3 Ga) and Bambuı´ (TDMZ1.4–1.9 Ga) Groups. This idea is compatible with the intermediate stratigraphic position of the Vazante Group between the Paranoa´ passive margin and the Bambuı´ foreland sequences (Pimentel et al., 2001). Thus, the Vazante Group may correspond to the top of the passive margin sequence (Pimentel et al., 2001) or represent sedimentation in a rapidly subsiding basin in the Brası´lia fold belt initial thrust fronts (Dardenne, 2000).
3. Analytical procedures All analytical procedures—except the U–Pb chemical preparation analyses conducted at the Isotope Lab of Sa˜o Paulo State University (UNESP)—were carried out at the Center of Geochronological Research of the University of Sa˜o Paulo. U–Pb isotopic analyses were carried out on zircon and titanite grains separated from metabasites collected in two different cores. The samples were crushed and sieved, and the !100 mesh heavy mineral fraction was concentrated. Sample VZ-1 yielded a reasonable quantity of small, pink, clear zircon grains. Another sample (VZ-2) had a very low yield of zircon (also small, pink, clear crystals); however, a great amount of titanite was recovered from it. The titanite grains are creamy brown in color, have a sugary texture, and are opaque. Mineral fractions were prepared and dissolved, and Pb and U were purified using procedures modified after Krogh (1973, 1982) and Parrish (1987). All samples were spiked with a 205Pb–235U tracer solution prior to dissolution. Isotopic ratios were measured using a Finnigan MAT 262 multicollector mass spectrometer equipped with an ioncounting system. Both Pb and U isotopic compositions were analyzed on single Re filaments using silica gel and phosphoric acid and corrected for average mass discrimination by a 0.12G0.05% per atomic mass unit factor for the multicollector mode and a 0.18G0.05% per atomic mass unit factor for the single-collector mode (based on replicate analyses of common Pb standard SRM 981). In the case of titanite fractions, U and Pb contents were determined for one sample; in the other fractions, only Pb isotopic ratios were measured because of their dominant common Pb contents. Uranium fractionation was monitored by replicate analyses of the standard SRM U-500. Uncertainties in U/Pb ratios due to uncertainties in fractionation and mass spectrometry were approximately G0.5% (2s); in some cases, weak signals caused uncertainties up to 2% (2s). Radiogenic Pb isotopes were calculated by correcting for blank Pb and nonradiogenic original Pb, in line with Stacey and Kramers’s (1975) model Pb, for the approximate age of the sample. Uncertainties in radiogenic Pb ratios are typically G0.1% unless the sample has an unusually low 206 Pb/204Pb ratio, in which case uncertainties in the common Pb correction may be greater. Decay constants and isotopic
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ratios used in the age calculations are those listed by Steiger and Ja¨ger (1977). Total procedure blanks are 30 pg for lead and 1 pg for uranium. The U–Pb data were regressed using the ISOPLOT/EX program (Ludwig, 1999). The uncertainty in the concordia intercept age is given at the 2-sigma level. Seven metabasite samples recovered from two drill cores were powdered and digested in acids. Ion exchange resin was used for primary separation of the rare earth elements, followed by a secondary HDEHP-coated Teflon powder column for Sm and Nd separation. The isotope ratios were measured on a Finnigan MAT 262 mass spectrometer, and the quoted errors are given at the 2-sigma level. Sm–Nd
model ages were calculated using the depleted mantle model (TDM) of DePaolo (1981). Sr isotopic analyses were performed on dolomite samples dissolved with HCl 0.1 N. Strontium was isolated by standard ion exchange techniques, and isotopic compositions were determined on a VG 354 thermal ionization mass spectrometer. Multiple NBS 987 standards (nZ28) analyzed during and prior to this work yielded an average value of 0.710241G8 (2s). Carbon and oxygen isotope ratios were obtained after reacting the powdered dolomites with 100% H3PO4 at 25 8C for 1–3 days (McCrea, 1950). The released CO2 was
Fig. 5. (a) Bleached dolomite showing S-C structures associated with the Vazante shear zone. (b) Hydrothermally altered metabasite with chrysotile, hematite, and dolomite veins. (c) Least altered metabasite. (d) Tectonic contact between metabasite and brecciated dolostone from the Vazante zinc mine. (e) Metabasite showing remains of subophitic igneous texture. Plagioclase is sericitized, and pyroxene is replaced by amphibole and chlorite (transmitted light, photo widthZ 3.2 mm). (f) Mylonitized metabasite cut by carbonate veins (transmitted light; photo widthZ3.2 mm). (g) Igneous ilmenite replaced by hydrothermal titanite (SEM image).
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
Mass balance calculations (Monteiro, 1997) indicate that the mylonitized metabasite displays a mass increase (3%) relative to the least mylonitized metabasite (Fig. 6a). This mass increase is related mainly to MgO (w60%), CO2 (w60%), Pb (w174%), and Zn (w212%) enrichments. Relative loss of Fe2O3 (w33%), Ba, and Sr (w60%) also are observed (Fig. 6a). In contrast, the host bleached dolostones exhibit a mass decrease of 6% relative to the least altered dolostone (Fig. 6b). The zinc ore, in tectonic contact with metabasic rocks, shows a different mineral association characterized by Zn-rich chlorite, hematite, talc, and apatite associated with willemite. Within the shear zone, the hydrothermal alteration is mainly fracture controlled, but the host dolostones are affected by pervasive alteration characterized by color alteration from gray or pinkish to red, which is associated with silicification, recrystallization, and replacement by dolomite, siderite, jasper, hematite, and chlorite. Along the contact with the metabasites, however, the dolostones are bleached and display a distinct mineral assemblage compared with those of the hydrothermalized dolostones, which are composed of dolomite, chrysotile, chlorite, and quartz and thereby indicate metasomatic alteration. These rocks also present great differences in their isotopic compositions (d18OVSMOWZC21.4 to C16.7‰, d13CPDBZK0.2 to K1.5‰) relative to unaltered dolomites (d18OVSMOW ZC26.2 to C27.2‰, d13CPDBZK1.0 to C2.7‰). The calculated 87Sr/86Sr initial ratios (tZ750 Ma) of the bleached rocks (87Sr/86SrZ0.7327) are more radiogenic than the unaltered ( 87 Sr/ 86SrZ0.7064–0.7081) and hydrothermalized (87Sr/86SrZ0.7130–0.7160) dolomites (Monteiro, 2002). The calcite of the bleached metadolomite has the most distinct d13CPDB value (K10.3‰) compared with the hydrothermal carbonates in the fault zone (d13CPDBZK5.9 to C1.7‰) (Monteiro et al., 1999).
analyzed by an Europa Geo 20–20 mass spectrometer. The CO2 oxygen isotopic compositions were corrected to calcite and dolomite by applying, respectively, the fractionation factors of 1.01025 and 1.01111. The uncertainties of the isotope measurements were 0.1‰ (2s) during the period of analyses.
4. Results and discussion 4.1. Petrography and hydrothermal alteration of the mafic dikes The metabasites exhibit relicts of subophitic igneous texture and remnants of igneous minerals, such as plagioclase, pyroxene, and ilmenite, replaced by a typical low greenschist facies assemblage composed of actinolite, clinozoisite, epidote, chlorite, talc, sericite, quartz, calcite, rutile, leucoxene, and apatite. Sericite, clinozoisite, chlorite, and carbonates replace plagioclase. Pyroxene exhibits strong chloritization, whereas ilmenite, initially zoned, is replaced by leucoxene and rutile. Biotite remnants indicate early potassic alteration, possibly preceding the mylonitization of this rock (Figs. 4 and 5). In addition to the low greenschist facies metamorphism, mylonitization and hydrothermal activity related to fluid flow within the Vazante shear zone also affected the metabasites. Chlorite (penninite), apatite, and leucoxene formation, related to S–C structures, overprints the potassic alteration. Late dolomitization associated with the formation of talc, chrysotile, lepidocrocite, and hematite within ductile brittle and brittle structures accompanies the total destruction of igneous textures and minerals (Fig. 4). Titanite occurs as fine-grained crystals on the chlorite vein borders and along S–C structures, thus representing a hydrothermal phase associated with mylonitization.
(b) 180
45
Zn
Zn
SiO2 Mylonitized and bleached dolostone
Mylonitized and hydrothermalized metabasite
(a)
299
30
Pb MgO Al2O3 15 LOI Y Fe2O3 Zr Cr Co Cu Sr CaO 0 0
10
20
CA=0.97 C˚ Mass increase of 3% wt.
120
CA=1.06 C˚ Mass loss of 6% wt.
60
CO2
LOI CaO
Ba SiO2
0 30
40
Least-altered metabasite
50
0
MgO 20
40
60
Least-altered dolostone
Fig. 6. Isocon diagrams (Grant, 1986). (a) Mass balance between least altered metabasite and mylonitized metabasite. Oxides in wt.%, and elements in 0.05 ppm. (b) Mass balance between least altered dolostone and bleached dolostone in contact with metabasite. Oxides in wt.%, and elements in 0.02 ppm (Monteiro, 1997).
300
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4.2. Relationships between the metabasites and zinc ore The metabasic rocks occur mainly within the Vazante shear zone, juxtaposed with dolomites and slates of the Vazante Group. Therefore, recognition of the relationship between the metabasites and the host sequence is difficult, but the strong metasomatic alteration of the bleached dolostone in contact with the metabasites—as evidenced by its distinct aspect, mineralogy, and isotopic signatures compared with hydrothermalized dolostones in the shear zone—may be related to alteration due to the dike emplacement. Metasomatic processes also affected the metabasites and nonsulfide zinc ore, which indicates that strong fluid-rock interactions took place along the tectonic contact zones. Mass balance calculations and stable isotopic studies (Monteiro, 1997; Monteiro et al., 1999) indicate that the metasomatic processes are related to Zn- and Pb-rich fluid circulation in the Vazante shear zone. Thus, metabasites were affected by hydrothermal processes similar to those responsible for the epigenetic nonsulfide zinc ore formation, which is coeval with the Vazante shear zone development (Monteiro, 1997; Monteiro et al., 1999). The low d 13C value of calcite in the bleached metadolomite could indicate a local carbon source, as represented by the metabasites. However, the predominance of positive d13C values in gangue carbonates of the zinc ore may reflect the minimal importance of this carbon source for mineralization processes. 4.3. Geochronology and isotope geochemistry of the mafic dikes Sm–Nd isotopic analyses were carried out on seven whole-rock samples from two mafic dikes (Table 1). The TDM model ages are 1.15–1.21 Ga, and the 3Nd(0) varies from C0.16 to K0.41. Because the Sm/Nd ratios are very fractionated (0.16), the TDM ages are overestimated and represent the maximum crystallization age of the dikes. In addition, crustal contamination during magma ascent, which could increase the TDM ages, cannot be ruled out. Five small zircon fractions with different morphologies (from euhedral to rounded grains) of sample VZ-1 were analyzed, and all are discordant (Table 2, Fig. 7).
The zircon fractions yield 207Pb/206Pb minimum ages of 2096–2398 Ma; the oldest age was determined for euhedral acicular zircon. The single fraction of euhedral zircon recovered from sample VZ-2 yielded a 207Pb/206Pb minimum age of 2092 Ma with a large error (110 Ma). However, this age is similar to ages obtained from two fractions of sample VZ-1 (Table 2, Fig. 7). The U–Pb ages determined for zircon grains recovered from the metabasites are too old and thereby indicate that zircon crystals were assimilated by the magma from the basement rocks. Although the results show some scatter on the U–Pb diagram, fractions define a discordia that yields an upper intercept age of 2100G25 Ma, which indicates the presence of a Paleoproterozoic basement for the Vazante Group. Furthermore, according to the older 207Pb/206Pb ages (2.2–2.4 Ga) determined through some fractions, the magma may have passed through crust composed of rocks older than 2.1 Ga. Another possibility is that the magma assimilated detrital zircon from younger metasedimentary rocks whose source rocks were Paleoproterozoic. Seven single-crystal fractions of titanite separated from a metamorphosed mafic dike (VZ-2) were analyzed for Pb isotopic compositions and U and Pb concentrations. Surprisingly, all the samples contained virtually all common Pb, as shown by the high Pb (878 ppm) and low U (1.2 ppm) concentrations determined from one sample (Table 3; Fig. 8a,b). The isotopic ratios are uniform and do not present a large variation: 206Pb/204Pb values range from 17.68 to 17.85, 207Pb/204Pb from 15.66 to 15.76, and 208Pb/204Pb from 37.07 to 37.34. Despite the large errors (Table 3), these isotopic ratios plot well above the Pb evolution curve proposed by the two-stage model of Stacey and Kramers (1975), which represents the average crustal Pb and has a m2 (238U/204Pb) value of 9.74 and k2 (232Th/238U) value of 3.78 (Fig. 8a,b). Our Pb isotope results fit better on an evolution curve with a m2 value of 10.4 (Fig. 8a) and k2 value of 3.58 (Fig. 8b), which suggests that the hydrothermal fluids originated from a source with a U/Pb ratio higher than the average crustal value. 207Pb–206Pb model ages (Table 3) determined through Stacey and Kramers’s (1975) evolution model range from 780 to 870 Ma. Although Pb–Pb model ages are questionable (Faure, 1986), they could indicate the time when the Pb-rich fluids separated from
Table 1 Sm–Nd isotopic data for mafic dikes from the Vazante Group Sample
Sm (ppm)
Nd (ppm)
147
Sm/144Nd
143
Nd/144Nd
Error (2s)
TDM (Ga)
3(0)
VZ-1a VZ-1b VZ-1c VZ-3a VZ-3b VZ-3c VZ-3d
5.596 5.399 5.548 4.355 4.215 4.406 4.080
21.15 20.37 20.77 16.45 15.999 16.78 15.65
0.1600 0.1603 0.1615 0.1601 0.1593 0.1588 0.1576
0.512625 0.512617 0.512646 0.512632 0.512631 0.512620 0.512617
0.000015 0.000013 0.000022 0.000014 0.000016 0.000011 0.000007
1.19 1.21 1.16 1.17 1.15 1.17 1.16
K0.25 K0.41 C0.16 K0.12 K0.13 K0.35 K0.41
13 33 15 3 10 110
0.42
2250 2150 2050 1950
0.34
1850 1750
2010 2080 2050 1854 1962 1974
206Pb/238U
2157 2398 2225 2097 2096 2092
301
Samples VZ-1 & VZ-2
0.38
0.30 1650
1870 1774 1880 1644 1836 1863
206Pb*/ 238U
Ages (Ma)
207Pb*/ 235U
207Pb*/ 206Pb*
Error 2s
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
0.26 3.5
Model 1 Solution on 4 points Lower intercept: 0 ± 30 Ma Upper intercept: 2100 ± 25 Ma MSWD = 26, P = 0.000
1550
4.5
5.5
6.5
7.5
8.5
0.74 1.94 0.87 0.19 0.55 6.13 0.134454 0.154658 0.139845 0.129950 0.129881 0.129521
Fig. 7. U-Pb concordia diagram of zircon from metabasites of the Vazante Group. Analyses represented by filled ellipses were not used for the regression. The analysis with the larger uncertainty is from sample VZ-2; all the other points were obtained from zircon fractions separated from sample VZ-1.
Zircon morphology: R, rounded; A, acicular (5:1); Eu, euhedral; SR, slightly rounded; and An, anhedral.
0.006 0.002 0.008 0.060 0.016 0.002 VZ-1 R VZ-1 An VZ-1 Eu VZ-1 SR VZ-1 An VZ-2 Eu
255.1 263.0 153.9 220.5 138.3 95.0
129.6 162.8 76.1 106.7 64.4 50.4
135.6 69.6 146.0 116.7 180.3 75.3
6.23688 6.75605 6.52770 5.20649 5.90223 5.98387
1.34 3.48 1.58 0.62 1.05 10.1
0.336428 0.316824 0.338545 0.290576 0.329587 0.335073
1.07 2.76 1.27 0.58 0.861 7.67
2s (%) 206Pb*/ 238U 2s (%)
206Pb/ 204Pb (obs.) Pb* (ppm) U (ppm) Size (mg) Sample fraction
Table 2 U–Pb zircon data for mafic dikes from the Vazante Group
207Pb*/ 235U
Radiogenic ratios
2s (%)
207Pb*/ 206Pb*
207Pb/235U
the source. If so, the maximum age for the mineralizing event would be 870 Ma. However, it is worth noting that, because our Pb data do not fulfill all the conditions of Stacey and Kramers’s Pb model (i.e. the data plot above the Pb growth curve), the ages presented here must be considered with caution. The Pb isotope compositions of titanite plot on the same field defined by the Pb isotopic ratios reported for galena from the Morro Agudo, carbonate-hosted Zn–Pb deposit and minor sulfide ore bodies from the Vazante deposit (Misi et al., 1999; Cunha et al., 2001, 2003). Thus, the hydrothermal metalliferous fluids responsible for the sulfide mineralization exhibit isotopic signatures similar to those that affected the metabasites (Fig. 8). The presence of common Pb in the titanite, with its unusually low U (1.2 ppm) and high Pb (878 ppm) concentrations (U/PbZ 0.001), confirms petrographic observations that the mineral is hydrothermal and coeval with the timing of the Pb-rich fluid percolation in the shear zone. Furthermore, comparison of the Pb isotopic compositions from the hydrothermal mineral phases, such as titanite (Vazante mine) and galena (Morro Agudo and Vazante mines), suggests similarities between the metalliferous fluids that affected the metabasic dikes and the fluids responsible for the galena formation. This finding reinforces that a similar lead source was responsible for the genesis of both nonsulfide and sulfide zinc deposits in the Vazante-Paracatu district. Furthermore, it may reflect the periodic expulsion of metalliferous fluids related to a long-lived hydrothermal system or the hydrothermal/metamorphic mobilization of common lead from syn- to late-diagenetic Zn–Pb mineralization in the district and its subsequent deposition into the epigenetic nonsulfide Zn mineralization.
– – – – – – 1.2 – – – – – – 878 Isotopic compositions corrected for mass fractionation factor of 0.12 amu-1. Errors are 2 sigma (%). a 207 Pb/206Pb model ages determined according to Stacey and Kramers’s (1975) two-stage Pb evolution model.
0.612 0.377 0.815 0.017 0.649 0.114 0.060 37.073 37.252 37.297 37.246 37.341 37.142 37.221 0.612 0.369 0.823 0.015 0.640 0.109 0.060 15.659 15.708 15.738 15.725 15.759 15.693 15.723 0.608 0.376 0.820 0.016 0.641 0.123 0.060 17.726 17.806 17.811 17.780 17.846 17.758 17.777 VZ-2a VZ-2b VZ-2c VZ-2d VZ-2e VZ-2f VZ-2g
U (ppm) Pb (ppm) Error Pb/204Pb 207
Error Pb/204Pb 207
Error Pb/204Pb 206
Sample
Table 3 Pb isotope data determined for titanite from mafic dikes from the Vazante Group
780 810 857 857 868 818 855
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
T (Ma)a
302
Fig. 8. (a) 207Pb/204Pb versus 206Pb/204Pb diagram and (b) 208Pb/204Pb versus 206Pb/204Pb diagram showing isotopic compositions determined for titanite from the metabasites from the Vazante Group. Ellipses represent 2-sigma errors. On the right side of (a), Pb isotope data of galena from the Vazante-Paracatu area (Field I, galena from N ore-body from Morro Agudo district; Field II, galena from other ore bodies from Morro Agudo and Vazante districts; Field III, galena from Ambro´sia and Fagundes districts; Cunha et al., 2003) are presented for comparison.
5. Conclusions The U–Pb investigation of zircon of the mafic dikes (metabasites) that cut the metasedimentary rocks of the Vazante Group and are affected by hydrothermal processes yields Paleoproterozoic ages (2.1–2.4 Ga), which suggests that the zircon crystals are xenocrystic grains. These zircon grains probably were assimilated by the magma during ascent through the continental crust; thus, the basement of the Vazante Group is either Paleoproterozoic or contains metasedimentary rocks formed by 2.1 Ga or older detrital sources. Mass balance calculations indicate that the metabasites were affected by hydrothermal processes responsible for the relative Zn and Pb enrichments and that these metasomatic processes are linked to Zn- and Pb-rich fluid circulation in the Vazante shear zone. Metasomatic processes also affected the dolostones and nonsulfide zinc ore in contact with
M. Babinski et al. / Journal of South American Earth Sciences 18 (2005) 293–304
metabasites, which provides evidence that strong fluid-rock interactions took place along the tectonic contact zones. These processes resulted in higher 18O-shifts and more radiogenic Sr signatures in bleached dolostones compared with those of hydrothermalized dolostones located in the shear zone. The similar Pb isotopic compositions of hydrothermal titanite from the metabasites and galena from the Vazante deposit suggest that the metamorphic mafic rocks were affected by epigenetic-hydrothermal processes comparable to those responsible for the Vazante nonsulfide zinc mineralization that is closely linked to the Vazante shear zone development. The titanite Pb isotopic signature also is similar to those reported for galena from other zinc sulfide deposits hosted by the Vazante Group, such as the diagenetic–epigenetic Morro Agudo deposit. This similarity may imply a common source of lead for both epigenetic nonsulfide and diagenetic–epigenetic sulfide zinc deposits in the Vazante-Paracatu district. It also could be related to the metamorphic/hydrothermal remobilization of preexisting Pb or deep-seated, basin-derived metalliferous fluids associated with a long-lived hydrothermal system related to the diagenesis and deformation of the Vazante Group during the Neoproterozoic. Acknowledgements We are grateful to the Companhia Mineira de Metais for continuous support and hospitality at the mine. This research was partially supported by Pronex funds (Project 167/96) and FAPESP grants (Proc. 98/0412-3, 96/3941-3, and 95/4652-2). MB and JSB thank CNPq for a Research Fellowship. We thank S.R.F. Vlach for microprobe analysis on titanite and V.T. Martins for preparing the Pb diagrams. We also thank Massimo Chiaradia (University of Leeds, U.K.) and an anonymous reviewer whose thorough reviews helped improve the article. This study is a contribution to IGCP 450-Proterozoic Sediment-Hosted Base Metal Deposits of Western Gondwana. References Almeida, F.F.M., 1967. Origem e evoluc¸a˜o da Plataforma Brasileira. Rio de Janeiro, Bol. DNPM 243, p. 36. Amaral, G., 1968. Geologia e depo´sitos de mine´rio na regia˜o de Vazante, Estado de Minas Gerais. Doctoral Thesis, Universidade de Sa˜o Paulo, p. 133. Amaral, G., Kawashita, K. 1967. Determinac¸a˜o da idade do Grupo Bambuı´ pelo me´todo Rb/Sr. Congresso Brasileiro Geologia 21, Anais, SBG. pp. 214–217. Azmy, K., Veizer, J., Misi, A., Oliveira, T.F., Sanches, A.L., Dardenne, M.A., 2001. Dolomitization and isotope stratigraphy of the Vazante Formation, Sa˜o Francisco Basin, Brazil. Precambrian Research 112, 303–329. Babinski, M., Kaufman, A.J., 2003. First direct dating of a Neoproterozoic post-glacial cap carbonate. South American Symposium on Isotope Geology 4, Salvador, Brazil. Short Papers 1, 321–323.
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