Phosphorites and the chemostratigraphic correlation of the Neoproterozoic sequences OF THE São FranciSco Craton AND THE brasilia fold belt

 

Misi, A.¹; Sanches, A.L.^1,2; Kaufman, A.J.³; Veizer, J.^4,; Azmy, K.⁵;

Powis, K.⁴; Teixeira, J.B.G.¹

 

1. Universidade Federal da Bahia, Grupo de Metalogênese, Centro de Pesquisa em Geofísica e Geologia, Instituto de Geociências, Campus da Federação, 40170-290, Salvador - BA, Brazil - misi@ufba.br, jbt@ufba.br

2. Departamento de Ciências Naturais, Universidade Estadual do Sudoeste da Bahia, Brazil - alsanches@yahoo.com.br

3. Department of Geology, University of Maryland, College Park, MD 20742, USA- kaufman@geol.umd.edu

4. Department of Geology, University of Ottawa, ON, Canada K1N 6N5 - veizer@science.uottawa.ca, kpowis@science.uottawa.ca

5. Department of Earth Sciences, Memorial University of Newfoundland

300 Prince Philip Drive St. John's, NL, Canada A1B 3X5 - kazmy@mun.ca

 

 

ABSTRACT
Phosphate deposits (phosphorites) of the Neoproterozoic sedimentary sequences of the São Francisco Craton are probably related to a global episode of phosphatization during the terminal Proterozoic Era. Although these phosphorites are intimately associated with stromatolites in some deposits, most of the stromatolitic structures present in these sequences are not mineralized.   This may suggest the involvement of other factors that controlled the phosphate concentration in these structures. The Neoproterozoic sedimentary sequences of the São Francisco Craton were accumulated during extensional events as a consequence of the breakup of the supercontinent Rodinia. They are represented by thick carbonate and siliciclastic strata in epicontinental seaways and shallow marine environments, and on passive cratonal margins, which have been intensely deformed in mobile belts surrounding the cratonic area.  Three mega-sequences are represented in the cratonal area: glaciogenic, carbonate and molasse. At least two transgressive-regressive sea level cycles occurred during the evolution of the carbonate mega-sequence, which lies above iron-cemented glacio-marine diamictite of probable Sturtian age. We report high resolution carbon and strontium isotope trends from analyses of well-preserved sample sets, and use from the best preserved samples in concert with lithostratigraphic observations to provide detailed correlations from the Neoproterozoic successions of the São Francisco Craton and surrounding fold belts. Phosphate accumulations are present in both of the cratonic non-deformed strata, and in the marginal fold belts. Most of these mineralizations are unequivocally above glaciogenic sequences, few others are not, but the presence of negative d¹³C signals in the carbonates below the phosphate, may suggest a further investigation of other related glaciogenic events.

 

Keywords: phosphorite, chemostratigraphy, Neoproterozoic, correlation, São Francisco Craton

 

Introduction

       Phosphate deposits (phosphorites) in the Neoproterozoic sedimentary sequences of the São Francisco Craton, are probably related to a widespread episode of phosphatization during the terminal Proterozoic Era. Although these phosphorites are intimately associated with stromatolites, most of the stromatolitic structures present in these sequences are not mineralized.   This suggests that some other controls or processes might have governed the phosphate concentration in these structures.Cook and Shergold (1986) showed that major phosphogenic event followed a period of glaciation. The glaciation “could produce a large volume of cold, nutrient-rich water, resulting in a major expansion of organic productivity in the photic zone following the glacial period”. Phosphate accumulations in the São Francisco Craton are found in both the cratonic non-deformed strata, and in the Brasilia Fold Belt, to the west of the craton. Some of these deposits rest unequivocally above glaciogenic sequences, some few others do not, but the presence of negative d¹³C signals immediately below these deposits may suggest that discrete glacial events could have occurred.

We report analyses of ⁸⁷Sr/⁸⁶Sr and d¹³C, compositions of carbonates and phosphorites in light of previously published data, in an attempt to better understand the evolution of Neoproterozoic sedimentary sequences and phosphorite formation. The stratigraphic position of glacio-marine diamictites along with the phosphate horizons is discussed.

 

Geotectonic setting and sequence stratigraphy

Neoproterozoic sedimentary basins in the São Francisco Craton evolved as a consequence of extensional events during the fragmentation of the Rodinia supercontinent, between 900 and 600 Ma, and the coincident closure of the Pan-African-Brasiliano rift.  The sedimentary sequences deposited during these events are distributed in the following geotectonic settings: (a) Mixed carbonate and siliciclastic strata deposited on tectonically stable cratons, represented by the Bambuí Group in the São Francisco basin, Una Group, in Irecê and Una-Utinga basins, and Rio Pardo Group in the Rio Pardo Basin; (b) Intensely deformed mixed carbonate and siliciclastic strata in basins around the stable cratons, including the Ibiá and Vazante groups (Brasília Fold Belt), Miaba, Canudos and Vasa Barris groups (Sergipe Fold Belt) and Macaúbas Group (Araçuaí Fold Belt),

Large-scale stratigraphic subdivisions are represented on the stable platforms and passive margin areas by Glaciogenic, Carbonate, and Molasse mega-sequences.  These units are separated from each other by first-order unconformities recognized across the Neoproterozoic basins, but within each of these mega-sequences there are additional parasequence boundaries that may be useful for regional correlation (Misi, 2001).

 

Chemostratigraphy

The detailed stratigraphy of the Neoproterozoic basins reviewed in this work is provided in several publications (see Misi, 2001 and references). Published strontium and carbon isotope data from the same successions were also compiled. Figure 1 shows the proposed correlation, based essentially on stable isotope composition of carbonate rocks and phosphorites. The current study of phosphorite is still in progress.

Only samples retaining the lowest Mn/Sr ratios (<0.2) and/or the highest Sr concentration (>300ppm) were used. The lowest ⁸⁷Sr/⁸⁶Sr values within any interval were considered to most likely reflect depositional conditions.  On the other hand, all of the published and new carbon isotope data from fine-grained carbonates (including both limestone and dolomite) were used for intra-basinal correlations.  In addition, the d³⁴S compositions of bedded and disseminated sulfate and carbonate associated sulfate (CAS) from the different successions were compared. 

 

Phosphate deposits

Phosphorites have been found in the Vazante, Bambuí and Una Groups. They are widespread over a large area and are stratigraphycally controlled (Figure 1). Coromandel, Rocinha and Lagamar, in the Vazante Group, as well as Campos Belos, Nova Roma, Monte Alegre and Cedro do Abaeté, in the Bambuí Group, are located at the lower section of the carbonate unit immediately above glaciogenic diamictites (Dardenne et al., 1986; Dardenne, 2001). Cabeceiras, in the Bambuí Group, is located at the boundary between the Lagoa do Jacaré and Serra da Saudade Formations (Dardenne et al., 1986). Irecê phosphorites, in the Una Group, occur in dolomitic facies at the top of laminated limestones, equivalent to the Sete Lagoas Formation of the Bambuí Group (Misi and Kyle, 1994).

 

Discussion and conclusion

The broad similarity of lithofacies and mega-sequences, as well as the occurrence of Pb-Zn and phosphate deposits restricted to a narrow stratigraphic interval in the Vazante, Bambuí, and Una groups suggest that these units may be correlative (Figure 1). The Sr isotope data from each of these successions presented in this study (ranging from 0.7074 to 0.7079) support the general correlation of these successions.  Carbon isotope data also reveal sharp negative excursions associated with post-glacial carbonate lithofacies and/or evidence of rapid transgression.  Carbonates in these units are moderately to extremely enriched in ¹³C, ranging from high values of +3 to +14‰ in the different sections. Outcrop samples of Serra do Garrote Formation limestone with high Sr contents yield low ⁸⁷Sr/⁸⁶Sr values of ca. 0.7069, which are quite similar to a limestone sample taken from core material of the Lapa Formation (~0.7068) immediately overlying the Morro do Calcário Formation.  These lower ⁸⁷Sr/⁸⁶Sr values (cf. Jacobsen and Kaufman, 1999) suggest the possibility that the Vazante succession may be older than the widespread Bambuí sediments. Nevertheless, carbonate fluorapatite from Rocinha and Lagamar deposits, both in stratigraphic units below the Serra do Garrote Formation, give consistent values in the same range of the other successions (Table 1). Additional chemostratigraphic studies and new radiometric constraints will be necessary, at least for the Vazante Group.

The formation of phosphate deposits throughout the Neoproterozoic interval has important economic

 

 

Figure 1.  Correlation between the Neoproterozoic succesions of the  São Francisco Craton and phosphorite deposits. 1 – Rocinha 2 – Lagamar 3 – Coromandel 4 –Campos Belos, Nova Roma, Monte Alegre and Cedro do Abaeté 5 – Cabeceiras 6 - Irecê

 

 

Table 1. Sr and C-isotope data of carbonate fluorapatite and associated micritic limestone from the Vazante Group Source: Misi et al., 1997; Sanches, 2001 (unpublished data)

 

 

significance. Phosphate concentrations are probably related to pre-glacial episodes that provided high  organic productivity.

References

Cook, P.J. and Shergold, J.H. 1986, Proterozoic and Cambrian phosphorite: nature and origin, In: Cook, P.J. & Shergold, J.H. (Eds.), Phosphate Deposits of the World. Proterozoic and Cambrian Phosphorites, Cambridge University Press, Cambridge:369-386.

Dardenne, M.A. 2001. Lithostratigraphic sedimentary sequences of the Vazante Group. In: Misi, A. and Teixeira, J.B.G., Proterozoic Base Metal Deposits of Africa and South America, IGCP 450 1st Field Workshop, Belo Horizonte and Paracatu, Brazil: 48-50.

Dardenne. M.A., Trompette, R., Magalhães, L.F., Soares, L.A. 1986, Proterozoic and Cambrian phosphorites – regional review: Brazil. In: Cook, P.J. and Shergold, J.H. (Eds.), Phosphate Deposits of the World. Proterozoic and Cambrian Phosphorites, Cambridge University Press, Cambridge:116-131.

Jacobsen, S. B., Kaufman, A. J. 1999. The Sr, C and O isotopic evolution of  Neoproterozoic seawater. Chemical Geology, v.  161:37-57.

Sanches, A.L. 2001. Discussão dos dados de Sm-Nd e Rb-Sr dos fosforitos de Rocinha (MG), Lagamar (MG) e Irecê (BA) e interpretação dos dados de Pb-Pb nos fosforitos de Lagamar, unp.preliminary report, 24 p.

Misi, A. 2001. Estratigrafia isotópica das seqüências do Supergrupo São Francisco, coberturas neoproterozóicas do craton do São Francisco. Idade e correlações. In: Pinto, C.P., Martins-Neto, M.A. (Eds.), Bacia do São Francisco. Geologia e Recursos Naturais. SBG, Núcleo de Minas Gerais: 67-92.

Misi, A. & Kyle, J. R. 1994. Upper Proterozoic carbonate stratigraphy, diagenesis, and stromatolitic phosphorite formation, Irecê Basin, Bahia, Brazil. Journal of Sedimentary Research. A 64:299-310.

Misi, A., Veizer, J., Kawashita, K., Dardenne, M.A. 1997. The age of the Neoproterozoic carbonate platform sedimentation based on ⁸⁷Sr/⁸⁶Sr determinations, Bambuí and Una Groups, Brazil. I South American Symposium on Isotope Geology, Campos do Jordão, São Paulo, Brazil, Extended Abstracts:199-200