GEODYNAMIC SETTING OF OROGENIC GOLD DEPOSITS IN THE ATLANTICA PALEOCONTINENT

João Batista G. Teixeira1, Paulo M. Vasconcelos2, and Aroldo Misi1

1Universidade Federal da Bahia, Salvador, Bahia, Brazil

2University of Queensland, Department of Earth Sciences, Queensland, Australia

 

SYNOPSIS

This paper discusses the age and tectonic setting for five of the major orogenic gold deposits in the Atlantica paleocontinent. The thermal record of the Atlantica collision zone is summarized, based mainly on available 40Ar/39Ar dating of hornblende and micas from granitoid intrusions and from hydrothermally altered zones of the actual lodes.
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THE SHAPE OF ATLANTICA
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The geological background for the Paleoproterozoic Atlantica continent[1] has quickly advanced in the last two years thanks to accurate rock dating and better correlation of the collision remnants [2, 3, 4]. This paper discusses the age and tectonic setting for five of the major orogenic gold deposits in the paleocontinent.

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Figure 1 is the Atlantica portrayal, in close agreement with K. C. Condie’s view[5], except for the absence of the Congo Craton and for the different positions of the São Francisco Craton and the Borborema Province. Here the great Paleoproterozoic continental collision comes into sight, established by the presumed root zone of a 4000 km long mountain chain.

This geotectonic unit is delineated by discontinuous granulite belts and granitoid intrusions that extend from Uruguay towards SSE Brazil, Venezuela, Guyana, NE Brazil, and reach Liberia and Ivory Coast. A large tract of ‘missing terrane’ is tentatively placed to the lower left side in order to stand for the appropriate accretionary scenario. The whole collision process likely took place between 2.1 and 1.85 Ga.

Peculiar features of Atlantica are the Birimian or Transamazonian (2.2- 2.1 Ga) greenstone belts composed of earlier erupted Fe-rich MORB-type tholeiite and later erupted IA andesite, associated with epiclastic to siliciclastic sediments. Most of these greenstone belts and companion intrusive granitoids are dislocated above gneiss-migmatite basement of the Guyana, São Francisco, São Luís and West African cratons (Figure 2).
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TECTONIC SETTING OF THE OROGENIC GOLD DEPOSITS

Orogenic gold deposits occur in the Birimian granite-greenstone terrains, e. g., Omai (Guyana), Fazenda Brasileiro (Brazil), Syama (Mali), Obuasi and Damang (Ashanti Belt, Ghana), hosted by various rock types. Metallogenic models for the Jacobina (Brazil) and Tarkwa (Ghana) deposits, which are largely hosted by siliciclastic metasediments (syngenetic or epigenetic?), remain under quest. Notwithstanding, all of these deposits (Figure 2) are meso- to epithermal and structurally controlled.

The thermal record of the Atlantica collision zone is summarized in Figure 3, based mainly on available 40Ar/39Ar dating of hornblende and micas from granitoid intrusions and from hydrothermally altered zones of the actual lodes. The diagram allows the perception of the symmetrical variation in the cooling ages, which become progressively younger as the distance from the main collision axis increases.
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The Syama[6], Fazenda Brasileiro[7] and Omai[8] deposits are syncollisional and relate to retrograde P- T paths following the 2100 Ga peak of emplacement of granodiorite- tonalite- diorite intrusions. The Jacobina deposits[9] are post-collisional and developed during a shortening event coeval with the emplacement of peraluminous granitic magmas. The Ashanti deposits[10] are also post-collisional, related with deep crustal fluids, with no relationship with magmatic events.

REFERENCES

[1] Rogers, J. J. W., 1996. A history of continents in the past three billion years. Journal of Geology, Vol. 104, pp. 91- 107.

[2] Hartmann, L. A., 2002. The Mesoproterozoic Supercontinent Atlantica in the Brazilian Shield - Review of Geological and U- Pb Zircon and Sm- Nd Isotopic Evidence. Gondwana Research, Vol. 5, No. 1, pp. 157- 163.

[3] Teixeira, J. B. G. et al., 2001. Evidence of Paleoproterozoic collage and dispersal of some Western Gondwana components, with implications for the Neoproterozoic metallogenesis in Misi, A. and Teixeira, J. B. G. (Orgs.) IGCP 450, Proterozoic Base Metal Deposits of Africa and South America – 1st Field Workshop. Extended Abstracts, pp. 98- 99.

[4] Fetter, A. H. et al., 2000. U- Pb and Sm- Nd geochronological constraints on the crustal evolution and basement architecture of Ceará State, NW Borborema Province, NE Brazil: implications for the existence of the Paleoproterozoic supercontinent "Atlantica". Revista Brasileira de Geociências, Vol. 30, No.1, pp.102- 106.

[5] Condie, K. C., 2002. Breakup of a Paleoproterozoic Supercontinent. Gondwana Research, Vol. 5, No. 1, pp. 41- 43.

[6] Vasconcelos, P. M.; Brimhall, G. H.; Becker, T. A.; Renne, P. R., 1994. 40Ar/39Ar analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa. Geochimica et Cosmochimica Acta, Vol. 58, pp. 401- 42

[7] Silva, M. G.; Coelho, C. E. S.; Teixeira, J. B. G.; Silva, F. C. A.; Silva, R. A.; Souza, J. A. B., 2001. The Rio Itapicuru greenstone belt, Bahia Brazil: geologic evolution and review of gold mineralization. Mineralium Deposita, Vol. 36, pp. 345- 357.

[8] Norcross, C; Davis, D. W.; Spooner, E. T. C.; Rust, A., 2000. U- Pb and Pb- Pb age constraints on Paleoproterozoic magmatism, deformation and gold mineralization in the Omai area, Guyana Shield. Precambrian Research, Vol. 102, pp. 69- 86.

[9] Teixeira, J. B. G.; Souza, J. A. B.: Silva, M. G.; Leite, C. M. M.; Barbosa, J. S. F.; Coelho, C. E. S.; Abram, M. B.; Conceição Filho, V. M.; Iyer, S. S. S., 2001. Gold mineralization in the Serra de Jacobina region, Bahia, Brazil: tectonic framework and metallogenesis. Mineralium Deposita, Vol. 36, pp. 332- 344.

[10] Oberthür, T.; Vetter, U.; Schmidt-Mumm, A; Weizer, T.; Amanor, J. A.;Gyapong, W. A.; Kumi, R.; Blenkinsop, T. G., 1994. The Ashanti Gold Mine at Obuasi, Ghana: Mineralogical, Geochemical, Stable Isotope and Fluid Inclusion Studies on the Metallogenesis of the Deposit in Oberthür, T. (ed.) Metallogenesis of Selected Gold Deposits in Africa. Geologisches Jahrbuch, Reihe D, Heft 100, Hannover, 31-129.

[11] Tassinari, C. C. G.; Bettencourt, J. S.; Geraldes, M. C.; Macambira; M. J. B.; Lafon, J. M., 2000. The Amazonian Craton in Cordani, U. G.; Milani, E. J.; Thomaz Filho, A.; Campos, D. A. (eds.) Tectonic Evolution of South America. Rio de Janeiro: 31st International Geological Congress, 41- 95.

[12] Vanderhaege, O.; Ledru, P.; Thiéblemont, E. E.; Cocherie., A.; Tegyey, M.; Milési, J. P., 1998. Contrasting mechanism of crustal growth: Geodynamic evolution of the Paleoproterozoic granite-greenstone belts of French Guyana. Precambrian Research, Vol. 92, pp. 165- 193.

[13] Barbosa, J. S. F.; Dominguez, J. M. L. (Coords.), 1996. Mapa Geológico do Estado da Bahia, Escala 1:.1.000.000. Estado da Bahia, Secretaria da Indústria, Comércio e Mineração.

[14] Milési, J. P. (Coord.), 1989. West African Gold Deposits in their Lower Proterozoic Lithostructural Setting. Éditions du BRGM, Chron. Rech. Minière, No. 497, 98 p., map.

[15] Nomade, S.; Féraud, G.; Chen, Y.; Pouclet, A., 2002. Thermal and tectonic evolution of the paleotroterozoic Transamazonian orogen as deduced from 40Ar/39Ar and AMS along the Oyapok river (French Guyana). Precambrian Research, Vol. 114, pp. 35- 53.

[16] Vasconcelos, P. M.; Becker, T. A., 1992. A idade da mineralização aurífera no depósito da Fazenda Brasileiro, Bahia, Brasil in Proceedings Workshop em Metalogênese. Pesquisas Atuais e Novas Tendências. UNICAMP, São Paulo, Boletim de Resumos.