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
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
SHAPE OF ATLANTICA
Figure 1 is the Atlantica portrayal, in close agreement with K. C. Condies view, 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).
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
The Syama, Fazenda Brasileiro and Omai 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 are post-collisional and developed during a shortening event coeval with the emplacement of peraluminous granitic magmas. The Ashanti deposits are also post-collisional, related with deep crustal fluids, with no relationship with magmatic events.