Implication of 40Ar/39Ar–petrochronology in the Buracão orogenic gold deposit for tectonic and low-temperature evolution of the northern Brasília Fold Belt, Brazil
Résumé
The 40Ar/39Ar ages (muscovite and biotite) and P-T estimations presented here allow us to reconstruct the Pressure–Temperature–time (P–T–t) path from the metamorphic peak to the generation of the orogenic Au (As, Bi, Ag) mineralization in the Buracão area. The application of software-based pseudosections for bulk phyllite composition allows us to better recognize three main stages in the P-T-t path. Rocks from the study area evolved from (i) syn-D1a greenschist metamorphic peak (T = 480–550 °C and P < 4.0 kbar) attained at ca. 538–530 Ma during the W-E shortening related to local D1 deformational stage, which is regionally reported in the external domain of the northern Brasilia Fold Belt. These younger ages, compared to those of mafic and felsic granulites from the northernmost internal domain west of the study area (ca. 610–570 Ma), indicate that the metamorphic peak in the external domain was attained after the internal domain. The (ii) syn-D1b barren type I quartz vein formation (T = 510–260 °C and P < 4.0 kbar) at ca. 531–525 Ma; and (iii) syn-D2a ore-bearing type II quartz vein formation and gold mineralization (T = 345–285 °C and P = 2.7–1.6 kbar) at ca. 522–515 Ma suggest that low temperature may represent cooling P-T conditions of the peripheral supracrustal cover. Moreover, the diachronicity between the local metamorphic peak and the gold mineralization (22 M.y.) is consistent with the structural relationship between gold veins and post-metamorphic peak D2a high deformation zones, which may have acted as fluid corridors through the metasedimentary pile. These features are coherent with the time span observed in several orogenic gold deposits worldwide, especially those associated with high-angle transpressive shear zones developed in the late stages of orogeny. The combination of the Ar–Ar petrochronology with previous published fluid reconstruction and tectonic data suggests metal-rich fluid percolated along regional shear zones through the Traíras Formation under post-metamorphic peak conditions. This regional tectonic and chronology might be useful for geodynamical reconstruction and as a prospection guide in poorly studied orogenic gold of the area.