6 research outputs found
Gossan mineralogy, textures, and gold enrichment over the Au (As, Bi, Ag) deposit in the BuracĂŁo Area (BrasĂlia Fold Belt, Brazil): Implications for gold prospecting in weathering profiles
International audienc
PâTâX conditions on the genesis of orogenic Au (As, Bi, Ag) deposit in metasedimentary rocks of the BuracĂŁo Area, AraĂ Group, BrasĂlia Fold Belt, Brazil
International audienceThe orogenic Au (As, Bi, Ag) mineralization (up to 31âŻg/t) of the BuracĂŁo area is hosted in the metasedimentary PaleoâMesoproterozoic TraĂras Formation, AraĂ Group, BrasĂlia Fold Belt, Brazil. In order to discuss the PâTâX conditions that generated this mineralization, we present mineralogical, geochemical and fluid inclusion data of veins, gold and wall rocks. Coupled with previous structural data these additional data allow us to link metamorphism and deformation with fluid migration and ore genesis. The integration of fluid inclusion microthermometry with mineral geothermobarometers and structures show that rocks of the study area evolved from: (i) synâD1a metamorphic peak (400â560âŻÂ°C, <6âŻkbar), through to the (ii) development of synâD1b type I barren veins from low salinity H2OâNaCl fluids during synâ to lateâmetamorphic peak; and to the (iii) development of synâD2a type II veins with the pyritic ore precipitation from low to high salinity H2OâCO2âCH4âNaClâŻÂ±âŻHCO3 fluids during postâmetamorphic peak (345â285âŻÂ°C, 2.7â1.6âŻkbar).The high fineness gold nuggets accompanied by As, Bi and Ag within pyriteâarsenopyriteâbismuthinite association in the pyritic ore allows the BuracĂŁo area to be classified as âa gold-only orogenic ore depositâ. Fluid inclusion compounds and the enrichment of As, Bi, Pb, Cu, Te, Se and Ag in pyrite from the pyritic ore point out to metamorphism of carbonaceous phyllites and carbonate rocks of the TraĂras Formation as a source for oreâforming fluids. We suggest that sulfur and gold in these fluids were derived by metamorphism of sedimentary rocks and conversion of pyrite into pyrrhotite in a metamorphicâhydrothermal system. The Au(HS)2ââbearing fluids percolated along intrabasinal D2a regional shear zones through the TraĂras Formation towards the BuracĂŁo area, where they precipitated type II oreâbearing veins within local structures. The ore precipitation was induced by contrasting fO2âfS2 related to the interaction between the sulfur-bearing fluid and host magnetiteâgraphite phyllites. The lithological control is represented by a series of reactions such as 2FeO(in silicates/magnetite)âŻ+âŻ4Au(HS)2â(aq)âŻ+âŻ4H+(aq)âŻââŻ2FeS2(s)âŻ+âŻ4Au(s)âŻ+âŻ4H2S(aq)âŻ+âŻ2H2O(l).The deposits of the BuracĂŁo area and other orogenic gold deposits hosted in metasedimentary rocks in the BrasĂlia Fold Belt and worldwide confirm the potential of ore genesis in zones that do not show any relation with (meta)igneous rocks, thus, expanding the exploration possibilities in metasedimentary successions
Implication of 40Ar/39Arâpetrochronology in the BuracĂŁo orogenic gold deposit for tectonic and low-temperature evolution of the northern BrasĂlia Fold Belt, Brazil
International audienceThe 40Ar/39Ar ages (muscovite and biotite) and P-T estimations presented here allow us to reconstruct the PressureâTemperatureâtime (PâTât) path from the metamorphic peak to the generation of the orogenic Au (As, Bi, Ag) mineralization in the BuracĂŁo 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 = 480â550 °C and P < 4.0 kbar) attained at ca. 538â530 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. 610â570 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 = 510â260 °C and P < 4.0 kbar) at ca. 531â525 Ma; and (iii) syn-D2a ore-bearing type II quartz vein formation and gold mineralization (T = 345â285 °C and P = 2.7â1.6 kbar) at ca. 522â515 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 ArâAr petrochronology with previous published fluid reconstruction and tectonic data suggests metal-rich fluid percolated along regional shear zones through the TraĂras 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