1. P–T–X conditions on the genesis of orogenic Au (As, Bi, Ag) deposit in metasedimentary rocks of the Buracão Area, Araí Group, Brasília Fold Belt, Brazil.
- Author
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Pires, G.L.C., Renac, C., Bongiolo, E.M., Neumann, R., and Barats, A.
- Subjects
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GOLD mining , *MINERALIZATION , *GEOCHEMISTRY , *SILVER ores , *ORE deposits - Abstract
Graphical abstract Highlights • Link between low–grade metamorphism and deformation on the ore genesis. • Carbonaceous phyllites and carbonate rocks as sources for ore–forming fluids. • P–T–t evolution from metamorphic peak to ore deposition based on fluid and mineral data. • Contrasting f O 2 – f S 2 as a chemical control for ore precipitation by fluid–rock interaction. • The Buracão area in the context of orogenic metasediment–hosted gold deposits. Abstract The orogenic Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is hosted in the metasedimentary Paleo–Mesoproterozoic Traíras Formation, Araí Group, Brasília Fold Belt, Brazil. In order to discuss the P–T–X 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) syn–D 1a metamorphic peak (400–560 °C, <6 kbar), through to the (ii) development of syn–D 1b type I barren veins from low salinity H 2 O–NaCl fluids during syn– to late–metamorphic peak; and to the (iii) development of syn–D 2a type II veins with the pyritic ore precipitation from low to high salinity H 2 O–CO 2 –CH 4 –NaCl ± HCO 3 fluids during post–metamorphic peak (345–285 °C, 2.7–1.6 kbar). The high fineness gold nuggets accompanied by As, Bi and Ag within pyrite–arsenopyrite–bismuthinite association in the pyritic ore allows the Buracão area to be classified as "a gold-only orogenic ore deposit". 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 Traíras Formation as a source for ore–forming 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 metamorphic–hydrothermal system. The Au(HS) 2 −–bearing fluids percolated along intrabasinal D 2a regional shear zones through the Traíras Formation towards the Buracão area, where they precipitated type II ore–bearing veins within local structures. The ore precipitation was induced by contrasting f O 2 – f S 2 related to the interaction between the sulfur-bearing fluid and host magnetite–graphite phyllites. The lithological control is represented by a series of reactions such as 2FeO (in silicates/magnetite) + 4Au(HS) 2 − (aq) + 4H+ (aq) ↔ 2FeS 2(s) + 4Au (s) + 4H 2 S (aq) + 2H 2 O (l). The deposits of the Buracão area and other orogenic gold deposits hosted in metasedimentary rocks in the Brasília 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. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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