Gold deposit formation during collapse tectonics: Structural, mineralogical, geochronological, and fluid inclusion constraints in the Ouro Preto goldmines, Quadrilatero Ferrifero, Brazil

The gold deposits of the Ouro Preto area (Minas Gerais, Brazil) occur along the main tectonic contact responsible for the thrusting of the Paleoproter ozoic units (Minas Supergroup) onto the gneissic and metasedimentary Archea n units (Rio das Velhas Supergroup) during the Brasiliano (ca. 600 Ma) tect onic event. These deposits are claimed to be the most complex of the Quadri latero Ferrifero, with orebodies lying parallel to the thrust-related stret ching lineation. Based on structural, mineralogical, geochronological, and fluid inclusion constraints, our study demonstrates that the gold-bearing q uartz veins were formed in response to late orogenic collapse tectonics tha t led, in this area, to gravity backsliding of the nappes. During these mov ements, sig moidal voids were created and filled by elongate quartz grains associated with sulfides, tourmaline, and carbonates. Infeered hydrothermal effects were responsible for the formation of illite, sulfides, carbonate, garnet, and green biotite within the matrix. Large arsenopyrites within th e veins were affected by three stages of fracturing during which the metall ic phases and associated gold were deposited. Gold, wherever it is in conta ct with Bi-related minerals, occurs in the form of electrum, maldonite, or aurostibite. Fluid inclusion studies show that this event was associated wi th the entrapment of both CO2-rich and NBO salt-rich fluids. The process wa s initialed after the Brasiliano thrusting event (ca. 500 degreesC, 8 kbars ) and in P-T conditions dose to 300 degrees to 400 degreesC and 2 kbars. An age of ca. 485 to 490 Ma has been obtained by Ar-40/Ar-39 dating of hydrot hermal biotite and muscovite. This study demonstrates that the Ouro Preto g old deposits were formed in a context of late orogenic collapse tectonics, drastically different from the conventional auriferous shear zone model. Th is could account for their relative paucity compared to other mesothermal g old deposits.