Castelo de Sonhos, located in the central-south sector of the Amazonian Craton, near the boundary between the Tapajós and Xingu-Iriri tectonic domains, is a gold deposit hosted in metaconglomerates and metasandstones of the Castelo dos Sonhos Formation (<2080 Ma U-Pb SHRIMP). Subvolcanic rocks and granitoids were identified in boreholes that drilled the deepest parts of the deposit area. Some of these rocks are intrusive into the Castelo dos Sonhos Formation, while for others rocks, the contact relationships could not be determined with confidence. In general, these rocks show calc-alkaline to alkaline affinities and their geochemical patterns indicate that they are related to volcanic arc or post-collisional tectonic settings. The subvolcanic rocks are represented by a porphyritic dacite with age of 2011 ± 6 Ma (U-Pb LA-ICP-MS).The granitoids were classified as biotite granodiorite, biotite monzogranite, muscovite monzogranite, respectively dated at 1976 ± 7 Ma, 1918 ± 9 Ma and 1978 ± 6 Ma (U-Pb SHRIMP), and an undated syenogranite. These ages represent three to four distinct magmatic events and indicate that the studied rocks are coeval to four major units from Tapajós Domain: the Cuiú-Cuiú Complex (2033-2005 Ma), the Comandante Arara Formation (2020-2012 Ma), the Creporizão Intrusive Suite (1998-1957 Ma), and the Tropas Intrusive Suite (1907-1892Ma). Despite the temporal correspondence, the geochemical data show no direct correspondence with the units cited above. Notwithstanding, the intrusion relationship between some of the studied rocks and the metasedimentary rocks of the Castelo dos Sonhos Formation establishes a temporal, spatial and possibly stratigraphic relationship between this formation and the Tapajós Domain. The intrusive contact relationship between the porphyritic dacite and metasandstones of the Castelo dos Sonhos Formation allowed us to determine at 2011 ± 6 Ma the minimum sedimentation age of this unit. The primary gold mineralization at Castelo de Sonhos deposit is stratabound and restricted to a metaconglomerate package and interlayered metasandstones. The mineralization distribution is erratic and does not seem to follow special features or structural control. In the matrix of the metaconglomerates, gold occurs as intergranular particles, occasionally associated with magnetite, and also within quartz grains (medium to coarse sand), which probably represent fragments of auriferous veins. In general, the gold particles show subrounded to rounded shapes, mild to moderately rough surfaces. The particles seldom contain inclusions, and only of magnetite. The chemical composition is homogeneous and characterized by high Au/Ag ratios. These characteristics indicate a syngenetic origin for gold within the metaconglomerates package. Therefore, the age of mineralization is limited by the time x interval of deposition of the Castelo dos Sonhos Formation (2011 ± 6 Ma to ca. 2080 Ma). On the other hand, the occurrence of gold in fracture planes of metasandstones indicates an epigenetic origin for this style of mineralization. The epigenetic mineralization is related to concurrent metamorphic, magmatic and deformational processes that affected the sedimentary sequence of the Castelo dos Sonhos Formation and caused the remobilization of gold originally hosted in metaconglomerates. It is likely that the interaction of these processes associated with infiltration of meteoric waters contributed to the generation and flow of oxidizing hydrothermal fluids, which have percolated through the metaconglomerates package and were able to solubilize some of the gold, and re-precipitate it accompanied by ferruginous films, in fracture planes of the metasandstones. As a conclusion, a modified paleoplacer model is proposed here to explain the hybrid nature (syngenetic and epigenetic) of the gold mineralization in the Castelo de Sonhos deposit.
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