The Castelo de Sonhos gold deposit (~1.5 Moz Au) is located in the southeastern Tapajós Gold Province and is hosted in variably deformed metaconglomerates and metasandstones of the Castelo dos Sonhos Formation, which was deposited between 2011 and 2050 Ma and considered to be a relic of a major foreland system. Gold is found mainly in the matrix of metaconglomerates and subordinately in fractures of metasandstones. The host rocks are strongly silicified, which was demonstrated to be the result of silica overgrowth during diagenesis, without relationship to hydrothermal alteration. The rocks are also impregnated with secondary iron oxides, which are interpreted to be a consequence of weathering of the metasedimentary sequence and total or partial dissolution of iron-bearing minerals and re-precipitation of iron during circulation of oxidized (meteoric and/or magmatic) fluids. The morphology of matrix gold particles indicates that they are not pristine, but only slightly modified and reshape, which suggests detrital provenance and relatively proximal distance of transport from the original sources. The chemical composition of the particles, with Au grading 94-97%, very high Au/Ag and lack of other significant metals, suggest a single source, or sourcing from a single class (orogenic?) of gold deposits. The gold particles from fractures are platy and show even higher Au/Ag ratios, given by very low Ag contents (<<1%), which are interpreted as epigenetic remobilization of the matrix gold. Oxygen isotope compositions of hydrothermal quartz-sulfide-muscovite veins cutting across the host metasedimentary sequence indicate a magmatic origin for the fluids, which might have been produced by granites that intruded the metasedimentary rocks and helped gold remobilization to fractures. As a consequence, Castelo de Sonhos is interpreted as a modified paleoplacer gold deposit.
