Abstract
The Castelo de Sonhos gold deposit (~1.5 Moz Au) is located in the southeastern Tapajos Gold Province and is hosted in variably deformed metaconglomerates (±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 also in fractures of metasandstones. The host rocks are strongly silicified, which was demonstrated to be the result of silica overgrowth during diagenesis, without relationship with hydrothermal alteration. The rocks are also impregnated with iron oxides, which is interpreted to be consequence of weathering of the metasedimentary sequence and secondary oxidation of Fe-bearing minerals during circulation of oxidized (meteoric?) fluids. The morphology of matrix gold particles indicates that they are not pristine, showing modified and reshaped textural characteristics, which suggests detrital provenance and relatively long 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 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 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.
Highlights
Gold deposits hosted in conglomerates are commonly found in Precambrian terranes worldwide, and a strong debate exists concerning the syngenetic, i.e., paleoplacer, versus modified paleoplacer and epigenetic origin of gold mineralization
Since all known magmatic-hydrothermal deposits from Tapajós are younger than Castelo de Sonhos, the sources of detrital gold must be external to the Tapajós Province, and are likely similar to those defined for the detrital zircons by Klein et al (2017), i.e., Rhyacian orogenic belts and Archean basement located to the east, northeast and southeast of Tapajós
Regarding to the Castelo de Sonhos deposit, it is likely that the interaction of the above described sedimentary, metamorphic, magmatic and deformation processes have contributed to the production and circulation of oxidizing hydrothermal fluids, which were capable of the solubilization of gold and iron from the matrix of the metaconglomerates and deposition of these elements in fractures in neighboring rocks
Summary
Gold deposits hosted in conglomerates are commonly found in Precambrian terranes worldwide, and a strong debate exists concerning the syngenetic, i.e., paleoplacer, versus modified paleoplacer and epigenetic origin of gold mineralization (see reviews in Law and Phillips 2005; Frimmel 2014). Deposits of this type are present in Brazilian Precambrian terranes, occurring in cratonic areas and in more or less reworked Archean-Paleoproterozoic basement segments within Neoproterozoic mobile belts (Fig. 1). Reconnaissance studies of the chemical composition of white mica and of stable isotope compositions of quartz veinlets that cut across the mineralized strata were performed in order to constrain potential sources of hydrothermal modification of the paleoplacer
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