Abstract

Gold-bearing placer conglomerates, dated between 2.2 and 2.8 Ga, have been identified at the base of the Moeda Formation in the Gandarela syncline. Intensive investigation of outcrop and underground exposures, both old and new, and borehole intersections around the western and southern margin of the syncline, has defined a paleoenvironment consisting of an upland fluvial drainage system in the north, leading southward to an alluvial fan and braided plain depository. Heavy mineral concentrates, composed predominantly of coarse rounded pyrite, are concentrated in the matrix of cobble conglomerates that occur in the form of discrete bars and sheets. The mineralogy and geochemistry of the ore closely resembles that of Witwatersrand placers as does the composition of the gold. Gold concentrations are equal to those of the average Witwatersrand gold deposit and therefore these deposits may make a significant contribution to Brazil's future gold production.

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