Sedimentology, geomorphology, structural controls, and detrital zircon ages of the Itiquira River diamond placer deposits, Mato Grosso, western Brazil

Abstract

The Itiquira River, Mato Grosso state (western Brazil), hosts several diamond placer deposits, mined intermittently over the last century. It runs over volcanic and sedimentary rocks of the Paraná Basin until it discharges in the northern Pantanal Basin. The bedrocks were deposited in marine, continental desertic, alluvial deltaic, and fluvial environments. The meanders of the Itiquira River are controlled by NE-SW, ENE-WSW, NNW-SSE, N–S, and NW-SE fractures and normal faults, developed in response to the evolution of the Paraná Basin and by neotectonics, linked with the development of the Pantanal Basin since the Paleogene. The Itiquira River middle valley, in which the diamondiferous placers are found, is controlled by NE-SW structures inherited from the Neoproterozoic Transbrasiliano Lineament. The landscape comprises dissected plateaus and structure-controlled valleys formed by Cretaceous, Paleogene, and Neogene planation processes. The diamonds occur in the muddy-sandy matrix of the current stream bed and older terraces gravels. In the Itiquira River, the diamond deposits are related to the following traps: point bars, cut-and-fill channels, pockets, and potholes. Sapphire, garnet (including kimberlitic), ilmenite, zircon, rutile, gold, and iron oxides are documented as heavy minerals in the gravels. Detrital zircon dating of grains extracted from the Itiquira River diamond deposits resulted in the ages of 2057, 1184, 873, 645–508, 307–207, and 144-142 Ma. The potential zircon sources are the Goiás Magmatic Arc granitoids, Paraguay Belt metavolcanics and granites, and Serra Geral Formation volcanics. The ages between 307 and 207 Ma are likely to be from an unknown (possibly kimberlitic) source. The Itiquira River tectonic, geomorphological, and sedimentological evolutions suggest potential sources for the diamond placers and paleoplacers.