Placeres diamantíferos do Rio Itiquira, MT, Brasil

Abstract

Itiquira is situated SE of Cuiabá, in the Mato Grosso State, and is inserted in the geological context of NW portion of the Paraná Basin, which from oldest to youngest includes the Cretaceous Marilia Formation, the Tertiary Cachoeirinha Formation and the Quaternary Pantanal Formation. Unconsolidated gravels from the latter unit host diamonds that are mined on the rivers known for rudimentary mining in Itiquira: Cambaúva, Cavoqueiro, Bode and Formosa and Velha farms. First records on the extraction of diamonds from these gravels date to the twentieth century, 1930, and took place in the artisanal mining Cavoqueiro, located on the right bank of the river, about 1 km downstream of the city. The region experienced high mining activity and the discovery of new mining areas by the year of 1940. Since then, the artisanal mining and the discovery of new fronts went into decline. We carried out geological mapping of abandoned fronts allowing the stratigraphic stacking of the placers, determination of their absolute age and study of the chemistry of mineral concentrates. The combination of geological and geochronological studies demonstrate the existence of three depositional events here informally termed T0 to T2, distributed along the channel, plains and older terraces on both sides. We used the LOE (Optically Stimulated Luminescence) dating technique for five samples collected from the channel to the highest terrace. The youngest age of 660 ± 90 years, was obtained in the sample EIT 13 and the oldest with 8400 ± 858 years in the sample EIT 05, both samples at T1 terrace. The ages show that part of these diamondiferous flats were deposited from the Holocene, and reworked until 660 ± 90 years. Heavy mineral samples collected for the study were examined with magnifying glass and analyzed by electron microprobe. The minerals identified were the garnets G0, G3, G4 and G4D, rutile, ilmenite, sapphire, zircon, and secondary iron oxides. Most of the identified mineral shows no close association with fertile diamond primary sources, with the exception of the garnet G4D.