MINERALOGIA E GEOQUÍMICA DE SEDIMENTOS LACUSTRES COM SUBSTRATO LATERÍTICO NA AMAZÔNIA BRASILEIRA

Abstract

The Lagoa Grande sedimentary basin, located on the Carajás mineral province (Northern Brazil), was developed over lateritic iron ore which constiitutes the N4-N5 plateau of Carajás. It presentespresents a unique opportunutyopportunity to observe in the field the complete sedimentary sequence from the top to the base until the bedrock, exposed through out a single pit bank. The 7m thick sequence is made mostly of fine detritus mud showing yellowish brown color with alternation of thin layers of gray to dark mud. Debris of the lateritic iron crust occur forming a conglomerate as well as dispersed in the mud layers. The sequence shows high content of organic matter in the dark layers and shows an acid pH. The sediments are composed of kaolinite, smectite, goethite, gibbsite, and quartz as main minerals, and hematite, illite, and anatase as acessory Ones. Vivianite was identified in the iron crust close to contact the with the zone (sediments). This contact is erosional and displays iron crust debris and clayey material. The crusts around the contact with sediments show very strong mineralogical and textural changing. This mineralogical composition includes minerals coming from the iron crust and mineral formed during the diagenesis. The chemical composition of the sediments can be well correlated to that of the underlyiedunderlined crusts and bear their geochemical signatures related to the banded iron formation and their lateritised products. The polenpollen data and the physical, mineralogical and chemical data indicate a sedimentation process during humid climate alternated with semi-arid short periods of time during the Later Pleistocene.