COBERTURA BAUXÍTICA E ORIGEM DO CAULIM DO MORRO DO FELIPE, BAIXO RIO JARI, ESTADO DO AMAPÁ

Abstract

The Morro do Felipe kaolin deposit, occurring on the left margin of the Jari river (Northern Brazil), constitutes the lower part of an ancient weathering profile which is bounded to a planation surface about 170 m high. The kaolin layer, up to 35 m thick, consists mainly of poorly ordered kaolinite with deferrification features and thin sand intercalations. In the lower part of the deposit, the kaolin is only slightly porous and disintegrates easily in water whereas in the upper part, it is more porous, firm, displays conchoidal fracture and resists disintegration in water. In both cases, the kaolin (mineral) is very similar to fire-clay type kaolinite. The kaolin deposit grades down into altered sandstones and mudstones, with conglomerate intercalations, of the Alter do Chao Formation and is overlain by an iron crust, lower nodular bauxite, upper nodular bauxite and a clayey overburden (Belterra Clay). The evolution of this alteration profile likely involved the following steps: 1) formation of an iron crust by podzolization or ground-water lateritization of Alter do Chao deposits; 2) regional bauxitization; 3) degradation of the bauxite horizon and formation of colluvial deposits; 4) deposition of the clayey overburden upon the colluvial deposits or directly on Alter do Chao sediments; 5) lowering of the base level and reactivation of weathering giving rise to the transformation of the overburden into a latosol (Belterra Clay) and the thickening and deferrification of the saprolite horizon which resulted in the present day kaolin deposit. As shows the evolution of the alteration profile, the Morro do Felipe kaolin represents a product of deep weathering which affected the Amazon region since the Lower Tertiary. The kaolinization, related firstly to bauxitization (Lower Tertiary) and triggered, at a later stage, by reactivation of tropical weathering (Upper Tertiary/Quaternary) and epirogenetic movements, resulted from the decomposition of clay minerals and possibly some feldspar, partial dissolution of quartz, neoformation of poorly ordered kaolinite and intense deferrification. It seems very likely that the process of kaolinization continues even today.