The present study deals with the supergene alteration of volcanic basic rocks of the precambrian Grão-Pará Group that occur at the N4 plateau of Carajás and provides evidences that strongly support the contiguous N5 bauxite deposit to be an extreme product of this alteration. The basic rocks are tholeiitic basalts with a primary mineral assemblage largely composed of labradorite and pigeonite-augite. Significant chemical and mineralogical transformations have been induced by the weathering on these "rocks and have been evaluated down to depths of 140 m. Distinct gains losses of major components characterize semide = composed and decomposed basalt in which new compatible minerals change from quartz-smectite-chlorite-kaolinite in the former to kaolinite-goethiteopaques-(gibbsite) in the latter. The N5 plateau consists of a ferruginous bauxitic layer underlain by 1)a lateritic crust; 2) a gibbsite-rich clay and 3) an argillaceous horizon of indefinite thickness, each displaying distinctive chemical and mineralogical compositions with varying proportions of kaolinite, gibbsite and Fe-hydroxides (oxides). Quartz is only present in the argillaceous horizon. The genetic link between N4 and N5 sequences is advocated on the basis of 1) the nature of the argillaceous horizon as probably being an intermediate stage between the semidecomposed and decomposed basalt; 2) the many similarities disclosed by the gibbsite-rich clay and the decomposed basalt; 3) the same heavy mineral suites for both profiles; anda) typical trace elements of basic rocks present in relatively high concentrations in N5 horizons and showing enrichment factors along a common trend from the unaltered basalt to the bauxite material.The present study deals with the supergene alteration of volcanic basic rocks of the precambrian Grão-Pará Group that occur at the N4 plateau of Carajás and provides evidences that strongly support the contiguous N5 bauxite deposit to be an extreme product of this alteration. The basic rocks are tholeiitic basalts with a primary mineral assemblage largely composed of labradorite and pigeonite-augite. Significant chemical and mineralogical transformations have been induced by the weathering on these "rocks and have been evaluated down to depths of 140 m. Distinct gains losses of major components characterize semide = composed and decomposed basalt in which new compatible minerals change from quartz-smectite-chlorite-kaolinite in the former to kaolinite-goethiteopaques-(gibbsite) in the latter. The N5 plateau consists of a ferruginous bauxitic layer underlain by 1)a lateritic crust; 2) a gibbsite-rich clay and 3) an argillaceous horizon of indefinite thickness, each displaying distinctive chemical and mineralogical compositions with varying proportions of kaolinite, gibbsite and Fe-hydroxides (oxides). Quartz is only present in the argillaceous horizon. The genetic link between N4 and N5 sequences is advocated on the basis of 1) the nature of the argillaceous horizon as probably being an intermediate stage between the semidecomposed and decomposed basalt; 2) the many similarities disclosed by the gibbsite-rich clay and the decomposed basalt; 3) the same heavy mineral suites for both profiles; anda) typical trace elements of basic rocks present in relatively high concentrations in N5 horizons and showing enrichment factors along a common trend from the unaltered basalt to the bauxite material.