Wet chemical analysis and instrumental neutron activation analysis (INAA) were performed on a suite of samples from three residual ferruginous bauxite profiles in the study area for abundances of Al2O3, SiO2, Fe2O3, Ti O2, Na, Sc, Hf, Cr, Co, La, Sm, Yb, Lu, and Th. In addition a mineralogical study accompanied this research, using petrographic, differential thermal, and x-ray diffraction techniques. In each profile, the relative mobility of these elements were calculated by empirical methods for comparison with the parent rock and mineral properties in the saprolite. This study indicates that the first original constituent of the basalt to decompose under the influence of weathering is interstitial glass. The second phase results in the decomposition of plagioclase and pyroxene. Plagioclase alters mainly to kaolinite and metahalloysite. Pyroxene and basaltic glass alters mainly to nontronite, hematite, limonite, and amorphous clay. Opaques remain nearly unaltered. The low grade ferruginous bauxite ore is not derived solely from basalt, but also forms by weathering of younger sedimentary strata that overlie the basalt flows of the Columbia River Group. In general, components that are progressively depleted under the influence of weathering are Si02, Na, La, Sm, and Lu; these losses result in greater concentration of AlO3, Fe2O3, TiO2, Sc, Cr, Th, and Hf. Cobalt behaves erratically. There seems to be no predictable relationship between the ratios of rare earth elements in ferruginous bauxite and the parent rock. In the most weathered zone, Fe2O3, TiO2, Al2O3, Sc, Hi, Cr, and Th are enriched. Na, SiO2, Sm, and Lu are depleted relative to the parent rock. Trace elements associated with iron-rich pisolites are Lu, Yb, Th, and Co. Only Th is associated with gibbsite.

Two major theories have been advanced to account for the scattered distribution of ferruginous bauxite deposits. Original workers proposed that ferruginous bauxite originally developed over all exposed Columbia River Basalt in western Oregon and was subsequently removed by erosion. Studies which followed have suggested that it may be locally favorable conditions, especially of drainage, which are responsible for deposit distribution. Field mapping in the Chapman Quadrangle shows a possible correlation between a series of sheared zones, which may have improved drainage, and the distribution of ferruginous bauxite deposits. Examination of the pisolitic zone ferruginous bauxite of the Chapman Quadrangle failed to show any evidence supporting the theory that this zone was produced by fluvial action. It appears, instead that the pisolitic zone of the deposits studied developed in place and that the structures seen in this zone are the result of authigenic processes. Mineralogical study of samples from the Chapman Quadrangle suggests that the ferruginous bauxite of the area probably developed under slightly acidic pH conditions and that the assemblage quartz, kaolinite, gibbsite may exist in ferruginous bauxite deposits because of the presence of iron oxide and hydroxide coatings on the quartz which may cut off contact between quartz and gibbsite. Chemical study shows that the lateral variation in elemental concentrations is much less than the vertical variation in concentrations seen by some previous workers, and that lateral variation appears to be randomly distributed for most elements. The behavior of elements during weathering can best be modeled by taking into account the various sorbtive reactions between ions formed during weathering and clays and hydroxides.

Ferruginous bauxite deposits developed from flows of the Columbia River Basalt Group in northwestern Oregon and southwestern Washington. Samples of the iron pisolite and the gibbsite nodular zones from the upper portion of the weathering profile of drill core from Columbia County, Oregon and Cowlitz County, Washington, were analyzed by instrumental neutron activation. The mineralogy was determined using Xray diffraction and Mossbauer spectroscopy. The gibbsite nodular zone is above the clay-rich saprolite or relic basalt zone. The nodules contain relic vesicles and well preserved relic plagioclase microlites. Gibbsite occurs with poorly crystalline goethite and hematite in the gibbsite nodular zone. Clays are absent in this part of the profile . The iron pisolite zone is at the top of the profile above the gibbsite nodular zone. Both maghemite and goethite pisolites occur in the lower part of the zone while maghemite pisolites are dominant in the upper part of the pisolite zone. The parent flow is the Frenchman Springs Member of the Wanapum Basalt for the Columbia County profile and the Pomona Member of the Saddle Mountains Basalt for the Cowlitz County profile. Distribution of the major, minor and trace elements through the profile shows three distinct sympathetic patterns consisting of lanthanide elements and Na; As, Sbi Th, Hf, and Ta; and transition metals Fe, Ti, V, and Cr. Ratios between the high-field strength elements Ta and Hf are nearly constant through the profile, and Hf appears to be the least mobile elements of the elements analyzed in the profile. Ratios of other elements were calculated against Hf, based on the assumption that it has remained largely immobile during weathering, to show element enrichment and depletion in the profile independent of mass-volume changes. Transition metals and Al show a progressive depletion through the upper gibbsite nodular and iron pisolite zone due to leaching in the profile. Lanthanide elements (except Ce), As, and Sb show an obvious enrichment in the iron pisolite zone relative to the gibbsite nodular zone. Volume reduction during weathering was calculated based on the immobility of Hf. In the gibbsite nodular zone, the volume reduction calculated for bulk samples is greater than for gibbsite nodule separates suggesting that a greater volume reduction occurred the matrix material surrounding the nodules. Ratios between gibbsite nodules and parent basalt of the immobile elements Hf, Ta, Fe, Ti, Th and Cr suggest that the nodules, where the relic textures are preserved, have undergone volume reduction. Based on the immobility of Hf, the gibbsite nodules lost approximately 40% of the original volume. The volume factors based on the immobility of Hf show that the pisolite zone experienced a greater volume reduction than the gibbsite nodular zone. Absolute gains and losses relative to the parent basalt show the following relative order of depletion: Na > La > Eu > Sm > Co > Mn > Ce > Sc > Ta > V > Cr > Lu > Th > Fe > Ga > Al. The lesser mobility of Ce and Lu relative to other lanthanide elements suggests fractionation of lanthanide elements in the bauxite profile. Aluminum is both enriched and depleted at different depths in the gibbsite nodular zone suggesting that Al is mobilized from the matrix and possibly the pisolite zone into Al enriched gibbsite nodules. Volume reduction and destruction of relic textures in the pisolite zone is accompanied by small-scale mobilization of Th, Cr, Fe, Hf, Ga, Sc, and Ta during the formation of iron pisolites. Formation of the iron pisolite zone above the gibbsite nodular zone may indicate a change in climate from a heavy year round to a seasonal rainfall pattern.