BAUXITE FORMATION ON PROTEROZOIC BEDROCK OF THE GUIANA SHIELD IN SURINAME

Duricrust paleosurfaces in Suriname developed on various parent rocks during Late Cretaceous – Early Tertiary times, both on sedimentary deposits in the coastal lowlands and on various crystalline rocks in inland regions of the Guiana Shield. Laterite-type bauxite deposits in the Bakhuis Mountains (W. Suriname) formed on Proterozoic high-grade metamorphic lithologies including banded charnokitic rocks, amphibolite, gneiss and anorthosite. The deposits have a variable thickness and represent isolated planation surfaces on flat-topped plateaus created after uplift and dissection of the Bakhuis horst structure. Petrological and geochemical studies were performed on drill core samples representing up to 7 m deep vertical profiles. The samples cover a classical sequence, including almost pristine, foliated charnockitic and leucocratic bedrock lithologies, saprolite, bauxite and lateritic duricrust. Secondary mineral assemblages have heterogeneous textures and are dominated by the presence of Al and Fe-(oxy)hydroxides, while kaolinite is a major phase in the saprolite horizons. EPMA analysis of Al-(oxy)hydroxide grains from selected samples revealed high Al₂O₃ contents (ca. 85 wt.%) that would point to (pseudo)boehmite as prominent Al-hosting phase. This finding is surprising since gibbsite is generally considered to be the principal Al-carrier of the local orthobauxite ores, in agreement with their lateritic character. Microtextures suggest various pathways for formation of the Al-(oxy)hydroxides including: (1) conversion of plagioclase into kaolinite, followed by desilication, (2) direct replacement of feldspar; (3) dehydration of another Al-(oxy)hydroxide precursor. Chemical profiles point to complete leaching of Na₂O, CaO, K₂O and MgO from surface layers, accompanied by a considerable loss of SiO₂. Zr, V and Cr are among the immobile trace elements showing enrichments towards the surface, roughly following the pattern for Fe₂O₃. The complexity of mineralogical, textural and geochemical properties presumably reflects multiple cycles of bauxitization associated with climatic changes during the weathering regime.