Variations in mineral composition and the weathering crust zoning of the REE-Nb chuktukon deposit (Chadobetsky uplift, Krasnoyarsk region)

Abstract

The Chuktukon REE-Nb deposit occurs within the carbonatite laterite supergene zone Chadobets upland, South of the Siberian craton. A unique laterite profile up to 600 m thick was identified by deep drilling. The laterite developed upon ultramafic lamprophyres and Ti-Th-REE-rich carbonatites. Here we report new mineralogical and geochemical data on carbonatites and laterites that reveal the evolution of supergene processes in the Chuktukon deposit within vertical zonation: (from top to bottom): 1) laterite horizon or ferruginous cap rock (ferricrete); 2) loose ochreous weathering products (ochre zone or regolith zone); 3) secondary phosphate enrichment horizon (apatite-rich zone); 4) unevenly altered disintegrated carbonatites. The bulk contents of CaO and P₂O₅ increases from primary carbonatite upwards due to the crystallization of carbonate-fluorapatite, and decrease in near-surface horizon due to the high leaching of rocks. The total REE₂O₃ content is highest in the rocks of the regolith zone and the enriched zone due to the redistribution and concentration of these elements both in the form of impurities and as a result of the redeposition of their own mineral phases. The variations in compositions of apatite, pyrochlore and monazite from different horizons of laterite profile were chosen as a proxy for the REE distribution in the supergene zone. Fluorapatite is gradually being replaced by hydroxyl-fluorapatite and carbonate-hydroxyl-fluorapatite, as the degree of weathering of primary carbonatites increases. Pyrochlore-Ca-Na-F is being replaced by the (BaSr)-variety (with Ce and Pb) during the same processes. Monazite changes from (Ce)-dominated to (La)-dominated. This is influenced by the oxidation factor, the consequence of which is also the formation of cerianite in the upper horizon. The study of the mineral and geochemical features of such an object as Chuktukon provides an improved understanding of the patterns of redistribution of the main and ore components of carbonatites in the supergene zone and can be useful in the study of other supergene deposits and delineation of dispersion halos of ore components.