The mobility of rare-earth and other elements in the process of alteration of rhyolitic rocks to bentonite (Lebombo Volcanic Mountainous Chain, Mozambique)

Abstract

Ca-type bentonite deposits of economic interest occur associated with rhyolitic rocks in the Lebombo volcanics. From previous geological studies on the deposits, besides the economic aspects, little is known about the main formation mechanism of beds of bentonite that can be more than 15 m thick, in some places. Chemical analyses of bentonite samples indicate that elements such as Ca, Mg, Sr, Zn, Cu and Sc are markedly enriched in bentonite in comparison to the parent rock. Other elements such as K, Na, Rb and Ba are depleted in bentonite. HREE (Yb, Lu) and Y are depleted in the bentonite samples, as well as Cs, Rb, Ta, U, Ba, Co, Cr and Pb. Concentrations of the LREE (La, Ce, Nd, Sm, Eu), Sc and Ga are slightly higher in the bentonites than in the parent rocks. The chondrite-normalized REE patterns show identical trends both in the rhyolites and bentonites samples. Alteration of the parent rocks to bentonite is associated with leaching and subsequent removal of, principally, K and Na in open-system conditions.