BENTONITE FROM MALIMAN, ANDEAN PRECORDILLERA ARGENTINA

GÓMEZ C., CRAVERO F., LUNA L., GARRIDO M.

Castellaneta Marina, Italia

Congreso; XIV International Clay Conference; 2009

AIPEA

Maliman (Hipo), an unusually white bentonite deposit, is located in the Andean Precordillera, San Juan province, Argentina. It is was formed from the alteration of volcanic rocks from Las Trancas Formation (Neocene). These rocks are calc-alkaline, varying in composition from andesites to rhyolites and weresubsequently faulted and intruded by andesitic/trachyandesitic dikes. The deposit is located in the central part of a 10-Km2 elliptical area. It was formed by the alteration of both vesicular and poorly vesicular interlayered rhyolitic glasses. The alteration products are montmorillonite, opal C-T and zeolites. The purpose of this paper is to determine the mineralogy, smectite distribution, and the chemical behavior of major and trace elements during the alteration. Emphasis is placed on the relative mobility of the elements and the role they played during the transformation of fresh vitreous rocks to bentonite. The main mineral is a low iron montmorillonite, with minor amounts of zeolites opal C-T, alkali feldspar and biotite. The conversion of volcanic rocks to bentonite results in the removal of Si, alkalis (K) and uptake of Mg and H O while Ti and Al are immobile. The trace element Th is immobile, while Nb and Y are leached, and Zr is enriched during the alteration to bentonite. There is no fractionation of rare earth elements (REE). 2Thus, removal of Si and alkalis from the glassy rock and Mg incorporation in smectite from a fluid phase must have taken place at a high water/rock ratio. This indicates the presence of high hydraulic conductivity areas. Field observations show that the alteration is structurally controlled, its intensity being higher around faults and trachytic / rachyandesitic dikes. Pockets with higher grade material show this interaction. In conclusion, the mobility of major and trace elements during alteration depends on the nature of the parent rock and the composition of the fluid phase and water/rock ratio. 2 elliptical area. It was formed by the alteration of both vesicular and poorly vesicular interlayered rhyolitic glasses. The alteration products are montmorillonite, opal C-T and zeolites. The purpose of this paper is to determine the mineralogy, smectite distribution, and the chemical behavior of major and trace elements during the alteration. Emphasis is placed on the relative mobility of the elements and the role they played during the transformation of fresh vitreous rocks to bentonite. The main mineral is a low iron montmorillonite, with minor amounts of zeolites opal C-T, alkali feldspar and biotite. The conversion of volcanic rocks to bentonite results in the removal of Si, alkalis (K) and uptake of Mg and H O while Ti and Al are immobile. The trace element Th is immobile, while Nb and Y are leached, and Zr is enriched during the alteration to bentonite. There is no fractionation of rare earth elements (REE). 2Thus, removal of Si and alkalis from the glassy rock and Mg incorporation in smectite from a fluid phase must have taken place at a high water/rock ratio. This indicates the presence of high hydraulic conductivity areas. Field observations show that the alteration is structurally controlled, its intensity being higher around faults and trachytic / rachyandesitic dikes. Pockets with higher grade material show this interaction. In conclusion, the mobility of major and trace elements during alteration depends on the nature of the parent rock and the composition of the fluid phase and water/rock ratio. Keywords: bentonite, element mobilization, Maliman-Argentina.