Alteration processes in the Maliman (hipo) bentonite deposit. Andean Precordillera, San Juan, Argentina

Abstract

In the San Juan province, northwestern Argentina, a white bentonite deposit was formed from Neogene volcanic rocks (Las Trancas Formation). These rocks were subsequently faulted and intruded by andesitic/trachyandesitic dikes. The deposit is located in the central part of a 10 km2 elliptical area, and results from the alteration of both vesicular and woody interlayered rhyolitic glasses. The main alteration products are montmorillonite, opal C-T, and zeolites. The highest concentration of montmorillonite is controlled by dikes and faults. The alteration of volcanic rocks to bentonite has taken place in an open hydrologic system where the removal of Si and alkalis led to the generation of opal C-T and zeolites, while Mg and H2O are taken up to form montmorillonite. During the alteration process, Th has low mobility, Nb and Y are leached, Zr is enriched, and there is no fractionation of rare earth elements (REE). In conclusion, the mobility of major and trace elements during alteration depends on the nature of the parent rock, on the composition of the fluid phase and on the water/rock ratio.