Secondary mineral assemblage as indicator of multistage alteration processes in basaltic lava flows: evidence from the Lessini Mountains, Veneto Volcanic Province, Northern Italy

Abstract

The secondary mineral assemblages in the Tertiary basalts from the Lessini Mountains are mainly clay minerals and zeolites, and result from multistage alteration processes. In the earliest Stage I, clay and silica minerals precipitate along the inner walls of the vesicles, followed by the deposition of the fine-grained zeolites of the Stage II (erionite, offretite, analcime, natrolite, heulandite and stilbite). The final Stage III is marked by a new generation of large, well-shaped zeolites (phillipstite-harmotomo, gmelinite, chabazite, willhendersonite and yugawaralite), followed by extensive crystallization of calcite. New morphological and chemical data are presented herein in order to reveal the chemical compositions of Lessini zeolite species, which have never been analyzed before. Textural and chemical observations indicate that the chemical elements for Stage I and II alteration minerals derived from the alteration of the basaltic glass and the primary phases in the volcanic host rocks. The final mineral assemblages of the Stage III suggest a variation in the crystallization conditions, probably related to the presence of new, late-stage fluids enriched in Ca and (CO₃)^2-_, leached from the surrounding calcareous rocks.