Paragenesis of alkaline zeolite in an outcrop of amigdaloidal olivine basalts in Junín de Los Andes, Neuquén, Patagonia, Argentina

Abstract

The optical characteristics, X-ray diffraction, unit cell, chemistry and crystallization conditions of an alkaline zeolite assemblage were studied. The zeolites have been found infilling amygdales and in the matrix of Tertiary olivine basalts from Junin de los Andes, Patagonia. The secondary minerals (albite, pumpellyite, interestratified chlorite/smectite (C/S), smectite, iddingsite, magnetite and hematite) are replacing phenocrysts of plagioclase, olivine and the matrix of the rocks. Zeolites are sodic, calco-sodic and potassic in composition. Na–chabazite, analcime, Ca-K phillipsite and paulingite are the first zeolites that crystallized. They were followed by acicular-fibrous zeolites as natrolite/Na-gonnardite, thomsonite, mesolite and scolecite. The formation of the first zeolites is consistent with low temperature (<100 ºC), low pressure (<0.1 GPa) and alkaline (pH 9–10) hydrothermal/geothermal meteoric fluids, which were favored by tensional fractures. The activity of Si decreased during the alteration process and the temperature increased toward 200 ºC allowing the analcime formation followed by the equilibrium assemblage natrolite/Na-gonnardite-thomsonite-scolecite and mesolite.