Heulandite-group zeolites in volcaniclastic deposits of the southern Basin and Range Province, Mexico

Heulandite-group zeolites occur as cement in volcanogenic sandstones and conglomerates of the molassic Baucarit formation of Sonora State, N Mexico; they are associated with smectites, calcite and silica minerals. The zeolites have been distinguished by their thermal behaviour and chemical composition. The Ca content is very high, frequently > 50% of the total exchangeable cations. These calcic heulandite-group zeolites were formed by reaction of acidic ashes and lithic fragments with interstitial water. Their composition varies both from one basin to another and between heulandite cementing sandstones and that replacing glassy volcanic fragments, the latter being more siliceous. The presence of heulandite suggests a widespread hydrothermal activity and a steep geothermal gradient in the region during the Miocene.