Characterisation and potential economic interest of authigenic zeolites in continental sediments from NW Mexico

In NW Mexico, zeolite deposits of potential economic interest occur in continental sediments related to mid-Tertiary basin and range tectonism. In central Sonora, where the stratigraphic column is thicker (e.g. in the Moctezuma Basin), two superposed sequences of sediments are distinguished on the basis of their lithology and stratigraphy. They are known as the Baucarit Formation. The lower sequence, Miocene in age and 300 to 400 m thick, is indurated and consists mainly of sandstones and conglomerates. The upper one, of variable thickness, consists of clays. Zeolites occur in both sequences as alteration products of volcanic glass in beds of white intercalated rhyolitic tuff. Previously known outcrops of zeolitites from Rio Batepito (erionite), Tetuachi and San Pedro Ures (heulandite group), Divisaderos (chabazite) and several other newly discovered occurrences are described in this work in terms of mineral association, physical properties (porosity, thermal behaviour) and chemical characteristics (major and trace elements for individual crystals and bulk rock). Heulandite-group minerals are also present as cement (30% by volume) of the sandstones from the lower sequence. Amygdales and veins from basaltic flows intercalated within the lowest sediments contain a large variety of zeolite species: chabazite, stilbite, phillipsite, mesolite, erionite, thomsonite, heulandite and analcime. Zeolites in the pyroclastic beds intercalated in the upper sequence represent the highest economic potential because they are essentially monomineralic and constitute up to 80% in volume of the whole rock. Because of the inhomogeneous distribution of amygdales, zeolites in the basaltic flows have a low economic interest.