CHARACTERIZATION 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 tectoni sm. In central Sonora, where the stratigraphic column is thicker (e.g. in the Moctezuma Basin), two superposed sequences of sediments are di stinguished on the basis of their lithology and stratigraphy. They are known as the Baucarit Formation. The lower sequence, Miocene in age a nd 300 to 400 m thick, is indurated and consists mainly of sandstones and conglomerates. The upper one, of variable thickness, consists of c lays. Zeolites occur in both sequences as alteration products of volca nic glass in beds of white intercalated rhyolitic tuff. Previously kno wn outcrops of zeolitites from Rio Batepito (erionite), Tetuachi and S an 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 behavi our) and chemical characteristics (major and trace elements for indivi dual crystals and bulk rock). Heulandite-group minerals are also prese nt as cement (30% by volume) of the sandstones from the lower sequence . Amygdales and veins from basaltic flows intercalated within the lowe st sediments contain a large variety of zeolite species: chabazite, st ilbite, phillipsite, mesolite, erionite, thomsonite, heulandite and an alcime. Zeolites in the pyroclastic beds intercalated in the upper seq uence represent the highest economic potential because they are essent ially monomineralic and constitute up to 80% in volume of the whole ro ck. Because of the inhomogeneous distribution of amygdales, zeolites i n the basaltic flows have a low economic interest.