ALTERATION OF SILICIC VITRIC TUFFS INTERBEDDED IN VOLCANICLASTIC DEPOSITS OF THE SOUTHERN BASIN AND RANGE PROVINCE, MEXICO - EVIDENCES FOR HYDROTHERMAL REACTIONS

In Northwestern Mexico, the Miocene basins that disrupted the Sierra M adre Occidental Province are filled with sandstones and conglomerates (the Baucarit Formation) cemented mainly by zeolites of the heulandite -clinoptilolite group. Few volcanic tuffs are intercalated in the sedi ments for which four different groups of samples have been defined. Th ese groups correspond to a gradation in the alteration of the glassy m atrix. Group 1 is characterized by the preservation of the glassy matr ix and the presence of disseminated patches of clay minerals with a co ntinuous variation between aluminous Al-montmorillonite and ferric sme ctite end-members. Heulandite-group zeolites and opal C-T are also pre sent. Group 2 is characterized by a nearly complete replacement of vol canic glass by a more homogeneous Al-montmorillonite. In some samples, heulandite-group zeolites are present as clusters on clay minerals. T he primary vitroclastic texture is generally preserved and relict glas s is present in small amounts. In group 3, the secondary assemblage is dominated by heulandite-group zeolite crystals as pseudomorphs of sha rds and pumiceous fragments. Discrete illite is present in all samples . Textures are exceptionally well-preserved. Group 4 is characterized by the presence of heulandite and clay minerals in which the Mg-Fe sme ctite end-member is more magnesian than in other groups. The original texture is not preserved. The following are deduced from the mass-bala nce calculations: the alteration of the tuffs leads to a strong Mg- an d Ca- and, to a lesser degree, Fe-enrichment, and to Na and K depletio n. Zeolites account for Ca-enrichment and clay minerals are host for F e and Mg. As a consequence, alteration may have occurred under open sy stem conditions and the most likely source for the high Ca and Mg gain s is a fluid circulating through the underlying volcaniclastic sedimen ts and underlying mid-Tertiary volcanics of the bimodal (basaltic-rhyo litic) sequence. However, those fluids may have been rather dilute and weakly alkaline. As estimated temperatures are between 85 and 125 deg rees C and as there is only a low burial, it is proposed that hot flui ds are responsible for the alteration of volcanic glass. A decrease wi th time in the initial permeability of the tuffs is consistent with th e observed evolution of the changing Al-smectite toward a more magnesi an composition.