Abstract: Volcanic and intrusive rocks (mainly andesite and monzonite) of Tertiary age are associated with gold- and silver-bearing quartz veins. Hydrothermal alteration took place in two stages. The first and less intense alteration stage is associated with economic mineralization; metallization during the second and more intense stage is limited to pyrite.
Alteration zones from the vein outward include: (1) dickite, (2) illite-kaolinite, (3) vermiculite-halloysite, and (4) chlorite-montmorillonite. Mixed-layer associations of 2:1 clay minerals are abundant and represent transitions between discrete clay minerals. Apparent occurrence of allophane in the illite-kaolinite zone indicates that amorphous aluminum silicates are intermediate phases in transitions from 2:1 to 1:1 clay minerals.
Alteration of ferromagnesian minerals and plagioclase resulted largely in the formation of 2:1 clay mineral types. Orthoclase is altered chiefly to illite and kaolinite. Dickite and quartz are end products of the most intense alteration in the area.
Chemical analyses of the altered rocks in general show a decrease in basic ions toward the vein. Less significant changes are shown by acidic ions. Analyses of illites indicate that substitution of K by Na, Fe, Ca and Mg has taken place near the vein as shown by an overall decrease in K Na , K Fe , K Ca and K Mg ratios. Both sodium and potassium seem to be instrumental in the formation of well-developed illite crystals.
Alteration stages operative in the Cochiti district were developed by the action of solutions probably ranging in pH from 4 to 10. Alteration zones were formed pene-contemporaneously with gradual outward migration of the least intense zones.