At the Nevada Test Site, zeolites, cristobalite, quartz, feldspar, and clay minerals were deposited by ground water that leached vitric volcanic rocks in the unsaturated zone and reacted with similar vitric rocks just above permeability barriers where the rocks are nearly saturated. Zeolite zoning occurred contemporaneously with zeolitization by a separation of cations according to solubility.
Zeolite zoning, chemistry of the zeolitized rocks, and lack of correlation of zones with depth below surface require that ground water moved downward through unsaturated vitric rocks. Vitric rocks were leached and the ground water increased in cation content and pH.
Water movement was reduced, saturation was increased, and reactions with vitric rocks occurred just above one of the following permeability barriers to form zeolites and other authigenic minerals: (1) Relatively impermeable welded tuffs, lava flows, and Paleozoic clastic rocks, (2) clay minerals formed by leaching of vitric rocks just above relatively permeable Paleozoic carbonate rocks, and (3) zeolitized rocks formed by ground-water reactions just above one of the aforementioned permeability barriers.
Zeolitization progressed upward through the vitric rocks by ground-water reactions just above impermeable zeolitized rocks. Ground-water movement through the zeolitized rocks, though greatly impeded, caused a redistribution of cations by charge and size and formed zeolite zones contemporaneously with the progressive thickening of the zeolitized rocks. A triangular plot of Ca plus Mg, K, and Na in zeolite-rich rocks shows that Ca and Mg are concentrated in the uppermost part of the zeolitized rocks, K in the middle part and Na in the lowermost part.