Alteration of Sandstone Pipes, Laguna, New Mexico

Abstract

Near Laguna, New Mexico, numerous vertical, cylindrical, collapsed, and brecciated bodies with down-faulted cores occur as distinctive geological features. They are believed to be breccia-pipe structures in sedimentary strata. Uranium-bearing pipes of the Woodrow and Jackpile deposits in the northern Laguna district are exposed in upper Morrison strata. Pipes in the southern Laguna district crop out predominantly in the Summerville and Bluff formations but thus far have not been found to be uranium-bearing. The structural similarity between the pipes of the two areas suggests a common origin. The red Summerville Formation within the breccia cores of the southern Laguna pipes is bleached white or greenish-white. Optical examination of thin sections and fragments discloses alteration of the potassic and sodic feldspar to kaolinite and sericite, accompanied by a removal of hematite cement and the introduction of abundant, upward-migratory calcite. Silicification may be observed in a few places. X-ray study of the clay matrices indicates that the Summerville siltstones are altered from predominantly montmorillonite-kaolinite mixtures to chlorite, illite (hydromica), mixed-layer montmorillonite-illite-chlorite, kaolinite mixtures. X-ray spectrochemical analyses confirm the optical and X-ray data. The structure, mineralization, and alteration features associated with the pipes indicate structural collapse with accompanying penetration by ascending hydrothermal solutions.