Abstract
On the Colorado Plateau, at the base of the Chinle Formation, siltstone silicification, bleaching, argillic alteration, and carbonatization are associated with uranium mineralization, as noted in three areas. (1) Near Cameron, Arizona, a silicified zone is accompanied by extensive bleaching, which extends downward into the top of the Shina-rump sandstone. Strata, ordinarily red, are turned white in spots and bands and along fracture borders —mainly by removal of iron. A mottled and variegated color pattern may be attributed to partial alteration and locally to oxidation of sulfide veins. (2) On the San Rafael Swell basal Chinle siltstones and mudstones show variegated bleaching, argillic alteration, silicification, and carbonatization under or near uranium ore bodies. The silicified rocks contain jasper, chert, and quartz in layers, veins, or irregular masses and are bordered by argillic alteration halos. Argillic alteration is accompanied by the removal of iron and by the transformation of the montmorillonitic component of mixed-layer clay into mica clay with a corresponding increase of the 2M1 over 1M mica polymorph. (3) Along the Colorado River gorge, 11 miles east of Moab, Utah, basal Chinle outcrops show similar mottled bleaching associated with silicification. Here a silicified zone contains jasper and massive or columnar quartz in pipelike structures perpendicular to the bedding planes.
Chinle alteration features on the Plateau are attributed to hydrothermal solutions, and the introduction of hypogene uranium minerals belongs to the alteration sequence. The accompanying carbonatization includes the emplacement of calcite and dolomite as nodules and veins in the siltstones. Argillic alteration and silicification are related; sulfide mineralization is associated.
