ABSTRACT

A thick sequence of upper Oligocene peralkaline ash-flow tuffs and an associated vent complex are located in Santa Clara Canyon, 91 km north of Chihuahua City, Mexico. The principal unit, the Cryptic Tuff, is a comendite and contains about 1% phenocrysts that include sanidine (Or 45–55) quartz, hedenbergite, and magnetite. A complete cooling unit consists of: (1) a basal vitrophyre with associated spherulitic pods; (2) greenish, densely welded tuff that contains stretched pumice lapilli; (3) a banded red and light pink zone that contains flowage features and shear folds; (4) a highly porous zone with abundant quartz and sanidine in cavities; and (5) a thick zone of micropoikilitic sanidine and quartz interspersed with riebeckite and aegirine. Flow breccias that contain fragments of the different Cryptic tuff varieties usually separate micropoikilitic riebeckite tuff units. A north-south-trending vent zone (at least 3 km long) is present in lower Santa Clara Canyon where over 400 m of Cryptic tuff is exposed. Flow foliation is vertical in the vent area. Overlying the Cryptic sequence is the bluish, comenditic Campana tuff. It is densely welded and contains about 10% sanidine and quartz phenocrysts set in a groundmass that includes riebeckite and matted aegirine needles.

A minimum loss of 8 mg U3O8 per gram of Cryptic tuff has been calculated using data for glassy and massive riebeckitic phases. These paralkaline tuffs could have been source rocks from which large amounts of uranium were mobilized.

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