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The elemental, isotopic, and mineralogic examination of core samples from the Tony M orebody, a typical Colorado Plateau tabular-type, uranium-vanadium deposit, located in the Henry structural basin of south-central Utah, allows a critical evaluation of proposed ore genesis models. Uranium mineralization is concentrated into two horizontally oriented tabular horizons hosted within the Salt Wash member of the Jurassic Morrison Formation. The ore zones are not strata bound and rise stratigraphically toward the center of the ancestral Henry basin. Preservation of fossil plant debris and the lack of oxidative destruction of iron disulfide minerals in the interval between the uranium-enriched units argue this zone is not the oxidized tongue formed by a roll-type mineralizing p ocess. The selenium-molybdenum trace element pattern is also inconsistent with the roll-front model.
The necessary interbedding of gray lacustrine mudstones and nearshore lacustrine sandstones, as required by the lacustrine-humate model, is present within the Tony M deposit. However, absence of quantitatively significant amounts of transported humic substances either associated or remote from mineralization suggests that mudstone-derived organic acids were not involved in uranium localization.
Vanadium oxide is the major vanadium-bearing phase within the lower uranium lense whereas vanadium is partitioned within chlorite in the supra-adjacent barren zone. Such a sharp vertical break in vanadium mineralogy implies vanadium deposition within two chemically different environments.
The lower ore zone is characterized by isotopically light (^dgr34S ^cong -26^pmil - 46^pmil) FeS2. Bacterial sulfate reduction is shown to be the most likely fractionating agent. The uniformity of the sulfur isotopic composition of the iron disulfide minerals requires a non-depletable sulfate reservoir. Dissolution of gypsum (^dgr34S ^cong + 14^pmil) occurring below the Tony M orebody is demonstrated to be a plausible sulfur source thereby establishing the presence of a sulfate brine. From this data we conclude that the solution-interface model, which postulates uraniferous meteoric fresh water flowing over a denser brine, best explains the genesis of this deposit.
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