The mineralogy and chemistry of samples from the cores obtained across Mariano orebody were determined and used to develop exploration tools for roll-type uranium deposits. Preliminary interpretations regarding the physicochemical conditions of ore deposition were made on the basis of paragenetic relations.

The host sandstones are confined between the bentonitic rock units, and contain scattered intercalations of detrital montmorillonitic material in the form of clay galls, stringers, and lenses derived from these bentonites. Authigenic clay minerals identified in the host rocks include cellular montmorillonite, platy chlorite, and pseudohexagonal “books” of kaolinite. The cellular montmorillonite is concentrated in the oxidized zone and appears to have formed prior to ore deposition. Authigenic chlorite is most abundant in the ore zone and has formed at the expense of cellular montmorillonite; its formation is interpreted to be related to the ore-forming processes. Kaolinite in sandstones is the last clay mineral to form, and is enriched in the reduced zone. Calcite, considered typical of such deposits, is found to be lacking in this orebody.

Iron-titanium oxides and their alteration products are the most abundant heavy-mineral species in the host rocks. In addition to anatase and rutile, the alteration products include hematite in the oxidized zone and pyrite in the ore and reduced zones. Carbonaceous material introduced later into the potential ore zone appears to have been responsible for the decomposition of Fe-Ti oxides and formation of pyrite.

The oxidation of pyrite by mineralizing solutions, resulting in reduction and subsequent deposition of uranium, is indicated by the paragenetic relationship. The positive correlation between organic carbon and uranium suggests that carbonaceous material also acted as a reductant for uranium.

A discriminant analysis was run using the chemical data to distinguish the geochemical zones (oxidized, ore, and reduced). Of the 15 variables used in this analysis, it was determined that the three zones could be separated using only six variables (Th, U, V, Zi, Ti, and Mn). The discriminant functions thus formulated could possibly be used to classify unknown samples in the area studied.

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