The Mariano Lake uranium deposit is located on the west side of the Smith Lake district of the Grants mineral belt. Mineralization is restricted to a basal arkosic sandstone of the Brushy Basin Member of the (Jurassic) Morrison Formation. This sandstone, called the Poison Canyon sandstone (economic usage), consists of a sequence of paleochannels in which mineralization has been deposited in a roll-type tabular deposit. This roll front is directly related to an oxidation-reduction interface.

Chemically, the deposit is somewhat different from other Grants mineral belt deposits. Calcium and CO₃ content are low, but V, Ba, and S are relatively abundant. Sulfur found in pyrite is also possibly associated with uranium sulfates. Titanium is found as a secondary oxide, derived from titanomagnetites of the originally deposited mineral assemblage. Molybdenum, arsenic, and other trace elements show a regular zoning across the deposit, but cerium is slightly depleted.

The mineralogy of the Mariano Lake deposit includes abundant disseminated pyrite in mineralized reduced areas and hematite in the oxidized barren areas. Calcite, barite, gypsum, and jordisite are rare. Clay mineralogy includes kaolinite, chlorite, illite, and mixed layer illite-montmorillonite. Contrary to what has been found in other deposits of the Grants mineral belt, zonation of the clays is reversed, with kaolinite being more abundant in the downdip reduced sediments. The phenomenon is thought to be the result of backwash off the south-dipping flank of the Mariano anticline.

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