The uranium deposits of Sierra Gomez, Chihuahua, consist of hexavalent uranium mineralization in middle Cretaceous

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limestones. Uranium was produced here in the 1960s, but work was discontinued in favor of the larger deposits of Pena Blanca.

The limestones of Sierra Gomez have undergone folding and thrusting. Tertiary volcanics were deposited and subsequently eroded from most of the area. Later faulting of the Basin and Range province left Sierra Gomez as a horst bounded by bolsons.

The uraniun deposits are in massive, sometimes fossiliferous or cherty limestones overlain by thin-bedded, fossiliferous limestones. Structurally the core of Sierra Gomez is a syncline bounded by anticlinal structures. Low-angle thrust faults are present in several mines in the area.

Hexavalent uranium is present along fractures and faults. Carnotite and tyuyamaunite have been identified. There is widespread calcite, silicification, hematization, and some fluorite, gypsum, and limonite. These replace limestone, either along low-angle faults, or in solution cavities.

Trace-element data available at this time indicate anomalous values for fluorine, vanadium, arsenic, molybdenum, mercury, chromium, nickel, copper, and zinc, in mineralized zones.

The uranium apparently was diagenetically leached from overlying volcanics and moved into the limestones, primarily along fault zones, until conditions favorable for precipitations were reached. Although evidence is supportive of a leaching hypothesis, it can neither be proven or disproven at this time.

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