ABSTRACT

The Shely cauldron is in southwestern Presidio County, Texas, in an area known as Pinto Canyon. The Shely Volcanic Group is an Oligocene volcanic sequence of rhyolite and trachyte ash-flow tuffs, lava flows, and volcaniclastic sediments and agglomerates. It represents an intracauldron facies which filled a collapse zone after the initial cauldron-forming eruptions (upper Colmena Tuff, Buckshot Ignimbrite, and lower Chambers Tuff).

The Allen intrusive complex represents a series of discontinuous early rhyolite porphyry domes and dikes related to the resurgence of the central cauldron block (previously known as the Loma Plata anticline), and a late nonporphyritic rhyolite flow dome associated with the final emplacement of a discontinuous ring dike along the outer margins of the ring-fracture zone.

The occurrence of economic mineralization associated with late-stage hydrothermal mineralization within the Shely caldera is highly probable considering the silicic nature of the late-stage rhyolite intrusions (e.g., the uranium at Organ Pipe Hill; fluorite at the Bienevides Ranch; lead, zinc, and silver mineralization at the abandoned Loma Plata mine; and molybdenum mineralization at the French Ranch).

Recently a great deal of interest has been shown in the anomalous uranium values found at Organ Pipe Hill. Several shallow exploratory holes were drilled but failed to detect any uranium of economic interest.

Analysis of the uranium mineralization at Organ Pipe Hill (an early rhyolite porphyry intrusion of the Allen complex) suggests that the uranium was leached from younger Allen volcanic units and deposited along fractures developed subsequent to the intrusive emplacement. I believe that hydrothermal convection cells associated with the emplacement of an adjacent nonporphyritic rhyolite flow dome and associated ash-flow tuffs (an offshoot of the major late stage ring dike emplacement) was the major contributing factor in the uranium concentrations at Organ Pipe Hill. Additional trace-element and geochemical analysis is still in progress.

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