Geological Circulars

Permanent URI for this collectionhttps://hdl.handle.net/2152/121888

Peer-reviewed geoscience research summaries, targeted on Bureau project areas in Texas and other locations, 1965–2003.

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Browsing Geological Circulars by Author "Duex, Timothy W."

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    Calderas and mineralization: volcanic geology and mineralization in the Chinati Caldera complex, trans-Pecos Texas
    (University of Texas at Austin. Bureau of Economic Geology, 1981) Duex, Timothy W.; Henry, Christopher D.
    This report describes preliminary results of an ongoing study of the volcanic stratigraphy, caldera activity, and known and potential mineralization of the Chinati Mountains area of Trans-Pecos Texas. Many ore deposits are spatially associated with calderas and other volcanic centers. A genetic relationship between calderas and base and precious metal mineralization has been proposed by some (Albers and Kleinhampl, 1970) and denied by others (McKee, 1976, 1979). Steven and others (1974) have demonstrated that calderas provide an important setting for mineralization in the San Juan volcanic field of Colorado. Mineralization is not found in all calderas but is apparently restricted to calderas that had complex, post-subsidence igneous activity. A comparison of volcanic setting, volcanic history, caldera evolution, and evidence of mineralization in Trans-Pecos to those of the San Juan volcanic field, a major mineral producer, indicates that Trans-Pecos Texas also could be an important mineralized region. The Chinati caldera complex in Trans-Pecos Texas contains at least two calderas that have had considerable postsubsidence activity and that display large areas of hydrothermal alteration and mineralization. Abundant prospects in Trans-Pecos and numerous producing mines immediately south of the Trans-Pecos volcanic field in Mexico are additional evidence that ore-grade deposits could occur in Texas.
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    Hydrology and water quality of the Eocene Wilcox group: significance for lignite development in east Texas
    (University of Texas at Austin. Bureau of Economic Geology, 1980) Henry, Christopher D.; Basciano, Joyce M.; Duex, Timothy W.
    Lignite development will place major demands on ground-water supplies. The Simsboro Formation and the Calvert Bluff Formation (a major lignite-bearing unit) of the Wilcox Group between the Colorado and Trinity Rivers constituted a test case to evaluate the availability and quality of ground water. Aquifer geometry (sand) was determined by comparing environmental geologic maps with subsurface sand-percent and net-sand maps constructed from electric logs (Kaiser, 1978). The combined maps correlate well and show that the Calvert Bluff Formation consists of a complex interfingering of coarse channel sands and fine interchannel muds. Sand outcrop areas occupying approximately 10 square miles separate much larger interchannel areas with few and minor sands. The Simsboro Formation consists of two parts - a thick multilateral sand (300 to 700 ft) in most of the southern outcrop belt and a series of channel sands (100 to 200 ft) interspersed with muds in the northern belt. Sands of the northern Simsboro belt are more like the Calvert Bluff channel sands than like the thick Simsboro sands. Available hydrologic data suggest that Simsboro and Calven Bluff sands have high hydraulic conductivity (6 to 20 mlday); interchannel muds have low hydraulic conductivity (1 to 2 m/day). Water compositions in the Simsboro and Calvert Bluff sands are similar and evolve similarly. Shallow ground water has a Ca-Mg-Cl-HC03 composition low in total dissolved solids (less than 500 mg/l). The water evolves over a depth range from 300 to 1,200 R to become a Na-HC03 water. Change in composition probably results from ion exchange with clays (caff for 2Na+) and solution of calcite (which contributes more Ca -- for exchange and increases HC03- concentration). Poor quality water is largely restricted to shallow wells (less than 100 ft) in muddy parts of the Calvert Bluff Formation.