Vermiculite in Central Texas
Vermiculite deposits in the Central Mineral region of Texas, chiefly in Precambrian metamorphic rocks, are situated in Llano County and adjacent parts of Mason, Gillespie, and Burnet counties with minor occurrences in Blanco and San Saba counties. All of the known deposits contain a lesser percentage of vermiculite than the deposits now being exploited in South Carolina and Montana; however, the deposits are substantial in size and will probably be mined when the richer domestic and foreign sources are exhausted. The bulk of the vermiculite is from weathering of biotite formed in the following suggested manner: (1) intrusion of sills and irregular masses of gabbro into the Valley Spring gneiss prior to or during regional metamorphism; (2) metamorphism of gabbro to produce amphibolite; and (3) partial conversion of amphibolite to biotite schist by metasomatism accompanying the emplacement of granites and pegmatites. In one deposit the process seems to have been arrested in the amphibolite stage with introduction of nonoriented biotite and feldspar by potash metasomatism. A third distinct type of vermiculite deposit associated with serpentine and soapstone appears to be primary hydrothermal rather than a weathering product of biotite. Suggested events leading to the formation of this type of deposit are: (1) emplacement of dunite and gabbro in Valley Spring gneiss; (2) metamorphism of gabbro to form amphibolite with deformation of dunite into lenticular masses; (3) alteration of dunite to nonfoliated serpentine during static conditions and through the influence of aqueous solutions; (4) alteration of serpentine to soapstone from periphery inward through influence of fluids from nearby granite intrusions; and (5) formation of vermiculite veins along fractures in soapstone and serpentine through the influence of fluids from pegmatites which closely followed the granite intrusion.
Clabaugh, S. E., and Barnes, V. E., 1959, Vermiculite in Central Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 40, 32 p.
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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation