Topaz rhyolites - distribution, origin, and significance for exploration.

Topaz rhyolites are fluorine-rich alkaline silicic lavas and shallow intrusive rocks, characterized by the presence of topaz crystals and other minerals rich in lithophile elements in gas cavities. Brief petrological descriptions of 22 occurrences of Cainozoic age in the western USA are given, and similar occurrences in Mexico and other countries are noted. The tectonic setting and geochemical characteristics of these topaz rhyolites suggest that they represent the extrusive equivalents of anorogenic or residual-type granites. Their petrogenesis presumably involved partial melting of the Precambrian continental crust, followed by further magmatic differentiation depending upon the particular environment of formation. Topaz rhyolites often exhibit spatial and genetic association with other types of volcanogenic mineral deposits, and hence may be valuable indicators of subsurface mineralization. -J.E.S.