Abstract

Topaz rhyolites are fluorine-rich alkaline silicic lavas and shallow intrusives that are characterized by the presence of topaz (Al 2 SiO 4 F 2 ) in gas cavities, commonly associated with Mn-Fe garnet, bixbyite, pseudobrookite, specularite, quartz, and other minerals. In the western United States, Cenozoic (0.5-50 m.y. old for dated examples) topaz rhyolites occur on both sides of the Colorado Plateau (in Colorado, New Mexico, Arizona, Utah, and Nevada) and in Idaho and Montana. They also occur in a single linear belt in Mexico. Their enrichment in lithophile (fluorophile) elements (Li, Rb, Cs, U, Th, Nb, Ta, Sn, W, Be, etc.) leads to the term rare metal rhyolites. Similar F-rich rocks from Mongolia and the Soviet Union have been called ongonites. Topaz rhyolites appear to represent a special class of the bimodal or high silica rhyolites of the western United States.Their extensional tectonic setting and geochemical characteristics suggest that topaz rhyolites are the extrusive equivalents of anorogenic or residual (A- or R-type) granites. Their petrogenesis presumably involves partial melting of Precambrian continental crust (they appear to be restricted to areas of such crust) in the presence of a high heat flow (which tends to enrich F in solids at the expense of H 2 O). Mafic magmas may provide the heat for melting. Further differentiation may depend on (1) zone refining during ascent, (2) extreme fractional crystallization, (3) dehydration due to early pyroclastic volcanism, and (4) apical enrichment of near-surface magma chambers due to liquid state processes.Practical interest in topaz rhyolites results, in part, from the spatial and genetic association with volcanogenic deposits of Be, U, Sn, and F. Spor Mountain, Utah, provides the best example of this type of mineralization. Topaz rhyolite lavas from productive districts tend to be relatively phenocryst rich, poor in miarolitic cavities, and granophyrically crystallized. Vitrophyres, where present, are exceptionally rich in F and fluorophile elements, and rare earth patterns are exceptionally fiat (low La/Yb ratios) and have pronounced negative Eu anomalies. Near-surface country rocks penetrated by the volcanic vents were reactive (carbonate rocks) and water tables were presumably high.Most topaz rhyolite lavas are not associated wth volcanogenic mineralization. These nonproductive lavas are generally phenocryst poor, distinctly flow banded, lithophysa rich, and spherulitically-crystallized. Vitrophyres are less enriched in F and fluorophile elements, country rocks are nonreactive, and water tables could have been low.Topaz rhyolites of both the productive and nonproductive type may be valuable indicators of subsurface mineralization. Their distribution coincides very closely with that of (1) fluorite and silver-base metal districts, (2) the central and eastern tungsten belts of Kerr (1946), and (3) topaz-rich porphyry Mo-W deposits of the Climax and Henderson type. Topaz rhyolite volcanic vent areas might then reflect the existence of large F-rich magma chambers below with (1) subvolcanic breccia, porphyry, and greisen vein deposits of Mo, W, Sn, and other elements, (2) base and precious metal veins and fluorite-rich replacements, and (3) more deeply seated rare metal pegmatites.

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