Economic Geology Vol. 77, 1982, pp. 1818-1836 Topaz Rhyolites-Distribution, Origin, and Significance for Exploration DONALD M. BURT, MICHAEL F. SHERIDAN, JAMES V. BIKUN~, AND ERIC H. CHRISTIANSEN~~ Department of Geology, Arizona State University, Tempe, Arizona 85287 Abstract Topaz rhyolites are fluorine.,.rich alkaline silicic lavas and shallow intrusives that are char­ acterized by the presence of topaz (AI2Si04F 2) in gas cavities, commonly associated with Mn­ Fe garnet, bixbyite, pseudobrookite, specularite, quartz, and other minerals. In the western United States, Cenozoic (O.5--50m.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 lithqphile (flu~rophile) elements (Li, Rb, Cs, U, Th, Nb, Ta, Sn, W,'Be, etc.) leads to the term rare metal rhyolites. SimilarF-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 petro­ genesis presumably involves partial melting of Precalnbrian 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 20). Mafic magmas may provide the heat for melting. Further differentiation may depend on (1) zone refining during ascent, (2) extreme fractional crys­ tallization, (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 thebest example of this type of mineralization. Topaz rhyolite lavas from. productive districts tend to be rel­ atively phenocryst rich, poor in miarolitic cavities, and granophyrically crystallized. Vitro­ phyres, where present, are exceptionally rich in F and fluorophile elements, and rare earth patterns are exceptionally flat (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 non­ productive lavas are generally phenocryst poor, distinctly flow banded, lithophysa rich, and spherulitically-crystallized. Vitrophyres are less enrichedin F and fluorophile elements, coun­ try 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 sea~ed rare metal pegmatites. Introduction occurrence in a lava flow near Nathrop, Colorado, and made a comparison with occurrences to topaz IN 1884 Whitman Cross described topaz, Al2Si04F 2, from a porphyritic rhyolite dome at Chalk Mountain, and garnet in samples of lithophysal rhyolite lava Colorado, less than 4 km west of what would 34 years from the Thomas Range, Utah, where topaz was first later becomethe Climax molybdenum mine (Wallace recognized in 1859. Exactly 100 years afterward, in et aI., 1968). In 1886 Cross reviewed this and a similar 1959, the Spor Mountain beryllium deposits were discovered in this same district- (Shawe, 1968). Since these early discoveries, Cenozoic rhyolite <:t Present address: Shell Oil Company, P.O. Box 2906, Houston, Texas 77001. domes and lava flows containing topaz have been <:t<:t Present address: Department of Geology, University of Iowa, described from more than 20 localities in the western Iowa City, Iowa 52242. United States (Christiansen, 1981). The topaz in the 0361-0128/82/103/1818-19$2.50 1818 TOPAZ RHYOLITES 1819 crystalline rhyolite presumably indicates fluorine en­ sidians) correlate well with the megascopic occur­ richment in the original magma. rence of topaz in cavities in associated crystalline la­ Available evidence increasingly suggests spatial vas. Topaz is unlikely if the vitrophyre contains less and genetic links between topaz rhyolite vent com­ than about 0.1 to 0.2 weight percent F; some vitro­ plexes and (1) Spor ryiountain-type volcanogenic de­ phyres, such as those from Spor Mountain, Utah, con­ posits of beryllium, lithium, uranium, and fluorine; tain more than 1.0 percent F (Bikun, 1980). Glassy (2) Climax-type subvolcanic breccia, porphyry, and portions of a topaz-bearing rhyolitic dike in I\1ongolia greisen deposits of molybdenum, tungsten, tin, and contain up to 3.2 percent F (Kovalenko et aI., 1971); possibly other lithophile elements; and (3) fluorite and the rock type has been called ongonite by the Soviets. manganese-rich, San Juan-type base and precious The value of 0.15 percent F isput as the lower limit metal veins in volcanic rocks, or fluorite-rich skarn for ongonite by Kovalenko and Kovalenko (1979, p. replacements in 'carbonate rocks. More tenuous 2~6). Soviet ongonites and U. S. topaz rhyolites there­ (mainly geochemical) links join topaz rhyolites tp fore appear to be equivalent rock types, although more deeply seated lithium and rare-metal granites three-fourths of the Mongolian and ?oviet occur­ and pegmatites. rences are intrusive and differ in some aspects of their This paper is, in part, derived from a more detailed geochemistry (Kovale~ko and Kovalenko, 1976; 1(10 ­ study of uranium deposits related to topaz ~hyolite valenko et aI., 1979; Antipin et aI. 1980). volcanism in the western United States (Burt et al., Volcanology 1980). Additional details on the petrology and geo­ chemistry of topaz ~hyolites are contained in a Ph.D. Small intrusive or extrusive domes and lava flows thesis by Christiansen (1981), which has been sub­ of rhyolite containing topaz do not appear very'dif­ mitted for publication as a Geological Society of ferent from those of other types of silicic magma. The American Special Paper. crystalline topaz rhyolite lavas range from distinctly gray or chalky, phenocryst-rich, massive, miarolitic Field Recognitioll types to pinkish-gray to tan, phenocryst-PQor, flow­ Numerous occurrences of topaz rhyolite lava 11n­ banded, lithophysal types. Both types of lava (and doubtedly remain to be discovered. In the field, their intermediate varieties) rpay develop a distinctive cav­ defining feature is the presence of topaz, which is ernous or honeycomb-like weathering pattern as in seen in miarolitic or lithophysal (concentrically lay­ the Honeycomb Hills, Utah (Linqsey, 1977). The ered) cavities in the lava. Topaz is most easily distin­ causes of this feature are complex, but silicified areas gllished frorn similarly appearing prismatic quartz by adjacent to fractures may be more resistant to weath­ its perfect basal cleavage and orthorhombic sym­ ering than the more friable fresh. rock, which weath- metry. Transparent crystals in freshly broken cavities ers to form cavities. are typically yellowish to pinkish brown; this color The effect of fluorine on rhyolitic magma is to gradually fades on exposure to sunlight. Where eu­ lower its solidus temperature anq visco'sity (cf. Wyllie, hedral quartz and topaz occur together~ the quartz 1979; Manning, 1981), as well ~s to expapd the field is normally present as small stubby crystals coating of stability of quartz. In this regard, fluorine has the I the walls of the cavity, whereas topaz is a large, single same qualitative effect as water; the difference is that crystal growing into its center. fluprine has a greater tendency to stay with the melt, Associated dark-colored minerals in the cavities can rather than escaping explosively, on release of pres­ q.lso be distinctive. The most commop are anhedral sure (see reviews by Bailey, 1977, and Burnham, to euhedral, red to black Mn-Fe garnet, black cubes 1979). This feature, in conjun~tionwith eruption tem­ of bixbyite, (Mn, Fe)20S, black acicular pseudobrook­ perature and water fugacity, permits some topaz rhy­ ite, Fe2TiOs, and black platy specular h'ematite (plus olite lavas (e.g., at Spar Mountain, Utah) to flow rather ilmenite?). Other common minerals are colorless far from their vent areas. crusts of sanidine, tabq.lar to prismatic, pink to red Fluorine-rich extrusions (domes or flows) typically hexagons of beryl, Golorless to purple fluorite, and have black basal vitrophres that grad~ upward into ruby red, stubby to platy cassiterite (Lufkin, 1976). thick devitrified zones. The lava bodies commonly Most of these are illustrated by Holfert (1978) and are underlain or bordered by various types of pyro­ Ream (1979). ~lastic deposits. These range from,near-vent explosion Topaz and associated minerals are not present in breccias and