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Topaz Rhyolites-Distribution, Origin, and Significance for Exploration

Topaz Rhyolites-Distribution, Origin, and Significance for Exploration

Economic Vol. 77, 1982, pp. 1818-1836

Topaz -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 rhyolites are .,.rich alkaline silicic and shallow intrusives that are char­

acterized by the presence of topaz (AI2Si04F 2) in gas cavities, commonly associated with Mn­ Fe , , , specularite, , and other . In the western , Cenozoic (O.5--50m.y. .old for dated examples) topaz rhyolites occur on both sides of the Plateau (in Colorado, New , Arizona, Utah~ and Nevada) and in 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.) to the term rare 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) . Their petro­ genesis presumably involves partial melting of Precalnbrian continental (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). 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 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 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 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 ( 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) and - districts, (2) the central and eastern belts of Kerr (1946), and (3) topaz-rich 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 , porphyry, and deposits of Mo, W, Sn, and other· elements, (2) base and veins and fluorite-rich replacements, and (3) more deeply sea~ed rare metal .

Introduction occurrence in a 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 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 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, 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, , , 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 in I\1ongolia greisen deposits of molybdenum, tungsten, , 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. -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 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 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 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 ?). Other common minerals are colorless far from their vent areas. crusts of , tabq.lar to prismatic, to red Fluorine-rich extrusions (domes or flows) typically hexagons of , Golorless to purple fluorite, and have black basal vitrophres that grad~ upward into red, stubby to platy (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 and cross-stratified surge deposits (Sheridan all gas cavities or in all portions of a topaz rhyolite and Updike, 1975) to minor, more distant plinian lava flow or dome. Nevertheless, if topaz is there, 15 pumice fall deposits, and nonwelded to partly welded to 30 minutes of diligent searching with a hand lens ash-flow deposits (cf. Sheridan, 1979; Self et aI., 1980). will generally reveal it. The well-sorted, layered, and even crossbedded na­ Fluorine contents of vitrophyres (porphyritic ob- ture of some of these pyroclastic units, particularly 1820 BURT, SHEnlDAN, B~KUN, AND CrIR~STlANSEN

the surge deposits, has leq to their misclassific~tion Sheep Cree~ Range) Nevada (Fries, 1942; Stewart e~' ~s water-Iai~ tuffs (especially at Spor Mountain, aI., 1977), ' , Utah). This usage is incorrect and should be dropped. Th~ small rat~o ~f pyroclastic products to lavas for Eruptions of topaz rhyolite lavas com'monly are most t,opaz rhyolites suggest~ 10 us that 'these magmas structurally controlled by earlier faults and fractlJres. ar~ relativ~ly dry and norma!ly do not erupt explo~ On a local scale, as In the Spor Mountain dis~rict, sively, ~nl~ss assisted ,by interactions with grounq Vtah (Lindsey, 1977, 1982), vents occur al9ng faults w~ter (phreatomagmatic explosions'; ~ee Sheridan ~nd allegedly related to the collapse of earli~r, unrel~ted Wohletz; 1981), the local accumulation of volatil~s, calderas. On' a more regional ~cale" they can occur or other special circumstances. An 01igoc~IierhyolItic along linear belts, such ~s the east-west~trend­ devitrified wel~ed . with minor topaz is reported i~g Deep Cre~k-Tintic and Wah Wah-Tushar min­ from north 9f Poncha Springs, Colot~do, by Van Al­ eral beltsin ~estern Utah ,(Hilpert and Rob~rts, i964; stine (1974, p,' 6). All other topaz occurrences ~is­ Stewart et al. 1977; '~owley et aI., 1978), On a still c~ssed b~low are pre~e~t ~n holocrystall~ne la v~s.,: ' . larger scale, they are commonly associated with rifts an~ ~ge (~~e following section). ' , pistribution Eruptive volumes of tqp~z rhyolites from si~gle Th~ di~tribution of ide~tified topaz rhyoli~es in the vents are g~nerally sm~ll(less than 10 km~; see Table ~estern'United States could most easily be desc:ribed 1), An exc~ptional ~ase is the l'op~z Mountain Rhy­ as surroundingthe Colorado Plateau (Fig, l)~Occ~r­ olite, T'homa's }la1?ge, Utah, that has an estimated rences in' New Mexico anq C91orado lie in or ne~r 'the minimum vol~me ~f 50 km3 (Turley and Nash, 1980) Rio Grande rift, and occurrences in' Arizona, Utah, that erupted from at least 12 separate vents (Lindsey, Nevada, ~nd' Idaho gen~rally he'in the 'eastern part 1979), l'wq areas of thick topaz rhyqlite lavas with of the Basin and Range province. Otherwise s~rp.ilar cassiterite mineralization a~e multiple vent fiefds with yo~ng, high-silica, alkaline rhyolites in the 'a~sin and higher eruptive volumes, 130 km3 for the Taylor Range province apparently lack topaz. Capsu'le'de­ {Creek Rhyolite, Black Range, New Mexico (Rho~es. scriptions of 22 occurrences of topaz rhyolite'lavas 1976), and pO km2 for th~ Izenh~od Ranch rhyolite, are given ~n l'able 1; we have been ~~able to locate

TABLE 1. Loc~Uons and Descriptions of Topaz Rhy()l~te ~avas from the Western United States'

Location Description ~ Heferences

1. Specimen Mountain. 'Lithophysal rhyolite flow with underlying) partly silicified. tuff; Conn. 1939; Wahlstrom, 1941, north-central Colorado overlies 'Precambrian ) granites) and pegmatites; , 19~4; Corbett, '1966," 2 ~4 km ) contemporaneous with basaltic and andesitic magmatism; 27 to 28 m.y. (?). . " '

2. Chalk Mountain. central Topaz-bearing rhyolite porphyry; Pennsylvanian ; Cross. 1884; , ~939 C~iorado 2 (4 k'm ) sanidine. quartz. ; Si02 = >74%; adjacent t~ porphyry Mo deposit at Cli~ax~ mid-Tertiary. '

3. Nathrop. central CQlora~o Dome an~ flow complex overlying partially argillized tuff; Cross, 1886; Epis and Chapin, overlies Precambrian and quartz monzonite; sanidinc. 1968; Van Alstine. 1969; quartz. plagioclase) biotite; Fe-Mn ~xid~s; topaz) garnet. Zielinski et ~l.! 1977 sanidine) quartz. Fe ) ', ; on ~argin of Thirtynine Mile volcanic field; calc-alkalic (40-34 and 19 m.y.); Si02 = >76%. F = '0.17-0.55%> U = 16 ppm, Th = 34 ppm, Be = 10 ppm, Li = 95 ppm, Nb =, 83 ppm. Mo = 5 ppm; 28 to 29 m.y. '

4. Tomichi Dome. central Dome and flow overlying tuff, small sill intrudes country rocks; Stark, 1934; Stark and l3ehre, ' 2 Color~d~ (6 km ) , Precambrian gnei'ss ov~rlain by Cretaceous sediments; 1936, Sanidine, pl~gioclase, quartz, biotite (, hornblende, sphene, Fe-Ti oxides); topaz, ga~nct, ma'g'~etite, "

5. Rosita I Iilis and Silver Domes with flows overlying tuff, erupted along of Cross> 1896; Siems, 1968; Cliff) southcentr~l small subsidence feature; rrec~rrtbrian gneisses, granites, and Sharp> 1978; Phair and Colorado older Tertiary volcanic rocks; sanidine,'quartz plagioclase ' J~nkins, 1975 . (""30 ~m2 each) «10%); topaz, garnet, quart~; contemporaneous with calc­ alkalic volcani~s in San )uan MountaIns> emplaced with

trachyte and andesjte; Si02 = >73%; Ag, Au, Ph. Zn, and Cu mineralizati~)(l in fissure veins and along fflults; 26 m.y. TOPAZ RHYOLITES 182i

TABLE l-(Continued)

Location Description References

6. Lake City, southeastern Plugs, dikes, sills, and laccoliths near margin of 28-in.y. caldera; Steven et aI., 1977; Lipman et Colorado Oligocene ash-flow tuffs of San Juan volcanic field; quartz, aI., 1978a sanidine, biotite; topaz, fluorite, quartz; contemporaneous with

basaltic (57% Si02) volcanism; Si02 = >76%, F. = 0.13%, U = 40 ppm, Th = 63 ppm, Be = 20 ppm, Nb = 80 ppm, Mo = 15 ppm; secondary U minerals occur on fractures; minor U removed; 18.5 m.y.

7. Grants Ridge, west-central Dome overylng tuff; emplaced through Cretaceous sediments; Kerr and Wilcox, 1963; New Mexico quartz,sanidine, plagioclase «10%); topaz, garnet (beryl); Bassett, 1963; Thaden et al., Contemporaneous with basaltic volcanism, part of Mt. Taylor 1967; Lipman and Mehnert, volcanic field; F = 0.5%, Rb = 680 ppm, Li = 180 ppm, Be 1980 = 19 ppm, Sn = 16 ppm; 3.3 m.y.

8. Black Range, southwestern boines occasionally with underlying tuffs; em pla~ed with oider Fties, i940; Fries et aI., 1942; New Mexico rhyolitic lavas and tuffs; sanidine, quartz, biotite, amphibolite, Ericksen et aI., 1970; S (f""o.J130 km ) plagioclase (25%); topaz, bixbyite, pseudobrookite; cassiterite, Ericksen and wedow,.i976; garnet, red berylj contemporaneous with basaltic andesite Elston et aI., 1976; Lufkin, lavas; SiOz = >75%, F = 0.38%, U = 13 ppm, Th = 32 ppm, 1972, 1976, 1977; Correa, Be = 12 ppm, Li = 50 ppm, Sn = 25 ppm; Sn (cassiterite and 1~80, 1981; Kimbler and wood tin) mineralization in lava; 24 m.y. Haynes, 1980

9. Saddle Mountain, RhY9lite with topaz, pseudobrookite, bixbyite, and garnet in Anthony et aI., 1977, p. 190 southeastern Arizona lithophysae.

10. Burro Creek, west-central Domes, plugs, and flows, with underlying tuffs; emplaced Burt et aI., 1981; Moyer, 1982 Arizona through Precambrian gneisses, granites, and pegmatites; sanidine, quartz, plagiocl~se (10%); topaz, garnet, quartz; F , = 0.2%, Rb = 630 ppm, Li = 195 ppm, Be = 8 ppm, Sn = 12 ppm; contemporaneous (?) with b~saltic volcanism.

11. Wah Wah Mountains, Domes and lava flows with underlying tuffs; plugs and dikes; Lindsey and Osmonson, 1978; southwestern Utah emplaced through Oligocerie volcanic rocks and locally Rowley et aI., 1978; Ream, 2 (f""o.J75 km ) through Paleozoic ; sanidine, quartz, plagioclase, Fe­ 1979; Keith, 1980; Best et Ti oxides ± biotite ± hornblende ± garnet (2-20%); topaz, aI., unpub. ms.; garnet, beryl, fluorite, Fe oxides, quartz, sanidine, opal; , Christiansen, 1980 trachyandesitel~vas contemporaneous with (60% Si02);' Si02 = >76%, F = 0.5%, U = 15-21 ,ppm, Th = 50-65 ppm', Be = 9 ppm, Li = 65 ppm, Nb = IpO-150 ppm, Me) = 10-70 ppm, Sn = 12 ppn:J.; U and F mineralization along margins of intrusions and iri altered tuffs, possibly associated with Mo mineralization; 22~13 m.y. and 12 m.y:

12. Mineral Mountains, Domes with associated tuffs; emplaced through Tertiary pluton \ Nash, 1976; Evans and Nash, southwestern Utah of granitic composition; Sanidine, quart~, plagioclase, biotite, 1978; Ward et aI., 1978; 2 (f""o.JI0 km ) Fe-Ti oxides (allanite, sphene, apatite) (3-15%); topaz, Lipman et aI., 1978b pseudobrookite, ; contemporaneous with basaltic volcanism; Si02 = >75%, F = 0.15-0.44%, Th = 10-40 ppm, Nb = 5-35 ppm; 0.5 m.y.

13. Smelter Knolls west-central Rhyolite domes and lava flows; emplaced through Quaternary Turley and Nash, 1980 S Utah (2.2 km ) alluvium and older volcanic rocks; sanidine, quartz, pl~gioclase, biotite, I:e-Ti oxides (IQ':'~O%); topaz, hematite;

contemponiheous with (48% Si02) and basaltic andesite (57% Si02); Si02 = >74%, F = 0.45-0.78%, U = 9-15 ppm, Th = 55 ppm, Be = 13 ppm, Li = 120 ppm, Nb = 50 ppm, Mo = 2-6 ppm; 3.4 m.y.

14. Keg Mountain, west­ Rhyolite domes and lava flows with underlying breccias and Erickson, 1963; Lindsey, 1975; 3 central Utah (20 km ) tuffs; emplaced through alluvium and Oligocene volcanic Lindsey et aI., 1975 rocks; sanidine, quartz, plagioclase, biotite ± hornblende; topaz (?), hematite; approximately contemporaneous with basaltic lavas in the region; Be = 5 ppm, Nb = 50 ppm, Rb = 340 ppm, Sr = 32 ppm, Y = 48 ppm, Zr = 160 ppm; 8 m.y. 1822 BURT, SHERIDAN, BIKUN1 AND Cl-IRr'STIANSEN

~ABLI': l-(Continued)

Location Description References

15. Thomas Range wcst­ Rhyolite domes and lava flows with underlying breccias and Cross, 1886; Pellfield and 3 central Utah (50 km ) tuffs; emplaced through alluvium and oJder tertiary volcanic Foote, 1897; Staatz and rocks; sanidine, quartz, plagioclase; Fe-Ti oxides ± biotite Carr, 1964; Lindsey, 1977, ± honlblende ± garnet (10-20%); topaz, garnet, fluorite, 1979, 1981, 1982; Turley bixbyite, pseudobrookite, bQryl, Fe oxides, opal; Si02 = >72%, and Nash, 1980; Hearn, 'F = 0.2-0.6%, U = 5-25 ppm, Th = 25-60 ppm, Be = 2-11 1979; Bikun, 1980 ppm, Li = 33 ppm, Nb = 50-100 ppm, Sri = 2 ppm; 6. m,'y.

16, Honeycomb I-lllIs, west­ 'Domes with underlying tuff; emplaced through alluvium and Montoya et al.~ 1964; 3 central Utah (1.5 km ) basaltic lavas; sanidine, quartz, plagioclase, biotite, Fe-Ti McAnulty and Levinson, oxides (40%); topaz, fluorite; Si02 = >75% (64%), F = 1.0% , 1964; Lindsey, 1977; Turley (8,0%), U = 17 ppm (150 ppm), Th = 30 ppm, Be = 11 ppm and Nash, 1980; (13 ppm),Li = 130 ppln (200 p~m),Nb = 50 ppm (80 ppm), Christiansen et al., 1980 Mo = 3 ppm (25 ppm) (concentrations in F-rich sample in parentheses); minor Be, Cs, and Rb mineralization in tuff; behoite ({:l-Be(OH)2) identified; 4.7 m.y.

17. Spor Mountain, west­ Lava flow with underlying altered tliff; ; emplaced through Stantz and Griffitts, 1961; 2 central Utah (5 kin ) Paleozoic ; locally intrudes Ollgocene rhyodacite; Staatz, 1963; Staatz and sanidine, quartz, plagioclase, bioti,te, J.'e..Ti oxides (apatite, Carr, 1964; Sha~e,. 1968; zirco.n) (40%); topaz, quartz, sanidine; contemporaneous Lindsey, 1977, 1979, 1981, volcanics lacking in immediate vicinity; same age as 1982; Bikun~ 1980; trachyandesites in Wah Wah Mountains; Si02 = >73%, F Christiansen, '1981 == 0.8-1.5%, U = 37 ppm, Th = 66 ppm; Be == 6~ ppm, Li = 90 ppm, Nb = 150 ppm, Sn = 30 ppm; ~e, U, F mineraHzation in altered tuff beneath lava; 21 m.y.

18. Buckhorn·area, Cortez Plugs and lava flows associated with tuff; emplaced through Gillulyand Masursky, 1965; Mountains, north-central Ordovician sediments; sanidine, quartz, plagioclase ± bIotite Wells et al., 1971; Rye et Nevada (3%); topaz, quartz fluorite; contemporaneous with diabase I al., 1974 and basaltic andesite (55% Si02); Be = 10 ppm, Sn = 20 ppm, Y = 70 ppm, Pb = 70 ppm; associated with rninor AU, Ag, Hg mineralization.

19. Izenhood Ranch, Sheep Domes and lava flows; intrudes 15 m.y. andesite; sanidine, Fries, 1942; Stewart et al., Creek ~ange, north­ quartz, plagioclase, Fe-Ti oxides (25%); topaz, garnet; nearly 1977; Christiansen et al., 2 central Nevada (50 km ) contempora~eouswith basaltic andesite (59% Si02) and 1980 basaltic (48% Si(2) lavas; 8i02 = >76%, F = 0.3%, U == 12 ppm (devitrified sample); cassiterite (wood tin) in fractures; 14 m.y.

20. Jarbidge, northern Nevada Domes and lava flows with underlying tuffs; emplaced through Coats et al., 1977 older volcanic rocks; sanidine, quartz, plagioclase ± biotite ± hornblende ± ; topaz (in asingle occurrence), garnet, fluorite; F = 500 ppm; 16 m.y.

21. Elkhorn Mountains, Plugs, dikes, and lava flows ove~lying tuffs; emplaced through Smedes, 1966; Chadwick, western Montana Butte quartz monzonite; sanidine, quartz, plagioclase., Fc-Ti i977; Greenwood et aI., oxides; topaz, quartz, fluorite; approximately contemporaneous 1978

with basaltic lavas and rhyolitic ash ,flows; Si02 = >75%, F = 0.2-0.5%, Be = 1-20 ppm, Nb = gO-100 ppm, Mo = 5-10 ppm, Sn = 10-50 ppm; Ag, Pb, Au mineralization locally within lavas and tuffs; 36 m.y.

22. Mountain, Little Plug (bysmalith); penetrates Cambrian sediments (, Pirsson, 1900; Witkind, 1973 Belt Mountain, central , shale, ); quartz, sanidlne, plagioclase, biotite, Fe-. Montana oxides; topaz; approximately contemporaneous with granite,

, , and shonkinit~; Si02 = >75%; Big Ben Mo deposit nearby; 50 m.y.

1 The descriptive information is ordered as follows: mode of-emplacement or structure; country rocks; phenocryst minerals (accessories) (vol. %); vapor-phase rriiherals that occur in cavities, in the groundmass, or along fractures; contemporaneous magmatism in the immediate vicinity; chemistry of vitrophyre (unless otherwise indicated); associated mineralization; age. Beneath each locality the approximate area 2 3 (km ) or volume (km ) of each occurrence is given if obtainable TOPAZ RHYOLITES 1823 descriptions of the other points plotted by Shawe (1976). -- . .------The distribution of topaz rhyolites roughly coin­ cides with the distribution of abundant fluorite de­ '22 posits and occurrences (Worl et aI., 1974; Shawe, 21

1976; Van Alstine, 1976; Van Alstine and Tooker, ...... ,., "', 1979). As mentioned, it also coincides with the dis­ 0.,' " tribution of Climax-type, Tertiary, topaz-bearing, I I high-grade porphyry molybdenum deposits in Colo­ I rado, New Mexico, Montana, Idaho, Nevada, and '/ ,2~, / Utah (Shawe, 1976; Woodcock and Hollister, 1978; ;19 '1 / '18 Keith, 1979, 1980; Mutschler et aI., 1981; White et 16.1.4••15 aI., 1981; Westra and Keith, 1981). These are the I : 17'13 '2 / .4 granite-type deposits defined by Mutschler et ai. ,',.I 3 • 5 (1981). They have also been called porphyry greisen r, . • 6 deposits (Burt, 1981), in order to emphasize the I ", \ unique characteristics of their alteration (rich in to­ \ \ ·7 paz, fluorite, and fluorine-rich ). In this regard, '10 topaz-bearing cavities in rhyolite might be considered as one-atmosphere greisen. .8 • 9 Not surprisingly, the distribution of high fluorine contents in silicic volcanic rocks (Coats et aI., 1963; Shawe, 1976, p. 15-16) also roughly coincides with that of topaz rhyolites. More of a surprise to us was the close spatial coincidence between topaz rhyolites and the central and eastern tungsten belts mapped FIG. 1. Identified occurrences of topaz rhyolite lavas, western United States. Points identified by number refer to Table 1; the 35 years ago by Kerr (1946). As noted by Kerr (1946), other points are from Shawe (1976, p, 20). Lines show the limits most of these occurrences are relatively young (Ter­ of the ancient Precambrian continental crust. Solid line = King tiary), consist of a high proportion of vein-type wol­ (1977, known outcrop limit); dashed line = Sears and Price (1978, framite deposits, and are related to small, near-sur­ inferred). face granitic plutons. These features contrast with those of the western belt of Sierran skarn and vein­ spread might indicate continued tapping of an ex­ type deposits related to deep-seated, batho­ tremely large evolving magma chamber or, more lithic, calc-alkaline intrusive rocks of Mesozoic age. probably, an episodic or continuous process of magma The significance of instead of scheelite as generation in a restricted area. an indicator of high fluorine activities was earlier Topaz rhyolites of apparently middle Tertiary age pointed out by Burt (1978, 1981). (approximately 30 m.y.; S. E. Kesler, pers. commun., Available radiometric ~ges of topaz rhyolites range 1981) are widely distributed in Durango, Zacatecas, from nearly 50 m.y. in the Little Belt Mountains, San Luis Potosi, and other states of Mexico (Foshag Montana (Witkind, 1973), to only 0.5 m.y. in the and Fries, 1942; Sinkankas, 1959, 1976). Occurrences Mineral Mountains, Utah (Evan and Nash, 1978; Lip­ that we have been able to locate on a map are shown man et aI., 1978). In the Rio Grande rift, the oldest in Figure 2. These and other occurrences are spatially, published age is 28 to 29 m.y. for the rhyolite at and presumably genetically, related to fluorite de­ Nathrop, Colorado (Van Alstine, 1969; F. E. Mut­ posits and to volcanogenic cassiterite deposits of the schler has provided us with an unpublished age of 38 Mexican type (Foshag and Fries, 1942; Ypma and m.y. for Tomichi Dome); many other rhyolites along Simons, 1969; Pan, 1974; Sillitoe, 1977), but. we have the rift are almost as old, but the age of the rhyolite been unable to find precise data on their radiometric at East Grants Ridge, New Mexico, is only 3.3 m.y. ages, geochemistry, or other features. (Lipman and Mehnert, 1980). One might seek topaz rhyolites in other fluorine­ In a relatively restricted area of west-central Utah, rich volcanic provinces, such as the British Tertiary ages range from 3.4 m.y. at Smelter Knolls (Turley province (cf. Meighan, 1979). They may once have and Nash, 1980), 4.7 m.y. at the Honeycomb Hills overlain fluorine-rich granites, as in , where (Turley and Nash, 1980), 6 to 7 m.y. in the Thomas a topaz-bearing tuffisite (intrusive tuff) has been de­ Range (Lindsey, 1979),8 m.y. in the Keg Mountains scribed (Wright, 1974), or in Missouri (Kisvarsanyi, (Lindsey et aI., 1975), and 21 m.y. at Spor Mountain 1981), or South China (Tu et aI., 1980, p. 191). Oc­ (Lindsey, 1979). This nearly 20-million-year age currences of ongonite in the Soviet Union and Mon- 1824 BURT, SHERIDAN, BIKUN, AND CHRIS1'IANSEN

has only been reported in topaz rhyolites from Mexico (Ypma and Simons, 1969), where it does not occur with topaz and is commonly altered to biotite. I-ligh fluorine and potassium activities presumably make fayalite unstable with respect to fluorine-rich biotite in other topaz rhyolites. Most topaz rhyolites, except the crystal-rich vari­ eties, are strongly flow banded. l'he matrix of this flow-banded rhyolite consists of a ·feIty iritergrowth of alkali , quartz arid" in many cases, acicular topaz. Biotite is rare, but garnet is commonly present as a vapor phase mineral in cavities. Spherulitic and lithophysal textures are common, especially along flow bands, and may be the result of local degassing of the rhyolite due to the thermal feedback mecha­ FIG. 2. Identified occurrences of topaz rhyolite lavas in Mexico nism described by Nelson (1981). (compiled from Foshag and Fries, 1942; Sinkankas, 1959, 1976). Smaller dots are tin rhyolites. 1 = America, Durango; 2 = Cerro Granophyric (equigranular) textures are mOre de los Henidios, Durango; 3 = Fresnillo, Zacatecas; 4 = Pinos, Za­ common in the phenocryst-rich, less flow-banded va­ catecas; 5 = Tepetates, San Luis Potosi; 6 = Guadalcazar, San Luis rieties of rhyolite, as at Spor Mountain and the Potosi; 7 = Cerritos, San Luis Potosi; 8 = Lourdes, San Luis Potosi; I-Ioneycomb I-lills, Utah. ~F'luorine-rich biotite and to­ 9 = Hacienda Sauceda, Guanajuato; 10 = San Felipe, Guanajuato; paz are common in the groundmass of these rhyolites, 11 = Tlachiquera, Guanajuato; '12 = Leon, Guanajuato; 13 = Te­ puxtepec, Guanajuato; 14 = Apulco, Hidalgo. and garnet is lacking. . Miarolitic cavities with abundant topaz are con­ centrated around large inclusions of mafic lavain the golia (Kovalenko' and Kovalenko, 1976, 1979) have topaz rhyolite of Spor Mountain. We speculate that already been mentioned; data examples are Mesozoic this text~re was caused by the quenching of hotter or older. mafic magma that mixed with the rhyolitic magma A Precambrian (1.7 b.y. old) metarhyolite (appar­ body very shortly before its eruption (Christiansen et ently a tuff) with topaz is present near beryllium vein aI., 1981). The miarolitic nature of the lava near the deposits (Meeves, 1966, p. 16-25) in the rronto Basin, mafic inclusions could be due to local heating and central Arizona (Conway, 1976, p. 319). A similar enhanced devolatilization. occurrence of anomalously radioactive topaz-bearing More details on mineralogy and petrology are rhyolite lavas and tuffs· has been found in the Kee­ given by Christiansen et aI., (1980), Bikun (1980), watin District, Northwest Territories, Canada (Le­ Christiansen (1981), and Turley and Nash (1980). Cheminant et ai. 1981, p.123). Obviously, such meta­ morphosed occurrences are much more' difficult to Geochemistry recognize than those in young volcanic rocks, so that Major element analyses of topaz rhyolites (sum­ they might be more common than the two known marized in Christiansen et aI., 1980, and Christiansen; occurrences would indicate. 1981) show them to be uniformly high in silica (gen­ erally >75%), NazO (3.5-4%), KzO/NazO (>1%), and Mineralogy and Petrography fluorine (0.1-1%) and low in TiOz «0.2%), CaO Miarolitic and lithophysal cavities in topaz rhyolites «0.9%), MgO «0.2%), and PZ0 5 «0.01%) r~lative contain varying amounts of topaz, quartz, sanidine, to most other rhyolites. These characteristics (except garnet, bixbyite, specularite, pseudobrookite, cassit­ the higher F content) are typical of rhyolites: ftom erite, beryl, and late fluorite and opal. The miner- . other bimodal (basalt-rhyolite) associations (Evans, alogy of the rest of the rock is much less·varied. Phe­ 1978; Ewart, 1979; Creecraft et aI.,1981). Normative nocrysts are :dominantly sanidine (about 01'50)' em­ corundurn (especially if fluorine is assigned to CaFz:: bayed quartz, and sodic plagioclase. Rarely these may Bikun, 1980) and the presence of vapor phase topaz constitute up to 40 percent of the rock, but normally and (commonly) garnet suggest that the. magmas were. they are only a few percent by volume. -rich metaluminous to slightly peraluminous. They defi­ hornblende, clinopyroxene, and garnet are unusual nitely were not peralkaline, as shown by the analyses as phenocrysts, but biotite is common in the more and by the lack of Na-Fe minerals such as aegirine phenocryst-rich lavas. rrrace minerals may include and riebeckite (minor aegirine is locally present in ilmenite, titanomagnetite, apatite, fluorite, titanite, intrusive ongonite at the type locality: Kovalenko and , and allanite. Fay~lite is common in other bi­ Kovalenko, 1976). Rhyolites with high F concentra­ modal rhyolites (Christiansen and Lipman, 1972) but tions (~1%) have slightly lower SiOz and KzO with TOPAZ RHYOLITES 1825

.------e _ 100 -- :-e..~....,..,_,_,_~~__ Spor Mtn.,UT ...... ~ ~ --.------... '~ --. ....•... Black Range,NM -...... •.\' / ..

10 Nathrop,CO . ".

0.1 La Ce Nd Sm Eu Tb Vb Lu

FIG. 3. Chondrite-normalized rare earth element pattern for topaz rhyolite from Spor Mountain, Utah, compared to other topaz rhyolites from the western United States (from Christiansen, 1981, p. 203). higher Al20 s and Na20 as a result of the effect of F (Badha~, 1982). Bimodal high silica rhyolites from on residual melts in the granite system (Manning, the Coso Range and Kern Plateau, (Bacon 1981). et aI., 1981; Bacon and Duffield, 1981), and Twin Trace element analyses (again summarized in Peaks, Utah (Creecraft et aI., 1981), have similar rare Christiansen et ai. , 1980; Christiansen, 1981) yield earth element patterns, however, but are lower in F unusually high values for Cs, Rb, Li, U, Th, Nb, Ta, and contain no reported topaz. . Sn, W, Mo, and Be. Most of these values show a pos­ Flat rare earth patterns with large negative Eu itive correlation with F content and with each other. anomalies also characterize fertile granttes believed Ratios such as K/Rb, Zr/Hf, Nb/Ta, and La/Yb sys­ to be parental tg rare metal pegmatites (Cerny, 1982; tematically decrease with increasing F content, an Trueman and Cerny, 1982). These in other respects observation based mainly on Soviet work on ongonites (high Si02, K, and Na; low Ca, Mg, Sr, and K/Rb, (Kovalenko and Kovalenko, 1976; Antipin et aI., etc.) are also similar to topaz rhyolites, as are the 1980). The high content of the above elements in Yenshanian (Mesozoic) granites parallel to W -Sn these and other rhyolites leads to the term "rare metal greisen vein deposits in South China (Tu et aI., 1980; rhyolites" (R. Wilson, pers. commun., 1980) and has also unpublished data obtained by D. M. Burt during important implications for prospecting. a visit to China in 1981). Strong depletions in Ba, Sr, Eu and, to a lesser Several vitrophyres have also been analyzed for extent, in light rare earth elements and Zr are also chlorine. Christiansen et ai. (1980) report values of characteristic of topaz rhyolites, especially those rich­ 700 to 1,700 ppm for samples from Utah, which are est in fluorine and rare . Typical rare earth much lower than those for fluoriI).e in the same sam­ patterns'such as those given in Christiansen et ai. ples. Vitrophyre analyses of CI by others (Turley and (1980) and Christiansen (1981) are exceptionally flat Nash, 1980; Moyer, 1982) range from less than 200 (low La/Yb ratios) with pronounced negative Eu to more than 900 ppm CI, again, much less than cor­ anomalies (Fig. 3). responding F values. These values are much lower Similar rare earth patterns have been reported than chlorine values (2,000-6,000 ppm) in most per­ from bimodal rhyolites associated with volcanogenic alkaline silicic glasses (Carmichael et aI., 1974). Per­ massive deposits (Campbell et aI., 1981). These alkaline rhyolites may be enriched in fluorine and deposits commonly contain tin (Mulligan, 1975, p.­ rare metals (e.g., Cepeda et aI., 1981), but the F /CI 70-71) and may conceivably be back-arc submarine ratio remains low, and they otherwise appear distinct equivalents of the subaerial deposits discussed here from the topaz rhyolites discussed here. Nevertheless, 1826 BURT, SHERIDAN, BIKUN, AND CHRISTIANSEN

I using the Nigerian younger granite province as an mafic magmas (Meighan;, 1979; Higgins, 1981; intrusive analogue '(where,'peraluminous-metalumi­ ·Thompson, 1982). 'These arguments do not easily ex­ nous and peralkalinegranites both occur: Jacobsen plain the apparent restriction of topaz rhyolites to et al.) 1958; Bowden and Jones);1978; Martin and contiJ:1ent~linteriors'(e?,cept for an alleged occurrence , Bowden, 1981), the two rock types ,might well be in Iceland: S.Ludington, perS. commuIl., 1980) and .associated. Similar as~ociations £ronl."Tertiary anorq­ their clearly'bimodal associations. Fractional melting genic volcanic.'suites in eastern ,have .been of a phlogopite.;.rich,'deep-subducted slab of oceanic described by ~wart (1981), but no topaz isreported crust (Westra and Keith, 1981) could r'estrict the.rhy­ in the K-rich high silic'a rhyolites. olites to the continental interior, b,ut it is difficult to Crystalline, rhyolites contain, in" general,,lower flu­ see how the melting or differenti~tionproduct would orine and some lithophile element contents t}lanvi­ be a high-silicarhyolite, or why topaz rhyolit~s'would trophyres (Zielinski, et aI., 1977)." This conclusion is be lacking in island. 'arcs. On balance, then, we shall also valid for topaz rhyolites (Steven et al., 1~77; continue to assume that the continental crust is the Bikun, 1980) and is, especially true for the, grano­ major source of magma and ,associated lithophile ele­ phyrically crystallized"phenocryst-rich varieties such ments, although we still canl?otexclude some of the ,as' those from 'Spar Mountain and the Honeycomb other possibilities. _ Hills, Utah. 'Vitrophyres from these two occurrences A second feature is that topaz rhyolites are common' contain' the highest fluorine and fluorophile trace ele­ in areas of regional extensiqn and" crustal thinning ment contents measured ·by us, and the crystalline (that is, the Basin and Range.province in the western rhyolites have compositions that aregenerally,lower United States). The initialcause of this extension was inU) Be, Sn, and F byapout20percent. Other trace probably back-arc spr,eading over a subducting slab eletnents,with the ,possible exception ofCs, are not that was also responsible.for calc-alkaline volcanism greatly affected. by devitrification. The conclusion is (Karig, 1971; Scholtz et aI., 1971; Thompson and , that considerable quantities of fluor-ineand the ore Burke, 1974; Stewart) 1978; Eaton, 1979). A concur­ elements cap be released during high .temperature rent increase in the angle of dip of the subduction . devitrification .and' crystallization" (in .addition to \ z()ne hasbeen suggested (Coney apd Reynolds, 1977; .

'whatever.quantities are released during',the initial Keith 1978; Lipma'h, 1981). " .,,' :'" "1' .eruption itself). The possible relations of these pro'­ These areas of extension are areas of increased seiSi cesses to, mineralization are discussed below. micity, high' regional heat flow , and abundant,,:l1Ia1;l:;' ,Isotope data for topaz rhyolites remain meager. tie-derived basaltic volcanism. (Smith, 1978;'131a,ck­ 'Initial 87Srj86Sr ratios range from 0.7120 {Wah Wah well, 1978). Basaltic magma (or earlier calc-alkaline Mountains, Utah; Christiansen, '198l} to 0.7054 (Lake .magma) 'could be the heat source'for fractionalrrielt~ City) Colorado; Liprnan etaL,:1978). Bqwman et al. ing of continental crust and the formation of the (1982) report 0180 values of 6.3 to, 6.9 per mil for rhyolitic magmas (Christiansen and Lipman, 1972; rhyolite from the Mineral·Mountains"Utah. Christiansen, ,1981; Hildreth 1981). Interestingly" in THese 'data do not demand but ,are,.suggestive. of, art west-central Utah, where the 21-m.y.-old SporMoun~ prigin of the, magmas frorn the IQwer continental tain ,.and numerous". younger topaz ,rhyolites occur crust. (discussion above), the crust is presently thinner and uppermost mantle seismic· velocities are lower'than Origin elsewhere in the eastern Basin and Range province Several features of the distribution of topaz rhyo­ (Prodehl~ 1979,' p.41).Of course, thecrus~ rnustha-ve lites relate to their probable origin. Afirs't feature been thicker21m.y. ago. " , (Christiansen et aI., 1980) is that U .. S. occurrences The formation 'of a topaz rhyolite magma preSQm­ are, restricted to areas underlain by continental crust ably begins with a small degree of partial meltipg,p£ ,of"Precambrian age (that is, the eastern half of the lower crustal rocks· of the Precambrian cra'ton~" rbe :··Basin and Range province; Fig. 1). The Precambrian presenc,e of 'a high heat flow would tend to expel , 'crust is therefore' a reasonable soUrce for the lluorine water, from .the solids, enriching' residual 'fluorine and lithophile elements' typically enriched in topaz 'prior to Imelting (Holloway, 1977;" cf.review by . rhyolites. However, jt should be, noted, 'that most .bi­ Bailey, 1977). Enrichmentinfluorine and lithophile modal rhyolites from this area?f the ,Basin and',Range elements should be enhanced if the rock that is being provinpe are not suffiqiently enriched in fluorine to melted has already undergqneat Jeast.oQe episode of ' contain topaz. anatexis, expelling a ,water~rich melt., (How

amphibole, minerals enriched in fluorine and litho­ 6 7 phile elements. Our model for topaz rhyolite petrogenesis is then ideritical to that of residual, or l:t-type, granites (White~ 1979), which also are called anorogenic, or A-type, granites (Martin and Piwinskii, 1972; Loiselle and Wones, 1979; Loiselle, M. C., and Wones, D. R., writteri commun., 1981; Wones, 1979; Higgins, 1981). It is also similar to that proposed for Climax-type porphyry molybdenum deposits by White et aI. (1981). Anorogen~c granites are tectonically and geo­ chemically distinct from the more familiar orogenic x x I x and S types (White, 1979). Mineralogic and geo­ x x x chemical features of anorogenic granites and topaz x x x x rhyolites are very similar (Christiansen et aI., 1980, x x x x fig. 13 and table 20). Correspondences include low FIG. 4. Surface and near-surface types of ore deposits possibly fH2 0 ; high fHF/fH20 ; low f02; high Si02; high Na20, associated with topaz rhyolite volcanism. (Nos. 1-3 are speculative; K20, arid K20/Na20; low CaO; high Fe/Fe + Mg; nos. 4-7 are not.) 1 :;::; (Li, W, etc.); 2 = tuffaceous hl~e sed­ and siJ11ilar trace element enrichments and deple­ iments (U, Li,Mn',etc.); 3 = hot spring deposits (W, Mn, Ag, efc.); tions. The occurence of both rock types in bimodal 4 = clastic rocks beneath tuffs (U); 5 = mineralized pyroclastic suites in extensional environments is another similar­ deposits (Be, F, Li, Cs, etc.); 6 = fractured lavas and vent breccias (Sn); 7 = vent and contact breccias (F, U, etc.). For more detail, ity. Nevertheless, it is possible that some anorogenic see Table',2. granites (peralkaline compositions) with low initial Sr isotopic ratios may be derived from extreme frac­ tional crystallization of a basaltic parent (Loiselle and The most important aspect of topaz rhyolite pet­ Wones, 1981; Higgins, 1981). rogenesis is probably the concentration of fluorine and The 1.4-b.y.~old Lawler Peak Granite, near Bag­ lithophile ,elements in a stagnant zone 3;t the top of dad, Arizona (Anderson et aI., 1955; Silver, 1968; Sil­ silicic magma chambers (Kovalenko and Kovalenko, ver et aI.? 1981), is a typical anorogenic granite batho­ 1976~ p. 104; Kovalen~o, 1978, p. 24~). The tendency lith, enriched in U and F, that contains lithophile for tin and associated metals to occur in this envi­ element mineral deposits (wolf~amite-beryl greisen ronment has been recognized since early in this cen­ veins and pegmatites. with beryl and reported topaz tury (Ferguson arid Bateman, 1912; Butler, 1915; and ainblygonite: Anderson et aI., 1955). Gabbro bod­ Emmons, 1933). This feature has generally been as­ ies occur in the vicinity. Within 16 km occurs a small cribed to roofward, streaming and accumulation of middle (?) Tertiary dome complex of topaz anq gar­ vplatiles and complexed metals in a separate vapor net-bearing rhyolite enriched in a similar suite of phase. Recently, the more general process of convec­ elements (Burt et aI., 1981; Moyer, 1982). Basalt flows tion-aided thermogravitatio~aldiffusion has been in­ cover nearby mesas. Tp.e obvious (if not necessarily voked (Shaw et aI., 1976; Hildreth, 1979). This process correct) c,onclusion is that the same. segment of un­ does 'not require the presence of a separate vapor derlying lithosphere was twice melted in an exten­ phase, although contributions by such a phase are not sional environment to produce two bimodal suites of excluded (Hildreth, 1981). The arguments and data magmas of very different ages. Similar arguments in favor of such liquid state differentiation in silicic might apply to Precambrian fluorine-rich anorogenic magma chambers have recently been reviewed by granite batholiths and Tertiary topaz rhyolites in several authors (Hildreth, 1979, 1981; Creecraft et aI., Colorado. 1981; Ludington, 1981). The possible importance of Partial melting is only the first stage of topaz rhy­ liquation (liquid immiscibility) in fluorine-rich melts olite petroge~esis. The later stages are conjectural, but (Kogarko et aI., 1974) has yet to be fully assessed in they may include dehydration and zone refining dur­ this context, although' its role in nature is believed to ing the magma's rise through the crust (Barker et aI., be minor (Kovalenko and Kovalenko, 1976, p. 120; 1975), large degrees of fractional crystallization in a 1979, p. 238). ne~r-surface magma chamber (Groves and Mc­ Mineral Deposits Carthy, 1978, plus num~rous other authors), dehy­ Volcanogenic near-surface deposits dration and rapid partial crystallization during pre­ cursor, explosive volcanism (Rhodes, 1976), and re­ Various types of volcanogenic near-surface deposits fusion due to renewed injections of mafic magma possibly associated with the eruption of topaz rhyolite (Thompson, 1982). tuffs and lavas are schematically indicated in Figure 1828 BURT, SHERIDAN, BIKUN, AND CIIRISTIANSEN

TABLE 2. Types of Deposits Depi~ted in F~gures 4 and 5 .

Number Type ~e~al(s) Examples Reference(s)

1. Playa lake brines Li(W) Silver Peak, Nevada Kunasz, 1975 . (Searles l~ake, Califorriia) Carpenter arid Gatrett, 1959

2. Tuffaceous lake sediments U (Li, Mn) Anderson Minc, Datc Creek Sherborne et aI., 1979 basin, Arizona

3. Hot springs deposits W, Mn, Ag Golconda, Nevad~ Kerr, 1940 4. Clastic rocks beneath tuffs U Yellow Chief Mine, Spar Lindsey, i978 Mountain district, Utah

5. Mineralized pyroclastic 'Be(F, Li, U, CS, etc.) Spot Mountain, Utah Lindsey, 1981 deposits beneath lava Horieycomb Hills, Utah McAnulty and Levinson, flows 1964

6. Fractured lavas arid vent Wood'Sn Black Rangc, New Mexico Lufkin, 1977 breccias DU,rango, Mexico Foshag and Fries, 1942 7. Vent and contact breccias F(U) Spor Mountain, Utah Staatz and Carr, 1964 in carbonate rocks,· Staats Mine, Utah Lindsey and Osrribnson, 1978

8. a. Fluorite-rich skarn Sn, W, Be (Fe) Iron Mountain, New Jahns, 1944 (ribbon rock elr . Mexico D. M. Burt, unpub. data wrigglite) Shizuyuan, Hunnn, China b. Sulfide-rich replacement Sn (Cu, Zn, Pb) Mt. Bischoff; Tasmania, Groves et aI., 1972 bodies Australia

9. Base and precious metal Ag, Au, Pb, Zn, (Sn, W) Rico, Colorado Naeser et aI., ·19~0 veins

10. Mineralized breccia pipes Mo Redwell Basin, Colorado Sharp, 1978 11. Stockwork and porphyry Mo, W, Sn Hendcrson Mine, Colorado Waliace ct al.: 1978 (porphyry grcisen) Pine Grove, Utah Keith, 1980 ... deposits Mt. Pleasant, New Parrish and Tully; '1978' Brunswick, Canada

12. a. Grelsen-bordered veins Sn, W, Be Boomer'Mine, Colorado I-iawley, 1969 Cornwall, England Hosking, 1969 b. Albitized aprogranite Ta, Nb, W, Sn, rare earth Echassiercs, Massif Central, Aubert, 1969 elements

13. Disseminated heavy Nb, Ta, Sn,W"U,Th, rare Jos Plateau, Nigeria Bowden and Jones, 1978 materials in granite e~rth 'elements, Zr, i-lf, (economic in placers etc. only)

14. Rare metal pegmatites Li, Be, Ta~ N~, Cs, Rb, Harding Mine, New Mexico Jahns and Ewing, 1977 ctc. 'Quartz Creek District, Staatz arid Tritoh, 1955· Colorado

4 and listed in Table 2. This list does not consider In Table 2, deposit types 1 (brines), 2 (lake Sedi~' i~dustrial mineral deposits such as pumice or perlite, men~s), and 3 (hot springs) are not definitely knqwn or topaz, garnet, arid red beryl as . or min­ to be associated, wit~ topaz rhyqlite volcanism; they eral specimens. It lik~wise does not consider placer are speculatively included here for the sak~ of com­ ,deposits of, for example, gemst9nes or wood tiri, or pleteness. Deposit types: 4 (uranium. in clastics), 5 the possible recovery of. geothermal energy from (beryllium in tuffs), and 7 (fluorite in. breccias) all young magma systems such as noosevelt I-Iot Springs, occur in the Spor Mountain district, Utah (Lindsey, Utah (Ward et aI., 1978). More speculatively, it does 1977, 1979, 1981; Bikun, 1980; Bullock, 1981), whereas not consider tin-containing volcanogenic massive sul­ type 6 (cassiterite or wood tin in lava) is most typical fide deposits as the possible result of submarine erup­ of the Mexican-type of deposit, (Foshag and Fries, tions, as mentioned above. 1942; Ypma and Simons, 1969; Pan, 1974). TOPAZ RHYOLITES 1829

The beryllium deposits at Spor Mountain, Utah

(type 5), are perhaps the most interesting and enig­ 2 matic from a genetic st~ndpoint. Their formation presumably involves the four stages of (1) emplace­ ment and apical enrichment of a fluorine-rich magma body, (2) explosive eruption of pyroclastic rocks, in­ cluding abundant carbonate fragments from around the vents, (3) passive eruption of fluorine-rich lava, and (4) mineralization, as discussed in detail by Burt and Sheridan (1981). The beryllium ore mineral ber­ trandite is present in altered tuffs just beneath the lava flow and occurs with fine-grained purple fluorite, chalcedony, and Mn-Fe oxides that replace Paleozoic GREI5EN SUB-VOLCANIC VOLCANIC- dolomite rock fragments erupted with the tuff (Lind­ PEGMATITIC "PNEUMATOLYIC" "PORPHRY" SURFICIAL sey et :iI., 1973). Lower tuff horizons, even those rich Early,wet partial melting products Late, dry partial melting products in carbonate fraginent~, are barren. In addition to Be, the zone immediately beneath the lava flow is greatly FIC. 5. Surface and subsurface types of ore deposits associated enriched in F, Li, Mn, Nb, Pb, Sn, U, W, and Zn, but with individ':lal batches of fluorine-rich magma bf varying water not Mo (Bikun, 1980). contents (and consequent depths of emplacement). Key: 1 through '7 same as in Figure 4; 8, fluorite-rich skarn (Sn, w., Be, etc.) and/ Is the source of these elements concentrated at the or sulfide-rich replacement bodies (Sn, eu, Zn, ph, etc.); 9, base top of the tuff (1) material' fractionated or leached and 'precious metal veins (Ag, Pb, Zn, Au, Sn, W, etc.); 10, min­ from the pluton presumably underlying the volcanic eralized breccia pipes (Mo); 11,. stockwork-porphyry deposits (Mo, vei~s vent complex, (2) material leached from the vent W, Sn, etc.); 12, greisen-bordered (Sn, W, Be, etc.) and/or albitized granite (disseminated Ta, Nb, W, Sn, rare earth elements); complex itself by throughgoing fluids that spread out 13, disseminated heavy minerals (weathered to placers of Nb, Ta, beneath the capping'flows; or (3) rriaterial released Sri, W, U, Th, rare earth elements, Zr, Hf, etc.); 14, rare metal by the tuffs anc;l/or the thick overlying lava flows ,on pegmatites (Li, Be, Ta, Rb, Cs, etc.). For more detail, see Table 2. devitri£icatioil-cry~tallization? More than one of these alternatives may have been a source, but only the third alternative accounts for the uniform distribution (1975, 1978). The figq.re depicts deposits formed by of mineralization just beneath the flows (Bikun, 1980). individual small magma batches of differeJ:?t water Nevertheless, the mineralization appears to have contents that crystallize at different depths in the formed at relatively low temperatures (200°C or less: crust (cf. Burnham, 1979). The more water-rich mag~ Burt and Sheridan, 1981), and uraniferous opal, at mas .crystallize at the ,greatest depths. the figut~ is least, formed over many millions of years (Ludwig an oversimplification for natural occurrences, where et aI., 1980). composite batholiths and multiple episodes of intru­ Subsurface deposits sion are the rule rather than the exception. The figure also does. not consider explosive eruptions of large, All of the volcanogenic deposit types depicted in water-rich· silicic magma chambers-only relatively Figure 4 are related extrusive rocks (subaerial). An small magma batches are depicted. Wetter ghosts are obvious variant is the formation of large, bulbous shown under each successive magma type to allow volcanic domes that form very shallow intrusidns for multiple episodes of intrusion. (Williams, 1932; Williams and McBirney, 1979, p. All of the subsurface types of mineral deposits in 118-197) ~ This geometry could account for the up­ Figure 5 are assumed to owe their. genesis primarily ward-spreading cross sections of rhyolite bodies that to devolatilizat~on of the parent magma (Jahns and host porphyry tin and tin-silver vein mineralization Burnham, 1969; Burnham, 1979). Topaz in gem pock­ at Potosi and Oturu in Bolivia (Sillitoe et a1., 1975; ets in pegniatites, in greisen veins, and influoririe­ Grant et aI., 1980). These Bolivian bodies, however, rich porphyry deposits, as well as in topaz rhyolite~, are not known to be topaz rhyolites and are rather is one of the conspicuous products of this devolatili­ poor in fluorine (Grant et aI., 1980). Topaz rhyolite zation; the mineral equilibria involved are analyzed intrusive plugs and laccoliths from the Lake City by Burt (1981) on acidity-salinity diagrams. The de­ district, Colorado (no. 6 in Table 1) are a better ex­ tailed nature of separations inyolving crystals, ample. They are weakly mineralized in uranium. melts, concentrated brines, and aqueous-vapor phases Other possible types of subsurface mineralization during crystallization and devolatilization of fl uorine­ related to fluorine-rich silicic magmas are depicted rich melts remains poorly understood. Experimental in Figure 5 and listed in Table 2. The figure is based studies summarized by Bailey (1977; cf. Manning, on concepts developed for tin deposits by Varlamoff 1981) indicate that fluorine causes a marked lowering 1880 BURT, SHERIDAN, BIKUN, AND CI-IRISTIANSEN

ELEMENTS CHARActERISTICS tinuity, if any, will likely remain speculative, due to the unE'avorable economics of deep drilling a~d LAVA mining.

Exploration Texture Flow-banded Volcanogenic surficial deposits Cavities Abundant IHhophysal cavities Most topaz rhyolite vent complexes ate not known to be associated, with volcanogenic mineralization of the Spor Mountain type. The characteristics of such Phenocrysts Crystal-poor noriproductive vent complexes are presented in a schematic cross ~ection through a lava flow and ,py­ Groundmass Spherulltlc or IIthophysal rQclastic deposit in Figure 6. This figure is based on observations at , Utah, Nathrop'; Col­ orado, East Grants Ridge, New Mexico, andl Bu'rro Low In F (0.1-0.3'£) and VITRQPHYRE lithophile elements Creek, Arizona (see l'able 1), as well as on literature ~~ descriptions of other areas. The low position of the TUFF AND --~ ~ Fresh paleowater table on this figure and its higher position L1THICS ~~~ Non-reactive in 'Figure 7 are conjectural. ' 1"---- ...... --=::-~- The contrasting characteristics of known produc':' BASEMENT ~~===-~Non-reactive tive vent complexes .(Bikun et aI., 1980) are presented

WATER TABLE -~~--- Low ----f in Figure 7. 1"'his figure is based maInly on Spor ~ Mountain, Utah (Be, U, F), but also on the Staats mine area, Wah Wah Mountains, Utah (U, F), the Honey­ F1C. ,6. Schematic cross section of an exposure of topaz rhyolite lava from a nonproductive vent complex. comb Hills, Utah (Be), the Black Range, New Mexico (Sn), and Izenhood llanch, Nevada (Sn). Ji:vidently, as compared with pro.ductive topaz rhy­ of the solidus in granitic melts, conceivably making olite vent complexes, most others had a lava char.:. possible a inclusion studies 'suggest a similar transition (see cri.t- , ical summary by Weisbrod, 1981; cf. l'homas and Baumann, 1980), but the evidence is ambiguous and CHARACTERISTICS phase, separation of one kind or another still seems to be dominant in alteration and ore deposition. LAVA The complete wet to dry magma sequence from Massive or blocky pegmatitic granites to topaz rhyolites could conceiv­ Texture ably be developed in a single district at different times u~der Sparse ~Iarolltlc by successive episodes of partial melting con­ Cavities cavities ditions of increasing temperature arid heat flow. Con­ versely, fractional crystallization of a single large, ini­ Phenocrysts Crystal-rich tially dry batholith might produce successively wetter magma batches ranging from topaz rhyolites to peg­ matidc granites~ l'his latter sequence would favor the Groundmass Granophyrlc emplacement of porphyry Mo-W plutons beneath or adjac~nt to topaz rhyolite vent complexes, ~s sug­ VITROPHYRE High In F('V1.0,£) and gested by the formation of successively deeper mo­ (If present) lithophile elements lybdenite orebodies at the Climax and Henderson deposits, Colorado (Wallace et aI., 1968> 1978). Altered Finally, in connection with Figure 5 and ~rable 2, Reactive we e~phasize that deposit types 1 through 3 alld especially 8 through 14 are only. speculativ,ely aSS07" Reactive ciated with topaz rhyolite volcanism. What is certain ------If------High ----1 is that they are associated with granitic magmas rich • in fluorin,e and lithophile metals at various levels of FIG. 7. Schematic cross section of an exposure of topaz rhyolite (depths of emplacement). l'heir ~ertical cO,n- lava from a productive vent complex, such as Spor Mountain, Utah. TOPAZ RHYOLITES 1831 more abundant lithophysae, less abundant pheno­ of the intrusion. Of course, the precious metal veins crysts, a higher eruption temperature (as implied by themselves might be an attractive target. two-feldspar geothermometry by Bikun, 1980), and In general, in the light of present ignorance, almost lessened geochemical enrichment or specialization any of the types of mineralization depicted in Figure (low La/Yb, K/Rb, Mg/Li, Zr/Nb, and Eu/Eu~ or 5 might be regarded as favorable for the occurrence high Rb/ Sr, etc.). In addition, the lavas crystallized of related types at depth. The deeper types, of course, more rapidly (as indicated by the crystallization tex­ might indicate the former existence of near-surface tures), and the vent areas consisted of nonreactive deposits that have since been eroded. In this case, rock types. only placer or paleoplacer deposits are of interest. Cassiterite (wood tin) deposits in New Mexico, Finally, metamorphosed Precambrian equivalents Nevada, and Mexico appear to be associated with to most of the deposit types discussed might well exist, unusually voluminous, thick, and crystal-rich topaz but prospecting for these would be much more dif­ rhyolite lava flows (and possibly more calcic lavas; ficult. data are sparse). In the vicinity of the deposits the Summary lavas are characteristically chalky (vapor phase al­ tered), and flow lineations tend to be steep or over­ Topaz rhyolite lavas, although known for more turned (Foshag and Fries, 1942; Ypma and Simons, than 100 years, are much more common than for­ 1969; Lufkin, 1972). Some bright red crystalline cas­ merly recognized. In the United States, they occur siterite in cavities and fractures is probably of high in zones of regional extension on·both sides of the temperature pneumatolytic origin (that is, the direct Colorado Plateau; a single belt is present in Mexico. product of magma devolatilization: Lufkin, 1976). On Their known ages practically span the Cenozoic Era the other hand, most botryoidal wood tin probably (0.5-50 m.y.); younger rhyolites are clearly geochem­ formed at temperatures below 150°C (Pan, 1974; ically specialized members of the bimodal basalt rhy­ Lufkin, 1977), possibly by dissolution and reprecip­ olite suite of the western United States. itation of earlier, high-temperature fumarolic depos­ Topaz, easily recognized in cavities in crystalline its (Correa, 1981). lava, indicates high fluorine and lithophile (fluoro­ phile) element contents in associated glasses and in Subsurface deposits the original magma. Enriched elements typically in­ clude Li, Rb, Cs, U, Th, Nb, Ta, Sn, W, Mo, and Be. The applicability of the above criteria to the search The formation of individual topaz crystals in gas for subsurface deposits is unknown. In the absence cavities in rhyolitic lava is analogous to the large-scale of any better data, any topaz-bearing rhy~lite could formation of topaz by devolatilization in subsurface be regarded as indicative of a fluorine-rich magma porphyry, greisen, and deposits (Burt, chamber and thus as favorable for the occurrence of 1981). If these fluorine-rich magmas share a cornman one or more types of subsurface mineralization. origin by partial crustal melting in heated extensional A fluorine-rich magma chamber might be quite environments, as here suggested (anorogenic, A-, or large and complex, and magma batches that produce, R-type granites, etc.), then topaz rhyolite lavas on the for example, porphyry Mo-W deposits might not rise surface could indicate subsurface mineralization of via the same passageways as topaz rhyolite lavas. Nev­ several types, including base and precious metal veins ertheless, in the absence of better information, the and fluorite-rich skarn replacements, porphyry 1\110­ vent area itself is probably the best drilling target, as, W -Sn deposits, greisen veins and disseminations apparently, would have been the case at the Hen­ (apogranites, etc.), or even pegmatites. derson deposit, Colorado (Wallace et aI., 1978), and Volcanogenic near-surface deposits of the Spor the Pine Grove deposit, Utah (Keith, 1980). Mountain type are important targets for F, Be, Li, The occurrence of a thermal anomaly (resetting U, Sn, and possibly other metals. Equivalent Precam­ the K-Ar ages), stable isotope and geochemical anom­ brian deposits may be found, as may back-arc sub­ alies, and base and precious metal veins (especially marine equivalents (tin-bearing volcanographic mas­ if they include sericitic alteration, fluorite, carbonates, sive sulfide deposits?). Further research is needed to manganese minerals, beryllium minerals, and wol­ test these speculations and to solve problems such as framite, as near Rico, Colorado, where altered, topaz­ the role of meteoric water in the eruption and min­ bearing alaskite porphyry dikes are also reported: eralization of topaz rhyolites, the stable and initial McKnight, 1974; Naeser et aI., 1980) would be ex­ strontium isotope characteristics of topaz rhyolites, tremely favorable indicators of a subsurface porphyry and the possible roles of thermogravitational diffusion target in the vent areas or elsewhere. Gravity and and various types of phase separation on element magnetic anomalies might be used to map the extent distributions in fluorine-rich melts. 1832 BURT, SlIERlDAN, BIKUN, AND CI-IRIS1'IANSEN

Acknowledgments Blackwell, D. D., 1978, Heat flow and energy loss in the western United States: Geol. Soc. America Mem. 152, p. ,175-208/ ,, We owe much to discussions with D. Lindsey, Bowden, P., and Jones, J. A., 1978, Mineralization in the Younger C. G. Cunningham, P. Goodell, B. Murphy, and B. Granite Province of northern Nigeria, in Stemprok> M., ed., Correa, and to readings from W. F. Nash, G. Westra, Metallization associated with acid magmatism: Prague, Cze­ D. Wones, and S. Ludington. We are especially grate­ choslovakia Geol. Survey, v. 3, p. 179-190. Bowman, J. R., Evans, S. 1I.; and Nash, W. P., 1982, isotope ful to R. Wilson for valuable discussions and for ­ geochemistry of Quaternary rhyolite from the Mineral Moun:­ ing us to several obscure publications. tains, Utah, USA: Univ. Utah Dept. Geology Geophysics Rept. We are also grateful to the Bendix Engineering DOE/II) 12079-61,' 21 p. ' 'I Corporation (0. S. Department of Energy Subcon­ Bullock, K. C., 1981, Geology of the fluorite occurrences, Spor Mountain, Juab County, Utahl Utah Geol. Mineral Survey, Spec. tract #79-270-E to D. M. Burt and M. F. Sheridan) Studies 53, 31 Pi and the O. S. National Science Foundation (Grant Burnham, ,C. W., 1979, Magmas and hydrothermal fluids, in EAR-7814785 to D. M. Burt) for their support of Barnes~ H. L., Geochemistry of hydrothermal ore deposits, 2nd different parts of this research. Finally, we are grate­ ed.: , Wiley> p. 71-136. ful to K. Gundersen and E. McGovern for typing the Burt> D. M., 1978, Wolframite-scheelite equilibria involving flu­ orine, phosphorus, and [abs.]: Geol. Soc. America, Abstracts manuscript and to S. Selkirk for drafting the figures. .with Programs, v. 10, p, 375. -- 1981, Acidity-salinity diagrams: Application to greisen and porphyry deposits: ECON. GI~OL., v. 76~ p. 832-843. September 21, 198.1; May 14, 1982 Burt, D. M., and Sheridan) M. 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