-'

Annual Report of the Director Geophysical Laboratory

28(11 UPTOK STHE!;T, NORTHWEST, WASHINGTON, D.C. ?OfJuS

1981- 1982

CARNEGIE

INSTITUTION

'-

Reprinted from ---

Camegie l!,slilutioh ofWashillgton Year Booh 81

for the year July 1, 1981-June 30, 1982

Issued December 1982

Paper;: from Uw Geophysica l Laboratory

Carnegie Institution

1"0. 1880

.~.

328

amphibole-rich retrograde alteration that contains the ore minerals. In sev­eral tin skarns fluorite-white mica veins carry appreciable quantities of beryl­lium minerals, very similar to the mus­covite-fluorite stage at McCullough Butte (e.g., Lost River, Alaska, see Dob­son, 1982; Moina, Tasmania, see Kwak and Askins, 1981).

The mineral assemblages, zoning, and geochemical data (Barton et al., this Re­port; M. D. Barton, in preparation) in­dicate low to", {S2' {C01, total Fe, Cu, and Pb and significant contents of acid, F­bearing species and the lithophile ele­ments. These data are consistent with a low-pressure, moderate-temperature origin for the alteration resulting from reaction of the carbonates with fluo­rine-rich fluids derived from a local granitic intrusive.

References

Dodson , D. C., Geology and alteration of the Lost River tin-tungsten-fluorine deposit, Alaska, E con. CeDI. , 77, 1033- 1052, 1982.

Einaudi, M. T., L. D. Meinert, and R. J. New­berry, Skarn deposits, Econ.. Ceol., 75th Anniu. Vol.. 317-391, ] 981.

Kwak, T. A. P., and P. W. Askins, Geology and genesis of the laminar F-Sn-W (-Be·ZnJ skarn at Moina, Tasmania, Australia,Ecan. Geol., 76 , 439-467. 1981.

TRACER STUDIES OF TH E FLUORlNE­RICH SKi\RN AT MCCULLOUGH BUTTE,

EUREKA COUNTY, NEVADA'"

M. D. Barton, J. Ruiz,i" E. Ito,·t and L. Jones~

A central problem in understanding the genesis of hydrothermal mineral deposits and some other metasomatic rocks is the origin of the fluids and non­volatile components that form the de­posits. Isotope and trace-element studies can place constraints on these origins

*Rcsearch supported in part by an American Selco grant.

tUniversity of Michigan , Ann Arbor, Michi­gan .

:i:Department of Terrestrial Magnetism. §CONOCO, Ponca City, Oklahoma.

CARNEGIE I NSTITUTION

and other parameters such as temper­atures, activities of species in aqueous solution, and fluid-rock ratios. The iso­topes ofH, C, 0, and 8 have been widely applied to the study of ore deposits (e.g., reviews by Taylor, 1979; Ohmoto and Rye, 1979) and to a lesser extent in the study of other problems in water-rock interaction (e.g., Taylor, 1977; Rumble et al., 1982). Strontium isotope mea­surements have recently been used to provide insight into transport and mix­ing mechanisms in igneous rocks (James, 1981) and hydrothermal trans­port in oceanic crust (McCullough et al., 1981), a granite (Dickin et al., 1980), and a fluorite deposit (Ruiz et al., 1980). James (1981) reviewed the use of com­bined oxygen and strontium isotope studies in resolving the sources of com­ponents in igneous systems. The rare earth elements (REE) are most com­monly used to decipher the evolution of igneous rocks (reviewed by Hanson, 1980), but recently Exley (1980) and Daigneault et al. (1982) used the ~EE in studies of fluid-rock interactions.

In this study C, 0, and 81' isotopes and the REE are used to evaluate the sources of components and to estimate fluid-rock ratios for the fluorine-rich skarn at McCullough Butte, Eureka County, Nevada. These results, in com­bination with those from ongoing geo­logical and geochemical studies of the McCullough Butte skarn (Barton, this Report), will provide a detailed descrip­tion of and model for the alteration that should bear on the mechanisms of for­mation of other skarns and fluorine-rich mineral deposi ts.

Carbon and Oxygen Results

The isotopes of carbon and oxygen are useful tracers of the ore-forming com­ponentR at McCullough Butte because the different possible sources of the components of the alteration (marine carbonate, granite porphyry, and me­teoric water) have significantly differ­ent initial isotopic ratios. The

CEDI-

comb cess, tion ( shoul reflec

At dol o IT.

teratic Barto bonat' o iso mque1 are pI o refe panso meteOl Ordov.

The of carl: stones orine-r those f ilar ag fresh c carbon. pleted 12C . In

Fig. 37. from the I\l carbon rail preliminar:

, i J (

-) I I

G EOPH YSICA L LA 1:l0RATOR Y

combination, during the alteration pro­cess, of equilibrium isotopic fractiona­tion among phases and mass transfer should produce isotopic variations that reflect the sources of components.

A total of 45 samples of calcite and dolomite, representing each of the al­teration types at McCullough Butte (see Barton, this Report) and the host car­bonates, were analyzed for their C and o isotope ratios with standard tech­niques. The results of these analyses are plotted in Fig. 37 with both C and o referred to the PDB scale. For com­parison, the ranges of magmatic and meteoric oxygen, magmatic carbon, and 'Ordovician limestone are shown.

,.-' The isotopic results separate groups of carbonates in the area. Fresh lime­stones and dolomites far from the flu­orine-rich alteration have values like those for unaltered carbonates of sim­ilar age elsewhere. Compared with the fresh carbonate rocks, the vein-filling carbonates from the Ag veins are de­pleted in 180 but little changed in 13C/ 12C. In contrast, the fluorine-rich alter-

2 o _ .....

329

aLion and its host carbonates show marked depletion in both J3C and 180 along a trend toward magmatic values. Generally, in the fluorite-bearing vein associations the vein carbonates are the most depleted, the skarn cm'bonates are less so, and the host carbonates are the least depl eted. The lightest value be­longs to a coarse calcite crystal from a pegmatitic vein cutting the granite por­phyry.

The depletion in 180 and not in 13C for the Ag veins is consistent with a fluid dominated by meteoric water and a high, cumulative fluid/rock value. In the skam, the depletion in both 180 and 13C is compatible with a dominantly magmatic fluid. Decarbonation reac­tions, which produce similar patterns, could have played a significant role. Large-scale interaction with pristine meteoric water is unlikely for the skarn because the 180 is insufficiently de­pleted. Fluid-rock ratios, however, are difficult to estimate because they de­pend on model assumptions and de­ta iled knowledge of the physical

o Veizer and HOe fs (1 976)

o

~ - 2 a.

U

'" -GO

-4

o o

o

'Magmatic carbanat~ 0>813C>-IO%.

Averoge Qr d·')v!cian Limestone 0

" 0 o 0'00

•

-. !i

e(} .~ l!I

• ,

o

• " • ••

o o

o

~ rl --------------------~ o Host ro cks for from

alteration

f Calc ite from SII • Hosl rocks neor alterat ion

Sk<:Hn and veins from F'r ich alleratian

-6 Meteori c water 8160< -30%.

-25

pegmaTIte vein--..m

-20 -!S SIBO

PDB

-1 0 -5 o

Fig. 37. Plot Ofo L3C (PDB) vs. 0180 (PDB) showing the isotopic compos itions of 45 carbonate samples from the McCullough Butte area. Th e range for meteoric water is from Taylor (] 979), the magmatic carbon range is from Ohmoto and RyE:' (1979). and the magmatic water range is estimated from preliminary isotopic analyses of the ignf:ous silicates.

7,..

!. • ... 1

330

conditions during petrogenes is. The magmatic source of oxygen and carbon for the skarn is similar to that found by Taylor and O'Neil (1977) for skarns in the Osgood Mountains.

Strontium and REE Results

Strontium and the REE usually sub­stitute for calcium in minerals , albeit to different degrees. Because calcium phases are abundant in the Mc­Cullough Butte rocks and because the different rock types should have differ­ent initial REE patterns and Sr isotope ratios, these elements can yield evi­dence about sources of components. The strontium isotopes have the added ad­vantage that they do not fractionate in geological processes; therefore, they can be used as absolute tracers independent of the physical conditions of ore for­mation (providing that corrections for radiogenic Sr are made). Neodymium isotope measurements could poten­tially be used in the same way as Sr ratios to determine the sources of the REE. ... ,

Strontium isotope analyse.3 for fif­teen samples are presented in Fig. 38; they were performed with stand ard techniques at the CONOCO laborato­ries, Ponca City, Oklahoma, and at the Carnegie Institution Department of Terrestrial Magnetism. Corrections for radiogenic strontium were made to sev­eral ofthe granite (Kg) ratios. Ten REE analyses were done by instrumental neutron activation analysis at the Uni­versity of Michigan. The REE patterns show strong light REE enrichments (relative to chondrites) in the granite porphyries, and flatter, less fracti on­ated patterns in the host carbonates. Fluorite REE patterns resemble those of the host rocks.

The intermediate strontium ratios for the fluorite (Fig. 38) require mixing of Sr from at least two sources. Multiple­source origin contrasts with the results of Ruiz et al. (1980) on the fluorite de­posit at Las Cuevas, where all the Sr

(' A fe, l:: G 1 E 1:\ S TI T U TI 0 N

4rl ---------------------------------, OJ> 4>

c. E o OJ>

o 2

... .D E =0

[2Jop lSJOh gjF iuorile

EB Kg

z 01 ~ II 11111 0 .708 0.709 0 .7 10 0.711 0 . 712

(87Sr /80 Sr)0

f ig. 38. In it ial strontitun ra tios for fifteen samples from the McC ullough Butte a rea. Kg, gran ite porph}Ties; Op. Pvgon ip Group carbon­ates; Oh, Hanson Creek Formation dolomites. The anomalouslv high Oh determi nation is fo r a sam­ple immedi~t E:ly adjacent to a mi ca- and hence Hb-nch '; ein; the Rb content was not determined, so , he value may be anomal{)usly high owing to radiogE-nic Sr.

was locally derived . The REE results suggest a more limited mobility for those elemen ts. in agreement with experi-

. mental partition coefficients (Flynn and Bur-nham , 1978) but somewhat differ­ent h om recent studi es on other hy­drother m al system s (Exley, 1980; Da igneault et al., 1982 '.

Conclusions

The C, 0 , and Sr isotopic results in­dicate th3.t magmatic fluids played a large pa rt in the fo rmation of the McCuliough Butte skarn. The patterns sugg'est that the REE were relatively immobil e_ e\·en under the F-rich (and possibly COrrich ! conditions of for­ma tion.

References

Da igneault. R., R. Charles. and·J . 1\. Ludden. Au­mineralization and ra re E'an h mobili t y labstr. ), GA C·.UAC Progr. lrith A bstr .. 1, 44.-1982.

Dick in , A . P .. R. A. Exl ey. and B. :\1. Smith , Iso­topic measurement of Sf a nd 0 exchange be· tween meteor ic·hydrothtr m.11 fluid and the Coi re l'aighE:ich granophyrE- . Isle of Skye. N. W. Scotl and, Earth Planet. Sci. Left ., 51 , 58-70. 19&0.

Exl ey, R. A .. :'l icroproLe 5ludie.- of REE·rich ac· cessory rnineral3: im plicat ions fo r Sky t granite petrogE:ne;; is and REf mobility in hyd rot her­mal systtms. Earth P lcn€ f. S ci. Lett ., 51. 97-1l0, 1950.

GEOPHY

Flynn , R. mental I

coeffici el po,· pha, moch im.

Hanson, G netic stu( Planet. E:.

J ames, D., diogenic taminati( 344,198.

McCulloug. burg, am 180 ;1°0 i, ta l seetio R es., 86,

Ohmoto, H. ca rbon , ir eral Depe Wil ey an< 1979.

Ruiz, J. , S. ter, Geolc vas fluori }.;con . Gee

Rumble, D., Fluid fl o\' Brook fos J. Sci., 26

Taylor, B. , a of meiaso ciated mel Mounta in. 63 , 1-49 ,

Taylor , H. P. orig in of I Soc. Lon d.

Taylor, H. P relationsh its, in Gee Deposits, ~ and Sons.

Veizer , J., a a nd 13CI'2( bonate roc 1387-1395

STABILlT' AND ~

The sub umene to r significant ments of observed i matites, a

TITUTION

HL1Bffi II 0.712

for fifteen area. Kg,

'il1p carbon-'mites. The for a sam­md hence termined , ,owing to

~ results I for those I . ~ expen-ynn and

at differ­[ther hy­i, 1980; I

suIts in­llayed a ! of the )atterns latively ch (and of for-

Iden, Au­. (abstr.l, 1982. 1ith, 150-lnge be­and the )kye. N. 51 , 58-

·rich ac­granite irother-5],97-

I I I

I

GEOPHYSICAL LABOJ{ATORY

Flynn, R. T. , and C. W. Burnham, An experi­mental determination of rare earth partition coefficients between a chloride-containing va­por phase and silicate melts, Geochim. Cos­mochim. Acta, 42, 685- 701, 1978.

Han~on, G. N., Rare earth elements in pelrog€:­netic studies of igneous systems, An II. Rev. Earth Planet. S ci., 8, 371-406, 1980.

James, D., The combined use of oxygen and ra­diogenic isotopes as indicators of crustal con­tamination, Ann. Rev. Earth Planet. S ci., 8, 311-344, 1981.

McCullough, M. T., R. T. Gregory, G. J. Wasser­burg, and H. P. Taylur, Jr., Sm-Nd, Rb-Sr, and 180 /160 isotopic systematics in an oceanic crus­tal section: the Samail ophiolite, J . Geophys. R es., 86, 2721-2735, 1981.

Ohmoto, H., and R. O. Rye, Isotopes of sulfur and carbon, in Geochemistry of Hydrothermal Min­eral Deposits, 2d ed., H. L. Barnes, ed., John Wiley and Sons, Inc., New York , pp. 509-567, 1979.

Ruiz, J., S. E. Kesler, L. M. Jones, and J . F. Sut­ter, Geology and geochemistry of the Las Cne­vas fluorite deposit, Sa n Luis Potosi, Mexico, EcolL. Geol., 75, 1200-1209, 1980.

Rumble, D., III, J . M. Ferry, and T. C. Hoering, Fluid flow during metamorphism at the Beaver Brook fossil locality , New Hampsh ire, Amer. J. Sci., 282, 886-919, 1982.

Taylur, B., andJ. R. O'N"il, Stable isotope studie5 of metasomatic Ca-Fe-Al-Si skarns and asso­ciated metamorphic and igneous rocks, Osgood Mountains, Nevada, Con/rib. Mineral. Petrel., 63, 1-49, 1977.

Taylor, H. P., Jr., Water/rock interactions and th~ origin of H~O in granitic batholiths, J. Geol. Soc. London, 133, 509-558, 1977.

Taylor, H. P. , Jr., Oxygen and hydrogen isotope rel at ionships in hydrothermal mineral depos­its, in Geochemistry of Hydrothermal .~1ineral Deposits, 2d ed., H. L. Barnes, ed ., J ohn Wiley and Sons, Inc. , New York , pp . 236-277 , 1979.

Veizer, J., and J. Hoefs, The nature of " 0 f160 and 13C/12C secular trends in sedimentary car­bonate rocks, Geochim. Cosmochim. _4cta. 40, 1387-1395, 1976.

STABILITY OF SPODUMENE IN ACIDIC AND SALINE FLUORINE-RICH

ENVIRONMENTS

David London

The subsolidus alteration of spod­umene to mica-bearing assemblages is significant for two reasons: (1) replace­ments of this type are commonly observed in spodumene-bearing peg­matites, and thus the processes in-

",

331

volved are of general significance to pegmatite evolution, and (2) economic deposits of tantalum are usually asso­ciated with mica-bearing assemblages that have replaced spodumene. Prelim­inary experimental studies of the ef­fects of aqueous H + , K + , and F - on the' stability of spodumene include those of Armstrong (1969), Munoz (1971), and Grubb (1973). Natural occurrences of altered spodumene and associated phases (Table 14 and Fig. 39) provide qualitative information that can be or­ganized in schematic isobaric, isother­mal J-LHF-J-LKF-J-LLiF acidity-salinity phase diagrams (e.g_, Burt, 1981). The phase diagrams depicted in Fig. 40

, present sequences of mineral reactions involving spodumene and other phases in the system LiAl02-Si02-HzO-HF-LiF­KF. The sequences of assemblages pro­duced by increasing fluid acidity (J-LHF) reflect the increasing capacity of an aqueous fluid to hydrolyze solid phases and leach alkali cations. Increasing sal­inity (J-LKF) also reflects increasing al­kalinity of the fluid phase, because some of the reactions listed in Table 15 and shown in Fig. 40 are actually exchange reactions. For example, the conversion of spodumene to K-feldspar with in­creasing J-LKF could be represented by the component KLi - I> an alkaline ex­change operator, which is equivalent to

J-L°KF - J-L°LiF + RT In(aKF!o,LiF)'

Figure 40 is constructed for an ar­bitrary P and T within the stability field of spodumene (e.g., 300°-400°C and 2-

TABLE 14. Names and Compositions of Phases under Consideration

Phase

Topaz Cookeite Muscovite Lepidolite Spodumene K-feldspar Quartz

Abbreviation Composition

Tpz AI2SiO~F2 Ckt LiAl:;Si30 ,o(OHla Mus KAl.lSi 30 1O\OH)~ Lpt K2Li2Al .. Si;OzoF3(OH) Spcl Li AlSi20 n l(;;p KAISi 30 a Qlz SiO~

I

1'1

I ~f I ·:·1 I,rel I ~i

I ~1. I L'i