The Formation of Subcalcic Garnet in Scheelite-Bearing

Canadian Minerologist Vol. 21, pp. 529-5dA(1983)

THE FORMATIONOF SUBCALCICGARNET IN SCHEELITE-BEARINGSKARNS

RAINER J. NEWBERRY.8 Deportmentof Applied Eorth Sciences,Stonford University,Stanford, Califurnia 94305,U.S.A.

ABSTRACT la cordillbre,et ils possOdentdes teneurs et desrdserves en tungstenesup6rieures aux exemplessans grenat subcalci- Compositionsofgarnet from 18tungsten-bearing skarns que.Pour trouver un tel grenatdans un skarn,il faut un in the westernCordillera of North America, with additional milieurelativement reducteur, et uneactivit6 6lev€e de I'ion examplesfrom the literature, indicate that solid solution Mn etr6duite de I'ion Cadans les fluides responsables. Ces betweengrandite and plralspite is far more extensivethan conditionsseraient le mieuxsatisfaites dans les skarns for- previously reported; garnet with up to 75 mole 9o andradite mdsi profondeurintermddiaire, dans les roches-hdtes enri- component can contain significant almandine - spessartine chiesen matibre organique et dansdes zones isol€es chimi- ("subcalcic") component. Detailed microprobe analyses quementdes marbres i calcite.Des calculs d'ordre semi show that garnets zoned quantitatifmontrent qu'i un niveaud'oxydation d6fini par skarn are typically toward more - - subcalciccompositions from core to rim and from periphery l'assemblagepyrite magndtite pyrrhotine,ou m6meinf6- of skarn toward the inferred fluid-source. l\mong tungsten rieurd ceniveau, un changementd'un facteurde deux dans skarns,those that possesssubcalcic garnet are more abun- I'activit6de I'hydroxyde de calcium en solutionpeut expli- dant, they generallypossess higher tungstengrades and ton- querles variations de composition observ6es. Un grenatsub- nages,and they appearto be more typical of occurrences calciqueest caractdristique des skarns i tungstbneparce que in the Cordillera. A model for the occurrenceof subcalcic detels skarns sont typiquement associ6s i des plutons ou gamet in a skarn requiresrelatively low oxidation-state,high rochesencaissantes relativement rdduits ou form6si grande manganese-ionactivity, and low calcium-ion activity in profondeur. skarn-forming fluids. These conditions are best met in (Traduit par la R€daction) skarns formed at moderate depths, in highly carbonaceous Mots-clds:grenat, skarn, scheelite, tungstbne, gites min6- host-rocks and in zones chemically isolated from calcite raux.m€tasomatisme. marble. Semiquantitativecalculations suggest that at oxidation states approximately at or below pyrite- magnetite- INTRoDUcTION pyrrhotite, changesin activity of calcium hydroxide in solution by a factor of 2 can producethe observedcomposi Garnet from tungsten-bearingskarns in Japan tional variations. Subcalcicgarnet generallyis characteristic (Shimazaki1977) covers much of the rangein com- of tungsten skarns becausesuch skarns are tlpically found position grossul€fite and almandine - associatedwith relativelyreduced plutons or wallrocks or between found at relatively great depths. spessartine,with reported compositionslimited to andradite contents of less than 15 mole q0, and Keywords: garnet, skarn, scheelite, tungsten, mineral typically lessthan 5 mole 90. Basedon thesedata, deposits,metasomatism. Shimazaki(1977) concluded that "the lack of Fe3* is essential for the formation of Fd+ and/or SovvAnn Mn2+-bearinggrossular under low temperatureand low pressureconditions." Similar occurrencesof La composition du grenat dans dix-huit skarns ir tung- garnet with appreciablepyralspite and low andradite stCnede la cordillbreOuest de I'Amdrique du Nord, ainsi ieported from the tungsten skarns at que pris que contents are dansles exemples dansla litt6rature, montre (Lee 1962),the MacMillan la entre grandite et pyralspite plus Victory mine, Nevada solution est de beaucoup (Dick dtenduequ'on ne le croyait auparavant.Un grenat qui con- Pass deposit, Yukon Territory 1970, the tient jusqu'd 7590 du p6le andradite (basemolaire) peut CanadaTungsten, Baker and Lened deposits,North- contenir une proportion appr€ciabled'une composante westTerritories @ick 1980),the Costabonnedeposit, "subcalcique" d almandin + spessartine.Des mesures France(Guy 1980),and Dchenitschke,central Asia d6tai[6esi la microsondemontrent que le grenat desskarns Ndmec 1967).From similar data, Zharikov (1970) est generalementzon6, avec un enrichissementen tennes and Shimazaki(1977) concluded that the chemical subcalciquesdu coeur vers la bordure d'un cristal et des environment favorable for scheelitedeposition is limites du skarn vers la source de la phase fluide (6tablie limited to one in which ferrous-iron-bearinggrossular par inf6rence), Les skarns i tungstdnequi contiennent un is Zharikov (1970)interpreted this to require grenat subcalciquesont plus repanduset plus typiques dans stable. acidic skarn-forming fluids, whereas Shimazaki (1977) suggested, instead, highly reduced en- EPresentaddress: Department of Geology, University of vironments characterizedby oxidation statesbelow Alaska, Fairbanks, Alaska 99701,U.S.A. the pyrrhotite - pyrite - magnetite buffer. 529 530 THE CANADIAN MINERALOGIST

In contrastwith the abovestudies, investigations electron microprobe at Queen's University, King- of the tungsten-bearingskarns at Lost Creek,Mon- ston, Ontario. Major elementswere determinedby tana (Collins 1977),King Island, Tasmania(Kwak energydispersion using standardswhose energy spec- & Tan 1981)and the OsgoodMountains, Nevada tra had beenpreviously stored on magnetictape, and (Iaylor l97Q showedthat only granditegarnet is pres- employing an acceleratingpotential of l5kV, a beam ent. Reconnaissancestudies of garnetcompositions current of 100-120pA, a samplecurrent of 0.034 from the Pine Creekmine, California (Wrighr 1973) pA, a beam diameterof 2-4 pm and a counting in- and from the Black Rock mine, California @lliot terval of 120seconds. Raw data for the elementswere l97l), on the otherhand, indicatedthe presenceof initially correctedon an Ns-880 minicomputer us- andraditic garnet with significant almandine + spes- ing a multiple least-squaresroutine. The intensity sartinecomponent. These data show that there is a ratios were correctedfor matrix effects following broad range of garnet compositions in tungsten Bence& Albee (1968)as well as for drift usinga For- skarns, that a significant almandine-spessartinecom- tran correctionprogram developedby P.L. Roeder ponent is not restrictedto the grossulariticend of (pers. comm.). Alpha factors used in the iterative the grandite series,and that some tungstenskarns correctionprocedure were taken from Albee & Ray lack subcalcicgarnet. (1970).Analytical accuracywas checkedby analyz- This paper summarizesan investigationinto the ing garnet and feldspar standards during each environmentsof tungsten-skarnformation, with par- session. ticular emphasison the occurrenceand compositions An additional 43 sampleswere preparedas grain of subcalcicgarnet and its relationship to scheelite mounts and analyzedon an 8-channelARL electron deposition. It is basedon a detailedstudy of tungsten microprobe at the University of California, Berkeley. skarns in the Sierra Nevada area of California Samples were analyzed using a 15 kV filament (Newberry 1980),with additional data and examples voltage, a sample current of 0.2-0.3 pA,.beam from Idaho, Nevada,Montana, Yukon Territory and diameterof 2 pm,and a countingtime of 50 seconds. Northwest Territories. In this paper, the term sub- Standards employed included grains of well- calcic garnetidentifies a skarn garnet that contains characterizedgarnet, pyroxene,rhodonite, anorthite, more than 5 mole 9o almandine + sDessartine hematiteand chromite.Raw data werecorrected for component. drift; intensity ratios were correctedfor matrix effects following Bence& Albee (1968)using a For- ANALYTICAL PRoCEDURES tran program developed by M.L. Rivers (pers. comm.). Severalgarnet samplesanalyzed at both Approximately 60 polishedthin sectionsrepresent- laboratories yielded negligible differences in ing 14 skarns were analyzedusing an ARL AMX composition.

TAELEI. REPRESENTATIVECOMPOSITIONS OF SUBCALCIC GARI{FI

No. la lb 2 3 4 5 6 7 8 9 t0 ll 12 t3 14 t5 (core) (rin) leight g

si02 36.12 36.82 36.67 36.73 37.20 36.35 36.37 37.88 36.83 36.94 38.36 37.60 37.48 38.48 37.12 37.56 Tiq 0.19 o.2o 0.35 0.16 0.00 0.16 0.55 0,49 0.12 0.18 0.17 0.00 0.45 0.39 0.36 0.37 A1203 14.67 19.60 9.19 17.36 la.m 15.10 6.42 19.38 4.56 21.51 18.56 21.75 18.81 18.18 19.04 20.13 Cr203 0.15 0.13 O.23 0.00 't6.210.00 0.05 0.36 0.@ 0.13 0.25 0.27 0.00 0.12 0.27 0.ll 0.14 Fe{F 17.61 16.19 18.68 14,28 14.94 22.71 9.40 24.56 16.72 7.33 12.A7 13.88 7.O2 6.92 13.86 lifno 12.76 l8.tl8 3.A2 19.39 16.36 13.17 3,98 p.50 2.26 4.49 2.35 6.60 10.62 3.79 Mso 0.31 0.86 0.20 0.46 o,29 0.35 0.31 0.39 0.09 0.19 0.16 0.35 0.31 0.40 0,06 0.18 Cao 16.23 9.48 30.51 11.94 il.07 19.47 24.57 ?3.42 3t.89 20.19 32.86 19.19 19.14 32.00 31.53 22.86 ToIAL .02 101.n 99.6s [email protected] 100.05 99.59 99.26 100.50 100.42 t00.48 100.06 98.34 100.81 100.53 98.55 98.87 ible jNgamet endffibers

Gr 17.8 14.6 27.8 14.4 21.7 24.1 l4.l 51.7 12.2 49.7 77.6'17.A 54.7 37.5 67.0 7t.3 57.8 Ad 29.6 lt,4 59.0 20.O 10.0 31.8 67.8 13.4 79.3 6.1 0.4 t6.3 20.'l 12.9 4.6 Sp 29.7 40.6 8.7 44.7 37.2 30.0 9.1 20.9 5.2 9.9 5.1 15.0 23.9 a.2 8.0 8.8 Alm 21.2 29.6 3.1 19.0 29.7 12.7 6.3 12.5 2.6 32.4 4.1 4.5 20.7 2.3 7.2 27.7 Py 1.2 3.3 0.8 1.9 l.t 1.4 'I1.2 1.5 0.3 o.7 0.6 1.4 1.2 1.5 0.2 0.7 UY 0.4 0.4 0.7 0.0 0.0 0.1 .l 0.1 0.4 0.8 0.8 0.0 0.4 0.8 0.4 0.4

r-Total ircn,as Fe0;-Gr-=grgssularite, nd = andradlte, Sp = spessrtine, Alm = almndine, py. pylpe, Uv = uvamvlte [os,l,2:Pinecreekmine,llS9lstsubleveldrlft,l=.6nimlntrusion,2=6mfmititroiion!,-5:BlackRocknine,3=7ll stope,_6700 level, 4 = 833.stope, 6600 l-eyel, 5 = 7ll drift, 6700 level; 6,7: Strawbemy mine, 6 . gamet = iein fm outcrop of $7 ntne, 7 gamet near quartz mnzonite dike, K lens, Znd level, fl olne; g: outcrcp of alpine mine, 7 m im lntrusion. subcalcic rir; 9-ll: outcmp of Gamet Dike-mine,8 =-LongRidge am, gimet vein in skarn 2 m rnirintrusi6n, 9 = upper mrkings ire;, skam adjacent to pegmtlte dike, l0 = lorer-mrkings-area, near inioskarn contacti lz,l3: Hardpointmine,'12 = ubil ttp-tt tsi.si, gi*"t vein in skan 3 m fm dike contact, 13 = outcrcp near portal, gamet vein ln sfim; 14: ;utcmp of Tungsten ,tlm nine, nasiiie gamet skam 2 n fm intnsion; 15: Lened prcspect, Cac Valley area, 3 m fm intrusion. Ska; lmati6ns shm on Fiqure l SUBCALCIC GARNET IN SCHEELITE.BEARING SKARNS 531

Calculationof molecularend-members was made following Rickwood (1968)after assignmentof ferric and ferrous iron. Ferric iron was determinedby subtracting the amount necessaryto complete trivalent occupancyfrom total iron per 24 oxygen atoms; the remaining iron was assumedto be ferrous. The analysiswas rejectedas unsatisfactoryif the cation total (ferrous iron * manganese+ calcium) differed from 6.00 gter 24 oxygenatoms) by more than 2t/0. Typical subcalcicgarnet compositions are listed in Table l.

Gsot-ocv AND GEocHEMISTRY oF SUBCALCICGanNgt OccunnsNcrs

Geology of subcalcic garnet occurrencesin skarns ol the North American Cordillera

Most of the hundredsof tungsten-bearingskarns in the WesternCordillera of Canadaand the United States ars located at or near contacts between calcareous metasedimentary rocks and coarse- gained calcalkalineplutons of Triassicto Cretaceous c>ll age. The locations of tungsten-bearingskarns dis- \ cussedin this paper are shown in Figure l. Of the thirty occurrencesshown, all but sevencontain sub- KEY: calcic garnet. In all cases,subcalcic garnet t quartz forms an assemblagewith a consistentparagenetic Scieellte skarns setting, proximal to the intrusive contact. The de- wlth subcalcic'garnets tailed occurrenceof subcalcicgarnet and space-time variations in its compositionvary with the type of .thls sludy hostrock. t othgr workers Tungsten-bearingskarns exist in host rocks rang- ing from pure marble to calcareoushornfels, and from massiveunits to thinly intercalatedor diamic- Scheellte skarns titic units. Thesedifferent host-rocksimpart char- lacklng acteristic morphologies and patterns of zonation subcalclc garnels upon the skarns. Wallrocks also vary from extremely 13" o12 carbonaceousto noncarbonaceousor evenhematitic, o thls study symbols.Deposits: l) Lost Rivdr, Ak. (Dobson 1982); 2) SpruceHen prospect,Fairbanks district, Ak.; 3) Mac- millan Pass,Yukon-Northwest Territories (Dick 1976); 4) Clea, Yukon (Dick 1980);5) Lened, N.W.T. (Dick 1980,this study); 6) CanadaTungsten, N.W.T. (Dick 1980);? Baker,N.W.T. @ick 1980);8) Emerald,B.C.; underllneddeposlts have 9) Colorado Gulch, Mont.; l0) Lost Creek/Brown's >.6 mt ore Lake, Mont. (Collins 1977);1l) TungstenJim, ld.; 12) OsgoodMountains, Nv. (Taylor 1976);13) Mill City, 0)roductlon + reserveo Nv.; 14) Victory mine, Nv. (Lee 1962); 15) Monte Cristo, Nv. (Sonnevil 1979); 16) Emerson, Nv.; 17) Alpine, Ca.; 18) Garnet Hill, Ca. @rock 1970); 19) BlackRock, Ca.;20) Hilton Creek,Ca.;21) Hardpoint, Ca.;22) Strawberry,Ca. (this study, Nokleberg l98l); 23) Scheelore,Ca. (Morgan 1975):U) Pine Creek,Ca. (this study, Brown 1980);25) TungstenHills, Ca.; 26) FIG. l. Location of tungsten-bearingskarns of the western Garnet Dike, Ca.;27) Consolidated,Ca.; 28) Tulare North American Cordillera discussedin this paper. Co., Ca.; 29) Darwin, Ca. (this study, Eastman1979); Skarnswith subcalcicgarnet indicated by closedsym- 30) El Jaralito, Sonora,Mexico (Peabody1979, Dunn bols; skarns lacking subcalcicgarnet indicated by open 1980). 532 THE CANADIAN MINERALOGIST

TABLE2. CON1MST1NGCHAMCTERISTICS OFREDUCED AND OXIDTZED TUNGSTEN SIGRNS

Redlcedskarns 0xldized Skarns Characteristlc Stronqly reduced Moderately reduced strongly carbon- mderately carbon- non-carbonaceous host rccks aceous ( ) 5U aceous(0.5 to 2, or hemtltlc carbon comn) cafbon typical ) associatedplutonic ilrenlte or mgiletlte mgnetlte bearlng mgne'lltebearing rccks bearing

endoskarnnlneralogy pyroxene-plagloclase pyrcxene-plagloclase quartz-epldote+ : quaru I quaftz-epldote plagloclase skarn nlneralogy prcgrade assemblages pyroxene/garnetratlo 1.0:1to 2:1 2,I to lt2 l.:2 to 1:10 pyroxenecomposition' mle Z hedenberglte 60-90 50-70 20-45 rcle%Johannsenlte 0-10 10-25 0- 5 garnet composltlon mle Z andradite 10-30 30-75 50-80 'late garnet composltlon rc]e Z andradite 0-30 10-75 80-98 mle Z spessartine 3-35 5-40 0-3 mle g almandlne 3-40 z-35 0 retrogradeassenblages typlcal mlneralogy biotlte, plagioclase blotite, hornblende, epidote, chlorite, (l quartz' calclte) hornblende,opaques chlorite,opaques actlnolite,opaques anphlbolecomposition g mle ferrctrerc'l lte 0-50 5-60 30-60 rcIe g trmlite 0-10 10-40 30-70 mle Z pargasite 50-98 5-70 5-20 diagnostic opaques pyfrhotite pyrite-regnetite- pyrite+mgnetite (Bisnuth ) pyrrhotite (btsmutfinlte) exanples Hardpolnt,Ca Pine Creek, Ca. osgoodMntns., Nv. lilacMillanpass, yT Black Rock,Ca. Lost Crcek, l,lont. CanadaTungsten, NLrf El ,laralito, Mexlco TungstenHills, Ca. IungstenJim, ld. ColoradoGulch, l,4ont. *reminde|ls diopslde i *rem,l nder i s grossularj te

indicating that the oxidation stateof host rocks can contacttoward marble. In thesesimple cases, zones be highly variable (Newberry 1980). containing subcalcicgarnet a quaftz are at the cen- Compositionsof skarn-mineralassemblages, com- tre of skarn veins and closeslto intrusive contacts, positions,and local geologicalsettings indicate that in contact with and replacingsalitic pyroxene- grzm- tungsten skarns can be classifiedinto three cate- dite garnetskarn. Zonesof subcalcicgarnet contain gories:"strongly reduced", "moderately reduced" variable amounts of quartz, typically between5 and and "oxidized". Characteristicsof thesethree types 80V0,but on averageabout 30V0.They contain no are listed in Table 2, adapted from Einaudi et a/. scheelite(Newberry & Einaudi l98l) and form a (1981). Reducedskarns, characterizedby ferrous- sharp cut-off in grade adjacent to ore-bearing iron-rich minerals,are found in carbonaceoushost garnet-pyroxeneskarn. rocks or associatedwith reducedplutons (crystallized Veins of subcalcicgarnet a quartz commonly ex- at depth or ilmenite-bearing,or both). Oxidized tend outward from massivegarnet-quartz, replacing skarns, characterizedby ferric-iron-rich minerals, are garnet-pyroxeneskarns several metres from the con- found in noncarbonaceousor hematitic host rocks tact (Fig. 2). Garnet compositionsvary systematically or associatedwith oxidized pluton$ (crystallizedat within these veins, becoming less subcalcic with shallowdepths or magnetite-bearing,or both). Sub- distanceinto the garnet-pyroxeneskarn @ig. 2) and calcicgarnet has only beenidentified in the reddced eventually approachthe composition of the wallrock skarns; andraditeand epidote-quartz urethe para- grandite. Grains of subcalcic garnet within the geneticallyequivalent assemblages in oxidized skarns. garnet-quartz zone may be slightly zoned, becom- Most tungsten-bearingskarns show either simple ing 5 to 20 mole 9o more subcalcicfrom core to rim or complex mineralogical zonation dependingon the (Table l). host lithology. Skarnsformed from relativelypure, Skarns formed from heterogeneouscarbonate thick marble beds, (e.g., the CanadaTungsten mine, lithologiesdo not showthe well-developedoutcrop- N.W.T., and the Pine Creek, Hilton Creek, and scalezoning characteristicsof skarns formed from Garnet Dike mines, Ca.), are characterized.by thick marble beds,except that a subcalcicgarnet -l- mineralogically simple time-equivalent metasomatic quartz zonemay be directly adjacentto the pluton. zonescontaining mineral assemblageswith increas- Calc-silicate grains and layers formed by meta- ing calciumcontents progressing from the intrusive morphic recrystallizationaxe typically overprinted by SUBCALCIC GARNET IN SCHEELITE-BEARING SKARNS 533

KEY: f-lenooskarn f garnet-pyroxeneskarn rtz monzonlte [..T]pyro r"ne hornf ets

!subcalcic garnet+ quartz

almandine + spessartine

2m +

Frc.2. Simplifiedmap of the E rib, 11894th sublevel workings, Pine Creek mine, showing distribution of 12 '. \ garnet a\ skarn types and variation in composition within a subcalcicgarnet - quartz vein. Note decreasinglysub- aa calcic composition of garnet with increasingdistance from the intrusive contact. Retrogmdeveins and alteration masses,which are structurally superimposedon the grossularite andradite early skarns, are not shown.

metasomaticcalc-silicates; individual garnet grains are found in garnet adjacent to quartz monzonite are typically zoned, in most caseswith grossularitic stocksand dykes;the sparsedata suggesta decrease or granditic cores and increasinglysubcalcic rims in the subcalciccomponent with distancefrom the @igs. 3, 4). Subcalcicgarnet typically coatsthe in- intrusivebody. At Black Rock, garnetcompositions sideof vugsformed by coalescinggrains of grandite are not a simple function of distancefrom the in- garnet (Fig. 5). Subcalcicgarnet grains also occur trusive body, but rather reflect the latest paths of as late, discontinuousveins in garnet-pyroxeneskarn fluid flow controlled by sedimentarylayering. and hornfels, and are not presentin marbleor other high-calciumrocks.Where subcalcic garnet occurs Most known oxidized skarns formed from in- adjacentto calcite,the calciteis alwaysa late, vug- homogeneous carbonate lithologies [e.81.,King filling mineral (Fie. 3). Thesetextures suggest that Island, Aust.: Edwards et sl. (1956)1,so that their subcalcicgarnet did not form in a given spot until zoning sequencesare difficult to decipher.In sev- the calcite originally presentwas entirely dissolved eral cases [Osgood Mountains, Nevada: Taylor by metasomaticfluids. (lW6), TungstenHills, California: Newberry(1980), Lost Creek,Montana: Collins (1977)1,however, the Zonation in garnet qompositionsalso occurs on sequenceappears to be: a narrow outer wollastonite- the scaleof hundredsof metres(e.9., the Black Rock idocrasezone, a thick main zoneof andraditic garnet mine, Fig. 6). The maximum amount of subcalcic t salitic pyroxene,and an inner epidote-quartz zone componentpresent in garnet at Black Rock varies of variable width adjacent to the intrusive body. from more than 8090near the quartz monzonite con- Garnetgrains within the garnet t pyroxenezone of tact to lessthan l07o in skarn exposedabout 250 oxidized skarnsare typically zoned from a grossu- metres from the contact. At the Strawberry and laritic core to a nearly pure andradite rim (Taylor Hardpoint skarnsthe maximum subcalciccontents 1976,Collins 1977). 534 THE CANADIAN MINERALOGIST

almandine +

grossularite andradite

FIc. 3. Sketchfrom a thin sectionof skarn from the 831 stope, 6600level, Black Rock mine, showingcompositional zoning and developmentof subcalcicrim on grandite garnet. Subcalcicgarnet lueasare indentified in thin section by pale orangecolor in transmittedlight and anomalousbirefringence with crossedpolarizers. Note pattern of zoning ol increasinglysubcalcic garnet compositionstoward rim. Late calcite - chlorite veinletsnot shown.

almandine+ spessarilne

fl F-:-l g l .lsubcalclc I lerandtte fi 6n"o"nbershtc I quartz gm ffilccheerrte grossularltlc i ffi."r*':- wcalclle FIG.4. Sketchof thin sectionfrom No, 2 orebodypit, Strawberrytungsten mine. Metamopphicgrossularitic core with quartz and diopside inclusions is surrounded by metasomatic grandite with scheelite and hedenbergitic pyroxene inclusions,in turn rimmed by subcalcicgarnet. Vug is filled with late, clear calcite, Garnet compositionsbecome more subcalcictoward rirn. Garnet types are indicated by color differencesin transmitted light and crossedpolarizers. SUBCALCIC GARNET IN SCHEELITE.BEARING SKARNS 535

Bulk-composition relationships in subcalcic grrnet

Compositionsof subcalcicgarnet determinedin this study are plotted in terms of mole 9o grossularite, andradite, and almandine + spessartinecom- ponentsin Figure 7. The plot clearly indicatesthe wide range in solid solution betweengrandite and py'ralspitefound in theseoccurrences. Almandine- spessartinesubstitution is clearlynot limited to very andradite-poor compositions: a garnet with as much as 75 mole 9o andradite can contain appreciable almandine + spessartine.Garnet compositions recorded from strongly reducedskarns, similar to thosereported by Shimazaki(1977), form a subgroup characterizedby low andradite and high grossularite contents. Garnet compositions recorded from moderatelyreduced skarns form a compositionally distinct group with a higher andraditecontent. The contrastin compositionsbetween the two groupsof reduced skarns is further illustrated in terms of relative almandineand total iron components(Fig. | , , :"' , , I 8). In the moderatelyreduced skarns, total-iron con- Key: tents are approximatelyindependent of almandine lll garnet-pyroxEne Efl nasnetlte content, whereasthe strongly reducedskarns show a linear increasein iron content with increasein 5tr subcabtcsamet V% pvrtte almandinecontent. The former trend reflectsan aD- calclre proximatelyfixed amount of total iron in garnet, E*lcu"'t I distributed betweenferrous and ferric forms, rather than a fixed ratio of ferrous to ferric iron. Ftc.5. Sketchof an unusuallysubcalcic-garnet-rich hand specimenfrom the 711 stope, 6700level, Black Rock Differences in relative almandine,/spessartine mine, showingthe complexityof the mineral zonation present.The protolith interlayered ratios also help to differentiate betweenthe two types skarn consistedof marble and calc-silicate hornfels beds; replacementof of skarn CIable l). Strongly reduced skarns are the marble interbeds from contacts with hornfels characterizedby high almandine/spessartineratios resulted in the formation of garnet-pyroxene skarn (l : I to 4: l), whereasmoderately reduced skarns have followed by subcalcic garnet. Vugs were filled with low ratios (l:l to l:5, averagingabout 1:2).These pyrite, pyrrhotite, magnetite, quartz and calcite during differencesare addressedin a subsequentsection. later retrogradealteration.

Compositional differencesbetween the two groups are summarizedin a plot of the molar ratios an- DtscusstoN dradite,/grossularite yersas almandine,/spessartine (Fig. 9). Strongly reducedskarns are characterized by low andradite,/grossularand variable, but generally high almandine,/spessartineratios, whereas Oxidation state and stabitity of subcalcic garnet moderatelyreduced skarns possess relatively high andradite and low almandine. Subcalcicgarnet com- The oxidation statepresent during early stagesof positions from Japanesetungsten skarns (Shimazaki skarn formation may be estimated by using ex- 1977)show similar trends, but witJrlower overall iron perimentally.derivedmineral-asssemblage buffers ap- contents.The strongly antithetic relationship observ- propriate for calcic garnet-pyroxeneassemblages ad- ed between almandine,/spessartineand andra- justed for mineral composition.Results of suchcal- dite,/grossulariteratios suggestsa causalrelationship culations (Newberry 1980, Einaudi et ql. l98l) betweenthe two ratios and merits further considera- demonstratethe contrastin oxidation statebetween tion. Clearly, if garnet compositionsare uniquely fix- skarnsthat lack and skarnsthat possessa subcalcic ed by a buffered /(O), then almandine-andradite garnet. Theseworkers suggestedthat tungstenskarns ratios should not show this strong inversecorrela- generally form at oxidation statesbelow or only tion. Other factors besidesoxidation statemust be slightly exceedingthe pyrrhotite - magnetite - pyrite important. buffer and that there is somecausal relationship be- 536 THE CANADIAN MINERALOGIST

mlne leYel

q.o o A-l OO o o o )l \f\o U o ooO.po ooooo ooo\\oo \-- oo \0Ooo oo o Ii:! ooooo o ooo o o 10o msters -\, ooo o r_J oo -l KEY:

. narlmuat alnaBdlno+ Poat- 8karn dlkes # apeaaartlne oonteot ot earnot *ffif,?|| quarrz at lhat Dolnt H'",0," ffi Q monronrrs

Ftc.6. Simplified geologicalcross-section through the Black Rock mine, showingdistribution of skarn and changein garnet compositionswithin the mine, Numbers indicate the maximum subcalciccomponent in garnet at various points, projectedinto the line of section.Compositional data presentedinclude values of cell dimensionand index of refrac- tion from Elliot (1971).Note systematicincrease in maximum subcalciccomponent of garnet toward the quartz monzonite contact. Late, high-angle faults (ess than 20 m displacements)have been deleted for clarity. tweenoxidation stateand skarn type. For example, data presentedby Liou (1973)indicates that over the biotite compositionsindicate that most plutons of temperature range of 750 to 600oc for the as- the easternand central SierraNevada cooled along semblageanorthite-spinel-garnet-quartz, the r atio a buffered path appoximately2-3 log units above of almandineto andradite componentin garnet is the Ni-NiO buffer @odgeet al. 1969,Dodee 1972). a function of /(O). The-restriction of subcalcic This log/(O) - T path correspondsto thelOt - garnet to oxidation statesat or.below Ni-NiO in T regime of moderately reduced tungsten skarns Liou's (1973)experiments, however, is not reflected (Newberry 1980)found in the central and eastern in natural assemblagesowing to two.differencesbe- SierraNevada of California. In the westernUfrited tweenthe experimentaland natural occurrences:(l) States,oxidized tungstenskarns (Fig. l) occur east variability in the calciumcontent of natural mineral of the deeper batholith terranes (Sierra Nevada, assemblagesand (2) the presenceof a substantial Aconchi, Idaho, Nelson); depth-of-formation manganesecomponent in the skarn-forming fluids. estimatesfor thesedeposits are generallyless than These effects are considered in the followine those to the west, and oxidation states were paragraphs. presumably higher (Newberry & Einaudi l98l). Thus, the mineralogyof a typical tungstenskarn in Calcium octivity and subcalcic garnet in skorns part simply reflects the imposed oxidation state. This evidencefrom natural assemblagesis cor- Subcalcicgarnet compositionsfrom moderately roborated by the experimentalstudies of Liou $nr, reduced skarns are characterized by variable who found that grandite garnet with a significant Fe+ /FE* ratios, independentof total iron content almandinecomponent could form only at an oxida- (Fie. 8). Garnet compositionsin theseskarns become tion state along or below the Ni-NiO buffer. more subcalcicwith proximity to the fluid source Graphical interpretation after Winchell (1958) of @igs.2, 6) and with time @igs.3, 4); subcalcicgarnet SUBCALCIC GARNET IN SCHEELITE.BEARING SKARNS 537 almandine+ spessartine FeaAlrSi"O.,, M%lbsieorz

strongly reduced A moderatelv reduced o . Garnol Dlke, Call{. Plne Greek, Ca[|. o a Hardpoint,Callf. Black Rock. Calll.A OTungslen Jim, ldaho .s Strawbsrry, Callf. o r Lened, NWT, Canada Hllton *?oo o Creek, Callf@ Tulare Callf. oAa Co. * +Japtneso Alplno, CaH. O Tung!ten ekarng o (Shlmlzakl t e77) .t ^Ao o ? n, e ol o^ i' oO Xpo " & '+t+t A tr - lr.

Ao ^ A +' o o oaaD o - ^ o n^ oAo .{* oo$ -9qom d ' % o a zro '$j "&";(d4:D' ooQ o ^ S Y{t andradite mole percent ClFerSi.O*

FIc. 7' Compositions of all subcalcicgarnet specimensanalyzed for this study, expressedin terms of mole go grossularite, andradite, and almandine + spessartine.Filled symbols representgarnet from strongly reduced skarns, open circles represent garnet from moderately reduced skarns. Nole in this figure, and in Figures 8 and 9, the close similarity betweenstrongly reducedskarns of this study and the Japaneseskarns of Shimazaki (1977). is not found as a replacement of high-calcium temperature, calcium ion is representedby a calcium minerals. Thesefacts suggestthat a low calcium-ion hydroxide complex; dependingon fluid composition, activity is a critical prerequisiteto formation of sub- chloride, fluoride and carbonatecomplexes would calcic garnet in skarns. For moderately reduced presumablybe presentas well.) The equilibrium a skarns,formation of subcalcicgarnet from grandite constant for the reaction, assuming skarn can be approximatedby a reaction that con- a(H2O) : a(SiOJ = 1, is: servesiron and aluminum: 3Ca3Fe2Si3O12+ 2CarAlrSirO,, + l5H2O+ o$:Eto"no U" K*= (2) in garnet in garnet aua3ar? 2Fe3Al2Si3O12+ /"O2+9SiO2 in garnet q:uaftz If oxygen fugacity, temperatureand pressureare ap- + l5Ca(oH)2(aq) (t) proximately constant, as might be expectedduring the formation of a single zoned grain of subcalcic (owing to complexation of aqueous ions at high garnet(e.g., Fig. 4), then the activity of the calcium 538 THE CANADIAN MINERALOGIST

o oo N EI -q oEo o oo o o€8oo o lr ooo o g o Eq A o 6 I ,,, o i"-\

to 20 mole percent almandlne compon€nt

FIc.8. Almatrdine content versus total moles iron per 12 oxygen atoms in analyzed subcalcicgarnet. Garnet compositions from strongly reduced skarns show an approximately linear relation betweentotal iron and almandine content, garnet com- positio$ from moderately reducedskarns show approximately constant total iron content, independentof almandine content. Dashedline through points from Japanesetungsten skarns describesthe equation total moles iron : 7 mole qo Ad + mole 9o Alm.

ion in solution, as expressedby aca(o11,,ought betweenthe calculatedvalues of Cresseyet al. (1978) to be directly proportional toi aad3ao2 / d*,2 and and thoseof Saxena(1973) yields aclivity coefficient designatedR. The calculation of the (uantity R is ver,rel,rmole-fraction relations that can be approx- complicatedby the fact that someof the components imated as simple functions. Theseare: have strongly nonideal mixing properties(Ganguly 1976)and thus, non-unit activity coefficient$(Ganexr- 'ys= ^tad: I +3(l - X**aJ for I (Xs*ot">0'4, ly & Kennedy 1974).Activity coefficients for the zL : :-s(t-Xo-o,J for I (Xu.-oir. ) 0.55' and almandine-grossularsystem at 850'C vary from 0.8 ?a- = 0.75 for 0.55( Xs-6x" ) 0.3. to 1.8 (Cresseyet al.1978) and are not known below 850oC,although calculationsby Saxena(1973) sug- The relationship betweengarnet composition and the gest that they may be as high as 3 or 4 at 400"C. activity of a garnet end-memberis modeled by a Use of regular-solution models for derivation of multisite mixing model (Wood & Nicholls 1978): pyrope-grossularsolid solutions(Newton 1977)has resultedin highly divergentestimates of interaction €.8., Qotm: Xp"z+3X6,l+2 ?a*, thus parameters;thus a rigorous derivation of activity coefficients is not employed here. Within the ac- Xs;2+r5XpS+u ^yudt ^l"r' curacyof the calculatedvalues, activity coefficients D- _ (3) can be approximatedas simple functions of the ap- Xp&*6^ra^2 plicable mole fractions. Because almandine and spessartine mix in a Values ofR calculated from equation 3 vary from relatively ideal manner (Ganguly& Kennedy1974), 105to 1.8 x l0-8 within a singlezoned grain of as do grossulariteand andradite (Ganguly 1976), ac- garnet(e.g., from the Black Rock mine). If calcium- tivity coefficients can be formulated in terms of the ion activity is approximately constant, then /(Oz) grandite and pyralspite components.Interpolation must vary at a single spot by approximately 6 orders SUBCALCIC GARNET IN SCHEELITE-BEARING SKARNS 539

of magnitude to account for the observedalman- dine,/andraditeratios. Suchvariation is exceedingly unlikely. The observedvariation in R, however, can be completelyaccounted for by variationsin the activity of calcium hydroxide by a factor of only 4.4 (equation2). Suchvariations, at constantfO) and T, are feasibleonce high-calcium minerals have been completely replacedor dissolved. Calculationsby Bird & Helgeson(1981), for example,indicate thar granditeis stableover a S-fold rangein calcium-ion activity at 400'C and I kbar. 2.6 In strongly reduced skarns, subcalcicgarnet a quartz appearsas a replacementof hedenbergitic plroxene t grossulariticgarnet + plagioclase.Com- positional relationshipsindicate that andraditecon- tents are roughly constantand independentof almandine content (Fig. 8). The reaction can be modeledas:

3 CaFeSirO6 + Ca3Al2Si3Op + 6 HrO + in pyroxene in garnet Fe3AlrSirO,, + 6 SiO2 + 6 Ca(oH), (4) in garnet q\afiz for which, assuminga(HrO) : a(SiOJ = l,

Qarn Qcu(og)r6 K".:----;_=--: (5) ia .o- q ohd'as, _a

Reductionof ferric iron is not requiredin this reaction, and it is thus independentof /(O). Calcula- tions using the previously described activity- coefficientsindicate that within a single deposit, a variation in aqueouscalcium-ion activity by a factor of 3 can account for the spectrum of molep€rcent almandlne/spessartlne grossular,/almandinevalues observed. Similar treat- ment of the data (1977) of Shimazaki indicatesthar Frc.9.Ratio of mole9o almandine to molel7o spessartine variationsin activity of the aqueouscalcium ion by versusratio of mole 9o andraditeto mole gogrossu- a factor of 2.2 could account for his observed ladtein analyzedspecimens of subcalcicgarnet. Note variations. sharp distinction betweenmoderately and strongly In summary,it is important to note that subcalcic reducedgroups and strong inverse correlation between garnet in tungstenskarns is not in equilibrium with the trilo ratios, a calcic clinopyroxene- grandite garnet - scheelirc assemblage,but rather replacesthat assemblage. When forming the assemblagepyroxene - garnet - tion assubcalcic garnet t quartz (Fig. l0), but pos- scheelite,skarn-forming fluids replacingmarble have sessingdominantly ferric, rather than ferrous,iron, sufficientlyhigh valuesof activity ofthe calciumion to prevent formation of a subcalcicgarnet. Con- Manganese activity and subcolcic garnet in skorns tinued addition of a high-silica, low-calcium metasomaticfluid to a garnet- pyroxene- scheelite Manganesealso plays a part in the observed skarn resultsin the depletionof calciumin the fluid almandine,/andraditerelations, as previously sug- and the concomitantdeposition of subcalcicgarnet gested(Fig. 9). Moderatelyreduced tungsten skarns + quartz @ig. 10).Further depletionof calciumion are characterized by a high spessartine,/almandine from the metasomaticfluid resultsin the formation ratio, always greater than 1:l and typically 2:1, of increasinglysubcalcic garnet (Fig. l0). This same whereas strongly reduced skarns are characterized trend of decreasingcalcium activity toward the fluid by a low ratio, lessthan 1:l and typically l:2. The sourcein oxidizedskarns results in the replacement ratio of spessartineto almandine also increaseswith of andraditic garnet by epidote + quaftz, an as- increasingandradite content (Fig. 9). Manganese,of semblagewith approximatelythe samebulk composi- intermediatesize between ferrous iron and calcium, 540 THE CANADIAN MINERALOGIST

GaO

FeOr.U

al!|trnd ln.-apca!altltt'Fr!Y-' !!!E FeO+MgOlMnO Frc. 10, Compositional triangle showing generalizedcompositions of mineral zones in tungsten skarns in terms of mole tlo CaO, SiOr, and FeOr.5+ Alol.i + FeO + MgO + MnO. Numbersrepresent distinct mineral zonesfrom marble (1) toward intrusiveiontact (3). Primed numbersrefer to oxidizedskarns, unprimed to reducedskarns. Zoning for both skarn types is from CaO-rich to increasingly CaO-poor assemblagesmoving away from the marble contact. appears to play a role in the stabilization of scheelitedeposition in skarnshas been suggested by andradirc-almandine combinations. Studies of severalworkers, e.9., Einaudi (1977)and Shimazaki garnet-stabilityrelations (Novak & Gibbs 1971)in- (1977).ln zonesof reducedcalcium-ion activity, this dicatethat increasingthe averagecation-size in the low oxidation state is reflected by the presenceof site occupiedby trivalent cations(i.e., increasingthe subcalcicgarnet. In fact, the bulk of the world's Fe3+to AF+ ratio) resultsin increasingdestabiliza- scheeliteresources are found in subcalcic-garnet- tion of small cationsin the divalentcation sites(i.e., bearing skarns (Fig. ll). In addition, of the seven decreasesthe stability of Fd+in the garnet). Thus, largestknown scheelite-bearingskarns (> 6 million in the andradite-richgarnets of moderatelyreduced tons of ore), only one, the King Island deposit (Kwak skarns, higher manganesecontents are apparentlyre- & Tan l98l), does not contain subcalcicgarnet. quired to stabilize a given amount of ferrous iron Becauseof the retrograde and pressure-dependent in the garnet, and increasingthe andradite/grossular solubility behavior of scheelite,tungsten skarns ratio in garnet increases the minimum spess€u- generally form at higher pressuresand temperatures tine,/almandineratio required for stability. Strong- than those that characterizecopper and base-melal ly reducedtungsten skarns, characterized by garnet skarns (Newberry & Einaudi l98l). Becausethe compositionswith a low andraditecontent, pos$ess oxidation state during skarn formation is in part highly variable almandine/spessartineratios that are relatedto depth (Shimazaki1980), deeper scheelite- independentof andradite content (Fig. 9). Thesedata bearing skarns should be characterizedby more and those of Shimazaki(1977) suggest that alman- reduced mineral-assemblages,including subcalcic dine and spessartinecontents can vary considerably garnet. Except where hematitic host-rocks in garnet from skarnswith lessthan 15mole 9o an- predominate or at depths transitional between dradite. The hyperbolic form of Figure I I is thus ex- scheeliteand basemetal skarns [e.9., Lost Creek: plained by a co-operativesubstitution mechanismin Collins (1977), Tungsten Hills: Newberry (1980)1, subcalcic garnet. At moderately reduced oxidation tungstenskarns were apparentlybuffered to oxida- states, a higher spessartine/almandineratio is re- tion statesthat would permit the formation of sub- quired as the andradite/grossularratio rises,whereas calcic garngt. Thus, although scheeliteskarns lack- at strongly reducedoxidation states,there is a limited ing a subcalcicgarnet do occur, they are lesscom- amount ofthe andraditecomponent that can be ac- mon and account for only a small fraction of known commodatedin a subcalcicgarnet. tungstenreserves. Oxidized and reducedtungsten skarns also con- Subcqlcic garnet and scheelite deposits trast in ore grades. Although grades are a function of tonnage, mining method, geographiclocation, ex- The relationshipbetween low oxidation stateand tent of retrograde196elilization of tungsten,host SUBCALCIC GARNET IN SCHEELITE-BEARING SKARNS 541 rocks and other variables, the reduced (subcalcic garnet-bearing)skarns are characterized,in general, c by higher tungstengrades (Fig. 12). This contrast can 3sql qt be found both between oxidized and reduced o!- =o tungsten-skarndistricts and also within districts that oo OE contain both types.In the Bishopdistrict of Califor- g5 nia, for example, the oxidized andradite - quartz - o epidote skarns of the Tungsten Hills area (Newberry o grades o 1980) contain of 0.2 to 0.5 Vo WO3 l! @atemanet ol. 1950),whereas t}te moderatelyreduc- t x ed subcalcic-garnet-bearingPine Creekand Adam- o 9 deposlts son mines are characterizedby averageskarn grades of 0.66to l.l9o WO, @ateman1945). Similarly, in 3 the Mount Morrison quadrangle,California, skarns ott :o adjacentto the Round Valley Peak granodioriteare :.o 6e in both highly eraphitic and nongraphitic marbles. ttE Reducedskarns, such as the Hardpoint and Pappas oo I I deposltg >L deposits, located in the highly graphitic Bloody Mountain Formation, are characterizedby assem- blagesdominated by hedenbergiticpyroxene, possesssubcalcic garnet compositions (Newberry 1980) Eg and contain tungstengrades in excessof l9o WO3 6g (Rinehart & Ross1964). Oxidized skarns, such as the o.25 0.50 0.7s Scheeloremine, developedin the noncarbonaceous Mount Baldwin marble, are characterizedby epidote Mllllon tons WOs - qtJarlz- andraditic grandite t pyrite assemblages, garnet do not contain subcalcic compositions Frc. ll. Approximateknown tonnages(production r- (Morgan 1975)and havetungsten grades of 0.25 to reserves,expressed in tonscontained WO3) of the three 0.490 WO3 (Rinehart & Ross 1964). classesof tungstenskarns discussed in this paper. Ton- nagedata from Einaudi el a/.(1981). Note that the bulk The relationship betweenskarn type and tungsten of knownWO3 tonnage is from reduced(subcalcic- grademight be explainedin terms of different P, T garnet-bearing)skarns. or X(CO) of formation; however, differencesin gradewithin skarnsadjacent to the samepluton are su'ggestiveof differencesdue to local stateof oxida- not possesssubcalcic garnet, Garnet compositions, tion. A proposedexplanation is basedon the obser- however, are far more variable and andradite-rich vation that oxidizedand reducedskarns are charac- than indicatedby previousstudies. In fact, the sub- terized by contrasting garnet/pyroxene ratios (Iable calcic garnet of skarns can simultaneouslycontain 2). In particular, abundant pyroxenecharacterizes more than 30 mole 9o andradite and 45 mole Vo reduced skarns, and abundant garngt characterizes almandine + spessartinecomponent, and a garnet oxidized skarns et al. l98l). If a skarn- @inaudi with as much as 65 mole 9o andraditemay contain forming systemdeposits hedenbergitic pyroxene [ow- an appreciablesubcalcic component. The occurrence ing in part to lower that contains only 25 /(Orl of suchgarnet compositionsrequires (l) a moderately mole VoCaO, then more calciumis availablein the reducedenvironment of formation, approximately fluid to deposit scheelitethan if the fluid deposits at or below the pyrrhotite - magnetite- pyrite buf- andraditicgarnet, which contains37.5 mole 9o CaO. fer, and (2) a relatively low activity of the calcium In other words, in skarnsformed from relatively pure ion in solution. Exceedinglyvariable almandine/an- marble, the higher the pyroxene/garnetratio, the dradite ratios occur in garnet compositionsfrom a higher the averagetungsten grade. given skarn, in part owing to variableactivity of the calcium ion and also to variable activity of the manganeseion. The presenceof manganeseis ap- CoNcLUSIoNS parently required to stabilize the simultaneous presenceof appreciablealmandine and andradite Garnet compositionsintermediate between gran- components.In general, higher relative andradite dite and pyralspiteare a characteristicfeature of most contents in garnet require higher spessartine/alman- tungsten-bearingskarns of westernNorth America. dine ratios for stability. Only thosetungsten skarns formed from hematite- Tungsten-bearingskarns typically possesssub- bearing host rocks or at relatively shallow depths do calcic garnet becausethey are characterizedby 542 THE CANADIANMINERALOGIST

a.Costabonne,Frarlce 0 - gcheeloro,CA o l! BrownbLake, Mont. I Tungstsn Hllls, GA x . o Osgood Mntns.,NV Mlll Ctty, NV Klng lsland, Tasmanla lt o o = Btack Rock, o CA o -El Ja.alllo, Merlco E Hllton Creek, CA o Plne Creek, CA qt Strawberry, CA o alrbanksdlotr.. AK It o Sang Dong, Korea

tt (,o J MacMlllanPass, Y-NWT. tt GarnetDlke, CA o tr Enerald, BC i Tungsten Jlm, lD - o tr Salau,France . o CanadaTungsten, NWT o o.5 i:o i.b 2'.o 2'.a ReportedAverage Skarn Grade (% WOs)

Flc. l2-, Reportedaverage tungsten grades from skarn ore yersasskarn type. Note higher averagegrades associated with reducedrather than with oxidizid skarns. Data from: Einaudi ef a/. (1981),Byers (1952), Bateman (1945), Rineharr & Ross (1964),Burchard (1977),Krauskopf (1953)and peabody (1979). moderateto low oxidation states,owing to wallrocks, developmentof this research.Early versionsof this an intrusion-bufferedoxidation stateand a depth of manuscript were reviewedby J.M. Schmidt, D.C. formation distinctly different from other skarn types. Dobson, M.T. Einaudi and J. Trammell. The Becausesuch skarns possessmoderate to high py- manuscript has also benefitted frorn extensive roxene,/garnetratios, the presenceof subcalcicgarnet reviews by R.F. Martin, L.T. Trembath and two in a skarn is also linked to higher averagetungsten anonymousreviewers. Financial support from NSF grades. Thus, the presenceof subcalcicgarnet in grant EAR 78-19588and from a granr by the Union skarnsis not only of petrologicaland mineralogical Carbide Corporation is gratefully acknowledged. interest,but also constitutesa potentially useful in- dicator of tungsten reserves. I{EFERENCES AcKNowLEDGEMENTS At-ses, A.L. & Rav, L. (1970): Correction factors for The author is grateful to the Union Carbide Corp., electron-probe microanalysis of silicates, oxides, Union CarbideCanada, Canada Tunsten, Teledyne carbonates,phosphates, and sulfates. Anal. Chem. Tungsten,and the Anaconda Company for permis- 42, 1408-7414. sion to map and sampletheir deposits.Microprobe BarsuaN, P.C. (1945): Pine Creek and Adamson analy$eswere performed at Queen,sUniversity with tungsten mines, Inyo County, California. Colif, J. the kind assistanceof L.A. Dick and at Universitv Mines Geol, 41, 231-249. of California, Berkeley,with the assistance of M.L. Emcrson, M.P. & Pnoc.ron, P.D. (1950): Rivers. Someof the analysesat Berkeleywere per- Geology and tungsten deposits of the Tungsten Hills, formedby D.C. Dobson.Discussions with Drs. M.T. Inyo County, California. Calif. J. Mines Geol.46, Einaudi, L.A. Dick and L.D. Meinert assistedin the 23-42. SUBCALCIC GARNET IN SCHEELITE-BEARING SKARNS 543

BrNce, A.E. & At-snr, A.L. (1968): Empirical correc- Eowenos,A.G,, Barnn, G. & Canow, K.J. (1956): tion factors for the electron microanalysis of silicates Metamorphism and metasomatism at King Island and oxides. J. Geol. 76,382-403. scheelitemine. ./. Geol. Soc.Aust. 3.55-100.

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Buncnanp, U. (1977): Genesis of the King Island & KnNr.nov, G.C. (1974): The energetics of (Tasmania) scheelite mine. .In Time- and Strata- natural garnet solid solution. I. Mixing of the Bound Ore Deposits (D.D. Klemm & H.J. alumino-silicateend-members. Contr. Mineral. Schneider, eds.). Springer-Verlag, New York. Petrology48, 137-148. Guv, B. (1980):Etude gdologiqueet pdtrologiquedu Bysns, F.M., Jn. (1957): Tungsten depositsin the Fair- gisementde Costabonne.Bur. Rech.Gdol. Minidres, banks districr, Alaska. U,S, Geol. Sum. Bull. l0?A-1. Mdm. 99,237-250. Corurs, B.I. (1977): Formation of scheelite-bearing Kneusxopr,K.B. (1953):Tunsgten deposits of Madera, and scheelite-barrenskarns at Lost.Creek, Pioneer Fresno,and Tulare counties,California. Cald. Dep. Mountains, Montana. Econ. Geol. 72, 1505-1523. Nat. ResourcesSpec. Rep.35. Cnsssev, G., Scxyro, R. & Woop, B.J. (1978): Kwer, T.A.P. & TnN,T.H. (1981):The geochemistry Thermodynamic properties of almandine-grossular of zoning in skarn mineralsat the King Island garnet solid solutions. Contr. Mineral. Petrologt 6f1, (Dolphin) mine. Econ. Geol. 16, 468-497. 397-404. Lrp, D.E. (1962):Grossularite-spessartite garnet from Drcr, L.A. (1976): Mptamorphism and Metasomatism the Victory mine, Gabbs,Nevada. Amer. Minerol. at the MacMillan pxs Tungsten Deposit, Yukon and 47. t4't-15t. District ol Ma$enzie, Canada. M.Sc. thesis, Lrou, J.G. (1973):Synthesis and stabilityrelations of Queen's Univ., Kingston, Ont. epidote, Ca2Al2FeSi3Orz(OH).J. Petrology 14, 381-413. (1980): z4 Comparative Study of the Geology, Mineralogy, ond Conditions of Formotion of Con- Moncer, B.A. (1975):Mineralogy and origin of skarns tact Metasomatic Mineral Deposits in the Northeast in the Mount Morrison pendant,Sierra Nevada, Canadian Cordillera. Ph.D. thesis, Queen's Univ., California. Amer, J. Sci.275, ll9-t42. Kingston, Ont. Nir"rnc,D. (1967\:The miscibility of the pyralspite DonsoN, D.C. (1982): Geology and alteration of the and granditemolecules in garnets.Mineral. Mag, Lost River tin-tungsten-fluorine deposit, Alaska. 36,389-402. Econ. Geol. 77, 1033-1052. Nrwspnnv,R.J. (1980):The Geology and Chemistryof in the (1972): Skarn Formation and TungstenDeposition Doocr, F.C.W. Yariation of ferrous-ferric California. Ph.D. thesis' Nevada batholith, U.S.A. Central SierroNevada, ratios in the central Sierra Stanford,Ca. Int. Geol. Congr. (24th) 10, 12-19. StanfordUniv., & Enieuot, M.T. (1981): Tectonic and Srvurn,V.C. & Mrvs,'R.E. (1969): Biotites -, geochemical setting of tungsten skarn deposits in the from granitic rocks of the central Sierra Nevada Cordillera. Ariz. Geol. Soc. Digest 14,99-112. batholith, California. J, Petrology 10, 250-271. Nnwros, R.C. (1977): Thermochemistry of garnet and Dur.n, D.P. (1980):, Petrology oJ the San Antonio aluminous pyroxenes in the CMAS system. .fn Scheelite Skarn, Baviacora, Sonora, Mexico. M.S. Thermodynamics in Geology (D.G. Fraser, ed.). D. thesis, Arizona State Univ., Tempe, Az. Reidel, Boston. Easrvrax, H.S. (1979): Skqrn Genesisand Sphalerite- Normsrnc, W.J. (1981): Geologic setting, petrology, Pyrrhotite-Pyrite Relationships in the Darwin Mine, and geochemistryof zoned tungsten-bearingskarns Inyo County Califurnia. Ph.D. thesis, Stanford at the Strawberry mine, central Sierra Nevada, Univ., Stanford, Ca. California. Econ. Geol. 76. lll-133. s44 THE CANADIAN MINERALOCIST

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