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 oxida- tion states approximately at or below pyrite- magnetite- INTRoDUcTION pyrrhotite, changesin activity of calcium hydroxide in solu- tion 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 ef- fects 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.